GaAs@TiO₂ Core-Shell Nanowire-Based Electrode and Co Catalyst for Enhanced Hydrogen Generation

Photoelectrochemical cells (PEC) are appealing devices for the production of renewable energy carriers. In this context, III-V semiconductors such as GaAs are very promising materials due to their tunable bandgaps, which can be appropriately adjusted for sunlight harvesting. Because of the high cost of these semiconductors, the nanostructuration of the photoactive layer can help to improve the device efficiency as well as drastically reduce the amount of material needed. III-V nanowire-based photoelectrodes benefit from the intrinsically high aspect ratio of nanowires, their enhanced ability to trap light as well as their improved charge separation and collection abilities, and thus are particularly attractive for PECs. However, III-V semiconductors often suffer from corrosion in aqueous electrolytes, preventing their utilization over long periods under relevant working conditions.<br/>In this study, the influence of the geometry of GaAs semiconductor as photocathodes for the water reduction was investigated. Photocathodes made of GaAs nanowires [1, 2] and protected with thin TiO₂ shells were prepared and studied under simulated sunlight irradiation to assess their photoelectrochemical performances in correlation with their structural degradation [3]. A strong improvement of the cell efficiency was demonstrated for the 1-dimensional NW geometry compared to its thin film counterpart, which is mainly due to a significant reduction of light reflected by the electrode. Morphological and electronic parameters, such as the aspect-ratio of the nanowires and their doping pattern were found to strongly influence the photocatalytic performances of the system. This work highlights the advantageous combination of nanowires featuring a buried radial p-n junction with Co-nanoparticles used as hydrogen evolution catalyst. The nanostructured photocathodes exhibit significant photocatalytic activities, comparable with previous noble-metal based systems tested under similar conditions (0 V vs. RHE, pH = 7).<br/>Furthermore, the stability of the NW-based photocathode over time was significantly improved with the deposition of a thin TiO₂ protective layer, leading to an increase of the Faradaic efficiency of the photocathodes by a factor 3 after 13 hours. The fabricated photocathodes display photocurrent density up to 1.15 mA/cm² at 0 V vs RHE under neutral conditions. Nevertheless, it would lead to solar-to-hydrogen performances below 1% in tandem photoelectrochemical cells. This calls for future tenfold improvement of the performances of such scalable materials to allow for a viable solar hydrogen technology. Still, such performance is at the state of the art for similar GaAs photoelectrodes, and demonstrates the potential of GaAs nanostructured semiconductor for photo-driven hydrogen production.<br/>[1] T. Dursap <i>et al.</i>, Nanoscale Advances (2020).<br/>[2] T. Dursap, <i>et al.</i>, Nanotechnology (2021).<br/>[3] T. Dursap, <i>et al.</i>, <i>under review</i> (2023).<br/><br/><u>Acknowledgments</u><br/>The authors acknowledge funding from the French <i>Agence Nationale de la Recherche </i>for funding (project BEEP 18-CE05-0017-01, Labex ARCANE and CBH-EUR-GS (ANR-17-EURE-0003)).