Highly efficient hydrogen evolution from water splitting on heptazine polymer with three types of defects

Abstract

Three types of defects were successfully introduced into heptazine polymer by calcinating the mixture of melamine and high proportion of KOH. The formation mechanism and effects of defects were studied in detail. In particular, cyano groups and sp2-nitrogen defects were introduced into samples when KOH content was low. With increasing usage of KOH, the heptazine rings tend to open at the position of sp2-nitrogen defects to form sp3-nitrogen defects. Different defects would introduce different defect level within the bandgap, thus adjusting the band structure of heptazine polymer and enhancing its response to visible light.The sp2-nitrogen defects acted as electron donor and introduced donor level at bottom of conduction band, while -C≡N groups and sp3-nitrogen defects acted as electron acceptor and introduced accept level above the valence band. The molecular dipole was increased due to three types of defects coexistence in heptazine polymer, which contributed to enlarge the built-in electric field and promote the separation of photogenerated carriers. As a result, the optimized photocatalyst 3MK-400 exhibits the highest hydrogen evolution rate of 4.37 mmol g-1h−1 under solar driven, which was 13.7 times higher than melem-400.