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A Green Chemical Approach to the Synthesis of Tellurium Nanowires Qingyi Lu, Feng Gao, and Sridhar Komarneni* Materials Research Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 Received March 4, 2005. In Final Form: April 12, 2005 Starch, an economical and safe carbohydrate, has been found to be not only an effective reducing agent but also a new morphology-directing agent for the synthesis of tellurium nanowires using commercial H2TeO4 precursor. The obtained tellurium nanowires are of single-crystal in nature, with an average diameter of ∼25 nm and length up to 10 µm. A possible synthetic mechanism involves the chain-shaped bioorganic molecule acting as a template for the one-dimensional growth of inorganic tellurium. The effects of different chain-shaped structures and concentrations of biomolecules on the nanowire morphology have been investigated and different one-dimensional structures, including thick rods, short nanowires, bunched nanowires, and assembled spikelet structures, have been fabricated. These experimental results have been found to be useful in substantiating the proposed synthetic mechanism.
Introduction Environmental protection is an essential issue for the chemical industry. It is well-known that to prevent waste is better than to treat it or clean it up after it has been created.1 Thus, it is important to design synthetic methods that will prevent or decrease waste. The concept of “green” chemistry aims to use and/or generate substances that possess little or no toxicity to people or the environment. The above concept is becoming one of the main goals of developing new techniques or improving current approaches.2,3 Nanomaterials, especially one-dimensional (1D) nanostructures, have distinct properties and are of great importance in understanding fundamental concepts and fabricating nanodevices.4-6 Scientists are touting these nanostructures to bring us a brand-new and fantastic world.7-9 So far, many effective approaches have been developed for the synthesis of nanomaterials, such as decomposition of metallorganic compounds for uniform nanowires,10,11 surfactant directing route for the shape control of nanoparticles,12-14 and chemical reduction for * Corresponding author. Phone: 814-865-1542. Fax: 814-8652326. E-mail:
[email protected]. (1) Devito, S. C.; Garett, R. L. Designing Safer Chemicals: Green Chemistry for Pollution Prevention; American Chemical Society: Washington, DC, 1996. (2) Anderson, D.; Anthony, J. L.; Chanda, A.; Denison, G.; Drolet, M.; Fort, D.; Joselevich, M.; Whitfield, J. R. Green Chem. 2004, 6, G5. (3) Anastas, P. T.; Warne, J. C. Green Chemistry: Theory and Practice; Oxford University Press: Oxford, UK, 1998. (4) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (5) Peng, X. G. Adv. Mater. 2003, 15, 459. (6) Rao, C. N. R.; Govindaraj, A.; Deepak, F. L.; Gunari, N. A.; Nath, M. Appl. Phys. Lett. 2001, 78, 1853. (7) Huang, M. H.; Mao, S.; Feick, F.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (8) Wang, Z. L.; Gao, R. P.; Gole, J. L.; Stout, J. D. Adv. Mater. 2000, 12, 1938. (9) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (10) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (11) Dinega, D. P.; Bawendi, M. G. Angew. Chem. 1999, 111, 1906; Angew. Chem., Int. Ed. 1999, 38, 1788. (12) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (13) Xiong, Y. J.; Xie, Y.; Li, Z. Q.; Li, X. X.; Gao, S. M. Chem. A Eur. J. 2004, 10, 654.
metal and semimetal nanomaterials with NaBH4,15,16 ethylene glycol,17 ethylenediamine,18 hydrazine,19,20 or hydrogen21 as reducing agents. Developing environmentally friendly methods to synthesize nanomaterials, especially 1D nanostructures, is still a challenge for chemists and materials scientists. Much current research has been directed toward the green synthesis of organic compounds,22 while a relatively limited amount of work has been done toward the green synthesis of inorganic nanomaterials. Recently, Wallen et al. reported the green synthesis of silver nanoparticles,23 and our group proposed an alginic acid-assisted method for tellurium nanowires.24 Elemental tellurium, as a semiconductor, exhibits a unique combination of many interesting and useful properties,25 such as a narrow band gap (∼0.35 eV),26 an effect of ultrafast electronic excitation on the A1 phonon frequency,27 a piezoelectric effect even stronger than that of quartz or CdS,28 and a high reactivity toward a wealth of chemicals that can be exploited to convert tellurium into other functional materials, such as CdTe29 and (14) Carswell, A. D. W.; O’Rear, E. A.; Grady, B. P. J. Am. Chem. Soc. 2003, 125, 14793. (15) Adhyapak, P. V.; Karandikar, P.; Vijayamohanan, K.; Athawale, A. A.; Chandwadkar, A. J. Mater. Lett. 2004, 58, 1168. (16) Scott, R. W. J.; Ye, H. C.; Henriquez, R. R.; Crooks, R. M. Chem. Mater. 2003, 15, 3873. (17) Komarneni, S.; Li, D. S.; Newalkar, B.; Katsuki, H.; Bhalla, A. S. Langmuir 2002, 18, 5959. (18) Gao, Y. H.; Niu, H. L.; Zeng, C.; Chen, Q. W. Chem. Phys. Lett. 2003, 367, 141. (19) Li, Y. D.; Wang, J. W.; Deng, Z. X.; Wu, Y. Y.; Sun, X. M.; Yu, D. P.; Yang, P. D. J. Am. Chem. Soc. 2001, 123, 9904. (20) Cao, M. H.; Hu, C. W.; Wang, Y. H.; Guo, Y. H.; Guo, C. X.; Wang, E. B. Chem. Commun. 2003, 1884. (21) Wu, Y.; Cui, Y.; Huynh, L.; Barrelet, C. J.; Bell, D. C.; Lieber, C. M. Nano Lett. 2004, 4, 433. (22) Thanh, G. V.; Pegot, B.; Loupy, A. Eur. J. Org. Chem. 2004, 5, 1112. (23) Raveendran, P.; Fu, J.; Wallen, S. L. J. Am. Chem. Soc. 2003, 125, 13940. (24) Lu, Q. Y.; Gao, F.; Komarneni, S. Adv. Mater. 2004, 16, 1629. (25) Liu, Z. P.; Hu, Z. K.; Xie, Q.; Yang, B. J.; Wu, J.; Qian, Y. T. J. Mater. Chem. 2003, 13, 159. (26) Anzin, V. B.; Eremets, M. I.; Kosichkin, Y. V.; Nadezhdinskii, A. I.; Shirokov, A. M. Phys. Status Solidi A 1977, 42, 385. (27) Tangney, P.; Fahy, S. Phys. Rev. B 2002, 65, 54302. (28) Araki, K.; Tanaka, T. Jpn. J. Appl. Phys. 1972, 11, 472. (29) Prieto, A. L.; Sander, M. S.; Martin-Gonzalez, M. S.; Gronsky, R.; Sands, T.; Stacy, A. M. J. Am. Chem. Soc. 2001, 123, 7160.
10.1021/la050594p CCC: $30.25 © 2005 American Chemical Society Published on Web 05/14/2005
Green Synthesis of Tellurium Nanowires
HgCdTe.30 It has been widely used to make thermoelectronics, photoconductors, and highly resistive and piezoelectronic devices.31-33 The availability of tellurium nanostructures with low dimensionality would no doubt bring new types of applications or enhance the performance of the currently existing devices.25 So far, tellurium nanowires have been synthesized by a hydrazine reducing route,31 Na2SO3 reducing (NH4)2TeS4 approach with sodium dodecyl benzenesulfonate assistance,25 and an acid/baseassisted disproportionation method with and without polymer surfactant poly(vinyl pyrrolidone).32,33 These methods are effective to produce tellurium nanorods or nanowires with harmful or dangerous chemicals as reducing agents or additions, which might bring difficulties to the equipment design and the product posttreatment. Starch is a polymer of glucose, the most abundant polysaccharide stored in plants, and has been widely used as a food source.34,35 This bioorganic molecule is an economical, safe, and green chemical. Its structure has been well-studied and confirmed to be chain-shaped with hydroxyl (-OH) on the surface.34,35 This 1D chain-shaped structure might be useful for the synthesis of 1D nanomaterials. Although starch is known as a nonreductive saccharide,34,35 we found here that this kind of polysaccharide could reduce H2TeO4‚2H2O powders under hydrothermal conditions to form tellurium nanowires with a high yield. The obtained tellurium nanowires are fine, long, and of single-crystal in nature. The formation mechanism of nanowires might go through a chain-shaped composite of inorganic-bioorganic molecules via hydrogenbonding interaction. The effects of the reactants’ concentrations/molar ratio and the type of the carbohydrates on the 1D structures’ size and assembled structure have also been investigated to explain the mechanism and different 1D structures, including thick rods, short nanowires, bunched nanowires, and assembled spikelet structures, could be fabricated.
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Figure 1. XRD pattern of the obtained Te sample with starch as reducing agent.
Experimental Section In a typical procedure for the synthesis of tellurium nanowires, 0.15 g of H2TeO4‚2H2O was mixed with 0.15 g of starch in 10 mL of distilled water in a Telfon-lined stainless steel autoclave. The autoclave was kept at 160 °C for 15 h and then cooled to room temperature. Solid and solution were separated by centrifugation at 2000 rpm for ∼10 min, and the collected solid product was washed with distilled water and alcohol several times by centrifugation, followed by drying in air at room temperature. Characterization. X-ray diffraction (XRD) was recorded on a Scintag diffractometer operated at 35 kV voltage and 30 mA current with Cu KR radiation (λ ) 1.541 78 Å). The transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) pattern were obtained with a Philips 420 transmission electron microscope with an accelerating voltage of 120 kV. A high-resolution TEM (HRTEM) micrograph was taken on a JEOL-20l0F transmission electron microscope operated at 200 kV.
Results and Discussion The X-ray diffraction (XRD) pattern shown in Figure 1 confirms the realization of tellurium crystallites from (30) Hong, K. J.; Jeong, J. W.; Baek, H. W.; Jeong, T. S.; Youn, C. J.; Moon, J. D.; Park, J. S. J. Cryst. Growth 2002, 240, 135. (31) Mayers, B.; Xia, Y. J. Mater. Chem. 2002, 12, 1875. (32) Mo, M. S.; Zeng, J. H.; Liu, X. M.; Yu, W. C.; Zhang, S. Y.; Qian, Y. T. Adv. Mater. 2002, 14, 1658. (33) Liu, Z. P.; Li, S.; Yang, Y.; Hu, Z. K.; Peng, S.; Liang, J. B.; Qian, Y. T. New J. Chem. 2003, 27, 1748. (34) Kimball, J. W. Biology, 6th ed.; Addison-Wesley Pub. Co.: 1994. (35) Hu, H. W. Organic Chemistry, 2nd ed.; High Education Publisher: Beijing, 1990.
Figure 2. TEM images, SAED pattern, and HRTEM image of the obtained Te sample with starch as reducing agent.
H2TeO4‚2H2O precursor powders using starch as the reducing agent. All peaks in this pattern can be indexed as the hexagonal phase of tellurium (Te, JCPDS, No. 361452), and no other peaks can be found, indicating that only elemental tellurium grains with high crystallinity and purity were obtained. Figure 2 displays transmission electron microscope (TEM) images and high-resolution TEM (HRTEM) image of the obtained sample, along with selected area electron diffraction (SAED) pattern. The TEM image with low magnification (Figure 2a) shows long nanowires with high yield. At higher magnification (Figure 2b), it can be clearly seen that the obtained tellurium crystallites have fine wirelike morphology and the average diameter of these nanowires is ∼25 nm and lengths are up to 10 µm. Its SAED pattern (Figure 2c) reveals several diffraction rings, which can be indexed to hexagonal tellurium, in agreement with the XRD result. This result confirms that crystalline tellurium nanowires could be synthesized in a large scale from commercial H2TeO4‚2H2O precursor by using starch as reducing agent. The preferred growth direction and the nature of single crystallinity of the nanowire could be verified by HRTEM image (shown in Figure 2d). The
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Scheme 1. Possible Mechanism for Creation of Te Nanowires
crystal planes perpendicular to the long axis of the nanowire have a spacing of 0.59 nm, which is consistent with that of (001) crystal planes. This means that the nanowires might grow in a preferred direction of [001], in agreement with the result reported previously in the literature.33 Although the exact mechanism of the formation of tellurium nanowires is difficult to know, we think that the chain-shaped structure of starch would play an important role in the formation of tellurium nanowires. Starch molecules have been reported to be chain-shaped structures with many hydroxyl (-OH) groups on their surfaces.34,35 These -OH groups might react with TeO42groups through hydrogen bonds to form chain-shaped intermediate compounds. Thus, the long chains of starch could serve as a directing template for the growth of tellurium nanowires (as shown in Scheme 1). Starch has been confirmed to be composed of chain-shaped amylopectin, a branched polymer, and a small amount of amylose, an unbranched polymer.34,35 We think the presence of the branched polymer would be important for the formation of separate and fine nanowires. As is wellknown, long and fine chains without any side chains usually have a tendency to assemble in parallel. The interaction between two unbranched chains might be much stronger than those with side chains. The former would not be beneficial to the growth of separate and fine nanowires and leads to the aggregation of nanowires or even the formation of thick nanowires or nanorods. In contrast, branched polymer might not have this problem and the short branches might decrease the interactions, which prevents the aggregation of the chains and leads to the formation of separate and fine nanowires. Accordingly, the possible synthetic mechanism of tellurium nanowires could be described as a chain-shaped inorganicbioorganic molecule intermediate templated process. This suggestion could be supported by our experimental results concerning the effects of biomolecule’s structure and concentration on the product’s morphology. When we substituted other chain-shaped structures of polysaccharides, such as cellulose and potato starch, for starch, we found that different chain-shaped structures of cellulose and starches result in products with different 1D structures. Compared with starch, cellulose also has chainshaped structure but no side chains.34,35 The absence of the side chains allows these linear molecules to lie close together, which brings more opportunities to form hydrogen bonds between adjacent chains.34,35 Thus, based on the possible mechanism, it could be predicted that the product would be bunched nanowires or even thick nanowires with the substitution of cellulose for starch. Figure 3a shows a TEM image of the product with cellulose as reducing agent. It confirms the prediction, and the product consists of nonuniform and thick nanowires. Compared with starch, potato starch is a less branched polymer but relatively more branched than cellulose.34,35 The obtained product prepared with potato starch as reducing agent consists of bunched wires (Figure 3b), which assemble but are separated from each other at the
Figure 3. (a) TEM images of the Te sample prepared with cellulose as reducing agent; (b) TEM images of the Te sample prepared with potato starch as reducing agent.
Figure 4. TEM images of the Te sample prepared with starch as reducing agent: (a) at a high starch concentration and (b) at a higher starch concentration.
ends (seen clearly in Figure 3b inset). This result might be attributed to the small amount of side branches of potato starch. The above results suggest that it is necessary to use branched, chainlike biomolecules as templates and reducing agents in order to get separate nanowires. To further study the synthetic mechanism we also investigated the effect of starch’s concentration on the product’s morphology. To form fine and long tellurium nanowires, an appropriate ratio between H2TeO4 and starch is essential. Our experimental results show that with the increase of the concentration of starch, the morphology of tellurium product changed from long and fine nanowires to relatively short nanowires (as shown in Figure 4a), and the further increase of starch concentration leads to the formation of assembled spikelet structures (as shown in Figure 4b). This is also rational according to the chain-shaped template mechanism. At a high starch concentration, the interaction between biomolecules increases, which decreases the growth rate of tellurium nuclei among chain-shaped starch molecules and thus limits the growth of tellurium 1D nanostructures to short nanowires. Higher starch concentration leads to the strong interaction between organic chains, which benefits the aggregation of 1D nanostructures and the formation of spikelet structures. Conclusion In summary, in this paper we report the synthesis of tellurium nanowires in a large scale with the assistance of starch, an economical and green chemical. The starch serves as not only an effective reducing agent but also a new morphology-directing agent for tellurium nanowires from H2TeO4 precursor powders. The obtained tellurium
Green Synthesis of Tellurium Nanowires
nanowires are long, fine, and of single crystal in nature. The formation of tellurium nanowires might go through a chain-shaped inorganic-bioorganic molecule intermediate with biomolecules acting as a template. The interactions between biomolecule-biomolecule, biomoleculeinorganic species, and inorganic-inorganic species affect the size, length, and the assembling behavior of nanowires. This method enlarges the application of biomolecules to the green synthesis of nanostructures with low dimensionality, and by using the above method possibly other
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nanowires could be synthesized under very mild and economical conditions. Acknowledgment. This work was supported by the NSF MRSEC under grant number DMR-0213623 and by the Huck Institutes of Life Sciences. TEM work was performed in the electron microscopy facility of the Materials Research Institute at Penn State University. LA050594P
