Crystallization kinetics study of In-doped and (In-Cr) co-doped TiO2 nanopowders using in-situ high-temperature synchrotron radiation diffraction

The influence of TiO2 nanopowder doping with 4 wt% indium and 2 wt% each of indium and chromium on phase transformation was studied. Samples were heated from ambient temperature to 950 °C in sealed quartz capillaries, and in-situ synchrotron radiation diffraction measurements were obtained. Capillary sealing yielded an increase in capillary gas pressure to 0.42 MPa at 950 °C in proportion to absolute temperature by Gay-Lussac’s Law. The initial synthesized samples were amorphous, and crystalline anatase appeared at 200 °C. Crystalline rutile appeared at 850 °C for the nanomaterials that were doped with In and In and Cr. A change in sealed-capillary oxygen partial pressure yielded a decrease and an increase in crystallization temperature, respectively, for the amorphous-to-anatase and anatase-to-rutile transformations. Crystalline titania (anatase and rutile) formed from the amorphous titania by 800 °C and 900 °C, for materials doped with In and In-Cr, respectively. The anatase concentration that was dominant in the In-doped materials up to 950 °C and the higher rutile concentration for the In-Cr doped materials from 900 to 950 °C results from the defect structure that was induced by doping. Cr-ions in the Ti sub-lattice retarded the transformation of anatase to rutile when compared with the retarding effect of mixed In/Cr ions. The transformation results because of the relatively smaller radius of Cr-ions when compared with the In-ions. The differences in phase-transformation kinetics for In, In-Cr and for undoped nanopowders in the literature agree with the calculated transformation activation energies.