TITLE:
Effect of Heat Treatment on Crystallite Size, Band-Gap Energy, and Photocatalytic Degradation of Brookite TiO2 Thin Films
AUTHORS:
Jariah Mohamad Juoi, Nur Dalilah Johari, Zulkifli Mohd Rosli
KEYWORDS:
TiO2, Crystallite Size, Band-Gap Energy, Thin Film, Methylene Blue, Photocatalytic Degradation
JOURNAL NAME:
Journal of Power and Energy Engineering,
Vol.14 No.9,
September
23,
2026
ABSTRACT: Brookite TiO2 is less extensively investigated in thin-film photocatalysis than anatase and rutile, and its response to thermal processing remains of interest. Here, brookite TiO2 coatings were produced by sol-gel spin coating and heat-treated at 200˚C, 300˚C, 400˚C, and 500˚C for 3 h. Glancing-angle X-ray diffraction (GAXRD), transmission electron microscopy with selected-area electron diffraction (TEM/SAED), UV-Vis spectroscopy, and methylene-blue (MB) degradation measurements were used to relate thermal history to structure, optical response, and photocatalytic behaviour. Brookite was the only detectable crystalline TiO2 phase at 200˚C - 400˚C, whereas the 500˚C film showed a predominantly amorphous XRD pattern. The calculated crystallite size changed from 47.9 nm ± 19.9 nm at 200˚C to 58.4 nm ± 12.0 nm at 300˚C and then to 7.8 nm ± 1.0 nm at 400˚C. The estimated optical band gap varied from 3.37 to 3.90 eV, reaching its minimum at 300˚C. This sample also gave the highest four-hour MB removal, 67.7% under UV-C irradiation and 97.8% under the reported visible-light condition. The results indicate that the photocatalytic response follows the combined evolution of crystallinity, crystallite size, and optical band gap with annealing temperature. Within the investigated processing window, 300˚C provided the most favourable overall combination of measured properties and MB degradation performance.