Formation of Aragonitic Layered Structures from Kaolinite and

Mar 15, 2013 - Clay materials have been an ever-present accoutrement of modern civilization; improvements to process these materials have quickened th...
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Formation of Aragonitic Layered Structures from Kaolinite and Amorphous Calcium Carbonate Precursors Jong Seto,*,† Thierry Azaı̈s,‡ and Helmut Cölfen† †

Physical Chemistry, Department of Chemistry, Universität Konstanz, Universitätstraße 10, 78457 Konstanz, Germany Laboratoire Chimie de la Matière Condensée de Paris, UMR 7574, UPMC Université Paris 6, CNRS, Collège de France, 11 place Marcelin Berthelot, 75005 Paris, France

‡

S Supporting Information *

ABSTRACT: Clay materials have been an ever-present accoutrement of modern civilization; improvements to process these materials have quickened their utilization for use in complex multiaxial load-bearing structures. Specifically, with better methods to organize the constituent metal oxide components in clay, the distribution of characteristic nematic and smectic phases can be controlled. In this work, we utilize the interactions of an amorphous calcium carbonate phase with kaolinite to form a complex composite that can be organized into distinct hierarchical structures. We demonstrate that these ACC− kaolinite composites can maintain characteristic long-range-ordered layer-by-layer structures across many length scales, from nano- to millimeter, through convenient and economical processing at room temperature.

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silicate hydrate (C−S−H) is well known.7 In combination with traditional cementious filler materials for concrete, C−S−H is able form a coherent, ordered concrete material for multiaxial load-bearing applications. However, the mechanism by which C−S−H is able to order and adhere to concrete filler particles and its primary interaction in these cementious materials remain unknown. Another application is the formation of layered polymer composites with smectic clays.8 It is well established that silicate-based materials are invaluable for today’s composite and structural materials found in numerous forms of clays and ceramics.4 Among well-known silicate-based clays used by the ton are montmorillonite and kaolinite. Montmorillonite, a well-known smectite clay with a single layer of octahedral alumina sandwiched in between by two tetrahedral adjacent layers of silicate, has a high adsorption capacity for polar charged molecules. It is well known for its characteristic swelling when exposed to hydration or other polar solvents. Specifically, montmorillonite possesses an aptitude for a high cation exchange capacity (CEC) on the order of 1000 mequiv of positive charge per kilogram of clay material.9,10 This simply means that the potential to retain cations such as H+ in montmorillonite clays is such that it is on the order of 1 mM per gram of clay. As a result, higher CEC occurs in the presence of lower pH, and lower CEC occurs in higher-pH regimes. This high capacity to retain cations is also

INTRODUCTION Through the use of clay materials, organized load-bearing structures can be assembled by bulk processing. These methods have included baking, kilning, and air drying, which are all various forms of dehydration. The dehydration process in clay materials plays two prominent roles in the formation of these materialsthe removal of a hydration component that primarily enabled a dispersion of reacting elements in solution and the orientation and formation of constituent materials into bulk materials that can withstand loading in compression. Furthermore, even after these dehydration processes, the constituent clay materials continue to be reactive and interact with various compounds in the environment because of their innate electrostatic surface behaviors and intermolecular organization. These are the characteristic behaviors that many groups have attempted to utilize to capture heavy metal pollutants in aquafiers and recently for use in carbon capture strategies.1−3 More specifically, many of these aforementioned clays are composed of silicate components, the most abundant geological mineral on earth, in either the main tetrahedron or octahedron form. The silicate constituents are responsible for the shortrange electrostatic interactions on which orthosilicate-, sorosilicate-, and phyllosilicate-based sheets are formed.4 Specifically, in the tetrahedral form the silicate component is able to form negative surface charges to create hydration layers as well as other binding partners on its surface, enabling for the diverse organization of the silicate subunits found in silicate minerals.5 Their natural abundance and surface reactivity make silicate-based minerals ideal for creating composite materials.6 Its application as a binder for cements in the form of calcium © 2013 American Chemical Society

Special Issue: Interfacial Nanoarchitectonics Received: January 31, 2013 Revised: March 14, 2013 Published: March 15, 2013 7521

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materials. In this case, the ACC component was present in excess and functions as a binder that allows for ACC−kaolinite interactions as well as ACC−ACC interactions to occur. The resulting mixture was then lyophilized for over 24 h. The raw kaolinite was provided by Lafarge Inc. and not produced synthetically in the laboratory. Kaolinite Samples without ACC. Raw kaolinite (kaolinite argile BS 3 (MCC-5-2002/02), Lafarge Inc., Saint Quentin Fallavier, France) was dispersed in doubly distilled H2O and allowed to air dry on SEM/ TEM/optical microscopy sample holders. ACC−Kaolinite Hybrid Composite Dispersions in Water. Four 1.5 mL Eppendorf tubes (Eppendorf AG, Hamburg Germany) each containing 1 mg of an ACC−kaolinite hybrid composite sample were filled with doubly distilled H2O such that the final volume in each tube was 1.0 mL. One of the four tubes was immediately spun in a microcentrifuge (centrifuge 5415R, Eppendorf AG, Hamburg Germany) at 9000 rpm for 2 min, the H2O was siphoned off, and the precipitate was allowed to air dry. Subsequently, the same centrifuge and air-drying procedure was performed on the remaining samples 2, 5, and 14 days after the initial exposure to H2O. The airdried samples were then examined under SEM and TEM. Polarized Light Microscopy. An optical microscope (Zeiss Imager M2m, Zeiss GmbH, Jena Germany) with polarized objectives of 5−100× with a Z automatic stage was used to analyze the samples. Samples were spread onto a glass slide and imaged in polarized reflective mode. Z slices were taken every 100 nm, and the slices were averaged over 1 mm to obtain convoluted images. Images were recorded digitally with a manufacturer-supplied digital camera (Zeiss mr5c, Zeiss GmbH, Jena, Germany). Zeta Potential Measurements. A zeta potential setup (Nano ZS Zetasizer, Malvern Instruments Ltd., Worcestershire, United Kingdom) was used to measure the zeta potential distribution at room temperature. Each measurement was performed with a 12 mm green, disposable square polystyrene zeta potential cuvette and was an average of 12 zeta runs. Subsequent data analysis and evaluation were performed with Zetasizer version 6.20 software (Malvern Instruments Ltd., Worcestershire, United Kingdom). SEM. Air-dried samples at room temperature were placed onto double-sided carbon tape for microstructural analysis. A desktop SEM system (Hitachi TM-3000, Hitachi High-Technologies Europe GmbH, Krefeld, Germany) at 15 kV was used to examine the morphology of crystalline components. Additional EDX analyses were performed with a solid-state EDX detector (Xcite detector, Bruker AXS GmbH, Berlin Germany) attached to the SEM setup. TEM. Samples were divided into aliquots on 400-mesh copper grids coated with a carbon film (Quantifoil Micro Tools GmbH, Jena, Germany) and allowed to air dry under clean-room conditions at the Nanostructure Laboratory of the University of Konstanz. The sample grids were imaged with an in-lens column filter TEM (Zeiss Libra 120, Zeiss SMT GmbH, Oberkochen, Germany) at 120 kV with a 1 mrad magnification series (8−100 kX) to examine the micro- and substructures of each sample. AFM. An AFM imaging setup (Nanowizard, JPK Instruments AG, Berlin, Germany) with silicon nitride AFM tips (Olympus Corp., Tokyo, Japan) was used to scan sample surfaces in contact mode. Using scan rates of 10 μm/s and the in-phase imaging mode, 100 × 100, 50 × 50, and 30 × 30 μm2 areas of the samples were imaged. ATR-FTIR. A standard ATR-FTIR spectrophotometer (Spectrum 100, Perkin-Elmer Life and Analytical Sciences, Bridgeport, CT, USA) was used to obtain spectra between 4000 and 650 cm−1 for each sample. The spectra were analyzed with a spectrum analysis program (Spectrum version 6.2.0, Perkin-Elmer Life and Analytical Sciences, Bridgeport, CT, USA) supplied by the FTIR manufacturer. XRD. A powder X-ray diffractometer (D8 Advance, Bruker AXS GmbH, Berlin, Germany) was used to measure the crystalline reflections of the samples with a scan time of 1 h each and a scan range of 2θ ≈ 10−80°. SAXS. A laboratory source SAXS/WAXS system (Nanostar, Bruker AXS GmbH, Berlin, Germany) with a copper anode generator operating at 40 kV and 35 mA measured samples for 1 h. Then data analysis and integration (both radial and azimuthal) of the SAXS

dependent on the exchangeable ion present in the clay. This property also facilitates the amount of hydration present in the intermolecular layers of the aluminum silicate platelets. By utilizing these mechanisms found in montmorillonite, several groups have attempted to optimize the CEC of montmorillonite under different conditions to intercalate heavy metals as well as toxic molecules in high concentrations into the interlayers of montmorillonite.11,12 Others more recently have simply attempted to functionalize this interaction with montmorillonite to tune the adsorption of specific molecules and nanoparticles.13−16 However, in the same respect that makes montmorrillonite a good material for adsorption and molecular retention, these same properties also inhibit the use of montmorrillonite for reaction chemistry because of the slow exchange rates as a result of the high CEC. In contrast, kaolinite having 2 orders of magnitude reduced CEC compared to that of montmorrillonite is a low shrink/ swell mineral often used as a filler component.9 Kaolinite is characterized by its hexagonal plates and phase transformations at high temperature under atmospheric conditions. Having subunits in the same structural configuration as that of montmorrillonite, it is a layered silicate−alumina−silicate whereby electrostatic interactions are also predominant on the surface.17 Because of these interactions, kaolinite is a reactive substrate of focus for many groups to bind or modify with other materials in forming unique composite structures that are often utilized in blood-clotting and paper-coating applications.18,19 It has been recently established that silica is responsible for the stabilization of “biomorphic” outgrowths found in geology.20 In an extension of this silica work, silicates similarly stabilize amorphous mineral phases traditionally known to be transient and to undergo quick amorphous-tocrystalline transformations.21−23 In this work, through the novel synthesis and addition of an amorphous calcium carbonate phase at room temperature in the presence of kaolinite, we find that the kaolinite is able to serve as a substrate that templates and subsequently reacts with the ACC phase to induce the crystallization of a preferred aragonitic polymorph of calcium carbonate. We demonstrate that the ACC−kaolinite interactions occur on short length scales but have ramifications for higher length scales, enabling the possibility to construct bulk materials that can be aligned and oriented for specific high loadbearing applications.

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MATERIALS AND METHODS

Synthetic ACC Production. Stable amorphous calcium carbonate was mass produced by the fast and continuous mixing of a mixture of 2 M CaCl2 + 2 M MgCl2 and 2 M Na2CO3 with two HPLC pumps (LC10AS liquid chromatograph, Shimadzu Co., Kyoto, Japan) connected via a Y mixer with a 200-μm-diameter hole at the junction of the two flowing streams with a flow rate of 5 mL/min (SI Figure 1). This is similar to a technique described by Volkmer and co-workers used to apply thin films as coatings.24 The ACC was washed several times in ethanol and subsequently in CaCO3-saturated doubly distilled H2O to get rid of the excess sodium chloride. Afterward, the ACC was spun down in a centrifuge for 5 min. The remaining supernatant was decanted, and the ACC fraction was recovered. While it was still in its aqueous form, the kaolinite component was added and rapidly mixed until the mixture was a homogeneous suspension of ACC and kaolinite. Using 1 mg of kaolinite argile (kaolinite argile BS 3 (MCC-52002/02), Lafarge Inc., Saint Quentin Fallavier, France), we added the aluminosilicate additive to the ACC and vigorously shook it to create a homogeneous suspension to obtain a final kaolinite argile concentration of 1 mg/mL with a pH of 10.1. This mixture was allowed to air dry, resulting in the aforementioned clay−calcium carbonate hybrid 7522

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patterns were accomplished using Fit2D (AP Hammersley, ESRF Internal Report, ESRF98HA01T, FIT2D V9.129 Reference Manual V3.1, 1998). TGA and DTA. Approximately 25−30 mg of sample was weighed and used for thermoanalysis in a standard thermogravimetric analyzer (STA 429, Netzsch GmbH, Selb, Germany) over a temperature range of 25−950 °C. Both TGA and DTA analyses of all samples were carried out under O2. Solid State NMR. The solid-state NMR measurements were performed on an Avance 300 Bruker spectrometer (7.0 T) using a 4 mm double-resonance MAS probe head. Samples were packed into a 4 mm (o.d.) NMR zirconia rotor and spun at a MAS frequency of νMAS = 14 kHz. The recycle delay in the 1H single-pulse experiment was 5 s. The 1H NMR spectra were referenced (δ = 0 ppm) to TMS.

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RESULTS AND DISCUSSION

Crystalline kaolinite plates have the capability to assemble along specific planes because of weak electrostatic and hydrophilic forces found on their [001] basal plane surfaces.18 Several groups have implicated the assembly process as a result of the surface charge density, suggesting that the charge densities of kaolinite plates are very dependent on the pH.25−28 In a similar fashion, negatively charged amorphous calcium carbonate (ACC, time-dependent point of zero charge at around pH 9.529) is attracted to these same kaolinite surfaces at pH 10.1 via cation bridging (ζ potentialACC = −6.33 mV) as a result of their high charge and hydration densities, enabling the diffusion and adsorption of ACC onto the kaolinite surfaces to occur more quickly than the assembly of kaolinite−kaolinite plates. A similar behavior of cation bridging specifically with Ca2+ has been observed in highly negatively charged C−S−H binding at high Ca2+ concentration.30 Thus, the ACC−kaolinite interaction inadvertently mediates an alternating layer-by-layer structure of ACC and kaolinite. As observed under polarized light microscopy, kaolinite platelets assemble with immediately adjacent platelets and demonstrate partial aggregation as well as a degree of polydispersity. In contrast to this kaolinite-plateletonly scenario, the addition of ACC to kaolinite platelets enables aggregation into independent, larger aggregate species (>100 μm) such that the constituents of kaolinite and ACC are organized along specific orientations (SI Figure 2). This orientation can be qualitatively observed under polarized light microscopy where kaolinite−kaolinite assemblies occur over 100−400 nm and ACC−kaolinite assemblies are seen to form over 400−1000 nm length scales (not shown). By examining these samples more closely under scanning electron microscopy (SEM), it can be confirmed that the kaolinite-only samples demonstrate a short-range order such that stacks of immediate neighboring kaolinite plates accrue together. This is in contrast to the ACC−kaolinite composite fractions where aggregates show orientation and assembly on a larger length scale as well as a connection of the kaolinite nanoplates so that they become undistinguishable in SEM (Figure 1). In comparison to kaolinite-only aggregation, the ACC− kaolinite samples demonstrate stacks that are thicker and cover a longer length scale. This observation implicates the ACC component as a surface-functionalizing agent such that adjacent kaolinite platelets are more prone to aggregate together with their {001} faces to result in the formation of larger species. Interestingly, the ACC found in the interplatelet regions cannot be detected by techniques such as ATR-FTIR and SAXS most likely because of the thickness of the samples (SI Figures 4 and 5). However, the attenuated signals from these methods most

Figure 1. Scanning electron microscopy of kaolinite and ACC + kaolinite hybrid composite samples. (A) Individual kaolinite platelets can be seen to be clustered together and stacked along the basal plane. (B) In the presence of ACC, kaolinite platelets appear to aggregate such that individual platelets can no longer be delineated and appear as one large complex (scale bars = 200 nm).

likely measure only ACC found on the surface of these hybrid composites. By approximations with EDX measurements, it is found that CaCO3 comprises roughly 88 and 12 atom % kaolinite (SI Figure 11). This concurs with the 87 atom % CaCO3 and 13 atom % kaolinite found from bulk 1H solid-state NMR (SI Figure 9). From Figure 1A, each platelet of kaolinite can be well defined and delineated. The platelets are approximately 100−400 nm in diameter and appear to be pseudohexagonal in morphology. In contrast, Figure 1b shows ACC−kaolinite species to be 100 nm−2 μm in diameter and assume a platelike clustering along the [001] face of each plate. This structuring at lower length scales lends itself to further organization on the macroscale (SI Figures 2 and 3). From transmission electron microscopy (TEM), the stacking of the kaolinite platelets occurs such that each stack contains two to four platelets in the kaolinite-only situation (Figure 2), with each platelet being approximately tens of nanometers thin because of the penetration of the electron beam and previous reports in the literature.17 Unlike the kaolinite-only samples, the ACC−kaolinite samples are observed to stack similarly such that the platelets are organized parallel to ⟨001⟩ but are also laterally organized such that neighboring stacks of platelets form a uniform superaggregate (Figure 2E). From the electron diffraction (ED) patterns, the kaolinite-only samples can be seen to display the orientational polydispersity observed in polarized light microscopy. Interestingly, this orientational polydispersity in the ab plane can also be witnessed in the ACC−kaolinite aggregates, suggesting that the ACC glues the kaolinite platelets together but does not lead to a perfect 3D mutual orientation of the platelets to a mesocrystal.31 In the ED pattern of the ACC−kaolinite sample, in addition to an amorphous halo indicating the existence of an ACC phase, the formation of an aragonitic structure can be observed and assigned, including the [001] zone axis typically found in aragonitic structures (Figure 2F). PXRD also confirms the existence of an aragonite species in the ACC−kaolinite 7523

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pH, it was determined that the surface potentials and charge densities dramatically decrease with increasing pH, possibly explaining the predominant attractive forces between kaolinite plates observed in high pH ranges.27,28,32 From the native charge densities on the kaolinite plate surfaces and through weak counterions such as Ca2+ and Mg2+ found in solution mediating the interaction, the platelets of kaolinite can accrue via a platelet-by-platelet fashion. Through the limited accumulation of these counterions on the surface, long-range stacking is limited, resulting in a rough aggregate surface, whereas the surface of the ACC−kaolinite hybrid material is smooth on the nanometer scale (SI Figure 7). Furthermore, these electrostatic interactions are very pH sensitive, and because of fluctuations in solution pH, the charge densities can also vary and result in various assemblies.17,28 When the solution pH of the forming ACC and kaolinite composite was measured, it was found that the ACC−kaolinite hybrid composites assemble in a pH 10.1 environment, suggesting that kaolinite−kaolinite plate stacking is the initially preferred orientation in the presence of ACC. This can be confirmed by both SEM and TEM, whereby longer-range aggregation and stacking can be observed when an ACC component is added to kaolinite (Figure 2). Specifically, the ACC is able to interact with the [001] surface of each kaolinite surface along a similar cation bridging mechanism found in the kaolinite-only stacking.28,37 In contrast, the ACC fraction contains high numbers of Ca2+ and Mg2+ ions, allowing bridging across kaolinite platelets over longer length scales (Figure 3, SI Figure 7). The ACC in the interplate spaces subsequently dehydrates, and in the presence of Mg2+, it undergoes an amorphous-to-crystalline transformation to aragonite.33,34,36,39 An aragonitic polymorph is

Figure 2. Transmission electron microscopy of kaolinite hybrid composites and corresponding electron diffraction patterns of ACC, kaolinite, and ACC−kaolinite. (A) ACC sample showing ACC without any additives. (B) ED pattern of ACC displaying diffraction spots of calcite and vaterite indicating the slow transformation of ACC to crystalline phases. (C) Kaolinite platelets without any additives dispersed in water. (D) ED pattern of kaolinite showing typical reflections from kaolinite. (E) ACC + kaolinite composite. (F) ED pattern of ACC + kaolinite displaying two diffraction spots, the zone axis of [001] and the twinning places of {110} of aragonite, in addition to the kaolinite electron diffraction pattern (scale barpanels A,C,E = 200 nm, scale barpanels B,D,F = 0.5 1/Å).

composite (SI Figures 6 and 14). The kaolinite crystalline structure does not match an aragonite {001}, ruling out the possibility that the kaolinite surface templates an aragonitic structure.27,32 This aragonite most likely arises from the excess amount of Mg2+ used in the stabilization of the ACC phase when forming the ACC−kaolinite hybrid composite. The excess Mg2+ subsequently causes the ACC to undergo a crystalline transformation to aragonite33−36 (SI Figures 12 and 13). Using the Debye−Scherrer relationship to analyze the PXRD [112] and [003] peaks of aragonite and kaolinite, respectively, we can calculate that the mean aragonite crystallite size is 5.48 nm. For kaolinite, it is calculcated to be 3.84 nm. In Figure 2B, when the ACC fraction is examined without the presence of any additives, it can be observed that the hydrated species slowly transforms into crystalline species, specifically calcite and vaterite (SI Figure 6). From the TEM and PXRD observation, we demonstrate that the ACC−kaolinite hybrid composite undergoes an ACC−crystalline phase transformation such that a predominant aragonitic phase is formed. The stacking phenomena observed in the kaolinite-only samples can be explained by the weak electrostatic forces on the [001] basal and [010] edge surfaces of each kaolinite platelet typically produced by an Al(III) substitution for Si(IV) in the silicate layer.17,26 Zeta potential measurements through various methods such as electrophoresis, electroacoustics, and titration have found that these quantities are inaccurate because of inhomogenous charge densities resulting from the edge surface.17,25,26,37,38 Instead, when scanning probe microscopy measurements on kaolinite plate surfaces were used at varying

Figure 3. Atomic force microscopy (AFM) of ACC−kaolinite hybrid composites. (A) Three-dimensional height profile of the ACC + kaolinite composite on a slide with subsequent higher magnifications. (B) 10 × 10 μm2 area scan of A. (C) 1.5 × 1.5 μm2 area scan of the region of interest highlighted in B. (See SI Figure 7 for the surface height profile, with the inset showing the step nature of the ACC− kaolinite interactions and a height profile of the area of interest.) 7524

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Figure 4. Elemental distribution of ACC−kaolinite hybrid composites. (A) End-on view of a stack of kaolinite plates in the presence of ACC and elemental maps of the stacks. (B) End-on view of a stack of kaolinite plates after the dissolution of the ACC component of an ACC−kaolinite sample (scale bar = 200 nm).

is removed through dissolution by water over time, the remaining stacked kaolinite platelets retain the long-rangeordered arrangements (Figure 4B). Through this stabilization mechanism by the ACC component, ACC−kaolinite can stack into structures with longer length scales. We observe an ACC−kaolinite composite material that selfassembles into long-range-ordered layered composites through shorter-range interactions. Present in the ACC are Mg2+ ions, an additive in ACC found to stabilize the amorphous calcium carbonate phase at low concentrations as a result of the high degree of hydration.41 By being able to assemble on short length scales, ACC constituents can be packed into the interplatelet spaces of kaolinite. A schematic of this ACC− kaolinite assembly is shown in Figure 5. However, the mechanism in which the ACC and kaolinite interact to form an aragonite−kaolinite hybrid composite is still not entirely clear. In the transformation reaction from ACC to crystalline aragonite, the Mg-doped ACC localized close to the kaolinite platelets are able to undergo a dehydration reaction such that the water is expelled and leaves the remaining magnesiumdoped calcium carbonate in a more concentrated, ordered form and with a preference for forming aragonite as a result of the high concentration of Mg present.36 We know from TGA that the contribution of CaCO3 to the mass of the entire hybrid composite is ∼2% (SI Figure 8). And through the interaction of the ACC on the surfaces of kaolinite platelets as well as filling the interplatelet spaces via cation bridging, the ACC is found to transform to an aragonitic polymorph. Even with this aragonitic crystalline component, the ACC−kaolinite composite still consists of a noticeable portion of an amorphous phase as revealed by dissolution experiments (Figure 4) as well as 1H solid-state NMR measurements of ACC−kaolinite hybrid composites (SI Figure 9). However, the relative amount of

preferentially formed instead of the more stable calcite polymorph because of the divalent Mg2+ substitution of Ca2+.35 This layer-by-layer aggregation of ACC and kaolinite enables long-range-ordered structures as observed frequently in biominerals whereby an organic template is able to direct the formation of aragonitic plate formation. From further analyses via AFM, we observe that the organization of the ACC−kaolinite composite stacks is indeed in the direction parallel to the [001] direction. Additionally, we find that the lateral interactions of the ACC−kaolinite species allow for the convergence of two neighboring stacks into one superstack, enabling the seamless joining of stacks resulting in a smooth surface on the nanometer scale (SI Figure 7), hence the long-range ordering of kaolinite plates. This enables the steplike growth plateau observed in the macrostructure of the ACC− kaolinite hybrid composites (Figure 3 inset). Specifically, ACC is able to act as a molecular binder via Ca2+/Mg2+ bridging such that kaolinite−kaolinite repulsion is reduced and mediates a growth layer where other ACC and kaolinite components can interact and further assemble seamlessly into more highly ordered structures (Figure 3). These observations reveal that ACC itself possesses the ability to fill in small spaces possibly to reduce interfacial energies between kaolinite−kaolinite platelets, acting similarly to molecular cement. This stabilization by ACC is significant in constructing larger kaolinite-related structures because the surface charge forces become extremely weak over large length scales.40 An ACC component can be observed to be distributed throughout all surfaces of the ACC−kaolinite composite (Figure 4A). Without an ACC component, kaolinite−kaolinite assembly does not occur beyond 400 nm, most likely because of the weakening of these aforementioned surface charge forces on this length scale (Figure 3B,C). Interestingly, when the ACC 7525

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and increased wear as a result of the stiffness of the aragonite46 (80−205 GPa) and kaolinite47 (50−255 GPa) components, allowing for better fracture toughness performance of the hybrid material. Last, because of the lack of a biological/organic component, the handling and manufacture of this hybrid material is also noteworthy. In the initial stages of mixing with the ACC and kaolinite components while the hybrid is still in the aqueous phase, the hybrid material can also be sprayed and applied to surfaces and interfaces. The assembly into nacreouslike structures after the dehydration of the sprayed ACC−kaolinite hybrid material allows for it to be used as a useful coating on mechanically sensitive surfaces or a binder for applications in the cement industry. The ease of applicability and lack of additional components further support this hybrid material as a cheap alternative to industrial cements used today while maintating high compressive and flexural strengths as well as high water permeability. By following biomimetic strategies to add custom functional groups to this ACC−kaolinite composite material, we can use replicate properties that are found in organismal systems such as graded stiffness and high specific adhesion to functionalize these composites. Specifically, with further modifications such as the monolayer addition of known organic components with specific catalytic or photothermal conversion properties, we can enable this ACC−kaolinite hybrid composite to be a model system for understanding the effect of specific molecules on the structure and function of a hierarchically self-assembled composite, perhaps endowing this composite not only with similar bulk material properties found in the biological world48−50 but also with properties with tunable organization and mechanical performance.

Figure 5. Schematic of kaolinite and ACC + kaolinite hybrid composite assembly along the basal plane. (A) The kaolinite organization is dependent on cation bridging interactions between other kaolinite platelets. (B) ACC is intercalated into the interplatelet space of kaolinite and increases the interactions at the platelet interface according to the following steps: (1) Attraction and adsorption of ACC onto the basal surfaces of kaolinite; (2) adsorption of ACC into the interplatelet spaces and further assembly of neighboring kaolinite plates; and (3) aggregation and filling in of residual ACC on kaolinite surfaces via cation bridging. (Note that trace amounts of Ca2+ and Mg2+ found in water also contribute to the cation bridging as seen in the case of local assembly between kaolinite platelets.)

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ASSOCIATED CONTENT

S Supporting Information *

Schematic of ACC formation via HPLC pumps. Macroscopic comparison of the packing of kaolinite and ACC + kaolinite hybrid composites. Dissolution of ACC−kaolinite hybrid composites in water at various durations. ATR-FTIR spectroscopy, small angle X-ray scattering, powder X-ray diffraction, and solid-state 1H NMR of the ACC−kaolinite hybrid composite. Surface height profile of kaolinite in comparison to that of the ACC−kaolinite hybrid composite from AFM. Thermogravimetric analysis of kaolinite in comparison to that of the ACC− kaolinite hybrid composite. Kaolinite platelet dispersions in water under various solution conditions. Approximation of the amount of kaolinite and ACC in the hybrid composites. Timeresolved ACC transformation in ACC−kaolinite hybrid composites. Time-resolved TEM aggegration and complexation of amorphous calcium carbonate nanoparticles. PXRD pattern of ACC only, kaolinite only, and the ACC−kaolinite hybrid composite after ACC dissolution treatment with water. This material is available free of charge via the Internet at http:// pubs.acs.org.

ACC to crystalline CaCO3 is indeed less than the crystalline phase, most likely aragonite as observed in SAED (Figure 2). In conclusion, this is the first report of an inorganic template for aragonite formation without the use of any organic additives in which aragonite formation and stabilization has been the domain of biologically derived minerals.42−45 The characteristic assemblies of these aragonite hybrid composites are sensitive to the solution conditions, specifically the pH and Ca2+ and Mg2+ concentrations (SI Figure 10). We demonstrate here that because of the synergistic interactions of ACC and kaolinite on the molecular scale an ordered hierarchical composite is formed that spans length scales larger than 1 mm. Most importantly, the preferential orientation of the kaolinite to organize into nacrelike sheets of platelets allows for this hybrid material to retain its anisotropic mechanical properties such that all of the kaolinite platelets are oriented with their basal planes parallel to the main loading direction. This enables this hybrid material to have better bulk mechanical properties and performance such as improved fracture toughness and wear (as observed in mollusk shells with nacre). Second, because there is a fraction of aragonite that is formed around the kaolinite platelets, this aragonite−kaolinite interface allows for efficient crack blunting

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS We thank Maria Helminger, Martin Spitzbarth, Ulrich Tritschler, and Marius Schmidt (Universität Konstanz) for their technical assistance in analyzing and preparing the ACC− kaolinite composite samples. Andreas Picker (Universität 7526

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Konstanz) is acknowledged for his fruitful discussions on the manuscript. Drs. Le-Chien Hoang, Jeffrey Chen, and Ellis Gartner (Lafarge Research and Development) are acknowledged for their discussion of this manuscript. This work was generously supported by Lafarge Inc.

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