Probing the Performance Limitations in Thin-Film FeVO4 Photoanodes

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C: Energy Conversion and Storage; Energy and Charge Transport

Probing the Performance Limitations in Thin Film FeVO Photoanodes for Solar Water Splitting 4

Jianyong Feng, Zhiqiang Wang, Xin Zhao, Guang Yang, Bowei Zhang, Zhong Chen, and Yizhong Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01330 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 22, 2018

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The Journal of Physical Chemistry

Probing the Performance Limitations in Thin Film FeVO4 Photoanodes for Solar Water Splitting Jianyong Feng,‡ Zhiqiang Wang,‡ Xin Zhao, Guang Yang, Bowei Zhang, Zhong Chen, and Yizhong Huang* School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798, Singapore

ABSTRACT FeVO4 is a potentially promising n-type multi-metal oxide semiconductor for photoelectrochemical water splitting based on its favorable optical band gap of ca. 2.06 eV that allows for absorption of visible light up to around 600 nm. However, the presently demonstrated photocurrent values on FeVO4 photoanodes are yet considerably low when comparing with αFe2O3, although FeVO4 can absorb comparable wavelengths of sunlight as α-Fe2O3. Donor-type doping and constructing nanoporous film morphology have afforded desirable (but far from satisfactory) improvements in FeVO4 photoanodes, while the fundamental properties, such as absorption coefficients and the nature of optical transition, and a quantitative analysis of the efficiency losses for FeVO4 photoanodes remain elusive. In the present study, we conduct a thorough experimental analysis of structural, optical, charge transport and surface catalysis properties of FeVO4 thin films to investigate and clarify how and where the efficiency losses occur. Based on the results, the charge recombination pathways and light harvesting loss in FeVO4 thin film photoanodes are identified and quantitatively determined. Our study will deepen

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the understanding on the photoelectrochemical behaviors of FeVO4 photoanodes and will also shed light on the optimization routes to engineer this material to approach its theoretical maximum.

INTRODUCTION The increasingly serious energy crisis and environmental problems require reliable solar energy conversion and storage technologies. Photoelectrochemical (PEC) water splitting represents an attractive way to the above end, via the direct conversion and storage of abundant and widespread solar energy into clean H2.1−5 The seminal work on TiO2 has stimulated significant research activity focused on visible-light responsive metal oxide photoelectrodes such as αFe2O3, WO3, and BiVO4.6−9 Among these metal oxides, Fe-based semiconductors have emerged as promising photoelectrode candadites because they hold several desirable merits for PEC water splitting, such as high theoretical solar-to-hydrogen efficiency and abundance in the earth’s crust.10,11 α-Fe2O3, the most important Fe-based metal oxide semiconductor, has been sufficiently optimized to produce a limited highest photocurrent of ca. 4 mA cm−2, a value far below its theoretical limit.12,13 Since Fe-based multi-metal oxides afford more atomic arrangements and more modification sites in crystal lattices than that of α-Fe2O3, they can provide more additional possibilities to achieve satisfactory performance for PEC water splitting. As a ternary Fe-based metal oxide, n-type FeVO4 is reported to possess a favorable band gap of 2.06 eV that allows for absorption of visible light up to around 600 nm, therefore it is suitable to act as the top or front electrode in the dual-absorber photoanode/photocathode tandem water splitting cells. Up to now, donor-type doping and constructing nanoporous film morphology have offered FeVO4 photoanodes remarkably enhanced photoactivity.14,15 However,


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the presently achieved photocurrent values on reported FeVO4 photoanodes are yet considerably low when comparing with α-Fe2O3, regardless of the fact that FeVO4 can absorb comparable wavelengths of sunlight as α-Fe2O3. Previous studies suggest that poor bulk charge transport and/or surface catalysis (bulk and surface recombination processes) are the main limitations for presently unsatisfying performance on FeVO4 photoanodes.14−16 However, a quantitative analysis of these efficiency losses is still elusive. In addition, the photophysical properties of FeVO4 thin films, such as absorption coefficients and the nature of optical transition (direct vs. indirect), remain to be addressed. In the present study, we conduct a thorough experimental analysis of structural, optical, charge transport and surface catalysis properties of FeVO4 thin films to investigate and clarify how and where the efficiency losses occur. Based on the results, the charge recombination pathways and light harvesting loss in FeVO4 thin film photoanodes are identified and quantitatively determined. Our study is helpful for future optimization of FeVO4 photoanodes to engineer this material to approach its theoretical maximum. Meanwhile, this study will provide valuable implications on the development of other important Fe-based metal oxide semiconductors for solar water splitting. EXPERIMENTAL SECTION Fabrication of FeVO4 Photoanodes. FeVO4 photoanode films were fabricated by using a sol-gel method. In a typical experiment, 0.5 mmol Fe(NO3)3·9H2O (Acros, 99+%), 0.5 mmol NH4VO3 (Sigma Aldrich, 99%), 0.5 mmol succinic acid (Sigma Aldrich, ≥ 99%) were dissolved in 9.7 mL ethylene glycol (Aldrich, 99.8%) and 0.3 mL HNO3 (min. 69%, Honeywell) under sonication. The above precursor solution (40 µL) was dropped onto fluorine doped tin oxide


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(FTO, 15 Ω) glass substrates (1 cm × 1 cm of the coated area), and dried on a hot plate at 80 °C for 20 minutes then at 120 °C for another 20 minutes. The films were then annealed at 500 °C (450 °C and 550 °C) for 1 hour in a box furnace (Nabertherm, W × D × H: 90 × 115 × 110 mm) with a ramp rate of 2 °C per minute. Electrochemical Characterization. PEC measurements were carried out in a threeelectrode configuration cell, with FeVO4 films as the working electrodes, a Ag/AgCl in saturated KCl electrode as the reference electrode, and a Pt foil as the counter electrode. The electrolyte was a 0.1 M potassium phosphate aqueous solution (pH = 7). The electrochemical measurements were conducted with a PCI4/300TM potentiostat (Gamry Instruments). Potentials were reported vs. the reversible hydrogen electrode (RHE) unless noted otherwise, with ERHE = EAg/AgCl + 0.197 + 0.0591 pH. Photocurrent densities were recorded under AM 1.5 G simulated sunlight (100 mW cm−2), from an Asahi HAL-320 EX3 simulator. The light intensity of the sunlight simulator was calibrated at 100 mW cm−2 by the standard reference of a Newport 91150V silicon cell before use. The irradiated area was a circle with a diameter of ca. 6 mm. The incident photon to current efficiency (IPCE) was measured under monochromatic light irradiation, provided by the xenon lamp equipped with band pass filters. The light intensity was obtained with a photometer (Newport, 840-C). Mott−Schottky plot was derived from the electrochemical impedance spectroscopy measurements. The measurements were performed using a PCI4/300TM potentiostat (Gamry Instruments) in a 0.1 M potassium phosphate aqueous solution (pH = 7) in the dark. The FeVO4 film was tested at potentials from −0.3 V to 1.4 V versus Ag/AgCl, with frequencies ranging from 1 Hz to 100000 Hz. An AC voltage amplitude of 10 mV was used during the electrochemical impedance spectroscopy measurements.


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Sample Characterization. The crystal structures of all the samples were measured by thin film X-ray diffraction (XRD, Shimadzu LabX-XRD-6000) with Cu Kα radiation (λ = 1.5418 Å). The optical absorption spectra of the thin film samples were obtained on an UV-visible-nearinfrared

(UV-Vis-NIR)

spectrophotometer

(PerkinElmer,

Lambda

950 UV/Vis/NIR

spectrophotometer). The morphology of the thin film samples was observed by field-emission scanning electron microscopy(FE-SEM; JEOL, JSM-7600F), and no conductive coating was deposited onto samples for these SEM measurements. Transmission electron microscopy (TEM) analyses were performed on a high-resolution transmission electron microscope (JEM-2010). RESULTS PEC Behaviors of FeVO4 Photoanodes. A sol-gel method was used to prepare FeVO4 thin films. Upon heating the precursor solution to evaporate excess ethylene glycol and promote the esterification reaction between ethylene glycol and succinic acid, a brownish gel layer was generated which was then heated in a box furnace to form a brownish yellow transparent film. Based on the annealing temperatures, the as-obtained FeVO4 films are denoted as FeVO4 450, FeVO4 500, and FeVO4 550, respectively. Figure 1 shows the photocurrent−potential curves of different FeVO4 photoanodes measured in a 0.1 M potassium phosphate electrolyte (pH = 7) under AM 1.5 G simulated sunlight (100 mW cm−2). For all the FeVO4 films studied here frontside illumination always yields slightly larger photocurrents than that by back-side illumination, the front-side photocurrent−potential curves are then used for comparison (Supporting Information Figure S1). Note that a small oxidation peak appears at around 0.8 VRHE for all FeVO4 electrodes, which may be ascribed to the oxidation of low valence-state vanadium ions (V4+) in FeVO4. Upon increasing the annealing temperatures, the concentration of V4+ will be reduced, resulting in less pronounced oxidation peaks. The onset potentials of FeVO4


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photoanodes are strongly dependant on the calcination temperatures, following the order FeVO4 550 < FeVO4 500

< FeVO4 450. This phenomenon may be partially rationalized by the

improved crystallinity of FeVO4 at higher temperatures. However, at high applied potentials FeVO4 500 and FeVO4 450 photoanodes produce larger photocurrents than FeVO4 550. This could be due to the undesirable contamination of FeVO4 by sodium diffusing from the glass substrate at high temperatures, as has been reported previously.17 At 1.6 VRHE, the FeVO4 500 photoanode produces a slightly larger photocurrent (ca. 0.28 mA cm−2) than the other two photoanodes of FeVO4 550 (ca. 0.24 mA cm−2) and FeVO4 450 (ca. 0.22 mA cm−2). Nevertheless, the presently achieved photocurrents (including the previously reported photocurrent values) are far below the predicted photocurrent of ca. 12.9 mA cm−2 for FeVO4, indicating one or several efficiency loss mechanisms are taking effect. In addition, the amounts of the precursor solution have been optimized to give the best result for FeVO4 500 photoanodes (Supporting Information Figure S1d), thus the present FeVO4 500 photoanode is best balanced among optical absorption, bulk charge transport and surface charge transfer (surface catalysis) characteristics. Therefore, the FeVO4 500 sample was chosen for further optical, structural, charge transport and surface catalysis property measurements. Structural and Morphological Investigations of FeVO4 Photoanodes. The morphological characteristics of FeVO4 photoanodes were analyzed using FE-SEM. As shown in Figure 2, the average particle sizes of FeVO4 increase gradually with the increase of annealing temperatures from 450 °C to 550 °C. The morphology of FeVO4 evolves from a pore-containing flat layer into an inter-connected worm-like particle network with elivated annealing temperatures. The above particle size increase and three-dimensional particle network formation are due to the particle growth and inter-particle sintering and necking of FeVO4 at higher


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temperatures. The XRD patterns of FeVO4 photoanode films show very weak diffraction peaks (Supporting Information Figure S2), regardless of the annealing temperatures, which can be roughly assigned as triclinic FeVO4 (JCPDS 38-1372). Cross-sectional SEM analysis of the FeVO4 500 photoanode shows a film thickness of ca. 130 nm, and the FeVO4 500 layer is smooth on the micrometer scale (Figure 3a). Therefore, it is possible that the limited thickness of FeVO4 500 contributes to its weak X-ray diffraction peaks. However, previously reported FeVO4 films with thicknesses up to about 1 µm still exhibit weak/negligible X-ray diffraction peaks, suggesting the intrinsic poor crystallinity of FeVO4 films formed on FTO substrates.14,15 The poor crystallinity of the prepared FeVO4 films suggests a significant amount of defects such as amorphorous regions and dislocations, are present in the resulting FeVO4 films. Because crystal lattice defects usually act as recombination centers for photo-generated electrons and holes, the poor crystalline features for FeVO4 photoanodes may constitute a major cause of efficiency loss. To gain further structural information with FeVO4 500, TEM measurements were conducted on FeVO4 500 particles scratched off the FTO substrate. Figure 3b shows that FeVO4 500 irregular particles in the size range of 30-50 nm can be observed. The interplanar spacing of 0.62 nm can be readily assigned as (001) crystal plane in triclinic FeVO4. The lattice fringe runs across the entire FeVO4 particle shown in Figure 3b, indicating its single crystal nature. However, porous particles, with the primary crystallites exhibiting different crystal orientations, coexist with the above single crystal FeVO4 particles (Figure 3c). The polycrystalline nature of these porous FeVO4 particle aggregates is further confirmed by the electron diffraction analysis shown in Figure 3d, which gives ring patterns. More high-resolution TEM images showing random crystal orientations in porous FeVO4 particle aggregates are provided in Figure S3. The formation of these macro- and mesopores is induced by the ester network template formed


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between succinic acid and ethylene glycol, and the released gases upon decomposition of NO3− and NH4+ ions. Optical Properties of FeVO4 500 Photoanodes. There are several reports on the investigation of FeVO4 based water splitting photoanodes, due to its favorable bulk band gap of 2.06 eV that allows absorbing a large portion of the solar spectrum.14,15 However, it is still unclear how much of the incident photons can be absorbed by FeVO4 thin films and the absorption coefficient spectra for FeVO4 remain unavailable. To clarify the above questions we conducted UV-vis measurements with an integrating sphere to gain the light harvesting efficiency spectra and absorption coefficient spectra of our 130-nm-thick FeVO4 500 film. Figure 4a shows the reflectance and transmittance spectra of FeVO4 500, photons with wavelengths larger than 500 nm are substantially transmitted or reflected. The light harvesting efficiency spectra in Figure 4a clearly show that significant light absorption occurs around 500 nm, with a small tail approaching 600 nm. The light harvesting efficiency spectra of FeVO4 500 resemble the extensively studied BiVO4 photoanode. By integrating the light harvesting efficiency spectra and the AM 1.5G one sun solar spectrum for wavelengths below 602 nm, the maximum attainable photocurrent for the present FeVO4 500 photoanode is 3.83 mA cm−2, as shown in Figure 4b. This light absorption photocurrent value is considerably low comparing with its theoretical maximum photocurrent of ca. 12.9 mA cm−2 judged from the reported bulk band gap of FeVO4. Based on the reflectance and transmittance spectra and the measured film thickness of 130 nm for FeVO4 500, we have calculated the absorption coefficient spectra (equation (1)).18

α(λ) = (

()

()

)

(1)


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where d is the film thickness, R(λ) and T(λ) are the reflectance and transmittance at specific wavelengths, respectively. As depicted in Figure 5a, the absorption of photons by FeVO4 500 begins at around 607 nm (2.04 eV), with absorption coefficients on the order of 103 cm−1 until approaching 530 nm (2.34 eV). In the wavelength range of 530 nm (2.34 eV) to 446 nm (2.78 eV), the absorption coefficients for FeVO4 500 are on the order of 104 cm−1. FeVO4 500 absorbs extensively photons with wavelengths less than 446 nm, on the order of 105 cm−1. The obtained absorption coefficient spectra in the present study is in good accordance with the very recent result on FeVO4.16 Further analysis of the optical transitions in FeVO4 500 comes from the establishment of Tauc plots assuming both indirect and direct band gaps. The intersection of the slops on either side of the transition is regarded as the band gap energy. As can be seen in Figure 5b, the indirect Tauc plot of FeVO4 500 shows several different slops with respect to photon energy, resulting in uncertainty of the indirect band gap determination. In contrast, the direct Tauc plot of FeVO4 500 in Figure 5c exhibits a sharp absorption onset occurring around 2.77 eV. Based on the available optoelectronic characteristics with α-Fe2O3 and BiVO4, we speculate that both direct and indirect transitions exist in FeVO4.12,19,20 The direct transition in FeVO4 is probably due to the charge transfer from O 2p orbitals to empty V 3d orbitals.19 Similarly, the indirect transitions for FeVO4 may arise from the electronic transitions between the O 2p orbitals and the Fe 3d orbitals or between the Fe 3d orbitals and empty V 3d orbitals, with the energy gap being around 2.5 eV; the weak indirect transition occurring around 610 nm can be assigned as the d−d transition states between electron energy levels of the Fe3+ ions in FeVO4.12 The penetration depths (α−1) for the above three photons at 2.77 eV, 2.5 eV, 2.03 eV are ca. 102 nm, 418 nm and 11.7 µm, respectively. Note that this is the upper estimation for the light penetration depths of FeVO4, because we use the measured thickness instead of effective thickness to


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calculate the absorption coefficients. In addition, the poor crystallinity of the prepared FeVO4 films suggests further increase in absorption coefficients is possible, as has been reported on hematite that small structural changes can affect its optical properties.20 Charge Transport and Surface Catalysis Properties of FeVO4 500 Photoanodes. Based on the optical properties analysis on FeVO4 500 photoanodes, we show that light absorbing procedure accounts for about 70% loss of theoretical maximum photocurrent (3.83 mA cm−2 vs. 12.9 mA cm−2). Despite the limited photon absorption, the expected photocurrent on FeVO4 500 photoanodes would be comparable to BiVO4, assuming an unity conversion efficiency of absorbed photons. The photocurrent of ca. 0.28 mA cm−2 at 1.6 VRHE delivered by FeVO4 500 suggests poor bulk charge transport and/or surface catalysis further lower the energy conversion efficiency. Then there is an urgent need to assess the surface and bulk recombination losses in FeVO4 500 photoanodes in a quantitative way. The water splitting photocurrent JH2O on FeVO4 500 can be expressed by equation (2): JH2O = Jabs × ηsep × ηinj

(2)

where Jabs is the light absorbing rate expressed as a current density, and by integrating the light harvesting efficiency spectra with respect to the AM 1.5G 100 mW cm−2 solar spectrum the Jabs value is determined to be 3.83 mA cm−2 (Figure 4b); ηsep is the charge separation efficiency that depicts the fraction of photogenerated holes that do not recombine with electrons in FeVO4 bulk, ηinj is the charge injection efficiency that shows the probability of surface holes that are injected into water. The charge separation efficiency ηsep is calculated by dividing the photocurrent in the presence of an appropriate hole scavenger (Jsca) by Jabs (Supporting Information Figure S4), since in the presence of an appropriate hole scavenger the ηinj becomes 100% (equation (3)). Similarly,


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the charge injection efficiency ηinj is obtained by dividing water splitting photocurrent JH2O by Jsca (equation (4)).21,22 Jsca = Jabs × ηsep

(3)

JH2O = Jsca × ηinj

(4)

With this strategy we quantified the contributions of the surface and bulk recombination processes, which are shown in Figure 6. The charge separation efficiency ηsep increases slowly with increasing potentials, reaching ca. 10% at 1.6 VRHE. As discussed previously on α-Fe2O3 the gradual enhancement in ηsep probably results from the widening of the space charge region.21 It is interesting that poor crystallinity of the prepared FeVO4 films does not significantly suppress the charge injection efficiency ηinj, at high potentials the ηinj reaches ca. 70%, as compared with ηinj values larger than 90% for α-Fe2O3 in the same potential range.21 Therefore, the poor ηsep predominantly contributes to the suppressed PEC performance on FeVO4 500 photoanodes. Band Structure of FeVO4. To gain more insights into the PEC behaviors of FeVO4 photoanodes, we have constructed the band structure of FeVO4 by the Mott−Schottky analysis according to the method developed previously.23−25 The frequency-independent space charge capacitances at each bias potential were used to construct the Mott−Schottky plot, which were determined by fitting the impedance spectra (Nyquist plots, Supporting Information Figure S5) with an equivalent circuit model of two parallel resistor-capacitor connected in series. One resistor-capacitor component is associated with the FeVO4 film bulk and the other one describes the space charge region of the FeVO4 film. The flatband potential EFB of FeVO4 500 is determined to be 0.8 VRHE, by the extrapolation of the Mott−Schottky plot (Figure 7). The positive slope of the Mott−Schottky plot suggests its n-type semiconducting nature of FeVO4,


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which then allows the calculation of donor density to be 3.7 × 1021 cm−3 using a FeVO4 dielectric constant of 1826 and equation (5).

=

( − −

)

(5)

where C is the space charge capacitance, e is the elementary charge, ND is the donor density, ε0 is the permittivity of vacuum, εr is the dielectric constant of FeVO4, A is the electrode surface area, E is the applied potential, EFB is the flatband potential, k is the Boltzmann constant, and T is the temperature. Assuming FeVO4 has an effective density of states in the conduction band (NC) of 5 × 1019 cm−3,27 an energy level for the conduction band edge ECB can be derived to be 0.11 V positive of flatband potential EFB, i.e., 0.91 VRHE (equation (6)).

ECB − EF =

ln ( )

(6)

This phenomenon that flatband enters the conduction band suggests that FeVO4 is heavily doped, which may result in a thin space charge layer thereby poor bulk charge separation. The schematic band structures of FeVO4 are shown in Figure 8. In order to give a comparison with another way to determine the band structure of FeVO4, we have performed X-ray photoelectron spectroscopy near the valence band. The result in Figure S6 shows that the difference between valence band edge EVB and Fermi level in FeVO4 500 is ca. 1.7 eV. Therefore, with the flatband potential EFB (0.8 VRHE) of FeVO4 500 obtained by Mott−Schottky analysis, the 1.7 eV value of EVB − EF, as well as the optical transitions, we have constructed another set of band structures for FeVO4 (Supporting Information Figure S7). It is interesting to note that the band structure and optical transition properties of FeVO4 are remarkably similar to those of copper vanadate.28−30 Therefore, the research achievements on copper vanadate could be valuable for understanding the PEC behaviors of FeVO4.


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With the above parameters, the space charge width W of FeVO4 500 at 1.6 VRHE is estimated to be 0.65 nm using equation (7). W=

( − −

ηsep,λ = 1 −

)

!"#

(7)

(8)

This ultra-small space charge width of FeVO4 500 is in line with its high doping level, which also indicates that charge separation in FeVO4 500 is mainly dominated by diffusion. The Gärtner model in equation (8) can be used to calculate the effective hole diffusion length Lp of FeVO4 500, where α is the absorption coefficient, W is the effective space charge width, and ηsep,λ is the charge separation efficiency for a specific photon (ηsep,λ ≠ ηsep), respectively.31 For photons with a wavelength of 420 nm, the absorption coefficient α is 1.73 × 105 cm−1, ηsep,420 is 13.3%, with these parameters equation (8) yields a hole diffusion length of ca. 8.1 nm. This Lp of 8.1 nm for FeVO4 500 is the upper limit due to the underestimated absorption coefficients. A reasonable Lp value for FeVO4 500 falls in the range of 3−5 nm, which is slightly larger than that of α-Fe2O3 (2−4 nm) but is smaller than those reported for BiVO4 (20−200 nm).12,22,32 DISCUSSION On the basis of the above optical, structural, charge transport and surface catalysis property measurements, the performance bottlenecks in FeVO4 500 photoanodes can be identified and quantified. Light absorption is the first step in water splitting process, which determines the maximum amounts of electron-hole pairs that can be produced. Therefore, to maximize the water splitting photocurrent on FeVO4, it must firstly absorb as much photons as possible. Previous


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studies provide only the optical absorption profiles for FeVO4 which are then utilized to obtain the band gap value of ca. 2.06 eV.14,15 Judging from this band gap value of 2.06 eV, FeVO4 is able to deliver a theoretical maximum photocurrent of ca. 12.9 mA cm−2, which is comparable to α-Fe2O3. However, the light harvesting efficiency spectra and the corresponding maximum attainable photocurrent of 3.83 mA cm−2 for our most efficient FeVO4 500 photoanode suggest that the first major efficiency loss (70%) in FeVO4 500 photoanodes is the poor optical absorption. Although the light harvesting efficiency for FeVO4 can be simply improved by applying thicker films, the absorption coefficient spectra show that a film thickness of over 26.9 µm is required to reach a 90% of optical absorption efficiency near the band edge of FeVO4. Even for the indirect band gap transition occurring around 2.5 eV, a 961-nm-thick FeVO4 is needed to realize 90% absorption of incident 496 nm photons. Considering the poor crystallinity of the prepared FeVO4 films, the substantial increase in absorption coefficients is theoretically accessible, thereby guaranteeing sufficient light absorption by FeVO4.20 The charge transport property measurements indicate that even for the limited absorbed photons the charge separation efficiency ηsep in FeVO4 photoanodes is generally lower than 10% (theoretical 3.83 mA cm−2 vs. < 0.384 mA cm−2 at potentials negative of 1.6 VRHE), suggesting extremely poor bulk charge transport. This poor bulk charge transport thereby establishes the second major efficiency loss (27%). Generally, poor bulk charge transport is induced by the presence of grain boundaries, large size of film particles, defects in the material bulk and low mobilities of charge carriers. As mentioned in the XRD analysis section, a significant amount of defects such as amorphorous regions and dislocations, as exemplified by the poor crystallinity, are present in FeVO4 films. Therefore, the poor crystalline features for FeVO4 photoanodes would constitute a major cause of bulk recombination, with crystal lattice defects acting as


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recombination centers for photo-generated electrons and holes. Correspondingly, constructing high quality crystalline FeVO4 films to improve both electron and hole transport, would be a desirable strategy to significantly increase the charge separation efficiency in FeVO4 photoanodes. The onset potentials of FeVO4 photoanodes are also found to depend largely on the calcination temperatures, which suggests the necessity of high temperature annealing for FeVO4 photoanodes. However, simply increasing the calcination temperatures does not guarantee high quality crystalline FeVO4 films, due to the undesirable reactions between vanadium oxide and sodium diffusing from the glass substrate at high temperatures.17 Therefore, high-temperature preparation of FeVO4 films on sodium-free FTO, to avoid sodium diffusion or any unexpected reactions from the regular glass, would ensure or improve the crystalline quality of resulting FeVO4 films. For the previously reported FeVO4 photoanodes with film thicknesses of ca. 1 µm that allow sufficient absorption of solar spectrum (reduced optical loss), they exhibit comparable photo-response with the present 130-nm-thick FeVO4 photoanodes, indicating much more serious bulk recombination in them.14 Besides the crystal defects in FeVO4, the intrinsic poor hole transport property in FeVO4 would be another major source of bulk recombination, considering the similar small hole diffusion length of FeVO4 with that of extensively studied αFe2O3. Under this condition, FeVO4 faces a similar or an even more serious problem as α-Fe2O3 when applying as water-splitting photoanodes, that is the disaccord between its long-required absorption length and its short hole diffusion length. Therefore, the explored strategies that have been proved to be effective for α-Fe2O3 photoanodes will parallel benefit further optimization of FeVO4 photoanodes.33,34 Among these important strategies, nanostructuring of photoelectrodes has been a prevailing trend over other methods to high-efficiency water splitting photoelectrodes.


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This is due to the fact that nanostructured photoelectrodes can simultaneously enlarge the volume ratio of space charge region to electrode bulk, shorten the minority carrier migration length and increase specific surface area, via which bulk recombination has been largely suppressed.12 For example, the concept of host-scaffold/guest-absorber as developed in dyesensitized solar cells, can be applied directly on FeVO4 photoanodes by applying extremely thin FeVO4 absorber on a high-surface area, nanostructured conductive scaffold.35 Very recently, Bi et al. propose a promising strategy of utilizing Fe3+/Fe2+ self-doped nanoporous FeVO4 photoanodes to realize efficient water splitting.15 We treated FeVO4 500 photoanodes with NaBH4 and H2O2 to achieve Fe2+ and Fe3+ doped film states, which showed almost no effects on photocurrent production (Supporting Information Figure S8). The reason could be the lower contact areas between our FeVO4 500 films and the NaBH4/H2O2 solutions when comparing with the reported highly porous FeVO4 photoanodes. We measured the incident photon to current efficiency (IPCE) values at 1.6 VRHE. As shown in Figure 9a, the IPCE curve peaks around 350 nm, reaching 10%. For photons with shorter wavelengths than 350 nm, the IPCE values decay sharply, possibly due to the inefficient collection of electrons generated by these photons that have very shallow light penetration depths. For photons with wavelengths larger than 450 nm the IPCE values are very low, which are generally lower than 2%. The IPCE curve begins to increase steeply from ca. 450 nm towards the UV range, and this photon energy is in good agreement with the direct optical band gap of ca. 2.77 eV in FeVO4 due to the electronic transition from O 2p orbitals to empty V 3d orbitals. We also compared our IPCE spectrum with the record FeVO4 photoanode, by which our 130-nm-thick FeVO4 500 photoanode performs much better than the record FeVO4 photoanode.15 Because the measured photocurrent on record FeVO4 photoanode is higher than 1 mA cm−2, the reported IPCE values


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on record FeVO4 photoanode may have been largely underestimated. The predicted photocurrent of 0.3 mA cm−2 at 1.6 VRHE for FeVO4 500 photoanode agrees well with the measured photocurrent of 0.28 mA cm−2 under AM 1.5G sunlight (100 mW cm−2), while predicted photocurrent of 0.2 mA cm−2 at ca. 1.6 VRHE on record FeVO4 photoanode confirms again the reported IPCE values on record FeVO4 photoanode have been largely underestimated (Figure 9b). Doping with foreign ions can increase the charge carrier density thereby improve charge transport to reduce bulk recombination in photoelectrodes, which has been a popular strategy to increase the photoelectrode performance.8,9 Introducing W into FeVO4 at V sites demonstrates a 2.5 times increase in photocurrent, as reported previously.14 In the present case, FeVO4 500 is identified as a highly doped semiconductor with an intrinsic high donor density of 3.7 × 1021 cm−3. Further introduction of donor type Mo ions into FeVO4 500 only leads to suppressed activity (Supporting Information Figure S8c). The poor bulk charge transport in FeVO4 500 is also supported by measuring the change in open circuit potential (∆OCP) in the dark versus under illumination.36 As shown in Figure 10, the equilibrium potential of the FeVO4 500 photoanode is observed to be ca. 0.75 VRHE, 0.48 V negative of the water oxidation potential of 1.23 VRHE. As in the dark and open circuit condition the photoanode is expected to equilibrate with the water oxidation potential, the reason for this mismatch is due to the presence of surface states, which has been extensively studied by previous works.36,37 Under illumination, electronhole pairs are generated and the quasi-Fermi level of the electrons shifts towards the conduction band edge while the quasi-Fermi level of the holes equilibrates with the water oxidation potential. If a considerable ∆OCP is obtained, this indicates that there is less recombination in this electrode. While for FeVO4 500, only 50 mV of ∆OCP is achieved even after 14K seconds’


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illumination, and this ∆OCP value is negligibly low when comparing with α-Fe2O3 and BiVO4.36,37 In addition, after turning off the light the initial equilibrium potential of 0.75 VRHE cannot be achieved again by the FeVO4 500 photoanode, for which the underlying reason needs further study. In summary, the ∆OCP measurement suggests a poor bulk charge transport in FeVO4 500 photoanodes, which is in good accordance with the above charge transport property measurement and the poor crystalline features for FeVO4 photoanodes. Finally, although Fe-based metal oxides usually exhibit promising catalytic activity towads water oxidation, especially for these that contain a significant amount of defects, the present FeVO4 500 photoanodes are inferior to α-Fe2O3 in catalyzing water oxidation reactions (ηinj = 40% for FeVO4 500 vs. ηinj > 70% for α-Fe2O3 at 1.4 VRHE).21 Therefore, deposition of efficient water oxidation catalysts to accelerate the charge transfer (hole transfer) at the solid/electrolyte interface would be a straightforward way to minimize the surface catalysis loss on FeVO4 potoanodes. CONCLUSIONS Sunlight conversion efficiency of water-splitting photoelectrodes depends on bulk optical band gaps, and assuming 100% absorption of the AM 1.5G spectrum for all photons with energies above 2.06 eV, FeVO4 is able to deliver a theoretically possible photocurrent of ca. 12.9 mA cm−2. Nevertheless, the presently achieved negligibly low photocurrents on FeVO4 photoanodes suggest one or several efficiency loss mechanisms are at play. The first challenge in using FeVO4 as a light absorber is its poor light absorption ability. Based on the light harvesting efficiency spectra, the corresponding maximum attainable photocurrent for our most efficient FeVO4 500 photoanode is only 3.83 mA cm−2, which contributes a 70% efficiency loss of its theoretical


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limit. This limitation in FeVO4 photoanodes can be potentially addressed/ameliorated by applying thicker films, or by modification of the absorption coefficients through crystallinity engineering. The second challenge of utilizing FeVO4 water-splitting photoanodes is their poor bulk charge transport, which establishes the second major efficiency loss (27%). Besides the crystal defects in FeVO4, the ultra-small space charge width and intrinsic poor hole transport property in FeVO4 constitute major causes of bulk recombination. Therefore, the corresponding crystallinity and doping control in FeVO4, and the existing strategies designed for α-Fe2O3 can be further explored to overcome the poor bulk charge transport limitation in FeVO4 photoanodes. For example, nanostructuring of the FeVO4 photoanodes can simultaneously shorten the migration length for minority carriers (holes), increase the volume ratio of space charge region to photoelectrode bulk and enlarge specific surface area, by which bulk recombination can be largely suppressed. The third limitation in FeVO4 photoanode is its poor ability of transferring holes into water, which can be improved by suitable surface modification with efficient water oxidation catalysts. We expect our efforts will lead to further improvement in PEC water splitting efficiency and deeper understanding of the limitations exist in these Fe-based metal oxide semiconductors. ASSOCIATED CONTENT Supporting Information. Additional photocurrent−potential curves of FeVO4 photoanodes synthesized at different temperatures and treated with H2O2 or NaBH4. XRD patterns, TEM images, impedance spectra, band structures of the FeVO4 samples. AUTHOR INFORMATION Corresponding Author


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*E-mail: [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. ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was support by Tier 1 (AcRF grant MOE Singapore M401992), Tier 2 (AcRF grant MOE Singapore M4011528), Chinese Natural Science Foundation (Grant 51271031 and 50701006) and National Basic Research Program (No. 2014CB6433000). REFERENCES (1) Grätzel, M. Photoelectrochemical Cells. Nature 2001, 414, 338-344. (2) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (3) Li, Z.; Luo, W.; Zhang, M.; Feng, J.; Zou, Z. Photoelectrochemical Cells for Solar Hydrogen Production: Current State of Promising Photoelectrodes, Methods to Improve their Properties, and Outlook. Energy Environ. Sci. 2013, 6, 347-370. (4) Kim, T. W.; Choi, K. Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990-994.


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(12) Sivula, K.; Le Formal, F.; Grätzel, M. Solar Water Splitting: Progress Using Hematite (αFe2O3) Photoelectrodes. ChemSusChem 2011, 4, 432-449. (13) Warren, S. C.; Voïtchovsky, K.; Dotan, H.; Leroy, C. M.; Cornuz, M.; Stellacci, F.; Hébert, C.; Rothschild, A.; Grätzel, M. Identifying Champion Nanostructures for Solar WaterSplitting. Nat. Mater. 2013, 12, 842-849. (14) Biswas, S. K.; Baeg, J. Enhanced Photoactivity of Visible Light Responsive W Incorporated FeVO4 Photoanode for Solar Water Splitting. Int. J. Hydrogen. Energy 2013, 38, 14451-14457. (15) Wang, W.; Zhang, Y.; Wang, L.; Bi, Y. Facile Synthesis of Fe3+/Fe2+ Self-Doped Nanoporous FeVO4 Photoanodes for Efficient Solar Water Splitting. J. Mater. Chem. A 2017, 5, 2478-2482. (16) Zhang, M.; Ma, Y.; Friedrich, D.; van de Krol, R.; Wong, L. H.; Abdi, F. F. Elucidation of the Opto-Electronic and Photoelectrochemical Properties of FeVO4 Photoanodes for Solar Water Oxidation. J. Mater. Chem. A 2018, 6, 548-555. (17) Dang, H. X.; Rettie, A. J. E.; Mullins, C. B. Visible-Light-Active NiV2O6 Films for Photoelectrochemical Water Oxidation. J. Phys. Chem. C 2015, 119, 14524-14531. (18) Johansson, M. B.; Zietz, B.; Niklasson, G. A.; Österlund, L. Optical Properties of Nanocrystalline WO3 and WO3-x Thin Films Prepared by DC Magnetron Sputtering. J. Appl. Phys. 2014, 115, 213510. (19) Park, Y.; McDonald, K. J.; Choi, K. Progress in Bismuth Vanadate Photoanodes for Use in Solar Water Oxidation. Chem. Soc. Rev. 2013, 42, 2321-2337.


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(20) Sivula, K.; Zboril, R.; Le Formal, F.; Robert, R.; Weidenkaff, A.; Tucek, J.; Frydrych, J.; Grätzel, M. Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach. J. Am. Chem. Soc. 2010, 132, 7436-7444. (21) Dotan, H.; Sivula, K.; Gratzel, M.; Rothschild, A.; Warren, S. C. Probing the Photoelectrochemical Properties of Hematite (α-Fe2O3) Electrodes Using Hydrogen Peroxide as a Hole Scavenger. Energy Environ. Sci. 2011, 4, 958-964. (22) Zhong, D. K.; Choi, S.; Gamelin, D. R. Near-Complete Suppression of Surface Recombination in Solar Photoelectrolysis by “Co-Pi” Catalyst-Modified W:BiVO4. J. Am. Chem. Soc. 2011, 133, 18370-18377. (23) Septina, W.; Prabhakar, R. R.; Wick, R.; Moehl, T.; Tilley, S. D. Stabilized Solar Hydrogen Production with CuO/CdS Heterojunction Thin Film Photocathodes. Chem. Mater. 2017, 29, 1735-1743. (24) Steier, L.; Herraiz-Cardona, I.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Tilley, S. D.; Grätzel, M. Understanding the Role of Underlayers and Overlayers in Thin Film Hematite Photoanodes. Adv. Funct. Mater. 2014, 24, 7681-7688. (25) Enache, C. S.; Lloyd, D.; Damen, M. R.; Schoonman, J.; van de Krol, R. PhotoElectrochemical Properties of Thin-Film InVO4 Photoanodes: The Role of Deep Donor States. J. Phys. Chem. C 2009, 113, 19351-19360. (26) Daoud-Aladine, A.; Kundys, B.; Martin, C.; Radaelli, P. G.; Brown, P. J.; Simon, C.; Chapon, L. C. Multiferroicity and Spiral Magnetism in FeVO4 with Quenched Fe Orbital Moments. arXiv 2008, 0812.4429V1.


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(27) Berglund, S. P.; Abdi, F. F.; Bogdanoff, P.; Chemseddine, A.; Friedrich, D.; van de Krol, R. Comprehensive Evaluation of CuBi2O4 as a Photocathode Material for Photoelectrochemical Water Splitting. Chem. Mater. 2016, 28, 4231-4242. (28) Seabold, J. A.; Neale, N. R. All First Row Transition Metal Oxide Photoanode for Water Splitting Based on Cu3V2O8. Chem. Mater. 2015, 27, 1005-1013. (29) Guo, W.; Chemelewski, W. D.; Mabayoje, O.; Xiao, P.; Zhang, Y.; Mullins, C. B. Synthesis and Characterization of CuV2O6 and Cu2V2O7: Two Photoanode Candidates for Photoelectrochemical Water Oxidation. J. Phys. Chem. C 2015, 119, 27220-27227. (30) Zhou, L.; Yan, Q.; Shinde, A.; Guevarra, D.; Newhouse, P. F.; Becerra-Stasiewicz, N.; Chatman, S. M.; Haber, J. A.; Neaton, J. B.; Gregoire, J. M. High Throughput Discovery of Solar Fuels Photoanodes in the CuO−V2O5 System. Adv. Energy Mater. 2015, 5, 1500968. (31) Gärtner, W. W. Depletion-Layer Photoeffects in Semiconductors. Phys. Rev. 1959, 116, 84-87. (32) Abdi, F. F.; Savenije, T. J.; May, M. M.; Dam, B.; van de Krol, R. The Origin of Slow Carrier Transport in BiVO4 Thin Film Photoanodes: A Time-Resolved Microwave Conductivity Study. J. Phys. Chem. Lett. 2013, 4, 2752-2757. (33) Lin, Y.; Xu, Y.; Mayer, M. T.; Simpson, Z. I.; McMahon, G.; Zhou, S.; Wang, D. Growth of p-Type Hematite by Atomic Layer Deposition and Its Utilization for Improved Solar Water Splitting. J. Am. Chem. Soc. 2012, 134, 5508-5511.


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Figure 1. Current−potential curves of FeVO4 photoanodes synthesized at different temperatures in the dark (dotted lines) and under AM 1.5G 100 mW cm−2 simulated sunlight (solid lines). The electrodes were illuminated from the front side, the electrolyte was a 0.1 M potassium phosphate aqueous solution (pH = 7), and the scan rate was 30 mV s−1.


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Figure 2. SEM images of (a, b) FeVO4 450, (c, d) FeVO4 500, and (e, f) FeVO4 550 films.


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Figure 3. (a) Cross-sectional SEM image of the FeVO4 500 photoanode, (b-d) TEM images of FeVO4 500 particles with different magnifications, and (e) electron diffraction pattern for panel (c).


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Figure 4. (a) Reflectance, transmittance and light harvesting efficiency spectra for the FeVO4 500 photoanode and (b) its maximum attainable photocurrent, obtained by integrating the light harvesting efficiency spectra with respect to the AM 1.5G 100 mW cm−2 solar spectrum.


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Figure 5. (a) Absorption coefficient spectra and Tauc plots evaluating the optical band gaps for the (b) indirect case and the (c) direct case for the FeVO4 500 photoanode.

Figure 6. Yields of charge separation and charge injection in FeVO4 500 photoanodes.


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Figure 7. Mott−Schottky plot of the FeVO4 500 photoanode obtained from the frequencyindependent space charge capacitances at each bias potential.


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Figure 8. Two possible band structures and corresponding optical transitions for FeVO4.


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Figure 9. (a) IPCE spectra for FeVO4 500 and the record FeVO4 photoanodes at ca. 1.6 VRHE. (b) Solar photocurrent spectra of FeVO4 500 and the record FeVO4 photoanodes at 1.6 VRHE as a function of wavelength, obtained by multiplication of the IPCE with the photon flux spectrum of global sunlight (100 mW cm−2 AM 1.5 G). The integration of solar photocurrent spectra with respect to wavelength gives the predicted photocurrents of 0.3 and 0.2 mA cm−2 at 1.6 VRHE for FeVO4 500 and the record FeVO4 photoanodes, respectively.


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Figure 10. Open-circuit potentials for FeVO4 500 measured in the dark and under AM 1.5G 100 mW cm−2 simulated sunlight. The electrolyte was a 0.1 M potassium phosphate aqueous solution (pH = 7).


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TOC Graphic


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Probing the Performance Limitations in Thin-Film FeVO4 Photoanodes

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