Ultrathin SiO 2 films in the range of 2−50-nm thickness were readily fabricated from inexpensive sodium silicate as starting material by its alternate adsorption with cationic polymer and subsequent treatment with O 2 plasma and calcination. Film thickness can be controlled by adjusting the number of adsorption cycles and the pH value of silicate solution. Film surface is generally smooth (small roughness) and remains unchanged after O 2 plasma treatment or calcination. Whereas a nanoporous thin film is obtained by O 2 plasma treatment, a dense silica film is produced through calcination at 450 °C. These preparative methods prove that inexpensive sodium silicates are converted to advanced silica-based materials, such as functional ultrathin films, coatings, capsules, and catalysts, by simple procedures.
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He et al. (2003) studied this question.