We describe self-assembled silica-carbonate aggregates that show a diverse range of morphologies, all of which display complex internal structure, orientational ordering of components, and well-organised, curved global morphologies that bear a strong resemblance to biogenic forms. The internal order is described as a liquid-crystallike organisation of colloidal particles. We discuss possible causes for the striking morphologies of these inorganic materials, including local nanocrystal packing constraints and global silica membrane templating.
We explore the use of tetraethoxysilane (TEOS) as a silica source for the formation of carbonate-silica composite materials known as 'biomorphs'. The basic hydrolysis of TEOS furnishes silica in a controllable fashion, allowing a significantly higher reproducibility of the obtained silica-barium and silica-strontium carbonate co-precipitates compared to commercial water glass silica used so far. We further discuss the influence of ethanol used as a co-solvent on the morphologies of biomorphs, which are examined by optical microscopy, field emission scanning electron microscopy (FESEM) and energy dispersive X-ray analysis (EDX).
ABSTRACT:The precipitation of calcium carbonate in alkaline silica solutions results in the formation of complex curvilinear forms if aragonite formation is encouraged by growth at an elevated temperature (80°C). The resulting coralline self-assembled silica-calcium carbonate particles are "biomorphs", bearing a striking resemblance to natural coral forms. These materials, comprised of calcium carbonate nanocrystals and an amorphous silica matrix, have a complex ultrastructure, made of clusters of gathered sheets of variable curvatures formed by successive curling. The nanocrystals within these "ruled surfaces" are thin, elongated, densely packed needles of aragonite. These clusters are outgrowths from central saddlelike cores that resemble developable petaloid surfaces. The size, shape, crystallography, and chemical composition of the resulting biomorphs were examined by optical microscopy, field emission scanning electron microscopy (FE-SEM), powder X-ray diffractometry (XRD), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM and HRTEM), and energy dispersive X-ray analysis (EDX).
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