Structure-specific synthesis processes are of key importance to the growth of polymorphic functional compounds such as TiO₂, where material properties strongly depend on structure as well as chemistry. The robust growth of the brookite polymorph of TiO₂, a promising photocatalyst, has been difficult in both powder and thin-film forms due to the disparity of reported synthesis techniques, their highly specific nature, and lack of mechanistic understanding. In this work, we report the growth of high-fraction (~95%) brookite thin films prepared by annealing amorphous titania precursor films deposited by pulsed laser deposition. We characterize the crystallization process, eliminating the previously suggested roles of substrate templating and Na helper-ions in driving brookite formation. Instead, we link phase selection directly to film thickness and average deposition rate, offering a novel, generalizable route to brookite growth that does not rely on the presence of extraneous elements or specific lattice-matched substrates. To further explore the growth process of brookite from amorphous precursors, in-situ XRD and in-situ optical microscopy/Raman spectroscopy are used to show that brookite is the first polymorph to form during annealing but that anatase growth is not far behind. Thus, the growth of these two polymorphs are difficult to separate via thermal treatment alone and suggests further that modifications to the amorphous precursor are needed to facilitate brookite formation. Then exploring different PLD parameters that could affect the amorphous precursor, such as average deposition rate and deposition oxygen pressure, we further show the combined structural and stoichiometric requirements for brookite stabilization. Once crystallized, high-fraction brookite films further exposed to higher annealing temperatures (up to 700°C) show little transformation of brookite regions and no large scale polymorphic transformations. In addition to providing a new synthesis route to brookite thin films, our results take a step towards resolving the problem of phase selection in TiO₂ growth, contributing to the further development of this functional material.
Funding Statement (additional comments about funding)
All research contained within was funded by the Department of Energy (DOE), in the Energy Frontier Research Center (EFRC) collaboration known as the Center for the Next Generation of Materials by Design: Incorporating Metastability (CNGMD).