Preparation of alumina films by the sol-gel method

This review describes our study on preparation of alumina films by a sol-gel process and their several applications that have been investigated since 1986. Alumina films were prepared from alkoxide or inorganic salt. Both as-prepared alumina films were transparent in ultraviolet, visible and near infrared regions. The alumina from inorganic salt (inorganic alumina) was structureless even after annealed at 300-700degreesC in air, while the alumina from alkoxide (alkoxide alumina) was in pseudo-boehmite at an annealing temperature lower than 400degreesC and was in gamma- or delta-type at 400-700degreesC. Both alumina films became opaque after annealed at temperatures above 1000degreesC. The inorganic alumina film annealed at 800degreesC showed a gas permeability that was influenced by physico-chemical properties of penetrant and alumina. Composite films of alumina and poly(vinyl alcohol) (PVA) were hydrophilic but insoluble in water, and removal of PVA from the composite films by annealing at 600degreesC led to formation of transparent alumina films. Such properties enabled us to use a counter diffusion method for fabricating gamma-Fe2O3-doped alumina films. Alumina films doped with organic dyes such as laser dyes, hole-burning dyes and non-linear optical dyes, which were fabricated by gelation of dye-added alumina sol, exhibited laser emission, hole-spectra and second- or third-harmonic generation properties, respectively. Hydrogenation of alkene was catalyzed by Ni nanoparticles doped alumina films that were prepared by gelation of Ni2+ solution-added alumina sol and annealing the Ni2+-doped alumina gel in hydrogen gas. Nonlinear optical properties were observed for alumina films doped with CdS, Au and Ag nanoparticles, which were fabricated by gelation of Cd2+, HAuCl4 and AgNO3 solution-added alumina sols and annealing the Cd2+-doped alumina gel in H2S gas and the Ag+- and Au3+-doped alumina gels in H-2 gas. Rare earth metal ion-doped alumina films, which were prepared by gelation of rare earth metal ion solution-added alumina sol and annealed the ion-doped alumina gel, exhibited not only normal luminescence but also up-conversion emission, energy transfer type luminescence and long lasting luminescence. (C) 2005 Springer Science + Business Media, Inc.