Boosting light harvesting and charge separation of WO₃ via coupling with Cu₂O/CuO towards highly efficient tandem photoanodes

Abstract

Photoanodes based on semiconductor WO₃ have been attractive due to its good electron mobility, long hole-diffusion length, and suitable valence band potential for water oxidation. However, the semiconductor displays disadvantages including a relatively wide bandgap, poor charge separation and transfer, and quick electron–hole recombination at the interface with the electrolyte. Here we present a significantly improved photoanode with a tandem structure of ITO/WO₃/Cu₂O/CuO, which is prepared first by hydrothermally growing a layer of WO₃ on the ITO surface, then by electrodepositing an additional layer of Cu₂O, and finally by heat-treating in the air to form an exterior layer of CuO. Photocurrent measurements reveal that the prepared photoanode produces a maximum current density of 4.7 mA cm⁻², which is, in comparison, about 1.4 and 5.5 times the measured values for ITO/WO₃/Cu₂O and ITO/WO₃ ones, respectively. These enhancements are attributed to (1) harvested UV, visible, and NIR light of the solar spectrum, (2) accelerated charge separation at the heterojunction between WO₃ and Cu₂O/CuO, (3) better electrocatalytic activity of formed Cu_xO than pure Cu₂O, (4) formation of a protective layer of CuO. This study thus may lead to a promising way to make high-performance and low-cost photoanodes for solar energy harvesting.

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1. Introduction

Photoelectrochemical (PEC) water splitting has been considered to be a promising pathway for solar energy conversion.^1,2 In general, a PEC cell consists of a semiconductor photoanode, a Pt counter electrode, and an aqueous electrolyte between them. In principle, the photoanode absorbs photons to generate electron–hole pairs, which are then separated and transferred to opposite electrodes for participating in water molecular splitting.³ Since the first discovery of a PEC cell with TiO₂ in 1972,⁴ various photoanode semiconductor materials, including transition metal oxides, Fe₂O₃, WO₃, BiVO₄, and ZnO, have been reported.^5–8

Among them, tungsten trioxide (WO₃) is a promising photoanode semiconductor used for PEC cells, due to its unique properties such as good electron mobility (∼12 cm² V⁻¹ s⁻¹), long hole diffusion length (∼150 nm), non-toxic, low cost, and appropriate valance band (VB) for water oxidation.^9,10 However, it captures only approximately 12% of the solar spectrum and suffers from rapid electron–hole recombination and slow transfer for photogenerated charge carriers at the photoanode/electrolyte interface, which seriously limits its PEC performance.¹¹ To overcome these drawbacks, studies with different approaches, such as doping with hetero-elements,^12,13 decorating with co-catalysts,^14,15 manipulating of nanostructured WO₃,^16,17 and coupling with another semiconductor to form heterojunction,^18,19 have been carried out. Among these studies, photoanodes that couple WO₃ with p-Cu₂O are considered to be one of the most promising strategies to promote charge separation and widen absorption.^9,20–22 For instance, Zhang J. et al. prepared WO₃/Cu₂O heterojunctions by electrodepositing different amounts of Cu₂O on the surface of the WO₃ film.²⁰ They found the highest photocurrent density was significantly improved by about 3.51 times higher than that of pristine WO₃, and this result is mainly attributed to the formation of p–n heterojunction and broaden solar absorption. We are aiming to extend these studies to achieve a more efficient photoanode by broadening the photoresponse range. The tandem cell is designed to achieve full use of the solar spectrum (UV, visible, and NIR regions).²³ As CuO has a narrower bandgap of ∼1.4 eV than Cu₂O of ∼2.1 eV,²⁴ it is expected to absorb lights in the NIR region. Thus, the tandem structure of WO₃/Cu₂O/CuO can harvest the whole solar spectrum due to WO₃, Cu₂O, and CuO layers are responsive to UV, visible, and NIR regions, respectively. The ITO/WO₃/Cu₂O photoanodes with electrodepositing different amounts of Cu₂O are investigated first in order to obtain an appropriate thickness of Cu₂O with optimal catalytic activity and then form Cu₂O/CuO by heat-treating.

In this work, a photoanode with the tandem structure of ITO/WO₃/Cu₂O/CuO is prepared and investigated. ITO/WO₃/Cu₂O/CuO structure is designed to enhance light harvesting and electron–hole pairs separation. It is systematically characterized in terms of structural, morphological, optical, and PEC properties. Its PEC performance is evaluated in comparison with these of photoanodes based on ITO/WO₃/Cu₂O and ITO/WO₃. As expected, significantly improved PEC performance of the as-prepared photoanode was achieved with a photocurrent density of 4.7 mA cm⁻² at 1.23 V versus reversible hydrogen electrode (RHE) under AM1.5G illumination, which is approximately 5.5 times that of pristine WO₃. These results demonstrate that ITO/WO₃/Cu₂O/CuO may be regarded as a promising, simple, and low-cost photoanode for solar energy harvesting.

2. Experimental details

2.1. Chemicals

Sodium tungstate dehydrates (Na₂WO₄–2H₂O), sodium chloride (NaCl), copper sulfate (CuSO₄), lactic acid (C₃H₆O₃), sodium hydroxide (NaOH), sodium sulfate (Na₂SO₄), and hydrochloric acid (HCl) were obtained from Sinopharm Chemical Reagent Co. Ltd. All the chemicals have been used directly without further purification.

2.2. Fabrication of ITO/WO₃ photoanodes

WO₃ film was directly synthesized on an indium tin oxide (ITO) substrate by the hydrothermal method reported previously.^25,26 As a cleaning procedure, ITO glass substrates were ultrasonicated consecutively in acetone, ethanol and DI water, and each process took 30 min. The solution was prepared, under vigorous stirring, first by dissolving 0.83 g Na₂WO₄–2H₂O in 19 mL DI water in a 50 mL beaker, then by adding 3 M HCl to reach pH 2.2, further by adding 0.29 g NaCl. The reaction took place in a Teflon-lined hydrothermal synthesis autoclave reactor at 180 °C for 150 min, where the cleaned ITO glass substrates (2.5 × 5 cm) were immersed with their conductive faces down in the precursor solution. After the reaction, uniform WO₃ films were obtained on the ITO substrates; they were rinsed extensively with deionized water and dried on a muffle furnace at 300 °C for a half-hour for usage.

2.3. Fabrication of ITO/WO₃/Cu₂O photoanodes

As intermediate samples, ITO/WO₃/Cu₂O photoanodes were made by electrodepositing a Cu₂O layer onto the prepared WO₃ surface. The bath solution for the process is an aqueous solution of 0.4 M copper sulfate and 3.0 M lactic acid, with an adjusted pH 12.0 by adding a controlled amount of 4.0 M NaOH. The electrochemical deposition was carried out with the three-electrode system where the prepared ITO/WO₃ substrate was used as the working electrode, a platinum plate as the counter electrode, and an Ag/AgCl wire as the reference electrode. The conditions set for the deposition include bath temperature at 60 °C, deposition potential of −0.4 V, and varied deposition times of 1, 5, 10, 15 min. Under such conditions, uniform Cu₂O films on the WO₃ substrates were obtained for all the samples in this experiment.

2.4. Fabrication of ITO/WO₃/Cu₂O/CuO photoanodes

WO₃/Cu₂O/CuO photoanodes were obtained by annealing the prepared WO₃/Cu₂O photoanodes in a muffle furnace at a suitable temperature (∼300 °C) for a suitable time (∼1 h) to oxidize their Cu₂O surfaces into CuO ones. The whole process for fabricating the WO₃/Cu₂O/CuO photoanodes is shown as a schematic in Fig. 1.

Fig. 1 Schematic diagram for fabricating ITO/WO₃/Cu₂O/CuO photoanodes.

2.5. Characterization

The prepared photoanodes were characterized for the crystal structures by using X-ray diffraction (X'Pert PRO; PANalytical B.V., Almelo, Netherlands), where Cu Kα (Kα = 1.5406 Å, 40 kV, 200 mA) was used as the X-ray source. Their elemental compositions were investigated by using X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Thermo-VG Scientific, East Grinstead, West Sussex, UK), with an Al Kα as the X-ray source. Their surface morphologies were measured with a scanning electron microscopy (SEM, Sirion 200, FEI, Hillsboro, Oregon, USA), equipped with an X-ray energy dispersive spectrometer, and a Field Emission Transmission Electron Microscope (FETEM, JEOL JEM-2100F). Their optical absorbance was measured by using a UV-vis-NIR spectrophotometer (V-670, Jasco, Tokyo, Japan). Their photoelectrochemical and photocurrent density measurements were evaluated in a three-electrode system by using the as-prepared photoanodes as the working electrode, Pt as a counter electrode and Ag/AgCl as a reference electrode in 0.5 M Na₂SO₄ (pH 7.0) electrolyte under simulated AM1.5 illumination. A 300 W Xe lamp (Perfect Light Company) was used as an illumination source.

The light source intensity was calibrated with an FZ-A irradiator-meter (Photoelectric Instrument Factory, China) to simulate AM1.5 illuminations (100 mW cm⁻²). The measured potential versus Ag/AgCl (E_Ag/AgCl) was transformed to that versus reversible hydrogen electrode (RHE) using Nernst equation:²⁷

E_RHE = E_Ag/AgCl + 0.0591 × pH + 0.1976 V

Electrochemical impedance spectroscopy (EIS) measurements were carried out at 20 mV s⁻¹ scan speed and frequency ranging from 0.1 Hz to 10⁵ Hz.

3. Results and discussion

The crystal structure of the fabricated ITO/WO₃/Cu₂O/CuO tandem photoanodes is revealed with X-ray diffraction (XRD) analysis, in comparison with these of the precursor photoanodes. The results are shown in Fig. 2. Apparently, WO₃ (♣) is identical in diffraction peaks among three different types of photoanodes, which is indexed as a hexagonal WO₃ (JCPDS no. 85-2460).²⁵ As well, the amorphous WO₃ is demonstrated by a broad hump between 2θ around 20° to 40°. Similarly, Cu₂O (◆) shows the same peaks between ITO/WO₃/Cu₂O and ITO/WO₃/Cu₂O/CuO photoanodes, it is indexed as cubic phase (space group: Pn3̄m (224); JCPDS no. 65-3288), due to the diffraction peaks located at 2θ of 29.6°, 36.5°, 42.4°, 61.5° and 73.7°, which reflects (110), (111), (200), (220), and (311), respectively. Additionally, CuO (●) is indexed as a monoclinic crystal as well (space group: C2/c (15); JCPDS no. 65-2309). These results demonstrate that the tandem photoanodes are constituted of layered crystals of WO₃, Cu₂O, and CuO.

Fig. 2 XRD patterns of (a) ITO/WO₃, (b) ITO/WO₃/Cu₂O and (c) ITO/WO₃/Cu₂O/CuO photoanodes. The mark ♠ is denoted for ITO, ♣ for WO₃, ◆ for Cu₂O, and ● for CuO.

To gain more insight into the chemical composition of photoanodes, XPS measurements were carried out for all three types of the obtained photoanodes. For the basic ITO/WO₃ photoanode, three well-resolved peaks at the binding energy of 35.5, 37.8 and 41.2 eV are observed (Fig. 3a), which are attributed to the spin–orbit split peaks of W 4f_7/2, W 4f_5/2 and W 4f_3/2, respectively.²⁵ On the other hand, its O 1s core-level XPS spectrum (Fig. 3b) contains one asymmetric peak, from which two fitted peaks centered at the binding energy of 530.6 and 532.5 eV can be obtained. The former corresponds to W–O bonds from the crystals, whereas the latter is related to W–OH bonds near the surface.⁹ These results indicate that this photoanode is fully covered with WO₃ crystals. Similarly, a full formation of the crystal Cu₂O layer on the basic ITO/WO₃ photoanode is also confirmed. Fig. 3c and d present the XPS spectra for Cu and O bonds of the prepared ITO/WO₃/Cu₂O photoanode, respectively. Obviously, only two peaks centered at 932.6 and 952.3 eV are observed, which corresponds to Cu 2p_3/2 and Cu 2p_1/2 of Cu₂O, respectively.²⁴ The fitted two peaks centered at 530.5 and 532.5 eV (Fig. 3d) correspond to the oxygen ions of the oxide and its surface group of OH⁻ and residual water that is left during wet processing. By heating the ITO/WO₃/Cu₂O photoanode in the open air, its surface Cu₂O is expected to be oxidized into CuO. As expected, other than the peaks for Cu₂O, two additional fitted peaks centered at 933.6 and 953.4 eV appear (Fig. 3e). The new peaks are assigned to the formation of CuO.²⁴ However, only one peak for oxygen ions is found for the heat-treated photoanode (Fig. 3f). This is attributed to the removal of residual water and prevention from surface hydroxylation of the subjected photoanode under temperature treatment.

Fig. 3 XPS spectra of the prepared photoanodes. (a) W 4f and (b) O 1s for ITO/WO₃, (c) Cu 2p and (d) O 1s for ITO/WO₃/Cu₂O, and (e) Cu 2p and (f) O 1s for ITO/WO₃/Cu₂O/CuO photoanode.

The surface morphology of the synthesized photoanodes is investigated. For the basic photoanode ITO/WO₃, its surface morphology was discussed in our previous report.^25,26 For the heterojunction photoanode of ITO/WO₃/Cu₂O, its surface morphology evolving with the formation of Cu₂O is studied. Fig. 4a–d shows the surface SEM image of the photoanode prepared with an electrodeposition time for 1, 5, 10 and 15 min, respectively. Obviously, cubic grains of Cu₂O are observed to form from the beginning and keep growing in density and size. At the deposition time of 15 min, the surface of the photoanode is fully covered with Cu₂O grains. The prepared photoanode at 15 min was oxidized when it tested for PEC measurements, then the photoanode at 10 min has been selected to form a tandem photoanode. The surface morphology of ITO/WO₃/Cu₂O photoanode before and after annealing was measured and results are shown in Fig. 5. It can be seen that surface morphology is slightly changed after the annealing. The morphology is a compacted and truncated pyramid before annealing. After annealed, irregular small particles are grown on the pyramid surface and the optical color was changed from reddish-brown to black due to CuO formation. The outline of the pyramid shape still could be seen, which is agreed with the mentioned XRD that the annealed photoanode has a mixture of Cu₂O and CuO. The cross-section SEM images of the ITO/WO₃/Cu₂O photoanode before and after annealing, with the highlight of WO₃, Cu₂O, CuO and ITO layers, are displayed in Fig. S1.† The annealed electrode slightly thicker than the as-prepared ITO/WO₃/Cu₂O due to the CuO particle formed on the top surface of the Cu₂O.

Fig. 4 SEM images of ITO/WO₃/Cu₂O photoanodes prepared at different electrodeposition times for Cu₂O layer formation: (a) 1 min, (b) 5 min, (c) 10 min and (d) 15 min. The scale bar is 2 μm.

Fig. 5 SEM images of ITO/WO₃/Cu₂O photoanode (a) before and (b) after annealing at 300 °C in the open air. The scale bar is 2 μm.

HRTEM analysis of the prepared photoanodes was measured to verify lattice fringes of the formed materials and the results are shown in Fig. 6. The regular spacing of 0.383 nm is observed in Fig. 6a, which is corresponded to the (002) plane of hexagonal WO₃. Moreover, a d-spacing of 0.246 nm is shown in Fig. 6b and c, which is correlated to the (111) plane of Cu₂O. Furthermore, the lattice spacing of 0.232 nm is assigned to the (111) plane of CuO. These results reveal that the lattice fringes of WO₃, Cu₂O and CuO are confirmed. HRTEM analysis has good agreement with XRD and XPS.

Fig. 6 HRTEM images of the prepared photoanodes. Lattice fringes of (a) WO₃ for ITO/WO₃, (b) Cu₂O for ITO/WO₃/Cu₂O, and (c) Cu₂O and CuO for ITO/WO₃/Cu₂O/CuO photoanode.

The optical absorbance spectra of the prepared photoanodes were measured by the UV-vis-NIR spectrophotometer and the results are shown in Fig. 7. Fig. 7a shows spectra of ITO/WO₃/Cu₂O photoanodes with varying electrodeposition times for the formation of the Cu₂O layer. The absorption edge wavelength is shifted to a higher wavelength (is redshifted) with increasing the deposition time, which is moved from around 320 nm to ∼520 nm for 1 min and 15 min, respectively that can highly increase the visible light absorption from the solar spectrum. As shown in Fig. 7b, the absorption edge wavelength is moved from about 500 nm to around 645 nm for ITO/WO₃/Cu₂O and ITO/WO₃/Cu₂O/CuO, respectively which can increase solar energy harvesting. These enhancements are attributed to the optical absorbance and bandgap (E_g) of functional layers with the tandem design. Optical absorbance and the corresponding calculated bandgap (E_g) from the Tauc plot of each bare layer have been implemented²⁸ and are presented in Fig. S2.† The absorption edges of WO₃, Cu₂O and CuO film are located at approximately 310 nm, 520 nm and 805 nm, which correlate to a bandgap of about 2.9 eV, 2.1 eV and 1.38 eV that responsive to UV light, visible light and NIR light, respectively.

Fig. 7 UV-vis absorption spectra of (a) ITO/WO₃/Cu₂O photoanodes prepared at different electrodeposition times for the formation of Cu₂O layers and (b) photoanodes with varied functional layers.

The linear sweep voltammetry (LSV) measurements were recorded to test the PEC performance and the results are displayed in Fig. 8. Fig. 8a shows the LSV curves of the ITO/WO₃/Cu₂O photoanodes with different electrodeposition times of the Cu₂O layer. Clearly, photoanodes exhibit significantly increased photocurrent at 1.23 V vs. RHE with increasing electrodeposition time compared with pure ITO/WO₃ photoanode. The highest achieved photocurrent density at 1.23 V vs. RHE is 3.4 mA cm⁻², which was recorded at deposition time of 10 min and 15 min, compared with 0.85 mA cm⁻² for ITO/WO₃ photoanode. This significant improvement could mainly be attributed to the high absorption coefficient of Cu₂O and the construction of a p–n heterojunction between Cu₂O and WO₃. Unfortunately, an excess of Cu₂O amount may produce lattice scattering and decrease of charge carrier concentration^20,21 that was happened for the photoanode at 15 min. An obvious peak is observed at 1.1 V vs. RHE and might be attributed to the oxidation of Cu₂O. For the investigation, SEM images of the photoanode were measured before and after the PEC test and the result is shown in Fig. S3.† A small particle is formed on a pyramid shape, which is attributed to the oxidation of a small amount of Cu₂O to Cu. Therefore, ITO/WO₃/Cu₂O at 10 min has been selected to fabricate a tandem photoanode. The ITO/WO₃/Cu₂O/CuO photoanode was formed by annealing of ITO/WO₃/Cu₂O in the open air. Fig. 8b presents the LSV plots of the functional photoanodes under light illumination (1 sun). As expected, ITO/WO₃/Cu₂O/CuO photoanode yields the highest photocurrent density of ∼4.7 mA cm⁻² at 1.23 V vs. RHE, which is approximately about 1.4 and 5.5 times of the measured values for ITO/WO₃/Cu₂O and ITO/WO₃ ones, respectively. These significant enhancements are attributed to the CuO layer that can extend light absorption and then increase the generation of electron–hole pairs²⁹ with the charge-separation improvement as well.

Fig. 8 Linear sweep voltammetry curves of (a) ITO/WO₃/Cu₂O photoanodes prepared at different electrodeposition times for the formation of Cu₂O layers and (b) photoanodes with varied functional layers.

The charge carriers separation improvement could be ascribed to the p–n junction effect, which could induce the directional transfer of photogenerated holes and electrons to the copper oxide and WO₃, respectively as shown in Fig. 9. The p–n junction influence can separate efficiently the generated electron–hole pairs. The photogenerated electron can transfer from the Cu₂O conduction band to that of WO₃, it would reduce hydrogen ion (H⁺) into hydrogen gas (H₂). On the other hand, the photogenerated hole can move from the valence band of WO₃ to that of Cu₂O, it would oxidize water into oxygen gas (O₂) and hydrogen ion (H⁺).^9,21 The substantial enhancement in charge separation on ITO/WO₃/Cu_xO photoanode indicates that Cu_xO coupling has a more obvious impact on the water oxidation kinetics of WO₃ than that on the surface-charge carriers' separation of ITO/WO₃ photoanode.³⁰

Fig. 9 Schematic diagram of the charge carrier transfer mechanism on ITO/WO₃/Cu₂O and ITO/WO₃/Cu₂O/CuO photoanodes under simulated sunlight illumination.

Electrochemical impedance spectroscopy (EIS) measurements were used to analyze the interfacial charge transfer properties of the prepared photoanodes. The experiments were carried out in a standard three-electrode system in a 0.5 M Na₂SO₄ aqueous solution with a 20 mV s⁻¹ scan speed at AC potential of 20 mV vs. Ag/AgCl with a frequency ranging from 0.1 Hz to 10⁵ Hz at room temperature under illumination. Fig. 10a shows Nyquist plots of ITO/WO₃/Cu₂O photoanodes at different electrodeposition times for the Cu₂O layer. The diameter of the semicircle is indicative of the charge transfer resistance, R_ct, between the photoanode and the electrolyte. It can be realized that the semicircle diameter is decreased with increasing electrodeposition time, which clearly indicates enhanced charge transfer and decreased the recombination rate of charge carriers at the interface between the photoanode and electrolyte, which agrees with previous results.²⁰ As shown in Fig. 10b, the ITO/WO₃/Cu₂O/CuO photoanode has the smallest semicircle diameter compared with ITO/WO₃/Cu₂O and ITO/WO₃ photoanode, which directs that the transfer of charge carrier from the photoanode to the electrolyte is increased due to the CuO layer formation and better electrocatalytic activity for water oxidation of Cu_xO compared with other photoanode.³⁰ EIS results have a great agreement with the absorbance of prepared photoanodes and their PEC performance. Overall, these results illustrate that enhanced electrocatalytic activity can facilitate better transport of charge carriers that is an important contributor to the superior PEC performance.

Fig. 10 Nyquist plots of (a) ITO/WO₃/Cu₂O photoanodes prepared at different electrodeposition times for the formation of Cu₂O layers and (b) photoanodes with varied functional layers.

The photocurrent stability of the prepared photoanodes was evaluated with photocurrent time (J–t) measurements under illumination and the results are displayed in Fig. 11. As shown in Fig. 11a and b, the photocurrent of the photoanodes rises greatly under chopped light illumination and drops largely to zero when the light is off, indicating their excellent transient photoresponse. The photocurrent shows intense spikes instantaneously with turning on the light. These photocurrent spikes are followed by slow decaying until the light turns off. The existence of a spike-shaped photocurrent is related to the accumulation of charge carriers at the interface of the photoanode with electrolyte. The photoanodes show a quick response once the light turned on, good stability, and recyclability for all cycles under chopped light. As shown in Fig. 11, the ITO/WO₃/Cu₂O/CuO photoanode shows a higher photocurrent density and more good stability than ITO/WO₃/Cu₂O and ITO/WO₃ photoanode, which is attributed to high generation of electron–hole pairs and fast charge carrier transfer. The stability enhancement of the photoanode may be correlated to the CuO layer served as a protected layer for the Cu₂O³¹ and better catalytic ability of Cu₂O/CuO with coupling with WO₃. Moreover, it can be realized that with the forming of the CuO layer on the top surface of the ITO/WO₃/Cu₂O, more accumulation of charges occurs at the interface of the photoanode and electrolyte. When the light was turned on, the rate of charge carrier generation inside the ITO/WO₃/Cu₂O/CuO is more than that for the ITO/WO₃/Cu₂O. Also, the photocurrent decay during sun illumination is lower for the tandem photoanode, which suggests that decreasing charge carriers' recombination rate.

Fig. 11 Evaluated stability of the prepared photoanodes. (a and b) Transient photocurrent of photoanodes was performed with a standard three-electrode system in a quartz glass beaker contained a 0.5 M Na₂SO₄ aqueous solution under chopped 1 sun illumination with period 60 s, on/off. (c) J–t and (d) relative stability curves of the photoanodes under continuous illumination for 3 hours.

For a long time stability investigation, the J–t curves of the functional photoanodes have been recorded under continuous illumination for 3 hours as shown in Fig. 11c. The relative photocurrent stability is defined as the percent of photocurrent density at any time divided by the initial photocurrent. The relative stability of the ITO/WO₃, ITO/WO₃/Cu₂O and ITO/WO₃/Cu₂O/CuO is 30%, 71% and 79%, respectively after illumination for 3 hours as presented in Fig. 11d. Photocurrent and stability of the ITO/WO₃ photoanode are both constrained by its low absorption, poor surface separation and slow transfer for photogenerated charge carriers. The ITO/WO₃/Cu₂O photoanode has a photocurrent higher than ITO/WO₃. But the chemical instability is an inherent drawback of Cu₂O in aqueous solutions because the redox potentials for the reduction and oxidation of monovalent copper oxide lie within the bandgap.⁹ Therefore, it is still not too stable. Higher photocurrent and better stability are observed for the ITO/WO₃/Cu₂O/CuO photoanode in comparison with other photoanodes. These improvements are attributed to (1) harvested UV, visible, and NIR light of the solar spectrum, (2) accelerated charge separation at the heterojunction between WO₃ and Cu_xO, (3) better electrocatalytic activity of formed Cu_xO, and (4) forming a protected layer of CuO.

4. Conclusions

The ITO/WO₃/Cu₂O/CuO tandem photoanode has been successfully fabricated for solar energy harvesting. WO₃ layer was deposited on an ITO glass by hydrothermal method and then electrodeposited Cu₂O on it, and finally oxidizing the Cu₂O layer into a partial CuO layer. The structural, morphological, optical and PEC properties of photoanode were systematically investigated. PEC performance is boosted to achieve a photocurrent of 4.7 mA cm⁻², which is about 1.4 times and 5.5 times higher than that for ITO/WO₃/Cu₂O and ITO/WO₃ ones, respectively. The improvement could be attributed to (1) harvested the whole solar spectrum due to WO₃, Cu₂O and CuO layers are responsive to UV, visible and NIR regions, respectively, (2) accelerated charge separation at the heterojunction between WO₃ and Cu₂O/CuO, (3) better electrocatalytic activity of formed Cu_xO than pure Cu₂O, (4) formation of a protective layer of CuO. Our results demonstrate that ITO/WO₃/Cu₂O/CuO could be a promising, low-cost and highly efficient photoanode for solar energy harvesting devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21774114), the National Key R&D Program of China (Grant No. 2017YFA0403403), and CAS-TWAS President's Fellowship (awarded to Mahmoud A. Khalifa).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ra00417d

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