K. Sasikumar, A. Ulvestad, J.W. Kim, R. Harder, E. Maxey, J.N. Clark, P. Mulvaney, B. Narayanan, S.A. Deshmukh, S.K.R.S. Sankaranarayanan, N. Ferrier, T. Peterka, O.G. Shpyrko
Argonne National Laboratory,
Keywords: lattice strain, gold, catalysis, chemisorption, reactions
Summary:Multi-electron transfer processes, such as ascorbic acid decomposition and hydrogen and oxygen evolution reactions, are important areas of research for energy and biological applications. These processes require the use of favorable catalysts to achieve the fast kinetics rates necessary for practical purposes. Nanostructured catalysts offer promise in this regard having shown improved activity, stability, and diverse properties relative to their bulk counterparts. However, several theoretical challenges such as understanding the strain and size dependent thermodynamics, and characterization challenges such as imaging catalytic activity at the single particle level still remain. In this work, we use ascorbic acid decomposition facilitated by a gold nanoparticle as a model multi-electron transfer process to investigate the strain field evolution in the nanocrystal lattice during catalysis. Integrating ultrafast imaging with molecular dynamics (MD) modeling can provide crucial insights on the catalytic activity of gold in such processes. Recently, experimental techniques have evolved to conduct time-dependent lattice dynamics measurements in nanomaterials. As part of this work, coherent x-ray diffractive imaging (CXDI), with sub twenty nanometer resolutions, was used to observe reversible lattice distortions in gold nanocrystals upon exposure to ascorbic acid solutions. Here, the lattice displacement dynamics are primarily concentrated on the surface of the gold nanoparticle during catalysis and are strongly size dependent. However, the magnitude of the observed effects is larger than predicted by electrowetting theory. Reactive MD is a suitable simulation model to capture the reaction chemistry near the gold-acid interface and to precisely determine lattice distortions in the gold lattice. Here, we present results from a series of reactive MD simulations, complementing the CXDI measurements. We reveal co-adsorbed acid-aided dissociation of water near the edges and corners of the nanocrystal facets. The local straining in the gold lattice originates from the chemisorption of the resultant hydroxyl ions. The magnitude of the simulated local lattice displacement is commensurate with experimental observations, indicating an alternate paradigm for lattice dynamics in addition to electrowetting theory. The results show the utility of characterizing the 3D displacement field evolution in identifying catalytically active nanoparticles during redox catalysis processes.