Hard-template Synthesis Of Several Mesoporous Materials And The Study Of Gas Sensing Properties

Ordered mesoporous materials have large application foreground in catalysis, sorption, separation, sensor, drug loading and release, electronic devices, and so on, because of their high surface area, regularly arranged mesopores with various topologies, tunable pore size, large pore volume. Since the discovery of the M41S family of ordered mesoporous silica materials reported by the Mobil's scientists, more and more researchers have devoted to the synthesis, characterization and applications of ordered mesoporous materials. Over the past twenty years, one could tailor and pre-design the pore structure, mesoscale arrangement and morphology of ordered mesoporous materials to some degree. Now, the research of ordered mesoporous materials has been focus on how to realize their facile and low-cost synthesis as well as real application in various fields. In this thesis, we chose several mesoporous solids as hard template for preparing some mesoporous metal oxides such as indium oxides, tin oxides, indium and tin oxide composite and copper oxide. We improved the original synthetic route, made it simple and reduced the synthetic cost. Moreover, we extended the hard-templated method to the preparation of some new mesoporous materials. Those synthesized mesoporous indium oxides were used to fabricate gas sensors and their gas sensing properties were investigatedMesoporous metal oxides have potential application in catalysis, sorption, chemical and biological separation, photonic and electronic devices, and drug delivery and so on. Compared to mesoporous metal oxides with amorphous walls, mesoporous metal oxides with crystallized walls can be expected to provide better thermal and mechanical stability, as well as superior electric and optical properties. Significant progress has been achieved in the preparation of ordered mesoporous metal oxides via using mesoporous silica as hard template. Mesoporous silica was first synthesized by using surfactant and the surfactant in mesoporous channel, as the most expensive synthetic material of the mesoporous silica, has to be removed from the pores via the decomposing method (such as calcination, photocalcination, microwave digestion, ozone treatment, H₂O₂-mediated oxidation, KMnO4-mediated oxidation) or the extraction method (such as solvent extraction and supercritical fluid extraction). The latter that could nondestructively remove surfactants and allow for the recovery and reuse, is especially favorable for cost-reduction. Unfortunately, the surfactant can not be usually removed completely by the extraction method, due to the smaller size of the connecting pore between the main pore channels of mesoporous silica. Consequently, the extracted mesoporous silica possesses a very poor interconnectivity between the main pore channels, and the resultant replicas usually are not ordered mesoporous materials but individual nanowires or nanoparticles when it is used as the template. In Chapter 2, we synthesized mesoporous silica SBA-15 with larger connecting pores by using higher temperature hydrothemal treatment. FTIR spectra confirmed the surfactant in SBA-15 with larger connecting pores could almost be removed completely via solvent-extraction method. Highly ordered mesoporous indium oxides, tin oxides and indium-tin oxide composites have been successfully synthesized by using this extracted SBA-15. The mesoporous indium oxide and tin oxides have high surface areas (53-126 m²Â·g^-1), large pore volumes (0.21-0.45 cm³Â·g^-1) and narrow pore size (3.7-6.5 nm) distributions.In the preparation of mesoporous metal oxide using mesoporous silica as hard template, the silica template was usually removed by NaOH or HF solution. Therefore, mesoporous silica is unsuitable for some metal oxides that is unstable in NaOH and HF solution, such as copper oxide and zinc oxide. Ordered mesoporous carbon CMK-3 is also a mesoporous solid with well-ordered mesostructure and rigidity. Therefore, it is possible to prepare mesoporous metal oxides by using mesoporous carbon as hard template. In Chapter 3, we used mesoporous carbon CMK-3 as hard template to prepare mesoporous copper oxide and indium oxide. The mesoporous copper oxide possesses an ordered mesostructure, high surface area (149 m²Â·g^-1), large pore volume (0.22 cm³Â·g(-1)) and narrow pore size (5.5 nm) distribution, whereas the mesoporous indium oxide possesses a disordered mesostructure.As described above, mesoporous carbon CMK-3 with 2D hexagonal mesostructure could be used as hard template to prepare mesoporous copper oxide. In principle, mesoporous carbon with other mesostructure could also be used as hard template. In Chapter 4, we investigated the possibility to use ordered mesoporous carbon CMK-8 with 3D cubic mesostructure (Ia3d) as hard template for the preparation of other mesoporous materials such as silica. Moreover, high temperature thermal treatment was employed to increase the polymerization degree of the Si-O-Si linkages in mesoporous silica and improve its hydrothermal stability. After boiled for 80 h, the mesoporous silica still possesses ordered mesostructure and large specific surface area (330 m²Â·g^-1), only reducing 12 % of its original specific surface area (375 m²Â·g^-1).The principle of semiconducting metal oxide gas sensors is based on surface-chemical interaction between gas molecules and the crystalline sensor material. For n-type semiconductors, such as indium oxide, intrinsic oxygen vacancies are responsible for the electronic conductivity. Chemical adsorption of oxygen species under ambient conditions (i.e. in the presence of air) creates extrinsic surface acceptor states that immobilize conduction band electrons from the near-surface region. In the classical model this results in an electron depletion layer. In granular systems Schottky barriers between the grains are formed because of these depletion layers. This leads to a lower conductivity in the surface near regions compared to the bulk, as electron transport is limited by such barriers. Therefore the overall conductivity is determined by the amount of surface oxygen species. Gases with reducing properties (such as formaldehyde) affect the amount of adsorbed oxygen which is why changes in the electronic conduction can serve as the measurable quantity for the presence and concentration of the target gases (''chemiresistors''). For a high sensitivity (i.e. strong change in conductivity) and a fast response rate a sensing material should exhibit large surface-to-volume ratios and facilitate high accessibility for the gas molecules. Therefore, mesoporous metal oxides are particularly suitable as materials for gas sensor. Formaldehyde (HCHO) is well known as a colorless, strong-smelling and toxic gas, and is extremely dangerous to the human body. Therefore, effective methods to monitor formaldehyde are of great importance and much demanded for both environmental protection and human health. In Chapter 5, gas sensors have been fabricated by using the mesoporous indium oxide and mesoporous tin oxide and their gas sensing properties have been investigated. The mesoporous indium oxide exhibited high sensitivity towards formaldehyde (HCHO).