Transformation of Bulk Alloys to Inorganic Nanowires
Authors: Magasinksi Alexandre 
Published: May 2020

One-dimensional (1D) nanomaterials, namely nanowires (NWs), nanotubes and nanofibers, have emerged as feasible building blocks for the next generation of devices and materials, with applications envisioned in electronics, sensors, energy technologies and water purification [1]. Because of their high strength and modulus, 1D materials are seen as reinforcement fillers for metal, polymer, epoxy, and glass composites. Remarkably, the reduction of the diameter of the 1D materials from 1000 nm by the factor of 10-100 can dramatically increase their mechanical properties. In particular, the Al2O3 1D materials with the diameters of the sub 50 nm are believed to exhibit superior tensile strength and modulus, as predicted theoretically (tensile strength 20 GPa) [2], which is of tremendous importance to the field of engineered composites, especially taking into account availability of alumina, its light weight and low cost. However, the synthesis of ceramic 1D materials embraces outstanding challenges with a use of complicated reaction techniques.
Here, we demonstrate an unexpected discovery of the synthesis of metal-organic Al alkoxide NWs with tunable diameters (40-1000 nm) and high aspect ratios (1,000+) upon exposure of bulk bimetallic Al-Li alloy to alcohols at ambient temperatures and pressures without the use of any catalysts, porous templates, corrosive chemicals or external stimuli [3, 4]. We further demonstrate a conversion of metal-organic Al alkoxide NWs to the corresponding ceramic Al2O3 NWs upon heating. We demonstrate the crucial parameters of controllable growth of the metal-organic NWs, such as composition of the alloy, molecular structure of the NWs, and the strain energy minimization of alloy grains at the reactive interface. We also show the cost-effective condition of the metal-organic alkoxide NWs synthesis by reducing the content of Li metal in alloy from 20 to 5wt.%. By utilizing the solution and solid-state nuclear magnetic resonance, we provide sufficient characterization rigor of the molecular structure of the metal-organic NWs. The conditions of the transformation of metal-organic NWs to oxide ceramic NWs and the microstructure of ceramic oxide NWs, which influence their mechanical properties will be additionally discussed.
We will also demonstrate the technology of the replacement of conventional polymer separators for Li-ion batteries (LIBs) by thermally stable, flexible, and wettable binder-free fabric from Al2O3-cellulose NWs. Because of the easiness in preparation and economical attractiveness, we foresee that our accessible and low-cost membranes will elicit as fundamental low-cost filler materials in LIBs.

References:
1.) Tian, B., et al., Coaxial silicon nanowires as solar cells and nanoelectronic power sources. Nature, 2007. 449: p. 885.
2.) Berger, M.-H., Fracture processes in oxide ceramic fibres, in Fiber Fracture, M. Elices and J. Llorca, Editors. 2002, Elsevier Science Ltd: Oxford. p. 89-105.
3.) Lei, D., et al., Transformation of bulk alloys to oxide nanowires. Science, 2017. 355(6322): p. 267-271.
4.) Wang, F., et al., Mechanisms of Transformation of Bulk Aluminum–Lithium Alloys to Aluminum Metal–Organic Nanowires. Journal of the American Chemical Society, 2018, 140(39): p.12493-12500.
Acknowledgement: This work was supported by Sila Nanotechnologies. GY is a shareholder of Sila.
Fig. 1. Transformation of bulk b-AlLi alloys to Al(OEt)3 NWs followed by the high temperature conversion to Al2O3 NWs. Demonstration of flexible properties of all ceramic Al2O3 NWs membrane.

Figure 1