Free-standing graphene oxide membrane with tunable channels for efficient water pollution control

In this study, a graphene oxide (GO) membrane with tunable interlayer spacing was fabricated by a facile method combining the inter-layer modification with external treatment. Congo red (CR), a negatively charged dye with π-orbital-rich groups, was adsorbed on nonoxide regions (G regions) of GO nano-sheets; thus, the interlayers were cross-linked by Ca²⁺ ions through chelating reaction. GO@CR nano-sheets π−π stacking interactions were changed by thermal reduction of the GO/Ca/CR membrane using a hot-pressing method. A broader effective inter-layer spacing control of the GO membrane in wet condition was achieved (from 7.7 ± 0.2 Å to 11.7 ± 0.25 Å). With the decrease of effective inter-layer spacing, the rejection of dyes and heavy metal ions gradually increased (i.e., methylene blue (99.5%), Cu²⁺ (98.6%), Ni²⁺ (97.2%), Pb²⁺ (97.2%) and Cd²⁺ (99.1%) at 7.7 Å) and a sufficient permeation flux was also achieved (17.1 L/m²·h·bar). Meanwhile, the diffusion mechanism of water molecules inside the interlayer gallery of GO laminates was explored by climbing image nudged elastic band (cNEB) method. The hydrogen bonding between water molecules and hydroxyl groups constrained the diffusion of water molecules; consequently, partially reduced hybrid GO membrane can show a better permeability for water and superior rejection for heavy metal ions.