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Growth of Silicalite Films on Pre-Assembled Layers of Nanoscale Seed Crystals on Piezoelectric Chemical Sensors** By Svetlana Mintova, Brian Schoeman, Valentin Valtchev, Johan Sterte, Shangyi Mo, and Thomas Bein* The synthesis of thin films and layers of zeolites has at- tracted increasing attention in the context of membrane separations, chemical sensors, and other applications.['] Clearly, the presentation of molecular sieving, intrazeolite sorption, diffusion and intrazeolite chemistry or catalytic functionality in thin-film architectures offers new opportu- nities. Several synthetic methods have been developed for the preparation of such structures, including the synthesis of rather thick films (several micrometers) of intergrown ZSM-5 or silicalite-1 materials on various substrates,"] de- position of pre-synthesized zeolites in sol-gel composite layers,['] attachment of crystals on molecular layer^,[^-^] and [*I Prof. T. Bein, S. Mo Department of Chemistry, Purdue University West Lafayette, IN 47907 (USA) Dr. S. Mintova, Dr. V. Valtchev Central Laboratory of Mineralogy and Crystallography Bulgarian Academy of Science 92 Rakovski Street, 1000 Sofia (Bulgaria) Dr. B. Schoeman, Prof. J. Sterte Division of Chemical Technology Lulei University of Technology S-971 87 Lulei (Sweden) [**I The authors gratefully acknowledge funding from the US National Sci- ence Foundation (T.B.), the National Research Council (Twinning Pro- gram, S.M. and T.B.), the Bulgarian Ministry of Science and Educa- tion, under contract X497 (V.V. and S.M.), and the Swedish Research Council for Engineering Sciences (TFR; B.S. and J.S.). S. Mo is a reci- pient of a Motorola Partnerships in Research Grant. growth of oriented aluminophosphate crystals on stable, molecular Zr-phosphonate m~ltilayers.[~~ In applications of the latter systems, it was demonstrated that zeolite layers on piezoelectric devices such as quartz crystal microba- lances (QCMS)'" or surface acoustic wave devices impart molecular selectivity to the sorption response of the device, thus creating highly selective and stable chemical sensors. While many types of zeolite layers and films have been prepared, it would be desirable to reduce the size of the zeolite crystals composing the film (faster transport and equilibration) combined with high crystal density (greater mass response with piezoelectric sensors, integrity of se- paration membranes). In this communication we report on the electrostatic assembly of nanoscale, colloidal silicalite-1 crystals (MFI structure) on organic layers supported on QCM electrodes, followed by growth of a dense film of sili- calite-1 from a clear precursor solution. The zeolite-modi- fied QCM devices were evaluated as chemical sensors by exposing them to highly dilute hydrocarbon vapors. The synthetic strategy used in the preparation of the si- licalite-1 film on the gold electrode of the QCM device is presented in Figure 1. For the formation of a molecular layer that can later serve to attract the negatively charged silicalite-1 colloid, a monolayer of y- mercaptopropyltrimethoxysilane (MPS) was adsorbed on the QCM, followed by hydrolysis in dilute HCI, thus forming a reactive partially crosslinked surface of siloxy groups on the substrate. Ellipsometry showed a resulting layer thickness of 9-lOA. The charge on the surface was reversed by adsorption of a cationic polymer with the re- peat unit (CH,CH(OH)CH2N(CH,)2)$ and an average molecular weight of 50000 g/mol. The thickness of the polymer layer was determined by ellipsometry to be ca. 13 A, suggesting deposition of molecular double or triple 1 ayer s zyxwvutsrq . Discrete colloidal silicalite-1 crystals of 60 nm diameter"] were adsorbed onto the modified QCM surface from an aqueous suspension.['"l This approach utilizes principles si- milar to those reported for the preparation of diverse alter- nating layer structures of polyelectrolytes and inorganic colloids, including amorphous silica,[''] exfoliated hectorite clay,[l2I and other lamellar and particulate metal com- pounds.["] Finally, growth of dense silicalite layers of con- trolled thickness was achieved by immersing the QCM crystals into a silicalite-1 synthesis solution to yield a silica- lite-1 film with 250 nm thickness.['41 X-ray diffraction patterns of the QCMs after silane mod- ification and adsorption of seed crystals, and after silicalite growth, and the pattern of a well-crystallized bulk silicalite- 1 powder are shown in Figure 2. The X-ray data demon- strate the presence of silicalite-1 on the substrate following the zeolite deposition and growth steps, when compared to the diffractogram of bulk silicalite-1 (Fig. 2d). On the QCM substrates, the peaks at 38" and 45" 28 are due to the Au electrodes, whereas the broad peak at 12" 26 is due to the plastic holder supporting the small QCM disks in these Adv. Marev. 1991, 9, No. 7 zyxwvutsrqp 0 V C H Verlugsgesellschaft mbH, 0-69469 Weinhelm, 1997 0935-9648/97/0706-0585 $I7.50+.50/0 585