Atomic Layer Deposition for the Conformal Coating of Nanoporous Materials

Atomic layer deposition (ALD) is ideal for applying precise, conformal coatings over nanoporous materials. We have recently used ALD to coat two nanoporous solids: anodic aluminum oxide (AAO) and silica aerogels. AAO possesses hexagonally ordered pores with diameters d∼10 nm and thicknesses L∼70 microns. The AAO membranes were coated by ALD with successive layers of A1 2 0 3 , TiO 2 and V 2 O 5 to fabricate catalytic membranes. SEM, TEM and EDAX analysis of the membranes demonstrate that the ALD layers uniformly coat the extremely high aspect ratio (L/d∼10 4 ) AAO pores. These catalytic membranes show remarkable selectivity in the oxidative dehydrogenation of cyclohexane. Additional AAO membranes coated with ALD Pd films show promise as hydrogen sensors. Silica aerogels have the lowest density and highest surface area of any solid material. Consequently, these materials serve as an excellent substrate to fabricate novel catalytic materials and gas sensors by ALD. In this study, both thin film and monolithic aerogels were coated by ZnO ALD and the properties of the aerogels were investigated as a function of the coating thickness.