The Research Of Three-dimensional Inverse Opal Oxide Semiconductor Based Gas Sensor

With the rapid development of the Internet of Things(Io T),gas sensor as the terminal device for gas information acquisition in the Io T system,which will play a greater role in the prevention and control of air pollution,air quality monitoring,clinical application of breath diagnosis,new energy safety and industrial production in the future.Among different kinds of gas sensors,The oxide semiconductor gas sensor is as a kind of important all-solid-state gas sensors,which has the advantages of simple device structure and manufacturing process,easy miniaturization and integration,and suitability for online monitoring.Thus,it has always been the focus attention in academia and industry.As well known that the oxide semiconductor sensitive material is the foundation and key components of oxide semiconductor gas sensor,and its gas sensing characteristics directly determine the sensing performance of gas sensors.As can be seen that designing and preparing high-efficiency oxide semiconductor sensitive material has important scientific significance for fabricating high-performance gas sensor.Therefore,this thesis aims to improve the receptor function,transducer function and utility factor of nanostructured oxide semiconductor.Combined with the micro/nano spatial advantages of three-dimensional inverse opal structure,firstly,the surface modification of noble metal catalyst is used to enhance the receptor function of sensitive materials and improve the sensitivity;Secondly,through the functional modification methods and strategies,in-situ doping of hetero-cation and constructing micro/nano scale heterogeneous contacts,in order to adjust the above mentioned sensing functions and achieve the improvement of sensitivity and selectivity;Finally,the use of visible light irradiation instead of thermal excitation,though using the “light trap effect” of the 3D inverse opal structure and inhibiting the recombination of photo-generated carriers,the performance of oxide semiconductor gas sensor at room temperature can be improved,thereby solving the intrinsic safety problem of oxide semiconductor gas sensors.The main research contents are as follows:1.Three-dimensional inverse opal(3D IO)structure possesses regularly arranged hole array,thin-walled ordered skeleton,and 3D interconnected channel,which resulting in larger specific surface area and good permeability.Besides,these structural features are conducive to the diffusion of gas molecules and inhibit the agglomeration of nanoparticles,and can significantly improve the utility factor,thereby achieving the improvement of the gas sensing characteristics of oxide semiconductors.The 3D IO In2O3 microspheres with an average diameter of ~ 750 nm were prepared by the self-assembly template assisted ultrasonic spray pyrolysis.Thus,the gas sensing measurement results showed that 3D IO In2O3 porous microspheres based gas sensor possessed higher sensitivity and better selectivity to acetone compared with the solid In2O3 microspheres based gas sensor.In order to further improve the performance of gas sensor,Pd O nanoparticles are uniformly loaded on the surface and hole inside of 3D IO In2O3 porous microspheres by the impregnation loading method,realizing the surface modification of nanocatalyst.The gas sensing measurements results showed that the acetone sensitivity of the 3D IO Pd O@In2O3 porous microspheres based gas sensors(S = 50.9 to 100 ppm acetone)was about 3.9 times higher than that of the 3D IO In2O3 based gas sensor.Furthermore,the 3D IO Pd O@In2O3 porous microspheres also exhibited low detection limit(500 ppb)and fast response/recovery speeds at 250 ?.These are attributed to the spill-over effect and the formation of Schottky barriers.2.Combined with the micro/nano spatial advantages of three-dimensional inverse opal structure,for further improving the receptor function,transducer function and utility factor of oxide semiconductor by the functional modification technologies of in-situ doping of hetero-cation,finally in order to achieve a significant improvement in the performance of 3D IO oxide semiconductor based gas sensors.Using surface sulfonated polystyrene(S-PS)microspheres as template,a series of Ga3+ doped 3D IO In2O3 porous microspheres((Gax In1-x)2O3,x = 0.1,0.2and 0.3)were prepared by ultrasonic spray pyrolysis.It was found that the Ga3+in-situ doping can decrease the crystal structure,effectively regulate the location of Fermi level,as well as the reaction activity and the concentration of surface chemical adsorbed oxygen,thus resulting in the improvement of gas sensing performance.The3 D IO Ga3+ doped In2O3((Ga0.2In0.8)2O3)porous microspheres based gas sensor exhibited higher sensitivity to 100 ppm formaldehyde(S = 47.2 ± 5),faster response/recovery speeds,better selectivity(the formaldehyde selectivity of pure In2O3 based gas sensor is worse),and ultra-low detect limit(50 ppb)at low operating temperature(200 ?),which means that this formaldehyde gas sensor can be used for monitoring indoor air quality.3.Designing and preparing heterogeneous composite sensitive materials composed of two different kinds of oxide semiconductor,through using the “synergy effect” between components and interface “heterojunction effect” to regulate the energy band structure,carriers concentration and mobility of composite sensitive materials,thereby significantly improving gas sensing characteristics.After introducing Zn O,compared with the 3D IO In2O3 multilayer films based gas sensors,the 3D IO Zn O-In2O3 multilayer films based gas sensors possessed higher sensitivity(the sensitivity to 100 ppm acetone increased by 2 times),better acetone selectivity,lower detection limit(1 ppm)and shorter recovery time(recovery speed increased by2 times),these are attributed to the synergistic effect of Zn O-In2O3,as well as the regulation of sensor's baseline resistance(Rair)and energy band structure by n-n heterojunction.Furthermore,we firstly prepared 3D IO Sn O2-Zn O hollow microspheres(3D IO Sn O2-Zn O HM)by one-step self-assembly template method,which was formed by periodic self-assembly of several hollow nanospheres(average diameter: ~ 170 nm).As shown in the gas sensing performance measurement results,the 3D IO Sn O2-Zn O(Sn/Zn = 1:1)HM based gas sensors showed high sensitivity(S= 3.1)to acetone at low concentration(1.8 ppm)in a high humidity atmosphere(98RH%)at lower operating temperature(275 ?),and fast response/recovery speeds(4s/17 s).In the simulation test of breath diagnosis,this sensor can clearly distinguished the difference between the exhaled breath samples of healthy volunteers and the diabetic patients,so as to realize the breath diagnosis of diabetes.4.Oxide semiconductor sensitive materials usually exhibit gas-sensing characteristics at higher temperature,it is mainly due to their catalytic activity requires thermal energy to excite.This not only leads to high power consumption,but also easily detonates flammable gases.In view of the above problems,this thesisproposes a strategy of using light energy instead of thermal energy to excite oxide semiconductor sensitive materials.The design and preparation of composite sensitive materials composed of photocatalyst and oxide semiconductor,and the enhancement of surface oxidation activity through the generation and transfer of photo-generated electrons,finally achieve the detection of gas at low temperature(or even room temperature).Meantime,the decomposition of photo-generated carriers on the surface adsorbed water significantly reduces the influence of environmental humidity on the sensing performance of sensor.On the one hand,the excitation wavelength of the Zn O-In2O3 composites is modulated to the visible light region by adjusting the proportion of each component in the composites.On the another hand,using the“light trap effect” of 3D IO structure and heterostructure to effectively inhibit the recombination of photo-generated electrons-holes,as well as to improve the utilization of light and the transfer of photo-generated electrons.Thus,under visible light illumination,compared with the Zn O(or In2O3)based gas sensor(S = 8.6 or 13 to 5 ppm NO2),the 3D IO Zn O-In2O3(Zn/In = 1:1)porous microspheres based visible light excitation gas sensor exhibited ultra-high sensitivity(S = 160.8 to 5 ppm NO2),low detection limit(250 ppb),fast response(188 s),better NO2 selectivity among different interference gases,better anti-humidity interference ability(well response-recovery characteristics at 80 RH%)and long-term stability at room temperature.