Hydrophilicity and surface charge modulation of Ti₃C₂T _x MXene based membranes for water desalination

Lamellar membranes obtained by stacking 2D layers possess ample transport pathways due to their intricate network of interlayer gaps. This makes them suitable for molecular separation applications. However, controlling the surface chemistry of the nanochannels within the membrane to tune the desired transport properties of water and ions is challenging. Ti₃C₂T _x has been considered for water desalination because of its hydrophilic surface and negative surface charge. Most of the studies of Ti₃C₂T _x membranes have presented promising salt rejection values in forward osmosis mode, which is less practical for water purification. Here, we investigate two types of reverse osmosis MXene-based lamellar membranes consisting of Ti₃C₂T _x nanosheets hybridized with (i) WS₂ nanosheets and (ii) polyvinyl phosphonic acid (PVPA). When hydrophilic and flexible Ti₃C₂T _x nanosheets are interleaved with softer and more hydrophobic WS₂ nanosheets in 2 : 1 mass ratio, nano capillaries with Janus chemistry are created with comparable rejection to bare Ti₃C₂T _x membrane and threefold higher permeance values. Further, we find that decorating Ti₃C₂T _x nanosheets with anionic polymers improves salt rejection. Our Ti₃C₂T _x /PVPA composite membranes reject ∼97% of divalent ions and ∼80% of monovalent ions with ∼0.2 Lm^-2 h^-1 bar^-1 of water permeance when tested with brackish water, and exhibit significantly improved chlorine resistance and cost benefits over the commercial Toray membranes.