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Crystal Growth Studies on Microporous Zincophosphate-Faujasite Using Atomic Force Microscopy Pablo Cubillas,* Mark A. Holden, and Michael W. Anderson Centre for Nanoporous Materials, School of Chemistry, The University of Manchester, Oxford Road, Manchester, M13 9PL, U.K. W Web-Enhanced b
ABSTRACT: In this paper we report the first in situ atomic force microscopy study on the crystal growth of zincophosphate-faujasite (ZnPO-FAU). Results show that growth at low to moderate supersaturation conditions takes place by a birthand-spread and/or spiral growth mechanism. At medium-tohigh supersaturation conditions, growth starts preferentially at crystal edges and corners forming macrosteps with a height of several nanometers that advance toward the center of the crystal. This behavior may be explained by nucleation inhibition on the crystal surface due to surface reconstruction and impurity adsorption after synthesis. ZnPO-FAU crystals were found to be metastable at the experimental conditions (25 C, 1 atm) as initial growth slowed down considerably during the duration of the experiment and was then followed by the dissolution of the crystals and the precipitation of a new phase. Linear defect formation, due to low angle dislocations, was also imaged for the first time and was found to be commonplace in the system. Defect formation was linked to the embedding of microcrystals of the same phase as growth takes place. Owing to the low angle nature of the misfit crystals were able to “heal” themselves after multiple layers have grown.
’ INTRODUCTION Crystalline microporous solids are some of the most versatile synthetic materials due to their wide range of industrial applications such as catalysis, ion-exchange, molecular sieving, and gas adsorption. The most widely known type of microporous solid is the zeolite, an aluminosilicate with a framework composed of corner sharing Si/Al tetrahedra. Si and/or Al can be substituted for other elements such as P, Ga, Ge, Fe, Zn, Co, and B, in which case the crystalline material is known as a zeotype. Owing to their extensive applications, a great number of studies have been devoted to study the nucleation and growth of these materials.1 Nevertheless, in situ studies remain elusive as most of these solids can only be synthesized under hydrothermal conditions (100250 C). In 1991 Gier and Stucky2 published the low temperature synthesis of several zeotypes, including two zincophosphate materials with the sodalite (SOD) and faujasite (FAU) structure. Several studies regarding different aspects of the synthesis of both molecular sieves have since been published,35 with special attention given to the synthesis of ZnPO-FAU by means of micelles.610 Singh et al.8 studied the differences of surface morphology between ZnPO-FAU grown by conventional hydrothermal methods and by reverse micelles, where water microdroplets are dispersed in a hydrocarbon medium with a surfactant. This paper included ex situ atomic force microscopy (AFM) images of end-of-synthesis crystals. Although they provide useful information, ex situ studies can be limited by several variables that are difficult to control, such as r 2011 American Chemical Society
possible surface rearrangement during crystal extraction and washing or the lack of knowledge of the supersaturation conditions at different synthesis times. In situ studies provide a wealth of information related to the crystal growth of these materials as has been recently shown for the ZnPO-SOD system.11 Holden et al.11 published the first in situ study on ZnPO-SOD. Results showed the formation of interlaced spirals on the {100} faces, due to a change in the growth anisotropy between alternating layers, which, in turn, is driven by a difference on the condensation rates for zinc and phosphorus. The goal of the present study was to further our understanding on the crystal growth of ZnPOFAU in the absence of additives by performing detailed in situ AFM observations.
’ EXPERIMENTAL SECTION ZnPO-FAU crystals were synthesized according to the method of Gier and Stucky.2 A precooled solution (4 C) containing 0.65 g of NaOH (Sigma-Aldrich), 12.1 g of TMAOH (Sigma-Aldrich), 3.71 g of H3PO4 (Fluka Chemika), and 37 g of deionized water was added to a precooled solution (4 C) of 7.14 g of Zn(NO3)2 3 6H2O (SigmaAldrich) in 8.54 g of water. The resultant gel was stirred for 5 min and then kept at 4 C for 5 h. After this time, crystals were filtered and washed with water. The dried product was identified as ZnPO-FAU by Received: April 1, 2011 Revised: May 16, 2011 Published: May 17, 2011 3163
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Figure 1. SEM (a) and ex situ AFM (b) images of representative ZnPOFAU crystals after 5 h of synthesis. means of powder X-ray diffraction. Long-term syntheses of up to 30 days were carried out to study the metastability of the crystals. AFM experiments were carried out using a NanoWizard II (JPK Instruments AG). All images were recorded in contact mode using Si3N4 cantilevers (NP-10, Veeco Probes) with a nominal spring constant of 0.58 N/m. In situ experiments were performed by mounting the crystals in an epoxy resin, cured for 24 h at 60 C. No effect on the crystals’ surface by the curing temperature could be observed, as seen by comparing with images taken on crystals kept at room temperature. The sample was then introduced into the AFM fluid cell (Biocell, JPK Instruments AG). Two types of growing solutions were used. In the first case (Type 1), a standard synthesis was carried out. Once a clear layer of solution had developed from the initial gel, it was removed with a syringe, filtered, and injected into the AFM fluid cell. In the second case (Type 2), solutions were prepared directly from fresh reagents and had the following molar composition: 0.0004 Zn/0.021 Na/2.13 TMAOH/ 0.033 P/100 H2O. All in situ growth experiments were carried out at room temperature (2530 C). Scanning electron micrographs (SEMs) were taken with a Quanta environmental scanning electron microscope (FEI Company). Samples were prepared by spreading zeolite powder on a carbon tape stuck on a metal stub followed by sputter coating with gold to reduce charging effects under electron beam. Powder X-ray diffraction patterns were obtained by means of a Philips X’pert diffractometer. Samples were ground prior to mounting into a sample holder. Data were collected in the 2θ range of 360 using Cu-KR radiation, with a wavelength of 1.54 Å.
’ RESULTS AND DISCUSSION Figure 1 shows a representative electron micrograph of ZnPOFAU crystals extracted after 5 h crystallization. It can be seen that
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they mainly possess the typical octahedral morphology,5,8 although a few crystals have a plate-like habit. These later crystals have been previously identified as twinned (spinel twinning) ZnPO-FAU crystals by Castagnola et al.5 In both cases, crystals are bound by the {111} faces only. AFM imaging of these crystals show that they possess a rough surface, at the nanometer scale, with no terraces or discernible steps (Figure 1b). This could be due to an incomplete synthesis when extracted (i.e., the synthesis solution was still supersaturated) or to the washing treatment. Crystal Growth of ZnPO-FAU. Two types of growth patterns were observed depending on the supersaturation of the solution used. In those experiments where the initial solution was highly supersaturated, growth took place initially by the formation of macrosteps up to several tens of monolayers deep (5060 nm) at the edge of the crystals, which then advanced toward the center of the crystal. In most cases, this was accompanied by twodimensional (2-D) nucleation over the central part of the crystal at a much slower rate. This process is illustrated in Figure 2. Figure 2ad shows four different crystals where macrosteps have formed due to preferential nucleation at the edge and corners. Figure 2e,f shows how these macrosteps (from crystal d) advance until they merge at the center of the crystal giving rise to an atomically smooth surface (as compared to the original rough surface). Once this surface has been created, growth proceeded by birth and spread or by spiral growth. An example of this is displayed in Figure 3. Figure 3a shows a growing ZnPO-FAU crystal where 2-D nucleation over the whole surface has set in. After a few minutes, a spiral appeared near one of the edges (Figure 3b, white arrow) and dominated the growth over the whole crystal. Later, a second spiral formed nearby (Figure 3c, white arrow) and overgrew the previous one (Figure 3d). Although it is not clear from the image, this spiral was probably a composite one, as it produced steps at a higher rate (as can be seen from the smaller separation between the steps) than the first one and therefore dominated the growth. On some occasions both processes, 2-D nucleation and spiral growth, were in operation at the same time on different crystals (Figure 4) indicating that the solution was still at medium supersaturation.12 In those experiments performed with type 2 solutions, crystal growth started by multiple 2-D nucleation over the surface of the seed crystals or by a combination of 2-D nucleation and spiral growth (Figure 5). This indicates that the solution was not as highly supersaturated as in type 1. The phenomena of preferential nucleation over (or close to) crystal edges has been observed before in different crystalline systems such as ionic crystals,13 metals,14 ice,14 or molecular crystals1417 and has been explained in terms of a fluctuating surface concentration, known as the Berg effect18 or polyhedral instability. All crystals growing from solution are surrounded by a layer of stagnant solution (boundary layer) where mass transport takes place by diffusion and whose width is controlled by the degree of agitation of the system. Berg18 observed directly the shape of the boundary layer around polyhedral crystals and determined that it did not follow that of the crystal. This results in edges and vertices better supplied with nutrients (i.e., a narrower boundary layer) than the centers of the facets. Since the relative supersaturation will be higher on the crystal edges, a higher nucleation rate is expected in these areas. For moderate supersaturation, this effect could be compensated and the shape of the face will remain flat.16,17,19 Nevertheless, at higher supersaturation the compensation mechanisms (e.g., face kinetic constant anisotropy, temperature gradients, nonuniform distribution of 3164
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Figure 2. In situ AFM deflection images showing the preferential nucleation and growth on step edges for four different ZnPO-FAU crystals (ad). (df) Sequence of images showing the macrostep advancement toward the center of the crystal, resulting in an atomically flat surface.
Figure 4. In situ AFM deflection image showing two different ZnPOFAU crystals, one growing by a spiral growth mechanism (left) and the other by “birth and spread” (right). Figure 3. Sequence of in situ AFM deflection images of a growing ZnPO-FAU crystals. The transition from a “birth and spread” mechanism to spiral growth is seen from images (a) to (c). Images (c) and (d) show the formation of a new spiral center (probably composite) that takes over the previously formed spiral (b).
impurities) will be unable to counteract the Berg effect, and a central depression will form on the crystal facets, resulting in a hopper or skeletal morphology. In the case of the ZnPO-FAU crystal growth initial observations seem to point toward the Berg effect as an explanation for the growth under high supersaturation conditions. Nevertheless a closer look at the data reveals some inconsistencies with this model. First, a varying degree of supersaturation over the crystal face will imply a decrease in the step advancement speed as these
move toward the center of the crystals.16,17 However, this was not observed in any of the experiments performed; in fact, preferential nucleation on the edges was mainly constrained to the initial stages of growth. Second, since this effect was observed in the AFM, there is no reason to expect it would not take place during the synthesis of the crystal seeds. However, the final crystals do not possess a skeletal morphology (Figure 1a). Moreover, from pure geometric considerations, owing to the position of the crystal facets (at a similar height as the main body of the resin in which the crystals are embedded) during the AFM experiments, a more uniform boundary layer across the surface is to be expected, minimizing the Berg effect. Finally, the importance of the Berg effect has been found to decrease with the size of the crystal,17 making it less likely to happen on the micrometer-sized crystals observed here. A more plausible explanation 3165
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Crystal Growth & Design to our observations may be found by considering the control of the original surface and the resin/crystal interface on the nucleation rate. After its removal from the growing solution and subsequent washing and storing, the external surface of the seeds crystals can be expected to suffer reorganization and/or become covered with a number of impurities. This will have the effect of slowing down surface nucleation for a given supersaturation. Additionally, nucleation at edges and corners is promoted by the presence of the interface resin/crystal and will become dominant at medium to high supersaturations. This hypothesis would imply that no growth or very limited nucleation would be observed over the main body of the “seed” crystals
Figure 5. In situ AFM deflection images showing two different ZnPOFAU crystals, before (a, c) and after (b, d) a type 2 solution was introduced in the AFM fluid cell. It can be seen that in both cases growth starts by 2-D nucleation and spiral growth (b, top right), in contrast to what is observed for type 1 solutions (Figure 2).
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for solutions below a certain supersaturation. This has been observed to happen during in situ AFM experiments on ZnPOSOD20 and metal organic frameworks, where growth could not be initiated on seed crystals using the same solutions that produce growth over atomically flat surfaces. Following this model, after a new layer of defect-free ZnPO-FAU has grown over the original surface, nucleation will proceed at its normal rate (for a given supersaturation), as observed in the AFM experiments (Figures 2 and 3). Metastability. In all experiments the rate of nucleation and step advancement decreased as time progressed until growth
Figure 7. Optical microscope images showing a ZnPO-FAU sample before (a) and at the end of (b) an in situ AFM experiment. During the course of the experiments new, needlelike, crystals are formed (b).
Figure 6. In situ AFM deflection images showing the growth and subsequent dissolution of a ZnPO-FAU crystal on the same experiment. Images were taken at (a) 8, (b) 14, (c) 22, (d) 32, (e) 42, (f) 52, (g) 66, and (h) 86 min after the introduction of the growing solution. 3166
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Crystal Growth & Design eventually stopped. This was invariably followed by dissolution of the crystals. The crystal growth deceleration varied from experiment to experiment and it was not related to the initial supersaturation of the growing solution. A time sequence (90 min) of a single experiment illustrating this effect is shown in Figure 6 (full movie in qt format is available in the HTML version of the paper). Figures 6ae show the growth regime which evolves from macro-step advancement from the corners to birth and spread and, finally, to step advancement. After approximately 52 min from the start of the experiment, growth stopped. This was immediately followed by an accelerating dissolution rate by step retreatment (Figure 6f,g). The decrease in the growth rate could be explained by a fast depletion of nutrients as the crystals grow, but the subsequent dissolution can only indicate that the ZnPO-FAU crystals are indeed metastable at the experimental conditions. This is confirmed by optical images of this and other experiments which show the nucleation and growth of a new phase, as shown in Figure 7. In order to further study the metastability of the ZnPO crystals, a series of syntheses were carried out over a period of 30 days at 4 C, and the products were studied by X-ray
Figure 8. SEM micrographs of a ZnPO-FAU synthesis taken after (a) 1, (b) 7, (c) 14, and (d) 30 days of reaction. The formation of a new phase is evident, as well as the appearance of dissolution features on the ZnPOFAU crystals, such as rounded edges and corners and etch pits.
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diffraction (XRD) and SEM. Figure 8 shows a series of SEM micrographs taken after 1, 7, 14, and 30 days of reaction. It can be seen how, after a week, a new phase starts to precipitate and increases its relative proportion with time. Powder XRD analysis of the sample after 30 days confirmed the formation of a hydrated-condensed zincophosphate phase. SEM micrographs also show signs of dissolution in the remaining ZnPO-FAU crystals, including the rounding of edges and corners and the
W (a) In situ AFM deflection image of a ZnPO-FAU crystal Figure 10. b at low supersaturation conditions. (b) Cross-section along two steps (black line in a) showing the monolayer step height. (c) Simplified faujasite-structure model showing the more plausible termination corresponding to the measured step height. A movie in qt format is available in the HTML version of the paper.
Figure 9. (a) SEM micrograph of a ZnPO-FAU crystal showing signs of dissolution. (b, c) Ex situ AFM deflection images of two different ZnPO-FAU crystals showing the formation of triangular etch pits. 3167
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Figure 11. In situ AFM deflection images of two composite spirals on a ZnPO crystal. (a) Shows two spirals with a Burgers’ vector of a single monolayer in length. (b) Single spiral with Burgers’ vector of two monolayers in length surrounded by five single step spirals.
formation of macroscopic etch pits (Figures 8bd and 9a). AFM analysis of these crystals confirmed the dissolution process by showing widespread formation of etch pits (Figure 9b,c). The fact that the transformation reaction is much slower in this case when compared to that observed in the AFM can be explained by the difference in temperature between both experiments. Therefore, the metastability of ZnPO-FAU is substantially increased at room temperature conditions (2530 C). 2-D Nucleation. As explained above, 2-D nucleation was observed in several experiments. In all cases, the shape of the nuclei is triangular, with the step directions forming a 60 angle with respect to the crystal edges. This is in agreement with what has been observed before on ZnPO-FAU grown hydrothermally8 and the aluminosilicate zeolite Y.21 Unfortunately, due to the rapidly changing supersaturation conditions in the experiments, it was not possible to obtain any meaningful value of nucleation rate (as this is supersaturation dependent), but it followed the expected trend, varying from tens of nuclei per square micrometer at high supersaturation to zero when approaching equilibrium. For the same reasons, no consequential values of step advancement speed could be measured. Height analysis of the steps observed reveals a step height of approximately 1.5 ( 0.1 nm (Figure 10). This value corresponds to the d111 spacing (Figure 10c) and is in agreement with the values reported by Singh et al.8 for ZnPO-FAU and aluminosilicate zeolite Y.21,22 Spiral Growth. Spiral growth was observed in several experiments, which means that this is a pervasive form of growth for these crystals. In all cases, the shape of the terraces produced by the spiral was in agreement with that formed by 2-D nucleation; that is, steps were parallel to the Æ110æ direction. Most spirals were produced by a screw dislocation with a Burgers’ vector of ca. 1.5 nm, that is, one monolayer (Figure 11), but in a few instances dislocations with a Burgers’ vector equivalent to two monolayers were observed (Figure 11b). This means that the structure is able to cope with a significant deformation, which will be required to produce a dislocation of almost 3 nm. Furthermore, on most occasions the spiral centers were of the composite type, with multiple dislocations located close to each other and generating multiple steps (Figure 11). Formation of composite spirals has been linked to inclusions in crystals.23,24 These inclusions produce an important mismatch and strain in the internal structure, which is relieved by the formation of multiple dislocations. In the case of ZnPO-FAU, it was possible to link, in one instance, composite spiral formation to the embedding of sub-
Figure 12. In situ AFM deflection images showing a linear defect across a growing ZnPO-FAU crystal (white arrows). The terrace growing from the top is stopped once it reaches the defect (ac), as well as the terrace growing below it (c, d).
micrometer crystallites onto a much larger ZnPO-FAU crystal. The spiral center shown in Figure 11b was observed on the same crystal shown in Figure 5ab where a ZnPO-FAU crystallite is shown to be embedded into the larger crystal (Figure 5a, top right). It is easy to assume that the crystallite will create a significant amount of stress into the structure which would explain the formation of the double step spiral. Linear Defects. Growth experiments on the ZnPO-FAU system show the formation of another type of linear defect that has not been identified before in any AFM study performed on microporous materials to date. An example of this defect is shown in Figure 12 where a horizontal line running across the crystal can be observed (highlighted by white arrows in Figure 12a). In subsequent images, it can be seen how the steps formed at either side of the line are not able to cross it as they grow. Height analysis across the defect shows a height of ca. 0.3 ( 0.1 nm, 3168
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Figure 13. (ah) Sequence of in situ AFM deflection images on a growing ZnPO-FAU crystal showing the formation and disappearance of a linear defect.
Figure 14. Simplified drawing showing the formation and healing mechanism of a low angle dislocation.
which explains why it is acting as a barrier to step movement. Further insight into the formation of these linear defects is shown in Figure 13. Figure 13a shows the initial stage of growth of a ZnPO-FAU crystal. At this point, growth takes place by 2-D nucleation over the body of the crystal, but principally by the advancement of macrosteps from the edge of the crystal toward its center. A pseudohexagonal crystal can be seen in the top right corner of the image (white box). As the main crystal is completely covered by the macrosteps, the outline of the smaller, pseudohexagonal crystal can be clearly seen in Figure 13b,c. This indicates that there is a certain degree of mismatch between the two crystals. Figure 13d shows a close-up view of the defect (white arrows) formed where it is clear how the steps on one side are not able to advance over it (i.e., there is no continuity of the steps across the defect). As time progresses, and further growth takes place by step advancement, the linear defect starts to withdraw from the left to the top of the image until it completely disappears (Figure 13eh). This indicates that at one point there is a match between the structures at both sides of the line defect. A possible explanation for the formation and disappearance of this defect could be a mismatch produced by the formation of different stacking sequences in different regions of the crystal. It is well-known that the faujasite structure can be thought as constructed by the stacking of layers with an ABCABC sequence.25 It has also been observed that sometimes this sequence can deviate into an ABAB type of stacking producing the EMT polymorph.25 If a ZnPO-FAU was to possess two different stacking sequences into two different areas, there will be a point in time (as the crystal grew) and location where they would meet. The difference in the type of layers falling alongside each other would create a boundary difficult to bridge by any
single growing step. Nevertheless, if the steps along both sides of the defect where to be of the same type, this boundary would disappear. Although this explanation seems, in principle, plausible it can be dismissed by two observations. First of all, the defect observed shows a small, but real, difference in height. If this was produced by a difference in stacking one would not expect to see a difference in height across the defect. Second, it was observed that the disappearance of the defects takes place over a great number of steps, not in a single layer as would be the case if a difference of stacking was the cause. With every step that grows the defect seems to “disappear” a little bit, in a way that resembles a “zipper” being closed. A different explanation for this defect is the formation of low angle dislocation with a Burgers’ vector of sub-monolayer magnitude, as reflected by the measured step of 0.3 nm when compared to the ca. 1.5 nm of a single monolayer. A cartoon showing the possible “healing” mechanism for this low angle dislocation is shown in Figure 14. Figure 14a shows a low angle (denoted by the dotted-line) dislocation of sub-monolayer step height. As new layers grow and reach the dislocation the length of the linear defect decreases (Figure 14bc) and will eventually disappear from the surface. This kind of model requires that some type of connectivity exists across the dislocation so that the ca. 0.3 nm step can be formed. This may be explained by looking at the faujasite structure itself. The faujasite structure along the {111} face is composed of rows of sodalite cages linked by double six rings, as shown in Figure 15a (highlighted in green box). These sodalite cages alternate between two heights as represented by two different colors in the cross-section shown in Figure 15b.The difference in height between the two types of cage positions is ca. 0.3 nm. If two {111} layers of faujasite where separated by a step of this height, there could be a 3169
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until it is “healed”. Since this plane will be populated by a large number of defects in the structure connectivity, it has the potential of acting as a low resistance barrier for diffusion. This has been proved for other types of high defect planes in other zeolite systems.26 Ultimately, the formation of this type of defect seems to be possible due to the very nature of the faujasite structure type that can accommodate a 0.3 nm step across its structure. This may explain why it has not been reported in AFM observations performed on other zeolite framework types.
’ CONCLUSIONS In situ observations on the crystal growth of ZnPO-FAU were carried out for the first time. Results show that at medium-to-high supersaturation conditions growth starts preferentially at crystal edges and corners forming macrosteps with a height of several nanometers that advance toward the center of the crystal. This behavior may be explained by nucleation inhibition on the crystal surface due to contamination. At low to moderate supersaturation conditions, growth takes place by a birth-and-spread and/or spiral growth mechanism. The formation of multiple and doublestep spirals could be linked to the entrapment of microcrystals of ZnPO-FAU during the growth process. ZnPO-FAU crystals were found to be metastable at the experimental conditions (25 C, 1 atm) as initial growth slowed down considerably during the duration of the experiment and was then followed by the dissolution of the crystals and the precipitation of a new phase. Metastability is reduced considerably at 4 C where ZnPO-FAU could still be observed after a 30day period. Finally, the formation of an unknown type of linear defect was observed in situ. This line defect is produced by a low angle dislocation forming a 0.3 nm step across ZnPO-FAU layers. Defect formation was linked to the embedding of microcrystals of the same phase as growth takes place. Because of the nature of the FAU structure, this kind of defect can be accommodated with a certain degree of connectivity in the structure which allows for it to repair itself as new layers of the structure are added. W b
Web Enhanced Feature. Full sequences of images recorded for the experiments displayed in Figures 6 and 12 are included as movies. These movies show a full growth to dissolution sequence as well as growth in the presence of a line defect.
Figure 15. (a) Simplified drawing showing the faujasite structure perpendicular to the Æ111æ direction. Box shows a row of sodalite cages linked through double six rings. Each color represents a different height. (b) Cross-section (along green line in a) showing the two distinct height positions of sodalite cages. (c) Structural diagram showing the connectivity of two different faujasite layers (in different color) across the linear defect (red line). Each colored sodalite cage represents a different height.
high degree of connectivity between them, as shown in Figure 15c. In this figure, each layer is represented by a different color, and the three cage colors correspond to three different heights for the sodalite cages, with blue being the highest, followed by orange and green. Across the red line, sodalite cages of the same height would be connected through double six rings (with a certain degree of rotation and loose bonds). The resultant structure will show a step height, across the red line, resulting from the difference between the highest cage positions at each layer, that is, of 0.3 nm. Then, as the crystal grows by the addition of new layers, the line defect will be projected as a plane upward,
’ AUTHOR INFORMATION Corresponding Author
*Tel: þ44 161 306 2770; fax: þ44 161 306 4559; e-mail: pablo.
[email protected].
’ ACKNOWLEDGMENT Funding was provided by EPSRC and ExxonMobil Research and Engineering. ’ REFERENCES (1) Cundy, C. S.; Cox, P. A. Microporous Mesoporous Mater. 2005, 82, 1–78. (2) Gier, T. E.; Stucky, G. D. Nature 1991, 349, 508–510. (3) Nenoff, T. M.; Harrison, W. T. A.; Gier, T. E.; Stucky, G. D. J. Am. Chem. Soc. 1991, 113, 378–379. (4) Harrison, W. T. A.; Gier, T. E.; Moran, K. L.; Nicol, J. M.; Eckert, H.; Stucky, G. D. Chem. Mater. 1991, 3, 27–29. 3170
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