Which kind of nitrogen chemical states doped carbon dots loaded by g-CN is the best for photocatalytic hydrogen production.

Recently, g-CN (CN) loaded N-doped carbon dots (NCDs) have been widely studied as promising metal-free photocatalysts due to their impressive performance in hydrogen production. However, deep understanding of the effect of nitrogen chemical states on photocatalytic activity is still lacked. In this work, NCDs doped with pyrrole nitrogen, graphite/pyrrole nitrogen, and pyrrole/pyridine nitrogen were prepared and hybridized with g-CN. The characterizations revealed that, incorporation of pyrrole N-doped CDs into g-CN (CN/NCDs-en) effectively enhanced the visible light absorption, facilitated electron-hole separation, and promoted the participation of photoexcited electrons in H evolution reaction. Moreover, theoretical calculation showed that, compared with graphite N and pyridine N, pyrrole N has the most appropriate H adsorption ability, which is conducive to the H formation. Under visible light irradiation, the CN/NCDs-en exhibited the best hydrogen evolution of 3028 μmol h g. These results shed a light on the design and optimization of N-doped metal-free photocatalysts for H evolution reaction.