- Email: [email protected]
MWCNT coatings obtained by thermal CVD using ethanol decomposition
Carbon 44 (2006) 718–723 www.elsevier.com/locate/carbon
MWCNT coatings obtained by thermal CVD using ethanol decomposition Marion Wienecke *, Mihaela-C. Bunescu, Klaus Deistung, Petra Fedtke, Erika Borchartd Institute for Surface and Thin Film Technology, Hochschule Wismar PF 1210, D-23952 Wismar, Germany Received 13 January 2005; accepted 21 September 2005 Available online 18 November 2005
Abstract A low-cost and reliable method to produce MWCNT coatings on large surfaces, with possibility to scale up to larger output, aiming at gas sensor applications is reported. The process was based on ethanol decomposition in the temperature range 700–900 C. Different qualities of carbon were produced depending on the experimental parameters. For the samples deposited on sapphire and using Ni as catalyst a high diversity of carbon products (amorphous carbon, graphite plates, MWCNTs, onion-like graphene, etc.) were registered, while for the samples deposited on quartz MWCNTs with better crystallinity were observed. Carbon nanofibres (about 50– 60 nm diameter) were observed only for the samples with Fe catalyst. Depending on temperature, nanotubes with different thickness of amorphous carbon coating occurred. According to our findings, the deposition of amorphous carbon phase can be minimized depending on the oxygen content in the process. 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Catalytic chemical vapour deposition; Electron microscopy; Catalytic properties
1. Introduction Since the discovery of single-walled carbon nanotubes (SWNTs) by Iijima in 1991 [1] this new nanostructured material found potential applications in various fields of materials research [2], based on their remarkable physical properties [3]. Among these applications are nanoelectronic devices, tips in atomic force microscopy, catalyst support in heterogeneous catalysis [4], and in electrodes for fuel cells [5]. However, the discovery of carbon nanotubes (CNTs) was first based on the carbon-arc method and the low yield and a high-graphitic content made the production costs very high. The decomposition of carbon-bearing precursors in the presence of catalysts to produce CNTs seems to be more suitable for large-scale synthesis [6]. Thus, nowadays various chemical vapour deposition (CVD) techniques
*
Corresponding author. Tel.: +49 3841 753 318; fax: +49 3841 753 136. E-mail address: [email protected] (M. Wienecke).
0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.09.020
became dominant in the mass synthesis of carbon nanotubes, such as thermal CVD [7,8], plasma enhanced CVD (PECVD) [9,10], alcohol catalytic CVD (ACCVD) [11], laser assisted CVD (LCVD) [12] or the high pressure CO-disproportionation (HiPCO) [13]. Although up to 450 mg/h SWNTs can be produced with the HiPCO method in a continuous flow [13], or a large output of zeolite supported SWNTs was reported with ACCVD [11], the most common and promising methods for large-scale carbon nanotube production are the thermal and plasma enhanced CVD [14], which results in multiwalled carbon nanotubes (MWCNTs) supported on substrates that were previously coated with catalytic nanoparticles or the catalyst was co-deposited from a precursor. Depending on the diameter of the catalytic particles MWCNTs with diameters up to 100 nm, in a narrow size distribution, with highly accessible surface area, low resistivity, and high stability are obtained. Using NH3 to control the catalytic reaction and the hot filament variant of the CVD method [10,15] or by application of an electric field [16] the MWCNTs can be highly vertically aligned.
M. Wienecke et al. / Carbon 44 (2006) 718–723
2. Experimental Quartz and sapphire plates (15 · 15 mm2) were used as substrates for thin films of either Fe or Ni that were deposited by DC plasma sputtering as catalyst for promoting the nanotubes growth. The catalyst film thickness (controlled by quartz oscillator-type monitor) was 5 nm. The substrates were cleaned and dried in the usual way for thin film deposition by rinsing in trichloroethylene, acetone and methanol and were plasma etched before sputtering. A quartz reactor tube (B = 35 mm) was placed in a three zone furnace providing a temperature plateau of about 40 cm length at temperatures between 700 and 900 C. The annealing of the catalyst films was made in situ in the reactor tube, followed by the growth of the MWCNTs by ethanol decomposition. The reactor tube was heated with about 20 C/min in air or with N2 flow until the desired temperature was obtained. The deposition was done by bubbling N2 through an ethanol reservoir at room temperature. The flow rate of the N2 could be varied between 10 and 200 sccm/min. All treatments in the reactor tube were made at atmospheric pressure. To investigate the
Furnace
Temperature controllers
Exhaust
Ethanol
Quartz tube
N2
Depending on the availability of a cheap and reliable method to coat large surfaces with MWCNTs we foresee a wide range of application for this kind of material in the field of electrochemical devices, fuel cells and electrochemical sensors in the near future. Several sensor applications have been previously reported in the literature. In [17] a sensor device that detects ppb concentrations of NO2 is described. The sensor is based on conductivity changes of MWCNT coatings on exposure to gases due to chemisorption. Another sensor type is described in [18] and consists in an array of highly ordered vertically aligned CNTs. It utilizes the ionization behaviour of different gases on carbon nanotube tips in an electric field and shows a high sensitivity and selectivity due to distinct breakdown behaviour for the different gases. In another kind of electrochemical application, based on the high specific surface area of the CNTs [19], porous tablets of carbon nanotubes were used as polarizable electrodes in an electric double-layer capacitor. In [20] MWCNTs were used as catalyst support material for direct methanol fuel cells (DMFC). In our own work [21], in order to increase the specific surface area of the electrodes for electrochemical oxygen gas sensors, we modified the PTFE membranes by coating with MWCNT suspensions. Reduction in the production costs for large area MWCNT coatings is a critical step in advancing electrochemical devices beyond basic research toward realizing new products. The aim of this work is to develop a low-cost and reliable method to coat substrates with highly graphitized MWCNTs and to find parameters that can be scaled up to larger outputs. Therefore, we have investigated the growth of MWCNTs by decomposition of ethanol in a thermal CVD process, similar to the ACCVD-method described in [11].
719
Fig. 1. Schematic of thermal CVD for ethanol decomposition.
influence of the different parameters, the time for ethanol decomposition and thus the deposition time, was set at 20 min for every experiment. A schematic drawing of the apparatus is shown in Fig. 1. The microstructure change of the catalyst films due to the annealing process was analyzed by transmission electron microscopy (TEM-Phillips CM 200) and electron diffraction. Fe or Ni thin films were deposited under the same conditions directly on TEM grids and were investigated first as deposited, and subsequently annealed either in N2 (sealed quartz glass tube) or in air up to 600 C. The morphologies of the resulted carbon coatings and nanotubes were analyzed by scanning electron microscopy (SEM-Tesla BS 340) and TEM. For TEM investigations the substrates with the carbon deposit were rinsed in acetone and a suspension of the deposit was dropped on a TEM grid. 3. Results and discussion Different morphologies of the carbon coatings were obtained. Fibril-like morphology as shown in Fig. 2 predominated only for Fe catalyst and was produced with 125 sccm/min N2 flow and 850 C decomposition temperature. Using Ni catalyst mixtures of different graphene structures, mostly onion-like as shown in Fig. 3 were obtained. As shown in Fig. 2, the fibrils are not aligned, exhibit occasional spiral shape, are relative long (about 10 lm
Fig. 2. SEM plan view image of coated surface.
720
M. Wienecke et al. / Carbon 44 (2006) 718–723
Fig. 3. Graphene with onion-like morphology obtained with Ni catalyst on sapphire.
length), and have narrow diameter distribution (50–60 nm as measured by high magnification TEM). By weighing the substrates before and after deposition we calculated a deposition rate of about 0.15 mg/cm2 h, which is comparable to the results in [22]. However, the deposition rate depends on the carbon yield in the ethanol feed, which in our experiments was fixed by the ethanol vapour pressure at room temperature. The morphology and crystallinity of the fibrils were investigated by TEM. Fig. 4 shows that the fibrils have a cavity, i.e. they are tubes and some of them have a bamboo like morphology. The inner graphitic layers may have amorphous carbon coatings depending on the process parameters (Fig. 5). Carbon nanotubes free of amorphous
Fig. 4. TEM image of MWCNTs grown on quartz without N2 flow before ethanol decomposition; the bamboo like structure is clearly seen.
Fig. 5. Amorphous coating on MWCNTs grown under N2 atmosphere on quartz glass.
coatings were observed only in experiments where the substrates were heated in air, without N2 flushing before deposition. The electron diffraction pattern produced by the small particles adjacent to the investigated MWCNTs revealed the Fe-oxide (Fe2O3) structure.
Fig. 6. Amorphous fibres grown in N2 atmosphere on sapphire.
M. Wienecke et al. / Carbon 44 (2006) 718–723
The experiments made with N2 flushing before deposition produced fibrils with varying thicknesses of amorphous coatings. In an extreme case the fibres were completely amorphous, although they still had a cavity and their diameter was equal to that of the MWCNTs obtained in other experiments (see Fig. 6). We have not observed other particular graphitic co-products in our experiments. Amorphous carbon coatings (like in Fig. 5) have to be prevented with respect to the above mentioned applications.
721
Amorphous coatings on nanotubes have previously been reported by several authors [23–26] depending on process parameters of the catalytic growth or post treatments, and in this sense the crystallinity or purity of CNT is discussed and can be optimized. Hernadi et al. [23] found that amorphous deposits depend on reaction conditions (time, temperature) and there is a narrow window for optimum conditions depending on the initial reactant used in the decomposition. Any deviation (higher or lower
Fig. 7. TEM bright field images and electron diffraction patterns (inserts) of Fe thin films: (a) as deposited, (b) annealed in N2 and (c) annealed in air at 600 C.
722
M. Wienecke et al. / Carbon 44 (2006) 718–723
temperature, longer times) results in amorphous deposits. Huang et al. [24] investigated the influence of different catalyst metals on the nanotube growth and found amorphous coatings only for Co catalyst. Similarly, they discussed the growth rates for the different catalysts and found the lowest nanotube growth rates for Co. Ci et al. [25] reported amorphous coating on as-grown nanotubes, due to secondary pyrolytic reactions. Using high resolution TEM study of the as grown and heat treated nanotubes they showed that amorphous coatings were transformed to graphitized layers by thermal annealing at 2500 C. According to our findings, the appearance and thickness of the amorphous deposits on the nanotubes depend on the balance between the catalytic growth and the secondary pyrolysis rates. However, in our experiments, this is particularly influenced by the initial O2-content in the reactor tube. Significantly, our experiments were therefore under oxidizing conditions, in contrast to most other authors using gas mixtures of C2H2 and ammonia or CH4/H2 [7– 10]. In our experiments, residual oxygen in the reactor tube could not be avoided and in some cases, the process was intentionally started in air. The influence of the OH-radicals is discussed in [26]. These radicals efficiently remove amorphous carbon (as is known from purification processes with H2O2) due to the reaction with nearby carbon atoms with dangling bonds to form CO. The authors reported high-purity singlewalled nanotubes produced by low temperature ethanol decomposition. However, in ethanol decomposition at 700–900 C no OH-radicals are produced. Chen et al. [27] used the decomposition of CH4–CO2 gas mixture in microwave plasma enhanced CVD (MPCVD) to obtain high-quality MWCNTs. Specifically, they investigated the reaction species in the plasma by optical emission spectroscopy and revealed that the oxygen presence in plasma enhanced the catalytic deposition of carbon. The effects of oxygen and nitrogen on carbon nanotube growth were also investigated by Yang et al. [28] using MPCVD. They found that the quality of the CNTs (diameter of 20–100 nm) was improved and the number density was greatly increased with addition of N2 and O2 and a nitride enhanced growth mechanism was proposed. However, we have reproducibly obtained MWCNTs without any amorphous coating, with bamboo like morphology and diameter of about 60 nm by ethanol decomposition at 850 C under atmospheric pressure and beginning the heating process intentionally in air, i.e. in the presence of O2, N2 and CO2. It is obviously that the absence of amorphous deposits is not determined by the ethanol pyrolysis chemistry, but by the oxidizing species in the air. We have investigated the behaviour of the Fe-catalyst thin film under these conditions by TEM analysis. We are aware that the behaviour of the catalyst thin films that are deposited on the TEM grids, particularly their wetting, growth mechanism and grain structure, differs
from that of the ones deposited on the oxide materials. The aim was not to analyze the grain structure, but the chemical reactions taking place under the experimental conditions. Fig. 7 shows the morphology of the as deposited Fe thin film (Fig. 7a) and after annealing at 600 C in N2 (Fig. 7b) and in air (Fig. 7c). The as-grown sputtered Fe thin film shows Fe crystallites (identified by electron diffraction) with diameters less then 5 nm. After heating at 600 C these crystallites coalesce. In N2 atmosphere a mixture of Fe and different Fe oxides was identified (residual oxygen content could not be prevented); the diameter of the particles increased to 25 nm. After annealing in air equiaxed Fe2O3 grains (identified by electron diffraction) with diameter of about 100–200 nm were produced. No iron carbide was found. Fe2O3 crystals were also identified between the grown MWCNTs, but we do not know how significant this is in describing the catalytic reaction mechanism. However, Prilutskiy et al. [29] investigated the growth of carbon nanomaterial by low temperature carbon monoxide disproportionation starting from Fe2O3 and NiO3, respectively and found similar results with ours: MWCNTs from Fe2O3, while mixtures of various graphene structures result from NiO3. 4. Conclusions We have demonstrated a process for coating quartz substrates with MWCNTs by ethanol decomposition at 850 C, atmospheric pressure, and under oxidizing atmosphere (i.e. with O2, N2 and CO2 present), using a predeposited Fe-catalyst thin film of 5 nm thickness. With this simple method we reproducibly obtained MWCNTs that are free of amorphous coating, exhibit bamboo type morphology, have an average diameter of 60 nm and a relatively narrow diameter distribution. The process parameters can be easily scaled up to larger outputs. Such a quality of carbon nanotubes is promising for electrochemical applications, such as fuel cells electrodes and electrochemical sensors. An amorphous coating would decrease the electrical conductivity of the CNT-based electrodes and composite materials by reducing the percolation. Moreover, electrochemical applications require activated surface sites, like carboxylic groups, and structural modification (opening, steps, and edges) [30], which could not be obtained on nanotubes with amorphous deposits. Acknowledgements The authors thanks the government of MecklenburgVorpommern in Germany for supporting this research within the TEM-FH Program under contract number 0201230 and the IT Gambert GmbH in Wismar for providing equipment.
M. Wienecke et al. / Carbon 44 (2006) 718–723
References [1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991; 354(6348): 56–8. [2] Ebbeson TW. Carbon nanotubes—preparation and properties. Boca Raton: CSC Press; 1997. [3] Saito R, Dresselhaus G, Dresselhaus MS. Physical properties of carbon nanotubes. London: Imperial College Press; 1998. [4] Rodriguez NM, Kim MS, Baker RTK. Carbon nanofibers: A unique catalyst support medium. J Phys Chem 1994;98(50):13108–11. [5] Che G, Lakshmi BB, Martin CR, Fisher ER. Metal-nanocluster-filled carbon nanotubes. Catalytic properties and possible applications in electrochemical energy storage and production. Langmuir 1999;15(3): 750–8. [6] Huczko A. Synthesis of aligned carbon nanotubes. Appl Phys A 2002; 74(5):617–38. [7] Park JB, Choi GS, Cho YS, Hong SY, Kim D, et al. Characterization of Fe-catalyzed carbon nanotubes grown by thermal chemical vapour deposition. J Cryst Growth 2002;244(2):211–7. [8] Lee CJ, Lyu SC, Kim HW, Park CY, Yang CW. Synthesis of bamboo-shaped carbon–nitrogen nanotubes using C2H2–NH3– Fe(CO)5 system. Chem Phys Lett 2002;359(1–2):109–14. [9] Chen M, Chen CM, Chen CF. Preparation of high yield multi-walled carbon nanotubes by microwave plasma chemical vapour deposition at low temperature. J Mater Sci 2002;37(17):3561–7. [10] Ren ZF, Huang ZP, Xu JW, Wang JH, Bush P, Siegal MP, et al. Synthesis of large arrays of well-aligned carbon nanotubes on glass. Science 1998;282(5391):1105–7. [11] Murakami Y, Chiashi S, Miyauchi Y, Maruyama S. Direct synthesis of single-walled carbon nanotubes on silicon and quartz-based systems. Jpn J Appl Phys 2004;43(3):1221–6. [12] Alexandrescu R, Crunteanu A, Morjan RE, Morjan I, Rohmund F, Falk LKL, et al. Synthesis of carbon nanotubes by CO2-laserassisted chemical vapour deposition. Infrared Phys Technol 2003; 44(1):43–50. [13] Bronikowski MJ, Willis PA, Colbert DT, Smith KA, Smalley RE. Gas-phase production of carbon single-walled nanotubes from carbon monoxide via the HiPCO process. A parametric study. J Vac Sci Technol A—Vac Surf Films, Pt. 2 2001;19(4):1800–5. [14] Endo M, Takeuchi K, Igarashi S, Kobori K, Shiraishi M, Kroto HW. The production and structure of pyrolytic carbon nanotubes (PCNTs). J Phys Chem Solids 1993;54(12):1841–8. [15] Huang ZP, Xu JW, Ren ZF, Wang JH. Growth of highly oriented carbon nanotubes by plasma-enhanced hot filament chemical vapour deposition. Appl Phys Lett 1998;73(26):3845–7. [16] Avigal Y, Kalish R. Growth of aligned carbon nanotubes by biasing during growth. Appl Phys Lett 2001;78(16):2291–3.
723
[17] Valentini L, Armentano I, Kenny JM, Canalini C, Lozzi L, Cantucci S. Sensors for sub-ppm NO2 gas detection based on carbon nanotube thin films. Appl Phys Lett 2003;82(6):961–3. [18] Modi A, Koratkar N, Lass E, Wei B, Ayajan PM. Miniaturized gas ionization sensors using carbon nanotubes. Nature 2003;424(6945): 171–4. [19] Zhang B, Liang J, Xu CL, Wei BQ, Ruan DB, Wu DH. Electric double-layer capacitors using carbon nanotube electrodes and organic electrolyte. Mater Lett 2001;51(6):539–42. [20] Li W, Liang C, Qiu J, Zhou W, Hongmei Han H, Wei Z, et al. Carbon nanotubes as support for cathode catalyst of a direct methanol fuel cell. Carbon 2002;40(5):791–4. [21] Wienecke M, Bunescu MC, Pietrzak M, Deistung K, Fedtke P. PTFE membrane electrodes with increased sensitivity for gas sensor applications. Synth Met 2003;138(1–2):165–71. [22] Andrews R, Jacques D, Rao AM, Derbyshire F, Qian D, Fan X, et al. Continuous production of aligned carbon nanotubes: a step closer to commercial realization. Chem Phys Lett 1999;303(5–6): 467–74. [23] Hernadi K, Fonseca A, Nagy JB, Bernaerts D. Catalytic synthesis of carbon nanotubes. In: Yoshimura S, Chang RPH, editors. Supercarbon. Springer series in materials science, vol. 33. New York: SpringerVerlag; 1998. p. 81–97. [24] Huang ZP, Wang DZ, Wen JG, Sennett M, Gibson H, Ren ZF. Effect of nickel, iron and cobalt on growth of aligned carbon nanotubes. Appl Phys A 2002;74(3):387–91. [25] Ci L, Zhu H, Wei B, Xu C, Liang J, Wu D. Graphitization behaviour of carbon nanofibers prepared by the floating catalyst method. Mater Lett 2000;43(5–6):291–4. [26] Maruyama S, Kojima R, Miyaichi Y, Chiashi S, Kohno M. Lowtemperature synthesis of high-purity single-walled carbon nanotubes from alcohol. Chem Phys Lett 2002;360(3–4):229–34. [27] Chen Mi, Chen Chieng-Ming, Chen Chia-Fu. Growth of carbon nanotubes by microwave plasma chemical vapour deposition using CH4 and CO2 gas mixture. Thin Solid Films 2002;420–421:230–4. [28] Yang DJ, Zhang Q, Yoon SF, Ahn J, Wang SG, Zhou Q, et al. Effects of oxygen and nitrogen on carbon nanotube growth using a microwave plasma chemical vapour deposition technique. Surf Coat Technol 2003;167(2–3):288–91. [29] Prilutskiy O, Katz EA, Shames AI, Mogilevsky D, Mogilko E, Prilutskiy E, Dub Pri SN. Synthesis of carbon nanomaterials by a catalytic disproportionation of carbon monoxide. Fullerenes, Nanotubes and Carbon Nanostructures 2005;13(1):53–66. [30] Rohland B, Mihaela-C. Bunescu, Marlies Pietrzak, Marion Wienecke, Barfels T, Mo¨ller S. CNT-based cathode material for DMFC. Fullerenes, Nanotubes and Carbon Nanostructures 2005;13(1): 511–22.
MWCNT coatings obtained by thermal CVD using ethanol decomposition Marion Wienecke *, Mihaela-C. Bunescu, Klaus Deistung, Petra Fedtke, Erika Borchartd Institute for Surface and Thin Film Technology, Hochschule Wismar PF 1210, D-23952 Wismar, Germany Received 13 January 2005; accepted 21 September 2005 Available online 18 November 2005
Abstract A low-cost and reliable method to produce MWCNT coatings on large surfaces, with possibility to scale up to larger output, aiming at gas sensor applications is reported. The process was based on ethanol decomposition in the temperature range 700–900 C. Different qualities of carbon were produced depending on the experimental parameters. For the samples deposited on sapphire and using Ni as catalyst a high diversity of carbon products (amorphous carbon, graphite plates, MWCNTs, onion-like graphene, etc.) were registered, while for the samples deposited on quartz MWCNTs with better crystallinity were observed. Carbon nanofibres (about 50– 60 nm diameter) were observed only for the samples with Fe catalyst. Depending on temperature, nanotubes with different thickness of amorphous carbon coating occurred. According to our findings, the deposition of amorphous carbon phase can be minimized depending on the oxygen content in the process. 2005 Elsevier Ltd. All rights reserved. Keywords: Carbon nanotubes; Catalytic chemical vapour deposition; Electron microscopy; Catalytic properties
1. Introduction Since the discovery of single-walled carbon nanotubes (SWNTs) by Iijima in 1991 [1] this new nanostructured material found potential applications in various fields of materials research [2], based on their remarkable physical properties [3]. Among these applications are nanoelectronic devices, tips in atomic force microscopy, catalyst support in heterogeneous catalysis [4], and in electrodes for fuel cells [5]. However, the discovery of carbon nanotubes (CNTs) was first based on the carbon-arc method and the low yield and a high-graphitic content made the production costs very high. The decomposition of carbon-bearing precursors in the presence of catalysts to produce CNTs seems to be more suitable for large-scale synthesis [6]. Thus, nowadays various chemical vapour deposition (CVD) techniques
*
Corresponding author. Tel.: +49 3841 753 318; fax: +49 3841 753 136. E-mail address: [email protected] (M. Wienecke).
0008-6223/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2005.09.020
became dominant in the mass synthesis of carbon nanotubes, such as thermal CVD [7,8], plasma enhanced CVD (PECVD) [9,10], alcohol catalytic CVD (ACCVD) [11], laser assisted CVD (LCVD) [12] or the high pressure CO-disproportionation (HiPCO) [13]. Although up to 450 mg/h SWNTs can be produced with the HiPCO method in a continuous flow [13], or a large output of zeolite supported SWNTs was reported with ACCVD [11], the most common and promising methods for large-scale carbon nanotube production are the thermal and plasma enhanced CVD [14], which results in multiwalled carbon nanotubes (MWCNTs) supported on substrates that were previously coated with catalytic nanoparticles or the catalyst was co-deposited from a precursor. Depending on the diameter of the catalytic particles MWCNTs with diameters up to 100 nm, in a narrow size distribution, with highly accessible surface area, low resistivity, and high stability are obtained. Using NH3 to control the catalytic reaction and the hot filament variant of the CVD method [10,15] or by application of an electric field [16] the MWCNTs can be highly vertically aligned.
M. Wienecke et al. / Carbon 44 (2006) 718–723
2. Experimental Quartz and sapphire plates (15 · 15 mm2) were used as substrates for thin films of either Fe or Ni that were deposited by DC plasma sputtering as catalyst for promoting the nanotubes growth. The catalyst film thickness (controlled by quartz oscillator-type monitor) was 5 nm. The substrates were cleaned and dried in the usual way for thin film deposition by rinsing in trichloroethylene, acetone and methanol and were plasma etched before sputtering. A quartz reactor tube (B = 35 mm) was placed in a three zone furnace providing a temperature plateau of about 40 cm length at temperatures between 700 and 900 C. The annealing of the catalyst films was made in situ in the reactor tube, followed by the growth of the MWCNTs by ethanol decomposition. The reactor tube was heated with about 20 C/min in air or with N2 flow until the desired temperature was obtained. The deposition was done by bubbling N2 through an ethanol reservoir at room temperature. The flow rate of the N2 could be varied between 10 and 200 sccm/min. All treatments in the reactor tube were made at atmospheric pressure. To investigate the
Furnace
Temperature controllers
Exhaust
Ethanol
Quartz tube
N2
Depending on the availability of a cheap and reliable method to coat large surfaces with MWCNTs we foresee a wide range of application for this kind of material in the field of electrochemical devices, fuel cells and electrochemical sensors in the near future. Several sensor applications have been previously reported in the literature. In [17] a sensor device that detects ppb concentrations of NO2 is described. The sensor is based on conductivity changes of MWCNT coatings on exposure to gases due to chemisorption. Another sensor type is described in [18] and consists in an array of highly ordered vertically aligned CNTs. It utilizes the ionization behaviour of different gases on carbon nanotube tips in an electric field and shows a high sensitivity and selectivity due to distinct breakdown behaviour for the different gases. In another kind of electrochemical application, based on the high specific surface area of the CNTs [19], porous tablets of carbon nanotubes were used as polarizable electrodes in an electric double-layer capacitor. In [20] MWCNTs were used as catalyst support material for direct methanol fuel cells (DMFC). In our own work [21], in order to increase the specific surface area of the electrodes for electrochemical oxygen gas sensors, we modified the PTFE membranes by coating with MWCNT suspensions. Reduction in the production costs for large area MWCNT coatings is a critical step in advancing electrochemical devices beyond basic research toward realizing new products. The aim of this work is to develop a low-cost and reliable method to coat substrates with highly graphitized MWCNTs and to find parameters that can be scaled up to larger outputs. Therefore, we have investigated the growth of MWCNTs by decomposition of ethanol in a thermal CVD process, similar to the ACCVD-method described in [11].
719
Fig. 1. Schematic of thermal CVD for ethanol decomposition.
influence of the different parameters, the time for ethanol decomposition and thus the deposition time, was set at 20 min for every experiment. A schematic drawing of the apparatus is shown in Fig. 1. The microstructure change of the catalyst films due to the annealing process was analyzed by transmission electron microscopy (TEM-Phillips CM 200) and electron diffraction. Fe or Ni thin films were deposited under the same conditions directly on TEM grids and were investigated first as deposited, and subsequently annealed either in N2 (sealed quartz glass tube) or in air up to 600 C. The morphologies of the resulted carbon coatings and nanotubes were analyzed by scanning electron microscopy (SEM-Tesla BS 340) and TEM. For TEM investigations the substrates with the carbon deposit were rinsed in acetone and a suspension of the deposit was dropped on a TEM grid. 3. Results and discussion Different morphologies of the carbon coatings were obtained. Fibril-like morphology as shown in Fig. 2 predominated only for Fe catalyst and was produced with 125 sccm/min N2 flow and 850 C decomposition temperature. Using Ni catalyst mixtures of different graphene structures, mostly onion-like as shown in Fig. 3 were obtained. As shown in Fig. 2, the fibrils are not aligned, exhibit occasional spiral shape, are relative long (about 10 lm
Fig. 2. SEM plan view image of coated surface.
720
M. Wienecke et al. / Carbon 44 (2006) 718–723
Fig. 3. Graphene with onion-like morphology obtained with Ni catalyst on sapphire.
length), and have narrow diameter distribution (50–60 nm as measured by high magnification TEM). By weighing the substrates before and after deposition we calculated a deposition rate of about 0.15 mg/cm2 h, which is comparable to the results in [22]. However, the deposition rate depends on the carbon yield in the ethanol feed, which in our experiments was fixed by the ethanol vapour pressure at room temperature. The morphology and crystallinity of the fibrils were investigated by TEM. Fig. 4 shows that the fibrils have a cavity, i.e. they are tubes and some of them have a bamboo like morphology. The inner graphitic layers may have amorphous carbon coatings depending on the process parameters (Fig. 5). Carbon nanotubes free of amorphous
Fig. 4. TEM image of MWCNTs grown on quartz without N2 flow before ethanol decomposition; the bamboo like structure is clearly seen.
Fig. 5. Amorphous coating on MWCNTs grown under N2 atmosphere on quartz glass.
coatings were observed only in experiments where the substrates were heated in air, without N2 flushing before deposition. The electron diffraction pattern produced by the small particles adjacent to the investigated MWCNTs revealed the Fe-oxide (Fe2O3) structure.
Fig. 6. Amorphous fibres grown in N2 atmosphere on sapphire.
M. Wienecke et al. / Carbon 44 (2006) 718–723
The experiments made with N2 flushing before deposition produced fibrils with varying thicknesses of amorphous coatings. In an extreme case the fibres were completely amorphous, although they still had a cavity and their diameter was equal to that of the MWCNTs obtained in other experiments (see Fig. 6). We have not observed other particular graphitic co-products in our experiments. Amorphous carbon coatings (like in Fig. 5) have to be prevented with respect to the above mentioned applications.
721
Amorphous coatings on nanotubes have previously been reported by several authors [23–26] depending on process parameters of the catalytic growth or post treatments, and in this sense the crystallinity or purity of CNT is discussed and can be optimized. Hernadi et al. [23] found that amorphous deposits depend on reaction conditions (time, temperature) and there is a narrow window for optimum conditions depending on the initial reactant used in the decomposition. Any deviation (higher or lower
Fig. 7. TEM bright field images and electron diffraction patterns (inserts) of Fe thin films: (a) as deposited, (b) annealed in N2 and (c) annealed in air at 600 C.
722
M. Wienecke et al. / Carbon 44 (2006) 718–723
temperature, longer times) results in amorphous deposits. Huang et al. [24] investigated the influence of different catalyst metals on the nanotube growth and found amorphous coatings only for Co catalyst. Similarly, they discussed the growth rates for the different catalysts and found the lowest nanotube growth rates for Co. Ci et al. [25] reported amorphous coating on as-grown nanotubes, due to secondary pyrolytic reactions. Using high resolution TEM study of the as grown and heat treated nanotubes they showed that amorphous coatings were transformed to graphitized layers by thermal annealing at 2500 C. According to our findings, the appearance and thickness of the amorphous deposits on the nanotubes depend on the balance between the catalytic growth and the secondary pyrolysis rates. However, in our experiments, this is particularly influenced by the initial O2-content in the reactor tube. Significantly, our experiments were therefore under oxidizing conditions, in contrast to most other authors using gas mixtures of C2H2 and ammonia or CH4/H2 [7– 10]. In our experiments, residual oxygen in the reactor tube could not be avoided and in some cases, the process was intentionally started in air. The influence of the OH-radicals is discussed in [26]. These radicals efficiently remove amorphous carbon (as is known from purification processes with H2O2) due to the reaction with nearby carbon atoms with dangling bonds to form CO. The authors reported high-purity singlewalled nanotubes produced by low temperature ethanol decomposition. However, in ethanol decomposition at 700–900 C no OH-radicals are produced. Chen et al. [27] used the decomposition of CH4–CO2 gas mixture in microwave plasma enhanced CVD (MPCVD) to obtain high-quality MWCNTs. Specifically, they investigated the reaction species in the plasma by optical emission spectroscopy and revealed that the oxygen presence in plasma enhanced the catalytic deposition of carbon. The effects of oxygen and nitrogen on carbon nanotube growth were also investigated by Yang et al. [28] using MPCVD. They found that the quality of the CNTs (diameter of 20–100 nm) was improved and the number density was greatly increased with addition of N2 and O2 and a nitride enhanced growth mechanism was proposed. However, we have reproducibly obtained MWCNTs without any amorphous coating, with bamboo like morphology and diameter of about 60 nm by ethanol decomposition at 850 C under atmospheric pressure and beginning the heating process intentionally in air, i.e. in the presence of O2, N2 and CO2. It is obviously that the absence of amorphous deposits is not determined by the ethanol pyrolysis chemistry, but by the oxidizing species in the air. We have investigated the behaviour of the Fe-catalyst thin film under these conditions by TEM analysis. We are aware that the behaviour of the catalyst thin films that are deposited on the TEM grids, particularly their wetting, growth mechanism and grain structure, differs
from that of the ones deposited on the oxide materials. The aim was not to analyze the grain structure, but the chemical reactions taking place under the experimental conditions. Fig. 7 shows the morphology of the as deposited Fe thin film (Fig. 7a) and after annealing at 600 C in N2 (Fig. 7b) and in air (Fig. 7c). The as-grown sputtered Fe thin film shows Fe crystallites (identified by electron diffraction) with diameters less then 5 nm. After heating at 600 C these crystallites coalesce. In N2 atmosphere a mixture of Fe and different Fe oxides was identified (residual oxygen content could not be prevented); the diameter of the particles increased to 25 nm. After annealing in air equiaxed Fe2O3 grains (identified by electron diffraction) with diameter of about 100–200 nm were produced. No iron carbide was found. Fe2O3 crystals were also identified between the grown MWCNTs, but we do not know how significant this is in describing the catalytic reaction mechanism. However, Prilutskiy et al. [29] investigated the growth of carbon nanomaterial by low temperature carbon monoxide disproportionation starting from Fe2O3 and NiO3, respectively and found similar results with ours: MWCNTs from Fe2O3, while mixtures of various graphene structures result from NiO3. 4. Conclusions We have demonstrated a process for coating quartz substrates with MWCNTs by ethanol decomposition at 850 C, atmospheric pressure, and under oxidizing atmosphere (i.e. with O2, N2 and CO2 present), using a predeposited Fe-catalyst thin film of 5 nm thickness. With this simple method we reproducibly obtained MWCNTs that are free of amorphous coating, exhibit bamboo type morphology, have an average diameter of 60 nm and a relatively narrow diameter distribution. The process parameters can be easily scaled up to larger outputs. Such a quality of carbon nanotubes is promising for electrochemical applications, such as fuel cells electrodes and electrochemical sensors. An amorphous coating would decrease the electrical conductivity of the CNT-based electrodes and composite materials by reducing the percolation. Moreover, electrochemical applications require activated surface sites, like carboxylic groups, and structural modification (opening, steps, and edges) [30], which could not be obtained on nanotubes with amorphous deposits. Acknowledgements The authors thanks the government of MecklenburgVorpommern in Germany for supporting this research within the TEM-FH Program under contract number 0201230 and the IT Gambert GmbH in Wismar for providing equipment.
M. Wienecke et al. / Carbon 44 (2006) 718–723
References [1] Iijima S. Helical microtubules of graphitic carbon. Nature 1991; 354(6348): 56–8. [2] Ebbeson TW. Carbon nanotubes—preparation and properties. Boca Raton: CSC Press; 1997. [3] Saito R, Dresselhaus G, Dresselhaus MS. Physical properties of carbon nanotubes. London: Imperial College Press; 1998. [4] Rodriguez NM, Kim MS, Baker RTK. Carbon nanofibers: A unique catalyst support medium. J Phys Chem 1994;98(50):13108–11. [5] Che G, Lakshmi BB, Martin CR, Fisher ER. Metal-nanocluster-filled carbon nanotubes. Catalytic properties and possible applications in electrochemical energy storage and production. Langmuir 1999;15(3): 750–8. [6] Huczko A. Synthesis of aligned carbon nanotubes. Appl Phys A 2002; 74(5):617–38. [7] Park JB, Choi GS, Cho YS, Hong SY, Kim D, et al. Characterization of Fe-catalyzed carbon nanotubes grown by thermal chemical vapour deposition. J Cryst Growth 2002;244(2):211–7. [8] Lee CJ, Lyu SC, Kim HW, Park CY, Yang CW. Synthesis of bamboo-shaped carbon–nitrogen nanotubes using C2H2–NH3– Fe(CO)5 system. Chem Phys Lett 2002;359(1–2):109–14. [9] Chen M, Chen CM, Chen CF. Preparation of high yield multi-walled carbon nanotubes by microwave plasma chemical vapour deposition at low temperature. J Mater Sci 2002;37(17):3561–7. [10] Ren ZF, Huang ZP, Xu JW, Wang JH, Bush P, Siegal MP, et al. Synthesis of large arrays of well-aligned carbon nanotubes on glass. Science 1998;282(5391):1105–7. [11] Murakami Y, Chiashi S, Miyauchi Y, Maruyama S. Direct synthesis of single-walled carbon nanotubes on silicon and quartz-based systems. Jpn J Appl Phys 2004;43(3):1221–6. [12] Alexandrescu R, Crunteanu A, Morjan RE, Morjan I, Rohmund F, Falk LKL, et al. Synthesis of carbon nanotubes by CO2-laserassisted chemical vapour deposition. Infrared Phys Technol 2003; 44(1):43–50. [13] Bronikowski MJ, Willis PA, Colbert DT, Smith KA, Smalley RE. Gas-phase production of carbon single-walled nanotubes from carbon monoxide via the HiPCO process. A parametric study. J Vac Sci Technol A—Vac Surf Films, Pt. 2 2001;19(4):1800–5. [14] Endo M, Takeuchi K, Igarashi S, Kobori K, Shiraishi M, Kroto HW. The production and structure of pyrolytic carbon nanotubes (PCNTs). J Phys Chem Solids 1993;54(12):1841–8. [15] Huang ZP, Xu JW, Ren ZF, Wang JH. Growth of highly oriented carbon nanotubes by plasma-enhanced hot filament chemical vapour deposition. Appl Phys Lett 1998;73(26):3845–7. [16] Avigal Y, Kalish R. Growth of aligned carbon nanotubes by biasing during growth. Appl Phys Lett 2001;78(16):2291–3.
723
[17] Valentini L, Armentano I, Kenny JM, Canalini C, Lozzi L, Cantucci S. Sensors for sub-ppm NO2 gas detection based on carbon nanotube thin films. Appl Phys Lett 2003;82(6):961–3. [18] Modi A, Koratkar N, Lass E, Wei B, Ayajan PM. Miniaturized gas ionization sensors using carbon nanotubes. Nature 2003;424(6945): 171–4. [19] Zhang B, Liang J, Xu CL, Wei BQ, Ruan DB, Wu DH. Electric double-layer capacitors using carbon nanotube electrodes and organic electrolyte. Mater Lett 2001;51(6):539–42. [20] Li W, Liang C, Qiu J, Zhou W, Hongmei Han H, Wei Z, et al. Carbon nanotubes as support for cathode catalyst of a direct methanol fuel cell. Carbon 2002;40(5):791–4. [21] Wienecke M, Bunescu MC, Pietrzak M, Deistung K, Fedtke P. PTFE membrane electrodes with increased sensitivity for gas sensor applications. Synth Met 2003;138(1–2):165–71. [22] Andrews R, Jacques D, Rao AM, Derbyshire F, Qian D, Fan X, et al. Continuous production of aligned carbon nanotubes: a step closer to commercial realization. Chem Phys Lett 1999;303(5–6): 467–74. [23] Hernadi K, Fonseca A, Nagy JB, Bernaerts D. Catalytic synthesis of carbon nanotubes. In: Yoshimura S, Chang RPH, editors. Supercarbon. Springer series in materials science, vol. 33. New York: SpringerVerlag; 1998. p. 81–97. [24] Huang ZP, Wang DZ, Wen JG, Sennett M, Gibson H, Ren ZF. Effect of nickel, iron and cobalt on growth of aligned carbon nanotubes. Appl Phys A 2002;74(3):387–91. [25] Ci L, Zhu H, Wei B, Xu C, Liang J, Wu D. Graphitization behaviour of carbon nanofibers prepared by the floating catalyst method. Mater Lett 2000;43(5–6):291–4. [26] Maruyama S, Kojima R, Miyaichi Y, Chiashi S, Kohno M. Lowtemperature synthesis of high-purity single-walled carbon nanotubes from alcohol. Chem Phys Lett 2002;360(3–4):229–34. [27] Chen Mi, Chen Chieng-Ming, Chen Chia-Fu. Growth of carbon nanotubes by microwave plasma chemical vapour deposition using CH4 and CO2 gas mixture. Thin Solid Films 2002;420–421:230–4. [28] Yang DJ, Zhang Q, Yoon SF, Ahn J, Wang SG, Zhou Q, et al. Effects of oxygen and nitrogen on carbon nanotube growth using a microwave plasma chemical vapour deposition technique. Surf Coat Technol 2003;167(2–3):288–91. [29] Prilutskiy O, Katz EA, Shames AI, Mogilevsky D, Mogilko E, Prilutskiy E, Dub Pri SN. Synthesis of carbon nanomaterials by a catalytic disproportionation of carbon monoxide. Fullerenes, Nanotubes and Carbon Nanostructures 2005;13(1):53–66. [30] Rohland B, Mihaela-C. Bunescu, Marlies Pietrzak, Marion Wienecke, Barfels T, Mo¨ller S. CNT-based cathode material for DMFC. Fullerenes, Nanotubes and Carbon Nanostructures 2005;13(1): 511–22.