Environment-Dependent Surface Dynamics of Supported Gold Nanoparticles Studied by High-Resolution Transmission Electron Microscopy

Nanostructured heterogeneous catalysts play an increasingly important role in contemporary society. Understanding their behavior under operational conditions has been the primary focus of many research groups over the past decades. This thesis presents time-resolved investigations of the dynamics of surfaces and interfaces of cerium dioxide supported gold nanoparticles under a variety of atmospheric conditions, at the atomic scale which is are of great importance to reveal the catalytic mechanisms. High-resolution transmission electron microscopy has the capability to image structures with sub-Ångstöm resolution and sub-second temporal resolution. Combined with environmental transmission electron microscopy, atomic surface dynamics such as column diffusion can be observed in the presence of gases.
The effect of the electron beam current on the surface dynamics of the sample has been evaluated by occupancy fluctuation of surface columns under a range of electron flux densities. The internal energy of the nanoparticles which most contributes to the kinetic energy of the diffusing atoms increases with both the electron flux density and the total time of exposure.
The surface and interface dynamics on the scale from sub-Ångström to nanometers on a model system of gold on cerium dioxide have been systematically investigated. Surface atom diffusion is observed under most conditions and normally reversible. Under some conditions the diffusing atoms on the surface move in a concerted manner, suggesting there is a variety of configurations which can be considered as local minima in potential energies in configurational space.
The relations between particles and substrate have been studied. Particle motion on the substrates are in three manners, such as rigid sliding, movement via mass transport and rigid rotation. Nanoparticles have two preferential crystallographic orientations with respect to the substrate, and they can transform between these two orientations under gas exposure.
Chemical environments and temperature influences the dynamics of supported nanoparticles. Firstly, the shape of the nanoparticle changes under exposure to different gases, and the surfaces are more active in oxygen than in hydrogen. Secondly, thermally activated layer appearance-disappearance fluctuations have been observed in different gases. In hydrogen and carbon monoxide the (100) facet fluctuates, while in oxygen the fluctuation is mainly on the (111) facet.
In the last part of the thesis, the reversibility and local strain field variation of the twinning-detwinning processes suggest twins play an important role in the catalytic reaction.