Abstract
Agricultural runoff, fuel spillages, urbanization, hospitalization, and industrialization are some of the serious problems currently facing the world. In particular, byproducts that are hazardous to the ecosystem have the potential to mix with water used for drinking. Over the last three decades, various techniques, including biodegradation, advanced oxidation processes (AOPs), (e.g., photocatalysis, photo-Fenton oxidation, Fenton-like oxidation, and electrochemical oxidation process adsorption), filtration, and adsorption techniques, have been developed to remove hazardous byproducts. Among those, AOPs, photocatalysis has received special attention from the scientific community because of its unusual properties at the nanoscale and its layered structure. Recently, bismuth based semiconductor (BBSc) photocatalysts have played an important role in solving global energy demand and environmental pollution problems. In particular, bismuth-based Z-scheme heterojunction (BBZSH) is considered the best alternative route to overhaul the limitations of single-component BBSc photocatalysts. This work aims to review recent studies on a new type of BBZSH photocatalysts for the treatment of contaminated water. The general overview of the synthesis methods, efficiency-enhancing strategies, classifications of BBSc and Z-scheme heterojunctions, the degradation mechanisms of Z- and S-schemes, and the application of BBZSH photocatalysts for the degradation of organic dyes, antibiotics, aromatics compounds, endocrine-disrupting compounds, and volatile organic compounds are reviewed. Finally, challenges and the future perspective of BBZSH photocatalysts are discussed.
Introduction
Currently, the world faces many serious problems such as poverty, climate change, disease, pollution, energy, etc., due to the expansion of industrialization, urbanization, agricultural runoff, fuel spills, etc.1,2 Various organic and inorganic toxins are released from different industries into the ecosystem without adequate treatment and are the main causes of surface and groundwater contamination.3,4 In low- and middle-income countries, more than 90% of deaths are caused by water contamination.5 Thus, pollution is not a local issue, but a planetary threat, that jeopardizes the sustainable development of modern society. Therefore, the control and prevention of water pollution require urgent global attention to have a healthy planet. So far, several techniques like filtration,6 biodegradation,7 physical adsorption,8 electrochemical oxidation,9 and advanced oxidation processes (e.g., photocatalysis, photo-Fenton oxidation, Fenton-like oxidation, etc.)10−13 have been used to remove hazardous pollutants from wastewater. In comparison to photocatalysis, other methods have their restrictions and disadvantages. Scientists have performed a great job in creating photocatalytic technology, which has surpassed the restrictions of other techniques. Photocatalytic technology has a low cost, excellent sustainability, high efficiency, and low environmental impact.14 Under mild and favorable conditions, photocatalytic technology can efficiently degrade and eventually convert to nonharmful minerals.15
Various types of nanostructured materials such as semiconductors,16,17 non-noble plasmonic metal,18 metal oxides,19,20 polymers,21,22 composites,23,24 etc., have been used for sunlight- based catalysis applications. Special focus has been placed on semiconductor-based photo- catalysts due to their unusual properties at the nanoscale.25 Recently, bismuth based semiconductor (BBSc) photocatalysts have been a new emerging research topic in photocatalytic technology. The layered structures, unique physicochemical properties, visible light responsiveness, tunable band structure, etc., make this an important class of photocatalyst materials under visible light.26−28 As a result of the hybrid orbitals of Bi 6s and O 2p in the valence band (enabling the separation of photogenerated carriers), BBSc photocatalysts are better in photocatalytic activities compared to conventional photocatalysts materials such as TiO2, ZnO, Fe2O3, etc.29
Moreover, bismuth metal is benign to the environment,30 cheap and easily made, and has been found to have plasmon resonance effect properties.31 Various compounds of BBSc photocatalysts, such as binary compounds like Bi2O3,32 Bi2S333 etc., ternary compounds like Bi2WO6,34 Bi2MoO6,35 BiPO4,36 BiOX, BiVO4,37 etc., and multicomponent oxides such as Bi3TiNbO9, Bi4Ti0.5W0.5O8Cl, Bi4NbO8Cl, etc., have shown superior efficiency under visible light.38 Aside from their unique properties, pristine BBSc photocatalysts have some limitations such as narrow visible light absorption, fast recombination rate of charge carriers induced by photons, small surface areas, and weak redox capacities that prevent them from being widely utilized.39−41 A variety of methods are used, for example, elemental doping, heterojunction formation,42,43 surface modification,44 and noble metal decorations, to overhaul their limitations.
BBSc heterojunction improves the charge carrier separation efficiency through improved kinetics and robust redox ability. Particularly, a Z-scheme heterojunction system can improve sunlight-powered photocatalysis efficiency compared to traditional heterojunction composites. There are two benefits of the Z-scheme heterojunction photocatalyst: increased charge separation efficiency and potent redox ability.45 As a result, isolated photogenerated electrons and holes can effectively lower the charge carriers’ recombination rate. Therefore, a bismuth-based Z-scheme heterojunction (BBZSH) is considered the best alternative route to eliminate the limitations associated with a pristine BBSc photocatalyst.31,46 Considering its benefits, several research groups have recently extensively worked on BBZSH photocatalysts for energy and environmental remediation applications.47−53 Despite their progress in removing pollutants and antibiotics from contaminated water, Z-scheme heterojunction photocatalysts have some challenges. For instance, material selection for both the electron donor and acceptor, designing the heterojunction architecture and optimizing the surface area and crystallinity, ensuring long-term stability and durability, and achieving high-performance photocatalysts at a large scale and reasonable cost are its challenges. The synthesis methods used to prepare photocatalytic materials have a significant impact on their photocatalytic activity. Different synthesis methods can alter the structure, morphology, composition, surface properties, and defect characteristics of the materials. These changes directly influence the materials’ optical absorption, charge carrier dynamics, surface reactivity, and catalytic performance. The most common methods used to synthesize BBSc photocatalysts include solvothermal/hydrothermal methods,54,55 coprecipitation methods,56 sol–gel method,21,57 solid-state thermolysis,58 stepwise deposition techniques,59 in situ growth,60 and photoreduction techniques. This work aims to review the recent studies on BBZSH photocatalysts for treating contaminated water. In particular, classifications of BBSc photocatalysts, the synthesis methods, efficiency-enhancing strategies, Z-scheme and S-scheme heterojunction photocatalytic mechanisms, and the use of BBZSH photocatalysts for the elimination of harmful pollutants, antibiotics, aromatics compounds, endocrine-disrupting compounds and volatile organic compounds from water are reviewed.
Classification of BBSc Photocatalysts
BBSc photocatalysts are a new generation of photocatalytic material with tunable band gaps that respond to visible light. They have layered structures, resulting in interlayer electric fields (IEFs) that facilitate the separation of photogenerated charge carriers during catalysis. It is possible to classify BBSc photocatalysts according to different aspects. In terms of their chemical composition, BBSc photocatalysts can be divided into metallic bismuth, binary compounds, ternary oxides, and multicomponent oxides. Bismuth is the heaviest and one of the pnictogens elements, with an electron configuration of [Xe]4f 145d106s26p3. It exists in a natural state and has two ores namely: bismuthinite (bismuth sulfide) and bismite (bismuth oxide). Since bismuth is found regularly as a byproduct of the mining of Cu, Pd, and Sn, it is relatively inexpensive for a rare metal.61 Bismuth and its compounds are nontoxic.30 Hence, bismuth is considered a green metal. It has a plethora of distinct characteristics, such as being very brittle, having a low melting point, having low thermal conductivity, etc. The unusual properties at the nanoscale make metallic Bi the best photocatalyst due to its plasmonic properties,31 and quantum confinement effect. Hence, it can be used directly in semiconductor photocatalysts or cocatalysts to speed up charge separation and hence boost photocatalytic activity.62
Bismuth oxide is a binary oxide of bismuth metal that exists in two forms: trivalent (Bi2O3) and pentavalent (Bi2O5). Bismuth trioxide is the most common and exists in different crystal phases, each with its unique properties and characteristics. The most commonly observed crystal phases of Bi2O3 are α-Bi2O3, β-Bi2O3 and γ-Bi2O3 as shown in Figure 1a–c. α-Bi2O3 is the most stable and well-known form α-Bi2O3 and it crystallizes in a cubic crystal structure, belonging to the space group Pa3. It has a high melting point and excellent stability and is commonly used in various applications, including catalysis, gas sensing, and optical devices. β-Bi2O3 phase of Bi2O3 has a hexagonal crystal structure, belonging to the space group P63/mmc. It is metastable and can be formed by high-temperature synthesis, specifically above 730 ◦C. It exhibits different properties compared to α-Bi2O3, such as improved photocatalytic activity, making it a promising material for solar energy conversion and environmental remediation.63,64 γ-Bi2O3 phase is less commonly observed and is generally obtained under specific synthesis conditions. It has a monoclinic crystal structure, belonging to the space group P21/c. The Bi–O bond distances in this crystal range from 2.14 to 2.57°A.65 It differs from the α and β phases in terms of its structural arrangement and properties. γ-Bi2O3 exhibits enhanced electrical conductivity and has shown potential in applications such as solid oxide fuel cells and electrochemical devices.64 Bismuth sulfide (Bi2S3) is another compound of bismuth that can exist in different crystal phases. The most commonly known crystal phases of Bi2S3 are orthorhombic and monoclinic. Orthorhombic is the most stable and often observed phase with Pnma space group. It has a layered structure, with bismuth and sulfur atoms arranged in alternating layers as plotted in Figure 2d. It is a direct bandgap semiconductor and exhibits interesting properties, such as a high absorption coefficient in the visible and near-infrared spectral region. There are two different sites where Bi3+ cations are found. The first site cations are bonded to six S2– atoms. The lengths of the bonds between Bi and S range from 2.68 to 3.07 Å. The second cation is bonded to seven S2– atoms, and the bond lengths between Bi and S range from 2.60 to 3.41 Å. S2– atoms are also present in three different sites. The five Bi3+ atoms form distorted edge-sharing SBi5 square pyramids with the first two S2– anions, while the third S2– anion binds to three Bi3+ atoms. These characteristics make orthorhombic a potential material for applications in optoelectronics, photovoltaics, and photocatalysis.66 A monoclinic phase of Bi2S3 has a different crystal structure compared to orthorhombic. It belongs to the space group C2/m. It is metastable and can be obtained through specific synthesis methods or under certain conditions. This phase may exhibit properties different from those of orthorhombic, and its characterization and potential applications are still being explored.
Figure 1.
Crystal structure of (a) α-Bi2O3 phase (cubic), (b) γ-Bi2O3 phase (monoclinic), (c) β-Bi2O3 phase (hexagonal), and (d) orthorhombic phase of Bi2S3.
Figure 2.
(a) Unit cell sillen structure of BiOX crystal, (b) unit cell aurivillius structure of Bi2WO6, (c) unit cell sillen-aurivillius structure of Bi4NbO8Cl.
Another class of BBSc-layered compounds is ternary oxides, and multicomponent oxides typically have Sillen, Aurivillius, and Sillen-Aurivillius phases. The best examples of a Sillen phase photocatalyst are BiOX (X= F, Cl, Br, or I) and bismuth oxide (Bi2O2) nanomaterial. The BiOX material is a ternary semiconductor with a unique layered structure that is made up of interwoven [Bi2O2] and double X slabs, as shown in Figure 2. The bond between Bi–O and Bi–X is strong and covalent. In contrast, the bond between X–X is weak and due to van der Waals forces. BiOX has attracted a great deal of research interest in both experimental and computational aspects because of its small bandgap, high electron density, and strong photooxidation ability. BiOX has an indirect band gap in its bulk state, but it has a high valence band edge because of the interaction between X np orbitals and O 2p orbitals. BiOX is widely used in photocatalysis, but its ability to absorb visible light depends on the type of halogen ion present. However, Sillen structure photocatalysts have a low light absorption and fast recombination rate, which affects their efficiency for environmental remediation and energy conservation applications.
Aurivillius oxide is a layered material with a general formula of [An–1BnO3n+1] [Bi2O2], where [An–1BnO3n+1] is perovskite slabs, [Bi2O2]2+ is bismuth oxide slab and n is the number of octahedral layers67 as indicated in Figure 2b. It is formed by bismuth oxide sheets swapped with one or more perovskite slabs. They can provide a suitable electronic system for visible light absorption and provide the opportunity to modify the electronic band structure by substituting the A or B sites.68 Swedish chemist Bengt Aurivillius described this crystal structure in 1949 for the first time, which was interesting for its ferroelectric properties. The VB in this crystallographic family primarily consists of O 2p states, while the CB is predominantly comprised of Bi 6p orbitals. The optical band gap varies from 2.5 to 2.8 eV. The most well-known and studied example of the structure of the Aurivillius phase is Bi4Ti3O12. Other examples include Bi2WO6, Bi2MoO6, Bi3TiNbO9, and Bi2WO6. Aurivillius oxides are ferroelectric materials and exhibit good ionic conductivity. This makes them effective in enhancing photogenerated charge separation, which is beneficial for various photocatalyzed reactions. Additionally, the small optical band gaps of specific Aurivillius structures enable them to absorb visible light efficiently.67
Another class of bismuth-based structures is the Sillen-Aurivillius phases, as shown in Figure 2c. These are layered oxyhalide materials with a general formula of [An–1BnO3n+1] [Bi2O2]2Xm. The number of perovskite layers is represented by n, while X represents the halide. [Bi2O2] is fluorite, and [An–1BnO3n+1] represents perovskite plates.69,70 The number of perovskite layers is an important parameter for controlling and improving photocatalytic activity because of the wide range of cations. The dimension and composition of perovskites affect their electronic and transport properties. Recently, bismuth-based Sillen-Aurivillius structures such as Bi4NbO8Cl, Bi4Ti0.5W0.5O8Cl, Bi7Fe2Ti2O17X (X = Cl, Br, I) have been reported for photocatalysis. Due to the small band gap, ferroelectric property, ionic conductivity, layered crystal structure, and stability, they are efficient for the degradation of organic pollutants.70 The presence of ferroelectricity can have both direct and indirect effects on photocatalytic activity. It influences photocatalytic activity by facilitating photoinduced charge carrier separation, promoting surface charge transport, increasing photocurrent and absorption, and modifying surface reactivity. Hence, ferroelectric materials offer exciting opportunities for developing efficient and sustainable photocatalysts for various applications in energy conversion and environmental remediation. Furthermore, ionic conductivity has also a significant impact on the photocatalytic activity of a material. Higher ionic conductivity can improve the photocatalytic activity by promoting charge carrier transport, enhancing the mass transport of reactants, and facilitating electrochemical reactions.
It is also possible to classify BBSc photocatalysts based on their Z-scheme heterojunction generations. The Z-scheme heterojunction photocatalysts have evolved into four generations. They are the first-generation (liquid mediator) Z-scheme, the second-generation (solid mediator) Z-scheme, the third-generation (direct Z-scheme) and the fourth-generation (S-scheme) heterojunction. For the first time, in 1979 Bard and his colleagues reported on the concept of the Z-scheme heterojunction in the semiconductor industry using two photosystems.71 The three elements that make up their model are semiconductor I (SC-I), semiconductor II (SC-II), and electron mediator. A model was derived from the photosynthesis mechanism in plants, in which SC-I and SC-II harvest the photon energy that is used to reduce CO2 to carbohydrates and oxidize H2O to O2 with a rough quantum yield of unity. The transfer between two semiconductors is facilitated by an electron shuttle mediator as shown in Figure 3a. The most popular redox mediators used in first-generation Z-scheme heterojunctions are IO–3/I–,72 Fe3+/Fe2+,73 VO2+/VO2+, [Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+.74 Electron shuttle mediators serve as electron acceptors and donors, separating the physical contact between the two photocatalysts. However, in the first-generation Z-scheme hetero- junction, the electron acceptor and donor are reacting with the electrons and holes generated in SC-I and II, as a result, the number of electrons and holes generated in SC-I and II decreases. Additionally, the reverse reaction and the shielding effect are other limitations of the first-generation BBZSH Photocatalysts. Thus, to conquer the drawbacks, the Tada group replaced the liquid mediator with a solid mediator in 2006, assuming that the photogenerated electron would flow smoothly between the two semiconductors and recombine across a low- contact resistance interface. They published the first study on a CdS/Au/TiO2 Z-scheme ternary heterojunction for photocatalytic water splitting.75 This year was a turning point for the development of second-generation Z-scheme heterojunction photocatalysts. Figure 3b shows the second-generation Z-scheme heterojunction. Various types of materials, such as reduced graphene oxide,43 carbon quantum dot,76 graphitic carbon nitride,77 and noble metals (Au, Ag)78,79 used as solid electron mediator. These materials act as a bridge, which serves as an electron shuttle mediator. The presence of a bridge between two photocatalysts enhances the process of separating electrons and holes generated by photons, increasing the redox capability and photocatalytic activities of the composites.80
Figure 3.
Schematic representation of (a) first generation, (b) second generation, and (c) third generation Z-scheme heterojunction photocatalyst. Reproduced from ref (40) under terms of the CC-BY license. Copyright 2022 The Authors.
Direct Z-scheme heterojunction, which describes the third-generation Z-scheme heterojunction. Direct Z-scheme systems share similar charge-transfer pathways with the first and second-generation Z-schemes but do not use a mediator as shown in Figure 3c. In direct Z-scheme heterojunctions, there is no use of media between the two photocatalysts of the photosystem. The charge carriers induced by photons are transported directly through the interface of two semiconductors, resulting in a shorter transport distance and higher photo- catalytic efficiency. This occurs when the holes in the photosystem with the least positive valence band combine with the weak electrons from the photosystem with the least negative conduction band. In this photocatalyst, an induced internal electric field participates in an oxidation and reduction reaction that separates the photoinduced pairs e––h+ on the surface of a photosystem. Consequently, the lifespan of the e––h+ pairs increases and maintains a superior redox capability between the photoinduced e– in the CB of photosystem I and the holes in the VB of photosystem II. Then after the two photosystems are in contact, e– transfers from SC-I to SC-II due to Fermi-level equilibration. This creates a positive charge in SC-I and a negative charge in SC-II. Consequently, the electron density decreases at the edges of the SC-I energy band, while accumulating at the edges of the SC-II energy band.81 Due to these benefits, direct Z-scheme heterojunctions can effectively use photon energy by increasing the lifespan of e––h+ and optimizing redox capacity. Hence, utilizing Z-scheme heterojunctions, one can improve the photocatalytic activity of photocatalysts by reducing e–-h+ recombination.40,42
The S-scheme heterojunction is a type of photocatalytic system that consists of two different semiconductors (reduction photocatalysts and oxidation photocatalysts) with staggered energy band alignments. In this configuration, one semiconductor acts as a photosensitizer, while the other acts as a catalyst. The S-scheme heterojunction improves photocatalytic activity through several mechanisms: It enables efficient charge separation, utilizes a redox potential gradient, offers synergistic effects, and broadens the spectral response. Collectively, these factors enhance the overall performance of the photocatalytic system, making the S-scheme heterojunction an effective strategy for improving photocatalytic activity.
The S- and Z-scheme heterojunctions are two different configurations of heterojunction systems commonly used in photocatalysis. Although both configurations aim to improve the efficiency and performance of photocatalytic reactions, they differ in their charge-transfer mechanisms, energy-level alignments, and redox potential gradients. The S-scheme hetero- junction relies on the spatial separation of charge carriers, staggered energy band alignments, and a redox potential gradient. On the other hand, the direct Z-scheme heterojunction utilizes an interfacial electron relay mechanism, and direct energy band alignment often does not establish a significant redox potential gradient. These differences in configuration affect the efficiency and characteristics of charge transfer and overall photocatalytic activity. Furthermore, in S-scheme heterojunctions, there is a Fermi energy difference, which contributes to charge transfer between two photocatalysts. Because of the presence of an internal electric field between two photocatalysts, charge transfer in S-scheme heterojunctions facilitates more charge separation, which increases photocatalytic activity. However, there is no charge transfer due to the Fermi energy difference in a Z-scheme heterojunction.82
Synthesis and Efficiency Enhancing Strategies of BBSc Photocatalysts
There are two general approaches to the preparation of photocatalytic materials, that is, top-down and bottom-up.83 Mechanical, electrical and thermal forces are used in the top-down approach to crushing bulk materials into microparticles or nanoparticles. Due to the chemical reactions and attractive forces small molecules and atoms are assembled in bottom-up processes. Specifically, the BBSc photocatalysts can be synthesized through physical, chemical, and biological (green) methods. In contrast to physical synthesis methods, chemical synthesis methods are profoundly dependent upon chemical solvents, while green synthesis methods are characterized by nontoxic chemicals and substrates.84 The performance of catalysts depends on various parameters such as morphological characteristics such as particle shape, size and surface area, electronic structures, catalyst dose, pH and pollutant concentration, and temperature.76 Therefore, synthesis methods may have a significant contribution to enhancing the noninherent parameters of the photocatalytic materials, such as morphology and surface properties. To date, the BBSc photocatalyst has received special research attention in the catalysis field owing to its strong redox reaction capacity.5,41,85 Particularly, the BBZSH photocatalysts facilitate the transfer of photogenerated charge carriers by forming more positive VB and negative CB potentials with stronger oxidation and reduction abilities. From a practical standpoint, two major difficulties in creating highly effective photocatalysts are their low quantum yield and their lack of efficiency in harvesting visible light. Several methods for synthesizing BBSc photocatalysts, such as solvothermal, hydrothermal, coprecipitation, template-assisted methods, etc., have been proposed to address these is- sues. In the next section, an overview of preparation techniques for BBSc photocatalysts is presented.
Synthesis Methods of BBSc Photocatalysts
The hydrothermal synthesis method belongs to synthesis by chemical methods. It is the most widely reported method for the synthesis of BBSc photocatalysts. Hydrothermal syn- thesis promotes the formation of well-crystallized BBSc photocatalysts and allows for the precise control of the size, shape, and morphology. The high-pressure and high-temperature conditions in hydrothermal reactions lead to the controlled growth of nanoparticles, resulting in improved crystallinity and reduced defects in the material. BBSc photocatalysts synthesized using hydrothermal methods often exhibit enhanced photocatalytic activity compared to other synthesis techniques. This is attributed to the controlled crystallinity, morphology, and composition achieved through hydrothermal synthesis. These advantages make hydrothermal synthesis a valuable technique for developing efficient photocatalytic materials. Besides, hydrothermal/solvothermal synthesis methods has also disadvantages such as requires high-temperature and high-pressure conditions, reaction time is often longer and not suitable for large-scale production due to the autoclave requirement. Recently, several types of Z-scheme heterojunction photocatalysts, such as In2S3/BiOBr,86 Ag2O/Bi2WO6,48 BiVO4/RGO/g-C3N4,87 etc., have been prepared by using the hydrothermal method. Hu et al.86 developed a Z-scheme indium In2S3/BiOBr heterojunction for photodegradation of organic pollutants under visible light. Li et al.87 constructed a Z-scheme heterojunction of BiVO4/RGO/g-C3N4 for the degradation of antibiotics. Xue et al.88 created a double Z-scheme g-C3N4/Bi2WO6/AgI photocatalyst via hydrothermal route for the degradation of organic pollutants in aqueous solutions. A direct heterojunction of Bi2S3 and Sb2S3 photocatalysts for dye degradation and reduction was also developed using the hydrothermal method by Li et al.53 Using a simple hydrothermal precipitation approach, a new Z-scheme flower-like Ag2WO4/Bi2WO6 photocatalyst with excellent efficiency was successfully pre- pared for the degradation of rhodamine B (RhB).89 Furthermore, Su et al.90 synthesized the direct Z-scheme heterojunctions of Bi2MoO6/UiO-66–NH2 using a facile solvothermal method. The synthesized sample was tested for the degradation of antibiotics such as fluoroquinolone, ofloxacin (OFL), and ciprofloxacin (CIP) under visible light. Compared to the pristine photocatalyst, all heterojunction photocatalysts have better photocatalytic degradation efficiency due to the formation of direct Z-scheme heterojunction. A series of BiPO4/Bi2MoO6 Z-scheme heterostructures were successfully synthesized by a one-pot solvothermal method.91
A solution combustion method is a chemical synthesis method of nanostructured materials. It is an exothermic reaction mechanism in which the precursor and solvents are mixed at the molecular level to form a solid-phase product. The solution combustion method offers simplicity, cost-effectiveness, and control over composition. But it may have limitations in terms of morphology control, potential impurities, high-temperature requirements, and scalability. Recently, Hezam et al.92 used a solution combustion method to create a direct Z-scheme heterojunction photocatalyst material composed of Cs2O/Bi2O3/ZnO. The small energy band gap of 1.92 eV of Cs2O was chosen to expand the range of light absorption. ZnO and Bi2O3 are ideal for direct Z-scheme systems because of their position between minimum conduction and maximum valence bands. The chemical bath deposition method is a chemical synthesis technique primarily used to prepare thin films of various materials from aqueous precursor solutions at temperatures below 100 ◦C. Recently, Sahnesarayi et al. prepared a photoelectrode of a Bi2O3 nanosheet using the chemical deposition method.93
The coprecipitation method is a chemical synthesis route commonly used for the preparation of inorganic and metal-based photocatalysts. The benefits of this approach include its convenient synthesis, cheapness, and low energy usage. However, its drawbacks include particle aggregation, small surface area, impurities, and uncontrolled morphology.94 Recently, a binary Z-scheme heterojunction of BiOI/(BiO)2CO3 photocatalyst was synthesized by using the coprecipitation method for photodegradation of sulfasalazine antibiotics.95 The ionic layer adsorption and reaction method is another chemical synthesis method commonly used to prepare uniform, large surface area thin films. In this technique, the substrate is individually submerged in a cationic and anionic solution. As an illustration, a new P–N junction of Bi4Ti3O12 nanofibers–BiOI nanosheets was created by the ionic layer adsorption and reaction approach.96 Sol–gel synthesis is another method of synthesis for BBSc photocatalysts. Sol–gel synthesis offers some advantages over other methods, such as being able to control composition, size, and morphology, mixing precursors homogeneously, and incorporating dopants and cocatalysts. However, it has the disadvantages of requiring multiple steps and longer processing times, requiring organic solvents, and being sensitive to reaction conditions. For examples, the Z-scheme heterojunction of Bi2O3/CuBi2O4 photocatalyst were synthesized by using sol–gel method for PMS activation and Lev degradation.97
In the field of nanomaterials preparation, green synthesis is a rapidly emerging technology. As a reducing and stabilizing agent, plants, bacteria, fungi, and algae are used. As a result, the green method of nanomaterials is an eco-friendly, biocompatible, cheap, and nontoxic method. Recently, a green synthesis method was used to synthesize Z-scheme heterojunction photocatalysts. For example, the Z-scheme heterojunction of CdSe/BiOCl photocatalysts was synthesized through the green synthesis method for organic dye degradation.98 Similarly, Li et al. used the green synthesis method to prepare heterostructured ternary Cd/CdS/BiOCl photocatalysts for the removal of persistent organic contaminants in wastewater.99
Efficiency Enhancing Strategies
BBSc photocatalysts have gained significant attention due to their unique electronic structure and excellent photocatalytic properties.100 Although BBSc photocatalysts show impressive photocatalytic performance, they have limitations such as weaker charge separation, poor charge carrier mobility, small surface area and high rates of recombination, low reducing ability, and limited visible-light absorption. To further enhance their efficiency, several strategies can be employed. Construction of heterojunction, band gap engineering, cocatalyst deposition, and morphology control are some key techniques to improve the photocatalytic performance of bismuth-based photocatalysts.25 In this section, the recent progress on BBSc photocatalyst heterojunction construction and elemental doping strategies are discussed.
Today extensive research efforts have been devoted to proposing heterojunction systems to solve the shortcomings of single-component BBSc photocatalysts. One example of improving charge separation efficiency and enhancing photocatalytic performance is by constructing Z- scheme heterojunction nanocomposites using two semiconductors. By designing a proper heterojunction-type photocatalytic system, it is possible to isolate electrons and holes in separate locations, which in turn extends the lifetime of photogenerated carriers.71,101,102 However, the design and construction of highly active heterojunction photocatalysts remain a challenging and new focus of research. In this section, various types of heterojunctions and other related junctions are discussed.
Type I Heterojunction: In a Type I heterojunction, the energy bands of two different semiconductor materials align in such a way that the conduction band minimum (CBM) of one semiconductor is lower than the CBM of the other semiconductor, while the valence band maximum (VBM) of the first semiconductor is higher than the VBM of the second semiconductor as shown in Figure 4a. This results in an overlapping of the energy bands at the interface. This type of heterojunction exhibits a staggered band alignment, where the CBM and VBM levels are misaligned between the two semiconductors. As a result, an electron can easily transfer from one semiconductor to the other without any energy barrier.103,104
Type II Heterojunction: In a Type II heterojunction, there is a staggered alignment of energy bands as shown in Figure 4b. The CBM of one material is higher than that of the other material, while the VBM of one material is lower than that of the other material. This band offset causes photoinduced electrons and holes to be spatially separated between the two materials, facilitating efficient charge separation and promoting photocatalytic activity. Type II heterojunctions are commonly used in various photocatalytic systems.103,104
Type III Heterojunction: Type III heterojunctions consist of two semiconductor materials with both the CBM and VBM of one semiconductor being higher than the respective levels of the other semiconductor as shown in Figure 4c. This leads to a discontinuity or offset between the energy bands of the two semiconductors. This type of heterojunction exhibits a broken-gap or broken-alignment band structure, where there is no overlap or misalignment of the CBM and VBM levels at the interface. Consequently, there is a significant energy barrier for carriers transferring from one semiconductor to the other.103,104
P–N Junction: A P–N junction is created by connecting a p-type semiconductor to an n-type semiconductor as plotted in Figure 4f. At the interface between the p-type and n-type regions, there is a depletion region due to the diffusion of charge carriers. The P–N junction allows for efficient charge separation and can be useful in various applications, including solar cells and light-emitting diodes.56
Schottky Junction: A Schottky junction is formed at the interface between a metal and a semiconductor. It creates a barrier at the interface, known as the Schottky barrier, which affects the charge carrier transport. Schottky junctions are commonly used in electronic and optoelectronic devices due to their unique characteristics and rectifying properties.38
Z-scheme and S-scheme Heterojunction: Z-scheme heterojunctions are a specific type of heterojunction design used in photocatalysis. This configuration involves two different semiconductor materials with suitable band alignments, typically referred to as photocatalysts 1 and 2 as shown in Figure 4d, e. In a Z-scheme heterojunction, photogenerated electrons in one photocatalyst (photocatalyst 1) can transfer to the conduction band of the other photocatalyst (photocatalyst 2), while photogenerated holes move in the opposite direction. This scheme typically utilizes Type II heterojunction. There is an efficient charge transfer occurs via a mediator material or a conductive bridge. The transferred electrons and holes participate in redox reactions on the respective photocatalyst surfaces, facilitating pollutant degradation or other desired photochemical transformations. Hence, the Z-scheme design enhances charge separation and provides a continuous redox cycle, resulting in efficient utilization of light energy for photocatalysis.50 The S-scheme is the fourth generation Z-scheme hetero- junction is a photocatalytic system that involves two different photocatalysts connected in a series to achieve efficient charge separation and enhance overall photocatalytic activity.55 An S-type heterojunction design was considered to be formed by connecting two n-type semiconductors. According to Shawky and Mohamed, it is also possible to create S-scheme heterojunctions from n-type or p-type semiconductors, provided that the reduction photocatalyst must have a higher Fermi level and conduction band position than the oxidation photocatalyst.105 In this scenario, the induced electric field plays a vital role in transferring charge carriers between the two photocatalysts. The reduction photocatalyst has a smaller work function and a higher Fermi level, while the oxidation photocatalyst has a greater work function and a lower Fermi level.106 The performance of S-scheme heterojunction photocatalysts is influenced by several key factors such as band alignments and level matching, light absorption, charge carrier dynamics (lifetime and diffusion length of electrons and hole) and catalyst loading and its morphology.107 Hence, understanding electronic processes is very important for photocatalytic applications. Because by studying the kinetics and pathways of electron and hole transfer, researchers can identify the key processes influencing overall photocatalytic performance and develop strategies to optimize them through mate- rial design and interface engineering.108 Careful selection and design of materials in S-scheme systems can significantly enhance charge transfer dynamics. That is the choice of a suitable electron acceptor and donor can help optimize the charge transfer process. Furthermore, to enhance the charge kinetics in the S-scheme heterojunction, manipulation of the interfaces between different materials is crucial. It can be done by modulating surface properties using doping, codoping, and surface decorating.109
Figure 4.
Physical diagram of heterojunction photocatalysts types (a), Type-I, (b), Type-II, (c), Type-III, (d, e) Z-scheme heterojunctions, and (f) Schottky junctions. Adapted in part with permission from ref (110). Copyright 2016 American Chemical Society.
Most single-component BBSc photocatalysts exhibit wide band gap, limiting their absorption of visible light. Band gap engineering techniques, such as doping with elements like nitrogen or sulfur, can narrow the band gap and extend light absorption into the visible range. By increasing the absorption of visible light, the photocatalytic efficiency can be significantly enhanced. Hence, doping is an effective technique for enhancing photocatalytic activity as it regulates the optoelectronic properties and dynamics of the materials.111 In photocatalysts, doping is one of the primary objectives to trap photoexcited charge carriers and to promote their separation. There are two distinct types of elemental doping: metal and nonmetal doping. Metal doping can impact the electronic structure of photocatalysts in two ways. First, it introduces defects in the crystal, which increases the absorption properties of the photocatalyst and enhances the lifespan of the photogenerated charge carrier. Second, it generates a surface plasmon resonance effect on a Bi-based semiconductor surface. The presence of surface plasmon resonance (SPR) can robust the spectral response of semiconductor photocatalysts and enhance its quantum yield.100 Noble metals such as Ag, Ni, Au, Pt, Pd, V, and others have been reported to be used to modify the electronic band structure of Bi-based semiconductor photocatalysts. These metals absorb photoinduced electrons on the surface of semiconductor materials and enhance the lifespan of photogenerated charge carriers thereby boosting photocatalytic reactions.
By introducing nonmetallic elements, energy levels can be created between the conduction and valence bands in BBSc photocatalyst photocatalysts. This leads to improved optical properties of pure BBSc photocatalysts. Furthermore, it enhances charge transmission, which promotes the effective separation of electron–hole pairs and hinders the recombination of photoinduced charge carriers. Liang et al. successfully conducted a simple hydrothermal synthesis to prepare a dual-doped Bi2WO6, decorated with metallic bismuth (Bi) and nonmetallic carbon (c) atoms. The synthesized material was instigated with different characterization techniques. The results showed that the codecorated C/Bi Bi2WO6 exhibited significantly higher photocatalytic activity than the undoped Bi2WO6 when decomposing various industrial pollutants under visible light, including phenol, ciprofloxacin, bisphenol A, rhodamine B, and methyl orange. The synergistic effects of SPR and C doping are attributed to the increased photocatalytic activities.112
Belousov et al.113 recently conducted a comparative study on the efficacy of doping and heterojunction methods using LED light, to enhance the efficiency of Bi2WO6 photocatalyst. Bi2W0.5Mo0.5O6 and Bi2WO6/g-C3N4 composites were synthesized using solid solution and hydrothermal synthesis methods, respectively. According to the findings, the approach of adding dopants proved to be effective. The most successful result was achieved when using Bi2W0.5Mo0.5O6 to break down methylene blue, resulting in a conversion rate of 52.9% and a TON of 0.022. The use of only a small amount of H2O2 also resulted in impressive degradation rates for methylene blue (97.9% conversion, TON of 0.040) and phenol (81.2% conversion, a TON of 0.056). Another alternative in band gap engineering is the deposition of cocatalysts on the surface of BBSc, as it enhances charge carrier separation and improves the efficiency of specific reactions. Co-catalysts, such as noble metals (e.g., Pt, Au) or co- catalysts like NiO or CoOx, can serve as active sites for charge transfer and promote specific reaction pathways. The appropriate choice and deposition of cocatalysts can accelerate the kinetics of photocatalytic reactions, leading to enhanced efficiency. The key advantages of doping on photocatalysts are (i) enhancing visible light absorption, (ii) efficient charge carrier separation, improving catalytic activity, (iv) extended lifespan and stability, and (v) impart selectivity and tunability to photocatalysts, enabling control over the types of reactions they can catalyze.
Photocatalytic Degradation Mechanisms in Z- and S-Scheme Heterojunctions
Photocatalytic degradation relies on the use of radiation and photocatalysts to produce ROS.40 The photocatalytic degradation mechanism in Z-scheme heterojunctions can be summarized as follows. (i) Both photocatalyst 1 and photocatalyst 2 should have suitable band gaps to absorb light in the desired range, usually in the visible or near-ultraviolet (UV) region. Photons with sufficient energy are absorbed by these materials, promoting electron transitions from the valence band to the conduction band. (ii) Upon photon absorption, both photocatalysts generate electron–hole pairs. For example, in photocatalyst 1, electrons move to the conduction band (CB1), leaving holes in the valence band (VB1). Similarly, in photocatalyst 2, electrons are excited to the conduction band (CB2), and holes are created in the valence band (VB2). (iii) The photogenerated electrons in the conduction band of photocatalyst 1 can transfer to the conduction band of photocatalyst 2 due to the formation of a suitable energy band alignment or via a conductive bridge (e.g., metal or carbon-based catalysts). This transfer occurs as a result of a suitable energy-level difference and efficient charge-transport pathways. Similarly, holes in the valence band of photocatalyst 2 can transfer to the valence band of photocatalyst 1. This hole transfer from photocatalyst 2 to photocatalyst 1 is necessary to complete the Z-scheme and create a continuous redox cycle for efficient pollution degradation. (iv) Once the photogenerated electrons and holes are separated and transferred between the two photocatalysts, they can participate in various redox reactions on the surface of the materials. Photogenerated holes in photocatalyst 1 (VB1) can oxidize adsorbed organic molecules on the surface, producing reactive oxygen species (ROS), such as hydroxyl radicals. These ROS are highly reactive and can effectively degrade the pollutants. At the same time, photogenerated electrons in photocatalyst 2 (CB2) can reduce oxygen molecules or other suitable electron acceptors, leading to the production of superoxide radicals or other reactive species that facilitate pollutant degradation. V) To increase the photocatalytic efficiency, the recombination of the photogenerated electron–hole pairs should be minimized. The appropriate band alignment and material properties of the Z-scheme heterojunction facilitate efficient charge separation and minimize electron–hole recombination. This ensures a continuous flow of charge carriers, allowing for sustained degradation of pollutants.80,114,115
The charge transfer mechanism in an S-scheme heterojunction can be explained as follows: When light (photons) is incident on the heterojunction, it generates electron–hole pairs in both semiconductors due to the absorption of photons. The absorption of photons leads to the excitation of electrons from the valence band to the conduction band, leaving behind holes in the valence band. The two semiconductors in the heterojunction have different energy levels, which create a barrier at the interface, called a band offset. This offset leads to a difference in the energy levels of the conduction and valence bands between the two semiconductors. Because of the band offset, the energy bands in the two semiconductors bend at the interface. In one semiconductor, the conduction band edge is higher, while in the other semiconductor, the valence band edge is higher. The photogenerated electrons in the semiconductor with a higher conduction band edge will transfer to the semiconductor with a lower conduction band edge, as electrons tend to move from higher to lower energy levels. This transfer of electrons is known as electron injection or electron transfer.116 On the contrary, photogenerated holes in the semiconductor with a lower valence band edge will transfer to the semiconductor with a higher valence band edge, because holes tend to move from higher to lower energy levels. This transfer of holes is known as hole injection or hole transfer. The enhanced photocatalytic activity of S-scheme heterojunctions can be attributed to several underlying mechanisms. These mechanisms are related to the unique structural and electronic characteristics of S-scheme heterojunctions and their ability to facilitate efficient charge carrier separation and utilization. The efficient charge separation, cascade electron transfer, dual cocatalyst effect, interface states, and synergistic effects all contribute to the improved utilization of photoinduced charges for photocatalytic reactions, leading to higher catalytic activity and efficiency.117,118 By understanding this general charge transfer mechanism in S-scheme heterojunctions, researchers can design and optimize these structures to enhance their efficiency in applications such as solar cells, photocatalysis, and photodetectors.
BBZSH Photocatalytic Degradation
Organic Dye Degradation
In the 21st century, due to the rapid development of industrialization, various toxic substances, such as synthetic chemicals, fertilizers, dyes, heavy metals, etc., are among the sources of water contaminants.119 Organic dyes are mostly used to impart color to various materials, including textiles, plastics, paper, and fabrics.120 Approximately 7 × 105 tons of pigments and organic dyes are discharged into the environment in industrial wastewater.121 Photocatalytic degradation using advanced catalysts offers a promising approach for the remediation and removal of organic dyes from contaminated water sources, contributing to environmental sustainability.5,22,25,85,122 Recently, BBZSH photocatalysts have gained considerable attention for dye degradation applications due to their advanced charge transfer and separation capabilities. For instance, Mosleh et al. synthesized the Z-scheme heterojunction of Bi2WO6/Ag2S/ZnS photocatalyst for degradation of methyl green (MG) and auramine-O (AO). These studies have shown that the construction of Bi-based Z-scheme heterojunctions can significantly improve the photocatalytic performance for dye degradation.123 In another studies reported by Bi et al. Bi2WO6/ZnIn2S4 a double Z-scheme configuration resulted in a dye degradation efficiency that was 12–63 times higher than that of a single component photocatalyst.124 This suggests that the incorporation of multiple semiconductors in a Z-scheme configuration can enhance the separation of charge carriers and improve the overall photocatalytic efficiency for dye degradation. Despite this, Mahalakshmi et al. also synthesized the Z-scheme Bi2O3/g-C3N4 heterojunction photocatalysts using in situ thermal polymerization method for degradation of methylene blue (MB) dye. The 1:1 ratio of Bi2O3/g-C3N4 heterojunction photocatalyst performs highest photocatalytic degradation of 91.2% after 120 min of visible-light irradiation.125 Furthermore, the optimized composition of g-C3N4/BiYWO6 heterojunction photocatalysts has shown significant improvement in the degradation of methylene blue under visible light exposure, indicating its potential for wastewater remediation works.126 Furthermore, Zhong and his colleagues utilized a simple hydrothermal process to synthesize heterojunction composites of BiVO4@g-C3N4(100). The formation of the heterojunctions was validated through various characterization techniques. Figure 5a displays the scanning electron microscopy (SEM) image. The images obtained from TEM depict (Figure 5b) a distinct interface between BiVO4(121) and g-C3N4(100), signifying the successful formation of the heterojunction between the two materials through the hydrothermal reaction. Figure 5c, demonstrates UV–vis diffuse reflectance spectra of the samples. The EDX elemental mapping analysis (Figure 5d–i) confirmed the presence of five constituent elements, including Bi, V, O, C, and N, indicating the successful synthesis of homogeneous BiVO4 and g-C3N4(100) composite structures. Using simulated visible light to irradiate RhB, photocatalytic activity was evaluated. Based on the experimental results, the composite structure of 10% BiVO4@g-C3N4(100) showed the highest photocatalytic performance, with photodegradation rates 2.36 and 30.58 times greater than those for pure g-C3N4 and pure BiVO4. As shown in Figure 5j, the absorption peak of RhB drastically decreased after 60 min of irradiation of the catalysts for RhB.
Figure 5.
(a) SEM image of 10 wt % BiVO4@g-C3N4 Z-scheme heterojunction, (b) TEM Image of 0 wt % BiVO4@g-C3N4 Z-scheme heterojunction, (c) UV–vis diffuse reflectance spectra of the samples, (d–i) SEM elements mapping of 10 wt % BiVO4@g-C3N4 heterojunctions and (j) UV–Vis spectra of RhB at different visible irradiation-times in the presence of 10 wt % BiVO4@g-C3N4. Adapted in part with permission from ref (127). Copyright 2019 Elsevier.
In another study reported by Hao et al. a ternary Z-scheme heterojunction of WO3/Ag3PO4/Bi2WO6 photocatalyst was fabricated via a one-pot hydrothermal method for degradation RhB.128 The result shows that, the Z-scheme heterojunction photocatalysts were shown an outstanding photocatalytic degradation rate constant of 1.9 and 1.3 times higher than that of pure Bi2WO6 and WO3–Bi2WO6 photocatalysts, respectively. Li et al. have also synthesized Z-scheme heterojunction of Bi2Fe4O9/Bi2WO6Z photocatalysts using a facile hydrothermal method. The Z-scheme heterojunction of Bi2Fe4O9/Bi2WO6Z was able to photodegrade 100% of RhB in 90 min, demonstrating significant catalytic activity.129 Din et al.102 conducted a study on the Z-scheme heterojunction of the 3-dimensional hierarchical Bi3O4Cl/Bi5O7I, using calcination and solvothermal methods. Figure 6a–d shows how Bi3O4Cl (BOC), Bi5O7I (BOI), and BOC/BOI-3 photocatalyst the decomposition of BPA and RhB dye in an aqueous solution under visible light. The heterojunction of BOC/BOI-3 showed enhanced degradation efficiency of 97% and 92% for RhB and bisphenol A (BPA), respectively. The untreated photocatalysts only achieved 20% and 10% efficiency for RhB dye, respectively, and 2.3% and 37% for aqueous pollutants containing BPA, respectively. The improved efficiency of this photocatalyst is attributed to its ability to effectively absorb visible light, its large surface area, and its enhanced separation and transport of photoexcited electron–hole pairs. Furthermore, BBSc Z-scheme heterojunction photocatalysts for the removal of organic dye pollutants are summarized in Table 1. Based on several report studies,124−126 it is evident that Bi-based Z- scheme heterojunctions hold great promise for the efficient degradation of dyes in wastewater treatment processes, offering a feasible solution to address the environmental concerns associated with dye pollution. The utilization of Bi-based Z-scheme heterojunctions has shown great potential in the degradation of dyes, with significantly improved photocatalytic performance and higher degradation efficiency compared to single component photocatalysts.130
Figure 6.
(a, b) RhB degradation by photocatalysis and its effectiveness with reaction rate constants applied to all samples; (c, d) BPA degradation by photocatalysis and its effectiveness with reaction rate constants applied to BOI, BOC, and BOC/BOI-3 samples. Adapted in part with permission from ref (102). Copyright 2022 The Authors.
Table 1. BBZSH Photocatalysts for the Removal of Organic Dyes.
| S.No. | Photocatalysts | Synthesis methods | Application | Light source | Irradiation time (min) | Eff. (%) | ref. |
|---|---|---|---|---|---|---|---|
| 1 | g-C3N4/Bi2WO6 | Hydrothermal method and heat treatment | Photodegradation of methylene blue | 180 | 100 | (131) | |
| 2 | SnO2/Bi2S3/BiOCl | One-spot hydrothermal | Photocatalytic degradation of rhodamine B | Visible light | 180 | 80.8 | (132) |
| 3 | Bi2MoO6@Co3O4 | Two-step hydrothermal method | Photoanode for photo | 110 | 88.43 | (41) | |
| 4 | Ag2O2/Bi2MoO6 | Hydrothermal and coprecipitation | Photocatalytic degradation | 86 | (133) | ||
| 5 | g-C3N4@Bi/Bi2O2CO3 | Simple solvothermal method | Photocatalytic degradation of rhodamine B | visible light | 120 | 93 | (101) |
| 6 | Bi2Fe4O/BiWO6 | Facile hydrothermal | Photocatalytic degradation of rhodamine B | 90 | 100 | (129) | |
| 7 | Ag/AgI/BiOI | Facile ion exchange and photoreduction | Photocatalytic degradation of methyl orange | 500 W xenon lamp | 180 | 93 | (134) |
| 8 | Cu2O/Au/BiPO4 | Hydrothermal method | Photocatalytic degradation of methyl orange | 300 W xenon lamp | 60 | 100 | (135) |
| 9 | KNbO3/Bi2O3 | In situ growth method | Photocatalytic degradation of methyl orange | 375 W mercury lamp | 50 | 90.8 | (136) |
| 10 | g-C3N4/Bi2O3/BiPO4 | Simple one-step hydrothermal process | Photocatalytic degradation of methylene orange | 160 | 90 | (137) | |
| 11 | g-C3N4/Bi4O7 | Calcination methods | Photocatalytic degradation of MB, phenol, rhodamine, bisphenol | 500 W halogen lamp | (138) | ||
| 12 | CdSe/BiOCl | Green synthesis method | photodegradation of RhB 40 100 98 | Ultraviolet-light | 40 | 100 | (98) |
| 13 | Cd/CdS/BiOCl | Green synthesis method | photodegradation of RhB | Visible light | 90 | (99) | |
| 14 | Bi2Fe4O9/Bi2WO6 | Facile hydrothermal method | Photodegradation of RhB | Visible light | 90 | 100 | (129) |
| 15 | Bi2WO6/Ag2S/ZnS | Hydrothermal method | Degradation of methyl green (MG) and auramine-O (AO) dyes | Visible light | 82.75, 77.41 | (123) | |
| 16 | Bi2WO6/C-Dots/TiO2 | Facile chemical wet technique | Degradation of fluoroquinolone levofloxacin | Sunlight | 88.3 | 90 | (139) |
| 17 | BiVO4/TiO2 | Hydrothermal method | RhB degradation | Visible light | 74.4 | (140) | |
| 18 | TiO2/BiVO4 | Sol–gel method | Degradation of azo dyes | Visible light | 10 | 98.75 | (141) |
| 19 | Bi2O3/MoSe2 | Hydrothermal method | Degradation of methylene blue dye | Visible light | 80 | 96.5 | (142) |
Antibiotics Degradation
Antibiotics are chemicals that are used to treat infections caused by bacteria and for livestock production. Particularly, wastewater from hospitals and pharmaceutical companies ares the primary sources of antibiotics.59,60 Hence, their accumulation in the environment hurts ecosystems. The elimination of antibiotics from the environment using different techniques such as biodegradation, membrane separation, coagulation, and absorption has been re- ported. Conventional methods of contaminated treatment are inefficient at eliminating antibiotics. This is mainly because of the low biodegradability of antibiotics, which makes their removal difficult. Furthermore, some antibiotics cannot be removed using conventional treatment methods, which are expensive. However, advanced oxidation processes (AOPs) have shown promise for breaking down antibiotics into smaller, harmless, and biodegradable compounds.143 Photocatalysis has great potential for eliminating antibiotics from aquatic environments. Bismuth oxides, which can absorb a broad range of light (approximately 780 nm), are commonly used in photocatalytic applications. Bi4O7 is one such oxide with a band gap of 1.99 eV. It has the potential to match the bandgap of In2O3 and improve the photocatalytic capability of In2O3 under visible light by constructing a Z-scheme heterojunction with In2O3.144 A solid-state synthesis method was used to prepare the hetero- junction of In2O3/Bi4O7 for the degradation of doxycycline hydrochloride. Based on the results, doxycycline hydrochloride photodegrades 92.1% more efficiently in 2 h than pure silicon dioxide and bichromate. Bai et al. synthesized Z-scheme Bi2S3/Bi2O2CO3 photocatalysts for the degradation of antibiotics and organic compounds in wastewater. The fabricated heterojunction has photocatalytic efficiency of 61% and 89% for degradation of ciprofloxacin antibiotic) and rhodamine B dye, respectively. In another studies report by Wen et al. a direct AgI/Bi4V2O11 photocatalyst was synthesized using a combination of coprecipitation and hydrothermal methods, for sulfamethazine drug photodegradation. The Z-scheme heterojunction photocatalysts have been found to be the most effective photocatalysts for sulfamethazine (SMZ) degradation, with 91.47% SMZ being degraded within 60 min.145
A recent study by Yin et al. introduced a Z-scheme heterojunction called MT-BiVO4/P@g-C3N4 that was developed to degrade tetracycline hydrochloride (TC-HCl) from water.146 In this study, the photocatalysts were composed of P-doped graphite carbon nitride nanosheets loaded with BiVO4 in two coexisting phases, monoclinic scheelite (M-BiVO4) and tetragonal zircon (T-BiVO4). This material was abbreviated as MT-BiVO4 and was produced using a hydrothermal method. Photocatalytic experiments confirmed that 5%MT–BiVO4/P@g-C3N4 had the highest reaction rate, which was 3.8 times higher than g-C3N4, 1.6 times higher than that of P@g-C3N4, 1.9 times higher than BiVO4, and 2.1 times higher than 5 of BiVO4/g-C3N4. The improved performance was attributed to several factors, including P-doping of the modified g-C3N4, isotype heterojunction effects of MT-BiVO4, and the Z-scheme heterojunction formed between MT-BiVO4 and P@g-C3N4. These factors increase the absorption range of visible light and accelerate the transfer and separation of charge carriers between the catalysts. Jin et al.57 created a Z-scheme Au@TiO2/Bi2WO6 heterojunction to break down antibiotics. Bi, Ti, and Ag contribute to the plasmon resonance effect which enhances the photocatalytic activity of the heterojunction. Under visible light irradiation, the optimal mass ratio of Au@TiO2 to Bi2WO6 degraded sulfamethoxazole (SMX) and tetracycline hydrochloride (TC) by 96.9% and 95.0% within 75 min, respectively. Furthermore, Liu et al. also synthesized a Z-scheme heterojunction of BiOI and exfoliated g-C3N4 to degrade tetracycline in wastewater.147 They used a combination of thermal exfoliation and chemical precipitation to synthesize the compounds. The researchers tested the Photocatalytic activities of different photocatalysts, including BCN, ECN, BiOI, and different ratios of BiOI in BiOI/ECN compounds. They found that the Z-scheme heterojunction of BiOI-0.4 and ECN-0.4 had the highest degradation rate compared with other photocatalysts, including BCN, with photocatalytic activity up to 10 times greater than that of BCN. The charge transfer pathways and the interface band alignment of the constructed Z-scheme of BiOI/ECN are shown in Figure 7a and b depicts the degradation efficiency of RhB by synthesized samples. Furthermore, BBSc photocatalysts for the removal of antibiotics from wastewater are summarized in Table 2.
Figure 7.
(a) Diagrams of the interfacial electron transfer between BiOI and ECN, (b) the degradation efficiency of RhB using all samples. Adapted in part with permission from ref (147). Copyright 2021 Elsevier.
Table 2. BBZSH Photocatalysts for the Removal of Antibiotics from Contaminated Water.
| S.No. | Photocatalysts | Synthesis methods | Application | Light source | Irradiation time (min) | Eff. (%) | ref. |
|---|---|---|---|---|---|---|---|
| 1 | MT-BiVO4/g-C3N4 | Hydrothermal method | Photocatalytic degradation of tetracycline hydrochloride from water | (82) | |||
| 2 | g-C3N4/Bi2MoO6/CeO2 | Facile solid-state thermolysis | Photocatalytic degradation of 4-chlomjnfc | 80 | 99.1 | (58) | |
| 3 | Bi2MoO6/UiO-66–NH2 | Facile solvothermal | Photocatalytic degradation of ofloxacin and ciprofloxacin | 90 | 100, 96 | (90) | |
| 4 | Ag3PO4/Bi2S3/Bi2O3 | Photocatalytic degradation of sulfamethazine and cloxacillin | 90 | 98.06, 90.26 | (148) | ||
| 5 | TiO2/Bi2O3 | Facile sol–gel | Photocatalytic degradation of tetracycline hydrochloride (TC) | visible light | 60 | 80 | (21) |
| 6 | Au@TiO2/Bi2WO6 | Sol–gel and hydrothermal method | Photocatalytic degradation of sulfamethoxazole and TC | 75 | 96.9, 95 | (57) | |
| 7 | CuBi2O4/Bi2Sn2O7/Sn2O7 | In situ growth method | Photocatalytic degradation of TC | 89.7 | (60) | ||
| 8 | CdS/Bi3O4Cl simple hydrothermal | Photocatalytic degradation of CIP and TC | 250 Xe lamp | 120 | 84.2 | (149) | |
| 9 | BiOCl-Au-CdS | Step-wise decomposition method | Photocatalytic degradation of sulfadiazine antibiotics | 300W Xe lamp | 240 | 100 | (59) |
| 10 | BiOCl/Bi–Bi2O3 | One-step hydrothermal method | Photocatalytic degradation of RhB and TC | 300W Xe lamp | 60 | 90, 70 | (150) |
| 11 | AgVO3/Bi4Ti3O12 | Simple precipitation method | Photodegradation of tetracycline (TC) | 60 | 57 | (151) | |
| 12 | BiOBr/Bi2O4 | Coprecipitation method | Photodegradation of 4-chlorophenol (4-CP) | (56) | |||
| 13 | CoAl–LDH/Bi2MoO6 | Simple hydrothermal method | Photocatalytic degradation of tetracycline and ciprofloxacin | 85.98, 72.69 | (152) | ||
| 14 | TiO2(B)/BiOCl | Simple hydrothermal method | Photodegradation of TC | 100 | 80 | (153) | |
| 15 | CdS/BiOBr | Solvothermal route | Photocatalytic degradation of norfloxacin and ciprofloxacin | 100 | (54) | ||
| 16 | SnS2/Bi2WO6 | Hydrothermal route | Photocatalytic degradation of tetracycline (TC) and ciprofloxacin (CIP) | 97, 93 | (55) | ||
| 17 | Bi2MoO6/TiO2 | Solvothermal-calcination process | Photocatalytic degradation of tetracycline hydrochloride | 120 | 90.8 | (154) | |
| 18 | AgI/Bi2WO6 | In situ precipitation method | Photocatalytic degradation of TC | 60 | 93.05 | (155) | |
| 19 | Bi2Fe4O9/Bi2MoO6 | Facile one-pot solvothermal | Photocatalytic degradation of MB and TC | 150, 180 | (156) | ||
| 20 | Bi2WO6/g-C3N4 and Bi2WO6/TiO2 | Hydrothermal methods | Photocatalytic degradation of cefixime | 135 | 94, 91 | (157) | |
| 21 | Bi@β-Bi2O3/g-C3N4 | In situ deposition and oxidation | Photocatalytic degradation of 2,3-dihydroxynaphthalene | 100 | 87 | (158) | |
| 22 | Bi2S3/Bi2O2CO3 | A simple chemical route | Degradation of antibiotics and organic compounds | Sunlight | 61 | 89 | (121) |
| 23 | ZnIn2S4/BiVO4 | Hydrothermal method | Photocatalytic mineralization of antibiotics | 180 | (60) | (159) |
Aromatic Compounds Degradation
Aromatic chemicals are the most common and enduring contaminants in the environment.160 The degradation of aromatic compounds is a crucial process in environmental remediation, particularly water. These compounds are commonly found in industrial wastewater and can have harmful effects on both human health and the ecosystem. To address this issue, the widely reported methods is biological degradation methods.160−162 Recently researchers have been investigating various photocatalytic methods for the degradation of aromatic compounds.163,164 Particularly, BBZSH photocatalysts have shown promising results in the degradation of aromatic compounds. Huang et al. synthesized BiOI/Bi2WO6 layered heterojunction using hydrothermal method for degradation of phenol. The construction of Z- scheme heterojunction photocatalysts has significantly improved the photocatalytic ability of degrading phenol.164 Lan et al. synthesized heterojunction photocatalysts in the Z-scheme Bi@β-Bi2O3/g-C3N4 using an in situ deposition and oxidation method for the degradation of 2,3-dihydroxynaphthalene. The results of the study show that a heterojunction photocatalyst of the Z scheme was degraded with a removal ratio of 870% after 100 min of irradiation.158 The efficient charge separation and enhanced photocatalytic activity of Z- scheme heterojunctions enable the effective degradation of these compounds into harmless byproducts. Furthermore, in the recent year a Z-scheme heterojunction of BiOBr/ZIF-8/ZnO photocatalyst was prepared for the removal of phenols in wastewater. The result demonstrates that the ideal BiOBr/ZIF-8/ZnO photocatalysts were shown to have the degradation efficiency of 90% of the removal of phenol or bisphenol in 2 h, with kinetic constants that were 3.8 and 2.3 times higher, respectively, than those of pure BiOBr.165
Endocrine-Disrupting Compound Degradation
It is believed that the endocrine system in the body is regulated by hormones through the endocrine system. Endocrine disrupting chemicals (EDCs) are synthetic or natural chemicals that can mimic, inhibit, or otherwise affect hormones. These chemicals are linked to numerous health problems.166 Some sources of EDCs such as BPA, phythoetogens, pesticides, phthalates, dioxins, etc., are commonly found in water sources and can have detrimental effects on aquatic ecosystems and human health.167 Bi-based heterojunction photocatalysts can help in the degradation of EDCs, breaking down their complex molecular structures and reducing their concentrations in contaminated water. For instance, a combined hydrothermal-calcination method was used to synthesize Z-scheme heterojunctions of P-doped g-3N4/α-Bi2O3 nanocomposites for degrading refractory endocrine disruptors-benzophenones.168 According to these studies, 25 wt % P-g-C3N4/α-Bi2O3 nanocomposite demonstrated superior degradation capability for 4-hydroxybenzophenone (4H-BP) and benzophenone-1. In a study by Li et al. they prepared an AgI/BiOBr heterojunction using a chemical bath deposition method. The photocatalytic performance of synthesized photocatalysts was checked with the degradation of 17α-ethinylestradiol (EE2). 97.0% of the compound EE2 was eliminated within 9 min following irradiation. Compared to pure photocatalysts, AgI/BiOBr heterojunctions have 16.0 and 138.7 times more rate constant than pure AgI and BiOBr, respectively.169 Bi/γ-Bi2O3/O-doped g-C3N4 heterojunction photocatalysts was synthesized using solvothermal method for degradation of bisphenol A (BPA). It has been shown that Bi/γ-Bi2O3/EtCN heterojunctions are more active in photocatalytic degradation of bisphenol A (BPA) than g-C3N4 and ethanol assisted carbon nitride (EtCN), with a degradation rate 15.67 times higher than g-C3N4. The catalyst exhibits a broad light absorption range due to the all-solid heterojunction formed between Bi2O3 and EtCN with Bi acting as an electron shuttle as shown in Figure 8.170
Figure 8.
Schematic diagram of Z-scheme heterojunction of Bi/γ-Bi2O3/EtCN and its BPA degradation rate. Adapted in part with permission from ref (170) .Copyright 2022 Elsevier.
Volatile Organic Compound (VOC) Degradation
Chemicals known as volatile organic compounds (VOCs) are frequently released from the burning of fuels, industrial production of waste gases, and the use of decorative paints and photochemical pollution.171−173 Among VOs, toluene threatens the ecosystem and human life. Hence, several methods such as adsorption, biofiltration, and thermal catalysis have been used for removal of toluene from environment.171 Nevertheless, those approaches have drawbacks, such as low efficiency and large energy requirements. Therefore, recent research indicates that photocatalytic oxidation is an effective method for removing VOCs from the environment. The Z-scheme heterojunction photocatalysts based on Bi have demonstrated potential for degrading VOCs via photocatalytic oxidation. For example, Shi et al. synthesized a ternary Z-scheme heterojunction BiVO4/BiPO4/g-C3N4 photocatalyst and high-specific surface area semicoke activated carbon (SAC) for degradation of volatile organic compounds. The surface area of the prepared semicoke activated carbon is 619.27 m2 g1. Under simulated sunshine irradiation, the composite of BiVO4/BiPO4/g-C3N4/SAC degrades toluene by 85.6% in 130 min, which is 2.43 times that of pure photocatalyst.173 Recently, Xu et al. prepared Bi2O3 quantum dots deco- rated TiO2/BiOBr dual Z-scheme heterojunction photocatalysts for degradation of gaseous toluene.172 These studies result shows that, the presences of Bi2O3 quantum dots and BiOBr in the photocatalysts were regulated and the optimized removal rate for gaseous toluene. In a recent study reported by Guo et al. heterojunction of Bi2WO6/black-TiO2 photocatalysts for degradation of toluene was synthesized by using a combination of hydrothermal and sol–gel methods. The heterojunction of Bi2WO6/black-TiO2 photocatalysts exhibits the best photocatalytic activity of converting 90% of toluene after 120 min, which is twice that of B-TiO2 and 55 times that of Bi2WO6.174 The mechanism of toluene degradation is shown in Figure 9. Recently Jia et al. synthesized a direct Z-scheme of WO3/Bi2WO6 using a simple hydrothermal process for degradation of toluene.171 The studies result demonstrates that, the highest rate of photocatalytic toluene conversion (92%) in 60 min, whereas the pure WO3 (45%) and Bi2WO6 (63%).
Figure 9.
Mechanism of toluene degradation using the Z-scheme heterojunction of Bi2WO6/black-TiO2 photocatalysts. Adapted in part with permission from ref (174). Copyright 2023 Elsevier.
Conclusion, Challenges, and Future Prospective
Over the last three decades, attempts have been made to tackle the serious planetary problems using different techniques such as biodegradation, advanced oxidation processes (AOPs) (e.g., catalysis, photo-Fenton oxidation, Fenton-like oxidation, and electrochemical oxidation process adsorption), filtration, and adsorption methods. However, most of these methods are expensive. This remains because of subtle hazardous pollutants. However, photocatalysis- based water treatment is considered to be a green technology because it is cost-effective and environmentally friendly. Owing to their unique layered structure and the internal electric fields between the layers, chemical stability, availability, and affordability, BBSc photocatalysts have received attention in photocatalytic applications for water treatment. Furthermore, compared to other metal oxide photocatalysts, BBSc has better photocatalytic efficiency owing to the hybrid orbitals in the valence band and the absorption of broad visible-light radiation. BBSc photocatalysts can be prepared using chemicals such as solvothermal/hydrothermal, coprecipitation, chemical bath deposition, solution combustion, etc., physical methods include ball milling, physical vapor deposition, and green synthesis methods.
While pure BBSc photocatalysts are more efficient than other metal oxide photocatalysts, they have some limitations, including a narrow visible light absorption range, high recombination rate, and low redox capacity, all of which affect their catalytic activity. Researchers have recently been working extensively to improve BBSc photocatalysts using various methods. In particular, the construction of heterojunctions and elemental doping have been widely utilized to overcome their drawbacks. Based on the VB and CB edge locations of the two coupled semiconductors, three types of heterojunctions can be constructed (type I, type II, and type III). There are three charge transfer mechanism are sensitization, Type-II and Z-scheme. Sensitization involves using a sensitizer to extend the light absorption range of a photocatalyst. Type II heterojunctions have staggered energy band alignments, enabling efficient charge separation and transfer. Z-scheme heterojunctions involve the transfer of electrons and holes between two photocatalysts, facilitating a continuous redox cycle and enhancing photocatalytic efficiency.
Generally speaking, the photocatalytic degradation mechanism involves the absorption of light, charge separation, transfer of electrons and holes between photocatalysts, and sub- sequent redox reactions. The Z-scheme design enhances charge separation and minimizes recombination, resulting in improved photocatalytic degradation efficiency. It is crucial to note that the efficiency of photoinduced charge transfer in heterojunction photocatalysts depends on various factors, including the energy band alignment, morphology, surface area, and interfacial charge transfer kinetics. Proper selection and design of the heterojunction materials are crucial for optimizing the charge transfer process and maximizing photocatalytic activity. The charge transfer mechanism in the Z-scheme and S-scheme is similar to that in the type II heterojunction, but the main difference is the way electron–hole formation. In a Z-scheme heterojunction, an electron hole is created by the sequential excitation of two coupled photocatalysts. The S-scheme heterojunction was created by simultaneous excitation of the two photocatalysts. Additionally, charge transfer occurs because of the Fermi energy difference in the S-scheme heterojunction, which facilitates more charge separation during the catalysis process. Despite their great potential for removing pollutants and antibiotics from contaminated water, Z-scheme heterojunction photocatalysts also have some challenges.175
One of the key challenges is selecting suitable materials for both the electron donor and acceptor in the Z-scheme photocatalysts. Finding materials with compatible band gaps and suitable band alignment is critical but can be a complex task.176
The separation and transfer of charge carriers across the heterojunction interfaces must be fast and efficient to minimize recombination losses. Designing the heterojunction architecture and optimizing the surface area and crystallinity of the materials can help improve charge transfer.
Ensuring the long-term stability and durability of Z-scheme heterojunction photocatalysts. Many photocatalysts may experience degradation or structural changes over time due to factors like photocorrosion, harsh reaction conditions, or surface passivation. Developing catalysts with high resistance to photocorrosion and maintaining the stability of the junctions under operational conditions are other challenges.
Achieving high-performance photocatalysts at a large scale and reasonable cost is a challenge. Some materials used in Z-scheme photocatalysts, such as noble metals or rare earth elements, can be expensive or scarce. Therefore, developing cost-effective and abundant alternatives is crucial for their widespread implementation.
Understanding the reaction kinetics and optimizing parameters to enhance selectivity toward the desired products is an ongoing challenge. This involves detailed characterization techniques, computational modeling, and optimization of reaction conditions.
To tackle these difficulties, it is necessary to bring together experts from different fields such as materials science, chemistry, physics, and engineering. If these obstacles can be overcome, Z-scheme heterojunction photocatalysts can be more efficient, reliable, and cost-effective, making them suitable for use in the production of clean energy and the remediation of the environment. The outlook for Z-scheme heterojunction photocatalysts is very encouraging, as they provide several benefits and potential for further growth and utilization.
Future research efforts will likely focus on developing new S-scheme heterojunction systems with optimized band alignments and improved interfacial charge transfer to achieve higher photocatalytic activity.
Further exploration of S-scheme heterojunction systems in different photocatalytic applications such as solar energy conversion, environmental remediation, water splitting, pollutant degradation, CO2 reduction, and sustainable chemistry. Researchers can explore diverse material combinations to tailor the band alignments and optimize the charge transfer pathways. This opens up opportunities for discovering new materials and designing high-performance S-scheme heterojunction photocatalysts.
Exploring the synergistic effects between the S-scheme configuration and cocatalysts or cocatalytic reactions can lead to improved catalytic performance and increased reaction selectivity. Integrating cocatalysts or cocatalytic reactions with S-scheme heterojunctions offers a versatile approach to enhance their catalytic performance. By promoting charge separation, facilitating surface reactions, and creating synergistic effects, cocatalysts effectively improve the overall efficiency and effectiveness of S-scheme heterojunction photocatalysts.116
Future research efforts will likely focus on deepening our understanding of the charge transfer dynamics, interfacial phenomena, and reaction kinetics involved in S-scheme systems. This fundamental knowledge will guide the rational design and optimization of S-scheme heterojunctions for enhanced photocatalytic activity.
- Future research will address the development of scalable and economically viable syn- thesis methods, as well as the integration of S-scheme heterojunction photocatalysts into practical devices and systems. The scalability and economic viability of synthesis methods for S-scheme heterojunction photocatalysts can be improved in several ways such as the following:109
- To reduce time, energy, and resource consumption, identify critical reaction parameters, such as temperature, pressure, and catalyst concentration, and optimizing them for improved efficiency and scalability.
- To produce high yield catalysts, choose synthesis methods that yield high product yields, which is crucial for economic viability. Designing environmentally friendly synthesis methods is essential for sustainability and cost-effectiveness.
- Exploring cost-effective and readily available precursors and raw materials can significantly impact the economic viability of the synthesis methods.
- Another strategy is collaboration between researchers and industrial partners, as well as the sharing of knowledge and best practices, which can accelerate the development of scalable and economically viable synthesis methods.
- Future research will address the development of scalable and economically viable synthesis methods, as well as the integration of S-scheme heterojunction photocatalysts into practical devices and systems. The practical implementation of S-scheme heterojunction photocatalysts in real-world devices and systems presents both challenges and opportunities.177
- Some challenges are (i) scalability, (ii) stability and durability, (iii) cost-effectiveness, and (iv) reactant selectivity and specificity.
- Some potential opportunities are (i) enhanced photocatalytic activity, (ii) extended light absorption range, and (iii) stability and durability compared to single component photocatalysts
Generally, to realize these opportunities in practical applications, further research and development are required to optimize the synthesis methods, enhance the stability, and improve the scalability of S-scheme heterojunction photocatalysts.
In conclusion, the future of S-scheme heterojunction photocatalysts holds considerable potential for advancing the field of photocatalysis. Their enhanced photocatalytic activity, a broader range of applications, material combinations, integration of cocatalysts, and fundamental understanding will drive further developments and pave the way for their practical implementation in various fields. Integrating cocatalysts or cocatalytic reactions with S-scheme heterojunctions helps in promoting charge separation, facilitating surface reactions, and creating synergistic effects. Continued research and exploration in this area will undoubtedly contribute to the advancement of sustainable and clean energy technologies.
Author Contributions
The following writers certify their contributions to the paper: Tadesse Lemma Wakjira: Conception, Draft manuscript writing, and Compilation of the final version. Abebe Belay Gemta: Conception, editing, and approval of the manuscript. Gashaw Beyene Kassahun: Conception and Editing. Dinsefa Mensur Andoshe: Formatting and Editing. Kumneger Tadele: Formatting and Editing.
The authors declare no competing financial interest.
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