HNb₃O₈ Nanosheet–Graphene Oxide Composite Membranes for Molecular Separation

Two-dimensional (2D) nanomaterials (nanosheets) are promising building blocks of functional membranes for molecular separation. The design of 2D nanochannels with a locally tuned interfacial layer is a key strategy for improving the separation performance. Here, we present a 2D laminar membrane with an interfacial dense heterolayer comprising HNb3O8 nanosheets (NbNs) and graphene oxide (GO) for application to a nanofiltration membrane, which we formed by a successive vacuum filtration process. Despite its simple double-layered structure, the GO/NbN membrane showed superior separation performance for Na2SO4 rejection (94.0%) to the GO membrane (58.1%) and NbN membrane (48.7%). The hydraulic resistance (Rm) of the layered GO/NbN membrane was approximately double the theoretical value, which was estimated by adding the Rm values of the two respective layers. Molecular dynamics simulations suggested a stable GO/HNb3O8 interlayer spacing with a minimum interaction energy of only 1.0 nm. We attribute the higher rejection with the layered GO/NbN membrane to a lower diffusivity of ions than in the GO membranes because of the narrower space and stronger interaction between nanosheet surfaces. We propose that the layered GO/NbN membrane has an interfacial dense heterolayer, which is fundamentally different from previously reported laminar membranes.