PHOTOCATALYSIS
g/C3N4-based photocatalysis
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Photocatalysis has been regarded as a sustainable and environmentally friendly technique for the degradation of contaminants. Graphitic carbon nitride (g/C3N4), an efficient metal-free polymeric semiconductor photocatalyst, has attracted extensive attention, but the practical applications of intrinsic g/C3N4 are usually limited by its insufficient visible-light absorption (the band absorption edge is about 460 nm), high recombination rate of charge carriers and low electrical conductivity. Various strategies, including doping with elements to narrow the bandgap, coupling with cocatalysts to enhance the reaction kinetics, and loading on substrates with excellent mobility of charge carriers to realize the efficient charge extraction, have been developed to overcome these drawbacks. However, the systematic optimization on the photocatalytic behaviors of g/C3N4 for degrading complicated chemicals is lack of deserved attention.
Deng et al. optimized the photocatalytic performance of g/C3N4-based metal-free photocatalysts, via doping of oxygen, decorating with CDs (carbon quantum dots), and loading on the rGO (reduced graphene oxide) for degradation of a representative antibiotic, lincomycin. The combination showed a synergistic effect both in the degradation rate and the degree of decomposition of lincomycin. The bandgap of g/C3N4 has been narrowed efficiently by doping of oxygen, which enhanced the utilization of visible light. CDs were introduced as cocatalysts to boost the production of reactive species of ·O2?. rGO was used to optimize the charge extraction. In comparison with the intrinsic g/C3N4, the structurally optimized photocatalyst showed a tenfold enhancement in degradation rate. In the degradation, the active species, including ·O2?, ·OH, and h+, had different contributions in the different photocatalysts. The intermediate, H2O2, played an important role in the photocatalytic process, and the detailed functions and originations were clarified for the first time. ·O2? was the main active species for all the systems, but its production pathways were different for g/C3N4 and O-g/C3N4(oxygen doped g/C3N4). It came from the O2 reduction in g/C3N4, while from the H2O2 decomposition in the systems containing O-g/C3N4. The H2O2 production was very important, and it originated from H2O oxidation in the systems of O-g/C3N4 and CD-O-g/C3N4, and from both H2O oxidation and O2 reduction in the systems of rGO-O-g/C3N4 and CD-rGO-O-g/C3N4. Another reactive species of ·OH was generated by the h+ in the active sites of rGO. In addition, decorating of CDs could promote the production of ·O2? from H2O2 by chemical catalysis without light on account of the peroxidase-mimetic activity of them. Thus, a new metal-free g/C3N4-based photocatalyst of high efficiency has been provided by engineering the components efficaciously.
Chao Zhao (Institute of Semiconductors, CAS, Beijing, China)
