An NEXAFS investigation of the reduction and reoxidation of TiO₂(001)

Abstract

The near-edge X-ray absorption fine structure (NEXAFS) technique was applied to characterize the oxidation states of titanium cations on TiO₂(001) surfaces reduced by argon-ion bombardment and reoxidized by thermal treatment. Although many characterization studies of reduced TiO₂ have been performed, none of these has applied both surface-sensitive (electron yield) and bulk-sensitive (fluorescence yield) NEXAFS to characterize reduced TiO₂(001) single-crystal surfaces. The fluorescence yield NEXAFS of polycrystalline samples of the suboxides TiO and Ti₂O₃ were used as standards to fingerprint reduced cations on the TiO₂(001) surface. NEXAFS has allowed us to estimate the concentration of oxygen and titanium in the near-surface region of reduced and reoxidized samples. The results of this study demonstrate that oxygen is preferentially removed during ion bombardment, that the depth of the altered layer is comparable to the ion penetration depth, and that the electronic environment of cations in the altered layer is comparable to that of cations in titanium suboxides. The extents of reduction calculated from NEXAFS results for reduced and reoxidized surfaces as a function of annealing temperature compare favorably with those previously determined by analysis of Ti 2p XPS data.