Plasmon assisted TiCT grafting and surface termination tuning for enhancement of flake stability and humidity sensing performance

Humidity sensors play a critical role in monitoring human activities, environmental health, food processing and storage, and many other fields. Recently, some 2D materials, particularly MXenes, have been considered as promising candidates for creating humidity sensors because of their high surface area, surface-to-bulk ratio, and excellent conductivity, arising from the high concentration and mobility of free electrons. In this work, we propose the plasmon-assisted surface modification and termination tuning of common MXene (Ti 3 C 2 T x ) to enhance their response to humidity and increase their stability against oxidation. Hydrophobic (-C 6 H 4 -CF 3 ) and hydrophilic (-C 6 H 4 -COOH) chemical moieties were covalently grafted to the Ti 3 C 2 T x surface using plasmon-mediated diazonium chemistry. In situ Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) measurements, performed at different humidity levels indicate that surface modification significantly affects penetration of water molecules in Ti 3 C 2 T x films. As a result, the sensitivity of the flakes to the presence of water molecules was significantly altered. Additionally, proposed surface grafting commonly proceeds on the less stable MXene surface sites, where flake oxidation commonly initiates. As a result of the modification, such "weak" and more chemically active sites were blocked and Ti 3 C 2 T x stability was significantly enhanced. Covalent surface modification of MXenes allows enhancement of their humidity-sensor performance.