NMR Relaxation Unveils the Promoting Effect of Surface–Water Interactions on Photocatalytic Degradation over Carbon Nitride in Aqueous Solutions

The role of interfacial water interactions with photocatalyst surfaces is a critical yet elusive aspect of photocatalysis. In this work, we uncover how temperature-driven structural evolution in carbon nitride controls water–surface interactions and, in turn, photocatalytic activity. A series of carbon nitride (CN-x) photocatalysts were prepared by thermal polycondensation of melamine at 450–650 °C to elucidate the effect of calcination temperature on their structure and photocatalytic behavior. Systematic characterization (FTIR, XRD, XPS, SEM, UV–vis DRS, N2 sorption, and 1H NMR relaxation) revealed progressive polymerization and structural ordering with increasing temperature, accompanied by an enlarged pore size and narrowed band gap. The photocatalytic degradation of Congo Red (CR) displayed a distinct valley-shaped trend with temperature, with CN-450 showing the highest activity despite its relatively low surface area. Using NMR relaxation measurements, we establish a direct correlation between the T1/T2 ratio of adsorbed water and photocatalytic activity, revealing that an optimal water–surface interaction facilitates the generation of reactive oxygen species. Density functional theory (DFT) calculations confirmed that temperature-dependent structural evolution modulates the surface polarity and water adsorption energy, corroborating experimental findings. These results highlight the pivotal role of surface hydrophilicity in photocatalytic processes and demonstrate the value of NMR relaxation as an effective probe for understanding interfacial dynamics and guiding the rational design of photocatalysts.