Hierarchical branched Cu₂O nanowires with enhanced photocatalytic activity and stability for H₂ production

Abstract

Hierarchical branched Cu₂O nanowires were synthesized under mild conditions and exhibit remarkable performance for photocatalytic H₂ generation from water. The obtained results open appealing perspectives for converting solar energy into storable chemical energy.

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Cuprous oxide (Cu₂O) is an attractive p-type oxide semi-conductor for photo-electrochemical (PEC) hydrogen production because of its direct band gap of 2.17 eV, a corresponding theoretical photocurrent of −14.7 mA cm⁻² and an optimal light-to-hydrogen conversion efficiency of 18% based on the AM 1.5 spectrum.¹ However, the practical application of Cu₂O in the PEC process is still limited by two main drawbacks: (1) the incommensurability of the intrinsic carrier diffusion length (20–100 nm)^2,3 with the light absorption depth near the band gap (approximately 10 μm)⁴ and (2) photo-instability in aqueous solution under illumination, as a result of self-photocorrosion.^5–7 Structures based on nanowires (NWs) are known to offer a potential solution to these problems, allowing light absorption along the long axial direction and the simultaneous efficient carrier separation and collection in the nanometre-scale radial direction.⁸ Recent reports also found that forming complex 3-D hierarchical branched structures of 1-D systems could further improve the solar energy harvesting from that of simpler 1-D NWs, due to the major advantages of tremendously increased surface area and enhanced hetero-junction density.^9,10 Up to now, extensive research efforts in photocatalytic H₂ generation from water have focused on combining Cu₂O with an appropriate n-type semiconductor such as TiO₂, NiO, etc.,^11–14 whereas the performance of Cu₂O as a function of its nanoscale structural organization, has scarcely been addressed. In fact, the development of effective and low-cost preparation routes for this material, particularly with complex architectures based on NWs, is still an ongoing challenge.^15,16

Herein we present an extremely simple strategy to synthesize a new type of hetero-structure that consists of 2-D nanosheets (NSs) and 1-D NWs, which builds on the success of the synthesis of NWs with Cu materials.¹⁷ This NS–NW hetero-structure is made by an epitaxial growth of NSs at pre-grown Cu NWs via proper oxidizing post-treatment (ambient atmosphere, 60 °C for 5 h) under humidity (65–70%), as illustrated in Fig. 1A. It exhibits a Euphorbia splendens-like structure with thin NWs as the backbone and NSs as the branches. As expected, such architectural Cu₂O has shown a strong structure-induced enhancement of photocatalytic performance, including much higher photocatalytic activity as well much better durability for H₂ evolution compared with the Cu₂O nanoparticles (NPs) and NWs without branching structures. The as-prepared hierarchical branched Cu₂O NWs could be a promising candidate for photocatalysis applications owing to its intrinsic low-cost, non-toxicity and scalability. Additionally, the primary finding of the superior photocatalytic performance of this architectural Cu₂O can inspire us to further develop it in photocathode device designs.

Fig. 1 (A) Schematic formation process of hierarchical branched Cu₂O NW nanostructures. The digital camera picture on the right is of the branched NWs produced in a single batch. (B) Typical high magnification SEM image and model (inset) of the as-grown Cu₂O NS–NWs. (C) STEM image and (D and E) corresponding elemental mapping of Cu and O of a single NS–NW. Scale bar = 100 nm. (F) Cu/O atomic ratio extracted from STEM along the diameter of the NWs and epitaxial NSs (indicated by the blue arrows in panel C). (G) Powder X-ray diffraction pattern of the products.

Fig. 1B presents the high magnification SEM image of the product, which suggests that many NSs have decorated the entire NW surface, forming the hierarchical branched morphology. These on-wire NSs form sharpened irregular shapes. The lateral size of the NSs varies in a narrow range from 8 to 12 nm. However, all of the NWs show similar aspect ratios. Elemental mapping of the NS–NW was carried out using scanning transmission electron microscopy (STEM) as shown in Fig. 1C–E. Different colors indicate the presence of different elements, where yellow refers to the presence of copper and green to oxygen. Spatial distribution of the color verifies the branched structures with an inside filled nature, instead of core–shell structures or nanotubes. In addition, no obvious impurities are found on the surfaces of the samples, indicating that the as-prepared samples are relatively pure. Energy dispersive X-ray spectroscopy (EDX, Fig. S3†) confirms the presence of copper and oxygen. Using the EDX line-scanning in STEM to probe the spatial distribution of the Cu and O compositions across this single NW, we observed that the Cu/O atomic ratio of the whole NS–NW was nearly 2 : 1 at all positions across the NW diameter and adjacent epitaxial NS branches (Fig. 1F). The powder X-ray diffraction (XRD) pattern of the as-obtained products confirms the material to be Cu₂O, in which a set of Bragg peaks can be perfectly indexed as the pure cubic cuprite structure (Pn3̄m, JCPDF no. 78-2076) with lattice constant a = 4.267 Å (Fig. 1G). The corresponding transmission electron microscopy (TEM) image in Fig. 2A shows that the diameters of the Cu₂O NWs are typically 90–130 nm and uniform along their length (6–8 μm). Burrs grow on the surfaces of NWs as demonstrated in the SEM characterization. Such additional branches are highly desirable for the improvement of photovoltaic performance owing to an increase in the wire surface area. Fig. 2C illustrates the corresponding selected area electron diffraction (SAED) pattern of a single NW from Fig. 2B, indicating that the Cu₂O NWs are consistently single crystalline. High-resolution TEM (HRTEM) images reveal that most NWs have well-resolved (111), (110) and (002) lattice planes (Fig. 2D and E). Moreover, both HRTEM images and the assignments of SAED consistently reveal that the preferential growth direction for the Cu₂O NWs is 〈110〉. The crystallographic relationship between the branched tips and the NW body were also investigated by further HRTEM imaging of the selected areas in Fig. 2F. It was found that lattice fringes mostly extend from the NW to the branched NS without any obvious lattice distortion (Fig. 2G and H). The corresponding FFT (fast Fourier transform) obtained from the junction and branch regions all show identical patterns (insets of Fig. 2G and H), suggesting that the branched hetero-structure is single crystalline throughout the entire structure. The FFT pattern also indicates that the growth direction of the NS is [002]. The consistently observed 90° angle between the branch and the body further demonstrates that the Cu₂O NSs epitaxially grow from the NW along the (110) plane, resulting in single crystalline junctions with limited structural defects upon the crystal interface. The as-obtained high surface area NS–NWs and their high quality interface will significantly promote the efficiency of charge separation and consequent transition under illumination, which have been demonstrated by a photoreduction reaction.

Fig. 2 (A) TEM characterization of Cu₂O NS–NWs. (B) TEM image with high magnification and (C) SAED pattern of a selected Cu₂O NS–NW from region a in panel A. The zone axis (ZA) is [110]. (D) The corresponding HRTEM image of region b. (E) Enlarged area of region c. (F) Partially amplified region in panel B. (G and H) HRTEM images of the junction and branched NS. The insets are corresponding FFT patterns.

The photocatalytic properties of the as-obtained Cu₂O NS–NWs and control samples (i.e. Cu₂O NWs, Cu₂O NPs) have been evaluated for their H₂ production from aqueous solution under UV-vis light irradiation. To accelerate the H₂ photogeneration process, a sacrificial agent (i.e. Na₂S/Na₂SO₃) was used in the current work to achieve alternatively photogenerated hole suppression. Furthermore, we also employed platinum (Pt) as co-catalyst in this study (see ESI† for Pt loading details). The loading amount of Pt is fixed at 1 wt%, which has been optimized by a corresponding test with varied loading rate (see ESI†). During the whole test process the closed gas circulation system was vacuumed every 5 hours. The corresponding results are summarized in Fig. 3A. The NS–NWs show the highest H₂ yield of up to ca. 263.8 μmol h⁻¹ as compared to ca. 89.7 and 12.9 μmol h⁻¹ of H₂ generated by Cu₂O NWs and NPs, respectively. It was also found that the Cu₂O NS–NWs exhibit remarkably improved photostability and long lifetimes. Cycling experiments (see ESI†) further confirmed that such a steady H₂ evolution rate can be maintained until 80 h of irradiation. By contrast, both of the Cu₂O NWs and NPs underwent deactivation upon prolonged utilization. In particular the Cu₂O NPs almost lost their photoactivity completely after 15 h of illumination, indicating the dramatic deactivation of the Cu₂O photocatalyst due to photocorrosion. The intrinsic stability of Cu₂O NS–NWs was examined by X-ray photoelectron spectroscopy (XPS) after the photocatalytic reaction (Fig. S9 and 10†). The results show that the Cu2p_3/2 and Cu2p_1/2 peaks still appear at binding energies of 932.6 and 952.9 eV, which can be assigned to Cu₂O in accordance with data in the literature.¹⁸ This suggests that Cu₂O was neither reduced nor oxidized after photocatalytic reaction. Correspondingly, there was no noticeable difference in the Cu LMM Auger spectra of the samples before and after irradiation. The Auger parameter value was determined to be 916.1 eV, revealing that only Cu₂O was present on the surface of tested samples.¹⁹ It is well known that the activities of the semiconductor photocatalysts depend on their structural and electronic properties, which are further governed by their dimensions, crystallinity and surface properties. For the NS–NWs, the structures have numerous sharply tapered ends and needle-like burrs. These unique geometrical features facilitate the buildup of an appreciable concentration of charged species and the rapid carrier separation within a photoexcited sample, as a result of the accumulation of electrons at pointed domains of the branched Cu₂O NWs and efficient carrier collection from the terminals to a central trunk. In addition, co-catalysts act as electron conductors to extract photogenerated electrons from the semiconductors, further enhancing charge separation efficiency and limiting the self-photoreduction process.²⁰ However, the deactivation of electrocatalyst-decorated semiconductors still remains the fundamental issue to combine efficiency with cost. The main limiting factor is likely due to less efficient interfacial interaction between co-catalysts and semiconductors. Employing complex 3-D hierarchical branched NW structures may be ideal to solve this problem. In the current study, the addition of branches to the Cu₂O NWs increases the surface area, which enhances the interface with Pt nanoparticles (NPs). In fact, sufficient contact between the co-catalyst Pt and Cu₂O NS–NW can be clearly observed, as shown in Fig. 3B–G. As a consequence, the photoelectrons generated at Cu₂O sites are more accessible for water reduction as the surface of Cu₂O becomes densely covered by Pt NPs, improving the solar energy harvesting.

Fig. 3 (A) Time courses of H₂ evolution under visible light (λ > 420 nm) irradiation and UV-vis irradiation over Pt/Cu₂O NS–NWs. Catalyst: 0.1 g, 0.5 M Na₂S–0.5 M Na₂SO₃ aqueous solution (200 mL). The reaction system was evacuated with light irradiation after each run. Time courses in runs 20–30 are omitted. (B) STEM image and (C–E) corresponding EDX elemental mapping images of as-obtained 1 wt% Pt loaded Cu₂O NS–NWs after 30 min irradiation by using a 300 W Xe lamp. Scale bar = 150 nm. (F) STEM image of Pt loaded Cu₂O NS–NWs. (G) high magnification TEM images of NSs embellished by Pt nanoparticles from the region marked with a red rectangle in panel F.

In summary, we have established a simple and facile strategy for the large-scale synthesis of hierarchical branched Cu₂O NWs via a solution based method. This NS–NW heterostructure is supposed to be beneficial for the large surface area and efficient separation of the photoexcited electrons and holes, thus demonstrating significantly improved activity and stability for H₂ production from water. We believe it not only holds great potential in applications for solar energy conversion, but also provides a novel architecture to integrate the extraordinary properties of 1-D NWs and 2-D NSs into a 3-D space.

The authors acknowledge the National Natural Science Foundation of China (no. 50901090, and no. 21106185) and China Scholarship Council (CSC, no. 201206450026) for their funding support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3nr04280d

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