Article pubs.acs.org/Langmuir
Oriented Attachment of Calcite Nanocrystals: Formation of SingleCrystalline Configurations as 3D Bundles via Lateral Stacking of 1D Chains Mihiro Takasaki, Yuya Oaki, and Hiroaki Imai* Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan S Supporting Information *
ABSTRACT: The formation of single-crystalline configurations by the oriented attachment of calcite was experimentally demonstrated as 3D bundles in a nonaqueous system. In the initial stage, 1D short chains elongated in the c direction were formed through the primary oriented attachment of calcite nanoblocks ∼30 nm in diameter. The 3D bundles were then produced through subsequent side-by-side oriented attachment of the 1D chains in the progressive stage. Finally, micrometer-sized single-crystalline architectures were constructed via large-scale oriented attachment of the nanoscale building blocks with a decrease in repulsion force due to the surface charge.
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INTRODUCTION Mesoscopic granular textures consisting of oriented nanocrystals have been observed on various CaCO3-based biominerals.1−11 Biogenic CaCO3 nanograins in the textures are arranged in the same crystallographic direction, and their formation is usually associated with specific growth that is controlled by organic molecules.12−14 Two kinds of production routes involving ion-based stepwise growth15 and particle-based growth through oriented attachment16,17 have been proposed for the development of textured crystals. Single-crystalline configurations that consist of nanocrystals aligned in specific directions are frequently observed in several biominerals.6 However, in particular, the particle-based formation mechanism of biogenic crystals has not been deeply discussed on the basis of detailed experimental results. Although crystals have been known to enlarge through ionby-ion addition routes for a long time, a nonclassical model, such as the oriented attachment of nanocrystals, has recently been regarded as an alternative pathway for crystal growth.18−21 Since the first report proposing a new mechanism that involves the attachment of nanocrystals,22 various experimental results have demonstrated crystal growth by the particle-by-particle addition route.23−29 Oriented attachment was clearly observed in several nanocrystals in nonaqueous systems.30−32 Specific crystal growth of CaCO3 in aqueous systems was reported to be associated with oriented attachment mediated by organic molecules.33,34 Although the attachment was occasionally assumed from the coexistence of the specific single-crystalline configuration and polycrystalline granular features, direct observation of the oriented attachment of a CaCO3 crystal © 2017 American Chemical Society
has rarely been shown in previous reports. Very recently, we demonstrated the 1D oriented attachment of CaCO3 nanocrystals in an aqueous system by changing the basicity and collision frequency at ambient temperatures.35 Nanometric calcite blocks were observed to be attached to each other in the c direction under basic conditions. However, lateral attachment of the nanorods consisting of nanoblocks has not been clearly observed in previous works. The stacking mode of oriented attachment would be essential to produce single-crystalline configurations as 3D bundles consisting of oriented nanograins similar to biogenic mesotextures.6 In the current study, we report the formation of bulky singlecrystalline configurations as 3D bundles via 1D chains from calcite nanoblocks through primary linear alignment and subsequent lateral attachment (Figure 1). The change in the morphology of calcite nanocrystals was monitored in an ethanol system. Elongation of the crystals occurred through the primary 1D oriented attachment of nanoblocks due to a dipole moment originating from the Coulombic interaction on the c faces. Bulky single-crystalline bundled architectures were then produced through the subsequent side-by-side attachment of 1D chains with a decrease in repulsion force due to the surface charge. Although biomineralization occurs in aqueous systems, ethanol systems with stirring were utilized to observe the selforganizing capability of nanocrystals. This report shows direct experimental evidence of the formation of bulky CaCO3 Received: December 22, 2016 Revised: January 28, 2017 Published: February 6, 2017 1516
DOI: 10.1021/acs.langmuir.6b04595 Langmuir 2017, 33, 1516−1520
Article
Langmuir
Figure 1. Schematic illustrations of the primary 1D oriented attachment of nanoblocks in the c-axis direction and the formation of 3D bundles by the subsequent side-by-side stacking of 1D chains.
crystals by the oriented attachment of calcite nanoblocks, which are regarded as building blocks of various biominerals.
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EXPERIMENTAL SECTION
Calcite nanoblocks were synthesized in a 200 cm3 aqueous dispersion of 42.5 g dm−3 Ca(OH)2 through carbonation by the introduction of CO2 at a rate of 3 dm3 min−1. The pH of the dispersion (∼13) started to decrease when the carbonation reaction was finished. Calcite nanoblocks were maintained at pH 12 and 25 °C for a certain period and then removed from the dispersion by centrifugation and washing with dewatered ethanol. We redispersed the calcite nanoblocks in ethanol and maintained them at 4 °C to monitor the morphology variation in the nonaqueous medium. We analyzed the nanograins obtained by drying the filtered dispersion with X-ray diffraction (XRD, Rigaku MiniFlex II), scanning electron microscopy (SEM, Hitachi S4700 and JEOL JSM-7600F, operated at 1.0−5.0 kV), and transmission electron microscopy (TEM, FEI Tecnai F20, operated at 200 kV) with selected area electron diffraction (SAED). The dispersion was dropped on a copper grid covered with a collodion film for TEM observation.
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Figure 2. TEM images of calcite nanoblocks as precursors of nanorods (a,b). TEM images of calcite nanocrystals in an aged ethanol dispersion after stirring for over 120 h (c,e,g). Panels d and f are enlarged images of c and e, respectively. (h) SAED pattern of a small bundle of calcite nanorods.
RESULTS AND DISCUSSION As shown in our previous article,35 calcite nanoblocks ∼30 nm in diameter were found to be synthesized in a dispersion at pH 12 through the carbonation of Ca(OH)2 according to the SEM image and the XRD pattern (Figure S1a,b). We found the gradual formation of 1D chains from the calcite nanoblocks in the basic aqueous system at pH 12 at 25 °C (Figure S1c−e). The nanometric chains elongated up to ∼500 nm in the dispersion stirred for 720 h. The collision frequency enhanced the elongation of the calcite rods, while the width of the rods was unchanged by elongation. Each calcite rod was confirmed to be a single crystal elongated in the c direction according to the presence of a continuous lattice in the high-resolution TEM (HRTEM) image and its fast Fourier transform (FFT) pattern (Figure S1f). These results clearly indicate that the singlecrystalline chains were formed by the oriented attachment of the primary nanocrystals in the c direction of calcite. On the contrary, 3D bundles of the calcite chains were not mainly produced in the aqueous system. In the present study, we demonstrated the formation of bulky single-crystalline configurations as 3D bundles from calcite nanoblocks in a nonaqueous medium. The nanocrystals ∼30 nm in diameter were separated by centrifugation and then redispersed in ethanol (Figure 2a, b). As reported in our previous article,35 the formation of 1D short chains consisting of several nanocrystals was observed in the nonaqueous medium by stirring at 4 °C. In our further research, we found that large-scale 3D oriented attachment also proceeded
in an aged dispersion after stirring for over 120 h (Figure 2). Figure 2c−f shows laterally attached nanorods. This means that the lateral attachment of nanorods occurred successively in an ethanol medium as the initial stage of the lateral stacking. We observed a small 3D bundle consisting of tens of nanorods (Figure 2g). The SAED spots assigned to two sets of singlecrystalline configurations (yellow and orange) were obtained from the bundle (Figure 2h). Thus, most of the nanorods elongated in the c direction (yellow) were suggested to be arranged in the same crystallographic directions. A tilted part located in the upper right of the bundle is regarded as another small set of single-crystalline configurations (orange) consisting of nanorods elongated in the c direction. Therefore, in the nonaqueous medium, the lateral attachment of the calcite chains, as well as the elongation in the c direction, was promoted by stirring. As shown in Figure 3, micrometer-scale architectures were formed as 3D bundles of calcite chains in the ethanol dispersion. The stacking of small chains occurred at the end and side of the large architectures (Figure 3a1,b1,c1). Finally, bulky, dense aggregates were formed in ethanol (Figure 3d1). According to the SAED patterns (Figure 3a2,b2,c2), the chains were aligned in the same crystallographic orientation within the 1517
DOI: 10.1021/acs.langmuir.6b04595 Langmuir 2017, 33, 1516−1520
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Figure 3. TEM images and SAED patterns of micrometer-scale architectures consisting of calcite nanoblocks formed in the progressive stage of the assembly in the ethanol system after stirring for over 120 h.
The mosaic architectures of single-crystalline configuration suggest that the bulky products were not produced through the dissolution−precipitation process. The lateral stacking was not observed in the aqueous system. Thus a trace amount of water is deduced to be negligible for the morphological evolution.
bundles, and the long axis of the large architectures was assigned to [001] (Figure 3a4,b4,c4,d4). Thus single-crystalline configurations of calcite are constructed by large-scale oriented attachment through the side-by-side stacking of 1D short chains. However, the alignment of the chains was not perfect in the architectures because diffraction arcs rather than defined spots were observed. The weak ring patterns in Figure 3c2 originated from the small 1D chains existing around the 3D bundles. Figure 1 shows schematic illustrations of the formation mechanism for large-scale single-crystalline configurations as 3D bundles via 1D chains by the oriented attachment of calcite nanocrystals. The anionic and cationic planes in the calcite lattice are alternatively stacked in the c direction (Figure S2). Because the c planes of the nanograins are deduced to be charged positively or negatively, Coulombic interaction promotes the head-to-tail attachment of the c faces of the calcite nanocrystals and then produces their 1D alignment. In an aqueous system, side-by-side stacking is suppressed because the faces of the uncharged side are stabilized with water molecules. In ethanol, the stabilization of the side faces with the organic molecules is weaker than that with water. Thus the side-by-side oriented attachment of 1D chains is promoted due to the reduction of the total surface energy. Finally, bulky single-crystalline configurations (Figure 3) are formed through the large-scale oriented accumulation of nanorods from calcite nanoblocks. The crystallinity evaluated from the SAED patterns for the large-scale architectures (Figure 3) was better than that for the small bundle (Figure 2g). This suggests that the alignment of the calcite nanoblocks is improved by the aging process.
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CONCLUSIONS We monitored specific morphological changes of calcite nanocrystals in an ethanol system. Micrometric singlecrystalline configurations were produced by primary oriented attachment in the c direction and subsequent lateral stacking of the chains. Here we found the self-organizing capability of calcite nanocrystals in the liquid medium, although biomineralization occurs in aqueous systems. This report shows direct experimental evidence of the formation of bulky CaCO3 crystals by the oriented attachment of calcite nanoblocks, which are regarded as building blocks of various biominerals. Our findings on the controllable nonclassical crystal growth of calcite would be helpful for the understanding of biogenic and biomimetic mineralization processes. The oriented attachment in biomineralization may be controlled by biogenic molecules even in aqueous systems.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b04595. Figure S1. SEM image and a typical XRD pattern of nanoblocks in an aqueous dispersion at pH 12 after carbonation. SEM and TEM images and an HRTEM 1518
DOI: 10.1021/acs.langmuir.6b04595 Langmuir 2017, 33, 1516−1520
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image with an FFT pattern of nanorods in an aqueous dispersion at pH 12 and at 25 °C after stirring. Figure S2. Schematic illustration of a calcite nanoblock with its ionic configuration. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Mihiro Takasaki: 0000-0001-9183-7698 Hiroaki Imai: 0000-0001-6332-9514 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by Kato foundation for Promotion of Science, Grant-in-Aid for Challenging Exploratory Research (15K14129), and Grant-in-Aid for Scientific Research (A) (16H02398) from Japan Society for the Promotion of Science.
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