The effects of drying time and relative humidity on the stability of sol-gel derived silicate films in solution

The stability of thin silicate films in solution has been evaluated as a function of drying time and drying conditions using scanning probe microscopy and an electrochemical probe technique. In these experiments, the silicate films were spin coated on various substrates using a sol formed by the acid catalyzed hydrolysis and condensation of tetramethoxysilane. The silicate films were then dried under a relative humidity of 25, 50, or 75% for 3, 12, 24, or 48 h. After drying, the films were immediately placed in a solution of potassium nitrate or a solution of a redox probe, specifically ferrocene methanol, ruthenium hexaammine, or cobalt phenanthroline. Cyclic voltammetry in conjunction with atomic force microscopy was used to monitor the disruption in gel structure as the films sat in aqueous solution. The stability of each film was found to be essentially independent of the substrate it was coated on but a strong function of the length of the drying time and the relative humidity under which it was dried. Films that were dried at 25% humidity for 48 h stayed on the electrode for several days. In contrast, films that were dried at 75% humidity for 3 h fell off the electrode within 24 h. (C) 2002 Elsevier Science B.V. All rights reserved.