Electro-assist deposition of binary sol-gel films with graded structure

Graded SiO₂-TiO₂ binary composite films are prepared by a unique electro-assist deposition approach from a mixture of sol-gel precursors containing tetramethoxysilane (TMOS) and titanium tetraisopropoxide (TTIP). A negative potential applied to either stainless steel or ITO substrates causes the reduction of the solvent, thus increasing the concentration of hydroxyl ions, which enhances the precursor condensation and film deposition. The Ti:Si ratio in the films depends on the deposition potential, time and composition of the precursor solution. This ratio decreases as the films grow thicker due to the more facile kinetics of silica deposition. Nevertheless, the Ti:Si ratio in the films is always lower than in the corresponding precursor solution. As the Ti:Si ratio in the deposition solution increases, the thickness of the electro-assist deposited films at the same conditions gradually decreases. SIMS and cross-section EDX analysis show that the Ti:Si ratio in the films increases from the surface to the substrate, suggesting electro-assist deposition as a potential method for one-step preparation of graded sol-gel films.