Atomic Engineering of Nanomaterials in Molten Salts and Liquid Metals

The use of inorganic molten salts and liquid metals as solvents opens up the possibility to synthesize hard-to-crystallize colloidal nanomaterials as well as lower the diffusion barriers in covalent lattices. In this work, we demonstrate how molten inorganic salts and liquid metals can be used to fabricate a new class of ceramic two-dimensional (2D) materials and metal matrix composites, respectively. We believe that detailed understanding of the solutes’ solvation by highly ionized media of inorganic melts is necessary for the rational design of nanomaterials in these unconventional media. Chapter 2 focuses on the detailed analysis of X-ray total scattering patterns of nanocrystals (NCs) as typical solutes in ionic melts. With the aid of molecular dynamics (MD) simulations, we provide evidence for the formation of a layered ionic solvation shell around NCs dispersed in molten inorganic salts and organic ionic liquids. The solvation shell has enhanced ion-ion correlations compared to the ions in the bulk liquid and extends far beyond the Debye screening length. The charge density wave consisting of the restructured ions generates an oscillatory potential between NCs and hence constitutes a fundamentally different mechanism of colloidal stabilization in addition to the standard electrostatic and steric mechanisms. In Chapter 3, we present versatile chemical transformations of surface functional groups in 2D transition-metal carbides (MXenes) which opens up a new design space for this broad class of functional materials. We introduce a general strategy to install and remove surface groups by performing substitution and elimination reactions in molten inorganic salts. Successful synthesis of MXenes with O, NH, S, Cl, Se, Br, and Te surface terminations, as well as bare MXenes (no surface termination) is demonstrated. These MXenes show distinctive structural and electronic properties. For example, the surface groups control interatomic distances in the MXene lattice, and Tin+1Cn (n = 1, 2) MXenes terminated with Te2- ligands show a giant, (> 18%) in-plane lattice expansion compared to the bulk TiC lattice. And in Chapter 4, we show that Nb2C MXenes synthesized in molten inorganic salts exhibit surface-group-dependent superconductivity. In Chapter 5, we focus on liquid metals as the nanomaterials’ dispersion media. For example, MXenes can be efficiently dispersed in liquid Ga and lightweight alloys of Al, Mg, Li. We show that the Lifshitz theory predicts strong van der Waals attraction between nanoscale objects interacting through liquid metals. However, a uniform distribution of MXenes in liquid metals can be achieved through colloidal gelation, where particles form self-supporting networks stable against macroscopic phase segregation. By choosing Mg-Li alloy as an example of ultra-lightweight metal matrix and Ti3C2Tx MXene as a nanoscale reinforcement, we apply liquid metal gelation technique to fabricate functional nanocomposites with up to 57 % increase in the specific yield strength without compromising the matrix alloy’s plasticity. This work expands applications for MXenes and shows the potential for developing MXene-reinforced metal matrix composites for structural alloys and other emerging applications with metal-MXene interfaces, such as batteries and supercapacitors