C to BN conversion in multi-walled nanotubes as revealed by energy-filtering transmission electron microscopy

We used a field emission high-resolution 300 kV electron microscope (Omega filter) to study the nanostructures appearing during consecutive stages of chemical reaction between multi-walled chemical-vapor-deposited (CVD) C nanotubes (NTs) and boron oxide in flowing nitrogen at >1573 K. We found that C→BN conversion in the tubular layers occurs through inhomogeneous crystallization of B/N domains onto and within undulating defective graphitic C shells opened by oxidation. The crystallization preferentially takes place at open edges, internal and external tube surfaces. Enrichment of shells with B and N at the expense of outflowing C gradually occurs. In the course of the transformation unique longitudinal nanotubular heterojunctions composed of C-rich and BN-rich domains form.