Glucose-Assisted Synthesis of CoTe Nanotubes in Situ Templated by Te Nanorods Hai Fan,† Yuanguang Zhang,‡ Maofeng Zhang,† Xuyang Wang,† and Yitai Qian*,† Hefei National Laboratory for Physical Sciences at Microscale and Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026 People’s Republic of China, and Department of Chemistry, Anqing Normal College, Anqing 246011, P. R. China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2838–2841
ReceiVed NoVember 18, 2007; ReVised Manuscript ReceiVed April 6, 2008
ABSTRACT: In this study, a glucose-assisted method has been developed to prepare CoTe nanotubes using Te nanorods as the in situ templates under hydrothermal conditions. It was found that the concentration of glucose has an important influence on the morphology of the products. The average inner diameter of CoTe nanotubes can be adjusted from 60 to 20 nm by changing the quantity of glucose from 0.5 to 1.0 g. The samples are characterized by means of X-ray power diffraction (XRD), field emission scanning electron microscopy (FESEM), X-ray photoelectron spectra (XPS), selected area electron diffraction (SAED), and transmission electron microscopy (TEM). The formation mechanism of the CoTe nanotubes is discussed based on the experimental results. Introduction Since the discovery of carbon nanotubes in 1991, there has been significant interest in synthesizing nanoscale onedimensional (1D) tubular forms of various solids because of the future technological advances in electronics, magnetics, optoelectronics, and memory devices.1 It is known that templating synthesis represents a convenient approach for fabricating 1D tubular nanostructures. The templates used in synthesis can be divided broadly into two classes.2 The first class is called soft templates. Usually, ligands, surfactants, organogelators, and polymers have been widely used as soft templates for nanomaterials. The second class is called hard templates. In solution reactions, hard templates exhibit superior aspects in preparing nanomaterials. By templating against currently existing nanowires (or rods, belts), nanomaterials with designed morphology and dimension can be prepared. For example, CdSe nanotubes can be synthesized by reacting cadmium salts with Se nanowires.3 Jiang et al. converted Se nanowires to Se/Ag2Se nanotubes by irradiating the nanocables by electron beams.4 Using trigonal selenium nanorods as templates, nickel selenide nanotubes were obtained by evaporation to remove Se cores.5 Usually, the unreacted Se cores need to be removed additionally through evaporation or irradiating. Up to now, there are few reports on the synthesis of telluride nanotubes templated by Te. Semiconducting tellurides are being actively researched due to their distinctive properties and now are widely used in many fields. For example, CdTe is a promising material for electronic and optical devices.6 Bi2Te3-based compounds have become major components of the thermoelectric industry.7 FeTe2 has been reported to have excellent magnetic properties.8 Although many nanorods,9 nanowires,10 and nanotubes11–13 of tellurides have been synthesized, up to now, comparatively few studies of cobalt telluride nanocrystals have been reported. Xie Y et al.14 prepared CoTe2 nanorods by the reaction of CoCl2 · 6H2O and tellurium in an autoclave with ethylenediamine as the solvent. Peng et al.15 reported the synthesis of CoTe semiconductor nanocluster wires through a coreduction method by using N2H4 · H2O as the reducing agent. Different from the above * To whom correspondence should be addressed. Fax: 86-551-3607402. E-mail:
[email protected]. † University of Science and Technology of China. ‡ Anqing Normal College.
synthetic routes, we selected ethanolamine as the reducing agent, assisted by glucose, to prepare CoTe nanotubes. To the best of our knowledge, few reports have been reported on the synthesis of CoTe nanotubes. Herein, we prepared CoTe nanotubes by a glucose-assisted method using Te nanorods as the in situ templates under hydrothermal conditions. The average inner diameter of CoTe nanotubes can be adjusted from 60 to 20 nm by changing the quantity of glucose from 0.5 to 1.0 g. These CoTe nanotubes would have potential applications in catalytic, electronic, and magnetic fields.
Experimental Section Chemicals. CoSO4 · 7H2O, TeO2, ethanolamine, and glucose were purchased and used as received without further purification. Synthesis. In a typical procedure for fabricating CoTe nanotubes, CoSO4 · 7H2O (0.8 g), TeO2 (0.5 g), and glucose (0.5 g) were put into a beaker together. Then a suitable volume of deionized water was added into the beaker so as to make powders completely dissolved by continuously stirring. After that, ethanolamine (20 mL) was added into the above solution under magnetic stirring at room temperature. At last the final solution was transferred into a 60-mL Teflon-lined stainless steel autoclave, which was filled with distilled water up to 80% of total volume. The autoclave was sealed and maintained at 180 °C for 30 h, and then allowed to cool to room temperature naturally. The final product was filtered and washed with absolute ethanol and distilled water several times respectively, and dried in a vacuum at 60 °C for 4 h. Characterizations. The phase and the crystallographic structure of the products were determined by X-ray diffraction (XRD) using a Japan Rigaku D/max-γA rotating-anode X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ ) 1.54178 Å). The morphology and size of the products were observed by using field emission scanning electronic microscopy (FESEM), which was performed on a JEOL JSM 6700 scanning electron microanalyzer. Transmission electron microscopy (TEM), taken with a Hitachi H-800 transmission electron microscope, was used to observe the morphologies of the as-prepared products. For the high-resolution transmission electron microscopy (HRTEM) observations, a JEOL-2010 transmission electron microscope was used with an accelerating voltage of 200 kV. Selected area electron diffraction (ED) was also conducted on the microscope. X-ray photoelectron spectra (XPS) were collected by using
10.1021/cg7011364 CCC: $40.75 2008 American Chemical Society Published on Web 07/03/2008
Glucose-Assisted Synthesis of CoTe Nanotubes
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Figure 1. XRD patterns of the CoTe nanotubes synthesized at 200 °C for 16 h. an ESCALab MKII X-ray photoelectron spectrometer with nonmonochromatized Mg KR X-ray as the excitation source.
Results and Discussion Figure 1 shows the XRD pattern of the sample prepared at 200 °C for 16 h; all the diffraction peaks can be readily indexed to the hexagonal CoTe with calculated lattice parameters a ) 3.875, c ) 5.360 Å, which is in good agreement with the literature values (JCPDS 34-0420). The average particle size as calculated by Debye Scherrer formula is 25 nm. No other peaks for impurities were detected. Further evidence for the composition of the product was obtained by XPS, and the XPS spectra of CoTe nanotubes were recorded. Figure 2 shows the XPS spectra obtained from the Co and Te regions of the CoTe nanotubes. There are two major peaks at 781.0 and 797.0 eV (Figure 2a), which are attributed to Co 2p3/2 and Co 2p1/2, respectively.16 The peaks at 368.30 and 572.25 eV shown in Figure 2b corresponded with the binding energies of Te3d3/2 and Te3d5/2, respectively. The quantification of peaks gave a ratio of Co:Te of 14.79:14.22, which is almost consistent with the stoichiometry of CoTe. Figure 3a shows the FESEM image of the as-obtained CoTe sample, which indicates the large yield of the CoTe nanotubes with length of up to 1 µm. Figure 3b shows the representative TEM images of the as-obtained CoTe nanotubes. It is seen that the sample is composed of well-defined CoTe nanotubes with an average outer diameter of 80-120 nm and wall thickness of 10-20 nm. Figure 4a shows the TEM image of an individual nanotube clearly exhibiting a tubular structure. The selected-area electron diffraction (ED) pattern (shown in the inset) confirms that the nanotube is a polycrystalline structure. However, the relatively brighter spots as indexed in the ED pattern can clearly show that the nanotube preferentially grew along the [210] direction and can be indexed as the [01j0] zone axis of the hexagonal CoTe phase. A lattice-resolved HRTEM image in Figure 4b indicates that the observed fringe spacings agree well with the spacings for (100) and (002) planes of CoTe, respectively, further confirming that the nanotubes grew preferentially along the [210] direction. The possible reason is that most of the CoTe nanoparticles that formed based on the Te nanorods have a tendency to attach together along a certain crystalline orientation with their [001] direction perpendicular to the surface of Te nanorods and [210] direction along the nanorods. Therefore,
Figure 2. XPS spectra of CoTe nanotubes: (a) Co region; (b) Te region.
Figure 3. (a) FESEM image of CoTe nanotubes. (b) TEM image of CoTe nanotubes.
the obtained CoTe nanotubes have a tendency to grow along the [210] direction. The concentration of glucose is found to play a key role in the formation of the nanotubes. In the controlled experiment conducted without using glucose and keeping other experimental conditions unchanged, only CoTe nanorods could be obtained (Figure 5a), which indicated that ethanolamine could reduce TeO2 to Te2- and further form CoTe. When the added amount of glucose is 0.5 g, CoTe nanotubes with an inner diameter of about 60 nm could be obtained (Figure 5b). When the quantity of glucose was increased to 1.0 g, the average inner diameter decreased to 20 nm (Figure 5c). When the quantity of glucose was further increased to 2.0 g, CoTe nanorods with a rough surface instead of nanotubes could be prepared (Figure 5d). On the basis of the above experimental results, the optimal
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Figure 4. (a) The TEM image of one CoTe nanotube (the inset is its SAED pattern). (b) The corresponding HRTEM image.
Figure 7. The TEM images of the products obtained at different reaction times with the quantity of glucose at 0.5 g: (a) 1 h; (b) 5 h; (c) 10 h; (d) 16 h.
Scheme 1. Schematic Illustration Describing the Formation of CoTe Nanotubes
Figure 5. TEM images of CoTe products obtained at different quantities of glucose: (a) 0 g, (b) 0.5 g, (c) 1.0 g, (d) 2.0 g.
Figure 6. XRD patterns of the products obtained at different reaction times with the quantity of glucose at 0.5 g: (a) 1 h; (b) 5 h; (c) 10 h. (d) 16 h.
conditions for the formation of CoTe nanotubes is that the quantity of glucose is 0.5-1.0 g, and the average inner diameter of CoTe nanotubes can be adjusted from 60 to 20 nm. In order to study the formation mechanism of the CoTe nanotubes, time-dependent experiments were carried out at different stages with glucose at 0.5 g, and the products we obtained were characterized by XRD and TEM. Figure 6a-d gives the XRD patterns of samples at different reaction times: 1, 5, 10, and 16 h. These XRD patterns indicate the gradually formation of the CoTe phase and disappearance of the Te phase.
When the reaction time is 1 h, only the hexagonal Te phase (JCPDS card number 36-1452) could be obtained (Figure 6a). When the reaction time is 5 and 10 h, both Te and CoTe phases could be found in the XRD patterns (Figure 6b,c). However, the relative intensity of CoTe peaks in Figure 6c is higher than that in Figure 6b. When the reaction time was prolonged to 16 h, pure CoTe phase (JCPDS card number 34-0420) could be obtained. The corresponding TEM images of the samples are shown in Figure 7a-d. When the reaction time is 1 h, Te nanorods with a smooth surface could be obtained (Figure 7a). While the reaction time is 5 h, the surface of the nanorods became rough, and Te/CoTe nanocables were formed. Furthermore, at the end of some nanocables, hollow structures have been observed (Figure 7b). Prolonging the reaction time to 10 h, the nanotubes became the majority (Figure 7c). Finally, almost pure CoTe nanotubes could be formed while prolonging the reaction time to 16 h (Figure 7d). On the basis of the above results, the formation mechanism of CoTe nanotubes in our experiment follows an in situ template-directed synthesis process. The possible schematic diagram is depicted in Scheme 1, which demonstrated three stages in the formation of CoTe nanotubes. First, Te nanorods could be formed when the reaction time is 1 h. Second, with a prolonged reaction time, the newly produced Te on the surface of Te nanorods would be further reduced to Te2- anions.17 These Te2- anions then combined with Co2+ cations in the solution to generate insoluble nanoparticles made of CoTe, which were deposited in situ as a conformal sheath around each Te nanorod to produce a Te/CoTe nanocable structure. These nanoparticles layers were not very compact. Although it would
Glucose-Assisted Synthesis of CoTe Nanotubes
influence the further formation velocity of CoTe to a certain extent, however, Co2+ and Te2- can still go through the CoTe nanoparticles layer freely. Furthermore, because the Te nanorods would be reduced to Te2-, there would be a certain space for CoTe to form inside the CoTe nanoparticles layer due to the high concentration of Te2-; therefore, CoTe can form inside the CoTe nanoparticles layer. Our experimental result indicated that the formation velocity of CoTe could be accelerated by the addition of glucose in the reaction solution, due to the reductive property of glucose. In order to indicate that glucose can also act as reducing agent and have the ability to reduce TeO2 to Te2- in our reaction solutions, we conducted a controlled experiment without ethanolamine while keeping glucose at 0.5 g and the other reaction conditions unchanged. Furthermore, a suitable NaOH solution was added to ensure the reaction solution has the same alkaline conditions. We found that the product obtained was only CoTe2 (JCPDS No. 74-0245) as indexed by XRD (Figure S1, Supporting Information). The above results sufficiently confirm that glucose can act as reducing agent in our experiment. When the concentration of glucose in the solution increased, the formation velocity of CoTe increased correspondingly, which led to increased wall thickness of CoTe layers around Te nanorods before most of the Te2- anions went out of CoTe nanoparticles layer. When the quantity of glucose was increased to 2.0 g, only CoTe nanorods could be obtained, as can be confirmed in Figure 5. Finally, the reaction time was further prolonged, the formation velocity of CoTe with glucose at 0.5 g was relatively slower; therefore, the majority of Te2- would go out of the CoTe nanoparticles layer before CoTe formed. Thus, pure CoTe nanotubes could be obtained after Te was completely reduced. Conclusion In summary, using Te nanorods as the in situ templates under hydrothermal conditions, CoTe nanotubes have been obtained after Te nanorods were removed. In our experiment, glucose was found to have an important influence on the morphology of the products. The average inner diameter of CoTe nanotubes can be adjusted from 60 to 20 nm by changing the quantity of glucose from 0.5 to 1.0 g. The possible mechanism is proposed
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based on our experimental results. This method is simple and easily controllable, which can have wide applications in the preparation of other selenide and telluride nanocrystals. Acknowledgment. This work was supported by National Natural Science Foundation of China (20431020) and the 973 Project of China (2005CB623601). Supporting Information Available: XRD pattern of the product obtained without ethanolamine while keeping the quantity of glucose at 0.5 g. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 125, 353–389. (2) Zeng, H. C. J. Mater. Chem. 2006, 16, 649–662. (3) Jiang, X, C.; Mayers, B.; Herricks, T.; Xia, Y. N. AdV. Mater. 2003, 15, 1740–1743. (4) Jiang, Z. Y.; Xie, Z. X.; Zhang, X. H.; Huang, R. B.; Zheng, L. S. Chem. Phys. Lett. 2003, 378, 313–316. (5) Zhao, A. W.; Xu, L. Q.; Luo, T.; Qian, Y. T. Chem. Lett. 2005, 34, 1136–1137. (6) Kanoun, M. B.; Sekkal, W.; Aourag, H.; Merad, G. Phys. Lett. A 2000, 272, 113–118. (7) Foos, E. E.; Stroud, R. M.; Berry, A. D. Nano Lett. 2001, 1, 693– 695. (8) Liu, A. P.; Chen, X. Y.; Zhang, Z. J.; Jiang, Y.; Shi, C. W. Solid State Commun. 2006, 138, 538–541. (9) Qiu, X. F.; Lou, Y. B.; Samia, A.; Devadoss, A.; Burgess, J.; Dayal, S.; Burda, C. Angew Chem. Int. Ed. 2005, 44, 5855–5857. (10) Kuno, M.; Ahmad, O.; Protasenko, V.; Bacinello, D.; Kosel, T. Chem. Mater. 2006, 18, 5722–5753. (11) Xiao, F.; Yoo, B.; Lee, K.; Myung, N. J. Am. Chem. Soc. 2007, 129, 10068–10069. (12) Tong, H.; Zhu, Y. J.; Yang, L. X.; Li, L.; Zhang, L. Angew Chem. Int. Ed. 2006, 45, 7739–7742. (13) Deng, Y.; Cui, C. W.; Zhang, N. L.; Ji, T. H.; Yang, Q. L.; Guo, L. Solid State Commun. 2006, 138, 111–113. (14) Xie, Y.; Li, B.; Su, H. L.; Liu, X. M.; Qian, Y. T. Nanostruct. Mater. 1999, 11, 539–544. (15) Peng, Q.; Dong, Y. J.; Li, Y. D. Inorg. Chem. 2003, 42, 2174–2175. (16) Yue, G. H.; Yan, P. X.; Liu, J. Z.; Fan, X. Y.; Zhuo, R. F. Appl. Phys. Lett. 2005, 87, 262505. (17) Zhou, B.; Zhao, Y.; Lin, P.; Zhu, J. J. Mater. Chem. Phys. 2006, 96, 192–196.
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