Article pubs.acs.org/cm
Bottom-Up Self-Assembly of Amorphous Core−Shell−Shell Nanoparticles and Biomimetic Crystal Forms in Inorganic Silica− Carbonate Systems Josef Eiblmeier,† Matthias Kellermeier,*,‡ Mao Deng,§ Lorenz Kienle,§ Juan Manuel García Ruiz,∥ and Werner Kunz*,† †
Institute of Physical and Theoretical Chemistry, University of Regensburg, Universitätsstrasse 31, D-93040 Regensburg, Germany; Physical Chemistry, University of Konstanz, Universitätsstraße 10, 78457 Konstanz, Germany; § Technical Faculty, Christian-Albrechts-University Kiel, Kaiserstrasse 2, D-24143 Kiel, Germany; ∥ Laboratorio de Estudios Crystalográficos, IACT (CSIC-UGR), Av. de las Palmeras 4, E-18100 Armilla, Spain ‡
S Supporting Information *
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 onepot route, which is based on self-organization of simple components and may be exploited for the design of novel advanced materials. KEYWORDS: core−shell nanoparticles, barium carbonate, silica, biomimetic self-assembly, biomorphs, layer-by-layer mineralization, amorphous precursor phases
1. INTRODUCTION
size and polymorphism of crystallites, and trigger concerted aggregation of stabilized particles. In the past years, research on biomimetic mineralization has demonstrated the existence of alternative crystallization pathways where, unlike the classical
One of the main goals of material science is to find new processing routes for the fabrication of advanced materials. A frequently used approach to achieve this is to mimic strategies applied by Nature to produce materials with hierarchical structure and superior properties. Living organisms control their crystallization processes by using certain biopolymers and functional matrixes,1−4 which may direct nucleation, regulate the © 2013 American Chemical Society
Received: January 31, 2013 Revised: March 26, 2013 Published: April 1, 2013 1842
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the case of the ACC particles, stabilization of nanosized building units is accomplished by pH-induced polymerization of silica all over their surface, preventing further growth and ripening. However, in contrast to silica coating of ACC (where all available carbonate is consumed during the initial precipitation step),35 the interplay between silica and carbonate is not terminated at this point as further material for growth is continuously provided by diffusion processes in the experiments (e.g., via uptake of atmospheric CO2).48 Consequently, new carbonate nucleation takes place, and these nucleation events indeed occur in the vicinity of previously formed crystallites because ongoing silica polymerization around them now increases the local pH, as acidic silanol (−Si−OH) groups vanish in the course of condensation. This leads to an increase in the local supersaturation of carbonate and eventually triggers nucleation of new BaCO3 nanoparticles, which again are covered by silica due to local pH gradients. The whole process is then re-iterated such that, overall, the pH at the growth front oscillates up and down, causing carbonate and silica to be alternately mineralized due to their reversed solubility trends with pH.36,45,55 In this way, elaborate ultrastructures of silica-decorated carbonate nanocrystals are generated by dynamic bottom-up self-assembly of simple components. Here we show that by slightly changing the experimental protocol, co-mineralization of barium carbonate and silica can be tuned to afford complex core−shell−shell particles in a straightforward one-pot synthesis. In particular, we have precipitated BaCO3 by mixing BaCl2 and Na2CO3 at nominally high supersaturation and in the presence of varying amounts of sodium silicate. As for calcium carbonate,35 sufficiently high silica concentrations lead to stabilization of amorphous nanoparticles, which either persist in solution or serve as precursors for the formation of biomorphic ultrastructures. However, unlike the CaCO3 case, these nanoparticles are found to consist of a silica core, which is surrounded by a layer of barium carbonate that in turn is coated by an outer skin of silica.
picture, crystal growth proceeds via (oriented) attachment of primary nanoparticle units,5−7 potentially leading to complex ultrastructures of small building blocks.8−10 For example, it was shown that synthetic organic polymers, capable of interacting with the as-formed particles, can induce self-organization of the building units on the mesoscale and prevent their fusion to a single crystal. This has been realized for a wide variety of minerals,11 including calcium carbonate12 and barium carbonate,13,14 with the aid of tailored block copolymers. Among the carbonate minerals, CaCO3 is the most intensively studied one as it represents the major inorganic constituent in many biogenic materials. During biomineralization of CaCO3, its metastable amorphous form (ACC) was often found to serve as a solid precursor for the mature structures, as for instance in the case of the spicules, spines, and teeth of sea urchins.15−17 The absence of crystalline order appears advantageous because amorphous matter can be molded into virtually any shape, which ideally will remain after crystallization and thus yield crystal morphologies far beyond euhedral symmetry restraints. While the temporary occurrence of ACC in vivo seems to be regulated by certain proteins (often in combination with magnesium ions)18−20 or via entrapment in vesicles,21,22 successful stabilization in synthetic systems was achieved by introducing organic additives like amphiphilic dendrimers,23 block copolymers,24 polycarboxylates,25,26 polyphosph(on)ates,27−29 and others.30−34 Recent studies have shown that the transformation of ACC into crystalline polymorphs can also be controlled in purely inorganic systems, namely by using sodium silicate as a soluble additive in alkaline reaction media.35,36 Under these conditions, ACC particles are kinetically stabilized because silica precipitates spontaneously on their surface, forming a skin that impedes re-dissolution. Coating of ACC by silica was ascribed to a local decrease of pH caused by dissociation of bicarbonate ions upon growth of CaCO3 in alkaline solutions (Ca2+ + HCO3− → CaCO3 + H+). This provokes successive condensation, polymerization, and precipitation of silica nearby the ACC surface (SiO(OH)3− + H+ → Si(OH)4 → “SiO2” + 2H2O), leading to composite particles with a carbonate core and a silica shell. Furthermore, the degree of ACC protection was found to depend on the amount of added silica, ranging from temporary stabilization over hours to permanent conservation in the long term.35 These findings are consistent with the results of independent studies on the role of the silica content for the stability of the ACC component in biomineralized plant cystoliths.37,38 pH-based interactions between growing carbonates and dissolved silicate species are however not limited to the formation of amorphous core−shell particles as described above, but can also be used to assemble nanosized crystalline units into complex meso- and micrometer-scale architectures. This is documented in great detail for the higher homologues in the alkaline-earth carbonate series, i.e., SrCO3, and in particular BaCO3.36,39−55 Indeed, gradual crystallization of these minerals in silica-rich media can yield unique non-crystallographic morphologies, which self-assemble if the supersaturation is kept at moderately high levels over extended periods of time. These so-called silica biomorphs consist of uniform carbonate nanorods, arranged according a specific long-range orientational order on the mesoscale.48,50,52,53 Crystal aggregates exhibiting a related mode of construction, but different morphologies, were obtained also for calcium carbonate.56,57 The morphogenetic mechanism underlying the formation of these complex materials has been unraveled recently:36,45,50,55 it was suggested that, as in
2. RESULTS 2.1. Fractal Branching at Low Silica Concentrations. Under the chosen conditions, direct mixing of barium chloride and sodium carbonate in the absence of silica results in the immediate formation of uniform rod-like crystals (Figure 1a), typically several micrometers in length and showing the pseudohexagonal habit characteristic of twinned witherite (BaCO3).41 Rapid growth of these crystals is evident from an instant clouding of the solutions upon combining reagents (see Figure S1 in the Supporting Information (SI)), indicating homogeneous nucleation of BaCO3 at the given high supersaturation. The fact that highly crystalline material is developed in the system within seconds is in sharp contrast to the behavior of calcium carbonate which, under the same conditions, precipitated in the form of spherical ACC nanoparticles that persisted in solution up to about one hour.35 This suggests that amorphous BaCO3 precursors are much more transient than their CaCO3 counterparts. Addition of silica at concentrations ranging from 135 to 750 ppm does not retard crystallization to a large extent (again unlike observations made for CaCO3),35 as crystalline precipitates were deposited at the bottom of the vials already after few minutes (see Figure S1 of the SI). Solids formed in this range of silica contents were all confirmed to be witherite by means of X-ray diffraction (Figure S2 of the SI). However, the presence of silicate species in the solutions provokes significant morphological changes in the final crystal products (Figure 1b−f). 1843
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silica content in the precipitates obtained at 540 and 750 ppm SiO2 is much higher (Si/Ba ≈ 0.3 and 0.4, respectively) and close-up views of their structure reveal that they are composed of numerous co-oriented nanoparticles (Figure S4 of the SI). This suggests that growth of these forms proceeds via assembly of small building units (as in the case of silica biomorphs) and, hence, that they are nanocrystalline aggregates rather than bifurcated single crystals. This scenario is supported by XRD patterns of the samples at 540 and 750 ppm SiO2 (cf. Figure S2 of the SI), where marked broadening of diffraction peaks is observed, indicating the presence of small primary particles. The high Si/Ba ratios detected for the assemblies may originate from coating of individual BaCO3 units in skins of silica, as proposed for the building blocks of silica biomorphs.45,50,55 In any case, it is worth noting that aggregates with such complex structure and composition can obviously be grown in a fast and efficient manner simply by mixing reagents at high supersaturation. 2.2. Core−Shell−Shell Nanoparticles at High Silica Concentrations. When the silica concentration is increased to 920 ppm, an initially clear solution is obtained that turns turbid after few minutes. With time, the precipitated material is deposited at the bottom of the vial (SI Figure S1), where it forms a gel-like sediment (rather than being crystalline). At a still higher silica content of 1870 ppm, clouding of the solution sets in instantaneously, but there is no longer any distinct settling of the precipitates discernible, indicating that generated particles remain dispersed in the solution. Particles present in the solutions were first characterized by means of transmission electron microscopy (TEM). Images taken from samples isolated right after mixing disclose fundamental differences between precipitates formed at 920/ 1870 ppm SiO2 and the crystals obtained at the same time for lower silica contents (Figure 2a,b). Indeed, nanoparticles with more or less distorted spherical shapes are observed, which tend to build interconnected networks owing to agglomeration and merging of primary grains. The degree of aggregation seems to be higher at 920 ppm SiO2 (Figure 2a) than at 1870 ppm (Figure 2b), as is confirmed quantitatively by dynamic light scattering measurements (see below). The average diameter of the particles was found to be 100 ± 15 nm, independent of the silica concentration. Electron diffraction (ED) patterns collected from the nanoparticles further demonstrate that they are amorphous (insets in Figure 2a,b), although reflections corresponding to witherite could be identified in few cases (Figure S5 of the SI); however, the presence of crystalline BaCO3 might well result from transformation of amorphous domains under the influence of the electron beam at high resolution. The composition of these early stage particles was further analyzed using EDX and infrared spectroscopy. While EDX profiles show that the precipitates contain both Ba and Si (Figure 2c), IR spectra evidence the coexistence of silica and barium carbonate (Figure 2d). Bands at 1045, 781, and 460 cm−1 can be assigned to amorphous silica (corresponding to stretching, bending, and rocking vibrations of the SiO 2 network, respectively), whereas the peak at 1643 cm−1 originates from water molecules entrapped in the siliceous matrix.61 For crystalline witherite, sharp IR bands at 694 (υ4-mode), 857 (υ2), 1065 (υ1), and 1449 cm−1 (υ3) would in principle be expected.62 However, the collected spectra display a split υ3-peak (1420/1450 cm−1), and the υ4-band at about 700 cm−1 is entirely absent (arrow in Figure 2d; the υ2-band is seen as a shoulder at 865 cm−1, whereas the υ1-band is hidden by the strong silica
Figure 1. FESEM images of BaCO3 crystals formed upon direct mixing of BaCl2 and Na2CO3 in (a) the absence of silica and the presence of (b) 135, (c) 270, (d) 370, (e) 540, and (f) 750 ppm SiO2. Precipitates were isolated after 24 h of aging. Arrows in (c) mark the onset of crystal splitting at 270 ppm silica. Scale bars are 2 μm.
While crystals grown at 135 ppm SiO2 still exhibit rod-like habits with no marked variation in size or shape (Figure 1b), increasing the silica content to 270 ppm induces splitting of the elongated crystal at their tips, where tilted projections emerge at non-crystallographic angles from the parent rod (indicated by red arrows in Figure 1c). This effect becomes more and more pronounced as the silica concentration is further increased (Figure 1d−f), and it is evident that branching proceeds in a selfsimilar manner, i.e., the projections split by themselves and produce second-generation outgrowths. This fractal branching pattern leads to the formation of dumbbell-shaped morphologies at 540 ppm SiO2 (Figure 1e) and finally gives rise to spherulitic, cauliflower-like structures with diameters around 20 μm at 750 ppm (Figure 1f). Thus, addition of silica induces progressive breaking of crystallographic symmetry, from initial pseudohexagonal rods toward increasingly bifurcated architectures. Similar growth behavior has been observed in the formation of silica biomorphs, where such fractal entities were found to be precursors of the more complex morphologies,45,50 as well as during the crystallization of fluorapatite in gelatin matrixes.58−60 The structure evolution seen in the present experiments with increasing silica concentration therefore reflects the morphological development displayed by biomorphs as a function of growth time (over hours). It has been suggested that branching of witherite crystals results from growth poisoning by oligomeric silicate species, which act as nonabsorbable impurities and generate misoriented 2D islands on the basal {001} faces of the crystal.45,50 EDX analyses show that the amount of silica incorporated into the crystals is fairly low (Si/Ba atomic ratio
