Highly dispersed Pd nanoparticles on chemically modified graphene with aminophenyl groups for formic acid oxidation

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1 Highly dispersed Pd nnoprticles on chemiclly modified grphene with minophenyl groups for formic cid oxidtion Yng Su-Dong( ) )), Shen Cheng-Min( ) ), Tong Ho( ) ), He Wei( ) ), Zhng Xio-Gng( ), nd Go Hong-Jun( ) ) ) College of Mteril Science nd Engineering, Nnjing University of Aeronutics nd Astronutics, Nnjing , Chin ) Beijing Ntionl Lortory for Condensed Mtter Physics, Institute of Physics, Chinese Acdemy of Sciences, Beijing , Chin (Received 20 July 2011; revised mnuscript received 18 August 2011) A novel electrode mteril sed on chemiclly modified grphene (CMG) with minophenyl groups is covlently functionlized y nucleophilic ring-opening rection etween the epoxy groups of grphene oxide nd the minophenyl groups of p-phenylenedimine. Plldium nnoprticles with n verge dimeter of 4.2 nm re deposited on the CMG y liquid-phse orohydride reduction. The electroctlytic ctivity nd stility of the Pd/CMG composite towrds formic cid oxidtion re found to e higher thn those of reduced grphene oxide nd commercil cron mterils such s Vulcn XC-72 supported Pd electroctlysts. Keywords: chemiclly modified grphene, fuel cell, electroctlysis, Pd nnoprticles PACS: E, Lp, Dn, Jn DOI: / /20/11/ Introduction Grphene, new clss of two-dimensionl cron nnostructure, hs recently een of specil interest to reserchers ctively engged in nno-science ecuse it possesses mny fscinting properties such s gint electron moility, extremely high therml conductivity nd extrordinry elsticity nd stiffness. [1 7] Therefore, it hs potentil pplictions in mny fields such s nnoelectronics, sensors, nnocomposites, tteries, supercpcitors nd hydrogen storge. [8] In prticulr, grphene is good heterogeneous ctlystsupport mteril for direct liquid fuel cells, due to its high specific surfce re, excellent electronic conductivity, nd high chemicl stility. [3,8,9] Much effort hs een devoted to the synthesis of metl nnoprticles (NPs)-grphene hyrid composites. [10 12] However, grphene lcks sufficient inding sites for nchoring precursor metl ions or metl NPs, so it is importnt to find wys to deposit stle, well dispersed metl NPs on its surfce. In order to disperse NPs homogeneously on the surfce, it must e modified vi suitle functionliztion. One promising route to chieve mss production of chemiclly modified grphene (CMG) is to generte it from grphene oxide (GO). Reserchers hve suggested tht GO contins plentiful, chemiclly rective epoxy groups on its sl plnes. [13,14] These epoxy groups cn e esily modified through ring-opening rections under vrious conditions. Wng et l. [15] demonstrted n epoxide ringopening rection y mens of dding octdecylmine to dispersion of GO, producing colloidl suspensions of CMG pltelets in orgnic solvents. In relevnt study, n ionic liquid with n mine end group ws ttched to GO pltelets vi the ring-opening rection with epoxy groups. [16] This strtegy could increse the numer of surfce nucletion sites for NPs. In this study, we report on the friction of CMG with minophenyl groups s ctlyst-support y mens of nucleophilic ring-opening rection etween the epoxy groups of GO nd the minophenyl groups of p-phenylenedimine (PPD). After this rection, Project supported y the Ntionl Bsic Reserch Progrm of Chin (Grnt No. 2007CB209700), the Grdute Student Innovtion Foundtion of Jingsu Province, Chin (Grnt No. CX09B 075Z), nd the Reserch Funding of Nnjing University of Aeronutics nd Astronutics, Chin (Grnt No. NS ). Corresponding uthor. E-mil: zhngxg@163.com Corresponding uthor. E-mil: hjgo@phy.iphy.c.cn c 2011 Chinese Physicl Society nd IOP Pulishing Ltd

2 plldium NPs re directly reduced onto the CMG supports. Our results show tht Pd/CMG exhiits excellent ctlytic ctivity nd stility for formic cid oxidtion in comprison with reduced grphene oxide (RGO) nd Vulcn XC-72 supported Pd electroctlysts. 2. Experiment 2.1. Mterils The grphite powder (SP grde), plldium chloride (PdCl 2 ), hydrzine hydrte, sodium orohydride (NBH 4 ) nd PPD were purchsed from Sinophrm Chemicl Regent Co. Ltd., Chin. Potssium hydroxide (KOH) ws otined from Nnjing Chemicls, Chin. Unless otherwise stted, other regents were of nlyticl grde nd were used s received Synthesis of CMG nd Pd/CMG composites The GO ws prepred ccording to modified Hummer s method. [17] CMG with minophenyl groups ws synthesized etween GO nd PPD. 100 mg of PPD ws dded into 100 ml of homogeneous, trnsprent GO (0.5 mg ml 1 ) dispersion in wter. Then, 100 mg of KOH ws dded into the mixture nd the mixture ws sujected to ultrsoniction for 30 min. Finlly, the homogeneous solution ws vigorously stirred t 80 C for 24 h. After centrifuging nd rinsing with wter nd ethnol severl times, the resulting product ws redispersed in 100 mg wter. Then, 1 ml hydrzine hydrte ws dded into the dispersion nd the rection mixture ws kept t 100 C for 24 h under constnt stirring. [18] The finl products were centrifuged, wshed nd finlly ir-dried. For comprison, RGO ws prepred y reduction of GO with hydrzine hydrte s descried ove. Pd/CMG composite ws synthesized y using NBH 4 s reductive gent t room temperture. 20 mg CMG ws dissolved in wter y ultrsonic tretment for 2 h nd 0.72 ml of 7 mg ml 1 PdCl 2 solution ws dded under stirring. The ph vlue of this mixture ws djusted to 6.5 using 1 M NOH. Then, 100 mg of NBH 4 ws slowly dded into the mixture with stirring for 12 h t room temperture. The resulting slurry ws centrifuged, wshed with deionized wter nd then dried in vcuum oven. For comprtive purposes, the sme procedure ws followed for the syntheses of Pd/RGO nd Pd/Vulcn XC-72(C) Preprtion of electrode Glssy cron (GC) electrodes, 5 mm in dimeter (electrode re 0.2 cm 2 ), polished with 0.05 µm lumin to mirror-finish efore ech experiment, were used s sustrtes for supported ctlysts. For the electrode preprtion, typiclly, 3 mg of ctlyst ws dded into 0.5 ml 0.05 wt.% Nfion solution nd then the mixture ws ultrsonicted for 1 h for uniform dispersion. A mesured volume (30 µl) of this mixture ws dropped y microsyringe onto the top surfce of the GC electrode. The s-otined ctlyst-modified GC electrode ws employed s the working electrode in our experiments Chrcteriztion Infrred spectr were recorded with model 360 Nicolet AVATAR Fourier trnsform infrred (FT IR) spectrophotometer. X-ry photoelectron spectroscopy (XPS) ws recorded on n ASTM E system. The nlysis of the composition of the ctlyst ws otined with Thermo IRIS Intrepid II inductively coupled plsm tom emission spectrometry (ICP-AES) system. X-ry diffrction (XRD) nlysis ws performed on Bruker D8-ADVANCE diffrctometer with Cu K rdition t wvelength of λ = nm. Trnsmission electron microscopy (TEM, JEOL JEM- 2100) ws used to investigte the morphology of smples Electrochemicl mesurement All electrochemicl mesurements were crried out with CHI 660c electrochemicl worksttion, using three-electrode test cell. A conventionl threeelectrode system ws used with modified GC electrode s the working electrode (5 mm in dimeter), Pt wire s the counter electrode nd sturted clomel electrode (SCE) s the reference electrode. All electrolytes were deerted y uling N 2 for 20 min nd protected with nitrogen tmosphere during the entire experimentl procedure. All experiments were crried out t temperture of (25 ± 1) C. 3. Results nd discussion We used FT IR nlysis of CMG to confirm covlent interctions for the nucleophilic ring-opening rection etween GO nd PPD. Before recording the FT IR spectr, the CMG ws wshed with wter nd

3 ethnol repetedly in order to get rid of ny impurities. Figure 1 shows the FT IR spectr of CMG nd PPD. By comprison with the infrred spectr, we see tht the CMG exhiits peks corresponding to the C=C stretching of quinonoid (1575 cm 1 ) nd enzenoid rings (1508 cm 1 ), C N stretching of the romtic mine (1220 cm 1 ), romtic C H in-plne ending (1160 cm 1 ) nd out-of-plne deformtion of C H in 1,4-disustituted enzene ring (829 cm 1 ). [19] Moreover, the peks t 3200 nd 3310 cm 1 cn e ssigned to the NH 2 of minophenyl groups. It is worth noting tht some nds of PPD re shifted towrd lower wvenumers in the CMG composition. The spectrl red-shift phenomen of chemiclly synthesized nnocomposite result from the interction etween the CMG nd PPD in ccordnce with recent report. [20] The spectrl red-shift phenomen of chemiclly synthesized nnocomposite result from the minophenyl groups grfted on the CMG nnosheets sl plne. Such shift could e n evidence for the CMG with minophenyl group formtion etween PPD nd GO cused y nucleophilic ring-opening rection etween the epoxy groups of GO nd the minophenyl groups of PPD. Thus, it is cler tht the minophenyl groups were well ttched to the CMG surfce. Trnsmitnce Wvenumer/cm -1 Fig. 1. FT IR spectr of CMG (curve ) nd PPD (curve ), indicting tht minophenyl groups hve een successfully modified on the surfce of CMG. To further illustrte the covlent functionliztion, the XPS of CMG smples is given in Fig. 2. As shown in Fig. 2(), the XPS survey scn spectrum exhiits distinct C 1s, N 1s nd O 1s peks. As exhiited in the inset of Fig. 2(), the N 1s spectrum shows single component pek t ev, which indictes tht the experimentl procedure ws effective for grfting minophenyl groups onto the CMG. [21,22] Consequently, the minophenyl groups re grfted to the surfce of the CMG. A shoulder of the min NH 2 pek is oserved t ev nd is ssocited with the presence of the mmonium species. The C1s XPS spectrum of CMG (Fig. 2()) exhiits n dditionl component t ev, nd the new component cn e ssigned to the comintion of the C ound to nitrogen, which ppered fter the ring-opening rection nd the C N groups forming from the minophenyl groups of PPD. These results, comined with the FT IR spectr, prove tht the covlent rection etween the minophenyl groups of PPD nd the epoxy groups on the GO sheets occurred successfully. Intensity Intensity N-C N-H C 1s N 1s O 1s () Binding energy/ev C in grphite C-N () C-OH C(epoxy/ether) C=O Binding energy/ev Fig. 2. XPS spectr of CMG: () XPS full spectrum of CMG, () higher resolution dt of the C1s of CMG, indicting tht CMG surfce hve een modified with minophenyl groups. In order to compre the influence of different supporting mterils on morphology of Pd NPs, we use TEM technique to nlyse the shpe nd size distriutions of the Pd NPs loded on the three supporting mterils. The three supporting mterils re modified grphene sheets, reduced grphene oxide sheets nd Vulcn XC-72 cron powder, seprtely. The preprtion of Pd NPs loded on supporting mterils is under the sme condition. The TEM imges nd ptterns of Pd NPs size distriution re shown in Fig. 3. From Fig. 3(), it is seen clerly tht Pd NPs

4 Chin. Phys. B Vol. 20, No. 11 (2011) re uniformly distriuted on the chemiclly modified grphene sheets. This oservtion contrsts with the results of control experiments in which the Pd NPs re deposited on RGO (Fig. 3(c)) surfces where prticles of vrious sizes exist mostly s ggregtes nd re distriuted rndomly. In Fig. 3(e), Pd NPs deposited on Vulcn XC-72 (Pd/C) lso show lrge size distriution of Pd prticles tht ggregted significntly in mny regions. Results show tht the men sizes of Pd NPs re (4.2 ± 0.8) nm, (4.6 ± 1.1) nm nd (5.5±1.3) nm for Pd/CMG, Pd/u-CMG nd Pd/C, respectively. The smll dimeter nd reltively nrrow dimeter distriution of Pd NPs on CMG is due to the immoiliztion of ifunctionl minophenyl groups, which offers uniformly distriuted ctive sites for nchoring metl ions. In ddition, the minophenyl groups on CMG cn effectively vert the ggregtion of CMG nd thus led to wter-dispersile grphene sheets. This composite provides highly effective surfce res of CMG for susequent deposition of Pd NPs. Highly dispersed metl NPs on supports with lrger surfce res hve dvntges in ctlytic ctivity nd sensor sensitivity. This unique structure ppers to provide suitle support for mny uniform Pd NPs, therey fvouring high performnce in formic cid oxidtion, s discussed elow. Fig. 3. TEM imges nd prticle size distriution histogrms of Pd/CMG (() nd ()), Pd/RGO ((c) nd (d)), nd Pd/C ((e) nd (f)), indicting Pd NPs hve een deposited on the surfce of CMG

5 Here, we propose possile formtion mechnism for Pd ctlyst NPs on CMG. When the ph of the solution is controlled t slight cidity, the minophenyl groups on the CMG surfce ecome wekly positive chrge. [23] It leds to the electrosttic interction etween the negtive PdCl 2 4 ions nd positively chrged functionl groups of minophenyl on the CMG, forming uniformly distriuted Pd ion precursors on the surfce of CMG. Finlly, sodium orohydride is dded into this mixed solution to reduce the Pd precursors. The electrosttic interction etween Pd ions nd GMG resulted in uniform nd dispersive formtion of Pd NPs on the CMG surfce. On the other hnd, this strtegy could increse the numer of surfce nucletion sites for NPs. The XRD ptterns of the different composites re shown in Fig. 4. The GO pttern (pttern in Fig. 4) hs strong diffrction pek t 2θ vlue of 9.7 with corresponding d-spcing of 0.91 nm. The incresed interlyer spcing for GO cn weken the vn der Wls interction etween lyers nd mke exfolition vi soniction esier. [24] However, it is worth noting tht no ovious peks ttriutle to GO could e found for the CMG smples (pttern in Fig. 4), confirming the reduction of the GO sheets nd indicting tht CMG ws otined successfully in this work. In conclusion, well-exfolited GO nd CMG were successfully otined in this work. The XRD ptterns of vrious Pd-cron nnocomposites re shown in ptterns c e of Fig. 4. Chrcteristic diffrction peks of Pd(111) t 39.6 nd Pd(200) t 45.5 for fcecentered cuic (fcc) structure were lso oserved. Pttern e in Fig. 4 shows tht the Pd (111) nd t 39.6 is very rod for the CMG nnomteril, which is in Pd(111) contrst to the shrp nd nrrow nds for Pd NPs on RGO nnomterils (pttern d in Fig. 4). The figure indictes the smller size of Pd NPs on the CMG sheets. The ppliction of Scherrer s [25] eqution to the rodening of the Pd(111) pek shows tht the verge sizes of the Pd NPs for Pd/CMG, Pd/RGO, nd Pd/C re 4.2, 4.6, nd 5.5 nm, respectively. The prcticl compositions of Pd in different smples were evluted y ICP-AES nlysis. The Pd/CMG, Pd/RGO nd Pd/C ctlysts hd the metl lodings of 30.1, 28.2, nd 27.6 wt%, respectively. The oxidtion currents of cyclic voltmmogrms nd chronomperometry tests were normlized to the mesured metl loding for further compring the ctivities of different ctlysts. The electrochemiclly ctive surfce re (ECSA) provides importnt informtion regrding the numer of ville ctive sites. [26] Figure 5 shows cyclic voltmmogrms of different electroctlysts tht re in contct with 0.25 M H 2 SO 4. All potentils were referred to the sturted clomel electrode (SCE) in this work. Typicl hydrogen nd oxygen dsorption/desorption ehviours on Pd cn e clerly oserved on the smples. The ECSA ws estimted y integrting the voltmmogrm corresponding to hydrogen desorption (Q H ) y dpting the ssumption of 212 µc cm 2 from the electrode surfce. The ECSA for Pd/C, Pd/RGO nd Pd/CMG were estimted to e 107.5, 123.2, nd m 2 g 1, respectively. This indictes tht the Pd/CMG ctlyst hs lrger ECSA, which cn e ttriuted to the uniform distriution nd smll prticle size of Pd NPs on CMG. This result demonstrtes tht Pd NPs deposited on CMG re electrochemiclly more ccessile, which is very importnt for electroctlytic pplictions in fuel cells. In ddition, the doule lyer cpcitnce ws lso otined from Intensity C(001) Pd(200) C(100) e d c i/ma{mg c θ/(Ο) Fig. 4. XRD ptterns of GO (curve ), CMG (curve ), Pd/C (curve c), Pd/RGO (curve d) nd Pd/CMG (curve e) E /V(vs.SCE) Fig. 5. Cyclic voltmmogrms of different composites in 0.25 M H 2 SO 4 solution t scn rte of 20 mv s 1 for Pd/CMG (curve ), Pd/RGO (curve ) nd Pd/C (curve c)

6 the cyclic voltmmetry dt. The electricl cpcitnce is mesure of the surfce re. The incresed doule-lyer thickness of the Pd/CMG sed electrodes reflects the higher specific surfce re of the support composite. The high ECSA for the Pd/CMG mteril is lso supported y the high electroctlytic ctivity for electro-oxidtion of formic cid. The otined cyclic voltmmetry (CV) curves exhiit very prominent chrcteristic peks for formic cid oxidtion. Figure 6 compres the formic cid oxidtion ctivities of the three Pd ctlysts in 0.25 M H 2 SO 4 contining 0.25 M HCOOH. As shown in Fig. 6, the highest HCOOH oxidtion current density cn e oserved in the Pd/CMG smple, indicting the highest ctlytic ctivity for HCOOH oxidtion. The current densities of Pd/CMG, Pd/RGO nd Pd/C, clculted from the forwrd-scn, were , 70.13, ma mg 1 t 0.11 V, respectively. The higher electroctlytic ctivity for the formic cid oxidtion on Pd/CMG thn those on Pd/RGO nd Pd/C shows the importnce of the distriution nd the dispersion of Pd electroctlysts on CMG supports. This is n importnt improvement for the Pd ctlyst ecuse the use of CMG s the support mteril cn significntly reduce the mount of unit loding to chieve the sme performnce s tht of the Pd/CMG. i/ma{mg E/V(vs.SCE) Fig. 6. Cyclic voltmmogrms of different composites in 0.25 M H 2 SO M HCOOH solution t scn rte of 50 mv s 1 for Pd/CMG (curve ), Pd/RGO (curve ) nd Pd/C (curve c). The chronomperometric technique is n effective method to evlute the electroctlytic stility of ctlyst mterils. Figure 7 shows the typicl current density-time responses of Pd-sed ctlysts for formic cid electro-oxidtion. The polriztion currents for the formic cid oxidtion rection on the three ctlysts ech shows significnt decy initilly nd rech stle vlue fter eing c polrized t 0.1 V for 600 s. The current decy for the HCOOH oxidtion rection indictes the slow dectivtion of Pd-sed electroctlysts y the dsorption of CO or CO-like intermedites. [27,28] However, in the stedy-stte region, Pd/CMG composites present current density of ma mg 1, while Pd/RGO nd Pd/C composites exhiit current densities of out 9.99 nd 2.15 ma mg 1, respectively. This indictes tht Pd/CMG ctlyst lso hs much higher stility towrd the poisoning y dsored CO or CO-like intermedite species during the HCOOH oxidtion s compred with Pd/RGO nd Pd/C ctlysts. The significnt enhncement in the ctlytic ctivity nd stility of Pd/CMG ctlyst s compred with Pd/RGO nd Pd/C ctlysts my e ttriuted to the etter uniformly distriuted nd smller Pd NPs supported on CMG. i/ma{mg c t/s Fig. 7. Chronomperometric curves for different composites in 0.25 M H 2 SO M HCOOH solution of 0.1 V for Pd/CMG (curve ), Pd/RGO (curve ) nd Pd/C (curve c). These fvourle properties my result from three fctors. First, CMG with minophenyl groups cn gretly enhnce the nucletion of nnocrystlline metls onto CMG y providing lrge numer of hydrophilic moleculr groups on the surfce. [29] These heterogeneous nucletion sites fcilitte the deposition of finer, etter dispersed Pd NPs, highly desirle feture for Pd ctlyst pplictions. Additionlly, the specific interctions etween the minophenyl groups on the CMG nd PdCl 2 4 could led to the deposition of Pd NPs in nrrower size distriution. [30] Well-dispersed Pd NPs on the Pd/CMG hyrid structure likely give rise to more phse oundries, which in turn improves the ctlytic efficiency. Finlly, the CMG sheets ech hve lrge surfce re nd the prticles cn e deposited on oth sides of these sheets. On the other hnd, the lrge surfce res my fcilitte the trnsmission of the electrolyte nd formic cid through the surfce of the ctlyst

7 4. Conclusion The CMG with minophenyl groups is successfully synthesized s ctlyst-supports y nucleophilic ring-opening rection etween GO nd PPD. The CMG sheets re used s support mterils for Pd NPs nd the electrochemicl properties of ssemlies for formic cid oxidtion re investigted. The superior performnce of Pd/CMG is ttriuted to the etter uniform distriution nd dispersion with nrrow prticle size distriution. It is envisged tht oth the ese nd the convenience of the strtegy developed in this study mke it n ttrctive lterntive for lrgescle nd cost-effective production of CMG, s well s for the deposition of vriety of metl NPs on CMG for other heterogeneous ctlyst pplictions. References [1] Kim K S, Zho Y, Jng H, Lee S Y, Kim J M, Kim K S, Ahn J H, Kim P, Choi J Y nd Hong B H 2009 Nture [2] Schedin F, Geim A K, Morozov S V, Hill E W, Blke P, Ktsnelson M I nd Novoselov K S 2007 Nture Mter [3] Stnkovich S, Dikin D A, Dommett G H B, Kohlhs K M, Zimney E J, Stch E A, Piner R, Nguyen S T nd Ruoff R S 2006 Nture [4] Stoller M D, Prk S J, Zhu Y W, An J H nd Ruoff R S 2008 Nno Lett [5] Novoselov K S, Geim A K, Morozov S V, Jing D, Zhng Y, Duonos S V, Grigoriev I V nd Firsov A A 2004 Science [6] Go M, Pn Y, Zhng C D, Hu H, Yng R, Lu H L, Ci J M, Du S X, Liu F nd Go H J 2010 Appl. Phys. Lett [7] Pn Y, Zhng H G, Shi D X, Sun J T, Du S X, Liu F nd Go H J 2009 Adv. Mter [8] Geim A K nd Novoselov K S 2007 Nture Mter [9] Behtu N, Lomed J R, Green M J, Higginothm A L, Sinitskii A, Kosynkin D V, Tsentlovich D, Prr-Vsquez A N G, Schmidt J, Kesselmn E, Cohen Y, Tlmon Y, Tour J M nd Psquli M 2010 Nture Nnotechnol [10] Li Y M, Tng L H nd Li J H 2009 Electrochem. Commun [11] Li Y J, Go W, Ci L J, Wng C M nd Ajyn P M 2010 Cron [12] Shng N G, Ppkonstntinou P, Wng P, Silv S nd Rvi P 2010 J. Phys. Chem. C [13] He H Y, Klinowski J, Forster M nd Lerf A 1998 Chem. Phys. Lett [14] Lerf A, He H Y, Forster M nd Klinowski J 1998 J. Phys. Chem. B [15] Wng S, Chi P J, Chu L L, Zho L H, Png R Q, Sivrmkrishnn S, Zhou M, Goh R G S, Friend R H, Wee A T S nd Ho P K H 2008 Adv. Mter [16] Yng H F, Shn C S, Li F H, Hn D X, Zhng Q X nd Niu L 2009 Chem. Commun [17] Hummers W S nd Offemn R E 1958 J. Am. Chem. Soc [18] Stnkovich S, Dikin D A, Piner R D, Kohlhs K A, Kleinhmmes A, Ji Y, Wu Y, Nguyen S T nd Ruoff R S 2007 Cron [19] Ellison M D nd Gsd P J 2008 J. Phys. Chem. C [20] Wng H L, Ho Q L, Yng X J, Lu L D nd Wng X 2009 Electrochem. Commun [21] Roodenko K, Gensch M, Rppich J, Hinrichs K, Esser N nd Hunger R 2007 J. Phys. Chem. B [22] Mrtin C, Alis M, Christien F, Crosnier O, Belnger D nd Brousse T 2009 Adv. Mter [23] Wng S Y, Wng X nd Jing S P 2008 Lngmuir [24] Hontori-Lucs C, López-Peindo A J, López-Gonzáez J de D, Rojs-Cervntes M L nd Mrtín-Arnd R M 1995 Cron [25] Ding H, Shi X Z, Shen C M, Hui C, Xu Z C, Li C, Tin Y, Wng D K nd Go H J 2010 Chin. Phys. B [26] Seger B nd Kmt P V 2009 J. Phys. Chem. C [27] Yu X W nd Pickup P G 2009 Electrochem. Commun [28] Miyke H, Okd T, Smjeske G nd Osw M F 2008 Phys. Chem. Chem. Phys [29] Guo D J nd Li H L 2005 Cron [30] Guo Z P, Hn D M, Wexler D, Zeng R nd Liu H K 2008 Electrochim. Act