ZnO–ZnGa2O4 core–shell nanowire array for stable photoelectrochemical water splitting

Abstract

A dense array of vertically aligned ZnO–ZnGa₂O₄ core–shell nanowires was synthesized on a large scale on an a-plane sapphire substrate by a simple two-step chemical vapor deposition method. The ZnO cores and ZnGa₂O₄ shells of the nanowires are of single crystal quality and have aligned crystallographic orientations as evidenced from XRD and TEM analyses. Mott–Schottky analysis and voltage onset from the photocurrent–voltage curve confirm an n-type semiconductor property, a flat-band potential of −0.4 V (versusNHE) and a carrier density of 7 × 10¹⁸ cm⁻³ for the ZnO–ZnGa₂O₄ core–shell nanowires. A stable and large photocurrent of 1.2 mA cm⁻² was obtained with the ZnO–ZnGa₂O₄ core–shell nanowire array when used as a photoanode at an applied bias of +0.7 V (versusAg/AgCl) under a 300 W xenon lamp illumination. Moreover, a low dark current of <10⁻⁴ mA cm⁻² was obtained at an applied bias of +0.7 V (versusAg/AgCl) without light illumination for the ZnO–ZnGa₂O₄ nanowire array. These results suggest that the dense array of ZnO–ZnGa₂O₄ core–shell nanowires provides enhanced electronic properties and stable anti-photocorrosion ability and, therefore, is promising as a photoanode in photoelectrochemical water splitting.