Polymorphic Evolution of TiO2 in Hydrothermal Reaction System of Layered Titanate Nanosheets

Abstract The hydrothermal formation mechanisms of TiO 2 polymorphs of brookite and anatase, and their nanocomposites were investigated systematically in a mixed solution reaction system of titanium isopropoxide (TTIP) and tetramethylammonium hydroxide (TMAOH) by using XRD, FE‐SEM and TEM observations, and SAED. The titanate nanosheets with lepidocrocite‐type layered structure are formed firstly, and then transformed to the TiO 2 polymorphs by topotactic structural transformation reactions in the reaction system. The pH value and the concentration of TMA + play the key roles in the formation of TiO 2 polymorphs. The small‐sized layered titanate nanosheets are formed and transformed to anatase phase under low pH and low TMA + concentration conditions, while the large‐sized layered titanate nanosheets are formed and transferred to brookite phase under high pH and high TMA + concentration conditions. The anatase‐brookite nanocomposites can be obtained from the mixture of small‐ and large‐sized nanosheets under middle pH conditions, in which the anatase nanocrystals are formed on the surface of brookite nanocrystal.