Ion-Exchange and Cation Solvation Reactions in Ti 3 C 2 MXene

Ti3C2 and other two-dimensional transition metal carbides known as MXenes are currently being explored for many applications involving intercalated ions, from electrochemical energy storage, to contaminant sorption from water, to selected ion sieving. We report here a systematic investigation of ion exchange in Ti3C2 MXene and its hydration/dehydration behavior. We have investigated the effects of the presence of LiCl during the chemical etching of the MAX phase Ti3AlC2 into MXene Ti3C2Tx (T stands for surface termination) and found that the resulting MXene has Li cations in the interlayer space. We successfully exchanged the Li cations with K, Na, Rb, Mg, and Ca (supported by X-ray photoelectron and energy-dispersive spectroscopy) and found that the exchanged material expands on the unit-cell level in response to changes in humidity, with the nature of expansion dependent on the intercalated cation, similar to behavior of clay minerals; stepwise expansions of the basal spacing were observed, with changes consistent with the size of the H2O molecule. Thermogravimetric analysis of the dehydration behavior of these materials shows that the amounts of H2O contained at ambient humidity correlates simply with the hydration enthalpy of the intercalated cation, and that the diffusion of the exiting H2O proceeds with kinetics similar to clays. These results have implications for understanding, controlling, and exploiting structural changes and H2O sorption in MXene films and powders utilized in applications involving ions, such as electrochemical capacitors, sensors, reverse osmosis membranes, or contaminant sorbents. ■ INTRODUCTION MXenestwo-dimensional transition metal carbonitrides of the Mn+1XnTx composition, where M is an early transition metal, X is carbon and/or nitrogen, n = 1, 2, or 3, and T represents a variable surface termination such as O, OH, or Fare a rapidly growing class of materials featuring hydrophilicity, high electrical conductivity, and tunable chemistry and structure. They have been explored in applications from energy storage to water purification to transparent conductive electrodes. Furthermore, spontaneous intercalation of organic molecules (urea, hydrazine, DMF, etc.), as well as spontaneous and electrochemical intercalation of various cations (Li, Na, K, NH4 , Mg, Al) were previously reported. Understanding the intercalation chemistry of 2D materials is a prerequisite to many applications. Based on the ability to intercalate cations and the reversible increase in the c lattice parameter (c-LP) with H2O uptake, MXenes have been compared with expanding clay minerals. Although the reversible expansion of clay minerals in H2O and the effects of cation exchange have been studied extensively, a fundamental study of the structural effects resulting from cation substitution in MXenes in aqueous, pH-neutral environments has not been undertaken. This is especially pertinent now that a number of MXene applications have been explored involving interactions with salt solutions; for example, when MXenes are used in supercapacitors with aqueous electrolytes, as sorbents for Pb or Cr ions, or membranes for ion sieving, the nature of the cation can play a large role in affecting the sorption efficiency or structure and permeability of the membranes. It has been shown that spontaneous cation intercalation described in reference may lead to changes in MXene surface chemistry, but no systematic studies have been reported. If intercalated cations are structurally inherent in MXenes, controlling their concentrations and understanding their effects on interlayer separation can be crucial for tuning Received: March 30, 2016 Revised: April 29, 2016 Article