Metal-Free Highly Stable and Crystalline Covalent Organic Nanosheet for Visible-Light-Driven Selective Solar Fuel Production in Aqueous Medium
Sandip Biswas, Anupam Dey, Faruk Ahamed Rahimi, Soumitra Barman, Tapas Kumar Maji
Abstract
Conversion of CO 2 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 CO 2 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 CO 2 reduction. The 2D-layered COF is exfoliated into ultrathin covalent organic nanosheets (CONs), which shows visible-light-driven photoreduction of CO 2 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 CH 4 (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 CO 2 reduction is studied by DFT-based theoretical calculation, which is further supported by in situ diffuse reflectance spectroscopy study.