Tunable atomic level surface functionalization of a multi-layered graphene oxide membrane to break the permeability-selectivity trade-off in salt removal of brackish water

Abstract

To enhance graphene oxide (GO) membrane performance, atomic level surface functionalization was applied via plasma-enhanced atomic layer deposition (ALD) to a GO membrane. Unlike conventional ALD approaches, to functionalize the surface without increasing the membrane thickness, we conducted only a few (3–9) ALD cycles, which allowed for the formation of a tunable ultra-thin (1.44 nm) and uniform hydrophilic metal oxide (Al2O3) layer on the multi-layered membrane. The ALD-treated GO membrane exhibited enhanced water permeability (from 32.9 to 68.0 LMH/bar) and NaCl rejection (from 46.6 up to 63.8%), successfully overcoming the typical trade-off between permeability and rejection efficiency in briskish water desalination. The formed atomic level Al2O3 layer was characterized via Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and d-spacing measurements. The results revealed that this enhanced water permeability can be mainly attributed to the increase in the surface hydrophilicity achieved without narrowing the GO membrane nanochannel structure (dry state 7.5 ~ 7.7 Å). Moreover, healed defects on the two-dimensional GO and improved electrostatic interaction (induced by the ALD treatment) resulted in improved salt rejection. Therefore, the distinctive features of ALD-treated GO membrane may contribute to enhancing its application in brackish water desalination.