Cobalt-Doped TiO 2 Nanocrystallites: Radio-Frequency Thermal Plasma Processing, Phase Structure, and Magnetic Properties

Co 2+ -doped TiO 2 nanopowders have been synthesized via one-step Ar/O 2 RF thermal plasma oxidizing mists of liquid precursors containing titanium tetra- n -butoxide and cobalt (II) nitrate. Co 2+ doping was found to steadily promote rutile crystallization. The solubility of Co 2+ in the TiO 2 lattice was determined to be around 2.0 atom %, above which CoTiO 3 and Co 2 TiO 4 may be formed as impurity phases. The existence of metallic Co or any other ferromagnetic impurities were excluded by a variety of characterization techniques, including high-power XRD, HR-TEM, XPS, whereas the nanopowders exhibit room-temperature ferromagnetism, which was indicated to be of intrinsic nature. The ferromagnetic performances of the TiO 2:Co samples tend to decrease with increasing the Co 2+ content up to ∼7 atom %, which has been ascribed to the enhanced rutile crystallization by co-doping and the formation of paramagnetic impurities.