Three-Dimensional Analysis of Carbon Nanofibers by Cross-sectional TEM Observations

Abstract

Cross-sectional TEM images of carbon nanofibers formed through methane decomposition over Ni/SiO 2 catalysts were observed by using an ultramicrotome. The TEM images indicated that carbon nanofibers have regular hexagonal prism structures.Carbonaceous materials with a nanoscale fibrous structure is generally called carbon nanofibers, which are the key material in the future nanotechnology due to their attractive properties. Carbon nanofibers can be produced through the catalytic decomposition of carbon-containing gases, such as hydrocarbons and carbon monoxide. Nickel, cobalt and iron metals are frequently used as catalysts, and the properties of carbon nanofibers are influenced by the types of carbon-containing gases or catalysts. The utilization of carbon nanofibers has been proposed in the various fields (e.g., electronic devices, 2 templates, 3 and catalysts 4 ). If the property of carbon nanofibers is different, the performance in their application must be different. Carbon nanofibers have a lot of characteristic properties, and their morphology is one of important properties. Until now, the morphology of carbon nanofibers has been investigated by using a transmission electron microscope (TEM). In conventional TEM studies, we can obtain their two-dimensional morphology such as straight, bent, twisted or helical, because they lie in the matrix with their axis perpendicular to the electron beam; however, the three-dimensional structure (i.e., cylinder or polygonal prism) is also important. To understand the information in the three-dimensional morphology of carbon nanofibers, it was deduced on the basis of the careful analysis of their two-dimensional images. Rodriguez et al. synthesized a variety shape of carbon nanofibers by choosing the appropriate reaction conditions, and proposed their three-dimensional structure on the basis of TEM images. 5 It is considered that the combination between the conventional TEM images and the cross-sectional images of carbon nanofibers would give the direct information of the three-dimensional structures. As far as we know, the cross-sectional TEM images of carbon nanofibers have not been measured. Thus, in the present study, we use an ultramicrotome to obtain the sliced sample of carbon nanofibers. Since an ultramicrotome can make an extreme thin sample, we could see the cross-sectional images of carbon nanofibers.Ni/SiO 2 catalysts for producing the carbon nanofibers were prepared by the impregnation of SiO 2 with aqueous solutions containing Ni(NO 3 ) 2 , followed by drying in air at 353 K and calcination in air at 873 K. The carbon nanofibers were formed through methane decomposition at 803 K over the catalysts, which had been previously reduced with H 2 at 773 K.6 Crosssectional specimens for TEM observation were prepared by embedding carbon nanofibers in an acrylic resin (LR White Resin; London Resin Company Ltd.). After the matrix was polymerized by heating at 333 K for 24 h, the resulting samples were cut in 57-nm thick sections on an ultramicrotome. TEM images were...