Selective Detection of Toluene Using Pulse-Driven SnO₂ Micro Gas Sensors

Improvement of gas selectivity, especially among volatile organic compound (VOC) gases, was attempted by introducing pulse-driven modes in semiconductor gas sensors. The SnO₂ microsensor was fabricated on a miniature sensor device constructed with a microheater and electrode. The gas-sensing properties were evaluated under a pulse-driven mode by switching the heater on and off. According to density functional theory calculations and temperature-programmed reaction measurements, toluene molecule, which is one of the VOC gases, was adsorbed on the SnO₂ surface by van der Waals forces. The conventional sensor response, <i>S</i>_e, defined as the change in the electrical resistance in air and target gas atmosphere, to toluene was four and eight times greater than that to CO and H₂, respectively. Moreover, the newly proposed sensor response, <i>S</i>_p, defined as the change in the electrical resistance of the device in the target gas atmosphere during the heater-on period, to toluene was 33 and 29 times greater than that to CO and H₂, respectively. This significant difference in the <i>S</i>_p to toluene was caused by the combustion reaction of condensed toluene within the sensing layer. Accordingly, the pulse-driven mode of the semiconductor gas sensor can be exploited to improve the gas selectivity of VOC gases based on these newly defined sensor response measures.