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Posted: Nov 02, 2006
A novel approach to control the microstructure of carbon nanotubes
(Nanowerk Spotlight) Various methods have been developed for growing well-aligned CNTs based on variant alignment mechanisms such as 'overcrowding growth', 'template hindrance growth' and 'electric field induced growth'. Compared to other methods, electric field induced growth has been considered to be a more effective and controllable method for producing well-aligned single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). Interestingly, while the alignment of CNTs became more controllable and repeatable with the assistance of an electric field, it was also shown that for CNTs grown in an electric field, the diameter uniformity and the crystallinity of graphite sheets of CNTs were clearly improved. This led Chinese researchers to develop an electric-field-induced method to not only improve CNT uniformity but also to create a new approach to control the microstructure of CNTs.
Generally, variant CNT microstructures determine their properties, for example, highly graphitized CNTs exhibit excellent mechanical and electrical properties; however, the CNTs with defects and poor crystallinity contribute to the field emission property and hydrogen storage capacity. Therefore, it is of vital importance to control the CNT microstructures effectively for desired applications.
Recently, for the purpose of growing CNTs with different microstructures and crystallinity, several methods have been attempted by changing growth parameters such as catalyst particle size, growth temperature, flow rate of carrier gas, plasma power and bias voltage, in a chemical vapor deposition (CVD) process.
"Compared to the regular arc discharge and CVD methods, our process provides an unique advantage for synthesizing carbon nanotubes in a simple experiment setup and investigating the effect of electric field on the growth of carbon nanomaterials" Professor Chunxu Pan from the Department of Physics at Wuhan University/PR China, explains to Nanowerk.
Pan and his colleagues found that the CNT microstructures can be changed by an electric field through the influence of the electrostatic force on the carbon surface and bulk diffusion on/in a deformed catalyst particle. Preliminary experiments revealed that anisomeric "graphite - nongraphite - graphite - nongraphite..." CNTs could be synthesized when a pulsed electric field is applied, which are expected to exhibit special properties and promising applications.
Pan describes the results of his work: "Firstly, our work has demonstrated that CNT diameter uniformity and crystallinity of graphite sheets can be improved through applying an electric field. Secondly, our work proposed a new approach to control the microstructure of carbon nanotubes and to fabricate isomeric structures. Thirdly, the experimental results provide new clues for discovering the growth mechanism of carbon nanotubes, which is important for theoretical work."
Specifically, the Chinese researchers experimentally observed the microstructural transformation of carbon nanotubes from the 'herringbone' into a highly crystalline structure in an electric field. These findings are described in a recent paper, titled "Electric-field-induced microstructural transformation of carbon nanotubes", that was published in the August 7, 2006 edition of Applied Physics Letters.
HRTEM micrographs of CNTs grown (a) and (b) without electric field and (c) and (d) with electric field. Arrows indicate the direction of tube axis. (Reprinted with permission from the American Institute of Physics)
The recent experiments revealed that CNTs grown without an electric field appeared with a kind of 'herringbone structure' which had a larger diameter (32 nm) and more graphite layers (>30) with less-ordered graphitic structure which were inclined to the tube axis (∼20°). For CNTs grown in an electric field, the diameter uniformity and the crystallinity of graphite sheets of CNTs were improved clearly, i.e., they exhibited smaller diameter (13 nm) and fewer graphite layers (13–16) parallel to the tube axis.
Pan explains the experimental results: "It is well known that the carbon diffusion in catalyst particle depends upon its stress and deformation states. Preliminary calculation and simulation show that the shape and interior stress states of the particle can be changed by an electrostatic force, which then influences the CNT microstructures. In the case of without an electric field, the carbon concentration gradient of bulk diffusion near the precipitating facets is higher, which implies that the surface diffusion and bulk diffusion on/in the particle are comparable, and then, the graphite layers precipitate along the conical facets to form a herringbone structure."
"However, when an electric field is applied, a strong electrostatic force acts upon the particle due to field enhancement which leads to particle elongation. Then, the surface diffusion becomes prominent and the bulk diffusion becomes negligible because of the tensile stress gradient. Furthermore, the
carbon concentration gradient in the particle suggests that CNT grown from an elongated particle has a larger proportion of carbon assembling in the outer shells. In addition, the electrostatic force induced deformation for the particle results in many active dislocation steps upon the nanoparticle surface which will also adsorb surface-diffused carbon and nucleated graphite layers parallel to the tube axis."
Pan points out that further theoretical studies based on molecular dynamics simulation or in situ observation of CNT growth are required to understand the microstructural transformation mechanism.
"A future direction of our work will be focused on applying this method to produce isomeric carbon nanotubes and to fabricate nano-devices based on this kind of nanotubes" he concludes.