Rapid Microwave-Assisted Synthesis of Uniform Ultralong Te

Apr 15, 2010 - tor, such as nanowires, nanobelts, and nanotubes, has attracted intensive ... However, all these synthetic methods either need harsh re...
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Rapid Microwave-Assisted Synthesis of Uniform Ultralong Te Nanowires, Optical Property, and Chemical Stability Jian-Wei Liu, Fang Chen, Meng Zhang, Hao Qi, Chuan-Ling Zhang, and Shu-Hong Yu* Division of Nanomaterials & Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, School of Chemistry & Materials, the National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, P. R. China Received February 22, 2010. Revised Manuscript Received April 5, 2010 Uniform and ultralong single-crystalline tellurium (Te) nanowires with a diameter of 20 nm and length of tens of micrometers can be rapidly synthesized by a microwave-assisted method. The formation process of high-quality Te nanowires is strongly dependent on the reaction conditions such as the amount of polyvinylpyrrolidone (PVP), pH value of initial solution, reaction time, and the choice of surfactant. The hydrophilic Te nanowires display a broadened luminescent emission from shortwave ultraviolet to visible region excited by vacuum-ultraviolet (VUV) under synchrotron radiation at room temperature. Based on the examination of the chemical stability of the as-prepared Te nanowires stored in water, the relationship between the synthetic methodology and chemical stability of Te nanostructures has been discussed.

1. Introduction Within the past few decades, one-dimensional (1D) semiconductor, such as nanowires, nanobelts, and nanotubes, has attracted intensive interest due to their fundamental significance in basic scientific research and potential applications.1 Semiconductor nanowires with a diameter of 1-100 nm and high aspect ratio have shown promising application in device fabrication. Furthermore, physical properties of semiconductor nanowires are significantly altered, and chemical reactivity is enhanced due to the confinement effect of low dimensionality. Elemental tellurium is a well-known p-type helical semiconductor with the narrow bandgap energy of 0.35 eV at room temperature2 and exhibits intriguing properties such as unique photoconductivity, nonlinear optical response, and high thermoelectric along with piezoelectric responses. Considering these novel properties, there are many potential applications in gas sensors, optoelectronic devices, and photonic crystal, self-developing holographic recording devices, radiative cooling devices, field-effect devices, and infrared acoustooptic deflector.3 So far, different approaches have been developed for the synthesis of 1D Te nanostructures, such as photolytic preparation,4 electrochemical and electrophoretic deposition,5 physical evaporation,6

decomposition of TeCl4 at 250-300 °C,7 nanoparticles as starting reagent,8 self-seeding solution process,9 polyol process,10 surfactant-assisted hydrothermal method in an ethanol/water media,11 and microwave-assisted synthesis in ionic liquid.12 However, all these synthetic methods either need harsh reaction conditions, complex procedures, or long reaction time. Microwave irradiation offering rapid and uniform heating of solvents, reagents, and intermediates should provide uniform nucleation and growth conditions. This rapid, efficient, environmentally friendly method has been implemented as a viable technique in the synthesis of a range of nanostructures.13 Zhu et al. have described the synthesis of Te nanowires by microwave in ionic liquids at 180 °C.12 Up until now, rapid microwave synthesis of high-quality Te nanowires in water phase in high yield without use ionic liquids has not been achieved yet. In this work, we report the rapid synthesis of uniform ultralong Te nanowires by microwave-assisted reaction in the presence of polyvinylpyrrolidone (PVP) using hydrazine hydrate as reducing agent and Na2TeO3 as tellurium source. The influence of the amount of PVP, reaction time, pH value of initial solution, and the choice of a surfactant on the formation of Te nanowires has been studied. The optical property and chemical stability of the Te nanowires have also been investigated.

*Corresponding author: Fax þ 86 551 3603040, e-mail [email protected].

(1) (a) 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, 15, 353. (b) Hochbaum, A. I.; Yang, P. D. Chem. Rev. 2010, 110, 527. (c) Fan, H. J.; Werner, P.; Zacharias, M. Small 2006, 2, 700. (2) (a) Kudryavstev, A. A. The Chemistry and Technology of Selenium and Tellurium; Collet’s Ltd.: London, 1974. (b) Tangney, P.; Fahy, S. Phys. Rev. B 2002, 65, 054302. (c) Wang, Y.; Tang, Z. Y.; Podsiadlo, P.; Elkasabi, Y.; Lahann, J.; Kotov, N. A. Adv. Mater. 2006, 18, 518. (3) (a) Frank, S.; Poncharal, P.; Wang, Z. L.; de Heer, W. A. Science 1998, 280, 1744. (b) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Nature 2001, 409, 66. (c) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y. Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. D. Science 2001, 292, 1897. (4) Webber, D. H.; Brutchey, R. L. Chem. Commun. 2009, 38, 5701. (5) Zhao, A. W.; Ye, C. H.; Meng, G. W.; Zhang, L. D.; Ajayan, P. M. J. Mater. Res. 2003, 18, 2318. (6) Li, X. L.; Cao, G. H.; Feng, C. M.; Li, Y. D. J. Mater. Chem. 2004, 14, 244. (7) Yu, H.; Gibbons, P. C.; Buhro, W. E. J. Mater. Chem. 2004, 14, 595. (8) Tang, Z. Y.; Wang, Y.; Sun, K.; Kotov, N. A. Adv. Mater. 2005, 17, 358. (9) (a) Gautam, U. K.; Rao, C. N. R. J. Mater. Chem. 2004, 14, 2530. (b) Mayers, B.; Xia, Y. N. J. Mater. Chem. 2002, 12, 1875.

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2. Materials and Methods 2.1. Materials. All chemicals were analytical grade. Poly-

(acrylic acid) (PAA, MW ≈ 1800) is purchased from Sigma-Aldrich. All other chemicals are commercially available from Shanghai Chemical Reagent Co. Ltd. Polyvinylpyrrolidone (PVP, MW ≈ 40 000), cetyltrimethylammonium bromide (CTAB), Na2TeO3, (10) Mayers, B.; Xia, Y. N. Adv. Mater. 2002, 14, 279. (11) (a) Qian, H. S.; Yu, S. H.; Gong, J. Y.; Luo, L. B.; Fei, L. F. Langmuir 2006, 22, 3830. (b) Liu, Z. P.; Li, S.; Yang, Y.; Hu, Z. K.; Peng, S.; Liang, J. B.; Qian, Y. T. New J. Chem. 2003, 27, 1748. (c) Liu, Z. P.; Hu, Z. K.; Liang, J. B.; Li, S.; Yang, Y.; Peng, S.; Qian, Y. T. Langmuir 2004, 20, 214. (12) (a) Zhu, Y. J.; Wang, W. W.; Qi, R. J.; Hu, X. L. Angew. Chem., Int. Ed. 2004, 43, 1410. (b) Zhu, Y. J.; Hu, X. L. Chem. Lett. 2004, 33, 760. (13) (a) Dallinger, D.; Kappe, C. O. Chem. Rev. 2007, 107, 2563. (b) Dahl, J. A.; Maddux, B. L. S.; Hutchison, J. E. Chem. Rev. 2007, 107, 2228. (c) Hu, B.; Wang, S. B.; Wang, K.; Zhang, M.; Yu, S. H. J. Phys. Chem. C 2008, 112, 11169.

Published on Web 04/15/2010

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Figure 1. (a) XRD pattern of the as-synthesized Te nanowires. The standard pattern of Te is presented at the bottom. (b) EDS analysis of the Te nanowires. The sample was prepared from 2 mL solution by microwave heating at 150 °C for 15 min, which was taken from a 38 mL mother solution made of 33 mL of deionized water, 1.0 g of PVP, 0.0922 g of Na2TeO3, 3.33 mL of aqueous ammonia, and 1.67 mL of hydrazine hydrate. hydrazine hydrate (85 wt %), aqueous ammonia solution (2528 wt %), acetone, N,N-dimethylformamide (DMF), and CHCl3 were used as received without further purification. 2.2. Synthesis of Te Nanowires. In a typical procedure, 0.0922 g of Na2TeO3 and 1.0000 g of PVP were added into 33 mL of deionized water under vigorous magnetic stirring at room temperature for 10 min. Then, 3.33 mL of aqueous ammonia and 1.67 mL of hydrazine hydrate were added into the previous solution and further stirred for 5 min. After that, 2 mL of such a solution was heated to 150 °C by microwave in a microwave sealed vessel made of quartz under magnetic stirring and was maintained at this temperature for 15 min (see Supporting Information S1). The microwave system was a CEM Discover microwave synthesizer (2.45 GHz, maximum power 300 W, Discover, CEM). The obtained dark blue product was deposited by acetone, then collected by centrifuging the mixture and washed several times with deionized water and ethanol, and then finally dried in a vacuum at 60 °C for characterization. An Innova 40 benchtop incubator shaker was used to chemical transformation reactions. 2.3. Characterization. The phase purity of the as-prepared products was determined by X-ray diffraction (XRD) using a Philips X’Pert Pro Super X-ray diffractometer equipped with graphite-monochromatized Cu KR radiation. High-resolution transmission electron microscope (HRTEM) images were performed on a JEOL-2010 transmission electron microscope operated at an acceleration voltage of 200 kV. The energy-dispersive X-ray spectroscopy (EDS) analysis was also done with a JEOL2010 TEM with an Oxford windowless Si (Li) detector equipped with a 4-pulse processor. Field-emission scanning electron microscopy (FESEM) was carried out with a field-emission scanning electron microanalyzer (Zeiss Supra 40 scanning electron microscope at an acceleration voltage of 5 kV). X-ray photoelectron spectra (XPS) were recorded on an ESCALab MKII X-ray photoelectron spectrometer, using Mg KR radiation as the exciting source. Raman scattering spectra were recorded with a Renishaw System 2000 spectrometer using the 514 nm line of Arþ for excitation. UV-vis spectra were recorded on a UV-2501PC/2550 at room temperature (Shimadzu Corp., Japan). Photoluminescence (PL) emission was performed at room temperature with a Perkin-Elmer LS55 luminescence spectrometer. The vacuum-ultraviolet (VUV) exciting PL measurement was performed in beamline Station of VUV spectroscopy at the National Synchrotron Radiation Laboratory (NSRL, Hefei, China).

3. Results and Discussion 3.1. Microwave-Assisted Synthesis of Te Nanowires and Characterization. The X-ray diffraction (XRD) pattern shown in Figure 1a confirms the products obtained from reduction of Langmuir 2010, 26(13), 11372–11377

Figure 2. TEM images of Te nanowires. (a) TEM image of the Te nanowires. (b) TEM image of a single nanowire. The inset of (a) displays the digital photograph of Te nanowires and inset of (b) shows the corresponding HRTEM and the corresponding SAED pattern obtained from the single nanowire. The electron beam was focused along [010] axis. The sample was prepared from 2 mL solution by microwave heating at 150 °C for 15 min, which was taken from a 38 mL mother solution made of 33 mL of deionized water, 1.0 g of PVP, 0.0922 g of Na2TeO3, 3.33 mL of aqueous ammonia, and 1.67 mL of hydrazine hydrate.

sodium tellurite (Na2TeO3) in the presence of hydrazine hydrate (85 wt %) via microwave-assisted process. All peaks in this pattern can be indexed to the hexagonal phase of tellurium with a cell constants a = 4.0 A˚ and c = 5.9 A˚, which are in good agreement with the standard literature data (JCPDF card number: 36-1452). An energy dispersive X-ray spectrometer (EDS) spectrum was used to analyze the composition of the nanowires (Figure 1b). Strong Te peaks undoubtedly confirmed that the product is Te. The signals of Cu and C peaks come from the carbon-coated copper grid, which is a normal observation for TEM samples. The Si peaks come from the microwave sealed vessel made of quartz. The transmission electron microscope (TEM) image in Figure 2a shows the long nanowires with high aspect ratios, the average diameter of 20 nm, and lengths up to tens of micrometers. The dispersion of Te nanowires in water is deep blue, which is a typical color as found previously for ultrathin Te nanowires synthesized by the hydrothermal method.11a The high-resolution transmission electron microscope (HRTEM) image inset in Figure 2b depicts a single Te nanowire, and its inset illustrates the lattice spacing of ca. 5.9, 4.0, and 3.2 A˚, corresponding to the lattice spacings of the (001), (100), and (101) planes for hexagonal tellurium, respectively. The crystal planes perpendicular to the long axis of the nanowire have a spacing of 0.59 nm, which is consistent with that DOI: 10.1021/la100772n

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Figure 3. TEM images of the samples prepared by microwave at 150 °C in 15 min in the presence of PVP. The addition of PVP was (a) 0, (b) 0.5, (c) 1.0, and (d) 1.5 g. The sample was prepared from 2 mL solution by microwave heating at 150 °C for 15 min, which was taken from a 38 mL mother solution made of 33 mL of deionized water, 0.0922 g of Na2TeO3, 3.33 mL of aqueous ammonia, 1.67 mL of hydrazine hydrate, and different amounts of PVP.

of (001) crystal planes. The selected area electron diffraction (SAED) pattern taken on a typical nanowires shows that it is single crystalline (inset in Figure 2b). These results suggest that the Te nanowires grow along [001] direction, which is in agreement with the result reported previously.11a The influence of the amount of PVP on the shape and size of the Te nanostructures has been studied. Figure 3 demonstrates that the amount of PVP played a crucial role in synthesis of Te nanowires. Some short nanorods and nearly spherical particles were obtained in the absence of PVP shown in Figure 3a. This is different from that reported previously by Zhu12a and Yu.11a When the amount of PVP increased to 0.5 g, high aspect ratio nanowires with a diameter of 20 nm began to come into sight (Figure 3b). The surface of the nanowires is rough, and a mass of tiny nanoparticles around nanowires appeared (see Supporting Information Figure S2). When the amount of PVP increased up to 1.5 g, the aspect ratio was remarkably decreased, and the diameter increased. As discussed above, the uniform, superlong nanowires can be obtained in the presence of 1.0 g of PVP, and the length of the nanowires was more than tens of micrometers. Moreover, a suitable surfactant and reaction time are also important parameters for the formation of uniform Te nanowires. If poly(acrylic acid) (PAA) or cetyltrimethylammonium bromide (CTAB) was used instead of PVP, only some nanorods and aggregated rods were found as shown in Figure 4a,b. The results demonstrated that using PVP is more favorable for producing Te nanowires with high aspect ratio. The color of the reaction solution turned to blue after reaction for only 30 s in the presence of PVP. The TEM image in Figure 4c shows that the short nanorods formed after microwave heating for 30 s. After microwave heating for 12 min, a dark blue product was found to be composed of long nanowires and small amount of Te nanoparticles (Figure 4d). Thus, the amount of PVP, reaction time, and a suitable surfactant played key roles in producing tellurium nanowires with high quality. The pH value of an initial reaction solution played another crucial role in the formation of uniform Te nanowires. Te nanowires can be prepared in appropriate alkaline conditions using the hydrazine as the reducing agent in the present microwave system. 11374 DOI: 10.1021/la100772n

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Figure 4. Influence of the surfactant on the synthesis of Te nanostructures. TEM images of the samples prepared under different conditions by microwave heating a 2 mL solution at 150 °C, which was taken from a 38 mL mother solution containing 33 mL of deionized water, 1.67 mL of hydrazine hydrate, 3.33 mL of aqueous ammonia, and 0.0922 g of Na2TeO3. (a, b) In the presence of 1.0 g of poly(acrylic acid) (PAA) and 1.0 g of cetyltrimethylammonium bromide (CTAB). (c, d) In the presence of 1 g of PVP. The microwave heating time was (c) 30 s and (d) 10 min, respectively.

The pH value of the a mixed solution of 1.0 g of PVP, 0.0922 g of Na2TeO3, 33 mL of H2O, and 1.67 mL of hydrazine is about 10. Adding 3.33 mL of aqueous ammonia to the solution and the optimal pH was 12, resulting in the formation of uniform Te nanowires as shown in Figure 2a. In order to investigate the influence of the pH value of initial solution on the growth of Te nanowires, we used 1 M HCl and 1 M NaOH to adjust the pH of the starting solution. When the pH was 8, Te nanorods with a diameter of about 35 nm and the length of 300 nm appeared (Figure 5a). When the pH value was adjusted to 9, the diameter of the Te nanowires was 20 nm and length was at least 2 μm, as shown in Figure 5b. When the pH value was adjusted to 14, a majority of particles are Te nanorods with a diameter of 50 nm and the length of 2 μm, accompanying with a small fraction of some tiny nanorods and aggregated nanoparticles (Figure 5c). Thus, 1-D Te nanostructures can be obtained under microwave conditions when the pH value of the starting solution is in the range of 8-14. Herein, the interaction between PVP and growing 1-D Te nanostructures was obviously affected by pH value during the nucleation and growth and Ostwald ripening process of tellurium nanowires. Figure 6 shows the X-ray photoelectron spectra (XPS) of the Te nanowire monolayer. Figure 6a depicts the survey of the sample, indicating that the peaks values at 285, 400, and 532 eV can be readily assigned to the binding energies of C 1s, N 1s, and O 1s, respectively. Survey of the Te 3d region is displayed in Figure 6b; the Te 3d peak shift displayed two strong peaks at 572.7 and 583.1 eV, corresponding to Te(0) 3d binding energy. 3.2. Optical Property of Te Nanowires. Figure 7a shows the UV-vis spectrum of the Te nanowires displaying two typical absorption peaks located at 266 and 630 nm. The wavenumbers of the two peaks are slightly different from that reported previously.11a This phenomenon may be due to the difference in the diameter and the length of the Te nanowires.14 The absorption peak at 266 nm (4.66 eV) is due to the allowed direct transition from the valence band (p-bonding triplet) to the conduction band (p-antibonding triplet), and another broad but strong absorption (14) Lin, Z. H.; Yang, Z. S.; Chang, H. T. Cryst. Growth Des. 2008, 8, 351.

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Figure 5. TEM images of as-prepared Te nanostructures obtained by microwave heating a 2 mL solution at 150 °C for 15 min, which was taken from a mother solution containing 0.0922 g of Na2TeO3, 1.67 mL of hydrazine hydrate, 1.0 g of PVP, and 33 mL of deionized water. The pH value of (a), (b), and (c) were 8, 9, and 14, respectively. The pH value was adjusted by 1 M HCl and 1 M NaOH.

Figure 6. XPS spectra for the obtained Te nanowires: (a) survey of the sample; (b) survey of the Te 3d region.

Figure 7. (a) UV-vis spectrum of Te nanowires dispersed in water. (b) Raman scattering spectrum of tellurium nanowires.

peak centered at 630 nm (1.97 eV) can be assigned to a forbidden direction transition.9a,15,16 The Raman scattering spectrum taken for the synthesized Te nanowires at room temperature is depicted in Figure 7b. The characteristic vibration peaks at 102.9, 122.5, 140.2, and 268.6 cm-1 were observed at room temperature, which are close to those reported previously, but have an obvious shift to high frequency.17 Photoluminescence (PL) of high-quality Te nanowires has been explored intensively because their optical properties are directly linked to their potential optoelectronic applications. Lin et al. have studied the photoluminescence intensity varies with the (15) Isom€aki, H. M.; Boehm, J. v. Phys. Scr. 1982, 25, 801. (16) Sanchez-Iglesias, A.; Grzelczak, M.; Rodriguez-Gonzalez, B.; AlvarezPuebla, R. A.; Liz-Marzan, L. M.; Kotov, N. A. Langmuir 2009, 25, 11431. (17) (a) Song, J. M.; Lin, Y. Z.; Zhan, Y. J.; Tian, Y. C.; Liu, G.; Yu, S. H. Cryst. Growth Des. 2008, 8, 1902. (b) Zhang, L.; Wang, C.; Wen, D. Y. Eur. J. Inorg. Chem. 2009, 22, 3291.

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alternation of the diameter and the aspect ratio of the Te nanowires.14 Qian et al. have reported two strong emission bands over the range of 390-550 nm.11a The emission peak around 700 nm reported by Gautam et al. was not observed here.9a Figure 8a shows the excitation spectrum of the uniform Te nanowires with the emission monitored at 420 nm. An excitation peak at 270 nm could be observed in this excitation spectrum. Figure 8b shows the photoluminescence emission spectrum of the uniform Te nanowires with an excitation wavelength of 270 nm and the emission peak is around 420 nm, which is a little different from that reported previously.11a Synchrotron radiation is a kind of electromagnetic radiation emitted by a charged particle beam in a circular accelerator.18 Photoluminescence properties of Te nanowires excited by (18) (a) Ternov, I M. Phys.-Usp. 1995, 38, 409. (b) Chen, J.; Wu, C. Y.; Tian, J. P.; Li, W. J.; Yu, S. H.; Tian, Y. C. Appl. Phys. Lett. 2008, 92, 233104.

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Figure 8. (a) Photoluminescence excitation spectrum of the Te nanowires with the emission at 420 nm. (b) Photoluminescence emission spectrum of the Te nanowires with an excitation wavelength of 270 nm.

Figure 9. Luminescent spectra of Te nanowires under VUV excitation. (a, b) Excitation at 120 and 193 nm, respectively. The sharp peaks at 386 and 579 nm in (b) are frequency-doubled peaks. All of the spectra were measured at room temperature.

Figure 11. Digital photograph of a fresh sample in water (a) and the samples stored for (b) 6, (c) 8, (d) 16, (e) 19, and (f) 70 h. The fresh nanowires were prepared as described in Figure 1.

Figure 10. Time-dependent UV-vis spectrum change during the oxidization of tellurium nanowires dispersed in water at room temperature. The fresh nanowires were prepared as described in Figure 1.

vacuum-ultraviolet (VUV) synchrotron radiation at the room temperature were investigated. Figure 9a shows the emission spectrum of the Te nanowires excited by 120 nm and the emission peaks located at 300, 420, and 584 nm. Figure 9b shows the similar emission peaks excited by 193 nm. From the excitation spectrum, excitation at 120 nm gives the maximum emission intensity, and 11376 DOI: 10.1021/la100772n

the excitation peaks 210, 195, and 292 nm are the frequencydoubled peaks (see Supporting Information Figure S3a-c). 3.3. Chemical Stability of the Te Nanowires. The chemical stability of the Te nanowires is crucial to their applications. Thus, the chemical stability of the Te nanowires synthesized by this microwave-assisted process was also examined. The chemical stability of the Te nanowires synthesized by hydrothermal method was studied previously by our group.19 Herein, the freshly synthesized Te nanowires by microwave-assisted synthesis have been found to be less stable than that found in the sample synthesized by hydrothermal process.19 The newly prepared Te nanowires were dispersed in water under ambient conditions, and the time-dependent UV-vis spectra (19) Lan, W. J.; Yu, S. H.; Qian, H. S.; Wan, Y. Langmuir 2007, 23, 3409.

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Figure 12. Time-dependent UV-vis spectra during the oxidization of tellurium nanowires dispersed in water: (a) stored in the air and in the dark; (b) bubbled with O2 and in the dark. The fresh nanowires were prepared as described in Figure 1.

of the solution were monitored as shown in Figure 10. The UVvis peaks for the Te nanowires dispersed in water shows weakened gradually as the time was prolonged. Figure 11 shows digital photographs of the Te nanowires after being stored in water for different periods. The color of the sample became gradually faded and finally became clear and colorless as shown in Figure 11. Even after storing for 16 h or longer time, the solution almost becomes colorless, indicating that the nanowires have been completely degraded because of oxidation (Figure 11d,e). Previous study indicates that it takes 2 days for the Te nanowires with a diameter of 4-9 nm, which were synthesized by hydrothermal process, to be degraded from dark blue to colorless at room temperature in water.19 However, the speed of the oxidation is much faster here. In addition, the time-dependent UV-vis spectrum was also measured during the oxidization of tellurium nanowires in water when the dispersion was stored in the dark either in the air or bubbled with O2. The tinfoil was used to wrap the bottle of nanowire dispersion to keep the light out. In both cases, the Te nanowires were oxidized. Once the nanowire dispersion is stored in air without light, a similar degradation rate was observed as indicated by the time-dependent UV-vis adsorption spectra (Figures 10 and 12a). However, the nanowires will be oxidized rapidly after 2.5 h if O2 is bubbled into the nanowire dispersion. The degradation rate of the Te nanowires dispersed in water but bubbled with O2 in dark place is much faster than that without bubbling O2 into the nanowire dispersion as indicated by the time-dependent UV-vis spectra shown in Figure 12b. Thus, we propose that Te nanowires dispersed in water are oxidized mainly because of oxygen in air. From the viewpoint of application, it certainly needs to consider the stability problem of the high-quality Te nanowires after synthesis in the future. It is well-known that Te nanostructures can act as an efficient template to synthesize other nanomaterials by taking advantage of their reactivity. The Te nanowires synthesized by the present approach could act as more reactive templates for controlled synthesis of other nanostructures as we recently reported on using

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Te nanowires synthesized by hydrothermal process as templates.20 The detailed study is ongoing.

4. Conclusions In summary, uniform and ultralong Te nanowires with a diameter of 20 nm and length of tens of micrometers can be rapidly synthesized by facile microwave-assisted synthesis in aqueous solution. The results demonstrated that the amount of PVP, reaction time, pH value, and a suitable surfactant are key parameters for successful synthesis of high-quality Te nanowires with high aspect ratio. The optical property of the hydrophilic Te nanowires has been investigated under the vacuum-ultraviolet (VUV) excitation. The chemical stability of the as-prepared Te nanowires dispersed in water has been systematically studied, showing that the Te nanowires are more easily oxidized while they are stored in water than that synthesized by hydrothermal process.11a The present study further emphasizes the important role of the different synthesis methodology played in the chemical stability of Te nanostructures, which should draw attention in the future from the viewpoint of application. Acknowledgment. S.H.Y. acknowledges the funding support from the National Basic Research Program of China (2010CB934700), the Program of International S & T Cooperation (S2010GR0314), and the National Natural Science Foundation of China (No. 50732006), the Principle Investigator Award from the National Synchrotron Radiation Laboratory (NSRL, Hefei, China), and the beamline Station of vacuum-ultraviolet (VUV) spectroscopy for PL measurement at NSRL. Supporting Information Available: Microwave reaction conditions, SEM, TEM images, the VUV excitation spectra, and XRD pattern measurements. This material is available free of charge via the Internet at http://pubs.acs.org. (20) (a) Liang, H. W.; Liu, S.; Gong, J. Y.; Wang, S. B.; Wang, L.; Yu, S. H. Adv. Mater. 2009, 21, 1850. (b) Liang, H. W.; Liu, S.; Wu, Q. S.; Yu, S. H. Inorg. Chem. 2009, 48, 4927.

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