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

Abstract

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 TiO-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 TiO(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 TiO-TiN interface after depositing the oxide onto TiN through the formation of interfacial Ti-O bonds. Our DFT+U study shows the presence of Ti cations in the TiO region, which are preferentially located next to the interface region as well as the rotation of the rutile TiO octahedra in the interface structure. The DFT+U TiO electronic density of states (EDOS) shows localized Ti 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 (Ti) ≥ 4.03 eV) or O in the oxide (E (O) ≥ 3.40 eV) are quite high relative to perfect TiO-TiN, defects are known to form during the oxide growth and can therefore be present after TiO 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 TiO introduces N 2p states localized between the top edge of the O 2p valence states and the midgap Ti 3d states, thus reducing the band gap in the TiO 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.

TiO2-HfN Radial Nano-Heterojunction: A Hot Carrier Photoanode for Sunlight-Driven Water-Splitting

“…Figure 6a showed the PDOS of selected atoms in the HfN-TiO2 system, where TiO2 and HfN slabs are kept far apart to ensure there is no interaction between them. As expected, the plot showed TiO2 to be a wide bandgap n-type semiconductor whose valence and conduction bands are mostly composed of O-2p and Ti-3d states [51]. These figures also showed the electronic properties of HfN to be metallic with overlapped valence and conduction bands that are mostly composed of Hf d-states, which is consistent with earlier findings [8].…”

Structure, Stability and Water Adsorption on Ultra-Thin TiO2 Supported on TiN

“…This correction is applied to account for the potential reduction of Ti atoms that can form in the TiO 2 -TiN interface. The U value was chosen in consistency with our previous works on TiO 2 -TiN interfaces 40,54 and surface modified TiO 2 . 63 We use Bader charge analysis 64 to assess the localization of reduced Ti cations.…”

“…The features observed in the structure of 2-layer thick TiO 2 -TiN interfaces are consistent with our previous model with a thicker oxide layer. 40…”

“…In our previous study, we found that the introduction of several O vacancies becomes more favourable, leading to the stability of highly nonstoichiometric systems. 40 Based on these results, in addition to the single vacancy system in the 2-layer thick TiO 2 model, we also consider the formation of highly non-stoichiometric interfaces, in which all of the bridging O are removed from the interface region. Figure 4 depicts the structure of 2-layer thick interface models with 6 interfacial oxygen vacancies, which results in an interface with the oxide composition of TiO 1.75 -TiN.…”

“…37 From experiments, it is observed that TiN deposited by CVD on Si (100) forms a uniform film with stoichiometric composition and predominance of (200) out-of-plane orientation at deposition temperatures in the range of 400-700 º C. 38 The epitaxial growth of TiO 2 on TiN leads to the formation of rutile TiO 2 (110) with the presence of an interfacial oxynitride (TiN x O y ) transition layer. 39 In a previous publication, 40 we used Hubbard corrected density functional theory (DFT+U) to study the structure and electronic properties of rutile TiO 2 (110)-TiN (100) interfaces. We have found that in the rutile (110)-TiN interface system defects such as Ti vacancies in TiN, O vacancies in TiO 2 or interdiffusion of O and N atoms within the interface may be present.…”