Covalent doping of g-C3N4 with the benzo[c][1,2,5]-chalcogenadiazole acceptor blocks: photocatalysis and electronic structure

The methodology of in situ thermal synthesis has been developed for the semiconductors based on graphitic carbon nitride (g-C3N4) doped by benzo[c][1,2,5]chalcogenadiazoles (chalcogen Ch = O, S, Se). Benzo[c][1,2,5]chalcogendiazoles were obtained by methods previously presented in the literature. The purity of the resulting organic structures was confirmed by 1H and 13C NMR, GC-MS, IR-spectroscopy, elemental analysis, and the melting point determination. The technique for obtaining g-C3N4 samples consists in sintering melamine and the required acceptor block mixture at 550 °C in a neutral atmosphere. For pure and doped g-C3N4 its structure formation fact was confirmed by PXRD, IR-spectroscopy and 13C NMR. Semiconductor and other properties of carbon nitride materials were studied by UV-spectroscopy, PL-spectroscopy, cyclic voltammetry technique, SEM conbined with EDS, as well as by plotting nitrogen sorption-desorption isotherms. A series of photocatalytic water-splitting experiments under the UV-light (λ = 365 nm) action in the presence of samples of pure and doped carbon nitride as a photocatalyst, hexachloroplatinic acid as a co-catalyst, and triethanolamine as a electron-sacrificial agent was carried out. The amount of hydrogen formed during the water-splitting experiment was determined for every hour using the GC-method. It was found that all three dopants positively affected photophysical and catalytic properties of the materials. Quantum chemical calculations confirmed that the benzo[c][1,2,5]chalcogenadiazoles served as acceptor blocks with accumulation of the most of the HOMO electron density.