Toward high surface area TiO₂ brookite with morphology control

TiO₂ materials have many practical applications due to their intrinsic physico-chemical properties. Research activities on the properties of TiO₂ brookite have been restrained by the difficulty of preparing such a phase. Here, we report on the synthesis of TiO₂ brookite prepared by thermal decomposition of a titanium oxalate hydrate compound. Since the characteristics of the prepared TiO₂ brookite are dictated by those of the precursor, the present work aims to understand the aqueous precipitation process of the oxalate hydrate phase. It was shown that the formation of the phase occurred via different steps that are affected by the synthesis conditions, i.e. the oxalate source and the duration time. At first, in agreement with the Ostwald's rule of stages, the formation of the oxalate phase implied a metastable intermediate that is a poorly crystallized TiO ₂ phase. The pH of the solution was shown to impact on the kinetic of transformation of this intermediate toward the final compound. In the presence of alkali ions, the oxalate phase was shown to undergo a dissolution/etching phenomenon that is dependent on the nature of the alkali ion used. The apparent difference in adsorption ability of the alkali ions on the different crystal planes of the titanium oxalate hydrate phase accounted for the variety of the obtained morphologies. Finally, it was suggested that the reaction was promoted by a coordination-assisted mechanism involving the complexing properties of the oxalate anions toward Ti⁴⁺ ions. The obtained TiO₂ brookite materials exhibit unreached high specific surface area lying between 150 and 400 m² g^-1 while displaying high packing density around 1-1.2 g cm^-3. The lithium insertion ability of the prepared material depends on the calcination temperature. Increasing the temperature led to a decrease of the lithium uptake properties but was shown to improve the kinetics of lithium insertion. This was due to an increase of the pore radius size that enabled a faster lithium diffusion transport to be achieved under high current density conditions.