(513e) Improvement of Metal Oxide Catalyst Reactivity by Modification of Surface Fermi Level

There is good reason to believe that the properties of semiconducting metal oxide catalysts can be improved when designed according to the principles of microelectronic devices. Since metal oxide semiconductors support space charge, it is possible for surface electronic properties to couple to bulk electronic properties. Consequently, electronic "band engineering" can be employed to optimize surface reactivity and affect either thermal catalysis or photocatalysis. For instance, hydroxyl group acidity on the TiO2 surface can be tuned via the electron richness of the semiconductor, which can be manipulated via controlled doping. Alternatively, the direction and magnitude of the near-surface electric field within the space charge region can be adjusted by bulk doping which, in turn, affects the flow of photogenerated charge carriers toward the surface in photocatalysis. The present work describes the applicability of photoreflectance (PR), a type of modulation spectroscopy, to understanding semiconductor surface-bulk coupling in the context of catalysis using TiO2 as an example metal oxide. The approach involves the synthesis of a thin film of the semiconductor on a silicon substrate by chemical vapor deposition or atomic layer deposition. N- and p-type dopants are introduced into TiO2 during deposition to produce samples of varying doping levels. The physical and chemical properties of the thin films are characterized using ellipsometry, x-ray diffraction, and x-ray photoelectron spectroscopy. Detailed electrical characterization employing a Schottky diode test structure is undertaken to obtain a precise estimate of carrier concentration. PR is then utilized to better understand the effect of film thickness and uniformity, crystal structure, and doping on the position of the surface Fermi level. The results shed light on the properties of the TiO2 surface as it relates to thermal catalysis and photocatalysis, and enable unprecedented precision in the tailoring of metal oxide catalysts to ensure optimal surface reactivity.