Reactive layer-by-layer films from solutions containing silicic acid and a Ti(IV) complex: preferential incorporation of silica and interactions of the obtained films with hexacyanoferrate anions.

The concept of reactive layer-by-layer (LBL) deposition allows the build-up of films containing polycations and oxide particles, namely, silica and poorly crystalline anatase. Because polyelectrolyte multilayer films have been produced from blended polyanions or polycations solutions and since preferential incorporation of one of the partners of the blend has been found in most cases, one should wonder if a preferential polycondensation of either silica or titania should occur when the reactive deposition is performed from a solution containing a precursor of both inorganic species. X-ray photoelectron (XPS) and UV-visible spectroscopies show that the reactive LBL films made from the blend and poly(diallyldimethylammonium chloride) (PDADMAC) incorporate predominantly silica over TiO(2) over the whole molar fraction range of the silicic acic/hydrosoluble Ti(IV) complex. The transparency of the films below 365 nm, corresponding to the band edge of TiO(2), can easily be modulated. The silica/TiO(2) films are all able to bind hexacyanoferrate owing to the presence of the polycation allowing the binding of the oxide particles to the substrate. However, the binding capacity of the film does not scale proportionally to its thickness. The films made from eight dipping cycles showed a sudden decrease in their binding capacity for hexacyanoferrate when the molar fraction of the titanium complex was higher than ∼0.6 in the blend. For the same films, electrochemical impedance spectra (EIS) showed marked differences with a change in film composition: the more TiO(2) in the film, the higher the resistance to electron and to mass transfer. Therefore, EIS helps to explain the reduced surface concentration measured by means of cyclic voltammetry for films rich in TiO(2).