Surfactant Drived Synthesis Of Mesoporous Titania, Silica And Studies Of Their Mircostruetures

Mesoporous titania, silica materials have been attracted considerable attentions because of their promising applications such as photovoltaic solar cells, molecular encapsulation, adsorbents, catalysts and catalyst supports, low-k dielectric materials, chemical sensors, host-guest chemistry, antireflective coatings and membrane separations. However, tailoring of porous structure is indispensable for these practical applications which benefit from highly ordered structures and high porosity. The addition of surfactants as structure-directing agents is an effective method to introduce nanometer-sized voids into materials. In this way, the amount of surfactant is one of the most important issues for determining the porous structures. Besides, porous structure is also influenced by heat treatment. In this thesis, different groups of porous materials, such as titania powders/films and silica films were synthesized and the dependences of the mesostructures on the surfactants and heat treatments were investigated systematically as follow:1?Different mesostructured titania (TiO2) powders were synthesized via a sol-gel method using a nonionic amphiphilic triblock copolymer F38as the structural template. Mesostructural evolution of TiO2powders depending on template loading and calcination were investigated systematically. Results show that the polymeric template as well as calcination plays an important role in tuning morphology, grain size, crystal phase, mesoporosity and specific surface area of the synthesized mesoporous TiO2. The critical nuclei size for the phase transformation from anatase to rutile is found likely in the range of26-30nm. Two types of mesopores, i.e. intercrystalline voids and templated mesopores, are found to form depending on the template loading. Mesoporosity of TiO2prepared with less F38is attributed to intercrystalline voids, and high specific surface area of TiO2synthesized with more F38is resulted from formation of templated mesopores. 2?Tunable mesoporous titania thin films were prepared by a sol-gel method and spin-coating technique using copolymer F38as the structure-directing agent. The dependence of the crystallization, mesoporous morphology, porosity, surface structure of mesoporous TiO2thin films on the loading of the polymeric template were studied. The porosity of the films is found to increase with increasing the template loading. With increasing the weight ratio of F38up to about15wt%, the pore percolation occurs and the mesopores are highly interconnected in a worm-like shape. The positron annihilation characteristics in mesoporous TiO2thin films are also investigated. The positron annihilation line shape parameter S decreases with increasing F38loading, which indicates that the crystallinity of TiO2thin films is enhanced with the increase of copolymer F38. With further increasing F38loading up to25wt%, S parameter increases, implying Ps atoms are probably formed in the highly interconnected long worm-like pores, and the self-annihilation of p-Ps contributes to S parameter. Both the S-W relationship and the positronium3y annihilation fraction results indicate that mesostructure change occurs at a F38loading of20wt%.3?Porous silica films were synthesized via a sol-gel method and dip-coating technique using a nonionic amphiphilic triblock copolymer F127as the structural template. For the mesoporous silica films, the variation of positron annihilation line shape parameter S reveals that the porosity of the silica films increases with loading more F127, which is also confirmed by the decrease of refractive index n. With increasing F127loading up to around15wt%, a remarkable increment in positronium3y-annihilation fraction is found, which shows the pore percolation in porous silica films occurs around a loading of F127with15wt%. Different linear relationships between positron annihilation line shape parameters S and W are found for the as-deposited films and calcined ones, indicative of the decomposition of the copolymer porogen in the as-deposited films upon calcination. This also reveals the variation of positron annihilation sites as a function of F127loading or porosity. Strong correlations between positronium3y annihilation fraction, S parameter and porosity of the mesoporous silica films with isolated pores are obtained, which may provide a complementary method to determine closed porosities of mesoporous silica films by Doppler broadening of positron annihilation radiation spectra based on a slow positron beam.Silica films with cage-like pores were synthesized via a sol-gel process using triblock copolymers F88and F127as the structure-directing agents. A silica film with random channel-like pores was prepared with F38. Silylation of these films was performed to modify the pore surfaces. Pore entrance size of cage-like pores in mesoporous silica films is difficult to be determined by conventional techniques. A simple expedient is proposed by using positron annihilation lifetime spectroscopy (PALS) based on a slow positron beam. Because of the nature of positronium in mesoporous silica, almost no Ps annihilates in the smaller connecting channels of cages. By trimethylsilylation of the silica, an appreciable fraction of Ps can be trapped and annihilate in the channels, which renders the possibility to estimate the pore entrance size from Ps lifetime in it.4?Porous silica films were synthesized via a sol-gel method and dip-coating technique using a cationic surfactant (hexadecyltrimethylammonium bromide, CTAB) as the structural template. The results indicate the following:(1) the nanoscopic pore structures of the silica films are altered by varying CTAB weight ratio;(2) the positron annihilation sites of the as-deposited films are influenced by CTAB weight ratio, leading to the change in the film composition; and (3) the nanoscopic pore structure change for the calcined films occurs around15wt%of CTAB weight ratio, i.e., isolated pores are likely formed in the silica films prepared with CTAB less than15wt%, beyond which mesopores become interconnected.The structural evolution of CTAB-silica nanocomposite film upon the multistep heat treatment was also investigated. The positron annihilation line shape parameters S and W are found to locate on two different representative lines depending on the films' physicochemical properties, revealing the surfactant decomposition in CTAB-silica nanocomposite film upon successive heat treatment from200?up to300?and the formation of porous structure. The S parameter of the porous silica film treated successively at lower temperature (300?) is comparable to that in the silica film by one-step calcination at450?. Moreover, the networks of the porous film reassemble during the heat treatment at lower temperatures. For the porous films treated successively, a remarkable decrease of S value is found at the dip of S-E curves, which might be associated with the reassembly of CTAB-silica nanocomposites near the substrate upon low temperature treatment.Distinct difference was observed for the W parameters of the same silica film prepared by CTAB after rotating it by90°around the positron beam axis, and the variation of W parameters becomes more pronounced with increasing CTAB loading, probably indicative of ordered aligned pores in the films with more CTAB. For the porous silica films prepared by F127, the W parameters near the film/substrate interface vary remarkably after rotating, implying aligned pores might be formed near the interface. This result signifies pore alignment may be probed by Doppler broadening of positron annihilation radiation spectra.