Study of concentration-dependent cobalt ion doping of TiO2 and TiO2-xNx at the nanoscale

Experiments with a porous sol-gel generated TiO2 nanocolloid and its corresponding oxynitride TiO2-xNx are carried out to evaluate those transformations which accompany additional doping with transition metals. In this study, doping with cobalt (Co-II) ions is evaluated using a combination of core level and VB-photoelectron and optical spectroscopy, complementing data obtained from Raman spectroscopy. Raman spectroscopy suggests that cobalt doping of porous sol-gel generated anatase TiO2 and nitridated TiO2-xNx introduces a spinel-like structure into the TiO2 and TiO2-xNx lattices. TEM and XPS data complemented by valence band-photoelectron spectra demonstrate that metallic cobalt clusters are not formed even at high doping levels. As evidenced by Raman spectroscopy, the creation of a spinel-like structure is commensurate with the room temperature conversion of the oxide and its oxynitride from the anatase to the rutile form. The onset of this kinetically driven process correlates with the formation of spinel sites within the TiO2 and TiO2-xNx particles. Despite their visible light absorption, the photocatalytic activity of these cobalt seeded systems is diminished relative to the oxynitride TiO2-xNx.