Furthermore, through surface engineering, brookite can become either oxidative or reductive 8 depending on the exposed surface ((201) or (210) respectively), adding versatility as a catalyst. Brookite is seeing renewed interest 5, 6, 7, with a rutile/brookite mixture serving as effective photocatalysts and outperforming Degussa P25 (an anatase/rutile mixture commonly used to degrade organic pollutants) at degrading both rhodamine B and methyl orange. Rutile, the ground state, and anatase are by far the most common and find many applications 3, 4, from paint pigments to transparent conductors and photocatalysts. 1, although many other synthetically prepared structures have been reported 1, 2. TiO 2 can form in many structures, the most prominent of which are the naturally-occurring rutile, anatase and brookite polymorphs depicted in Fig. 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 TiO 2 growth, contributing to the further development of this promising functional material. Instead, we link phase selection directly to film thickness, offering a novel, generalizable route to brookite growth that does not rely on the presence of extraneous elements or particular lattice-matched substrates. We characterize the crystallization process, eliminating the previously suggested roles of substrate templating and Na helper ions in driving brookite formation. 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. The robust growth of the brookite polymorph of TiO 2, 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. Structure-specific synthesis processes are of key importance to the growth of polymorphic functional compounds such as TiO 2, where material properties strongly depend on structure as well as chemistry.
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