Engineering surface states of hematite based photoanodes for boosting photoelectrochemical water splitting

Abstract

Hematite-based photoanodes are promising candidates for photoelectrochemical water splitting. However, the performance of pristine hematite semiconductors is unsatisfactory due to charge recombination occurring at different interfaces: back contact, bulk and semiconductor/electrolyte interfaces. Increasing efforts have been focused on enhancing the performance of hematite based photoanodes via nanostructure control, doping, heterojunction construction, and surface modification with a secondary semiconductor or an oxygen evolution electrocatalyst. Most of the previous studies attributed the enhanced PEC water splitting performance to the changes in the donor density via doping, the formation of type II heterojunction via a secondary semiconductor coating and the improved water oxidation kinetics via coating oxygen evolution electrocatalysts. However, the role of surface states presented at the semiconductor/electrolyte interfaces of hematite-based photoanodes has been overlooked in previous investigations, which virtually play a critical role in determining the photoelectrochemical water oxidation process. In this review, we summarize the recent progress of various techniques employed for the detection of surface states of hematite photoanodes and highlight the important role of modifying surface states in the development of high performance hematite based photoanodes for photoelectrochemical water splitting application. The challenges and future prospects in the study of hematite based photoanodes are also discussed.

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1. Introduction

The increasing global energy demand of modern society drastically conflicts with the finite fossil fuel supply in nature, motivating plenty of research efforts for the development of sustainable and environmentally friendly energy sources.¹ Inexhaustible solar energy is among the most promising candidates, and has been utilized widely in photovoltaic approaches.² However, the fluctuant and intermittent nature of insolation make such applications unsuitable for the storage and dispatch of solar energy for consumption.³ Accordingly, conversion of solar energy into the form of chemical bonds is an attractive approach for efficient, economical, and convenient utilization of solar energy.^4–6 Among these strategies, photoelectrochemical (PEC) water splitting devices, using earth abundant semiconductors, have for long been considered to be the ‘Holy Grail’ of the solar energy conversion revolution.^7–9

As an earth abundant semiconductor, hematite photoanodes have been intensively investigated as photoanodes for PEC water splitting because of several promising properties, like high abundancy in nature, environmental-friendliness, high photochemical stability, a narrow bandgap (1.9–2.2 eV), and a theoretical maximum solar-to-hydrogen (STH) efficiency of 15.4%.^10,11 However, their relatively low absorption coefficient, short excited-state lifetime (10⁻⁶ s),^12,13 poor oxygen evolution reaction kinetics, short hole diffusion length, and poor electrical conductivity lead to multiple electron–hole recombination pathways occurring in the bulk, interfaces, and surfaces, which significantly limits the PEC activity of hematite photoanodes.¹¹ A rapid charge transport and transfer between the back substrate, the photoactive semiconductor, the electrocatalyst and the electrolyte are necessary for an efficient STH performance.¹⁴ Recently, plenty of researchers have devoted their efforts to the development of various nanostructures, doping, heterojunction construction, and surface modification with a secondary semiconductor or oxygen evolution electrocatalyst for improving the PEC performance of hematite based photoanodes.^11,15–35 Typically, they attributed the enhanced PEC water splitting performance of hematite composite electrodes to the changes in the donor density via doping, improved charge separation efficiency via constructing type II heterojunctions, and the enhanced water oxidation kinetics via coating an oxygen evolution electrocatalyst.^11,15–35 However, the investigation about the surface state mediated charge transfer at the semiconductor–electrolyte interfaces of hematite based photoanodes has not been properly considered by the research community of PEC water splitting, which is critical for understanding the PEC mechanism and further improving their PEC performance.

In this review, we will correct this deficiency and highlight the role of surface states present at the hematite/electrolyte interfaces, which is important for further development of hematite-based photoanodes for PEC water splitting.^36,37 Specifically, we will review the development of hematite-based photoanodes following this outline: (i) the fundamental concept of photoelectrochemistry and surface states; (ii) detecting the surface states of hematite via different techniques; (iii) engineering of surface states of hematite photoanodes.

2. The fundamental concept of photoelectrochemistry and surface states

The free energy required for the conversion of 1H₂O molecule to H₂ and 1/2O₂ molecules under standard conditions is ΔG = 237.2 kJ mol⁻¹.³ According to the Nernst equation, it corresponds to ΔE° = 1.23 V per transferred electron. Theoretically, a semiconductor with a band gap energy (E_g) larger than 1.23 eV can drive the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) using electrons/holes generated under illumination (Fig. 1) if it has a conduction band edge energy (E_cb) and a valence band edge energy (E_vb) that straddle the electrochemical potentials of E°(H⁺/H₂) and E°(O₂/H₂O). To drive these two reactions, photoinduced electrons or holes in the semiconductor bulk must travel to the semiconductor/electrolyte interfaces (SEI), and then react with the electrolyte species directly at the semiconductor surface without recombination. The charge-transfer process at SEI results in losses because of the concentration and kinetic overpotentials for driving the HER and the OER. Therefore, the energy required for photoelectrochemical water splitting at a photoelectrode is usually reported as 1.6–2.4 eV per generated electron–hole pair.^38,39

Fig. 1 Photoelectrochemical cell basics. (a) Schematic of a ‘wired’-type tandem cell for water splitting with incident solar illumination striking the photoanode and transmitting to the photocathode. (b) Working principle of the tandem cell for water splitting using a photoanode with bandgap energy E_g,1, and a photocathode with E_g,2 (where E_g,1 > E_g,2). Briefly, on absorption of a solar photon, an electron (e⁻) from the valence band (VB) is promoted to the conduction band (CB) leaving the corresponding electron hole (h⁺). The electric field in the depletion layer physically separates these charges and, in the photocathode, the electrons in the CB drift to the semiconductor–liquid junction, increasing the quasi-Fermi energy of the cathode, E^C_QF, to drive the reduction of H⁺ to H₂ at a water reduction catalysis (WRC) site. Analogously, in the photoanode, electron holes in the VB drift to the semiconductor–liquid junction, increasing the photoanode's quasi-Fermi energy, E^A_QF, sufficiently to surmount the overpotential for oxidation (η_O) and oxidize water to O₂ at a water oxidation catalysis (WOC) site. Photogenerated electrons in the CB of the photoanode travel through the external circuit to recombine with the holes in the VB of the photocathode. E_F, Fermi energy; E(V_RHE), electronic potential with respect to the reversible hydrogen electrode; η_R, overpotential for reduction. Reproduced with permission from ref. 10.

Whereas, at the surface of a practical semiconductor, the periodic crystal symmetry is broken and thus, produces electronic states within the bandgap, which are named surface states/mid-gap states.⁴⁰ The surface states present at the surface of semiconductors can either be intrinsic or extrinsic, which relies on the environmental conditions of the semiconductors. In the field of semiconductor photoelectrochemistry, surface states play a vital role in the kinetics of interfacial reactions at illuminated semiconductor electrodes. Recently, the investigation of surface states affecting the kinetics of Fe₂O₃, BiVO₄, CuWO₄, GaN, CuFeO₂, Ta₃N₅, and Cu(In,Ga)(Se,S)₂ photoelectrodes for PEC water splitting has drawn plenty of attention.^41–48

Typically, in photoanodes, surface states can act as recombination centres for holes generated by light, therefore, surface recombination competes intensively with charge transfer from the semiconductor to the electrolyte. Specifically, under illumination, the valence band of a photoanode obtains abundant excess of holes, which can be transferred to the electrolyte by a direct charge transfer mechanism to launch the solar fuel production reaction. However, surface states trap hole carriers, leading to another favourable pathway for indirect charge transfer and a new undesirable recombination pathway.^49,50 A simple kinetic model neglecting any electrostatic influence, like the presence of an electric field has been presented in Fig. 2 to show the effect of surface state-assisted electron–hole recombination and indirect charge transfer processes on photoelectrochemical water splitting. At low bias potential, direct hole transfer from the valence band can be neglected and only three processes occur at this interface: trapping/detrapping of electrons, trapping/detrapping of holes and charge transfer of holes. Hence, from this picture, it is clear that surface state-assisted recombination and hole transfer to the solution have a common factor, which is hole trapping/detrapping by surface states.^51,52 Therefore, it is of great interest to distinctly detect the surface states present on the photoelectrodes and to deeply understand the critical role that the surface states play in the PEC water splitting process.

Fig. 2 (a) Energy diagrams of an n-type semiconductor of thickness L where electrons and holes are generated at a rate G and recombine at a rate U_r. At the interface of length δL, hole transfer to the redox level E_redox can take place (b) directly from the valence band (kinetic constant k_vb) and (c) indirectly from surface states at low bias and directly from the valence band at higher bias. In this latter model, we consider trapping of electrons and holes (β_n and β_p) and detrapping (ε_n,ε_p) and hole transfer from surface states (k_s) at the energy level E_ss, in competition with direct hole transfer. In these schemes E_c and E_v are the energies of the lower edge of the conduction band and the higher edge of the valence band, respectively. E_Fn and E_Fp are the quasi-Fermi levels, Φ_n and Φ_p are the injection barriers of electrons and holes, E_vac is the local vacuum level, ϕ is the local electrostatic potential and V is the applied voltage. Reproduced with permission from ref. 51.

3. Detecting the surface states of hematite via different techniques

3.1 Photoelectrochemical impedance spectroscopy (PEIS)

The photoelectrochemical oxygen evolution process on hematite involves 4-electron transfer/O₂ molecule. It is hypothesized that the OER reaction steps involve the formation of higher-valent iron states at the surface by hole capture. Eqn (1)–(4) illustrate the complications arising from the fact that the oxidation of water is a 4-electron process, therefore, holes need to be ‘stored’ in intermediate states. The Fe(IV) and Fe(V) intermediates in this scheme can also act as electron acceptors, so that surface recombination reactions of the kind shown in eqn (5) and (6) are likely to take place. The Fe(IV) and Fe(V) states can be thought as ‘surface-trapped holes’, which may have sufficient surface mobility to allow second order reactions of the type illustrated by the last step shown in eqn (1)–(4).⁵³

hν → h⁺+ e⁻ (1)

Fe(III)_surf + h⁺ → Fe(IV)_surf (2)

Fe(IV)_surf + h⁺ → Fe(V)_surf (3)

2Fe(V) + 2H₂O → 2Fe(III) + O₂ + 4H⁺ (4)

Fe(IV) + e⁻ → Fe(III) (5)

Fe(V) + e⁻ → Fe(IV) (6)

Based on the assumed possible mechanism in eqn (1)–(6), L. M. Peter et al. firstly discussed and derived the possibility of using photoelectrochemical impedance spectroscopy (PEIS) to illustrate the surface states present at the surface of hematite photoanodes, as displayed in Fig. 3.⁵³

Fig. 3 (a) Phenomenological kinetic scheme for PEIS analysis. (b) Typical PEIS response predicted by eqn (7). Reproduced with permission from ref. 53.

Fig. 3a is a simplified version of the model used in the previous discussions to describe the surface state mediated charge transport in hematite photoanodes, which excludes direct hole transfer from the valence band to solution redox; and a typical PEIS response shown in Fig. 3b predicted by L. M. Peter et al. based on the derivation of eqn (7):

where R_ser is the series resistance, C_sc is the space charge capacitance, j_h is the current density corresponding to the flux of holes reaching the interface, and ω is the radial frequency. k_t and k_r are the first order rate constants for interfacial transfer and recombination, respectively.⁵³

Furthermore, J. Bisquert and T. Hamann et al. systematically investigated the selection of applicable electrical equivalent circuits for fitting the PEIS data to the interpretation of the surface states present on photoelectrodes.^54–59 In order to illustrate the PEC measurements of the hematite photoanodes under illumination and dark conditions, J. Bisquert and T. Hamann et al. adopted a classical view, as depicted in Fig. 4a, to interpret the electrochemical impedance spectrum (EIS) measurements.⁶⁰ The equivalent circuit model (ECM) shown in Fig. 4b highlights the central role of the surface states acting as a recombination center, trapping electrons from the conduction band and holes from the valence band, as given by R_trapping. The surface states affecting the charge transfer of holes to the donor species in solution are described by R_ct,trap, and another pathway for direct charge transfer from the semiconductor bands is included as R_ct,bulk.^54,61

Fig. 4 (a) Proposed physical model for the charge carrier dynamics in hematite electrodes, showing their generation, G, by light absorption, surface-state trapping, and interfacial charge-transfer reactions. (b) Equivalent circuit corresponding to the physical model in (a). (c) Simplified model used for IS interpretation created by removing R_ct,trap. (d) Simplified model used for IS interpretation created by removing R_ct,bulk. (e) Randles circuit. (f) J(V) curve (green solid line) and C_trap (orange triangles) and R_ct,trap (red circles) values obtained for a 60 nm hematite electrode under 1 sun illumination and pH 6.9 with the schematic showing the surface state mediated hole transportation at the hematite/electrolyte interface. Reproduced with permission from ref. 54.

However, the general ECM proposed in Fig. 4b, including the surface-state hole-trapping process, cannot unambiguously fit the EIS because it does not discriminate between R_ct,bulk and R_ct,trap. Therefore, two simplifications of this general ECM have been employed, as shown in Fig. 4c and 4d. In these simplifications, either R_ct,trap or R_ct,bulk is eliminated. The simplified ECs are excellent approximations if charge transfer for PEC water oxidation is dominated by one route, either from the valence band (Fig. 4c) or surface states (Fig. 4d). They fitted the EIS data under illumination with these simplified models and found that more consistent results were obtained for the ECM displayed in Fig. 4d.⁵⁴ Therefore, the simplified ECM in Fig. 4d is employed to derive the fitting parameters from all the EIS measurements under illumination for the EIS analysis, while the Randle circuit in Fig. 4e is utilized for the fitting of EIS data obtained under dark conditions. Fig. 4f reveals that the strong correlation between the C_trap peak with the R_ct,trap valley and the photocurrent onset clearly indicates that the hole-transfer step leading to water oxidation takes place predominantly from surface trapped holes, and not directly from valence band holes. The PEIS methodology proposed by J. Bisquert and T. Hamann et al. not only provides new insight into the PEC water oxidation process of pristine hematite photoanodes, but also will be of great utility in further investigations of complex hematite composite systems to determine the density and the energy level of the surface states and charge transfer efficiency.⁵⁴

3.2 Intensity modulated photocurrent spectroscopy (IMPS), intensity modulated photovoltage spectroscopy (IMVS) and distribution of relaxation times (DRT) analysis

IMPS and IMVS are relatively uncommon techniques that probe the dynamic relation between light illumination and the electrochemical response of the photoelectrodes. IMPS is a powerful small amplitude perturbation technique that can deconvolute the rate constants for charge transfer and recombination at illuminated semiconductor electrodes mediated by the surface states.^62–66 L. M. Peter et al. proposed a generalized theoretical model for intensity modulated photocurrent spectroscopy (IMPS) and derived the analytical equations for the frequency dependence of the photocurrent and photovoltage responses.⁶² This theory treats the influence of the space charge, the surface states and the Helmholtz capacitances on the IMPS response. In the model, the charge transfer and recombination at the SEI are described by kinetic equations, whereas the surface states charging and the current through the external circuit are treated using linear circuit equations.⁶² In this way, the kinetic constant of the surface state-mediated charge transfer and charge recombination in the photoelectrode system can be readily obtained from the IMPS plot. IMVS is conceptually similar to IMPS, but it is not as popular as IMPS as a measurement technique. It probes the relationship between light intensity and photovoltage at a constant photocurrent response. These photoelectrochemical immittance triplets (PIT), consisting of three electrochemical techniques (PEIS, IMPS and IMVS) based on electrochemical impedance and optical modulation used to probe the dynamic performance of photoelectrodes for PEC water splitting, have been mathematically and phenomenologically compared by A. Rothschild et al.⁶⁷

In addition to these established PIT analysis methods, there is another approach, DRT-based empirical analysis, which is a distribution function that can be calculated for any impedance spectrum without any a priori assumption.^67,68 The most useful characteristic of DRT analysis is its capability to separate polarization processes more clearly than in common Nyquist or Bode plots, where they usually appear convoluted. Since it circumvents the construction of ECM, which always depend on presumptions and are never unique, DRT analysis is a powerful tool to support impedance data analysis.^67,68 For instance, A. Rothschild et al. employed DRT analysis to study hematite photoanodes in alkaline electrolytes, where they identified two dominant polarization processes: the first process is surface recombination, which is dominant at low potentials and is suppressed by adding a hole scavenger (H₂O₂) to the electrolytes; the second process is suspected to be the formation of double-bonded oxygen intermediates (Fe=O), which dominates the water oxidation reaction at high potentials (without H₂O₂) and the H₂O₂ photo-oxidation reaction at all potentials.^68,69

3.3 Cyclic voltammetry

T. W. Hamann et al. demonstrated that the surface state trapped holes in hematite photoanodes could be measured by using cyclic voltammetry in the dark.⁶³ Specifically, holes could accumulate on the hematite photoanode surface by applying a positive potential under illumination followed by measuring the cathodic current in the dark as the potential is scanned negatively and the surface states are reduced. As shown in Fig. 5, the cathodic current peak observed in the dark cyclic voltammetry scan is well consistent with the C_ss peak obtained in the fitted EIS data, indicating the presence of surface states in hematite photoanodes.⁶³

Fig. 5 Left: Equivalent circuit parameters obtained from fitting EIS data for a hematite electrode in contact with H₂O (a and b) under 0.1 sun (orange triangles), 0.33 sun (yellow squares) and 1 sun (green diamonds) illuminations. Right: (a) Cyclic voltammetry of hematite in H₂O scanned at 200 mV s⁻¹ after holding the electrode at a potential of 2 V vs. RHE for 60 seconds under 1 sun illumination. The first cycle is a solid red line and the second cycle is a dashed orange line. (b) CVs measured under the same conditions as in (a) at different scan rates: 20 mV s⁻¹ (red, solid), 50 mV s⁻¹ (orange, long dash), 200 mV s⁻¹ (yellow, medium dash), 500 mV s⁻¹ (green, short dash) and 1000 mV s⁻¹ (blue, dots). Reproduced with permission from ref. 63.

3.4 Operando UV-vis

Considering that the hematite surface plays a significant role in determining the efficiency of water oxidation, it is of great interest to directly identify the identity of the relevant hematite surface species via in situ techniques to evidence the surface state mediated charge transfer mechanism hypotheses. T. Hamann and H. Irie et al. detected the intermediate species during the oxygen evolution reaction via in situ UV-vis absorption spectroscopy (Fig. 6a) for atomic layer deposition and spray deposition hematite photoanodes.^70,71 For instance, T. Hamann et al. quantitatively measured the formation of the Fe(IV) intermediate states by integrating the absorption peak at 572 nm via operando UV-vis experiments, as displayed in Fig. 6b and c; the absorption features at 572 nm could be correlated with the C_ss obtained in PEIS, which is assigned to the oxidation of a low-valent Fe–H₂O or Fe–OH to a high-valent Fe=O group at the hematite photoanode surface.⁷⁰ Additionally, the coating of a single layer of alumina by atomic layer deposition onto the bare hematite photoanode reduced the UV-vis absorption features because there are fewer exposed iron atoms at the hematite surface, and fewer iron hydroxyls are able to be generated upon oxidation by a photogenerated hole.⁷⁰

Fig. 6 (a) Experimental setup of spectroelectrochemical measurements performed under monochromatic illumination. (b) Change in absorption spectra measured vs. 1.0 V vs. RHE. Spectra measured in the dark and under applied potentials of 1.2 (red line), 1.8 (orange line), 2.0 (yellow line) and 2.2 (green line) V vs. RHE. (c) Change in absorption spectra measured at 0.82 V (red), 1.22 V (orange), and 1.62 V versus RHE (yellow) measured under 0.1 mW cm⁻² 405 nm illumination with respect to the absorption spectra measured at 0.62 V. Reproduced with permission from ref. 70.

3.5 In situ infrared (IR) spectroscopy

It is well established that infrared spectroscopy is a powerful technique for the determination of intermediate species in catalytic reactions.^72–76 Additionally, the utilization of attenuated total reflectance infrared (ATR-IR) spectroscopy is supposed to overcome challenges including the low concentration of intermediates on electrode surfaces and the competitive infrared absorption by water.⁷⁵ T. Hamann et al. investigated the water-oxidation reaction on hematite via in situ ATR-IR spectroscopy, and directly observed the existence of the Fe^IV=O intermediate during the PEC water splitting process, as shown in Fig. 7.⁷² As displayed in Fig. 7(c) and (d), a potential- and light-dependent absorption peak located at 898 cm⁻¹ in in situ ATR-IR spectra is assigned to a Fe^IV=O group, which is an intermediate in the PEC water oxidation reaction. These results provide direct evidence of high-valent iron–oxo intermediates as the product of the first hole transfer reaction on the hematite surface and represent an important step in establishing the mechanism of PEC water oxidation on semiconductor electrodes.⁷² J. C. Zhao et al. investigated the reaction order of interfacial hole transfer by rate law analysis based on EIS measurements and probed the high-valent Fe^IV=O intermediates by operando Fourier-transform infrared (FT-IR) spectroscopy, finding that in highly alkaline electrolytes, coupling of adjacent surface trapped holes becomes the dominant mechanism with a reaction orders of ∼2.⁷⁶

Fig. 7 Experimental set-up and results of infrared spectroscopy measurements during electrochemical and PEC water oxidation. (a) This schematic of the set-up used for operando PEC infrared (IR) measurements depicts the hematite working electrode (WE), platinum counter electrode (CE), Ag/AgCl reference electrode (RE) and ZnSe ATR crystal. A thin layer of electrolyte was introduced between the hematite WE and the ATR crystal. (b) J–V curves of a hematite electrode in the operando PEC infrared set-up measured in contact with D₂O in the dark (blue) and under illumination (dark red). (c) Infrared spectra of hematite scanned at constant applied potentials, from 1.43 to 2.03 V versus RHE, in the dark. (d) Infrared spectra of hematite scanned at constant applied potentials, from 1.13 to 1.63 V versus RHE, under illumination. Peaks at 898 and 743 cm⁻¹ appear at potentials positive to the onset of the water-oxidation current, 1.7 V versus RHE in the dark and 1.25 V versus RHE under illumination, which indicates that they may be species involved in the water-oxidation reaction. (e) The ATR-IR cell assembly parts. Reproduced with permission from ref. 72.

3.6. Four dimensional transmission electron microscopy (4D TEM)

Nonetheless, the aforementioned mechanistic investigations of hematite based photoanodes are limited to obtain macroscopic information of the Fe^IV=O intermediates via in situ spectroscopies, like UV-vis, in situ ATR-IR spectroscopy, PEIS, IMPS, IMVS, and DRT, but without spatially disclosing the location of the surface intermediates or active sites.⁷⁸ The shortage of an accurate spatial identification of the OER active sites (Fe^IV=O) in hematite photoanodes currently hinders the clear determination of the OER mechanism of hematite photoanodes for water splitting. Recently, by employing core-loss electron energy loss spectroscopy (EELS) signals (of several hundred eV), coupled to femtosecond temporal resolution as well as ultrafast energy-filtered 4D TEM, A. H. Zewail et al. have identified that Fe^IV=O has a lifetime of a few picoseconds; they also studied the associated photoinduced electronic transitions and charge transfer processes in hematite photocatalysts, as shown in Fig. 8.⁷⁷ It demonstrates that the direct observation of Fe^IV=O intermediates at hematite photoanodes with spatial, temporal and energy resolution is possible via 4D TEM.

Fig. 8 Left: (a) 3D intensity-energy-time dynamics of the overall Fe L-edge FEELS. (b) Intensity contour map of (a). (c) Top view of Fe L3-edge (above 700 eV) and O K-edge (around 540 eV) FEELS. Right: (a) Electrons centred at 709.5, 699.5 and 689.5 eV energy loss are selected by a slit window of 4 eV for the elemental mapping and the jump-ratio images. (b) Time-dependent EELS of Fe L3-edge with background subtracted. (c) Ultrafast TEM image of a typical Fe₂O₃ particle without energy filtering. Temporal change of iron elemental mapping images (d–f) and jump-ratio images (g–i) of the α-Fe₂O₃ particle obtained by a three-window method with two pre-edge (699.5 ± 2 eV, 689.5 ± 2 eV) and one post-edge (709.5 ± 2 eV), and a two-window method with one pre-edge (699.5 ± 2 eV) and one post-edge (709.5 ± 2 eV), respectively. Reproduced with permission from ref. 77.

3.7 Density functional theory (DFT)

Some other theoretical researchers investigated the nature of surface states in hematite based photoanodes via DFT calculations.^79–81 B. Iandolo et al. investigated the surface states on hematite under photoelectrochemical conditions and calculated the density of states for two relevant hematite atomic terminations via DFT calculations, where the presence and the role of surface states were rationalized. It predicted a Nerstian dependence of the OER onset potential on pH, which was to a very good extent confirmed by PEC measurements and EIS characterization.⁷⁹ Meanwhile, M. C. Toroker et al. performed first-principles calculations with DFT+U on several possible adsorbates at the α-Fe₂O₃(0001) surface, ruling out the existence of a stable peroxo Fe–O–O–Fe adsorbate. They demonstrated that the origin of the surface absorption peak at 580 nm could be a Fe–O˙ type bond that acts as an essential intermediate for water oxidation; the projected density of states (PDOS) of the *O intermediate shows a mid-gap state, which is located far below the conduction band edge, but close to the valence band edge, as shown in Fig. 9.⁸⁰

Fig. 9 Left: Intermediates and free energies at zero bias for water oxidation over pure Fe₂O₃(0001). In the frames, side and top views of the intermediates’ surface: (a) *vac, (b) *OH₂, (c) *OH, (d) *O, and (e) *OOH. Spheres in red, gold, and white represent O, Fe, and H atoms, respectively. Marked in yellow are the surface oxygen atoms. The location of the active site with adsorbates is circled in black. The location of the surface state is circled in blue at the *O intermediate. Right: Projected density of states (PDOS) of five water oxidation intermediates of the pure Fe₂O₃(0001) surface: (a) *vacancy, (b) *OH₂, (c) *OH, (d) *O, (e) *OOH intermediates, and (f) bulk Fe₂O₃. O, Fe, and H total DOS is in red, blue, and green plain lines, respectively. Enlarged is the mid-gap states for the *O intermediate, with the p orbital of O atom at the active site and the d orbital of the neighbouring Fe atom in red and blue broken lines, respectively. DOS is normalized to a maximum total density of one. Reproduced with permission from ref. 80.

3.8 Scanning electrochemical microscopy (SECM)

SECM is a versatile electrochemical platform used in the characterization of electrocatalysts and photoelectrodes to collect products and measure electron transfer kinetics, which is useable in the PEC water splitting field.^82–84 For instance, A. J. Bard et al. employed surface interrogation scanning electrochemical microscopy (PEC SI-SECM) for the quantitative determination of the reaction intermediates, where a light pulse to a hematite film at a given potential generates intermediates that are then analysed by a tip generated titrant at known times after the light pulse.⁸³ The density of photoactive sites on a hematite surface, proposed to be Fe⁴⁺ species in a redox titration, was successfully quantified to be 18 Fe atoms nm⁻²; the pseudo-first order reaction rate constants were determined as 0.03 to 0.19 s⁻¹ based on time-dependent redox titration experiments.⁸³

3.9 Kinetic isotope effect (KIE)

Understanding the mechanism of water oxidation to O₂ represents the bottleneck towards the design of an efficient PEC water splitting device. It is well established that water oxidation on hematite is mediated by surface trapped holes, characterized to be the high valent –Fe=O species. However, the mechanism of the subsequent rate-limiting O–O bond formation step is still a missing piece. Recently, mechanistic studies of water oxidation at hematite photoanodes via kinetic isotope effects have drawn researchers's attention.^76,85,86 For instance, J. C. Zhao et al. demonstrated the pivotal role of proton transfer in water oxidation on hematite photoanodes using PEC characterization, the H/D kinetic isotope effect, and PEIS. They found that the rate-determining interfacial hole transfer to water molecules at the hematite surface is a concerted proton–electron transfer process at pH < 12. A mechanistic transition in the rate-determining step from concerted proton–electron transfer (CPET) to electron transfer (ET) occurred after OH⁻ became the dominant hole acceptor.⁸⁵

4. Engineering of surface states on hematite photoanodes

4.1 Intrinsic modification engineering

4.1.1 Sintering parameters. The sintering parameters, including temperature, period and atmosphere, play a vital role in the PEC performance of hematite based photoanodes because they are supposed to reduce the crystal defects, to affect the donor density via Sn doping from FTO substrates, to create oxygen vacancies (V_O) or to modify the surface states of the photoelectrodes.^87–99 For instance, J. S. Jang et al. examined the role of FTO deformation during the activation of hematite photoanodes on FTO substrates, as displayed in Fig. 10a.⁸⁹ The incorporation of Sn dopants from the FTO substrates into the hematite lattice was confirmed by X-ray photoelectron spectroscopy and was found to increase with sintering time. Specifically, under 800 °C annealing, the FTO substrates underwent a stoichiometric change that directly affected their electrical conductivity; their resistivity was doubled after 20 min of sintering at 800 °C. Activation of hematite photoanodes by high-temperature sintering entails a kinetic competition between Sn dopant diffusion from the FTO substrate into the hematite and the resulting thermal deformation and conductivity loss in the FTO substrates.⁸⁹ Therefore, the increasing photocurrent response is attributed to the enhanced Sn doping and thus the improved donor density in the hematite based photoanodes.^87–89

Fig. 10 (a) Activation of hematite photoanodes by high-temperature sintering entails a kinetic competition between Sn dopant diffusion from the FTO substrate into the hematite, and the resulting thermal deformation and conductivity loss in the FTO substrate. Reproduced with permission from ref. 89 (b) simplified band diagram of hematite electrodes under conditions with (left) and without (right) Fermi level pinning by the sub-conduction band surface states. Reproduced with permission from ref. 90.

It is reported that controlling the sintering temperature may not only induce Sn diffusion into the hematite matrix, but also alter the surface states present at the surface of the hematite based photoanodes.^90–94 For instance, T. W. Hamann et al. demonstrated that annealing hematite at 800 °C substantially improves the water oxidation efficiency of the ultrathin film hematite electrodes, which is ascribed to the removal of the surface states close to the conductance band of the hematite photoanodes and the reduction in recombination and Fermi level pinning, as displayed in Fig. 10b.⁹⁰ Moreover, Y. Li et al. reported a novel fire treatment, which was adopted to activate two types of hematite photoanodes (sol–gel thin films and hydrothermal nanowires), which successfully removed the surface trap states (surface hydroxyls) on hematite and thus resulted in an optimized photocurrent density of 2.63 mA cm⁻².⁹² Additionally, Y. D. Li et al. found that the cooling rate during the sintering process is an important factor deciding PEC water splitting of hematite photoanodes. Rapid cooling of the hematite photoanodes increased the photocurrent from 0.635 mA cm⁻² for the normally cooled sample to 0.856 mA cm⁻² at 1.23 V vs. RHE. The rapidly cooled hematite nanowires showed improved C_trap and decreased R_ct,trap values, indicating the improvement of the hole transfer capability and the acceleration of the surface charge transfer via modifying the surface states, respectively.⁹³

On the other hand, the sintering atmosphere is another important parameter that might affect the PEC performance of hematite photoanodes. It is well established that oxygen vacancies (V_O) in hematite photoanodes can be readily obtained via sintering the electrodes in an oxygen deficient atmosphere.^95–97 Y. Li et al. reported a novel method for the preparation of highly conductive hematite photoanodes through thermal decomposition of FeOOH in an oxygen-deficient atmosphere (N₂ + air). They showed substantially enhanced photoactivity compared to the pristine hematite prepared in air. The oxygen content during thermal activation significantly affects the formation of V_O and thus the photoactivity of hematite nanowires for PEC water oxidation.^95,96 A. J. Cowan et al. proposed that V_O may act as a ‘mixed blessing’, with the enhanced electrical properties being balanced with a potential increase in trap-mediated electron–hole recombination at the surface defect sites introduced.⁹⁶ Recently, L. Z. Wang et al. carried out a comprehensive analysis of the key charge transfer and surface reaction steps in PEC processes on hematite photoanodes to understand the role of V_O in hematite photoanodes. Moreover, they also clarified that V_O can facilitate the surface electrocatalytic processes while leading to severe interfacial recombination at the semiconductor/electrolyte interface, in addition to the well-reported improvements in bulk conductivity.⁹⁷ Additionally, H₂ treatment during the sintering process could introduce vacancies as well as mid-gap states in the photoactive semiconductor for water splitting.^98–100 For instance, G. A. Ozin et al. presented a means of activating hematite towards PEC water splitting via thermal treatment of ultrathin films of hematite under an atmosphere of 5% H₂; it was determined that V_O is the most likely structural change following hydrogen treatment. Surface state analyses using electrochemical techniques revealed a probable change in the mechanism of PEC water oxidation as well as the appearance of a new surface state coincident with the onset of photocurrent.⁹⁹

4.1.2 Acid/alkali media surface treatments. It has been reported that the surface states of hematite electrodes can be modified via treating the hematite photoanodes in acidic or alkaline media, leading to enhanced charge separation efficiency and thus improved PEC water splitting performance.^101–103 Y. Li et al. investigated the effect of acidic media (including acetic acid, HCl, HNO₃, and H₃PO₄) treatment on hematite photoanodes. They demonstrated that a simple acid treatment method modifies either the nature or position of the available trap sites, which substantially enhances the efficiency of electrons moving out of traps in hematite nanowire photoanodes, and thus reduces the electron–hole recombination loss and improves PEC performance.¹⁰¹ Meanwhile, J. H. Ye et al. found that the photocurrent densities increased 3- and 2-times for α-Fe₂O₃ and Ti:α-Fe₂O₃ photoanodes after KOH treatment, respectively. They demonstrated that a conformal thin layer grafted with hydroxyl (–OH) groups was formed on the hematite surface, which behaved like an electrocatalyst to accelerate the water oxidation kinetics on hematite photoanodes.¹⁰²

4.1.3 Re-growth. Disordered hematite structures can be synthesized at low cost and on a large scale; however, they typically suffer from charge recombination and poor PEC water splitting performance. Additionally, the structural disorders on or near the hematite surfaces are important causes of the low photovoltages. Recent investigations reveal that the surface disordered hematite structures can be corrected by a re-growth strategy, which pave the way for producing efficient photoanodes based on hematite.^104,105 For instance, D. W. Wang et al. developed a facile regrowth strategy to reduce surface disorders and as a consequence, a turn-on voltage of 0.45 V (versus reversible hydrogen electrode) was achieved.¹⁰⁴ Moreover, A. Tricoli et al. employed capillary force-induced re-structuring and chemically-induced re-growth methods, to drastically increase the photocurrent by 24 times from 0.032 mA cm⁻² of the initially disordered hematite photoanodes to 0.79 mA cm⁻² at 1.23 V vs. RHE under simulated 1 sun irradiation.¹⁰⁵

4.2 Extrinsic engineering

4.2.1 Doping/a secondary semiconductor coating. Considerable attention has been drawn to the development of composite hematite photoanodes with doping or secondary semiconductors, to further enhance the PEC water oxidation efficiency.^106–116 Meanwhile, other researchers spared no effort in correlating PEC performance improvement with the surface state evolution upon doping or coating a secondary semiconductor onto hematite photoanodes.^117–128 For instance, D. Monllor-Satoca et al. prepared mesoporous hematite–titania composite films by mixing the respective preformed nanoparticles obtained by a non-aqueous sol–gel route in a wide range of loading levels (0–20 mol%). Voltammetric and EIS studies were performed observing an optimum 10% doping, with a 15-fold photocurrent increase (up to 1.3 mA cm⁻² at 1.23 V vs. RHE) and a 100-fold decrease of the charge transfer resistance. The roles of surface states and donor densities were highlighted, assuming a charge transfer mechanism through hole trapping at surface states and its isoenergetic transfer to water; an optimum 10–15% doping range was obtained, where the maximum overlapping between surface and water states was prevalent (Fig. 11). The tailoring of the donor states (conductivity), phase coexistence (solubility), heterojunctions (energetics) and surface states (kinetics) in the hematite composite photoanodes plays a decisive role in determining the PEC water oxidation performance.¹¹⁷

Fig. 11 Kinetic scheme of the charge generation and transfer processes at the biased (1.23 V_RHE) semiconductor electrolyte interface (SEI) under illumination and for different titania doping levels, including: (a) no doping (0%), (b) low or large doping (5% and/or 20%) and (c) optimum doping (10–15%). Grey and white areas refer to electron filled or empty states, respectively. The dotted lines inside the conduction band (CB) filled states denote photogenerated electrons with the same relative area as the empty states at the valence band (VB); the enlarged grey areas highlight the bulk (core) doping levels, which are shown intentionally oversized for comparison purposes. The green arrow refers to the charge generation process upon visible photon absorption (hν); the purple arrow refers to the hole trapping process at surface states (SS); the orange arrow refers to the hole transfer process from SS to water molecules; the red arrow refers to (photo)electron transfer from semiconductor CB states to the underlying conducting substrate (FTO). The thickness and shape of the arrows indicate the relative rates of the charge transfer processes, where the dotted line represents the slowest rate (a) and the thickest line represents the fastest rate (c). The light blue shaded areas refer to the relative overlapping of the SS and water density of states (DOS). Note that the relative size of the SS distribution for the non-doped sample (a) has been intentionally enlarged to highlight the overlapping between the semiconductor and the redox couple states. D_redox: normalized DOS of the redox couple in the electrolyte; D_sc: DOS of the semiconductor; E: electrode potential; E_c,s: surface CB edge potential; E_F: Fermi level of the semiconductor that matches the electrode potential (E) and the O₂/H₂O couple thermodynamic potential (1.23 V_RHE); E_SS: center potential of the SS distribution; E_v,s: surface VB edge potential; HP: Helmholtz plane at the SEI, across which the charge transfer process occurs; l: redox couple reorganization energy. Reproduced with permission from ref. 117.

C. Li et al. systematically investigated the energy distribution of surface states (C_ss) and dj/dV on pure Fe₂O₃, Ti–Fe₂O₃, Al₂O₃/Ti–Fe₂O₃ and H₂O₂/Ti–Fe₂O₃ electrodes.¹¹⁸ As displayed in Fig. 12, there is only one surface state present in pure Fe₂O₃, Ti–Fe₂O₃ and H₂O₂/Ti–Fe₂O₃, whereas there are two kinds of surface states appearing on the Al₂O₃/Ti–Fe₂O₃ electrode; additionally, compared to the energy levels of the surface state peaks in pure Fe₂O₃, those of Ti–Fe₂O₃, Al₂O₃/Ti–Fe₂O₃ and H₂O₂/Ti–Fe₂O₃ have been shifted close to the valence band edge.¹¹⁸ Therefore, these results reveal that surface Ti modification (Ti–Fe₂O₃ electrode), Al₂O₃ coating by ALD (Al₂O₃/Ti–Fe₂O₃ electrode), and H₂O₂ treatment (H₂O₂/Ti–Fe₂O₃ electrode) indeed modify the energy levels and density of the surface states and the corresponding PEC water splitting performance of hematite based photoanodes. B. X. Zhou et al. revealed that coating of ZnFe₂O₄ and ZnO layers onto hematite modified the surface states of hematite and thus affected its PEC performance.¹²⁶ C. M. Li et al. employed a co-doping method by sequential in situ Ti-doping and ex situ Mg-doping in order to tailor the surface states of hematite photoanodes for great improvement of the charge transfer at the interface and the subsequent transport ability by the suppressed charge recombination. This resulted in about 11-fold higher photocurrent than that of undoped Fe₂O₃ at 1.23 V vs. RHE. This was mainly due to Mg and Ti-doping into Fe₂O₃ modifying the surface states enabling participation of more holes in water oxidation and better kinetics that enhanced charge transfer and suppressed charge recombination for efficient water oxidation.¹²¹ Additionally, T. A. Kandiel et al. reported that Ba- and Sr-modified hematite photoanodes exhibited higher density of surface states and higher charge transfer rate constants at low bias potential than those of bare hematite, which might be the reason for the higher photocurrent observed for Ba- and Sr-modified hematite photoanodes.¹²⁸

Fig. 12 The relation between surface states and photocurrent based on the results of CV (black), C_ss (blue) and dj/dV (orange) resulted from the electrodes of (a) Ti–Fe₂O₃, (b) pure Fe₂O₃, (c) Al₂O₃/Ti–Fe₂O₃, and (d) H₂O₂/Ti–Fe₂O₃ photoanodes. The C_ss is fitted with Gauss plots (green line) and the iR-drop is compensated. (e) The schematic illustration of photo-electrochemical water oxidation on a hematite photoanode with surface states as intermediates. Reproduced with permission from ref. 118.

4.2.2 Oxygen evolution catalyst (OEC) overlayer deposition. The deposition of oxygen evolution catalysts (OEC) onto the hematite photoanodes would alter the surface states and thus enhance the PEC performance of hematite photoanodes.^129–139 For instance, G. A. Ozin et al. demonstrated that for the case of hematite/iridium catalyst composite photoanodes, the iridium catalyst not only improved the kinetics of the water oxidation reaction both in the dark and under illumination, but also increased the density of photoholes at the surface and decreased the rate of recombination of majority charge carriers with these surface-bound charges at moderate to high applied potentials.¹²⁹ A. B. F. Martinson et al. reported that hematite photoanodes coated with an ultrathin Co(OH)₂/Co₃O₄ layer by atomic layer deposition (ALD) resulted in significantly enhanced photoelectrochemical water oxidation performance. The enhanced PEC performance (100–200 mV cathodic shifts in the photocurrent onset potential, 0.81 mA cm⁻² photocurrent responses at 1.23 V vs. RHE) is attributed to modified surface states which enable faster charge transfer from the surface states of Co(II)/Co₃O₄-modified hematite photoanodes than the charge transfer from the surface states of bare hematite electrodes.¹³⁰ It was reported by T. W. Hamann and D. R. Gamelin et al. that the deposition of cobalt phosphate (Co-P_i) onto the hematite photoanode surface changes the nature of the surface states in hematite and thus could introduce an alternative pathway for water oxidation catalyzed by Co-P_i, which allows the composite photoanode to avoid positive charge accumulation at the a-Fe₂O₃ surface.^131,132

Meanwhile, D. W. Wang et al. reported that when a thin layer of a heterogenized molecular Ir catalyst (het-WOC) is applied onto a hematite photoanode, the photocurrent response is improved from ca. 0.25 mA cm⁻² to 0.66 mA cm⁻² at 1.23 V vs. RHE in an acidic electrolyte.¹³³ A detailed mechanistic investigation about the system via PEIS and IMPS reveals that the enhanced PEC performance is primarily due to the improved charge transfer (42 fold), while the surface recombination rate remains unchanged. In stark contrast, heterogeneous oxide catalyst (IrO_x) coatings improve the PEC performance of hematite by significantly reducing the surface recombination rate. These results suggest that the het-WOC provides additional charge-transfer pathways across the Fe₂O₃|H₂O interface, while IrO_x and similar bulk metal-oxide catalysts replace the Fe₂O₃|H₂O interface with a fundamentally different one, as displayed in Fig. 13.¹³⁴ Recently, S. Eslava et al. unravelled the nature of the interactions between different loadings of an electrocatalyst (CoFeO_x) and a hematite semiconductor via a combination of electrochemical approaches of PEIS and IMPS. They found that a thin CoFeO_x layer mainly reduces surface charge recombination, while an extremely thin CoFeO_x layer enhances charge transfer kinetics; an interlayer of GaO_x modifies the surface state distribution and increases the charge transfer rate.¹³⁷

Fig. 13 Top: Schematic illustrations of the kinetic models for (a) bare hematite (b) hematite with het-WOC and (c) hematite with IrO_x. Bottom: Mechanistic perspective of bare hematite (a), hematite with het-WOC (b) and hematite with IrO_x (c). Reproduced with permission from ref. 134.

4.2.3 Doping, secondary semiconductors and OEC overlayers. Other researchers have devoted their efforts to the combination of secondary semiconductors and OEC overlayers to modify the donor density and the surface states of hematite photoanodes for further enhancing the PEC performance of hematite-based photoanodes.^{14,120,125,140} X. L. Zheng et al. reported a facile and rapid three-step approach to simultaneously reduce the bulk, interface, and surface recombinations for a hematite nanorod array based photoanode, leading to a greatly improved photocurrent density at low bias potential.¹⁴⁰ In this strategy, flame-doping enhances the Ti doping concentration without hampering the morphology and surface properties of the hematite nanorods, which effectively reduces both the bulk and surface recombinations. Then, the addition of a dense layer between the hematite nanorods and the FTO glass substrate reduces the interfacial recombination by suppressing the electron back-injection into the electrolyte. Finally, sequential oxalic acid etching and FeOOH deposition improve both the interface quality between the FeOOH electrocatalyst and the hematite nanorods and the surface catalytic activity.¹⁴⁰

Meanwhile, J. T. Ma et al. fabricated a NiO/P-Fe₂O₃ composite photoanode which produced a high photocurrent density of 2.08 mA cm⁻² at 1.23 V vs. RHE and a 110 mV cathodic shift of the onset potential, which was ascribed to the improved electrical conductivity and the modified surface states because of the P doping and NiO coating.¹²⁰ Moreover, J. T. Ma et al. further demonstrated that F-doping and NiOOH coating onto hematite photoanodes resulted in enhanced PEC water splitting performance, fulfilling the requirements of low surface trapping sites, high conductivity, efficient charge separation and injection efficiency, as shown in Fig. 14.¹²⁵

Fig. 14 Top: (a) Nyquist plots of the samples at 1.05 V vs. RHE under illumination and (b) chronoamperometry of all photoanodes in a period of 2 h. Bottom: Energetic schemes for (a) Fe₂O₃, (b) F-Fe₂O₃, (c) Fe₂O₃/F-Fe₂O₃ NRs and (d) NiOOH/Fe₂O₃/F-Fe₂O₃ photoanode, where k_rec refers to the surface recombination and k_ct represents charge transfer through surface states. Reproduced with permission from ref. 125.

Recently, we have reported the fabrication of ITO/Fe₂O₃/Fe₂TiO₅/FeNiOOH multi-layer nanowires (Fig. 15) for PEC water splitting and elucidated the mechanism underlying the interfacial coupling effect of the quaternary hematite composite photoanode.¹⁴ In this work, secondary semiconductors (ITO, Fe₂TiO₅) and OECs were combined to modify the donor density and surface states in the composite photoanodes, delivering 2.2 mA cm⁻² at 1.23 V vs. RHE and 1 sun, which is 10 times higher than that of pristine hematite photoanodes. A deep and careful investigation into the mechanism responsible for the enhanced PEC performance of ITO/Fe₂O₃/Fe₂TiO₅/FeNiOOH multi-layer nanowires shows that: (i) the ITO underlayer is used as a high quality electrical back contact to reduce the back-contact interface charge recombination and as a Sn doping source for tuning the donor density, (ii) the atomic level Fe₂TiO₅ coating serves as the surface state density and energy level modulation layer, effectively suppressing the charge recombination at the semiconductor junction interface, (iii) the FeNiOOH nanodots increase the surface active sites, and thus accelerate the OER kinetics at the SEI. The interfacial coupling effect between the ITO underlayer, the ultrathin Fe₂TiO₅ layer and the FeNiOOH nanodots in the ITO/Fe₂O₃/Fe₂TiO₅/FeNiOOH photoanodes simultaneously enhances the charge separation and water oxidation efficiency.¹⁴ Most recently, we integrated Fe₂O₃ nanowires with an ultrathin Fe₂TiO₅ heterojunction and CoFe-Prussian blue analogue (PBA) decoration for enhanced PEC water splitting in an acidic electrolyte (pH = 1). Thanks to the combination of core–shell Fe₂O₃/Fe₂TiO₅ type II heterojunction nanowires and the catalytic function of CoFe–PBA, Fe₂O₃/Fe₂TiO₅/CoFe–PBA composite photoanodes are able to deliver 1.25 mA cm⁻² photocurrent at 1.23 V vs. RHE, almost one order of magnitude photocurrent increment in comparison to the pristine Fe₂O₃ nanowires. A systematic electrochemical investigation reveals that the enhanced PEC performance of the Fe₂O₃/Fe₂TiO₅/CoFe–PBA composite electrode can be attributed to the modified surface state density after successive coatings.¹⁴¹

Fig. 15 Top: Thermodynamic scheme for the charge transfer processes at 1.23 V vs. RHE of SEI under illumination for the Fe₂O₃, ITO/Fe₂O₃, ITO/Fe₂O₃/Fe₂TiO₅, and ITO/Fe₂O₃/Fe₂TiO₅/FeNiOOH electrodes. The black arrows indicate the bandgap of In₂O₃ (ITO matrix), hematite and Fe₂TiO₅. The dark blue arrow refers to the hole transfer process at the heterojunction interfaces. The purple arrow refers to the electron transfer process present at the heterojunction interfaces. For simplicity, interfacial charge transfer is considered to occur through the E_v and/or E_c states directly, without the intervention of interfacial SS. In all photoanodes, 4 electron–hole couples per visible active semiconductor are depicted. In₂O₃ (main ingredient of ITO matrix): green; Fe₂O₃: red; Fe₂TiO₅: indigo; FeNiOOH: yellow. Bottom: The atomic supercell models illustrate the interfaces existing at In₂O₃ (ITO matrix), hematite, Fe₂TiO₅ and FeNiOOH species in the integrated electrodes. Reproduced with permission from ref. 14.

5. Conclusions and prospects

Their earth abundant nature and excellent visible-light response endow hematite based photoanodes with promising photoelectrochemical water oxidation activity in the field of PEC water splitting. Studies centred on hematite based photoanodes will be beneficial for the future development of other photoelectrodes. Considerable work has been performed to control the composition, morphology, and nanostructure of hematite based photoanodes to improve their PEC water splitting performance. Although significant progress has been achieved, great efforts are still required to further explore the surface state mediated charge transfer mechanism of hematite based photoanodes for PEC water oxidation. In this review, we have summarized the recent progress in hematite photoanodes for PEC water splitting and highlighted the critical role of surface states in the development of high-performance hematite based photoanodes for PEC water splitting application. As a model system, the approach summarized here of hematite composite photoanodes might also be employed for the investigation of other surface state mediated photoelectrodes for PEC water splitting, including TiO₂, BiVO₄, CuWO₄, GaN, CuFeO₂, Ta₃N₅, and Cu(In,Ga)(Se,S)₂.^{41–48,98,142,143} The following three aspects deserve special attention in the future investigations of hematite based photoanode systems in order to definitely improve their PEC efficiency:

(1) Typically, previous investigations attributed the enhanced PEC water splitting performance of hematite composite photoanodes either to the improved donor density or the changed surface states. Indeed, it is essential to simultaneously monitor the evolution of the donor density and surface states of hematite based photoanodes upon doping, type II heterojunction construction and OEC deposition.

(2) Previous studies on hematite based photoanodes have mainly focused on the strategy for the improvement of PEC water oxidation performance via the design of nanostructures, doping, heterojunction and OEC deposition. Few studies have reported the investigation about the mechanisms that are responsible for the enhanced PEC performance. Several previous mechanistic investigations have focused on simple hematite based photoanode systems. However, the main obstacle is to correlate the charge transfer efficiency at the SEI with the evolution of surface states upon coating with a secondary semiconductor or OEC. More hematite based composite photoanode systems should be extensively explored to overcome these obstacles.

(3) Most of the designing of composite hematite based photoanodes are limited to pristine or binary systems rather than ternary or quaternary composite photoelectrode systems. The integration of ternary or quaternary composite photoelectrodes with a secondary underlayer, overlayer and OEC is favourable for the improvement of the hematite-based photoanodes because the charge recombination simultaneously occurs at the different interfaces, including the back contact, bulk interfaces and SEI interfaces. Therefore, extending exploration of more complex hematite-based photoanodes might be of high interest.

(4) Various techniques have been employed to probe the nature of the surface states in hematite-based photoanode systems, including PEIS, CV, IMPS, IMVS, DRT, in situ UV-vis, in situ ATR-IR, 4D TEM, DFT, SECM and KIE. Nevertheless, the accurate observation of surface states/surface intermediates of hematite photoanodes with spatial, temporal and energy resolution is still limited. Further deep investigations about PEC water oxidation in hematite photoanodes will require emerging in situ techniques, like atomic force microscopy,¹⁴⁴ single-molecule super-resolution fluorescence microscopy,^78,145in situ (S)TEM,¹⁴⁶ surface photovoltage microscopy (SPVM),¹⁴⁷ on-line inductively coupled plasma mass spectrometry (ICP-MS),¹⁴⁸ and dual-working-electrode photoelectrochemistry techniques.^149–151

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors acknowledge funding from Generalitat de Catalunya 2017 SGR 327, the Spanish MINECO project ENE2017-85087-C3 and the BIST Ignite project InWOC2. ICN2 is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706) and is funded by the CERCA Programme/Generalitat de Catalunya. Part of the present work was conducted in the framework of the Universitat Autònoma de Barcelona Materials Science PhD program.

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