Fabrication of Color Visualized Sensing System for NOx Gas Using Dry Deposition Method

As the emission of toxic gas continues to increase, the importance of monitoring the emission of toxic gas through a gas sensor is increasing continuously. Since most of these harmful gases are invisible, a system to give quick information about leakage is crucial. Conventionally, gas sensors should be heated to a high temperature to sense toxic gas by measuring its change in resistance. In this study, a gas sensor operable at low temperature with an electrochromic device that can display its presence of gas by changing its color at low voltage are implemented in one circuit to visualize sensing of toxic gas. When toxic gas is inserted, resistance of sensor is increasing, and the electrochromic device changes its color by changing its current. Therefore, the gas sensor to be used in this study should have high sensitivity at low temperature, while the electrochromic device operates at low voltage.<br/>Ceramic oxides such as WO₃, ZnO, and TiO₃ are mainly used as active materials for gas sensors. In this study, a thin film was fabricated using a nanoparticle deposition system (NPDS) as a thin film deposition method to fabricate sensing films. NPDS is a method of dry deposition on a substrate by accelerating the powder in the nozzle to supersonic speed due to the pressure difference between the chamber and the nozzle in a low-vacuum atmosphere at room temperature. NPDS can control roughness of thin film to control reactive surface area. In addition, MoS₂ was coated on the surface of the thin film using a hydrothermal synthesis method after thin film production through NPDS. Through this, we tried to fabricate a gas sensor that has a highly reactive surface which can be operated even at low temperatures through MoS₂ coating.<br/>In this study, three thin films of WO3, ZnO, and TiO3 were fabricated by NPDS method, followed by MoS₂ coating on each thin film through hydrothermal synthesis. We have used 200 ppm of NOx gas while we monitor the behavior of gas sensing while we vary temperature. The material with the best sensing sensitivity was selected. Finally, a sensor using the selected material with the highest sensitivity, and an electrochromic device were implemented in one circuit to build color visualized sensing system. We have used electrochromic gel (EC gel) based on mono-heptyl viologen (MHV, 1-heptyl-4-(4-pyridyl) pyridinium) which is capable of color-changing at low voltage for electrochromic device.