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Electrochimica Acta 81 (2012) 301– 307

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

oderately reduced graphene oxide as transparent counter electrodes forye-sensitized solar cells

ye-Su Janga, Jin-Mun Yunb, Dong-Yu Kimb, Dong-Won Parkc, Seok-In Naa, Seok-Soon Kimd,∗

Professional Graduate School of Flexible and Printable Electronics, Department of Flexible and Printable Electronics, Chonbuk National University, 664-14, Deokjin-dong, Deokjin-gu,eonju-si, Jeollabuk-do 561-756, Republic of KoreaSchool of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of KoreaResearch Institute for Solar and Sustainable Energies (RISE), Gwangju Institute of Science and Technology, Gwangju 500-712, Republic of KoreaDepartment of Nano & Chemical Engineering, Kunsan National University, Kunsan, Jeollabuk-do 753-701, Republic of Korea

r t i c l e i n f o

rticle history:eceived 3 February 2012eceived in revised form 5 July 2012ccepted 5 July 2012vailable online 16 July 2012

a b s t r a c t

Moderately reduced graphene oxide (GO) films were fabricated by simple and fast thermal treatmentof solution processed GO, and their application as an alternative to conventional Pt counter electrode indye-sensitized solar cells (DSSCs) was investigated. GO without thermal treatment and thermally treatedGO at 150 ◦C showed low efficiency of ∼0.5%, whereas cell performance was significantly improved byapplying thermal treatment over 250 ◦C. In particular, the DSSC with GO thermally treated at 350 ◦C

eywords:ye-sensitized solar cellsraphene oxideeductionhermal treatmentounter electrode

exhibited the highest performance with open-circuit voltage (VOC) of 0.66 V, short-circuit current density(JSC) of 16.35 mA/cm2, F.F. of 33.33%, and overall power conversion efficiency (PCE) of 3.60%. Moderatereduction of GO by simple thermal treatment over 250 ◦C was confirmed through the measurements ofX-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). Enhancement of efficiencyafter high temperature thermal treatment might be attributed to the improved electrical conductivitiesand higher catalytic activities, resulting from the reduction of GO.

. Introduction

Since the first demonstration of highly efficient mesoporousiO2 based dye-sensitized solar cells (DSSCs) by O’Regan andrätzel, interest in DSSCs as a promising alternative to conven-

ional silicon or compound semiconductor solar cells has grownonsiderably due to their affordable cost, simple fabrication pro-ess, and their environmental compatibility [1–5]. General DSSCsomprise a dye-modified wide band semiconductor electrode, aounter electrode, and an electrolyte containing a redox coupleI−/I3−). Photoexcitation of the dye molecules leads to the injec-ion of an electron from the dye to the TiO2 and the flow throughhe external circuit. Simultaneously, the oxidized dye is reducedy a redox mediator in the electrolyte and returns to the groundtate. The role of counter electrode, which is one of the most impor-ant components in DSSCs, is catalyzing the reduction of the redoxpecies in the electrolyte used as a mediator in regeneration of dyesfter electron injection [6–8]. For high performance DSSC, counter

lectrode with high catalytic activity towards the I3− + 2e− = 3I−

eaction and electrical conductivity is necessary. Transparency forhe special applications such as power-producing windows and

∗ Corresponding author.E-mail addresses: nsi12@jbnu.ac.kr (S.-I. Na), sskim@kunsan.ac.kr (S.-S. Kim).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.07.021

© 2012 Elsevier Ltd. All rights reserved.

metal-foil based DSSCs is also one of the most important issues[9]. Hence, the platinized transparent conductive counter electrodeprepared by thermal decomposition of H2PtCl6 is commonly usedas counter electrode due to its superior catalytic activity. How-ever, due to the scarcity of noble Pt inducing increase in the cost ofDSSCs and Pt corrosion by the redox species in the electrolyte, it isimportant to develop novel cost-efficient candidates that are simul-taneously anti-corrosive and capable of yielding DSSCs of relativelyhigh performance.

As low cost alternative counter electrodes, conducting polymerssuch as poly(3,4-ethylenedioxythiophene) (PEDOT) and polyani-line (PANI), various carbonaceous materials such as carbon black,activated carbon, and single or multi-wall carbon nanotubes, andpolymer-carbonaceous material composites have been developed[10–16]. Although carbonaceous materials are cost-efficient andresistant to corrosion and have good electrocatalytic activity, for thecomparable high efficiency to Pt counter electrode systems, mostof the carbonaceous materials have to have thickness of several �mand thus opaque [17].

Recently, two-dimensional grapheme, with a single layer of sp2

network of carbon atoms arranged in a flat honeycomb structure,

has attracted considerable attention as potential counter electrodedue to its single-atom-thick layer structure, excellent electricalproperties, large specific surface area, and inertness against oxygenand water vapor [18–25]. Graphene has been prepared by several

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pproaches such as micromechanical exfoliation of graphite,hemical vapor deposition, epitaxial progress, and the reductionf graphene oxide (GO) [26–29]. The low output of the first threeethods makes them unsuitable for wide-ranging applications,hereas the chemical and thermal reduction of GO is considered

s promising route for the mass production of graphene [30].n GO, because most of carbon atoms bonded with oxygen isp3 hybridized, it disrupts the sp2 conjugation of the hexagonalraphene lattice. Hence, the substantial sp3 fraction in GO makest an insulating material. Jung et al. reported that incrementalemoval of oxygen in GO induces transition of GO from electricalnsulator to semiconductor and ultimately to a graphene-likeemimetal [31]. Although the reduction of GO can be effectivelyerformed by chemical process using a hydrazine reagent, it is

nadequate due to the toxicity of chemical reducing agent andultiple-steps. In addition, the dispersion concentration of the

educed GO produced using hydrazine is low, which could alsoe disadvantageous for practical applications to the devices [32].ecently, it is reported that GO can be reduced by thermal methodshich are believed to be green methods. There have been two

inds of thermal methods: one is solvothermal reduction methodnd the other is solid heating reduction. In the case of solvother-al reduction, harsh solvents such as N,N-dimethylformamide

r N-methyl-2-pyrrolidinone, high pressure, and/or long react-ng time are needed and solid heating reduction necessarilyequires ultra-high vacuum under Ar and H2, and/or rapid heating>200 ◦C min−1) up to 1050 ◦C under Ar gas or up to 800 ◦C under2 gas [33–37]. This complicate and high temperature process over00 ◦C is not suitable for counter electrode in cost-efficient DSSCs.

Herein, in this study, we demonstrate a facile, fast, and cost-fficient way to fabricate efficient DSSCs containing moderatelyeduced GO counter electrode by thermal treatment of solutionrocessed GO films in air. GO films were prepared via solutionrocess on fluorine-doped SnO2 (FTO) coated glass substrates andhermally treated under the various conditions of temperature, at50, 250, and 350 ◦C in air. Effect of temperature on the proper-ies of GO based counter electrode and performance of DSSCs werenvestigated.

. Experimental

For the preparation of solution processed GO based counterlectrodes, the production of GO was performed using the mod-fied Hummer’s method, which is general synthetic method [38].raphite powder (1 g) was added to concentrated H2SO4 (98%,0 ml) with stirring, and then KMnO4 (3 g) was slowly added tohe mixture to prevent a rapid increase in the temperature. Theeaction mixture was then kept at 40 ◦C for 6 h and subsequentlyuenched by adding ice (500 g) containing H2O2 (30 wt.%, 10 ml).he resultant suspension of GO was filtered over a PTFE (polyte-rafluoroethylene) filter membrane and the remaining solid wasequently washed with 1 M HCl, acetone, and water. To obtain anqueous dispersion of GO, the gel-like GO was freeze-dried andesultant GO powder was redispersed in deionized water. Then,queous dispersion of GO was diluted with dimethylformamideDMF) to make a concentration of ∼1.5 mg/ml for the use as aoating solution.

Conductive fluorine-doped tin oxide (FTO) coated glass sub-trates (Philkington, sheet resistance: 8 �/�) were cleaned with

special detergent followed by ultrasonication in distilled-water,cetone, and isopropyl alcohol. Then, the substrates were kept in an

00 ◦C oven for 30 min to remove residual solvent. For the fabrica-ion of DSSCs, nanocrystalline TiO2 based electrodes were preparedy spreading a TiO2 paste (ENB Korea, TTP-20N) onto the FTO glasssing the doctor blade coating and subsequently sintered at 450 ◦C

Acta 81 (2012) 301– 307

for 30 min to convert anatase phase and make interparticle net-works. After then the nanostructured TiO2 films were immersedinto in an ethanolic solution of 5 × 10−4 M Ru 535 dye (SolaronixCo., Ltd) for 12 h and rinsed with absolute ethanol to removephysically adsorbed dye molecules. GO based counter electrodeswere prepared by spin coating of GO dispersion in DI water/DMFand thermally treated at 150, 250, and 350 ◦C for 10 min in air.General Pt counter electrode was also prepared by spin-coatingof 0.5 mM chloroplatinic acid (H2PtCl6) in isopropanol followingby thermal treatment at 400 ◦C. Nanocrystalline TiO2 based elec-trodes were sandwiched with thermally treated GO and Pt counterelectrodes using a 25 �m-thick hot-melt sealing material and theinner space was filled with an ionic liquid containing electrolyteof 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 0.03 M I2,0.1 M LiI, 0.1 M guanidium thiocyanate (GSCN), and 0.5 M 4-tert-butylpyridine (TBP) in a mixture of acetonitrile and valeronitrile(85:15 v/v).

Photocurrent density–voltage (J–V) measurements were per-formed using a Keithley 2400 instrument under 100 mW/cm2

illumination from a xenon light source with an AM 1.5 G filter.A calibrated silicon reference solar cell certified by the NationalRenewable Energy Laboratory (NREL) was used to confirm the accu-rate measurement conditions. Formation of uniform GO layers onthe relatively rough FTO surface and structural properties of ther-mally treated GO were investigated through the measurement offield emission scanning electron microscope (FE-SEM, Hitachi S-4800) and atomic force microscopy (AFM, Veeco Dimension 3100).Cyclic voltammetry measurement of I−/I3− redox mediator on thecounter electrodes was carried out in an acetonitrile solution of10 mM LiI, 1 mM I2 and 0.1 M LiClO4 with Pt wire counter electrodeand Ag/AgCl reference electrode. X-ray photoelectron spectroscopy(XPS) measurements providing the direct evidence of the reductionGO by thermal treatment were carried out using an AXIS-NOVA(Kratos) system with a monochromatized Al K� under a pressureof 5 × 10−8 Torr. Finally, the optical properties of various GO andPt counter electrodes were investigated via. UV–vis spectroscopy(Varian, AU/DMS-100S).

3. Results and discussion

As shown in the schematics of DSSC structure in Fig. 1, to replacehigh cost Pt counter electrode catalyzing the reduction of the redoxspecies in the electrolyte, we fabricated transparent GO films with athickness of ∼4 nm by simple spin-coating. Because for high perfor-mance DSSC, counter electrode with high catalytic activity towardsreduction reaction of redox mediator and excellent electrical con-ductivity is necessary, we observed change of electrical propertiesand catalytic activity of GO films after post thermal treatment andthe effect of thermal treatment on the performance of DSSCs.

Before applying GO films to DSSCs, to confirm successful for-mation of 2-dimensional GO on FTO by simple spin-coating ofdispersion of synthesized GO in DI water/DMF mixture solvent, sur-face morphologies of FTO and GO films on FTO were investigatedusing FE-SEM. Fig. 2(a) shows irregular surface of the FTO com-posed of many grains with variable shapes and sizes. It means thatcommercialized FTO we used in this study was produced by sput-tering method. The inset of Fig. 2(b) clearly shows 2-dimensionalGO covered on FTO substrate, and an uniform covering of large areaof FTO with 2-dimensional GO was confirmed through the surfaceimage shown in Fig. 2(b).

As we mentioned, in GO, because most of carbon atoms bonded

with oxygen is sp3 hybridized, it disrupts the sp2 conjugation ofthe hexagonal graphene lattice. In other words, the substantial sp3

fraction in GO makes it an insulating material. Hence, to removeoxygen groups in GO inducing transition of GO from electrical

H.-S. Jang et al. / Electrochimica Acta 81 (2012) 301– 307 303

Fig. 1. A schematic of the device structure fabricated with

Fig. 2. FE-SEM images of FTO (a) and solution processed GO on FTO (b).

GO and moderately reduced GO counter electrodes.

insulator to semiconductor or ultimately to a graphene-likesemimetal, we applied post thermal treatment on the spin-coated GO films. Although usual thermal reduction of GO hasbeen reported by several groups and known as green technology,because it requires complicate conditions such as ultra-high vac-uum under Ar and H2 and high temperature of ∼800–1000 ◦C underAr gas or under H2 gas, it is not adequate to the simple productionof DSSCs based on FTO/glass substrate. Hence, we tried to fabricatemoderately reduced GO by simple post thermal treatment of GOfilms and investigate the possibility of thermally treated GO filmsas counter electrodes.

To evaluate the possibility of GO with and without post thermaltreatment as counter electrode in DSSCs, conventional nanocrys-talline TiO2 based DSSCs were fabricated by using GO counterelectrodes with and without post-thermal treatment and com-pared with DSSC fabricated with typical Pt counter electrodeproduced by thermal decomposition. Fig. 3 shows representative

J–V data of the DSSCs, and the performance characteristics weresummarized in Table 1. Here, GO films were thermally treatedat 150, 250, and 350 ◦C for 10 min in air. As shown in Fig. 3 and

0.60.40.20.0

0

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20 FTO

GO N.A

GO 150oC

GO 250oC

GO 350oC

Pt

Cu

rren

t d

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sit

y (

mA

/cm

2)

Voltage (V)

Fig. 3. Representative J–V characteristics of devices including GO with and withoutthermal treatments and Pt as counter electrodes.

304 H.-S. Jang et al. / Electrochimica Acta 81 (2012) 301– 307

Fig. 4. AFM topography image of FTO (a) and solution processed GO on FTO (b); GO without thermal treatment (c) and thermally treated GO at 150 ◦C (d), 250 ◦C (e), and3

stottHitVA

TT

50 ◦C (f) on flat glass.

ummarized in Table 1, when we applied GO to DSSC without posthermal treatment, it showed poor performance with a low PCEf 0.50%, especially F.F. was very low. The device with thermallyreated GO at 150 ◦C also showed low efficiency of ∼0.51%, similaro DSSC including GO counter electrode without thermal treatment.owever, cell performance was significantly improved by apply-

ng thermal treatment over 250 ◦C. In particular, the DSSC with GO

hermally treated at 350 ◦C exhibited the highest performance withOC of 0.66 V, JSC of 16.35 mA/cm2, F.F of 33.33%, and PCE of 3.60%.lthough this value is relatively low compared to conventional Pt

able 1he photovoltaic characteristics of DSSCs based on various counter electrodes.

Counter electrodes VOC (V) JSC (mA/cm2) F.F. (%) � (%)

FTO 0.40 5.10 8.40 0.20GO N.A./FTO 0.50 9.60 9.32 0.50GO 150 ◦C/FTO 0.38 10.36 12.80 0.51GO 250 ◦C/FTO 0.64 16.50 26.90 2.85GO 350 ◦C/FTO 0.66 16.35 33.33 3.60Pt/FTO 0.67 16.20 60.00 6.44

based DSSC having VOC of 0.67 V, JSC of 16.20 mA/cm2, F.F. of 60.00%,and PCE of 6.44%, we emphasize that solution processed GO film fol-lowed by facile and fast thermal treatment can be a candidate forcost-efficient Pt-less counter electrode.

To find evidence for the enhancement of performance and char-acterize fundamental structural and electrical properties of GOafter thermal treatment, we carried out several analysis such asAFM, XPS, and conductivity. Fig. 4(a) and (b) shows AFM morpholo-gies of FTO and GO coated FTO, respectively. After spin-coating ofGO on FTO, the root mean square (RMS) roughness was decreasedfrom 16.3 nm to 12.16 nm as a result of uniform covering of GOlayers on FTO. Because sputtered FTO used as general transpar-ent electrode for DSSCs has typical columnar structure, it has veryrough surface as shown in Fig. 4(a). Hence, it is difficult to obtainaccurate information on the morphology of GO films composed ofsingle layer of GO nanosheet, which have the thickness of ∼1 nm.Therefore, we prepared GO layers on flat glass substrates via spin

coating and applied thermal treatment at different conditions (at150, 250, and 350 ◦C for 10 min in air). Fig. 4(c), (d), (e), and (f)shows the AFM images of GO prepared by spin coating withoutand with thermal treatment at 150, 250, and 350 ◦C, respectively.

H.-S. Jang et al. / Electrochimica Acta 81 (2012) 301– 307 305

(d) (c)

(a) (b)

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nd the

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ductivities.As mentioned earlier, counter electrode with high catalytic

activity towards reduction reaction of redox mediator and excel-lent electrical conductivity is required for high performance DSSC.

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GO 25 0oC

GO 35 0oC

Pt

Cu

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Fig. 5. XPS spectra of GO without thermal treatment (a) a

s shown in Fig. 4(c), (d), and (e), the GO films without and withhermal treatment at 150 and 250 ◦C showed a well formed 2-imensional nanosheets. On the other hands, in the case of GOlms thermally treated at 350 ◦C, some aggregations of nanosheetsnd relatively non-uniform morphology were observed. The thick-esses of nanosheets of GO without and with thermal treatment at50 ◦C has similar value of ∼1.1 nm, which is well consistent withrevious reports on the thickness of single layer of GO, whereas GOlms thermally treated at 250 and 350 ◦C showed decreased thick-ess of nanosheets of ∼0.9 nm [39,40]. Decrease in the thickness ofingle sheet, resulting from the removal of oxygen group, indicateshat solution processed GO films are moderately reduced by simplehermal treatment at 250 and 350 ◦C. In other words, our facile ther-

al treatment of solution processed GO films can efficiently induce moderate reduction of GO resulting in the improvement of elec-rical properties. For further confirmation on the improvement oflectrical properties of GO as a result of reduction of GO, the averageonductivity values of GO films without and with thermal treat-ents were measured through the 4-point probe measurement.

he conductivity values of GO film without thermal treatment andhermally treated GO at 150 ◦C were not measured due to theirnsulating properties. On the other hand, the similar average con-uctivity values of ∼9.5 S/cm were obtained in the case of thermallyreated GO films at 250 and 350 ◦C. Consequently, the increasedfficiency might be attributed to the improvement of electricalroperties of solution processed GO by high temperature thermalreatment over 250 ◦C resulting in the removal of oxygen group inO.

The XPS analysis providing information on the degree of reduc-ion of GO resulted from the removal of oxygen group was also

erformed. Fig. 5 shows the C 1s spectrum of GO without and withhermal treatment at 150, 250, and 350 ◦C, respectively. As shownn Fig. 5(a) and (b), C 1s spectrum GO without and with thermalreatment at 150 ◦C is composed of four components assigned to

Binding energy (eV)

rmally treated GO at 150 ◦C (b), 250 ◦C (c), and 350 ◦C (d).

non-oxygenated ring C (284.9 eV), the C in C O bonds (286.4 eV),the carbonyl C (C O, 287.7 eV), and the carboxyl group (C O O H,289.0 eV). In the case of GO after thermal treatment at 250 and350 ◦C, dramatic decrease in the intensity of the peaks correspond-ing to C O, C O, and C O O H was observed as shown in Fig. 5(c)and (d). It clearly indicates that oxygen functional groups of solu-tion processed GO films can be efficiently removed during facilethermal treatment at 250 and 350 ◦C [32]. These results were inclose agreement with the AFM analysis and measurement of con-

1.40.70.0-0.7

Potential (V vs. Ag/AgCl)

Fig. 6. Cyclic voltammograms of the electrodes in acetonitrile solution of 10 mM LiI,1 mM I2 and 0.1 M LiClO4.

306 H.-S. Jang et al. / Electrochimica Acta 81 (2012) 301– 307

ated w

TwwAsostaocsomfiorcGr[Gnc

pactG∼d

td

Fig. 7. Transmittance spectra of bare FTO and FTO co

o evaluate electrocatalytic activity of GO and Pt counter electrodes,e obtained CVs of I−/I3− redox mediator on the GO without andith thermal treatment and Pt counter electrodes, respectively.lthough the exact charge transfer mechanisms are not fully under-tood, many literatures report that two pairs of redox peaks can bebserved in the CVs of various counter electrodes for the iodidepecies [22]. The pair of peaks at low potential can be attributed tohe oxidation and reduction of I−/I3− (I3− + 2e− ↔ 3I−) and the pairt high potential can be attributed to the oxidation and reductionf I2/I3− (3I2 + 2e− ↔ 2I3−). As shown in Fig. 6, in the case of our Ptounter electrode, clear two pairs of reversible peaks are observed,imilar to other report for comparable systems [22,42]. While nobvious redox peaks were observed for GO film without post ther-al treatment, demonstrating poor catalytic activity. When the GO

lms were thermally treated over 250 ◦C, two pairs of peaks werebserved, and current densities and charges were increased as aesult of improved catalytic activity. Smaller current densities andharges for GO than those for Pt counter electrode indicate that ourO based counter electrodes have lower catalytic activity to the

eduction of triiodide ion than conventional Pt counter electrode41–43]. To improve catalytic activity of our moderately reducedO counter electrode, studies on the optimization of film thick-ess or morphology, incorporation of second catalytic materials areurrently underway.

Finally, because for the special applications such as power-roducing windows and metal-foil based DSSCs transparency islso one of the most important issues, optical transparency wasonfirmed through the measurement of optical transmission spec-ra. As shown in transmittance spectra and photographs of variousO and Pt in Fig. 7, GO based counter electrodes with a thickness of4 nm are very transparent in the all range of solar spectrum and

o not significantly alter the transparency of FTO electrode.

All these results suggest that although the efficiency is still lowerhan Pt based DSSC, the moderately reduced GO, which is pro-uced by facile and fast thermal treatment of solution processed

ith Pt and GO without and with thermal treatments.

GO film, can be a promising candidate for cost-efficient and highlytransparent DSSCs.

4. Conclusion

In conclusion, we demonstrated the application of moderatelyreduced GO as a highly transparent counter electrode for DSSCs. Tosolve the problems of thermal reduction process requiring ultra-high vacuum under special atmosphere (Ar and H2) and hightemperature of ∼800–1000 ◦C, moderately reduced GO were pre-pared by the thermal treatment of solution processed GO films at∼250 and 350 ◦C for 10 min in air. Reduction of GO by simple ther-mal treatment was confirmed through the decreased thickness ofnanosheet in the AFM and decreased intensities of C O, C O, andC (O) O in the XPS spectra, resulting from the removal of oxygengroup in GO during thermal treatment. Improved catalytic activitytowards reduction reaction of redox mediator was also observedthrough the measurements of CVs of I−/I3− redox mediator on theGO thermal treated over 250 ◦C. As a result of improved electri-cal property and catalytic activity after thermal treatment over250 ◦C, the device with thermally treated GO over 250 ◦C showeddramatic improvement of efficiency, compared to GO without andwith thermal treatment at 150 ◦C. In particular, the DSSC with GOthermally treated at 350 ◦C exhibited the highest performance withVOC of 0.66 V, JSC of 16.35 mA/cm2, F.F. of 33.33%, and PCE of 3.60%.Although this value is relatively low compared to conventional Ptbased DSSC, we emphasize that solution processed GO film fol-lowed by facile and fast thermal treatment can be a candidate forcost-efficient Pt-less and transparent counter electrode.

Acknowledgements

This research was supported by Basic Science Research Programthrough theNational Research Foundation of Korea (NRF) funded

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eferences

[1] B. O’Regan, M. Grätzel, Nature 353 (1991) 737.[2] L.M. Peter, K.G.U. Wijayantha, Electrochimica Acta 45 (2000) 4543.[3] N.G. Park, K.M. Kim, M.G. Kang, K.S. Ryu, S.H. Chang, Y.J. Shin, Advanced Mate-

rials 17 (2005) 2349.[4] K. Lee, S.W. Park, M.J. Ko, K. Kim, N.-G. Park, Nature Materials 8 (2009) 665.[5] W. Lee, S.-J. Roh, K.-H. Hyung, R.S. Mane, S.-H. Lee, S.-H. Han, Solar Energy 83

(2009) 690.[6] M.Y. Song, D.K. Kim, K.J. Ihn, S.M. Jo, D.Y. Kim, Nanotechnology 15 (2004)

1861.[7] S.H. Kang, S.-H. Choi, M.-S. Kang, J.-Y. Kim, H.-S. Kim, T. Hyeon, Y.-E. Sung,

Advanced Materials 20 (2008) 54.[8] B. Liu, E.S. Aydil, Journal of the American Chemical Society 131 (2009) 3985.[9] T.N. Murakami, M. Grätzel, Inorganica Chimica Acta 361 (2008) 572.10] J.-G. Chen, H.-Y. Wei, K.-C. Ho, Solar Energy Materials and Solar Cells 91 (2007)

1472.11] Z. Li, B. Ye, X. Hu, X. Ma, X. Zhang, Y. Deng, Electrochemistry Communications

11 (2009) 1768.12] T. Kitamura, M. Maitani, M. Matsuda, Y. Wada, S. Yanggida, Chemistry Letters

30 (2001) 1054.13] K. Imoto, K. Takatashi, T. Yamaguchi, T. Komura, J. Nakamura, K. Murata, Solar

Energy Materials and Solar Cells 79 (2003) 459.14] K. Suzuki, M. Yamamoto, M. Kumagai, S. Yanagida, Chemistry Letters 32 (2003)

28.15] H. Choi, J. Shin, G.-W. Lee, N.-G. Park, K. Kim, S. Hong, Current Applied Physics

10 (2010) S165.16] W. Hong, Y.G. Lu, C. Li, G. Shi, Electrochemistry Communications 10 (2008)

1555.17] T.N. Murakami, S. Ito, Q. Wang, M.dK. Nazeeruddin, T. Bessho, I. Cesar, P. Liska, R.

Humphry-Baker, P. Comte, P. Pechy, M. Grätzel, Journal of the ElectrochemicalSociety 153 (2006) A2255.

18] L. Wan, S. Wang, X. Wang, B. Dong, Z. Xu, X. Zhang, B. Yang, S. Peng, J. Wang, C.Xu, Solid State Science 13 (2011) 468.

19] C.-T. Hsieh, B.-H. Yang, J.-Y. Lin, Carbon 49 (2011) 3092.

[

[

[

Acta 81 (2012) 301– 307 307

20] D.W. Zhang, X.D. Li, H.B. Li, S. Chen, Z. Sun, X.J. Yin, S.M. Huang, Carbon 49 (2011)5382.

21] H. Choi, H. Kim, S. Hwang, W. Choi, M. Jeon, Solar Energy Materials and SolarCells 95 (2011) 323.

22] J.D. Roy-Mayhew, D.J. Bozym, Christian Punckt, I.A. Aksay, ACS Nano 4 (2010)6203.

23] L. Kavan, J.H. Yum, M. Gratzel, ACS Nano 5 (2011) 165.24] T. Battumur, S.H. Mujawar, Q.T. Truong, S.B. Ambade, D.S. Lee, W. Lee, S.-H. Han,

S.-H. Lee, Current Applied Physics 12 (2012) e49.25] R. Cruz, D. Tanaka, A. Mendes, Solar Energy 86 (2012) 716.26] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V.

Grigorieva, A.A. Firsov, Science 306 (2004) 666.27] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P. Kim, J.Y. Choi,

B.H. Hong, Nature 457 (2009) 706.28] P.W. Sutter, J.I. Flege, E.A. Sutter, Nature Materials 7 (2008) 406.29] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y.

Wu, S.T. Nguyen, R.S. Ruoff, Carbon 45 (2007) 1558.30] Z.-l. Wang, D. Xu, Y. Huang, Z. Wu, L.-M. Wang, X. Zhang, Chemical Communi-

cations 48 (2012) 976.31] I. Jung, D.A. Dikin, R.D. Pinter, R.S. Ruoff, Nano Letters 8 (2008) 4283.32] J.M. Yun, J.S. Yeo, J. K, H.G. Jeong, D.Y. Kim, Y.J. Noh, S.S. Kim, B.C. Ku, S.I. Na,

Advanced Materials 23 (2011) 4923.33] H.L. Wang, J.T. Robinson, X.L. Li, H.J. Dai, Journal of the American Chemical

Society 131 (2009) 9910.34] C. Nethravathi, M. Rajamathi, Carbon 46 (2008) 1994.35] K.H. Liao, A. Mittal, S. Bose, C. Leighton, K.A. Mkhoyan, C.W. Macosko, ACS Nano

5 (2011) 1253.36] X.L. Li, H.L. Wang, J.T. Robinson, H. Sanchez, G. Diankov, H.J. Dai, Journal of the

American Chemical Society 131 (2009) 15939.37] M.J. McAllister, J.L. Li, D.H. Adamson, H.C. Schnlepp, A.A. Abdalam, J. Liu, I.A.

Aksay, Chemistry of Materials 19 (2007) 4396.38] S. Stankovich, R.D. Piner, X. Chen, N. Wu, S.T. Nguyen, R.S. Ruoff, Journal of

Materials Chemistry 16 (2006) 155.39] V. Pham, T. Cuong, T.-D. Nguyen-Phan, H. Pham, E. Kim, S. Hur, E. Shin, S. Kim,

J. Chung, Chemical Communications 46 (2010) 4375.40] J. Zhang, H. Yang, G. Shen, P. Cheng, J. Zhang, S. Guo, Chemical Communications

46 (2010) 1112.

41] S.-S. Kim, Y.-C. Nah, Y.-Y. Noh, J. Jo, D.-Y. Kim, Electrochimica Acta 51 (2006)

3814.42] M. Wu, X. Lin, Y. Wang, L. Wang, W. Guo, D. Qi, X. Peng, A. Hagfelt, M. Gratzel,

T. Ma, Journal of the American Chemical Society 134 (2012) 3419.43] G. Wagn, W. Xing, S. Zhuo, Electrochimica Acta 66 (2012) 151.