One-step fabrication of PVDF Janus membranes by template-induced texturing and Janus nanosheet self-orientation for membrane distillation

Membrane distillation (MD) is a promising thermal desalination technology; however, achieving high permeate flux, excellent wetting resistance, and long-term operational stability simultaneously remains challenging, often requiring multistep surface-modification strategies. Herein, a one-step phase-inversion strategy is proposed to simultaneously engineer both membrane surfaces by integrating template-induced surface texturing with the migration and self-orientation of Janus graphene oxide (JGO) nanosheets. A mechanically roughened glass substrate generated hierarchical surface roughness, while JGO nanosheets spontaneously migrated and oriented during non-solvent-induced phase separation, resulting in asymmetric surface wettability within a single fabrication step. The resulting PVDF Janus membrane exhibited water contact angles of 135.9° and 73.9° on the template-contacting and opposite surfaces, respectively, with porosity increased from 70% to 86%, a thickness of approximately 82 μm, and an average pore size of 0.35 μm. Accordingly, the permeate flux nearly doubled from 7.75 to 16.25 L·m⁻²·h⁻¹ using 3.5 wt% NaCl at 60 °C while maintaining >99.9% salt rejection. The membrane also demonstrated superior anti-scaling performance during 12 h of operation with a scaling-prone feed containing 7 wt% NaCl and 1000 ppm Ca²⁺/SO₄²⁻ at 65 °C. During 100 h of continuous AGMD operation with Caspian Sea water (TDS: 15,929 ppm) at 70 °C, the membrane maintained an average permeate flux of 22.8 L·m⁻²·h⁻¹, with salt rejection decreasing only from >99.9% to 98.5%.