Spectroscopic investigation of the anatase-to-rutile transformation of sol-gel-synthesized TiO₂ photocatalysts

Among the three major phases in titania, anatase is reported to be a better photocatalytically active phase. Anatase to rutile transformations, under normal conditions, usually occurs at a temperatue range of 600-700 °C. Various chemical additives have previously been employed to extend the anatase transition to higher temperatures. The effect of employing various concentrations of formic acid and water on phase transition has systematically been studied by XRD, FTIR, and Raman spectroscopy. A considerably higher anatase phase (41%) has been obtained at 800 °C, and 10% anatase composition is retained after annealing the materials at 900 °C for the optimized composition. On comparison, a control sample which has been prepared without formic acid showed that the rutile phase formed at a temperature of 600 °C, FTIR and Raman studies indicated that the formate group favored a bridging (syn-anti or syn-syn) mode of chelation depending on the reaction conditions. It has been concluded that the resulting syn-anti binding hinders cross-linking of the gel network, resulting in a weakened structure and thus causing the anatase to rutile transformation temperature to occur at a lower temperature than with the syn-syn mode of binding where more ordered gel networks are formed.