Metal-Free Highly Stable and Crystalline Covalent Organic Nanosheet for Visible-Light-Driven Selective Solar Fuel Production in Aqueous Medium

Conversion of CO2 into solar fuels via artificial photosynthesis is one of the most promising and sustainable approaches to mitigate global warming and worldwide energy shortage. Covalent organic frameworks (COFs) exhibit well-defined arrangements of building blocks, tunable porosity, and high thermal and chemical stability in harsh conditions. The tunable band gaps of COFs by suitably introducing chromophoric light-harvesting units make them a unique class of metal-free heterogeneous photocatalysts for the successful conversion of CO2 to solar fuel. In this work, we report a simple, efficient, and low-cost 2D COF (TTA-Tz) composed of 1,3,5-tris-(4-aminophenyl)triazine (TTA) and 4,4′-(thiazolo[5,4-d]thiazole-2,5-diyl)dibenzaldehyde (Tz) for photocatalytic CO2 reduction. The 2D-layered COF is exfoliated into ultrathin covalent organic nanosheets (CONs), which shows visible-light-driven photoreduction of CO2 to CO (yield = 2.8 mmol g–1, rate = 82 μmol h–1 g–1, and selectivity >99%) in aqueous medium without an external sacrificial electron donor. Interestingly, for a mixed solvent system, the CO evolution rate (163 μmol g–1 h–1) is found double than the aqueous medium case with 99% selectivity. By introducing both BNAH and TEA as sacrificial electron donors, the significant amount of CH4 (499 μmol g–1) is produced and the rate of CO evolution (310 μmol g–1 h–1) is further enhanced. The mechanistic insight of CO2 reduction is studied by DFT-based theoretical calculation, which is further supported by in situ diffuse reflectance spectroscopy study.