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Rafael Silva and Tewodros Asefa *

Noble Metal-Free Oxidative Electrocatalysts: Polyaniline and Co(II)-Polyaniline Nanostructures Hosted in Nanoporous Silica

Electrocatalysts that are capable of catalyzing oxidation of organic compounds have enormous appeal because of their potential applications in fuel cells as well as organic syntheses. [ 1 , 2 ] More-over, when used in chemical reactions for organic synthesis, oxidation electrocatalysts have advantages compared to conven-tional oxidation catalysts because the former enable ‘greener’ processes by avoiding the use of sacrifi cial oxidizing agents that often lead to undesired byproducts. [ 3–5 ] However, despite these potential advantages, the use of electrochemical catalysts in oxidation reactions for synthetic purposes is not a very well explored research fi eld.

On the other hand, in the past few years, a number of fuel cells based on oxidative electrocatalytic systems have been designed and demonstrated to work using methanol, [ 6–9 ] methane, [ 10 , 11 ] ethanol, [ 12 , 13 ] hydrazine, [ 14 , 15 ] formic acid [ 16 , 17 ] and L-ascorbic acid [ 18 , 19 ] as fuels. However, in all of these cases, high effi ciency in the fuel cells was obtained only when expen-sive noble metal and bimetallic nanoparticles such as PtRu and PtSn supported on carbon were used as the electrocatalysts. [ 20 , 21 ] Furthermore, most of these nanoparticle-based electrocatalysts have been known to easily aggregate or quickly get poisoned by the oxidation reaction products and thus lose their electro-catalytic activities in the presence of reactants/products. [ 22 , 23 ] Moreover, many of these electrocatalysts do involve expensive and precious metals such as Pt, Au, Pd and Ru. [ 24 , 25 ]

We here propose and demonstrate successfully a new in situ synthetic method to highly active oxidation electrocatalysts that are composed of electroactive/conducting polyaniline (PANI) nanostructures that are controllably polymerized within the channels pores of mesoporous silica. There are clearly some benefi ts and advantages to be expected by confi ning or synthe-sizing PANI within the pores of the high surface area meso-porous silica materials such as SBA-15. This is also supported, to some extent, by experimental results in our case (see below). Generally, when polymers such as PANI are prepared as bulk

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R. Silva , Prof. T. Asefa Department of Chemistry and Chemical Biology, RutgersThe State University of New Jersey610 Taylor Road, Piscataway, New Jersey 08854, USA E-mail: tasefa@rci.rutgers.edu Prof. T. AsefaDepartment of Chemical and Biochemical Engineering, RutgersThe State University of New Jersey98 Brett Road, Piscataway, New Jersey 08854, USA

DOI: 10.1002/adma.201104126

Adv. Mater. 2012, DOI: 10.1002/adma.201104126

materials by themselves, they often possess very low porosity, low surface areas, and small contact areas to interact with solu-tions. Consequently, such polymers exhibit poor electrocatalytic activities. Conversely, when the polymerization of PANI is con-trollably performed on the channel walls of nanoporous silica material such as SBA-15, it forms a uniform and high surface area polymeric structure that is supported over the channel walls of the very high surface area SBA-15. This subsequently produces a PANI/SBA-15 composite material with lots of acces-sible redox active sites in it and a large contact area to interact with solutions, and hence better electrocatalytic activities.

Figure 1 shows the schematic illustration of the synthetic procedure we employed to make PANI/SBA-15 and Co(II)-doped PANI/SBA-15 materials. First, the internal channel walls of the SBA-15 were modifi ed with diamine groups. Diamine groups were chosen because of their ability to anchor different redox active transition metal ions on the pore walls of the SBA-15 mesoporous material. Furthermore, the amine groups could easily be converted into ammonium ions, which in turn could be used to electrostatically anchor persulfate ions—the oxidizing agents required for polymerization of aniline into PANI. By subsequent in situ polymerization of aniline using the immobilized persulfate ions on the channel walls of SBA-15 as oxidizing agents, PANI/SBA-15 was produced. The electro-chemical and electrocatalytic properties of the material were then investigated. In addition, the PANI/SBA-15 material was immobilized with other electroactive species, such as Co 2 + ions, to study the redox properties of metal ions within PANI/SBA-15 and the electrocatalytic activity of metal-doped PANI/SBA-15 materials.

In previously reported synthetic routes to PANI and other polymers within nanoporous materials, including mesoporous silica, [ 26 , 27 ] the monomers (such as aniline) were typically added into the pores of the nanoporous materials prior to the addition of the oxidizing agents (e.g., persulfate ions) in form of solu-tions. Since the oxidizing agents existed predominantly in the solutions, the polymerization took place both within the pores as well as on the walls of the SBA-15 material. In contrast, in our synthesis, the oxidizing agents were fi rst placed on the channel walls of the SBA-15 material as uniformly as possible, prior to addition of aniline. Thus the polymerization took place only on the channel walls of the the mesoporous material.

The reaction in the synthesis of PANI/SBA-15 involved oxida-tive polymerization, where persulfate ions within SBA-15 were consumed stoichiometrically by aniline to produce small seg-ments of PANI. These small segments of PANI crosslinked to

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Figure 1 . Schematic representation of the synthesis of polyaniline/SBA-15 (PANI/SBA-15) and Co(II)-doped PANI/SBA-15 materials or electrocatalysts by in situ polymerization of aniline within the pores of mesoporous SBA-15, followed by doping of the PANI/SBA-15 with Co(II) ions.

Figure 2 . (a) UV/Vis spectra of undoped and H + -doped PANI/SBA-15 materials.

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one another via a process akin to condensation polymerization and formed a higher molecular weight PANI. [ 28 , 29 ] Because, in our synthetic approach, the in situ polymerization process took place only on channel walls within the one-dimensional long cylindrical shaped pores of SBA-15, it was expected to result in chains of PANI nanostructures with low degree of branching. This was supported indirectly by electrochemical measure-ments and wide-angle X-ray diffraction (XRD) analysis (vide infra). It has already been documented that the specifi c con-ductivity of PANI depends greatly on the degree of branching of the poly mer, where PANI with low degree of branching gives higher specifi c conductivity. [ 30 , 31 ] Therefore, PANI with low degree of branching is often preferred for various electro-chemical applications. Interestingly, our PANI/SBA-15 material exhibited good conductivity and electrocatalytic activities (see below), presumably because of the low degree of branching or cross-link density of its PANI. This was corroborated by wide-angle X-ray diffraction (XRD) analysis (see Supporting Informa-tion), which showed the PANI to have an amorphous or nano-scale structure, suggesting the presence of low cross-link den-sity in it (please note that bulk PANI materials with high degree of branching and high cross-link density, e.g., prepared by mag-netic stirring of aniline in solutions, typically show sharp Bragg refl ections). [ 32–35 ]

The structures and composition of the PANI/SBA-15 mate-rial was thoroughly characterized by various methods (see Sup-porting Information for more details). For instance, the well-known absorption bands and absorption shifts corresponding to undoped and H + -doped PANI in the PANI/SBA-15 com-posite materials were confi rmed by UV-Vis absorption spectros-copy ( Figure 2 ).

Next up, the electrochemical properties and electrocatalytic activities of PANI/SBA-15 and Co(II)-doped PANI/SBA-15 in oxidation of L-ascorbic acid were characterized. Successful cat-alytic oxidation of L-ascorbic acid is appealing because it has recently been shown that L-ascorbic acid can be used as fuel in direct fuel cells operating at room temperature. [ 18 ] The elec-trochemical characterization of PANI/SBA-15 and Co(II)-loaded

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PANI/SBA-15 was performed with cyclic voltammetry on a sample prepared by placing 0.5 mg of the composite materials on the surface of a carbon paste electrode (CPE). Figure 3 a shows the cyclic voltammograms (CV) of a bare CPE and a CPE coated with PANI/SBA-15. The CV of PANI/SBA-15 shows two main redox processes (I A /I C and II A /II C ), with their anodic peaks centered at − 0.325 and 0.256 V, respectively. These two redox processes are due to the typical inter-conversion reactions of PANI that take place upon varying the potential. While the I A /I C redox process corresponds to the leucoemaldine/emeral-dine transition, the II A /II C redox process corresponds to emer-aldine/pernigraniline transition ( Scheme 1 ). [ 36 ] Moreover, the observed strong signals associated with PANI in the PANI/SBA-15 indicated that the PANI hosted within the pores of SBA-15 were highly electroactive.

Some previous reports showed that transition metals (such as Ni, Zn, Fe, and La) as well as rare-earth metals (such as Eu, Sm,

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Figure 3 . (a) Cyclic voltammetric (CV) responses of bare carbon paste electrode (CPE) (—) and CPE coated with PANI/SBA-15 (- - -) in 0.5 M H 2 SO 4 solutions at scan rate of 100 mV s − 1 . (b) CV responses of Co(II)-doped PANI/SBA-15 in 0.5 M H 2 SO 4 in the presence of 1 mM L-ascorbic acid at dif-ferent scan rates: 100 mV s − 1 (—), 50 mV s − 1 (- - -) and 20 mV s − 1 (- .. -). (d) CV responses of PANI/SBA-15 in 0.5 M aqueous H2SO4 solution in the presence of 10 mM L-ascorbic acid at different scan rates: 100 mV s − 1 (—), 50 mV s − 1 (- - -) and 20 mV s − 1 (- .. -). (e,f) CV responses of Co(II)-doped PANI/SBA-15 in 0.5 M aqueous H 2 SO 4 solution in the presence of (e) 1 mM and (f) 10 mM L-ascorbic acid at different scan rates: 100 mV s − 1 (—), 50 mV s − 1 (- - -) and 20 mV s − 1 (- .. -). (g) CV responses of bare CPE in 0.5 M aqueous H 2 SO 4 solution in the presence of 10 mM L-ascorbic acid at different scan rates: 100 mV s − 1 (—), 50 mV s − 1 (- - -) and 20 mV s − 1 (- .. -). (h) chronoamperometric results of L-ascorbic acid oxidation in a 25 mL solution containing 10 mM L-ascorbic and 0.5 M H 2 SO 4 using PANI/SBA-15 (—) and Pt/C (- - -) as electrocatalysts. The current densities are reported considering the geometric area of the electrodes.

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Scheme 1 . Redox processes in different forms of PANI.

and Nd) interact with PANI and form pseudo-protonated struc-tures having improved conductivity. [ 37 ] With this motivation in mind, i.e., with the goal of improving the conductivity as well as the electrocatalytic activity of PANI/SBA-15, we prepared Co(II)-doped PANI/SBA-15 material and investigated its electrochem-ical and electrocatalytic properties towards anodic oxidation of L-ascorbic acid. The material was synthesized by letting the nitrogen atoms of PANI as well as the residual diamine groups within the channels pores of PANI/SBA-15 chelate to Co(II) ions from excess aqueous Co(II) solution. This gave 2.52 mmol Co(II) per gram of PANI/SBA-15. However, in a control experiment, where excess aqueous Co(II) solution was stirred with SBA-15/Diamine, only 0.61 mmol Co(II) in a gram of SBA-15/Diamine was obtained. Thus, the presence of PANI in the mesoporous SBA-15 material clearly led to the chemisorption of more than four-fold Co(II) ions in material. This is most likely due to the presence of relatively larger density of N-atoms in PANI, which form favorable interactions with Co(II). This result further sug-gests that the Co(II) ions are predominantly around PANI, and thus potentially able to form pseudo-protonated structures.

Cyclic voltammetry of the resulting Co(II)-doped PANI/SBA-15 material is presented in Figure 3 b. In the voltammo-gram, three redox processes are clearly observable. The peaks at − 0.302 and 0.245 V are attributed to the transition in oxidation states of PANI (cf., the corresponding values for PANI/SBA-15 are − 0.325 and 0.256 V, respectively). Thus, these results indi-cate that doping PANI/SBA-15 with Co(II) makes the redox potentials for PANI to shift to lower values. In other words, doping PANI/SBA-15 with Co(II) makes the PANI to undergo oxidation/reduction processes more easily. The third redox signal, whose anodic process is shown at ∼ 0.0 V, is attributed to the inter-conversion of Co 2 + /Co 3 + . It is worth noting that the inter-conversion of Co 2 + /Co 3 + in the Co(II)-doped PANI/SBA-15 material took place while PANI was in its highest conductive form, i.e. , the emeraldine salt.

In Figures 3 c and 3 d, the cyclic voltammograms (CV) of PANI/SBA-15 in the presence of diluted solution of L-ascorbic acid at different sweep rates are shown. The cyclic voltammograms

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are composed of three oxidation peaks and a very broad reduction signal. The asymmetry between the anodic and cathodic currents suggests that the two processes are not revers-ible. More importantly, the observed stronger anodic current compared to cathodic current in the CV indicates the occurrence of net oxi-dation in the redox cycle, or the oxidation of a substance (i.e., L-ascorbic acid, in our case). Furthermore, when the concentration of L-ascorbic acid was increased, the anodic cur-rent became much stronger in the positive sweep direction as shown in Figure 3 d.

By comparing the cyclic voltammograms in Figures 3 c and 3 d, the central oxidation peak is confi rmed to be a signal associated with the oxidation of L-ascorbic acid. Fur-thermore, it can be seen that the potential at which the oxidation of L-ascorbic acid occurs shifts from low to high as the concentration of L-ascorbic acid increases. For instance, the

oxidation of L-ascorbic acid in the presence of PANI/SBA-15 occurs at 0.134 V in 1 mM L-ascorbic acid solution, but at 0.209 V in 10 mM L-ascorbic acid solution.

The result in Figures 3 d further shows that PANI/SBA-15 exhibits a strong catalytic activity towards oxidation of L-ascorbic acid, with very low overpotential and strong current density. Interestingly, the potential required for oxidation of L-ascorbic acid and the current density obtained when using PANI/SBA-15 as an electrocatalyst was actually found to be very comparable to those of conventional Pt/C electrocatalysts (Supporting Information). For instance, the potential required for oxidation of 10 mM L-ascorbic acid solution with Pt/C was found to be 0.160 V at the same sweep rate of 100 mV s − 1 (cf., it is 0.209 V for PANI/SBA-15).

The electrocatalytic property of Co(II)-doped PANI/SBA-15 in oxidation of L-ascorbic acid was also investigated in the same way as above. The results are presented in Figures 3 e and 3 f. The shapes of the voltammograms for Co(II)-doped PANI/SBA-15 (Figure 3 e) appeared very similar to those obtained for PANI/SBA-15 (Figure 3 c), except for the slightly narrow cen-tral anodic peak in the former. Furthermore, a sharp cathodic peak at 0.050 V superposed on the broad signal in the nega-tive sweep direction was seen in the case of Co(II)-doped PANI/SBA-15, demonstrating the reversibility of the Co 2 + /Co 3 + transi-tion in this sample. This peak was not observed in the metal-free PANI/SBA-15, as expected.

Since the position of the anodic oxidation signal of L-ascorbic acid in the presence of Co(II)-doped PANI/SBA-15 electrocata-lyst was virtually the same as the one obtained in the presence of the metal-free PANI/SBA-15 electrocatalyst, there was no further improvement in the electrocatalytic activity of the mate-rial as a result of the presence of Co(II) ions in it. Despite no improved electrocatalytic activity was observed as a result of doping of PANI/SBA-15 with Co(II), the obtained results them-selves were interesting for the following reasons. Firstly, the electrochemical experiment revealed that the Co(II) ions immo-bilized within the channels pores of the PANI/SBA-15 material effi ciently underwent redox processes. Secondly, the reported

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synthetic method was shown to produce a PANI/SBA-15 mate-rial capable of holding up high density of electrochemically accessible metal ions in it. Thirdly, the immobilized Co(II) ions made PANI’s redox potential to shift to lower values or PANI to undergo reversible oxidation/reduction more easily. Fourthly, the structure of Co(II) species in the Co(II)-doped PANI/SBA-15 material mimics different metal ions stabilized by proteins within the nanosize active sites of many enzymes. [ 38 ] Finally, the experimental demonstration of successfully accessing the redox properties of the metal ions within the pores of the Co(II)-doped PANI/SBA-15 material via PANI was akin to the synergistic activities exhibited by two or more groups in many bifunctional enzymes during various physiologically relevant processes in the body. [ 38 ]

In order to further confi rm the involvement of the PANI/SBA-15 system as an electrocatalyst, a control experiment involving bare CPE electrode as an electrocatalyst for the oxi-dation of L-ascorbic acid was performed (Figure 3 g). How-ever, the CV did not show the strong signal associated with oxidation of L-ascorbic acid at ∼ 0.2 V. The effectiveness of the PANI/SBA-15 material as an electrocatalyst was further evalu-ated by comparing its electrocatalytic activity with that of com-mercially available Pt/C electrocatalyst. Figure 3 h shows the chronoamperometric curves for the oxidation of L-ascorbic acid at constant potential of 0.2 V using PANI/SBA-15 and Pt/C as electrocatalysts. The results indicated that the cur-rent densities remained reasonably stable for several hours for both materials. The results in Figure 3 h further revealed that the average current density obtained from PANI/SBA-15 was about 82% as much as that of Pt/C when both materials are used as electrocatalysts in L-ascorbic acid oxidation under similar conditions.

In conclusion, SBA-15 mesoporous silica, having diamine groups in its internal channel pores, was successfully used for controlled polymerization of PANI nanostructures within the channel-pores. The synthesis produced a high surface area PANI/SBA-15 nanocomposite material composed of approxi-mately 4.7 wt% PANI. The resulting PANI/SBA-15 material showed high electroactivity. In addition, the PANI/SBA-15 mate-rial successfully chemisorbed large amounts of redox active Co(II) ions, presumably through the interaction between PANI/diamine groups and the Co(II) ions. By applying cyclic sweep potential over both PANI/SBA-15 and Co(II)-doped PANI/SBA-15 with cyclic voltammetry, reversible redox properties associated with PANI (as well as reversible redox signals cor-responding to Co 2 + /Co 3 + transition) were observed. The redox inter-conversion of Co 2 + /Co 3 + occurred while PANI was in its most conducting form, i.e. , emeraldine salt. Accessing the redox activity of the metal ions (Co 3 + ) entrapped within the nano-pores of the PANI/SBA-15 via the conducting PANI could be advantageous for the potential utilization of these materials as electrocatalysts in fuel cells. Indeed, both the metal-free (PANI/SBA-15) as well as Co(II)-loaded PANI/SBA-15 nanocomposite materials showed high activity in oxidation of L-ascorbic acid at an extremely low overpotential, yielding high current den-sity. Furthermore, the electrocatalytic activity of PANI/SBA-15 toward oxidation of L-ascorbic acid was found to be very compa-rable to that obtained from a conventional and more expensive Pt/C electrocatalyst. Thus, the PANI/SBA-15 material could

© 2012 WILEY-VCH Verlag GAdv. Mater. 2012, DOI: 10.1002/adma.201104126

potentially serve as an effi cient and inexpensive substitute cata-lyst or electrocatalyst in fuel cells and other catalytic reactions, where expensive Pt-based materials have found wide ranges of applications, including fuel cells. Furthermore, because of the presence of its robust inorganic support material (SBA-15), the PANI/SBA15 system could have greater stability to high temperatures and in different solutions. Moreover, because the SBA-15 support material contains residual surface silanol groups, the PANI/SBA15 could easily be modifi ed with other functional groups, [ 39 ] generating even more versatile electro-catalytic systems.

Experimental Section Synthesis of PANI/SBA-15 : As-synthesized SBA-15 was prepared as per

literature report. [ 40 ] SBA-15 was externally modifi ed with methyl groups using hexamethyldisilazane (HMDS). The internal walls of SBA-15 were modifi ed using N -(2-aminoethyl)-3-aminopropyltrimethoxysilane, generating diamine groups on the surfaces of its mesopores (SBA-15/Diamine). The diamine groups in the inner walls were protonated using dilute HCl solution, and the resulting ammonium ions were used for chelation of persulfate ions. When the resulting SBA-15/Diamine-Persulfate material was dispersed in aniline solution under acidic condition, polymerization of PANI occurred on the channel walls of the SBA-15 material as a result of the reaction between aniline and surface persulfate groups. The PANI/SBA-15 was recovered by centrifugation and washed with 1:1 ratio of acetone:ethanol solution. Co(II)-doped PANI/SBA-15 was prepared by treating the PANI/SBA-15 with aqueous Co 2 + solution and washing the precipitate with suffi cient amount of ethanol to remove the residual Co 2 + ions that are not chelated. The resulting PANI/SBA-15 and Co-doped PANI/SBA-15 materials were well characterized and then their electrochemical and electrocatalytic properties towards L-ascorbic acid oxidation were performed (see supporting information for details).

Supporting Information The Supporting Information contains detailed synthetic procedures for the materials and additional characterization results. Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements TA gratefully acknowledges the partial fi nancial assistance provided by the United States National Science Foundation (NSF) (under Grant Nos: CAREER CHE-1004218, NSF DMR-0968937, NSF NanoEHS-1134289, NSF-ACIF for 2010, and NSF Special Creativity grant in 2011) for his research in nanocatalysis. RS acknowledges the CAPES, Brazil, and the Fulbright Agency, USA for offering him Fellowships for his graduate study.

Received: October 26, 2011 Revised: February 3, 2012

Published online:

[ 1 ] a) G. Liu , C. F. Sun , D. Li , S. P. Song , B. W. Mao , C. H. Fan , Z. Q. Tian , Adv. Mater. 2010 , 20 , 2148 ; b) W. Li , M. Waje , Z. Chen , P. Larsen , Y. Yan , Carbon 2010 , 48 , 995 ; c) Z. P. Shao , S. M. Haile , J. M. Ahn , P. D. Ronney , Z. L. Zhan , S. A. Barnett , Nature 2005 , 435 , 795 ; d) I. Dumitrescu , J. P. Edgeworth , P. R. Unwin , J. V. Macpherson , Adv. Mater. 2009 , 21 , 3105 ; e) B. M. Leonard , Q. Zhou , D. Wu , F. J. DiSalvo , Chem. Mater. 2011 , 23 , 1136 ; f) Z. Zhang , M. Li ,

5wileyonlinelibrary.commbH & Co. KGaA, Weinheim

6

www.advmat.dewww.MaterialsViews.com

CO

MM

UN

ICATI

ON

Z. Wu , W. Li , Nanotechnology 2011 , 22 , 015602 ; g) L. Zhang ,

J. J. Zhang , D. P. Wilkinson , H. J. J. Wang , Power Sources 2006 , 156 , 171 ; h) A. Sarkar , M. A. Vadivel , A. Manthiram , J. Phys. Chem. C 2008 , 112 , 12037 ; i) R. Ganesan , J. S. Lee , Angew. Chem. Int. Ed. 2005 , 44 , 6557 ; j) Z. Shao , J. Mederos , C. Kwak , S. M. Haile , J. Fuel Cell Sci. Tech. 7 , 021016 .

[ 2 ] a) V. Rosca , M. Duca , M. T. de Groot , M. T. M. Koper , Chem. Rev. 2009 , 109 , 2209 ; b) C. Coutanceau , S. Brimaud , C. Lamy , J.-M. Léger , L. Dubau , S. Rousseau , F. Vigier , Electrochim. Acta 2008 , 53 , 6865 ; c) M. N. Elinson , A. S. Dorofeev , F. M. Miloserdov , A. I. Ilovalsky , S. K. Feducovich , P. A. Belyakov , G. I. Nikishin , Adv. Synth. Catal. 2008 , 350 , 591 .

[ 3 ] G. Palmisano , R. Ciriminna , M. Pagliaro , Adv. Synth. Catal. 2006 , 348 , 2033 .

[ 4 ] C. Comninellis , Electrochim. Acta 1994 , 39 , 1857 . [ 5 ] R. N. Biboum , B Keita , S. Franger , C. P. N. Njiki , G. Zhang , J. Zhang ,

T. Liu , I.-T. Mbomekalle , L. Nadjo , Materials 2010 , 3 , 741 . [ 6 ] D. R. Ou , T. Mori , H. Togasaki , M. Takahashi , F. Ye , J. Drennan ,

Langmuir 2011 , 27 , 3859 . [ 7 ] J. Kua , W. A. Goddard , J. Am. Chem. Soc. 1999 , 121 , 10928 . [ 8 ] G. Yu-Guo , H. Jin-Song , W. Li-Jun , Adv. Mater. 2008 , 20 ,

2878 . [ 9 ] a) Z. Jusys , J. Kaiser , R. J. Behm , Langmuir 2003 , 19 , 6759 ; b) W. Li ,

Q. Xin , Y. Yan , Int. J. Hydrogen Energy 2010 , 35 , 2530 . [ 10 ] Z. Shao , J. Mederos , W. C. Chueh , S. M. Haile , J. Power Sources

2006 , 168 , 589 . [ 11 ] B. C. Chan , R. X. Liu , K. Jambunathan , H. Zhang , G. Y. Chen ,

T. E. Mallouk , E. S. Smotkin , J. Electrochem. Soc. 2005 , 152 , A594 .

[ 12 ] a) X. Y. Zhang , W. Lu , J. Y. Da , H. T. Wang , D. Y. Zhao , P. A. Webley , Chem. Commun. 2009 , 2 , 195 ; b) C. Mahendiran , T. Maiyalagan , K. Scott , A. Gedanken , Mater. Chem. Phys. 2011 , 128 , 341 .

[ 13 ] F. Vigier , C. Coutanceau , A. Perrard , E. M. Belgsir , C. Lamy , J. Appl. Electrochem. 2004 , 34 , 439 .

[ 14 ] K. Asazawa , K. Yamada , H. Tanaka , A. Oka , M. Taniguchi , T. Kobayashi , Angew. Chem. Int. Ed. 2007 , 46 , 8024 .

[ 15 ] J. C. Chinchilla , K. Asazawa , T. Sakamoto , K. Yamada , H. Tanaka , P. Strasser , J. Am. Chem. Soc. 2011 , 133 , 5425 .

[ 16 ] a) X. Ji , K. T. Lee , R. Holden , L. Zhang , J. Zhang , G. A. Botton , M. Couillard , L. F. Nazar , Nat. Chem. 2010 , 2 , 286 ; b) O. Winjobi , Z. Zhang , C. Liang , W. Li , Electrochim. Acta 2010 , 55 , 4217 .

[ 17 ] X. W. Yu , P. G. Pickup , J. Power Sources 2008 , 182 , 124 . [ 18 ] N. Fujiwara , S.-I. Yamazaki , Z. Siroma , T. Ioroia , K. Yasuda , J. Power

Sources 2007 , 167 , 32 . [ 19 ] G. Raman , G. Aharon , Nanotechnology 2008 , 19 , 435709 . [ 20 ] a) C. Dupont , Y. Jugnet , D. Loffreda , J. Am. Chem. Soc. 2006 , 128 ,

9129 ; b) H. Ataee-Esfahani , L. Wang , Y. Nemoto , Y. Yamauchi , Chem. Mater. 2010 , 22 , 6310 .

wileyonlinelibrary.com © 2012 WILEY-VCH Verlag Gm

[ 21 ] J. Melke , A. Schoekel , D. Dixon , C. Cremers , D. E. Ramaker , C. Roth , J. Phys. Chem. C 2010 , 114 , 5914 .

[ 22 ] R. Mu , Q. Fu , H. Xu , H. Zhang , Y. Huang , Z. Jiang , S. Zhang , D. Tan , X. Bao , J. Am. Chem. Soc. 2011 , 133 , 1978 .

[ 23 ] a) S. Gottesfeld , J. Pafford , J. Electrochem. Soc. 1988 , 135 , 2651 ; b) T. Saida , Y. Takasu , W. Sugimoto , Electrochemistry 2011 , 79 , 371 .

[ 24 ] a) C. X. Xu , L. Q. Wang , R. Y. Wang , K. Wang , Y. Zhang , F. Tian , Y. Ding , Adv. Mater. 2009 , 21 , 2165 ; b) Y. Y. Chu , Z. B. Wang , Z. Z. Jiang , D. M. Gu , G. P. Yin , Adv. Mater. 2011 , 23 , 3100 ; c) C. Pan , Y. Li , Y. Ma , X. Zhao , Q. Zhang , J. Power Sources 2011 , 196 , 6228 .

[ 25 ] a) S. J. Jiang , Y. W. Ma , G. Q. Jian , H. S. Tao , X. Z. Wang , Y. N. Fan , Y. N. Lu , Z. Hu , Y. Chen , Adv. Mater. 2009 , 21 , 4953 ; b) A. R. Malheiro , J. Perez , H. M. Villullas , J. Power Sources 2010 , 195 , 7255 ; c) T. Maiyalagan , K. J. Scott , Power Sources 2010 , 195 , 16 , 5246 ; d) H. Ataee-Esfahani , L. Wang , Y. Nemoto , Y. Yamauchi , Chem. Mater. 2010 , 22 , 6310 .

[ 26 ] Y. Q. Dou , Y. Zhai , F. Zeng , X. X. Liu , B. Tu , D. Zhao , J. Coll. Inter. Sci. 2010 , 341 , 353 .

[ 27 ] S. Zhu , J. Gu , Z. Chen , J. Dong , X. Liu , C. Chen , D. Zhang , J. Mater. Chem. 2010 , 20 , 5123.

[ 28 ] Y. He , Appl. Surf. Sci. 2006 , 252 , 2115 . [ 29 ] a) R. Silva , E. C. Muniz , A. F. Rubira , Polymer 2008 , 49 , 4066 ;

b) R. Silva , E. C. Muniz , A. F. Rubira , Langmuir 2009 , 25 , 873 ; c) R. Silva , G. M. Pereira , E. C. Muniz , A. F. Rubira , Cryst. Growth. Des. 2009 , 9 , 3307 .

[ 30 ] J. J. Langer , Synt. Met. 2000 , 113 , 263. [ 31 ] N. Gospodinova , L. Terlemezyan , P. Mokreva , A. Tadjer , Polymer

1996 , 37 , 4431 . [ 32 ] H. Liu , X. B. Hu , J. Y. Wang , R. I. Boughton , Macromolecules 2002 ,

35 , 9414 . [ 33 ] D. S. Sutar , N. Padma , D. K. Aswal , S. K. Deshpande , S. K. Gupta ,

J. V. Yakhmi , J. Colloid Interf. Sci. 2007 , 313 , 353 . [ 34 ] J. B. M. Krishna , A. Saha , G. S. Okram , A. Soni , S. Purakayastha ,

B. Ghosh , J. Phys. D: Appl. Phys. 2009 , 42 , 095404 . [ 35 ] K. Gupta , G. Chakraborty , S. Ghatak , P. C. Jana , A. K. Meikap ,

R. Babu , J. Appl. Phys. 2010 , 108 , 073701 . [ 36 ] a) G. D’Aprano , M. Leclerc , G. Zotti , Macromolecules 1992 , 25 , 2145 ;

b) F. F. C. Bazito , L. T. Silveira , R. M. Torresi , S. I. Córdoba de Torresi , Phys. Chem. Chem. Phys. 2008 , 10 , 1457 .

[ 37 ] S. Smertenko , O. P. Dimitriev , S. Schrader , L. Brehmer , Synt. Met. 2004 , 146 , 187 .

[ 38 ] a) M. J. Wiester , P. A. Ulmann , C. A. Mirkin , Angew. Chem. Int. Ed. 2011 , 50 , 114 ; b) M. W. Hosseini , J. M. Lehn , K. C. Jones , K. E. Plute , K. B. Mertes , M. P. Mertes , J. Am. Chem. Soc. 1989 , 111 , 6330 .

[ 39 ] K. K. Sharma , A. Anan , R. P. Buckley , W. Ouellette , T. Asefa , J. Am. Chem. Soc. 2008 , 130 , 218 .

[ 40 ] Y. Xie , S. Quinlivan , T. Asefa , J. Phys. Chem. C 2008 , 112 , 9996 .

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