Synthesis of Copolymerized Carbon Nitride Nanosheets from Urea and 2-Aminobenzonitrile for Enhanced Visible Light CO₂ Reduction with a Ruthenium(II) Complex Catalyst

Copolymerized carbon nitride nanosheets (NS-C₃N₄) are synthesized by heating a mixture of urea and 2-aminobenzonitrile (ABN) at 823 K for 2 h in air. The visible light-absorption capability of the copolymerized NS-C₃N₄ can be improved with an increase in the ABN content in the starting material, while maintaining the rather negative conduction band potential of NS-C₃N₄. With the aid of a ruthenium(II) complex catalyst, the copolymerized NS-C₃N₄ becomes active for CO₂ reduction into formate (>90% selectivity) under visible light (λ > 400 nm) in the presence of triethanolamine as an electron donor. The activity is enhanced with increasing the starting ABN content to reach a maximum at a certain amount, beyond which it declines. The optimized material, modified with a silver promoter and a phosphonate-functionalized ruthenium(II) catalyst, gives a high turnover number of 6000 (vs Ru catalyst) for formate production. Physicochemical analyses indicate that increasing the starting ABN concentration improves the visible light-absorption capability of the copolymerized NS-C₃N₄, but increases the number of trap states, which can work as recombination centers of photogenerated electrons and holes. Therefore, an appropriate adjustment of the ABN comonomer amount is essential to obtain copolymerized NS-C₃N₄, which shows high photocatalytic activity.