Enhanced Charge Separation and Collection in High-Performance

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Enhanced Charge Separation and Collection in High Performance Electrodeposited Hematite Films Omid Zandi, Abraham R. Schon, Hamed Hajibabaei, and Thomas W. Hamann Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03707 • Publication Date (Web): 22 Dec 2015 Downloaded from http://pubs.acs.org on December 26, 2015

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Chemistry of Materials

Enhanced Charge Separation and Collection in High Performance Electrodeposited Hematite Films Omid Zandi, Abraham R. Schon, Hamed Hajibabaei, Thomas W. Hamann* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824-1322, United States

Abstract: Hematite electrodes with variable morphologies were prepared via a simple electrodeposition (ED) method. The photoelectrochemical (PEC) properties of planar and nanostructured electrodes were examined under PEC water oxidation and compared to that of planar analog prepared by atomic layer deposition (ALD). The water oxidation performance of electrodeposited planar thin films was comparable to the nanostructured films; surprisingly both ED films significantly outperformed the ALD made planar films. The superior performance is attributed to variations in the crystallographic properties which results in enhanced hole transport and collection as confirmed by photoelectrochemical and electrochemical impedance spectroscopy measurements and structural analysis. These results indicate a non-zero hole diffusion length for the electrodeposited hematite thin films in contrast to the ALD counterparts. Electrodeposited hematite thin films modified with Co-Pi exhibit near unity hole collection efficiencies and produce the highest photocurrent among reported planar electrodes. This approach thus provides a simple and scalable approach to prepare high performance thin film absorber hematite electrodes for solar water splitting.

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Introduction Hematite (α-Fe2O3) is an attractive photoanode material for photoelectrochemical (PEC) water splitting.1–4 The unique combination of hematite’s good light absorption, suitable energetics, stability, and elemental abundance could potentially enable efficient solar water splitting in a sustainable manner. Efforts to prepare efficient hematite photoanodes have been unsuccessful, however, mainly because of high rates of bulk and surface recombination coupled with a low hole mobility.5,6 Nonetheless, following the seminal report by Grätzel and co-workers in 2006 on mesoporous hematite nanostructured electrodes, remarkable progress has been made in the performance, reflected in both the photocurrent and photovoltage.3 This improvement can be attributed to the advancement of our understanding of the performance limiting mechanisms which can be effectively utilized in devising strategies to mitigate these efficiency-loss processes. For example, due to a very short hole collection length, nanostructuring is generally adopted to decouple the feature size and light absorption depth and thus maximize light absorption while maintaining the bulk within hole collection length.3,7–11 Surface modification strategies, on the other hand, are generally employed in order to suppress surface recombination and/or enhance water oxidation kinetics.12–16 The combination of bulk and surface modification strategies has resulted in a promising photocurrent density of 3-4 mA cm-2 at 1.23 V vs. RHE in the state of the art systems.12,17 The water oxidation photocurrent onset of bare hematite in these systems however, has been consistently very positive of the flat band potential which is generally attributed to surface recombination.5,18,19 We recently demonstrated a near unity hole collection efficiency for hematite thin film electrodes prepared via atomic layer deposition (ALD) which exhibit a photocurrent onset potential just positive of the flat band potential.20 Most recently Jang and


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Wang et al. employed a combination of high temperature annealing and a NiFeOx catalyst which produced a breakthrough photocurrent onset potential of 0.45 V vs. RHE on a solution processed hematite electrodes.21 Wang’s system enabled unassisted water splitting when coupled with an amorphous Si at an efficiency of 0.91%. Even with recent progresses however, the photocurrent generated with hematite electrodes is far less than what could potentially be generated with a band gap of 2.1 eV (~12 mA cm-2) especially at low applied bias where high photocurrent is desired for a tandem PEC cell design.22 Although low photocurrent has usually been associated with a short hole diffusion length23 and thus bulk recombination, it has been demonstrated that even for very thin films where the entire bulk of the electrode is within the depletion region, the absorbed photon to current conversion efficiency (APCE) is much less than unity.20,24 This indicates that the diffusion length is essentially zero and there is significant depletion region recombination. Depletion region recombination, which was also evident from the voltage dependent photocurrent of hematite thin film electrodes in the presence of a fast hole scavenger,20,25 further reduces the fill factor of the J-V curves resulting in a low photocurrent at low applied bias. Realizing high photocurrents at low applied potential thus requires effective reduction of bulk/depletion region recombination or an increase of the hole mobility or electric field at the electrode interface, any combination of which will improve the charge separation efficiency. There are several strategies to enhance charge separation presumably by enhancing hole conductivity. One strategy is the deposition of highly crystalline hematite electrodes. This was demonstrated by Warren et al.26 for nanostructures with reduced density of high-angle grain boundaries and Kim et al.17 for single crystal mesoporous hematite, which both produced high photocurrents. Another strategy is fabrication of highly doped hematite electrodes. High dopant



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densities can enhance charge separation in the bulk as well as hole transport in the depletion region by inducing sharper band bending at the interface.17,27 Here we report a comparative study of hematite electrodes prepared via two different routes exhibiting either planar or nanostructured morphology. An electrodeposition (ED) method was utilized to fabricate planar thin film and nanostructured electrodes simply by tuning the deposition pH and temperature, following a modified procedure previously reported by the Choi group.28 Analog hematite thin films with comparable thicknesses were also prepared by ALD following a previously established method.24,29 The hematite electrodes were examined under PEC water oxidation conditions. A combination of electrochemical and photoelectrochemical measurements and structural analysis were employed to elucidate the differences in water oxidation performance.

Experimental Electrode preparation Hematite electrodes were prepared via electrodeposition of FeOOH from FeCl2 (aq) solution using a modified version of previously reported method.28 Briefly F:SnO2 (FTO)–coated aluminoborosilicate glass substrates (Solaronix, 10 Ω/sq) were cleaned by sonication in soap, water and isopropyl alcohol each for 10 min followed by blow drying with a stream of N2. Cleaned FTO substrates were used as the working electrode in a custom made electrochemical cell along with a Pt mesh and an Ag/AgCl as counter and reference electrodes, respectively. Two pHs (acidic and neutral) were chosen for deposition as the solution pH strongly affected the deposition mode and thus the electrode morphology.


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Electrodeposition of planar thin films in acidic condition (a-ED). Acidic deposition was performed in 0.1 M FeCl2·4H2O (pH = 4.2) at 60 °C by applying 1.2 V vs. Ag/AgCl reference electrode under gentle stirring. The film thickness was determined by the deposition time (i.e. the total amount of charge passed). Acidic depositions produced planar films with excellent uniformity and reproducibility over studied substrates size as large as ~10 cm2 (Figure 1S in the SI). Electrodeposition of nanostructured FeOOH in neutral conditions (n-ED). Neutral deposition was performed in slightly different conditions as the Fe2+ ions are not stable and soluble at neutral and basic pHs. Deposition solutions contained 0.02 M FeCl2·4H2O and 3M NH4Cl. A high concentration of NH4Cl was used to stabilize the Fe2+ ions according to the previous report.28 This solution was purged with N2 for at least 30 min before adjusting the pH to 7.5 by addition of KOH. Electrodeposition was then performed at room temperature under constant N2 atmosphere (to minimize the oxidation of Fe2+ ions) and gentle stirring. The FTO working electrode was biased to 0.0 V vs. Ag/AgCl for neutral deposition. The lower applied potential used herein (compared to 0.3 V in utilized by Spray et al.28) was found to result in better uniformity and a more controllable morphology. The amount of the FeOOH was controlled by the deposition time. ALD of planar Fe2O3 thin films. Comparable thicknesses of Fe2O3 were deposited by ALD on FTO substrates using the previously reported method.24 Annealing. As deposited films were annealed either at 500 °C (2h) or 800 °C (10 min) in air to convert the amorphous electrodeposited FeOOH to crystalline α-Fe2O3. Annealing at 500 °C was done by ramping the furnace temperature at 20 degree/min to 500 °C, holding for 2h and then cooling to room temperature over 2h. For annealing at 800 °C, electrode were fixed on a flat Si



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wafer surface which were then placed in a pre-heated furnace at 800 °C for a short annealing time (10 min) followed by quenching at room temperature. Electrocatalyst deposition. Co-Pi deposition was carried out using a photoelectrodeposition method reported previously.13,30 Hematite electrodes were immersed in a solution containing 0.5 mM Co(NO3)2·6H2O in a 0.1 M phosphate buffer (pH 6.9). A bias of 0.1 V vs. RHE was applied under 1 sun illumination. The thickness of the Co-Pi layer was controlled by varying the deposition time. Deposition times of 30 and 360 s were found to produce the highest improvement for nanostructured and planar electrodes, respectively. Characterization The surface morphology of the prepared films was examined by scanning electron microscopy, SEM (Carl Zeiss Auriga, Dual Column FIBSEM). Absorbance measurements were made using a Perkin-Elmer Lambda35 UV-vis spectrometer equipped with a Labsphere integrating sphere. The absorbance spectra of the films were measured by illuminating from the substrate-electrode interface. The incident light was corrected for passing through and being reflected by the substrate using a previously reported approach.24 Raman spectroscopy measurements were made using a Renishaw inVia instrument equipped with 532 nm laser operated at 5% of the source power (45 W). X-ray diffraction (XRD) spectra were obtained on a Bruker D8 Advanced diffractometer using Cu radiation with a Kα1 wavelength of 1.5418 Å. Spectroscopic ellipsometry measurements were performed using a Smart-SE instrument (Horiba Jobin Yvon) over the wavelength range of 600-900 nm for the Fe2O3 films on FTO substrates. Photoelectrochemical measurements The electrodes were examined in contact with aqueous solutions of 1 M KOH electrolyte (pH = 13.6). A homemade saturated Ag/AgCl electrode was used as the reference electrode and high


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surface area platinum mesh was used as the counter electrode. The reference electrode was regularly calibrated vs. saturated calomel electrode (SCE) (Koslow Scientific) and all potentials were converted to the reversible hydrogen electrode (RHE) scale by the equation VRHE = VAg/AgCl + 0.197 V + pH (0.059 V). Photoelectrochemical measurements were made with an Eco Chemie Autolab potentiostat coupled with Nova electrochemical software. The light source was a 450 W Xe arc lamp (Horiba Jobin Yvon). An AM 1.5 solar filter was used to simulate sunlight at 100 mW cm-2 (1 sun). Unless otherwise mentioned, all photoelectrochemical tests were carried out by shining light on the electrodes through the electrolyte side (front side illumination) in a custom-made electrochemical cell. All steady state and chopped light J-V curves were measured at a rate of 20 mV/s. A computer controlled ThorLabs solenoid shutter was used for the chopped light measurements. EIS data were collected using a 10 mV amplitude perturbation between 10kHz-10mHz. Data were fit using Zview software (Scribner Associates).

Results and discussion SEM micrographs of typical electrodes prepared via the three routes (planar electrodeposited, aED, nanostructured electrodeposited, n-ED, and planar ALD) are shown in Figure 1. Asdeposited films by ED are amorphous FeOOH according to a previous report28 and our structural characterization (Figure 3S in the SI). As it can be seen the a-ED produced uniformly coated film which is converted to a compact planar film upon annealing. The n-ED films exhibit a sheet-like nanostructured texture. The cause of the pH-dependent morphology of electrodeposited films has been described elsewhere.28,31,32 The morphology of the nanostructured films undergo significant changes in the feature size upon annealing. Also shown in Figure 1 are the SEM micrographs of



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the ALD hematite films. As expected from the conformal growth mechanism of ALD, the general topography is very similar to the FTO substrate.24 a

b

c

Figure 1. SEM images of as-deposited (top panel) and annealed (bottom panel) hematite electrodes prepared via a) ALD b) a-ED and c) n-ED. Scale bars are 400 nm.

In order to test the PEC activity of the electrodeposited films, current density vs applied voltage (J-V) measurements were performed on electrodes annealed at 500 and 800 °C. Electrodeposited films annealed at 500 °C produced little to no photocurrent, irrespective of the film morphology (Figure 2S in the SI). This is often observed for solution processed hematite electrodes annealed below 500 °C,21,33 which is attributed to low crystallinity (Figure 3S) and poor solid-solid contact at the FTO-hematite interface.34 Annealing at 800 °C for 10 min resulted in dramatic improvement in the water oxidation performance. Electrodeposited hematite electrodes also showed significantly better water oxidation performance compared to ALD films, both annealed at 800 °C (Figure 2). The planar electrodeposited films in particular showed comparable photocurrent to the nanostructured electrodes, producing ~1 mA cm-2 at 1.23 V (Figure 5S), the


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highest reported value for a planar film to the best of our knowledge. The high photoactivity along with the simplicity and scalability of the a-ED thus make it an ideal method to deposit high performance hematite thin films on TCO substrates with variable morphologies.

Figure 2. Chopped light J-V curves of hematite electrodes prepared via ALD (orange), a-ED (dark red) and n-ED (green) in contact with 1 M KOH a) and with 0.5 M H2O2 b). Also shown is a photograph of the electrodes which produced the J-V responses. Additional plots are shown in the SI.

J-V responses were also measured in the presence of H2O2 as a hole scavenger (Figure 2b). Higher photocurrents were observed at low applied potentials in all cases compared to H2O oxidation J-V curves (Figure 4S). Assuming unity hole collection with the hole scavenger, the discrepancy in the J-V curves under H2O and H2O2 oxidation can be attributed to surface



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recombination. This effect is more prominent for ALD films as seen from its more positive H2O oxidation onset potential (Figure 4S). Electrodes were also modified with the Co-Pi water oxidation catalyst which is expected to enhance surface hole collection. Co-Pi modification produced ~100-150 mV cathodic shift in the current onset potential and an overall increase in the photocurrent at low applied potentials (Figure 5S) consistent with previous reports.13,14,17 The ALD films still suffer from an additional 200 mV positive onset compared to ED films. Thus, the ED electrodes exhibit better hole collection efficiencies compared to the ALD electrodes, which is indicative of superior surface properties (see below). In addition to a better photocurrent onset potential, a much higher photocurrent was measured for the planar a-ED films (~1 mA cm-2 at 1.23 V) compared to ALD films (~0.6 mA cm-2 at 1.23 V) in contact with the hole scavenger. This indicates a higher flux of holes reaching the electrode surface, given that the light harvesting efficiency of these electrodes were comparable (see Figure 6S). The larger photocurrent indicates a better charge separation for the ED films. The somewhat higher photocurrent of the nanostructured n-ED electrode compared to planar films can be attributed to the three-dimensional structure placing a large fraction of the bulk within the hole collection length. The feature size in the nanostructured hematite plays a key role in determining the overall charge separation efficiency, however it has proven difficult to control under the high annealing temperatures employed.7,33 The planar a-ED electrodes clearly exhibited superior charge separation and hole collection compared to the ALD films, despite having nominally the same morphology, thickness and light absorption. Further experiments were therefore conducted to elucidate the cause of the enhanced water oxidation performance of electrodeposited electrodes compared to ALD electrodes. The structural properties of hematite films were characterized by X-ray diffraction (XRD) and Raman


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spectroscopy. Plots of XRD patterns are shown in Figure 3. Several diffraction peaks were observed along [110], [104], [300], [012], [024] directions of the electrodeposited hematite films annealed at 800 °C. For the ALD hematite film only two peaks were resolved corresponding to [104] and [110] diffractions with significantly weaker intensities. Given that the thicknesses of ALD and a-ED films are comparable (56 ± 4 nm and 59 ± 2 nm were measured by spectroscopic ellipsometry for the ALD and a-ED films respectively), significantly sharper peaks in the XRD spectrum of the electrodeposited film indicate increased crystallinity. Sharper and more intense peaks were also observed in the Raman spectra of the ED films (Figure 7S), which is also consistent with increased crystallinity which has been shown to correlate with increased water splitting performance.29 The higher crystallinity of the ED films compared to ALD films can be explained in terms of the deposition conditions. ED results in amorphous FeOOH (Figure 2S) films which are converted to crystalline α-Fe2O3 through annealing at elevated temperatures. The initial amorphous nature of the films allows Fe and O atoms to diffuse easily during annealing and form larger crystallites. For the ALD made films, however, the as-deposited films are already somewhat crystalline and annealing at 800 °C results in a negligible effect on the diffraction peaks. This is consistent with a prior report that showed 800 °C annealing of hematite thin films prepared via ALD only affects surface states, and not the bulk properties.20 The crystallite sizes were calculated from the [110] diffraction peak using the Scherrer equation35 and found to be 27 and 43 nm for ALD and a-ED films, respectively (a ~200 nm thick ALD film was used for crystallite calculation as the film discussed herein showed a very weak diffraction peak). Larger crystallites are associated with a reduced density of grain boundaries which are known to reduce hole mobility and act as recombination centers.26,36 Grain boundaries strongly affect the hole mobility considering that hole transport is happening via small polaron hopping, i.e. the



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distances and type of the neighboring atoms could strongly modify hole conductivity.36 We therefore attribute the improvement in charge separation in the bulk to enhanced hole conductivity in the electrodeposited samples. Further, the higher crystallinity of the a-ED films can produce a reduced density of surface defects, resulting in decreased surface recombination and better photocurrent onset potential.

Figure 3. XRD spectra of hematite electrodes prepared via ALD (orange), a-ED (dark red) and n-ED (green) annealed at 800 °C.

Electronic and kinetic effects resulting from the increased crystallinity corresponding to the different preparation routes were investigated by performing EIS measurements. Mott-Shottky (M-S) plots of planar a-ED and ALD films measured in the dark are shown in Figure 4. Dopant densities were calculated by fitting the linear slope to the M-S equation37 using a dielectric constant of 32 for hematite and the geometric surface area of the electrodes.38 Nearly one order of magnitude higher carrier concentration was found for the a-ED film (7.35 × 1020 cm-3) compared to the ALD film (1.47 × 1020 cm-3). We note that such high carrier densities are consistent with degenerate doping levels. In such cases changes in applied potential changes the


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capacitance across the space charge layer as well as the Helmholtz layer. The slope of M-S plot however, is still given by the M-S equation and thus is a function of dopant density assuming a constant dielectric constant.39 The effect of Helmholtz capacitance shifts the intercept with the V axis by a constant value which should be accounted for when calculating the flat band potential from the M-S plots, however.39,40 This surprisingly large increased carrier concentration especially in the case of a-ED films results in sharper band-bending. The associated high drift potential thus enhances the hole transit time in the depletion region, which reduces depletion region and surface recombination.6,41 We note that the dopant density of the ALD film is somewhat higher than the previously reported values for hematite electrodes prepared by ALD which is due to the higher annealing temperature employed herein (800 °C) compared to the previous reports of electrodes annealed at 500 °C.24,42 The higher annealing temperature can increase the density of oxygen vacancies as well as Sn diffusion-doping from the FTO substrate, and thus the carrier concentration.33,43

Figure 4. Mott-Schottky plots of bare planar hematite electrodes prepared via ALD (orange circles) and a-ED (dark red triangles). EIS data were measured in dark and fitted to the Randles circuit model shown in the SI.



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One significant consequence of a higher dopant density, however, is the substantial reduction of the depletion width, W. Equation 1 is used to calculate W for the a-ED and ALD films at an applied bias of 1.23 V vs. RHE using the dopant densities determined from the M-S plots. Since, as noted above, the M-S intercept is not a reliable measurement of the flat band potential for such highly doped materials, the photocurrent onset potential in the presence of a hole scavenger was used as an estimate of the flat band potential, which we have previously shown to be in good agreement with the flat band potential determined from M-S plots of non-degenerately doped hematite electrodes.19,20 The flat band potential for both films was thus taken as 0.75 V for the calculation of the built-in potential drop, Vbi. At 1.23 V Vbi is therefore approximately 0.48 V for both films assuming no surface Fermi level pining for the electrodes in contact with hole scavenger. 2ߢε0 Vbi 1/2 ൰ W= ൬ qNd

(1)

The depletion widths were calculated to be 1.52 and 3.40 nm for a-ED and ALD films, respectively. A very small depletion width, especially in the case of the a-ED film, results from the high dopant density as expected. Note that a 0.1 V error in the estimate of Vbi corresponds to less than 10% error in W, and would not affect the interpretations below. Therefore, although a higher dopant density results in the sharper band bending which facilitates charge separation, it also reduces the depletion width significantly so that the number of the carriers generated inside of this layer is significantly lower. In order to gain a more quantitative measure of this effect, the absorptance of the depletion width (i.e. the light harvesting efficiency of the depletion layer for each electrode) was calculated using the absorptivity of hematite films, assuming no reflection losses.6 The absorptance of depletion


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widths is shown in Figure 5a for 1.52 and 3.40 nm of hematite, corresponding to the W of ED and ALD films, respectively. The APCE in the depletion layer was then calculated using the incident photon-to-current efficiency (IPCE) measured at 1.23 V (Figure 9S) and the calculated absorptances of W. Assuming zero hole diffusion length, quantitative charge collection in the depletion layer should produce a 100% APCE at all wavelengths. As seen in Figure 5b, the APCE of the ALD film depletion width reached a maximum of ~30 %. This indicates a hole diffusion length of zero and that the significant depletion region recombination limits the charge separation and overall efficiency of such electrodes. Strikingly different behavior was observed for the calculated APCE of the depletion region of the a-ED film, however, with over 100% values for photons > 400 nm. This indicates that not only the APCE of depletion width is 100% but there is a significant diffusion length of holes outside of the depletion region which contribute to the IPCE (scheme 1). Quantitative hole collection in the depletion region is not surprising given that the thickness is only 1.52 nm. Over 100% APCE implies that there is a nonzero hole diffusion length for photons shorter than 450 nm in contrast to the measurements on ALD films.25 Figure 5b indicates that the diffusion length is zero for holes resulting from the absorption of green photons (< 400 nm), however. This photon energy-dependent APCE and hole diffusion length appears to be general for hematite electrodes,3,17,23,24,44 which can be attributed to differences in hole mobility and reactivity of holes localized on Fe (shallow holes resulting from a d-d transition) in the valence band.23,24,44 This is a major loss in photocurrent for hematite as it covers a spectral region where the solar spectrum is most intense. Finally, we note that while the geometric surface area is a very good estimate for the uniform planer a-ED film, it is likely an underestimate for the ALD film which conforms to the FTO substrate morphology as depicted in figure 1. Considering the surface roughness of FTO is ~1.5, the resultant dopant



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density of the ALD film would be 6.53 × 1019 cm-3. This would produce a larger value for W, a concomitant larger absorptance for this region, and hence even smaller APCEs of W for the ALD films. The effect of error in determining W thus would magnify the difference in behavior depicted in figure 5, but not affect the overall interpretation of hole collection lengths discussed above.

Figure 5. a) Calculated absorptance of the depletion width for a-ED (dark red) and ALD (orange) thin films. b) APCE of the depletion width calculated from the IPCE data measured under 1 Sun illumination at 1.23 V vs. RHE. Not that the APCE values here does not reflect the overall quantum efficiency of the electrodes and are only a measure of charge collection efficiency in the depletion region.

Scheme 1. Schematic band bending diagram and charge transfer processes in hematite electrodes prepared by ALD and a-ED under PEC water oxidation. Red arrows indicate recombination.


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EIS measurements were also carried out under PEC water oxidation conditions in order to gain insight into the dynamics of charge carriers at the electrode surface. The impedance data were fit to an equivalent circuit model previously established for hematite-electrolyte interface under illumination (Figure 10S).45 The equivalent circuit model contained a chemical capacitance due to hole accumulation on the surface, surface state capacitance (Css), in addition hematite space charge capacitance (Cbulk). The model also includes resistances due to trapping in surface states, Rtrap, and hole transfer at the surface, Rct (i.e. charge transfer resistance). Plots of these parameters vs applied potential are shown in Figure 11S for a-ED and ALD electrodes. Cbulk is slightly higher for the a-ED electrode consistent with a higher dopant density as determined through the M-S analysis in dark. The striking differences were in the charge transfer resistances, however. As seen in Figure 11S (c), Rtrap is more than two orders of magnitude lower at potentials around the photocurrent onset for the a-ED hematite electrode compared to the ALD electrode. A lower hole trapping resistance would result in a higher density of surface holes at given potential. Note that the surface states here are interpreted as reaction intermediates as opposed to recombination centers.19,20 More than one order of magnitude lower charge transfer



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resistance was also observed for the a-ED electrode around the photocurrent onset potential, which is consistent with the early photocurrent onset due to the higher surface hole collection efficiency on these electrodes. It has been shown from a detailed kinetics analysis of PEC water oxidation reaction that the hole collection efficiency is proportional to the ratio Rtrap/Rct.46 The ratio of Rtrap/Rct thus can be used to get insight into the kinetics of water oxidation versus recombination at the hematite electrode surface. Plots of these values are shown in Figure 6. It can be seen that Rtrap/Rct plots resemble the general trend of the J-V curves shown in Figure 2a with the minimum points corresponding to the photocurrent onset potential of these electrodes. Lower Rct and earlier photocurrent onset of the a-ED electrode thus can be attributed to a more catalytically active hematite surface. This has been shown in recent works for preferentially oriented hematite electrodes along [110] plane.47,48 This also is consistent with calculation which showed [110] provides the most catalytically active surface for water oxidation.49 A more catalytically active hematite surface can be explained in terms of a higher fraction of [110] crystal orientation in the ED films, which would produce iron terminated hematite surfaces (–Fe– OH). Although the XRD data do not conclusively support a [110] preferred orientation, the higher crystallinity and more pronounced [110] diffraction peaks for the ED films indicates a larger fraction of crystals with such orientation and thus a larger fraction of –Fe–OH terminations.47 We note that –Fe–OH groups on the surface are considered as active sites crucial for the H2O oxidation reaction.19,20 We note a more catalytically active surface toward H2O oxidation is also supported by the early and sharp dark current onset potential for ED electrodes shown in Figure 2a. We note that the early water oxidation onset and lower Rct of the ED electrode can alternatively result from a slower surface recombination rate. Faster water oxidation kinetics is a more likely explanation, however, for the following reason. The onset


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potential of H2O oxidation is determined by band bending which reduces the surface electron concentration hence surface recombination. For a given applied potential, the potential drop in the electrode is the same for both films. The bulk carrier density of the ED film however, is higher which results in a higher surface electron density at a given applied potential, which would produce a faster surface recombination rate for the ED electrode compared to the ALD electrode, assuming a constant rate constant. Since in this case the recombination is occurring to the same species (water oxidation intermediate), such a dramatic change is unlikely.

Figure 6. Rtrap/Rct for ALD (orange circles) and a-ED (dark red triangles) thin film hematite electrodes.

Conclusion High performance hematite electrodes of variable morphology were prepared via a simple electrodeposition method. The overall water oxidation performance of the ED planar thin films reported here is significantly higher than the state of the art planar hematite electrodes prepared via ALD. We note that this result is not necessarily general to electrodeposition, and may be constrained to the specific methodology reported here. Nevertheless, the better performance of the ED films shown here can be attributed to enhanced charge separation and surface hole 19 Environment ACS Paragon Plus


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collection resulting from a highly doped crystalline hematite that can be fabricated via electrodeposition. These results partially explain the failure of previous attempts at utilizing the thin layer absorber approach to fabricate high performance photoelectrodes. For the ED films, however, over 100% APCE was achieved over most of hematite’s absorption spectrum in the depletion region, which implies a non-zero hole diffusion length at these wavelengths. In principle, therefore, electrodeposited hematite electrodes with an optimized hematite thickness corresponding to the depletion width + one hole diffusion length, i.e. ~ 3 nm, on high surface area transparent conducting substrates offers the possibility to finally push the photocurrent of hematite close to its maximum of 12 mA cm-2. Fabricating such a thin, uniform high quality thin film electrodes via electrodeposition is a challenging task which we are currently pursuing in order to achieve this goal.

ASSOCIATED CONTENT Supporting Information. Addition data including detailed experimental conditions, additional J-V curves, absorption measurements, and EIS equivalent circuits can be found in the supporting information (SI). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions


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OZ and TWH designed the experiments. OZ, AS and HH carried out the measurement. OZ and TWH wrote the manuscript. All authors have given approval to the final version of the manuscript. Funding Sources TWH thanks the National Science Foundation (CHE-1150378) for the support of this research.

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Enhanced Charge Separation and Collection in High-Performance

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