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2008, 112, 13342–13345 Published on Web 08/13/2008
Ag-Pt Nanoplates: Galvanic Displacement Preparation and Their Applications As Electrocatalysts Chien-Liang Lee* and Chun-Ming Tseng Department of Chemical and Materials Engineering, National Kaohsiung UniVersity of Applied Science, Kaohsiung 807, Taiwan, R. O. C. ReceiVed: July 03, 2008; ReVised Manuscript ReceiVed: August 01, 2008
Ag-Pt circular, hexagonal, and triangular nanoplates are successfully prepared by the galvanic displacement reaction and used as electrocatalysts for hydrogen evolution reaction. As an electrochemical analyses, fair comparison regarding the activity can be thus made among these newly electrocatalysts. The triangular Ag-Pt nanoplates are measured to have maximum electrochemical activity. Introduction Metal nanoparticle-assisted catalytic properties on chemical1,2 and electrochemical3,4 reaction has attracted attention because it has high surface area with active sites. The shape of nanoparticles has been further found to have a promising effect on catalysis5-9 and electrocatalysis.3,10 The reason that shape effect promotes catalytic reaction is that higher atomic fractions are located at corners, edges and defects of nanocatalysts5,6,11 when metal nanomaterials of different shapes are used as catalyst. Tetrahedral, cubic, and spherical Pt nanoparticles are used to catalyze the electron-transfer reaction between hexacyanoferrate ions and thiosulfate ions. Pt nanoparticles of tetrahedral microstructures with corner atoms of higher fraction are measured to have higher activity.5,6 Spherical Pt12 and Pt-based alloy13 nanoparticles are often used as catalysts in hydrogen evolution reaction which is the key reaction in fuel cell. Recently, hollow Pt nanospheres were found to have enhanced activities on electrochemical reaction.14 In this investigation, circular, hexagonal, and triangular Ag-Pt nanoplates are prepared and then used as electrocatalysts for hydrogen evolution reaction. Ag-Pt nanoplates with circular (Ag-Ptcircular), hexagonal (Ag-Pthexangular), and triangular (Ag-Pttriangular) morphology are synthesized via a galvanic displacement reaction in which the Ag nanoplates act as templates and react with the added platinum ions. Compared to the morphology of Ag-Ptcircular and Ag-Pthexangular, small assembled islands are formed and observed on Ag-Pttriangular. As an electrochemical analyses, fair comparison regarding the activity can be thus made among these newly electrocatalysts. Experimental Section Preparation and Shape Control of Ag-Pt Nanoplates. The synthesis of Ag-Pt nanoplates in aqueous solution is described below. The method of synthesizing the circular, hexagonal and triangular Ag nanotemplates was modified from the seed growth method which was used for the preparation of triangular Ag nanoplates.15 Initially, 50 µL of a 0.05 M silver nitrate (AgNO3) * To whom correspondence should be addressed. E-mail address:
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aqueous solution was added to 10 mL of a 2.5 × 10-4 M sodium citrate aqueous solution. Then, 25 µL of 0.1 M NaBH4 solution was gradually added to a stirred mixed solution of sodium citrate and AgNO3. A Ag seed solution of light yellow was then obtained. Furthermore, 10 mL of 0.05 M AgNO3 was added to 200 mL of 0.1 M hexadecyltrimethy ammonium bromide (C16TAB) aqueous solution. Ten milliliters of 0.1 M ascorbic acid and 0.408 mL of the prepared Ag seed solution were slowly dropped into the C16TAB aqueous solution. The circular Ag nanotemplates then were prepared after 0.8 mL of 2 M NaOH aqueous solution was added. If the amounts of the seed solution are changed to 0.266 and 0.182 mL in the synthesis pathway, the hexagonal and triangular Ag nanoplates are then obtained, respectively. A 200 mL solution of the synthesized Ag circular nanoplates was precipitated by centrifugation at 4000 rpm and redispersed using 3 mL of deionized water to reduce the interaction of free C16TAB with the synthesis of the Ag-Pt triangular nanoplates. In order to eliminate the Cl- interaction upon the galvanic synthesis, 13.9 mg K2PtCl4 was added and slowly dissolved in 1 mL aqueous solution of 25 mM AgNO3 to form white AgCl solid. The white precipitates were removed by the centrifugation method and then formed the 1 mL solution with Pt2+ concentration of 33.5 mM. A solution of 8.3 µL Pt2+ was added to 3 mL of stirred solution of Ag nanotemplates at a fixed controlled temperature (60 °C). After 70 min, the circular Ag-Pt nanoplates were obtained. On the basis of the same method, hexagonal and triangular Ag-Pt nanoplates are then obtained if hexagonal and triangular Ag nanoplates were used as templates for the galvanic displacement reaction. The characteristic size, shape, and composition of the prepared Ag-Pt nanoplates in solution, which were dipped onto the copper grid that was covered with a carbon film and dried naturally, were observed under a high-resolution transmission electron microscope (HRTEM; JEOL JEM-3000F), and an energy dispersive X-ray spectroscope (EDX). The optical properties of surface plasmon resonance (SPR) of the Ag–Pt nanoplates prepared from Ag nanoplates were measured by UV-vis spectrophotometer (Agilent 8453). As for the analysis 2008 American Chemical Society
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Figure 1. HRTEM images, XRD patterns, and element mapping EDX patterns of prepared Ag-Pt nanoplates (mole ratio: Ag/Pt ) 18:1). (A) HRTEM image of Ag-Ptcircular; (B) HRTEM image of Ag-Pthexangular; (C) HRTEM image of Ag-Pttriangular; (D) XRD patterns; (E) element mapping EDX patterns of Ag-Ptcircular; (F) element mapping EDX patterns of Ag-Pthexangular; (G) element mapping EDX patterns of Ag-Pttriangular.
of X-ray diffraction patterns (XRD) of alloy nanoplates, XRD spectroscopy (Shimadzu XD-3A, Cu anode) was employed. Electrochemical Measurement. A 0.772 mg sample of carbon powder (XC-72) was added and further dispersed into 1 mL of aqueous solution via ultrasonic vibration. A 30 µL sample of the resulting aqueous solution was dropped onto 0.07 cm2 glassy carbon electrode (GCE) and heated at 70 °C to evaporate H2O. On the other hand, in order to compare with the electrochemical activity of Ag-Pt nanoplates, spherical Ag-Pt nanoparticles and Pt nanoparticles are prepared with the same Pt and Ag concentration. A 50 µL solution concentrated from 1 mL solution of the prepared Ag-Pt nanoplates, Ag-Pt
nanoparticles, Pt nanoparticles, and commercial Pt catalysts was dropped onto carbon powder/GCE electrode. The weight of Ag-Ptcircular, Ag-Pthexangular, Ag-Pttriangular, spherical Pt nanocatalysts, Ag-Pt nanocatalysts, and commercial Pt catalysts are estimated to be 6.67 × 10-2, 5 × 10-2, 8.33 × 10-3, 7.5 × 10-2, 1.33 × 10-1, and 1.05 × 10-1 mg, respectively. In order to prevent the catalyst from falling down in the electrolyte during the measurement, the glass carbon electrode was rinsed with 3 µL of 5 wt % Nafion solution and heated at 70 °C for 20 min. Electrochemical measurement was carried out by using a potentiostate (Autolab PGSTAT30). A three-electrode cell, consisting of a GCE working electrode, a Pt counter electrode,
13344 J. Phys. Chem. C, Vol. 112, No. 35, 2008
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and a Ag/AgCl (3 M KCl) reference electrode, was used for the linear scanning voltammetry (LSV) measurement. For the measurement of activity for hydrogen evolution, the LSV experiment was performed in 1 M H2SO4(aq) solution at a scan rate of 20 mV/s. Results and Discussion As depicted in HRTEM images of Figure 1A-C, the Ag-Pt nanoplates with different shapes have been successfully prepared via a galvanic displacement reaction, in which platinum ions were added and then reacted with templates using the Ag nanoplates. This indicates that the shape of Ag-Pt nanoplates can be controlled by using this method. Figure 1A,B clearly reveals that the circular and hexagonal nanoplates with continuous and uniform edges are formed, respectively. As for the case of the prepared triangular nanoplates, which are shown in Figure 1C, the small and assembled solid islands are observed significantly on the surface and edge. XRD spectroscopic analysis of the Ag-Pt nanoplates yields information on the composition architecture. Figure 1D presents the XRD spectrum of the prepared Ag-Ptcircular, Ag-Pthexangular, and Ag-Pttriangular nanoplates. The diffraction peaks in the three cases of the prepared nanomaterials are obtained at the same locations, which are between silver’s (PCPDS No. 089-3722) and platinum’s peaks (PCPDS No. 089-3722). This indicates that the prepared nanomaterials via this displacement method are composed from a Ag-Pt alloy. Four peaks located at 38.25, 44.65, 64.85, and 77.55° detected from the dry Ag-Ptcircular powders are assigned to be (111), (200), (220), and (311) diffraction planes of facecentered cubic (fcc) structure, respectively. Comparing with the stand spectrum of Ag and Pt, it is worthy to note that the diffraction peaks of Ag-Pt nanoplates prepared by the displacement method are located closely to the peaks of silver. Additionally, element mapping of EDX spectroscopic analysis upon the prepared Ag-Pt nanoplates yields further information on the composition architecture and the atomic distribution. Figure 1 E-G presents the EDX spectrum obtained by element mapping of the prepared Ag-Pt nanoplates, as the image of the single Ag-Ptcircular, Ag-Pthexangular, and Ag-Pttriangular nanoplates are captured, respectively. The two strong signals from the nanomaterials identify them as silver and platinum. Random atomic distributions presenting Ag and Pt signal at the Ag-Pt nanoplates are obtained. The exact composition and mixed alloy of the prepared nanoplates is thus determined. The optical properties of the prepared bimetallic nanotemplates are provided by surface plasmon resonance (SPR) spectra. Figure 2 A-C shows a comparison between the SPR spectra for Ag-Ptcircular, Ag-Pthexangular, and Ag-Pttriangular nanoplates and their corresponding Ag nanotemplates, respectively. Theoretically, SPR optical peak of Pt nanoparticles can only be detected at Uv region.16 As for the optical properties of Ag triangular nanoplates, the position of SPR band of long wavelength site presenting the interplane dipole plasmon resonance can redshift with increasing size.17 Note that, in this experimental observation, the SPR positions of long wavelength sites of the prepared Ag nanotemplates with circular, hexagonal, and triangular morphology upon the kinetic-control proceeding with shape change are detected and redshift sensitively from ∼559 nm, and ∼654 to ∼879 nm in the UV-vis spectra, respectively, as shown in Figure 2A-C. Furthermore, after Pt2+ ions are added into Ag nanoplate solution, the significant and broaden SPR bands of Ag-Ptcircular, Ag-Pthexangular, and Ag-Pttriangular nanoplates are detected and then shown in Figure
Figure 2. Surface plasmon resonance spectra of prepared Ag-Pt nanoplates and their corresponding Ag nanotemplates. (A) Ag-Ptcircular; (B) Ag-Pthexangular; (C) Ag-Pttriangular.
Figure 3. The comparative linear scanned voltammogram (LSV) of the triangular, hexangular, and circular Ag-Pt nanoplates for hydrogen evolution. Inset: the comparative LSV of triangular Ag-Pt nanoplates, spherical Ag-Pt nanoparticles, Pt nanoparticles, and commercial Pt catalysts for hydrogen evolution. Electrolyte, 1 M H2SO4(aq); scan rate, 0.02 V/s.
2A-C, respectively. It indicates that the Ag still exists in the nanomaterials after the displacement reaction. Figure 3 reveals the comparison activities of Ag-Ptcircular, Ag-Pthexangular, Ag-Pttriangular, Ag-Pt nanoparticles, Pt nanoparticles, and commercial Pt catalysts upon an electrochemical LSV measurement in the hydrogen evolution reaction, respectively. In a comparison made with Ag-Pt nanoparticles and Pt nanoparticles, the high activity of the prepared Ag-Pt nanoplates all are observed. In contrast to the other nanocatalysts, the Ag-Pttriangular shows its higher activity starting from ∼-0.23V in the acid solution. It is worthy to note that the activity order of Ag-Ptcircular < Ag-Pthexangular < Ag-Pttriangular toward hydrogen evolution is obtained. The active sites of the formed small island on Ag-Pttriangular can adequately react with electrolyte species and enhance the activity. Note that, as shown in the inset of Figure 3, the activity of triangular Ag-Pt nanoplates is found to be much higher than those of spherical Ag-Pt, Pt nanoparticles, and commercial Pt catalysts. This mentions that the prepared Ag-Pt nanoplates have the excellent electroactivities.
Letters Conclusion Circular, hexagonal, and triangular Ag-Pt nanoplates are successfully synthesized in the galvanic displacement reaction, in which added Pt2+ ions slowly reacted with the prepared Ag nanoplates as templates. As supported results by the XRD and EDX spectrum, the prepared nanoplates are mixed alloy structure. Additionally, the prepared Ag-Pt nanoplates are successfully applied as new electrocatalysts on hydrogen evolution. The activity of Ag-Pt nanoplates shows the order of Ag-Ptcircular < Ag-Pthexangular < Ag-Pttriangular nanoplates. Acknowledgment. The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 96-2218-E-151-001. References and Notes (1) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362. (2) Campbell, C. T. Science 2004, 306, 234. (3) Tian, N.; Zhou, Z. Y.; Sun, S. G.; Ding, Y.; Wang, Z. L. Science 2007, 316, 732.
J. Phys. Chem. C, Vol. 112, No. 35, 2008 13345 (4) Ullah, M. H.; Chung, W. S.; Kim, I.; Ha, C. S. Small 2006, 2, 870. (5) Narayanan, R.; El-Sayed, M. A. Nano Lett. 2004, 4, 1343. (6) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2004, 108, 5726. (7) Xu, R.; Wang, D. S.; Zhang, J. T.; Li, Y. D. Chem.sAsian J. 2006, 1, 888. (8) Xiong, Y. J.; Wiley, B.; Xia, Y. N. Angew. Chem., Int. Ed. 2007, 46, 7157. (9) Park, K. H.; Jang, K.; Kim, H. J.; Son, S. U. Angew. Chem., Int. Ed. 2007, 46, 1152. (10) Subhramannia, M.; Ramalyan, K.; Pillal, V. K. Langmuir 2008, 24, 3576. (11) Adzic, R. R.; Tripkovic, A. V.; Ogrady, W. E. Nature 1982, 296, 137. (12) Ma, C.; Sheng, J.; Brandon, N.; Zhang, C.; Li, G. Int. J. Hydrogen Energy 2007, 32, 2824. (13) Wu, M.; Shen, P. K.; Wei, Z. D.; Song, S. Q.; Nie, M. J. Power Sources 2007, 166, 310. (14) Liang, H. P.; Zhang, H. M.; Hu, J. S.; Guo, Y. G.; Wan, L. J.; Bai, C. L. Angew. Chem., Int. Ed. 2004, 43, 1540. (15) Lee, C. L.; Tseng, C. M.; Wu, R. B.; Yang, K. L. Nanotechnology 2008, 19, 215709. (16) Garcia-Gutierrez, D. I.; Gutierrez-Wing, C. E.; Giovanetti, L.; Ramallo-Lopez, J. M.; Requejo, F. G.; Jose-Yacaman, M. J. Phys. Chem. B 2005, 109, 3813. (17) Pastoriza-Santos, I.; Liz-Marzan, L. M. J. Mater. Chem. 2008, 18, 1724.
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