Zn-doped tri-s-triazine crystalline carbon nitrides for efficient hydrogen evolution photocatalysis

Graphitic carbon nitride (g-CN) typifies organic semiconducting photocatalyst, but it suffers from inefficient charge transfer and unconsummated polymerization at a certain level. In order to develop an optimal setup of charge distribution in a specific crystal plane, we attempted to realize both crystallinity and doping at the same time. Taking account into three different types of melem-based precursors (melem, oligomelem and melon), we systematically investigated a Zn-doped conjugated system using a two-step thermal polymerization strategy of a molten salts bath. The as-prepared Zn-doped crystalline carbon nitrides (Zn-CCN) show a highly ordered crystal structure with an extended tri-s-triazine-based intraplanar network, improved light harvesting and significantly suppressed electron-hole recombination. In addition, the density functional theory (DFT) calculations revealed that the Zn atoms prefer to dwell in the cave of the (100) plane, and their 4s-orbital electrons directly participate in the formation of the conduction band. This unique Zn-doped electronic structure notably reduces the in-planar charge density when Zn atoms act as a rope ladder urging electrons to escape from the N-encircled cave. When Zn-CCN is tailored with these unique layouts, it exhibits dramatically enhanced visible-light photocatalytic hydrogen evolution and excellent cycling stability, exceeding most of other metal-induced crystalline carbon nitrides. It is anticipated that this work can pave a novel research path in future studies of g-CN-based and other two-dimensional photocatalysts with ameliorated performances.