Bottom-Up Self-Assembly of Amorphous Core–Shell–Shell Nanoparticles and Biomimetic Crystal Forms in Inorganic Silica–Carbonate Systems

Abstract

Mineralization of alkaline-earth carbonates in silica-rich media at high pH leads to fascinating crystal morphologies that strongly resemble products from biomineralization, despite the absence of any organic matter. Recent work has demonstrated that elaborate CaCO3 structures can be grown in such systems even at high supersaturation, as nanoparticles of amorphous calcium carbonate (ACC) were spontaneously coated by skins of silica and thus served as temporary storage depots continuously supplying growth units for the formation of crystalline calcite. In the present study, we have precipitated barium carbonate under similar conditions and found surprisingly different behavior. At low silica concentrations, there was no evidence for an amorphous carbonate precursor phase and crystallization 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 crystallization was in turn prevented and amorphous nanoparticles were stabilized in solution. However, in contrast to previous observations made for CaCO3, the particles were found to be 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. These self-assembled core–shell–shell nanoparticles were characterized by different techniques, including high-resolution transmission electron microscopy and elemental analyses at the nanoscale. Time-dependent studies further evidence that the carbonate component in the particles can either be permanently trapped in an amorphous state (high silica concentrations, leading to impervious outer silica skins), or be released gradually from the interstitial layers into the surrounding medium (intermediate concentrations, 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 solubility of carbonate and silica, which link their chemistry in solution and provoke coupled mineralization events. Our findings depict a promising strategy for the production of multilayered nanostructures via a facile one-pot route, which is based on self-organization of simple components and may be exploited for the design of novel advanced materials.