Characterisation of sol-gel TiO2 films by etching

Thin TiO2 films that have been prepared from a hydrolysed tetra-ethyl-ortho-titanate (TEOTi) solution are characterised by etching the films in a 0.25% HF solution. The etch rate is influenced by a number of process parameters that are applied for the preparation of the coating solutions. The etch rare of the TiO2 film increases when the hydrolysis is performed in a more diluted system. However, after a hydrolysis time of 30 min, further dilution of the hydrolysis mixture has only a minor effect on the etch rate of the final coating. An increase of the water versus TEOTi molar ratio and an increase of the HCl concentration in the hydrolysis mixture result in a decrease of the etch rate of the TiO2 film. The etch rate of the film is highly influenced by the morphology of the film. TEM measurements of the films have shown that a low etch rate is found for films consisting of small particles. In particular, the hydrolysis stage of the processing of the coating solution has a large effect on the morphology of the final film.