Passivation-driven speciation, dealloying and purification.

Thin passivating surface oxide layers on metal alloys form a dissipation horizon between dissimilar phases, hence harbour an inherent free energy and composition gradient. We exploit this gradient to drive order and selective surface separation (speciation), enabling redox-driven enrichment of the core by selective conversion of low standard reduction potential (E°) components into oxides. Coupling this oxide growth to volumetric changes during solidification allows us to create oxide crystallites trapped in a metal ('ship-in-a-bottle') or extrusion of metal fingerlings on the heavily oxidized particle. We confirm the underlying mechanism through high temperature X-ray diffraction and characterization of solidification-trapped particle states. We demonstrate that engineering the passivating surface oxide can lead to purification via selective dealloying with concomitant enrichment of the core, leading to disparate particle morphologies.

Autonomous Thermal-Oxidative Composition Inversion and Texture Tuning of Liquid Metal Surfaces.

Droplets capture an environment-dictated equilibrium state of a liquid material. Equilibrium, however, often necessitates nanoscale interface organization, especially with formation of a passivating layer. Herein, we demonstrate that this kinetics-driven organization may predispose a material to autonomous thermal-oxidative composition inversion (TOCI) and texture reconfiguration under felicitous choice of trigger. We exploit inherent structural complexity, differential reactivity, and metastability of the ultrathin (∼0.7-3 nm) passivating oxide layer on eutectic gallium-indium (EGaIn, 75.5% Ga, 24.5% In w/w) core-shell particles to illustrate this approach to surface engineering. Two tiers of texture can be produced after ca. 15 min of heating, with the first evolution showing crumpling, while the second is a particulate growth above the first uniform texture. The formation of tier 1 texture occurs primarily because of diffusion-driven oxide buildup, which, as expected, increases stiffness of the oxide layer. The surface of this tier is rich in Ga, akin to the ambient formed passivating oxide. Tier 2 occurs at higher temperature because of thermally triggered fracture of the now thick and stiff oxide shell. This process leads to inversion in composition of the surface oxide due to higher In content on the tier 2 features. At higher temperatures (≥800 °C), significant changes in composition lead to solidification of the remaining material. Volume change upon oxidation and solidification leads to a hollow structure with a textured surface and faceted core. Controlled thermal treatment of liquid EGaIn therefore leads to tunable surface roughness, composition inversion, increased stiffness in the oxide shell, or a porous solid structure. We infer that this tunability is due to the structure of the passivating oxide layer that is driven by differences in reactivity of Ga and In and requisite enrichment of the less reactive component at the metal-oxide interface.

Rapid One-Step Synthesis of Complex-Architecture Block Polymers Using Inductively "Armed-Disarmed" Monomer Pairs.

A facile method is reported for rapid, room-temperature synthesis of block copolymers (BCP) of complex morphology and hence nontraditional spherical assembly. The use of solvated electrons generates radical anions on olefinic monomers, and with a felicitous choice of monomer pairs, this species will propagate bimechanistically (via radical and the anion) to form BCPs. Molecular weight of the obtained BCP range from Mw = 97 000-404 000 g mol-1 (polydispersity index, PDI = 1.4-3.0) depending on monomer pairs. The composition of the blocks can be controlled by changing monomer ratio, with the caveat that yield is affected. Detailed characterization by 2D nuclear magnetic resonance spectroscopy, differential scanning calorimetry (DSC), and analysis of the mechanisms involved indicate the structure of obtained block copolymers to be at least a triblock with a complex central unit. Evaluating trends in the Hammett parameter segregates monomer pairs into "armed and disarmed" groups with respect to radical or anionic polymerization akin to oligosaccharides synthesis.