In Situ Observation of the Stability of Anatase Nanoparticles and Their Transformation to Rutile in an Acidic Solution

Thermodynamic stability of anatase nanoparticles and their transformation behaviors to rutile phase in an acidic solution was investigated in situ at two different peptization temperatures using a freeze-drying method. When peptized at 30 °C, the initial product was anatase with a significantly distorted atomic structure, a significant amount of hydroxyl group and Ti3+ ions, and, thus, a thermodynamically unstable state. The instability of 30 °C-peptized anatase was responsible for a suitable transformation to rutile later via dissolution of the anatase to form a titanium hydroxylate, followed by reprecipitation into rutile. On the other hand, 80 °C-peptized anatase had a relatively more ordered atomic structure, a much reduced amount of hydroxyl group, negligible Ti3+ ions, and, thus, a thermodynamically more stable state. Plausible reasons why the 80 °C-peptized anatase does not transform to rutile were deduced.