Double-Step Modulation of the Pulse-Driven Mode for a High-Performance SnO2 Micro Gas Sensor: Designing the Particle Surface via a Rapid Preheating Process.

To improve the sensing properties toward volatile organic compound gases, a preheating process was introduced in a miniature pulse-driven semiconductor gas sensor, using SnO2 nanoparticles. The miniature sensor went through a short preheating span at a high temperature before being cooled and then experienced a measurement span under heating; this is the double-pulse-driven mode. This operating profile resulted in the modification of the surface conditions of naked SnO2 nanoparticles to facilitate the adsorption of O2- and ethanol-based adsorbates. Temperature-programmed reaction measurement results show that ethanol gas was adsorbed onto the SnO2 surface at 30 °C, and the adsorption amount of ethanol and its byproducts was increased after ethanol exposure at high temperatures followed by cooling. The electrical resistance of the sensor in synthetic air increased as the preheating temperature increased. The sensor responses, Si and Se, to 1 ppm ethanol at 250 °C were enhanced by introducing the preheating process; Si values at 250 °C with and without preheating at 300 °C are 40 and 15, respectively. The obtained improvements were attributed to an increase in O2- adsorption onto the SnO2 surface during the preheating phase. During the cooling phases, the adsorption of ethanol-based molecules onto the SnO2 surface and their condensation in the sensing layer contributed to the enhanced performance. In addition, the double-pulse-driven mode improves the recovery speed in the electrical resistance after gas detection. These improvements made in the sensing properties of the double-pulse-driven semiconductor gas sensors provide desirable advantages for healthcare and medical devices.

Pulse-Driven Semiconductor Gas Sensors Toward ppt Level Toluene Detection.

Improvements in the responses of semiconductor gas sensors and reductions in their detection limits toward volatile organic compounds (VOCs) are required in order to facilitate the simple detection of diseases, such as cancer, through human-breath analysis. In this study, we introduce a heater-switching, pulse-driven, micro gas sensor composed of a microheater and a sensor electrode fabricated with Pd-SnO2-clustered nanoparticles as the sensing material. The sensor was repeatedly heated and allowed to cool by the application of voltage to the microheater; the VOC gases penetrate into the interior of the sensing layer during its unheated state. Consequently, the utility factor of the pulse-driven sensor was greater than that of a conventional, continuously heated sensor. As a result, the response of the sensor to toluene was enhanced; indeed, the sensor responded to toluene at levels of 1 ppb. In addition, according to the relationship between its response and concentration of toluene, the pulse-driven sensor in this report can detect toluene at concentrations of 200 ppt and even lower. Therefore, the combination of a pulse-driven microheater and a suitable material designed to detect toluene resulted in improved sensor response, and facilitated ppt-level toluene detection. This sensor may play a key role in the development of medical diagnoses based on human breath.