Research Update : Photoelectrochemical water splitting and photocatalytic hydrogen production using ferrites ( MFe 2 O 4 ) under visible light irradiation

The utilization of solar light for the photoelectrochemical and photocatalytic production of molecular hydrogen from water is a scientific and technical challenge. Semiconductors with suitable properties to promote solar-driven water splitting are a desideratum. A hitherto rarely investigated group of semiconductors are ferrites with the empirical formula MFe2O4 and related compounds. This contribution summarizes the published results of the experimental investigations on the photoelectrochemical and photocatalytic properties of these compounds. It will be shown that the potential of this group of compounds in regard to the production of solar hydrogen has not been fully explored yet.

Molecular hydrogen is considered as a substitute of fossil fuels but its solar powered production from water remains a challenge.5][16][17][18] One of these classes are ferrites with the general empirical formula M 1 x M 2 y Fe 3−x−y O 4 .These ferrites have been employed as photocatalysts to form molecular hydrogen and/or oxygen from aqueous suspensions usually containing a sacrificial reagent as well as electrodes to investigate their potential use as photocathodes or photoanodes.To ensure direct comparison, all electrochemical potentials are given herein with respect to the normal hydrogen electrode (NHE).
Most of the ferrites with the empirical formula MFe 2 O 4 (i.e., M 1 x M 2 y Fe 3−x−y O 4 , with x = 1 and y = 0) crystallize in the spinel structure.In the spinel ferrites, the oxide anions are arranged in a cubic close-packed lattice and the cations M and Fe occupy some or all of the octahedral and tetrahedral sites.Although the charges of M and Fe in the prototypical spinel structure (x = 1) are +2 and +3, respectively, other combinations are also possible.
Simple spinels of the type MFe 2 O 4 can be regarded as compounds where the iron ions of the spinel magnetite, T  corresponds to the degree of inversion (0 ≤ y ≤ 1).Spinel compounds with the empirical formula  [19][20][21] Other ferrites such as CuFe 2 O 4 form crystalline solids with cubic or tetragonal unit cells depending on the synthetic conditions. 22errites are regarded to be chemically and thermally stable in aqueous systems. 23Most of them are semiconductors with bandgap energies allowing the excitation by visible light, and energetic positions of the conduction and the valence bands are suitable for either reduction of protons and/or oxidation of water.The bandgap energies E g as well as the energetic positions of the valence band E VB and the conduction band E CB of some simple ferrites are tabulated in Table I.The variation of M 1 , M 2 , x, and y in M 1 x M 2 y Fe 3−x−y O 4 is known to affect the resistivity (conductivity), [24][25][26][27][28][29] the optical properties (reflectivity, bandgap energy), [24][25][26][27][28][29] and the p-/n-type behavior 30,31 of the semiconductor.][34][35] Fundamental (photo)electrochemical investigations of ferrite electrodes such as Fe 3 O 4 , 36 Li 0.5 Fe 2.5 O 4 , 37,38 MgFe 2 O 4 , [37][38][39][40] p-CaFe 2 O 4 , [41][42][43][44][45][46][47][48][49] 31,52 n-ZnFe 2 O 4 , 24,37,38,48,53,54 Zn x Ti y Fe 3−x−y O 4 , 24 and CdFe 2 O 4 55 have been published.The results of these studies as they relate to the determination of the flatband potentials and the energetic positions of the valance and conduction bands of the semiconductors are as well summarized in Table I.
As becomes obvious from Table I, several cubic and orthorhombic ferrites show p-type conductivity.Consequently, these compounds are suitable to act as photocathodes for the hydrogen evolution reaction (HER) in cells being designed for photoelectrochemical water splitting.Although there is a great scientific and technical interest in photocathodes, only p-type CaFe 2 O 4 , CoFe 2 O 4 , and NiFe 2 O 4 have been installed in photoelectrochemical cells (PECs) yet.Ye and co-workers have investigated the visible light induced water splitting reaction employing a |Pt|Na 2 SO 4 (0.1M) |p-CaFe 2 O 4 | PEC. 47The photocathodes have been fabricated by depositing CaFe 2 O 4 thin films on fluorine-doped tin oxide (FTO) coated glass employing a pulsed laser deposition method.A hydrogen evolution rate of ∼4.8 µmol m −2 h −1 was observed under visible light irradiation (300 W Xe) without applying any additional bias.In a three-electrode configuration, a cathodic photocurrent was observed at values more negative than +0.64 V.A photocurrent density of −117 µA cm −2 at −0.06 V was reported being significantly larger than the values reported by Matsumoto et al. 41,42 for metal-loaded CaFe 2 O 4 photoelectrodes probably due to shorter electron transfer distances in the thinner films used in the work of Ye and co-workers and to their higher electric conductivity. 47urthermore, Ye and co-workers have compared the photoelectrochemical properties of p- x photoelectrodes with the same single-layer thickness of 10-15 nm but an increasing number x (x = 10, 15, 20, and 25), a remarkable effect on the photocurrent density and the onset potential was observed.The 20-junction photoelectrode showed the highest photocurrent density (−25.23 µA cm −2 at +0.4 V) and the most positive onset potential (+1.3 V) of all four samples.Furthermore, the 20-junction photoelectrode-based PEC exhibited a high open circuit photovoltage of up to +0.97 V, which was much higher than that reported for a cell having a single junction photoelectrode that exhibited an open circuit photovoltage of +0.13 V. 48 The water splitting quantum efficiency of a pristine p-CaFe 2 O 4 electrode was found to be relatively low, which has been assumed to be due to the poor mobility of the photogenerated charge carriers. 43Therefore, efforts have been made to improve the conductivity of CaFe 2 O 4 electrodes by doping the material.Doping with Na and Mg yielding oxides of the type Ca 1−x Na x Fe 2−y Mg y O 4 resulted in semiconductors exhibiting higher electronic conductivity but still very small photocurrents. 43In an attempt to improve the low quantum efficiency for the light-induced water splitting reaction, Sekizawa et al. have prepared various metal-doped CaFe 2 O 4 electrodes by radio frequency magnetron co-sputtering onto glass substrates coated with antimony-doped tin oxide followed by post-annealing at a low temperature.The doping metals were aggregated in the films after annealing as revealed by scanning transmission electron microscopy.Doping of CaFe 2 O 4 with Au and Ag resulted in an enhancement of the photocurrent without affecting the p-type conductivity.Doping with Ag resulted in an improvement of the carrier mobility together with a red-shift of the photoabsorption.Ag-doped CaFe 2 O 4 showed a 23-fold higher photocurrent than undoped CaFe 2 O 4 . 49][46] Open-circuit voltages and short-circuit current densities of these PECs are presented in Table II.Molecular hydrogen was generated from these cells under irradiation with visible light without applying a bias (500 W Xe lamp).However, the expected H 2 /O 2 ratio of 2 was not achieved (Table II) showing that O 2 evolution is suppressed and an additional suitable oxygen evolution catalyst is needed.The semiconducting materials for the photocathode were prepared by a solid state route from Fe and Ca salts varying the Fe/Ca ratio (1.8-2.1).The highest photocathodic current was observed for an electrode having a Fe/Ca ratio of 1.9, for which XRD measurements revealed the presence of Ca   .A photon-to-electron conversion efficiency of 0.28 was calculated.However, corrosion of the semiconductor electrode was observed under illumination as well as in the dark. 52p to now, mainly PECs of the type |Pt|electrolyte|p-ferrite| have been studied.These cells employing ferrites as photocathodes are able to split water under visible light irradiation without applying an external bias.However, the reported photocurrents are well below those that are required for a technical application 65,66 indicating the inhibition of the interfacial charge carrier transfer.
As seldom as p-conducting ferrites as photocathodes, bare ferrites MFe 2 O 4 with n-type conductivity have been investigated as photoanodes in PECs.One of the exceptions is the n-ZnFe 2 O 4 electrode the photoanodic behavior of which was investigated by Wijayantha and co-workers. 53,54The electrode was prepared by aerosol-assisted chemical vapor deposition of alcoholic solutions of a bimetallic precursor on FTO.The thickness, morphology, and nanostructure of the electrode were controlled by altering the solvent for dissolution of the bimetallic precursor and the physical deposition parameters. 53,54The photocurrents were found to be dependent on the solvent, as well as on the deposition temperature and the deposition time.The maximum photocurrent density of 350 µA cm −2 at 0.44 V was obtained with a ZnFe 2 O 4 electrode synthesized using a 0.1 M solution of the bimetallic precursor in ethanol, an optimum deposition temperature of 450 • C, and a deposition time of 35 min.This electrode showed an incident-photon-to-electron conversion efficiency of 13.5 ×10 −2 at 350 nm and an applied potential of 0.44 V. 53 Varying the methanol/ethanol ratio of the solvent resulted in a change in the texture of the ZnFe 2 O 4 electrode.The textured electrodes exhibited a significantly higher photocurrent under AM1.5 illumination compared to their compact counterparts.The authors attributed this behavior to the improved collection of the photogenerated minority carriers at the ZnFe 2 O 4 /electrolyte interface as the average feature size gradually decreased. 54n general, ferrites as photoelectrodes need high temperatures to crystallize (>1000 • C).This limits the choice of support materials and poses a critical challenge to maintain desired electrode material properties like surface area and porosity.Recently, Kim et al. introduced a hybrid microwave annealing (HMA) post-synthetic heat treatment with graphite powder as a susceptor which is compatible to most transparent conducting glasses.They treated solution processed β-FeOOH nanorods with a Zn nitrate solution and obtained ZnFe 2 O 4 nanorods after thermal treatment at 550 • C for 3 h and at 800 • C for 20 min.Some unwanted ZnO on the nanorods was removed in NaOH.Subsequently, the ZnFe 2 O 4 nanorods were subjected to HMA (5 min) to increase their crystallinity.The HMA treated ZnFe 2 O 4 nanorods (550 • C) exhibited at +0.4 V (1M NaOH) and AM 1.5G illumination a photocurrent of 240 µA cm −2 , which was 10-15 fold increased in comparison to conventional thermally treated electrodes, and was stable for at least 3 h.The authors reported that stoichiometric amounts of H 2 and O 2 can be measured with Faradaic efficiencies (=actual gas evolution rate/rate expected from the current) of 90%-100%.The improved performance after the HMA treatment was attributed to better crystallinity and reduced surface defects as evinced by electrochemical impedance spectroscopy.For sometime now, the scientific interest focuses on the study of heterojunction electrodes such as the ZnFe 2 O 4 /Fe 2 O 3 n-n-heterojunction [68][69][70] 73 4 p-n-heterojunctions as photoanodes for the oxygen evolution reaction (OER).Borse and coworkers have prepared ZnFe 2 O 4 /Fe 2 O 3 layers on stainless steel by depositing an aqueous solution of Zn and Fe salts employing a plasma spray method and investigated the photoelectrochemical hydrogen production from water in a |ZnFe 2 O 4 /Fe 2 O 3 |NaOH aq (1M) |graphite| cell under simulated solar light (AM1.5G, 100 mW cm −2 ) employing a bias of 0.6 V. Again the composite photoanode exhibited a significantly higher photoactivity than a bare ZnFe 2 O 4 photoelectrode.The rates of HER at the ZnFe 2 O 4 and the ZnFe 2 O 4 /Fe 2 O 3 photoanodes were calculated to be 46.3 and 99.0 µmol cm −2 h −1 , respectively, resulting in solar-to-hydrogen conversion efficiencies of 0.60 × 10 −2 and 1.25 × 10 −2 , respectively.However, no data were given for the formation of molecular oxygen; thus, stoichiometric water splitting was not proven.The results of electrochemical impedance spectroscopy evinced a significantly lower interfacial charge transfer resistance of the ZnFe 2 O 4 /Fe 2 O 3 composite electrode than of the ZnFe 2 O 4 electrode. 68nFe 2 O 4 /Fe 2 O 3 photoanodes have also been prepared via electrodeposition and chemical growth of FeOOH films on FTO and subsequent conversion into α-Fe 2 O 3 by heat treatment.A Zn-containing solution was deposited on top of this α-Fe 2 O 3 film yielding a Zn-rich layer at the top surface of the α-Fe 2 O 3 layer after annealing. 69,70McDonald and Choi employing the electrodeposition route obtained photoelectrodes composed of particles with an α-Fe 2 O 3 (hematite) core and a ZnFe 2 O 4 shell as confirmed by XRD.The highest photocurrent was obtained with a composite electrode exhibiting a ZnFe 2 O 4 /Fe 2 O 3 ratio of 1.The increase in the photocurrent of the heterojunction electrodes compared to bare α-Fe 2 O 3 electrode was claimed by the authors to be due to an enhanced electron hole separation at the ZnFe 2 O 4 /Fe 2 O 3 interface.A further enhancement in photocurrent was obtained by a treatment of the composite electrodes with an Al 3+ solution yielding thin layers of a solid solution after heat treatment.With this, probably the number of surface states that serve as electron hole recombination centers is reduced.However, it was also observed that both the formation of a ZnFe 2 O 4 layer and the incorporation of Al 3+ into the surface resulted in a surface being less catalytic for the OER.However, when Co 2+ was introduced into the surface of the ZnFe 2 O 4 /Fe 2 O 3 composite electrodes as oxygen evolution catalyst, the onset of the photocurrent was shifted to more negative voltage and the overall photocurrent was improved. 69 related photoanode has been prepared 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 and 800 • C. The authors assumed that this procedure induces the formation of a p-CaFe 2 O 4 /n-Fe 2 O 3 heterojunction photoanode.The presence of Ca in the Fe 2 O 3 film and the formation of CaFe 2 O 4 have been evinced by XPS measurements.The photoanode showed a 100% higher photocurrent response than that obtained using a bare α-Fe 2 O 3 electrode under illumination (AM 1.5G, 100 mW cm −2 ).Based on the results of electrochemical impedance spectroscopy, the photocurrent enhancement has again been attributed to an enhanced charge carrier separation and a reduced resistance of the interfacial charge transfer between the electrolyte and the electrode. 71ee and co-workers reported the preparation and characterization of p-CaFe 2 O 4 modified TaON and BiVO 4 p-n-heterojunction photoanodes. 72,73Both n-type semiconductors are known to be suitable anode materials for solar driven water splitting in PECs.The position of valence band of p-CaFe 2 O 4 is more positive than the water oxidation potential, and both semiconductors, TaON and BiVO 4 , form staggered relative band positions with the ferrite as required for an effective heterojunction photoanode.In both cases, p-CaFe 2 O 4 , which has been synthesized by a conventional solid state reaction, was deposited on top of the n-semiconductor/FTO electrode by electrophoresis.The pristine TaON electrode showed an anodic photocurrent density of 230 µA cm −2 at +0.42 V (0.5 M NaOH, λ >420 nm).The CaFe 2 O 4 layer on the surface of the TaON electrode resulted in a significant increase of the photocurrent density (1260 µA cm −2 ).The observed photocurrent was found to be the result of overall water splitting yielding H 2 and O 2 in a ratio of 1.5, however, accompanied by a deterioration of the TaON.Impedance spectroscopic analysis indicated that the formation of the heterojunction increased the photocurrent density by reducing the resistance of the charge carrier transport and, consequently, enhancing the electron-hole separation. 72 1.5G 100 mW cm −2 ); the bare BiVO 4 electrode showed a photocurrent density of 580 µA cm −2 at +0.82 V, while the CaFe 2 O 4 /BiVO 4 heterojunction photoanode exhibited 960 µA cm −2 being an increase of 65% as compared to the BiVO 4 electrode.The formation of the heterojunction was found to reduce the recombination of the photogenerated charge carriers on the electrode surface with little effect on the bulk recombination as evinced by an investigation of the interfacial transfer of charge carriers using hydrogen peroxide as an electron donor. 73][73] Employing a CoPi/CaFe 2 O 4 /TaON photoanode in 0.1 M potassium phosphate buffer solution of pH 11.5 in a three electrode system with an applied bias of ∼0.55 V, total amounts of H 2 and O 2 of 123 and 59 µmol were released within 3 h of illumination with λ ≥ 400 nm (500 W Hg lamp with long-pass cutoff filter) resulting in a nearly stoichiometric ratio of 2.1.The Faradaic efficiency reached ∼80% for both HER and OER.The slight excess of molecular hydrogen has been attributed to the slow kinetics of oxygen evolution and the self-oxidation of TaON.However, the stability of the electrode was not optimum.The initial current decreased with time and reached about a half after 3 h.The beneficial role of the CoPi co-catalyst was revealed by performing the gas evolution experiment with a CaFe 2 O 4 /TaON photoanode in the absence of CoPi.Under these experimental conditions, no constant photocurrent was obtained, the Faradaic efficiencies decreased (50%-70%) and the H 2 /O 2 ratio became surprisingly significantly less than stoichiometric ratio (1.5:1). 72imilar results have been obtained with BiVO 4 -based photoanodes.In comparison to a bare BiVO 4 electrode, the CoPi/BiVO 4 photoanode showed a higher photocurrent density.On the other hand, the CoPi/CaFe 2 O 4 /BiVO 4 electrode exhibited a lower photocurrent density when compared with the CaFe 2 O 4 /BiVO 4 heterojunction electrode, but an improved stability of the current density was observed in the presence of CoPi, indicating that the presence of the OER co-catalyst is beneficial for the stabilization of the CaFe 2 O 4 /BiVO 4 heterojunction.The evolution of H 2 and O 2 during the photoelectrochemical water splitting reaction was measured employing a three electrode |CoPi/CaFe 2 O 4 /BiVO 4 | 0.1M potassium phosphate, pH7|Pt| cell applying a bias.The total amount of H 2 and O 2 evolved within 2 h of illumination with visible light (λ ≥ 400 nm) was 297 and 140 µmol, respectively.The resulting H 2 /O 2 ratio of 2.1 confirmed that the generation of the photocurrent was mainly due to the water splitting reaction.The Faradaic efficiency during this reaction was reported to be about 78%-88%.The photocurrent density being initially ∼4000 µA cm −2 dropped rapidly during the first 30 min of illumination and decreased slowly afterwards. 73ust as the PECs containing ferrite photocathodes, even cells with ferrite-containing photoanodes are able to split water under visible light irradiation without external bias.However, the measured photocurrents of PECs with bare ferrite photoanodes are low, again indicating the inhibition of the interfacial charge carrier transfer and, consequently, an enhanced rate of charge carrier recombination.Higher photocurrents were observed with heterojunction photoanodes due to an enhanced electron/hole separation at the interface between the two semiconductors.However, photocurrents in the order of 1000 µA cm −2 were only measured with efficient n-semiconductor electrodes having a p-CaFe 2 O 4 layer on the surface.1][82][83][84][85][86] Thus, cheap and varied electrode materials for PECs are a reachable target without the use of precious OER catalysts such as IrO 2 or RuO 2 .It is therefore evident that more photoelectrochemical investigations employing ferrite/n-semiconductor heterojunctions are desirable.New synthesis routes leading to high surface area materials and low temperature processes (e.g., hybrid microwave annealing) are likely to improve the existing performance.In addition, thin underlayers or overlayers of TiO 2 and Al 2 O 3 (but also of other metal oxides) are expected to enhance the photocurrent either through adjusting band alignments or by passivating surface recombination centers as it was revealed for other iron-based photoelectrodes. 871,12,[14][15][16][17]  Fe 3 O 4 nanoparticles were synthesized using a co-precipitation method.The reported XRD data of the synthesized compound are consistent with magnetite with spinel structure.The authors observed the evolution of molecular hydrogen in the dark at the Fe 3 O 4 surface at a temperature of 100 • C (3.8 µmol h −1 ) while no hydrogen evolved at 30 • C and 75 • C.Under illumination, an increasing temperature of the ethanol-water suspension resulted in an increasing HER rate (60 µmol h −1 at 85-100 • C).However, under visible light illumination at this temperature, the photocatalyst was found to be stable only up to 22 h. 88agnesium ferrite, p-MgFe 2 O 4 , derived from layered double hydroxides (molar ratio Mg/Fe = 2), synthesized by a co-precipitation method, was found to be active for photocatalytic HER under visible light illumination (200 W lamp, 29 mW cm −2 ).The best performance was achieved at pH 10 with a quantum efficiency of 0.5 × 10 −2 in the presence of the hole scavenger Na 2 SO 3 (0.025 M). 40 Spinel Pt and RuO 2 co-catalysts for the reduction and the oxidation reaction, respectively, showed only low photocatalytic activity (quantum yield QY < 1 × 10 −2 ), while the co-catalysts loaded heterojunction photocatalyst consisting of both ferrites was found to be highly active for hydrogen production from a water-methanol mixture under visible light illumination (quantum yield = 10.1 × 10 −2 ).The HER activity was found to be higher than that of a CaFe 2 O 4 /MgFe 2 O 4 photocatalyst prepared by a conventional solid-state reaction method at 1300 • C probably due to the higher crystallinity of the former. 56xpanding their work on the MgFe 2 O 4 /CaFe 2 O 4 heterojunction photocatalyst, Kim and coworkers have investigated the effect of substitution of Fe 3+ by Ti 4+ in the semiconductor lattice.Again, from water-methanol mixture and under visible light illumination, the Ti 4+ ion doped photocatalysts, CaFe 2−x Ti x O 4 /MgFe 2−x Ti x O 4 , with 0.01 ≤ x ≤ 0.06, loaded with Pt as co-catalyst, showed a slightly enhanced quantum yield for this reaction (13.3 × 10 −2 for x = 0.03) as compared to its Ti-free counterpart (10.1 × 10 −2 ). 89isible light induced hydrogen and oxygen formation employing a CaFe 2 O 4 /PbBi 2 Nb 1.9 W 0.1 O 9 p-n-heterojunction loaded with Pt was investigated by Kim et al.The authors reported a quantum yield of 38 × 10 −2 for the oxygen formation in the presence of AgNO 3 as the sacrificial reagent under illumination with λ ≥ 420 nm.The rate for hydrogen evolution from a water-methanol mixture was found to be lower than that for oxygen evolution by a factor of at least 10. 57 Orthorhombic phase BaFe 2 O 4 powder synthesized employing a solid state reaction was also investigated by Kim and co-workers.The ability of photocatalytic hydrogen evolution from a water-methanol mixture was tested after depositing both, Pt and RuO 2 nanoparticles on the semiconductor surface.Quantum yields of 6.24 × 10 −2 and 1.73 × 10 −2 were reported for the irradiation with λ > 210 nm and λ > 420 nm, respectively.Significantly lower quantum yields were calculated for only Pt-loaded BaFe 2 O 4 showing that RuO 2 assists the charge separation. 58hotocatalytic hydrogen evolution induced by p-NiFe 2 O 4 was studied by Peng and co-workers, 90 Hong et al., 91 Kang and co-workers, 92 and Rekhila and co-authors. 52eng and co-workers prepared mesoporous spinel NiFe 2 O 4 nanoparticles via a hydrothermal process followed by calcination.Employing the spinel ferrite as the photocatalyst without any cocatalyst, they observed hydrogen evolution under visible light illumination of an aqueous suspension in absence of any sacrificial reagent.However, no evolution of molecular oxygen was detected but the pH of the reaction increased during the hydrogen evolving photoreaction.Addition of methanol into the aqueous suspension resulted in an increased hydrogen evolution rate.The pH of the suspension was found to decrease during the reaction due to the oxidation of methanol yielding formic acid.Apparent quantum efficiencies of 0.07 × 10 −2 and 0.52 × 10 −2 were calculated for the HER in the absence and in the presence of methanol, respectively.Increasing the calcination temperature employed during the nanoparticle synthesis resulted in increasing crystal size, but decreasing surface area and pore volume of the photocatalyst which ultimately impacts on the hydrogen evolution rate.The highest rate (∼15 µmol h −1 ) was found with nanoparticles calcined at 500 • C having a surface area of 76 m 2 g −1 and a total pore volume of 0.26 cm 3 g −1 .Samples calcined at 300 • C and 700 • C exhibited lower rates.To evaluate the photostability of NiFe 2 O 4 , the photocatalyst was reutilized in repeated experimental runs after separation, washing, and drying.The respective amounts of molecular hydrogen formed in the second and third runs were ∼96% and ∼93% of the first run.Neither Ni 2+ nor Fe 3+ were detected in the residual solution after illumination indicating NiFe 2 O 4 nanoparticles to be stable visible-light active photocatalysts. 90ong et al. have synthesized submicron-sized mesoporous NiFe 2 O 4 spheres employing an aerosol spray pyrolysis method using Pluronic F127 as the structure-directing agent.Amorphous and crystalline photocatalysts were obtained depending on the amount of the structure-directing agent and the calcination conditions.For the purpose of comparison, the authors synthesized a reference NiFe 2 O 4 employing the above-mentioned method of Peng and co-workers.The photocatalytic performance for hydrogen evolution was examined by visible light illumination of aqueous suspensions containing methanol.The relative hydrogen evolution rates as calculated from reported rates are 1.0, 1.5, and 4.5 for the amorphous NiFe 2 O 4 (≥235 m 2 g −1 ), the reference sample (81 m 2 g −1 ), and the crystalline spinel NiFe 2 O 4 (121 m 2 g −1 ), respectively.These data indicate that the photocatalytic activity for hydrogen evolution depends primarily on crystallinity.The surface area seems to be of secondary importance.The formation of CO 2 was observed indicating oxidation of the sacrificial reagent.Repetitive experimental runs showed no change of the HER (within experimental error) indicating the stability of the spinel photocatalysts. 91ang and co-authors determined the hydrogen evolution ability of core-shell NiFe 2 O 4 /TiO 2 nanoparticles in water-methanol mixtures under UV irradiation.They prepared cubic NiFe 2 SO 4 by a precipitation method followed by a calcination step at 500 • C. Subsequently, the spinel was coated with TiO 2 by hydrolysis of a titanium alkoxide.Molecular hydrogen was photocatalytically generated from methanolic suspensions of the core-shell nanoparticles.In contrast to the experimental results reported by Peng et al. 90 and Hong et al. 91 no hydrogen was formed from suspensions containing the bare NiFe 2 O 4 .Little conclusive, the authors argued that the enhanced hydrogen production by NiFe 2 O 4 /TiO 2 in comparison to the bare NiFe 2 O 4 was mainly due to the effective electron/hole separation caused by the core-shell structure.Differences in the XPS spectra recorded before and after the photocatalytic reaction reveal changes in the oxidation states of the metal cations not being reversible in the time scale of the experimental runs. 92ekhila et al. synthesized a p-conducting NiFe 2 O 4 spinel photocatalyst by a sol-gel method followed by calcination at 850 • C and investigated its ability to produce molecular hydrogen from aqueous S 2 O 3 2− suspensions under visible light illumination.The amount of NiFe 2 O 4 , the pH, and the concentration of S 2 O 3 2− were varied to identify optimal reaction conditions.The highest quantum efficiency was reported to be 0.53 × 10 −2 , i.e., in the range reported by others employing methanol as sacrificial agent. 52ang et al. synthesized CuFe 2 O 4 nanoparticles via a citric acid-assisted sol-gel method with a final calcination at 850 • C. For the purpose of comparison, CuFe 2 O 4 materials were also fabricated by a co-precipitation method followed by calcination at 850 • C and a solid-state reaction at 1000 • C. All prepared samples exhibited tetragonal structure.The photocatalytic activity of the as-prepared photocatalysts was evaluated by measuring the amount of evolved molecular hydrogen from aqueous oxalic acid solutions under visible light illumination.Under identical experimental conditions, the highest photocatalytic HER was found using the CuFe 2 O 4 nanoparticles synthesized via the sol-gel method.The authors did not notice a decrease of the photocatalytic HER in repeated runs (4 × 10 h); therefore, they concluded that CuFe 2 O 4 is a stable photocatalyst for the photocatalytic H 2 evolution under visible light illumination. 93hotocatalytic, visible light induced HER with the spinel CuFe 2 O 4 was also evaluated with alkaline suspensions containing sulfide as the sacrificial reagent.The spinel has been synthesized by solid-state reactions at ≥1000 • C employing a mixture of CuO and Fe 2 O 3 as the reactants.A quantum yield of H 2 evolution of 0.1 × 10 −2 was reported.With illumination time, the evolution of molecular hydrogen slowed down because the oxidation products competed with the adsorbed water for the light generated electrons.However, the initial performance of catalyst was almost restored using a fresh sulfite solution. 60he photocatalytic properties of p-CuFe 2−x Mn x O 4 with 0 ≤ x ≤ 2 with respect to hydrogen evolution from visible light illuminated suspensions containing sulfite were determined by Helaïli   C, by a co-precipitation method with a final calcination step at 850 • C and by a sol-gel method with a final heat treatment at 900 • C. All the employed synthetic procedures led to the formation of tetragonal CuFe 2 O 4 .The maximum rate of H 2 evolution was found to be 15 µmol g −1 min −1 with a quantum efficiency of 1.3 %.The photocatalytic activity of the heterostructure was found to be dependent on the synthesis method of the ferrite (sol-gel reaction > solid state reaction > co-precipitation method).The authors reported that CuFe 2 O 4 exhibits an excellent stability in alkaline media. 61orse and co-authors have reported the successful photocatalytic hydrogen evolution at ZnFe 2 O 4 surfaces. 62,64,94Employing a water-methanol mixture and using Pt-loaded ZnFe 2 O 4 under visible light illumination, a quantum yield of 0.15 × 10 −2 was calculated.The spinel ferrite was synthesized by using the polymer complex method at the relatively low temperature of 900 • C. 62 However, also with Pt-loaded ZnFe 2 O 4 prepared by a conventional solid-state reaction at temperatures ≥900 • C, a quantum yield of 0.11 ×10 −2 was obtained under slightly different experimental conditions. 94Iron sites in the spinel-phase ZnFe 2 O 4 have been substituted by Ti 4+ forming the spinels ZnFe 2−x Ti x O 4 , with x ≤ 0.08.For the HER with Pt-loaded ZnFe 2−x Ti x O 4 (0.02 ≤ x ≤ 0.08) from a water-methanol mixture under visible light illumination, quantum yields between 0.27 × 10 −2 and 0.77 × 10 −2 were reported.The highest quantum yield was obtained with ZnFe 1.94 Ti 0.06 O 4 .No change of the bandgap energy of ZnFe 2 O 4 by substituting Fe 3+ by Ti 4+ was observed.The authors supposed the higher electron density by n-type doping to be responsible for a more efficient charge separation in ZnFe 2−x Ti x O 4 resulting in an increased photocatalytic activity. 94n a more recent paper, Borse and co-workers reported the synthesis of uniformly sized nanocrystalline ZnFe 2 O 4 photocatalysts by a rapid microwave solid-state synthesis.The thus synthesized photocatalysts, possessing Brunauer-Emmett-Teller (BET) surface areas between 2.3 and 5.6 m 2 g −1 depending on the time of microwave irradiation, generated molecular hydrogen from a water-methanol mixture even without any loading with a co-catalyst.A maximum quantum yield of 0.19 × 10 −2 at λ = 420 ± 10 nm was determined for a material with a BET surface area of 4.6 m 2 g −1 .This quantum yield was approximately 3.8 times higher than that determined with a ZnFe 2 O 4 prepared by a conventional solid-state method. 64eng and co-workers have prepared a "floriated" ZnFe 2 O 4 via a hydrothermal treatment of iron() and zinc() salts in the presence of sodium oxalate and cetyltrimethylammonium bromide followed by ion-exchange to remove the organic ions, washing, drying, and calcination at 500 • C. A "flaky" ZnFe 2 O 4 was prepared under the same synthetic conditions without adding cetyltrimethylammonium bromide.Cubic spinel structure was confirmed by XRD for both ferrite samples.The BET surface area of the "floriated" and the "flaky" photocatalyst was reported to be 52 and 51 m 2 g −1 , respectively.The photocatalytic activity with respect to visible light induced HER was tested employing these photocatalysts in both, water and water-methanol mixtures.Approximately 85 µmol g −1 molecular hydrogen was evolved during 5 h of visible light illumination of suspensions of the "floriated" ZnFe 2 O 4 in water without added sacrificial reagent.However, no formation of molecular oxygen was observed leaving the question open what kind of oxidation process occurred.The pH of the suspension steadily increased from 7.3 to 9.1 during illumination.In the presence of methanol acting as sacrificial agent, 238 µmol g −1 molecular hydrogen was evolved under otherwise identical experimental conditions.In this case, a slight decrease of the pH from 7.5 to 7.3 was observed.This decrease was attributed to the formation of some formic acid.The evolved amount of molecular hydrogen (23.8 µmol) was found to be much lower than the theoretical value (ca.494 µmol) calculated on the basis of the methanol reaction pathway.Although exposing almost the same surface area, the "flaky" photocatalyst exhibits significantly lower photocatalytic activity indicating lower crystallinity.No oxygen evolution in the presence of AgNO 3 was observed over visible light irradiated ZnFe 2 O 4 . 95u and co-workers compared the ability of the spinels ZnFe 2 O 4 , ZnFeGaO 4 , and ZnGa 2 O 4 to generate molecular hydrogen photocatalytically from aqueous sodium sulfite solutions under both UV-vis (λ ≥ 250 nm) and visible light illumination (λ ≥ 420 nm).The photocatalysts were synthesized by conventional solid-state methods at 1000 • C employing the oxides as reactants.The results of the photocatalytic experiments showed that ZnFeGaO 4 shows to a reasonable HER under visible light irradiation exhibiting a slightly improved performance compared with ZnFe 2 O 4 under full range irradiation (971 vs. 862 µmol h −1 g −1 with λ ≥ 250 nm).The authors concluded that substituting Ga into the ZnFe 2 O 4 structure enhanced the light absorbance in the UV region and modified the electronic structure. 96rari and co-workers investigated the light-induced hydrogen evolution in the presence of a ZnFe 2 O 4 /SrTiO 3 photocatalyst with Na 2 S 2 O 3 being the sacrificial agent at 50 • C. ZnFe 2 O 4 alone was found to be an effective photocatalyst for hydrogen generation under visible illumination, but the photoactivity increased significantly when the spinel was combined with the wide bandgap semiconductor SrTiO 3 . 63he ferrite photocatalysts employed for HERs in the presence of sacrificial reagents and the obtained quantum yields are summarized in Table IV.As it becomes obvious from the data given in this table, the quantum yield (defined as two times the amount of evolved molecular hydrogen divided by the number of absorbed photons) of the HER catalyzed by ferrites under illumination with visible light is usually around 1 × 10 −2 .Only for CaFe 2 O 4 /MgFe 2 O 4 and related p-n-heterojunctions, quantum yields exceeding 10 × 10 −2 have been reported.It has to be mentioned that the activity of a given photocatalytically active compound seems to depend on material properties influenced by the method employed to synthesize the compound (cf. the cited papers of Hong et  ).It is well known that the rate of photocatalytic hydrogen evolution depends inter alia on the amount of photocatalyst as well as on the amount of co-catalyst(s), the spectral distribution and the light intensity, the concentration of the sacrificial reagent, the pH, and the temperature of the suspension.This is of course valid also for ferrite photocatalysts. 52,61,88,93Since it is not sure that all photocatalytic runs have been performed under optimized conditions, direct comparability of the reported quantum yields is not given.Consequently, the quantum yields tabulated in Table IV can serve only as a rough guide.
It has recently been shown that photocatalytic water splitting can be an economically attractive route if the decomposition of water is achieved stoichiometrically with a solar-to-hydrogen conversion efficiency of 10 × 10 −2 . 66This efficiency has not yet been achieved.However, NiFe 2 O 4 is one of the few materials where photocatalytic hydrogen production under irradiation with visible light in the absence of sacrificial reagents was reported.It will be necessary to examine whether (i) the simultaneous oxygen generation becomes possible by combining NiFe 2 O 4 with a suitable co-catalysts, and (ii) the efficiency can be improved.The data in Table IV suggest that an increase in the conversion efficiency is possible by forming heterojunctions between ferrites M x 1 M 2 y Fe 3−x−y O 4 with optimized composition.
Desirable properties for semiconductors employed as electrodes in photoelectrochemical cells and as photocatalysts are narrow bandgaps near 2 eV that allow the absorption of a large part of visible solar light, suitable energetic position of the valance and the conduction bands, high conductivities, and resistance to photocorrosion in aqueous solutions.As the above summary shows, many   ferrites of the type MFe 2 O 4 seem to fulfill these conditions since PECs containing photocathodes as well as photoanodes with a bare ferrite of the type MFe 2 O 4 are able to split water under visible light irradiation without external bias.However, the reported photocurrents are well below those that are required for a technical application. 65,66Nevertheless, the measured photocurrents of some ferrites are in the same order of magnitude as those reported for other, much more intensively studied semiconductor electrodes (cf.Table 1 in Ref. 97).The improved electrical conductivity as compared with the corresponding single component metal oxide (iron oxides) mainly attributed to the presence of different metal cations facilitating the electron transport process and/or supporting the redox chemistry at the electrolyte/semiconductor interface has significant importance in designing efficient photoelectrodes. 98urrently, heterostructures of different ferrites or ferrites with other semiconducting oxides show typically the best performance with respect to the measured photocurrents due to an enhanced electron-hole separation. 17Extensive studies with electrodes made from synthetically readily accessible ferrites of the type M x 1 M 2 y Fe 3−x−y O 4 , are still lacking.As shown, inclusion of a third metal ion into the lattice of a ferrite MFe 2 O 4 yielding M x 1 M 2 y Fe 3−x−y O 4 ferrites alters the photocatalytic properties of the material.Therefore, it seems possible that a systematic investigation of these ferrites will result in electrodes having an improved performance in photoelectrochemical devices.
It has been shown that the variation of the metal ions 1 M and 1 M as well as the variation of x and y is influencing the bandgap energy, the energetic positions of the valence and of the conduction band, as well as the p-/n-semiconducting properties.
Thus, the ferrites M x 1 M 2 y Fe 3−x−y O 4 seem to be a group of compounds that allows the targeted adjustment of desired material properties (bandgap engineering).Tailoring of semiconductor properties is one key to improve the performance of photoelectrochemical and photocatalytic devices.Another important aspect is the surface composition of both, normal and inverse spinel ferrites which is known to be different from their bulk composition.Low energy ion scattering (LEIS) and angle resolved XPS studies on ferrites showed that the surface is dominated by octahedral O M cations, while the tetrahedral T M cations prefer to occupy sites below the surface.These findings are of paramount importance in designing future ferrite photocatalysts since photocatalytic reactions are initiated at the electrolyte/semiconductor interface. 99,100is work has been funded in part by the Deutsche Forschungsgemeinschaft under the program SPP 1613 (Nos.BA 1137/13-1, BA 1137/22-1, BR 1768/9-1, WA 1116/23-1, and WA1116/28-1).

2 Dillert
[Fe] O [Fe 2 ]O 4 , are substituted by other metal ions according to T [M 1−y Fe y ] O [M y Fe 2−y ]O 4 where the superscripts T and O identify the tetrahedral and octahedral sites, respectively, and y 104001-et al. APL Mater.3, 104001 (2015)

7 Dillert
Several semiconducting ferrites of the types MFe 2 O 4 and M x 1 M 2 y Fe 3−x−y O 4 have been studied as possible water splitting photocatalysts due to their ability to absorb visible light and their claimed resistance against photocorrosion.The ability of Pt-loaded Fe 3 O 4 to evolve molecular hydrogen from water-ethanol mixtures under visible light illumination was investigated by Mangrulkar et al.The employed single-phase 104001-et al. APL Mater.3, 104001 (2015)

TABLE I .
Bandgap energies and energetic positions of the bands of some selected MFe 2 O 4 compounds.
g /eV a E VB /V b E CB /V b E fb /Va The values of the indirect and direct bandgaps are labeled by i and d, respectively.b All values are given vs. NHE; if necessary, the values given in the reference have been converted.c Determined by electrochemical measurement in suspension, S, or at an electrode, E.
CaFe 2 O 4 , n-ZnFe 2 O 4 , p-CaFe 2 O 4 /n-ZnFe 2 O 4 , and multiple p-n-junction CaFe 2 O 4 /ZnFe 2 O 4 photoelectrodes.The electrodes have been prepared by a pulsed laser deposition method using CaFe 2 O 4 and ZnFe 2 O 4 pellets as the targets and FTO as the substrate.The authors observed a photocathodic current assigned to the reduction of water on a single-layer p-CaFe 2 O 4 thin film and a photoanodic current due to the oxidization of water on a single-layer n-ZnFe 2 O 4 thin film.A FTO/ZnFe 2 O 4 /CaFe 2 O 4 photoelectrode exhibited a negative photocurrent and a positive open circuit photovoltage (+0.025V, λ = 430 nm, 118 µW cm −2 ) indicating that this electrode with a p-CaFe 2 O 4 layer at the surface contacting the electrolyte acts as a photocathode.Investigating the photoelectrochemical properties of four multiple-junction FTO/(ZnFe 2 O 4 /CaFe 2 O 4 ) 2 Fe 2 O 5 as an impurity in CaFe 2 O 4 .Yang et al. have investigated the photoelectrochemical performance of porous CoFe 2 O 4 nanosheets on FTO.The electrodes have been prepared from an aqueous solution of Co and Fe nitrate via a template-free electrochemical deposition followed by a heat treatment at 933 K.The

TABLE III .
22parent quantum efficiency of the photocatalytic hydrogen evolution employing Cu-containing spinels.22Theirphotocatalystswereprepared by a solid-state reaction employing appropriate amounts of CuO, Fe 2 O 3 , and Mn 2 O 3 at 850 • C. The energetic position of the bands and consequently the bandgap energy are found to be affected by substituting Fe by Mn (TableIII).All prepared spinels were photocatalytically active (TableIII); the highest apparent quantum efficiency (1.59 × 10 −2 ) was reported for CuFe 1.6 Mn 0.4 O 4 .22Trariand co-workers determined the visible light induced hydrogen evolution from alkaline thiosulfate solutions in the presence of CuFe 2 O 4 and CuFe 2 O 4 /TiO 2 as photocatalysts.CuFe 2 O 4 was synthesized by solid state reaction of CuO and Fe 2 O 3 at 850 al. on NiFe 2 O 4 91 and of Yang et al. on CuFe 2 O 4 93

TABLE IV .
Photocatalytic hydrogen evolution from aqueous ferrite suspensions containing sacrificial reagents.