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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. 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 ºC and 10% anatase composition is retained after annealing the materials at 900 ºC for the optimised composition. On comparison a control sample which has been prepared without formic acid showed that rutile phases started to form at a temperature of 600 C, FTIR and Raman studies indicated that the formate group favoured 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 crosslinking 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.
Nolan, N., Pillai, S., Seery, M.: Spectroscopic Investigation of the Anatase-to-Rutile Transformation of Sol-Gel Synthesised TiO2 Photocatalysts. Journal of Physical Chemistry C, 113, 2009, 16151-16157.