A highly electropositive ReS2 based ultra-sensitive flexible humidity sensor for multifunctional applications

Even though there are reports on Transition Metal Dichalcogenide (TMD) based humidity sensors, one crucial thing that remains unnoticed is the electropositivity of the transition metal as it plays a vital role in humidity sensing. It is found that Re has an elevated value of electropositivity in comparison to widely explored transition metals, which signifies that rhenium disulfide has higher surface energy to attain water molecules and attach to the substrate. Surprisingly, there are fewer reports on ReS2 based humidity sensors and hence there is an urgent need to explore them. This work demonstrates the utilization of ReS2 on cellulose paper as a flexible, cost-effective, and environmentally friendly humidity sensor and its real time multifunctional applications. The ReS2 based humidity sensor exhibited an enhanced sensitivity (similar to 18.26), a quicker response time (similar to 142.94 s), and improved long-term stability (similar to 1300 s) in comparison to other TMDs as well as a commercially available humidity sensor. Furthermore, to validate different real time applications, the 2D ReS2 based flexible humidity sensor was utilized as a respiration sensor to detect slow, fast, and normal breath patterns, to estimate human skin conditions (dry, moist), and also to monitor baby diaper wetting. To distinguish various breath patterns, the frequency and amplitude of breath patterns are used which hold promising applications in non-invasive healthcare. Furthermore, a detailed sensing mechanism in terms of tunneling, proton hopping, and electron transfer is explained in the context of electropositivity of Re. The successful demonstration of an economical humidity sensor based on 2D ReS2 deposited on bio-degradable and flexible cellulose paper provides promising applications in flexible and wearable electronics.