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

Abstract

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 OHspecies 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 TiO 2 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.

Brookite, the Least Known TiO2 Photocatalyst

“…On the other hand, an unusual single (121) peak was found for a film containing neither anatase nor rutile obtained by a modified sol-gel method [4]. Hu et al [5] used one characteristic peak of anatase at 2θ = 62.57°, which does not overlap with any diffraction peak of brookite, to ascertain the existence of anatase. According to Bokhimi and Pedraza [6] the only way to prove that the samples are single brookite phase is to run their XRD patterns with a good statistic to refine the crystalline structure of brookite in order to demonstrate that the modeling of only this phase is enough to reproduce the experimental diffraction pattern.…”

“…High quality brookite flowers were obtained by Hu et al [5] by hydrothermal treatment at 220 °C for 48 h of a suspension containing TiOSO 4 and NaOH at pH 12.5. Time-resolved experiments revealed that layered titanate transformed into spindle-like brookite nanoparticles that aggregated together to yield the flower morphology.…”

Synthesis and characterization of undoped and cobalt-doped TiO2 nanoparticles via sol–gel technique

“…Among these polymorphs, rutile and anatase have been widely studied. Brookite is rarely studied due to its complicated structure and difficulties in sample preparation (Hu et al 2009). These three phases can be commonly described as constituted by arrangements of the same building block-Ti-O 6 octahedron in which Ti atom is surrounded by six oxygen atoms situated at the corners of a distorted octahedron.…”

Electrosynthesized TiO2 films: dependence of the brookite–anatase ratio on the applied potential

“…This variation indicates that the least negative synthesis potential is the most favorable condition for brookite polymorph, in accordance with XRD analysis. Figure 6d shows the positions of anatase and brookite fitted modes for all TT temperatures as function of the electrosynthesis potential, as well as the calculated position of these modes (dashed lines) [6,14,15]. There is no significant shift of the position of B mode for 400 and 600°C.…”