Optical properties of a nanocrystalline Co-doped TiO$_{2}$ after various treatments

The absorption spectra of cobalt-doped anatase TiO$_{2}$ nanopowders in the infrared and visible ranges have been studied after various oxidative and reductive treatments. The annealing leads to the appearance of the Drude-like contribution in the infrared region and significant change in the absorption in an visible range due to the formation of defects of the oxygen-vacancy-type and Ti$^{3+}$ ions. The observed additional contribution in the absorption spectra of TiO$_{2}$:Co nanopowders as compared to the spectra of undoped powders is ascribed to the $d$–$d$ transitions in ions Co$^{2+}$. The change in the absorption related to cobalt after annealing is explained by a change in the local environment of Co ions from the octahedral to the tetrahedral environment.