Synthesis of Carbon Nanotubes Using a Butane−Air Bunsen Burner

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Synthesis of Carbon Nanotubes Using a Butane-Air Bunsen Burner and the Resulting Field Emission Characteristics Chih-Che Hsieh,† Meng-Jey Youh,‡ Hung-Chih Wu,† Li-Chieh Hsu,† Jin-Cheng Guo,† and Yuan-Yao Li*,† Department of Chemical Engineering, National Chung Cheng UniVersity, Chia-Yi 621, Taiwan, and Department of Information Technology, Hsing Wu College, Taipei County 244, Taiwan ReceiVed: July 3, 2008; ReVised Manuscript ReceiVed: October 13, 2008

Multiwalled carbon nanotubes (MWCNTs) with uniform diameters (10-20 nm) and high densities per unit area were synthesized in a 5-10 s combustion process using a mixture of butane and air at 880 °C. The growth mechanism of MWCNTs was studied via time-sequence experiments to reveal that the catalyst nanoparticles were formed on a 3 nm thick Ni-coated wafer after 3 s in the flame. Short and uniform MWCNTs were synthesized using a 5 s process, whereas MWCNTs with a maximum density and length were obtained using a 10 s process. It is believed that both the synthesis temperature and the combustion products, such as CO, H2O, CnHm (n ) 1 or 2), and polycyclic aromatic hydrocarbons, play an important role in the growth of the MWCNTs. Furthermore, the field emission properties of the combustion-generated carbon nanotube (CNT) films were studied, and an emission current density of 0.18 mA/cm2 at 7 V/µm was obtained. These results suggested that this fabrication method provided rapid and direct growth of field-emission CNTs on a desired substrate. Introduction 1

Since the carbon nanotube (CNT) was first reported in 1991, CNTs have been recognized as one of the most promising nanomaterials of the 21st century. CNTs not only possess unique characteristics in terms of their electrical, mechanical, optical, thermal, and chemical properties, but also have the potential for a variety of applications in electronic devices,2 field emission displays,3 composites,4 sensors,5 electrodes,6 adsorbents,7 etc. There are two general fabrication methods for CNTs.1,8-10 One of these is already in mass production, and the products are powdered for use in composites, electrodes, adsorbents, etc. The other method is direct growth of CNTs. This growth can be spatially targeted to create devices such as CNT random access memory devices (NRAM),11 single-electron transistors (SETs),12 or tips for scanning probe microscopy (SPM).13 CNTs fabricated using the latter method do not require high rates of synthesis (tons per year) but instead require higher quality CNTs and stricter synthesis conditions, such as a low-temperature synthesis environment. Many fabrication methods have been developed for the aforementioned purposes. Chemical vapor deposition (CVD) is a well-established technology for the production of CNTs.14-16 In this method, hydrocarbon and/or carbon monoxide is pyrolyzed in the presence of a catalyst at an adequate temperature, resulting in CNT formation. CNT production by direct flame combustion could be a low-cost mass-production alternative to CVD methods.17,18 This reduction in expenditures results from a continuous mass production process that does not require external energy for synthesis (e.g., electricity for a CVD furnace, laser, or arc-discharge). Furthermore, equipment costs are low and operation is simple. Two types of flames are typically used * To whom correspondence should be addressed. Fax: +886 5 2721206. E-mail: [email protected]. † National Chung Cheng University. ‡ Hsing Wu College.

for CNT synthesis, diffusion and premixed. In the case of premixed flames, gaseous fuel is mixed with air prior to combustion. By adjusting the air-fuel mixture, complete or incomplete combustion can be controlled and steady flames with different luminosities, colors, and forms can be generated. For diffusion flames, combustion occurs at the point where the fuel mixes with the ambient air. Such flames are generally smoky and flicker excessively, because the heat intensity of combustion is low. In recent years, many approaches to the flame-based synthesis of CNTs have been reported. In 1996, Richter et al.17 were first to attempt to synthesize CNTs with premixed, low-pressure hydrocarbon flames. Vander Wal et al. used both diffusion19,20 and premixed flames21to synthesize multiwalled carbon nanotubes (MWCNTs). A few special types of flames have also been studied for CNT formation as well, such as inverse diffusion flames,22 flames with opposed fuel and oxider flows,23 trumpetshaped flames,24 and others. In spite of all these studies, important properties of the resulting flame-generated CNTs, such as purity, uniformity, and feasibility for mass production, have not yet been fully studied. The Bunsen burner, which was invented by Robert Bunsen in 1855, is a common laboratory instrument used for heating, sterilization, and combustion. The device safely burns a continuous stream of a flammable gas, such as natural gas (principally composed of methane), or a liquefied petroleum gas, such as propane, butane, or a mixture of both. The fuel gas flows up through a needle valve at the bottom of the barrel, where it mixes with air drawn through adjustable open slots on the sides of the bottom of the burner barrel by the Venturi effect.25 The gas burns at the top of the barrel once ignited by a flame or spark. The amount of air (oxygen) mixed with the gas stream affects the completeness of the combustion reaction in the flame. Less air yields an incomplete reaction, which produces carbon monoxide, water, and small soot particles in the flame. In contrast, a gas stream that is well-mixed with air provides

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Figure 3. TEM image of unmodified MWCNT.

Figure 1. (a) Temperature profile of the flame. (b and c) Image of the combustion experiment.

Figure 4. Raman spectrum of unmodifid MWCNT (10 s synthesis, 3 nm thick Ni film).

(HRTEM). The graphitization of the CNTs was also studied using Raman spectrometry in order to understand the effect of combustion time on CNT quality. In addition, the field emission properties of the resulting CNT films were also studied. We believe that the butane-air Bunsen combustion process provides a rapid (5 to 10 s), easy (simpler reaction setup and easier operation relative to CVD, arc discharge, and laser procedures), cost-effective, and safe method for the direct synthesis of CNTs onto a desired device. Figure 2. FESEM image of CNTs formed at 880 °C from a 10 s exposure to combustion process.

oxygen in an equimolar amount and therefore results in a complete and ultimately hotter reaction. Carbon monoxide is recognized as an excellent carbon source for the synthesis of CNTs,26,27 while water acted as a weak oxidant28 for removing amorphous carbon during the synthesis process. During the experiments for CNT synthesis, H2O vapor is generated from both incomplete combustion and the catalyst-containing aqueous solution. In this study, a butane-air Bunsen burner was used to create a premixed flame for CNT synthesis. The Bunsen burner provided a stable flame and a controllable temperature profile by adjusting the fuel/air input ratio. Under the appropriate experimental conditions, CNTs were uniformly grown on a substrate within 5-10 s of combustion. These CNTs were characterized by field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy

Experimental Methods A commercial Bunsen burner (REKROW, RK4102) loaded with liquefied butane (C4H10) fuel was used to synthesize CNTs. A 5 mm × 5 mm p-type silicon wafer served as the substrate onto which a nickel (Ni) catalyst film was deposited either by sputtering or by spin coating the catalyst precursor (0.1 M Ni(NO3)2(aq)) at 5000 rpm. A typical synthesis process followed. Butane gas mixed with air was admitted into the burner, and the burner was lit with an electric spark to generate a flame. The fuel flow rate was adjusted via the needle valve at the base of the burner. The air baffle was adjusted to its half-open state in order to control the fuel-air mixture to yield incomplete combustion as desired. The gas mixture at the burner outlet was collected, and its composition was analyzed by gas chromatography (GC, SHIMADZU GC-14B). The total flow rate of the mixture was 6.0 standard liters per minute (SLM) with a butane/air ratio of 1.0. The flame was yellow, with a bushy outer flame surrounding a bright blue inner cone. This is a typical flame pattern for

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Figure 5. FESEM images of CNTs grown on substrate surfaces after combustion exposure times of (a) 3, (b) 5, (c) 10, (d) 20, and (e) 40 s. (880 °C, 3 nm Ni film).

incomplete combustion. The flame was about 13 cm long. The temperature profile of the flame was measured using a K-type thermometer (Figure 1a). The temperature increased rapidly from the blue region at the base of the flame and reached a maximum of 940 °C at 6 cm above the burner outlet. A wafer coated with a layer of either catalyst or catalyst precursor was placed in the flame at 880 °C, about 5 cm above its base. After the desired exposure time (3-40 s), the wafer was removed from the flame and allowed to cool to ambient temperature. The radial temperature distribution of the flame was difficult to measure by thermal couple because of small size in radial dimension. However, the entire substrate was located in the inner

corn region which ensures an even thermal/combustion process. Images of the combustion experiment are provided in Figure 1b. The resulting materials were characterized by FESEM (Hitachi S4800), HRTEM (Philips/FEI Tecnai G2), and microRaman spectrometry (Tokyo Instruments, INC HeNe Laser 632.8 nm). To prepare samples for HRTEM, CNTs were removed from the substrate and suspended in ethanol by ultrasonic agitation. A drop of the suspension was applied on a carbon grid and then the sample was dried prior to TEM analysis. The surface morphology of catalyst/catalyst-precursor films was examined via atomic force microscopy (AFM, Universal SPM).

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Figure 6. Raman spectra of the unmodified MWCNTs.

For field emission experiments, a simple, planar, diode-like device was used as the cathode and indium tin oxide (ITO)/glass was used as the anode. The field emission properties of the diodetype emitter were analyzed in a vacuum chamber at 5 × 10-7 torr. The field emission current was measured using an electrometer (Keithley 2410) by applying a positive voltage to the anode. The spacer’s thickness was approximately 150 µm. Results and Discussion Combustion Synthesis of CNTs with Ni Thin Films. FESEM images in Figure 2 were acquired of a sample wafer coated with a 3 nm thick Ni film after a 10 s exposure to combustion processes at 880 °C. Uniform growth of onedimensional (1D) carbon nanomaterials on the substrate was observed. The diameter and length of the nanomaterials fell within the ranges of 10-20 and 170-230 nm, respectively. The HRTEM image of the 1D carbon nanomaterials (Figure 3) revealed hollow tube morphology with walls that were parallel to the long axis of the tube. The inner and outer diameters of the tubes were 4 and 10 nm, respectively. In accordance with FESEM and HRTEM analyses, the 1D carbon nanomaterials were identified as MWCNTs. The Raman spectrum of these MWCNTs (Figure 4) depicted a ratio of G-band (1580 nm-1) to D-band (1350 cm-1) intensities (IG/ID) of 0.86, which was a reasonable value for MWCNTs and reflected their graphing. Similar FESEM images were acquired of a set of samples coated with 3 nm thick Ni-catalyst films after they were exposed to combustion reactions at 880 °C for 3, 5, 10, 20, or 40 s (Figure 5). The 1D nanomaterial was observed to be uniformly distributed across the substrate surface following 5 s flame exposure (Figure 1(b)). Energy-dispersive X-ray spectrometry (EDS) identified the bright spots (10 nm) in these FESEM images as Ni compounds. These compounds were the catalyst seeds responsible for MWCNT formation. It was believed that these seeds were formed as a result of thermal nucleation of the Ni film upon exposure to the flame. When combustion exposure time was increased to 10 s (Figure 5c), longer MWCNTs (∼200 nm) were synthesized at higher density per unit area. When combustion exposure time was increased to 20 s (Figure 5d), however, the density of MWCNTs per unit area decreased, resulting in a rougher surface. This roughness was attributed to combustion of the MWCNTs after prolonged exposure to the flame, which likely burned off or damaged the MWCNTs. The longer stay in the flame, the more oxidation occurred. The outer graphene layers of the MWCNTs were oxidized leading to creation of defects (not perfect structure of graphene on the surface of the MWCNTs). The Raman spectra of all of these samples (3, 5, 10, 20, and 40 s exposures)

Figure 7. FESEM images of CNTs grown on substrate surfaces coated with different thicknesses of Ni film. (a) 3, (b) 20, and (c) 40 nm.

supported these findings (Figure 6). As the combustion exposure time was increased from 10 to 40 s, the IG/ID ratio decreased from 0.86 to 0.71. This decrease indicated that the degree of MWCNT graphitization had decreased and that the number of MWCNT defects had increased. Another series of experiments was conducted in which the thickness of the Ni film coating the silicon substrate was varied. Sample substrates were prepared with 3, 20, and 40 nm thick Ni films and were subsequently exposed to combustion reactions at 880 °C. FESEM images were acquired of the resulting MWCNTs, which grew on all three types of substrates (Figure

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Figure 8. AFM images of film roughness generated using (a) sputtering and (b) spin-coating techniques.

7). The distribution of MWCNT diameters was similar for all three samples, but increasing the film thickness decreased the total number of CNTs formed. Larger catalyst particles were observed on the samples with thicker Ni films (20 and 40 nm). The average catalyst particle diameters were about 10, 150, and 300 nm for the 3, 20, and 40 nm thick films, respectively. It was hypothesized that thermal migration and nucleation processes occurred within the Ni films during exposure to the combustion reaction. The Ni catalysts were therefore formed. Because of the volume difference, the thicker Ni film (large volume) formed bigger catalysts. The catalysts were lifted up from the substrate by the CNTs during synthesis. The reaction sequences within a butane-air flame are very complicated and poorly understood. During combustion, butane is first dehydrogenated to generate one of four major resonancestabilized free radicals, propargyl (C3H3-), allyl (C3H5-), 1-methylallenyl (CdCdCsCH3), or cyclopentadienyl (C5H5-).29 After a sequence of reactions in which these free radicals recombine, combustion products, such as polycyclic aromatic hydrocarbons (PAHs), H2O, H2, CO, CO2, CH4, C2H2, C2H4, and C2H6, are generated.30 Carbon soot can then be formed from the degradation of PAHs. Polycylic aromatic hydrocarbons, soot, and the aforementioned byproduct play different roles in the formation of CNTs. Carbon monoxide31 and the various hydrocarbon species (CH4, C2H2, etc.)32 serve as the carbon sources. These compounds were decomposed on the surface of the catalyst particles to promote the formation of CNTs.33 Molecular and atomic hydrogen acted as strong etchants,34 and

Figure 9. FESEM images of CNTs grown on substrate surfaces spincoated with 0.1 M aqueous Ni(NO3)2 and exposed to combustion processes at 880 °C for (a) 5, (b) 10, and (c) 20 s.

H2O acted as a weak oxidant28 to remove amorphous carbon from the catalyst21 for a better growth of MWCNTs. Polycylic aromatic hydrocarbons are thermodynamically stable materials, and they inactivate catalysts, which is not favorable to CNT growth.35 Many factors affect the growth of CNTs during flamebased synthesis, such as the combustion products (PAHs, CO, CO2, hydrocarbons, etc.), the synthesis temperature, the reaction time, and the type of catalyst or catalyst precursors used.36 In particular, the fuel-to-oxygen (air) ratio plays an important role on the growth of CNTs. If the air baffle was fully open, the

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Figure 10. (a) Emission current density vs electric field for MWCNTs synthesized from Ni metal and aqueous Ni(NO3)2 films. (b) FowlerNordheim plots of field emissions from MWCNTs synthesized from Ni metal and aqueous Ni(NO3)2 films.

TABLE 1: Field Emission Characteristics of MWCNTs Ni film Ni(NO3)2 film (aq)

Eturn-ona (V/µm)

JE)7 V/µmb (mA/cm2)

βc

4.9 6.2

0.18 0.04

1338 1062

a Eturn-on corresponds to an emission current density of 10 µA/cm2. Current density at E ) 7 V/µm. c β is the field enhancement factor calculated from Fowler-Nordheim plots by setting the work function of the emitters to 4.7 eV. b

excess oxygen will cause the completed combustion so that there is no sufficient carbon source for the growth of CNTs. In contrast, if insufficient oxygen is admitted (air valve closed), this will increase the generation carbon soot (or PAH), which “poison” the catalyst for CNT growth. Combustion Synthesis of CNTs with a Ni(NO3)2 (aq) Film. Flame-based synthesis of CNTs was also attempted by coating a substrate wafer with a layer of 0.1 M aqueous nickel nitrate. Nickel nitrate (Ni(NO3)2) was hypothesized to decompose at high temperatures to form Ni or NiO catalyst seeds.37 AFM was used to probe the morphology and roughness of the films formed using sputtering and spin-coating techniques (Figure 8, panels a and b). As expected, the film formed by sputtering was smoother than that formed by spin coating. The roughness of the sputtered film was about 0.22 nm, and that of the spincoated film was about 4.11 nm. Sample substrates were prepared with 0.1 M aqueous Ni(NO3)2 film, and FESEM images of the samples were acquired following 5, 10, or 20 s exposures to combustion processes at 880 °C (Figure 9). The results were similar to those obtained with the 3 nm Ni film. MWCNTs were successfully synthesized on the substrate exposed to combustion processes for 10 s. In terms of MWCNT uniformity, however, the tubes formed on

the substrates spin-coated with aqueous Ni(NO3)2 were not as uniform as those formed on substrates sputtered with Ni metal. This difference in density per unit square was attributed to differences between the catalytic films’ roughness and to differences between the mechanisms of catalyst particle formation. FESEM analysis revealed that, following 5 s exposure to combustion processes, many catalyst particles had formed on the substrate, but MWCNTs had not been synthesized (Figure 9a). This result was different from that achieved under similar reaction conditions with the substrate coated with a 3 nm Ni film in which uniform growth of MWCNTs was observed (Figure 5b, 5 s reaction). Clearly, the formation of the MWCNTs was slower with the Ni(NO3)2 film, probably because it took extra time for the Ni(NO3)2 to decompose and nucleate to form active NiO or Ni catalyst particles.37,38 Thus, the overall MWCNT synthesis process was slower. Field Emission Characteristics of the As-Produced MWCNTs. The relationship between current density and electric field was measured for the unmodified MWCNTs (Figure 10a), and the corresponding Fowler-Nordheim (FN) plots were also prepared based on this relationship (Figure 10b). Both the MWCNT prepared with the Ni metal catalyst and those prepared with 0.1 M aqueous Ni(NO3)2 precatalyst were studied in this manner. A diode-like device, which included a cathode composed of MWCNTs, and an ITO glass anode, was loaded into a vacuum chamber at a pressure of 5 × 10-7 torr. The turn-on and threshold voltages were defined as the electrical field strength that was required to generate current densities of 10 and 1 mA/cm2, respectively. The turn-on voltages of the MWCNTs synthesized from the Ni metal and aqueous Ni(NO3)2 films were 4.9 and 6.2 V/µm, respectively (Figure 10a). As expected, the MWCNTs synthesized from the Ni film demonstrated a higher current density (0.18 mA/cm2 at 7 V/µm). This greater value was attributed to the differences between the coating techniques applied to the samples. The Ni metal film not only provided nucleation sites for MWCNT growth, but also served as an electrically conductive layer between the MWCNTs and the substrate. According to the FN equation,39 emission current density (J) can be described as a function of the electric field in the following manner:

J)A

(

-Bφ3/2 (βE)2 exp φ βE

)

(1)

where J is expressed in units of A/cm2, E is the applied electric field given in units of V/cm, β is the field-enhancement factor, and φ is the work function of the emission surface expressed in units of eV. The work function φ of the CNTs was assumed to be 4.7 eV.40 The constants A and B were 1.54 × 10-6 AV-2 eV and 6.83 × 107 VeV-3/2 cm-1, respectively. The linear relationships observed in the materials’ FN plots indicated that the measured current originated from field emissions. The FN equation’s field-enhancement factor, β, was calculated from the slope of the plot of ln(J/E2) vs (1/E). The field emission characteristics of the two samples are summarized inTable 1. The MWCNTs synthesized from Ni films have higher β values (β) 1338) than those synthesized from the aqueous Ni(NO3)2 films (β ) 1062). These values were in accord with previous reports of the β-values for MWCNTs. In some reports,41-43 the turn-on voltages of the CNT emitters were in the range of 2-3 V/µm, which were relatively low then our experimental result (4.9 V/µm). However, the turn-on voltages are depended on the various structure (diode or triode type), types of emitters, and gap of anode/cathode plates, etc. Although the largest

19230 J. Phys. Chem. C, Vol. 112, No. 49, 2008 current density in this study (0.18 mA/cm2) is not as good as in other reports, we believe that the synthesis method can be modified to fabricate a high performance field emission device. Conclusions Synthesis of CNTs using a conventional Bunsen burner and the field emission properties of the resulting CNTs were studied. MWCNTs were synthesized on silicon substrates coated with 3 nm thick Ni films during a 5-10 s exposure to combustion processes at 880 °C. A series of experiments revealed that catalyst seeds had formed from the Ni film after just 3 s of exposure to the flame. MWCNTs were synthesized during a 5 s exposure, and they reached their maximum lengths within 10 s. FESEM analysis demonstrated that when combustion time was increased, MWCNT defects increased, and the density of MWCNTs per unit area decreased dramatically. Longer exposure to flame appears to damage and/or burn off MWCNTs. The Raman spectra of the samples revealed that the degree of MWCNT graphitization, as indicated by the G- to D-band intensity ratio (IG/ID), decreased as combustion time increased. An aqueous solution of 0.1 M Ni(NO3)2 was also tested as a catalytic precursor for use in the flame synthesis of CNTs. When silicon substrates were coated with a film of this precatalyst solution, MWCNTs were synthesized within 10 s. The field emission properties of the unmodified MWCNT-coated surfaces were also investigated. Catalyst coating techniques were found to have an effect on the nanotubes’ field emission performance. In conclusion, we demonstrated a rapid (5-10 s), cost-effective (inexpensive reactor and fuel), and safe method for synthesizing MWCNTs directly onto substrates or devices using a conventional Bunsen burner. Acknowledgment. The financial support of this work by the National Science Council of the Republic of China under Contract No. NSC 97-2120-M-006-005 is gratefully acknowledged. References and Notes (1) Iijima, S. Nature 1991, 354, 56–8. (2) Ravindran, S.; Chaudhary, S.; Colburn, B.; Ozkan, M.; Ozkan, C. S. Nano Lett. 2003, 3, 447–53. (3) Choi, W. B.; Chung, D. S.; Kang, J. H.; Kim, H. Y.; Jin, Y. W.; Han, I. T.; Lee, Y. H.; Jung, J. E.; Lee, N. S.; Park, G. S.; Kim, J. M. Appl. Phys. Lett. 1999, 75, 3129–31. (4) Ramasubramaniam, R.; Chen, J.; Liu, H. Y. Appl. Phys. Lett. 2003, 83, 2928–30. (5) Chopra, S.; McGuire, K.; Gothard, N.; Rao, A. M.; Pham, A. Appl. Phys. Lett. 2003, 83, 2280–2. (6) Frackowiak, E.; Metenier, K.; Bertagna, V.; Beguin, F. Appl. Phys. Lett. 2000, 77, 2421–3. (7) Darkrim, F. L.; Malbrunot, P.; Tartaglia, G. P. Int. J. Hydrog. Energy 2002, 27, 193–02. (8) Wang, Y. H.; Chiu, S. C.; Lin, K. M.; Li, Y. Y. Carbon 2004, 42, 2535–41. (9) Guo, T.; Nikolaev, P.; Thess, A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1995, 243, 49–54. (10) Zhu, H. W.; Xu, C. L.; Wei, B. Q.; Wu, D. H. Carbon 2002, 40, 2023–5. (11) Rueckes, T.; Kim, K.; Joselevich, E.; Tseng, G. Y.; Cheung, C. L.; Lieber, C. M. Science 2000, 289, 94–7.

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