Oxygen-Functionalized and Ni+x (x=2, 3)-Coordinated Graphitic Carbon Nitride Nanosheets with Long-Life Deep-Trap States and their Direct Solar-Light-Driven Hydrogen Evolution Activity.

Graphitic carbon nitride, a 2 D layered photocatalyst coupled with transition metal oxides often shows promising photocatalytic hydrogen evolution activity. However, low surface area and poor charge separation greatly hinder its photocatalytic efficiency. A Ni+x (x=2, 3)/O-g-C3 N4 photocatalyst with a very high specific surface area (199 m2  g-1 ) has been prepared by thermal condensation and wet-impregnation methods. The oxygen-functionalized and Ni+x (x=2, 3)-coordinated g-C3 N4 produced 1664 μmol g-1 of hydrogen evolution from water under direct solar light irradiation in 4 h, which is 23 times higher than that over O-g-C3 N4 . This significant enhancement results from the combined effects of large surface area, the formation of long-life deep-trap states, effective charge carrier separation, and extended visible light absorption. The separation and transport behavior of the charge carriers are investigated by photoluminescence, time-resolved photoluminescence, photocurrent and Mott-Schottky measurements. Additionally, the interaction between Ni+x (x=2, 3) and O-g-C3 N4 is studied by X-ray photoelectron spectroscopy, X-ray diffraction, and FTIR spectroscopy. The Ni+x (x=2, 3)/O-g-C3 N4 photocatalyst shows remarkable reusability over a period of two months (six cycles). This study may provide a pathway to simultaneously overcome the challenges of low surface area and poor charge separation in g-C3 N4 -based photocatalysts.