Langmuir 2006, 22, 5217-5219
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Colloidal Crystal Layers of Hexagonal Nanoplates by Convective Assembly J. Alex Lee, Linli Meng, David J. Norris, L. E. Scriven, and Michael Tsapatsis* Department of Chemical Engineering and Materials Science, UniVersity of Minnesota, Minneapolis, Minnesota 55455-0132 ReceiVed January 12, 2006. In Final Form: March 19, 2006 Particles of the zeolite ZSM-2 prepared as nearly hexagonal nanoplatelets were coated onto flat substrates by a convective assembly technique. On the submillimeter scale, coatings ranged in patterns from striped to continuous. Particles were preferentially oriented out-of-plane, as supported by X-ray diffractometry. The novel observation is that where the particle coating was only a monolayer thick, particles were locally close-packed and uniformly oriented both in and out of plane in a hexagonal colloidal crystalline arrangement that may be described as being tiled (observations by scanning electron microscopy). This is the first documented demonstration of convective assembly applied to anisometric nanoparticles that resulted in particulate coatings with locally ordered microstructure, i.e., colloidal crystallinity.
Supported particle arrays possessing any degree of structural order at the macroscopic and/or nanoscopic scale1-11 are important for diverse applications including optical materials,9 chemical sensors,12 and zeolite-based separations.3,13 Especially of interest are dry, densely packed, and thin particle films on solid substrates, and their fabrication methods. For example, synthetic opals are precursors to photonic band gap materials,9,11 and close-packed oriented zeolite coatings may be useful in membrane separations and other applications.3,14,15 Although the mechanisms at work are still under study, synthetic opals are readily produced on flat substrates by convective assembly8 (also known as vertical deposition9,11) from evaporating suspensions of spherical particles. This process is essentially dip coating with simultaneous drying, and is similar to deposit formation from drying and receding suspension drops.16,17 Spherical particles can closely pack into crystalline structures (opals) regardless of their orientations, but analogous close-packed oriented structures comprising zeolites * Corresponding author. E-mail:
[email protected]. (1) Pieranski, P. Phys. ReV. Lett. 1980, 45, 569-572. (2) Murray, C. MRS Bull. 1998, 23, 33-38. (3) Hedlund, J.; Sterte, J.; Anthonis, M.; Bons, A.-J.; Carstensen, B.; Corcoran, N.; Cox, D.; Deckman, H.; Gijnst, W. D.; de Moor, P.-P.; Lai, F.; McHenry, J.; Mortier, W.; Reinoso, J.; Peters, J. Microporous Mesoporous Mater. 2002, 52, 179-189. (4) Bowden, N.; Terfort, A.; Carbeck, J.; Whitesides, G. M. Science 1997, 276, 233-235. (5) Ha, K.; Lee, Y.-J.; Lee, H. J.; Yoon, K. B. AdV. Mater. 2000, 12, 11141117. (6) Zheng, H.; Rubner, M. F.; Hammond, P. T. Langmuir 2002, 18, 45054510. (7) Coutinho, D.; Balkus, K. J. Microporous Mesoporous Mater. 2002, 52, 79-91. (8) Dimitrov, A. S.; Nagayama, K. Langmuir 1996, 12, 1303-1311. (9) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132-2140. (10) Abkarian, M.; Nunes, J.; Stone, H. A. J. Am. Chem. Soc. 2004, 126, 5978-5979. (11) Norris, D. J.; Arlinghaus, E. G.; Meng, L. L.; Heiny, R.; Scriven, L. E. AdV. Mater. 2004, 16, 1393-1399. (12) Bein, T. Chem. Mater. 1996, 8, 1636-1653. (13) Lai, Z.; Bonilla, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.; Trasaki, O.; Thompson, R. W.; Tsapatsis, M.; Vlachos, D. G. Science 2003, 300, 456-460. (14) Huang, L.; Wang, Z.; Sun, J.; Miao, L.; Li, Q.; Yan, Y.; Zhao, D. J. Am. Chem. Soc. 2000, 122, 3530-3531. (15) Lee, J. S.; Lee, Y.-J.; Tae, E. L.; Park, Y. S.; Yoon, K. B. Science 2003, 301, 818-821. (16) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Nature (London) 1997, 389, 827-829. (17) Deegan, R. D.; Bakajin, O.; Dupont, T. F.; Huber, G.; Nagel, S. R.; Witten, T. A. Phys. ReV. E 2000, 62, 756-765.
are challenging to fabricate because the constitutive particles are anisometric. Until now, ordered arrays of anisometric zeolite particles have been made by covalent linkers5,15 or stamping.14 Simpler processes such as convective assembly are attractive because they can easily be scaled up and adapted to continuous processing. To our knowledge, however, there is no indication in the literature that the convective assembly technique has ever been applied to anisometric (orientable) particles to produce oriented and close-packed, i.e., colloidal crystalline, monolayers on solid supports. Here we present convective assembly by evaporation assisted withdrawal coating as a simple and promising technique to make colloidal crystal layers of anisometric particles from dilute aqueous suspensions. To demonstrate the concept, we coated clean flat glass with nanoscopic crystals of the zeolite ZSM-218 prepared as nominally hexagonal plates with aspect ratio about 4:1.19,20 This shape was chosen because of their potential for hexagonal close-packing of the particles if they could be deposited in a colloidal crystalline arrangement. Suspensions of ZSM-2 particles between 150 and 200 nm in the largest dimension were synthesized from clear suspension according to literature procedures.20,21 The coatings were made directly onto an angled wall of the coating bath cell (Figure 1). As confirmed by video light microscopy observations, liquid flow in the meniscus region toward the contact line forced the particles to convect and subsequently deposit there. As the suspension evaporated, the total bath level fell and caused the contact line to eventually sweep down the substrate and coat it with particles; the substrate was effectively “withdrawn” relative to the bath. Evaporation was assisted by dry gas, usually nitrogen, in controlled laminar flow over the bath. In a typical experiment, the coating suspension was diluted to between 0.002 wt % and 0.05 wt %, or concentrations of about 8 to 200 trillion particles/ ml. A particle concentration of 0.002 wt % is sufficient to hypothetically coat the entire vessel with a continuous 100-layer of particles. Low magnification scanning electron microscopy (SEM) shows that the coatings from the most dilute suspensions are not (18) Ciric, J. U.S. Patent 3,411,874, 1968. (19) Barrer, R. M.; Sieber, W. J. Chem. Soc., Dalton Trans. 1977, 10, 10201025. (20) Schoeman, B. J.; Sterte, J.; Otterstedt, J.-E. J. Colloid Interface Sci. 1995, 170, 449-456. (21) Nikolakis, V.; Xomeritakis, G.; Abibi, A.; Dickson, M.; Tsapatsis, M.; Vlachos, D. G. J. Membr. Sci. 2001, 184, 209-219.
10.1021/la0601206 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/05/2006
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Figure 1. Coating cell. Suspended particles (a) convect to the contact line (b) where they deposit and begin to assemble a colloidal crystal (d), which then grows along the substrate (c) as the suspension evaporates and the entire bath surface (e) recedes.
Figure 2. Typical coatings range from striped to continuous. Stripes widen and the coating eventually becomes continuous as the suspension concentration is increased.
continuous, but rather appear as regularly alternating stripes of particle deposits and void spaces that run along the direction parallel to the contact line (Figure 2a,b and Supporting Information). Similar macroscopic patterning of deposits from suspension, not restricted to striping, are reported in the literature.10,22-24 Thick continuous coatings could be made from more concentrated suspensions (Figure 2c). When the substrate angle is made steeper, the stripes are qualitatively wider while their spacing remains similar (see Supporting Information). The effect of gas flow rate is unclear because it affects both the rates at which the bath level moves (withdrawal rate) and the particles move toward the contact line. The latter is qualitatively confirmed by video light microscopy, where lower gas flow rates corresponded to slower particle convection. Video microscopy of the coating process for striped coatings also revealed that the contact line, visible as a front toward which the particles move, periodically jumps off the growing deposit and starts a new deposition farther down the substrate. Adachi et al. describe this phenomenon as “stick-and-slip” of the contact line, governed by a balance between the increasing component of surface tension in the receding direction at the pinned contact line and some slipping yield force due to viscous effects.22 Alternatively, Abkarian et al. suggest that such a motion may be governed by equilibrium capillary rise on heterogeneous surfaces.10 The stripes, having been reported previously in coatings from isometric particles, are not the primary observations we wish to report here, although we have included the brief discussion above for completeness. The novel observation of primary interest is (22) Adachi, E.; Dimitrov, A. S.; Nagayama, K. Langmuir 1995, 11, 10571060. (23) Deegan, R. D. Phys. ReV. E 2000, 61, 475-485. (24) Ray, M. A.; Kim, H.; Jia, L. Langmuir 2005, 21, 4786-4789.
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Figure 3. Representative image of the beginning of a coating stripe. Where the coating is monolayered, the particles exhibit hexagonal close packing. Near the bottom of this image, the coating is developing a second layer, where the close packing is lost.
that within the monolayered regions of each stripe (always at the leading edge, or the beginning of the stripe), the particles are nearly uniformly oriented and close-packed according to their anisometry, i.e., hexagonally; in essence, they form a locally tiled structure (Figure 3 and Supporting Information) that extends the entire breadth of the stripe, about several cm, until edge effects become significant. The tiling implies preferential orientation in-the-plane of the monolayer, which can be represented by a nearest-neighbor particle misalignment distribution (Figure 4), where the angle of the main diagonal of every particle was compared to that of each immediately bordering particle, so that the “misalignment” is expected to have a sharp peak at 0° in a perfect lattice, and bounded by (-30°,30°) in a random lattice. The distribution from roughly 100 particles (600 measurements) is normal with mean -0.2° and standard deviation 7.3°, indicating preferential orientation in-the-plane. Interpretation of this distribution should take into account a distribution of each particle’s asymmetry, which can be characterized similarly to the nearest-neighbor misalignment by taking the angles between the main diagonals of each particle, and examining the distribution of their deviation from 60° (Figure 5). Interestingly, the two distributions have similar widths. It should also be noted that in addition to deviations from the perfectly hexagonal shape, SEM images also indicate a distribution in particle size, which can further contribute to misalignment. Where the particle deposit grows thicker than a monolayer, there is no more visible local ordering, and colloidal crystallinity is lost (bottom of Figure 3). Particles also start to lose their flatness against the substrate, or out-of-plane preferential orientation, which the above discussion has so far taken for granted, although visual inspection by SEM suggests that it may remain largely intact, at least on the deposit surface (see Supporting Information). To investigate the possibility that particles were overall preferentially oriented out-of-plane, we used X-ray diffractometry (XRD) and the crystallographic preferential orientation (CPO) index.25,26 Here we show the pattern for two coatings (Figure 6). Between the powder18 and coating (25) Jeong, H. K.; Krohn, J.; Sujaoti, K.; Tsapatsis, M. J. Am. Chem. Soc. 2002, 124, 12966-12968. (26) CPO is calculated from peak intensities for the appropriate reflections. (S) refers to coating samples, and (P) refers to the powder: CPO ≡ [I400/I005|S - I400/I005|P]/(I400/I005|P).
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Figure 4. Distribution of the degree of a particle’s misalignment from its nearest neighbors. The distribution is normal and centered around -0.2° (practically 0° or “aligned”) with a standard deviation of 7.3°. The distribution indicates preferential orientation. See Supporting Information for details.
XRD patterns for ZSM-2, the CPO index comparing the (400) and (005) reflections is larger than 8 for the thickest coating (and immeasurably large for the thinner), indicating strong (h00) outof-plane orientation (patterns for more coatings in Supporting Information). In summary, we have explored the potential of convective assembly by evaporation assisted withdrawal coating to produce particulate coatings from suspensions of hexagonal nanoparticles with regions of highly ordered microstructure. Where the coatings are monolayer in thickness, the particles are very clearly preferentially oriented out-of-plane. The novel observation is that locally, the particles are also preferentially oriented in-theplane to a degree that seems to be determined by the imperfection in each particle’s shape. As the coating becomes thicker, it loses preferential orientation in-the-plane, but seems to retain preferential orientation out-of-plane. Eventually, the deposit ends, followed by a void space, and a new deposit begins as a monolayer, where local order is observed. The formation of this close-packed, locally ordered monolayer, or tiles, is observed over essentially the entire breadth of the coating. This remarkable order, though short-lived, is a promising feature of the coating process, potentially useful as precursors to ultrathin selective membranes. Acknowledgment. We would like to acknowledge support from the Department of Energy under award no. DE-FG26-
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Figure 5. Particle shape distribution characterized by a distribution of single particle asymmetry. The distribution is normal and centered around 0° with a standard deviation of about 7.4°, and is as wide as that for the nearest-neighbor particle misalignment distribution. See Supporting Information for details.
Figure 6. Reflection XRD patterns for coatings from varying suspension concentrations compared to the pattern for a powder sample indicate strong preferential out-of-plane orientation.
04NT42119, the National Science Foundation under award no. CTS-0332484, and the members of the Industrial Partnership for Research in Interfacial and Materials Engineering. Supporting Information Available: ZSM-2 synthesis recipe, detailed coating procedure, details of characterization, and more images (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. LA0601206
