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Jan 26, 2017 - Photoelectrochemical measurements of bare CuO thin films prepared by ..... Production with CuO/CdS Heterojunction Thin Film Photocathod...
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Stabilized Solar Hydrogen Production with CuO/CdS Heterojunction Thin Film Photocathodes Wilman Septina, Rajiv Ramanujam Prabhakar, René Wick, Thomas Moehl, and S. David Tilley* Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland

Chem. Mater. 2017.29:1735-1743. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 12/22/18. For personal use only.

S Supporting Information *

ABSTRACT: Cupric oxide (CuO) is a promising material for large-scale, economic solar energy conversion due to the abundance of copper, suitable band gap, and ease of fabrication. For application as a photocathode for water splitting, the main challenge is prevention of the inherent photocorrosion in aqueous media. Photoelectrochemical measurements of bare CuO thin films prepared by oxidation of electroplated Cu indicated that the vast majority of the photocurrent in 1 M phosphate buffer solution (pH 7) comes from photocorrosion of the CuO into metallic Cu, with a faradaic efficiency for hydrogen evolution of ∼0.01%. We found that deposition of an n-type CdS buffer layer underneath a protective TiO2 layer yielded a stable and efficient photoelectrode, with the champion electrode giving 1.68 mA cm−2 at 0 VRHE and an onset potential of ca. 0.45 VRHE. We attribute a favorable band alignment of CuO/CdS for the record photovoltage obtained with this material and a high conformality of the TiO2 layer on the sulfide surface for the high stability of hydrogen-producing photocurrents (faradaic efficiency ∼100%).

1. INTRODUCTION Photoelectrochemical (PEC) water splitting is an appealing method for large scale conversion and storage of solar energy in the form of hydrogen fuel. In this approach, sunlight is absorbed by semiconducting materials in contact with an aqueous electrolyte, generating a photovoltage that is used to drive the energetically uphill water splitting reaction. The photovoltage could be generated from a single semiconductor, such as the well-known Honda-Fujishima cell featuring a TiO2 photoanode,1 or from a combination of two or more semiconductors in a so-called tandem system.2,3 In order to reduce the cost of solar hydrogen, new Earth-abundant light absorber materials are needed that can be fabricated with less cost than crystalline silicon, while maintaining high solar-tohydrogen (STH) efficiency and stability of at least several years.4 Cupric oxide (CuO) is one material that can potentially meet the stringent demands enumerated above. It consists of only Earth-abundant elements and can be fabricated with low energy intensity techniques. Various reports suggest that the band gap of this material is around 1.4−1.7 eV, enabling it to harvest a significant portion of the solar spectrum.5,6 With a band gap of 1.5 eV, a tandem PEC system with efficiency of over 15% could be achieved when coupled, for example, with a photoanode semiconductor having a band gap of 2 eV.7 The nature of the bandgap (direct or indirect) has been unclear,5 but a recent joint experimental and theoretical study strongly suggests that the CuO has a direct band gap at 1.46 eV.8 Although for solar applications CuO is not as intensively studied as the closely © 2017 American Chemical Society

related cuprous oxide (Cu2O), there are a few reports on its application in photovoltaic devices9−11 and several reports on its application as a photocathode for hydrogen production.12−14 One of the main challenges in using CuO as a light absorber in a liquid system is its tendency toward photocorrosion in the presence of protons. Even in the absence of light, the Pourbaix diagram of the Cu−O system suggests that CuO has a limited stability range in aqueous electrolytes, making dark electrochemical reduction likely. Although there are several reports on the use of CuO as a photocathode for hydrogen production, it is unclear how much of the observed current actually corresponds to H2 evolution, as the inevitability of photocorrosion is usually not addressed. A rare example where this issue was addressed found that the photocurrent arising from CuO/electrolyte junctions was purely due to corrosion.15 To the best of our knowledge, only a report by Jang et al. attempted to quantify the H2-faradaic efficiency of their photocurrent, which found ca. 82% faradaic efficiency over 1 h.14 The faradaic efficiency is of crucial importance since most literature reports show an appreciable dark current of at least 0.1 mA cm−2 in their current density−potential curves, indicating photoelectrode corrosion. It is clear that the application of CuO as a photocathode requires some protective strategy, in the same manner as other corrosion-prone materials such as Cu2O,16,17 Si,18 and III−V Received: December 12, 2016 Revised: January 26, 2017 Published: January 26, 2017 1735

DOI: 10.1021/acs.chemmater.6b05248 Chem. Mater. 2017, 29, 1735−1743

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Chemistry of Materials materials,19 if it is to stably generate hydrogen on the order of years. TiO2 deposited by atomic layer deposition (ALD) has emerged as a widely used protective layer for photocathode materials owing to the chemical stability of TiO2 over a wide pH range, coupled with the high conformality of the ALD process. As a few representative examples, Lee et al. showed stable operation of InP nanopillars coated with a thin layer of ALD-TiO2 for more than 4 h.20 Azevedo et al. employed ALDTiO2 to Cu2O/Al:ZnO films, and the photoelectrode showed 90% stability over more than 50 h of light chopping after steam treatment of the multilayer structures. 21 Seger et al. demonstrated good stabilities for >2 weeks for TiO2-coated Si photocathodes.18 These results are encouraging as to the feasibility of the protection layer strategy for PEC water splitting for real world applications. The deposition of overlayers on CuO also provides the opportunity to improve the photovoltage and photocurrent through judicious choice of an appropriate buffer layer. Light absorbing materials featuring protective overlayers represent a class of photoelectrodes termed “buried junctions,” where the photovoltage is generated from an internal junction and is decoupled from the redox potential of the electrolyte. These photoelectrodes are essentially heterojunction solar cells where one of the current collectors has been replaced with the aqueous electrolyte. Photoelectrochemical water splitting cells employing buried junctions with n-type overlayers have been intensively investigated for various common photocathode materials such as CIGS,22,23 Cu2O,16,24 Si,25 and GaP26 and proven to be effective in enhancing the performance and stability of the photoelectrodes for H2 production. Thus, in this study, we sought to investigate different buffer layers between the CuO photoabsorber and TiO2 protective layer to maximize the photovoltaic properties while stabilizing it in aqueous electrolyte. To minimize the fabrication costs, CuO was made by a simple electrodeposition-annealing approach. We demonstrate that ALD-TiO2 is able to suppress the corrosion in CuO photocathode and that insertion of a CdS buffer layer between the CuO and TiO2 significantly increases the photocurrent with minimal dark current over a wide-range of potentials. We also confirm faradaic efficiencies close to 100%, as expected for a platinized TiO2 surface. To the best of our knowledge, this is the first report demonstrating stabilized H2 production with CuO-based photocathodes.

Supra 50 VP microscope. For cross-sectional imaging of the device, ca. 3 nm of Pt was sputtered onto the side of the photocathode stack to avoid charging during the SEM measurement. Crystal structures of the fabricated films were evaluated by X-ray diffraction (XRD) using a Bruker AXS D8Advance diffractometer. Diffuse reflectance spectra were obtained using a Shimadzu UV-3600Plus spectrometer with integrating sphere. Tauc plots were constructed from diffuse reflectance data using the Kubelka−Munk equation. Deposition of CdS. A CdS layer was deposited on top of the CuO film using chemical bath deposition (CBD). In a typical synthesis procedure, a beaker containing 59 mL of stirred deionized water was heated inside a water bath. When the temperature of the solution reached 50 °C, 195 mg of CdSO4 powder was added to the solution (final concentration of 12.5 mM). When the solution reached 60 °C, 16 mL of an 11 M solution of NH4OH was added, causing the temperature of the solution to decrease slightly. When the temperature of the solution again reached 60 °C, 1.256 g of thiourea was added (final concentration of 0.22 M), followed by immersion of the CuO films into the solution for 5 min. The CdS-coated CuO films were then rinsed thoroughly with water and dried under a stream of N2. Atomic Layer Deposition of TiO2. TiO2 was deposited by atomic layer deposition (ALD) using a Picosun R200 tool. Before deposition, the CuO sample was rinsed thoroughly with deionized water and dried under a stream of N2 before placing in the ALD reaction chamber. The deposition was carried out at 120 °C using sequential pulses of tetrakis(dimethylamino)titanium (Tprecursor: 85 °C) and H 2O (Tprecursor: 25 °C). One thousand cycles were used, yielding amorphous TiO2 thin films with a thickness of 52 nm (0.52 Å/cycle), as determined from ellipsometric measurements on a piece of silicon witness wafer (alpha-SE ellipsometer, J.A. Woolam Co.). Deposition of Catalyst. Platinum catalyst was photoelectrodeposited from an aqueous solution of 1 mM H2PtCl6 in 0.1 M Na2SO4, using a constant current of −0.14 mA cm−2 for 9 min 35 s with constant irradiation from simulated 1 sun illumination, to give 23 mC of charge passed during deposition of the platinum catalyst. The photodeposition was carried out in a two-electrode configuration with a platinum mesh counter electrode. Photoelectrochemical Measurements. The photoelectrochemical measurement of the photocathodes was evaluated in a threeelectrode configuration using a BioLogic SP-200 potentiostat under irradiation from simulated 1 sun illumination calibrated using a silicon diode. A 1 M phosphate buffer (K2HPO4/KH2PO4, pH 7) solution was used as the electrolyte. The reference electrode was Ag/AgCl (sat’d KCl), and a Pt mesh was used as the counter electrode. Before measurement, the electrolyte was sparged with N2 for ca. 1 h to remove dissolved oxygen. The sample was mounted into a photoelectrochemical cell equipped with an O-ring to fix the area (0.28 cm2). Incident photon-to-current efficiency (IPCE) was measured on a home-built system featuring a halogen light source with double monochromator and white light bias and compared with a calibrated silicon photodiode. Electrochemical Impedance Spectroscopy (EIS). EIS measurements were used to determine the space charge capacitance. The space charge capacitance in the depletion regime of the semiconductor can be used to obtain Mott−Schottky plots and determine the n- or p-type character of a semiconductor, the doping, and the flat band potential. The measurements were performed using a BioLogic SP-200 potentiostat in three-electrode configurations in the dark. The potential range was chosen to be in the depletion region and where the CuO samples were stable against oxidation and reduction reactions. The CuO film deposited on Au-coated FTO was scanned in the positive direction with bias potentials from 0.7 to 1.0 VRHE in 0.1 M KOH (pH 13) and with frequencies ranging from 1 Hz to 200 kHz. The ALD-TiO2 on FTO was scanned in the negative direction from 1.0 to −0.2 VRHE in 1 M H2SO4 (pH 0) and with frequencies ranging from 200 mHz to 200 kHz. An AC amplitude of 10 mV was applied as voltage perturbation during the measurements. The impedance spectra were fitted with the ZView software from Scribner. Mott−Schottky plots were obtained by plotting Csc−2 against the applied bias potential. The linear extrapolation of the data in the

2. EXPERIMENTAL SECTION Fabrication of CuO Thin Films. CuO thin films were fabricated by electrodeposition of Cu metal onto Au-coated FTO (150 nm sputtered Au with 10 nm Cr adhesion layer) from an aqueous solution containing 10 mM CuSO4 and 100 mM trisodium citrate (pH adjusted to 2.5 by the addition of concentrated H2SO4). The electrodeposition was carried out in a two-electrode configuration with a Pt mesh counter electrode using a BioLogic SP-50 potentiostat in galvanostatic mode (−1 mA cm−2 for 60 min). Two annealing methods were investigated. First, the electroplated Cu was placed directly into an open tubular furnace at 600 °C and annealed for 2 h exposed to ambient air (“one-step annealing”). For the “two-step annealing”, the electrodeposited Cu was first placed under an argon atmosphere inside the tubular furnace by three sequential pump-flush cycles (pumping down to a base pressure of 10−3 mbar and then flushing with Ar until a base pressure ca. 10 mbar) before finally leaving the pressure at 10−2 mbar. The electroplated Cu was then heated from room temperature to 600 °C at a ramp rate of 20 °C/min and kept for 1 h, followed by 2 h of annealing at the same temperature with exposure to ambient air. Characterizations of CuO Thin Films. Morphologies of the films were examined by scanning electron microscopy (SEM) using a Zeiss 1736

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Figure 1. (a) XRD patterns and (b) top-view and cross-sectional SEM images of electroplated Cu and CuO films (both one-step and two-step annealing). depletion regime yields the flat band potential, and the slope in this region gives the majority charge carrier concentration according to the Mott−Schottky equation below:

⎛ 1 2 kT ⎞ ⎜E − E − ⎟ = fb 2 2⎝ e ⎠ Csc eεε0NDA

at 600 °C inside an evacuated furnace with a base pressure of argon before the film was converted to CuO by further annealing at the same temperature with exposure to atmospheric pressure of air. Figure 1a shows XRD patterns of electroplated Cu and the oxidized films. The XRD pattern of the electroplated film confirmed the deposition of metallic Cu with the appearance of diffraction peaks at 43.1° and 50.1° (JCPDS 04-0836). SEM images of this film (Figure 1b) show that the metallic Cu deposited in round-shaped grains (200 to 400 nm in diameter) with a thickness of ca. 500 nm. After annealing, the diffraction peaks due to Cu disappeared and were replaced by peaks assigned to CuO (JCPDS 48-1548). The CuO diffractogram shows the successful formation of the monoclinic structure, which is the most stable crystal structure of this compound, having two main orientations along the (002) and (111) directions at 35.2° and 38.4°, respectively. The two-step annealing procedure resulted in films that were slightly more oriented in the (111) direction (Figure S1) compared to the one-step annealing. The most dramatic difference between the two annealing procedures is the much larger grain size in the two-step method as revealed by SEM (Figure 1b), with the majority being 600 nm to micron-size grains while the ones from one-step annealing method had all less than 300 nm grain sizes. In the two-step annealing method, the grain structure of the intermediate CuOx film appears to play an important role in obtaining CuO thin films with relatively large grains, as the intermediate CuOx film (Figure S2) also showed relatively large grains, the size of which was maintained in the second step of the annealing process under air. Another important observation is that, in some cases, the CuO fabricated from the one-step annealing process suffered from inhomogeneity over a large area (Figure S3), which hampered the reproducibility. We also noticed that the Au layer in the one-step annealed case has a different morphology and looks slightly thicker compared to that of unannealed and two-step annealed films, which could be related to the thermal shock from placing the electroplated Cu on Au-coated FTO directly into a furnace at 600 °C. Nevertheless, the two-step annealing method enabled better

(1)

where Csc is the space-charge capacitance, e is the charge of an electron, ε is the relative dielectric constant of the materials, ε0 is the permittivity of free space, ND is the acceptor or donor density (depending if it is a p- or n-type semiconductor), A is the electrode area (accounting for the roughness of the semiconductor surface, we assumed a factor of 2 times the geometric area), E is the potential applied, Efb is the flat band potential, k is the Boltzmann constant, and T is the temperature. Faradaic Efficiency. The faradaic efficiency of the photoelectrodes was measured in a glass gastight three-compartment cell in a threeelectrode configuration using Ag/AgCl (sat’d KCl) as the reference electrode and Pt wire as the counter electrode. The photoelectrodes were masked with epoxy to fix the area (0.15 cm2). The measurement was performed using 1 M phosphate buffer (pH 7) as the electrolyte. The solution was stirred and constantly sparged with argon at a rate of 7 mL/min. The gas outlet from the cell was fed into a 450-GC Gas Chromatograph (Bruker Daltonics GmbH) for hydrogen analysis. During measurement, the exposed area of the photoelectrode was constantly illuminated with a white-light LED. The intensity of the light reaching the outer part of the cell measured by silicon photodiode was ca. 0.15 sun. Before measurement of the photoelectrode, a calibration curve of current vs measured hydrogen was constructed using a Pt-wire working electrode by applying galvanostatic current steps of 20 to 200 μA (6 steps) for 2 h each, in order to correct for any small leaks in the system.

3. RESULTS AND DISCUSSION Fabrication and Structural Characterizations of CuO Thin Film. CuO films were fabricated from electroplated copper on Au-coated FTO electrodes using two different annealing approaches. In the one-step annealing method, the electroplated Cu film was annealed at 600 °C for 2 h in air. According to the Cu−O phase diagram,27 CuO is the most stable compound at temperatures below 1000 °C under air at atmospheric pressure; thus, it was expected that CuO would be easily formed with this annealing condition. In the two-step annealing method, the electroplated Cu film was first annealed 1737

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Figure 2. (a) Current density−potential curve of bare CuO in 1 M phosphate buffer (pH 7) under chopped illumination from simulated 1 sun illumination (AM 1.5 G, 100 mW cm−2), (b) current density−time profile of a bare CuO film measured at 0.4 VRHE under constant 1 sun illumination, (c) top view SEM image of a CuO film after measurement at 0.4 VRHE for 30 min (inset shows a photograph of the measured CuO film), and (d) faradaic efficiency of a bare CuO film measured at 0.4 VRHE.

color of the CuO film transformed into the reddish color typical of metallic copper. XRD analysis of the measured region (Figure S5a) confirmed the partial decomposition of the CuO film to metallic Cu. It is clear that CuO undergoes photoinduced corrosion in an aqueous solution, as expected due to the position of the electrochemical potentials for reduction of CuO to Cu2O as well as Cu2O to Cu inside the bandgap of CuO (Figure S5b). The photoexcited electrons created upon illumination reduce the semiconductor rather than water. Indeed, measurement of the hydrogen generated from this film at 0.4 VRHE gave ∼0.01% faradaic efficiency (Figure 2d), confirming that the vast majority of the observed photocurrent resulted from photocorrosion. ALD-TiO2 for Protection Layer and CdS as n-Type Buffer Layer. As the results above show, a protective layer is required to avoid direct contact of the CuO film with the solution and prevent corrosion. The application of a protective overlayer moves us into the realm of buried junctions, and thus, different n-type heterojunction materials (or “buffer layers”) can be explored in order to maximize the performance. We explored the use of different n-type materials coupled with a protective layer to enhance the stability as well as the performance of CuO-based photocathode (Figure 3a). First, we assessed the TiO2 protective layer itself as the junction material, by depositing 52 nm of amorphous TiO2 onto the CuO film by atomic layer deposition (ALD), followed by photoelectrodeposition of Pt as a hydrogen evolution catalyst. The current density−potential curve of this photocathode (Figure 3b, black line) shows that the dark current at potentials more negative than 0.25 VRHE was significantly suppressed, indicating that the ALD-TiO2 layer inhibited the corrosion of the CuO film. However, the photocurrent

control of the film appearance on a macroscopic scale and resulted in a much more reproducible performance. Therefore, CuO films fabricated from the two-step annealing method were used throughout this study. The band gap of these films were estimated from a Tauc plot of the diffuse-reflectance data (Figure S4). We used the formula for a direct transition and found ca. 1.57 eV, similar to reported values.5,8 Photoelectrochemical Measurements of Bare CuO Thin Film. Photoelectrochemical measurements were performed on the bare CuO film in order to investigate its behavior in an aqueous solution. Figure 2a shows a current density−potential curve of a bare CuO film measured in 1 M phosphate buffer (pH 7) under chopped illumination from simulated AM 1.5 G solar irradiation (100 mW cm−2). The scan was performed toward negative potentials with a scan rate of 10 mV/s. Photocathodic currents as high as ca. 1.16 mA cm−2 were observed before the onset of dark current (at 0.4 VRHE). Appreciable dark current appeared at potentials more negative than 0.25 VRHE, indicating corrosion of the CuO film, as expected from its Pourbaix diagram. When a fixed bias of 0 VRHE was applied to the CuO film under constant illumination, the photocurrent rapidly decreased to zero in less than 3 min, as CuO was converted into metallic Cu. In order to determine the origin of the cathodic photocurrent at more positive potentials, a current density−time profile at a fixed potential of 0.4 VRHE was recorded, as shown in Figure 2b. Upon illumination, the photocurrent rapidly decreased to 50% of the original photocurrent in less than 90 s, finally reaching a relatively stable plateau at around 0.1 to 0.2 mA cm−2 after 15 min of measurement. After this experiment, the macroscopic appearance as well as the morphology of the measured film were drastically changed, as can be seen in Figure 2c. The gray 1738

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nm thick CdS/TiO2 layers (ca. 70 to 80 nm thick CdS) deposited onto a ca. 650 nm thick CuO film. As can be seen in the current density−potential curve of this photocathode in Figure 3b (red and blue lines), the insertion of a CdS layer between the CuO and ALD-TiO2 layers improved the photocurrent, even in the absence of Pt catalyst. The appearance of spikes in the photocurrent of the photocathode without the catalyst is due to capacitive charging and discharging and likely some amount of electron trapping in the TiO2 overlayer, accompanied by proton intercalation. Upon addition of a platinum catalyst, the dramatic improvement of the photocurrent and photovoltage is revealed. It is interesting to note that the plateau photocurrents observed (ca. 1.35 mA cm−2 at 0 VRHE with minimal dark current) are close to the maximum photocorrosion currents obtained in Figure 2a. This observation implies that all photoelectrons that reach the CuO interface, which previously participated in the corrosion reaction in the absence of overlayers, are successfully injected into the CdS, leading to hydrogen evolution. The presence of the TiO2 protection layer on the top of CuO/CdS junction is still important as a significant dark current was observed in the absence of the TiO2 (Figure S6), suggesting direct contact of the electrolyte solution with the CuO. Reproducibility of the performance of the CuO/CdS/TiO2/Pt photocathode is shown in Figure S7 where one of the photocathodes reached a maximum photocurrent of 1.68 mA cm−2 at 0 VRHE. The effective stabilization of the photocathode with ALDTiO2 indicated a conformal ALD layer growth on the surface of CdS, thus blocking direct contact of CuO with the solution. The evolution of the top-view SEM images from CuO/CdS to CuO/CdS/TiO2 in Figure S8a clearly shows complete coverage of the fine grains of CdS by TiO2. To further confirm the conformality of the oxide film growth on a sulfide surface, we deposited CdS onto an FTO substrate and coated it with ALDTiO2. Since CdS is a photoactive n-type semiconductor, we measured its current density−potential curve under chopped illumination in aqueous electrolyte with a hole scavenger (0.1 M Na2SO3). As can be seen in Figure S8b, anodic photocurrent was observed from FTO/CdS due to oxidation of SO32− into SO42− by photogenerated holes, accompanied by significant dark current at potentials more positive than 0 V vs Ag/AgCl (ca. 0.9 VRHE), which is likely due to partial oxidation of the CdS. After coating with ALD-TiO2, we observe that the photoanodic current is greatly diminished, with the small remaining currents likely due to the photoactivity of the TiO2. Most notably, the dark current at potentials more positive than 0 V vs Ag/AgCl is completely suppressed, indicating complete coverage of the sulfide surface. For the application of a photocathode in a tandem PEC system, another important parameter is the photovoltage (or the onset potential). The onset potential is not obvious from the current−density potential curve of the photocathodes in Figure 3b, as small cathodic photocurrent was detected as early as 0.8 VRHE. Thus, we performed cyclic voltammetry of a photocathode under dark and constant illumination in the potential range of 0.4−0.8 VRHE, where the onset potential was thought to be located (Figure 4). Under illumination, it is now clearly visible where the photocurrent crosses the axis and an oxidative peak is observed, attributed to H2 oxidation. The photovoltage of the CuO/CdS junction is therefore estimated to be ca. 0.45 V (assuming zero overpotential for proton reduction on Pt), which is far greater than previous reports with other junctions where CuO is the primary light absorber (Table

Figure 3. (a) Structure of the photocathode with the role of each of the layers. (b) Current density−potential curves of CuO/TiO2 (with Pt) and CuO/CdS/TiO2 with and without Pt-catalyst measured in 1 M phosphate buffer (pH 7) under chopped illumination from simulated 1 sun illumination. (c) Cross-sectional SEM images of the CuO/CdS/TiO2 photocathode.

generated from this photocathode was very small (ca. 0.04 mA cm−2 at 0 VRHE), indicating severe recombination due to a nonideal interface between CuO and TiO2. A recent study on the band alignment of CuO/TiO2 junctions by Morasch et al.11 revealed a high conduction band offset, which could favor high interface recombination and therefore inhibit charge extraction from the CuO to generate hydrogen. To improve the extraction of photoexcited electrons from the CuO film, we explored the deposition of n-type buffer layers between the CuO film and the TiO2 protective overlayer. Cadmium sulfide (CdS) was thought to be a good candidate since it has been successfully implemented as an n-type buffer layer for high-efficiency thin film solar cells28 and is expected to have a better conduction band alignment with CuO than TiO2. In addition, the deposition of a CdS layer by chemical bath deposition (CBD) is a relatively low cost and gentle method, which might minimize damage to the CuO layer during deposition. Figure 3c shows the cross-sectional SEM of the CuO/CdS/TiO2 photocathode used in this study, with ca. 130 1739

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photocathode was highly stable over 30 min, maintaining close to 100% of its initial current. The improved stability of the platinized ALD-TiO2 versus previous reports16 is due to the larger amount of platinum deposited in the present study (∼10×). Hydrogen measurements by gas chromatography for a photocathode biased at 0 VRHE showed close to 100% faradaic efficiency, following an induction period during which the equilibrium of hydrogen between the electrolyte and headspace was established (Figure 5b). To the best of our knowledge, this is the first demonstration of ∼100% faradaic efficiency for a CuO-based photocathode for water splitting. Another common heterojunction partner for CuO is zinc oxide (Table S1), which has also proved to be a good junction material for Cu2O photocathodes and solar cells.16,30 We therefore deposited ZnO by ALD to assess the performance. In our hands, the CuO/ZnO behaved poorly, with very low photocurrent (Figure S9) and appreciable dark current at high bias potential. The zinc precursor used in the ALD process, diethylzinc, has been shown to reduce a CuO surface layer on Cu2O photocathodes,31 and this process likely happens on the CuO photocathode as well, which may result in the poor performance. To gain a deeper insight into the electronic structure of our photocathode, we determined the band positions of each of the layers by impedance spectroscopy and Mott−Schottky analysis. We multiplied the geometric surface area by a factor of 2 to give an estimate of the real surface area, due to the roughness of the CuO sample (see Figure 1b). To circumvent frequency dispersion in the Mott−Schottky plots, we determined the frequency-independent space charge capacitance at each bias potential by a complete impedance analysis (see the Supporting Information). Extrapolation of the Mott−Schottky plot of the CuO film on Au (Figure S10) gave a flat band potential of ca. 0.90 VRHE and acceptor density (Na) of 3.2 × 1021 cm−3, assuming a CuO dielectric constant of 18.1.32 This relatively high acceptor density indicates a large amount of copper vacancies in the CuO (see the Supporting Information for a discussion of the acceptor density).5 For the ALD-TiO2, Mott− Schottky analysis gave a flat band potential of ca. −0.02 VRHE and donor density (Nd) of 5 × 1020 cm−3, consistent with previous reports.33 Since both the CuO and TiO2 have high doping densities, the Fermi level should be very close to the valence band for CuO and to the conduction band for TiO2. Therefore, the valence and conduction bands for CuO are estimated to be approximately 0.90 and −0.67 VRHE, whereas for TiO2 the band positions are approximately 3.18 and −0.02 VRHE. Mott−Schottky plot analysis of CdS films deposited onto FTO was problematic and always resulted in probing the FTO underlayer. Thus, we used a literature value, which reported the conduction band offset between CdS and TiO2 to be ca. 0.3 eV.34 With these data, the band alignment of the CuO/CdS/ TiO2 was estimated as illustrated in Figure 6, with a type-II staggered band alignment between CuO and CdS. An important parameter is the conduction band offset (ΔECB) between CuO and CdS, which is 0.35 eV, much smaller than with TiO2 (0.65 eV), and also ZnO, which has a similar conduction band position as TiO2.33 This relatively small offset of CuO and CdS makes the buried CuO/CdS junction more favorable for obtaining a high photovoltage compared to the CuO/TiO2 or CuO/ZnO junctions. The better band alignment of the CuO/CdS induces a stronger band bending in the CuO, which reduces the interfacial recombination as a result of the smaller concentration of holes near the interface. Indeed, we

Figure 4. Cyclic voltammetry of the CuO/CdS/TiO2/Pt photocathode near the onset potential under dark and constant illumination from simulated 1 sun illumination, measured in 1 M phosphate buffer (pH 7) at a sweep rate of 5 mV/s. The sample was swept two cycles in the dark to stabilize the dark current before being illuminated and swept for three cycles. Dashed arrows indicated the direction of the sweep.

S1). Photocurrents at more positive potentials are likely due to reduction of the platinum precatalyst and not hydrogen evolution:29 no hydrogen was detected when faradaic efficiency measurements were carried out at 0.5 VRHE. The photostability of the device under operation was also tested by applying a bias potential of 0 VRHE under constant illumination of AM1.5 G, as shown in Figure 5a. The

Figure 5. (a) Current density−time behavior of CuO/CdS/TiO2/Pt measured at 0 VRHE. (b) Faradaic efficiency of the photocathode measured at 0 VRHE after an induction period of 35 min. 1740

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spectrum indicates that nanostructuring should have a large effect on the photocurrent (as can be seen in Table S1). It is noteworthy that these results can be applied to CuObased solar cells due to the similarity of the device structure. There are only a few reports on solar cell devices using CuO as the photoabsorber. The majority of these reports use ZnO as the heterojunction partner and generally show quite low solar conversion efficiencies apart from a p-CuO/n-Si heterojunction where the n-Si is the main photoabsorber (Table S2). Thus, the achievement of record photovoltages with a CuO/CdS heterojunction could also be further developed for CuObased solar cells.

4. CONCLUSION In this report, we showed that photocurrents generated from bare unprotected CuO films were mainly derived from photocorrosion of CuO and not from H2 production, as commonly claimed by the majority of the reports on CuObased photocathodes. We have demonstrated, for the first-time, H2 production from stabilized CuO-based photocathodes with ∼100% faradaic efficiency, through deposition of a CdS layer by chemical bath deposition followed by ALD-TiO2 onto the CuO thin film. The CdS layer formed a favorable heterojunction with CuO for a good extraction of the photogenerated carriers in the film, while the conformality of the ALD-TiO2 layer on CuO/ CdS junction was able to protect the photocathode from the inherent photocorrosion problem of CuO. Further work will focus on the reduction of the doping density in CuO in order to effectively harvest the longer wavelength photons. Our findings also indicate that CuO/CdS is a promising p−n junction for solar cells based on earth-abundant CuO films.

Figure 6. Relative band positions of CuO/CdS/TiO2 heterojunctions.

achieved record photovoltage with the CuO/CdS junction and 100× the photocurrent compared to CuO/TiO2. The obvious improvement that needs to be made in the performance of this photocathode is the photocurrent, considering that, with a bandgap of 1.5 eV, CuO can potentially produce 25 mA cm−2 (not accounting for parasitic absorption of the overlayers and catalyst) under AM 1.5 G solar irradiation (100 mW cm−2). To better understand the shortcomings of this photocathode, we measured the incident-photon-to-currentefficiency (IPCE) of the device and used it to calculate the expected photocurrent under AM 1.5 G (100 mW cm−2) solar irradiation (Figure 7). The IPCE spectrum shows that the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05248. Additional information including experimental details, XRD patterns of electroplated Cu and annealed films (close up view), top-view SEM image of the intermediate CuOx film, an example of the inhomogeneous CuO film that was sometimes obtained from the one-step annealing method (including SEM images), Tauc plot to estimate the band gap of CuO, XRD pattern of CuO film after photoelectrochemical measurement at fixed bias of 0.4 VRHE for 30 min, possible chemical reactions within the gap of CuO, current density−potential curves of CuO/CdS/Pt, reproducibility of the CuO/CdS/ TiO2/Pt photocathode, top-view SEM images of CuO/ CdS and CuO/CdS/TiO2, current density−potential curve of FTO/CdS film with and without ALD-TiO2 measured using 0.1 M Na2SO3, current density−potential curves of CuO/ZnO/TiO2/Pt, Mott−Schottky plots of CuO and ALD-TiO2, faradaic efficiency of the CuO/ CdS/TiO2/Pt photocathode measured at 0 VRHE, additional current density−potential curves of CuO/ CdS/TiO2/Pt, extended current density−time profile of photocathode CuO/CdS/TiO2/Pt, current density−time profile of CuO/CdS/TiO 2 (without Pt), current density−potential curve of bare CuO at more positive potentials, and tables of the relevant performance data

Figure 7. IPCE spectrum of the CuO/CdS/TiO2/Pt photocathode measured at 0.3 VRHE with white light bias (∼0.07 sun). The photocurrent integrated from the spectrum is 0.80 mA cm−2.

majority of the photocurrent comes from charge collection at wavelengths