Preparation And Gas-sensing Properties Of Hierarchical Oxides

With the rapid development of artificial intelligence technology,we are entering the era of the Internet of Things.As a key sensing node,gas sensors have been widely concerned in industrial safety,environmental monitoring,smart home,medical and health fields,such as industrial toxic gas and volatile substance detection,indoor and outdoor air pollution detection,food deterioration and gas leakage alarm,breath sensor analysis and disease diagnosis.The development of high performance gas-sensing materials and novel semiconductor sensors has become a research focus in related fields,which has attracted the attention of researchers at home and abroad.Metal oxide is a typical resistive gas sensitive material with high response,stable performance and low cost,but it has disadvantages such as low surface reactivity,high power consumption and poor selectivity.The construction of hierarchical structure is beneficial to increase the specific surface area of gas-sensing materials,and then increase the adsorption of gas molecules on the surface of gas-sensing materials.In this thesis,semiconductor gas sensors are constructed based on hierarchical oxide materials,and their gas-sensing performance is regulated by surface modification and heterostructure construction.The main research contents are as follows:1.Triethylamine response of the hierarchical tungsten oxide materials was improved by surface modification of carbon polymer dots.Carbon polymer dots with particle size of 2.5±0.5 nm were prepared by hydrothermal synthesis using sodium alginate as raw material.The carbon polymer dots have rich surface functional groups that facilitate their interaction with gas molecules through covalent or hydrogen bonds.The defective oxide/carbon polymer heterostructures were obtained by loading carbon polymer dots on the surface of hierarchical tungsten oxide by ultrasonic chemical method and heat treatment.The morphology and structural characteristics of the composites were characterized,and the effect of carbon polymer dots on the gas-sensing properties of hierarchical tungsten oxide was systematically studied.Compared with pure tungsten oxide,the response of the carbon polymer dots/tungsten oxide to 40ppm triethylamine is increased by 4.3 times,the response/recovery time is increased by4.3 times/2.1 times,the operating temperature is reduced by 30℃,and the detection limit is up to 46 ppb.The simulation results show that the adsorption-desorption kinetics of gas molecules on the surface of tungsten oxide is enhanced 67 times by the addition of carbon polymer dots.In addition,the hierarchical structure and defect structure of carbon polymer dots/tungsten oxide materials provide a large number of channels for gas adsorption and diffusion.2.Based on soft-hard interface design,a high-performance flexible ammonia gas sensor was constructed based on hierarchical bismuth oxide and flexible seaweed fiber paper.Hierarchical bismuth oxide with size of about 5±1μm were prepared by hydrothermal synthesis,which were loaded on seaweed fiber paper and a large number of defects were produced by low temperature heat treatment.The morphology,structure,chemical composition and surface chemical states of the composite were analyzed by SEM,TEM,XRD and XPS.The flexible gas sensor exhibited outstanding ammonia sensing performance at room temperature,including ultra-high sensitivity（improved16 times vs existing ammonia sensor）,short response/recovery time（improved 2-8times/16-34 times vs existing gas sensor based on Bi₂O₃）,and after the different bending angle and repeated bending can still keep the original gas sensitive performance.The flexible gas sensor can detect ammonia at a lower limit of 117 ppb and demonstrate a significant response to simulated expiratory breath of helicobacter pylori infected patients.After the sensor is connected with green LED in circuit,the visual detection of ammonia concentration can be realized under the resistance change before and after gas adsorption,and a gas alarm system is successfully made.This study provides the possibility of real-time analysis of ammonia exhaled by human body and opens up new applications for flexible wearable devices for non-invasive medical diagnosis.The flame-retardant performance and ammonia-sensing performance of the composite paper based flexible gas sensor were systematically studied,and the application of the flexible gas sensor in gas leakage alarm and human breath detection was verified.3.Based on the interface barrier regulation of heterojunction,The gas-sensing performance of hierarchical oxides at low working temperature is improved.Bismuth oxide and tungsten oxide with hierarchical structure were synthesized by hydrothermal method and ultrasonic chemical method,respectively.Then,tungsten/bismuth hetero-oxides were prepared by blending method.The appearance of electron depletion layer and electron accumulation layer on both sides of heterojunction leads to band bending and improves electron transfer ability.Heterojunction can also improve the catalytic activity,increase more adsorption sites,which is conducive to gas transport.By changing the molar ratio of bismuth oxide to tungsten oxide in tungsten/bismuth hetero-oxide,the effect of different molar ratio on gas sensitivity was investigated.The differences of tungsten/bismuth hetero-oxide sensors were compared from five dimensions of operating temperature,sensitivity,response time,recovery time and selectivity,so as to obtain the optimal gas sensitive components of the heterojunction.Compared with pure tungsten oxide,the sensitivity of tungsten/bismuth hetero-oxide to triethylamine is increased by 2.1 times,the response/recovery time is shortened by 1.2times/1.7 times,and the operating temperature is reduced by 30℃.At the same time,the gas-sensing material has good performance in anti-interference,stability and cycling test,suggesting its excellent performance in practical application.In addition,the sensitivity has high linearity in different concentrations of triethylamine,indicating that the sensor has the advantage of detecting low concentration triethylamine.This study not only offers a rational solution to design advanced gas sensors by tuning interfacial energy barriers of nanoscale heterojunctions but also provides a fundamental understanding of bismuth oxide in gas-sensing field.