PHOTOCATALYSIS

Reactive facet of carbon nitride single crystals

Nat. Catal., 3, 649 (2020)



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Reactive facets of heterogeneous photocatalysts play an important role during the water photolysis process since they afford the active sites/planes for the photoredox splitting of water or loading of co-catalysts. Recent studies indicated that the facets also play additional roles during charge separation. Therefore, identifying and exploring how the reactive facets facilitate the reactivity is considered to be a crucial step for developing high-performance photocatalysts for solar to chemical energy conversion. Investigations of the reactive facets have mainly focused on inorganic photocatalysts. There are only few reports about the exact reactive facets of polymeric carbon nitride (PCN) due to its low crystallinity and instability under the irradiation of high-energy electron beams. The PTI (polytriazine imide) intercalated with LiCl (PTI/Li⁺Cl^–), which is synthesized by ionothermal methods, forms an ideal system for exploring the reactive facets of conjugated polymers because of its high crystallinity.



Wang et al. studied the surface structures of PTI/Li⁺Cl^– under an extremely low electron dose rate via aberration-corrected integrated differential phase contrast (AC-iDPC) imaging. The photodeposition of Co and Pt co-catalysts was used to probe the reactive facets of PTI/Li⁺Cl^– in overall water photolysis. First-principles calculations were performed to understand the electronic structures and confirm the electron–hole transition among the prismatic {$10bar 10$} planes of the PTI/Li⁺Cl^–. Unlike conventional view on PCN nanosheets, all those experiments demonstrated that the side prismatic planes {$10bar 10$} were the major reactive facets because of two factors: the photogenerated electron–hole pairs can easily migrate along with the conjugated layers to the {$10bar 10$} facets, and the co-catalysts are mostly photodeposited on the {$10bar 10$} facets due to the more electrons or holes available on the prismatic surfaces. Upon this discovery, PTI/Li⁺Cl^– crystals with different aspect ratios were prepared at different synthetic temperatures. It is found that the overall water splitting performance followed a linear correlation with the relative surface areas of the {$10bar 10$} and {0001} planes. The photocatalytic overall water splitting performance on PTI-550 (samples synthesized at 550 °C) showed an AQY (apparent quantum yield) of 8% at 365 nm. In contrast with the conventional application of PCN nanosheets, this work provides a new design strategy to boost the photoactivity of layered conjugated polymers in a one-step-excitation overall water splitting reaction.



Kong Liu (Institute of Semiconductors, CAS, Beijing, China)



doi: 10.1088/1674-4926/41/9/090202