Hydrothermal rutile to anatase reverse phase transformation

TiO2 sols peptised with oxalic acid were synthesised with oxalic acid ratios from 0.25 to 1.0 oxalic acid per titanium. The resulting transparent colloidal sols of TiO2 contained a mixture of phases and sizes of TiO2 particles depending on the oxalic acid content. The sols were hydrothermally treated at different temperatures ranging from 85 degrees C to 275 degrees C. Treatment below 100 degrees C produced mostly anatase, while the decomposition of the oxalic acid between 120 degrees C and 170 degrees C allowed a phase change to rutile, with a rapid crystallite size increase from around 10 nm to 40 nm. AT-FTIR confirmed the complete decomposition of organic acids within the sols treated at 220 degrees C. Very limited rutile crystallite growth was observed above this temperature. However, sols produced with the lower oxalic acid content remained as rutile while those produced using higher initial oxalic: Ti ratios (0.5 and 1.0 M equivalents) induced a reverse phase transformation back to anatase phase TiO2. High resolution SEM showed that the anatase phase TiO2 crystallites grew to around 30 nm at the highest temperature used (275 degrees C) having indistinct morphology, while the rutile phase TiO2 were elongated rods growing up to 100 nm.