Surface Modification Of WO₃ Nanosheets And Their Selective Detection Of Gas

With the rapid development of human society,the emission of volatile organic compounds?VOCs?such as acetone,toluene and formaldehyde is increasing,which seriously harms the atmospheric environment and human health.Therefore,it is increasingly important to study various methods and techniques for rapid detection and monitoring of such VOCs.At present,gas sensors are widely used and cost-effective methods of detection.However,the commonly used gas sensors are difficult to accurately detect the presence of VOCs due to their low sensitivity and poor selectivity.Studies have shown that increasing the specific surface area of the sensitive material and modifying the surface of the material can increase the sensitivity of the gas sensor.Based on this,in this thesis,WO₃ nanosheets with large specific surface area were prepared by using ammonium metatungstate and sodium tungstate as raw materials in this paper,and the surface modification of WO₃ nanosheets was carried out to enhance the gas sensitivity from multiple angles.The main research work is as follows:?1?Synthesis of WO₃ nanosheets:WO₃ nanosheets were prepared by oil bath method,hydrothermal method with oxalic acid directing agent,glucose reducing agent hydrothermal method and potassium citrate monohydrate as dispersant hydrothermal method.The morphology and gas-sensitive properties of WO₃ nanosheets synthesized by the four methods were compared.The results show that the structure of WO₃ nanosheets synthesized by hydrothermal method with glucose reducing agent is more uniform and dispersed,the nanosheets are thinner,the optimal working temperature is lower,and it has good selectivity and high sensitivity to acetone.It has good stability and reproducibility.Therefore,this method was chosen for surface modification of subsequent WO₃nanosheets.?2?Pd-modified WO₃ nanosheets:Pd-WO₃ nanosheets were synthesized by hydrothermal method using Na₂Pd Cl₄ solution as palladium source and sodium tungstate as tungsten source,and XRD was used to detect metal after doping.The peak of palladium indicates that Pd has been supported on the surface of the WO₃ nanosheet and is mainly present in the form of a metal.The BET test results before and after doping showed that the specific surface area increased after doping.The gas sensitivity test results showed that the flake Pd-WO₃ sensor showed good gas sensitivity to acetone,namely fast response,high selectivity and low detection limit?50 ppb?.In addition,the effect of doping amount on gas sensitivity was investigated.The results show that the gas sensitivity is better when the doping amount is 2 at%Pd-WO₃.?3?SnO₂ modified WO₃ nanosheets:SnO₂-WO₃ composites were prepared by two-step hydrothermal method and characterized by XRD.The presence of SnO₂diffraction peaks indicates the successful preparation of SnO₂-modified WO₃nanoparticles.SEM test results show that the spherical and surface-loaded nanosheets are formed.This structure makes the surface of the material form an n-n heterojunction,which can effectively improve the sensitivity of the material to acetone.The gas sensitivity test results show that the composite WO₃ nanosheets exhibit good gas sensitivity,high sensitivity,high selectivity and low detection limit?200 ppb?.In addition,the effect of the composite amount on the gas sensitivity was investigated.The results show that the gas sensitivity is better when the compounding amount is 0.2 mmol SnO₂-WO₃.