Extended View on the Vapor-Liquid-Solid Mechanism for Oxide Compound Nanowires: The Role of Oxygen, Solubility, and Carbothermal Reaction

摘要

The Si nanowire growth can be well explained by the classical vapor-liquid-solid (VLS) process taking into account the respective Au-Si phase diagram. For oxide-based compound materials, no phase diagram with gold exists because of the insolubility of these materials into the Au catalyst material. Hence, it is not correct to claim a simple VLS mechanism for the respective growth. In this study, a more complex model for the growth of oxide nanowires (NWs) is proposed by analyzing the influence of oxygen concentration and timing of oxygen inflow into the furnace while growing SnO2 NWs by a carbothermal chemical vapor deposition process. It is shown that a controlled amount of oxygen is mandatory to grow the SnO2 NWs. However, either too low or too high oxygen concentration strongly suppresses the nanowire growth. On the basis of the here-presented experiments, we propose the formation of solid oxide flakes on the catalyst surface and their respective concurrence as guided by the Sn/O balance feeding the liquid catalyst surface. A new model is discussed, taking into account the effect of surface transport and the respective transport of SnO2 solid flakes, the effect of the Sn gradient in the catalyst droplet, and a possible viscosity gradient at the droplet-solid nanowire interface.