Hierarchical Sm2O3/MXene nanoflower composites with ionic regulation for high-performance flexible printed humidity sensors

Due to the high electrical conductivity, large specific surface area, and abundant hydrophilic functional groups, MXene-based composites have garnered significant attention as candidate functional materials for humidity sensing. Nevertheless, it remains challenging to simultaneously achieve superior sensitivity, fast response kinetics, and reliable printability in flexible humidity sensors. Herein, a flexible printed humidity sensor based on a Sm 2 O 3 -modified MXene composite was developed, which features a hierarchical nanoflower architecture constructed via solvothermal synthesis. This architecture effectively suppresses MXene restacking while providing abundant mesoporous channels and interfacial sites, facilitating the adsorption and diffusion of water molecules. To further enhance the humidity response, NaCl was introduced to promote ionic conduction. The resulting sensor exhibits rapid response and recovery times (4.9   s and 4.3   s, respectively) and a large capacitance variation of 3357.7% over a wide relative humidity range (11-97% RH). In addition, the sensor demonstrates ultralow hysteresis, favorable repeatability, and excellent long-term stability, with negligible capacitance fluctuation after 10 days of storage. Moreover, proof-of-concept demonstrations were performed for non-contact finger proximity sensing, human respiration monitoring, plant microenvironment tracking, and skin-interfaced humidity detection, demonstrating the potential versatility of the proposed sensor. Collectively, this study presents a promising strategy for fabricating high-performance flexible humidity sensors, offering broad prospects for wearable health monitoring and intelligent human-machine interaction in next-generation Internet of Things systems.