DFT Simulations of Titanium Oxide Films on Titanium Metal

Titanium surfaces in implants are covered by titanium oxide passivation layers of 1.5-25 nm thickness. At the surface TiO2 phases form with anatase and rutile structures. To improve the understanding of the initial phases of passivation, we investigated the interfaces between Ti and the most common oxide surfaces rutile (110) and (100) and anatase (101) and (001). Simulations based on a DFT+U approach revealed the presence of metal induced gap states (MIGS) caused by oxygen atoms that have moved toward the metal phase. We discuss the structural disorder around the interface and its effect on the oxide crystal structure. The computed work of separation is 3.4-4.3 J/m(2) which is much higher than the surface energies of Ti metal and TiO2 and results in a negative interface energy. Charge transfer takes place from the metal to the oxide and appears to be less dependent on the oxide phase than on the thickness of the slabs. We computed a charge transfer of 0.07 e per atom, most of which remains located in the first interface layers. The metal work function changes as a consequence of the formation of the passivation layer, but Delta Phi is positive for rutile and negative for anatase.