In situ TEM/STEM of the Coarsening of Nanoporous Gold

A.A. El-Zoka, J. Howe, R.C. Newman, D. Perovic
University of Toronto,
Canada

Keywords: nanoporous gold, characterization, in situ TEM, coarsening, nanomaterials, dealloying

Summary:

Nanoporous gold (NPG) is usually fabricated by the selective electrolytic dissolution (dealloying) of Ag from a binary Ag-Au alloy, or sometimes from a ternary alloy such as Ag- Au-Pt. The resulting ligament-pore structure is bicontinuous and has many interesting properties and potential applications [1]. Coarsening of NPG after dealloying can alter its catalytic and functional properties considerably. Driven by curvature, Au atoms diffuse quickly along step edges, acting to smoothen the freshly formed surface [2]. Understanding the structural and morphological changes that occur during this process is vital to define the thermal and chemical environments where NPG can be utilized most successfully. Several notable studies demonstrated the significance of surface diffusion of Au in the coarsening of ligaments at temperatures as low as 200 oC [3,4]. However, the presence and/or formation of voids and vacancies during coarsening adds to the complexity of the multi- faceted mechanism involved [5,6]. The effect of the addition of small amounts of Pt to NPG precursors has been proven not only to limit the size of ligaments during dealloying, but to also limit the coarsening of NPG postdealloying [2,7]. The atomistic action of Pt is not understood, beyond the likelihood that slowly diffusing Pt atoms accumulate at step edges. In an effort to provide a more in-depth understanding on this topic, we have carried out a comparative in situ TEM/STEM study of the morphological evolution of nanoporous layers with different precursors during thermal coarsening. A reductive atmosphere of hydrogen has been used to ensure minimal surface contamination before or during the microscopy investigation [8]. Effects of sample size and initial pore size-to-ligament size ratio on the coarsening behaviour will also be discussed. The results reveal how the presence of Pt causes a significant change in the coarsening mechanism, thermal onset of coarsening and coarsening rate. The distinction between the two main coarsening mechanism observed, Particle Migration Coalescence (PMC), and Ostwald Ripening, is shown for NPG [9].