Fundamentals and environmental applications of bismuth vanadate through photoelectrocatalysis

Abstract

This review outlines the fundamentals of photoelectrocatalysis (PEC) using BiVO₄ as a promising photoelectrode for environmental applications. Key PEC factors—light absorption, semiconductor properties, potential, temperature, and pH—are discussed. BiVO₄’s optical, structural, and stability traits are compared with other vanadates. Synthesis methods (electrodeposition, SILAR, sputtering, and others) and performance-enhancing strategies (doping, heterojunctions, high-exposed facets) are examined. BiVO₄’s pollutant degradation pathway and dual-function PEC systems are explored, emphasizing the balance between properties for efficient energy conversion and environmental cleanup.

Introduction

The global increase in environmental contamination—driven by population growth, agricultural expansion, industrial activity, and poor waste management—has led to the widespread pollution of air^1,2,3,4, soil⁵, surface water^6,7, and groundwater^8,9. Organic pollutants, including dyes, surfactants, plastics, compounds of emerging concern (CECs), and human waste, have accumulated in aquatic and terrestrial ecosystems, presenting critical challenges to environmental and public health. In response, significant research has been devoted to developing efficient methods for the degradation or removal of these persistent contaminants, particularly in aqueous environments¹⁰. Among the most effective technologies, advanced oxidation processes (AOPs) have gained prominence due to their ability to generate highly oxidizing reactive oxygen species (HOROS), which initiate complex oxidation cascades that break down organic molecules into less harmful or value-added products or mineralized forms¹¹. Within this class, photoelectrocatalysis (PEC) has emerged as a promising approach for degrading recalcitrant organic compounds, offering enhanced performance through the combination of photocatalysis and electrochemical assistance. As shown in Fig. 1, research in PEC has rapidly expanded in recent years, with continuing growth anticipated.

Fig. 1: Number of annual publications from 2000 to 2023.

Initial search terms: (“photoelectrode” OR “photoanode” OR “photoelectrochemistry” OR “photocathode” OR “photoelectrochemical”); second search added: AND (“BiVO4” OR “Bismuth vanadate”). Search conducted on Scopus, July 29, 2024.

A PEC system typically consists of a photoactive electrode (photoanode or photocathode), usually a semiconductor metal oxide, operated under an external bias potential or current¹². Common materials for photoanode fabrication include TiO₂¹³, WO₃¹⁴, ZnO¹⁵, Cu₂O¹⁶, CdS¹⁷, FeS¹⁷, Fe₂O₃¹⁸, Bi₂WO₆¹⁹, BiVO₄²⁰, as well as perovskite-based oxides (ABO₃)^21,22. Among these, TiO₂ has been extensively studied due to its non-toxicity and affordability. However, its photocatalytic activity is restricted to the ultraviolet region of the spectrum²³, prompting the search for visible-light-photocatalytic alternatives. Bismuth vanadate (BiVO₄) has gained considerable attention due to its relatively narrow bandgap (~2.4 eV) and visible-light photoactivity, which make it suitable for applications such as organic dye degradation and water splitting^24,25. As illustrated in Fig. 1, BiVO₄ has become one of the most widely studied materials in PEC research. Given that Earth receives approximately 8.6 × 10⁴ TW year⁻¹ of solar energy—vastly exceeding the ~13.2 TW consumed globally in 2019^26,27—PEC represents a compelling route for solar-to-chemical energy conversion.

BiVO₄ is particularly attractive for such applications due to its low cost, environmental safety, and photochemical stability, making it well-suited for large-scale implementations in environmental remediation and hydrogen generation.

In PEC systems, the application of an external bias facilitates charge separation and suppresses electron–hole recombination, thereby improving overall photocatalytic efficiency. Further enhancements through doping, morphology control, and heterojunction engineering have continued to improve the performance of BiVO₄-based electrodes. Current research trends and degradation efficiencies in PEC systems are critically evaluated²⁸.

Despite the extensive knowledge surrounding BiVO₄, recent reviews have primarily focused on water-splitting and photocatalytic applications. In contrast, this review provides a comprehensive analysis of the physicochemical principles governing PEC systems based on BiVO₄, with particular emphasis on environmental remediation. We examine charge generation, separation, and transfer mechanisms in BiVO₄ photoanodes and assess their impact on the degradation of various pollutants. Additionally, we explore synthesis strategies and surface modification techniques—including doping and structural optimization—highlighting their role in improving charge transfer, reducing recombination, and enhancing overall system performance. These insights underscore the potential of BiVO₄ as a sustainable, multifunctional material for efficient solar-to-chemical energy conversion and environmental applications

Catalysis, electrocatalysis, photocatalysis, and photoelectrocatalysis

Catalysis, electrocatalysis (EC), photocatalysis (PC), and PEC are distinct yet interrelated processes that utilize catalysts or catalyst-like materials to facilitate chemical transformations under varying conditions. Catalysts work by lowering the activation energy of a reaction, thereby increasing the reaction rate without altering the thermodynamic equilibrium. These reactions typically require external inputs such as elevated temperature or pressure to initiate or sustain reaction kinetics (Fig. 2a). EC involves an electrode—typically a metal—that enhances electrochemical reactions at its surface, driven by an external electrical potential or current (Fig. 2b). PC involves the absorption of photons by the photocatalyst, which generates electron-hole pairs. These pairs then participate in redox reactions to degrade pollutants or produce useful chemicals (Fig. 2c). Lastly, PEC combines light irradiation and an applied electrical potential to enhance reaction rates; the catalyst functions under both conditions, promoting redox reactions and reducing recombination losses, making it particularly effective for processes like hydrogen production and environmental remediation (Fig. 2d). BiVO₄ can function as both a photocatalyst and photoelectrocatalyst. Therefore, while this discussion covers PC, it primarily focuses on PEC.

Fig. 2: Types of Catalytic Processes.

a heterogenous catalysis, b electrocatalysis, c photocatalysis, and d photoelectrocatalysis (the flow of e⁻ and h⁺ depends on the type of semiconductor (n or p) so the charge balance is defined at another electrode with the complementary reaction).

Catalysis

Catalysis (Fig. 2a) is a phenomenon where traces of a foreign material (the catalyst) increase the rate of chemical reactions²⁹. The catalytic process typically involves the following elementary steps: (i) Diffusion of the substrate to the catalyst surface; (ii) Adsorption of the substrates onto the active sites of the catalyst; (iii) Chemical reaction forming products on the catalyst surface; (iv) Desorption of the products from the active sites; (v) Catalyst regeneration for new chemical reactions (though catalysts are not indefinitely reusable)^29,30.

At the molecular level, a chemical reaction occurs when reactants collide with sufficient energy to overcome the activation energy barrier, leading to bond-breaking and the formation of new, more stable bonds, molecules, or atoms (i.e., products). A catalyst facilitates this process by spontaneously adsorbing one of the reactants, which causes the molecules to lose rotational and translational degrees of freedom. This process helps break the bonds of the adsorbed reactant, guiding the reaction down a more energetically favorable pathway. As shown in Fig. 3a, the free energy of the catalyzed (blue line) and uncatalyzed (green line) reactions is the same, meaning the catalyst does not affect the equilibrium constant, which depends only on the reactants and products. In other words, the catalyst changes reaction kinetics by generating intermediates (IM–n, related to adsorption, reaction, and desorption processes) to bypass traditional transition state (TS) (Fig. 3) but does not alter the thermodynamics of the chemical reaction^29,30,31.

Fig. 3: Reaction Energy Profile and Degradation Performance.

a Typical reaction energy diagram of the uncatalyzed reactions (green line) and catalysis reaction (blue line), where TS is a transition state, IM-n are intermediates. Adapted from Kakaei et al.²⁹ b Removal efficiency percentage of polystyrene microplastics by electrooxidation. Taken from Kiendrebeogo et al.³⁹.

Electrocatalysis

EC is a heterogenous catalytic process involving electrochemical reactions on the electrode–electrolyte interface (Fig. 2b). In this process, the electrode serves as both an electron donor/acceptor and catalyst, hence it is termed “electrocatalyst”^32,33. EC can also be defined as the introduction of catalytic materials on the electrode surface to increase the rate of electrochemical reactions while using a potential perturbation value equal to or less than that used without a catalyst; this generally results in a substantial increase in electron transfer efficiency³⁴. This latter definition closely resembles that of heterogeneous catalysis but incorporates additional assistance by electric potential perturbations.

A helpful example used to explain EC is the hydrogen evolution reaction (HER) in Reaction 1³⁵. As reactions with an outer-sphere mechanism are generally not considered electrocatalytic, we will focus on the typical HER pathway³⁶. Thus, the formation of H₂ occurs according to (i) the Volmer reaction, electrosorption with formation of ^•H (Reaction 2), followed by (ii) the Heyrosky reaction, electrochemical reaction between ^•H and H⁺ (Reaction 3) or (iii) the Tafel reaction (chemical reaction between two adjacent ^•H).

As in heterogenous catalysis, here, the final two steps involve product desorption and electrode reactivation. Reaction 1 does not occur spontaneously, as the electrode must donate an electron to the proton for the reaction to progress.

$$2{{\rm{H}}}^{+}+2{{\rm{e}}}^{-}\rightleftarrows {{\rm{H}}}_{2}{\qquad}{{\rm{E}}^\circ }_{{\rm{H}}^{+}/{{\rm{H}}}_{2}}={\ }0.0{\ }{vs}.{\ }{\rm{NHE}}$$

(1)

$${\rm{H}}^{+}+{{\rm{e}}}^{-}+{\rm{M}} \rightleftarrows {{\rm{M-H}}}^{\bullet }{\qquad}{\rm{(Where\ M\ is\ the\ electrode\ material)}}$$

(2)

$${{\rm{M-H}}}^{\bullet }+{{\rm{H}}}^{+}+{{\rm{e}}}^{-} \rightleftarrows {\rm{M}}+{\rm{H}}_{2}$$

(3)

$${2}{{\rm{M-H}}}^{{\bullet{}}}\rightleftarrows 2{\rm{M}}+{{\rm{H}}}_{2}$$

(4)

The interaction between the electrode and the reactant significantly influences the reaction rate. For instance, in the case of H₂ production, the electrode material reduces the reaction kinetics; if Pt is replaced with Hg, the reaction rate decreases^35,37. This relationship is illustrated in Table 1, where the exchange current j_o represents the charge exchange rate at equilibrium per unit area. A larger j_o values indicates more a favorable charge transfer and faster reaction, whereas a smaller value corresponds to a slower rate³⁶.

Table 1 Exchange current densities for hydrogen evolution using different metals as electrodes in 1.0 M H₂SO₄

Another crucial factor in EC is the overpotential. In electrochemical systems, increasing the overpotential generally increases the reaction rate; however, the work potential is based on the objective of the assay. For instance, in electrosynthesis in aqueous media, if the overpotential exceeds the potential of O₂ or H₂ production, the Faradaic efficiency (FE) may decrease or by-products could be produced.

The electrocatalytic process has gained significant attention in the field of wastewater decontamination. Its versatility has enabled the development of complex systems. A notable example is the anodic oxidation of polystyrene microplastics on a boron-doped diamond (BDD) electrode, which exemplifies the potential of electrocatalysis in addressing persistent pollutants. In this process, both the electrolyte and water undergo electrooxidation via the abstraction of one electron, initiating the formation of reactive species capable of attacking and breaking down the polymer matrix. This process can generate HOROS such as hydroxyl radical (^•OH) and sulfate radical (${\mathrm{SO}}_{4}^{\bullet -}$) by mean reactions of single electron transfer promoted by the external supply. In the studies of Lu et al.³⁸ and Kiendrebeogo et al.³⁹ show that these oxidized species have the most important contribution in the degradation^38,39. Since these radicals are what degrades the polystyrene microplastics, it is referred to as indirect oxidation. Additionally, by changing the material nature, the degradation present and important change, too. BDD achieved up to ~90% of removal efficiency while electrodes such as iridium oxide (IrO₂) and mixed metal oxides (MMO) just achieved up to ~20% and ~10%, respectively³⁹ (Fig. 3b). This significant contrast in the degradation percentages is because BDD is a pour catalyst for O₂ evolution (the oxygen evolution reaction is slow), which makes it in a good catalyst to produce ^•OH^40,41,42. It reveals how the electrocatalyst can form “exotic species” and how the material influences in their formation, as discussed in this section.

Photocatalysis

PC is a type of heterogenous catalytic process in which a chemical reaction is accelerated in the presence of a photocatalyst under light irradiation, typically described in terms of photon energy (hν), Fig. 2c⁴³. Depending on the photocatalyst’s physical state, PC can be classified as homogenous or heterogenous. In heterogenous PC, the photoactive material is usually in the solid state and can take various forms⁴⁴. The PC process involves the following four steps: (i) Irradiation of the photocatalyst, using either sunlight or artificial light sources (e.g., LEDs or traditional lamps) whose energy exceeds that of the material’s bandgap energy (hν > E_bg); (ii) Excitation of electrons (e⁻) from the valence band (VB) to the conduction band (CB), generating holes (h⁺) in the VB; these charge carriers are referred to as “photogenerated ${{\rm{e}}}_{{\rm{CB}}}^{-}$/${{\rm{h}}}_{{\rm{VB}}}^{+}$ pairs”⁴⁵; (iii) Adsorption of the substrate onto the photoactivated catalyst surface; (iv) Redox reaction of the substrate⁴⁶. The efficiency of the process primarily depends on the photocatalyst material, which must satisfy the following criteria⁴⁷: (i) an appropriate energy difference between the CB minimum (E_CB) and VB maximum (E_VB); (ii) suitable band-edge positions of E_CB and E_VB, that provide the required redox potentials to drive the desired reduction/oxidation reactions; and (iii) favorable dynamics of the photogenerated ${{\rm{e}}}_{{\rm{CB}}}^{-}$/${{\rm{h}}}_{{\rm{VB}}}^{+}$ pairs, including low recombination rates, sufficient diffusion lengths, high mobility, and efficient charge transfer.

In PC, charge transfer reactions occur simultaneously, with reduction taking place in the CB and oxidation in the VB. However, one of these half-reactions typically limits the overall process, which can be kinetically described as: d[h⁺]/dt = d[e^-]/dt. It is therefore essential to distinguish PC processes based on their energy balance. When the oxidation of organic compounds is coupled with hydrogen (H₂) generation, the reaction is classified as photosynthetic, owing to its positive Gibbs free energy change under dark conditions. In contrast, if charge neutrality is maintained via O₂ reduction, the process is considered conventional photocatalysis. Nevertheless, photoelectrochemical (PEC) systems provide the most direct and accurate means of characterizing and quantifying these processes⁴⁸. PC is currently regarded as one of the most promising approaches for wastewater treatment, as it facilitates the generation of ^•OH, superoxide radicals (${{\rm{O}}}_{2}^{\bullet -}$), and photogenerated holes (${h}_{{VB}}^{+}$) under light irradiation⁴⁹. These HOROS act as strong oxidizing agents that attack a wide range of organic compounds, initiating sequential reactions that lead to extensive oxidation and, in some cases, complete mineralization. Consequently, PC is classified as an AOP⁴⁶.

For instance, in the photocatalytic degradation of Basic Red 46 using an Fe-doped BiVO₄/SnO₂ photocatalyst, the formation of excitons in the composite triggers the formation of HOROS. Initially, the system undergoes a 30-minute adsorption–desorption equilibrium stage in the dark, during which the pollutant concentration in the bulk solution slightly decreases. Upon light irradiation, oxygen molecules in the medium react with ${{\rm{e}}}_{{\rm{CB}}}^{-}$ in CB from the SnO₂ component to form ${{\rm{O}}}_{2}^{\bullet -}$, while water molecules are oxidized by ${{\rm{h}}}_{{\rm{VB}}}^{+}$ in VB of the Fe-doped BiVO₄ to yield ^•OH. According to Abrishami-Rad et al.⁵⁰, both radical species significantly contribute to the degradation of Red 46 (Fig. 4a).

Fig. 4: BiVO₄ on PC and PEC.

a Photocatalytical degradation of Red 46 using Fe-doped BiVO₄/SO₂ composite in several percentages of dopping⁵⁰. b Glycerol-to-DHA reaction on BiVO₄, illustrating the general PEC mechanism on a photoanode. Original schematic illustration by Liu et al.²⁸¹.

Photoelectrocatalysis

PEC is a heterogenous PC process assisted by the application of a relatively small external potential or current, Fig. 2d^12,51. In a typical PEC system, the working electrode is a photoelectrode consisting of a semiconductor-based photocatalyst supported on a conductive substrate (electrode). Upon illumination with light of energy greater than the photocatalyst’s bandgap, the photoelectrode becomes photoactivated, while the applied external potential facilitates charge separation and transport by minimizing recombination losses. The specific perturbation depends on the characteristics of the photoelectrode (n-type or p-type) and the desired reaction (photoelectrooxidation or photoelectroreduction). Instead of potential perturbation, external current perturbations can also be used, either cathodic (i_c) or anodic (i_a)⁴⁵. PEC is considered an AOP when the photoelectrogenerated holes in the VB have an oxidation potential high enough to produce ^•OH from water oxidation. ^•OH is one of the strongest oxidants in nature, chemically green, and non-selective (E°_•OH/H2O = 2.74 V vs. SHE), making it suitable for degrading (oxidizing) persistent organic compounds, either to achieve complete mineralization or to generate products useful for circular economy purposes¹². Photoelectrochemical oxidation occurs through both ^•OH and photoelectrogenerated ${h}_{{VB}}^{+}$. For instance, the proposed PEC mechanism of glycerol (substrate) on BiVO₄ is outlined in Fig. 4b: (i) Light generates ${e}_{{CB}}^{-}$/${h}_{{VB}}^{+}$ pairs; (ii) The substrate adsorbs onto the surface; (iii) Holes react with the substrate; (iv) The generated oxidizing agents react with the organic compound (degradation pathway) or with itself (recombination); (v) The molecule undergoes dehydration; (vi) The derivatized substrate desorbs from the surface.

Factors affecting the photoelectrocatalytic process based on BiVO₄

Effect of light in PEC systems

For BiVO₄, which has a bandgap energy (E_bg) of 2.4 eV, photons with a wavelength of at least 517 nm (within the visible spectrum) are required to initiate excitation, according to the relation E = hc/λ. Photons with energy greater than E_bg can effectively induce charge separation through light absorption⁵². In a PEC system, Tateno et al.⁵³ investigated the oxidation of 1-phenyl ethyl alcohol under illumination with various wavelengths. As shown in Fig. 5b, lower-energy (longer-wavelength) light required approximately 2.5 times more applied potential to achieve comparable FE. This behavior correlates with the light-harvesting efficiency of the BiVO₄ photoanode (Fig. 5a), which exhibits a maximum response around 460 nm. At longer wavelengths, the efficiency declines sharply due to reduced photon absorption.

Fig. 5: Effect of incident wavelength and applied potential on PEC processes.

a Light-harvesting efficiency spectra of BiVO₄/WO₃ composite, BiVO₄, and WO₃ photoanodes. b Effect of wavelength on the photoelectrocatalytic oxidation of 1-phenyl ethyl alcohol. Adapted from Tateno et al.⁵³.

The reduction in FE under low-energy illumination can be rationalized by fundamental principles of semiconductor physics. When a light beam with energy close to the semiconductor’s E_bg irradiates its surface, an electron in the VB is excited to the CB (Fig. 6a). However, such an electron may recombine with the hole via radiative recombination, emitting a photon with energy close to E_bg (Fig. 6c). On the other hand, when the light energy is significantly greater than E_bg, the excited electron transitions to a higher energy level, typically a non-bonding molecular orbital in the CB. In this case, instead of remaining at the CB edge, the electron undergoes a process called “relaxation,” descending to the CB edge while releasing energy as heat (Fig. 6b)⁵⁴. Therefore, PEC efficiency depends on the overlap between the charge carrier energy levels and the allowed states in the molecular orbitals of the reactive species at the interface. In the PEC process, maximizing the number of available charge carriers is essential, as they are driven toward the external circuit with the aid of an external potential.

Fig. 6: Fundamental photophysical processes in semiconductors.

a one photoexcited electron jumps from the valence band to the conduction band; b relaxation of the electron from a non-bonding molecular orbital to a lower-energy non-bonding molecular orbital; c exciton recombination.

Light intensity is another crucial parameter that directly influences the efficiency of PEC systems. Zainal et al.⁵⁵ examined the PEC degradation of methyl orange using TiO₂ under illumination from a 300 W tungsten lamp. By adjusting the lamp’s intensity to 100%, 75%, 50%, 25%, and 0%, they monitored dye degradation over 2 hours. The results demonstrated that the degradation rate increased with light intensity, the greater the photon flux incident on the semiconductor surface, the higher the generation rate of photogenerated ${e}_{{CB}}^{-}$/${h}_{{VB}}^{+}$ pairs, thereby promoting the formation of HOROS and accelerating dye degradation. The relationship between degradation efficiency and light intensity exhibited an exponential trend, highlighting the importance of considering this parameter in the optimization of PEC processes^55,56. Notably, any factor that affects light intensity or wavelength—such as solution turbidity or excessive absorbance—can impair charge carrier generation and thus reduce PEC performance. Optimizing the direction of light irradiation is a practical strategy to mitigate these losses. For instance, semi-transparent BiVO₄-modified electrodes have shown improved performance when illuminated from the back side, as this configuration enhances charge carrier generation at the electrode–semiconductor interface, facilitating their transfer into the external circuit⁵⁷. Moreover, placing the light source inside the reactor ensures more direct illumination of the photoelectrode, minimizing losses due to scattering and absorption by the solution⁴⁵. Thus, PEC cell design must meet two key requirements: (i) maximizing photoelectrode light absorption and (ii) enhancing mass transfer within the reactor system⁵⁸.

Semiconductor properties and photoelectrode behavior

Semiconductors are classified into n-type and p-type based on their majority charge carriers. In n-type semiconductors, the Fermi level (E_F) lies closer to the CB, making holes generation in the VB more favorable. Conversely, p-type semiconductors have E_F near the VB, thereby facilitating electron production. In simpler terms, n-type semiconductors have electrons as majority carriers and holes as minority carriers, while p-type semiconductors exhibit the opposite behavior⁵⁹. Based on this classification, PEC electrodes can be categorized as photoanodes (n-type semiconductors), which facilitate oxidative processes under appropriate illumination, and photocathodes (p-type semiconductors), which promote reductive processes.

The photoelectrode type plays a crucial role in the PEC process and its applications. This distinction is evident when comparing the photocurrent response of p-type and n-type BiVO₄ under various applied potentials. In one study, tetragonal zircon phase-type BiVO₄ -tz-BiVO₄ (commonly referred to as p-BiVO₄ due to its p-type characteristics) was synthetized via a hydrothermal method and deposited onto a fluorine-doped tin oxide (FTO) electrode. Subsequent annealing at different temperatures to obtain monoclinic scheelite-type BiVO₄−ms-BiVO₄ (n-BiVO₄) (Fig. 7a). As confirmed in the Mott–Schottky plots (Fig. 7c); a clear shift in slope at 450 °C indicates a transition from p-type to n-type conductivity. This transition substantially impacts the lineal sweep voltammetry (LSV) test under chopped light illumination (Fig. 7b). Specifically, p-BiVO₄ exhibited enhanced photoactivity at cathodic potentials, consistent with its reductive behavior, while n-BiVO₄ showed superior performance under anodic conditions, supporting oxidative reactions⁶⁰.

Fig. 7: Structural and electrochemical characteristics of BiVO₄ synthesized at different temperatures.

a X-ray diffraction patterns, b LSV curves under chopped light, and c Mott–Schottky plots. Adapted from Wang et al.⁶⁰.

Semiconductor morphology a critical role in determining PEC performance. For instance, a porous BiVO₄/FTO photoanode exhibited an improvement of over 99% in FE and 87.4% in selectivity compared to a conventional BiVO₄/FTO photoanode. These enhancements are attributed to reduced charge recombination, improved charge separation, extended light absorption, and a larger effective surface area^61,62,63. Unlike conventional PC, where the semiconductor is typically dispersed in suspension, PEC offers greater operational efficiency by suppressing charge recombination, enhancing the synergy between electrochemical and photochemical pathways, and eliminating the need for photocatalyst recovery thus enabling the reuse of durable photoelectrodes. In PEC systems, however, it is important to consider both the exposed facet of the semiconductor—commonly addressed in PC—and the orientation of the crystal on the electrode surface, which is uniquely relevant to PEC. With respect to facet exposure, Zhang et al. investigated BiVO₄ photoanodes with dominant (010), (010)/(110), and (110) facets. Their findings demonstrated that photoanodes dominated by the (010) facet exhibited superior photoelectrochemical performance, attributed to the deeper valence band energy associated with this crystallographic orientation^64,65.

The impact of crystal orientation was further explored by Lu et al.⁶⁶, who compared two fabrication strategies: a seed-layer method to control facet exposure and conventional electrodeposition to generate a nanoporous surface. A randomly oriented surface, produced via hydrothermal synthesis, showed a low photocurrent density of 0.3 mA cm⁻², while exposure of the high-exposed (040) facet led to a substantial increase in photocurrent—reaching 2.2 mA cm⁻²—surpassing even the classical electrodeposition approach (1.3 mA cm⁻²). These findings are consistent with the work of Zhang et al.⁶⁷, who prepared BiVO₄ powders with preferential (010) facet growth and demonstrated that, despite well-defined crystalline facets, random orientation on the electrode surface significantly limited PEC activity. Lastly, the adsorption of reagents and products on the photoelectrode surface is a crucial parameter influencing PEC performance. For example, when a TiO₂ electrode was used in the synthesis of azobenzene, the FE decreased to 62.0% compared to that observed with a BiVO₄ electrode⁶³. Such limitations highlight the challenges of continuous PEC operation, as many pollutants and organic substances remain adsorbed on the photoelectrode, hindering its continuous use⁵⁸. Moreover, the surface composition of the semiconductor also plays a role. In water-splitting reactions, variations in the interfacial concentration of Bi or V have been shown to affect band edge positioning and charge separation efficiency⁶⁸.

Effect of applied potential in PEC systems

The high energy consumption in EC and the high charge carrier recombination in PC make PEC a promising alternative for water decontamination⁶⁹. Under constant light intensity, the generation rate of charge carriers in a semiconductor remains constant until perturbed by an external factor. When an external potential or current is applied, electrons in the CB are efficiently extracted into to the external circuit, thereby suppressing charge carriers’ recombination. Additionally, an electron donor compound (i.e., a reducing agent) must be available at the semiconductor–electrolyte interface to fill the holes created by photoexcitation. As the applied potential increases, the photocurrent correspondingly rises^69,70. However, if the reducing agent does not fill the holes fast enough, or the surface becomes saturated with oxidized products, the photocurrent will be limited by the charge carrier recombination^56,71. Despite the recombination limitations that may exist in PEC, its synergistic nature allows it to achieve high decontamination efficiencies even under low overpotential conditions.

Applying an external potential not only drives charge separation, but also directly influences the semiconductor itself. When a photoelectrode is immersed in an electrolyte solution, electrostatic interactions between the semiconductor and the solution cause a shift of the E_F of the semiconductor. This displacement induces band bending and establishes a quasi-Fermi level (E_F,equil.), which reaches equilibrium with the redox potential of the solution (E_F,redox). This phenomenon, knows as “band bending,” originates at the semiconductor/electrolyte interface and extends into the bulk of the semiconductor, forming a space charge layer (SCL) or space charge region (Fig. 8a). The SCL plays a crucial role in PEC by promoting charge separation and suppressing charge carrier recombination⁴⁵. The SCL thickness depends on the potential and varies in relation to the flat band potential (E_fb), E_fb defining the potential at which no excess charge exists at the semiconductor/electrolyte interface, the semiconductor/electrolyte interface is in electrostatic equilibrium⁷². When the applied potential exceeds E_fb, the SCL widens, further shifting the semiconductor’s E_F and enhancing charge separation. Conversely, if the applied potential (E) is equal to the E_fb, the SCL approaches zero thickness, and E_F aligns with the quasi-Fermi level. With knowledge of a semiconductor’s bandgap, it become possible to estimate the absolute position of the conduction and valence bands^47,73. Overall, the applied potential modulates the delicate balance between carrier recombination and charge transfer.

Fig. 8: Space charge layer and charge dynamics on BiVO₄.

a Formation of the space charge layer (SCL) at the semiconductor−solution interface. b Charge transfer rate constant (red) and recombination rate constant (black) for unmodified (open symbols) and CoPi-modified BiVO₄ (closed symbols). The error bars represent the variability in measurements for different samples at selected potentials. In total, three bare BiVO₄ electrodes and two BiVO₄/CoPi electrodes were analyzed. Figure and description adapted from Zachäus et al.¹⁹⁶.

Band bending not only enhances charge separation but also facilitates electron extraction and diminishes recombination losses. Intensity-modulated photocurrent spectroscopy (IMPS) studies have shown that BiVO₄ photocurrent response is predominantly limited by surface recombination rather than surface catalysis²². By quantifying the charge recombination and transfer rate constants—both determined by IMPS—for bare BiVO₄ and BiVO₄ photoanodes modified with cobalt phosphate (CoPi), it has been shown that CoPi suppresses surface recombination by a factor of 10 to 20 (Fig. 8b). Surprisingly, the charge transfer rate constant is minimally affected, despite the well-established electrocatalytic properties of CoPi. This reduction in recombination leads to an overall increase in charge transfer efficiency, which is consistent with the observed improvement in photocurrent. This topic will be further discussed in a later section. In summary, Fig. 8 provides evidence—based on kinetic constants—that the applied potential reduces recombination much more significantly than the enhancement achieved through modification. This finding underscores the complex interdependence between charge transfer dynamics and recombination processes in PEC systems.

Fig. 9: Effect of pH on band bending.

Adapted from Tayyebi et al.⁵⁷.

Effect of temperature on PEC performance

As discussed in previous sections, temperature change occurs naturally through relaxation or recombination. The quasi-Fermi levels of holes (${E}_{F,{h}^{+}}^{q}$) and electrons (${E}_{F,{e}^{-}}^{q}$) are affected by temperature variations and described by Eqs. (5) and (6):

$${E}_{F,{h}^{+}}^{q}={E}_{{VB}}-{k}_{B}* T* {\text{ln}}\left(\frac{{N}_{{eff},V}}{{n}_{h}}\right)$$

(5)

$${E}_{F,{e}^{-}}^{q}={E}_{{CB}}+{k}_{B}* T* {\text{ln}}\left(\frac{{N}_{{eff},C}}{{n}_{e}}\right)$$

(6)

where k_B, is the Boltzmann constant (1.380649×10⁻²³ J/K); ${N}_{{eff},V/C}$ represents the effective density of states in the CB or VB; n_e and n_h are the electron and hole concentrations, respectively; and T is temperature. These parameters determine the potential of quasi-Fermi levels in a pristine semiconductor^74,75.

In the case of TiO₂, energy band positions shift with temperature. Specifically, the VB moves toward more positive potentials (vs. vacuum), while the CB shifts toward more negative potentials (vs. vacuum). Consequently, the E_bg of the semiconductor narrows as the temperature increases⁷⁶. Generally, the performance of PEC systems improves with increasing temperature. Chanyu et al. investigated this effect using BiVO₄ thin-film photoanodes as a model system⁷⁷. Electrochemical impedance spectroscopy revealed that the improvement is primarily due to improved charge transport efficiency within the bulk of the material, rather than increased interfacial charge injection. Polarization curves revealed for both water and sulfite oxidation showed a shift toward less positive onset potentials, attributed to enhanced charge separation and reduced recombination⁷⁷. However, the enhancement in PEC performance by temperature is not unlimited. Biswas et al. reported that the photocurrent in TiO₂ reaches a maximum at 45 °C in NaOH and KOH solutions, attributing the subsequent decline to pH changes at higher temperatures⁷⁶. Similarly, the α-SnWO₄ photoanode exhibited its maximum photocurrent at 50 °C in a KOH/H₃BO₃ buffer (pH = 9) containing 0.2 M Na₂SO₃. Beyond this temperature, the photocurrent declined due to photoanode degradation⁷⁸. On the other hand, Liu et al.⁷⁹ found that for CuWO₄, the photocurrent decrease at elevated temperatures was primarily due to increased recombination rates. As the temperature rises, carrier mobility increases, which extends the minority carrier diffusion length, allowing carriers to reach the electrode faster and increasing the photocurrent. However, this increased carrier mobility also reduces carrier lifetime compared to that at room temperature, which can affect the photocurrent at higher temperatures if mobility does not counteract recombination^59,79. Surprisingly, BiVO₄-based PEC systems exhibit an irreversible enhancement in photocurrent following high-temperature treatment, even upon returning to room temperature. At high temperatures, the kinetics of interfacial reactions also increase, leading to a drop in localized surface pH due to oxidation reactions (e.g., citrate or sulfite oxidation), which produce H⁺ and contribute to BiVO₄ photo-corrosion. This results in the reprecipitation of BiVO₄ as an amorphous surface layer. Since amorphous BiVO₄ possesses a more energetic VB and a comparable E_bg to its crystalline counterpart, electron extraction is facilitated, thereby improving the photocurrent response⁷⁷.

In summary, temperature influences PEC dynamics in both the SCL and the semiconductor-electrolyte interface, primarily influencing reaction kinetics, charge transfer rates, and species adsorption. These effects can significantly impact conversion efficiency and recombination dynamics. Additionally, temperature-induced changes in the interactions among water, ionic species, and the semiconductor surface may alter electrode composition and increase susceptibility to photocorrosion.

Effect of pH on PEC performance

Beyond its well-known influence on electrochemical processes, pH also plays a crucial role in photochemical reactions, influencing the species produced and the reaction pathways in PEC systems. In an alkaline medium, oxygen evolution on a BiVO₄ photoanode proceeds via the oxidation of OH⁻ by h⁺, producing H₂O and O₂. In contrast, under acidic conditions, the O₂ evolution pathway involves the direct oxidation of H₂O by h^{+ 57}.

The influence of pH on photocurrent response has been studied using BiVO₄/FTO as a model photoelectrode⁵⁷. Tayyebi et al.⁵⁷ systematically examined photocurrent generation at acidic (pH 2.5), neutral (pH 6.5), and alkaline (pH 9.5) conditions. LSV assays revealed a distinct trend across the potential range. At low applied potentials (up to ~0.3 V vs. Ag/AgCl), the photocurrent was highest in acidic media up to ~0.3 V (vs. Ag/AgCl), consistent with previously reported behavior for NiO/ZrO₂ electrodes⁸⁰. From 0.3 V to 1.4 V (vs. Ag/AgCl), the highest photocurrent occurred in alkaline media. However, at higher potentials (1.4–1.8 V vs. Ag/AgCl), the trend reversed, with acidic media again yielding the highest photocurrent. This behavior is attributed to potential-dependent modifications in the Helmholtz layer and shifts in the quasi-Fermi level, both of which are influenced by the differential adsorption of H⁺ and OH⁻ species at the semiconductor–electrolyte interface, as described in Eq. (7)⁵⁷:

$${E}_{F}^{{pH}}={E}_{F}^{p{H}_{0}}-0.059* {pH}\,\left({at}25^\circ C\right)$$

(7)

These interface variations induce band bending within the semiconductor, which either facilitates or impedes charge separation. For instance, in alkaline media, the BiVO₄ photoanode exhibits an upward shift in energy band positions, resulting in increased band bending that enhances charge separation efficiency (Fig. 9)⁵⁷.

In the case of TiO₂, pH plays a pivotal role in the PEC degradation of organic compounds by modulating surface charge and molecular interactions. Under highly acidic media, protons (H₃O⁺) adsorb onto the TiO₂ surface, rendering it positively charged and thereby repelling cationic species. On the other hand, in alkaline media, the adsorption of OH⁻ results in a negatively charged surface that enhances the attraction of cationic species. A key parameter governing this behavior is the point of zero charge (pH_pzc)⁷⁰. For BiVO₄, which has a pH_pzc ~3.4, the surface becomes positively charged below this value due to H₃O⁺ adsorption, leading to electrostatic repulsion of cationic species. Conversely, above this pH, the surface repels anionic species^81,82. However, in PEC systems, the surface charge of the semiconductor is also influenced by the applied potential. For n-type semiconductors, anodic polarization induces a positive surface charge, which alters electrostatic interactions and, in turn, influences the degradation efficiency of organic molecules.

In addition to its effect on surface charge, pH can significantly alter the chemical structure and speciation of target compounds in solution. For instance, the PC degradation of p-nitrophenol (PNP, pKa ~ 7.1) on TiO₂ (pH_pzc ~ 6.2) is strongly pH-dependent, altering its adsorption behavior⁸³. At pH 5, the TiO₂ has a positive charge, while at pH 8, it becomes negatively charged, thus modifying the adsorption behavior of PNP. Various interaction mechanisms between PNP and TiO₂ surface have been proposed⁸⁴, including hydrogen bonding with Ti–H₂O complexes, which become stronger at higher pH and enhancing degradation efficiency⁸³. The most effective degradation occurs at pH > 8, making alkaline conditions particularly favorable for environmental water treatment applications.

Similar effects are expected for BiVO₄-based systems, where pH is anticipated to influence the adsorption and degradation of organic pollutants through surface charge modulation. As the evidence suggests, the profound and varied effects of pH exerts multifaceted effects on PEC processes, underscoring the complexity of understanding and optimizing BiVO₄ photoelectrochemical performance. These intricacies will be explored in greater detail in the following subsection.

Effect of pH on PEC performance and stability of BiVO₄

The pH modulates the flat-band potential and surface proton activity, thus altering the interfacial energetics and kinetics of water oxidation on BiVO₄. As the pH increases, the valence band edge shifts ~−59 mV/pH, affecting the thermodynamic driving force for hole-mediated oxidation in the VB. However, this also increases the local concentration of OH⁻, which can contribute a greater quantity of ^•OH after the oxidation of water, but if there is saturation, it can also favor the chemical recombination of radicals. The oxygen evolution reaction (OER) proceeds via multi-step proton-coupled electron transfers (PCET), typically involving adsorbed intermediates such as S–OH*, S–O*, and S–OOH*, whose formation barriers are pH-dependent⁸⁵. Here, S refers to the surface and * denotes reactive oxygen species (ROS) intermediates formed in situ.

The overall half-reaction remains:

$$2{H}_{2}O\,\to \,{O}_{2}\,+\,4H+\,+\,4{e}^{-}\,({E}^{0}=1.23\,V\,{vs}.\,{RHE})$$

(8)

but the reaction kinetics and surface coverage of intermediates vary with pH, influencing the steady-state photocurrent and product selectivity.

The long-term chemical stability of BiVO₄ under PEC operation is also strongly pH dependent. In acidic media, BiVO₄ undergoes pronounced dissolution, especially via vanadium leaching, due to the higher solubility of vanadate species and the increased chemical potential for metal release. A representative dissolution pathway is:

$${{\rm{BiVO}}}_{4}\,+\,3{{\rm{H}}}^{+}\,\to \,{{\rm{Bi}}}^{3+}\,+{{\rm{VO}}}_{2}^{+}+\,{{\rm{H}}}_{2}{\rm{O}}$$

(9)

At near-neutral pH, dissolution is kinetically hindered, and surface passivation can occur through the formation of Bi₂O₃-like layers, which may impede hole transfer. In alkaline media, stability improves due to the lower solubility of Bi³⁺ and VO₄³⁻ species, although the accumulation of OH⁻ at the interface can still foster recombination. Additionally, the surface formation of BiO(OH) may either protect or resist charge transfer depending on its structural order and coverage. These competing mechanisms highlight the intricate balance between performance and degradation, where pH simultaneously modulates charge carrier dynamics, photocorrosion rates, and interfacial reactivity. For a more detailed description of the multiple dissolution and transformation reactions of BiVO₄, as well as mechanistic insights and analysis of the Pourbaix diagram, we recommend the reader to consult the comprehensive studies published in the literature^86,87.

Theoretical work by Ambrosio et al. offers atomic-level insight into these pH effects by developing a methodology that combines ab initio electronic structure calculations, molecular dynamics, and thermodynamic integration⁸⁸. This approach enables the determination of the pK_a values of specific surface adsorption sites and the calculation of the pH_pzc, as well as the dominant water adsorption mode (molecular vs. dissociative) on ₍₀₁₀₎BiVO₄. The resulting speciation diagram indicates that from pH 2 to 8, the interface is predominantly covered by molecularly adsorbed water, while HO⁻ becomes the main adsorbed species above pH 8.2. Proton adsorption is negligible except under strongly acidic conditions. These findings support the view that interfacial site occupancy, dictated by pH, strongly influences hole transfer kinetics and surface recombination, and thus governs the observed PEC response.

Experimental studies confirm that BiVO₄ exhibits slow, uniform dissolution under operating conditions, mainly due to the buildup of photogenerated holes at the surface and the lack of spontaneous passivation via Bi oxides⁸⁷. Complementary work by Tao et al. supports a degradation mechanism involving preferential V leaching, with surface enrichment of Bi₂O₃ or Bi₄O₇ acting as passivating layers that reduce photocorrosion but also impede charge transport⁸⁶. Furthermore, Singh et al. introduced a first-principles thermodynamic metric (Gibbs free energy difference with respect to stable Pourbaix domains, ΔG_pbx) to quantify the metastability of BiVO₄ (~0.59 eV/atom)⁸⁹. Although BiVO₄ falls in a metastable regime, its operational stability at near-neutral pH is attributed to kinetic barriers and dynamic surface reconstruction, underscoring the limitations of purely thermodynamic descriptors.

Beyond its well-documented influence on semiconductor band edge positions and surface charge, pH profoundly dictates the speciation of organic pollutants and affects their interfacial dynamics during PEC water treatment using BiVO₄ photoanodes. Many organic contaminants exist as acid-base pairs, and their protonation state, defined by solution pH, critically impacts their adsorption characteristics, oxidation mechanisms, and ultimately, electron transfer kinetics at the semiconductor-electrolyte interface^90,91,92. While the BiVO₄ surface typically carries a negative charge above its isoelectric point⁹⁰, under PEC operation (where the photoanode is illuminated and polarized) a spatial charge separation occurs. This generates hole accumulation at the surface, resulting in positive local charge regions that can attract anionic species, thereby facilitating interfacial electron transfer. It’s worth remembering that, given the proven generation of ^•OH from water oxidation on BiVO₄, the indirect oxidation of organic compounds by these radicals is the typically expected mechanism.

The interaction behavior of selected organic compounds with BiVO₄ is expected to conform to trends predicted and experimentally observed across diverse PEC systems. For instance, deprotonated species such as carboxylates and phenolates tend to interact more strongly with the BiVO₄ surface under near-neutral to alkaline conditions, often resulting in enhanced PEC activity. Consider the degradation of carboxylic acids (e.g., acetic acid, benzoic acid) and phenolic compounds (e.g., phenol, bisphenol A): at low pH (acidic conditions, below their respective pKa values), these compounds exist predominantly in their neutral, protonated forms (R-COOH and Ar-OH), which exhibit limited adsorption due to weaker electrostatic interactions or even repulsion. This limits direct hole transfer from the valence band and ultimately slows down the degradation kinetics. In contrast, under alkaline conditions (pH above the pK_a), these species deprotonate into their anionic forms, carboxylates (R-COO⁻) and phenoxides (Ar-O⁻), which are electrostatically attracted to the positively polarized BiVO₄ surface under illumination and bias. This facilitates interfacial adsorption, improves charge transfer efficiency, and promotes oxidative degradation. Additionally, the increased electron density on the oxygen atoms in these anions enhances their susceptibility to oxidation by valence band holes or surface-generated hydroxyl radicals, further accelerating PEC reactivity. Oppositely, organic compounds containing amine groups (e.g., aniline) exhibit an inverse pH dependence. At low pH, amines are protonated to form cationic ammonium species (R-NH₃⁺), which are repelled by the positively charged photoanode, hindering adsorption and reactivity. As the pH increases, these molecules deprotonate to their neutral forms (R-NH₂), which, while not strongly attracted electrostatically, remain susceptible to oxidation by photogenerated h⁺ or ^•OH.

These ideas underscore the intricate interplay between pH, surface charge, and PEC-induced polarization in modulating pollutant adsorption and oxidation kinetics. Deviations from these behaviors may result from non-ideal interfacial interactions or surface modifications associated with solid-state transformations. In this context, in situ interfacial characterization using spectroelectrochemical techniques is strongly encouraged as a key tool for mechanistic elucidation.

Future advancements in PEC water treatment require integrated approaches that combine speciation modeling of representative pollutants with experimental photocurrent and degradation measurements. Such efforts will clarify the pH-dependent interfacial behavior of organic contaminants on BiVO₄, enabling optimized operational conditions that maximize degradation efficiency and photoanode stability. Understanding the precise mechanistic pathways, whether direct hole oxidation, indirect radical attack, or mediated electron transfer, as functions of pH and organic speciation are crucial for designing more effective and selective PEC systems.

As highlighted in this section, careful control and documentation of factors affecting the PEC process are essential for future reproducibility. A summary of the catalytic processes is provided in Table 2.

Table 2 General summary of four catalytic processes

Properties of BiVO₄ and comparison with other metal vanadates

Metal vanadates are a big family conformed for structures such as MVO₄, MV₂O₄, MV₂O₆, MV₃O₈, M₂V₂O₇, and M₃V₂O₈ (where M is an alkaline earth/transition/rare earth/other metal). Promising photocatalyst based on TiVO₄⁹³, SmVO₄⁹⁴, Ag₃VO₄⁹⁵ and V₂O₅⁹⁶ have to be studied in contaminant elimination and hydrogen production; nevertheless, the photoelectroactivity of these materials is poor. For instance, photoanodes based on Ag₃VO₄ showed the maxima photocurrent of 19 μA cm⁻² at 1.23 V (vs. RHE) while BiVO₄-photoanodes can achieve up to 130 μA cm⁻² even at less potentials (0.20 V vs. Ag/AgCl or around 0.40 V vs. NHE)^95,97. Also, not all vanadate presents photoactivity to make photoelectrodes. For instance, the Ni₂V₂O₇ has a band gap of 2.17 eV and a band position to promote oxidation reaction; however, it does not happen. This vanadate has to undergo a hydrogen treatment to have oxygen vacancies (NiV₃O₈) to present a little of photocurrent⁹⁸. Although several metal vanadates shown roughly the same band positions as BiVO₄ (Fig. 10), photoelectrochemical application has not studied deep.

Fig. 10: Band Structure of Metal Vanadates.

Conduction band (CB) and valence band (VB) of several metal vanadates used in photocatalysis such as Ni₃(VO₄)₂²⁸², FeVO₄²⁸³, V₂O₅⁹⁶, Ca₂V₂O₇²⁸⁴, quantum dots (QD)-Ni₃(VO₄)₂²⁸⁵, GdVO₄²⁸⁶, CaV₂O₆²⁸⁴, SmVO₄⁹⁴, TiVO₄⁹³, Ag₃VO₄⁹⁵, BiVO₄⁹⁷, NiV₃O₈⁹⁸, Ni₂V₂O₇⁹⁸, InVO₄²⁸⁷, LaVO₄²⁸⁸, Zn₃(VO₄)₂²⁸⁹, YVO₄²⁹⁰, and Mg₃V₂O₈²⁹¹.

Crystalline structure and optical properties

BiVO₄ naturally occurs as mineral pucherite; however, when synthesized in the laboratory, it can adopt different crystal structures. As shown in Fig. 11a–c, four main crystal forms have been observed: orthorhombic (o-BiVO₄, pucherite), monoclinic scheelite (ms-BiVO₄, clinobisvanite), tetragonal zircon-type (tz-BiVO₄, dreyerite), and tetragonal scheelite (ts-BiVO₄, clinobisvanite), which features modifications in atomic positions compared to ms-BiVO₄^49,99. Clinobisvanite can exist in either of the two scheelite structures—tetragonal or monoclinic—with the key difference being a greater distortion in the local environments of V and Bi ions in the monoclinic structure^12,100. Although more than eight crystalline phases of BiVO₄ have been reported, the monoclinic phase exhibits the best PC performance due to its bandgap energy of approximately 2.4 eV. Consequently, ms-BiVO₄ is commonly used to produce photoanodes^97,101. More recently, tz-BiVO₄ has been employed in photoanode fabrication, and a combination of ms-BiVO₄ and tz-BiVO₄ has been explored for water splitting without the need for an external bias^102,103.

Fig. 11: Crystalline forms and band structure of BiVO₄.

a o-BiVO₄, b tz-BiVO₄, and c ms-BiVO₄. Each crystalline form was obtained and adapted from the Crystallography Open Database. d Variation in the band positions of ms-BiVO₄ facets. Adapted from Yu et al.¹⁰⁶.

The monoclinic structure consists of vanadium ions coordinated by four oxygen atoms and bismuth ions coordinated by eight oxygen atoms from eight different VO₄ units, forming layers²⁵. In the tetragonal structure of BiVO₄, the V–O bond lengths are equal at 1.72 Å, while the Bi–O bond lengths are 2.453 Å and 2.499 Å. Due to significantly distortion, the ms-BiVO₄ structure loses its tetragonal symmetry, leading to changes in its bonds lengths: V–O (1.77 Å and 1.69 Å) and Bi–O (2.354 Å, 2.372 Å, 2.516 Å, and 2.628 Å)¹⁰⁰. These distortions in the monoclinic structure enhance charge mobility due to the hybridization and overlap of Bi³⁺ (6 s²) orbitals with O²⁻ (2p⁶) orbitals at the top of the VB^12,104.

Density functional theory studies have shown that the VB is primarily composed of non-bonding O 2p_π states, with minor contributions from Bi 6s orbital, while the CB consists of V 3 d states¹⁰⁴. The band positions of monoclinic BiVO₄ (m-BiVO₄) are approximately 0.32 eV for the CB and 2.47 eV for the VB⁹⁷. Although the minimum bandgap, defined by the VB maximum and CB minimum, is an indirect bandgap of 2.17 eV, some studies classify BiVO₄ as a direct bandgap semiconductor because it exhibits larger direct band gaps that are closer to experimental values¹⁰⁵.

In recent years, the engineering of materials has made efforts to grow anisotropic materials in one direction or materials of one highly exposed facet. The bismuth vanadate is one of these anisotropic materials. The facets most studied of this material are {010} and {110}, which present slight differences. The (110) facet presents a CB energetically higher than the CB of the (010) facet, and vice versa, the VB of the (110) facet is energetically lower than the VB of the (010) facet (Fig. 11d). This variation in energy produces a movement of carriers resulting in an accumulation of holes and electrons in the (110) facet and (010) facet, respectively^106,107,108. Strategies used to synthesize photoanodes with a preferred facet are explained in the Fabrication of BiVO₄ Photoanodes section.

Stability of BiVO₄ in photoelectrochemical systems

Despite its notable advantages, BiVO₄ has several limitations, including susceptibility to photocorrosion, low electrical conductivity, and a sluggish charge transfer kinetics^86,109. Under PEC processes in aqueous media, a passivation layer—primarily composed of bismuth oxides—tends to form on the electrode surface, which progressively reduces the photocurrent density. Notably, several studies have demonstrated that the use of saturated solutions of V⁵⁺ ions can substantially improve the operational stability of the BiVO₄ photoanode⁸⁶. Photocorrosion of BiVO₄ becomes particularly pronounced in acidic environments. For instance, in H₂SO₄, a BiVO₄/WO₃/FTO photoanode can lose up to 74% of its photoactivity after only 120 minutes of PEC operation¹¹⁰. This phenomenon is likely due to the leaching of Bi³⁺ ions into the acidic medium, a process that can occur even in the absence of an external bias¹¹⁰. To further investigate this phenomenon, Li et al. studied the photocorrosion behavior of BiVO₄ electrodes using both aqueous (0.1 M NaHCO₃/H₂O) and non-aqueous (0.1 M LiClO₄/MeCN) media. Electrodes were first subjected to irradiation under applied potentials of 1.0 V or 1.6 V (vs. Ag/AgCl) for 24 hours, followed by 24 hours in the dark at open-circuit conditions, and finally exposed to another 24-hour PEC cycle under the same potentials. Photocorrosion was assessed through UV–Vis absorption spectra and photocurrent measurements. After approximately 96 hours of operation (four full cycles), electrodes in the aqueous medium exhibited a marked reduction in absorption between 400–600 nm (Fig. 12a) compared to those in acetonitrile (CH₃CN), consistent with a greater degree of structural degradation. Similarly, photocurrent responses showed significantly diminished performance for the aqueous system (Fig. 12b). In contrast, the CH₃CN-based medium effectively mitigated degradation, as evidenced by minimal losses in both absorption and photocurrent intensity⁷¹.

Fig. 12: Optical and Electrochemical Stability of BiVO₄.

a UV–Vis spectra and b photocurrent vs. potential of a BiVO₄ electrode before and after ~96 hours of photoelectrocatalysis. Adapted from Li et al.⁷¹.

Another critical factor influencing BiVO₄ photocorrosion is the applied potential. Higher anodic potentials in acidic electrolytes tend to accelerate the corrosion process of the photoanode⁵⁶. To overcome these challenges, recent efforts have focused on doping strategies aimed at enhancing the structural and photoelectrochemical stability of BiVO₄, as discussed in the following sections.

Synthesis of powder BiVO₄

The synthesis of BiVO₄ powders commonly employs NH₄VO₃ as the VO₄³⁻ source and Bi(NO₃)₃‧5H₂O as the Bi³⁺ source. However, various other organic and inorganic precursors can be used (Table 3). Precise pH control is crucial for the successful formation ms-BiVO₄, as hydroxyl and vanadate ions compete in aqueous solution. Under strongly alkaline conditions, especially during hydrothermal synthesis, undesired by-products such as tetragonal zircon-type (tz-BiVO₄) and other vanadium-rich derivatives (e.g., Bi₉VO₁₆, Bi₄V₂O₁₁, Bi₁₂V₂O₂₃, Bi₁₄,V₅O₃₃, and Bi₂O₃) can form, all of which exhibit lower PC activity compared to ms-BiVO₄¹⁰¹. Despite these challenges, careful optimization of hydrothermal conditions—such as temperature, reaction time, and precursor concentration—can yield high-purity BiVO₄ with minimal formation of secondary crystalline phases. Owing to its versatility and scalability, the solvo/hydrothermal route has emerged as the most prevalent method for producing high-quality BiVO₄ powders for photocatalytic and photoelectrochemical applications.

Table 3 Overview of precursors and synthesis methods for BiVO₄

Annealing has become a key step to obtaining ms-BiVO₄, the thermodynamically favored phase with superior photoactivity. Conversion to ms-BiVO₄ is typically induced by thermal treatment at temperatures exceeding 397–497 °C, promoting an irreversible phase transformation from other less active crystalline forms¹¹¹. This transition is facilitated by oxygen loss during annealing, which generates oxygen vacancies that stabilize the monoclinic structure and enhance photocatalytic performance. For instance, annealing under nitrogen atmosphere increases the density of oxygen vacancies compared to air treatment, thereby improving the photocurrent response of the resulting photoanode⁶⁰. In a comparative study, Wang et al. demonstrated that BiVO₄ annealed in nitrogen (BiVO₄-N₂) achieved a photocurrent density of 1.09 mA cm⁻², slightly surpassing that of the air-annealed counterpart (0.88 mA cm⁻²)¹¹². Likewise, BiVO₄ prepared by various synthetic methods followed by annealing has consistently shown enhanced photoactivity⁵⁷.

The tz-BiVO₄ can also be synthesized via hydrothermal routes through the addition of EDTA-2Na to the precursor solution. EDTA-2Na chelates Bi³⁺ ions, thereby inducing Bi vacancies in the crystal lattice. These cationic vacancies typically confer p-type semiconductor behavior, as observed in BiVO₄⁶⁰. This strategy has been successfully applied to prepare tz-BiVO₄ powder and tz-BiVO₄/FTO photocathodes, which display excellent PC performance under reductive potentials^60,103,113.

Another promising avenue in BiVO₄ engineering is the development of two-dimensional (2D) architectures. The cetyltrimethylammonium bromide (CTAB)-induced assembly technique plays a crucial role in the formation of nanosheets¹¹⁴. This method has previously been used to synthesize Bi-nanotubes through the formation of lamellar surfactant/inorganic composite (BiCl₄⁻-CTA⁺)¹¹⁵. In the case of BiVO₄, VO₄³⁻ ions react with Bi³⁺ within the lamellar structure via hydrothermal treatment. After washing, impurities such as CTAB and Cl⁻ are removed, yielding pure BiVO₄. However, this method primarily yields orthorhombic-BiVO₄¹¹⁴, which, as previously discussed, does not exhibit the highest photoactivity.

In recent years, the synthesis of 0D BiVO₄ quantum dots (QDs) has gained increasing attention. QD-BiVO₄ has been synthesized via the hydrothermal method using sodium oleate as a surfactant, Bi(NO₃)₃·5H₂O as the Bi³⁺ source, and either Na₃VO₄·12H₂O or NH₄VO₃ as the VO₄³⁻ source^116,117.

Alternatively, QDs can be obtained through the successive ionic layer adsorption and reaction (SILAR) method, which will be discussed in detail later. Although heterojunctions incorporating BiVO₄-QD enhance photo(electro)catalytic activity, pristine BiVO₄ QDs have not yet been evaluated in PEC processes or their applications^118,119. Moreover, QD-BiVO₄ can be synthesized within large metal-organic frameworks, where the precursors are confined within the network, facilitating the formation of BiVO₄ crystals with reduced particle size¹²⁰.

A comprehensive summary of BiVO₄ precursors, synthesis methods, and processing conditions is presented in Table 3.

Fabrication of BiVO₄ Photoanodes

Electrodeposition

Kim and Choi¹²¹ developed an electrodeposition method for BiOI using a plating solution containing Bi(NO₃)₃·5H₂O, KI, and p-benzoquinone at pH 1.75. BiOI is synthesized on the electrode through cathodic potentials, as illustrated in Fig. 13. The reaction between the Bi salt and KI generates the [BiI₄]⁻ complex, enhancing solubility. Meanwhile, the reduction of p-benzoquinone to hydroquinone consumes protons, leading to local pH change at the working electrode, thereby facilitating BiOI precipitation. The BiOI-coated electrode is then drop-cast with a VO(acac)₂ solution in dimethyl sulfoxide (DMSO) and subsequently calcined to form BiVO₄. Any excess V₂O₅ is removed by washing the electrode with NaOH solution^61,121. This method offers several advantages, including the synthesis of nanoporous BiVO₄ and the formation of 2D layers using a system relatively simple^121,122. Also, Huang et al. reported the formation of twin structures of BiVO₄ using this method¹²³. This kind of structure in photocatalysis is of high interest due to its inherent charge separation, which also improves the photoelectrocatalytic behavior.

Fig. 13: Schematic representation of the BiOI electrodeposition method.

Adapted from Wang et al.¹²².

Another electrochemical approach for synthesizing BiVO₄ photoanodes uses Bi(NO₃)₃·5H₂O in acetic acid and sodium acetate as a plating solution. Under anodic overpotential, (BiO)₄(OH)₂CO₃ (bismutite) is electrodeposited onto the electrode surface via Kolbe electrolysis. The electrode is then drop-cast with a VO(acac)₂ solution and annealed at 520 °C for 2 hours. Any residual V₂O₅ from the vanadium precursor is removed using a NaOH solution¹²⁴.

Alternatively, Bi electrodeposition can be used as a precursor step in BiVO₄ synthesis. In this approach, Bi(NO₃)₃·5H₂O is dissolved in DMSO as the plating solution, and a reductive potential is applied to deposit Bi particles onto the electrode. The electrode is then dried under a N₂ atmosphere with heat before undergoing the same drop-casting and annealing steps as in the previous method¹²⁵.

Seed layer method

The seed layer technique has emerged as an effective strategy for fabricating high-crystalline-purity BiVO₄ photoanodes, leveraging the advantages of the solvo/hydrothermal method. This approach involves the initial deposition of a thin BiVO₄ film—designated as the seed layer—onto a conductive substrate, followed by subsequent crystal growth using a secondary method such as hydrothermal treatment. The seed layer is typically prepared from a precursor solution containing Bi(NO₃)₃·5H₂O, NH₄VO₃, citric acid, and PVA in HNO₃. This solution is spread evenly on the electrode by spin coating and then calcined. This process generates the first thin film, appointed as seed (Fig. 14a, step 1). The prepared electrode is then immersed in a hydrothermal growth solution—usually of lower concentration than the seed precursor—to promote layer growth and control film thickness (Fig. 14a, Step 2; Table 3).

Fig. 14: Seed layer and SILAR methods for BiVO₄.

a the two steps of the seed layer method and b the successive ionic layer adsorption and reaction method for BiVO₄ photoelectrode fabrication.

Several variations of this technique have been explored. Notably, replacing the hydrothermal step with a reflux reaction has been shown to significantly influence surface morphology. While hydrothermal treatment typically results in a granular surface, reflux-grown films often exhibit faceted structures such as pyramids or rods^126,127,128. In the study of Xia et al., chemical bath deposition (CBD) with NaCl was used to promote the formation of BiVO₄ photoanodes with dominant (040) facets. The presence of chloride ions selectively adsorbs onto specific crystal planes, thereby directing preferential facet growth¹²⁹. Further control over crystal orientation has been achieved through the addition of polyvinylpyrrolidone (PVP) during hydrothermal synthesis. Wang et al. reported that PVP facilitates the alignment of the (040) facet parallel to the FTO substrate¹³⁰. However, Zhang et al. observed that PVP may lead to mixed exposure of (010) and (110) facets, suggesting that the molecular weight of PVP plays a critical role in determining facet selectivity⁶⁴. Additionally, the same group demonstrated that adjusting the pH of the growth solution to 0.9 under otherwise identical conditions favored the exclusive formation of (110)-oriented BiVO₄ photoanodes⁶⁴.

Successive Ionic Layer Adsorption and Reaction (SILAR) technique

The SILAR method is a widely adopted technique for the surface modification and synthesis of semiconductor materials, including BiVO₄. This approach involves the alternate immersion of a substrate into two separate precursor solutions. The first solution typically contains Bi³⁺ ions as the bismuth source, while the second provides vanadate species (VO₄³⁻) as the vanadium source. Through sequential dipping, ions are adsorbed onto the substrate surface in a controlled layer-by-layer fashion. Upon exposure to the vanadate-containing solution, the previously adsorbed Bi³⁺; ions undergo in situ reaction to form BiVO₄. Repeating this cycle allows the gradual and uniform deposition of the semiconductor film (Fig. 14b).

An essential post-deposition step in the SILAR process is thermal annealing, which promotes crystallization and enhances the structural integrity of the resulting BiVO₄ film¹³¹. This high-temperature treatment is critical for achieving high crystallinity and optimizing the material’s photoelectrochemical performance. One of the major advantages of the SILAR method lies in its precise control over film thickness and particle size, both of which play a pivotal role in determining photocatalytic efficiency. For instance, Xie et al. successfully synthesized BiVO₄ quantum dots (QD-BiVO₄) on SnO₂ nanostructures via SILAR, demonstrating the method’s capacity to produce nanoscale materials with tailored properties¹³¹.

Beyond BiVO₄, the SILAR technique has been effectively employed for the deposition of other photoactive materials such as WO₃¹³² and TiO₂^133,134, further highlighting its versatility. These studies collectively underscore SILAR’s potential for tuning surface morphology, crystallinity, and optoelectronic properties across a range of photoanode systems. The combination of process simplicity, precision, and compatibility with diverse substrates makes SILAR a compelling method for the fabrication of high-performance photoelectrodes for PEC applications.

Electrospinning and electrospraying techniques for BiVO₄ electrode fabrication

Electrospinning is an emerging and effective method for modifying electrodes, enabling the production of nanofibrous materials under the influence of a high-voltage electric field. The technique is considered relatively simple and efficient, as it facilitates the formation of well-structured nanofibers with minimal chemical processing¹³⁵. In a typical procedure, Bi(NO₃)₃·5H₂O and vanadium(III) tris(acetylacetonate) (C₁₅H₂₁O₆V) are dissolved in acetic acid to form a spinnable precursor solution (Fig. 15). The resulting fibers are directly deposited onto a fluorine-doped tin oxide (FTO) substrate and subsequently calcined to yield BiVO₄ films^136,137.

Fig. 15: Schematic illustration of FTO electrode modification via electrospinning.

FESEM image adapted from Rajini et al.¹³⁹.

An alternative approach involves the use of vanadium(IV) oxyacetylacetonate (VO(acac)₂) in place of C₁₅H₂₁O₆V, often combined with polyvinylpyrrolidone (PVP) as a binding agent. This formulation results in a viscous, homogeneous solution that is readily electrospun onto FTO substrates. Following calcination, the fibrous BiVO₄ or co-doped BiVO₄ nanostructures exhibit enhanced structural integrity and photocatalytic potential^{138,139,140,141,142}. Among available precursors, VO(acac)₂ is currently the most widely used vanadium source in BiVO₄ electrospinning.

In addition to electrospinning, electrospraying has also been explored for photoanode fabrication. Swansborough-Aston et al. reported the synthesis of BiVO₄ films using bismuth(III) 2-ethylhexanoate as the Bi³⁺ source and VO(acac)₂ as the (VO₄)³⁻ source⁵⁶.

The two precursor solutions were introduced into a mixing chamber and electrosprayed onto a pre-heated FTO substrate. While electrospraying offers control over surface coverage and uniformity, it requires a more sophisticated setup compared to electrospinning. Specifically, a precision syringe pump and high-voltage equipment are essential for generating uniform coatings. Moreover, the use of organic solvents and the relatively high complexity of the apparatus make this technique less environmentally friendly and less economically viable for large-scale electrode modification.

Sputtering and other surface modification techniques

Magnetron sputtering (MS) has recently gained attention as a versatile physical vapor deposition (PVD) technique for modifying photoelectrodes. This method has been employed to fabricate BiVO₄-based photoanodes on various substrates, including BDD, demonstrating promising results for PEC applications^143,144. This physical deposition method enables the formation of thin layers of materials (e.g., semiconductors) onto various substrates¹⁴⁵. MS enables the controlled deposition of thin films by bombarding a target material with high-energy ions—typically argon or helium—generated in a plasma. These ions eject atoms from the target, which then condense onto a substrate to form a uniform coating^146,147,148. For BiVO₄ deposition, typical precursors include metallic Bi or Bi₂O₃ as the Bi³⁺ source, and metallic V or V₂O₅ for the (VO₄)³⁻ source^49,149,150.

In addition to direct BiVO₄ film formation, sputtering can be used to modify electrode surfaces with high precision^143,144. The crystalline phase of the deposited BiVO₄ can be tailored using radio frequency (RF) sputtering, a variation of MS that does not require a magnetron. Specifically, high RF power and elevated substrate temperatures favor the formation of ms-BiVO₄, whereas lower RF power and temperatures promote the formation of tz-BiVO₄. Intermediate conditions can yield a mixed-phase material containing ms-BiVO₄, tz-BiVO₄, and ts-BiVO₄¹⁵¹. This method, the same the last one, requires an expensive equipment to develop it which implies a disadvantage. Despite its advantages in phase control and film uniformity, sputtering techniques require sophisticated and costly equipment, which can be a limiting factor for widespread adoption.

Other less commonly used deposition techniques have also been reported for electrode modification. These include pulsed laser deposition⁷⁷, hot spin coating¹⁵², Langmuir–Blodgett deposition^67,153, screen printing for ITO substrates¹⁵⁴, and aerosol pyrolysis¹⁵⁵. While these methods offer specific advantages, the most commonly adopted approaches for BiVO₄-based photoelectrode fabrication remain those discussed in the previous sections.

Doping and heterojunction engineering in BiVO₄-based photoelectrodes

To enhance the PEC performance of BiVO₄, several strategies have been investigated, including ionic doping, heterojunction formation, and morphology control. This section focuses on the effects of doping and heterojunction design on improving charge transport and light-harvesting properties in BiVO₄ photoanodes.

Doped BiVO₄

Ionic doping—either with cations or anions—is a well-established approach for modifying the optical and electronic properties of BiVO₄. In particular, the substitution of V⁵⁺ with Mo⁶⁺ or W⁶⁺ has been extensively investigated. These dopants possess oxidation states of +6 and ionic radius comparable to V⁵⁺, allowing them to integrate into the vanadium sublattice with minimal disruption to the host structure^156,157. The resulting materials, Mo:BiVO₄ and W:BiVO₄, maintain the monoclinic scheelite-type phase while exhibiting significantly improved PEC activity (Fig. 16a). Since the doping concentration is relatively low, the monoclinic phase of pristine BiVO₄ is preserved with minimal structural alterations¹⁵⁷.

Fig. 16: Oxygen evolution and photocurrent of doped BiVO₄.

a oxygen evolution by photocatalysis with several metal-doped BiVO₄¹⁵⁸, and photocurrent of b Mo:BiVO₄ and c W:BiVO₄ at 3, 5, 7, and 9 at.%. Adapted from Talasila et al.¹⁵⁷.

The incorporation of Mo⁶⁺ and W⁶⁺ introduces donor levels within the band structure, thereby increasing the charge carrier concentration and enhancing electron mobility^156,157. As illustrated in Fig. 16b,c, photoanodes doped with 3–9% Mo or W demonstrate photocurrent densities nearly double that of pristine BiVO₄. However, performance declines beyond the optimal doping concentration, likely due to lattice distortion or a reduction in the number of catalytically active surface sites¹⁵⁷.

Although both dopants significantly enhance water-splitting performance, Mo:BiVO₄ reaches its maximum photocurrent at a lower dopant concentration, as Mo⁶⁺ has been identified as a more efficient donor^158,159. Additionally, co-doping with Mo⁶⁺ and W⁶⁺ (Mo/W:BiVO₄) has shown synergistic effects, leading to further improvements in PEC performance under optimized conditions. These findings underscore the potential of rational dopant selection and concentration tuning as powerful tools for optimizing BiVO₄-based photoelectrodes¹⁶⁰.

Beyond Mo and W doping, other elements have been incorporated into BiVO₄. For instance, Co:BiVO₄¹⁶¹, Co/La:BiVO₄¹⁶¹, Er:BiVO₄¹⁶², Fe/W:BiVO₄¹⁶³, Fe:BiVO₄¹⁶³, La:BiVO₄¹⁶¹, Nb:BiVO₄¹⁶⁴, Ni:BiVO₄¹⁶⁵, Ta:BiVO₄¹¹⁰, Y:BiVO₄¹⁶², and Zn:BiVO₄¹⁶⁶ has been reported to enhance conductivity^156,166, stability^{161,163,165,166}, wettability (hydrophilicity)^166,167, absorption coefficient^161,162,165, photocorrosion resistance⁸⁶, and acidic media resistance^86,110. Additionally, anionic doping with halides (F⁻, Cl⁻, Br⁻) has shown slight improvements in PEC performance, with Cl-BiVO₄ exhibiting the best results¹⁶⁸. Moreover, surface treatments with [B(OH)₄]⁻ anions have been found to enhance BiVO₄ photoanode performance despite not being incorporated into the crystal lattice¹⁶⁹. The presence of such anions in BiVO₄ may also create synergic effects when combined with other semiconductors in heterojunctions.

Heterojunctions

Heterojunction engineering is a widely adopted strategy to enhance the PEC performance of BiVO₄-based photoanodes. Heterojunctions are formed by coupling two or more semiconductors with distinct bandgap energies (E_bg) and VB and CB alignments. This configuration improves charge separation and transport, thereby minimizing electron–hole recombination and boosting the efficiency of charge utilization^74,170. From a physicochemical standpoint, heterojunctions modulate the band edge positions of the constituent semiconductors, enhancing both the oxidation and reduction potential of the system. The resulting band alignment facilitates interfacial charge transfer, enabling the spatial separation of photogenerated charge carriers and extending their lifetime. Depending on the energy band configuration, different heterojunction types—such as Type I, Type II, Z-scheme, etc.—can be designed to optimize redox processes and light-harvesting capabilities. These heterostructures also broaden the light absorption range by combining semiconductors with complementary absorption spectra, allowing more efficient utilization of the solar spectrum. This enhancement translates to higher photocurrent densities and improved PEC performance in applications such as water splitting and photocatalytic pollutant degradation. Notably, the built-in electric field generated at the interface of the heterojunction—within the space charge region—plays a crucial role in driving the photogenerated electrons and holes in opposite directions. This internal field significantly reduces charge recombination and promotes efficient charge migration to reactive sites¹⁷¹.

A detailed overview of heterojunction types and their corresponding charge transfer mechanisms is presented in Table 4, highlighting their structural features and roles in enhancing PEC activity.

Table 4 Different types of heterojunctions

Since the degradation of contaminants in photoelectrochemical (PEC) systems is predominantly driven by oxidation processes, heterojunctions are typically engineered on the photoanode. In this configuration, photogenerated electrons are directed toward the cathode via an external circuit, where they participate in reduction reactions. The charge carrier dynamics in PEC systems generally follow similar pathways to those in photocatalytic (PC) heterojunctions, with the notable distinction of being influenced by an applied external potential. For instance, the BiVO₄/BiOI interface forms a Type II heterojunction. Following the Type II mechanism, e⁻ in the CB of BiVO₄ (CB-BiVO₄) are transferred to the external circuit, while h⁺ in the VB of BiVO₄ (VB-BiVO₄) migrate to the VB of BiOI, where they engage in oxidation reactions (Table 4, Entry 2)¹⁷². In contrast, the BiVO₄/Co₃Se₄ junction exemplifies an S-scheme heterojunction. In this system, electrons in CB-BiVO₄ recombine with holes in VB-Co₃Se₄, leaving the remaining holes in VB-BiVO₄ to drive oxidation reactions, while electrons in CB-Co₃Se₄ are transferred to the external circuit or reduce species at the cathode (Table 4, Entry 7). Although schematic representations often show the CB-Co₃Se₄ spatially distant from the electrode, this is not a literal depiction; the materials used by Yusuf et al. were powders rather than large-area films, implying that certain Co₃Se₄ particles are in direct contact with the electrode, enabling electron transfer¹⁷³. Similarly, the BiVO₄/g-C₃N₄ photoanode operates via a Z-scheme mechanism (Table 4, Entry 6)¹⁷⁴, as described by its authors. In this configuration, h⁺ accumulate in VB-BiVO₄ and electrons in CB-g-C₃N₄. The photocatalytic performance of this system can be further enhanced by doping g-C₃N₄ with redox-active metals, as demonstrated by Mafa et al.¹⁷⁵. As detailed in BiVO₄ properties section, BiVO₄ presents anisotropic characteristics, which have effects on the photoelectrochemical behavior. The monoclinic BiVO₄ presents a (010) acts as a reductive surface, while the (110) facet is oxidative. The reason these facets have these properties is the surface heterojunction they undergo in the BiVO₄ crystal, whereby electrons preferentially migrate to the (010) facet and holes to the (110) facet. This facet-selectivity enables the design of facet-specific heterojunctions, such as Ag/₍₀₁₀₎BiVO₄¹⁰⁷. Additionally, the formation of a twin structure of BiVO₄ introduces a series of internal homojunctions that promote charge separation via a “back-to-back” potential mechanism, analogous to that of surface heterojunctions (Table 4, Entry 8)¹²³.

All the aforementioned examples involve binary heterojunctions. In more complex systems containing three or more components, multiple heterojunction types can coexist. For instance, Malefane et al. reported a quadruple-component triple S-scheme structure comprising BiOBr@LaNiO₃/CuBi₂O₄/Bi₂WO₆¹⁷⁶. Dual type II heterojunctions, such as ZnIn₂S₄/FeOOH/BiVO₄ have also been developed by Li et al.¹⁷⁷, as well as C-scheme architectures like Co₃O₄/Bi₄O₅I₂/Bi₅O₇I by Malefane¹⁷⁸, and so on¹⁷⁹.

Given that pristine BiVO₄ photoanodes are prone to self-passivation⁸⁶, using a protective layer to prevent direct contact with the solution is an effective strategy to enhance stability and maintain photocurrent. For instance, constructing a heterojunction with NiFeO_x on B-doped BiVO₄ (B-BiVO₄) significantly reduces the loss of photocurrent compared to the bare B-BiVO₄ photoanode¹⁶⁹. Likewise, incorporating CoPi onto Nb-doped BiVO₄ suppresses photocorrosion by preventing the leaching of VO_x specie¹⁶⁴. Nakajima et al. investigated the BiVO₄/WO₃ heterojunction in acidic media and proposed that the stability arises from the diffusion of W⁶⁺ ions into BiVO₄. This induces the migration of Bi³⁺ to the photoanode surface, forming BiO_x nanoparticles that contribute to corrosion resistance¹⁸⁰.

Coating BiVO₄ with co-catalysts has also been shown to enhance both activity and durability. Jian et al. studied a BiVO₄/Fe-MOF/Ni-MOF photoanode, which retained 96% of its initial photocurrent after 1 h of chronoamperometry at 1.23 V (vs. RHE) in potassium borate buffer (pH = 9), with only 8.87% decay after 10 h, compared to a 50% decay in the uncoated BiVO₄ photoanode¹⁸¹. Similarly, Xia et al. demonstrated that coating BiVO₄ with a N and S co-doped Fe–Co MOF (BiVO₄/N,S–FeCo–MOF) reduced the photocurrent decay from 46.6% to just 4% over 5 h under the same conditions¹⁸². Metal oxides have also proven effective as protective layers. Yu et al. reported that a ZnCo₂O₅-coated BiVO₄ photoanode maintained a photocurrent of 1.2 mA cm⁻², whereas the pristine BiVO₄ showed a steady decrease from 0.50 to 0.30 mA cm⁻² after 4 h at 1.23 V (vs. RHE)¹⁸³. In another example, the NiVO_x/BiVO₄ photoanode developed by Cao et al. exhibited only a 14% photocurrent loss after 100 h of chronoamperometry at 0.8 V (vs. RHE)¹⁸⁴.

Additionally, p-n heterojunctions such as BiVO₄/BiFeO₃ have been shown to significantly enhance the PEC degradation of pharmaceutical contaminants^185,186. For instance, during the degradation of ciprofloxacin at +2 V in 0.1 M Na₂SO₄ (pH = 6.8) under xenon lamp illumination, the BiVO₄/BiFeO₃ photoanode achieved 80.3% degradation, compared to 49.0% and 56.0% for BiVO₄ and BiFeO₃ alone, respectively¹⁸⁵. This clearly demonstrates the synergistic improvement in PEC performance achieved through heterojunction engineering.

Pollutant degradation

A PEC cell utilizing a BiVO₄ photoanode operates by absorbing incident visible light, promoting electrons from the VB to the CB, thereby generating photogenerated e⁻/h⁺ pairs. The photogenerated h⁺ accumulate at the surface of the BiVO₄ photoanode, where they oxidize water molecules to produce ^•OH. These HOROS subsequently degrade nearby organic pollutants. Concurrently, an externally applied bias or current drives the extraction of photogenerated e⁻ from CB-BiVO₄ toward the cathode. The cathode must function as an electrocatalyst capable of facilitating a suitable reduction reaction—such as the hydrogen evolution reaction (HER), the electrochemical reduction of metal ions resulting in metal deposition, or the reduction of organic and inorganic species—to maintain charge neutrality within the system⁴⁸. This synergistic coupling of photooxidation at the anode with electrochemical reduction at the cathode enhances the overall PEC efficiency, enabling simultaneous pollutant degradation and energy conversion. The mechanism is schematically illustrated in Fig. 17.

Fig. 17: BiVO₄ photoanodes for wastewater treatment.

Schematic representation of photoelectrocatalysis systems using BiVO₄ photoanodes for organic matter degradation. CB conduction band, VB valence band, E_bg bandgap energy, EG exciton generation, RC recombination, OP organic pollutant.

Owing to its favorable band edge positions and visible-light responsiveness, BiVO₄ has emerged as one of the most widely studied photocatalysts for wastewater treatment. In PEC applications, BiVO₄-based photoanodes play a pivotal role in generating HOROS that facilitate the degradation of organic contaminants in aqueous environments. The underlying mechanism of this process is outlined below¹⁸⁷:

• i. First, upon light irradiation, the photocatalytic behavior of the semiconductor induces the separation of e⁻/h⁺ pairs at the BiVO₄ photoanode:

  $${{\rm{BiVO}}}_{4}+{\rm{hv}}\to {{\rm{BiVO}}}_{4}({{\rm{h}}}_{{\rm{VB}}}^{+}+{{\rm{e}}}_{{\rm{CB}}}^{-})$$

  (10)

• ii. This step can be followed by the movement of charge carriers, as in a heterojunction, which is addressed in previous sections. Photoexcited electrons in the CB are transported to the cathode under an applied anodic potential, where they participate in reduction reactions. Unlike PC, PEC effectively separates charges, reducing the probability of recombination (Reactions 11 and 12):

  $${{\rm{e}}}_{\mathrm{CB}}^{-}+{{\rm{h}}}_{\mathrm{VB}}^{+}+\to \mathrm{Semiconductor}+\mathrm{heat}\,(\mathrm{or\; light})$$

  (11)
  $${\mathrm{BiVO}}_{4}({{\rm{e}}}_{\mathrm{CB}}^{-})+{}^{\bullet}{\mathrm{OH}}\to {}^{-}\mathrm{OH}.$$

  (12)

• iii. The holes in BiVO₄ drive oxidation reactions. The primary reaction involves the generation of ^•OH (Reaction 13), which is chemisorbed on the photocatalyst. Under alkaline conditions (pH > 7), ^•OH production may follow an alternative pathway (Reaction 14). If CB electrons are not effectively separated from the semiconductor (as in PC), they can react with ^•OH, forming ⁻OH (Reaction 12), which decreases the quantum yield of the degradation process and represents a chemical recombination pathway¹⁸⁸. When electrons are efficiently extracted by the external potential, they can react with oxygen to generate another HOROS of interest (Reaction 15). If the highest occupied molecular orbital of the organic pollutant (OP) has a lower energy level than the hole, it will also react (Reaction 16).

  $${\mathrm{BiVO}}_{4}({{\rm{h}}}_{\mathrm{VB}}^{+})+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{H}}}^{+}+{}^{\bullet}\mathrm{OH}$$

  (13)
  $${\mathrm{BiVO}}_{4}({{\rm{h}}}_{\mathrm{VB}}^{+})+{}^{-}{\text{OH}}\to{}^ {\bullet}\mathrm{OH}$$

  (14)
  $${\mathrm{BiVO}}_{4}({{\rm{e}}}_{\mathrm{CB}}^{-})/{\text{cathode}}+{{\rm{O}}}_{2}\to{}^{\bullet}{{\rm{O}}}_{2}^{-}$$

  (15)
  $${{\rm{BiVO}}}_{4}({{\rm{h}}}_{{\rm{VB}}}^{+})+{\rm{OP}}\to {\rm{oxidized\; product}}$$

  (16)

  In the presence of dissolved ions in the solution, additional oxidizing species can be generated, such as ^•ClO, ^•Cl, ^•Cl₂⁻, HClO^189,190, and SO₄^{•− 51,189,191}, as well as ^•Br, ^•BrOH⁻, and ^•Br₂^{− 192}, contributing to water decontamination. The formation of these reactive species leads to further reactions, such as^74,193:

  $${{\rm{O}}}_{2}^{\bullet -}+{{\rm{H}}}^{+}\to {}^{\bullet}{\mathrm{HO}}_{2}$$

  (17)
  $$2{}^{\bullet}{{\rm{O}}}_{2}^{-}+2{{\rm{H}}}^{+}\to {{\rm{O}}}_{2}+{{\rm{H}}}_{2}{{\rm{O}}}_{2}$$

  (18)
  $$2{}^{\bullet}{\mathrm{HO}}_{2}\to {{\rm{H}}}_{2}{{\rm{O}}}_{2}+{{\rm{O}}}_{2}$$

  (19)
  $${{\rm{H}}}_{2}{{\rm{O}}}_{2}+{{\rm{O}}}_{2}^{\bullet -}\to {}^{\bullet}{\mathrm{OH}}+{}^{-}{\mathrm{OH}}+{{\rm{O}}}_{2}$$

  (20)
  $${{\rm{H}}}_{2}{{\rm{O}}}_{2}+{}^{\bullet }{\text{OH}}\to {{\rm{H}}}_{2}{\rm{O}}+{}^{\bullet}{{\rm{O}}}_{2}{\rm{H}}$$

  (21)
  $${\scriptstyle{\,\,\bullet}\atop }\!\!\!{\mathrm{HO}}_{2}+{}^{\bullet}\mathrm{OH}\to {{\rm{H}}}_{2}{\rm{O}}+{{\rm{O}}}_{2}$$

  (22)
  $${\mathrm{BiVO}}_{4}({{\rm{e}}}_{\mathrm{CB}}^{-})+{{\rm{H}}}_{2}{{\rm{O}}}_{2}\to {}^{\bullet}{\mathrm{OH}}\,+\,{}^{-}{\mathrm{OH}}$$

  (23)

  If the BiVO₄ photoanode is coupled with another semiconductor to form a heterojunction, additional reactions can occur at the interface between the two materials. For instance, in the BiVO₄/Co₃O₄ heterojunction, reactions with Co^2+/3+ have been reported, contributing to overall PC activity and enhancing the degradation process¹³⁶:

  $${{\rm{Co}}}^{2+}+{{\rm{H}}}_{2}{\rm{O}}\to {{\rm{CoOH}}}^{+}+{{\rm{H}}}^{+}$$

  (24)
  $${{\text{CoOH}}}^{+}+{{\text{HSO}}}_{5}^{-}\to {{\text{CoO}}}^{+}+{}{{{\text{SO}}}_{4}^{\bullet }}^{-}+{{\rm{H}}}_{2}{\rm{O}}$$

  (25)
  $${\rm{Co}}{{\rm{O}}}^{+}+2{{\rm{H}}}^{+}\to {{\rm{Co}}}^{3+}+{{\rm{H}}}_{2}{\rm{O}}$$

  (26)
  $${\mathrm{Co}}^{3+}+{{\text{HSO}}}_{5}^{-}\to {\mathrm{Co}}^{2+}+{}{{\mathrm{SO}}_{5}^{\bullet }}^{-}+{{\rm{H}}}^{+}$$

  (27)
  $${\mathrm{Co}}^{3+}+{\mathrm{BiVO}}_{4}({{\rm{e}}}_{\mathrm{CB}}^{-})\to {\mathrm{Co}}^{2+}$$

  (28)

• iv. Finally, as the reaction progresses, pollutants can generate inorganic salts (IS) such as NO₃⁻, SO₄²⁻, and PO₄²⁻, as well as carbon dioxide (CO₂), water, and short-chain oxidized organic compounds like CH₃OH and CHOOH¹⁸⁹:

$${\mathrm{BiVO}}_{4}({{\rm{h}}}_{\mathrm{VB}}^{+})/^{\,\!\bullet}\!\!\!{\text{OH}}/{{{\rm{O}}}_{{2}}^{{\bullet }-}}/\ldots +{\text{OP}}\to \mathrm{by}-{\text{products}}+{\text{IS}}\,+{\mathrm{CO}}_{2}+{{\rm{H}}}_{2}{\rm{O}}$$

(29)

Although the primary interest in PEC water treatment lies in the generation of ^•OH/ O₂^•−—the main oxidants produced—some OPs degrade through alternative pathways involving species such as reactive chlorine species¹⁸⁹ or ${{\rm{h}}}_{{\rm{VB}}}^{+}$¹⁷⁴. However, the generation of a “cocktail” of oxidizing species enhances PEC’s versatility, making it a highly promising technology for the degradation of diverse pollutants.

Identifying the main reactive species responsible for oxidation is essential for evaluating the efficiency of PC and PEC processes and guiding their optimization. Furthermore, correlating the formation of these species with the semiconductor’s band edge positions provides valuable insights into the underlying photo-redox mechanisms. For instance, doping-induced shifts in band potentials can significantly alter reactivity. A notable example is Zn doping in Bi₂WO₆, which causes a shift in the band edges toward more negative potentials. This shift enables the formation of O₂^•⁻ via direct O₂ reduction—an otherwise thermodynamically unfavorable process in pristine Bi₂WO₆¹⁹⁴. Complementary kinetic studies based on the solvent isotope effect further revealed that O₂ reduction in Bi₂WO₆ proceeds via a proton-assisted mechanism involving a coordinated water molecule¹⁹⁵. Such investigations offer mechanistic clarity on the photo-redox pathways operative in both PC and PEC systems.

In the case of BiVO₄, several mechanistic pathways have been proposed, including direct excitation from the VB to the CB, as well as mechanisms involving oxygen vacancies and surface states that facilitate charge separation and enhance photocatalytic activity. However, recent intensity-modulated photocurrent spectroscopy (IMPS) studies underscore the critical role of surface modification in improving PEC performance. For instance, the application of cobalt phosphate (CoPi) on sprayed BiVO₄ photoanodes was found to suppress surface recombination by a factor of 10–20, without significantly altering the charge transfer rate constant. This result indicates that CoPi primarily functions as a surface passivation layer rather than an enhancer of water oxidation kinetics. In contrast, modification with ruthenium oxide (RuO_x), a well-known oxygen evolution reaction (OER) catalyst, does not lead to a measurable increase in photocurrent¹⁹⁶. These findings strongly suggest that surface recombination, rather than sluggish OER kinetics, constitutes the principal performance bottleneck in BiVO₄ photoanodes.

This highlights a critical challenge in PEC systems—surface recombination—which is well recognized in photovoltaics but often underappreciated in photoelectrochemistry. Moreover, it points to the broader implication that oxide semiconductors may universally suffer from surface recombination issues, reinforcing the need for effective passivation strategies. Alternative materials or surface treatments may thus offer superior potential for stabilizing BiVO₄ and other photoanode materials.

At the illuminated BiVO₄–electrolyte interface, dynamic charge transfer processes result in the formation of various oxidative species that play a pivotal role in degrading a wide array of waterborne contaminants. These applications have been extensively evaluated under diverse experimental conditions, as summarized in Table 5.

Table 5 Degradation of pollutants using BiVO₄-based photoanodes in photoelectrocatalysis

In addition to degradation efficiency, the inclusion of energy consumption and cost analysis has been increasingly recognized as essential for the comprehensive evaluation of AOPs. BiVO₄-based PEC systems typically exhibit energy consumption values below 20 kWh per gram of TOC removed (< 20 kWh/g-TOC)^197,198, although this varies depending on reactor design, system efficiency, and other operational parameters¹⁹⁷. Due to the heterogeneity of experimental setups listed in Table 5, emphasis has been placed on pollutant removal efficiency and treatment duration as a first-level comparison metric. Nevertheless, future studies are encouraged to systematically report energy-related parameters to facilitate more meaningful and standardized comparisons across PEC technologies.

Kinetic constant and BiVO₄ degradation efficiency

To accurately evaluate the enhancement in contaminant degradation achieved through PC and/or PEC processes—and to ensure meaningful comparisons with alternative treatment methods—Kisch and Bahnemann recommend that, for solid–liquid systems such as semiconductor powder suspensions or photoanodes in dissolved substrates, reaction rates be determined under standardized conditions¹⁹⁹. Specifically, the use of identical photoreactor configurations, incident light intensities, and wavelength ranges is essential. Furthermore, it is important to ensure that kinetic constants correspond to the same extent of oxidation in order to enable fair comparisons across different systems.

As illustrated in Fig. 18a, pseudo-first-order kinetic analysis of ciprofloxacin degradation under PC, EC, and PEC conditions shows a markedly higher rate constant for PEC (6.17 × 10⁻³ min⁻¹) compared to PC (1.63 × 10⁻³ min⁻¹) and EC (3.32 × 10⁻³ min⁻¹). Similarly, after 4 hours of treatment (Fig. 18b), degradation efficiency was higher for PEC (80.3%) than for PC (31.0%) and EC (54.9%)¹⁸⁵. This trend is consistent with the degradation efficiencies observed after 4 hours of treatment (Fig. 18b), where PEC achieved an 80.3% removal, compared to 54.9% for EC and 31.0% for PC. Similar PEC > EC > PC patterns have been reported for other organic contaminants, including phenol⁹⁷, tetracycline²⁰⁰, and bisphenols²⁰¹, underscoring the synergistic interaction between light and applied bias in PEC systems for enhanced degradation of waterborne pollutants. Ultimately, the comparison of kinetic constants under identical experimental conditions provides valuable insight into the intrinsic efficiency of different treatments and informs the rational design of future PEC reactor systems.

Fig. 18: Ciprofloxacin degradation via photocatalysis, electrocatalysis, and photoelectrocatalysis.

a Kinetics of Ciprofloxacin degradation, b Decreased concentration of Ciprofloxacin. Adapted from Nkwachukwu et al.¹⁸⁵.

Building upon the significance of reaction rate comparisons in solid/liquid systems, the application of BiVO₄-based photocatalysts for the degradation of OPs and simultaneous hydrogen production highlights key kinetic factors that govern process efficiency. In both PC and PEC systems, BiVO₄-driven reactions are predominantly mediated by reactive species such as ^•OH, superoxide anion radicals (O₂^•−), and photogenerated h⁺, which facilitate the degradation of emerging contaminants including caffeine (CAF) and tetracycline (TC). For instance, the incorporation of CoPi as a co-catalyst has been shown to enhance the photodegradation rate constant of CAF from 2.1 × 10⁻³ min⁻¹ to 4.6 × 10⁻³ min⁻¹. More notably, under PEC conditions with an applied potential of 0.6 V (vs. Ag/AgCl), the rate constant increased 26-fold, reaching 5.5 × 10⁻² min^−1 202. Similarly, a BiVO₄/BiOBr nanocomposite demonstrated excellent PC performance, yielding a degradation rate constant of 0.0159 min⁻¹ for TC and achieving a decomposition efficiency of 90.4%²⁰³.

Kinetic analyses further underscore the impact of heterostructure engineering and applied bias on performance. For instance, apparent rate constants of 0.0924 h⁻¹ and 0.845 h⁻¹ were reported for CNP/B-BiVO₄ and CNP/B-BiVO₄/WO₃ photoanodes, respectively, in the degradation of Orange II dye. This nearly tenfold enhancement is corroborated by a 58% reduction in COD after 3 hours of electrolysis, indicating not only the degradation of the parent pollutant but also the effective removal of intermediate by-products²⁰⁴. Such outcomes demonstrate the environmental advantages of BiVO₄-based PEC systems, which contribute to both pollutant mineralization and reduction of overall organic load. Collectively, these kinetic insights emphasize the dual functionality of BiVO₄-based materials in addressing environmental pollution while concurrently enabling hydrogen generation. This synergy positions BiVO₄ as a promising candidate for integrated water treatment and renewable energy production applications.

Degradation pathways

To evaluate the degradation mechanism of the given contaminant, it is essential to identify the reactive species generated at the photoanode surface. Since the radicals have unpaired electrons, they can absorb microwaves when exposed to a magnetic field. Among the most informative techniques for this purpose is electron spin resonance (ESR), also known as electron paramagnetic resonance (EPR). This method exploits the property of radicals—species with unpaired electrons—to absorb microwave radiation when subjected to an external magnetic field. HOROS such as O₂^•− and ^•OH can be readily detected using spin-trapping agents such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO)^136,205 or 2,2,6,6-tetramethyl-4-piperidone (TEMP)²⁰⁶. Figures 19a, b illustrate typical ESR spectra of DMPO–O₂^•− and DMPO–^•OH adducts, demonstrating the photoanode’s capacity to generate these radicals upon light irradiation^201,207. In the absence of light (dark conditions), no radical formation is observed, and the ESR spectra display a flat baseline. Upon illumination, the emergence of characteristic peak patterns—four peaks with 1:1:1:1 intensity for O₂^•− and four peaks with 1:2:2:1 intensity for ^•OH —confirms the generation of these reactive species.

Fig. 19: Reactive species and pathways in PEC degradation.

ESR spectra of a DMPO-O₂^•−, b DMPO-^•OH, and c effect of the scavenger in the photoelectrochemical degradation of bisphenol A. Adapted from Wang et al.²⁰¹. d The time-resolved in-situ Fourier-transform infrared spectra of NiFeO_x/BiVO₄ photoanode recorded at 1.23 V (vs. RHE) under illumination. Taken from Gunawan et al.²⁰⁹, and e two pathways in the degradation of ciprofloxacin. Taken from Nkwachukwu et al.¹⁸⁵.

In addition to ESR, the formation of surface species can be probed using synchrotron-radiated in situ Fourier-transform infrared (SR-FTIR) spectroscopy, a powerful technique for investigating transient intermediates in photoelectrochemical systems²⁰⁸. As shown in Fig. 19d, SR-FTIR spectra reveal the evolution of hydroperoxo (HOO^•) and O₂^•− species on the surface of NiFeO_x/BiVO₄ photoanodes. Other species, such as FeOOH, may also form depending on the specific composition and surface chemistry of the electrode material²⁰⁹.

A straightforward and effective approach to identify the dominant reactive species involved in pollutant degradation is through free radical trapping experiments. These tests involve adding specific radical scavengers to the degradation medium and monitoring changes in either the kinetic constant or degradation efficiency^201,210. In Table 6, compiles common scavengers used in such assays, including their targeted species and limitations. Notably, certain scavengers—such as hydrogen peroxide (H₂O₂), methanol (MeOH), and isopropyl alcohol (IPA)—are known to interact with multiple radical species, which can lead to ambiguous or misleading interpretations of the reaction mechanism²¹¹.

Table 6 Scavengers and some species formed in PEC

The specificity and reactivity of scavengers are better understood by examining their reaction rate constants with key radicals. For instance, tert-butyl alcohol (TBA) reacts with ^•OH at a rate of 3.8–7.6 × 10⁸M⁻¹ s^{−1136,206,212}, MeOH reacts with ^•OH at 1.2–2.8 × 10⁹M⁻¹ s^{−1136,206,212} and with ${\mathrm{SO}}_{4}^{\bullet -}$ at 1.6–7.8 × 10⁷M⁻¹ s^−1 206; IPA reacts with ^•OH at 2.0 × 10⁹M⁻¹ s^{−1136,206,212}; ethanol (EtOH) with ^•OH at 2.2×10⁹M⁻¹ s^−1 212; p-benzoquinone (p-BQ) with O₂^•− at 1.1 × 10⁹M⁻¹ s^{−1 213,214}. These high reactivity values ensure that the added scavengers efficiently compete with the pollutant for reaction with the radicals, thus suppressing their participation in degradation and revealing their individual contributions.

Understanding the dominant oxidizing species is critical for proposing accurate degradation mechanisms. Although ^•OH are often referred to as the “holy radical” in water treatment due to their high reactivity, they are not always the primary contributors. For example, Wang et al. demonstrated that 5 mg L⁻¹ of bisphenol A was degraded up to 94% within 120 minutes at an applied potential of +1 V using a BiVO₄ photoanode. When p-BQ, IPA, and EDTA were added as scavengers for O₂^•⁻, ^•OH, and photogenerated h⁺, respectively, only p-BQ caused a significant drop in degradation efficiency (see Fig. 19c), indicating that O₂^•⁻ was the primary active species²⁰¹. A similar trend was observed by Mane et al. for the degradation of orange using a BiVO₄/g-C₃N₄ photoanode¹⁷⁴. In their radical trapping experiments, the degradation efficiency dropped from 90% to just 21.46% upon the addition of p-BQ, again highlighting the predominant role of superoxide radicals.

Conversely, in other systems, photogenerated h⁺ have been identified as the main oxidative agents. Orimolade et al. reported that a Ag:BiVO₄/BiOI photoanode achieved 92% degradation of diclofenac sodium in 2 hours at +1.0 V (vs. Ag/AgCl). When EDTA, a hole scavenger, was introduced, the efficiency dropped sharply to 27%, indicating the central role of h⁺ in this degradation process²¹⁵.

Similarly, Shao et al. reported a substantial decrease in the kinetic constant for peroxymonosulfate-assisted bisphenol A degradation—from 0.041 min⁻¹ to 0.001 min⁻¹—when ammonium oxalate was used as a h⁺ scavenger, further confirming the importance of h⁺ in the system⁵¹. However, it is important to recognize that scavengers such as EDTA and ammonium oxalate do not exclusively inhibit direct reactions between h⁺ and pollutants. These agents also suppress hole-initiated secondary reactions, such as the formation of ^•OH or SO₄^•⁻ radicals. Therefore, while decreases in degradation efficiency upon scavenger addition provide strong evidence for the involvement of specific species, they do not quantify the exact contribution of direct vs. indirect pathways.

The interaction between organic pollutants and oxidizing agents in AOPs typically results in a cascade of chemical transformations. Contaminants such as heterocycles, aromatic rings, and aliphatic rings are often subject to ring-cleavage during the initial stages of degradation^185,216. Aliphatic carbon atoms may be oxidized through hydroxylation or undergo C–C bond cleavage²⁰¹. Functional groups such as halogens and amines can be substituted by oxygen-containing species—most commonly ^•OH—via nucleophilic substitution reactions^185,200. Moreover, secondary and tertiary amines (R_n–N) may undergo dealkylation reactions, further promoting molecular fragmentation and oxidation²¹⁷. Alcohol moieties on the pollutant structure can be oxidized to ketones (in the case of secondary and tertiary alcohols), aldehydes, or carboxylic acids (from primary alcohols)^200,203. A representative example is the degradation of ciprofloxacin. According to Nkwachukwu et al., fluoride substitution by a hydroxyl group and cleavage of the aliphatic heterocycle are among the proposed degradation pathways (green arrow, Fig. 19e). In an alternative pathway, cleavage of a double bond leads to the formation of an aldehyde group (red arrow, Fig. 19e)¹⁸⁵.

Carboxylic acids formed during these transformations may undergo decarboxylation, resulting in the release of CO₂²⁰⁰. Small organic molecules such as methanol (CH₃OH), methane (CH₄), butyl groups (C₄H₉), and carboxylic fragments (e.g., COOH) may appear as terminal byproducts or intermediates resulting from C–C or C–X bond cleavage^216,218.

Despite the structural degradation of contaminants, there remains ongoing debate regarding the term “elimination of contaminants.” In many studies, the transformation of pollutants is equated with their removal, yet the toxicity of intermediate products is often overlooked. For example, in the PEC degradation of bisphenol AP, most of the identified intermediates exhibited increased toxicity in at least one of three tested organisms—fish, Daphnia, and green algae²⁰¹. This finding underscores the need for a more comprehensive assessment of AOP efficacy. If the toxicity of degradation products is not monitored, “elimination” should be redefined either as complete mineralization (i.e., conversion to CO₂ and H₂O without the formation of harmful intermediates) or as transformation into value-added compounds in the context of organic synthesis.

BiVO₄-based photoanodes for dual-function PEC systems

BiVO₄ has emerged as one of the most promising photoanode materials for visible-light-driven PEC water splitting, particularly when optimized through structural modifications and heterojunction engineering. For instance, the incorporation of TiO₂ interlayers has been shown to improve charge separation, achieving photocurrent densities of approximately 5.5 mA cm⁻² at 2.08 V (vs. RHE)²¹⁹. Doping with niobium (Nb) further enhances the incident photon-to-current efficiency (IPCE) and significantly increases the hydrogen evolution rate. Solar-to-hydrogen (STH) conversion efficiencies as high as 1.1% at 1.08 V (vs. RHE), with FEs reaching 64%, highlight the strong potential of BiVO₄-based systems for renewable hydrogen production²²⁰. However, key challenges remain, particularly high bulk recombination rates and limited charge transport, which call for continued advancements in material design and surface passivation strategies²²¹.

Beyond hydrogen production, BiVO₄-based photoanodes offer a compelling platform for dual-function PEC, enabling the simultaneous generation of clean energy and degradation of organic pollutants. Nb-modified BiVO₄ films have shown enhanced photocatalytic oxidation activity toward organic compounds, underscoring BiVO₄’s capability to facilitate both water oxidation and the breakdown of persistent contaminants²²⁰. Integrating BiVO₄ into PEC systems that couple hydrogen evolution with organic oxidation presents a sustainable route to concurrently address energy and environmental concerns. To maximize these dual benefits, optimization of operational parameters is critical. One proposed approach involves applying an external current to enhance power density, ensuring efficient utilization of photogenerated charges at both the photoanode and photocathode²²². Nevertheless, hydrogen production rates remain strongly dependent on the specific architecture of the BiVO₄-based system. For instance, a BiVO₄ photoanode operating at 1.4 V (vs. Ag/AgCl) with an added chemical bias of 0.37 V achieved a hydrogen generation rate of 0.15 mmol h^−1 221. In contrast, a BiVO₄/TiO₂ layered photoanode produced a cumulative 0.6 mmol H₂ after 10 hours at 2.08 V (vs. RHE)²¹⁹. Furthermore, BiVO₄/BiOBr nanocomposites have demonstrated effective dual functionality, achieving a tetracycline (TC) degradation rate constant of 0.0159 min⁻¹ and a 90.4% decomposition efficiency. Under solar irradiation, these systems generated hydrogen at a rate of 1.864 mmol cm⁻² s⁻¹, corresponding to a photocurrent density of 0.2198 mA cm^−2 203. Looking ahead, future research should focus on improving interfacial charge transfer kinetics, optimizing material–electrolyte interactions, and refining photoanode/cathode coupling and reactor design. Addressing these challenges will be essential to unlock the full potential of BiVO₄-based systems for scalable, efficient, and practical PEC devices. Achieving this goal will require a balanced consideration of the physicochemical trade-offs inherent in dual-function PEC operation—simultaneously targeting hydrogen fuel generation and effective water purification.

Physico-chemical trade-offs in dual-function PEC systems

Dual-function PEC systems are designed to simultaneously achieve two coupled half-reactions: (i) oxidative degradation of organic pollutants at the photoanode through the photoelectrochemical oxygen transfer reaction (POTR), and (ii) reductive hydrogen evolution at the cathode⁴⁸. The system’s overall performance arises from a delicate interplay between these competing and sometimes synergistic reactions, which are controlled by electrode potentials, interfacial charge transfer kinetics, and the dynamics of photogenerated carries.

At the photoanode, the anodic current (I_anode) includes contributions from: (i) The oxidation of organic pollutants, occurring through both direct h⁺ transfer and indirect pathways mediated by ^•OH, which are generated by water oxidation reaction. This radical-mediated pathway is central to the POTR mechanism; (ii) The OER itself, which consumes a fraction of the current and produces molecular oxygen. The Faradaic efficiency for pollutant degradation (FE_pollutant) is defined as:

$${{\rm{FE}}}_{{\rm{pollutant}}}={{\rm{nFr}}}_{{\rm{pollutant}}}/{{\rm{I}}}_{{\rm{anode}}}$$

(30)

where r_pollutant is the molar degradation rate (including direct and radical-mediated pathways), n the number of electrons transferred per molecule degraded, and F the Faraday constant.

The FE associated with oxygen evolution is:

$${{\rm{FE}}}_{{\rm{OER}}}=4{{\rm{Fr}}}_{{\rm{O}}_2}/{{\rm{I}}}_{{\rm{anode}}}$$

(31)

where r_O2 is the molar rate of oxygen evolution.

Charge conservation requires:

$${{\rm{FE}}}_{{\rm{pollutant}}}+{{\rm{FE}}}_{{\rm{OER}}}=1$$

(32)

The presence of organic pollutants enhances scavenging of ^•OH, improving FE_pollutant by reducing radical recombination and OER. However, increasing the applied anodic bias to accelerate oxidation kinetics also intensifies OER and radical chemical recombination, which detracts from the overall FE of degradation.

At the cathode, in the absence of dissolved oxygen (i.e., no parasitic oxygen reduction reaction, ORR), the FE for hydrogen evolution is:

$${{\rm{FE}}}_{{\rm{H}}_2}=2{{\rm{Fr}}}_{{\rm{H}}_2}/{{\rm{I}}}_{{\rm{cathode}}}$$

(33)

where r_H2 is the molar hydrogen production rate.

Operating the cathode at lower potentials favors selective proton reduction and maximizes FE_H2, thus minimizing side reactions. However, this means the anode operates with lower anodic polarization, which can reduce the photoanode’s oxidative capacity and limit the rate of contaminant degradation.

Therefore, optimal operation of the PEC requires developing at the potential that maximizes the power density (E_M) at the photoanode^19,222. In this way, the applied external energy (applied current) is balanced to satisfy the following equations:

$$\left|{{\rm{I}}}_{{\rm{anode}}}\right|=\,\left|{{\rm{I}}}_{{\rm{cathode}}}\right|$$

(34)

$${{\rm{E}}}_{{\rm{anode}}}={{\rm{E}}}_{{\rm{FB}}}+{{\rm{E}}}_{{\rm{M}}}$$

(35)

$${{\rm{E}}}_{{\rm{cathode}}}={{\rm{E}}}_{{\rm{HER}}}-{{\rm{E}}}_{\left(\left|{{\rm{I}}}_{{\rm{cathode}}}\right|=\left|{{\rm{I}}}_{{\rm{anode}}}\right|\right)}$$

(36)

to ensure selective oxidation of pollutants with the concomitant hydrogen production.

Beyond thermodynamics, factors such as charge carriers recombination in the photoanode, surface reaction kinetics, adsorption-desorption equilibria, mass transport constraints, and overpotentials losses significantly influence system behavior, especially in flow or porous electrode configurations.

This integrated physico-chemical framework clarifies the inherent trade-offs between degradation kinetics and hydrogen production efficiency in dual-function PEC systems and serves as a guide for their rational design and operational optimization. These physicochemical considerations represent the initial framework for understanding the system. What follows is a more in-depth exploration that bridges the gap between laboratory research and industrial application, specifically concerning the design and scaling of PEC reactors²²³. The development of an appropriate model for photoelectrochemical reactor optimization is paramount²²⁴, as it will ultimately improve the accuracy of photocurrent and H₂ yield predictions crucial for the widespread adoption of PEC technologies, particularly when considering the added benefit of treating contaminated water. Given these critical advancements, further investigation into these areas is clearly warranted.

Conclusions

This review has outlined the key features and advances of BiVO₄-based materials for photoelectrochemical (PEC) applications, with a focus on the degradation of organic pollutants (OPs) in aqueous environments. BiVO₄ photoanodes have played a pivotal role in the development of more efficient and stable photoelectrodes for solar-driven processes. Their ability to generate reactive oxidative species under various operational conditions makes them highly competitive with other advanced oxidation processes (AOPs), offering reduced treatment times and improved water quality.

As demonstrated throughout this review, BiVO₄ has emerged as a benchmark material for visible-light-driven PEC systems. With a suitable bandgap (~2.4 eV), tunable electronic structure, and relative photostability, BiVO₄ stands out among vanadate semiconductors for solar-to-chemical energy conversion, particularly in environmental remediation and hydrogen evolution. Its performance is strongly influenced by physicochemical properties and operational parameters, including applied potential, pH, temperature, light intensity, and crystal orientation. Among these, light source and pH have been identified as critical factors that significantly affect charge transfer kinetics, Faradaic efficiency, and photocorrosion resistance—highlighting the importance of system optimization.

BiVO₄-based PEC systems have shown superior performance compared to standalone photocatalytic (PC) or electrochemical (EC) processes, particularly for the degradation of persistent contaminants such as pharmaceuticals and dyes. The synergy between photoexcitation and external bias enhances charge carrier separation and reduces recombination, leading to improved degradation efficiencies and faster reaction kinetics. Moreover, these systems offer dual functionality by enabling both pollutant removal and simultaneous hydrogen production, positioning BiVO₄ as a promising candidate for integrated water treatment and renewable energy generation under solar irradiation.

Despite its advantages, pristine BiVO₄ faces intrinsic limitations, including photocorrosion and bulk charge recombination. To overcome these challenges, strategies such as ionic doping and heterojunction engineering have been extensively explored. Incorporation of metal ions (e.g., Mo⁶⁺, W⁶⁺, Co²⁺) and the formation of heterojunctions with other semiconductors or co-catalysts have demonstrated marked improvements in charge carrier dynamics, light absorption, electrical conductivity, and overall stability. Conventional architectures such as type II, Z-scheme, and S-scheme heterojunctions continue to evolve, while more advanced configurations—such as triple S-scheme systems—are garnering increased attention for their enhanced PEC capabilities.

A wide range of synthesis and fabrication techniques enables morphological and structural tuning of BiVO₄ photoanodes. Methods such as electrodeposition, successive ionic layer adsorption and reaction (SILAR), seed-layer approaches, and sputtering allow precise control over crystal orientation, facet exposure, and nanoscale architecture, thereby enhancing PEC performance. Among these, hydrothermal synthesis, particularly when combined with seed-layer methods, is notable for promoting crystallinity and the exposure of {010} facets under optimized conditions. Nevertheless, the anisotropic behavior of BiVO₄ under PEC conditions remains insufficiently understood and warrants further investigation.

The progress discussed herein indicates that the deployment of BiVO₄-based photoanodes for real-world wastewater treatment is increasingly feasible, even under non-ideal conditions. Transitioning from lab-scale efficiencies to industrial-scale application will require innovative reactor designs, improvements in light distribution, fluid dynamics, electrode integration, and scalable fabrication processes—guided by kinetic modeling and robust techno-economic evaluations.

Furthermore, the integration of coupled PEC cells presents a promising route for multifunctional systems. On the cathodic side, valuable reduction products—such as hydrogen—can be generated with high Faradaic efficiency and minimal external energy input. Optimization of operating conditions could enable near-autonomous operation using only solar energy, thereby reducing both economic and energy costs and extending system longevity.

Despite the promising outlook, several challenges remain. Future research should prioritize enhancing the generation of reactive species, improving long-term material stability, and maximizing solar energy conversion efficiency in both pollutant degradation and hydrogen evolution. Equally important are efforts to improve charge carrier separation, minimize recombination losses, and facilitate the integration of PEC systems into practical, large-scale applications.

Ongoing research continues to expand the boundaries of PEC technology, revealing both new challenges and opportunities. As this review has only addressed a portion of its potential, further interdisciplinary exploration is encouraged to fully realize the impact of BiVO₄-based PEC systems in environmental and energy applications.