Reducing dielectric confinement effect in ionic covalent organic nanosheets to promote the visible-light-driven hydrogen evolution

Ultra-thin two-dimensional (2D) organic semiconductors are promising candidates for photocatalysts because of the short charge diffusion pathway and favorable exposure of active sites plus the versatile architecture. Nonetheless, the inherent dielectric confinement of 2D materials will induce a strong exciton effect hampering the charge separation. Herein, we demonstrated an effective way to reduce the dielectric confinement effect of 2D ionic covalent organic nanosheets (iCONs) by tailoring the functional group via molecular engineering. Three ultra-thin CONs with different functional groups and the same ionic moieties were synthesized through Schiff base condensation between ionic amino monomer triaminoguanidinium chloride (TG) and aldehyde linkers. The integration of the hydroxyl group was found to significantly increase the dielectric constant by enhancing the polarizability of ionic moieties, and thus reduced the dielectric confinement and the corresponding exciton binding energy (E_b). The champion hydroxyl-functional iCON exhibited promoted exciton dissociation and in turn a high photocatalytic hydrogen production rate under visible-light irradiation. This work provided insights into the rationalization of the dielectric confinement effect of low-dimensional photocatalysts.