The Transient Photocurrent and Photovoltage Behavior of a Hematite

Article pubs.acs.org/JPCC

The Transient Photocurrent and Photovoltage Behavior of a Hematite Photoanode under Working Conditions and the Influence of Surface Treatments Florian Le Formal,* Kevin Sivula, and Michael Graẗ zel Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne, Laboratory of Photonics and Interfaces, Station 6, CH-1015 Lausanne, Switzerland S Supporting Information *

ABSTRACT: Hematite (α-Fe2O3) is widely recognized as a promising candidate for the production of solar fuels via water splitting, but its intrinsic optoelectronic properties have limited its performance to date. In particular, the large electrochemical overpotential required to drive the water oxidation is known as a major drawback. This overpotential (0.4 − 0.6 V anodic of the flat band potential) has been attributed to poor oxygen evolution reaction (OER) catalysis and to charge trapping in surface states but is still not fully understood. In the present study, we quantitatively investigate the photocurrent and photovoltage transient behavior of α-Fe2O3 photoanodes prepared by atmospheric pressure chemical vapor deposition, under light bias, in a standard electrolyte, and one containing a sacrificial agent. The accumulation of positive charges occurring in water at low bias potential is found to be maximum when the photocurrent onsets. The transient photocurrent behavior of a standard photoanode is compared to photoanodes modified by either a catalytic or surface passivating overlayer. Surface modification shows a reduction and a cathodic shift of the charge accumulation, following the observed change in photocurrent onset. By applying an electrochemical model, the values of the space charge width (5−10 nm) and of the hole diffusion length (0.5−1.5 nm) are extracted from photocurrent transients’ amplitudes with the sacrificial agent. Characterization of the photovoltage transients also suggests the presence of surface states causing Fermi level pinning at small applied potential. The transient photovoltage and the use of both overlayers on the same electrode enable differentiation of the two overlayers’ effects and a simplified model is proposed to explain the roles of each overlayer and their synergetic effects. This investigation demonstrates a new method to characterize water splitting photoelectrodesespecially the charge accumulation occurring at the semiconductor/electrolyte interface during operation. It finally confirms the requirements of nanostructuring and surface control with catalytic and trap passivation layers to improve iron oxide’s performance for water photolysis.

1. INTRODUCTION One attractive approach to convert solar energy into a solar fuel is to use a semiconductor to directly perform photoelectrochemical water splitting, consequently producing hydrogen.1 An ideal semiconductor for water photolysis must exhibit high chemical stability in water, low production costs, good visible light absorption, conduction and valence band edges straddling the reduction and oxidation potentials of water, good charge transport, and catalytic ability. Despite the large amount of research effort since the pioneering work of Boddy2 and Fujishima3 with TiO2 in 1968 and 1972, respectively, these requirements have not yet been fulfilled by any candidate. Hematite (α-Fe2O3) is nonetheless one of the most promising materials for water splitting, in regards to its availability and its good light absorption of the solar spectrum with a band gap of about 2.0−2.2 eV. The issue originating from the inadequate position of its conduction band, which is too low for water reduction, can be surmounted by assembling the photoanode in a tandem device. In this way, a © 2012 American Chemical Society

photocathode or an inexpensive photovoltaic (PV) cell (e.g., a dye-sensitized solar cell, DSC) is connected in series with the photoanode to generate sufficient chemical potential for completing the water splitting reaction.4 A second drawback of hematite is the dilemma brought by the poor electronic properties (low electron and hole diffusion lengths) compared to the light absorption depth at large wavelengths (on the order of 100 nm above λ = 500 nm). This creates a disaccord between the depth where charge carriers are photogenerated (in the bulk) and the distance they diffuse before recombining. Doping with elements such as Si, Ge, Nb, or Ti can substantially increase the electronic conductivity by increasing the number of carriers, but also decreases the space charge layer width (where an electric field drives charge separation), causing only a thin material slab located within a few nanometers from Received: August 29, 2012 Revised: November 22, 2012 Published: November 28, 2012 26707

dx.doi.org/10.1021/jp308591k | J. Phys. Chem. C 2012, 116, 26707−26720

The Journal of Physical Chemistry C

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the electrolyte to be active.5 As a result, several nanostructures, such as nanowires,6,7 nanotubes,8,9 nanoparticles assemblies,10 or those with an extremely thin absorber (ETA) approach consisting in the deposition of an hematite thin layer on a mesoporous host,11 have been produced to decrease the distance photogenerated positive carriers need to diffuse to reach the surface where they can react with water. Indeed, nanostructuring efforts increased the photoactivity of hematite photoanodes. One particularly successful nanostructure is obtained from another technique, atmospheric pressure chemical vapor deposition, using Fe(CO)5 and tetraethyl orthosilicate (TEOS; Si doping source) as precursors and forming dendritic nanostructures with a feature size of 5−10 nm at the top.12,13 The observed photocurrent onset at 1.0 V versus the reversible hydrogen electrode (RHE) is, however, in stark contrast to the flat band potential (Vfb) determined for this material (0.55 V vs RHE as measured by impedance spectroscopy for the nanostructures studied here).14 This corresponds to an overpotential of about 0.4 − 0.6 V for the water splitting photocurrent to onset, which has been assigned to poor oxygen evolution (OER) kinetics on α-Fe2O3 surfaces. A partial reduction of the overpotential has been attained with deposition of an OER catalyst such as Co,13 CoPi,15 or IrO2 nanoparticles.16 In our previous studies, we have demonstrated an alternative origin of the large overpotential as the deposition of an Al2O3 or a Ga2O3 passivating layer atop the iron oxide layer also produces a decrease of the onset potential.17,18 Indeed, the origin of the late onset potential of iron oxide photoanodes has been previously assigned by several research groups to unfavorable surface properties,19 corresponding to poor charge transfer properties at the surface20 and leading to enhanced interfacial charge recombination. In terms of energy, the accumulation of holes has been assumed to occur in localized states (traps), such as higher oxidation states of iron (FeIV, FeV, or FeVI),21 mediating the recombination of holes with conduction band electrons or with reaction intermediates (redox species).22 Wilhelm et al. also assumed these traps to provide the primary path of electron transfer at the surface through resonance tunneling.23 The hole accumulation has been characterized by several advanced electrochemical methods through the direct or indirect measurement of a surface state capacitance with (photo)electrochemical impedance spectroscopy (EIS or PEIS),20,24−27 or intensity modulated photocurrent spectroscopy (IMPS).28 The behavior of the photogenerated hole on the semiconductor surface has also been characterized with transient absorption spectroscopy (TAS), leading to the measurement of a hole lifetime in the 1− 10 s range at potentials where photocurrent flows.29−31 However, these previous studies still leave questions about the nature of the overpotential, and, furthermore, no method has yet enabled quantification of the charges accumulating in the overpotential region. Transient phototcurrent (TPC) measurements have shown great potential as a tool to gain information on this specific issue, as transient features are commonly observed in the same applied potential range. TPCs have been qualitatively described before, assigning the anodic peaks observed when light is switched on, to holes accumulating at the semiconductor−liquid junction (SCLJ) and perturbing the space charge layer.32 This accumulation has also been previously supposed to occur in surface states,17,19,20,23 which offer the possibility for electrons to recombine with accumulated holes or with redox species.22

This has been assumed due to the effect of sample preparation on surface quality and transient behavior.19,33 The disappearance of the transient behavior with different redox species also emphasized the role of hydroxyl groups on the formation of surface states (intermediates species) or the origin of accumulation due to slow water oxidation.34 However, none of these investigations have developed a method to quantitatively determine the number of charges perturbing the semiconductor surface. In this study, we present an unprecedented analysis of the TPC and transient photovoltage (TPV) of photoanodes allowing the quantification of accumulated charges at the SCLJ under different working conditions (bias current or voltage and illumination). The transient behavior has been recorded in a standard electrolyte and with a sacrificial agent, leading to the determination of important material parameters and to the identification of charge accumulation effects. A comparison between a bare iron oxide photoanode and those modified with a catalytic or a surface passivation coating is also given, bringing new insights into the respective role of the different overlayers.

2. EXPERIMENTAL SECTION 2.1. Hematite Photoanode Preparation. Nanostructured iron oxide films were prepared in a home-built atmospheric pressure chemical vapor deposition (APCVD) chamber according to the method presented at first by Kay et al.13 and optimized in 2010 by Cornuz et al.35 The deposition technique and setup are described comprehensively in the two references indicated above. Hematite samples used in this study were deposited with a carrier gas flow of 6 L min−1 and a 3 min deposition time resulting in a nanostructured layer with a thickness of 600−800 nm and crystallite top feature size of about 5−10 nm. The silicon doping was assumed to be about 1.5 atom % (according to the last optimization).13 An interfacial layer of SiOx was first deposited on the FTO substrate by starting the TEOS flux 3 min before adding the Fe(CO)5 flux to the carrier gas. The role of the underlayer has been already discussed previously.33 Some of the APCVD photoanodes were covered with a thin layer (approximately 0.45−0.5 nm) of amorphous aluminum oxide by atomic layer deposition (ALD, Cambridge Nanotech S100) as previously reported.17 Deposition of alumina by ALD from trimethyl aluminum (TMA) and water has been thoroughly discussed by Puurunen.36 Samples were annealed at 300 °C after ALD. Due to the small amount of Al2O3 deposited and to the low annealing temperature, the alumina layer is likely to not show the exact same characteristics than the Al2O3 crystal and is assumed to be amorphous. A cobalt(II) post-treatment was performed on bare hematite photoanodes and on ALD-treated photoanaodes using the method developed previously in our laboratory.13 2.2. Photoelectrochemical Characterization. The hematite photoanodes were characterized photoelectrochemically in a closed cell allowing contact with a sufficient volume of electrolyte (about 20 mL) and a 0.5 cm2 aperture for illumination. The total surface immersed into the electrolyte was about 2.5 cm2. The sample was always illuminated at the hematite/electrolyte interface. Two different electrolytes were used in this study: first a “standard electrolyte”, i.e., 1 M NaOH (Reactolab SA) solution is prepared in distilled water (proanalysi Milli-Q water, 25 °C), pH = 13.6,37 and a second one, containing a sacrificial agent, is 26708



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photocurrent transient and photovoltage transient measurements, the applied current was fixed for all measurements (at 0.1, 0.5, and 1 sun in 1 M NaOH and in 1 M NaOH + 0.5 M H2O2 electrolytes) with the current obtained in the JV characterization (see Figure 1) at the same potentials where

prepared with 0.5 M H2O2 (Reactolab SA) and 1 M NaOH in distilled water. The sample was measured within the 2 h following the preparation of the second electrolyte to avoid any bubble formation. Electric currents and voltages were applied or measured, depending on the characterization, with a Keithley 2602 Sourcemeter. The Keithley is controlled by a computer with custom software and was used in a 4-wire configuration, with the sense wires (Hi-S and Lo-S) connecting the working electrode and the reference electrode (to control/measure the voltage), the wires Hi and Lo connecting the working electrodes, and the counter electrode (to control/measure the current). Potentials are applied to the Ag/AgCl reference electrode but are reported here against the RHE according to the Nernst equation.13 Our transient measurements are performed while placing the sample under applied potential (for TPC) or current (for TPV) in a white light bias and by pulsing a colored light perturbation at the electrode. The white light bias is brought by an array of white photodiodes (Philips Lumileds, Model white star LXHLNWE8). The power of illumination has been set to 99.9 mW cm−2, 54.5 mW cm−2, and 11.3 mW cm−2 as measured by a calibrated Si diode. These white light bias power densities are labeled as 1 sun, 0.5 sun, and 0.1 sun in the following text. The white LEDs have an emission spectrum with fewer UV photons but a larger blue component as compared to the xenon arc lamp typically used. Therefore photocurrent densities measured here are slightly higher than the ones measured with a xenon lamp. Overall, this setup is a slight modification of the one used in the study of DSCs.38,39 The light perturbation used to measure the transients is also induced by an array of blue photodiodes (Philips Lumileds, Model LXHL-NB98). The blue photodiodes presents a narrow emission around 470 nm and were used due to the high absorption coefficient of hematite at short wavelengths. The intensity of the diodes was manually controlled and set in a way to observe a photocurrent and photovoltage transient signal for each applied potential or current and for each white light bias. This power has been converted to a maximum possible additional photocurrent of 4.34 mA cm−2 with the measurement of a Si diode photocurrent. Unless otherwise stated, the power of the blue pulse perturbation has been maintained constant for all experiments. A pulse width of 0.5 s was produced after a stabilization of 4 s under the base experimental conditions (i.e., under white light bias and applied potential or current bias). Photocurrent or photovoltage data were recorded every 1 ms approximately for 2.2 s after the beginning of the measurement. For the photocurrent measurements, the sample was held in the electrolyte under a fixed potential and a fixed white light bias (1 sun, 0.5 sun or 0.1 sun) for 4 s before the blue light pulse was emitted for 0.5 s. The intensity of the colored diode, used for the pulse, was minimized as much as possible, in a way that a current transient is still observable in 1 M NaOH at 0.7 V vs RHE. Photocurrent transients are recorded every 50 mV from 0.7 to 1.2 V vs RHE and every 100 mV from 1.2 to 1.4 V vs RHE. The photovoltage transients were recorded while the photoelectrode was held in the electrolyte at a fixed current flowing between the working electrode and the counter electrode, and again white light bias (1 sun, 0.5 sun and 0.1 sun). The blue diode pulse is switched on after 4 s and was kept on for 0.5 s. In order to allow comparison between

Figure 1. Current densities, in mA cm−2 of the prepared photoanode tested in 1 M NaOH electrolyte (red circles) and in 1 M NaOH + 0.5 M H2O2 electrolyte (blue squares) are shown as a function of the applied potential, with respect to the RHE. Solid lines (and filled markers) correspond to the photocurrent measured under 1 sun (AM 1.5G) conditions; broken lines (and open markers) correspond to the steady state photocurrent obtained when the blue light pulse was added to the white light bias. The onset potentials of the photocurrent have been determined to be 1.055 V in 1 M NaOH and 0.72 V in 1 M NaOH + 0.5 M H2O2 electrolyte (see text for details).

photocurrent transients are measured. The dark current obtained for both hydrogen peroxide and water oxidation is close to zero at applied potentials lower than 1.5 V vs RHE. Therefore, it was impossible to fix a current corresponding to the fixed potential, where the photocurrent transient is measured. To allow comparisons of photocurrent and photovoltage transient in similar conditions, this study has consequently not been performed in dark conditions.

3. RESULTS AND DISCUSSION 3.1. Current−Voltage Properties. The current−voltage dependence of the APCVD hematite photoanode is shown in Figure 1, under illumination brought by a white diode array and corresponding to 1 sun, in a standard electrolyte (1 M NaOH, red circles) and in one containing a sacrificial agent (with H2O2, blue squares). Dark currents are not shown in the figure, but can be found in another study of the same nanostructured hematite photoanodes in the two electrolytes investigated here.14 According to this same study, a possible current doubling mechanism does not occur on the same type of hematite photoanode under anodic polarization in hydrogen peroxide containing electrolyte and will therefore be neglected. The measured photoelectrode exhibits a typical response of an APCVD grown hematite sample. The photocurrent shows an onset potential at about 1.0 V vs RHE, a rapid increase to reach ca. 3 mA cm−2 at 1.23 V vs RHE and a saturation (photcurrent plateau) of 3.0 − 4.0 mA cm−2 from 1.43 V vs RHE to the onset of the dark current. The slightly better photocurrent plateau shown in this study, i.e., 3.5 mA cm−2 at 1.43 V vs RHE, as compared to the previous reports (3.0−3.2 mA cm−2)16,17,35 is explained by the solar illumination source used in this study (see Experimental Section). JV character26709



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Figure 2. Schemes explaining the data treatment of the photocurrent and photovoltage transients for the following cases: (a) Photocurrents transients measured in 1 M NaOH. (b) Photocurrents transients measured in 1 M NaOH + 0.5 M H2O2. (c) Photovoltage transients measured in 1 M NaOH at low bias light intensity and low applied current. (d) Photovoltage transients measured in 1 M NaOH at high bias light intensity and large applied current or in 1 M NaOH + 0.5 M H2O2.

izations of the same photoanode under lower white light intensity (0.5 sun and 0.1 sun) are shown in Figure S1 (see Supporting Information). The photocurrent plateau is found to be almost linear vs the solar illumination power with the following current density measured at 1.43 V vs RHE: 0.38, 1.89, and 3.51 mA cm−2 for 0.1 sun, 0.5 sun, and 1 sun, respectively. The onset potential of the water splitting photocurrent, defined by the potential where the tangent of the photocurrent at the onset inflection point crosses the linear extrapolation of the exchange current observed before the photocurrent onset, is measured to be similarly around 1.05 V vs RHE for all bias light (1.055, 1.053, and 1.049 V for 1 sun, 0.5 sun, and 0.1 sun, respectively). The photocurrents of the photoanode recorded in the aqueous electrolyte containing H2O2 (blue squares in Figure 1) show a different behavior. The anodic photocurrent density onsets (defined here as the potential where the photocurrent become positive) at ca. 0.72 V vs RHE and reaches a plateau (with still a slight increasing slope), similar to the one obtained in the aqueous electrolyte without hydrogen peroxide. This plateau is attained at 1.0 V vs RHE, followed by a slow and linear increase (see derivation of the blue pulse originated photocurrent later) up to the onset of the dark current observed at 1.5 V vs RHE. Under lower bias light (see Figure S1), the onset potentials of the photocurrent appear at 0.74 and 0.79 V vs RHE for 0.5 sun and 0.1 sun illumination, respectively. The photocurrent plateau attained in the hydrogen peroxide electrolyte is similar to the one obtained in 1 M NaOH. This is expected as the photocurrent saturation value has been shown to be related to the nanostructure and

especially the number of carriers generated within a certain proximity to the SCLJ.14 The dotted lines (empty markers) shown in Figure 1 correspond to the steady-state photocurrent density measured with the same electrode in the same respective electrolyte and under illumination corresponding to the blue diode pulse added to the white light bias. As expected, the plateau photocurrent is increased in both electrolytes as a greater number of free charges are photogenerated inside the material. The onset potential of the photocurrent is slightly improved in the electrolyte containing hydrogen peroxide but remains constant in 1 M NaOH. Two features characterize the different behavior of the photoanode in the different electrolytes tested here. While the onset potential is theoretically related to the open circuit potential (which depends logarithmically on the light intensity),32 in our experiment the onset does not vary with the incident light intensity in NaOH but does vary in the electrolyte containing H2O2. Second, the difference between the onset potential and the flat band potential is 0.5−0.6 V in NaOH but only 0.1−0.2 V when using the sacrificial agent (assuming the flat band potential of hematite in these electrolytes is the same as previously measured: around 0.50 to 0.55 V vs RHE).14,17,40 Next, we will show that these key contrasts in behavior are strongly correlated to the photoanode’s transient behavior. 3.2. Methodology. The TPC and TPV were measured in the two electrolytes previously mentioned, under white light bias and applied potential (current for TPV) bias during a blue light pulse for 0.5 s. Examples of these measurements obtained 26710



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exponential function (eq 3, dotted line/broken lines in Figure 7.4, c and d) in two distinct regions: one during the blue light pulse, and the second after the pulse.

at representative conditions (key potentials and their related currents) are shown in the Supporting Information (Figures S2 and S3). A schematic representation of the photocurrent transient recorded in the standard electrolyte is also provided in Figure 2a. When the blue diode pulse is switched on, the photocurrent density exhibits a sharp anodic peak before an exponential drop to eventually reach a new steady state photocurrent (only when the applied potential is over the onset of the photocurrent; see Figure 1). Immediately after the light perturbation is switched off, a cathodic current peak is observed, and then the current density is restored to the level attained before perturbation. Four different time domains can be defined to characterize this type of curve: t1 is set between the beginning of the light pulse and the transient maximum, t2 is set between the transient maximum and end of the light pulse, t3 is defined between the end of the light pulse and the transient minimum, and finally t4 is set between the transient minimum and the last point of the measurement. The current densities recorded during these different regions are fitted with monoexponential functions (t0 being a fixed value equal to the time of the first point of the relative region). J(t ) = J0 + A e−(t − t0)/ τ

V (t ) = V0 + Aae−(t − t0)/ τa + Abe−(t − t0)/ τb

Four time constants were obtained for each photovoltage transient: two for the photovoltage rise (τV12a and τV12b), and two for the photovoltage decay (τV34a and τV34b). The value of V0 obtained during the light pulse was used to quantify the photovoltage amplitude (due to the light pulse, ΔVSS). When measured at relative high applied potential in sodium hydroxide and for all bias potential in hydrogen peroxide, the drift before the light pulse is not observed (Figure 2d) and are treated in a similar way to the one presented in Figure 2c without the drift correction. The parameters obtained from the data treatment are first discussed for a non-treated hematite photoanode in the following in terms of photocurrent transient amplitude, density of accumulated charges, photovoltage transient, and time constant. 3.3. Photocurrent Transients. The amplitudes of the photocurrent transients, ΔJSS, are displayed versus the applied potential referenced to the RHE in Figure 3a (plain lines). In similarity with the photocurrent in 1 M NaOH, ΔJSS increased steeply from an applied voltage of ca. 1.0 V vs RHE (close to the onset potential of the photocurrent) and seems to saturate at about 1.4 V (in the potential range where the photocurrent plateaus) as an inflection point is observed around 1.15−1.25 V vs RHE. The different ΔJSS obtained at different bias light intensities cross around the inflection point as ΔJSS has a more anodic onset but a larger value at 1.4 V vs RHE with decreased bias light intensity. After a similar and modest increase up to 0.85 V, ΔJmax increases to a larger extent for lower bias light (blue > orange > red) before attaining saturation at 1.0 V for 1 sun, 1.05 V for 0.5 sun and 1.15 V vs RHE for 0.1 sun bias light measurement (see broken lines in Figure 3a). The disappearance of the anodic and cathodic is attained (1.4 V for 1sun illumination) when ΔJmax reaches ΔJSS. According to the tendency observed in Figure 3a, the disappearance of the transient peaks will be attained at a potential slightly higher than 1.4 V vs RHE for 0.5 sun bias light measurement and at even higher potential for 0.1 sun measurements. The difference between the height of the transient peak and the rise in steady state (ΔJmax − ΔJSS) has been previously used by other groups to give a qualitative characterization of the accumulated charges, but this method suffers from considerable inaccuracy as it depends critically on the time delay between points recording. Our method of data fitting and integration allows for quantitative analysis. In the electrolyte containing hydrogen peroxide, the α-Fe2O3 film also exhibits a behavior similar to what is observed in Figure 1. ΔJSS,H2O2 reaches a photocurrent plateau at about 0.8−0.9 V vs RHE and shows a slight increase with the applied potential. The relationship between ΔJSS,H2O2 with the applied potential can be derived according a classical model, detailed previously by Gerischer.41 According to this theory, the space charge layer is responsible for charge separation when the semiconductor absorbs light, and its width WSC is given by eq 4.

(1)

This equation describing the current density can also be found under another form for the rise of the photocurrent (eq 2).34 J(t ) − JSS J(t = 0) − JSS

(3)

= e −t / τ (2)

Four different time constants are thus obtained for the defined time domains: τJ1, τJ2, τJ3, and τJ4. J0 (found in eq 1, for the second time domain) defines the new steady state photocurrent (white light bias + blue pulse), and the difference between J0 and the previous steady state photocurrent (due to only white light) is called ΔJSS. By integrating the measured photocurrent density minus the steady state value of the photocurrent (C cm−2 s−1) with respect to time, we obtain a value proportional to the number of positive charges accumulated at the surface (i.e., C(cm−2 s−1) × s = C(cm−2)) in analogy to calculating the charge accumulated at a capacitor. The areas corresponding to the number of charges are shown in Figure 2a for the anodic and cathodic transients. When the photocurrent is recorded in the electrolyte containing the sacrificial agent (Figure 2b), no charge accumulation is observed during the light pulse. This type of curve is fitted to a monoexponential function in two regions (eq 1): the first one is between the beginning and the end of the light pulse and defines a new steady state photocurrent (J0) and one time constant (τJ12−H2O2). The difference between the previous and the new steady state photocurrent is denoted ΔJSS,H2O2. The second time region, between the end of the light pulse and the last measured point, is fitted in order to define a second time constant (τJ34−H2O2). Figure 2c shows the typical photovoltage transient obtained under low bias potential in the standard electrolyte (blue curve). This curve exhibits a drift before the blue light pulse is switched on. To correct this, the measured photovoltage curve has been fitted between t = 0 and the beginning of the blue light pulse to an exponential function (eq 1, dotted blue line) and the corrected photovoltage (red curve) was obtained with subtracting the exponential part of the fitting function (Ae−(t−t0)/τ). The corrected curve was then fitted to a two

WSC = W0 Vappl − Vfb =

2εε0 eND

Vappl − Vfb

(4)

where ε is the relative dielectric constant (taken as 80 for iron oxide),37 ε0 is the permittivity of vacuum (= 8.85 × 10−12 F 26711



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charge accumulation is measured (Figure 2b). Thus we can consider that all positive charges reacting at the surface of the hematite photanode with H2O2 are photogenerated between the surface and a distance D inside the semiconductor. This distance D corresponds to the total thickness where photogenerated carriers are separated and harvested. This occurs both in the depletion region, WSC, and in the bulk of the semiconductor at a distance where a photogenerated hole can diffuse to the depletion region. The distance D is described by eq 5. D = WSC + L D,eff =

2εε0 eND

Vappl − Vfb + L D,eff

(5)

where LD,eff is the effective hole diffusion length, assumed to be independent of applied potential. As no charge accumulation (anodic transient peak) is observed in the electrolyte containing H2O2, the current measured in the external circuit is assumed to correspond exactly to the number of positive charges reaching the surface to react. The distance inside the iron oxide film responsible for ΔJSS,H2O2 has been calculated as follows: first, the amount of additional photocurrent brought by the blue diode pulse was normalized to the pulse intensity (corresponding to 4.34 mA cm−2). A ratio of reacted photogenerated holes to the number of incident photons as a function of the applied potential is thus obtained (between 20 and 30%, not shown here). Giving a lambertian absorption and considering negligible reflection losses (with Transmittance = 1 − Absorptance), one can obtain the distance D (eq 6). ⎛ ΔJSS,H2O2 ⎞ ⎟ D = −α −1log(1 − A) = −α −1log⎜⎜1 − ⎟ J ⎝ pulse intensity ⎠

(6)

where A is the sample absorptance, α is the absorption coefficient of hematite at 470 nm (corresponding to the blue diode emission) and taken as 46 nm−1 at this wavelength,40 and Jpulse intensity is the photon flux converted in terms of current density with a Si photodiode. Figure 3b shows the distance D (full markers plain lines) versus the square root of the difference of the applied potential and the flat band potential (taken as Vfb = 0.55 V vs RHE).14 For all white light bias intensities, the distance D is found to be linear with (Vappl − Vfb)0.5 in the applied potential range 0.8−1.1 V vs RHE with the coefficient of determination, R2, of the linear regression analysis over 0.999. Another representation of the parameter D with the applied potential is shown in the Supporting Information (see Figure S4). The parameters obtained from the fitting of D with eq 5 are shown in Table 1 with figures estimated from a measurement performed at a lower intensity of the blue pulse (corresponding to 1.96 mA cm−2). Considering a flat band potential of 0.55 V, the effective hole diffusion length obtained with this model is between 0.6 and 1.4 nm. These values are lower than the numbers presented by Kennedy and Frese (2−4 nm)42 but in the same order of magnitude. This difference may arise from the bias illumination conditions used for our experiment or from the assumption of a lambertian light absorption on a flat interface, whereas the samples studied here are nanostructured. If the nanostructure was accounted for, the calculated effective hole diffusion length would, in fact, be smaller than the value obtained with a flat interface because in the geometry of a nanostructure the photogenerated holes have a greater

Figure 3. (a) Steady-state amplitudes of the photocurrent transient and transient peak height (ΔJSS in filled markers/solid lines and ΔJmax in open markers/dotted lines) measured in two different electrolytes are shown as a function of the applied potential, with respect to the RHE. (b) Distance D (in nm) representing the thickness inside the hematite film thickness where active photogenerated holes are originated from (see text for details), is shown as a function of the square root of the difference between the applied potential Vappl. and the flat band potential Vfb; linear fits of this distance D are shown as dotted lines. (c) Accumulated charge density (filled markers, solid lines) and dissipated charge density (empty markers, broken lines) are shown as a function of applied potential with respect to the RHE. All data (a, b and c) are shown for three different intensities of the white light bias: 1 sun (red circles), 0.5 sun (orange squares) and 0.1 sun (blue triangles).

m−1), e is the electronic charge (= 1.6 × 10−19 C), ND is the donor density, Vappl is the applied potential, and Vfb is the flat band potential. This model neglects charge recombination in the space charge layer and on the surface (electrons should be repelled from the surface by the space charge layer field). These assumptions seem relevant in the H2O2 electrolyte, as no 26712



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cm−2 at 1.15 V vs RHE for 0.1 sun, 0.5 sun, and 1 sun bias light, respectively. At this stage, it is impossible to determine in which energy levels the positive charges are accumulating or if charge transfer from the semiconductor to the electrolyte occurs through the valence band or through intraband-gap trap states. The experiments performed with different bias light nevertheless show interesting features that could be explained by two means. First, the blue light pulse implies a greater change in the hole quasi Fermi level (QFL) when the sample is under low light bias condition as the hole QFL depends on the logarithm of the ratio of the hole concentration under illumination (p* + p0, p* being the additional hole density due to illumination) on the thermodynamic equilibrium concentration of holes in the dark (p0).43 The perturbation light pulse would cause a more significant shift in the QFL at low bias light intensity and allow more holes to be stored in intraband gap states. Another possibility is that the higher bias light instensity involves a higher number of accumulated holes on the surface before the perturbation light pulse (in steady state conditions under bias light). This may cause less positive charges to be required to accumulate at the surface of the semiconductor to attain the new equilibrium (space charge layer vs accumulated charges repulsion). In both cases, the depletion layer width will be altered by the presence of holes accumulated at the surface and at low voltage bias. The charge separation effect of the space charge layer may even be canceled with the charge repulsion at the surface, especially accounting for the nonzero probability of electrons tunneling through the thin space charge layer (less than 3 nm in the H2O2 electrolyte at 0.8 V vs RHE) to recombine with stored holes. This may explain why charge accumulation is only observed at potentials more anodic than 0.8 V vs RHE. 3.4. Photovoltage Transients. Alternative information can be obtained from the study of photovoltage transients and the effect of the blue light pulse on those has been qualitatively described in Figure 2. The drift observed before the light pulse at low bias light and low applied voltage in 1 M NaOH electrolyte has been corrected with an exponential function, which has been subtracted, to obtain a flat steady state before the pulse (see red curve in Figure 2c). This steady state photopotential attained when applying a fixed current under 0.1, 0.5, or 1 sun white light bias conditions is shown versus the expected photopotential (based on the J−V characterization, Figure 1) in Figure 4a for measurements performed in the two electrolytes. Unlike in the electrolyte containing a sacrificial agent, the photopotentials recorded in 1 M NaOH did not show a good linearity with the expected potential. At applied current below the ones corresponding to high expected potentials, 1.3 and 1.4 V vs RHE, the measured photopotential deviates from the ideal case (when measured photopotential = expected photopotential). This deviation is more significant for lower white light bias, and the photopotential could not reach a value under 1.15 V vs RHE at 0.1 sun light bias (full blue triangles) whereas a photopotential of 1.0 V vs RHE is attained at low applied current with a light bias of 0.5 sun (full orange squares) or 1 sun (full red circles). The inability of the photoanaode to reach a photopotential close to the expected value occurs in the same potential range of positive charge accumulation at the surface, i.e., 0.8−1.2 V vs RHE. The deviation could possibly be a manifestation of Fermi level pinning induced by the presence of surface states under the conduction band that play the role of

Table 1. Effective Hole Diffusion Length LD,eff (in nm), Depletion Width at an Applied Potential of One Volt over the Flat Band Potential, W0 in nm V−0.5 and the Donor Density ND in cm−3 Obtained from Fitting the Data Presented in Figure 3b with a Classical Model (Eq 5)a blue light diode powerb/ mA cm−2 4.34 mA cm

−2

1.96 mA cm−2

white light bias intensity 0.1 sun 0.5 sun 1 sun 0.1 sun 0.5 sun 1 sun

LD, Eff./ W0/ nm nm V−0.5 1.12 0.98 0.61 1.40 1.24 0.69

9.08 8.77 8.76 8.49 8.22 8.51

ND/cm−3 1.16 1.15 1.15 1.23 1.31 1.22

× × × × × ×

1020 1020 1020 1020 1020 1020

a These data were obtained at two different blue light diode powers (characterized with the maximum photocurrent obtained with a Si diode). bThe power has been converted in terms of maximum photocurrent reachable considering a quantum efficiency of unity and measured with a Si diode.

probability to reach the surrounding depletion layer. The figures obtained from this study confirm the very small diffusion length of holes in hematite, which is mentioned in most of hematite photoanode investigations. The effective hole diffusion length is also found to be shorter at higher bias light intensity than at lower bias light intensity, possibly caused by the increased number of photogenerated electrons located in the diffusion length area at high bias light intensity, increasing charge recombination as they are not repelled by the space charge field. The depletion layer width and the donor density calculated with this method are also in good agreement with the literature reports, with W0 on the order of 8−9.5 nm (at 1 V vs Vfb) and ND in the range of 0.1−2 × 1020 cm−3.17,40 The depletion layer width and the hole diffusion length have been calculated in the electrolyte containing a sacrificial agent, and these parameters would be reduced in a standard electrolyte as positive charge accumulation at the surface will decrease the band bending and therefore reduce the depletion layer width. However, if we can prevent the charge accumulation at small applied potential in a standard electrolyte, these figures should be valid and justify the utilization of nanostructures with feature size of about 5−10 nm for water splitting photoanodes based on hematite. In order to quantitatively characterize the accumulated charge density observed during water oxidation, we considered the quantity defined in Figure 2a. The accumulated charges and the dissipated charges are shown versus the applied potential in Figure 3c. Unlike photocurrent densities, which are normalized to the geometric area (0.503 cm2, corresponding to the illuminated area), the measured charge densities have been normalized to the real Fe2O3 surface area, which is equal to the geometric area times the roughness factor, taken as 21 for these nanostructures.40 First, one can observe that the charge density of the cathodic peak corresponds to the inverse of the accumulated charge density. It therefore confirms that a part of the charges are accumulated in the material during the light pulse and that the cathodic current observed after the light pulse arises to bring electrons to recombine with the stored holes. The charge accumulation starts at an applied potential of 0.8 V vs RHE for all the different light bias applied. At higher potential, it rises to reach a maximum, which appears to be decreasing and cathodically shifted with increasing white light bias: 6.3 μC cm−2 at 1.05 V, 3.2 μC cm−2 at 1.10 V and 2.3 μC 26713



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involved) may influence the presence of charged trap states (due to reaction intermediates, FeIV or FeV states) as well as the density of accumulated charges at the interface with electrolyte, involving the reduction of the depletion layer width. 3.5. Time Constants. Time constants obtained from the fitting of TPC and TPV measured under 1 sun white light bias are shown in Figure 5. Time constants obtained from the fitting of TPC in the standard electrolyte and in the one containing a sacrificial agent are compared in Figure 5a,b. These six time constants can be separated in two different ranges: first the rise and the decay of the photocurrent transient (red squares and orange circles for both experiments) is around 0.01 s at low applied potentials and decrease to a value of 0.005−0.006 s with increasing applied potential. As these time constants do not vary with the electrolyte, they are likely to be related to the same phenomena. The two other time constants (τJ2 and τJ4, in NaOH) are found to increase from 0.02 to 0.2 s with increasing potential and correspond to the decay of the photocurrent during the positive charge accumulation and to the rise of the photocurrent during the recovery of the prepulse equilibrium. It is interesting to note that they saturate when charge accumulation is maximum and starts to decrease. It suggests a relation between these time constants with the processes related to the accumulation/recombination of the stored charges on the surface. Figure 5c,d shows the time constants obtained from the fitting of the photovoltage transients when measured in the standard electrolyte (c) and the one containing a sacrificial agent (d). The fastest process (τV12a and τV34a) has been determined to occur within a time of 0.01 s, whereas the slowest process (τV12b and τV34b) occurs within a time of 0.1−1 s. The four time constants determined in the H2O2 electrolyte (Figure 5d) are also in this range (0.01−1 s) but do not show any trends. It is possible that the different processes involved occur at a similar time scale in this electrolyte so that it is difficult to separate them, and therefore mixing or exchange of the time constants happens during the fitting. From this experiment, it seems that two separate phenomena can be identified. As the slower process cannot be determined from photocurrent transients in H2O2, this phenomenon can be related to the surface events that are claimed to cause the large overpotential, whereas the faster process, measured in both electrolytes, may be related to bulk phenomena. However, it is interesting to compare the time constants measured here with ones found in the literature. Glasscock et al. determined two time constants from the fit of photocurrent transients with mean values of 0.03 and 3 s for doped films (Ti and Si doping).20 The fast process is in the same range as the one found in this study, but the slow process is 1 order of magnitude higher than the one measured here, possibly due to the different surface properties of their reactive magnetron sputtered films. Recently, the dynamics of photogenerated holes has also been measured by TAS in the microsecond to second time scale.29,46 The decay of the transient absorption signal has been fitted with a stretched exponential function with a lifetime of 3 s (±1 s) and assigned to long-lived holes that become active for water splitting. For the oxidation of methanol, the lifetime has been measured at 0.4 s. Peter and co-workers have developed a model including two time domain-separated processes to discuss the PEIS28 and IMPS21 characterization of their hematite thin films, deposited by aerosol assisted chemical vapor deposition. The slower process (between 0.1 and 10 s) is assigned to the hole transfer

Figure 4. (a) Measured steady-state photopotentials (in V vs RHE) are shown as a function of the expected potential in V vs RHE for measurements performed in 1 M NaOH electrolyte (full markers, plain lines) and in 1 M NaOH + 0.5 M H2O2 electrolyte (empty markers, broken lines). Measured photopotentials are recorded while applying a fixed current between the photoanode and the counter electrode and have been corrected when a drift was observed to obtain a stable photopotential before the pulse (see Figure 2). (b) Photovoltage transient amplitudes in V are shown as a function of the measured steady state photopotential in V vs RHE for measurements performed in 1 M NaOH electrolyte (ΔVSS, filled markers, solid lines) and in 1 M NaOH + 0.5 M H2O2 electrolyte (ΔVSS, H2O2 open markers, dotted lines). Three different white light biases have been used for this experiment: 0.1 sun (blue triangles), 0.5 sun (orange squares) and 1.0 sun (red circles).

recombination centers. Fermi level pinning has already been observed for hematite photoanodes in several investigations.32,44,45 Figure 4b shows the TPV amplitude (ΔVSS and ΔVSS,H2O2) induced by the blue light pulse as a function of the measured steady state photopotential before the light pulse. In the electrolyte containing H2O2 (broken lines, empty markers), the linear increase in ΔVSS,H2O2, observed up to 1.3 V, is consistent with a rise in the Fermi level of electrons due to the increase in photogenerated electrons by light absorption. Similar to the TPC experiment and in both electrolyte, the hematite photoanode seems to be more reactive in low white light bias intensity, probably due to the greater relative increase in photogenerated charge density implied by the blue light pulse in the sample held under 0.1 sun bias than in the sample held in 1 sun bias. The measured amplitudes in NaOH (ΔVSS) are much less significant compared to those observed in the H2O2 containing electrolyte (approximately 10 times less) especially in the potential range where charge accumulation occurs. It suggests that at potentials cathodic of 1.2 V vs RHE, the additional charges brought by the light pulse are not used to raise the Fermi level of electrons but are lost due to another process, possibly surface recombination. The different reactions involved in each electrolyte (with different number of electrons 26714



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Figure 5. Time constants obtained from the fitting of photocurrent and photovoltage transients measured under 1 sun white light bias and applied potential (vs RHE) or applied current. Time constants from the fitting of photocurrent transients in a standard electrolyte (a) and in one containing a sacrificial agent (b) are shown as a function of the applied potential with respect to RHE. Time constants obtained from the fitting of photovoltage transients in a standard electrolyte (c) and in one containing a sacrificial agent (d) are shown as a function of the steady state potential measured before the beginning of the pulse. See text for details.

are shown in Figure 6. The deposition of cobalt on the surface of the hematite photoanode has been obtained by dipping the sample in a cobalt nitrate solution and results in a cathodic shift of the photocurrent by 70 mV (Figure 6a). The deposition of the alumina overlayer (0.5 nm approximately) has been obtained by ALD and results in an overpotential decrease of 99 mV (Figure 6b). The subsequent deposition of cobalt onto the alumina-modified anode further shifted the JV curve by 74 mV toward negative potentials. The onset potentials of the different photoanodes have been listed in Table S1 (Supporting Information). The plateau photocurrent is attained at smaller applied potentials, but is not modified by any of these surface modifications. As shown previously for the electrode without surface modification, this plateau scales almost linearly with the light intensity (see Table S1 in the Supporting Information). The effects of the cobalt and alumina layer have been rationalized as a better catalysis of the OER and as a surface passivation, respectively, but cannot be distinguished on current voltage characterization.17 The accumulated charge density and the dissipated charge density, defined previously (see Figure 2), of a photoanode before (black circles) and after surface treatment (blue squares for cobalt nitrate, red diamonds for alumina and violet triangles for alumina/cobalt combinatory layers) are displayed in Figure 7. These charge densities have been measured with 0.1 sun white light and applied potential biases. As observed previously in Figure 3c, the accumulated charge density corresponds exactly to the dissipated charge density, i.e., the electron density that diffuses from the external circuit after the end of the light pulse. Additionally, this amount of accumulated charges is reduced by a factor of 3 when the

from surface states to the electrolyte, while the faster process (0.01−1 s) has been assigned to the recombination of photogenerated charges via these same surface states. Finally, in conventional EIS (in dark conditions), two features occurring at different frequencies are also observed.5 The fast process is characterized at high frequencies (in the kHz to 100 Hz frequency range, corresponding to 0.001−0.01 s) and is assigned to space charge events. The slow process (from 10 to 0.1 Hz, corresponding to 0.1−10 s) has been assigned to surface events17 or to the influence of intra band gap states in the bulk or on the surface.24−26,37 Overall, with all these different analytical techniques, two time domains are distinguishable in the microsecond to second time scale. The faster process, measured from 0.001 to 0.1 s (at 0.01s in our study) is likely to be related to a bulk or space charge process, possibly the electron transport from the charge separation area to the collection electrode. The slower one determined between 0.01 s to the 3 s, and measured between 0.1 and 1s in our study, could be related to surface events such as charge recombination via surface states, charge transfer to the electrolyte, or formation of a reaction intermediate. 3.6. Influence of Surface Modification. In a second set of experiments, we measured TPC and TPV of hematite photoanodes with a surface modification of cobalt(II) or with the ALD of an alumina overlayer. A 0.5 nm thick alumina layer was used in this study, as it has been shown previously that the effect of the alumina overlayer saturates from 3 ALD of alumina (corresponding to 0.5 nm).17 The influence of the cobalt treatment,13 the alumina overlayer, and the combination of alumina with cobalt17 on the current voltage characteristics of the hematite photoanode have been described previously and 26715



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Figure 6. Current densities, in mA cm−2 of the prepared photoanode tested in 1 M NaOH electrolyte in 1 M NaOH, under 1 sun illumination (AM 1.5G, solid lines) and in the dark (dotted lines) are shown as a function of the applied potential, with respect to the reversible hydrogen potential. (a) An APCVD hematite photoanode post-treated with cobalt nitrate is compared to a reference sample (APCVD sample without overalyers, black circles). (b) An APCVD hematite photoanode post-treated with an alumina overlayer (deposited with ALD, red diamonds) is compared to an APCVD hematite photoanode post-treated with alumina and cobalt nitrate subsequently (violet triangles) and to a reference sample (black circles). The onset potentials of the photocurrent have been determined to be 1.03 V for the first reference (a), 0.96 V for the cobalt post-treated sample (a), 1.07 V for the second reference (b), 0.97 V for the alumina post-treated sample (b), and 0.89 V for the sample post-treated with alumina and cobalt nitrate (b). See text for details.

Figure 7. 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. (a) An APCVD hematite photoanode post-treated with cobalt nitrate is compared to a reference sample (APCVD sample without overalyers, black circles). b) An APCVD hematite photoanode post-treated with an alumina overlayer (deposited with ALD, red diamonds) is compared to an APCVD hematite photoanode post treated with alumina and cobalt nitrate subsequently (violet triangles) and to a reference sample (black circles). All data (a and b) are shown for measurements performed with a white light bias intensity of 0.1 sun.

The effect of the cobalt deposition on a Al2O3 covered hematite sample (violet triangle in Figure 7b) is different than when deposited on a bare hematite electrode. The accumulation charge density is not reduced by the subsequent cobalt deposition but only shifted toward negative potential by ca. 200 mV. This suggests that cobalt has a different role on a surfacepassivated electrode than on a bare hematite photanode. The photovoltage characterization of the same samples, measured with a white light bias of 0.5 sun, is shown in the Supporting Information (Figure S5). Some difference arises between the different overlayers as the Fe2O3 photoanode covered with alumina shows a less significant deviation of the measured steady state photopotential (before the pulse) compared to an hematite sample covered with cobalt and to a reference. With the addition of alumina on the surface, the steady state photovoltage does decrease when the bias potential is lower than 1 V vs RHE unlike the photovoltage response of the sample covered with cobalt. The subsequent cobalt treatment of the alumina treated sample does not change the photovoltage characterization in terms of deviation. In terms of photovoltage amplitude, both the cobalt and alumina overlayer exhibit an increase in ΔVSS (Figure S5c and d) as compared to a reference. Nevertheless, we can note that the increase was more significant for the sample coated with cobalt than for the

sample is recovered with a cobalt layer or an alumina layer (from 6 μC cm−2 to 2 μC cm−2). It confirms that a certain amount of holes that reach the surface during the blue light pulse is stored and that this storage is likely to occur at the surface of the sample as only the surface of the sample has been modified with cobalt or alumina. The maximum accumulated charge density is also cathodically shifted by 100 mV approximately, corroborating the assumption that charge accumulation increases until the onset of the photocurrent. From this experiment, the effect of the cobalt overlayer can still not be distinguished from the effect of the alumina overlayer. In Figure 7a, the charge accumulation on the cobalt treated sample seems to occur at low bias potential before it decreases. This accumulation is likely to relate to the oxidation/reduction of cobalt species, and capacitive current has been observed in this potential region with cobalt in an investigation of the CoPi catalyst deposited on hematite photanodes.47 This accumulation will therefore be neglected as it is assumed to be caused by a thick layer of cobalt on the iron oxide surface. 26716



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Scheme 1. Schematic Representation of the Effect of Bias Potential (a,b) and Overlayers (c,d) on the Band Configuration of the Photoanode and the Relative Rates of Electron−Hole Recombination and Interfacial Charge Transfer in a Hematite Photoanode for Water Photoelectrolysis (Solid Lines: Majority Events; Dotted Lines: Minority Events)a

a

The applied positive bias increases the depletion layer width, decreasing the electon/hole recombination probability in the space charge layer whereas the alumina and the cobalt overlayers are respectively assumed to passivate the surface traps mediating recombination and to extract the photoholes, decreasing in both case photogenerated charge recombination.

one with an alumina overlayer and the cobalt post treatment on an alumina post treated sample showed a further enhancement in PV amplitude, even if it did not show any influence on the initial PV deviation. The effect of the overlayers may be summarized as follows: The deposition of cobalt or alumina on the surface of hematite cathodically shifts the onset potential of the photocurrent by 70 and 99 mV, respectively. The subsequent depositon of cobalt on alumina further decreases the onset potential by 74 mV. Both alumina and cobalt on the surface lead to a decrease in the charge accumulation at the surface of the hematite by a factor of 3 without canceling it completely. The maximum charge accumulation has been also shifted by 100 mV for both post treatment confirming the assumption of a relationship between this maximum and the onset potential. The additional cobalt treatment on the alumina overlayer does not further decrease the charge accumulation but shifts its maximum drastically by 200 mV toward negative potentials. The drift observed in the photovoltage measurement is attenuated more significantly with the alumina overlayer compared to the cobalt one, even if the overpotential reduction is the same. The cobalt treatment lead to the measurement of a greater photovoltage amplitude compared to the sample coated with alumina or to the reference. A significant increase in the PV amplitude has also been observed when cobalt is deposited over the alumina overlayer, even if this treatment has no effect on the steady state drift. 3.7. Discussion. The large overpotential observed with hematite has been previously attributed to a bad interfacial charge transfer rate, possibly due to charge recombination mediated at the interface by surface states.

Surface states causing Fermi level pinning have been observed through several analytical techniques, at energetic levels between the flat band potential and a potential 0.5 V more anodic, corresponding to applied voltages between 0.5 and 1.0 V vs RHE.32,44,45 These energetic traps are localized too high in energy for holes to react with water, and it is therefore unlikely that charge transfer occurs from these surface states. Nevertheless, the presence of these surface states is detrimental for the onset of the photocurrent, even if charge transfer for water photolysis with hematite is occurring with holes localized energetically in the valence band or in other intraband gap states (closer to the valence band). Indeed recombination of photogenerated holes can possibly be mediated by these same surface states even in the case of a space charge region depleting the surface from electrons, considering the increasing probability of electrons tunneling through an ultrathin space charge layer (at small potentials). With a donor density estimated to 1020 cm−3,12 and taking in account a dielctric constant of 80,37 the depletion layer width is less than 3 nm at 0.5 V vs Vfb (ca. 1 V vs RHE) according to eq 4. This small depletion layer width has been confirmed with the investigations of the photocurrent in H2O2 in this report. It is worth noting here that the width of this layer would be further reduced in water by positive charge accumulated at the surface by electrostatic repulsion of holes (or attraction of electrons), consequently increasing the recombination probability. Within this study the charge accumulation has been observed when the applied potential (i.e., the bulk fermi level of electrons) is less than 0.8 V vs Vfb with a maximum at Vappl = 0.5 V vs Vfb, justifying the crucial role of accumulated charges at low bias potentials. 26717



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passivating a surface not passivated enough by the Al2O3 layer. However, it is unlikely because no improvement in the cathodic shift was observed with depositing thicker layers of alumina (2 nm).17 Therefore our results indicate that the effect of the alumina and cobalt layers are synergetic and not additive (involving the same process). Moreover, the effects of the two overlayers are distinguished with the photovoltage charaterization of the samples (Figure S5). The sample covered with an alumina layer exhibits a less significant deviation of the measured steady state photopotential, implying that the surface states, assumed to be responsible of this phenomenon, are inactive. On the other hand, deposition of a cobalt layer seem to only increase the photovoltage amplitude, possibly due to increase in charge separation at the SCLJ. The surface passivation effect can be related to the photoluminescence observed in a hematite photanode covered with alumina, presumably caused by an increase of direct recombination, whereas this was not observed with the sample covered with cobalt.17 Consequently, the role of the alumina overlayer is assumed to passivate surface states, eliminating charge recombination through these same surface states that are assumed to cause Fermi level pinning (see Scheme 1c). The decrease in charge accumulation is therefore related to the smaller probability of charges recombining directly, whereas the cathodic shift can be rationalized considering that a smaller applied potential is required to spatially separate the photogenerated charges enough. These surface traps are possibly oxygen vacancies or higher valence iron atoms (FeIV or FeV) on the surface. Alumina is an oxide with a stoichiometry very close to the ideal Al2O348 unlike hematite, which is known to deviate from ideality due to oxygen vacancies. Thus alumina may compensate the oxygen vacancies that can exist on the hematite surface. The relatively high oxygen content of the alumina layer can be perceived as an electron-rich layer as there are more O2− anions in its lattice as compared to hematite. The passivation of the surface traps by the alumina layer can also be understood as a layer reducing the attraction of electrons toward the surface. The cobalt layer is assumed to extract the hole from the iron oxide surface. The cobalt layer formed will increase the distance between the electrons from the conduction band and the holes localized on the cobalt, as shown in Scheme 1d. This will decrease the charge recombination (the accumulation maximum) as well as the potential required to separate the charges enough to not observe electron tunneling through the depletion layer. Several studies have recently shown the positive effect of the cobalt oxide layers onto semiconductors for water splitting, and its positive effect has been related to the ability of the cobalt oxide layer to store holes in proximity (with the stability of cobalt at several oxidation states Co2+/Co3+/ Co4+).13,47,49 With the model described in Scheme 1, the synergetic effect of the alumina and cobalt layers is possible and occurs with increasing the distance between the electron, in the conduction band and repulsed by the alumina layer, and the hole located in the cobalt layer. The accumulation is consequently cathodically shifted but not further reduced maybe caused by still a too small space charge width obtained with these highly doped nanostructured hematite photonanodes.

The accumulated charge determined herein is in accordance with the surface charge capacitance determined in an EIS study of hematite thin films by Klahr et al.24 However, it was not possible to determine whether holes were reacting from intraband gap states (instead of valence band), as it has been recently assumed in several electrochemical models.21,24,28 The decrease in accumulated charges with increasing bias light could be explained by the activation/deactivation of surface states by the position of the hole QFL, but the effect of the QFL would also influence other parameters such as the driving force for hole transfer. Without the assumption that holes are located in intraband gap states, our results can be explained with the model depicted in Scheme 1. At low potential over the flat band potential, positive charges migrate to the interface with water and accumulate, canceling or reducing the effect of the space charge field. With such small depletion layer width, electron can tunnel through and recombine with accumulated holes (Scheme 1a). With increasing the applied potential, the space charge layer width becomes bigger. At large enough applied potential, the depletion layer is too large to allow electrons and positive charges to recombine through the space charge layer (see Scheme 1b with Vappl = 0.8 V vs Vfb). This simplified model implies recombination through surface states as Fermi level pinning has been already evidenced in several studies, higlighting the existence of these states. Direct recombination of electrons and holes from the conduction and valence band, respectively, could also occur but has been neglected in Scheme 1 for clarity. From this model, the process involving photogenerated holes on the surface of the semiconductor can be described as follows: at low anodic potential versus flat band potential, the accumulated charges mesured with our TPC experiment correspond to the amount of positive charges required to cancel the depletion layer field or to reduce the depletion layer enough to observe electron tunneling. At intermediate potentials, charge transfer to the electrolyte and charge recombination are competing processes (see photocurrent transient description). The measured accumulated charge is the amount of holes that modify the band bending (and therefore the amount of separated charges) to equilibrate the amount of charges diffusing to the surface and the amount that can react with the electrolyte species. At high enough potentials, only reaction with the electrolyte occurs and charge accumulation cannot be measured. With the investigation of TPCs and TPVs, we have also shown that charge accumulation is attenuated and occurs at more cathodic potentials when covering the photoanode surface with an alumina layer or with a cobalt layer. The combination of these two treatments leaded to a further shift in the accumulation maximum but not to a reduction in magnitude of charge accumulation obtained after the first treatment. As accumulated charges have been detected at potentials close to flat band with any overlayers, our observation possibly indicates that a certain concentration of holes is necessary for the reaction to occur or that the best passivating/catalyst layers has not been discovered yet for αFe2O3. The role of the different overlayers can not be distinguished from the curent voltage characterization as a similar cathodic shift is observed. The same cathodic shift is also observed with the accumulation maximum, supporting the possible relationship between the onset of the photocurrent and the accumulation maximum. The further cathodic shift due to the combination of the two overlayers could be due to the cobalt

4. CONCLUSION This work provides a quantitative analysis of TPC and TPV observed on iron oxide photoanodes during water oxidation. 26718



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Photocurrent and photovoltage transients were obtained with a short blue light pulse on an APCVD hematite sample held at working conditions. The sharp anodic peak measured between a blue light pulse and the establishment of the new steady state photocurrent is related quantitatively to the density of positive charges accumulated at the SCLJ, also corresponding to the amount of electrons that are diffusing from the external circuit when the light pulse is stopped. The accumulation of positive charges has been measured to be maximum between 2 and 6 μC cm−2 at the potential where photocurrent onsets, depending on the applied potential and on the white light bias intensity. This amount of positive charge is assumed to significantly perturb the space charge region, especially considering the small width measured with photocurrent transients in H2O2 based electrolyte (W0 = 8−9 nm at 1 V vs Vfb). The hole diffuson length has also been measured with this method to be between 0.5 and 1 nm, a reasonable value compared to the value calculated by Kennedy and Frese (2−4 nm). All these measurements confirmed the requirement of nanostructuring doped hematite photoanode with features size inferior or equal to 10 nm. The different behavior of the photovoltage transient in the two electrolytes tested is assumed to be due to the charging/ discharging of surface states causing Fermi level pinning, and two processes are distinguished with the determination of their time constants. The first process occurring at a time scale of of 0.01 s has been assigned to electron transport in the bulk, and the second process (in the range of 0.1 to 1 s) has been assigned to the hole trapping in surface states and/or to the transfer of holes from surface states to the electrolyte. Finally, this method has also been applied to hematite thin films covered with alumina and a cobalt layer. The surfacemodified photoanodes show a decreased and cathodically shifted accumulation maximum, which is correlated to the shift in the photocurrent onset. Moreover, the further cathodic shift of the accumulation maximum with the sample covered with both alumina and cobalt, as well as the difference observed in the photovoltage characterization enables one to differentiate the two effects. According to a simplified model derived in the last part of the study, the alumina layer passivates surface states, which mediates recombination between accumulated holes and electrons tunneling through the space charge layer. The cobalt layer was assumed to extract the holes from the surface, enhancing spatial separation of the photogenerated charge carriers. Both overlayers would imply that the potential to apply to obtain a space charge layer thick enough to avoid electron tunneling is less, rationalizing the cathodic shift observed in the charge accumulation maximum and in the photocurrent onset. Positive charge accumulation has been observed even with the combination of the two overlayers (alumina and cobalt) as well as in several other semiconductor candidates for water splitting application. This study provides a new tool to compare these electrodes and especially the quality of their surface with the charge accumulation quantification. The use of different electrolytes with different catalyst or surface passivation layer would help in assigning the processes observed during this transient. The better understanding of surface events brought by this new analysis technique should ultimately lead to the decrease of the overpotential for water splitting, i.e., the additional energy given by the second solar system in a tandem cell, enhancing the performance of hematite for the efficient and sustainable storage of solar energy.

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ASSOCIATED CONTENT

S Supporting Information *

Additional characterizations of the photoanodes (full JV characterizations) in both electrolytes, typical examples of photocurrent and photovoltage transients measured, a table listing the onset potential and the plateau photocurrents of each prepared electrode, a comparison between two electronic models of the photoanode behavior with potential, and photovoltage transients of the surface-modified samples are presented in the Supporting Information. This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Now a postdoctoral fellow at the chemistry department of Imperial College, London. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Swiss Federal Office of Energy (Project number 102326, PECHouse) through the Energy Center of EPFL for financial support. We also acknowledge M. Cornuz for the hematite film preparation and for the alumina deposition, and Dr. R. Humphry-Baker for technical support on the transient measurement setup and for useful discussion. The authors also thank Pr. James Durrant, Dr. S. Pendlebury, and Dr. M. Barroso from Imperial College, London for useful discussions.

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REFERENCES

(1) Gratzel, M. Chem. Lett. 2005, 34, 8−13. (2) Boddy, P. J. J. Electrochem. Soc. 1968, 115, 199−203. (3) Fujishima, A.; Honda, K. Nature 1972, 238, 37−38. (4) Brillet, J.; Cornuz, M.; Le Formal, F.; Yum, J. H.; Grätzel, M.; Sivula, K. J. Mater. Res. 2010, 25, 17−24. (5) Sivula, K.; Le Formal, F.; Grätzel, M. ChemSusChem 2011, 4, 432−449. (6) Fan, Z. Y.; Wen, X. G.; Yang, S. H.; Lu, J. G. Appl. Phys. Lett. 2005, 87, 013113. (7) Vayssieres, L.; Beermann, N.; Lindquist, S. E.; Hagfeldt, A. Chem. Mater. 2001, 13, 233−235. (8) Spray, R. L.; Choi, K.-S. Chem. Mater. 2009, 21, 3701−3709. (9) Mohapatra, S. K.; John, S. E.; Banerjee, S.; Misra, M. Chem. Mater. 2009, 21, 3048−3055. (10) Sivula, K.; Zboril, R.; Le Formal, F.; Robert, R.; Weidenkaff, A.; Tucek, J.; Frydrych, J.; Grätzel, M. J. Am. Chem. Soc. 2010, 132, 7436− 7444. (11) Sivula, K.; Le Formal, F.; Grätzel, M. Chem. Mater. 2009, 21, 2862−2867. (12) Cesar, I.; Sivula, K.; Kay, A.; Zboril, R.; Grätzel, M. J. Phys. Chem. C 2009, 113, 772−782. (13) Kay, A.; Cesar, I.; Grätzel, M. J. Am. Chem. Soc. 2006, 128, 15714−15721. (14) Dotan, H.; Sivula, K.; Grätzel, M.; Rothschild, A.; Warren, S. C. Energy Environ. Sci. 2011, 4, 958−964. (15) Zhong, D. K.; Cornuz, M.; Sivula, K.; Grätzel, M.; Gamelin, D. R. Energy Environ. Sci. 2011, 4, 1759−1764. (16) Tilley, S. D.; Cornuz, M.; Sivula, K.; Grätzel, M. Angew. Chem., Int. Ed. 2010, 49, 6405−6408. (17) Le Formal, F.; Tétreault, N.; Cornuz, M.; Moehl, T.; Grätzel, M.; Sivula, K. Chem. Sci. 2011, 2, 737−743. (18) Hisatomi, T.; Le Formal, F.; Cornuz, M.; Brillet, J.; Tetreault, N.; Sivula, K.; Grätzel, M. Energy Environ. Sci. 2011, 4, 2512−2515. (19) Dare-Edwards, M.; Goodenough, J.; Hamnett, A.; Trevellick, P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 2027−2041. 26719



Article

(20) Glasscock, J. A.; Barnes, P. R. F.; Plumb, I. C.; Savvides, N. J. Phys. Chem. C 2007, 111, 16477−16488. (21) Peter, L. M.; Wijayantha, K. G. U.; Tahir, A. A. Faraday Discuss. 2012, 155, 309−322. (22) Iwanski, P.; Curran, J.; Gissler, W.; Memming, R. J. Electrochem. Soc. 1981, 128, 2128−2133. (23) Wilhelm, S.; Yun, K.; Ballenger, L.; Hackerman, N. J. Electrochem. Soc. 1979, 126, 419−424. (24) Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Hamann, T.; Bisquert, J. J. Am. Chem. Soc. 2012, 134, 4294−4302. (25) Leduc, J.; Ahmed, S. J. Phys. Chem. 1988, 92, 6661−6665. (26) Horowitz, G. J. Electroanal. Chem. 1983, 159, 421−436. (27) Aroutiounian, V.; Arakelyan, V.; Shahnazaryan, G.; Stepanyan, G.; Turner, J.; Kocha, S. Electrochim. Acta 2000, 45, 1999−2005. (28) Upul Wijayantha, K. G.; Saremi-Yarahmadi, S.; Peter, L. M. Phys. Chem. Chem. Phys. 2011, 13, 5264−5270. (29) Pendlebury, S. R.; Cowan, A. J.; Barroso, M.; Sivula, K.; Ye, J.; Grätzel, M.; Klug, D. R.; Tang, J.; Durrant, J. R. Energy Environ. Sci. 2012, 5, 6304−6312. (30) Barroso, M.; Cowan, A. J.; Pendlebury, S. R.; Grätzel, M.; Klug, D. R.; Durrant, J. R. J. Am. Chem. Soc. 2011, 133, 14868−14871. (31) Cowan, A. J.; Barnett, C. J.; Pendlebury, S. R.; Barroso, M.; Sivula, K.; Grätzel, M.; Durrant, J. R.; Klug, D. R. J. Am. Chem. Soc. 2011, 133, 10134−10140. (32) Sanchez, C.; Sieber, K.; Somorjai, G. J. Electroanal. Chem. 1988, 252, 269−290. (33) Le Formal, F.; Grätzel, M.; Sivula, K. Adv. Funct. Mater. 2010, 20, 1099−1107. (34) Abrantes, L.; Peter, L. J. Electroanal. Chem. 1983, 150, 593−601. (35) Cornuz, M.; Grätzel, M.; Sivula, K. Chem. Vap. Deposition 2010, 16, 291−295. (36) Puurunen, R. J. Appl. Phys. 2005, 97, 121301. (37) Kennedy, J.; Frese, K. J. Electrochem. Soc. 1978, 125, 723−726. (38) Zhang, P.; Chi, M.; Sharma, S.; McFarland, E. J. Mater. Chem. 2010, 20, 2013−2017. (39) Zhang, Z.; Zakeeruddin, S.; O’Regan, B.; Humphry-Baker, R.; Grätzel, M. J. Phys. Chem. B 2005, 109, 21818−21824. (40) Cesar, I.; Sivula, K.; Kay, A.; Zboril, R.; Grätzel, M. J. Phys. Chem. C 2009, 113, 772−782. (41) Gerischer, H. J. Electroanal. Chem. 1975, 58, 263−274. (42) Kennedy, J.; Frese, K. J. Electrochem. Soc. 1978, 125, 709−714. (43) Duret, A. Mesoscopic Thin Films of Hematite as Photoanode for Water Photoelectrolysis; EPFL: Lausanne, Switzerland, 2005. (44) Duret, A.; Grätzel, M. J. Phys. Chem. B 2005, 109, 17184− 17191. (45) Shinar, R.; Kennedy, J. J. Electrochem. Soc. 1983, 130, 392−396. (46) Pendlebury, S. R.; Barroso, M.; Cowan, A. J.; Sivula, K.; Tang, J.; Grätzel, M.; Klug, D.; Durrant, J. R. Chem. Commun. 2011, 47, 716− 718. (47) Zhong, D. K.; Sun, J.; Inumaru, H.; Gamelin, D. R. J. Am. Chem. Soc. 2009, 131, 6086−6087. (48) Prescott, R.; Graham, M. J. Oxid. Met. 1992, 38, 233−254. (49) Barroso, M.; Mesa, C. A.; Pendlebury, S. R.; Cowan, A. J.; Hisatomi, T.; Sivula, K.; Grätzel, M.; Klug, D. R.; Durrant, J. R. Proc. Natl. Acad. Sci. U.S.A. 2012, 109, 15560−15564.

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The Transient Photocurrent and Photovoltage Behavior of a Hematite

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