Dawson-type Polyoxometalates-based vacancies g-C3N4 composite-nanomaterials for efficient photocatalytic nitrogen fixation

Fixing nitrogen (N2) is the second vital chemical process preceded only by photosynthesis in nature. It is still a bottleneck problem to construct efficient photocatalysts for reduction N2. Herein, we firstly report four kinds of Dawson-type polyoxometalates (POMs)-based nitrogen vacancies graphitic carbon nitride (V-g-C3N4) (POMs@V-g-C3N4) composite-nanomaterials, which have outstanding photocatalytic N2 fixation activities under mild conditions. V-g-C3N4 can capture N2 molecules strongly and accurately. The doped POMs into V-g-C3N4 play a major role in promoting the activation and dissociation N2 by the following three reasons: (1) establishing a wide spectrum from 200-900 nm to enhance the utilization of light energy; (2) POMs are easily reduced under light conditions with the advantage of stronger pre-reduction properties to provide abundant electrons to activate N2. The reduced POMs further react with oxygen returning to the original POMs, and the reaction forms a self-healing, recyclable photocatalysis N2-fixing system. (3) suppressing effectively the recombination of photo-generated carriers and weakening charge-transfer resistance. Surprisingly, α2-K8P2W17O61(Co2+•OH2)•16H2O (P2W17Co) P2W17Co@V-g-C3N4 shows the best photocatalytic N2 fixation efficiency of 214.6 μmol L-1 h-1, which is increased by 91.05% and 95.99% compared to the solely P2W17Co and V-g-C3N4. It provides a new view in design of sustainable, stable and high-performance photocatalytic N2 fixation nanomaterials.