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.

Metal-oxide thin films find many uses in (opto)­electronic and renewable energy technologies. Their deposition by solution methods aims to reduce manufacturing costs relative to vacuum deposition while achieving comparable electronic properties. Solution deposition on temperature-sensitive substrates (e.g., plastics), however, remains difficult due to the need to produce dense films with minimal thermal input. Here, we investigate combustion thin-film deposition, which has been proposed to produce high-quality metal-oxide films with little externally applied heat, thereby enabling low-temperature fabrication. We compare chemical composition, chemical structure, and evolved species from reactions of several metal nitrate [In­(NO3)3, Y­(NO3)3, and Mg­(NO3)2] and fuel additive (acetylacetone and glycine) mixtures in bulk and thin-film forms. We observe combustion in bulk materials but not in films. It appears acetylacetone is removed from the films before the nitrates, whereas glycine persists in the film beyond the annealing temperatures required for ignition in the bulk system. From analysis of X-ray photoelectron spectra, the oxide and nitrate content as a function of temperature are also inconsistent with combustion reactions occurring in the films. In­(NO3)3 decomposes alone at low temperature (∼200–250 °C) without fuel, and Y­(NO3)3 and Mg­(NO3)2 do not decompose fully until high temperature even in the presence of fuel when used to make thin films. This study therefore distinguishes bulk and thin-film reactivity for several model oxidizer-fuel systems, and we propose ways in which fuel additives may alter the film formation reaction pathway.

We describe a process to produce aqueous precursor solutions of the flat -Al 13 hydroxo cluster (Al13(μ3-OH)6(μ2-OH)18(H2O)24(NO3)15) via stoichiometric dissolution of bulk Al­(OH)3(s) in HNO3(aq). We highlight its facility by demonstrating high yields and large-scale synthesis. X-ray diffraction confirms formation of a single-phase product, and Raman spectra show characteristic O-Al-O vibrational modes, both techniques confirming the identity of the flat -Al 13 cluster in the bulk. 27Al NMR spectroscopy and dynamic light scattering also confirm the presence of the cluster in aqueous solution. We show the as-prepared solution produces smooth and continuous thin films via spin-coating. In capacitors, the films exhibit low leakage currents (near 10 nA/cm2) and dielectric constants expected for amorphous Al2O3. Because the precursor preparation requires no postsynthesis purification, it is readily scalable to large volumes.

Structure−property relationships were determined for the family of three-layer Aurivillius materials Bi 2 Sr(A)TiNb 2 O 12 (A = Ca 2+ , Sr 2+ , Ba 2+ ). X-ray and neutron diffraction along with selected area electron diffraction indicate that Bi 2 SrBaTiNb 2 O 12 crystallizes in the nonpolar I4/mmm space group, whereas the polar B2cb space group best describes Bi 2 SrCaTiNb 2 O 12 and Bi 2 Sr 2 TiNb 2 O 12 . Despite the different space groups, all three compositions show relaxor behavior as evidenced through P(E) and dielectric measurements. These relaxor properties are derived from the extensive amount of disorder in each composition that is found at every cationic crystallographic site and do not depend on the space group. This disorder is so extensive that it disrupts the ferroelectric properties allowed by symmetry in the B2cb space group. This work demonstrates the important role of cation substitution and site disorder in these three-layered Aurivillius materials and its significant effect on both ferroelectric and dielectric properties.

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