1.
Introduction

Since Fujishima and Honda initially, in 1972, reported hydrogen generation in a photoelectrochemical water splitting device by introducing the TiO₂ electrode^[1]. The conversion from solar energy into chemical fuels like hydrogen is a most practicable method to solve the world-wide sustainable energy challenges. However, as has been discussed often, until now, the reported conversion efficiency is still too low for application in reality^[2–10]. To solve this problem, building novel morphologies has been an effective method in fundamentally improving the efficiency of photocatalytic water splitting, based on their structural advantage depending on the size and shape. Nanowire, as a typical nanostructure, has been successfully manufactured in various semiconductors for enhancing the PEC performances, such as TiO₂, ZnO^[10].



Cu₂O, as an attractive candidate for light energy conversion, possessing a favorable direct band gap (E_g ~ 2.1 eV), which the maximum photocurrent in theory is of 15 mA/cm² and the light-to-H₂ conversion efficiency is 18% under a standard solar radiation^[10–14]. Besides, compared to other p-type PEC materials, Cu is much abundant and the preparation method is low-cost by industrial proven. Recently, the Cu₂O thin films photocathode with highly active of was proposed^[14], which reached the large photocurrents of ~7.6 mA/cm² at 0 V versus RHE and maintained working for one-hour testing time. In our previous work, we reported an efficacious surface modification for Cu₂O photocathodes by trisodium citrate to realize an impressive PEC performance^[15]. These results indicate significant feasible of Cu₂O in photocatalysis water splitting. In particular, for efficiency improvement, various Cu₂O nanostructures with high specific surface area and short charge carrier diffusion lengths compared with bulk materials, have been successfully fabricated, including nanorods, nanowires (NWs), nanotubes, nanocubes, and nanospheres^[16–19] and different synthesis^{[20, 21]}. However, super-long Cu₂O NW arrays have been not well reported, for PEC water splitting. In this paper, we synthesize successfully super-long Cu₂O NW arrays by the convenient and low-cost anodic alumina oxide (AAO) template technology. In comparison with the photocathode based on Cu₂O films, the super-long Cu₂O NWs photocathode demonstrates a remarkable enhancement in photocurrent, from –1.00 to –2.75 mA/cm² at –0.8 V versus Ag/AgCl. Since the photoelectrochemical system was optimized by depositing Pt NPs with ALD on the Cu₂O NWs, the plateau photocurrent has been improved to –7.00 mA/cm² at –0.8 V versus Ag/AgCl and the external quantum yield (EQY) is up to 34% at 410 nm. The research results of this work offer an inexpensive, naturally resourceful nanowire material element for applications in photoelectrochemical cells.

2.
Experimental

2.1
Preparation of Cu₂O NWs and films

Synthesized from high purity Al foils, AAO templates were fabricated with a two-step anodization in oxalic acid (0.3 M) at 40 V during eight-hours anodization time for the first step and one-hour anodization time for the second step^{[22, 23]}. The AAO pores were broadened in H₃PO₄ solution (5 wt%) for 50 min at 30 °C. After that, a 25 nm-thickness layer of gold was deposited with the process of physical vapor deposition (PVD). Subsequently, Ni film layer was synthesized on the surface of AAO template at a current density of 5 mA/cm². The Ni plating solution has NiSO₄ (0.38 M), NiCl₂ (0.12 M), and H₃BO₃ (0.5 M). The AAO template with gold was utilized as the working electrode, while Ni foil was used as the counter electrode. Then, the backside alumina was removed by CuCl₂ solution, and the AAO with Ni layer was obtained. Later on, the sample was soaked in H₃PO₄ (5%) for 30 min in 60 °C in order to remove the barrier layer before the deposition of Cu₂O. At the end, Cu₂O NWs were electrodeposited catholically in a copper sulphate bath (0.4 M) which contained 3 M lactic acid. The pH value of the bath was configured to 12 with caution adding 3 M NaOH. Then Cu₂O NWs were synthesized under a steady potential (–0.40 V versus Ag/AgCl) with a standard three-electrode system for a 90 min. By a heating stage with a real-time temperature probe, the bath temperature was set at 45 °C the whole time. After deposition, the AAO skeleton was removed by immersion in sodium hydroxide aqueous solution (0.1 M) aim time 1 hour and long Cu₂O ordered nanowires were hence obtained. As a comparison, Cu₂O thin films directly on FTO/Au was prepared by the same process.

2.2
Fabrication of photocathodes of Cu₂O NWs and films

The strip of conductive Cu tapes was connect to the uncovered FTO and Ni area of the FTO/Au/Cu₂O and Ni/Cu₂O NWs, respectively, in order to extend the conducting circuit. The Cu tapes were later put through the glass tubes, and then waterproofed with the a/b mixed epoxy adhesive. The area of samples was measured by a computer with the help of Photoshop software.

2.3
Measurements of PEC characteristics

The EQY measurement of the sample was obtained under a xenon lamp (150 W) with a grating monochromator from Newport corporation. Photocurrent–potential (J–V) curves and impedance characteristic were recorded by the high accuracy Bio-Logic potentiostat (SP-200) in sodium sulphate solution (0.1 M). A reference electrode (Ag/AgCl) and a counter electrode (Pt) were utilized during the measurements. As the light source, a standard 300 W xenon lamp (Newport) was used, and the optical intensity was set to 100 mW/cm² with a standard Si solar cell from Newport corporation.

2.4
Characterizations of samples

The crystal structures and mapping of the Cu₂O NWs and films were evaluated by measuring X-ray diffraction (XRD) spectra with Ni filtered Cu Kα radiation. The surface and cross-section morphologies of the samples were investigated using a scanning electron microscope (SEM, Hitachi S-570).

3.
Results and discussion

The whole fabrication procedure of Cu₂O NWs can fall into 5 detailed processes that are presented schematically in Fig. 1, in which the colors of the Al, AAO, Au, Ni and Cu₂O are illustrated as gray, blue, yellow, black and orange, respectively. The process includes gold layer deposition (I), Ni electrodeposition (II), aluminum and barrier layer removal (III), Cu₂O growth (IV), and template removal (V). The AAO templates were employed as the main template during the preparation for the dramatic advantages, such as highly adjustable structural parameters, uniform and well oriented nano-porous structures, large surface area, cost-effective and good thermal stability and high mechanical strength^[23–25].






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Figure1.
(Color online) Schematic illustration of the whole fabrication procedure of Cu₂O NWs by AAO template: gold layer deposition (I), Ni electrodeposition (II), aluminum and barrier layer removal (III), Cu₂O growth (IV), and template removal (V).




Fig. 2(a) clearly illustrates that bare AAO template, which was fabricated with two times oxidization^{[24, 25]}, shows quite uniform distribution of nanopores with ~150 nm in diameter and ~5 μm in length. The Cu₂O NWs and films were prepared by anodic electrodeposition, which was in a Cu sulphate bath added 3 M 2-hydroxy propionic acid, by following the previously reported processes^[15]. Fig. 2(b) shows the representative scanning electron microscopic (SEM) images of the as-grown nanowires of Cu₂O on nano-Ni substrate. And the Cu₂O nanowires are arranged vertically and neatly on the substrate. The size of the nanowires is gauged as 4.5 μm around in length and 85 nm around in diameter, respectively. The films of Cu₂O grown on the substrates of Au/FTO glass are fabricated by continuously distributed polyhedral particles, as shown in Fig. 2(c)^[15]. Numerous attentions make the promotion and/or utilizations of PEC materials, especially in nanostructure to be used in splitting water (H₂O → $frac{1}{2}$ O₂ + H₂) systems. Also involving these photocatalysts to light-absorbers, this is unique since catalysts utilization in standard electrolysis. The most noteworthy alteration is that light-coupled electrolysis needs absorbers across on large areas to raise irradiation flux capture. On the other hand, when a catalyst is straight released on the absorber surface that will harshly inferior the requirements for current detection per unit geometric area.






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Figure2.
(Color online) SEM images of (a) the as-prepared AAO template, (b) Cu₂O NWs (inset is cross-sectional SEM image of Cu₂O NWs) and (c) Cu₂O films. (d) The corresponding XRD patterns of Cu₂O NWs and films (inset is mapping of Cu₂O NWs).




In addition, the band gap of the grown films of Cu₂O is measured as 2.1 eV by the UV–vis spectrums reported in our previous paper^[15], which is consistent with the reported values^{[22, 23]}.



The corresponding XRD patterns of the Cu₂O NWs and films are given in Fig. 2(d), where the diffraction peaks at 29.6°, 36.5°, 42.4°, 61.4° and 73.6° can be no doubtfully classified to the reflection of planes (110), (111), (200), (220) and (311) of the polycrystalline Cu₂O, which are consistent with the No. 05-0667 in JCPDS card^[26]. Phases of Cu and CuO are not distinguishable, suggesting a pure Cu₂O structure in the nanowires and films. For further determination of the composition of the material elements, we characterized the mapping of Cu₂O NWs with SEM, as shown in inset of Fig. 2(d), which sustains that just Cu and O elements are appear in the nanowires.



Fig. 3(a) demonstrates the typical stable state EQY curves of the photoelectrodes of Cu₂O NWs and films. Comparing to Cu₂O thin films photoelectrode, the Cu₂O NWs photoelectrode on the nano-Ni exhibits an increscent EQY below the absorption threshold of Cu₂O (600 nm). An enhancement of 2-fold is obtained, indicating that Cu₂O NWs based electrode has a larger photo-to-current efficiency than that with Cu₂O thin films. This result is consistent with those reported in recent studies that have focused on the outstanding light and electric properties of nanowires and improvement in photocatalytic performance of semiconductor nanowires^{[27, 28]}.






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Figure3.
(Color online) (a) EQY spectra, (b) photocurrent–potential profiles, (c) time-dependent photocurrent density spectra and (d) impedance spectra of the Cu₂O NWs and films photoelectrode.




J–V curves were characterized by immersing the PEC electrodes in 0.1 M sodium sulfate solutions, with the reference of Ag/AgCl and the counter electrode of Pt wire. Each plot represents a typical photo-response obtained under simulative light source (AM 1.5 G, 100 mW/cm²), as illustrated in Fig. 3(b). These data reveal a cathodic photocurrent and imply a obvious p-type characteristic of both samples of Cu₂O NWs and films, which are consistent with our previous report^[15]. In the system, photo-induced electrons move from films and nanowires to electrolyte to drive PEC reactions, and the corresponding holes transport from films and nanowires to ITO electrode during the PEC measurement. To be exciting, compared to the Cu₂O films electrode, the photocathode of Cu₂O NWs possesses a remarkable improvement in PEC performance, in good agreement with EQY measurement. To be noted, both Cu₂O films and NWs photoelectrode have the photoreduction peak at 0 V versus Ag/AgCl, owing to the weak chemical stabilization of Cu₂O, which is consistent with other literature reported^[14].



As illustrated in Fig. 3(c), to study the PEC performance and stability under visible light of samples, the time-dependent photocurrent (J–t) profiles of the Cu₂O NWs and films was given with chopped light illumination (on: 250 s; off: 250 s) at –0.3 V versus Ag/AgCl. The results demonstrated that both samples exhibit photocurrent responses under each illumination quickly and reproducibly. The plateau photocurrent density of the Cu₂O films electrode was –0.11 mA/cm², while that of the Cu₂O NWs was –0.71 mA/cm², which represents a more 500% increase from the Cu₂O films. The result demonstrates that nanostructure of materials can achieve the improvement of the photoelectric conversion, owing to the capability of offering large surface area and more reaction sites, decoupling light absorption and the collection of charge carrier, shorting diffusion distance of charge carrier compared to the bulk structures^{[29, 30]}.



To elucidate the strong correlation between nanostructures and the enhanced photocurrent and EQY values, electrochemical impedance measurement was carried out. Fig. 3(d) shows Nyquist Impedance spectra of Cu₂O NWs and films at DC frequency (200 kHz – 200 Hz) performed at open circuit voltage. In the electrolyte, Rct can be figured with fitting the semi-arc in 1 kHz region^[31], which reflects the interfacial charge-transfer resistance between the working electrode and electrolyte. Smaller circular radius results in a lower resistance of electron transport and higher efficiency of separation between photo-excited positive and negative charges. As shown in Fig. 3(d), the charge transfer resistance of Cu₂O NWs electrode is much smaller than that of Cu₂O film sample, indicating a much smaller electron transport resistance and a larger photo response, leading to a larger photocurrent in Cu₂O NWs electrode.



To further improve the performance of PEC in Cu₂O nano-device, Pt NPs were deposited on the nanowire surrounds, which is the catalyst by a procedure of the ALD^{[32, 33]}. Fig. 4(a) displays the top view SEM image of the catalyst on Cu₂O NWs, where the particles’ size can be gauged as 5–10 nm, and Pt NPs are uniformly and high density coated on the Cu₂O NWs. The photocurrent-potential curves of the improved photocathode are shown in Fig. 4(b). By comparing with the bare Cu₂O NWs electrode, the one modified with Pt NPs catalyst has an effective enhancement in photocurrent, which is amplified to –7 mA/cm². Moreover, the photocurrent-time profile of the photoelectrode at –0.3 V versus Ag/AgCl (Fig. 4(c)) shows that there is no apparent decrease in current density during about 2500s time span after several light–dark period. To be surprising, by modifying the Cu₂O NWs surface with Pt NPs, the EQY is amplified to 34%, which is 2-fold of the bare Cu₂O NWs and 4-fold of Cu₂O films, which showing a great latent application of Cu₂O in water splitting, as shown in Fig. 4(c).






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Figure4.
(Color online) (a) Top-view SEM image of Cu₂O NWs with Pt NPs. (b) Photocurrent–potential curves and (c) photocurrent-time profile at –0.3 V versus Ag/Ag and (d) EQY spectra of the photoelectrode based on Cu₂O NWs with Pt NPs. The inset is impendence spectra.




To shed light on the enhancement mechanism of PEC performance in Cu₂O NWs with Pt NPs, the schematic illustration of Cu₂O NWs/Pt photoelectrode and the total band energy spectrum of the Cu₂O/Pt/electrolyte structure are schematically presented in Fig. 5. The bending band of Cu₂O/Pt/electrolyte is similar to that case for p-Si/Pt/ electrolyte contact in a photoelectrochemical cell^[34]. Photons absorbed by Cu₂O NWs generate minority-carriers (electrons), which drifts to the interface of photoelectrode/electrolyte where water was decomposed (from H₃O⁺ to H₂). As an electrocatalyst for the photocathodic reaction (hydrogen generation), Pt NPs greatly increase the specific surface area and catalytic activity during photochemical reaction^[35].






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Figure5.
(Color online) (a) Schematic diagram of Cu₂O NWs/Pt photoelectrode and (b) energy band-gap spectrum of the Cu₂O NWs with/without Pt NPs.

4.
Conclusion

In conclusion, we have shown that a novel photocathode based on super-long Cu₂O NWs was successfully fabricated via the anodic alumina oxide template method. In comparison with the photocathode of Cu₂O films, the Cu₂O NWs photocathode demonstrates a significant enhancement in photocurrent, from –1.00 to –2.75 mA/cm², attributing to advantages of the nanostructure including high specific surface area, light-trapping and short carriers transfer. After optimization of the photoelectrochemical electrode through depositing Pt NPs with ALD technology on the Cu₂O NWs, the plateau of photocurrent has been enlarged to –7 mA/cm² furthermore. This work provides a low-cost, naturally abundant nanowire material for application in photoelectrochemical cells.

Acknowledgements

This work was supported by European Research Council (HiNaPc: 737616), European Research Council (ThreeDsurface: 240144), BMBF (ZIK-3DNanoDevice: 03Z1MN11), DFG (LE 2249_4-1), BMBF (Meta-ZIK-BioLithoMorphie: 03Z1M511), National Natural Science Foundation of China (Nos. 21577086, 51702130, 21503209), Natural Science Foundation of Jiangsu Province (BK 20170550), Jiangsu Specially-Appointed Professor Program, Hundred-Talent Program (Chinese Academy of Sciences), Beijing Natural Science Foundation (No. 2162042) and Key Research Program of Frontier Science, CAS (No. QYZDB-SSW-SLH006).