Nb2O5, LiNbO3, and (Na, K)NbO3 Thin Films from High-Concentration Aqueous Nb-Polyoxometalates

Synthesizing functional materials from water contributes to a sustainable energy future. On the atomic level, water drives complex metal hydrolysis/condensation/speciation, acid-base, ion pairing, and solvation reactions that ultimately direct material assembly pathways. Here, we demonstrate the importance of Nb-polyoxometalate (Nb-POM) speciation in enabling deposition of Nb2O5, LiNbO3, and (Na, K)NbO3 (KNN) from high-concentration solutions, up to 2.5 M Nb for Nb2O5 and similar to 1 M Nb for LiNbO3 and KNN. Deposition of KNN from 1 M Nb concentration represents a potentially important advancment in lead-free piezoelectrics, an application that requires thick films. Solution characterization via small-angle X-ray scattering and Raman spectroscopy described the speciation for all precursor solutions as the [HxNb24O72]((x-24)) POM, as did total pair distribution function analyses of X-ray scattering of amorphous gels prior to conversion to oxides. The tendency of the Nb-24-POM to form extended networks without crystallization leads to conformal and well-adhered films. The films were characterized by X-ray diffraction, atomic force microscopy, scanning electron microscopy, ellipsometry, and X-ray photoelectron spectroscopy. As a strategy to convert aqueous deposition solutions from {Nb-10}-POMs to {Nb-24}-POMs, we devised a general procedure to produce doped Nb3O5 thin films including Ca, Ag, and Cu doping.

Automated summary

Synthesizing functional materials from water is crucial for the future of sustainable energy. This study demonstrates the importance of Nb-POM speciation in enabling deposition of Nb2O5, LiNbO3, and KNN from high-concentration solutions. The films were characterized using various techniques, and a new method was proposed to convert aqueous deposition solutions from {Nb-10}-POMs to {Nb-24}-POMs.