A sol-gel combustion synthesis method for TiO₂ powders with enhanced photocatalytic activity

TiO₂ photocatalytic powders were synthesized by a sol-gel combustion synthesis method in which urea was used as the fuel and titanyl nitrate was used as the oxidizer. Various fuel-to-oxidizer ratios were studied for their effects on the combustion phenomena and the properties of the synthesized TiO₂. The fuel-to-oxidizer ratio was found to determine the maximum combustion temperature, which in turn affects the specific surface area, crystallite size, and weight fraction of anatase phase of the synthesized TiO₂. The synthesized TiO₂ all contain carbonaceous species and are either pure anatase or anatase-rutile mixed phase in crystalline structure. The photocatalytic activity of the TiO₂ was found to correlate to a certain degree with the specific surface area, crystallite size, weight fraction of anatase phase, and visible and IR absorbances. The mixed phase TiO₂ shows a higher photocatalytic activity than the pure anatase phase TiO₂ when containing a small fraction (<~25 wt%) of rutile phase but a lower phoyocatalytic activity when containing a large fraction (>~25 wt%) of rutile phase. The synthesized TiO₂ all show higher photocatalytic activity than Degussa P25 TiO₂. The enhanced photocatalytic activity was attributed mainly to sensitization by the carbonaceous species and larger amounts of hydroxyl group adsorbed on the TiO₂ surface.