Article pubs.acs.org/JPCC
Preparation of PtAu Alloy Colloids by Laser Ablation in Solution and Their Characterization Jianming Zhang, Daniel Nii Oko, Sébastien Garbarino, Régis Imbeault, Mohamed Chaker, Ana C. Tavares, Daniel Guay, and Dongling Ma* Institut National de la Recherche Scientifique, INRS-Énergie, Matériaux et Télécommunications, 1650 Boulevard Lionel-Boulet, Varennes, Québec J3X 1S2, Canada ABSTRACT: Stable PtAu alloy colloids with a wide range of compositions were prepared using pulsed laser ablation on single metal-mixture targets in water. The concentration of Pt in the alloys can be tuned by varying the Pt/Au ratio in the targets, which are made by compression molding a mixture of Pt and Au powders at different ratios. Such fabricated PtAu alloy nanoparticles (NPs) show a face-centered cubic structure, and their composition basically follows that of their corresponding targets. The effect of aqueous solution pH and ablating laser fluence on the formation and structure of alloy NPs was further investigated. It is found that PtAu alloy colloids of identical composition can be achieved over a pH range extending from 4.0 to 11.0 and at fluences varying from 4 to 150 J cm−2 as long as the targets of the same composition are used. This finding suggests that alloy formation is essentially insensitive to both factors in certain ranges, and the method developed herein for the alloy NP formation is quite robust. Moreover, the surface composition, estimated from electrochemical measurements, is identical to the overall composition of the NPs estimated from Vegard’s law and X-ray diffraction data, which is a strong indication of the uniform composition on the surface and in the interior of these alloy NPs.
1. INTRODUCTION In fuel cells, it is well-known that platinum (Pt) is one of the most effective catalysts for the oxidation of hydrogen and small organic molecules like formic acid, methanol, and ethylene glycol1−6 and the reduction of oxygen.5,6 Recently, platinum− gold (PtAu) bimetallic nanoparticles (NPs) have received increasing attention as fuel cell electrocatalysts due to their higher activity than pure Pt NPs.6−24 The presence of Au can modify the surface adsorption energy and consequently reduces the poisoning problem of Pt catalysts induced by the adsorption of CO-like intermediate species.7,23 Despite the miscibility gap between Pt and Au, PtAu alloy nanostructures with various compositions, sizes, and morphologies have been synthesized successfully by different approaches, such as coreduction of Pt and Au precursors in solution,6−9,13−24 dealloying of PtMAu (M = Al, Cu) bulk alloys,10,11 and pulsed laser deposition (PLD) using separate Pt and Au targets in a closed chamber filled with gas.12 In those preparation methods where chemical precursors and/or stabilizing agents are used, an additional washing or thermal annealing process is needed to remove the chemical residues as they may poison the surface of the PtAu alloy catalysts. Although the use of chemical reagents is not required in the gas-phase PLD approach, in practice the instrument setup and its operation are complicated and timeconsuming. An effective yet simple approach is thus desired for fabricating PtAu alloy NPs. © 2012 American Chemical Society
Pulsed laser ablation in liquid solution (PLAL) offers an alternative approach to the generation of pure colloidal NPs of various materials without the use of chemical precursors.25−39 It has been shown that, in contrast to the plasma plume formed in the classical PLD process involving a gas environment, the plume formed during PLAL has very distinct characteristics: (i) plasma plume expansion is limited by ambient liquid and (ii) the size and lifetime of plasma in liquid are much smaller.37−39 The high density of the plasma with high temperature in liquid is highly favorable for chemical reactions, and the short lifetime of the ablated species favors the formation of metastable phases.38 An additional benefit could be that the surface of NPs formed in the absence of any stabilizing agents is clean and does not necessitate additional washing or thermal treatment. As a result, the metal atoms at the surface of the NPs are expected to be readily accessible, leading to the increase of the number of active surface sites for catalytic reactions, which is of high relevance to catalysis (including electrochemical catalysis). To date, to the best of our knowledge, only alloy targets have been largely utilized to produce alloy NPs using PLAL. Nonetheless, the fabrication of some alloy targets is very challenging due to the miscibility gap of certain metals, such as Pt and Au in the current study. It is thus attractive to develop a Received: March 14, 2012 Revised: May 23, 2012 Published: May 24, 2012 13413
dx.doi.org/10.1021/jp302485g | J. Phys. Chem. C 2012, 116, 13413−13420
The Journal of Physical Chemistry C
Article
attenuate the laser beam to vary the laser fluence on the target in the range of ∼4.0−150.0 J/cm2. The pH values were measured with an Orion 230A pH meter and adjusted by using NaOH or HCl solutions. Characterization. (i). XRD. Drops of highly concentrated colloidal solution were placed on freshly cleaved nondoped, double-side polished single crystalline (311) silicon substrates and dried in a vacuum desiccator to form a film. The measurements were performed on the film samples using a Panalytical XRD X-pert Pro diffractometer at a grazing incidence angle of 5° with step size of 0.1° and counting time of 15 s per step. (ii). XPS. Samples for XPS measurements were prepared in the same way as those for XRD measurements. XPS spectra were taken using ESCA Escalab 220i XL with a monochromated Al Kα X-ray source (1486.6 eV). High-resolution spectra were obtained using a scan step of 0.1 eV with pass energy of 20 eV for Au 4f and Pt 4f. Curve fitting of the XPS data was carried out after linear background subtraction using casaXPS version 2.2.107. The binding energies were calibrated by referencing them to the C 1s binding energy (284.6 eV) originating from adventitious contamination of the sample surface. (iii). TEM, Selected Area Electron Diffraction (SAED), and Energy Dispersive X-ray Spectroscopy (EDS). A small drop of colloidal solution was deposited onto a Cu/C grid, and the excess solution was wicked away by a filter paper. The grid was subsequently dried in air and imaged with a JEOS-2100F TEM (École Polytechnique de Montréal, Montréal, Canada). Meanwhile, the SAED pattern and EDS spectra were measured at different locations on the sample. The particle size was determined by measuring more than 100 individually dispersed particles identified in TEM images. (iv). Neutron Activation Analysis (NAA). Elemental analysis with NAA was performed on 1 mL of as-prepared colloidal solution using a SLOWPOKE nuclear reactor (É c ole Polytechnique de Montréal, Montréal, Canada) to quantify the concentrations of Au and Pt. (v). Electrochemical Characterization. An adequate volume of the PtxAu100−x colloidal suspension, corresponding to metal loading of 45 ± 3 μg/cm2, was deposited on glassy carbon (GC) disks (5 mm in diameter, PineChem Inc.). After solvent evaporation, 3 μL of 5 wt % Nafion solution was pipetted onto the electrode and allowed to dry overnight. Preceding every electrode preparation, the GC disk was systematically polished with alumina slurries (1.0 and 0.05 μm in diameter) and then ultrasonically cleaned in pure water for 5 min. Prior to each electrode characterization, the glassware and the electrochemical cell were cleaned according to a wellestablished method. Auxiliary and reference electrodes were a platinum gauze and a Reversible Hydrogen Electrode (RHE), respectively. All electrochemical studies were carried out at room temperature in 0.5 M H2SO4 solution. The electrolyte was initially purged with high purity Argon (N5.0, Praxair) for at least 30 min, followed by a continuous light flow of Argon over the solution throughout electrochemical testing. Cyclic voltammograms (CVs) were recorded at 50 mV s−1 using a VSP potentiostat (Biologic Science Instruments).
general strategy that can potentially lead to the synthesis of a variety of alloy NPs, including those that are not miscible in the bulk. In this context, we present an improved PLAL approach that uses targets made of a mixture of two types of metal powders (instead of homogeneous alloy targets, which are not available for Pt and Au) to prepare PtAu alloy colloids over the whole composition range. To do so, the targets containing various Pt contents (0, 30, 50, 70, and 100 at. %) were made by compression molding a mixture of Pt and Au powders at different ratios. For laser ablation, the targets were then immerged into an aqueous solution to make samples denoted as PtxAu100−x (x: 100 × Pt atomic content in the target). X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were used to characterize the structure of as-prepared NPs. It is found that PtAu alloy NPs with a face-centered cubic (fcc) structure are formed over a broad composition range by varying the composition of the targets. The composition of the alloy NPs is similar to that of the targets. Over wide ranges, the solution pH and ablation fluence do not affect the formation of alloy NPs, whose composition is mainly determined by that of the target. Electrochemical characterization, by means of cyclic voltammetry, confirms the presence of both Pt and Au atoms on the NP surface, with content similar to that in the bulk. The approach developed herein is based on the use of targets that are simply made of mixtures of (metallic) powders targets and can be readily extended to the fabrication of other alloy NPs, whose alloy targets are not available. This could prove to have great technical importance for the preparation of alloy NPs with exotic composition and structure in a controlled fashion and make a contribution to the field of catalysis, where alloy NPs have found important applications.
2. EXPERIMENTAL SECTION Materials. Platinum (Pt) micropowder (≥99.9%, 0.5−1.2 μm), gold (Au) micropowder (≥99.9%,
