Conductivity hysteresis in MXene driven by structural dynamics of nanoconfined water

Abstract

Water under 2D confinement exhibits unique structural and dynamic behaviors distinct from bulk water, including phase transitions and altered hydrogen-bonding networks, making it of great scientific interest. While confinement in 2D materials like graphene, mica, or hexagonal boron nitride has been reported, their lack of intrinsic hydrophilicity or metallic conductivity limits their suitability for probing the interplay between confined water and electronic transport. MXenes, a family of 2D transition metal carbides and nitrides, overcome these limitations by combining high metallic conductivity (~10⁴ S cm^-1) with hydrophilicity, offering a unique platform to investigate confined water dynamics and their influence on electronic properties. Here, we show that temperature and confinement drive structural transitions of water within MXene interlayers, including the formation of localized ice clusters, amorphous ice, and dynamic hydrogen-bonded networks. These transformations disrupt stacking order, inducing a reversible metal-to-semiconductor transition and conductivity hysteresis in MXene films. Upon heating to 340 K, the dissociation of ice clusters restores interlayer spacing and metallic behavior. Our findings experimentally establish MXenes as an exceptional platform for studying the phase change of confined water, offering new insights into how nanoscale water dynamics modulate electronic properties and enabling the design of advanced devices with tunable interlayer interactions.

Fig. 1. Schematic illustration of the interaction between confined water, intercalated ions, and MXene.

Protons and other cations are intercalated between the MXene sheets with one layer of confined water. MXene with Li⁺ intercalation has the most water molecules confined in the interlayer, followed by Na⁺ and K⁺. Note that protons eliminate the water layer after drying.

Fig. 2. Temperature dependence of electrical resistivity and interlayer spacing of MXene films with Li⁺, Na⁺, K⁺, and H⁺ intercalants, noted as Li-Ti₃C₂, Na-Ti₃C₂, K-Ti₃C₂, and H-Ti₃C₂.

a Electrical resistivity of MXenes as a function of temperature during the third thermal cycle. b Interlayer spacing before and after the third cooling-heating cycle. c 002 reflections of pristine MXenes and after the third cooling-heating cycles of the MXenes at 300 K. d Integrated resistivity hysteresis for ion-intercalated Ti₃C₂T_x samples and their corresponding interlayer spacing at 300 K for all thermal cycles. The circled (dashed line) points are MXenes from the first thermal cycle. The color darkness of the points increases with the number of cycles, and the errors are within the order of the size of the symbols.

Fig. 3. Role of confined water within MXene interlayer on thermal hysteresis during heating/cooling cycles.

Thermal hysteresis of electrical resistivity of (a) Li-Ti₃C₂T_x MXene and (d) H-Ti₃C₂T_x MXene between 200 K and 300 K in the first five thermal cycles followed by heating to 375 K and beginning the fifth thermal cycle with cooling and heating between 400 K and 200 K, at dT/dt = 3 K min⁻¹. Cycle 6 began with cooling from 375 K to 200 K, followed by reheating to 380 K. The same legend of cycles is shared by (a, d). X-ray Diffraction (XRD) patterns of (b) Li-Ti₃C₂T_x and (e) H-Ti₃C₂T_x MXene between 200 K and 300 K at repeating thermal cycles followed by heating to 380 K and cooling back to 300 K. c, f Differential scanning calorimetry (DSC) study of a Li-Ti₃C₂T_x MXene and b H-Ti₃C₂T_x MXene between 200 K and 300 K recorded at dT/dt = 10 K min⁻¹.

Fig. 4. Temperature-dependent XPEEM of Li-Ti₃C₂T_x MXene.

a Schematic of the in situ X-ray Photoemission Electron Microscopy (XPEEM) imaging upon heating. b XPEEM micrographs of a Li-Ti₃C₂T_x film at 260 K. The color scale is based on the respective contribution of the three water bands (I: red, II: green, III: blue) over the MXene film. Ice clusters are circled by dashed red lines. c X-ray absorption spectroscopy (XAS) at the oxygen K-edge of the MXene film (green region) and ice clusters (purple region) are shown with their peak fitting. d Temperature-dependent XAS at the oxygen K-edge of the ice clusters (purple) and MXene film (black) between 220 and 590 K. The spectra are normalized and offset vertically for clarity. The XA spectra from confined water are obtained by subtracting the XA spectrum at 240 K from those recorded at 260 K and 300 K. e XPEEM micrographs over the 220–590 K temperature series. The scale bars are 2 µm.

Fig. 5. Schematic summarizing the proposed temperature-dependent structural transitions of confined water correlated with electronic changes.

Based on experimental results, water within MXene interlayers is proposed to transition during cooling (top path) from a quasi-2D liquid-like layer (~300 K) to localized water clusters (~240 K) and an amorphous ice-like state (~200 K). Dissociation of these structures upon heating (bottom path) restores the initial state, with the overall cycle correlating with observed changes in interlayer spacing, stacking order, and electronic conductivity hysteresis.