Bimetallic core-shell nanoparticles, including Au-Pt nanoparticles (NPs), exhibit interesting electrocatalytic properties for fuel cell reactions.¹ For this application, the ability to control the size of the NPs, as well as their composition and surface crystallinity, has been shown to be important to enhance the utilization efficiency of the catalysts.² NP arrays have been previously achieved by fabrication methods such as “top-down” or “bottom-up” approaches, which typically involve lithography and self-assembly, respectively. Top-down methods are often costly and time consuming, while bottom-up methods suffer from poor long range (> µm) order. The present study has focused on the formation of bimetallic nanostructures on Ta templates, where the large surface energy difference between a metallic thin film (Au and Pt) and the air-formed oxide on the Ta surface cause the dewetting of a sputter-coated metallic film to form NPs. This approach has been previously used to form Au NPs of controllable size, with a linear relationship observed between the metal film thickness prior to dewetting and the NP diameter.³ Here, we have attempted to form useful Au/Pt nanostructures and the electrochemistry of these materials, formed using a range of fabrication conditions, will be discussed in detail.

Methods and Results

Solid-state dewetting of thin metal films on oxide substrates is a known method of fabricating metal NP arrays over relatively large dimensions (e.g., cm²) without requiring costly lithography processes.^4,5 Recent work in our group has demonstrated the ability to form ordered Au NP arrays on Ta templates, which are covered by a thin, air-formed Ta oxide film. This was achieved by the sputter-deposition of a thin film (3-4 nm) of Au, followed by thermal annealing at 450 ^oC for 30 minutes.⁴ This method was then adapted to form Au-Pt nanostructures, where two metal films (3-4 nm each of Au and Pt) were sequentially sputtered (Au then Pt, or Pt then Au) on chemically polished Ta templates, and then thermally annealed at 450 ^oC or 600 ^oC for 90 minutes. These Au-Pt nanostructures were characterized by FESEM (field emission scanning electron microscopy) to determine the extent of dewetting.  It was observed that higher annealing temperatures of 600 ^oC (well below the melting point of both Au and Pt) were needed to further dewet the thin metal films (Figure 1 (a) and (b)), as heating at 450 ^oC was only able to partially dewet these Au-Pt films (Figure 1 (c) and (d)).

The electrochemical behaviour of these bimetallic nanostructures was investigated by cyclic voltammetry (CV) in 0.5 M H₂SO₄ solution and then compared to the response from the individual Au vs. Pt thin films. The CVs show that the bimetallic thin films (before annealing) exhibit characteristics of both metals, indicating that the second sputter-coated metal does not form a conformal coating on the first sputter-coated film.  After annealing at 450 ^oC, the CVs exhibit only Au characteristics, regardless of the deposition sequence. This suggests that, regardless of the order of sputter-coating, Au is coating the Pt structures after thermal annealing (i.e., these are Pt@Au nanostructures), in contrast to the Pt enrichment observed at the surface of Au@Pt core-shell NPs, formed by the electrochemical reduction of ionic precursors in solution. The characteristics of the nanostructures were shown to be optimized by varying the fabrication conditions (film thickness and annealing conditions), which also aided in understanding the mechanism of dewetting of these thin bimetallic films.  

Acknowledgements

We gratefully acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC).

References

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