Letter pubs.acs.org/NanoLett
Tailoring Charge Recombination in Photoelectrodes Using Oxide Nanostructures Beniamino Iandolo,†,‡ Björn Wickman,† Elin Svensson,† Daniel Paulsson,† and Anders Hellman*,† †
Department of Physics, Chalmers University of Technology, 412 96 Göteborg, Sweden Center for Electron Nanoscopy, Technical University of Denmark, 2800 Kongens Lyngby, Denmark
‡
S Supporting Information *
ABSTRACT: Optimizing semiconductor devices for solar energy conversion requires an explicit control of the recombination of photogenerated electron−hole pairs. Here we show how the recombination of charge carriers can be controlled in semiconductor thin films by surface patterning with oxide nanodisks. The control mechanism relies on the formation of dipole-like electric fields at the interface that, depending on the field direction, attract or repel minority carriers from underneath the disks. The charge recombination rate can be controlled through the choice of oxide material and the surface coverage of nanodisks. We provide proof-of-principle demonstration of this approach by patterning the surface of Fe2O3, one of the most studied semiconductors for light-driven water splitting, with TiO2 and Cu2O nanodisks. We expect this method to be generally applicable to a range of semiconductor-based solar energy conversion devices. KEYWORDS: Energy conversion, charge recombination, hematite, photoelectrodes, water splitting
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semiconductor, and recombination of the photogenerated charges inside the electrode. Transition metal oxides suffer heavily from the latter drawback, as compared for instance to III−V semiconductors. Thus, there is a need to develop methods and designs to deterministically, and spatially, reduce charge recombination in oxides. For example, Fe2O3 is a very promising candidate for the PEC half-cell reaction leading to O2 evolution, being an abundant n-type semiconductor that shows excellent stability in neutral and alkaline electrolytes and is able to absorb a considerable portion of the solar spectrum thanks to its bandgap of ∼2.0 eV.13−15 It is not surprising, therefore, that considerable effort has been made to alleviate the poor charge transport and slow O2 kinetics that characterize Fe2O3.16−18 Here, we focus on the charge transport issue and report a method to control the recombination of photogenerated charges in Fe2O3 by surface patterning with nanodisks of an additional oxide. In particular, the recombination is regulated
o utilize the potential of solar energy to replace fossil fuels, which are the main drivers of climate change,1,2 it requires efficient technologies to capture sunlight but also to store and release the energy on demand. To this end, photosynthetic (PS) devices for conversion and storage of sunlight into fuels3 have attracted high interest.4 One particularly promising type of PS devices relies on hydrogen generation via water splitting,5,6 given the abundance of water and that the reverse reaction can release the chemical energy in a CO2 neutral way, e.g., with the help of fuel cell technology. Research on monolithic devices for photoelectrochemical (PEC) water splitting (i.e., devices in which electrodes are immersed in aqueous electrolyte and at least one of them is photoactive) has intensified over the last two decades.7,8 However, selecting suitable material(s) for PEC water splitting is an intricate dilemma9,10 because materials with high solar-tohydrogen conversion efficiency are typically not stable in aqueous environment and/or are scarce,11 whereas stable and abundant materials usually exhibit poor performance.12 Limiting factors include: large optical bandgap that inhibits absorption of the photon-richest part of the solar spectrum, slow reaction kinetics, small photovoltage sustained by the © XXXX American Chemical Society
Received: December 17, 2015 Revised: February 27, 2016
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DOI: 10.1021/acs.nanolett.5b05154 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters by dipole-like electric fields that form thanks to charge transfer at the surface between the two oxides,19 as schematically shown in Figure 1a. The direction of the charge transfer, and as a
measured as a function of electrochemical potential, E, determines the activity toward the oxygen evolution reaction (OER). jphoto is directly proportional to the amount of O2 produced, provided that the Faradaic efficiency of the OER is unity; i.e., that no other products are evolved or electrode corrosion takes place.23 We note that none of the photoanodes tested here showed sign of (photo)corrosion over a total PEC characterization time of at least 4 h per sample. Furthermore, we have previously measured the amount of O2 produced on our Fe2O3 and Fe2O3 + TiO2 photoanodes and found the Faradaic efficiency to be unity.24 As shown in Figure 2, the OER is initiated at a potential, Eonset, of 1.2 V vs RHE for all samples. Reference Fe2O3
Figure 2. Steady state photocurrent density, jphoto, as a function of applied potential E and under AM 1.5 solar simulated illumination. Inset: top-view scanning electron microscope (SEM) image of a Fe2O3 + TiO2 disks photoanode (disk nominal average diameter: 40 nm; disk thickness: 5 nm; surface coverage: 0.1). The scale bar is 200 nm.
Figure 1. (a) Left: as result of introducing TiO2 nanodisks on top of Fe2O3, charge redistribution creates an additional dipole-induced electric field that repels holes from underneath the nanodisks. Right: as result of introducing Cu2O nanodisks, the additional dipole-induced electric field attracts holes underneath the nanodisks. (b) Position of the bottom of the conduction band and the top of the valence band for TiO2, Fe2O3, and Cu2O, at pH 13. The electrochemical potentials at standard conditions for the hydrogen and oxygen evolution reactions are marked with dashed lines.
electrodes yield a maximum anodic photocurrent of about 0.1 mA cm−2, which is commensurate with previous results on flat, polycrystalline Fe2O3 films where the electron conductivity is determined by the concentration of oxygen vacancies.25−28 Such photocurrent per geometric unit area is indeed more than an order of magnitude lower than what achieved on nanostructured, Si-doped champion photoanodes characterized by a much larger solid−liquid contact area.29 Such a performance difference is partly due to the fact that our samples are much flatter and therefore have a much smaller Fe2O3/electrolyte contact area. More importantly, we stress that we focus here on only one issue affecting the performance of a photoelectrode, namely, recombination of photogenerated charge carriers before reaching the solid/liquid interface, rather than on the total energy conversion efficiency of a PEC device. Our samples are an excellent model system given the focus of this work, thanks to a less complicated interpretation of PEC datain particular that from electrochemical impedance spectroscopy (EIS), as compared to that obtained on nanostructured photoelectrodes.30 The Fe2O3 + TiO2 disks outperforms the other photoanodes, with an increase in jphoto of about 3.8 times for E = 1.45 VRHE and about 4.5 times at E = 1.65 VRHE compared to reference sample consisting of a bare Fe2O3 film. We obtained an almost identical performance by replacing TiO2 with SiO2 disks (see Figure S3), and we notice that the top of the valence band in SiO2 lies at a lower energy than the one of Fe2O3, which is also
consequence of the dipole-like field, is determined by the relative Fermi level of the involved oxides; however, for undoped to weakly doped oxides it can be deduced by the relative position between the top of the valence band in Fe2O3 and in the additional oxide (see Figure S1 in the Supporting Information, SI, where we describe the limitations of the use of the “top of the valence band” as compared to the one of the relative Fermi level of the oxides). In our proof-of-principle demonstration, we chose TiO2 to achieve a dipole orientation that repels holes from underneath the nanodisks, since the top of the valence band in TiO2 is lower in energy than the one of Fe2O3 (see Figure 1b for details).9 We employed Cu2O, whose top of the valence band is higher in energy than the one of Fe2O3,5 to obtain a dipole orientation that attracts holes to the bottom of the nanodisks. We prepared thin-film photoanodes consisting of polycrystalline, 25 nm thick Fe2O3 films on top of indium−tin oxide (ITO) coated glass, a model system well-characterized in previous work.20,21 Then we deposited disks of various oxides on top of the Fe2O3 films by means of hole-mask colloidal lithography.22 The disks have an average diameter of 40 nm and a thickness of 5 nm, and they cover approximately 10% of the surface of Fe2O3. The steady-state photocurrent density, jphoto, B
DOI: 10.1021/acs.nanolett.5b05154 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters the case for TiO2. In other words, the photocurrent enhancement mechanism is not restricted to the Fe2O3:TiO2 system. On the other hand, introducing Cu2O disks reduces the photocurrent by about 3.2 times for E = 1.45 VRHE and about 2.4 times at E = 1.65 VRHE compared to the bare Fe2O3 film. Finally, the addition of Fe2O3 disks has a negligible effect on jphoto, as the latter is merely 8% larger than for the bare Fe2O3 film, proving that neither the film texturing, nor the fabrication procedure have an appreciable influence on the photoactivity. To gain deeper understanding of the observed variation in performance, we break jphoto into the product31 jabs × ηtr × ηct. jabs is the maximum photocurrent density in the ideal case of no losses; ηtr is the fraction of photogenerated holes that reach the solid/liquid interface; and ηct is the fraction of holes that, once they reach the interface, are consumed in the O2 evolution. jabs is determined by the optical absorption in the semiconductor, A, ηtr by the charge transport, and ηct by the catalytic activity toward O2 evolution. In this framework, an increase of jphoto can therefore originate from (i) an increase of A, and thus of jabs; (ii) a decrease of electron−hole recombination leading to enhanced ηtr; and/or (iii) a decrease of surface recombination or an increase of the reaction rate, leading to enhanced ηct. It is clear from Figure 3a that A is practically identical for all samples. We rationalize the fact that the optical spectra are dominated by the absorption in Fe2O3 by recalling that the amount of material added is much smaller (the thickness of the nanodisks is 20% of the Fe2O3 film, and they cover around 10% of the surface, resulting in a total volume increase of around 2%). To extract ηtr, we measured the photocurrent in the presence of a hole scavenger, i.e. a molecule or ion whose oxidation potential is much lower than that of H2O, and whose oxidation kinetics is much faster than the one of H2O,31,32 so that ηct can be considered unity. j−E plots for electrolytes containing H2O2 as hole scavenger are shown in Figure S5−S6. Setting ηct = ηctH2O2 = 1 we obtain jphotoH2O2 = jabs × ηtr, from which we obtained ηtr. As shown in Figure 3b, the charge transport inside Fe2O3 is indeed affected by the surface patterning and in excellent agreement with the photoactivity measurements. Then, we obtained ηct for the OER by taking the ratio between the photocurrent measured without and with the hole scavenger: ηct = jphoto/jphotoH2O2. For all samples, ηct is practically equal to zero when E is more cathodic than Eonset, and then it rises steeply to about 0.8 at 1.55 VRHE (see Figure 3c). Measuring the photocurrent density in the presence of a hole scavenger enabled us to discriminate between losses occurring inside the Fe2O3 film and those occurring at the semiconductor/electrolyte interface. In particular, the additional oxides dramatically affect the charge transport inside Fe2O3, and more specifically the amount of holes that are able to reach the solid/liquid junction, as revealed by electrochemical impedance spectroscopy (EIS). The key to obtain useful information from this technique is the choice of the lumped elements equivalent circuit used to fit the impedance spectra.33−35 Examples of these spectra are shown in Figure S7 in the form of Nyquist plots. Under illumination and for potential increasingly more anodic, the Nyquist plot first shows two semicircles, one of small and one of larger radius, associated with high and with low frequency processes, respectively. The large semicircle shrinks in size with increasing E until it disappears for very anodic E. In order to fit the Nyquist plots for E < 1.6 VRHE, we employed the Hamann’s equivalent-circuit, first discussed for Fe2O3 based photoanode by Klahr and co-workers,36 which has
Figure 3. Efficiency of photoelectrochemical processes in Fe2O3 based photoanodes. (a) Optical absorption, A, as a function of photon wavelength, λ. (b) Charge transport efficiency, ηtr, within the photoanode as a function of E. The Fe2O3 + TiO2 disks photoanode shows higher ηtr than the bare Fe2O3 sample and the Fe2O3 + Fe2O3 control sample. For the Fe2O3 + Cu2O photoanode, the situation is reversed, and ntr is lower than the reference samples. The trend is in quantitative agreement with that of the photocurrent related to O2 evolution. (c) Semiconductor/electrolyte charge transfer efficiency, ηct, as a function of E. It is clear that patterning the surface with the nanodisks does not affect ηct, which rises from 0 to about 0.8 of higher within 0.3 V from the photocurrent onset potential, Eonset.
proven to accurately describe the O2 evolution related photoelectrochemistry of our photoanodes within the relevant potential range. The Hamann’s equivalent circuit is shown in Figure S8a, and its components are the series resistance, RS, between ITO and Fe2O3, the capacitance of the space-charge layer, Cbulk, the charge recombination resistance, Rtrap, the capacitance associated with charges localized on surface states, CSS, and the charge transfer resistance across the Fe2O3/H2O interface, RSS. For E ≥ 1.6 VRHE, we have shown37 that the OER takes place via direct transfer of holes from the valence band. In this case it is appropriate to use a Randle’s circuit to fit the results (Figure S8b), with Cbulk as only capacitance in the system and with Rtrap and RSS lumped in one resistance. While the ITO-Fe2O3 contact resistance, RS, is practically constant (Figure 4a), the bulk electron−hole recombination resistance Rtrap is lower for Fe2O3 + TiO2 and higher for the Fe2O3 + Cu2O as compared with the Fe2O3 + Fe2O3 nanodisks (Figure 4b), respectively. This is the same qualitative trend as for the charge transport efficiency ηtr (i.e., reducing and C
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Figure 4. Results from electrochemical impedance spectroscopy (EIS) characterization. Panels a−e show the results of fitting Nyquist plots obtained under illumination with the equivalent circuit containing surface states at the Fe2O3/electrolyte interface. Solid lines are a guide to the eye. (a) The ITO-Fe2O3 contact resistance, RS, is practically constant for all three samples and independent of the applied potential. (b) The bulk transport resistance, Rtr, is lower for the sample with TiO2 disks and higher for the sample with Cu2O disks, as compared with the control Fe2O3 + Fe2O3 sample, in agreement with the trend showed by the charge transport efficiency ηtr. (c) The Fe2O3-electrolyte charge transfer resistance, Rct decreases for all samples for more anodic E, until it reaches a dip around Eonset and then levels off. (d) Space-charge-layer capacitance measured under illumination, Cbulk,light. (e) The surface state capacitance, CSS is higher for the sample with TiO2 disks and lower for the sample with Cu2O disks, as compared with the Fe2O3 + Fe2O3 sample, in agreement with the trend shown in the photocurrent measurements. (f) Mott−Schottky plots obtained from measurement of Cbulk in the dark. The almost perfect overlap between data for Fe2O3 + TiO2 and Fe2O3 + Fe2O3 confirms that the concentration of majority charge carriers (i.e., electrons) is unaffected by the presence of the TiO2 disks, in other words: significant Ti doping into Fe2O3 can be excluded.
Figure 5. Measurements on photoanodes with varying surface coverage of TiO2 nanodisks, θ. (a) Photocurrent density jphoto as a function of E for θ = 0 (back circles), θ = 0.01 (blue squares), θ = 0.05 (light blue upward triangles), θ = 0.1 (green diamonds), θ = 0.14 (orange downward triangles), θ = 0.21 (magenta rectangular triangles), and θ = 1 (fully covering TiO2 film on Fe2O3, no symbols). We note that the fully covered photoelectrode shows negligible photoactivity. (b) jphoto as a function of θ at 1.45 VRHE and 1.65 VRHE. The solid lines are a guide to the eye. jphoto first increases for increasing θ, then reaches a maximum around 0.1 coverage, and then decreases with further increase in θ, until it drops back at almost the same performance as the control sample without nanodisks.
reaches a peak for E slightly more anodic than Eonset and then decreases (Figure 4e), indicating that the surface states are charged and then discharged with holes before and after OER is initiated, respectively. We determined the total charge density on the surface states, QSS, by integrating CSS with respect to E. We obtained QSS values of 62.96 μC cm−2, 19.04 μC cm−2, and 4.72 μC cm−2 for Fe2O3 + TiO2, Fe2O3 + Fe2O3, and Fe2O3 + Cu2O, respectively, in good agreement with the behavior of ηtr. Furthermore, by measuring the bulk capacitance in the dark, we performed Mott−Schottky (MS) analysis, where Cbulk−2 is plotted against E (for further details, see the SI). The MS plots
enhancing charge recombination by introducing TiO2 and Cu2O nanodisks, respectively). At the same time, we found only negligible difference in the Fe2O3-electrolyte charge transfer resistance, RSS (Figure 4c), which is also consistent with the previous determination of ηct. The capacitance of the space-charge layer, Cbulk, is also very similar for all samples (Figure 4d), with highest values approaching 1 μC cm−2. This is about 20 times smaller than the typical capacitance of the Helmoltz layer in the electrolyte, CH, which justifies a posteriori the choice of neglecting CH in the equivalent circuit. For all samples, the surface states capacitance, CSS, first increases, D
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of TiO2, Fe2O3, Cu2O, and SiO2 were deposited on top of the Fe2O3 films by hole-mask colloidal lithography22 (further details are given in the SI). The average disk diameter was 40 nm, the thickness 5 nm, and surface coverage was ranging from 0.05 to 0.01 depending on the sample). SEM of the as-prepared samples was performed in a Supra 60 VP microscope (Zeiss), at an acceleration voltage of 5 kV. The optical absorption in the photoanodes was measured with a Cary 5000 spectrophotometer (Varian) equipped with the external diffuse reflectance accessory (DRA) 2500. A blank ITO substrate was used as reference, after being exposed to the same thermal treatment as the hematite covered substrates. XPS spectra were acquired in a PerkinElmer PHI 5000C ESCA system (base pressure 1.1 VRHE, in order to avoid corrosion of the Cu2O nanodisks as predicted by the Cu Pourbaix diagram. Cyclic voltammetry was performed using a Gamry Ref600 potentiostat, first in the dark for at least 10 cycles at a scan rate of 50 mV s−1, in order to remove organic contaminants from the surface, and then at a scan rate of 10 mV s−1, both in the dark and under illumination. The measured current was divided by the geometric area of the electrode to obtain the current density j. The forward scans corresponding to the third cycle are plotted without resistance compensation. Steady state photocurrent density was obtained by chronoamperometry, where the voltage was changes in steps of 50 mV from more cathodic to more anodic potentials. After starting illumination, the photocurrent typically stabilized within 20 s, and the current was averaged over the last 30 s (of about total 90) of the measurement. The maximum photocurrent in the presence of no losses in the system, jabs, was obtained using the optical absorption of Fe2O3, A, and the spectral emission of the solar simulator, according to a standard procedure available in the literature.43 For EIS characterization, AC voltage of root-meansquare amplitude of 10 mV and frequency varying between 105 Hz and 0.1 Hz was superimposed to the DC bias. Nyquist plots obtained under illumination were fitted using the software Echem Analyst (Gamry). MS analysis was performed in dark and under illumination, in the same potential window with AC voltage of 104 Hz. Here, the DC voltage was swept in steps of 50 mV from more anodic to more cathodic potentials to avoid polarization effects.
shown in Figure 4f are linear between 0.6 and 1.2 VRHE. The nonlinearity observed for more anodic potentials is due to the fact that the space-charge layer extends throughout the Fe2O3 and then partly into the underlying ITO layer.21,38 By fitting the linear region of the MS plot, we found that neither the donor density ND, nor the flat band potential Efb is affected by the presence of the nanodisks, which confirms that neither the electronic conductivity, nor the position of the conduction band edge is modified. This allows us to rule out surface doping (either with Ti or by modifying the density of oxygen vacancies) as mechanism of increased charge transport efficiency. Finally, from measurements on samples with surface coverages of TiO2 nanodisks, θ, ranging from 0 to 0.21 (see Figure 5), we found that jphoto reaches a maximum for θ ∼ 0.1, then decreases until it almost reaches the value of jphoto for the reference sample for θ = 0.21. Comparison of jphoto in the presence and in the absence of H2O2 as hole scavenger reveals an excellent agreement between O2 evolution activity and charge transport efficiency (see Figure S10). Further work is needed to understand the performance decrease for θ > 0.1, which may for instance be related to a detrimental influence of increasing surface coverage on transport of electrons. In any case, Fe2O3 coated by a 5 nm TiO2 film (θ = 1) showed negligible photocurrent, confirming that O2 evolution takes place only on the Fe2O3 in our system (we recall that the TiO2 obtained by oxidation of Ti at ambient temperature, as in our case, leads to an amorphous structure,12 which is inactive toward water oxidation). In conclusion, we reported a method to control the recombination of photogenerated charge carriers in a thinfilm photoelectrode by patterning its surface patterning with nanodisks of an additional oxide. In our proof-of-principle demonstration, we showed that charge recombination in a weakly doped n-type semiconductor such as Fe2O3 is reduced when the top of the valence band in the additional oxide lies at lower energy than that in Fe2O3, and vice versa. This method is general, since it only relies on introducing interfaces between materials having different positions of the top of their valence band, and thus could be used to tune charge recombination in a broad range of semiconductor-based solar energy conversion devices. We envision that this approach may be combined with other strategies to improve charge collection in Fe2O3-based photoanodes, for instance doping with higher valence atoms to increase the electronic conductivity.29,39 Moreover, we consider it to be a promising complement to nanostructuring Fe2O3, which is an established strategy toward photocurrent enhancement,29,40,41 in photoelectrode architectures based on stacked thin films.42 To this respect, the effect of oxide patterning on the performance of photoanodes whose performance is closer to state-of-the-art needs to be investigated. Finally, the approach introduced here relies only on the relative position of the top of the valence band in the oxide with respect to that of Fe2O3. Therefore, we expect this strategy to be of interest for tuning charge recombination in most photoelectrodes, and not to be restricted to n-type semiconductors but to be applicable to p-type semiconductors used for light-driven reduction half reactions. Experimental Section. Photoelectrodes were fabricated on top of ITO coated glass substrates. The thickness of the iron oxide films was measured using a surface profiler (Dektak 150, Veeco) and found to be 25 ± 1.9 nm. Fe2O3 was the only iron oxide phase detected by XRD in a previous work.20 Nanodisks E
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(23) Dotan, H.; Mathews, N.; Hisatomi, T.; Grätzel, M.; Rothschild, A. J. Phys. Chem. Lett. 2014, 5, 3330−3334. (24) Iandolo, B.; Wickman, B.; Seger, B.; Chorkendorff, I.; Zorić, I.; Hellman, A. Phys. Chem. Chem. Phys. 2014, 16, 1271−1275. (25) Hiralal, P.; Saremi-Yarahmadi, S.; Bayer, B. C.; Wang, H.; Hofmann, S.; Upul Wijayantha, K. G.; Amaratunga, G. a. J. Sol. Energy Mater. Sol. Cells 2011, 95, 1819−1825. (26) Kumari, S.; Singh, A. P.; Deva, D.; Shrivastav, R.; Dass, S.; Satsangi, V. R. Int. J. Hydrogen Energy 2010, 35, 3985−3990. (27) Singh, A. P.; Mettenbörger, A.; Golus, P.; Mathur, S. Int. J. Hydrogen Energy 2012, 37, 13983−13988. (28) Franking, R.; Li, L.; Lukowski, M. A.; Meng, F.; Tan, Y.; Hamers, R. J.; Jin, S. Energy Environ. Sci. 2013, 6, 500−512. (29) Kay, A.; Cesar, I.; Graetzel, M. J. Am. Chem. Soc. 2006, 128, 15714−15721. (30) Photoelectrochemical Hydrogen Production; van de Krol, R., Grätzel, M., Eds.; Electronic Materials: Science & Technology; Springer US: Boston, MA, 2012; Vol. 102. (31) Dotan, H.; Sivula, K.; Grätzel, M.; Rothschild, A.; Warren, S. C. Energy Environ. Sci. 2011, 4, 958−964. (32) Klahr, B. M.; Hamann, T. W. J. Phys. Chem. C 2011, 115, 8393− 8399. (33) Vanmaekelbergh, D.; Cardon, F. J. Phys. D: Appl. Phys. 1986, 19, 643−656. (34) Leng, W. H.; Zhang, Z.; Zhang, J. Q.; Cao, C. N. J. Phys. Chem. B 2005, 109, 15008−15023. (35) Bisquert, J. J. Electroanal. Chem. 2010, 646, 43−51. (36) Klahr, B.; Gimenez, S.; Fabregat-Santiago, F.; Hamann, T.; Bisquert, J. J. Am. Chem. Soc. 2012, 134, 4294−4302. (37) Iandolo, B.; Hellman, A. Angew. Chem., Int. Ed. 2014, 53, 13404. (38) van de Krol, R. J. Electrochem. Soc. 1997, 144, 1723−1727. (39) Ling, Y.; Wang, G.; Wheeler, D. A.; Zhang, J. Z.; Li, Y. Nano Lett. 2011, 11, 2119−2125. (40) Lin, Y.; Zhou, S.; Sheehan, S. W.; Wang, D. J. Am. Chem. Soc. 2011, 133, 2398−2401. (41) Warren, S. C.; Voïtchovsky, K.; Dotan, H.; Leroy, C. M.; Cornuz, M.; Stellacci, F.; Hébert, C.; Rothschild, A.; Grätzel, M. Nat. Mater. 2013, 12, 842−849. (42) Dotan, H.; Kfir, O.; Sharlin, E.; Blank, O.; Gross, M.; Dumchin, I.; Ankonina, G.; Rothschild, A. Nat. Mater. 2012, 12, 158−164. (43) Murphy, A.; Barnes, P.; Randeniya, L.; Plumb, I.; Grey, I.; Horne, M.; Glasscock, J. Int. J. Hydrogen Energy 2006, 31, 1999−2017.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b05154. Further details on preparation of photoelectrodes, detailed band diagram of investigated oxides, XPS measurements, cyclic voltammetry measurements in the presence of hole scavenger, Nyquist plots and equivalent circuit for analysis of EIS measurements, and SEM images of samples with varying surface coverage of nanodisks (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Igor Zoriċ for valuable discussions. B.I. and A.H. thank the Swedish Research Council, Formas (project numbers 219-2011-959 and 229-2009-772), and the Chalmers Area of Advance Material and Energy for financial support. B.W. thanks Formas (project number 219-2011-959).
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REFERENCES
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