In Situ Visualization of Site-Dependent Reaction Kinetics in Shape-Controlled Nanoparticles: Corners vs Edges

Corner and edge sites have long been predicted to play a dominant role in the chemistry of nanomaterials due to their low metal-metal coordination numbers. However, due to the difficulty in directly observing chemical reactions, a detailed understanding of how a material's crystallographic sites and morphology affect those sites' reaction kinetics is limited. Using environmental scanning transmission electron microscopy (ESTEM), we report direct observations under continuous reaction conditions of site-dependent chemical reactivity critical to corrosion and heterogeneous catalysis. A range of fcc nickel nanocrystal morphologies bound by the low index {111} facets (triangular plates, hexagonal plates, and decahedral and icosahedral nanoparticles) were studied during oxidation to visualize in situ changes using atomic number (Z) contrast ESTEM. Oxidation is shown to occur preferentially at the corner sites and then the edges. The enhanced oxidation rate of the corners persists until corrosion has progressed to a depth of similar to 2-4 nm, depending on the nanoparticle geometry. The nanoparticle systems showed that the triangular plates were the most reactive followed by the hexagonal plates, decahedra, and icosahedra. Time-resolved measurements of oxidation rates show a size-dependent induction period for the edges, hypothesized to be due to oxygen diffusion along the surface of the smaller particles to react preferentially at the more reactive corner sites.