Synergistic modulation of metal-free photocatalysts by the composition ratio change and heteroatom doping for overall water splitting

Graphitic carbon nitride (g-C3N4) based metal-free photocatalysts have attracted growing attention in recent years, while the efficiency is largely limited by the slow reaction rate of the oxygen evolution reaction (OER) and fast recombination of photo-generated electron–hole pairs. In this study, we show that the photocatalytic activity for water oxidation can be better improved by the synergistic effect on electronic structures, including the N ratio change and heteroatom doping. Pristine g-C3N4 presents poor OER activity due to strong binding with the intermediates of *OH and *O, which can be weakened significantly by reducing the ratio of N in g-C3N3 and g-C2N. Furthermore, binding with all the intermediates during the OER process can be further modulated by heteroatom doping. In particular, low overpotentials of 0.37 V and 0.38 V are obtained on S-doped g-C3N3 and g-C2N, respectively. More importantly, the introduced impurity bands by S-doping can effectively trap the photo-generated holes for water oxidation and thus improve the photocatalytic efficiency, which is demonstrated by the nonadiabatic molecular dynamics simulations. Besides, other characteristics for photocatalysts, including band edges, light absorbance, thermal stability and so on, are also evaluated. The systematic study provides insight into modulation mechanisms for electronic structures by commonly applied approaches in experiments and guidance on the performance optimization of carbon nitride based metal-free photocatalysts.