Large-Area Single Wall Carbon Nanotubes: Synthesis

J. Phys. Chem. C 2007, 111, 1601-1604

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Large-Area Single Wall Carbon Nanotubes: Synthesis, Characterization, and Electron Field Emission Yu H. Yang, Chih Y. Wang, Uei S. Chen,* Wei J. Hsieh, Yee S. Chang, and Han C. Shih* Department of Materials Science and Engineering, National Tsing Hua UniVersity, 101, Section 2, Kuang Fu Road, Hsinchu, Taiwan 300, R.O.C. ReceiVed: September 5, 2006; In Final Form: NoVember 2, 2006

Large-area, high-uniformity, and high-density single wall carbon nanotubes (SWNTs) were fabricated by ethanol at 900 °C in a tubular furnace. The Fe(NO3)3‚9H2O and the ICP/DCP standard solution of Mo were reacted to deposit SWNTs on the surface of the MgO sol-gel nanopowders. SWNTs have diameters in the range of 0.6-1.6 for SWNTs in bundles, and diameters in the range of 2-6 nm for isolated SWNTs were identified by the high-resolution transmission electron microscope and the breathing mode of the Raman spectrrum. A high yield of the as-synthesized SWNTs is over 500 wt % relative to the Fe-Mo metal in the catalyst. The extremely low turn-on field Eto ) 0.008 V/µm and threshold field Ethr ) 0.07 V/µm show that the SWNTs have an excellent performance on the electron field emission.

Introduction Since their discovery in 1993,1 single wall carbon nanotubes (SWNTs) have been studied becase of their unique chemical and physical properties.2,3 Many important applications of SWNTs such as sensors,4 field emitters,5 transistors,6 and supercapacitors7 have been demonstrated. To realize various important applications of SWNTs, the production of high yield and high quality SWNTs is an important issue. Various methods such as electric arc discharge,8 laser ablation,9 and catalytic chemical vapor deposition (CCVD)10-16 have been developed for synthesizing the SWNTs. A significant effort has been devoted to the studying of carbon nanotubes (CNTs) by this group.17-21 To realize the superior property of the SWNTs, a CCVD method has been utilized to synthesize the SWNTs. Recent reports have shown that the CCVD method is a high yield, high purity, and low cost method for synthesizing SWNTs. Many carbon-containing molecules such as ethylene,11 acetylene,12 alcohol,13 methane,14 hexane,15 and benzene16 have been used as the precursors. Among them, alcohol is found to be a special carbon source. Because the etching effect of the decomposed OH radicals attack the carbon atoms with a dangling bond, impurities such as amorphous carbon and carbon nanoparticles are eliminated,13 which is a key to get high purity SWNTs. Alcohol is the precursor that can synthesize the SWNTs and etch the amorphous carbon species on the catalyst surface. Alcohol is also an economic carbon source. However, there are only few reports on the synthesis of SWNTs using alcohol as a carbon source. Maruyama et al.13 announced the synthesis of SWNTs from alcohol over zeolite supported Fe and Co catalyst with high purity at 700-900 °C. In their results, the quantity of SWNTs is too low for other researches. To get a large-scale synthesis of high-quality SWNTs, some groups have shown that MgO powder is very good support. It is because MgO powder has a large surface area, which provides a huge reaction area. The other advantage of using MgO powder as a support is that * Corresponding author. E-mail: [email protected] (H.C.S.); [email protected] (U.S.C.). Tel: +886-3-5715131 ext 3845. Fax: +886-3-5710290.

MgO can be removed by mild acid treatment. In this study, MgO nanopowder has been fabricated by sol-gel method,22 because it has a better viscosity which can promote physical adsorption of the catalyst particles and provide a larger area than the conventional MgO nanopowder. Colomer et al.14 have demonstrated the large-scale fabrication of SWNTs from methane over MgO powder supported metal catalyst at a hightemperature 1000 °C. Their results showed that the carbon products have a lot of amorphous carbon materials. In this experiment, we used ethanol as a carbon source and MgO nanopowder as a catalyst support to get high-quantity and highquality SWNTs. Recent works have shown that CNTs have a good electron field emission performance.5,23-27 Because it is difficult to fabricate well-aligned SWNTs arrays, most groups focused on the vertically well-aligned multiwalled CNTs (MWNTs). Studies on the electron field emission property of the SWNTs are therefore relatively few. In the present work, we describe the synthesis of SWNTs from alcohol over sol-gel MgO nanopowder supported Fe and Mo at 900 °C in a furnace. Such produced carbon materials contain 80-90% SWNTs, some double wall CNTs and few MWNTs. Our results show that ethanol is a very effective carbon source for production of high-quality and high-purity SWNTs in high yield. The result of electron field emission analysis shows that the SWNTs is a potential candidate for applications on the field emission display (FED).28 Experimental Section Catalysts are prepared by impregnation of MgO with DI water solution of metal salts. A mixture of Fe(NO3)3‚9H2O (99.99%, Aldrich) and Mo solution (ICP/DCP standard solution, 9.8 mg/ mL of Mo in water, Aldrich) was dissolved in DI water and sonicated for 1 h. In this experiment, MgO nanopowder was fabricated by the sol-gel method22 as shown in Figure 1. To embed the Fe-Mo bimetal catalysts onto the MgO nanopowder, MgO nanopowder was introduced to the mixed Fe-Mo solution and sonicated for 1 h. The weight ratio of Fe:Mo:MgO ) 1:0.4:

10.1021/jp065755s CCC: $37.00 © 2007 American Chemical Society Published on Web 01/06/2007

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Figure 1. Flowchart describing the sol-gel MgO nanopowder preparation.

40 was used to fabricate the catalyst embedding on the support material. The catalyst solution was filtered out the DI water and dried at 150 °C for 4 h. Then, the catalyst powder was grounded to fine powders. The grounded catalyst material (∼1 g) was put into a quartz boat, and the boat was inserted into the reactor. The reactor is made of Mullite tube (99.5% Al2O3) of 65 mm in inner diameter and 120 cm in length mounted in an electrical tube furnace. The reactor was heated up to 900 °C at 10 °C /min with a flow rate of 200 sccm Ar. When the boat reaches 900 °C, Ar gas was evacuated and ethanol was supplied to the tube for typically 20 min from a room-temperature reservoir. Keeping the vacuum pump on, the pressure of ethanol in the tube was controlled at ∼5 Torr. After the reaction, the tube was cooled down to room temperature in an Ar atmosphere. Finally, the crude sample was immersed in 50 mL of concentrated HCl under sonication for 20 min. The solution was filtered through a VCTP filter (Isopore membrane filters, porosity 0.1 µm, Millipore), washed by DI water to fix the pH at 6-7, and dried at 70 °C. The product CNTs are characterized by a field emission scanning electron microscope (FESEM, JEOL JSM-6500F), high-resolution transmission microscope (HRTEM, JEOL JEM3000F), Raman spectrometer (He-Ne laser 632.8 nm, Jobin Yvon LABRAM HR Micro-Raman system), and high-voltage source meter (Keithley 237) for measuring the emission current versus the applied voltage. Catalyst was analyzed by the X-ray diffraction (XRD, Shimadzu SRD-600). For the electron field emission measurement, the anode electrode made of glass coated with indium tin oxide (ITO) was separated from the cathode electrode by an insulator spacer (∼180 µm). The emission distance between the coated ITO and the tip of SWNTs is approximated to the spacer thickness of ∼180 µm. The definition of a whole cycle is that voltage increased by steps of 0.25 V/s from 0 to 50 V. Electron field emission measurements were conducted under a pressure of 1 × 10-5 Torr. The sample was prepared by placing the SWNTs on a conductive copper tape for a detecting area of 0.5 cm × 0.5 cm. Results and Discussion The XRD pattern from catalyst shows the composition of MgO and Fe3O4, as the major components, as shown in Figure 2a. The amount of Mo is apparently too small to be detected by the XRD analysis. The sample is prepared after heating up to 900 °C with an Ar atmosphere in the furnace. By this way,

Figure 2. (a) XRD spectrum of the Fe-Mo/MgO catalyst after heating up to 900 °C in an Ar atmosphere, (b) FESEM image of the as-synthesized carbon filaments fabricated by pyrolysis alcohol over the Fe-Mo/MgO catalyst at 900 °C after Hcl wash treatment.

the real catalyst before the fabrication of SWNTs is known. As a result, the XRD pattern shows that Fe transforms to iron oxide and Mo must also transform to molybdenum oxide. For this reason, it may suggest that metal oxide catalyst must play an important role in yielding SWNTs in this work. Observations by FESEM showed that the as-synthesized product is extremely abundant with carbon filaments, as shown in Figure 2b. The nanoscale diameter of the MgO powder leads the diameter limit of the Fe and Mo catalyst particles. The smaller diameters of catalyst particles promote the yield of SWNTs. The lengths of carbon filaments are up to several microns with diameters of 5-25 nm (Figure 2b). On the right side of Figure 2b, carbon filament flakes are found. They come from highly aggregated carbon filaments. The carbon filaments in this picture have a clean and a smooth surface without amorphous carbon deposits. It is worth noting that the image of carbon filaments is obtained from unpurified as-synthesized carbon product after acid treatment, which means that the carbon product is of high putity. They also indicate that the huge quantities of carbon filaments have been synthesized over the Fe-Mo/MgO catalyst by the catalytic pyrolysis of ethanol at 900 °C. The result of the SEM image indicated that this method can be used for the production of large scale and high purity carbon filaments. However, SEM observation only showed a roughly qualitative result of the carbon filament yield. A further measurement of the as-synthesized carbon filament yield is necessary to be established. A weight gain measurement for the as-synthesized carbon filaments indicates a high carbon

Large-Area Single Wall Carbon Nanotubes

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Figure 5. Electron field emission J-V curve and Fowler-Noderheim plot (inset) of the as-synthesized SWNTs.

TABLE 1: Turn On (Eto) and Threshold (Ethr) Fields for Various CNTs Field Emitters Figure 3. HRTEM images of the as-synthesized SWNTs fabricated by pyrolyzing alcohol over the Fe-Mo/MgO catalyst at 900 °C. (a and b) SWNTs in bundles with the diameter in the range of 0.6-1.6 nm. (c) an isolated SWNT with ∼6 nm in diameter. (d) HRTEM image showing a DWNT in addition to the SWNTs.

Figure 4. Raman spectrum of the as-synthesized SWNTs. The inset shows the magnitude frequencies for RBM.

filaments yield of ∼500 wt % relative to the weight of the FeMo metal in the Fe-Mo/MgO catalyst. TEM studies give further information about the morphology and the microstructure of the as-synthesized carbon filaments over the Fe-Mo/MgO catalyst by the catalytic pyrolysis of alcohol. Figure 2 shows the HRTEM images of the assynthesized carbon filaments which are consisted of bundles and isolated SWNTs. Panels a and b in Figure 3 are the HRTEM images of SWNTs bundles with clear graphite layers between SWNTs. The diameter of SWNTs in bundle is in the range of 0.6-1.6 nm. Figure 3, panels c and d, shows the HRTEM images of isolated SWNTs with the diameters in the range of 2-6 nm. This result tells us that the diameter of SWNTs fabricated by catalytic CVD has a wide distribution compared with SWNTs fabricated by the arc discharge method. In Figure 3d, one double wall carbon nanotube (DWNT) is also observed. The result of HRTEM observation shows that the carbon filaments consist of 80-90% SWNTs and a few DWNTs and

emitter

Eto (V/µm)

Ethr (V/µm)

ref

arc MWNT film arc SWNT fim catalytic MWNT array aligned and opened MWNT array MWNT flake SWNT in ths work

2.6 >1.7 >2.0 0.8 0.19 0.008

4.6 6.5 4.8 2.3 0.26 0.07

25 5 23 26 27 this work

MWNTs. In this work, we can hardly find any amorphous carbon covering the surface of SWNTs, which indicates that the as-synthesized SWNTs are of high purity. Raman spectroscopy is a well-known useful tool for the characterization of SWNTs. The spectrum exhibits unambiguously the characteristic frequencies of SWNTs, and different laser excitation sources lead to different intensity ratios of the low-frequency peaks (100-350 cm-1) and high-frequency peaks (1500-1600 cm-1).29 Figure 4 shows a typical Raman spectrum obtained with a He-Ne 638.4 nm laser as an excitation source on the SWNTs fabricated on the Fe-Mo/MgO catalyst by the catalytic pyrolysis of ethanol at 900 °C. The high-resolution spectrum in the low-frequency region shows 11 peaks; they are at 146.5, 166.4, 188.0, 212.8, 219.8, 252.2, 285.6, 308.4, 324.0, 343.5, and 359.0 cm-1. This frequency region is associated with the radical breathing mode (RBM). RBM is the phonon vibration of carbon molecules on the circumference of SWNTs, and it is also sensitive to the diameter of tubes. Some theoretical formulas had been created to calculate the diameter of SWNTs. Considering the interaction effect between tube and tube in one bundle of SWNTs, the expression ω ) 6.5 + 223.75/d is used to calculate the diameter of SWNTs, where ω is the RBM frequency in cm-1 and d is the diameter of the SWNTs in nm. According to the above formula, these peacks at 146.5, 166.4, 188.0, 212.8, 219.8, 252.2, 285.6, 308.4, 324.0, 343.5, and 359.0 cm-1 correspond to the diameters of 1.60, 1.40, 1.23, 1.08, 1.05, 0.91, 0.80, 0.74, 0.70, 0.66, and 0.63 nm. The diameter distribution of SWNTs from the Raman spectrum analysis is in the range of 0.6-1.6 nm, and it conforms to the result of HRTEM observation. Between 1500 and 1600 cm-1, the peak center at 1572.0 cm-1 is referred to as the G-band. In our results, the G-band has a narrow and strong spectrum peak, indicating a good arrangement of hexagonal lattice of graphite. There is the other shoulder peak in the 1500-1600 cm-1 range, which centers at 1525.2 cm-1. This shoulder peak is more important

1604 J. Phys. Chem. C, Vol. 111, No. 4, 2007 in the case of SWNTs produced by the arc discharge, where a separate peak can be observed and can be used to realize the electric property of SWNTs.30 The other peak in the Raman spectrum that has to be realized is the peak in the 1200-1400 cm-1 range (D-band), which indicates that the level of disorder is small, and a weak D-band peak indicates that the amount of amorphous carbon in the as-synthesized SWNTs sample is very small. The small ratio of ID/IG indicates that the prepared sample has less defects in the carbon structure. In this work, high quality SWNTs are synthesized and demonstrated by the Raman spectrum as well as HRTEM observations. The emission current density versus electric field of the SWNTs is shown in Figure 5. For each sample, five J-E curves are measured to verify the reproducibility of the measurement. The field emission properties of our SWNTs are excellent. The electron emission turn-on field Eto defined as producing a current density of 10 µA/cm2 is 0.008 V/µm, and the threshold field Ethr defined as producing a current density of 10 mA/cm2 is 0.07 V/µm. Both Eto and Ethr are much lower than the observed in other studies, as shown in Table 1. These amazing results are mainly contributed to the small diameter of the SWNTs (∼0.6-1.6 nm) demonstrated by HRTEM and the extremely high density of the SWNTs (∼1010-1011/cm2) observed in FESEM. For the issue of electron field emission, there is an important effect which is the screen effect. From the FESEM observation, collections of the SWNTs were found to be clustered in the form of flakes which imply that the SWNTs are very close to each other due to the van der Waals force. For this reason, the screening effect31 must be taken into consideration and the subsequent performance of the FE property must be poor. However, due to the performance of the SWNTs on the FE property, there must be some other effects in this case. Tong et al. shows a great FE performance of the MWCNTs (Eto ) 0.19 V/µm, Ethr ) 0.26 V/µm) and the morphology of which is also flake-like.27 The screening effect seems to be small and/or does not occur. For this reason, the result from Tong et al. was taken as a reference to explain the great performance of the SWNTs. It can be also found from the inset of the Figure 4 that the plot of ln(I/V2) versus 1/V yields straight lines in the region of high voltage and low voltage, which indicates that the emission behavior follows that of the Fowler-Nordheim model and then can be defined as cold electron field emission. It can be used as field emitters for applying extremely small work voltage. Owing to the extremely small voltage applied, the corresponding thermal effect accompanied through the field emission is small. In other words, lower damage caused by thermal effect could enhace the reliability of the device. The electron field emission measurements show SWNTs are suitable to be used as field emitters. Conclusion In this work, high-quality SWNTs are synthesized by pyrolysis of ethanol over an Fe-Mo/MgO catalyst. The SWNTs have a high yield of over 500% relative to Fe-Mo metal in the catalyst. Such producd SWNTs are composed of 80-90% SWNTs, some double-walled CNTs and few MWNTs. There are very small amounts of amorphous carbon materials are synthesized accompanied with SWNTs. The SWNTs have the diameter in the range of 0.6-1.6 for SWNTs in bundles and in the range of 2-6 nm for isolated SWNTs. The extremely small turn-on field Eto ) 0.008 V/µm and threshold field Ethr ) 0.07 V/µm show that the SWNTs have an excellent performance on the electron field emission and can be used for field emission displays (FEDs) by screen printing method.28 Our result

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