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Research Article
Polymer-Assisted Deposition of Composite Catalysts for the Growth of Vertical Aligned Carbon Nanotubes
Fei L1, Li QW2, Jia QX3 and Luo HM1*
1Department of Chemical Engineering, New Mexico State University, USA
2Suzhou Institute of Nanotech and Nanobionics, Chinese Academy of Science, China
3Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States

ABSTRACT
We report a chemical solution approach, polymer-assisted deposition (PAD) to deposit metal composite as catalyst for the growth of vertically aligned carbon nanotubes (VACNTs). The catalyst composition profoundly affects the quality of VACNTs. The length of VACNTs depends on the molar ratio of metal ions in the composite catalyst. In detail, the prepared CNTs have 2 to 7 walls with diameters ranging from 7 to 15 nm. The length of CNTs can be tuned in the range of 150 to 650 μm. The VACNTs grown from the catalysts deposited by this simple solution method are comparable in quality to those prepared with catalysts grown by vacuum deposition techniques.
KEYWORDS
Vertically aligned carbon nanotubes; Polymer-assisted deposition; Catalyst

INTRODUCTION
Carbon nanotubes (CNTs) have attracted a lot of interest due to their excellent mechanical, electrical, and thermal properties,great chemical stability, and large surface area [1-3]. Particularly, vertically aligned CNT arrays (VACNTs) with highly ordered structure and long length are ideal for a variety of applications including gas and water separation membranes [4,5], filter emitters in microelectronic devices [6], biosensors [7], and electrodes for lithium ion batteries [8]. Although a variety of methods such as arc discharge and laser ablation are available to synthesize highly ordered VACNTs, catalytic thermal chemical vapor deposition (CVD) has been proven the most effective approach for growing dense and large-area VACNTs [9-11]. Highdensity metallic catalysts on a flat substrate and feeding gas are always involved in a typical thermal CVD process for growing VACNTs. Among various synthetic parameters (e.g. gas flow rate, growth time, and temperature), the catalyst shows the most profound impact on the length, diameter, and morphology of CNTs [12,13].
Well-controlled diameter and length of VACNTs are required in many applications [1]. Currently, one of the biggest challenges is to grow VACNTs in a large quantity with excellent reproducibility and great controllability in the diameter and lengths [14]. Herein, in this paper, we report the growth of VACNTs on silica substrates by thermal chemical vapor deposition. The catalyst was prepared by a reproducible, controllable, and cost-effective chemical solution technique, polymer-assisted deposition (PAD) [15], rather than expensive physical vapor deposition method. We found that Fe:Mg catalysts can be used to produce high quality of VACNTs. The length of as-prepared CNT arrays can be modified by varying the molar ratios of Fe and Mg catalysts coated on silica substrates.
MATERIALS AND METHODS
The catalysts were prepared by PAD, where polyethylenimine (PEI) was used as a binding agent to bind metal ions. The metal precursors used were FeCl3, MgCl2, and Al(NO3)3. In detail, 2 g ethylenediaminetetraacetic acid (EDTA) was dissolved in 40 mL H2O, followed by the addition of 2 g FeCl3. To the solution, 2 g PEI were further introduced. The final solution was agitated until a homogeneous one was obtained. Then the solution was placed in an Amicon ultrafiltration unit with a 10,000 MW cutoff membrane, purified by repeatedly washing with 200 mL of water, and finally concentrated to 30 mL. Inductively coupled plasmaatomic emission spectroscopy (ICP-AES) showed that the final Fe concentration in the Fe-polymer solution was 0.209 M. The solution of Mg bound to PEI and EDTA, having a concentration of 0.233 M, was prepared in a similar way. While Al was bound to fluorinated PEI polymer (PEIF) which was prepared by slowly adding 5 mL 48 % HF to PEI solution (10g in 40 mL H2O). Specifically, 2 g Al(NO3)3 was dissolved in 40 mL H2O, followed by adding 3 g PEIF into the solution. The final Al concentration was 0.201 M. The precursor solutions were prepared by mixing two solutions with the desired molar ratios of Fe/Mg = 5, 10, 30, 50, 70, and 100 %, Fe/Al = 30 %. In order to decrease the film thickness, the solution was diluted by H2O to 3 times of the original volume amount, which then was spin- coated on the Si substrates (with SiO2 on the top of substrates) at 5000 rpm for 30 s. Standard chemical vapor deposition approach was applied to grow VACNTs. In detail, ethylene was used as the carbon source, and Ar with 6 % H2 (forming gas) as carrying gas. The VACNTS were grown at 750°C for 30 min. The surface morphology of the films was analyzed by scanning electron microscopy (SEM). The microstructure of the films was analyzed by
Scheme 1 Schematic illustration of the growth process of VACNTs.



Scheme 1

Schematic illustration of the growth process of VACNTs.

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RESULTS AND DISCUSSION
Procedure for the preparation of VACNTs is illustrated in Schme 1. The polymer-bonded metal ion solution was spincoated on silica substrate first. Dense VACNTs films were grown by CVD. To optimize the compositions of the catalysts for the growth of VACNTs, the growth parameters for VACNTs were the same except the catalyst composition. We have found that Fe:Mg is better than Fe:Al, under the same molar ratio of Fe in the composite catalysts, for the growth of VACNTs arrays. Figure 1 shows SEM images of CNTs grown from different composite catalysts. As can be seen in (Figure 1a), dense VACNTs arrays were grown on the Fe catalysts on silica substrate. However, when Al was introduced to Fe catalyst (30 % Fe: 70 % Al, molar percentage of metal ions), rather than the formation of perpendicularly aligned CNTs arrays, only CNTs clusters or islands (Figure 1a) were formed. While in the case of 30 % Fe: 70 % Mg catalyst, well aligned CNTs with length of around 150 μm (Figure 1b) were observed. Increasing the Fe concentration to 50 %, the as produced CNTs remain densely packed and vertically aligned on the surface of silica wafer (Figure 1d), suggesting that catalytically active sites for the growth of CNTs are uniformly formed in the Fe:Mg composite catalysts.
Figure 1 Effect of catalyst compositions on the growth of CNTs (a) 100 % Fe;(b) 30 % Fe: 70 % Al ; (c) 30 % Fe: 70 % Mg; (d) 50 % Fe: 50 % Mg.


Figure 1

Effect of catalyst compositions on the growth of CNTs (a) 100 % Fe; (b) 30 % Fe: 70 % Al ; (c) 30 % Fe: 70 % Mg; (d) 50 % Fe: 50 % Mg.

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The effect of Fe:Mg molar ratio in the composite catalysts on the length of as-grown VACNTs was also studied. (Figure 2) shows the dependence of CNTs' length on the molar percentage of Fe in Fe:Mg composite. As can be seen from the figure, no VACNTs can be formed if the Fe concentration is 5 % and 10 % in the composites. Once the Fe molar ratio gets larger, the length of VACNTs increases linearly with Fe concentration. With the catalyst composition of 70 % Fe: 30 % Mg, VACNTs with a length of 650 μm are achieved. When the catalyst is 100 % Fe, VACNTs with a length of 300 μm are formed. This result, together with the fact that the Fe:Mg catalyst is able to produce high-quality VACNTs, indicates that the addition of Mg doesn't degrade the performance of Fe catalyst, but offers the opportunity to tune the length of VACNTs array ranging from 150 to 650 μm.
Figure 2 Dependence of VACNTs length on the Fe molar percentage in the Fe:Mg composite catalysts.



Figure 2

Dependence of VACNTs length on the Fe molar percentage in the Fe:Mg composite catalysts.

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Figure 3 displays the SEM and TEM images of the 650 μm VACNTs synthesized based on the composite 70 % Fe: 30 % Mg catalyst. In figure 3a, the VACNTs are very dense and well aligned. The TEM image of several CNTs drawn from the vertical arrays shows that the CNTs have 2 to 7 walls, and their diameters are in the range of 7 to 15 nm. The inset of (Figure 3b) shows the lowmagnification image of the VACNTs
Figure 3 (a) SEM image of the VACNTs film with a length of 650 mm, synthesized with 70 % Fe: 30 % Mg catalyst (b) TEM image of the CNTs extracted from the film. The inset shows the low-magnification TEM image of the CNTs.

Figure 3

(a) SEM image of the VACNTs film with a length of 650 mm, synthesized with 70 % Fe: 30 % Mg catalyst (b) TEM image of the CNTs extracted from the film. The inset shows the low-magnification TEM image of the CNTs.

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CONCLUSIONS
In conclusion, composite catalysts coated via PAD method have been demonstrated to be very effective to grow vertically aligned CNTs. It is also confirmed that the catalyst composition plays a crucial role on the quality of VACNTs. The Fe:Mg catalyst is better than Fe:Al catalyst for the growth of VACNTs. Additionally, the length of VACNTs can be tuned by varying the molar ratios of the Fe:Mg in the catalyst composites.
ACKNOWLEDGEMENTS
This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Los Alamos National Laboratory, an affirmative action equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under contract DE-AC52-06NA25396.

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Cite this article: Fei L, Li QW, Jia QX, Luo HM (2013) Polymer-Assisted Deposition of Composite Catalysts for the Growth of Vertical Aligned Carbon Nanotubes. Chem Eng Process Tech 1(2): 1013.
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