Deposition of oriented zeolite A films: in situ and secondary growth

Abstract

Zeolite A suspensions with a monomodal, narrow particle size distribution have been prepared. The suspended particles in a TMAOH water solution at pH 9 are negatively charged with a zeta potential of −43 mV. Modification of the external surface of the zeolite particles by a silylation reaction produces particles that, when they are suspended in water, are positively charged and have a zeta potential of +40 mV. The suspensions of the negatively or positively charged particles can be used for the preparation of adsorbed layers of particles on oppositely charged substrates by electrostatic attraction. This deposition process leads to a high coverage of the substrate with well-adhered particles. The cubic morphology of the zeolite particles results in preferential orientation after deposition. The particles are oriented with their {h 0 0} planes (cube faces) parallel and perpendicular to the substrate (out-of-plane orientation). The particles are randomly oriented with respect to the direction perpendicular to the substrate (in-plane orientation). Although, under optimized conditions, the coverage is high and only one adsorption cycle is necessary, the particles are not closely packed. Alternately, the zeolite particle suspensions can be used to deposit close-packed arrays of particles by convective particle transport during dip coating on substrates bearing the same charge as the zeolite particles. Using monodispersed zeolite A suspensions and slow speed dip coating close-packed hexagonal colloidal crystals were prepared. The type of colloidal crystal deposits formed range from continuous sublayers, monolayers, or multilayers to isolated discoidal clusters consisting of few zeolite particles. Factors affecting the deposited layer(s) structure are particle concentration of the suspension and withdrawal speed. In addition to close packing, the layers prepared by dip coating exhibit preferred orientation with the particle faces lying parallel and perpendicular to the substrate surface. Moreover, this second route of precursor film formation by colloidal crystallization leads to domains of well-aligned zeolite particles in three dimensions, i.e. with their faces parallel to each other. The oriented domains span the length of several particles; however, low angle boundaries and other defects during colloidal crystallization prevent the formation of macroscopically three-dimensionally ordered zeolite particles. The precursor layers were subjected to secondary growth in order to prepare continuous intergrown films. Secondary growth proceeds initially by local epitaxy on the deposited particles. Later in the process, deposition proceeds by incorporation of particles from solution along with re-nucleation on the growing film. The intergrown films have predominately [h 0 0] out-of-plane orientation; however, after extended secondary growth treatment a population of [h h h] grains appears on the surface of the regrown films.