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 lepidocrocitetype 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.

Structure, Properties, Preparation, and Application of Layered Titanates

“…In addition, the Langmuir–Blodgett method is also a commonly used nanosheet design method . Organic/inorganic nanocomposites and thin films have been successfully prepared by these methods. − For example, manganese-intercalated layered titanate was prepared by the exfoliation flocculation method, and stronger photocatalytic activity than protonated layered titanate was obtained . Reasonable selection of different nanosheets to construct superlattice structures can make full use of the synergistic properties of these two-dimensional materials .…”

“…143,144 There are two different conversion methods for the conversion of layered titanate into TiO 2 : calcination in air 145 and hydrothermal treatment. 182 Generally, there are two mechanisms for the formation of TiO 2 crystal forms in hydrothermal processes: in situ topological transformation and dissolution−recrystallization. 146 For example, under milder reaction conditions (in water 147 or 0.05 M HNO 3 148 ), the transformation of H 2 Ti 3 O 7 to anatase undergoes a topological chemical reaction.…”

“…In addition, the preparation of TiO 2 with layered titanate as the precursor is also a research hotspot. , There are two different conversion methods for the conversion of layered titanate into TiO 2 : calcination in air and hydrothermal treatment . Generally, there are two mechanisms for the formation of TiO 2 crystal forms in hydrothermal processes: in situ topological transformation and dissolution–recrystallization .…”