Degradation of catalytic thin films revealed using transmission electron microscopy
Keywords: Materials Science and Engineering.

As we face an urgent need to combat climate change and achieve a long-term goal of "carbon neutral" in the modern world, hydrogen as a renewable clean energy source has become an essential solution for energy storage and conversion. A major challenge in a future hydrogen economy is to find economical ways of storing, producing, and using hydrogen at a massive scale. Although much research effort has been devoted to finding acid-stable and active nanoscale catalysts for the production and consumption of hydrogen, such as in the oxygen evolution reaction (OER) of water splitting and oxygen reduction reaction (ORR) of fuel cells, it has been shown challenging owing to the high applied voltage and corrosive environment during electrochemical testing. In order to develop catalysts with good activity and long-term stability, it is important to understand the relationship between performance and materials properties as well as their degradation behavior in testing conditions. These goals can be achieved by applying transmission electron microscopy (TEM) with additional advanced functions, which characterizes the changes in structure and chemistry at atomic and nano scales. For reproducible electrochemical characterization, thin films of the materials of interest are deposited onto substrates and subsequently characterized by TEM methods in cross-section. With conventional cross-section TEM specimen preparation methods, this dissertation describes the degradation of catalytic thin films using high-resolution TEM (HR-TEM) imaging while maintaining the nature of the surface catalyst. The power of HR-TEM for this sort of study is firstly demonstrated by the characterization of an iridium oxide thin film for an OER catalyst. The film, deposited on yttria-stabilized zirconia (YSZ) substrate, was confirmed to be a mixture of predominately (100)-oriented orthorhombic columbite structure and co-stabilized tetragonal rutile structure. The orientation relationship between the two polymorphs and the epitaxial growth of the columbite phase on the YSZ substrate are also revealed. The random distribution of the rutile phase within the film is determined using fast Fourier transformation (FFT) filtering and inverse FFT images. Previous studies have reported a highly active strontium iridate catalyst for OER in acid, while the stability of the material needs more evaluation. As the second part of this thesis, cross-sectional TEM experiments were conducted to investigate the structure and the degradation of the catalyst. Measurements confirm the epitaxial growth of the films from perovskite strontium titanate substrate from HR-TEM images and selected area diffraction patterns (SADP). The loss of the materials following electrochemical testing in a sulfuric acid environment for several hours can be demonstrated by comparing pre- and post-test thin film thickness. While the thickness of the film decreases from 40 nm to 18 nm, losing more than 50\% of its materials, the smooth surface can be confirmed to change to a rough one while its bulk structure is maintained. The results provide evidence for a layer-wise degradation model. Furthermore, the surface phase structure on the post-test thin film is analyzed using the measurement from HR-TEM images. The surface region shows 3-5 nm crystalline domains corresponding to pseudocubic perovskite strontium iridate and possible amorphous materials. The final part of this dissertation focuses on molybdenum nitride thin films as a catalyst for ORR. The polycrystalline structures of the thin films with different catalytic activity synthesized under different conditions were studied using both HR-TEM and SADP. It is shown that the thin film consists of rocksalt and hexagonal molybdenum nitride has better activity performance than the film of only rocksalt crystals. The depth-dependent structure and composition of the films are determined by both fast Fourier transformation (FFT) analysis of HR-TEM images and dark field (DF) TEM images. Finally, the impact of various electrochemical testing setups on the degradation process of the Mo-N thin films is discussed by comparing the results of thin films grown under the same conditions but tested with different treatments. The results show the dissolution of the thin films mostly happens during cycling of electrochemical testing, although the structures and chemical compositions remain unchanged. In summary, unlike the common method to study the degradation of nanoparticle catalysts with TEM, this thesis shows the usefulness of advanced cross-section TEM characterization to illuminate changes in catalytic thin films after being tested in a harsh acidic environment, which is rarely carried out. For materials such as the metal oxide and nitride catalysts in this study that are often considered stable, they are shown to suffer significant degradation during the electrochemical tests