Controlled Synthesis And Gas-Sensing Properties Study Of Nanostructured ZnO, In₂O₃ And SnO₂

Gas sensors technology is one of the most important key technologies of the future with a constantly increasing number of applications both in the industrial and in the private sectors, which are usually used to detect noxious and flammable gas in environmental monitoring, healthcare, and automobiles. Consequently, the development of fast and sensitive gas sensors with small cross sensitivity is the subject of intense research, propelled by strategies based on nanoscience and -technology. The gas-sensing reatction as a gas-solid interface reaction occurs on the surface of semiconductor metal oxides. The working principle of a gas sensor is based on the changes in the electrical conductivity of sensing materials brought about by the chemical interaction of gas molecules with the surface of semiconductor metal oxides. Accordingly, for a given type of base material, the sensor property sensitively depends on various factors, such as the working temperature, exposed crystal planes, microsturctures, and surface doping, and so on. Therefore, to study effect of various factors on the gas-sensing property is of important not only for basic theoretical research in the nanomaterials synthesis field but also for the pursuit of surperior gas-sensing materials. This paper focused on the controlled synthesis of various semiconductor metal oxides nanostructures such as ZnO, SnO2, and In2O3 through liquid-chemical routes and the detection of toxic volatile organic compounds. In addition, the effects of morphologies and structures of as-prepared nanostructures on the gas sensing properties to volatile organic compounds (VOCs) were also investigated in terms of exposed crystal planes, ordered structures, porosity, and grain size of sensing materials. The detailed information of the dissertation is listed as follows.1. Morphology-dependent gas-sensing properties of ZnO nanostructures for chlorophenolSingle-crystalline ZnO nanostructures with different morphologies exhibited discrepant gas sensing properties to 2-chlorophenol due to their different arrangement manner of surface atoms and the number of dangling bonds on different crystal planes. Three kinds of single-crystalline ZnO nanostructures including nanoawls, nanorods, and nanodisks were prepared by using glycerin, ethylenediamine (EDA), and NaOAc as capping agents via simple hydrothermal routes. Different exposed crystal planes were expected for these ZnO nanostructures. For ZnO nanoawls and nanorods, the dominating surfaces are {1010} planes. But the difference between the two samples lies in that ZnO nanorods have exposed polar (0001) planes. For ZnO nanodisks, the dominating surfaces are±(0001) planes. Gas sensing properties to 2-chlorophenol of as-prepared ZnO nanostructures were also investigated. Results demonstrated that the response value for detection of 2-chlorophenol were heavily dependent on their exposed crystal planes and the percentage of polar (0001) planes. ZnO nanodisks showed greatest sensitivity towards 2-chlorophenol due to the highest percentage of (0001) planes. ZnO nanoawls had the lowest sensitivity because there are no (0001) planes in the ZnO nanoawls. In addition, density functional theory (DFT) calculations were employed to simulate the gas sensing reaction involving surface reconstruction and charge transfer both of which result in the increase of charge density on ZnO surface and the increase of conductivity. Zn atoms performed the function of reactive sites and played important roles in gas sensing reactions. ZnO (0001) planes are fully exposed with ZnO, so nanodisks have the greatest sensitivity.2. F127-directed preparation of In2O3 microbundles and their enhanced gas sensing propertiesHierarchical In2O3 rod-like microbundles were fabricated via the Pluronic F127-(EO106PO70EO106-) assisted hydrothermal reaction followed by calcining the In(OH)3 precursors. The results revealed that the In2O3 microarchitectures were constructed with well-aligned one-dimensional (1D) single-crystalline nanorods with highly uniform morphologies and particular exposed facets. Structural analysis suggested that the In2O3 nanorods were enclosed by {110} and {001} facets. The triblock copolymer acted as a structure-directing agent and played a key role in the formation of In(OH)3 microbundles. The formation of the precursors In(OH)3 microbundles was studied through contrastive experiments and computational simulation, which can be contributed to the soft-template-directed self-assembly mechanism. The gas sensing properties of the as-prepared In2O3 microbundles were investigated. Compared to the samples prepared in the absence of F127, the In2O3 microbundles exhibited a superior sensing performance toward 2-chloroethanol vapor, which can be explained by hierarchically ordered structures and exposed crystal surfaces.3. Porous SnO2 hollow nanospheres as highly sensitive gas sensors for volatile organic compounds detection Hollow and porous nanostructures usually possess high surface area. This effect means that a significant fraction of atoms are located at surface of sensing materials that can participate in sensing reactions, providing more active sites and enhanced sensitivity. Porous SnO2 hollow nanospheres with high surface areas were synthesized through a solvothermal method by using SnCl2 and NaClO as raw materials. By tuning the concentration of hydrochloric acid (HCl) in ethanol solution, regular porous SnO2 hollow nanospheres can be obtained with the diameters ranging from 90 to 150 nm, which are composed of small nanocrystals with average sizes of less than 10 nm. Time-dependent experiments results demonstrated that the formation of porous SnO2 hollow nanospheres is ascribed to etching the center part of the nanospheres. It was found that hydrochloric aced and NaCIO also played important roles in determining the final morphologies of the final products. The as-prepared porous SnO2 hollow nanospheres exhibited high sensitivity and fast response for detection of formaldehyde and 2-chloroethnol due to the novel porous structures and high surface areas. The lowest detection limit was down to 0.5ppm. Therefore, the porous SnO2 hollow nanospheres are expected to have promising applications in detecting toxic volatile organic compounds (VOCs).4. In2O3 nanofibers and nanoribbons:Preparation by electrospinning and their formaldehyde gas sensing propertiesIn2O3 nanofibers and nanoribbons were prepared by electrospinning combined with poly (vinyl pyrrolidone)-assisted sol-gel technique. By tuning the experimental parameters, the morphology transformation of In2O3 from nanofibers to nanoribbons was achieved. The average diameter of In2O3 nanofibers is 180 nm. The nanoribbons have an average width of 1μm and a thickness of about 150 nm. Both of their lengths can reach millimetres. It was found that both the rapid evaporation of solvent and the concentration of precursor played important roles in the formation process of In2O3 nanoribbons. In electrospinning process, the products were nanofibers when there was less ethanol and lower concentration of PVP. In2O3 nanoribbon can be obtained using the precursor with more ethanol and higher concentration. The formation of ribbon-like structures can be contributed to the rapid gelation on the surface of the electrospun jet and the buckling of skin associated with the bending instability. The average grain size composing nanofibers and nanoribbons is 18.6 and 11.2nm, respectively. The gas sensor based on the In2O3 nanoribbons exhibits higher and faster sensor response to formaldehyde vapor than that based on nanofibers at a relatively operating temperature owing to the size-dependent property.