In situ structural modification of graphitic carbon nitride by alkali halides and influence on photocatalytic activity

Abstract

A series of graphitic carbon nitride photocatalysts were prepared by thermally polymerizing the mixtures of melamine and alkali halides (MCl, M = Na, K, Rb, Cs) in air. XRD, FTIR, Raman, SEM and TEM were used to investigate the effect of alkali halides on the structure and morphology of g-C₃N₄. It is found that such prepared samples exhibit reduced in-plane long-range periodic ordering while maintaining the framework of melon strands. However, no significant increase in surface area was observed by BET measurements. Moreover, PL and EIS measurements show that alkali halide modification results in improved separation and transfer of photogenerated electron and holes. Furthermore, the results of UV-vis absorption spectra and ultraviolet photoelectron spectra (UPS) indicate that the presence of alkali halides during the thermal polymerization process can decrease band gap energy and shift the conduction/valence bands of g-C₃N₄ upward. The variation of structural, electronic, and optical properties depends on the type of cationic ions. Potassium chloride modification gives rise to the best photocatalytic performance among all the samples, which is attributed to the reduced charge carrier recombination, narrower band gap, and higher CB edge. These findings may help in the design of visible-light-driven photocatalysts with enhanced photocatalytic activity.