Ultra-Thin Porous Film Coating Via Molecular Layer Deposition

Porous thin films/coatings have wide applications in separations, corrosion protective coatings, gas storage, controlled drug delivery, and catalyst encapsulation for controlling reaction selectivity and maintaining catalytic activity. Ultra-thin porous films/coatings are especially important for some applications. Inorganic films/coatings with few defects, such as zeolite membranes and mesoporous films, have been prepared by hydrothermal synthesis and sol-gel method. While these films/coatings have well defined uniform porous structures, it is difficult to control their thickness with the precision of angstroms. In this paper, we demonstrate a general method to create porous inorganic films/coatings with precisely controlled thickness down to several angstroms from dense organic/inorganic hybrid aluminum alkoxide (alucone) films grown by molecular layer deposition (MLD). The organic constituents in the film can be removed either by mild water etching at room temperature, or calcination in air at elevated temperatures. Because of the layer by layer growth process for MLD, the deposited polymer films have regular structure and the oxidation of MLD polymer films produces uniform interconnected highly porous structures with high surface area. The porous structure has both micropores and mesopores with BET surface area as high as 1200 m²/g. The porous alumina films were used to stabilize highly dispersed Pt nanoparticles (< 2 nm) supported on high surface silica gel particles. The Pt catalyst with a porous ultra-thin alumina layer was much more stable to calcination in air, even at 800 °C. We expect this porous thin film/coating growth technique, which offers both precise control of the thickness down to several angstroms and well defined porous structure, to have wide applications in catalyst functionalization, controlled drug delivery and nano-filtration.