Abstract
The comparative analysis of atomic structure and morphology of fumed silica nanoparticles prepared under different synthesis conditions are studied using TEM, FTIR, quantum chemistry, and low-temperature nitrogen adsorption methods. It was determined that the structure of amorphous silica nanoparticles is formed by branched chain-like clusters of the length of 0.6-2.4 nm that correspond to proto-particles or nuclei of nanoparticles. A linear part of the smallest clusters is consisted of two tetrahedra SiO4 with common oxygen atom and oppositely directed vertices. The inter-tetrahedral average angle of Si ‒ O ‒ Si bonds is about 180˚. It is shown that textural porosity of fumed silica powder depends on the initial degree of aggregation of nanoparticles. The average mesopores (1 nm < R < 25 nm) volume is in the rage of 0.26-0.60 cm3∙g-1 for materials pretreated at different temperatures. It is found out that the mean mesopores radius decreases from 34 nm to 10 nm with decreasing average size of silica nanoparticles. The micropores (R < 1 nm) contribution to the total pore volumes is small for all materials (0.003-0.029 cm3∙g-1).
Highlights
The main feature both of natural and synthetic forms of disordered amorphous silica is the absence of longrange ordering of the oxygen and silicon atoms arrangement, the isotropy of physical and chemical properties, and the difference from zero of the entropy of these materials at 0 K
Fumed silica was synthesized as a result of high temperature oxidation of silicon tetrachloride in the oxygen-hydrogen flame at industrial conditions [13]
The formation of fumed silica nanoparticles can be divided into several steps: protoparticles nucleation → primary particles → aggregates → agglomerates [14]
Summary
The main feature both of natural and synthetic forms of disordered amorphous silica is the absence of longrange ordering of the oxygen and silicon atoms arrangement, the isotropy of physical and chemical properties, and the difference from zero of the entropy of these materials at 0 K. The structure of quartz glass formed by three-dimensional disordered grid consisted of vertices-joined [SiO4] tetrahedra has been well investigated [1]. 0.162 nm and the second one at about 0.265 nm [2] These distances correspond to the Si–O and O–O bond lengths in [SiO4] tetrahedra and are close to similar characteristics of the structure of crystalline silica form with tetrahedral coordination of silicon atoms [3]. The blurred character of subsequent maxima indicates shortrange ordering (0.6 - 0.7 nm) for quartz glass and other amorphous forms of silica. There are conclusions on the presence of ultrafine crystallites in amorphous silica of quartz or tridymite structures [6] based on FTIR data. According to NMR spectroscopy data, the structure of silica gel consists of fragments of cristobalite [8], but for fumed silica both quartz and cristobalite motives were observed [9]
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