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

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 g_cat⁻¹ h⁻¹ during the photocatalytic CO₂RR, which is absent in Cu/b-C₃N_4.