Two-dimensional, few-layer NiPS₃ for flexible humidity sensor with high selectivity

Chemically and electrically sensitive two-dimensional (2D) nanomaterials are of immense interest as probing electrodes for wearable electronic devices. A new family of two dimensional (2D) layered materials, namely metal phosphochalcogenides (MPX₃), are potential candidates towards the development of sensors for various analytes. Herein, we demonstrate the ability of few-layer NiPS₃ nanosheets for humidity sensing by fabricating a cost-effective, flexible sensor device. The results indicate that the NiPS₃ nanosheet-based humidity sensors possess high sensitivity with a responsivity of ∼10⁶, superior selectivity, and most importantly, rapid response, recovery times and good reproducibility. Response times of ∼1–2 s at low humidity levels and ∼3 s at high humidity levels with recovery times of ∼2–3 s are observed. The device was tested in both flat and bent states, causing no prominent changes in the response; hence, the sensor is an excellent candidate for use in flexible devices. The characteristics of the NiPS₃-based sensor were further investigated using complex impedance studies and in situ Raman spectroscopy to understand the sensing mechanism. The fast response and recovery associated with the NiPS₃-based humidity sensors allow real time monitoring of human respiration and water evaporation on skin. These humidity sensors can also be utilized for non-contact analysis and hence will be an attractive candidate for health and environmental monitoring.