Construction of an enhanced built-in electric field in S-doped g-C3N4/NiCo2O4 for boosting peroxymonosulfate activation

Transition metal oxide-mediated heterogeneous peroxymonosulfate (PMS) activation on organic pollutant degradation has attracted much attention, where continuous redox cycling between high and low valence metal ions is the key step as it relates to the radical generation. Herein, a composite PMS activator of S-doped g-C3N4/NiCo2O4 (CNS_NCO) was constructed and exhibited excellent tetracycline (TC) degradation performance with 96.9% removal efficiency within 30 min. Experimental and theoretical calculations indicated that S doping not only widened the difference in the work function of CNS and NCO, but also enhanced the built-in electric field intensity of CNS_NCO heterojunctions, resulting in more electron transfer from CNS to NCO in a directional manner and accelerating the Co(III)/Co(II) and Ni(III)/Ni(II) redox cycle to form more reactive oxygen species (ROS). It was demonstrated that 1O2 (main ROS) was formed mainly through SO4•−, •OH and •O2− (minor ROS) conversion. Meanwhile, high-valent cobalt-oxo (CoIV = O) species also played an important role in TC degradation.