CRYSTAL GROWTH & DESIGN
Synthesis, Characterization, and Growth Mechanism of Tellurium Nanotubes Guangcheng
Xi,†
Yiya
Peng,†
Weichao
Yu,†
and Yitai
2005 VOL. 5, NO. 1 325-328
Qian*,†,‡
Department of Chemistry and Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China Received April 6, 2004;
Revised Manuscript Received August 10, 2004
ABSTRACT: A simple hydrothermal reduction method, employing sodium tellurate (Na2TeO4‚2H2O) as tellurium source and formamide (HCONH2) as a reductant, was used to prepare and investigate tellurium nanotubes. The diameters of the nanotubes range from 200 to 600 nm and their lengths from 4 to 15 µm. Unlike studies reported previously,1,2 a series of electron microscopy characterization results suggests that the growth of tellurium nanotubes under the present experimental conditions is governed by a nucleation-dissolution-recrystallization growth mechanism: sphere-like tellurium nanoparticles initially formed in the hydrothermal system; the sphere-like nanoparticles were gradually dissolved to generate free tellurium atoms in the solution; these tellurium atoms were renewedly transferred onto the surfaces of the sphere-like nanoparticles and evolved into groove-like nanorods; the groove-like nanorods could be grown into tellurium nanotubes eventually. Introduction It is well-known that the shape and size of inorganic nanocrystals have much influence on their physical properties.3,4 Thus, the synthesis of inorganic nanocrystals of controlled size and shape are of special interest. In the past decade, there has been an increasing number of reports on the synthesis of novel colloid nanocrystals such as rods,5 cubes,6,7 prisms,8,9 triangles,10 disks,11 and tubes;12 however, the challenge of synthetically controlling the shape of nanomaterials has been met with limited success. Understanding the growth mechanism and the shape-guiding process of nanocrystals is technologically important in the shapeand size-controlled synthesis of nanocrystals and will make it possible to program the system to yield the nanomaterials with a desired shape and/or size.13 As an example, herein, we discuss the preparation and shape evolution of tellurium nanotubes. Trigonal tellurium (t-Te) and related materials have attracted more and more attention. As a semiconductor, tellurium exhibits a wealth of unique useful properties, such as nonlinear optical responses, photoconductivity, and thermoelectric properties, which result in their potential applications in electronic and opticalelectronic devices.14,15 As those properties within the nanometer regime might be associated with their morphologies16,17 (thus, if trigonal tellurium (t-Te) and related materials were obtained in a nanotubular form), they might act as highly functionalized materials. Recently, Mayers et al. and Mo et al. reported solution-phase approaches to the synthesis of tellurium nanotubes.1,2 Their studies showed that the growth processes of tellurium nanotubes are not alike under different prepared conditions. So it is meaningful to investigate the morphological growth process of tellurium nanotubes prepared under different experimental conditions, which can provide important information to the fields of crystal growth and * Corresponding author. E-mail:
[email protected]. † Department of Chemistry. ‡ Structure Research Laboratory.
design and morphology-controlled synthesis of tellurium nanotubes and other inorganic nanomaterials. In this paper, a hydrothermal preparation of tellurium nanotubes was developed by using sodium tellurate (Na2TeO4‚2H2O) as tellurium source and formamide (HCONH2) as a reductant. To study the growth mechanism of tellurium nanotubes, we have systematically surveyed the growth process of tellurium nanotubes by analyzing the samples at different growth stages, which makes it possible to arrest the tellurium crystals in different stages of their growth. On the basis of experimental observations, a nucleation-dissolutionrecrystallization growth mechanism has been proposed to explore the formation of tellurium nanotubes, which may provide more comprehensive insights for the formation mechanism of nanotubes. Experimental Procedures All the reagents used in the experiment were of analytical purity and were purchased from Shanghai Chemical Reagent Company and used without further purification. In a typical procedure, 0.0005 mol of sodium tellurate (Na2TeO4‚2H2O), 25 mL of NaOH (1 M), and 30 mL of formamide (HCONH2) were added into a Teflon-lined stainless steel autoclave of 60 mL capacity, which gave final concentrations of 0.0091 mol L-1 TeO42-, 0.45 mol L-1 OH- , and 3.66 mol L-1 HCONH2. The autoclave was sealed and maintained at 160 °C for 20 h. After that, the autoclave was allowed to cool to room temperature naturally. It was found that a large quantity of dark gray particles floated on the top of the solution. The dark gray particles were filtered off and washed several times with distilled water and absolute ethanol to remove impurities and then dried in a vacuum at 50 °C for 4 h. The chemical reaction can be formulated as
TeO42- + 3HCONH2 + 4H2O f TeV + 3CO2v + 3NH4- + 5OHX-ray powder diffraction (XRD) pattern of the products was recorded on a Rigaku (Japan) D/max-γA X-ray diffractometer equipped with graphite monochromatized CuK_1 radiation (λ ) 1.54178 Å). The SEM images of the products were examined by a scanning electron microscope (Hitachi X-650) and a field-
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Figure 1. (a) XRD patern of the as-synthesis tellurium nanotubes. (b) XRD pattern of the sphere-like tellurium nanocrystals. emission-scanning electron microscope (JEOL-6300F, 15 kV). The TEM images, SAED pattern, and HRTEM image were recorded on a JEOL 2010 microscope. The samples used for SEM, TEM, and HRTEM characterization were dispersed in absolute ethanol and were ultrasonicated before observation.
Results and Discussion The phase and purity of the as-obtained products were examined by XRD (Figure 1a). All of the reflections in Figure 1a can be readily indexed with a pure trigonal phase (space group: P3121 (152), with infinite, helical chains of tellurium atoms packed parallel to each other along the c-axis) of tellurium with lattice constants a ) 0.4462 nm and c ) 0.5881 nm, which are in agreement with the reported values of a ) 0.4458 nm and c ) 0.5927 nm (JCPDS 36-1452). Compared with the standard pattern of trigonal phase tellurium, the unusually strong (hk0) reflection peaks and weak (hkl) reflection peaks (l * 0) were seen in the XRD pattern, which suggests that the as-synthesized tellurium crystals might have a preferential growth orientation of [001]. This is further demonstrated by SAED and HRTEM studies. The XRD pattern confirms that well-crystallized trigonal tellurium was obtained under current synthetic conditions. The morphology and dimension of the as-prepared products were examined by SEM. From the SEM image of the tellurium nanotubes (Figure 2a), one can see that the panoramic morphology of tellurium powder consists of needlelike crystals that are usually 200-600 nm in diameter and 4-15 µm micrometers in length. Closer inspection demonstrates that they all had tubular structures. Figure 2b gives a magnified view of one of the tubular crystals, revealing that it consists of hollow hexagonal prisms, ca. 240 nm in diameter and ca. 50 nm in wall thickness. Different from Xia’s work,1 the TEM image shown in Figure 2c clearly indicates that the inside of the tellurium nanotube has no block (socalled cylindrical seeds). The SAED pattern inserted in Figure 2c indicates that the tube grew along the [001] direction. An HRTEM image taken from this nanotube is shown in Figure 2d. As seen from the image, this particular nanotube is a structurally uniform single crystal. The observed interplanar spacing is about 0.58
Figure 2. Characterizations of the tellurium nanostructures formed by the hydrothermal process for 20 h. (a) Lowmagnification SEM image of as-prepared tellurium nanotubes. (b) High-magnification SEM image of a tellurium nanotube, showing a tubular hexagonal prism. (c) TEM image and SAED pattern (inset) of an individual tellurium nanotube. (d) HRTEM image of a part of the wall of tellurium nanotube.
nm, which corresponds to the separation between (001) lattice planes of trigonal tellurium. More individual nanotubes have also been examined by this method, and the resulting diffraction patterns suggest that the tellurium nanotubes grow along the [001] direction. All the previous characterizations have confirmed that tellurium nanotubes have been successfully prepared, so how do the nanotubes form? Recently, the concentration depletion at the surfaces of cylindrical seeds mechanism 1 and the helical belt template mechanism2 were used to account for the formation of tellurium nanotubes. However, we do not think the two mechanisms work in the formation of the present nanotubes because our tellurium nanotubes are genuinely hollow without cylindrical seeds inside and there are no tellurium nanobelts observed in the samples. To substantially understand the growth mechanism of the tellurium nanotubes, we have systematically surveyed the growth process of tellurium nanotubes by analyzing the samples at different growth stages. Figure 3a-f shows the SEM images of five samples taken at different stages of the hydrothermal reaction: (a) 5 h, (b) 8 h, (c and d) 12 h, (e) 16 h, and (f) 20 h. These images clearly exhibit the evolution of tellurium nanostructures from nanoparticles to nanotubes over time at 160 °C. The detailed growth process of the tellurium nanotubes may be described as follows. After heating for 5 h, a large amount of black solid product was generated; Figure 3a shows the initial product mainly consisted of spherelike nanoparticles with diameter of 500-1000 nm. XRD pattern of the sphere-like nanoparticles can be indexed as a trigonal phase of tellurium (Figure 1b), indicating the formation of trigonal tellurium crystals through the reduction of Na2TeO4 by formamide. After 8 h reaction, some nanorods growing from the surface of the sphere-
Tellurium Nanotubes
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Figure 4. Crystal structure of trigonal tellurium.
Figure 3. SEM images of five samples collected after hydrothermal treating for (a) 5 h, (b) 8 h, (c and d) 12 h, (e) 16 h, and (f) 20 h.
like nanoparticles can be observed (Figure 3b). From further investigation of a high-magnification image of the nanorods (inset in Figure 3b), it can be seen that the nanorods are not solid nanorods but some groovelike nanorods. The length of these groove-like nanorods is over 2 µm, and the diameter is about 300 nm. Figure 3b clearly demonstrates that, after 8 h reaction, groovelike tellurium nanorods began to grow out from the surfaces of the sphere-like nanoparticles. After 12 h reaction, the product mainly consisted of two different forms of tellurium nanostructures, groove-like rods (∼40%), and sphere-like particles (∼50%) (Figure 3c), indicating many sphere-like nanopaticles had developed into groove-like nanorods. Besides the nanoparticles and the groove-like nanorods, there are small quantities (∼10%) of intermediary nanotubes (Figure 3d) in the sample, which have unclosed segments at the ends of the nanotubes (indicated by an arrow). When the reaction was lengthened to 16 h, relative complete tellurium nanotubes were obtained (Figure 3e). However, the nanotubes still have prongs at the ends. Finally, as the hydrothermal treatment proceeded long enough (20 h), perfect tellurium nanotubes with diameters of 200-600 nm were formed (Figure 3f). Almost no tellurium nanoparticles were observed. On the basis of the experimental observations, we believe that formation of hollow structures can be
rationally expressed as a mechanism of nucleationdissolution-recrystallization. First, when the reduction reaction was processed in the alkaline solution at 160°C, it directly gave t-Te sphere-like crystals instead of a-Te colloids owing to the very slow rate of the nucleation.18 Then, under the present hydrothermal condition, the tellurium sphere-like nanoparticles were gradually dissolved to generate free tellurium atoms in the solution. These tellurium atoms were renewedly transferred onto the surfaces of the sphere-like nanoparticles, and there was an intrinsic tendency to grow into nanorods due to its anisotropic crystal structure.19 Nevertheless, from Figure 3b it can be seen that the nanorods are not solid but groove-like, so how do the groove-like nanorods form? Since the dissolution speed of tellurium in solution processed very slowly, it could not provide enough tellurium atoms for the growth of the growing rodlike crystals. This would lead to undersaturation in the central part of the growing regions of the sphere-like nanoparticles.20 Additionally, the continuous feeding tellurium on the surface of sphere-like nanoparticle could diffuse into two directions: circumferential diffusion and diffusion parallel to the tuber axis ([001]), which will induce tellurium crystals growth along the circumferential direction and the tuber axis direction. Because of the proper anisotropic crystal structure of tellurium, we speculate that the growth rate of the latter is faster than the former. (The tellurium structure is shown in Figure 4, which consists of a spiral chain of atoms with three atoms per turn and corresponding atoms in each chain forming a hexagonal network. The bonds between atoms on the same chain are covalent, whereas between chains they are thought to be mixed interactions of electronic and van der Waals.)21,22 If the growth rate of the former is faster than the latter, closed nanotubes could form on the surfaces of the sphere-like nanoparticles instead of the groove-like nanorods. On the basis of the previous analysis, it can be seen that the formation of groove-like nanorods is rational. Following the incessant dissolution of the sphere-like tellurium nanoparticles, the groove-like nanorods gradually developed into incomplete nanotubes with unclosed segments at the ends (Figure 3d), which further demonstrated that the tuber axis directional growth rate is faster than the circumferential directional growth rate. The growth of the incomplete nanotubes continued by consuming tellurium atoms in the solution. Furthermore, we noted that the two growth rates would tend to balance, following the reduction of the tellurium atoms. Eventually, complete tellurium nanotubes formed (Figure 3f).
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Figure 5. (a) SEM image of tellurium porous microtubes. (b) SEM image of tellurium rodlike microcrystals.
It was noteworthy that many factors such as the reaction temperature and NaOH concentration had a great influence on the growth of tellurium nanotubes. At high temperature (200 °C), Na2TeO4 was rapidly reduced to tellurium by formamide, and the reaction only gave tellurium porous microtubes and particles (Figure 5a). When reaction temperature is lower than 120 °C, however, the reduction reaction cannot be initiated. Therefore, it is important to select an optimum temperature for the formation of tellurium nanotubes. We found that the suitable temperature range is from 160 to 180 °C. By performing the hydrothermal reaction in this temperature range, bulk tellurium nanotubes could be successfully produced. We also investigated the influence of NaOH concentration on the formation of tellurium nanotubes. Keeping other reaction conditions unchanged, we found that the concentration of NaOH in the range of 0.45-0.8 M was favorable for the formation of tellurium nanotubes. At 0.1 M NaOH solution, the product was dominated by tellurium rodlike microcrystals (Figure 5b), and no nanotubes could be detected. As the concentration of NaOH was increased beyond 0.8 M, tellurium nanotubes were still obtained as a major product. However, the quantity of the yield of tellurium nanotubes was lower as compared with that when the concentration of NaOH in the range of 0.45-0.8 M. These results may be rationally explained in chemical reaction, in which the concentration of OH- had a great influence on the redox rate of Na2TeO4 with formide. Apparently, at a higher concentration of OH-, the reduction of Na2TeO4 was more difficult, and this led to the formation of little tellurium nanotubes. At a very low concentration of OH-, the Na2TeO4 could be reduced very rapidly, and this would lead to a very high supersaturation of tellurium atoms in the reaction system; this was unfavorable for the growth of tellurium nanotubes. Conclusion In summary, with formamide as a reductant, single crystalline tellurium nanotubes in high quantity were successfully synthesized via a simple hydrothermal reduction process. The SEM investigation indicated that the growth process of tellurium nanotubes was rationally illustrated by a nucleation-dissolution-recrys-
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tallization growth mechanism. In the process of formation of tellurium nanotubes, sphere-like tellurium nanoparticles initially formed in the hydrothermal system. Then, the sphere-like nanoparticles were gradually dissolved to generate free tellurium atoms in the solution. These tellurium atoms were renewedly transferred onto the surfaces of the sphere-like nanoparticles and evolved into groove-like nanorods. The groove-like nanorods could be grown along the circumferential direction and the tuber axis direction until all spherelike tellurium nanoparticles had been completely consumed, eventually resulting in the formation of nanotubes having well-defined hollow interiors. It should be pointed out that the exact mechanism for the formation of tellurium nanotubes by the method presented herein should be further probed, which is now underway of our work. Importantly, the tellurium nanotubes might be useful as the templates to generate other 1-D functional nanomaterials. Acknowledgment. This work was supported by National Science Foundation of China and the 973 Project of China. References (1) Mayers, B.; Xia, Y. N. Adv. Mater. 2002, 14, 279-282. (2) Mo, M. S.; Zeng, J. H.; Liu, X. M.; Yu, W. C.; Zhang, S. Y.; Qian, Y. T. Adv. Mater. 2002, 14, 1658-1662. (3) Alivisatos, A. P. Science 1996, 271, 933-937. (4) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257-264. (5) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanish, A.; Alivisato, A. P. Nature 2000, 404, 59-61. (6) Sun, Y.; Xia, Y. Science 2002, 298, 2176-2179. (7) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; ElSayed, M. A. Science 1996, 272, 1924-1925. (8) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901-1903. (9) Pastoriza-Santos, I.; Liz-Marzan, L. M. Nano Lett. 2002, 2, 903-905. (10) Yang, J.; Fendler, J. H. J. Phys. Chem. 1995, 99, 55055511. (11) Maillard, M.; Giorgio, S.; Pileni, M.-P. J. Phys. Chem. B 2003, 107, 2466-2470. (12) Li, Y. D.; Li, X. L.; He, R. R.; Zhu, J.; Deng, Z. X. J. Am. Chem. Soc. 2002, 124, 1411-1416. (13) Lee, S.-M.; Cho, S.-N.; Cheon, J. Adv. Mater. 2003, 15, 441444. (14) Kudryavstev, A. A. The Chemistry and Technology of Selenium and Tellurium; Collet’s Ltd.: London, 1974. (15) Tangney, P.; Fahy, S. Phys. Rev. B 2002, 65, 054302054314. (16) Nie, S.; Emory, S. R. Science 1997, 275, 1102-1106. (17) Link, S.; Sayed, M. A. E. L. J. Phys. Chem. B 1999, 103, 8410-8426. (18) Liu, Z. P.; Li, S.; Yang, Y.; Hu, Z. K.; Peng, S.; Liang, J. B.; Qian, Y. T. New J. Chem. 2003, 27, 1748-1752. (19) Mayers, B.; Gates, B.; Yin, Y.; Xia, Y. N. Adv. Mater. 2001, 13, 1380-1384. (20) Tang, Q.; Liu, Z. P.; Li, S.; Zhang, S. Y.; Liu, X. M.; Qian, Y. T. J. Cryst. Growth 2003, 259, 208-214. (21) Lu, J.; Xie, Y.; Xu, F.; Zhu, L. Y. J. Mater. Chem. 2002, 12, 2755-2761. (22) Li, X. L.; Cao, G. H.; Feng, C. M.; Li, Y. D. J. Mater. Chem. 2004, 14, 244-247.
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