Preparation And Characteristic Research Of Metallic Oxide Based Composite Nitrogen Dioxide Gas Sensor

As one of the toxic air pollutants,nitrogen dioxide?NO₂?is generated during combustion process in the power plants,factories,and internal combustion engines.It will damage the respiratory and lead to pharyngeus disease even in low concentration and it is the main source resulting in acid rain as well.Thus it is great hazardous to environment and human health.Therefore,it is urgent to develop a convenient and practicable NO₂ sensor with high sensitivity,fast response,low-cost,which could work at room-temperature in the field of public health,environmental quality and industrial safety applications.In this dissertation,metal oxide nanomaterials?such as cerium oxide?CeO₂?and zinc oxide?ZnO??and their composites with reduced graphene oxide?rGO?or ?-sexithiophene??-6T?were used to develop high-performance NO₂ senors at room temperature.Furthermore,the NO₂ sensing mechanism was discussed based on characteristic measurement results and sensing performances of sensors.The main content includes following two parts:1.A novel rGO/CeO₂ bilayer film was prepared on gold interdigital electrodes by facile spray and thermal reduction process with optimized technological parameters.The features of composite films were characterized by morphology analysis?SEM?and spectral analysis?UV-Vis,FTIR and XPS spectra?.The SEM results showed CeO₂ nanoparticles plays an effectively supporting role for rGO film in rGO/CeO₂ bilayer film.Moreover,the results of UV-Vis,FTIR and XPS spectral analyses futher proved that GO was partially reduced to rGO.It was also revealed by the results that heterojunction was effectively formed at the interface of rGO sheets and CeO₂ nanoparticles because of the strong interaction between them.Otherwise,the result of Ce 3d XPS spectra indicated that Ce4+ in GO/CeO₂ was partly reduced to Ce3+ in rGO/CeO₂ after the annealing.The room-temperature NO₂ sensing properties results showed that the response of rGO/CeO₂ based sensor to 10 ppm NO₂ is 20.5%,which is 8.2 times higher than that of pure rGO.In order to achieve optimal sensing performance,a series of experiments based on different content of CeO₂ in rGO/CeO₂ bilayer film were investigated.The result revealed that the rGO/CeO₂ sensor with 1.5 ml CeO₂ and 1.0 ml GO exhibited the best gas sensing properties,which showed a response value of 20.5% and response time of 92 s to 10 ppm NO₂,respectively.Furthermore,the detection limit of optimized rGO/CeO₂ sensor to NO₂ was lower than 1 ppm and obtained excellent repeatability and good selectivity.The NO₂ sensing mechanism model of rGO/CeO₂ sensor was established based on its characteristics and sensing properties.It could be concluded that more oxygen vacancies was generated due to the more existed Ce3+ in rGO/CeO₂,which was beneficial to the synergistic effect of the heterojunction at the interface between r GO and CeO₂.In addition,the supporting effect of CeO₂ nanoparticles in rGO/CeO₂ bilayer film could facilitate the adsorption process of NO₂ molecules.Therefore,the rGO/CeO₂ bilayer film sensor achieved rapid response and significantly enhanced NO₂-sensing properties.2.A novel ZnO NR\?-6T bilayer film was developed on gold interdigital electrodes by hydrothermal method and vacuum vapor deposition.The height of fabricated ZnO NR prepared by hydrothermal method was linearly correlated with the hydrothermal time?Rh2=0.99356?.The hydrothermal time of ZnO NR in the bilayer film was 10 min,in which the height and diameter of ZnO NR were about 400 nm and 215 nm,respectively.Through SEM morphological analyses,it was found that ?-6T nanoparticles were uniformly deposited on the surfaces of hexagonal columnar ZnO NR.ZnO NR\?-6T film showed the similar porous structure as the ZnO NR.The Raman,UV-Vis and XRD spectral analyses futher proved the strong interaction between ZnO NR and ?-6T in ZnO NR\?-6T bilayer film,resulting in the charge transfer from ZnO NR to ?-6T molecules.The heterojunction was effective formed at the interface between ZnO NR sheets and ?-6T.The room-temperature NO₂ sensing measurements results showed that the response of ZnO NR\?-6T to 8 ppm NO₂ is 76.9%,which is 2.1 times higher than that of pure ?-6T?25.5%?,whereas pure ZnO NR did not recovery after exposing to NO₂.Furthermore,the effect of thickness of ?-6T film on the gas sensing performance of bilayer film was investigated.The sensing properties results revealed that the ZnO NR\?-6T sensor with 25 nm ?-6T owns the best NO₂ gas sensing properties at room temperature.The response of the optimized sensor was linearly increased with the concentration of NO₂ within 0² ppm,then tended to saturate within 4⁸ ppm.Otherwise,the response value and response time of proposed sensor to 8 ppm NO₂ was 76.9% and 41 s,respectively,and the proposed sensor also exhibited low detection limit?ppb magnitude?and good repeatability.The NO₂ gas-sensing mechanism model of ZnO NR\?-6T sensor was established based on characteristics and gas sensing properties.It could be inferred that the excellent NO₂ gas sensing performance was contributed to the combination of synergistic effect of the formed heterojunction at the interface of ZnO NR and ?-6T and the porous structure on the surface of the ZnO NR\?-6T bilayer film.