catalytic gas-phase synthesis and oxygen electrocatalysis of multi

Catalytic Gas-Phase Synthesis and Oxygen Electrocatalysis of Multi-Walled Carbon Nanotubes and Nitrogen-Doped

Carbon Nanotubes

DISSERTATION

Submitted to the Graduate School of Chemistry and Biochemi-

stry in the fulfillment of the requirements for the degree of

''Doktors der Naturwissenschaften'' (Ph.D.)

by

Kunpeng Xie

from Fujian, China

Laboratory of Industrial Chemistry Faculty of Chemistry and Biochemistry

Ruhr-University Bochum

Bochum, November 2014

The following research work has been carried out during the period from July 2011 to

July 2014 in the Laboratory of Industrial Chemistry, Ruhr-University Bochum, Germa-

ny.

Date of submission: 19.11.2014

Date of Ph.D. defense: 12.12.2014

Chair of the examination board: Prof. Dr. Roland Fischer

First examiner: Prof. Dr. Martin Muhler

Second examiner: Prof. Dr. Wolfgang Schuhmann

Acknowledgement

I would like to express my deepest acknowledgement to my supervisor Prof. Dr. Mar-

tin Muhler for giving me the opportunity to work in this fascinating research area, leav-

ing me the freedom to approach the numerous questions open-mindedly, and trusting in

my research and its results. With his outstanding expertise and intuition he always gave

me valuable guidance on my research and motivated me to think independently as a re-

searcher. I sincerely thank my co-supervisor Prof. Dr. Wolfgang Schuhmann for allowing

me to do the electrochemical measurements in his group, giving me suggestions during

our scientific meetings, and offering me valuable guidance on writing this dissertation. I

would like to express my deepest gratitude to my group leader Dr. Wei Xia who has con-

tinuously guided and supported me throughout my Ph.D. career. He introduced me to

various interdisciplinary projects as part of my Ph.D. and gave me useful and critical

suggestions whenever I approached him. I would also like to thank Prof. Dr. Wolfgang

Grünert and Prof. Dr. Michael Wark for scientific discussions and suggestions. I appre-

ciate the friendly support from Mrs. Sigrid Kalender and Mrs. Gundula Talbot.

I want to thank all my past and present group members of LTC. I am deeply grateful

to Dr. Zhenyu Sun for sharing his scientific experience to me, to Dr. Michael Becker, Dr.

Peirong Chen, and Dr. Anqi Zhao for their experimental guides, to Dr. Stephan Kolf for

his help and encouragement at the beginning of my Ph.D. study. I would also thank Dr.

Ankur Bordoloi, Dr. Holger Ruland, Dr. Bastian Mei, Dr. Ilja Sinev, Dr. Marie Holz, Dr.

Kevin Kähler, Dr. Stefan Kaluza, Dr. Dennis Grossmann, Mr. Hendrik Düdder, Mr. Jo-

han Anton, Mr, Ahmet Becerikli, Mr. Munir Chaar, and Ms. Larissa Schwertmann for

their gainful discussions and suggestions. My special thanks are credited to those who I

have worked with and gained helps from during my Ph.D not only at work but also in

life: Dr. Ly May Chew, Mr. Thomas Emmerich, Dr. Mariam Salazar, Ms. Anna Pougin,

Mr. Arne Dittmer, Ms. Rosemary Puls, Dr. Peirong Chen, Dr. Freddy Oropeza, Ms. Wei-

wen Dong, Mr. Fengkai Yang, Dr. Sheng Chu, Dr. Ping Wang, Ms. Haoyu Shi and Mr.

Huiqing Song.

I appreciate the involvement and contribution of the colleagues who have accom-

plished most of the measurements of the characterization: Mrs. Sigrid Plischke (TG),

Mrs. Noushin Arshadi (BET and TPR), Mr. Philipp Weide (XPS), Mrs. Susanne Buse

(BET) and Mr. Fengkai Yang (SEM). I thank Dr. Volker Hagen, Mr. Brune Otto, Mr.

Horst Otto, Mr. Heinz Pfeiffer, Mrs. Dagmar Scholz for technical support.

I want to thank the people outside LTC: Mrs Kirsten Keppler, Mrs Angelika Mücke

and Mrs. Ute Gundert for experimental analysis, Dr. Thomas Reinecke for XRD mea-

surement, discussion and helps in data analysis, Dr. Rolf Neuser, Mr. Matthias Born

and Mrs. Sandra Schmidt for providing SEM measurements.

I also appreciate the help and support from ELAN group. I deeply thank Dr. Justus

Masa, Dr. Xingxing Chen and Dr. Artjom Maljusch for scientific discussion on electro-

chemical measurements, Dr. Thomas Erichsen for technical support.

I acknowledge the International Max-Planck Research School for Surface and Inter-

face Engineering in Advanced Materials (IMPRS-SurMat) for a research grant. I am

grateful to Dr. Rebekka Loschen, Dr. Elke Gattermann, and Dr. Andreas Erbe for orga-

nizing the various courses and annual seminars which were quite helpful to my Ph.D.

study. Thanks to SurMat again for giving me the opportunity to know all the other Sur-

Mat students who have shared joyful moments in the T-courses and SurMat seminars. I

also want to thank Dr. Andreas Erbe and Mrs. Petra Ebbinghaus for giving me the

chance to learn and accomplish Raman measurements at MPIE.

I also acknowledge the German Federal Ministry of Education and Research (BMBF)

for the projects: CarboScale (grant No. 03X0040G) and CarboElch (03X0207C) within the

scope of the Inno.CNT alliance, and project SusHy (03X3581D).

During my Ph.D. career, I have made many friends, both related and unrelated to

work. I am deeply grateful to all my friends who have shared my joy, frustration, setback

and breakthrough. I will always cherish the special moments that we spent with.

Last but not the least, I am deeply grateful for my parents, sister and my dear wife

who have fuelled me with their unconditional love, care and encouragement and gave me

strength that help me to achieve where I am today.

Abstract

Driven by worldwide commercial interest from various fields, the carbon nanotube

(CNT) industry has been growing rapidly. Several crucial issues regarding the large-

scale production and application remain the great challenges. The aim of this disserta-

tion is to understand the growth kinetics of CNTs, to correlate the synthesis parameters

and the structure and properties of nitrogen-doped carbon nanotubes (NCNTs), and to

use the residual growth catalysts as electrocatalysts for the oxygen reduction and evolu-

tion reactions.

The large-scale production of CNTs suffers from low yield due to the fast catalyst

deactivation caused by the deposition of amorphous carbon on active sites. Water and

ammonia were investigated with respect to their influence on the CNT growth kinetics

at the initial stage. The initial growth rate of CNTs and the mean lifetime of the active

sites, i.e., two essential parameters in evaluating the growth behaviour of CNTs were

derived. The results demonstrate that CNT growth can be either promoted or inhibited

by H2O and NH3 depending on their concentrations, as well as the applied growth tem-

peratures. The optimal conditions for CNT growth in the scope of this study were deter-

mined to be 650 °C at 200 ppm H2O or 1000 ppm NH3 in a mixture of C2H4 and H2 with

the volumetric ratio of 57 : 43.

The influence of synthesis parameters on the morphology, yield, composition, graphi-

tization and oxidation resistance of NCNTs were systematically investigated. In addition

to the synthesis parameters like growth time, precursor concentration and growth tem-

perature, the surface etching through the carbon gasification showed significant impact

on the structure and property of NCNTs. The optimal conditions for NCNT growth in the

scope of this study were determined to be 120 min, 5.9 vol.% ethylenediamine in a He

flow with the total flow rate of 100 sccm and 650 °C.

With respect to the utilization of residual growth catalysts in CNTs, HNO3 vapor oxi-

dation was employed for the synthesis of MnO2-OCNTs hybrids containing Co3O4 nano-

particles. Electrocatalytic studies show that MnO2 and Co3O4 nanoparticles act as active

sites for the oxygen evolution and reduction reactions. Oxygen functional groups on CNT

surface favor the release of generated oxygen bubbles. The excellent OER and ORR ac-

tivities of obtained catalysts can be assigned to the strong coupling between metal oxides

and carbon nanotubes.

Table of Content

Acknowledgement ........................................................................................................... III Abstract ............................................................................................................................. V

Table of Content ............................................................................................................. VII Chapter 1 Introduction ................................................................................................... 1 Chapter 2 Literature review ........................................................................................... 5

2.1 Progress in the synthesis of CNTs ................................................................... 5 2.1.1 Synthesis of CNTs ..................................................................................... 5 2.1.2 Water-assisted CVD synthesis of CNTs ................................................. 13 2.1.3 Ammonia in the synthesis of CNTs ........................................................ 17

2.2 Progress in the synthesis of NCNTs .............................................................. 19 2.2.1 Structure and properties of NCNTs ....................................................... 19 2.2.2 Synthesis of NCNTs ................................................................................ 23

2.3 Progress in oxygen electrocatalysis ................................................................ 25 2.3.1 Reaction mechanism ................................................................................ 25 2.3.2 O2 electrocatalysts ................................................................................... 26 2.3.3 CNTs and NCNTs in O2 electrocatalysis ................................................ 29

Chapter 3 Experimental ............................................................................................... 33 3.1 Synthesis .......................................................................................................... 33

3.1.1 Assisted growth of CNTs ......................................................................... 33 3.1.2 Synthesis of NCNTs ................................................................................ 36 3.1.3 Synthesis of metal oxide-CNTs hybrids ................................................. 39

3.2 Characterization .............................................................................................. 41 3.3 Electrochemical tests ...................................................................................... 43

Chapter 4 Synthesis of CNTs and NCNTs ................................................................... 45 4.1 Growth kinetics of CNTs ................................................................................. 45

4.1.1 Introduction ............................................................................................. 45 4.1.2 Evaluation of kinetic data ....................................................................... 47 4.1.3 Results and discussion ............................................................................ 48 4.1.4 Summary .................................................................................................. 58

4.2 Synthesis and characterization of NCNTs ..................................................... 59 4.2.1 Introduction ............................................................................................. 59 4.2.2 Results and discussion ............................................................................ 60

VIII TABLE OF CONTENTS

4.2.3 Summary .................................................................................................. 81 Chapter 5 CNTs with residual metals as bifunctional electrocatalysts ..................... 83

5.1 Introduction ..................................................................................................... 83 5.2 Results and discussion .................................................................................... 84

5.2.1 OER .......................................................................................................... 84 5.2.2 ORR ........................................................................................................ 103

5.3 Summary ....................................................................................................... 108 Chapter 6 Conclusions and outlook ............................................................................ 109 Bibliography ................................................................................................................... 113 Appendix ......................................................................................................................... 125

Abbreviations .......................................................................................................... 125 List of Figures ......................................................................................................... 126 List of Tables ........................................................................................................... 130 Publications ............................................................................................................. 131 Presentations and conference contributions ......................................................... 132

Chapter 1 Introduction

Field of interest

The energy crisis caused by limited availability of fossil fuels has boosted the global

demand for renewable energy in the past few decades. One of the challenges in the de-

velopment of renewable energy is the fluctuation of the electric power generated from

renewable energy which is strongly dependent on weather conditions. Electrochemical

energy conversion and storage is considered as one of the most promising technologies to

resolve the aforementioned problem.

Water electrolyzers and fuel cells are among the most promising techniques in elec-

trochemical energy conversion and storage, in which hydrogen can be produced by water

electrolysis and further utilized in fuel cells to generate electricity and water. As a con-

sequence, an energy cycle can be established. The sluggish kinetics and large overpoten-

tials in the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the

bottlenecks in the development of these techniques. The state-of-the-art catalysts for

ORR and OER are Pt-based and Ru/Ir-based noble metal catalysts, respectively.[1-3]

However, the high costs and low availabilities of these precious metals inhibit the devel-

opment and commercialization of fuel cells and electrolyzers. To decrease the depen-

dence on these precious metal catalysts, it is essential to develop alternative catalysts

that are less expensive, abundant, comparably active and durable. Non-precious metal

oxides, such as MnOx, Co3O4, as well as Mn- and Co-based mixed oxides have proved to

be promising alternatives to noble metal catalysts.[4-10] However, the poor electrical con-

ductivity of these oxides restrains their large scale applications in electrocatalysis.

Carbon materials, especially carbon nanotubes (CNTs) and nitrogen-doped carbon

nanotubes (NCNTs), have been extensively studied as catalyst supports and electrocata-

lysts for both ORR and OER due to their outstanding electrical conductivity, high sur-

face area and high chemical and thermal stabilities.[11-13] The superior performance in

these studies have attracted more research interests in CNTs and NCNTs, which, as a

result, accelerated the development for large-scale production of CNTs and NCNTs.[14]

2 CHAPTER 1 INTRODUCTION

Problem statement

Since the pioneering research of CNTs in the early 1990s,[15-17] significant progress

has been made in their synthesis. Considering the wide range applications and the in-

creasing demands of CNTs in various fields, the large-scale production of CNTs is still a

great challenge. The deposition of amorphous carbon on the active sites of catalysts often

results in fast deactivation of the catalyst, thus leading to a low CNT yield. Water-

assisted chemical vapor deposition (CVD)[18] has proved to be an effective method for the

growth of CNTs at high rate due to the reactivation of the active sites by etching the de-

posited amorphous carbon ( ).[18-19] It is known that ammonia can be

used not only in the pretreatment of the catalysts but also in the CNT growth stage.

Most of the studies only examined the influence of NH3 on CNT morphology, while few

studies analyzed the effect of NH3 on the growth kinetics of CNTs. Therefore, a funda-

mental understanding on the roles of H2O and NH3 from a growth kinetic perspective of

view is still required. Recently, the CNT growth kinetics has been studied by using gas

chromatography to measure the consumption of the carbon source or by ex situ electron

microscopy to determine the height of CNT forests.[20-21] Unfortunately, these techniques

are insufficient for monitoring the initial stage of the CNT growth.

As to NCNTs, although much progress has been made in the synthesis by catalytic

CVD,[22-26] important questions remain in the growth kinetics and mechanism. A com-

prehensive study is still needed for understanding the influence of synthesis parameters

such as growth time, precursor concentration and growth temperature on the yield, com-

position, morphology, graphitization and thermal stability of NCNTs.

Although CNTs can be produced in the industrial-scale by CVD process,[14, 27] their

further applications may suffer from multi-step purification processes in order to remove

the carbonaceous impurities (e.g. amorphous carbon) and residual metal catalysts (e.g.

Fe, Ni and Co). Moreover, oxygen functionalization of carbon nanotubes by chemical oxi-

dation is an essential step in creating anchoring sites for the deposition of metal or metal

oxides. Our research group has been developing electrocatalysts based on CNTs for ORR

and OER.[28-36] Recently, Masa et al. demonstrated that trace metal residues promote the

activity of carbon catalysts for ORR.[37-38] Zhao et al. developed a novel electrocatalyst

based on spinel Mn-Co oxide in NCNTs for bifunctional oxygen electrocatalysis.[28] These

studies stimulated the idea of utilizing other than removing the residual metal catalysts

from the CNTs, for example, as bifunctional electrocatalysts. To date, there has not been

sufficient research in these fields.[28, 39]

3

Aims and scope

The aim of this dissertation is to understand the growth kinetics of CNTs, to correlate

the synthesis parameters and the structure and properties of NCNTs, and to use the re-

sidual growth catalysts as electrocatalysts for the oxygen reduction and evolution reac-

tions.

Co-MnOx is used as catalyst for the synthesis of CNTs and NCNTs in this study,

which is known to be highly active for the catalytic CVD growth of CNTs from

ethylene.[40-41] To gain reliable data for the growth kinetics of CNTs, a tubular fixed-bed

reactor under plug-flow conditions and fast on-line analysis are established. Ethylene-

diamine (EDA) is widely used as building block for large-scale production of industrial

chemicals. Considering the abundance of EDA and its compositional similarities to ethy-

lene, EDA is used as N and C precursors for NCNT synthesis by injection-CVD. To de-

sign the CNTs-based catalysts for bifunctional oxygen electrocatalysis, the catalyst

should fulfill the tasks of growing CNTs and as active sites for OER and OER simulta-

neously. Hence, a Co-Mn oxide is chosen for these purposes.

The key questions addressed in this dissertation are:

1. What is the influence of water vapor and ammonia on the growth kinetics of

CNTs? To what extent can the CNT yield be affected by H2O and NH3?

2. How are the yield, composition, morphology, graphitization and thermal stability

of NCNTs influenced by the synthesis parameters (i.e. growth time, precursor

concentration and growth temperature) in the injection-CVD? Is it possible to de-

velop a growth model to explain their correlations?

3. Is it possible to directly utilize the residual metal catalysts from the CNT synthe-

sis for bifunctional oxygen electrocatalysis? Are there any simple and efficient

methods that can be used for the modification of the catalyst to achieve a better

activity?

Significance of the study

The study of the influence of water and ammonia on the growth kinetics of CNTs may

give a guide for industrial production of CNTs with high yield from a kinetic perspective.

The study of the influence of synthesis parameters on the structure of NCNTs may add

new understanding on the controlled synthesis of NCNTs. Furthermore, the utilization

4 CHAPTER 1 INTRODUCTION

of residual metal catalysts for bifunctional oxygen electrocatalysis may provide a new

approach to design the oxygen electrodes.

Structure of the dissertation

This dissertation consists of five chapters in addition to this one. In Chapter 2, the lit-

erature related to the current study is comprehensively reviewed, including the synthe-

sis of CNTs and NCNTs, oxygen electrocatalysis, and the application of CNTs and

NCNTs in oxygen electrocatalysis. Chapter 3 provides general information on the mate-

rials and experimental methods, including descriptions of the experimental setups, syn-

thesis procedures of the catalysts, and the electrochemical tests. The results obtained in

the studies are divided into two parts (Chapter 4 and Chapter 5). Firstly, Chapter 4 pre-

sents the results of the synthesis of CNTs and NCNTs, in which the influence of water

and ammonia on the growth kinetics of CNTs and the effects of growth parameters on

the synthesis of NCNTs are examined, respectively. The possibilities to increase the

CNT yields by water-assistance and ammonia-assistance are demonstrated. Moreover,

an etching effect of nitrogen-containing species on the morphology and structural order-

ing of NCNTs is discussed. Secondly, Chapter 5 reports the electrocatalytic activities of

bifunctional oxygen electrocatalysis. The possibilities to utilize the residual catalysts as

bifunctional oxygen electrocatalysts are demonstrated. Finally, some concluding remarks

of the main results obtained within the scope of this dissertation and an outlook are

given in Chapter 6.

Chapter 2 Literature review

2.1 Progress in the synthesis of CNTs

In the last 30 years, carbon nanomaterials including fullerene, carbon nanotubes

(CNTs) and graphene have drawn tremendous attention in the scientific community. The

discovery of C60 fullerene and pioneering research of graphene has been awarded with

Nobel Prizes in 1985 and 2010, respectively.[42-43] CNTs, of which the impact is as high as

fullerene and graphene, have reached a more mature development and have achieved

large-scale production in industry. As for the history of CNTs, it can be dated back to

more than 100 years ago.[44] The first possible application of CVD for producing carbon

filaments by decomposing methane was reported by Hughes and Chambers in 1889.[44]

The first observation of the carbon filaments by electron microscopy was in the 1950s.

The research interest of the synthesis of carbon filaments or carbon fibers by CVD was

boosted in the early 1970s. Since the Nature publication by Iijima in 1991, the research

interest in CNTs has been growing rapidly.

2.1.1 Synthesis of CNTs

2.1.1.1 Structure of CNTs

CNTs are cylindrical structures of allotropic carbon. As schematically shown in Figure

2.1a, the structure of CNTs can be regarded as rolling of graphene sheets into concentric

cylinders terminated with fullerene-like caps with hexagonal and pentagonal rings.[45-46]

CNTs can be categorized as single-walled carbon nanotues (SWCNTs),[16, 47] double-

walled carbon nanotubes (DWCNTs),[48-49] and multi-walled carbon nanotubes

(MWCNTs),[15] depending on the number of rolled graphene sheets. When rolling a single

graphene sheet with varying degrees of twist along its length, SWCNTs have different

structures – armchair, zigzag and chiral structures – which possess different physical

and chemical properties.[45, 50] According to the synthesis conditions and catalysts, differ-

ent inner structures of MWCNTs can be obtained: hollow, bamboo-like and herringbone

(Figure 2.1b).[51] Due to the curvature of the graphene sheets in the specific cylindrical

6 CHAPTER 2 LITERATURE REVIEW

structure, this distance in MWCNTs is stretched to 3.40 Ǻ, which is 3.35 Ǻ in single

crystalline graphite.[52]

Figure 2.1: (a) Wrapping of graphene sheet to form SWNT. Adapted from ref. [46]. (b) A

schematic cross-section through a “hollow-tube”, “bamboo” and “herring-

bone” MWCNT. Reproduced from ref. [51].

2.1.1.2 Properties and applications of CNTs

Over the last decade, CNTs have been extensively studied in various fields.[53-55] For

example, CNTs are promising materials in electronic devices and polymer reinforcement

due to their outstanding electronic and mechanical properties.[56-57] Due to the structural

difference, SWCNTs and MWCNTs exhibit quite different properties. The variation of

the chiral structure of SWCNTs leads to special optical and electronic properties. The

SWCNTs can be metallic or semiconducting depending on their chiral angle and tube

diameter. All armchair SWCNTs are expected to be metallic, around 30 % of zigzag and

chiral SWCNTs are metallic, and the rest are semiconducting.[50] The properties of indi-

vidual CNT and bundling CNTs are different as well. Most of the bonds in CNTs are co-

valent sp2, making CNTs the strongest and stiffest materials yet discovered in terms of

tensile strength and elastic modulus. For the individual CNT, the elastic and shear

moduli were found to be as high as 1 TPa (around 5 times higher than steel) and 100

GPa, respectively.[58-61] However, the strengths of MWCNTs and CNT bundles are down

to a few GPa because of the weak shear interaction between adjacent shells and tubes.[62]

In recent years, CNTs have been increasingly used as catalyst supports or as catalysts

in heterogeneous catalysis[63-64] and electrochemical energy conversion and storage[34, 65-66]

owing to their high surface area, chemical stability and high electrical-conductivity. The

defects and heteroatom substitutions can also change the properties of CNTs. For in-

stance, substitutional doping of nitrogen in CNTs can alter the electronic properties of

CNTs by changing the electron donor states near the Fermi level.[67] Owing to the unique

(a)

(b)

1.2 PROGRESS IN THE SYNTHESIS OF CNTS 7

electronic properties, NCNTs exhibited great potential in electrochemical applications.[68]

Heterogeneous catalysis is currently a growing field for CNT applications. Traditionally,

CNTs are more used as supports for catalysts. Recently, a considerable number of re-

ports have revealed that CNTs themselves can be active for some reactions. However,

the assignment of the active sites is still under debate. Some studies assigned the active

sites to heteroatoms like O- and N-induced defect sites or surface functional groups,[69-70]

while others believed that the residual catalysts used from catalytic CVD synthesis can

also impact the catalytic reactions.[71-72] In addition, CNTs also show potential applica-

tions in biomaterials and biomedicine due to their unique biological and medical proper-

ties.[73-75] Several reviews are devoted to their properties and applications in these

fields.[54, 76-78]

The properties of CNTs mainly depend on their structure, which can be tuned by va-

rying the catalysts, precursors and synthesis conditions. Since the pioneering research

on CNTs in the early 1990s,[15-17] significant progress has been made in their synthesis.

Despite of the wide range potential applications and increasing demands of CNTs, the

large-scale production of CNTs and chirality-controlled synthesis of SWCNTs remain as

the major challenges for the large-scale application of CNTs.

2.1.1.3 Growth mechanism of CNTs

The commonly used techniques for CNT synthesis include arc-discharge,[15, 47, 79] laser

ablation,[80] and chemical vapor deposition.[81] Although the arc-discharge and laser abla-

tion can produce CNTs (in particular MWCNTs) with superior crystallinity due to the

high reaction temperatures (the former: > 3200 ºC and the latter: ~1200 ºC), the high

impurities in the products (such as fullerene, soot and amorphous carbon) and the poor

control of growth conditions restrain their practical applications. Catalytic CVD is the

most popular and widely used method for the large-scale synthesis of CNTs due to its

low cost, low operation temperature, high production yield and easy scale-up.[41, 82] The

composition of the feedstock and wide window of reaction conditions of CVD enable the

synthesis of CNTs with desired structures and agglomeration states for specific applica-

tions. The SWCNTs fabricated by CVD possess crystallinity close to those grown by arc

and laser techniques, which is particularly attractive to the industry. The synthesis me-

thods of CNT have been comprehensively reviewed by Prasek et al. and Journet et al.[46,

83]

In order to control the synthesis of CNTs, it is essential to understand the growth me-

chanism of CNTs. The catalytic CVD process of CNT growth, in general, involves (1)


formation of active sites on catalyst particles, (2) dissociative adsorption of carbon-

containing gas precursors on the active sites, (3) diffusion of carbon atoms, (4) initial

formation of graphite-like cups, (5) continuous formation of nanotubes and (6) termina-

tion of growth.

(1) Catalysts

Transition metals, typically, Fe, Co, Ni, are commonly used for conventional CNT syn-

thesis. A noticeable number of studies have demonstrated that CNT synthesis can also

be implemented from ceramics (e.g. SiC and Al2O3),[84] which challenge the current con-

cepts of the role of catalyst and popular growth mechanism of CNT synthesis. With re-

spect to mass production, only Fe, Co and Ni-based catalysts can achieve high CNT

yield,[14, 27] owing to the high solubility and diffusion rate of carbon in these metals at

high temperatures.[85] It is worth to note that the state-of-the-art catalysts for commer-

cial CNT mass production are Fe/Mo/Al2O3, Co/Fe/Al2O3 and Co-Mn-Al-Mg mixed

oxide.[14]

The agglomeration behavior of CNTs can be tuned by designing the catalyst structure

to obtain either particle aggregates with randomly entangled CNTs (by mixed catalysts)

or arrays with nearly paralleled CNTs (by substrate supported catalysts or floating cata-

lysts).

The particle size of the active catalyst directs the formation of SWCNTs or MWCNTs.,

The synthesis of SWCNTs can be achieved by synthesizing small catalyst particles (i.e. a

few nm) with a narrow distribution, or tuning the catalyst elemental composition to mod-

ify the work of adhesion of carbon on the catalyst particle in order to exclusively grow

SWCNTs on the smallest particles, or to decrease the reaction temperature only for the

purpose of growing the smallest particles.[82] On the other hand, larger particles (i.e. a

few tens nm) grow MWCNTs.

(2) Precursors

The precursors widely used for CNT synthesis are methane, ethylene, acetylene, ben-

zene, carbon monoxide and alcohol. The morphology of CNTs is strongly affected by the

molecular structure of the precursor, meaning that it is possible to perform controllable

growth CNTs with desired structure from certain precursors. Linear hydrocarbons, such

as methane, ethylene, and acetylene are utilized to produce a straight hollow CNT struc-

ture. Cyclic hydrocarbons like benzene, xylene, cyclohexane, and fullerene are used to

produce relatively curved/hunched CNTs with bridged inner tube walls.[86-87]


Ethylene is widely used for the production of CNTs with extremely high yield by CVD

method. Recent studies found that the active species are actually the carbon species con-

taining triple-bond (i.e. acetylene) which were produced by the gas-phase pyrolysis of

ethylene at 750 °C.[88-89] Inspired by this finding, acetylene has been re-examined and

demonstrated as the most suitable precursor for growing SWCNTs among various car-

bon precursors.[90-94]

(3) Diffusion of carbon atoms

The two classic growth models, vapor-liquid-solid model (VLS)[95] and vapor-solid-solid

model (VSS), were proposed and developed based on the pathways of carbon diffusion to

explain the formation mechanisms of CNTs in catalytic CVD. In the VLS model, the dis-

solved carbon atoms were proposed to diffuse via the bulk of catalyst particle in the form

of liquid metastable carbides. However, the driving force for carbon bulk diffusion and

formation of hollow nanotubes were unclear. The VSS model was developed from the

VLS model. Owing to the great development of in situ HRTEM and in situ XPS, the ini-

tial formation of CNTs from solid particles via surface or subsurface diffusion of carbon

was identified.[96-97]

The growth of SWCNTs is well accepted to follow the VSS model. The growth of

MWCNTs is rather complex. It is still unclear how carbon atoms can diffuse from the

outer surface to the inner walls of a MWCNT to continue the growth, if the catalyst par-

ticle is already encapsulated by the outer walls. A reasonable explanation could be that

carbon atoms diffuse on the surface of the metal particle to form outer walls of nano-

tubes and simultaneously diffuse via sub-surface (in the first few atomic layers) to form

the inner walls of nanotubes.[82] The growth mechanism has been reviewed by Tessonnier

et al. and Journet et al.[82-83]

(4) Initial growth

The initial growth behavior of CNTs is strongly influenced by the strength of metal-

support interactions, which, in general, can be described in the "tip-growth model" and

the "base-growth model".[85, 95, 98] In the "tip-growth model", the metal-support interaction

is weaker. The dissolved carbon diffuses down through the metal and deposits at the bot-

tom of the metal, lifting the whole metal particle off the substrate. In the "base-growth

model", the metal-support interaction is stronger. The diffusion of the dissolved carbons

is limited to the top surface of the metal particle, forming fullerene-like cups. With con-

tinuous hydrocarbon feeding, the CNTs grow on the top of the catalyst particles that are

anchored on the substrate.


In addition, the particle size might also influence the catalyst-support interaction

leading to different growth behaviors of CNTs, e.g. large particles (≥ 5 nm) may lead to

the "tip-growth mode" and small particles (< 5 nm) may follow the "base-growth

mode".[99]

(5) Continuous growth

The growth time is an important factor for controlling the morphology and purity of

CNTs. During the CNT growth, the carbon precursor molecules or the active carbon spe-

cies from the gas-phase pyrolysis (e.g. > 730 ºC for ethylene[89]) may react with the

formed nanotubes, meaning lengthening and thickening of the nanotubes are competi-

tive processes during the CNT growth.[100] The TEM images in Figure 2.2 illustrate that

the thickening of nanotubes originated from the increased wall numbers while the inner

diameter of nanotube was hardly changed. Even the growth of CNT forest height

stopped, whereas the CNT forest weight still steadily increased due to the surface accu-

mulation of carbonaceous impurities from the gas-phase pyrolysis of the precursor,[101]

which were observed on the outermost layer (Figure 2.2). Feng et al.[102] proposed a so-

called epitaxial growth mode in which the epitaxial growth of graphene layers occurred

on the outer surface of CNTs at the slow growth stage. It is very likely that these epitax-

ial graphene layers can be used as seeds for self-assembling the outer walls of nano-

tubes,[92] resulting in thickening of nanotubes with poor graphitization (Figure 2.2).

Therefore, the longer the growth time, the larger the outer diameter of the CNTs can be

produced, which was also reported in other investigations.[25, 101-104] The approach to pre-

vent the thickening of nanotubes or formation of surface carbonaceous species is to in-

troduce etching reagents, such as H2O or H2 plasma,[105-106] which can suppress epitaxial

growth of graphene layers by selective gasification of the non-tubular carbons to produce

highly pure CNTs.

However, one should be aware that the thickening of the nanotubes can also be in-

duced by the catalysts during the nucleation steps.[105] It is also known that the outer di-

ameter of CNTs is predetermined by the particle size of the catalysts.[107-109] If the cata-

lyst particles are well stabilized by the supports (e.g. the mixed oxide catalysts), the in-

ner diameter was hardly changed and only the outer diameter became larger during the

CNT growth. If the catalyst particles are free of any supports (e.g. the floating catalysts),

both outer and inner diameters of CNTs increase due to the agglomeration of catalyst

particles.[110-111]


Figure 2.2: HRTEM images of the CNT arrays grown on 6 min-pretreated Fe (1 nm) cat-

alyst films (scale bar: 5 nm). Adapted from ref. [100].

(6) Growth termination

In both the VLS and VSS mechanisms, catalyst particles are proposed to be encapsu-

lated with carbon species. Most of the dissolved carbons form nanotubes, a small amount

will transform to amorphous carbon which is believed to be able to deactivate the cata-

lyst and cease the CNT growth.[112-115] Some studies reported that the catalyst deactiva-

tion and the growth termination resulted from the phase transformation from cementite

(Fe3C) to Hägg carbide (Fe5C2),[116] or sub-surface diffusion[117-118] or Ostwald ripening of

catalyst particles.[119-120] Besides the catalyst deactivation, additional reasons for CNT

growth termination have been considered to be structural disorder,[121-122] graphene layer

formation around the catalyst particles,[123] steric hindrance,[124] defect diffusion to the

growth front,[125] or the loss of the active carbon sites for incorporation of more acetylene

molecules due to the close up of nanotubes,[92] and so on.

A better understanding of the growth deactivation mechanisms is essential for better

controlling the CNT production. Concerning the catalyst deactivation, introducing addi-

tional environmental gas has been demonstrated to be an effective route to reactivate

the catalyst, thus prolonging the lifetime of the catalysts. The additional environmental

gases can be divided into two groups. One group comprises the oxidative oxygen-

containing molecules such as H2O,[18, 21] CO2,[126-127] O2,[128-129] and ethanol,[130-131] which

can effectively remove the coating of amorphous carbon by selective oxidation to produce

CNTs with high yield and quality. Another one is reductive NH3. Researchers have dem-

onstrated the positive effect of ammonia on CNT growth. Hence, NH3 is widely used in

the pretreatment step to activate catalyst before CNT growth.[132-135] Moreover, addition-

al environmental gas affects the formation of catalyst particles by changing the interac-

tion between the catalyst and the support or the surface properties, causing surface re-

construction or faceting of the catalyst. The influence of H2O and NH3 in the CVD syn-

thesis of CNTs will be discussed in the sections 2.1.2 and 2.1.3.


The influence of growth temperature is complex. On the one hand, temperature influ-

ences the precursor dissociation, carbon diffusion and lifting of the cap of nanotubes. On

the other hand, it also affects the particle size of the catalyst and further the diameter of

CNTs. It is well accepted that most of MWCNTs can be synthesized from most of the

precursors at a lower temperature range (400 − 900 °C), but SWCNT synthesis can only

be achieved with some selected precursors (like carbon monoxide, methane, ethylene,

acetylene etc.) at higher temperatures (900 − 1200 °C). The selection of precursor and

growth temperature is important for CNT production because the deposition of amorph-

ous carbon (carbonaceous compound) not only causes high impurity but also deactivates

the catalyst rapidly. Water-assisted CVD might be a promising solution to address the

aforementioned question.[18]

Large-scale production of CNTs is a complex engineering process. Before really

achieving large-scale production of CNTs, CNT growth has been first performed in small

laboratory reactor, and then gradually applied to industrial reactors. During this engi-

neering process, lots of studies on the growth behavior are usually carried at different

scales, for example, from atomic scale, nanoscale, mesocale, to reactor scale, pilot scale

and final production scale. As the space scale of the reactor increase, more and more fac-

tors need to be considered, for instance, the issues of heat and mass transfer become

more severe in the industrial reactor and the stress on the catalyst powder increases

from the laboratory scale to industrial scale leading to different growth behaviors. No-

wadays, there are several successful CNT production process by CVD method (Table 2.1).

Table 2.1: Scalable CNT production by CVD method.[27]

Project Affiliation “Carbon Multiwall Nanotubes” Hyperion Company Endo process Shinshu University “CoMoCATProcess at SWeNT” University of Oklahoma“HiPco Process” Rice University “Nano Agglomerate Fluidized” process Tsinghua University “Baytube” process Bayer Company Supergrowth of SWCNT arrays Hata research group Super-aligned CNTs for yarn preparation Tsinghua University

2.1.1.4 Recent breakthroughs in SWCNT synthesis

SWCNTs are interesting materials for electronic devices because they can be either

metallic or semiconducting. However, the bottleneck in the development of SWCNTs is


that the common routes for the synthesis of SWCNTs are poorly controllable for the chi-

rality. Therefore, the chirality-controlled synthesis is essential for achieving the desired

properties for SWCNTs. Very recently, two studies published in Nature have made

breakthroughs in the chirality-controlled synthesis of SWCNTs by optimizing the struc-

ture of high-melting-point tungsten-based alloy nanocrystals as template for selective

growth of SWCNTs with high specific chirality (> 99 %) and composition, or through bot-

tom-up strategy using template molecules to produce targeted SWCNTs with single chi-

rality.[136-137]

2.1.1.5 Unconventional routes for CNT synthesis

Besides these conventional techniques, there are some unconventional techniques for

producing CNTs, such as liquid pyrolysis,[138-139] solid-state pyrolysis[140-141] and bottom-

up organic approach.[142-143] Recently, several studies reported a simple and efficient

route to produce CNFs or CNTs from solid carbon materials. Shen et al.[140] investigated

the formation of CNFs from various pre-oxidized solid carbon sources, like artificial gra-

phite and activated carbon. However, the temperature for carbonization was as high as

1500 ºC. Another interesting work was reported by Su and coworkers.[141] CNTs were di-

rectly produced on a Fe-exchanged resin via a solid-phase process at a much lower tem-

perature range of 400 − 800 ºC. Such CNT-composed resin spheres could be a good solu-

tion in handling the problems during the mechanical post-treatment of CNTs for large-

scale applications.

2.1.2 Water-assisted CVD synthesis of CNTs

In 2004, Hata et al.[18] reported the so-called “super-growth” of SWCNT forests by add-

ing a small amount of H2O to the ethylene feed. In the same year, Liu et al.[144] investi-

gated the effect of H2O on SWCNT synthesis from CH4. They found that the formation of

amorphous carbon and carbides can be suppressed by adding a suitable amount of H2O

vapor. By statistical and macroscopic analysis, the optimal catalyst activity in water-

assisted chemical vapor depostion (WACVD) growth of SWCNTs was estimated to be 84

± 6 %.[145] The super-growth by WACVD brings the hope for large-area production of

SWCNT forests. To achieve this dream, a precise control in the delivery gas is needed to

enhance the reproducibility and stability. Compared with horizontal gas-flow direction, a

vertical flow direction could be a better choice. Based on this idea, Hata and coworkers [146] developed a different approach for synthesizing SWCNT forest in WACVD using a

shower head to dispense feeding gases, which resulted in a slower initial growth rate but

much longer lifetime of catalyst compared with other feeding directions, yielding a 1 cm


tall forest with 1×1 cm size. By optimizing the growth conditions, an A4 size SWCNT

forest was grown in a 6 m×1.5 m lateral batch CVD furnace. The WACVD not only favors

SWCNT growth but also enhances the selectivity and purity of DWCNTs.[147]

2.1.2.1 Mechanisms

The mechanisms of WACVD of CNT growth are summarized as followed:

(1) Selective etching of amorphous carbon on the active sites

(2.1)

(2) Inhibition of Ostwald ripening of catalyst particles

The first mechanism of WACVD is based on the chemical reaction of carbon gasifica-

tion by H2O vapor (Eq. 2.1). It has been illustrated that water vapor in the feed gas pro-

motes the CNT growth by selectively etching the amorphous carbon. The carbon gasifica-

tion reaction was confirmed by monitoring CO in the effluent gas.[115, 144] To reveal the

secret of water-assisted growth of CNTs, Yamada et al.[19] examined catalyst re-

activation the catalyst deactivation by using H2O to remove the carbon coating through

ex situ microscopic and spectroscopic methods. In WACVD synthesis of MWCNTs, H2O

was found not only to gasify the amorphous carbon on catalyst, but also to maintain the

catalyst activity by suppressing the thickening of CNTs and simultaneously promoting

the lengthening of nanotubes.[105]

The second mechanism was proposed in a recent study conducted by Amama et al.[119]

The addition of H2O in both reduction and growth steps was found to stabilize the cata-

lyst particles. Water was proposed to promote the formation of hydroxyl groups on the

substrate to limit the diffusion of catalyst atoms and subsequently inhibit catalyst ripen-

ing resulting in smaller catalyst particles (see Figure 2.3).[119] On the other hand, the

catalyst particles were agglomerated during the CNT growth without H2O, implying that

except the poisoning by amorphous carbon, Oswald ripening also has an impact on the

self-termination of CNT growth. Based on this theory, H2O has also been used to narrow

the diameter distributions of SWCNTs and DWCNTs.[148-149] It is clear that the catalyst-

support interaction strongly influences the CNT growth manner. Choosing a suitable

support or substrate is of importance for ‘supergrowth’. Such a support is supposed to

enhance the stability and number density of the catalyst particles[117] and limit the sur-

face and sub-surface diffusion of catalyst particles. To date, Al2O3-supported Fe has been

demonstrated to be the best support for WACVD synthesis of CNTs.[150]


Figure 2.3: Scheme of Ostwald ripening of catalysts, and how it is expected to affect car-

pet growth with and without the addition of water vapor. Adapted from ref.

[119].

In addition, the catalyst deactivation by amorphous carbon has been challenged by

different opinions. A recent study by Schünemann et al.[151] showed that CNTs can be

formed on amorphous carbon-coated catalysts. The present of amorphous carbon did not

prevent the CNT growth in catalytic hydrocarbon decomposition and graphitization

process.

2.1.2.2 Kinetics

The study of the growth kinetics of CNTs aims at providing information about the

reaction mechanism and also optimizing the experimental conditions in a cost-saving

way. The CNT growth is a complex process. Therefore, building a growth kinetic model

to describe all the stages including initial carburization-nucleation and growth termina-

tion is rather complex.[114, 152-153] The simplified kinetic models are always built based on

some specific assumptions.

(1) If the catalyst possesses a constant activity, the growth behavior of aligned CNTs

is linear. The kinetic model based on the weight or length of CNTs can be simply ex-

pressed as:

(2.2)

(2.3)


where is the mass accumulation of CNTs over catalyst, is the length of

aligned CNTs, is the growth rate of aligned CNTs, is the growth time.[152]

(2) If the rate-determining step is the carbon source decomposition, the length of

aligned CNTs can be described as:

(2.4)

where is the effective diffusion coefficient, is the surface rate constant for react-

ing the carbon source to CNT, 0 is the feedstock concentration, and is a structure-

dependent constant of the CNT array.[154]

(3) If the CNT growth termination is only caused by the catalyst deactivation, the

rate-determining step is the catalyst deactivation. The growth behavior of CNTs based

on carbon accumulation and forest height can be expressed as:

1 exp (2.5)

1 (2.6)

where is the mass accumulation of CNTs over catalyst, is intrinsic CNT

growth rate for the fresh catalyst, is deactivation kinetic functions, L is the length of

the CNT forest, is the initial growth rate, and is the mean lifetime of the catalytical-

ly active sites.

In addition, Wang et al.[116] proposed another kinetic model based on the growth ter-

mination caused by the formation of iron carbide.

2.1.2.3 Unconventional views

Interestingly, CNTs can also be produced from graphite by water assistance.[155-157] In

2003, Kang et al.[155] reported one-step water-assisted synthesis of MWCNTs from gra-

phite without using any catalysts at the ambient pressure. A pre-heated red-hot graphite

rod (800 °C) was rapidly dipped into cool water (0 °C) to obtain a suitable power to crimp

the graphite sheets and wrap the honeycomb pattern back on top of itself and join the

edge carbons with C−C σ bonds. The CNT yield was found to be 40 %.

Moreover, H2O can also be utilized for surface functionalization of CNTs with oxygen

functional groups,[158] and to fabricate free standing VACNT lifted from the substrate.[159]


2.1.3 Ammonia in the synthesis of CNTs

Ammonia has been used not only in the pretreatment of the catalysts and CNT

growth stage but also as nitrogen sources for NCNT synthesis.

2.1.3.1 NH3 pretreatment

Studies have demonstrated that NH3 pretreatment is critical for forming aligned na-

notube arrays. One role of NH3 is the formation of catalyst particles by etching/reduction

of the metal oxides, offering a route to control the diameter, growth rate and structure of

the vertically aligned CNTs (VACNTs) by properly altering the NH3 pretreatment condi-

tions.[132-134] The enhanced CNT growth was related to the formation of surface nickel ni-

tride in some studies.[133] Recently, Kim et al.[135] carried out an investigation on the re-

structuring of surface morphology of catalyst during the NH3 pretreatment prior to CNT

growth. However, their experiment suffered from the contamination of H2O which has an

etching effect on catalyst particles. So far, the mechanism of NH3 on the control of cata-

lyst composition and size is still lacking direct evidence.

2.1.3.2 NH3 during the growth

It is generally accepted that mixing NH3 in the feed gas can yield high quality

VACNTs. The role of NH3 is assumed to reactivate the catalytic activity of the catalyst

particles by removing the deposited amorphous carbon.[160] Moreover, the statistical

analysis of the diameter distribution of the CNTs grown with and without the presence

of NH3 indicated that the addition of NH3 during CNT growth might also enlarge the

particle size, which is consist with the observations in other studies.[135, 161] Li et al.[161]

found that the vertically aligned carbon nanofibers arrays can be synthesized on carbon

substrate over Cu foil at 350 °C only by feeding with C2H2 and NH3 during the CVD

process. They claimed that NH3 can accelerate the diffusion of Cu atoms to form larger

spherical Cu nanoparticles and help to form VACNTs (Figure 2.4). Moreover, NH3 was

found to promote the graphitization of the deposition of graphene layer on the outer sur-

face of CNTs during the thickening stages.[162] In addition, several recent progresses had

been made by using NH3 for chirality-control of SWCNTs and MWCNTs.[163-164] Zhu et al.

demonstrated that it is possible to selective control the synthesis of SWCNTs with a nar-

row (n,m) distribution assisted by NH3 with a specific concentration (i.e. 500 ppm).[163]

The influence of NH3 is not only on the catalyst but also on the structure of formed

CNTs. A well-known structure of CNTs grown in NH3-containing environment is the so-

called bamboo-like structure. In order to reveal the role of NH3 in CNT growth, Pattin-


son et al.[164] provided in-depth analysis of a series of nitrogen sources for chiral angle

controlled-synthesis of CNTs, showing that intermediate NH3 plays a key role on chiral

angle control. The results of high resolution TEM images and electron diffraction pat-

terns revealed that CNTs grown with 3 mL·min–1 NH3 was via epitaxial growth resulting

in armchair chiral angle, while with 30 mL·min–1 NH3 the grown CNTs possessed mixed

chrial angles (Figure 2.5).

Although ammonia has been widely investigated in both pretreatment and growth

stages, most of the studies only examine the CNT morphology in the presence and ab-

sence of ammonia. Few reports analyze of the growth kinetics of CNT growth in the

presence of NH3.

Figure 2.4: Schematic illustration of the different growth mechanisms in different at-

mospheres. Adapted from ref. [161].

Figure 2.5: TEM Images and electron diffraction pattern of CNTs produced via epitaxial

growth (a-c) and non-epitaxial growth (d-e), respectively. Adapted from ref.

[164].

(a)

(b) (c)

(d)

(e) (f)

1.2 PROGRESS IN THE SYNTHESIS OF NCNTS 19

2.2 Progress in the synthesis of NCNTs

CNTs have shown great potential for applications in various nanoelectronic devices

such as transistor channels and interconnects due to their unique electronic properties.

As mentioned above, the properties of CNTs are determined by their structures. Doping

and functionalizing CNTs with heteroatoms, including N, B, P, Si, Si, can alter the prop-

erties of CNTs.[165-168] Due to their similar atomic sizes to carbon, nitrogen and boron are

widely utilized as dopants for CNTs. N-doping leads to strong electron donor states near

the Fermi level (n-type),[67] while boron-doping causes the down-shift of Fermi level to

the valence band (p-type).[165, 169]

2.2.1 Structure and properties of NCNTs

A structural feature of NCNTs is the so-called bamboo-shape (Figure 2.6). However,

this is not a unique feature of NCNTs, because such bamboo-shape can also be found in

non-N-doped CNTs. The metal-carbon binding strength is critical for the formation of

CNTs. A too low or too high metal-carbon binding energy will not favor the growth of the

nanotubes or only form carbides to terminate the growth. Therefore, the metal-carbon

binding energy must be in a suitable range to promote the growth of CNTs.[170-173] The

bamboo structure can be assigned to the strong metal-carbon binding strength during

the formation of CNTs. It has been demonstrated by density functional theory (DFT) cal-

culation that the driving force for forming bamboo structure in NCNTs is the nitrogen-

metal interactions, which causes prolonged bonding at local "pin points" and thus a de-

formation of the catalyst nanoparticles.[24]

Figure 2.6: TEM images of compartment layers of NCNTs with various N concentrations.

Adapted from ref. [174].


Previous research has revealed that the physicochemical properties of NCNTs mainly rely

on the amount and types of the N-doping atoms. N incorporates into the CNT lattice by

several bonding structures. Figure 2.7 displays the main nitrogen species in a carbon lat-

tice: (i) pyridine-like, (ii) pyrrole-like, (iii) quaternary, (iv) pyridine-N-oxide, (v) nitrogen

oxides. In addition, amine functionalization is possible. The first three groups are com-

monly observed in NCNTs by XPS. Both type (i) and type (ii) add p-electrons to the π

system and have N-C sp2 coordination. Type (iii) N substitutes a graphitic C atom.

Figure 2.7: Different types of N functionalities in graphite. Reproduced from ref. [175].

Figure 2.8: (A) EELS spectra involving C and N K edges recorded at the points marked in the inset

image. (B) Chemical mapping of C and N profiles extracting from an EELS line-scan

across the CNx nanotube as labeled in (A). (C) Scheme of CNx nanotube showing inho-

mogeneous distribution of C and N. Adapted from ref. [176].

Identification of the chemical environment of the nitrogen atoms in NCNTs, e.g. local-

ized elemental distribution and types of N-C bonding, is essential for understanding the

properties of NCNTs and further exploring their applications. Great efforts have been

devoted to uncover the role of nitrogen in the rearrangement of nanotube structures.


Suenaga et al.[176] analyzed the element distribution in a single CNx nanotube by

chemical mapping the radial distribution of C and N in the nanotube (Figure 2.8A-B)

and proposed a radial nanotube structure model (Figure 2.8C): i) the outer shell is a ni-

trogen rich sheath; ii) the inner shell consists of graphite layers with small amount of

nitrogen; iii) the innermost hollow is constitutive of amorphous CNx with high N/C ratio.

This model represents an inhomogeneous distribution of N atoms within NCNTs. How-

ever, this model is too simple and cannot explain the common bamboo-like structure of

NCNTs.

Owing to the great development in electron microscopy tomography, which is powerful

to model 3D morphology of nanomaterials and analyze the elemental distribution in

3D.[177] Florea et al.[178] determined the 3D distribution of nitrogen within NCNTs by ap-

plying the energy-filtered transmission electron microscopy (EFTEM). It is found that

nitrogen is preferentially incorporated inside the transversally bended arches inside the

nanotube, whereas the curled arches have relatively low nitrogen incorporations (Figure

2.9 top). In addition, the analysis of the cross-section views of 3D reconstructed nanotube

revealed that nitrogen incorporation disturbed the circular shape of the nanotube

(Figure 2.9 bottom). The reconstructed 3D nanotube model well represents the bamboo-

like structure of NCNTs and provides a 3D distribution of nitrogen. However, the resolu-

tion of EFTEM is insufficient to identify the atomic configuration of N-C bonds.

In a recent study, Arenal et al.[179] configured the nitrogen substitution with in N-

doped SWCNTs at atomic level for the first time. By using a combination of high-angle

annual dark field (HAADF), EELS and DFT simulations, they identified only a small

fraction of nitrogen incorporated into the wall of individual SWCNT in the form of gra-

phitic-like nitrogen and pyrrolic nitrogen, respectively (Figure 2.10). This is consistent

with the conclusions in some reported XPS results. In addition, the majority of nitrogen

was found to be in an amorphous CNx sheath that partially covered the outer surface of

nanotubes. This finding, on the one hand, is partially in good agreement with the model

in Figure 2.8C; on the other hand, it may influence the conclusions drawn on the proper-

ties probed on these samples,[179] in particular, when linking the total N content with the

properties of NCNTs.

The substitutional doping of N has been reported to change the hardness and electric-

al conductivity of NCNTs, which were confirmed from both experimental and theoretical

perspectives.[180] N-doping also changes the chemical behavior of NCNTs, offering an al-


ternative method to the conventional surface functionalizaton of CNTs. For instance,

NCNTs can improve the dispersion for metal nanoparticles.[181-182]

Figure 2.9: Top: typical longitudinal slices extracted at the same depth and orientation

from the shape-sensitive reconstruction (left), C and N 3D elemental maps

(middle), and C-to-N 3D relative map where nitrogen in green and carbon in

red (right). Bottom: combined morphological and chemical analysis of the

highly doped N-CNT in cross-section: (left) longitudinal slice extracted from

the mean-density (ZL) tomographic reconstruction; (middle) two transversal

sections through the 3D mean-density and chemical relative (C-to-N) recon-

structions; (right) cross-sectional views by the 3D model of the analyzed tube.


Figure 2.10: Atomic model of a (22, 0) nanotube including two a graphitic-like substitu-

tional nitrogen and a pyrrolic-like substitutional nitrogen, respectively.



2.2.2 Synthesis of NCNTs

There are two common routes to modify of CNTs: in situ substitutional doping[183-184]

and post surface functionalization.[185-186] The former route enables the in situ incorpora-

tion of N into bulk structure of CNTs, while the latter route needs a multi-step treat-

ment, involving first oxidation of pristine CNTs and further functionalization. The latter

can only modify the outer walls of CNTs.

2.2.2.1 Growth mechanism

Different from the conventional CNT growth models, NCNT growth involves the for-

mation of regular internal bamboo cavities in the nanotubes. One explanation is that the

presence of nitrogen atoms increases the stress in the catalyst surface and the accumu-

lated stress forces the metal catalyst to eject C and N atoms to lower the surface stress,

forming a cave. As the N and C atoms continue to dissolve into the catalyst, the catalyst

will continue to eject new layers. The repeated ejection of layers from the catalyst sur-

face finally produces the bamboo-like inner structure.[187]

2.2.2.2 In situ substitutional doping

It is needless to emphasize that CVD synthesis is also the most common technique for

growing NCNTs with conventional metal catalysts (e.g. Fe, Co, Ni) and N-containing

precursors. There are some common CVD-based methods: classical CVD,[188] aerosol-

assisted CVD,[189] gas-phase pyrolysis of a mixture containing catalyst and C/N precursor

(floating catalyst CVD).[183] The highest nitrogen content has been reported to be nearly

20 at.% using aerosol-assisted CVD.[189]

A summary of nitrogen contents in CNTs using different methods, nitrogen/carbon

precursor and catalysts is given in Table 2.2. It can be seen that many nitrogen-

containing organic compounds can be used for NCNT production. These compounds al-

ways self-decompose before reaching the catalyst surface. Then the question is, what are

active species for NCNT growth? It is known that acetylene and ethylene have been

demonstrated to be efficient precursors for CVD synthesis of undoped CNTs.[90-93, 190]

However, due to the high reactivity of N-containing species at high temperature, analy-

sis of the active species in NCNT growth remains a challenge. Recently, Pint et al.[191]

employed in situ mass spectrometry to investigate the decomposition of C5H5N and

CH3CN during the SWCNT growth and finally identified HCN and C2H2 as the active

species. Considering the precursors used in the past works (e.g benzylamine, prrizine,


ethylenediamine), HCN and C2H2 are very likely to be the universal active species for

NCNTs.

Some recent studies reported the self-assembly of NCNTs without any assistances of

metal atoms.[192-193] Nitrogen atoms were found to promote the formation of graphitic

structure through a transient formation of –C≡N terminal groups, which can thermody-

namically assemble into more stable sp2-hybrid structure.[192]

Table 2.2: A summary of nitrogen contents in NCNTs synthesized using different me-

thods, nitrogen/carbon precursor and catalysts.

Method / precursor / catalyst / temperature N content / ref. Pyrolysis / pyridine / Fe / 1000 °C 1 − 2 %[183] Aerosol CVD / CH3CN, C4H8O / Fe / 950 °C ~20 at.%[189] Thermal CVD / NH3, C2H2 / Fe / 850 °C 0.4 − 2.4%[174] Plasma-enhanced CVD / NH3, C2H2 / Ni / 650 °C 13 at.%[194] Pyrolysis / FePc, CoPc, NiPc / 750 − 1000 °C 0.7 − 7.8 at.%[195] Floating catalyst CVD / NH3, xylene, pyridine / Fe / 800 °C 0 − 10 at.%[196] CVD / monoethanolamine / Fe / 700 − 900 °C 4.8 − 5.8 at.%[197] Spray pyrolysis / CH3CN / Fe/ 850 − 950 °C 3.20 − 8.3 at.%[198] Aerosol CVD / Benzylamine, toluene / Fe / 800 − 900 °C 0 − 2.2 at.%[23] Annealing / polyacrylonitrile microspheres / 1000 °C 5 − 7 at.%[193] CVD / NH3, CH4, / Fe / 550 − 950 °C 2 − 5.4 %[22] Water-assisted pyrolysis / CH3CN / Fe / 800 °C 0.9 − 2.2 %[199]

2.2.2.3 Post surface functionalization

Post surface functionalization of CNTs has been widely implemented by post-

treatment and/or chemically decoration with heteroatom-containing group.[33, 186, 200-201]

This strategy always require multiple steps, for example, the CNTs need to be oxidized

first to create oxygen-containing functional groups, which can further react with ammo-

nia or nitrogen-containing precursors to obtain N-functional groups on the CNT surface.

2.3 PROGRESS IN OXYGEN ELECTROCATALYSIS 25

2.3 Progress in oxygen electrocatalysis

Electrochemical energy conversion and storage is considered as one of the most prom-

ising technologies for the development of renewable energy. Oxygen electrocatalysis,

which involves the oxygen reduction and evolution reactions, plays a vital role in several

techniques in electrochemical energy conversion and storage, such as conventional fuel

cells, regenerative fuel cells, water electrolyzers, and metal-air batteries.[3, 202-209] The

counter reactions of ORR and OER are hydrogen oxidation reaction and hydrogen evolu-

tion reaction, which possess fast reaction rates on commercial catalysts, i.e., platinum

(Pt) or Pt-based catalysts. However, the ORR and OER suffer from the sluggish kinetics

and large overpotentials and consequently restrain the development of these techniques.

To data, the state-of-the-art catalysts for ORR and OER are Pt-based and Ru/Ir-based

noble metal catalysts, respectively.[1-3]

2.3.1 Reaction mechanism

The electrochemical redox reactions of O2 and corresponding thermodynamic equilib-

rium potentials in acidic (Eq. 2.7) and alkaline (Eq. 2.8) media are listed below:

4 4 2 1.229 (2.7)

2 4 4 0.401 (2.8)

These are only the overall reactions of ORR and OER. Both reactions involve a num-

ber of reaction intermediates and several elementary steps, such as adsorp-

tion/desorption steps, four or two electron-transfer steps, dissociation or recombination

steps, which vary with the types of the electrocatalysts and electrolytes. Here, three

well-known reaction mechanisms in alkaline electrolyte are summarized as following:[3,

210]

(1) Associative mechanism

(2.9)

(2.10)

2 (2.11)

(2.12)

(2) Dissociation/recombination mechanism

2 (2.13)

2 2 2 2 2 2 (2.14)

2 2 2 (2.15)


(3) Peroxo mechanism

In the peroxo mechanism, ORR can proceed through two electron-transfer steps to

form (Eq. 2.9) and then (Eq.2.16), which later breaks into (Eq. 2.17).

Subsequently, the formed is reduced to (Eq. 2.12).

(2.16)

2 2 2 (2.17)

, and are the surface intermediates, where * represents the active sites

on the surface. The ORR proceeds through the forward reactions (→) and OER proceeds

through the backward reactions (←).

2.3.2 O2 electrocatalysts

The electrocatalysts can be generally categorized into seven groups:[3, 209, 211]

(1) Noble metals, alloys, oxides and their mixtures, i.e., Pt, Ag, PtAu, RuO2, IrO2.

(2) Transition metal oxides, e.g., single-metal oxides and mixed-metal oxides (mainly

spinel-type, perovskite-type, pyrochlore-type);

(3) Functional carbon materials, i.e., doped carbons and nanostructured carbons;

CNTs, graphene and mesoporous carbons;

(4) Metal oxide-nanocarbon hybrid oxides;

(5) Metal−N complex, i.e., non-pyrolyzed and pyrolyzed complex;

(6) Transition metal nitrides;

(7) Conductive polymers;

On the basis of availability and cost, the electrocatalysts can be divided into precious

metal catalysts and non-precious metal catalysts.

2.3.2.1 ORR electrocatalysts

The state-of-the-art commercial catalysts for ORR in fuel cells are carbon supported

Pt catalysts. One area in the development of ORR catalysts is the optimization of plati-

num catalysts, which are the best catalysts for the ORR (Figure 2.11). The activity peaks

at the top of the volcano where the binding to oxygen species is neither too strong nor too

weak. Among the pure metals labeled in Figure 2.11, Pt is located at the closest region to

the top of the volcano, which means it is the most active catalyst. Despite the high cost,

Pt maintains its momentum in the academic society. The idea is not only to reduce the

usage of Pt in fuel cells to lower the cost, but also to seek for the possibility to push Pt

more closer to the top of the volcano in Figure 2.11. It is evident that alloying Pt with


transition metals like Cu, Co or Ni shows enhanced activity toward ORR because the

combination of strain and ligand effects modifies the electronic and surface properties of

Pt particles.[212] Stability of the catalyst is an important factor for long-term performance

of fuel cells. The challenges for carbon-supported Pt or Pt based catalysts are Pt dissolu-

tion, Ostwald ripening, corrosion of the carbon supports, agglomeration, particle de-

tachment and leaching of alloying metal atoms.[3] However, the annually increasing

price, limited production amount and poor stability of the Pt catalysts hinder their large-

scale commercialization in fuel cells.

Figure 2.11: Volcano-type relationship for the ORR activity versus the oxygen binding

energy. Adapted from ref. [213].

Another area in the development of ORR catalyst is to explore the less expensive ma-

terials with comparable performances to Pt catalysts. Recent breakthroughs in the de-

velopment of novel non-precious metal catalysts (NPMC) including N−coordinated tran-

sition metal macromolecules (M−N−C), conductive polymers, chalcogenides, oxynitrides,

carbonitrides, metal oxides and carbon materials with high activity and practical dura-

bility have been considered to be the alternative materials to Pt catalysts. Among these

candidates, nitrogen-doped carbon materials such as NCNTs[68] and N-doped

graphene,[214] have shown their outstanding electrocatalytic activities for ORR, making

them the milestone in the development of the ORR catalysts (Figure 2.12).[202] The supe-

rior performance of metal-free NCNTs can be assigned to the positive shift of the charge

density of the carbon lattice with the incorporation of the electron-rich N atoms.[68] Sev-

eral excellent reviews cover the recent development of NPMC.[211, 215-222]


Figure 2.12: Brief history of ORR catalysts for N−C based catalysts. Adapted from ref.

[202].

2.3.2.2 OER electrocatalysts

The best anode materials for OER so far are noble (Ru, Ir) oxides and transition metal

(Ni, Co, Mn-containing) oxides. Matsumoto et al.[223] investigated the electrocatalytic ac-

tivity of transition metal oxides for OER and compared the overpotentials of thesis cata-

lysts. The corresponding activity sequence was determined to be RuO2 > IrO2 > Co oxides

≈ Ni oxides > Fe oxides ≈ Mn oxides ≈ Pb oxides. Three factors were proposed to impact

the electrocatalytic activity: electron transfer, strength of metal−oxygen (M−O) bond and

work function. The secret of the sequence was unclear until a universality of the OER

activity on various oxides was revealed by Nørskov and coworkers[2] using first principles

periodic DFT calculation. The possible limitations of potential are the binding energies of

HO*, HOO* and O* species on the oxide surfaces. According to Sabatier’s principle, if the

M−O binding is either too week or strong, the oxidation of HO* or formation of HOO*

will be the rate-limiting steps.[224-225] It is found that ∆ ∆ can be used as a uni-

versal descriptor for the OER activity, as shown in Figure 2.13. The volcano curve was

establish by using the scaling relation between and . The general

trend of the theoretical OER activity is in line with the experimental observations de-

spite that the descriptors are slightly different.[1-2, 223, 226] In contrast to metal catalysts

for ORR, the catalysts for anodic OER have to survive the oxidation conditions. It is ex-

tremely challenging for one catalyst possessing both high activity and high stability.


Figure 2.13: (a) Theoretical overpotentials for the OER on various oxides versus the dif-

ference in the free binding energy between O* and OH*. (b) Theoretical

overpotentials versus the experimental overpotentials in acidic (filled

squares) and in alkaline media (empty circles). Adapted from ref. [2].

2.3.3 CNTs and NCNTs in O2 electrocatalysis

A statistic analysis of the annual amount of the published papers on CNTs from ISI

Web of Knowledge shows a continuously growing interest in CNTs since 1991 (Figure

2.14). It is noteworthy that the number and proportion of papers about CNTs in electro-

catalysis increases linearly since 2001.

1991 1994 1997 2000 2003 2006 2009 2012

1000

3000

5000

7000

9000

11000

13000

15000

CNTs CNTs in electrocatalysis

Num

ber o

f pap

ers

Publication year

0

2

4

6

8

10

Proportion of CNTs in electrocatalysis

Pro

porti

on /

%

Figure 2.14: The number of annually published papers on carbon nanotubes and their

applications in electrocatalysis from 1991 to 2013. Data from ISI Web of

Knowledge.


2.3.3.1 Metal-free electrocatalysts

Carbon materials including graphite, glassy carbon, active carbon and CNTs have al-

ready been studied for ORR activities in alkaline electrolytes for several decades.[227]

CNTs are a new class of emerging material for ORR. The electrochemical redox proper-

ties of the edge plane sites and the tube ends on CNTs were first demonstrated by Comp-

ton and coworkers.[228] NCNTs are considered to be the most promising metal-free cata-

lysts for ORR. The preliminary successful attempts have shown superior intrinsic activi-

ty and stability of NCNTs for ORR in alkaline solutions.[68, 229-230] Maldonado and Steven-

son[230] found that N doping not only favor the O2 adsorption and reduction to H2O2, but

also dramatically accelerated the H2O2 decomposition on carbon fibers. Gong et al.[68] re-

ported excellent ORR activity and stability on VA-NCNTs. A positive charge density on

adjacent carbon atoms caused by the incorporation of N atoms was claimed to be benefi-

cial for ORR. Moreover, the NCNTs synthesized by in-situ CVD growth were found to be

more stable in highly corrosive media (industrially relevant conditions, 10 M KOH, 80

ºC) than the NCNTs obtained by post-treatment.[29]

The excellent metal-free NCNTs catalysts have boosted great interest to explore more

so-called "metal-free" carbon-based catalysts. However, a significant question has recent-

ly been raised that whether these "metal-free" catalysts are 100 % free of any metals,

even though they are confirmed by XPS, inductively coupled plasma-optical emission

spectroscopy (ICP-OES) or electron diffraction X-ray spectroscopy, which may rise

another question whether these techniques are sensitive enough to detect the trace met-

al concentrations? Because looking at the development of ORR catalysts (Figure 2.12),

one will easily notice that most of the catalysts are metal or metal−N based materials. It

is widely accepted that Metal−N4 is the active site.[231]

Most of the reported "metal-free" catalysts were purified by acid leaching. However,

the carbon-encapsulated metals in CNTs are extremely difficult to be removed by normal

acid washing. For the CNTs produced by catalytic CVD process, the residual metals were

found to be electroactive for the reduction of hydrogen peroxide.[232-233] As for NCNTs,

the residual metals (e.g. FexOy/Fe) can influence the chemical disproportionation of

to form species and molecular .[234] Masa et al.[38] found that the addition of Fe to

a completely metal-free NC catalyst can significantly affect the activity and selectivity of

the catalyst for ORR, even the concentration of additional Fe is as low as 0.05 %. Simi-

larly, another work reported by Wang et al.[235] illustrated that the so-called "metal-free"


doped graphene for ORR had significant contamination of Mn and other impurities

which mainly came from the fabrication process (i.e. Hummers method).

In order to obtain 100 % metal-free CNTs, Zhang et al.[236] prepared MWCNT films by

a dihexadecyl hydrogen phosphate (DHP) method on a glassy carbon electrode and found

that the films can catalyze ORR by two 2-eletron processes. Wang et al.[192] prepared

NCNTs by self-assembly method without metal assistance. The transferred electron

numbers per oxygen molecule of NCNTs was found to be 2.6, indicating a two- and four

electron combined pathway. Their results are different from the four-electron reaction

pathway reported by Dai and coworkers.[68]

As for OER, Cheng et al.[39] reported metal-CNTs hybrids with high metal oxidation

catalyst loading (> 50 wt.%) produced by arc-discharge and CVD exhibiting excellent ac-

tivity and stability for OER in alkaline solution. However, the working mechanism is

still unclear. To date, there is a lack of studies about direct utilization of as-grown CNTs

and NCNTs for OER.

In addition, several studies reported the utilization of nitrogen-containing carbon ma-

terials for OER without metal oxides.[237-240] In these studies, high nitrogen content, effi-

cient mass and charge transfer were claimed to be favorable for OER, which may provide

some guides for designing CNTs- or NCNTs-based carbon materials for bifunctional oxy-

gen electrocatalysis.[241-242]

2.3.3.2 CNTs and NCNTs as supports

CNTs have been extensively studied as catalyst supports owing to their outstanding

electrical conductivity, high surface area and high chemical and thermal stabilities.[11-13]

It is generally accepted that direct deposition of metal or metal oxide nanoparticles is

difficult on the pristine CNTs due to the inert nature of carbon. In practice, surface func-

tionalization with oxygen-containing or nitrogen-containing groups is required to tune

the surface property of CNTs and create anchoring sites for depositing metal or metal

oxide particles.[36, 243-249] However, it is worth to remember that the pristine CNTs pro-

duced by CVD always have high concentrations of surface defects and carbonaceous spe-

cies which might serve as anchoring sites. Yang et al.[250] prepared MnOx-doped CNTs

(MnOx-CNTs) by electrochemical deposition method using pristine CNTs as support. The

MnOx-CNTs exhibited impressive ORR activity due to the beneficial electron transfer

between Mn ions and CNTs, and the high positive charged MnOx-CNT surface.


Direct growth of CNTs with substitutional doping with nitrogen provides a different

approach to change the local chemical reactivity of CNTs and simultaneously creating

anchoring sites for metal or metal oxides.[29, 64, 251-252] Chetty et al.[245] found the pyridinic

and pyrrolic nitrogen species served as the anchoring sites for the deposition of PtRu

particles. Surface defects related to nitrogen could also be used for anchoring metal na-

noparticles.[253] First-principles calculations revealed that nitrogen can assist the deposi-

tion of Pt by activating the nitrogen-neighboring carbon atoms due to the large electron

affinity of nitrogen.[254]

Pt/C is the state-of-the-art electrocatalyst for ORR. As compared to the most widely

used carbon black supports (e.g. Vulcan XC 72R), CNT-supported Pt catalysts have

shown better performance, which can be assigned to the enhanced metal-support inte-

raction by nitrogen doping.[255]

Ru/Ir-based oxides catalysts are the best known electrocatalysts for the OER.[1-2] CNT-supported non-precious metal oxides, such as MnOx, Co3O4, as well as Mn- and Co-

based mixed oxides have proved to be promising alternatives to the precious metal cata-

lysts.[256-257] On the one hand, these metal oxides serve as active sites for OER. On the

other hand, CNTs can promote the dispersion and electron transfer of the metal oxides

owing to their high surface area and excellent electrical conductivity.

Liang et al.[258] reported enhanced ORR and OER activities of NCNTs-supported CoO,

compared with the N-doped graphene-supported Co3O4 and commercial Pt/C. Liu et

al.[259] found that the spinel cobalt oxide/MWCNT composites prepared by hydrothermal

method outperformed the physical mixture of spinel oxide and MWCNTs toward OER

and ORR. The improved activity can be assigned to synergistic coupling effect at the in-

terface between metal oxide and carbon. However, the mechanism of the aforementioned

synergistic effect is still unclear.

Chapter 3 Experimental

3.1 Synthesis

3.1.1 Assisted growth of CNTs

3.1.1.1 Chemicals and gases

The following chemicals and gases were used in this study:

Co(NO3)2⋅6H2O (99.999 %, Sigma-Aldrich), Mn(NO3)2⋅xH2O (x = 4 – 6, 99.99 %, Sigma-

Aldrich), Al(NO3)3⋅9H2O (99.997 %, Sigma-Aldrich), Mg(NO3)2⋅6H2O (99.999 %, Sigma-

Aldrich), synthetic air (99.999 %, Air Liquide), Ar (99.999 %, Air Liquide), NaOH (99.3

%, VWR International), H2 (99.999 %, Air Liquide), C2H4 (99.95 %, Air Liquide), 1.0

vol.% NH3/Ar (99.999 %, Air Liquide), HPLC water (Sigma-Aldrich).

3.1.1.2 Water-assisted CVD setup

The CNT growth experiments were carried out in a fixed-bed reactor using of a U-

shaped quartz tube with a length of 534 mm and an inner diameter of 7 mm. The flow

scheme of the CVD setup is shown in Figure 3.1. The gas flows were regulated by mass

flow controllers, and a LabVIEW interface was used to control the system. The isother-

mal reactor block was heated by high performance heating cartridges (Horst) controlled

by optical control relays (Eurotherm 2416). The pressure drop in the fixed-bed reactor

was monitored by a digital pressure gauge (Wika, Eco Tronic). The quantitative analysis

of C2H4 and CO in the effluent gas was performed using two infrared detectors connected

in series. The concentration of C2H4 was measured by a flow-cell detector (Emerson

Process Management, Rosemount NGA 2000 MLT 4) equipped with a nondispersive

infrared cuvette for the range of 0 − 100 %. A similar nondispersive infrared detector

(BINOS) was used for CO measurement in the range of 0 − 4000 ppm.

34 CHAPTER 3 EXPERIMENTAL

Figure 3.1: Flow sheet of the water-assisted CVD set-up used in the CNT growth expe-

riments.

3.1.1.3 Ammonia-assisted CVD setup

The ammonia-assisted CVD setup possessed similar structure to the aforementioned

water-assisted CVD setup, where the Ar (line 2) was replaced by H2/Ar (1.0 vol.% in Ar)

and the water bubbler was disconnected. The flow scheme of the CVD setup is shown in

Figure 3.2.

Figure 3.2: Flow sheet of the ammonia-assisted CVD set-up used in the CNT growth ex-

periments.

3.1 SYNTHESIS 35

3.1.1.4 Synthesis of Co-Mn-Al-Mg oxide by co-precipitation

The Co-Mn-Al-Mg oxide (Co3Mn3Al0.8Mg1Ox) catalyst was prepared by the co-

precipitation method at pH = 10 using nitrates.[41] After drying, calcination, pressing,

and sieving catalyst particles, those with a grain size of 300 μm ± 50 μm were used in

this study. Typically, about 5 mg of the catalyst were loaded and fixed in the effluent

side of the U-tube reactor using quartz wool plugs. A quartz rod was used in the other

branch of the U-tube reactor to minimize the dead volume of the system.

3.1.1.5 Synthesis of CNTs by water- and ammonia assistances

Water-assistance

In a typical water-assisted CNT growth experiment, the loaded catalyst was pre-

reduced by heating to 650 ºC at a heating rate of 5 ºC min–1 in a 1 : 1 mixture of H2 and

Ar at a total flow rate of 150 sccm (line 1 in Figure 3.1). Subsequently, feed gases consist-

ing of C2H4 (57 sccm), H2 (43 sccm), Ar (line 1 + line 2, 50 sccm) and H2O vapor (e.g., 200

ppm) were introduced to the reactor. The Ar line 2 was used as carrier gas for H2O va-

por, and Ar line 1 was used to balance the total flow at 150 sccm. Before each growth ex-

periment, the reactor was bypassed for about 5 min until a stable gas mixture with the

desired H2O concentration was achieved. The concentrations of C2H4 and CO in the ex-

haust gas were recorded by two infrared detectors with a sampling rate of 1.0 s. After

CNT growth, the reactor was cooled to room temperature in Ar atmosphere. Blank expe-

riments without catalyst in the reactor were performed by following the same procedure

and reaction conditions.

Ammonia-assistance

In a typical ammonia-assisted CNT growth experiment, the reduction of catalyst was

performed following the same procedure as described in ''water-assisted CNT growth

experiment''. Prior to the CNT growth, feed gases consisting of C2H4 (57 sccm), H2 (43

sccm), Ar and NH3 (e.g., 1000 ppm) were introduced to the reactor. The NH3/Ar was used

as NH3 supply and Ar was used to balance the total flow at 150 sccm. Before each growth

experiment, the reactor was bypassed for about 5 min until a stable gas mixture with the

desired NH3 concentration was achieved. The concentration of C2H4 in the exhaust gas

was recorded by the infrared detector with a sampling rate of 1.0 s. After the CNT

growth, the reactor was cooled to room temperature in Ar atmosphere.

To minimize the experimental error, each growth experiment was repeated 3 − 5

times, depending on the quality of data obtained, and the average was used for kinetic


analysis. The mean standard deviation of the experimental data confirms the high re-

producibility of each experiment. The reactor was weighed before and after the CNT

growth for 300 s to determine the overall carbon yield (Y300s), which was used to confirm

the calculated yield based on the conversion of ethylene.

Variation of the H2O concentration

The CNT growth was performed at 650 ºC. H2O vapor was introduced into the reactor

by passing Ar (line 2 in Figure 3.1) through a saturator at 0 ºC achieved by using a wa-

ter-ice bath. The concentration of H2O vapor was calculated from the flow rate of Ar (0 –

50 sccm, line 2) by assuming saturated vapor pressure at 0 ºC. The total flow was always

fixed at 150 sccm by varying the flow rate of Ar line 1, correspondingly. The concentra-

tions of H2O vapor were set at 0, 200, 250, 300 and 500 ppm. It is worth noting that high

H2O concentrations were achieved by increasing the flow rate of Ar (line 2) used as carri-

er gas. The H2O stream reached the reactor within a shorter period of time at higher

concentrations, and subsequently the reaction products reached the detector earlier.

Variation of growth temperature

The catalysts were first reduced at 650 ºC prior to the CNT growth at 550, 650 and

750 ºC with a constant H2O concentration of 200 ppm. To ensure comparability, the cool-

ing or heating step after reduction was carried out in Ar (150 sccm) always for 60 min to

allow the reactor reaching the desired growth temperature.

Variation of the NH3 concentration

The CNT growth was performed at 650 ºC. The concentration of NH3 was calculated

at the ratio of the flow rate of NH3/Ar (0 – 50 sccm) to the total flow rate of 150 sccm. The

total flow was always fixed at 150 sccm by varying the flow rate of Ar, correspondingly.

The concentrations of NH3 were set at 0, 1000, 2000 and 3300 ppm.

3.1.2 Synthesis of NCNTs



ethylenediamine (EDA, > 99 %, VWR International), nitric acid (HNO3, 65 %, Sigma-

Aldrich), helium (99.999 %, Air Liquide), hydrogen (99.999 %, Air Liquide).

3.1 SYNTHESIS 37

3.1.2.2 Injection-CVD setup

The experimental setup used in this study is schematically shown in Figure 3.3. The

injection CVD synthesis of NCNTs was performed in a horizontal quartz tube reactor

with a length of 70 cm and an inner diameter of 3 cm. The reactor was heated in a three-

zone electrical furnace (length 45 cm, Carbolite GmbH). Mass flow controllers (Bronk-

horst) were installed for flow regulations of hydrogen and helium. The inlet of the reac-

tor was coupled with a spiral tube which was heating up to 100 – 120 ºC to vaporize the

liquid precursor, which was injected into the inlet of the spiral tube by a syringe pump

(Legato 100, KD Scientific, USA). Helium flow was used to carry the vaporized precursor

into the reactor. A LabView interface software was used to control temperature and gas

flow.

Figure 3.3: Flow diagram of the injection CVD set-up used in the synthesis of NCNTs.

3.1.2.3 Synthesis of NCNTs by injection-CVD

In a typical NCNT growth experiment, a quartz boat containing about 60 mg of Co-

Mn-Al-Mg oxide catalyst (section 3.1.1.4) was loaded to the center of the tube reactor in

the furnace. The distance between the quartz boat and the inlet of the reactor was kept

constant for all the growth experiments, which is important to obtain highly reproduci-

ble results. The catalyst was firstly reduced in a gas mixture of H2 (50 sccm) and He (50

sccm) while the furnace was heated from room temperature to a desired temperature

(e.g. 650 °C) at a rate of 10 °C min–1. The NCNT growth was started by switching the

flowing gas to He (100 sccm) and injecting EDA to the inlet of the spiral tube by the sy-

ringe pump at a specific rate. The injection was terminated after a specific duration. He

flow was kept constant at 100 sccm until cooling to room temperature. Finally, the sam-

ple was collected from the quartz boat for further studies.


In this study, the influence of three main process parameters, i.e., growth time, pre-

cursor concentration (gas-phase), and growth temperature were investigated in the CVD

synthesis. The concentration of EDA in the gas-phase was calculated by converting the

liquid feeding rate to gas feeding rate.

The weight-based NCNT yield (Y) was calculated by Eq. 3.1.

Y = mproduct–mcat.mcat.

(3.1)

where mproduct is the total weight of collected NCNTs right after growth, mcat. is the ini-

tial weight of the used catalyst before reduction.

Variation of the growth time

The reaction temperature and EDA concentration were fixed at 650 °C and 4.6 vol.%.

Growth times of 30, 60, 90, 120, 150 and 180 min were investigated.

Variation of the ethylenediamine concentration

The reaction temperature and duration were fixed at 650 °C and 120 min. The varia-

tion of the EDA concentration in gas-phase was adjusted by varying the injection rate of

the syringe pump. EDA concentrations of 3.4, 4.6, 5.9 and 7.1 vol.% were investigated.

Variation of the growth temperature

The EDA concentration and growth time were fixed at 5.9 vol.% and 120 min. Growth

temperatures of 550, 650, 750, and 850 °C were investigated. An additional cooling or

heating step after reduction was always carried out in He (100 sccm) for 30 min to allow

the reactor reaching the desired growth temperature, which was to ensure comparabili-

ty.

3.1.2.4 Purification of NCNTs

The as-produced NCNTs were purified by acid-washing before further characteriza-

tion. In detail, The NCNTs were washed in 1.0 M HNO3 solution by a ratio of 100 mg

(NCNTs)/100 mL (HNO3) under magnetically stirring for 72 h at room temperature.

Subsequently, the NCNTs were filtered and washed with deionized water for 3 times,

then dried at 60 – 80 °C in air overnight.

3.1.2.5 Synthesis of CNTs

In order to establish a fair comparison of growth kinetics between NCNTs and CNTs,

it is essential to perform the CNT growth with similar C and H concentrations to those

3.1 SYNTHESIS 39

contained in the NCNT growth. For example, compared with the NCNT growth at 650 °C

with 5.9 vol.% EDA, the CNT growth was carried out at 650 °C in a gas flow containing

6.0 vol.% C2H4, 12.0 vol.% H2 and 82.0 vol.% He with a total flow of 100 sccm. Growth

times of 30, 60, 90, 120, 150 and 180 min were investigated.

3.1.3 Synthesis of metal oxide-CNTs hybrids



commercial spinel manganese cobalt oxide (Co1.5Mn1.5O4, particle size: 0.10 – 0.50 μm,

SA: 10.0 – 14.0 m2 g–1, Sigma-Aldrich), nitric acid (HNO3, 65 %, Sigma-Aldrich), HPLC

water (Sigma-Aldrich), helium (99.999 %, Air Liquide), hydrogen (99.999 %, Air Li-

quide,), ethylene (C2H4, 99.95 %, Air Liquide), synthetic air (99.999 %, Air Liquide).

3.1.3.2 Synthesis of CNTs

The synthesis of CNTs was performed according to the procedure used in our previous

studies.[40-41] The growth experiments were carried out in a flow setup equipped with a

three-zone electrical furnace and a horizontal quartz tube reactor with a length of 84 cm

and an inner diameter 3 cm. The flow scheme of the CVD setup is shown in Figure 3.4.

The gas flows were regulated by mass flow controllers, and a LabVIEW interface was

used to control the system. Typically, about 10 mg of catalyst were placed in a quartz

boat (length: 15 cm) located at the center of the quartz reactor. The catalyst was reduced

in diluted H2 flow (50 vol.% H2 in He, total 100 sccm) by heating to 650 °C at a heating

rate of 10 °C min–1. After reduction the CNT synthesis was performed at 650 °C in a feed

gas mixture of 57 sccm C2H4 and 43 sccm H2 for a specific duration. After the reaction

the sample was removed from the reactor and weighted. The CNT yield YCNT was deter-

mined by the Eq. 3.2:

Y = mproduct–mcat.mcat.

(3.2)

where mproduct is the total mass of the sample after reaction and mcat. is the mass of the

catalyst before reduction. The as-grown CNT samples were ground in an agate mortar

before further applications.


Figure 3.4. Flow sheet of the CVD setup used in the CNT growth experiments.

3.1.3.3 HNO3 vapor treatment

The evaporation temperature of HNO3 was maintained at 115 °C for all the treat-

ments. The as-grown samples were partially oxidized via HNO3 vapor treatment at 200

°C for 8, 24, 48 and 72 h, respectively.

3.1.3.4 Oxidation by thermal oxidative cutting

A thermal method published by Shaffer et al.[260] and developed by Zhao et al.[28] was

used for thermal oxidative cutting of CNTs. About 500 mg of as-grown samples were

loaded in a quartz boat (length: 15 cm) and placed into the inlet part of a tubular reactor

with an inner diameter of 20 mm. The sample was heated at specific temperature for 5

min, and then withdrawn from the heating zone and kept for 12 min. Thermal oxidative

cutting of NCNTs was achieved by repeating this two-step procedure three times while

maintaining the flow rate of the synthetic air at 100 sccm. The oxidation temperatures of

400, 500 and 600 °C were investigated.

3.1.3.5 Purification of samples

The as-grown and oxidized samples were purified by washing in 1.5 M HNO3 by a ra-

tio of 100 mg (NCNTs)/100 mL (HNO3) under magnetically stirring for 72 h at room

temperature. Subsequently, the NCNTs were filtered and washed with deionized water

for 3 times, then dried at 60 – 80 °C in air overnight.

3.1.3.6 Synthesis of MnOx/OCNTs

Oxygen-functionalized OCNTs (OCNTs) were obtained by oxidizing the purified CNTs

(Baytubes C 150 P) with HNO3 vapor at 200 °C for 72 h. MnOx/OCNTs with 7.88 wt.%

3.1 SYNTHESIS 41

Mn loading which was prepared by incipient wetness impregnation using Mn(NO3)2

aqueous solution.[261]

3.2 Characterization

SEM: The morphology and homogeneity of the CNTs was examined with a LEO

(Zeiss) 1530 Gemini scanning electron microscope (SEM) or a Quanta 3D FEG scanning

electron microscope. The diameter distribution was obtained by counting 500 – 600

CNTs.

Elemental analysis: The contents of carbon, nitrogen and oxygen were determined by

a VarioEL analyzer (Elementar Analysensysteme GmbH). The contents of metals (Co,

Mn, Al, Mg) were determined by inductively coupled plasma-optical emission spectrosco-

py (ICP-OES, PU701 UNICAM).

BET surface area: N2 physisorption measurements were carried out at 77 K using

an Autosorb–1 MP Quantachrome system. Prior to the measurements, all samples

were degassed at 200 °C for 2 h. The surface areas were calculated from the linear

part of the Brunauer-Emmett-Teller (BET) plots.

XRD: X-ray diffraction (XRD) was performed using a PANalytical theta-theta

powder diffractometer with a Cu Kα source. Scans were run from 10 to 80 ° with a

step width of 0.03 ° and a collection time of 20 s per step.

Raman spectroscopy: Raman spectra were collected by employing a Horiba Jobin

Yvon LabRam 2 confocal Raman Microscope with a laser of wavelength at 632.817 nm

(He/Ne, 1.96 eV) as excitation sources. The spectral resolution was 2.04 cm–1 per pixel

with 600 groove/mm grating. A Si wafer was used as sample holder. A 50 × objective was

used. The impinged laser power was kept constant at 4.1 mW for CNT samples and 0.41

mW for NCNT samples, respectively. Each spectrum was collected by an integration

time of 90 s and five times of accumulation in order to achieve spectrum with good sign-

noise ratio. Each sample was measured three times at different location. The data

process was performed by the following steps: (1) Baseline correction and spectrum

smooth by LabSpec (Version 5.0); (2) Spectral normalization to the Raman shift of 1581

cm–1 by Origin software (OriginLab® corporation); (3) Averaging spectra of each sample;

(4) Spectrum evaluation through a five-peak fitting for first order spectrum with the

range of 850 – 1850 cm–1 and a three-peak fitting for second order spectrum with the

range of 2300 – 3029 cm–1, as developed by Sadezky et al.[262]


TPO: Temperature-programmed oxidation (TPO) experiments were carried out in a

horizontal quartz tube reactor (Figure 3.5). An online infrared detector (Bühler Technol-

ogies, Germany) was used to quantitatively determine the release of CO and CO2, which

was calibrated in the range 0 − 4000 ppm by standard gas mixtures prior to the mea-

surements. For each measurement, 5.0 mg of sample was heated from room temperature

to 800 °C at a heating rate of 1 °C min–1 under flowing diluted O2 (5 vol.% O2 in He) at a

flow rate of 30 sccm.

Figure 3.5: Scheme of the tube reactor used for TPO and TPD experiments.

TPD: Temperature-programmed desorption (TPD) experiments were carried out in

the same reactor used in TPO (Figure 3.5). Helium (99.999 %, 30 sccm) is used as carrier

gas. Typically, about 50 mg OCNTs were used for each measurement. The reactor was

heated from room temperature to 1000 °C at a heating rate of 2 °C min−1 and held at

1000 °C for 60 min. The CO-CO2 detector was used for quantitative analysis of CO and

CO2 released during the decomposition of surface oxygen groups.

TPR: Temperature-programmed reduction (TPR) measurements were performed in a

flow set-up using 4.734 vol.% H2 in Ar at a flow rate of 84.1 sccm. The H2 concentration

in the exhaust gas was monitored by a thermal conductivity detector (Hydros, Fisher-

Rosemount). The reactor loading with about 50 mg of the sample was heated from room

temperature to 750 °C with a heating rate of 5 °C min–1 and held for 120 min at the

maximum temperature before cooling down.

quartz rod quartz wolle

gas flow directionØ1: 7 mm

Ø2: 3 mmsample

3.3 ELECTROCHEMICAL TESTS 43

3.3 Electrochemical tests

Electrochemical evaluations were performed at room temperature in a conventional

three-electrode electrochemical glass cell controlled by a Autolab potentiostat /

galvanostat (PGSTAT12, Eco Chemie, Utrecht, The Netherlands) combining with a ro-

tating disc electrode rotator (EDI101; Radiometer, Villeurbanne, France) and its speed

control unit (CTV101). A home-made Ag/AgCl/3 M KCl electrode (+0.210 V vs NHE) and

a platinum wire served as reference electrode (RE) and counter electrode (CE),

respectively. Several glassy carbon disc electrode (diameter: 3.8 mm) encapsulated in

Teflon cylinder were employed as working electrodes (WE).

Preparation of working electrodes

The glassy carbon electrodes were polished by using different alumina pastes (3.0, 1.0,

0.5, 0.1, and 0.05 μm) to obtain a mirror-like surface and subsequently cleaned by water

in the ultrasonic bath. The catalyst suspension was prepared by ultrasonically dispers-

ing 5.0 mg of the catalyst in a mixture of water (490 μl), ethanol (490 μl) and Nafion® (5

%, 20 µl). Subsequently, 5.3 μl of the resulting catalyst suspension were dropped onto the

polished glassy carbon electrode to obtain a catalyst loading of 233 µg cm–2. The electrode

was dried in air at room temperature before measurement.

Electrochemical measurements

(1) Oxygen evolution reaction

Cyclic voltammetry and linear sweep voltammetry

Prior to the measurement, the KOH electrolyte (0.1 M) was purged with synthetic air

for 20 min to reach the O2-saturated state. Cyclic voltammetry (CV) experiments were

carried out in the potential range of 0.0 – 1.0 V (vs Ag/AgCl/3 M KCl) at a scan rate of

100 mV s–1 and a rotation speed of 1600 rpm. Linear sweep voltammetry (LSV) experi-

ments were carried out in the potential range of 0.0 – 1.0 V (vs Ag/AgCl/3 M KCl) at a

scan rate of 5 mV s–1 and a rotation speed of 1600 rpm.

Chronoamperometry

The durability of the catalyst was assessed by chronoamperometry at 0.65 V (vs

Ag/AgCl/3 M KCl) for 72 h in quiescent air-saturated KOH (0.1 M).


(2) Oxygen reduction reaction

RDE voltammetry

Prior to the rotating disk electrode (RDE) voltammetry, the catalyst was scanned in

Ar-saturated KOH electrolyte (0.1 M) in the potential range of + 0.2 to – 0.8 V (vs

Ag/AgCl/3 M KCl) at scan rates of 100 mV s–1 and 5 mV s–1 without rotation, respective-

ly. Subsequently, the KOH electrolyte (0.1 M) was purged with synthetic air for 20 min

to reach the O2-saturated state. CV experiments were carried out in the potential range

of + 0.2 to – 0.8 V (vs Ag/AgCl/3 M KCl) at scan rates of 100 mV s–1 and 5 mV s–1 without

rotation, respectively. RDE experiments were performed in same potential range at a

scan rate of 5 mV s–1 and a rotation speed of 100, 400, and 900 rpm, respectively. The

voltammograms measured in Ar-saturated were subtracted from those measured in the

O2-saturated electrolyte.

Data processing

The RHE calibration was converted using the furmula: ERHE = EAg/AgCl/3 M KCl + 0.210 +

0.0592 × pH, where EAg/AgCl/3 M KCl is the experimental potential vs Ag/AgCl/3 M KCl. All

the potentials were corrected by IR drop, where R was measured by i-interrupt.

Therefore, all the potentials mentioned in the section RESULTS AND DISCUSSION

were reported against the RHE. Moreover, all the current densities were calculated

using the geometric surface area of the electrode (s = 0.1134 mm2).

Overpotential (η) was calculated using the formula η = ERHE – 1.23.

Tafel slops

According to the Bulter-Volmer equation,

where i is, i0 is, α is, f =F/RT, η is overpotential,

ln ln

The plot of ~ ln gives a linear relationship, and the slope is . This slope is called

the Tafel slope, which provides the information about the electron transfer in the rate-

determining step.

Chapter 4 Synthesis of CNTs and NCNTs

4.1 Growth kinetics of CNTs*

4.1.1 Introduction

Among various methods, catalytic chemical vapor deposition (CVD) has been widely

used for the large-scale synthesis of CNTs due to its easy control of feedstock and wide

window of reaction conditions.[41, 82] However, the deposition of amorphous carbon on ac-

tive sites during the CNT growth process often causes a fast deactivation of the catalyst,

leading to a low yield of CNTs.[19] Considering the wide range of applications and the in-

creasing demand for CNTs in various fields, the large-scale production of CNTs with uni-

form properties remains a challenge.[82]

The addition of etching agents to the feed during the CNT growth, such as H2O[18, 21]

and NH3[161-163, 263] was investigated and proved to be beneficial for the CNT yield and

quality. Water vapor in the feed gas promotes the growth by selective etching of amorph-

ous carbon deposits through the gasification reaction (Eq. 4.1), which was confirmed by

monitoring CO in the effluent gas[115] and ex situ microscopic and spectroscopic

analysis.[19] Recent studies indicate that water promotes the formation of hydroxyl

groups on carbon, which subsequently inhibits catalyst ripening by reducing the diffu-

sion of catalyst particles.[119]

(4.1)

1 (4.2)

The large-scale production of CNTs with a high growth rate calls for a fundamental

understanding of the growth kinetics under different conditions. A simple kinetic model

*A part of this chapter is based on the publication ‘Influence of Water on the Initial Growth Rate of Carbon Nanotubes from Ethylene over a Cobalt-Based Catalyst’ by K. P. Xie, M.

Muhler and W. Xia in Ind. Eng. Chem. Res., 2013, 22, 312 – 320.

46 CHAPTER 4 SYNTHESIS OF CNTS AND NCNTS

(Eq. 4.2) was developed by Hata and coworkers[21] to study the growth of vertically

aligned single-walled CNTs. This model was very suitable to describe the relation be-

tween the height of the CNT forest (H), the initial growth rate (β), and the mean lifetime

of the catalytically active sites (τ0). The initial growth rate reflects the initial carburiza-

tion-nucleation process, and the mean lifetime mirrors the deactivation of catalyst.

Most of the literature studies conclude that the major contribution to the enhanced

yield of CNTs by water-assisted CVD is an extended mean lifetime of the catalytically

active sites through etching the deposited amorphous carbon or inhibiting the sintering

of catalyst particles.[150] With regard to the influence of ammonia in both pretreatment

and growth stages, most of the studies only examine the CNT morphology in the pres-

ence and absence of ammonia. Few reports analyze of the growth kinetics of CNT growth

in the presence of NH3.

H2O in the feed gas not only gasifies amorphous carbon,[18-19] but also reacts with me-

tallic catalysts forming oxides.[115] Similarly, NH3 is believed to either etch amorphous

carbon[134, 162] or induce the reconstruction of catalyst particles during the CNT

growth.[164, 264] Hence, the initial growth rate and the mean lifetime can be affected by

adding H2O and NH3 to the feed gas. Therefore, it is necessary to investigate the roles of

H2O and NH3 in the growth of MWCNTs from a kinetic perspective. The CNT growth

kinetics was studied by using gas chromatography to measure the consumption of the

carbon source[20] or by ex situ electron microscopy to determine the height of CNT fo-

rests.[21] These techniques are insufficient for monitoring the initial stage of the CNT

growth. Recently, our research group has investigated the initial CNT growth kinetics

using Co-based mixed oxide catalysts.[112] The initial growth rate and the mean lifetime

of the catalytically active sites were derived for different growth temperatures and ethy-

lene concentrations.

In this study, the influence of H2O vapor and NH3 on the initial CNT growth kinetics

was investigated. Gas detectors with a sampling rate of 1.0 s were employed for fast on-

line quantitative analysis to achieve reliable data within the first few minutes of the

CNT growth. Carbon mass accumulation was derived from the degrees of C2H4 consump-

tion, and CO generation due to carbothermal reduction of the mixed oxide catalyst and

gasification of amorphous carbon deposit was monitored online. The initial growth rate

and the mean lifetime of the catalytically active sites derived from the kinetic model

were used to evaluate the formation of CNTs and the catalyst deactivation. The CO gen-

eration provided additional information on the initial formation of CNTs and the deacti-

4.1 GROWTH KINETICS OF CNTS 47

vation of the Co catalyst. The influence of H2O vapor on the CNT morphology and NH3

on the nitrogen doping of the CNTs was also investigated.

4.1.2 Evaluation of kinetic data

The carbon mass accumulation (mt) was described by Eq. 4.3.[112] The fitting of Eq. 4.3

was performed with the Origin software, which yielded the initial growth rate (r0) and

the mean lifetime (τ) of the catalytically active sites. The product of r0 and τ yielded the

theoretically predicted maximum CNT yield (Ymax) assuming that all the obtained carbon

material consisted of CNTs, as previously confirmed.[112]

1 (4.3)

As a typical example, the C2H4 concentration profiles ( ~ ) obtained from the gas

detector with or without catalysts are shown in Figure 4.1a. The concentration was con-

verted to molar flow rate as shown in Figure 4.1b. The C2H4 consumption rate (∆ )

was obtained by subtracting the C2H4 flow rate ( ) of the blank experiment (Figure

4.1c). Subsequently, the carbon mass accumulation (mt) was calculated by integrating

the C2H4 consumption rate, which was averaged from 3 – 5 repeated experiments (Figure

4.1d).

The calibration recorded a time delay of 16 s from the reactor to C2H4 detector, and

another time delay of 2 s from C2H4 detector to CO detector, which were considered dur-

ing data processing by shifting the time scale correspondingly. Hence, relative time

( ) was used in the ethylene consumption rate profile (∆ ~ , Figure

4.1c) and the carbon mass accumulation profiles ( ~ , Figure 4.1d).


Figure 4.1: (a) C2H4 concentration profiles ( ~ ) during the CNT growth and the

blank experiment; (b) corresponding profiles of the molar flow rates

( ~ ) derived from (a); (c) time-resolved consumption rate of ethylene

(∆ ~ ) during the CNT growth calculated by subtracting the mo-

lar C2H4 flow rate from the blank experiment; (d) time-resolved accumula-

tion of the carbon mass ( ~ ) during the CNT growth under stan-

dard conditions calculated from theC2H4 consumption rate. The initial

growth rate (r0) is the initial slope of the fitting curve.

4.1.3 Results and discussion

4.1.3.1 Influence of H2O vapor during the initial growth step

The Co-based mixed oxide catalyst is highly active for CNT growth, and the product is

proved to be of high purity without other carbon materials.[40-41] Figure 4.2 shows the

CNT mass accumulation (mt) at 650 °C at different H2O concentrations obtained from the

measured C2H4 consumption profiles. It can be seen that the initial mt within the first 21

s was slightly influenced by H2O vapor (Figure 4.2a). At least two processes occurred at

this stage, namely, carbothermal reduction of the catalyst and dissolution of carbon in

the catalyst. It is known that the mixed oxide catalyst was not fully reduced at the ap-

plied temperature of 650 °C.[40-41] H2O as a weak oxidant can influence both processes at


the initial catalyst activation stage. In contrast, clear differences in mt were observed at

different H2O concentrations during further growth up to 300 s (Figure 4.2b). Enhanced

mt was observed for a H2O vapor concentration of 200 ppm

Figure 4.2: CNT mass accumulation ( ) as a function of relative time ( ) at dif-

ferent H2O concentrations. The growth was performed at 650°C. (a) First 21

s and (b) up to 300 s.

Figure 4.3: (a) The initial growth rate (r0) and (b) the mean lifetime (τ) of the Co catalyst

as a function of the H2O concentration. The growth was performed at 650 ºC.

The initial growth rate (r0) and the mean lifetime of the catalytically active sites (τ)

were derived using the kinetic model (Eq. 4.3). Without H2O the r0 and τ were deter-

mined to be 0.125 gCNTs gcat.–1 s–1 and 481 s, respectively. Figure 4.3 shows that both r0 and

τ pass through a maximum with increasing the H2O concentration. At 200 ppm of H2O

both r0 and τ reached maxima at 0.130 gCNTs gcat.–1 s–1 and 1065 s, respectively. A further

increase in the H2O concentration led to a decrease of the two parameters. H2O as a

weak oxidant can change the oxidation state of catalysts and effectively remove depo-


sited amorphous carbon on the active sites of the catalyst. Thus, H2O at appropriate con-

centrations can enhance the initial growth rate and extend the mean lifetime of active

sites resulting in a higher yield of CNTs. However, when exceeding a certain level, H2O

showed an overall negative effect on the growth of CNTs, likely due to the partial oxida-

tion of the metallic Co catalyst lowering its catalytic activity

The CNT yield obtained by weighing the reactor before and after growth for 300 s

(Y300s) is shown in Table 4.1, which is compared with the calculated carbon accumulation

after 300 s (m300s), and the theoretically predicted maximum CNT yield (Ymax). It can be

seen that the CNT yields determined by weighing and calculation show only small devia-

tions. All of the three types of yield increase when feeding 200 ppm H2O vapor. However,

a further increase of the H2O concentration led to a decrease of all the three yields. The

CNT yields with more than 300 ppm of H2O are lower than those without H2O.

Table 4.1: CNT yield by weighing the reactor before and after growth for 300 s (Y300s),

carbon accumulation by calculation after growth for 300 s (m300s) and theoreti-

cal predicted maximum CNT yield (Ymax) derived from curve fitting.a

H2O / ppm Y300s / gCNTs gcat.–1 m300s / gCNTs gcat.

–1 Ymax / gCNTs gcat.–1

0 27.2 27.8 60.1 200 31.8 33.7 138.6 250 27.9 28.5 77.2 300 26.6 26.9 66.1 500 23.6 24.0 54.3

a The growth was performed with different H2O concentrations at 650 ºC.

To verify the assumption that H2O as a weak oxidant can remove amorphous carbon

through gasification reaction (Eq. 4.1), the generation of CO was monitored during the

CNT growth. As can be seen from Figure 4.4a, only a single CO peak was detected with-

out H2O, which can be assigned to the carbothermal reduction of the mixed oxide cata-

lyst containing cobalt oxides by C2H4 (Eq. 4.4). In the presence of H2O, an additional

broad contribution was observed following the first carbothermal reduction peak. It was

found that the intensity of these two CO peaks increased with increasing H2O concentra-

tions from 0 to 500 ppm. On the one hand, H2O can oxidize amorphous carbon species,

especially carbon atoms at defect sites, forming CO.[265] On the other hand, extra H2O as

weak oxidant can convert metallic cobalt catalysts to oxides (Eq. 4.5), which can be fur-

ther reduced by C2H4 releasing CO. These two steps correspond to the overall steam re-


forming of ethylene over cobalt (Eq. 4.6). The carbothermal reduction was dominant

within the first 12 s, while the gasification of carbon was dominant during the further

growth.

2 2 2 2 (4.4)

2 (4.5)

2 4 (4.6)

Figure 4.4: CO generation during the CNT growth at 650 ºC with different concentra-

tions of H2O vapor. (a) up to 300 s, (b) first 21 s.

To investigate the influence of H2O vapor on the CNT morphology, the CNTs grown

without and with 200 ppm of H2O were analyzed by SEM. Figure 4.5 shows the diameter

distribution obtained by measuring 500 CNTs. It can be seen that 95 % of the CNTs

grown without H2O vapor were in the range of 11 − 21 nm. In contrast, the CNTs grown

with 200 ppm H2O were in the range of 11 − 19 nm with a narrower diameter distribu-

tion. It is known that the diameter of CNTs is related to the particle size of the cata-

lysts.[40-41] Literature results showed that the presence of reactive gases like H2 or NH3

can improve the homogeneity of nanoparticles leading to a narrower size distribution.[251]

Hence, the narrower diameter distribution suggests a similar effect for the presence of

H2O in the feed gas.


Figure 4.5: Outer diameter distribution of CNTs grown at 650 ºC with 0 and 200 ppm of

H2O in the feed gas.

Figure 4.6: Time-resolved CNT mass accumulation ( ) as a function of relative time

( ) obtained at different temperatures. (a) First 21 s, (b) up to 300 s.

4.1.3.2 Influence of the growth temperature

The optimized H2O concentration of 200 ppm was used for temperature variation ex-

periments, and the growth without H2O was studied for comparison. While varying the

growth temperature, the catalysts were always reduced at 650 °C. The corresponding

CNT mt were derived from consumed C2H4. Figure 4.6a shows mt vs relative time in the

first 21 s. It can be seen that at the initial stage, mt increases with increasing growth

temperature from 550 to 750 °C, which is in good agreement with our earlier studies.[112]

Interestingly, the initial CNT growth was depressed by H2O at 550 and 650 °C but pro-

moted at 750 °C. It is known that the gasification needs a higher temperature than 550

°C.[265] Hence, H2O in the feed gas may dominantly react with the Co catalyst at 550 °C


lowering the growth rate. During further growth for up to 300 s, mt was strongly inhi-

bited by H2O vapor at 550 °C, but dramatically promoted at 750°C (Figure 4.6b). Surpri-

singly, mt at 650 °C in the presence of H2O was even higher than that at 750 °C. The re-

sults show that H2O has the highest promoting effect at 650 °C, whereas at 550 °C it acts

as an inhibitor for the CNT growth.

Figure 4.7 shows the trends of the initial growth rate (r0) and the mean lifetime of the

catalytically active sites (τ) determined by fitting the results shown in Figure 4.6. It can

be seen that both r0 and τ reached maxima at 650 °C for the growth without H2O. The r0

and τ were enhanced by feeding H2O at higher growth temperatures of 650 and 750 °C,

whereas at 550 °C both r0 and τ decreased in the presence of H2O. The decrease of r0 at

750 °C without H2O can be related to the sintering of catalyst particles.[82] Furthermore,

the concentration of C2H4 decreased at 750 °C due to homogeneous decomposition,[266]

which was also observed in the blank experiment as indicated by the decrease of C2H4

concentration at 750 °C. The r0 was accelerated dramatically at 750 °C by feeding H2O,

which is even slight larger than r0 at 650 °C. The higher r0 at 750 °C compared with 650

°C can be related to the less severe sintering of catalyst particles due to the presence of

H2O. In contrast to the slight increase of r0, τ decreased significantly at 750 °C. The ho-

mogeneous decomposition of C2H4 generates additional amorphous carbon deposits,

which cover the active sites of the catalyst leading to a faster deactivation.[40]

Figure 4.7: (a) The initial growth rate (r0) and (b) the mean lifetime of the catalytically

active sites (τ) as a function of growth temperature derived from Figure 4.6.

The CNT yield by weighing the reactor after growth for 300 s (Y300s), the carbon mass

accumulation by calculation after growth for 300 s (m300s) and the theoretical predicted

maximum CNT yields (Ymax) are summarized in Table 4.2. Small deviations of CNT yields

were observed between Y300s and m300s. The results confirmed that 650 °C is the optimal

temperature with the maximum yield. The comparison with the yields without H2O


shows that the addition of H2O led to a decrease of Y300s by 33 % at 550 °C, but to an in-

crease by 39 % at 750 °C. The theoretical predicted maximum CNT yields show a similar

trend.

Table 4.2: CNT yields by weighing the reactor after growth for 300 s (Y300s), the carbon

mass accumulation by calculation after growth for 300 s (m300s) and theoretical

predicted maximum CNT yield (Ymax) derived from curve fitting. The growth

was performed at different temperatures without and with H2O.

Temperature / ºC

H2O / ppm

Y300s

/ gCNTs gcat.–1

m300s

/ gCNTs gcat.–1

Ymax / gCNTs gcat.

–1 550 0 19.1 19.8 36.0 550 200 12.8 13.6 18.9 650 0 27.8 28.0 91.4 650 200 28.9 30.3 119.7 750 0 17.6 17.9 25.0 750 200 24.5 25.4 49.0

Figure 4.8: CO profiles obtained under different growth temperatures with 200 ppm of

H2O vapor. (a) First 21 s, (b) up to 300 s.

Similarly, the time-resolved CO generation at different growth temperatures in the

presence of 200 ppm H2O was determined (Figure 4.8). It can be seen that the intensity

of the first CO peak increases with increasing growth temperature from 550 to 750 °C.

As mentioned above, the first CO peak can be attributed to the carbothermal reduction

of the oxides in the catalysts. A higher temperature leads to a higher activity of carbo-

thermal reduction of oxides (Figure 4.8a). The second CO peak mainly related to the ga-

sification of amorphous carbon shows the lowest intensity at 650 °C and the highest in-


tensity at 750 °C. A too low growth temperature, for instance 550 °C, may result in the

deposition of amorphous carbon or polyaromatics on CNTs, which can be oxidized by

H2O. As discussed above, thermal or homogeneous decomposition of C2H4 occurred at 750

°C leading to the deposition of amorphous carbon. These results agree well with the

short mean lifetimes of the active sites at 550 and 750 °C. On the contrary, the long

mean lifetime of active sites at 650 °C can be assigned to a minimum amount of carbon

deposits.

The diameter distribution of CNTs grown at different temperatures without and with

H2O was obtained by SEM studies (Figure 4.9). It can be seen that the mean diameter

increases with increasing growth temperature. Furthermore, the H2O vapor in the feed

gas narrowed the CNT outer diameter, which is related to the metal particle size.[107-109,

267] On the one hand, the Co particle size increases with temperature. On the other hand,

H2O vapor seems to favor the formation of smaller Co particles during the activation of

the pre-reduced mixed oxide precursor under growth conditions in the presence of C2H4

and H2.[40]

Figure 4.9: Outer diameter distributions (a-c) and mean diameter (d) of CNTs grown at

different temperatures with 0 and 200 ppm of H2O vapor.


4.1.3.3 Influence of NH3 during the initial growth step

Figure 4.10 shows mt at 650 °C at different NH3 concentrations obtained from the

measured C2H4 consumption profiles. It can be seen that the initial mt within the first 21

s was depressed by NH3 (Figure 4.10a), indicating negative effect of NH3 on the initial

activation of catalyst. With further growth up to 300 s (Figure 4.10b), mt was slightly im-

proved by the NH3 concentration of 1000 ppm, but strongly inhibited by the NH3 concen-

trations of 2000 and 3300 ppm, respectively.

Figure 4.10: CNT mass accumulation ( ) as a function of relative time ( ) at dif-

ferent NH3 concentrations. The growth was performed at 650°C. (a) First 21

s and (b) up to 300 s.

Figure 4.11a shows a downrend of r0, which slowed down from 0.117 gCNTs gcat.–1 s–1 at

1000 ppm to 0.088 gCNTs gcat.–1 s–1 at 3300 ppm. In contrast, Figure 4.11b presents an up-

down trend of τ, which was extended to 774 s at 1000 ppm and then withdrawn to 532 s

at 3300 ppm.

The CNT yield by weighing the reactor after growth for 300 s (Y300s), the carbon mass

accumulation by calculation after growth for 300 s (m300s), the theoretical predicted max-

imum CNT yields (Ymax) and nitrogen content determined from the CNTs grown for 300 s

are summarized in Table 4.3. Again, small deviations of CNT yields were observed be-

tween Y300s and m300s. All of the three types of yield increase when feeding 1000 ppm NH3.

However, a further increase of the NH3 concentration caused a dramatic decrease of all

the three yields. Moreover, the nitrogen content determined from the CNTs grown for

300 s shows an increasing trend with NH3 concentration. The presence of nitrogen in the

grown CNTs reflected the participation of nitrogen during the CNT formation.


Figure 4.11: (a) The initial growth rate (r0) and (b) the mean lifetime (τ) of the Co catalyst

as a function of NH3 concentration. The growth was performed at 650 ºC.

Table 4.3: CNT yields by weighing the reactor before and after growth for 300 s (Y300s),

carbon accumulation by calculation after growth for 300 s (m300s) and theoreti-

cal predicted maximum CNT yield (Ymax) derived from curve fitting. Nitrogen

content determined from the CNTs grown for 300 s.

NH3 / ppm

Y300s / gCNTs gcat.

–1 m300s

/ gCNTs gcat.–1

Ymax / gCNTs gcat.

–1 N contenta

/ wt.% 0 27.2 27.8 60.1 -

1000 28.3 28.8 90.8 0.13 2000 23.1 23.7 61.4 0.24 3300 19.5 20.1 47.1 0.34

aThe N content was measured by VarioEl analyzer.

The kinetic results strongly suggest that the addition of NH3 slows down the initial

growth rate, as also reported by Pittinson,[264] by suppressing the carbon precipitation

during the CNT nucleation. On the other hand, the addition of NH3 with appropriate

concentration, which was not high enough to significantly suppress the initial growth

rate, can extend the mean lifetime of active sites (e.g. 1000 ppm) by carbon gasification

(Eq. 4.7), leading to the enhanced CNT yield.

(4.7)

Furthermore, exceeding a certain level, NH3 showed an overall negative effect on the

growth of CNTs, likely due to the poison effect on the metallic Co catalyst lowering its

catalytic activity, as indicated by the low initial growth rate and short mean lifetime of

active sites.


4.1.4 Summary

The influence of H2O and NH3 on CNT growth kinetics during the first 300 s was in-

vestigated by monitoring the C2H4 conversion and CO generation at different concentra-

tions and growth temperatures. The obtained carbon mass accumulation was fitted with

a kinetic model yielding the initial growth rate and the mean lifetime of the catalytically

active sites.

Water-assistance

The kinetic results revealed that H2O vapor not only affected the mean lifetime of

the catalytically active sites but also significantly influenced the initial growth rate.

Feeding 200 ppm H2O vapor at 650 °C accelerated the initial growth rate and extended

the mean lifetime of the catalytically active sites. Higher water concentrations of up to

500 ppm led to lower growth rates and lower CNT yields. At 550 °C, 200 ppm H2O were

found to be an inhibitor decreasing both the initial growth rate and the mean lifetime,

whereas at 650 °C and 750 °C, both the initial growth rate and the mean lifetime were

significantly enhanced by feeding H2O.

CO generation was attributed to either the carbothermal reduction of the catalysts

or to the carbon gasification by water. CO generation obtained at different tempera-

tures revealed that 650 °C was the optimal growth temperature in the scope of this

study, which was neither too low nor too high to cause the severe deposition of amorph-

ous carbon. Moreover, SEM studies show the thinning of the outer diameters of CNTs

in the presence of H2O.

Ammonia-assistance

At all the concentrations in the scope of this study, NH3 was found to lower the ini-

tial growth rate. At low NH3 concentration (≤ 1000 ppm), the initial growth rate was

slightly suppressed, therefore, the extended mean lifetime of the catalytically active

sites mainly contributed to the enhanced CNT yield. However, at the high NH3 concen-

tration (≥ 2000 ppm), both initial growth rate and mean lifetime of the catalytically ac-

tive sites were strongly suppressed, leading to the inhibited CNT yield. Moreover, the

results of elemental analysis confirmed the nitrogen doping in the as-grown CNTs.

4.2 SYNTHESIS AND CHARACTERIZATION OF NCNTS 59

4.2 Synthesis and characterization of NCNTs

4.2.1 Introduction

Over the last decade, CNTs have been extensively studied in various fields, such as

electronic devices and polymer reinforcement due to their outstanding electronic and

mechanical properties.[268-270] Due to their outstanding surface properties, high conduc-

tivity and chemical stability, CNTs have been increasingly used as catalyst support or as

catalysts in heterogeneous catalysis and electrochemical energy conversion and

storage.[11, 13, 271-272]

In-situ substitutional doping with nitrogen can effectively tune the physicochemical

properties of CNTs. The introduction of N atoms into the carbon lattice modifies the ar-

rangement of the hexagnol rings and lead to the formation several types of N-containing

groups like pyridinic, prrolic, and quaternary nitrogen groups. On the other hand, the

electron-rich N can activate the inert π electrons, resulting in significant changes in the

electronic and chemical properties of CNTs.[67] A structural feature of NCNTs has been

reported as being bamboo-shaped nanotubes which has been assigned to the stronger

metal-graphene binding during the formation of NCNTs than the formation of CNTs.[24]

The unique surface and structural properties of NCNTs have significantly boosted the

interest in exploring their applications in electrocatalysis and catalysis. The superior

performance in ORR makes the NCNTs promising alternatives to precious platinum cat-

alysts.[68] The NCNTs synthesized by in-situ CVD growth were found to be more stable in

highly corrosive media under industrially relevant conditions (10 M KOH, 80 ºC) than

the NCNTs obtained by post-treatment.[29] In addition, the NCNTs have been used as

metal-free catalysts in acetylene hydrochlorination reaction, where the activity and se-

lectivity of the NCNTs outperformed un-doped CNTs thanks to the enhancement in the

formation of the covalent bond between C2H2 and NCNTs.[70]

Catalytic CVD has been demonstrated to be a relatively mature technique for indus-

trial scale production of CNTs due to its easy control of feedstock and wide window of

reaction conditions.[27, 41, 82] Recently, the large-scale production of CNTs by CVD method

has been achieved in industry,[14, 27] making CNTs cheap and abundant. Hence, CVD is

the most promising for industrial production of NCNTs.[273]

Various nitrogen-containing precursors have been used in NCNT synthesis, for exam-

ple, ammonia, pyridine, acetonitrile, melamine, benzylamine, and ethylenediamine

(EDA). EDA is widely used as building block for the large-scale production of industrial

chemicals. Our research group has studied the catalytic growth of CNTs from ethylene

over a Co-based oxide catalyst and achieved very high CNT yields.[40-41] Considering the

abundance of EDA and its compositional similarities to ethylene, EDA is considered as a

promising candidate as N and C precursor for NCNT synthesis, especially, for the under-

standing of NCNT growth mechanism. For example, it has been reported that N inhibits

the growth[274] and decreases the oxidation resistance.[275]

Recently, there have been a number of reports on the catalytic growth of NCNTs.[22-26]

Koós et al.[23] studied the effect of the synthesis of NCNTs by aerosol CVD, where the

concerns were mainly on the effect of benzylamine/toluene ratio on the structure and N

contents of the obtained samples. Sharifi et al.[25] studied the N-doping effect on the

structure and the relationship between the type and amount of nitrogen species incorpo-

rated in NCNTs by XPS and Raman spectroscopy. A detailed study on the effect of tem-

perature on the morphology, dimension, defect, nitrogen inclusion and thermal stability

of NCNTs was reported by Chizari et al.[22] Most studies reported that the N content or

N/C ratio decreased with increasing growth temperature.[22, 26]

The purpose of this study is to understand the influence of synthesis parameters on

the NCNT yield, composition, morphology, graphitization and thermal stability in air.

Among others, the growth time, precursor concentration, and growth temperature were

systematically varied. A benchmarked Co-Mn-Al-Mg mixed oxide catalyst[14, 27, 40-41] was

employed for the synthesis of NCNTs with ethylenediamine as carbon and nitrogen pre-

cursors. Moreover, an etching effect of the N precursor is discussed.

4.2.2 Results and discussion

4.2.2.1 Influence of growth time

For the variation of growth time, the NCNTs were synthesized at 650 °C with 4.6 vol.%

EDA for 30 – 180 min with a time interval of 30 min. The NCNTs mentioned in this sec-

tion are all referred to these growth conditions.

The morphology of the NCNTs was examined by SEM in order to understand the ef-

fect of the growth time on the dimension of the grown NCNTs in terms of the size distri-

bution of the outer diameter and mean outer diameter of the nanotubes. It was found

that all the samples consisted of NCNT agglomerates with entangled nanotubes, as pre-

sented in Figure 4.12a-f. The homogeneity of the NCNTs grown for different durations

was investigated by a statistical analysis of the size distribution of the outer diameter of


600 NCNTs from several SEM images. A comparison of the obtained outer diameter dis-

tributions of all the samples is presented Figure 4.12g. The samples grown from 30 to

120 min showed a broader distribution, indicating poorer homogeneity of the grown na-

notubes. In contrast, the samples grown from 150 to 180 min presented narrower distri-

butions, indicating better homogeneity of the grown nanotubes. The corresponding stan-

dard deviations (no shown) indicate that more than 95 % of NCNTs of all the samples

were in a range of 12.5 – 35.0 nm. It can be seen from Figure 4.12h that the mean outer

diameter of the NCNTs as a function of growth time is in a volcanic shape. The mean

outer diameter of the NCNTs increases as the growth time increases from 30 to 90 min

(region 1), then decreases as the growth time further increases to 180 min (region 2).

Sharifi et al.[25] observed a similar trend of the outer diameter of NCNTs as a function of

growth time, where the thickening of the nanotubes was assigned to the coating of car-

bon species. The decreasing outer diameter after prolonged growth was seldom reported.

The thinning of nanotubes is assumed to be related to the surface etching through car-

bon gasification. A detailed discussion of the etching effect is given in section 4.2.2.6.

Figure 4.12: SEM images of purified NCNTs grown with 4.6 vol.% EDA at 650 °C as a

function of growth time (a-f). The corresponding size distribution of outer

diameter (g) and the mean outer diameter (h) determined by measuring 600

NCNTs.

Figure 4.13 shows that the yields of as-grown NCNTs and the contents of carbon, ni-

trogen as well as residual metals in purified NCNTs as a function of growth time. It can

be seen that the NCNT yield rises quasi-linearly as the growth time increases from 30

min to 120 min, and the trend terminates at 120 min (Figure 4.13a). As the growth time

further increases, a slight decrease in the yield was observed, which might be attributed

to the carbon gasification by the nitrogen-containing species or H2 decomposed from EDA.

The yields of the NCNTs were determined to be 1.5 ± 0.1 gNCNTs gcat.–1 at 30 min, 6.8 ± 0.1

gNCNTs gcat.–1 at 120 min, and 6.7 ± 0.0 gNCNTs gcat.

–1 at 180 min.

Figure 4.13b presents the contents of carbon, nitrogen, and the sum of residual metals

(i.e. Co, Mn, Al, Mg) in the purified NCNTs as a function of growth time. It is worth to

note that residual metals always exist in nanotubes despite of acid-washing.[276] The

100×N/C is the atomic percentage of nitrogen compared to carbon derived from the

weight ratio. The nitrogen content increased with increasing growth time for up to 150

min, and kept constant when further increasing growth time to 180 min. In contrast, the

carbon content rose gradually within the first 120 min. After peaking at 120 min, the

carbon content decreased with further growth to 180 min. As mentioned above, the car-

bon loss might be related to the carbon gasification by the nitrogen-containing species or

H2 decomposed from EDA (detailed discussion in section 4.2.2.6). The sum of residual

metal contents shows a converse trend as compared to the carbon content. The 100×N/C

as a function of growth time reaches a plateau at 150 min that is similar to the trend of

the nitrogen content. It implies that the ratio of N/C is not constant during the growth.

The reason is unclear: the varying N/C ratio may be due to the change in the precipita-

tion rates of nitrogen and carbon atoms or to the etching-induced carbon loss.


Figure 4.13. (a) The yield of as-grown NCNTs and (b) the contents of carbon, nitrogen

and residual metals in purified NCNTs as a function of growth time at

650 °C with 4.6 vol.% EDA. The C and N contents in weight percent (wt.%)

were determined from elemental analysis. The 100×N/C is the atomic per-

centage of nitrogen compared to carbon derived from the weight ratio.

Raman spectroscopy has been widely used for analyzing the disordered-induced de-

fects, amorphous carbon and electronic properties of NCNTs. The normalized Raman

spectra of purified NCNTs are compared in Figure 4.14a. The spectra show two first-

order modes: the G-band (~1581 cm–1) and the D-band (~1332 cm–1), as well as two

second-order modes: the 2D-band (~2650 cm–1) and the D+G-band (~2913 cm–1). The G-

band is a Raman active mode (E2g) originating from the tangential vibration of sp2 car-

bon networks.[277] The D-band is a Raman-inactive mode (e.g. A1g) but activated by the

crystalline edges. Therefore, the D-band is generally used as an indicator for edge defects

in carbon lattice.[277-279] The G'-band corresponds to the overtone of the D-band that ori-

ginates from the two-phonon scattering around the K point of the Brillouin zone. Even

though the G'-band is independent of the defects, a number of studies found that the G'-

band is sensitive to the change of graphitic structural order of CNTs.[280-282] The weak

peak at around 2913 cm–1 is a combination of the D- and G-band, also indicating the dis-

order-induced feature of CNTs.[283] A shoulder peak of G-bands at around 1620 cm–1 is

assigned to the D'-band which originates from a double resonant Raman process, imply-

ing structural disorder and defects in graphitic materials.[283]

In this work, a 5-peak fitting model, which was developed by Sadezky et al.[262] has

been successfully applied in characterizing CNTs[284-285] was utilized for spectral deconvo-

lution of first-order modes. According to the previous studies, two more bands, the I-band

(1200 cm–1) and the D''-band (1490 cm–1), which are relevant to the presence of ionic im-

purities, amorphous carbon and disordered graphitic material,[286] are included for

achieving accurate spectral deconvolution, as suggested by Cuesta et al.[286] and Rouzaud

et al.[287] In addition, a 2-peak fitting with Lorentz functions was performed for the

second-order modes. Peak fitting of the sample grown for 30 min is given as an example

(Figure 4.14a). The R2 (no shown) of the curve fitting were all above 0.99, revealing high-

ly reliable results from peak-fitting. The integrated intensity ratio of the D-band and G-

band (D/G) is widely used for characterizing the defect quantity in sp2 carbon based ma-

terials. The D''/G ratio is expected to provide information on the amorphous carbon (Cx-

Ny). The G'/G ratio is related to the graphitic structural order and electronic properties of

NCNTs. Moreover, based on the peak assignment, the sum (D/G, D'/G, D''/G) is defined

and suggested to be a descriptor for overall disorder in the structure of NCNTs including

edge defects, amorphous carbon (including amorphous CxNy).

Figure 4.14b presents the growth time dependence of the integrated peak intensity

ratios. It is clear that there is a transition in the trend of these integrated peak intensity

ratios at round 90 – 120 min. In the time region 1, D/G, D''/G and the sum (D/G, D'/G,

D''/G) decreased and G'/G rose with incrasing growth time, indicating an increase in the

overall graphitization of the grown NCNTs. In the time region 2, all the ratios show in-

verse tendency compared with region 1, implying that a longer growth time than about

100 min caused a poorer degree of graphitization. Additionally, there is an inverse rela-

tionship between D/G and G'/G, which is in good agreement with earlier studies.[288-289]

Comparing with the changes of N content as well as N/C ratio, it is found that the in-

crease of nitrogen content does not always lead to the increase of disorder in the struc-

ture or the decreasing of graphitization of NCNTs. Correlating Raman results with the

results of mean outer diameter, yields, element contents, it is clear that the growth time

strongly influences structure and graphitization of the NCNTs. Meanwhile, it is also

possible to control the structure and graphitization of the NCNTs by optimizing the

growth time. A too short or too long growth time might produce NCNTs with a large

amount of defects while a proper growth time can produce NCNTs with better graphiti-

zation and considerably high N doping level.


Figure 4.14: (a) Normalized Raman spectra of purified NCNTs grown at 650 °C with 4.6

vol.% EDA as a function of growth time. Peak-fitting is given for the sample

30 min as an example. (b) The corresponding integral peak intensity ratios

as a function of growth time.

Temperature-programmed oxidation (TPO) is known to be a sensitive technique for

the characterization of the structure and purity of CNTs.[29, 247] The oxidation of NCNTs

has two stages: (1) initial oxidation and (2) bulk oxidation. The initial oxidation is re-

lated to the oxidation of amorphous CxNy.[290] It is worth to note here that the dimension

of the carbon material also strongly influences its oxidation rate. Moreover, it is well

known that the curvature of graphene sheets at the tips of CNTs is larger than that on

the walls, which results in lower activation energy for the oxidation reaction. Therefore,

oxidation of MWCNTs is expected to start at the tip rather than at the cylindrical

walls.[291] It is important to point out that the opening of the tips of nanotubes makes the

encapsulated metals (in this work mainly Co, Mn) exposed to air, which are then oxi-

dized to metal oxides (e.g. Co3O4, MnO2). These oxides are known to be effective catalysts

for the catalytic oxidation of carbon at low temperatures.[292-295] It is believed that the re-

sidual metals decrease the activation energy of carbon oxidation and strongly lower the

oxidation resistance of CNTs.[41, 296] Due to the relatively low yield, the as-synthesized

NCNTs contained high concentrations of residual catalysts. Hence, a purification step

was performed with nitric acid (1.0 M) and most of the metal catalysts can be removed,

although there is always a certain amount left especially encapsulated in carbon. Fur-

thermore, the activation energy of oxidation of NCNTs is determined to be lower than

that of normal CNTs due to the poor degree of graphitization by nitrogen doping.[297] As a

result, the bulk oxidation of NCNTs is highly relevant to nanotube dimension, residual

metal content and graphitization degree.

Figure 4.15 shows the TPO results with CO2-CO TPO profiles of NCNTs grown for

120 min as an example. Mainly CO2 was produced during oxidation, and only traces of

CO were observed despite of the low O2 concentration applied during TPO. The initial

oxidation temperature (Tonset) and bulk oxidation temperature (Tmax) were obtained from

the CO2 profiles, following a similar process reported by Kissinger,[298] as labeled by

green lines in Figure 4.15a. Tonset and Tmax of NCNT samples as a function of growth time

are presented in Figure 4.15b. It can be seen that both Tonset and Tmax increased as the

growth time increases in time region 1, but decreased in time region 2. The range of ini-

tial oxidation temperature was determined to be around 320 – 335 °C, which is in good

agreement with the reported oxidation temperature of amorphous carbon (355 °C).[290]

By combining the results of mean outer diameter (Figure 4.12), elemental analysis

(Figure 4.13) and Raman spectroscopy (Figure 4.14), the following conclusions can be

drawn:

• 0 – 60 min: smaller mean outer diameter, higher residual metal content, and poor

graphitization lead to lower oxidation resistance;

• 90 – 120 min: larger mean outer diameter, lower residual metal content, and bet-

ter graphitization result in higher oxidation resistance;

• 150 – 180 min: smaller mean outer diameter, higher residual metal content, and

poorer graphitization lead to lower oxidation resistance.

Figure 4.15: (a) CO2/CO-TPO profile of the purified NCNTs grown for 120 min. (b) The

onset and maximum oxidation temperatures of the purified NCNTs as a

function of growth time at 650 °C with 4.6 vol.% EDA.


4.2.2.2 Influence of precursor concentration

For the variation of EDA concentration, the reaction temperature and duration were

fixed at 650 °C and 120 min. EDA concentrations of 3.4, 4.6, 5.9 and 7.1 vol.% were in-

vestigated.

The morphologies for the NCNTs with different EDA concentrations are displayed in

Figure 4.16a-d, where the entangled nanotubes with typical tubular structure can be

identified. The comparison of the presented SEM images and size distributions of the

outer diameter (Figure 4.16e) shows that the NCNTs grown with 4.6 vol.% EDA possess

larger outer diameter and broader size distribution than the NCNTs grown with other

concentrations. The mean outer diameter of the NCNTs as a function of EDA concentra-

tion has a maximum at 4.6 vol.%. Some researchers have reported the thinning of

NCNTs by increasing the concentration of nitrogen-containing precursors,[23, 299] which is

due to the nitrogen-induced tube closure.[274] However, the thinning of nanotubes could

be due to the surface etching through the carbon gasification (detailed discussion in to

section 4.2.2.6).

Figure 4.16: SEM images of purified NCNTs grown at 650 °C for 120 min as a function of EDA

concentration (a-d). The corresponding size distribution of outer diameter

(e) and the mean outer diameter (f) measured from 600 NCNTs.

Figure 4.17a shows that the NCNT yield increased from 5.6 ± 0.4 gNCNTs gcat.–1 to 8.2 ±

0.4 gNCNTs gcat.–1 when the EDA concentration increases from 3.4 to 4.6 vol.%, and then

decreased to 8.0 ± 0.5 gNCNTs gcat.–1 , when the EDA concentration was further increased to

7.1 vol.%. Combined with the element contents presented in Figure 4.17b, the decrease

in NCNTs yield with 7.1 vol.% EDA was found to be correlated to the decrease in the

carbon content. The nitrogen content of the NCNTs seems to be independent of the EDA

concentration, which were all round 4.5 wt.%. The 100×N/C slightly increased with the

increasing EDA concentration. Surprisingly, a dramatic decrease in the residual metal

content was observed for the NCNT sample grown with 7.1 vol.% EDA. It is likely that

the etching was severe enough to open the holes on the tips or walls of the nanotubes,[300]

exposing the encapsulated metals to the acid solution during the acid-washing. There-

fore, more residual metals were removed by acid-washing, as compared with the NCNTs

grown with lower EDA concentration.

Figure 4.17: (a) The yield of as-grown NCNTs and (b) the contents of carbon, nitrogen

and residual metals in purified NCNTs as function of EDA concentration at

650 °C for 120 min.

The normalized Raman spectra of the NCNTs grown at 650 °C in 120 min with differ-

ent concentrations of EDA precursor are compared in Figure 4.18a. The most pro-

nounced difference among these spectra is the intensity change of the G'-band in the

second-order spectra. It again proves that the G'-band is sensitive to nitrogen doping in

CNTs. Figure 4.18 depicts the EDA concentration dependence of the integrated peak in-

tensity ratios. It can be seen that increasing the EDA concentration from 3.4 vol.% to 4.6

vol.% led to a decrease in the D/G, D''/G, and sum (D/G, D'/G, D''/G,), indicating the en-

hancement in graphitization of NCNTs. When further increasing the EDA concentration

to 7.1 vol.%, these peak ratios rose, indicating a decrease in the degree of graphitization.

The inverse tendency G'/G provided additional proof for the dependence of the degree of

graphitization of NCNTs on the EDA concentration.


Figure 4.18: (a) Normalized Raman spectra of NCNTs grown at 650 °C in 120 min with

different EDA concentrations. (b) The corresponding integral peak intensity

ratios as a function of EDA concentration.

The effect of EDA concentration on the oxidation resistance of the NCNTs was studied

using TPO. Both initial- and maximum oxidation temperatures as a function of EDA

concentration have a maximum at 4.6 vol.% (Figure 4.19). As concluded in section

4.2.2.1, the bulk oxidation of NCNTs strongly depends on the nanotube dimension, resi-

dual metal content and degree of graphitization. Again, by combining the results of mean

outer diameter (Figure 4.16), elemental analysis (Figure 4.17) and Raman spectroscopy

(Figure 4.18), the following conclusion can be drawn:

• 3.4 vol.% EDA: smaller mean outer diameter, higher residual metal content, and

poor graphitization lead to lower oxidation resistance;

• 4.6 vol.% EDA: largest mean outer diameter, higher residual metal content, and

best graphitization result in highest oxidation resistance (graphitization degree

was the dominant factor);

• 5.9 vol.% EDA: smaller mean outer diameter, higher residual metal content, and

poorer graphitization result in lower oxidation resistance;

• 7.1 vol.% EDA: smallest mean outer diameter, lowest residual metal content, and

poorest graphitization result in higher oxidation resistance (graphitization degree

was the dominant factor);

Figure 4.19: The onset and maximum oxidation temperatures of the purified NCNTs as

functions of EDA concentration at 650 °C for 120 min.

4.2.2.3 Influence of growth temperature

For the variation of growth temperature, the EDA concentration and duration were

fixed at 5.9 vol.% and 120 min. The NCNTs mentioned in this section are all referred to

these growth conditions. An additional cooling or heating step after reduction was al-

ways carried out in He (100 sccm) for 30 min to allow the reactor reaching the desired

growth temperature, which was necessary to ensure comparability.

The effect of growth temperature on the morphology of the NCNTs was examined by

SEM (Figure 4.20a-d) showing the typical nanotubular structure of the entangled

NCNTs. It can be seen that the outer diameter of NCNTs become larger as the growth

temperature increased from 550 °C to 850 °C. The size distribution of the outer diameter

(Figure 4.20e) shows that 99 % of the NCNTs grown at 550, 650, 750 and 850 °C were in

the range of 4.5 – 25.5 nm, 10.5 – 34.5 nm, 10.5 – 49.5 nm, 10.5 – 73.5 nm, respectively.

The corresponding peak frequency decreases from 28.7 % to 11.2 % and the peak position

shifts from 13.5 nm to 19.5 nm with increasing growth temperature. The mean outer di-

ameter shows a similar trend (Figure 4.20f).

It is known that the diameter of CNTs is related to the catalyst particle size.[107-109]

The statistical analysis of the outer diameter of the NCNT grown at different tempera-

tures reveals that the higher growth temperature resulted in larger outer diameters,

broader distribution of outer diameter and larger mean outer diameter of the grown na-

notubes, which is in good agreement with previous studies.[22, 111, 198, 301] The temperature

seems to determine the initial formation of the size of Co nanoparticles during H2 reduc-


tion. Maintaining the growth temperature at higher temperature may cause larger par-

ticles due to sintering. On the other hand, a high temperature (e.g. ≥ 750 °C) might also

cause severe deposition of amorphous carbon due to the gas-phase pyrolysis of the pre-

cursor, also leading to thickening of nanotubes.[22]

Figure 4.20: SEM images of NCNTs (washed) grown with 5.9 vol. % EDA for 120 min at:

(a) 550, (b) 650, (c) 750 and (d) 850 °C. The corresponding size distribution

of the outer diameter (e) and the mean outer diameter (f) of the NCNTs

were derived measuring 600 NCNTs. To ensure comparability, a cooling or

heating step after reduction was carried out in He always for 30 min to al-

low the reactor reaching the desired growth temperature.

The yield of NCNTs as a function of growth temperature is shown in Figure 4.21a. The

growth temperature also showed a significant impact on the yields of NCNTs: the NCNT

yield rose as the growth temperature increased. The NCNT yield dramatically increased

by nearly 3 times with increasing growth temperature from 550 to 650° C, which might

be attributed to the higher activation energy at higher temperature for a higher forma-

tion rate of NCNTs and longer mean lifetime of the active sites on catalyst.[112] However,

a too high temperature (≥ 750 °C) not only yielded larger catalyst particles, but also

caused fast deactivation of catalyst. The deposition of amorphous carbon by precursor

pyrolysis at high temperature not only thickened the nanotubes, but also blocked the ac-

tive sites and then deactivated the catalyst leading to a lower yield.[22, 112] Moreover, the

gas-phase pyrolysis of the EDA resulted in a lower initial concentration of EDA for

NCNT growth. These reasons may explain the constant yield when increasing tempera-

ture from 750 °C to 850 °C.

The carbon and nitrogen contents and the 100×N/C ratio as a function of growth temperature

are exhibited in Figure 4.21b. Increasing the growth temperature resulted in an increase of the

carbon content and a decrease of the nitrogen content in NCNTs, resulting in a decrease of the

100×N/C ratio. The residual metal content showed a similar trend as the nitrogen content in

the temperature range of 550 – 750 °C. However, at 850 °C the residual metal content showed

a remarkable increase due to the severe deposition of amorphous carbon preventing the leach-

ing of residual metals during the acid washing.

Figure 4.21: The yield of as-grown NCNTs and (b) the contents of carbon, nitrogen and

residual metals in purified NCNTs as function of the growth temperature

with 5.9 vol.% EDA for 120 min.

The normalized Raman spectra of the NCNTs produced at different growth tempera-

tures are shown in Figure 4.22a. The most remarkable difference among these spectra is


the up-shift in the mode frequencies of the G-band and the D-band, and the change in

the intensities of the G-band. The D-band and G-band peaks became sharp and narrow

as the growth temperature increased. The integrated peak ratios as a function of growth

temperature show a transition at a growth temperature of 750 °C (Figure 4.22b), which

is consistent with the results reported by Chizari et al.[22] In the temperature range of

550 – 750 °C, D/G, D''/G, and the sum (D/G, D'/G, D''/G,) decreased and G'/G rose with

growth temperature, implying an enhancement in the degree of graphitization of the

NCNTs.[22, 188] However, these peak ratios show an inverse trend when the temperature

increased to 850 °C, indicating a decrease of the graphitization due to the severe deposi-

tion of amorphous carbon. It is necessary to remind that the yield of NCNTs grown at

850 °C was similar to that obtained at 750 °C. Therefore, it can be concluded that the

larger outer diameter of NCNTs grown at 850 °C was due to the surface coating with

amorphous carbon.

Moreover, previous studies revealed that nitrogen doping can cause the up-shift of the

G-band frequency.[302-303] Our results show that nitrogen doping caused the up-shift of

the G-band frequencies as well as of the D-band and the G'-band frequencies (Figure

4.22c). The up-shift of the frequency did not correlate to the nitrogen content (Figure

4.21b), but correlates to the outer diameters of the NCNTs in the temperature range of

550 – 750 °C, which is in agreement with Nii et al.[304] However, the G-band frequency of

the NCNTs grown at 850 °C shifted to a lower frequency. The FWHMs of the G-, D- and

G'-bands show a similar trend. The Raman results suggest that the mode frequency and

FWHM of NCNTs are not only related to the diameter but also to the degree of graphiti-

zation of the nanotubes.

The initial and maximum oxidation temperatures of the NCNTs grown at different

temperature are shown in Figure 4.23. The initial oxidation temperature increases li-

nearly with the increase in growth temperature. The maximum oxidation temperature of

NCNTs increased as the growth temperature increased up to 750 °C, and then decreased

slightly when the growth temperature increased to 850 °C, which can be assigned to the

relative higher residual metal content and poorer graphitization of the nanotubes.

Figure 4.22: (a) Normalized Raman spectra of NCNTs grown in 120 min with 9.0 vol.%

EDA as a function of the growth temperature. (b) Intensity ratios, (c) the

corresponding integral area ratios and (d) the frequencies and FWHM of G-

bands of the NCNTs determined by curve-fitting as a function of the growth

temperature.

Figure 4.23: The onset and maximum oxidation temperatures of the purified NCNTs as

functions of growth temperature with 5.9 vol.% EDA for 120 min.


4.2.2.4 Influence of nitrogen on the growth kinetic

A comparison of the growth kinetics between NCNTs and CNTs was performed to un-

derstand the role of nitrogen in the NCNT growth. The CNT growth was performed over

the same catalyst at 650 °C with 6.0 vol.% C2H4 and 12.0 vol.% H2 in order to achieve

comparable concentrations of C and H atoms and similar impact of carbon gasification by

H2. The CNT yield ~ t profile shows a linear growth behavior (Figure 4.24). Catalyst

deactivation and growth termination were not observed in the studied time scope due to

the rather low ethylene concentration. The growth rate of CNTs was determined to be

0.0792 gCNTs gcat.–1 min–1 (Eq. 4.8).

0.0792 6.0 . % 12.0 . % ; 30 180 (4.8)

0.0573 4.6 . % ; 30 120 (4.9)

0.0771 5.9 . % ; 30 90 (4.10)

In contrast, NCNTs showed quasi-linear growth behavior in time region 1 and growth

termination in time region 2. The quasi-linear growth behavior is similar to a polymeri-

zation-like growth behavior reported by Plata et al.[190] The growth rates can be obtained

from the slopes of the yield ~ t curves by linear fitting within the range of time region 1

(Figure 4.24). It was found that a higher EDA concentration led to a high growth rate

(Eq. 4.9 and Eq. 4.10). The comparison of the growth rate between the NCNTs (5.9 vol.%

EDA) and the CNTs (6.0 vol.% C2H4 + 12.0 vol.% H2) shows the inhibition of the growth

rate by the nitrogen-doping. A similar inhibition effect of N-doping on the growth kinet-

ics of NCNTs was reported by Pittinson et al.[264]

The decreased NCNT yield in the time region 2 again confirmed the surface etching of

NCNTs, which become more severe with 5.9 vol.% EDA. The etching rate can be ob-

tained by establishing the relationship between the yield and time after the growth ter-

mination region (Eq. 4.11 and 4.12), which can be derived from the yield ~ t profiles:

0.00125 6.98 4.6 . % ; 120 180 (4.11)

0.0232 12.4 5.9 . % ; 150 180 (4.12)

Figure 4.24: The yields of NCNTs grown with 4.6 vol.% and 5.9 vol.% EDA at 650 °C as a

function of growth time. The yield of CNTs grown at 650 °C with a feed gas

of 6.0 vol.% C2H4 and 12.0 vol.% H2 at a total flow of 100 sccm.

4.2.2.5 Influence of nitrogen doping on the properties of NCNTs

MWCNTs (Baytubes C150P, Bayer Material Science, Leverkunsen, Germany), which

were produced using the same Co-based catalyst, were used as reference for Raman and

TPO studies. The as-received CNTs were purified by 1.5 M HNO3 at room temperature

for 3 days.

Figure 4.25a presents the Raman spectra of the purified CNTs and NCNTs (650 °C,

120 min, 4.6 vol.% EDA). The NCNTs show dramatic broader peaks in the first order

spectrum and much weaker peaks in the second order spectrum, in particular the D''-

band and G'-band. The integrated intensity ratios reveal a poorer graphitization of the

NCNTs, which can be attributed to the nitrogen-doping (Figure 4.25b).

The TPO study (Figure 4.25c) shows that the nitrogen-doping lowered the oxidation

temperature of NCNTs, as already reported.[305] It is worth to note that the CO peak was

not high enough to be observed for NCNTs. Full combustion of 5 mg carbon needed 0.417

mmol O2, and the consumption of the O2 in the TPO process was only 0.224 mmol. There-

fore, the product of TPO should be a mixture of CO2 and CO. However, the content of re-

sidual metals in NCNTs was determined to be 3.5 wt.%. During the TPO process, these

residual metals were converted to oxides (i.e. Co3O4, MnO2),[306] which are known to be

effective catalysts for catalytic oxidation of CO to CO2. In contrast, the sum of the con-

tent of residual metals (Co, Mn, Al, Mg) of purified CNTs was determined to be 0.2 wt.%,

which was rather low to contribute to the catalytic oxidation of CO .


Figure 4.25: (a) Normalized Raman spectra of NCNTs and CNTs. (b) The onset and max-

imum oxidation temperatures of the purified NCNTs and CNTs.

4.2.2.6 Understanding the surface etching effect by carbon gasification

The growth time is an important factor for controlling the morphology and purity of

CNTs. During the CNT growth, the carbon precursor molecules or the active carbon spe-

cies from the gas-phase pyrolysis (e.g. > 730 ºC for ethylene[89]) may react with the

formed nanotubes, implying that lengthening and thickening of the nanotubes are com-

petitive processes during the CNT growth. The TEM studies illustrated that the thicken-

ing of nanotube originates from the increased wall numbers while the inner diameter of

nanotube was hardly changed.[100] Even when the growth of a CNT forest height stopped,

the CNT forest weight still steadily increased due to the surface accumulation of carbo-

naceous impurities from the gas-phase pyrolysis of precursor,[101] which occurred on the

outermost layer (Figure 2.2). Feng et al.[102] proposed a so-called epitaxial growth mode

in which the epitaxial growth of graphene layers occurred on the outer surface of CNTs

at the slow growth stage. It is very likely that these epitaxial graphene layers could be

used as seeds for self-assembling the outer walls of nanotubes,[92] resulting in thickening

of nanotubes (Figure 2.2). Therefore, the longer the growth time, the larger the outer di-

ameter of the CNTs can be produced, which was also reported in other investigations.[25,

101-104] The approach to prevent the thickening of nanotubes or formation of surface car-

bonaceous species is to introduce etching reagents, such as H2O or H2 plasma,[105-106]

which can suppress epitaxial growth of graphene layers by selective gasification of the

non-tubular carbons to produce highly pure CNTs. However, one should be aware that

the thickening of the nanotubes can also be induced by the catalysts on the nucleation

steps.[105] It is also known that the outer diameter of CNTs is determined by the particle

size of the catalysts.[107-109] If the catalyst particles are well stabilized by the support (e.g.

the mixed oxide catalysts), the inner diameter was hardly changed and only the outer

diameter became larger during the CNT growth. If the catalyst particles are free of any

supports (e.g. the floating catalysts), both outer and inner diameters of CNTs increase

due to the agglomeration of catalyst particles.[110-111]

The decrease in outer diameter of nanotubes during the growth has been rarely re-

ported. Figure 4.26 illustrates two distinct stages of NCNT growth: the thickening stage

(before 90 min) and thinning (after 90 min), according to the SEM results. Two possible

processes, namely catalyst-induced and gas phase-induced thickening, are proposed for

the thickening process of nanotubes.[105] Surface etching through the carbon gasification

is proposed for the thinning process of nanotubes.

The catalyst-induced thickening is likely due to the ammonia-induced reconstruction

of catalyst particles. It is known that the reshaping of particles always occurs during the

CNT growth. The outer diameter of nanotubes correlates to the largest diameter of the

catalyst particle.[96-97] In this work, ethylenediamine was used as carbon and nitrogen

source, which at 650 °C mainly pyrolyzed to HCN, C2H2, H2, and NH3 (Eq. 4.13, 4.14 and

4.15).[89, 307-308] Both C2H2 and HCN are known to be the active precursors for CNT and

NCNT growth, (Eq. 4.16 and 4.17, where M* represents the metallic active sites).[92-93, 191]

Ammonia can influence the reconstruction of catalyst particles to form larger

particles,[161, 163] nucleating new outermost graphitic layer, leading to the thickening of

nanotubes. A similar result was also reported by Cui et al.[105] The nucleation of the out-

ermost graphitic layer might follow a polymerization-like formation mechanism of CNTs,

as proposed by Plata et al.[190] Acetylene and HCN could react via the metallic active

sites to form metallacycle intermediates for further nucleation of graphitic CxNy

layers.[89, 190]

H2N–CH2–CH2–NH2 → CH2=CH–NH2 + NH3 (4.13)

CH2=CH–NH2 → CH≡CH + NH3 (4.14)

H2N–CH2–CH2–NH2 → 2H–C≡N + 3H2 (4.15)


xCH≡CH + M* → → M@Cx + xH2 (4.16)

CH≡CH + H–C≡N + M* → → M@CxNy + NH3 +H2 (4.17)

The gas phase-induced thickening (Figure 4.26) process is likely to follow a self-

assembly process of the outermost graphene layer. According to the pyrolysis theory,

acetylene at 650 °C mainly pyrolyzed to vinyl acetylene (CH2=CH–C≡CH), benzene or Cx

(x > 6) by radical reactions (Eq. 4.18 and 4.19).[89] It is known that alkynes like acetylene,

methyl acetylene, and vinyl acetylene can assist the CNT growth.[190] Due to the struc-

tural similarity to acetylene, HCN could follow similar reactions to form acrylonitrile

(CH2=CH–C≡N), pyridine, and CxNy (x > 6) (Eq. 4.20, 4.21 and 4.22). On the CNT sur-

face, the defects in the carbon lattice might act as active carbon sites (A*) for the precipi-

tation of carbon.[92] Acetylene and HCN can self-assemble via the active carbon sites to

form graphitic layers by the assistance of nitrogen atoms (Eq. 4.23 and 4.24),[192] similar

to the epitaxial formation process of graphene layers.[102] This can be called nitrogen-

induced self-organization of the outer walls. A similar self-assembly process of CNTs was

also reported by Eres et al.[92]

2 CH≡CH → CH2=CH–C≡CH (4.18)

CH2=CH–C≡CH + CH≡CH → → Cx (4.19)

CH≡CH + H–C≡N → CH2=CH–C≡N (4.20)

CH2=CH–C≡CH + H–C≡N → N

→ CxNy (4.21)

CH2=CH–C≡N + CH≡CH → N

→ CxNy (4.22)

3 CH≡CH + A* → A* → Cx (4.23)

CH≡CH + H–C≡N + A* → A*N→ CxNy (4.24)

The thinning of nanotubes (Figure 4.26) can be attributed to the surface etching

through the carbon gasification by NH3 or H2 etching (Eq. 4.25, 4.26 and 4.27),[309-310] re-

sulting in the decrease of the graphitization of the grown NCNTs (Figure 4.14). The sur-

face etching may first occur on the amorphous surface carbon species, defects on the

walls and the tips of nanotubes. However, the etching-induced thinning of nanotubes has

been rarely reported or discussed in the studies concerned the synthesis of NCNTs. It is

important to remember that the N/C ratio in ethylenediamine is 1, which is nearly the

highest among the common used precursors, e.g. acetonitrile (CH3CN) and pyridine

(C6H5N). Therefore, the amount of NH3 produced from the gas-phase pyrolysis of EDA

would be large enough to cause remarkable etching of NCNTs.

2NH3 → N2 + 3H2 (4.25)

NH3 + C → HCN + H2 (4.26)

C + 2H2 → CH4 (4.27)

Moreover, one should be aware that the catalyst-induced polymerization of CxNy, gas-

phase self-assembly of CxNy and surface etching are competitive processes during the

entire NCNT growth. In the active growth stage (time region 1), either the catalyst-

induced polymerization process or the nitrogen-promoted self-assembly process result in

the thickening of outer diameter of nanotubes as well as the increased nitrogen content,

as demonstrated by the SEM and elemental analysis studies. The surface etching by ga-

sification of the amorphous carbon not only enhances the graphitization of the grown

NCNTs, but also reactivates the catalysts, as uncovered by the Raman and growth kinet-

ic studies. In the passive growth stage (time region 2), the surface etching becomes the

dominant process, resulting in the thinning of nanotubes, larger concentration of defects

and lower degree of graphitization of nanotubes.

Figure 4.26: Schematic diagram illustrating the thickening and thinning of the nano-

tube.

In addition, previous studies have strongly suggested that nitrogen atoms are prefe-

rentially either concentrated in the inner walls of the nanotubes[176, 178] or in the form of

polyaromatic species grafted on the outer surface of nanotubes.[179] However, the forma-

tion mechanism is still unclear. Our experimental results suggest that the accumulation

of nitrogen on the outer surface of nanotubes may be due to the catalyst-induced polyme-

rization of CxNy and gas phase-induced self-assembly of CxNy.

thickening thinning


4.2.3 Summary

Nitrogen-doped multi-walled carbon nanotubes have been produced by injection CVD

from EDA over the highly active Co-Mn-Al-Mg mixed oxide catalyst. The influence of the

growth time, ethylenediamine concentration and growth temperature on the morpholo-

gy, yield, composition, graphitization and oxidation resistance of the NCNTs has been

systematically investigated. Increasing growth time within time region 1 resulted in

thickening of nanotubes, carbon and nitrogen accumulation, better graphitization of na-

notubes and higher oxidation resistance, whereas increasing the growth time within

time region 2 caused inverse trends in these properties. The nitrogen content was inde-

pendent of the used EDA concentration. However, increasing the EDA concentration led

to severe surface etching through carbon gasification, outputting thinning of nanotubes,

lower yield, poor graphitization and lower oxidation resistance. It was found that the

graphitization level was always independent of the nitrogen content. The effect of sur-

face etching through the carbon gasification strongly influenced the graphitization level

of NCNTs. Moreover, the oxidation resistance was strongly related to the content of resi-

dual metals and graphitization level of the NCNTs. The growth temperature controlled

the particle size and activity of the catalyst, reflected by the outer diameter and yield.

High temperature not only affected the nitrogen incorporation during the nanotube for-

mation, but also caused the gas-phase pyrolysis of the precursor leading to the deposition

of amorphous carbon. In view of high yield and high nitrogen content (> 4.0 wt.%), the

optimal conditions for NCNT growth in the scope of this study were determined to be 120

min, 5.9 vol.% EDA and 650 °C.

Chapter 5

CNTs with residual metals as bifunc-

tional electrocatalysts

5.1 Introduction

Renewable energy has been considered as the solution to the increasing energy crisis.

Recent developments in this field have heightened the need for electrochemical energy

conversion and storage,[3, 202] boosting a renewed interest in seeking new catalytic mate-

rials with high activities and stabilities. Fuel cells, rechargeable metal-air batteries and

water electrolyzer are among the most promising techniques in electrochemical energy

conversion and storage. The sluggish kinetics in oxygen reduction reaction (ORR) and

large over-potentials in oxygen evolution reaction (OER) are the most significant ob-

stacles in these fields. Carbon-supported Pt and Pt-based nanocrystals are known as the

most active catalysts for ORR, but their practical applications suffer from high cost and

fast deactivation.[3] Ti-supported RuO2 or IrO2, or their mixtures are used as dimensio-

nally stable anodes for water electrolysis in current industrial processes.[311] However,

none of these catalysts possess good activities in both ORR and OER.

Carbon materials incorporated with non-precious metal oxides, such as MnOx, Co3O4,

as well as Mn- and Co-based mixed oxides have proved to be promising alternatives to

the precious noble metal catalysts.[8, 10, 28, 258, 312-313] On the one hand, these non-precious

oxides can provide active sites for either ORR or OER; on the other hand, the carbon

support, such as carbon nanotubes (CNTs) and graphene, can favor the electrical conduc-

tivities of these non-precious metal oxides.

CNTs are promising supports for nanostructured metal or metal oxide electrocatalysts

due to their outstanding electrical conductivity, high surface area, and high chemical

and thermal stabilities.[11] Although CNTs can be produced in the industrial scale by

chemical vapor deposition (CVD) processes,[14, 27] their further applications may suffer

from multi-step purification processes in order to remove the inevitable carbonaceous

84 CHAPTER 5 CNTS WITH RESIDUAL METALS AS BIFUNCTIONAL ELECTROCATALYSTS

impurities (e.g. amorphous carbon) and residual metal catalysts (e.g. Fe, Ni, and Co).

Moreover, oxygen functionalization of CNTs by chemical oxidation is an essential step in

creating anchoring sites for the deposition of metal or metal oxides.[30, 66, 245]

Here, a two-step vapor-phase method was developed for the synthesis of MnO2-

OCNTs hybrids containing nanoparticles (Co3O4-MnO2-CNTs). CNTs were produced over

a Co-Mn oxide by CVD. The obtained CNTs and the residual metal catalysts were fur-

ther oxidized by HNO3 vapor oxidation and utilized as electrocatalysts for ORR and OER

under alkaline conditions.

5.2 Results and discussion

5.2.1 OER

5.2.1.1 Co-Mn oxides

The SEM image shows that the commercially available Co-Mn oxide consits of par-

ticles with diameters in the range of hundred nanometers (Figure 5.1a-b). The bulk crys-

talline phases in the Co-Mn oxide were investigated by XRD. The XRD pattern reveals

the presence of cubic MnCo2O4 and tetragonal CoMn2O4 spinels. The reduction characte-

ristics of the Co-Mn oxide was studied by temperature-programmed reduction (TPR) us-

ing 4.71 vol.% H2 in Ar. The three peaks in the TPR profile imply that the Co-Mn oxide

was reduced in three steps. Peak maxima were identified at 233, 401 and 555 °C. Accord-

ing to Becker et al.,[41] the peak at low temperature can be assigned to the reduction of

Mn4+ to Mn3+. The peak at medium temperature is related to the reduction of Mn3+ to

Mn2+ and Co3+ to Co2+. The peak at high temperature can be attributed to the reduction

of Co2+ to metallic Co. Therefore, the phases in the reduced oxide are suggested to be me-

tallic Co and MnO.

5.2 RESULTS AND DISCUSSION 85

Figure 5.1: (a, b) SEM images, (c) XRD pattern and (d) H2-TPR profile of the as-received

Co-Mn oxide.

5.2.1.2 CNT growth and kinetic analysis

The CNT growth over the commercial Co-Mn oxide was carried out following an opti-

mized procedure described in ref.[41] A benchmark catalyst, Co-Mn-Al-Mg oxide, was used

as reference for evaluating the activity of the Co-Mn oxide. The CNT growth was per-

formed in a horizontal hot-wall reactor with a variation of duration in order to analyze

the growth kinetics. The obtained yields over these two kinds of catalysts as a function of

the growth time are shown in Figure 5.2. The optical images show the remarkable in-

crease in volume during the CNT growth. The kinetic analysis of the growth behavior

was performed by using the kinetic model described in Chapter 4 (Eq. 4.3). The initial

growth rate (r0) and the mean lifetime (τ) of the catalytically active sites were derived

from the curve-fitting. The theoretical maximum CNT yields (Ymax) were predicted by r0

× τ, assuming all the obtained carbon material consisted of CNTs.[112] The kinetic para-

meters presented in Table 5.1 show that the Co-Mn oxide possessed a slower initial

growth rate and a lower theoretical maximum yield but a longer mean lifetime of the ac-

tive sites as compared with the Co-Mn-Al-Mg oxide. The obtained sample was denoted as

MCC-growth time, e.g., MCC-5 refers to the sample obtained at the growth time of 5


min. The focus was on the sample MCC-5 and MCC-120, which were expected to possess

greatly different metal contents. The two samples were used as starting materials in the

synthesis of electrocatalysts.

Figure 5.2: CNT yields of the different oxides as a function of growth time. Optical im-

ages of the catalyst before (left) and after CNT growth for 5 min (middle) and

120 min (right), respectively.

Table 5.1: Theoretical maximum yield, initial growth rate and mean lifetime of CNTs

grown with Co-Mn oxide and the Co-Mn-Al-Mg oxide catalysts.

Sample Ymax / gCNTs gcat.–1 r0 / gCNTs gcat.

–1 min–1 τ / min

Co-Mn oxide 137 2.63 52 Co-Mn-Al-Mg oxide 201 6.77 30

5.2.1.3 MCC for OER

The SEM images in Figure 5.3 show that the MCC-5 and MCC-120 consist of large-

diameter carbon nanotubes with relatively wide diameter distributions. The CNT growth

process splits the large particles of Co-Mn oxide (see Figure 5.1) into small particles,

yielding a homogeneous structure of entangled CNTs with oxide support (i.e. MnO).[112]

The compositions of the catalyst before and after CNT growth were determined by ele-

mental analysis (see Table 5.2). As expected, a longer growth time yielded a higher con-

tent of carbon and consequently lower contents of Co, Mn and O.

The size distributions of the outer diameters of the CNTs in the MCC-5 and MCC-120

samples are presented in Figure 5.3c. A widening in the size distribution can be observed

when the growth time is prolonged to 120 min. The mean outer diameters of MCC-5 and


MCC-120 were determined to be 29.4 and 33.5 nm, respectively. The BET surface areas

of the catalyst before and after CNT growth are summarized in Table 5.2. As expected,

the CNT formation led to a dramatic increase in the BET surface areas.

Table 5.2: Elemental analysis, BET surface area of the samples before and after CNT

growth.

Sample Elemental analysis / wt.% BET surface area (N2)

/ m2 g–1 Coa Mna Cb Ob Co-Mn oxides 34.81 25.77 - 39.42 9 MCC-5 2.10 1.72 93.69 2.49 87 MCC-120 0.26 0.21 98.54 0.99 85

aThe contents of Mn and Co were measured by ICP-OES. bThe contents of C and O were measured by VarioEl analyzer.

The XRD patterns in Figure 5.3d show the crystal structures of the samples after

CNT growth. The diffraction peaks at ≈ 26.1 and ≈ 42.8 ° originate from the (002) and

(100) reflections of graphite, respectively. The peaks appearing at 35.0, 40.7, 58.9, and

70.4 ° are indexed as the (111), (200), (220) and (311) planes of cubic MnO. The peaks

around 44.3 and 51.5 ° can be assigned to the (111) and (200) reflections of metallic Co.

The mean particle sizes of MnO and Co were calculated from the characteristic peak

(200) using the Scherrer equation (d = K·λ/(β1/2·cos θ), where d is the mean size of the

crystalline domains, K is dimensionless shape factor, λ is the X-ray wavelength, β1/2 is

the line broadening (FWHM), θ is the Bragg angle). The mean particle sizes of MnO and

Co in MCC-5 were estimated to be 25 nm and 2 nm, respectively.


Figure 5.3: SEM images of MCC-5 (a) and MCC-120 (b) and the corresponding size dis-

tributions of the outer diameter (c) and (d) XRD patterns.

Combining the TPR and XRD results, the phases in the MCC-5 and MCC-120 can be

concluded to be carbon nanotubes with metallic Co and MnO. According to the CNT

growth mechanism,[82] metallic Co particles are encapsulated in the walls of nanotubes

and remain in the metallic state after growth. During the CNT growth, the original cata-

lyst particles (Co-Mn oxide) can be separated by the formation of CNTs and finally form

homogeneous structure of entangled CNTs with MnO particles.[314] Therefore, the struc-

ture of the sample after CNT growth is suggested to be MnO-CNTs hybrid materials

with encapsulated Co nanoparticles (MnO-CNTs@Co).

Figure 5.4a shows cyclic voltammograms (CV) of OER recorded at a scan rate of 100

mV s–1 and a rotation rate of 1600 rpm in 0.1 M KOH for the samples before (Co-Mn


oxide) and after CNT growth (MCC-5 and MCC-120). Each sample shows one anodic oxi-

dation peak. The close overlay of the CV curve suggests rather low capacity for each

sample. It is worth to note that the redox features of MnO are not clear for MCC-5 and

MCC-120 due to its low content and encapsulation by carbon (Table 5.2). The anodic cur-

rents at lower potential could originate from the oxidation of the manganese oxide and

that at higher potential can be related to the electrooxidation of water to oxygen.[315]

To identify the origin of the OER activity, the sample MCC-120 was washed in 1.5 M

HNO3 for 72 h at room temperature to remove the un-encapsulated MnO. The washed

MCC-120 exhibited a poor activity for OER, indicating that the exposed MnO is the ac-

tive species in MCC-5 and MCC-120 before washing.

Linear sweep voltammetry (LSV) was carried out with a slow scan rate (5 mV s–1) in

order to achieve steady state at the electrode surface for a precise evaluation of the OER

activity of these samples. Figure 5.4b shows the anodic LSV for the samples including

Co-Mn oxide, MCC-5, MCC-120 and MCC-120 (washed). It can be seen that the OER ac-

tivities of MCC-5 and MCC-120 outperformed that of the Co-Mn oxide and purified

MCC-120.

The onset potentials (Eonset), potential (EOER) at 10 mA cm–2 and the corresponding

overpotentials (η) are listed in Table 5.3 for a better comparison of catalytic activity of

the samples. Eonset was determined from the region with exponentially increasing current

densities. EOER was obtained at a current density of 10 mA cm–2 (the horizontal dashed

line in Figure 5.4b) which has been proposed for estimating the efficiency of driving the

OER.[316-318] η was calculated by subtracting the standard potential of the half-reaction of

water oxidation (1.23 V) from EOER. It can be seen that the MCC-5 and MCC-120 samples

possessed similar catalytic activities, with the MCC-5 being slightly more active than

MCC-120, which can be attributed to a higher MnO content in MCC-5.


Figure 5.4: (a) First cycle of CVs recorded at 100 mV s–1 and 1600 rpm and (b) Anodic

LSV recorded at 5 mV s–1 and 1600 rpm in 0.1 M KOH. (c) The corresponding

Tafel plots.

Table 5.3: Electrochemical OER performance derived from LSV measured at 5 mV s–1

and 1600 rpm in 0.1 M KOH

Material Eonset / V

EOER / V at 10 mA cm–2

η / V at 10 mA cm–2

Tafel slope / mV decade–1

Co-Mn oxides 1.66 - - 134 MCC-5 1.64 1.73 0.50 58 MCC-120 1.65 1.74 0.51 54 MCC-120 (washed) 1.75 - - 160

Tafel plots have been widely applied for evaluating the catalytic electrode kinetics.[319]

The Tafel slope of each sample was obtained from the linear region on the Tafel plot (sol-

id line in Figure 5.4c) by using the Tafel equation (η = b log j + a, where η is overpoten-

tial, j is the current density, and b is the Tafel slope). As expected, the samples MCC-5

and MCC-120 show favorable kinetics with smaller Tafel slopes than that obtained for

Co-Mn oxide and washed MCC-120.


However, the large overpotentials of MCC-5 and MCC-120 at 10 mA cm–2 indicate

that MnO-CNTs hybrid material is insufficient in driving water oxidation.[317] In addi-

tion, the inert nature of CNTs is unfavorable for OER and ORR in aqueous electrolytes.

A higher degree of oxidation of Mn and hydrophilic CNTs are expected to enhance the

OER activities of MCC-5 and MCC-120. However, by traditional liquid-phase oxidation

in HNO3 it is difficult to obtain high-valent manganese oxides and hydrophilic CNTs in

one step due to the severe leaching of metal oxides. A simple, highly effective HNO3 va-

por method, which was developed by our group,[320] was employed for the oxidation.

5.2.1.4 HNO3 vapor oxidation

The MCC-5 and MCC-120 were treated by HNO3 vapor oxidation at 200 °C for 72h.

The oxidized samples were designated as MCC-growth time-V(oxidation time), (where ‘V’

stands for vapor phase), such as MCC-5-V72 and MCC-120-V72.

The compositions and BET surface areas of the samples before and after the oxidation

are displayed in Table 5.4. The oxidized samples show significant increases in metal and

oxygen contents and BET surface areas. Moreover, the oxidation treatment also caused

the loss of carbon, which amounts to 71 wt.% and 25 wt.% for MCC-5-V72 and MCC-120-

V72, respectively. These results are in good agreement with previous results.[247]

Significant changes in the morphology of carbon nanotubes were not observed for

MCC-5-V72 and MCC-120-V72 by comparing the SEM images in Figure 5.3 and Figure

5.5. Figure 5.5c presents the XRD patterns of the samples MCC-5-V72 and MCC-120-

V72. The peaks at 19.1 (111), 36.9 (311), 38.6 (222), 44.9 (400), 59.3 (511), and 65.2 °

(440) show the overlapping of the reflections of the cubic MnO2 and cubic Co3O4. Addi-

tional peaks at around 31.3 and 55.7 ° can be assigned to the (220) and (422) reflections

of the Co3O4. The formation of Co3O4 can be explained by the creation of defects in the

walls or tips of the nanotubes that allowed the HNO3 vapor to pass through the channels

of nanotubes.

Therefore, it can be concluded that MnO was oxidized to MnO2 by vapor phase oxida-

tion and meanwhile the encapsulated metallic Co was also oxidized to Co3O4. The sam-

ples after HNO3 vapor oxidation were identified to be hybrids of MnO2 nanoparticles and

OCNTs decorated with Co3O4 nanoparticles (Co3O4-MnO2-OCNTs).


Figure 5.5: SEM images of MCC-5-V72 (a) and MCC-120-V72 (b) and the corresponding

XRD patterns (c).

Table 5.4: Elemental analysis, BET surface area and weight changes of the samples after

HNO3 vapor oxidation.

Sample Elemental analysis / wt.% BET surface

area (N2) / m2 g–1

Weight change / wt.%a Co Mn C O

MCC-5-V72 6.98 5.40 68.40 19.22 125 –71 MCC-120-V72 0.31 0.28 93.34 6.07 157 –25 aWeight change (wt.%) = (mfinal – minitial) × 100 / minitial.

The CV scans in Figure 5.6a show that the OER occurred at the potential above 1.6 V

on the samples before and after HNO3 vapor oxidation. The anodic peaks of the oxidized

samples shifted to less positive potentials. In addition, remarkable background currents

can be observed for the oxidized samples, which can be contributed to capacitive charac-

ters of MnO2 and oxygen functional groups.

The oxidized samples exhibit larger current densities and earlier onset of anodic oxi-

dation current as compared with the non-oxidized samples (Figure 5.6b). The onset po-

tentials of OER were determined to be 1.56 V for MCC-5-V72 and 1.62 V for MCC-120-

V72, respectively (Table 5.5). The OER potentials and overpotentials at 10 mA cm–2 show


that MCC-5-V72 outperformed the other samples (Table 5.5), which is likely due to the

higher contents of MnO2 and Co3O4.

Figure 5.6: (a) First cycle of CVs recorded at 100 mV s–1 and 1600 rpm and (b) Anodic

LSV recorded at 5 mV s–1 and 1600 rpm in 0.1 M KOH for the samples before

and after HNO3 vapor oxidation. (c) The corresponding Tafel plots.


and 1600 rpm in 0.1 M KOH for the samples before and after HNO3 vapor

oxidation.

Material Eonset / V




MCC-5 1.64 1.73 0.50 58 MC-120 1.65 1.74 0.51 54 MCC-5-V72 1.56 1.65 0.42 68 MCC-120-V72 1.62 1.68 0.45 50 MnOx/OCNTs 1.53 1.67 0.44 105

In addition, the OER activity of MCC-5-V72 was compared with MnOx/OCNTs (7.88

wt.% Mn), which was prepared by incipient wetness impregnation.[261] The comparison of

Eonset, EOER and η at 10 mA cm–2 indicates a better OER activity for MCC-5-V72, which


results from the strong coupling at interfaces between metal oxides and carbon nano-

tubes.[28] Moreover, compared with the large Tafel slope of MnOx/OCNTs, the small Tafel

slopes of MCC-5-V72 and MCC-120-V72 imply an improved electron transfer at the in-

terfaces between metal oxides and carbon nanotubes.

5.2.1.5 Time effect

To elucidate the formation of MnO2 and Co3O4, as well as oxygen functional groups by

HNO3 vapor oxidation, the oxidation time was varied for sample MCC-5.

Figure 5.7a presents the XRD patterns of the oxidized samples as a function of oxida-

tion time. It can be seen from the vanishing of the characteristic peaks of MnO and the

emerging of the characteristic peaks of MnO2 that MnO was directly oxidized to MnO2 in

the first 8 h. Furthermore, the relative intensities of metallic Co peaks decreased indi-

cating the partial oxidation of metallic Co. When the oxidation time was extended to 72

h, the characteristic peaks of Co3O4 and MnO2 can be identified.

The oxidation-induced structural changes of the carbon nanotubes were assessed by

Raman spectroscopy. Figure 5.7b shows the first-order and second-order Raman spectra

of the oxidized samples. The spectral deconvolution was performed via curve-fitting de-

veloped by Sadezky et al.[262] A detailed peak assignment is shown in section 4.2. Similar

to Likodimos et al.,[285] the dependence of the integrated intensity ratios of the D, D', D'',

G', and D+G bands (D/G, D'/G, D''/G, G'/G and D+G/G) relative to the tangential G mode

for all the samples were investigated. The down and up behavior of the D/G, D'/G and

D+G/G ratios indicate the removal of amorphous carbon during the early oxidation stage

and formation of oxygen-containing groups at the later stage. The decreasing trend of

D''/G ratio is additional proof for the removal of amorphous carbon. On the other hand,

the HNO3 vapor oxidation caused slight changes in the graphitic structural order of the

oxidized nanotubes, as illustrated by the slight decrease in the G'/G ratio.


Figure 5.7: (a) XRD patterns and (b) normalized Raman spectra of the samples oxidized

by HNO3 vapor as a function of oxidation time in vapor phase. (c) The corres-

ponding integrated intensity ratio calculated from the peak areas derived via

spectral deconvolution.

TPD experiments were carried out to quantitatively analyze the release of CO2 and

CO from the oxidized samples (Table 5.7). Different peaks of released CO2 and CO can be

assigned to different oxygen-containing functional groups.[244, 285] CO2 originates mainly

from the decomposition of carboxyl groups and carboxylic anhydrides at lower tempera-

tures. CO is associated with the decomposition of hydroxyl, carbonyl, and ether groups at

higher temperatures. The content of oxygen obtained from CO2 and CO can be used as

an indicator for the degree of oxygen functionalization. It can be seen that the degree of

oxygen functionalization rises with oxidation time, which is in line with the Raman re-

sults. The oxygen functionalization can considerably improve the wettability of carbon

nanotubes,[321] and a hydrophilic surface promotes the release of gas bubbles and reduces

the overpotential.


Table 5.6: TPD studies of the samples oxidized by HNO3 vapor as a function of oxidation

time.

Sample Oxidation time / h

CO2

/ mmol g–1 CO

/ mmol g–1 Oa

/ wt.% MCC-5 - 0.026 0.25 0.37 MCC-5-V8 8 0.77 1.12 3.19 MCC-5-V24 24 1.00 1.31 4.00 MCC-5-V48 48 1.22 1.68 4.95 MCC-5-V72 72 1.77 3.68 8.66

aO content calculated from the amounts of CO2 and CO decomposed from the surface oxygen-containing functional groups.

Table 5.7: Weight change and BET surface area as of the samples oxidized by HNO3 va-

por as a function of oxidation time.

Sample Oxidation time / h

Weight change / wt.%

BET surface area / m2 g–1

MCC-5 0 0 87 MCC-5-V8 8 +1.8 96 MCC-5-V24 24 –19.7 109 MCC-5-V48 48 –41.4 110 MCC-5-V72 72 –71.0 125

A further investigation of the weight change and BET surface area of oxidized sam-

ples shows that a short oxidation time (i.e. 8 h) caused slight increases in the weight and

surface area of MCC-5-V8 (Table 5.7), which can be attributed to the oxygen functionali-

zation and oxidation of MnO to MnO2 as confirmed by TPD and XRD results. A long oxi-

dation time (≥ 24 h) resulted in a large weight loss and increased the surface area due to

the carbon corrosion and opening of the carbon nanotubes by the oxidative nitric acid

vapor.[247]

The cyclic and linear sweep voltammograms of OER for the oxidized samples are

shown in Figure 5.8a-b. The oxidized samples show extraordinary activities, as demon-

strated by much lower onset potentials and overpotentials needed to reach 10 mA cm–2

(Table 5.8), compared with non-oxidized MCC-5. Moreover, MCC-5-V48 and MCC-5-V72

outperform MCC-5-V8 and MCC-5-V24 in the OER. In addition, the redox features in

Figure 5.8a can originate from the oxidation Mn3+ ↔ Mn4+ + e–.[322] This feature can only

be observed for the samples MCC-5-V8, -V24 and -V48, implying that the manganese


oxides were not fully oxidized. The Tafel plots are presented in Figure 5.8c. A shorter

oxidation time (≤ 24h) was found to favor the OER kinetics, while a longer oxidation time

(≥ 48h) yielded slower the OER kinetics.

The OER activity of MCC-5-V72 was compared with commercial Pt/C, RuO2 and IrO2

catalysts, which have been widely used for OER activity benchmarking.[323] A

MnCo2O4/NCNTs reported by Zhao was also tested for comparison.[28] The onset poten-

tials and overpotentials at 10 mA cm–2 of these commercial catalysts obtained from Ma-

sa’s work were tested following the same procedure as the OER measurements per-

formed in this work (Table 5.8). It was found that the OER activity of MCC-5-V72 is

comparable to that of commercial RuO2 and MnCo2O4/NCNTs, and better than that of

commercial Pt/C and IrO2 catalysts.

Figure 5.8: (a) First cycle of CVs recorded at 100 mV s–1 and 1600 rpm and (b) anodic

LSV of OER recorded at 5 mV s–1 and 1600 rpm in 0.1 M KOH for the sam-

ples oxidized by HNO3 vapor as a function of time in vapor phase. (c) The

corresponding Tafel plots.


Table 5.8: Electrochemical OER performance derived from LSV measured at 5 mV s –1

and 1600 rpm in 0.1 M KOH for the samples oxidized HNO3 vapor as a func-

tion of time.

Material Eonset / V




MCC-5 1.64 1.73 0.50 58 MCC-5-V8 1.61 1.67 0.44 46 MCC-5-V24 1.61 1.66 0.43 45 MCC-5-V48 1.56 1.65 0.42 66 MCC-5-V72 1.56 1.65 0.42 68 MCC-5-V72 (∆)a 1.57 1.66 0.43 64 Commercial Pt/C[313] - 1.90 0.67 - Commercial RuO2[313] - 1.64 0.41 - Commercial IrO2[313] - 1.70 0.47 - MnCo2O4/NCNTs[28] - 1.66 0.43 -

a∆: thermal treatment in He flow at 300 °C for 2 h.

5.2.1.6 Hydrophilic properties

It is known that carboxyl groups mainly contribute to the hydrophilic properties of

CNTs. To understand the influence of hydrophilic properties on the OER activity, MCC-

5-V72 was thermally treated in He flow at 300 °C for 2 h in order to partially remove the

carboxyl groups, which decompose at 300 °C as demonstrated in previous TPD

studies.[243-244] The XRD patterns show similar crystalline structures in the samples be-

fore and after thermal treatment (Figure 5.9a). The OER study reveals that the removal

of carboxyl groups by thermal treatment resulted in a slight increase in the onset poten-

tial and overpotential at 10 mA cm–2 (shown in Table 5.8), indicating a positive effect of

carboxyl groups for OER. In addition, the removal of carboxyl groups yielded a smaller

Tafel slope, which benefited the OER kinetics. However, these results are insufficient to

draw a conclusion on the influence of hydrophilic property on the OER activity. Further

thermal treatment at higher temperature is in progress to gain additional proof. Howev-

er, high temperature treatment can also affect the crystallinity of MnO2, which is a key

factor for the electrochemical activity.[324-326]


Figure 5.9: (a) XRD patterns and (b) anodic LSV recorded at 5 mV s–1 and 1600 rpm in

0.1 M KOH for MCC-5-V72 obtained before and after thermal treatment at

300 °C for 2 h. The corresponding Tafel plots shown in the inset.

5.2.1.7 Formation mechanism

Based on the above results, the formation mechanism involving three steps is summa-

rized in Scheme 5.1:

(1) Formation of metallic Co and MnO (Co/MnO) nanoparticles during the hydrogen

reduction of Co-Mn oxides;

(2) CNT growth over Co-MnO nanoparticles from ethylene at 650 °C yielding MnO-

CNTs@Co;

(3) HNO3 vapor oxidation at 200 °C yielding Co3O4-MnO2-OCNTs.

Scheme 5.1: Steps of synthesis process of Co3O4-MnO2-OCNTs hybrids (black plates: Co-

Mn oxides; blue sphere: Co particles; light grey cubes: MnO particles; cap-

sule-like shape: carbon nanotubes; dark grey cubes: MnO2 particles; green

spheres: Co3O4 particles).

(1) (2) (3)

-CO

OH

-CO

OH

-CO

OH

HO

OC

-

HO

OC

-

HO

OC

-


5.2.1.8 Thermal oxidative cutting

To further demonstrate the advantages of HNO3 vapor oxidation, which not only re-

sults in high valent manganese oxide (i.e. MnO2) but also creates surface oxygen-

containing functional groups on CNTs, thermal oxidative cutting was chosen to oxidize

sample MCC-5 by air flow, as described in section 3.1.3.4. The obtained samples were

designated as MCC-growth time-C(oxidation temperature), (where ‘C’ stands for cut-

ting), such as MCC-5-C400.

Thermogravimetry (TG) was used to determine the starting oxidation temperature of

MCC-5. The TG curve shows a slight increase in weight in the temperature range of

200–400 °C due to the oxidation of manganese monoxide (inset of Figure 5.10a). The oxi-

dation of CNTs occurred at about 400 °C. The weight loss of the oxidized sample in-

creased with increasing oxidation temperature, resulting in the relative increase of the

mass fraction of Co and Mn.[28]

XRD patterns reveal the phase transformation of the oxidized samples obtained at dif-

ferent temperatures (Figure 5.10b), which is summarized as following:

MnO, Co400

MnO, Mn3O4, Co

MnO, Co500

Mn3O4, Mn2O3, CoMn2O4, Co3O4

MnO, Co600

Mn3O4, Mn2O3, CoMn2O4, Co3O4

CoMn2O4 was formed at the interface/contact between MnO and metallic Co.[28] Sur-

prisingly, MnO2 was not detected by XRD.


Figure 5.10: (a) TG weight loss of MCC-5. (b) XRD patterns and (c) normalized Raman

spectra of the samples obtained by thermal oxidative cutting as a function

of temperature. (d) Integrated intensity ratios derived from the spectral de-

convolution.

Table 5.9: Mass loss and amounts of desorbing CO2 and CO derived from TPD experi-

ments for the samples oxidized by oxidative oxidation.

Sample Temperature / °C

Carbon loss / wt.%

TPD CO2

/ mmol g–1 CO

/ mmol g–1 O

/ wt.% MCC-5 -

400 - 0.026 0.25 0.37

MCC-5-C400 –0.14 0.077 0.31 0.56 MCC-5-C500 500 –30.2 0.21 0.75 1.40 MCC-5-C600 600 –41.8 0.26 0.93 1.73

aO content calculated from the amounts of CO2 and CO decomposed from the surface oxygen-containing functional groups.

To investigate the influence of thermal oxidative cutting by air on defect concentra-

tion and the graphitic structure of CNTs, Raman measurements were performed with

the oxidized samples. The decrease in the D/G, D'/G and D''/G ratios indicates the re-

moval of amorphous carbon from the CNTs and the decreases of the overall structural


ordering of CNTs. Although the weight losses caused by thermal oxidative cutting were

remarkable, the amounts of generated oxygen-functional groups (Table 5.9) were much

lower than those created by HNO3 vapor oxidation (Table 5.7). This observation indicates

that the thermal oxidative cutting mainly occurred at the tip of the carbon nanotubes.

Figure 5.11 and Table 5.10 show that slight enhancements in OER activities were

achieved with the oxidized samples by thermal oxidative cutting. It is worth to mention

that the manganese oxide obtained by HNO3 vapor oxidation is mainly MnO2, while

those obtained by thermal oxidative cutting are mainly Mn3O4 and Mn2O3. Based on the

OER results, the OER activity order can be concluded that MnO2 > Mn3O4 > Mn2O3 ≈

MnO, which is in good agreement with the DFT calculations reported by Rossmeisl’s

group.[210]

Figure 5.11: Anodic LSV recorded at 5 mV s–1 and 1600 rpm in 0.1 M KOH for the sam-

ples obtained by thermal oxidative cutting as a function of time in air flow.


and 1600 rpm in 0.1 M KOH for the samples obtained by thermal oxidative

cutting as a function of time in air flow.

Samples EOER / V at 10 mA cm–2


MCC-5 1.73 0.50 MCC-5-C400 1.71 0.48 MCC-5-C500 1.73 0.50 MCC-5-C600 1.73 0.50


5.2.1.9 Long-term stability

The samples MCC-5-V48 and MCC-5-V72 were identified to be the best catalysts for

OER in the scope of this work. The former was further investigated for long-term stabili-

ty in the OER. The test was performed using chronoamperometry at a constant potential

of 1.62 V for 72 h in 0.1 M KOH. It can be seen that the current response rose within the

first 2 h (Figure 5.12) due to the oxidation of Mn3+ to Mn4+. As the time further in-

creased, the current response started to decrease gradually due to the anodic dissolu-

tions of carbon (Eq. 5.1)[272] and MnO2 (Eq. 5.2).[210, 327]

2 4 4 (5.1)

2 4 3 (5.2)

Figure 5.12: Chronoamperometric response recorded at 1.62 V in 0.1 M KOH for MCC-5-V48.

5.2.2 ORR

To explore the activity as bifunctional catalysts, oxygen reduction reaction (ORR) of

the samples oxidized by HNO3 vapor was investigated.

Figure 5.13 shows the CV cycles recorded in argon-saturated and oxygen-saturated

KOH (0.1 M) for the samples before and after HNO3 vapor oxidation, respectively. In ar-

gon-saturated KOH electrolyte, the non-oxidized samples (MCC-5 and MCC-120) only

show small background currents. The oxidized samples (MCC-5-V72 and MCC-120-V72)

show larger background currents from the electrochemical double layers and the redox

features of manganese oxides,[210] which further confirmed the formation of high valent

manganese and oxygen functional groups of CNTs.


Figure 5.13: CV recorded at 5 mV s–1 without rotation in argon-saturated and oxygen-

saturated KOH (0.1 M) for the samples before and after HNO3 vapor oxida-

tion. (a) MCC-5, (b) MCC-120, (c) MCC-5-V72 and (d) MCC-120-V72).

LSV of ORR obtained at a scan rate of 5 mV s–1 and a rotation speed of 900 rpm for

the samples before and after HNO3 vapor oxidation are presented in Figure 5.14a. The

onset potential (Eonset) and half-wave potential (E1/2) are summarized in Table 5.11. It is

clear that the onset potentials and half-wave potentials for MCC-5-V72 and MCC-120-

V72 shifts positively around 50 and 70 mV, respectively, indicating dramatically im-

proved ORR activities by HNO3 vapor oxidation. Among these catalysts, MCC-5-V72

possesses the best ORR activity, which is comparable to the MnOx/NC reported by Masa

et al.[313] The excellent ORR activity of MCC-5-V72 can be ascribed to the strong coupling

at interfaces between metal oxides and carbon nanotubes.

The kinetics of oxygen adsorption was analyzed by Tafel plots. As shown in Figure

5.14, two Tafel slopes can be obtained for each sample. The slopes obtained at lower

overpotentials are related to the high coverage of oxides and/or adsorbed oxygen inter-

mediates (Temkin isotherm) and the slopes obtained at higher overpotentials can be at-

tributed to the low coverage of oxides and/or adsorbed oxygen intermediates (Langmuir


isotherm).[328-329] A smaller Tafel slope indicates faster kinetics of oxygen adsorption and

vice versa.

Figure 5.14: Cathodic LSV recorded with 5 mV s–1 and 900 rpm in oxygen-saturated

KOH (0.1 M) for the samples before and after HNO3 vapor oxidation.

Table 5.11: Electrochemical ORR performance derived from LSV measured at 5 mV s–1

and 1600 rpm in 0.1 M KOH for the catalysts before and after HNO3 vapor

oxidation.

Material Eonset / V

E1/2 / V

EORR / V at 1.0 mA cm–2

Tafel slope (1) / mV decade–1

Tafel slope (2) / mV decade–1

MCC-5 0.84 0.72 0.75 57 192 MC-120 0.74 0.67 0.68 97 133 MCC-5-V72 0.89 0.82 0.84 38 110 MCC-120-V72 0.81 0.73 0.75 61 125 MnOx/NC [313] - - 0.81 - - Pt/C [313] - - 0.96 - -

It is known that ORR in alkaline electrolytes can follow either a direct 4-e- reduction

mechanism (Eq. 5.3) or a series of 2e– × 2e– reduction mechanism (Eq. 5.4 and Eq. 5.5).[37]

2 4 4 (5.3)

2 2 (5.4)

2 3 (5.5)

The ORR kinetics was analyzed using the Koutecky-Levich (K-L) equation (Eq. 5.6):

√ (5.6)


where j is the measured current density, jk is the kinetic current density, B is the Le-

vich slope, ω is the rotation speed. The Levich slope (Eq. 5.7) is defined as:

0.62 / / (5.7)

where n is the number of electron transferred, F is the Faraday constant, is the

bulk oxygen concentration, is the diffusion coefficient of oxygen, υ is the viscosity of

the solution. The following parameters were used for the calculation: for an oxygen-

saturated 0.1 M KOH solution, the oxygen concentration = 1.2 × 10–3 mol L–1, the

oxygen diffusion coefficient = 1.9 × 10–5 cm2 s–1 and the viscosity of the electrolyte υ =

1.1 × 10–2 cm2 s–1.[37]

Figure 5.15 shows the RDE voltammograms in O2-saturated KOH electrolyte. As ex-

pected, the diffusion current increased with increasing rotation speed for all the samples.

K-L plots were derived from the potentials within the diffusion-limited regions (0.3 – 0.6

V).

The obtained number of electrons transferred as a function of potential is displayed in

Figure 5.16. It can be seen that the number of electrons transferred were increased for

the oxidized samples. Surprisingly, the number of electrons transferred for each sample

was not constant at different potentials, implying the ORR kinetics of these samples was

different from a first-order reaction.[330-331]


Figure 5.15: RDE voltammograms recorded at 5 mV s–1 in oxygen-saturated KOH (0.1 M)

and Koutecky-Levich plots for (a, b) MCC-5, (c, d) MCC-120, (e, f) MCC-5-

V72, and (g, h) MCC-120-V72, respectively at different rotation speeds.


Figure 5.16: The calculated number of electrons transferred as a function of potential.

5.3 Summary

A two-step vapor-phase synthesis method has been developed for using of the residual

CNT growth catalysts to prepare MnO2-OCNTs hybrid materials containing Co3O4 nano-

particles (MnO2-OCNTs@Co3O4). First, a spinel Co-Mn oxide was used to grow CNTs,

yielding MnO-CNT hybrid materials with encapsulated Co nanoparticles. Second, HNO3

vapor oxidation was further employed to oxidize the MnO nanoparticles to MnO2 nano-

particles and to create abundant oxygen functional groups. The obtained results demon-

strate that MnO2 nanoparticles acted as active sites for oxygen evolution and reduction

reactions and the oxygen functional groups on CNT surface favor the release of generat-

ed oxygen bubbles. The CNT-encapsulated Co nanoparticles were also oxidized to Co3O4

nanoparticles due to the opening of carbon nanotubes by oxidation. The excellent OER

and ORR activities of obtained catalysts can be attributed to the strong coupling at inter-

faces between metal oxides and CNTs. The long-term stability test revealed the inevita-

ble anodic dissolution of metal oxides and CNTs at high working voltages.

Chapter 6

Conclusions and outlook

This dissertation deals with three fundamental questions: (1) the influence of water

and ammonia on the growth kinetics of CNTs, (2) the influence of synthesis parameters

on the structure and property of NCNTs, and (3) the use of the residual growth catalysts

as bifunctional electrocatalysts for the oxygen reduction and evolution reactions.

Growth kinetics of CNTs. The influence of H2O and NH3 on CNT growth kinetics at

the initial growth stages was investigated over a Co-based mixed oxide catalyst under

plug-flow conditions. By means of fast on-line analysis, the C2H4 conversion and CO gen-

eration at different feed gas compositions and growth temperatures were monitored

within the first 300 s. CO was generated either by the carbothermal reduction of the cat-

alysts, or by the gasification of deposited carbon. The obtained time-resolved carbon

mass accumulation was fitted with a kinetic model yielding the initial growth rate and

the mean lifetime of the catalytically active sites. The results of this investigation show

that H2O vapor can be a promoter for the CNT growth at low concentrations (≤ 250 ppm),

but an inhibitor at high concentrations (≥ 300 ppm) at 650 °C. At 550 °C 200 ppm H2O

were found to be an inhibitor decreasing both the initial growth rate and the mean life-

time, whereas at 650 °C both the initial growth rate and the mean lifetime were signifi-

cantly enhanced by feeding H2O. The results demonstrate the dual role of H2O during

the CNT growth: the removal of deposited carbon by gasification and the partial oxida-

tion of the Co catalyst. Depending on the temperature and H2O concentration, the CNT

growth can be either promoted or inhibited. NH3 was shown to be a promoter for the

CNT growth at low concentrations (≤ 1000 ppm), but an inhibitor at high concentrations

(≥ 2000 ppm). As a whole, this work provides additional information about the optimal

growth temperature (e.g. 650 °C), which should be neither too low to achieve a sufficient

growth rate, nor too high to cause the severe deposition of amorphous carbon.

However, this work still has open questions to be answered. First, the minimum of the

water concentration investigated was limited down to 200 ppm. Hence, this study was

not able to optimize the water concentration in a wider range. Second, this study is li-

110 CHAPTER 6 CONCLUSION AND OUTLOOK

mited by the lack of information about the outer diameter of the CNTs grown with am-

monia or particle size of the catalyst after CNT growth, which would be helpful to estab-

lish the relationship between the initial growth rate, nitrogen doping and particle size.

Growth of NCNTs. In order to evaluate the influence of the synthesis parameter (i.e.

growth time, ethylenediamine concentration and growth temperature) on the morpholo-

gy, yield, composition, graphitization and oxidation resistance of the NCNTs, the ob-

tained NCNTs were systematically characterized by using SEM, elemental analysis,

Raman spectroscopy and TPO. One of the most significant findings is that the surface

etching through the carbon gasification strongly influences the structure and graphitiza-

tion degree of the grown NCNTs. In view of high yield and high nitrogen content (> 4.0

wt.%), the optimal conditions for NCNT growth in the scope of this study were deter-

mined to be 120 min, 5.9 vol.% EDA in a He flow with a total flow rate of 100 sccm and

650 °C.

To elucidate the thickening and thinning of nanotubes, catalyst-induced and gas

phase-induced thickening processes and surface etching through the carbon gasification

were proposed. The evidence of this study suggests that unlike the conventional CVD

process of CNT and NCNT growth, the NCNTs grown with the precursor containing high

N/C ratio should involve (1) adsorption and dissociation of carbon-nitrogen-containing

gas precursor, (2) diffusion of carbon and nitrogen atoms, (3) initial formation of gra-

phite-like cups, (4) competition between the continuous formation of nanotubes (includ-

ing thickening and thinning stages) and surface etching through carbon gasification, (5)

termination of growth, and (6) surface etching through carbon gasification.

Regarding the surface etching through carbon gasification, additional analysis tech-

niques, such as transmission electron microscope (TEM) and X-ray photoelectron spec-

troscopy (XPS) will be helpful to clarify the graphitic structure of nanotubes and surface

composition as well as the nitrogen configuration. With respect to the high yield produc-

tion of NCNTs, influence of additional gases (e.g. ethylene, hydrogen, ammonia, water)

on the yield and nitrogen content should be further investigated. Among these gases,

ethylene is believed to be able to enhance the yield and simultaneously maintain the ni-

trogen content. The results would contribute to the understanding of the dependence of

the properties of NCNTs on the synthesis parameters and provide guidelines for the

mass production of NCNTs.

CNTs with residual metals as bifunctional electrocatalysts. A simple, easily scalable,

and novel method has been developed for the synthesis of MnO2-OCNTs hybrids contain-

111

ing Co3O4 nanoparticles (MnO2-OCNTs@Co3O4). A spinel Co-Mn oxide was used to grow

CNTs yielding MnO-supported CNTs with Co nanoparticles encapsulated in the nano-

tubes. HNO3 vapor oxidation was further employed to oxidize the MnO nanoparticles to

MnO2 nanoparticles which acted as active sites for oxygen evolution and reduction reac-

tions, and to create abundant oxygen functional groups which favor the release of gener-

ated oxygen bubbles. Simultaneously the encapsulated Co nanoparticles were oxidized to

Co3O4 nanoparticles due to the opening of carbon nanotubes by oxidation. The excellent

OER and ORR activities of the obtained catalysts can be assigned to the strong coupling

between metal oxides and carbon nanotubes. The two-step vapor-phase synthesis tech-

nique shows a high potential in preparing CNT-oxide hybrid for electrochemical energy

conversion.

As inspired by the obtained results, several questions can be raised: What kind of

structure is formed at the interfaces between the metal oxides and the carbon nano-

tubes? What is the physicochemical meaning of the strong coupling? To elucidate the

strong coupling at interfaces between the metal oxides and CNTs in the obtained hybrid

matertials, temperature-programmed reduction (TPR) and XPS experiments are

planned. XPS is expected to provide information not only on the chemical oxidation state

of metals but also on the possible binding between metals and oxygen or carbon. TEM

will help to identify the location of the obtained MnO2 nanoparticles and provide addi-

tional information about the crystalline structure. The long-term stability test indicates

that further efforts are essential to overcome the inevitable anodic dissolution of metal

oxides and CNTs at high working voltages.

It is interesting to see the outcome of the ongoing and planned experiments. It is ex-

pected that the carbon nanotubes-based materials will prove to be a promising material

for oxygen electrocatalysis.

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Appendix

Abbreviations

symbol denotion BET Brunauer-Emmett-Teller CNTs Carbon nanotubes CV Cyclic voltammetry CVD Chemical vapor deposition DFT Density functional theory DWCNTs Double-walled carbon nanotubes EDA Ethylenediamine ICP-OES inductively coupled plasma-optical emis-

sion spectroscopy LSV Linear sweep voltammetry MWCNTs Multi-walled carbon nanotubes NCNTs Nitrogen-doped carbon nanotubes NPMC Non-precious metal catalysts OCNTs Oxygen functionalized carbon nanotubes OER Oxygen evolution reaction ORR Oxygen reduction reaction RDE Rotating disk electrode SEM Scanning electron microscope SWCNTs Single-walled carbon nanotubes TEM Transmission electron microscope TPD Temperature-programmed desorption TPO Temperature-programmed oxidation TPR Temperature-programmed reduction VACNTs Vertically aligned carbon nanotubes VLS Vapor-liquid-solid VSS Vapor-solid-solid WAVCD Water-assisted chemical vapor deposition XPS X-ray photoelectron spectroscopy XRD X-ray diffraction

126 APPENDIX

List of Figures Figure 2.1: (a) Wrapping of graphene sheet to form SWNT. Adapted from ref. [46]. (b)

A schematic cross-section through a “hollow-tube”, “bamboo” and “herringbone” MWCNT. Reproduced from ref. [51]. ..................................................................................... 6

Figure 2.2: HRTEM images of the CNT arrays grown on 6 min-pretreated Fe (1 nm) catalyst films (scale bar: 5 nm). Adapted from ref. [100]. ................................................... 11

Figure 2.3: Scheme of Ostwald ripening of catalysts, and how it is expected to affect carpet growth with and without the addition of water vapor. Adapted from ref. [119]. ... 15

Figure 2.4: Schematic illustration of the different growth mechanisms in different atmospheres. Adapted from ref. [161]. ................................................................................ 18

Figure 2.5: TEM Images and electron diffraction pattern of CNTs produced via epitaxial growth (a-c) and non-epitaxial growth (d-e), respectively. Adapted from ref. [164]. ...................................................................................................................................... 18

Figure 2.6: TEM images of compartment layers of NCNTs with various N concentrations. Adapted from ref. [174]. ............................................................................. 19

Figure 2.7: Different types of N functionalities in graphite. Reproduced from ref. [175]. ...................................................................................................................................... 20

Figure 2.8: (A) EELS spectra involving C and N K edges recorded at the points marked in the inset image. (B) Chemical mapping of C and N profiles extracting from an EELS line-scan across the CNx nanotube as labeled in (A). (C) Scheme of CNx nanotube showing inhomogeneous distribution of C and N. Adapted from ref. [176]. .............................................................................................. 20

Figure 2.9: Top: typical longitudinal slices extracted at the same depth and orientation from the shape-sensitive reconstruction (left), C and N 3D elemental maps (middle), and C-to-N 3D relative map where nitrogen in green and carbon in red (right). Bottom: combined morphological and chemical analysis of the highly doped N-CNT in cross-section: (left) longitudinal slice extracted from the mean-density (ZL) tomographic reconstruction; (middle) two transversal sections through the 3D mean-density and chemical relative (C-to-N) reconstructions; (right) cross-sectional views by the 3D model of the analyzed tube. Adapted from ref. [178]. .................................................................... 22

Figure 2.10: Atomic model of a (22, 0) nanotube including two a graphitic-like substitutional nitrogen and a pyrrolic-like substitutional nitrogen, respectively. Adapted from ref. [179]. ....................................................................................................................... 22

Figure 2.11: Volcano-type relationship for the ORR activity versus the oxygen binding energy. Adapted from ref. [213]. .......................................................................................... 27

Figure 2.12: Brief history of ORR catalysts for N−C based catalysts. Adapted from ref. [202]. ...................................................................................................................................... 28

Figure 2.13: (a) Theoretical overpotentials for the OER on various oxides versus the difference in the free binding energy between O* and OH*. (b) Theoretical overpotentials versus the experimental overpotentials in acidic (filled squares) and in alkaline media (empty circles). Adapted from ref. [2]. ................................................................................. 29

Figure 2.14: The number of annually published papers on carbon nanotubes and their applications in electrocatalysis from 1991 to 2013. Data from ISI Web of Knowledge. ... 29

Figure 3.1: Flow sheet of the water-assisted CVD set-up used in the CNT growth experiments. .......................................................................................................................... 34

127

Figure 3.2: Flow sheet of the ammonia-assisted CVD set-up used in the CNT growth experiments. .......................................................................................................................... 34

Figure 3.3: Flow diagram of the injection CVD set-up used in the synthesis of NCNTs. ............................................................................................................................................... 37

Figure 3.4. Flow sheet of the CVD setup used in the CNT growth experiments. ......... 40 Figure 3.5: Scheme of the tube reactor used for TPO and TPD experiments. .............. 42 Figure 4.1: (a) C2H4 concentration profiles ( 2 4 ~ ) during the CNT growth and the

blank experiment; (b) corresponding profiles of the molar flow rates ( 2 4 ~ ) derived from (a); (c) time-resolved consumption rate of ethylene (∆ 2 4 ~ ) during the CNT growth calculated by subtracting the molar C2H4 flow rate from the blank experiment; (d) time-resolved accumulation of the carbon mass ( ~ ) during the CNT growth under standard conditions calculated from theC2H4 consumption rate. The initial growth rate (r0) is the initial slope of the fitting curve. ................................... 48

Figure 4.2: CNT mass accumulation ( ) as a function of relative time ( ) at different H2O concentrations. The growth was performed at 650°C. (a) First 21 s and (b) up to 300 s. ............................................................................................................................ 49

Figure 4.3: (a) The initial growth rate (r0) and (b) the mean lifetime (τ) of the Co catalyst as a function of the H2O concentration. The growth was performed at 650 ºC. .. 49

Figure 4.4: CO generation during the CNT growth at 650 ºC with different concentrations of H2O vapor. (a) up to 300 s, (b) first 21 s. ................................................ 51

Figure 4.5: Outer diameter distribution of CNTs grown at 650 ºC with 0 and 200 ppm of H2O in the feed gas. .......................................................................................................... 52

Figure 4.6: Time-resolved CNT mass accumulation ( ) as a function of relative time ( ) obtained at different temperatures. (a) First 21 s, (b) up to 300 s. .................. 52

Figure 4.7: (a) The initial growth rate (r0) and (b) the mean lifetime of the catalytically active sites (τ) as a function of growth temperature derived from Figure 4.6. .................. 53

Figure 4.8: CO profiles obtained under different growth temperatures with 200 ppm of H2O vapor. (a) First 21 s, (b) up to 300 s. ............................................................................ 54

Figure 4.9: Outer diameter distributions (a-c) and mean diameter (d) of CNTs grown at different temperatures with 0 and 200 ppm of H2O vapor. ............................................ 55

Figure 4.10: CNT mass accumulation ( ) as a function of relative time ( ) at different NH3 concentrations. The growth was performed at 650°C. (a) First 21 s and (b) up to 300 s. ............................................................................................................................ 56

Figure 4.11: (a) The initial growth rate (r0) and (b) the mean lifetime (τ) of the Co catalyst as a function of NH3 concentration. The growth was performed at 650 ºC. ........ 57

Figure 4.12: SEM images of purified NCNTs grown with 4.6 vol.% EDA at 650 °C as a function of growth time (a-f). The corresponding size distribution of outer diameter (g) and the mean outer diameter (h) determined by measuring 600 NCNTs. ........................ 61

Figure 4.13. (a) The yield of as-grown NCNTs and (b) the contents of carbon, nitrogen and residual metals in purified NCNTs as a function of growth time at 650 °C with 4.6 vol.% EDA. The C and N contents in weight percent (wt.%) were determined from elemental analysis. The 100×N/C is the atomic percentage of nitrogen compared to carbon derived from the weight ratio. ................................................................................. 63

Figure 4.14: (a) Normalized Raman spectra of purified NCNTs grown at 650 °C with 4.6 vol.% EDA as a function of growth time. Peak-fitting is given for the sample 30 min

128 APPENDIX

as an example. (b) The corresponding integral peak intensity ratios as a function of growth time. .......................................................................................................................... 65

Figure 4.15: (a) CO2/CO-TPO profile of the purified NCNTs grown for 120 min. (b) The onset and maximum oxidation temperatures of the purified NCNTs as a function of growth time at 650 °C with 4.6 vol.% EDA. ........................................................................ 66

Figure 4.16: SEM images of purified NCNTs grown at 650 °C for 120 min as a function of EDA concentration (a-d). The corresponding size distribution of outer diameter (e) and the mean outer diameter (f) measured from 600 NCNTs. .................................................. 67

Figure 4.17: (a) The yield of as-grown NCNTs and (b) the contents of carbon, nitrogen and residual metals in purified NCNTs as function of EDA concentration at 650 °C for 120 min. ................................................................................................................................. 68

Figure 4.18: (a) Normalized Raman spectra of NCNTs grown at 650 °C in 120 min with different EDA concentrations. (b) The corresponding integral peak intensity ratios as a function of EDA concentration. .................................................................................... 69

Figure 4.19: The onset and maximum oxidation temperatures of the purified NCNTs as functions of EDA concentration at 650 °C for 120 min. ................................................. 70

Figure 4.20: SEM images of NCNTs (washed) grown with 5.9 vol. % EDA for 120 min at: (a) 550, (b) 650, (c) 750 and (d) 850 °C. The corresponding size distribution of the outer diameter (e) and the mean outer diameter (f) of the NCNTs were derived measuring 600 NCNTs. To ensure comparability, a cooling or heating step after reduction was carried out in He always for 30 min to allow the reactor reaching the desired growth temperature. ................................................................................................ 71

Figure 4.21: The yield of as-grown NCNTs and (b) the contents of carbon, nitrogen and residual metals in purified NCNTs as function of the growth temperature with 5.9 vol.% EDA for 120 min. ........................................................................................................ 72

Figure 4.22: (a) Normalized Raman spectra of NCNTs grown in 120 min with 9.0 vol.% EDA as a function of the growth temperature. (b) Intensity ratios, (c) the corresponding integral area ratios and (d) the frequencies and FWHM of G-bands of the NCNTs determined by curve-fitting as a function of the growth temperature. ............................. 74

Figure 4.23: The onset and maximum oxidation temperatures of the purified NCNTs as functions of growth temperature with 5.9 vol.% EDA for 120 min. .............................. 74

Figure 4.24: The yields of NCNTs grown with 4.6 vol.% and 5.9 vol.% EDA at 650 °C as a function of growth time. The yield of CNTs grown at 650 °C with a feed gas of 6.0 vol.% C2H4 and 12.0 vol.% H2 at a total flow of 100 sccm. .................................................. 76

Figure 4.25: (a) Normalized Raman spectra of NCNTs and CNTs. (b) The onset and maximum oxidation temperatures of the purified NCNTs and CNTs. .............................. 77

Figure 4.26: Schematic diagram illustrating the thickening and thinning of the nanotube. ............................................................................................................................... 80

Figure 5.1: (a, b) SEM images, (c) XRD pattern and (d) H2-TPR profile of the as-received Co-Mn oxide. ........................................................................................................... 85

Figure 5.2: CNT yields of the different oxides as a function of growth time. Optical images of the catalyst before (left) and after CNT growth for 5 min (middle) and 120 min (right), respectively. .............................................................................................................. 86

Figure 5.3: SEM images of MCC-5 (a) and MCC-120 (b) and the corresponding size distributions of the outer diameter (c) and (d) XRD patterns. ........................................... 88

129

Figure 5.4: (a) First cycle of CVs recorded at 100 mV s–1 and 1600 rpm and (b) Anodic LSV recorded at 5 mV s–1 and 1600 rpm in 0.1 M KOH. (c) The corresponding Tafel plots. ............................................................................................................................................... 90

Figure 5.5: SEM images of MCC-5-V72 (a) and MCC-120-V72 (b) and the corresponding XRD patterns (c). .......................................................................................... 92

Figure 5.6: (a) First cycle of CVs recorded at 100 mV s–1 and 1600 rpm and (b) Anodic LSV recorded at 5 mV s–1 and 1600 rpm in 0.1 M KOH for the samples before and after HNO3 vapor oxidation. (c) The corresponding Tafel plots. ................................................. 93

Figure 5.7: (a) XRD patterns and (b) normalized Raman spectra of the samples oxidized by HNO3 vapor as a function of oxidation time in vapor phase. (c) The corresponding integrated intensity ratio calculated from the peak areas derived via spectral deconvolution. ......................................................................................................... 95

Figure 5.8: (a) First cycle of CVs recorded at 100 mV s–1 and 1600 rpm and (b) anodic LSV of OER recorded at 5 mV s–1 and 1600 rpm in 0.1 M KOH for the samples oxidized by HNO3 vapor as a function of time in vapor phase. (c) The corresponding Tafel plots. 97

Figure 5.9: (a) XRD patterns and (b) anodic LSV recorded at 5 mV s–1 and 1600 rpm in 0.1 M KOH for MCC-5-V72 obtained before and after thermal treatment at 300 °C for 2 h. The corresponding Tafel plots shown in the inset. ......................................................... 99

Figure 5.10: (a) TG weight loss of MCC-5. (b) XRD patterns and (c) normalized Raman spectra of the samples obtained by thermal oxidative cutting as a function of temperature. (d) Integrated intensity ratios derived from the spectral deconvolution. . 101

Figure 5.11: Anodic LSV recorded at 5 mV s–1 and 1600 rpm in 0.1 M KOH for the samples obtained by thermal oxidative cutting as a function of time in air flow. .......... 102

Figure 5.12: Chronoamperometric response recorded at 1.62 V in 0.1 M KOH for MCC-5-V48. ......................................................................................................................... 103

Figure 5.13: CV recorded at 5 mV s–1 without rotation in argon-saturated and oxygen-saturated KOH (0.1 M) for the samples before and after HNO3 vapor oxidation. (a) MCC-5, (b) MCC-120, (c) MCC-5-V72 and (d) MCC-120-V72). .................................................. 104

Figure 5.14: Cathodic LSV recorded with 5 mV s–1 and 900 rpm in oxygen-saturated KOH (0.1 M) for the samples before and after HNO3 vapor oxidation. ........................... 105

Figure 5.15: RDE voltammograms recorded at 5 mV s–1 in oxygen-saturated KOH (0.1 M) and Koutecky-Levich plots for (a, b) MCC-5, (c, d) MCC-120, (e, f) MCC-5-V72, and (g, h) MCC-120-V72, respectively at different rotation speeds. ............................................ 107

Figure 5.16: The calculated number of electrons transferred as a function of potential. ............................................................................................................................................. 108

130 APPENDIX

List of Tables Table 2.1: Scalable CNT production by CVD method.[27] ............................................... 12 Table 2.2: A summary of nitrogen contents in NCNTs synthesized using different

methods, nitrogen/carbon precursor and catalysts. ............................................................ 24 Table 4.1: CNT yield by weighing the reactor before and after growth for 300 s (Y300s),

carbon accumulation by calculation after growth for 300 s (m300s) and theoretical predicted maximum CNT yield (Ymax) derived from curve fitting.a ..................................... 50

Table 4.2: CNT yields by weighing the reactor after growth for 300 s (Y300s), the carbon mass accumulation by calculation after growth for 300 s (m300s) and theoretical predicted maximum CNT yield (Ymax) derived from curve fitting. The growth was performed at different temperatures without and with H2O. ................................................................... 54

Table 4.3: CNT yields by weighing the reactor before and after growth for 300 s (Y300s), carbon accumulation by calculation after growth for 300 s (m300s) and theoretical predicted maximum CNT yield (Ymax) derived from curve fitting. Nitrogen content determined from the CNTs grown for 300 s. ....................................................................... 57

Table 5.1: Theoretical maximum yield, initial growth rate and mean lifetime of CNTs grown with Co-Mn oxide and the Co-Mn-Al-Mg oxide catalysts. ....................................... 86

Table 5.2: Elemental analysis, BET surface area of the samples before and after CNT growth. ................................................................................................................................... 87

Table 5.3: Electrochemical OER performance derived from LSV measured at 5 mV s–1 and 1600 rpm in 0.1 M KOH ................................................................................................ 90

Table 5.4: Elemental analysis, BET surface area and weight changes of the samples after HNO3 vapor oxidation.................................................................................................. 92

Table 5.5: Electrochemical OER performance derived from LSV measured at 5 mV s–1 and 1600 rpm in 0.1 M KOH for the samples before and after HNO3 vapor oxidation. ... 93

Table 5.6: TPD studies of the samples oxidized by HNO3 vapor as a function of oxidation time. ...................................................................................................................... 96

Table 5.7: Weight change and BET surface area as of the samples oxidized by HNO3 vapor as a function of oxidation time. .................................................................................. 96

Table 5.8: Electrochemical OER performance derived from LSV measured at 5 mV s –1 and 1600 rpm in 0.1 M KOH for the samples oxidized HNO3 vapor as a function of time. ............................................................................................................................................... 98

Table 5.9: Mass loss and amounts of desorbing CO2 and CO derived from TPD experiments for the samples oxidized by oxidative oxidation. ......................................... 101

Table 5.10: Electrochemical OER performance derived from LSV measured at 5 mV s–

1 and 1600 rpm in 0.1 M KOH for the samples obtained by thermal oxidative cutting as a function of time in air flow. ................................................................................................ 102

Table 5.11: Electrochemical ORR performance derived from LSV measured at 5 mV s–

1 and 1600 rpm in 0.1 M KOH for the catalysts before and after HNO3 vapor oxidation. ............................................................................................................................................. 105

131

Publications

(1) K. P. Xie, M. Muhler, W. Xia, Influence of water on the initial growth rate of carbon nanotubes from ethylene over a cobalt-based catalyst, Industrial & Engineering Chemistry Research, 2013, 52(39), 14081– 4088.

(2) Z. Y. Sun, K. P. Xie, Z. A. Li, I. Sinev, P. Ebbinghaus, A. Erbe, M. Farle, W. Schuhmann, M. Muhler, E. Ventosa, Hollow and Yolk-shell iron oxide nanostructures on few-layer grapheme in Li-ion batteries, Chemistry – A European Journal, 2014, 20, 2022–2030.

(3) Z Y. Sun, N. N. Dong, K. P. Xie, W. Xia, D. König, T. Nagaiah, M. D. Sánchez, P. Ebbinghaus, A. Erbe, X. Y. Zhang, A. Ludwig, W. Schuhmann, J. Wang, M. Muh-ler, Nanostructured few-layer graphene with superior optical limiting properties fabricated by a catalytic steam etching process, Journal of Physical Chemistry C, 2013, 117(22): 11811–11817.

(4) E. Ventosa, A. Tymoczko, K. P. Xie, W. Xia, M. Muhler, W. Schuhmann, Low temperature hydrogen reduction of high surface area anatase TiO2 and anatase/(B) TiO2 applied in high charging rate Li-ion batteries. ChemSusChem, 2014, 7(9): 2584–2589.

(5) M. J. Becker, W. Xia, K. P. Xie, A. Dittmer, K. Voelskow, T. Turek, M. Muhler, Se-parating the initial growth rate from the rate of deactivation in the growth kinetics of multi-walled carbon nanotubes from ethene over a cobalt-based bulk catalyst in a fixed-bed reactor, Carbon, 2013, 58: 107–115.

(6) A. Q. Zhao, J. Masa, W. Xia, A. Maljusch, M.-G. Willinger, G. Clavel, K. P. Xie, R. Schögl, W. Schuhmann, M. Muhler, Spinel Mn-Co oxide in N-doped carbon nano-tubes as a bifunctional electrocatalyst synthesized by oxidative cutting, Journal of the American Chemical Society, 2014, 135(21): 7551–7554.

(7) P. R. Chen, L. M. Chew, A. Kostka, K. P. Xie, M. Muhler, W. Xia, Purified oxygen- and nitrogen-modified multi-walled carbon nanotubes as metal-free catalysts for selective olefin hydrogenation, Journal of Energy Chemistry, 2013, 22(2): 312–320.

132

Presentations and conference contributions

(1) K. P. Xie, W. Xia, J. Masa, W. Schuhmann, M. Muhler, Highly dispersed RuO2 on ni-trogen-functionalized carbon nanotubes for oxygen evolution reaction, JCS-Ruhr, Bochum, 2014.

(2) K. P. Xie, W. Xia, J. Masa, W. Schuhmann, M. Muhler, Highly dispersed RuO2 on ni-trogen-functionalized carbon nanotubes for oxygen evolution reaction, Electrochemi-stry 2014, Mainz, 2014.

(3) K. P. Xie, W. Xia, J. Masa, W. Schuhmann, M. Muhler, Nitrogen-doped carbon nano-tubes as bifunctional electrocatalysts for oxygen reduction and evolution, SurMat Evalution, Düsseldorf, 2014.

(4) K. P. Xie, C. Seidler, W. Xia, M. Muhler, Parametric study on the synthesis of nitro-gen-doped carbon nanotubes from ethlyenediamine over a cobalt-based catalyst, 47. Jahrestreffen Deutscher Katalytiker, Weimar, 2014.

(5) K. P. Xie, M. Muhler, W. Xia, Influence of water on the initial growth rate of carbon nanotubes from ethylene over a cobalt-based catalyst, RUB Research Day, Bochum, 2013.

(6) K. P. Xie, W. Xia, M. Muhler, Parametric study on the synthesis of nitrogen-doped carbon nanotubes from ethylenediamine over a cobalt-based catalyst, RUB Materials Day, Bochum, 2013.

(7) K. P. Xie, W. Xia, M. Muhler, Kinetic study on the role of H2O in the initial stage of carbon nanotube growth over cobalt-based catalyst, 46. Jahrestreffen Deutscher Ka-talytiker, Weimar, 2013.

(8) K. P. Xie, W. Xia, M. Muhler, Kinetic study on the water-assisted growth of carbon nanotubes over cobalt-based catalyst, Inno. CNT 2013, Fellbach, 2013.

(9) K. P. Xie, W. Xia, M. Muhler, Kinetic study on the water-assisted growth of carbon nanotubes at the initial stage, SurMat workshop & RUB Materials Day, Bochum, 2012.

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