(9g) Shaping Single-Walled Metal Oxide Nanotubes From Precursors of Controlled Curvature
In this talk, we will discuss new molecular-level concepts [1,2] for constructing nanoscopic metal oxide objects, specifically single-walled metal oxide nanotubes. These materials are attractive emerging candidates for a variety of 'molecular recognition' applications in separations, catalysis, and other areas. The capability to engineer the shape and size of such objects with molecular-scale precision is a sought-after goal in nanomaterials design and synthesis.
First, we describe the identification and elucidation of the mechanistic role of molecular precursors and nanoscale (1-3 nm) intermediates with intrinsic curvature, in the formation of single-walled aluminosilicate nanotubes. We characterize the structural and compositional evolution of molecular and nanoscale species over a length scale of 0.1-100 nm, by electrospray ionization (ESI) mass spectrometry, nuclear magnetic resonance (NMR) spectroscopy (27Al liquid-state, 27Al and 29Si solid-state MAS) and dynamic light scattering (DLS). Together with structural optimization of key experimentally identified species by solvated density functional theory (DFT) calculations, this study reveals the existence of intermediates with bonding environments, as well as intrinsic curvature, similar to the structure of the final nanotube product.
Then, we show that “proto-nanotube-like” intermediates with inherent curvature form in aqueous synthesis solutions immediately after initial hydrolysis of reactants, disappear from the solution upon heating to 95°C due to condensation accompanied by an abrupt pH decrease, and finally form ordered single-walled aluminosilicate nanotubes. Integration of all our results leads to the construction of the first molecular-level mechanism of single-walled metal oxide nanotube formation, incorporating the role of monomeric and polymeric aluminosilicate species as well as larger nanoparticles.
Using this mechanistic knowledge, we demonstrate how the diameters of metal oxide nanotubes can be shaped with Ångstrom-level precision by controlling the shape of nanometer-scale precursors. Furthermore, we measure (at the molecular level) the subtle relationships between precursor shape and structure, and final nanotube curvature. Anionic ligands are used to exert fine control over precursor shapes, allowing assembly into nanotubes whose diameters relate directly to the curvatures of the ‘shaped’ precursors.
 G. I. Yucelen, R. P. Choudhury, A. Vyalikh, U. Scheler, H. W. Beckham, and S.Nair, "Formation of Single-Walled Aluminosilicate Nanotubes from Molecular Precursors and Curved Nanoscale Intermediates", Journal of the American Chemical Society, 133(14), p. 5397-5412 (2011).
 G. I. Yucelen, D.-Y. Kang, R. C. Guerrero-Ferreira, E. R. Wright, H. W. Beckham, and S. Nair, "Shaping Single-Walled Metal Oxide Nanotubes from Precursors of Controlled Curvature", Nano Letters, 12 (2), p. 827-832 (2012).