Solar photocatalytic treatment of model and real oil sands process water naphthenic acids by bismuth tungstate: Effect of catalyst morphology and cations on the degradation kinetics and pathways

Bitumen extraction from oil sands produces large quantities of oil sands process water (OSPW), which contains recalcitrant naphthenic acids (NAs). In this study, three different morphologies of bismuth tungstate (Bi₂WO₆) photocatalysts were prepared by hydrothermal method. The prepared catalyst was characterized to obtain its structural, textural and chemical properties and tested for the degradation of model NAs and real OSPW under simulated solar irradiation. Nanoplate, flower-like and swirl-like Bi₂WO₆ were prepared and the results showed that the flower-like structure exhibited the highest specific surface area and total pore volume. The highest photocatalytic activity for the degradation of NAs was also demonstrated by the flower-like Bi₂WO₆, achieving complete degradation of cyclohexanoic acid (CHA) at fluence-based rate constant of 0.0929 cm²/J. Superoxide radicals (O₂^•-) and holes were identified as the major reactive species generated during the photocatalytic process. The effect of metallic ions on the degradation rates of S-containing and N-containing NAs differed and the heteroatom was found to be the main reactive site. The by-products of heteroatomic NAs were identified and degradation pathways were reported for the first time. The concentration changes of each byproduct were further estimated by mass balance. This research provides valuable information for the treatment of NAs by engineered passive solar-based approaches.

Understanding the hierarchical behavior of Bi2WO6 with enhanced photocatalytic nitrogen fixation activity

Hierarchical Bi2WO6 nanostructures self-assembled with planar arranged nanosheets and dispersed Bi2WO6 nanosheets were synthesized with different dosages of EG via a simple hydrothermal route. The Bi2WO6 photocatalysts were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS). A control experiment was conducted to test the effect of EG dosage on the growth mechanism and behavior of the highly (010) exposed hierarchical lamellar nanostructures and dispersed nanosheets. The photocatalytic nitrogen fixation rate of the hierarchical Bi2WO6 nanostructures was estimated to be 948 μmol g-1 h-1 across the full spectrum, which was 23% higher than that of the dispersed nanosheets (770 μmol g-1 h-1) due to chemisorption on the hierarchical structures and enhanced surface oxygen vacancies (OVs).

Morphology Regulation Mechanism and Enhancement of Photocatalytic Performance of BiOX (X = Cl, Br, I) via Mannitol-Assisted Synthesis

BiOX (X = Cl, Br, I) photocatalysts with dominant (110) facets were synthesized via a mannitol-assisted solvothermal method. This is the first report on the exposed (110) facets-, size-, and defects-controlled synthesis of BiOX achieved by solvothermal synthesis with mannitol. This polyol alcohol acted simultaneously as a solvent, capping agent, and/or soft template. The mannitol concentration on the new photocatalysts morphology and surface properties was investigated in detail. At the lowest concentration tested, mannitol acted as a structure-directing agent, causing unification of nanoparticles, while at higher concentrations, it functioned as a solvent and soft template. The effect of exposed (110) facet and surface defects (Bi(3−x)+, Bi4+, Bi5+) of BiOX on the photocatalytic activity of nanomaterials under the UV–Vis irradiation were evaluated by oxidation of Rhodamine B (RhB) and 5-fluorouracil (5-FU), an anticancer drug, and by reduction of Cr(VI). Additionally, the influence of crucial factors on the formation of BiOX in the synthesis with mannitol was discussed extensively, and the mechanism of BiOX formation was proposed. These studies presented a new simple method for synthesizing BiOX without any additional surfactants or shape control agents with good photocatalytic activity. The study also provided a better understanding of the effects of solvothermal conditions on the BiOX crystal growth.

Insights into the impurities of Bi2WO6 synthesized using the hydrothermal method

Bismuth tungstate (Bi2WO6) nanomaterials are widely used as visible-light driven photocatalysts. However, limited attention has been paid to the purity of prepared Bi2WO6 nanoparticles, which may affect the photocatalytic performance and hinder in-depth study of Bi2WO6. In this work, the impurities of Bi2WO6 formed during the hydrothermal process under a wide range of acid-base conditions (from 1.5 M HNO3 to 0.5 M NaOH) were qualitatively analyzed and accurately quantified for the first time. After confirmation of Bi2WO6 stability, the impurities were dissolved using acid or base treatment, followed by measurements of the ion concentrations using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Furthermore, various characterization techniques including XRD, FE-SEM, TEM, UV-Vis DRS, XPS and FTIR were implemented to explore the change in morphology and optical properties of Bi2WO6 prepared in different acid-base environments, and to facilitate qualitative analysis of impurities. The hydrolytic properties of raw materials used for the synthesis of Bi2WO6 were also analyzed with UV-Vis transmittance observation. Following these analyses, the types and contents of impurities in Bi2WO6 prepared by the hydrothermal method under different acid-base conditions were determined. Results show that the primary impurity is WO3·0.33H2O (41.09%) for the precursor prepared in 1.5 M nitric acid solution. When the pH of the precursor was in the range of 0.97-7.01, the synthesized Bi2WO6 has relatively high purity, and the impure products were identified as BiONO3. Bi2O3 began to appear when pH reached 9.01 and it reached 18.88% when pH was 12.98. The final product was Bi2O3 exclusively for the precursor conditioned in 0.5 M NaOH solution. In addition, the accuracy of the proposed quantitative method using ICP-MS was validated for several scenarios by weight difference experiments.

Influence of cobalt substitution on the crystal structure, band edges and photocatalytic properties of hierarchical Bi2WO6 microspheres

An efficient Bi₂WO₆ catalyst with Co²⁺ substitution was synthesized via a facile hydrothermal route.

Review on nanoscale Bi-based photocatalysts

Nanoscale Bi-based photocatalysts are promising candidates for visible-light-driven photocatalytic environmental remediation and energy conversion. However, the performance of bulk bismuthal semiconductors is unsatisfactory. Increasing efforts have been focused on enhancing the performance of this photocatalyst family. Many studies have reported on component adjustment, morphology control, heterojunction construction, and surface modification. Herein, recent topics in these fields, including doping, changing stoichiometry, solid solutions, ultrathin nanosheets, hierarchical and hollow architectures, conventional heterojunctions, direct Z-scheme junctions, and surface modification of conductive materials and semiconductors, are reviewed. The progress in the enhancement mechanism involving light absorption, band structure tailoring, and separation and utilization of excited carriers, is also introduced. The challenges and tendencies in the studies of nanoscale Bi-based photocatalysts are discussed and summarized.

Facile fabrication of well-polarized Bi2WO6 nanosheets with enhanced visible-light photocatalytic activity

A universal and facile strategy is proposed to fabricate polarized Bi₂WO₆ nanoparticles with the assistance of a soluble organic–inorganic composite film.