Evolution and Control of Complex Curved Form in Simple Inorganic Precipitation Systems

Abstract

Crystal architectures delimited by sinuous boundaries and exhibiting complex hierarchical structures are a common product of natural biomineralization. However, related forms can also be generated in purely inorganic environments, as exemplified by the existence of so-called "silicacarbonate biomorphs". These peculiar objects form upon coprecipitation of barium carbonate with silica and self-assemble into aggregates of highly oriented, uniform nanocrystals, displaying intricate noncrystallographic morphologies such as flat sheets and helicoidal filaments. While the driving force steering ordered mineralization on the nanoscale has recently been identified, the factors governing the development of curved forms on global scales are still inadequately understood. In the present work, we have investigated the circumstances that lead to the expression of smooth curvature in these systems and propose a scenario that may explain the observed morphologies. Detailed studies of the growth behavior show that morphogenesis takes crucial advantage of reduced nucleation barriers at both extrinsic and intrinsic surfaces. That is, sheets grow in a quasi-two-dimensional fashion because they spread across interfaces such as walls or the solution surface. In turn, twisted forms emerge when there is no foreign surface to grow on, such that the evolving aggregates curve back on themselves in order to use their own as a substrate. These hypotheses are corroborated by experiments with micropatterned surfaces, which show that the morphological selection intimately depends on the topology of the offered substrate. Finally, we demonstrate that, with the aid of suitable template patterns, it is possible to directly mold the shape (and size) of silica biomorphs and thus gain polycrystalline materials with predefined morphologies and complex structures.

Silica Biomorphs: Complex Biomimetic Hybrid Materials from “Sand and Chalk”

“…In the past few years, considerable progress has been made towards the understanding of the crucial stages in the morphogenesis of silica biomorphs, and a model was proposed [72,73] and elaborated [53,55,56,74,75] to explain the processes leading to structured mineralization on different length scales, which is consistent with most of the experimental observations. In the following, we will summarize the central aspects of this current picture, beginning with a phenomenological description of morphology development on the micron scale before addressing the discussion on the driving force for self-assembly at nanometric dimensions.…”

“…After a certain time, small laminar tongues begin to extrude from some of these objects ( Figure 13b); this step marks a transition in the mode of growth, as all subsequent morphologies are polycrystalline aggregates consisting of the uniform nanosized building units described above (cf. [75] Copyright 2012 American Chemical Society. [75] Copyright 2012 American Chemical Society.…”

New insights into the early stages of silica-controlled barium carbonate crystallisation

“…[5][6][7] Such studies are typically inspired by natural biomineralisation, where an organic matrix controls the crystallisation of inorganic matter to produce superior hybrid structures. [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] Under the influence of dissolved silicate as a crystallisation modifier, metal carbonates (usually BaCO 3 ) can indeed assemble spontaneously into a range of sinuously shaped non-crystallographic aggregates (such as regular helicoids), which mimic products from biomineralisation closely in terms of morphology and internal hierarchy, and therefore were termed "silica-carbonate biomorphs". Among these, the processes observed during precipitation of alkaline-earth carbonates into alkaline, silica-containing media are probably the most prominent and well-studied case.…”

“…Among these, the processes observed during precipitation of alkaline-earth carbonates into alkaline, silica-containing media are probably the most prominent and well-studied case. 17,24,27 After a recent surge of interest in this field, [18][19][20][21][22][23][24][25][26][27][28][29][30][31] it has been proposed that the formation of biomorphs is driven by an autocatalytic co-precipitation cycle, in which the components are alternately mineralised due to their opposite trends of solubility with pH. 16,32,33 The structural complexity of these peculiar materials relies on the fact that they are constituted of a multitude of uniform carbonate nanocrystals, which are largely co-oriented and thus generate mesoscopic order in the mature aggregates (as a third level of hierarchy in addition to molecular ordering on the nanometrescale and morphological control on the micron-scale).…”

“…Indeed, such core-shell structures could be experimentally verified for hybrid nanoparticles of amorphous calcium carbonate (ACC) 34 or barium carbonate 35 and silica produced at high supersaturation. 17,[24][25][26][27][28]44,45 During addition, the Ba 2+ potential was recorded in situ with the aid of an ion-selective electrode, while the pH was kept constant by back-titration with sodium hydroxide. 24 Consequently, the detailed role of the silica species in the stabilisation of BaCO 3 nanoparticle units during growth of biomorphs has not yet been clarified.…”