Article Cite This: J. Phys. Chem. C 2018, 122, 23554−23563
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Particle Aggregation Modifies Crystallization: Extending the Hierarchical Order of a Polycrystalline Material to the Macroscale Pamela Knoll and Oliver Steinbock* Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, United States
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ABSTRACT: Hierarchical architectures can transform inferior substances into high-performance materials. Although this effect is prevalent among biominerals, few examples in chemistry match the complexity of such ordered materials. One exception is the formation of “biomorphs”, which consist of co-aligned, crystalline nanorods that assemble into smoothly curved microstructures. These lifelike sheets, helices, and urns form when CO2 reacts with aqueous solutions of alkaline earth metals and silicate. For the case of SrCO3−silica biomorphs, micrometer-scale sheets form at the solution−air interface and begin to move due to convection with speeds of 1−10 μm/s. Short-range interaction via capillary forces induces the formation of clusters and eventually a single, centimeter-scale aggregate. Within this pattern, biomorphs are arranged into simple chains with some branching points yielding fractal dimensions of 1.2−1.3. Importantly, crystallization continues within the aggregate and diminishes internal motion. We show that the resulting structures are qualitatively different from those formed from solitary units and often include very large sheets connected to several biomorph units. Both aggregation and collective growth extend the hierarchical order to the centimeter scale and constitute a novel aspect of the system’s crystallization dynamics.
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INTRODUCTION The nontrivial extension of chemical order from the molecular to the macroscopic length scale is a profound scientific challenge and also offers intriguing engineering opportunities. The most prominent examples are the self-assembly of molecules, nanoparticles, and even macroscopic units into larger structures, such as colloidal crystals and virus capsules.1,2 These processes typically occur near the thermodynamic equilibrium and hence differ qualitatively from chemical and biological self-organizations that create complexities in both structure and dynamics far from the equilibrium. Control of these intriguing phenomena can be achieved by selecting structural features, such as specific binding sites,2 but also via the temporal “programming” of certain events. The latter option is clearly the less studied one, but can be expected to yield ultimately greater control over and variability in the resulting products. Key to this approach is the creation of hierarchical order akin to characteristics of certain biominerals or even more complicated biological systems. For example, Venus flower baskets (E. aspergillum) grow intricate siliceous skeletal systems with features ranging from nanoparticles to complex fibers and finally the 20−30 cm large glass cages.3 Other important materials include shells, bones, and teeth, all of which turn inferior and greatly abundant substances into high-performance materials.4−6 Considering these systems, the question arises whether only living organisms can accomplish these © 2018 American Chemical Society
sophisticated transformations or whether biology tightly controls processes that could also occur in the absence of life. To date, chemistry offers only few examples that suggest the latter. These examples tend to involve precipitation reactions, such as the classic “chemical garden” experiment, which creates hollow tubes at length scales between 10 μm to several millimeters.7−9 Equally intriguing, but lesser studied systems are so-called “biomorphs”, which are the focus of our study.10 Biomorphs are inorganic, polycrystalline structures with lifelike morphologies, such as leaf-shaped sheets, helices, urns, funnels, and coral-like structures.11−16 These smoothly curved, noneuhedral objects form when earth alkaline metal ions, silicate, and carbonate react in an aqueous solution or gel at high pH.10,17,18 The 10−500 μm large structures consist of coaligned nanorods with very high aspect ratios. These orthorhombic, single or twinned nanocrystals have a typical width of 5−50 nm, and their length can exceed 200 nm. The nanorods consist primarily of metal carbonates (e.g., BaCO3 in the form of witherite), but X-ray diffraction patterns reveal strain caused by silicon compounds19 that account for a few percent of the overall nanorod composition.20 Energydispersive X-ray spectroscopy (EDX) measurements of the Received: July 26, 2018 Revised: September 14, 2018 Published: September 19, 2018 23554
DOI: 10.1021/acs.jpcc.8b07212 J. Phys. Chem. C 2018, 122, 23554−23563
Article
The Journal of Physical Chemistry C
Figure 1. Biomorphs at the solution−air interface. (a) SEM image of a biomorph with downward extending micropillars. The lower surface, submersed in solution, exhibits a curved, flower-like appearance whereas the side exposed to the air is flat. (b) SEM image of a biomorph micropillar showing a multitude of co-aligned nanorods. (c, d) Optical micrographs recorded through crossed polarizers showing a forming aggregate after (c) 142 min and (d) 182 min of reaction. The yellow and cyan circles mark two globules for reference. Scale bars are (a) 20 μm, (b) 2 μm, and (c) 200 μm.
reaction system containing 5.0 mM strontium nitrate, 8.4 mM sodium metasilicate, and 0.15 mM sodium carbonate with the pH adjusted to a value between 10.6 and 11.0 by addition of 0.5 M HCl. All experiments are carried out at room temperature. Petri Dish Experiments. Solution volumes of 4.8 and 14.1 mL are transferred into polystyrene Petri dishes of 35 or 60 mm diameter, respectively. In both cases, the corresponding average solution height is 5 mm. A lid is placed on top of the dishes, but the system is not sealed. The rate of globule nucleation increases with increasing pH and decreasing volume of the solution. For analysis, biomorphs are extracted from the solution using a spatula, rinsed with deionized water, and transferred to a microscope slide. Channel Experiments. Experiments in thin, vertical channels are performed using an acrylic frame machined from 95 × 30 × 2 mm OPTIX acrylic sheeting (Plaskolite). A 65 × 20 × 2 mm U-shaped void is cut using a Roland SRM-20 desktop milling machine. For each experiment, two 25 × 75 × 1 mm glass microscope slides (VWR) are glued to each side to form a thin channel. The channel is filled to a solution height of around 5 mm (
