Iron Oxide-Based Nanoparticles for Fast-Response Humidity Sensing, Real-Time Respiration Monitoring, and Noncontact Sensing

Nanosize
$\alpha $
-Fe2O3 is one of the best humidity sensing materials for medical diagnosis as well as noncontact resistive sensing applications. In this study, we synthesized
$\alpha $
-Fe2O3 nanoparticles using the hydrothermal method without any precipitating agent at different operating conditions for humidity sensing property. Initially, we synthesized nanosize
$\alpha $
-Fe2O3 particles using the hydrothermal method using heat treatment of 10 h (FO1). The X-ray diffraction (XRD) of the developed material confirms their crystalline phase formation having a grain size of 27 nm as observed in the FESEM image. Further to see the size effects on the sensing property of
$\alpha $
-Fe2O3 particles, we synthesized the same composition of
$\alpha $
-Fe2O3 with the same method but increasing the duration of heat treatment to 20 h (FO2) so as to obtain large-size particles having a grain size of around 48 nm. The sensing property of both these developed materials was studied, and it was found out that the FO1 sensor showed a better response/recovery time of 9/4 s as compared with FO2, i.e., 13/24. In addition to this, material dispersion stability, porosity, and surface area of the sample were confirmed using zeta potential and Brunauer-Emmett-Teller (BET) analysis. With ultrafast response and excellent sensitivity, the fabricated device employing FO1 can be used in medical diagnoses like asthma, coughing, and apnea detection. In additiona, the device exhibits a very low hysteresis loss of 0.4% and a higher level of stability.