Selective Gas Response of MXene Surface Functional Groups Revealed by Gas-Phase Transmission Electron Microscopy

Two-dimensional (2D) transition metal carbides, nitrides, or carbonitrides known as MXenes form a class of inherently functionalized layers. The large variety of surface terminations plays a pivotal role in MXene properties and governs the interactions with their environment. In particular, numerical simulations suggest that these functional groups could be key players in gas sensing applications (toward, e.g., humidity, volatile organic compounds─VOCs─or NH₃) for which MXenes have been identified as highly promising. Focusing on the benchmark Ti₃C₂T_x MXene (T being OH, O, F, or Cl), we here use electron energy-loss spectroscopy (EELS) in environmental transmission electron microscopy (ETEM) to characterize in situ, and on the nanometer scale, the selective interactions of different standard surface terminations with two model gases: ethanol (a typical VOC) and water vapor. The quantitative analysis of the core-edge fine structure, supported by density functional theory simulations, demonstrates the much higher affinity of chlorine terminations toward ethanol than water vapor and their superior response for ethanol as compared to oxygen terminations. In addition, the analysis of the carbon K-edge brings evidence of the different modifications of the Ti₃C₂ conducting core electronic structure upon ethanol or water vapor adsorption, bringing fundamental elements for the understanding of the different sensing mechanisms in Ti₃C₂T_x layers. Our results highlight the benefits of MXene surface engineering for their rational design as gas sensors, as well as the high relevance of EELS in gas-phase TEM to reveal the intrinsic mechanisms at play in gas adsorption on nanomaterials.