Grazing incidence synchrotron X-ray diffraction study of crystal orientation in microporous films

Thin microporous films with MFI type structure were deposited on silicon wafers via the seed method and spin coating of a stable colloidal mixture containing nanosized zeolites. The seed method includes adsorption of nanosized MFI seed crystals followed by hydrothermal synthesis of the seeds in order to obtain an intergrown film with a thickness of about 250 nm. In addition, thin MFI porous films with similar thicknesses were prepared by spin coating of stable colloidal suspensions and are compared with the seeded and grown layers. The diffractograms of the MFI films at grazing incidence and exit geometry were measured, which reflect the distribution of the crystal orientation along the film thickness. Preferred orientation of porous materials in the films is attributed to the deposition techniques and morphological features of the zeolite nanoparticles. The MFI adsorbed seeds and spin-deposited crystals are preferentially oriented with their a-axis perpendicular to the substrate, while the crystals in the grown film show an orientation change from the a- to the b-axis perpendicular to the substrate as a function of the film thickness. (C) 2002 Elsevier Science B.V. All rights reserved.