Limited crystallite growth upon isothermal annealing of nanocrystalline anatase

The crystalline growth kinetics during isothermal sintering of two nanotitania powders synthesized by similar routes, but with and without the presence of chlorine in the synthesis batch, were studied by X-ray powder diffraction and modeled by several grain growth models. Both nanopowders contained anatase as the initial titania phase with similar crystallite dimension. Crystal growth curves at three isotherms per sample were sampled over a period of time up to 40 h. Temperature steps within different ranges (375, 425, and 475 °C for Cl-free sample; 500, 550, and 600 °C for Cl-containing sample) were chosen. The XRD analysis of samples heated at 900 °C revealed that, while the entire Cl-free sample was converted to rutile, only 10% of the Cl-containing sample had transformed to rutile with a very limited crystal growth. Direct comparison of the crystal growth curves showed different behavior which was modeled by different grain growth kinetic equations including a growth limiting factor. Although the generalized parabolic grain growth model with time exponent (so-called "Höfler-Averback equation") provided good fits for both the Cl-free and Cl-containing nanopowders at all temperatures, only the isothermal curve at 500 °C of the Cl-containing sample was satisfactorily fitted with a modified KJMA equation. The activation energy values of the grain growth are very similar and in line with the previously reported values. We conclude that the crystal size locking phenomenon observed for Cl-bearing anatase can be ascribed to the effects of chlorine ions adsorbed on grain surface as previously suggested. The blocked crystal growth of nanoanatase obtained by the reflux synthesis of organic solvents in the presence of hydrochloric acid as catalyst makes this material very appealing for devices that require a given nanosize and for the anatase phase in spite of the high temperature processing.