Experimental investigation on carbon nanotube grown by thermal chemical vapor deposition using non-isothermal deposited catalysts

Materials Chemistry and Physics 97 (2006) 511–516

Experimental investigation on carbon nanotube grown by thermal chemical vapor deposition using non-isothermal deposited catalysts Ta-Tung Chen a , Yih-Ming Liu a , Yuh Sung b , Ha-Tao Wang a , Ming-Der Ger c,∗ a

Department of Mechanical Engineering, Chung Cheng Institute of Technology, National Defense University, Ta-His, Tao-Yuan 33502, Taiwan, ROC b Chung Shan Institute of Science and Technology, P.O. Box 90008-17, Tao-Yuan 32552, Taiwan, ROC c Electrochemical Microfabrication Laboratory, Department of Applied Chemistry, Chung Cheng Institute of Technology, National Defense University, Ta-His, Tao-Yuan 33502, Taiwan, ROC Received 19 April 2005; received in revised form 27 July 2005; accepted 29 August 2005

Abstract The CNTs growth by thermal chemical vapor deposition, incorporating a novel plating technique called non-isothermal deposition (NITD) for preparing catalysts on silicon substrate, was investigated in this study. The aim of this research is to examine the effects of particle size and distribution density of Ni catalysts and Ni-Pd co-catalysts on structure morphology of CNTs. The CNTs were grown by thermal chemical vapor deposition (CVD) using C2 H2 at various temperatures (ranged from 700 to 900 ◦ C). The size and density of Ni catalysts prepared by NITD process were observed to be increased with the increasing deposition time. The diameter and density of the grown CNTs were noted to be increased with the increasing of the Ni catalyst particle size. The results also proved that using Ni-Pd as a co-catalyst had the effect on reducing the growth temperature for the synthesis of CNTs. © 2005 Elsevier B.V. All rights reserved. Keywords: Carbon nanotube; Chemical vapor deposition; Non-isothermal deposition; Co-catalyst

1. Introduction The need to develop nanosized materials had been increased tremendously during the last decade due to their excellent quantum effect and physical properties. Carbon nanotube (CNT) is one of the promising nanomaterials leading to special applications such as energy storage, display, opto-electronics and bio-medicine because of its perfect structures exhibiting unique electronic and excellent mechanical properties [1]. Synthesis of CNTs has been under significant investigation by a number of R&D groups since its first observation using electron microscope by Iijima [2]. Various methods such as arc-discharge, laser ablation, thermal chemical vapor deposition (CVD) have been tried to synthetically fabricate CNTs [3–5]. Among the above methods, thermal CVD is suitable to produce large quantities of vertically well-aligned CNTs for commercial applications such as electronic devices and field emitters [6–8].

∗

Corresponding author. Tel.: +886 3 3900714; fax: +886 3 3900714. E-mail address: [email protected] (M.-D. Ger).

0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.08.076

The process of synthesis of CNTs by thermal CVD includes catalyst preparation and growth of carbon nanotube. Transition metals are often applied as catalysts for CNTs growth. These metals can be deposited onto the substrate, from solution containing them using chemical techniques such as electroplating, electroless plating, non-isothermal deposition and spin coating or by direct deposition using physical techniques such as sputtering and vacuum evaporation [9]. Physical techniques for the preparation of catalysts are usually complex processes with the necessity of using expensive equipment. On the contrary, the chemical catalyst preparation techniques are suitable for the deposition of catalysts onto the substrate at low cost. During the fabrication of CNTs, transition metals such as Fe, Co and Ni are coated on the substrate as thin films. These films are often chemical etching using NH3 gas or thermal treated in order to form nanosized catalyst particles acting as the initial growth sites for the rearrangement and pile-up of carbon atoms. The CNTs are grown by thermal decomposition of hydrocarbon gas such as C2 H2 at high temperatures. The catalyst preparation method involved during this research is the non-isothermal deposition (NITD) technique proposed by Sung et al. [10]. By using this technique, nanosized well-arrayed

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catalyst particles can be obtained on the substrate by selfassembly of the homogeneous nucleus due to the increase of reaction temperature in the interface and the reactivity of chemical solution by providing a local high temperature area on the substrate. The temperature gradient achieved by NITD results in thermal convection inducing the diffusion process for metal particles to be deposited onto the substrate. The advantage of NITD technique is its capability of being operated at low cost without the need to etch the deposited films. Based on the relevant theories and experimental results reported in recent studies of synthesis of CNTs, there is little articles discussed about the effect of adding Pd, by chemical plating onto the catalysts, on the CNTs growth process, such as the effect on lowering the growing temperature. In researches regarding to the synthetic mechanism for CNTs growth by thermal CVD technique, it has been reported that some catalysts appear at the top of CNTs or are present on the substrate after the CNTs growth process. Which of the above two growth mechanisms being likely to occur is considered to depend on the strength of the adhesion force between catalysts and substrate. The CNTs synthesized mechanism can be theoretically categorized as ‘top growth model’ mechanism and ‘base growth model’ mechanism. This paper is aiming to investigate the effects of Ni catalyst size, density depending on the deposition time and the growth temperature on the synthesis growth of CNTs using thermal CVD technique. Some literatures indicated that it is viable to fabricate CNTs at lower temperature by putting a Pdcoated plate near the catalyst-coated sample with a distance of 3 mm set between them [11–13]. Therefore, the effect of Ni-Pd co-catalysts on lowering the growth temperature is also investigated. 2. Experimental setup A schematic diagram of the newly developed reactor used in our work was as shown in Fig. 1. The reactor is constructed of a heater, a temperature controller, a thermostat and a glass vessel including a helix cooler. The reactive substrate bonded closely with the heater was fixed well on the bottom of the reactor so that its operating temperature allowed to be precisely controlled.

Fig. 1. A schematic diagram of NITD equipment.

The temperature of plating bath can be flexibly adjusted with the cooling system inside the reactor. Based on this design, the deposition temperature of the substrate (Ts ) and bath (Tb ) can be operated independently, and Ts , in general, was operated much higher than Tb . High substrate temperature on the other hand could enhance the deposition rate; meanwhile, on the other the bath with a lower temperature can keep its stability for a long bath-life. From Fig. 1, it can be clearly seen that the clearance between the Teflon® -coated heater and the plate substrate is adjustable. Mass and heat transfer will show a big difference as compared with the bath since the clearance was set between 30 and 200 ␮m. Hence, we believe that the metal ions in this space can be reduced to metallic nanoparticles by reductant at a higher temperature. The resulting metallic primary nuclei are then forced to deposition on the substrates. Furthermore, due to a higher temperature of the substrates and a higher reactivity of the metallic nanoparticles, these deposited primary particles can bond with the substrates so as to enhance the adhesions and form reactive sites. Afterward, they grow over and over again and finally resulted in the formation of the metallic film on the surface of the substrate. The substrates used in this experiment are Si plate with the dimension of 1 cm × 1 cm cut from a p-type silicone wafer with the diameter of 100 mm. The sample was degreased for 10 min and chemical etched for 10 min to dissolve the oxide layer. The Ni catalysts were prepared onto the silicone substrate using nonisothermal deposition (NITD) technique (as shown in Fig. 1) for 30, 60 and 90 s using the plating solution of NiSO4 ·6H2 O, NaH2 PO2 ·H2 O, C2 H5 O2 N, C3 H5 O3 Na, H14N (100 ml). The Ni-Pd co-catalysts were prepared by electroless plating technique with the solution of PdCl2 . The CNTs were grown by thermal CVD technique in a horizontal quartz reaction chamber with the length of 200 cm and the diameter of 15 cm (as shown in Fig. 2). The chamber was heated to the growth temperature ranged from 700 to 900 ◦ C under Ar flow. The substrate was pre-treated by NH3 for 10 min. The CNTs were fabricated using acetylene (C2 H2 ) at a flow rate of 133 sccm for 10 min. The chamber was

Fig. 2. A schematic diagram showing the setup of thermal CVD equipment.

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Fig. 3. SEM photographs of Ni catalysts NITD coated for 30, 60 and 90 s with the density of 2.3, 2.9, 3.1 particles ␮m−2 shown in (a), (c), (e) and the average size of 20 ∼ 30, 40 ∼ 60, 60 ∼ 80 nm shown in (b), (d) and (f).

cooled to room temperature under Ar atmosphere. The structure of prepared catalysts and grown CNTs were examined by SEM. 3. Results and discussion Fig. 3 shows Ni catalytic particles deposited on Si substrates by NITD technique for 30, 60 and 90 s. The average particle size and density calculated using Matrox Inspector 2.2 increased with the increasing of deposition time. The calculation results of particle size and density are 20–40 nm (2.3 particles ␮m−2 ) for 30 s of plating 40–60 nm (2.9 particles ␮m−2 ) for 60 s of plating and 60–80 nm (3.1 particles ␮m−2 ) for 90 s of plating. It was also found that the uniformity of Ni catalysts was improved as the plating time was increased. The microstructure of CNTs grown at 900 ◦ C with the flow rate of (C2 H2 ) set at 133 sccm for 10 min is shown in Fig. 4. Fig. 4(a, c and e) indicate that the diameter and density of CNTs produced on the Ni-catalyst coated sample prepared by NITD technique for 30, 60 and 90 s are shown to be increased (the diam-

eter ranged from 80–100 to 150–250 nm) with the increasing of catalyst size and density, which are increased as the plating time is increased. Fig. 4(b, d and f) shows that the density of CNTs grown on Ni-Pd co-catalysts plated sample is increased, however, the diameter of CNTs is becoming thinner ranged from 30 to 80 nm. The CNTs grown at 800 ◦ C with the same parameters of catalysts preparation, flow rate of (C2 H2 ) and growth duration as those at 900 ◦ C is shown in Fig. 5. It can be shown in Fig. 5(a, c and e) that the growth temperature reduced to 800 ◦ C resulted in less density and the reduction of the diameter ranged from 100 to 200 nm of CNTs grown on Ni-catalyst coated Si substrate. The structure morphology of CNTs grown on Ni-Pd co-catalyst coated Si sample shown in Fig. 5(b, d and f) indicates that their grown density are denser than those obtained on Ni-catalyst coated sample. The CNTs is shown to be distributed uniformly throughout the substrate. This result also implies that the Ni-Pd co-catalyst might have the effect on increasing the CNTs’ density during the synthesis process, which is in agreement with some literatures, suggesting that the catalytic Pd particles with high

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Fig. 4. SEM photographs of CNTs grown on (a), (c) and (e) Ni catalyst samples and on (b), (d) and (f) Ni-Pd co-catalyst samples prepared by NITD for 30, 60 and 90 s, at 900 ◦ C and C2 H2 flow rate of 133 sccm.

Fig. 5. SEM photographs of CNTs grown on (a), (c) and (e) Ni catalyst samples and on (b), (d) and (f) Ni-Pd co-catalyst samples prepared by NITD for 30, 60 and 90 s, at 800 ◦ C and C2 H2 flow rate of 133 sccm.

Fig. 6. The morphology of CNTs grown on Ni-Pd co-catalyst-deposited substrate at the growth temperature of 700 ◦ C.

activity is able to encourage the decomposition of C2 H2 providing the carbon source [11–13]. However, the detailed reasons for this grown mechanism needed further research and development. Fig. 6 shows the structure morphology of CNTs grown on Ni-Pd co-catalyst-deposited substrate at 700 ◦ C. The substrates were prepared by NITD for 30, 60 and 90 s to initially form Ni catalysts and then the formation of Ni-Pd catalyst by adding Pd was followed using isothermal plating of Pd for 20 s. It can be shown from Fig. 6 that the synthesis of large areas and dense CNTs can be achieved at 700 ◦ C by using Ni-Pd co-catalysts. On the contrary, the samples with only Ni catalysts resulted in the grown carbon clusters or fibers during the synthesis process conducted at 700 ◦ C (as shown in Fig. 7). This implies that the addition of Pd onto the Ni catalysts substrate acting as cocatalysts had the effect in promoting the synthesis of CNTs at

Fig. 7. The formation of carbon clusters or fibers the samples with only Ni catalysts achieved at 700 ◦ C.

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Fig. 8. TEM images of multi-wall CNTs produced on Ni-Pd co-catalyst-deposited substrate at 700 ◦ C.

lower temperatures, which can be further proved by the TEM examination results, as shown in Fig. 8(a and b), indicating that the multi-wall CNTs, with the diameter ranging from 20 to 25 nm, were capable of being produced on Ni-Pd co-catalystdeposited substrate at 700 ◦ C. The CNTs growth processes can be divided into the following three steps according to Baker’s investigations [14]: (1) The carbon atoms produced from thermal decomposition of hydrocarbon gas such as C2 H2 or CH4 are absorbed by the heated catalytic metal particles. While the Cn Hm moleculars contact with catalysts, the bonding between C and H is broken. The carbon atoms then diffuse into the catalysts via the surface and bulk of metal catalysts and hydrogen atoms diffuse out of the catalyst surface. This process is a strong exothermic reaction for the Cn Hm molecular, which can increase the temperature on the absorbance site between catalyst and carbon atom and the saturation of carbon atoms in catalysts (N.S. Kim, 2002). (2) While the carbon atoms are arrived at their supersaturated point in metal catalysts, carbon precipites vertically out of the surface of catalysts in a crystalline structure, forming the tube. The precipitation process is an endothermic process which builds up a temperature gradient during the process of carbon atom diffusion into and out of the catalyst surface. This temperature gradient provide the required driving force for the activation energy dependent process of CNTs growth. (3) If the metal catalysts loose their activity due to the prolong growing time or too many carbon atoms adhering to catalyst surface resulting in too low growth rate, the top areas of CNTs will be closed leading to the stopping of the CNTs growth process. The above discussion of CNTs growth mechanism implies that the activity of the metal catalyst surface dominate the process of thermal decomposition of carbon source gas and the extent of temperature gradient effect. Pd has effects in the improvement of activity of metal catalyst-coated substrate, due to its excellent ability as an activator, and the increasing the catalytic time of Ni. Therefore, the CNTs grown on the substrate with Ni-Pd co-catalysts are thought to be both denser and longer than those grown on the substrate with Ni catalysts at the same growing temperature.

4. Conclusion This paper investigates the effects of particle size and distribution density of Ni catalysts and Ni-Pd co-catalysts prepared by NITD technique on the structure morphology of CNTs grown by thermal CVD with C2 H2 at the flow rate of 133 sccm and the growing temperatures of 700, 800 and 900 ◦ C. The results can be summarized as below. The size and density of Ni catalysts were observed to be increased with the increasing deposition time after the NITD process. The distribution of Ni catalysts was also more uniform with the increasing plating duration. The diameter and density of the grown CNTs were increased as the prepared Ni catalysts were increased. The CNTs growing at higher temperature had larger diameter than those growing at lower temperature. The results also showed that, at the same growth temperature, the synthesized CNTs on Ni-Pd co-catalyst prepared substrate are denser but with a thinner diameter than those produced on Ni-catalyst prepared substrate. The synthesis of large areas and dense CNTs was shown to be achieved by using Ni-Pd cocatalysts at 700 ◦ C, however the samples with only Ni catalysts resulted in the grown carbon clusters or fibers, which implies that the addition of Pd onto the Ni catalysts substrate acting as co-catalysts was proved to have the effect in promoting the synthesis of CNTs at lower temperatures.

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