On crystallite size dependence of phase stability of nanocrystalline TiO2

Stability of anatase phase component present in commercial grade nanocrystalline powder of TiO2, synthesized from gas phase, is studied. Powders are heated at elevated temperatures under ambient conditions. X-ray and electron diffractometric studies are done to measure the microstructural parameters like crystallite size and rms strain. With annealing crystallite sizes of both anatase and rutile phases show a marginal increase up to 600 degreesC, beyond this temperature anatase phase transforms to rutile phase and the crystallite size of rutile phase increases rapidly. Annealing at 1000 degreesC resulted in the growth of hexagonal shaped crystallite of rutile phase oriented parallel to (110) plane. A detailed analysis of the dislocation arrangements in these nanosized crystallites is made from the x-ray diffraction data. The results of analysis show that at higher temperatures polygonization of the dislocations with lowering of dislocation density at the grain boundary possibly favors the growth of larger sized crystallites of rutile phase. High values of dislocation density obtained for anatase give evidence of piling up of dislocations and thereby increases the strain energy of the lattice. The increase in stored energy may be responsible for the observed changes in the lattice parameter for the anatase phase. Thus, it appears that the disordered lattice of anatase favors its transformfiation to rutile structure. (C) 2003 American Institute of Physics.