Journal of Molecular Liquids 191 (2014) 172–176

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Journal of Molecular Liquids

j ourna l homepage: www.e lsev ie r .com/ locate /mol l iq

Electrochemical studies on graphene oxide-supported metallic andbimetallic nanoparticles for fuel cell applications

Vinod Kumar Gupta a,⁎, Mehmet Lütfi Yola b, Necip Atar c, Zafer Üstündağ c, Ali Osman Solak d,e

a Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, Indiab Sinop University, Faculty of Arts and Science, Department of Chemistry, Sinop, Turkeyc Dumlupinar University, Faculty of Arts and Science, Department of Chemistry, Kutahya, Turkeyd Ankara University, Faculty of Science, Department of Chemistry, Ankara, Turkeye Kyrgyz-Turk Manas University, Faculty of Engineering, Department of Chemical Engineering, Bishkek, Kyrgyzstan

⁎ Corresponding author. Tel.: +91 1332285801; fax: +E-mail addresses: vinodfcy@iitr.ac.in, vinodfcy@gmail.

0167-7322/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.molliq.2013.12.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 November 2013Received in revised form 7 December 2013Accepted 9 December 2013Available online 21 December 2013

Keywords:Fuel cellGraphene oxideNanoparticleCharacterizationMethanol oxidation

A fuel cell is an electrochemical cell that converts a source fuel into an electrical current. It generates electricityinside a cell through reactions between a fuel and an oxidant, triggered in the presence of an electrolyte. Fuelcells have been attractingmore andmore attention in recent decades due to high-energy demands, fossil fuel de-pletions, and environmental pollution throughout world. In this study, different sized metallic and bimetallicnanoparticles (AuNPs, Fe@AuNPs, Ag@AuNPs) were synthesized on graphene oxide sheets and their electrocat-alytic activities for methanol oxidation were investigated. All the catalysts were characterized by transmissionelectron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). Electrochemical measurements areperformed to compare the catalytic efficiencies of methanol oxidation. Experimental results demonstrated thatgraphene oxide-supported bimetallic nanoparticles enhanced electrochemical efficiency for methanol electro-oxidation with regard to diffusion efficiency, oxidation potential and forward oxidation peak current. Grapheneoxide-supported Ag@AuNPs, in comparison to graphene oxide-supported Fe@AuNPs and AuNPs, showed themore efficiency for methanol electro-oxidation.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Fuel cells have gained attention owing to the consumption of fossilfuels and the increase in environmental pollution. Many studies havebeen reported on this topic and it has been found that the suitable fuelis methanol. A sulfuric acid or perchloric acid such as supportingelectrolyte uses the direct methanol/air fuel cell due to the fact that itremoves the CO2 produced during the electro-oxidation of methanol.In an acidic medium, the methanol oxidation reaction can be writtenas follows [1–4].anode reaction:

CH3OHþH2O→CO2 þ 6Hþ þ 6e− E�

¼ 0:043V standard hydrogen electrode SHEð Þð Þ

cathode reaction:

3=2O2 þ 6Hþ þ 6e−→3H2O E� ¼ 1:229V SHEð Þ

overall reaction:

CH3OHþ 3=2O2→CO2 þ 2H2O E� ¼ 1:186V SHEð Þ

91 1332273560.com (V.K. Gupta).

ghts reserved.

The electroanalytical and potentiometric nanosensors includingdifferent nanomaterials have been reported in the last decades [5–26].However, there are still some crucial difficulties such as including lowelectrocatalytic activity for methanol and ethanol oxidation reactions.Therefore, in order to increase the catalytic activity, the novel carbonma-terials are needed. Vulcan XC-72R carbon black is one of the most usedcarbon material for fuel cell applications due to its electrical conductivityand surface area [27,28]. During the past several years, carbon nanotubeswere investigated as catalyst and it has been demonstrated that carbonnanotubes can more effectively improve electrocatalytic activity of Ptnanoparticles for methanol and ethanol oxidations compared to VulcanXC-72R carbon black [29–34].

Recently, graphene/graphene oxide, a mono-atomic sheet of hexag-onally carbon atom, has been considered as a “rising star” carbon mate-rial because of its special properties: including mechanical strength[35–37], low density and heat conductance [38,39]. Hence, graphene/graphene oxide should be investigated as catalyst to improve electrocat-alytic activity for methanol and ethanol oxidations [40–42]. In addition,important developments have been provided in terms of a series ofcarbon-supported catalysts for fuel cell electrodes with high catalyticactivity, low cost, and high resistance to carbon monoxide (CO)poisoning [43–47].

The aimof this study is preparation and characterization of grapheneoxide-supported metallic and bimetallic nanoparticles (AuNPs, Ag@

173V.K. Gupta et al. / Journal of Molecular Liquids 191 (2014) 172–176

AuNPs and Fe@AuNPs) (Scheme 1) and investigation of their effects infuel cell applications.

2. Experimental

2.1. Materials

Graphite powder (Sigma-Aldrich), hydrogen tetra-chloroauratehydrate (HAuCl4) (Sigma-Aldrich, USA), p-aminothiophenol (PATP)(Sigma-Aldrich, USA), trisodium citrate dihydrate (Na3C6H5O72H2O)(Sigma-Aldrich, USA), 1-ethyl-3-[3-dimethylaminopropyl]carbodiimidehydrochloride (EDC) (Sigma-Aldrich, USA), HPLC grade acetonitrile(MeCN) (Sigma-Aldrich, USA), ethanol (Sigma-Aldrich, USA), methanol(Sigma-Aldrich, USA), isopropyl alcohol (IPA) (Sigma-Aldrich, USA), hy-drochloric acid (HCl) (Sigma-Aldrich, USA), hydrogen peroxide (H2O2)(Sigma-Aldrich, USA), potassium permanganate (KMnO4) (Sigma-Aldrich, USA), sulfuric acid (H2SO4) (Sigma-Aldrich, USA), iron(III) ni-trate (Fe(NO3)3) (Merck, Germany), ascorbic acid (Merck, Germany),potassium persulfate (K2S2O8) (Merck, Germany), phosphorus pentox-ide (P2O5) (Merck, Germany), perchloric acid (HClO4) (Sigma-Aldrich,USA), activated carbon (Sigma-Aldrich, USA), acetic acid (Merck,Germany), sodium acetate (Merck, Germany) and other chemicalswere reagent grade quality and were used as received.

2.2. Instrumentation

All electrochemical experiments were performed using a BAS-100Belectrochemical analyzer (Bioanalytical System Inc., Lafayette, IL, U.S.)and Gamry Reference 600 work-station. Argon gas was passed throughthe solutions during experiments for 10 min.

TEM measurements were performed on a JEOL 2100 HRTEMinstrument (JEOL Ltd., Tokyo, Japan) to examine the morphology ofnanocomposites.

An ES300 electron spectrometer (ΦULVAC-PHI, Inc., Japan/USA)withMgKα X-rays was utilized for XPS measurements, which was performedat 90° electron take-off angle. The pressure was maintained as 10−7 Pa.

2.3. Synthesis of graphene oxide

Graphene oxide was synthesized by a modified Hummers method[48–50]. In the procedure, graphite powders were pre-oxidized by

Scheme 1. Chemical structure of graphene oxide-su

H2SO4, K2S2O8 and P2O5. Then, the resulting material was oxidized byconcentrated H2O2. After it was centrifuged, it was washed with 0.1 MHCl and by ultra-pure water. The graphene oxide was dried in air.

2.4. Synthesis of AuNPs, Ag@AuNPs and Fe@AuNPs

TheAuNPswere synthesized according to a procedure reported [51].In a typical procedure, HAuCl4 was reduced by Na3C6H5O72H2O as a re-ducing agent in aqueous solution at pH 10. The reaction mixture wasboiled until a change of color was observed. The color of solutionchanged to wine red, indicating the formation of AuNPs.

The initial silver colloids were synthesized according to the follow-ing procedure [52,53]. AgNO3 was reduced by β-CD in aqueous media.The reaction mixture was heated until the color of solution turnedpale yellow. The pale yellow color states the formation of silver nano-particles (AgNPs). After that, HAuCl4 solution (1 mM) was added tothe colloidal silver solution for the preparation of Ag@AuNPs. Then,the solution was incubated for 15 min at room temperature. Afterincubation, the solution was heated on a water bath. The color ofsolution was changed from pale yellow to pink. This situation showsthe formation of Ag@AuNPs.

For preparation of Fe@AuNPs, Fe(NO3)3 (4 mL, 0.01 M)was reducedby 20 mLof 0.1 Mascorbic acid for 20 min at room temperature under anitrogen atmosphere. The pHof the solutionwas adjusted to 4.0 and theHAuCl4 (4 mL, 0.01 M)was added at room temperature for 1 hwithoutusing any dispersing agents. A dark solid indicating that the iron corewas coated with a gold shell was then separated using a magnet andwashed with water. The Fe@AuNPs was dried overnight in a vacuumoven at 25 °C [54].

2.5. Preparation of graphene oxide-supported AuNPs, Ag@AuNPs andFe@AuNPs

The synthesized graphene oxide was dissolved in ethanol at aconcentration of 2 mg mL−1 with the aid of ultrasonic agitation for1 h, resulting in a homogeneous black suspension. To ensure the surfaceactivation of carboxylate groups of graphene oxide, the graphene oxidesuspension was interacted with 0.2 M EDC solution for 8 h. The activat-ed graphene oxide suspension was well mixed with 1.0 mM PATP at a1:1 volume ratio for 2 h. In a typical experiment of self-assembly, theaqueous dispersion of AuNPs, Ag@AuNPs and Fe@AuNPs (1 mg mL−1)

pported metallic and bimetallic nanoparticles.

174 V.K. Gupta et al. / Journal of Molecular Liquids 191 (2014) 172–176

was mixed with the aqueous dispersion of PATP functionalizedgraphene oxide sheets (0.1 mg mL−1) at a 1:1 volume ratio andsonicated for 15 min to form a homogeneous mixture. The mixturewas then kept undisturbed under ambient condition for 12 h.

2.6. Procedure for the electrode preparation

Glassy carbon electrodes (GCEs)were cleaned by polishingwith finewet emery paper. Theywere polishedwith 0.1 μmand 0.05 μmaluminaslurries, respectively onmicrocloth pads. The electrodes were sonicatedtwice in ultra pure water and then in 50:50 (v/v) IPA and MeCN solu-tion. After washing with water, glassy carbon electrode was washedwith MeCN to eliminate any physisorbed materials. Finally, 20 μL ofgraphene oxide-supported nanoparticle suspensions was droppedonto the GCE and then evaporating the solvent under an infrared heatlamp.

2.7. Electrochemical measurements

Electrocatalytic oxidation of 0.5 mol L−1 methanol on grapheneoxide-supported Ag@AuNPs, Fe@AuNPs and AuNPs was investigatedin 0.1 mol L−1 HClO4 by cyclic voltammetry (CV) between −0.5 and+1.5 V. The potentials were measured with respect to the Ag/AgClelectrode as a reference electrode. The counter electrode was a Pt wire.

3. Results and discussion

3.1. Characterizations of graphene oxide-supported Ag@AuNPs, Fe@AuNPsand AuNPs

Themorphologies of thenanocompositeswere investigated byusingthe JEOL 2100HRTEMwith an accelerating voltage of 200 keV. A drop ofsample solution was deposited on a polymeric grid dried at room

Fig. 1. TEM image of (a) graphene oxide, (b) graphene oxide-supported AuNPs, (c) gra

temperature under an argon gas stream. The transparent and wrinkledgraphene oxide sheets (Fig. 1a) have exhibited a few layer planar sheet-like morphology. The TEM image of the AuNPs shows (Fig. 1b) that thesizes of the AuNPs are very similar with a mean diameter of 8 to 10 nmon a lighter shaded substrate corresponding to graphene oxide sheet. Asshown in Fig. 1c and d, Ag@AuNPs and Fe@AuNPs have been seen asdark dots with a mean diameter of 5 to 10 nm.

In the XPS spectrum (Fig. 2) of the nanocomposites, C, N, S, Au, Feand Ag peaks are prominent, showing that AuNPs, Ag@AuNPs and Fe@AuNPs have been functionalized on the PATP functionalized grapheneoxide sheets. The C1s core-level spectra of the nanocomposites werecurve-fitted in Fig. 2. The peaks at 281.6 eV, 283.1 eV and 285.4 eV areassigned to CH and CN and CONH, respectively [55–57]. The peak locat-ed at 398.3 eV in the N1s narrow region XPS spectrum is correspondedto C–N groups in the covalent attachment of the carboxyl group of thegraphene oxide with the amino group of the PATP [55]. S2p region wascurve-fitted with two components by a doublet (2p1/2 and 2p3/2),owing to the spin-orbit coupling. The peak at 163.3 eV indicates thatthe sulfur atom in the nanocomposite was grafted with the AuNPs.The peak at 162.2 eV can be assigned to free mercapto group inunreacted PATP. The peak signals at 82.7 eV and 87.8 eV arecorresponded to Au 4f7/2 and 4f5/2, respectively, indicating thefunctionalization of Au with sulfur [58]. Fe 2p1/2 and Fe 2p3/2 peaks ap-pear at 724.9 and 712.7 eV, respectively, indicating the presence of ironnanoparticles [59]. Ag3d region is characterized by doublet 3d5/2

and 3d3/2 signals that appear at 365.8 and 372.4 eV, respectively, corre-sponding the presence of Ag° [58].

3.2. Electrocatalytic activity for methanol oxidation reaction

The electrocatalytic activities of graphene oxide-supported Ag@AuNP, Fe@AuNP and AuNP modified GCE were investigated by CV in0.1 mol L−1 HClO4 at 100 mV s−1. The voltammograms are shown in

phene oxide-supported Ag@AuNPs and (d) graphene oxide-supported Fe@AuNPs.

Fig. 2. The narrow region XPS spectra of the graphene oxide-supported Ag@AuNPs, Fe@AuNPs and AuNPs for the deconvolution spectra of the C1s, N1s, S2p, Au4f, Fe2p, and Ag3d.

175V.K. Gupta et al. / Journal of Molecular Liquids 191 (2014) 172–176

Fig. 3. The peak current of 0.5 mol L−1 methanol on graphene oxide-supported Ag@AuNPs increases slowly at lower potentials and thenquickly increases at potentials higher than 500 mV, with oxidation oc-curring at approximately 580 mV on the forward potential sweep. Thecurrent density is directly proportional to the amount of methanol oxi-dized at the electrode. The current density observed at graphene oxide-supported modified GCE, in comparison to GCE, is approximately0.61 ± 0.7 mA cm−2. According to this result, graphene oxide playsan important role to catalyze methanol. The observed current densityon graphene oxide-supported Ag@AuNPs is much higher than theother graphene oxide-supported nanoparticles, which confirms thatgraphene oxide-supported Ag@AuNPs generate a more completeoxidation of methanol to carbon dioxide. The efficiencies of the Ag@AuNPs, Fe@AuNPs and AuNPs on methanol oxidation were comparedwith regard to oxidation potential and current density, these data aresummarized in Table 1. As shown in Table 1, the oxidation peak poten-tial of methanol for graphene oxide-supported Ag@AuNPswas 580 mV,compared with an oxidation potential of 660 mV for graphene oxide-

Fig. 3. The electrocatalytic activities of graphene oxide-supported (a) Ag@AuNP, (b) Fe@AuNP, (c) AuNP modified, (d) graphene oxide, and (e) bare GCE.

supported Fe@AuNPs and an oxidation potential of 620 mV forgraphene oxide-supported AuNPs. This observation shows that thegraphene oxide-supported Ag@Au catalyst can significantly decreasethe barrier to methanol oxidation and that graphene oxide-supportedAg@AuNPs perform better than the other graphene oxide-supportedcatalysts (Fig. 3). As given in Fig. 4, for all graphene oxide-supported cat-alysts, the forward oxidation current (I) is proportional to the squareroot of the scan rate, suggesting that the oxidation behavior ofmethanolat all electrodes is controlled by diffusion processes. The slope forgraphene oxide-supported Ag@Au catalyst is larger than those for theother graphene oxide-supported catalysts, indicative of a faster diffu-sion process of methanol on the surfaces of graphene oxide-supportedAg@AuNPs.

In the literature, several catalysts were prepared for the fuel cellapplication. For example, pure Pt and nitrogen containing carbonnanotubes as platinum catalyst (Pt/N-CNT) showed an activity of0.167 mA cm−2 and an activity of 13.3 mA cm−2, respectively [60].Noblemetal (Pt, Pd) electrocatalysts supported on carbonmicrospheres(CMSs) were used for methanol oxidation in alkaline media [61]. Pt/CMS and Pd/CMS electrodes showed an activity of 11.4 mA cm−2

and 2.5 mA cm−2, respectively. Recently, the PtPd alloy porous filmscomprised of nanodendrites showed enhanced electrocatalyticactivities toward the electrooxidation of methanol in acidic solution[62–70]. The prepared Pt3Pd1, Pt1Pd1, and Pt1Pd3 alloy porousfilms showed an activity of 1.36 mA cm−2, 0.94 mA cm−2 and0.82 mA cm−2, respectively. In addition, the conventional 20% Pt/

Table 1Comparison of electrocatalytic activity of methanol oxidation on graphene oxide-supported Ag@AuNPs, Fe@AuNPs and AuNPs (scan rate: 100 mV s−1).

Electrode Peak current density (mA cm−2) E (mV)

Graphene oxide-Ag@Au 14.79 ± 0.7 580Graphene oxide-Fe@Au 8.75 ± 0.2 606Graphene oxide-Au 1.21 ± 0.5 620Graphene oxide 0.61 ± 0.7 655GCE – –

Fig. 4. The relationship of I vs. the square root of scan rate on graphene oxide-supportedcore–shell nanoparticle modified GCE.

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Vulcan (E-TEK) electrode shows an activity of 1.3 mA cm−2. So we cansay that the anodic current densities of graphene oxide-Ag@Au andgraphene oxide-Fe@Au catalysts are found to be higher than those ofthe mentioned electrodes and Pt/Vulcan (E-TEK) electrode, which indi-cate that the catalysts (graphene oxide-Ag@AuNPs and graphene oxide-Fe@AuNPs) preparedwith grapheneoxide as the support have excellentcatalytic activity on methanol electrooxidation.

4. Conclusions

Ag@AuNPs, Fe@AuNPs and AuNPs were successfully synthesized ongraphene oxide sheets. The results show that the Ag@AuNPs, Fe@AuNPsand AuNPs were highly dispersed on the graphene oxide nanosheetsand graphene oxide canmore effectively enhance electrocatalytic activ-ity of Ag@AuNPs, Fe@AuNPs and AuNPs for the oxidation of methanolinto CO2. Notably, the peak potential of methanol oxidation at grapheneoxide-supported Ag@AuNP modified GCE is lower than Fe@AuNPs andAuNPs. Thus, it can be said that graphene oxide-supported Ag@AuNPsshow better electrocatalytic performance.

References

[1] K. Kardesch, G. Simader, Fuel Cells and Their Applications, VCH, Weinheim,Germany, 1996.

[2] B.D. Mcnicol, J. Electroanal. Chem. 118 (1981) 71.[3] V.S. Bagotsky, A.M. Skundin, Chemical Power Sources, Academic Press, London,

1980.[4] A.S. Arico, S. Srinivasan, V. Antonucci, Fuel Cells 1 (2001) 133.[5] V.K. Gupta, A.K. Jain, G. Maheshwari, H. Lang, Z. Ishtaiwi, Sens. Actuators B Chem.

117 (2006) 99.[6] R.N. Goyal, V.K. Gupta, N. Bachheti, R.A. Sharma, Electroanalysis 20 (2008) 757.[7] R.N. Goyal, V.K. Gupta, N. Bachheti, Anal. Chim. Acta. 597 (2007) 82.[8] V.K. Gupta, S. Chandra, H. Lang, Talanta 66 (2005) 575.[9] V.K. Gupta, R. Prasad, A. Kumar, Talanta 60 (2003) 149.

[10] R.N. Goyal, V.K. Gupta, S. Chatterjee, Biosens. Bioelectron. 24 (2009) 3562.[11] V.K. Gupta, S. Chandra, R. Mangla, Electrochim. Acta 47 (2002) 1579.[12] V.K. Gupta, A.K. Jain, P. Kumar, S. Agarwal, G. Maheshwari, Sens. Actuators B Chem.

113 (2006) 182.[13] V.K. Gupta, A.K. Jain, P. Kumar, Sens. Actuators B Chem. 120 (2006) 259.[14] V.K. Gupta, A.K. Singh, M. Al Khayat, B. Gupta, Anal. Chim. Acta. 590 (2007) 81.[15] V.K. Gupta, R. Jain, K. Radhapyari, N. Jadon, S. Agarwal, Anal. Biochem. 408 (2011)

179.[16] V.K. Gupta, A.K. Singh, S. Mehtab, B. Gupta, Anal. Chim. Acta. 566 (2006) 5.[17] R.N. Goyal, V.K. Gupta, S. Chatterjee, Electrochim. Acta 53 (2008) 5354.[18] V.K. Gupta, M.R. Ganjali, P. Norouzi, H. Khani, A. Nayak, S. Agarwal, Crit. Rev. Anal.

Chem. 41 (2011) 282.

[19] V.K. Gupta, A. Nayak, S. Agarwal, B. Singhal, Comb. Chem. High ThroughputScreening 14 (2011) 284.

[20] V.K. Gupta, L.P. Singh, R. Singh, N. Upadhyay, S.P. Kaur, B. Sethi, J. Mol. Liq. 174(2012) 11.

[21] V.K. Gupta, B. Sethi, R.A. Sharma, S. Agarwal, A. Bharti, J. Mol. Liq. 177 (2013) 114.[22] B.J. Sanghavi, A.K. Srivastava, Electrochim. Acta 55 (2010) 8638.[23] B.J. Sanghavi, A.K. Srivastava, Anal. Chim. Acta. 706 (2011) 246.[24] B.J. Sanghavi, S.M. Mobin, P. Mathur, G.K. Lahiri, A.K. Srivastava, Biosens. Bioelectron.

39 (2013) 124.[25] B.J. Sanghavi, G. Hirsch, S.P. Karna, A.K. Srivastava, Anal. Chim. Acta. 735 (2012) 37.[26] R.N. Goyal, V.K. Gupta, S. Chatterjee, Biosens. Bioelectron. 24 (2009) 1649.[27] Y.H. Lin, X.L. Cui, C.H. Yen, C.M. Wai, Langmuir 21 (2005) 11474.[28] Y.L. Guo, Y.Z. Zheng, M.H. Huang, Electrochim. Acta 53 (2008) 3102.[29] T. Maiyalagan, Appl. Catal., B 80 (2008) 286.[30] B. Rajesh, K.R. Thampi, J.M. Bonard, N. Xanthopoulos, H.J. Mathieu, B. Viswanathan,

J. Phys. Chem. B 107 (2003) 2701.[31] H. Tang, J.H. Chen, Z.P. Huang, D.Z. Wang, Z.F. Ren, L.H. Nie, Y.F. Kuang, S.Z. Yao,

Carbon 42 (2004) 191.[32] G. Girishkumar, T.D. Hall, K. Vinodgopal, P.V. Kamat, J. Phys. Chem. B 110 (2006)

107.[33] A. Halder, S. Sharma, M.S. Hegde, N. Ravishankar, J. Phys. Chem. C 113 (2009) 1466.[34] R.V. Hull, L. Li, Y.C. Xing, C.C. Chusuei, Chem. Mater. 18 (2006) 1780.[35] 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.[36] C. Lee, X.D. Wei, J.W. Kysar, J. Hone, Science 321 (2008) 385.[37] M. Freitag, M. Steiner, Y. Martin, V. Perebeinos, Z.H. Chen, J.C. Tsang, P. Avouris, Nano

Lett. 9 (2009) 1883.[38] M.A. Panzer, K.E. Goodson, J. Appl. Phys. 103 (2008) 094301.[39] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zimney, E.A. Stach, R.D.

Piner, S.T. Nguyen, R.S. Ruoff, Nature 442 (2006) 282.[40] B. Seger, P.V. Kamat, J. Phys. Chem. C 113 (2009) 7990.[41] E. Yoo, T. Okata, T. Akita, M. Kohyama, J. Nakamura, I. Honma, Nano Lett. 9 (2009)

2255.[42] M.S. Wietecha, J. Zhu, G.H. Gao, N.Wang, H. Feng, M.L. Gorring, M.L. Kasner, S.F. Hou,

J. Power Sources 198 (2012) 30.[43] X.Z. Cui, L.M. Guo, F.M. Cui, Q.J. He, J.L. Shi, J. Phys. Chem. C 113 (2009) 4134.[44] S. Kim, H.J. Sohn, S.J. Park, Curr. Appl. Phys. 10 (2010) 1142.[45] T. Takeguchi, Y. Anzai, R. Kikuchi, K. Eguchi,W. Ueda, J. Electrochem. Soc. 154 (2007)

B1132.[46] G.Q. Lu, W. Chrzanowski, A. Wieckowski, J. Phys. Chem. B 104 (2000) 5566.[47] Z. Jusys, R.J. Behm, J. Phys. Chem. B 105 (2001) 10874.[48] W.S. Hummers, R.E. Offeman, J. Am. Chem. Soc. 80 (1958) 1339.[49] S.F. Hou, S.J. Su, M.L. Kasner, P. Shah, K. Patel, C.J. Madarang, Chem. Phys. Lett. 501

(2010) 68.[50] A.P. Tuan, B.C. Choi, K.T. Lim, Y.T. Jeong, Appl. Surf. Sci. 257 (2011) 3350.[51] D.T. Nguyen, D.J. Kim, M.G. So, K.S. Kim, Adv. Powder Technol. 21 (2010) 111.[52] M.H. Ullah, K. II, C.S. Ha, Mater. Lett. 60 (2006) 1496.[53] A. Kumar, V. Chaudhary, Nanotechnology 20 (2009) 095703.[54] M.N. Nadagouda, R.S. Varma, Cryst. Growth Des. 7 (2007) 2582.[55] F. Montagne, J. Polesel-Maris, R. Pugin, H. Heinzelmann, Langmuir 25 (2009)

983.[56] R. Schlapak, P. Pammer, D. Armitage, R. Zhu, P. Hinterdorfer, M. Vaupel, T. Fruhwirth,

S. Howorka, Langmuir 22 (2006) 277.[57] E. Bekyarova, M.E. Itkis, P. Ramesh, C. Berger, M. Sprinkle, W.A. de Heer, R.C. Haddon,

J. Am. Chem. Soc. 131 (2009) 1336.[58] R. Guzel, Z. Ustundag, H. Eksi, S. Keskin, B. Taner, Z.G. Durgun, A.A.I. Turan, A.O.

Solak, J. Colloid Interface Sci. 351 (2010) 35.[59] A.P. Grosvenor, B.A. Kobe, M.C. Biesinger, N.S. McIntyre, Surf. Interface Anal. 36

(2004) 1564.[60] T. Maiyalagan, B. Viswanathan, U.V. Varadaraju, Electrochem. Commun. 7 (2005)

905.[61] C. Xu, L. Cheng, P. Shen, Y. Liu, Electrochem. Commun. 5 (2007) 997.[62] J. Liu, L. Cao, W. Huang, Z. Li, J. Electroanal. Chem. 686 (2012) 38.[63] A.K. Jain, V.K. Gupta, A. Bhatnagar, Suhas 38 (2003) 463–481.[64] A. Mittal, V.K. Gupta, A. Malviya, J. Mittal, J. Hazard. Mater. 151 (2008) 821–832.[65] V.K. Gupta, R. Jain, A. Mittal, M. Mathur, S. Sikarwar, J. Colloid Interface, Sci. 309

(2007) 464–469.[66] A. Mittal, J. Mittal, A. Malviya, D. Kaur, V.K. Gupta, J. Colloid Interface, Sci. 343 (2010)

463–473.[67] V.K. Gupta, A. Rastogi, A. Nayak, J. Colloid Interface, Sci. 342 (2010) 533–539.[68] V.K. Gupta, S. Agarwal, T.A. Saleh, Water Res. 45 (2011) 2207–2212.[69] V.K. Gupta, A. Mittal, A. Malviya, J. Mittal, J. Colloid Interface, Sci. 335 (2009) 24–33.[70] V.K. Gupta, N. Atar, M.L. Yola, M. Eryılmaz, H. Torul, U. Tamer, I.H. Boyacı, Z.

Üstundağ, J. Colloid Interface, Sci. 406 (2013) 231–237.