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Oxygen Vacancies Engineering of Iron Oxides Films for Solar Water Splitting Maxime Rioult, Dana Stanescu, Emiliano Fonda, Antoine Barbier, and Helene Magnan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00552 • Publication Date (Web): 25 Mar 2016 Downloaded from http://pubs.acs.org on March 30, 2016

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The Journal of Physical Chemistry

Oxygen Vacancies Engineering of Iron Oxides Films for Solar Water Splitting Maxime Rioult1,†, Dana Stanescu1, Emiliano Fonda2, Antoine Barbier1, Hélène Magnan1,* 1: SPEC, CEA, CNRS, Université Paris-Saclay, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France. 2: Synchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, 91192 Gif-sur-Yvette, France. †

: Current affiliation : Synchrotron SOLEIL, L’Orme des Merisiers Saint-Aubin, 91192 Gif-surYvette, France.

ABSTRACT. Hematite (α-Fe2O3) can be considered as one of the top candidates to act as photoanode in the framework of clean hydrogen production through solar water splitting. The O:Fe ratio, that in this material plays a crucial role in the definition of its photoelectrochemical properties, has been investigated in details. For this purpose, we examined thermal magnetite oxidation and hematite reduction as two possible routes to produce semiconducting iron oxides layers with controlled stoichiometry. We report on properties of single crystalline nanometric films elaborated by atomic oxygen plasma assisted molecular beam epitaxy as model systems to disentangle structural phase transition effects from pure stoichiometry ones. We provide new insights on the mechanisms related to hematite properties modifications and their correlation with photocurrent changes upon the presence of oxygen vacancies and phase mixing with

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magnetite, with respect to the vacancies concentration regimes. We show on one hand that crystallographic structure mixing appears as strongly detrimental for photoanodes synthesis whatever the oxygen vacancies concentration. On the other hand oxygen vacancies in the optimal concentration range, while preserving the α-Fe2O3 corundum phase, is highly favorable for solar water splitting, inducing a substantial reduction of 0.2 V for the onset potential and an overall photocurrent increase of 50 % with respect to stoichiometric hematite. The present study demonstrates more generally the possibility of using oxygen vacancies as a degree of freedom for the optimization of hematite photoanodes.

I.

INTRODUCTION

Hydrogen production through solar water splitting is very appealing within the framework of renewable energy development. As a matter of fact, hydrogen is an energy carrier of choice which does not lead to any greenhouse gas production when produced with solar light. This seductive idea is very challenging although many materials science issues have to be solved in order to reach a reasonable efficiency. During the process, electron-hole pairs are generated in illuminated semiconductors dipped in an aqueous solution, and the water oxido-reduction reactions (oxygen production at the photoanode and hydrogen production at the photocathode) are initiated.1,2 Since the pioneering discovery of water photoassisted electrolysis using TiO2 in 1972 by Fujushima and Honda1, several materials were investigated as photoanodes3 where water ଵ

oxidation occurs (2ܱ‫ ି ܪ‬+ 2ℎା → ଶ ܱଶ + ‫ܪ‬ଶ ܱ). Hematite (α-Fe2O3) is one of the most promising candidates. The quasi-ideal 2.2 eV band gap of this n-type semiconductor grants a


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theoretical solar−to−hydrogen yield of 13 %.3 The abundance and stability in aqueous environments of this material are additional assets in the framework of environment friendly energy production.4 Moreover, the valence band edge of hematite, located below the H2O/O2 redox potential, favors the water oxidation reaction and makes it a photoanode of choice in the case of dual photoelectrodes water splitting devices.5 Unfortunately, great efforts are still needed to overcome hematite drawbacks that are mainly low conductivity, low carrier lifetime6,7, poor surface kinetics8, and a conduction band edge positioned below the water reduction potential (2‫ܪ‬ଶ ܱ + 2݁ ି → ‫ܪ‬ଶ + 2ܱ‫) ି ܪ‬9. Several strategies have been reported to improve the water splitting performances. We can cite doping with transition metals7,10,11 such as Ti or Si, semiconductor heterojunctions12, surface engineering through the deposition of overlayers13, cocatalysts14 or chemical etching15. In the present work we examine the use of oxygen vacancies as a degree of freedom to optimize hematite thin films photoanode performances. Oxygen vacancies are expected to act as shallow donor dopants in hematite (i.e. energy states close to the conduction band minimum)16-21. The creation of an oxygen vacancy leaves two electrons per missing oxygen atom. This will increase the overall majority carrier electron concentration and therefore the conductivity. However the introduction of two extra electrons may also reduce Fe3+ into Fe2+.22 Iron reduction can be beneficial or detrimental in the framework of solar water splitting. The conduction mechanism in non-ideal hematite (i.e. oxygen deficient) being attributed to polaron hopping conduction thanks to Fe3+/Fe2+ mixed valence (electrons “hop” from Fe2+ to Fe3+)23, oxygen vacancies can be expected to indirectly increase the conductivity17 and the carrier mobility24 whereas at too high concentrations, Fe2+ sites may serve as recombination centers for photoexcited holes because of the presence of electronic states within the band gap25, which


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could lower the photocurrent4,16. The evolution in photocurrent will be the result of the competition between these two concomitant phenomena. In addition, changes in the oxygen vacancies concentrations can lead to structural phase transition from hematite (α-Fe2O3) to magnetite (Fe3O4). Importantly, the semi-metal Fe3O4 is not expected to be photoactive16. Previous

studies

considering the effect

of

oxygen

stoichiometry,

through

various

reducing/oxidizing treatments, on the photoelectrochemical properties of iron oxide films, have considered rather complex sample configurations like nanoparticles24, nanowires16,20, polycrystalline samples17 and nanostructures26,27. For example, Sun et al. reported on the possibility of optimizing the oxygen vacancies concentration in hematite nanostructures by sintering in various oxygen-deficient atmospheres26 or by reducing hematite through a hydrogen treatment27. These studies have shown an improvement of the photocurrent attributed mainly to an improvement of the electrical conductivity, through enhanced polaron hopping induced by moderate oxygen vacancies concentrations whereas too high concentrations lead to a decrease of the photocurrent26. However, due to the intrinsic nature of the samples, the crystallographic structure modifications induced by the increase of the oxygen vacancies concentration could not be studied in details. As a matter of fact, in the case of polycrystalline samples, treatments used to generate oxygen vacancies can result in a wealth of additional parasitic changes like (i) crystallites or particles size distribution modifications, (ii) predominant crystallographic orientation development, (iii) surface roughness and morphology changes as well as (iv) preferential ion migrations along grain boundaries leading to inhomogeneous compositions etc. Hence mingled conclusions may be drawn because of the existence of intrinsically entangled properties.


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In the present work we studied controlled single crystalline samples of iron oxides with oxygen stoichiometries ranging from Fe3O4 to α-Fe2O3. These samples were nanometric single crystalline films deposited by atomic oxygen assisted molecular beam epitaxy (AO-MBE) on Pt (111) single crystals11. AO-MBE allows a high control of the stoichiometry and of the crystalline quality of the thin films. The use of an atomic oxygen plasma cell as the oxidation source grants a precise control of the oxygen content in the material, allowing for the direct elaboration of the full range of possible iron oxides. We already validated the feasibility of this model samples approach using AO-MBE in order to study the effects of Ti-doping10,11, film thickness10 and HCl-etching15 on the properties of hematite films within the framework of water photoelectrolysis. In order to elaborate semiconducting iron oxides photoanodes with oxygen stoichiometries deviating from either magnetite or hematite, two complementary routes are possible: oxidation of magnetite and reduction of hematite. We realized both approaches in order to explore the O:Fe ratio compositions between Fe3O4 and Fe2O3 for electronic properties, phase composition, crystallographic structure and photoelectrochemical performances. These two complementary strategies will subsequently structure this paper. The composition region close to Fe3O4 (from Fe3O4 to Fe2O3-δ,) is explored using thermal oxidation of Fe3O4 films annealed in ambient air at various temperatures. It should be noted that this process may lead to the formation of other iron oxides like maghemite (γ-Fe2O3), which appears as a promising material for solar water splitting24,28. The second region, next to Fe2O3 (from Fe2O3 to Fe2O3-δ.), is examined by reducing α-Fe2O3 films through annealing in ultra-high-vacuum (UHV) at different temperatures.


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II.

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EXPERIMENTAL SECTION

Sample preparation. The iron oxide layers were deposited on single-crystalline Pt (111) substrates using AO-MBE, a technique that makes possible the deposition of single crystalline layers of controlled composition and thickness10,11,15. High purity metals (99.99% grade) were evaporated from dedicated Knudsen cells in the presence of an atomic oxygen plasma (350 W power) in order to obtain well defined oxides under good vacuum conditions (i.e. 10-7 mbar working pressure, 10-10 mbar base pressure). During the deposition, the samples were rotated continuously around their normal to ensure a homogeneous deposit, and radiatively heated at a temperature of ca. 900 K. The oxide deposition rate was about 0.15 nm/min. The thickness of our films was 15 nm, i.e. large enough to yield acceptable photocurrents. Air annealings were performed in a high temperature furnace for 23 hours at 180°C and 220°C in ambient air. Annealings in UHV were realized inside the EXAFS apparatus at the SAMBA beamline29 at synchrotron SOLEIL (Saint-Aubin, France), for 15 min at various temperatures (270°C, 350°C, 400°C, 450°C). A dedicated sample was used for each temperature avoiding any mingled results that could derive from successive annealings. By convention, samples annealed in air (resp. UHV) will be noted T-air (resp. T-vac) where T is the annealing temperature.

Crystallographic structure and chemistry. In situ Reflexion High Energy Electron Diffraction (RHEED) patterns were observed and acquired during film growth to monitor the crystal quality and structure of the samples. In situ XPS spectra were systematically recorded just after deposition in order to determine the stoichiometry and the electronic structure of the films. More precisely, we recorded Fe2p, O1s


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core levels and the valence band region using the Al Kα radiation. Ex situ spectra were also recorded after the annealing procedures. In the case of hematite films annealed in UHV, Extended X-ray Absorption Fine Structure (EXAFS) spectra at the Fe K-edge were acquired in situ after the annealing procedure on the SAMBA beamline29 at synchrotron SOLEIL (Saint-Aubin, France). The spectra were recorded at room temperature (after the down cooling) in normal incidence (cf. figure 1.a, with linear polarization of photons parallel to the film surface, i.e. more sensitive to the Fe in-plane neighboring) and in grazing incidence (cf. figure 1.b, with linear polarization of photons perpendicular to the film surface, i.e. more sensitive to the Fe out-of-plane neighboring). The kχ(k) EXAFS oscillations spectra were fitted according to the same method than in ref. 11, using HORAE and FEFF 8.4 codes30-32 assuming a hematite (corundum) structure (cf. figure 1.c). More precisely, we fitted the two incidences together and used single scattering paths (distance and atom type) from the absorbing Fe atom of length up to 3.8 Å, corresponding to the octahedral shell of oxygen (O1 and O2) and the more distant shells of Fe neighbors (among which the two first neighbors noted Fe1 and Fe2), as illustrated on figure 1.c. We checked that multiple scattering was negligible. The fitting parameters for each path corresponding to a shell of neighbors are the amplitude for the path, the distance between the absorbing atom and the neighbor and the Debye-Waller (DW) factor to take into account additional structural disorder. In order to reduce the number of fitting parameters and to take into account the strain due to epitaxy, we linked the change in interatomic distances to a structural expansion by introducing two expansion factors: α parallel to the surface of the film and β perpendicular to it; with the exception of the first Fe-O shells for which Fe-O1 and Fe-O2 distances were allowed to float.


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Figure 1. EXAFS measurements geometries using linear photon polarization of synchrotron radiation: (a) normal incidence (more sensitive to in-plane neighboring) and (b) grazing incidence (more sensitive to out-of-plane neighboring). The black dashed (resp. green) arrow stands for the propagation (resp. polarization) direction of the incident photon beam. (c) Crystal structure of α-Fe2O3 used in the FEFF calculations, detailing the terminology of the neighbors’ shells. Small and large spheres represent Fe and O atoms respectively.

Photocurrent measurements. The photocurrent of our films was measured using a three electrodes cell10,11,15. All measurements were performed at room temperature using a NaOH 0.1M (pH = 13) solution as electrolyte, a platinum wire as counter electrode and an Ag/AgCl electrode for the potential reference (VAg/AgCl = + 0.197 V vs. SHE). The sample was mounted as anode (working electrode) using a dedicated sample holder that allows the contact only between the hematite surface and the electrolyte. The illumination source was a Newport 1000 W Xe Arc Lamp with an infrared filter, providing an incident light flux measured with a Newport 1918-R Power Meter around 100 mW/cm². Potential control and current acquisition were done using a


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Princeton Applied Research 263A potentiostat. For photocurrent – voltage curves, the potential was swept from - 0.2 to + 0.8 V vs. Ag/AgCl at a speed of 50 mV/s. The photocurrent is defined as the difference between the current recorded under light and the one in the dark.

III.

RESULTS AND DISCUSSION

Magnetite oxidation: from Fe3O4 (111) to Fe2O3-δδ : The RHEED pattern of the 15 nm thick Fe3O4 (111) film grown by AO-MBE on a Pt (111) single crystal, shown in figure 2.a (bottom), features straight streaks accounting for an epitaxial bidimensional growth of the (111) spinel structured overlayer. Upon air annealing, the RHEED patterns evolve as presented on figure 2.a, where we included also a RHEED pattern of a reference α-Fe2O3 (0001)/ Pt (111) film33 (figure 2.a, top). For an easy comparison, we show the RHEED patterns for a given angular in surface plane azimuth corresponding to the (11)∗௦ (reciprocal direction indexed using the 2D surface lattice) direction for Fe3O4 (111) (spinel structure) and to the (10)∗௦ direction for α-Fe2O3 (0001) (corundum) (figure 2.a and 2.b). It is clear that upon air annealing of Fe3O4: (i) additional lines corresponding to the hematite lattice are increasingly visible and (ii) the lines specific to Fe3O4 tend to disappear. This is even clearer when one analyses the profile of the RHEED patterns integrated along the streaks direction (figure 2.c). On these profiles the peaks specific to Fe3O4 (111) and α-Fe2O3 (0001) are highlighted with black solid and red dashed arrows respectively. We confirm thus that upon air annealing at moderate temperature the initially pure Fe3O4 thin film progressively incorporates α-Fe2O3 features while remaining epitaxial and single crystalline.


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Figure 2. (a) RHEED patterns obtained along the azimuth given by the arrows depicted on (b). (b) Corresponding surface reciprocal lattices. (c) Profiles obtained by integrating the RHEED patterns along the streak direction using the integration box highlighted with red dashes on (a) bottom. From bottom to top: as-grown Fe3O4 (black solid line), 180-air (green solid line) and 220-air (blue dotted line) samples and a reference α-Fe2O3 film (red dashed line). The elementary cell (in the reciprocal space) is also shown on (b) with the same color than the corresponding reciprocal lattice (red for the (111) spinel (S) surface, and green for the (0001) corundum (C) surface). On (c), the black solid (resp. red dashed) arrows highlight RHEED streaks specific to Fe3O4 spinel structure (resp. α-Fe2O3, corundum structure).

These results evidence that upon annealing, the crystallographic structure changes from the pure Fe3O4 spinel structure to a mix of iron oxides consisting of corundum and spinel structures, likely leading to a pure Fe2O3 corundum structure. However it is not straightforward to conclude


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if upon air annealing the Fe3O4 film becomes a mix of Fe3O4 and α-Fe2O3 or a mix of γ-Fe2O3 (maghemite) and α-Fe2O3 since Fe3O4 and γ-Fe2O3 share both the same spinel structure. This question can be qualitatively clarified thanks to the XPS measurements. Fe2p and valence band spectra are shown on figure 3.a and 3.b respectively. For pure α-Fe2O3 (0001) and Fe3O4 (111) films, the XPS results confirm characteristics reported in previous works34, i.e. pure Fe3+ valence state and semiconducting behavior for the hematite film, and mixed Fe2+/Fe3+ valence state and metallic-like behavior for the magnetite film. The latter is due to a high number of electronic states in the band gap, close to the Fermi level (figure 3.b). Upon air annealing, the Fe2p core levels analysis (figure 3.a) shows that the oxidation state of iron moves closer to only Fe3+, like in pure hematite. This is deduced from concomitant appearance and increase of the typical Fe3+ shake-up satellite (as it is observed in pure hematite, see e.g. ref. 34) around 719.2 eV binding energy and from the energy shift of the Fe2p1/2 and Fe2p3/2 lines (resp. 0.5 eV and 1 eV) toward higher binding energies. In addition, the valence band evolution (figure 3.b) shows a clear gap opening consistent with the evolution of the metallic film towards a semiconductor state. For air-annealed samples as well as for the reference hematite film we highlighted in shaded grey in figure 3.b the integral of the valence band spectra between the valence band minimum cut-off and the baseline (no photoelectron recorded). The bigger this integral is, the higher the number of electronic states available within the band gap is. In Fe3O4, electronic states in the gap are induced by the presence of Fe2+ ions25. Air annealing oxidizes these Fe2+ into Fe3+ (cf. Fe2p core level analysis), explaining why the number of electronic states close to the Fermi level decreases. However, the grey area remains larger on annealed samples as compared to pure hematite showing that the oxidation is incomplete. The quantitative analysis of this area as a function of the annealing temperature is shown on figure S1 (see Supplementary Information)


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and allows to establish the relative percentage of Fe2O3 and Fe3O4 in the air-annealed samples, as reported in table 1 which is around 85 %. Moreover the RHEED data shown on figure 2 evidences that the relative percentage of corundum and spinel structures in the air-annealed samples is also around 85 %. This is more visible on figure S2 (see Supplementary Information), where a linear combination of the RHEED patterns profiles for reference α-Fe2O3 and Fe3O4 films (with 85% and 15% coefficients respectively) is shown together with the profiles for the 180-air and 220-air samples. One observes that the profiles are very similar. Therefore for these samples the ratio between corundum and spinel structures deduced from the RHEED analysis is the same than the ratio between Fe2O3 and Fe3O4 obtained from XPS. Thus we can qualitatively conclude that the Fe2O3 created by magnetite air-annealing is mainly in the corundum structure and that the amount of maghemite inclusions (Fe2O3 in the spinel structure) is marginal (< 5%) if any.

Figure 3. (a) Fe2p core level and (b) valence band XPS spectra. From bottom to top: as-grown Fe3O4 (black solid line, M stands for magnetite), 180-air (green solid line) and 220-air (blue dotted line) samples and a reference α-Fe2O3 film (red dashed line, H stands for hematite).


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Table 1. Percentage of Fe2O3 and Fe3O4 derived from the area of the valence band region (see figure S1) for as-grown Fe3O4, 180-air and 220-air samples and a reference α-Fe2O3 film. Sample

% of Fe2O3

% of Fe3O4

As-grown Fe3O4

0

100

180-air

84

16

220-air

88

12

As-grown α-Fe2O3

100

0

Let us now discuss the photo-electrochemical properties of these films. Figure 4 presents the photocurrent of the two air-annealed Fe3O4 films, additionally compared with Fe3O4 and α-Fe2O3 reference films. First we observe that hematite shows the best photocurrent at any bias, whereas the Fe3O4 sample features almost zero photocurrent, which confirms its metallic character. The photocurrent of air-annealed samples gradually increases from the Fe3O4 film photocurrent when the annealing temperature increases, converging toward the photocurrent of the reference hematite film. However the photocurrents of air annealed samples are very low and do not match at all the values that one would expect from a linear combination corresponding to the film composition previously determined using XPS, i.e. ca. 85% of α-Fe2O3 photocurrent for 180-air and 220-air samples. Therefore the mixing of the two different structures (corundum / spinel) even at low percentage appears to be unfavorable for solar water splitting. This result is different from observations made on titanium dioxide where anatase / rutile mixtures were reported having better photocatalytic activity than single phases35. It is maybe due in our case to the detrimental metallic character of Fe3O4. Indeed, magnetite inclusions in hematite may act as traps for holes and electrons, as it is illustrated on scheme 1. This effect will be exacerbated at the boundaries between the two crystallographic structures. As a matter of fact, in the case of


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hematite reduction to magnetite, the boundaries between hematite and magnetite domains have a dendritic shape36. If we consider that in the case of magnetite oxidation to hematite we encounter the same process, the density of domains boundaries (which are likely to act as recombination centers) will be higher for lower air-annealing temperatures. Hence in these samples with mixing phases, the photocurrent will be lowered with respect to pure hematite.

Figure 4. Photocurrent for air-annealed 15 nm thick Fe3O4 films grown on Pt (111): as-grown (black solid line, M stands for magnetite), 180-air (green solid line) and 220-air (blue dotted line) samples and a reference α-Fe2O3 film (red dashed line, H stands for hematite).


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Scheme 1: Illustration of the recombination behavior at the hematite / magnetite interface showing magnetite inclusions acting as traps for the photogenerated carriers, inducing recombination. Full and open circles stand for photogenerated electrons and holes respectively.

In the light of the results of this section, we can conclude that the presence of residual magnetite inside fairly majority hematite is highly detrimental for photoelectrochemical properties whatever the oxygen vacancy concentration. One may note that this first sample realization route produces samples with a crystallographic structure mix between spinel and corundum. As a consequence, it is not possible to disentangle strictly and quantitatively structural and chemical effects using this strategy. To do so, it is thus necessary to consider hematite samples with an amount of oxygen vacancies small enough to avoid crystallographic changes so that we can study, as a single parameter, the effect of oxygen deficiency on the photocatalytic properties. Since this state is unlikely reachable using magnetite oxidation, we have to examine the reduction of hematite. It is the purpose of the next session.


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Hematite reduction: from α-Fe2O3 (0001) to Fe2O3-δδ : To investigate the low oxygen vacancy regime within the corundum structure we studied αFe2O3 (0001) films deposited on Pt (111) annealed in ultra-high-vacuum (UHV). The stoichiometry and the electronic structure of the film after annealing were determined by means of XPS measurements. The Fe2p core level XPS spectra of vac-annealed samples are shown on figure 5, along with the data for an as grown hematite film. The temperatures were well below the ones required to produce Pt-Fe intermixing; consistently no additional signal coming from Pt could be observed after annealing.

Figure 5: XPS after 15 min UHV annealing of 15 nm hematite films grown on Pt (111). From bottom to top: as-grown (black solid line), 270-vac (blue dashed line), 350-vac (red solid line), 400-vac (olive dotted line) and 450-vac (orange solid line) samples.


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As a matter of fact, an oxygen vacancy created by UHV annealing is likely to reduce Fe3+ into Fe2+. For UHV annealing temperatures up to 350°C, we saw no changes in the Fe2p spectra, meaning that the introduced oxygen vacancies concentration, if any, is not sufficient to induce a detectable amount of Fe2+ by XPS. The area of the shake-up satellite typical for a Fe3+ oxidation state34 starts to decrease for an annealing temperature of 400°C (see also figure S3 in Supplementary Information), accounting for Fe2+ species in a sufficient amount to be detected. For a temperature of 450°C, we can also notice a very slight shift of the Fe2p lines toward lower binding energies, of 0.2 eV in the case of the Fe2p3/2 peak. This difference is smaller than what we observed earlier by comparing the Fe2p3/2 peaks of pure α-Fe2O3 and Fe3O4 films (figure 3.a). The crystallographic structure of the films was studied by EXAFS. EXAFS is a very well suited technique to determine the local environment of a given ion with great sensitivity. The experimental kχ(k) EXAFS oscillations spectra for all samples and the two incidences are shown on figure 6.a and 6.b.


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Figure 6: In situ experimental kχ(k) EXAFS oscillations spectra in (a) normal and (b) grazing incidences for 15 nm thick hematite films annealed in UHV at various temperatures. From bottom to top: as-grown (AG, black solid line), 270-vac (blue dashed line), 350-vac (red solid line), 400-vac (olive dotted line) and 450-vac (orange solid line) samples. Asterisks highlight the main modifications with increasing the annealing temperature.

Upon UHV annealing, the structure slightly changes with respect to as-grown hematite, as highlighted with asterisks. This is more visible for grazing incidence spectra (figure 6.b), meaning that (qualitatively) the out-of-plane neighborhood of Fe atoms is more modified than the in-plane neighborhood upon UHV annealing. Also when increasing the annealing temperature, the differences between normal and grazing incidence spectra are less and less important, meaning that the structure is less and less anisotropic. The spectra recorded in normal


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and grazing incidences are very different for the as-grown sample, accounting for a strong anisotropic structure (which is expected for hematite)11, whereas for the 450-vac sample differences are lighter, accounting for a more isotropic structure with respect to the as-grown structure. This is in agreement with observations of the XANES and pre-edge regions (cf. figure S4, see Supplementary Information). The EXAFS oscillations for all the vac-annealed samples were fitted with the method detailed in the experimental section, assuming a hematite (corundum) structure for all samples (shown on figure 1.a). Examples of fit curves are shown in figure S5 (see Supplementary Information) for the as-grown and 450-vac samples. The obtained structural parameters (interatomic distances and Debye-Waller (DW) factors) for all samples are given in table S1 and the evolution of the interatomic distances vs. the annealing temperature is depicted on figure 7. The good agreement of the fits with the experimental data clearly shows that the hematite crystallographic structure is confirmed for all samples. Consistently with what was observed qualitatively, visible changes occur for an annealing temperature higher than 400°C. Other samples feature structural parameters very close to the ones of pure hematite. The DW factors always lie within 1.10-3 and 1.10-2 Å-2, which shows that the structure is well-ordered. From figure 7 we see that upon UHV annealing the Fe-O distances (figure 7.a) tend to converge toward the interatomic distance in pure Fe3O4 between the Fe in octahedral site (Oh site) and its oxygen neighbors. This is more visible for the Fe-O2 distance (second oxygen neighbor). Analogously, the Fe-Fe distances (figure 7.b) converge toward the distance between two Fe atoms in Oh sites in pure Fe3O4, especially for the Fe-Fe1 distance. The largest variation upon annealing is recorded for the FeFe1 distance (i.e. the distance between the absorbing Fe and its first in-plane Fe neighbor, cf. figure 1.c) which is 2.90 Å for the as-grown sample and 2.97 Å for the 450-vac sample.


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Figure 7: (a) Fe-O and (b) Fe-Fe interatomic distances obtained by EXAFS simulations at the Fe K edge for as-grown and UHV-annealed 15 nm thick hematite films for the first (O1, Fe1, empty symbols) and second (O2, Fe2, full symbols) layers of neighbors (see figure 1.c for a sketch of the lattice and the definition of the sites). The simulated structure was hematite. Interatomic distances for bulk hematite and bulk magnetite (for the two first neighbor shells of an absorbing Fe atom in Oh site) obtained from FEFF calculations are also shown with red (short and wide) dashed lines and black solid line respectively.

Our results suggest that a crystallographic phase transition from hematite to magnetite is initiated at a UHV annealing temperature between 350°C and 400°C, as it can also be seen from the XPS spectra. Bertram et al. studied the reduction upon UHV annealing of maghemite (γFe2O3) thin films (thickness around 8 nm) deposited on MgO (001) by molecular beam epitaxy37.


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The authors reported that the critical temperature for the reduction from γ-Fe2O3 to Fe3O4 is 360°C. Our findings are consistent with these results. Lastly, one should note that attempts to fit the EXAFS oscillations considering a magnetite (spinel) structure were unsuccessful for all samples; in particular the output fit parameters were physically meaningless (negative DW factors). This validates the choice of the hematite structure for all vac-annealed samples and shows that even for the 450-vac sample the crystallographic structure is mainly corundum, with a negligible amount of magnetite inclusion. Photocurrent results for vac-annealed samples are shown on figures 8.a and 8.b.

Figure 8: (a) Photocurrent vs. voltage curves for vac-annealed α-Fe2O3 films grown on Pt (111): as-grown (black open diamonds), 270-vac (blue plus), 350-vac (red triangles), 400-vac (green open circles) and 450-vac (orange open squares). (b) Photocurrent at a bias of 0.6 V vs. Ag/AgCl as a function of the UHV annealing temperature with the same symbol coding as (a). The grey dashed line is only a guide for the eye.


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By comparison with the as-grown sample, a 270°C UHV-annealing temperature does not induce significant changes in the photocurrent. For an annealing temperature of 350°C we observe a reduction of 0.2 V for the onset potential and a photocurrent gain of 50 % with respect to the as-grown sample, which corresponds to a substantial improvement as compared to asgrown hematite. Above this temperature, i.e. for the 400-vac and 450-vac samples, the photocurrent decreases. For these two last samples, Fe2+ species were detected in XPS and structural modifications were evidenced by EXAFS. Consistently with our first section, the photocurrent decrease above 350°C can be explained by the onset of magnetite inclusions (as detected by EXAFS) induced by a too large Fe2+ concentration (as detected by XPS) which cause charge recombination at the hematite/magnetite interfaces. The results shown in this section indicate that there is a critical oxygen vacancies concentration above which: •

Fe2+ species are detected in XPS by a decrease of the Fe3+ shake-up satellite and a slight shift of the Fe2p3/2 peak toward lower binding energies;

•

A phase transition toward the Fe3O4 inverse spinel structure is initiated likewise by the presence of magnetite inclusion in the layer;

•

The photocurrent decreases.

From the photocurrent point of view, the optimal O vacancy concentration is obtained by UHV annealing at 350°C for 15 min, as it can be seen on the figure 8.b. This optimal concentration lies just below the detection limit of Fe2+ in hematite as observed by (i) XPS (through the analysis of the Fe3+ satellite), (ii) Fe K pre-edge analysis or (iii) crystallographic changes recorded in EXAFS (through the determination of the interatomic distances). It thus likely corresponds to the highest concentration of O vacancies possible without changing the crystallographic structure or


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initiating a phase transition. We have therefore found a new possibility to optimize hematite films for solar water splitting: the introduction of a controlled rate of oxygen vacancies. The question of stability of this optimized sample against new supplementary oxidation or reduction has also been addressed. Indeed, we checked the reproducibility of photocurrent measurements several times and that EXAFS spectra remain identical after photocurrent measurements (oxidation condition) or stay in air.

Discussion: We can now merge the two parts of the present study considering the correlation between the photocurrent and the annealing temperatures for air and vacuum annealing (figure 9). In this figure we highlighted in red the zone where inclusions of Fe3O4 were detected by XPS and/or EXAFS (labelled zone II) in the sample. We can evidence that the presence of magnetite inclusions is correlated with a low photocurrent. In the zone highlighted in blue (labelled zone I), the hematite layer accommodates the incorporation of O vacancies without any crystalline structure changes and one observes an increase of the photocurrent of 50 % for the best case. This result highlights the beneficial role of oxygen vacancies for solar water splitting but also reveals the detrimental role of magnetite inclusions.


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Figure 9: Photocurrent at 0.6 V vs. Ag/AgCl as a function of the annealing temperature for vacannealed samples (blue open circles, left curve) and air-annealed samples (red open squares, right curve). The different samples set data are separated by the dashed black line. Zones I and II (separated wide dashed-double dotted black line) correspond respectively to the absence and the existence of magnetite inclusions within the sample.

Considering that our optimal sample corresponds to the maximal homogeneous concentration of oxygen vacancies prior detectable bulk Fe2+, our observations are consistent with recent results reported by Li et al. showing improved photocurrent of H2 treated sintered hematite nanostructures attributed to the presence of near surface only Fe2+.27 One may also note that previous works showing the beneficial role of surface Fe2+ in the improvement of the photocurrent of hematite photoanodes26,27 correspond to samples with large specific surfaces, as compared to our flat films in this study. This is consistent with the photocurrent increase we observed in a previous work for etched single crystalline layers15. Manipulating the bulk oxygen vacancies concentration appears thus as an extra possible parameter to improve hematite based photoanodes.


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IV.

CONCLUSIONS

Our single crystalline nanometric thin films grown by AO-MBE proved to be suitable systems to study independently the role of the structure and oxygen content effects on iron oxides physical and photoelectrochemical properties. We explored the two possible routes consisting of magnetite oxidation and hematite reduction. The gap opening of Fe3O4 layers annealed in ambient air occurs through an iron oxidation (Fe2+→Fe3+) is observed as accompanied by crystallographic structure changes leading to a magnetite/hematite mix that appears highly detrimental to the photocurrent. Oxygen vacancies introduced in stoichiometric hematite through annealing in UHV allowed studying the small oxygen vacancies content regime. Importantly, at small concentration they do not destabilize the corundum lattice and their concentration can be optimized. This optimal concentration, inducing a 50 % photocurrent gain and reducing significantly the onset potential by 0.2 V with respect to as-grown hematite, is exceeded when Fe2+ species start to be detected by XPS, Fe K pre-edge analysis or by crystallographic changes in EXAFS. In this study we disentangled the relative role of the stoichiometry and phase changes in iron oxide thin films in the framework of photoanode optimization. We demonstrated the possibility to use bulk oxygen vacancies as a degree of freedom to optimize the photocurrent properties of iron oxide films provided the corundum structure is preserved. We also correlated the photocurrent properties to crystallographic and electronic structures changes upon oxygen vacancies generation or disappearance, thanks to the use of perfectly controlled single crystalline systems and relatively low temperatures annealing procedures.


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ASSOCIATED CONTENT Supporting Information. Supporting Information document includes: area of the valence band region as a function of air annealing temperature, comparison of the RHEED patterns between air-annealed samples and a linear combination of spinel and corundum structures, normalized Fe2p satellite area as a function of vacuum annealing temperature, experimental XANES spectra for UHV-annealed samples , examples of fitting curves and details of structural parameters (interatomic distances and DW factors) obtained by EXAFS simulations at the Fe K edge for asgrown and UHV-annealed 15 nm thick hematite films. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail : [email protected], +33 (0)1 69 08 94 04. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This work was funded in part by the CEA program DSM-Energie under the grant “Hemaphoto”.


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