Three-dimensional ZnO/Si broom-like nanowire heterostructures as photoelectrochemical anodes for solar energy conversion

We report a low-cost solution fabrication of three-dimensional (3D) ZnO/Si broom-like nanowire (NW, nanobroom) heterostructures, consisting of Si NW backbones and ZnO NW stalls, and their application as photoelectrochemical anodes for solar water splitting and energy conversion. The nanobroom morphology and atomic structure are characterized using the scanning, transmission, and scanning transmission electron microscopies. Both Si NW backbones and ZnO NW stalls are defect-free, single-crystalline, and their surfaces are smooth. The optical absorption and photocurrents from nanobroom array electrodes with different Si and ZnO NW dimensions are studied. The longer Si NW backbones and smaller ZnO NW stalls lead to better light absorption and larger photoanodic current. The ZnO/Si nanobrooms show much higher photoanodic current than the bare Si NWs due to the effective Si/ZnO junction and increased surface area. The nanobroom electrode stability is also investigated and using a thin TiO2 coating layer protecting the NWs against dissolution, long-term stability is obtained without any change in shape and morphology of nanobrooms. Finally, the effect of catalyst to improve the oxygen evolution reaction (OER) at the electrode surface is studied resulting in large enhancement in photoanodic current and significant reduction in anodic turn-on potential. This study reveals the promise of the use of simply fabricated and low-cost 3D heterostructured NW photoelectrodes for clean solar energy harvesting and conversion.