TiO2 thin films were spin coated on soda-lime-silica glass substrates under identical conditions and then annealed for 2h 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 ∼255nm. 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 3.49eV to 3.43eV 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

WO₃/CeO₂ heterostructured nanocomposites containing different WO₃ ratios (0.1, 0.3, 0.5, and 1.0 wt %) were synthesized by a precipitation method. The coupling of CeO₂ and WO₃ with a high specific surface area noticeably enhanced the photocatalytic activity of indigo carmine (IC) degradation under visible-light irradiation. The degradation rate constants (k) of 0.5 wt % WO₃/CeO₂ nanocomposites reached 4 and 5 times higher than those of CeO₂ and WO₃, respectively. Regarding the experimental results, the X-ray diffraction (XRD) patterns of the CeO₂ spherical nanoparticles and rod-shaped WO₃ were assigned to the cubic fluorite and orthorhombic phase structures, respectively. The increasing photocatalytic activity of nanocomposite samples could be attributed to the heterojunction of the photocatalysts with efficient charge separation and strong oxidative ability, which were confirmed by the photoluminescence spectra and diffuse reflectance spectrometry. The staggered heterojunction of the nanocomposite promoted efficient electron transfer and suppressed the recombination of photogenerated electrons and holes during the process.

The present work describes the effects of water on Fe-doped nanoparticulate CeO₂, produced by flame spray pyrolysis, which is a critical environmental issue because CeO₂ is not stable in typical atmospheric conditions. It is hygroscopic and absorbs ~29 wt % water in the bulk when exposed to water vapor but, more importantly, it forms a hydrated and passivating surface layer when immersed in liquid water. In the latter case, CeO₂ initially undergoes direct and/or reductive dissolution, followed by the establishment of a passivating layer calculated to consist of ~69 mol % solid CeO₂·2H₂O and ~30 mol % gelled Ce(OH)₄. Under static flow conditions, a saturated boundary layer also forms but, under turbulent flow conditions, this is removed. While the passivating hydrated surface layer, which is coherent probably owing to the continuous Ce(OH)₄ gel, would be expected to eliminate the photoactivity, this does not occur. This apparent anomaly is explained by the calculation of (a) the thermodynamic stability diagrams for Ce and Fe; (b) the speciation diagrams for the Ce⁴⁺-H₂O, Ce³⁺-H₂O, Fe³⁺-H₂O, and Fe²⁺-H₂O systems; and (c) the Pourbaix diagrams for the Ce-H₂O and Fe-H₂O systems. Furthermore, consideration of the probable effects of the localized chemical and redox equilibria owing to the establishment of a very low pH (<0) at the liquid-solid interface also is important to the interpretation of the phenomena. These factors highlight the critical importance of the establishment of the passivating surface layer and its role in photocatalysis. A model for the mechanism of photocatalysis by the CeO₂ component of the hydrated phase CeO₂·2H₂O is proposed, explaining the observation of the retention of photocatalysis following the apparent alteration of the surface of CeO₂ upon hydration. The model involves the generation of charge carriers at the outer surface of the hydrated surface layer, followed by the formation of radicals, which decompose organic species that have diffused through the boundary layer, if present.