On the Preferred Orientation and Microstructural Manipulation of Molecular Sieve Films Prepared by Secondary Growth

MFI-supported films were prepared using secondary growth of precursor layers. The precursor layers were prepared free of additives and no calcination step was used before secondary growth. Combined microstructural control over film thickness, orientation, continuity, and surface roughness was achieved. Hydrothermal conditions were identified, such as film growth proceeds predominantly by direct growth of the particles of the precursor layer. The prepared polycrystalline films have a columnar microstructure. The out-of-plane grain orientation can be manipulated by secondary growth conditions, and is such that the c -axes of the grains range from perpendicular to the substrate to an angle of ∼34° from the direction normal to the substrate, whereas the in-plane grain orientation is random. Initial film growth rates were sustained at a constant value for extended periods of time with an activation energy of 70 kJ/mol. The degree of out-of-plane orientation increases with film thickness up to a point where changes in concentrations in the secondary growth solution lead to surface roughening and change in the preferred growth direction. Sustained growth preserving film orientation and columnar microstructure is achieved during repeated secondary growth, allowing for control of the film thickness without compromising the deposit orientation and surface smoothness. As a result of their microstructure exhibiting well-intergrown columnar texture with small surface roughness, oriented films with thickness well exceeding the wavelength of visible light are optically transparent.