Advances in Synthesis and Applications of Bismuth Vanadate Based Structures

1. Introduction

Water pollution is one of the most serious of all environmental problems with critical effects on the human live. Photocatalytic degradation technology, including advanced oxidation processes (AOPs), has become increasingly more useful and necessary in the past few decades due to degradation a wide range of organic and inorganic contaminants. Obtaining photocatalysts with ideal performances is the core problem for photocatalysis [1, 2].

In recent decades, an immense research attention has been focused on solving the pollution problem and developing different kinds of semiconductor photocatalysts. A series of photocatalysts have been designed for this purpose. Since the traditional photocatalyst TiO₂ is limited in ultraviolet (UV) range applications, alternative materials have been widely explored. Owing to their unique properties, Bi-based semiconductor photocatalytic materials have been widely studied [2]. Among of different materials with photocatalytic properties, bismuth-vanadate (BiVO₄) showed great photocatalytic features to extend beyond the UV region due to its suitable band-gap of 2.4 eV and favorable band edge alignment to water splitting. BiVO₄ shows interesting physicochemical and dielectric properties, ferroelelasticity, semiconductivity, pigmentation, and photocatalytic features. BiVO₄ is polymorphous and occurs in three forms which can be prepared synthetically; scheelite-type structure with monoclinic (ms-) and tetragonal (ts-) systems, and zircon-type structure with tetragonal system (tz-). All three crystalline phases, ts-BiVO₄, ms-BiVO₄ and tz-BiVO₄, are n-type semiconductors with band gap energies of 2.34, 2.40, and 2.90 eV, respectively. A fourth form, i.e., an orthorhombic structure of BiVO_4, only occurs naturally as the mineral pucherite. As one of the promising non-titania (TiO₂) visible-light-driven photocatalysts, ms-BiVO₄ is studied intensively and there are many ways to prepare ms-BiVO₄ micro- and nanoparticles of varying morphologies and sizes. Also, due to yellow color, non-toxic ms-BiVO₄ is a good commercially available substitute to toxic cadmium- and lead-based yellow pigments [3, 4]. Following the work of Kudo’s group in 1998 on photocatalytic evolution of O₂ under visible light irradiation of BiVO₄ in aqueous AgNO₃ solution [5-7], considerable research efforts have been devoted to the BiVO₄-based material. This material is an excellent candidate for use in photocatalytic water splitting and photocatalytic degradation of air/water pollutants [8, 9]. The ms-BiVO₄ is known to exhibit excellent photocatalytic activity under visible light; on the other hand, pure tz-BiVO₄ has been studied to a much lesser extent as a photocatalyst [10-12].

In order to improve the photocatalytic performance of BiVO₄, promoting the separation and transfer of photogenerated carriers, namely, the photoinduced electron (e⁻) and hole (h⁺) pairs, is necessary [13, 14]. There are a lot of BiVO₄-based heterojunction photocatalysts including: n-BiVO₄@p-MoS₂ [15], CaFe₂O₄/BiVO₄ [16], TiO₂/BiVO₄ [17, 18], rGO/BiVO₄ [19], Bi₂WO₆/BiVO₄ [20], Co₃O₄/BiVO₄ [21], BiVO₄/B_i4V₂O₁₁ [22], Ag₃PO₄/BiVO₄ [23], Cu₃Mo₂O₉/BiVO₄ [24], BiVO₄/CdS [25], Bi₂S₃/BiVO₄/MgIn₂S₄ [26] and 2D Zn-MOF/BiVO₄ [27], which have been developed for photocatalytic decomposition of water, degradation of organic pollutants, reduction of CO₂ and heavy metal ions [28-30]. Enhanced photocatalytic performance has been reported for numerous doped BiVO₄ materials with zircon-type structure. Bi³⁺-based compounds can be easily doped with rare ions (RE³⁺) due to the equal valence and similar ionic radius. This means that RE³⁺ ions could be regarded as active co-catalysts and dopants in order to enhance the photocatalytic activity of BiVO₄ [31-33]. Also, other dopants like of Cu²⁺, Yb³⁺, Er³⁺, Nd³⁺ and Sm³⁺ ions can induce modification of the BiVO₄ shape, increase of its active area and change drastically optical properties., while molybdenum (VI), Mo⁶⁺ and tungsten (VI), W⁶⁺, can improve the electrical characteristics, the electron mobility and electrical conductivity of the BiVO₄ [34-39].

This review will be focused on the state of art of the basic and applied research on BiVO₄-based structures. A comprehensive systematic study of developments, preparation methods, structure, properties and advances in everyday applications over the past decades in research on BiVO₄-based structures will be described.

2. Discussion

2.1. Advances in Synthesis of BiVO₄-Based Structures

Different methods of synthesis have been reported for preparation of monoclinic ms-BiVO₄ or tz-BiVO₄ with various morphologies and sizes. Several approaches have been used: co-precipitation and micro-emulsion [40-48], hydrothermal synthesis with and without employing surfactant or template [49-56], rapid microwave-assisted process and microwave-assisted hydrothermal method [57-64] and solvothermal approach [65-71]. Using a precipitation method, the BiVO₄ can be obtained from an aqueous solution of NH₄VO₃ and an aqueous nitric acid solution of Bi(NO₃)₃ [58, 72-75] or from an aqueous nitric acid solution of Bi₂O₃ and V₂O₅ [12, 76]. By using microwave-assisted or hydrothermal technique, either an aqueous NaVO₃ or NH₄VO₃ solution with nitric acid of Bi(NO₃)₃ solution or Bi₂O₃ and V₂O₅ in a molar ratio in HNO₃ were mixed [77-82].

Several specialized methods have also been developed for the synthesis of BiVO₄. These include: (i) a reaction involving layered potassium vanadate powders (KV₃O₈ and K₃V₅O₁₄) and Bi(NO₃)₃ in aqueous solution, using a controlled vanadium-to-bismuth ratio [83]; (ii) a high-temperature colloidal synthesis in which Bi(NO₃)₃, NH₄VO₃, and polyethylene glycol (PEG) are dissolved in water [84]; and (iii) a low-temperature molten salt method employing a LiNO₃–NaNO₃ eutectic mixture (heated to 200 °C) as the solvent and a BiVO₄ precursor, obtained as a precipitate from Bi(NO₃)₃ and NH₄VO₃ solutions, as the solute.[85]. Structural phase transition in BiVO₄ (from monoclinic to tetragonal) can occurs under high pressure [86, 87].

The Bi(NO₃)₃x5H₂O was used in most syntheses of the BiVO₄ as a bismuth precursor owing to its wide availability and cheapness. However, Bi(NO₃)₃x5H₂O readily hydrolyzes into basic salts, BiO(NO₃) and Bi(OH)₂NO₃, and other nitrates with numerous complicated compositions [88, 89]. Recently, a novel and non-conventional way of synthesis was attempted, through a straightforward room-temperature non aqueous preparation method for producing nanocrystalline tz-BiVO₄: NH₄VO₃ and Bi(NO₃)₃·5H₂O have been employed as precursors, with ethylene glycol serving as both the solvent and reaction medium for precipitation. Additionally, it functions as a capping agent, limiting particle growth and preventing agglomeration [90,91,92].

The Figure 1 schematically illustrates the formation process of the monoclinic/tetragonal BiVO₄ heterostructure (m–t BiVO₄), which involves nucleation followed by aggregation-driven growth. The phase transition from the monoclinic to the tetragonal structure is attributed to the stability of the MEOH–Bi complex formed during solvothermal treatment. In this process, Bi³⁺ ions coordinate with both the oxygen atom of the C–O–C bond and that of the O–H bond in the 2-methoxyethanol (MEOH) molecule. Notably, the relative intensity of the (200) diffraction peak, associated with the tetragonal phase, increases with prolonged thermal treatment [93].

2.2. Advances in Morphologies and Sizes of BiVO₄-Based Structures

The morphology of BiVO₄ material has a significant impact on the photocatalytic efficiency and other applications. The final morphology of BiVO₄ particle is strongly influenced by the preparation method and reaction parameters such as concentration and pH of precursor solution, solvent, reaction temperature, duration, molar ratio of Bi³⁺/V⁵⁺, surfactant and dopant. The morphology of BiVO₄ as photocatalysts significantly enhances the performance of ceramic membranes in oily wastewater treatment [94]. The initial pH of the precursor solution is found to be a critical parameter in defining the phase and final morphology of BiVO₄ particles [95-99]. The BiVO₄ samples prepared by different methods of synthesis have different morphologies and size, such as: i) highly uniform monodisperse nanospheres of 125 nm in diameter [100]; ii) irregular spheres (20-100 nm or few micrometers) [101,102]; iii) hollow spheres (~700 nm) [103, 104]; iv) nanorods (length of 300 nm) [105, 106]. Other BiVO₄ particles with unusual morphologies have been realized, like: needle (50-400 nm) and irregular dog-bone (300-600 nm) [107], butterfly (4-10 µm) [108], leaf peanut (1 – 10 µm) and roundish aggregates (1 – 5 µm) [109-111], polyhedral (6 – 8 µm) [112, 113], decagonal shape rods (2–3 μm) [114], potato and broccoli-like (150 - 500 nm) [115], bowknot (5µm) and dumbbell-like (~3 μm) [116]. Additionally, there are several publications about tz-BiVO₄ spherical nanoparticles with diameters in a range of 10-40 nm and ellipsoidal shape with the radius of about 20 nm [58, 117]. Compared to larger particles or bulk material, nanostructured materials with larger surface areas showed a significantly enhanced reactivity and higher photocatalytic activities for water splitting under UV light irradiation [118, 119].

Field emission scanning electron microscopy (FESEM) technique was generally used to observe the morphologies of BiVO₄ samples. Figures 2a) and 2b) show the prepared BiVO₄ samples like microspheres and dumbbell-like, according to the use of different molecular weight of PEG in the synthesis. The results indicated that BiVO₄ with different microstructures can be selectively synthesized by simply changing the molecular weight of PEG. Also, the morphologies of samples can be controlled through varying the pH value of hydrothermal process, to get as spindle and wheat like BiVO₄ samples, as shown in Figures 2c) and 2d) [116].

The scanning electron microscopy (SEM) images of the BiVO₄ samples with different morphologies obtained for various Bi³⁺/V⁵⁺ molar ratios are shown in Figure 3. When the Bi³⁺/V⁵⁺ molar ratio is 1.0, micrometer-sized dumbbells (constructed from the assembly of many nanorods) with length of about 4–7 μm appear (Figures 3a and 3b), while when the Bi³⁺/V⁵⁺ molar ratio is 0.77, the obtained samples are composed of microrods with average diameter of about 1.4 μm (Figures 3c and 3d). As the molar ratio is reduced to 0.67 and 0.56, the BiVO₄ ellipsoids with diameter of 1.0–1.3 μm and length of 1.5–2.0 μm, consisting of many small nanoparticles, and microspheres with an average diameter of 1.1 μm are obtained, respectively (Figures 3e – 3h). As the Bi³⁺/V⁵⁺ molar ratio is adjusted to 0.5, the BiVO₄ particles with cake-like morphology, constituted by many small nanoparticles, and uniform size about 1.1 μm are formed (Figures 3k and 3l). In order to obtain detailed information on the structure and morphologies of as-synthesized samples, transmission electron microscopy (TEM) additionally can be performed.

2.3. Crystal and Electronic Structure of BiVO₄

This section discusses various crystal structures and the electronic structure of BiVO₄ related to photo-electrochemical properties. BiVO₄ is an n-type semiconductor and can be synthesized in three crystal phases: monoclinic-scheelite (ms), tetragonal-zircon (tz), and tetragonal-scheelite (ts), as mentioned before and shown in Figure 4. The natural structure of BiVO₄ as a mineral is pucherite with the orthorhombic crystal, and this structure cannot be obtained in laboratory [121, 122]. The scheelite structure has a tetragonal crystal system (space group: I41/a where a = b = 5.1470, c = 11.7216 Å) or a monoclinic crystal system (space group: I2/b with a=5.1935, b = 5.0898, c = 11.6972 Å, and β= 90.3871°) [123] whereas the zircon-type structure consists of a tetragonal crystal system (space group: I41/amd with a = b = 7.303 and c = 6.584 Å). In the BiVO₄ scheelite structure, each Bi atom, similar as Gd³⁺ in GdVO₄ matrix, [123] is coordinated by eight oxygen atoms from different VO₄ tetrahedral units and each V atom is coordinated by four oxygen atoms at the tetrahedral site [124], as shown in Figure 4a and 4b, where Bi and V centers are coordinated along the [001] direction. Each oxygen atom is coordinated by two Bi and one V centers, forming a three-dimensional network bridging Bi and V centers together. However, monoclinic scheelite structure shows differences such as more distortion in the local environment of Bi and V ions, two different V–O bond lengths (1.69 and 1.76 Å) and four different Bi-O bond lengths (2.35, 2.37,2.52 and 2.63 Å) that lead to loss of four-fold symmetry [124]. Whereas for tetragonal scheelite structure, all four V-O bond lengths are equal to 1.73 Å, two different bond lengths Bi-O (2.4 and 2.47 Å) exist [124]. The observed significant distortion in the monoclinic scheelite structure enhances the local polarization leading to better electrons and holes separation and superior photo-electrocatalytic activity compared to tetragonal scheelite structure [125]. The local environment of Bi and V centers for zircon-type structure is shown in Figure 4, where eight oxygen atoms coordinate Bi by six different VO₄ tetrahedral units because two VO₄ tetrahedral units provide two oxygen atoms to Bi atom. Each V atom is coordinated by four oxygen atoms [126].

The band gap energy of BiVO₄ allows it to be active in the visible region, with values of 2.4 eV for the scheelite structure and 2.9 eV for the zircon type structure. BiVO₄ does not yet attain practical conversion efficiency. The most limiting factor for BiVO₄ conversion efficiency is the fast recombination of photogenerated electron-hole pairs [127-133]. The density functional theory (DFT) calculations indicated changes in bandgap and density of states and showed that the smaller band gap of monoclinic bismuth vanadate, compared with the zircon type, is coming from hybridization between the Bi 6s state and the O 2p states at the top of valence band [134]. The conduction band is primarily composed by V 3d states, with additional contribution of O 2p and Bi 6p orbitales [135]. The coupling of states results in an upward dispersion of the valence band, and a lowering of the conduction band to a minimum, causing symmetric electron and hole masses, which facilitate a relatively efficient charge carrier separation and extraction [136]. The first-principles band structure calculations demonstrate the direct character of the BiVO₄ band gap [90-92].

3. Advances in Applications of BiVO₄-Based Structures

3.1. Degradation of Organic Compounds: Role of BiVO₄ Based Composites Photocatalysts

This section discusses the degradation of various organic compounds in the presence of the BiVO₄-based structures and composites photocatalysts. In recent years, researchers have been making great efforts to develop effective semiconductor photocatalysts to harness the visible spectrum of sunlight in photocatalytic applications. A method for the degradation of toxic organic compounds/pollutants from environment using semiconducting materials is an attractive approach. Nano-sized ball-like structure of BiVO₄ nanoparticles can act as a good photocatalyst, sensor and heavy metal detector. Photocatalytic efficiency was assessed through degradation studies using methylene blue (MB) dye under visible light irradiation, demonstrating an impressive 93% degradation rate [82, 137, 138]. In the case of the BiVO₄/graphene nanocomposite, effectively degradation of methyl orange (MO) was spotted while the photocatalytic activity increased, resulting in different composites, after excitation of BiVO₄ photocatalyst and generation of the electron–hole pairs [139]. On another side, the BiVO₄@MWCNT photocatalysts were synthesized by incorporation of the synthesized BiVO₄ nanoparticles with various percentages of multi-walled carbon nanotube (MWCNT) and were used as probes for the photocatalytic removal of atrazine (AZ) under visible light illumination [140]. The p-n heterojunction photocatalyst prepared by decorating CuO micro-planks with spherically shaped BiVO₄ was proved to be efficient in degradation MB and Cr(VI) reduction under visible light irradiation. Moreover, two-dimensional (2D) TiO₂ aerogel powder decorated with BiVO₄ (TiO₂/BiVO₄) was used for reduction of toxic Cr(VI) to Cr(III) [141, 142]. The Bi/BiVO₄ microstructures with novel hollow chainlike morphology were successfully fabricated. Owing to the synergistic interaction between them, improved photocatalytic activity was observed for photodegradation of Rhodamine B (RhB) under visible-light illumination, compared to the BiVO₄ and Bi photocatalytic single action. [143]. The synergistic effects of oxygen vacancies and built-in electric fields in GdCrO₃/BiVO₄ and 2D/2D InVO₄/BiVO₄ heterostructures effectively enhanced their photocatalytic performance for nitrate reduction in water and improved their photoelectrochemical activity, respectively, by facilitating charge separation and suppressing recombination [144, 145].

Pharmaceuticals and antibiotics have been classified as critical water pollutants. In order to find a suitable technique for removing them from contaminated water, the photoelectrocatalytic oxidation method has attracted much attention in recent years. The BiVO₄/Ag₂S p-n heterojunction fabricated by using electrodeposition and successive ionic layer adsorption on fluorine-doped tin oxide glass (FTO) or graphitic carbon nitride (g-C₃N₄) decorated with Pt@BiVO₄ have been used for degradation of ciprofloxacin and sulfamethoxazole [146, 147].

Figure 5 shows the UV–visible absorption peak spectra related to the degradation of MB dye in 24 hours, as a function of the magnetic stirring rotation speed.

The degradation rate efficiency (D) can be calculated using Eq. (1). The degradation of dye catalyzed by BiVO₄ is analyzed using the pseudo-first-order kinetics model (Eq. (2)), and the rate constant of dye degradation is calculated [149].

$$D\left(\mathrm{\%}\right)={\displaystyle \frac{Co-Ct}{Co}}\times 100$$

Eq. (1)

$$-ln\left({\displaystyle \frac{Ct}{Co}}\right)=Kt\times t$$

Eq. (2)

where C₀ is the initial solute concentration, $\mathrm{C}\mathrm{t}$ is the solute concentration at a certain reaction time (t), and k is the pseudo-first-order rate constant.

On other side, the sonocatalysis activity of BiVO₄/FeVO₄ composites was investigated by analyzing the removal process of the drug model tetracycline (TETR). In comparation to BiVO₄, the composites exhibited significantly enhanced sonocatalytic activity in degrading TETR due to the formation of type-II heterojunctions, which promoted the effective electron-hole pairs separation [150].

3.2. Antimicrobial Activity of the BiVO₄-Based Structures

BiVO₄ has also been studied as potential non-toxic material for biomedical applications due to its antibacterial activity against various pathogenic bacteria. The antibacterial activity of this material was estimated by using pathogenic microbes Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) [151]. The photo-induced high antimicrobial activity of monoclinic-BiVO₄ nanoparticles has also been tested in antimicrobial efficiency and environmental remediation applications based on its photocatalytic activity targeting organic dyes and different water pollution [152]. The BiVO₄@activated carbon fiber can be used as antibacterial agent against both E. coli and S. aureus with enhanced recyclability [153]. Antibacterial efficacy was assessed against by Gram-positive (E. coli and S. aureus) and Gram-negative (E. coli and Pseudomonas aeruginosa) bacteria using BiVO₄ and Ta-doped BiVO₄ nanoparticles at various concentrations [154]. Bacterial cultures were incubated with Ta-BiVO₄ nanoparticles under visible light illumination and in dark for the antibacterial assays. The antimicrobial activity of Ta-doped BiVO₄ nanoparticles was successfully assessed indicating their potential as strong antimicrobial agents against both Gram-positive and Gram-negative bacteria [154-156]. Antimicrobial activity of BiVO₄ nanoparticles against six different bacterial strains is presented in Figure 6.

The antibacterial activity and the generation of reactive oxygen species (ROS) of BiOCl/BiVO₄ heterojunction can be improved compared to BiVO₄ [157]. The ROS produced in the PDA-rGO/BiVO₄ heterojunction blocked the transmembrane transport of bacteria, as confirmed by using ROS fluorescence detection [158]. The antibacterial and UV protection properties of the prepared CuO/BiVO₄@cotton were also studied. The CuO/BiVO₄ nanocomposite has overcome the low quantum efficiency of pure BiVO₄. The nanocomposite possesses very good recyclability of the photocatalyst on cotton fabrics/flexible textile with multiple functions beside degradation of pollutants from waste water [159]. The normal inactivated tested bacterial strains with a smooth surface attributed to the production of ROS species such as OH and O²⁻, which easily cross the cell membrane and enter the interior of the strains [160, 161].

3.3. Application of the BiVO₄-Based Structures in Food Industry

The BiVO₄ is an ideal starting material for antioxidant surveillance under visible light irradiation. Due to unsatisfactory charge collection and utilization in practical applications, the BiVO₄ is usually doped to exploit the effects of dopants on the photocatalytic behavior under visible light illumination. The substitution of Bi³⁺ or V⁵⁺ ions with other M³⁺ or M⁶⁺ metal ions leads to significant changes in physical properties such as structural distortions of crystal unit, formation of a new phase, and to evident changes in morphological, optical and electrical properties due to the different ionic radii of the involved ions. This proof-of concept is useful in detection of antioxidant capacity in the foodstuff industry, opening up a bright future in cosmetic and healthcare areas [162]. A label-free photoelectrochemical (PEC) sensor based on BiVO₄@GO composites was prepared for detection of antioxidants and antioxidant capacity of food. Large surface area and good conductivity make the BiVO₄@GO composites an unique promising sensor for detection of antioxidant capacity in food, which will help the organism to take enough antioxidants to defend against free radicals [163], while the crystal-reconstructed BiVO₄ PEC biosensor can be applied in the fields of multi-tumor or viral biomarker detection [164]. The composition and pH of the electrolyte, applied bias, as well as surface morphology of the photoactive layer can have significantly effect on the selectivity and use of the PEC sensors [165]. The synthesized BiVO₄ PEC sensor with unique carnation-like morphology and high specific surface resulted in high possibility for Cr(VI) detection, with a wide linear range of 2–210 μM and a very low limit of detection (LOD) of 0.01 μM. This BiVO₃ based PEC sensor can be used, for instance, in food safety monitoring for Cr(VI) detection in peanuts, rice, soil and tap water, with satisfactory recovery rates of 90.3 to 103.0% [166].

Propyl gallate is widely used in the food industry as one of the most important additives to prevent the oxidation processes. The synthesized Cu₃(PO₄)₂/BiVO₄ composites and GCE/BiVO₄/ZrO₂@graphene electrode can be used for determination of propyl gallate and of acetaminophen, phenylephrine hydrochloride and cytosine, respectively, in different food products. These materials are beneficial for food quality monitoring and reduce the risk of propyl gallate overuse in food and are recommended for potential medical applications [167, 168]. A new photosensitive sensor was developed for the successful electrochemical analysis of quercetin from natural samples using ITO/MWCNT@PC@BiVO₄ composite due to the photosensitivity and stable structure of BiVO₄ high electron permeability of MWCNT and advantageously electron transfer [169, 170].

The synergic effects between g-C₃N₄ and BiVO₄ in the g-C₃N₄/BiVO₄ composites used as a photoactive material produced photocurrent signals resulting in an increased photocurrent response after the composites fix on the surface of the FTO electrode [171]. The illustration of g-C₃N₄/BiVO₄ PEC sensor for tetracycline (TC) residue detection in food samples (honey) and animal products (kidney, milk, pork) is presented on the Figure 7.

3.4. Applications of the BiVO₄-Based Structures as Photoelectrode in Water Splitting

In recent years, in addition to the application in the environmental protection of the BiVO₄-based structures for organic pollutant degradation, superior antimicrobial properties and food safety monitoring, attention has been also paid to the research focused on multipotential applications of these structures [172]. BiVO₄ -based photoelectrodes show great potential in several practical applications. Due to good visible light responsiveness and stable optoelectrical properties, the BiVO₄ -based structure can be widely used for efficient water splitting and hydrogen production [173, 174].

To fabricate photoanodes with large-area possessing excellent and uniform PEC activities, it is very important to include defect states of the material. Following this way, highly efficient BiVO₄ photoanodes containing adjustable concentrations of bismuth and oxygen vacancies, Bi_vac. and O_vac, were fabricated [175]. With the aim to promote charge separation in the bulk material, the BiVO₄ based photoanode with lattice strain was prepared by generating Bi vacancies [176]. In addition, BiVO₄ photoanodes were also used in PEC cells for the production of various chemicals, as well as for PEC water splitting [177].

Water splitting based on numerous visible-light-responsive photocatalysts is one of the most important and cost-effective approaches for the conversion of solar energy into clean and renewable hydrogen energy and production of green H₂ on a large scale. This result can be easily achieved in a one-step excitation system using a single photocatalyst or Z-scheme strategies based on a pair of photocatalysts [178, 179]. Because photoelectrochemical water splitting techniques often require a corrosive electrolyte, limiting their stability and environmental sustainability, the alternatively way for clean production of hydrogen can be obtained directly from sunlight and water by photocatalytic water splitting [180].

The BiVO₄ material possesses several limitation parameters including low charge mobility, high bulk recombination rates and oxygen evolution reaction (OER) kinetics at the surface for its use as high-performance PEC photoanodes for solar water splitting. Numerous strategies have been applied to improve these performances and the texture engineering has emerged as a promising approach. One approach for improving the PEC efficiency is based on the controlling the crystallographic orientation and exposed facets which enhancing charge transport and reduce surface recombination [181]. Following this direction in preparation, the octadecahedral-BiVO₄ photoanodes was successfully produced with exposed {040}, {011} and high-reactivity {121} facets. It was shown the charge separation is dramatically improved and the {121} facets show better oxygen evolution reaction (OER) activity for triggering water oxidation than the {040} and {011} facets [182]. Other approaches for improving the PEC efficiency are based on different morphologies, doping, modification and making different BiVO₄-based composites. The BiVO₄ nanowires due to negative surface photovoltage signal are suitable for the construction of membranes for solar energy conversion [183]. A suitable doping concentration of Cu in BiVO₄ resulted in enhanced electronic conductivity and improved charge transfer dynamics compared to un-doped BiVO₄ [184, 185].

Multi-interfacial optimization of BiVO₄-based composites to improve charge separation efficiency due to synergistic effect within the material matrix has emerged as a main strategy for improving PEC performance [186]. Ti₃C₂ quantum dots-modified BiVO₄ photoelectrodes, BiVO₄/Ti₃C₂ QDs, for water splitting H₂ production, showed improved photoelectron-hole pairs separation and photocurrent density of about 2.5 times higher than that of bare BiVO₄ [187]. On other side, due to the synergistic effect of CuSCN and Ni: FeOOH, the photocurrent density of the optimized BiVO₄/CuSCN/Ni: FeOOH photoanode is 3.39 times higher than that of pure BiVO₄ [188]. Similar ratio in photocurrent density is obtained for photoanode BiVO₄/Co,Fe-NTMP (nitrilotris methylene phosphonic acid) and BiVO₄ [189]. The RGO@g-C₃N₄/BiVO₄ photocatalysts has dual applications in photo electrocatalytic H₂ production and antibiotics tetracycline chloride degradation. Triple composites of g-C₃N₄/RGO/BiVO₄, formed by the synergistic effect between BiVO₄, RGO and g-C₃N₄, show significant photocatalytic activity compared to pure BiVO₄ or g-C₃N₄ [190]. Recently, the Ag/BiVO₄ composite for application in H₂O₂ fuel cell was fabricated. Compared with BiVO₄ nanoplates, the Ag/BiVO₄ composite has a narrower band gap, enhanced visible light absorption and high photocatalytic activity and it provides a new strategy model for the efficient conversion and utilization of solar energy [191]. On other hand, 10 cm² perovskite-BiVO₄ tandem PEC devices were fabricated with a selective Cu92In8 alloy catalyst which could demonstrate syngas production coupled to O₂ evolution over 36 hours [192].

An example of the photocatalytic system based on the BiVO₄ composites is presented in the scheme shown in Figure 8. The system is composed of two separate reaction parts: a hydrogen evolution cell containing halide perovskite photocatalysts (MoSe₂-loaded CH(NH₂)₂PbBr_3-xI_x) and an oxygen evolution cell containing NiFe-layered double hydroxide modified BiVO₄ photocatalysts mediated by the I₃⁻/ I⁻ redox shuttle [193].

4. Conclusions

This review has provided a systematic study and a related bibliography concerning developments, preparation methods, structures, different properties applications and recent advances of the research on BiVO₄-based structure. Additionally, a detailed discussion was given regarding the preparation approaches, processes, morphology, crystal and electronic structure, performance, and application perspective of these materials. A method of degradation of toxic organic compounds/pollutants in the environment using semiconducting materials is a very attractive approach. Photodegradation of various organic compounds methylene blue, Rhodamine B, atrazine, as well as pharmaceutical species, such as ciprofloxacin and sulfamethoxazole, in the presence of the BiVO₄-based photocatalysts under visible light irradiation was explained in detail. BiVO₄ has been studied as the non-toxic material with high potential for biomedical applications due to antibacterial activity against various pathogenic bacteria microbes, e.g., Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Additionally, BiVO₄ has been studied as an ideal starting material for antioxidant surveillance under visible light irradiation, for detection of antioxidant capacity in the foodstuff monitoring, opening up a bright future for these materials also in the cosmetic and healthcare areas. Moreover, the BiVO₄ as biosensor can be applied in the fields of multi-tumor or viral biomarker detection. Due to good visible light responsiveness and stable optoelectrical properties, BiVO₄-based structures have also been studied for photoanode for efficient water splitting and hydrogen production, as well as high-performance photoelectrochemical sensors.

Looking into the future, research on the BiVO₄-based structures should be focused on developing novel synthesis techniques for nanostructure design and for the industrial-scale production processes, with the aim of improving the charge separation efficiency, the photocatalytic performance, the antibacterial activity and photoelectrochemical efficiency. These goals may be achieved by controlling the crystallographic orientation and long-term stability of BiVO₄ -based photoanodes. As to the existing concern for the preservation of the environment, continuous exploration and optimization of BiVO₄-based structures in the field of PEC water splitting will certainly significantly contribute to the development of a global system in the direction of clean, renewable and sustainable energy.