Ultrathin films of inorganic materials (SiO2 nanoparticle, montmorillonite microplate, and molybdenum oxide) prepared by alternate layer-by-layer assembly with organic polyions

We have been investigating alternate assembling of inorganic materials, SiO2 nanoparticles, clay microplate, and polyoxometalates with oppositely-charged polyions. In this paper, previously reported results are summarized and compared in order to establish a unified interpretation. Reproducible film growth is observed in assembly of anionic SiO2 particles and cationic poly(diallyldimethylammonium chloride) (PDDA). The adsorption time required for successful assembly of SiO2 nanoparticles is only 2 s. Scanning electron microscopic (SEM) observation shows that the obtained film has a surprisingly flat surface. Similarly montmorillonite clay microplate is successfully assembled with cationic PDDA and poly(ethyleneimine) (PEI), but its adsorption required 5–6 min. These adsorption kinetics are interpreted based on the nature of the adsorbent. Adsorption of rigid and spherical SiO2 particles is diffusion-limited and very fast, probably because the relaxation process is not conceivable during the adsorption. In contrast, relaxation processes may play an important role in the adsorption of planar clay plates, as suggested previously for adsorption of organic polyions. Multilayer films of molybdenum oxide are prepared by alternate adsorption of ammonium octamolybdate ((NH4)4[Mo8O26]) and poly(allylamine) hydrochloride (PAH). Adsorption of molybdenum oxide does not show saturation. Protonation and condensation of the molybdenum oxide are probably accelerated on the pre-adsorbed molybdenum surface, resulting in film growth without saturation.