Green and Scalable Synthesis of Atomic-Thin Crystalline Two-Dimensional Triazine Polymers with Ultrahigh Photocatalytic Properties

Scalable and eco-friendly synthesisof crystalline two-dimensional(2D) polymers with proper band gap and single-layer thickness is highlydesired for the fundamental research and practical applications of2D polymers; however, it remains a considerable and unresolved challenge.Herein, we report a convenient and robust method to synthesize a seriesof crystalline covalent triazine framework nanosheets (CTF NSs) witha thickness of similar to 80 nm via a new solvent-free salt-catalyzednitrile cyclotrimerization process, which enables the cost-effectivelarge-scale preparation of crystalline CTF NSs at the hundred-gramlevel. Theoretical calculations and detailed experiments revealedfor the first time that the conventional salts such as KCl can notonly act as physical templates as traditionally believed but alsomore importantly can efficiently catalyze the cyclotrimerization reactionof carbonitrile monomers as a new kind of green solid catalysts toachieve crystalline CTF NSs. Upon simple liquid-phase sonication,these CTF NSs can be easily further exfoliated into abundant single-layercrystalline 2D triazine polymers (2D-TPs) in high yields. The obtainedatomically thin crystalline 2D-TPs with a band gap of 2.36 eV andrich triazine active groups exhibited a remarkable photocatalytichydrogen evolution rate of 1321 mu mol h(-1) undervisible light irradiation with an apparent quantum yield up to 29.5%at 420 nm and excellent photocatalytic overall water splitting activitywith a solar-to-hydrogen efficiency up to 0.35%, which exceed allmolecular framework materials and are among the best metal-free photocatalystsever reported. Moreover, the processable 2D-TPs could be readily assembledon a support as a photocatalytic film device, which demonstrated superiorphotocatalytic performance (135.2 mmol h(-1) m(-2) for hydrogen evolution).