Facile Synthesis of Bimetallic Pt-ag/graphene Composite and Its Electro-photo-synergistic Catalytic Properties for Methanol Oxidation

A Pt-Ag/graphene composite (Pt-Ag/GNs) was synthesized by the facile aqueous solution method, in which Ag + was first transformed into Ag 2 O under UV light irradiation, and then Ag 2 O, Pt 2+ , and graphene oxide (GO) were simultaneously reduced by formic acid. It was found that Pt-Ag bimetallic nanoparticles were highly dispersed on the surface of graphene, and their size distribution was narrow with an average diameter of 3.3 nm. Electrocatalytic properties of the Pt-Ag/GNs composite were investigated by cyclic voltammograms (CVs), chronoamperometry (CA), CO-stripping voltammograms, and electrochemical impedance spectrum (EIS) techniques. It was shown that the Pt-Ag/GNs composite has much higher catalytic activity and stability for the methanol oxidation reaction (MOR) and better tolerance toward CO poisoning when compared with Pt/GNs and the commercially available Johnson Matthey 20% Pt/C catalyst (Pt/C-JM). Furthermore, the Pt-Ag/GNs composite showed efficient electro-photo-synergistic catalysis for MOR under UV or visible light irradiation. Particularly in the presence of UV irradiation, the Pt-Ag/GNs composite exhibited an ultrahigh mass activity of 1842.4 mA·mg −1 , nearly 2.0 times higher than that without light irradiation (838.3 mA·mg −1).


Introduction
Platinum has attracted significant attention as a catalyst for the methanol oxidation reaction (MOR) [1].However, the monometal Pt catalyst is not only expensive, but also easily poisoned by the adsorption of CO-like intermediate species, reducing the electrocatalytic activity of the catalyst during methanol oxidation [2].Binary or ternary Pt-based catalysts containing other transition metals, such as NiAuPt [3], PtRu [4], PtCu [5], FePt [6], PtSn [7], PtCo [8], PtRhNi [9], or PtPd [10] can evidently improve the performance in catalyzing MOR compared with the monometallic Pt catalysts.Particularly, they can alleviate CO ads poisoning and expose more Pt active sites, resulting in lower noble metal dosages and lower costs.Among the transition metals, Ag can provide rich surface oxygen-containing species to promote the oxidation of CO-like species and prevent the catalyst from poisoning [11].Ag has a high degree of free electron mobility and exhibits molecular-like excited-state properties with well-defined absorption and emission features, and its surface plasmon resonance (SPR) effect has a prominent contribution to the enhanced photocatalytic activity [12,13].Therefore, Pt-Ag bimetallic nano-structures are expected to provide an efficient synergistic effect between the electro-catalytic and photo-catalytic properties for MOR.
The unique properties of graphene nanosheets (GNs), such as larger specific surface areas, excellent optical transmittance, high chemical and thermal stability, and outstanding electrical conductibility, make it a superior support material for Pt-based catalysts [14,15].Referring to our previously reported catalysts, such as Pt/SnO 2 /GNs [16] and Pt/TiO 2 /GNs [17], it is known that GNs can not only improve the dispersion of Pt nanoparticles, but also can act as the acceptor and the transfer body of the photo-electron, giving rise to charge transfer and separation efficiency of the photo-generated electron-hole pairs which leads to a better photocatalytic activity.
Herein, we explored a facile method to synthesize Pt-Ag/graphene composite (Pt-Ag/GNs) with the use of formic acid as a reducing agent (Scheme 1).In the synthetic process, HCOOH has the advantages of simple operation, environmental friendliness, and mild reaction conditions, and the Ag 2 O intermediate species formed under UV light irradiation contribute to the good dispersion of the bimetallic Pt-Ag nanoparticles on GNs with narrow size distribution.In comparison with commercially available Johnson Matthey 20% Pt/C (denoted as Pt/C-JM) and Pt/GNs, the as-synthesized Pt-Ag/GNs exhibit much higher catalytic activity and stability for MOR and better resistance to CO poisoning.More importantly, with the aid of the integrated photo-response of Ag and GNs, methanol electro-catalytic oxidation and photo-catalytic oxidation reactions are synergistically coupled on Pt-Ag/GNs under UV irradiation or visible light irradiation, bringing about increased catalytic activity and stability.
Therefore, Pt-Ag bimetallic nano-structures are expected to provide an efficient synergistic effect between the electro-catalytic and photo-catalytic properties for MOR.
The unique properties of graphene nanosheets (GNs), such as larger specific surface areas, excellent optical transmittance, high chemical and thermal stability, and outstanding electrical conductibility, make it a superior support material for Pt-based catalysts [14,15].Referring to our previously reported catalysts, such as Pt/SnO2/GNs [16] and Pt/TiO2/GNs [17], it is known that GNs can not only improve the dispersion of Pt nanoparticles, but also can act as the acceptor and the transfer body of the photo-electron, giving rise to charge transfer and separation efficiency of the photo-generated electron-hole pairs which leads to a better photocatalytic activity.
Herein, we explored a facile method to synthesize Pt-Ag/graphene composite (Pt-Ag/GNs) with the use of formic acid as a reducing agent (Scheme 1).In the synthetic process, HCOOH has the advantages of simple operation, environmental friendliness, and mild reaction conditions, and the Ag2O intermediate species formed under UV light irradiation contribute to the good dispersion of the bimetallic Pt-Ag nanoparticles on GNs with narrow size distribution.In comparison with commercially available Johnson Matthey 20% Pt/C (denoted as Pt/C-JM) and Pt/GNs, the assynthesized Pt-Ag/GNs exhibit much higher catalytic activity and stability for MOR and better resistance to CO poisoning.More importantly, with the aid of the integrated photo-response of Ag and GNs, methanol electro-catalytic oxidation and photo-catalytic oxidation reactions are synergistically coupled on Pt-Ag/GNs under UV irradiation or visible light irradiation, bringing about increased catalytic activity and stability.
Scheme 1. Schematic of the preparation procedure of the Pt-Ag/GNs composite.

Results and Discussion
Figure 1 shows the XRD patterns for the following samples: (a) Ag/GNs, (b) Pt/GNs and (c) Pt-Ag/GNs.The diffraction pattern for the Pt/GNs catalyst shows the typical peaks of the face centeredcubic (fcc) structure of Pt (0).Peaks are located at 39.8°, 46.3°, 67.6° and 81.4° corresponding to the (111), (200), ( 220) and (311) planes of Pt (0) [18,19].This indicates that the reduction of the Pt 2+ precursor to Pt (0) by formic acid has been effective.Pt-Ag/GNs (curve c) shows broader and asymmetrical peaks in comparison to both Ag/GNs and Pt/GNs.The reason for this is that these peaks are the combination of the peaks for both Pt and Ag, which indicates that both elements are not alloyed.This is also supported by the the Powder Diffraction File of International Centre for Diffraction Data (ICDD PDF) of Pt (No. 04-0802) and Ag (No. 04-0783) in Figure 1.Furthermore, the representative diffraction peaks assigned to the (111) and (220) planes of Ag2O located at 32.8° and 54.9°, respectively, are not observed (Figure 1) [20], suggesting that the intermediate species Ag2O was completely transformed into Ag after the reduction with formic acid.Scheme 1. Schematic of the preparation procedure of the Pt-Ag/GNs composite.220) and (311) planes of Pt (0) [18,19].This indicates that the reduction of the Pt 2+ precursor to Pt (0) by formic acid has been effective.Pt-Ag/GNs (curve c) shows broader and asymmetrical peaks in comparison to both Ag/GNs and Pt/GNs.The reason for this is that these peaks are the combination of the peaks for both Pt and Ag, which indicates that both elements are not alloyed.This is also supported by the the Powder Diffraction File of International Centre for Diffraction Data (ICDD PDF) of Pt (No. 04-0802) and Ag (No. 04-0783) in Figure 1.Furthermore, the representative diffraction peaks assigned to the (111) and (220) planes of Ag 2 O located at 32.8 • and 54.9 • , respectively, are not observed (Figure 1) [20], suggesting that the intermediate species Ag 2 O was completely transformed into Ag after the reduction with formic acid.The morphologies of the Pt-Ag/GNs and Pt/GNs composites were characterized by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM).The TEM image of Pt/GNs shows apparent aggregation of the Pt particles on the surface of GNs (Figure 2A).By contrast, the spherical bimetallic nanoparticles of the Pt-Ag/GNs composite are uniformly dispersed on the surface of the GNs with an average size of about 3.3 nm (Figure 2D,E), and a relatively narrow size distribution peak (the inset of Figure 2E).HRTEM images of Pt/GNs and Pt-Ag/GNs (Figure 2C,F) show lattice fringes with spacing of 0.225 nm, which is in good agreement with the (111) planes of fcc Pt (0) [21].In addition, the lattice spacing of 0.235 nm shown in Figure 2F can be assigned to the (111) planes of metallic Ag [22], further proving the formation of separate Ag nanoparticles in Pt-Ag/GNs.This also indicates that the Pt and Ag nanoparticles are separately deposited on the GNs, instead of forming a Pt-Ag alloy.The reason for this may be that Ag + was first transformed into Ag2O under UV irradiation [23], and then the heterogeneous phase between the precursor Pt 2+ and Ag2O can prevent the formation of a binary alloy structure, which results in the formation of the heterostructured Pt-Ag nanoparticles.It is worth mentioning that (111) Pt planes are adjacent to (111) Ag planes, suggesting that Ag2O provides an anchor for the Pt 2+ precursor during the reduction process, resulting in a proper dispersion of Ag and Pt nanoparticles on the GNs after reduction by formic acid.The morphologies of Pt and Ag nanoparticles on GNs were further analyzed by high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) and elemental mapping images.As shown in Figure 3A, Pt and Ag nanoparticles are highly dispersed onto the GNs surface, and it is difficult to discriminate Pt from Ag because of their full integration.Elemental mapping (Figure 3B,C) of the Pt-Ag/GNs composite show that the Pt and Ag nanoparticles are in good co-existence and in intimate contact, which further verifies that Ag2O provides an anchor for the precursor of Pt 2+ .The selected area electron diffraction (SAED) pattern demonstrates that the Pt-Ag/GNs composite has a typical polycrystalline structure (the inset of Figure 2F).The morphologies of the Pt-Ag/GNs and Pt/GNs composites were characterized by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM).The TEM image of Pt/GNs shows apparent aggregation of the Pt particles on the surface of GNs (Figure 2A).By contrast, the spherical bimetallic nanoparticles of the Pt-Ag/GNs composite are uniformly dispersed on the surface of the GNs with an average size of about 3.3 nm (Figure 2D,E), and a relatively narrow size distribution peak (the inset of Figure 2E).HRTEM images of Pt/GNs and Pt-Ag/GNs (Figure 2C,F) show lattice fringes with spacing of 0.225 nm, which is in good agreement with the (111) planes of fcc Pt (0) [21].In addition, the lattice spacing of 0.235 nm shown in Figure 2F can be assigned to the (111) planes of metallic Ag [22], further proving the formation of separate Ag nanoparticles in Pt-Ag/GNs.This also indicates that the Pt and Ag nanoparticles are separately deposited on the GNs, instead of forming a Pt-Ag alloy.The reason for this may be that Ag + was first transformed into Ag 2 O under UV irradiation [23], and then the heterogeneous phase between the precursor Pt 2+ and Ag 2 O can prevent the formation of a binary alloy structure, which results in the formation of the heterostructured Pt-Ag nanoparticles.It is worth mentioning that (111) Pt planes are adjacent to (111) Ag planes, suggesting that Ag 2 O provides an anchor for the Pt 2+ precursor during the reduction process, resulting in a proper dispersion of Ag and Pt nanoparticles on the GNs after reduction by formic acid.The morphologies of Pt and Ag nanoparticles on GNs were further analyzed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and elemental mapping images.As shown in Figure 3A, Pt and Ag nanoparticles are highly dispersed onto the GNs surface, and it is difficult to discriminate Pt from Ag because of their full integration.Elemental mapping (Figure 3B,C) of the Pt-Ag/GNs composite show that the Pt and Ag nanoparticles are in good co-existence and in intimate contact, which further verifies that Ag 2 O provides an anchor for the precursor of Pt 2+ .The selected area electron diffraction (SAED) pattern demonstrates that the Pt-Ag/GNs composite has a typical polycrystalline structure (the inset of Figure 2F).X-ray photoelectron spectra (XPS) were used to further analyze the valence states and surface composition of the as-synthesized composites, displayed in Figure 4.The binding energies of Ag 3d 5/2 and Ag 3d 3/2 for Pt-Ag/GNs are 368.1 eV and 374.0 eV (Figure 4A), respectively, demonstrating that the deposited Ag nanoparticles are in a zero-valent state [22,24].Figure 4B shows Pt 4f spectra of Pt/GNs (curve a) and Pt-Ag/GNs (curve b).The peaks located at 71.08 and 74.38 eV for Pt-Ag/GNs and at 71.38 and 74.79 eV for Pt/GNs can be assigned to the binding energies of Pt 4f 7/2 and Pt 4f 5/2 , respectively.Their values are in line with that of platinum in the zero-valent state, which further verifies the formation of Pt (0) nanoparticles in Pt/GNs and Pt-Ag/GNs composites [14].However, the binding energy of Pt 4f for Pt-Ag/GNs is significantly lower than that for Pt/GNs.The reason may be that Pt has a higher electro-negativity than Ag, so Pt can withdraw electrons from the neighboring Ag atoms [25].Furthermore, there exists an electrostatic interaction between the metal nanoparticles and the graphene sheets because of the remaining oxygen containing functional groups on the surface of the GNs, which allows a high dispersion of Pt-Ag nanoparticles on GNs to produce stronger electronic interactions between metal nanoparticles and supports [26,27], inducing their binding energy shift.

Results and Discussion
Catalysts 2016, 6, 144 5 of 13 the deposited Ag nanoparticles are in a zero-valent state [22,24].Figure 4B shows Pt 4f spectra of Pt/GNs (curve a) and Pt-Ag/GNs (curve b).The peaks located at 71.08 and 74.38 eV for Pt-Ag/GNs and at 71.38 and 74.79 eV for Pt/GNs can be assigned to the binding energies of Pt 4f7/2 and Pt 4f5/2, respectively.Their values are in line with that of platinum in the zero-valent state, which further verifies the formation of Pt (0) nanoparticles in Pt/GNs and Pt-Ag/GNs composites [14].However, the binding energy of Pt 4f for Pt-Ag/GNs is significantly lower than that for Pt/GNs.The reason may be that Pt has a higher electro-negativity than Ag, so Pt can withdraw electrons from the neighboring Ag atoms [25].Furthermore, there exists an electrostatic interaction between the metal nanoparticles and the graphene sheets because of the remaining oxygen containing functional groups on the surface of the GNs, which allows a high dispersion of Pt-Ag nanoparticles on GNs to produce stronger electronic interactions between metal nanoparticles and supports [26,27], inducing their binding energy shift.Raman spectra were used to discuss the structural changes of the graphene based composites during the preparation process.As shown in Figure 5, the D band (associated with a breathing mode of k-point phonons of A1g symmetry) and the G band (corresponding to the E2g phonon of the C sp 2 atom) are observed [28].ID/IG, namely the intensity ratio of the D and G bands, can be used to predict the extent of defects in carbonaceous materials [29].It is found that the ID/IG value of the Pt-Ag/GNs composite (curve b) is obviously smaller than that of Pt/GNs (curve a), which indicates that there is a lower degree of disorder for the graphene structure.Raman spectra were used to discuss the structural changes of the graphene based composites during the preparation process.As shown in Figure 5, the D band (associated with a breathing mode of k-point phonons of A 1g symmetry) and the G band (corresponding to the E 2g phonon of the C sp 2 atom) are observed [28].I D /I G , namely the intensity ratio of the D and G bands, can be used to predict the extent of defects in carbonaceous materials [29].It is found that the I D /I G value of the Pt-Ag/GNs composite (curve b) is obviously smaller than that of Pt/GNs (curve a), which indicates that there is a lower degree of disorder for the graphene structure.
Catalysts 2016, 6, 144 5 of 13 the deposited Ag nanoparticles are in a zero-valent state [22,24].Figure 4B shows Pt 4f spectra of Pt/GNs (curve a) and Pt-Ag/GNs (curve b).The peaks located at 71.08 and 74.38 eV for Pt-Ag/GNs and at 71.38 and 74.79 eV for Pt/GNs can be assigned to the binding energies of Pt 4f7/2 and Pt 4f5/2, respectively.Their values are in line with that of platinum in the zero-valent state, which further verifies the formation of Pt (0) nanoparticles in Pt/GNs and Pt-Ag/GNs composites [14].However, the binding energy of Pt 4f for Pt-Ag/GNs is significantly lower than that for Pt/GNs.The reason may be that Pt has a higher electro-negativity than Ag, so Pt can withdraw electrons from the neighboring Ag atoms [25].Furthermore, there exists an electrostatic interaction between the metal nanoparticles and the graphene sheets because of the remaining oxygen containing functional groups on the surface of the GNs, which allows a high dispersion of Pt-Ag nanoparticles on GNs to produce stronger electronic interactions between metal nanoparticles and supports [26,27], inducing their binding energy shift.Raman spectra were used to discuss the structural changes of the graphene based composites during the preparation process.As shown in Figure 5, the D band (associated with a breathing mode of k-point phonons of A1g symmetry) and the G band (corresponding to the E2g phonon of the C sp 2 atom) are observed [28].ID/IG, namely the intensity ratio of the D and G bands, can be used to predict the extent of defects in carbonaceous materials [29].It is found that the ID/IG value of the Pt-Ag/GNs composite (curve b) is obviously smaller than that of Pt/GNs (curve a), which indicates that there is a lower degree of disorder for the graphene structure.Cyclic voltammograms (CVs) of Pt/C-JM (curve a), Pt/GNs (curve b) and Pt-Ag/GNs (curve c) in a 0.5 M H 2 SO 4 solution were collected to evaluate the electrochemical properties of the different composites, shown in Figure 6.In general, the hydrogen adsorption/desorption peaks of the CV curves can be utilized to measure the electrochemically active surface area (ECSA) of Pt-based catalysts on the basis of Equation (1) [30].
Catalysts 2016, 6, 144 6 of 13 Cyclic voltammograms (CVs) of Pt/C-JM (curve a), Pt/GNs (curve b) and Pt-Ag/GNs (curve c) in a 0.5 M H2SO4 solution were collected to evaluate the electrochemical properties of the different composites, shown in Figure 6.In general, the hydrogen adsorption/desorption peaks of the CV curves can be utilized to measure the electrochemically active surface area (ECSA) of Pt-based catalysts on the basis of Equation (1) [30].
The ECSA values for the Pt-Ag/GNs (curve c) is 120.3 m 2 •g −1 , which is about 1.5 times and 2.5 times higher than that of the Pt/GNs (curve b) (80.5 m 2 •g −1 ) and the Pt/C-JM (curve a) (48.9 m 2 •g −1 ), respectively.The larger ECSA mainly arises from better dispersion of the Ag and Pt nanoparticles on the GNs (as shown in TEM analysis), which can offer more active sites.
In order to further investigate their catalytic activities for methanol oxidation reaction (MOR), CVs in 1 M CH3OH + 0.5 M H2SO4 solution were conducted (Figure 7).As shown in Figure 7, all of the curves have two significant oxidation peaks: the forward scan oxidation peak is assigned to the peak current of methanol oxidation, and the reverse scan oxidation peak is attributed to the peak current of the CO intermediate species oxidation.As presented in Figure 7, the forward scan current density (mass activity) increased up to 838.3 mA•mg −1 on Pt-Ag/GNs without light irradiation (t = 0 min), which is significantly higher than that of Pt/GNs (653.8 mA•mg −1 ) and Pt/C-JM (402.6 mA•mg −1 ).The improved catalytic activity may originate from the role of Ag.First, the introduction of Ag into Pt-Ag/GNs brings about the good distribution of the Ag and Pt nanoparticles on the GNs, which generates more active sites for MOR (as shown in Figure 6).Secondly, Ag has a lower electronegativity than Pt, so Pt can withdraw electrons from the neighboring Ag atoms, which produces stronger electronic interactions between the Pt and Ag nanoparticles (as analyzed in the Pt 4f spectra) [25].The interaction may effectively weaken the capacity to bind to adsorbed intermediates of Pt [31].Thirdly, the coexistence of Ag and Pt nanoparticles on the surface of GNs can increase the electron transfer efficiency via an interfacial interaction among the Ag, Pt and GNs, which was verified by Electrochemical Impedance Spectra (EIS) analysis.As shown in Figure 8, the Nyquist plots of EIS for Pt-Ag/GNs, Pt/GNs and Pt/C-JM were obtained in 0.5 M H2SO4 + 1.0 M CH3OH solution at 0.5 V.The diameter of the primary semicircle can be used to analyze the charge transfer resistance of the catalyst, which describes the rate of charge transfer during the methanol oxidation reaction [32].The semicircle radius of Pt-Ag/GNs (curve c) is much smaller than that of Pt/GNs (curve b) and Pt/C-JM (curve a), which illustrates that Pt-Ag/GNs have better charge transfer and a faster methanol oxidation reaction rate [33,34].The ECSA values for the Pt-Ag/GNs (curve c) is 120.3 m 2 •g −1 , which is about 1.5 times and 2.5 times higher than that of the Pt/GNs (curve b) (80.5 m 2 •g −1 ) and the Pt/C-JM (curve a) (48.9 m 2 •g −1 ), respectively.The larger ECSA mainly arises from better dispersion of the Ag and Pt nanoparticles on the GNs (as shown in TEM analysis), which can offer more active sites.
In order to further investigate their catalytic activities for methanol oxidation reaction (MOR), CVs in 1 M CH 3 OH + 0.5 M H 2 SO 4 solution were conducted (Figure 7).As shown in Figure 7, all of the curves have two significant oxidation peaks: the forward scan oxidation peak is assigned to the peak current of methanol oxidation, and the reverse scan oxidation peak is attributed to the peak current of the CO intermediate species oxidation.As presented in Figure 7, the forward scan current density (mass activity) increased up to 838.3 mA•mg −1 on Pt-Ag/GNs without light irradiation (t = 0 min), which is significantly higher than that of Pt/GNs (653.8 mA•mg −1 ) and Pt/C-JM (402.6 mA•mg −1 ).The improved catalytic activity may originate from the role of Ag.First, the introduction of Ag into Pt-Ag/GNs brings about the good distribution of the Ag and Pt nanoparticles on the GNs, which generates more active sites for MOR (as shown in Figure 6).Secondly, Ag has a lower electro-negativity than Pt, so Pt can withdraw electrons from the neighboring Ag atoms, which produces stronger electronic interactions between the Pt and Ag nanoparticles (as analyzed in the Pt 4f spectra) [25].The interaction may effectively weaken the capacity to bind to adsorbed intermediates of Pt [31].Thirdly, the coexistence of Ag and Pt nanoparticles on the surface of GNs can increase the electron transfer efficiency via an interfacial interaction among the Ag, Pt and GNs, which was verified by Electrochemical Impedance Spectra (EIS) analysis.As shown in Figure 8, the Nyquist plots of EIS for Pt-Ag/GNs, Pt/GNs and Pt/C-JM were obtained in 0.5 M H 2 SO 4 + 1.0 M CH 3 OH solution at 0.5 V.The diameter of the primary semicircle can be used to analyze the charge transfer resistance of the catalyst, which describes the rate of charge transfer during the methanol oxidation reaction [32].The semicircle radius of Pt-Ag/GNs (curve c) is much smaller than that of Pt/GNs (curve b) and Pt/C-JM (curve a), which illustrates that Pt-Ag/GNs have better charge transfer and a faster methanol oxidation reaction rate [33,34].
Most interestingly, it was found that the peak current value of the Pt-Ag/GNs composite was significantly increased under UV or visible light irradiation.Particularly, the Pt-Ag/GNs composite exhibited a remarkably increased value (about 1842.4 mA•mg −1 ) after UV irradiation for 30 min, which is nearly 2.0 times higher than that without light irradiation (838.3 mA•mg −1 ) (Figure 7A).The value is also far more than that of reported commercial PtRu-C (E-TEK, Secaucus, New Jersey, NJ, USA) [35] or PtRu-C prepared by the ethanol reduction method [36].However, the current density of Pt/GNs and Pt/C-JM are only slightly increased (Figure 7B,C).The obvious improvement in the catalytic activity of Pt-Ag/GNs under external light irradiation apparently stem from the introduction of Ag, which may endow Pt-Ag/GNs with an efficient electro-photo synergistic catalysis for MOR under UV or visible light irradiation.In order to further check whether the Pt-Ag/GNs composite can effectively respond to ultraviolet light or visible light irradiation, chronoamperometric curves of Pt-Ag/GNs were measured by turning on/off UV or visible light every 10 s in 1 M CH 3 OH + 0.5 M H 2 SO 4 solution (presented in Figure 9).When UV or visible light was turned on, the current density increased rapidly and temporarily up to a steady state; as the light was turned off, the current density decreased instantly.In the first illumination cycle, the current intensities were boosted from 0.325 to 2.171 (mA•cm −2 ) (568.0% increase) under UV irradiation (curve c) and from 0.325 to 1.522 (mA•cm −2 ) (368.3% increase) under visible irradiation, respectively (curve b).The increase of current density in the first illumination cycle is apparently higher than that of the reported silicon-based Pt-Ag nano-forests catalyst under visible irradiation [37].Therefore, the as-synthesized Pt-Ag/GNs should be a more efficient Pt-Ag bimetal electro-photo synergistic catalyst for MOR under solar irradiation.
Catalysts 2016, 6, 144 7 of 13 [35] or PtRu-C prepared by the ethanol reduction method [36].However, the current density of Pt/GNs and Pt/C-JM are only slightly increased (Figure 7B,C).The obvious improvement in the catalytic activity of Pt-Ag/GNs under external light irradiation apparently stem from the introduction of Ag, which may endow Pt-Ag/GNs with an efficient electro-photo synergistic catalysis for MOR under UV or visible light irradiation.In order to further check whether the Pt-Ag/GNs composite can effectively respond to ultraviolet light or visible light irradiation, chronoamperometric curves of Pt-Ag/GNs were measured by turning on/off UV or visible light every 10 s in 1 M CH3OH + 0.5 M H2SO4 solution (presented in Figure 9).When UV or visible light was turned on, the current density increased rapidly and temporarily up to a steady state; as the light was turned off, the current density decreased instantly.In the first illumination cycle, the current intensities were boosted from 0.325 to 2.171 (mA•cm −2 ) (568.0% increase) under UV irradiation (curve c) and from 0.325 to 1.522 (mA•cm −2 ) (368.3% increase) under visible irradiation, respectively (curve b).The increase of current density in the first illumination cycle is apparently higher than that of the reported silicon-based Pt-Ag nanoforests catalyst under visible irradiation [37].Therefore, the as-synthesized Pt-Ag/GNs should be a more efficient Pt-Ag bimetal electro-photo synergistic catalyst for MOR under solar irradiation.Catalysts 2016, 6, 144 7 of 13 [35] or PtRu-C prepared by the ethanol reduction method [36].However, the current density of Pt/GNs and Pt/C-JM are only slightly increased (Figure 7B,C).The obvious improvement in the catalytic activity of Pt-Ag/GNs under external light irradiation apparently stem from the introduction of Ag, which may endow Pt-Ag/GNs with an efficient electro-photo synergistic catalysis for MOR under UV or visible light irradiation.In order to further check whether the Pt-Ag/GNs composite can effectively respond to ultraviolet light or visible light irradiation, chronoamperometric curves of Pt-Ag/GNs were measured by turning on/off UV or visible light every 10 s in 1 M CH3OH + 0.5 M H2SO4 solution (presented in Figure 9).When UV or visible light was turned on, the current density increased rapidly and temporarily up to a steady state; as the light was turned off, the current density decreased instantly.In the first illumination cycle, the current intensities were boosted from 0.325 to 2.171 (mA•cm −2 ) (568.0% increase) under UV irradiation (curve c) and from 0.325 to 1.522 (mA•cm −2 ) (368.3% increase) under visible irradiation, respectively (curve b).The increase of current density in the first illumination cycle is apparently higher than that of the reported silicon-based Pt-Ag nanoforests catalyst under visible irradiation [37].Therefore, the as-synthesized Pt-Ag/GNs should be a more efficient Pt-Ag bimetal electro-photo synergistic catalyst for MOR under solar irradiation.The catalytic stability of Pt-Ag/GNs, Pt/GNs, and Pt/C-JM for methanol oxidation was further tested by chronoamperometry in 0.5 M H2SO4 containing 1.0 M CH3OH solution at 0.70 V for 2000 s (Figure 10).As shown in Figure 10, rapid initial current density decay is observed because of CO-like intermediate species (mainly COads) poisoning during the methanol electrooxidation reaction [38].The slower decay slope and the significantly higher final steady-state current density of Pt-Ag/GNs without irradiation (curve c), compared with that of Pt/GNs (curve b) and Pt/C-JM (curve a), indicate that the introduction of Ag into Pt-Ag/GNs improves its electro-catalytic stability for MOR.The enhanced tolerance ability to CO can be reflected in the CO-stripping voltammograms test.As shown in Figure 11, CO-stripping voltammograms of Pt/C-JM (curve a), Pt/GNs (curve b), and Pt-Ag/GNs (curve c) were measured by the oxidation of pre-adsorbed and saturated CO in 0.5 M H2SO4 solution at a scan rate of 100 mV•s −1 .The peak potential of CO oxidation for Pt-Ag/GNs, Pt/GNs, and Pt/C-JM was 0.67, 0.70 and 0.71 V, respectively.Obviously, Pt-Ag/GNs exhibited a more negative peak potential.The negative shift of the CO oxidation peak demonstrates easier oxidation of CO on the surface of the Pt-Ag/GNs catalyst, indicative of a better anti-poisoning ability.Ag nanoparticles are oxophilic species and can remove the CO-like adsorbing species on Pt active sites at a lower potential through facile formation of Ag(OH)ads [39], contributing to better tolerance towards CO poisoning for Pt-Ag/GNs.The catalytic stability of Pt-Ag/GNs, Pt/GNs, and Pt/C-JM for methanol oxidation was further tested by chronoamperometry in 0.5 M H 2 SO 4 containing 1.0 M CH 3 OH solution at 0.70 V for 2000 s (Figure 10).As shown in Figure 10, rapid initial current density decay is observed because of CO-like intermediate species (mainly CO ads ) poisoning during the methanol electrooxidation reaction [38].The slower decay slope and the significantly higher final steady-state current density of Pt-Ag/GNs without irradiation (curve c), compared with that of Pt/GNs (curve b) and Pt/C-JM (curve a), indicate that the introduction of Ag into Pt-Ag/GNs improves its electro-catalytic stability for MOR.The enhanced tolerance ability to CO can be reflected in the CO-stripping voltammograms test.As shown in Figure 11, CO-stripping voltammograms of Pt/C-JM (curve a), Pt/GNs (curve b), and Pt-Ag/GNs (curve c) were measured by the oxidation of pre-adsorbed and saturated CO in 0.5 M H 2 SO 4 solution at a scan rate of 100 mV•s −1 .The peak potential of CO oxidation for Pt-Ag/GNs, Pt/GNs, and Pt/C-JM was 0.67, 0.70 and 0.71 V, respectively.Obviously, Pt-Ag/GNs exhibited a more negative peak potential.The negative shift of the CO oxidation peak demonstrates easier oxidation of CO on the surface of the Pt-Ag/GNs catalyst, indicative of a better anti-poisoning ability.Ag nanoparticles are oxophilic species and can remove the CO-like adsorbing species on Pt active sites at a lower potential through facile formation of Ag(OH)ads [39], contributing to better tolerance towards CO poisoning for Pt-Ag/GNs.The catalytic stability of Pt-Ag/GNs, Pt/GNs, and Pt/C-JM for methanol oxidation was further tested by chronoamperometry in 0.5 M H2SO4 containing 1.0 M CH3OH solution at 0.70 V for 2000 s (Figure 10).As shown in Figure 10, rapid initial current density decay is observed because of CO-like intermediate species (mainly COads) poisoning during the methanol electrooxidation reaction [38].The slower decay slope and the significantly higher final steady-state current density of Pt-Ag/GNs without irradiation (curve c), compared with that of Pt/GNs (curve b) and Pt/C-JM (curve a), indicate that the introduction of Ag into Pt-Ag/GNs improves its electro-catalytic stability for MOR.The enhanced tolerance ability to CO can be reflected in the CO-stripping voltammograms test.As shown in Figure 11, CO-stripping voltammograms of Pt/C-JM (curve a), Pt/GNs (curve b), and Pt-Ag/GNs (curve c) were measured by the oxidation of pre-adsorbed and saturated CO in 0.5 M H2SO4 solution at a scan rate of 100 mV•s −1 .The peak potential of CO oxidation for Pt-Ag/GNs, Pt/GNs, and Pt/C-JM was 0.67, 0.70 and 0.71 V, respectively.Obviously, Pt-Ag/GNs exhibited a more negative peak potential.The negative shift of the CO oxidation peak demonstrates easier oxidation of CO on the surface of the Pt-Ag/GNs catalyst, indicative of a better anti-poisoning ability.Ag nanoparticles are oxophilic species and can remove the CO-like adsorbing species on Pt active sites at a lower potential through facile formation of Ag(OH)ads [39], contributing to better tolerance towards CO poisoning for Pt-Ag/GNs.Particularly, the current density of Pt-Ag/GNs decay more slowly in the presence of light irradiation, and the steady-state current density is significantly enhanced under durative UV (curve e) or visible light irradiation (curve d), which are respectively nearly 2.1 times and 1.4 times higher than that without light irradiation (Figure 10).The results further confirm that the incorporation of Ag brings about the positive effects of light irradiation on the catalytic stability and tolerance to CO poisoning.This is also supported by the ratio of the forward anodic peak current (If) to the reverse anodic peak current (Ib) under irradiation.It is well known that the If/Ib ratio can also be used to describe the catalyst tolerance to the accumulation of CO-like intermediate species [40][41][42].The If/Ib of Pt-Ag/GNs is 0.85 after UV irradiation for 30 min, which is higher than that of Pt/GNs (0.80) and Pt/C-JM (0.72) (Figure 7), respectively, implying more valid oxidation of methanol to carbon dioxide during the anodic scan and the removal of poisoning species from the surface of Pt-Ag/GNs under UV irradiation.It should be noted that Pt-Ag/GNs not only exhibited better catalytic activity, stability and tolerance to CO under UV irradiation, but its visible light assisted electro-photo synergistic catalysis performance is also more effective than that of the reported silicon-based Pt-Ag nano-forests catalyst [37].The efficient synergistic catalysis under UV and visible light irradiation is in favor of harvesting the natural sunlight (UV and visible light) to produce clean energy and provides very promising practical applications.It is well known that noble metal Ag nanoparticles (NPs) show strong visible light absorption due to their surface plasmon resonance (SPR), which is produced by the collective oscillations of surface electrons [43,44].The unique characteristics of Ag contribute to the beneficial effects of electro-photo-synergistic catalysis under visible light irradiation.Furthermore, GNs can effectively extend the light absorption edge to the visible light region [45].By integrating with Pt-Ag nanoparticles, GNs are helpful for endowing Pt-Ag/graphene composites with the visible light response for photocatalytic methanol oxidation.Due to the local electromagnetic field and the excellent UV-Visible light absorption capabilities of Ag NPs, the excited electrons can be transferred quickly to GNs, and GNs can be used as the photocatalytic carriers to extract the photo-generated electrons and immediately transfer them to Pt particles, to limit self-photoreduction processes and more easily release Pt active sites [17].Therefore, the efficient synergistic effects of photo-catalysis and electro-catalysis for MOR are achieved under external light irradiation.

Materials
Graphite powder (~325 mesh, 99.9995%) was purchased from Alfa Aesar (Ward Hill, Massachusetts, MA, USA).5% Nafion 117 solution and commercial Pt/C (20%) catalyst (denoted as Pt/C-JM) were purchased from Johnson Matthey (London, UK).AgNO3, Pt(NO3)2, NaOH, KMnO4, H2O2 (30%), K2S2O8, P2O5, H2SO4, acetone, methanol, and ethanol were all analytical reagent and Particularly, the current density of Pt-Ag/GNs decay more slowly in the presence of light irradiation, and the steady-state current density is significantly enhanced under durative UV (curve e) or visible light irradiation (curve d), which are respectively nearly 2.1 times and 1.4 times higher than that without light irradiation (Figure 10).The results further confirm that the incorporation of Ag brings about the positive effects of light irradiation on the catalytic stability and tolerance to CO poisoning.This is also supported by the ratio of the forward anodic peak current (I f ) to the reverse anodic peak current (I b ) under irradiation.It is well known that the I f /I b ratio can also be used to describe the catalyst tolerance to the accumulation of CO-like intermediate species [40][41][42].The I f /I b of Pt-Ag/GNs is 0.85 after UV irradiation for 30 min, which is higher than that of Pt/GNs (0.80) and Pt/C-JM (0.72) (Figure 7), respectively, implying more valid oxidation of methanol to carbon dioxide during the anodic scan and the removal of poisoning species from the surface of Pt-Ag/GNs under UV irradiation.It should be noted that Pt-Ag/GNs not only exhibited better catalytic activity, stability and tolerance to CO under UV irradiation, but its visible light assisted electro-photo synergistic catalysis performance is also more effective than that of the reported silicon-based Pt-Ag nano-forests catalyst [37].The efficient synergistic catalysis under UV and visible light irradiation is in favor of harvesting the natural sunlight (UV and visible light) to produce clean energy and provides very promising practical applications.It is well known that noble metal Ag nanoparticles (NPs) show strong visible light absorption due to their surface plasmon resonance (SPR), which is produced by the collective oscillations of surface electrons [43,44].The unique characteristics of Ag contribute to the beneficial effects of electro-photo-synergistic catalysis under visible light irradiation.Furthermore, GNs can effectively extend the light absorption edge to the visible light region [45].By integrating with Pt-Ag nanoparticles, GNs are helpful for endowing Pt-Ag/graphene composites with the visible light response for photocatalytic methanol oxidation.Due to the local electromagnetic field and the excellent UV-Visible light absorption capabilities of Ag NPs, the excited electrons can be transferred quickly to GNs, and GNs can be used as the photocatalytic carriers to extract the photo-generated electrons and immediately transfer them to Pt particles, to limit self-photoreduction processes and more easily release Pt active sites [17].Therefore, the efficient synergistic effects of photo-catalysis and electro-catalysis for MOR are achieved under external light irradiation.

Preparation of Pt-Ag/GNs
Graphene oxide (GO) was prepared from purified natural graphite using Hummers' method with some minor modification [46,47].The product was collected by centrifugation after thoroughly washing with dilute hydrochloric acid and deionized water.After that, the obtained precipitate was dried in vacuum for further use.The overall synthetic route of Pt-Ag/GNs is illustrated in Scheme 1.In a typical synthetic process, 50 mg of GO was dispersed in 50 mL of distilled water by sonication for 30 min.Then, 30 mg of AgNO 3 was added to the mixture under stirring for 30 min.Subsequently, 50 mg of solid sodium hydroxide was added under UV irradiation and the mixture was stirred for another 30 min, which caused that Ag + was transformed into Ag 2 O [23].After removing the ultraviolet lamp and adjusting the pH to 7, 3 mL of Pt(NO 3 ) 2 (1g/100 mL) were added.After stirring for 30 min, 3 mL of HCOOH (0.5 M) were added.Subsequently, the mixture was stirred for 48 h under ambient condition.After repeated centrifugation (10,000 rpm, 20 min) and thorough washing, the final Pt-Ag/GNs precipitate was lyophilized for 48 h.The counterpart Pt/GNs was prepared by a similar method in the absence of AgNO 3 .The Pt content was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES).The actual loading amount of Pt on the GNs for Pt-Ag/GNs and Pt/GNs was about 20.7 wt % and 25.9 wt %, respectively.

Characterization Method
The microscopic features and morphology of the samples were investigated through TEM and HRTEM with a TECNAI G2 high-resolution transmission electron microscope (FEI, Hillsboro, Oregon, OR, USA) operating at 200 K. Elemental distribution mapping was performed in a HAADF-STEM with an Oxford INCA energy dispersive X-ray detector (FEI).XPS was conducted on a VG ESCALAB 250 spectrometer (Thermo Scientic, Pleasanton, California, CA, USA) with Al Kα radiation (1486.6 eV).XRD patterns were obtained with an X'pert Pro X-ray diffractometer (PANalytical B.V., Holland, The Netherlands), equipped with a Cu Kα radiation source.Raman spectra were recorded on a Renishaw-in-Via Raman (Renishaw, London, UK) micro-spectrometer equipped with a 514 nm wavelength laser line.The actual amount of Pt loading in the catalysts was determined by ICP-AES (ICAP6300, Thermo Scientic).

Electrochemical Measurements
A conventional three electrode cell was used for the electrochemical measurement on a CHI 660C electrochemical workstation (ChenHua, Shanghai, China), with a platinum column as the counter electrode, a standard Ag|AgCl (3 M KCl) electrode as the reference, and a modified glassy carbon electrode (GCE, 3.0 mm in diameter) as the working electrode, respectively.To prepare the working electrode, 5 mg of catalyst sample were first dispersed into 1 mL of ethanol by ultrasonication, and then 5 µL of the previously prepared ethanol suspension were dropped onto the pre-cleaned surface of the GCE.Subsequently, an aliquot of Nafion solution (7.5 µL, 0.5%) was dropped on the coated GCE.CVs were recorded in 0.5 M H 2 SO 4 or 0.5 M H 2 SO 4 solution containing 1.0 M CH 3 OH at room temperature with a scan rate of 100 mV•s −1 .CO-stripping voltammograms were performed in a 0.5 M N 2 -saturated H 2 SO 4 solution, and then CO was bubbled for 30 min to allow the complete adsorption of CO onto the composites while the working electrode was kept at 0.1 V vs. Ag|AgCl.Excess CO in the electrolyte was then purged out with N 2 for 15 min.Chronoamperometric curves were obtained for 2000 s in 0.5 M H 2 SO 4 solution containing 1.0 M CH 3 OH at 0.70 V. EIS was carried out in 0.5 M H 2 SO 4 in the presence of 1.0 M CH 3 OH with a potential of 0.5 V, and the Nyquist plots were recorded between 0.1 and 100,000 Hz.

Photo-Electrochemical Measurements
A 15 W UV lamp (ZW3D15WZ105, Cnlight, Guangdong, China) was used to provide UV light (200 < λ < 400 nm) or visible light (λ > 400 nm) via different optical filters.It was placed at the bottom of the electrochemical cell, facing the working electrode at a distance of 15 cm.The irradiating power density was measured as 0.3 mW•cm −2 .Cyclic voltammograms and chronoamperometric curve analyses were carried out after external light irradiation for 30 min.

Conclusions
In summary, Pt-Ag bimetallic nanoparticles with small size and uniform size distribution were highly dispersed on the surface of GNs in a Pt-Ag/GNs composite with the aid of synchronous reduction by HCOOH.The good dispersion of the Pt-Ag nanoparticles, the strong interaction between the Pt and the Ag nanoparticles, and the good electron transfer capacity of the Pt-Ag/GNs composite contributed to its higher electrocatalytic activity, better tolerance of CO, and better stability for MOR.Under UV or visible light irradiation, Pt-Ag/GNs showed an efficient synergistic photoelectrocatalysis for MOR, which provides an effective strategy to utilize natural sunlight (UV and visible light) to produce clean energy, and favors potential applications of electro-photo catalysts in direct methanol fuel cell anodic reactions.

Figure 1
Figure 1 shows the XRD patterns for the following samples: (a) Ag/GNs, (b) Pt/GNs and (c) Pt-Ag/GNs.The diffraction pattern for the Pt/GNs catalyst shows the typical peaks of the face centered-cubic (fcc) structure of Pt (0).Peaks are located at 39.8 • , 46.3 • , 67.6 • and 81.4 • corresponding to the (111), (200), (220) and (311) planes of Pt (0)[18,19].This indicates that the reduction of the Pt 2+ precursor to Pt (0) by formic acid has been effective.Pt-Ag/GNs (curve c) shows broader and asymmetrical peaks in comparison to both Ag/GNs and Pt/GNs.The reason for this is that these peaks are the combination of the peaks for both Pt and Ag, which indicates that both elements are not alloyed.This is also supported by the the Powder Diffraction File of International Centre for Diffraction Data (ICDD PDF) of Pt (No. 04-0802) and Ag (No. 04-0783) in Figure 1.Furthermore, the representative diffraction peaks assigned to the (111) and (220) planes of Ag 2 O located at 32.8 • and

Figure 2 .
Figure 2. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of Pt/GNs (A, B, C) and Pt-Ag/GNs (D, E, F); particle size distributions of Pt-Ag/GNs in the inset of (E); SAED of Pt-Ag/GNs in the inset of (F).

Figure 2 .
Figure 2. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of Pt/GNs (A, B, C) and Pt-Ag/GNs (D, E, F); particle size distributions of Pt-Ag/GNs in the inset of (E); SAED of Pt-Ag/GNs in the inset of (F).

Figure 2 .
Figure 2. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images of Pt/GNs (A, B, C) and Pt-Ag/GNs (D, E, F); particle size distributions of Pt-Ag/GNs in the inset of (E); SAED of Pt-Ag/GNs in the inset of (F).