Molecular Origin and Electrochemical Influence of Capacitive Surface

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Molecular Origin and Electrochemical Influence of Capacitive Surface States on Iron Oxide Photoanodes Yelin Hu, Florent Boudoire, Iris Herrmann-Geppert, Peter Bogdanoff, George Tsekouras, Bongjin Simon Mun, Giuseppino Fortunato, Michael Grätzel, and Artur Braun J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08013 • Publication Date (Web): 04 Jan 2016 Downloaded from http://pubs.acs.org on January 23, 2016

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The Journal of Physical Chemistry

Molecular Origin and Electrochemical Influence of Capacitive Surface States on Iron Oxide Photoanodes

Yelin Hua,b, Florent Boudoirea,c, Iris Hermann-Geppertd,e, Peter Bogdanofff, George Tsekourasa, Bongjin Simon Mung, Giuseppino Fortunatoh, Michael Graetzelb, Artur Brauna*

1

Laboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dübendorf, Switzerland 2

Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland

3

Department of Chemistry, University of Basel, Spitalstr. 51, CH-4056 Basel, Switzerland 4

Institute for Materials Research, Sustainable Energy Technology, Helmholtz-Zentrum Geesthacht, D-21502 Geesthacht, Germany 5

Institute for Materials Technology, Helmut-Schmidt University, D-22043 Hamburg, Germany 6

Institute for Solar Fuels, Helmholtz-Zentrum Berlin für Materialien und Energie, D-14109 Berlin, Germany

7

Department of Physics and Photon Science, Ertl Center for Electrochemistry and Catalysis, Gwangju Institute of Science and Technology, Gwangju, Korea

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8

Protection and Physiology, Empa, Swiss Federal Laboratories for Materials Science and Technology, CH-9014 St. Gallen, Switzerland

*Corresponding author: [email protected]

Abstract The origin, the nature and the electronic structure of surface defects causing surface states on metal oxides, and their role in solar water splitting have been under scrutiny for several decades. In the present study, the surface of hematite films is treated with an oxygen plasma and then subject to a detailed investigation with electroanalytical methods and element orbital specific x-ray spectroscopy. We observe a systemic variation of photoelectrochemical properties with oxygen treatment time. Fe 2p and O 1s core level X-ray photoelectron spectra and resonant valence band photoemission at the Fe 3p edge reveal the filling of prevalent oxygen vacancies with concomitant oxidation of Fe2+ to Fe3+ upon the oxygen treatment. DC bias dependent impedance spectra confirm how a prevalent capacitive surface state, which evolves parallel with the photocurrent onset potential, becomes diminished upon oxygen treatment. Surface states of iron induce higher reactivity towards water oxidation than oxygen surface states. The correlation between oxygen vacancy filling, concentration of surface states and photocurrent density in the course of treatment confirms that the surface defects are of a capacitive nature and that the onset of water splitting can be considered as a result of dielectric breakdown in an interfacial hydroxide layer between photoanode and water.

Introduction


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Hydrogen production by solar water splitting in photoelectrochemical cells (PECs) has attracted much attention in artificial photosynthesis1 owing to its promise for direct conversion of sunlight into chemical fuels. Semiconductor photoanodes are utilized to absorb photons which generate electron holes via photoexcitation, which are expected to oxidize the water at the semiconductor surface and thus produce hydrogen. Photoanode materials must satisfy a number of physical requirements related to visible light absorption, charge carrier separation and transportation, oxygen generation kinetics, electronic band structure (conduction and valence band energy levels) and stability in alkaline electrolytes. No single material has met all such requirements; consequently, research efforts have been carried out in aspects of nanotechnology and catalysis in order to improve photoanode properties. Hematite (α-Fe2O3) is a prominent photoanode material for water splitting due to its abundance, stability, environmental compatibility, valence band edge position and suitable band gap for solar applications. Despite these attractive characteristics, hematite photoanodes suffer particularly from a short hole diffusion length (2-4 nm)2 in comparison to long light absorption length (α-1 = 118 nm at λ = 550 nm). Because of this, only those photogenerated holes which are generated close to the surface can reach the semiconductor-liquid junction (SCLJ). Considerable efforts to optimize the nanostructured morphologies of hematite have resulted in significant improvements in performance3. While nanostructuring of hematite is a successful strategy to increase the performance of hematite photoanodes, an additional problem seems to be relevant, namely a low oxygen evolution rate constant at the SCLJ4 attributed to recombination of holes and electrons at surface states. Surface states5 can act adversely as recombination centers, but may also have a positive effect such as intermediate states promoting charge transfer. This dualism requires henceforth a detailed understanding of surface electronic states and is crucial in devising strategies to control and optimize the water splitting properties of hematite photoanodes. It is believed that high efficiencies can only be achieved by minimizing recombination of electrons and holes 3 ACS Paragon Plus Environment


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particularly on the surface6. During the process of water oxidation, photoexcited holes accumulate on surface states which eventually transfer to water molecules which are then oxidized. From the electrochemical point of view, the charging/discharging process on surface states can be identified by quantitative modelling of electrochemical impedance spectroscopy (EIS)7, elucidating the causal relationship between surface states and photocurrent density8-9. We have recently conducted an operando near edge X-ray absorption fine structure (NEXAFS) study under actual PEC water splitting conditions. This study produced two different photogenerated hole states in the valence band near the Fe3d-O2p hybridized states, the spectral weight of both scaled with a parabolic profile and with a Gaussian profile, respectively, during scanning the DC bias potential10. Such two hole states have been postulated over 35 years ago by Kennedy et al.2. Entirely independent from this and at about the same time when we published our NEXAFS study, an impedance study was published which produced a similar pattern of surface states on hematite, matching part of our NEXAFS data astonishingly well11. Dare-Edwards et al.4 have postulated that valence band (VB) holes at the hematite surface occur in Fe3+ 3d energy levels, rather than in O 2p energy levels. Although the above-cited “old” works have pointed to the crucial role of surface hole accumulation on the photocurrent, it is as yet unclear how the distribution of surface states and resultant photoelectrochemical properties are altered by surface composition. In this study hematite photoanodes were prepared by dip coating and modified by oxygen plasma with varying treatment periods in order to investigate photoelectrochemical properties under water-splitting conditions. Oxygen plasma etching is a common cleaning method that may influence the chemistry of surface layers12-15 and ultimately PEC performance13, 15,16. The surface chemistry and electrical properties of as-prepared and of oxygen plasma-treated hematite films were investigated by X-ray photoelectron spectroscopy (XPS), resonant photoemission spectroscopy (RPES) and EIS. We found that upon oxygen plasma treatment 4 ACS Paragon Plus Environment

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oxygen vacancies are filled, the concentration of surface states is decreased and the photocurrent density is lowered. These observations demonstrate the crucial importance of surface structure with regard to light-driven water oxidation.

EXPERIMENTAL SECTION Hematite Films Preparation Hematite sample preparation was carried out in an extracted fume hood and based on the method detailed in Figure 1 in

17

and rewritten here for the convenience of the reader. As shown in

17

, this coating method is reproducible and yields typically around 160 nm

hematite thickness per dip. Briefly, iron(III) nitrate (28.0 g) was gradually added to a beaker containing oleic acid (17.0 g) at 120 °C with stirring to obtain a homogeneous solution. This solution was heated at 120 °C until the evolution of brown-colored NO2(g) ceased, which took ~ 2h. A reddish brown mass was formed, which was then cooled to room temperature and subsequently dried for 24 h. The resultant dried mass was then mixed with 70 mL of tetrahydrofuran (THF) until completely dissolved, followed by high speed centrifugation at 4000 rpm for 0.5 h. Excess THF from the supernatant solution was allowed to evaporate over several hours until a concentrated iron-oleate precursor was achieved. Iron-oleate films were deposited on fluorine-doped tin oxide (FTO)-coated glass (12 × 30 × 2 mm, 10 Ω/sq, XOP FÍSICA, S.L, España) using a dip coating machine (DipMasterTM-50, Chemat Technology Inc., USA). After removal of the precursor solution from the back side of the FTO glass with acetone, wet iron-oleate films were dried at 75 °C for 5 min followed by direct transfer into a furnace at 500 °C for 30 min calcination in air. The photoelectrode geometrical area was typically around 1.0 to 1.5 cm2. Dip-coating and calcination procedures were repeated in order to obtain a total of four hematite layers, which was known from previous experience to be a reproducible method to give optimal water splitting photocurrent17. This procedure 5 ACS Paragon Plus Environment


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makes films which can be indexed with the hematite phase Bragg reflections in XRD (Figure S1).

Oxygen Plasma Treatment of Hematite Films Oxygen plasma treatment was performed in a plasma etcher by Diener electronics at Optotransmitter Umweltschutz Technologie e.V. (OUT e.V. Berlin, Germany). The part of the FTO glass not coated with hematite was covered with glass during treatment in order to avoid modification of the FTO and thereby maintain good electrical back contact to the hematite film. After evacuation of the process chamber an oxygen partial pressure of 0.2 mbar was achieved by flowing oxygen at a rate of 20 sccm. Samples were treated in oxygen plasma for 2.5, 5, 10, or, 20 min with power of 100 W.

Photoelectrochemical Characterization and Electrochemical Impedance Spectroscopy Unless otherwise stated, the electrochemical characterization followed the suggestions made in a recent standardization document18. Photocurrent and EIS measurements were performed using a Voltalab80 PGZ 402 potentiostat and a three-electrode photoelectrochemical cell (socalled “ Cappuccino cell” manufactured after a design from EPFL) containing 10 mL of 1 M KOH (pH = 13.6) electrolyte, an Ag/AgCl reference electrode, and a 0.5 cm × 0.5 cm platinum sheet counter electrode. Working electrode potentials are reported relative to the reversible hydrogen electrode (RHE) and were calculated according to the equation ERHE = EAg/AgCl + 0.197 V + (0.059 x pH) V (25 °C). EIS spectra were recorded between 100 kHz and 1 Hz frequency range using a 10 mV AC perturbation amplitude, with 10 data points per decade. Least square fitting of the spectra was carried out using ZView v2.80 software (Scribner Associates). The film was illuminated from the electrolyte side through a fused silica window with 0.52 cm2 aperture, while the geometric area of hematite working 6 ACS Paragon Plus Environment

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electrodes immersed in the electrolyte was ~ 1.2 cm2. The photocurrent density was thus obtained by accounting the aperture area. The photocurrent densities thus refer to an electrode area of around 1.2 cm2.The simulated light is generated by a 1 sun Oriel Lamp by L.O.T-Oriel AG.. An Air Mass 1.5 filter was used in order to remove infrared (> 800 nm) and short wavelength UV light (< 280 nm).

X-ray Photoelectron Spectroscopy Surface properties of hematite films were assessed using an XPS spectrometer (PHI LS 5600) equipped with a standard MgKα X-ray source. The energy resolution of the spectrometer was set to 0.8 eV/step at a pass energy of 187.85 eV for survey scans and 0.125 eV/step and 29.35 eV pass energy for region scans. The X-ray beam was operated at a current of 25mA and an acceleration voltage of 13kV. Spectral shifts from charging effects were corrected for by refereeing to the C 1s signal of surface hydrocarbon observed at 285.0 eV binding energy. The photoelectron of C 1s, O 1s and Fe 2p were selected for the determination of elemental concentrations and chemical shifts. The concentration of surface elements and species and associated fittings were performed using CasaXPS 2.3.15 software. Binding energies of presented spectra were corrected according to the C 1s peak at 285.0 eV, which was due to surface carbon. Resonant Photoemission Spectroscopy Resonant photoemission spectroscopy (RPES) experiments were performed at Beamline 8.2 of the Stanford Synchrotron Radiation Lightsource (SSRL), Menlo Park, USA. Valence band (VB) spectra were recorded under ultra-high vacuum conditions as incident photon energies were swept across the Fe 3p absorption threshold centered at around 56 eV. Photoemission in the resonant mode allows for x-ray contrast enhancement and thus to discriminate spectral 7 ACS Paragon Plus Environment


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contributions in the valence band which arise from those elements and orbitals which are resonant excited with the particular x-ray photon energy.

RESULTS AND DISCUSSION Photoelectrochemical Properties of Hematite Photoanodes The systematic influence of oxygen plasma treatment time on the photocurrent density of hematite photoanodes is presented in Figure 1. Plots of photocurrent density (J) vs. applied potential (VRHE) for pristine and oxygen plasma-treated hematite photoanodes are shown in Figure 1(a). In the case of the untreated pristine hematite, the dark current is negligible within the measurement potential range. Upon illumination the onset of the water oxidation photocurrent is observed at 0.95 VRHE, above which J increased to 0.3 mA/cm2 at 1.23 VRHE and 0.48 mA/cm2 at 1.43 VRHE. Except for the hematite photoanode treated in oxygen plasma for the shortest time of 2.5 min, the water oxidation photocurrent onset of oxygen plasmatreated hematite photoanodes is shifted anodically compared to pristine hematite to ~ 1.2 VRHE. The photocurrent density at 1.23 VRHE decreases from 0.3 mA/cm2 for pristine hematite to 0.22, 0.07, 0.05, and 0.075 following oxygen plasma treatment for 2.5, 5, 10, and 20 min, respectively. Similarly, photocurrent densities at 1.43 VRHE decreases following oxygen plasma treatment. The changes in photocurrent density at 1.23 VRHE and 1.43 VRHE applied potential with duration of oxygen plasma treatment are summarized in Figure 1(b), which approaches a minimum steep at around 5 minutes and then settles on a low intensity plateau which roughly extends from 5 minutes to 20 minutes. The trend in photoelectrochemical properties of hematite synthesized in aqueous environment as observed here is quite contrary to the trend observed for sol-gel derived hematite films in previous studies13-15, where dark current onset is shifted anodically and the photocurrent onset is shifted cathodically following the oxygen plasma treatment. This is likely a manifestation of the importance of synthesis 8 ACS Paragon Plus Environment

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and processing conditions, which determine the crystallographic and electronic structure and microstructure, which determine the functionality and interaction with environment parameters. Possible reasons for this discrepancy may be the possibility of increased disorder on the hematite surface and enhanced recombination following oxygen plasma treatment, which is investigated below by considering the results of XPS, RPES and EIS analyses.

Chemical Composition and Surface Structure of Hematite First, XPS analysis is carried out in order to investigate the chemical composition of oxygen plasma-treated hematite. Figure 2 shows the XPS survey spectrum of untreated (pristine) hematite recorded between 0-800 eV binding energy, revealing the valence band (VB) and features corresponding to Fe 3p, C 1s, O 1s and Fe 2p core levels. Higher resolution Fe 2p and O 1s core level spectra corresponding to different oxygen plasma treatment times are shown in Figure 3. The Fe 2p3/2 features in Figure 3(a) display peak binding energies of ~ 711.3 eV, similar to Fe3+ in Fe2O3 (710.9 eV19, 711.2 eV20), Fe2+/Fe3+ in Fe3O4 (709.7/711.1 eV19) and Fe3+ in FeOOH (711.2 eV20, 711.3-711.9 eV21). Fe 2p3/2 features are observed to be distinctly made up of a peak and a shoulder separated by ~ 1 eV, indicative of the α form of iron oxides22. The binding energy of the shoulder in the Fe 2p3/2 features is similar to that known for Fe2+ in FeO (709.6 eV, 709.1-709.5 eV). The Fe 2p1/2 features in Figure 3(a) are located at ~ 724.8 eV binding energy and are broadened likely due to the simultaneous presence of Fe3+ (725 eV) and Fe2+ (723 eV) species23. These observations are consistent with our previous Fe 2p resonant photoemission study which revealed the presence of Fe2+ (V୓••) on the surface of asprepared (pristine) dip-coated hematite samples24.



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Figure 3(b) shows the O 1s spectra of pristine and oxygen plasma-treated α-Fe2O3 films. These spectra are asymmetric and show two contributions at binding energies of 529.5 eV and 531.4 eV, which can be attributed to hematite lattice oxygen (O2-), and to nonstoichiometric oxygen (O22-/O-) or surface hydroxyls (OH-), respectively21. We interpret the peak at 531.4 eV as originating from OH-. This structure is rather broad and could include also spectroscopic response from not fully coordinated oxygen, such as from oxygen near oxygen vacancies (Figure 3 in ref. 25). In our previous studies on ceramic proton conductors 25

and WO3 films (Figure 6 in ref 26) it was necessary to include a third structure for quality

least square peak fitting. We made similar observations in low nitrogen doped TiO2 where we observed a third structure between the eg-t2g multiplett in the oxygen NEXAFS spectra 27. As we have put it recently in a paper on WO3, 28, “Shpak et al. used four molecular species for deconvolution of their O 1s XPS spectra, i.e., O2−, OH−, O−, and H2O,29 whereas Azimirad et al. 29 did not include the O− species. Inclusion of the O− species in our deconvolution was not significant and thus was not applied in our further analysis.” 29,30. No peak is observed around 536 eV, revealing the absence of surface-adsorbed water on sample surfaces. Absence of water is expected because of to the ultra-high vacuum conditions24. The two oxygen peaks were analyzed and fitted with pseudo-Voigt functions31. Dramatic variation in the relative intensity of the two contributions occurs with oxygen plasma treatment, with the higher binding energy component becoming more pronounced up to 20 min treatment time. For the untreated hematite the O2- peak is more intense compared to the OH- peak, giving an O2-/OH- intensity ratio of 3.05. After 2.5 min oxygen plasma treatment the O2- contribution decreases compared to the OH- contribution, leading to a reduced O2-/OH- intensity ratio of 1.65. The O2-/OH- intensity ratio reaches a minimum value of 0.76 following 20 min of oxygen plasma treatment. The change in the O2-/OH- intensity ratio with oxygen plasma treatment time is graphically illustrated in Figure 4. 10 ACS Paragon Plus Environment

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According to previous works on metal oxides

25,26,28,32-37

, the higher binding energy

contribution in an O 1s spectrum may be related to oxygen vacancy (V୓•• ) concentration. According to this, the relative increase of the O22-/O- peak intensity with oxygen plasma treatment observed here could be explained by the formation of surface oxygen vacancies. This, however, seems unlikely in the oxidizing environment of the oxygen plasma. A more plausible explanation is that the enhancement of the OH- contribution arises from the formation of oxyhydroxide species following oxygen plasma treatment. A previous study by a different research group concerning the oxygen plasma treatment of NiOx films revealed the introduction of the dipolar surface species nickel oxyhydroxide (NiOOH), which resulted in changes to the VB and band gap energy.12 The necessary hydrogen for the NiOOH formation was believed to result from residual adsorbed water from ambient humidity in the samples. A more recent air plasma study on iron oxide provided films with higher oxygen vacancy concentration after plasma treatment 16. This difference would support the suggestion that the lower oxygen concentration in air for plasma treatment results in the higher concentration of oxygen vacancies. In our case we believe that oxygen plasma treatment of hematite results in the formation of FeOOH and/or Fe(OH)3 on the surface. This idea is further tested below with interpretation of iron oxidation state from RPES spectra. An additional observation from Figure 3(b) is that the O2- peak is shifted to ~ 0.5 eV higher binding energy after 20 min treatment. Binding energy shifts in XPS are related to characteristics of the initial state and final state, which can be explained by charge transfer (fingerprint of covalency/ionicity)38,39 and Madelung potential (electrostatic interaction between probed atom and its neighbors)40. Due to counterbalancing of these effects, the binding energy shift in O 1s spectra is equal to the charge-transfer contribution and has a linear relationship with oxygen charge41. Here the shift to higher binding energies of the O 1s signal in plasma-treated hematite is believed to reflect the relaxation of negative charges on 11 ACS Paragon Plus Environment


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the hematite surface following surface treatment42. Such shift is consistent with the presence of OOH- or OH- groups on the hematite surface, which attract electrons away from structural oxygen, making this lattice site less negatively charged. The VB indicated in the XPS survey spectrum in Figure 2 consists of Fe 3d, O 2p and hybridized Fe 3d-O 2p states. RPES is widely used to study the hybridization of electronic states and defects in the VB of metal oxides 43. We have previously studied Fe 2p RPES of the lower VB and suggested the formation of electron-hole doping features on the hematite surface following anodization24,44. Here, we used the Fe 3p absorption edge as opposed to the Fe 2p absorption edge partly since the former has a shorter associated electron escape depth compared to the latter, thereby making the RPES measurements more surface sensitive45. Figure 5(a) shows the Fe 3p resonant VB spectra of an as-prepared hematite film, recorded with 1 eV steps in incident photon energies between 50-60 eV. The features at ~ 2.5, 5.5 and 9.0 eV in the main band region may be ascribed to O 2p-Fe 3d hybridized states. Additionally a feature at ~ 13.1 eV is observed in the satellite region, while no clear M23M45M45 Auger emission above the Fe 3p core threshold is observed. These results are in agreement with the energy distribution curve (EDC) studied by Fujimori46. In the main band region, a distinct feature was observed at ~ 2.5 eV, highlighted by the red circles in Figure 5(a). The likely origin of this feature according to ligand-field (LF) theory47 or molecular orbital (MO) calculation48 is the highest occupied orbital of FeO69- (Fe3+), 3eg↑, with two electrons, and this contributes to one single 5Eg symmetry component. FeO69- cluster configurationinteraction (CI) calculation, however, indicates that the feature at ~ 2.5 eV is actually composed of two adjacent peaks with 5E and 5T2 symmetry42. The highest occupied orbital of FeO610- (Fe2+) is 2t2g↓ with one electron49, resulting in a 5T2 symmetry peak at ~ 0 eV and giving rise to the feature highlighted by the green circles in Figure 5(a). The 2t2g↓ occupied orbital is absent for Fe3+ in FeO69- (Fe2O3), so that the appearance of such a feature indicates 12 ACS Paragon Plus Environment

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the presence of Fe2+ on the surface24. This lends credibility to the suggestion that our pristine hematite films are at the surface sub-stoichiometric and thus contain Fe2+. Resonant photoemission yield may show a significant enhancement at incident photon energies above the Fe 3p absorption threshold, and is used to distinguish the Fe 3d-derived valence states from the overlapping non-hybridized O 2p states of iron oxide39, 46, 50-52. Figure 5(b) shows photoemission intensities at binding energies of 0 eV and 2 eV as functions of excitation energy. At 0 eV binding energy, a broad peak between 52-56 eV is observed, which shows peak resonance at 54 eV incident photon energy and becomes off-resonant at incident photon energies exceeding 55 eV. According to Fujimori52, the lowest-bindingenergy feature of FexO shows only a peak around 55 eV, which is a characteristic resonance behavior and which is absent and replaced by a dip for the maximum valence of iron in Fe2O3.46 This may be utilized as a fingerprint to distinguish Fe2+ in mixed-valence iron compounds. Photoemission intensity at the binding energy of 2 eV shows no peak at 54 eV, which indicates the presence of Fe2+ at the surface. Based on the resonant feature at ~ 0 eV binding energy for excitation at 54 eV observed in Figure 6, we purposely select this photon energy in order to maximize x-ray spectroscopic contrast and thus sensitivity to Fe2+. The VB spectra in Figure 6(a) show the influence of oxygen plasma treatment time. No overt changes in the VB spectra are observed following treatment, however closer inspection of the 2t2g↓ feature intensity at ~ 0 eV binding energy (highlighted by the green circles) does indeed reveal important changes, as summarized in Figure 6(b). Upon oxygen plasma treatment, the intensity of the 2t2g↓ feature decreases, owing to the oxidizing effect of the plasma and indicating a systematic lowering of surface Fe2+ concentration. Thus the overall trends in the 2t2g↓ feature intensity from RPES measurements (Figure 6(b)) and in O2-/OH- peak spectral weight ratio from O 1s core level spectra (Figure 4) are in direct agreement. 13 ACS Paragon Plus Environment


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We have already reported that Fe2+ and V୓•• defects are present at the hematite surface prior to oxygen plasma treatment24, 44. The above spectroscopic analysis points to iron oxyhydroxide (FeOOH) formation during oxygen plasma treatment, which we illustrate in a simple sketch for the hematite (0001) surface in Figure 7. Initially, there exists one oxygen vacancy (V୓••) in the considered oxygen-terminated surface layer. The Fe cation closest to this oxygen vacancy is less ionic and less positively charged (lower oxidation state) compared to a conventional Fe3+ cation in the bulk of hematite. Following oxygen plasma treatment – in the presence of residual humidity -, the oxygen vacancy reacts to form OOH or OH surface groups, and the closest Fe cation is concurrently oxidized to Fe3+. Oxygen in the OOH and OH surface groups has less negative charge compared to lattice oxygen in the bulk, which accounts for the observed chemical shift in O 1s spectra to higher binding energy observed in Figure 3(b).

Trap Chemical Capacitance and Density of Surface States The electric transport properties of metal oxides are at large determined by their electronic structure. In order to further investigate the effect of oxygen plasma treatment on hematite transport properties in relation to the surface states, we carried out EIS under 1 Sun illumination and DC bias between – 400 mV to + 700 mV (vs. RHE). A relevant selection of EIS spectra is shown in Figure 8(a), which can be rationalized by high and low frequency processes corresponding to electron trapping/releasing between the conduction band and surface states, and to charge transfer between iron oxide surface states and water, respectively9. We use a simplified equivalent circuit8-9 to fit impedance spectra, as presented in Figure 8(b), where RS is the series resistance, Cbulk is the bulk capacitance of the hematite electrode, Rtrap is the resistance associated with recombination of conduction band electrons and valence band holes at surface states, Rct,trap is the charge transfer resistance associated


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with the transfer of holes from surface states to adsorbed water, and Ctrap represents the trap chemical capacitance attributed to the charging and discharging of surface states for current generation8. The variation in Ctrap vs. oxygen plasma treatment time is presented in Figure 9(a). Ctrap has highest capacitance for untreated hematite and decreases with oxygen plasma treatment. Ctrap values can be used to calculate the energy distribution of interfacial density of states according to the relation Ctrap = q DSS

53

, where q is the elemental charge and DSS is the

concentration (area specific density) of hematite surface states. Detailed observation of the plasma treated samples (2.5 – 20 min) (Figure S2) shows there exist three sub-peaks in the screened potential range: the first centered around 0.9 VRHE, the second around 1.4 VRHE, and the third around 1.65 VRHE. Our previous operando O NEXAFS study on hematite10,11 shows the presence of two electron holes states, which we believe correspond to the two peaks in the potential window from 0.8 to 1.5 VRHE.11 Detailed work is ongoing in our lab to further explain these surface states. Figure 8(b) shows the variation in DSS values calculated from Ctrap peaks in Figure 9(a) as a function of oxygen plasma treatment time. The concentration of surface states on hematite is decreased upon oxygen plasma treatment, with a minimum observed after 10 min treatment. The trend in DSS (Figure 9(b)) with oxygen plasma treatment time is paralleled by the trends observed in photocurrent density (Figure 1(b)), O2-/OH- ratio (Figure 3(c)) and 2t2g↓ feature intensity (Figure 6(b)), revealing the intimate correlation between surface molecular structure, surface states and water splitting activity. Through this combined X-ray and impedance spectroscopy study, we show the effect of surface structure on surface electronic states and photoelectrochemcial properties of hematite. Prior to oxygen plasma treatment, the pristine film shows more iron sites from a spectroscopy point of view and higher surface state capacitance, suggesting higher charge transfer through the SCLJ, from the electrochemistry point of view. Upon plasma treatment, a decrease in iron 15 ACS Paragon Plus Environment


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surface occupancy is observed, and at the same time the surface state capacitance is decreased. This is the proof that iron surface states induce higher reactivity towards water oxidation than oxygen surface states. Computational studies54-55 of the water oxidation mechanism on hematite (0001) have been carried out by other groups, and density functional theory (DFT) investigations of Feterminated or defective hematite (0001) surfaces predict that surface defects or oxygen vacancies are reactive towards H2O56-57. It is generally believed that photoelectrocatalytic water splitting starts with H2O adsorption at surface oxygen vacancy sites and includes the formation of surface hydroxyl intermediates. The above spectroscopic and electrochemical analyses indicate that the surface of oxygen plasma-treated hematite is oxidized and that oxygen vacancies are filled, leading to higher energy barrier for water adsorption. Moreover, VB holes arrive at the hematite anode surface as Fe3+ (3d5)4. Unfavorable charge transfer substantially limits hole concentration on the oxygen ligand (3d6L), likely limiting the charging and discharging of surface states and accumulated hole concentration, in turn lowering water oxidation activity. CONCLUSION As-prepared hematite films are inherently oxygen deficient at the surface and thus contain Fe2+ species. Oxygen plasma treatment fills oxygen vacancies, oxidizes the Fe2+ to Fe3+, and forms FeOOH on the surface when residual water is present. The result is a decrease in the concentration of surface states and impeded water adsorption. An intimate correlation is established between the presence of surface Fe2+ or V୓•• , surface states and photocurrent density. This further improves understanding of the water oxidation mechanism on the surface of hematite. Surface states of iron induce higher reactivity towards water oxidation than oxygen surface states.


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ACKNOWLEDGMENTS We thank the Swiss National Science Foundation (project numbers 132126 for Y.H., 137868 for F.B., R’Equip 121306, and NanoTera SHINE 20NA21-145936) for financial support. Financial support from the Strategic Korean-Swiss Cooperative Program in Science and Technology within the project “Spectroscopy on photoelectrochemical electrode materials: SOPEM” for A. Braun and from NRF-2013K1A3A1A14055158 for B.S. Mun is gratefully acknowledged. The photoelectrochemical cell was built by Peter Wyss and Marc Zollinger (Empa Machine Shop) after the original design provided by the Laboratory for Photonics and Interfaces, EPFL Lausanne. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. We thank Dr. Jun-Sik Lee (SSRL) for help with RPES measurements.

SUPPORTING INFORMATION X-ray diffractogram of hematite film on FTO substrate Trap state capacitance vs. potential Tables with impedance least square fit parameters



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Table of contents graphic. 297x210mm (220 x 220 DPI)

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Figure 1. Current-voltage (J-V) characteristics of hematite electrodes in 1 M KOH following oxygen plasma treatment for indicated periods (min) in dark (dotted lines) and under 1 Sun illumination (solid lines); scan rate = 10 mV/s. (b) Variation of photocurrent density with oxygen plasma treatment time under 1.23 VRHE and 1.43 VRHE applied bias. 166x247mm (100 x 100 DPI)


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Figure 2. XPS survey spectrum of untreated hematite. 288x201mm (300 x 300 DPI)



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Figure 3. (a) XPS Fe 2p and (b) O 1s core level spectra of hematite with indicated oxygen plasma treatment times (min). 73x213mm (220 x 220 DPI)


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Figure 4. Variation of O2-/OH- peak ratio from XPS O 1s core level spectra of hematite dependent on oxygen plasma treatment time. 166x124mm (100 x 100 DPI)



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Figure 5. (a) Valence band XPS spectra of untreated hematite with excitation energy stepped at 1 eV intervals between 50-60 eV. (b) XPS intensities at binding energies of 0 and 2.5 eV of untreated hematite as functions of excitation energy. 153x219mm (100 x 100 DPI)


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Figure 6. (a) Fe 3p resonant XPS valance band spectra measured at 54 eV excitation energy of hematite samples after 0, 2.5, 5, 10, 20 and 30 min oxygen plasma treatment time. (b) Variation in intensity of Fe2+ 2t2g↓ orbital signal with oxygen plasma treatment time. 155x223mm (100 x 100 DPI)



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Figure 7. Schematic of FeOOH formation on hematite (0001) surface upon oxygen plasma treatment. 221x145mm (150 x 150 DPI)


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Figure 8. (a) Nyquist plots of untreated and 20 min oxygen plasma-treated hematite electrodes under 1 Sun illumination and indicated applied DC bias in 1 M KOH. Fitting results indicated as solid lines and selected AC perturbation frequencies labeled as filled black symbols. (b) Equivalent circuit used to interpret spectra. 175x182mm (100 x 100 DPI)



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Figure 9. (a) Trapped surface state capacitance of hematite electrode vs. applied potential in 1 M KOH under 1 Sun illumination with oxygen plasma treatment times (min) indicated (b) density of surface states as a function of oxygen plasma treatment time for hematite electrodes at 1.43 VRHE applied potential. 176x252mm (100 x 100 DPI)


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Figure S2. Trapped surface state capacitance of hematite electrode vs. applied potential in 1 M KOH under 1 Sun illumination with oxygen plasma treatment times (min). 403x577mm (72 x 72 DPI)



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Figure S2. Trapped surface state capacitance of hematite electrode vs. applied potential in 1 M KOH under 1 Sun illumination with oxygen plasma treatment times (min). 288x201mm (300 x 300 DPI)


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Molecular Origin and Electrochemical Influence of Capacitive Surface

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