The Research On Semiconductor Metal Oxides Nanomaterial For NO₂ Gas Sensors

This dissertation focuses on the NO₂ gas sensors fabricated using metal oxide semiconductive nanomaterials as sensing materials, mainly including four parts: Firstly, solid-state Taguchi-type NO₂ gas sensors were fabricated with pure In2O₃ and SiO₂-doped crystalline In2O₃ synthesized by sol-gel method. The sensing characteristics, such as response time, response linearity, sensitivity, working temperature, and working range were studied in detail. Sensors prepared with these sensing nanomaterials show good sensing characteristics to NO₂ gas, and SiO₂-doping improves sensitivity. Response and recovery are rapid. Secondly, Thin film NO₂ sensors were fabricated using pure WO₃ and SiO₂-doped WO₃ nanoparticles with various SiO₂ contents (0-10%, wt%). WO₃ nanoparticles were obtained by the means of thermal pyrolysis of white tungstic acid starting from Na2WO₃ then followed with ultrasonic treatment, and SiO₂ powder was synthesized via the sol-gel route. SiO₂-doped WO₃ nanoparticles were synthesized by solid-state reaction under the proper temperature. The NO₂ responses were found to be very high and quite reversible, while the response and recovery times were fast and the reproducibility was satisfactory. The sensors show good long-term stability during 110 days. Thirdly, NO₂ gas sensors were fabricated with pure WO₃ powder obtained under different calcinating temperature from tungstic acid which in turn was prepared by passing an aqueous Na2WO4 solution through a cation exchange resin protonated in advance. Various types of WO₃ powder show different sensing characteristics, such as the sensitivity and the best working temperature range. The experimental results show the possibility of improving the NO₂ gas sensor's characteristics through controlling the synthesis parameters. Lastly, the metal oxide composites semiconductor NO₂ sensing materials were prepared and examined, including SnO₂-WO₃, TiO₂-WO₃ and In2O₃-WO₃ by the use of solid-state doping reactions. The sensing characteristics of the WO₃-based composites improved greatly compared with the former sensors. The environmental protection policies of most countries are oriented towards the regulation of measurement and control of toxic gases in the atmosphere. Nitrogen oxides (NOx: NO₂ and NO) are typical toxic gases released from combustion facilities and automobiles. The on-site and real-time detection of hazardous gas calls for gas sensors with low energy consumption, fast response, and rapid recovery. The advent of nanostructured materials with enhanced properties paved the way to improve the gas detection. Thus, the fabrication of gas sensors using novel nanomaterials has recently been an active topic. This paper presents the results obtained with metal oxides gas sensors showing sensitivity to NO2 in the parts per million (ppm) range. These metal oxide semiconductor, Taguchi-type NO2 gas sensors prepared with the obtained nanomaterials as sensing materials show the fastest response and recovery time, good linearity, low resistance, low working temperature, high sensitivity, and good long-term stability.