Probing Aluminum-Oxo 2D Cluster Assembly Pathways to Gibbsite Nanoplates

Aluminum is the third-most abundant element in Earth’s crust. As a result, aluminum hydroxide polymorphs such as gibbsite and bayerite are abundant minerals in soils and dominate aluminum ores. They have also been widely used as adsorbents, fire retardants, coatings, catalysts, and luminescence powders, as well as comprising important precursors to various alumina products. Understanding the crystallization pathways of the aluminum hydroxide polymorphs is thus important to geochemistry, environmental science, energy storage, catalysis, biomedicine, industrial processing, and even nuclear waste treatment. Here we use <i>in situ</i> magic angle spinning nuclear magnetic resonance (MAS-NMR), scanning electron microscopy (SEM), scanning/transmission electron microscopy (S/TEM), atomic force microscopy (AFM), small angle X-ray scattering (SAXS), X-ray absorption spectroscopy (XAS), electrospray ionization (ESI)-mass spectrometry, X-ray diffraction (XRD), and X-ray pair distribution function (PDF) techniques to probe the nucleation and crystal growth mechanisms of gibbsite nanoplates in detail. By focusing on understanding the role of aluminum coordination change dynamics from tetrahedral in solution to octahedral in solids and vice versa, and by quantifying intermediate polyoxoaluminate cluster formation, some unifying principles governing these transformations emerge. Furthermore, various advanced techniques reveal the transformation and assembly of aluminum-oxo 2D clusters during the hydrothermal process, which indicate cluster assembly pathways to gibbsite nucleation and crystal growth. These findings are important for developing new methods for morphology and size-controlled synthesis of 2D aluminum hydroxide materials and may aid in the design of novel metal oxide 2D materials.