Metal Oxide Photoelectrodes for Solar Fuel Production, Surface Traps

Perspective pubs.acs.org/JPCL

Metal Oxide Photoelectrodes for Solar Fuel Production, Surface Traps, and Catalysis Kevin Sivula* Laboratory for Molecular Engineering of Optoelectronic Nanomaterials, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland ABSTRACT: The photoelectrochemical reduction of water or CO2 is a promising route to sustainable solar fuels but hinges on the identification of a stable photoanode for water oxidation. Semiconductor oxides like Fe2O3 and BiVO4 have been gaining significant attention as promising materials. However, they exhibit a major drawback of a large required overpotential for solar water oxidation. In this Perspective, recent efforts to characterize and reduce the overpotential are critically examined. The accumulation of photogenerated holes at the semiconductor−liquid interface, recently observed with multiple techniques, is rationalized with surface state models. Transient absorption spectroscopy and electrochemical impedance spectroscopy suggest that surface treatments designed to either passivate surface traps or increase reaction rates (as catalysts) actually perform identically. This calls into question the definition of a catalyst when coupled to a semiconductor photoelectrode. In contrast, results from transient photocurrent spectroscopy suggest that two separate loss mechanisms are indeed occurring and can be addressed separately.

S

olar fuels produced by light-to-chemical energy conversion in a photoelectrochemical (PEC) device are promising substances for transportable, renewable energy use and chemical feedstocks. The direct interface of a semiconductor and a liquid electrolyte present in a PEC device offers an elegant and potentially more efficient mechanism of solar-tochemical energy conversion compared to brute-force photovoltaic + electrochemical approaches.1 As such, tremendous research efforts have been put forth developing PEC devices based on III−V compounds, amorphous silicon, and oxide semiconductors. Devices have been designed to produce promising solar fuels via the reduction of water into hydrogen2 or of CO2 into formate,3 CO,4 or methanol.5 To complete a reaction in a PEC device, the fuel-producing reduction reactions must occur with a corresponding oxidation reaction, which is most conveniently water oxidation to produce H+ as both H2O and CO2 reduction require protons.

The efficient operation of the water oxidation reaction is a fundamental requirement to obtain PEC solar fuels at high solarto-chemical efficiency. The goal of this Perspective is to examine the state-of-the-art knowledge in understanding and overcoming the limitations of the water oxidation reaction on PEC electrodes for solar fuel production. reaction has become a central topic in the pursuit of efficient solar fuel production. The goal of this Perspective is to examine the state-of-the-art knowledge in understanding and overcoming the limitations of the water oxidation reaction on PEC electrodes for solar fuel production. First, to give the reader a sense of the context that the water oxidation reaction plays in state-of-the-art PEC solar fuel production devices, it is helpful to first briefly summarize the operation mechanism of a PEC device. Taking PEC water splitting as the prototypical reaction, in an ideal case, a single semiconductor can be used to split water by absorbing two photons to produce one molecule of H2 in the

Fuel producing reduction reactions: 2H+ + 2e− → H 2 CO2 + H+ + 2e− → HCOO− CO2 + 2H+ + 2e− → CO + H 2O CO2 + 6H+ + 6e− → CH3OH + H 2O

Corresponding oxidation reaction: 2H 2O + 4h+ → O2 + 4H+

Thus, the efficient operation of the water oxidation reaction is a fundamental requirement to obtain PEC solar fuels at high solar-to-chemical efficiency. Indeed, the water oxidation © 2013 American Chemical Society

Received: February 10, 2013 Accepted: April 25, 2013 Published: April 29, 2013 1624

dx.doi.org/10.1021/jz4002983 | J. Phys. Chem. Lett. 2013, 4, 1624−1633

The Journal of Physical Chemistry Letters

Perspective

been accomplished. While many research efforts continue on this path, alternative approaches are also being pursued. A dual-absorbing tandem cell requires four photons to produce one molecule of H2 (a D4 approach) and can both absorb a substantial fraction of the solar spectrum and generate sufficient Δμex for water splitting. The schematic for this approach is shown in Figure 1b for a photoanode/photocathode combination. Here, the two semiconductor band gap energies, Eg1 and Eg2, harvest complementary portions of the solar spectrum. Assuming reasonable losses, a solar-to-chemical conversion efficiency of at least 18% should be entirely feasible with this approach by optimizing the band gap energies.6 More absorbers can be added to further increase the light-harvesting capability of the cell but with added device complexity and fabrication costs. PEC devices with state-of-the-art solar-to-hydrogen conversion efficiency use this tandem concept and have been reported at efficiencies near the feasible limits using materials with tuned energy levels.7,8 However, the stability of these devices is poor. Indeed, a further challenge for PEC devices has been the identification of materials with suitable stability in the harsh environment of an electrolyte. Semiconducting oxides are particularly attractive in this regard, especially for the photoanode performing the oxidation half reaction. Oxide semiconductors like TiO2,9 WO3,10 Fe2O3,11 and BiVO4,12 have been thoroughly investigated for their performance as n-type photoanodes. In general, the advent of nanotechnology has seen a dramatic increase in the photocurrent delivered by these oxides through careful nanostructuring and doping. These strategies enhance light harvesting by increasing the probability of a photogenerated hole to reach the SCLJ and improve the transport of the majority carriers (electrons) to the conductive substrate. Improvements in both electrode nanostructure and interfacial charge transfer have led to demonstrations of tandem devices with oxide semiconductors capable of unassisted solarto-chemical conversion efficiencies of around 3−4% at standard illumination conditions.2 However, the aforementioned limitations for water oxidation that give rise to a high ηox still represent a major drawback for these devices. For example, the water-splitting photocurrent generated by state-of-the-art Fe2O3 electrodes corresponds to a solar-to-hydrogen conversion efficiency of around 5%,13 but to actually achieve a device with that performance, two additional absorbing devices are needed in tandem to overcome the overpotential.14 This prohibitively complicates device fabrication. To give a sense of how this high overpotential can be quantified, the usual method for characterizing the performance of a photoelectrode using a three-electrode potentiostat (apart from the tandem cell configuration) is outlined in Figure 2 for an n-type semiconductor. When the photoelectrode is polarized by a potentiostat at the so-called flat band potential (Vfb), the conduction and valence bands are flat throughout the semiconductor. The flat band potential is essentially defined by the energy of the conduction and valence bands at the SCLJ (i.e., the surface species). For oxides, these surfaces species typically exhibit Nernstian behavior with respect the pH of the electrolyte.15 When the potential of the electrode is moved cathodically (more negative) or anodically (more positive) with a potentiostat, the Fermi level of the electrode and the conduction and valence bands move along with the vacuum (reference) level, but the energy levels at the surface stay fixed. This causes the band bending and the formation of the

so-called S2 scheme (Figure 1a). Here, an n-type semiconductor forms a Schottky junction with the aqueous

Figure 1. Electron energy scheme of (a) S2 PEC water splitting using a photoanode and (b) D4 PEC water splitting using a photoanode and photocathode in tandem. The absorption of a photon (hν) by the semiconductor with a band gap of Eg creates an electron−hole pair that can be separated by the space charge layer, W, to generate a free energy of Δμex. This free energy must be greater than the energy needed for water splitting (1.23 eV) plus the overpotential losses at both the anode and the cathode, ηox and ηred, for the water-splitting reaction to occur. Two photons must be absorbed in the S2 mechanism to produce one H2, while four are needed for the D4 approach.

electrolyte due to the equilibration of the semiconductor Fermi energy with the oxidation/reduction potential of the electrolyte. This results in a depletion zone of width W at the semiconductor−liquid junction (SCLJ). The electric field in the depletion zone causes the bending of the conduction and valence bands and drives the separation of photogenerated electrons (in the conduction band) from their hole counterparts (in the valence band). This splits the Fermi level into separate quasi-Fermi levels for the holes and the electrons and generates a (photoelectro)chemical potential Δμex between the SCLJ and the counter electrode. If Δμex per electron is greater than the 1.23 V needed to split water plus the electrochemical overpotentials required for the oxidation, ηox, and reduction reactions, ηred, the PEC reaction will proceed. Because of the overpotentials and other loss processes, the band gap energy (Eg) required to generate sufficient Δμex precludes also harvesting a large fraction of the solar spectrum, and the identification of a single material capable of performing S2 water splitting at high solar-to-chemical efficiencies has not yet 1625



Perspective

chemical vapor deposition (APCVD), it was found that at potentials only slightly greater than Vfb, photogenerated holes are indeed available for oxidation, but the rate for water oxidation is very low and limits the photocurrent. This is clearly seen by comparing the photocurrents with the hole scavenger (red curves) and without (blue curves) in Figure 3. At high

Figure 3. Current density versus applied potential (J−V) plots in the dark (dotted curves) and under simulated solar illumination (solid curves) for USP (A) and APCVD (B) hematite electrodes. The red curves correspond to a 0.5 M H2O2 + 1 M NaOH electrolyte, while the blue corresponds to 1 M NaOH. Reprinted from ref 20 with permission from RSC, copyright 2011.

Figure 2. (top) Electron energy scheme of an n-type semiconductor photoanode under polarization using a potentiostat at different potentials relative to the flat band potential, Vfb. An accumulation of majority carriers (purple shaded region) occurs when V < Vfb, and a depletion layer (of width W, yellow shaded region) forms when V > Vfb. (bottom) Idealized photocurrent density versus applied potential curves are shown relative to Vfb, and the overpotential for the oxidation reaction, ηox, is shown for a typical electode with an onset of photocurrent at Von.

potentials, however, without the hole scavenger, the water oxidation rate increases substantially, and the yield of charges that reach the SCLJ that successfully undergo water oxidation reached more than 90% (under these conditions, the photocurrent becomes limited by bulk recombination). While determining the actual photogenerated hole current possible at potentials near the flat band potential is difficult due to a reverse reaction occurring with conduction band electrons that competes with water oxidation at these potentials (which is the reason for the cathodic current seen in Figure 3 at potentials less than 0.7 V versus RHE), the maximum possible photocurrent for water oxidation can be estimated at more anodic potentials where the competing electron-transfer reaction is slow. Overall, the work of Dotan et al. showed clearly that photogenerated charges are available for water oxidation at potentials anodic of the flat band potential in hematite photoanodes, but water photo-oxidation is not typically observed with this material until about 1.0 V versus RHE, suggesting ηox = 0.5−0.6 V at one sun illumination. A more recent study using thin films of Fe2O3 deposited by atomic layer deposition (ALD) by Klahr et al. confirms this value of ηox using Fe(CN)6]3−/4− as a one-electron redox shuttle.21 In addition to hematite, it has also been shown with BiVO4 using H2O2 that photogenerated holes are present at the SCLJ at potentials just slightly over the flat band potential but do not oxidize water due to a large overpotential (0.4−0.5 V) in this material as well.22 These observations with Fe2O3 and BiVO4 have led to much debate on the cause of the overpotential for water oxidation on oxide semiconductors. In general, the electrochemical overpotential on metallic electrodes is a well-studied phenomenon and is known to arise from many sources including the electrical resistance in the electrode or electrolyte or due to the depletion of charge carriers at the surface caused when an electrochemical reaction is sufficiently rapid to lower the surface concentration of the charge carriers below that of bulk solution. However, results from using the H2O2 scavenger eliminate these as possibilities. Rather, the overpotential has been most commonly ascribed to kinetic limitations in the water oxidation reaction. In general, this is expected as the water oxidation reaction must involve

Schottky junction under anodic polarization that drives the separation of electrons and holes produced by the absorption of illumination. For n-type photoanodes, photogenerated holes will move to the SCLJ and are available for electrochemical reaction. In the ideal case where the overpotential for the electrochemical reaction approaches zero, photocurrent can be measured at all potentials more anodic than Vfb (green curve in Figure 2). In the limit of a thin film where the light absorption rate can be considered independent of depth into the film and the diffusion length of photogenerated carriers is small (often the case in oxides), the photocurrent should rise in proportion to the depletion width, W, which grows proportional to the square root of the applied voltage.16 In systems that exhibit an overpotential, photocurrent is not observed directly anodic of Vfb but after an additional potential has been applied. This is shown schematically by the red curve in Figure 2 where the magnitude of the overpotential for the oxidation reaction, ηox, is also indicated. In practice, the flat band potential can be measured by electrochemical impedance spectroscopy, which can directly measure the capacitance of the Schottky junction and is usually found to be around 0.5 V versus the reversible hydrogen electrode (RHE) for Fe2O3,17 0.3 V for WO3,18 and 0.0 V for BiVO4.19 A demonstration of the effect of overpotential can be directly seen by comparing the photocurrent of an electrode performing water oxidation to the same electrode preforming an electrochemically less demanding oxidation reaction, the oxidation of H2O2 for example. The oxidation of a “hole scavenger” like H2O2 by n-type electrodes can indeed be a useful tool for understanding the effects of the overpotential, as shown recently by Dotan et al.20 In this work, it was shown that the hole scavenger can be used as a probe to diagnose the factors limiting the photoelectrochemical splitting of water by hematite electrodes. For the two types of nanostructured hematite electrodes examined, one produced by ultrasonic spray pyrolysis (USP) and the other by atmospheric pressure 1626



Perspective

In terms of overpotential, iridium oxide (IrO2) is the best (dark) water oxidation catalyst.34,35 Small nanoparticles (∼2 nm diameter) deposited onto a glassy carbon electrode achieve quantitative Faradaic efficiency of water oxidation at overpotentials as low as 0.25 V to achieve a current density of 0.5 mA cm−2.36,37 For comparison, the cobalt phosphate catalyst requires an overpotential of 0.40 V to achieve 0.5 mA cm−2.29 The surface of a nanostructured hematite photoanode was modified with IrO2 nanoparticles by Tilley et al. using an electrophoretic technique, and a shift in the water oxidation photocurrent of 160 mV was observed.13 While this work demonstrated that high photocurrents could be obtained with an applied bias of only 1.23 V versus RHE (the thermodynamic water oxidation potential), the inability of this high-performance catalyst to eliminate most of the overpotential suggests that photogenerated holes at the SCLJ do not have enough Δμex for water oxidation despite the known valence band position, which should be 1.3 V more anodic than the water oxidation potential (1.23 V versus RHE). Thus, factors other than slow water oxidation kinetics must play an important role to the onset of photocurrent in photoanode materials like Fe2O3. Indeed, even in the earlier work on using hematite as a photoanode, several groups suggested that electronic surface traps can be an additional loss mechanism.25,27,38−41 Electronic states on the surface of an oxide photoanode due to oxygen vacancies or crystal defects could cause a degree of the so-called Fermi level pining.42 This concept is shown schematically in the extreme case in Figure 4. Given the presence of electronic states

four photogenerated holes to evolve one molecule of O2. While this is mainly thought to occur in steps, second-order or higher rates due to a requirement of vicinal sites and high activation energies can be present in intermediate steps. This does not explain the large difference in overpotential observed for photoanodes like WO3 and TiO2 (reported to be about 0.218,23,24 for both under standard illumination conditions) and photoanodes like Fe2O3 or BiVO4. Because of this, other factors have been suggested. For example, for Fe2O3, it has been pointed out that valence band holes created by visible light illumination have mainly Fe 3d rather than O 2p character as in TiO2. This can result in a smaller rate constant for water oxidation25 (while a O 2p hole band is thought to exist in Fe2O3 at deeper energies, suggesting that there are two types of holes possible in this material, a photogenerated hole generated in the O 2p would rapidly thermalize to the Fe 3d band). In the limiting case of an extremely retarded water oxidation rate of the Fe 3d holes, the resulting effect on the performance of the photoanode would be to prevent the onset of the photocurrent until a sufficient concentration of photoholes was present at the SCLJ. If this hypothesis is true, then two aspects about the performance can be predicted. First, when polarizing the photoelectrode at potentials between the Vfb and the observed onset of photocurrent, photogenerated holes that are separated by the space charge field will travel to the SCLJ, but as their reaction rate is extremely slow, they will accumulate at this interface, analogous to charges accumulating in a capacitor, until their accumulation creates a field that opposes the space charge field and stops the further separation of charges. This should be observable as a transient photocurrent, and in fact, this was described in early reports on hematite25−27 and is a common observation in many electrode materials. Second, if the holes have a slow reaction rate due to the activation energy of one of the intermediates, then it should be possible to lower that activation energy by adding an appropriate catalyst. Indeed, a general strategy for increasing the rate of the water oxidation reaction and reducing the overpotential on hematite and other oxide semiconductors has been to add a catalyst at the SCLJ. One of the first reports of using a catalyst on nanostructured Fe2O3 to split water used a monolayer of surface-absorbed Co2+ from Co(NO3)2.28 It is known that CoOx exhibits a low overpotential for water oxidation, and it has been suggested that on the cobalt-modified hematite surface, water oxidation can occur with a smaller overpotential. Indeed, a shift of the photocurrent curve in the cathodic direction consistent with a reduction of overpotential of 80 mV was observed. More recently, the cobalt oxyhydroxide based catalyst prepared from phosphate buffer (Co−Pi) reported by Nocera and Kanan29 has been applied to hematite photoanodes by a simple electrodeposition technique30 and a photodepostion technique,31 which yielded an impressive overpotential reduction of 120 mV on average over a few films compared to 90 mV for the electrodeposted Co−Pi. The Co−Pi deposition has also shown a reduction of overpotential in BiVO4 films.22,32 In these reports, the photocurrent exhibited by the Co−Pi-functionalized BiVO4 is almost the same as the photocurrent found by using the sacrificial H2O2 electrolyte, suggesting that other limiting factors, like the transport of majority carriers, is limiting the ultimate performance of this material.33 This is in stark contrast to Fe2O3, where even with the photodepostion of Co− Pi, 300−500 mV of overpotential remains for the water oxidation.

Figure 4. Electron energy scheme of an n-type semiconductor photoanode under polarization using a potentiostat at different potentials and with the presence of surface states that cause Fermi level pinning. (a) Electrode at Vfb. (b) An anodic potential empties the surface states but does not induce band bending. Band bending commences at (c) when the surface states are emptied.

at the SCLJ with energies in the band gap energy of the semiconductor, an applied bias to the electrode does not induce the bending of the bands of the semiconductor but rather causes the emptying of these surface states (Figure 4a,b). The potential drop occurs over the Helmholtz layer instead of in the semiconductor photoelectrode, and as a result, no space charge field is present to separate photogenerated charges. It is not until the surface states are emptied that the photoelectrode can develop band bending and a space charge field, Figure 4c. This scheme represents what would happen with a very large amount of surface states that are localized near one value of energy; however, in real situations, surface states would more likely have a small concentration and a broad energy distribution. This would lead to a nonideal behavior of the space charge width with applied potential, which can be 1627



Perspective

quantified by measuring the open-circuit photopotentials as a function of the redox potential of the electrolyte employed.43 For an ideal SCLJ forming a Schottky junction, the (opencircuit) photopotential, VOC, should follow the relation

VOC = Vfb − Vredox where Vredox is the redox potential of the redox couple present in the electrolyte in contact with the photoelectrode. A plot of VOC versus Vredox should thus exhibit a slope of 1 in the ideal case. A slope of 1 can be observed for Fe2O3 in carefully prepared single-crystalline electrodes,27 but for electrodes prepared via solution-based approaches or other nanostructuring techniques, slopes less than 1 are observed.44 For example, in hematite films prepared by spray pyrolysis, a slope of only 0.54 was observed when varying the Vredox from −0.1 to 0.22 V versus Ag/AgCl. This suggests that the Fermi level of the electrode is partially pinned; a change of 0.1 V in the redox potential induces a change of 0.05 V in the photopotential. This represents good evidence of the existence of surface states. However, in general, there is a lack of understanding of the precise nature, concentration, and the energy of surface states on oxide photoelectrodes as there are many difficulties characterizing electronic surface states under device operation conditions. Very recently, some progress has been made using different characterization techniques, in situ soft X-ray spectroscopy and electrochemical impedance spectroscopy. In situ soft X-ray spectroscopy has recently shown that surface-specific electronic states can be characterized under electrode operating conditions,45 and for hematite, it has strongly suggested that surface traps can be directly related to the intrinsic electronic structure of hematite. In a report by Braun et al., a novel electrode architecture was employed (using a 100 nm thick SiN window) to enable for the first time the study of the electronic states of the valence band by near-edge X-ray absorption fine structure (NEXAFS) spectra under simulated sunlight and in the dark while exposed to aqueous electrolyte and an applied potential.46 Under illumination, two additional features appeared at energies corresponding to the valence band edge. This is consistent with the two predicted types of holes thought to exist in hematite and in direct contrast to the assumption that photogenerated holes would quickly thermalize to the lowest valence band energy. On the basis of this expected electronic structure of hematite, these extra transitions were assigned to the O 2p type hole transition into the charge-transfer band (CTB) and the higher-energy peak to the more Fe 3d type hole transition into the upper Hubbard band (UHB), respectively. Because the Fe 3d holes are predicted to have a slower rate of water oxidation, these states are prime candidates for surface trapping states. To examine this hypothesis, the appearance of the spectral features was compared to the photocurrent of the electrode. Interestingly, the appearances of these spectral features correlated well with the photocurrent onset. This is shown in Figure 5a where the spectral weights of the CTB and the UHB signatures are shown as a function of the applied potential under illumination and superimposed on the photocurrent curve. As the potential is scanned anodically from the flat band potential, the spectral weight of both bands is observed to increase in intensity until the point of photocurrent onset, where they then decrease rapidly. This behavior is expected even without invoking a “surface state” explanation due to the previously mentioned accumulation of holes at SCLJ resulting from slow oxidation kinetics. The NEXAFS results do show that Fe 3d type holes in

Figure 5. Hole accumulation in hematite photoanodes is shown in two separate cases. (a) Comparison of the photocurrent (blue line) and NEXAFS spectral weight of the t1u↑ CTB (circles) and a1g↑ UHB (triangles) spectral features and their sum (squares). The potential is measured versus the RHE. Reprinted from ref 46. (b) Photocurrent curve, J (green solid line), the capacitance of the trapping states, Ctrap (orange triangles), and the resistance of charge transfer from the trapping states, Rct,trap (red circles), as a function of the applied potential versus Ag/AgCl. Values obtained for a 60 nm hematite electrode under 1 sun illumination and pH 6.9. Reprinted form ref 60.

the UHB require more applied bias to reach their peak concentration (the bias where the accumulation and reaction rates are the same), compared to their counterparts in the CTB. In addition, even at high potentials, the UHB spectral feature does not completely vanish. These observations suggest that Fe 3d type holes in the UHB are kinetically hindered (due to a disparate reaction mechanism) but are not completely consistent with them being a source of Fermi level pinning. In that case, the spectral signature should be observed at potentials cathodic of the onset of the CTB signature (upon scanning anodically, photogenerated holes would first populate the surface states and then accumulate in the valence band). As the identification of these holes was at the detection limit of the characterization technique, it is possible that more sensitive measurements will reveal that the UHB states are indeed populated before the CTB. Until then, it can be stated only that this study does suggest that two types of holes exist simultaneously in hematite under operational conditions, in contrast to the assumption that a rapid thermalization of CTB holes to the UHB would occur. Perhaps this is due to a small density of states of the UHB states at the SCLJ. The complete consequences of this dual hole existence toward solar water splitting are still not fully explored. Evidence of surface states that could contribute to Fermi level pinning and the late onset of water oxidation has been 1628



Perspective

of surface states is still needed. Indeed, it is not well understood if the trap states found in these photoelectrodes are intrinsic surface states (due to crystalline defects at the surface of the semiconductor material) or induced by illumination. More detailed characterization of the surface is needed under operating conditions to better describe the processes occurring at this interface. Despite the lack of precise characterization of surface trapping states, strategies to reduce the effects of these potential loss channels have recently been reported. Surface treatments offer an obvious approach to passivate trapping states, and given the nanostructured morphology of many oxide photoelectrodes, conformal overlayers applied by ALD are particularly attractive to investigate passivating surface states. This approach was first reported for hematite recently.52 While TiO2 overlayers showed no beneficial effect, it was found that an ultrathin coating of Al2O3 reduced the overpotential by as much as 100 mV under standard illumination conditions (before dissolving in the basic electrolyte). Importantly, this effect was distinguished from a catalytic effect by the subsequent addition of Co2+ ions (as a well-known catalyst). The cobalt treatment further decreased the overpotential, suggesting that surface passivation strategies can be combined with catalytic approaches. Importantly noted in that work was the absence of shift in the photocurrent onset observed when a bare electrode was treated with Al3+ ions (in an analogous way to the Co2+). Given the known catalytic ability of cobalt, this strengthened the argument that two separate loss processes can occur at the SCLJ in an oxide semiconductor. Subsequent analysis of electrochemical impedance and photoluminescence spectra in this system revealed a significant change in the surface capacitance and radiative recombination, respectively, which further supported the conclusion that surface states are passivated by the Al2O3 overlayer. Recently, investigation of other overlayers (e.g., Ga2O3, In2O3) have strengthened this argument,53 but in the last year, compelling evidence from transient techniques has caused the field to reconsider the paradigms of surface traps and catalysts. Durrant’s group has established transient absorption spectroscopy as a powerful technique to study photoelectrodes under operating conditions. Their efforts have correlated the photocurrent in many water-splitting oxide photoelectrodes (e.g., TiO2, WO3, and Fe2O3) to the presence of long-lived (100 ms−1 s) photoholes.24,54,55 It should be noted that due to the requirement to employ four holes in the water oxidation reaction, the mechanism can proceed via a multihole mechanism requiring vicinal surface sites or via energetically more difficult single-electron-transfer steps on a single site. Durrant and co-workers’ recent results use kinetic arguments to show that the water-splitting reaction proceeds as a chain of single steps on oxide photoanodes and also that the rate of the water-splitting reaction depends mainly on the photogenerated holes’ ability to avoid recombination at the SCLJ.56 The longlived holes found in these reports are suggested to be required for appreciable oxidation rates as the water oxidation reaction proceeds slowly. For hematite, these long-lived holes were found to remain at the SCLJ for up to 4 s at the potentials necessary to split water.54 This surprisingly long time scale implies that the water oxidation rate is slow enough to make the rates of competing recombination reactions with conduction band electrons in the space charge region (Jdr and Jss in Figure 6) significant even under relatively high band bending conditions (where electron concentration in the space charge

more precisely described using electrochemical impedance spectroscopy (EIS). EIS is a well-established tool for studying electrochemical systems and has been used extensively on PEC systems to indicate that surface states are present.47−49 One difficulty with using EIS is that the data from the measurement must be fit to an “equivalent circuit” model, and an accurate physical model that includes surface states cannot always unambiguously fit the impedance spectra. Nevertheless, a recent and careful study by Hamann and Bisquert has suggested that the existence of surface states can be clearly seen using this technique.50 In this work, EIS spectra of hematite photoanodes prepared by ALD are examined in the dark and under various illumination intensities. An additional capacitive process is noted under illumination, which supports the model that charges are accumulating at the SCLJ. The EIS spectra were further fit to a model that assumes charge transfer to the electrolyte occurs through surface trap states and a model that assumed charge transfer rather occurred through the valence band. More consistent fits were obtained with the model that assumes surface-trap-mediated charge transfer dominates. The further observation of a good correlation between the increase of the capacitance of the trapping states (Ctrap) and the decrease of the charge-transfer resistance (Rct,trap) to the onset of photocurrent (as shown in Figure 5b) provided convincing evidence that the hole-transfer reaction for the water oxidation process takes place through the surface trapping states. Indeed, the peak of the capacitive trapping matches well with the accumulation of charges found in the NEXAFS study. In addition, very good evidence of Fermi level pinning was obtained in the EIS study with the hematite electrodes in pH 6.8 electrolyte. A plot of the bulk capacitance as a function of the applied voltage (Mott−Schottky plot) showed a flattening of the slope under illumination at potentials when the surface trap capacitance was at its peak indicated that additional applied potential was going into the charging of surface states rather than increasing the band bending of the depletion layer. However, this effect was less observed in the electrolyte pH more commonly used for water splitting (13.3−13.6) where a large ηox is still present. This suggests that the assumption that only charges from surface trap states can participate in the water oxidation reaction may be a less accurate description of the actual case. Indeed, it should also be pointed out that the energy of the trapping states calculated in the EIS work places these states below a potential of 1.45 V versus RHE for the case of pH 13.3 electrolyte and 1 sun illumination. If all charge transfer occurred through these surface states, then no O2 bubbles would be observed at the surface of the hematite photoelectrode at any applied potential as 1.45 V (not 1.23 V) is the necessary potential for bubbles to form (the upper heating valve must be considered instead of the lower heating value for bubble formation due to the energy of the phase change51). Because bubbles are commonly observed from hematite photoanodes, some combination of surface-trapmediated charge transfer and direct valence band transfer must be occurring if surface states are indeed at the suggested energy levels. Because a larger overpotential is observed at lower pH, perhaps the branching ratio of surface state transfer and valence band charge transfer also changes significantly. In a broader perspective, the EIS study does agree very closely with the NEXAFS study in that a measurable amount of charges accumulates at the SCLJ in Fe2O3 photoelectrodes and that this charge accumulation is closely related to the onset of photocurrent. However, a convincing and precise description 1629



Perspective

shift in the onset of photocurrent is due to the reduction of Fermi level pinning and a correspondingly greater depletion layer width compared to that of a nonfunctionalized photoanode under the same operating conditions. However, this view is in stark contrast to results obtained using a combination of surface-passivating Al2O3 overlayers in combination with a cobalt “catalyst” where an additional shift was observed and suggested that two separate loss processes were indeed occurring.52 Indeed, an important and urgent question for this field remains, are there actually two distinct methods of reducing the overpotneial on oxide photanodes, or do all treatments act to reduce surface traps? The answer to this question is critical to directing the design of surface treatments for oxide photoelectrodes and for ultimately enabling inexpensive materials like Fe2O3 and BiVO4 as high-performance photoelectrodes. A recent study using photocurrent transients gives important insight into this point. Le Formal et al. exposed operating photoandes to short light pulses superimposed on a steady background illumination and measured the change of the photocurrent as a function of time in the micosecond to second time scale.61 At potentials cathodic of the photocurrent onset potential, anodic transients spikes of photocurrent are observed when a light pulse is incident on the electrode, and when the light pulse ceases, a cathodic current spike is measured. By fitting the transient curves to a decay model and integrating, the amount of charges accumulating at the SCLJ as a function of the applied potential and steady-state light intensity could be quantified. The data for hematite electrodes prepared by APCVD are shown in Figure 7a for both the anodic (charging) peaks and cathodic (discharging) peaks along with the steady-state photocurrent data at 1 sun illumination. The amount of charges accumulating at the SCLJ was found to be equal to the amount of charges discharging after the light pulse was removed, indicting that the reversible accumulation was indeed occurring. In addition, the amount and the peak position of the accumulated charges changed in a way consistent with the variation expected by a continuous change in the quasi-Fermi level. Also as expected, the peak of the accumulation occurs at the onset of photocurrent, exactly as was observed for the NEXAFS and EIS results (Figure 5). While the transient technique cannot differentiate between holes that charge empty trap states and those that accumulate at the valence band edge due to the slow water oxidation kinetics, the addition of surface treatments and subsequent analysis gave additional insight. Consistent with the results of Barroso et al.,57 Peter et al.,59 and Klahr et al.,60 it was found that both the treatments of the cobalt catalyst and the Al2O3 passivating overlayer showed a similar effect on the accumulated charges (Figure 7b and c) when the treatment was applied to a bare electrode. If indeed there was a significant concentration of surface traps, the addition of a surface trap passivation layer would be expected to primarily reduce the amount of charges that accumulate at the SCLJ (reducing the magnitude of the peak), while a small cathodic shift of the peak would be also expected due the reduction of Fermi level pinning (as equal amounts of band bending would be obtainable with less applied potential in the case of a surface-passivated electrode compared to an untreated one). Indeed, another report shows similar behavior for the treatment of bare Fe2O3 electrodes. Riha et al. recently reported the ALD of a submonolayer of Co(OH)2/Co3O4 on hematite that reduced the overpotential similar to the Co2+ treatment.62 They observed a decrease in surface state capacitance (as

Figure 6. Qualitative energy diagram showing the favorable pathways (green) of minority carriers being generated by light absorption, G, driven to the electrolyte interface by the electric field in the depletion region (drift) and collected, Jhc, by a redox couple, D+/D, in solution. Also shown are the deleterious recombination pathways (red) including bulk recombination, Jbr, depletion region recombination, Jdr, surface state recombination, Jss, and electron transfer, Jet. From Klahr, B. M.; Hamann, T. W. J. Phys. Chem. C 2011, 115, 8393−8399.

region should be very small). This suggests that an effective strategy to improve photoanode performance would be to prevent these recombination pathways. Furthermore, Barroso et al.’s study of Co−Pi (referred to as CoOx in their work) functionalized (supposedly catalyzed) and Ga2O3 functionalized (supposedly surface-passivated) Fe2O3 electrodes has strongly suggested that both of these surface treatments improve the onset of the photocurrent in the same manner, by reducing the electron recombination at the SCLJ.57 This is in stark contrast to how the Co−Pi species is known to operate when applied to a dark electrode and brings up an important question, how should a true catalyst be expected to perform when coupled with a semiconductor photoelectrode? At a given operation condition (light intensity and applied bias), a true catalyst at the SCLJ would be expected to increase the rate of the water oxidation reaction and correspondingly decrease the time that a photogenerated hole resides at the SCLJ. Conversely, passivating surface traps would be expected to increase the lifetime of photogenerated holes as trapmediated recombination would be suppressed. What was found by Barroso et al.57 using the transient absorption spectroscopy technique is that both of the treatments seem to have the same effect, increasing the lifetime of the photogenerated holes. This observation surprisingly suggests that electron/hole recombination is decreased due to the suppression or passivation of surface recombination events when using cobalt and not due to an increase in catalysis. Indeed, in a recent analysis of this work, Gamelin pointed out that the water oxidation need not occur on the Co−Pi in this scenario and suggested that it may be a spectator.58 Recent electrochemical studies by Peter et al.59 and Klahr et 60 al. on Fe2O3 and by Zhong et al.22 on BiVO4 reach similar conclusions using cobalt-based catalysts. These studies have further suggested that the cobalt layer could be also operating as a “hole sink” or a “hole reservoir” that moves photogenerated holes away from recombination centers on the surface of the oxide photoanode. In either case, these studies consistently report that while a cobalt-based material like Co−Pi (Co2+ or CoOx) can operate as an effective catalyst for water oxidation in the dark, its operation when coupled to an oxide photoanode is that of a surface trap (recombination) passivation agent. The 1630



Perspective

carry over to a lower number of accumulated charges in the Al2O3 + Co2+ treated electrode. This was not observed. Rather, identical peak heights were observed, and with the assumption that all surface trap states are passivated by the Al2O3, these results suggest that the Al2O3 treated electrode and the Al2O3 + Co2+ treated electrode have the same amount of band bending at the potential where the charge accumulation peaks (the onset of water splitting). Because it is known from many reports that the flat band potential does not change when adding the Co-based catalysts or surface-passivating layers, the alternative suggestion that the cobalt is acting to reduce electron−hole recombination by increasing the band bending and enhancing the space charge field at the SCLJ, is more consistent with the observed results. However, it should be noted that the expected 15% change of the peak height is at the limit with respect to the relatively large experimental error accrued with the complicated measurement technique. Observing this change with more careful measurements would support an explanation of improved OER kinetics. Indeed, in the work from Riha et al.,62 the authors argue that their capacitance results do suggest that an increase of OER catalysis is occurring. While it is not clear how their results can be interpreted with respect to a possible change in Fermi level pinning between the bare and treated electrode or how their electrodes would behave with a combination of surfacepassivating and catalytic overlayers, it is very possible that further studies may reveal true catalytic behavior (i.e., a photocurrent onset with less accumulation of valence band charges at the same amount of band bending). Until more evidence is gathered, it can be stated that the majority of reports suggest that cobalt-based surface treatments are not behaving as true catalysts when coupled with Fe2O3 or BiVO4. Very recent evidence suggests the same for IrOx based surface treatments as well.63 The difference between the mechanism of operation of the Co−Pi catalyst on a dark electrode and that on a photoelectrode may be due to the difference of the electron-transfer reaction from the catalyst to the electrode. In the case of a dark electrode, the electrons are transferred from the catalyst to the electrode at the set Fermi level of the electrode, while in a photoelectrode, electrons must be injected into the valence band of the semiconductor; the latter may be kinetically limiting depending on the specific energy states available. This suggests that an appropriate catalyst with the right energy levels could be identified and employed to increase the rate of the water oxidation reaction independent of reducing the recombination with conduction band electrons. Under the right conditions, the Co-based catalysts or other overlayers may be shown to do this. Overall, the recent examination of results for surface traps and catalysis on oxide photoelectrodes for water oxidation strongly suggest that it is possible to address the problems of surface trap recombination and sluggish reaction kinetics independently but that surface traps must be passivated before a catalyst can be employed. However, convincing evidence that a surface treatment can perform as a true catalyst is still elusive. Rather, it seems more likely that overlayers like Co−Pi and IrOx can act to passivate surface states if they are present or, in the case of already-passivated electrodes, decrease electron− hole recombination by increasing band bending. For a material like BiVO4, this is currently inconsequential as the application of the Co−Pi treatment gives photocurrent onsets similar to those when a hole scavenger is used, showing that surface losses

Figure 7. Transient photocurrent spectroscopy with hematite photoelectrodes. (a) Accumulated charge density (full markers, plain lines) and dissipated charge density (empty markers, broken lines) are shown as a function of applied potential with respect to the RHE and the electrode photocurrent at 1 sun (green line). Three different intensities of the white light bias are shown, 1 sun (100 mW cm−2, red circles), 0.5 sun (50 mW cm−2, orange squares), and 0.1 sun bias (10 mW cm−2, blue triangles). (b) Accumulated and dissipated charge densities are shown for an electrode post-treated with cobalt nitrate compared to a reference sample (sample without overlayers, black circles). (c) A photoanode post-treated with an Al2O3 (deposited with ALD, red diamonds) is compared to a photoanode post-treated with Al2O3 and cobalt nitrate subsequently (violet triangles) and to a reference sample (black circles). All data (b and c) are shown for measurements performed with a white light bias intensity of 0.1 sun (10 mW cm−2). Adapted from ref 61.

measured by EIS) that matches well the behavior of the transients before and after the surface treatments. This strengthens the view that the Co-based catalysts reduce the overpotential for water oxidation in a similar way to the surfacepassivating Al2O3. However, when the Co2+ catalyst was added to the alreadypassivated electrode, a further reduction of the peak height was not observed (as would be expected if the Co catalyst further reduced the concentration of surface states). Instead, a large cathodic shift of the peak was seen (Figure 7c). At a minimum, these results show that the cobalt treatment can act in a distinct way, which indicates that two separate loss processes do in fact occur at the SCLJ. Do these results also imply that the cobalt is acting as a true catalyst? With the definition that a catalyst is an agent that increases the rate of a reaction by lowering the activation energy of a transition state, a material acting as a true catalyst on the surface would lower the Δμex needed to drive the water oxidation reaction, and a cathodic shift of the charge accumulation peak would be observed. No peak height change would be expected if the amount of charges reaching the SCLJ was independent of the applied potential in range of interest (0.8−1.2 V versus RHE). This range is indeed where the photocurrent “plateaus” in the H2O2 experiment (Figure 3b). However a ∼15% change in photocurrent does occur over this range when H2O2 is used, and this change would be expected to 1631



Perspective

(8) Khaselev, O.; Turner, J. A. A Monolithic Photovoltaic− Photoelectrochemical Device for Hydrogen Production via Water Splitting. Science 1998, 280, 425−427. (9) Nowotny, J.; Bak, T.; Nowotny, M. K.; Sheppard, L. R. Titanium Dioxide for Solar-Hydrogen I. Functional Properties. Int. J. Hydrogen Energy 2007, 32, 2609−2629. (10) Sartoretti, C. J.; Alexander, B. D.; Solarska, R.; Rutkowska, W. A.; Augustynski, J.; Cerny, R. Photoelectrochemical Oxidation of Water at Transparent Ferric Oxide Film Electrodes. J. Phys. Chem. B 2005, 109, 13685−13692. (11) Sivula, K.; Le Formal, F.; Grätzel, M. Solar Water Splitting: Progress Using Hematite (α-Fe2O3) Photoelectrodes. ChemSusChem 2011, 4, 432−449. (12) Sayama, K.; Nomura, A.; Zou, Z.; Abe, R.; Abe, Y.; Arakawa, H. Photoelectrochemical Decomposition of Water on Nanocrystalline BiVO4 Film Electrodes under Visible Light. Chem. Commun. 2003, 0, 2908−2909. (13) Tilley, S.; Cornuz, M.; Sivula, K.; Grätzel, M. Light-Induced Water Splitting with Hematite: Improved Nanostructure and Iridium Oxide Catalysis. Angew. Chem., Int. Ed. 2010, 49, 6405−6408. (14) Brillet, J.; Cornuz, M.; Le Formal, F.; Yum, J. H.; Graetzel, M.; Sivula, K. Examining Architectures of Photoanode−Photovoltaic Tandem Cells for Solar Water Splitting. J. Mater. Res. 2010, 25, 17−24. (15) Nozik, A. J.; Memming, R. Physical Chemistry of Semiconductor−Liquid Interfaces. J. Phys. Chem. 1996, 100, 13061−13078. (16) Memming, R. Semiconductor Electrochemistry; Wiley-VCH: New York, 2001. (17) Lindgren, T.; Vayssieres, L.; Wang, H.; Lindquist, S. E. PhotoOxidation of Water at Hematite Electrodes. Chemical Physics of Nanostructured Semiconductors; VSP: Lorton, VA, 2003; pp 83−110. (18) Santato, C.; Ulmann, M.; Augustynski, J. Photoelectrochemical Properties of Nanostructured Tungsten Trioxide Films. J. Phys. Chem. B 2001, 105, 936−940. (19) Sayama, K.; Nomura, A.; Arai, T.; Sugita, T.; Abe, R.; Yanagida, M.; Oi, T.; Iwasaki, Y.; Abe, Y.; Sugihara, H. Photoelectrochemical Decomposition of Water into H2 and O2 on Porous BiVO4 Thin-Film Electrodes under Visible Light and Significant Effect of Ag Ion Treatment. J. Phys. Chem. B 2006, 110, 11352−11360. (20) 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. (21) Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. W. Electrochemical and Photoelectrochemical Investigation of Water Oxidation with Hematite Electrodes. Energy Environ. Sci. 2012, 5, 7626−7636. (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) Mi, Q.; Zhanaidarova, A.; Brunschwig, B. S.; Gray, H. B.; Lewis, N. S. A Quantitative Assessment of the Competition between Water and Anion Oxidation at WO3 Photoanodes in Acidic Aqueous Electrolytes. Energy Environ. Sci. 2012, 5, 5694. (24) Cowan, A. J.; Tang, J.; Leng, W.; Durrant, J. R.; Klug, D. R. Water Splitting by Nanocrystalline TiO2 in a Complete Photoelectrochemical Cell Exhibits Efficiencies Limited by Charge Recombination. J. Phys. Chem. C 2010, 114, 4208−4214. (25) Dareedwards, M. P.; Goodenough, J. B.; Hamnett, A.; Trevellick, P. R. Electrochemistry and Photoelectrochemistry of Iron(III) Oxide. J. Chem. Soc., Faraday Trans. 1983, 79, 2027−2041. (26) Yeh, L. S. R.; Hackerman, N. Iron-Oxide Semiconductor Electrodes in Photoassisted Electrolysis of Water. J. Electrochem. Soc. 1977, 124, 833−836. (27) Sanchez, C.; Sieber, K. D.; Somorjai, G. A. The Photoelectrochemistry of Niobium Doped Alpha-Fe2O3. J. Electroanal. Chem. 1988, 252, 269−290.

Overall, the recent examination of results for surface traps and catalysis on oxide photoelectrodes for water oxidation strongly suggest that it is possible to address the problems of surface trap recombination and sluggish reaction kinetics independently but that surface traps must be passivated before a catalyst can be employed. can be essentially eliminated in this material. However, for a material like Fe2O3, even with a surface-passivating overlayer and a cobalt treatment, a large (300 mV) overpotential for water oxidation under standard conditions remains. Further work on identifying the precise mechanisms for charge transfer and surface trapping at the SCLJ is expected to direct advanced strategies to further reduce the overpotential losses in this promising material. Greater insight gained from these and other future works will be critical to directing innovative surface treatments and eventually enabling the commercial application of oxide semiconductor-based solar fuel devices.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: kevin.sivula@epfl.ch. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The author would like to thank F. Le Formal for helpful discussions and the FSB at EPFL for financial support. REFERENCES

(1) 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. (2) Brillet, J.; Yum, J. H.; Cornuz, M.; Hisatomi, T.; Solarska, R.; Augustynski, J.; Graetzel, M.; Sivula, K. Highly Efficient Water Splitting by a Dual-Absorber Tandem Cell. Nat. Photonics 2012, 6, 824−828. (3) Sato, S.; Arai, T.; Morikawa, T.; Uemura, K.; Suzuki, T. M.; Tanaka, H.; Kajino, T. Selective CO2 Conversion to Formate Conjugated with H2O Oxidation Utilizing Semiconductor/Complex Hybrid Photocatalysts. J. Am. Chem. Soc. 2011, 133, 15240−15243. (4) Woolerton, T. W.; Sheard, S.; Reisner, E.; Pierce, E.; Ragsdale, S. W.; Armstrong, F. A. Efficient and Clean Photoreduction of CO2 to CO by Enzyme-Modified TiO2 Nanoparticles Using Visible Light. J. Am. Chem. Soc. 2010, 132, 2132−2133. (5) Barton, E. E.; Rampulla, D. M.; Bocarsly, A. B. Selective SolarDriven Reduction of CO2 to Methanol Using a Catalyzed p-GaP Based Photoelectrochemical Cell. J. Am. Chem. Soc. 2008, 130, 6342−6344. (6) Weber, M. F.; Dignam, M. J. Efficiency of Splitting Water with Semiconducting Photoelectrodes. J. Electrochem. Soc. 1984, 131, 1258−1265. (7) Licht, S.; Wang, B.; Mukerji, S.; Soga, T.; Umeno, M.; Tributsch, H. Over 18% Solar Energy Conversion to Generation of Hydrogen Fuel; Theory and Experiment for Efficient Solar Water Splitting. Int. J. Hydrogen Energy 2001, 26, 653−659. 1632



Perspective

(28) Kay, A.; Cesar, I.; Gratzel, M. New Benchmark for Water Photooxidation by Nanostructured α-Fe2O3 Films. J. Am. Chem. Soc. 2006, 128, 15714−15721. (29) Kanan, M. W.; Nocera, D. G. In Situ Formation of an OxygenEvolving Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072−1075. (30) Zhong, D. K.; Gamelin, D. R. Photoelectrochemical Water Oxidation by Cobalt Catalyst (“Co−Pi”)/α-Fe2O3 Composite Photoanodes: Oxygen Evolution and Resolution of a Kinetic Bottleneck. J. Am. Chem. Soc. 2010, 132, 4202−4207. (31) Zhong, D. K.; Cornuz, M.; Sivula, K.; Gratzel, M.; Gamelin, D. R. Photo-Assisted Electrodeposition of Cobalt−Phosphate (Co−Pi) Catalyst on Hematite Photoanodes for Solar Water Oxidation. Energy Environ. Sci. 2011, 4, 1759−1764. (32) Abdi, F. F.; van de Krol, R. Nature and Light Dependence of Bulk Recombination in Co−Pi-Catalyzed BiVO4 Photoanodes. J. Phys. Chem. C 2012, 116, 9398−9404. (33) Liang, Y.; Tsubota, T.; Mooij, L. P. A.; van de Krol, R. Highly Improved Quantum Efficiencies for Thin Film BiVO4 Photoanodes. J. Phys. Chem. C 2011, 115, 17594−17598. (34) Kiwi, J.; Gratzel, M. Oxygen Evolution from Water Via Redox Catalysis. Angew. Chem., Int. Ed. 1978, 17, 860−861. (35) Kiwi, J.; Gratzel, M. Colloidal Redox Catalysts for Evolution Of Oxygen and for Light-Induced Evolution of Hydrogen from Water. Angew. Chem., Int. Ed. 1979, 18, 624−626. (36) Nakagawa, T.; Beasley, C. A.; Murray, R. W. Efficient Electrooxidation of Water near Its Reversible Potential by a Mesoporous IrOx Nanoparticle Film. J. Phys. Chem. C 2009, 113, 12958−12961. (37) Nakagawa, T.; Bjorge, N. S.; Murray, R. W. Electrogenerated IrOx Nanoparticles as Dissolved Redox Catalysts for Water Oxidation. J. Am. Chem. Soc. 2009, 131, 15578−15579. (38) Iwanski, P.; Curran, J. S.; Gissler, W.; Memming, R. The Photoelectrochemical Behavior of Ferric Oxide in the Presence of Redox Reagents. J. Electrochem. Soc. 1981, 128, 2128−2133. (39) Horowitz, G. Capacitance Voltage Measurements and Flat-Band Potential Determination on Zr-Doped α-Fe2O3 Single-Crystal Electrodes. J. Electroanal. Chem. 1983, 159, 421−436. (40) Ahmed, S. M.; Leduc, J.; Haller, S. F. Photoelectrochemical and Impedance Characteristics of Specular Hematite. 1. Photoelectrochemical, Parallel Conductance, and Trap Rate Studies. J. Phys. Chem. 1988, 92, 6655−6660. (41) Leduc, J.; Ahmed, S. M. Photoelectrochemical And Impedance Characteristics of Specular Hematite. 2. Deep Bulk Traps in Specular Hematite at Small Alternating-Current Frequencies. J. Phys. Chem. 1988, 92, 6661−6665. (42) Bard, A. J.; Bocarsly, A. B.; Fan, F. R. F.; Walton, E. G.; Wrighton, M. S. The Concept of Fermi Level Pinning at Semiconductor−Liquid Junctions  Consequences for Energy-Conversion Efficiency and Selection of Useful Solution Redox Couples in Solar Devices. J. Am. Chem. Soc. 1980, 102, 3671−3677. (43) Duret, A.; Gratzel, M. Visible Light-Induced Water Oxidation on Mesoscopic α-Fe2O3 Films Made by Ultrasonic Spray Pyrolysis. J. Phys. Chem. B 2005, 109, 17184−17191. (44) Shinar, R.; Kennedy, J. H. Open-Circuit Photopotentials at Doped α-Fe2O3 Electrodes in Aqueous Solution. J. Electrochem. Soc. 1983, 130, 392−396. (45) Jiang, P.; Chen, J.-L.; Borondics, F.; Glans, P.-A.; West, M. W.; Chang, C.-L.; Salmeron, M.; Guo, J. In Situ Soft X-ray Absorption Spectroscopy Investigation of Electrochemical Corrosion of Copper in Aqueous NaHCO3 Solution. Electrochem. Commun. 2010, 12, 820− 822. (46) Braun, A.; Sivula, K.; Bora, D. K.; Zhu, J.; Zhang, L.; Grätzel, M.; Guo, J.; Constable, E. C. Direct Observation of Two Electron Holes in a Hematite Photoanode during Photoelectrochemical Water Splitting. J. Phys. Chem. C 2012, 116, 16870−16875. (47) Kelly, J. J.; Memming, R. The Influence of Surface Recombination and Trapping on the Cathodic Photocurrent at pType III−V Electrodes. J. Electrochem. Soc. 1982, 129, 730−738.

(48) Allongue, P.; Cachet, H. Band-Edge Shift and Surface Charges at Illuminated n-GaAs/Aqueous Electrolyte Junctions: Surface-State Analysis and Simulation of Their Occupation Rate. J. Electrochem. Soc. 1985, 132, 45−52. (49) Lincot, D.; Vedel, J. Recombination and Charge Transfer at the Illuminated n-CdTe/Electrolyte Interface: Simplified Kinetic Model. J. Electroanal. Chem. Interfacial Electrochem. 1987, 220, 179−200. (50) 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. (51) Kwak, H.-Y.; Panton, R. L. Gas Bubble Formation in Nonequilibrium Water−Gas Solutions. J. Chem. Phys. 1983, 78, 5795−5799. (52) 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. (53) Hisatomi, T.; Le Formal, F.; Cornuz, M.; Brillet, J.; Tetreault, N.; Sivula, K.; Gratzel, M. Cathodic Shift in Onset Potential of Solar Oxygen Evolution on Hematite by 13-Group Oxide Overlayers. Energy Environ. Sci. 2011, 4, 2512−2515. (54) Pendlebury, S. R.; Barroso, M.; Cowan, A. J.; Sivula, K.; Tang, J.; Gratzel, M.; Klug, D.; Durrant, J. R. Dynamics of Photogenerated Holes in Nanocrystalline α-Fe2O3 Electrodes for Water Oxidation Probed by Transient Absorption Spectroscopy. Chem. Commun. 2011, 47, 716−718. (55) Pesci, F. M.; Cowan, A. J.; Alexander, B. D.; Durrant, J. R.; Klug, D. R. Charge Carrier Dynamics on Mesoporous WO3 during Water Splitting. J. Phys. Chem. Lett. 2011, 2, 1900−1903. (56) Cowan, A. J.; Barnett, C. J.; Pendlebury, S. R.; Barroso, M.; Sivula, K.; Grätzel, M.; Durrant, J. R.; Klug, D. R. Activation Energies for the Rate-Limiting Step in Water Photooxidation by Nanostructured α-Fe2O3 and TiO2. J. Am. Chem. Soc. 2011, 133, 10134− 10140. (57) 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 Photoanodes for Solar Water Splitting. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 15640−15645. (58) Gamelin, D. R. Water Splitting: Catalyst or Spectator? Nat. Chem. 2012, 4, 965−967. (59) Peter, L. M.; Wijayantha, K. G. U.; Tahir, A. A. Kinetics of Light-Driven Oxygen Evolution at α-Fe2O3 Electrodes. Faraday Discuss. 2012, 155, 309−322. (60) Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Bisquert, J.; Hamann, T. W. Photoelectrochemical and Impedance Spectroscopic Investigation of Water Oxidation with “Co−Pi”-Coated Hematite Electrodes. J. Am. Chem. Soc. 2012, 134, 16693−16700. (61) 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. (62) 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. (63) Badia-Bou, L.; Mas-Marza, E.; Rodenas, P.; Barea, E. M.; Fabregat-Santiago, F.; Gimenez, S.; Peris, E.; Bisquert, J. Water Oxidation at Hematite Photoelectrodes with an Iridium-Based Catalyst. J. Phys. Chem. C 2013, 117, 3826−3833.

1633


Metal Oxide Photoelectrodes for Solar Fuel Production, Surface Traps

Recommend Documents