Infrared spectroscopy studies of adsorption and photochemistry on TiO2 surfaces : From single crystals to nanostructured materials

TiO2 based photocatalysis is a green nanotechnology that can be used for removal of pollutants from water and air, as well as making synthetic fuels from water and carbon dioxide. Said photocatalysis has received major research interests during the last decades. Despite these efforts, many elementary processes that occur on the photocatalyst surface are not fully understood and, therefore, limit our ability to purposefully manufacture more efficient photocatalytic materials. The objective of this thesis is to provide new understanding at a molecular level of important adsorbate species on the TiO2 surfaces. Fundamental properties of adsorption and photochemistry of primarily formic acid on different TiO2 surfaces, ranging from single crystals to nanoparticles, have been studied using infrared spectroscopy. A method to simulate IR spectra have been developed and, combined with experimental data, has been proven to be a powerful tool to identify different adsorbate geometries on the surface. In the presence of oxygen, a thermally activated and irreversible reaction between formate and oxygen adatoms takes place on the single crystal rutile (110) surface to yield hydrogen bicarbonate surface complexes. For disordered single crystal surfaces, the adsorption geometry of formate changes due to exposure of Ti3+ atoms on the surface, and the adsorption spectra shows resemblances with that observed for formate adsorption on nanocrystalline surfaces. Illumination with UV light results in small changes of the formate coverage on the disordered single crystal and nanocrystalline rutile surfaces, whereas on the rutile (110) surface only miniscule changes in formate coverages are seen. This is due to the lack of oxygen electron acceptors and OH/H2O electron donors in the vacuum environment, which results in a much lower degradation rate compared to measurements made at ambient conditions. Furthermore, it is shown that the coordination of the formate molecule on various TiO2 surfaces has a profound effect on the photocatalytic degradation rate, with bidentate coordinated formate molecules being most resilient towards oxidation. The results presented here shows that additional insight in the processes on the TiO2 photocatalyst surface can be obtained by combining spectroscopic studies of single crystals and nanocrystalline films and that it is possible to unravel adsorption geometries on surfaces by combining experimental and simulated IR spectra