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

Abstract

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.