Phases and crystallization of encapsulated cobalt nanorods inside BN nanotubes

Abstract Continuous filling of BN nanotubes with Co has been obtained using high-temperature substitution synthesis from C nanotubes; this involves filling of C nanotubes with a Co melt accompanied with simultaneous C→BN conversion within tubular layers. Analytical transmission electron microscopy shows that 3d-transition metals, like Co, do not form compounds inside BN nanotubes; rather they form solid solutions with marginal C, N and/or B doping. Ordering of interstitial impurities in a Co crystal lattice has been frequently observed, which exclusively exhibits a certain threefold superlattice. Structural orientation relationship always exists between fillings and BN nanotubes, indicative of an epitaxial crystallization mode of an encapsulated Co filling on a BN innermost wall template. A prevailing crystallization phenomenon shows a certain {0 1 1} plane of cubic Co coincides BN filament with an orientation relation of 〈0 1 1〉Co//〈 1 2 1 0〉BN and 〈4 5 5〉Co//〈0 1 1 0〉BN. This phenomenon possibly interprets the transition metals, Fe, Co and Ni, to be the most effective catalysts for the formation of C and BN nanotubes.