Structure and reactivity of highly reduced titanium oxide surface layers on TiO₂: A first-principles study

Titanium oxide structures featuring highly reduced TiO_x films on top of nearly stoichiometric TiO₂ hold promise for applications ranging from photocatalysis to resistance switching devices. Here, we focus on titanium monoxide (TiO) layers on anatase TiO₂ (001) and use first principles calculations to investigate their structure and properties as well as their interface with liquid water. We find that only a single TiO layer can grow epitaxially on anatase (001) and subsequent growth leads to the formation of TiO islands. The TiO layers decrease the work function and enhance the surface conductivity in comparison to pure anatase, two features that can improve the TiO₂ performance in photocatalytic hydrogen evolution but are thermodynamically unstable relative to pure TiO₂ in humid/aqueous environment. Furthermore, first principles molecular dynamics simulations of the TiO (001)-water and anatase (001)-water interfaces show that unlike the multilayer structure of interfacial water on the anatase surface, a very dense and tightly packed first water layer is present on the surface of TiO, which could represent the first stage of partial surface reoxidation.