Oxygen vacancy induced structural phase transformation in TiO2 nanoparticles

Nanosize TiO2 powders prepared by the sol-gel technique at pH of precipitation 4.5 and 6.5 show the anatase phase after calcining at 500 degrees C. Anatase to rutile phase transformation (ART), however, occurs at 650 degrees C in the case of pH 6.5 while 850 degrees C is found to be the ART temperature for the lower pH sample. pH of precipitation dependent ART temperature has not been reported so far to the best of our knowledge. It is known that the smaller the particle size, the lower the ART temperature, and vice versa. The observation of higher crystallite size and lower ART temperature in the case of the higher pH sample contradicts the reported result. We realized from x-ray photoelectron spectroscopic studies that oxygen vacancy concentration drives the ART temperature to lower values in the higher pH sample compared with the sample synthesized at lower pH; even the particle size is found to be higher in the former one.