Structural impact of Cr doping in titanium oxide thin films grown by co-sputtering and flash-lamp annealed

Titanium dioxide (TiO2) is used in many applications as a photocatalyst. However, TiO2 activity is mostly limited to the UV spectral region due to its wide band-gap (~3eV). For this reason, many efforts1 have been focused on band-gap narrowing to achieve visible-light (VISL) response in TiO2, mostly by doping. Metal (cation) doping increases VISL absorption significantly but, unfortunately, it introduces structural distortions in the host matrix that result in a large number of defects acting as carrier recombination centers.1 Post-processing thermal treatments are normally employed here to improve the structural order.2 In this work, we study the impact of rapid non-contact thermal processes as flash-lamp annealing (FLA) on the electronic structure of Cr-doped TiO2. For this purpose, (amorphous) thin films with different Cr contents were produced at room temperature by magnetron co-sputtering. The dopant concentration was quantified by Rutherford backscattering spectrometry (RBS) whereas the resulting structural phases after FLA were assessed by Raman and X-ray diffraction (XRD). Due to the disordered nature of the samples, the structural characterization has been complemented with local-order information around host and dopant sites from the X-ray near-edge structure (XANES). Finally, the optical properties have been studied by spectroscopic ellipsometry (SE). It is found that FLA can selectively tune the anatase/rutile phase formation in pure TiO2. In addition, films
with low doping (Cr < 6 at.%) display a rutile structure. For higher doping level, the formation of high-valence Cr sites is observed, which seems to be detrimental for the structural promotion. Nonetheless, these sites are thermally unstable and annihilated upon FLA. REFs: 1Asahi et al. SCI 293, 269 (2001); 2W. Zhu et al. PRL 103, 226401 (2009).