Humidity Sensing Mechanism and Respiration Monitoring of WO₃ Nanoplates with a Monoclinic Crystal Phase

Functioning of hydrothermally synthesized WO₃ nanoplates was investigated for humidity sensing and respiration monitoring under different breathing conditions. The monoclinic phase was identified by the X-ray diffraction (XRD) technique. The average crystallite size was calculated by Williamson-Hall (W–H) plot (27 nm) and modified Scherrer equation (24 nm). The optical band gap was calculated as 2.7 eV using UV–visible spectroscopy. The field emission electron microscopy (FE-SEM) and high resolution transmission electron microscopy (HR-TEM) micrographs of readied WO₃ have confirmed the formation of microstructured nanoplates having an average diameter of 216 nm. Fluorine-doped tin oxide (FTO) substrate was used for the deposition of film and also used as an electrode. The investigation of humidity was carried out at different relative humidity (RH)−11%, 33%, 44%, 54%, 74%, and 95%. The fabricated humidity sensor has shown excellent reversibility, stability and very small humidity hysteresis (<2%) at room temperature. The maximum response was observed as 41.95% at 95% RH with response and recovery time as 2 s and 134 s respectively. During the 30 d of observation, only a 0.4% decrease in response was observed. The fabricated WO₃-based humidity sensor was investigated for human respiration having respiration rates of 2.51 min⁻¹, 3.09 min⁻¹ and 3.74 min⁻¹.