Construction Of In₂O₃ Micro-Nano Structure And Study On O₃ Sensing Properties And Applications

As the strong oxidizing gas,ozone（O₃）is the product of photochemical pollution and can induce haze.In addition,O₃ is one of the six main pollutants in the current GB3095-2012 Environmental Air Quality Standard in China.When the concentration of O₃ in the air exceeds the standard,it will cause serious damage to people and plants,so it is very important to monitor the concentration of O₃ in the environment in real time.For O₃ detection,metal oxide based gas sensors have attracted much attention for their advantages of low price,simple process and high sensitivity.However,in order to accurately and efficiently detect O₃ in the atmospheric environment,the problems of low sensitivity,high working temperature,poor selectivity and low detection limit of the current metal oxide based O₃ gas sensor should be further solved.In this dissertation,the sensing material In₂O₃ with low working temperature was taken as the research object.Through the regulation of morphology and crystal structure,heterostructure design,heterogeneous cation doping and loading bimetallic catalysts,the sensitivity of In₂O₃ sensor towards O₃ was improved and the detection limit was lowered.In addition,the relationship between the structure of sensing materials and the performance of the sensor was deeply analyzed,and the sensing mechanism of the sensor was proposed.Furthermore,the sensor system integrated with multiple devices could solve the problem of poor selectivity and realize the selective O₃ detection.Finally,the sensor application circuit was designed and the environmental O₃ concentration monitoring system with alarm function was developed.The specific research contents of this dissertation include:1.The controllable preparation of In₂O₃ micro-nano structure and the study of gas sensing performance.The study mainly focuses on the following aspects:（1）The effect of In₂O₃ crystal structure on O₃ sensing performances of sensors was studied.In₂O₃ with cubic structure,hexagonal corundum structure and mixed structure were prepared by solvothermal method.It is found that the sensitivity of cubic-type In₂O₃sensor to 100 ppb of O₃ at 120°C was 5.7,which were 3.8 and 2.5 times higher than that of hexagonal-type and mixed-type In₂O₃ sensor,respectively.The enhanced sensitivity of cubic-type In₂O₃ sensor to O₃ was mainly due to the narrow band gap and strong oxygen adsorption capacity of sensing materials.（2）The effect of In₂O₃morphology on sensing performances of O₃ sensors was studied.Branch-like hierarchical In₂O₃ was synthesized by co-precipitation method.The untreated commercial In₂O₃ powder was used as the contrast material.It is found that the branch-like In₂O₃ sensor was more sensitive to O₃,and its sensitivity was 4.9 times higher than that of commercial In₂O₃ powder at 70°C.This study revealed the close relationship between sensor performance and the morphology of sensing materials.By constructing hierarchical structure,more reactive sites could be provided for gas adsorption,which was beneficial to achieve the enhanced gas response.（3）The effect of the adsorbed oxygen contents on sensing performances of O₃ sensors was studied.By changing the hydrothermal reaction time,the flower-like hierarchical In₂O₃ with different surface adsorbed oxygen contents were designed.It is found that when the reaction time was 12 h,the content of adsorbed oxygen was significantly increased,and the corresponding sensor showed the best O₃ sensing performance.The sensitivity of the optimal sensor to 100 ppb of O₃ was 16.7 at 80°C.This work verified that enhancing the oxygen adsorption capacity of the material could improve the sensitivity of the sensor to O₃.2.The influence of the composition and interface of materials on sensing performances of sensors was studied.N-n heterojunction Zn O-In₂O₃ materials were constructed with MIL-68（In）and ZIF-8 as sacrificial templates.Through material characterization,it is found that Zn O-In₂O₃ composites were based on In₂O₃hexagonal prism with Zn O nanoparticles uniformly grown on the surface.With the introduction of Zn O,the response of Zn O-In₂O₃ sensor（S-3 sensor）to 160 ppb of O₃was significantly improved,which was about 2.5 times than that of the pristine In₂O₃sensor（110°C）.The detection limit of S-3 sensor was reduced to 15 ppb.The improvement of gas sensing performances of the sensor was mainly due to:（1）the modulation effect of n-n heterojunction on carrier concentration;（2）the improvement of oxygen adsorption capacity of composites;（3）the increase of the specific surface area and the number of reactive sites.The above experimental results proved that multi-component composite modification was an effective way to develop high-response O₃ sensors.3.The influence of Fe³⁺,Sn⁴⁺or Sb⁵⁺doping on the O₃ sensing performance of the sensor was studied.Fe³⁺,Sn⁴⁺or Sb⁵⁺doped In₂O₃ nanostructures were synthesized by hydrothermal method and calcination method.When the doping amount of Fe³⁺was 0.5 wt%,the response of 5Fe In O sensor to 50 ppb of O₃ at 90°C was 15.5,which was 5.7 times than that of undoped In₂O₃ sensor.Heterogeneous ion doping could introduce impurity levels into the substrate material,regulate the adsorbed oxygen content on the surface of the sensing material,and thus improve the sensitivity of the sensor to O₃.In addition,the optimized four sensors（5Fe In O,10Fe In O,30Fe In O and In₂O₃）were integrated into the sensor system for O₃ detection.The sensitivity normalization analysis method was combined with the data processing.Finally,the selective O₃ detection was realized.4.The influence of loading bimetal Au Pt on O₃ sensing performance of the sensor was studied.Bimetallic Au Pt particles with the uniform size（9 nm）were synthesized by oleylamine reduction method,and then loaded into In₂O₃ nanofibers to prepare Au Pt-In₂O₃ composites.It is found that the trace level and dual-selective detection of O₃ and acetone（C₃H₆O）could be realized by adjusting the working temperature of the sensor.At 90°C,the optimized sensor was the most sensitive and selective to O₃.The detection limit was 20 ppb.At 240°C,the sensor was the most sensitive and selective to C₃H₆O.The detection limit was 500 ppb.The enhanced gas sensing performances was mainly attributed to the component and structure design of bimetallic catalysts as well as the design of"metal-semiconductor"heterogeneous interface.In addition,we also developed an environmental O₃ concentration monitoring system with alarm function based on the optimized sensor.When the concentration of O₃ in the air exceeded the safety threshold（50 ppb）recommended by the World Health Organization（WHO）,the diode could warn to users.Aiming to develop practical metal oxide based O₃ sensors,this dissertation systematically explores the relationship between the morphology,crystal structure and chemical composition of In₂O₃ materials and O₃ sensing performance.The sensing mechanism of metal oxide based sensors is clarified.This dissertation provides experimental and theoretical basis for developing high-performance O₃ sensors.