Influence of ferroelectric substrates on the photochemical reactivity of titanium dioxide thin films

The efficiency of particulate photocatalysts used to split water is limited by the recombination of photogenerated electrons and holes and the back reaction of intermediate species to reform water. This efficiency could be improved by better spatial separation of charge carriers and reaction sites. The primary goal of this work is to test the hypothesis that dipolar fields in ferroelectric substrates can drive photogenerated electrons and holes in opposite directions, thus creating spatially distinct locations for photochemical oxidation and reduction reactions on the surfaces of supported TiO2 thin films.;To investigate this, thin films of TiO2 (10 to 100 nm) were grown on polycrystalline, ferroelectric BaTiO3 and BiFeO3 substrates by pulsed laser deposition. The films grown on BaTiO 3 were characterized by electron backscatter diffraction, which showed that some substrate orientations stabilize the anatase phase while others stabilize rutile. Specifically, orientations within about 25° of (100) BaTiO3 promote the growth of biaxially orientated anatase such that [001] BaTiO3 is parallel to [001] anatase and [110] BaTiO3 is parallel to [110] anatase. Orientations further from (100) stabilize the growth or rutile. While the rutile growth is epitaxial, there is no single orientation relationship.;The photochemical properties of the surface were probed using the photochemical reduction of Ag+ to Ag0 and the photochemical oxidation of Pb2+ to Pb4+ These specific oxidation and reduction reactions were chosen because they leave behind insoluble reaction products (Ag and PbO2) on the surface that allow the location of the reaction to be determined. The same locations of the surface were imaged by atomic force microscopy before and after the reaction and the heights of the reaction product on the surface were used as a measure of the extent of the reactivity.;In this study, we found that the ferroelectric substrate does influence reactivity on the surface of the thin film. Oxidation and reduction reactions on the thin film surface were observed to occur in separate locations. This effect was observed for films grown on both n-type BaTiO3 and p-type BiFeO3. The observed spatial localization of reactions diminished as the thickness of the film was increased from 10 nm to 100 nm. The effect also diminished as the carrier concentration in the film was increased. Experiments in which the reactivity of the bare substrate was compared to the reactivity of the supported film showed that domains that promoted reduction (or oxidation) on the substrate surface also promoted reduction (or oxidation) on the film surface.;Because the BaTiO3 substrates were polycrystalline, the effect of film and BaTiO3 substrate orientation on the reactivity was also explored. No significant differences in the reactivity of {100}, {110} and {111} BaTiO3 were observed. For some rutile films orientations near the type (hk0), the presence of the ferroelectric substrate appeared to increase the photochemical reactivity of the TiO2 as compared much thicker films with the same structure and orientation.