In this study, photocorrosion of ZnO is inhibited by doping Ni in the ZnO nanostructure and electron-hole recombination was solved by forming a heterostructure with S-g-CN. Ni is doped into ZnO NPs from 0 to 10% (w/w). Among the Ni-decorated ZnO NPs, 4% Ni-doped ZnO NPs (4NZO) showed the best performance. So, 4% Ni-ZnO was used to form heterostructure NCs with S-g-CN. NZO NPs were formed by the wet co-precipitation route by varying the weight percentage of Ni (0-10% w/w). Methylene blue (MB) was used as a model dye for photocatalytic studies. For the preparation of the 4NZO--SCN nanocomposite, 4NZO NPs were formed in the presence of various concentrations of S-g-CN (10-50% (w/w)) by using the coprecipitation route. The electron spin resonance (ESR) and radical scavenger studies showed that O and OH free radicals were the main reactive species that were responsible for MB photodegradation.