Self-Powered Photodetectors Based on Core–Shell ZnO–Co₃O₄ Nanowire Heterojunctions

Self-powered photodetectors operating in the UV–visible–NIR window made of environmentally friendly, earth abundant, and cheap materials are appealing systems to exploit natural solar radiation without external power sources. In this study, we propose a new p–n junction nanostructure, based on a ZnO–Co₃O₄ core–shell nanowire (NW) system, with a suitable electronic band structure and improved light absorption, charge transport, and charge collection, to build an efficient UV–visible–NIR p–n heterojunction photodetector. Ultrathin Co₃O₄ films (in the range 1–15 nm) were sputter-deposited on hydrothermally grown ZnO NW arrays. The effect of a thin layer of the Al₂O₃ buffer layer between ZnO and Co₃O₄ was investigated, which may inhibit charge recombination, boosting device performance. The photoresponse of the ZnO–Al₂O₃–Co₃O₄ system at zero bias is 6 times higher compared to that of ZnO–Co₃O₄. The responsivity (R) and specific detectivity (D*) of the best device were 21.80 mA W^–1 and 4.12 × 10¹² Jones, respectively. These results suggest a novel p–n junction structure to develop all-oxide UV–vis photodetectors based on stable, nontoxic, low-cost materials.