The role of nitrogen defects in graphitic carbon nitride for visible-light-driven hydrogen evolution

Abstract

The introduction of nitrogen (N) defects (N vacancies labeled as V_ns and cyano groups) has been demonstrated as one of the promising strategies to extend the light absorption range of graphitic carbon nitride (CN), thus improving the photocatalytic activity for hydrogen (H₂) evolution. However, the photocatalysis mechanism of such N-deficient CN (DCN) has not been fully understood. In this study, N defects are introduced into CN by a KOH-assisted thermal polymerization method. On the basis of experimental investigations and density functional theory (DFT) calculations, it is found that the extension of the absorption range of DCN is attributed to both the valence band (VB) tailing induced by V_ns and bandgap narrowing induced by cyano groups. Moreover, the conduction band (CB) is lowered by the N defects, indicating a reduced driving force for H₂ evolution. Transient absorption (TA) spectroscopy reveals that when the electrons in the intrinsic VB of DCN are excited to the CB, the separation efficiency of these electrons and as-generated holes is seriously restricted by their low mobility. While when the electrons in VB tail states (V_n states) are excited to the CB, the separation efficiency of these electrons and as-generated holes could be almost maintained thanks to the improved mobility of the holes. As a result, DCN shows a limited enhancement of the H₂ evolution rate compared with CN under visible light irradiation. This work points out that extending the light absorption range of a given photocatalyst by doping (or self-doping) may be accompanied by some negative factors, which restrict the overall photocatalytic activity.