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J. Phys. Chem. C 2007, 111, 18888-18891
Synthesis and Characterization of Single-Phase TiC Nanotubes, TiC Nanowires, and Carbon Nanotubes Equipped with TiC Nanoparticles Tomitsugu Taguchi,* Hiroyuki Yamamoto, and Shin-ichi Shamoto Quantum Beam Science Directorate, Japan Atomic Energy Agency, Tokai-mura, Ibaraki-ken 319-1195, Japan ReceiVed: July 20, 2007; In Final Form: October 4, 2007
Single-phase TiC nanotubes were successfully synthesized for the first time, by the reaction of carbon nanotubes (CNTs) with Ti powder at 1300 °C for 30 h. CNTs equipped with TiC nanoparticles and TiC nanowires were synthesized along with these single-phase TiC nanotubes, and the number ratio of single-phase TiC nanotubes to these other products was very small. According to the core and low-energy electron energy loss spectra taken from CNTs and the CNTs reacted with Ti powder, the ratio of Ti to C in TiC nanowire was higher than that in TiC nanotubes and the ratio in CNT equipped with TiC nanoparticles was the lowest. Three different types of TiC nanomaterials, TiC nanowires, TiC nanotubes, and CNTs equipped with TiC nanoparticles, as well as unreacted CNTs, were observed in this study, possibly because the concentration of Ti vapor decreased with increase in depth from the surface of CNT aggregates. A partly single-crystalline TiC nanotube was also observed.
1. Introduction Since the discovery of carbon nanotubes (CNTs) in 1991,1 a significant number of studies have been carried out to prepare new one-dimensional (1-D) nanostructured materials such as nanotubes, nanowires, and nanowhiskers for potential applications. In particular, nanotubes represent an interesting and important class of nanomaterials with wide applications such as electronic devices and drug delivery due to their singular shape and properties. There have been many reports about synthesis of ceramic nanotubes from such compounds as BN, TiO2, MgO, ZnO, AlN, SiO2, VOx, and ZnS.2 There are also reports about synthesis of many nanomaterials such as TiC, NbC, BN, B/C/N, SiO2, and GaN nanostructures2a,b,3 using CNTs as the template materials. The authors reported that singlephase SiC nanotubes were synthesized by the heat treatment of CNTs with Si powder at 1200 °C.4 There have been, however, few reports about synthesis of carbide ceramic nanotubes. The carbide ceramics, in general, have high melting points, excellent resistance to oxidation and corrosion, and excellent mechanical properties.5 In particular, TiC has been used as a dispersion phase and coating layer because TiC has ultrahardness, high elastic modulus, and low density.5 Some researchers have reported the synthesis of TiC nanowires.3a,6 Wong et al. reported about the synthesis of TiC nanorods and CNTs coated with TiC by reacting CNTs with Ti metal and iodine in sealed quartz and the possibility of synthesis of new TiC nanotubes.6b Furthermore, Enyashin and Ivanovskii reported that contrary to the metallic-like crystalline TiC, the TiC nanotubes were semiconducting using a density functional-based tight binding method.7 Up to now, nobody has, however, reported the synthesis of single-phase TiC nanotubes. The objective in this study was, therefore, to synthesize singlephase TiC nanotoubes by the reaction of CNTs with Ti powder. The microstructure observation and characterization of the TiC nanomaterials synthesized in this study were carried out by * Corresponding author. Phone: +81-29-282-5479. Fax: +81-29-2843813. E-mail:
[email protected].
transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and X-ray diffractometry. 2. Experimental Procedures Carbon nanotubes (GSI Creos Corporation, Tokyo, Japan), the template material, and Ti powder (Nilaco Corporation, Tokyo, Japan) were used. The diameter of the CNTs is widely distributed in the diameter range from 50 to 200 nm. The wall thickness of the CNTs is also distributed in the range from 10 to 50 nm. There were no graphitic or amorphous carbon rests on the surface of the CNTs.4 Details of the reaction apparatus are described in our previous reports.4 Both the CNTs on the graphitic foil and the Ti powder were put in a BN crucible. A lid made of BN was put on the crucible. The Ti powder did not contact directly with the CNTs in this study. The crucible was heated at 1300 °C in a vacuum of around 5 × 10-4 Pa for 30 h. The X-ray diffraction measurements were carried out at 40 kV and 20 mA using Cu KR radiation with a step-scanning technique in the θ-2θ mode. The microstructural observation was performed by TEM (model 2000F, JEOL Ltd., Akishima, Japan) operated at 200 kV. A holey-carbon copper grid sample holder was used for TEM observations. The EELS (model 607, Nippon Gatan, Nishi-Tokyo, Japan) analyses were also carried out in order to characterize the materials transformed in this process. 3. Results and Discussion The X-ray diffraction patterns of the CNTs and the CNTs reacted at 1300 °C with Ti powder are shown in Figure 1. The results of X-ray diffraction measurements revealed that TiC was formed from CNTs by this process. The carbon peaks were observed, but no titanium peaks were observed in the reacted material. The TEM microphotographs of CNTs reacted at 1300 °C for 30 h with Ti powder and the electron diffraction patterns of the corresponding selected area are shown in Figure 2. Three
10.1021/jp0756909 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/05/2007
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(
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MTiC FC VTiC ) VC 1+ FTiC MC
Figure 1. X-ray diffraction patterns of carbon nanotubes and the carbon nanotubes reacted at 1300 °C for 30 h with Ti powder.
Figure 2. TEM microphotographs and corresponding selected area electron diffraction patterns of the carbon nanotubes reacted at 1300 °C for 30 h with Ti powder: (a) low-magnification and (b) highmagnification TEM microphotographs of TiC nanowire; (c) lowmagnification and (d) high-magnification TEM microphotographs of the CNTs equipped with TiC nanoparticles; (e) low-magnification and (f) high-magnification TEM microphotographs of single-phase TiC nanotube.
different types of TiC nanomaterials were synthesized: TiC nanowires (Figure 2, parts a and b), the CNTs equipped with TiC nanoparticles (Figure 2, parts c and d), and single-phase TiC nanotubes (Figure 2, parts e and f). A lot of unreacted CNTs were also observed. TEM observation indicated that TiC nanowires were made of concatenated TiC grains of 50-200 nm diameter. In the case of perfect transformation of carbon to TiC, the volume of TiC formed from CNTs, VTiC, is given by the following equation:4b
(1)
where VC, FC, and MC are, respectively, the volume, density, and molecular weight of CNT. Further, FTiC and MTiC are, respectively, the density and molecular weight of TiC. On the basis of the above equation, the reacted nanotubes swell up to approximately 2.4 times their initial volume in transforming all the carbon in the CNT to TiC. Furthermore, the transformed TiC grains grew bigger during heat treatment. The CNTs with thicker walls had the smaller inner cavities. Through the volume swelling of reacted nanotubes, it is easier to fill up the inside cavities of CNTs and thus form the TiC nanowires when the CNTs have thick walls than when they have thin walls. It is, therefore, likely that the existence of CNTs with different wall thicknesses influences the ratio of formation of TiC nanowires to TiC nanotubes. The CNTs equipped with TiC nanoparticles consisted of CNTs and TiC nanoparticles 20-100 nm in diameter. The high-resolution TEM observation of CNTs equipped with TiC nanoparticles revealed that TiC nanoparticles gnawed the wall of CNTs by transforming the carbon in CNTs to TiC. Each TiC nanoparticle was single-crystalline with angular shape. A few single-phase TiC nanotubes were successfully synthesized for the first time as shown in Figure 2, parts e and f. The number ratio of single-phase TiC nanotubes to the other products was, however, very small. The highresolution TEM observation indicated that the crystal orientation of the formed TiC grains was quite random in both the singlephase TiC nanotubes and the CNTs equipped with TiC nanoparticles. The single-phase TiC nanotubes consisted of only fine TiC grains without any carbon phase and were polycrystalline. The core EEL spectra taken from CNT, TiC nanowire, TiC nanotube, and CNT equipped with TiC nanoparticles are shown in Figure 3. The EEL spectra taken from TiC nanowire, TiC nanotube, and TiC nanoparticles had two features, corresponding to the Ti L23 and C K edge. All the EEL spectra in this study had the C K edge. All the C K edges were dominated by wellseparated π and σ peaks. Metal-nonmetal bonds in cubic refractory compounds such as TiC are formed in π-bonds and σ-bonds.8 Therefore, the C K edge taken from TiC had a bigger π peak than that from CNT.8b The low-energy EEL spectra taken from CNT, TiC nanowire, TiC nanotube, and CNT equipped with TiC nanoparticles are shown in Figure 4. The most prominent peak appeared at 2124 eV corresponding to bulk plasmons.8a Every spectrum of TiC showed a second broad peak at 43-45 eV that corresponded to excitations from the Ti 3p level.8a Soto reported that the most prominent peak gradually shifted to lower energy with increase in the ratio of Ti to C in TiCx thin film.8c,d The most prominent peak of TiC nanowire has its maximum at lower energy loss than that of TiC nanotube. Also, the most prominent peak of CNT equipped with TiC nanoparticles appears at the highest energy loss among the TiC products. These results revealed that the ratio of Ti to C in TiC nanowire was higher than that in TiC nanotube and the ratio in CNT equipped with TiC nanoparticles was the lowest. The schematic of the growth mechanism of the TiC nanomaterials synthesized in this study is shown in Figure 5. During the heat treatment at 1300 °C in a high vacuum with Ti powder, TiC phase was formed on the surface of CNTs by reaction with Ti vapor:
C(CNTs) + Ti(vapor) f TiC(s)
(2)
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Figure 3. Electron energy loss spectra of (a) as-received CNT, TiC nanowire, and single-phase TiC nanotube and (b) the CNT equipped with TiC nanoparticles.
Figure 4. Low-loss electron energy loss spectra of (a) as-received CNT, TiC nanowire, and single-phase TiC nanotube and (b) the CNT equipped with TiC nanoparticles.
Figure 5. Schematic of the growth mechanism of TiC nanowires, single-phase TiC nanotubes, and the CNTs equipped with TiC nanoparticles.
The concentration of Ti vapor must be the highest at the surface of CNTs aggregates and then decrease gradually at greater depths into the CNT aggregate, because Ti is progressively consumed through reaction with the carbon in CNTs. At the surface of CNTs aggregates, it would be easy to transform CNTs to TiC by the above reaction because the highest Ti vapor concentration is there. As mentioned above, the reacted nanotubes swelled up to approximately 2.4 times the initial volume if all the carbon in the CNT is transformed to TiC. Furthermore,
the grain growth caused by uniting fine TiC grains with each other might be accelerated due to the existence of excess Ti vapor. A lot of TiC nanowires were, therefore, formed by filling up the inside cavities of CNTs with the TiC grains formed at the surface of CNT aggregates. The concentration of Ti vapor was slightly lower a little below the surface of CNT aggregate than at the surface. However, there still might be Ti vapor enough to transform all the carbon in the CNT to TiC. The inside cavities of CNTs with thick walls were filled with formed TiC grains due to the volume swelling caused by the transforming of carbon to TiC. On the other hand, the inside cavities of CNTs with thin walls were not filled with the transformed TiC grains, the grain growth caused by uniting fine grains together being limited because there was less Ti vapor. Therefore, singlephase TiC nanotubes were synthesized. Also, probably, all the carbon inside the CNT aggregate could not be transformed to TiC because of insufficient Ti vapor. The CNTs equipped with TiC nanoparticles were, therefore, formed probably inside the CNT aggregate. Deeper inside the CNT aggregate, there were many unreacted CNTs since the concentration of Ti vapor was considerably decreased. Because the concentration of Ti vapor decreased with increased depth below the surface of the CNT aggregate, three different types of TiC nanomaterials, i.e., TiC
TiC Nanomaterials
J. Phys. Chem. C, Vol. 111, No. 51, 2007 18891 terials were synthesized: TiC nanowires, single-phase TiC nanotubes, and the CNTs equipped with TiC nanoparticles. TiC nanotubes were successfully synthesized for the first time. The number ratio of single-phase TiC nanotubes to the other products was, however, very small. According to the core and low EELS of CNTs and the CNTs reacted with Ti powder, the ratio of Ti to C in TiC nanowire was the highest and the ratio in TiC nanotube was higher than that of CNT equipped with TiC nanoparticles. Because the concentration of Ti vapor decreased with increasing depth below the surface of the CNT aggregates, three different types of TiC nanomaterials and unreacted CNTs were observed in this study. A partly single-crystalline TiC nanotube was also observed. This result suggests that singlecrystalline TiC nanotubes could be synthesized by appropriately controlling the reaction time, temperature, and vapor concentration in this process and taking into consideration the thickness of CNTs walls in the starting material.
Figure 6. TEM microphotographs and corresponding selected area electron diffraction pattern of partly single-crystalline TiC nanotube: (a and b) low-magnification TEM microphotographs and (c and d) highmagnification TEM microphotographs.
nanowires, TiC nanotubes, and CNTs equipped with TiC nanoparticles, as well as unreacted CNTs, were obtained in this study. A partly single-crystalline TiC nanotube was also observed. Its TEM microphotographs are shown in Figure 6. This nanotube was single-crystal in a region 300 nm in length. The hole of this tube was observed to be connected to the holes in the right and left polycrystalline TiC nanotubes. TEM observation indicated that interplanar d spacing was 0.25 nm, corresponding well with that of the {111} lattice plane spacing of TiC. This partly single-crystalline TiC nanotube grew along the [110] direction. This result corresponded to a previous study about synthesis of single-crystal TiC nanorods by reacting CNTs with Ti metal and iodine in sealed quartz.3a Dai et al. reported that the morphology of single-crystal TiC nanorods, whose axis was [110], was smooth or regular saw-tooth.3a However, the outermost and innermost surface of partly single-crystalline TiC nanotube in this study had irregular nanosize {111} facets. The {111} nanosize faceted areas are shown by the white dotted line in Figure 6b. These results support the possibility of the synthesis of single-crystalline TiC nanotubes by this process. Further studies are needed controlling the reaction time, temperature and vapor concentration and using the CNTs with thin walls. 4. Conclusions The microstructure observation and characterization of CNTs reacted with Ti powder at 1300 °C for 30 h in vacuum were carried out by TEM, EELS, and X-ray diffractometry. The TEM observation revealed that three different types of TiC nanoma-
Acknowledgment. This work was partly supported by Grants-in-Aid for Young Scientist (B) (No. 19760479) from the Ministry of Education, Science, Sports and Culture of Japan. References and Notes (1) Iijima, S. Nature 1991, 354, 56-58. (2) (a) Chopra, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G.; Zettl, A. Science 1995, 269, 966-967. (b) Golberg, D.; Bando, Y.; Mitome, M.; Kurashima, K.; Sato, T.; Grobert, N.; ReyesReyes, M.; Terrones, H.; Terrones, M. Physica B 2002, 323, 60-66. (c) Kasuga, T.; Hiramatsu, M.; Hosono, A.; Sekino, T.; Niihara, K. Langmuir 1998, 14 (12), 3160-3163. (d) Zhan, J.; Bando, Y.; Hu, J.; Golberg, D. Inorg. Chem. 2004, 43 (8), 2462-2464. (e) Xing, Y. J.; Xi, Z. H.; Xue, Z. O.; Zhang, X. D.; Song, J. H.; Wang, R. M.; Xu, J.; Song, Y.; Zhang, S. L.; Yu, D. P. Appl. Phys. Lett. 2003, 83 (9), 1689-1691. (f) Yin, L. W.; Bando, Y.; Zhu, Y. C.; Golberg, D.; Li, M. S. AdV. Mater. 2004, 16 (11), 929-933. (g) Zhang, M.; Bando, Y.; Wada, K. J. Mater. Res. 2000, 15 (2), 387-392. (h) Niederberger, M.; Muhr, H. J.; Krumeich, F.; Bieri, F.; Gunther, D.; Nesper, R. Chem. Mater. 2000, 12, 1995-2000. (i) Wang, X.; Gao, P.; Li, J.; Summers, C. J.; Wang, Z. L. AdV. Mater. 2002, 14 (23), 1732-1735. (3) (a) Dai, H.; Wong, E. W.; Lu, Y. Z.; Fan, S.; Lieber, C. M. Nature 1995, 375, 769-772. (b) Zhang, M.; Bando, Y.; Wada, K.; Kurashima, K. J. Mater. Sci. Lett. 1999, 18, 1911-1913. (c) Zhu, J.; Fan, S. J. Mater. Res. 1999, 14, 1175-1177. (4) (a) Taguchi, T.; Igawa, N.; Yamamoto, H.; Jitsukawa, S. J. Am. Ceram. Soc. 2005, 88 (2), 459-461. (b) Taguchi, T.; Igawa, N.; Yamamoto, H.; Shamoto, S.; Jitsukawa, S. Physica E 2005, 28 (4), 431-438. (5) (a) Carbides: Properties, Production, and Applications; Kosolapova, T. Y., Kalish, H. S., Hausner, H. H., Vaughan, N. B., Eds.; Plenum Press: New York, London, 1971. (b) Hintermann, H. E. Tribol. Int. 1980, 13 (6), 267. (c) Dallaire, S.; Cliche´, G. Surf. Coat. Technol. 1992, 50 (3), 233-239. (6) (a) Liang, C. H.; Meng, G. W.; Chen, W.; Wang, Y. W.; Zhang, L. D. J. Cryst. Growth 2000, 220 (3), 296-300. (b) Wong, E. W.; Maynor, B. W.; Burns, L. D.; Lieber, C. M. Chem. Mater. 1996, 8 (8), 2041-2046. (7) Enyashin, A. N.; Ivanovskii, A. L. Physica E 2005, 30 (1-2), 164168. (8) (a) Martinesz-Martinez, D.; Sanchez-Lopez, J. C.; Rojas, T. C.; Fernandez, A.; Eaton, P.; Belin, M. Thin Solid Films 2005, 472, 64-70. (b) Sanchez-Lopez, J. C.; Martinez-Martinez, D.; Lopez-Cartes, C.; Fernandez-Ramos, C.; Fernandez, A. Surf. Coat. Technol. 2005, 200, 40-45. (c) Soto, G. Appl. Surf. Sci. 2004, 230, 254-259. (d) Soto, G. Appl. Surf. Sci. 2004, 233, 115-122.
