Oxidation of a polycrystalline titanium surface by oxygen and water

Reactions of a well-characterized polycrystalline titanium surface with oxygen and water molecules at 150–850 K were studied in UHV by X-ray photoelectron spectroscopy (XPS), thermal desorption spectroscopy (TDS) and Fourier transform reflectance–absorption infrared spectroscopy (FT-RAIRS). At 150 K, O2 oxidizes Ti0 to TiIV, TiIII and TiII, but Ti exposure to H2O at this temperature produces only TiII species. At temperatures above 300 K, further oxidation of Ti by H2O was observed. Maximum oxidation by either molecule is achieved at 550–600 K. Upon heating the oxidized titanium above 850 K, the oxide layer is completely reduced to Ti0. Hydroxyl species are identified on the Ti surface after reaction with H2O; they appear to be mostly hydrogen bonded between 250 and 350 K, and isolated in the 450–650 K surface temperature range. Depth profiling of the O2-oxidized Ti surface shows that TiIV/TiIII species account for about 20% of the total thickness of the oxide layers and are located near the surface, while TiII has a broader distribution, and is concentrated close to the oxide–metal interface. The OH group concentration is maximized at 550 K on the sample surface and accounts for about 16% of the total surface oxygen, with a decreasing concentration of OH into the bulk of the titanium oxides.