Zusammenfassung
Silica-barium carbonate core-shell nanoparticles have been prepd. and characterized. At low silica concns., there was no evidence for an amorphous carbonate precursor phase and crystn. occurred immediately, resulting in elongated crystals that showed progressive self-similar branching due to the poisoning influence of silicate oligomers on the growth process. Above a certain threshold in the ...
Zusammenfassung
Silica-barium carbonate core-shell nanoparticles have been prepd. and characterized. At low silica concns., there was no evidence for an amorphous carbonate precursor phase and crystn. occurred immediately, resulting in elongated crystals that showed progressive self-similar branching due to the poisoning influence of silicate oligomers on the growth process. Above a certain threshold in the silica content, rapid crystn. was in turn prevented and amorphous nanoparticles were stabilized in soln. The particles were hybrids consisting of a silica core that was surrounded by a layer of amorphous barium carbonate, which was then again covered by a an outer shell of silica. Time-dependent studies further evidence that the carbonate component in the particles can either be permanently trapped in an amorphous state (high silica concns., leading to impervious outer silica skins), or be released gradually from the interstitial layers into the surrounding medium (intermediate concns., giving porous external shells). In the latter case, enhanced particle aggregation induces segregation of silica hydrogel with embedded amorphous BaCO3 precursors, which later crystallize in the matrix to yield complex ultrastructures consisting of uniform silica-coated nanorods. The spontaneous formation of core-shell-shell nanoparticles and their subsequent development in the system is discussed on the basis of local pH gradients and inverse pH-dependent trends in the soly. of carbonate and silica, which link their chem. in soln. and provoke coupled mineralization events. [on SciFinder(R)]
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