Nb₂O₅ and Ta₂O₅ Thin Films from Polyoxometalate Precursors: A Single Proton Makes a Difference

Thin film materials from water-based precursors follow the principals of green chemistry, leading to a more sustainable future in the energy intensive era in which we currently reside. While simple in practice, aqueous metal-oxide chemistry is complex at the molecular level. Here we develop the first water-based formation of Nb2O5 and Ta2O5 thin films; utilizing tetramethylammonium salts of [H2Ta6O19]6– and [H3Nb6O19]5– polyoxometalates. Although the clusters are structurally identical group V analogues and differ only by a single proton, this difference has a considerable influence on the quality of the films that are obtained. Through characterization of the solid-state precursor (single-crystal X-ray diffraction), the aqueous precursor solution (X-ray scattering), and the thin films (atomic force and scanning electron microscopies, X-ray diffraction, and reflectivity), we rationalize the important roles of cluster protonation that carry through all chemical processes from the precursor to the metal oxide coating.