Interaction of Water with Atomic Layer Deposited Titanium Dioxide on p‐Si Photocathode: Modeling of Photoelectrochemical Interfaces in Ultrahigh Vacuum with Cryo‐Photoelectron Spectroscopy

This study combines cryo‐photoelectron spectroscopy and electrochemical analysis techniques to investigate the p‐Si/SiO₂/TiO₂/H₂O system in the context of water‐splitting. Atomic layer deposition is used for the preparation of a TiO₂ thin film coating for a p‐Si/SiO₂ photocathode. First, an interface experiment is performed to study the contact properties of the interface between p‐Si/SiO₂ and TiO₂. For the p‐Si/TiO₂ heterojunction, a downward band bending of 0.3 eV is found for the p‐Si toward the interface. Second, a water adsorption experiment is conducted, which allows the investigation of the surface chemistry of the TiO₂ coating in contact to water. A direct correlation between the amount of surface hydroxide species, formed due to water dissociation, and Ti³⁺ defect state density is found. Furthermore, a surface water species can be identified in addition to the commonly found bulk molecular water. Together with the results from a Mott–Schottky analysis, a complete energy level alignment can be constructed.