Iron(III)-doped TiO2 nanopowders, with controlled Fe to Ti atomic ratios, (R(Fe/Ti)) ranging from nominal 0 to 20%, were synthesized using oxidative pyrolysis of liquid-feed metalorganic precursors in a radiation-frequency RF thermal plasma. The valence of Fe doped in the TiO2 phase formation, defect structures, band gaps, and magnetic properties of the resultant nanopowders were systematically investigated by using Mössbauer spectroscopy, X-ray diffraction, Raman spectroscopy, TEM/high-resolution transmission electron microscopy, UV-vis spectroscopy, and measurements of magnetic properties. The Fe doped in TiO2 was trivalent in a high-spin state as determined by the isomer shift and quadrupole splitting from the Mössbauer spectra. No other phases except anatase and rutile were identified in the resultant nanopowders. Thermodynamically metastable anatase predominated in the undoped TiO2 nanopowders. This was explained from a kinetic point of view, based upon classical homogeneous nucleation theory. With Fe doping, the formation of rutile was strongly promoted because rutile was more tolerant than anatase to defects such as O vacancies resulting from the substitution of Fe3+ for Ti4+ in TiO2. The concentration of O vacancies reached a maximum at R(Fe/Ti) = 2%, above which excess O vacancies tended to concentrate. As a result of this concentration, an extended defect-like crystallographic shear structure was established. With Fe doping, a red-shift of the absorption edges occurred in addition to the d-d electron transition of Fe in the visible light region. The as-prepared iron-doped TiO2 nanopowders were paramagnetic in nature at room temperature.

Pyrogenic Iron(III)-Doped TiO2 Nanopowders Synthesized in RF Thermal Plasma - Phase Formation, Defect Structure, Band Gap and Magnetic Properties. X.H.Wang, J.G.Li, H.Kamiyama, M.Katada, N.Ohashi, Y.Moriyoshi, T.Ishigaki: Journal of the American Chemical Society, 2005, 127[31], 10982-90