Journal of Colloid and Interface Science 433 (2014) 156–162

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science

www.elsevier .com/locate / jc is

A green one-pot synthesis of Pt/TiO2/Graphene composites and itselectro-photo-synergistic catalytic properties for methanol oxidation

http://dx.doi.org/10.1016/j.jcis.2014.06.0120021-9797/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author at: College of Chemistry & Chemical Engineering, FujianNormal University, Fuzhou 350007, China. Fax: +86 591 22867399.

E-mail address: shenlin@fjnu.edu.cn (S. Lin).

Lingting Ye a, Zhongshui Li a,b, Lian Zhang a, Fengling Lei a, Shen Lin a,b,⇑a College of Chemistry & Chemical Engineering, Fujian Normal University, Fuzhou 350007, Chinab Fujian Key Laboratory of Polymer Materials, Fuzhou 350007, Fujian, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 January 2014Accepted 6 June 2014Available online 20 June 2014

Keywords:GrapheneTitanium dioxideElectrocatalyticPhotoelectrocatalyticMethanol oxidation

A facile and green one-pot method was used to synthesize Pt/TiO2/Graphene composites with ethanol asa reducing agent under microwave irradiation. The as-prepared composites were characterized by SEM,TEM, EDX, XPS, XRD and Raman. Electrocatalytic performance of the Pt/TiO2/GNs composites was inves-tigated by cyclic voltammetry (CV), chronoamperometric (CA), COad stripping voltammetry and electro-chemical impedance spectrum (EIS). All experimental data have revealed that TiO2 (P25) not onlyenhanced the reduction ability of ethanol under microwave irradiation but also promoted Pt heteroge-neous nucleation to form Pt nanoclusters which are around P25 and loaded on graphene nanosheets(GNs) surface. Electrochemical experiments showed that Pt/TiO2/GNs had much higher catalytic activityand stability toward methanol oxidation reaction (MOR) and better resistance to CO poisoning comparedwith Pt/GNs and the commercially available Johnson Matthey 20% Pt/C catalyst (Pt/C-JM). Especiallyunder UV irradiation with 20 min, Pt/TiO2/GNs composites showed an ultrahigh forward peak currentdensity of 1354 mA mg�1, nearly 2.5 times higher than that of Pt/C-JM, which indicated that theelectrocatalytic and photocatalytic properties of Pt/TiO2/GNs had been integrated to boost the catalyticperformance for MOR.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

It is well known that Pt is the most effective catalyst for MOR[1]. However, the pure Pt catalyst is rather expensive and can berapidly poisoned by the adsorption of intermediate CO duringmethanol oxidation [2]. In this regard, binary and ternary Pt-basedalloys such as Pt–Pd [3], Pt–Zn [4], Pt–Ni [5], Pt–Cu [6], Pt–Ru [7],and Pt–Pd–Ru [8], and transition metal oxides, including TiO2

[9,10], MnO2 [11], SnO2 [12], CeO2 [13], and ZnO–TiO2 [14] havebeen studied to improve stability and CO tolerance for MOR.Among these metal oxides, TiO2 is of great interest for its charac-teristics such as low cost, environmental friendliness and high sta-bility in acidic environment. Moreover, TiO2 is a semiconductorwith a band gap of about 3.2 eV, and can use as an effective photo-catalyst under UV illumination [8]. However, the low electricconductivity prevents it from using in electrocatalysis field.Fortunately, TiO2 supported on carbon materials will improve itselectron conductivity and corrosion resistance so that TiO2/carbon

composites can be proposed as a potential catalyst support [15]. Inorder to make the TiO2/carbon composites act as an active anddurable support, it is crucial to construct a good interfacial contactbetween TiO2/carbon materials and Pt nanoparticles.

In recent years, the emergence of graphene with its interestingproperties such as high electrical conductivities, unique mechani-cal properties, and large specific surface areas, has opened a newavenue for utilizing two-dimensional carbon material as a promis-ing carrier for Pt and Pd [16]. However, the combination of graph-ene with Pt alone does not contribute to achieving the maximumutilization of metallic platinum [17]. In order to further maximizethe catalyst activity and minimize the amount of noble Pt, anotherefficient way is to disperse them onto functionalized graphene,which cannot only resolve the irreversible agglomerates betweengraphene layers, but also introduce the additional functionalcomponent to bring about the synergistic effect [18].

Herein, a facile and green one-pot method is used to synthesizePt/TiO2/Graphene composites, in which K2PtCl4 and GO undergo asynchronous reduction process with ethanol as a reducing agentunder microwave irradiation. For the first time, TiO2 (P25) wasused to directly anchor Pt nanoparticles onto graphene nanosheets(GNs) surface. It is worthwhile to say that the synergistic reduction

L. Ye et al. / Journal of Colloid and Interface Science 433 (2014) 156–162 157

mechanism is in favor of making Pt and P25 intimately contact,which helps to strengthen the synergistic effects of them. Andthe combination of metal oxide and graphene materials allowsthe optimization of both their dispersion and electrical conductiv-ity. Pt/TiO2/GNs composites have much higher catalytic activityand stability toward methanol oxidation and improved toleranceof CO compared to the Pt/GNs and Pt/C-JM. The interesting pointis that the methanol electrocatalytic oxidation and methanolphotocatalytic oxidation reactions are synergistically coupled onPt/TiO2/GNs under UV irradiation, resulting in a significantlyimproved activity and stability.

2. Experimental

2.1. Materials

Graphite powder (�325 mesh, 99.9995%) was purchased fromAlfa Aesar. Nano-TiO2 (P25), K2PtCl4 (99%), KMnO4, H2O2 (30%),K2S2O8, P2O5, H2SO4, methanol, and ethanol were all purchasedfrom Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) andused without further purification. The water used in all experi-ments was deionized to a resistivity of 18 MX cm.

2.2. Synthesis of Pt/TiO2/GNs

Graphene oxide (GO) were synthesized using a modified Hum-mers’ method [19,20]. In brief, graphite powders were reacted withconcentrated H2SO4, K2S2O4, P2O5 and KMnO4. Then H2O2 wasadded to the mixture after completion of the reaction. The productwas washed and centrifuged with dilute hydrochloric acid and dis-tilled water several times. After that, the precipitate was lyophi-lized. Pt/TiO2/GNs are prepared via a green synchronousreduction process, in which ethanol was used as a reducing agentunder microwave irradiation. The overall synthetic route was illus-trated in Scheme 1(a). Typically, 50 mg of GO were dispersed in50 mL of distilled water by sonication for 30 min. Then, 10 mg ofTiO2 were added, and the mixture was sonicated for another30 min to form homogeneous dispersion. After that, the mixturewas transferred into a round bottom flask, then 5 mL of K2PtCl4

(1 g/100 mL) and 2 mL of C2H5OH were added. The mixture wasput into a microwave reactor and treated with microwave irradia-tion power of 240 W. Subsequently, the product was cooled toroom temperature and purified by repeated centrifugation(10,000 rpm, 20 min) and washing cycles. Finally, the black precip-itate was lyophilized, and Pt/TiO2/GNs were collected. For compar-ison, Pt/GNs were prepared by similar procedure without P25. The

Scheme 1. The synthetic route of Pt/TiO2/GNs composites (a)

actual Pt loadings of the Pt/TiO2/GNs and Pt/GNs were determinedto be about 19.7 and 18.5 wt%, respectively.

2.3. Characterization

Field emission scanning electron microscopy (FESEM) imageswere performed on a JSM-7500F field emission scanning electronmicroanalyzer (JEOL, Japan). Transmission electron microscopic(TEM) images, high-resolution transmission electron microscopic(HRTEM) images and selected area electron diffraction (SAED) pat-terns were obtained with a TECNAI G2 high-resolution transmis-sion electron microscope (FEI, USA) operating at 200 k. X-rayphotoelectron spectroscopy (XPS) was performed with monochro-matic Al Ka radiation (1486.6 eV) using a Quantum 2000 system(PHI, USA). X-ray diffraction (XRD) patterns were carried out onan X’pert Pro diffractometer (Philips, USA), using Cu Ka radiationand a scanning rate 5� min�1. Raman spectra were measured usinga Renishaw-in-via Raman micro-spectrometer equipped with a514 nm diode laser excitation on a 300 lines mm�1 grating. Theactual amount of Pt loadings of the catalysts was determined byinductively coupled plasma-mass spectroscopy (ICP-MS, X Series2, Thermo Scientific USA).

2.4. Electrochemical measurements

Electrochemical experiments were carried out in a standardthree-electrode cell using a CHI 660C electrochemical workstation(ChenHua, Shanghai, China), including an Ag |AgCl (3 M KCl) elec-trode, a platinum column and a glassy carbon electrode (GCE). In atypical procedure, 5 mg as-prepared composites was ultrasonicallymixed in 1 mL ethanol to form homogeneous ink, followed bypipetting 5 lL of ink onto the pre-cleaned glassy carbon disk(3 mm in diameter). Subsequently, 7.5 lL diluted 0.5% Nafion solu-tion was pipetted to fix the composites on the electrode. Cyclic vol-tammetry (CV) was recorded in 0.5 M H2SO4 solution with orwithout 1.0 M CH3OH at room temperature with a scan rate of100 mV s�1. The CO stripping voltammograms were measured byoxidation of preadsorbed CO (COad) in the 0.5 M H2SO4 solutionat a scan rate of 100 mV s�1. CO was bubbled for 30 min to allowthe complete adsorption of CO onto the composites when theworking electrode was kept at 0.1 V. Excess CO in the electrolytewas then purged out with N2 for 15 min. Chronoamperometric(CA) curves were collected for 2000s at 0.67 V in a mixture of1 M CH3OH and 0.5 M H2SO4 solution. The electrochemical imped-ance spectra (EIS) were carried out under 0.5 V in a mixture of 1 MCH3OH and 0.5 M H2SO4 solution, and the Nyquist plots wereobtained in the frequency range of 100 kHz to 0.1 Hz.

and schematic representation of Pt/TiO2/GNs function (b).

158 L. Ye et al. / Journal of Colloid and Interface Science 433 (2014) 156–162

2.5. Photo-electrochemical measurements

In the electrochemical setup described above, a quartz windowwas introduced at the bottom of the electrochemical cell, facingtoward the GCE. A 15 W UV lamp (ZW3D15W-Z105, Cnlight,China) was employed to provide the UV light which can penetratethrough the quartz window and illuminate the working electrode.The used lamp emits shortwave UV radiation with the wavelengthsranging from 250 to 270 nm. The distance between the lamp andGCE is 15 cm, and the irradiation intensity is 0.3 mW cm�2.

3. Results and discussion

3.1. Characterization of Pt/TiO2/GNs

SEM was used to survey the difference in the surface morphol-ogy between Pt/TiO2/GNs and Pt/GNs. As shown in Fig. 1a, Pt nano-particles are found to be sparsely dispersed on the surface of GNs.In comparison, the dense clusters composed of Pt and P25 can beobserved (Fig. 1b), and it is difficult to distinguish Pt from P25due to their full integration. The morphology of the Pt/TiO2/GNsand Pt/GNs were further investigated by TEM and HRTEM. Fig. 2ashows the TEM image of the Pt/GNs, revealing embryonic Ptnanoclusters apart from each other. By contrast, Pt nanoclustersare obviously formed around P25 in Pt/TiO2/GNs (Fig. 2b). It is evi-dent that P25 offers distributed active sites for anchoring Pt nano-particles. Fig. 2c shows HRTEM image of a Pt multipod for Pt/GNs.Apparently, (111) lattice fringes (0.225 nm) are observed, suggest-ing that embryonic Pt nanoclusters for Pt/GNs are in good crystal-linity. HRTEM image of Pt/TiO2/GNs presents the lattice fringe withinterplanar spacing of 0.196 and 0.225 nm, corresponding to (200)and (111) planes of face-centered-cubic (fcc) Pt (Fig. 2d). Theselected-area electron diffraction (SAED) (the inset of Fig. 2d) indi-cates that the Pt nanoparticles are typically polycrystalline struc-tures. The Pt lattice fringes (0.225 nm) are close to the anataseTiO2 lattice fringes (0.351 nm) corresponding to its (101) plane,indicating that the isolated primary Pt nanoparticles with thediameter of about 3 nm (red circle in Fig. 2d) are near P25.Evidently, the presence of P25 promotes the heterogeneous nucle-ation of Pt nanoparticles to forming the cluster structure, and theisolated primary Pt nanoparticles may be with higher surface-to-volume ratios and possess more enough absorption sites than Ptmultipod. In addition, the selected field was used to carry outEDS analysis (red circle in Fig. 2b). The peaks corresponding to C,O, Ti and Pt elements are found in Fig. 2e, further confirming theintimate attachment between P25 and Pt on GNs.

X-ray photoelectron spectroscopy (XPS) is employed to furthercharacterize the chemical states of different elements in the com-posites. Pt 4f spectra of Pt/TiO2/GNs and Pt/GNs composites areshown in Fig. 3a and b. The two peaks at 74.52 eV, 71.17 eV for

Fig. 1. Typical SEM images of Pt/

Pt/TiO2/GNs and at 74.65 eV, 71.25 eV for Pt/GNs are consistentwith the binding energies of Pt 4f5/2 and Pt 4f7/2, respectively, indi-cating the presence of Pt (0) state [21–23]. Evidently, the bindingenergy of Pt 4f for Pt/TiO2/GNs shifts to a slightly lower value,due to the addition of TiO2. The shift of Pt 4f binding energy forPt/TiO2/GNs indicates the strong metal-support interaction (SMSI)between Pt and TiO2 [24]. As shown in Fig. 3c two characteristicpeaks assigned to Ti 2p (Ti 2p1/2 at 465.5 eV and Ti 2p3/2 at459.8 eV) for Pt/TiO2/GNs are observed [25].

The similar results can be found in XRD patterns for Pt/GNs andPt/TiO2/GNs (Fig. 4). As shown in curve a of Fig. 4, the peak for(101) plane of anatase TiO2 is found at the 2h value of 25.3� [26].The representative diffraction peaks for the face-centered cubiclattice of Pt (0), namely (111), (200), (220), (311) and (222)planes, are observed at 39.94�, 46.38�, 67.74�, 81.55� and 86.08�,respectively [27]. The XRD patterns are in agreement with theSAED (the inset of Fig. 2d). Compared with the 2h value of39.89�, 46.30�, 67.71�, 81.54� and 86.05� in curve b of Fig. 4 (Pt/GNs), they are slightly shifted to higher angles, which can beattributed to intimate attachment of Pt nanoclusters to the surfaceof P25.

The results from SEM, TEM, XPS and XRD analysis show that theone-pot synthesis of Pt/TiO2/GNs make Pt and GNs intimately con-tact, and P25 may play an important role in the synchronousreduction of platinum precursor and GO with ethanol as a reduc-tant under microwave irradiation. This can be proven by the reac-tion time used to complete the synthesis of different composites.The reaction time to synthesize Pt/TiO2/GNs is 100 s, which is obvi-ously shorter than the time used to synthesize Pt/GNs (150 s). Thissuggests that the introduction of P25 will accelerate the reactionprocess, which may be ascribed to the synergistic aiding reductionmechanism between ethanol and P25 under microwave irradia-tion. The polarization effect of P25 and the thermal effect ofethanol in microwave electromagnetic field are integrated toenhance ethanol reaction activity on the surface of TiO2.

The successful integration of GNs, P25 and Pt is also character-ized by Raman spectra. As shown in Fig. 5, the two peaks appearingat around 1320 and 1600 cm�1 correspond to the structural defectsin the graphitic plane (D-band) and the E2g vibrational mode pres-ent in the sp2 bonded graphitic carbons (G-band), respectively [28].The D/G intensity ratio is inversely proportional to the average sizeof the sp2 domains. As for Pt/TiO2/GNs and Pt/GNs, the increased D/G intensity ratio suggests a decrease in-plane sp2 domains arebecoming during the reduction of GO [29]. Compared with Pt/GNs, the position and intensity of D and G band of graphene forPt/TiO2/GNs do not change obviously after combination with P25and Pt nanoclusters. It suggests that the introduction of P25 intoGNs does not change the size of in-plane sp2 domains greatlyand guarantee high electrical conductivity of the graphene sheets[30,31].

GNs (a) and Pt/TiO2/GNs (b).

Fig. 2. Typical TEM images of Pt/GNs (a) and Pt/TiO2/GNs (b); HRTEM image of Pt/GNs (c) and Pt/TiO2/GNs (d); EDX analysis of Pt/TiO2/GNs (e); SAED in (d) inset.

L. Ye et al. / Journal of Colloid and Interface Science 433 (2014) 156–162 159

3.2. Electrochemical behavior

CV curves of Pt/TiO2/GNs, Pt/GNs and Pt/C-JM in 0.5 M H2SO4

solution is collected to evaluate the electrochemical surface areas(ECSA) in Fig. 6. All the composites show characteristic hydrogenadsorption–desorption peaks in both forward and backward scans,suggesting the presence of electrochemically active Pt. The ECSAwere calculated by the integrated charge (QH) in the hydrogenadsorption region. According to the equation ECSA = QH/(210 lC cm�2 � Pt loading), the ECSA of Pt/TiO2/GNs is96.7 m2 g�1, which is significantly higher than that of Pt/GNs

(46.5 m2 g�1) and that of Pt/C-JM (48.9 m2 g�1). Also, the value ishigher than other Pt-based catalysts, including Pt/Graphene(27.84 m2 g�1) [32], Pt-7% CeO2/GN (66.4 m2 g�1) [13], Pt-WO3/C(89 m2 g�1) [33], Pt/C + 20 wt% TiO2 (61.2 m2 g�1) [2], core–shellPt-TiO2/C (84.9 m2 g�1) [15]. This indicates that the intimatelycombination of P25 and GNs allows the optimization of both theirdispersion and electrical conductivity, accordingly, increases theECSA of Pt in Pt/TiO2/GNs.

Subsequently, to further survey the catalytic activities of thetitled composites toward MOR, CV curves in 1 M CH3OH + 0.5 MH2SO4 solution are carried out. Fig. 7 presents the CV curves of

Fig. 3. Pt 4f XPS profile for Pt/TiO2/GNs (a) and Pt/GNs (b) and Ti 2p XPS spectrum of Pt/TiO2/GNs (c).

Fig. 4. XRD patterns of Pt/TiO2/GNs (a) and Pt/GNs (b).

Fig. 5. Raman spectra of GO (a), Pt/GNs (b) and Pt/TiO2/GNs (c).

Fig. 6. CV curves of Pt/TiO2/GNs (a), Pt/GNs (b) and Pt/C-JM (c) in the N2 saturatedsolution of 0.5 M H2SO4 at 100 mV s�1.

160 L. Ye et al. / Journal of Colloid and Interface Science 433 (2014) 156–162

Pt/TiO2/GNs, Pt/GNs and Pt/C-JM during MOR process under UVirradiation at 5 min interval. It can be perceived that the forwardpeak current density reaches 820 mA mg�1 on the Pt/TiO2/GNswithout UV irradiation (t = 0 min). The value is higher thanthat of Pt/GNs, Pt/C-JM and other Pt/TiO2-based catalysts,including Pt/TiO2-C (106.8 mA mg�1) [24], Pt/TiO2@N-doped

C-900 (490 mA mg�1) [10], Pt/C + 20 wt% TiO2 (647.6 mA mg�1)[2]. It clearly reveals that TiO2 provides a promotion effect on theelectrooxidation of methanol, which results from the coactionamong the Pt nanoclusters, P25 and GNs [25]. Most importantly,the forward peak current density for Pt/TiO2/GNs increases accord-ingly with the increase of UV irradiation time (from 0 to 20 min).After UV irradiation for 20 min, Pt/TiO2/GNs composites showremarkably increased forward peak current density with an ultra-high value of 1360 mA mg�1, nearly 2.5 times higher than that ofPt/C-JM. However, only slight increase of current density for Pt/GNs and Pt/C-JM is observed. Evidently, the significant increasesin methanol oxidation current density can be attributed to the syn-ergistic effect between the electro-catalytic and photo-catalyticproperties of Pt/TiO2/GNs [15]. TiO2, decorated with Pt nanoparti-cles, greatly improves its photocatalytic performance. It is worth-while to say that the intimately connected structure of Pt/TiO2/GNs is helpful for this synergistic effect. With UV illumination,GNs acts as the electron acceptor to extract photogeneratedelectrons from TiO2 and transfer to Pt particles (Scheme 1(b)),thus resulting in increasing charge separation efficiency withinthis oxide semiconductor-GNs system, which limits self-photoreduction process and simultaneously releases more Pt activesites [34]. With this, the electro-catalytic and photo-catalytic prop-erties of Pt/TiO2/GNs are found to be significantly improved.

In addition, the ratio of the forward peak current density (If)to the reverse peak current density (Ib), If/Ib, can be used to

Fig. 7. CV curves under UV irradiation at 5-min interval of Pt/TiO2/GNs (a), Pt/GNs (b) and Pt/C-JM (c) in the N2 saturated solution of 1 M CH3OH + 0.5 M H2SO4 at 100 mV s�1.

Table 1Electrocatalytic parameters of different composites.

Composites Pt/TiO2/GNs Pt/GNs Pt/C-JM

If (mA mg�1) 1354 930 540Ib (mA mg�1) 1325 946 649If/Ib 1.02 0.98 0.83On potential of CO oxidation a (V) 0.45 0.48 0.50

a The onset potential of CO oxidation is defined as the potential at which 20% ofthe current value at the peak potential was reached in CO stripping curves.

Fig. 8. CO stripping curves of Pt/TiO2/GNs (a), Pt/GNs (b) and Pt/C-JM (c) in the N2

saturated solution of 0.5 M H2SO4 at 100 mV s�1.

Fig. 9. Chronoamperometry curves of Pt/TiO2/GNs (a), Pt/GNs (b), Pt/C-JM (c) andPt/TiO2/GNs under durative UV irradiation (d) in the N2 saturated solution of 1 MCH3OH + 0.5 M H2SO4 at 0.67 V for 2000s.

Fig. 10. Nyquist plots of EIS for MOR in the N2 saturated solution of 1 MCH3OH + 0.5 M H2SO4 at 0.5 V in the frequency range of 100 kHz to 0.1 Hz: Pt/TiO2/GNs (a), Pt/GNs (b) and Pt/C-JM (c).

L. Ye et al. / Journal of Colloid and Interface Science 433 (2014) 156–162 161

investigate the catalyst tolerance to the intermediate carbonaceousspecies (such as COad) accumulation [13,35]. As listed in Table 1,the If/Ib ratio of Pt/TiO2/GNs is 1.02 after UV irradiation for20 min, which is higher than that of Pt/GNs (0.98) and Pt/C-JM(0.83), respectively, implying more valid removal of the poisoningspecies (such as COad) from the Pt/TiO2/GNs surface. It is specu-lated that the added TiO2 on graphene sheets can offer abundanthydroxyl sources for oxidation of intermediate carbonaceous spe-cies at lower potentials, alleviating CO poisoning toward Pt parti-cles [2,17,25]. The improved tolerance of CO is also be verified byCOad stripping voltammetry presented in Fig. 8. The peak potentialof CO oxidation was 0.59 V for Pt/TiO2/GNs, but 0.62 and 0.64 V forPt/GNs and Pt/C-JM, respectively. Furthermore, the onset potentialof CO oxidation on Pt/TiO2/GNs is also lower (0.45 V) than that onPt/GNs and Pt/C-JM, as shown in Table 1. The negative shift of

onset potential and peak potential for the Pt/TiO2/GNs indicatesthat the CO species on the Pt/TiO2/GNs interfaces are more easilytransformed to CO2 due to the oxidation effect of TiO2. So the activesites on Pt are released for further electrochemical reaction [2].

162 L. Ye et al. / Journal of Colloid and Interface Science 433 (2014) 156–162

CA measurement is used to evaluate the electrocatalytic stabil-ity of different composites. After finishing 20 min UV irradiation,the CA curves of Pt/TiO2/GNs, Pt/GNs and Pt/C-JM are measuredin 1 M CH3OH + 0.5 M H2SO4 at 0.67 V for 2000 s. As shown inFig. 9, an initial rapid current density decay is observed, due tothe formation of some intermediate species (mainly COads) duringthe methanol oxidation reaction [36]. Throughout the measure-ment, although the current density continues to decay gradually,Pt/TiO2/GNs composites (curve a) shows a lower declining rate incompare with the Pt/GNs (curve b) and Pt/C-JM (curve c). It is evi-dent that the Pt/TiO2/GNs are a better catalyst for methanol oxida-tion due to both superior catalytic activity and stability. Especially,the current density declining rate is much slower under durativeUV irradiation (curve d), further affirming that the introductionof P25 are helpful for the catalyst stability and the tolerance ofCO due to the synergistic effects between the electro-catalyticand photo-catalytic properties. EIS was used to further investigatethe intrinsic behavior of the anodic process. The Nyquist plots ofEIS for Pt/TiO2/GNs, Pt/GNs and Pt/C-JM in 1 M CH3OH + 0.5 MH2SO4 at 0.5 V are shown in Fig. 10. The diameter of the primarysemicircle can be used to analyze the charge transfer resistanceof the catalyst, describing the rate of charge transfer during themethanol oxidation reaction [37]. The semicircle radius on theNyquist plots of EIS for Pt/TiO2/GNs are much smaller than thatof Pt/GNs and Pt/C-JM, clearly authenticating that the incorpora-tion of TiO2 results in the improved conductivity of Pt/TiO2/GNs.It was reported that high Pt utilization and good electron conduc-tivity can be obtained when a certain amount of TiO2 nanoparticleswith proper size was added evenly [2]. Herein, the well-interfacialcontact between TiO2-GNs and Pt gives benefits for improving theconductivity of Pt/TiO2/GNs. EIS analysis has good coherence withthe CV curves for MOR on Pt/TiO2/GNs, Pt/GNs and Pt/C-JM.

4. Conclusions

In conclusion, a facile and green synchronous reduction methodwas used to synthesize Pt/TiO2/GNs composites and the abundantand nontoxic P25 provide an additional cost benefit of the catalysts.With the aid of the synergistic reduction mechanism, unique Ptnanoclusters around P25 on GNs were obtained. The well-orga-nized structure among Pt, P25 and GNs and the alleviating CO poi-soning role of P25 is responsible for the remarkable improvement ofelectrocatalytic activities and stability for MOR. Interestingly, underUV irradiation for 20 min, Pt/TiO2/GNs show an ultrahigh catalyticactivity (the forward peak current density of 1354 mA mg�1) andstability for MOR, which may be attributed to the synergistic effectbetween the electro-catalytic and photo-catalytic properties. Thestudy above will be of importance for the fundamental understand-ing of photo-electrocatalysis and helpful to the searching of highperformance catalysts for methanol oxidation.

Acknowledgments

This work was financially supported by the National NaturalScience Foundation of China (No. 21171037) and Research Founda-tion of the Education Department of Fujian Province (Nos. JA13082and JB13010).

References

[1] S. Yin, L. Luo, C. Xu, Y. Zhao, Y. Qiang, S. Mu, J. Power Sources 198 (2012) 1.[2] L. Yu, J. Xi, Electrochim. Acta 67 (2012) 166.[3] Y. Lu, Y. Jiang, H. Wu, W. Chen, J. Phys. Chem. C 117 (2013) 2926.[4] Y. Kang, J.B. Pyo, X. Ye, T.R. Gordon, C.B. Murray, ACS Nano 6 (2012) 5642.[5] L. Tamasauskaite-Tamasiunaite, A. Balciunaite, A. Vaiciukeviciene, A. Selskis, E.

Norkus, J. Power Sources 225 (2013) 20.[6] M. Ammam, E.B. Easton, J. Power Sources 222 (2013) 79.[7] T. Maiyalagan, J. Solid State Electrochem. 13 (2009) 1561.[8] H. Zhang, W. Zhou, Y. Du, P. Yang, C. Wang, J. Xu, Int. J. Hydrogen Energy 35

(2010) 13290.[9] Y.-N. Wu, S.-J. Liao, H.-F. Guo, X.-Y. Hao, J. Power Sources 224 (2013) 66.

[10] X. Zhao, J. Zhu, L. Liang, J. Liao, C. Liu, W. Xing, J. Mater. Chem. 22 (2012) 19718.[11] R. Liu, H. Zhou, J. Liu, Y. Yao, Z. Huang, C. Fu, Y. Kuang, Electrochem. Commun.

26 (2013) 63.[12] D. Wang, X. Li, J. Wang, J. Yang, D. Geng, R. Li, M. Cai, T.-K. Sham, X. Sun, J. Phys.

Chem. C 116 (2012) 22149.[13] S. Yu, Q. Liu, W. Yang, K. Han, Z. Wang, H. Zhu, Electrochim. Acta 94 (2013) 245.[14] V. Manthina, J.P.C. Baena, G. Liu, A.G. Agrios, J. Phys. Chem. C 116 (2012)

23864.[15] W. Li, Y. Bai, F. Li, C. Liu, K.-Y. Chan, X. Feng, X. Lu, J. Mater. Chem. 22 (2012)

4025.[16] L. Gao, W. Yue, S. Tao, L. Fan, Langmuir 29 (2013) 957.[17] H. Huang, Q. Chen, M. He, X. Sun, X. Wang, J. Power Sources 239 (2013) 189.[18] Z. Li, X. Huang, X. Zhang, L. Zhang, S. Lin, J. Mater. Chem. 22 (2012) 23602.[19] N.I. Kovtyukhova, P.J. Ollivier, B.R. Martin, T.E. Mallouk, S.A. Chizhik, E.V.

Buzaneva, A.D. Gorchinskiy, Chem. Mater. 11 (1999) 771.[20] Q. Zeng, J. Cheng, L. Tang, X. Liu, Y. Liu, J. Li, J. Jiang, Adv. Funct. Mater. 20

(2010) 3366.[21] P. Kundu, C. Nethravathi, P.A. Deshpande, M. Rajamathi, G. Madras, N.

Ravishankar, Chem. Mater. 23 (2011) 2772.[22] P. Wang, F. Li, X. Huang, Y. Li, L. Wang, Electrochem. Commun. 10 (2008) 195.[23] Z.-S. Li, S. Lin, Z.-L. Chen, Y.-D. Shi, X.-M. Huang, J. Colloid Interface Sci. 368

(2012) 413.[24] Q. Lv, M. Yin, X. Zhao, C. Li, C. Liu, W. Xing, J. Power Sources 218 (2012) 93.[25] B.Y. Xia, B. Wang, H.B. Wu, Z. Liu, X. Wang, X.W. Lou, J. Mater. Chem. 22 (2012)

16499.[26] L. Liu, Q. Fan, C. Sun, X. Gu, H. Li, F. Gao, Y. Chen, L. Dong, J. Power Sources 221

(2013) 141.[27] X. Huang, Z. Li, X. Zhang, X. He, S. Lin, J. Colloid Interface Sci. 393 (2013) 300.[28] Q. Guo, Z. Zheng, H. Gao, J. Ma, X. Qin, J. Power Sources 240 (2013) 149.[29] R. Liu, S. Li, X. Yu, G. Zhang, S. Zhang, J. Yao, L. Zhi, J. Mater. Chem. 22 (2012)

3319.[30] S. Guo, S. Dong, E. Wang, ACS Nano 4 (2009) 547.[31] J.-J. Shi, G.-H. Yang, J.-J. Zhu, J. Mater. Chem. 21 (2011) 7343.[32] X. Liu, J. Duan, H. Chen, Y. Zhang, X. Zhang, Microelectron. Eng. 110 (2013) 354.[33] C. Yang, N.K. van der Laak, K.-Y. Chan, X. Zhang, Electrochim. Acta 75 (2012)

262.[34] P.D. Tran, S.K. Batabyal, S.S. Pramana, J. Barber, L.H. Wong, S.C.J. Loo, Nanoscale

4 (2012) 3875.[35] B. Luo, X. Yan, S. Xu, Q. Xue, Electrochim. Acta 59 (2012) 429.[36] M. de Dios, V. Salgueirino, M. Perez-Lorenzo, M.A. Correa-Duarte, J. Chem.

Educ. 89 (2012) 280.[37] J.J. Wang, G.P. Yin, J. Zhang, Z.B. Wang, Y.Z. Gao, Electrochim. Acta 52 (2007)

7042.