Study Of The In Situ Fabrication Of Gas Sensors By Electrostatic Spray Method

With the rapid development of intellectual technology in the modern society,the request for quality of gas sensors which can detect and monitor the quality of the living environment is becoming stricter.In recent years,many nanomaterials and nanotechnologies have been developed to develop advanced gas sensors which meet the demands of the future.In situ growth technique is a new technique which can make the target products grow directly on a specific substrate.This method has attracted much attention due to its highly automated preparation and the resulting multidimensional products anchored on substrates with high specific surface areas.However,the existing methods for in situ preparation of gas sensors are often limited by their own limitations.In this paper,we successfully synthesized a variety of 3D porous sensitive materials on the surfaces of alumina ceramic tubes by improving the traditional electrostatic spray deposition(ESD)technique.These in situ prepared metal oxide sensors exhibited excellent gas sensing properties.The main contents and innovations are as follows:(1)In chapter 2,taking the solvent formula as the breakthrough point,we studied and explored the general rule of the preparation of 3D porous nanostructures by ESD.It was found that in the solvent system of ethylene glycol and ethanol mixture,the strong hydrogen bonding force of ethylene glycol made the evaporation of ethylene glycol slowly.In the ESD process,some solvents which were unable to evaporate in time can easily converge into "water beach" and finally formed a cracked thick film.When some of the ethylene glycol components were replaced by 1,2-propanediol,the strong hydrogen bond force was weakened and the mixed solvent was more likely to be evaporated,resulting in the 3D porous structure.Based on the physical characterization and electrochemical analysis of the synthesized 3D porous materials,we established the mechanism model of nanomaterials with different morphologies on the substrate by ESD.(2)Because the alumina ceramic tube electrode showed a tubular structure,we modified the ESD experimental platform in chapter 3 and successfully deposited Ag/Fe2O3 on the outer surface of the ceramic tube uniformly.In the modified ESD platform,we used the rotary electric motor to rotate the ceramic tube around the shaft at a constant speed,to guarantee the uniformity of the deposited film.Meanwhile,a far-infrared carbon fiber heating tube was used as the heat source to heat the physical/chemistry reactions occurred on the surface of the rotating ceramic tube.We found that the Ag doping amounts had a significant effect on the performances of the sensors.When the mass ratio of Ag in the sensing film was 6%,the sensitivities of Ag/Fe2O3 sensor to 1,10,and 50 ppm H2S at an optimal operating temperature of 200? were 6.76,21.13,and 29.52,respectively.When the concentrations of H2S were as low as 0.1 and 0.2 ppm,the sensitivities of the sensor maintained at 1.48 and 2.03,respectively.We compared the gas sensing properties of the in situ fabricated sensors and the conventional brush coated ones and found that the former showed better sensing properties because of the structural advantages.(3)According to the mechanism model of the in situ preparation established in chapter 2,we synthesized different 3D porous Fe2O3/C sensing materials on the surface of the ceramic tubes by changing the environmental temperature in chapter 4.We found that when the environmental temperature was 28 ?,the obtained 3D porous Fe2O3/C was assembled by many nanosheets,and the mass percentage of carbon was 3.6%.In the 0.3,1,3,5,and 10 ppm H2S atmosphere,this sensor can output the sensitivities as 1.89,4,7.58,10.18,and 14.21,respectively.This performance was better than that of the reported Fe2O3-based H2S gas sensors.In addition,we also studied the role of carbon in the sensing materials.The presence of carbon can reduce the operating temperature required for the sensors to a certain extent.We ascribed the superiority of the 3D porous Fe2O3/C sensor to the high specific surface area and rich reaction sites.(4)Following the general rules of constructing 3D materials in chapters 2 and 4,we prepared the sensors with the most classical n-type semiconductor ZnO as the sensing materials in chapter 5.With the change of the environmental temperature,the deposited ZnO film exhibited a variety of structures.At an environmental temperature of 30 ?,the sensing film was assembled by many crosslinked nanosheets.According to the TEM images,these nanosheets were composed of many nanoparticles in a size of 10 nm.In the gas sensing test,the sensitivities of the nanosheet-cross-linked ZnO in the 1,3,5,7,and 10 ppm H2S atmosphere were 2.3,6.3,10.8,13.6 and 20.2,respectively.It is worth noting that after annealing at 400 ? in air,the sensor would fail to detect H2S.Through the analogy experiment,we concluded that based on the in situ fabrication technology established in this paper,the number of physical gaps had great influence on the performance of the sensing films.