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Thermal Reduction Route to the Fabrication of Coaxial Zn/ZnO Nanocables and ZnO Nanotubes J. Q. Hu, Q. Li, X. M. Meng, C. S. Lee, and S. T. Lee* Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials, City University of Hong Kong, 83, Tat Chee Avenue, Kowloon Tong, Hong Kong, SAR, China Received June 7, 2002. Revised Manuscript Received October 17, 2002
Coaxial Zn/ZnO nanocables and ZnO nanotubes have been fabricated via a thermal reduction route using ZnS powder as the source material. The samples were characterized using X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectrometry. The as-synthesized Zn/ZnO nanocables consisted of a metallic core (Zn) ≈50 nm in diameter and a semiconductor outer shell (ZnO) ≈5 nm in thickness and several micrometers in length. A good epitaxial relationship between the Zn core and ZnO shell was observed, and misfit dislocations were observed at the Zn/ZnO interface, which accommodated the relatively large lattice mismatch. The outer diameter and wall thickness of the ZnO nanotubes are ≈60 and ≈10 nm, respectively. The possible formation mechanisms for the Zn/ZnO nanocables and ZnO nanotubes are discussed.
1. Introduction Since the discovery of carbon nanotubes,1 onedimensional (1D) nanostructured materials have attracted much attention.2-7 Recently, the unusual electric properties of the 1D nanostructures have aroused great interest in the nanomaterials community.8-13 If a layered structure of semiconducting, insulating, or metallic materials in the radial direction can be achieved in the 1D nanostructure, the utility of these materials may be further enhanced by combining uniform electronic properties in the axial direction.14 This idea has prompted the synthesis of coaxial-layered nanostructures, namely, nanocables, in which the component phases are separated in the radial direction. Nanocables are expected to have great technological potential in fabricating nanometer-scale electronic devices possessing various interesting functions.15 Zhang et al.15,16 have * To whom correspondence should be addressed. E-mail: apannale@ cityu.edu.hk. (1) Iijima, S. Nature 1991, 354, 56. (2) Dai, H. J.; Wong, E. W.; Lu, Y. Z.; Fan, S. S.; Lieber, C. M. Nature 1995, 375, 769. (3) Yang, P. D.; Lieber, C. M. Science 1996, 273, 1836. (4) Han, W. Q.; Fan, S. S.; Li, Q. Q.; Hu, Y. D. Science 1997, 277, 1287. (5) Cheng, G. S.; Zhang, L. D.; Zhu, Y.; Fei, G. T.; Li, L.; Mo, C. M.; Mao, Y. Q. Appl. Phys. Lett. 1999, 75, 2455. (6) Shi, W. S.; Peng, H. Y.; Zheng, Y. F.; Wang, N.; Shang, N. G.; Pan, Z. W.; Lee, C. S.; Lee, S. T. Adv. Mater. 2000, 12, 1343. (7) Yin, A. J.; Li, J.; Jian, W.; Bennett, A. J.; Xu, J. M. Appl. Phys. Lett. 2001, 79, 1039. (8) Alivisatos, A. P. Science 1996, 271, 933. (9) Wang, Q. H.; Setlur, A. A.; Lauerhaas, J. M.; Dai, J. Y.; Seeling, E. W.; Chang, R. P. H. Appl. Phys. Lett. 1998, 72, 2912. (10) Garg, A.; Han, J.; Sinnott, S. B. Phys. Rev. Lett. 1998, 81, 2260. (11) Wang, Z. L.; Dai, Z. R.; Gao, R. P.; Bai, Z. G.; Gole, J. L. Appl. Phys. Lett. 2000, 77, 3349. (12) Li, Y.; Meng, G. W.; Zhang, L. D.; Phillipp, F. Appl. Phys. Lett. 2000, 76, 2011. (13) Arias, R.; Mills, D. L. Phys. Rev. B 2001, 63, 134439. (14) Szuromi, P. Science 1998, 281, 881. (15) Zhang, Y.; Suenaga, K.; Colliex, C.; Iijima, S. Science 1998, 281, 973. (16) Zhang, Y.; Gu, H.; Suenaga, K.; Iijima, S. Chem. Phys. Lett. 1997, 279, 264.
synthesized coaxial nanocables consisting of a silicon carbide core, an amorphous silicon dioxide layer, and an outer sheath of carbon and boron nitride using reactive laser ablation. Shi et al.17 have obtained similar nanocables with a silicon core, silicon dioxide interlayer, and carbon outer shell by combining laser ablation and thermal evaporation. Suenaga et al.18,19 have also prepared a three-layer coaxial nanocable with a carbon core, boron nitride interlayer, and carbon sheath by an arc discharge method. Several other two-layer nanocables have also been fabricated, including SiC/SiO2 (SiC-cored) nanocables by carbothermal reduction20 and CdSe/polymer (CdSe-cored) nanocables by the γ-irradiation method.21 Efforts have also been devoted to encapsulate foreign materials inside the hollow cavities of carbon nanotubes to produce new nanocables or nanowires. Various metals and compounds have been successfully filled inside carbon nanotubes by chemical insertion, physical insertion, in situ arc encapsulation, and so on.22 So far, all of the existing nanocables are composed of a semiconducting core and a semiconducting or insulting outer shell, and normally no specific orientation relationship is observed at the core/shell interface. (17) Shi, W. S.; Peng, H. Y.; Xu, L.; Wang, N.; Tang, Y. H.; Lee, S. T. Adv. Mater. 2000, 12, 1927. (18) Suenaga, K.; Colliex, C.; Demoncy, N.; Loiseau, A.; Pascard, H.; Willaime, F. Science 1998, 278, 652. (19) Suenaga, K.; Willaime, F.; Loiseau, A.; Colliex, C. Appl. Phys. A: Mater. Sci. Process. 1999, 68, 301. (20) Meng, G. W.; Zhang, L. D.; Mo, C. M.; Zhang, S. Y.; Qin, Y.; Feng, S. P.; Li, H. J. J. Mater. Res. 1998, 13, 2533. (21) Xie, Y.; Qiao, Z. P.; Chen, M.; Liu, X. M.; Qian, Y. T. Adv. Mater. 1999, 11, 1512. (22) (a) Ajayan, P. M.; Stephan, O.; Redlich, P.; Colliex, C. Nature 1995, 375, 564. (b) Ugarte, D.; Chatelain, A.; Deheer, W. A. Science 1996, 274, 1897. (c) Guerret-Pie´court, C.; Bouar, Y. Le.; Loiseau, A.; Pascard, H. Nature 1994, 372, 159. (d) Tsang, S. C.; Chen, Y. K.; Harris, P. J. F.; Green, M. L. H. Nature 1994, 372, 159. (e) Terrones, M.; Hsu, W. K.; Schilder, A.; Terrones, H.; Grobert, N.; Hare, J. P.; Zhu, Y. Q.; Schwoerer, M.; Prassides, K.; Kroto, H. W.; Walton, D. R. M. Appl. Phys. A: Mater. Sci. Process. 1998, 66, 307. (f) Sloan, J.; Cook, J.; Green, M. L. H.; Hutchison, J. L.; Tenne, R. J. Mater. Chem. 1997, 7, 1089.
10.1021/cm020649y CCC: $25.00 © 2003 American Chemical Society Published on Web 12/05/2002
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In the present work, coaxial Zn/ZnO nanocables and ZnO nanotubes were fabricated via a thermal reduction route using ZnS powder as the source material. The assynthesized cables consisted of a metallic core (Zn) and a semiconducting outer shell (ZnO). A good epitaxial relationship between the Zn core and ZnO shell was observed, and misfit dislocations were detected at the Zn/ZnO interface, which accommodated the relatively large lattice mismatch (17%). The formation of the Zn/ ZnO nanocables is believed to involve a VS growth mechanism, while the ZnO nanotubes are formed by thermal evaporation of the Zn core of the Zn/ZnO nanocables during the annealing process. Figure 1. XRD pattern of the synthesized product.
2. Experimental Procedure The synthesis of Zn/ZnO nanocables and ZnO nanotubes were carried out in a high-temperature tube furnace.6,17 Briefly, an alumina tube was mounted inside a high-temperature tube furnace. Two grams of analytical-grade ZnS (Goodfellow, 98+%) powders were placed on an alumina wafer. After placing the wafer at the center of the tube, the tube was sealed and evacuated by a mechanical rotary pump to a pressure of 6 × 10-2 Torr. The furnace was heated at a rate of 10 °C/min to 800 °C for 30 min, and then further to 1300 °C for 8 h. During the experiment, a constant flow of Ar mixed with 5% H2 was introduced at a flow rate of 50 sccm through the tube, and the tube pressure was maintained at 300 Torr by pumping. The deposited product was characterized and analyzed by scanning electron microscopy (SEM; Philips XL 30 FEG), powder X-ray diffraction (XRD; Siemens D-500 with Cu KR radiation), transmission electron microscopy (TEM; Philips, CM20 and CM200 FEG, at 200 kV) and energy-dispersive X-ray spectrometry (EDS; equipped on a CM20 TEM).
Figure 2. SEM image of the general morphology of the synthesized product.
3. Results and Discussion The resulting product, which appeared like a piece of gray-black wool, was deposited on the inner wall of the tube downstream, after the furnace was cooled to room temperature. A high yield (400 mg for 8-h deposition) of the product was achieved. A powder X-ray diffraction (XRD) pattern of the product is shown in Figure 1. The peaks marked with “c” can be indexed as the hexagonal structured Zn with lattice constants a ) 2.662 Å and c ) 4.945 Å, and the peaks marked with “s” can be identified as the wurtzite-structured ZnO with lattice constants a ) 3.250 Å and c ) 5.206 Å. These values are in accordance with the JCPDS data within experimental error (the lattice constants for Zn and ZnO from JCPDS files are a ) 2.665 and c ) 4.947 Å and a ) 3.2498 and c ) 5.2066 Å, respectively). No characteristic peaks corresponding to the starting material ZnS are detected. The scanning electron microscopy (SEM) image shown in Figure 2 gives a general view of the morphology of the as-prepared product. The product is observed as entangled and curved wire-like structures several micrometers in length. On the basis of the XRD and SEM results, the as-synthesized product was a composite material of Zn and ZnO with wire-like morphology. Detailed microstructures of the product will be further investigated using transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive X-ray spectroscopy (EDS) attached to the TEM. Figure 3 shows one typical TEM image of the prepared wire-like product (the inset showing a high-
Figure 3. TEM image of the prepared Zn/ZnO nanocables (the inset showing a high-magnification TEM image of a segment of single cable).
magnification TEM image of a segment of a single wire). The different contrast (a thin sheath with a lighter contrast formed outside the surface) observed along the direction normal to the “wire” axis suggests a layered structure with different phase composition along the radial or lateral direction. The product is not a monolithic “nanowire”, but a wrapped composite nanowire, so-called “coaxial nanocable”, which consists of a central core and an outer shell. The diameter of the central core and the thickness of the outer shell are ≈50 and ≈5 nm, respectively. Figure 4a-c shows the other typical morphologies of the final product. The clear contrast (the outer part is darker than the inner part of these wirelike structures) shown in Figure 4b suggests some of the wire-like nanostructures display a hollow internal core along the length, assuming the form of nanotubes. Figure 4c shows a single nanostructure that consists of a junction formed between a nanotube and a nanocable (the nanotube was connected with the nanocable). The
Coaxial Zn/ZnO Nanocables and ZnO Nanotubes
Figure 4. (a) A low-magnification TEM image of the synthesized ZnO nanotubes, (b) a high-magnification TEM image of a single nanotube, and (c) a TEM image showing a segment of the nanotube connected with a segment of the nanocable.
Figure 5. HRTEM image recorded from a single Zn/ZnO nanocable exhibiting the epitaxial relationship between the Zn core and ZnO shell (misfit dislocations are marked with arrows) and its SAED pattern (inset) of the nanocable corresponding to the Zn [011] zone axis and ZnO [011] zone axis.
TEM image reveals that the wall of the tube and the sheath layer of the cable have the same thickness and a similar contrast. It suggests that the nanotube is the extension of the outer shell of the coaxial nanocable. Figure 5 shows one of the many high-resolution TEM images taken from one single nanocable. The large number of high-resolution TEM images of single synthesized nanocables confirmed that the core and the shell were both well-defined crystalline materials and the absence of any amorphous materials at the interface. The d-spacing (marked in the Figure 5) of the core
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material has a measured value of 0.23 nm, which agrees well with the (100) spacing of wurtzite-Zn, while the d-spacing of the outer shell material is measured to be at 0.28 nm, which agrees with the (100) spacing of wurtzite-ZnO. These values are consistent with the X-ray results, revealing the existence of both Zn and ZnO. A good epitaxial relationship between the Zn core and ZnO shell is clearly revealed by the high-resolution TEM image. A regular array of misfit dislocations (marked with arrows) with an average repeating distance corresponding to five (100) ZnO planes (or six (100) Zn planes) is observed at the Zn/ZnO interface. These dislocations possibly serve to accommodate the relatively large lattice mismatch between Zn and ZnO (17.99%). The interface regions between two adjacent misfit dislocations are smooth and coherent, and no amorphous material exists at the interface. The insetting SAED patterns taken from the same nanocable show two sets of diffraction spots, which correspond to those from the Zn [011] zone axis and ZnO [011] zone axis, respectively. An epitaxial relationship between the Zn and ZnO, as revealed by the high-resolution TEM image, is also suggested by the SAED pattern, that is, h 1)Zn//(11 h 1)ZnO. (100)Zn//(100)ZnO; (11 To further confirm the composition of the prepared products, EDS spectra were recorded with an electron beam probe size of ≈2 nm. The spectra shown in parts a and b of Figure 6were obtained, respectively, from the outer shell and the core along the radial direction of a single nanocable. They both reveal the presence of Zn and O (Cu and C signals come from TEM grids). However, quantitative analysis shows that the oxygen content of the sheath is much higher than that of the core. This suggested that the sheath consisted of ZnO and the core was Zn. The spectrum shown in Figure 6c is recorded from a single nanotube, which indicated the presence of the Zn and O with an approximate atomic ratio of 1:1.17. So we can conclude that the synthesized nanotubes are ZnO nanotubes. In our experiment, the starting material ZnS powder was placed at the center of the tube, while the resulting Zn/ZnO nanocables are always found downstream of the tube near the coldfinger. This suggests that the growth of the nanocables involves the formation and transport of the vapor; otherwise, the nanocables will nucleate and grow directly on the alumina wafer. In fact, the formation of nanocables may originate from the reduction of ZnS powder by hydrogen (from the processing gas) at high temperatures (around 1300 °C). Considering the low boiling point (911 °C) of metallic Zn, the reduced zinc should be in the vapor state due to the high processing temperature. The vaporized Zn is then carried by the processing gas and deposits as Zn nanoparticles through homogeneous nucleation at a low-temperature region in the tube. It has been widely recognized that the surface activity of nanoparticles can be much higher than that of the bulk materials. These Zn nanoparticles thus serve as energetically favorable sites, which will enhance atomic absorption and diffusion, and further direct the one-dimensional growth of the Zn nanowires. As the reduction proceeds, Zn vapor is continually generated to fuel further growth of Zn nanowires along the axis. During the growth process, the surface of the as-grown Zn nanowires would be slightly oxidized and covered by a thin layer of ZnO. The oxygen in the ZnO shell might originate from the residual oxygen in the
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mechanism appears to provide a reasonable explanation of the growth of the nanocables. In this mechanism, the liquid-forming agents (or so-called catalysts) are not required for the growth of one-dimensional structures.25 It is well-known that Zn and ZnO both have the hexagonal (hcp) structure. From the growth kinetics point of view, when ZnO is formed, rearrangement of the sublattices of zinc from hcp (Zn) to hcp (ZnO) has to occur only at the Zn reaction front. Thus, the nucleating ZnO grain has a template to follow the orientation of Zn exactly. This results in the formation of a hexagon on a hexagon and the epitaxial relationship between the Zn core and the ZnO shell in the cable. At the same time, the large number of misfit dislocations at the Zn/ ZnO interface will impede the progress of the Zn reaction front. This will also contribute to the formation of the Zn/ZnO nanocable. Regarding the formation of ZnO nanotubes, it is believed that they are formed by the thermal evaporation of the Zn core of the Zn/ZnO nanocable during the annealing process. Since the surface melting temperature of nanoparticles can be much lower than that of their bulk materials,26 the materials at the cored Zn nanowire tips (similar to the case of nanoparticles) may be in or near their molten states (Zn: mp 410 °C). This would lead to evaporation of the Zn core out of the nanocable, whereas the ZnO shell with a high melting point (ZnO: mp 1975 °C) would remain intact. As a result, a partial hollow cavity with the ZnO sheath, namely, ZnO nanotube, was formed. 4. Conclusion
Figure 6. EDS spectra from the sheath (a) and the core (b) along the radial direction of a single nanocable and a single nanotube (c), respectively.
alumina tube and the alumina wafer at high growth temperatures. Due to the presence of hydrogen and high vapor pressure of Zn in the system, the axial growth of Zn nanowires will be more dominant compared to the surface oxidation of Zn nanowires. Otherwise, extensive ZnO formation would inhibit the wire growth and result in the formation of Zn particles covered with ZnO. The formation of the ZnO shell will stop the oxygen penetration and further oxidation of the Zn nanowire core. Consequently, the formed ZnO shell will retard the lateral growth of the Zn nanowire. On the basis of the above discussion, it may be expected that all growth conditions will promote the formation of the nanocable structure. In the present case, no evidence of the existence of metallic catalyst particles was observed on any tips of the synthesized nanocables, as shown in Figures 3 and 4. The growth of the nanocables appears not to follow the vapor-liquid-solid (VLS) growth mechanism.24 Rather, the vapor-solid (VS) growth (23) Bai, Z. G.; Yu, D. P.; Zhang, H. Z.; Ding, Y.; Wang, Y. P.; Gai, X. Z.; Hang, Q. L.; Xiong, G. C.; Feng, S. Q. Chem. Phys. Lett. 1999, 303, 311.
Coaxial Zn/ZnO nanocables and ZnO nanotubes were fabricated via a thermal reduction route using ZnS powder as the source material. The as-synthesized nanocables consisted of a metallic core (Zn) of about 40 nm in diameter and a semiconductor outer shell (ZnO) of about 5 nm in thickness. A good epitaxial relationship between the Zn core and ZnO shell was observed, and misfit dislocations were detected at the Zn/ZnO interface. These dislocations served to accommodate the strain caused by the large lattice mismatch (17%). The outer diameter of the ZnO nanotubes is ≈60 nm with a wall thickness of ≈10 nm. The formation of the Zn/ZnO nanocables is believed to involve a VS growth mechanism. The ZnO nanotube was supposed to form from the Zn/ZnO nanocable by thermal evaporation of its Zn core during an annealing process. Compared to others, the present nanocables have a unique structure that consists of a metallic core and semiconducting sheath having a specific epitaxial relationship between them. The synthesis method in the current study is simple and the final products are free of external catalyst contamination. Acknowledgment. The work was supported by a grant from the Research Grants Council of the Hong Kong SAR, China [Project No. CityU 3/01C (8730016)] and a Strategic Research Grant of the City University of Hong Kong [Project No. 7001175]. CM020649Y (24) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 89. (25) Klimovskaya, A. I.; Ostrovskii, I. P.; Ostrovskaya, A. S. Phys. Status Solidi 1996, A153, 465. (26) Lee, S. T.; Wang, N.; Zhang, Y. F.; Tang, Y. H. J. Mater. Res. 1999, 24, 36.
