Stability of Adsorbed Water on TiO2-TiN Interfaces. A First Principles and Ab Initio Thermodynamics Investigation.

Titanium Nitride (TiN) surfaces can oxidise and the growth of a TiO<sub>x</sub> layer on the surface along with the likely presence of water in the surrounding environment can modify the properties of this widely used coating material. The present Density Functional Theory study, including Hubbard +U correction (DFT+U), investigates the stability of adsorbed water at TiO<sub>2</sub>-TiN interfaces with different defects, that serve as a model for an oxide layer grown on a TiN surface. Surface free energy calculations show the stability of perfect TiN-TiO<sub>2</sub> interface at regular O pressures, while oxygen vacancy-rich TiO<sub>1.88</sub>–TiN is more favourable at reducing conditions. An isolated water is preferentially adsorbed dissociatively at perfect and oxygen defective interfaces while molecular adsorption is more stable at higher coverages. The adsorption energy is stronger at the oxygen defective interfaces which arises from the high concentration of reduced Ti<sup>3+</sup> and strong interfacial atomic relaxations. Ab initio atomistic thermodynamics show that water will be present at high coverage on TiO<sub>2</sub>-TiN interfaces at ambient conditions and the pristine interface is only stable at very low pressure of O and H<sub>2</sub>O. The results of these DFT+U simulations are important for the fundamental understanding of wettability of interfacial systems involving metal oxides.