Ag Alloy Nanoparticles: Surface-Enhanced Raman


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ARTICLE pubs.acs.org/JPCC

Coreduced Pt/Ag Alloy Nanoparticles: Surface-Enhanced Raman Scattering and Electrocatalytic Activity Kwan Kim,*,† Kyung Lock Kim,† and Kuan Soo Shin*,‡ † ‡

Department of Chemistry, Seoul National University, Seoul 151-742, Korea Department of Chemistry, Soongsil University, Seoul 156-743, Korea

bS Supporting Information ABSTRACT: In an effort to develop highly effective Pt-based substrates for surface-enhanced Raman scattering (SERS) and electrocatalysis, we have synthesized 10 nm-sized Pt/Ag alloy nanoparticles, preparing first 7 nm-sized seed particles of Pt, followed by the coreduction of Ag and Pt precursors onto them. The formation of Pt core Pt/Ag alloy shell nanoparticles was evident from their UV visible extinction characteristics, X-ray diffraction patterns, and high-resolution transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy images. According to the phase diagram, an alloy is possible only with a very high atomic content of either Ag or Pt, but the formation of alloys herein, even with an apparent composition of Pt0.70Ag0.30, can be understood by presuming that due to the presence of seed Pt particles, the reduction potentials of Pt and Ag are modified to allow the formation of Pt/Ag alloys onto them. The formation of alloys of Pt with Ag has resulted in the enhancement of not only the SERS activity but also the electrocatalytic activity of Pt alone. The SERS activity was confirmed to increase as more Ag was incorporated into Pt to form Pt/Ag alloy nanoparticles. As a methanol electrooxidation catalyst, a pure Pt surface was poisoned by incompletely oxidized species such as CO and was thus unable to recover its electrocatalytic activity. In contrast, methanol oxidation peaks were observed in repeated cycles when Pt/Ag alloyed electrodes were used. In particular, the ratio of the forward oxidation current peak to the reverse current peak was as large as 2.52 for the Pt0.95Ag0.05 electrode, which is more than 3 times larger than that of a commercially available pure Pt catalyst, suggesting that the Pt/Ag alloy catalysts are superior in their ability to tolerate poisoning species.

1. INTRODUCTION Pt is a well-known catalyst that has a high catalytic activity, especially for methanol electrooxidation and oxygen electroreduction reactions.1 9 However, many factors can affect the catalytic activity of platinum particles. The use of Pt nanoparticles in fuel cells, for example, is severely limited due to poisoning of the surface by strongly adsorbed intermediates (CO) that block the catalytically active sites.1 To circumvent these limitations, Ptbased alloys are frequently synthesized as alternatives. Among numerous possible metal combinations, Pt/Au and Pt/Ag nanoparticles have been studied intensively for their catalytic properties coupled also with their unique optical properties.7 10 In particular, an improved methanol electrooxidation has been confirmed by using Pt/Ag alloy nanoislands fabricated on Au nanorods.8 An alloyed Pt/Ag catalyst has also been reported to be advantageous in oxygen reduction in hydrochloric acid electrolysis.11 Pt itself is unstable in the highly corrosive conditions of aqueous HCl electrolysis, and the adsorption of Cl ions at the Pt surface can cause kinetic limitations, increasing the formation of undesirable H2O2.11 Once alloyed with Ag, Pt can be retained even under these corrosive conditions, maintaining the catalytic activity, because of the lower standard potential of Ag compared to that of Pt.8 r 2011 American Chemical Society

Since the discovery of surface-enhanced Raman scattering (SERS) in the 1970s, there have been extensive theoretical and experimental studies of this effect.12 15 In recent years, it has been reported that even single molecule spectroscopy is possible by SERS, suggesting that the enhancement factor (EF) can reach as much as 1014 1015.16 19 One can therefore readily acquire vibrational spectra from adsorbates on roughened noble metals such as silver and gold. However, this is not the case for Pt. One of the challenging issues in nanoscience is then the fabrication of nanostructures that exhibit a large SERS effect.20 In line with this, Tian and his colleagues reported that even transition metals like Pt could be made to be SERS-active, via an electrochemical roughening process, with the EFs ranging from 1 to 3 orders of magnitude.21 The aggregates of laser-ablated Pt nanoparticles are also known to exhibit an EF of 1.4  103.22 Nevertheless, it is still difficult to obtain the Raman spectra of molecules adsorbed onto Pt, especially in nonelectrochemical environments. To overcome this difficulty, a “borrowing” method has been developed, in Received: July 6, 2011 Revised: September 26, 2011 Published: November 08, 2011 23374

dx.doi.org/10.1021/jp2063707 | J. Phys. Chem. C 2011, 115, 23374–23380

The Journal of Physical Chemistry C

ARTICLE

Figure 1. Upper panel: Low-magnification TEM images of (a) Pt0.97Ag0.03, (b) Pt0.95Ag0.05, (c) Pt0.90Ag0.10, (d) Pt0.70Ag0.30, and (e) Pt0.0Ag1.0 grown on Pt seed particles. Low panel: Their corresponding high-magnification HR-TEM images. The insets are the FFT patterns.

which a less SERS-active material is deposited onto a highly SERS-active substrate in order to benefit from the strong local field generated in the latter material.23 The incorporation of Ag or Au atoms into Pt would then be a sound alternative. One of our eventual goals is the development of Ag-doped Pt nanoparticles that will function efficiently not only as a Ptsubstitute SERS substrate but also as a Pt-substitute electrocatalyst. Perhaps the most challenging hurdle to overcome would be to make Pt and Ag miscible. According to the phase diagram, an alloy is possible only with a very high atomic content of either Ag or Pt.8,24 For example, the atomic compositions of Pt/Ag alloys stable at 400 °C are limited to Age2Ptg98 or Agg95Pte5. In this case, well-mixed Pt/Ag alloy nanoparticles may not form through the simultaneous reduction of Pt and Ag precursors. Despite this, He et al. succeeded in preparing alloyed Pt/Ag nanoparticles on Au nanorods by depositing Pt nanoislands beforehand to guide the growth of the alloy particles.8 A similar tactic has been used to prepare Pt/Ag nanoparticles in this work, first by synthesizing 7 nm-sized seed particles of Pt and then coreducing Ag and Pt precursors onto them. The formation of Pt core Pt/Ag alloy shell nanoparticles could be evidenced from the UV visible (UV vis) extinction and X-ray diffraction characteristics. Compared with pure Pt nanoparticles, the alloyed nanoparticles exhibited improved catalytic performance for methanol oxidation, as well as higher SERS activity.

2. EXPERIMENTAL SECTION Chloroplatinic acid hexahydrate (H2PtCl6 3 6H2O), silver nitrate (AgNO3, 99+%), sodium citrate, sodium borohydride, L-ascorbic acid, 3-aminopropyltrimethoxysilane (3-APS), 4-nitrobenzenethiol (4-NBT, 80%), and 4-aminobenzenethiol (4ABT, 97%) were purchased from Aldrich and used as received. Other chemicals unless specified were reagent grade, and triply distilled water of resistivity greater than 18.0 MΩ 3 cm (Millipore Milli-Q System) was used in preparing aqueous solutions. The platinum seeds of ∼7 nm diameter were prepared as follows.25 First, 3 mL of a 0.2% solution of chloroplatinic acid hexahydrate was added to 39 mL of boiling deionized water. After 1 min, 0.92 mL of 1% sodium citrate was added, followed half a minute later by a quick injection of 0.46 mL of a freshly prepared 0.08% solution of sodium borohydride also containing 1% sodium citrate. After 10 min, the sol solution was cooled down

to room temperature. The seeds obtained in this way were used in preparing ∼10 nm-sized Pt core Pt/Ag alloy shell nanoparticles. Specifically, to 30 mL of deionized water were consecutively added 1 mL of the seed solution, 0.05 mL of a mixture solution of H2PtCl6/AgNO3, and 0.5 mL of 1.25% L-ascorbic acid solution also containing 1% sodium citrate under vigorous stirring and boiling at 80 °C: The mixture solution was composed of 0.25 mM H2PtCl6 and 0.25 mM AgNO3 in a molar ratio of 0.97:0.03 or 0.95:0.05 or 0.90:0.10 or 0.70:0.30 or 0.0:1.0. After further boiling for 40 min, the reaction mixtures were cooled down to room temperature. The reaction products were gathered by centrifugation (8000 rpm), washed three times with deionized water, and finally redispersed in deionized water. Pt/Ag nanoparticle films were prepared by dropping the above sol solutions onto indium tin oxide (ITO) or silicon wafer substrates. Initially, ITO and Si substrates were subjected to ozonolysis to render them hydrophilic at a water contact angle of