High Temperature Transformations of Transition Metal Oxide Nanocrystals Studied by In-Situ Transmission Electron Microscopy

Abstract

The scientific study described in this thesis aims to achieve understanding of the thermal behavior and thermal stability of nanostructured transition metal oxides (TMOs) by means of in-situ experimentation in the transmission electron microscope (TEM). TMO nanocrystals are proper candidate materials for catalysis, gas sensing, and biomedical applications. The properties at the nanoscale are different from those of the corresponding bulk materials due to surface effects and confinement effects. The thermal evolution and stability of nanocrystals in different size ranges and in various shapes consequently affect their performance in applications. This study used primarily in-situ TEM as the main method to achieve real-time inspection of changes taking place at elevated temperatures while imaging at high spatial resolution. The TMO nanocrystals investigated in this thesis include Co3O4, TiO2, MoO3 and WO3. The common trend found for all TMO samples is that transformations to lower-valence state oxides or eventually to pure metals always occurred when heated at elevated temperatures in the electron microscope. The same transformations also occurred in the reference ex-situ experiments for most of the samples. We conclude that the (very low) partial oxygen pressure in the vacuum of the TEM column is of importance for the specific temperatures at which these transformations take place. Considering changes in nanoparticle morphology, processes of coalescence and sublimation were often observed, and were sometimes found to be dependent on the areal concentration of dropcast nanoparticles. The study described in this thesis confirms the wildly diverse behavior of nanoparticles.