BiVO4 As a Sustainable and Emerging Photocatalyst: Synthesis Methodologies, Engineering Properties, and Its Volatile Organic Compounds Degradation Efficiency

Bismuth vanadate (BiVO4) is one of the best bismuth-based semiconducting materials because of its narrow band gap energy, good visible light absorption, unique physical and chemical characteristics, and non-toxic nature. In addition, BiVO4 with different morphologies has been synthesized and exhibited excellent visible light photocatalytic efficiency in the degradation of various organic pollutants, including volatile organic compounds (VOCs). Nevertheless, the commercial scale utilization of BiVO4 is significantly limited because of the poor separation (faster recombination rate) and transport ability of photogenerated electron–hole pairs. So, engineering/modifications of BiVO4 materials are performed to enhance their structural, electronic, and morphological properties. Thus, this review article aims to provide a critical overview of advanced oxidation processes (AOPs), various semiconducting nanomaterials, BiVO4 synthesis methodologies, engineering of BiVO4 properties through making binary and ternary nanocomposites, and coupling with metals/non-metals and metal nanoparticles and the development of Z-scheme type nanocomposites, etc., and their visible light photocatalytic efficiency in VOCs degradation. In addition, future challenges and the way forward for improving the commercial-scale application of BiVO4-based semiconducting nanomaterials are also discussed. Thus, we hope that this review is a valuable resource for designing BiVO4-based nanocomposites with superior visible-light-driven photocatalytic efficiency in VOCs degradation.


Introduction
In the 21st century, environmental protection and remediation are the greatest challenges for human beings due to massive population increases and the growth of industrialization [1]. Our natural water has been significantly damaged and continues to deteriorate due to human activity and the growth of chemical, agriculture, and pharmaceutical industries. Textile industries, in particular, discharge annually 15% (one thousand tons) of hazardous dyes as effluents [2]. Along with these, a vast amount of toxic and harmful ments in BiVO 4 -based materials and their potential visible light photocatalytic applications for VOCs degradation. Specifically, the synthesis methodologies of BiVO 4 -based materials, the engineering of BiVO 4 to achieve changes in its structural and electronic properties, and the correlation of these properties with improvement in visible light photocatalytic activity are also discussed. Finally, further prospects for BiVO 4 -based materials are presented, which may provide a better understanding and encourage the field-scale application of BiVO 4 -based materials for VOCs degradation.

Advanced Oxidation Process and Semiconductor Photocatalysis
Advanced oxidation processes (AOPs) are amongst the widely accepted eco-friendly processes for the treatment of different wastewaters. They involve the in-situ generation of hydroxyl ( • OH) and sulphate (SO 4 • ) radicals, which are strong oxidants for the oxidation of various types of toxic organic pollutants. Among these, the hydroxyl ( • OH) radical is the most efficient species in AOPs. Some AOPs, including photolysis (UV) and photochemical (UV/H 2 O 2 , UV/O 3 ) reactions, the Fenton reaction (Fe 2+ /H 2 O 2 ), photo-Fenton reactions (light/Fe 2+ /H 2 O 2 ), cavitation (ultrasonic irradiation), and electrochemical and photocatalysis have been utilized effectively for the oxidation of pollutants [44,45]. Among these, photocatalytic processes have been effectively utilized in a series of oxidation and reduction reactions on the surface of semiconductor materials in the presence of light irradiation. A Web of Science bibliometrics ( Figure 1) analysis showed that photocatalytic oxidation was an efficient process among AOPs for the degradation of hazardous pollutants, especially volatile organic compounds (VOCs). different modifications to BiVO4, such as metal and non-metal loading, the control of morphologies, and the formation of heterojunctions with bismuth and non-bismuth-based oxides, have been performed, which has led to various reviews and research articles on the degradation of water pollutants. However, review articles on BiVO4-based materials for VOCs degradation are rarely reported. Therefore, the present review covers recent developments in BiVO4-based materials and their potential visible light photocatalytic applications for VOCs degradation. Specifically, the synthesis methodologies of BiVO4-based materials, the engineering of BiVO4 to achieve changes in its structural and electronic properties, and the correlation of these properties with improvement in visible light photocatalytic activity are also discussed. Finally, further prospects for BiVO4-based materials are presented, which may provide a better understanding and encourage the field-scale application of BiVO4-based materials for VOCs degradation.

Advanced Oxidation Process and Semiconductor Photocatalysis
Advanced oxidation processes (AOPs) are amongst the widely accepted eco-friendly processes for the treatment of different wastewaters. They involve the in-situ generation of hydroxyl ( • OH) and sulphate (SO4 • ) radicals, which are strong oxidants for the oxidation of various types of toxic organic pollutants. Among these, the hydroxyl ( • OH) radical is the most efficient species in AOPs. Some AOPs, including photolysis (UV) and photochemical (UV/H2O2, UV/O3) reactions, the Fenton reaction (Fe 2+ /H2O2), photo-Fenton reactions (light/Fe 2+ /H2O2), cavitation (ultrasonic irradiation), and electrochemical and photocatalysis have been utilized effectively for the oxidation of pollutants [44,45]. Among these, photocatalytic processes have been effectively utilized in a series of oxidation and reduction reactions on the surface of semiconductor materials in the presence of light irradiation. A Web of Science bibliometrics ( Figure 1) analysis showed that photocatalytic oxidation was an efficient process among AOPs for the degradation of hazardous pollutants, especially volatile organic compounds (VOCs). When the semiconductor absorbs the photon at not less than the band gap energy of the semiconductor (Eg), the electrons (e − ) from the valence band (VB) are excited towards the conduction band (CB), and holes (h + ) are left behind in the VB. The photogenerated electrons and holes pairs move into the surface of the semiconductor where they react with surface-adsorbed water or hydroxyl ( − OH) groups or dissolved oxygen in the When the semiconductor absorbs the photon at not less than the band gap energy of the semiconductor (E g ), the electrons (e − ) from the valence band (VB) are excited towards the conduction band (CB), and holes (h + ) are left behind in the VB. The photogenerated electrons and holes pairs move into the surface of the semiconductor where they react with surface-adsorbed water or hydroxyl ( − OH) groups or dissolved oxygen in the reaction medium and produce reactive radical species, i.e., superoxide radical anions (O 2 •− ) and • OH radicals. The reactive radical species undergo redox reactions with surface-adsorbed pollutant molecules and completely degrade them. Furthermore, the presence of holes in the VB directly oxidizes the surface-adsorbed pollutants and electrons in the CB, contributing to the indirect oxidation of pollutants by • OH radicals generated through the photo-splitting of hydrogen peroxide (H 2 O 2 ) formed in situ. The corresponding steps involved in the photocatalytic oxidation process using semiconductor materials, and their schematic representation, are shown in Equations (1)- (12) and Figure 2. Semiconductor + hυ (λ ≥ band gap energy) → Semiconductor (h + ) + Semiconductor (e − ) (1) h + + H 2 O → Semiconductor + H + + • OH (2) Semiconductor (h + ) + -OH → Semiconductor + • OH Semiconductor (e − ) + O 2 → Semiconductor + O 2  Initially, various metal oxides (TiO2, ZnO, WO3, etc.) and metal sulfide-based (e.g., ZnS, CdS, etc.) semiconductor materials were utilized as catalysts for the photocatalytic oxidation of pollutants, as shown in Table 1. TiO2 is the best among the semiconductor materials as it has the characteristics of chemical and biological stability, low toxicity, high Initially, various metal oxides (TiO 2 , ZnO, WO 3, etc.) and metal sulfide-based (e.g., ZnS, CdS, etc.) semiconductor materials were utilized as catalysts for the photocatalytic oxidation of pollutants, as shown in Table 1. TiO 2 is the best among the semiconductor materials as it has the characteristics of chemical and biological stability, low toxicity, high durability, resistance to photocorrosion, high photocatalytic activity, and low cost. Although TiO 2 has these merits, it has some disadvantages, such as high bandgap energy (~3.2 eV), poor adsorption capacity, low surface area, and a high recombination rate of photogenerated charge carriers that limit the practical applicability of TiO 2 materials. Furthermore, the high bandgap energy restricts its usage under simulated or natural solar light irradiation, and the high recombination rate of charge carriers reduces the photocatalytic degradation efficiency. Approaches such as metal or non-metal doping, coupling with high surface area adsorbents (e.g., activated carbon, graphene oxide, etc.) and other semiconductors (e.g., WO 3 , g-C 3 N 4 , etc.), reduction in size of the semiconductor, changes to the morphology and dye sensitization, etc., have been used to enhance the visible light response and the lifetime of photogenerated charge carriers, consequently improving the photocatalytic degradation efficiency of TiO 2 materials. Nevertheless, modified TiO 2 materials are limited in their industrial application because of the poor stability (leaching or natural degradation) and reusability of the materials, the high cost of dopants, harmful modifying agents, and poor re-production/re-synthesis of material with similar properties. In order to overcome this, various non-TiO 2 -based visible-light-responsive semiconductor materials have been developed for the photocatalytic oxidation of various pollutants, including VOCs. Among non-TiO 2 semiconducting materials, bismuth-based semiconducting materials [BiVO 4 , Bi 2 WO 6 , Bi 2 MoO 6 , BiOX (X-Cl, Br, I), etc.,] have received much attention because of their narrow band gap energy, excellent visible light absorption, considerable chemical and thermal stability, and suitable band potentials for the generation of reactive radical species [46,47]. BiVO 4 is one of the best bismuth-based semiconducting materials due to its low band gap energy (E g = 2.3-2.4 eV) and high visible light absorption. The discovery of BiVO 4 by Kudo et al. [43] for O 2 evolution under visible light irradiation has significantly influenced the development of BiVO 4 -based materials for various applications, including VOCs degradation. Nevertheless, the faster rate of photogenerated electron-hole pair recombination, poor charge carrier transport ability, and the water oxidation kinetics of BiVO 4 limit its industrial application. A large number of modifications have been performed to improve the photocatalytic performance of BiVO 4 so that commercial requirements would be fulfilled. These are described in the forthcoming sections.

Fundamental Aspects of BiVO 4 Photocatalyst
BiVO 4 is an n-type semiconductor and has been identified as one of the most efficient visible-light-responsive photocatalysts with a band gap energy of 2.4 eV. Naturally, BiVO 4 occurs as the pucherite mineral with an orthorhombic crystal structure. However, laboratory-prepared BiVO 4 crystallizes either in a scheelite or zircon-type structure. Furthermore, the scheelite structure has a monoclinic and tetragonal crystal system, and the zircon-type structure has a tetragonal crystal system [48]. The crystal structures are shown in Figure 3. als [BiVO4, Bi2WO6, Bi2MoO6, BiOX (X-Cl, Br, I), etc.,] have received much attention because of their narrow band gap energy, excellent visible light absorption, considerable chemical and thermal stability, and suitable band potentials for the generation of reactive radical species [46,47]. BiVO4 is one of the best bismuth-based semiconducting materials due to its low band gap energy (Eg = 2.3-2.4 eV) and high visible light absorption. The discovery of BiVO4 by Kudo et al. [43] for O2 evolution under visible light irradiation has significantly influenced the development of BiVO4-based materials for various applications, including VOCs degradation. Nevertheless, the faster rate of photogenerated electron-hole pair recombination, poor charge carrier transport ability, and the water oxidation kinetics of BiVO4 limit its industrial application. A large number of modifications have been performed to improve the photocatalytic performance of BiVO4 so that commercial requirements would be fulfilled. These are described in the forthcoming sections.

Fundamental Aspects of BiVO4 Photocatalyst
BiVO4 is an n-type semiconductor and has been identified as one of the most efficient visible-light-responsive photocatalysts with a band gap energy of 2.4 eV. Naturally, BiVO4 occurs as the pucherite mineral with an orthorhombic crystal structure. However, laboratory-prepared BiVO4 crystallizes either in a scheelite or zircon-type structure. Furthermore, the scheelite structure has a monoclinic and tetragonal crystal system, and the zircon-type structure has a tetragonal crystal system [48]. The crystal structures are shown in Figure 3.  Similarly, the band structures of scheelite and zircon-type BiVO 4 are shown in Figure 4. In zircon-type BiVO 4 , the valence and conduction bands are comprised of O 2p and V 3d orbitals, whereas in the case of scheelite-type BiVO 4 , the valence band consists of Bi 6s and O 2p orbitals and the conduction band consists of a V 3d orbital. Therefore, the monoclinic (m-BiVO 4 ) scheelite-type system has a relatively smaller band gap energy (2.4 eV) than the tetragonal zircon-type system (2.9 eV); therefore, it shows high visible-light-driven activity [50].
Nanomaterials 2023, 13, x FOR PEER REVIEW 7 Similarly, the band structures of scheelite and zircon-type BiVO4 are shown in F 4. In zircon-type BiVO4, the valence and conduction bands are comprised of O 2p a 3d orbitals, whereas in the case of scheelite-type BiVO4, the valence band consists of and O 2p orbitals and the conduction band consists of a V 3d orbital. Therefore, the oclinic (m-BiVO4) scheelite-type system has a relatively smaller band gap energy (2. than the tetragonal zircon-type system (2.9 eV); therefore, it shows high visible-l driven activity [50].  [43]; 1999, copyright from A can Chemical Society).
However, the utilization of BiVO4 for catalytic activity is not impressive becau suffers from a high recombination rate of electron-hole pairs and poor charge tran properties. Therefore, the modification of BiVO4 materials, such as by doping of B with metals and non-metals, to control its morphology and the synthesis of comp However, the utilization of BiVO 4 for catalytic activity is not impressive because it suffers from a high recombination rate of electron-hole pairs and poor charge transport properties. Therefore, the modification of BiVO 4 materials, such as by doping of BiVO 4 with metals and non-metals, to control its morphology and the synthesis of composite BiVO 4 (heterojunction, S-scheme, and Z-scheme) materials has been performed to overcome the abovementioned shortcomings, which are discussed in forthcoming sections.

Synthesis Methodologies of BiVO 4 Photocatalyst
It is clear that the crystallinity, particle size, and shape of photocatalysts have a significant impact on the photocatalytic activity of a catalyst. Furthermore, it is well known that photocatalytic processes take place on the photocatalyst's surface. A substantial increase in a photocatalyst's surface-to-volume ratio will increase its specific surface area, increasing the number of active sites that are available for photocatalytic reactions [50,51]. The morphology and particle size are directly proportional to the greater surface recombination of photogenerated charge carriers. The photocatalyst produces electrons and holes in the diffusion time; the diffusion time of the photocatalyst from the bulk to the surface is represented by Equation (13).
where r is the grain radius and D is the diffusion coefficient of the charge carrier. Therefore, when the grain radius decreases, a large number of electrons and holes will travel to the surface for photocatalytic reaction. The synthesis of small and uniform particle sizes plays a vital role in enhancing the photocatalytic activity of semiconducting materials [49][50][51].
The various synthesis methodologies of BiVO 4 with different morphologies and their photocatalytic activity and degradation efficiency are summarized in Table 2.

Hydrothermal Method
Kamble and Ling synthesized truncated square, 18-sided morphological BiVO 4 nanomaterials using the hydrothermal method. Figure 5a-d show the different sizes and shapes of m-BiVO 4 nanoparticles (NPs). Figure 5c shows the truncated square (18-sided) hexagonal bipyramidal shape with exposed {040} facets exhibiting a strongly revealed surface phenomenon and a facet effect for the visible-light-driven photocatalytic degradation of MB dye [42]. nomaterials using the hydrothermal method. Figure 5a-d show the differen shapes of m-BiVO4 nanoparticles (NPs). Figure 5c shows the truncated squar hexagonal bipyramidal shape with exposed {040} facets exhibiting a strongly re face phenomenon and a facet effect for the visible-light-driven photocatalytic d of MB dye [42].  [42]; copyright from Na Sun et al. [51] synthesized a BiVO4 nanoplate-stacked star morphologic via a hydrothermal method using ethylenediamine tetraacetic acid (EDTA). ratio of EDTA to Bi 3+ played an important role in the star morphology of BiVO like BiVO4 structure showed a higher efficiency for the photodegradation of M under visible light irradiation. The dendritic structure of BiVO4 was fabricate al. [52] using an additive-free hydrothermal method at various hydrotherm tures, including 100 °C, 140 °C, and 180 °C ( Figure 6). The synthesized dendrit Sun et al. [51] synthesized a BiVO 4 nanoplate-stacked star morphological structure via a hydrothermal method using ethylenediamine tetraacetic acid (EDTA). The molar ratio of EDTA to Bi 3+ played an important role in the star morphology of BiVO 4 . The star-like BiVO 4 structure showed a higher efficiency for the photodegradation of MB in 25 min under visible light irradiation. The dendritic structure of BiVO 4 was fabricated by Lei et al. [52] using an additive-free hydrothermal method at various hydrothermal temperatures, including 100 • C, 140 • C, and 180 • C ( Figure 6). The synthesized dendritic structure of BiVO 4 was used as a photocatalyst for the degradation of RhB dye. The dendritic BiVO 4 synthesized at 140 • C (99.3%) showed superior photocatalytic activity to material synthesized at 100 (58.3%) and 180 • C. The higher activity may be attributed to the high surface area (2.1 m2/g) and crystallinity compared to the other BiVO 4 samples.
An olive-like BiVO 4 hierarchical morphology via a template-free hydrothermal method was synthesized by Wang et al. [53]. The photocatalytic activities of olive-like BiVO 4 was estimated against MB under visible light irradiation. The BiVO 4 hierarchical morphology was prepared at different pH values. At a pH of 2.05, the BiVO 4 appeared as an olivelike structure but steadily changed into a spherical structure when the pH value varied from 2.05 to 4.02, and at pH 6.00, it adopted a cuboid structure. The authors reported that the olive-like BiVO 4 resulted in ∼95.7% degradation of MB within 1 h under visible light conditions. S. Obregon et al. prepared BiVO 4 hierarchical heterostructures using a surfactant-free hydrothermal method [54]. The m-BiVO 4 prepared at pH 9 showed needle-like morphology with {110} and {002} planes. The synthesized m-BiVO 4 showed good photoactivities for the degradation of methylene blue (MB) under UV-Vis irradiation.
BiVO4 was estimated against MB under visible light irradiation. The BiVO4 hierarchica morphology was prepared at different pH values. At a pH of 2.05, the BiVO4 appeared a an olive-like structure but steadily changed into a spherical structure when the pH valu varied from 2.05 to 4.02, and at pH 6.00, it adopted a cuboid structure. The authors re ported that the olive-like BiVO4 resulted in ∼95.7% degradation of MB within 1 h unde visible light conditions. S. Obregon et al. prepared BiVO4 hierarchical heterostructures using a surfactant-fre hydrothermal method [54]. The m-BiVO4 prepared at pH 9 showed needle-lik Lu et al. [55] fabricated core-shell-structured (CSS) BiVO 4 via a surfactant-and template-free hydrothermal method using bismuth nitrate/ammonium vanadate/ethanol/ acetic acid as precursors. They also synthesized different morphologies using PVP and CTAB surfactants. Figure 7a-f show that the shell of the BiVO 4 hollow spheres became thinner as the hydrothermal reaction time increased. The BiVO 4 plate morphology and biscuit morphology showed ∼88% degradation of RhB at a very slow rate, while the BiVO 4 biscuit morphology with a core shell structure showed ∼99% degradation of RhB in 4.5 h.
Lu et al. [55] fabricated core-shell-structured (CSS) BiVO4 via a surfactant-and template-free hydrothermal method using bismuth nitrate/ammonium vanadate/ethanol/acetic acid as precursors. They also synthesized different morphologies using PVP and CTAB surfactants. Figure 7a-f show that the shell of the BiVO4 hollow spheres became thinner as the hydrothermal reaction time increased. The BiVO4 plate morphology and biscuit morphology showed ∼88% degradation of RhB at a very slow rate, while the BiVO4 biscuit morphology with a core shell structure showed ∼99% degradation of RhB in 4.5 h. Meng et al. [56] fabricated various nanoparticles with polyhedral, rod-like, tubular, leaf-like, and spherical morphological structures using a hydrothermal method in the presence of triblock copolymer P123 as a surfactant ( Figure 8). Their photocatalytic activities were estimated towards the decomposition of MB dye under visible light stimulation. At different pH = 1, 6, 9, or 10 conditions, the 2D nanoentities were annealed at 400 °C and showed the abovementioned different morphologies. Among the various BiVO4 morphologies, those synthesized hydrothermally with P123 at pH 6 or 10 showed excellent photocatalytic activity due to their greater surface areas and high concentrations of surface oxygen defects. Two-dimensional BiVO4 single-crystal nanosheets were prepared by Zhang Meng et al. [56] fabricated various nanoparticles with polyhedral, rod-like, tubular, leaf-like, and spherical morphological structures using a hydrothermal method in the presence of triblock copolymer P123 as a surfactant ( Figure 8). Their photocatalytic activities were estimated towards the decomposition of MB dye under visible light stimulation. At different pH = 1, 6, 9, or 10 conditions, the 2D nanoentities were annealed at 400 • C and showed the abovementioned different morphologies. Among the various BiVO 4 morphologies, those synthesized hydrothermally with P123 at pH 6 or 10 showed excellent photocatalytic activity due to their greater surface areas and high concentrations of surface oxygen defects. Two-dimensional BiVO 4 single-crystal nanosheets were prepared by Zhang et al. [57] with thicknesses of ∼10-40 nm via a hydrothermal route using sodium dodecyl benzene sulfonate (SDBS) as an anionic surfactant. SDBS formed micelles in aqueous solution and enabled the synthesis of BiVO 4 nanoparticles with controlled growth. The as-synthesized BiVO 4 nanoparticles were used for the photocatalytic degradation of the RhB dye. et al. [57] with thicknesses of ∼10-40 nm via a hydrothermal route using sodium dodecyl benzene sulfonate (SDBS) as an anionic surfactant. SDBS formed micelles in aqueous solution and enabled the synthesis of BiVO4 nanoparticles with controlled growth. The assynthesized BiVO4 nanoparticles were used for the photocatalytic degradation of the RhB dye. A monoclinic BiVO4/sepiolite nanocomposite was fabricated by H. Naing et al. [58]. The nanocomposite BiVO4/sepiolite exhibited excellent visible light photocatalytic performance against antibiotic tetracyclines (TCs) and methylene blue (MB). In the monoclinic BiVO4-30%-sepiolite, fibrous or needle-like sepiolite was distributed on the peanutshaped monoclinic BiVO4 surface. The photocatalytic efficacies of pure BiVO4, pure sepiolite, and serial monoclinic BiVO4/sepiolite (BVO/S) nanocomposites were studied using the remediation of MB dye and TCs in aqueous solution under visible light irradiation. About 96% of the MB pollutants and 78% of the antibiotic TCs were degraded by BVO-30% S after 4 h of visible light irradiation. A synergistic effect between sepiolite and monoclinic BiVO4 enhanced the separation of the photo-generation carriers, promoting high adsorption, and restrained the regrouping of electron-hole pairs, enhancing photocatalytic activity. Moreover, the hydrophobic nature of sepiolite nanofiber possibly enabled holes generated on the BiVO4/sepiolite nanocomposites to react with pollutants and degrade to smaller molecules.
Chen et al. [59] synthesized snow-like BiVO4 using a cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal method. In the snow-like BiVO4 morphology, oxygen vacancies depended upon the concentration of CTAB. The snow-like BiVO4 morphology coexisted with counter-Br − ions, inducing high-concentration surface oxygen defects, A monoclinic BiVO 4 /sepiolite nanocomposite was fabricated by H. Naing et al. [58]. The nanocomposite BiVO 4 /sepiolite exhibited excellent visible light photocatalytic performance against antibiotic tetracyclines (TCs) and methylene blue (MB). In the monoclinic BiVO 4 -30%-sepiolite, fibrous or needle-like sepiolite was distributed on the peanut-shaped monoclinic BiVO 4 surface. The photocatalytic efficacies of pure BiVO 4 , pure sepiolite, and serial monoclinic BiVO 4 /sepiolite (BVO/S) nanocomposites were studied using the remediation of MB dye and TCs in aqueous solution under visible light irradiation. About 96% of the MB pollutants and 78% of the antibiotic TCs were degraded by BVO-30% S after 4 h of visible light irradiation. A synergistic effect between sepiolite and monoclinic BiVO 4 enhanced the separation of the photo-generation carriers, promoting high adsorption, and restrained the regrouping of electron-hole pairs, enhancing photocatalytic activity. Moreover, the hydrophobic nature of sepiolite nanofiber possibly enabled holes generated on the BiVO 4 /sepiolite nanocomposites to react with pollutants and degrade to smaller molecules.
Chen et al. [59] synthesized snow-like BiVO 4 using a cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal method. In the snow-like BiVO 4 morphology, oxygen vacancies depended upon the concentration of CTAB. The snow-like BiVO 4 morphology coexisted with counter-Br − ions, inducing high-concentration surface oxygen defects, which produced more highly reactive oxygen species (ROS), i.e., superoxide and hydroxyl radicals, which resulted in the superfast degradation of ciprofloxacin (CIP).

Electro-Spinning Method
Various 1D nanostructured BiVO 4 materials, such as nano-fibers [60] and microribbon [61], have been synthesized for photocatalysis applications. Cheng et al. [60] prepared BiVO 4 porous 1D nanofibers by an electro-spinning method using polyvinyl pyrroli-done (PVP)/acetic acid/ethanol/N, N-dimethylformamide/bismuth nitrate/vanadium (IV) oxy acetylacetonate as a precursor, and the photocatalytic efficiency was estimated towards the photodegradation of RhB dye. The authors reported that the 500 • C calcined BiVO 4 sample displayed a higher photocatalytic efficiency for RhB than that for other calcinating temperatures. Liu et al. [61] reported micro-ribbon BiVO 4 of an ∼2-3 µm width for the visible-light-driven photocatalytic degradation of MB dye. They also studied the impact of a calcinating temperature of 500 • C on the morphology of BiVO 4 .

Solvothermal Method
Red blood cell, flower-like microsphere and dendrite BiVO 4 morphologies were prepared by Chen et al. [62] via a facile solvothermal method by adjusting the solution pH and using bismuth nitrate/ammonium vanadate/citric acid/ethylene glycol/ethanol/water as precursors. Figure 9 illustrates the various morphologies and microstructure of the BiVO 4 samples using FE-SEM micrographs. Figure 9a,b show the morphology of BiVO 4 red blood cell (S-BiVO 4 ), which was achieved using Na 2 CO 3 as a pH-controlling agent. The flowerlike microsphere BiVO 4 nanoparticles (A-BiVO 4 ) were obtained using NH 3 ·H 2 O, as shown in Figure 9c,d. Figure 9e shows the BiVO 4 dendrite-like morphology (N-BiVO 4 ), which was achieved without the addition of citric acid and Na 2 CO 3 under similar conditions. The BiVO 4 red blood cell (S-BiVO 4 ) catalyst exhibited greater catalytic activity than the flower or dendritic morphologies of BiVO 4 .
Various 1D nanostructured BiVO4 materials, such as nano-fibers [60] and micro bon [61], have been synthesized for photocatalysis applications. Cheng et al. [60] prep BiVO4 porous 1D nanofibers by an electro-spinning method using polyvinyl pyrroli (PVP)/acetic acid/ethanol/N, N-dimethylformamide/bismuth nitrate/vanadium (IV acetylacetonate as a precursor, and the photocatalytic efficiency was estimated tow the photodegradation of RhB dye. The authors reported that the 500 °C calcined B sample displayed a higher photocatalytic efficiency for RhB than that for other calcin temperatures. Liu et al. [61] reported micro-ribbon BiVO4 of an ∼2-3 μm width fo visible-light-driven photocatalytic degradation of MB dye. They also studied the im of a calcinating temperature of 500 °C on the morphology of BiVO4.

Solvothermal Method
Red blood cell, flower-like microsphere and dendrite BiVO4 morphologies were pared by Chen et al. [62] via a facile solvothermal method by adjusting the solutio and using bismuth nitrate/ammonium vanadate/citric acid/ethylene glycol/ethanol/w as precursors. Figure 9 illustrates the various morphologies and microstructure o BiVO4 samples using FE-SEM micrographs. Figure 9a-b show the morphology of B red blood cell (S-BiVO4), which was achieved using Na2CO3 as a pH-controlling a The flower-like microsphere BiVO4 nanoparticles (A-BiVO4) were obtained u NH3·H 2O, as shown in Figure 9c-d. Figure 9e shows the BiVO4 dendrite-like morpho (N-BiVO4), which was achieved without the addition of citric acid and Na2CO3 under ilar conditions. The BiVO4 red blood cell (S-BiVO4) catalyst exhibited greater catalyt tivity than the flower or dendritic morphologies of BiVO4.

Co-Precipitation Method
Mesoporous monoclinic BiVO 4 photocatalysts with different morphologies were prepared by Suwanchawalit et al. [63]. In the synthesis of m-BiVO 4 , TX100 was used as a surfactant in the co-precipitation method. The TX100 molecules play an important role in the synthesis of BiVO 4 . An m-BiVO 4 structure was prepared by Lai et al. [64] via a precipitation method using a visible light catalyst for the photocatalytic degradation of thiobencarb (TBC). TBC was efficiently degraded by approximately 97% within 5 h. The as-prepared BiVO 4 photocatalyst had a polyhedral morphology with a 6-8 µm edge length ( Figure 10).

Co-Precipitation Method
Mesoporous monoclinic BiVO4 photocatalysts with different morphologie pared by Suwanchawalit et al. [63]. In the synthesis of m-BiVO4, TX100 wa surfactant in the co-precipitation method. The TX100 molecules play an impo the synthesis of BiVO4. An m-BiVO4 structure was prepared by Lai et al. [64] v itation method using a visible light catalyst for the photocatalytic degradation carb (TBC). TBC was efficiently degraded by approximately 97% within 5 h. pared BiVO4 photocatalyst had a polyhedral morphology with a 6-8 μm edge ure 10).

Sol-Gel Method
The sol-gel method is a promising approach for synthesising metal oxid ide composites as this methodology is capable of controlling the morphologi face properties of the materials at the nanoscale [65]. Min et al. synthesized doped BiVO4 photocatalysts using the sol-gel method and utilized them for catalytic degradation of MO dye [66]. Co-doped BiVO4 photocatalysts may ena ergistic effects of lanthanum and boron to separate the photogenerated holes an in BiVO4 composite. Although all synthesized bismuth vanadate composites ical structures, some La-doped composites show a decrease in particle size, and also inhibits particle growth. In this study, due to La and B doping, BiVO4 (L composites showed a higher photocatalytic degradation of MO dye in 60 min and B-BiVO4, and the specific surface area and surfaces for oxygen vacancie enhanced, reducing the crystallite size, and also reducing the band gap and t of absorbed light in the visible region. Mousavi-Kamazani synthesized composite nanostructures of copper oxid muth (Cu/Cu2O/BiVO4/Bi7VO13) using the Pechini sol-gel method [67]. By alte action conditions, different morphological structures were synthesised. The ethylenediamine as a gelling agent, tannic acid as a chelating agent, and a 1:

Sol-Gel Method
The sol-gel method is a promising approach for synthesising metal oxide/mixed oxide composites as this methodology is capable of controlling the morphological and surface properties of the materials at the nanoscale [65]. Min et al. synthesized La-and B-doped BiVO 4 photocatalysts using the sol-gel method and utilized them for the photocatalytic degradation of MO dye [66]. Co-doped BiVO 4 photocatalysts may enable the synergistic effects of lanthanum and boron to separate the photogenerated holes and electrons in BiVO 4 composite. Although all synthesized bismuth vanadate composites have spherical structures, some La-doped composites show a decrease in particle size, and La-doping also inhibits particle growth. In this study, due to La and B doping, BiVO 4 (La-B-BiVO 4 ) composites showed a higher photocatalytic degradation of MO dye in 60 min than BiVO 4 and B-BiVO 4 , and the specific surface area and surfaces for oxygen vacancies were also enhanced, reducing the crystallite size, and also reducing the band gap and the intensity of absorbed light in the visible region.
Mousavi-Kamazani synthesized composite nanostructures of copper oxides and bismuth (Cu/Cu 2 O/BiVO 4 /Bi 7 VO 13 ) using the Pechini sol-gel method [67]. By altering the reaction conditions, different morphological structures were synthesised. The addition of ethylenediamine as a gelling agent, tannic acid as a chelating agent, and a 1:1:1 ratio for Cu:Bi:V enabled a rectangular cube-like morphology to be formed. When the gelling agent changed to polyethene glycol instead of ethylenediamine, plate-like microstructures were formed. By changing the chelating agent from tannic acid to fumaric acid, a pseudo-spherical morphology was obtained. In the absence of Cu and 0.5 mole Cu, platelike nanostructures, nanorods, and quasi-spherical structures were observed, respectively. Moreover, the composite structure (Cu/Cu 2 O/BiVO 4 /Bi 7 VO 13 )) with rectangular cubelike morphology (size of about 30-100 nm) exhibited excellent photocatalytic oxidative desulfurization of the oil derivatives under visible light (92%) than BiVO 4 and Cu 2 V 4 O 11 . The results suggest that the addition of Cu and Cu 2 O species into the composites increases the electrical conductivity, capable of electron-hole separation, and alters the morphology and also particle size, which might be the reason for the enhanced photocatalytic activity.

Engineering/Modification Processes of BiVO 4 Properties
As discussed earlier, BiVO 4 is one of the promising photocatalysts for many practical applications ranging from water treatment, the removal of dyes and organic pollutants, H 2 generation, cargo and biomedical deliveries, etc. [69,70]. The better photocatalytic performance of BiVO 4 is accredited to their visible-light-responsive band gap (2.4 eV), layered structure, suitable valance band maximum, chemical stability, and nontoxicity. BiVO 4 has four different polymorphic forms, including orthorhombic, zircon-tetragonal, monoclinic (m), and tetragonal (t) BiVO 4 [71]. Among them, m-BiVO 4 is more active for photo-related applications. Generally, a low temperature reaction yields zircon-tetragonal BiVO 4 phase, which can be transformed into m-BiVO 4 by inducing calcination reactions and reversibly back to t-BiVO 4 phase at a temperature of 528 • K. The band gaps energies of t-BiVO 4 and m-BiVO 4 were reported to be indirect of 2.3 eV and 2.4 eV, respectively [72]. Previous experimental studies with BiVO 4 have some shortcomings, such as poor charge carrier transfer (bulk carrier mobility of 0.05-0.2 cm 2 V −1 s −1 ) and charge recombination before being captured by targeted molecules for photochemical reactions [73]. To overcome these issues, several strategies, for example, the nano-scaling [74], morphology engineering [75], crystal facet control [76], and crystal structure control [77] of BiVO 4 , have been demonstrated to improve the optical and electronic properties to some extent, resulting in high photocatalytic performances towards organic pollutant degradation.
Many recent studies have also proposed multiple advantages of doping and mixedphase BiVO 4 systems over single-phase BiVO 4 photocatalysts [78][79][80]. This resulted in the extended light absorption capability, carrier mobility, and higher efficiency of BiVO 4 -based systems for photochemical reactions. This is mainly attributed to the effect of donor defects or through adding excess electrons to the BiVO 4 model system through doping with metals (Mo, W, Sn) [69,81,82], nonmetals (S, F), and the creation of heterostructures with other semiconductors. In this section, an overview of different strategies such as (1) metal doping, (2) noble metal doping, (3) nanocomposite structures, (4) composite with carbon analogs, and (5) heterostructures related to BiVO 4 photocatalyst have been briefly discussed.

Metal/Nonmetal-Doped BiVO 4
Metal doping is one of the conventional and effective strategies for modifying the electronic properties of semiconductors, i.e., p-and n-type conductivity behaviors. In semiconductor photocatalysis, doping with metal/nonmetal can improve the charge carrier separation, tune the band gap energy, and enhance visible light absorption. For example, Liu et al. [83] synthesized Mo-doped BiVO 4 using an electrospun method and studied its morphology, crystal structure, and optoelectronic properties. A stoichiometric amount of bismuth and vanadium solutions were prepared, and varied amounts of ammonium molybdate (0.4, 1, 1.5, 3 mol%) were incorporated and stirred (12 h) to obtain homogeneous solution. Then, an electrospinning reaction was performed at a temperature of 60 • C and 15 kV voltages. According to the formation mechanism ( Figure 11A(i)), pure BiVO 4 can form homogenous, well-dispersed particles together with many pores. When a small amount of Mo (1%) was incorporated into BiVO 4 , the particle size increased, and the existing pores disappeared ( Figure 11A(iii-v)). When further increasing the Mo content (3%), the BiVO 4 particle size increased and Mo saturated, forming secondary-phase MoO 3 on the surface of BiVO 4 . As a result, photocatalytic tests revealed that 1% Mo-BiVO 4 shows excellent photoactivity~three times higher than reference BiVO 4 ( Figure 11A(ii)), indicating that a small content of Mo dopant is crucial to improving the separation of charge carriers and electronic conductivity.
On the contrary, a higher Mo content led to crystal phase transformation from monoclinic to tetragonal together with secondary-phase formation, which might act as recombination centers causing a reduction in the photocatalytic activity. A theoretical study by Zhang et al. [84] purports the effects of Mo/W co-doping for the photocatalytic activity of monoclinic BiVO 4 . They found that Mo or W atom doping preferably occurs at the V site to generate continuum states directly above the conduction band (CB) level of BiVO 4 , and this decreases the band gap, which is beneficial for photochemical reactions. Particularly, they found that W-doped BiVO 4 exhibits a smaller band gap than the Mo-doped BiVO 4 , and the electronic properties of BiVO 4 are quite different. Additionally, Mo/W/Mo and W/Mo/W co-doping in BiVO 4 requires low formation energies and reduced bandgaps compared to other doping systems, which may extend the light absorption and could be more suitable for visible-light-driven photocatalysis. Yao et al. [85] fabricated Mo-doped BiVO 4 via a solid-state reaction by grinding stoichiometric amounts of Bi 2 O 3 , V 2 O 5 , and MoO 3 in an agate mortar and heating at 600•C for 5 h, followed by calcination at 800 • C for 2 h. The Mo-doped BiVO 4 showed much more photocatalytic activity for water oxidation and MB dye degradation compared to pure BiVO 4 , due to the higher surface acidity of Mo-doped BiVO 4 (2 atom%) stemmed from the existence of Lewis and Brønsted acidic sites associated with Mo 6+ doping, which offer greater adsorption feasibility for water molecules and organics contaminants. Gao et al. [86] reported the synthesis of Ni-doped BiVO 4 and Z-scheme BiVO 4 -Ni/AgVO 3 nanofibers using a strategy which combined an electrospinning and hydrothermal strategy ( Figure 11B(i)). First, a nanofibrous Ni-doped BiVO 4 was obtained using an electrospinning precursor solution of Bi(NO 3 ) 3 ·5H 2 O, Vo(acac) 2 , and Ni(NO 3 ) 2 ·6H 2 O (prepared in mixed solvent of DMF, CH 3 COOH, and CH 3 CH 2 OH). PVP was used a matrix during the electrospinning process. Next, hydrothermally grown AgVO 3 on the surface of Ni-doped BiVO 4 forms a Z-scheme heterojunction of BiVO 4 -Ni/AgVO 3 . Due to the synergetic effect of Ni doping and AgVO 3 assembly, the specific surface area and light absorption ability of BiVO 4 -Ni/AgVO 3 was significantly improved compared to BiVO 4 . Ni-doping adds impurity energy levels and replaces V sites on the {121} plane ( Figure 11B(ii)), producing the structural distortion of tetrahedral VO 4 3+ , which can be confirmed from a right shift in diffraction peaks [87]. The lowering of the diffraction peak intensity also suggests successful Ni doping, which leads to a reduction in crystallinity. A Z-scheme optimal BiVO 4 -Ni-1/AgVO 3 -25 photocatalyst showed superior photocatalytic Cr 6+ reduction efficiency (99.7%) in 80 min. The comparison of apparent rate constants for Cr 6+ reduction over different photocatalysts is shown in Figure 11B(iii). The inset shows the HRTEM image of BiVO 4 -Ni/AgVO 3 and the presence of three different phases.
Bashir et al. [88] fabricated Gd-doped BiVO 4 using a simple hydrothermal method. The resulting Gd-doped BiVO 4 ultrasonically treated with rGO to form Gd-doped BiVO 4 /rGO of the nanocomposite nanostructures ( Figure 12A(i)). According to SEM analysis, Gd doping does not change the surface morphology. As shown in TEM images ( Figure 12A(ii,iii)), three different phases of components promote significantly improved electron/hole pair separation, excellent photocatalytic MB degradation efficiency (97%) than BiVO 4 (53%), and Gd/BiVO 4 (69%) within 100 min ( Figure 12A(iv)). The higher photoactivity of rGO/Gd/BiVO 4 is due to a heterojunction effect between Gd/BiVO 4 and rGO sheets, which not only enhances the light absorption but also enlarges the surface area in the presence of rGO. Bashir et al. [88] fabricated Gd-doped BiVO4 using a simple hydrothermal method. The resulting Gd-doped BiVO4 ultrasonically treated with rGO to form Gd-doped BiVO4/rGO of the nanocomposite nanostructures (Figure 12(Ai)). According to SEM analysis, Gd doping does not change the surface morphology. As shown in TEM images (Figure 12(Aii,iii)), three different phases of components promote significantly improved Unlike transition metals, doping with a rare earth metal is also found to improve the photocatalytic properties; however, this finding is rarely reported [89]. These metal ions possess excellent luminescence properties and therefore endow several benefits such as light absorption, modified surfaces, and acting as electron traps that can help minimize the recombination of photoinduced charge carriers [90]. Moscow et al. [91] reported erbium (Er) and yttrium (Y)-doped BiVO 4 using a simple microwave-assisted approach ( Figure 12B(i)). In synthesis, Bi and V precursors were dissolved under magnetic stirring. Later, different amounts of Er and Y precursors were introduced, followed by microwave irradiation forming Er 3+ -and Y 3+ -doped BiVO 4 nanostructures. According to SEM and XRD results, Er 3+ and Y 3+ doping led to a reduction in the particle sizes of BiVO 4 and formed mixed-phase BiVO 4 . Raman spectra analysis revealed (V-O) band shift from 820 cm −1 to 850 cm −1 and disappeared δ (VO 4 + ) doublet due to the conversion of monoclinic BiVO 4 to tetragonal BiVO 4 phase ( Figure 12B(ii)). Photocatalytic tests suggested that Y-doped BiVO 4 reportedly has the highest degradation efficiencies of 93%, 85%, and 91% for MB, MO, and RhB dyes, respectively, in 180 min under light irradiation. The possible photocatalytic electron transfer mechanism is shown in Figure 12B(iii). A photocatalytic degradation of acetaldehyde was also achieved at an impressive rate using Y-doped BiVO 4 . Because of the formation of the inner energy state Er 3+ and Y 3+ metal, the band gap reduced, light absorption extended, and the recombination of electron-hole pairs suppressed. Next, the co-doping of Gd and Y into BiVO 4 was successfully accomplished by simple hydrothermal synthesis [92]. Upon sunlight illumination (90 min), a Bi 0.92 Gd 0.07 Y 0.01 VO 4 photocatalyst exhibited 94% degradation efficiency for methylene blue dye (MB), which is four times larger than pure BiVO 4 . Recently, Sudrajat and Hartuti [93] used a one-step hydrothermal preparation method to prepare B-doped BiVO 4 (B-BiVO 4 ) with an oval-shaped morphological structure. The B dopant acts as mid-gap-state electron donors, allowing more excitations of the band gap to be produced and the conduction band of BiVO 4 . The light-induced infrared absorption measurement confirmed that there were more electrons available for reduction reactions and more holes available for oxidation reactions, leading to greater photocatalytic activity for the mineralization of phenoxyacetic acid (PAA) in the presence of simulated sunlight.
Similarly, doping with nonmetals such as nitrogen (N) and fluorine (F) in BiVO 4 has been adopted to promote light absorption, band gap tuning, and catalytically active surfaces. For instance, Wang et al. [94] prepared N-doped BiVO 4 using a sol-gel technique with hexamethylene tetramine (C 6 H 12 N 4 ) as a N source. N-doping was found to not change the morphology and surface area of BiVO 4 significantly. However, N was found to be doped into crystal lattice O-Bi-N-V-O bonds, creating highly active V 4+ species, oxygen vacancies, and a red shift in the absorption band. The photoactivity of BiVO 4 in this system mainly depends on two factors: (i) the content N-doping and (ii) heat treatment temperature. The N-BiVO 4 calcined at a temperature of 500 • C showed the highest activity. Li et al. [95] reported the synthesis of F-doped BiVO 4 nanospheres via a simple two-step hydrothermal method. NaF was employed as the fluoride source which helped to modify the crystal lattice of BiVO 4 , suppressing the charge carrier recombination, resulting in high photoactivity. Wang et al. [96] synthesized B-doped-BiVO 4 photocatalysts using a CS-template-assisted sol-gel method. According to these authors, B doping can form a monoclinic crystal structure, large surface areas, a smaller band gap value, and higher V 4+ species. This results in the best photoactivity of 0.04B CS-BiVO 4 for the degradation of MO dye, which is not only because of B doping but also due to the cellular morphology, which stems from the template as a similar 0.04B-BiVO 4 sample prepared without a template showed much lower photoactivity. Similarly, doping with nonmetals such as nitrogen (N) and fluorine (F) in BiVO4 has been adopted to promote light absorption, band gap tuning, and catalytically active surfaces. For instance, Wang et al. [94] prepared N-doped BiVO4 using a sol-gel technique

Noble-Metal-Doped BiVO 4 as Photocatalyst
Noble metals (Au, Ag, Pt) are very attractive in photocatalysis because of their surface plasmon resonance (LSPR), which can provoke the oscillation of conduction band electrons and plasmonic energy transfer. This mainly occurs via two different mechanisms: (i) direct electron transfer and (ii) plasmon-directed resonant transfer of energy. Cao et al. [97] have reported the synthesis of Au-BiVO 4 nanosheets using hydrothermal and a cysteine-linking strategy. Au precursor in the presence of cysteine evolves Au-doped BiVO 4 , as shown in Figure 13A(i). Interestingly, the surface plasmon resonance (SPR) of Au enables excellent visible-light-driven photocatalytic activity related to pure BiVO 4 for the degradation of MO dye ( Figure 13A(ii)). In this system, Au acts as an electron sink retarding the recombination of photoinduced electrons and holes. The contribution from Au nanoparticles was clarified by comparing the experimental results with Pt-BiVO 4 (prepared through a similar strategy), showing no SPR in the range of (500 ± 20 nm) as Au-BiVO 4 under visible light illumination. Moreover, electron trapping on Au raises the Fermi level (E f ) of Au to more negative potentials (E f *), leading to band alignment for an effective charge transfer. In addition, the photogenerated electrons transfer to Au nanoparticles can reduce the adsorbed sacrificial agent S 2 O 8 2− to SO 4 2− on the Au surface. As a result, the holes remain on the BiVO 4 surface and have a considerably higher lifetime to perform the water oxidation process ( Figure 13A(iii)). Reddy et al. [98] reported Au-doped BiVO 4 photocatalyst synthesis via a sonication and calcination method and applied it as electrodes for water splitting and electrochemical storage. This study suggests that, due to Au doping, the photocurrent density increased 25 times related to reference BiVO 4 , demonstrating the synergistic role of Au, while BiVO 4 increased the electrical conductivity and charge transfer at the interface. The synergistic effects of Ag nanoparticles (LSPR) and N-doped graphene (upconversion effect) with BiVO 4 have also been reported for the degradation of tetracycline hydrochloride (TC•HCl), as shown in Figure 13B(i) [99]. A ternary photocatalyst N-GNDs/Ag/BiVO 4 was prepared through the solvothermal and hydrothermal process, and it exhibits distinct behavior from reference BiVO 4 . According to the experimental results, possible charge transfer mechanisms using N-GNDs/Ag/BiVO 4 have been schematically proposed ( Figure 13B(ii)). Under light illumination, N-GNDs can absorb NIR light and contribute to the light upconversion phenomenon, which can help to extend light absorption. Since the potential of O 2 /O 2 − (−0.33 eV, NHE) is a higher negative value than the conduction band of BiVO 4 (+0.47 eV, NHE), the conduction band electron of BiVO 4 cannot reduce O 2 to produce • O 2 − radicals. As a result, the photoinduced electron from BiVO 4 migrates to N-GNDs and Ag-NPs, leading to band alignment. Thus, a Schottky barrier is formed at the interface to facilitate more efficient electron transfer; N-GNDs serve as electron acceptors to capture photoexcited electrons from BiVO 4 , and hot electrons from Ag-NPs (LSPR effect) reduce O 2 to • O 2 − , which later oxidizes TC•HCl to smaller molecules.
Wei et al. [111] fabricated N-doped Biochar (N-Biochar)@BiVO 4 nanocomposite via an easy hydrothermal method and evaluated its photocatalytic performance for the degradation of triclosan (TCS). Figure 14A(i,ii) shows the fern-like morphology of BiVO 4 , while the composite shows well-dispersed N-biochar in intimate contact with BiVO 4 . Upon light irradiation (60 min), the BiVO 4 @N-Biochar catalysts showed 94.6% TCS degradation efficiency, which is much higher than pure BiVO 4 (56.7%). As per LSMS and the E. coli (Escherichia coli) colony assessment studies, a detoxification efficiency of 72.3 ± 2.6% was determined, signifying a remarkable reduction in biotoxicity during photodegradation. The schematic representation of the possible charge transfer mechanism on BiVO 4 @N-Biochar is illustrated in Figure 14A(iii). Cao et al. [112] have reported the synthesis of Al-doped BiVO 4 composites with the use of a simple hydrothermal method and evaluated for photocatalytic decomposition of MB dye ( Figure 14B(i)). Different molar ratios of Bi to Al were used and calcined at a temperature of 500 • C under Ar gaseous environment (1h) to obtain Al-doped BiVO 4 . Based on optical spectroscopy, the band gap energies of pristine BiVO 4 , Al-0.03-BV, and Al-0.3-BV samples were determined to be 2.36, 2.40, and 2.41 eV, respectively. As shown in Figure 14B(ii), when isopropanol and potassium dichromate were used as scavengers, the degrading efficiency for MB was decreased, demonstrating that e − and · OH − radicals are the active species in the degradation of dye molecules. Their experimental results confirm that optimal 30 mol% Al-BiVO 4 (Al-0.3-BV) showed excellent photocatalytic activity for MB degradation due to the synergistic effect of appropriate Al doping and Al 2 O 3 surface passivation. Based on transient photovoltage (TPV) and surface photocurrent (SPC) results, the coexistence of Al 3+ and Al 2 O 3 evolved, causing a synergistic effect for advancing e − transfer and extending the lifetime of charge carriers. Al doping can result in transforming the surface morphology of BiVO 4 , as the polyhedron structure of BiVO 4 becomes thinner and there is a reduced grain size ( Figure 14B(iii-v)). Similarly, Wetchakun et al. [113] studied BiVO 4 /CeO 2 nanocomposites through the co-precipitation and hydrothermal method. The different molar concentration of semiconductors' constituent was fixed and evaluated for the degradation of dyes pollutants in water. The XRD results suggest that two different kinds of diffraction peaks confirmed the coexistence of mixed-phase, indicating BiVO 4 /CeO 2 nanocomposite formation ( Figure 14C(i)). Under light irradiation (>400 nm), the molar ratio 0.6:0.4 for BiVO 4 /CeO 2 nanocomposite displayed the highest photocatalytic degradation activity for the removal of MB dye in water ( Figure 14C(ii)).

Activated Carbon, Carbon, and Other Adsorbents-Based Composites
Patil et al. [73] reported the synthesis of BiVO 4 /Ag/rGO hybrid architectures using a cost-effective hydrothermal method exhibiting impressive reaction rates for water oxidation and organic pollutant degradation reactions. Fern-like BiVO 4 nanostructures were prepared and decorated on the surface of reduced graphene oxide (rGO) sheets ( Figure 15A(i)), which can offer a large surface area and excellent electron transfer properties. They discovered that Ag nanoparticles can be reduced during hydrothermal reaction and deposited on the surface of BiVO 4 , forming a Schottky junction between Ag and BiVO 4 . Figure 15A(iii,iv)) depicts SEM and TEM images of nanocomposites, demonstrating different phases of Ag, rGO, and BiVO 4 . As shown in Figure 15A(ii), a complete degradation of MB dye within 120 min was achieved using a hybrid BiVO 4 /Ag 2%/rGO catalyst under simulated light irradiation, which is~2.18 and~1.25 times larger than pure BiVO 4 and BiVO 4 /Ag photocatalysts. On the basis of PL and PEC results, the highest photocatalytic performance in this system is attributed to the combined effect of Ag and rGO, enabling the efficient promotion of e − /h + separation across the interface and visible light absorption. The effect of graphene oxide on BiVO 4 photocatalyst was also demonstrated by Zhang et al. by incorporating graphene oxide (GO) between the BiVO 4 and NiOOH oxygen evolution catalysts (OEC).
The results indicate that GO served as hole extraction layer due to its hole storage capability and improved the stability of the material. Meanwhile, GO employs the formation of p/n heterointerface with BiVO 4 and encouraged the hole transfer from BiVO 4 to NiOOH [114].
Graphitic carbon nitride (g-C 3 N 4 ) is one of the best 2D semiconductor materials due to its large surface area and intriguing electronic properties. Incorporating g-C 3 N 4 with BiVO 4 can significantly modify the physicochemical properties and showed impressive photoactivity towards the degradation of organic pollutant dyes, CO 2 reduction, and H 2 generation reactions. Alhaddad et al. [115] reported a g-C 3 N 4 -incorporated Pt@BiVO 4 nanocomposites catalyst for the detoxification of ciprofloxacin. A sol-gel reaction between Bi (NO 3 ) 3 .5H 2 O and NH 4 VO 3 was adopted using CH 3 COOH and HCl, forming BiVO 4 nanoparticles. Then, using C 6 H 14 as a solvent, solid dispersions of BiVO 4 in different mass contents of 1.0, 2.0, 3.0, and 4.0 wt% and g-C 3 N 4 were prepared, where it was agitated for 4h, resulting in the formation of heterostructures. At last, solid dispersions were then prepared via photoreduction. As shown in the TEM image ( Figure 15B(i)), Pt nanoparticles can be photoproduced and deposited on the surface of BiVO 4 and g-C 3 N 4 . A comparison study revealed that the 0.5 wt% Pt@4 wt% BiVO 4 -g-C 3 N 4 heterojunction is optimal, displaying 5.0-and 3.7-times higher photocatalytic efficiency than pure g-C 3 N 4 and BiVO 4 for the decomposition of ciprofloxacin ( Figure 15B(ii)). The plausible electron transfer mechanism over Pt@BiVO 4 -g-C 3 N 4 during the photocatalytic removal of ciprofloxacin is illustrated in Figure 15B(iii). The creation of the p-n heterojunction can facilitate charge carrier separation, while Pt nanoparticles further assist in the increase in the light absorption and photocatalytic efficiency.

Heterojunction Construction
Interestingly, monoclinic (m) BiVO 4 and tetragonal zircon (tz) BiVO 4 can form a heterojunction. For example, Dabodiya et al. [116] prepared mixed-phase BiVO 4 (m:tz-60:40) using a microwave-hydrothermal method, which displays a 95% degradation efficiency for Rhodamine B dye. As shown in Figure 16A(ii), the effect of phase transition from tz-BiVO 4 to m-BiVO 4 was investigated in terms of its photocatalytic efficiency for the decomposition of RhB dye. On the basis of UV-reflectance and PL results, they discovered a reduction in the bandgap energy and facilitated e-/h + separation at the m-BiVO 4 /tz-BiVO 4 interfaces and enhanced photoactivity under visible light irradiation. Similarly, Patil et al. [117] reported the controlled synthesis of BiVO 4 (pillars-like, dendrite-like, and microgranule-like) and the m-BiVO 4 /tz-BiVO 4 heterojunction using simple a hydrothermal-solvothermal and solid-state reaction ( Figure 16B(i)). The mixed-phase m-BiVO 4 /tz-BiVO 4 heterojunction prepared through solvothermal reaction displayed the highest photodegradation efficiency of 95% for MB dye related to single-phase BiVO 4 prepared through a hydrothermal (BVO-HDR; 79%) and solid-state reaction (BVO-SSR; 88%). Experimental results confirmed that temperature plays a critical role in the phase transformation of BiVO 4 . On the basis of PEC and EIS results, the high photocurrent density and reduction in internal resistance is confirmed, demonstrating that special dendritic architectures and the heterojunction effect is crucial to promote the e-/h + separation and utilization. Figure 16B  Recently, Li et al. [103] developed a ZnO/BiVO4 heterojunction thin films catalyst using a simple chemical bath deposition and electrodeposition method. The as-formed three-dimension choral-like ZnO/BiVO4 displayed an excellent photoelectrocatalytic tetracycline degradation efficiency of 84.5%. Figure 17(Ai-ii) shows the schematic representation of the BiVO4/ZnO electrode and charge transfer process during photoelectrocatalysis. According to the radical scavenger's test, •O2 − and •OH were found to be major active Recently, Li et al. [103] developed a ZnO/BiVO 4 heterojunction thin films catalyst using a simple chemical bath deposition and electrodeposition method. The as-formed three-dimension choral-like ZnO/BiVO 4 displayed an excellent photoelectrocatalytic tetracycline degradation efficiency of 84.5%. Figure 17A(i,ii) shows the schematic representation of the BiVO 4 /ZnO electrode and charge transfer process during photoelectrocatalysis.
According to the radical scavenger's test, •O 2 − and •OH were found to be major active species responsible for tetracycline degradation. As shown in Figure 17A(iii), ZnO/BiVO 4 showed a tetracycline degradation efficiency 84.5%. and two sets of lattice fringes, indicating an intimate interface between the two semiconductor and heterojunction creation. As shown in Figure 17(Bii), the difference in the molar ratio showed varied photocatalytic activity. Among them, a 10:1 mol ratio of the BiVO4:Ag3VO4 heterojunction displayed the highest 95% degradation efficiency for RhB dye, which is 10-and 3.4-times higher than pure BiVO4 and Ag3VO4, respectively. The schematic representation of the photocatalytic reaction mechanism is illustrated in Figure  17(Biii). On the basis of electrochemical impedance analysis (Figure 17(Biv)), a small semicircle for BiVO4:Ag3VO4 suggests a lowest internal resistance and faster electron transfer process due to the heterojunction effect and the effective separation of the photo-induced charges carriers. Yan et al. [118] fabricated the BiVO 4 /Ag 3 VO 4 heterojunction using a simple hydrothermal and coprecipitation method and evaluated its photocatalytic performance towards RhB dye degradation. Figure 17B(i) shows the TEM image of hybrid BiVO 4 /Ag 3 VO 4 and two sets of lattice fringes, indicating an intimate interface between the two semiconductor and heterojunction creation. As shown in Figure 17(Bii), the difference in the molar ratio showed varied photocatalytic activity. Among them, a 10:1 mol ratio of the BiVO 4 :Ag 3 VO 4 heterojunction displayed the highest 95% degradation efficiency for RhB dye, which is 10-and 3.4-times higher than pure BiVO 4 and Ag 3 VO 4 , respectively. The schematic representation of the photocatalytic reaction mechanism is illustrated in Figure 17B(iii). On the basis of electrochemical impedance analysis ( Figure 17B(iv)), a small semicircle for BiVO 4 :Ag 3 VO 4 suggests a lowest internal resistance and faster electron transfer process due to the heterojunction effect and the effective separation of the photo-induced charges carriers.
Bao et al. [119] designed a facet-heterojunction Z-scheme photocatalyst AgBr-Ag-BiVO 4 {010} to increase the photoactivity of BiVO 4 for the inactivation of pathogenic bacteria and the degradation of organic dyes from wastewater. First, facet-controlled BiVO 4 {010} was obtained using the facile hydrothermal method, and then Ag nanoparticles were deposited through photoreduction, while in situ chemical treatment in the presence of KBr and Fe(NO 3 ) 3 enabled the transformation of the outermost layer of Ag nanoparticles to AgBr. Interestingly, Ag nanoparticles can be selectively deposited on the BiVO 4 {010} facets and transform. Figure 18i shows the FESEM image of the AgBr-Ag-BiVO 4 {010} heterojunction and highly dispersed AgBr nanoparticles, which showed an increase in particle sizes upon transformation. AgBr-Ag-BiVO 4 {010} displayed the highest photocatalytic inactivation for Escherichia coliK-12, which is ∼4 and 15 times the reference Ag-BiVO 4 {010} and BiVO 4 , respectively (Figure 18iii). The possible charge transfer mechanism during the photocatalytic inactivation of bacterium is illustrated in Figure 18ii. The photoluminescence (PL) spectroscopy and PEC results suggested a suppression in the charge recombination upon the heterojunction, and the electron paramagnetic resonance (EPR) results revealed that h + , • OH, and • O 2 − , are the major active species for the degradation of RhB dyes in wastewater. Reprinted with permission from Ref. [118].
Bao et al. [119] designed a facet-heterojunction Z-scheme photocatalyst AgBr-Ag-BiVO4 {010} to increase the photoactivity of BiVO4 for the inactivation of pathogenic bacteria and the degradation of organic dyes from wastewater. First, facet-controlled BiVO4 {010} was obtained using the facile hydrothermal method, and then Ag nanoparticles were deposited through photoreduction, while in situ chemical treatment in the presence of KBr and Fe(NO3)3 enabled the transformation of the outermost layer of Ag nanoparticles to AgBr. Interestingly, Ag nanoparticles can be selectively deposited on the BiVO4 {010} facets and transform. Figure 18i shows the FESEM image of the AgBr-Ag-BiVO4 {010} heterojunction and highly dispersed AgBr nanoparticles, which showed an increase in particle sizes upon transformation. AgBr-Ag-BiVO4 {010} displayed the highest photocatalytic inactivation for Escherichia coliK-12, which is ∼4 and 15 times the reference Ag-BiVO4 {010} and BiVO4, respectively (Figure 18iii). The possible charge transfer mechanism during the photocatalytic inactivation of bacterium is illustrated in Figure 18ii. The photoluminescence (PL) spectroscopy and PEC results suggested a suppression in the charge recombination upon the heterojunction, and the electron paramagnetic resonance (EPR) results revealed that h + , • OH, and • O2 -, are the major active species for the degradation of RhB dyes in wastewater.

BiVO4-Based S-Scheme and Z-Scheme Nanocomposite Materials
As mentioned above, it is difficult for single BiVO4 semiconducting materials to offer a strong visible light response and high redox capability simultaneously. So, the coupling of BiVO4 with another suitable semiconducting material to synthesize S-scheme-and Zscheme-type composite materials has received significant attention. The band structure of both the S-scheme-and Z-scheme-type composite materials is shown in Figure 19.

BiVO 4 -Based S-Scheme and Z-Scheme Nanocomposite Materials
As mentioned above, it is difficult for single BiVO 4 semiconducting materials to offer a strong visible light response and high redox capability simultaneously. So, the coupling of BiVO 4 with another suitable semiconducting material to synthesize S-scheme-and Zscheme-type composite materials has received significant attention. The band structure of both the S-scheme-and Z-scheme-type composite materials is shown in Figure 19. Nanomaterials 2023, 13, x FOR PEER REVIEW 30 of 40 Figure 19. The band structure of (A) Z-scheme-and (B) S-scheme-type composite materials.
In both composite materials, the high CB semiconductor materials combine with high VB semiconducting materials, whereas they follow two different pathways for photo-generated electron-hole pairs separation and transfer. In Z-scheme-type materials, the photogenerated electron-hole pairs are separated by an internal electric field between the two semiconductor interfaces, whereas in the case of S-scheme-type materials, the charge carrier separation and transfer occur through an internal electric field, energy band bending, and Coulomb gravity. Furthermore, there are two types of Z-scheme-type materials, such as direct Z-scheme-type materials and mediator (indirect)-based Z-scheme-type materials. In indirect Z-scheme-type materials, metals and non-metals, carbon materials, and quantum dots, etc., are used as mediators. For example, Li et al. coupled high-valence-bandedge BiVO4 with high-conduction-band-edge g-C3N4 material through a wet impregnation-calcination method which yielded Z-scheme BiVO4/g-C3N4 materials. The calculated CB and VB potentials of g-C3N4 and BiVO4 are 1.20 and 1.54 eV and 0.46 and 2.86 eV, respectively. Under visible light irradiation, the electron-hole pairs are generated in both the semiconductors. Subsequently, the electrons present in the CB of BiVO4 combines with holes present in the VB of g-C3N4, so the electrons in the CB of g-C3N4 and holes in the VB of BiVO4 are efficiently separated and possess a higher potential than the generation potential of the reactive radicals ( • OH and O2 •-). Consequently, a higher concentration of reactive radicals is generated, which showed higher photocatalytic activity in the degradation of malachite green (MG) dye in the presence of visible light irradiation and H2O2 [120]. Similarly, Hu et al. developed a g-C3N4/BiVO4-based S-scheme system using hydrothermal methods for the degradation of paraben preservative in the presence of visible light and natural solar light irradiation. The CB and CB of g-C3N4 were located at-1.3 and + 1.44 V vs. RHE at pH 0, respectively, and the CB and VB of BiVO4 were sited at + 0.09 and + 2.4 V vs. RHE, respectively. The energy difference present in the mixed g-C3N4/BiVO4 system would allow the transfer of electrons of g-C3N4 to BiVO4, which leads to the positively charged region on the g-C3N4 side and negatively charged region on the BiVO4 side. Therefore, there is a generation of an inner electric field at the interface of g-C3N4/BiVO4, with the direction from BiVO4 to g-C3N4. Under irradiation, the photoexcited electrons on the CB of BiVO4 combined with the holes on the VB of g-C3N4 lead to the In both composite materials, the high CB semiconductor materials combine with high VB semiconducting materials, whereas they follow two different pathways for photogenerated electron-hole pairs separation and transfer. In Z-scheme-type materials, the photo-generated electron-hole pairs are separated by an internal electric field between the two semiconductor interfaces, whereas in the case of S-scheme-type materials, the charge carrier separation and transfer occur through an internal electric field, energy band bending, and Coulomb gravity. Furthermore, there are two types of Z-scheme-type materials, such as direct Z-scheme-type materials and mediator (indirect)-based Z-scheme-type materials. In indirect Z-scheme-type materials, metals and non-metals, carbon materials, and quantum dots, etc., are used as mediators. For example, Li et al. coupled high-valence-band-edge BiVO 4 with high-conduction-band-edge g-C 3 N 4 material through a wet impregnationcalcination method which yielded Z-scheme BiVO 4 /g-C 3 N 4 materials. The calculated CB and VB potentials of g-C 3 N 4 and BiVO 4 are 1.20 and 1.54 eV and 0.46 and 2.86 eV, respectively. Under visible light irradiation, the electron-hole pairs are generated in both the semiconductors. Subsequently, the electrons present in the CB of BiVO 4 combines with holes present in the VB of g-C 3 N 4 , so the electrons in the CB of g-C 3 N 4 and holes in the VB of BiVO 4 are efficiently separated and possess a higher potential than the generation potential of the reactive radicals ( • OH and O 2 •-). Consequently, a higher concentration of reactive radicals is generated, which showed higher photocatalytic activity in the degradation of malachite green (MG) dye in the presence of visible light irradiation and H 2 O 2 [120]. Similarly, Hu et al. developed a g-C 3 N 4 /BiVO 4 -based S-scheme system using hydrothermal methods for the degradation of paraben preservative in the presence of visible light and natural solar light irradiation. The CB and CB of g-C 3 N 4 were located at-1.3 and + 1.44 V vs. RHE at pH 0, respectively, and the CB and VB of BiVO 4 were sited at + 0.09 and + 2.4 V vs. RHE, respectively. The energy difference present in the mixed g-C 3 N 4 /BiVO 4 system would allow the transfer of electrons of g-C 3 N 4 to BiVO 4 , which leads to the positively charged region on the g-C 3 N 4 side and negatively charged region on the BiVO 4 side. Therefore, there is a generation of an inner electric field at the interface of g-C 3 N 4 /BiVO 4 , with the direction from BiVO 4 to g-C 3 N 4 . Under irradiation, the photoexcited electrons on the CB of BiVO 4 combined with the holes on the VB of g-C 3 N 4 lead to the efficient separation of electrons and holes on the CB of g-C 3 N 4 and on the VB of BiVO 4 for the production of reactive radicals, which could participate in the degradation of paraben [121]. Similarly, various researchers have developed BiVO 4 -based Z-scheme and S-scheme materials for the degradation of dyes and other compounds; however, this has been scarcely studied for the application of VOC degradation.

Volatile Organic Compounds Degradation Application
As described in the above sections, volatile organic compounds (VOCs) pose a serious threat to environment and human health. VOCs are mainly BTEX (benzene, toluene, ethylbenzene, xylene), acetylene, acetone, ethylene, trichloroethylene, benzaldehyde, acetaldehyde, isopropanol, hexane, etc., and are mainly released by human activities through outdoor and indoor sources. VOCs create significant health issues; they specifically cause allergies, cancer, and they slow down and damage the nervous and respiratory system. Therefore, significant research activities are put forward for controlling and degradation of VOCs. Among those, the photocatalytic oxidation/degradation of VOCs has received great attention because of the simple operation and reaction conditions, low cost, and the fact that it completely degrades VOCs, and renewable solar energy can be used for their degradation. Among photocatalysts, BiVO 4 is a better visible-light-responsive system for VOCs degradation, which are described in this section and Table 3. Hu et. al. synthesized a BiVO 4 /TiO 2 heterojunction photocatalyst using the sol-gel method and evaluated the photocatalytic oxidation of gaseous benzene under UV light and simulated solar light irradiation. The BiVO 4 with a loading percentage of 0.5% presented a higher photocatalytic oxidation of benzene (66.8% conversion, Figure 20a) and a high amount of CO 2 production (Figure 20b) compared to the other percentage loaded materials and bare TiO 2 and BiVO 4 materials. The improved visible light absorption by the introduction of BiVO 4 and the activated species (•OH) are responsible for the high activity of the BiVO 4 /TiO 2 heterojunction photocatalyst [123]. Furthermore, Zhao et al. developed CuO/BiVO 4 hollow nanospheres using the sol-gel method followed by the impregnation method and demonstrated visible light photocatalytic activity by the degradation of gaseous toluene. The results revealed that 5% CuO-loaded BiVO 4 hollow nanospheres showed higher visible light photocatalytic activity (85%) in toluene degradation compared to other percentage-loaded composites and bare materials [124]. Chen [131]. However, still the binary composites showed low degradation efficiency due to the low separation and transfer rate of photogenerated charge carriers. In order to further enhance the photocatalytic efficiency of BiVO 4 , BiVO 4-based Z-scheme and ternary nanocomposites were prepared, and also metal nanoparticles were introduced into BiVO 4 . For example, a coral-like direct Z-scheme BiVO 4 /g-C 3 N 4 was synthesized by Sun et al. for the degradation of toluene under visible light irradiation. The visible light absorption of BiVO 4 was significantly increased after g-C 3 N 4 loading and also promoted the separation of photogenerated electron hole pairs. The improved separation of photogenerated electron hole pairs on the direct Z-scheme BiVO 4 /g-C 3 N 4 material led to a higher toluene degradation efficiency compared to bare materials [130]. Furthermore, Li et al. studied automobile exhaust gas purification by improving the adsorption capacity of volatile compounds (automobile exhaust gases HC, NO, and CO) onto g-C 3 N 4 /BiVO 4 composites through introducing tourmaline powder into g-C 3 N 4 /BiVO 4 composites. The introduction of tourmaline powder considerably increases the adsorption capacity of the automobile exhaust gas molecules by releasing negative ions, which enhances the contact between the automobile exhaust gas molecules and the g-C 3 N 4 /BiVO 4 composite material. The enhancement in the adsorption capacity improves the hydrocarbon, CO, and NO purification efficiency by 1.73, 1.74, and 2.52 times compared to pure g-C 3 N 4 [134]. In addition, the charge carrier's separation and transport efficiency was enhanced by introducing an oxygen vacancy (OVs) on BiVO 4 using the electrochemical reduction process . The ternary nanocomposite (BiVO 4 /WO 3 /TiO 2 ) was prepared by coupling OVs-BiVO 4 with WO 3 /TiO 2 nanotubes for toluene gas degradation. The OVs-BiVO 4 /WO 3 /TNTs displayed a 28-times higher photocurrent intensity compared to pristine BiVO 4 /WO 3 /TNTs, which leads to higher photocatalytic toluene gas degradation. Furthermore, the stability of the composite materials was also enhanced by introducing OVs [135]. Recently, Zhu et al. developed triangular Ag nanoplates (AgNPs)-loaded BiVO 4 for the degradation of gaseous formaldehyde (HCHO) under the irradiation of daylight lamp as a visible light source. The loading of triangular Ag nanoplates significantly decreases the recombination rate of photogenerated electron-hole pairs that extends the lifetime of charge carriers, which leads to a high HCHO oxidation efficiency. The plasmonic effect of AgNPs was also the reason for the enhancement of the catalytic activity [137]. Similarly, Shi et al. prepared activated carbon from semi-coke waste generated during the processing of coal and was loaded into a ternary BiVO 4 -BiPO 4 -g-C 3 N 4 Z-scheme heterojunction photocatalyst using a one-step sol-gel method for the degradation of toluene under visible light irradiation. The activated carbon-loaded ternary composites showed a 2.43-times higher photocatalytic activity (85.6%) than the pure photocatalyst in the degradation of toluene under 60% relative humidity and 0.5 g/cm 3 of composite material. The enhanced adsorption of toluene by activated carbon loading and the improved visible light response leads to higher activity in toluene VOC degradation [139]. Likewise, the development of BiVO 4 -based visible light active materials for VOC decomposition is still growing; however, there are some limitations that affect the possible utilization of BiVO 4 on a commercial scale, which are described in the forthcoming sections.
Nanomaterials 2023, 13, x FOR PEER REVIEW 33 of 4 photocatalytic activity (85.6%) than the pure photocatalyst in the degradation of toluen under 60% relative humidity and 0.5 g/cm 3 of composite material. The enhanced adsorp tion of toluene by activated carbon loading and the improved visible light response lead to higher activity in toluene VOC degradation [139]. Likewise, the development of BiVO4 based visible light active materials for VOC decomposition is still growing; however, ther are some limitations that affect the possible utilization of BiVO4 on a commercial scale which are described in the forthcoming sections. Figure 20. Photocatalytic oxidation of benzene (a) and the amount of CO2 production (b) usin BiVO4/TiO2 and bare nanomaterials under visible light irradiation (reprinted with permission from [123]; 2011, copyright from Elsevier.

Summary and Outlook
BiVO4 has been identified as one of the most promising visible-light-responsive pho tocatalytic materials (low bandgap energy, ~2.4 eV) for the degradation of various pollu tant molecules, including VOCs. However, the materials are facing significant issues suc as the high recombination rate of photogenerated charge carriers, the inappropriate posi tion of CB of BiVO4, and the low redox ability of charge carriers. So, various engineerin modifications have been performed to improve these limitations, which leads to high cat alytic efficiency. Therefore, in summary, we have reviewed the synthesis methodologie of BiVO4 and various engineering modifications of BiVO4, such as changes in the mor phology, metal and non-metal loading, heterojunction formation, Z-scheme-and S scheme-type materials development and support on high-surface-area adsorbents, etc followed by their application in VOCs degradation, which was reviewed in detail in thi paper, for which it showed a greater performance.
Though the modified BiVO4 material showed an efficient visible light photocatalyti performance, it is still lagging behind in terms of commercial applications because of it poor reusability and low lifetime. Furthermore, for the better commercial utilization and recovery of materials, there is a need for an immobilized photocatalytic reactor. However the presently available BiVO4-based immobilized reactor systems have a low mass trans fer effect (both external and internal). In addition, morphologies-controlled synthesis gen erally results in a relatively larger size of BiVO4 material, which is easier to agglomerat and it decreases the active surface sites, thereby reducing the catalytic performance Hence, future research needs to focus on advanced synthesis techniques, such as atomi layer deposition methods, plasma treatment, and other micro techniques, to achieve greater precision in the control of the morphology and the production of an appropriat size of the BiVO4. This should be followed by smaller size BiVO4-based materials as thes could be effectively used to make an efficient photocatalytic surface with a high mass transfer effect. Future research also needs to be focused on the production of concret

Summary and Outlook
BiVO 4 has been identified as one of the most promising visible-light-responsive photocatalytic materials (low bandgap energy,~2.4 eV) for the degradation of various pollutant molecules, including VOCs. However, the materials are facing significant issues such as the high recombination rate of photogenerated charge carriers, the inappropriate position of CB of BiVO 4 , and the low redox ability of charge carriers. So, various engineering modifications have been performed to improve these limitations, which leads to high catalytic efficiency. Therefore, in summary, we have reviewed the synthesis methodologies of BiVO 4 and various engineering modifications of BiVO 4 , such as changes in the morphology, metal and non-metal loading, heterojunction formation, Z-scheme-and S-scheme-type materials development and support on high-surface-area adsorbents, etc., followed by their application in VOCs degradation, which was reviewed in detail in this paper, for which it showed a greater performance.
Though the modified BiVO 4 material showed an efficient visible light photocatalytic performance, it is still lagging behind in terms of commercial applications because of its poor reusability and low lifetime. Furthermore, for the better commercial utilization and recovery of materials, there is a need for an immobilized photocatalytic reactor. However, the presently available BiVO 4 -based immobilized reactor systems have a low mass transfer effect (both external and internal). In addition, morphologies-controlled synthesis generally results in a relatively larger size of BiVO 4 material, which is easier to agglomerate and it decreases the active surface sites, thereby reducing the catalytic performance. Hence, future research needs to focus on advanced synthesis techniques, such as atomic layer deposition methods, plasma treatment, and other micro techniques, to achieve a greater precision in the control of the morphology and the production of an appropriate size of the BiVO 4 . This should be followed by smaller size BiVO 4 -based materials as these could be effectively used to make an efficient photocatalytic surface with a high mass-transfer effect. Future research also needs to be focused on the production of concrete evidence for the photogenerated separation of electron-hole pairs and transfer pathways/mechanism in heterojunction and Z-scheme-and S-scheme-type materials. Finally, the toxicity assessment of the synthesized materials as well as degraded solution requires increased attention in the near future for the commercial utilization of a developed photocatalytic system. We hope that this review article has created a solid foundation for the development of BiVO 4 -based composite materials with a high performance, as well as their associated technologies, for the decomposition of VOCs.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.