Controlling the Interlayer Spacing of Two-Dimensional Titanium Carbide (MXene) Membranes for Selective Ion Separations in Water

MXenes are an emerging class of two-dimensional nanomaterials with promising applications in critical resource recovery, energy storage, and desalination. Specifically, MXene membranes are able to selectively separate cations based on charge and ionic radius¹, with low permeation rates for large and/or multivalent cations but high permeation rates for alkali-metal cations such as Li⁺. Fine-tuning of this ion sieving behavior could enable the energy-efficient recovery of lithium and other critical metals from wastewater. More recently, aluminum ions (Al³⁺) have shown the ability to "lock" the interlayer spacing of MXene membranes²via intercalation between MXene nanosheets, preventing other cations from intercalating and preserving the selectivity of these membranes over time. Other ions³, as well as water molecules⁴, can also readily intercalate between MXene layers; furthermore, intercalation can strongly affect water adsorption/desorption³.<br/><br/>The exchange rates and mechanisms of ions and water within these confined interlayer spaces remain poorly understood, yet understanding these mechanisms is critical to the use of MXenes for advanced ion sieving technologies. To explore these processes, we have examined the changes in MXene membranes' interlayer spacing in response to intercalation (and exchange after pre-intercalation) with a variety of metal cations, including hydrophilic ions such as Mg²⁺ as well as more hydrophobic cations such as Cs⁺. We have also investigated the effects of water intercalation on interlayer spacing, especially the ability of hydrophilic cations to introduce additional layers of water between MXene nanosheets. Understanding how to reliably control the interlayer spacing (i.e., pore size) of MXene thin films will improve our ability to design selective ion sieves for metal recovery from water.<br/><br/>[1] Ren, C.E. et al., <i>J. Phys. Chem. Lett. </i><b>6</b>, 4026-4031 (2015).<br/>[2] Ding, L. et al., <i>Nat. Sustain. </i><b>3</b>, 296-302 (2020).<br/>[3] Zaman, W.<i> </i>et al., <i>Proc. Natl. Acad. Sci. U.S.A.</i> <b>118</b>, e2108325118 (2021).<br/>[4] Muckley, E. S.<i> </i>et al<i>.,</i> <i>ACS Nano</i> <b>11</b>, 11118-11126 (2017).