Engineering reactive facets of highly crystalline carbon nitride guided by molecular-level insights

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    As early as 1875, Jules Verne depicted a picture that water would be the coal of the future in his science fiction. The discovery of water photolysis effect in 1972 opened the possibility to fulfil the challenging dream. However, photocatalytic water splitting is still hindered by its low efficiency. Developing highly efficient photocatalysts is therefore a critical step towards the practical application.

    At 2009, our group introduced polymeric carbon nitride (PCN) as a new generation of metal-free photocatalyst (Wang, X. et al., Nat. Mater. 8, 76-80 (2009).), which stimulated great and general interests in the field of artificial photosynthesis. PCN photocatalyst integrating the merits of easy preparation, low cost and non-toxicity is considered as a better alternative to metal oxide based photocatalysts. After a decade’s research effort, the intrinsic properties of PCN has been improved and overall water splitting has been achieved in our group (Wang, X. et al., Chem. Sci. 7, 3062-3066 (2016).), albeit with relatively low activity. The low crystallinity of PCN prepared by the typical thermal-polycondensation method brings in structural defects that comes along with serious recombination of the photogenerated electrons and holes, deteriorating the water splitting performance. Hence, high degree of crystallinity with minimum numbers of structural defects are highly desirable.

    Starting from 2017, we picked up Poly-triazine imide (PTI) with high crystallinity as a candidate for realizing overall water splitting. Unlike the conventional “trial and error” strategy, the current study represents a new research method for chemists by first identifying the reactive sites and charge migration pathways of crystalline PTI crystals, following with engineering of the crystal morphologies to expose more active sites and finally achieving an optimized apparent quantum yield of 8%@365nm. It is the highest quantum yield value for the polymeric photocatalysts in overall water splitting reaction. It demonstrates the great powder of the combination of advanced characterizations (i.e., aberration corrected integrated differential phase contrast (AC-iDPC) imaging) and density function theory (DFT) calculations in providing molecular-level insights on the reactive sites/facets of crystalline photocatalytic materials. Moreover, the molecular-level insight serves as a guideline for boosting the photocatalytic performance by increasing of surface areas of reactive {10-10} planes.

    The revealed reactive sites/facets are the side planes of crystalline PCN crystals, differing from the traditional view that photocatalyst reaction mainly occurs on the basal planes of ultrathin PCN crystals. Hence, systematic reactive facet engineering of PCN crystals is highly desirable in the future to optimize the photocatalytic performance of a large family of crystalline conjugated polymers. For more details, please see our recent article “Molecular-level insights on the reactive facet of carbon nitride single crystals photocatalyzing overall water splitting” in Nature Catalysis.

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