High-fraction brookite films from amorphous precursors

Abstract

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, offering a novel, generalizable route to brookite growth that does not rely on the presence of extraneous elements or particular lattice-matched substrates. 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 promising functional material.

Figure 1

Crystal structures of TiO₂ rutile (tetragonal, P4₂/mmm), brookite (orthorhombic, Pbca) and anatase (tetragonal, I4₁/amd) polymorphs.

Figure 2

Computed low-temperature free energies of (a) Na_xTiO₂ and (b) Na_2yTiO_2+y compounds, with Ti-O frameworks constrained to those of known TiO₂ phases, as a representative sample of structure selection in the Na_xTiO_2+y chemical space. The dotted lines denote the global thermodynamic equilibrium in each composition space.

Figure 3

Room temperature XRD patterns of TiO₂ films grown on a-SiO₂ and EXG substrates annealed following a protocol similar to that shown in Fig. 4. Optical images of these films appear in Fig. S3. The brookite and anatase phases appear in similar proportions in films on both substrates.

Figure 4

The integrated peak areas for brookite (121) [blue] and anatase (101) [green] on a 65-nm film, grown on EXG, as a function of time and temperature during the anneal indicated by the gray line. Both polymorphs crystallize at ~290 °C.

Figure 5

(a) Optical image (100x magnification) of a 65-nm TiO₂ film on EXG glass. Regions marked B yield the Raman spectrum for brookite in (b), while those marked A yield the anatase spectrum in (c). The 2D Raman maps at 319 cm⁻¹ (d) and 144 cm⁻¹ (e) show that the color variations in the optical image correlate with a particular polymorph. The brookite:anatase ratio in this film is 50:50 and no rutile is observed. The dotted lines in (b) and (c) indicate the wavenumber range over which the Raman intensity maps in (d) and (e), respectively, are acquired.

Figure 6

HRTEM image of a 50-nm TiO₂ film on EXG glass annealed following a protocol similar to that shown in Fig. 4. The selected area electron diffraction patterns in the insets indicate that the grain on the left is anatase and on the right, is brookite. The S-shaped grain boundary is approximately 20 nm wide.

Figure 7

(a,b) Elemental EDS map and plot trace across the substrate/film interface of the film shown in Fig. 4. (c,d) Elemental EDS map and plot trace showing an anatase/brookite phase boundary in the film shown in Fig. 6. Na (green) is below the detection limit in the TiO₂ films and accumulates near the film-substrate interface.

Figure 8

Phase fraction of brookite (blue), rutile (red) and anatase (green) polymorph or amorphous component (gray) in TiO₂ films as a function of thickness. Na-free a-SiO₂ and Si/SiO₂ (circles), low-Na EXG (squares) and high-Na SLS (diamonds) substrates are represented. The uncertainty in thickness is ±5 nm for d <50 nm and ±3 nm for d >50 nm.