Growth of oriented molecular sieves on organic layers

H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.

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Growth of Oriented Molecular Sieves on Organic Layers Sue Feng and Thomas Bein* Department of Chemistry, Purdue University, West Lafayette, IN 47907-1393, USA The oriented growth of zincophosphate molecular sieves on modified gold substrates is reported. Self-assembled layers of thiols with alcohol terminating groups were phosphorylated with phosphoryl chloride. A series of mixed layers containing alkyl chains and phosphate groups of similar chain lengths was prepared and the resulting mixed layers were examined as substrates for the room temperature growth of zincophosphate molecular sieves with FAU structure. The mixed functional films show profound effects on the density and orientation of surface-grown crystals. At high phosphate concentration, the crystals are highly oriented with their (111) faces parallel to the substrate, while both density and order decreased with decreasing phosphate content. The crystal density changes with phosphate density in a nonlinear fashion, suggesting a minimum surface concentration of phosphate groups for successful growth. 1. INTRODUCTION The great potential of zeolite-based thin films as a means to organize matter and to manipulate molecules has been increasingly recognized. 1 While the 'traditional' uses of zeolites include gas separation and a great variety of catalysis, the shape selectivity of these structures for molecular sorption and diffusion has inspired the creation of highly selective separation membranes and chemical sensors. Our interest in nanoporous assemblies on surfaces originates from the desire to construct highly selective chemical sensors. If a zeolite microcrystal is considered to function as a component with selective sorption properties, it will be desirable to assemble arrays of such microcrystals on appropriate sensor substrates such as piezoelectric oscillators. 2 We expect a great increase in surface area compared to the bare substrate, and a steep sorption isotherm at low partial pressure (amplification), as well as the molecular sieve selectivity and specific surface interactions of the zeolite (selectivity). Another advantage of zeolites is their considerable thermal and chemical stability, which is of significance for practical applications. With the goal of constructing chemically selective sensors, we have previously deposited zeolite/sol-gel composite films by dip-coating a suspension of the zeolite crystals in a tetraethylorthosilicate-derived sol on piezoelectric

2148 oscillators.3, 4 A second approach involved the bonding of zeolite crystals to a molecular coupling layer on the gold electrode of a piezoelectric oscillator (such as a quartz crystal microbalance, QCM). A layer of (3-mercaptopropyl)triethoxysilane was attached to the gold, followed by exposure to a suspension of the zeolite crystals. We have demonstrated the high molecular selectivity of chemical sensors based on this concept.5,6, 7 If oriented molecular sieve channels on surfaces are desired, more stringent requirements regarding the assembly must be met. Oriented molecular sieve channel structures are of interest for size-selective chemical sensors, separation membranes, and other novel devices. Our approach, displayed in Figure 1, is based on the expectation that certain functional organic layers can assist in the surface-nucleation and growth of molecular sieves. We have recently discovered that single component organo-phosphonate films can promote the growth of oriented molecular sieves, specifically zinco-phosphate and aluminophosphate crystals.8, 9 The crystals are attached to the surface with one of their triangular faces in the case of zinco-phosphate, and with oriented vertical channels in the case of aluminophosphate molecular sieves, and important factors such as film order, stability, and nature of the terminal functional group were discussed. The previous results led us to further investigate these systems, to address issues such as the zeolite crystal growth mechanism, and the interface reactions between the organic films and the inorganic zeolitic components. An understanding of how the molecular packing of phosphonate films can influence the molecular sieve crystal growth is important for the development of other oriented molecular sieve crystal films. However, the tri-layer films used initially contain a rather complicated structure which makes it more difficult to study s t r u c t u r e / r e a c t i v i t y relationships. In contrast, single layer organo phosphate films prepared by adsorption of 11-mercapto-1-undecanol (MUD) followed by phosphorylation of the hydroxyl group produce simpler structures than the tri-layer films. MUD self-assembles on gold surfaces to form densely-packed, monomolecular films. Because of their ease of preparation, and controllable surface chemical functionality, alkanethiol self-assembled monolayers have served as model organic surfaces in many fundamental studies of molecular adsorption, wetting, and lubrication.l~ The densely-packed nature and higher order of the organo phosphate single layers makes them even candidates for atomic imaging. Although the single layer organo phosphate films are expected to be more ordered, they have less thermal stability compared to the tri-layer films, thus the types of molecular sieves that can be grown on such surfaces are more limited13,14,15,16,17A8,19,20 In this article, we will describe studies using singlelayer organo phosphate films for the promotion of surface growth of zincophosphate molecular sieve crystals. We also describe the effect of mixed single organo phosphate monolayers of HS-(CH2)ll-CH3, and HS-(CH2)ll-O-PO3H2 on surface growth.

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Figure 1. Growth of zincophosphate molecular sieves on mixed alkyl/organophosphate single layers on gold (schematic). 2. EXPERIMENTAL SECTION A 100% phosphate self-assembled monolayer was formed on the gold substrate (on Cr/Si) via adsorption (for 48 h) of 11-mercapto-1-undecanol (MUD) from a 1.0 mM solution in ethanol. The MUD monolayers were phosphorylated for 1 h in a solution of 0.2 M phosphorus oxychloride (POC13) (Aldrich, 99%) and 0.2 M 2,4,6-collidine (Aldrich, 99%) in dry acetonitrile in a nitrogen filled glovebox, and then rinsed thoroughly with acetonitrile. The mixed films were prepared by immersing the gold substrates in pre-mixed solutions (at different ratios) of lmM 1-dodecanethiol (Aldrich, 98%) and lmM MUD solution in ethanol for 24 hours, followed by phosphorylation of the hydroxyl groups. For surface crystal growth, the substrates with different surface concentrations of organo-phosphate were placed into a zinco-phosphate molecular sieve synthesis bath. The zeolite X analog (Na,dabco)96Zn96P960192.192H20 21 was

2150 prepared from a clear solution containing 32 mmol NaOH, 134 mmol 1,4diazabicyclo[2.2.2] octane (dabco) and 64 mmol H3PO4, in 75 mL of water that was cooled to about 7 oC. To this was added a pre-cooled solution of 48 mmol Zn(NO3)2 in 10 mL of water to give a gel that converts to a milk on shaking. The crystallization was completed after 5 h at 7 oC. For thin film depositions, the gold substrate pre-coated with single layer organo phosphate films was carefully placed face-down into the zinco-phosphate gel mixture. The above systems were characterized with reflection-absorption infrared spectroscopy (RAIR), contact angle, ellipsometry, X-ray photoelectron spectroscopy (XPS), grazing-angle X-ray diffraction, and scanning electron microscopy (SEM). 3. RESULTS AND DISCUSSION 3.1. Characterization of Organic Mixed Films The Reflection Absorption IR (RAIR) spectrum of the pure HS-(CH2)11-CH3 monolayer on gold (not shown) exhibits five absorption bands in the C-H stretching region, as commonly found for a HS-(CH2)11-CH3 monolayer (with the following assignments of the C-H stretching peaks: CH2, Ua 2920 cm -1 and Us 2850 cm-1; for CH3, ua(ip) 2965 cm -1 and ua(FR) 2937 and Us 2878 cm-1). The exact frequencies of the CH2 stretching peaks have been used to differentiate between crystalline-like (ordered) and liquid-like (disordered) conformation of the alkyl chains.22, 23 A generally observed trend is that the CH2 vibrational frequency increases from 2918 to 2924 cm -1, when going from crystalline-like to a disordered conformation of the alkyl chain. Earlier infrared studies on gold substrates indicated that the thiols with a chain length greater than 11 methylene groups favor the better ordered structure where the Ua CH2 < 2920 cm -1. The band positions of the mixed layers therefore indicate a moderate degree of order, compared to the most crystalline films on gold. The CH3 bands are still present, but with reduced intensity in the spectrum of the 1:1 mixture of the HS-(CH2)llCH3:HS-(CH2)ll-O-PO3H2 monolayer. In the 0:1 mixture of the HS-(CH2)ll-CH3: HS-(CH2)ll-O-PO3H2 monolayer, the only bands present in the spectrum are the expected CH2 stretching modes. X-ray photoelectron spectroscopy (XPS) of the single layer mixed organo phosphate films of HS-(CH2)ll-CH3:HS-(CH2)ll-O-PO3H2 showed that S, C, O, and P are the only elements detected in the films. Apparent elemental ratios were obtained from the integrated signal intensities of the P(2p) and S(2p) peaks that were corrected for instrument and atomic sensitivity factors (we did not attempt to correct for different electron mean free paths).24, 25 The P(2p) intensity decreased as the concentration of the phosphonate component decreased in the mixture solution while the S(2p) intensity remained about constant. A plot of the P/S ratios indicates that the P/S ratio on the surface is only slightly lower than the P/S ratio in the solution.

2151 The cosine of the advancing water contact angle on the mixed layers shows a linear relationship with the molar ratios in the adsorption solution, suggesting (i) that there is no strong preference for adsorption of either of the two components HS-(CH2)11-CH3 or HS-(CH2)ll-OH (and by implication of the phosphate), and (ii) that Cassie's Law stating cos 0 = X1 cos 01 + X2 cos 02, 26 appears to hold reasonably well (X1 and X2 are the mole fractions of the two components in the mixed layer, and 01 and 02 are the contact angles on pure layers of the two components). Because the observed relationship is linear, we assume that the solution molar ratio is approximately reproduced on the surface.

3.2. Effects on zinco-phosphate crystal growth The single layer 100% organo-phosphate films promote the nucleation and growth of zinco-phosphate molecular sieve crystals from the hydrothermal synthesis mixture (see Figure 1). Scanning electron micrographs of the films show that single layers of zeolite crystals grow on the surface. The majority of the crystals grow to similar sizes and with their (111) faces oriented parallel to the substrate, and the basal faces truncated at the corners. In order to investigate the effect of the surface organo-phosphate molar ratio on the zinco-phosphate molecular sieve crystal growth density, we studied four different mixture ratios of single-layer mixed films composed of HS-(CH2)11-CH3: HS-(CH2)11-O-PO3H2.

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Figure 3. Grazing-angle x-ray diffraction patterns normalized to the (111) peak of zinco-phosphate molecular sieve crystals on mixed organo-phosphate films where the phosphate molar ratios are: (a) 1.00, (b) 0.50, (c) 0.333, and (d) 0.167. 20 = 8-38 ~ We observe three significant effects: (i) Scanning electron micrographs (SEMs) reveal that the zinco-phosphate crystal density increases as the mole fraction of the phosphate functional groups increases (Figure 2). We observe a steep initial increase up to a 'saturation' value at about 50% phosphate in the layer. (ii) Two different sizes of crystals are found on the surface. The large crystals are larger than l~tm, and smaller crystals are smaller than 0.5~tm. Finally, the crystals are less oriented on mixed organic films than on the 100% phosphate films, as shown by electron microscopy and grazingangle x-ray diffraction. Figure 3 shows normalized (vs. intensity of the strong (111) peak at 5.9 ~ 20, 28,700 cps) grazing-angle x-ray diffraction patterns of zinco-phosphate molecular sieve crystals on films where the phosphate molar ratio (Xp) ranges from 1.00 to 0.167. With the 100% phosphate film, the almost perfect orientation of the

2153 crystals is confirmed by the high intensity of the (111) peak (at 5.9 ~ 20, 28,700 cps) compared to all other peaks. In the mixed layer, the orientation decreases with decreasing phosphate content. For example, the intensity of the (311) peak at 11.6 ~ 20 increases from (a) to (d); this indicates an increasing disorder in the system. 4. CONCLUSIONS Self-assembled single layer pure and mixed organo-phosphate films on gold were used for the promotion of zinco-phosphate molecular sieve crystal growth. Spectroscopic and wetting data suggest that the compositions of mixed single layer films of HS-(CH2)ll-CH3 and HS-(CH2)11-OH (and the phosphate) closely reflect the compositions of the liquid precursor solution. Based on structural and microscopic evidence it appears that the surface crystallization requires only very small aggregates of phosphate groups, for example a triangle or a group of seven with hexagonal symmetry. This could explain the rapid drop of crystallization density at low phosphate concentration, where only very few aggregates would remain on the surface. On the other hand, small aggregates would be abundant beyond moderate phosphate concentrations, thus leading to a constant maximum crystal density (provided that the crystal density has an upper limit even on pure phosphate layers). The results obtained from these model systems can provide guidance for extended control of molecular sieve crystal growth at the inorganic-organic interface. Oriented channel structures are promising candidates for applications such as catalytic membranes with true molecular selectivity, or selective access of molecules of pre-selected size to sensor interfaces. 5. ACKNOWLEDGMENTS The authors greatly appreciate funding from the U.S. National Science Foundation, the Exxon Education Foundation, and from the Purdue Research Foundation.

REFERENCES

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Ozin, G. A.; Kuperman, A.; Stein, A. Angew. Chem. Int. Ed. Engl., 28 (1989) 359. Ward, M. D. and Buttry, D. A., Science, 249 (1990) 1000-1007. Bein, T., Brown, K., Frye, G. C. and Brinker, C. J., J. Am. Chem. Soc., 111 (1989) 7640-7641. Yan, Y., Bein, T., Brown, K. D., Forrister, R. and Brinker, C. J., Mat. Res. Soc. Symp. Proc. Vol. 271 (1992) 435-441. Yan, Y. and Bein, T., Chem. Mater., 4 (1992) 975-977.

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7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26.

Yan, Y. and Bein, T., J. Phys. Chem., 96 (1992) 9387-9393. Yan, Y. and Bein, T., J. Am. Chem. Soc., 117 (1995) 9990-9994. Feng, S. and Bein, T., Nature, 368 (1994) 834-836. Feng, S. and Bein, T., Science, 265 (1994) 1839-1841. Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem., 43 (1992) 437. Chidsey, C. E. D.; Loiacono, D. N. Langmuir, 6 (1990) 682. Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc., 112 (1990) 558. Ulman, A. Introduction to Thin Organic Films: From Langmuir-Blodgett to Self-Assembly, Academic: Boston, MA, 1991. Lee, H.; Kepley, L. J.; Hong, H.-G.; Mallouk, T. E. J. Am. Chem. Soc., 110 (1988) 618. Lee, H.; Kepley, L. J.; Hong, H.-G.; Akhter, S.; Mallouk, T. E. J. Phys. Chem., 92 (1988) 2597. Akhter, S.; Lee, H.; Hong, H.-G.; Mallouk, T. E.; White, J. M. J. Vac. Sci. Technol., 7 (1989) 1608. Hong, H.-G.; SackeR, D. D.; Mallouk, T. E. Chem. Mater., 3 (1991) 521. Kepley, L. J.; Sackett, D. D.; Bell, C. M.; Mallouk, T. E. Thin Solid Films, 208 (1992) 132. Cao, G.; Hong, H.-G.; Mallouk, T. E. Acc. Chem. Res., 25 (1992) 420. Bent, S. F.; Schilling, M. L.; Wilson, W. L.; Katz, H. E.; Harris, A. L. Chem. Mater., 6 (1994) 122. Gier, T. E.; Stucky, G. D. Nature, 349 (1991) 508. Snyders, R. G.; Strauss, H. L. J. Phys. Chem., 86 (1982) 5145. Snyders, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. J. Phys. Chem., 90 (1986) 5623. Scofield, J. H. J. Electron. Spectrosc., 8 (1976) 129. Seah, M. P. Practical Surface Analysis, 2nd ed.; Briggs, D.; Seah, M. P., Eds.; Auger and X-Ray Photoelectron "Spectroscopy, John Wiley and Sons: Chichester, 1990, 2, 240. Adamson, A. W. (1990) 'Physical Chemistry of Surfaces', Fifth Ed., Ch. X, Wiley Interscience: New York. See also: Cassie, A. B. D., Discuss. Faraday Soc., 3 (1948) 11.