Engineering Surface States of Hematite Based Photoanodes for Boosting Photoelectrochemical Water Splitting

: Hematite-based photoanodes are promising candidates for photoelectrochemical water splitting. However, the performance of pristine hematite semiconductors is unsatisfactory due to the charge recombination occurring at the 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 oxygen evolution electrocatalyst. Most of the previous studies attributed the enhanced PEC water splitting performance to the changes on 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. Albeit, the role of surface states presented at the semiconductor/electrolyte interfaces of hematite-based photoanodes has been overlooked in the previous investigations, which virtually plays 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 sates at hematite photoanodes and highlight the important role of modifying surface sates in the development of high performance hematite based photoanodes for photoelectrochemical water splitting application. The challenges and future tendencies in the study of hematite based photoanodes are also discussed.


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 environmental 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 natures 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][5][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][8][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 photochemically stability, a narrow bandgap (1.9-2.2 eV), and a theoretical maximum solar-to-hydrogen (STH) efficiency of 15.4%. [10][11] However, its 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, 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 oxygen evolution electrocatalyst. [11, However, the investigation about the surface states mediated charge transfer at the semiconductorelectrolyte 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 its 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 the 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 at hematite via different techniques; (iii) Engineering of surface states at hematite photoanodes.

The Fundamental Concept of Photoelectrochemistry and Surface
States Figure 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 Eg,1, and a photocathode with Eg,2 (where Eg,1>Eg,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, , 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, , sufficiently to surmount the overpotential for oxidation (ηO) 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. EF, Fermi energy; E(VRHE), electronic potential with respect to the reversible hydrogen electrode; ηR, overpotential for reduction. Reproduced with permission from ref. [10] The free energy required for the conversion of 1 H2O molecule to H2 and ½O2 molecules under standard conditions is ∆G= 237.2 kJ mol -1 . [3] According to the Nernst equation, it corresponds to ∆E° = 1.23 V/transferred electron. Theoretically, a semiconductor with a band gap energy (Eg) larger than 1.23 eV can drive the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) using electrons/holes generated under illumination ( Figure 1) if it has a conduction band edge energy (Ecb) and valence band edge energy (Evb) that straddles 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 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/generated electron-hole pair. [38][39] 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 as surface states/mid-gap states. [40] The surface states presented 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 on 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, Cu(In,Ga)(Se,S)2 photoelectrodes for PEC water splitting has drawn plenty of attention. [41][42][43][44][45][46][47][48]  (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 Ur. At the interface of length δL, hole transfer to the redox level Eredox can take place (b) directly from the valence band (kinetic constant kvb) 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 (βn and βp) and detrapping (εn, εp) and hole transfer from surface states (ks) at the energy level Ess, in competition with direct hole transfer. In these schemes Ec and Ev are the energies of the lower edge of the conduction band and the higher edge of the valence band, respectively. EFn and EFp are the quasi Fermi levels, Φn and Φp are the injection barriers of electrons and holes, Evac is the local vacuum level, ϕ is the local electrostatic potential and V is the applied voltage. Reproduced with permission from ref. [51] 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 Figure 2 to show the effect of surface states-assisted electron-hole recombination and indirect charge transfer processes on photoelectrochemical water splitting. At low bias potential, direct holes 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 states-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 presented at the photoelectrodes and to deeply understand the critical role the surface states play in the PEC water splitting process.

Photoelectrochemical Impedance Spectrum (PEIS)
The photoelectrochemical oxygen evolution process on hematite involves the 4 electrons transfer/O2 molecule. It is hypothesized that the OER reaction steps involve the formation of the higher-valent iron states at the surface by hole capture. Equations and Fe(V) intermediates in this scheme can also act as electron acceptors, so that surface recombination reactions of the kind shown in equations (5-6) 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 equations (1)(2)(3)(4). [53] hv → h + e (1) Fe(IV) + e → Fe(III)   (7): Where, Rser is the series resistance, Csc is the space charge capacitance, jh is the current density corresponding to the flux of holes reaching the interface, ω is the radial frequency. kt and kr 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 sates present at photoelectrodes. [54][55][56][57][58][59] In order to illustrate the PEC  Reproduced with permission from ref. [54] However, the general ECM proposed in Figure 4b, including the surface-states holetrapping process, cannot unambiguously fit the EIS because it does not discriminate between Rct,bulk and Rct,trap. Therefore, two simplifications of this general ECM have been employed, as shown in Figure 4c  In addition to these established PIT analysis methods, there is another approach, DRTbased 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 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 (yellow, medium dash), 500 mV s -1 (green, short dash) and 1000 mV s -1 (blue, dots). Reproduced with permission from ref. [63] T. W. Hamann et al. demonstrated that the surface states trapped holes in hematite photoanodes could be measured by using cyclic voltammetry in the dark. [63] Specifically, holes could accumulate on the hematite photoanodes 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 Figure 5, the cathodic current peak observed in the dark cyclic voltammetry scan is well consistent with the Css peak obtained in the fitted EIS data, indicating the presence of surface states in hematite photoanodes. [63] 3.4 Operando UV-vis  [70] 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    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 postedge (709.5 ± 2 eV), respectively. Reproduced with permission from ref. [77] Nonetheless, the aforementioned mechanism 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

Kinetic Isotope Effect (KIE)
Understanding the mechanism of water oxidation to O2 represents the bottleneck towards the design of 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-  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][88][89] It is reported that controlling the sintering temperature may not only induce the Sn diffusing into the hematite matrix, but also alter the surface states present at the surface of the hematite based photoanodes. [90][91][92][93][94] Figure 11. Kinetic scheme of the charge generation and transfer processes at the biased (1.23 VRHE) 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 than the empty states at the valence band (VB); the exceeding 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 photons absorption (hv); 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. Dredox: normalized DOS of the redox couple in the electrolyte; Dsc: DOS of the semiconductor; E: electrode potential; Ec,s: surface CB edge potential; EF: Fermi level of the semiconductor that matches the electrode potential (E) and the O2/H2O couple thermodynamic potential (1.23 VRHE); ESS: center potential of the SS distribution; Ev,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] Plenty of 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][107][108][109][110][111][112][113][114][115][116] Meanwhile, other researchers spared no effort in correlating PEC performance improvement with the surface states evolution upon doping or coating of a secondary semiconductor onto hematite photoanodes. [117][118][119][120][121][122][123][124][125][126][127][128] For  dj/dV on pure Fe2O3, Ti-Fe2O3, Al2O3/Ti-Fe2O3 and H2O2/Ti-Fe2O3 electrodes. [118] As displayed in Figure 12, there is only one surface state present at the pure Fe2O3, Ti-  Recently, we have reported that the fabrication of ITO/Fe2O3/Fe2TiO5/FeNiOOH multilayer nanowires ( Figure 15) for PEC water splitting and elucidated the mechanism underlying the interfacial coupling effect of the quaternary hematite composite photoanode. [14] In

Conclusions and prospects
An earth abundant nature and excellent visible-light response endow hematite based photoanodes with promising photoelectrochemical water oxidation activity in the fields of PEC water splitting. Studies centred on the 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 its PEC water splitting performance. Although significant progress has been achieved, great efforts are still required to further explore the surface states mediated charge transfer mechanism of hematite based photoanodes for PEC water oxidation. In this review, we have summarized the recent progresses on hematite photoanodes for PEC water splitting and highlighted the critical role of surface sates in the development of high-performance hematite based photoanodes for the 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 sates mediated photoelectrodes for PEC water splitting, including TiO2, BiVO4, CuWO4, GaN, CuFeO2, Ta3N5, Cu(In,Ga)(Se,S)2. [41-48, 98, 142-143] The following three aspects deserve special attention in the future investigation of hematite based photoanodes system in order to definitely improve their PEC efficiency: (1) Typically, the previous investigation 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. in hematite photoanodes will require emerging in-situ techniques, like atomic force microscope, [144] single-molecule super-resolution fluorescence microscopy, [78,145] insitu (S)TEM, [146] surface photovoltage microscopy (SPVM), [147], on-line inductively coupled plasma mass spectrometer (ICP-MS), [148] and dual-working-electrode photoelectrochemistry techniques. [149][150][151]