A novel methodology and its mechanism to synthesize ZnO/ZnS core–shell nanostructure by hydrothermal-supported co-precipitation method are presented. Chemical precursors Na2S and thioacetic acid were used as chemical conversion agents. Detailed structural, morphological, compositional and optical studies were carried out. Powder X-ray diffraction analysis confirms the presence of both wurtzite and cubic phase in ZnO/ZnS core–shell, respectively. OH absorption capacity of as-grown ZnO/ZnS nanostructures was found to be enhanced due to interfacial roughness ZnO/ZnS as compared to pristine ZnO. Morphological studies confirm the formation of irregular spherical nanocrystals of size ~ 50 nm. Band gap of ZnO nanocrystals was found to be increased upon sulphidation process. Room temperature EPR studies also confirmed the ZnS shell over ZnO nanocrystals suppresses the paramagnetic defects in ZnO. Interestingly, defect-related visible light emission from ZnO nanocrystals was found to be suppressed completely due to the presence of the larger band gap of ZnS as a shell over ZnO core. ZnS shell restricts the photogenerated charge carriers within the ZnO nanocrystal core, making ZnO/ZnS core–shell a potential candidate for optoelectronic applications such as UV photodetectors. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH, DE part of Springer Nature.