Theoretical and experimental study of charge transfer processes in photocatalysis on anatase-TiO2

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Our basic understanding of photocatalysis is quite limited, because the assumed reaction pathways mostly lack direct confirmation. E.g., it is often not known, whether a molecule gets adsorbed or desorbed after receiving the photo-excited carrier. Charge transfer between the solid and the gas phase is of great importance also beyond photocatalysis, however, appropriate theoretical and experimental techniques to study it are lacking. This proposal aims to develop such methods and to apply them in case studies of practical significance. Theory often applies periodic models which require corrections because of the interaction between artificially repeated charges. Charge correction at a gas/solid interfaces, where the charge can be located in either phase, remain a challenge. Solutions so far are either making assumptions on the location of the charge, or are not self-consistent. We propose here to develop a general and self-consistent correction scheme, at various levels of complexity, for supercell calculations in charged 1D, 2D and 3D systems, and implement it into a standard electronic structure package (VASP). Considering the complexity of the problem, simple convergence tests are insufficient. Since we are not aware of any theoretical method where the avoidance of the charge correction problem is not in a trade-off for size-convergence problems, the only acceptable test is experiment.Reactions on isolating surfaces can be followed by atomic resolution using atomic force microscopy (AFM). The assignment of intensity changes to the charging of surface species is, however, mostly speculative, because it cannot be observed directly. We will develop a method based on the charge sensitive Kelvin-probe force microscopy (KPFM), in conjunction with non-contact atomic force microscopy on the atomic scale, for unambiguous identification of the charge state of surface species. TiO2 is probably the best know photocatalyst but, in the overwhelming majority of the cases, rutile was used, while the anatase form is known to be superior in photocatalysis. Therefore, the theoretical and experimental tools, developed here, will be tested on anatase-TiO2, which, according to our preliminary theoretical results, shows chemical pathways different form rutile. After an atomic scale characterization of the anatase surface by AFM, and the necessary method developments in atomic scale KPFM, we will concentrate on the transformation of CO and NO into compounds without health and environmental hazards. In these cases, charge transport across the interface plays a particularly important role.