Boosting exciton dissociation and charge transfer by regulating dielectric constant in polymer carbon nitride for CO2 photoreduction

The high binding energy of excitons and their slow charge transfer due to a low dielectric constant greatly limits the further improvement of photocatalytic efficiency in polymer carbon nitride (PCN). Herein, PCN samples with uniformly doped phosphorus (P-PCN) with stable and efficient CO2 photoreduction performance were prepared by the solid-state chemical reaction of PCN with sodium hypophosphite (NaH2PO2). With NaH2PO2 as both the molten salt and dopant precursor, this doping approach ensures that P is doped from the surface into the bulk, resulting in intimate P-OH bonds at the in-planar interstitial sites of the PCN melon. After polarization by the P-OH sites with high dipole moments, 0.7 at% P-PCN shows a significantly increased dielectric constant. Thus, a low exciton binding energy is achieved compared with pure PCN. This enhances the spatial separation and transport efficiency of photogenerated charge carriers over the optimal 0.7 at% P-PCN sample, leading to better selectivity for CO2-to-CO photoreduction with a three-fold higher yield compared to that of pure PCN without any cocatalysts or sacrificial agents. The controllable P doping method utilizing a thermodynamically feasible diffusion-controlled solid-state reaction reported in this work can be adopted to design other cost-effective photocatalytic systems.