Establishment Of Low Temperature Gas Sensor And Its Characteristic Studies

As gases exist everywhere in our daily life and industrial manufacture,highly sensitive,reliable and highly selectivity gas sensor are in great demand.Among various gas sensors,low dimensional semiconductor gas sensor present interesting advantages in their simple implementation,low cost and good reliability for real-time monitoring.However,most of these sensors can only function at elevated operating temperatures.This leads to high power consumption and poor durability.Moreover,as some detective gases are highly flammable,if the input energy for sensor operation is high enough,the sensor,itself,can possibly trigger an explosion.Thus,the key point and challenge is to reduce the operating temperature of MOSs sensors.As for majority solid materials,the surface state is more stable at lower temperature,it is difficult for gas molecules to break this balance.Meanwhile,response time become longer at the low temperatures,which is not suitable for the practical applications.In this thesis,we achieved flammable and poisonous gas sensors,which work at low temperature and present highly sensitive,good selectivity and fast response time.In the introduction part,we discussed the gas adsorption process on solid surface and the charge transfer rules,summarized the development tendency,and tried our best to find the method based on fundamental laws of physics for realizing low temperature gas sensors.1.In Chapter 2,new,highly sensitive room temperature hydrogen gas sensors were fabricated by coating Pt decorated-cube-like-In2O3 on Au electrodes.The loading of the Pt nanoparticles enhanced the catalytic dissociation of oxygen molecules,adsorbed a substantial quantity of hydrogen,and the sensor exhibited a dramatic decrease in working temperatures to 25?.Responding to 0.5%vol hydrogen,the sensor achieved response and recovery times of approximately 36 s and 70 s,respectively.The sensor also achieved excellent stability and high sensitivity to hydrogen at RT.In addition,it also showed a slow response to CO.The gas response to 1.5%vol CO was 10 times lower than that to hydrogen.For other VOCs,the response can be neglected.Such a large discrepancy reveals the sensor's outstanding selectivity to IL.Grain boundary theory and Shottky barrier effect are applied to explain the gas sensing effect of the Pt-coated Ih2O3 nanocubes sensor.2.In Chapter 3,a catalytically activated hydrogen sensor is obtained based on Pd decorated WO3 nanoplates constructed by a solvothermal method.The pure WO3 sensor exhibits poor selectivity and low sensitivity to hydrogen.In contrast,the observed response of the as-produced sensor is up to 843 at a low operating temperature of 80?;the response value is even greater than that of WO3 sensors at high temperatures(250-400?).In addition,the Pd-loaded WO3 sensors show excellent selectivity towards H2 in comparison to other common gases(CH4,C3H6O,C2H6,C3H8O and NH3).The significantly improved performance is thoroughly explained in terms of the adsorption-desorption mechanism and chemical kinetics theories.Furthermore,an interfacial model demonstrated in this chapter indicates that the interfacial barrier between WO3 nanoparticles can be a novel effect for excellent gas sensing performance3.In Chapter 4,hierarchical Cr-doped WO3 microspheres have been successfully synthesized for efficient sensing of H2S gas at low temperatures.The temperature and gas concentration dependence on the sensing properties elucidate that Cr dopants remarkably improve the response and lower the sensor' operating temperature down to 80?.Under 0.1%vol H2S,the response of Cr-doped WO3 sensor is 6 times larger than pristine WO3 sensor at 80?.We suggest the increasing number of oxygen vacancies created by Cr dopants to be the underlying reason for enhancement of charge carrier density and accelerated reactions with H2S.4.In Chapter 5,horizontally aligned CNT arrays and random CNT network are synthesized by thermal CVD at 925? in an ethanol atmosphere.With appropriate functionalization applied to the aligned CNTs,a high sensitivity of 8.48%for ambient CO2 gas concentration of 500 ppm is achieved.In addition,these aligned CNT array sensors show much faster response and recovery time than a random CNT network.Moreover,good selectivity against NO2 and NH3,and good repeatability are demonstrated.These results pave the way for a deeper understanding of the physical and electrical properties of single-wall CNT and inter-tube junctions.