Sensing Performances And Gas-Solid Interfacial Molecular Interactions Of Cobalt Oxide Gas Sensors

Gas sensing technologies have extensive applications in various fields.Metal oxide gas sensors are gaining more attention due to their advantages,including small volume,low cost,high sensitivity,easy integration,and relatively simple structure and operation.The gas-solid interfacial interactions during gas sensing play a critical role in the response performance of metal oxide gas sensors.Monitoring the dynamic evolutions of gas molecules and sensing materials using advanced in situ characterization technologies can provide profound insights into the sensing processes at the molecular level.This could significantly contribute to the theoretical understanding of gas-sensing mechanisms and the development of high-performing sensing materials.The Co₃O₄ material has an advantage in detecting low-reactive gases due to its good thermocatalytic activity and abundance of oxygen species.This dissertation focuses on optimizing the sensing performances of the Co₃O₄ sensor,constructing in situ characterization platforms,investigating gas-solid interfacial interactions during gas sensing,establishing the correlation between gas-solid interfacial interactions and the gas-sensing features of sensors,and utilizing machine learning algorithms to differentiate gas types and concentrations.The main contents and conclusions are presented as follows:（1）The Co₃O₄ material is selected as the sensing material,and its gas responses are enhanced through Ag-loading,Ti（Ⅳ）-,and Al（Ⅲ）-doping strategies.Various methods,including scanning and transmission electron microscopy,X-ray diffraction,X-ray photoelectron spectroscopy,Raman spectroscopy,and N₂ adsorption-desorption tests,are employed to assess the physical and chemical properties of the sensing materials.Ag nanoparticles are loaded onto the surface of the Co₃O₄ material,while Ti（Ⅳ）and Al（Ⅲ）substitute the Co（Ⅱ）and Co（Ⅲ）sites of the Co₃O₄ material,respectively.The Ag-loaded strategy can decrease the optimal working temperature of Co₃O₄ sensors from 250 to150°C and increase the ethanol gas response from 24 to 257.Meanwhile,Ti（Ⅳ）-or Al（Ⅲ）-doped approaches can improve the toluene gas response by about threefold compared to that of the Co₃O₄ sensor.The response/recovery times of 3-Ti-Co₃O₄ and5%-Al-Co₃O₄ sensors at 280°C toward 50 ppm toluene gas are 68/87 s and 75/45 s,respectively.（2）Utilizing the advantages of in situ infrared spectroscopy,in situ Raman spectroscopy,and in situ X-ray diffraction（XRD）for monitoring the sensing reactions,surface adsorption species,and structural dynamics of sensing materials during gas sensing,in situ cells and gas line systems are constructed to meet the requirements of characterization instruments.The states of the sensing material in the in situ cells correspond to the conditions of gas-sensing processes,such as the gas response-recovery processes of ethanol,acetone,and toluene gases at 50-300°C,which can be achieved by adjusting the working temperatures and gas atmospheres of the in situ cells.The gas-solid interfacial interactions,including the sensing reactions of gas molecules,adsorption interactions,and structural dynamics of sensing materials,can be observed through these constructed in situ characterization platforms.（3）Based on the response features of sensors and the monitoring results of in situ characterization platforms,the correlations between the sensing features of sensors and the gas-solid interfacial interactions during gas sensing can be clarified.In detail,in situ infrared spectroscopy results indicate a high consistency between the dynamic evolution of adsorption species on the surface of sensing materials and the resistance changes of sensors during gas sensing.The resistance changes of sensors are determined by the sensing reaction of target gases and the adsorption interactions between surface species and sensing materials.Moreover,the sensing reaction rate of target gases and the dissociation speed of the surface-adsorbed species could be accelerated at elevated temperatures.In situ Raman spectroscopy results confirm that the working temperature significantly affects the surface stress and phonon relaxation rate of the Co₃O₄ material.Gas molecules consume the surface-adsorbed oxygen species of sensing materials,resulting in changes in the compositions and contents of the adsorption species and the surface oxidation states of sensing materials.These changes further cause alterations in the electron density and bond energy of Co-O bonds in the Co₃O₄ material,leading to different sensing features of the sensor toward targets.Interestingly,ethanol gas can induce significant changes in the electron scattering of Ag-Co₃O₄ materials while not affecting the Co₃O₄,which could be the reason for the highly enhanced gas-sensing responses of Ag-Co₃O₄ sensors.In situ XRD results indicate that the Co₃O₄ undergoes thermal-induced lattice expansion at high-temperature working states,leading to a slight increase in the particle size of the material.The Co₃O₄ material can maintain the same crystal structure under different temperatures and atmospheres.Theoretical calculation results suggest that the Lewis acid activity on the surface of Co₃O₄ is modulated by Ti（Ⅳ）-and Al（Ⅲ）-doped strategies.This modulation enhances the interfacial adsorption interactions between gas molecules and sensing materials and accelerates the sensing reaction rates of toluene molecules,leading to differences in gas-response features.（4）The response-recovery features of sensors toward targets stem from the accumulated gas-solid interfacial interactions during gas sensing.The more pronounced these differences in interfacial interactions are,the more significant the response features of sensors toward targets become.Machine learning algorithms can utilize these differences in sensing features to predict gas types and concentrations,with prediction accuracy depending on the dataset features and algorithm types.Machine learning algorithms offer new solutions for the selectivity issue of metal oxide gas sensors,thereby advancing the research and development of intelligent gas sensors.This dissertation systematically investigates the gas-solid interfacial interaction mechanisms of Co₃O₄-based gas sensors under different working states using in situ characterization platforms.It reveals the gas-solid interfacial interaction mechanisms of gas-sensing processes from the perspectives of gas molecules and sensing materials,respectively.Additionally,this research establishes the relationship between gas-solid interfacial interactions and gas-sensing features.This could enhance the understanding and recognition of gas-sensing processes and hold significant theoretical value and practical significance for developing high-performance sensing materials.