Observation and formation mechanism of stable face-centered-cubic Fe nanorods in carbon nanotubes
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David Jacques and Rodney Andrews Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40511-8410 (Received 13 November 2002; accepted 4 February 2003)
The crystallographic structure and orientation of iron nanoparticles present in carbon nanotubes (CNTs) was studied when iron was used as a catalyst. It was found that while most of the nanoparticles encapsulated inside the CNTs had the expected ␣–Fe (body-centered-cubic) phase, a significant number of them formed and retained the ␥–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). It was also found iron particles at the tips of the nanotubes were either ␣–Fe or cementite (Fe3C). On the basis of these observations and thermodynamics, a mechanism for the formation of these particles and insights into CNT growth is proposed.
I. INTRODUCTION
Materials confined in micro- or nanocapillaries have been studied for years because they can exhibit a variety of phenomena that do not occur in the bulk state, such as layering and commensurate–incommensurate transitions.1–3 Substances encapsulated by carbon nanotubes (CNTs) have also been studied ever since their discovery;4 many oxides and halides have been found to fill CNTs by various methods.5–9 A recent paper reports by simulations that water inside a CNT may show new polymorphic phases of ice, a critical point in the fusion curve, and a volume decrease upon its transformation into ice.10 Therefore, a detailed investigation into materials confined inside CNTs is of great importance, as it may lead to significant improvements in exploiting materials in an abnormal state not found in bulk materials. II. EXPERIMENTAL
The multiwalled nanotubes (MWNTs) used in this study were fabricated by chemical vapor deposition (CVD) using ferrocene [Fe(C5H5)2] as a catalyst and xylene [C6H4(CH3)2] as the carbon source. The reactor temperature was raised to 700 °C, and the nanotubes were removed after furnace-cooling to 50–100 °C. A high yield of CNTs was produced, and many of them contained iron-rich particles normally in the shape of a
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J. Mater. Res., Vol. 18, No. 5, May 2003 Downloaded: 11 Mar 2015
nanorod both internally and at their growth fronts. These CNTs with particles were examined after being stored in ambient air for over five months.
III. RESULTS
A transmission electron microscopy (TEM) image of an iron-rich nanoparticle at the center of a MWNT is shown in Fig. 1(a) along with accompanying microdiffraction patterns (DPs) [Figs. 1(b) and 1(c)]. The diameter of the CNT is around 50 nm. The diameter of the particle is up to 10 nm, and the length is 45 nm. As can be seen, both DPs contain a single row of closely spaced reflections superimposed on single-crystal patterns. In both DPs, the row of arc-shaped diffraction spots is oriented orthogonal to the axis of the MWNT while their spacing correspon
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