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Electrochimica Acta 121 (2014) 215– 222

Contents lists available at ScienceDirect

Electrochimica Acta

jo u r n al hom ep age: www.elsev ier .com/ locate /e lec tac ta

hape-controlled synthesis of Pt nanoparticles via integration ofraphene and �-cyclodextrin and using as a noval electrocatalyst forethanol oxidation

hongshui Lia,b, Lian Zhanga, Xiaomei Huanga, Lingting Yea, Shen Lina,b,∗

College of Chemistry & Chemical Engineering, Fujian Normal University, Fuzhou 350007, ChinaFujian Key Laboratory of Polymer Materials, Fuzhou 350007, Fujian, China

r t i c l e i n f o

rticle history:eceived 23 October 2013eceived in revised form2 December 2013ccepted 28 December 2013vailable online 8 January 2014

a b s t r a c t

The integration of graphene nanosheets (GNs) and �-cyclodextrin (CD) may bring about large surfacearea and high conductivity of GNs and unique supramolecular recognition capability of CD together.In this paper, for the first time, GNs-CD was used as a supporting material to construct nanodendritichydrangea-like Pt nanoparticles, which was implemented by a facile wet chemical procedure. �-CDon GNs surface was used to regulate the final morphology of Pt nanoparticles without additional cap-

eywords:raphene-cyclodextrint nanoparticleslectrocatalyst

ping agent and seeds, and GNs functioned as a matrix to obtain a relatively uniform distribution of Ptnano-clusters. Electrocatalytic performance of the as-prepared Pt/GNs-CD hybrid was investigatedby cyclic voltammetry (CV), chronoamperometry, COad stripping voltammetry, and electrochemicalimpedance spectroscopy (EIS). The Pt/GNs-CD hybrid exhibited much higher catalytic activity and stabil-ity than Pt/GNs and Pt/Vulcan X72R towards methanol oxidation, suggesting that Pt/GNs-CD hybrid couldbe a promising electrocatalyst for high-performance direct methanol fuel cells (DMFCs) applications.

ethanol oxidation

. Introduction

DMFCs are attractive energy-conversion devices for poweringortable electronics because methanol is an abundant and inex-ensive liquid fuel that has a high volumetric energy density andan be handled, stored, and transported easily [1,2]. Until now, purelatinum is the most active anode catalyst for methanol oxidationecause of its relative high catalytic activity and long-term oper-tion stability [3,4]. However, the reaction of methanol oxidations slow, involving the adsorption of methanol followed by succes-ive dehydrogenation steps, yielding intermediate species, mainlys CO and poisoning to the catalysts. So, it limits the performancef DMFCs to the commercial application [1,3]. Decreasing the poi-oning effect from intermediate and enhancing the efficiency ofatalyst are the crucial issues in the research of DMFCs. It is widelyccepted that the electrocatalytic efficiency is highly dependentn the shape and size of the Pt materials [1,3]. In order to achieveigher electrocatalytic activity and better precious-metal utiliza-ion, the construction of functional Pt nanostructures with designed

hapes and sizes is proven to be an effective strategy.

GNs is the most promising candidate as the supporting materialo load platinum nanoparticles owing to its high surface area,

∗ Corresponding author. Tel.: +86 591 22867399; fax: +86 591 22867399.E-mail address: shenlin@fjnu.edu.cn (S. Lin).

013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.12.174

© 2014 Elsevier Ltd. All rights reserved.

excellent mechanical and electrical properties [3,5,6]. Unfortu-nately, strong �-� stacking interaction between the GNs leads to itsirreversible aggregation, which will not only ruin the advantage ofhigh surface area of graphene, but also obstruct the dispersion of Ptparticles on GNs [7,8]. A particularly attractive option is to function-alize GNs by covalent or noncovalent methods in order to obtainaqueous dispersion graphene nanosheets (AD-GNs) [6,9]. Theavailability of AD-GNs not only drastically reinforces its interac-tion with the metal, helping to control the morphology of platinumafter reduction process, but also effectively avoids irreversibleaggregation of Pt/graphene hybrids. Luo et al. [10] functionalizedgraphene nanosheets with poly(diallyldimethylammonium chlo-ride (PDDA) to support unifromly distributed and highly densePt nanoparticles. Mayavan et al. [11] used poly sodium styrenesulfonate (PSS) as a stabilizer not only to prevent re-stacking ofreduced graphene sheets but also to bind Pt NPs intimately. We hadreported that the linear soluble polymer modified graphene basedmultilayer films were used as a novel matrix for electro-depositionof Pt particles in situ [4,12]. However, cyclic soluble polymerfunctionalized GNs to load Pt particles has been rarely reportedyet. And with the aid of cyclic soluble polymer, how to furtherconstruct the desirable Pt nanoparticles with unique structures on

GNs is still a critical challenge.

�-Cyclodextrins (�-CD) are oligosaccharides consisting ofseven glucose units and being toroidal in shape with a hydropho-bic inner cavity and a hydrophilic exterior [13]. These interesting

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16 Z. Li et al. / Electrochimi

haracteristics enable them to bind selectively various organic,norganic, and biological guest molecules into their cavities toorm stable host-guest inclusion complexes or nanostructureupramolecular assemblies in their hydrophobic cavity [14,15].he integration of GNs and �-CD (GNs-CD) may bring aboutarge surface area and high conductivity of GNs and uniqueupramolecular recognition capability of CDs together, whichrouses extensive research interest in the fields of biosensors andlectronics [9,14,16–19]. However, to the best of our knowledge,tilizing the interaction between GNs-CD and metal for load-

ng shape-controlled noble metallic nanoparticles has not beeneported hitherto.

Herein, we firstly used GNs-CD as a supporting material toonstruct Pt nano-clusters with unique morphologies. The inter-ction affinity of aqueous dispersion GNs-CD with platinumas responsible for the formation of so-called nanodendriticydrangea-like particles. Most importantly, it was found thathe as-prepared Pt/GNs-CD hybrid exhibited higher electrocat-lytic activities regarding methanol oxidation and better toleranceowards CO poisoning than Pt/GNs and Pt/Vulcan X72R. More-ver, the preparation method reported here was contributed to theanodendritic hydrangea-like Pt clusters growing directly on theurface of functionalized graphene nanosheets without an addi-ional surfactant or seeds. On the other hand, Pt/GNs-CD (NaBH4)as prepared using NaBH4 as a reductant instead of other poi-

onous reductants at room temperature in one-pot.

. Experimental

.1. Materials

Graphite powder (-325 mesh, 99.9995%) were purchased fromlfa Aesar. H2PtCl6 (99%), KMnO4, H2O2 (30%), K2S2O8, P2O5, �-CD,2SO4, NaBH4, ammonia, acetone, methanol, and ethanol were allurchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai,hina) and used without further purification. The water used in allxperiments was deionized to a resistivity of 18.25 M� cm.

.2. Characterization

TGA was carried out by using a TGA/SDTA851e Mettler toledohermogravimetric analyzer at a heating rate of 10 ◦C min−1 underir ambience from 40 to 800 ◦C. Ramam spectra were measuredsing a Renishaw INVIA Reflex Raman micro-spectrometer with

514 nm diode laser excitation on a 300 lines mm−1 gratingt room temperature. X-ray photoelectron spectroscopy (XPS)as performed at room temperature with monochromatic Al Ka

adiation (1486.6 eV) using a Quantum 2000 system (PHI, USA).ransmission electron microscopic (TEM) images, high-resolutionransmission electron microscopic (HRTEM) images and selectedrea electron diffraction (SAED) patterns were obtained with aECNAI G2 high-resolution transmission electron microscope (FEI,SA) operating at 200 k. X-ray diffraction (XRD) was carried outn an X’pert Pro diffractometer (Philips, USA), using Cu Ka radi-tion. Ultraviolet–visible absorption spectroscopy (UV–vis) wasecorded using a TU-1810DPC spectrophotometer (PUXI, Beijing,hina).

.3. Electrochemical measurements

Electrochemical experiments were carried out at the CHI 660 Clectrochemical workstation (ChenHua, Shanghai, China). A con-

entional three-electrode cell was used, including an Ag/AgClsaturated KCl) electrode as reference electrode, a platinum columns counter-electrode and the modified glassy carbon electrode asorking electrode. Prior to the surface coating, the glassy carbon

a 121 (2014) 215– 222

electrode was polished successively by alumina powder with 1.0,0.3, and 0.05 �m, respectively, and rinsed with deionized water,followed by sonication in acetone and doubly distilled water suc-cessively. Then the electrode was allowed to dry under nitrogenambience. 5 mg as-prepared hybrids were ultrasonically mixedin 1 mL ethanol to form homogeneous ink, followed by dropping5 �L of ink onto the surface of the glass carbon electrode (3 mmin diameter). Subsequently, 7.5 �L diluted 0.5% Nafion solutionwas dropped to fix the samples on the electrode. CVs were mea-sured at the potential range between -0.1 and 1.0 V in a mixingsolution containing 1 M CH3OH and 0.5 M H2SO4. The scan ratewas set at 100 mV s−1. Chronoamperometric curves were collectedfor 2500 s in a mixture of 1 M CH3OH and 0.5 M H2SO4 solutionat room temperature. The COad stripping voltammograms weremeasured by oxidation of preadsorbed CO in the 0.5 M H2SO4 solu-tion at a scan rate of 100 mV s−1. CO was purged into the 0.5 MH2SO4 solution for 30 min to allow the complete adsorption of COonto the catalyst when the working electrode was kept at 0.1 V.Excess CO in the electrolyte was then purged out with N2 for15 min. EIS measurements of the different hybrids were performedunder 0.5 V in 1 M CH3OH + 0.5 M H2SO4 solution. The Nyquistplots of EIS were obtained in the frequency range of 100 kHzto 0.1 Hz at 25 ◦C and the amplitude of modulation potentialwas 5 mV.

2.4. Synthesis of GNs-CD

GO nanosheets were synthesized from natural graphite by amodified Hummers’ method [20,21]. GNs-CD has been synthesizedby reduction of GO in the presence of �-CD in situ [9,18]. In atypical synthesis, 20.0 mL of the homogeneous graphene oxide dis-persion (0.5 mg mL−1) was mixed with 20.0 mL of 80 mg mL−1 �-CDaqueous solution and 300.0 �L of ammonia solution, followed bythe addition of 20 �L of hydrazine solution. After being vigorouslyshaken and stirred for a few minutes, the vial was put in a waterbath (60 ◦C) for 3.5 h. The stable black dispersive solution (GNs-CD)was obtained. Excess �-CD and hydrazine hydrate were removedby repeated centrifugation (10, 000 rpm, 20 min) and washing pro-cedures.

2.5. Synthesis of Pt/GNs-CD

The as-prepared GNs-CD was dispersed in de-ionized (DI) waterby ultrasonication, and then platinum precursor (PtCl62−) wasadded. The resulting mixture was sonicated in order to achieve thesufficient contact between GNs-CD and PtCl62−. Pt nano-clusterswere obtained by reduction with NaBH4 or HCOOH as a reduc-ing agent. The overall synthetic route of Pt/GNs-CD hybrids wasillustrated in Scheme 1. In a typical example, 50 mg GNs-CD wasdispersed in de-ionized water to form a stable dispersive solu-tion by ultrasonication for 60 min, then 6 mL of 19.3 mM H2PtCl6aqueous solution was added. After sonicating for anther 60 min,5 mL of NaBH4 aqueous solution (240 mM) was added to theresulting dispersion under magnetic stirring. Subsequently, themixed solution was stirred for 48 h at room temperature, and thenwas centrifuged to remove excess NaBH4. Finally, the as-obtainedPt/GNs-CD (NaBH4) were purified by repeated centrifugation (10,000 rpm, 20 min) and washing procedures. The hybrid was thenlyophilized for further characterization and application. Pt/GNs(NaBH4), Pt/Vulcan (NaBH4), and Pt/GNs-CD (HCOOH) were pre-

(GNs, Vulcan X72R, etc) or different reducing agent (HCOOH). Ptcontent in the hybrids was determined with TGA. The actual Ptloading was in the range of 28% to 31%, which was close to theinitial designed loading amounts.

Z. Li et al. / Electrochimica Acta 121 (2014) 215– 222 217

Scheme 1. Synthetic Route

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Fig. 1. Raman spectra of GNs (1), GNs-CD (2), Pt/GNs (3) and Pt/GNs-CD (4).

. Results and discussion

TGA analysis showed that the loading amount of CD moleculesn GNs was about 32 wt%. It was this big loading that made GNs-D be easy soluable and dispersive in water. The combination ofNs-CD and Pt was characterized by raman spectra. As shown

n Fig. 1, the D band (about 1313 cm−1) was assigned to defectsn the curved graphene sheet and staging disorder, while the Gand (about 1595 cm−1) was related to the graphitic hexagon-inch mode [22]. It was found that the position and intensity of

and G band for graphene did not change obviously after modif-ng with �-CD, and even loading Pt nanodendrite. This suggestedhat the functionalization of GNs and the reduction process of plat-num precursor (NaBH4 as a reductant) did not change the size of

Fig. 2. XPS profiles of Pt4f for Pt/

of Pt/GNs-CD Hybrids.

in-plane sp2 domains greatly and guarantee high electrical conduc-tivity of the graphene sheets [22,23]. Fig. 2 showed Pt 4f XPS profilefor Pt/GNs (NaBH4) and Pt/GNs-CD (NaBH4) (NaBH4 as a reduc-tant). The Pt4f7/2 and Pt4f5/2 binding energy was found at 71.36 eVand 74.69 eV for Pt/GNs-CD (NaBH4) and 71.19 eV and 74.56 eV forPt/GNs (NaBH4), indicating the presence of Pt (0) state [24]. Evi-dently, the binding energy of Pt 4f for Pt/GNs-CD (NaBH4) shiftedto higher value, suggesting a stronger interaction between the Ptparticles and GNs-CD supporting material [25].

Fig. 3 exhibited TEM and HRTEM images of Pt/GNs (NaBH4),Pt/GNs-CD (NaBH4), and Pt/GNs-CD (HCOOH) (HCOOH as a reduc-tant). As shown in Fig. 3(a) and 3(e), Pt nanoparticles supportedon the surface of GNs have a size of 3-5 nm. The defect sites of GNsmay be the reason for the formation of nonuniform Pt nanoparticlesafter quick reduction with NaBH4 [10]. By contrast, hydrangea-likePt nanoclusters were observed when GNs-CD was used as the sup-porting material and NaBH4 as a reductant (Fig. 3(b)). The size ofthese hydrangea-like nanoclusters was about 80 nm, showing a rel-atively uniform distribution. As indicated in Fig. 3(d), the highermagnification image displayed 3D hydrangea-like nanoclusters,whose surfaces were covered by a densely packed array of isolatedprimary nanoparticles (circled in red). HRTEM image focused on theedge of hydrangea-like nanoclusters showed distinct fringes corre-sponding to d spacing of 0.226 nm close to the expected d(111)value of 0.227 nm for fcc Pt (Fig. 3(f)). Furthermore, the primarynanoparticles with the sizes of 3 nm exhibited a disordered attach-ment with uncorrelated alignment of planes (circled in red ofFig. 3(f)). The selected-area electron diffraction (SAED) pattern ofPt/GNs-CD (NaBH4) showed four distinct diffraction rings standed

for the (111), (200), (220), and (311) crystalline planes (Fig. 3(h)),further confirming the highly crystalline feature of hydrangea-likePt nanoclusters. This result was in agreement with the XRD pat-tern of Pt/GNs-CD (NaBH4) (Fig. 4). The approach above presented a

GNs (a) and Pt/GNs-CD (b).

218 Z. Li et al. / Electrochimica Acta 121 (2014) 215– 222

Fig. 3. TEM and HRTEM images.(a), (e): Pt/GNs (NaBH4); (b), (d), (f): Pt/GNs-CD (NaBH4); (c), (g):Pt/GNs-CD (HCOOH); (h): SAED pattern of Pt/GNs-CD (NaBH4).

Z. Li et al. / Electrochimica Acta 121 (2014) 215– 222 219

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Fig. 4. XRD patterns of (a) Pt/GNs-CD (NaBH4) and (b) Pt/GNs (NaBH4).

reener and one-step strategy to obtain Pt nanodendritic morphol-gy, offsetting the weakness of traditional methods such as cappinggent aid method [26], seed-mediated growth approach [22] oromplicated galvanic replacement procedure [27]. In order to gainurther insight into the mechanism of nanodendritic formation,COOH was used as a reductant to prepare Pt/GNs-CD (HCOOH).s shown in Fig. 3(c), nanodendritic assemblies also appeared, withn incomplete and less organized aggregation. The nanoclustersainly consisted of nanorods, with its length and diameter of about

-20 nm and 3-4 nm, respectively. The HRTEM image in Fig. 3(g) dis-layed the crystallographic alignment of an isolated nanorod andhe lattice spacing between the (111) planes, 0.227 nm, being simlarith Pt/GNs-CD (NaBH4).

It was evident that the formation of nanodendritic structure didot depend on the reducants, and GNs-CD played an important role

n controlling the morphology of Pt nanoparticles. The aqueous dis-ersion GNs-CD offered a very open structure, and PtCl62− ionsere easily anchored to the cavity of GNs-CD by the enrichment

apability of �-CD. This can be proven by UV-Vis absorption spec-ra for PtCl62-, �-CD and �-CD + PtCl62− (Fig. 5). The absorbancet 260 nm was assigned to the electronic absorption from PtCl62−

ons [28], vanishing after the mixture of �-CD and PtCl62− wasonicated for 30 min. It suggested that the free PtCl62− ions wasaptured by �-CD. During the reduction process, the enrichmentf platinum precursor was in favor of seed nucleation, resulting in

he continuous formation of new nuclei. The freshly formed nucleiere thermodynamically unstable because of their high surface

nergy, and they tended to aggregate driven by the minimizationf interfacial energy [26]. GNs-CD regulated the final morphology

ig. 5. UV-Vis absorption spectra for �-CD (a), �-CD + PtCl62− after sonicating for0 min (b) and PtCl62-(c). The metal concentration is 2 mM.

Fig. 6. The CV curves of different hybrids in 0.5 M H2SO4 solution: Pt/Vulcan (NaBH4)(1), Pt/GNs (NaBH4) (2), Pt/GNs-CD (NaBH4) (3).

of nanoparticles by restraining their final agglomeration, whichresulted from hydrophobic-hydrophobic interactions between �-CD and Pt nanoparticles [13]. The strong hydrophobic interactionwould partly block the 2D surface diffusion and epitaxial match-ing, leading to an assembly of ordered aggregated nanoclustersrather than to an ordered oriented growth. In the whole process, thetwo dimensional GNs functioned as a matrix to load the nanoclus-ters, resulting in the relatively uniform distribution. In addition, theextremely slow rate for the weaker reduction of HCOOH favored Ptparticle growth, decreasing [Pt seed]/[Pt growth] ratio to form therelatively bigger nanorods.

Inspired by the attractive structure, Pt/GNs-CD hybrids weresubsequently tested as electrocatalysts for methanol oxidation. Thedifferent supporting materials (Vulan X72R, GNs and GNs-CD, ect)were used for comparison. Fig. 6 depicted the cyclic voltammetry(CV) curves of the different hybrids recorded at room temperaturein N2-purged H2SO4 solution (0.5 M) at a scan rate of 100 mV s−1.All the CV curves exhibited obvious peaks associated with hydrogenadsorption/desorption and Pt oxide formation/reduction. The elec-trochemical surface areas (ECSA) were calculated by the integratedcharge (QH) in the hydrogen adsorption region. According to theequation ECSA = QH/(210 mC cm−2 × Pt loading), the specific ECSAof Pt/GNs-CD (NaBH4) was 36.20 m2 g−1, which was higher thanthat of Pt/GNs (NaBH4) (22.52 m2 g−1), and was about 88% of that ofPt/Vulcan (NaBH4) (Vulan X72R as supporting materials and NaBH4

as a reductant) (Table 1). As indicated in Fig. 7, it was found themass activity of Pt/GNs-CD (NaBH4) (curve 4) was obviously higherthan that of Pt/Vulcan (NaBH4), Pt/GNs-CD (HCOOH), and Pt/GNs(NaBH4), respectively. The detailed electrochemical parameters of

Fig. 7. The CV curves of different hybrids in 1 M CH3OH + 0.5 M H2SO4 solution:Pt/Vulcan (NaBH4) (1), Pt/GNs-CD (HCOOH) (2), Pt/GNs (NaBH4) (3), Pt/GNs-CD(NaBH4) (4).

220 Z. Li et al. / Electrochimica Acta 121 (2014) 215– 222

Table 1Electrochemical parameters of as-prepared different hybrids.

Hybrids Onset potentiala

/VECSA/m2 g−1

Mass activity/mA mg−1

Specific activity/mA cm−2

Pt/Vulcan (NaBH4) 0.43 41.33 290 0.70Pt/GNs (NaBH4) 0.36 22.52 287 1.27Pt/GNs-CD (NaBH4) 0.35 36.20 478 1.32

a The onset potential is defined as the potential at which 10% of the current value at the peak potential was reached in this work.

Table 2Comparison of Pt/GNs-CD (NaBH4) with some similar Pt-based catalysts.

Catalysts Mass activity/mA mg−1

Morphologies of Pt Preparation conditions

Pt/GNs-CD(NaBH4) 478 hydrangea-like nanoclusters wet chemical procedure at room temperaturePt nanoclusters/GNs [3] 182 nanoclusters refluxing at high temperatureBPtNW/GNs [29] 299 branched nanowires wet chemical procedure at room temperature

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Pt@PDDA-GNs [10] 185 naPt NPs/functionalized GNs [5] 70 naPtRu-NP/CNT-PCA [30] 390 na

s-prepared hybrids obtained from the CV curves were listed inable 1. It was observed that the onset potential value for methanolxidation on Pt/GNs-CD (NaBH4) was 0.35 V, which was lowerhan that for Pt/GNs (NaBH4) and Pt/Vulcan (NaBH4). Lower onsetotential indicated superior electro-catalytic activity for methanolxidation. Furthermore, the specific activity of Pt/GNs-CD (NaBH4)1.32 mA cm−2) was nearly two times higher than that of Pt/VulcanNaBH4) (0.70 mA cm−2). It should be noted that the mass activity oft/GNs-CD (NaBH4) was also higher than those of recently reportedt graphene-based nanomaterials or CNT-supported Pt NPs. As indi-ated in Table 2, compared with Pt nanoclusters/GNs and branchedlatinum nanowires/GNs (BPtNW/GNs), Pt/GNs-CD (NaBH4) withhe similar Pt clusters structure exhibited notably higher activity.he reasons may be that the aqueous dispersive GNs-CD was help-ul for the dispersion of hybrid on electrode surface. In contrast withimetallic PtRu-NP/CNT-PCA, the as prepared Pt/GNs-CD (NaBH4)lso provided a cost benefit for the electrocatalyst. It is worthy ofoting that the preparation conditions for Pt/GNs-CD (NaBH4) areery facile. It was the facile one-pot strategy that nanodendriticydrangea-like Pt nanoparticles are obtained, in which �-CD onNs is used to regulate the final morphology of Pt nanoparticlesithout additional capping agent and seeds.

Chronoamperometry tests were carried out at 0.70 V for 2500 so assess the electrocatalytic stability of different hybrids. As shownn Fig. 8, the currents decayed over time in a parabolic style. Gen-

rally, the curves featured a sharper decrease during the earlierinutes, because of the formation of intermediate species (e.g.,

Oad) [31]. Afterward, the currents diminished much more slowlynd reached a quasi-stationary within 2500 s. The current decay

ig. 8. Chronoamperometric curves of different hybrids in 1 M CH3OH + 0.5 M H2SO4

olution: Pt/Vulcan (NaBH4) (1), Pt/GNs (NaBH4) (2), Pt/GNs-CD (NaBH4) (3).

ticles refluxing at high temperatureticles thermal treatment at high temperatureticles microwave-assisted reduction

percentages of Pt/GNs-CD (NaBH4), Pt/GNs (NaBH4) and Pt/Vulcan(NaBH4) were 70.9%, 72.5%, and 90.1%, respectively. The lowercurrent declining rate and obviously higher quasi-stationary cur-rent clearly indicated better electrocatalytic stability of Pt/GNs-CD(NaBH4) [32]. The COad stripping voltammograms were measuredby oxidation of preadsorbed and saturated CO in the 0.5 M H2SO4solution at a scan rate of 100 mV s−1 (Fig. 9). It was found that theonset potential and peak potential for COad oxidation on Pt/GNs-CD (NaBH4) (curve 3) are 0.58 and 0.66 V, respectively, showing anobviously negative shift compared with those on Pt/Vulcan (NaBH4)(curve 1) and Pt/GNs (NaBH4) (curve 2). This clearly confirmed thatCOad species could be more effectively oxidized to be removed onPt/GNs-CD (NaBH4), demostrating improved tolerance of CO [33].Fig. 10 displayed the Nyquist plots of EIS for Pt/Vulcan (NaBH4)and Pt/GNs-CD (NaBH4). Both electrodes exhibited a semicircle athigher frequency region. At high frequencies, the diameter of thesemicircle has been considered as the charge transfer resistancerepresenting the rate of charge exchange between ions in aque-ous and composite at electrochemical interface [34–36]. It couldbe seen that the diameter of the semicircle of Pt/GNs-CD (NaBH4)(curve 2) was much smaller than that of Pt/Vulcan (NaBH4) (curve1), suggesting that Pt/GNs-CD (NaBH4) has lower charge-transferresistance and faster reaction rate for the methanol oxidation reac-tion. This further confirmed that the functionalization of GNs still

good coherence with the CV curves of methanol oxidation.The higher catalytic activity and improved tolerance for CO poi-

soning of Pt/GNs-CD (NaBH4) can probably be attributed to three

Fig. 9. CO-stripping voltammograms of different hybrids in 0.5 M H2SO4 solution:.Pt/Vulcan (NaBH4) (1), Pt/GNs (NaBH4) (2), Pt/GNs-CD (NaBH4) (3).

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ajor factors: (i) The size of primary particles in hydrangea-liket nanoclusters was in proper scale (with the sizes of 3 nm), withigher surface-to-volume ratios and enough absorption sites; (ii) Ptanodendritic clusters were uniform in morphology and well dis-ersed on GNs-CD, and Pt/GNs-CD (NaBH4) can effectively avoided

rreversible aggregation due to the role of �-CD functionalization;iii) GNs-CD used as the catalyst support offered the superior elec-ric conductivities, helping to enhance electron transfer on thelectrode interface.

. Conclusion

In summary, a facile wet chemical procedure was used toynthesize Pt/GNs-CD nanodendrite hybrids. By the aid of com-ined effects of graphene nanosheets and �-cyclodextrin, GNs-CDffectively loaded nanodendritic hydrangea-like Pt clusters. Nan-dendrite Pt/GNs-CD (NaBH4) possessed obviously enhancedatalytic activities and better tolerance of CO towards methanolxidation owing to Pt nano-clusters with unique morphologies andts uniform distribution on GNs-CD support. Moreover, the efficentt utilization and the abundant and inexpensive �-CD provided additional cost benefit of the electrocatalysts. The present studyill be helpful to explore novel high performance catalysts forMFCs applications.

cknowledgments

This work was financially supported by the National Naturalcience Foundation of China (No.21171037), Research Fundationf the Education Department of Fujian Province (No.JA13082 &o.JB13010), and “333” Research Promotion Project of Fujian Nor-al University.

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