Controlled Interlayer Engineering of Urea-Cross-Linked Reduced Graphene Oxide Membranes for High-Performance Ion Sieving

Intercalated graphene oxide (GO) membranes hold significant promise for molecular separation applications; however, their practical performance is often constrained by excessive swelling and limited water permeability. In this study, we introduce a straightforward so far effective strategy to fabricate mechanically robust urea-cross-linked reduced graphene oxide (UrGO) membranes through controlled hydrothermal reduction of GO (0–4 h), followed by urea-mediated cross-linking to enhance ion selectivity. The extent of reduction critically influences the interlayer spacing, structural integrity, and transport properties of the membranes. X-ray photoelectron spectroscopy (XPS) confirms the formation of chemical bonds between urea, reduced GO nanosheets, and the PVDF support, resulting in a reinforced laminar structure. Notably, the wet-state interlayer spacing decreases systematically from approximately 1.18 nm in UrGO-0 to 0.91 nm in UrGO-4, effectively reducing membrane swelling while maintaining high permeability. The optimized UrGO-4 membrane demonstrates exceptional ion rejection rates 41.5% for KCl, 61.1% for NaCl, 89.4% for CaCl2, 98.2% for MgSO4 and 98.6% for CuSO4 and maintains a high water permeance of 105.31 L m–2 bar–1 h–1. The observed selectivity enhancement is attributed to cooperative contribution of nanochannel confinement, electrostatic interactions, and improved interlayer structural integrity. This scalable and practical approach covers the way for the development of high-performance laminar membranes tailored for water purification and advanced treatment processes.