High efficiency visible-light-driven Fe2O3-xSx/S-doped g-C3N4 heterojunction photocatalysts: Direct Z-scheme mechanism

Several nanoporous Fe2O3-xSx/S-doped g-C3N4 (CNS) Z-scheme hybrid heterojuctions have been successfully synthesized by one-pot in situ growth of the Fe2O3-xSx particles on the surface of CNS. The characterization results show that S-doping in the g-C3N4 backbone can greatly enhance the charge mobility and visible light harvesting capability. In addition, porous morphology of hybrid composite provides available open pores for guest molecules and also improves light absorbing property due to existence of multiple scattering effects. More importantly, the Fe2O3-xSx nanoparticles formed intimate heterojunction with CNS and developed the efficient charge transfer by extending interfacial interactions occurred at the interfaces of both components. It has been found that the Fe2O3-xSx/CNS composites have an enhanced photocatalytic activity under visible light irradiation compared with isolated Fe2O3 and CNS components toward the photocatalytic degradation of methylene blue (MB). The optimal loaded Fe2O3-xSx value obtained is equal to 6.6 wt% that provided 82% MB photodegradation after 150 min with a reaction rate constant of 0.0092 min−1 which was faster than those of the pure Fe2O3 (0.0016 min−1) and CNS (0.0044 min−1) under the optimized operating variables acquired by the response surface methodology. The specific surface area and the pore volume of Fe2O3(6.6)/CNS hybrid are 33.5 m2/g and 0.195 cm3/g, which are nearly 3.8 and 7.5 times greater compared with those of the CNS, respectively. The TEM image of Fe2O3(6.6)/CNS nanocomposite exhibits a nanoporous morphology with abundant uniform pore sizes of around 25 nm. Using the Mott-Schottky plot, the conduction and valence bands of the CNS are measured (at pH = 7) equal to −1.07 and 1.48 V versus normal hydrogen electrode (NHE), respectively. Trapping tests prove that OH and O2− radicals are major active species in the photocatalytic reaction. It has been established that formation of the Z-scheme Fe2O3(6.6)/CNS heterojunction between CNS and Fe2O3 directly produces OH as well as O2− radicals which is consistent with the results obtained from trapping experiments.