| Semiconductor metal oxide gas sensors have many advantages such as high sensitivity,excellent repeatability,long-term stability,low cost,easy manufacturing and easy integration,and are,widely used in the detection and real-time monitoring of toxic and harmful,flammable and explosive gases.Currently,semiconductor metal oxide gas sensors still have shortcomings such as high operating temperature,poor selectivity,and sensitivity to be further improved,which hinder the development and application of semiconductor metal oxide gas sensors.Therefore,it is of great significance to conduct research on semiconductor metal oxide gas sensitive materials with high sensitivity,good selectivity and low operating temperature.In this research,the microstructure of ZnO based gas-sensitive materials are adjusted by means of heterostructure construction,hierarchical structure assembly and noble metal modification in order to improve the specific surface area,enrich the pore structure,reduce the grain size and increase the number and activity of surface active sites of materials,to promote the diffusion,adsorption and activation of oxygen and the test gas to facilitate the surface reaction,thereby reducing the operating temperature,improving sensitivity and selectivity.The specific experimental content and research results of this research are as follows:First,the SnO2/ZnSn(OH)x ultrathin nanosheet precursors with Zn/Sn molar ratios of 2,3,and 4 were prepared by urea decomposition method,and then were calcined at 500℃ under high temperature to obtain mesoporous ZnO-SnO2 n-n heterojunction ultrathin nanosheets.The ZnO-SnO2 n-n heterojunction nanosheets were characterized by means of XRD,SEM,HRTEM,BET,and XPS.The results showed that the ZnO-SnO2 n-n heterojunction ultrathin nanosheets had smaller grain size,larger specific surface area(97.4 m2 g-1)and abundant mesoporous structure.The formation of n-n heterostructure significantly improves the specific surface area and mesoporous pore volume of ZnO-SnO2 nanosheets,at the same time increases the chemically adsorbed oxygen content on the surface of nanosheets.Gas-sensitive performance test results showed that ZnO-SnO2 n-n heterojunction ultrathin nanosheets has the highest response value to ethanol gas when Zn/Sn molar ratio was 3 and exhibits a maximum response value of 80 to 50 ppm of ethanol at an optimal operating temperature 240℃,which is higher than about 10 times the response value of the pure ZnO sample.The response and recovery time are 7.s and 42 s respectively.At the same time,ZnO-SnO2 n-n heterojunction ultrathin-nanosheets shows the excellent selectivity,a wide detection range,good repeatability and long-term stability to ethanol gas.ZnO-SnO2 n-n heterojunction ultrathin nanosheets exhibit excellent gas sensitivity can be attributed to the large specific surface area,small grain size and electronic effect of ZnO-SnO2 n-n heterojunction.Secondly,the hierarchical structure Zn(OH)2 precursors assembled by nanosheets were synthesized by solvothermal method,and were calcined at 350℃ to obtain the hierarchical structure mesoporous ZnO assembled from nanosheets,and then ZnO was modified by the noble metal Au to obtain a hierarchical structure Au/ZnO gas-sensitive material.The structure and morphology of the hierarchical structure Au/ZnO by means of XRD,SEM,HRTEM,BET and XPS were used to analyze.Gas-sensitive performance tests showed that the response value of the hierarchical structure 1%Au/ZnO to 50 ppm of ethanol gas reached 80 at 270℃,which is about 2.7 times higher than that of the hierarchical structure ZnO.It is worth noting that the gas-sensing response value of the hierarchical structure 1%Au/ZnO reached 62 at 210℃,which is about 3.9 times higher than that of pure ZnO nanomaterials,and the response and recovery time of the hierarchical structure 1%oAu/ZnO to ethanol gas were 10 s and 43 s at 210℃,respectively,which is obviously smaller than that of hierarchical structure ZnO.In addition,the hierarchical structure 1%Au/ZnO exhibited high selectivity and excellent long-term stability.The excellent gas-sensing performance of the hierarchical structure 1%Au/ZnO can be attributed to its open three-dimensional structure and the electronic and catalytic effects of Au nanoparticles,which promotes the formation of surface active oxygen species and the progress of surface chemical reactions.Finally,the hierarchical structure ZnIn(OH)x precursors with a Zn/In molar ratio of 1,2,3 was prepared by urea co-precipitation method,and mesoporous hierarchical structure assembled from ZnO-In2O3 n-n heterojunction nanosheets were prepared by calcination at 350℃,and then obtained Au-modified ZnO-In2O3 n-n heterojunction hierarchical structure(Au/ZnO-In2O3)by sol immobilization method.XRD,SEM,HRTEM,BET and XPS were used to characterize and analyze the ZnO-In2O3 n-n heterojunction hierarchical structure and the hierarchical structure Au/ZnO-In2O3.This study found that the ZnO-In2O3 n-n heterojunction(Zn2InOx)hierarchical structure with a Zn/In molar ratio of 2 has the largest specific surface area(94.5 m2 g-1)and the highest response value to ethanol gas.The response value of the Zn2InOx to 50 ppm of ethanol gas reached 52 at the optimal working temperature of 210℃,which is 10.4 and 3.3 times than that of the hierarchical structure ZnO and In2O3,respectively,and the response and recovery-time were 50 s and 5 1 s,respectively.It indicated that the formation of ZnO-In2O3 n-n heterojunction significantly improves the ethanol gas sensing performance of the material.Furthermore,Au nanoparticles were used to modify the Zn2InOx hierarchical structure,and it was found that the 1%Au-modified Zn2InOx hierarchical structure(1%Au/Zn2InOx)had the highest gas-sensing response value.At the optimal operating temperature of 210℃,the response of the 1%Au/Zn2InOx hierarchical structure to 50 ppm of ethanol gas reached 150,and the response and recovery time are shortened to 44 s and 36 s.In addition,the 1%Au/Zn2InOx hierarchical structure also shows high selectivity,wide detection range,good repeatability and excellent long-term stability for ethanol gas.The analysis found that the electronic effects of the ZnO-In2O3 n-n heterojunction and the electronic and catalytic effects of Au nanoparticles promoted the improvement of the gas-sensing performance of the 1%1%Au/Zn2InOx hierarchical structure. |
