Research On NO₂ Gas Sensors Based On Indium Oxide With Hierarchical Structure

With the rapid development of Chinese economy, the urbanization of our country is speeding up, the size and the population of the cities grow dramatically. At the same time, with the increase of household income, the car ownership and the car exhaust emissions of our country keep growing. In this case, more and more people live in the cities where are being polluted by more and more harmful gases exhausted by cars. So, the urban environment pollution problem become more and more serious. Besides, due to the high density of buildings and the changes of traffic flow, the urban environment pollution shows clear regional and timing characteristics. NO₂ is one of the most important harmful gases exhausted by cars. It is active and caustic, and it could react with moisture or hydrocarbons in the atmosphere and cause acid rain or photochemical smog. So NO₂ is quite harmful to our health and the environment we live in. In order to solve the new urban environmental problems, automatic monitoring system with small size, low-cost, high efficiency and could be set widely and densely which could monitor the harmful gases such as NO₂ in the atmospheric environment in long term is quite necessary because it could provide timely and effective air quality information which has important reference value for people’s daily life and urban environment treatment.The current methods for atmospheric NO₂ monitoring needs instruments with high-cost and big size, and the detection process is complex and requires a lot human intervention. Thus, they cannot meet the needs of current atmospheric NO₂ monitoring due to their poor real-time performance, high detection cost, low degree of automation and the shortage of monitoring stations. Meanwhile, monitoring system based on gas sensors has become one of the most promising alternatives for the atmospheric NO₂ monitoring because of the fascinating characteristics of gas sensors based on MOS（Metal Oxide Semiconductor）: easy manufacture, small size, low cost, low power consumption, wide detection range, simple detection process, tractable signal as well as easy to be integrated and networked. Though gas sensors based on MOS have many merits, there are still some deficiency in their sensitivity and detection limit,so they still cannot meet the requirements of current National Ambient Air Quality Standard very well. In order to solve these problems, to make gas sensors based on MOS can be used in atmospheric NO₂ monitoring,some kinds of In₂O₃ hierarchical nanostructures have been synthesized by solvothermal method to make high performance NO₂ gas sensors after the investigation of the sensing mechanism of these gas sensors. Doping technology has also been used to improve the sensing performance of the fabricated gas sensors. The main achievements of this thesis are summarized as follows.（1） In₂O₃ with spindle-like hierarchical structure was synthesized via solvothermal method combined with the subsequent thermal treatment and the structural and morphological characteristics of the obtained sample were characterized. Sensors based on the obtained sample were fabricated and the sensing properties of these sensors were tested. The test results indicated that the response of the fabricated sensors to 1 ppm NO₂ at the optimum operation temperature, 80 oC could reach to 1395.4 and there was still a certain response to 50 ppb NO₂（about 1.5）. Besides, it can also be observed that the fabricated gas sensors exhibited excellent selectivity to NO₂.（2）Hollow In₂O₃ microspheres constructed by primary nanoparticles were successfully prepared by thermal treatment of the precursor which was synthesized via a facile solvothermal method without use of any templates or surfactants. A series experiments were carried out at different reaction time to observe the evolutions of structure and morphologies of the calcined samples. The hollow structure and rough surface of the obtained sample gave more chance for the NO₂ molecules to react with the sensing material. When the sensing properties of sensors based on the hollow In₂O₃ microspheres were tested, the test results indicated that the optimum operation temperature of the fabricated gas sensor to NO₂（80 oC） was rather low, and the responses to 500 ppb and 50 ppb NO₂ were about 323.5 and 2.0, respectively. Apart from the high sensitivity, the test results also indicated that the fabricated sensors also exhibited excellent selectivity to NO₂. Besides, the correlation of the response of the fabricated gas sensors to 500 ppb NO₂ with the relative humidity was also tested and expounded.（3） Flower-like In₂O₃ microspheres have been successfully synthesized via a facile and efficient solvothermal route combined with the subsequent thermal treatmentand and Cu-doping was carried out to improve the sensing properties of the obtained sample. It can be found from the FESEM images that the flower-like microspheres were built up from abundant irregular-shaped nanosheets and the obvious open spaces existed between those nanosheets provide convenience for the target gas NO₂ to diffuse into the inner part and react with it. Furthermore, the concentration of the major carrier, electron were reduced by Cu-doping. It made the obtained Cu-doped sample more sensitive to the electron capturing caused by NO₂ and therefore improved the sensing properties of the fabricated gas sensors. But in the meanwhile, the integrity and the homogeneity of the morphology and nanotructure were destroyed gradually with the increase of the Cu doping amount. The test result indicated the 1.0 mol% Cu-doped sample found a balance between the reducing of the electron concentration and the preservation of the hierarchical nanostructure and exhibited excellent sensing properties to NO₂. The response of sensors based on the 1.0 mol% Cu-doped sample to 400 ppb NO₂ at the optimum operation temperature（60 oC） was 1800.1 which was about 13.5 times higher than that of the sensors based on the undoped sample. Besides, when the NO₂ concentration reduced to 25 ppb the response of the sensors based on the 1.0 mol% Cu-doped sample could still reach to 11.8 while there was nearly no any response for sensors based on the undoped sample. So sensors based on the 1.0 mol% Cu-doped sample could meet the requirements of the National Ambient Air Quality Standard and could be used in atmospheric NO₂ monitoring. Moreover, as to demonstrate the effect of structure on sensing properties, 1.0 mol% Cu-doped In₂O₃ nanoparticles were prepared by grinding the corresponding flower-like In₂O₃ microspheres, and the sensing properties of gas sensors based on the 1.0 mol% Cu-doped In₂O₃ before and after being grinded were both tested. And the important role that hierarchical structure played in gas sensing properties improvement has been confirmed by the test results. Moreover, a 48-channel aging equipment for gas sensor based on MOS with aging current of each channel could be adjusted independently was designed for the sensor aging before sensing properties test.