A Flexible Wearable Humidity Sensor with Fast Response and High Responsivity Based on GOQDs/WS₂ Heterojunctions

Humidity sensors are widely used in human health detection, human-computer interaction, motion detection, Internet of Things, and so on. In this work, we report highly sensitive and fast response flexible humidity sensors based on graphene oxide quantum dots (GOQDs)/WS₂ hybrid films, which were deposited on a flexible polyimide (PI) substrate by a facile drop-coating technique. The prepared humidity sensor exhibits high responsivity (99.57%), fast response/recovery time (1.55 s/41.1 s), wide working range (11-98% relative humidity (RH)), excellent repeatability, low hysteresis, and excellent stability. The performance of the GOQDs/WS₂-based humidity sensor is significantly improved compared to the pure WS₂-based humidity sensor, with a 41-fold improvement in response time. Ultraviolet-visible (UV-vis) absorption spectra indicate that the formation of impurity energy levels is the major cause of band gap changes of GOQDs/WS₂ nanocomposites, thereby facilitating the transfer of electrons in the semiconductor. Further, the Mott-Schottky measurement result indicates that the GOQDs/WS₂ nanocomposites show a bipolar semiconductor structure, confirming the formation of P-N heterojunctions. In addition, the prepared humidity sensors were integrated with flexible printed circuit (FPC) and successfully applied for human respiration monitoring, motion monitoring, and nontouch switches, showing the great potential in multifunctional application scenarios.