Effect of Annealing Temperature on the Photocatalytic Activity of TiO₂ Thin Films

TiO2 thin films were spin coated on soda-lime-silica glass substrates under identical conditions and then annealed for 2 h in air in the range of 300°-500 °C in increments of 50 °C. The mineralogical, morphological, optical, and photocatalytic properties then were assessed for the films. The techniques used were glancing-angle X-ray diffraction (GAXRD), field emission gun transmission electron microscopy (FEGTEM), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), UV-VIS spectrophotometry (UV-VIS), and methylene blue (MB) degradation. The films had a consistent thickness of ∼255 nm. Anatase peaks recrystallised at the annealing temperature of 400 °C, with the crystallinity increasing with increasing annealing temperatures. Although recrystallisation caused a significant increase in grain size, the crystalline films showed only a slight increase in the grain size with increasing annealing temperatures. In contrast, the surface roughness of all of the films increased significantly with increasing annealing temperatures. This was associated with increased grain faceting, which was supported by the X-ray diffraction data.

All of the films showed high transparency in the visible region, with the optical indirect band gap of the crystalline films decreasing slightly from 3.49 eV to 3.43 eV with increasing annealing temperatures. Four regimes of photocatalytic performance could be identified, which depended principally on the degree of crystallinity and the level of contamination. In short, a blank was used to negate heating effects, the amorphous films were inert, the onset of crystallisation established photoactivity, and the photoactivity of the films annealed at 400°-500 °C decreased in a trend consistent with the trends in increasing grain size, increasing surface roughness, increasing crystallinity, decreasing band gap, and increasing contamination. Since this consistency was the case, these variables could not be decoupled.For the samples that were fabricated using the specified materials and methods, characterised, and tested, the optimal temperature for annealing was found to be 400 °C.