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, O-2 oxidizes Ti-0 to Ti-IV Ti-III and Ti-II, but Ti exposure to H2O at this temperature produces only Ti-II 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 Ti-0. 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 O-2-oxidized Ti surface shows that Ti-IV/Ti-III species account for about 20% of the total thickness of the oxide layers and are located near the surface, while Ti-II has a broader distribution, and is concentrated close to the oxide-metal interface. The OH group concentration is maximized at 550 K oil 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. (C) 2000 Published by Elsevier Science B.V. All rights reserved.