Nanoscale iron particles in carbon nanotubes

In this study, the crystallographic structure and orientation of iron nanoparticles in carbon nanotubes (CNTs) were revealed. It was found that while many of the nanoparticles encapsulated inside as-synthesized CNTs had the expected α-Fe (body centered-cubic) phase, a significant number of them formed and retained the γ-Fe (face-centered-cubic) phase that is not the normal bulk phase at room temperature (nor even expected to form at the growth temperature used (700°C)). Through characterization by transmission electron microscopy, the proportion of γ-Fe inside as-synthesized CNTs was found to be almost as high as 40%. It was also found that iron particles at the tips of the nanotubes were either α-Fe or cementite (Fe ₃C). Based on these observations and thermodynamics, a mechanism for the formation of these particles and insights into CNT growth are proposed. It was also found that stable γ-Fe could be selectively synthesized by transforming α-Fe confined in CNTs under electronic irradiation in transmission electron microscopy. By transforming α-Fe into γ-Fe with electronic irradiation, the portion of a CNT containing the iron particle can modify its physical properties due to the difference in the strain and charge transfer induced by the transformation. Therefore, the synthesis of γ-Fe by this method may suggest not only a way of using γ-Fe as a catalyst, but a way of engineering a CNT for electronic and optoelectronic applications. By annealing CNTs at various temperatures, it was found that CNTs, heated above 1400°C and cooled to room temperature, contain twice as many γ-Fe particles as α-Fe particles. Also, it was found that iron particles are aligned in CNTs in specific directions. That is, α-Fe is oriented along the <111> or <001> directions of its bcc structure, and γ-Fe is along the <110> direction of its fcc structure. These orientations of the iron particles were not changed with heat treatment up to 1600°C. A mechanism for transformation of α-Fe into γ-Fe by annealing or electron irradiation is suggested.