Synthesis of TiO2 nanocrystals controlled by means of the size of magnetic elements and the level of doping with them

TiO2 nanocrystals were synthesized by a hydrolysis method combined with a thermal treatment. TiO2 nanocrystals with rutile and anatase structure were selectively synthesized by controlling the pH level in the precursor solution, and the crystallite size was controlled by changing the reaction temperature. Moreover, Co-doped TiO2 nanocrystals with rutile structure were also synthesized by means of the addition of Co to the precursor solution. Secondary phases such as Co precipitates and Co oxide were not present in the sample tested, with [Co] < 10 mol%. With an increase in the Co doping level, the Eg-phonon signal at 447 cm-1 was broadened and shifted to a lower frequency, indicating the incorporation of Co into the rutile TiO2 host lattice and lattice expansion. Optical absorption spectra showed that the absorption edge at ~3.0 eV corresponded to the band gap of rutile TiO2 and shifted to the lower energy side upon Co doping. These results indicated the possibility of band gap engineering of rutile TiO2 via Co doping. On the other hand, the charge transfer gap between O 2p and Co 3d orbitals was also observed for samples with Co, suggesting the possibility of photo-induced magnetism in rutile TiO2 nanocrystals, obtained by visible light irradiation.