High-Quality Brookite TiO2 Flowers: Synthesis, Characterization, and Dielectric Performance

High-quality brookite flowers were fabricated via a facile solution chemistry technique. The synthetic conditions to the flower-like brookite were monitored by a series of time-resolved experiments and further optimized by adjusting the concentrations of the Na+ and OH− species involved in the reaction system. Careful sample characterizations by the combined techniques of X-ray diffraction, Raman, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectra indicate the formation of highly phase-pure and well-crystallized brookite with an extremely low defect concentration. Different from the natural brookite mineral with an indirect transition (Zallen, R.; Moret, M. P. Solid State Commun. 2006, 137, 154), the present high-quality brookite flowers showed a direct transition with a bandgap energy of 3.4 ± 0.1 eV, which is larger than those of its two other polymorphs, that is, a direct band gap of 3.0 ± 0.1 eV for rutile and indirect band gap of 3.2 ± 0.1 eV for anatase. Room-temperature alternative current impedance measurements indicate that the permittivity for the brookite flowers is 93 at 40 Hz, which is much higher than that for anatase but slightly lower than rutile as opposed to what is theoretically predicted in the literature. Strikingly, the flower shape also enables high quality brookite TiO2 with a high structural stability up to 900 °C in air, impossibly accessible when using other preparation methods. These observations pave the way for high-quality brookite flowers to find a broad class of technological uses.