A nanocomposite material including iron sulfide (FeS.sub.2) nanoparticles, iron oxide (-Fe.sub.2O.sub.3) nanoparticles, titanium dioxide (TiO.sub.2) nanoparticles, and graphitic carbon nitride (C.sub.3N.sub.4) nanosheets and a method of its preparation. The nanocomposite is used in a method of forming oxygen gas from water using an applied voltage and photoirradiation, a method of forming hydrogen gas from water using an applied voltage and photoirradiation, and a method of photodegrading organic pollutants using visible light photoirradiation.