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Effects of high-energy ball milling on the morphology and the field emission property of multi-walled carbon nanotubes
Materials Letters 58 (2004) 3410 – 3413 www.elsevier.com/locate/matlet
Effects of high-energy ball milling on the morphology and the field emission property of multi-walled carbon nanotubes Zhendong Tao a,b,*, Haoran Geng a, Ke Yu c, Zhongxi Yang a,b, Yingzi Wang b a
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, China b School of Materials Science and Engineering, Jinan University, 106 Jiwei Road, Jinan, Shandong Province 250022, PR China c Department of Electron Engineering of East China Normal University, Shanghai 200062, China Received 12 September 2003; received in revised form 29 April 2004; accepted 10 May 2004 Available online 17 August 2004
Abstract The effects of grinding time in a vibration mill on the morphology and the field emission property of multi-walled carbon nanotubes (MWNTs) were studied using XRD, Raman spectra and high-resolution transmission electron microscopy (HRTEM). The results show that the closed ends of MWNTs are partly broken when ground for 4 h. The number of broken ends increases with grinding time. The field emission measurement results indicate that the presence of Fe particles can improve the field emission property of ground MWNTs. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Powder technology; Carbon nanotubes; High-energy ball milling; Structure
1. Introduction Carbon nanotubes have shown promising potential for application in many engineering fields due to their extraordinary electronic and physical properties [1,2]. Generally, the ends of multi-walled carbon nanotubes (MWNTs) are closed. Properties of MWNTs will undergo novel changes if the ends are broken by suitable means. High-energy ball milling is an effective method to crack and to open the closed ends of carbon nanotubes with the help of mechanical force. Li et al. [3] studied the morphology change of the carbon nanotubes by grinding the mixture of carbon nanotubes and Fe nano-powder. It was observed that carbon nanotubes with large aspect ratios could become nanoparticles when ground for 60 min in a shaker mill. Gao et al. [4] obtained carbon nanotubes with saturated Li composition by grinding the mixture of carbon nanotubes and Li powder. Kim et al. [5] investigated the effect of ball milling on the morphology of cup-stacked
* Corresponding author. School of Materials Science and Engineering, Jinan University, 106 Jiwei Road, Jinan, Shandong Province, 250022, PR China. E-mail address: [email protected] (Z. Tao). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.05.045
carbon nanotubes by means of high-resolution transmission electron microscopy (HRTEM), and discovered that the grinding process resulted in the shortening of carbon nanotubes and the formation of nano-barrels that exhibit an increased number of accessible active sites and anisotropy at both ends. In the present study, the morphology, structure change, and the field emission property of the mixture of MWNTs and Fe nano-powder were investigated using high-energy ball milling.
2. Experiments MWNTs with diameters of 20 – 40 nm and Fe powder with the particle sizes of 60 –80 nm were mixed at a weight ratio of 6:5. The mixture was ground in a vibrating mill that used ZrO2 balls with diameters of 6, 4, and 2 mm, respectively, as grinding medium. The effective diameter and effective length of the milling pan was 80 and 90 mm, respectively. The weight ratio of ball/material was 25. The amplitude and the vibration frequency of the vibrating mill were 3 and 50 Hz, respectively. XRD, Raman spectra, HRTEM and field emission measurements were performed on the mixture after grounding for 1, 2, 4, and 8 h,
Z. Tao et al. / Materials Letters 58 (2004) 3410–3413
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respectively. Particle size distributions of the mixture were measured using a laser size meter.
3. Results and discussion The XRD patterns of the samples ground for different times are shown in Fig. 1. It can be seen that in the ground MWNT samples have wider diffraction peaks compared to the control. The longer the grinding time, the wider the diffraction peaks, which means that the amount of amorphous carbon increases with the grinding time. Furthermore, in the left side of the major diffraction peak of Fe in the ground samples, there are several shoulders and the locations of the major diffraction peaks change slightly, the 2h angles of major diffraction peaks of Fe ground for 0, 1, 2, 4, 8 and 16 h are 45.06j, 44.84j, 44.82j, 44.70j, 44.66j and 44.24j. This suggests that the crystal structures of Fe particles may have changed to a certain degree. The amorphism of the MWNTs could be confirmed by Raman spectra shown in Fig. 2. It can be observed that the height of the third peak of ground samples decreases significantly with grinding time and the width of the peak increases simultaneously. Another observation is that the D peak is lower than the G peak for the case of shorter ground time, whereas the D peak is higher than the G peak for the case of longer ground time. Both of the above observations prove that the amount of amorphous structure in ground MWNTs increases compared to the control MWNTs. This is consistent with the results of XRD measurements. High-energy ball milling not only makes part of MWNTs amorphous, but also decreases their particle sizes. Table 1 lists the changes of volume equivalent diameters of MWNTs measured by laser size meter. Evidently, all of the volume equivalent diameters decrease with grinding time. It should be pointed out that the specific surface area of the ground mixtures in this study is smaller than that of normal MWNTs. This is due to the higher density and larger particle sizes of the Fe powder in the mixture. The
Fig. 2. Raman spectra of samples ground for different times: (a) 0 h; (b) 1 h; (c) 2 h; (d) 4 h.
decrease of the volume equivalent diameter means that the aspect ratio of MWNTs also decreases because of the action of the mechanical force during ball milling. In other words, high-energy ball milling shortens the longer MWNTs entangling each other. This proves that some MWNTs are broken and therefore their closed ends become opened during ball milling. The HRTEM photos of the ground samples in Fig. 3 also confirm this result. Another reason causing MWNTs to be broken is the presence of Fe powder in the mixture. It is believed that Fe particles can play a role of grinding medium in the grinding process of MWNTs [3]. In the present study, the average volume equivalent diameters of MWNTs with and without Fe powder ground for 16 h are 0.75 and 1.09 Am, respectively. The HRTEM photos in Fig. 4 indicate smaller aspect ratio of MWNTs and more open ends in the case of the presence of Fe powder. The opened structure of MWNTs provides favorable conditions for modification. For example, the opened cavities could be filled up with some substances such as Se, Cs, etc., which possess lower surface tension and lower melting point [6]. In addition, the exposed inter-layer surface area can significantly increase absorption surface area and consequently enhance the hydrogen storing capability of MWNTs [7,8]. Field emission measurements were performed with diode structure in a vacuum chamber under a pressure of 5 10 5
Table 1 Change of volume equivalent diameters of MWNTs ground for different times
Fig. 1. XRD patterns of samples ground for different times: (a) 0 h; (b) 1 h; (c) 2 h; (d) 4 h.
Sw (m2 g
Grinding time (h)
Volume equivalent diameter (Am) D10
D25
D50
D75
D90
0 1 4 8 16 50
0.45 0.36 0.31 0.25 0.22 0.16
0.61 0.56 0.54 0.48 0.40 0.31
1.05 0.96 0.88 0.81 0.72 0.49
1.88 1.47 1.36 1.28 1.13 0.82
3.15 2.20 2.14 1.98 1.75 1.24
7.67 8.58 8.96 10.90 12.55 15.04
1
)
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Z. Tao et al. / Materials Letters 58 (2004) 3410–3413
Fig. 3. HRTEM photos of ground samples: (a) 0 h; (b) 0.5 h; (c) 1 h; (d) 2 h; (e) 4 h; (f) 8 h.
Fig. 4. HRTEM photos of MWNTs ground for 16 h with and without Fe powder: (a) with Fe; (b) without Fe.
Pa at room temperature. The MWNT film, which was fabricated on nickel substrates (as a cathode) using a screen-printed method, was separated by two Teflon spacers with thickness of 200 Am from a phosphors/ITO/glass anode. The measured emission area was 10 7 mm2. Fig. 5 shows the curves of electron emission current density versus electric field from two MWNT films with and without grinding. Fig. 6 shows the corresponding Fowler – Nordheim (FN) plots. The obtained straight lines indicate that the emission is indeed caused by a vacuum tunneling. The turn-on fields of the two samples with grinding for 8 h and without grinding were estimated to be 2.67 and 4.02 V/ Am, respectively. In this study, the turn-on field is defined as the electric field required to produce a current density of 0.1 AA/cm2. The emission current fluctuations of the sample
Fig. 5. Emission current density versus electric field curves of the MWNTs films with and without grinding.
Fig. 6. Fowler – Nordheim plots.
Z. Tao et al. / Materials Letters 58 (2004) 3410–3413
Fig. 7. Emission images of the CNT films on the fluorescent screen: (a) with grinding; (b) without grinding.
ground for 8 h was within F 4%, and the sample without grinding F 7% when the current density was kept at 350 AA/cm2. The emission images from the MWNTs emitter films with and without grinding are shown in Fig. 7a and b. The light dots of the ground sample emerged more closely and uniformly, compared to the ungrounded one. The emission site density (ESD) could be estimated to be 104/ cm2 from the magnified photograph. It is indicated that ball milling can improve the field emission of the sample. The improvement is probably due to sharpening of the emitter end.
4. Conclusion Under high-energy ball milling, some of the MWNTs experienced structural and morphological changes. XRD
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and Raman spectra measurements confirmed that the amorphous content of MWNTs increases with grinding time. The closed ends of MWNTs can be broken partly and become opened after 4 h of grinding. The resulting cavities can be filled with substances with lower melting point and lower surface tension. At the same time, the exposed interlayer surface area can enhance the hydrogen absorption and storing capabilities of MWNTs. Fe particles in the mixture of MWNTs and Fe powder can play a role in the grinding medium during ball milling. Intensive mechanical action breaks the longer entangling MWNTs into shorter ones that have smaller aspect ratios. At the same time, the presence of Fe particles can improve the field emission property of ground MWNTs.
References [1] T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Nature 382 (1996) 54. [2] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Nature 381 (1996) 678. [3] Y.B. Li, B.Q. Wei, J. Liang, Q. Yu, D.H. Wu, Carbon 37 (1999) 493 – 497. [4] B. Gao, C. Bower, J.D. Lorentzen, L. Fleming, A. Kleinhammes, X.P. Tang, L.E. McNeil, Y. Wu, O. Zhou, Chem. Phys. Lett. 327 (2000) 69 – 75. [5] Y.A. Kim, T. Fukai, M. Endo, T. Yanagisawa, M.S. Dresselhaus, Chem. Phys. Lett. 355 (2002) 279 – 284. [6] Hui-ming Cheng, Carbon Nanotubes: Synthesis, Microstructure, Properties and Applications, Chemistry Industry Press, Beijing, China, 2002, p. 8. [7] H. Imamura, S. Tabata, N. Shigetomi, Y. Takesue, Y. Sakata, J. Alloys Compd. 330 – 332 (2002) 579 – 583. [8] S. Bouaricha, J.P. Dodelet, D. Guay, J. Huot, S. Boily, R. Schulz, J. Alloys Compd. 307 (2000) 226 – 233.
Effects of high-energy ball milling on the morphology and the field emission property of multi-walled carbon nanotubes Zhendong Tao a,b,*, Haoran Geng a, Ke Yu c, Zhongxi Yang a,b, Yingzi Wang b a
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, China b School of Materials Science and Engineering, Jinan University, 106 Jiwei Road, Jinan, Shandong Province 250022, PR China c Department of Electron Engineering of East China Normal University, Shanghai 200062, China Received 12 September 2003; received in revised form 29 April 2004; accepted 10 May 2004 Available online 17 August 2004
Abstract The effects of grinding time in a vibration mill on the morphology and the field emission property of multi-walled carbon nanotubes (MWNTs) were studied using XRD, Raman spectra and high-resolution transmission electron microscopy (HRTEM). The results show that the closed ends of MWNTs are partly broken when ground for 4 h. The number of broken ends increases with grinding time. The field emission measurement results indicate that the presence of Fe particles can improve the field emission property of ground MWNTs. D 2004 Elsevier B.V. All rights reserved. Keywords: Nanomaterials; Powder technology; Carbon nanotubes; High-energy ball milling; Structure
1. Introduction Carbon nanotubes have shown promising potential for application in many engineering fields due to their extraordinary electronic and physical properties [1,2]. Generally, the ends of multi-walled carbon nanotubes (MWNTs) are closed. Properties of MWNTs will undergo novel changes if the ends are broken by suitable means. High-energy ball milling is an effective method to crack and to open the closed ends of carbon nanotubes with the help of mechanical force. Li et al. [3] studied the morphology change of the carbon nanotubes by grinding the mixture of carbon nanotubes and Fe nano-powder. It was observed that carbon nanotubes with large aspect ratios could become nanoparticles when ground for 60 min in a shaker mill. Gao et al. [4] obtained carbon nanotubes with saturated Li composition by grinding the mixture of carbon nanotubes and Li powder. Kim et al. [5] investigated the effect of ball milling on the morphology of cup-stacked
* Corresponding author. School of Materials Science and Engineering, Jinan University, 106 Jiwei Road, Jinan, Shandong Province, 250022, PR China. E-mail address: [email protected] (Z. Tao). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.05.045
carbon nanotubes by means of high-resolution transmission electron microscopy (HRTEM), and discovered that the grinding process resulted in the shortening of carbon nanotubes and the formation of nano-barrels that exhibit an increased number of accessible active sites and anisotropy at both ends. In the present study, the morphology, structure change, and the field emission property of the mixture of MWNTs and Fe nano-powder were investigated using high-energy ball milling.
2. Experiments MWNTs with diameters of 20 – 40 nm and Fe powder with the particle sizes of 60 –80 nm were mixed at a weight ratio of 6:5. The mixture was ground in a vibrating mill that used ZrO2 balls with diameters of 6, 4, and 2 mm, respectively, as grinding medium. The effective diameter and effective length of the milling pan was 80 and 90 mm, respectively. The weight ratio of ball/material was 25. The amplitude and the vibration frequency of the vibrating mill were 3 and 50 Hz, respectively. XRD, Raman spectra, HRTEM and field emission measurements were performed on the mixture after grounding for 1, 2, 4, and 8 h,
Z. Tao et al. / Materials Letters 58 (2004) 3410–3413
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respectively. Particle size distributions of the mixture were measured using a laser size meter.
3. Results and discussion The XRD patterns of the samples ground for different times are shown in Fig. 1. It can be seen that in the ground MWNT samples have wider diffraction peaks compared to the control. The longer the grinding time, the wider the diffraction peaks, which means that the amount of amorphous carbon increases with the grinding time. Furthermore, in the left side of the major diffraction peak of Fe in the ground samples, there are several shoulders and the locations of the major diffraction peaks change slightly, the 2h angles of major diffraction peaks of Fe ground for 0, 1, 2, 4, 8 and 16 h are 45.06j, 44.84j, 44.82j, 44.70j, 44.66j and 44.24j. This suggests that the crystal structures of Fe particles may have changed to a certain degree. The amorphism of the MWNTs could be confirmed by Raman spectra shown in Fig. 2. It can be observed that the height of the third peak of ground samples decreases significantly with grinding time and the width of the peak increases simultaneously. Another observation is that the D peak is lower than the G peak for the case of shorter ground time, whereas the D peak is higher than the G peak for the case of longer ground time. Both of the above observations prove that the amount of amorphous structure in ground MWNTs increases compared to the control MWNTs. This is consistent with the results of XRD measurements. High-energy ball milling not only makes part of MWNTs amorphous, but also decreases their particle sizes. Table 1 lists the changes of volume equivalent diameters of MWNTs measured by laser size meter. Evidently, all of the volume equivalent diameters decrease with grinding time. It should be pointed out that the specific surface area of the ground mixtures in this study is smaller than that of normal MWNTs. This is due to the higher density and larger particle sizes of the Fe powder in the mixture. The
Fig. 2. Raman spectra of samples ground for different times: (a) 0 h; (b) 1 h; (c) 2 h; (d) 4 h.
decrease of the volume equivalent diameter means that the aspect ratio of MWNTs also decreases because of the action of the mechanical force during ball milling. In other words, high-energy ball milling shortens the longer MWNTs entangling each other. This proves that some MWNTs are broken and therefore their closed ends become opened during ball milling. The HRTEM photos of the ground samples in Fig. 3 also confirm this result. Another reason causing MWNTs to be broken is the presence of Fe powder in the mixture. It is believed that Fe particles can play a role of grinding medium in the grinding process of MWNTs [3]. In the present study, the average volume equivalent diameters of MWNTs with and without Fe powder ground for 16 h are 0.75 and 1.09 Am, respectively. The HRTEM photos in Fig. 4 indicate smaller aspect ratio of MWNTs and more open ends in the case of the presence of Fe powder. The opened structure of MWNTs provides favorable conditions for modification. For example, the opened cavities could be filled up with some substances such as Se, Cs, etc., which possess lower surface tension and lower melting point [6]. In addition, the exposed inter-layer surface area can significantly increase absorption surface area and consequently enhance the hydrogen storing capability of MWNTs [7,8]. Field emission measurements were performed with diode structure in a vacuum chamber under a pressure of 5 10 5
Table 1 Change of volume equivalent diameters of MWNTs ground for different times
Fig. 1. XRD patterns of samples ground for different times: (a) 0 h; (b) 1 h; (c) 2 h; (d) 4 h.
Sw (m2 g
Grinding time (h)
Volume equivalent diameter (Am) D10
D25
D50
D75
D90
0 1 4 8 16 50
0.45 0.36 0.31 0.25 0.22 0.16
0.61 0.56 0.54 0.48 0.40 0.31
1.05 0.96 0.88 0.81 0.72 0.49
1.88 1.47 1.36 1.28 1.13 0.82
3.15 2.20 2.14 1.98 1.75 1.24
7.67 8.58 8.96 10.90 12.55 15.04
1
)
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Z. Tao et al. / Materials Letters 58 (2004) 3410–3413
Fig. 3. HRTEM photos of ground samples: (a) 0 h; (b) 0.5 h; (c) 1 h; (d) 2 h; (e) 4 h; (f) 8 h.
Fig. 4. HRTEM photos of MWNTs ground for 16 h with and without Fe powder: (a) with Fe; (b) without Fe.
Pa at room temperature. The MWNT film, which was fabricated on nickel substrates (as a cathode) using a screen-printed method, was separated by two Teflon spacers with thickness of 200 Am from a phosphors/ITO/glass anode. The measured emission area was 10 7 mm2. Fig. 5 shows the curves of electron emission current density versus electric field from two MWNT films with and without grinding. Fig. 6 shows the corresponding Fowler – Nordheim (FN) plots. The obtained straight lines indicate that the emission is indeed caused by a vacuum tunneling. The turn-on fields of the two samples with grinding for 8 h and without grinding were estimated to be 2.67 and 4.02 V/ Am, respectively. In this study, the turn-on field is defined as the electric field required to produce a current density of 0.1 AA/cm2. The emission current fluctuations of the sample
Fig. 5. Emission current density versus electric field curves of the MWNTs films with and without grinding.
Fig. 6. Fowler – Nordheim plots.
Z. Tao et al. / Materials Letters 58 (2004) 3410–3413
Fig. 7. Emission images of the CNT films on the fluorescent screen: (a) with grinding; (b) without grinding.
ground for 8 h was within F 4%, and the sample without grinding F 7% when the current density was kept at 350 AA/cm2. The emission images from the MWNTs emitter films with and without grinding are shown in Fig. 7a and b. The light dots of the ground sample emerged more closely and uniformly, compared to the ungrounded one. The emission site density (ESD) could be estimated to be 104/ cm2 from the magnified photograph. It is indicated that ball milling can improve the field emission of the sample. The improvement is probably due to sharpening of the emitter end.
4. Conclusion Under high-energy ball milling, some of the MWNTs experienced structural and morphological changes. XRD
3413
and Raman spectra measurements confirmed that the amorphous content of MWNTs increases with grinding time. The closed ends of MWNTs can be broken partly and become opened after 4 h of grinding. The resulting cavities can be filled with substances with lower melting point and lower surface tension. At the same time, the exposed interlayer surface area can enhance the hydrogen absorption and storing capabilities of MWNTs. Fe particles in the mixture of MWNTs and Fe powder can play a role in the grinding medium during ball milling. Intensive mechanical action breaks the longer entangling MWNTs into shorter ones that have smaller aspect ratios. At the same time, the presence of Fe particles can improve the field emission property of ground MWNTs.
References [1] T.W. Ebbesen, H.J. Lezec, H. Hiura, J.W. Bennett, H.F. Ghaemi, T. Thio, Nature 382 (1996) 54. [2] M.M.J. Treacy, T.W. Ebbesen, J.M. Gibson, Nature 381 (1996) 678. [3] Y.B. Li, B.Q. Wei, J. Liang, Q. Yu, D.H. Wu, Carbon 37 (1999) 493 – 497. [4] B. Gao, C. Bower, J.D. Lorentzen, L. Fleming, A. Kleinhammes, X.P. Tang, L.E. McNeil, Y. Wu, O. Zhou, Chem. Phys. Lett. 327 (2000) 69 – 75. [5] Y.A. Kim, T. Fukai, M. Endo, T. Yanagisawa, M.S. Dresselhaus, Chem. Phys. Lett. 355 (2002) 279 – 284. [6] Hui-ming Cheng, Carbon Nanotubes: Synthesis, Microstructure, Properties and Applications, Chemistry Industry Press, Beijing, China, 2002, p. 8. [7] H. Imamura, S. Tabata, N. Shigetomi, Y. Takesue, Y. Sakata, J. Alloys Compd. 330 – 332 (2002) 579 – 583. [8] S. Bouaricha, J.P. Dodelet, D. Guay, J. Huot, S. Boily, R. Schulz, J. Alloys Compd. 307 (2000) 226 – 233.