Effect of pH in the Hydrothermal Preparation of Bi2WO6 Nanostructures

Bismuth Tungstate Nanoplates-Vis Responsive Photocatalyst for Water Oxidation

The development of visible-light-responsive (VLR) semiconductor materials for effective water oxidation is significant for a sustainable and better future. Among various candidates, bismuth tungstate (Bi2WO6; BWO) has attracted extensive attention because of many advantages, including efficient light-absorption ability, appropriate redox properties (for O2 generation), adjustable morphology, low cost, and profitable chemical and optical characteristics. Accordingly, a facile solvothermal method has been proposed in this study to synthesize two-dimensional (2D) BWO nanoplates after considering the optimal preparation conditions (solvothermal reaction time: 10-40 h). To find the key factors of photocatalytic performance, various methods and techniques were used for samples' characterization, including XRD, FE-SEM, STEM, TEM, HRTEM, BET-specific surface area measurements, UV/vis DRS, and PL spectroscopy, and photocatalytic activity was examined for water oxidation under UV and/or visible-light (vis) irradiation. Famous commercial photocatalyst-P25 was used as a reference sample. It was found that BWO crystals grew anisotropically along the {001} basal plane to form nanoplates, and all properties were controlled simultaneously by tuning the synthesis time. Interestingly, the most active sample (under both UV and vis), prepared during the 30 h solvothermal reaction at 433 K (BWO-30), was characterized by the smallest specific surface area and the largest crystals. Accordingly, it is proposed that improved crystallinity (which hindered charge carriers' recombination, as confirmed by PL), efficient photoabsorption (using the smallest bandgap), and 2D mesoporous structure are responsible for the best photocatalytic performance of the BWO-30 sample. This report shows for the first time that 2D mesoporous BWO nanoplates might be successfully prepared through a facile template-free solvothermal approach. All the above-mentioned advantages suggest that nanostructured BWO is a prospective candidate for photocatalytic applications under natural solar irradiation.

Bioinspired hierarchical 3D flower-in-ridge hybrid structure for the photodegradation of persistent organic pollutants

The development of next-generation photocatalysts has consistently gained inspiration from the evolution of natural nanostructures regarding their fabrication and application for the elimination of persistent organic pollutants (POPs). Herein, we synthesized blue-colored oxygen-vacant Bi₂WO_6-x inside butterfly wing architectures (BW-Bi₂WO_6-x) via modified functionalization and solvothermal techniques. Given that the (WO₄)^2- layer in Bi₂WO₆ structurally resembles the structure of WO₃, the introduction of oxygen vacancies (OVs) boosts the solar light absorption in comparison to the short visible light absorption range (<450 nm) in pristine Bi₂WO₆ (P-Bi₂WO₆). Hence, the fabricated BW-Bi₂WO_6-x sample exhibited broadened photo-absorption over the visible to NIR wavelength range, improved semiconductor attachment on the wing architecture and heightened surface area with numerous active sites for the adsorption of POP molecules. The performance of the BW-Bi₂WO_6-x photocatalyst was monitored for the elimination of methylene blue (MB), rhodamine B (RhB) and 4-chlorophenol (4-CP) under UV light exposure, yielding 91%, 92% and 94% degradation, respectively, in 60 min. Similarly, the degradation efficiencies of 94%, 98% and 98% for the photodegradation of MB, RhB and 4-CP under visible light for 60 min, respectively, were observed. Under NIR light, 80%, 79% and 85% degradation efficiencies were observed for MB, RhB and 4-CP, respectively, after 60 min. Therefore, the proposed BW-Bi₂WO_6-x sample can provide insights and inspire the development of photo-responsive materials for applications in energy, defense and water treatment.

Synthesis and Characterization of Bi2WxMo1−xO6 Solid Solutions and Their Application in Photocatalytic Desulfurization under Visible Light

Photocatalytic oxidative desulfurization has attracted much attention in recent years due to the continuous tightening of the sulfur content requirements in motor fuels and the disadvantages of the industrial hydrodesulfurization process. This work is devoted to the investigation of the photocatalytic activity of Bi2WxMo1−xO6 solid solutions (x = 1, 0.75, 0.5, 0.25, 0) in the oxidative desulfurization of hydrocarbons under visible light irradiation using hydrogen peroxide as an oxidant. The synthesized photocatalysts were characterized in detail using XRD, SEM, EDS, low-temperature nitrogen adsorption–desorption, and DRS. It was shown that the use of solid solutions Bi2WxMo1−xO6 with x = 0.5–0.75 leads to the complete oxidation of organosulfur compounds to CO2 and H2O within 120 min. The high photocatalytic activity of solid solutions (x = 0.5–0.75) is attributed to their ability to absorb more visible light, the presence of the corner-shared [Mo/WO6] octahedral layers, which may promote the generation and separation of photogenerated charges, and the hierarchical 3D flower-like structure. The reaction mechanism of the desulfurization was also analyzed in this work.

pH-induced hydrothermal synthesis of Bi2WO6 nanoplates with controlled crystal facets for switching bifunctional photocatalytic water oxidation/reduction activity

Bifunctional photocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have attracted growing interest to understand the mechanisms governing different evolution reactions, and the bifunctional activity of a single type of crystalline photocatalyst has gained especial attention. We herein report the high photocatalytic OER and HER activities of Bi₂WO₆ nanoplates (BWO NPs) which are synthesized by a simple hydrothermal method, and the switchable OER and HER performances controlled by the pH value of the precursor solvent. In the pH range from 4 to 9, the thickness of BWO NPs along the [001] direction exhibits interesting dependence on the pH value, which decreases as the pH value increases. Correspondingly, the BWO NPs obtained at the pH value of 7 (BWO-7) show the highest photocatalytic OER activity, while the BWO NPs synthesized at the pH value of 9 (BWO-9) exhibit the highest photoactivity towards HER. The electronic band structure analysis indicates that the highest photocatalytic OER activity is related to the band alignment of the valence band maximum of Bi₂WO₆, which determines the efficient separation of photogenerated electrons and holes as well as the fast charge transfer kinetics. The crystal facet evolution resulting from thickness reduction promotes the exposure of {001} facets for HER and decreases the exposure of {100} and {010} facets for OER. This work provides new insights into the combined effects of crystal facets and electronic band structures on photocatalysis.