A Comprehensive Review on the Boosted Effects of Anion Vacancy in the Heterogeneous Photocatalytic Degradation, Part II: Focus on Oxygen Vacancy

Environmental problems, including the increasingly polluted water and the energy crisis, have led to a need to propose novel strategies/methodologies to contribute to sustainable progress and enhance human well-being. For these goals, heterogeneous semiconducting-based photocatalysis is introduced as a green, eco-friendly, cost-effective, and effective strategy. The introduction of anion vacancies in semiconductors has been well-known as an effective strategy for considerably enhancing the photocatalytic activity of such photocatalytic systems, giving them the advantages of promoting light harvesting, facilitating photogenerated electron-hole pair separation, optimizing the electronic structure, and enhancing the yield of reactive radicals. This Review will introduce the effects of anion vacancy-dominated photodegradation systems. Then, their mechanism will illustrate how an anion vacancy changes the photodegradation pathway to enhance the degradation efficiency toward pollutants and the overall photocatalytic performance. Specifically, the vacancy defect types and the methods of tailoring vacancies will be briefly illustrated, and this part of the Review will focus on the oxygen vacancy (OV) and its recent advances. The challenges and development issues for engineered vacancy defects in photocatalysts will also be discussed for practical applications and to provide a promising research direction. Finally, some prospects for this emerging field will be proposed and suggested. All permission numbers for adopted figures from the literature are summarized in a separate file for the Editor.

Visible-light-driven AgI/Bi4O5I2 S-scheme heterojunction for efficient tetracycline hydrochloride removal: Mechanism and degradation pathway

The existence of excessive tetracycline hydrochloride (TCH) in the ecological environment has seriously threatened human health, so there is an urgent need to develop a high-performance photocatalyst that can efficiently and greenly remove TCH. Currently, most photocatalysts have the problems of fast recombination of photogenerated charge carriers and low degradation efficiency. Herein, S-scheme AgI/Bi₄O₅I₂ (AB) heterojunctions was constructed for TCH removal. Compared with the single component, the apparent kinetic constant of the 0.7AB is 5.6 and 10.2 time as high as the AgI and Bi₄O₅I₂, and the photocatalytic activity only decreases by 3.0% after four recycle runs. In addition, to verify the potential practical application of the fabricated AgI/Bi₄O₅I₂ nanocomposite, the photocatalytic degradation of TCH was performed under different conditions by regulating the dosage of photocatalyst, the TCH concentration, pH, and the existence of various anions. Systematical characterizations are conducted to investigate the intrinsic physical and chemical properties of the constructed AgI/Bi₄O₅I₂ composites. Based on the synergetic characterizations by in situ X-ray photoelectron spectroscopy, band edge measurements, as well as reactive oxygen species (ROS) detections, the S-scheme photocatalytic mechanism is proved. This work provides a valuable reference for developing efficient and stable S-scheme AgI/Bi₄O₅I₂ photocatalyst for TCH removal.

Novel fabrication of the recyclable Bi7O9I3/chitosan and BiOI/chitosan heterostructure with improved photocatalytic activity for degradation of dimethyl phthalate under visible light

Among the bismuth oxyhalides, bismuth oxide has the shortest band gap and high absorption power in the visible light region. Dimethyl phthalate (DMP) has been identified as endocrine-disrupting plasticizer and emerging pollutant, which was selected as the target pollutant to evaluate the efficacy of the studied catalytic process. In this work, Bi₇O₉I₃/chitosan and BiOI/chitosan were efficaciously synthesized by the hydrothermal process method. Characterizing prepared photocatalysts was done by employing transmission electron microscopy, X-ray diffraction, scanning electron microscopy energy-dispersive spectroscopy, and diffuse reflectance spectroscopy. For this study, the test design was performed using the Box-Behnken Design (BBD) method in which the variables of pH, Bi₇O₉I₃/chitosan dose, and dimethyl phthalate concentration were examined for the catalytic removal of dimethyl phthalate in the presence of visible light. Our detected results disclosed that the order of efficiency in DMP removal was as follows: Bi₇O₉I₃/chitosan > BiOI/chitosan > Bi₇O₉I₃ > BiOI. Also, the maximum pseudo-first-order kinetic coefficient for Bi₇O₉I₃/chitosan was 0.021 (min)^-1. When the synthesized catalysts were exposed to visible light irradiation, the predominant active species were O₂^- and h⁺ for degradation of DMP. The study on the reuse of Bi₇O₉I₃/chitosan showed that this catalyst could be reused 5 times without significant reduction in efficiency, which indicates the cost-effectiveness and environmental friendliness of using this catalyst.

Enhancement of solar-driven photocatalytic activity of oxygen vacancy-rich Bi/BiOBr/Sr2LaF7:Yb3+,Er3+ composites through synergetic strategy of upconversion function and plasmonic effect

For better use of solar energy, the development of efficient broadband photocatalyst has attracted extraordinary attention. In this study, a ternary composite consisting of Sr₂LaF₇:Yb³⁺,Er³⁺ upconversion (UC) nanocrystals and Bi nanoparticles loaded BiOBr nanosheets with oxygen vacancies (OVs, SLFBB) was designed and synthesized by multistep solvent-thermal method. Mechanisms of in-situ formation of Bi nanoparticles and OVs in BiOBr/Sr₂LaF₇:Yb³⁺,Er³⁺ composites (SFLB) are clarified. The Bi metal and OVs enhanced the light-harvesting capacity in the region of visible-near-infrared (Vis-NIR), and promoted the separation of electron-hole (e^-/h⁺) pairs. Furthermore, the surface plasmon resonance (SPR) effect of Bi metal can improve the energy transfer from Sr₂LaF₇:Yb³⁺,Er³⁺ to BiOBr via nonradiative energy transfer process, resulting in enhancing the light utilization from upconverting NIR into Vis light. Due to the synergistic effects of UC function, SPR and OVs, the SFLBB exhibited obviously enhanced photocatalytic ability for the degradation of BPA with a rate of 8.9 × 10^-3 min^-1, which is about 2.78 times higher than 3.2 × 10^-3 min^-1 of BiOBr (BOB) under UV-Vis-NIR light irradiation. This work provides a novel strategy for the project of high-efficiency Bismuth-based broadband photocatalysts, which is helpful to further understand the mechanism of enhanced photocatalysis by UC function and plasmonic effect.

New insights into the role of sulfite in BiOX photocatalytic pollutants elimination: In-operando generation of plasmonic Bi metal and oxygen vacancies

Photocatalysis has been regarded as a sustainable strategy for wastewaters remediation, and sulfite addition could significantly accelerate the photocatalytic performances. However, the related mechanisms are still not well understood. Here, we for the first time found that plasmonic Bi and oxygen vacancies were in-operando generated on BiOX (X = Cl, Br, I) in the presence of sulfite under light irradiation. The oxidative degradation rate constants of 4-nitrophenol, bisphenol A, and phenol were improved by about 11.5, 4.7, and 12.2 times on BiOBr and 9.1, 1.6, and 3.1 times on BiOCl with addition of 5 mM sulfite, while the photocatalytic reduction rate of 4-nitrophenol to 4-aminophenol was promoted by approximate 31.7 times on BiOI. The results indicated that sulfite could improve the photooxidation ability of BiOBr and BiOCl and the photoreduction performance of BiOI, resulted from the improved light absorption and separation of photogenerated charge carriers. This work can provide exploratory platforms for understanding and maximizing the sulfite-assisted BiOX photocatalysis.

Phase transformation and heterojunction construction of bismuth oxyiodides by grinding-assisted calcination in the presence of thiourea and their photoactivity

Bismuth-rich oxyhalides are promising photocatalysts due to their special layered structure and adjustable band gap energy. In this work, a series of bismuth oxyiodides were fabricated by grinding-assisted calcination in the presence of thiourea, where grinding-induced mechanical force could accelerate the decomposition reaction and thiourea could prohibit the crystal particles from growing due to coordination action. The combined effect of grinding and thiourea could decrease the temperature of phase transformation of bismuth oxyiodides. Among these, heterojunction Bi4O5I2/Bi5O7I containing uniform flower-like microspheres assembled by ultra-thin nanosheets exhibited the highest photocatalytic activity and favorable stability for the degradation of the antibiotic tetracycline under visible light irradiation. This work could provide a good reference for the design of bismuth-rich oxyhalide heterojunction for photocatalytic applications.

Construction of NH2-MIL-125(Ti)/CdS Z-scheme heterojunction for efficient photocatalytic H2 evolution

Designing efficient semiconductor-based photocatalysts for hydrogen production is a challenging but promising prospect in energy conversion. Herein, a novel Z-scheme CdS/NH₂-MIL-125(Ti) heterojunction is successfully fabricated through a facile solvethermal method. The detailed characterizations reveal that CdS nanoparticles are in-suit archored on NH₂-MIL-125(Ti) nanoplates. Benefited from the intrinsic band alignment and intimate contact of two species, this established structure gives a positive effect regarding charge separation. In consequence, the optimal CdS/NH₂-MIL-125(Ti) nanocomposites exhibit excellent photocatalytic performance with hydrogen evolution rate of 6.62 mmol·h^-1·g^-1 under visible light illumination, which was 3.5 times higher than that of the pristine CdS. We believe that this work will provide a new avenue to develop high-efficiency heterojunction catalyst for solar-driven energy conversions and other application.

Reduced graphene oxide/Bi4O5Br2 nanocomposite with synergetic effects on improving adsorption and photocatalytic activity for the degradation of antibiotics

A photocatalyst based on the integration of reduced graphene oxide (rGO) with Bi₄O₅Br₂ nanosheets was facilely prepared and was confirmed by transmission electron microscope, scanning electron microscope, X-ray diffraction and Raman spectroscopy. The integration of rGO can effectively improve the adsorption and the photocatalytic properties of Bi₄O₅Br₂ nanosheets towards the target antibiotics under visible light irradiation. rGO/Bi₄O₅Br₂ nanocomposite containing 1.0 wt% of rGO exhibits the optimal adsorption and photocatalytic activity towards ciprofloxacin (CIP), norfloxacin (NOR) and tetracycline (TC). The removal efficiencies of CIP, NOR and TC are 97.6%, 80.7% and 98.7%, which are higher than that obtained with Bi₄O₅Br₂ nanosheets. The capture experiments and ESR data show that ·O₂^-, OH· and h⁺ are the main active species that participated in the photodegradation system. This work provides a simple strategy to integrate rGO with Bi_xO_yX_z (X = Cl, Br, I) nanosheets to construct effective photocatalysts for the degradation of antibiotics.

Bi4O5I2/nitrogen-doped hierarchical carbon (NHC) composites with tremella-like structure for high photocatalytic performance

BiOI is a visible photocatalyst towards organic pollutant. In this work, biomass waste (withered typha grass) was used to fabricate nitrogen-doped hierarchical carbon (NHC) by an one-step carbonization route. Then NHC provided a good carrier to load the BiOI semiconductor materials by a green simple co-precipitation method, after adding NaOH solution, the irregular microspheres BiOI/NHC was gradually etched by OH^- to form the tremella-like Bi₄O₅I₂/NHC. The well-defined tremella-like Bi₄O₅I₂/NHC invested adequate interface and high particular surface range (S_BET: 66 m² g^-1), which is higher than pure BiOI (22 m² g^-1) and Bi₄O₅I₂ (17 m² g^-1). Multiple synergistic effects, such as high S_BET can give more dynamic destinations, the special tremella-like structure can assimilate more reflected occurrence light of other nanosheets, low I content can increase the conduction/valence band gap of semiconductor materials and NHC can act as an electron acceptor, making as-prepared Bi₄O₅I₂/NHC composite ideal candidates for photocatalysis. The degradation rate of Bi₄O₅I₂/NHC reaches up to 87.4% of methyl orange in 2 h, which is about 2 times higher than BiOI and Bi₄O₅I₂. Therefore, this work gives a technique to link NHC derived from biomass waste to Bi₄O₅I₂ with highly-efficiency photocatalytic performance.

AgI loading BiOI composites with enhanced photodegradation efficiency for bisphenol A under simulated solar light

Bismuth oxyiodide (BiOI) is a narrow band gap semiconductor which can be driven by visible irradiation. In order to efficiently separate photo-generated carriers and utilization of visible light, a facile solvothermal approach was used to synthesize a novel AgI loading BiOI 3D hierarchical composite (AgI-BiOI). The AgI-BiOI with Ag and Bi molar ratio of 1:8 (AgI-BiOI (1-8)) showed great enhancement for photocatalytic degradation of bisphenol A (BPA) with pseudo-first degradation rate constant about 3.7 or 14.5 times than that of pristine BiOI or AgI under simulated solar light. This synergistic enhancement for BPA degradation on AgI-BiOI(1-8) is mainly ascribed to enhancing the light absorption intensity and accelerating photo-generated carriers separation due to the formation of AgI-BiOI heterojunction. Free radical quenching experiments proved that positive holes (h⁺) and superoxide (O₂^•-) radicals were dominantly responsible for the degradation of BPA rather than singlet oxygen (¹O₂) or hydroxyl radicals (•OH). The AgI-BiOI(1-8) hardly showed any ecotoxicity to C. elegans through lethal experiments. The luminance bacteria acute toxicity of degradation intermediates of BPA increased before 30 min then reduced significantly with reaction. The good durability and environmental-friendly characteristics make AgI-BiOI(1-8) catalyst to be a good solar light-driven candidate.

Efficient photocatalytic reactions of Cr(vi) reduction and ciprofloxacin and RhB oxidation with Sn(ii)-doped BiOBr

Efficient photocatalytic reactions of Cr(VI) reduction, ciprofloxacin and RhB oxidation with Sn(II) doped BiOBr.

Generalized Synthesis of Ternary Sulfide Hollow Structures with Enhanced Photocatalytic Performance for Degradation and Hydrogen Evolution

A series of ternary sulfide hollow structures have been successfully prepared by a facile glutathione (GSH)-assisted one-step hydrothermal route, where GSH acts as the source of sulfur and bubble template. We demonstrate the feasibility and versatility of this in situ gas-bubble template strategy by the fabrication of novel hollow structures of MIn₂S₄ (M = Cd, Zn, Ca, Mg, and Mn). Interestingly, with the reaction time varying, the hierarchical CdIn₂S₄ microspheres with controlled internal structures can be regulated from yolk-shell, smaller yolk-shell (yolk-shell with shrunk yolk), hollow, to solid. Under visible-light irradiation, all of our prepared CdIn₂S₄ samples with different morphologies were photoactivated. In virtue of the appealing hierarchical hollow structure, the yolk-shell-structured CdIn₂S₄ microspheres exhibited the optimal photocatalytic activity and excellent durability for both the X₃B degradation and H₂ evolution, which can be ascribed to the synergy-promoting effect of the small crystallite size together with the unique structural advantages of the yolk-shell structure. Thus, we hypothesize that this proof-of-concept strategy paves an example of rational design of hollow structured ternary or multinary sulfides with superior photochemical performance, holding great potential for future multifunctional applications.

Synthesis of Bi2O3–Bi4V2O11 heterojunctions with high interface quality for enhanced visible light photocatalysis in degradation of high-concentration phenol and MO dyes

Bi₂O₃–Bi₄V₂O₁₁ heterostructures with high interface quality were synthesized by calcining Bi₂VO_5.5–Bi(OHC₂O₄)·2H₂O precursors. The Bi₂O₃–Bi₄V₂O₁₁ heterostructure exhibits outstanding photocatalytic activity for degrading phenol and MO dyes with high concentration under visible light irradiation.

Lantern-like bismuth oxyiodide embedded typha-based carbon via in situ self-template and ion exchange–recrystallization for high-performance photocatalysis

Well-defined lantern-like hierarchical Bi₇O₉I₃/NTC with superior photocatalytic activity is successfully obtained via a one-step route using in situ ion exchange–recrystallization.

Preparation of Z-scheme AgI/Bi5O7I plate with high visible light photocatalytic performance by phase transition and morphological transformation of BiOI microspheres at room temperature

Z-scheme AgI/Bi₅O₇I plate composite fabricated at room temperature through phase transition and morphological transformation process with high photocatalytic activity.

Investigation on enhanced photocatalytic degradation of bisphenol A with bismuth oxyiodide catalyst using response surface methodology

In this study, Bi₇O₉I₃ photocatalyst was successfully synthesized via a simple and rapid microwave irradiation method. The characterization of prepared photocatalysts was determined by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectrometry (EDS), UV-vis diffuse reflectance spectroscopy (DRS) and photoluminescence (PL). The photocatalytic performance was determines by the degradation of bisphenol A (BPA) under xenon lamp illumination. The Bi₇O₉I₃ catalyst exhibited superior photocatalytic performance and the first-order kinetic rate constant of Bi₇O₉I₃ was about 4.2 times greater than that of BiOI. The enhanced photocatalytic activity was associated with surface morphology, suitable band gap energy and low recombination rate of electron–hole pairs. Furthermore, the photocatalytic efficiency of BPA with Bi₇O₉I₃ was systematically investigated using a three factor, three level Box–Behnken experimental design and response surface methodology (RSM). A quadratic polynomial model was proposed. Experimental and predicted values exhibited a good correlation with a predicted R² value of 0.9016. A relative significance study of three independent variables showed that catalyst dosage had the most significant positive effect on the degradation of BPA, followed by initial concentration of BPA and pH value. The prepared Bi₇O₉I₃ is a promising photocatalyst for practical application in organic pollutant decomposition.

Bi₇O₉I₃ was synthesized via a microwave irradiation method and the influence of different parameters on BPA degradation was investigated using a response surface methodology.