Cutinho Joel, Chang Boyce S, Oyola-Reynoso Stephanie, Chen Jiahao, Akhter S Sabrina, Tevis Ian D, Bello Nelson J, Martin Andrew, Foster Michelle C, Thuo Martin M
Department of Materials Science and Engineering , Iowa State University , 2220 Hoover Hall , Ames , Iowa 50011 , United States.
Microelectronics Research Center , Iowa State University , 133 Applied Sciences Complex I, 1925 Scholl Road , Ames , Iowa 50011 , United States.
ACS Nano. 2018 May 22;12(5):4744-4753. doi: 10.1021/acsnano.8b01438. Epub 2018 Apr 16.
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.
液滴捕获了由环境决定的液体材料平衡状态。然而，平衡通常需要纳米级界面组织，特别是钝化层的形成。在此，我们证明，在适当选择触发因素的情况下，这种动力学驱动的组织可能使材料易于发生自主热氧化成分反转（TOCI）和纹理重构。我们利用共晶镓铟（EGaIn，75.5% Ga，24.5% In w/w）核壳颗粒上超薄（约0.7 - 3 nm）钝化氧化层的固有结构复杂性、不同反应性和亚稳定性来说明这种表面工程方法。加热约15分钟后可产生两层纹理，第一次演变表现为起皱，而第二次是在第一层均匀纹理之上的颗粒生长。第一层纹理的形成主要是由于扩散驱动的氧化物堆积，正如预期的那样，这增加了氧化层的硬度。这一层的表面富含Ga，类似于在环境中形成的钝化氧化物。第二层在较高温度下出现，是由于现在厚而硬的氧化壳受到热触发断裂。这个过程导致表面氧化物成分反转，因为第二层特征上的In含量更高。在更高温度（≥800 °C）下，成分的显著变化导致剩余材料固化。氧化和固化过程中的体积变化导致形成具有纹理表面和多面核心的中空结构。因此，对液态EGaIn进行可控热处理会导致表面粗糙度可调、成分反转、氧化壳硬度增加或形成多孔固体结构。我们推断这种可调性是由于钝化氧化层的结构，该结构由Ga和In的反应性差异以及金属 - 氧化物界面处反应性较低成分的必要富集驱动。
