Alkali metal cation adsorption-induced surface polarization in polymeric carbon nitride for enhanced photocatalytic hydrogen peroxide production.

Photocatalytic hydrogen peroxide (H₂O₂) generation on the catalyst surface from oxygen is an electron-demanding process, making the construction of an electron-rich surface highly advantageous. In this study, a localized electric field was observed on the surface of polymeric carbon nitride (g-C₃N₄) when alkali metal cations were adsorbed onto it. These fields effectively inhibited surface carrier recombination and extended their lifespan, thereby enhancing H₂O₂ production. As a result, g-C₃N₄ achieved a superior H₂O₂ yield of 2.25 mM after 1 h in a 0.25 M K⁺ solution, which was 2.06 times greater than that (1.09 mM) achieved in a pure solvent. Notably, the increase in photocatalytic efficiency showed a remarkable dependence on ion species. At low concentrations, H₂O₂ generation efficiency was in the order of Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺. However, after optimizing the ion concentration, the highest H₂O₂ production was achieved in a solution containing K⁺ instead of Cs⁺. Molecular dynamics simulations and temperature-dependent photocatalysis experiments revealed that the synergistic interaction between adsorption energy and adsorption distance was crucial in governing the extent to which alkali metal cation adsorption enhanced g-C₃N₄ photocatalytic H₂O₂ production. This study provides theoretical insights for the design of materials for electron-demanding photocatalysis and aids in understanding variations in photocatalytic behavior in natural waters.