Preparation Of Metal Modified Co₃O₄ Matrix Composites And Their Gas Sensitive Properties To NO₂

Nitrogen dioxide（NO₂）is a major environmental pollutant that has a significant impact on water,soil,and the atmosphere.It not only damages the natural environment but also poses a threat to human health,with severe cases even endangering human life.Therefore,the development of NO₂ gas sensors that operate at low concentrations,exhibit high sensitivity,have a short response time,are selective,and are easy to use is of great significance.Metal oxides have natural advantages in gas detection.Their simple structure,ease of preparation,and low cost make them widely used in the field of gas sensors.Although p-type metal oxide semiconductor-based sensors are not as sensitive as n-type metal oxide semiconductor-based sensors,they have clear advantages in humidity resistance stability and fast response time,which make them worthy of further research.This thesis described the synthesis of spherical Co₃O₄ using a hydrothermal method and the preparation of Au/Co₃O₄ composite materials by combining a new,green,and pollution-free method for synthesizing Au nanoparticles with Co₃O₄.Modified methods involving doped metal oxides and loaded precious metals were used to produce Au-Ag/Co₃O₄ and Au-Co₃O₄/Cu O composite materials.Various characterization techniques were used to characterize the sensitive materials and identify the best response value for the composition ratio.Finally,a theoretical model for analyzing the NO₂ gas sensing mechanism is proposed.1.The main research of this paper includes the following parts:First,the use of a hydrothermal method and calcination to prepare spherical Co₃O₄ material.Secondly,a new green method that used a peptide（glutathione）to reduce and synthesize Au nanoparticles was combined with Co₃O₄ to produce Au/Co₃O₄ composite materials.The synthesis of Au/Co₃O₄ was confirmed by various materials characterization techniques（XRD,XPS,Mapping）,and the spherical shape of Co₃O₄ was confirmed by SEM.XPS analysis was used to analyze the oxygen species content,which was found to be related to the enhanced gas sensitivity and oxygen adsorption content.To investigate the effect of gas sensing test environment on the performance,tests were conducted under two different environments:room temperature ultraviolet light excitation and low-temperature heating.The results showed that the sensor had a better response and recovery ability in the low-temperature heating environment,and the response value of 20 ppm NO₂ was 23.6 at 120℃.Additionally,Au/Co₃O₄ exhibited good selectivity and stability in a 120℃environment.Finally,a reliable NO₂ gas sensing mechanism was proposed.2.The paper further investigated whether various noble metals have a synergistic effect on improving the gas sensitivity of Co₃O₄ through a hydrothermal method and a two-step calcination method.Ag-modified Co₃O₄ composite materials were first prepared,followed by the addition of Au to form Au-Ag-modified Co₃O₄ composite materials,and sensors were made.Based on the gas sensing test results,different mass fractions of Au and Ag-modified Co₃O₄ had different responses to NO₂.In this study,it was found that 2 wt%Ag-Au nanoparticles modified Co₃O₄ had a higher response,with a response of 10 to 3.5 ppm NO₂ at 120℃.The material also exhibited excellent repeatability and selectivity.Finally,the reasons for the enhancement of gas sensitivity by bimetallic modification were analyzed.3.In this paper,the influence of precious metals on heterogeneous structure was studied.Firstly,Co₃O₄/Cu O composites were prepared by hydrothermal reaction and multiple calcination.It was confirmed that there were no other impurities present in the sample.Test results showed that the sensing response of different proportions of Co₃O₄and Cu O was different.The 1%Co₃O₄/Cu O nanocomposite exhibited the best sensing performance.At low temperatures（100°C）,the response value to 3 ppm NO₂ reached24.5.Furthermore,the addition of Au nanoparticles reduced by glutathione to the composite material further improved its response.The response reached 28.3 and exhibited good stability,selectivity,and excellent humidity resistance.