Preparation And Gas Sensing Properties Of In₂O₃-based Nanomaterials With Different Structures

Nanomaterials and their structures have attracted significant attention of scientists in a few decades. In particular, they have become a hot research spot in nanotechnology research area because of their novel physical, optical, chemical and biological characteristics and potential applications in nano-deviecs. Hence, it is of great importance to study the relationship between the morphology and physical properties of nanomaterials by controlling the size and dimensions of nanomaterials, in order to design and synthesize functional materials with various requirements.Semiconductor metal oxides In₂O₃ is an important optoelectronic functional materials, because of their excellent physical and chemical properties. It has been widely used in solar cells, transparent electrodes, photoelectric sensors, antistatic coatings and gas sensors. As a gas sensitive material, its sensitivity is strongly dependent on the structure of nano-materials (ig, grain size, surface area, dimension, and the network and pore structure) and the dopant species.In this paper, we prepared various In₂O₃-based nanomaterials (including nanopartices,one-dimensional nanofibers and mesporous structures by sol-gel method, electrospinning method and zeolite template method. The effect of various synthesis conditions on the morphology and physical properties of nanomaterials was also studied in detail. In addition, the gas-sensing properties of indirect heatingstructure sensors with different In₂O₃ structures were also investeged. The relationship between the gas-sensing properties and nanostructure of In₂O₃-based nanomaterials was discussed.In order to improve the sensitivity of these gas sensors, we doped additive with In₂O₃ nanomaterials. The influence of the additive on the sensing properties of the sensor was studied. Based on these studies, a few hazardous gas sensors have been developed and the performaces of the sensors have been optimized.In chaper I, we introduced the concept of semiconductor gas sensors and their properties and applications. A couple of new synthesis methods of metal oxides were described. In addition, the significance of our research and main results were also summarized.In chaper II, Ag-doped In₂O₃ nanocrystalline powders with different doping concentrations have been prepared by a sol-gel method. The nanocrystallines are characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and X-ray photoelectron spectrum (XPS). The results indicated that these powders had a good crystalline structure with an average crystallite size of 12 nm. The indirect heating structure sensors were fabricated by loading these powders on ceramic tubes with Au electrodes. Gas sensing measurements indicated that the sensor fabricated with 8 wt.% Ag-doped In₂O₃ showed the highest sensitivity to HCHO among the nanoscrystallines with different Ag doping concentrations. The detection limit can reach 2ppm with a short response time (10-15 s) and an excellent selectivity at 100°C. However, these gas-sensors often suffer from a serious degradation due to the aggregation of nanoparticels. It is still a challenge to improve the the stability and repeatability of these gas sensors.In chaper III, Ag-doped In₂O₃ nanofibers with different Ag doping concentrations have been prepared by an electrospinning method. The results of XRD, TEM, SEM and FESEM indicated that these nanofibers had a good crystalline structure with diameters ranging from 60 to 120 nm and lengths of several tens of micrometers; Gas sensing measurements indicated that the sensor fabricated with 8 wt.% Ag-doped In₂O₃ nanofibers showed the best HCHO sensing properties. At 115°C, the sensitivity of this sensor is up to 3 when the sensor was exposed to 5 ppm HCHO. Good selectivity and stability were also observed in our investigations. These results indicated that the Ag-doped In₂O₃ nanofibers avoid degradation compared to Ag-doped In₂O₃ nanocrystilline powders.In chaper IV, Ni-doped In₂O₃ nanofibers with different doping concentrations have been prepared by an electrospinning method. The results of XRD, HRTEM and XPS indicated that Ni2+ ions substitute In3+ ions in the cationic sites of In₂O₃ crystal lattice with the formation of In_2-xNi_xO₃ solid solution. The results of TEM, SEM, FESEM indicated that the nanomaterias were made of a large area of nanofibers which had a good crystalline structure with diameters of 70 nm and lengths of several tens of micrometers diameter; The nanofiber is made up of ultrafine particles of about 20 nm. The result of XPS indicated that a large number of adsorbed oxygen exist in our solid solution nanofibers.The sensor fabricated with 10 mol% Ni doped In₂O₃ nanofibers exhibits the best trimethylamine sensing characteristics. The sensitivity is up to 48 when the sensor is exposed to 50 ppm trimethylamine, and the response time and recovery time are within 10 s and 20 s, respectively. The ccalibration curve for the trimethylamine is in the range of 3-100 ppm. The sensing characteristics of these nanofibers are attributed to the solid solution system combined with the one-dimensional nanostructure.In chaper V, Ni-doped In₂O₃ mesoporous materials were synthesized by a KIT-6 tempate method. The results of XRD, HRTEM and BET indicated that these powders had a high ordered mesoporous structure. Gas test experiments of TMA showed that the mesoporous materials have higher sensitivity compared to the nanofiber structure. This is mainly attributed to its ordered mesoporous structure, uniform pore size and large specific surface area. However, our research is still at the intial stage. Other influencing factors such as different pore size, different doping concentration, calcination temperature, stability and selectivity of this gas sensor is still under investigation now.