Nitrogen vacancies in polymeric carbon nitrides promote CO2 photoreduction

Photocatalytic CO2 reduction into valuable chemicals and fuels is thought to be an effective way to reduce greenhouse gas emissions and achieve sustainable carbon cycling. Recent studies have confirmed that polymeric carbon nitride (PCN) can drive CO2 reduction reaction under visible light irradiation. However, the pristine PCN typically suffers from moderate efficiency. Herein, nitrogen-vacancies-functionalized PCN (NVS-PCN) photocatalysts are successfully prepared via a novel formic acid-assisted etching protocol for the first time. It is found that the NVs are mainly stemmed from the selectively breaking of surface N-(C)3 sites within PCN matrix. Steady-state photoluminescence (PL), time-resolved PL, and photocurrent results approve that the NVs can significantly suppress the recombination of photogenerated charge carriers. CO2 temperature programmed desorption and DFT calculations determine that the NVs modification can optimize the electronic properties of NVs-PCN photosystem, and thus considerably improve the activation of CO2, while lower the formation barrier of COOH* intermediates. Consequently, the NVs-PCN photosystem exhibits a 279-fold improvement in CO2 reduction activity over the pristine PCN under 400 nm light irradiation. Moreover, we successfully prepare another NVs-modified PCN-T (trithiocyanuric acid as a precursor) under the same preparation conditions. This NVs-PCN-T photosystem also shows exceptional performances for photocatalytic CO2 reduction and hydrogen production with visible light. The apparent quantum yields (AQYs) for CO2 reduction and hydrogen production reach as high as 6.17 % and 31.08 % at 400 nm, respectively. The insights provided in this work are expected to provide a novel clue for manipulating catalytic properties and advancing the in-depth understanding on the role of defect structure of photocatalysts during CO2 reduction.