Root-Growth of Boron Nitride Nanotubes: Experiments and \textit{Ab Initio} Simulations

TL;DR

This study combines experiments and ab initio simulations to investigate the root-growth mechanism of boron nitride nanotubes, highlighting the active role of liquid boron and the formation of stable caps that facilitate nanotube growth.

Contribution

It provides the first combined experimental and computational evidence supporting the root-growth mechanism of BNNTs without transition metals.

Findings

BNNTs are attached to boron nanoparticles, indicating root-growth.

Nitrogen atoms form boron nitride islands on liquid boron surfaces below 2400 K.

Stable cap structures facilitate further nanotube growth.

Abstract

We have synthesized boron nitride nanotubes (BNNTs) in an arc in presence of boron and nitrogen species only, without transition metals. We find that BNNTs are often attached to pure boron nanoparticles, suggesting that root-growth is a likely mechanism for their formation. To gain further insight into this process we have studied key mechanisms for root growth of BNNTs on the surface of a liquid boron droplet by ab initio molecular dynamics simulations. We find that nitrogen atoms reside predominantly on the droplet surface where they organize to form boron nitride islands below 2400 K. To minimize contact with the liquid particle underneath, the islands assume non-planar configurations that are likely precursors for the thermal nucleation of cap structures. Once formed, the caps are stable and can easily incorporate nitrogen and boron atoms at their base, resulting in further growth. Our simulations support the root-growth mechanism of BNNTs and provide comprehensive evidence of the active role played by liquid boron.