Characterization of Zr-doped TiO2 nanocrystals prepared by a nonhydrolytic sol-gel method at high temperatures

Highly crystalline and surface-modified Zr-doped TiO2 nanorods were successfully prepared using a nonhydrolytic sol-gel method that involves the condensation of metal halides with alkoxides in anhydrous trioctylphosphine oxide (TOPO) at either 320 or 400 degrees C. In addition, the interaction of the cross-condensation between the Ti and Zr species was studied by characterizing the morphologies, crystalline structures, chemical compositions, surface properties, and band gaps of the nanocrystals obtained at different reaction temperatures and Zr-to-Ti stoichiometric ratios. Increases in the concentration of Zr4+ and in the reaction temperature led to large nanorods and regular shapes, respectively. In addition, only the anatase form was observed in the Zr-doped TiO2 nanorods. The Zr-to-Ti ratios obtained ranged from 0.01 to 2.05, all of which were far below the stoichiometric ratios used during the preparation of the samples (0.25-4). Moreover, the Zr4+ units accumulated mainly at the surface of the TiO2 nanocrystals. The band gaps of the Zr-doped TiO2 nanorods ranged from 2.8 to 3.8 eV, which are smaller than those of pure TiO2 (3.7 eV) or ZrO2 (5.2 eV). The Zr-doped anatase TiO2 nanorods prepared at 400 degrees C at an initial stoichiometric Zr-to-Ti ratio of 2: 3 exhibited the highest photoactivities for the decomposition of rhodamine B because of the presence of trace amounts of Zr4+ (Zr/Ti = 0.03) in the TiO2 and the regular shapes of these particles. DSC analysis indicated that the temperatures for forming nanocrystalline TiO2 and ZrO2 were 207 and 340 degrees C, respectively. Moreover, the reactivities of condensation between the Ti species were reduced when Zr species were involved in the NHSG reactions. The results obtained in this study clearly demonstrate that the faster kinetics for the generation of TiO2 controls the material properties as well as the photoactivities of the nonhydrolytic sol-gel-derived nanocrystals.