Recent advances on Bi2WO6-based photocatalysts for environmental and energy applications

Degradation of Dye Wastewater by a Novel mBT-MPR Visible Light Photocatalytic System

The high efficiency and low consumption green wastewater treatment technology has important practical significance for the recycling of printing and dyeing wastewater. The efficiency of visible light catalytic degradation of organics is greatly affected by the performance of the catalyst and the photo reactor. Therefore, Bi₂WO₆/TiO₂/Fe₃O₄ (mBT) visible light photocatalyst was accurately prepared by the ammonia iron double drop method. In order to improve the photodegradation efficiency, a tubular magnetic field-controlled photocatalytic reactor (MPR) was developed. The novel mBT-MPR visible light photocatalytic system was proposed to treat RhB simulated wastewater. The experimental results showed that when the dosage of mBT catalyst was 1 g/L and visible light was irradiated for 60 min, the average removal rate of rhodamine B (RhB) with initial an concentration of 10 mg/L in the simulated wastewater for four times was 91.7%. The mBT-MPR visible light photocatalysis system is a green and efficient treatment technology for organic pollutants in water with simple operation, low energy consumption, and no need for catalyst separation.

Advances in Bi2WO6-Based Photocatalysts for Degradation of Organic Pollutants

With the rapid development of modern industries, water pollution has become an urgent problem that endangers the health of human and wild animals. The photocatalysis technique is considered an environmentally friendly strategy for removing organic pollutants in wastewater. As an important member of Bi-series semiconductors, Bi₂WO₆ is widely used for fabricating high-performance photocatalysts. In this review, the recent advances of Bi₂WO₆-based photocatalysts are summarized. First, the controllable synthesis, surface modification and heteroatom doping of Bi₂WO₆ are introduced. In the respect of Bi₂WO₆-based composites, existing Bi₂WO₆-containing binary composites are classified into six types, including Bi₂WO₆/carbon or MOF composite, Bi₂WO₆/g-C₃N₄ composite, Bi₂WO₆/metal oxides composite, Bi₂WO₆/metal sulfides composite, Bi₂WO₆/Bi-series composite, and Bi₂WO₆/metal tungstates composite. Bi₂WO₆-based ternary composites are classified into four types, including Bi₂WO₆/g-C₃N₄/X, Bi₂WO₆/carbon/X, Bi₂WO₆/Au or Ag-based materials/X, and Bi₂WO₆/Bi-series semiconductors/X. The design, microstructure, and photocatalytic performance of Bi₂WO₆-based binary and ternary composites are highlighted. Finally, aimed at the existing problems in Bi₂WO₆-based photocatalysts, some solutions and promising research trends are proposed that would provide theoretical and practical guidelines for developing high-performance Bi₂WO₆-based photocatalysts.

2D/2D Phosphorus-Doped g-C3N4/Bi2WO6 Direct Z-Scheme Heterojunction Photocatalytic System for Tetracycline Hydrochloride (TC-HCl) Degradation

Bi₂WO₆-based heterojunction photocatalyst for antibiotic degradation has been a research hotspot, but its photocatalytic performance needs to be further improved. Therefore, 2D/2D P-doped g-C₃N₄/Bi₂WO₆ direct Z-scheme heterojunction photocatalysts with different composition ratios were prepared through three strategies of phosphorus (P) element doping, morphology regulation, and heterojunction, and the efficiency of its degradation of tetracycline hydrochloride (TC-HCl) under visible light was studied. Their structural, optical, and electronic properties were evaluated, and their photocatalytic efficiency for TC-HCl degradation was explored with a detailed assessment of the active species, degradation pathways, and effects of humic acid, different anions and cations, and water sources. The 30% P-doped g-C₃N₄/Bi₂WO₆ had the best photocatalytic performance for TC-HCl degradation. Its photocatalytic rate was 4.5-, 2.2-, and 1.9-times greater than that of g-C₃N₄, P-doped g-C₃N₄, and Bi₂WO₆, respectively. The improved photocatalytic efficiency was attributed to the synergistic effect of P doping and 2D/2D direct Z-scheme heterojunction construction. The stability and reusability of the 30% P-doped C₃N₄/Bi₂WO₆ were confirmed by cyclic degradation experiments. Radical scavenging experiments and electron spin resonance spectroscopy showed that the main active species were •O₂^- and h⁺. This work provides a new strategy for the preparation of direct Z-scheme heterojunction catalysts with high catalytic performance.

Facile preparation of visible light-sensitive layered g-C3N4 for photocatalytic removal of organic pollutants

The graphite-phase carbon nitride (g-C₃N₄) photocatalytic materials were prepared by one-step calcination method to degrade methylene blue (MB) and potassium butyl xanthate (PBX) under visible light irradiation. The prepared g-C₃N₄ photocatalytic materials were investigated in detail by various characterizations, and the experiments showed that the graphitic phase carbon nitride photocatalytic materials were successfully prepared by the one-step calcination method. The material possesses excellent optical properties and strong visible light absorption, thus achieving photocatalytic degradation of MB and PBX. The catalyst dosage, pH, the initial concentration of pollutants have important effects on photocatalytic activity of MB and PBX. The photocatalytic degradation efficiency was 98.99% for MB and 96.83% for PBX under the optimal conditions (catalyst dosage, initial pollutant concentration and pH value were 500 mg L^-1, 20 mg L^-1 and 7, respevtively). The photocatalytic mechanisms on MB and PBX were elucidated. ·OH was the key specie for MB, while ·O₂^- was the key specie for PBX. This study advances the development of photocatalytic technology for mineral wastewater.

Self-doping induced oxygen vacancies and lattice strains for synergetic enhanced upconversion luminescence of Er3+ ions in 2D BiOCl nanosheets

Rare earth (RE) ions combined with two-dimensional (2D) semiconductors can exhibit unexpected optical properties. However, fluorescence quenching has always been inevitable due to defects associated with the synthesis and doping of 2D materials. In this work, we reported an efficient upconversion (UC) enhancement of Er³⁺ doped BiOCl nanosheets, utilizing a defect engineering strategy conversely rather than eliminating defects. Experiments and theoretical calculations provide evidence that oxygen vacancies (OVs) and lattice strain are simultaneously formed in the BiOCl:Er³⁺ nanosheets through self-doping of Cl^- ions. Under 980 nm excitation, samples doped with 300 mol% Cl^- ions exhibit the best luminescent emission, and the green and red UC emissions are enhanced 3.5 and 15 times, respectively. We showed that the OVs in the 2D semiconductor can act as energy bridges to transfer charges to the Er³⁺ energy level, enriching the electron population at the excited levels; while, the lattice strain enhances the energy transfer and charge accumulation, which synergistically enhance the UC luminescence. This research provides a new insight into the development of defect engineering for UC PL in 2D nanomaterials.

Enhancing Photocatalysis of Ag Nanoparticles Decorated BaTiO3 Nanofibers through Plasmon-Induced Resonance Energy Transfer Turned by Piezoelectric Field

Revealing the charge transfer path is very important for studying the photocatalytic mechanism and improving photocatalytic performance. In this work, the charge transfer path turned by the piezoelectricity in Ag-BaTiO3 nanofibers is discussed through degrading methyl orange. The piezo-photocatalytic degradation rate of Ag-BaTiO3 is much higher than the photocatalysis of Ag-BaTiO3 and piezo-photocatalysis of BaTiO3, implying the coupling effect between Ag nanoparticle-induced localized surface plasmon resonance (LSPR), photoexcited electron-hole pairs, and deformation-induced piezoelectric field. With the distribution density of Ag nanoparticles doubling, the LSPR field increases by one order of magnitude. Combined with charge separation driven by the piezoelectric field, more electrons in BaTiO3 nanofibers are excited by plasmon-induced resonance energy transfer to improve the photocatalytic property.

Construction of a binary S-scheme S-g-C3N4/Co-ZF heterojunction with enhanced spatial charge separation for sunlight-driven photocatalytic performance

A step-scheme (S-scheme) photocatalyst made of sulfurized graphitic carbon nitride/cobalt doped zinc ferrite (S-g-C3N4/Co-ZF) was constructed using a hydrothermal process because the building of S-scheme systems might increase the lifespan of highly reactive charge carriers. Utilizing cutting-edge methods, the hybrid photocatalyst was evaluated by employing TEM, XPS, XRD, BET, FTIR, transient photo-response, UV-vis, EIS and ESR signals. In order to create a variety of binary nanocomposites (NCs), nanoparticles (NPs) of 6% cobalt doped zinc ferrite (Co-ZF) were mixed with S-g-C3N4 at various concentrations, ranging from 10 to 80 wt%. For photocatalytic dye removal, a particular binary NC constructed between S-g-C3N4 and Co-ZF produces a huge amount of catalytic active sites. The findings showed that loading of S-g-C3N4 on 6% Co-ZF NPs serves as a good heterointerface for e-/h+ separation and transportation through the S-scheme S-g-C3N4/Co-ZF heterojunction. By boosting the hybrid system's BET surface area for the photocatalytic process, the addition of 6% Co-ZF improves the system's ability to absorb more sunlight and boosts its photocatalytic activity. The highest photo-removal effectiveness (98%), which is around 2.45 times higher than that of its competitors, was achieved by the hybrid photocatalyst system with an ideal loading of 48% Co-ZF. Furthermore, the trapping studies showed that the primary species involved in the MB aqueous photo-degradation were ˙OH- and h+.

Synthesis of Tin-Doped Three-Dimensional Flower-like Bismuth Tungstate with Enhanced Photocatalytic Activity

Photocatalytic degradation of harmful organic matter is a feasible and environmentally friendly method. Bi2WO6 has become a hotspot of photocatalysts because of its unique layered structure and visible light response. In the present study, Sn doping was adopted to modified Bi2WO6 by hydrothermal method. The Sn-doped Bi2WO6 photocatalysts were characterized by XRD, SEM, TEM, BET, XPS, PL, and DRS, respectively. The results show that Sn-doped Bi2WO6 shows three-dimensional (3D) flower-like morphology, which is composed of two-dimensional (2D) nanosheets. Sn4+ ions enter into the Bi2WO6 lattice, producing a degree of Bi2WO6 lattice distortion, which is in favor of reducing the recombination of photogenerated electrons and holes. Moreover, the specific surface area of Bi2WO6 is significantly increased after doping, which is beneficial to providing more active sites. The photocatalytic results show that 2%Sn-Bi2WO6 exhibits the highest photocatalytic activity. After 60 min of irradiation, the photocatalytic degradation degree of methylene blue (MB) increases from 80.6% for pure Bi2WO6 to 92.0% for 2%Sn-Bi2WO6. The first-order reaction rate constant of 2%Sn-Bi2WO6 is 0.030 min−1, which is 1.7 times than that of pure Bi2WO6.

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.

Recent Progress in the Synthesis and Applications of Composite Photocatalysts: A Critical Review

Photocatalysis is an advanced technique that transforms solar energy into sustainable fuels and oxidizes pollutants via the aid of semiconductor photocatalysts. The main scientific and technological challenges for effective photocatalysis are the stability, robustness, and efficiency of semiconductor photocatalysts. For practical applications, researchers are trying to develop highly efficient and stable photocatalysts. Since the literature is highly scattered, it is urgent to write a critical review that summarizes the state-of-the-art progress in the design of a variety of semiconductor composite photocatalysts for energy and environmental applications. Herein, a comprehensive review is presented that summarizes an overview, history, mechanism, advantages, and challenges of semiconductor photocatalysis. Further, the recent advancements in the design of heterostructure photocatalysts including alloy quantum dots based composites, carbon based composites including carbon nanotubes, carbon quantum dots, graphitic carbon nitride, and graphene, covalent-organic frameworks based composites, metal based composites including metal carbides, metal halide perovskites, metal nitrides, metal oxides, metal phosphides, and metal sulfides, metal-organic frameworks based composites, plasmonic materials based composites and single atom based composites for CO₂ conversion, H₂ evolution, and pollutants oxidation are discussed elaborately. Finally, perspectives for further improvement in the design of composite materials for efficient photocatalysis are provided.

Unique Cd0.5Zn0.5S/WO3-x direct Z-scheme heterojunction with S, O vacancies and twinning superlattices for efficient photocatalytic water-splitting

Photocatalytic water-splitting employing the Z-scheme semiconductor systems mimicking natural photosynthesis is regarded as a promising way to achieve efficient soalr-to-H₂ conversion. Nevertheless, it still remains a big challenge to design high-performance direct Z-scheme photocatalysts without the use of noble metals as electron mediators. Herein, a unique Cd_0.5Zn_0.5S/WO_3-x direct Z-scheme heterojunction was constructed for the first time, which consisted of smaller O-vacancy-decorated WO_3-x nanocrystals anchoring on Cd_0.5Zn_0.5S nanocrystals with S vacancies and zinc blende/wurtzite (ZB/WZ) twinning superlattices. Under visible-light (λ > 420 nm) irradiation, the Cd_0.5Zn_0.5S/WO_3-x composites exhibited an outstanding H₂ evolution reaction (HER) activity of 20.50 mmol h^-1 g^-1 (corresponding to the apparent quantum efficiency of 18.0% at 420 nm), which is much superior to that of WO_3-x, Cd_0.5Zn_0.5S, and Cd_0.5Zn_0.5S loaded with Pt. Interestingly, the introduced O and S vacancies contributed to improving the HER activity of Cd_0.5Zn_0.5S/WO_3-x significantly. Moreover, the cycling and long-term HER measurements confirmed the robust photocatalytic stability of Cd_0.5Zn_0.5S/WO_3-x for H₂ production. The excellent light harvesting and efficient spatial charge separation induced by the ZB/WZ twinning homojunctions and defect-promoted direct Z-scheme charge-transfer pathway are responsible for the exceptional HER capability. Our study could enlighten the rational engineering and optimization of semiconductor nanostructures for energy and environmental applications.

Fabrication of recyclable reduced graphene oxide/graphitic carbon nitride quantum dot aerogel hybrids with enhanced photocatalytic activity

Recyclable photocatalysts that can efficiently respond to visible light must be developed for practical application. Herein, three-dimensional (3D) reduced graphene oxide (rGO)/graphitic carbon nitride quantum dot (CNQD) aerogel hybrids for harvesting visible light were synthesized via a hydrothermal method. The graphitic CNQDs were not only decorated on but also integrated onto the surface of rGO. The CNQDs produced photogenerated charge under visible light. 3D rGO could serve as an acceptor of the photogenerated electrons and stereoscopically facilitated the charge transfer through aerogel networks owing to its high conductivity. The ciprofloxacin removal ratio of the aerogel hybrids was about 6.1 times higher than that of bulk g-C₃N₄.

Recyclable photocatalysts that can efficiently respond to visible light must be developed for practical application.

Roles of Mo dopant in Bi2WO6 for enhancing photocatalytic activities

We present the investigation of the roles of molybdenum (Mo) dopant with a concentration of 0.0625% to 1.0% Mo into bismuth tungstate (Bi₂WO₆) by a one-step hydrothermal method for the enhancement of photocatalytic activities. The obtained materials and doping effects were characterized by the morphology, crystal structure, chemical states, and optical properties. By combining XRD, XANES, and EXAFS studies, the distortion of the local structure with substitutional doping was revealed as doping with Mo ions was located at the lattice sites of the tungsten ions. Photocatalytic reactions of Mo-doped Bi₂WO₆ were studied by the degradation of methyl orange dye under visible light irradiation. The results show that the optimal concentration of Mo dopant is 0.25%, with the highest photocatalytic activity up to ∼2-fold compared to the bare Bi₂WO₆. From our investigation, we propose that the impurity level is located below the conduction band edge of Bi₂WO₆ after doping with Mo⁶⁺ ions. This impurity level acts as an electron trapping site to prevent the transition of excited electrons from the conduction band to the valence band. By trapping experiments, the superoxide anion radicals (O₂˙^-) as the main active species to enhance photocatalytic efficiency was established.

Rational design of n-Bi12TiO20@p-BiOI core-shell heterojunction for boosting photocatalytic NO removal

Bismuth titanate (Bi₁₂TiO₂₀) with unique sillenite structure has been shown to be an excellent photocatalyst for environmental remediation. However, the narrow light-responsive range and rapid recombination of photoinduced electrons-holes limit the photocatalytic performance of Bi₁₂TiO₂₀. To overcome the limitations, a practical and feasibleway is to fabricate heterojunctions by combining Bi₁₂TiO₂₀ with suitable photocatalysts. Here, using a facile chemical precipitation method, a novel and hierarchical core-shell structure of n-Bi₁₂TiO₂₀@p-BiOI (BTO@BiOI) heterojunction was rationally designed and synthesized by loading BiOI nanosheets on BTO nanofibers. The constructed BTO@BiOI composites exhibited significant charge transfer ability due to the synergistic effects of the built-in electric field between BTO and BiOI as well as close interfacial contacts. In addition, the narrow bandgapcharacteristics of the BiOI led to wide light absorption ranges. Therefore, the BTO@BiOI heterojunction exhibited an improved photocatalytic performance under visible light irradiation. The NO removal efficiency of optimal BTO@BiOI was 45.7%, which was significantly higher compared tothat of pure BTO (3.6%) or BiOI (23.1%). Moreover, the cycling experiment revealed that BTO@BiOI composite has a good stability and reusability. The possible mechanism of photocatalytic NO oxidation over BTO@BiOI was investigated in detail.