Adsorption and Photocatalysis of Organic Dyes by g-C3N4 In Situ Doped with S

Thiourea was used not only as an organic precursor but also as a sulfur source to achieve in situ synthesis of S-doped g-C3N4 (g-C3N4/S) with high adsorption capacity and good photocatalytic activity. XRD and FT-IR investigations indicated that the doping of S might result in the substitution of C or N in the unit cell of g-C3N4 without structural damages. SEM observations showed that g-C3N4/S particles exhibited irregular shape and desultorily aggregates. EDS analyses showed that all the samples contained C, N, 0 and S elements. In addition, DRS spectra revealed a slight adsorption edge around 645 nm with corresponding band gaps of 1.8-1.9 eV, attributable to the formation of impurity energy level by S doping. The discoloration ratio of MB could approach 90% within 60 min adsorption in dark and next 120 min photocatalysis under visible light irradiation. The effects of initial MB concentration, g-C3N4/S dosage and solution pH on MB discoloration ratio were analyzed. Comparative test on the discoloration of different dyes (MB, RhB, and MO) revealed that the electrostatic attraction and repulsion played important roles in the adsorption of dye molecules by g-C3N4/S surface. The synergistic effects of adsorption and photocatalysis governed the dye discoloration process. Finally, the mechanisms underlying synergy is discussed.