In Situ Generation of Two-Dimensional Au–Pt Core–Shell Nanoparticle Assemblies
© to the authors 2009
Received: 8 July 2009
Accepted: 24 September 2009
Published: 14 October 2009
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© to the authors 2009
Received: 8 July 2009
Accepted: 24 September 2009
Published: 14 October 2009
Two-dimensional assemblies of Au–Pt bimetallic nanoparticles are generated in situ on polyethyleneimmine (PEI) silane functionalized silicon and indium tin oxide (ITO) coated glass surfaces. Atomic force microscopy (AFM), UV–Visible spectroscopy, and electrochemical measurements reveal the formation of core–shell structure with Au as core and Pt as shell. The core–shell structure is further supported by comparing with the corresponding data of Au nanoparticle assemblies. Static contact angle measurements with water show an increase in hydrophilic character due to bimetallic nanoparticle generation on different surfaces. It is further observed that these Au–Pt core–shell bimetallic nanoparticle assemblies are catalytically active towards methanol electro-oxidation, which is the key reaction for direct methanol fuel cells (DMFCs).
Nanoparticle assemblies have gained significant attention recently with the intention of comprehending the true potential applications of their unique physical, optical, and electronic properties . The ultimate aim is to interface these assemblies to microscale and subsequently to macroscale by organizing them into higher-level structures, devices, and systems with well-defined functionality. Gold–platinum (Au–Pt) bimetallic nanoparticles as alloy  or core–shell structure , in particular, has attracted increased interest due to its superior performance as fuel cell catalyst over conventional platinum (Pt) based catalyst [4–6]. The major problem for Pt-based catalyst is their poisoning by CO-like intermediates [7, 8]. The unexpected finding of catalytic activity of gold at the nanoscale  has opened up various new possibilities of catalyst development and it is well-known today that presence of Au in AuPt system enhances the catalytic activity for electrochemical methanol oxidation reaction (MOR) as a result of electronic interaction between Au and Pt or from the lattice parameter contraction . Currently, these catalysts are mostly synthesized through direct deposition of Pt on the preformed gold nanoparticle (Au NP) seeds in solution [3, 10–13] or on solid supports [14, 15]. However, problem arises from their tendency of coagulation, which occurs due to the unusual high surface energy. While different capping agents such as thiols, amines, phosphines, polymers etc. are normally used in synthesis methods to stabilize and disperse these nanoparticles, interaction with these stabilizing agents may profoundly alter the catalytic properties of these systems .
Although assemblies of monometallic nanoparticles are reported extensively [16–19], alloy or core–shell bimetallic nanoparticle has received very limited attention as building blocks until recently. Therefore, new methods of assembling these bimetallic nanoparticles on appropriate surfaces are necessary to integrate them as possible components for future nanodevices or as novel catalysts for fuel cell applications. Early attempt of generating assembly of Au–Pt core–shell nanoparticle (Au–Pt NP) employs the deposition of Pt layer onto preformed self-assembled seed Au NPs on silicon surfaces through the reduction of PtCl6 2− in presence of NH2OH as the mild reducing agent . In another recent approach, Au NP film at the air–water interface was transferred to a solid surface to build a three dimensional nanoporous structure and finally coated with a Pt layer to form the desired core–shell structure . Apart from the limited methods available for assembling core–shell nanoparticles, stabilization against coagulation and fabrication over a large surface area still remains a challenge. As an alternative strategy, nanoparticle assemblies can be generated in situ inside a suitable template on a solid surface to avoid number of sequential steps involved in the methods discussed before. Moreover, the template will essentially act as a reaction chamber that provides scaffold for immobilization of specific metal ions, prevent aggregation, and further act as capping agent to control the growth of the desired nanoparticle structure. It is also possible to create lithographically defined structures of a suitable template to generate nanoparticle patterns without the multiple steps of synthesis and absorption of the corresponding nanoparticles from solution.
In this paper, we report the in situ synthesis of bimetallic core–shell Au–Pt NP assemblies on silicon and indium tin oxide (ITO) coated glass surfaces and also demonstrate that these nanoparticle assemblies are catalytically active towards methanol electro-oxidation which is one of the key electrode reactions in direct methanol fuel cells (DMFCs).
Water used in these experiments was purified though a Millipore system with a resistivity of 18 MΩ cm. Boron doped p-type silicon wafers polished on one side (resistivity 10–30 ohm cm) were purchased from Virginia Semiconductor (Fredericksburg, VA). Indium tin oxide (ITO) coated glass substrates were obtained from Delta Technologies, Ltd. (Stillwater, MN) with a resistance of 4–8 Ω. Gold (III) chloride trihydrate (HAuCl4·3H2O), chloroplatinic acid hexahydrate (H2PtCl6·6H2O), silver nitrate, 200 proof (absolute) ethanol, absolute methanol, and sodium citrate tribasic dihydrate were purchased from Sigma–Aldrich (St. Louis, MO) and were used as received. Trimethoxysilylpropyl modified polyethylenimine (TSPEI) as 50% solution in isopropanol was obtained from Gelest, Inc. (Morrisville, PA) and used without further purification. Concentrated sulfuric acid, 70% nitric acid, and concentrated hydrochloric acids were obtained from Fisher scientific (Pittsburgh, PA).
Silicon and ITO surfaces were cleaned before proceeding to surface modification with TSPEI and subsequent nanoparticle formation. Silicon surfaces, cut into required sizes for different characterization, were placed in aqua regia solution (3:1 v/v HCl:HNO3) (Caution! Aqua regia is a strong oxidizing agent and should be handled with extreme care) for at least 4 h, rinsed with Millipore water, dried under a stream of argon, and then heated briefly on a hot plate to evaporate any residual water. ITO surfaces were sequentially rinsed in acetone, ethanol, and water, followed by 1.0 M HCl for 10 min and then treated with 1:1:5 (v/v) H2O2:NH4OH:H2O for an hour. Finally, these surfaces were thoroughly rinsed with Millipore water and dried under flow of argon.
For surface modification, the cleaned samples were immersed in a 2% solution (v/v) of TSPEI in 95% ethanol for 6 min . Surfaces were then rinsed with absolute ethanol, dried under a flow of argon, and left overnight for curing of the silane layer. In a subsequent step, [AuCl4]− ion adsorption was done by exposing the surfaces in a 1 × 10−2 M solution of HAuCl4·3H2O for 8 h followed by rinsing with water and finally drying the surfaces under a flow of argon. In situ reduction to generate Au NPs on the surface was achieved by exposing the above surfaces to a freshly prepared 1% (w/v) aqueous sodium citrate solution for an additional 8 h. For Au–Pt bimetallic core–shell NP generation, 1 × 10−2 M HAuCl4 and 1 × 10−2 M H2PtCl6 solutions were made separately and mixed in different volumes to create the desired Au:Pt mole ratio in the final solution, ranging from 1:0,1:0.25, 1:0.75 to 1:1. After incubating the surface for 8 h, rinsing with water and drying under flow of argon, surfaces with adsorbed ions were placed in a freshly prepared 1% (w/v) aqueous solution of sodium citrate for 8 h to generate the Au–Pt NPs.
Contact angle was measured by Pocket Goniometer, PG-1 model (Paul N. Gardner Company Inc., Pompano Beach, FL), which is a battery-operated instrument for manual measurements of static contact angles at “equilibrium.” The plunger was filled with Millipore water, which was slowly pressed out until it formed a droplet on the surface (1.5 cm wide × 2.5–3.0 cm long). At least five different spots were chosen for measuring the contact angle for each surface and an average of the values was reported.
UV–visible spectra of gold nanoparticles on ITO surfaces were collected using USB4000-UV-VIS spectrophotometer from Ocean Optics (Dunedin, FL). Surfaces with nanoparticles were directly introduced into the cuvette holder (1 cm path length) and spectra were recorded with corresponding clean unmodified surface as the reference.
Atomic force microscopy (AFM) imaging was performed using Veeco (Santa Barbara, CA) Multimode system, equipped with a Nanoscope IIIa controller, in tapping mode. Cantilevers were phosphorous-doped silicon specific for tapping mode imaging. The local root-mean-square (RMS) surface roughness was determined using height data from at least four representative 2 μm × 2 μm scan areas through roughness analysis program included in the AFM analysis software. Surface coverage of different nanoparticles was estimated by analyzing the mean grain area and number of grains present in a typical 2 μm × 2 μm (total area = 4 μm2) AFM image of the respective nanoparticle.
Cyclic voltammetric (CV) measurements were performed in a Teflon® electrochemical cell (3 ml maximum volume) using standard three-electrode configuration, which was controlled by a CHI660C electrochemical workstation (CH Instruments, Inc. Austin, TX). A coiled platinum wire was used as the counter electrode, aqueous Ag/AgCl was used as the reference electrode, and ITO surfaces were used as the working electrodes.
Water contact angle measurements are used to follow the change in surface character during different steps of Au–Pt NP generation on the surface. Figure 1d shows the overall trend in the contact angle change from bare silicon surface to the nanoparticle formation on the surface for Au:Pt mole ratio of 1:1 in the final solution. Contact angle increased from 25 ± 2.1° for bare silicon to 54 ± 0.5° with adsorbed [AuCl4]− and [PtCl6]2− on the surface and eventually decreased to 34 ± 0.57° due to formation of Au–Pt NPs after the reduction step. This indeed implies the enhanced hydrophilic character of the surface due to nanoparticle formation. A similar trend in change of hydrophilic/hydrophobic character has been observed for Au NP formation on the surface by the present method and also reported in the literature for Ag NP formation [23, 24]. Controlling the hydrophilic and hydrophobic properties of a surface is significant due to different potential application areas like self-cleaning surfaces and sensors.
Generation of Au–Pt core–shell structure from simultaneous reduction of surface bound [AuCl4]− and [PtCl6]2− ions, evident from the UV–Visible and electrochemical results presented above and the preferred elemental distribution of the core and shell materials warrant further discussion. Since the formation of nanoparticles in solution essentially proceeds through nucleation and growth steps, it is expected that the metal ion which is easier to reduce will nucleate first and serve as the nucleation site for the second one during simultaneous reduction of the two metal ions in solution. In this case, platinum will have to be first reduced to Pt2+ from Pt4+ and then to Pt0 with a standard redox potential of 0.775 V for [PtCl6]2−/[PtCl4]2− and 0.68 V for [PtCl4]2−/Pt0 compared to a single step reduction for gold from Au3+ to Au0 with a standard reduction potential of 1.002 V for [AuCl4]−/Au0, reported at room temperature . Considering the reduction potentials, it is obvious that Au will preferably nucleate first to form the core followed by Pt to form the shell. However, the situation for surface bound ions is rather different as the respective ions are pinned down to the surface through electrostatic attraction of the template and will have limited mobility. The underlying mechanism of in situ core–shell structure formation on the surface is a subject of our on-going investigation.
In summary, we have reported an elegant method for in situ generation of two-dimensional Au–Pt bimetallic nanoparticle assemblies on solid surfaces functionalized with polyethyleneimine template. AFM results reveal the formation of Au–Pt NP assemblies on silicon surface without much long range ordering and also show an increase in size of these bimetallic nanoparticles compared to their monometallic Au equivalents. Comparison of UV–Visible and electrochemical response of Au–Pt NPs to that of Au NPs generated on ITO surfaces authenticate the core–shell structure of these bimetallic nanoparticles. Moreover, these Au–Pt NP assemblies are active towards methanol oxidation, demonstrating their potential as catalyst for DMFCs. This in situ synthetic approach relies on self-assembly employing wet chemical technique at an ambient condition and can also be extended to create other bimetallic NP assemblies. Further, it offers a flexible method to generate bimetallic core–shell NPs for site selective deposition, nanoparticle patterning for nanoelectronic applications and can also be combined with the conventional lithographic techniques.
We thank the American Chemical Society, Petroleum Research Fund (ACS-PRF) and National Science Foundation (NSF) for financial support. Office of the Vice President for Research (OVPR), UM-Ann Arbor and the Office of Research and Sponsored Programs, UM-Dearborn are also gratefully acknowledged for additional funding.