Graphitic carbon nitride nanomaterials integrating optical absorption and membrane performance for solar distillation

Graphitic carbon nitride (g-C₃N₄) nanomaterials offer high potential for solar-driven membrane distillation (MD), yet balancing optical absorption with mass transport stability remains challenging. Herein, nanocrystalline g-C₃N₄ thin films were fabricated via spray pyrolysis and integrated into polyvinyl alcohol (PVA)-coated polytetrafluoroethylene (PTFE) membranes to form photothermally active antiwetting barriers. Microstructural analysis revealed a polycrystalline structure with an average crystallite size of 36.6 nm and a compressive microstrain (ε = 0.0047), which promotes non-radiative energy dissipation for photothermal heating. Spectroscopic modeling using the Wemple-DiDomenico single-oscillator approach yielded an oscillator energy (E _o) of 5.65 eV, dispersion energy (E _d) of 16.5 eV, and a high-frequency dielectric constant (ε _∞) of 4.28. Crucially, embedding g-C₃N₄ into the PVA matrix narrowed the optical bandgap from 2.49 eV to 2.13 eV, shifting the absorption edge toward 582 nm and enabling enhanced solar spectrum utilization. Under photothermal testing, the PVA/g-C₃N₄ coating achieved a localized surface temperature of 62.7 °C under low irradiation. In MD tests, although the PVA/g-C₃N₄ coating introduced transport resistance (reducing the permeability coefficient from 0.58 to 0.23 L m^-2 h^-1 °C^-1), it dramatically enhanced long-term durability. While pristine PTFE suffered catastrophic pore wetting and complete flux loss within 10 h, the PVA/g-C₃N₄ membrane maintained stable operation for 20 h (1200 min) with ∼54% flux retention and excellent salt rejection (>99.5%). This study establishes a structure-property relationship demonstrating that sacrificing initial permeability for photothermal stability and antiwetting resistance is essential for practical solar desalination.