Olivier Monfort
email: olivier.monfort@ensc-rennes.fr

Gustav Plesch

Bismuth vanadate-based semiconductor photocatalysts: a short critical review on the efficiency and the mechanism of photodegradation of organic pollutants

Keywords: BiVO 4, photocatalysis, efficiency, mechanism, organic pollutant Acknowledgement

The number of publications on photocatalytic bismuth vanadate-based materials is constantly increasing. Indeed, bismuth vanadate is gaining stronger interest in the photochemical community since it is a solar-driven photocatalyst. However, the efficiency of BiVO 4 -based photocatalyst under sunlight is questionable: in most of the studies investigating the photodegradation of organic pollutants, only few works identify the by-products and evaluate the real efficiency of BiVO 4 -based materials. This short review aims to (i) present briefly the principles of photocatalysis and define the photocatalytic efficiency, (ii) discuss the formation of reactive species involved in the photocatalytic degradation process of pollutants and thus the corresponding photodegradation mechanism could be determined. All these points are developed in a comprehensive discussion by focusing especially on pure, doped and composite BiVO 4 . Therefore, this review exhibits a critical overview on different BiVO 4based photocatalytic systems with their real efficiency. This is a necessary knowledge for potential implementation of BiVO 4 materials in environmental applications at larger scale than laboratory conditions.

Introduction to semiconductor photocatalysis 1.Generalities and principles

Since the last industrial revolution, our natural environment has been considerably damaged and is still being deteriorated due to the human activity, in particularly factories in the field of textile, chemistry, agriculture and pharmacy [START_REF] Azenha | Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste[END_REF][START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF][START_REF] Hinojosa-Reyes | Semiconductor Materials for Photocatalytic Oxidation of Organic Pollutants in Wastewater[END_REF]. For example, the quality of water (underground and at the surface) is alarming because the production of dyes, pesticides, fertilizers, drugs and synthetic chemicals from these industries releases organic pollutants such as aromatic compounds, azo and sulfur derivatives and active pharmaceutical ingredients [START_REF] Azenha | Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste[END_REF][START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF][START_REF] Hinojosa-Reyes | Semiconductor Materials for Photocatalytic Oxidation of Organic Pollutants in Wastewater[END_REF][START_REF] Larsson | Pollution from drug manufacturing: review and perspectives[END_REF]. These contaminants could be toxic not only for human beings but also for plants and animals, and this concern has pushed environmental agencies and national governments to legislate and impose stringent measures in order to limit the diffusion of pollutants in our environment [START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF]. However, limiting the pollution does not remediate the damages already caused to the planet. Therefore, for the abatement of pollutants in water, photocatalysis has been found to be a viable solution and a good alternative to traditional biological, chemical and physical decontamination technologies [START_REF] Azenha | Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste[END_REF][START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF].

The photocatalysis can be classified into two categories: homogeneous and heterogeneous photocatalysis. In this review, we focus on heterogeneous photocatalysis using solid inorganic materials. Heterogeneous photocatalysis is an eco-friendly process where semiconductors are considered, so far, as the most promising photocatalysts [START_REF] Azenha | Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste[END_REF][START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF]. A photocatalyst is a material that can induce reaction upon direct light absorption or photosensitisation [START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF]. Under an irradiation of energy hν equal or larger than the energy band-gap E g (separating the conduction band CB and the valence band VB) of the semiconductor, electrons (e -) migrate to the CB leaving holes (h + ) in the VB, thus the material is photoactivated due to the formation of e -/h + pairs. The photogenerated e -and h + are the basis of the heterogeneous photocatalysis using semiconductors (SC) [START_REF] Azenha | Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste[END_REF][START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF]Hermann 1999;[START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF][START_REF] Linsebigler | Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results[END_REF].

However, after photon absorption by the SC, the formation of photogenerated e -/h + pair is in competition with charge recombination leading to the release of absorbed energy in the form of heat or light [START_REF] Azenha | Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste[END_REF][START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF]Hermann 1999;[START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF][START_REF] Linsebigler | Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results[END_REF]. The fate of electrons and holes can follow several pathways that affect the photocatalytic efficiency. For a photocatalytic reaction, it is mandatory that the photogenerated charges reach the surface of the photocatalyst where they react with adsorbed species behaving either as electron donor (D) or acceptor (A) [START_REF] Azenha | Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste[END_REF][START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF]Hermann 1999;[START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF][START_REF] Linsebigler | Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results[END_REF].

It is worth to notice that, at the photocatalyst surface, h + and e -exist in the form of defects or are trapped. The resulted redox reactions give rise to intermediates or final products (A -and D + ) as seen in Fig. 1. The feasibility of a photocatalytic reaction depends on the position of the CB and VB edges of the photocatalyst and the redox potential energy of the adsorbed reactants [START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF]Hermann 1999;[START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF][START_REF] Linsebigler | Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results[END_REF][START_REF] Litter | Heterogeneous photocatalysis: Transition metal ions in photocatalytic systems[END_REF]. Indeed the potential of adsorbed donor should be lower (more negative) than the energy of the valence band maximum (VBM), and similarly, the potential of adsorbed acceptor should be higher (more positive) than the energy of the conduction band minimum (CBM) as illustrated in Fig. 1 [START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF]Hermann 1999;[START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF][START_REF] Linsebigler | Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results[END_REF][START_REF] Litter | Heterogeneous photocatalysis: Transition metal ions in photocatalytic systems[END_REF]. Usually, the degradation of organic pollutants catalysed by a semiconductor is an oxidative process using either directly the oxidising power of photogenerated holes or indirectly through oxidation by reactive oxygen species (ROS). These ROS are formed by reaction of oxygen and/or water with photogenerated e -/h + pair [START_REF] Azenha | Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste[END_REF][START_REF] Koltsakidou | Cytarabine degradation by simulated solar assisted photocatalysis using TiO2[END_REF]. The photooxidation through h + is not kinetically favored in the presence of ROS since these strong oxidising species react much faster. Therefore, for efficient pollutant degradation, i.e. its full mineralisation, the formation of ROS radicals (eqs.

1-8) is crucial [START_REF] Azenha | Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste[END_REF][START_REF] Edge | Radiolytic and Photolytic production of Free Radicals and Reactive Oxygen Species: Interactions with Antioxidants and Biomolecules[END_REF][START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF][START_REF] Hinojosa-Reyes | Semiconductor Materials for Photocatalytic Oxidation of Organic Pollutants in Wastewater[END_REF][START_REF] Lazar | Photocatalytic Water Treatment by Titanium Dioxide: Recent Updates[END_REF][START_REF] Lelario | Effectiveness of different advanced oxidation processes (AOPs) on the abatement of the model compound mepanipyrim in water[END_REF]. The use of ROS radicals for the photooxidative degradation of organic pollutants is so-called advanced oxidation process (AOP). The most powerful ROS are hydroxyl radicals (OH • ) that can be formed from photogenerated h + and surface hydroxyl (on photocatalyst) or even water (hydroxide anions). Hydroxyl radicals have a strong redox potential vs. Normal Hydrogen Electrode (NHE) and they react non-selectively and rapidly with most organic substrates [START_REF] Azenha | Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste[END_REF][START_REF] Edge | Radiolytic and Photolytic production of Free Radicals and Reactive Oxygen Species: Interactions with Antioxidants and Biomolecules[END_REF][START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF][START_REF] Hinojosa-Reyes | Semiconductor Materials for Photocatalytic Oxidation of Organic Pollutants in Wastewater[END_REF][START_REF] Lazar | Photocatalytic Water Treatment by Titanium Dioxide: Recent Updates[END_REF][START_REF] Polczynski | Stabilization and strong oxidizing properties of Ag(II) in a fluorine-free solvent[END_REF][START_REF] Teoh | Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors[END_REF][START_REF] Wood | The potential diagram for oxygen at pH 7[END_REF].

SC + hν

→ e -+ h + (1)

OH - ads + h + → OH • E(OH - ads /OH • ) = 2.8 V vs. NHE (2) H 2 O + h + → OH • + H + E(OH - water /OH • ) > 2.8 V vs. NHE (3)
Other radical species can be formed by reaction between photogenerated charges, already formed radicals and other chemical species present in the photocatalytic system as, for instance, in the following equations:

O 2 + e - → O 2 •- E(O 2 /O 2 •- ) = -0.33 V vs. NHE (4) O 2 •-+ H + → HO 2 • (5) 2 OH • → H 2 O 2 (6) H 2 O 2 + e - → HO • +OH - (7) H 2 O + O 2 •- → HO 2 • + OH - (8)
Semiconductor photocatalysts can be used not only for the degradation/mineralisation of organic pollutants but also in other various processes such as water splitting for the production of O 2 and H 2 (Fig. 2), partial oxidation, reductive dehalogenation, etc. [START_REF] Teoh | Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors[END_REF].

Fig. 2 Photocatalytic processes occuring in semiconductor material

Semiconductor photocatalysts and performance issue

By far, among the semiconductor materials including metal oxides (TiO 2 , WO 3 , ZnO, α-Fe 2 O 3 , SrTiO 3 , …) and sulfides (ZnS, CdS, …), titania-based photocatalysts are the most used due to their low cost, non-toxicity, chemical stability, ease of synthesis and relative high photocatalytic efficiency compared to other inorganic semiconductors that are toxic, photocorrosive or less efficient like CdS, ZnO and α-Fe 2 O 3 , respectively [START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF].

However, pure TiO 2 is photoactive only under ultra-violet (UV) radiations, which represent only about 5% of the natural solar light [START_REF] Hashimoto | TiO2 Photocatalysis: A Historical Overview and Future Prospects[END_REF]Hermann 1999;[START_REF] Lazar | Photocatalytic Water Treatment by Titanium Dioxide: Recent Updates[END_REF][START_REF] Ni | A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production[END_REF]). As a consequence, scientists have focused their research either on the improvement of TiO 2 photocatalyst by shifting its photoactivity toward the visible region (which represents about 50% of the sunlight) or on the elaboration of new photocatalysts that are activated under visible light [START_REF] Hashimoto | TiO2 Photocatalysis: A Historical Overview and Future Prospects[END_REF][START_REF] Ni | A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production[END_REF]. For the latter case, bismuth vanadate (BiVO 4 ) is a serious candidate (presented in section 3) and this material is discussed throughout this short review. Since the last decade, the number of works devoted to BiVO 4 has considerably increased and even some reviews are partly devoted to this promising semiconductor photocatalysts, especially in the field of water oxidation [START_REF] Bhat | Recent Advances in Bismuth-Based Nanomaterials for Photoelectrochemical Water Splitting[END_REF][START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF]Huang et al. 2014a;Li et al. 2013;[START_REF] Lianos | Review of recent trends in photoelectrocatalytic conversion of solar energy to electricity and hydrogen[END_REF][START_REF] Liao | Hydrogen Production from Semiconductor-based Photocatalysis via Water Splitting[END_REF][START_REF] Moniz | Visible-Light Driven Heterojunction Photocatalysts for Water Splitting -A Critical Review[END_REF][START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF]). However, the real efficiency of photooxidative degradation of organic pollutants using BiVO 4 is questionable: in most of the works on BiVO 4 photocatalysis, only the concentration in initial pollutant is followed and not that of by-products that are formed and that could be more toxic than the original targeted pollutant (discussed in section 4). Therefore, for the comparison of BiVO 4 photocatalyst with the most popular and commercially available TiO 2 , the results should be taken with caution since the superiority of bismuth vanadate (and more generally any promising semiconductor photocatalyst) over titania is often based on tests using a single organic substrate or on a narrow selection of test reactions which are not representative for adequate comparison: indeed many factors cannot be validated in a single test assay [START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF][START_REF] Teoh | Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors[END_REF]). Thus, this short review is devoted to a critical discussion of the real efficiency of visible light-driven BiVO 4 -based photocatalysts based mainly on the photodegradation mechanism involved in such photooxidative processes -reflecting the ability of full mineralisation of organic pollutants.

Notions of photocatalytic efficiency and mechanism

Photocatalytic efficiency

The photocatalytic efficiency can be expressed in two different ways that are intimately linked. The first one is the quantum yield Φ that measures the efficiency of a photocatalytic process and which is defined as the number of events occurring per photon absorbed by the system (Serpone and Salinaro1999):

𝜙𝜙 = 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 (𝐴𝐴𝐴𝐴𝑚𝑚)𝐴𝐴𝑜𝑜 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝐴𝐴𝑟𝑟𝐴𝐴𝐴𝐴 𝑟𝑟𝐴𝐴𝐴𝐴𝑐𝑐𝐴𝐴𝐴𝐴𝑟𝑟𝑐𝑐 𝐴𝐴𝑟𝑟 𝑝𝑝𝑟𝑟𝐴𝐴𝑐𝑐𝐴𝐴𝑟𝑟𝐴𝐴 𝑜𝑜𝐴𝐴𝑟𝑟𝐴𝐴𝑟𝑟𝑐𝑐 𝑖𝑖𝐴𝐴 𝐴𝐴ℎ𝑟𝑟 𝑐𝑐𝑠𝑠𝑐𝑐𝐴𝐴𝑟𝑟𝐴𝐴 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 (𝑟𝑟𝑖𝑖𝐴𝐴𝑐𝑐𝐴𝐴𝑟𝑟𝑖𝑖𝐴𝐴)𝐴𝐴𝑜𝑜 𝑝𝑝ℎ𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝑐𝑐 𝑟𝑟𝑎𝑎𝑐𝑐𝐴𝐴𝑟𝑟𝑎𝑎𝑟𝑟𝑐𝑐 𝑎𝑎𝑠𝑠 𝐴𝐴ℎ𝑟𝑟 𝑝𝑝ℎ𝐴𝐴𝐴𝐴𝐴𝐴𝑟𝑟𝑟𝑟𝐴𝐴𝑟𝑟𝑚𝑚𝑠𝑠𝑐𝑐𝐴𝐴 𝑟𝑟𝐴𝐴 𝑤𝑤𝑟𝑟𝑤𝑤𝑟𝑟𝑚𝑚𝑟𝑟𝐴𝐴𝑒𝑒𝐴𝐴ℎ 𝜆𝜆 (9)
It needs to be stressed that, as expressed in eq. 9, it is the absorbed photons and not the incident ones that initiate and drive a photocatalytic process. However, the ability to measure the actual absorbed photons is very difficult in heterogeneous systems due to scattering of light by the semiconductor surface. Therefore, it is usually assumed that all the light is absorbed and the efficiency is quoted as apparent quantum yield. In addition, e -/h + recombination and back-donation process -after charge transfer from the adsorbed species to the semiconductor surface -limit the overall quantum yield of photocatalytic reactions [START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF]Hermann 1999;[START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF][START_REF] Linsebigler | Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results[END_REF][START_REF] Litter | Heterogeneous photocatalysis: Transition metal ions in photocatalytic systems[END_REF].

The second possibility to define the photocatalytic efficiency is to calculate the percentage of mineralisation. Indeed the mineralisation efficiency is a concrete way to evaluate the total abatment of organic pollutant, which is the focus of section 4. In addition, the determination of the photocatalytic efficiency in the removal of organic pollutants depends on the type of irradiation and semiconductor photocatalyst, the susceptibility of the pollutant to photooxidative degradation, and it is also related to the photocatalytic mechanism [START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF]). Among the parameters just cited previously, the targeted pollutant is one of the most important. Indeed, its chemical structure as well as its electronic properties can influence the efficiency of a photooxidative degradation [START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF][START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF][START_REF] Teoh | Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors[END_REF]), sometimes requiring longer irradiation time [START_REF] Lazar | Photocatalytic Water Treatment by Titanium Dioxide: Recent Updates[END_REF][START_REF] Lopez-Alvarez | Solar photocatalitycal treatment of carbofuran at lab and pilot scale: Effect of classical parameters, evaluation of the toxicity and analysis of organic by-products[END_REF]. For instance, aliphatic compounds are more susceptible to photooxidation than aromatic substances since a single bond is easier to break than a double bond [START_REF] Gaya | Heterogeneous Photocatalysis Using Inorganic Semiconductor Solids[END_REF][START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF][START_REF] Teoh | Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors[END_REF]). In addition, the degree of substitution of a pollutant is also a crucial parameter in its ability to undergo photooxidation [START_REF] Fang | Transformation of polychlorinated biphenyls by persulfate at ambient temperature[END_REF][START_REF] Lowry | Congener-Specific Dechlorination of Dissolved PCBs by Microscale and Nanoscale Zerovalent Iron in a Water/Methanol Solution[END_REF][START_REF] O'carroll | Nanoscale zero valent iron and bimetallic particles for contaminated site remediation[END_REF][START_REF] Quiroga | Chemical degradation of PCB in the contaminated soils slurry: Direct Fenton oxidation and desorption combined with the photo-Fenton process[END_REF][START_REF] Rybnikova | Removal of PCBs in contaminated soils by means of chemical redeuction and advanced oxidation processes[END_REF]. Indeed, in the case of polychlorinated biphenyls (PCBs), which are classified as persistent organic pollutant (resistance to natural degradation and thus contaminating all natural compartments -especially soil, ground and surface water), photooxidation is difficult to achieve due to chlorine substituents hindering the aromatic ring, which cannot be attacked by ROS radicals [START_REF] Fang | Transformation of polychlorinated biphenyls by persulfate at ambient temperature[END_REF][START_REF] Lowry | Congener-Specific Dechlorination of Dissolved PCBs by Microscale and Nanoscale Zerovalent Iron in a Water/Methanol Solution[END_REF][START_REF] O'carroll | Nanoscale zero valent iron and bimetallic particles for contaminated site remediation[END_REF][START_REF] Quiroga | Chemical degradation of PCB in the contaminated soils slurry: Direct Fenton oxidation and desorption combined with the photo-Fenton process[END_REF][START_REF] Rybnikova | Removal of PCBs in contaminated soils by means of chemical redeuction and advanced oxidation processes[END_REF]). However, many scientists have already successfully mineralised pollutants such as pesticides, BTX, pharmaceutical compounds that are a threat for human health [START_REF] An | Photocatalytic degradation kinetics and mechanism of antivirus drug-lamivudine in TiO2 dispersion[END_REF][START_REF] Lopez-Alvarez | Solar photocatalitycal treatment of carbofuran at lab and pilot scale: Effect of classical parameters, evaluation of the toxicity and analysis of organic by-products[END_REF][START_REF] Zhang | Heterogeneous photocatalytic decomposition of benzene on lanthanum-doped TiO2 film at ambient temperature[END_REF]). Thus, the study of photocatalytic mechanism and mineralisation efficiency is an important issue to evolve from laboratory research to pilot scale experiments, i.e. from fundamental to applied science.

Photocatalytic mechanism

Identification of oxidative species

Apart from the nature of photocatalyst, irradiation and pollutant, the photocatalytic mechanism is probably the most crucial information since it governs the photocatalytic efficiency. Indeed the mechanism gives a real idea on the probability of mineralisation. By studying the mechanism, it is possible to identify the main oxidative species that are associated to an oxidising power. From this identification, it is possible to deduce the ability (or not) to degrade a target pollutant and its degradation products into CO 2 and H 2 O. The oxidative species e.g. h + and ROS can be identified by several means directly and indirectly.

As direct determination method, electron paramagnetic resonance (EPR) is the most used technique since particular species such as DMPO (5,5-dimethyl-1-pyrroline-N-oxide) are used as spin trapping agents for ROS radicals [START_REF] Chen | Photodegradation of tetracycline and formation of reactive oxygen species in aqueous tetracycline solution under simulated sunlight irradiation[END_REF][START_REF] Hu | BiVO4/TiO2 nanocrystalline heterostructure: A wide spectrum responsive photocatalyst towards the highly efficient decomposition of gaseous benzene[END_REF][START_REF] Hu | One-pot template-free synthesis of heterophase BiVO4 microspheres with enhanced photocatalytic activity[END_REF][START_REF] Kanigaridou | Solar photocatalytic degradation pf bisphenol A with CuOx/BiVO4: Insights into the unexpectedly favorable effect of bicarbonates[END_REF][START_REF] Lin | Highly Efficient Photocatalytic Degradation of Organic Pollutants by PANI-Modified TiO2 Composite[END_REF][START_REF] Mohapatra | Dramatic activities of vanadate intercalated bismuth doped LDH for solar light photocatalysis[END_REF][START_REF] Saison | New Insights Into BiVO4 Properties as Visible Light Photocatalyst[END_REF][START_REF] Sun | Quantum-sized BiVO4 Modified TiO2 Microflower Composite Heterostructure: Efficient Production of Hydroxyl Radicals towards Visible Light-Driven Degradation of Gaseous Toluene[END_REF][START_REF] Zhang | Selective oxidation of sulfides on Pt/BiVO4 photocatalyst under visible light irradiation using water as the oxygen source and dioxygen as the electron acceptor[END_REF][START_REF] Zhang | Co-Pd/BiVO4: High-performance photocatalysts for the degradation of phenol under visible light irradiation[END_REF]. By using EPR method, superoxide and hydroxyl radicals are especially identified in the photocatalytic system due to their characteristic 6-line and 4-line peaks, respectively [START_REF] Chen | Photodegradation of tetracycline and formation of reactive oxygen species in aqueous tetracycline solution under simulated sunlight irradiation[END_REF][START_REF] Hu | BiVO4/TiO2 nanocrystalline heterostructure: A wide spectrum responsive photocatalyst towards the highly efficient decomposition of gaseous benzene[END_REF][START_REF] Hu | One-pot template-free synthesis of heterophase BiVO4 microspheres with enhanced photocatalytic activity[END_REF][START_REF] Kanigaridou | Solar photocatalytic degradation pf bisphenol A with CuOx/BiVO4: Insights into the unexpectedly favorable effect of bicarbonates[END_REF][START_REF] Lin | Highly Efficient Photocatalytic Degradation of Organic Pollutants by PANI-Modified TiO2 Composite[END_REF][START_REF] Mohapatra | Dramatic activities of vanadate intercalated bismuth doped LDH for solar light photocatalysis[END_REF][START_REF] Saison | New Insights Into BiVO4 Properties as Visible Light Photocatalyst[END_REF][START_REF] Sun | Quantum-sized BiVO4 Modified TiO2 Microflower Composite Heterostructure: Efficient Production of Hydroxyl Radicals towards Visible Light-Driven Degradation of Gaseous Toluene[END_REF][START_REF] Zhang | Selective oxidation of sulfides on Pt/BiVO4 photocatalyst under visible light irradiation using water as the oxygen source and dioxygen as the electron acceptor[END_REF][START_REF] Zhang | Co-Pd/BiVO4: High-performance photocatalysts for the degradation of phenol under visible light irradiation[END_REF]. Another technique which can be considered as direct method for the identification of OH • radicals is the photoluminescence technique involving terephtalic acid (TA) or coumarin [START_REF] Hu | One-pot template-free synthesis of heterophase BiVO4 microspheres with enhanced photocatalytic activity[END_REF][START_REF] Lamdab | InVO4-BiVO4 composite films with enhanced visible light performance for photodegradation of methylene blue[END_REF][START_REF] Lin | Highly Efficient Photocatalytic Degradation of Organic Pollutants by PANI-Modified TiO2 Composite[END_REF][START_REF] Lu | Photocatalytic degradation of bis(2-chloroethoxy)methane by a visible light-driven BiVO4 photocatalyst[END_REF][START_REF] Mohapatra | Dramatic activities of vanadate intercalated bismuth doped LDH for solar light photocatalysis[END_REF]. By introducing TA in the presence of hydroxyl radicals, a fluorescent compound (2-hydroxyterephtalate) is formed and a signal at 426nm appears by prior excitation at 315nm. The fluorescence intensity in the photoluminescent spectrum increases with irradiation time.

Indirect methods are usually chosen for the identification of oxidative species during a photocatalytic reaction. Indeed such methods are easy to implement as, for example, the charge scavenging technique. Chemical substances that scavenge particular oxidative species are deliberately introduced in the photocatalytic system and the overall influence on the degradation rate is followed [START_REF] Chen | Photodegradation of tetracycline and formation of reactive oxygen species in aqueous tetracycline solution under simulated sunlight irradiation[END_REF][START_REF] Hu | Facile synthesis of double cone-shaped Ag4V2O7/BiVO4 nanocomposites with enhanced visible light photocatalytic activity for environmental purification[END_REF][START_REF] Huang | Enhanced photocatalytic performance of BiVO4 in aqueous AgNO3 solution under visible light irradiation[END_REF][START_REF] Katsumata | Photocatalytic degradation of bisphenol A by Ag3PO4 under visible light[END_REF][START_REF] Li | Facile synthesis and high activity of novel BiVO4/FeVO4 heterojunction photocatalyst for degradation of metronidazole[END_REF][START_REF] Lin | Highly Efficient Photocatalytic Degradation of Organic Pollutants by PANI-Modified TiO2 Composite[END_REF][START_REF] Liu | Enhanced photocatalytic mechanism of the Nd-Er codoped tetragonal BiVO4 photocatalysts[END_REF][START_REF] Liu | Enhanced photocatalytic activity over flower-like sphere Ag/Ag2CO3/BiVO4 plasmonic heterojunction photocatalyst for tetracycline degradation[END_REF][START_REF] Xue | An interesting Eu,F-codoped BiVO4 microsphere with enhanced photocatalytic performance[END_REF][START_REF] Zhai | Novel Magnetically Separable BiVO4/Fe3O4 Photocatalyst: Synthesis and Photocatalytic Performance under Visible-light Irradiation[END_REF][START_REF] Zhang | Co-Pd/BiVO4: High-performance photocatalysts for the degradation of phenol under visible light irradiation[END_REF]. By this mean, hydroxyl radicals can be identified using iso-propanol or tert-butanol; superoxide radicals can be highlighted by using p-benzoquinone while photogenerated hole can be directly scavenged by ammonium oxalate or EDTA (ethylene-diamine-tetra-acetate). Another technique to identify oxidative species involved in photocatalytic mechanism is to make assumptions based on theoretical calculation of photocatalyst band structure, but such a method is not viable [START_REF] Guo | One-dimensional spindle-like BiVO4/TiO2 nanofibers heterojunction nanocomposites with enhanced visible light photocatalytic properties[END_REF][START_REF] Lamdab | InVO4-BiVO4 composite films with enhanced visible light performance for photodegradation of methylene blue[END_REF].

Identification of by-products

In order to state the photocatalytic efficiency and to confirm the ability of full mineralisation, the final state of the photocatalytic system should be analysed to search for degradation products (if any). For this, a commonly used method is the total organic carbon (TOC) analysis from which the amount of totally oxidised carbon (CO 2 ) at a time t in the photocatalytic system is calculated [START_REF] Hu | Oxidation of sulfamethoxazole and related antimicrobial agents by TiO2 photocatalysis[END_REF][START_REF] Koltsakidou | Cytarabine degradation by simulated solar assisted photocatalysis using TiO2[END_REF][START_REF] Lin | Highly Efficient Photocatalytic Degradation of Organic Pollutants by PANI-Modified TiO2 Composite[END_REF][START_REF] Lopes | Controlled synthesis of BiVO4 photocatalysts: Evidence of the role of heterojunctions in their catalytic performance driven by visible light[END_REF][START_REF] Luo | A comprehensive investigation of tetragonal Gd-doped BiVO4 with enhanced photocatalytic performance under sun-light[END_REF][START_REF] Mohapatra | Dramatic activities of vanadate intercalated bismuth doped LDH for solar light photocatalysis[END_REF][START_REF] Lu | Photocatalytic degradation of bis(2-chloroethoxy)methane by a visible light-driven BiVO4 photocatalyst[END_REF]). Such analysis does not give information on the identity of the different organic molecules present in the system at time t. Therefore, the remaining degradation products should be identified using other methods such as gas and liquid chromatography (GC and LC), mass spectrometry (MS), nuclear magnetic resonance (NMR), and other analytical methods [START_REF] Hu | BiVO4/TiO2 nanocrystalline heterostructure: A wide spectrum responsive photocatalyst towards the highly efficient decomposition of gaseous benzene[END_REF][START_REF] Kanigaridou | Solar photocatalytic degradation pf bisphenol A with CuOx/BiVO4: Insights into the unexpectedly favorable effect of bicarbonates[END_REF][START_REF] Koltsakidou | Cytarabine degradation by simulated solar assisted photocatalysis using TiO2[END_REF][START_REF] Lelario | Effectiveness of different advanced oxidation processes (AOPs) on the abatement of the model compound mepanipyrim in water[END_REF][START_REF] Liu | Enhanced photocatalytic activity over flower-like sphere Ag/Ag2CO3/BiVO4 plasmonic heterojunction photocatalyst for tetracycline degradation[END_REF][START_REF] Lopes | Controlled synthesis of BiVO4 photocatalysts: Evidence of the role of heterojunctions in their catalytic performance driven by visible light[END_REF][START_REF] Lu | Photocatalytic degradation of bis(2-chloroethoxy)methane by a visible light-driven BiVO4 photocatalyst[END_REF][START_REF] Sun | Quantum-sized BiVO4 Modified TiO2 Microflower Composite Heterostructure: Efficient Production of Hydroxyl Radicals towards Visible Light-Driven Degradation of Gaseous Toluene[END_REF][START_REF] Zhang | Selective oxidation of sulfides on Pt/BiVO4 photocatalyst under visible light irradiation using water as the oxygen source and dioxygen as the electron acceptor[END_REF][START_REF] Zhang | Co-Pd/BiVO4: High-performance photocatalysts for the degradation of phenol under visible light irradiation[END_REF].

Chromatographic techniques (LC and GC) are based on the difference in retention times of the products. The retention time is characteristic for each chemical compound for given stationary and eluent phases under given conditions. In addition, MS can identify chemical products and by-products based on their mass-to-charge ratio (m/z). Indeed, mass spectrometry implies to ionise molecules in order to identify the mass of their different ion fragments with accuracy depending on the technique used e.g. high resolution MS or MS/MS tandem. However, these analytical techniques can be coupled in order to improve the identification of degradation products that are eventually present in the treated system, especially if TOC analysis confirms presence of organic matter.

All these analytical methods cited just before can be also used to study the kinetic of photocatalytic processes. However, the study of kinetics is more complex since during the photocatalytic process, many additional parameters should be taken into account such as the formation of secondary radical species and the competition between initial and formed species [START_REF] Hu | Oxidation of sulfamethoxazole and related antimicrobial agents by TiO2 photocatalysis[END_REF][START_REF] Teoh | Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors[END_REF]. With respect to this degree of complexity, the kinetics of photooxidative degradation processes are still the matter of discussion. Usually, many scientists claim that photocatalytic processes obey the Langmuir-Hinshelwood model, which is a model based on the adsorption of reactants onto the photocatalyst surface, while other researchers reclaim for proper experimental evidences to validate this model [START_REF] Kumar | Langmuir-Hinshelwood kinetics -A theoretical study[END_REF][START_REF] Lazar | Photocatalytic Water Treatment by Titanium Dioxide: Recent Updates[END_REF].

Brief background of bismuth vanadate (BiVO 4 )

Bismuth vanadate fulfills several requirements that are necessary for an ideal photocatalyst [START_REF] Gan | Towards highly efficient photoanodes: boosting sunlight-driven semiconductor nanomaterials for water oxidation[END_REF][START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF][START_REF] Tolod | Recent Advances in the BiVO4 Photocatalyst for Sun-Driven Water Oxidation: Top-Performing Photoanodes and Scale-Up Challenges[END_REF]). Among them, non-toxicity, resistance to chemical corrosion and visible light harvesting are the most important. The latter parameter is crucial for potential applications in renewable technologies since the energy coming from the Sun could be used to trigger green photocatalytic processes.

Although further studies seem necessary to evaluate the benefits of BiVO 4 , bismuth vanadate and its derivative materials are considered as good candidates for environmental applications.

Structure of bismuth vanadate

Concerning its structural characteristics, BiVO 4 exists in three polymorphs: monoclinic scheelite-like, tetragonal scheelite-like and tetragonal zircon-like structures with [START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF]. The difference between monoclinic and tetragonal scheelite structures is based on the local environment of Bi and V that is more distorted in the monoclinic structure [START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF]). In addition the tetragonal structure is a high temperature phase exhibiting reversible transition to monoclinic scheelite at 255 °C [START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF][START_REF] Tolod | Recent Advances in the BiVO4 Photocatalyst for Sun-Driven Water Oxidation: Top-Performing Photoanodes and Scale-Up Challenges[END_REF]. Moreover, irreversible transition between tetragonal zircon and tetragonal scheelite happens at about 400-500 °C [START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF][START_REF] Tolod | Recent Advances in the BiVO4 Photocatalyst for Sun-Driven Water Oxidation: Top-Performing Photoanodes and Scale-Up Challenges[END_REF]). Of the two BiVO 4 polymorphs stable at room temperature, monoclinic scheelite-like structure exhibits the best photoactivity due to better photon absorption. This improved light absorption results from smaller E g and more distorted V and Bi units promoting charge carrier transport compared to tetragonal-zircon structure [START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF][START_REF] Tolod | Recent Advances in the BiVO4 Photocatalyst for Sun-Driven Water Oxidation: Top-Performing Photoanodes and Scale-Up Challenges[END_REF][START_REF] Tokunaga | Selective Preparation of Monoclinic and Tetragonal BiVO4 with Scheelite Structure and Their Photocatalytic Properties[END_REF][START_REF] Xu | A review in visible-light-driven BiVO4 photocatalysts[END_REF]. Bismuth vanadate has a direct band gap where the upward of VB and the lowering of CB is the result of hybridization of O 2p with Bi 6s orbitals and predominant contribution of V 3d orbitals, respectively [START_REF] Chen | Semiconductor based Photocatalytic Hydrogen Generation[END_REF][START_REF] Gan | Towards highly efficient photoanodes: boosting sunlight-driven semiconductor nanomaterials for water oxidation[END_REF]Li et al. 2013a;[START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF][START_REF] Tolod | Recent Advances in the BiVO4 Photocatalyst for Sun-Driven Water Oxidation: Top-Performing Photoanodes and Scale-Up Challenges[END_REF].

Photophysical properties

The photocatalytic activity of BiVO 4 depends strongly on its crystalline phase but also on the exposed crystal facets [START_REF] Gan | Towards highly efficient photoanodes: boosting sunlight-driven semiconductor nanomaterials for water oxidation[END_REF]. Indeed the different crystal facets of BiVO 4 influence the thermodynamic and kinetic factors of a photoreaction by modifying important properties such as the preferential adsorption of a reactant, the migration of charge carriers and the desorption of products [START_REF] Gan | Towards highly efficient photoanodes: boosting sunlight-driven semiconductor nanomaterials for water oxidation[END_REF][START_REF] Teoh | Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors[END_REF]. For monoclinic BiVO 4 , the {010} and {110} crystal facets provide reduction and oxidation sites, respectively, thus these facets have redox functions in which photogenerated electrons and holes are available (Li et al. 2013;[START_REF] Tachikawa | Crystal-Face-Dependent Charge Dynamics on a BiVO4 Photocatalyst Revealed by Single-Particle Spectroelectrochemistry[END_REF][START_REF] Tan | BiVO4 {010} and {110} Relative Exposure Extent: Governing Factor of Surface Charge Population and Photocatalytic Activity[END_REF][START_REF] Tan | Enhancing the Photoactivity of Faceted BiVO4 via Annealing in Oxygen-Deficient Condition[END_REF][START_REF] Thalluri | Green-Synthesized BiVO4 Oriented along {040} Facets for VisibleLight-Driven Ethylene Degradation[END_REF][START_REF] Xi | Synthesis of bismuth vanadate nanoplates with exposed {001} facets and enhanced visible-light photocatalytic properties[END_REF]). However, [START_REF] Tan | BiVO4 {010} and {110} Relative Exposure Extent: Governing Factor of Surface Charge Population and Photocatalytic Activity[END_REF][START_REF] Tan | Enhancing the Photoactivity of Faceted BiVO4 via Annealing in Oxygen-Deficient Condition[END_REF] observed that the photooxidation properties of monoclinic bismuth vanadate are better for {010} dominant-BiVO 4 (facet with reduction sites). They explained this fact by fast electron transfer to acceptor species favored by more e - available at larger {010} surface, therefore e -/h + is promoted and photogenerated h + at {110}

surface reacts more efficiently with donor species [START_REF] Tan | BiVO4 {010} and {110} Relative Exposure Extent: Governing Factor of Surface Charge Population and Photocatalytic Activity[END_REF][START_REF] Tan | Enhancing the Photoactivity of Faceted BiVO4 via Annealing in Oxygen-Deficient Condition[END_REF]). For better understanding, Fig. 3 shows the different charge transfer in both {010} and {110} dominant BiVO 4 . tetrahedra are not connected to each other, and also to the strong localisation of V 3d orbitals that constitute the CB [START_REF] Tolod | Recent Advances in the BiVO4 Photocatalyst for Sun-Driven Water Oxidation: Top-Performing Photoanodes and Scale-Up Challenges[END_REF]). In addition, BiVO 4 has a short hole diffusion length (70-100 nm) that compromises the optimisation of light harvesting according to the optical penetration depth principle [START_REF] Tolod | Recent Advances in the BiVO4 Photocatalyst for Sun-Driven Water Oxidation: Top-Performing Photoanodes and Scale-Up Challenges[END_REF].

Synthesis of bismuth vanadate

The way of preparation of bismuth vanadate is crucial in the performance of BiVO 4 photocatalyst [START_REF] Chiarello | Photocatalytic Hydrogen Production[END_REF][START_REF] Gan | Towards highly efficient photoanodes: boosting sunlight-driven semiconductor nanomaterials for water oxidation[END_REF]. Indeed, many parameters such as phase formation, morphology, crystal facets, surface area and surface defects depend directly on the synthesis procedure. Thus, the synthesis influences significantly e -/h + transport and interfacial kinetics [START_REF] Chiarello | Photocatalytic Hydrogen Production[END_REF][START_REF] Gan | Towards highly efficient photoanodes: boosting sunlight-driven semiconductor nanomaterials for water oxidation[END_REF][START_REF] Xu | A review in visible-light-driven BiVO4 photocatalysts[END_REF]. Bismuth

Pure BiVO 4

BiVO 4 is a relatively recent material in the field of photocatalysis, therefore most of the literature is devoted to optimisation of its physical characteristics (morphology, porosity, etc…) for potential photocatalytic applications. Consequently only basic photocatalytic tests are performed. To introduce the photooxidative properties of BiVO 4 , many works discussed the photocatalytic efficiency by following initial concentration of model pollutants without taking care of detailed photodegradation pathways. However, to discuss the photocatalytic efficiency, a statement cannot be made based on rough observation (such as the monitoring of initial pollutant concentration) or even from assumptions on photocatalytic mechanism.

Table 1 summarises published articles devoted to photodegradation of organic pollutants using pure BiVO 4 (this list is not exhaustive). Only few groups have studied in details the mineralisation of pollutant photodegradation using pure BiVO 4 photocatalyst. For instance [START_REF] Lopes | Controlled synthesis of BiVO4 photocatalysts: Evidence of the role of heterojunctions in their catalytic performance driven by visible light[END_REF] have studied by TOC and MS the photooxidative degradation of methylene blue under visible light. They have found out that pure BiVO 4 mineralised 65% of the dye while the degradation products are compounds containing hydroxylated aromatic or cleaved chromophore functions [START_REF] Lopes | Controlled synthesis of BiVO4 photocatalysts: Evidence of the role of heterojunctions in their catalytic performance driven by visible light[END_REF]. From one work to another, differences between photocatalytic efficiencies can appear, but they arise from many reasons such as the way of synthesis or the parameters of photocatalytic system (type of irradiation, kind of pollutant, concentrations in photocatalyst and pollutant, etc.). Deeper studies are therefore necessary to clear out all these uncertainties on BiVO 4 efficiency before considering its use in environmental applications.

Table 1. Summary of photocatalytic degradation of organic pollutants using pure BiVO 4 . Concerning the mechanism of pollutant degradation, numerous works can be found, but authors often confuse efficiency and mechanism (refs. in Table 1). For example, they evaluate the efficiency in mineralisation by extrapolating the discussion on the mechanism and the formation of final by-products such as CO 2 . Vice versa, mechanism is often determined based on the efficiency of mineralisation (the produced CO 2 ) suggesting a hypothetical mechanistic degradation pathway. To evaluate the mechanism in a safer way, the most popular technique is the use of charge scavengers. However, the results could be very different from one work to another, where some scientists have stated the main oxidation species as hydroxyl radicals while other researchers have found that superoxide radicals have the higher contribution in the degradation of the studied pollutant (refs. in Table 1). In addition, Monfort et al. (2017a) have identified, also by charge scavenger, the photogenerated holes as the main photooxidative species of pure BiVO 4 . This observation is in adequation with the work of [START_REF] Saison | New Insights Into BiVO4 Properties as Visible Light Photocatalyst[END_REF] whom has proven this fact by EPR results which is even a more reliable method. But why different mechanisms are identified for the same photocatalyst? This is a critical issue since many parameters can affect the observed photochemical properties. Under normal conditions (25 °C; pH = 0), BiVO 4 is not able to produce hydroxyl and superoxide radicals (Fig. 4). Indeed, the conduction band minimum However, the pH of photocatalytic medium can shift the energetic position of redox potentials for a given reaction while the presence of a dye in the photocatalytic system can sensitise the photocatalyst. In the latter case, the observed mechanism does not result from the intrinsic properties of the photocatalyst. Indeed, the excited electrons of the dye can be transferred to the CB of BiVO 4 and the injected electrons can subsequently produce O 2 •-(Fig. 5). Therefore superoxide radicals could become the main oxidative species in the photodegradation of organic dye. In order to solve this issue, the study of photodegradation and sensitisation mechanisms based on organic dye should be coupled to another pollutant in order to evaluate intrinsic properties of pure bismuth vanadate (Monfort et al. 2017b;[START_REF] Odling | BiVO4-TiO2 Composite Photocatalysts for Dye Degradation Formed Using the SILAR Method[END_REF].

Fig. 5 Sensitization of a photocatalyst by a dye and its subsequent degradation

Despites the works attempting to discuss photocatalytic mechanism based on experimental observations in presence of BiVO 4 , many other published works often base their discussion on assumptions, calculations, already published data or even speculations [START_REF] Hofmann | Nonaqueous Synthesis of a Bismuth Vanadate Photocatalyst By Using Microwave Heating: Photooxidation versus Photosensitized Decomposition in Visible-Light-Driven Photocatalysis[END_REF][START_REF] Liu | Solvothermal synthesis of nanostructured BiVO4 with highly exposed (010) facets and enhanced sunlight-driven photocatalytic properties[END_REF]. But without further experimental data to complete these theoretical results, a reliable mechanism cannot be determined.

Doped BiVO 4

Generalities on doped-BiVO 4

Since the photocatalytic efficiency of pure bismuth vanadate is low and leads to reasonable photodegradation rates only after long irradiation time, modification of BiVO 4 is necessary. The development of modified BiVO 4 includes the increase of surface area, number of photocatalytic sites, number of absorbed photons, charge carrier separation and also the decrease in energy band gap E g [START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF]. Doping is a technique commonly used to improve the photocatalytic performance of BiVO 4 . The principle of this method is based on the insertion of electron donor or acceptor species (referred as n-and ptype dopants, respectively) in the crystalline structure of BiVO 4 . As a result discrete electronic energy levels are inserted within the E g of the semiconductor (Fig. 6). At optimal dopant concentration, an excess of hole and electron population is present in the VB and CB, respectively. The beneficial effects that are usually observed in a doped semiconductor photocatalyst are (1) reduction of E g , ( 2) enhancement of electrical conductivity, (3) increase of e -/h + pair separation and (4) improvement in surface adsorption of target molecule (Huang et al. 2014a).

Fig. 6 Illustration of n-and p-type doping in a semiconductor

However the doping techniques should be distinguished from decoration and surface plasmon resonance (SPR) phenomena. Indeed, many researchers considered such methods as doping [START_REF] Long | Effect of Gold Nanoparticles on the Photocatalytic and Photoelectrochemical Performance of Au Modified BiVO4[END_REF][START_REF] Zhang | Preparation of Ag doped BiVO4 film and its enhanced photoelectrocatalytic (PEC) ability of phenol degradation under visible light[END_REF][START_REF] Zhang | Co-Pd/BiVO4: High-performance photocatalysts for the degradation of phenol under visible light irradiation[END_REF] but the principles of these techniques are different from those using "real" dopants [START_REF] Obregon | Heterostructured Er3+ doped BiVO4 with exceptional photocatalytic performance by cooperative electronic and luminescence sensitization mechanism[END_REF][START_REF] Shan | Er3+, Yb3+ doping induced core-shell structured BiVO4 and nearinfrared photocatalytic properties[END_REF]. For example, noble metals are often used to decorate BiVO 4 and they can trap photogenerated e -(Fig. 7). Thus, e -/h + separation is favored [START_REF] Long | Effect of Gold Nanoparticles on the Photocatalytic and Photoelectrochemical Performance of Au Modified BiVO4[END_REF][START_REF] Zhang | Preparation of Ag doped BiVO4 film and its enhanced photoelectrocatalytic (PEC) ability of phenol degradation under visible light[END_REF]). On the other hand, noble metal nanoparticles can also shift the photoactivation of BiVO 4 toward longer wavelength due to SPR, and thus a transfer of electrons occurs from the decorating nanoparticle to the BiVO 4 CB as seen in Fig. 7 [START_REF] Long | Effect of Gold Nanoparticles on the Photocatalytic and Photoelectrochemical Performance of Au Modified BiVO4[END_REF]. The use of metal and non-metal doping is not the easiest technique for improving the properties of a material. Indeed, to the determination of their optimal concentration, the dopants should be concentrated at the surface of the photocatalyst because they favor charge recombination in the material bulk. In the case of BiVO 4 , which is a n-type semiconductor, tunsgten(VI) and molybdenium(VI) have been found to be the most promising n-type dopants among numerous transition and rare earth metals [START_REF] Berglund | Incorporation of Mo and W into nanostructured BiVO4 films for efficient photoelectrochemical water oxidation[END_REF]Huang et al. 2014a;Li et al. 2013a;[START_REF] Luo | Solar hydrogen generation from seawater with a modified BiVO4 photoanode[END_REF][START_REF] Park | Factors in the Metal Doping of BiVO4 for Improved Photoelectrocatalytic Activity as Studied by Scanning Electrochemical Microscopy and First-Principles Density-Functional Calculation[END_REF][START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF][START_REF] Parmar | Photocatalytic and Photoelectrochemical Water Oxidation over Metal-Doped Monoclinic BiVO4 Photoanodes[END_REF][START_REF] Ye | Rapid Screening of BiVO4-Based Photocatalysts by Scanning Electrochemical Microscopy (SECM) and Studies of Their Photoelectrochemical Properties[END_REF]. Mo(VI) and W(VI) improve the electronic properties of BiVO 4 (especially for water oxidation) while other dopants such as Cu, Yb, Er, Nd and Sm ions are beneficial to the morphology and surface area of BiVO 4 , but also to better visible light absorption associated with a decrease of E g . Table 2 summarises the effect of doped BiVO 4 on the efficiency of pollutant photodegradation. It is worth to notify that, in some works relatively, high amount of dopants is used (Huang et al. 2014b;[START_REF] Zhou | Visible-light sensitive cobalt-doped BiVO4 (Co-BiVO4) photocatalytic composites for the degradation of methylene blue dye in dilute aqueous solutions[END_REF]. Therefore, the metal impurity is not inserted in BiVO 4 lattice, but it is surely present in another form such as a decorated oxide.

Efficiency of doped-BiVO 4

The efficiency of doped-BiVO 4 photocatalyst compared to pure BiVO 4 is undoubtedly better due to the different reasons discussed just above. However, since the photocatalytic properties are often tested for the degradation of an organic dye (Table 2), the real efficiency of the modified photocatalyst is disputable. Indeed, most of the works only follow the decolorisation process (refs. in Table 2). By this way, only the degradation of the chromophore function is proved, but no information about the mineralisation of the entire molecule is given. Fortunately, some works are devoted to the real degradation efficiency using doped photocatalyst, especially [START_REF] Regmi | Visible-light-induced Fe-doped BiVO4 photocatalyst for contaminated water treatment[END_REF] whom followed by HPLC the degradation process of ibuprofen using Fe-doped BiVO 4 . They discussed the gradual degradation of this pharmaceutical into the formation of by-products that are themselves also degraded until the mineralisation [START_REF] Regmi | Visible-light-induced Fe-doped BiVO4 photocatalyst for contaminated water treatment[END_REF]. The degradation process is discussed in details: the first step of photooxidation is a multi-hydroxylation of the pollutant followed by a cycle-opening until the oxidation into ketones and carboxylic acids before full mineralisation into CO 2 and H 2 O [START_REF] Regmi | Visible-light-induced Fe-doped BiVO4 photocatalyst for contaminated water treatment[END_REF]. [START_REF] Zhang | Co-Pd/BiVO4: High-performance photocatalysts for the degradation of phenol under visible light irradiation[END_REF] also discussed the photooxidative properties of Co/Pd-BiVO 4 for phenol removal by supporting on HPLC results while Luo et.

al ( 2016) discussed the efficiency of Gd-doped BiVO 4 by using TOC analysis. The TOC removal is measured to be only 15% while the decolorisation rate reaches 95% indicating that the mineralisation degree in the removal of an organic dye is very low [START_REF] Luo | A comprehensive investigation of tetragonal Gd-doped BiVO4 with enhanced photocatalytic performance under sun-light[END_REF].

Therefore, all the works focused on photocatalytic properties using an organic dye as model pollutant should be interpreted with caution.

On the other hand, the reproducibility of photocatalytic measurements and the reusability of photocatalysts are also crucial parameters in the efficiency of photodegradation processes since they can be considered as limiting factors. With this respect, the use of photocatalytic layer is better than powder suspension system for practical reasons. Many researchers have studied the reusability of modified BiVO 4 (refs. in Table 2), which generally shows an excellent reproducibility in pollutant degradation. This characteristic should be always tested since some materials cannot be reused as for example Nb-BiVO 4 (Monfort et al. 2017b). Indeed, such a photocatalyst exhibits excellent photooxidative properties in the degradation of Rhodamine B (Monfort et al. 2017b), but after repeated use, the photocatalytic efficiency decreases and it is even worse than pure BiVO 4 , which has good stability over time (Fig. 8). This observation is probably due to by-products obstruction in the porous and hierarchical structure of Nb-BiVO 4 material since the degradation of organic dye is not complete [START_REF] Luo | A comprehensive investigation of tetragonal Gd-doped BiVO4 with enhanced photocatalytic performance under sun-light[END_REF]Monfort et al. 2017b).

Mechanism using doped-BiVO 4

The photooxidative degradation mechanism of organic pollutants could show differences between various doped-BiVO 4 due to the wide range of tested dopants. Thus, it is extremely important to study deeply and experimentally the photodegradation mechanism for a better understanding of each doped system. Charge scavenger technique and photoluminescence spectroscopy are generally used to detect the formation (or not) of oxidative radical species, but most of the works base their discussion on calculations and theory which are often not correlated by the experiments (refs. in Table 2). For Cu-doped BiVO 4 , [START_REF] Gao | Effects of pH on the hierarchical structures and photocatalytic performance of Cu-doped BiVO4 prepared via hydrothermal method[END_REF] have found that the main oxidative species are the hydroxyl radicals (OH • ) and holes while O 2 •-play a minor role. This is due to copper that acts as electron scavenger; thus it increases the e -/h + pair separation and consequently the photocatalytic efficiency. On the other hand, [START_REF] Xue | An interesting Eu,F-codoped BiVO4 microsphere with enhanced photocatalytic performance[END_REF] have studied the photocatalytic properties of Eu-and F-codoped BiVO 4 . They have found that the main oxidation species in the degradation of Rhodamine B are superoxide radicals since almost no effect is observed in the presence of holes and OH • scavengers. In this case, they ascribe the excellent photocatalytic properties of doped BiVO 4 to Eu(III) which plays the role of e -trap for efficient formation of O 2

•- [START_REF] Xue | An interesting Eu,F-codoped BiVO4 microsphere with enhanced photocatalytic performance[END_REF]. In addition, [START_REF] Xue | An interesting Eu,F-codoped BiVO4 microsphere with enhanced photocatalytic performance[END_REF] ascribe also to Eu and F dopants the better crystallinity and morphology of the photocatalyst (compared to bare BiVO 4 ). Another example of mechanism using doped BiVO 4 is Co/Pd-decorated bismuth vanadate [START_REF] Zhang | Co-Pd/BiVO4: High-performance photocatalysts for the degradation of phenol under visible light irradiation[END_REF]. In their work, [START_REF] Zhang | Co-Pd/BiVO4: High-performance photocatalysts for the degradation of phenol under visible light irradiation[END_REF] Therefore, studies on mechanism should be deepened because it is a necessary knowledge for the development of doped-BiVO 4 systems in potential environmental applications.

BiVO 4 composites

Generalities on BiVO 4 composites

The elaboration of composite is also an efficient way to improve the photocatalytic properties of BiVO 4 [START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF]. Usually, a semiconductor photocatalyst like bismuth vanadate can be assembled with several types of materials such as another semiconductor or a metal to form a heterojunction or a decorated photocatalyst, respectively, or BiVO 4 can even be assembled with carbon materials (carbon nanotube, graphene oxide, etc.) [START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF]Huang et al. 2014a;[START_REF] Moniz | Visible-Light Driven Heterojunction Photocatalysts for Water Splitting -A Critical Review[END_REF][START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF][START_REF] Xu | Preparation of BiVO4-graphene Nanocomposites and Their Photocatalytic Activity[END_REF][START_REF] Zhang | Carbon-modified BiVO4 microtubes embedded by Ag nanoparticles with high photocatalytic activity under visible light[END_REF]Zhu et al. 2017a). In this section, we will focus on semiconductor heterojunctions which are of three types defined according to their electronic band structure (Fig. 9) [START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF]Huang et al. 2014a;[START_REF] Moniz | Visible-Light Driven Heterojunction Photocatalysts for Water Splitting -A Critical Review[END_REF].

Fig. 9 Illustration of the 3 different types of semiconductor heterojunction

In type I, both photogenerated e -and h + are transferred from semiconductor 1 (SC1) to semiconductor 2 (SC2) due to position of E g2 , which is within the E g1 [START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF].

This kind of heterojunction does not improve the photocatalyst since all the charge carriers are transferred and accumulated in one semiconductor [START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF]. In type II heterojunction, the VBM and CBM of semiconductor 1 are lower in energy than those of semiconductor 2, respectively [START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF]Huang et al. 2014a;[START_REF] Moniz | Visible-Light Driven Heterojunction Photocatalysts for Water Splitting -A Critical Review[END_REF][START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF]). As a result, the photogenerated e -move from SC1 to SC2 while photogenerated h + migrate from SC2 to SC1 [START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF]Huang et al. 2014a;[START_REF] Moniz | Visible-Light Driven Heterojunction Photocatalysts for Water Splitting -A Critical Review[END_REF][START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF]. If both semiconductors are in sufficient contact, efficient charge carrier separation occurs during the photoactivation process [START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF]Huang et al. 2014a;[START_REF] Moniz | Visible-Light Driven Heterojunction Photocatalysts for Water Splitting -A Critical Review[END_REF][START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF]. Finally, the type III heterojunction consists of SC1 which has E g1 position higher in energy than E g2 of SC2. Consequently, recombination between h + from VB of SC1 and e -from CB of SC2 occurs [START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF]. This type of heterojunction could be interesting for designing indirect Z-scheme system with an appropriate electron mediator [START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF]. In the rest of this section, we will discuss type II semiconductor heterojunction since it is the simplest way to limit e -/h + pair recombination i. e. to improve charge carriers lifetime, to reach efficient charge transport and to enhance light absorption [START_REF] Gholipour | Nanocomposite heterojunctions as sunlightdriven photocatalysts for hydrogen production from water splitting[END_REF][START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF].

Concerning BiVO 4 , one of the most frequently used semiconductor for type II heterojunction is WO 3 (E g = 2.6 eV) mainly for the purpose of water splitting [START_REF] Chatchai | Photoelectrocatalytic performance of WO3/BiVO4 toward the dye degradation[END_REF][START_REF] Fujimoto | WO3/BiVO4 composite photoelectrode prepared by improved auto-combustion method for highly efficient water splitting[END_REF]Huang et al. 2014a;[START_REF] Moniz | Visible-Light Driven Heterojunction Photocatalysts for Water Splitting -A Critical Review[END_REF][START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF][START_REF] Pilosh | Nanostructured WO3/BiVO4 Photoanodes for Efficient Photoelectrochemical Water Splitting[END_REF]. Another interesting composite is BiVO 4 /TiO 2 , which is also a type II semiconductor heterojunction with similar electronic band configuration to BiVO 4 /WO 3 . However, one could say that the E g of TiO 2 should energetically encompass the E g of BiVO 4 to form a type I heterojunction since CBM of TiO 2 and BiVO 4 is -0.2 V and 0 V, respectively, while VBM is at 3.0 V and 2.4 V for TiO 2 and BiVO 4 , respectively [START_REF] Guo | One-dimensional spindle-like BiVO4/TiO2 nanofibers heterojunction nanocomposites with enhanced visible light photocatalytic properties[END_REF][START_REF] Hu | BiVO4/TiO2 nanocrystalline heterostructure: A wide spectrum responsive photocatalyst towards the highly efficient decomposition of gaseous benzene[END_REF]Sun et al. 2015). But this is only valid for the single components [START_REF] Guo | One-dimensional spindle-like BiVO4/TiO2 nanofibers heterojunction nanocomposites with enhanced visible light photocatalytic properties[END_REF][START_REF] Hu | BiVO4/TiO2 nanocrystalline heterostructure: A wide spectrum responsive photocatalyst towards the highly efficient decomposition of gaseous benzene[END_REF][START_REF] Sun | Quantum-sized BiVO4 Modified TiO2 Microflower Composite Heterostructure: Efficient Production of Hydroxyl Radicals towards Visible Light-Driven Degradation of Gaseous Toluene[END_REF]. Indeed, in the heterojunction, the energy band gap position of BiVO 4 and TiO 2 is shifted by homogenisation of their Fermi levels [START_REF] Guo | One-dimensional spindle-like BiVO4/TiO2 nanofibers heterojunction nanocomposites with enhanced visible light photocatalytic properties[END_REF][START_REF] Hu | BiVO4/TiO2 nanocrystalline heterostructure: A wide spectrum responsive photocatalyst towards the highly efficient decomposition of gaseous benzene[END_REF][START_REF] Sun | Quantum-sized BiVO4 Modified TiO2 Microflower Composite Heterostructure: Efficient Production of Hydroxyl Radicals towards Visible Light-Driven Degradation of Gaseous Toluene[END_REF]. Therefore, after the thermodynamical equilibrium in BiVO 4 /TiO 2 , the VBM and CBM of BiVO 4 are lower in energy than those of TiO 2 , respectively [START_REF] Guo | One-dimensional spindle-like BiVO4/TiO2 nanofibers heterojunction nanocomposites with enhanced visible light photocatalytic properties[END_REF][START_REF] Hu | BiVO4/TiO2 nanocrystalline heterostructure: A wide spectrum responsive photocatalyst towards the highly efficient decomposition of gaseous benzene[END_REF][START_REF] Sun | Quantum-sized BiVO4 Modified TiO2 Microflower Composite Heterostructure: Efficient Production of Hydroxyl Radicals towards Visible Light-Driven Degradation of Gaseous Toluene[END_REF]. This is an important feature of the heterojunction that should be taken into consideration for discussing the real charge transfer in a composite. The Table 3 summarises various BiVO 4 composites for pollutant photodegradation. In addition, Fig. 10 illustrates the energetic positions of CBM and VBM of the semiconductor components in different heterojunctions calculated from the Mulliken electronegativity theory (eqs. 10 -12).

This theory does not take into account the thermodynamical equilibrium of the Fermi level in the heterojunction, but the Fig. 10 gives an overview of the possibility of designing semiconductor heterojunction photocatalyst with BiVO 4 .

E CB = Χ -E e -0.5E g (10)

E VB = E g + E CB (11) Χ = [χ(A) a χ(B) b χ(C) c ] 1/(a+b+c) (12)
where E VB and E CB are the VB and CB edge potentials, Χ is the absolute electronegativity of the semiconductor (a, b and c are the atomic number of the compounds A, B and C respectively), E g is the energy band gap of the semiconductor and E e is the energy of free electrons on the hydrogen scale (4.5 eV). 3)

Efficiency of BiVO 4 composites

It can be seen from Table 3 that the use of composite systems increase the photocatalytic efficiency by more than a factor 2 compared to bare BiVO 4 . However, the recurrent problem in photocatalytic study is that only the initial concentration of the target pollutant is followed. Thus few of the reported works in Table 3 focuses on the real efficiency in full mineralisation of organic pollutants. For instance, in gas system such as benzene and toluene, the analysis of efficiency is relatively simple since a gas chromatograph is necessary.

In water matrix, [START_REF] Kanigaridou | Solar photocatalytic degradation pf bisphenol A with CuOx/BiVO4: Insights into the unexpectedly favorable effect of bicarbonates[END_REF] [START_REF] Liu | Enhanced photocatalytic activity over flower-like sphere Ag/Ag2CO3/BiVO4 plasmonic heterojunction photocatalyst for tetracycline degradation[END_REF]. In light of the small amount of works focused on the real efficiency of BiVO 4 composites, supplementary studies are necessary to deepen the knowledge to a wide range a pollutants.

In addition, the reproducibility in pollutant degradation using regenerated photocatalyst is often a limiting factor for heterogeneous photocatalytic process. In the case of the composites containing BiVO 4 (refs. in Table 3), most of the photocatalysts exhibit excellent reproducibility in photocatalytic process and can be reuse without significant changes in the degradation rate until 5 times. Before reusing the photocatalyst, the photoactive material should be regenerated by cleaning process followed by reannealing.

Mechanism using BiVO 4 composites

Different mechanisms could be observed according to the nature of the semiconductor associated with BiVO 4 . Therefore, it is crucial to investigate experimentally the mechanism of photodegradation for a given composite system. For example, [START_REF] Kanigaridou | Solar photocatalytic degradation pf bisphenol A with CuOx/BiVO4: Insights into the unexpectedly favorable effect of bicarbonates[END_REF] have detected hydroxyl radicals using EPR measurements for the degradation of bisphenol A in presence of CuO x /BiVO 4 composite, but they do not detailed the electronic band structure of the photocatalyst. Similarly, Yuan et. al (2014) have found by photoluminescence measurements that OH • radicals are probably the main oxidising species in the degradation of an organic dye using Cu 2 O/BiVO 4 . In many works (Table 3), the mechanism of photooxidative degradation is often determined using charge scavengers, photoluminescence and calculations using Mullinken electronegativity. By this way, the corresponding band structure of the composite can be determined. However some of the works reported in Table 3 only focuses on the detection of hydroxyl radicals. Therefore, a large gap in mechanism discussion is left and the rest of the mechanism is explained based on the theory.

Various photooxidative mechanisms have been identified using different composites.

For instance, Li et. al (2015) have found that the main oxidising species in the degradation of metronidazole using FeVO 4 /BiVO 4 are O 2 3). Therefore, the lifetime of e -/h + pair increases. The explanation of the differences between mechanisms is due to the relative position of E g of the components in the composite. Indeed, the band structure of the heterojunction governs the charge transfer within the composite, where accumulation of e -in CB of BiVO 4 and h + in VB of the other side is often observed (refs. in Table 3). In addition, after the thermodynamical equilibrium of Fermi level, the redox potentials of VBM and CBM differs according to the nature of the component associated with BiVO 4 . Finally, other phenomena can influence the mechanism such as sensitisation, surface plasmon resonance and adsorption (refs. in Table 3). Therefore the existing and actual studies on mechanism using BiVO 4 -based composites should be taken with caution.

A particular attention is given to TiO 2 /BiVO 4 system since many works are devoted to this composite. By reporting to other works, the mechanism of photooxidative reactions using BiVO 4 /TiO 2 composite is still a matter of discussion due to incomplete or lack of experimental data. Indeed, even if the experimental and theoretical methods used in these studies often lead to a type II BiVO 4 /TiO 2 heterojunction, the reported energy level of E g and the identity of oxidative species are different from one work to another, suggesting that additional work is needed (refs. in Table 3). Many authors describe a photooxidation process but through different radicals as main oxidising species, either O 2 •-or OH • or even h + (Guo et al. 2016;[START_REF] Hu | Facile synthesis of double cone-shaped Ag4V2O7/BiVO4 nanocomposites with enhanced visible light photocatalytic activity for environmental purification[END_REF][START_REF] Sun | Quantum-sized BiVO4 Modified TiO2 Microflower Composite Heterostructure: Efficient Production of Hydroxyl Radicals towards Visible Light-Driven Degradation of Gaseous Toluene[END_REF][START_REF] Wetchakun | Efficient photocatalytic degradation of methylene blue over BiVO4/TiO2 nanocomposites[END_REF], while [START_REF] Odling | BiVO4-TiO2 Composite Photocatalysts for Dye Degradation Formed Using the SILAR Method[END_REF] preconised a sensitisation of the photocatalyst when a dye is the targeted pollutant. Finally, Monfort et. al (2017a) have studied the mechanism of dye degradation using different layered BiVO 4 /TiO 2 composites. Such a layered system is considerably different from powder suspension and the photocatalytic properties are probably more interesting to investigate. In this study, they notice that the top-coated layer of the composite has an influence on the photocatalytic mechanism since the interface could be different in respect with the layer configuration i.e. the nature of semiconductor at the surface of the composite (Monfort et. al 2017a). Indeed, the photooxidative degradation of rhodamine B under solar irradiation runs through h + for top-coated BiVO 4 (on TiO 2 ) while O 2 •-is identified as main oxidative species for top-coated TiO 2 (on BiVO 4 ) (Monfort et. al 2017a). All these different results on TiO 2 /BiVO 4 but also on the other BiVO 4 composite systems exhibit that deepened studies are needed to really understand the mechanism of photooxidative process which is not trivial.

Summary and perspectives

This short review summarised the most important results collected using BiVO 4 -based photocatalysts. Bismuth vanadate is a semiconductor that gains stronger interest in the photochemical community since the number of publications devoted to photocatalytic systems composed of BiVO 4 increases promptly. However, many uncertainties about the photocatalytic efficiency and mechanism have been highlighted in this critical review. This issue reflects the quick increase of interest for this promising material where researchers attempt to present new insights of BiVO 4 -based photocatalysts rather than to focus on deepened study. Therefore, such a photocatalyst cannot be actually implemented in environmental applications although BiVO 4 is highly promising.

Indeed, BiVO 4 -based photocatalyst can be activated under solar light; thus designed in the form of film, which presents many advantages compared to powder systems, sunlightdriven bismuth vanadate materials can be deposited on foams or membranes for simultaneous filtration and degradation of pollutants in air and surface water. Such a material, which can be also used as electrode in water splitting processes as well as solar cells, would gain in popularity, especially for the design of pilot scale test, if the knowledge on BiVO 4 -based systems is deepened.

Fig. 1

 1 Fig. 1 Band structure of a semiconductor and its photoactivation under irradiation of energy hν where A and D are adsorbed reactants having different redox potentials

Fig. 3

 3 Fig. 3 Charge transfer mechanism in BiVO 4 with (a) {010}-dominated facet and (b) {110}dominated facet

  vanadate was first synthesised by solid state and melting reactions in 1963[START_REF] Chen | Semiconductor based Photocatalytic Hydrogen Generation[END_REF][START_REF] Roth | Synthesis and stability of bismutotantalite stibiotantalite and chemically similar ABO4 compounds[END_REF]. To date, the most commonly used synthesis methods for the preparation of BiVO 4 in the form of film and powder are metal organic decomposition (MOD), precipitation and hydrothermal methods[START_REF] Galembeck | BiVO4 thin film preparation by metalorganic decomposition[END_REF][START_REF] Lg | Some trends in the design of homo-and heterometallic molecular precursors of high-tech oxides[END_REF][START_REF] Ke | Photocatalytic water splitting for O2 production under visible-light irradiation on BiVO4 nanoparticles in different sacrificial reagent solutions[END_REF][START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF][START_REF] Sayama | Photoelectrochemical decomposition of water on nanocrystalline BiVO4 film electrodes under visible light[END_REF][START_REF] Thurston | Toward a General Strategy for the Synthesis of Heterobimetallic Coordination Complexes for Use as Precursors to Metal Oxide Materials: Synthesis, Characterization, and Thermal Decomposition of Bi2(Hsal)6.M(Acac)3[END_REF][START_REF] Xu | A review in visible-light-driven BiVO4 photocatalysts[END_REF]. Other techniques can be also used such as electrochemical and sol-gel processes but they are less common[START_REF] Hofmann | Nonaqueous Synthesis of a Bismuth Vanadate Photocatalyst By Using Microwave Heating: Photooxidation versus Photosensitized Decomposition in Visible-Light-Driven Photocatalysis[END_REF][START_REF] Pookmanee | Bismuth Vanadate (BiVO4) Powder Prepared by the Sol-gel Method[END_REF][START_REF] Seabold | Efficient and Stable Photo-Oxidation of Water by a Bismuth Vanadate Photoanode Coupled with an Iron Oxyhydroxide Oxygen Evolution Catalyst[END_REF][START_REF] Wang | Influence of heat treatment on photocatalytic performance of BiVO4 synthesised by sol-gel method[END_REF]). Most of these preparation methods allow the use of dopants and/or surfactants into the reactive mixture in order to tune the chemical composition and to control the morphology of BiVO 4 materials[START_REF] Park | Progress in bismuth vanadate photoanodes for use in solar water oxidation[END_REF]. A surfactant, also called structure directing agent, is often a polymer that affects the micro-and nano-structure of a material by adjusting its morphology, surface area, porosity, crystallinity, and crystal phase formation[START_REF] Garcia-Perez | Selective Synthesis of Monoclinic Bismuth Vanadate Powders by Surfactant-Assisted Co-Precipitation Method: Study of Their Electrochemical and Photocatalytic Properties[END_REF][START_REF] Xu | A review in visible-light-driven BiVO4 photocatalysts[END_REF]. Examples of surfactant are poly-ethylene glycol (PEG), Triton X-100, Pluronic P123 and F127, cetyl-trimethyl-ammonium bromide (CTAB) and poly-vinyl pyrrolidone (PVP).4 BiVO 4 -based photocatalystsAs a semiconductor photocatalyst, BiVO 4 -based materials are mainly studied in the form of powder or film. Most of the published works on BiVO 4 photocatalyst are actually devoted to systems containing powder suspension. For environmental applications, immobilisation of photocatalyst on rigid supports such as glass, polymer or ceramic is crucial in order to avoid the post-separation difficulties associated with the slurry form[START_REF] Azenha | Photodegradation of Pesticides and Photocatalysis in the Treatment of Water and Waste[END_REF][START_REF] Ibhadon | Heterogeneous Photocatalysis: Recent Advances and Applications[END_REF] Fitzpatrick 2013, Lazar et al. 2012).

  g/L) Methylene Blue (10 mg/L) Solar C/C 0 (120min) = 0.1 Lu et al. (2015) Powder (0.5 g/L) Methyl Orange (10 -5 M) Visible C/C 0 (120min) = 0.2 Jiang et al.

(Fig. 4

 4 Fig. 4 Band structure of pure BiVO 4 associated with the energetic value of band edge positions and ROS radicals formation

Fig. 7

 7 Fig. 7 Decorated BiVO 4 with Au nanoparticle. The different role of gold involved either in (a) electron trapping or (b) surface plasmon resonance. It is important to notice that these two processes occur under different irradiation energies i.e. wavelength (a) greater and (b) smaller than E g

  B; MB = methylene blue; MO = methyl orange

Fig. 8

 8 Fig.8Reproducibility of Rhodamine B photodegradation using Nb-BiVO 4 under solar light (under identical experimental conditions as inMonfort et al. 2017b)

Fig. 10

 10 Fig. 10 Energetic position of the VBM and CBM calculated from Mulliken electronegativity theory in different heterojunction materials (refs. in Table3)

Table 2 .

 2 Summary of photocatalytic degradation of different pollutants using doped BiVO 4 .

	Dopant	Form	Pollutant	Irradiation	Photocatalytic efficiency (from "pure" to "doped")	Ref.
	Eu(III) + F	Powder (0.6 g/L)	RhB (10 mg/L)	Solar	From 65 to 100 % after 60 min	Xue et al. (2017)
	Nd(III) + Er(III)	Powder (1 g/L)	RhB (10 mg/L)	Solar	From 57 to 96 % after 150 min	Liu et al. (2017)
	0.01% Ag(I)	Powder (1 g/L)	MB (10 ppm)	Visible	From 55 to 100 % after 60 min	Huang et al. (2017)
	10 at% Gd(III)	Powder (1 mg/L)	RhB (5 mg/L)	Solar	from 40 to 95 % after 120 min	Luo et al. (2016)
	10 at% Nb(V)	Film (1.5 cm²)	RhB (10 -5 M)	Solar	From 51 to 72 % after 180 min	Monfort et al. (2017b)
	2.5 mol% Fe(III)	Powder (1 g/L)	MB (20 mg/L)	Visible	From 25 to 55 % after 180 min	Chala et al. (2014)
	6 at% Sm(III)	Powder (1 g/L)	RhB (5 mg/L)	Solar	from 40 to 95 % after 120 min	Luo et al. (2015a)
	10 at% Nd(III)	Powder (1 g/L)	RhB (5 mg/L)	Solar	from 40 to 95 % after 120 min	Luo et al. (2015b)
	Er(III) + Yb(III)	Powder	RhB (20 mg/L)	Infra-Red	Until 25 % after 180 min	Shan and Liu (2016)
	8 at% Yb(III)	Powder (1 g/L)	RhB (5 mg/L)	Solar	from 50 to 95 % after 120 min	Huang et al.

  have shown that the main oxidative species in phenol degradation are holes and O 2 •-using both charge scavenger and EPR. They explain the observed mechanism by Co/Pd which traps e -. Therefore, it enhances the formation of O 2 •while h + can oxidise directly phenol since the redox potential for OH • formation is not reached. These few examples show that the mechanism varies from one dopant to another, but it shows also that degradation mechanism is not clearly determined since different authors have identified different oxidative species for a given doped material (refs. in Table 2).

Table 3 .

 3 Summary of photocatalytic degradation of different pollutants using BiVO 4 composite.

	Composite with	Form	Pollutant	Irradiation	Photocatalytic efficiency (from "pure" to "composite")	Ref.
	TiO 2	Powder (1.35 g)	Benzene (20 mL/min)	Visible	from 5 to 60 % after 8 h	Hu et al. (2011)

  have identified by liquid chromatography-mass spectroscopy the by-products from the degradation of bisphenol A. Using CuO x /BiVO 4 , hydroxylation of bisphenol A occurs until the scission of the molecule and subsequently to ring-opening until mineralisation is reached (Kanigaridou et al. 2016). On the other hand, Li et. al (2016b) have identified the by-products from the photooxidation of a pharmaceutical (tetracycline) in presence of Bi 2 Ti 2 O 7 /BiVO 4 using HPLC. However, further studies on openring molecules have not been performed. Similarly, Liu et. al (2018) have used HPLC and GC-MS to study the efficiency in the degradation of tetracycline using Ag/Ag 2 CO 3 /BiVO 4 composite photocatalyst. They have found out that tetracycline is degraded by successive oxidations resulting in open-ring molecule, and finally mineralisation into CO 2 , H 2 O and

	NH 4	+ is reached

  •-while for Ag 4 V 2 O 7 /BiVO 4 and Bi 2 Ti 2 O 7 /BiVO 4 , the main oxidative species in the degradation of MB and tetracycline, respectively, are hydroxyl radicals[START_REF] Hu | Facile synthesis of double cone-shaped Ag4V2O7/BiVO4 nanocomposites with enhanced visible light photocatalytic activity for environmental purification[END_REF] Li et al. 2016b). Another example, Liu et al. (2018) have studied the mechanism of tetracycline using Ag/Ag 2 CO 3 /BiVO 4 and they have found that OH • and h + are the main oxidising species. Atypical systems have the advantages to be decorated such as Ag/Ag 2 CO 3 /BiVO 4 (by silver), BiVO 4 /Ag 2 O and BiVO 4 /Fe 3 O 4 , where the decorated compounds play a role of e -trap (refs. in Table

The authors wish to acknowledge the financial support provided by the Scientific Grant Agency of the Slovak Republic through the project VEGA 1/0276/15.


Selma Souihel

Frederic Chavane

Olivier Marre

Bruno Cessac

Processing various motion features and measuring RGCs pairwise correlations with a 2D retinal model

Introduction

Most models of the early visual system consider simple and decorrelated retino-thalamic entries. However, studies have shown the retina is able to perform complex tasks using for instance motion anticipation to compensate delays in retino-cortical transmission [START_REF] Jordan | Anticipation of moving stimuli by the retina[END_REF], [START_REF] Fisher | Alert response to motion onset in the retina[END_REF]. There exists models of retina anticipation, based on gain control at the level of retinal ganglion cells (RGCs) and bipolar cells, able to reproduce several motion features. These models consider 1D receptive field kernels, and thus does not take into account the anisotropy of RGCs receptive fields. Furthermore, they only simulate isolated RGCs whereas these cells are connected in the retina via amacrine cells. This raises the question of which part of the complex motion processing and anticipation is performed by the retina and which part is processed by the visual cortex. In this work, we propose a 2D retina model anticipation, implementing a realistic connectivity, allowing us to reproduce responses to different stimuli.The connectivity allows us to reproduce RGC's responses to stimuli occurring outside their receptive fields. We also present measures of correlations that emphasize the role of connectivity in motion response.

Anticipation model variability

The cascade model used in this work, from Chen & al. [START_REF] Fisher | Alert response to motion onset in the retina[END_REF], is able to reproduce several motion processing features such as anticipation, alert response to motion onset and motion reversal. We first generalize the model in 2D and study anticipation variability, in an isotropic framework.

Anisotropy implementation

Within the isotropic framework, when the receptive field is smaller than the bar's size, anticipation produces shape deformation. Recent studies have shown that some retinal neurons are tuned to the orientation of elongated visual stimuli. In order to account for direction selectivity and study its effect on the object shape, we implement anisotropic filtering by rewriting the Gaussian of std σ u and σ v and rotation angle θ in a non orthogonal system of axes (x,φ). Integral separability and invariance along x axis allow the use of error function for half the computation.

σ x = σ u σ v σ 2 v cos θ 2 + σ 2 u sin θ 2 σ φ = σ 2 v cos θ 2 + σ 2 u sin θ 2 sin φ tan(φ) = σ 2 v cos θ 2 + σ 2 u sin θ 2 (σ 2 u -σ 2 v ) cos θ sin θ

Connectivity model

In the retina, cells with different types are connected and interact in order to produce a visual output. The connectivity at the level of ganglion cells is due to interneurons called amacrines. Other cells, such us Parasol RGCs, are directly connected through gap junctions. We developed a connectivity model where a cell has X branches, each of which has length Y and angle Z. X and Y follow exponential distributions and Z a uniform distribution. Correlation of means and mean correlation

C 1 (t) = E((Π 1 ∩ Π 2 )(t)) -E(Π 1 (t))E(Π 2 (t)) E(Π 1 (t))E(Π 2 (t)) C 2 (t) = E( (Π 1 ∩ Π 2 )(t) -Π 1 (t)Π 2 (t) Π 1 (t)Π 2 (t) )

Correlations measurement

When the retina is presented with moving bar stimulus, transient and non stationary changes in the firing rates can occur. Thus, measuring correlations is not straight forward. Under the assumption that spike trains Π 1 and Π 1 are non-homogeneous Poisson processes, we compute correlation using a moving window algorithm. We then generated correlated Poisson processes to test our method.

Correlations in the model

We want to compare correlation between two simulated cells with correlations in the data. For that we generate 200 sets of independent rasters. We find that the anti-correlation could be just an artifact related to the level of activity.

Conclusion

We generalized Chen model, which reproduces anticipation and other motion features, in 2D, implementing anisotropy and a biologically inspired connectivity between bipolar and ganglion cells. We studied the effect of anticipation on the object shape in 2D and the effect of connectivity on anticipation. The study of correlations in recorded data doesn't provide any decisive conclusion, since the anti-correlation seem to be an artifact due to low levels of firing rate. Next step will be to implement inhibitory connections between RGCs through amacrine cells to reproduce differential motion. RGCs spike trains responding to motion stimuli can be then used as an input to a primary visual cortex model to study motion processing.

Figure 1 :

 1 Figure 1: Schematic of the isotropic model.

Figure 2 :

 2 Figure 2: Variability of the model. A) Anticipation increases with the center std of the receptive field (RF), and is invariant with regards to the surround. B) Anticipation is maximum when the bar spans the whole RF and follows the RF shape. C) Anticipation increases with contrast and D) decreases with velocity.

Figure 3 :

 3 Figure 3: Schematic of the isotropic model.

Figure 4 :

 4 Figure 4: Filter transformation.

Figure 5 :

 5 Figure 5: Tuning curves of anisotropic bipolar cells.

Figure 6 :

 6 Figure 6: Anisotropy reduces shape deformation.

Figure 7 :

 7 Figure 7: A) Morphology of Parasol ganglion cells. B) Geometry of the model C) Connection probability decreases with distance. Analytic distribution probability meets geometric simulation.

Figure 8 :

 8 Figure 8: Long range connectivity between bipolar and ganglion cells increases anticipation. However, it puts the network in a highly excited state. Inhibitory connections, if involved in motion, have also to be taken into account.

Figure 9 :

 9 Figure 9: Computing the firing rates and correlation between two correlated Poisson processes with a moving window algorithm.

Figure 10 :

 10 Figure 10: Pairwise correlation in experimental data (200 trials).

Figure 11 :

 11 Figure 11: The effect of firing rate on correlation in simulated data (200 trials). A) Simulated data with a level of activity similar to recorded cells. B) Simulated data with higher firing rate.

This reasearch is entirely funded by Agence Nationale de la Recherche, in the context of the ANR project "TRAJECTORY". We would like to thank our collaborators, Olivier Marre from Institut De la Vision for providing us with retina recordings and Frédéric Chavane from Institut des Neurosciences de la Timone, both for their significant help and their insightful suggestions.

Acknowledgements