Enhanced photocatalytic H₂O₂ production over carbon nitride by interlayer expansion effect

Carbon nitride (CN) possesses a densely stacked layered architecture with an interlayer spacing of 3.22 Å, which is comparable to the molecular diameters of O₂ (2.96∼3.46 Å), H₂O (2.65∼2.90 Å) and H₂O₂ (3.30 Å). Therefore, even a subtle variation in the interlayer spacing can strongly influence photocatalytic H₂O₂ production. To maximize the interlayer spacing, CN was treated with NH₄Cl and LiCl/NaCl molten-salt, achieving an enlarged spacing of 3.32 Å. Mechanistic investigation revealed that the lower eutectic melting point favored ion intercalation, thus increasing the interlayer spacing·H₂O₂ production rate significantly improved to 5.47 mmol·g⁻¹·h⁻¹, 218-fold higher than that of pristine CN. The two-step single-electron reduction was confirmed as the dominant pathway. Experimental and theoretical calculation studies demonstrated that interlayer expansion promoted H₂O₂ generation via four crucial effects: (i) The amount of adsorbed O₂ was increased owing to reduced steric hindrance. (ii) Weakened interlayer interactions elongate O-O bonds and improve O₂ activation. (iii) Suppressed H₂O₂ self-decomposition via accelerated desorption. (iv) Optimized Gibbs free energies and inhibited H₂O formation side reactions, thus enhancing H₂O₂ selectivity. This work provided a feasible strategy for regulating the interlayer spacing of CN and deepened the understanding of interlayer expansion effect in photocatalytic H₂O₂ production.