Diameter control of multiwalled carbon nanotubes using experimental strategies

Carbon 43 (2005) 2760 2768 www.elsevier.com/locate/carbon Diameter control of multiwalled carbon nanotubes using experimental strategies Chien-Sheng Kuo a, Allen Bai b, Chien-Ming Huang a, Yuan-Yao Li a, *, Chi-Chang Hu c, Chien-Chong Chen c a Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan, ROC b Department of Biochemical Engineering, Kao Yuan Institute of Technology, Kao-Hsiung 1821, Taiwan, ROC c Center for Nanotechnology Design and Prototyping, National Chung Cheng University, Chia-Yi 621, Taiwan, ROC Received 9 October 2004; accepted 22 May 2005 Available online 5 July 2005 Abstract Multiwalled carbon nanotubes (MWNTs) were synthesized using a chemical vapor deposition floating feed method in a vertical reactor. Effects of the preparation variables on the average diameter of carbon nanotubes were systematically examined using the fractional factorial design (FFD), path of the steepest ascent, and central composite design (CCD) coupled with the response surface methodology. From the FFD study, the main and interactive effects of reaction temperature, methane flow rate, and chamber pressure were concluded to be the key factors influencing the diameter of MWNTs. Two empirical models, representing the dependence of the diameter of carbon nanotubes at the vicinities around maximum (420 nm) and minimum (15 nm) on the reaction temperature and methane flow rate, were constructed in two independent CCD studies. These models, shown as contour diagrams, indicated that the diameter of carbon nanotubes generally increased with increasing reaction temperature and methane flow rate. Based on both models, the diameter of MWNTs from 15 to 420 nm can be controlled precisely by using a continuous CVD fabrication method. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Chemical vapor deposition; Electron microscopy 1. Introduction Mass production of carbon nanotubes in various forms has been widely developed and studied by many research teams using various methods. For instance, an arc plasma jet technique with the cathode placed at an oblique angle of about 30 from the anode was proposed to produce the single-walled carbon nanotubes by Ando et al. [1]. Lee et al. [2] employed plasma rotating arc discharge method with the anode rotated at speeds from 0 to 10,000 rev/min (rpm) for the large-scale synthesis of carbon nanotubes. In addition, the floating * Corresponding author. Tel.: +886 5 2720411x33403; fax: +886 5 2721206. E-mail address: chmyyl@ccu.edu.tw (Y.-Y. Li). catalyst CVD method was employed by several research groups for the mass production of carbon nanotubes [3]. Lee et al. [4] reported the vapor phase growth of aligned carbon nanotubes in a horizontal quartz tube reactor by heating the flowing mixtures of acetylene and iron pentacarbonyl. Satishkumar et al. [5] fabricated singlewalled carbon nanotubes from acetylene with organometallic mixtures in a two-stage furnace. Nikolaev et al. [6] investigated the gas-phase synthesis of singlewalled carbon nanotubes from carbon monoxide with iron pentacarbonyl in a horizontal quartz tube reactor under a high-pressure and elevated-temperature operation. Zhu et al. [7] employed mixtures of n-hexane or benzene with ferrocene and thiophene in a vertical furnace for the production of double-walled carbon nanotubes. Ci et al. [8,9] studied the fabrication of 0008-6223/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.05.022 13

C.-S. Kuo et al. / Carbon 43 (2005) 2760 2768 2761 carbon nanofibers through a floating catalyst method using mixtures of benzene, thiophene and ferrocene in a vertical furnace. All the above studies demonstrate the possibility of mass production for CNTs, while variables of fabrication, such as composition of mixtures, pressure, and temperature, were found to significantly influence the yield and quality of carbon nanotubes [10]. In addition, the amount of carbon nanotubes obtained from all the above methods for a few hours was limited (from milligrams to a few grams). Accordingly, more reliable and economic techniques improving the above drawbacks of CNT mass production were worthy being developed. In this research, MWNTs in large quantity were prepared by a floating catalyst method using more economic and safe chemicals. For example, e.g. methane as the carbon source (in comparison with acetylene, benzene, and thiophene) and nitrogen as the carrier gas (in place of hydrogen). A vertical reactor was employed to carry out the continuous fabrication process in the absence of any substrate, which is beneficial to the mass production of MWNTs. In addition, preparation variables such as the compositions of mixture gases for the CVD reaction, operational temperature, and pressure were systematically studied to find an optimal condition for the growth of MWNTs. On the other hand, the possible factors influencing the growth rate and quality of MWNTs should be complicated, and are hard to clarify under the one-factor-at-a-time procedure, from the above brief review. Accordingly, a sequential procedure of experimental strategies [11 13], including the fractional factorial design (FFD), path of the steepest ascent, and central composite design (CCD), was employed to achieve the statistically significant regression equations modeling for the dependence of the MWNT diameter on the preparation variables. In addition, the CCD coupled with the response surface methodology was used to construct the relationship between the diameter of MWNTs and the preparation variables around the conditions of maximum and minimum diameters. 2. Experimental A diagram of the experimental setup is shown in Fig. 1. It consists of stainless steel gas flow lines, mass flow controllers, a syringe pump, as well as a vertical electric furnace fitted with a quartz tube (75 mm OD, 70 mm ID, and 1200 mm long) as the reactor. The hot zone was 410 mm long. The flow rates of gases and catalysts were controlled by means of mass flow controllers and a syringe pump, respectively. Feed gases were introduced to the top inlet of the reactor, and the products and exhaust gases were released from the bottom of the quartz tube. Fig. 1. A schematic diagram of the reactor for the synthesis of MWNTs. Nitrogen was introduced into the vertical tube during the initial heating step. The CVD experiments were started when the temperature of the furnace reached the desired reaction temperature (between 1050 and 1150 C). A mixture of Fe(CO) 5 as the catalyst precursor, methane as the carbon source, and nitrogen as the carrier gas was then admitted into the vertical furnace reactor for pyrolysis. The flow rate of methane was controlled in the range from 125 to 250 sccm, and the feeding of Fe(CO) 5 was from 0.5 to 2.5 ml/h when the flow rate of nitrogen was controlled from 1000 to 2000 sccm. After the 1-h reaction, the reactor was cooled down naturally to room temperature under a nitrogen flow. Effects of the following preparation variables on the diameter of MWNTs were investigated in the FFD study: (A) reaction temperature; (B) flow rate of methane; (C) flow rate of nitrogen; (D) flow rate of catalyst; and (E) chamber pressure. Fixed levels of these five variables are given in Table 1. The design matrix with levels and the diameters of MWNTs are shown in Table 2. Note that the lower limit of chamber pressure was set at 650 Torr. The average diameter of MWNTs in the CCD study was estimated from three SEM pictures. Note that the average diameters of MWNTs shown in Table 1 Factors and levels for a 2 5 1 fractional factorial design Factor Level + A Reaction temperature ( C) 1050 1150 B Flow rate of methane (sccm) 125 250 C N 2 carried gas flow rate (sccm) 1000 2000 D Flow rate of catalyst (ml/h) 0.5 2.5 E Chamber pressure (Torr) 650 755 14

2762 C.-S. Kuo et al. / Carbon 43 (2005) 2760 2768 Table 2 The design matrix and experimental data of the average diameter of MWNTs from the 2 5 1 FFD with the defining relation I = ABCDE Run Factors Average diameter (nm) A B C D E 1 + 50 2 + 120 3 + 140 4 + + + 270 5 + 80 6 + + + 210 7 + + + 150 8 + + + 170 9 + 60 10 + + + 190 11 + + + 150 12 + + + 170 13 + + + 70 14 + + + 80 15 + + + 170 16 + + + + + 170 this work are acceptable because they were estimated from 50 random-selected MWNTs. In addition, the standard deviation for every sampling is less than 1.5 nm. The yield of MWNTs was more than 50% for every sample. 3. Results and discussion 3.1. Fractional factorial design The fractional factorial design (FFD) is introduced to efficiently screen out the key preparation variables affecting the diameter of MWNTs. This experimental design can find the influences of each preparation variable as a variety of other variable levels, as well as the interactions among these variables on the diameter of MWNTs. The observations of the diameter (nm) of MWNTs with the design matrix in the 2 5 1 FFD experiments are shown in Table 2. According to the defining relation I = ABCDE introduced by Box et al. [11], the main effect of the factor E, for example, and the interaction effect of the factors A, B, C, and D were said to be confounded [11 13]. From the principle of the sparsity of effects [12,13], a system or process is likely to be driven primarily by some main and low-order interaction effects. Thus, the effects of the high-order (e.g. three-, four-, and five-order) interactions were assumed to be negligible. Based on this assumption, the effect of A B C D interaction on the diameter of the MWNTs can be ignored and hence, the main effect of factor E can be isolated in this FFD study. The above argument was also applicable for Factors A D. Data of the MWNTs diameter shown in Table 2 were subjected to regression analysis to estimate the effects of preparation variables. The analysis of variance (i.e., ANOVA) on the diameter of MWNTs in nanometer is summarized in Table 3. The test statistic, F, is defined as F = MSF/MSE, where MSF and MSE were the mean squares of factors or interactions, and errors, respectively. If the calculated value of F is greater than the value in the F table at a specified probability level (e.g., F 0.1 (1,8) = 3.46), a statistically significant factor or interaction is obtained. After the test, factors A, B and E, and interactions A B, A C, A D and A E exhibit statistically significant effects on the diameter of MWNTs. From a combination of the estimates for the preparation variables and the ANOVA results, a polynomial model with statistical significance can be generated. This model, quantitatively elucidating the effects of preparation variables with statistical significance, is presented as follows: y ¼ 140.63 þ 31.88x A þ 33.12x B þ 16.88x E 10.62x A x B 11.88x A x C 11.88x A x D þ 20.63x A x E ð1þ Table 3 Analysis of variance for the average diameter of MWNTs from the 2 5 1 fractional factorial design Factor df SS MS F A 1 16,256.25 16,256.25 36.1 B 1 17,556.25 17,556.25 39.0 E 1 4556.25 4556.25 10.1 AB 1 1806.25 1806.25 4.0 AC 1 2256.25 2256.25 5.0 AD 1 2256.25 2256.25 5.0 AE 1 6806.25 6806.25 15.1 Error 8 3600 450 Total 15 55,093.75 R 2 = 0.94; F 0.1 (1,8) = 3.46. 15

C.-S. Kuo et al. / Carbon 43 (2005) 2760 2768 2763 where x i represents the coded variables for factor i (i.e., A, B, C, D, and E). The coded variables, x i, employed in Eq. (1) are defined as follows [13]: x i;high ¼ðX i;high X i;mean Þ=S i ð¼ þ1þ x i;low ¼ðX i;low X i;mean Þ=S i ð¼ 1Þ X i;mean ¼ðX i;high þ X i;low Þ=2 S i ¼ðX i;high X i;low Þ=2 ð2þ ð3þ ð4þ ð5þ Diameter / nm 180 170 160 150 140 130 where X i,high and X i,low are the high and low levels of factor i in the nature units, respectively. Note in Eq. (1) that the term(s) without statistical significance was deleted from the full-effect model based on the analysis of variance. These effects were considered as errors in the experiments and their variances were accordingly pooled into the sum of squares of errors (i.e., SSE). Therefore, the multiple correlation coefficient squared, R 2 =1 (SSE/SST) equal to 0.94 for this model, indicates a good fitting for the experimental data. A more detailed description for the fractional factorial design has been reported in our previous work [14 17]. The main effects (i.e., A, B, C, D, and E) and two-factor interactions effects (i.e., A B, A C, A D, and A E) are shown in Fig. 2(a) and (b), respectively. From Fig. 2(a), the diameter of MWNTs was increased by increasing the reaction temperature (A), flow rate of carbon source (B), and chamber pressure (E). Note in Fig. 2(b) that the effect of factor E was negligible when factor A was set at the low level. On the other hand, a sharp increase in the diameter, from 135 to 210 nm was found for factor E as factor A was kept at the high level. This phenomenon is referred to as an interaction between factors A and E (denoted as A E). Similarly, synergistic effects on increasing the diameter of MWNTs are also visible for interactions A C and A D. Based on the above results and discussion, factors A, B, and E were considered in the path of the steepest ascent for approaching the maximum diameter of MWNTs, while factors C and D were maintained at the low level. 3.2. Path of steepest ascent Based on the results and discussion in Section 3.1, factors A (reaction temperature), B (methane flow rate), and E (chamber pressure) as well as interactions A B, A C, A D, and A E were concluded to be the key variables influencing the diameter of MWNTs. The effects of these variables were further verified in the study of the path of steepest ascent. According to Fig. 2(a), the directions of the steepest ascent should be positive for factors A (increasing reaction temperature), B (increasing methane flow rate), and E (increasing chamber pressure). In addition, the flow rate of nitrogen has to be fixed at the low level (i.e. 1000 sccm) in order to possess the synergistic effect of A C. Furthermore, the flow a Diameter / nm b 120 110 100 220 200 180 160 140 120 100 80 60 40 20 A(-) A(+) B(-) B(+) C(-) C(+) D(-) D(+) E(-) E(+) C(-) C(+) D(+) A(-) A(+) A(-) A(+) A(-) A(+) rate of catalyst was kept at the low level in order to increase the diameter of MWNTs. Therefore, an increase in diameter of the MWNTs should be found when the above three variables were simultaneously changed in the positive directions of the steepest ascent. On the contrary, the diameter of MWNTs should be decreased significantly when the factors A, B, and E were moved in the negative directions. Because the chamber pressure is limited by the system, the pressure can be only adjusted from 760 to 650 Torr. Accordingly, the study of the path of steepest ascent neglects the E factor, and the chamber pressures (E) are set as 650 and 755 Torr for growing minimum and maximum diameter of MWNTs. Moreover, the flow rates of N 2 and catalyst must be set at high level if the MWNTs with small diameters are preferred. From all the above results and discussion, it is very easy to control the diameter of MWNTs by varying the directions and the step size of all preparation variables investigated in this work. 16 B(-) B(+) D(-) E(+) Fig. 2. (a) Main effects of (A) reaction temperature, (B) methane flow rate, (C) N 2 carried gas flow rate, (D) catalyst flow rate, and (E) chamber pressure on the average diameter of MWNTs. (b) Interaction effects of (A C); (A D); and (A E) on the average diameter of MWNTs; where (+) and ( ) indicate the high and low levels of the these factors, respectively. A(-) E(-) A(+)

2764 C.-S. Kuo et al. / Carbon 43 (2005) 2760 2768 Table 4 Paths of the steepest ascent (runs 1 7) and decent (runs 8 13) for the growth of MWNTs Run Factor Average diameter (nm) A B 1 1100 187.5 100 2 1120 212.5 149 3 1140 237.5 181 4 1160 262.5 223 5 1180 287.5 353 6 1200 312.5 423 7 1220 337.5 342 8 1100 187.5 89 9 1090 175 78 10 1080 162.5 68 11 1070 150 52 12 1060 137.5 31 13 1050 125 41 Remark: The nitrogen flow rate is 1000 sccm, the chamber pressure is 755 Torr and the flow rate of catalyst is 0.5 ml/h for runs 1 7. The nitrogen flow rate is 2000 sccm, the chamber pressure is 650 Torr and the flow rate of catalyst is 2.5 ml/h for runs 8 13. On the basis of the steepest ascent methodology [12] the direction for increasing the diameter of MWNTs was +1.0S A (e.g. 10 C in this work) in the x A direction for every +1.0S B (e.g. 12.5 sccm) in the x B direction (i.e. the ratio of step size = 1:1). Similar situations are applicable for the path of the steepest decent study (i.e., a decrease in the diameter) although the directions are completely reversed. Typical results for the path of the steepest ascent and decent are shown in Table 4 for runs 1 7 and 8 13, respectively. From this table, the maximum and the minimum diameters of MWNTs should be located in the vicinities of the experimental settings of run 6 and run 12, respectively. Hence, the experimental settings of run 6 in this table were employed as the central point settings in the first CCD study in order to find the maximum diameter of MWNTs. The experimental settings of run 12 in this table were employed as the central point settings in the second CCD study to approach the minimum diameter of MWNTs. 3.3. Central composite design In order to identify the relationship between the preparation variables and the diameter of MWNTs around the vicinities of both maximum and minimum diameters of MWNTs, two central composite designs were carried out. The central point of the first CCD study is the experiment settings of run 6 in Table 4 while for the second CCD study, the experimental settings of run 12 is used as the central point. The design factors and levels for both CCD studies are shown in Table 5; meanwhile the design matrix with their corresponding results is listed in Table 6. Due to the fact that factor E (chamber pressure) is the least significant factor among these three variables and that the upper limit of the chamber pressure is 760 Torr, factor E is set at 755 Torr in the first CCD study in order to approach the maximum diameter of MWNTs. On the other hand, factor E is set at 650 Torr in the second CCD study to achieve the minimum diameter of MWNTs. Note that for both CCD studies, three replicates at the central point were performed to evaluate the pure error between each experiment. Data for the average diameter of all MWNTs in Table 6 were thus subjected to regression analysis. The resultant second-order models corresponding to the maximum and minimum diameters of MWNTs are generated as Eqs. (6) and (7), respectively: D ðnmþ ¼420 þ 3.14x A 0.7x B 49.63x 2 A 54.87x 2 B þ 39.75x Ax B D ðnmþ ¼30 þ 13.62x A þ 14.83x B þ 10.44x 2 A þ 9.44x 2 B 10.75x Ax B ð6þ ð7þ where x A and x B are the coded variables for factors A and B, respectively. The magnitudes of coefficients on the same regression equation illustrated the relative effects of linear, quadratic, and interaction for the reaction temperature (x A ) and the flow rate of methane (x B ) on the diameter of MWNTs. The term(s) without statistical significance was deleted from the full second Table 5 Factors and levels at the vicinity of maximum and minimum diameters of MWNTs for the (1) first and (2) second CCD studies, respectively Level Maximum diameter (1) Minimum diameter (2) Factor A ( C) Factor (sccm) Factor A ( C) Factor (sccm) pffiffi 2 1228.2 347.9 1074 155.2 1 1220 337.5 1070 150 0 1200 312.5 1060 137.5 1p 1180 287.5 1050 125 ffiffiffi 2 1171.8 277.2 1046 120.8 Remark: The nitrogen flow rate, catalyst flow rate, and chamber pressure are 1000 sccm, 0.5 ml/h, and 755 Torr, respectively, for the maximum diameter study. The nitrogen flow rate, catalyst flow rate, and the chamber pressure are 2000 sccm, 2.5 ml/h, and 650 Torr, respectively, for the minimum diameter study. 17

C.-S. Kuo et al. / Carbon 43 (2005) 2760 2768 2765 Table 6 Design matrixes and experimental data of the average diameter for MWNTs in the (1) first and (2) second CCD study with a quadratic form fit Run Maximum diameter (1) Diameter (nm) Minimum diameter (2) Diameter (nm) A B A B 1 1220 337.5 358 1070 150 68 2 1220 287.5 275 1070 125 62 3 1180 337.5 284 1050 150 64 4 1180 287.5 360 1050 125 15 5 1228.2 312.5 312 1074 137.5 69 6 1171.8 312.5 322 1046 137.5 28 7 1200 347.9 306 1060 155.2 69 8 1200 277.2 307 1060 120.8 24 9 1200 312.5 418 1060 137.5 30 10 1200 312.5 422 1060 137.5 31 11 1200 312.5 421 1060 137.5 29 order model from the analysis of variance. The R 2 value of Eq. (6) is 0.996, indicating that this regression model is an excellent representation for the dependence of MWNT diameters on factors A and B in the first CCD study. The R 2 value of Eq. (7) is 0.987, also indicating that this regression model is a very good representation for the dependence of the diameter on factors A and B. Accordingly, the regression models, Eqs. (6) and (7), obtained in this work can be used to precisely predict the average diameter of MWNTs around the vicinities of maximum and minimum diameters, respectively. 3.4. Contour plots Eqs. (6) and (7) were used to construct the contour plots for the diameter (in nm) of MWNTs against the reaction temperature (X A ) and methane flow rate (X B ) of CVD, as shown in Fig. 3. These contour plots facilitate a straightforward comparison of the dependence of the diameter on the key preparation variables. From Fig. 3(a), the diameter is gradually increased with the simultaneous movements of the reaction temperature and the methane flow rate to the central (original) point. In addition, when the growth temperature, the flow rate of methane, N 2, and catalyst, as well as the chamber pressure are equal to 1200 C, 312.5 sccm, 1000 sccm, 0.5 ml/h, and 755 Torr, the diameter of MWNTs reaches a maximum, 422 nm. In addition, the maximum diameter of MWNTs has been confirmed to be 420 nm. In Fig. 3(b), the extreme region with the minimum diameter (15 nm) occurs at a reaction temperature close to 1050 C and a flow rate of methane close to 125 sccm. In addition, the average diameter of MWNTs is gradually decreased with the simultaneous decrease in the reaction temperature and the methane flow rate. Accordingly, the diameter of MWNTs prepared at a reaction temperature <1050 C and a flow rate of methane <125 sccm was expected to be smaller than 15 nm. Unfortunately, as the reaction temperature is lower than 18 Fig. 3. Contour plots for the average diameter (nm) of MWNTs against the reaction temperature (x A ) and methane flow rate (x B ) for (a) the first (maximum diameter) and (b) the second (minimum diameter) CCD studies.

2766 C.-S. Kuo et al. / Carbon 43 (2005) 2760 2768 1050 C, the amount of CNTs is very limited (<10%). However, when the preparation variables were set at 1050 C with the flow rate of methane = 125 sccm and the chamber pressure of 650 Torr, the MWNTs with the smallest diameter, 15 nm, were successfully fabricated. Note that the decrease in both reaction temperature and the flow rate of methane caused a regular decrease in the diameter of MWNTs from 89 to 15 nm. In addition, the MWNTs with an average diameter of 100 nm were obtained when the reaction temperature, the flow rate of methane, the chamber pressure, and the flow rate of nitrogen are 1100 C, 187.5 sccm, 755 Torr, and 1000 sccm, respectively. Moreover, the diameter of MWNTs was regularly increased from 120 to 423 nm when the reaction temperature, the flow rate of methane, and the chamber pressure were regularly increased (from 1100 C, 187.5 sccm, and 755 Torr) to 1200 C, 312.5 sccm and 755 Torr, respectively under a constant flow rate of 1000 sccm for the carrier gas and a constant catalyst flow rate of 0.5 ml/h (i.e., kept at the low level for both factors). Interestingly, the flow rates of carrier gas and catalyst determine the direction for increasing/decreasing the diameter of MWNTs and the average diameters of MWNTs varying from 15 to 423 nm can be precisely controlled in this CVD system. From the above results and discussion, the key variables affecting the diameter of MWNTs can be easily identified by means of the statistically experimental methodology. The diameter of MWNTs can be simply controlled by the simultaneous change in the reaction temperature and the flow rate of methane. 3.5. Morphology and characterization of MWNTs Fig. 4 shows an example of MWNTs products obtained from the CVD floating process. Based on the distribution of MWNTs observed by SEM, the yields of any kind of MWNTs is more than 20% and the lengths Fig. 5. The HR-TEM image of a MWNT obtained by the pyrolysis of Fe(CO) 5 and CH 4 (150 sccm) at 1070 C in a flow of nitrogen (500 sccm) with the chamber pressure of 650 Torr; (a) the graphitic structure of the MWCNT and (b) a catalyst particle on the top of the MWCNT. Fig. 4. A SEM image of MWNTs. 19 are longer than 5 lm. Fig. 5(a) shows a high-resolution TEM (HRTEM) image of the MWNT, revealing that the structure of this MWNT is a waving structure of graphite sheets at a short range. Most inner graphite sheets have straight fringes but the outer graphite sheets have waving fringes, indicating a defective crystalline structure. The space of graphite layers in the MWNT is 0.34 nm meanwhile the outer and inner diameters of MWNT are 30 nm and 5 nm, respectively. Fig. 5(b) shows a MWNT with a 10-nm particle (catalyst) on the top. The crystalline information of MWNTs with average diameters of 50, 100, and 200 nm, is shown in Fig. 6, where these XRD patterns were measured at a scan rate of 1 /min from 20 to 80. Two main crystalline peaks,

C.-S. Kuo et al. / Carbon 43 (2005) 2760 2768 2767 C(002) C(101) c b a 20 25 30 35 40 45 50 55 60 65 70 75 80 2θ Fig. 6. XRD patterns of MWNT S with the average diameter of (a) 50, (b) 100, and (c) 200 nm. C(101) and C(0 02), are clearly centered at 45 and 26 on these XRD patterns, indicating the presence of a graphite crystalline structure on these MWNTs. Note that the intensity of C(0 0 2) increases with decreasing the diameter from 200 to 50 nm. This phenomenon suggested that the crystalline quality of the graphite structure on the MWNTs becomes poor with an increase in the average diameter. In addition, the broad diffraction peak (centered at 45 ) between 43 and 46 may be attributed to the contribution of Fe(1 1 0) (centered at 44.7 ). This result suggests the presence of Fe particles, Intensity 0.0007 0.0006 0.0005 0.0004 0.0003 0.0002 0.0001 0 1000 1200 1400 1600 1800 Raman shift (cm -1 ) Fig. 7. A Raman spectrum of MWNTs grown from the pyrolysis of Fe(CO) 5 and CH 4 (150 sccm) at 1070 C in a flow of nitrogen (500 sccm) with the chamber pressure of 650 Torr. 20 possibly due to the aggregation of Fe atoms from the catalyst precursor. Fig. 7 shows the Raman spectra to evaluate the crystalline degree of MWNTs, where two main Raman bands are found at 1335 cm 1 (D band) and 1580 cm 1 (G band). Note that the peaks of Raman spectra of these MWNTs are not sharp and the ratio of G/D bands is only about 1. The results suggest the relatively poor graphite structure of the MWNTs since the G band indicates the effective structure of graphite sheets [7]. The above finding agrees well with the result from TEM observation. 4. Conclusions Using sequential experiment strategies i.e. fractional factorial design, path of the steepest ascent, and central composite design coupled with the response surface methodology, the predominant factors affecting the diameter of MWNTs were the reaction temperature and methane flow rate. A very strong interaction on the diameter of MWNTs was found between the reaction temperature and the chamber pressure in the CVD process. The diameter of MWNTs was regularly decreased from 89 to 15 nm with a regular decrease in the reaction temperature and the flow rate of methane in a lower chamber pressure (650 Torr) when the flow rates of carrier gas and catalyst were 2000 sccm and 2.5 ml/h, respectively. The diameter of MWNTs also

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