Ionic Salt-Mediated Tuning of the Morphology and Band Structure of Graphitic Carbon Nitride for NO Removal under Visible Light

Abstract

Graphitic carbon nitride (g-C3N4, CN) as a popular photocatalyst has been investigated for the removal of air pollutants, but its photocatalytic efficiency and selectivity for ionic species (Si) can be enhanced through chemical or microstructural modification. The combinatorial study of six salts, Na+, K+, and NH4+ (per)sulfate, as mediators in the melamine precursor for the CN synthesis indicates that the thermal decomposition behaviors of the salts affect the CN polymerization process and the functional groups on CN samples, though all of them are found to increase the surface area. The mediation with Na+ and K+ (per)sulfate increases the dangling −C≡N bonds and surface moieties at a higher oxidation state. The sodium and ammonia salts caused an upward shift of the conduction band energy for the final photocatalyst, while K2SO4 and K2S2O8 induced a downward shift of the conduction band. The NO removal efficiency of CN with sodium persulfate and sulfate reached 80% and near 100%, respectively, much higher than that of the pristine CN (60%). Moreover, the samples with persulfates showed higher Si than those with sulfate salts. The high surface area and the keen hierarchical pores down to 2 nm in the salt-mediated CN samples increased the residence time of the gaseous species and the resultant NO removal efficiency, and Si. NO2 can still be produced in the dark for 15 min after the photocatalysis, indicating that there are long-lived species related to H2O2 produced in the prior illumination. The systematic investigation of the effects of inorganic salt addition was presented on CN in terms of the microstructure, band structure, and correlated photocatalytic oxidation efficiency, pathway, and product selectivity.