Water Oxidation on Hematite Photoelectrodes: Insight into the Nature

Article pubs.acs.org/JPCC

Water Oxidation on Hematite Photoelectrodes: Insight into the Nature of Surface States through In Situ Spectroelectrochemistry Benjamin Klahr† and Thomas Hamann*,‡ †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States Department of Chemistry, Michigan State University, 578 South Shaw Lane, East Lansing, Michigan 48824-1322, United States

‡

S Supporting Information *

ABSTRACT: Uniform planar films of hematite (α-Fe2O3), deposited by atomic layer deposition, were examined using in situ spectroelectrochemistry during photoinduced water oxidation. A change in the absorption spectrum of hematite electrodes during water oxidation was measured under illumination and applied potentials. The absorption was correlated to a charge measured by cyclic voltammetry and with a capacitance measured by impedance spectroscopy. Modification of the hematite surface with alumina reduced the absorption feature and the associated capacitance, suggesting that these features are associated with the surface. Comparing the spectral change of hematite to absorption features of molecular analogues allowed us to tentatively assign the absorbance and capacitive features to the oxidation of a low valent iron-aqua or iron-hydroxyl species to a high valent iron-oxo chemical species at the surface.

■

INTRODUCTION As the need for abundant, inexpensive and clean sources of energy becomes increasingly apparent, collecting and storing the vast energy of the sun remains a high priority for the scientific community. One route of achieving direct collection and storage of solar energy is through photoelectrochemical water splitting, which results in the energy-dense fuel, hydrogen. However, the search continues for materials that can satisfy the stringent requirement for widespread adoption including low cost, high efficiency, and high stability. Hematite is one of the most promising candidates for the oxidation half of the water splitting reaction due to its earth abundance, chemical stability, and strong light absorption at wavelengths more energetic than the ∼2.1 eV bandgap.1−5 Despite these benefits, efficient transfer of a photogenerated hole at the surface of hematite to water (hole collection) occurs only under high applied potentials, that is, when band bending is large. This is typically attributed to the large amount of recombination between electrons in the conduction band and oxidized surface states at low applied potentials.6−10 While this general concept is largely agreed upon, the nature and role of these surface states remain a topic of debate, as highlighted in several recent perspectives.11,12 For example, we have suggested that the surface states often measured at bare hematite electrodes during water oxidation are actually iron-based intermediates that are directly involved in the water oxidation mechanism.7,12 This is in line with the hypothesis of Peter and coworkers.13,14 In contrast with this, Le Formal et al. have portrayed surface states as more of an electronic state.9 In this case, the surface states are not chemically involved in water oxidation; rather, they are only a source of deleterious recombination. Durrant and coworkers assign the slow decay © 2014 American Chemical Society

of an absorption feature corresponding to water oxidation, and thus the rate-limiting process, to bulk recombination.15 Clearly, the precise role of surface states and other potential ratelimiting factors in water oxidation at hematite electrodes must be fully understood to design strategies to overcome this barrier. In an attempt to clarify this discrepancy, we studied the absorption of surface species through in situ UV−vis absorption spectroscopy under varying illumination and applied potentials. To further probe the role of surface states, we modified hematite electrodes with alumina oxide layers. Hematite electrodes were prepared and further modified by atomic layer deposition (ALD), a gas-phase technique that employs alternating metal precursor and oxidation pulses. ALD metal precursors are designed so that they react with oxidized (hydroxylated) substrate termini but not with themselves. Once reacted with the surface, the attached metal precursor is oxidized, generating a free hydroxide that can now react with a subsequent metal precursor molecule. This self-limiting, gasphase mechanism results in the highly controllable and conformal growth of metal oxides on both flat and high aspect ratio substrates; a beneficial characteristic for creating ideal, multicomponent nanostructures.16 Nanostructuring is the most widely used strategy when trying to overcome the discrepancy between light absorption depths (200 nm at λ = 500 nm) and charge collection lengths in hematite (0−10 nm). While overcoming the poor collection length of hematite is required Received: January 16, 2014 Revised: April 18, 2014 Published: April 21, 2014 10393

dx.doi.org/10.1021/jp500543z | J. Phys. Chem. C 2014, 118, 10393−10399

The Journal of Physical Chemistry C

Article

for obtaining large photocurrents, this study employs thin films to better understand the interfacial processes.

Impedance spectroscopic and photoelectrochemical measurements were made with an Eco Chemie Autolab potentiostat coupled to Nova electrochemical software. Impedance data were gathered using a 10 mV amplitude perturbation of between 10 000 and 0.01 Hz. Data were fit using Zview software (Scribner Associates). The light source was a 450 W Xe arc lamp. An AM 1.5 solar filter was used to simulate sunlight at 100 mW cm−2. All photoelectrochemical measurements were performed by shining light from the substrate− electrode (SE) interface. Light chopping J−V were made at a scan rate of 75 mV/s. The light was chopped using a computer controlled ThorLabs solenoid shutter, which was set to activate every 266 ms such that the light was turned on or off every 20 mV. Steady-state J− V curves were measured at a scan rate of 5 mV/s. Cyclic voltammetry used to investigate capacitive features was measured at a scan rate of 200 mV/s. Spectroelectrochemistry. Electrodes were prepared in the same fashion as those prepared for photoelectrochemical experiments and clamped to a 1 cm quartz cuvette with a hole cut in to allow the contact of electrolyte to the electrode. An aqueous 1 M NaOH solution was used in all spectroelectrochemical experiments. A 2 mm “no-leak” Ag/ AgCl electrode (Warner) was used as the reference electrode, and a 0.5 mm platinum wire was used as the counter electrode. This setup was placed inside a PerkinElmer Lambda 35 UV−vis spectrometer. Electrical contacts from a μATUOLAB III were placed inside the UV−vis chamber. A 405 nm, 40 mW laser diode (Sanyo) controlled by a ThorLabs benchtop current controller was used to illuminate the hematite sample inside the UV−vis with monochromatic light. A 475 nm long pass filter was used to prevent the laser from going into the UV−vis detector, thereby maximizing the signal-to-noise ratio. Figures of the experimental setup can be seen in the Supporting Information.

■

EXPERIMENTAL SECTION Electrode Fabrication. Fluorine-doped tin oxide (FTO) (Hartford Glass, 12 Ω cm−2) coated glass was cleaned by sonicating sequentially in detergent/water mixture, deionized water, acetone, and isopropyl alcohol for 15 min each. A layer of Ga2O3 was deposited onto FTO by ALD (Savannah 100, Cambridge Nanotech). This layer has been shown to facilitate the deposition of hematite by ALD in our lab. This strategy has also been observed to enhance the photocatalytic efficiency of very thin films deposited by spray pyrolysis.17 In this case, Ga2O3 is hypothesized to act as a crystalline template for the hematite that prevents nonproductive amorphous thin films.17 The gallium precursor, bis(μ-dimethylamino) tetrakis(dimethylamino)digallium, was heated to 150 °C, and the substrate was heated to 200 °C. Water, the oxidant in the ALD process, was kept at room temperature. The gallium precursor was introduced into the ALD chamber with a 0.2 s pulse and an 8 s wait or “exposure” to allow the precursor to react with the substrate. The water pulse consisted of a 0.015 s pulse and an 8 s exposure. Each pulse was separated by a 12 s nitrogen purge. One gallium precursor and one water pulse comprise a single ALD cycle. The Ga2O3 layers were prepared by 18 ALD cycles, which were measured to be ∼2 nm on silicon wafers as a control by ellipsometry (Horiba Jobin Yvon, Smart-SE). Hematite was deposited onto the Ga2O3 layer by ALD using ferrocene as the metal precursor and ozone as the oxidation source. Deposition conditions have been described in detail elsewhere.6,7 The hematite films were annealed by increasing the temperature to 500 °C over 30 min and soaking at 500 °C for 30 min. Films were prepared by 800 ALD cycles and measured to be ∼48 nm by absorption (PerkinElmer, Lambda 35 with a Labsphere integrating sphere) and as ellipsometric measurements as previously described.18 Films were characterized by Raman spectroscopy and XRD as previously described.18 Al2O3 was deposited on the hematite films by ALD using trimethylaluminum and water as the precursors. Pulses of both precursors were each 0.015 s separated by a 6 s nitrogen purge time. Precursors were used at room temperature, and the substrate was heated to 200 °C. The growth rate of the deposition was measured to be 1.1 Å per cycle by ellipsometric analysis on control silicon wafers. The growth rate is assumed to be the same on the hematite electrodes. Samples were annealed after Al2O3 deposition by heating to 300 °C for 20 min.19 Electrodes were masked with a 60 μm Surlyn film (Solaronix) with a 0.28 cm2 hole to define the active area and to prevent scratching of the thin films when clamping to a custom-made glass electrochemical cell. Surlyn films were adhered to the electrodes by heating to 120 °C. A homemade saturated Ag/AgCl electrode was used as a reference, where potentials were converted to the reversible hydrogen electrode (RHE) scale by the equation ERHE = EAg/AgCl + 0.197 + 0.059 pH. The reference electrode was frequently calibrated to a SCE electrode (Koslow Scientific). A high surface area platinum mesh was used as the counter electrode. Photoelectrochemical Analysis. The water oxidation properties of the hematite films were examined in contact with an aqueous solution containing 1 M NaOH. The pH was determined to be 13.9 by a Fisher Scientific Accumet pH meter.

■

RESULTS Spectroelectrochemistry. The visible-light absorption spectra of hematite electrodes were measured as a function of applied potential under monochromatic illumination. The monochromatic illumination, generated by a 405 nm diode (∼0.1 mW cm−2), was chosen because it is strongly absorbed by the hematite film. Furthermore, this light source is easily focused and directed into the UV−vis spectrometer. Figure 1 shows the absorption difference spectra for various applied potentials, relative to a reference applied potential of 0.62 V

Figure 1. Change in absorption spectra measured at 0.82 V (red), 1.22 V (orange), and 1.62 V versus RHE (yellow) measured under 0.1 mW cm−2 405 nm illumination with respect to the absorption spectra measured at 0.62 V. 10394



Article

Figure 2. (a) CV measured at 200 mV/s after holding 1.4 V versus RHE for 60 s in the dark (red solid line) and under 0.03 (orange dotted), 0.1 (yellow dashed), and 0.3 (green dotted dashed) mW cm−2 of 405 nm light. (b) Change in absorbance of 572 nm light measured while performing the CV in panel a.

versus RHE. This reference potential was used because it is sufficiently cathodic that negligible positive charge, attributed to oxidizing surface states, is stored according to our previous impedance spectroscopic measurements.6,7 As the applied potential becomes increasingly anodic, additional absorption peaks grow in at 480 and 572 nm. The peak at 480 is not welldefined in the absorption spectra measured under monochromatic illumination because of the low resolution at this wavelength resulting from the 475 nm long pass filter. Spectra that include a well-defined peak at 480 nm, measured at high applied potentials in the dark, can be found in the Supporting Information. The observation that the same absorption feature can be generated both in the dark and under illumination is in agreement with both potential and light modulated absorption spectroscopy studies performed by Cummings et al.13 The main feature at 572 nm is also consistent with the difference spectra of hematite observed by Barroso et al. measured under applied bias under dark conditions.15,20 To investigate the origin of the absorption peaks, we monitored the absorption at 572 nm while performing electrochemical measurements. Under varying amounts of monochromatic light intensity, cyclic voltammetry (CV) measurements were performed by holding an applied potential of 1.4 V versus RHE for 60 s and then scanning cathodically at 200 mV/s. Relatively low light intensities of 0.03, 0.1, and 0.3 mW cm−2 of the 405 nm laser were used to avoid significant oxygen bubble formation at the hematite surface. The cathodic wave observed in Figure 2a has been correlated to a surfacestate capacitance and attributed to the reduction of oxidized surface species in a previous study.7 The changes in absorption at 572 nm measured during cyclic voltammetry are shown in Figure 2b. As the potential is scanned cathodically, the absorbance decreases and creates a valley with time. In the anodic scan of the cyclic voltammetry, the absorbance is recovered. The magnitude of the change in absorbance (Figure 2b) is plotted against the cathodic charge from the cyclic voltammetry and is shown in Figure 3. The cathodic charge was calculated by integrating all of the cathodic current in the cathodic scans shown in Figure 2a. The linear relationship of Figure 3 clearly shows that the absorption measured at 572 corresponds to the charge measured by cyclic voltammetry. Through the slope of the line in Figure 3, the absorption (abs) can be related to the charge (Q) by the equation Δabs = (Q/ zq)[σO(λ) − σR(λ)] where σO(λ) and σR(λ) are the absorption cross sections of the oxidized and reduced species, respectively, z is the number of elections transferred per oxidation/ reduction, and q is the elementary charge of an electron.13

Figure 3. Change in absorbance measured at 572 nm versus cathodic charge calculated from CVs measured under varying illumination intensities. The error bar represents the noise in the absorption measurement.

Assuming that the charge is associated with the surface (justified and discussed later), and adjusting the charge density for the geometric surface area by estimating a roughness factor of 1.5,21 a difference in the absorption cross section between the oxidized and reduced species at the surface, measured at 572 nm, is calculated to be 4.8 × 10−17 cm2. This corresponds to a difference in molar absorption coefficient, Δε, of 2.9 × 104 M−1 cm−1.22,23 The significance of this value is further analyzed in the Discussion. Water Oxidation on Alumina-Coated Hematite Electrodes. To validate our assumption that the absorbance and capacitance are indeed surface species, as opposed to bulk trap states, the surface of hematite electrodes was modified by adding alumina via ALD. It should be noted that a single ALD cycle will not necessarily form a complete monolayer of “alumina” because of the geometric limitations of the trimethylaluminum (TMA) precursor. Additional ALD cycles are expected to result in an essentially completely coated surface, however. In initial measurements, water oxidation was examined on hematite electrodes coated with alumina under 1 sun illumination. The stability of these electrodes was investigated at pH 13.9 (1 M NaOH) by holding a constant potential and measuring the current for 10 min (Supporting Information). The effect of the alumina layers in the pH 13.9 electrolyte appeared to be stable, and thus further experiments were conducted in this basic solution. The surprising finding that alumina layers are stable over the course of minutes is in agreement with Le Formal et al.19 J−V curves of water oxidation on hematite electrodes coated with alumina in the pH 13.9 solution are shown in Figure 4. With only one ALD cycle 10395



Article

5a was selected because it best illustrates two clear semicircles, which is indicative of two capacitances. Interpretation of impedance spectroscopy and the justification of an equivalent circuit for water oxidation at bare hematite electrodes has been previously discussed in detail.6,7 The simplified equivalent circuit is shown in Figure 5b and includes a series resistance, Rs, a capacitance of the depletion layer of the hematite, Cbulk, a resistance representing charge trapping in surface states, Rtrap, a capacitance representing the charge stored in the surface states, Css, and a charge transfer resistance from those surface states, Rct,ss. Because of a similar impedance response of the bare and alumina-coated hematite electrodes (two clear semicircles), the same equivalent circuit is used to fit the spectra from all electrodes. The most informative parameter derived from the impedance spectroscopy is the surface-state capacitance, which can be found in Figure 6a for hematite electrodes with varying amounts of alumina. For the bare electrode, a sharp peak in the capacitance is observed, a feature discussed in detail previously.6 The surface-state capacitance decreases significantly with only one cycle of alumina and forms two peaks. The capacitance decreases to a minimum reached with three cycles of alumina. This trend of the capacitance was also measured by cyclic voltammetry, a method previously described that has been shown to agree with the surface-state capacitance measured for water oxidation at bare hematite electrodes.7 In this experiment, a high positive bias (1.8 V vs RHE) was initially applied under 1 sun illumination to oxidize the surface states. The light was turned off, and the potential was immediately scanned for several cycles. On the first scan, cathodic peaks were observed, which were not observed in subsequent scans because the surface states are not able to be reoxidized at these potentials, in the dark, and on this time scale. For clarity, only the first scans are shown in Figure 6b. Subsequent scans are shown in the Supporting Information. This feature, which has been previously shown to be highly dependent on the scan rate, is clearly a capacitive peak.7 The capacitive peaks measured by cyclic voltammetry are observed at the same potentials and the same relative magnitudes as those observed in the Css. This agreement helps to corroborate the equivalent circuit used for the impedance interpretation. A reduction in the surface state capacitance with the addition of alumina is consistent with experiments performed by Le Formal et al., who quantified a decrease in the charge measured in current transients for alumina-coated hematite electrodes compared with bare

Figure 4. Steady-state J−V curves measured at 5 mV/s under 1 sun illumination for bare hematite electrodes (red), hematite coated with 1 (orange dotted), and 3 (yellow dashed) and 5 (green dashed dotted) ALD cycles of alumina in pH 13.9 solution. Steady state J−V curves measured in the dark are shown in black for reference.

of alumina, the current density decreases slightly and further decreases with increasing cycles of alumina. This is in contrast with the results found by Le Formal et al., who also applied alumina layers by ALD to nanostructured hematite prepared by APCVD.19 It should be noted, however, that these nanostructures likely have more exposed crystal facets compared with the planar films used herein, which may account for the observed difference for the same surface modification. Electrochemical impedance spectroscopy was employed to understand the effect of the alumina surface modification. Typical Nyquist plots for hematite electrodes coated with alumina are shown in Figure 5a. The potential shown in Figure

Figure 5. (a) Characteristic Nyquist plot for water oxidation under 1 sun illumination at a bare hematite electrode (red circles) and one coated with one (orange triangles), three (yellow squares), and five (green diamonds) cycles of alumina. (b) Equivalent circuit used to fit Nyquist plots.

Figure 6. (a) Css for a bare hematite electrode (red circles), a hematite coated with one (orange triangles), three (yellow squares), and five (green diamonds) ALD cycles of alumina measured under 1 sun illumination. Error bars represent error in the fitting (b) CV measured in the dark immediately after 1 sun illumination for a bare hematite electrode (red solid) and hematite coated with one (orange dotted), three (yellow dashed), and five (green dotted, dashed) ALD cycles of alumina. CVs were measured at 200 mV/s. 10396



Article

Figure 7. (a) Steady-state J−V curves measured at 5 mV/s under 0.1 mW cm−2 405 nm illumination for a bare hematite electrode (red solid line) and a hematite electrode coated with 1 cycle of alumina (orange dotted line). A J−V measured in the dark is shown for the bare electrode (black solid line). Overlaid is the change in absorption measured as a function of potential for the same bare (red circles) and alumina-coated (orange triangles) electrodes. Error bars represent noise in the absorption measurment (b) Css measured by impedance spectroscopy under 1 sun illumination (solid lines, filled in shapes) and the rate of change of absorbance with potential (dashed lines, hollow shapes) of a bare electrode (red circles) and one coated with one cycle of alumina (orange triangles).

electrodes.9 Additional impedance parameters can be found in the Supporting Information. Because the observed absorption feature in Figure 1 was shown to be correlated to the charge stored in surface states, the effect on the absorption was also examined for hematite electrodes with alumina-modified surfaces. The absorption at 572 nm was measured as a function of applied potential under 0.1 mW cm−2 405 nm illumination. These absorption values are overlaid with the J−V curve measured under the same conditions for a bare hematite electrode and one coated with one cycle of alumina (Figure 7a). While the change in absorption becomes significantly lower for alumina-modified hematite electrodes, the J−V is only slightly affected. Because the absorbance is related to the charge, a pseudocapacitance can be calculated by the equation C = (dQ/dV) ∝ (dabs/dV). When comparing the pseudocapacitance measured by the absorption of species at 572 nm and to the capacitance measured by impedance spectroscopy, it is again clear that the species measured by absorption measurements and impedance spectroscopy are the same.

supported by analyzing Css for different electrode thicknesses. The Css and absorption feature should scale with thickness if they are associated with the bulk or they should be independent of the electrode thickness if they are associated with the surface. As shown in the Supporting Information, we observe a constant Css as a function of electrode thickness, showing that Css, and thus the absorption feature at 572 nm, are indeed measuring the surface. The assignment of this absorption feature to the surface is in contrast with the conclusions made by Barroso et al., who measure a similar absorption spectra to (Figure 1) but assign it to processes in the bulk based on the insensitivity of the transient absorption decay dynamics to electrolytes including MeOH and KI.15 Additional insight can be gained about these surface features when examining the shape of the difference absorption spectra shown in Figure 1 and comparing it to spectra of potential molecular analogues. For example, de Oliveira et al. prepared and isolated an Fe(III)B*(H2O) and its corresponding oxidized species, Fe(V)B*(O).29,30 The visible absorption spectra of this isolated Fe(V)B*(O) species contain striking similarities with the absorption spectra displayed in Figure 1. In addition, the molar absorption coefficient, ε, for the Fe(III)B*(H2O) species at 572 nm was measured to be near 0, while the analogous Fe(V)B*(O) species was measured to be ∼4 × 104 M−1 cm−1.30 This corresponds to a difference in molar absorption coefficient, Δε = 4 × 104 M−1 cm−1, which is in close agreement with the value calculated from the slope of the line in Figure 3, Δε = 2.9 × 104 M−1 cm−1.22,23 Whereas this is not conclusive evidence of the identity of impedance spectroscopy and in situ absorption spectroscopy features measured herein, we tentatively assign them to oxidation of a low valent Fe−H2O or Fe−OH to a high valent FeO group at the surface. More detailed in situ analytical techniques are underway in our lab to identify these species. It has recently been discussed how the assignment of water oxidation occurring through an FeO surface species is consistent with theoretical calculations, the study of analogous molecular water oxidation catalysts, and surface studies on the water oxidation at TiO2.12 The addition of a single layer of alumina by ALD reduced the change in absorption measured at bare hematite electrodes (Figure 7a). The addition of alumina is expected to alter the chemical identity of the surface. For example, a surface that was

■

DISCUSSION The absorption feature at 572 nm has been clearly correlated with capacitances labeled Css from impedance spectroscopy. This absorption feature has also been correlated with the current onset of water oxidation at hematite electrodes.15,20 Thus, it is very important that the absorption feature and capacitance are correctly assigned to a physical process so that correct interpretations can be made and corresponding solutions can be developed. To help identify this physical process, we modified the hematite surface with alumina, which was shown to affect the absorption feature and the associated capacitance, as previously discussed. We therefore attribute these features to oxidation and absorption of surface species. Further support of this assignment is that the charge measured from the capacitance peak is a consistent magnitude as if it were at the surface. For example, integrating the capacitance peak shown in Figure 7b measured under 1 sun illumination, the charge is calculated to be 2.9 × 1014 cm−2, once corrected for the roughness factor of 1.5.7,21 Estimating one reaction site per unit cell, for the 0001 surface of hematite, the surface coverage is calculated to be ∼64%.7,24−28 Finally, the assignment of the absorption feature and capacitance to the surface is further 10397



■

once dominated by iron hydroxyls and iron oxo groups is now covered predominantly by aluminum atoms (likely aluminum hydroxyls and oxo groups). Thus, a potential explanation for a reduction in the change in absorption with the addition of alumina is that because there are fewer iron atoms at the surface, fewer iron hydroxyls are able to be generated upon oxidation by a photogenerated hole. Aluminum hydroxyl and oxo intermediates are not expected to absorb light in the visible region, resulting in a net decrease in absorption. In addition, the alumina modification was observed to reduce the surface state capacitance measured by impedance spectroscopy. Despite a significant reduction in the measured surface state capacitance with the addition of alumina, the J−V curve is not greatly affected (Figure 7a). We have previously attributed the surfacestate capacitance of hematite electrodes to the requirement of a buildup of iron-based water oxidation intermediates. It is possible that the modification of the surface with aluminum atoms alters the water oxidation mechanism, which would likely affect the buildup of charged species.

CONCLUSIONS The absorption properties of hematite films were examined during water oxidation. Under anodic potential and illumination, a change in the absorption spectrum of hematite was observed that is correlated with a capacitive charge measured by both cyclic voltammetry and impedance spectroscopy. Modification of the hematite surface with alumina reduced the change in absorbance and the associated capacitance, suggesting that these features are associated with the surface. The similarities between the difference absorption spectrum (Figure 1) and absorption features of potential molecular analogues allowed us to tentatively assign the absorbance and capacitive features to the oxidation of a low valent iron-aqua or iron hydroxyl species to a high valent Fe-oxo chemical species at the surface. These species are hypothesized to be an intermediate of the water oxidation mechanism on bare hematite electrodes. While the exact identity of the surface intermediates is still unknown, we believe this work constitutes strong evidence of the general mechanism of water oxidation at hematite electrodes, which includes water oxidation occurring through surface states. Work is ongoing in our lab to further confirm the identity of the surface states. ASSOCIATED CONTENT

S Supporting Information *

Experimental setup of spectroelectrochemistry measurements, difference absorption spectra in response to large applied bias, stability tests of alumina coated electrodes, EIS fit results, Mott−Schottky plots, and cyclic voltammograms. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

(1) Finklea, H. O. Semiconductor Electrodes; Elsevier: Amsterdam, 1988. (2) Kennedy, J. H.; Frese, K. W. Photo-Oxidation of Water at αFe2O3 Electrodes. J. Electrochem. Soc. 1978, 125, 709−714. (3) Hamann, T. W. Splitting Water with Rust: Hematite Photoelectrochemistry. Dalton Trans. 2012, 41, 7830−7834. (4) Goodenough, J. B. Metallic Oxides. Prog. Solid State Chem. 1971, 5, 145−399. (5) Sivula, K.; Le Formal, F.; Grätzel, M. Solar Water Splitting: Progress Using Hematite (α-Fe2O3) Photoelectrodes. ChemSusChem 2011, 4, 417−417. (6) Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Hamann, T.; Bisquert, J. Water Oxidation at Hematite Photoelectrodes: The Role of Surface States. J. Am. Chem. Soc. 2012, 134, 4294−4302. (7) Klahr, B.; Giménez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. Electrochemical and Photoelectrochemical Investigation of Water Oxidation with Hematite Electrodes. Energy Environ. Sci. 2012, 5, 7626−7636. (8) 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. (9) Le Formal, F.; Sivula, K.; Grätzel, M. The Transient Photocurrent and Photovoltage Behavior of a Hematite Photoanode under Working Conditions and the Influence of Surface Treatments. J. Phys. Chem. C 2012, 116, 26707−26720. (10) Pendlebury, S. R.; Cowan, A. J.; Barroso, M.; Sivula, K.; Ye, J.; Gratzel, M.; Klug, D. R.; Tang, J.; Durrant, J. R. Correlating LongLived Photogenerated Hole Populations with Photocurrent Densities in Hematite Water Oxidation Photoanodes. Energy Environ. Sci. 2012, 5, 6304−6312. (11) Sivula, K. Metal Oxide Photoelectrodes for Solar Fuel Production, Surface Traps, and Catalysis. J. Phys. Chem. Lett. 2013, 4, 1624−1633. (12) Young, K. M. H.; Klahr, B. M.; Zandi, O.; Hamann, T. W. Photocatalytic Water Oxidation with Hematite Electrodes. Catal. Sci. Technol. 2013, 3, 1660−1671. (13) Cummings, C. Y.; Marken, F.; Peter, L. M.; Upul Wijayantha, K. G.; Tahir, A. A. New Insights into Water Splitting at Mesoporous αFe2O3 Films: A Study by Modulated Transmittance and Impedance Spectroscopies. J. Am. Chem. Soc. 2012, 134, 1228−1234. (14) Peter, L. Energetics and Kinetics of Light-Driven Oxygen Evolution at Semiconductor Electrodes: The Example of Hematite. J. Solid State Electrochem. 2013, 17, 315−326. (15) Barroso, M.; Pendlebury, S. R.; Cowan, A. J.; Durrant, J. R. Charge Carrier Trapping, Recombination and Transfer in Hematite (α-Fe2O3) Water Splitting Photoanodes. Chem. Sci. 2013, 4, 2724− 2734. (16) Riha, S. C.; Klahr, B. M.; Tyo, E. C.; Seifert, S.; Vajda, S.; Pellin, M. J.; Hamann, T. W.; Martinson, A. B. F. Atomic Layer Deposition of a Submonolayer Catalyst for the Enhanced Photoelectrochemical Performance of Water Oxidation with Hematite. ACS Nano 2013, 7, 2396−2405. (17) Hisatomi, T.; Brillet, J.; Cornuz, M.; Le Formal, F.; Tetreault, N.; Sivula, K.; Gratzel, M. A Ga2O3 Underlayer as an Isomorphic Template for Ultrathin Hematite Films Toward Efficient Photoelectrochemical Water Splitting. Faraday Discuss. 2012, 155, 223−232. (18) Klahr, B. M.; Martinson, A. B. F.; Hamann, T. W. Photoelectrochemical Investigation of Ultrathin Film Iron Oxide Solar Cells Prepared by Atomic Layer Deposition. Langmuir 2011, 27, 461−468. (19) Le Formal, F.; Tetreault, N.; Cornuz, M.; Moehl, T.; Gratzel, M.; Sivula, K. Passivating Surface States on Water Splitting Hematite Photoanodes with Alumina Overlayers. Chem. Sci. 2011, 2, 737−743. (20) Barroso, M.; Mesa, C. A.; Pendlebury, S. R.; Cowan, A. J.; Hisatomi, T.; Sivula, K.; Grätzel, M.; Klug, D. R.; Durrant, J. R. Dynamics of Photogenerated Holes in Surface Modified α-Fe2O3

■

■

Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 517-355-9715. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS T.W.H. thanks the National Science Foundation (CHE1150378) for support of this research. 10398



Article

Photoanodes for Solar Water Splitting. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 15640−15645. (21) A roughness factor of 1.5 was used, which is typical of the underlying FTO. (22) Cameron, P. J.; Peter, L. M. Characterization of Titanium Dioxide Blocking Layers in Dye-Sensitized Nanocrystalline Solar Cells. J. Phys. Chem. B 2003, 107, 14394−14400. (23) Fattori, A.; Peter, L. M.; McCall, K. L.; Robertson, N.; Marken, F. Adsorption and Redox Chemistry of cis-RuLL′(SCN)2 with L=4,4′dicarboxylic acid-2,2′-bipyridine and L′=4,4′-dinonyl-2,2′-bipyridine (Z907) at FTO and TiO2 Electrode Surfaces. J. Solid State Electrochem. 2010, 14, 1929−1936. (24) The number of active sites per unit cell is estimated by assuming one Fe atom in a given plane per unit cell. Each atom is estimated to have only one active site because the formation of two oxo groups on a single Fe atom is energetically unfavorable. (25) Hellman, A.; Pala, R. G. S. First-Principles Study of Photoinduced Water-Splitting on Fe2O3. J. Phys. Chem. C 2011, 115, 12901−12907. (26) Eggleston, C. M.; Hochella, M. F. The Structure of Hematite (001) Surfaces by Scanning Tunneling Microscopy - Image Interpretation, Surface Relaxation, and Step Structure. Am. Mineral. 1992, 77, 911−922. (27) Eggleston, C. M.; Stack, A. G.; Rosso, K. M.; Higgins, S. R.; Bice, A. M.; Boese, S. W.; Pribyl, R. D.; Nichols, J. J. The Structure of Hematite (α-Fe2O3) (001) Surfaces in Aqueous Media: Scanning Tunneling Microscopy and Resonant Tunneling Calculations of Coexisting O and Fe terminations. Geochim. Cosmochim. Acta 2003, 67, 985−1000. (28) Blake, R. L.; Hessevick, R. E. Refinement of the Hematite Structure. Am. Mineral. 1966, 51, 123−129. (29) B* = 3,3,6,6,9,9-hexamethyl-3,4,8,9-tetrahydro-1H-1,4,8,11benzotetraazacyclotridecine-2,5,7,10(6,H,11,H)-tetraone. Ligand is in a class called tetraamido macrocyclic ligand (TAML). (30) de Oliveira, F. T.; Chanda, A.; Banerjee, D.; Shan, X.; Mondal, S.; Que, L.; Bominaar, E. L.; Münck, E.; Collins, T. J. Chemical and Spectroscopic Evidence for an FeV-Oxo Complex. Science 2007, 315, 835−838.

10399


Water Oxidation on Hematite Photoelectrodes: Insight into the Nature

Recommend Documents