Research Of Gas Sensors Based On Tin Oxide And Zinc Oxide With Hierarchical Structures

With the development of social economy,industrial production continuously discharges some poisonous,harmful,inflammable and explosive gases into the air,including methane,chlorine,ammonia,nitrogen oxides,hydrogen sulfide,acetone,carbon monoxide and so on.The emission of these gases has caused serious enviromental safety and human health problems.Therefore,various gas sensors have been researched and exploited to effectively monitor the air quality of our enviroment and prevent some dangerous situations.Among these gas sensors,metal oxide semiconductor gas sensors have been widely used in our life is due to their small size,simple manufactring process and high sensitivity.But the metal oxide semiconductor gas sensors also have a series of problems in application.For example,bad response and recovery rate?high working temperature and low gas sensitivity,so we need to further improve the gas sensing performance of metal oxide gas sensors to solve these problems.As well known,the most important part of gas sensor is gas sensing material,so it is very necessary to prepare high quality gas sensing materials.This thesis takes SnO₂ and ZnO as research objects.We used hydrothermal method and the gas sensing performance of metal oxide semiconductor gas sensing materials was improved by fabricating hierarchical structure,hierarchical porous structure and heterojunction structure.Meanwhile,we also conducted a detailed analysis about the gas sensing mechanism of metal oxide semiconductor gas sensing materials.This thesis reads as follows:?1?We used hydrothermal method and calcination to prepare SnO₂nanomaterials with different morphologies.Meanwhile,we analyzed and compared the gas sensing properties of SnO₂ nanomaterials with different morphologies.The results show that the morphology of SnO₂ nanomaterial will be changed when the annealing temperature is increased.The SnO₂ nanomaterial shows a better hierarchical flower-like nanostructure,excellent methanol gas sensitivity and selectivity when the annealing temperature is 400?.The response of the hierarchical SnO₂ nanoflowers to 100 ppm methanol at 200?is 58,and the response and recovery time are 4 s and 8 s.The methanol detection limit of hierarchical SnO₂ nanoflowers at200?is 1ppm,and the gas sensitivity is 1.6.The gas sensor based on the hierarchical SnO₂ nanoflowers owns good selectivity?repeatability and stability.The good methanol gas sensing performance of hierarchical SnO₂ nanoflowers can be attributed to the unique hierarchical structure.?2?To further improve the gas sensing performance of the hierarchical SnO₂nanoflowers,we synthesized hierarchical NiO/SnO₂ nanoflowers by a simple hydrothermal method.The results indicate that the hierarchical NiO/SnO₂nanoflowers to 100 ppm ethanol shows a wonderful gas sensing performance.The response of hierarchical SnO₂ nanoflowers to 100 ppm ethanol is 36 when the working temperature is increased to 164?.But at the same working temeperature,the response of hierarchical NiO/SnO₂ nanoflowers to 100 ppm ethanol is 243.And the ethanol detection limit of hierarchical NiO/SnO₂ nanoflowers at 164?is 500 ppb,the response value is 1.2,the response and recovery time are 4 s and 6 s.The superior ethanol gas sensing performance of hierarchical NiO/SnO₂ nanoflowers can be attributed to the unique hierarchical structure?the formation of p-n heterojunction and the catalysis of NiO.In addition,the reason why the hierarchical NiO/SnO₂nanoflowers shows a ultra-high gas response to ethanol and bad response to methanol is that different working temperatures can cause the gas sensor displays the different gas sensing responses to the same target gas,and the response of gas sensor to target gas is related with many factors,such as the stability and decomposition rate of gas molecules on the surface of the materials?the carrier concentration?the Debye length and the catalytic activity of materials.?3?Subsequently,we prepared hierarchical porous and non-porous zinc oxide microflowers by a simple hydrothermal method after annealed at different temperatures.The response of hierarchical porous zinc oxide microflowers to 50 ppm ethanol at 260?is 110,the response and recovery time are 4 s and 12 s.At the same working temperature,the response of hierarchical non-porous zinc oxide microflowers to 50 ppm ethanol is 60,the response and recovery time are 4 s and 18 s.The resposne of hierarchical porous zinc microflowers to 50 ppm ethanol is 1.8 times that of the hierarchical non-porous zinc oxide microflowers under the same conditions.Meanwhile,the ethanol detection limit of hierarchical porous zinc oxide microflowers at 260?is 0.1 ppm,and the gas sensitivity is 1.6.And the gas sensor based on the hierarchical porous zinc oxide microflowers owns good selectivity?repeatability and stability.The excellent ethanol gas sensing performance of hierarchical porous zinc oxide microflowers is due to the hierarchical porous structure has many active sites.These active sites can effectively promote the contact between the material and the target gas,and then improve the ethanol gas sensitivity of the material.?4?To reduce the working temperature of the hierarchical porous zinc oxide microflowers gas sensor,we combined SnO₂ and ZnO by a simple hydrothermal method and annealed at 400?for 3 h.The results show that the response of hierarchical SnO₂/ZnO microflowers to 100 ppm ethanol at 225?is 73.7.The response and recovery time are 4 s and 6 s.Meanwhile,the detection limit of hierarchical SnO₂/ZnO microflowers to ethanol at 225?is 1 ppm,and the gas sensitivity is 1.2.The response of hierarchical non-porous zinc oxide microflowers to100 ppm ethanol at 225?is 38.8,the response and recovery time are 5 s and 8 s.The response of hierarchical tin oxide nanoflowers to 100 ppm ethanol at 225?is 5.7,the response and recovery time are 5 s and 10 s.The good ethanol gas sensing performance of hierarchical SnO₂/ZnO microflowers can be ascribed to the unique hierarchical structure and the formation of n-n heterojunction.