Influence of Water on the Initial Growth Rate of Carbon Nanotubes

Article pubs.acs.org/IECR

Influence of Water on the Initial Growth Rate of Carbon Nanotubes from Ethylene over a Cobalt-Based Catalyst Kunpeng Xie,† Martin Muhler,† and Wei Xia†,* †

Laboratory of Industrial Chemistry, Ruhr-University Bochum, Universitätsstrasse 150, 44780 Bochum, Germany ABSTRACT: Water-assisted growth of multiwalled carbon nanotubes (CNTs) was studied over a Co-based catalyst under plugflow conditions. The influence of water concentration and temperature on the growth kinetics within the first 300 s was analyzed by measuring the conversion of ethylene. Feeding 200 ppm H2O vapor at 650 °C accelerated the initial growth rate and extended the mean lifetime of the catalytically active sites. Higher water concentrations of up to 500 ppm led to lower growth rates and lower CNT yields. Water of 200 ppm showed a promoting effect at 650 °C, but an inhibiting effect at 550 °C. The CO generated by steam gasification of deposited carbon was monitored online indicating coking of the catalyst. The results demonstrate that water plays a dual role: the removal of amorphous carbon on the catalyst by gasification and partial oxidation of the metallic Co catalyst. Water also influenced the diameter distribution of the CNTs.

1. INTRODUCTION Carbon nanotubes (CNTs) are promising materials for various applications, including electrochemical energy storage1−3 and catalysis4,5 due to their high surface area, high electrical and thermal conductivity, and high mechanical strength.6,7 Among various methods, catalytic chemical vapor deposition (CVD) has been widely used for the large-scale synthesis of CNTs due to its easy control of feedstock and wide window of reaction conditions.8,9 However, the deposition of amorphous carbon on active sites during the CNT growth process often causes a fast deactivation of the catalyst, leading to a low yield of CNTs.10 Considering the wide range of applications and the increasing demand for CNTs in various fields, the large-scale production of CNTs with uniform properties remains a challenge.9 In 2004, Hata et al.11 reported the so-called “super-growth” of single-walled carbon nanotube forests by adding a small amount of H2O to the ethylene feed. The addition of weak oxidants to the feed during the CNT growth, such as H2O,11,12 CO2,13,14 or ethanol,15,16 was investigated and proved to be beneficial for the CNT yield and quality. Water vapor in the feed gas promotes the growth by selective etching of amorphous carbon deposits through the gasification reaction (eq 1), which was confirmed by monitoring CO in the effluent gas17 and ex situ microscopic and spectroscopic analysis.10 Recent studies indicate that water promotes the formation of hydroxyl groups on carbon, which subsequently inhibits catalyst ripening by reducing the diffusion of catalyst particles.18 C + H 2O → CO + H 2 (1) H(t ) = βτ0(1 − e−t / τ0)

the catalytically active sites (τ0). A different kinetic model was proposed by Pérez-Cabero et al.19 to describe the CNT growth with two diffusion parameters, rC0 and kB, and two deactivation parameters, kd and kr. Recently, Latorre and co-workers20 reported a so-called “phenomenological kinetic model” to describe all of the relevant stages involved in the CNT growth by catalytic CVD. The initial carburization-nucleation and the growth cessation steps were explained by different mechanisms. Most of the literature studies conclude that the major contribution to the enhanced yield of CNTs by water-assisted CVD is an extended mean lifetime of the catalytically active sites through etching the deposited amorphous carbon or inhibiting the sintering of catalyst particles.21 The initial growth rate reflects the initial carburization-nucleation process, and the mean lifetime mirrors the deactivation of catalyst. H2O in the feed gas not only gasifies amorphous carbon,10,11 but also reacts with metallic catalysts forming oxides.17 Hence, the initial growth rate and the mean lifetime can be affected by adding H2O to the feed gas. Therefore, it is necessary to investigate the role of H2O in the growth of multiwalled CNTs from a kinetic perspective. The CNT growth kinetics were studied by using gas chromatography to measure the consumption of the carbon source22 or by ex situ electron microscopy to determine the height of CNT forests.11 These techniques are insufficient for monitoring the initial stage of the CNT growth. Recently, we have investigated the initial CNT growth kinetics using Co-based mixed oxide catalysts.23 The initial growth rate and the mean lifetime of the catalytically active sites were derived for different growth temperatures and ethylene concentrations. In this work, we report on the influence of H2O vapor on the initial CNT growth kinetics. Gas detectors with a sampling rate of 1.0 s were employed for fast online quantitative analysis,

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The large-scale production of CNTs with a high growth rate calls for a fundamental understanding of the growth kinetics under different conditions. A simple kinetic model (eq 2) was developed by Hata and co-workers12 to study the growth of vertically aligned single-walled CNTs. This model was very suitable to describe the relation between the height of the CNT forest (H), the initial growth rate (β), and the mean lifetime of © 2013 American Chemical Society

Received: Revised: Accepted: Published: 14081

June 9, 2013 August 27, 2013 August 30, 2013 August 30, 2013 dx.doi.org/10.1021/ie401829e | Ind. Eng. Chem. Res. 2013, 52, 14081−14088

Industrial & Engineering Chemistry Research

Article

Figure 1. Flow sheet of the water-assisted CVD setup used in the CNT growth experiments.

which allowed us to achieve reliable data within the first few minutes of the CNT growth. Carbon mass accumulation was derived from the degrees of C2H4 consumption, and CO generation due to carbothermal reduction of the mixed oxide catalyst and gasification of amorphous carbon deposit was monitored online. The initial growth rate and the mean lifetime of the catalytically active sites derived from the kinetic model were used to evaluate the formation of CNTs and the catalyst deactivation. The CO generation provided additional information on the initial formation of CNTs and the deactivation of the Co catalyst. The influence of H2O vapor on the morphology of CNTs was also investigated.

2.3. CNT Growth Procedure. In a typical CNT growth experiment, the loaded catalyst was prereduced by heating to 650 °C at 5 °C min−1 in a 1:1 mixture of H2 (99.999%) and Ar (99.999%) at a total flow rate of 150 sccm (line 1 in Figure 1). Subsequently, feed gases consisting of C2H4 (57 sccm, 99.95%), H2 (43 sccm), Ar (line 1 + line 2, 50 sccm) and H2O vapor (e.g., 200 ppm) were introduced into the reactor. The Ar line 2 was used as carrier gas for H2O vapor, and Ar line 1 was used to balance the total flow at 150 sccm. Before each growth experiment, the reactor was bypassed for about 5 min until a stable gas mixture with the desired H2O concentration was achieved. The concentrations of C2H4 and CO in the exhaust gas were recorded by two infrared detectors with a sampling rate of 1.0 s. After the CNT growth, the reactor was cooled to room temperature in Ar atmosphere. Blank experiments without catalyst in the reactor were performed by following the same procedure and reaction conditions. To minimize the experimental error, each growth experiment was repeated 3−5 times, depending on the quality of data obtained, and the average was used for kinetic analysis. The mean standard deviation of the experimental data confirms the high reproducibility of each experiment. The reactor was weighed before and after the CNT growth for 300 s to determine the overall carbon yield (Y300s), which was used to confirm the calculated yield based on the conversion of ethylene. 2.4. Variation of the H2O Concentration. H2O vapor was introduced into the reactor by passing Ar (line 2 in Figure 1) through a saturator at 0 °C achieved by using a water-ice bath. The concentration of H2O vapor was calculated from the flow rate of Ar (0−50 sccm, line 2) by assuming saturated vapor pressure at 0 °C. The total flow was always fixed at 150 sccm by varying the flow rate of Ar line 1, correspondingly. The concentrations of H2O vapor were set at 0, 200, 250, 300, and 500 ppm. It is worth noting that high H2O concentrations were achieved by increasing the flow rate of Ar (line 2) used as carrier gas. The H2O stream reached the reactor within a shorter period of time at higher concentrations, and subsequently the reaction products reached the detector earlier. 2.5. Variation of Growth Temperature. The catalysts were first reduced at 650 °C prior to the CNT growth at 550, 650, and 750 °C with a constant H2O concentration of 200

2. EXPERIMENTAL SECTION 2.1. Water-Assisted CVD Setup. The CNT growth experiments were carried out in a fixed-bed reactor using of a U-shaped quartz tube with a length of 534 mm and an inner diameter of 7 mm. The flow scheme of the CVD setup is shown in Figure 1. The gas flows were regulated by mass flow controllers, and a LabVIEW interface was used to control the system. The isothermal reactor block was heated by highperformance heating cartridges (Horst) controlled by optical control relays (Eurotherm 2416). The pressure drop in the fixed-bed reactor was monitored by a digital pressure gauge (Wika, Eco Tronic). The quantitative analysis of C2H4 and CO in the effluent gas was performed using two infrared detectors connected in series. The concentration of C2H4 was measured by a flow-cell detector (Emerson Process Management, Rosemount NGA 2000 MLT 4) equipped with a nondispersive infrared cuvette for the range of 0−100%. A similar nondispersive infrared detector (BINOS) was used for CO measurement in the range of 0−4000 ppm. 2.2. Catalyst. Cobalt-based mixed-oxide catalysts containing Co, Mn, Mg, and Al were prepared by the coprecipitation method at pH = 10 using nitrates.8 After drying, calcination, pressing, and sieving catalyst particles, those with a grain size of 300 μm ± 50 μm were used in this study. Typically, about 5 mg of the catalyst were loaded and fixed in the effluent side of the U-tube reactor using quartz wool plugs. A quartz rod was used in the other branch of the U-tube reactor to minimize the dead volume of the system. 14082

dx.doi.org/10.1021/ie401829e | Ind. Eng. Chem. Res. 2013, 52, 14081−14088


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Figure 2. (a) C2H4 concentration profiles during the CNT growth and the blank experiment; (b) correspondinsg profiles of the molar flow rates derived from (a); (c) time-resolved consumption rate of ethylene during the CNT growth calculated by subtracting the molar C2H4 flow rate from the blank experiment; (d) time-resolved accumulation of the carbon mass during the CNT growth under standard conditions calculated from the C2H4 consumption rate. The initial growth rate is the initial slope of the fitting curve.

Figure 3. CNT mass accumulation (mt) as a function of relative time at different H2O concentrations. The growth was performed at 650 °C. (a) First 21 s and (b) up to 300 s.

shown in Figure 2a. The concentration was converted to molar flow rate as shown in Figure 2b. The C2H4 consumption rate was obtained by subtracting the C2H4 flow rate of the blank experiment (Figure 2c). Subsequently, the carbon mass accumulation (mt) was calculated by integrating the C2H4 consumption rate, which was averaged from 3−5 repeated experiments (Figure 2d). The calibration recorded a time delay of 16 s from the reactor to C2H4 detector, and another time delay of 2 s from C2H4 detector to CO detector, which were considered during data processing by shifting the time scale correspondingly. Hence, relative time was used in the ethylene consumption rate profile (Figure 2c) and the carbon mass accumulation profiles (Figure 2d).

ppm. To ensure comparability, the cooling or heating step after reduction was carried out in Ar (150 sccm) always for 60 min to allow the reactor reaching the desired growth temperature. 2.6. Characterization. The obtained CNTs were investigated with a LEO (Zeiss) 1530 Gemini scanning electron microscope (SEM). The diameter distribution was obtained by counting 500 CNTs. 2.7. Evaluation of Kinetic Data. The carbon mass accumulation (mt) was described by eq 3.23 The fitting of eq 3 was performed with the Origin software, which yielded the initial growth rate (r0) and the mean lifetime (τ) of the catalytically active sites. The product of r0 and τ yielded the theoretically predicted maximum CNT yield (Ymax), assuming that all of the obtained carbon material consisted of CNTs, as previously confirmed.23 ⎡ ⎛ t ⎞⎤ mt = r0·τ ⎢1 − exp⎜ − ⎟⎥ ⎝ τ ⎠⎦ ⎣

3. RESULTS AND DISCUSSION 3.1. Influence of H2O Vapor during the Initial Growth Step. The Co-based mixed oxide catalyst is highly active for CNT growth, and the product is proved to be of high purity without other carbon materials.8,24 Figure 3 shows the CNT

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As a typical example, the C2H4 concentration profiles obtained from the gas detector with or without catalysts are 14083



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Figure 4. (a) The initial growth rate (r0) and (b) the mean lifetime (τ) of the Co catalyst as a function of the H2O concentration. The growth was performed at 650 °C.

mass accumulation (mt ) at 650 °C at different H2O concentrations obtained from the measured C2H4 consumption profiles. It can be seen that the initial mt within the first 21 s was slightly influenced by H2O vapor (Figure 3a). At least two processes occurred at this stage, namely, carbothermal reduction of the catalyst and dissolution of carbon in the catalyst. It is known that the mixed oxide catalyst was not fully reduced at the applied temperature of 650 °C.8,24 H2O as a weak oxidant can influence both processes at the initial catalyst activation stage. In contrast, clear differences in mt were observed at different H2O concentrations during further growth up to 300 s (Figure 3b). Enhanced mt was observed for a H2O vapor concentration of 200 ppm. The initial growth rate (r0) and the mean lifetime of the catalytically active sites (τ) were derived using the kinetic model (eq 3). Without H2O the r0 and τ were determined to be −1 and 481 s, respectively. Figure 4 shows that 0.125 gCNT g−1 cat s both r0 and τ pass through a maximum with increasing the H2O concentration. At 200 ppm of H2O both r0 and τ reached −1 and 1065 s, respectively. A further maxima at 0.130 gCNT g−1 cat s increase in the H2O concentration led to a decrease of the two parameters. H2O as a weak oxidant can change the oxidation state of catalysts and effectively remove deposited amorphous carbon on the active sites of the catalyst. Thus, H2O at appropriate concentrations can enhance the initial growth rate and extend the mean lifetime of active sites resulting in a higher yield of CNTs. However, exceeding a certain level, H2O showed an overall negative effect on the growth of CNTs, likely due to the partial oxidation of the metallic Co catalyst lowering its catalytic activity. The CNT yield obtained by weighing the reactor before and after growth for 300 s (Y300s) is shown in Table 1, which is compared with the calculated carbon accumulation after 300 s (m300s), and the theoretically predicted maximum CNT yield (Ymax). It can be seen that the CNT yields determined by weighing and calculation show only small deviations. All of the three types of yield increase when feeding 200 ppm H2O vapor. However, a further increase of the H2O concentration led to a decrease of all the three yields. The CNT yields with more than 300 ppm of H2O are lower than those without H2O. To verify the assumption that H2O as a weak oxidant can remove amorphous carbon through gasification reaction (eq 1), the generation of CO was monitored during the CNT growth. As can be seen from Figure 5a, only a single CO peak was detected without H2O, which can be assigned to the carbothermal reduction of the mixed oxide catalyst containing

Table 1. CNT Yield by Weighing the Reactor before and after Growth for 300 s (Y300s), Carbon Accumulation by Calculation after Growth for 300 s (m300s) and Theoretical Predicted Maximum CNT Yield (Ymax) Derived from Curve Fittinga H2O (ppm)

Y300s (gCNTs gcat−1)

m300s (gCNTs gcat−1)

Ymax (gCNTs gcat−1)

0 200 250 300 500

27.2 31.8 27.9 26.6 23.6

27.8 33.7 28.5 26.9 24.0

60.1 138.6 77.2 66.1 54.3

a

The growth was performed with different H2O concentrations at 650 °C.

cobalt oxides by C2H4 (eq 4). In the presence of H2O, an additional broad contribution was observed following the first carbothermal reduction peak. It was found that the intensity of these two CO peaks increased with increasing H 2 O concentrations from 0 to 500 ppm. On the one hand, H2O can oxidize amorphous carbon species, especially carbon atoms at defect sites, forming CO.25 On the other hand, extra H2O as weak oxidant can convert metallic cobalt catalysts to oxides (eq 5), which can be further reduced by C2H4 releasing CO. These two steps correspond to the overall steam reforming of ethylene over cobalt (eq 6). The carbothermal reduction was dominant within the first 12 s, while the gasification of carbon was dominant during the further growth. 2CoOx + xC2H4 → 2Co + 2xCO + 2x H 2

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Co + x H 2O → CoOx + x H 2

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C2H4 + 2H 2O → 2CO + 4H 2

(6)

To investigate the influence of H2O vapor on the CNT morphology, the CNTs grown without and with 200 ppm of H2O were analyzed by SEM. Figure 6 shows the diameter distribution obtained by measuring 500 CNTs. It can be seen that 95% of the CNTs grown without H2O vapor were in the range of 11−21 nm. In contrast, the CNTs grown with 200 ppm H2O were in the range of 11−19 nm with a narrower diameter distribution. It is known that the diameter of CNTs is related to the particle size of the catalysts.8,24 Literature results showed that the presence of reactive gases like H2 or NH3 can improve the homogeneity of nanoparticles leading to a narrower size distribution.26 Hence, the narrower diameter 14084



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Figure 5. CO generation during the CNT growth at 650 °C with different concentrations of H2O vapor. (a) Up to 300 s and (b) first 21 s.

During further growth for up to 300 s, mt was strongly inhibited by H2O vapor at 550 °C, but dramatically promoted at 750 °C (Figure 7b). Surprisingly, mt at 650 °C in the presence of H2O was even higher than that at 750 °C. The results show that H2O has the highest promoting effect at 650 °C, whereas at 550 °C it acts as an inhibitor for the CNT growth. Figure 8 shows the trends of the initial growth rate (r0) and the mean lifetime of the catalytically active sites (τ) determined by fitting the results shown in Figure 7. It can be seen that both r0 and τ reached maxima at 650 °C for the growth without H2O. The r0 and τ were enhanced by feeding H2O at higher growth temperatures of 650 and 750 °C, whereas at 550 °C both r0 and τ decreased in the presence of H2O. The decrease of r0 at 750 °C without H2O can be related to the sintering of catalyst particles.9 Furthermore, the concentration of C2H4 decreased at 750 °C due to homogeneous decomposition,27 which was also observed in the blank experiment as indicated by the decrease of C2H4 concentration at 750 °C. The r0 was accelerated dramatically at 750 °C by feeding H2O, which is even slight larger than r0 at 650 °C. The higher r0 at 750 °C compared with 650 °C can be related to the less severe sintering of catalyst particles due to the presence of H2O. In contrast to the slight increase of r0, τ decreased significantly at 750 °C. The homogeneous decomposition of C2H4 generates additional amorphous carbon deposits, which cover the active sites of the catalyst leading to a faster deactivation.24 The CNT yield by weighing the reactor after growth for 300 s (Y300s), the carbon mass accumulation by calculation after growth for 300 s (m300s) and the theoretical predicted maximum CNT yields (Ymax) are summarized in Table 2. Small deviations of CNT yields were observed between Y300s and m300s. The results confirmed that 650 °C is the optimal temperature with the maximum yield. The comparison with the yields without H2O shows that the addition of H2O led to a decrease of Y300s by 33% at 550 °C, but to an increase by 39% at

Figure 6. Outer diameter distribution of CNTs grown at 650 °C with 0 and 200 ppm of H2O in the feed gas.

distribution suggests a similar effect for the presence of H2O in the feed gas. 3.2. Influence of the Growth Temperature. The optimized H2O concentration of 200 ppm was used for temperature variation experiments, and the growth without H2O was studied for comparison. While varying the growth temperature, the catalysts were always reduced at 650 °C. The corresponding CNT mt were derived from consumed C2H4. Figure 7a shows mt vs relative time in the first 21 s. It can be seen that at the initial stage, mt increases with increasing growth temperature from 550 to 750 °C, which is in good agreement with our earlier studies.23 Interestingly, the initial CNT growth was depressed by H2O at 550 and 650 °C but promoted at 750 °C. It is known that the gasification needs a higher temperature than 550 °C.25 Hence, H2O in the feed gas may dominantly react with the Co catalyst at 550 °C lowering the growth rate.

Figure 7. Time-resolved CNT mass accumulation (mt) as a function of relative time obtained at different temperatures. (a) First 21 s, (b) up to 300 s. 14085



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Figure 8. (a) The initial growth rate (r0) and (b) the mean lifetime of the catalytically active sites (τ) as a function of growth temperature derived from Figure 7.

C2H4 occurred at 750 °C leading to the deposition of amorphous carbon. These results agree well with the short mean lifetimes of the active sites at 550 and 750 °C. On the contrary, the long mean lifetime of active sites at 650 °C can be assigned to a minimum amount of carbon deposits. The diameter distribution of CNTs grown at different temperatures without and with H2O was obtained by SEM studies (Figure 10). It can be seen that the mean diameter increases with increasing growth temperature. Furthermore, the H2O vapor in the feed gas narrowed the CNT outer diameter, which is related to the metal particle size.28−31 On the one hand, the Co particle size increases with temperature. On the other hand, H2O vapor seems to favor the formation of smaller Co particles during the activation of the prereduced mixed oxide precursor under growth conditions in the presence of C2H4 and H2.24

Table 2. CNT Yield by Weighing the Reactor after Growth for 300 s (Y300s), the Carbon Mass Accumulation by Calculation after Growth for 300 s (m300s) and Theoretical Predicted Maximum CNT Yield (Ymax) Derived from Curve Fittinga temp. (°C)

H2O (ppm)

Y300s (gCNTs gcat−1)

m300s (gCNTs gcat−1)

Ymax (gCNTs gcat−1)

550 550 650 650 750 750

0 200 0 200 0 200

19.1 12.8 27.8 28.9 17.6 24.5

19.8 13.6 28.0 30.3 17.9 25.4

36.0 18.9 91.4 119.7 25.0 49.0

a

The growth was performed at different temperatures without and with H2O.

750 °C. The theoretical predicted maximum CNT yields show a similar trend. Similarly, the time-resolved CO generation at different growth temperatures in the presence of 200 ppm H2O was determined (Figure 9). It can be seen that the intensity of the first CO peak increases with increasing growth temperature from 550 to 750 °C. As mentioned above, the first CO peak can be attributed to the carbothermal reduction of the oxides in the catalysts. A higher temperature leads to a higher activity of carbothermal reduction of oxides (Figure 9a). The second CO peak mainly related to the gasification of amorphous carbon shows the lowest intensity at 650 °C and the highest intensity at 750 °C. A too low growth temperature, for instance 550 °C, may result in the deposition of amorphous carbon or polyaromatics on CNTs, which can be oxidized by H2O. As discussed above, thermal or homogeneous decomposition of

4. CONCLUSIONS The influence of H2O on CNT growth kinetics during the first 300 s was investigated by monitoring the C2H4 conversion and CO generation at different H2O concentrations and growth temperatures. CO was generated either by the carbothermal reduction of the catalysts, or by the gasification of deposited carbon. The obtained carbon mass accumulation was fitted with a kinetic model yielding the initial growth rate and the mean lifetime of the catalytically active sites. At 650 °C, H2O vapor was shown to be a promoter for the CNT growth at low concentrations (≤250 ppm), but an inhibitor at high concentrations (≥300 ppm). At 550 °C, 200 ppm H2O were found to be an inhibitor decreasing both the initial growth rate and the mean lifetime, whereas at 650 °C, both the initial

Figure 9. CO profiles obtained under different growth temperatures with 200 ppm of H2O vapor. (a) First 21 s and (b) up to 300 s. 14086



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Figure 10. Outer diameter distributions (a−c) and mean diameter (d) of CNTs grown at different temperatures with 0 and 200 ppm of H2O vapor. (4) Bitter, J. H. Nanostructured Carbons in Catalysis a Janus Material-Industrial Applicability and Fundamental Insights. J. Mater. Chem. 2010, 20, 7312. (5) Schulte, H. J.; Graf, B.; Xia, W.; Muhler, M. Nitrogen- and Oxygen-Functionalized Multiwalled Carbon Nanotubes Used as Support in Iron-Catalyzed, High-Temperature Fischer−Tropsch Synthesis. ChemCatChem 2012, 4, 350. (6) Atthipalli, G.; Wang, H.; Gray, J. L. Catalyst-Assisted Vertical Growth of Carbon Nanotubes on Inconel Coated Commerical Copper Foil Substrates versus Sputtered Copper Films. Appl. Surf. Sci. 2013, 273, 515. (7) Dai, H. J. Carbon Nanotubes: Synthesis, Integration, and Properties. Acc. Chem. Res. 2002, 35, 1035. (8) Becker, M. J.; Xia, W.; Tessonnier, J. P.; Blume, R.; Yao, L. D.; Schlögl, R.; Muhler, M. Optimizing the Synthesis of Cobalt-Based Catalysts for the Selective Growth of Multiwalled Carbon Nanotubes under Industrially Relevant Conditions. Carbon 2011, 49, 5253. (9) Tessonnier, J.-P.; Su, D. S. Recent Progress on the Growth Mechanism of Carbon Nanotubes: A Review. ChemSusChem 2011, 4, 824. (10) Yamada, T.; Maigne, A.; Yudasaka, M.; Mizuno, K.; Futaba, D. N.; Yumura, M.; Iijima, S.; Hata, K. Revealing the Secret of WaterAssisted Carbon Nanotube Synthesis by Microscopic Observation of the Interaction of Water on the Catalysts. Nano Lett. 2008, 8, 4288. (11) Hata, K.; Futaba, D. N.; Mizuno, K.; Namai, T.; Yumura, M.; Iijima, S. Water-Assisted Highly Efficient Synthesis of Impurity-Free Single-Walled Carbon Nanotubes. Science 2004, 306, 1362. (12) Futaba, D. N.; Hata, K.; Yamada, T.; Mizuno, K.; Yumura, M.; Iijima, S. Kinetics of Water-Assisted Single-Walled Carbon Nanotube Synthesis Revealed by a Time-Evolution Analysis. Phys. Rev. Lett. 2005, 95, 056104. (13) Magrez, A.; Seo, J. W.; Kuznetsov, V. L.; Forro, L. Evidence of an Equimolar C2H2−CO2 Reaction in the Synthesis of Carbon Nanotubes. Angew. Chem., Int. Ed. 2007, 46, 441. (14) Wen, Q.; Qian, W. Z.; Wei, F.; Liu, Y.; Ning, G. Q.; Zhang, Q. CO2-Assisted SWNT Growth on Porous Catalysts. Chem. Mater. 2007, 19, 1226. (15) Zhang, Y. Y.; Gregoire, J. M.; van Dover, R. B.; Hart, A. J. Ethanol-Promoted High-Yield Growth of Few-Walled Carbon Nanotubes. J. Phys. Chem. C 2010, 114, 6389. (16) Guellati, O.; Janowska, I.; Begin, D.; Guerioune, M.; Mekhalif, Z.; Delhalle, J.; Moldovan, S.; Ersen, O.; Pham-Huu, C. Influence of

growth rate and the mean lifetime were significantly enhanced by feeding H2O. The results demonstrated the dual role of H2O during the CNT growth: the removal of deposited carbon by gasification and the partial oxidation of Co catalyst. Depending on the temperature and H2O concentration, the CNT growth can be either promoted or inhibited. The optimal conditions for CNT growth in the scope of this study were determined to be 650 °C and 200 ppm H2O. SEM studies found that the diameters of CNTs grown in the presence of H2O are smaller, likely indicating the presence of smaller Co particles formed during the carbothermal activation of the catalyst under growth conditions.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49 234 32 14115. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS K.X. thanks the IMPRS-SurMat for a research grant. The authors thank the German Federal Ministry of Education and Research for its financial support through the Carboscale project (BMBF Grant 03X0040G). Fruitful discussions with Robert Schlögl are gratefully acknowledged.

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REFERENCES

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Influence of Water on the Initial Growth Rate of Carbon Nanotubes

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