Layer-by-Layer Growth and Condensation Reactions of Niobate and Titanoniobate Thin Films

Abstract

Thin films were grown on amine-primed Si and glass substrates by sequential adsorption reactions of polyallylamine hydrochloride (PAH) and anionic colloids derived from HTiNbO5 and HCa2Nb3O10. The acid−base chemistry of polycation/polyanion adsorption was studied in detail for PAH/HTiNbO5. The pKa of PAH, defined as the pH at which it is half protonated, is 8.7. Titanoniobate colloids, prepared by reaction of HTiNbO5 with tetra(n-butylammonium) hydroxide, TBA+OH-, are unilamellar at pH ≥ 8.5 and restack below pH 7.0. Efficient tiling of a PAH-terminated surface by a layer of unilamellar titanoniobate sheets occurs only at intermediate pH values (8.5−9.0). At lower pH, the colloid restacks on the surface, and at higher pH, only partial coverage by single sheets is observed by atomic force microscopy (AFM). At pH 8.5, high-quality multilayer films can be grown by sequentially adsorbing PAH with either the titanoniobate or niobate colloid. TGA/DTA studies of bulk PAH/titanoniobate intercalation compounds show that they decompose oxidatively to form HTiNbO5 at 310−350 °C and that this decomposition is followed by interlayer condensation to make Ti2Nb2O9. A similar process occurs in the PAH/titanioniobate multilayer films at 350 °C.