Co and (Co,Mo) doping effects on the properties of highly reduced TiO2 anatase thin films

This work reports on the structural, optical, electrical and magnetic properties of Co:TiO2 and (Co,Mo):TiO2anatase thin films grown onto (0001) sapphire substrates by Pulsed Laser Deposition in highly reducing conditions and at a low temperature of 350 °C. Undoped TiO2−δ as well as doped films with nominal compositions of Ti0.95Co0.05O2−δ, Ti0.94Co0.05Mo0.01O2−δ, and Ti0.92Co0.05Mo0.03O2−δ were studied. They all show similar microstructures, with smooth surfaces and RMS roughness values less than 0.5 nm. The optical band gap energies of the doped films are red shifted with respect to that deduced for the undoped TiO2−δ. The correlation between the band gaps and the Urbach energies of the films is discussed. All samples show semiconductor behavior with n-type conduction. The Co:TiO2 sample is ferromagnetic with a saturation magnetization of 1.3 μB/Co, a high electrical conductivity of 123 S cm−1 and a carrier density of 1.88 × 1021 cm−3 at room temperature. The ferromagnetic order of the Co:TiO2 system is suppressed when carriers are added by codoping with Mo.