ORCID Patricia Garcia-Muñoz
email: garciamunoz@unistra.fr

Fernando Fresno

Víctor A De La Peña O'shea

ORCID Nicolas Keller

V A De La Peña O'

Shea Orcid

FERRITE MATERIALS FOR PHOTOASSISTED ENVIRONMENTAL AND SOLAR FUELS APPLICATIONS

Ferrites are a large class of oxides containing Fe 3+ and at least another metal cation that have been investigated for and applied to a wide variety of fields ranging from mature technologies like circuitry, permanent magnets, magnetic recording and microwave devices to the most recent developments in areas like bioimaging, gas sensing, and photocatalysis. In the last respect, although ferrites have been less studied than other types of semiconductors, they present interesting properties such as visible light absorption, tuneable optoelectronic properties and high chemical and photochemical stability. The versatility of their chemical composition and of their crystallographic structure opened a playground for developing new catalysts with enhanced efficiency. This article reviews the recent development of the application of ferrites to photoassisted processes for environmental remediation and for the synthesis of solar fuels. Applications in the photocatalytic degradation of pollutants in water and air, photo-Fenton, and solar fuels production, via photocatalytic and photoelectrochemical water splitting and CO2 reduction, are reviewed paying special attention to the relationships between the physico-chemical characteristics of the ferrite materials and their photo-activated performance.

KEYWORDS: ferrite; photocatalyst; heterojunction; water treatment; air detoxification; solar fuels 1. Ferrites: structure, synthesis and properties

Structure of ferrites

Ferrites -with an ethymology from the latin word 'ferrum' meaning iron -are a large class of oxides containing Fe 3+ and at least another metal cation that have been investigated and applied as powder, films or ceramic bodies for the last 50 years. They are classified according to their crystal structure and the way the oxygen anions are arranged around the metal cations, as spinel, garnet, magnetoplumbite and orthoferrite. While spinel and garnet ferrites both crystallize within a cubic structure, magnetoplumbites and orthoferrites crystallize within a hexagonal and orthorhombic structure, respectively.

Spinel ferrites

Spinel ferrites are compounds with a general chemical formula MFe2O4, where M refers to a divalent metal cation, and that crystallize in a crystallographic structure isomorph with that of the naturally-occurring mineral spinel MgAl2O4 (AB2O4 as general composition). They crystallize in a cubic symmetry structure containing eight formula units A[B]2O4, with a rigid sub-lattice of 32 closely packed oxygen anions giving rise to 64 tetrahedral (A)-type and 32 octahedral [B]-type interstitial sites partially occupied by 24 M 2+ and Fe 3+ metal cations (Figure 1). Depending on the synthetic conditions, tetragonal unit cells can be obtained as well [START_REF] Helaïli | Synthesis and physical properties of the CuFe2-xMnxO4 (0 ≤ x ≤ 2) solid solution[END_REF]. In the spinel ferrites, the electrical neutrality is maintained by M 2+ and Fe 3+ cations occupying both tetrahedral and octahedral interstitial sites. The interstitial fourfold and sixfold sites have rt and ro radii in the 0.055 nm < rt < 0.067 nm and 0.070 nm < ro < 0.075 nm range, so that they can be occupied by many transition metal cations with d 0 to d 10 electronic configurations. with permission from ref. [START_REF] Sickafus | Structure of Spinel[END_REF] Copyright SPIE The octahedral vs. tetrahedral site occupation -and therefore the spinel ferrite structure -is reported to be driven by the electrostatic contribution to the lattice energy, the cation radii, the cation charge and the crystal field effect. The reader can refer to the work of Sickafus et al. [START_REF] Sickafus | Structure of Spinel[END_REF]. Following the M 2+ and Fe 3+ metal cation balance between both tetrahedral and octahedral sites, the spinel ferrites are categorized as:

-normal spinels (M #$ ) &' [Fe +$ ] -. # O4, in which the smaller divalent M 2+ cations occupy only the tetrahedral A-sites and the trivalent Fe 3+ cations occupy the octahedral B-sites. Eg. Zn-or Cd-Fe2O4.

-inverse spinels (Fe +$ ) &' [M 2+ Fe 3+ ] Oh O4 in which the divalent M 2+ cations are located in the octahedral B-sites, while the trivalent Fe 3+ cations are equally distributed between both tetrahedral A-sites and octahedral B-sites. Eg. Mg-, Co-, Cu-, Ni-, NiCo-, CuMn-or MgCu-Fe2O4.

-mixed spinels /M 0 #$ Fe 1 +$ 2 &' 3M 450 #$ Fe #51 +$ 6 -. O 8 , in which both the divalent M 2+ and trivalent Fe 3+ cations are distributed between both tetrahedral A-sites and octahedral B-sites. e. g. NiZn-, MgZn-, ZnMn-, MnNi-, CuZn-or NiCuZn-Fe2O4.

Some spinel ferrites, such as MgFe2O4, CaFe2O4 and BaFe2O4, are also reported to crystallize as orthorhombic phases [START_REF] Levy | Equation of state, structural behaviour and phase diagram of synthetic MgFe2O4, as a function of pressure and temperature[END_REF][START_REF] Candeia | Synthesis and characterization of spinel pigment CaFe2O4 obtained by the polymeric precursor method[END_REF][START_REF] Candeia | Monoferrite BaFe2O4 applied as ceramic pigment[END_REF] (Fig 1c).

Garnet ferrites

Garnets are ferrites with a general chemical formula M3Fe5O12, where M is a rare-earth cation.

They crystallize in the structure of the X3Y2(SiO4)3 silicate mineral garnet, in which the X and Y sites are usually occupied by divalent (Ca 2+ , Mg 2+ , Fe 2+ ) and trivalent (Al 3+ , Fe 3+ , Cr 3+ ) cations, respectively, while the (SiO4) 4-units are providing a tetrahedral framework [START_REF] Valenzuela | Novel Applications of Ferrites 2012[END_REF]. They crystallize in the cubic system, within a relatively complex structure based on a unit cell composed of eight formula units. Three kinds of cations sites coexist within this structure, and the cation distribution is usually expressed as {M3}(Fe3)[Fe2]O12. The M rare-earth cations occupy the largest dodecahedral (eightfold) sites, while the Fe 3+ cations distribute among both tetrahedral (fourfold) and octahedral (sixfold) sites. By contrast to the spinel structure, the oxygen sublattice is better described as a polyhedral arrangement rather than as a closepacked one.

Magnetoplumbite ferrites

Magnetoplumbites are synthetic ferrites first prepared at Philips Lab in the 50s, and mainly sharing a general chemical formula M 2+ Fe12O19, where M 2+ can be Ba 2+ , Pb 2+ , Sr 2+ or Ca 2+ .

Although they are labelled also as hexaferrites, they crystallize in a hexagonal or rhombohedral symmetry structure. In addition to the MFe12O19 form (labelled as M type ferrite, Figure 2), they can be categorized within five other groups, consisting in M1-4Me2Fe12-36O22-60 type ferrites. While M 2+ is considered as a large divalent cation, Me 2+ represents usually a smaller divalent first-row transition metal cation such as Co 2+ , Fe 2+ , Zn 2+ or Ni 2+ . They are consequently categorized as Z-type (Ba3Me2Fe24O41), Y-type (Ba2Me2Fe12O22), X-type (Ba2Me2Fe28O46), W-type ferrites (BaMe2Fe16O27), and U-type (Ba4Me2Fe36O60) ferrites [START_REF] Pullar | Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics[END_REF]. The most studied magnetoplumbites are ferrites containing Ba and Co as divalent cations, but a large range of cation-substituted hexaferrites are investigated, especially M, W, Z and Y ferrites containing Sr, Zn, Ni and Mg. Hexaferrites can also form many solid solutions, and partial substitution with trivalent or tetravalent cations can be performed. The reader can refer to the review of Pullar for in-depth analysis [START_REF] Pullar | Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics[END_REF].

Figure 2. (left)

Perspective view [START_REF]BaFe12O19[END_REF] and (right) polyhedra of the unit cell in the case of the Mferrite structure (BaFe12O19) (Fe blue, Ba green, O red). Reproduced with permission from ref.

[9] Copyright Elsevier and Wiley.

Orthoferrites

Orthoferrites are ferrites with a general chemical formula MFeO3, where M is very often a rare-earth cation, an alkaline or an alkaline earth metal cation. They crystallize in a distorted perovskite structure with an orthorhombic symmetry and a ABO3 formula unit, in which FeO6 as a rotary tilted BO6 element fills the empty space left around the A cation (Figure 3). In this structure, the Fe cations occupy octahedral sites and are sixfold coordinated to oxygen anions, while the larger M cation is twelvefold coordinated to anions. While the ideal perovskite-type structure displays a cubic symmetry with a unit cell composed of four formula units in which the atoms are touching one another, the deviation from the ideal perovskite structure to give pseudo-cubic -mainly orthorhombic -symmetry has been rationalized by Goldschmidt [START_REF] Goodenough | Crystallographic and Magnetic Properties of Perovskite and Perovskite-Related Compounds[END_REF].

He introduced a tolerance factor t applicable at room temperature to the empirical ionic radii and materializing the stability and the distortion of the crystal structure within the following range:

0.75 < > = (@ A + @ -) √2 (@ E + @ -) < 1

For instance, lanthanum orthoferrite LaFeO3 shows a high Goldschmidt tolerance factor of 0.954, indicating that the distorted structure maintains a high level of symmetry, while the Yttrium orthoferrite YFeO3 structure displays a far lower symmetry with a lower tolerance factor of 0.855 [START_REF] Dixon | Thermal evolution of the crystal structure of the orthorhombic perovskite LaFeO3[END_REF]. (orthorhombic crystal symmetry). Adapted and reproduced with permission from ref. [START_REF] Yuan | First-order spin reorientation transition and specific-heat anomaly in CeFeO3[END_REF].

Copyright AIP Publishing.

Preparation methods

Progress in the synthesis of orthoferrites and spinel ferrites has been driven in the last decades by the technological interest aroused by the ferrite materials in various interdisciplinary application fields and by that derived from their tunable magnetic properties. Whether they O Fe M are categorized as bottom-up or top-down approaches, or as chemical or physical methods, a large variety of preparation methods have been investigated for synthesizing ferrite nanoparticles. Among them, co-precipitation, sol-gel methods, thermal methods and solidstate reactions remain the most popular strategies implemented for elaborating catalysts for heterogeneous photo-Fenton or photocatalysis, whereas a large (non-exhaustive) panel of methods such as sonochemical, microemulsion, reverse micelle, electrospinning, polyol, electrochemical, (auto)combustion, vapor deposition, laser ablation or flame-spray pyrolysis methods has been also reported (Figures 4 and5) [START_REF] Kefeni | Application of spinel ferrite nanoparticles in water and wastewater treatment: A review[END_REF][START_REF] Kefeni | Ferrite nanoparticles: Synthesis, characterisation and applications in electronic device[END_REF][START_REF] Tatarchuk | Spinel Ferrite Nanoparticles: Synthesis, Crystal Structure, Properties, and Perspective Applications[END_REF][START_REF] Masunga | Recent advances in copper ferrite nanoparticles and nanocomposites synthesis, magnetic properties and application in water treatment: Review[END_REF]. where less volatile precursors are in liquid form, with significantly higher combustion enthalpy (>50% of total energy of combustion), usually in an organic solvent ; and (c) electrospraying technique, that is a technique similar to electrospinning with the exception that it forms nanoparticles rather than nanofibers. Reproduced with permission from ref. [START_REF] Dhand | Methods and strategies for the synthesis of diverse nanoparticles and their applications: a comprehensive overview[END_REF]. Copyright Royal Society of Chemistry. For applications in photocatalysis or photo-Fenton processes, the specific surface area exposed, the crystallinity as well as the possibility to tune the intrinsic properties of the ferrites (such as the surface redox properties, the band-gap values or the light absorption profile) are the main criteria to be considered for selecting the synthesis method. The cationic or anionic mono-or bi-substitution within the defined structure of the ferrites is an elegant and powerful way to tailor those properties. Further, as far as the catalysts need to operate in a manner consistent with the constraints of industrial implementation, whether it concerns liquid or gas phase applications, the synthesis method should allow the ferrite catalyst to be immobilized on macroscopic supports. In addition, since most of the methods require a thermal treatment for increasing the crystallinity of the ferrite materials, the methods enabling a lower temperature to be achieved has to be preferred for not impacting too strongly on the specific surface area. Therefore, the chemical syntheses in solution remain the preparation methods of choice, among which co-precipitation, sol-gel methods and thermal methods are the most popular ones.

Co-precipitation method

The co-precipitation is a very common and fast synthesis method consisting in favoring the simultaneous precipitation of insoluble or low solubility products formed under high supersaturation conditions, from aqueous solutions of metal precursors, through pH adjustment and use of precipitating agents. The overall process in solution consists in nucleation, growth, coarsening and/or agglomeration processes that are highly difficult to uncouple, while a final thermal treatment is needed for decomposing the intermediate precipitate into the usable ferrite as well as for crystallizing the oxide. The co-precipitation method has been successfully used for synthesizing a large variety of spinel ferrites [START_REF] Amiri | Magnetic and structural properties of RE doped Co-ferrite (REåNd, Eu, and Gd) nano-particles synthesized by co-precipitation[END_REF][START_REF] Zi | Synthesis and magnetic properties of CoFe2O4 ferrite nanoparticles[END_REF][START_REF] Harzali | Structural and magnetic properties of nano-sized NiCuZn ferrites synthesized by coprecipitation method with ultrasound irradiation[END_REF][START_REF] Rashmi | Solar light responsive Sm-Zn ferrite nanoparticle as efficient photocatalyst[END_REF][START_REF] Vinosha | Investigation on the role of pH for the structural, optical and magnetic properties of cobalt ferrite nanoparticles and its effect on the photofenton activity[END_REF][START_REF] Pereira | Superparamagnetic MFe2O4 (M = Fe, Co, Mn) Nanoparticles: Tuning the Particle Size and Magnetic Properties through a Novel One-Step Coprecipitation Route[END_REF] or orthoferrites, mono-, bi -or non-substituted.

Sol-gel methods

The sol-gel method is a versatile approach in which a metal alkoxide solution undergoes hydrolysis and condensation polymerization reactions, with the formation of a sol and subsequent cross-linking to form a gel. It requires usually a final heat treatment for removing volatile byproducts and crystallizing the ferrite material. It takes advantage of a relatively low annealing temperature compared to solid state reactions for maintaining a relatively high surface area. It globally benefits from its simplicity, low implementation costs and tuneability for preparing a large range of spinel ferrites [START_REF] Tatarchuk | Chapter 22: Spinel Ferrite Nanoparticles: Synthesis, Crystal Structure, Properties, and Perspective Applications[END_REF][START_REF] Sharma | Tailoring the photo-Fenton activity of spinel ferrites (MFe2O4) by incorporating different cations (M = Cu, Zn, Ni and Co) in the structure[END_REF][START_REF] Zielinska-Jurek | Design and application of magnetic photocatalysts for water treatment. The effect of particle charge on surface functionality[END_REF][START_REF] Kurian | Heterogeneous Fenton behavior of nano nickel zinc ferrite catalysts in the degradation of 4-chlorophenol from water under neutral conditions[END_REF][START_REF] Li | Construction of heterostructured CuFe2O4/g-C3N4 nanocomposite as an efficient visible light photocatalyst with peroxydisulfate for the organic oxidation[END_REF] and orthoferrite structures [START_REF] Peng | Perovskite LaFeO3/montmorillonite nanocomposites: synthesis, interface characteristics and enhanced photocatalytic activity[END_REF][START_REF] Sannino | Structured catalysts for photo-Fenton oxidation of acetic acid[END_REF][START_REF] Soltani | Improving heterogeneous photo-Fenton catalytic degradation of toluene under visible light irradiation through Ba-doping in BiFeO3 nanoparticles[END_REF][START_REF] Li | Nanocomposite LaFe1-xNixO3 /Palygorskite catalyst for photo-assisted reduction of NOx: Effect of Ni doping[END_REF][START_REF] Li | Palygorskite Immobilized Direct Z-Scheme Nitrogen-Doped Carbon Quantum dots/PrFeO3 for Photo-SCR Removal of NOx[END_REF] with controlled composition, structure and morphology. One can note that the sol-gel method combined with combustion was used for preparing BaFe12O19 and CoFe12O19 hexaferrites (magnetoplumbite structure) [START_REF] Zielinska-Jurek | Design and application of magnetic photocatalysts for water treatment. The effect of particle charge on surface functionality[END_REF][START_REF] Ansari | Eco-friendly synthesis of cobalt hexaferrite and improvement of photocatalytic activity by preparation of carbonic-based nanocomposites for waste-water treatment[END_REF].

Pechini developed a modified sol-gel method for materials which do not have favorable hydrolysis equilibria, and in which the metallic ions undergo complexation through the addition of bi-or tri-dentate organic chelating agents [START_REF] Pechini | Method of preparing lead and alkaline earth titanates and niobates and coating method using the same to form a capacitor[END_REF][START_REF] Danks | The evolution of 'solâ€"gel' chemistry as a technique for materials synthesis[END_REF]. The method builds on the principles of sol-gel chemistry involving small-molecule chelating ligands that in the initial step form a solution of metal/chelate complexes. It takes this further to convert the mixture into a covalent polymer network to entrap the metal ions. The use of chelating agents allows stable complexes to be formed with a large variety of metals, and to get rid of the requirement that the metal has to form stable hydroxo species. The underlying strategy of the method lies in the delay of the thermal decomposition of the organic matrix in order to afford more control over the growing ceramic phase. While citric acid is the most popular chelating agent (i.e. citrate method), it can be also replaced by ethylenediaminetetraacetic acid (EDTA), which has the advantage of chelating most metals and, with four carboxylate groups, it is easily cross-linked to form a gel. When ethylene glycol is used in association with citric acid, the key-reaction is the transesterification between citrate and ethylene glycol (Figure 6). The metal salt dissolved in water with citric acid and ethylene glycol forms a homogeneous precursor solution containing the metal-citrate chelate complexes, while the heat-induced polyesterification between the citrate and ethylene glycol creates an extended covalent network. The method requires a final calcination step for combusting the organic matrix and forming the oxide. It became very popular for synthesizing ferrite materials, notably due to its ability to form a polymeric precursor where two or more metals may be dispersed homogeneously throughout the network [START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF][START_REF] Jamil | Visible light activity of BaFe1-xCuxO3-d as photocatalyst for atrazine degradation[END_REF].

Thermal methods

The thermal methods are mainly based on thermal decomposition [START_REF] Yin | High performance of magnetic BiFeO3 nanoparticle-mediated photocatalytic ozonation for wastewater decontamination[END_REF], as well as on hydrothermal [START_REF] Luo | Rational design of Z-scheme LaFeO3/SnS2 hybrid with boosted visible light photocatalytic activity towards tetracycline degradation[END_REF][START_REF] Liu | Fabrication of Ag/AgCl/ZnFe 2 O 4 composites with enhanced photocatalytic activity for pollutant degradation and E. coli disinfection[END_REF][START_REF] Liu | The triplecomponent Ag 3 PO 4 -CoFe 2 O 4 -GO synthesis and visible light photocatalytic performance[END_REF][START_REF] Diao | Magnetic multi-metal co-doped magnesium ferrite nanoparticles: An efficient visible light-assisted heterogeneous Fenton-like catalyst synthesized from saprolite laterite ore[END_REF], solvothermal and microwave-assisted methods. While the thermal decomposition consists in the simple decomposition of precursors, organic solvents and surfactants during heat treatment to get the ferrite, the other methods are driven by the need to lower the temperature used for crystallization in order for the ferrite to maintain a suitable specific surface area. Hydrothermal and solvothermal usually differ in terms of solvent, aqueous vs. non-aqueous, respectively, in which the soluble metal salts are dissolved.

Temperature, pressure, synthesis duration as well as choice of solvent, of precursors and of additional surfactant, are the main synthesis parameters impacting on the physico-chemical properties of the ferrites [START_REF] Kefeni | Application of spinel ferrite nanoparticles in water and wastewater treatment: A review[END_REF][START_REF] Kefeni | Ferrite nanoparticles: Synthesis, characterisation and applications in electronic device[END_REF][START_REF] Casbeer | Synthesis and photocatalytic activity of ferrites under visible light: A review[END_REF][START_REF] Ameer | Ultra low permittivity/loss CoFe2O4 and CoFe2O4-rGO nanohybrids by novel 1-hexanol assisted solvothermal process[END_REF]. While the temperature can be as high as 500 °C for the thermal decomposition method, it can be decreased to the 100-200 °C range for solvothermal, hydrothermal and microwave-assisted methods. However, although the microwave-assisted syntheses allow shorter reaction durations to be achieved, they suffer usually from low synthesis yields.

Solid-state reaction methods

Besides the bottom-up chemical syntheses in solution above-reported, the solid-state reaction methods are a suitable top-down approach consisting in treating solid precursors at high temperature, usually oxides or carbonates, for inducing solid-solid diffusion, with a further stabilization/crystallization of the ferrite defined structure. CoFe2O4, CuFe2O4, NiFe2O4 and Ni1-xMnxFe2O4 spinel ferrites have notably been synthesized [START_REF] Marinca | Structural and magnetic properties of the copper ferrite obtained by reactive milling and heat treatment[END_REF][START_REF] Zhang | Synthesis and characterization of nickel ferrite nanoparticles via planetary ball milling assisted solid-state reaction[END_REF][START_REF] Manova | Mechano-Synthesis, Characterization, and Magnetic Properties of Nanoparticles of Cobalt Ferrite, CoFe2O4[END_REF][START_REF] Yan | Hydrothermal synthesis and structure evolution of metal-doped magnesium ferrite from saprolite laterite[END_REF]. However, the obtained ferrites usually suffer from low specific surface area when compared to chemical synthesis in solution. Therefore, the mechanical milling method has been implemented as a high-energy collision top-down approach method using planetary ball mill, high-energy shaker or tumbler mill for inducing proper conditions in which solid state chemical reactions can take place, mostly through mechanical activation or mechano-chemistry. However, while the method is low-cost and time-saving with easy up-scaling, it suffers notably from strong contamination problems during prolonged milling, non-stoichiometrical structure, nonuniform size distribution and low crystallinity.

Further, many more or less complex and versatile bottom-up or top-down methods have been implemented for preparing ferrite materials, alone or as part of the main popular methods above-described. Without being exhaustive, one can list notably microemulsion, flame spray pyrolysis, auto-combustion, vapour deposition, liquid exchange, self-reactive quenching, or electrospinning. The reader can refer to the reviews of Kefeni et al. [START_REF] Kefeni | Application of spinel ferrite nanoparticles in water and wastewater treatment: A review[END_REF] and Tatarchuk et al. [START_REF] Tatarchuk | Chapter 22: Spinel Ferrite Nanoparticles: Synthesis, Crystal Structure, Properties, and Perspective Applications[END_REF], or directly to the original research cited therein.

Finally, it is worth noting that the optimal physico-chemical properties that the ferrite materials need to exhibit are highly depending on their application fields, so that putting into perspective the advantage vs disadvantage of the different preparation methods remain in many cases inappropriate.

Ferrite properties

Ferrites are globally considered as being thermally and chemically stable in aqueous systems [START_REF] Casbeer | Synthesis and photocatalytic activity of ferrites under visible light: A review[END_REF], and the Pourbaix diagrams indicate that most of the ferrites are stable in most of the alkaline or near-neutral media used in photoelectrochemical cells [START_REF] Dereje | Photoelectrochemical and theoretical investigations of spinel type ferrites (MxFe3âˆ'xO4) for water splitting: a mini-review[END_REF]. However, they are reported to suffer from corrosion in acidic media [START_REF] Thomson | Uhlig's Corrosion Handbook[END_REF][START_REF] Hemmi | Electrochemical Considerations Regarding General Corrosion of Materials in a BWR Primary Circuit[END_REF]. Independently of the crystallographic structure, the mono-or bi-substitution of the cations within the ferrite materials is known to affect many of their physico-chemical properties, among others, the resistivity (conductivity), the optical properties (reflectivity), the band-gap energy, the energy position of both valence and conduction bands, or the p/n-type behaviour [START_REF] Dereje | Photoelectrochemical and theoretical investigations of spinel type ferrites (MxFe3âˆ'xO4) for water splitting: a mini-review[END_REF]. Therefore, the controlled cationic single-or multi-substitution in the ferrite network, whether A-or B-sites are concerned, became an elegant, versatile and promising way to tune both bulk and surface physico-chemical properties of the ferrite-based photocatalysts used for environmental (water and air depollution) and solar fuel applications.

The specific surface area exposed by the ferrite materials is strongly depending on the preparation method. However, the ferrite catalysts globally exhibit low to medium specific surface area (<100 m 2 /g), in contrast to lab-made or commercially available titania photocatalysts, that can reach high surface areas within the 200-350 m 2 /g range thanks to the implementation of adequate synthesis approaches. Without being obviously exhaustive, Table 1 gathers some selected spinel ferrites and orthoferrites, substituted or not, with their specific surface area, crystallite size and isoelectrical point being reported in relationship with their preparation method and final calcination temperature. It points out to which extent the synthesis method impacts on some properties of the materials. It has to be mentioned that maintaining a relatively low or moderate calcination temperature does not necessarily prevent failing to obtain high specific surface area ferrites, while, by contrast, it may result in materials with low crystallinity and therefore with a low photoactivity due to a high recombination rate.

Most of the ferrite materials are semiconductors and take advantage of being capable to absorb a part of visible light due to a band gap of about 2 eV (corresponding to wavelengths lower than 621 nm). This contrasts with the very popular anatase TiO2 reference material, whose band gap of 3.2 eV allows only the absorption of UV light, corresponding to wavelengths lower than 388 nm. Most of them display energy positions of both valence and conduction bands suitable for either the oxidation of water, the reduction of protons and/or the generation of active hydroxyl radicals from water and the reduction of dioxygen into the superoxide radical, as necessary for environmental and water splitting applications. Figure 7 gathers the band positions of the most popular ferrite materials with AB2O4 spinel structure.

Similarly to its influence on the reactivity in thermal catalysis [START_REF] Ramankutty | Surface properties and catalytic activity of ferrospinels of nickel, cobalt and copper, prepared by soft chemical methods[END_REF][START_REF] Reddy | Cr-and Ce-Doped Ferrite Catalysts for the High Temperature Waterâˆ'Gas Shift Reaction: TPR and Mossbauer Spectroscopic Study[END_REF], the chemical nature of the substituted metals and the magnitude of the mono-and bi-substitution ratios is known to influence some of the physico-chemical properties directly affecting the reactivity in photocatalysis, such as the optical properties via the band gap energy and the material reflectivity, the conductivity [START_REF] Šimša | Optical and magneto-optical properties of magnetite and manganese ferrites[END_REF][START_REF] Balaji | Combustion synthesis and characterization of Sn4+ substituted nanocrystalline NiFe2O4[END_REF][START_REF] Harish | Synthesis, enhanced optical and photocatalytic study of Cdâ€"Zn ferrites under sunlight[END_REF], as well as the n/p-type behavior of the semiconductor [START_REF] Archer | Electrochemical approaches to solar energy conversion: A brief overview and preliminary results obtained with n-type cobalt ferrite[END_REF][START_REF] Antonious | Photoelectrochemical characteristics of p-and n-type polycrystalline Ni-ferrite electrodes in aqueous solutions[END_REF].

The p or n nature of the spinel ferrite influences the positions of both conduction and valence bands, and consequently the band gap value as well. This can be evidenced in the case of the MgFe2O4 spinel ferrite, with a conduction band for the p-ferrite being 1.8 eV more cathodic compared to its n-counterpart, and band gap being smaller from ca. 0.3 eV. for the p-ferrite. By contrast, the p-or n-nature of the ZnFe2O4 ferrite has been reported to only affect the band position, for the p-ferrite being ca. 1.2 eV more cathodic compared to its ncounterpart. Adapted from [START_REF] Dereje | Photoelectrochemical and theoretical investigations of spinel type ferrites (MxFe3âˆ'xO4) for water splitting: a mini-review[END_REF].

H 2 O/OH°O 2 /O 2 °-
Growing interest has been paid to ABO3 transition-metal oxide orthoferrites thanks to a small band gap (2.0-2.7 eV), that allows the catalyst to be activated by visible light, and that results from the relative location of valence and conduction bands, mainly constituted of strongly mixed eg states of Fe 3d and O 2p states, and of the t2g states of Fe 3d, respectively. For instance, LaFeO3, one of the most common and promising perovskite-like ferrite catalyst, is usually reported with a band gap between 2.0 and 2.6 eV, depending on the synthesis method and their resulting physico-chemical properties [START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF][START_REF] Ismael | Perovskite-type LaFeO3: Photoelectrochemical Properties and Photocatalytic Degradation of Organic Pollutants Under Visible Light Irradiation[END_REF]. Ansari et al. used CoFe12O19 hexaferrites as photocatalyst with a 3 eV band-gap [START_REF] Ansari | Eco-friendly synthesis of cobalt hexaferrite and improvement of photocatalytic activity by preparation of carbonic-based nanocomposites for waste-water treatment[END_REF]. By contrast Zielinska et al. graphically estimated the band-gap of the BaFe12O19 semiconductors material at 1.0 eV [START_REF] Zielinska-Jurek | Design and application of magnetic photocatalysts for water treatment. The effect of particle charge on surface functionality[END_REF].

Towards more efficient ferrite-based photocatalytic materials

Ferrite semiconductor-based photocatalysis and photoelectrochemistry have attracted attention because of the ability of the ferrite materials to absorb and consequently use visible light -and therefore to directly convert the solar energy -for performing surface redox reactions allowing environmental remediation to be implemented and solar fuels to be synthesized. During the photocatalytic process, the recombination of the photogenerated electron-hole pairs plays a highly negative role. Thus, similarly to the wide bandgap semiconductor TiO2, considered as the most frequently used semi-conductor photocatalyst under UV-A light, the photocatalytic efficiency of the ferrites is usually defined by the abundance and stability of the photo-generated charges (electron and holes). Therefore, efforts have been devoted to extending the life time of such charge carriers, i.e. to slowing down their recombination rate. Many approaches have been investigated for that purpose, including the modification of the shape of the nanoparticles or of their chemical compositions through the design and fabrication of semiconductor heterostructures, hybrid nanocomposites, doped-nanostructures or more globally multi-phase materials.

The design of semiconductor heterojunctions is a general and very promising strategy for elaborating ferrite-based photocatalysts with improved efficiency, whether the reactions concern environmental applications (water and air depollution) or the solar fuel field for the energy-related applications (CO2 conversion or H2 production). Basically, a heterojunction can in general be schematically described as the interface between two different semiconductors with unequal band structure, that can result in band alignments [START_REF] Vinodgopal | Enhanced Rates of Photocatalytic Degradation of an Azo Dye Using SnO2/TiO2 Coupled Semiconductor Thin Films[END_REF][START_REF] Ranjit | Synthesis, characterization and photocatalytic properties of iron-doped TiO2 catalysts[END_REF]. In the recent years, most of the studies were devoted to the creation of controlled heterojunctions with anatase TiO2 of different morphologies absorbing UV-A light, or with the smaller gap graphitic g-C3N4

semiconductor absorbing visible light, as well as to the design of visible light responsive ferrite/ferrite interface heterojunctions. Whatever the configuration investigated, the heterojunction strategy was mainly implemented for use as photocatalysts and photoelectrodes for energy applications. Details are given in the further corresponding sections.

In some cases like for conventional Z-scheme heterojunctions, there is not physical contact between both systems, the charges migrating through donor/acceptor pairs [START_REF] Bard | Photoelectrochemistry and heterogeneous photo-catalysis at semiconductors[END_REF]. Depending on the band gaps and on the relative position of the valence and conduction bands of both semiconductors, the engineering of ferrite-based heterojunctions aims (i) at separating spatially the photogenerated charges for extending the charge carrier life time thanks to efficient semi-conductor/semi-conductor interfacial charge transfers, and/or (ii) at using the ferrite semiconductor as solid visible light absorption/harvesting system for the photosensitization of wider band gap semiconductors (ie. TiO2).

The reader can refer to the critical reviews of Low et al. [START_REF] Low | Heterojunction Photocatalysts[END_REF] and Jang et al. [START_REF] Jang | Heterojunction semiconductors: A strategy to develop efficient photocatalytic materials for visible light water splitting[END_REF] that extensively depicted and systematically discussed the basic principles of various heterojunction photocatalysts, namely conventional heterojunctions of type I-II-III depending on the relative positions of both bandgaps, p-n heterojunctions, direct Z-Scheme heterojunctions (including solid-state Z-scheme) as well as graphene-based heterojunctions.

The elaboration of metal-ferrite semiconductor junctions is another strategy followed by different research groups reported in the next sections of this review. The supported metals consists usually in nanoscale metal islands on the semiconductor. The junction forms an electric field that facilitates the separation the photogenerated electrons and holes [START_REF] Jang | Location and State of Pt in Platinized CdS/TiO2 Photocatalysts for Hydrogen Production from Water under Visible Light[END_REF][START_REF] Jang | Development of a Potential Fe2O3-Based Photocatalyst Thin Film for Water Oxidation by Scanning Electrochemical Microscopy: Effects of Agâˆ'Fe2O3 Nanocomposite and Sn Doping[END_REF],

together with the formation of local space charge separation zones at the interphase. Beside a role for enhancing the lifetime of the charge carries, the supported metallic nanoparticles act as well as co-catalyst in different innovative ferrite-based systems used as photocatalysts or photoelectrodes for hydrogen production and CO2 reduction (see section 3).

The association of ferrites nanoparticles with adsorbent materials such as montmorillonite or palygorskite clays is also reported in the field of environmental remediation for trying to take advantage of the synergistic effect of adsorption and catalysis (section 2).

Finally, the cationic doping of ferrite -usually depicted as partially substituted ferrites -is a very promising approach investigated by many research groups for improving the efficiency of the ferrite photocatalysts and photoelectrodes, and reported in the next sections. Indeed, a, b and c correspond to crystallite sizes derived from XRD, SEM and TEM analysis, respectively.

whatever

Ferrite photocatalysts for environmental applications: water detoxification and air depollution

Introduction

Photocatalysis as an Advanced Oxidation Process has been applied for water treatment and for air depollution. These processes are based on the formation of highly oxidative species that can convert recalcitrant organic pollutants into mineral compounds, i.e. CO2, H2O and mineral acids. Among the catalysts that can be employed are found the ferrites. The main properties of those materials are their high stability and the good mobility of the oxygen in the network and so the facility to form vacancies and to stabilize metals with non-usual oxidation states [START_REF] Kefeni | Application of spinel ferrite nanoparticles in water and wastewater treatment: A review[END_REF][START_REF] Masunga | Recent advances in copper ferrite nanoparticles and nanocomposites synthesis, magnetic properties and application in water treatment: Review[END_REF]. In this scenario, ferrites have been employed in both fields (water and air treatment), taking advantage of their activation by visible light to generate the species that carry out the photocatalytic process [START_REF] Casbeer | Synthesis and photocatalytic activity of ferrites under visible light: A review[END_REF]. The catalytic applications for environmental remediation in which ferrites result relevant include not only their use as photocatalysts , but also, as catalyst in intensified processes by simultaneously applying single processes with the purpose of obtaining a higher efficiency by reducing the inherent drawbacks of each single treatment. [START_REF] Soltani | Improving heterogeneous photo-Fenton catalytic degradation of toluene under visible light irradiation through Ba-doping in BiFeO3 nanoparticles[END_REF][START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF].

Ferrite photocatalysts for water detoxification

There are several works dealing with the use of ferrite catalysts for the treatment of water by photocatalysis (Table 2). The first ones were reported in 2015 by Zaharieva et al. [START_REF] Zaharieva | Preparation, characterization and application of nanosized copper ferrite photocatalysts for dye degradation under UV irradiation[END_REF] and Mahto et al. [START_REF] Mahto | Citric acid fuctionalized magnetic ferrite nanoparticles for photocatalytic degradation of azo dye[END_REF]. In the first case, the authors synthesized Cu0.25Fe2.75O4 by a co-precipitation method with a further thermal treatment and applied it for the degradation of malachite green. They obtained better activity than the reference TiO2-P25. In the second one, Fe3O4, MnFe2O4 and ZnFe2O4, obtained by co-precipitation too, were employed to degrade azo dyes in aqueous solution. Materials were functionalized by citric acid in order to get an improvement in the photoactivity.

Recently, other methods, as mentioned in the section 1, have been employed for the synthesis of ferrites such as combustion route, electrospinning and sol-gel [START_REF] Ansari | Eco-friendly synthesis of cobalt hexaferrite and improvement of photocatalytic activity by preparation of carbonic-based nanocomposites for waste-water treatment[END_REF][START_REF] El-Rafei | Electrospun magnetically separable calcium ferrite nanofibers for photocatalytic water purification[END_REF][START_REF] Patil | Sugarcane juice mediated eco-friendly synthesis of visible light active zinc ferrite nanoparticles: Application to degradation of mixed dyes and antibacterial activities[END_REF]. Ansari et al.

synthesized a cobalt hexaferrite, CoFe12O19, by sol-gel method and using a natural agent as reducing agent: maltose and fructose. In order to increase the activity, the nanoparticles were supported on carbon nanotubes or graphene sheets. The effect of the reducing agent as well as the calcination temperature was evaluated. The band gap value of 3.0 eV let their use in the photocatalytic degradation of methyl orange under visible light. The authors obtained 75% of methyl orange degradation in 50 min of reaction as well as a higher activity when the ferrite was supported due to an improved charge separation and a lower charge recombination.

CaFe2O4, with spinel structure, was obtained by electrospinning by El-Rafei's group [START_REF] El-Rafei | Electrospun magnetically separable calcium ferrite nanofibers for photocatalytic water purification[END_REF]. The material showed a good magnetization, a key-parameter in water treatment for a recovery purpose, as well as photoactivity under simulated sunlight. The study was focused on the effect of heat-treatment temperature and showing the needed of higher than 800°C treatment to get a well-formed ferrite phase.

The last technique employed for the synthesis of ferrite as photocatalysts for water treatment has been the autocombustion route performed by Patil et al [START_REF] Patil | Sugarcane juice mediated eco-friendly synthesis of visible light active zinc ferrite nanoparticles: Application to degradation of mixed dyes and antibacterial activities[END_REF]. In this case, the authors got ZnFe2O4 by using sugar cane as fuel. The resulted material showed a band gap value of 2.8 eV and was used for two objectives. On one hand, as photocatalyst for dye removal and, on the other hand, for antibacterial purposes. The data indicated that a complete removal of mixed dyes after 150 min was reached as well as good inhibition of Escherichia Coli, Staphylococcus Aureus and Pseudomonas Aeruginosa. ZnFe2O4 showed good reusability over 4 cycles minimum.

However, the perovskite specific surface areas are usually quite low, and, in most cases, the activity can be limited by this parameter. In order to increase the activity, they have been immobilized on different supports like montmorillonite and active carbon. A few works have been developed in this sense. It is worth highlighting the works of Peng et al. [START_REF] Peng | Perovskite LaFeO3/montmorillonite nanocomposites: synthesis, interface characteristics and enhanced photocatalytic activity[END_REF] and Roonasi et al. [START_REF] Roonasi | Synthesis and application of barium ferrite/activated carbon composite as an effective solar photocatalyst for discoloration of organic dye contaminants in wastewater[END_REF]. In the former, LaFeO3 was deposited on montmorillonite and employed to oxidize Rhodamine B in water [START_REF] Peng | Perovskite LaFeO3/montmorillonite nanocomposites: synthesis, interface characteristics and enhanced photocatalytic activity[END_REF]. The composite photoactivity was higher than that of pristine LaFeO3, which was associated to the adsorption onto the surface. They got a complete degradation of Rhodamine B in 75 min, but the main drawback was that the TOC evolution was not followed during the time-course of the photoreactions. Roonasi et al. [START_REF] Roonasi | Synthesis and application of barium ferrite/activated carbon composite as an effective solar photocatalyst for discoloration of organic dye contaminants in wastewater[END_REF] supported barium ferrite on active carbon for the discoloration of dyes in water. When comparing the unsupported ferrite with the supported one, data showed that the photoactivity improved when adding an the active carbon due to a synergetic effect.

Other strategies have been developed to improve the activity too. Among them, coupling semiconductors in heterojunctions has gained importance. In this frame, it is worth highlighting the employment of LaFeO3/SnS2, LaFeO3/AgBr, BiFeO3/BiOI and Ag3PO4-CoFe2O4-GO, the latest with a double improvement by the addition of an adsorbent. In the first case, LaFeO3 was obtained by a hydrothermal method and well-dispersed in SnS2 nanosheets [START_REF] Luo | Rational design of Z-scheme LaFeO3/SnS2 hybrid with boosted visible light photocatalytic activity towards tetracycline degradation[END_REF].

Better activity than pristine LaFeO3 was observed for tetracycline oxidation. In parallel, Song et al. [START_REF] Song | Enhanced photocatalytic decomposition of an organic dye under visible light with a stable LaFeO3/AgBr heterostructured photocatalyst[END_REF] obtained a LaFeO3/AgBr composite, used for Rhodamine B removal. They got a faster movement of charges with the 10% LaFeO3/AgBr. In these two cases, the enhancement of the photoactivity could be ascribed to the prevalence of Z-scheme mechanism of charges transfer. The synthesis of a BiFeO3/BiOI composite by wet impregnation was carried out by Malathi et al. [START_REF] Malathi | An efficient visible light driven bismuth ferrite incorporated bismuth oxyiodide (BiFeO3/BiOI) composite photocatalytic material for degradation of pollutants[END_REF]. The authors designed a photocatalyst that resulted to be quite active and stable after several runs. The 2% BiFeO3/BiOI got the maximum activity for a complete Rhodamine B depletion within the first 60 min of reaction and after 3 sequential cycles of reaction. The good activity was related to the efficient charge separation of photoinduced hole-electron pairs by transferring the electrons from BiOI to BiFeO3 surface [START_REF] Malathi | An efficient visible light driven bismuth ferrite incorporated bismuth oxyiodide (BiFeO3/BiOI) composite photocatalytic material for degradation of pollutants[END_REF]. Finally, the last work with composites was performed by Zielinska et al. [START_REF] Zielinska-Jurek | Design and application of magnetic photocatalysts for water treatment. The effect of particle charge on surface functionality[END_REF]. In this case, the role of ferrites was to provide a magnetic core to a TiO2/SiO2 coated material for a recovery purpose after application. Figure 8 shows the HAADF images of the catalyst as example.

The last recently reported strategy deals with the partial substitution of cations in the ferrite structure. There are a few works that have developed this method until now. Jamil et al.

synthesized a BaFe1-xCuxO3-δ material [START_REF] Jamil | Visible light activity of BaFe1-xCuxO3-d as photocatalyst for atrazine degradation[END_REF], the group of Rashmi et al. ZnSmxFe2-xO4 [START_REF] Rashmi | Solar light responsive Sm-Zn ferrite nanoparticle as efficient photocatalyst[END_REF] and recently Garcia-Muñoz et al. obtained a La1-xTiXFeO3 ferrite by sol-gel method [START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF].

A Pechini synthesis of BaFe1-xCuxO3-δ catalyst led to a material rich in oxygen vacancies that was successfully employed for the abatement of atrazine in aqueous solution. The best photoactive catalysts resulted in BaFe0.95Cu0.05O3-δ sample which provoked a 90% of atrazine removal (in 180 min of reaction) that resulted not only from the vacancies created during synthesis but also from the higher surface area [START_REF] Jamil | Visible light activity of BaFe1-xCuxO3-d as photocatalyst for atrazine degradation[END_REF]. In contrast, ZnSmxFe2-xO4 nanoparticles were synthesised by co-precipitation method. In this case, the insertion of Sm 3+ ions in the ZnFe2O4 spinel matrix shows an increase in lattice parameter and crystallite size leading to an extension of the light absorption spectrum toward the visible range as Sm content increases up to x = 1.5 with narrow band gap (1.42 eV). This fact reduced the recombination of electron and holes improving the degradation of Methyl Orange [START_REF] Rashmi | Solar light responsive Sm-Zn ferrite nanoparticle as efficient photocatalyst[END_REF]. A modified Pechini method was used for the synthesis of Ti modified LaFeO3 catalysts [START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF]. The best performance was reached by adding a 10% of nominal TiO2 to the synthesis. However, a scarce activity was seen for 4-Cl-Phenol depletion, whether solar light or visible light was used as activation source, what led to the implementation of other strategies that will be studied in the next part of this review. Fe3O4, SiO2 and TiO2 (blue is Ti, red is Fe and green Si). Reproduced with permission from [START_REF] Zielinska-Jurek | Design and application of magnetic photocatalysts for water treatment. The effect of particle charge on surface functionality[END_REF].

Copyright MDPI.

Ferrite photocatalysts for air depollution

In contrast to water treatment, in the scope of air depollution, only a few works have been recently detailed, all of them treating NOx (Table 3). The requirement of immobilizing ferrites onto supports improves the dispersion (avoiding agglomeration of particles) of those kind of catalysts characterized by relatively small values of surface area. Therefore, more complex further studies are needed.

Only two works with pure single ferrite have been published up to now, to the best of our knowledge. Agdham et al. [104] reported the synthesis of BiFeO3 by a sol-gel procedure. The resulted material showed a band gap value of 2.1 eV, and it was employed for the removal of NOx under UV light though. The authors got 36% conversion by using a reusable catalyst.

Other strategies have also been applied in the air treatment. Xiazhang's group developed the immobilization of several ferritic composites onto a palygorskite support [START_REF] Li | Nanocomposite LaFe1-xNixO3 /Palygorskite catalyst for photo-assisted reduction of NOx: Effect of Ni doping[END_REF][START_REF] Li | Palygorskite Immobilized Direct Z-Scheme Nitrogen-Doped Carbon Quantum dots/PrFeO3 for Photo-SCR Removal of NOx[END_REF]. The first immobilized ferrite was LaFeO3 where Ni was replacing Fe inside the perovskite structure; and the second was a ternary material composed of N-CQDots (Nitrogen-Doped Carbon Quantum dots) and of PrFeO3 acting as consistent structure to be further supported. In the first case, the LaFe1-xNixO3 structure was conditioned by the fraction of Ni added to the synthesis. While ratios below 0.4 gave rise pure LaFe1-xNiXO3, when x >0.4 the material resulted in a heterojunction due to the co-precipitation of some LaNiO3. However, it must be said that 90% conversion was obtained when employing a ferrite with x=0.5 [START_REF] Li | Nanocomposite LaFe1-xNixO3 /Palygorskite catalyst for photo-assisted reduction of NOx: Effect of Ni doping[END_REF]. In the second case, the ternary composite with a 5% of CQDs (Carbon Quantum dots)/PrFeO3 got 93% conversion with a total selectivity to N2 (Figure 9). The good results obtained are associated to the formed Zscheme that not only enhanced the separation of charges, but also promoted the absorption of visible light [START_REF] Li | Palygorskite Immobilized Direct Z-Scheme Nitrogen-Doped Carbon Quantum dots/PrFeO3 for Photo-SCR Removal of NOx[END_REF].

Combination of ferrite photocatalysts and other oxidizing agents

To take advantage of the redox surface properties of ferrite materials and to improve the reactivity, the ferrite catalysts started to be employed simultaneously as Fenton catalysts, with the addition of H2O2 as oxidizing agent, under light irradiation [START_REF] Vinosha | Investigation on the role of pH for the structural, optical and magnetic properties of cobalt ferrite nanoparticles and its effect on the photofenton activity[END_REF][START_REF] Sannino | Structured catalysts for photo-Fenton oxidation of acetic acid[END_REF][START_REF] Bhoi | Photocatalytic degradation of alachlor using type-II CuS/BiFeO3 heterojunctions as novel photocatalyst under visible light irradiation[END_REF]105,106]. In this approach, materials with different compositions have been used for this purpose, like LaFeO3, CoFe2O4, mesoporous CuFe2O4, NiFeMO4 (M=La, Sm, Gd, Dy), CuS/BiFeO3.

By adding H2O2, a larger variety of pollutants have been degraded than in the case of pure photocatalysis using ferrites (Table 4). In the first case, Sannino et al. [START_REF] Sannino | Structured catalysts for photo-Fenton oxidation of acetic acid[END_REF] employed LaFeO3

for the removal of acetic acid. Only 60% depletion in 300 min was obtained for the best case.

Then CoFe2O4 ferrite was used to oxidize methylene blue, with almost complete oxidation in 75 min of reaction [START_REF] Vinosha | Investigation on the role of pH for the structural, optical and magnetic properties of cobalt ferrite nanoparticles and its effect on the photofenton activity[END_REF]. The combined mechanism in presence of H2O2 led to 90% of pesticide removal in 1 h of reaction.

Several efforts have been made to intensify the reactivity of ferrite materials in water in simultaneous application of AOPs processes. In this sense, the addition of H2O2 to the reaction media was not enough to improve substantially the activity. Lately, the research has been moved towards the material modification taking into account the composition of a ferrite and its ability to exchange A or B cation and so to tune its catalytic properties. In this scenario, recently the last trend has been the doping of the materials with transition metals [START_REF] Soltani | Improving heterogeneous photo-Fenton catalytic degradation of toluene under visible light irradiation through Ba-doping in BiFeO3 nanoparticles[END_REF][START_REF] Diao | Magnetic multi-metal co-doped magnesium ferrite nanoparticles: An efficient visible light-assisted heterogeneous Fenton-like catalyst synthesized from saprolite laterite ore[END_REF][START_REF] Mandal | Photocatalytic and antimicrobial activities of zinc ferrite nanoparticles synthesized through soft chemical route: A magnetically[END_REF]107,[START_REF] Li | Enhanced visible light assisted Fenton-like degradation of dye via metal-doped zinc ferrite nanosphere prepared from metal-rich industrial wastewater[END_REF]. ZnxFe3-XO4 materials were synthesized by a soft chemical route by Mandal et al [START_REF] Mandal | Photocatalytic and antimicrobial activities of zinc ferrite nanoparticles synthesized through soft chemical route: A magnetically[END_REF].

This magnetic catalyst was employed for the degradation of Acid Blue 113 and also for an antimicrobial purpose. The pristine and modified ferrites showed very good photoactivity when H2O2 was added in the reaction media. The ferrites were quite stable after 8 cycles of reaction and its magnetic properties allow recovery after each sequential run. Simultaneously, Soltani et al. [START_REF] Soltani | Improving heterogeneous photo-Fenton catalytic degradation of toluene under visible light irradiation through Ba-doping in BiFeO3 nanoparticles[END_REF] obtained BiFeO3 substituted by barium via sol-gel method. The ratio of substitution of Bi1-XBaXFeO3 was between 0.03 and 0.12. The experimental results indicate that the increase of Ba until 12% affected the redox cycle and provoked oxygen vacancies that significantly enhanced the toluene degradation under pure visible light.

In 2018, Cu-doped LaFeO3 has been obtained by Phan et al. [107]. The catalysts were employed for a dye decoloration, in this case Methyl Orange. The best reactivity resulted from the catalyst with 15% of copper. The authors attributed this improvement to the fact that it generates much more HO° than pristine LaFeO3. This behavior was associated to an enhancement of the H2O2 decomposition by the two metals, Cu and Fe, which provoke additional reactions, as well as to an improvement of the availability of Fe(II) at the surface.

The greater generation of HO° was corroborated by ESR technique. In the same way, Co-doped MgFe2O4 was synthesized from saprolite laterite ore [START_REF] Diao | Magnetic multi-metal co-doped magnesium ferrite nanoparticles: An efficient visible light-assisted heterogeneous Fenton-like catalyst synthesized from saprolite laterite ore[END_REF]. The low addition of Co (~1%) greatly improved the activity. 97% of the dye was abated and 68% of TOC mineralized after 180 min.

The authors pointed out a synergistic effect between photocatalysis and heterogeneous photo-Fenton mechanism with this stable catalyst. The measurement of the iron leached to the solution showed that the activity was associated to the surface iron of the catalyst and not to iron in homogeneous solution. Besides this, the ferrite lost 10% of activity during its reuse.

Recently, Yang Li et al. [START_REF] Li | Enhanced visible light assisted Fenton-like degradation of dye via metal-doped zinc ferrite nanosphere prepared from metal-rich industrial wastewater[END_REF] has obtained a metal-doped zinc ferrite material by a modified microwave hydrothermal method. The authors compared synthesis using metals in wastewater and also using chemical precursors. The catalytic test showed that the doped material synthesized by using metals contained in wastewater was highly more active than the one using chemical precursors. Also, the substituted ferrite exhibited better efficiency than the pristine ZnFe2O4. The authors postulated that HO° are formed through photogenerated h + and H2O2 decomposition by Fe 3+ and Mn 3+ on the catalyst surface. After the first cycle of reaction, 95% of Congo red was depleted within 3 h of reaction which let to 60% of TOC conversion. After the third cycle of reaction, 75% of decoloration was reached indicating a slight loss of activity of the material.

Finally, the La1-xTiXFeO3 material synthesized through a modified Pechini sol-gel method by Garcia-Munoz et al. [START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF] was employed as a combined photocatalyst and heterogeneous photo-Fenton catalyst without any homogeneous contribution (Figure 10). The authors demonstrated that the strategy combining both AOPs within a single dual catalyst allowed La1-xTixFeO3 to simultaneously take advantage from the higher reaction rate of the photo-assisted heterogeneous Fenton process and of the higher mineralization yield of photocatalysis. This Ti-substituted LaFeO3 dual catalyst was reported to be very active in water treatment, by reaching complete mineralization and total pollutant removal with higher efficiencies than via single AOPs, whether the process operates under UV-A light, simulated solar light or pure visible light (l>420 nm). The dual catalyst operates with pure heterogeneous surface reactions with an increase in the catalyst robustness by more than two orders of magnitude compared to the unsubstituted ferrite. This was symbolized by the absence of any Fe release and by the stability of catalytic performance with test cycles with no loss of activity [109].

Ferrite-based photocatalysts and photoelectrodes for energy applications: hydrogen production and CO 2 reduction

Introduction

Energy applications of photocatalysis, mainly CO2 reduction and hydrogen production by water splitting or photoreforming, are technologically less developed than environmental ones (viz. pollutant degradation), although their scientific interest has increasingly grown in the last decades as a result of their appealing possibilities in a hopefully not-so-long-term circular economy based on solar energy [START_REF] Yang | Photocatalysis: From Fundamental Principles to Materials and Applications[END_REF].

The production of hydrogen from renewable feedstock and energy sources is crucial for its implementation as a clean energy vector in a future non-carbon-based energy system [115].

Photocatalysis and photoelectrochemistry offer the possibility of directly converting sunlight into chemical energy by splitting the water molecule into hydrogen and oxygen, making use at the surface of semiconductor photocatalysts, of the reductive and oxidative power of photogenerated electrons and holes, respectively. Alternatively, due to the kinetic constraints of water oxidation, hydrogen can be produced using organic sacrificial agents, ideally derived from biomass feedstock, in the process called photoreforming [116,117]. In the last decades, a vast amount of semiconductors has been tested as photocatalysts for photocatalytic or photoelectrochemical water splitting [START_REF] Yang | Photocatalysis: From Fundamental Principles to Materials and Applications[END_REF][START_REF] Kudo | Heterogeneous photocatalyst materials for water splitting[END_REF]119], among which iron-based oxides and particularly ferrites have been less explored than other types of materials. This is generally related to low charge carrier mobilities and hence short exciton lifetimes, which nevertheless can be improved by doping or by other chemical, structural or surface modifications [120,121]. Some ferrites have indeed the adequate band positions for at least one of the halfreactions (proton reduction and water oxidation) involved in water splitting and, furthermore, these positions can be tuned by means of the inclusion of different metals into the ferrite structure [121]. As described below, some ferrites have been reported active for photocatalytic hydrogen evolution. Selected examples are summarized in Table 5.

Regarding photoelectrochemical hydrogen production, varied chemical composition, multiple valence states and choice of metal cation, narrow band gaps and both n-type and p-type behaviour convert ferrites into attractive candidates as electrodes for photoelectrochemical cells (PEC). However, even if they are considered as promising earth abundant and non-toxic alternative photoactive materials for PECs, only a few works have been published using ferrites as photocathodes and photoanodes [121,122]. Actually, the measured photocurrents of PECs with ferrite photoelectrodes are still low, which is mainly related to the charge mobility limitations mentioned above. Similarly to the case of photocatalysis, different strategies proved that higher photocurrents can be achieved, including nanostructuring, In turn, CO2 reduction is probably the lowest-TRL application of photocatalysis. In spite of its scientific interest, providing a route for CO2 valorization coupled to solar energy harnessing [143], it still faces a number of issues mainly related to the inherent difficulty associated to the stability and relative inertness of the CO2 molecule and to the complex nature of water oxidation. Indeed, this latter is in principle the preferred reaction to close the electron balance as Nature does in natural photosynthesis [144]. Among the different semiconductors that have been reported as CO2 reduction photocatalysts [145-147], ferrite-based ones do not play a prominent role, even if some of these oxides have adequate band positions for some of the reactions involved in CO2 reduction [148]. Nevertheless, a few works have reported photocatalytic and photoelectrochemical CO2 reduction using ferrite materials, as will be described in the next sections.

Ferrite photocatalysts for water splitting and CO2 reduction

Single-phase ferrite photocatalysts

Magnetite, the chemically simplest ferrite (Fe3O4 or Fe II Fe III 2O4), can be employed for water oxidation in the presence of the appropriate co-catalyst, as shown by Neudeck and coworkers, who prepared Fe3O4 nanoparticles using xyloglucan as stabilizing agent [START_REF] Neudeck | General route to functional metal oxide nanosuspensions, enzymatically deshelled nanoparticles, and their application in photocatalytic water splitting[END_REF]. This method yields size controllable nanoparticles with narrow size distributions, which are able to oxidize water into oxygen under irradiation with a blue-light LED, with the participation of the (Ru(bipy)3) 3+ complex as sensitizer/co-catalyst and using sodium persulfate as electron scavenger. In the presence of other nanoparticle-stabilizing polymer such as dextran, magnetite also showed oxygen evolution although lower than that attained with the xyloglucan. This reveals an effect of the latter that, according to the authors, is beyond surface area and might be related to the lifetime of the photo-generated excited Ru complex that would favour charge transfer to water molecules.

Sacrificial hydrogen production over magnetite was reported by Mangrulkar and co-workers in the presence of ethanol as sacrificial reagent, with platinum as co-catalyst, and under UVAvis (tungsten filament lamp) irradiation [150]. The material was synthesized in the form of 10-12 nm nanoparticles, and the band gap was roughly estimated from the UV-vis spectrum as 2.7 eV, considerably larger than the value of ca. 1.9 eV determined from photoelectrochemical measurements [START_REF] Büchler | Comparison of the semiconductive properties of sputter-deposited iron oxides with the passive film on iron[END_REF]. The Pt/Fe3O4 photocatalytic system was active for hydrogen evolution in the presence of different sacrificial reagents, with ethanol giving the best results over methanol and ethanolamine. Experiments carried out in different conditions revealed that hydrogen evolved only above a certain value of irradiation power, and that, in the dark, there was hydrogen evolution when the temperature was raised. This suggest that the reaction has a thermal component, and indeed experiments in photo-thermal conditions revealed a synergistic effect of the photocatalytic and the thermocatalytic effects, even at temperatures as low as 100 °C. Deactivation of the catalyst was observed in all cases, and photo-thermal conditions also enhanced this deactivation. hydrogen evolution without the concourse of any co-catalyst and with stable rates along 72 h. All three ferrites led to higher hydrogen evolution rates than magnetite in the order Zn < Cd < Ni, although in the case of the cadmium sample this should be taken cautiously because of the presence of the mentioned iron oxide phase. The superior activity of nickel ferrite was attributed to the affinity of Ni for hydrogen. Nonetheless, the NiFe2O4 sample displayed the highest metal surface area as determined by hydrogen chemisorption.

Indeed, NiFe2O4 is probably the most extensively explored ferrite photocatalyst for water splitting reactions. As summarized in Table 5, some works have reported its activity in different experimental conditions, with a quite wide range of hydrogen production rates. An apparent quantum yield of 0.53% was obtained by Rekhila et al. with a sol-gel prepared nickel ferrite using thiosulfate as sacrificial reagent without any co-catalyst, under the full spectrum of a halogen lamp [153] and in optimum conditions of pH, S2O3 2-concentration and catalyst mass.

An electrochemical impedance spectroscopy analysis revealed the p-type semiconductivity of NiFe2O4, as well as the drastic increase of the lifetime of electrons under illumination in the presence of the sacrificial reagent, emphasizing its crucial role in improving the charge separation by hole scavenging. Peng and co-workers obtained NiFe2O4 nanoparticles with homogeneous size distribution and high surface area by means of a surfactant-assisted hydrothermal method [START_REF] Peng | Preparation of NiFe2O4 nanoparticles and its visiblelight-driven photoactivity for hydrogen production[END_REF]. These nanoparticles gave rise, under visible light (λ ≥ 420 nm) and in the absence of any co-catalyst, to hydrogen evolution from an aqueous methanol solution with an apparent quantum yield of 0.52 %, after an optimum calcination temperature of 500 °C in which a compromise situation between crystallinity and surface area is reached.

The effect of particle size and distribution is revealed by the lower efficiency of NiFe2O4 synthesized without any structure-directing agent, which was formed by inhomogeneous agglomerations of particles. XRD and XPS studies of the solid together with ICP-AES analyses of the liquid reaction medium and consecutive activity runs confirmed the stability of the ferrite in reaction conditions. In the absence of any sacrificial reagent, some hydrogen evolution was observed in water spitting experiments, although without any oxygen production, which suggests the possibility of self-oxidation or the possible role of organic matter remaining from the synthesis as hole scavenger. A structure-directing agent, in this case Pluronic F127, was also used by Hong and co-workers to obtain mesoporous NiFe2O4 photocatalysts with high crystallinity and large surface area by a self-assembly associated aerosol spray pyrolysis method [155], which were tested for methanol photoreforming under visible light irradiation (λ > 420 nm). Again, the highest activity of all different catalyst appears as a compromise between crystallinity and surface area, with well-crystallized mesoporous samples resulting in higher H2 evolution rates than amorphous ones with small pore size and high BET values, but also than a non-mesoporous, highly crystalline ferrite with relatively low surface area. The photocatalytic character of the reaction was demonstrated by an action spectrum study that showed that the dependence of hydrogen evolution on the irradiation wavelength closely matched the absorption spectrum of NiFe2O4 (Figure 11). This mesoporous ferrite was shown to be stable up to three photocatalytic runs without any loss of activity and without structural or morphological modifications. A modification of the Pechini method was employed by Domínguez-Arvizu et al. to obtain single-phase nickel ferrite with average crystal size of 25 nm and 50 m 2 g -1 of BET surface area after calcination temperature of 400 °C, which was determined as the minimum one to remove all the organic matter after the chelation step [156]. The authors studied some optical characteristics such as the absorption spectrum and scattering, absorption and extinction zinc ferrite may also lead to improved photocatalytic hydrogen production, as shown by Boudjemaa and co-workers, who prepared M0.2Zn0.8Fe2O4 spinels by a co-precipitation method, being M Co, Ni or Cu [START_REF] Boudjemaa | Msubstituted (M = Co, Ni and Cu) zinc ferrite photo-catalysts for hydrogen production by water photo-reduction[END_REF]. In this work, a considerable improvement of the activity of the zinc ferrite is reported in the case Co-and Ni-doping, with the former giving the best results of the series. In contrast, Cu-doping resulted in a decrease of activity with respect to ZnFe2O4. On the basis of electrochemical, photoelectrochemical and optical characterization, the authors mainly ascribe the effect of the guest cation on activity to its influence on the electronic structure of the ferrites, with the band gap values of the different samples decreasing in the same order as the obtained hydrogen evolution rates increase.

Ortega López et al. report a significant hydrogen production with the cobalt spinel ferrite CoFe2O4 [START_REF] Ortega López | Synthesis method effect of CoFe2O4 on its photocatalytic properties for H2 production from water and visible light[END_REF]. They compared two different synthetic methods, namely co-precipitation, followed by a relatively low temperature crystallization, and ball-milling of a stoichiometric mixture of iron metal and Co3O4 previously activated y solid-state reaction at 700 °C. The latter method yielded a final CoFe2O4 cubic spinel with 5 nm crystal size vs. 20 nm with the former synthesis. This, however, resulted in a lower surface area of the ball-milled sample than the co-precipitated one and, contrarily to what may be expected from the quantum-size effect, to a considerably lower band gap (1.15 vs. 1.38 eV). The authors ascribe both results to the differences in the secondary particle size (100-500 vs. 25 nm), which is more plausible in the case of the surface area than in that of the electronic structure. In any case, the balled-milled sample exhibited a higher adsorption capacity which the authors relate to oxygen vacancies created by the high-energy mechanochemical process and invoke to explain the higher hydrogen production per surface area unit displayed by this catalyst.

Copper ferrite (CuFe2O4) has also been shown active for photocatalytic hydrogen evolution.

Yang et al. obtained CuFe2O4 nanoparticles by sol-gel, solid-state reaction and coprecipitation methods, among which the first one yielded the highest hydrogen formation rate using oxalic acid as sacrificial reagent [163]. The use of citric acid in the sol-gel method resulted in a relatively homogeneous particle size distribution, which according to the authors had a key influence on the higher activity of the so-obtained photocatalyst, due to the shorter path travelled by photogenerated charges to the surface. The CuFe2O4 photocatalyst was active in four consecutive cycles, although a decline in the hydrogen evolution rate can be observed in each of them.

Regarding ferrites of non-transition metals, some alkaline and alkali earth metal ferrites have been tested for photocatalytic hydrogen evolution. For example, Boudjemaa and co-workers synthesized the MgFe2O4 spinel ferrite by calcination of a Mg-Fe layered double hydroxide obtained by co-precipitation [138]. This material gave rise to hydrogen evolution under visible light from an aqueous sulphite solution, although production declined after 20 min of reaction.

This deactivation was ascribed by the authors to the saturation of catalytic sites and competitive reduction of the end product S2O6 2-for conduction band electrons. Dom et al.

reported the synthesis of orthorhombic CaFe2O4 nanoparticles using different preparation methods, namely solid state reaction, polymer complex, microwave sintering and selfpropagating combustion (SPC) [START_REF] Dom | Photo Chemical Hydrogen Generation from Orthorhombic CaFe2O4 Nanoparticles Synthesized by Different Methods[END_REF]. As they already reported for zinc ferrite [160], the microwave-directed method yielded interesting results, outperforming by nearly one order of magnitude the activities obtained with the rest of syntheses, which gave very similar results [START_REF] Dom | Photo Chemical Hydrogen Generation from Orthorhombic CaFe2O4 Nanoparticles Synthesized by Different Methods[END_REF]. The authors ascribed this difference to an optimum combination of high crystallinity with monodispersed morphology, together with efficient visible light absorption, suitable band energetics and relatively low charge transfer resistance. Photodeposition of platinum as a hydrogen evolution co-catalyst resulted in a further improvement of the activity by a factor of ca. 2.5, resulting in an apparent quantum yield of 1.57 % under 420 nm irradiation. Different strontium ferrites, namely the cubic, hexagonal and orthorhombic SrFe2O4, SrFe12O19 and Sr7Fe10O22, respectively, were selectively obtained by Jiménez-Miramontes et al. by modifying the crystallisation conditions in the Pechini synthetic method [165]. Among the three crystalline phases, the activity towards hydrogen production under UV irradiation was maximum for the cubic one, with the hexagonal phase yielding a two orders of magnitude lower amount of hydrogen as a result of unsuitable band positions.

The perovskite-type lanthanum ferrite, LaFeO3, has been studied for photocatalytic hydrogen evolution in a number of articles. It is considered as a promising catalyst for this application by combining adequate band positions, visible light absorption and (photo)chemical stability.

Parida and co-workers actually reported that LaFeO3 was active both for hydrogen evolution from water by using an electron donating (methanol) sacrificial agent, and for oxygen evolution with an electron scavenging (Ag + ) agent. High productions were obtained in both cases (see table 5 for the case of hydrogen), although no overall water splitting experiments were reported [START_REF] Parida | Fabrication of nanocrystalline LaFeO3: An efficient sol-gel auto-combustion assisted visible light responsive photocatalyst for water decomposition[END_REF]. Using ethanol as sacrificial agent, Tijare and co-workers reported a relatively similar hydrogen evolution rate (even if the usual difficulties to quantitatively compare different works must be considered) that could be further boosted by using platinum as co-catalyst [167]. However, in this case the improvement caused by platinum was considerably low if compared to its effect on other semiconducting catalysts [START_REF] Núñez | Ga-Promoted Photocatalytic H2 Production over Pt/ZnO Nanostructures[END_REF]. Indeed, Iervolino and co-workers studied LaFeO3 as a promising noble-metal-free hydrogen evolution photocatalyst [START_REF] Iervolino | Production of hydrogen from glucose by LaFeO3 based photocatalytic process during water treatment[END_REF]. Interestingly, they reported stoichiometric overall water splitting (i.e.

without any sacrificial reagent) under UV light over LaFeO3 photocatalysts synthesized using the solution combustion synthesis method. The hydrogen yield increased by a factor of 4 when using glucose as sacrificial electron donor. Under purely visible light irradiation (440 nm), the activity of the best-performing catalyst, obtained from an optimized synthetic method, for glucose photoreforming was above 70% of that under UV, revealing the interest of this material as visible-light-active photocatalyst. Using a similar synthetic procedure, Chen et al.

obtained a homogeneously nanocrystalline LaFeO3 that was further modified by encapsulation with a conductive polyaniline (PANI) aerogel (Figure 12) as a means to control the hydrophilicity of the surface and to improve charge mobility. An improvement of hydrogen evolution activity of about 4 times with respect to the unmodified ferrite was observed, using in this case platinum as co-catalyst and triethanolamine as electron donor [START_REF] Chen | Interface engineering: Surface hydrophilic regulation of LaFeO3 towards enhanced visible light photocatalytic hydrogen evolution[END_REF]. Doping Ru 3+ into the Fe 3+ positions of the LaFeO3 structure improved the hydrogen evolution rate in this reaction by a factor of ca. 4.5 under UV irradiation [START_REF] Iervolino | Production of hydrogen from glucose by LaFeO3 based photocatalytic process during water treatment[END_REF], which was ascribed to the role of the dopant cation as electron scavenger, preventing electron-hole recombination. The optimum doped catalyst was also active in glucose photoreforming under visible light and, in an especially interesting result, in the valorization of a real waste water from a brewing company by using it as sacrificial agent for the production of hydrogen. Partial substitution of Sr 2+ for La 3+ and Ti 4+ for Fe 3+ , using a flux growth synthetic method, was studied by Domen and coworkers as a way of further improving the photocatalytic activity of LaFeO3 for hydrogen evolution by methanol photoreforming, using Pt as co-catalyst [170]. Simultaneous cation substitution led to improvement in photocatalytic activity attributed to the concurrence of modified structural properties by Sr-doping and electronic characteristics due to Ti-doping, as

CaFe2O4 irradiated with light of different wavelengths [START_REF] Matsumoto | Photocatalytic reduction of carbon dioxide on p-type CaFe2O4 powder[END_REF]. The CH3OH production followed the same trend against irradiation wavelength than the photocurrent in a CaFe2O4 photocathode, proving the photocatalytic character of the reaction. However, in spite of the narrow band gap of the ferrite (1.9 eV), both photocurrents and CO2 conversions were significantly higher under UV light. Results with the addition of suspended solid BaCO3 led the authors to propose a mechanism in which CO3 2-/CO2 activation occurs at the interface between the solid catalyst, the solid carbonate and water. Matsumoto reported two years later a screening of photoelectrochemical response and CO2 reduction to methanol with different ferrites [148]. Both properties were not necessarily related, with some ferrites showing electrochemical but not catalytic activity under irradiation and vice versa. Not only the mentioned calcium ferrite was photocatalytically active but also some compositions containing magnesium, strontium, barium, lead and bismuth were. The elements forming the oxides as well as the positions of the band edges were found to determine the photocatalytic activity, with CaFe2O4 and (Bi, Pb)2Sr2BiFe2O9+x giving the best results.

Apparently, as reported by Xiao and co-workers from a combination of UV-vis and XPS spectroscopies, ZnFe2O4 has adequate band positions to conduct, under visible light, CO2 reduction but not water oxidation [START_REF] Xiao | Fabrication of ultrafine ZnFe2O4 nanoparticles for efficient photocatalytic reduction CO2 under visible light illumination[END_REF]. Therefore, triethanolamine was used in that study as a sacrificial agent to scavenge photoproduced holes, while conduction band electrons were utilized to reduce aqueous bicarbonate into mainly acetaldehyde and ethanol. It remains unclear though, whether these products result from the oxidation of triethanolamine.

Ferrite-based heterojunction photocatalysts

Either Copyright Royal Society of Chemistry.

Different ferrites have been combined with TiO2 in coupled photocatalysts for energy applications. An et al. reported the preparation of Fe3O4/TiO2 composites with varying magnetite loadings supported on nanofibrilated cellulose, and the activity of the obtained systems for hydrogen production from aqueous methanol solutions [START_REF] An | Synthesis of nano-fibrillated cellulose/magnetite/titanium dioxide (NFC@Fe3O4@TNP) nanocomposites and their application in the photocatalytic hydrogen generation[END_REF]. An increase of electron-trapping sites, reducing recombination, is invoked to account for the improved photocatalytic activity of the composite under UV light with respect to cellulose-supported TiO2, although no scheme of the electronic coupling of both phases is reported. According to previous works though, it is feasible that electrons photogenerated in titania are transferred into the lower lying conduction band of magnetite [177], although this would mean that those transferred electrons would lose reduction potential. In addition to electronic features, these composites were reported to be recyclable by means of an external magnetic field.

Recyclability tests showed a loss of activity of ca. 8% after 15 photocatalytic runs [START_REF] An | Synthesis of nano-fibrillated cellulose/magnetite/titanium dioxide (NFC@Fe3O4@TNP) nanocomposites and their application in the photocatalytic hydrogen generation[END_REF].

A p-n heterojunction of CuFe2O4 and TiO2 supported on reduced graphene oxide was reported by Hafeez and co-workers for hydrogen evolution under UV-vis light using glycerol as hole scavenger [178]. Based on optical and electrochemical measurements, a mechanism was proposed in which electrons would flow from the high-energy conduction band of the ferrite phase to the lower-energy one of titania, and from there to the reduced graphene oxide sheets, where they would be transferred to protons to produce hydrogen. In turn, holes would migrate from the valence band of TiO2 VB to that of CuFe2O4 and there be trapped by glycerol molecules. Therefore, CuFe2O4 and rGO acting as hole traps and electron mediators, respectively, would improve photocatalytic activity with respect to TiO2, as shown by the hydrogen evolution results, by largely inhibiting charge carrier recombination. This inhibition is actually supported by the practical disappearance of photoluminescence at wavelengths associated with TiO2 in the composite system. A similar mechanism, without the concourse of graphene, was proposed by Uddin and co-workers to account for the visible-light (λ > 400 nm) activity of CuFe2O4 heterojunctions for CO2 reduction into methanol from an aqueous solution of potassium bicarbonate, using sodium sulphide as hole scavenger [179]. The ferrite phase would in this case act as a visible-light sensitizer for titania, which would be inactive in those irradiation conditions, and which does, on the other hand improve the activity with respect to the ferrite single phase. Again, photoluminescence spectroscopy measurements under UV excitation suggested recombination to be diminished in the composite, although the difference between TiO2 and the two-phase system was not as dramatic as in the previous case. As is common in photocatalytic CO2 reduction experiments [144], deactivation of the catalyst was observed in all cases, which the authors associated to the depletion of active sites.

A Z-scheme electronic mechanism was proposed by Son et al. in the photocatalytic reduction of CO2 in cyclohexanol solution by ZnFe2O4/TiO2 composites [START_REF] Song | Photocatalytic reduction of carbon dioxide over ZnFe2O4/TiO2 nanobelts heterostructure in cyclohexanol[END_REF]. With a similar relative band position arrangement to that of the previous heterojunction, ZnFe2O4 and TiO2 may couple in such a way that electrons from the conduction band of the latter are transferred into the valence band of the former, while electrons in the ferrite conduction bands reduce CO2 and holes in titania VB oxidize the sacrificial agent, in a kind of all-solid Z-scheme without an electron mediator between the distinct phases. This heterojunction, formed by TiO2 nanobelts decorated with ferrite nanoparticles, showed higher photocatalytic activity than either of the two single phases. Cyclohexyl formate and cyclohexanone were detected as the main products, together with lower amounts of formic acid. This led the authors to propose a mechanism in which cyclohexane is oxidized to cyclohexanol, while CO2 is reduced into formic acid. Esterification between these two intermediates would form cyclohexyl formate, while part of the alcohol would be further oxidized to the ketone (Figure 14). This research group has also reported the combination of the zinc ferrite with bismuth oxychloride, BiOCl, for the same reaction [START_REF] Song | ZnFe2O4 deposited on BiOCl with exposed (001) and (010) facets for photocatalytic reduction of CO2 in cyclohexanol[END_REF]182]. Apparently, the same type of heterojunction as in the case of the ZnFe2O4/TiO2 can be formed with the oxychloride instead of titania, leading to improved photocatalytic activity with respect to the separate semiconductors. The preparation method and the resulting morphology of the particles affected the final photocatalytic activity, in such a way that hydrothermally obtained samples displayed a better performance than physical mixtures of the semiconductors, probably due to a more extended interphase contact [182].

In addition, systems with hierarchical microsphere microstructure showed higher photocatalytic activities, apparently due to a better charge carrier separation, than those with non-hierarchical nanosheet morphology [182]. Furthermore, photocatalytic activity in the ZnFe2O4/BiOCl system is also influenced by the exposed facets of BiOCl. Thus, the yields of cyclohexyl formate and cyclohexanone were higher for the samples with more exposed (001) facets than for the samples with more exposed (010) facets, which the authors attribute to the higher density of oxygen atoms on the (001) facets which enriches the production of electrons under UV irradiation [START_REF] Song | ZnFe2O4 deposited on BiOCl with exposed (001) and (010) facets for photocatalytic reduction of CO2 in cyclohexanol[END_REF] Figure 14. Schematic diagram for photocatalytic reduction of CO2 in cyclohexanol over a Zscheme ZnFe2O4/TiO2 heterostructure photocatalyst under UV light irradiation. "CH" stands for cyclohexanone and "CF" for cyclohexyl formate. Reproduced with permission from ref. [START_REF] Song | Photocatalytic reduction of carbon dioxide over ZnFe2O4/TiO2 nanobelts heterostructure in cyclohexanol[END_REF]. Copyright Elsevier.

Soto-Arreola and co-workers combined ZnO with either CuFe2O4 or NiFe2O4 to form type-II heterojunctions aimed at improved photocatalytic activity for water splitting without any sacrificial reagent under UV light [183]. With an optimum amount of 3 wt.% ZnO, both composites, obtained by physical mixture of the single phases, surpassed the activity of the corresponding single-phase ferrites and that of ZnO under the same reaction conditions. In spite of the absence of any sacrificial agent, the reported results are in the same order of magnitude as other works with similar materials that make use of organic hole scavengers.

Based of fluorescence spectroscopy and (photo)electrochemical measurements, the between CO2 and photocatalyst surfaces, which occurs via the formation of monodentate and bidentate carbonate and bidentate bicarbonate as the main surface species.

In a similar approach, Khan et al. reported the use of another wide-band gap alkaline oxide like SrO to promote both charge separation and CO2 adsorption [186]. However, the electronic matching here is slightly different from those in the previous examples, with the conduction band of SrO lying considerably above that of the ferrite, so that it is only able to extract hot electrons generated in the latter by absorption of relatively high-energy (although still visible)

photons. Charge separation is demonstrated by increased signals in the surface photovoltage spectra upon the addition of the strontium oxide phase, as well as by the increased generation of hydroxyl radicals followed by the formation of the fluorescent 7-hydroxycoumarin from coumarin solutions. On the other hand, TPD studies reveal that the alkaline oxide promotes the adsorption of CO2 via carbonate species, increasing not only the amount of CO2 adsorbed but also the strength of the interaction. In this case, carbon monoxide is the main product of According to static photoluminescence spectra and time-resolved fluorescence decay curves, the ferrite is able extract photoinduced electrons and holes from carbon nitride, which per se leads to a poor activity for hydrogen production due to the low energy of the conduction band of MgFe2O4. However, if the nitride is modified with both ferrite and platinum, the latter can extract electrons more efficiently, according to the greatly improved activity with respect to both bare C3N4 and MgFeO4/C3N4. In turn, electrochemical measurements revealed the ferrite to be a good oxidation catalyst, which led the authors to propose a dual effect of MgFe2O4 in as co-catalyst. The introduction of 2 wt.% of Ni2S improved the photocatalytic activity of the LaFeO3/C3N4 heterojunction in the same conditions by a factor of 60, with an apparent quantum yield of 2 % at 420 nm [START_REF] Xu | Photocatalytic hydrogen evolution performance of NiS cocatalyst modified LaFeO3/g-C3N4 heterojunctions[END_REF], although the activity was still lower than that obtained in the previous work with platinum as co-catalyst. Ida et al. developed a tandem cell (Figure 15a) without applying an external voltage using (hk0)-oriented p-type CaFe2O4 deposited over FTO (100 nm thick) by pulse layer deposition (PLD) as photocathode and TiO2 as photoanode with an onset potential of -0.75 V (Figure 15b) [START_REF] Ida | Preparation of p-type CaFe 2 O 4 photocathodes for producing hydrogen from water[END_REF]. The open-circuit voltage was 0.97 V and the short-circuit current was about 200 μA/cm 2 (Figure 15c), and a H2 and O2 evolution of 70 and 4 μmol, respectively, in 48 h was attained (Figure 15d). After this time, many cracks were observed on the CaFe2O4 surface as shown in Figure 15e and the solution contained 2.3 μmol of Fe, indicating a slight corrosion of the CaFe2O4 electrode. Later, the same authors, described the use of n-ZnO as photocathode generating a photovoltage of 0.82 V [START_REF] Ida | Photoelectrochemical hydrogen production from water using p-type CaFe 2 O 4 and n-type ZnO[END_REF]. In this case, only H2 was detected and Zn was partially dissolved, which accounted for the absence of O2. In a following work, they also described a modification of p-type CaFe2O4 with the presence of a Ca2Fe2O5 impurity enhancing the short circuit photocurrent density (0.55 mAcm -2 ) and slightly increasing the photovoltage (1.09 V) [START_REF] Ida | Photoelectrochemical hydrogen production from water using p-type and n-type oxide semiconductor electrodes[END_REF]. During the reaction a H2∕O2 ratio of 3.7 was observed which is lower than in the previous paper. In addition, O2 formation was improved. Reproduced with permission from [START_REF] Ida | Preparation of p-type CaFe 2 O 4 photocathodes for producing hydrogen from water[END_REF]. Copyright American Chemical Society.

One strategy to improve the low quantum efficiency of a pristine p-CaFe2O4 electrode due to the poor mobility of the photogenerated charge carriers is to dope it with different elements.

In this way, Matsumoto et al. used Na and Mg which have similar ionic radii than Ca and Fe respectively to prepare type Ca1-xNaxFe2-yMgyO4 photocathodes [START_REF] Matsumoto | Improvement of CaFe2O4 photocathode by doping with Na and Mg[END_REF]. This led to the formation of acceptor levels within the bandgap and to higher electronic conductivity but still low photocurrents. Fan and co-workers reported the preparation of MgFe2O4 nanofibers and nanowires by electrospinning [START_REF] Li | Fabrication of MgFe2O4/MoS2 Heterostructure Nanowires for Photoelectrochemical Catalysis[END_REF][START_REF] Jiang | Rod-intube nanostructure of MgFe 2 O 4 : electrospinning synthesis and photocatalytic activities of tetracycline[END_REF]. By CVD-coating with MoS2, a 1D heterostructure with enhanced charge carrier mobility was created, which showed 92 % photoelectrochemical tetracycline degradation after two hours. A photoelectrochemical hydrogen evolution rate of 5.8 mmol h - 1 g -1 was found at 0.5 V bias under Xe arc lamp irradiation. Guijarro et al. [196] used a β-FeOOH nanostructure as a solid precursor for MgFe2O4, CuFe2O4

and ZnFe2O4 thin films. The infiltration with metal nitrate solutions and subsequent calcination at 800 °C led to the formation of MgO, CuO and ZnO impurities, which were etched under highly acidic (7 M HNO3 for CuFe2O4, MgFe2O4) or alkaline (5 M NaOH for ZnFe2O4) conditions.

They were able to improve the performance by NiFe2Ox deposition and post-synthetic calcination in H2 atmosphere. Promising faradaic efficiencies (97 %), but also a high degree of bulk recombination and Fermi level pinning at 0.9 V vs. RHE, were reported.

In the case of CO2 photoreduction, the use of ferrites as photoelectrodes is scarcer. Rezaul Karim et al. [197] developed a p-type CuFe2O4 photocathode, synthesized by the sol-gel method, for CO2 photoelectroreduction in a CO2 saturated NaHCO3 solution under visible light irradiation (470 nm). Methanol was found as the only product in the liquid phase with a faradaic efficiency of 62% of and a quantum efficiency of 14.4%, using a bias potential of 0.5 V (vs RHE). The relatively lower quantum efficiency was ascribed to the formation of competitive gas products during CO2 reduction.

Ferrite photoanodes

Ferrites can also act as n-type semiconductors being suitable candidates as photoanodes for PEC. Among them, ZnFe2O4 is a remarkable example. propose the use of ZnFe2O4 prepared by aerosol-assisted chemical vapour deposition (AACVD) over FTO as photoanode for water splitting. The obtained photocurrents have a high dependence with the synthesis conditions, with a maximum photocurrent density of 0.35 mA cm -2 at 1.23 V versus RHE using ethanol as solvent and a calcination temperature of 450 °C.

An incident photon-to-current conversion efficiency of 13.5 % at 350 nm at an applied potential of 1.23 V versus RHE was observed [141]. The authors attributed this behaviour to the improved collection of the photogenerated minority carriers at the ZnFe2O4∕electrolyte interface as the average feature size gradually decreased with the solvent from ∼500 nm (methanol) to ∼100 nm (ethanol) [142].

To decrease the synthesis temperature, Kim et al. propose the use of hybrid microwave annealing (HMA) post-synthetic heat treatment with graphite powder as the susceptor. The synthesis is based on the treatment of β-FeOOH nanorods with a zinc nitrate solution with a thermal treatment at 550 °C for 3 h. After that, ZnFe2O4 nanorods were calcined at 800 °C (20 min) or subjected to HMA (5 min) to increase the crystallinity. The ZnFe2O4 nanorods from HMA treatment show a 10-to 15-fold increase in activity compared to conventional thermally treated electrodes and an enhancement of the stability, which is attributed to a higher crystallinity and lower amount of surface defects [START_REF] Kim | Awakening solar water-splitting activity of ZnFe2O4 nanorods by hybrid microwave annealing[END_REF]. In a later work, these authors modified the synthesis atmosphere (vacuum, air, or hydrogen) after the first thermal stage (800 °C) [199]. In both H2 and vacuum an increase of the photoactivity about 20-fold is observed. Hufnagel et al. [200] prepared mesoporous ZnFe2O4 thin films on a macroporous ATO scaffold using atomic layer deposition (ALD). These photoanodes show a more negative photocurrent ∼1.2 V versus RHE, which was almost twice as high as that for a Fe2O3 electrode (0.24 mAcm -2 ). On the other hand, Borse et al. used plasma spray method to deposited an aqueous solution of Zn and Fe salts over stainless steel leading to a ZnFe2O4∕Fe2O3 electrode obtaining a photocurrent five-fold higher than for bare Zinc ferrite. In addition, the rates for HER were determined to be 46.3 μmol•cm -2 (STH conversion = 0.06) and 99.0 μmol•cm -2 (STH conversion = 0.0125) for ZnFe2O4 and the ZnFe2O4∕Fe2O3 photoanode respectively [START_REF] Dom | Eco-friendly ferrite nanocomposite photoelectrode for improved solar hydrogen generation[END_REF].

Kim et al. [START_REF] Kim | Single-crystalline, wormlike hematite photoanodes for efficient solar water splitting[END_REF]207] reported the preparation of heterojunction photoanodes using p-CaFe2O4

with TaON and BiVO4. The selection of these oxides is due to their staggered relative band positions with the ferrite leading to an effective heterojunction for water oxidation reactions.

The CaFe2O4 layer on the surface of a TaON lead to an increase of in the photocurrent density (1.26 mAcm -2 ) it is about 5.5 times greater than bare TaON. This enhancement in the photoelectrochemical performance is due to a reduction of the resistance of the charge carrier transport and, therefore, an improved electron-hole separation [START_REF] Kim | Single-crystalline, wormlike hematite photoanodes for efficient solar water splitting[END_REF]. In the case of CaFe2O4∕BiVO4 an increase of 65% over that measured at the BiVO4 electrode was observed [207]. In order to improve the photoelectrochemical performance, an OER co-catalyst (cobalt phosphate, Co-Pi) was deposited on CaFe2O4∕TaON. After deposition of Co-Pi, H2 and O2 were generated with a nearly stoichiometric ratio of 2.1 (123 μmol H2 and 59 μmol O2) applying a bias of 1.23 V versus RHE under 3 h of illumination with λ ≥ 400 nm. The STH efficiency was 0.053% at 1.0 V versus RHE, but reached 0.55% when a PV device is coupled in a tandem configuration (assuming the applied voltage is zero). However, the initial current decreased within 3 h to about 50%. Similar results have been obtained with CaFe2O4∕BiVO4 composite photoanodes, but with a lower photocurrent and a higher stability. H2 and O2 production were 297 and 140 μmol, respectively (within 2 h of illumination with visible light, λ ≥ 400 nm), with a faradic efficiency around 80%.

Another example with a calcium ferrite was reported by Ahmed and co-workers, who prepared a p-CaFe2O4∕n-Fe2O3 heterojunction photoanode by anisotropic growth of a β-FeOOH film on FTO from an aqueous solution containing Fe and Ca ions followed by two-step thermal annealing at 550 °C and 800 °C [START_REF] Ahmed | Enhanced photoelectrochemical water oxidation on nanostructured hematite photoanodes via p-CaFe2O4/n-Fe2O3 heterojunction formation[END_REF]. This heterojunction showed a photocurrent density of 0.53 mAcm -2 at 1.23 V versus RHE, being 100% higher than that obtained with Fe2O3. This behaviour is explained by a reduction of the resistance of the interfacial charge transfer between the electrolyte and the electrode and an improvement of the charge separation. Chen et al. [START_REF] Chen | Ruddlesden-Popper compounds (SrO)(LaFeO3)n (n=1 and 2) as p-type semiconductors for photocatalytic hydrogen production[END_REF]

Figure 1 .

 1 Figure 1. Crystallographic structure of AB2O4 spinel ferrites schematizing the different crystallographic sites: (a) normal spinel, (b) inverse spinel, and (c) orthorhombic. Reproduced

Figure 3 .

 3 Figure 3. Crystallographic structure of MFeO3 orthoferrites with distorted perovskite structure

Figure 4 .

 4 Figure 4. Schematic diagrams of some selected less conventional synthesis methods (a) laser pyrolysis, that uses a laser for the decomposition of materials by heating gas-phase reactants in an oxygen-poor environment, (b) flame spray pyrolysis, that is the latest of the flame aerosol technologies, and consists in a one-step combustion process with self-sustaining flame

Figure 5 .

 5 Figure 5. Schematic diagrams depicting microemulsion-based syntheses : (a) typical reversemicelle system, that is a water-in-oil microemulsion where the polar head groups of the surfactants creating the aqueous core reside towards the inside, whereas the organic tails of the surfactant molecules are directed towards the outside; (b) one-microemulsion synthesis method, and (c) two-microemulsion synthesis method in which the microemulsions carrying the separate reactants are mixed together in the appropriate ratio. Reproduced with permission from ref.[START_REF] Dhand | Methods and strategies for the synthesis of diverse nanoparticles and their applications: a comprehensive overview[END_REF]. Copyright Royal Society of Chemistry.

Figure 6 .

 6 Figure 6. (top) Transesterification reaction occurring between citric acid and ethylene glycol in a typical Pechini process; (bottom) Proposed schematic of the process of making metal/organic gels. Reproduced with permission from ref. [39]. Copyright Royal Society of Chemistry.

Figure 7 .

 7 Figure 7. Band positions of the most popular spinel ferrites in contact with aqueous solution referenced to normal hydrogen electrode (RHE) (right pH=10 and left pH=14) relative to the standard potentials for the oxidation and reduction of water, as well as for the reduction of dioxygen into the superoxide radical and the oxidation of water into the hydroxyl radical.

  photoelectrodes requires investigating charge trapping and recombination as well as understanding the factors affecting the related processes. Furthermore, deeper understanding of the relationship between the physico-chemical properties of the ferritebased systems and the catalytic behavior and efficiency indicators under visible or solar light is absolutely necessary. Especially, understanding the physico-chemical phenomena taking place in the bulk, and at the surface and the interfaces of the multi-component systems, is of high interest, as well as understanding the catalytic mechanism(s) taking place on such systems, and how the material design can orientate the efficiency indicators.

Figure 8 .

 8 Figure 8. HAADF images of Fe3O4/SiO2 (a) and Fe3O4/SiO2/TiO2 (b) combined with mappings of

Figure 9 .

 9 Figure 9. NOx conversion of N-CQDs/PrFeO3/Palygorskite with various amounts of N-CQDs (a) and 5 wt% N-CQDs/PrFeO3/Pal during several lamp-on/off tests (b). reproduced with permission from ref. [33]. Copyright ACS.

Figure 10 .

 10 PHOTOCATALYSIS + PHOTO-CWPO

  heterojunctions, co-catalyst loading and control of the defect chemistry [121, 122]. Pioneer work on the fundamental investigation using ferrites as electrodes in (photo) electrochemical reactions dates back to the late 1970s and early 1980s and has continued in the following years, including different compositions. P-type CaFe2O4 [123-130], CdFe2O4 [131], p-and ntype CoxFe3-xO4 and CoTixFe2-xO4 [132], p-type CoFe2O4 [133], p-and n-type NiFe2O4 [134, 135], Li0.5Fe2.5O4 [136], MgFe2O4 [137, 138], TixFe3-xO4 [139] and n-type ZnFe2O4 [136-142] are remarkable examples.

Figure 11 .

 11 Figure 11. Dependence of H2 evolution rates (RH2) on the wavelength of monochromatic light irradiation of a 25 vol.% methanol aqueous solution with a mesoporous NiFe2O4 photocatalyst. Reproduced with permission from ref. [155]. Copyright American Chemical Society.

  by combining different ferrites or by joining one ferrite another semiconductor like TiO2, ZnO or C3N4, some works have dealt with the improvement of photocatalytic hydrogen production or carbon dioxide reduction by incorporating the materials subject of this review into different kinds of heterostructures. Regarding the combination of different ferrites, Chen et al. reported the fabrication of a CaFe2O4/MgFe2O4 bulk p-n heterojunction for photocatalytic hydrogen evolution from CaFe2O4 (white) and MgFe2O4 (gray). (d) Energy band diagram showing band gap energies and band positions of p-CaFe2O4and n-MgFe2O4. (e) Operating principle of the formed heterojunction promoted by co-catalysts. Reproduced with permission from ref. [174].

  photocatalyst[188], and some works have extended this studies to its combination with ferrites for photocatalytic water splitting. Thus, Chen and co-workers enhanced the photocatalytic activity of graphitic carbon nitride for hydrogen evolution from triethanolamine solutions through the incorporation of MgFe2O4 by annealing a mixture of ferrite particles obtained by the sol-gel method and melamine as the C3N4 precursor [189].

3. 3 . Ferrite-based photoelectrode materials for solar fuels production 3 . 3 . 1 .

 3331 photoelectrode configuration, a negative photocurrent and a positive open circuit photovoltage (+0.025 V, λ = 430 nm, 118 μWcm -2 ) was described, indicating that p-CaFe2O4 layer acts as photocathode. In the case of four multiple-junction FTO∕(ZnFe2O4∕CaFe2O4)x photoelectrodes with the same single-layer thickness of 10 to 15 nm, an improvement in the photocurrent density and the onset potential was observed with increasing number of layers x (x = 10, 15, 20, and 25). The 20-junction photoelectrode showed the highest photocurrent density (-0.025 mAcm -2 at +0.4 V) and the most positive onset potential (+1.3 V) of all four samples.

Figure 15 .

 15 Figure 15. (a) Reaction and band model in photovoltaic cells using p-type and n-type semiconductor electrodes. (b) Current-potential curve and SEM image of the TiO2 electrode. (c) Current-potential curve of a photocell with CaFe2O4 (2 cm 2 ) and TiO2 (0.5 cm 2 ) electrodes and model structure of measurement cell. (d) Amount of hydrogen and oxygen gases generated from the photocell short-circuited by connecting the CaFe2O4 and TiO2 electrodes as a function of illumination time. (e) Current-time evolution for the photocell short-circuited by connecting the CaFe2O4 and TiO2 electrodes and SEM image of CaFe2O4 after 48 h reaction. All reactions were carried out in 0.1 M NaOH (aq) under illumination (500 W Xe lamp).

Table 1 .

 1 Selected spinel ferrites and orthoferrites with their specific surface area, crystallite size and isoelectrical point following their preparation method and final calcination temperature

	Ferrites	Specific	Crystallite	Isoelectric	Preparation method (calcination temperature)	References
		surface area	size (nm)	Point
		(m 2 /g)
	CuFe2O4	48-69	30-110 b	7.3	Combination of acid leaching, chemical exchange,	Tu et al. (2013), Tu
					and ferrite process from Printed Circuit Board	Etal. (2012) and Tu
					sludge	et al. (2014) [67-69]
	CuFe2O4	70	72 a	-	Micro-emulsion (650 °C)	Rehman et al.
						(2015) [70]
	CoFe2O4	149	20-30 c	8.1	Hydrothermal (100 °C)	Sun et al (2019)
						[71]
	CoFe2O4	202	10-20 c	8.0	Hydrothermal (100 °C)	Sun et al (2019)
						[71]
	Co0.6Fe0.4Fe2O4	97	14 c	2.9	Thermal decomposition (400 °C)	Duan et al. (2015)
						[72]
	MnFe2O4	48	50-80 c	6	Hydrothermal (100 °C)	Sun et al. (2015)
						[71]
	ZnFe2O4 hollow fibers 132	50	6.5	Sol-gel using kapok fibers as bio-template (700 °C) Li et al. (2015) [73]
	MgFe2O4	54	25-35 c	11	Co-precipitation (500 °C)	Srivastava et al.
						(2015) [74]
	MnFe2O4	3-156	200-290 b 6.6	Co-precipitation (400 °C)	Yang et al. (2014)
						[75]

Table 2 . Summary of ferrite materials used as photocatalyst for water treatment.

 2

	Catalyst	Method of synthesis	Pollutant	Conversion	Advantages	Disadvantages Reference
					92% MO (80	Rashmi et al.
	ZnSmxFe2-xO4	Co-precipitation	Methyl orange	min)		[21]
					90% alachlor
					removed (60
	CuS/BiFeO3	Two-step	Alachlor		min)		Bhoi et al. [93]
							better rates with
							10% LaFeO3; Z
	LaFeO3/SnS2	Hydrothermal method Tetracycline			scheme	Luo et al. [94]
			Dye,	methyl			better	no	complete
	LaFeO3/AgBr	Precipitation	orange				photoactivity	degradation	Song et al. [91]
					100%
					Rhodamine
					removed (75
	LaFeO3/montmorillonite Assembling sol-gel	Rhodamine B	min)		Peng et al. [29]
					total removal
	ZnFe2O4/PMS	Soft chemical process Orange II		(90 min)		Zhu et al [95]
					20%	phenol
		Two-Spinneret			removed (210
	TiO2/CoFe2O4	Electrospinning	Phenol		min)		Li et al. [96]

Table 3 . Summary of ferrite catalysts for air detoxification.

 3

		Method	of
	Catalyst	synthesis		Pollutant	Conversion	Advantages	Disadvantages	Reference
		sol-gel
	BiFeO3	autocombustion	NOX	36%		UV light	Rahimi et al.[104]
								Xiazhang Li et al.
	LaFe1-XNiXO3/palygorskite sol-gel		NOX	>90%	Visible light		[32]
		Sol-gel						Xiazhang Li et al.
	CQDs/PrFeO3/Palygorskite	impregnation		NOX				[33]

Table 4 . Summary of ferrite catalysts employed in simultaneous application of oxidation systems.

 4

		Method	of
	Catalyst	synthesis		Pollutant	Conversion	Advantages	Disadvantages	Reference
					90% MO with
	Cu doped LaFeO3 Hydrothermal		Methyl orange	15%Cu (60 min)	Visible light	No TOC data	Phan et al. [110]
					60% TOC with Fe
					substitution (300	Higher effective
	LaFeO3	Sol-gel		Acetic acid	min)		use of H2O2	Sannino et al. [30]
					98.6-99.3% MB		Vinosha et al.
	CoFe2O4	Co-precipitation	Methylene Blue	(75 min)			[111]
					95% Rhodamine
					B; TOC>85% (180
	Co-MgFe2O4	Hydrothermal		Rhodamine B	min)		Visible light	Diao et al. [44]
		Synthesis
	mesoporous	including
	CuFe2O4	template		Sulfonamide	31% TOC (40 min) Good reusability	no mineralization Gao et al. [112]
					30%	undoped
	NiFeMO4 (M=La,	Sol-gel			material; 90% La		Samoila et al.
	Sm, Gd,Dy)	autocombustión	Dyes, orange II	doped (30 min)		No TOC data	[113]
					Total
					degradation;
	(Ba,Bi)FeO3	Sol-gel		Toluene	85%TOC (40 min) Visible light	Soltani et al. [31]

  The band gaps decreased upon metal substitution from 2.8 eV in Fe3O4 to 1.62, 1.90 and 2.23 eV in Ni, Cd and Zn ferrites, respectively. All samples gave rise to

	Gobara et al. compared the activity of magnetite for hydrogen production from methanol
	aqueous solutions under simulated solar radiation with those of different ferrites resulting
	from the substitution of several divalent cations (Zn 2+ , Cd 2+ or Ni 2+ ) for Fe 2+ [152]. All catalysts

were prepared by co-precipitation methods which resulted in nanocrystalline cubic phases except for the cadmium ferrite which was considerably less crystalline and contained a segregated Fe2O3 phase.

  Rekhila et al proposed the use of p-NiFe2O4 pellets prepared by sintering sol-gel synthesized particles at 850°C[134]. The open-circuit voltage, short-circuit current and efficiency were reported to be, respectively, 0.43 V, 0.71 mA cm -2 and 0.28 under irradiation with visible light (50 mW cm -2 ) with Pt as counter electrode.Yang et al. prepared porous CoFe2O4 nanosheets on FTO from aqueous solutions of Co and Fe nitrate using a template-free electrochemical deposition followed by a heat treatment at 933 K[START_REF] Yang | Electrochemical synthesis of CoFe2O4 porous nanosheets for visible light driven photoelectrochemical applications[END_REF]. The photocathodes exhibit a small photocurrent of ∼0.3 μAcm-2 in 0.1 M aqueous Na2S at zero bias voltage under wavelength ≥ 390 nm (30 mWcm -2 ).

  Tahir et al., for instance [141, 142],

Table 5 .

 5 Photocatalytic hydrogen production with selected single-phase ferrite photocatalysts

	Catalyst	Band	Co-	Sacrificial agent Light source	H2 evolution rate	Apparent		Reference
		gap (eV)	catalyst				(mmol h -1 gcat -1 )	quantum yield (%)
	Fe3O4	2.7	Pt	Ethanol	2 × 200 W	8.275	-		Mangrulkar et
					tungsten lamp				al. [150]
	Fe3O4	2.84	-	Methanol	Simulated	1.25	-		Gobara et al.
					solar					[152]
	NiFe2O4	1.7	-	Methanol	250 W Xe	0.155	0.52 (λ ≥ 420	Peng et al. [154]
					lamp			nm)
					λ ≥ 420 nm
	NiFe2O4	1.56	-	S2O3 2-	500	W	1.529	0.53	(full	Rekhila et al.
					halogen lamp		spectrum)		[134]
	NiFe2O4	1.7	-	Methanol	Xe	lamp	0.045	7.5 × 10 -3 (λ=	Hong et al.
					λ ≥ 420 nm		450 ± 10 nm)	[155]
	NiFe2O4	1.73	Au	Triethanolamine 300 W Xe	3.162	-		Zeng et al. [158]
					lamp
					λ ≥ 420 nm
	NiFe2O4	1.62	-	Methanol	Simulated	4.95	-		Gobara et al.
					solar					[152]
	NiFe2O4	1.6	-	Methanol	250 W metal	0.040	-		Domínguez-
					halide lamp				Arvizu et al.
										[156]

Acknowledgements.

The authors want to thank the European Fund for regional development (EFRE/FEDER) for the financial support of the PHOTOPUR project which is performed within the framework of Interreg V and the Sciences Offensive. Financial support from project SOLPAC: ENE2017-89170-R, MCIU/AEI/FEDER, EU from the Spanish Ministry of Science, Innovation and Universities is also gratefully acknowledged.

coefficients, as a basis for future designs of photoreactors using this catalyst. Under white light and without any co-catalyst present, NiFe2O4 produced ca. 5 times larger amount of hydrogen than TiO2 P25 after 8 h from an aqueous methanol solution.

In photocatalytic hydrogen evolution reactions, the presence of a co-catalyst, most generally a noble metal, improves the hydrogen yield by boosting charge separation and acting as catalytic sites for H-H bond formation [START_REF] Núñez | Ga-Promoted Photocatalytic H2 Production over Pt/ZnO Nanostructures[END_REF]. In addition, in the case of some metals with appropriate particle size, the localized surface plasmon resonance effect may enhance light absorption in the visible range [START_REF] Yang | Photocatalysis: From Fundamental Principles to Materials and Applications[END_REF]. In a recent work, Zeng and co-workers examined the combined effect of plasmonic gold nanoparticles and fluorescein sensitization on the activity of NiFe2O4 photocatalysts for visible-light hydrogen production from water using triethanolamine as sacrificial reagent [START_REF] Zeng | In situ plasmonic Au nanoparticle anchored nickel ferrite: An efficient plasmonic photocatalyst for fluorescein-sensitized hydrogen evolution under visible light irradiation[END_REF]. The presence of gold increased the hydrogen yield by a factor of 5 at an optimum metal loading with respect to the pure ferrite, which the authors ascribed to stronger absorption in the visible range and slower recombination on the basis of UV-vis and photoluminescence spectra, respectively. A synergistic effect of fluorescein sensitizer and plasmonic Au co-catalyst further improved the photoactivity by one order of magnitude, with stable hydrogen production throughout 6 photocatalytic cycles.

In contrast to the above-cited work by Gobara et al. [152], Rodríguez-Rodríguez and coworkers obtained a higher hydrogen yield with the spinel zinc ferrite ZnFe2O4 compared to NiFe2O4 [159]. The authors prepared nanosized Ni, Co and Zn ferrites in oil-in-water microemulsions and tested them without any co-catalyst for photocatalytic hydrogen evolution from methanol aqueous solutions. The photocatalytic activity followed the order ZnFe2O4 > NiFe2O4 > CoFe2O4, which is mainly attributed to conduction band energies increasing in the reversed order on the basis of UV-vis spectra. A higher conduction band energy would thus imply a stronger driving force for the transfer of electrons to protons. Dom and co-workers used a microwave solid-state synthesis method to improve the activity of zinc ferrite by a factor of 4 with respect to the one obtained by a conventional solid state reaction [160]. Electrochemical measurements together with physicochemical and optical characterizations let the authors conclude that the absence of agglomeration of nanoparticles in the microwave method compared to the conventional one modified the electronic band positions and the electron mobility, resulting in improved photocatalytic behaviour. Doping well as reduced grain boundaries and lattice defects, which relates to the synthesis method.

The combination of the different factors led to an optimal activity in La0.925Sr0.075Fe0.925Ti0.075O3. Related perovskite lanthanum strontium ferrites in the Ruddlesden-Popper series (SrO)(LaFeO3)n (n: 1 and 2) were reported a hydrogen evolution photocatalysts from Na2SO3 solutions by Chen and co-workers [START_REF] Chen | A promising synergistic effect of nickel ferrite loaded on the layered double hydroxide-derived carrier for enhanced photocatalytic hydrogen evolution[END_REF], with the n= 1 compound (SrLaFeO4) giving rise to a higher hydrogen evolution rate with Pt as co-catalyst. Anisotropic charge transportation phenomena are invoked to account for the activity of this kind of solids. Reproduced with permission from ref. [START_REF] Chen | Interface engineering: Surface hydrophilic regulation of LaFeO3 towards enhanced visible light photocatalytic hydrogen evolution[END_REF]. Copyright Elsevier.

As mentioned earlier, photocatalytic CO2 reduction studies with ferrite catalysts are much scarcer in the literature than hydrogen production ones. Matsumoto and co-workers reported in 1994 the production of methanol and formaldehyde from a CO2-saturated solution on aqueous methanol solutions under visible light, with the concourse of two co-catalysts, namely RuO2 and Pt [174]. By bulk heterojunction, the authors mean that there are junctions of the two phases distributed along the whole composite material, as investigated by highresolution transmission electron microscopy, which was obtained by a polymer-complex synthetic method that allowed good crystallinity to be obtained at low temperature as well as an adequate distribution of the composing phases. This heterojunction, which band structure is shown in Figure 13, gave rise to a remarkable hydrogen production activity under light with λ ≥ 420 nm, with reported quantum yield of ca. 10%, in contrast with the single phases that showed little activity in the same conditions. Compared to a layered heterojunction, the authors ascribed the improved photocatalytic activity to the higher probability of excitons reaching the interfaces and dissociate, particularly for semiconductor particles with sizes in the same range as the exciton diffusion length. In a related work, Vijayaraghavan and coworkers reported a series of photocatalysts based on magnesium and calcium ferrites composed by single phases and mixed compositions of up to four different crystalline structures -MgFe2O4, cubic and orthorhombic CaFe2O4 and Fe2O3 [175], with activities for hydrogen evolution from glycerol solutions depending mainly on the phase composition of the photocatalyst system, in the absence of co-catalysts. A kind of electron cascade mechanism is proposed by the author to account for the highest activity displayed by a catalyst composed of the four mentioned phases. ZnO reduce protons to produce hydrogen, while holes remaining in the valence band of the ferrite phase would oxidize water to oxygen. However, no oxygen evolution was reported in the work to account for the electron balance and, as shown by the authors in their electronic scheme, holes in the valence band of both copper and nickel ferrites do not have enough reduction potential to oxidize water. A similar charge transfer mechanism was proposed by Karamian and Sharifnia to account for the visible-light activity of BiFeO3-ZnO p-n junctions for photocatalytic CO2 reduction using methane as hole scavenger [184], although in this case only the ferrite component is activated by the used irradiation source, thereby acting as an inorganic visible-light sensitizer of the otherwise inactive, wide-band-gap ZnO phase. The charge separation was supported by photoluminescence spectroscopy results, which suggested a hindered recombination in the mixed phases. With an optimized 1:1 molar ratio between the components, ca. 20% CO2 conversion was reached, which declined to ca. 14% after four runs. However, no other efficiency measure than conversion is given to make the results comparable to other works, nor is it clear what products are obtained.

Regarding other oxides that have been coupled to ferrites, Guo and co-workers [START_REF] Guo | Photocatalytic reduction of CO2 with H2O vapor under visible light over Ce doped ZnFe2O4[END_REF] reported the preparation of Ce-doped ZnFe2O4 via a sol-gel method that actually gave rise to bi-phasic samples composed of a Ce 3+ -doped ZnFe2O4 spinel phase together with an additional CeO2 phase with fluorite structure. These composites displayed higher activity than the pristine zinc ferrite for photocatalytic CO2 reduction under visible light, with CH4 and CO as main products, together with hydrogen from the competing reduction of water and oxygen from water oxidation (not shown though). The formation of a p-n junction between the two phases induced the establishment of a type-II heterojunction that promoted charge separation and reduced recombination, as suggested by a combined study of photoluminescence and UV-vis absorption spectroscopies with (photo)electrochemical measurements. In addition, TPD and FTIR studies revealed that the alkalinity of cerium oxide favours the chemical interaction the photocatalytic system as hole extractor and oxidation co-catalyst, with platinum acting as the reduction co-catalyst. The situation is different with nickel and copper ferrites, since both have conduction band levels above that of C3N4, according to the same authors [START_REF] Chen | Ferrites boosting photocatalytic hydrogen evolution over graphitic carbon nitride: a case study of (Co, Ni)Fe2O4 modification[END_REF].

Therefore, under visible irradiation both ferrites can donate electrons from their conduction bands to that of the nitride, while holes can be move from the valence band of the latter to that of the ferrite. In this line, steady-state photoluminescence spectra and fluorescence decay curves suggest in both cases (CuFe2O4/C3N4 and NiFe2O4/C3N4) an inhibited electronhole recombination, while electrochemical impedance spectroscopy shows a greatly enhanced internal conductivity in the composites compared to the bare nitride. As a result, the activity of the heterojunctions for H2 production from aqueous triethanolamine increases with respect to the single phases, and is further enhanced in the presence of platinum as reduction co-catalyst. As in the previous work, the authors highlight the dual role of the ferrite as charge separation promoter and oxidation co-catalyst.

The junction of C3N4 with the perovskite-structured lanthanum ferrite LaFeO3 has been studied in a few works for photocatalytic hydrogen production. In principle, the relative band positions of both phases would be adequate for the formation of a type-II heterojunction. However, as previously described for the zinc ferrite heterojunctions, an all-solid Z-scheme without any electronic mediator seems to occur in the case of LaFeO3/C3N4 photocatalysts.

Acharya et al. proposed this kind of junction based on thermodynamic considerations [191],

given that, in the case of a type-II heterojunction, electrons would fall into a conduction band (that of LaFeO3 in this case) with reduction potential slightly below that of the H + /H2 redox couple. Even if the presence of Pt as co-catalyst is able to overcome this limitation in the single-phase ferrite, the observed improvement in hydrogen evolution with respect both to the ferrite and to C3N4 would not occur. This mechanism is, according to the authors, supported by the observed increase in photoluminescence in the heterojunction due to the recombination of the electrons in the conduction band of C3N4 with the holes in the valence band of the ferrite. However, this evidence does not seem so obvious from the photophysics point of view. The same kind of heterojunction between these two components was proposed by Xu and co-workers [192], by which they surpassed the activity of both single phases for hydrogen evolution from triethanolamine solutions under visible light, using platinum (3 wt.%) onset (0.9 V versus RHE) compared to reported values. In addition, these electrodes exhibit a photoresponse 4 to 5 fold higher than the same material in film conformation.

In order to improve the diffusion length of minority carriers, several strategies have been developed. One of them is the use of structured transparent conductive oxide current collectors to capture and tunnel the photogenerated electrons readily while the large interfacial area allows efficient transfer of the holes to the solution. In this way, Fu et al.

Conclusions

Even if ferrites have displayed some appealing properties that make them promising materials for light-activated applications for energy and environment like photocatalysis, photo-Fenton and photoelectrochemistry, the amount of scientific literature dealing with such materials is still lower compared to other types of catalysts or semiconductors. Nevertheless, as we have tried to point out here, some works have reported interesting results in the above-mentioned processes. However, there is still room for advancement in the way towards the full understanding of the relationships between the physico-chemical properties of these materials and their performance in photoassisted reactions. The possibilities that they display for the modification of their chemical composition, crystalline structure and optoelectronic properties, as well as their versatility for the creation of heterojunction and doped systems, are in our opinion the most relevant characteristics of ferrites for their further development in photoassisted environmental and solar fuels applications.