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–1and 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.