Sulfur-doped photocatalysts with iron-nitrogen coordination bonds by modifying graphitic carbon nitride obtained from ammonium thiocyanate pyrolysis with ferrous sulfate heptahydrate in ethanol

Abstract The photocatalytic agent graphitic carbon nitride (g-C3N4) was synthesized by pyrolysis of ammonium thiocyanate (NH4SCN) which is commonly produced via extraction of desulfurization waste liquids as the main salt. The g-C3N4 photocatalyst was modified by treatment with different amounts of ferrous sulfate heptahydrate in ethanol, which resulted in Fe-doping and a certain degree of S-doping of the material. According to the characterization results, the modified sample eg-C3N4/FeSO4-10% with superior photocatalytic efficiency exhibited the highest specific surface area and pore volume, poor crystallinity, and a good sample polymerization degree. The existence of more C defect vacancies in the modified sample structure increased the electron density around C and N atoms in the triazine ring structure and becomes the electron capture center. At the same time, the presence of ethanol solvent promotes the formation of more Fe–N coordination bonds between a large number of lone pair electrons and metal Fe ions. In addition, with the increase of the doping amount of S element, a new impurity level is introduced, and the band gap widths of the samples become narrower. The changes of surface structure and energy band structure of the modified samples led to a significantly reduced carrier recombination rate and further promoted the production of more reactive oxygen groups. In the process of Rhodamine B (RhB) degradation as organic pollutant, the degradation can reach 98% rapidly after 60 min of visible light irradiation, which is 78% higher than g-C3N4, at a simultaneous increase of the photocatalytic reaction rate constant from 0.007 to 0.305 per min. Meanwhile, the sample eg-C3N4/FeSO4-10% also showed excellent photocatalytic activity in the NOx removal process of air pollution. Compared with the original g-C3N4, the NO removal rate increased from 64% to 75%, and the production amount of harmful intermediate NO2 decreased from 0.15 to 0.08 mg/m3.