Thin Films from Porous Nanoparticles (2008)

Porous materials in the nanometer range are interesting materials in many fields of science and technology. Especially in terms of colloidal suspensions, these materials are promising candidates for applications such as gas sensing, host-guest chemistry, drug delivery and coatings in the semiconductor industry. In this work, syntheses of monodisperse, stable, colloidal suspensions of different materials for the preparation of porous thin films were described. Colloidal suspensions of zeolite Beta nanocrystals with low aluminum content were prepared and the crystallization kinetics was studied. The crystalline Beta was stabilized in colloidal suspensions by addition of inorganic silica-based binders and applied for the preparation of films by a spin-on process. The thickness of the films was controlled by repeated coating steps, speed of deposition and Beta concentration of the coating suspensions. The Beta films exhibit good mechanical properties, smooth surfaces and show a low refractive index, which is typical for highly porous silica based materials. Additionally, a synthetic approach for the preparation of colloidal mesoporous silica spheres and functionalized colloidal suspensions of nanoscale mesoporous materials with high yields from concentrated solutions is presented. Narrow particle size distributions in the range of about 50 to 150 nm were established with Dynamic Light Scattering measurements and electron microscopy before and after template extraction. Discrete nanoscale mesoporous particles with functionalized pore surfaces resulted when adding functional organoalkoxysilanes directly to the initial silica precursor solutions. Nitrogen sorption analysis indicated that the functional groups were located at the inner surfaces of the mesoporous channel systems. By further decreasing the diameter of these mesoporous silica spheres, their scattering ability for visible light was also drastically decreased. We have demonstrated the synthesis of extremely small mesoporous silica nanoparticles via a specific co-condensation process with phenyl groups. If the size of the particles falls below about 1/10 of the wavelength of the incoming light (0.1 ), the colloidal suspensions show optical transparency. Because of an easy handling and a good reproducibility, the suspensions are ideal for the production of thin film by spin-coating. The films showed excellent optical qualities, exhibited good diffusion properties and a highly accessible pore system. Thanks to the small particle size and the resulting low surface roughness, the formation of multilayers was possible without transmitting defects on the surface with every following coating step. The availability of such homogeneous porous thin films made it possible to use ellipsometry as analysis method. Ellipsometric porosimetry (EP) is a convenient method to determine the effective porosity of a thin film on its original support without destroying it. It was possible to record sorption isotherms of the thin films with ellipsometry and to correlate the data with nitrogen sorption data of dried powders of the same material. The thin films showed very low refractive indices around 1.2 in the case of both, zeolites and functionalized mesoporous silica nanoparticles. Besides, a synthesis procedure for TiO2 particles in an acidic medium by a simple sol-gel process was investigated. The material showed a high surface area and the thin films prepared from the colloidal suspensions had a high refractive index combined with a certain porosity. Thus, the preparation of wavelength specific Bragg mirrors could be realized by a simple and reproducible spin-coating approach using colloidal suspensions of functionalized porous silica nanoparticles and titania sols. The Bragg reflectors show a sensitivity towards specific relative pressures of organic vapors like isopropanol or toluene.