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Highly efficient nanoporous graphitic carbon with tunable textural propertiesfor dye-sensitized solar cells†

Pavuluri Srinivasu,*ab Ashraful Islam,c Surya P. Singh,a Liyuan Han,c M. Lakshmi Kantamab

and Suresh K. Bhargavabd

Received 30th May 2012, Accepted 14th August 2012

DOI: 10.1039/c2jm33498d

Nanoporous graphitic carbon (NGC) is synthesized by direct

carbonization of iron phthalocyanine (Fe-PC) and 3D mesoporous

silica (KIT-6) mixture for the first time, which exhibits tunable

surface area and pore volume, as well as superior energy conversion

efficiency for dye-sensitized solar cells compared to activated carbon

(AC) and large porous carbon (LPC).

Carbon based materials such as fullerenes, carbon nanotubes1 and

nanoporous carbons2 have received much attention due to their

potential applications in fuel cells,3 hydrogen storage materials,4

electrochemical double-layer capacitors,5 and electrode materials in

lithium batteries.6 In particular, nanoporous carbons with high

surface area, large pore volume, tunable pore diameter, chemical

inertness, good mechanical stability and electrical conductivity have

been paid great interest, which derive from a well-defined two- or

three-dimensional pore structure. Since the first report of nanoporous

carbon materials by the inverse replica technique using mesoporous

silica as a template and a soluble carbon precursor, various meso-

porous structures have been reported from different silica templates

such as CMK-2,7 CMK-3,8 and CMK-4 (ref. 9) with cubic Pm3n,

Ia3d and hexagonal p6mm symmetries. The potential applications of

nanoporous carbons especially for fuel cells and solar cells often

require a graphitic pore wall with controlled textural properties.

However, creating a graphitic pore wall with an ordered pore struc-

ture and controllable textural properties of mesoporous carbons is

very difficult using traditional methods. Traditional synthesis

methods for graphitic nanoporous carbon by high temperature

treatment and chemical vapor deposition methods lead to collapse of

the pore structure unless special care is taken.10 Therefore, develop-

ment of a graphitic nanoporous carbon material with a highly

ordered pore structure is desirable for practical applications.

aInorganic and Physical Chemistry Division, Indian Institute of ChemicalTechnology, Hyderabad-500607, India. E-mail: pavuluri.srini@iict.res.inbRMIT-IICT Research Centre, Indian Institute of Chemical Technology,Hyderabad-500607, IndiacPhotovoltaicMaterials Unit, National Institute forMaterials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, JapandAdvanced Materials and Industrial Chemistry Group, School of AppliedSciences, RMIT University, Melbourne, Australia

† Electronic supplementary information (ESI) available. See DOI:10.1039/c2jm33498d

20866 | J. Mater. Chem., 2012, 22, 20866–20869

Moreover, a little attention has been given for development of a new

synthetic route for graphitic nanoporous carbon materials using

mesoporous silica as a template. Interestingly, there have been no

reports available in the literature on direct synthesis of ordered

graphitic nanoporous carbon with tunable textural properties using

KIT-6 mesoporous silica as a template. Therefore, it would be

interesting to develop a synthetic route for fabrication of graphitic

nanoporous carbon with mesoporous silica as a template to ensure

that the desired mesoporosity is achieved. Recently, carbon nano-

materials have emerged as a new class of promising materials for

potential applications including energy conversion and storage.11–13 In

particular for dye-sensitized solar cells (DSSCs), carbon materials

such as carbon nanotubes,14 graphite,15 activated carbon,16 hard

carbon spherule,17 and mesoporous carbon18 have been used as a

counter electrode, which is one of the important components of

DSSC. The traditional DSSC system consists of a platinized fluorine-

tin oxide (FTO) glass as a counter electrode. The cost and corrosion

properties of platinum in triiodide-containing solutions retard

commercialization of DSSCs technology. Therefore, it is highly

desirable to fabricate a metal free counter electrode for low cost

DSSCs by replacing platinum with nanoporous carbon. However,

the energy conversion efficiency obtained for the metal-free counter

electrode using porous carbon as the counter electrode and N3 dye is

low (ca. 6.8%). Therefore, low-cost, highly efficient and metal-free

carbon counter electrodes are still highly desirable for photovoltaic

applications. In the present study, we report direct synthesis of

graphitic nanoporous carbon using KIT-6 mesoporous silica as a

template and iron phthalocyanine as the carbon precursor. In this

approach, the direct addition of the carbon precursor to the template

allows for the formation of an ordered nanoporous carbon material

with tunable textural properties.

The graphitic nanoporous carbon materials were synthesized by

direct heating of the mixture of iron phthalocyanine (1.1 g) and KIT-

6 (1.5 g) at 800 �C. The composite obtained after the thermal treat-

ment was treated with HF solution to dissolve the template. The

resulting product is denoted as NGC-X-Z, where X and Z

indicate the temperature and time used for the thermal treatment. A

series of materials are prepared by changing the heating temperature

and time.

The pore structure of nanoporous graphitic carbon (NGC)

materials is examined by nitrogen sorption measurements. Fig. 1a

shows the nitrogen sorption isotherms of NGC-800-2, NGC-800-5

This journal is ª The Royal Society of Chemistry 2012

Fig. 1 (a) Nitrogen adsorption–desorption isotherms of NGC materials

prepared at different pyrolysis temperatures (open symbols: desorption;

closed symbols: adsorption): (O)NGC-800-2, (,) NGC-800-5 and (B)

NGC-1000-5, (inset) respective BJH adsorption pore size distributions.

(b) Small-angle XRD patterns of NGC materials. (c) Wide-angle XRD

patterns of NGC materials.

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and NGC-1000-5 materials along with BJH (Barrett–Joyner–

Halenda) pore sized distributions derived from the adsorption

branches of the isotherms. All thematerials possess a classical type IV

isotherm with H1 type hysteresis loop and steep capillary conden-

sation steps occurring at high relative pressures, characteristic of

mesoporous materials. The textural properties of the NGCmaterials

Table 1 Textural properties of NGC materials synthesized at differentpyrolysis temperatures

Sample a0 (nm)ABET

(m2 g�1)Vp

(cm3 g�1)dp, BJH (nm)adsorption

NGC-800-2 23.5 1278 1.7 4.2NGC-800-5 22.8 1152 1.4 3.9NGC-1000-5 22.7 967 1.1 3.9

This journal is ª The Royal Society of Chemistry 2012

are summarized in Table 1. It is interesting to note that the specific

surface area of the NGC materials prepared using the KIT-6

template systematically decreases with increasing temperature of

thermal treatment. The NGC-800-2 has a specific surface area of

1278 m2 g�1 and a total pore volume of 1.7 cm3 g�1, whereas the

NGC-1000-5 material exhibits a lower surface area and pore volume

than the NGC-800-2 material. This decreasing trend is attributed to

the graphitized crystalline nature of carbon frameworks. BJH pore

size distribution of NGCmaterials (Fig. 1, inset) calculated from the

adsorption branch of the isotherm shows that the materials have

uniform pore size distribution, which confirms that the replication

process was successful. Furthermore, it is interesting to note that the

capillary condensation step shifts to lower relative pressures, which

directly reveals that the mesopore diameter significantly decreases

from 4.2 nm to 3.9 nmwith increasing pyrolysis temperature of NGC

materials. These results clearly indicate that the textural parameters

of NGC materials can be simply tuned by choosing a suitable

pyrolysis temperature for the KIT-6 and Fe-PC composite.

The powder X-ray diffraction patterns of NGCmaterials prepared

using KIT-6 and Fe-PC materials at different pyrolysis temperatures

are shown in Fig. 1b. All the samples exhibit sharp distinct reflections

of 2q below 3.0�, which are characteristic for highly ordered meso-

structures belonging to a 3D cubic Ia3d space group. The XRD

patterns of NGC materials exactly match with the patterns of the

KIT-6 silica template, which indicates that the NGC materials are

inverse replicas of the template. It is interesting to note that all the

materials exhibit clear higher order peaks that can be indexed as

(211), (220) and (420) diffractions. The structural order of these

materials is independent of the pyrolysis temperature used for the

synthesis. It is observed that the lower angle peak of NGC materials

shifts to a higher order with increasing the pyrolysis temperature,

indicating the shrinkage of the NGC framework during pyrolysis

treatment and the etching process, which affect the cubic porous

matrix. The length of the cubic cell a0 forNGCmaterials is calculated

using the equation a0 ¼ (h2 + k2 + l2)1/2d211 (Table 1). It should be

noted that the unit cell parameter of NGC can be readily tailored by

changing the pyrolysis temperature. The unit cell parameter value of

23.5 nm for NGC-800-2 decreases to 22.8 nm for NGC-1000-5

material.When theKIT-6 andFe-PC compositematerial is treated at

800 �C for 2 h, the obtained carbon product is amorphous in nature

(Fig. 1b). The graphitic nature of nanoporous carbon increases with

increasing the pyrolysis temperature and a highly graphitic nano-

porous carbon is obtained at 1000 �C in 2 h. In addition, the intensity

of the peak at 26�, which represents graphitic carbon, increases

sharply with increasing pyrolysis time. The overall process for the

formation of graphitic carbon in the nanoporous matrix can be

explained by ‘in situ’ catalysis of metal nanoparticles. After heating

the Fe-PC andKIT-6 mixture, above the sublimation temperature of

Fe-PC (�500 �C), the obtained composite contains iron nano-

particles dispersed graphitic carbon and KIT-6, under an inert

atmosphere. The produced iron nanoparticles during the heat treat-

ment mainly catalyze the amorphous carbon to form graphitic

carbon. The powder XRD pattern and HR-TEM data strongly

support the graphitic nature of carbon materials.

To investigate the effect of pyrolysis temperature on nature of

carbon materials and graphitization behaviour, the carbon materials

are characterized by Raman spectroscopy. The Raman spectra of

NGC-800-2, NGC-800-5 and NGC-1000-5 materials are shown in

Fig. 2. All three materials exhibit two peaks centred around

J. Mater. Chem., 2012, 22, 20866–20869 | 20867

Fig. 2 Raman spectra of NGC materials synthesized at different

pyrolysis temperatures.

Fig. 4 Photocurrent–voltage characteristics of dye-sensitized solar cells

of AC, LPC and NGC-1000-5 materials.

Table 2 Photovoltaic parameters of DSSCs based on NGC, LPC andAC electrodes

Sample Jsc (mA cm�2) Voc (V) FF (%) h (%)

NGC-1000-5 15.15 0.75 0.71 8.1LPC 13.41 0.80 0.66 7.1AC 12.07 0.76 0.15 1.4

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1575 cm�1 (G band) and at 1355 cm�1(D-band), which correspond

to sp2 bonded graphene layers in the carbon lattice and are associated

with in-planar termination of the disordered graphene-like structure,

respectively. Moreover, a sharp increase of the G-band intensity

together with decrease of the D-band intensity with increasing

pyrolysis temperature and a systematic increase in the relative

intensity of the G band and D band (IG/ID) are indicative of the high

degree of structural order or higher graphitization of carbon mate-

rials, which are consistent with the XRD results. The above results

clearly confirm that the carbon materials indeed possess an ordered

graphitic structure and the degree of graphitization can be controlled

by tuning the pyrolysis temperature. In addition, iron nanoparticles

dispersed in graphitic carbonmaterials can also be preparedwhen the

composite is treated with NaOH instead of HF. The EDS elemental

mapping of the NGC-1000-5 material in Fig. 3 shows C, O and Fe

Fig. 3 HR-TEM elemental mapping of iron nanoparticles dispersed on

the NGC material.

20868 | J. Mater. Chem., 2012, 22, 20866–20869

element peaks only, demonstrating that the KIT-6 silica template

used for the synthesis of graphitic carbon materials is completely

removed during the etching process with 2MNaOH solution and Fe

nanoparticles are uniformly distributed throughout the NGC mate-

rial. Photovoltaic performances of DSSCs based on the graphitic

nanoporous carbon, mesoporous carbon and activated carbon using

N719 dye under the illumination of air mass (AM) 1.5 sunlight (100

mW cm�2, WXS-155S-10) are shown in Fig. 4. The short circuit

current (Jsc), the open-circuit voltage (Voc), the fill factor (FF) and the

over-all conversion efficiency (h) of devices based on these counter

electrodes are summarized in Table 2. A graphitic nanoporous

carbon basedDSSC shows better device performance than both large

porous carbon (LPC) and activated carbon (AC) based DSSCs. The

graphitic nanoporous carbon counter electrode, which is synthesized

at a carbonized temperature of 1000 �C, shows a Jsc of 15.15 mA

cm�2, which is superior over Jsc of 13.6 and 12.07 mA cm�2 of

mesoporous carbon and activated carbon respectively. The improved

photocurrent of the graphitic nanoporous carbon based DSSC,

relative to mesoporous carbon (LPC) and activated carbon (AC)

basedDSSCs, is consistent with themonochromatic incident-photon-

to-current conversion efficiency (IPCE) measurements result (SI).

The over-all conversion efficiency achieved is 8.1%, 7.0% and 1.44%

for graphitic nanoporous carbon, mesoporous carbon and activated

carbon respectively. The higher FF and h indicate that graphitic

nanoporous carbon is more efficient than large porous carbon (LPC)

and activated carbon (AC) materials.

In summary, high surface area and graphitic nanoporous carbon

with an ordered pore structure has been explored as the metal-free

counter electrode for DSSCs. Nitrogen adsorption isotherm

This journal is ª The Royal Society of Chemistry 2012

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measurement indicates that the graphitic nanoporous material shows

a very high surface area and large pore volume than the mesoporous

silica KIT-6 material. The graphitic nanoporous structure of this

material helps in the penetration of electrolyte, and high specific

surface area and interconnected pore structure facilitate high energy

conversion efficiency compared to other large porous carbon and

activated carbon.

Notes and references

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