Effect of brookite presence on nanocrystalline anatase – rutile phase transformation

TiO2 nanocrystals were prepared by hydrolysis and peptisation of titanium isopropoxide under different pH values. The as-prepared powder of very fine anatase crystallites ranges from 8 nm in acidic solution of pH 3 to 10 nm in basic solution of pH 8. Heat treatment of the powders leads to grain growth and anatase-rutile transformation. Experimental results have shown that the anatase to rutile phase transformation in the heat-treated powders depend not only on primary crystallite size but also the presence of brookite phase. It is observed that in the powders with the same size the presence brookite phase would accelerate the anatase to rutile transformation. Furthermore it has been found that the transition route is different among samples prepared in acidic or basic solutions. It is also proved that in case of acidic solutions, the transition route follows the brookite-anatase-rutile path, but in case of basic solutions, it would be done via anatase-brookite-rutile. Apart from these observed paths, the main route of A-R remains unchanged for both acidic and basic conditions. So it could be justified that brookite phase together with anatase grain size in titania powder plays very important and different roles and accelerates phase transformation.