Morphology of sol–gel produced composite films for optical oxygen sensors

The results reported concern the characterization of thin layer SiO2-based matrices with an oxygen sensing component Ru(II)-tris(4,7-diphenyl-1,10-phenanthroline) immobilized, when a sol–gel process along with dip- and spin-coating deposition methods are used. SEM, TEM and AFM study, assisted by X-ray energy dispersive microanalysis reveals the influence of the precursors used, sol treatment and the coating conditions on the films morphology and Ru distribution in the matrices. Uniform and smooth surface is produced from tetraethoxysilane (TEOS). The presence of ormosils (methyltriethoxysilane, MtEOS and octyltriethoxysilane, OtEOS) significantly increases the surface roughness exhibited as dots on the SEM image. Their surface concentration and size depend on the number of immersions and withdrawal speed at the dip coating. Spin deposition leads to rather different morphology of the films, based on TEOS/OtEOS. Following commonly used sol preparation procedure (with 1.25–2.5 g Ru-complex/dm3 sol) microcrystallization of the complex occurs with formation of randomly distributed crystals 100–400 nm in size. The ultrasound treatment of the sol by means of ultrasound disintegrator leads to homogeneous distribution of the complex without observable crystallization and significant improvement of the film sensing properties (increase of Stern–Volmer constant and better linearity of the Stern–Volmer plots both in gaseous and aqueous media).