Co-precipitation of silica and alkaline-earth carbonates using TEOS as silica source

Abstract

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).

Additive-induced morphological tuning of self-assembled silica–barium carbonate crystal aggregates

“…In the absence of any further additive, silica-barium carbonate self-assembly results in this case typically in racemates of regularly wound helical filaments (Fig. 2), curvilinear sheets as well as thicker croissant-like braids (''worms''), as documented in detail in earlier work [18,23,24].…”

“…Previous studies on the Si/Ba ratio in silica biomorphs led to a diverse range of values, depending on the type of morphology as well as on the respective position within an aggregate. For instance, Si/Ba percentages of 15-29% were reported for biomorph helices, while 6-35% was found for worms [18,37]. Coralline CaCO 3 biomorphs were claimed to contain 35-40% Si relative to Ca 26 .…”

“…The self-assembly of alkaline-earth metal carbonates in the presence of silicate is an outstanding example in this context, as purely inorganic precursors cooperate in this case to constitute hierarchical structures and shapes strongly reminiscent of biological forms [15][16][17][18][19]. Precipitation of barium carbonate (witherite) from alkaline silica solutions or gels results in a variety of complex curved, non-crystallographic ultrastructures, including helicoidal filaments and sinuous sheets.…”

The epitaxial role of silica groups in promoting the formation of silica/carbonate biomorphs: A first hypothesis

“…Strictly speaking, Voinescu et al [22,23] reported the formation of silica-calcium carbonate biomorphs showing ''coralline'' self-assembled shapes, these synthetic aggregates being built by aragonite nano-crystals and an amorphous silica matrix. Nevertheless, the thermal conditions for precipitation were sharply different from those used to obtain the usual silica biomorphs [12,14], since the authors operated far from the room temperature (80 1C).…”

Growth Behavior and Kinetics of Self‐Assembled Silica–Carbonate Biomorphs

“…EDX data (lower panels in Figure 4 bd) confirm the presence of appreciable amounts of silica, with Si/Ba atomic ratios of skin-bearing biomorphs ranging from about 0.15 to 0.4, in good agreement with values reported previously for syntheses run under similar conditions. [21,22] Coherent results were obtained when determining the average Si content of bulk samples prepared by homogenizing a great deal of aggregates. On the other hand, highmagnification FESEM micrographs and AFM images of biomorphs grown for 8 h or less reveal a bare carbonate core instead of a smooth silica surface ( Figure 5).…”