Synthesis And Properties Of Functional Nanometer Silica Materials

The synthesis and properties of functional nanometer silica materials were studied. These studies on phytoliths. nano-structured silica and mesoporous silica are helpful to synthesis and application of silica materials, and provide a potential application for Gramineae family and others.In most plant species, particularly those belonging to the Gramineae family, aqueous silicic acid, Si(OH)4, is transported along the transpiration stream in the plant, where polymerization always occurs as a combined result with the increase of the concentration of silicic acid through transpiration. At the end of polymerization, silicon is deposited in cells and intercellular spaces, known as phytoliths. It is a porous opal (SiO2·nH2O), and its morphology differ with different species in Gramineae family, depending on the species of plant, between the cells, within the cell walls, or even sometimes completely infilling the cells thermselves.The phytoliths can give some useful information about human-living environment, food source and others in archaeological field, and can be used as the cheap Si resource to prepare nanometer silica.In this paper, amorphous hydrated silica in Gramineae plant, named as phytoliths, was extracted and investigated from wheat straw. Porous nano-structured silica was prepared from agricultural waste materials through combustion and acid leaching. The results show that:i) the phytoliths in epidermal cells of wheat straw are round with the diameter of 14～22μm, while those in trachea are oblong with length of 18～40μm and width of 12～18μm. These different phytoliths are all core-shell structures with the silica shell and the organism core of the plant cells; ii) The distribution of particle size, surface area, pore diameter and pore volume of nano-structured silica samples decreases with the increase of calcining temperature, and at higher temperature, some agglomerates are formed. The results of this work are useful for scientists pursuing new synthetic route for valuable and widely applicable nanoscale silica materials, also helping to solve disposal and pollution problems.Mesoporous silica MS-MCM-41 was synthesized using sodium silicate prepared from wheat straw ash as a silica source and cetyltriammonium bromide (CTABr) as a surfactant, via a hydrothermal treatment in the hydrolysis-condensation stage.The molar composition was 1.0 SiO2:1.1 NaOH:0.27 CTABr:110 H2O. The effect of the temperature in this stage on the mesoporous silica MCM-41 structure and physical peoperties was investigated, and compared with those synthesized by simple method without hydrothermal treatment. The mesoporous silica products were characterized by small-angle X-ray diffraction, nitrogen adsorption-desorption measurement, high resolution transmission electron microscopy and scanning electron microscope. It has been shown that more ordered 2D hexagonal structure of mesoporous silica was obtained with the hydrothermal aging temperature increasing from 80℃to 120℃. There existed a type IV adsorption isotherm and an H1 hysteresis loop in N2 adsorption-desorption curves.The BET surface area was between 724 and 1312m2/g, the pore volume and average pore size increased with the increase of the temperature.