Synthesis And Gas Sensing Properties Of Several Specially Shaped Wide Band GAP N-Type Oxide Semiconductors

In recent years,more and more toxic,harmful and dangerous gases have filled people's living space,which seriously threatens human health,industrial safety and environmental protection.In addition,some gases can also be used as key biomarkers for the diagnosis of specific diseases,they can monitor the condition in real time under painless and non-invasive conditions and promptly treat them.Some gases can be used as evidence of illegal crimes,such as the detection of ethanol,it can effectively control drunk driving and reduce traffic accidents.Therefore,rapid and accurate monitoring of related gases is of great significance.Oxide semiconductor gas sensors have developed rapid and used widely since their appearance.As the key part of the oxide semiconductor gas sensor,the sensitive material plays a vital role in the gas sensing performance of the sensor.Wide band gap semiconductors have been given high attention to its physical and chemical properties,such as excellent ability to withstand high temperatures,good thermal conductivity,high breakdown voltage,extraordinary electron saturation rate.In this paper,several common N-type wide band gap oxide semiconductor materials(SnO2,ZnO,TiO2)with good stability and low cost are studied,and the morphological adjustment,surface modification and doping of sensing materials are used as modification means to improve the sensitivity,selectivity and response/recovery properties of the sensor.The gas-sensitive properties of sensing materials are optimized by intentionally selecting different metals and metal oxides.The details are as follows:(1)A uniform and independent leaf-like SnO2 hierarchical architectures were successfully synthesized via a facile template-free hydrothermal synthesis method,and each leaf was consisted by a main stem with two groups of highly symmetric and parallel branch.To investigate the effect of diammonium phosphate and the formation process of hierarchical SnO2 leaf-like microstructure,the controlled experiments of the hydrothermal process with different amounts of diammonium phosphate and different reaction times have been carried out systematically,and a possible formation mechanism of SnO2 hierarchical architecture was speculated.In addition,the gas sensing performances of the as-prepared products were investigated and the results revealed that the dendritic SnO2 material provides a rich active site for gas diffusion,chemisorption and reaction,and promotes the improvement of gas sensitivity.The sensor based on the unique leaf-like SnO2 hierarchical architectures exhibited high response and good selectivity properties to NO2 at low working temperature.(2)The Sm2O3 loaded mulberry-like SnO2 samples were successfully synthesized by facile hydrothermal synthesis method and simple isometric impregnation method,and the mulberry-shaped architectures was compoed of nanoparticles with diameters of approximately 4-10 nm,endowing the material with aporous structure,which is potentially beneficial for the diffusion and transport of gas molecules.The 2.5 mol% Sm2O3/SnO2 exhibited the highest response(41.14)to 100 ppm acetone,the response was 2.29 times higher than that of pure SnO2(18).In addition,with 2.5 mol% Sm2O3 loading,the low detection threshold of the sensor dropped from 500 ppb to 100 ppb.The enhanced gas sensing performance was mainly bacause of the increased oxygen vacancies created by the substitution of samarium in the SnO2 lattice,which enhanced the adsorption of oxygen and the exceptional catalytic effect of Sm2O3.(3)The Au nanoparticles were loaded with pecan-kernel-like TiO2 graded materials by hydrothermal method combined with precipitation method.The effect of Au loading on the gas sensing properties of TiO2-based gas sensors was studied.The test results indicated that the noble metal Au improved the catalytic efficiency which played crucial role in enhancing the performance of the gas sensor.The sensor with 5 wt% Au-loaded TiO2 exhibited the highest response,short response/recovery time,excellent repeatability and stability to toluene.(4)A novel TiO2 hedgehog-like structure composed of hundreds of onedimensional nanorods was synthesized by a simple hydrothermal method combined with sintering process.Different amounts of Ag were successfully supported on TiO2 by an equal volume impregnation route and the gas sensing properties of the sensors based on the hedgehog-like TiO2 and Ag/TiO2 materials were systematically studied.The high electron mobility of 1D nanorods on the surface of TiO2 and the high porosity of Ag loaded hedgehog-like TiO2 architectures enable the sensor with fast responsive and recovered properties.TiO2 loaded with 0.5 at% Ag exhibited the highest response to xylene with low response/recovery time at the operating temperature of 375?.In addition,the sensitivity and selectivity of the TiO2 sensor were enhanced markedly with Ag loading.(5)Based on the previous research work,we found that the gas sensing properties of metal oxide semiconductor gas sensors are closely related to the morphology of the sensitive materials and the distribution of oxygen.The high electron mobility of the one-dimensional nanomaterial can improve the response/recovery rate of the sensor to the target gas;in addition,increasing the ratio of oxygen vacancies to chemisorbed oxygen can improve the gas sensitivity of the sensitive material.Compared with SnO2 and TiO2,the morphology of ZnO is easier to control.Ce doped flower-liked ZnO materials were successfully synthesized by a simple room-temperature precipitation route,and their sensing performances were investigated systematically.XPS analysis showed that the amount of Ce doping had a great influence on the distribution of oxygen species.After Ce doping,the proportion of oxygen vacancies and chemisorbed oxygen is significantly increased.The increase in the proportion of oxygen vacancies and chemisorbed oxygen species greatly promotes the gas adsorption and chemical reaction on the surface of sensing materials,thereby improving the gas sensing properties of surface resistance metal oxide semiconductors.The response of the sensor based on pure ZnO to 100 ppm ethanol was about 13.2,and 0.5 at% Ce/ZnO exhibited the highest response to ethanol,it's about 72.6.Thus,the doping of flower-liked ZnO with Ce should be a promising approach for designing and fabricating the high performance gas sensor.