Assessing the Suitability of Iron Tungstate (Fe2WO6) as a

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Assessing the Suitability of Iron Tungstate (FeWO) as a Photoelectrode Material for Water Oxidation Fatwa F. Abdi, Abdelkrim Chemseddine, Sean P. Berglund, and Roel van de Krol J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10695 • Publication Date (Web): 15 Dec 2016 Downloaded from http://pubs.acs.org on December 20, 2016

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The Journal of Physical Chemistry

Assessing the Suitability of Iron Tungstate (Fe2WO6) as a Photoelectrode Material for Water Oxidation Fatwa F. Abdi*, Abdelkrim Chemseddine, Sean P. Berglund and Roel van de Krol Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute for Solar Fuels, HahnMeitner-Platz 1, 14109 Berlin, Germany

ACS Paragon Plus Environment

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ABSTRACT

Orthorhombic iron tungstate (Fe2WO6), with a reported bandgap of ~1.5-1.7 eV, is a potentially attractive material as the top absorber in a tandem photoelectrochemical (PEC) device. Few studies have been carried out on this material, and most of the important optical, electronic, and PEC properties are not yet known. We fabricated thin film Fe2WO6 photoanodes by spray pyrolysis and identified the performance limitations for PEC water oxidation. Poor charge separation is found to severely limit the photocurrent, which is caused by a large mismatch between the light penetration depth (~300 nm) and carrier diffusion length (< 10 nm) of the material. In addition, the conduction band of Fe2WO6 lies 0.65 V positive of the reversible hydrogen electrode potential, which means that a large external bias potential is required for water oxidation. Based on these observations, we critically discuss the suitability of Fe2WO6 as a novel photoelectrode material for photoelectrochemical and photocatalytic applications.


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TEXT Introduction The development of metal oxide photoelectrodes for solar water splitting is particularly attractive mainly due to their general (photo-)chemical stability in water and relatively low cost. So far, most efforts have been devoted to simple binary metal oxides. Nearly all candidates have been studied, but only a few have shown photocurrents larger than 3 mA/cm2: Fe2O3, WO3 and Cu2O.1-3 Of these, WO3 is limited by its relatively large bandgap (~2.6 eV), and progress on Fe2O3 is severely hampered by its extremely short carrier lifetime and diffusion length. Cu2O appears to be a more viable candidate, showing a nearly ideal bandgap (2.0 eV) and highly promising photocurrents (~10 mA/cm2 in a very recent report).4 But it requires extensive modification with suitable corrosion protection layers and still falls short of its theoretical potential.2,5 To have a realistic chance of finding novel light absorbers that meet the demanding requirements for practical applications, we urgently need to expand our material database and explore more complex metal oxides. More than 8,000 and 700,000 combinations are possible for ternary and quarternary metal oxides, respectively, which clearly opens up a new realm of possibilities in finding an ideal semiconducting material for solar water splitting. Learning from the discoveries of high-temperature superconductors, such as mercury barium calcium copper oxide (a quintenary oxide!),6 the development of multinary (‘complex’) metal oxides in solar water splitting cannot be avoided. An example of a complex metal oxide that has gained a considerable success in the last few years is bismuth vanadate (BiVO4). While the material initially showed poor catalytic activity and extensive bulk recombination, efforts in depositing co-catalysts, doping, and nanostructuring have successfully improved the photoelectrochemical performance.7-11 Photocurrents larger than


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3 mA/cm2 have been reported by many research groups,10-16 and solar water splitting devices with solar-to-hydrogen (STH) efficiencies higher than 5%—the highest for oxide-based solar water splitting devices—have been fabricated based on tandem configurations of a BiVO4 photoelectrode and a variety of different solar cell technologies (e.g., thin film Si, perovskites, and III-V materials).13-17 However, the material suffers from one intrinsic limitation: the bandgap is indirect at 2.4-2.5 eV (the direct bandgap is ~2.7 eV),18 which is too large for practical solarto-fuel applications. Only ~9% STH efficiency can be theoretically achieved, assuming that all solar photons with energies >2.4 eV are absorbed and that 100% of the charge carriers reach the interface. Smaller bandgap oxides are therefore desired to push the STH efficiency further. A recent study by Seitz et al. outlined that for dual-absorber tandem systems, taking into account the overpotentials for the H2 and O2 evolution reactions, a top absorber with a bandgap of 1.7-1.9 eV and a bottom absorber with a bandgap of ~1.2 eV can achieve STH efficiencies above 20%.19 While there are already several good options for the bottom absorber (e.g. Si, WSe2),20-23 there are currently few—if any—obvious candidates for the top absorber. Finding an efficient and stable complex metal oxide absorber with bandgap of 1.7-1.9 eV would therefore be a major breakthrough for the field. One possible candidate is iron tungstate, Fe2WO6. It has an orthorhombic crystal structure; columbite at lower temperature (< 800 °C), and tri-α-PbO2 at temperatures higher than 800 °C.2429

The crystal structure of the phase is shown in Figure 1. At present, the material is relatively

unexplored. There have been only ~30 publications on Fe2WO6 (7 in the last decade), and only 2 are relevant to photoelectrochemistry or photocatalysis.29,30 As a result, many key material parameters are not known. The dominant conductivity type is still not clear, with both p- and ntype conductivity reported, and little is known about the carrier mobility and lifetime.28,30,31


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However, it is known that the bandgap is ~1.5-1.7 eV,30,31 which means that more than 20% STH efficiency can potentially be reached with this material. In addition, some degree of photocatalytic activity has been reported29,30—although it is unclear how the different crystal structures or phases of Fe2WO6 affects the photocatalytic activities—which makes this a potentially interesting candidate as a photoelectrode material for water splitting.

Figure 1. (a) Crystal structure of Fe2WO6, which consists of FeO6 and WO6 octahedrons. Views along the (100) and (001) planes are shown in (b) and (c), respectively.

In this study, we have developed a spray pyrolysis recipe for the deposition of crystalline thin film Fe2WO6 photoelectrodes on FTO-coated glass (F-doped SnO2). The sprayed films show ntype conductivity and have an indirect bandgap of 1.7 eV. The photocurrent density is modest, although a heat treatment at 800 °C improves it by a factor of > 2. Using a combination of photo-


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electrochemical techniques, we have identified the underlying causes for these improvements and were able to pinpoint the remaining bottlenecks. Based on these findings, we will provide a realistic assessment of the suitability of Fe2WO6 as a photoelectrode material for solar water splitting.

Experimental Section Thin films of Fe2WO6 were prepared using a low-cost and facile spray pyrolysis process. For details of the experimental setup we refer to our previous reports.8,32 The precursor solution was made by dissolving 5 mM Fe(AcAc)3 (99.9%, Alfa Aesar) and 2.5 mM W(OC2H5)6 (99.8%, 5% w/v in ethanol, Alfa Aesar) in 100 mL of ethanol. 1 vol% triethyl orthoformate (TEOF, 98%, Fluka Analytical) was added to each solvent prior to the addition of the metal precursors in order to remove trace amounts of water and prevent premature hydrolysis of the metal precursors. The substrate (FTO-coated glass, 7 Ω per square, TEC-7, Pilkington) was placed on a heating plate, and the temperature was set at 500 °C. A total solution volume of 100 mL was sprayed in 60 cycles, with each cycle consisting of a 5 s on, 55 s off sequence to allow the solvent to evaporate from the substrate and the acetylacetonate ligands to be pyrolyzed. Some of the samples were post-annealed at 800 °C in technical air (20% O2, 80% N2) for 10 hours; for these samples, FTOcoated quartz (15 Ω per square, Solaronix) was used as a substrate. In addition, several films were deposited on bare quartz substrates (Spectrosil 2000, Heraeus) to study the optical properties, crystal structure, and microwave conductivity. Photoelectrochemical characterization was carried out in a three-electrode configuration. Electrical contact to the sample was made using a copper wire and conducting tape (tin-plated copper foil with conductive adhesive, 3M 1183). The potential of the working electrode (i.e.,


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the sample) was controlled by a potentiostat (EG&G PAR 273A). A Pt wire and an Ag/AgCl electrode (XR300, saturated KCl and AgCl solution, Radiometer Analytical) were used as the counter and reference electrodes, respectively. The electrolyte was a 0.1 M potassium hydroxide (KOH) solution, pH ~13, in demineralized and deionized water (18.2 MΩ.cm). Cyclic voltammetry measurements were performed with a scan rate of 50 mV/s. White light photocurrent measurements were performed under AM1.5 solar illumination (100 mW/cm2) with a solar simulator (WACOM, type WXS-50S-5H, class AAA). Monochromatic photocurrents were measured with a 300 W Xenon lamp (Oriel) coupled into a grating monochromator (Acton Spectra Pro 2155). Long pass filters (3 mm thick, Schott) were used to block any 2nd-order diffracted light. The monochromatic light intensities were measured with a calibrated photodiode (PD300UV diode read by a Nova II controller, Ophir Photonics) and ranged between 0.1 – 0.8 mW/cm2. Mott-Schottky (1/C2 vs V) measurements were done using Zahner Elektrik IM6 potentiostat at a frequency of 1 kHz. Intensity modulated photocurrent spectroscopy (IMPS) measurements were done at a fixed bias potential applied by an EG&G 283 potentiostat. The sample was illuminated by a 455 nm LED driven by a DC2100 LED driver, both from Thorlabs. The LED provided a constant 4 mW/cm2 illumination intensity, onto which a sinusoidal 0.6 mW/cm2 amplitude modulation was superimposed by connecting the signal output of a frequency response analyser (FRA, Solartron 1250) to the LED driver. The light intensity was measured by reflecting part of the incident light off a quartz window onto a high-speed reference Si photodiode (Thorlabs PDA10A-EC). The photodiode’s signal and the photocurrent generated by the sample were fed into the FRA to determine the opto-electrical impedance, given by Z = Φlight


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/ jphoto. The measured impedance was then normalized to the absolute value of the incident photon flux, which was measured separately with the PD300UV photodiode. Structural analysis was performed with a Bruker D8 Advance x-ray diffractometer (Cu-Kα, λ = 0.154056 nm) in the grazing incidence configuration. Scanning electron micrographs were taken with a LEO GEMINI scanning electron microscope at an accelerating voltage of 2-5 kV. Ultraviolet-visible absorption data were measured with a Perkin Elmer Lambda 950 spectrometer. X-ray photoelectron spectroscopy (XPS) was performed with a monochromatic Xray source (SPECS FOCUS 500 monochromator, Al Kα radiation, 1486.74 eV) and a hemispherical analyzer (SPECS PHOIBOS 100). The pass energy was set to 10 eV with step sizes of 0.05 eV. Ultraviolet photoelectron spectroscopy (UPS) was performed with He-I source (E = 21.218 eV) and the same hemispherical analyzer as in the XPS measurement. Both XPS and UPS measurements were performed in a UHV system with a base pressure in the low 10-8 mbar range.

Results and Discussions Figure 2a shows a photograph of a thin film Fe2WO6 photoelectrode deposited on an FTOcoated glass substrate. The film has a dark brown color, indicating good visible light absorption. The morphology of as-deposited films consist of grain-like structures with some degree of surface roughness (Supplementary Fig. S1). Upon annealing at 800 °C, the morphology of the film changes, as shown by the scanning electron micrograph (SEM) in Fig. 2b. The particles seem to coalesce, resulting in reduced surface roughness and denser films, as also confirmed by the cross-section SEM shown in the inset of Fig. 2b. From the inset, the thickness of the film is found to be ~400 nm.


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Figure 2. (a) Photograph of a Fe2WO6 thin film deposited on an FTO-coated glass substrate. (b) Scanning electron micrograph (SEM) of Fe2WO6 thin film deposited on FTO, after annealing at 800 °C in air. Inset shows the cross-section SEM of this film. (c) X-ray diffractogram for the asdeposited (black) and annealed (red) Fe2WO6 films deposited on FTO substrate.

To confirm and quantify the reduction in surface roughness, the electrochemically active surface area was determined with capacitive current measurements. Briefly, this consists of measuring current-voltage curves as a function of scan rate; in the potential range where the forward and reverse currents are constant and equal in magnitude (but opposite in sign), the current is entirely due to capacitive charging of the surface. By plotting this current as a function of scan rate a linear curve is found with a slope given by the capacitance, which is proportional to the electrochemically active area. The analysis for our films shows that the electrochemically


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active surface area is reduced by a factor of 1.8 ± 0.1 after annealing at 800 °C (Supplementary Fig. S2). X-ray photoelectron spectroscopy (XPS) reveals that the chemical nature of Fe and W on the surface does not change significantly (i.e., no shift in peak positions), but the stoichiometry of the surface is improved upon annealing at 800 °C; the Fe/W ratio increases from ~1.3 at 500 °C to ~1.8 at 800 °C (Supplementary Fig. S3). X-ray diffractograms (Fig. 2c) confirm that both as-deposited and annealed films have the orthorhombic Fe2WO6 phase, despite the relatively low deposition temperature of 500 °C. Interestingly, this is much lower than previous reports on Fe2WO6, which always required synthesis temperatures above 750 °C.25,27,30,31 Since spraying under the same conditions on quartz substrates yields amorphous films (Supplementary Fig. S4), we conclude that the underlying FTO is responsible for the crystallization at these comparatively low temperatures. Only after annealing at 800 °C did we observe the Fe2WO6 peaks for the film on quartz. Annealing the films deposited on FTO at 800 °C appears to further improve the crystallinity of the Fe2WO6 films. However, it should be noted that the apparent growth of the Fe2WO6 peaks may be partially due to a decrease and broadening of the FTO peaks at these high temperatures.


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Figure 3. Absorption coefficient (α) of Fe2WO6 film annealed at 800 °C in air as a function of photon energy and wavelength.

The optical characteristics of the films are analyzed by UV-Vis spectroscopy. Based on the thickness (d) of ~400 nm, the absorption coefficient (α) of the films can be calculated according to the following equation:

α=

− ln(1 − A) d

(1)

where A is the absorption of the films. Figure 3 shows α of the Fe2WO6 film upon annealing at 800 °C as a function of photon energy. In the region of the reported bandgap of 1.5-1.7 eV,30,31 α is in the order of 103 cm-1, indicating relatively weak absorption. The absorption coefficient reaches a maximum value of 3 × 104 cm-1 above 3.0 eV, which corresponds to a penetration depth of α-1 = 333 nm. However, between the bandgap region and the maximum absorption, α increases with a relatively modest slope, which suggests that the optical transition in the Fe2WO6 film is indirect.


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Figure 4. (a) Tauc plot for bandgap analysis of as-deposited (open squares) and annealed (closed squares) Fe2WO6 films. Indirect and direct bandgaps are estimated from the intercept of the (αhν)0.5 and (αhν)2 curves with the x-axis, respectively, and shown in the inset table. (b) Incident-photon-to-current conversion efficiencies (IPCE) of the annealed (800 °C) Fe2WO6 film as a function of monochromatic illumination wavelength at applied potentials of 1.2 and 1.45 V vs RHE. The inset shows the photocurrent response under chopped illumination at wavelengths close to the indirect bandgap of Fe2WO6.

Based on the absorption coefficient spectra, the resulting Tauc plots are calculated and shown in Fig. 4a. The indirect and direct bandgaps for the as-deposited film (500 °C) are found to be 1.6 and 2.3 eV, respectively. Annealing at 800 °C increases the bandgap slightly, resulting in values of 1.7 and 2.45 eV, respectively. We attributed this small change of bandgap to the presence of small sub-stoichiometric x-ray amorphous phases at low deposition temperature, which are annealed out at high temperature. This is in agreement with our XPS results described above.


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An open-circuit potential (OCP) measurement under chopped AM1.5 illumination reveals that the films show n-type conductivity, i.e., the OCP shifts to a more negative value upon illumination (Supplementary Fig. S5). The incident photon-to-current conversion efficiency (IPCE, also called external quantum efficiency—EQE) of the annealed film is shown in Fig. 4b. The IPCE values are relatively low; at a wavelength of 350 nm, we achieve only ~2% at 1.2 V vs RHE and ~9% at 1.45 V vs RHE. Although it seems from Fig. 4b that the onset of the IPCE lies close to 600 nm (~2.1 eV), we observe photocurrents up to 700 nm (see inset of Fig. 4b), which corresponds well with the indirect bandgap of the material. This confirms that the films are indeed photoactive under illumination with photon energies above the bandgap, although quantum efficiencies are low. Figure 5a shows the AM1.5 photocurrent-voltage curve for both the as-deposited (500 °C) and annealed (800 °C) films. The photocurrent of the as-deposited film is modest, 30 µA/cm2 at 1.23 V vs RHE and 50 µA/cm2 at 1.45 V vs RHE. Moreover, the photocurrent only starts above ~1 V vs RHE, which is a rather positive value compared to most other photoanode materials. For the as-deposited film a pseudo-plateau photocurrent between 1.3 and 1.5 V vs RHE is observed, beyond which the photocurrent increases steeply. This indicates extensive surface or bulk recombination that is quickly suppressed as soon as the potential exceeds ~1.5 V vs RHE. A possible explanation for such behavior is Fermi level pinning, but more detailed studies would be needed to confirm this. Annealing at 800 °C increases the photocurrents to 70 µA/cm2 at 1.23 V vs RHE and 250 µA/cm2 at 1.45 V vs RHE, improvements of a factor of 2 and 5, respectively. The onset potential is not significantly affected. Furthermore, the pseudo-plateau photocurrent regime is now no longer observed. If our tentative explanation of Fermi level pinning is true, this


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may indicate that the high temperature treatment has passivated the states that are responsible for pinning the Fermi level.

Figure 5. (a) Three-electrode AM1.5 photocurrent-voltage (J-V) curve of as-deposited (500 °C, black) and annealed (800 °C, red) Fe2WO6 films on FTO substrates. The dark currents are shown as dashed curves. The electrolyte used is 0.1 M potassium hydroxide (KOH) and the scan rate is 50 mV/s. (b) Efficiencies for charge injection (closed symbols) and charge separation (open symbols) for the as-deposited and annealed films as a function of applied bias.

To understand the reason behind the photocurrent improvement, we performed detailed photoelectrochemical analysis of our Fe2WO6 films. First, the photocurrents are measured in the presence of a hole scavenger, H2O2, to distinguish charge separation from charge injection limitations, as described in previous reports.8,33 From this, the charge separation efficiency in the semiconductor (ηsep) and charge injection efficiency from the surface of the semiconductor to the electrolyte (ηinj) can be calculated:


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ηsep =

J H 2O2

ηinj =

J H 2O

J abs

J H 2O2

(2)

(3)

Jabs is the photon absorption rate expressed as a current, JH2O is the photocurrent for water oxidation and JH2O2 is the photocurrent in the presence of H2O2 as a hole scavenger. Figure 5b shows the calculated charge separation and charge injection efficiencies for both the as-deposited (500 °C, black curves) and annealed (800 °C, red curves) films. First, both the charge separation and injection efficiencies increase upon annealing; the cause of improvement is detailed below. The charge injection efficiency is modest (0.2-0.3 at 1.23 V vs RHE), but can reach values up to 0.9 at potentials exceeding 1.6 V vs RHE. Alternatively, one could try to deposit surface passivating or co-catalyst layers to improve the injection efficiency at low potentials.7,34,35 To understand why the charge injection efficiency (ηinj) improves upon annealing at 800 °C, an intensity modulated photocurrent spectroscopy (IMPS) analysis was carried out. With IMPS, the films are subjected to modulated illumination, and the modulated photocurrent response is measured. From the in- and out-of-phase (real and imaginary) components, and assuming a simplified model for charge injection and surface recombination,36-38 these two surface processes can be de-convoluted and quantified. Further details on the theory and analysis of IMPS measurements can be found in the supplementary information (Supplementary note 1) and in the literature.36-42 The first order charge injection (kinj) and surface recombination (krec) rate constants of both asdeposited and annealed films determined by IMPS are shown semi-logarithmically in Figure 6a. Starting with the as-deposited film (500°C), we observe two distinct potential regimes. At potentials lower than ~1.4 V vs RHE, the surface recombination rate constant decreases slightly


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as a function of applied potential, whereas the charge injection rate constant remains more or less constant. At potentials > 1.4 V vs RHE, the surface recombination decreases much more rapidly, and the charge injection rate constant makes a modest but pronounced step to higher values. The behavior of kinj and krec is indicative of the presence of a high density of surface states causing Fermi level pinning,43,44 consistent with our interpretation of the intermediate photocurrent plateau in the I-V curve of Fig. 5a. At potentials above 1.4 V vs RHE, the Fermi level unpins, and further changes in the applied potential now drop mostly across the space charge layer. This explains the rapid decrease of the surface recombination rate constant at potentials > 1.4 V vs RHE.

Figure 6. (a) Semi-logarithmic plot of charge injection (open symbols) and surface recombination (closed symbols) rate constants of both as-deposited (500 °C, black) and annealed (800 °C, red) Fe2WO6 films as a function of bias potential, obtained from IMPS measurements. The vertical dashed lines distinguish two potential regimes for each of the films, as described in the text. (b) Charge injection efficiency from the IMPS measurement, defined as kinj/(kinj + krec) as a function of bias potential of both as-deposited and annealed Fe2WO6 films.


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Annealing at 800 °C has significant effects on both curves: the charge injection curve is shifted cathodically by ~400 mV and the recombination rate constant is suppressed by more than 1 order of magnitude across the entire potential range. These effects are presumably correlated, since a cathodic shift of the band edges (Fermi level unpinning) is expected to increase the band bending at any value of the applied potential. We tentatively attribute the cathodic shift to a decrease of the surface state density. Part of this reduction is due to the decrease in the effective surface area upon annealing at 800 °C (vide supra). The main factor, however, is likely to be a direct consequence of annealing out defects at such a high temperature, analogous to the decrease of the bulk donor density observed from the Mott-Schottky results. The charge injection efficiency based on these IMPS data, kinj/(kinj+krec), is shown in Fig. 6b for both films. These curves track the current-voltage curves shown in Fig. 5, which indicates that the improved performance is indeed caused by an enhanced injection efficiency at lower potentials. To confirm that a reduction in surface state density can indeed cause a cathodic shift of the band edges, we simulated the effect of surface state density on the photocurrent density using a modified Gärtner equation.40,41 We found that a reduction of surface state density indeed leads to a cathodic shift in the current-potential curve (Supplementary note 2), in agreement with the interpretation of our results. The charge injection is however not the main problem with our films; charge separation is. Figure 5b shows that even after annealing at 800 °C the charge separation efficiency remains lower than 7%. This means that more than 93% of the charge carriers recombine before they reach their respective interfaces. Enhancing the charge separation efficiency is therefore the first


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bottleneck that needs to be addressed when attempting to improve the overall photoelectrochemical performance of sprayed Fe2WO6 films. A key factor that determines the charge separation efficiency is the presence of the internal electric field, which in turn depends on the donor density. This can be determined from a MottSchottky plot (C-2 vs. applied potential), provided that certain criteria are met; the main criterion is that the overall response of the system is dominated by the capacitance of the space charge layer. As shown in the Bode plot of our sprayed Fe2WO6 films (Supplementary Fig. S6), this is the case for frequencies around ~1 kHz, where the real part of the impedance is constant, and the imaginary part has a slope close to -1 when plotted against frequency on a log-log scale. A MottSchottky plot of the film, measured at 1 kHz, is shown in Fig. 7. For the linear part of the slope, the following equation applies:

1 2 kT   = V − VFB −  2 2  appl C eN Dε 0ε r As  e 

(4)

Here, e is the elementary charge, ND is the donor density, ε0 is the permittivity of vacuum, εr is the dielectric constant (~410 for Fe2WO6 at 300 K31), As is the surface area of the photoelectrode, Vappl is the applied potential, VFB is the flatband potential, k is Boltzmann constant, and T is temperature. From the slope of Fig. 7, a donor density of ~1 × 1020 cm-3 is calculated for the asdeposited film, which decreases to ~2 × 1019 cm-3 after annealing. This 5-fold reduction in donor density is equivalent to an increase of the space charge layer width by a factor of ~2.2 (i.e., √5). The wider space charge layer means that an internal electric field is present in a larger part of the film. The charge separation efficiency (Fig. 5b) indeed improves by a factor of ~2 after annealing, and we thus attribute the enhanced photocurrent to a decrease in the carrier density.


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Figure 7. Mott-Schottky plot of the as-deposited (500 °C, black) and annealed (800 °C, red) Fe2WO6 films, measured at 1000 Hz.

We will now estimate the positions of the conduction and valence bands relative to the water reduction and oxidation potentials. According to equation 4, one can estimate the flatband potential of a semiconductor by taking the x-axis intercept of the linear region of a MottSchottky curve (the thermal energy kT/e at room temperature is relatively small, ~26 mV, and can be neglected). The flatband potentials of the as-deposited and annealed samples are ~0.6 and 0.65 V vs RHE, respectively. From the measured donor density values, the difference between the Fermi level (EF) and the conduction band (EC) can be estimated with the expression

 −( EC − EF )  N D = n = N C exp   kT  

(5)

where NC is the effective density-of-states (DOS) at the bottom of the conduction band. Values for NC typically range from 1019 – 1020 cm-3, which places EF within less than 60 meV of the conduction band edge. We can therefore estimate that the conduction band and valence band of Fe2WO6 lie at (0.60 ± 0.06) V vs RHE and (2.30 ± 0.06) V vs RHE, respectively. Ultraviolet


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photoelectron spectroscopy (UPS) reveals that the valence band maximum is located ~1.7 eV below the Fermi level (Supplementary Fig. S7), which means that the Fermi level lies very close to the conduction band. This is consistent with the high donor density obtained from the MottSchottky measurement. While the holes in the valence band of Fe2WO6 are thermodynamically able to oxidize water, the conduction band lies at much too positive potentials for hydrogen evolution. Consequently, a relatively large external bias potential needs to be applied before a photocurrent can be observed. This is consistent with the very positive photocurrent onset potential observed in Fig. 5a. Based on the donor density and flatband potential, the width of the space charge layer can be calculated according to the following equation:

W=

2ε 0ε r  kT  Vappl − VFB −  eN D  e 

(6)

Even at potentials as high as 1.6 V vs RHE, a space charge layer of only ~40 nm exists within the annealed film. Since the incident light penetrates the film much further, this means that the majority of the photo-excited carriers in these ~400 nm thick films are transported via diffusion, i.e., there is no internal electric field to separate the carrier transport via a drift mechanism. In an attempt to measure and quantify the carrier diffusion length, time-resolved microwave conductivity (TRMC) measurements have been performed. However, no TRMC signal could be detected from our films (Supplementary Fig. S8). Based on the detection limit of our setup, this means that either the carrier lifetime is less than 1 ns, the carrier mobility is lower than 10-3 cm2/Vs, or both. From these values an upper limit of 10 nm is calculated for the carrier diffusion length. This very small value is consistent with the fact that we observe very low charge separation efficiencies (Fig. 5b).


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It is important to note that the donor density of our film is very high for a typical semiconductor, even for the annealed sample (2 × 1019 cm-3). The exact origin of this is unknown, but it is presumably a result of the presence of impurities or other types of point defects in the film. A donor density of 2 × 1019 cm-3 corresponds to a concentration of 0.02% (200 ppm), so even a small amount of defects can explain the high donor density. Possible impurity sources are the Fe and W precursors, with purities of 99.9% and 99.8%, respectively. The inevitable limitation on weighing accuracy during the preparation of the precursor solution may also play a role, and can easily lead to deviations exceeding 0.02% of the ideal 2 : 1 (Fe : W) stoichiometry. The high donor density after extensive annealing at 800 °C suggests that Fe2WO6 is a rather unforgiving material, in the sense that its tolerance for defects that affect the performance of the photoelectrode is low.

Conclusions In summary, we have assessed the suitability of Fe2WO6 as a candidate photoanode material. We successfully deposited orthorhombic n-type Fe2WO6 via spray pyrolysis at relatively low temperature (500 °C). Optical analysis yields an indirect bandgap value of 1.7 eV, which is ideal as a top absorber in a stacked tandem configuration. Unfortunately, many challenges have to be overcome to make this a viable photoabsorber. Firstly, the bandgap is indirect, and the absorption coefficient is rather small (< 104 cm-1 up to 2.4 eV). Moreover, bulk recombination was found to limit the photoelectrochemical performance. The absence of a TRMC signal indicates that the photogenerated charge carriers have either a short lifetime ( 1.0 V). Since the optimum bandgap for a bottom absorber in combination with Fe2WO6 (Eg = 1.7 eV) is ~1.2 eV,19,45 it is highly unlikely that such a large open circuit potential can be achieved, unless the bottom cell itself is a tandem junction. Although strategies exist to address any one of these challenges individually (e.g., doping to overcome the low carrier mobility, nanostructuring to address the short diffusion length, plasmonic enhancement to improve the absorption), the efforts needed to solve all of them at the same time are likely to be prohibitive. Instead, these efforts are better spent on other complex metal oxides such as the metal vanadates, niobates, cuprates or other tungstates, several of which have shown promising intrinsic properties.46-53 Having said that, Fe2WO6 may still be considered for other applications in photocatalysis that would benefit from the narrow bandgap, the strong oxidation power of the photo-generated holes, and the abundance and non-toxicity of the constituents. Typical examples are in environmental remediation, such as air and waste-water purification. Finally, we argue that the methodology presented in this work represents a robust method for the critical evaluation of photoelectrode materials. Our work reveals many of the important fundamental properties of a promising low bandgap complex metal oxide. There are many candidate materials and limited research resources, so we have to ‘fail quickly’ while avoiding false negatives. Establishing the key intrinsic properties—and associated bottlenecks—is an


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essential first step in this process that will expedite the selection and development of new materials. This rigorous methodology should therefore be widely applied to other promising materials for photoelectrochemical energy conversion applications. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Supplementary Fig. S1-10, Supplementary note 1 and 2 (PDF).

AUTHOR INFORMATION Corresponding Author * [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We thank Christian Höhn for performing the UPS and XPS measurement. Part of this work was supported by the German Federal Ministry of Education and Research (BMBF project “MeOx-4H2”, #03SF0478A). REFERENCES


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Schilling, A.; Cantoni, M.; Guo, J.; Ott, H. Superconductivity Above 130 K in the Hg-BaCa-Cu-O System Nature 1993, 363, 56-58.

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Table of Contents Graphic:


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Figure 1. (a) Crystal structure of Fe2WO6, which consists of FeO6 and WO6 octahedrons. Views along the (100) and (001) planes are shown in (b) and (c), respectively. 88x45mm (220 x 220 DPI)


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Figure 2. (a) Photograph of a Fe2WO6 thin film deposited on an FTO-coated glass substrate. (b) Scanning electron micrograph (SEM) of Fe2WO6 thin film deposited on FTO, after annealing at 800 °C in air. Inset shows the cross-section SEM of this film. (c) X-ray diffractogram for the as-deposited (black) and annealed (red) Fe2WO6 films deposited on FTO substrate. 292x278mm (300 x 300 DPI)



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Figure 3. Absorption coefficient (α) of Fe2WO6 film annealed at 800 °C in air as a function of photon energy and wavelength. 214x190mm (96 x 96 DPI)


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Figure 4. (a) Tauc plot for bandgap analysis of as-deposited (open squares) and annealed (closed squares) Fe2WO6 films. Indirect and direct bandgaps are estimated from the intercept of the (αhν)0.5 and (αhν)2 curves with the x-axis, respectively, and shown in the inset table. (b) Incident-photon-to-current conversion efficiencies (IPCE) of the annealed (800 °C) Fe2WO6 film as a function of monochromatic illumination wavelength at applied potentials of 1.2 and 1.45 V vs RHE. The inset shows the photocurrent response under chopped illumination at wavelengths close to the indirect bandgap of Fe2WO6. 237x165mm (300 x 300 DPI)



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Figure 5. (a) Three-electrode AM1.5 photocurrent-voltage (J-V) curve of as-deposited (500 °C, black) and annealed (800 °C, red) Fe2WO6 films on FTO substrates. The dark currents are shown as dashed curves. The electrolyte used is 0.1 M potassium hydroxide (KOH) and the scan rate is 50 mV/s. (b) Efficiencies for charge injection (closed symbols) and charge separation (open symbols) for the as-deposited and annealed films as a function of applied bias. 178x81mm (300 x 300 DPI)


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Figure 6. (a) Semi-logarithmic plot of charge injection (open symbols) and surface recombination (closed symbols) rate constants of both as-deposited (500 °C, black) and annealed (800 °C, red) Fe2WO6 films as a function of bias potential, obtained from IMPS measurements. The vertical dashed lines distinguish two potential regimes for each of the films, as described in the text. (b) Charge injection efficiency from the IMPS measurement, defined as kinj/(kinj + krec) as a function of bias potential of both as-deposited and annealed Fe2WO6 films. 191x82mm (300 x 300 DPI)



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Figure 7. Mott-Schottky plot of the as-deposited (500 °C, black) and annealed (800 °C, red) Fe2WO6 films, measured at 1000 Hz. 122x85mm (300 x 300 DPI)


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Assessing the Suitability of Iron Tungstate (Fe2WO6) as a

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