Preparation And Performances Of Porous Graphene Oxide/Metal Oxide Based Gas Sensing Materials

Gas sensors are known to detect not only toxic and dangerous gases in the surrounding environment,but also certain trace gases exhaled by the human body,such as acetone,hydrogen sulfide,ethanol,nitrogen dioxide,etc.Analysis and detection based on typical biomarker gas types and concentrations can enable noninvasive and fast disease diagnosis,which is a promising medical non-destructive detection technology.Medical studies have shown that acetone,which is exhaled by the human body,serves as a biomarker to initially determine whether a patient has diabetes.The human exhaled gas has high humidity,low acetone content and relatively complex components,so the acetone gas sensor used for respiratory detection needs to meet the following conditions:(1)good anti-humidity performance;(2)high gas sensing response and low detection limit;(3)excellent acetone selectivity;(4)fast response recovery rate and excellent stability.In recent years,graphene oxide(GO)-based gas sensing materials have been widely used in gas detection due to their high gas response and good stability,but there are still bottlenecks such as high humidity interference,poor selectivity,and slow response recovery rate.Porous GO(PGO)was prepared by defecation of GO,and a controllable pore structure was generated on its 2D substrate surfaces,which further overcame the shortcomings of slow response recovery and poor selectivity of GO based gas-sensitive materials,and made it more practical application value.In this thesis,PGO/metal oxide based gas sensing materials are composed of different metal oxides(α-Fe2O3,SnO2,ZnO)by hydrothermal method,using PGO as matrix.The PGO/ZnO gas sensing materials with excellent homogeneous morphology and gas sensing properties was chosen as the substrate,and then its gas sensing performances was further improved by surface loading of noble metal Pt nanoparticles and surface modification of MOFs materials,and the enhanced gas sensing mechanism was investigated to finally obtain acetone gas sensors with high sensitivity,fast response,excellent selectivity and moisture resistance for low-temperature breath detection.The main research contents of this thesis are as follows.(1)PGO was firstly prepared by a combination of ultrasonication and strong alkali etching with some hydrophobicity.Using this as the substrate,hydrothermal method was used to compound different metal oxides(α-Fe2O3,SnO2,ZnO)to form PGO/metal oxide gas-sensitive materials.The reaction temperature,reaction time and molar addition of metal salts were adjusted to obtain homogeneous and well-dispersed composite gas sensing materials,and their gas sensing properties were tested and analyzed.The results show that the PGO/ZnO gas sensing materials have superior gas sensing properties compared with the PGO/a-Fe2O3 and PGO/SnO2 gas sensing materials.The PGO/ZnO gas sensing materials exhibited a higher gas sensing response(42.9),lower detection limit(0.193 ppm),faster response recovery rate(3/7 s)and good stability for 50 ppm acetone at an optimum operating temperature of 240℃.These were mainly attributed to the unique porous structure and surface characteristics of PGO,which results in relatively excellent gas sensing properties.Although the introduction of PGO improves the gas sensing performance of single metal oxides,it still suffers from poor selectivity and humidity insensitivity.(2)To address the problems of PGO/ZnO gas sensing materials,attempts were made to ameliorate these problems by surface loading of the noble metal Pt.The PGO/ZnO(abbreviated as PGZ)gas sensing composite was selected as the matrix,and the noble metal Pt nanoparticles with the size of about 5 nm were loaded on the surface of the PGZ gas sensing materials by decomposition of chloroplatinic acid(H2PtCl6·6H2O)through a simple oil bath heating method.The gas sensing properties were tested and analyzed by regulating the addition amount of H2PtCl6·6H2O.The results show that the Pt0.8/PGZ gas sensing materals(abbreviated as PPGZ)have the optimal gas sensing properties when the addition amount of H2PtCl6·6H2O is 0.8 mM.The gas sensing response to 50 ppm acetone at the optimum operating temperature(220℃)is up to 150.2,which is about 3.5 times higher than that of the PGZ gas sensing materials,mainly due to the excellent catalytic activity of the noble metal Pt.At the same time,the fast response recovery rate(4/7 s)and good stability are still maintained,and the humidity insensitivity is also improved,but the problem of poor selectivity remains unresolved.(3)According to the problem of poor selectivity of Pt/PGZ(abbreviated as PPGZ)gas sensing materials,subsequently this work will improve this problem by surface modification of MOFs materials(with large specific surface,tunable pore,framework structure for selective adsorption of gas molecules,and hydrophobic properties).The PPGZ gas sensing materials were selected as the substrate and 2-methylimidazole as the growth agent of MOFs materials,and MOFs materials with different thickness and size(ZIF-8,ZIF-CoZn)were fabricated by regulating the reaction temperature and time to modify their surfaces,and the effects of MOFs materials(type,size and thickness)on their gas sensing properties were investigated.The effects of MOFs materials(type,size and thickness)on their gas-sensitive properties were investigated.Firstly,ZIF-8 was selected to modify the surface of PPGZ gas sensitive material.When the addition amount of dimethyimidazole was 1.5 mM,ZIF-8 on the surface of PPGZ@1.5ZIF8 gas sensing materials displayed excellent homogeneity and uniform distribution of the "nuclear sheath" structure with the best gas sensing performances.The gas sensing response of PPGZ@1.5ZIF-8 gas sensor to 50 ppm acetone is 92.8 under the optimal operating temperature of 200℃.Compared to the PPGZ gas sensing materials,the gas response is reduced,but the selectivity of PPGZ@1.5ZIF-8 gas sensing materials is greatly improved,as well as the operating temperature and the humidity insensitivity,but still does not meet the performance requirements of acetone gas sensors for breath detection.Then,the PPGZ surface was modified by bimetallic MOFs(ZIF-CoZn).When the addition amount of dimethylimidazole was 1.5 mM,the gas sensing response(114.2)of the prepared PPGZ@1.5ZIFCoZn gas sensor was further improved compared with PPGZ@ZIF-8 gas sensor.The operating temperature(160℃)is greatly reduced,and the selectivity,stability and humidity insensitivity are considerably improved,which is mainly attributed to the superior catalytic activity of Co2+ in ZIF-CoZn.With the introduction of Co2+,a large number of reactive oxygen species can be generated by dissociating and activating oxygen molecules in air at relatively low operating temperatures.At the same time,the porous structure of ZIF-CoZn can play the role of screening gas molecules,which plays a crucial role in the improvement of selectivity.Besides,the PPGZ@1.5ZIF-CoZn gas sensor exhibits a high gas sensing response(7.2)and good humidity insensitivity towards lower concentrations of acetone(1 ppm).In conclusion,PPGZ@1.5ZIF-CoZn gas sensing materials have promising applications for the analysis and detection of exhaled gases in high humidity environments or in the human body.