Structure, stability and water adsorption on ultra-thin TiO₂ supported on TiN

Interfacial metal-oxide systems with ultra-thin oxide layers are of high interest for their use in catalysis. The chemical activity of ultra-thin metal-oxide layers can be substantially enhanced compared to interfacial models with thicker oxide. In this study, we present a Density Functional Theory (DFT) investigation of the structure of ultra-thin rutile layers (one and two TiO₂ layers) supported on TiN and the stability of water on these interfacial structures. The rutile layers are stabilized on the TiN surface through the formation of interfacial Ti-O bonds. Charge transfer from the TiN substrate leads to the formation of reduced Ti³⁺ cations in TiO₂. The concentration of Ti³⁺ is proportionally higher in the ultra-thin oxide, compared to interfacial models with thicker oxide layers. The structure of the one-layer oxide slab is strongly distorted at the interface while the thicker TiO₂ layer preserves the rutile structure. The energy cost for the formation of a single O vacancy in the one-layer oxide slab is only 0.5 eV with respect to the ideal interface. For the two-layer oxide slab, the introduction of several vacancies in an already non-stoichiometric system becomes progressively more favourable, which indicates the stability of the highly defective interfaces. Isolated water molecules dissociate when adsorbed at the TiO₂ layers. At higher coverages, the preference is for molecular water adsorption. Our ab initio thermodynamics calculations show the fully water covered stoichiometric models as the most stable structure at typical ambient conditions. This behaviour is similar to that observed on thicker oxide in TiO₂-TiN interfaces or pure TiO₂ surfaces. In contrast, interfacial models with multiple vacancies are most stable at low (reducing) oxygen chemical potential values. The high concentration on reduced Ti³⁺ introduces significant distortions in the O-defective slab. Whereas, a water monolayer adsorbs dissociatively on the highly distorted 2-layer TiO_1.75-TiN interface, where the Ti³⁺ states lying above the top of the valence band contribute to a significant reduction of the energy gap compared to the stoichiometric TiO₂-TiN model. Our results provide a guide for the design of novel interfacial systems containing ultra-thin TiO₂ with potential application as photocatalytic water splitting devices.