Gould, AL; (2017) Properties of bimetallic AuAg nanoparticles for H2 production. Doctoral thesis , UCL (University College London).

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Abstract

Nanoparticles exhibit distinctly different properties from those of bulk matter, as a result of their large surface area to volume ratio. They have been shown to be catalytically active for a number of reactions, and therefore hold great interest for use in industry, where a fine balance of turnover and cost must be achieved. This thesis focuses on bimetallic gold/silver nanoparticles, where the introduction of a secondary metallic species can be used to not only control nanoparticle properties, but also reduce scale up costs. For gold/silver bimetallic nanoparticles (nanoalloys), it is unclear as to how the two metals will mix: based on thermodynamic arguments alone, it is unclear whether similarities in atomic size and number of valence electrons lead to the formation of an alloy or a segregated core@shell arrangement. In this work, we investigate the most energetically favourable and stable chemical arrangements based on interatomic potential basin-hopping algorithms, re-optimised using density functional theory (DFT), evaluating their potential as co-catalysts for hydrogen production. Diffusion is particularly important in catalyst stability, and therefore we examine both Au-Ag interdiffusion and the interaction of Ag nanoparticles and amorphous Si (a-Si). We examine the influence of calcination processes, often used in experimental synthesis, on differing AuAg nanoparticle chemical arrangements using classical molecular dynamics simulations. Our calculations show Ag@Au nanoparticles are the most promising in terms of achieving a higher catalytic turnover; however, we also find that Ag@Au nanoparticles are particularly unstable due to the energetically favourable formation of a ‘rosette-like’ icosahedral geometry, which exposes core Ag atoms. In addition, diffusion of Ag nanoparticles into a-Si is studied for parallel comparison with experiment, performed at the University of Utrecht. Experimental observations suggest the diffusion of Ag atoms into the a-Si matrix, however, we do not observe the same computationally, suggesting that experimental voids in a-Si may facilitate this diffusion. Finally, we investigate the applicability of CO as a probe molecule for determining changes in surface composition through vibrational stretching frequencies, both experimentally and theoretically, using diffuse reflectance infrared Fourier transform spectroscopy and DFT modelling.

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