In the present study, we report a novel nanocatalyst prepared by the modification of g-C₃N₄ nanosheets with a NiO/WO₃ nanohybrid via a simple ultra-sonication method. A novel method was employed for the synthesis of the NiO/WO₃ nanohybrid. The photocatalytic efficiency of the catalysts was explored in C–H activation and degradation of a product commonly found in pharmaceutical waste, namely metronidazole. The photocatalyst recorded a rapid and highly selective conversion of benzene to phenol under irradiation using a LED bulb. Degradation of metronidazole was carried out both in the presence of sunlight and LED light and monitored using UV-vis spectroscopy. It was observed that the degradation process was more efficient under solar light irradiation, where complete degradation was recorded in 90 minutes with a rate constant of 0.0165 min⁻¹. The modified photocatalyst showed much higher efficiency when compared to the individual components of g-C₃N₄ and nanostructured NiO/WO₃. The structural features of the photocatalyst were thoroughly investigated using powder XRD, SEM, TEM, XPS, UV-DRS, PL and BET analysis. This enhanced efficiency is attributed to a larger surface area, lower band gap and delayed recombination of photogenerated charge carriers at the heterojunction. The critical involvement of photoactive radicals in the degradation process was thoroughly investigated by trapping experiments using photoluminescence spectroscopy.