UCLA

We report a microscopic study of p-GaAs/TiO2 heterojunctions using cross-sectional high resolution transmission electron microscopy (HRTEM). The photocatalytic performance for both H2 evolution and CO2 reduction of these heterostructures shows a very strong dependence on the thickness of the TiO2 over the range of 0-15 nm. Thinner films (1-10 nm) are amorphous and show enhanced catalytic performance with respect to bare GaAs. HRTEM images and electron energy loss spectroscopy (EELS) maps show that the native oxide of GaAs is removed by the TiCl4 atomic layer deposition (ALD) precursor, which is corrosive. Ti3+ defect states (i.e., O vacancies) in the TiO2 film provide catalytically active sites, which improve the photocatalytic efficiency. Density functional theory (DFT) calculations show that water molecules and CO2 molecules bind stably to these Ti3+ states. Thicker TiO2 films (15 nm) are crystalline and have poor charge transfer due to their insulating nature, while thinner amorphous TiO2 films are conducting.