Cu2O Nanowire Photocathodes for Efficient and Durable Solar Water

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Cu2O Nanowire Photocathodes for Efficient and Durable Solar Water Splitting Jingshan Luo, Ludmilla Steier, Min-Kyu Son, Marcel Schreier, Matthew T. Mayer, and Michael Grätzel Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04929 • Publication Date (Web): 11 Feb 2016 Downloaded from http://pubs.acs.org on February 14, 2016

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Cu2O Nanowire Photocathodes for Efficient and Durable Solar Water Splitting Jingshan Luo,* Ludmilla Steier, Min-Kyu Son, Marcel Schreier, Matthew T. Mayer, Michael Grätzel Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1015 Switzerland *E-mail: [email protected] Abstract Due to its abundance, scalability and non-toxicity, Cu2O has attracted extensive attention towards solar energy conversion, and it is the best performing metal oxide material. Until now, the high efficiency devices are all planar in structure, and their photocurrent densities still fall well below the theoretical value of 14.5 mA cm-2 due to the incompatible light absorption and charge carrier diffusion lengths. Nanowire structures have been considered as a rational and promising approach to solve this issue, but due to various challenges, performance improvements through the use of nanowires have rarely been achieved. In this work, we develop a new synthetic method to grow Cu2O nanowire arrays on conductive fluorine-doped tin oxide substrates with well-controlled phase and excellent electronic and photonic properties. Also, we introduce an innovative blocking layer strategy to enable high performance. Further, through material engineering by combining a conformal nanoscale p-n junction, durable protective overlayer and uniform catalyst decoration, we have successfully fabricated Cu2O nanowire array photocathodes for hydrogen generation from solar water splitting delivering unprecedentedly high photocurrent densities of 10 mA cm-2 and stable operation beyond 50 hours, establishing a new benchmark for metal oxide based photoelectrodes. Table of contents graphic:

0 -2

Current density (mA cm )

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-2 -4 -6 -8 -10 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Potential vs. RHE (V)

Keywords: Cu2O, Nanowire, Photocathode, Solar Water Splitting

ACS Paragon Plus Environment

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The seasonal, regional and diurnal cycle variations of solar radiation incident on Earth require an efficient way to store solar energy. Converting solar energy directly into chemical fuels is considered as one of the most promising strategies to solve this issue1-4. To meet the global energy demand at the terawatt scale, the materials used must be Earth-abundant, scalable and compatible with low cost fabrication processes. Besides these, non-toxicity is important considering the human health and environment. Among the materials used for solar fuel generation, Cu2O is the only candidate that meets all of these demands while also delivering impressive performance in terms of photocurrent and photovoltage. With a band gap of 2.0 eV, Cu2O could theoretically deliver a solar to hydrogen conversion efficiency of 18% for water splitting and a power conversion efficiency of 20% as a solar cell5-7. Though Cu2O has favorable band energy positions for water splitting, there are two main challenges inhibiting its application as an efficient and durable photocathode for water splitting. The first issue is stability, as the redox potentials for the reduction and oxidation of the monovalent copper oxide lie within the water splitting potentials, rendering the photocorrosion of the material in the electrolyte more favorable compared to the water splitting reactions5. This issue has been improved by surface protection through overlayers formed by atomic layer deposition (ALD)5, 8. The second issue is the unfavorable ratio of the carrier diffusion length over the light absorption depth9, 10. In order to efficiently absorb sunlight, Cu2O films must typically be at least one µm thick. However, the minority carrier (electron) diffusion length is limited to about 200 nm or less, depending on the synthetic method5, 11. This results in inefficient collection of photogenerated carriers. Even though various attempts have been made to enhance the carrier collection in Cu2O, including doping12, high temperature processing and overlayer engineering6, 13, the photocurrent density still remains low for this material. Nanostructured photoelectrodes14, especially nanowire (NW) arrays, constitute an attractive approach to solve this problem through morphology control, combining efficient light harvesting along the full length of the wire with short radial diffusion distance for the minority carriers towards the electrolyte solution15, 16. Furthermore, nanostructures offer a large surface area, leading to increased exposure of catalytic sites and accelerated reaction kinetics16, 17. Though nanostructuring is a promising concept toward enhancing the efficiency, making a real nanostructured device that truly delivers higher performance compared to planar architecture has proven to be challenging18. Taking the dominant solar material Si for example, the most efficient devices are still planar in structure19. The challenges are manifold: The primary one is maintaining material quality, as the processes for nanostructure synthesis usually introduce more defects in the materials, either due to the growth method or simply from the higher surface area to volume ratio. The second challenge is the precise control of doping and the engineering of overlayers to make an efficient p-n junction20. Finally, it is difficult to make a charge-collecting contact along the nanostructure surface without interfering with light absorption, especially for devices illuminated from the front side. For photoelectrochemical (PEC) water splitting applications employing a semiconductor– electrolyte interface, this contact is facilitated by the electrolyte penetrating into the porous nanostructure, thereby naturally creating a conformal contact15, 21.


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Thus, although several groups have attempted Cu2O NW photocathodes for solar water splitting22-27, the use of nanostructured morphologies has failed to produce performance benefits compared to the compact film devices that were developed by our group5, 8. Most works report the NW synthesis and the structure along with the basic PEC response for water splitting without overlayer protection and catalyst loading24, 27. A few studies did carry out surface protection22, 23, 25, 26, but still were not able to reach the benchmark water splitting performance of the best planar Cu2O device5, 8. It is worth mentioning that the reducing photocurrent from Cu2O samples in direct contact with water is mainly expected to be due to the photo-corrosion of Cu2O itself28, necessitating product gas quantification to validate reported hydrogen evolution currents. In this work, we report a highly efficient and durable Cu2O NW photocathode that outperforms the best reported planar Cu2O for solar water splitting. The success of this electrode relies on the following three efforts. First, we developed a facile and highly reproducible method to grow high quality Cu2O NW arrays on Cu-coated fluorine-doped tin oxide (FTO) substrates. Second, we introduced a blocking layer strategy to inhibit the direct contact of exposed Cu metal surface with the n-type overlayer enabling the formation of a high quality p-n junction on the surface. Lastly, we adopted a judicious overlayer protection and catalyst decoration strategy to create uniform and conformal heterostructure interfaces. Previous reports of Cu2O NW synthesis were mostly based on Cu foil substrates subjected to electrochemical anodization or chemical oxidation followed by thermal annealing in Ar or N2 atmospheres22, 23. Generally, the foils used are thin and flexible, which is advantageous for making flexible devices. However, for surface-protected Cu2O electrodes, the flexibility of the substrate can lead to cracks in the protection layer, resulting in corrosion of the samples when exposed to electrolyte. To avoid this damage, we chose conductive FTO glass as a rigid substrate which was subsequently coated with a homogenous Cu film by sputtering. The Cu film serves both as a precursor and an electronic contact in the resulting device while the FTO provides long-range charge collection. The Cu2O NW array was prepared through electrochemical anodization of the Cu layer in 3 M KOH to form Cu(OH)2 NWs, followed by thermal annealing at 600 °C in Ar gas atmosphere22. A more detailed description of the synthesis can be found in the Methods section. Our initial trials failed to produce the characteristic orange colored Cu2O, rather forming black CuO instead. We reasoned that excess Cu from the substrate might serve as an electron source for the transformation of Cu(OH)2 into Cu2O. For samples grown on Cu foils, abundant Cu is present and these substrates typically produce Cu2O. However, our initial experiments using sputtered Cu film substrates employed a thin 500 nm layer of Cu, which was fully converted into Cu(OH)2 during the anodization process. Due to the absence of elemental Cu, the annealing the Cu(OH)2 NWs resulted in the Cu-deficient CuO phase. To better understand the role of the Cu layer, we prepared substrates with various thicknesses of Cu (0.5, 1.0 and 1.5 µm) and subjected them to a fixed duration of anodization at identical current densities followed by annealing. For a layer thickness of 0.5 µm or less, CuO was the final product. In contrast, for films with ≥1.5 µm thickness, pure Cu2O was formed. With Cu


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thicknesses between 0.5 µm and 1.5 µm, mixed phase samples of Cu2O and CuO were formed. Figure 1 summarizes these results. The phase of the samples prepared from three different starting thicknesses of Cu can be predicted by visual inspection of their color (Figure 2a, inset), due to the different band gaps of CuO and Cu2O of ~1.5 eV and ~2.0 eV, respectively. We confirmed these values from absorption spectra derived from the diffuse reflectance data according to the Kubelka-Munk theory, Figures 2a and 2b. The sample of mixed phase has two shoulders in the absorption spectrum, corresponding to the two phases present. To further evaluate the phase of the products, X-ray diffraction (XRD) measurements were performed, as presented in Figure 2c. The original Cu(OH)2 has an orthorhombic structure (Figure S1), whereas Cu2O and CuO exhibit cubic and monoclinic structures, respectively, allowing them to be well-distinguished by XRD. The diffraction patterns confirmed that a mixture of phases was formed from the intermediate thickness of Cu, whereas phase-pure CuO and Cu2O resulted from the thin and thick Cu films, respectively. Importantly, the absence of peaks attributable to metallic Cu in the CuO and mixed phase samples, and their clear presence in the Cu2O sample, support the notion that excess Cu from the substrate is key in determining the product phase. Raman spectroscopy of each sample further confirmed the individual and mixed phases present (Figure 2d)29, 30. The synthetic steps and identification of intermediate phases allow us to propose reaction mechanisms for the processes involved. The key concept is based on the reduction of the Cu(II) in the Cu(OH)2 that is only possible if a reductant (here the Cu metal) is present. Hence, the initial thickness of the Cu(0) substrate layer dictates the materials formed during the annealing step. The first step consists of the anodization of the sputtered Cu film, wherein the electrochemical oxidation of the metal in strongly alkaline solution results in the hydroxyl phase spontaneously forming a NW array morphology22, as described by Equation 1. anodization

Cu + 2 OH- →

Cu(OH)2 + 2 e-

∆

Cu(OH)2 → CuO + H2 O ∆

(1) (2)

Cu(OH)2 + Cu → Cu2 O + H2 O

(3)

Subsequent heating of these Cu(OH)2 samples induces dehydration and conversion to oxide phases. It is known that in oxygen-rich environments the predominant resulting phase will be the copper (II) phase, CuO31. Interestingly, under the anaerobic conditions employed here, heating in the absence of excess Cu (following its complete consumption by the anodization process) had a similar outcome, producing the Cu-deficient phase (Equation 2). When a supply of excess Cu from the substrate is available, however, the Cu-rich phase is formed (Equation 3). In this process, the oxidation of Cu metal is compensated by reduction of the hydroxide form, resulting in the desired Cu(I) oxide. For the intermediate case, where the substrate Cu is fully consumed before the dehydration is complete, the combination of the processes of Equations 2 and 3 produces a mixture of both phases as observed in Figure 2. The reaction in Equation 3 starts with the excess Cu forming Cu2O at the interface. Once the


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initial layer Cu2O is formed, the reaction proceeds with the supply of Cu diffused through this layer. As the diffusion coefficient of Cu in Cu2O is fairly large32, this reaction could proceed up to several tens of micrometers distance with abundant Cu supply, enabling the complete conversion of Cu(OH)2 into Cu2O, in agreement with previous report29. This procedure therefore demonstrates how a simple two-step process, with control of the copper supply during anodization and conversion processes, can produce nanostructure arrays of pure or mixed phase copper oxide. Having developed a reliable method for synthesis of nanostructured Cu2O on a rigid substrate, we examined its structural and electronic properties in more detail. The Cu(OH)2 sample formed through the initial Cu anodization step exhibits a NW array structure, with diameters of 100-300 nm and lengths of 3-5 µm, shown in Figure S1. After annealing, the resulting Cu2O sample inherits the nanowire structure from the transformation of Cu(OH)2, as shown in Figures 3a and 3b. The Cu2O NWs exhibit larger diameters than the original Cu(OH)2 NWs due to the incorporation of the excess Cu from the substrate resulting in volume expansion. For comparison, we show in Figure 3c and 3d images of an electrodeposited Cu2O film prepared in the manner previously optimized to fabricate high-performance photocathodes, hereafter referred to as “planar” samples5, 8. As mentioned above, nanostructured materials are prone to defect formation that influence their physical properties. In order to diagnose the presence of electronic defects in the phasepure Cu2O NWs, we carried out photoluminescence (PL) and electrochemical impendence spectroscopy (EIS) measurements of both the Cu2O NW samples and conventional planar samples grown by electrochemical deposition (Figure 3e and 3f)5. The Cu2O NW and planar samples show strikingly different PL spectra. For NW samples, there is only one sharp PL peak, located at 635 nm, corresponding to the exciton luminescence33. However, several PL peaks are detected with planar samples, located at 580 nm, 635 nm, 790 nm and 930 nm and attributed to G2, Y1, 𝑉02+ , VCu luminescence signals, respectively34. 𝑉02+ and VCu are the luminescence signals resulting from defect states33, 34. Clearly, the NW sample shows a defect free room temperature exciton luminescence response as compared to the planar Cu2O, an observation confirming the high quality of the material formed by this conversion process. We derived charge carrier densities (NA) and flat band potentials (Efb) of Cu2O NWs and the planar Cu2O electrodes from EIS measurements using the Mott−Schottky equation (𝐶

𝐴

𝑏𝑢𝑙𝑘

2

) = 𝑒𝜀

2 𝑟 𝜀0 𝑁 𝐴

(𝐸 − 𝐸𝑓𝑏 +

𝑘𝑇 𝑒

),

where 𝑒 is the electronic charge, 𝜀𝑟 = 7.5 is the relative permittivity of Cu2O35, 36, 𝜀0 is the permittivity of vacuum, 𝑘 is Boltzmann constant, and 𝑇 is the absolute temperature9. The active area, A, is roughness corrected for both sample types. The roughness factor was estimated from scanning electron microscopy (SEM) images taken in cross-sectional and top view and was on average 4 for the nanowire electrode and 1.03 for the planar Cu2O electrode. The measured flat band potentials of Cu2O NW arrays and planar samples were 0.65 and 0.73 V versus reversible hydrogen electrode (RHE), respectively, and their charge carrier densities were found to be 1.5×1019 cm-3 and 2.0×1019 cm-3, respectively. If we assume that intrinsic


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doping of the material results from cation vacancies, VCu, the slightly lower charge carrier density of the NWs is in good agreement with the PL measurements. The above experiments confirm the reproducible growth of high quality Cu2O NW samples. Next, we focus on incorporation of Cu2O NW arrays into photoelectrodes for sunlight-driven water splitting. As Cu2O is highly susceptible to photo-corrosion, protection layer engineering is mandatory to enable durable water reduction. Carbon, NiOx and WO3 coatings have previously been investigated as protection layers for Cu2O NW samples22, 23, 26. However, with performance declining within 30 minutes, their stability is insufficient. Here we use atomic layer deposition (ALD) technique as a tool to coat the surface protection layers, which is one of the best choices for this purpose as it can provide precise thickness control at the angstrom or monolayer level and deposition on high aspect ratio nanostructures with excellent step coverage37. Before depositing 100 nm of TiO2 as the protection overlayer, a 20 nm thick layer of Al-doped ZnO (AZO) was coated on the Cu2O nanowire to form a buried p-n junction5. Figure 4a and 4b show the top view and cross-sectional SEM images of Cu2O NW samples with overlayers, respectively. It can be clearly seen that the over-layer coatings are homogeneous and conformal across the whole cross section of the sample. Figure 4c shows a magnified top view of the sample after overlayer coating. To enable efficient hydrogen evolution, we photo-electrodeposited a thin layer of RuOx catalyst on the TiO2 coated NWs8. Comparing Figure 4c with 4d, the roughness of the TiO2 surface is slightly increased due to RuOx deposition. For a high performance Cu2O based PEC electrode, the quality of the surface p-n junction, the protection layer and the catalyst are the most important parameters. The p-n junction must be contiguous and uniform, the protection layer must be conformal and pinhole-free and the catalyst must be robust and uniformly deposited on the electrode surface. For a detailed examination of the Cu2O NW-based hetero-nanostructure, Figure 5 depicts scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray spectroscopy (EDX) analysis on a single Cu2O nanowire wrapped by the AZO, TiO2 and RuOx layers. The high-angle annular dark field (HAADF) image (Figure 5a) and the combined EDX elemental mapping image (Figure 5b) show clearly the distribution of each layer, illustrating the high quality coatings achieved by ALD. In the HAADF image, the Cu2O core and RuOx out-layer grains appear brighter than the TiO2 protection layer as Cu and Ru atoms have a higher atomic number than Ti. The detailed elemental mapping images (Figure 5c-f) illustrate separately the elements in each layer, from the Cu2O core to the RuOx catalyst grains on the surface. The high quality Cu2O and AZO interface forms a nanoscale radial p-n junction across the whole nanowire surface, enabling efficient charge separation and collection38. Furthermore, the conformal and pinhole free TiO2 protection layer enables the stable performance of the sample. Finally, the homogeneous coating of RuOx catalysts enables the effective utilization of the high surface area for efficient proton reduction at minimized overpotential. Due to the large surface area of the nanostructured electrode, the optimized catalyst loading used here (per geometric area) was approximately half of the optimized amount for the planar Cu2O device previously reported8. This reduced loading is


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another advantage of NW photoelectrodes, especially when rare or expensive catalysts are used16, 17. Although the overlayer engineering on the Cu2O nanowires appeared of high quality, the initial attempts to use the nanowire photocathodes for light-driven water splitting resulted in low efficiencies. This is revealed by the RuOx catalyst photo-electrodeposition process, where only a small photovoltage was detected when the sample was exposed to simulated AM 1.5G solar illumination (Figure S2). After the RuOx catalyst deposition, the sample showed very small photocurrent response under chopped light illumination with large dark current. After careful investigation, we hypothesized that this poor photoelectrochemical response might be due to the occurrence of a direct contact between the AZO layer and the Cu substrate, which will induce severe recombination of charge carriers and shunt the device. This is very difficult to directly confirm with a Cu2O NW device due to the restricted access by SEM to the bottom of the long Cu2O NW array. Therefore we used planar Cu2O films directly electrodeposited on FTO substrates, comparing samples in which the Cu2O exhibited either incomplete or complete coverage of the FTO surface (representative SEM images are provided in Figure S3). After overlayer protection and catalyst deposition, samples in which the exposed FTO is in contact with the overlayers again showed small photocurrent, similar to what was observed above for the NW devices. Furthermore, both device types exhibited significant dark current, supporting a shunt in the charge transport which bypasses the photoactive layer. Meanwhile, samples with a continuous Cu2O film between the FTO and the overlayers showed decent performance (Figure S3). We infer from these observations that the poor photoresponse of the Cu2O NW sample was caused by exposed substrate, which shunts the device after overlayer coating. To overcome this problem, we sought to passivate any exposed Cu substrate by incorporating a thin Cu2O blocking layer in the NW device. The Cu2O blocking layer is formed by electrochemical deposition after the Cu2O NW conversion step but before the ALD overlayer deposition. The incorporation of this blocking layer caused no apparent change in morphology for the NW arrays (Figure S4), but the photoresponse improved dramatically, as revealed in Figure S5. A deposition time of 10 min was enough to significantly enhance the Cu2O NW electrode performance, although the persistence of noticeable dark current suggested the blocking layer was imperfect. After 30 min we obtained the highest photocurrents with negligible dark currents. These results show the importance of a substrate blocking layer in forming nanostructured heterojunction devices on conductive substrates, indicating a possible explanation for the limited performances achieved on previous reports of Cu2O NW photocathodes. After optimization, these heterostructure Cu2O NW photocathodes reproducibly achieved photocurrent densities reaching 10 mA cm-2 at -0.3 V vs RHE and photocurrent onset potentials of approximately +0.48 V vs RHE, as shown in Figure 6a, approaching the best performing Si NW photocathodes39. In contrast, the best planar Cu2O device formed by electrochemical deposition on the same substrate reached photocurrent densities of at most 7.8 mA cm-2 at the same bias potential, showing that a significant enhancement in photoresponse resulted from the NW array morphology. The deposition time for the


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champion planar film is 100 min. To eliminate the contribution of the blocking layer to the NW samples, we also measured the performances of planar samples electrodeposited for 10 and 30 min. As is shown in Figure S6, the planar samples deposited at 10 and 30 min only show a photocurrent density of 2.5 and 3.5 mA cm-2, respectively. In contrast, the NW sample with 10 min blocking layer deposition already showed a photocurrent density of more than 9 mA cm-2. These results show that the performance contribution from the blocking layer to the NW sample is negligible, and the electrodeposited layer on NW sample mainly serves as blocking layer to reduce recombination. The 10 mA cm-2 photocurrent density represents a new benchmark for metal oxide based photoelectrodes towards solar water splitting. As long as each step is well controlled, the high performing samples can be reproducibly achieved with small deviations in photocurrent density and onset potential, see Figure S7 of three representative samples. For the planar film device, there is a tradeoff between light absorption and charge collection which depends on the absorber thickness. When the film is thin, the short transport distance enables efficient collection of holes at the back contact and electrons at the p-n junction where they are driven to the electrolyte to reduce water. However, the optical absorption of thin Cu2O is too low to obtain efficient devices. To achieve adequate light absorption, film thicknesses of several micrometers are typically employed40, but since the minority carrier (electron) diffusion length in Cu2O is relatively small (approximately 20-200 nm)9, 11 the carrier collection is poor as they recombine before reaching the p-n junction. This is especially problematic for photons in the wavelength range of 500-600 nm (with energies corresponding to forbidden transitions in Cu2O7) which are absorbed deep in the films, far away from the depletion region induced by the p-n junction. The key advantage of the nanostructure approach is the ability to decouple the length scales of light absorption and charge transport by enabling the processes to occur in orthogonal directions15, 41. A nanowire’s large aspect ratio provides a long optical path for improved light absorption along the vertical axis, while its much smaller diameter facilitates radial collection of minority carriers over a shorter distance, minimizing the probability of recombination within the absorber material. This effect was clearly achieved for the Cu2O NW array device, where the incident-photon-to-current efficiency (IPCE) in the nanostructure experienced a major enhancement. As shown in Figure 6b, the IPCE for the planar film device is reasonably high across wavelengths shorter than 500 nm, but drops sharply towards longer wavelengths. The NW array device, on the other hand, showed a greatly improved ability of transforming these lower-energy photons in photocurrent. The ~3–5 µm long nanostructures proved capable of strong absorption while their radii of approximately 150 nm are on the order of the expected minority carrier diffusion length and therefore allow efficient charge collection. Because of the significant flux of photons in this range from the incident solar spectrum, this improved photoresponse is crucial for achieving high current densities from this ~2 eV band gap material. Figure S8 shows the AM1.5G integrated current density based on the IPCE measurements at 0 V versus RHE, confirming the measured J–V curves in Figure 6a. To evaluate the stability of the Cu2O NW sample, we measured the current density and the Faradic efficiency for hydrogen generation with an in-line gas chromatograph under


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continuous simulated AM 1.5G illumination and an applied potential of 0 V versus RHE. The NW array photocathode showed excellent stability over 55 hours (Figure 6c) with a constant Faradic efficiency of ~100% towards hydrogen evolution, and the amount of hydrogen generated is also quantified with time, Figure S9a. Interestingly, after exchanging the electrolyte, the performance of the sample recovered close to its initial level, as revealed by the photoelectrochemical measurements before and after extended testing (Figure S9b). Furthermore, no observable pinholes or electrode surface damage could be detected, suggesting that the Cu2O NW device is highly stable in aqueous pH 5 solution. The decreased photocurrent might result from the decomposition of the electrolyte, which is still under investigation. In theory, the nanostructuring of materials should enable simultaneous optimization of light absorption and charge separation through morphological control, thereby enabling higher overall quantum efficiencies than their planar counterparts. Practically, however, this concept has proven difficult to demonstrate, with many nanostructured device efficiencies failing to show significant enhancement over planar ones16, 17, 39. A primary reason for this lies in the challenge of fabricating the desired nanostructures while preserving material quality, since parameters like dopant density, crystallinity, and impurity content can be very difficult to control by known synthetic methods. This work demonstrated a simple anodization and annealing procedure to synthesize phase-pure crystalline Cu2O NW arrays which exhibit superior optoelectronic properties. The supply of Cu from the substrate was found to be crucial for creating the desired phase. Combining this material with an innovative blocking layer strategy and the deposition of robust overlayers enabled a device exhibiting an unprecedented combination of performance and stability for metal oxide photoelectrodes. An important benefit of employing copper as substrate is avoiding the use of rare and expensive gold, moving the device one step closer to being composed entirely of Earthabundant materials. In fact, the only rare material used was the RuOx catalyst, chosen for its activity and robust nature in pH 5 conditions. It is largely understood that operation in extreme pH electrolytes (either acidic or alkaline) is necessary for efficient photoelectrolysis in a complete water splitting device3, 42. To this end, adapting the overlayer strategy for stability in these conditions, as well as developing suitable hydrogen evolution catalysts for alkaline operation43, 44, is presently under investigation. As this device is capable of photocurrent densities which push the limit for predicted photocurrent achievable for Cu2O films7, 38, further performance improvement will largely depend on the ability to increase the photovoltage produced by the heterojunction. Recent developments using Ga2O3 and other interface materials with optimized conduction band offsets should be applicable to this Cu2O NW system13, 25, 45, provided that a high-quality and conformal interface can be formed with the nanowires. Furthermore, since a nanostructured electrode leads to smaller local currents per real catalytic surface area16, 39, the resulting decrease in catalytic overpotential should enable the use of more abundant catalyst materials. In this configuration, catalyst transparency remains important46, and we have already demonstrated here the ability to employ significantly reduced catalyst loadings on nanostructured electrodes.


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For implementation in a two-absorber tandem device capable of complete water splitting, Cu2O would serve as the wide bandgap top absorber and therefore transparency to longwavelength photons is a desirable trait21, 47, 48. This presents a challenge to the NW device reported here, which depends on a sufficient supply of Cu from the substrate to target the desired Cu2O phase during the synthesis. Further optimization of the Cu thickness and anodization time, targeting a Cu2O device with minimal residual Cu, should enable the possibility of long-wavelength transparency. Conclusions A new method to synthesize high quality Cu2O nanowire samples has been developed. These samples show superior physical properties compared to the conventional planar sample by electrochemical deposition, and demonstrate 25% enhancement in photocurrent density toward sunlight-driven water splitting. The combination of a high quality surface p-n junction, conformal protection layer, excellent catalyst decoration and the introduction of an innovative blocking layer is the key to efficient and stable performance.

Methods Cu coated FTO substrates: Cu was coated on FTO substrates in different thicknesses by DC sputtering at room temperature with Alliance-Concept DP 650 system. The purity of the target is 99.995% and the average growth rate is 2.65 nm s-1. Only this process was done in the clean room, and all the following sample preparation processes were carried out in normal ambient lab conditions. Synthesis of Cu(OH)2 NW precursors: The Cu(OH)2 NW precursors were prepared by anodizing Cu coated FTO substrates at constant current density (10 mA cm-2) mode at room temperature in 3 M KOH solution for 3 min. Transformation of Cu(OH)2 into copper oxide: The Cu(OH)2 NW precursors were transformed into copper oxide samples by thermal annealing at 600 °C in Ar gas atmosphere for 4h. Depending on the initial thickness of Cu films, Cu2O, CuO or their mixed phase samples were resulted, see main manuscript for detailed information. Cu2O planar film and blocking layer deposition: The Cu2O was electrodeposited from a basic solution of lactate stabilized copper sulfate electrolyte. For the preparation of the electrolyte, 7.98 g CuSO4, 21.77 g K2HPO4 and 67.5 g of lactic acid were dissolved in 250 ml H2O, and the pH of electrolyte was adjusted to 12 by KOH (2 M) solution, the total resulted solution is around 1L in the end. Cu2O layers were deposited at a constant current density of 0.1 mA cm-2 (Galvanostatic mode) for certain using a source meter (Keithley 2400) in a twoelectrode configuration (a Pt mesh served as the counter electrode). For champion planar film for water splitting, the deposition time is 100 min. And 10, 30 and 50 min were used for the blocking layer deposition. During deposition, the electrolyte was maintained at 30 ºC using a hot plate with an in situ temperature probe.


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Atomic layer deposition of overlayers: The overlayers were deposited by atomic layer deposition (ALD) (Savannah 100, Cambridge Nanotech). Aluminum-doped zinc oxide (AZO) was deposited by running 5 super-cycles 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 tetrakis-dimethylamino 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. RuOx catalyst deposition: For the RuOx catalysts deposition, a 12 ml solution containing 3.0 mg of KRuO4 was used. The catalyst solution was fresh made before each deposition. Typically, samples were catalyzed at a current density of -30 μA cm-2 for 6-8 min under simulated one sun illumination. Material characterizations: The XRD patterns were acquired with a Bruker D8 Discover diffractometer in Bragg–Brentano mode, using Cu Kα radiation (1.540598 Å) and a Ni βfilter. 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 1 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. Raman and Photoluminescence spectra were carried out on LabRAM HR Raman spectrometer with 532 nm Laser. The morphology of the films was characterized using a high-resolution scanning electron microscope (ZEISS Merlin), and a high-resolution transmission electron microscope (Technai Osiris, FEI). The composition of the nanowire samples with overlayers were characterized by the energy-dispersive X-ray (EDX) spectra obtained in STEM mode with Technai Osiris. Electrochemical measurements: EIS measurements were carried out in the dark in a three-electrode configuration in 0.1 M sodium acetate solution (pH 8.2). Full impedance spectra were measured with a SP-200 (BioLogic Science Instruments) at frequencies from 1 MHz to 0.1 Hz with a sinusoidal potential perturbation of 25 mV. The range of the bias potential was 0.40–0.68 V vs. RHE. The capacitance was extracted from the fitting according to the standard Randles circuit fitted with Zview (Scribner Associates). 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 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)


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in three-electrode configuration at 0 V versus reversible hydrogen electrode (RHE). Comparison with a calibrated Si photodiode allowed the calculation of the IPCE. All potentials have been referenced to the RHE by the expression: VRHE = VAg/AgCl + 0.197 V + 0.059 V ×pH. Stability and Faradic efficiency measurement: The stability of the samples was measured at 0 V vs. RHE in three electrode configuration using a potentiostat (Iviumstat, Ivium) in a homemade gas tight cell with a quartz window (Edmund Optics). A short Pt wire was used as counter electrode in the same compartment. Simulated AM 1.5G illumination calibrated to 1 sun intensity was supplied by a 450 W Xe light source (LOT Oriel) combined with an AM 1.5G filter (LOT Oriel). Ar gas (99.9999%, Carbagas) was sparged into the electrolyte at a constant rate of 5.00 (+/-0.5%) mL min-1, set by means of a mass flow controller (Bronkhorst HIGH-TECH). The resulting product gases passed through the sample loop of a gas chromatograph and analysis was carried out at an interval of 4 minutes. The chromatograph (Trace ULTRA, Thermo Scientific) was equipped with a ValcoPLOT column (FS, Molsieve 5A, 30 m, 0.53 mm, 20 um film, Vici) and a PDD detector (Vici). Helium (99.9999%, Carbagas) was used as carrier gas. A certified hydrogen standard (Carbagas) was used to calibrate the measurement. The measured molar flow of hydrogen was compared to the observed photo-current density, yielding the faradaic efficiency. Due to the initial accumulation of hydrogen in the measurement cell’s headspace and electrolyte, a steady state is only reached after a few measurements.

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Supporting Information Supporting Information Available: Additional material characterization and photoelectrochemical performance data. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements The authors would like to thank Xiaoyun Yu, Dr. Néstor Guijarro and Prof. Kevin Sivula for for the Raman, PL measurements and the discussions. This work is supported by the following financial sources: EPFL Fellowship (awarded to J.L.) co-funded by Marie Curie from the European Union’s Seventh Framework Programme for research, technological development and demonstration (no. 291771); the PECDEMO project co-funded by Europe's Fuel Cell and Hydrogen Joint Undertaking (FCH JU) (no. 621252); the Nano-Tera NTF project (TANDEM); the PHOCS project supported under the Future and Emerging Technologies programme of the European Commission (no. 309223); the PECHouse project funded by the Swiss Federal Office for Energy. Author Contributions J.L. conceived the project and performed the experiments. L.S., M.K.S., M.S. and M.T.M. contributed to the structural, electrochemical and spectroscopic measurements. M.G. supervised the project. J.L. wrote the manuscript. All authors discussed the results and commented on the manuscript. Competing Financial Interests The authors declare no competing financial interests.


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Cu(OH)2

CuO

Cu ≤ 0.5 µm

Cu(OH)2 Cu thin

Cu2O-CuO

Cu 0.5 µm ~ 1.5 µm FTO

Glass Cu(OH)2 Cu thick

Cu2O

Cu ≥ 1.5 µm

Cu sputtering

Anodization 10 mA cm-2, 3 min, 3M KOH

600 ˚C , 4h, Ar atmosphere

Figure 1. Schematic diagram of phase controlled synthesis of copper oxide nanowire arrays. Copper oxide nanostructures are synthesized from Cu thin films on FTO/glass by a two-step process. Cu films are first anodized to form Cu(OH)2 nanowires, and subsequent annealing in Ar leads to dehydration and formation of oxide phases. For a fixed anodization treatment, the starting thicknesses of Cu ultimately dictate whether pure CuO, mixed Cu2OCuO, or pure Cu2O nanowires are formed.


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b

a

Normalized absorbance (a.u.)

Cu2O

30 Cu2O-CuO

20 10

CuO

0

400

500 600 700 Wavelength (nm)

c 3

 CuO  Cu2O

0.6

CuO

0.4 Cu2O-CuO

0.2 Cu2O

0.0 400

800

2

Cu2O-CuO

 

  

 

1

 

 

  



  

 

CuO











30



40

 

 



50 60 2(degree)

Cu2O

  



70

 

 

  



 

800







500 600 700 Wavelength (nm)

d



  

20

0.8



 FTO Cu

0

1.0

80





Cu2O-CuO



Intensity (a.u.)

Diffuse reflectance (%)

40

Intensity (a.u.)

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



CuO





100



 

200





300 400 500 600 -1 Raman shift (cm )

Cu2O

700

800

Figure 2. Characterization of copper oxide samples of different phases. Samples prepared from 0.5, 1.0 and 1.5 µm thick Cu substrates are labeled as their resulting compositions of CuO, Cu2O-CuO, and Cu2O, respectively. a, Diffuse reflectance spectra. b, Absorbance spectra derived from diffuse reflectance via Kubelka-Munk theory. c, X-ray diffraction patterns. Indexes were taken from the following patterns: Cu2O PDF#05-0667, CuO PDF#450937, Cu PDF#04-0836, FTO PDF#41-1445. d, Raman spectra.


Nano Letters

a

e Cu2O NW

Photoluminescence (a.u.)

b

Wavelength (nm)

f

d

c

Cu2O planar

550 600 650 700 750 800 850 900 950 1000

400 nm

1 µm

Cu2O NW

3

NFB=0.65 V

3 4

2

11

2

-2

-2

Cbulk (10 cm F )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1

19

ND=1.5*10 cm

-3

Cu2O planar NFB=0.73 V 19

ND=2.0*10 cm

-3

1 0

200 nm

0.40

100 nm

0.45

0.50

0.55

0.60

0.65

0.70

0.75

Potential vs. RHE (V)

Figure 3. Comparison of Cu2O nanowire and planar samples. a, Top view and b, cross sectional SEM images of Cu2O NWs. c, Top view and d, cross sectional SEM images of electrodeposited Cu2O planar sample. e, Photoluminescence spectra. f, Mott-Schottky plots obtained from impedance measurements.


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a

b

1 µm

c

2 µm

d

400 nm

200 nm

Figure 4. SEM characterization of Cu2O NW sample with overlayers. A NW array device following ALD overlayer deposition of AZO and TiO2 is examined by a, top view, b, cross section, and c, magnified top view SEM images. d, Top view SEM image of complete Cu2O NW device following electrodeposition of RuOx catalyst.


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a

b

c

d

e

f

Figure 5. TEM and EDX characterization of the Cu2O NW with overlayers. a, HAADF image. b, Combined elemental mapping image. c, d, e and f, Element mapping of Cu, Zn, Ti and Ru, respectively.


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b 80

0

-2

Current density (mA cm )

a

-2

60 IPCE (%)

-4 -6 -8

40

20 Cu2O NW on Cu coated FTO

-10

Cu2O Compact film on Cu coated FTO

0 350

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6

400

450

500

550

600

650

Wavelength (nm)

Potential vs. RHE (V)

c 125 FE (%)

100 75 50 25 0 10 8

J (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6 4 2 0 0

5

10

15

20

25

30

35

40

45

50

55

60

Time (h)

Figure 6. Photoelectrochemical performance, stability and Faradic efficiency measurements. a, J-V curves under simulated AM 1.5G chopped illumination and b, IPCE spectra under monochromatic illumination of Cu2O NW and planar devices. c, Measured photocurrent and Faradic efficiency for hydrogen evolution of Cu2O NW device under constant bias at 0 V vs. RHE and simulated AM 1.5G illumination. All measurements were performed in pH 5 electrolyte.


Cu2O Nanowire Photocathodes for Efficient and Durable Solar Water

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