Abstract
Vertically aligned carbon nanotubes (VA-CNTs) were successfully grown on silicon substrates by atmospheric pressure chemical vapor deposition at 750∘C using acetylene as a carbon source and Fe-Co nanoparticle thin films as a cocatalyst. Preparation of the cocatalyst was operated by an economical sparking process. A small amount of water vapor was introduced into the reactor by controlling diffusion by heated water in a flask. The CNTs were characterized by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The VA-CNTs were obtained only when the water vapor fraction in the introduced gas was in the range of 310–440 ppm. The length of the VA-CNTs reached 0.8 mm with a growth rate of 17 μm/min. Moreover, D/G-band ratio suggests that the optimum fraction of water vapor decreases defects in CNTs.
1. Introduction
Carbon nanotubes (CNTs) are regarded as one of the most promising materials for various potential applications, such as electrodes for electrochemical double-layer capacitors [1–3], field emitters [4, 5] in flat panel displays, electron source in X-ray tubes [6], and nanoelectronic devices [7, 8]. Thus, numerous synthetic methods for production of CNTs such as arc discharge [9, 10], laser ablation [11], pyrolysis of hydrocarbon [12–14], and chemical vapor deposition (CVD) [15, 16] were developed. The CVD technique has attracted particular attention because of its ability to produce a high yield of very pure CNTs at low costs, while being easily scaled up [17].
Nevertheless, long vertically aligned CNTs remain challenging to produce. Hata and coworkers reported that the introduction of a small fraction of water into the CVD growth environment increases dramatically the achievable growth height of single-wall nanotubes (SWNTs) [18]. We believe that a simple and economical fabrication system is essential for further industrial manufacturing. In this view, we aimed at eliminating the traditional costly equipment for catalyst deposition and water vapor insertion. We used an iron-cobalt cocatalyst, prepared from a sparking process [19, 20] in conjunction with a diffusion-controlled water insertion method. CNTs were synthesized using CVD with acetylene as a carbon source in an argon carrier gas. The amount of vapor is essential in order to achieve high-purity and long vertically aligned CNTs [21] and the diffusion-controlled insertion allows precise control of water vapor even for very low gas flows. Nasibulin and coworkers have demonstrated the effect of CO2 and H2O vapor on the synthesis of CNT grown in situ using premade Co and Fe catalyst nanoparticles and particles deposited by a hot wire generator [21].
Here, we focused on an alternative way of preparing the catalyst in a cost-effective and simple manner, using a high-voltage sparking process that can produce evenly distributed 5–30 nm in diameter nanoparticles. The effect of different amount of vapor or the ratio of carbon, hydrogen, and oxygen in the synthesis process also reported in this paper.
2. Experimental Details
The CNTs are synthesized on a Si substrate with a Fe-Co catalyst. Si substrates are sonically cleaned in acetone ethanol and dried. The Fe-Co nanoparticles are prepared using a high-voltage sparking process. Iron wire (Advent Research Materials Ltd., ϕ 0.25 mm, purity 99.95%) is used for the anode tip and cobalt wire (Advent Research Materials Ltd., ϕ 0.25 mm, purity 99.95%) for the cathode tip. The tips are placed 1 mm apart and approximately 1 mm above the horizontally placed 5 × 7 mm Si substrate (Ted Pella, Inc.) to ensure even distribution of the particles. A capacitor (25 nF) is charged up to 12 kV after which the voltage is applied to the tips creating a spark. Fe and Co nanoparticles are torn from the tips and deposit on the Si surface. Consequently, Fe and Co nanoparticles deposited onto the substrate. Figure 1 shows the typical Fe-Co nanoparticles with a diameter ranging from 6 to 59 nm as estimated from the AFM measurement. Dense and homogenous Fe/Co nanoparticle distribution is desired in order to obtain vertically aligned CNTs growing from the base.
The CVD apparatus is set up for CNTs synthesis as shown in the schematic diagram in Figure 2. The ceramic porcelain ceramic furnace tube of 4 mm diameter is cleaned using alcohol, and the processed Si substrate is placed at the center of the furnace. Then, the furnace is heated up to 750°C in Ar gas flowing at 100 sscm. When temperature is reached, acetylene is added to the gas flow at 2 sscm and water vapor is introduced into the reactor by controlled diffusion. Using Fick’s law of diffusion, the vapor flow rate was calculated to be in the range of 200–350 ppm when the water flask is heated at (30–65°C). After the growth time (15 min to 4 hr), the reactor was cooled to room temperature in Ar atmosphere and the reaction is terminated. The as-synthesized products were characterized by scanning electron microscopy (SEM, JEOL JSM-6335F), transmission electron microscopy (TEM, JEOL JEM-2010), and Raman Spectroscopy (HORIBA JOBIN YVON T64000) directly on the substrate.
3. Results and Discussion
Typically, a deposited film can be visually observed after 15 seconds in growth conditions once the acetylene is introduced. The carbon nanotube films are dull black in color and can be grown in the areas covered by the sparking process. Figures 3(a) and 3(b) show SEM images of the CNTs synthesized at 750°C with 50 to 1 (100 : 2 sccm) Ar to acetylene ratio in water-vapor-free condition.
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On the other hand, water vapor plays a critical role in the length of vertically aligned CNT synthesis. In its absence, the CNTs curl up in bundles as shown in Figures 3(a) and 3(b), rather than growing upwards. CNTs grow in rapid rate initially, and, because of its high density of CNTs, they control each other to grow vertically. The evenly distributed VA-CNTs shown in Figures 3(c) and 3(d) are formed in the presence of the right amount of water vapor (1.7 μmol/min in our case). We control the flux of water vapor delivered through control of the diffusive flux out of the flask. The diffusive flux is given by Fick’s law of diffusion as illustrated in where is the diffusion coefficient and is the position. In the case of a tube the flux can be expressed with , the cross section of the tube, , the length of the tube from the flask outlet to the furnace, and , the saturated water vapor concentration at a temperature . Therefore, the flux can be controlled via geometrical parameters, such as the length or the cross-section of the tubing, or by the absolute value of the vapor concentration in the source. The latter can be readily varied by heating the water as shown in Figure 4.
In our system, the diffusion coefficient of water vapor is 2.5 × 10−5 m2/s, the length of the tube is 0.3 m, and for a flask heated at 45°C is 100 g/m3. Hence, the flow of water vapor obtained from this approach is 0.9 μmol/min. Furthermore, in the presence of water vapor, the CNT yields appear to contain less amorphous carbon potentially due to its etching by an oxidizer like oxygen (Figure 3) [22–25]. However, the high content in H radicals can etch the preformed CNTs [26] causing CNTs growth rate reduction. However, when growing vertical multiwalled nanotubes (MWNTs) the H-blocking effect should be lesser due to higher stability of larger tubes [26].
In contrast, the added water vapor has an insignificant effect on the nanotube diameter which result in CNT diameters ranging from 7 to 22 nm with an average size of approximately 13 ± 3 nm. We found that the optimum water vapor delivery rate for obtaining the longest CNTs is of 1.7 μmol/min (370 ppm) at 75°C (Figure 5). At lower delivery rates (up to 1.2 μmol/min or 260 ppm), the process yields shorter CNTs only approximately 20 um long. However, by increasing the delivery rate to more than 1.8 μmol/min (400 ppm), the length of CNTs is reduced due to the cleavage effect of excessive H and O radicals. Since the flux of water vapor is central to CNT growth, we suggest an optimum ratio of water/acetylene for vertically aligned MWNT synthesis of (370/17000). This is based on our best result in which the flow rates of argon, acetylene, and water vapor are 4.5 mmol/min (17000 ppm), 77 μmol/min, and 1.7 μmol/min (370 ppm), respectively. Zhang and coworkers suggested that CNT synthesis in a CVD process without O2 yields no VA-SWNTs while the absence of H2 still produces VA-SWNT [25]. This supports our results demonstrating that a small amount of oxygen can promote VA-CNT growth. Hence, small amount of water vapor can increase CNT yields.
In the optimal conditions aforementioned, the fraction of water vapor and acetylene delivered to the reaction are 370 ppm and 17,000 ppm, respectively. These values are in the same range as other reports which mention water vapor fraction in the range of 100–400 ppm and a carbon source fraction in the range of 1500–10,000 ppm, [27–29]. This also asserts the soundness of the diffusive delivery approach we employed for adding water vapor, as it provides accurate control in the small ranges flux rates involved as well as a simple electronic free method.
The peaks of the Raman spectra performed on the CNTs are centered at 1339 and 1573 cm−1 and correspond to the disorder-induced vibrational mode (D-band) and the in-plane carbon stretching mode (G-band), respectively (Figure 6). The D-band/G-band intensity ratio () decreased with the addition of water vapor. This indicates that the introduction of small amounts of water vapor to the growth environment decreased defect levels in the atomic carbon structure.
Moreover, TEM images and (inset) the associate selected area electron diffraction pattern (SADP) of the individual VA-CNTs are shown in Figure 7 providing further information on the morphology and microstructure of as-synthesized VA-CNTs over Fe-Co/Si catalyst. Figure 7(a) shows the uniformity of VA-CNTs without metal catalyst nanoparticles or amorphous carbon at lower resolution. The outer diameter of the CNT is 8–20 nm and inner diameter is 3–5 nm with 10 walls on average as presented in Figure 7(b). The fringes of graphite layers show the VA-CNT structure with less of the degree of disorder and defects. The hexagonal structure of MWNTs has been confirmed by the corresponding crystallographic (002), (101), and (110) planes.
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Figure 8 shows a comparison of the ratio as the degree of structural defects and the CNTs diameter of as-grown CNTs which add water vapor in difference temperature. The result illustrated that increasing the temperature of water vapor, decrease the defect of atomic structure of CNTs.
Our best condition for CNT synthesis, water heated at 65°C (444 ppm), was found to be nanotubes grew rapidly at the early stage and then slowed down to reach their saturation stage. The relationship between CNTs length and synthesis time is illustrated in Figure 9 showing rapid growth of 17.3 μm/min in the second stage from 20th to 40th minute, then the growth rate reduces to 2.3 μm/min. From this relationship, only 30 min would be adequate to grow VA-CNTs up to 500 μm.
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4. Conclusions
VA-CNTs were successfully synthesized using Fe/Co catalyst deposited by a cost-effective high-voltage sparking process. Growth was effectuated using CVD with acetylene as a carbon source with a small amount of water vapor addition to the growth environment through a simple diffusion process. The vapor-assisted conditions provided increased quantity and quality of CNTs. Furthermore, the D-band/G-band ratio signifies that the defect of atomic carbon structure was decreased. These results demonstrate that water vapor is essential for long VA-CNTs synthesis.
Acknowledgment
The authors thank the National Research University Project under Thailand’s Office of the Commission on Higher Education (CHE) for financial support.