In-situ vacuum studies of photocatalytic oxidation of isopropanol on nanometer thick TiO$_{2}$ films grown on silicon

We report on measurements of the photocatalytic activity of ultra-thin TiO$_{2}$ films grown on $n$ and $p$ type Si wafers.~Using the oxidation of isopropanol to acetone as a model system, photocatalytic studies were carried out in-situ, in a high vacuum chamber equipped with leak valves for injecting isopropanol, oxygen and water vapor onto the TiO$_{2}$ sample.~The sample was irradiated through a quartz widow with a UV strobe light source.~The reaction was monitored with a line-of-sight mass spectrometer coupled to a lock-in amplifier tuned to the strobe frequency.~We find that the photocatalytic efficiency is enhanced as the TiO$_{2}$ thickness is reduced from 50\textit{nm} to 2\textit{nm}.~We also find that the efficiency is enhanced by lowering the substrate Fermi level in going from $n$ type to $p$ type Si.~The results strongly point to the hypothesis that only near surface electron-hole pair generation is relevant to the photocatalytic process; and that the reaction rate can be controlled by varying the substrate Fermi level which in turn changes the electrostatic potential variation within the heterostructure.