Minerals to Materials: Bulk Synthesis of Aqueous Aluminum Clusters and Their Use as Precursors for Metal Oxide Thin Films

Abstract

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.

Visualization of Aluminum Ions at the Mica Water Interface Links Hydrolysis State-to-Surface Potential and Particle Adhesion

“…17 There are also implications for materials synthesis, where solution-based deposition of aluminum ions and clusters is an area of growing interest for the production of Al 2 O 3 thin films. 18,19 The adsorption of aluminum at phyllosilicate interfaces (such as the mica−water interface) is especially complex, since these surfaces can concentrate aluminum ions by many orders of magnitude relative to bulk, and are known to promote the formation of hydrolyzed polymeric aluminum species 20 and even induce the growth of crystalline aluminum-hydroxide films. 21−23 These complexities preclude the development of a fully general model for aluminum adsorption, because traditional surface-complexation models such as the TLM, which can be used to describe molecular adsorption, are unable to readily describe the formation of surface films or polymers.…”

“…For example, aluminum salts are widely employed as industrial coagulants and mordants due to their effectiveness at inducing particle coagulation, and aluminum ions are ubiquitous in environmental systems, where they interact with phyllosilicate minerals such as clay and mica to influence phenomena such as nutrient cycling, , soil mechanics, aluminum toxicity, mineral dissolution, , and nanoparticle/colloid transport . There are also implications for materials synthesis, where solution-based deposition of aluminum ions and clusters is an area of growing interest for the production of Al 2 O 3 thin films. , …”

Complexity of Uranyl Peroxide Cluster Speciation from Alkali-Directed Oxidative Dissolution of Uranium Dioxide

“…Factors affecting the dissolution of metal oxides under conditions conducive to formation of nanoscale metal oxide clusters in solution have not been extensively studied. Typically, transition-metal and actinide polyoxometalates (POMs) are synthesized in one-pot reactions using soluble precursors, with an emphasis on crystallizing clusters for structure characterization. − Rarely have studies directly synthesized POMs by starting with solid materials, , but one recent example provides initial evidence that formation of uranyl peroxide clusters encourages dissolution of UO 2 in alkaline solutions . There have been no detailed studies of the factors encouraging dissolution of uranium solids to form POMs.…”

The Role of Bi3+ in Promoting and Stabilizing Iron Oxo Clusters in Strong Acid

“…Molecular metal oxo clusters serve as models to understand metal oxide growth from solution in both nature and synthesis, in particular when a metastable nanophase precedes aggregation and crystallization . Metal oxo clusters giving rise to metal oxides through self‐assembly in water require strategic control over hydrolysis and condensation reactions, which depend mainly on pH, concentration, and temperature .…”

“…Molecularmetaloxoclustersserveasmodelstounderstand metal oxide growth from solution in both nature and synthesis,i np articular when am etastable nanophase precedes aggregation and crystallization. [1][2][3] Metal oxo clusters giving rise to metal oxides through self-assembly in water require strategic control over hydrolysis and condensation reactions, which depend mainly on pH, concentration, and temperature. [4] Identifying cluster forms on the reaction pathway to iron oxyhydroxides is of particular interest as iron is abundant in the geosphere and biosphere,a nd controls Eh-pH conditions in natural waters.I ron speciation in both nature and synthesis has remained particularly enigmatic [5] owing to high exchange rates of water bound to Fe 3+ [6] and the difficulty in harnessing the very rapid hydrolysis and condensation reactions.…”