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

Titanium nitride (TiN) surfaces can oxidize, and the growth of a TiO_x 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 TiO₂-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 a perfect TiN-TiO₂ interface at regular O pressures, while oxygen vacancy-rich TiO_1.88-TiN is more favorable 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 arise from the high concentration of reduced Ti³⁺ and strong interfacial atomic relaxations. Ab initio atomistic thermodynamics show that water will be present at high coverage on TiO₂-TiN interfaces at ambient conditions, and the pristine interface is only stable at very low pressure of O and H₂O. The results of these DFT+U simulations are important for the fundamental understanding of wettability of interfacial systems involving metal oxides.