Synergy of nanocrystalline carbon nitride with Cu single atom catalyst leads to selective photocatalytic reduction of CO2 to methanol

Abstract

Carbon nitride (C3N4) possesses both a band gap in the visible range and a low-lying conduction band potential, suitable for water splitting and CO2 reduction reactions (CO2RR). Yet, bulk C3N4 (b-C3N4) suffers from structural disorder leading to sluggish reaction kinetics. This can be improved by graphitisation; however, current processes in the literature, lead to a variety of graphitised C3N4 (g-C3N4), making it difficult to link the degrees of graphitisation with the functional properties. Herein, we employ complementary analyses, including electrochemical impedance, photoluminescence, and photocurrent, to elucidate structure-property-function relationships. Guided by the descriptors, we developed a facile two-step annealing method that yields nanocrystalline carbon nitride (nc-C3N4), comprising nanoscale graphitic domains within an amorphous matrix. The nanocrystalline grains of nc-C3N4 allow effective immobilisation of Cu atoms and stabilisation of low oxidation states (Cu(i)). Electron microscopy and energy-dispersive X-ray spectroscopy demonstrate that Cu is atomically dispersed. Importantly, the addition of only 0.11 wt% of copper to nc-C3N4 drastically decreases the charge recombination and resistance to change transfer. The synergy of the Cu single-atom catalyst and nanocrystalline domains in carbon nitride (Cu/nc-C3N4) leads to a remarkable 99% selectivity towards methanol production with a rate of 316 μmol gcat−1 h−1 during the photocatalytic CO2RR, which is absent in Cu/b-C3N4