Low hysteresis, high sensitivity, fast response, and recovery time of humidity sensor based on Schiff bases material

The Schiff bases compound 2-((2-chloro-4-methylphenylimino)methyl)-5diethylamino)phenol (hereafter L4 compound) has been synthesized and was used for the first time in the fabrication of sensor and to test its humidity sensing properties. The solution of L4 compound was deposited on interdigitated Ag–Pd electrodes using drop-casting technique. The scanning electron micrographs (SEM images) revealed that the surface morphology consisted of flat surface along with pores and pore channels of various shapes and sizes. Porosity and conduits facilitated and accommodated more H₂O molecules and played key role in the sensing mechanism. When applied frequency was increased from 25 Hz to 2 MHz, the capacitance of the device was decreased from 48.8 to 3.92 pF. However, when humidity was changed from 39.4 to 90%RH the capacitance of the device was enhanced from 8.68 × 10⁻¹⁰ to 4.1 × 10⁻⁹ Farad. The regression or goodness of fit (R²) values were 0.96 and 0.94 which are very close to unity and represented the best fit of both the curves to improve linearity. The L4 compound-based sensor showed rapid response time (3.5 s) and recovery time (9.4 s), which are much quicker than our previous published results. The present L4-based sensor showed high sensitivity of 47,299%, which is higher than conventional humidity sensor. The hysteresis of the sensor was low (3.63%). Thus, the rapid response/recovery time, high sensitivity, and low hysteresis of L4 compound appear to be very promising candidate for efficient capacitive humidity sensor.