Two-dimensional (2D) transition metal carbides and nitrides (MXenes) are a family of materials synthesized by top-down selective etching of an atomic layer from layered ternary carbides (MAX phases). The resulting materials have demonstrated flexibility, mechanical stability, hydrophilicity, high electronic conductivities and intercalation of cations and/or large organic molecules, spurring interest in their application for ion sieving, gas separation, filtration and water purification. Additionally, MXenes are solution-processed, so they can be synthesized in scalable quantities up to 100 g batches at high concentrations. Consequentially, the as-produced colloidal solutions contain flakes with a wide lateral size distribution (from hundreds of nanometers to several micrometers). The polydispersity in lateral size results in heterogeneous properties, necessitating processing techniques which can isolate 2D materials based on size or thickness. Here, solution-based size selection and separation techniques, such as sedimentation-based differential and density gradient centrifugation, will be discussed for fabrication of free-standing MXene membranes with controlled flake sizes. The lateral size-property dependence is observed for example in free-standing membranes fabricated with smaller flakes, which exhibit a higher resistance than membranes with larger flakes. Furthermore, membranes composed of Ti3C2 nanosheets have already shown potential in selectively sieving gases, such as H2, and various alkali, alkaline earth ions (Li+, Na+, K+, Mg2+, Ca2+), metals (Ni2+ and Al3+), and dye cations (MB+) based on charge and size without sacrificing water flux rate. The high aspect ratio of the flakes allows for the fabrication of membranes with uniform and narrow 2D nanochannels, which in conjunction with control over the lateral size provides valuable insight on the mechanism of transport of ions and molecules for various applications.

Membranes with controlled 2D MXene lateral flake sizes

256th National Meeting (2018)

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