Elsevier

Carbon

Volume 47, Issue 4, April 2009, Pages 938-947
Carbon

High-yield synthesis of carbon nano-onions in counterflow diffusion flames

https://doi.org/10.1016/j.carbon.2008.11.054Get rights and content

Abstract

High-yield synthesis of carbon nanotubes (CNTs) and nano-onions (CNOs) on a catalytic nickel substrate using counterflow diffusion flames was investigated. With ethylene fixed at 5%, methane varied from 15% to 45% in the upper flow, and air supplied in the lower flow, only a moderate CH4 concentration (25%) could yield high density CNTs. However, when oxygen was increased to 50% in the lower flow, only CNOs were synthesized. An increase in methane concentration from 15% to 45% led to a higher yield and a greater diameter (ranging from 5 to 60 nm) of CNOs. To examine the role of mixed fuel, it was observed that as ethylene was removed and only 45% methane and nitrogen were supplied in the upper flow, no CNOs could be generated. While, as methane was increased to 50% or 55%, high-yield CNOs were synthesized and the yield increased with methane concentration. Note that the key parameters influencing the formation and yield of CNOs are both the oxygen and fuel concentrations. There was a critical threshold value of oxygen concentration, 30%, for onset of CNOs synthesis. Also, the critical threshold value of methane concentration for onset of CNOs formation decreased with increasing oxygen concentration or ethylene concentration.

Introduction

Carbon nano-onions are quasi-spherical carbon nanoparticles and are made of concentric graphitic shells. They have been observed to either encapsulate metals [1], [2] or to consist only of carbon layers [3]. Recently, carbon nano-onions have attracted much attention from the research community owing to their outstanding chemical and physical properties. Currently, numerous conventional methods have been employed successfully for their production, such as dc arc-discharge [1], [4], high energy electron irradiation [5], thermal treatment of carbonaceous materials [6], high-dose carbon ion implantation into metals [7], and plasma-enhanced chemical vapor deposition [8]. However, all these methods for carbon onions synthesis require high energy input, and their carbon onions are of low-yield or by-products, leading to the difficulty in separating them from the product [9]. Therefore, large-scale applications require a simple, continuous and energy-efficient method to synthesize carbon nano-onions.

A flame can naturally and easily produce an appropriate high-temperature environment with high radical concentrations required for the growth of CNTs. Therefore, flame synthesis shows a more promising potential for inexpensive and mass production of high-purity CNTs than other synthesis methods. Recently, great efforts have been devoted to studying synthesis of CNTs in flames [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Different morphologies and structures of CNTs have been grown successfully in diffusion flames or premixed flames using various flow configurations (e.g. co-flow, counterflow and wall stagnation flow).

Previous studies [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23] on flame synthesis of CNTs utilized either diffusion flames or premixed flames. It has been recognized that the formation of CNTs using flame synthesis strongly depends on the catalytic metal, hydrocarbon reactants and proper environments. Several parameters affecting CNTs growth, such as temperature, strain rate, and equivalence ratio, have been studied extensively. However, to our knowledge, little attention has been paid on the effects of mixed fuel and oxygen concentrations upon the fabrication of carbon nanostructures, especially carbon nano-onions. Therefore, in this article we aimed at exploring the influence of mixed fuel and oxygen concentrations on the synthesis of carbon nanomaterials using counterflow diffusion flames and a catalytic Ni substrate. A large quantity of CNTs or CNOs could be synthesized successfully by varying the flame parameters, namely, fuel concentrations and oxygen concentrations.

A brief comparison of literature related to the current work is made as follows. Most of the studies on diffusion flame synthesis of CNTs used methane–air, ethylene–air, or acetylene–air diffusion flames in co-flow and counterflow configurations [10], [11], [12], [16], [19], [23]. Therefore, in these studies, neither were the effects of both mixed fuel and oxygen concentrations on CNTs growth studied, nor were results available for the synthesis of CNOs. Only a few investigations have been conducted to examine the effects of O2 concentration on CNTs growth [13], [22]. However, the authors examined the role of O2 in flame synthesis of CNTs, not in flame synthesis of CNOs. In addition, the role of mixed fuel in flame synthesis of CNTs or CNOs has not yet been well documented. For instance, in Merchan-Merchan et al.’s study [13], the molar concentration in the fuel stream was fixed at 96% methane + 4% acetylene (C2H2) for all experiments, and the oxygen content in the oxidizer stream (O2 + N2) had a minimum of 50%. Therefore, the effect of mixed fuel was not studied. The authors reported that CNTs were only observed for oxygen enrichment of 50% oxygen or greater. Observed carbon structures only included CNTs and soot particles. They examined the role of O2 in flame synthesis of CNTs, not in flame synthesis of CNOs. In other related work [14], [15], the flame parameter was kept constant with fuel and oxidizer compositions of 4%C2H2 + 96%CH4 and 50%O2 + 50%N2, respectively. An electric field was applied to control CNT alignment and morphology [15]. Even though different carbon nanoforms were observed, their types only included multi-walled carbon nanotubes, bundles and coiled nanofibers. It is obvious that, in the two studies [14], [15], the effects of both mixed fuel and oxygen concentrations on flame synthesis of CNTs were not studied, nor were results available for the synthesis of CNOs.

Although, CNOs are only one type of carbon nanomaterials, their morphologies, microstructures, and growth mechanisms are different from those of CNTs and other carbon nanomaterials. Clearly, synthesis conditions for CNOs can be quite different from those for CNTs or other carbon nanostructures. However, the effects of ethylene and methane concentrations as well as those of oxygen on the nature of carbon nano-onions formation using flame synthesis have not been previously documented in literature. Hence, the role of O2 that governs the formation of CNOs in flames still remains poorly understood and needs clarification because it can not be expected to be the same as in flame synthesis of CNTs. In addition, what role mixed fuel plays in flame synthesis of CNOs is of great interest and much more study is needed. In view of the above considerations, in this article we put particular focus on the influence of mixed fuel and oxygen concentrations on the synthesis of carbon nano-onions using counterflow diffusion flames and a catalytic Ni substrate. The flame conditions were C2H4/CH4/N2 mixture (with CH4 ranged from 15% to 55% and C2H4 ranged from 0% to 10%) in the fuel stream and O2/N2 mixture (with O2 ranged from 21% to 50%) in the oxidizer stream. Accordingly, the influence of fuel mixture (ethylene and methane concentrations) and oxygen concentrations on the synthesis of carbon nano-onions were examined. The results of this study are of interest and importance since they show how the effects of ethylene and methane as well as oxygen concentration on carbon nano-onions formation.

Section snippets

Experimental

A schematic of the counterflow is shown in Fig. 1. The counterflow system consisted of two identical vertically aligned cylindrical burners with an inner diameter of 46 mm and an outer diameter of 50 mm. Each burner contained series of small wire-mesh screens and honeycombs to produce uniform velocity profile at the exit plane of burner. The separation distance between the burner exit planes, L, was adjustable but kept at a constant value (22 mm) in the experiment. Oxygen, nitrogen, methane and

Flame synthesis of CNTs

In this section of discussion, the influences of the CH4 concentrations on the production of the carbon nanotubes were investigated. The CH4 concentration (ΩM) ranged from 15% to 45%, while C2H4 concentration (ΩE) and oxygen concentration (ΩO) were fixed at 5% and 21%, respectively. Fig. 2 demonstrates the temperature profiles and CNTs yield for different axial positions z at the flow centerline, r = 0. In Fig. 2, the positions of the upper edge of blue flame front and the soot layer are

Conclusions

High-yield synthesis of carbon nanotubes and nano-onions on a catalytic nickel substrate using counterflow diffusion flame method was investigated. A stable quasi-one-dimensional diffusion flame was utilized by introducing two opposite streams of gases in the upper and lower burners. The mixture of ethylene and methane and nitrogen was introduced from the upper burner and the oxidizer (oxygen and nitrogen) was supplied from the lower burner. The influences of fuel concentrations and oxygen

Acknowledgement

This work was supported by the National Science Council, Taiwan, ROC, under contract NSC96-2221-E-168-019. Valuable comments by the reviewers of this report are kindly appreciated.

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