Substrate Fermi level effects in photocatalysis on oxides: Properties of ultrathin TiO2/Si films

Photocatalysis has widespread applications from solar cells to photolithography. We studied the photocatalytic properties of TiO2 films of thicknesses down to 2 nm, grown on n-type and p-type silicon wafers, using the oxidation of isopropanol as a model system. Direct in vacuo mass spectrometry measurements were performed under irradiation above the TiO2 bandgap. We present a model consistent with our experimental results, which indicate that only near-surface electron-hole pair generation is relevant and that the reaction rate can be controlled by varying the substrate Fermi level in going from n-type to p-type silicon, by approximately a factor of 2.