Kinetic study of Pt nanocrystal deposition on Ag nanowires with clean surfaces via galvanic replacement
© Shen et al.; licensee Springer. 2012
Received: 7 February 2012
Accepted: 6 May 2012
Published: 6 May 2012
Without using any templates or surfactants, this study develops a high-yield process to prepare vertical Ag-Pt core-shell nanowires (NWs) by thermally assisted photoreduction of Ag NWs and successive galvanic replacement between Ag and Pt ions. The clean surface of Ag nanowires allows Pt ions to reduce and deposit on it and forms a compact sheath comprising Pt nanocrystals. The core-shell structural feature of the NWs thus produced has been demonstrated via transmission electron microscopy observation and Auger electron spectroscopy elemental analysis. Kinetic analysis suggests that the deposition of Pt is an interface-controlled reaction and is dominated by the oxidative dissolution of Ag atoms. The boundaries in between Pt nanocrystals may act as microchannels for the transport of Ag ions during galvanic replacement reactions.
One-dimensional nanostructures have attracted much interest because of their fascinating properties and extensive use in electronics, sensing, catalysis, and electrochemical applications [1–4]. For example, changing the morphology of Pt nanostructure from nanoparticle to nanowire (NW) has been regarded as an important strategy to improve the performance of Pt-based catalysts, which have been widely used as the anode of direct methanol fuel cells for catalyzing the dehydrogenation of methanol [5–7]. By doing so, an enhancement in electrocatalytic activity can be obtained due to the large side surface, which is able to provide additional catalytic active facets. A great deal of effort has been devoted to the synthesis of Pt nanowires; however, it still remains a huge challenge to synthesize long and oriented single-crystalline Pt NWs [8–11]. Using template- and surfactant-free processes, the Pt NWs produced are extremely fine (mostly less than 10 nm in diameter) but exhibit a limit in length of about 200 nm.
A recently developed process, thermally assisted photoreduction, [16, 17] has been successfully applied to prepare Ag nanowires with the length up to 10 μm on TiO2-coated substrates in large quantities without using templates and surfactants. The substrate could be rigid like Si wafers or flexible like carbon cloths. Ag nanowires thus produced are single-crystalline with a preferred <110> growth direction. The approach of this work is to use these clean-surface Ag NWs as sacrificial templates for the galvanic exchange with Pt ions to synthesize ultra-long Ag-Pt core-shell nanowires in large yield. In addition to the optical properties and microstructural characteristics, the mechanisms and reaction kinetics are also discussed.
Synthesis of Ag NWs on TiO2
Galvanic exchange between Ag and Pt salts
Ag NWs thus prepared were removed from the TiO2 substrate by ultrasonic oscillation and then immersed in 0.05 M aqueous Na2Pt(OH)6 solution for the exchange of Pt (step 4 in Figure 1). Instead of commonly used precursor, H2PtCl6, Na2Pt(OH)6 was adapted in this study because Pt ions can be reduced more easily mainly due to the smaller electronegativity of the OH group (3.02) [18, 19] compared with Cl (3.16). The solution was then isothermally heated to reflux at 100°C, 170°C, and 200°C. After a certain reaction period, the samples were rinsed with deionized water and dried under N2 before analysis.
Characterizations of core-shell NWs
The structure and phase of the NWs were characterized by transmission electron microscopy (TEM) (G2, Philips Tecnai, Amsterdam, The Netherlands) with an accelerating voltage of 200 kV and also a grazing incidence X-ray diffraction meter (D/MAX2500, Rigaku Corporation, Tokyo, Japan) (incidence angle of 0.5°) with graphite monochromatic CuKα radiation (λ = 0.15418 nm) at a scanning rate of 2° per min from 30° to 80°. The morphology and size distribution were obtained by scanning electron microscopy (SEM) (JSM-6700, JEOL Ltd., Akishima, Tokyo, Japan) with an accelerating voltage of 20 kV. The measurements of the UV-visible (vis) absorption spectra were carried out at room temperature using a Hitachi-4001 spectrophotometer (Hitachi High-Tech, Minato-ku, Tokyo, Japan). Auger electron spectroscopy (AES) (MICROLAB 350, Thermo VG Scientific, West Sussex, England) equipped with an Ar ion gun was used to investigate the elemental distribution of the cross-sectioned nanowires.
Results and discussion
Morphology and structure of Ag NWs
Optical and structural characteristics of Ag-Pt core-shell NWs
Reaction kinetics and mechanisms
Activation energies of the reactions in galvanic replacement
Decrease in Ag
Gain of Pt
where SHE is the standard hydrogen electrode.
In this study, surfactant-free single crystalline Ag NWs prepared by thermally assisted photoreduction were used as sacrificial templates for the galvanic deposition of Pt. Through this simple route, large quantities of vertical Ag-Pt core-shell NWs could be prepared. Due to the great reduction potential, Pt ions could be reduced by the electrons generated from the oxidative dissolution of Ag. A compact shell comprising Pt nanocrystals can thus be formed on the surface of Ag NWs. TEM observation and AES elemental mapping both verify the core-shell structural feature of these NWs. According to the kinetic analysis and estimated activation energy, the galvanic exchange of Ag by Pt ions was an interface-controlled process, which was dominated by the dissociation of Ag atoms.
Y-LS is a Ph.D. student at the Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, Taiwan. S-YC is an associate professor of the Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan. J-MS is an associate professor of the Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan. I-GC is a distinguished professor at the Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan.
The authors thank the National Science Council of Taiwan for funding, NSC 100-2120-M-006-006, to support this work.
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