Ab Initio Study of the Atomic Level Structure of the Rutile TiO₂(110)-Titanium Nitride (TiN) Interface

Titanium nitride (TiN) is widely used in industry as a protective coating due to its hardness and resistance to corrosion and can spontaneously form a thin oxide layer when it is exposed to air, which could modify the properties of the coating. With limited understanding of the TiO₂-TiN interfacial system at present, this work aims to describe the structural and electronic properties of oxidized TiN based on a density functional theory (DFT) study of the rutile TiO₂(110)-TiN(100) interface model system, also including Hubbard +U correction on Ti 3d states. The small lattice mismatch gives a good stability to the TiO₂-TiN interface after depositing the oxide onto TiN through the formation of interfacial Ti-O bonds. Our DFT+U study shows the presence of Ti³⁺ cations in the TiO₂ region, which are preferentially located next to the interface region as well as the rotation of the rutile TiO₂ octahedra in the interface structure. The DFT+U TiO₂ electronic density of states (EDOS) shows localized Ti³⁺ defect states forming in the midgap between the top edge of the valence and the bottom of the conduction band. We increase the complexity of our models by the introduction of nonstoichiometric compositions. Although the vacancy formation energies for Ti in TiN (E_vac (Ti) ≥ 4.03 eV) or O in the oxide (E_vac (O) ≥ 3.40 eV) are quite high relative to perfect TiO₂-TiN, defects are known to form during the oxide growth and can therefore be present after TiO₂ formation. Our results show that a structure with exchanged O and N can lie 0.82 eV higher in energy than the perfect system, suggesting the stability of structures with interdiffused O and N anions at ambient conditions. The presence of N in TiO₂ introduces N 2p states localized between the top edge of the O 2p valence states and the midgap Ti³⁺ 3d states, thus reducing the band gap in the TiO₂ region for the exchanged O/N interface EDOS. The outcomes of these simulations give us a most comprehensive insight on the atomic level structure and the electronic properties of oxidized TiN surfaces.