Multilayer deposition of silica sol-gel films by electrochemical assisted techniques

In the past decade, electro-assisted sol−gel deposition was employed by different groups in the world to deposit thin coatings on a variety of substrates. By applying a negative potential, the reduction reactions of water and oxygen occur at the working electrode, and the concurring generation of hydroxide makes the pH basic, thus enabling a faster condensation of sol−gel precursors. In this work, we present two new methods that allow one to modulate the thickness of the deposited layer. The first one consists in an electrochemically assisted deposition of multilayers, with a linear growth of the film up to ten layers. A final thickness of 1 ± 0.1 μm is achievable, without cracks and with no heat treatment required. The second method uses a pulsed potential in order to control the diffusion of hydroxide ions and, as a consequence, the growth of the silica layer on the substrate, determining a good homogeneity of the film. The starting solution contains a mixture of tetraethyl orthosilicate (TEOS), triethoxymethylsilane (MTES), water, ethanol, hydrochloric acid, and potassium nitrate as electrolyte. The samples are characterized by ellipsometery, using Cauchy model.