Structurally Stable Hollow-Fiber-Based Porous Graphene Oxide Membranes with Improved Rejection Performance by Selective Patching of Framework Defects with Metal–Organic Framework Crystals

Graphene oxide (GO)-based membranes have demonstrated great potential in water treatment. However, microdefects in the framework of GO membranes induced by the imperfect stacking of GO nanosheets undermine their size-sieving ability and structural stability in aqueous systems. This study proposes a targeted growth approach by growing zeolitic imidazolate framework-8 (ZIF-8) nanocrystals precisely to patch microdefects as well as to cross-link the porous graphene oxide (PGO) flakes coated on the outer surface of the hollow fiber (HF) alumina substrate (named the hybrid PGO/ZIF-8 membrane). This method simultaneously improves their structural stability and size-sieving performance without compromising their water permeance. Various structural and elemental analyses were used to elucidate the targeted growth of the ZIF-8 crystals. The X-ray photoelectron spectroscopy (XPS) analysis confirmed the targeted coordination interaction of oxygen moieties on the edges of PGO nanosheets with metal ions of ZIF-8 crystals, allowing for the precise growth of the ZIF-8 nanocrystals in the PGO membranes. The XPS depth profile analysis revealed the uniform distribution of the ZIF-8 precursor throughout the PGO/ZIF-8 membrane. The resultant membrane showed a water permeance of 4 L m^-2 h^-1 bar^-1 and maintained a very stable performance under pressure and prolonged cross-flow operation. Notably, the molecular weight cutoff (MWCO) improved considerably from 570 to 320 g/mol without sacrificing any water permeance after the targeted growth of ZIF-8. This research paves the way for the preparation of highly selective and stable PGO-based membranes for applications in industrial wastewater treatment.