Propelling solar-to-H2O2 conversion of a molecularly tunable covalent heptazine skeleton with boosted spatial charge separation and awakened n–π* electron transitions

Abstract

The covalent heptazine skeleton has garnered increasing attention in artificial photosynthesis of H₂O₂, yet its limited light absorption and facile recombination of photogenerated carriers restrict its overall performance. Herein, a molecularly tunable covalent heptazine skeleton with boosted spatial charge separation and awakened n–π* electron transitions is fabricated via molecular doping integrating molten salt assisted calcination. The solar-to-H₂O₂ conversion performance of the optimized 30-P-KPHI sample reaches 5565.79 μmol g⁻¹ h⁻¹, which is 131 times higher than that of the original CN, surpassing that of most reported similar photocatalysts. A series of characterization tests (e.g., transient absorption and transient/steady state fluorescence spectroscopy) and theoretical calculations (e.g., HOMO/LUMO and absorption energy) demonstrate that the n–π* electron transition broadens the visible light absorption beyond 500 nm, and the meticulously designed covalent heptazine skeleton facilitates spatial charge separation. More importantly, the introduction of the cyanide group significantly enhances the adsorption and activation of oxygen. A two-electron oxygen reduction mechanism is proposed by combining the rotating disk electrode test, capture experiments, EPR tests and theoretical calculations. This work provides a new insight for the simple and novel construction of high-activity and cost-effective CN photocatalysts holding great prospects for solar-to-H₂O₂ conversion.