Mechanism Of Surface Oxygen Species On SnO₂ Gas Sensing Performance

Metal oxide gas sensors have been widely used in industrial and agricultural production,environmental monitoring and military security due to their excellent performance,low cost,simple fabrication,and easy integration.Among them,tin dioxide（SnO₂）based gas sensors have been the focus of research in this field.At present,the research on this type of sensor mainly focuses on the preparation of special nanostructured SnO₂ and the preliminary research on sensing characteristics,focusing on the novelty of its structure and the innovation of preparation methods and technologies,ignoring the exploration of its sensitive mechanism,which has become an obstacle to the research and application of such sensors.In the process of gas detection,the adsorption and dissociation of oxygen on SnO₂ surface is the first step which provides active oxygen for the subsequent sensing reaction of reducing gas on the surface.Therefore,the formation,types,properties and effects of oxygen species have always been the focus of research on the gas sensing mechanism of SnO₂-based sensors.In this paper,the oxygen species on SnO₂ surface are studied within the framework of ionosorbed model,and the ion adsorption model is improved and supplemented.Meanwhile,from the perspective of surface adsorption of oxygen,combined with XPS,CO-TPR characterization methods and first-principles calculations,the sensitization mechanisms of high-valent,low-valent and homovalent doping were systematically studied.It can be seen that the research in this paper is of great significance to the development of chemical sensor science and technology,and also provides a theoretical basis for the practical application of sensors.The main research contents are as follows:（1）A sensing-model based on O^2-species is proposed.In this model,the O^2-species was identified as the main oxygen species on the SnO₂ surface.The change of surface oxygen vacancies in gas detection is taken as a direct factor causing surface energy band bending and surface conductance changes.Based on this,a function model of sensor resistance,oxygen partial pressure and CO concentration is established.According to the band bending on SnO₂ surface under different oxygen partial pressures,the functional model is deduced and analyzed in the case of depletion layer,accumulation layer and the flat-band,respectively.In the case of the depletion layer（corresponding to high oxygen partial pressure）,the function model can well fit the variation of the resistance of SnO₂-based sensor with the oxygen partial pressure and CO concentration.Furthermore,it can clearly explain the previous experimental phenomenon that the response of SnO₂-based sensors to reducing gases increases with decreasing oxygen partial pressure.In the case of the accumulation layer（corresponding to low oxygen partial pressure）,the model predicts that the gas response of the SnO₂-based sensor will no longer increase with the decrease of the oxygen partial pressure,but will decrease with the further decrease of the oxygen partial pressure,which is verified in the subsequent experiments.（2）Ion doping is an important means to improve the sensitive performance of SnO₂-based gas sensors.It is very important to understand how ion doping affects the species and proportion of oxygen adsorbed on SnO₂ surface to clarify the doping sensitization mechanism.Ion doping can be divided into heterovalent doping and homovalent doping.For heterovalent ion doping,we take high-valent Sb doping and low-valent Co-doping SnO₂ as the research objects.Through XPS,power-law response,CO-TPR experiments conducted a comparative study on the types,proportions and quantities of oxygen species on the surface of the two doped SnO₂.At the same time,combined with DFT calculation,the reasons for the difference of oxygen adsorption on different doped surfaces are discussed.The study found that the electron concentration on the SnO₂ surface is more important for the adsorption and dissociation of oxygen on the surface than the oxygen adsorption sites（oxygen vacancies）at the working temperature.Therefore,the amount of adsorbed oxygen on the Sb-doped SnO₂ surface（234.2μmol/g）is significantly higher than that of the pure SnO₂ sample（154.5μmol/g）.On the contrary,the amount of oxygen species on the surface of Co-doped SnO₂（127.1μmol/g）decreased compared with that before doping.In the process of gas detection,the more adsorbed oxygen on the surface of the Sb-doped SnO₂-based sensor can enhance the recognition function of the sensor.However,the gas sensing test results show that the response of Sb-doped SnO₂-based sensor to reducing gases（CO,ethanol,acetone,methanol,benzene,and formaldehyde）is lower than that of Co-doped SnO₂.This indicates that during gas detection,although there is less oxygen adsorbed on the surface of low-valent Co-doped SnO₂（weaker recognition function）,the baseline resistance R_a increases sharply（342 MΩ）due to the ionization of acceptor Co _Sn defects,which is Larger resistance changes can be produced with less surface reaction,that is,Co doping enhances the switching function of the sensor.In contrast,the ionization of the donor Sb _Sn resulted in a baseline resistance of only 22.3 kΩfor the sensor,and the huge surface electron concentration instead weakened the sensor’s ability to convert surface reactions into resistance changes.It can be seen that single heterovalent ion doping cannot coordinate the recognition function and switching function of the sensor at the same time.（3）In response to the above problems,we designed and prepared a co-doped SnO₂ sensitive material with Sb-doped SnO₂ as the core and Co-doped SnO₂ as the shell.CO-TPR test found that the amount of oxygen species on the surface of co-doped SnO₂ was 171.5μmol/g,which was 44.4μmol/g higher than that of single Co-doped SnO₂.At the same time,the gas sensing test showed that the co-doped SnO₂-based sensor had a better effect on 100 ppm ethanol.The response is 117,which is 2.3times that of Co-doped SnO₂.The excellent gas-sensing performance of co-doped SnO₂ is derived from its unique core-shell structure:on the one hand,low-valent Co doping enables more oxygen vacancies（oxygen adsorption sites）to be formed on the surface of the shell,and at the same time,the electron-rich core moves toward the shell.The electron diffusion provides charge for the adsorption and dissociation of oxygen in the shell,and the synergistic effect of the core and shell improves the recognition function of the sensor.On the other hand,due to the Fermi level difference between the core and shell,an electronic potential barrier is formed between the core and shell,and the electron transport in the adjacent electron-rich core must cross this potential barrier.Due to the abundant oxygen species on the surface of the shell,the barrier height between the core and shell is very sensitive to the surface sensing reaction.Therefore,the sensing reaction of the surface can not only change the resistance of the sensor through the traditional surface charge transfer,but also control the electron conduction in the adjacent electron-rich core by adjusting the height of the potential barrier between the cores and shells,so that the switching function of the sensor can be enhanced.It can be seen that the heterovalent co-doped core-shell structure can coordinate the surface oxygen adsorption and electron conduction,and realize the simultaneous optimization of the sensor recognition function and the conversion function.（4）For homovalent ion doping,we prepared Se（+4）doped SnO₂ nanospheres with an average size of 55.6 nm and good dispersibility through a simple hydrothermal reaction.The power-law responses of XPS and oxygen show that the species and proportion of oxygen species on the surface of Se-doped SnO₂ do not change significantly compared with those before doping.However,the gas sensing test found that the response of the Se-doped SnO₂-based sensor to 100 ppm acetone was R_a/R_g=167,which was twice that of the pure SnO₂-based sensor.CO-TPR experiments show that the onset temperature of CO₂ generation（148°C）of Se-doped SnO₂ is much lower than that of pure SnO₂（206°C）,which enables the Se-doped SnO₂-based sensor to operate at a lower temperature corresponding to the maximum baseline resistance.It can also ensure a sufficient surface sensing response to improve the sensitivity.For pure SnO₂,it is necessary to increase the working temperature,in exchange for more surface sensing reactions at the expense of lower Ra.Therefore,the sensitization mechanism of Se-doped SnO₂ originates from the improvement of the activity of oxygen species on the surface of SnO₂ by Se-doping.