Anatase and rutile TiO2-δ powders are synthesized by the sol-gel method. The hysteresis loops and the zero field-cooled and the field-cooled magnetization curves indicate that reduced TiO2-δ powders exhibit the room-temperature ferromagnetism that becomes stronger with prolonging annealing time in H2/Ar mixture. Analysis of Ti 2p x-ray photoelectron spectroscopy spectra indicates that Ti ions are all in the Ti4+ state and that Ti3+ or Ti2+ ions do not exist in all samples. In addition, analysis of O 1s x-ray photoelectron spectroscopy spectra indicates that the concentration of oxygen vacancies increases with prolonging annealing time. Analysis of ultraviolet-visible absorption spectra also further confirms that the concentration of oxygen vacancies increases with prolonging annealing time. These results indicate that ferromagnetism in pure TiO2-δ powders stems from oxygen vacancies. The possible mechanism on ferromagnetism is discussed.
Zinc oxide ͑ZnO͒ tetrapodlike nanostructures were synthesized by thermal evaporation of Zn powder at different flow rates of argon. Scanning electron microscopy, x-ray diffraction, transmission electron microscopy, and photoluminescence were employed to study the structural features and optical properties of the product. Results show that the flow rate of argon gas has a comparative great influence on the morphology of ZnO nanostructures. As the flow rate increases, nano-ZnO morphology changes from an initial mixture of tetrapod, nanowire, nanosheet, and nanodendritic forms to a uniform tetrapodlike morphology. The fact that the ultraviolet emission weakens and the blue emission strengthens with increase of the argon flow rate suggests that the oxygen vacancy is most likely responsible for the blue emission. This was confirmed by the experimental results: the blue light emission disappears after the products are oxidized in air at 700°C, and then reappears after the oxidized products are deoxidized in H 2 at 700°C. The growth mechanism of the ZnO tetrapods is also discussed.
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