We report a facile and large-scale fabrication of three-dimensional (3D) ZnO/CuO heterojunction branched nanowires (b-NWs) and their application as photocathodes for photoelectrochemical (PEC) solar hydrogen production in a neutral medium. Using simple, cost-effective thermal oxidation and hydrothermal growth methods, ZnO/CuO b-NWs are grown on copper film or mesh substrates with various ZnO and CuO NWs sizes and densities. The ZnO/CuO b-NWs are characterized in detail using high-resolution scanning and transmission electron microscopies exhibiting single-crystalline defect-free b-NWs with smooth and clean surfaces. The correlation between electrode currents and different NWs sizes and densities are studied in which b-NWs with longer and denser CuO NW cores show higher photocathodic current due to enhanced reaction surface area. The ZnO/CuO b-NW photoelectrodes exhibit broadband photoresponse from UV to near IR region, and higher photocathodic current than the ZnO-coated CuO (core/shell) NWs due to improved surface area and enhanced gas evolution. Significant improvement in the photocathodic current is observed when ZnO/CuO b-NWs are grown on copper mesh compared to copper film. The achieved results offer very useful guidelines in designing b-NWs mesh photoelectrodes for high-efficiency, low-cost, and flexible PEC cells using cheap, earth-abundant materials for clean solar hydrogen generation at large scales.

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We report the fabrication of a three dimensional branched ZnO/Si heterojunction nanowire array by a two-step, wafer-scale, low-cost, solution etching/growth method and its use as photoelectrode in a photoelectrochemical cell for high efficiency solar powered water splitting. Specifically, we demonstrate that the branched nanowire heterojunction photoelectrode offers improved light absorption, increased photocurrent generation due to the effective charge separation in Si nanowire backbones and ZnO nanowire branching, and enhanced gas evolution kinetics because of the dramatically increased surface area and decreased radius of curvature. The branching nanowire heterostructures offer direct functional integration of different materials for high efficiency water photoelectrolysis and scalable photoelectrodes for clean hydrogen fuel generation.

We report the fabrication of three-dimensional (3D) branched nanowire (NW) heterostructures, consisting of periodically ordered vertical Si NW trunks and ZnO NW branches, and their application for solar water splitting. The branched NW photoelectrodes show orders of magnitudes higher photocurrent compared to the bare Si NW electrodes. More interestingly, selective photoelectrochemical cathodic or anodic behavior resulting in either solar water oxidation or reduction was achieved by tuning the doping concentration of the p-type Si NW core. Specifically, n-ZnO/p-Si branched NW array electrodes with lightly doped core show broadband absorption from UV to near IR region and photocathodic water reduction, while n-ZnO/p(+)-Si branched NW arrays show photoanodic water oxidation with photoresponse only to UV light. The photoelectrochemical stability for over 24 h under constant light illumination and fixed biasing potential was achieved by coating the branched NW array with thin layers of TiO2 and Pt. These studies not only reveal the promise of 3D branched NW photoelectrodes for high efficiency solar energy harvesting and conversion to clean chemical fuels, but also developing understanding enabling rational design of high efficiency robust photocathodes and photoanodes from low-cost and earth-abundant materials allowing practical applications in clean renewable energy.

This Communication reports a low-cost solution fabrication of wafer-scale ZnO/Si branched nanowire heterostructures and their high photodetection sensitivity, with an ON/OFF ratio larger than 250 and a peak photoresponsivity of 12.8 mA/W at 900 nm. This reported unique 3D branched nanowire structure offers a generic approach for the integration of new functional materials for photodetection and photovoltaic applications.