The present study reported the preparation of BiVO₄ by co-precipitation method. The as-prepared BiVO₄ photocatalyst were deposited on rGO sheets to form BiVO₄/rGO via the hydrothermal method. The crystalline structure, morphological, optical properties, and surface properties of the synthesized pure BiVO₄ compared to BiVO₄/rGO composite were studied using X-ray diffraction (XRD), scanning electronmicroscopy (SEM), photoluminescence (PL) spectrophotoscopy, UV–vis spectrophotometer with an integrating sphere, and N₂ adsorption-desorption isotherm based on BET theory. The photocatalytic activity of the prepared samples were evaluated by the degradation of MB dye in aqueous medium under visible light irradiation. The result showed that the BiVO₄/rGO composite exhibited greater photocatalytic efficiency compared to pure BiVO₄ with the photocatalytic degradation efficiency remains stable up to fifth cycle. The improved activity of the BiVO₄/rGO composite might be attributed to the high surface area available to adsorb more MB molecules, and efficient charge separation of BiVO₄ through π electron on the rGO structure. According to experimental results, the possible photocatalytic mechanism of the BiVO₄/rGO composite were determined and the active species hydroxyl radical were reported. Based on photocatalytic activity inhibition in the presence of both h⁺ (VB) and O₂^•− (CB) scavengers over the BiVO₄ photocatalyst, it can be proposed that the hydroxyl radical generated during the photocatalytic degradation mechanism is mainly responsible by the main active species of h⁺ and O₂^•− at VB and CB positions, respectively.

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.

In this work, metal-doped titanium dioxide (TiO₂) was synthesised with the aim of improving photocatalytic degradation and antimicrobial activities; TiO₂ was doped with copper (Cu) ranging from 0.1 to 1.0 wt%. The physical and chemical properties of the Cu-doped TiO₂ nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), the Brunauer–Emmett–Teller method (BET) and diffuse reflection spectroscopy (DRS). The results revealed that the anatase phase of TiO₂ was maintained well in all the Cu-doped TiO₂ samples. No significant difference in the particle sizes or the specific surface areas was caused by increasing Cu doping. However, the band gap decreased continuously from 3.20 eV for undoped TiO₂ to 3.12 eV for 1.0 wt.% Cu-doped TiO₂. In addition, the 0.1 wt.% Cu-doped TiO₂ displayed a much greater photocatalytic degradation of methylene blue (MB) and excellent antibacterial ability for Escherichia coli (E. coli) compared to undoped TiO₂. On the other hand, the high Cu doping levels had negative impacts on the surface charge of nanoparticles and charge transfer for OH• generation, resulting in decreasing MB degradation and E. coli photokilling for 1.0 wt.% Cu-doped TiO₂.