Molecular Cross-Linking of MXenes: Tunable Interfaces and Chemiresistive Sensing

Abstract

MXenes, a family of 2D transition metal compounds, have emerged as promising materials due to their unique electronic properties and tunable surface chemistry. However, the translation of these nanoscale properties into macroscopic devices is constrained by suitable cross-linking strategies that enable both processability and controlled inter-flake charge transport. Herein, this study demonstrates the tunability of interfaces and the inter-layer spacing between Ti₃C₂T_x MXene flakes through molecular cross-linking with homologous diamines. Oleylamine is first used to stabilize MXenes in chloroform, followed by diamine-mediated cross-linking to tune precisely the interlayer spacing. Grazing incidence X-ray scattering (GIXRD/GIWAXS) confirmed the correlation between ligand chain length and inter-layer spacing, which is further supported by Density Functional Theory (DFT) calculations. Furthermore, the charge transport properties of thin films consisting of diamine-cross-linked Ti₃C₂T_x MXenes are investigated and a strong dependence of the conductivity on the cross-linker length is observed. The dominating charge transport mechanism is variable range hopping (VRH) in accordance with the structure of the films. Finally, chemiresistive vapor sensing is probed using the MXene composites, and a pronounced sensitivity and selectivity for water is observed, highlighting their potential for use in humidity sensors. Insights into molecular cross-linking and its impact on charge transport open avenues for next-generation MXene-based electronic devices.