X-ray Diffraction and X-ray Spectroscopy Studies of Cobalt-Doped Anatase TiO2:Co Nanopowders

Co-doped TiO2 is one of the most extensively studied oxides for applying as dilute magnetic semiconductors due to its room temperature magnetism. Here we present results of the studies of Ti0.97Co0.03O2 nanopowders synthesized by microwave-assisted hydrothermal method by means of X-ray diffraction, soft X-ray absorption spectroscopy (Ti L-2,L-3 and Co L-2,L-3 spectra), hard X-ray absorption spectroscopy (Co K spectra), and 1s3p resonant inelastic Xray scattering at the Co K edge. According to X-ray diffraction data and Ti L-2,L-3 X-ray absorption spectra, all the samples before the thermal treatment exhibit anatase structure with substantial amount of amorphous phase. After annealing the Ti0.97Co0.03O2 samples in vacuum or hydrogen at 700 degrees C, the anatase structure persists while amorphous phase contribution is eliminated. Surface-sensitive soft X-ray absorption Co L-2,L-3 spectroscopy revealed only Co2+ ions tetrahedrally coordinated by oxygen ions and no sign of metallic Co. Co2+ tetrahedral sites (instead of typical octahedral ones) are an additional evidence for Co2+ localization at the distorted TiO2 particle surface. Bulk-sensitive X-ray diffraction, Co K X-ray absorption spectroscopy, and 1s3p resonant inelastic X-ray scattering at the Co K edge revealed clustering of metallic cobalt inside of the large agglomerates formed by TiO2 nanoparticles in annealed TiO2:Co nanopowders.