Letter pubs.acs.org/NanoLett
Solution Transformation of Cu2O into CuInS2 for Solar Water Splitting Jingshan Luo,*,† S. David Tilley,† Ludmilla Steier,† Marcel Schreier,† Matthew T. Mayer,† Hong Jin Fan,‡ and Michael Graẗ zel*,† †
Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland ‡ Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University (NTU), 637371 Singapore S Supporting Information *
ABSTRACT: Though Cu2O has demonstrated high performance as a photocathode for solar water splitting, its band gap is too large for efficient use as the bottom cell in tandem configurations. Accordingly, copper chalcopyrites have recently attracted much attention for solar water splitting due to their smaller and tunable band gaps. However, their fabrication is mainly based on vacuum evaporation, which is an expensive and energy consuming process. Here, we have developed a novel and low-cost solution fabrication method, and CuInS2 was chosen as a model material due to its smaller band gap compared to Cu2O and relatively simple composition. The nanostructured CuInS2 electrodes were synthesized at low temperature in crystalline form by solvothermal treatment of electrochemically deposited Cu2O films. Following the coating of overlayers and decoration with Pt catalyst, the as-fabricated CuInS2 electrode demonstrated water splitting photocurrents of 3.5 mA cm−2 under simulated solar illumination. To the best of our knowledge, this is the highest performance yet reported for a solution-processed copper chalcopyrite electrode for solar water splitting. Furthermore, the electrode showed good stability and had a broad incident photon-to-current efficiency (IPCE) response to wavelengths beyond 800 nm, consistent with the smaller bandgap of this material. KEYWORDS: Solution transformation, Cu2O, CuInS2, solar water splitting
S
strated as a highly efficient photocathode for water splitting.11,12 However, its band gap is too large to be used as the bottom electrode in a stacked tandem device, as the top electrode must have an even larger bandgap, which results in low overall efficiency due to inefficient light harvesting. Besides Cu2O, copper is a component of various other p-type semiconductors, most commonly copper chalcopyrites, with CuIn1−xGaxSe2 known for photovoltaic applications.13,14 Recently, copper chalcopyrites have received great attention as photocathodes for water splitting, mainly due to their relatively low costs and band gaps that are widely tunable through varying the composition of the materials.15−23 Until now, the main fabrication method of copper chalcopyrites is vacuum evaporation, which is an expensive and energy consuming process.20 Thus, low-cost solution processes are highly demanded for reducing the expense of synthesis. Currently, there are mainly two methods to do this, direct electrochemical film deposition and film formation by drop-casting the presynthesized nanoparticles or nanoinks.24−27 Though solution process synthesis of chalcopyrites has been investigated
olar energy is the green and renewable energy source that could provide sufficient energy to power humanity’s needs.1 However, its intermittent nature necessitates an efficient method of storage. Hydrogen fuel generation by photoelectrochemical (PEC) water splitting has been considered as one of the most promising methods to store solar energy.2 To split water, a thermodynamic potential of 1.23 V is required. However, due to the overpotential losses associated with the reaction kinetics, usually 1.7 V or more is needed for practical rates to be achieved.3 Requiring a large band gap to provide enough photovoltage while keeping the band gap small enough to absorb sufficient sunlight is a dilemma for a singleabsorber water-splitting device. To solve this problem, tandem configurations have been proposed that use multiple absorbers that cover a larger portion of the solar spectrum and together generate enough photovoltage to drive the overall water splitting reaction.4−6 Usually, two absorbers are used for the tandem cell with the n-type semiconductor serving as the photoanode and the p-type semiconductor as the cathode. To build a low cost tandem cell, many metal oxides can be chosen as the photoanode, such as TiO2, WO3, BiVO4, and Fe2O3,6−10 while there are few corresponding low cost photocathodes.11 Copper is an Earth-abundant and relatively inexpensive material, and recently Cu2O has been demon© XXXX American Chemical Society
Received: December 10, 2014 Revised: January 9, 2015
A
DOI: 10.1021/nl504746b Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. XRD and optical characterizations of the Cu2O film before and after transformation into CuInS2. (a) XRD patterns of Cu2O and CuInS2 films. (b) Photos of Cu2O and CuInS2 films. (c) Diffuse reflectance spectra of the Cu2O and CuInS2 films. (d) Absorption spectra of the Cu2O and CuInS2 films by transforming the diffuse reflectance spectra according to Kubelka−Munk equation.
deposited to a thickness of around 500 nm. To transform Cu2O into CuInS2, the films were solvothermally treated in an autoclave containing InCl3 and thioacetamide in ethylene glycol solution. During the treatment, thioacetamide reacts with water in the heated solution, which releases H2S, (CH3CSNH2 + H2O → CH3CONH2 + H2S). Then H2S decomposes, providing S2− and H+ ions, (H2S → 2H+ + S2−). The resulted H+ ions attack Cu2O and make it slowly dissolve and release Cu+ ions, (Cu2O + 2H+ → 2Cu+ + H2O). Finally, Cu+ ions react with In3+ and S2− ions in the solution, forming CuInS2 in situ (Cu+ + In3+ + 2S2− → CuInS2).29,31,43 The reaction results in vertically aligned nanosheet arrays of CuInS2 on the substrate, a morphology that may be favorable for light absorption and charge transport, potentially yielding improved device performance. The transformation of the Cu2O into CuInS2 was first confirmed by X-ray diffraction (XRD), as shown in Figure 1a. The as-deposited Cu2O film had a cubic crystal structure corresponding to cuprite (JCPDS card No. 05-0667). After transformation, the dominant peaks of Cu2O at 36 and 42° (corresponding to lattice planes (111) and (200)) disappeared entirely while peaks for CuInS2 emerged at 28, 47, and 55° (planes (112), (220), and (132), respectively), indicating the complete conversion of Cu2O into CuInS2. The XRD patterns also showed that the resulting CuInS2 had a tetragonal crystal structure (JCPDS card No. 47-1372). Because of the bandgap difference between Cu2O and CuInS2, the color of the samples also distinctly changed as a result of the transformation, as shown in Figure 1b. Absorption spectra (Figure 1d) calculated from the diffuse reflectance data (Figure 1c) according to the Kubelka−Munk theory showed absorption up to wavelengths of 820 nm for the CuInS2 after transformation from the Cu2O, which absorbs only up to 620 nm. The broader light absorption
extensively previously, the efficiency still lags far behind the vacuum evaporation method. For the nanoink approach, the charge recombination at the boundaries of the nanocrystals inhibits the improvement of the efficiency. The problem could be addressed by direct growth of the chalcopyrite film on the substrate by electrochemical deposition. However, it is very challenging to control the stoichiometry and composition of the films with this approach.24 Other approaches have also been tried, such as direct solvothermal synthesis and ZnO template growth.28−31 However, no appreciable photoelectrochemical performance has been achieved. The overall low performance of the solution-processed copper chalcopyrite films demands a new preparation method. Here, we develop a new solution-based method for fabricating copper chalcopyrite films, and CuInS2 was chosen as the model material due to its narrower bandgap compared to Cu2O and relatively simple composition. Instead of forming CuInS2 film directly, we devised an indirect strategy by transformation of electrochemically deposited Cu2O films, which are facile to prepare in simple aqueous solutions at low temperature. The transformation reaction concept encompasses a broad range of ideas for fabricating target materials indirectly from other materials, such as by dissolution−precipitation,32−34 ion exchange,35−37 galvanic replacement,38 or other reactions,39,40 and it has been widely used for material synthesis for energy applications including solar cells and energy storage. Here we broaden this concept to solar water splitting by transforming Cu2O into CuInS2, resulting in an efficient solution-processed copper chalcopyrite photocathode. Electrochemical deposition of Cu2O film has been studied for decades and represents a facile and mature technique.41,42 The electrolyte commonly used is a basic solution of lactate stabilized copper sulfate. The Cu2O films used here were B
DOI: 10.1021/nl504746b Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 2. SEM characterizations of the Cu2O film before and after transformation into CuInS2. (a) Top view and (b) cross-section view SEM images of the electrochemically deposited Cu2O film on Mo coated FTO substrate. (c) Top view and (d) cross-section view SEM images of the CuInS2 film following the transformation reaction.
Figure 3. TEM and EDX characterizations of the CuInS2 nanosheets. (a) Low-magnification TEM image. (b) High-resolution TEM image and (c) SAED pattern of a single CuInS2 nanosheet. (d−f) Element mapping of Cu, In, and S, respectively, for a single CuInS2 nanosheet by EDX.
C
DOI: 10.1021/nl504746b Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 4. SEM characterizations of the CuInS2 film after overlayer coating. (a) Front view and (b) cross-section view SEM images of CuInS2 after CdS coating. (c) Front view and (d) cross-section view SEM images of CuInS2 film after CdS, AZO, and TiO2 coating.
fabricated CuInS2 films were measured by linear sweep voltammetry under chopped simulated AM 1.5G illumination (see below), showing a photocurrent of around 100 μA cm−2 at higher bias potential, which was comparable or superior to the other reports using similar photocathodes.24,26,44,45 The lowperformance of pure CuInS2 electrode is mainly due to the poor charge separation at the electrode and electrolyte interface and the slow reaction kinetics, as neither overlayers nor catalyst were used here. It should be noted that the photocurrents in this unprotected state likely correspond to photocorrosion. In order to enhance the performance and stability of the material, a thin layer of CdS was coated on the surface by chemical bath deposition (CBD) followed by Al-doped ZnO (AZO) and TiO2 layers deposited by atomic layer deposition (ALD). A generalized band energy position for each layer is provided in the Supporting Information (Figure S1). From the SEM and TEM images in Figure 4 and 5, it can be seen that the CdS, AZO, and TiO2 layers conformally coated on the surface of the CuInS2 sheets, and the overlayer coating made the whole film more compact. SAED patterns were taken from both the edge and inside of the sample in Figure 5b to characterize the crystallinity of the overlayers. Focusing on the edge, the SAED pattern reveals the amorphous nature of the ALD TiO2 overlayer, Figure 5c. By moving the beam more toward the sheet center, diffraction spots appeared in the SAED patterns (Figure 5d), indicating the polycrystalline nature of the AZO and CdS layers, which is in good agreement with the previous reports.46 The CdS layer forms a p−n junction with CuInS2 at the interface, which enhances the charge separation,16 whereas AZO and TiO2 act as window layers, and they further enhance the charge separation as in photovoltaics.14 At the same time, the AZO and TiO2 served as the protection layer to enhance
range is in good agreement with the 1.5 eV band gap of CuInS2, enabling it to harvest a broader range of solar spectrum. Besides changes in crystal structure and optical properties, the morphology of the samples was also altered, as evidenced by both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) imaging. Before transformation, the Cu2O was a compact film with cubic texture on the surface, and the thickness of the film was around 500 nm (Figure 2a). After transformation into CuInS2, the thickness of the film expanded around 10 times to 5 μm with a porous nanosheet array structure, resulting in a great increase in surface area (Figure 2c,d). More surface area could provide a higher density of active sites for water splitting reaction (per unit projected area), which is favorable for the improvement of the efficiency. The CuInS2 sheets were very thin, likely only a few atomic layers, as evidenced by the high electron transmittance observed in Figure 3a, and showed a single crystalline nature, as revealed by the high-resolution TEM image and selected area electron diffraction (SAED) pattern, Figure 3b,c. As the charge transfer along the sheets within the basal plane should be more favorable than across the sheets, the vertical alignment of the CuInS2 sheets to the substrate is beneficial for the overall device performance. Furthermore, the composition of the nanosheets was characterized by the energy-dispersive X-ray (EDX) mapping obtained in STEM mode (Figure 3d−f). The data showed only copper (Cu), indium (In), and sulfur (S) element signals, which were distributed homogeneously through the sheets, further indicating the complete transformation of Cu2O to CuInS2. After the confirmation of the transformation of Cu2O into CuInS2, the photoelectrochemical properties of CuInS2 electrodes for solar water splitting were investigated in detail. The asD
DOI: 10.1021/nl504746b Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 5. TEM and SAED characterizations of the CuInS2 nanosheet after overlayer coating. (a) TEM image after CdS coating. (b) TEM image after CdS, AZO, and TiO2 coating. (c) SAED pattern of spot 1 in b. (d) SAED pattern of spot 2 in b.
the stability.47 To improve the charge transfer from electrode to electrolyte, standard Pt catalyst was deposited on the surface by photoelectrochemical deposition. The photoelectrochemical properties of the CuInS2 alone and after deposition of CdS, AZO, and TiO2 overlayers and Pt catalyst are shown in Figure 6. The sample reached a photocurrent of around 3.5 mA cm−2 at −0.3 V versus RHE in pH 5 electrolyte (Figure 6b), which was improved 35 times compared to bare CuInS2 electrode (Figure 6a). Incident photon to current efficiency (IPCE) measurement was carried out at 0 V versus RHE to show the spectral distribution of photocurrent generation (Figure 6c), revealing a broad response from the ultraviolet to the near-infrared range, confirming the extended light-harvesting capability compared to Cu2O, which is in agreement with the light absorption range. Decreased IPCE value from the ultraviolet to 500 nm might be due to the absorption of the AZO, TiO2, and CdS overlayers, because the device was illuminated through them before reaching CuInS2 absorber. Lastly, the stability of the sample was characterized by chronoamperometry measurements at 0 V versus RHE under chopped illumination with 80% photocurrent retention after 2 h of testing, which consists of a significant improvement over comparable electrochemically deposited electrodes.24 Furthermore, the consistently low dark currents point to the avoidance of corrosion reactions and therefore the effectiveness of the protection strategy.
The results above demonstrate the promising application of solution transformation methods toward solar water-splitting devices. Though only the transformation of Cu2O into CuInS2 was introduced in this work, the concept can be readily generalized to other copper chalcopyrites, such as CuIn1−xGaxSe2 and Cu2ZnSnS4,43,48,49 which could offer different choices of photoelectrodes when building a tandem device. Fine-tuning of the band gap by varying the compositions is strongly desirable in view of recent efforts to build efficient tandem devices. Further improvement of device performance could be achieved through annealing in sulfur vapor or by surface modification. In conclusion, CuInS2 was successfully fabricated by solution transformation of electrochemically deposited Cu2O, and its application as a photocathode for water splitting was evaluated. The IPCE proved that the resulting CuInS2 devices showed a photo response to wavelengths of 800 nm, indicating their superior light-harvesting capability and potentially more efficient solar energy to hydrogen conversion compared to Cu2O. Though here only the transformation of Cu2O into CuInS2 is demonstrated, there is no barrier to generalize the method to other copper chalcopyrites, which may have numerous applications in photovoltaic and optoelectronic devices. Methods. Cu2O Film by Electrochemical Deposition. The Cu2O films were electrodeposited from a basic solution of lactate stabilized copper sulfate electrolyte. 11 For the E
DOI: 10.1021/nl504746b Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 6. Photoelectrochemical characterizations of the as-fabricated CuInS2 electrodes in pH 5 electrolyte. (a) Current versus potenial (J−V) curve of pure CuInS2 electrode under choped AM 1.5 G (100 mW cm−2) simulated solar illumination. (b) J−V curve of CuInS2 electrode after CdS, AZO, and TiO2 coating in the dark and under AM 1.5 G (100 mW cm−2) simulated solar illumination. (c) IPCE spectrum of the CuInS2 electrode after CdS, AZO, and TiO2 coating at 0 V versus RHE under three electrode configuration. (d) Stability test of the CuInS2 electrode after CdS, AZO, and TiO2 coating at 0 V versus RHE under choped AM 1.5 G (100 mW cm−2) simulated solar illumination for 2 h.
with CuInS2 substrates fixed against the wall by kapton tape. Then, the mixture was mildly stirred for 10 min with heating, and the substrates were collected after the solution cooled to room temperature naturally. Atomic Layer Deposition of the Protecting Layers. The protecting layers were deposited by atomic layer deposition (ALD) (Savannah 100, Cambridge Nanotech). Aluminumdoped zinc oxide (AZO) was deposited by running five supercycles consisting of 1 cycle of trimethyl-aluminum and water after 20 cycles of diethyl zinc and water, at 120 °C, which gives a film of approximately 20 nm in thickness. Titanium dioxide (TiO2) was deposited at 150 °C using tetrakisdimethylamino titanium (TDMAT) and H2O2 as the Ti and O precursors, respectively. To ensure appreciable vapor pressure, TDMAT was heated to 75 °C. Typically, 1700 cycles deposition gives 100 nm TiO2 film. Material Characterizations. The XRD patterns were acquired with a Bruker D8 Discover diffractometer in Bragg− Brentano mode, using Cu K α radiation (1.540598 Å) and a Ni β-filter. Spectra were acquired with a linear silicon strip “Lynx Eye” detector from 2θ = 20°− 80° at a scan rate of 1° min−1, step width of 0.02°, and a source slit width of 4 mm and the reflection patterns were matched to the PDF-4+ database (ICDD). The optical properties of the films were characterized by the diffuse reflectance spectra from front side illumination with a Shimadzu UV-3600 UV−vis-NIR spectrophotometer (Shimadzu) using an integrating sphere. Absorbance spectra were calculated according to Kubelka−Munk theory. The morphology of the films was characterized using a highresolution scanning electron microscope (ZEISS Merlin), a
preparation of the electrolyte, 7.98 g of CuSO4, 21.77 g of K2HPO4 and 67.5 g of lactic acid were dissolved in 250 mL of H2O, and the pH of the electrolyte was adjusted to 12 by KOH (2 M) solution, the total resulted solution is around 1 L in the end. Cu2O thin films were deposited at a constant current density of −0.1 mA cm−2 (Galvanostatic mode) for 100 min using a source meter (Keithley 2400) in a two-electrode configuration (a Pt mesh served as the counter electrode), which resulted in a film thickness of 500 nm, as revealed by the cross-section SEM image. During deposition, the electrolyte was maintained at 30 °C using a hot plate with an in situ temperature probe. Transformation of Cu2O Film into CuInS2. The Cu2O films were transformed into CuInS2 by solvothermal treatment in 0.25 M InCl3 and 0.5 M thioacetamide in ethylene glycol solution at 200 °C for 10 h. Typically, the Cu2O film substrate was put into a Teflon autoclave angled against the vessel wall, facing down, and the as-made solution was poured into the autoclave to immerse the Cu2O film. The autoclave was then put into an oven heating at 200 °C for 10 h and cooled down to room temperature naturally. Chemical Bath Deposition of CdS. The CdS layer was done by chemical bath deposition in an aqueous mixture of Cd(CH3 COO)2 , ammonium hydroxide, and (NH2 ) 2 CS according to a previous report.24 The Cd(CH3COO)2 solution was prepared by dissolving 666 mg of Cd(CH3COO)2 in 20 mL of deionized water and adding 50 mL of ammonium hydroxide solution. The (NH2)2CS solution was prepared by dissolving 2.663 g of (NH2)2CS in 30 mL of deionized water. Both solutions were heated to 90 °C and poured into a beaker F
DOI: 10.1021/nl504746b Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
for financial support and Dr. Duncan Alexander from Interdisciplinary Centre for Electron Microscopy (CIME) in EPFL for the assistance in TEM characterization. S.D.T. and M.G. thank the Swiss Federal Office for Energy (PECHouse Competence Center, contract number SI/500090-02) for support. L.S. and M.T.M. acknowledge financial support from the FP7 Future and Emerging Technologies (FET) collaborative project “PHOCS” (Contract No. 309223). H.J.F. thanks the support from Singapore-Berkeley Research Initiative for Sustainable Energy (SinBeRISE) program.
transmission electron microscope (Philips, FEI CM12), and a high-resolution transmission electron microscope (Technai Osiris, FEI). Additionally, the selected area electron diffraction (SAED) patterns were acquired with Philips FEI CM12, and the composition of the nanosheets was characterized by the energy-dispersive X-ray (EDX) spectra obtained in STEM mode with Technai Osiris. Photoelectrochemical Measurements. The photoelectrochemical performance of the photocathodes was studied using an Ivium electrochemical workstation to acquire the photoresponse under simulated AM 1.5G illumination (100 mW cm−2) from a 450 W Xe-lamp (Lot-Oriel, ozone-free) equipped with an AM 1.5G filter (Lot-Oriel), calibrated with a silicon diode. Current−voltage measurements were carried out in a three-electrode configuration with the CuInS2 photocathodes as the working electrode, a Pt mesh as the counter electrode and Ag/AgCl/sat. KCl as the reference electrode, in an electrolyte solution of 0.5 M Na2SO4 and 0.1 M KH2PO4 at pH 5.0. A scan rate of 10 mV s−1 in the cathodic direction was used to acquire the data. IPCE measurements were performed under light from a 300 W xenon lamp with integrated parabolic reflector (Cermax PE 300 BUV) passing through a monochromator (Bausch & Lomb, bandwidth 10 nm fwhm) in three-electrode configuration at 0 V versus reversible hydrogen electrode (RHE). Comparison with a calibrated Si photodiode allowed the calculation of the IPCE. The stability of the electrodes was characterized with chronoamperometry at 0 V versus RHE under chopped and simulated AM 1.5G illumination (100 mW cm−2).
■
■
ASSOCIATED CONTENT
S Supporting Information *
Generalized band position diagram for the multilayer photocathode and redox levels of the involved chemical reactions. This material is available free of charge via the Internet at http://pubs.acs.org.
■
REFERENCES
(1) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729−35. (2) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori, E. A.; Lewis, N. S. Chem. Rev. 2010, 110, 6446−6473. (3) Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Grätzel, M. Science 2014, 345, 1593−1596. (4) Prévot, M. S.; Sivula, K. J. Phys. Chem. C 2013, 117, 17879− 17893. (5) Seger, B.; Castelli, I. E.; Vesborg, P. C. K.; Jacobsen, K. W.; Hansen, O.; Chorkendorff, I. Energy Environ. Sci. 2014, 7, 2397−2413. (6) Brillet, J.; Yum, J.-H.; Cornuz, M.; Hisatomi, T.; Solarska, R.; Augustynski, J.; Graetzel, M.; Sivula, K. Nat. Photonics 2012, 6, 824− 828. (7) Luo, J.; Ma, L.; He, T.; Ng, C. F.; Wang, S.; Sun, H.; Fan, H. J. J. Phys. Chem. C 2012, 116, 11956−11963. (8) Karuturi, S. K.; Luo, J.; Cheng, C.; Liu, L.; Su, L. T.; Tok, A. I. Y.; Fan, H. J. Adv. Mater. 2012, 24, 4157−4162. (9) Sivula, K.; Le Formal, F.; Grätzel, M. ChemSusChem 2011, 4, 432−449. (10) Abdi, F. F.; Han, L.; Smets, A. H. M.; Zeman, M.; Dam, B.; van de Krol, R. Nat. Commun. 2013, 4, 2195. (11) Paracchino, A.; Laporte, V.; Sivula, K.; Grätzel, M.; Thimsen, E. Nat. Mater. 2011, 10, 456−461. (12) Tilley, S. D.; Schreier, M.; Azevedo, J.; Stefik, M.; Graetzel, M. Adv. Funct. Mater. 2014, 24, 303−311. (13) Chirilă, A.; Buecheler, S.; Pianezzi, F.; Bloesch, P.; Gretener, C.; Uhl, A. R.; Fella, C.; Kranz, L.; Perrenoud, J.; Seyrling, S.; Verma, R.; Nishiwaki, S.; Romanyuk, Y. E.; Bilger, G.; Tiwari, A. N. Nat. Mater. 2011, 10, 857−861. (14) Chirilă, A.; Reinhard, P.; Pianezzi, F.; Bloesch, P.; Uhl, A. R.; Fella, C.; Kranz, L.; Keller, D.; Gretener, C.; Hagendorfer, H.; Jaeger, D.; Erni, R.; Nishiwaki, S.; Buecheler, S.; Tiwari, A. N. Nat. Mater. 2013, 12, 1107−1111. (15) Yokoyama, D.; Minegishi, T.; Jimbo, K.; Hisatomi, T.; Ma, G.; Katayama, M.; Kubota, J.; Katagiri, H.; Domen, K. Appl. Phys. Express 2010, 3, 101202. (16) Moriya, M.; Minegishi, T.; Kumagai, H.; Katayama, M.; Kubota, J.; Domen, K. J. Am. Chem. Soc. 2013, 135, 3733−3735. (17) Marsen, B.; Cole, B.; Miller, E. L. Sol. Energy Mater. Sol. Cells 2008, 92, 1054−1058. (18) Jacobsson, T. J.; Platzer-Björkman, C.; Edoff, M.; Edvinsson, T. Int. J. Hydrogen Energy 2013, 38, 15027−15035. (19) Jacobsson, T. J.; Fjallstrom, V.; Sahlberg, M.; Edoff, M.; Edvinsson, T. Energy Environ. Sci. 2013, 6, 3676−3683. (20) Kaneshiro, J.; Gaillard, N.; Rocheleau, R.; Miller, E. Sol. Energy Mater. Sol. Cells 2010, 94, 12−16. (21) Kumagai, H.; Minegishi, T.; Moriya, Y.; Kubota, J.; Domen, K. J. Phys. Chem. C 2014, 118, 16386−16392. (22) Gunawan; Septina, W.; Ikeda, S.; Harada, T.; Minegishi, T.; Domen, K.; Matsumura, M. Chem. Commun. 2014, 50, 8941−8943. (23) Zhao, J.; Minegishi, T.; Zhang, L.; Zhong, M.; Gunawan; Nakabayashi, M.; Ma, G.; Hisatomi, T.; Katayama, M.; Ikeda, S.; Shibata, N.; Yamada, T.; Domen, K. Angew. Chem., Int. Ed. 2014, 53, 11808−11812.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: jingshan.luo@epfl.ch (J.L.). *E-mail: michael.gratzel@epfl.ch (M.G.). Present Address
(S.D.T.) Department of Chemistry, University of Zurich, CH8057, Zurich, Switzerland Author Contributions
J.L. and S.D.T. conceived the project. J.L. designed and carried out the experiments. S.D.T. fabricated the substrates and assisted with Cu2O electrochemical deposition. L.S. assisted with the TEM characterizations. M.S. assisted with the diffuse reflectance and absorption spectra measurements. M.T.M. assisted with the IPCE measurement. H.J.F. contributed to the planning of the experiment and the analysis of the results. J.L. wrote the manuscript. M.G. supervised the project and corrected the manuscript. All authors contributed to the discussion and commented on the manuscript. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS J.L. would like to thank Nano-Tera NTF project (TANDEM) and PECDEMO project, cofunded by Europe’s Fuel Cell and Hydrogen Joint Undertaking (FCH JU) under Grant 621252 G
DOI: 10.1021/nl504746b Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters (24) Rovelli, L.; Tilley, S. D.; Sivula, K. ACS Appl. Mater. Interfaces 2013, 5, 8018−8024. (25) Zhong, J.; Xia, Z.; Zhang, C.; Li, B.; Liu, X.; Cheng, Y.-B.; Tang, J. Chem. Mater. 2014, 26, 3573−3578. (26) Tang, Y.; Ng, Y. H.; Yun, J.-H.; Amal, R. RSC Adv. 2014, 4, 3278−3283. (27) van Embden, J.; Chesman, A. S. R.; Della Gaspera, E.; Duffy, N. W.; Watkins, S. E.; Jasieniak, J. J. J. Am. Chem. Soc. 2014, 136, 5237− 5240. (28) Shi, L.; Yin, P.; Wang, L.; Qian, Y. CrystEngComm 2012, 14, 7217−7221. (29) Tang, M.; Tian, Q.; Hu, X.; Peng, Y.; Xue, Y.; Chen, Z.; Yang, J.; Xu, X.; Hu, J. CrystEngComm 2012, 14, 1825−1832. (30) Wu, J.-J.; Jiang, W.-T.; Liao, W.-P. Chem. Commun. 2010, 46, 5885−5887. (31) Xu, J.; Luan, C.-Y.; Tang, Y.-B.; Chen, X.; Zapien, J. A.; Zhang, W.-J.; Kwong, H.-L.; Meng, X.-M.; Lee, S.-T.; Lee, C.-S. ACS Nano 2010, 4, 6064−6070. (32) Luo, J.; Liu, J.; Zeng, Z.; Ng, C. F.; Ma, L.; Zhang, H.; Lin, J.; Shen, Z.; Fan, H. J. Nano Lett. 2013, 13, 6136−6143. (33) Luo, Y.; Luo, J.; Jiang, J.; Zhou, W.; Yang, H.; Qi, X.; Zhang, H.; Fan, H. J.; Yu, D. Y. W.; Li, C. M.; Yu, T. Energy Environ. Sci. 2012, 5, 6559−6566. (34) Luo, J.; Xia, X.; Luo, Y.; Guan, C.; Liu, J.; Qi, X.; Ng, C. F.; Yu, T.; Zhang, H.; Fan, H. J. Adv. Energy Mater. 2013, 3, 737−743. (35) Luo, J.; Karuturi, S. K.; Liu, L.; Su, L. T.; Tok, A. I. Y.; Fan, H. J. Sci. Rep. 2012, 2, 451. (36) Xu, J.; Yang, X.; Wang, H.; Chen, X.; Luan, C.; Xu, Z.; Lu, Z.; Roy, V. A. L.; Zhang, W.; Lee, C.-S. Nano Lett. 2011, 11, 4138−4143. (37) Mayer, M. T.; Simpson, Z. I.; Zhou, S.; Wang, D. Chem. Mater. 2011, 23, 5045−5051. (38) Oh, M. H.; Yu, T.; Yu, S.-H.; Lim, B.; Ko, K.-T.; Willinger, M.G.; Seo, D.-H.; Kim, B. H.; Cho, M. G.; Park, J.-H.; Kang, K.; Sung, Y.E.; Pinna, N.; Hyeon, T. Science 2013, 340, 964−968. (39) Jiang, J.; Luo, J.; Zhu, J.; Huang, X.; Liu, J.; Yu, T. Nanoscale 2013, 5, 8105−8113. (40) Wang, Y.; Zhou, T.; Jiang, K.; Da, P.; Peng, Z.; Tang, J.; Kong, B.; Cai, W.-B.; Yang, Z.; Zheng, G. Adv. Energy Mater. 2014, 4, 1400696. (41) Tench, D.; Warren, L. F. J. Electrochem. Soc. 1983, 130 (4), 869−872. (42) Golden, T. D.; Shumsky, M. G.; Zhou, Y.; VanderWerf, R. A.; Van Leeuwen, R. A.; Switzer, J. A. Chem. Mater. 1996, 8, 2499−2504. (43) Xu, J.; Lee, C.-S.; Tang, Y.-B.; Chen, X.; Chen, Z.-H.; Zhang, W.-J.; Lee, S.-T.; Zhang, W.; Yang, Z. ACS Nano 2010, 4, 1845−1850. (44) Gunawan; Septina, W.; Ikeda, S.; Harada, T.; Minegishi, T.; Domen, K.; Matsumura, M. Chem. Commun. 2014, 50, 8941−8943. (45) Ikeda, S.; Nonogaki, M.; Septina, W.; Gunawan, G.; Harada, T.; Matsumura, M. Catal. Sci. Technol. 2013, 3, 1849−1854. (46) Azevedo, J.; Steier, L.; Dias, P.; Stefik, M.; Sousa, C. T.; Araujo, J. P.; Mendes, A.; Graetzel, M.; Tilley, S. D. Energy Environ. Sci. 2014, 7, 4044. (47) Paracchino, A.; Mathews, N.; Hisatomi, T.; Stefik, M.; Tilley, S. D.; Gratzel, M. Energy Environ. Sci. 2012, 5, 8673−8681. (48) Wu, X.-J.; Huang, X.; Qi, X.; Li, H.; Li, B.; Zhang, H. Angew. Chem., Int. Ed. 2014, 53, 8929−8933. (49) Lesnyak, V.; George, C.; Genovese, A.; Prato, M.; Casu, A.; Ayyappan, S.; Scarpellini, A.; Manna, L. ACS Nano 2014, 8, 8407− 8418.
H
DOI: 10.1021/nl504746b Nano Lett. XXXX, XXX, XXX−XXX
