The development of new architectures and materials combination for optoelectronic devices, like photodetectors, is boosting the research in the field. The main target is the development of low-cost processes to produce highly efficient and stable devices not containing harmful or expensive materials. An extended range of heterostructures for the photo detection application have been reported  like BiOCl/TiO2 , grapheme/GaAs , SnO2/NiO , SrTiO3/CuS-ZnS , ZnO/NiO , Ag-doped ZnO , ZnO/Ga2O3 .
It is worth to note mention here that these devices are limited by slow photo response ranging from millisecond to seconds and short wavelength sensing. Thus, it is highly desirable to sense the wide spectral range with swift switching response for the photodetector by using photosensitive compounds. The materials of earth abundant and nontoxicity are highly critical for the large scale application and mass production of devices in diverse ongoing research fields. The metal oxides of wide bandgap are found very potential materials for the design of efficient photodetector geometries. Among them, zinc oxide (ZnO) is n-type, wide band gap (∼3.37eV), emiconducting metal oxide, nontoxic and earth abundant in nature has been capitalized as active UV light absorber and used in in different highly sensitive photodetectors . For making the p-n junction with ZnO, the large utilization of p-type metal oxides are (NiO), cuprous oxide (Cu2O), spinel cobalt oxide (Co3O4) and so on [10–13].
Cobalt oxide (Co3O4) has been highly investigated in various applications such as gas sensing , supercapacitors , Lithium ion batteries,  and in solar cells . Like Cu2O, Co3O4 is a p-type, earth abundant, non-toxic, semiconducting material. Its absorption spectrum is characterized by two direct transitions in the visible spectral range, corresponding to optical bandgaps of 1.5 eV and 2.2 eV [18, 14]. Thanks to the low bandgap, and the possibility to further optimize it. Generally it is believed that the Co3O4 exhibits spinel structure with mixed valence states. The Co3O4 being a p-type it is allowing the acceptor states which accelerates the trapping of charge thus it creates the charge transport. Because of the dual and indirect bandgaps of Co3O4 2.2 eV and 1.6 eV and it in spinel structure it contains Co3p and Co 2p spin states. It is a common observation that the dual bandgaps lead to the design of efficient photodetectors [19, 20]. These unique features of ZnO and Co3O4 altogether share the development of wide spectral photodetectors. The Co3O4 nanostructures have been produced through different methods like solution, spray pyrolysis, and chemical vapor deposition [21–24].
The p-n junction of ZnO-Co3O4 can be applied as efficient photodetector. A photosensor is a device, which generates photocurrent as a response to light. A photodiode is a device that converts light into electric current, commonly termed as photodetector, light detector or photosensor. Photodiodes typically work in reverse bias; the efficiency of these devices is mainly depending on the high surface charge carrier concentration . Thus, in order to maximize the photo response, different kind of heterostructures were investigated, to exploit increased light absorption due to multiple scattering, and to increase the interface area, and maximize exciton dissociation rate at the interface [26–32]. Moreover, there are numerous heterostructures both inorganic and inorganic-organic photodetectors, which employ transparent n-type semiconducting metal oxides including ZnO because of its capability to absorb UV light efficiently [33–36]. Beside this, Co3O4 as p-type metal oxide can be a good light absorber for visible photodetectors due to its direct optical transitions in the visible range.
It is the simplicity, cost, processing nature and energy consumption during a growth method which are keep in mind for the mass production of nanostructured materials. For this purpose, there is no report about the p-n junction of Co3O4/ZnO using low temperature aqueous chemical growth method for the development of visible light photodetector.
In this work, we investigated for the first time the system composed of hydrothermally grown ZnO nanorods covered by Co3O4 nanowires on fluorine doped tin oxide (FTO) glass, to be applied as photodetector. At the same time, the all oxide p-n junction can act as visible-light photodetector with fast response and good stability.