Langmuir 2006, 22, 1955-1958
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Crystallization on Surfaces of Well-Defined Topography Nicola B. J. Hetherington, Alex N. Kulak, Katherine Sheard, and Fiona C. Meldrum* School of Chemistry, UniVersity of Bristol, Cantock’s Close, Bristol, BS8 1TS, United Kingdom ReceiVed August 26, 2005. In Final Form: October 20, 2005 Single crystals of calcite with regular patterned surfaces comprising close-packed arrays of hemispherical cavities or domes were produced by crystallization on colloidal monolayers or PDMS replicas of these monolayers, respectively. Perfect replication of the substrate topography was achieved for all colloidal particles, irrespective of their size and surface chemistry when the substrate geometry permitted unrestricted ion flow to the growing crystal. This work demonstrates that crystallization within a mould provides a very general route to producing single crystals with curved surfaces and unusual morphologies and that such patterning can be applied from the micro- to the nanoscale.
Introduction Biology provides many beautiful examples of how mineralization can be controlled under ambient conditions to produce remarkable structures.1-3 Biominerals frequently exhibit unique morphologies, hierarchical organization on levels ranging from the nanometer to the macroscopic scale,4,5 and impressive mechanical properties,6 and it therefore remains an attractive strategy to apply bio-inspired methodologies to the synthesis of many technologically important materials. The morphologies of biominerals have long been a source of fascination. While many biominerals with complex forms are amorphous, with no preferred morphology, or are alternatively polycrystalline aggregates,1-3 it is undoubtedly the single crystals with noncrystallographic morphologies and curved surfaces that are the most striking. There is strong experimental evidence that soluble macromolecules occluded within biominerals are involved in minor modulation of crystal morphologies,7 whereas the gross form is dictated by the environment in which the crystal forms.2 Amorphous calcium carbonate (ACC), which can act as a transient precursor phase to single-crystal calcite in vivo has also been used synthetically as a precursor to calcite single crystals with complex morphologies.8-10 However, although this approach can certainly yield single crystals with complex form, an ACC precursor phase is certainly not required for morphological control. Indeed, we have recently shown that precipitation of calcite within spongelike polymer membranes can yield single crystals with identical spongelike structures.11,12 Single crystals with extremely complex morphologies can therefore be simply produced by shape constraint only, without the requirement for soluble additives, an amorphous precursor phase or the complex interplay of many time-dependent variables which inevitably operate during crystal growth in vivo.
We here describe an investigation of crystallization on surfaces with well-defined topographies, resulting in the production of crystals with patterned surfaces. Although use of organic matrixes in controlling crystal growth has received considerable attention,13-17 very few studies have addressed the influence of surface topography on crystal growth. Rough surfaces, constructed by the adsorption of 8 nm gold colloids on planar substrates have been used to support the growth of calcium carbonate.18,19 Compared with planar gold substrates, a higher density of crystals and a significantly higher proportion of aragonite was observed on the rough surfaces. Substrates were constructed from close-packed monolayers of either polystyrene or silica spheres, with the surface chemistry of the silica particles being selectively modified via silylation. This system provides an extremely flexible route to investigating the role of the surface topography and chemistry in controlling the crystal morphology. Previously, templating of the morphologies of calcite single crystals has only been demonstrated in the micron size range.8,11,12 The particle sizes were systematically varied across the range from 50 nm to 5 µm and replicas of these surfaces in PDMS (poly(dimethylsiloxane)) with curvature of equal magnitude but opposite sign were also used to support crystal growth. Colloidal particle monolayers and multilayers have been used as templates for the production of polycrystalline photonic solids20,21 but have not been previously employed to support growth of single crystals. Experimental Section
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Colloidal Particles. Polystyrene spheres and silica particles were either purchased commercially (Bangs Labs Inc., USA) or were synthesized according to the Sto¨ber synthesis.22 The Sto¨ber synthesis was undertaken by combining an ethanolic solution of tetraethoxysilane with a solution containing ethanol, water, and ammonium hydroxide under vigorous mechanical stirring and allowing the
(1) Lowenstam, H. A.; Weiner, S. On Biomineralization; Oxford University Press: New York, 1989. (2) Mann, S. Biomineralization: Principles and Concepts in Bioinorganic Materials Chemistry; Oxford University Press: New York, 2001. (3) Wilt, F. H. DeV. Biol. 2005, 280, 15. (4) Fantner, G. E.; Hassenkam, T.; Kindt, J. H.; Weaver, J. C.; Birkedal, H.; Pechenik, L.; Cutroni, J. A.; Cidade, G. A. G.; Stucky, G. D.; Morse, D. E.; Hansma, P. K. Nat. Mater. 2005, 4, 612. (5) Aizenberg, J.; Weaver, J. C.; Thanawala, M. S.; Sundar, V. C.; Morse, D. E.; Fratzl, P. Science 2005, 309, 275. (6) Currey, J. D. J. Exp. Biol. 1999, 202, 3285. (7) Weiner, S.; Addadi, L. J. Mater. Chem. 1997, 7, 689. (8) Aizenberg, J.; Muller, D. A.; Grazul, J. L.; Hamann, D. R. Science 2003, 299, 1205. (9) Loste, E.; Meldrum, F. C. Chem. Commun. 2001, 10, 901. (10) Loste, E.; Park, R. J.; Warren, L.; Meldrum, F. C. AdV. Funct. Mater. 2004, 14, 1211. (11) Park, R. J.; Meldrum, F. C. J. Mater. Chem. 2004, 14, 2291.
(12) Park, R. J.; Meldrum, F. C. AdV. Mater. 2002, 14, 1167. (13) Heywood, B. R.; Mann, S. AdV. Mater. 1994, 6, 9. (14) Donners, J. J. J. M.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. J. Am. Chem. Soc. 2002, 124, 9700. (15) Volkmer, D.; Fricke, M.; Agena, C.; Mattay, J. J. Mater. Chem. 2004, 14, 2249. (16) Travaille, A. M.; Kaptijn, L.; Verwer, P.; Hulsken, B.; Elemans, J. A. A. W.; Nolte, R. J. M.; Kempen, H. H. v. J. Am. Chem. Soc. 2003, 125, 11571. (17) Aizenberg, J. J. Chem. Soc. Dalton Trans. 2000, 21, 3963. (18) Ku¨ther, J.; Seshadri, R.; Nelles, G.; Butt, H.-J.; Knoll, W.; Tremel, W. AdV. Mater. 1998, 10, 401. (19) Ku¨ther, J.; Seshadri, R.; Nelles, G.; Assenmacher, W.; Butt, H.-J.; Mader, H.; Tremel, W. Chem. Mater. 1999, 11, 1317. (20) Sun, F. Q.; Cai, W. P.; Li, Y.; Cao, B. Q.; Lu, F.; Duan, G. T.; Zhang, L. D. AdV. Mater. 2004, 16, 1116. (21) Bartlett, P.; Baumberg, J.; Coyle, S.; Abdelsalam, M. Faraday Discuss. 2004, 125, 117. (22) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.
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1956 Langmuir, Vol. 22, No. 5, 2006 reaction to proceed overnight. Reaction temperatures of between 40 and 60 °C were applied to produce smaller particle sizes. Particles were isolated by centrifugation and were washed with water at least three times using a centrifugation/redispersal process. Particles with a size distribution with standard deviation less than 8% are required for formation of high quality, close-packed monolayers.23 Improvement of particle size distributions was achieved using seeding techniques, taken from the method of Bogush.24 Particle Monolayers. Particle monolayers were formed using an accelerated evaporation technique.25 An ethanol solution of either silica or polystyrene spheres was prepared, and a glass slide was mounted vertically in the solution. This was then placed in a vacuum desiccator at 20-40 mbar until all solvent had evaporated, yielding a monolayer coverage of particles on the glass slide. Adherence of the silica particle monolayers to the glass substrate was improved by heating to 520 °C for approximately 5 h. These monolayers were then directly used to support CaCO3 growth, or were chemically modified to render the surfaces hydrophobic. Briefly, the heat-treated silica particle monolayers were immersed in a 1% ethanolic solution of hexadecyltrimethoxysilane at 30 °C overnight and were then rinsed 5 times with ethanol. PDMS Replica of Particle Monolayers. A replica of the polystyrene sphere monolayers was formed in poly(dimethylsiloxane) (PDMS). Silicone elastomer Sylgard 184 was mixed thoroughly with the curing agent in a 20:3 ratio. The mixture was poured over a prepared monolayer and was degassed under vacuum before being heated at 60 °C for 2 h. The PDMS mould was then peeled off the glass slide, and the polystyrene particles were removed from the PDMS by immersing the stamp in toluene. Contact Angle Measurements. Contact angle measurements were made of the silica particle monolayers immediately after heattreatment, and after silanisation, and of the PDMS replicas using a KR_SS Drop Shape Analyzer DSA 10 Mk2. CaCO3 Precipitation. A slide supporting a particle monolayer was placed in a 2 mM supersaturated solution of CaCO3, formed by mixing 4 mM solutions of CaCl2 and Na2CO3. The slide was placed at an angle in the solution with the particle monolayer facing downward, to ensure that the crystals which formed on the particle monolayer had nucleated on the slide. PDMS substrates were floated on the surface with the moulded surface in contact with the solution. Crystallization was typically allowed to proceed for 4 h, after which time the substrates were removed from the solution, washed with Millipore water and dried in air. Analysis. The CaCO3 crystals were studied by SEM both on the monolayer and after removal from the slide. To view the crystal face in contact with the monolayer/PDMS replica, an adhesive carbon pad was pressed lightly against the upper surface of the substrate, and the crystals were pulled off to reveal their undersides. The carbon pad was then mounted on an SEM stub. Removal of polystyrene particles from the crystals was also achieved by immersing the slides supporting the crystals in toluene and sonicating for a few minutes. The crystals were allowed to sediment and were then transferred onto a glass slide using a pipet and dried in air. Samples for SEM were sputter-coated with Pt/Pd and imaged in a JEOL 6330 FEGSEM operating at 10 kV.
Results and Discussion Both the silica and polystyrene monolayers supported the growth of rhombohedral calcite crystals. Viewed in situ the size, morphology, and orientation of the crystals grown on the monolayers (Figure 1a) showed no significant variation as compared with control experiments using glass slides as substrates. However, that the particle monolayer had influenced crystallization was immediately apparent when the calcite crystals were lifted off the monolayer, and the crystal face originally (23) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132. (24) Bogush, G. H.; Zukoski, C. F. J. Colloid Interface Sci. 1991, 142, 1. (25) Kitaev, V.; Ozin, G. A. AdV. Mater. 1993, 15, 75.
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Figure 1. (a) Calcium carbonate precipitated on monolayer; (b) calcite crystals grown on 0.5 µm colloidal polystyrene monolayer after dissolution of the polystyrene spheres, showing the crystal face growing in contact with the monolayer; and (c) calcite single-crystal grown on monolayer of 0.5 µm silica particles, showing encapsulation of the particles within the crystal.
adjacent to the monolayer was observed. For both monolayer types, the crystal face apposed to the monolayer had grown intimately around the particles, resulting in the particles becoming embedded in the crystal. This effect was most clearly demonstrated after removal of the particles from the crystals. This procedure was easily achieved in the case of the polystyrene spheres, which were readily removed from the calcite crystals by dissolution in toluene. Thermal treatment of the silica sphere monolayers prior to crystal growth significantly improved their adhesion to the glass slide, such that many remained on the slide after lifting off the CaCO3 crystals. Removal of particles from the crystals showed that the monolayer structure was perfectly replicated in the nucleating crystal face, with a motif of close-packed hemispherical cavities patterning this face (Figure 1b shows a crystal after removal of polystyrene particles). Imaging of the crystal development over time suggested that the calcite crystals initially grew with a planar face in contact with the particle monolayers and that growth then continued through the monolayer to produce the hemispherical cavities. Further, longer precipitation times resulted in growth of the crystal through the monolayer, causing the colloidal particles to become encapsulated within the single crystal of calcite (Figure 1c). The ability of calcite to occlude foreign bodies within single crystals is well-documented, examples being provided by gel incorporation
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Figure 2. (a) Calcite crystal grown in association with 5 µm silica colloid and (b) face of calcite crystal grown on monolayer of 100 nm polystyrene particles, after dissolution of the polystyrene spheres.
Figure 4. Calcite crystals precipitated on PDMS replicas of (a) 0.5 µm and (b) 1.1 µm particle monolayers. The arrows in (b) indicate parallel faces. (c) High magnification image of the surfaces of the hemispherical indentations of calcite crystal precipitated on thermally treated silica colloids.
Figure 3. Schematic diagram of process used to form PDMS replica of colloidal particle monolayers and its use in templating crystal growth
within gel-grown calcite single crystals26 and the occlusion of organic macromolecules within biogenic crystals.7,27 A report of the inclusion of polymer spheres within ZnO crystals has also been made,28 and a very recent paper has used poly(St-ranMAA) latex particles as additives to control calcium carbonate (26) Henisch, H. K. Crystals in Gels and Liesegang Rings; Cambridge University Press: Cambridge, U.K., 1988. (27) Albeck, S.; Aizenberg, J.; Addadi, L.; Weiner, S. J. Am. Chem. Soc. 1993, 115, 11691.
precipitation and indicated that some inclusion of the particles within crystals occurs.29 Particle monolayers were also prepared from silica and polystyrene colloids in the size range 5 µm to 50 nm. In all cases, successful templating of the nucleating crystal face was observed. The large 5 µm particles were similar in size to the calcite crystals, so only one or two particles were typically associated with each crystal (Figure 2a). In contrast, small particles templated entire crystal faces in a close-packed motif (Figure 2b shows templating by 100 nm particles). Previous examples of morphological templating of single crystals of calcite have shown patterning on the micron-scale only.8,11,12 This work provides the first demonstration that the replication of complex morphologies into a single crystal can be achieved on the nanometer scale, a result consistent with biological systems, as exemplified by the ∼50 nm mineral bridges linking the single-crystal stacks of aragonite tablets in abalone nacre.30 (28) Wegner, G.; Baum, P.; Mu¨ller, M.; Norwig, J.; Landfester, K. Macromol. Symp. 2001, 175, 349. (29) Shen, Q.; Chen, Y.; Wei, H.; Zhao, Y.; Wang, D.; Xu, D. Cryst. Growth Des. 2005, 5, 1387. (30) Schaffer, T. E.; Ionescu-Zanetti, C.; Proksch, R.; Fritz, M.; Walters, D. A.; Almqvist, N.; Zaremba, C. M.; Belcher, A. M.; Smith, B. L.; Stucky, G. D.; Morse, D. E.; Hansma, P. K. Chem. Mater. 1998, 10, 946.
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Figure 5. Schematic diagram of the growth mechanism of crystals on the particle monolayers and on the PDMS replica.
Calcium carbonate growth on surfaces of opposite curvature was also investigated using PDMS replicas formed of the polystyrene monolayers (Figure 3). The polystyrene monolayers proved more suitable for this purpose than those constructed from the silica particles as the polystyrene particles could readily be removed from the final PDMS mould by dissolution in toluene. Immersion of the PDMS replica in a supersaturated solution of calcium carbonate again resulted in precipitation of rhombohedral calcite crystals on the substrate. Viewing of the nucleating crystal faces showed that the PDMS replica also directed the morphology of the crystal, resulting in an array of hemispheres, of identical size to the original colloids, patterning the crystal surfaces. Figure 4a shows an example of a calcite crystal precipitated on the monolayer of 0.5 µm particles. That the hemispheres are not perfectly close-packed on the crystal surface is likely to reflect minor faults in the PDMS replica. Comparison of the surfaces of the hemispherical cavities with those of the raised hemispheres generated on the PDMS surfaces showed some marked differences. The PDMS-templated hemispheres displayed some well-defined crystal faces, as is particularly clear in crystals templated from 1.1 µm particle replicas in which the planar faces on the raised hemispheres lie parallel to external faces of the main body of the crystal (arrowed in Figure 4b). In contrast, characterization of the surfaces of the hemispherical indents at high magnification with field-emission SEM showed evidence of some surface roughness at a 10-20 nm level, but no well-defined steps or crystal faces (Figure 4c). Samples were tilted in the SEM to aid viewing of the indent surfaces. While we cannot rule out that small steps may be present on these surfaces, they must be significantly smaller than those present on the raised hemispheres, giving the appearance of a smooth surface on the micron scale. That this did not derive from the different surface chemistries of the polystyrene spheres and PDMS was shown through crystallization on hydrophilic and hydrophobic silica particle surfaces. After thermal treatment, the silica particles were hydrophilic and completely wetting, while after silanisation with hexadecyltrimethoxysilane, they were hydrophobic with a measured contact angle of ∼108°. The PDMS replicas exhibited a similar contact angle of ∼110°. Identical
hemispherical indents were produced in the nucleating crystal faces on both of these silica substrates, indicating that the regular crystal faces produced in contact with the PDMS replica cannot be attributed to the surface hydrophobicity. These regular crystal faces may derive from the ability of ions to diffuse to the growing crystal (Figure 5). For the crystals growing directly on the colloidal particles, diffusion of ions is possible through the particle array, permitting continuous growth of the crystal through the particle monolayer. In contrast, growth of a calcite crystal on the PDMS replica results in blocking of the cavities in the PDMS by the planar nucleation face to further ion diffusion. Crystal growth into the cavities is therefore limited, resulting in termination of growth prior to the crystal impinging on the template surface, and the exhibition of some stable planar crystal faces rather than the much smoother surfaces which appear to arise when the morphology is restricted by the template.
Conclusions In conclusion, we have demonstrated that single crystals of calcite with regular patterned surfaces comprising either hemispherical cavities or domes can be produced by simple crystallization on colloidal monolayers or a PDMS replica of these monolayers, respectively. The pattern of features produced on the crystal face perfectly replicated the topography of the substrate when the flow of ions to the growing crystal was not restricted. That all colloidal particles, independent of their surface chemistry appeared to support this morphological replication demonstrates that growth of crystals within a mould provides a very general route to producing single crystals with curved surfaces and complex morphologies; all that is required is a mould of suitable form. For the first time we also show that this approach can be applied to the formation of nanoscale structural features. It should also be straightforward to extend our methodology to template single crystals with colloidal multilayers. Acknowledgment. Financial support from the Engineering and Physical Sciences Research Council (EPSRC, Grant GR/ S79732/01) is gratefully acknowledged. LA0523356
