Abstract
In-situ high-temperature synchrotron radiation diffraction was used to determine the effect of air and argon, calcination time, ion implantation, and pressure on phase transformations of nanostructured TiO2. The as-synthesized TiO2 nanomaterials were amorphous initially, as shown by the diffraction results. Crystalline anatase and rutile phases formed at elevated temperatures after thermal annealing. The anatase-to-rutile transformation (as determined from phase analysis in different atmospheres) is more rapid in argon than in air because of oxygen vacancies. The quantity of anatase decreased slightly with an increase in isothermal heating, whereas the quantity of rutile increased by phase transformation. The implantation of TiO2 nanostructures with different dopants affects the phase crystallization temperature. The implantation of V and Al ions reduces the crystallization temperature because smaller ions substituted for Ti in the TiO2 crystal structure by the dopant. A decrease in structural rigidity results in a relaxation of the Ti bonding environment, but Cr and In ions implantation resulted in a phase transformation inhibition. Cr and In ions substitute into the Ti sublattice and inhibit the anatase-to-rutile transformation. An improved structural rigidity is provided by In3+ and Cr3+ ions, which are larger than the Ti4+ ion. This increased rigidity restricts relaxation in the Ti bonding environment. An increased capillary gas pressure resulted with an increase in temperature. The transformation of the amorphous phase to anatase was enhanced by nanopowder heating under pressure and that of anatase-to-rutile transformation was reduced. This behavior is believed to occur in an oxygen-rich environment and interstitial titanium is expected to form when the material is heated under pressure. This suggests that atmospheric oxygen appears to accelerate the amorphous-to-anatase transformation, whereas interstitial titanium inhibits TiO2 structure relaxation, which is required for the anatase-to-rutile transformation.