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Pt Nanorods Aggregates with Enhanced Electrocatalytic Activity toward Methanol Oxidation Yun-Bo He, Gao-Ren Li,* Zi-Long Wang, Yan-Nan Ou, and Ye-Xiang Tong MOE Laboratory of Bioinorganic and Synthetic Chemistry/School of Chemistry and Chemical Engineering/ Institute of Optoelectronic and Functional Composite Materials, Sun Yat-sen UniVersity, Guangzhou 510275, P. R. China ReceiVed: May 31, 2010; ReVised Manuscript ReceiVed: September 16, 2010
Pt as one of the most promising electrochemical catalysts has attracted much attention because of its superior electrochemical performance. A facile electrochemical route was utilized to synthesize Pt nanorods aggregates at room temperature. The morphologies and structures of the prepared Pt deposits were characterized by scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometer (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The electrocatalytical activities of Pt nanorods aggregates were evaluated using methanol as model molecules. Pt nanorods aggregates showed highly improved electrocatalytical activity toward methanol oxidation compared with Pt nanoparticles aggregates and commercial Pt. The present method provides a new and facile strategy toward the synthesis of nanorods aggregates of noble metals with extensive applications. Introduction Recently, the development of the electrocatalysts for alternative energy sources such as hydrogen and fuel cells are attracting more and more attention as energy sources in an attempt to relieve the pollution and energy crises. Among the various electrocatalysts, Pt and Pt-based materials are still indispensable and are the most practical catalysts for fuel cell applications.1-4 Unfortunately, the expensive nature of Pt is a critical problem and has limited its technological viability. Therefore, there is a strong appetency to make more-efficient Pt and Pt-based catalysts. Up to now, many attempts have been contributed to the development of techniques to produce Pt catalysts with a high catalytic performance and utilization efficiency.5-7 As the catalytic property depends on both the size and the shape of Pt, so people can increase their catalytic efficiency and reduce the dosage by the fabrication of Pt nanostructures.8,9 Recently, many efforts have been made toward the synthesis of morphologycontrolled Pt nanoparticles and their influence on catalytic activity. Various Pt nanoparticles shapes have been successfully synthesized, such as Pt polyhedral,10 nanowires,11 nanorods,12 nanotubes,13 nanocubes,14 and flowerlike nanostructures.15 Among various nanostructures, the hierarchical nanostructures have attracted intensive research interest because they have higher surface area and supply enough absorption sites for all involved molecules in a narrow space. In general, the synthesis methods of Pt nanostructures involve the reduction of Pt salts in the presence of organic surfactants or polymeric stabilizers at the elevated temperature.16 In addition, the templates, such as the channels of porous materials17 and self-assembled structures of surfactants,18 were usually used for the synthesis of Pt nanostructures. On the basis of these facts, the development of mild, surfactant-free, and template-free methods for the synthesis of novel Pt nanostructures still remains a challenge. In addition, almost all of the previously mentioned synthetic methods produce powders that must be suspended in * To whom correspondence should be addressed. E-mail: ligaoren@ mail.sysu.edu.cn.
binders to evaluate their catalytic performance. However, the electrochemical deposition route is not plagued by these drawbacks. The distinct advantage of the electrochemical deposition described here is that the active material (Pt) is deposited directly onto a conducting substrate with excellent electrical contact without requiring post treatment. Herein, we reported the synthesis of the monodisperse Pt flowerlike nanorods aggregates in one step without any templates by the electrochemical route. In this process, the electrochemical deposition shows a simple, quick, and economical method for the synthesis of Pt nanorods aggregates. The deposition rate can be well controlled by deposition potential, current density, or salt concentrations. These prepared Pt flowerlike nanorods aggregates have a high electrochemically active surface area (EASA), exhibit predominant catalytic properties for the electrochemical oxidation of methanol, and have potential application in designing a membrane assembly for direct methanol fuel cells (DMFCs). Experimental Section Electrochemical deposition of Pt nanorods aggregates was carried out in solution of 0.005 M H2PtCl6 with different additives at potential of -0.4 V for 650 s. A simple threeelectrode cell was used in our experiments. A graphite electrode was used as a counter electrode (spectral grade, 1.8 cm2). A saturated calomel electrode (SCE) was used as the reference electrode that was connected to the cell with a double salt bridge system. All potential values determined in this study were the values versus SCE. A titanium sheet with surface area of 1.2 cm2 was used as a working electrode during electrodeposition. The electrodeposition experiments were carried out by potentiostatic electrolysis at room temperature. The obtained Pt deposits were analyzed by X-ray diffraction (XRD, PIGAKU, D/MAX 2200 VPC) to determine the film structures. The compositional information was obtained with an energy dispersive X-ray spectrometer (EDS). The surface morphologies of the deposited films were observed by field emission scanning electron microscopy (FE-SEM, JSM-6330F), thermal field
10.1021/jp104991k 2010 American Chemical Society Published on Web 10/22/2010
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Figure 1. (a) Low- and (b) high-magnification SEM images of the obtained Pt nanoflowers prepared in solution of 0.005 M H2PtCl6 + 0.01 M succinic acid at potential of -0.4 V for 650 s, (c) TEM image of a single Pt nanoflower. Inserts: SAED pattern of nanorods in the nanoflowers and HRTEM image of the tip of an individual nanorod, (d) EDS pattern of as-obtained Pt nanoflowers.
emission environment scanning electron microscope (TFE-SEM, FEI, Quanta 400), and transmission electron microscope (TEM, JEM-2010HR). The samples of the prepared Pt flowerlike nanorods aggregates and Pt nanoparticles aggregates were prepared by mechanically scratching Pt films for TEM. In addition, for TEM characterization, the scratched specimens were mixed with a small drop of ethanol, and then were put onto a holey-copper grid and dried in air at room temperature. The X-ray photoelectron spectroscopy of Pt sample was used to determine the chemical bonding state and surface composition of the deposits. X-ray photoelectron spectroscopy (XPS) analysis was carried out in a VG ESCALAB 250 (Thermo VG), using a nonmonochromated Al KR radiation in a twin anode and the distance between X-ray gun and sample was about 1 cm. The analysis chamber pressure was about 2 × 10-7 Pa. Highresolution spectra, at an energy resolution of ∼0.85 eV, were obtained at a perpendicular take off angle, using a pass energy of 20 eV steps. The binding energies were calibrated by placing the C1s peak of adventitious carbon at 284.6 eV. After Shirley background removal, the component peaks were separated by the XPS Peak Fit program version 4.1, using peak widths and shapes previously determined. The electrochemical properties of the prepared Pt nanorods aggregates were studied by cyclic voltammetry in a standard three-electrode cell at room temperature. The loading level was obtained by weighting the electrodes before and after electrodeposition. To decrease the errors, we do it as follows: before electrodeposition, the Ti substrates were dried at 500 °C in vacuum and then we weighted it; after electrodeposition, Pt
deposits on Ti substrates were also dried at 500 °C in vacuum and then weighted them. Then 30 µL of Nafion (0.2%) was placed on the surface of samples and dried before electrochemical experiments. A Pt foil served as the counter electrode. A saturated calomel electrode (SCE) was used as the reference electrode. Cyclic voltammetry measurements were carried out on a CHI 750 dual-channel electrochemical workstation (CH instruments, Inc.). Pt loading of both catalysts on electrode was 1.06 mg cm-2. The region for hydrogen adsorption (-0.37-0 V vs SCE on the backward potential scan) was used to estimate the EASAs. For CVs and chronoamperometry of methanol oxidation reaction, an air-free aqueous solution containing 1.0 M methanol and 0.5 M H2SO4 was used. CV curve was recorded between -0.28 and 1.2 V versus SCE with a scan rate of 50 mV s-1. Before recording chronoamperometry curves, the potential of working electrode (samples) was cycled several times then preset at 0 V for 30 s to remove any contaminants or oxidants on the surface. Results and Discussion First, the electrodeposition of Pt was carried out in solution of 0.005 M H2PtCl6 + 0.01 M succinic acid at potential of -0.4 V for 650 s. The morphology of the obtained Pt nanostructures was characterized using SEM, and it was shown in parts a and b of Figure 1 with different magnifications. The low-magnification SEM image in part a of Figure 1 indicates the obtained Pt deposits are uniform and dispersed flowerlike nanorods aggregates with size of about 600-800 nm. The low magnification
Pt Nanorods Aggregates
Figure 2. XRD pattern of Pt nanoflowers prepared in solution of 0.005 M H2PtCl6 + 0.01 M succinic acid at potential of -0.4 V for 650 s.
image also confirms the production of large-scale and narrow size distribution. The flowerlike characters of the structures are evident in the high-magnification micrograph, and they are built up by large numbers of spearlike nanorods with an average bottom diameter of about 50 nm and an average top diameter of about 5 nm (part b of Figure 1). These spearlike nanorods are interconnected with each other to form bigger secondary 3D flowerlike architectures. The selected-area electron diffraction (SAED) and HRTEM image shown in part c of Figure 1 indicate Pt nanorod has a single-crystal structure. The chemical composition of the obtained Pt flowerlike nanorods aggregates was determined by EDS analysis. In the EDS spectrum (part c of Figure 1), only Pt peaks are observed. This result indicates the obtained 3D flowerlike nanorods aggregates are composed exclusively of pure Pt. Part a of Figure 2 shows XRD pattern of Pt flowerlike nanorods aggregates, and it reveals that the obtained Pt flowerlike nanorods aggregates possess cubic structure with high crystallinity. XPS analysis was also used for the surface characterization of Pt catalysts. Figure 3 shows XPS spectra of Pt 4f and Pt 4d. The binding energies of all spectra are referenced to a C 1s value of 284.6 eV. Pt 4f displays two doublets from the spin orbital splitting of 4f5/2 and 4f7/2 states as shown in part a of Figure 3. Pt(4f) signals at 71.5 and 74.8 eV reveal that the deposited Pt nanoparticles all are in a metallic form.16a,19,20 The metallic state platinum on the surface can provide catalytically available sites, and the fraction of metallic state platinum on the surface can be a good indication of catalyst activity. On the
J. Phys. Chem. C, Vol. 114, No. 45, 2010 19177 basis of the high Pt(0)s value, the catalyst is expected to be an effective electrocatalyst for fuel cells. It should be noticed that the positions of these peaks are affected by the nanostructures. For bulk Pt, Pt (4f7/2) and Pt (4f5/2) peaks appear at about 71.0 and 74.3 eV, respectively.21 Therefore, a shift of about 0.5 eV to a higher binding energy was found in both Pt (4f7/2) and Pt (4f5/2) electrons of Pt flowerlike nanorods aggregates compared with those of bulk Pt. As XPS peak position is determined by the electron states of Pt, so XPS peak shift of Pt flowerlike nanorods aggregates can be attributed to the effect of special flowerlike nanostructures in modifying the electronic properties of Pt. Herein, we investigated the effect of additives on the morphologies of Pt nanostructures. When electrochemical deposition was carried out in solution of 0.005 M H2PtCl6 + 0.01 M thiourea +5 × 10-4 M polyethylene glycol (PEG) at potential of -0.4 V for 650 s, Pt nanoparticles aggregates were prepared, and their SEM images were shown in parts a and b of Figure 4. Parts c and d of Figure 4 show TEM images of Pt nanoparticles aggregates. The sizes of nanoparticles aggregates are about 100-600 nm, and the average size of nanoparticles is about 4 nm. SAED pattern of Pt nanoparticles is shown in inset in part d of Figure 4, and four kinds of crystalline orientations are clearly observed, which are indexed to be {111}, {200}, {220}, and {311} reflections of face-centered cubic (fcc) Pt.22 XRD pattern of Pt nanoparticles aggregates was shown in part e of Figure 4, which also reveals that the obtained Pt samples possess cubic structures. The formation of above different nanoaggregates may be attributed to the different deposition rates. The higher deposition rate favors the formation of nanoparticles aggregates, suppressing the growth of nanorods.23 The deposition rate of Pt was changed to be slow because of the stabilizing role of succinic acid for H2PtCl6 caused by complexing action when the succinic acid was added to the solution of H2PtCl6. The stabilizing role of succinic acid for H2PtCl6 can be explained by the cyclic voltammograms measured in aqueous solutions of 0.005 M H2PtCl6 + 0.01 M succinic acid, 0.005 M H2PtCl6, and 0.005 M H2PtCl6 + 0.01 M thiourea + 5 × 10-4 M PEG as shown in parts a, b, and c respectively of Figure 5. In all of the above CVs, two redox couples can be observed, indicating that Pt has been successfully deposited. The cathodic waves corresponding to the reduction of H2PtCl6 shown in part a of Figure 5 shifted negatively comparing with that shown in part b of Figure 5 when succinic acid was present in solution of H2PtCl6. The cathodic shift
Figure 3. XPS spectra of the obtained Pt nanoflowers prepared in solution of 0.005 M H2PtCl6 + 0.01 M succinic acid at potential of -0.4 V for 650 s.
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Figure 4. (a) Low- and (b) high-magnification SEM images of the obtained Pt nanoparticles aggregates prepared in solution of 0.005 M H2PtCl6 + 0.01 M thiourea + 5 × 10-6 M PEG at potential of -0.4 V for 650 s; (c, d) TEM images of Pt nanoparticles aggregates; (e) XRD pattern of Pt nanoparticles aggregates; (f) SEM image of Pt deposits preapred in solution of 0.005 M H2PtCl6 at potential of -0.4 V for 650 s.
suggests a restraining role of succinic acid for the electroreduction of H2PtCl6. However, when the thiourea and PEG were added into deposition solution, the cathodic wave corresponding to the reduction of H2PtCl6 shown in part c of Figure 5 shifted positively, and the current density was larger. Therefore, with the addition of thiourea and PEG in deposition solution, the electrodeposition rate of Pt was increased and more nucleuses were formed, which leads to the formation of nanoparticles aggregates as shown in parts a and b of Figure 4. On the basis of the above results, a schematic illustration for the evolution of Pt nanostructures is presented in Figure 6. First, H2PtCl6 ions are electro-reduced, and then Pt nanonuclei are formed. The electrochemical deposition generally gives rise to the isolated nuclei on the substrate, and these nuclei are random.19,20 With deposition time increasing, these isolated nuclei will evolve into the nanoparticles aggregates driven by the minimization of interfacial energy as these freshly formed nuclei are thermo-
dynamically unstable because of their high surface energy. As we all know, Pt has a face-centered cubic structure, and the order of surface energies is (110) > (100) > (111).24 Therefore, the slow electrodeposition rate favors the anisotropic growth and the growth along the closed-packed 〈111〉 direction has the priority according to the lowest energy principle. In addition, when succinic acid was added into deposition solution, the newly formed side surfaces, (110), (100), and (111) facets, will be stabilized through chemical interactions with the oxygen atoms of succinic acid. In comparison, the interaction between succinic acid and (111) facets should be much weaker compared with that between succinic acid and (110) facets and that between succinic acid and (100) facets,25 and this will favor to the anisotropic growth of Pt along 〈111〉 direction. On the basis of the growth superiority of Pt along 〈111〉 direction, Pt nanorods are finally formed. In the end, these formed Pt nanorods agglomerate to form the flowerlike nanorods aggregates.
Pt Nanorods Aggregates
Figure 5. Cyclic voltammograms for electrodeposition of Pt in solutions of (a) 0.005 M H2PtCl6 + 0.01 M succinic acid, (b) 0.005 M H2PtCl6, and (c) 0.005 M H2PtCl6 + 0.01 M thiourea + 5 × 10-4 M PEG.
However, when thiourea and PEG were added into deposition, the deposition rate of Pt obviously increased, and a large number of nuclei are formed simultaneously, which finally leads to the formation of isotropic Pt nanoparticles aggregates. However, when the electrodeposition was carried out without any additives, namely, in solution of 0.005 M H2PtCl6 at potential of -0.4 V for 650 s, out-of-order of Pt nanoparticles were obtained as shown in part f of Figure 4. The EASA was studied based on hydrogen adsorption using cyclic voltammetry (CV). Part a of Figure 7 shows the cyclic voltammogram of Pt flowerlike nanorods aggregates in the deaerated H2SO4 solution (pH 1) at a scan rate of 50 mV s-1. Characteristic peaks in the negative region (below 0 V) are attributed to atomic hydrogen adsorption on Pt surface and it can reflect the EASA of Pt. Remarkably large peaks were observed on Pt flowerlike nanorods aggregates, reflecting their high surface area. The typical Pt oxide formation and its reduction above 0 V were also observed. In addition, it should be noted that the multiple peaks associated with hydrogen adsorption and desorption were observed, indicating the multiple exposed crystallographic planes. The H adsorption/desorption peaks at about -0.13 V can be attributed to Pt(100) crystal faces.26-28 The more negative H adsorption features at about -0.26 V can be attributed to Pt(110) crystal faces.26-28 The H adsorption/desorption peaks at potentials < -0.30 V were not observed, indicating a low presence of (111) facets. EASAs of Pt flowerlike nanorods aggregates were estimated to be 56 m2 g-1 by EASA ) QH/[Pt] × 0.21 {[Pt]: Pt loading, mg/cm2; QH: the charge for hydrogen desorption, mC/cm2}, which is higher than 35 m2 g-1 of Pt nanoparticles aggregates as shown in Figure 7. This indicates that more Pt is available electrochemically at the surface of Pt flowerlike nanorods aggregates than at the surface of Pt nanoparticles aggregates. So, such flowerlike nanorods aggregate structures largely enhance the active sites for the electro-oxidation of methanol. In addition, compared with Pt nanoparticles aggregates, Pt flowerlike nanorods aggregates also can improve mass diffusion efficiency of guest molecules because of the interstices between the nanorods. To investigate the electrocatalytic activity of Pt flowerlike nanorods aggregates, the cyclic voltametric measurements of methanol oxidation were carried out in solution of 0.5 M H2SO4+2.0 M CH3OH at room temperature at a scan rate of 50 mV s-1 (Figure 8). In parts a and b of Figure 8, the forward
J. Phys. Chem. C, Vol. 114, No. 45, 2010 19179 anodic peaks at around 0.61 V are due to methanol oxidations, and in the backward scan the oxidation peaks at about 0.43 V can be attributed to the oxidations of adsorbed CO or CO-like species. The high electrocatalytic activity of Pt flowerlike nanorods aggregates is indicated by its superior performance for methanol oxidation. For Pt flowerlike nanorods aggregates, the onset potential for methanol oxidation is about 0.360 V, which is 65 mV more negative than 0.425 V obtained on Pt nanoparticles aggregates. This indicates a significant enhancement in the kinetics of methanol oxidation reaction. In addition, the maximum peak current density of methanol oxidation on Pt flowerlike nanorods aggregates is about twice that of Pt nanoparticles aggregates. This fact shows Pt flowerlike nanorods aggregates exhibited higher electrocatalytic activity for the oxidation of methanol compared with Pt nanoparticles aggregates. If the maximum peak current density in the forward is designated as jf and the maximum peak current density in the backward is designated as jb, the ratio of jf/jb is generally used to evaluate the tolerance of the catalysts to incompletely oxidized species accumulated on the surface of the electrode.29,30 A larger ratio of jf/jb represents more complete methanol oxidation, less accumulation of CO or CO-like species on the catalyst surface.19a From part a of Figure 8, the jf/jb ratio for Pt flowerlike nanorods aggregates is about 2.42 that is larger than 1.68 obtained on Pt nanoparticles aggregates. Therefore, Pt flowerlike nanorods aggregate electrode can lead to more complete methanol oxidation and less accumulation of CO or CO-like species than Pt nanoparticles aggregates. The durability of Pt samples is examined by repeating CV scan for 500 cycles at a scan rate of 50 mV/s. Parts b and d of Figure 8 show CVs of Pt flowerlike nanorods aggregates and nanoparticles aggregates after 500 cycles, repectively. For Pt nanoparticles aggregates catalyst, we observed a decrease in oxygen reduction reaction activity by 26.6%, which could be due to the aggregation and/or dissolution of Pt species. But for Pt flowerlike nanorods aggregates, a small decrease with 7.6% was observed, which shows Pt flowerlike nanorods aggregates have much higher stability than Pt nanoparticles aggregates. Part a of Figure 8 also clearly shows that the oxidation current density of Pt flowerlike nanorods aggregates is considerably higher than that of the commercial E-Tek catalyst in part e of Figure 8. This significant improvement in the catalytic performance can be attributed to the high level of dispersion of Pt nanoparticles and their special nanostructures. The inserts (1) and (2) in Figure 8 are SEM images of Pt flowerlike nanorods and Pt nanoparticles aggregates, respectively. After 500 cycles, Pt flowerlike nanorods and Pt nanoparticles aggregates almost keep the same morphologies as before. The rate of surface poisoning of the prepared Pt flowerlike nanorods aggregates was examined using chronoamperometry. Figure 9 shows the chronoamperometric curves of Pt flowerlike nanorods aggregates in solution of 0.5 M H2SO4 + 2.0 M CH3OH at a constant potential of 0.55 V versus SCE for 1200 s. At the beginning, the current density of Pt flowerlike nanorods aggregates electrode is 47 mA cm-2, which is much higher than 18 mA cm-2 obtained on Pt nanoparticles aggregates and 31 mA cm-2 obtained on the commercial E-Tek catalyst, indicating higher electrocatalytic activity for methanol oxidation. Pt flowerlike nanorods aggregates exhibited a slower current decay over time in comparison to Pt nanoparticles aggregates and the commercial E-Tek catalyst, indicating a larger catalytically active surface area and higher tolerance to CO-like intermediates. According to the results presented above, it is obvious that the particular structure of the flowerlike nanorods aggregates
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Figure 6. Schematic illustration of the formation and shape evolution of Pt flowerlike nanorods aggregates and nanoparticles aggregates.
Figure 7. Cyclic voltammograms for (a) Pt nanoflowers and (b) Pt nanoparticles recorded in the deaerated H2SO4 solution (pH 1) at a scan rate of 50 mV s-1.
Figure 9. Chronoamperometry curves of various Pt samples recorded in solution of 0.5 M H2SO4 + 2.0 M CH3OH at a constant potential of 0.55 V versus SCE for 1200 s: (a) Pt flowerlike nanorods aggregates, (b) Pt nanoparticles aggtegates, (c) E-Tek Pt/C catalyst.
nanorods aggregates can lead to higher surface area and provide more absorption sites for involved molecules in a limited space; fourth, the interconnected structures in flowerlike nanorods aggregates also can provide a faster electron transmission, which is favor for the enhancement of Pt electrocatalystic property. In addition, the good tolerance of Pt flowerlike nanorods aggregates to CO-like intermediates also has an advantage for the enhancement of the catalytic activities. Conclusions
Figure 8. Cyclic voltammograms of methanol oxidation on Pt flowerlike nanorods aggregates: (a) initial; (b) after 500 cycles; on Pt nanoparticles aggregates: (c) initial; (d) after 500 cycles; and on E-Tek Pt/C catalyst (e) recorded in solution of 0.5 M H2SO4 + 2.0 M CH3OH at room temperature at a scan rate of 50 mV s-1. The insets (1) and (2) are SEM images of Pt flowerlike nanorods and Pt nanoparticles aggregates after 500 cycles, respectively.
results in its predominant catalytic properties for the electrochemical oxidation of methanol. First, the unique properties of Pt nanorods can bring the improved catalysis; second, Pt nanorods can create channels for the effective transport of electrolyte; third, the interconnected structures in flowerlike
In summary, here we have developed a facile electrochemical procedure to synthesize 3D Pt superstructures, in which the large-scale assembly of flowerlike nanorods aggregates occurs at room temperature without any template. The electrochemical deposition is a promising cost-effective technique to fabricate 3D superstructures. The growth of various Pt nanoaggregates could be well controlled by manipulating electrodeposition parameters, such as additives. XPS revealed the shift of the d-band center of Pt to a lower level, which enhances the catalyst activity of Pt. Compared with nanoparticles aggregates, Pt flowerlike nanorods aggregates show predominant catalytic properties for methanol electro-oxidation of because of their special nanostructures. The use of Pt flowerlike nanorods aggregates as catalysts are believed to open the exciting possibilities for enhancing the performance of DMFC fuel cells. This simple approach is hoped to extend to prepare the
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