Synthesis, Characterazition And Performance Of Hierarchical Hollow Structured Silica Composites

With the rapid development of global industry, the problem of environmental pollution is becoming more and more serious, which has seriously affected the sustainable development of the environment for human survival and social economy. Therefore, the environmental protection and effective treatments of pollutants have become the one of the most important issues for all of the governments worldwide. As an effective method for degradation of organic pollutants, the photocatalytic degradation with low toxicity, low cost, high efficiency degradation and environment friendly has attracted a great many attentions of scientists. However, the lower specific surface area and wide band gap of widely used titania photocatalysts greatly restricted its applications. A large number of efforts have been done in order to overcome these shortcomings during the past decades. The preparations of TiO2 photocatalysts with cavity structures and studies of their structure influence on the catalytic performance has become a hot research field in recent years. At the same time, silver halides with high photocatalytic activities and stabilities have attracted more and more attentions, and a great many researches have been made recently. What is more, the removal efficiency of organic dye pollutants in water through effective adsorption has been an effective way to deal with water pollution. However, most of traditional adsorbents have shortcomings such as difficulties of recycle, high costs of recovery and weakness of repeat-ability. Thus, the design of magnetic adsorbents with high repeat-ability is very meaningful.In the present thesis, a series of novel photocatalysts with hollow structures such as TiO2@hierarchical hollow silica sphere, foamed titania-silica composite and Ag-AgBr@hierarchical hollow silica sphere were fabricated by microemulsion method using hexadecyl trimethyl ammonium bromide as template. And their photocatalytic activities in degradation of dyes were studied. The effects of hollow sphere structure of synthesized photocatalysts were discussed. A novel magnetically separable Fe3O4@HHSS material was synthesized and its dye adsorption performance was studied. The synthesis mechanisms of Ag-AgBr@hierarchical hollow silica sphere and magnetic hierarchical hollow silica sphere were proposed. The research mainly has following several aspects:1. A series of novel TiO2@hierarchical hollow silica sphere (TiO2@HHSS) photocatalysts with different TiO2 contents were synthesized by microemulsion and sol-gel method. The TiO2@HHSS photocatalysts were constructed by the spherical particles with a diameter between 90-150 nm having a small-ball@big-ball structure. The shell of the big-ball was composed of small-balls (15-20 nm). The photocatalytic activities of the TiO2@HHSS samples were much higher than that of commercial P25 photocatalyst. The high photocatalytic activities were attributed to the good dispersion of TiO2 crystallites and large specific surface area of TiO2@HHSS photocatalysts. Among the TiO2@HHSS photocatalysts with different TiO2 contents, the 40TiO2@HHSS photocatalyst shows the best photocatalytic activity. What is more, the 40TiO2@HHSS catalyst has an excellent reuse ability, maintaining 91.5% of the initial photodegradation activity after five cycles. In addition, the novel hierarchical hollow spheres of TiO2@HHSS may allow of multi-reflections of irradiation light and endow photocatalyst with high photocatalytic activity.2. A novel foamed titania-silica composite with hierarchical porous structures is prepared by one-pot microemulsion method using CTAB and octane as templates. The obtained composite possesses mesoporous and hollow structures with specific surface area of 354 m2/g and average pore diameter of 7.6 nm. Due to the smaller TiO2 particle size (3.9 nm), larger specific surface area and special foam like structure, the obtained FTSC shows superior photocatalytic activity on the degradation of MB compared with the commercial P25 and 40TiO2@HHSS photocatalyst.3. A novel plasmonic Ag-AgBr@hierarchical hollow silica sphere (Ag-AgBr@HHSS) photocatalyst with hierarchical hollow structure has been successfully prepared for the first time. The obtained photocatalyst possesses a high specific surface area of 540 m2/g and a hollow spherical shell structure. The hollow spherical shells are composed of the smaller hollow nanospheres with a diameter of about 15-20 nm. The Ag-AgBr nanoparticles with crystallite size of about 7.5 nm are dispersedly encapsulated in the Ag-AgBr@HHSS photocatalyst. The synthesis mechanism of Ag-AgBr@HHSS was proposed. Because of the surface plasma resonance of methl Ag nanoparticles deposited on the surface of Ag-AgBr@HHSS, the photocatalyst owns high efficient visible light photocatalytic activity and excellent stability. What is more, due to its special structure and smaller Ag-AgBr crystallite size, the Ag-AgBr@HHSS shows much higher photocatalytic activity than Ag-AgBr@MCM-41.4. A magnetically separable hierarchical hollow silica sphere (Fe3O4@HHSS) material was successfully prepared. The Fe3O4@HHSS possesses hollow spherical shell structure. The shell of Fe3O4@HHSS is composed of smaller hollow nanospheres (20 nm) and the Fe3O4 particles are dispersed within the HHSS matrix. The specific surface area of the Fe3O4@HHSS sample is 451.6 m2/g. Vibrating Sample Magnetometer (VSM) analysis of Fe3O4@HHSS shows a magnetic moment of 7.5 emu/g, and the Fe3O4@HHSS can be easily separated by a magnetic field within 2 min. The Fe3O4@HHSS also showed great ability to adsorb MB in aqueous solutions with maximum MB sorption capacity of 71.45 mg/g. The adsorption process was chemisorption in nature, while the adsorption isotherm data were well fitted to Langmuir model and the kinetic data were well fitted to pseudo-second-order kinetic model. The enthalpy of adsorption is relatively high, indicating that interaction between sorbent and MB molecules is chemical. Furthermore, the adsorbed MB could be efficiently desorbed from Fe3O4@HHSS using acidic ethanol and the Fe3O4@F?SS showed excellent recycling ability. From a practical point of view, the Fe3O4@HHSS nanocomposite is promising for waste water treatment.