Research On Ethanol And Acetone Gas Sensors Based On Hierarchical ZnO And Its Composite Microstructures

In recent years,with the rapid development of information technology and internet technology,gas sensors play a vital role in environmental monitoring,industrial production,food safety management,health diagnosis and atmospheric quality monitoring.As a class of all-solid-state sensor devices,semiconductor oxide gas sensors have advantages of small size,simple device structure,adjustable device performance and low cost,which is the focus of research in the field of gas sensors.The semiconductor metal oxide material is the core and basis of gas sensor development.Therefore,constructing superior semiconductor oxide gas-sensitive materials is of great significance for developing high-performance gas sensors.In this thesis,the gas-sensitive properties of gas sensors are improved and the gas-sensitive mechanism of gas sensors is investigated by regulating the sensitivity utilization,recognition and conversion functions of semiconductor oxides.Firstly,the ZnO double-shell spheres and porous hierarchical microsphere structures were prepared,and these two microstructures improved the sensitivity of ethanol gas sensors by increasing the utilization rate of the sensing materials.Then,the recognition and conversion function of the material were regulated by introducing Sn⁴⁺ or Sn²⁺ to achieve the separation of ethanol and acetone detection and further investigated the reasons for this phenomenon.Finally,based on the porous ZnO hierarchical structure,the introduction of In³⁺ doping achieved the improvement of gas sensing properties such as sensitivity,detection limit and moisture resistance of ethanol gas sensors.The main research contents are as follows:（1）Porous structure materials have a large specific surface area due to loose and rich pore structure.These properties facilitate the diffusion of gas molecules and improve the utilization of gas-sensitive material thus optimizing the gas sensing performances of gas sensor.The solid sphere,core-shell sphere and double-shell sphere ZnO materials were prepared by controlling the reaction temperature and time of co-precipitation reactions.Material characterization results showed that double-shell sphere ZnO material had larger specific surface area and pore structure distribution.Gas sensitivity test results showed that the gas sensor exhibited high response（47.4）and good selectivity for 100 ppm ethanol at optimal operating temperature of 275℃.Compared with the solid sphere,ZnO double-shell sphere sensor had a 4.8 times higher sensitivity for ethanol detection.The ethanol gas sensor also had short response/recovery time（5 s/52 s）and low ethanol detection limit（1ppm）with an air baseline resistance of 15 Mohm.In addition,the ZnO porous hierarchical microsphere structure was prepared by a facile one-step hydrothermal method.Material characterization results showed that the sensing material is assembled from porous nanosheets with a sparse hierarchical structure and a large specific surface area as well as rich pore structure.Gas sensing investigation results showed that the gas sensor exhibited high gas response（58.4）as well as good selectivity to 100 ppm ethanol at low operating temperature（250℃）,and the sensor is9.4 times higher in detecting ethanol compared to that of dense nanosheet sensitive material.The ethanol gas sensor also had a short response/recovery time（6 s/56 s）and a low detection limit（500 ppb）with a sensor air baseline resistance of 10 Mohm.The improved gas sensitivities could be accounted for the large specific surface area and pore structure which not only improve the utilization of the sensitive body but also provide more active sites for gas-sensitive reactions.（2）Combining the advantages of high sensitive body utilization provided by the porous hierarchical structure of ZnO,the gas-sensitive materials were functional modified by introducing Sn⁴⁺ with a smaller ionic radius and modulating the amount of Sn⁴⁺ addition to investigate the effect on the recognition function of sensing materials.The structural characterization of the materials showed that all materials were hierarchical structures assembled from porous nanosheets and had high specific surface area as well as rich pore structure.Gas-sensitive test results showed that the gas sensor had good selectivity for acetone at low Sn⁴⁺ introduction,and the gas sensor of 10 at% Sn⁴⁺ doping had high sensitivity,better selectivity to 100 ppm acetone at low working temperature（250℃）,a short response recovery time（2 s/63 s）,and low detection limit of acetone（100 ppb）.However,the gas sensors had better selectivity to ethanol at high Sn⁴⁺ introduction,and the gas sensor based on 35 at%Sn⁴⁺ doping had the highest sensitivity（58）to 100 ppm ethanol detection at 200℃.After further characterization,it was found that the reason for the change in selectivity for ethanol and acetone was the introduction of Sn⁴⁺ changed the acid-base of gas-sensitive material surface.In addition,the optimal operating temperature of gas sensor for ethanol and acetone detection was same at low Sn⁴⁺ introduction,while the optimal operating temperature of the gas sensor for ethanol and acetone detection was different at high Sn⁴⁺ introduction.（3）As two types widely used gases,the detection of ethanol and acetone is often difficult to distinguish between them due to their similar properties,and the moisture resistance of the sensor has been an important gas-sensitive characteristic to improve.Therefore,the construction of moisture-resistant gas sensor for the separated detection of ethanol and acetone is of great importance in practical applications.Combine the phenomenon of different optimal operating temperatures for ethanol acetone detection with the high doping amount of Sn⁴⁺.To further investigate the reason for this phenomenon in order to achieve ethanol and acetone detection separation,Sn²⁺ with large ionic radius was introduced for material functional modification and SnO₂-ZnO composite structures were synthesized by regulating the amount of Sn²⁺.The material structural characterization results showed that all microstructures were porous hierarchical structures,and the SnO₂ nanoclusters on the nanosheets surface gradually increased with the increase of Sn²⁺ introduction.Gas sensitivity test results showed that all Sn²⁺ introduction gas sensors had different optimal operating temperatures for ethanol（250℃）and acetone（300℃）detection.Therefore,it could be tentatively concluded that the difference in the optimal working temperature for ethanol acetone detection can be attributed to the SnO₂-ZnO composite structure,and the presence of SnO₂ nanoclusters on the surface may play an important role.In addition,the sensor with high Sn²⁺ introduction not only had higher sensitivity,better selectivity,and low detection limit（200 ppb）for ethanol and acetone detection at 250℃ and 300℃,respectively,but also had significantly improved moisture resistance of the sensor.The improved moisture resistance was due to the adsorption and block effect to water vapor of the SnO₂ nanoclusters on the hierarchical structure surface.（4）The construction of gas sensors with excellent overall gas-sensitive characteristics is important for practical applications.Combining the advantages of high sensitive body utilization provided by the porous hierarchical structure of ZnO and the optimization of sensors moisture resistance by surface nanoclusters,the In₂O₃-ZnO composite structure was prepared for material functional modification by introducing In³⁺.Structural characterization of the materials showed that all materials were hierarchical structures assembled from porous nanosheets,and the In₂O₃ nanoclusters were also attached on the nanosheets surfaces.Gas sensing test results showed that gas sensor based on 15at%In₂O₃-ZnO composite structure had low air baseline resistance（ 1 Mohm）,low ethanol detection limit（200 ppb）and better moisture resistance than the pure phase ZnO material.The gas sensor also had high ethanol sensitive performances.