Ions in MXene
Authors:   , Ghidiu Michael John , Barsoum Michel W. 
Published: July 2021

MXene, a new family of materials comprised of ternary carbonitrides of the early transition metals, has exploded into the field of two-dimensional materials since their discovery in 2011. They draw interest due especially to their unique combinations of hydrophilicity and high electrical conductivity with extremely large compositional variability. Since their introduction, they had been explored in applications of ion-intercalation-based energy storage, i.e. lithium-ion batteries and electrochemical capacitors (supercapacitors), and had been hailed as 'conductive clays', but no major work had been undertaken to provide a more detailed picture of the structural effects of chemically-intercalated ions and what exactly 'clay-like' behavior meant in the context of MXene. This work endeavored to fill this gap in the literature, focusing primarily on Ti₃C₂T_x, the most well-studied MXene (where T is a variable surface termination of the nanosheets). Based on a review of various literatures, we define 'clay-like' properties to include intercalated ions that are exchangeable and that can influence dynamic structural responses to the host material to stimuli, based on the chemistry of the intercalated ions. We successfully intercalated, by chemical means alone, cations of alkali metals (Li⁺, Na⁺, K⁺, Rb+, Cs+), alkaline earth metals (Mg²⁺, Ca²⁺), transition metals (Mn2+, Fe2+, Co2+, Ni2+), and alkylammonium cations of the form [(CH3)3NCnH2n+1]+, where n ranges from 1 to 16. We have also demonstrated control of exchange of initially-intercalated cations to other cations; X-ray photoelectron spectroscopy and X-ray diffraction results confirm that the process is a true exchange, as experienced in clay minerals. For the alkali and alkaline earth metal cations, we have found that the basal planes of the host MXene expand and contract their interlayer distance when H₂O enters and leaves the structure, and that the magnitude of this response correlates with the hydration enthalpy of the intercalated cation. This is a quantitative link to the same phenomena experienced in clays. The transition metal-intercalated samples also demonstrated similar expansion from intercalated H₂O. In addition, in Ti₃C₂T_x without added cations, we found an effect of pseudonegative compressibility along the c axis; that is, when pressure was applied in the presence of H₂O, the basal spacing expanded. This was determined to be due to the forced intercalation of H₂O, and the presence of intercalated K⁺ was found to hamper this effect. Such expansion was known in the literature of other materials, but we have found shearing of the nanosheets with respect to one another to be a potential cause. For the alkylammonium cation-intercalated samples, we observed a discontinuous expansion in the basal spacing correlated to the chain length of the intercalated organocations; this was quantitatively described both by simple volume-packing arguments as well as by more sophisticated computational results. This further led to a determination of the ion-exchange capacity of Ti₃C₂T_x. The results were found to be analogous to those of the clay minerals, providing further strength in the connection between MXenes and clays. Finally, the intercalation of alkylammonium cations was expanded to both Ti2CT2 and Nb2CTx, to demonstrate compatibility and universality with both changes to n (of Mn+1XnTx) as well as with changes to the transition metal M; these samples demonstrated the same kind of discontinuous expansion in basal spacing.