Microscopic Study of Atomic Layer Deposition of TiO₂ on GaAs and Its Photocatalytic Application

We report a microscopic study of p -GaAs/TiO 2 heterojunctions using cross-sectional high resolution transmission electron microscopy (HRTEM). The photocatalytic performance for both H 2 evolution and CO 2 reduction of these heterostructures shows a very strong dependence on the thickness of the TiO 2 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 TiCl 4 atomic layer deposition (ALD) precursor, which is corrosive. Ti 3+ defect states (i.e., O vacancies) in the TiO 2 film provide catalytically active sites, which improve the photocatalytic efficiency. Density functional theory (DFT) calculations show that water molecules and CO 2 molecules bind stably to these Ti 3+ states. Thicker TiO 2 films (15 nm) are crystalline and have poor charge transfer due to their insulating nature, while thinner amorphous TiO 2 films are conducting.