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

Abstract

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 ∼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 [H x Nb24O72](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 Nb24-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 {Nb10}-POMs to {Nb24}-POMs, we devised a general procedure to produce doped Nb2O5 thin films including Ca, Ag, and Cu doping.

Computational Prediction of Speciation Diagrams and Nucleation Mechanisms: Molecular Vanadium, Niobium, and Tantalum Oxide Nanoclusters in Solution

“…Metal oxides and molecular-oxo-clusters are key materials for the development of both current and future technologies. − These materials are usually prepared by crystallizing a solution containing molecular metal-oxo-clusters . However, how these compounds are formed in aqueous media and how high nuclearities are obtained remain unclear .…”

“…1−4 These materials are usually prepared by crystallizing a solution containing molecular metal-oxo-clusters. 5 However, how these compounds are formed in aqueous media and how high nuclearities are obtained remain unclear. 6 This is because the self-assembly of metal-oxo-clusters in water is affected by several variables: temperature, pH, ionic force, concentration, and counterions.…”

Decaniobate: The Fruit Fly of Niobium Polyoxometalate Chemistry

“…Flat and smooth films were obtained from 2.5 M Nb (as predominantly Nb 24 ), whereas all concentrations of Nb 10 (i.e., >0.5 M) yielded rough and nonconformal films (Figure 3B,C). 55 We attribute this to the gelation behavior of Nb 24 vs the crystallization behavior of Nb 10 .…”

“…SAXS of these solutions showed an impressive "structure factor", in addition to features indicative of Nb 24 clusters in solution (Figure 3D). 1,55 The structure factor, also called a Coulombic peak, indicates the ordering of clusters in solution, and the position of the peak on the q scale (Å −1 ) provides the average distance between clusters. This phenomenon is also described as "repulsion" between same-charge species, and in fact it can be diminished by adding shielding electrolytes.…”

Computational Prediction of Speciation Diagrams and Nucleation Mechanisms: Molecular Vanadium, Niobium and Tantalum Oxide Nanoclusters in Solution

“…[1][2][3][4] These materials are usually prepared by crystallization of a solution containing molecular metal-oxo clusters. 5 However, how these compounds are formed in aqueous media, and how high nuclearities are obtained remain unclear. 6 This is because the self-assembly of metal-oxo clusters in water is affected by several variables: temperature, pH, ionic force, concentration and counterions.…”