Anomalous Microwave Absorption Hysteresis in Ultrathin MXene Films with Confined Water.

MXenes, a family of two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides, are promising materials for electromagnetic interference (EMI) shielding due to their electronic conductivity, tunable surface functionalization, and adjustable interlayer spacing. In many practical environments, MXene films host water molecules between the carbide or nitride nanosheets. However, the influence of confined water on their microwave absorption remains unclear. Here, we investigate the role of water confinement in governing microwave absorption in Ti3C2Tx MXene films. In situ humidity-dependent measurements reveal that protonated films exhibit a 56% increase in absorption even without measurable changes in interlayer spacing. In contrast, Li+-intercalated films, which undergo interlayer swelling upon water uptake, show a 167% increase in absorption. The observed enhancement in microwave absorption upon water uptake is approximately 4 orders of magnitude larger than what would be expected from the increase in bulk water content alone. These changes indicate that water confined within MXene films produces microwave absorption behavior, unlike that of conventional microwave shielding materials where impedance matching is achieved by varying material thickness. The results clarify the role of confined water in EMI absorption in MXenes and indicate that water confinement can serve as a tunable parameter for engineering high-performance EMI shielding materials.