Mechanism of titanium electrochemical oxidation via isotopic labeling, high resolution ion depth profiling, and impedance spectroscopy

Abstract

The electrochemical oxidation mechanism of titanium between 0 and 10 V vs saturated calomel reference electrode (SCE) was examined for ultra-thin Ti films sputtered onto Si(001) substrates and exposed in-situ to H218O, and then anodized in D216O. The effects of this isotopic labeling procedure were studied using medium energy ion scattering (MEIS) and nuclear reaction profiling (NRP). Both MEIS and NRP results are consistent in showing that the titanium oxide layer is composed of two distinct regions, Ti16O2/Ti18O2/Ti/Si(001) for the entire range of the formation voltages (0–10 V vs SCE). The outermost region consists entirely of 16O, and the 18O region is always adjacent to the Ti metal. The two distinct structures observed can be consistent with both the point defect model (PDM) or high-field model (HFM), assuming the mobility of titanium cations is much higher compared to the mobility of oxygen ions. No Ti or 18O loss into the electrolyte during anodization is detected. Linear growth rate is observed in 0–10 V range vs SCE with experimental anodization ratio of 24.5±0.6 Å V−1. Mott–Schottky (MS) analyses show positive slopes, indicating formation of an n-type TiO2 semiconductor, with O vacancies (or Ti interstitials) as major charge carriers in the 0–10 V range. Charge carrier densities, ND = (0.8-5.0 )×1021 cm−3 were calculated from Mott–Schottky analysis and were well within the range of results reported in the literature. We observe derease in the charge carrier densities above ∼4 V vs SCE, that can be connected to defects annihilation or minor modification in the structure of the growing TiO2 film.