In situ investigation of water on MXene interfaces

A continuum of water populations can exist in nanoscale layered materials, which impacts transport phenomena relevant for separation, adsorption, and charge storage processes. Quantification and direct interrogation of water structure and organization are important in order to design materials with molecular-level control for emerging energy and water applications. Through combining molecular simulations with ambient-pressure X-ray photoelectron spectroscopy, X-ray diffraction, and diffuse reflectance infrared Fourier transform spectroscopy, we directly probe hydration mechanisms at confined and nonconfined regions in nanolayered transition-metal carbide materials. Hydrophobic (K⁺) cations decrease water mobility within the confined interlayer and accelerate water removal at nonconfined surfaces. Hydrophilic cations (Li⁺) increase water mobility within the confined interlayer and decrease water-removal rates at nonconfined surfaces. Solutes, rather than the surface terminating groups, are shown to be more impactful on the kinetics of water adsorption and desorption. Calculations from grand canonical molecular dynamics demonstrate that hydrophilic cations (Li⁺) actively aid in water adsorption at MXene interfaces. In contrast, hydrophobic cations (K⁺) weakly interact with water, leading to higher degrees of water ordering (orientation) and faster removal at elevated temperatures.

Geologic clays are minerals with variable amounts of water trapped within the bulk structure (1) and are routinely used as hydraulic barriers where water and contaminant transport must be controlled (2, 3). These layered materials can exhibit large degrees of swelling when intercalated with a hydrated cation (4). Fundamentally, water adsorption at exposed interfaces and transport in confined channels is dictated by geometry, morphology, and chemistry (e.g., surface chemistry, local solutes, etc.) (5). Understanding water adsorption and swelling in natural clay materials has significant implications for understanding water interactions in nanoscale layered materials. At the nanoscale, the ability to control the interlayer swelling and water adsorption can lead to more precise control over mass and reactant transport, resulting in enhancement in properties necessary for next-generation energy storage (power and capacity) (6–8), membranes (selectivity, salt rejection, and water permeability), catalysis (9–13), and adsorption (14).Two-dimensional (2D) and multilayered transition-metal carbides and nitrides (MXenes) are a recent addition to the few-atom-thick materials and have been widely studied in their applications to energy storage (6, 9, 15, 16), membranes (13), and adsorption (17). MXenes (Mn+1XnTx) are produced via selective etching of A elements from ceramic MAX (Mn+1 AX_n) phase materials (11, 18). The removal of A element results in thin Mn+1 X_n nanosheets with negative termination groups (Tx −). MXene’s hydrophilic and negatively charged surface properties promote spontaneous intercalation of a wide array of ions and compounds. Cation intercalation properties in MXenes have been vigorously explored due to their demonstrated high volumetric capacitance, which may enable high-rate energy storage (6, 19). In addition, their unique and rich surface chemistry may enable selective ion adsorption, making them promising candidates for water purification and catalytic applications (20–22).Water and ion transport within multilayered MXenes is governed by the presence of a continuum of water populations. The configuration of water in confined (interlayer) and nonconfined state (surface) influences the material system’s physical properties (13, 23–27). However, our current understanding of water–surface interactions and water structure at the molecular scale is incomplete due to limited characterization approaches (28). Most modern observations are limited to macroscopic measurements (e.g., transport measurement, contact angle, etc.), which do not capture the impact of local heterogeneity due to surface roughness, surface chemistry, solutes, etc. (29). Herein, we address this gap via combining theory with an ensemble of direct and indirect interrogation techniques. Water structure and sorption properties at MXene interfaces are directly probed by using ambient-pressure X-ray photoelectron spectroscopy (APXPS), X-ray diffraction (XRD), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). APXPS enables detection of local chemically specific signatures and quantitative analysis at near-ambient pressures (30). This technique provides the ability to spatially resolve the impact of surface chemistry and solutes on water sorption/desorption at water–solid interfaces. Model hydrophobic (e.g., K⁺) and hydrophilic (e.g., Li⁺) cations were intercalated into the layers via ion exchange to systematically probe the impacts of charged solutes on water orientation and sorption. Prior reports suggest that water within the confined interlayer transforms from bulk-like to crystalline when intercalated with bulky cations (31, 32). Furthermore, it has been demonstrated that water ordering is correlated with ion size (33, 34). Here, we expand upon this early work and examine the role that solute hydrophobicity and hydrophilicity impacts water adsorption on solid interfaces. Water mobility within the interlayer is impacted by the hydration energy of that cation. Results shed light on the intertwined role that surface counterions and terminating groups play on the dynamics of hydration and dehydration.