Immiscibility of Nucleating Aluminum Oxide Nanoparticles in Vapor

Multistep pathways in nucleation are now known to be ubiquitous and thus crucial to understand crystal formation processes. In a supersaturated mother phase, many types of source molecules are coagulated to form an amorphous or liquid-like intermediate, which subsequently transforms to a crystal. Such an intermediate possibly has a chemical formula that is inconsistent with that of the bulk crystal. Even in relatively simple vapor-phase homogeneous nucleation, the roles of the multicomponent mixtures at the nanoscale remain unknown. Here, by combining an experimental approach using in situ infrared (IR) measurements of homogeneous nucleation in the vapor phase and automatic quantum chemical explorations of reaction routes, we show that a liquid-like nature of nucleating nanoparticles in the Al-O binary system induces immiscible phase separation. In an oxygen-deficient atmosphere, aluminum oxide nanoparticles form from supersaturated vapor with a unique shape composed of an Al metal head and an anisotropic Al2O3 crystalline tail. The anisotropic nanoparticles are larger than spherical Al2O3 nanoparticles formed in an atmosphere with sufficient oxygen. This indirectly shows that fewer nuclei are available in the precursor gas, indicating that homogeneous nucleation is initiated by oxygen-bearing species. We propose that miscible O-rich species act as seeds, and subsequently, the phase separation is induced in the liquid-like nanoparticles. This is supported by quantum-chemical calculations that show aggregation of Al atoms in oxygen-deficient (AlO)(n) clusters of as few as 16 molecules. In situ IR measurements revealed that the degree of anisotropy continues to increase even after the source gas molecules are exhausted, suggesting that oxygen-bearing species migrate to the tail through the molten Al-rich head. Liquid-like nature of nucleating nanoparticles is the key to control shapes and structures of nanomaterials and to elucidate the origin of cosmic dust.