The atomic level journey from aqueous polyoxometalate to metal oxide

Abstract

Aqueous precursors tailored for the deposition of thin film materials are desirable for sustainable, simple, low energy production of advanced materials. Yet the simple practice of using aqueous precursors is complicated by the multitude of interactions that occur between ions and water during dehydration. Here we use lithium polyoxoniobate salts to investigate the fundamental interactions in the transition from precursor cluster to oxide film. Small-angle X-ray scattering of solutions, total X-ray scattering of intermediate gels, and morphological and structural characterization of the lithium niobate thin films reveal the atomic level transitions between these states. The studies show that 1) Lithium-[H 2 Nb 6 O 19 ] 6has drastically different solution behaviour than lithium-[Nb 6 O 19 ] 8-, linked to the precursor salt structure 2) in both compositions, the intermediate gel preserves the polyoxoniobate clusters and show similar local order and 3) the morphology and phases of deposited films reflect the ions behaviour throughout the journey from cluster solution to metal oxide.

An overview of selected current approaches to the characterization of aqueous inorganic clusters

“…First, amorphous domains, disorder, and impurities present in samples are often still observable by NMR, unlike X-ray diffraction (XRD) which is generally unable to discern these due to the lack of long-range order. While 69 Ga is more naturally abundant at 60.4% compared to 71 Ga at 39.6%, its quadrupole moment is approximately twice as large as 71 Ga which results in much broader NMR resonances. 59 ssNMR is also a nondestructive technique, where the integrity of the sample remains intact after analysis, allowing other characterization techniques to be employed on the identical sample.…”

“…With these advancements, ssNMR offers a unique modality for the characterization of molecular clusters for multiple reasons. 59,61,62 Gallium has two NMR active isotopes, 69 Ga and 71 Ga, which are quadrupolar, and both have spin I = 3/2. ssNMR is both element-selective and a quantitative spectroscopic technique where the NMR resonances can be recorded such that the signal is proportional to the number and type of sites present for the isotope being probed in the material.…”

“…ssNMR is both element-selective and a quantitative spectroscopic technique where the NMR resonances can be recorded such that the signal is proportional to the number and type of sites present for the isotope being probed in the material. 69 Ga is useful to confirm spectroscopic assignments, but 71 Ga is typically the isotope of choice to acquire and model NMR spectra. The most familiar experiments performed with NMR involve nuclei with nuclear spin quantum numbers I = 1/2, such as 1 H and 13 C. However, the vast majority of the periodic table contains quadrupolar nuclei, i.e.…”

“…R g is a shape independent root mean square measure of all mass weighted vectors in the particle from the center of mass. 71 This method transforms the reciprocal space data to real space and yields a geometrical representation of the scattering particle as a probability histogram ( p(r) vs. radius). Scattering data is dependent on form factors (size, shape, and scattering contrast) and structure factors (interactions between particles).…”

First investigation of polyoxoniobate and polyoxotantalate aqueous speciation by capillary zone electrophoresis

“…Nonetheless, it has to be underlined that protonated alkali hexaniobate salts can be isolated whereas only non-protonated alkali salts have been isolated for hexatantalate 29,30. This suggests that the hexaniobate ions are easier to protonate than the hexatantalate ions.…”