Chemical vapor deposition of pyrolytic carbon on carbon nanotubes. Part 2. Texture and structure

doi:10.1016/j.carbon.2004.12.023

Carbon

Volume 43, Issue 6, May 2005, Pages 1265-1278

https://doi.org/10.1016/j.carbon.2004.12.023 Get rights and content

Abstract

In a previous study we showed both the formation of genuine vapor grown carbon fibers (VGCFs) and of new and peculiar carbon nanotube-supported morphologies using a chemical vapor deposition process. Briefly, the latter are an association of beads (or fiber segments) with a more or less rough surface and more or less extended cone-based sub-morphologies with a smooth surface. The investigation of these materials regarding their texture, nanotexture and structure by transmission electron microscopy is reported, as a first step to understanding the formation mechanisms. It is shown that VGCFs exhibit a concentric texture, however with a variable microporous character and nanotexture quality. On the other hand, beads and related morphologies have a coarsely concentric microporous texture, as opposed to the cones and related morphologies, which exhibit a perfectly concentric and dense texture similar to that of perfect multiwall carbon nanotubes. Cross-sections performed with ultra-microtomy have revealed the spatial and textural relationship between cones and beads.

Introduction

In the past, carbon fibers in micrometer range sizes were preferred to nanometer range sizes, and depositing pyrolytic carbon onto primary nanofilaments prepared by catalyst-enhanced chemical vapor deposition processes (C-CVD) was a common way to thicken them into larger vapor grown carbon fibers (VGCFs) [1], [2], [3]. Various features could be obtained with variations in surface roughness and morphologies (straight, distorted, crenulated, …) for reasons which were not always understood. On the other hand, the relationship with results obtained by pyrocarbon deposition on a flat surface was not clear.

Thoroughly investigating the CVD-based thickening mechanisms of VGCFs is therefore interesting because it basically consists in depositing pyrolytic carbon onto non-flat substrate (multiwall nanotubes), with a radius of curvature in the nanometer range. Peculiar phenomena are therefore expected with respect to deposition onto regular flat substrates or larger diameter (micrometer range) fibers, likely to bring valuable data to understand better the overall carbon CVD mechanisms.

In previous studies [4], [5] we studied the effect of both temperature, gas phase composition, and time of flight, on morphologies, growth, and characteristics of the thickened VGCFs. Several unexpected morphologies based on bead (or fiber segment) and cone associations were found (Fig. 1) and their synthesis conditions were determined. In this paper we focus on the description of the microtexture, nanotexture and structure of beads and cones using various transmission electron microscopy (TEM) techniques for better understanding of the formation mechanism of these new carbon morphologies. Textural study is carried out using both bright field contrast and lattice fringe modes (HRTEM) whereas structural study is carried out using the selected area electron diffraction mode. Structure and texture of regular VGCFs were also investigated.

Section snippets

Synthesis conditions

Synthesis of VGCFs was performed as described previously [5]. Briefly, multiwall carbon nanotubes (MWNTs) were grown by the decomposition of methane on nanometric iron catalyst particles. The catalyst particles were supported by Grafoil^® coupons, placed along a horizontal mullite tube furnace in both the isothermal and non-isothermal regions of the furnace and able to reach 1400 °C. As a common lengthening step for all experiments, CH₄/H₂ = 1/9 (flow values are 60 sccm and 515 sccm for methane and

Topology, texture, and microtexture

The standard VGCFs have a smooth surface, are slightly distorted and have a ∼10 μm diameter (cf. Fig. 6 in [5]).

On the other hand, non-standard VGCFs exhibit a regular slightly distorted overall morphology similar to that of standard VGCFs, however with a rough surface (similar to that of the bead in Fig. 1) and a diameter of ∼8 μm. The surface roughness is due to the same porous and distorted texture that will be revealed within the body of the morphology (see the study of cross-section below).

Texture and structure

The structural variability of the cones, from turbostratic to graphitic, revealed from the cone center to the cone surface, is directly related to the radius of curvature. When the latter is below a threshold value, two successive concentric layers are not commensurable, because the difference in the related radii of curvature does not allow the atoms of both layers to be disposed with a suitable arrangement according to the AB sequence of graphite. Beyond the threshold value, the radius of

Conclusion

The structure, texture and nanotexture of CVD-derived morphologies such as VGCFs and remarkable morphologies based on conical and spherical sub-morphologies have been determined by TEM investigations.

All morphologies studied have in common a dense center, anisotropic, and concentric, whose the cross-section diameter never exceeds ∼200 nm. Other similarities are a systematic concentric texture, and a turbostratic structure. Cones partially make an exception with respect to the latter point,

Acknowledgments

We are grateful to the French Ministry of Foreign Affairs for providing the Ph.D. grant for H. Allouche, and Comité Mixte de Coopération Universitaire (CMCU), Centre Régional des Oeuvres Universitaires et Scolaires (CROUS) for partial financial support.

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Chemical vapor deposition of pyrolytic carbon on carbon nanotubes. Part 2. Texture and structure

Abstract

Introduction

Section snippets

Synthesis conditions

Topology, texture, and microtexture

Texture and structure

Conclusion

Acknowledgments

Carbon

Carbon

Acta Metall

Chem Phys Lett

Carbon

Carbon

Thin Solid Films

The formation of filamentous carbon

A review of catalytically grown carbon nanofibers

J Mater Res

Carbon beads with protruding cones

Nature

High-resolution TEM studies of carbonization and graphitization