N and Cr (co-)doping of TiO2 Thin Films Prepared by Reactive Magnetron Sputtering

The photoactivity of TiO2 has been exploited in many applications ranging from photocatalysis, hydrogen production, pigments or solar cells [1]. However, optical absorption in TiO2 is mostly limited to the ultraviolet region of the solar spectrum (band-gap > 3 eV), triggering strong efforts to achieve visible-light (VISL) response by band-gap narrowing [1]. Non-metal (anion) doping seems to be a promising approach, as shown for the case of nitrogen (N) doped films [2]. However, it is unclear if the effective optical absorption of N-doped TiO2 is based on real band-gap narrowing or the formation of intragap localized states [3]. Recently, it has been argued that narrow-gap TiO2 would require heavy doping, relating VISL absorption to oxygen vacancies and color centers [4]. Another obstacle is the low thermodynamic solubility of dopants at substitutional sites [4]. This situation does not only compromise the effectiveness of band-gap narrowing but also provide recombination centers that are responsible for the loss of photogenerated electron-hole pairs [4]. A recent concept relies on N and Cr co-doping [5] to increase the solubility limit by non-compensated dopants where the opposite charge state of p- and n-type sites substantially enhances the thermodynamic kinetics of dopant pairs. In any case, a critical aspect of cation (co)doping relies in the introduction of large structural distortions in the host TiO2 matrix [4], needing processing or post-processing thermal treatments at moderate temperatures (~500ºC). In this work, we address the production and characterization of TiO2 (co)doped films by magnetron sputtering. We also compare different thermal annealing methods for further dopant activation and enhancement/design of the structural order, with special attention to the influence of as-grown films. The potential of novel rapid thermal processing such as flash-lamp annealing is also explored. The electronic structure of as-grown and modified films is assessed by means of X-ray absorption fine-structure and photoelectron spectroscopy, which permits the analysis of either (nano)crystalline or disordered structures. The optical response is derived from spectroscopic ellipsometry and transmission measurements. Finally, the structural, optical and electronic properties are correlated with the photocatalytic response of the samples. REFS: [1] M.A. Henderson, Surf. Sci. Rep. 66, 185 (2011); [2] R. Asahi et al. Science 293, 269 (2001) ; [3] M. Batzill et al. Phys. Rev. Lett. 96, 026103 (2006); [4] N. Serpone et al., J. Phys. Chem. B 110, 24287 (2006); [5] W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)