Influence of metal ions intercalation on the vibrational dynamics of water confined between MXene layers

Two-dimensional (2D) carbides and nitrides of early transition metals (MXenes) combine high conductivity with hydrophilic surfaces, which make them promising for energy storage, electrocatalysis, and water desalination. The effects of intercalated metal ions on the vibrational states of water confined in ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{T}_{x}$ MXenes have been explored using inelastic neutron scattering (INS) and molecular-dynamics simulations to better understand the mechanisms that control MXenes' behavior in aqueous electrolytes, water purification, and other important applications. We observe an INS signal from water in all samples, pristine and with lithium, sodium, or potassium ions intercalated between the 2D ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{T}_{x}$ layers. However, only a small amount of water is found to reside in ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{T}_{x}$ intercalated with metal ions. Water in pristine ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{T}_{x}$ is more disordered, with bulklike characteristics, in contrast to intercalated ${\mathrm{Ti}}_{3}{\mathrm{C}}_{2}{T}_{x}$, where water is more ordered, irrespective of the metal ions used for intercalation. The ordering of the confined water increases with the ion size. This finding is further confirmed from molecular-dynamics simulation, which showed an increase in interference of water molecules with increasing ion size resulting in a concomitant decrease in water mobility, therefore providing guidance to tailor MXene properties for energy and environmental applications.