Understaning the tip-growth and root-growth vapor-liquid-solid mechanism in SnO2 nanowires by Comsol simulation

Abstract The conventional vapor-liquid-solid (VLS) mechanism has been widely accepted for understanding the tip-growth process of many metal nanowires. However, that model should be more comprehensive to explain the growth process of metal oxide. In this article, we produce tin oxide nanowires with gold nanoparticles and gold film as catalysts, which follow a tip-growth and root-growth process. Experimental results, such as transmission electron microscope images, X-ray photoelectron spectroscopy, and X-ray diffraction patterns, demonstrate that the tip-growth process occurs on Au nanoparticles, and the root-growth process appears on Au thick film. Both methods can be observed when nanowires develop in the presence of both Au nanoparticles and Au film. In this case, the size and length of root-growth nanowires are much larger than that of tip-growth nanowires. For example, the length of the earlier is about 2 micrometers, whereas that of the latter ranges from 20 to 100 micrometers. The irregular shape of particles on the tip of nanowires is very different from other studies, which are usually spherical droplet-like. This suggests a VLS mechanism where there is the existence of partly melted particles playing the role of catalyst. The explanation based on the Au-Si and Au-Sn phase diagram reveals an essential role of the wettability of tin oxide to the Au-Sn alloy liquid. The non-wetting property of tin oxide is necessary for the tip-growth VLS and the root-growth VLS mechanism. Comsol simulation allows calculating the concentration and the flux of Sn atoms arriving on the Au-Sn surface. It suggests that long nanowires in the samples come from both tip-growth and root-growth processes.