Self-Template Synthesis of AgâPt Hollow Nanospheres as

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Self-Template Synthesis of Ag–Pt Hollow Nanospheres as Electrocatalyst for Methanol Oxidation Reaction Yehui Zhang, Jiajing Li, Heng Rong, Xiaowei Tong, and Zhenghua Wang Langmuir, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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Self-Template Synthesis of Ag–Pt Hollow Nanospheres as Electrocatalyst for Methanol Oxidation Reaction

Yehui Zhang, Jiajing Li, Heng Rong, Xiaowei Tong, and Zhenghua Wang*

Key Laboratory of Functional Molecular Solids, Ministry of Education, College of Chemistry and Materials Science, Anhui Normal University, Wuhu 241000, P. R. China

*Corresponding author. Tel.: +86-553-3869303; Fax: +86-553-3869302; E-mail: [email protected].

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Abstract Ag–Pt bimetallic hollow nanospheres have been prepared through a one-pot wet-chemical route. The formation of the hollow nanostructure can be explained by a self-template mechanism in which initially formed silver nanoparticles serve as the template. The Ag–Pt hollow nanospheres with an Ag/Pt ratio of 0.89:1 show the best electrochemical catalytic performances in the methanol oxidation reaction. Furthermore, the catalytic activity of the Ag–Pt hollow nanospheres is also much better than commercial Pt/C catalyst. The superior electrochemical performance of the Ag–Pt hollow nanospheres can be ascribed to the hollow nanostructure and the synergistic effect of Ag and Pt. Keywords:

Hollow

nanosphere;

Bimetallic;

Methanol

Electrocatalyst

2


oxidation

reaction;

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1. Introduction With the rapid development of the world economy, the demands for energies are more and more urgent, and many world-class problems such as energy crisis and environment pollution arise as a result of excessive consumption of fossil energy. Therefore, the searching for new and clean energies has become an urgent task. Direct methanol fuel cells (DMFCs) with the advantages of high energy conversion efficiency, low operation temperature, simple cell structure and low pollutant emissions have been one of the most promising energy conversion devices in recent years. Platinum (Pt) has been proved to be the most effective catalyst for DMFCs.1–10 However, the disadvantages of Pt catalysts such as the low reserves, high costs and easily to be poisoned by carbon monoxide has been the bottleneck of the massive application of DMFCs. Therefore, much effort is still needed to improve the catalytic efficiency of Pt-based catalysts while decrease the Pt utilization.11–15 Among various nanostructures, hollow nanostructures have attracted a great deal of attentions due to their unique structure as well as their prospective applications in many areas. As for electrocatalysis, hollow structures exhibit enhanced activities because they can exposure more active sites for catalysis reactions.16,17 Except for the composition, the size and wall thickness of hollow structures can also affect the catalytic activities. Pt-based catalysts with hollow nanostructures are effective for methanol oxidation reaction (MOR) as well as improved utility efficiency of Pt. For instance, Liu et al. synthesized bimetallic Ag-hollow Pt heterodimers which displayed a significant difference in optical and catalytic properties;18 Khashab and co-workers

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prepared hollow Au@Pd and Au@Pt core–shell nanoparticles with better catalytic activities than metallic Pt;19 Wang and co-workers synthesized hollow Pt–Pd nanospheres supported on reduced graphene oxide with improved electrocatalytic activity and better stability for methanol oxidation in alkaline media.20 Template method is widely used for the fabrication of hollow nanostructures. The templates applied for the synthesis of hollow nanostructures include anodic aluminum oxide,21 carbohydrate spheres,22 silica,23 etc. Some of the templates are still retained in the final product and the removal of the template is inconvenient and time consuming. In this work, Ag–Pt bimetallic hollow nanospheres were prepared by a convenient self-template method. The initially formed silver nanoparticles serve as the template and were consumed during the succeeding reacting process. Therefore removal of the template is unnecessary. The as-obtained Ag–Pt bimetallic hollow nanospheres can be applied as an efficient catalyst for methanol oxidation reaction. Control experiments indicate that the Ag–Pt bimetallic hollow nanospheres with an Ag/Pt ratio of 0.89:1 show higher electrocatalytic activity than other Ag–Pt nanospheres and the commercial Pt/C catalyst. The enhanced catalytic performances of the Ag–Pt hollow nanospheres can be attributed to the synergistic effect of Ag and Pt as well as the unique hollow nanostructure. In comparison to solid structure, such hollow nanostructure can improve mass transport and enhance the utility efficiency of the active materials.

2. Experimental methods

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2.1 Synthesis of Ag–Pt hollow nanospheres Typical procedure for the synthesis of Ag–Pt hollow nanospheres is as following. Firstly, a KOH solution (1 mol L–1, 0.05 mL), a fresh ascorbic acid solution (0.01 mol L–1, 3 mL) and PVP (0.05 g) were sequentially added to distilled water (2 mL) in a beaker under continuous stirring to obtain a clear solution. Then, an AgNO3 solution (0.002 mol L–1, 4 mL) was added dropwise into the above solution under continuous stirring. The solution turned dark yellow immediately. After that, a H2PtCl6 solution (0.002 mol L–1, 4 mL) was added dropwise into the above solution under stirring. Finally, the beaker was heated in a water bath at 60 °C with constant shaking at 60 rpm for 1 h. The as-prepared sample, named as AgPt-1, was collected by centrifugation and washed repeatedly with water and absolute ethanol. For comparison, AgPt-2 and AgPt-3 samples were prepared in the same manner by simply adjusting the ratio between AgNO3 and H2PtCl6 while kept the total amount of AgNO3 and H2PtCl6 constant. The starting Ag/Pt ratios for preparing AgPt-2 and AgPt-3 samples were 1:2 and 2:1, respectively. 2.2 Characterizations The morphology of the samples was observed by scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM, FEI Tecnai G2 20). The phase and crystalline of the samples was characterized by X-ray powder diffraction (XRD, Bruker D8 Advance) with Cu Kα radiation. Elemental mappings together with energy dispersive X-ray spectroscopy (EDS) were analyzed by a TEM (JEOL JEM-2100F) equipped with an EDS

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spectrometer. The composition of the samples was determined by inductive coupled plasma

atomic

emission

spectrometer

(ICP-AES;

OPTIMA

5300DV).

Electrochemical performances were analyzed with an electrochemical working station (CHI-660D, ChenHua Corp., Shanghai, China). 2.3 Electrode preparation and electrochemical measurements All the electrochemical measurements were carried out with a three-electrode configuration at room temperature using a Pt plate electrode as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The working electrodes were prepared according to the following procedure. The catalysts were dispersed in 1000 µL deionized water by ultrasonication. A glassy carbon electrode (GCE) which has an active surface area of 0.071 cm2 (diameter of 3 mm) was polished with slurry of alumina powder followed by water washing. Then, 5 µL of the catalyst suspension was dropped onto the GCE and dried in air naturally. After that, 6 µL of 0.5% Nafion was dropped onto the catalyst film, and the electrode was dried in air naturally. The mass loading of Pt is about 8 µg for all the catalysts, as listed in Table 1.

3. Results and discussion 3.1. Materials characterizations The composition of the as-obtained Ag–Pt samples is characterized by ICP-AES and XRD. ICP-AES results indicate that the Ag/Pt atomic ratio of the AgPt-1sample is 0.85:1. The Ag/Pt ratio in the final sample is a little lower than the original Ag/Pt ratio

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in the starting material, which can be attributed to the loss of Ag during the synthesis process. The crystallographic information of the sample is characterized by XRD technique, and a typical XRD pattern of the AgPt-1 sample is shown in Figure 1a. As can be seen from this pattern, three broad peaks at 38.8°, 45.4° and 66.3° are present. These peaks lay between the diffraction peaks of face-centered cubic Ag (JCPDS card no. 4-783) and face-centered cubic Pt (JCPDS card no. 4-802). The broadening of the diffraction peaks can be attributed to the small size of the sample. The diffraction peaks of Ag and Pt are merged together because they are very close to each other. The information on the surface elemental oxidation states of the AgPt-1 sample are evaluated by the XPS technique. The XPS survey spectrum of the AgPt-1 sample is shown in Figure 1b. The peaks correspond to Pt, Ag, O and C elements can be seen in this figure. The presence of C and O elements is due to the exposure to air. Figure 1c presents the core level spectrum of Pt 4f. This spectrum can be deconvoluted into two couples of peaks. The peaks at binding energies of 71.2 and 74.6 eV are assigned to Pt 4f7/2 and Pt 4f5/2 of Pt0, while the peaks at binding energies of 72.1 and 75.4 eV are assigned to Pt 4f7/2 and Pt 4f5/2 of Pt2+.24 The integrated area of Pt0 peaks is much larger than that of Pt2+, which indicates that the Pt0 species are the main form of Pt on the surface of AgPt-1 sample. Figure 1d shows the deconvoluted core level spectrum of Ag 3d, the peaks at binding energies of 368.8 and 374.6 eV are assigned to Ag 3d5/2 and Ag 3d3/2 of Ag0, while the peaks at binding energies of 367.8 and 373.8 eV are assigned to Ag 3d5/2 and Ag 3d3/2 of Ag+.24 The upshift of binding energies for Pt in the bimetallic Ag–Pt sample can be attributed to the modification of electronic

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structures and a downshift of the d-band center position.25 The change in electronic structures can decrease the CO adsorption energy on Pt and favor for C–H cleavage on Pt, thus contributes to an increase of CO tolerance on Pt.26 The morphologies of the AgPt-1 sample are observed by using SEM and TEM. Figure 2a shows a SEM image of the AgPt-1 sample, from which many monodisperse sphere-like nanoparticles can be seen. The sizes of these nanospheres are not so uniform. As counted from Figure 2a, the diameter of these nanospheres is in the range of 55–110 nm, and the average diameter of these nanospheres is about 75 nm. A TEM image of these nanospheres is shown in Figure 2b. This image clearly reveals that the nanospheres are hollow interior. The shell thickness of the hollow nanospheres is about 18 nm, as measured from the TEM photograph. What's more, the hollow nanospheres are comprised of many tiny nanoparticles with diameter of only several nanometers. A HRTEM image of the hollow nanosphere is shown in Figure 2c. The lattice fringes of 0.204 nm and 0.196 nm correspond to Ag (200) and Pt (200), respectively. This result shows that the AgPt-1 hollow nanospheres are composed of many Ag and Pt nanoparticles and these nanoparticles are attached to each other. An EDS spectrum of the AgPt-1 hollow nanospheres is shown in Figure 2d, which further confirms the presence of Ag and Pt elements in the sample. Meanwhile, the atomic ratio of Ag/Pt is measured to be 0.89:1, which matches well with the result from ICP-AES. The elemental distribution of Ag and Pt in the AgPt-1 hollow nanospheres is characterized by EDS elemental mapping. Figure 3 presents typical elemental maps of

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Ag and Pt, which shows that both Au and Pt elements are evenly distributed in the shell of the AgPt-1 hollow nanospheres. The merged image indicates that the distribution of Pt in the AgPt-1 hollow nanospheres is closer to the outer region. Such a structure is benefit for the catalysis of methanol oxidation reaction. AgPt-2 and AgPt-3 samples were also prepared in the same manner. ICP-AES analysis reveals the Ag/Pt ratio of 0.44:1 and 1.76:1 for the AgPt-2 and AgPt-3 samples, respectively. Figure 4 shows SEM and TEM images of the AgPt-2 and AgPt-3 samples. All of these samples show hollow nanosphere morphologies that alike to the AgPt-1 sample. With the changing of Ag/Pt ratio, the nanospheres size becomes non-uniform. 3.2. Formation mechanism A possible formation mechanism of the Ag–Pt hollow nanospheres is presented in Figure 5. Ag nanoparticles were firstly formed by the reduction of Ag+ with ascorbic acid. The freshly formed Ag nanoparticles are chemically reactive, and can serve as a reducer for the reduction of PtCl62–. The reaction between Ag nanoparticles and PtCl62– firstly occurred on the surfaces of the Ag nanoparticles, which lead to the formation of a thin shell constructed by tiny Pt nanoparticles. According to the Kirkendall effect,27 with the reaction going on, the Ag nanoparticles continuously dissolved and the Pt shell continuously grown and finally hollow nanospheres were obtained. During the reacting process, the produced Ag+ ions and some of the PtCl62– ions can also be reduced by ascorbic acid, and the produced Ag and Pt tiny nanoparticles can also deposited onto the shell. As a result, the shell of the hollow

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nanospheres is composed of both Pt and Ag. Control experiments show that with the absence of H2PtCl6 solution, only solid Ag nanoparticles were obtained (Figure 6a). The appearance of these Ag nanoparticles is similar to the Ag–Pt hollow nanospheres. With the absence of AgNO3 solution, dendritic Pt nanoparticles were obtained (Figure 6b). 3.3. Electrocatalytic tests The electrocatalytic performances of the as-prepared AgPt-1, AgPt-2 and AgPt-3 hollow nanospheres were evaluated. As a comparison, commercial Pt/C (Johnson Matthey, 20%) which acted as the standard catalyst was also explored. Figure 7a shows typical cyclic voltammogram (CV) profiles of the Ag–Pt hollow nanospheres and commercial Pt/C. These CV profiles are measured in a 0.5 mol L–1 nitrogen saturated sulphuric acid solution in a potential range between –0.2 and 1.2 V with a scan rate of 50 mV s–1. All of these CVs exhibit typical regions of the hydrogen adsorption and desorption, electrochemical double layer and metal redox. According to these CV profiles, the specific electrochemical surface areas (ECSA) of the catalysts are calculated based on the following equation [Eq. (1)]:28 ECSA = QH/(m Qref)

(1)

Here QH is the coulombic charges for hydrogen adsorption (mC cm−2), m is the loading mass of Pt (mg cm−2) on the glassy carbon electrode, and Qref is the charge required for a monolayer adsorption of hydrogen on Pt catalyst surface.28,29 The ECSA values of AgPt-1, AgPt-2, AgPt-3 nanospheres and commercial Pt/C are 56.14, 45.18, 46.21 and 43.77 m2 g–1, respectively (Table 1). It is obvious that the AgPt-1

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hollow nanospheres have the largest ECSA. The catalytic efficiencies of the catalysts for MOR were measured in a mixed solution containing 0.5 mol L–1 sulphuric acid and 0.5 mol L–1 methanol at room temperature. Figure 7b shows typical CV profiles of methanol oxidation on different catalysts, which possess two obvious peaks in the forward and reverse sweeps, respectively. Both the two peaks can be ascribed to the characteristic methanol electrooxidation on the catalyst surfaces.30,31 The peak current density in the forward sweep can be applied to evaluate the catalytic activities of the catalysts.32,33 The mass activities of the catalysts calculated by dividing the peak current densities with Pt mass are shown in table 1. The mass activities of AgPt-1, AgPt-2, AgPt-3 and commercial Pt/C are 354.8, 236.0, 276.4 and 167.6 mA mgPt−1, respectively. It is clear that all the bimetallic Ag–Pt samples exhibit higher catalytic activity than commercial Pt/C, and the AgPt-1 sample shows the highest activity. Long-term catalytic performances of the catalysts are also investigated. Chronoamperograms of the catalysts for MOR were recorded at 0.60 V for 3600 s at room temperature, and the resultant chronoamperometric curves are revealed in Figure 8a. It can be seen that the current densities of all the catalysts quickly decreased at the initial stage. The decrease of current densities can be explained by the decrease of methanol concentration gradient at the catalyst surface as well as the poisoning of the catalysts.34 After the tests continued for 3600 s, the current density of the AgPt-1, AgPt-2, AgPt-3 and commercial Pt/C catalysts are 54.47, 33.45, 54.59 and 5.93 mA mgPt−1, respectively. The AgPt-1 sample shows the highest catalytic activities

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over the entire period, indicating its best catalytic efficiencies. In order to investigate the stability of the catalyst, the CV curves of the AgPt-1 catalyst in 0.5 mol L–1 N2-saturated sulphuric acid solution before and after chronoamperogram test for 3600 s are compared, as shown in Figure 8b. There is a little decrease of the peak area of hydrogen desorption after chronoamperogram test, which may due to the fact that Ag atoms are not stable in acid electrolyte. In order to verify the above speculation, the morphology and composition of the AgPt-1 catalyst after chronoamperogram test was characterized by TEM and EDS, as shown in Figure 8c and d. It can be seen that the morphology of the AgPt-1 catalyst is almost the same as before. However, the atomic ratio of Ag/Pt is decreased to 0.85:1. The improved catalytic activities of the bimetallic Ag–Pt hollow nanospheres are ascribed to the following aspects. Firstly, the hollow nanostructure can provide more active sites, which can be proved by the larger ESCA of the Ag–Pt hollow nanospheres than commercial Pt/C. Secondly, as indicated from XPS results, the co-existance of Ag and Pt in the Ag–Pt hollow nanospheres changed the surface electronic structure of Pt and modified the d-band center position of Pt. As a result, the CO adsorption energy on Pt is decreased, and the anti-poisoning ability of the Ag–Pt catalysts is increased.35,36 Such a synergistic effect of Pt and Ag result in higher catalytic activity of Ag–Pt catalysts than Pt-only catalysts. The difference of Ag/Pt ratio, particle size and wall thickness of the Ag–Pt hollow nanospheres may also affect their catalytic activities for MOR. As indicated from EDS elemental mapping (Figure 3) results, the distribution of Pt in the Ag–Pt hollow nanospheres is closer to

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the surface. The ratio of Pt atoms on the surface of hollow nanospheres varies with the change of Ag/Pt ratio. As a result, the ESCA of the Ag–Pt hollow nanospheres is different. Furthermore, according to previous reports, the shift of Pt 4f XPS peaks is different with the change of Pt ratio in the alloys.37,38 Therefore, the CO tolerance ability of the Ag–Pt hollow nanospheres can be affected by the Ag/Pt ratio. The difference in particle size may result in different specific surface areas, which affects the ESCA of the catalysts. The wall thickness of the Ag–Pt hollow nanospheres may affect mass transport. According to the above discussions, the Ag–Pt hollow nanospheres with difference Ag/Pt ratio, particle size and wall thickness exhibit different MOR performances. All in all, the bimetallic Ag–Pt hollow nanospheres show good catalytic activities for MOR and may have potential applications for DMFCs.

4. Conclusions In summary, a simple one-pot strategy was developed for the synthesis of bimetallic Ag–Pt hollow nanospheres. The Ag nanoparticles which were created at the earlier stage of the synthesis process serve as a template for the formation of the hollow nanostructure, and were consumed at the succeeding reacting process. Electrocatalytic performances of the Ag–Pt hollow nanospheres with different Ag/Pt ratio were investigated, and the Ag–Pt hollow nanospheres with a Ag/Pt atomic ratio of 0.89:1 shows larger ECSA and better catalytic activities than other Ag–Pt hollow nanospheres as well as commercial Pt/C catalyst. The superior electrocatalytic

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activities can be attributed to the hollow nanostructure and the synergetic effects between Ag and Pt. This method is promising for the preparation of other metallic hollow nanomaterials which may have potential applications in areas such as catalysts for DMFCs.

Acknowledgement Financial support from the National Natural Science Foundation of China (No. 21671007) is gratefully acknowledged.

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25. Li, H. H.; Zhao, S.; Gong, M.; Cui, C. H.; He, D.; Liang, H. W.; Wu, L.; Yu, S. H. Ultrathin PtPdTe Nanowires as Superior Catalysts for Methanol Electrooxidation. Angew. Chem. Inter. Ed. 2013, 52, 7472–7476. 26. Wang, D.Y.; Chou, H. L.; Lin, Y. C.; Lai, F. J.; Chen, C. H.; Lee, J. F.; Hwang, B.J.; Chen, C. Simple Replacement Reaction for the Preparation of Ternary Fe1-xPtRux Nanocrystals with Superior Catalytic Activity in Methanol Oxidation Reaction. J. Am. Chem. Soc. 2012, 134, 10011–10020. 27. Fan, H. J.; Gösele, U. Zacharias, M. Formation of Nanotubes and Hollow Nanoparticles Based on Kirkendall and Diffusion Processes: A Review. Small 2007, 3, 1660–1671. 28. Garsany, Y.; Baturina, O. A.; Swider-Lyons, K. E.; Kocha, S. S. Experimental Methods for Quantifying the Activity of Platinum Electrocatalysts for the Oxygen Reduction Reaction. Anal. Chem. 2010, 82, 6321–6328. 29. Sun, L. T.; Wang, H. J.; Eid, K.; Alshehri, S. M.; Malgras, V.; Yamauchi, Y.; Wang, L. One-Step Synthesis of Dendritic Bimetallic PtPd Nanoparticles on Reduced Graphene Oxide and Its Electrocatalytic Properties. Electrochim. Acta 2016, 188, 845–851. 30. Hofstead-Duffy, A. M.; Chen, D. J.; Sun, S. G.; Tong, Y. Y. J. Origin of the current peak of negetive scan in the cyclic voltammetry of methanol electro-oxidation on Pt-based electrocatalysts: a revisit to the current ratio criterion. J. Mater. Chem. 2012, 22, 5205–5208. 31. Pan, D. Y.; Wang, X. Y.; Li, J. H.; Wang, L.; Li, Z.; Liu,Y.; Liao, H. B.; Feng, C.

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Tables

Table 1 Summary result for the electrooxidation of methanol catalyzed by the Ag–Pt hollow nanosphere and the commercial Pt/C catalyst.

Catalyst

Pt loading

ECSA

Mass activity

[µg]

[m2 g−1]

[mA mgPt−1]

AgPt-1

8

56.14

354.8

AgPt-2

8

45.18

236.0

AgPt-3

8

46.21

276.4

Commercial Pt/C

8

43.77

167.6

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Figure captions Figure 1. (a) XRD pattern of the AgPt-1 sample; (b–d) survey, Pt 4f and Ag 3d XPS spectra of the AgPt-1 sample. Figure 2. (a) SEM image, (b) TEM image, (c) HRTEM image and (d) EDS spectrum of the AgPt-1 sample. Figure 3. (a) STEM image and (b–d) EDS elemental mappings of the Ag–Pt bimetallic hollow nanospheres. Figure 4. (a,b) SEM and TEM images of AgPt-2 sample; (c,d) SEM and TEM images of AgPt-3 sample. Figure 5. Formation mechanism of the Ag–Pt bimetallic hollow nanospheres. Figure 6. (a) TEM image of Ag nanospheres, (b) TEM image of pure Pt dendritic nanoparticles. Figure 7. (a) CV curves of the catalysts in 0.5 mol L–1 N2-saturated sulphuric acid solution, (b) CV curves of the catalysts in 0.5 mol L–1 sulphuric acid and 0.5 mol L–1 methanol solution. Figure 8. (a) Chronoamperograms of different catalysts for MOR at 0.60 V for 3600 s, (b) CV curves of the AgPt-1 catalyst in 0.5 mol L–1 N2-saturated sulphuric acid solution before and after chronoamperogram test for 3600 s, (c,d) TEM image and EDS spectrum of the AgPt-1 catalyst after chronoamperogram test for 3600 s.

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Figure 1. (a) XRD pattern of the AgPt-1 sample; (b–d) survey, Pt 4f and Ag 3d XPS spectra of the AgPt-1 sample.

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Figure 2. (a) SEM image, (b) TEM image, (c) HRTEM image and (d) EDS spectrum of the AgPt-1 sample.

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Figure 3. (a) STEM image and (b–d) EDS elemental mappings of the Ag–Pt bimetallic hollow nanospheres.

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Figure 4. (a,b) SEM and TEM images of AgPt-2 sample; (c,d) SEM and TEM images of AgPt-3 sample.

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Figure 5. Formation mechanism of the Ag–Pt bimetallic hollow nanospheres.

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Figure 6. (a) TEM image of Ag nanospheres, (b) TEM image of pure Pt dendritic nanoparticles.

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Figure 7. (a) CV curves of the catalysts in 0.5 mol L–1 N2-saturated sulphuric acid solution, (b) CV curves of the catalysts in 0.5 mol L–1 sulphuric acid and 0.5 mol L–1 methanol solution.

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Figure 8. (a) Chronoamperograms of different catalysts for MOR at 0.60 V for 3600 s, (b) CV curves of the AgPt-1 catalyst in 0.5 mol L–1 N2-saturated sulphuric acid solution before and after chronoamperogram test for 3600 s, (c,d) TEM image and EDS spectrum of the AgPt-1 catalyst after chronoamperogram test for 3600 s.

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