Incorporating multivalent metal cations into graphene oxide: Towards highly-aqueous-stable free-standing membrane via vacuum filtration with polymeric filters

Aqueous stability is an essential prerequisite for solution-based applications of graphene oxide (GO) membranes. In this work, we rationally explore the possibility of incorporating ppm-level multivalent metal cations to enhance the aqueous stability of GO membranes via vacuum filtration with low-cost polymer filters and focus on the effect of incorporation approach. Based on two basic ideas of introducing metal cations into GO or polymer filters during membrane preparing and processing, four metal cations, including Al³⁺, Cr³⁺, Mg²⁺, Cu²⁺, were incorporated into GO membrane prepared using nitrocellulose filter by pre-, post-, and filter-incorporation approaches. It is found that the neat GO membranes disintegrate in water within 4 h. When using pre-incorporation approach, the studied four metal cations showed very similar results, crack and swelling appear for GO membranes in 24 h in water. As for the post-and filter-incorporation approaches, the GO membranes showed superior aqueous stability for at least one month. The corresponding models and mechanisms were further proposed to understand the effect of metal cations on filtration process and GO membrane aqueous stability. The results demonstrate the successful attempts of incorporating ppm-level multivalent metal cations into GO to achieve highly-aqueous-stable GO membranes via vacuum filtration with low-cost polymeric filters.