Highly efficient visible-light-driven CdS-loaded ZnO-GaN nanowire photoanode fabricated on Si for H2 evolution

Abstract

GaN nanowires (NWs) are considered promising photoelectrode materials owing to their direct bandgap, which a high-efficiency photon absorption. Additionally, their conduction and valence bands straddle the water redox level, making them ideal candidates for solar water splitting without external biasing. Moreover, they have a large reactive surface area, which promotes charge separation. However, their practical applicability is limited because of their poor stability and low solar-to-hydrogen conversion efficiency, which can be mitigated by loading a passivation layer and cocatalyst onto the GaN NW surfaces. In this study, CdS/ZnO/GaN NW photoanodes were fabricated on Si substrates for photoelectrochemical water splitting. Vapor–liquid–solid GaN NWs were used as the core material on a Si substrate passivated with a ZnO conformal layer, and CdS was adopted as a cocatalyst. Compared to a ZnO/GaN NW photoanode, the CdS/ZnO/GaN NW photoanode showed a 10-fold increase in photocurrent density and a 47-fold increase in photoconversion efficiency under 1-Sun illumination. We propose that ZnO and CdS inhibit the photocorrosion of bare GaN NWs and thus reduce the surface defects, thereby lowering the photogenerated charge recombination rate. As a result, the stability of GaN NWs in a high pH electrolyte increases, leading to an efficient H2 evolution.