The effect of gold on the phase transitions of titania

The microstructure and phase transformation behavior of sol–gel derived Au–TiO2 has been studied. X-ray powder diffraction (XRD), and BET surface area measurements were used in order to understand the microstructural development of Au–TiO2 systems for possible regulation of the phase composition of TiO2. For TiO2, brookite transforms to anatase at T < 600 °C; brookite transforms to rutile and anatase transforms to rutile at T ≥ 600 °C were observed. For Au–TiO2, brookite transform to anatase which transforms to rutile. Gold enhanced the brookite to anatase phase transformation at T ≤ 500 °C, but retarded the anatase to rutile phase transformation. There is no evidence of the substitution of Ti4+ ions by gold/or gold ions in TiO2 and thus gold is bound mostly on the surface of TiO2, consistent with SEM measurements. Although, gold retards the anatase to rutile phase transformation, it is unable to maintain the porous structure of TiO2, leading to a collapse in the BET surface area to ≤1 m2 g−1 at 700 °C. The possibility of incorporation of Au3+ in the TiO2 matrix in an interstitial site is discussed. The phase transformations are discussed in terms of the nucleation mechanisms previously proposed.