Evolution and Control of Complex Curved Form in Simple Inorganic Precipitation Systems Matthias Kellermeier, †,⊥ Josef Eiblmeier, † Emilio Melero-García, ‡ Melanie Pretzl, § Andreas Fery, § and Werner Kunz* ,† † Institute of Physical and Theoretical Chemistry, University of Regensburg, Universitä tsstrasse 31, D-93040 Regensburg, Germany ‡ Laboratorio de Estudios Crystalogra ́ ficos, IACT (CSIC-UGR), Avda. del Conocimiento s/n, P.T. Ciencias de la Salud, E-18100 Armilla, Spain § Physical Chemistry Department, University of Bayreuth, Universitä tsstrasse 30, D-95445 Bayreuth, Germany * S Supporting Information 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 “silica- carbonate 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. 1. INTRODUCTION In recent years, interest in polycrystalline aggregates comprising ordered arrays of miniaturized building blocks has been fuelled due to their relevance for diverse fields of research. Archetypes for these materials frequently occur in the course of biomineralization, where concerted assembly of small crystal components into higher-order architectures is a well-established strategy to produce inorganic matter with complex morphol- ogy, hierarchical structure, and superior properties. 1 This mode of construction is facilitated by the beneficial influence of an organic matrix on the crystallization process, which may control nucleation and growth of the crystallites, enable their stabilization, and direct oriented attachment of individuals. 2 Modern approaches of morphosynthesis are often meant to mimic the concepts observed in vivo, so as to design novel materials and at the same time shed light on the physicochemical principles underlying the biological process. 3 In particular, crystallization reactions involving a nonclassical pathway, along which crystal growth is driven by aggregation of nanoparticle units instead of ion-by-ion addition, 4 were found to be readily modified under certain conditions and thereby exploited to generate manifold superstructures. 5 For this purpose, organic additives are usually employed that are capable of interacting with the particles, stabilizing them at colloidal dimensions and thus preventing fusion to continuous single crystals. In this case, self-organization of the as-formed building blocks on the mesoscale, guided to a greater or lesser extent by the chemistry of the adsorbed additive, can yield in polycrystalline assemblies with delicate morphologies free of crystallographic symmetry, 6 especially when the primary units are featured by high shape anisotropy. 7 Such biomimetic modulations of classical crystallization scenarios were realized successfully, among many others, for calcium and barium carbonate with the aid of specialized block copolymers. 8 Received: April 5, 2012 Revised: May 31, 2012 Published: June 6, 2012 Article pubs.acs.org/crystal © 2012 American Chemical Society 3647 dx.doi.org/10.1021/cg3004646 | Cryst. Growth Des. 2012, 12, 3647-3655