controllable synthesis of leaf‐like cuo nanosheets for
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Accepted Article
A Journal of
Title: Controllable synthesis of leaf-like CuO nanosheets for selectiveCO2 electroreduction to ethylene
Authors: Zhonghao Tan, Tingyue Peng, Xiaojie Tan, Wenhang Wang,Xiaoshan Wang, Zhongxue Yang, Hui Ning, Qingshan Zhao,and Mingbo Wu
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To be cited as: ChemElectroChem 10.1002/celc.202000235
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Controllable synthesis of leaf-like CuO nanosheets for selective
CO2 electroreduction to ethylene
Zhonghao Tan[a], Tingyue Peng[a], Xiaojie Tan[a], Wenhang Wang[a], Xiaoshan Wang[a], Zhongxue
Yang[a], Hui Ning*[b], Qingshan Zhao[b], Mingbo Wu*[a]
[a] Z.H. Tan, T.Y. Peng, X.J. Tan, W.H. Wang, Dr. X.S. Wang, Dr. Z.X. Yang, Prof. M.B. Wu
College of Chemical Engineering, College of New Energy, Institute of New Energy, State Key Laboratory of Heavy Oil Processing, China University of
Petroleum
No. 66, West Changjiang Road, Huangdao District, Qingdao, China, 266580
E-mail: [email protected]
[b] Prof. H. Ning, Dr. Q.S. Zhao
College of chemical engineering, China University of Petroleum
No. 66, West Changjiang Road, Huangdao District, Qingdao, China, 266580
Supporting information for this article is given via a link at the end of the document.
Abstract: Carbon dioxide reduction reaction (CO2RR) driven by
renewable electricity is a promising way to tackle the CO2 emission
woes and recycle use of CO2. The synthesis of electrocatalyst with
high activity and selectivity towards CO2RR to ethylene remains a
great challenge. Herein, the leaf-like CuO nanosheets are in-situ
fabricated on nitrogen doped graphene (NG) by a novel reduction-
oxidation-reconstruction process. Used as catalyst for CO2RR in 0.1
M KHCO3, a high Faradaic efficiency of ca. 30% towards ethylene with
an ultra-high ethylene/methane ratio of 190 was achieved at -1.3 V vs.
reversible hydrogen electrode. The SEM and TEM imagines confirm
the leaf-like CuO nanosheets display high-curvature structures while
multiple distinguished grain boundaries constructed by CuO(110) and
CuO(111) planes are verified by HRTEM. For the first time, we
present a facile method to combine the high-curvature structure and
grain boundary together to enhance the selectivity of CO2RR to
ethylene over CuO catalyst.
Introduction
Excessive CO2 emission form combustion of fossil fuels has
caused a series of environmental problems. Electrocatalytic CO2
reduction to value added chemicals by renewable electricity is of
great significance for tackling the carbon emissions and recycle
use of CO2.[1-7]
Due to the inert chemical activity of CO2 molecules, highly efficient
catalysts are essential for the efficient CO2 reduction reaction
(CO2RR) .[8-10] The mechanism of CO2RR are quite complicated
and the products are usually diverse, including C1 (CO, CH4,
CH3OH, HCOOH), C2 (C2H4, C2H6, C2H5OH, CH3COOH) and C3+
molecules.[11-16] Until now, the exploration of highly selective
catalysts is still a great challenge. For the C1 products, screens of
catalysts have been investigated and some excellent works have
been reported. For example, Wang et al.[17] synthesized a Ni-N-C
material with coordinatively unsaturated nickel–nitrogen sites,
achieving a high CO Faradaic efficiency (FE) of 92.0–98.0% over
a wide potential range of -0.53 to -1.03 V vs. reversible hydrogen
electrode (RHE). Li et al.[18] reported an ultrathin bismuth
nanosheets prepared via in situ topotactic transformation of
bismuth oxyiodide (BiOI) nanosheet template, where the single
crystallinity and enlarged surface areas affords this material an
excellent electrocatalytic performance for CRR to formate with
~100% selectivity. Wang et al.[19] investigated three copper
complex materials for CO2RR and found under the working
conditions copper(II) phthalocyanine undergoes reversible
structural and oxidation state changes to form ~2 nm metallic
copper clusters, which catalyzes the carbon dioxide to methane
with a high FE of 66%.
Compared to the C1 products, only few kinds of catalysts have
been reported to exhibit considerable selectivity to C2 products, of
which the copper-based materials are the most attractive,
especially for ethylene.[20-24] Many pioneer researchers have
studied the crystal structure effects of pristine copper towards
CO2RR, where crystal planes, nanoparticle size and edge/corner
morphologies are mostly discussed.[25-27] Chen et al.[23]
demonstrated that the atomic arrangements on Cu single crystal
surfaces is crucial toward product selectivity of electrocatalytic
CO2 reduction. The Cu(100) facets could facilitate ethylene
generation, while the lower stability of adsorbed *O on the
Cu(110) surface could cause the spilt of the reduction pathway
from ethylene to ethanol. Buonsanti’s study[28] reveals a non-
monotonic size-dependence of the selectivity in cube-shaped
copper nanocrystals and an optimal ratio of edge sites over (100)
plane-sites is crucial to maximize CO2RR to ethylene selectivity.
In addition, oxide-derived copper (ODCu) has shown enhanced
CO2RR activity and increased selectivity towards multi-carbon
products, which is closely related to the high curvature structures
derived from the preparation or pretreatment procedure. Cuenya
et al.[29] developed oxidized copper catalysts with lower
overpotentials for carbon dioxide electroreduction and record
selectivity towards ethylene (60%) through plasma treatments.
They found the roughness structure of oxide-derived copper
catalysts facilitates the stabilization of Cu+ species on the surface
during reactions, which may drive the reaction of negatively
charged CO2 reduction intermediates to C2 products. Sargent et
al.[30] presented an electro-redeposition, dissolution and
redeposition of copper from a sol–gel. By in situ X-ray
spectroscopy and density functional theory simulations, they
revealed the beneficial interplay between the sharp morphologies
of ODCu and Cu+ oxidation state. It was found the sharp features
of ODCu kinetically limit methane formation through local pH
effects while the presence of Cu+ stabilizes ethylene
intermediates. Furthermore, the copper nanowire arrays were
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also reported to promote the CO dimerization by resulting an
increased local pH near the copper active sites for ethylene
generation.[31]
Recently, grain boundaries have been designed to stabilize
unique active surfaces for CO2RR to C1 products on Au,[32-33]
SnO2[34-35] and Bi-Zn,[36] etc. However, novel studies on the grain
boundaries of copper materials towards CO2RR to C2 products
are still limited. Lee et al. [37] prepared Ag-incorporated biphasic
Cu2O-Cu catalysts through a facile electrochemical co-deposition
method. They assume that the host Cu2O was partially substituted
by Ag to form a solid Ag-Cu grain boundary, which relatively
suppresses the H2 evolution reaction and encourages the reaction
of mobile CO generated on Ag to a residual intermediate on a Cu
site. Consequently, by varying the elemental arrangement
(phase-separated and phase-blended) of Ag and Cu, they
realized the selectivity control over ethanol and ethylene. Inspired
by Lee’s work, we believe the grain boundary effects toward
ethylene selectivity may also be applicable for ODCu materials.
To our best knowledge, the combining of high-curvature
morphology and grain boundaries to promote the selectivity of
CO2RR to ethylene over copper oxide has yet to be explored.
Herein, we proposed a facile method to synthesize leaf-like CuO
nanosheets with both high-curvature structure and grain
boundaries. A novel procedure of first reduction and then
oxidation method was invented to synthesize the leaf-like CuO
nanosheets and a thermally spontaneous surface reconstruction
strategy was applied to construct the grain boundaries on the
edge of CuO nanosheets. Moreover, all above process can be
successfully carried out when using nitrogen doped graphene
(NG) as supports to obtain CuO/NG composites, which manifest
high selectivity towards ethylene as catalysts for electroreduction
of carbon dioxide.
Results and Discussion
The synthesis procedure of CuO/NG_AN is shown in Scheme 1.
Firstly, the copper hydroxide nanowire was in situ formed on the
NG to obtain Cu(OH)2/NG composite (Fig. S1a) by adding alkali
to a solution containing Cu2+ and NG. Then, using NaBH4 as
reduction reagent, the Cu(OH)2/NG was reduced to Cu/NG (Fig.
S1b). After oxidation in air, the Cu/NG was transformed to
CuO/NG_A, which was then transferred to CuO/NG_AN through
annealing in N2 atmosphere. Following the same procedure,
CuO_A and CuO_AN were prepared without NG as supports,
where A represents air and N represents nitrogen gas.
Scheme 1. Schematic illustration for the synthesis of CuO/NG_AN.
The XRD patterns of all the samples were shown in Figure 1. The
diffraction peak at 26° was attributed to NG, which is consistent
with previous reports.[38] In CuO/NG_A and CuO/NG_AN, the
position of the characteristic peak of NG has not changed,
indicating the NG is stable during the synthesis of composites. In
the Raman spectra (Figure S2), we can see two typical peaks
around 1350 cm-1 and 1590 cm-1, corresponding to the disordered
sp3C (D band) and in-plane vibrational sp2C (G band). It is
generally agreed that the intensity ratio of the D to G-band (ID/IG)
reveals the disordered structure and defects of the graphitic
carbon material. [39] Here in this work, the value of ID/IG is almost
the same for NG, CuO/NG_A and CuO/NG_AN, confirming the
stability of NG during the synthesis process.
The peaks at 32°,35°,38°,48°,53°,61° and 65° are accurately
indexed to the (110), (002), (111), (202), (020), (113) and (311)
planes of CuO (JCPDS PDF#80-0076) respectively. It was noting
that CuO/NG_A and CuO/NG_AN has the exactly same
characteristic peaks in the XRD patters, indicating the bulk crystal
structure of CuO has no significant change after the annealing
treatment in N2 atmosphere. As control experiments, the XRD
patterns of CuO_A and CuO_AN were also depicted in Figure 1.
It can be seen that all the CuOs have the exactly same
characteristic peaks, proving the bulk crystal phase of CuO was
not changed by NG supporting or annealing treatment.
Figure 1. XRD patterns of NG, CuO_A, CuO_AN, CuO/NG_A and CuO/NG_AN.
In order to further determine the valence state of copper in the
composite, XPS characterization was performed on CuO_A,
CuO_AN, CuO/NG_A and CuO/NG_AN, as shown in Figure S3.
For CuO (Figure S3a, b), no carbon peak was found while an
obvious C 1s peak exists in CuO/NG_A (Figure S3c) and
CuO/NG_AN (Figure S3d). The high-resolution Cu 2p spectra of
all the materials as made were deconvoluted to several Gaussian
peaks corresponding to Cu 2p 1/2 at 954.3 eV and Cu 2p 3/2 at
933.5 eV, confirming the Cu2+ state in the CuOs as made.[40]
Combining the results of XRD and XPS, it can be concluded that
the CuO is successfully synthesized and in situ fabricated on NG.
10 20 30 40 50 60 70
311113020
202
111
002
110
JCPDS PDF#80-0076
2 Theta (degree)
Inte
ns
ity
(a.u
)
CuO/NG_AN
CuO/NG_A
CuO_AN
CuO_A
NG
NG
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Figure 2. Cu2p spectra of a) CuO_A, b) CuO_AN, c) CuO/NG_A and d)
CuO/NG_AN.
The morphologies of all the samples were shown in Figure 3. It
can be seen that all the CuOs are leaf-like nanosheets with a clear
high-curvature edge. In addition, the CuO/NG_A and CuO/NG
_AN behave a smaller degree of aggregation compared to CuO
_A and CuO_AN, which is attributed the supports effects of NG.
Figure 3. SEM images of a) CuO_A, b) CuO_AN, c) CuO/NG_A and d)
CuO/NG_AN.
The TEM images afford more distinguished morphologies of
CuO/NG_A and CuO/NG_AN, as shown in Figure 4a, b. It can be
seen that the leaf-like morphology of CuO nanosheets are
maintained after the annealing treatment in N2 atmosphere.
However, when we look into the fine structures of CuO edge by
HRTEM, only Cu(110) crystals are displayed in the CuO/NG_A
with a uniformly arranged lattice fringes (Figure 4c). However,
Cu(110) and Cu(111) planes simultaneously present in CuO/NG
_AN, forming multiple disordered grain boundaries (Figure 4d).
Because no other substances were added during the annealing
process, it can be inferred that some of the CuO(110) planes were
transferred to CuO(111) planes spontaneously. Li et al.[41]
proposed the oxygen in CuO can be replaced by nitrogen to form
N-doped CuO at 300 °C under N2 atmosphere. We believe this
reaction may also happens in our experiments when we anneal
the CuO nanosheets in N2 atmosphere. Besides, due to the ultra-
thin and high-curvature structure, our CuO nanosheets is more
sensitive to oxygen, leading to the re-oxidation of N-doped CuO
to CuO in air circumstance. During there process, due to the
switchable nitridation-oxidation reaction associating with the
thermal stress, the Cu(110) planes with higher surface energy will
spontaneously transfer to CuO(111) planes with lower surface
energy,[42] leading to the generation of grain boundaries
eventually.
Figure 4. TEM images of a) CuO/NG_A, b) CuO/NG_AN; HRTEM images of c)
CuO/NG_A, d) CuO/NG_AN.
The activity of all the catalysts towards CO2RR were evaluated by
linear sweep voltammetry (LSV) in an N2 and CO2 saturated 0.1M
KHCO3 electrolyte (Fig. S5). When the electrolyte is saturated by
CO2, the reduction current of all the catalysts increased compared
to those in N2 saturated electrolyte starting from −0.4 V vs. RHE,
indicating the reduction of CO2 suppress the HER when the
applied potential was more negative than -0.4 V vs. RHE. Among
all the catalysts, the CuO/NG_AN has the lowest onset potential
and the highest current density under the same potentials,
indicating it has a better activity than other catalysts as made.
In order to investigate the selectivity of various materials towards
CO2RR to ethylene, the potentiostatic electrolysis of carbon
dioxide was monitored under various potentials from -1.0 V to -
1.4 V vs RHE. The FE of all the products were shown in Figure 6.
For all the catalysts as made, the highest FE of ethylene was
obtained at -1.3 V vs RHE. The CuO/NG_A and CuO/NG_AN
have higher selectivity of ethylene compared to the pristine CuO
_A and CuO_AN respectively, proving NG as support can
enhance the selectivity of ethylene with respect to its dispersing
effect. As control experiment, the catalytic performance of NG for
CO2RR was tested and the results were shown in Fig. S6. It can
be seen that no C2 molecular was found in the gas or liquid
products. However, a high FE of H2 was detected from 86%-87%
while the FE of CO was less than 3.0% under the whole range of
applied potentials. Obviously, NG presents good activity for the
hydrogen evolution reaction (HER) but a gentle ability for CO2-to-
CO. Even so, NG as supports are also reported to facilitate the
CO2RR to ethylene.[43] The HER reaction on NG is conducive to
increase the local pH near CuO nanostructures and facilitate the
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surface coverage of CO2 and CO on the CuO surface, resulting
the accelerating of CO2-to-C2H4 reaction.[44] Both with NG as
supports, CuO/NG_AN presents higher selectivity towards
ethylene than CuO/NG_A at -1.3 V vs RHE, which is attributed to
the grain boundaries formed after the annealing treatment of CuO
under N2 atmosphere. The copper atoms near the gain boundary
are presumed to expose more unsaturated bonds for the
stabilization of C2 intermediates, resulting in the accelerating
generation of ethylene. Finally, among all the catalyst as made,
CuO/NG_AN gives the best performance for electrocatalytic CO2
reduction to ethylene. The highest FE of ethylene is 29% and the
selectivity of ethylene in the carbon contained products is 78%.
The grain boundary associating with the high-curvature structure
and NG supports are all contributed to the excellent catalytic
performance of CuO/NG_AN.
Figure 5. Faradaic efficiency of a) CuO_A, b) CuO_AN, c) CuO/NG_A and d)
CuO/NG_AN.
At last, the durability of all the catalysis was evaluated at -1.3V vs
RHE and the results are shown in Fig. S7. All the catalysts behave
an increasing current density during a long electrocatalysis of CO2
for 15,000 s, which is related to the reduction of CuO under
negative potentials.[45] The samples with NG as supports have a
better stability than those without NG, confirming that NG is an
useful supports to stabilize copper oxide species under negative
potentials.[43] As the optimal material in this work, the FE of
ethylene over CuO/NG_AN shows a minor attenuation from 29%
to 26% after the longtime electrocatalysis of CO2. Fig. S8 shows
the SEM image of CuO/NG_AN after the longtime CO2RR,
proving that the morphology of CuO was maintained to some
extent. However, The XRD pattern of CuO/NG_AN after
electrocatalysis in figure S9 and the XPS pattern of CuO/NG_AN after electrocatalysis in figure S10 show that CuO is reduced to
Cu2O and Cu in the CO2RR process, leading to the increasing
current density and decreasing selectivity. All in all, the stability of
our materials still needs to be improved in the future works.
Conclusion
In summary, we present a facile method to synthesize a leaf-like
CuO by reduction-oxidation method. With an annealing treatment
in N2 atmosphere, we successfully controlled the reconstruction
of surface copper atoms to form grain boundaries of CuO(110)/
CuO(111) while keeping the leaf-like structure maintained. By in
situ fabricated on the nitrogen doped graphene, a novel CuO/NG
composite is prepared and used as electrocatalyst for carbon
dioxide reduction. A new stretagy by combination of high
curvature and grain boundary is applied to imporve the selectivity
of ethylene by electrocatalytic reduction of carbon dioxide on CuO.
At -1.3 V vs RHE, the selectivity of ethylene in the carbon
contained products reaches 78% with an with an
ethylene/methane ratio of 190. This work offered a facile strategy
to exploit morphological effects for elevating the selectivity of
copper-based catalysts for CO2-to-C2H4, which may be widely
used to prepare other highly selective CO2 reduction catalysts.
Experimental Section
Chemicals and materials
Graphene oxide (GO) was supplied from Nanjing XFNANO Materials Tech
Co., Ltd., P.R. China. Copper chloride dihydrate (purity, 99.0%), sodium
hydroxide (purity 96%), melamine (purity 99.99%), sodium borohydride
(purity, 98.0%), and isopropyl alcohol were purchased from Sinopharm
Chemical Reagent Co., Ltd., P. R. China. All the reagents were used as
received without further purification. The deionized water (15 MΩ) in this
work was made by a Millipore system in our lab.
Synthesis of NG
Typically,100 mg GO and 500 mg melamine were fully mixed and then
heated to 700 °C under nitrogen atmosphere for 3 h with a rate 5 °C/min
in a tube furnace. After cooling to room temperature, the solids were
washed with deionized water and recollected by filtration and dried at 60 °C
overnight for further characterizations.
Synthesis of CuO/NG composites
In a typical synthesis, 60 mg copper chloride dihydrate was firstly scattered
into 20 mL NG aqueous suspension (1.0 mg/mL), then 0.5 M of sodium
hydroxide was slowly added to neutralize the solution. After vigorous
stirring for 10 min, 20 mL of 0.03 M sodium borohydride solution was slowly
added, followed by vigorous stirring for 5 h. The above solution was suction
filtered and washed three times with deionized water, the solid was
collected and heated to 200 °C for 3h under air atmosphere to obtain
CuO/NG_A. Then, the as-obtained CuO/NG_A was heated to 300 °C
under nitrogen atmosphere for 3 h. After cooling down to room temperature,
the obtained solid was named as CuO/NG_AN. As control experiments,
the CuO_A and CuO_AN were also prepared using the same method
respectively without NG.
Characterizations
The crystal structure parameters were characterized by X-ray
diffraction( X'Pert PRO MPD) with Cu Ka radiation at 40 kV. SEM images
were carried on a Hitachi S4800 scanning electron microscope. TEM and
HRTEM images were obtained by a JEOL JEM-2100F field-emission
transmission electron microscope. The XPS spectra was recorded by X-
ray photoelectron spectroscopy (Thermofisher Escalab 250Xi) using Al Kα
radiation and 500 μm X-ray spot. Raman was detected by a Renishaw RM-
2000 laser Raman spectrometer (514 nm).
Electrochemical measurements
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In this work, all electrochemical measurements were carried out in a H-
type cell with a three-electrode system and 0.1 M KHCO3 as electrolyte.
Ag/AgCl electrode (KCl saturated) and Pt plate (1×1 cm-2) were used as
the reference electrode and counter electrode, respectively. All
electrochemical tests were performed on an electrochemical workstation
(CHI 760, Shanghai CH Instruments Co., China). All working potentials
referred to Ag/AgCl electrode (KCl saturated) were converted to reversible
hydrogen electrode (RHE) using the following equation: E (vs. RHE) = E
(vs Ag/AgCl) + 0.21 V+ 0.0591×pH.[46]
The working electrode was prepared as follows: 1.0 mg sample as made
and 5 μL 5% Nafion solution were dispersed into 200 μL isopropyl alcohol
to form a uniform catalyst ink. Then the catalyst ink was dropped on a L-
type glassy-carbon (GC, ø =10 mm) as a working electrode. Before each
test, high purity carbon dioxide gas (99.999%) was purged into the reaction
cell to saturate the electrolyte for at least 30 min. During the CO2
electrocatalysis tests, the CO2 flow rate was kept at 20 mL/min by a mass
flowmeter till the end of the experiment.
Analysis of products
The gas phase products were detected by an on-line gas chromatography
(BFRL-3420A, China), which was equipped with a thermal conductivity
detector (TCD) for hydrogen detection and a hydrogen flame detector
(FID) for the detection of carbon monoxide and other hydrocarbons. The
liquid phase products were detected by liquid chromatography (LC-2030
Plus, SHIMADZU, Japan).
Acknowledgements
This work is financially supported by the National Natural Science
Foundation of China (21808242); the Shandong Provincial
Natural Science Foundation (ZR2018BB070, ZR2018ZC1458);
the Fundamental Research Funds for the Central Universities of
China (19CX02042A).
Keywords: copper oxide • carbon dioxide • ethylene •
electroreduction • nitrogen doped graphene
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Entry for the Table of Contents
The leaf-like CuO nanosheets were synthesized by a facile method and show a high selectivity towards ethylene as catalysts for
electroreduction of carbon dioxide in aqueous solution due to it high-curvature structures and rich grain boundaries.
10.1002/celc.202000235
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