Structure of the carbon nanofilaments formed by liquid phase carbonization in porous anodic alumina template

doi:10.1016/j.matchemphys.2007.04.074

Materials Chemistry and Physics

Volume 105, Issues 2–3, 15 October 2007, Pages 367-372

https://doi.org/10.1016/j.matchemphys.2007.04.074 Get rights and content

Abstract

Platelet structure carbon nanofilaments of ∼30 nm in diameter have been prepared by heating a mixture of porous anodic alumina template and poly(vinyl)chloride (PVC) powders in an argon atmosphere, and the change in their structure and morphology with heat treatment temperature, ranging from 600 to 2800 °C, has been examined using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen gas adsorption measurements. The diameter of the carbon nanofilaments formed does not change with heat treatment temperature, being in agreement with the pore diameter of the template, while their length is reduced with the temperature. The platelet-type orientation of graphene layers is evident even at 600 °C with the layer structure further developing with increasing heat treatment temperature. The carbon nanofilaments formed at lower temperatures have micropores, while those formed at higher temperatures do not have porosity. Highly graphitized carbon nanofilaments have been obtained after heat treatment at 2800 °C, with another characteristic structural feature being presence of loops at the edge of graphene layers formed at 2800 °C.

Introduction

Carbon nanofilaments, often also referred to as carbon nanofibers, have attracted much attention due to their unique chemical and physical properties as well as their potential widespread applications. Preparation of carbon nanofilaments with well-controlled diameter as well as controlled orientation and crystallization degree is of technical great importance. There are basically three types of carbon nanofilaments: platelet, herringbone and tubular structures [1], although recently a new type of carbon nanofilaments, i.e., cup-stacked type, has also been reported [2], [3], [4]. Among a range of preparation methods of carbon nanofilaments, including arc discharge, laser abrasion and catalytic chemical vapor deposition techniques, a template carbonization technique has an advantage for the preparation of nanocarbons with well-controlled size and morphology. Porous anodic alumina is a suitable template for the preparation of carbon nanofilaments. When chemical vapor deposition has been applied to deposit carbonaceous materials into the straight pores of porous alumina, multi-walled carbon nanotubes, whose outer diameter is in agreement with pore diameter of the template, are generally formed [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17].

Recently, liquid phase carbonization in the pores of porous anodic alumina template has been demonstrated to form platelet carbon nanofilaments. Jian et al. have used mesophase pitch as a carbon precursor [18], while some of the present authors have utilized poly(vinyl)chloride (PVC) and poly(vinyl)alcohol (PVA) [19], both of which form pitch-like liquefied intermediate during thermal decomposition of the polymers. The template materials play an important role on orientation of graphene layers in carbon nanofilaments. The use of metallic aluminum template with square tunnel pits of 1 μm square as a template and PVC precursor results in the formation of carbon filaments with a concentric structure [19], and similar structure has been obtained from a carbon-lined porous alumina template and mesophase pitch precursor [20].

In order to get further insights into the development of highly oriented carbon nanofilaments by liquid phase carbonization using the porous alumina template, structural change in the carbon nanofilaments formed by this technique with heat treatment temperature has been examined in this study using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen gas adsorption measurements. Since the platelet carbon nanofilaments have numbers of exposed graphene edge sites, the materials are of considerable interest as catalysts [21] and electrodes for lithium ion batteries [22]. In fact, the authors have found that the carbon nanofilaments prepared by this method reveals a high rate capability as an anode of lithium ion batteries [23]. Thus, the structural characterization of the carbon nanofilaments formed at several temperatures is also of great importance for such applications.

Section snippets

Experimental

The template was prepared by anodizing of 99.99% pure aluminum foil of 0.1 mm thickness at 25 V in 0.3 mol dm⁻³ sulfuric acid electrolyte at 15 °C, followed by pore widening treatment in 5% phosphoric acid at room temperature for 30 min. To get relatively uniform pore diameter and less branched pore channels, two-step anodizing was carried out [24]; the porous films formed by anodizing for 1 h was once dissolved in a chromic acid–phosphoric acid solution at 80 °C, and again porous alumina films were

Results

Fig. 1 shows scanning electron micrographs of carbon nanofilaments heat-treated at several temperatures. It is clearly seen the formation of carbon nanofilaments with high aspect ratio. The diameter of the carbon nanofilaments is about 30 nm, which is in agreement with pore diameter of the template. The carbon nanofilaments heat-treated at 600–2800 °C have a similar diameter, but those become shorter, from more than several micrometers to less than 1 μm, with increasing heat treatment temperature.

Formation of platelet carbon nanofilaments

The present TEM observations of the carbon nanofilaments have confirmed the development of platelet structure even at temperature as low as 600 °C. In the pitch-like liquid formed during carbonization of PVC, it is believed that poly-aromatic compounds are developed [26]. Hurt et al. have examined interaction of liquid phase poly-aromatic compounds in pitch with various substrates at 325 °C, reporting that large poly-aromatic compounds prefer edge-on molecular orientation at the interface with

Conclusions

From a mixture of PVC powders and porous anodic alumina template with pore diameter of ∼30 nm, carbon nanofilaments with platelet structure are developed even as low as 600 °C. The filament diameter is independent of heat treatment temperature up to 2800 °C with their length shortening with increasing the temperature. Spherical hollow regions are present in the carbon nanofibers heat-treated at low temperatures, possibly formed by gas generated during thermal decomposition of PVC. The carbon

Acknowledgments

Thanks are due to Professor Y. Kaburagi, Musashi Institute of Technology, for the heat treatment of the carbon nanofilaments at 2800 °C. The present work was supported in part from the Ministry of Education, Culture, Sports, Science and Technology (Exploratory Research No. 16651053) and Tokuyama Science Foundation.

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Structure of the carbon nanofilaments formed by liquid phase carbonization in porous anodic alumina template

Abstract

Introduction

Section snippets

Experimental

Results

Formation of platelet carbon nanofilaments

Conclusions

Acknowledgments

Carbon

Nanostruct. Mater.

Chem. Phys. Lett.

Carbon

Carbon

Carbon

Carbon

Carbon

Carbon

Carbon

Carbon

Carbon

J. Mater. Res.

Mol. Cryst. Liq. Cryst.

Appl. Phys. Lett.

Chem. Mater.

Chem. Mater.