Photo-induced interfacial charge transfer in solar-responsive PVDF composite membranes

Solar-driven membrane distillation (SDMD) using advanced composite membranes offers promise for sustainable water purification. Here, we present an atomistic computational study of polyvinylidene fluoride (PVDF) membranes incorporating hexagonal boron nitride (BN) and tungsten nitride (WN) nanofillers to elucidate the mechanisms underlying photoinduced charge transfer and photothermal excitation. We use a hierarchical multiscale framework integrating density functional theory (DFT), time-dependent DFT perturbation theory, and classical molecular dynamics to assess the electronic, optical, and structural properties of PVDF/BN/WN composites. Our calculations reveal a dramatic narrowing of the band gap from pristine PVDF (5.41 eV) to 0.57 eV in the composite, driven by orbital hybridization and enhanced interfacial charge transfer. Low-energy optical absorption (~ 0.85 eV) associated with charge-transfer transitions and collective excitations in WN clusters, alongside broadband activity from the near-infrared to the ultraviolet, is predicted. Molecular dynamics simulations confirm structural stability and strong interfacial ordering, with realistic atomic densities and constrained polymer mobility. These findings provide quantitative insight into the interplay between nanofiller-induced electronic structure modulation and membrane stability, establishing a theoretical foundation for designing PVDF-based composite membranes for efficient solar-driven water purification and related energy applications.