pubs.acs.org/Langmuir © 2010 American Chemical Society
Multisegmented Se-Te-Se Hybrid Nanowires: A Building Unit with Inbuilt Block and Glue Functionality T. P. Vinod, Misun Park, Sang-Ho Kim,* and Jinkwon Kim* Department of Chemistry and GETRC, Kongju National University, Kongju, Chungnam 314-701, Korea Received April 13, 2010. Revised Manuscript Received May 11, 2010 A novel hybrid nanomaterial incorporating Te and Se components within a multisegmented nanowire morphology is synthesized through a facile aqueous phase reaction at room temperature. Te nanowires were used as templates to grow Se segments at their terminal locations. The Se-Te-Se structures obtained exhibit a self-organization property thereby enabling the formation of “nanotweezers” at elevated temperatures. The physical and chemical properties of its individual components are expected to provide interesting functionality and promising utility to these nanostructures. The inbuilt block and glue features associated with its components make it a potential building unit toward nanoarchitectures of higher sophistication.
Introduction One-dimensional nanomaterials such as nanowires, nanorods, and nanotubes are perceived as promising building units for future generation nanoscale devices and sensors.1 Recently, more sophisticated structures generated by the combination of multiple components in a single nanostructure, especially hybrid onedimensional nanomaterials, have drawn substantial attention because they enhance the functionality along with the effectuation of improved and novel properties which are not present in single component nanostructures.2 However, the studies on the interaction between these hybrid one-dimensional nanomaterials, especially on their assembled structures and their physical and chemical properties, are still in the very early stage. The assembled structures of nanomaterials have been demonstrated by utilizing the chemical affinity between organic functional groups.3,4 In those studies, the assembly was driven by the interactions of organic moieties present on the surfaces of nanomaterials. However, less effort has been made to utilize the van der Waals interactions between inorganic nanomaterial itself. In the case of hybrid nanomaterials, the van der Waals interactions can be tailored by selecting different materials for consisting segments, where the differences in chemical affinities among those segments can bring about a variety of directional assemblies. A hybrid material can be designed in such a way that one segment with strong chemical affinities can behave as a glue for superstructures whereas the other segment has certain functionalities such as unique optical, electrical, or magnetic properties. One can expect unique physical or chemical properties which depend not only on the material properties but also on the assembled structures. Trigonal-tellurium (t-Te), which has a narrow band gap, has attracted considerable interest due to its photoconductivity,
photoelectricity, thermoelectricity, catalysis, nonlinear optical properties, and high piezoelectricity. These characteristics make it suitable for various applications.5 Selenium has a lower melting point (144 °C) than tellurium, which makes it a suitable candidate as a sacrificial glue segment in the aforementioned design of segmented hybrid nasnostructures. Crystalline nanorods of Se/Te alloys (Se0.5Te0.5) has been previously reported with both components present homogeneously within the one-dimensional nanostructure.6 As per our best knowledge, there has been no report about a hybrid nanostructure composed of tellurium and selenium components with multisegmented morphology. In this Letter, we report an aqueous phase synthesis of a hybrid nanomaterial coalescing tellurium and selenium components within a multisegmented nanowire structure. We also report their assembled structures and physical consolidation using selective melting of Se segments.
Experimental Section Synthesis of Te Nanowires. Te nanowires were synthesized according to a reported procedure.7 The Te precursor (NH4)2TeS4 was reduced with sodium sulfite in presence of a surfactant solution (dodecylbenzenesulfonic acid, sodium salt (SDBS) and poly(sodium 4-styrenesulfonate) (PSS) in the molar ratio 1:1). The solution was kept for 30 h to get the growth of Te nanowires. It was then centrifuged to remove the unbound surfactants. Synthesis of Na2SeSO3. Na2SeSO3 was prepared with a procedure reported elsewhere.8 Se powder (1.43 mmol) and sodium sulfite (2.86 mmol) were mixed in 10 mL water and refluxed until all Se disappeared. The colorless solution of Na2SeSO3 was used for further reactions. Synthesis of Se-Te-Se Segmented Nanowires. A volume of 8 mL of an aqueous solution of tellurium nanowires was mixed with 0.137 mL of HCl (1M) in a round-bottom flask, and 0.0685 mmol
*To whom correspondence should be addressed. E-mail:
[email protected] (J.K.);
[email protected] (S.-H.K.). (1) Hu, J. T.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. Fan, H. J.; Werner, P.; Zacharias, M. Small 2006, 2, 700. Kong, J.; Franklin, N. R.; Zhou, C. W.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. J. Science 2000, 287, 622. (2) Cong, H. P.; Yu, S. H. Curr. Opin. Colloid Interface Sci. 2009, 14, 71. (3) Fava, D.; Nie, Z.; Winnik, M. A.; Kumacheva, E. Adv. Mater. 2008, 20, 4328. Nie, Z.; Fava, D.; Rubinstein, M.; Kumacheva, E. J. Am. Chem. Soc. 2008, 130, 3683. DeVries, G. A.; Brunnbauer, M.; Jackson, A. M.; Long, B.; Neltner, B. T.; Uzun, O.; Wunsch, B. H.; Stellacci, F. Science 2007, 315, 358. Zhang, Q.; Gupta, S.; Emirick, T.; Russel, T. P. J. Am. Chem. Soc. 2006, 128, 3898. (4) Nakashima, H.; Furukawa, K.; Kashimura, Y.; Torimitsu, K. Langmuir 2008, 24, 5654. Wang, Y.; Tang, Z.; Tan, S.; Kotov, N. A. Nano Lett. 2005, 5, 243.
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(5) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayer, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Q. Adv. Mater. 2003, 15, 353. Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897. Duan, X.; Huang, Y.; Cui, Y.; Wang, J.; Lieber, C. M. Nature 2001, 409, 66. (6) Mayers, B.; Gates, B.; Xia, Y. N. Adv. Mater. 2001, 13, 1380. Liu, Z.; Hu, Z.; Xie, Q.; Yang, B.; Wu, J.; Qian, Y. J. J. Mater. Chem. 2003, 13, 159. (7) Vinod, T. P.; Yang, M.; Kim, J.; Kotov, N. A. Langmuir 2009, 25, 13545. (8) Fan, S.; Li, G.; Zhang, M. X.; Mu, H.; Zhou, B.; Gong, L.; Liang, H.; Guo, L.; Guo, J. Cryst. Growth Des. 2009, 9, 95. Stroyuk, A. L.; Raevskaya, A. E.; Kuchmiy, S. Y.; Dzhagan, V. M.; Zahn, D. R. T.; Schulze, S. Colloids Surf., A. 2008, 320, 169.
Published on Web 05/17/2010
DOI: 10.1021/la101455z
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Figure 2. Dark field TEM image and EDS analysis at five different regions of the nanostructure showing the relative atomic percentages of Te and Se. Figure 1. (a) TEM image of Se-Te-Se multisegmented nanowires, (b) HRTEM image of Se segment, and (c) HRTEM image of Te segment.
Na2SeSO3 was added to it with vigorous stirring. The reaction mixture was mildly stirred for another 30 min. It was then kept for aging at room temperature for 12 h. After this, the reaction mixture was centrifuged to remove unreacted reagents and then it was washed with water. Characterization. TEM images and EDS data were obtained from a Tecnai G2F30 field emission transmission electron microscope. Scanning electron microscopy (SEM) images were obtained from a MIRA LMH (TESCAN) microscope. Powder XRD data were was obtained from a Rigaku DMAX 2000 X-ray diffractometer.
Results and Discussion It has been reported that the dismutation of Na2SeSO3 under acidic condition can yield t-Se nanostructures.8 Our attempt was to carry out this reaction in the presence of t-Te nanowires to obtain growth of Se on Te nanowire templates. All the Te nanowires converted to Se-Te-Se within 30 min of the reaction (Figure S2 in the Supporting Information). The transmission electron microscopy (TEM) images of the product obtained are shown in Figure 1. As displayed in Figure 1a, two one-dimensional segments are grown at terminal positions of each Te nanowire. Dark field TEM and energy-dispersive X-ray spectroscopy (EDS) analysis on the nanostructure (Figure 2) revealed the terminal areas of each nanostructure as being composed exclusively of Se. The middle segment of the nanostructure showed high percentages of Te and low amounts of Se. The Se segments have more width compared to the Te nanowires. The Se segments have an average length of 144 nm and an average thickness of 25 nm. High resolution TEM (HRTEM) images of the Te part (Figure 1c) show lattice fringes with spacing of 5.9 A˚ along the [001] direction, which corresponds to t-Te planes. A d spacing of 4.9 A˚ was observed in the HRTEM image of the Se segment (Figure 1b), which is attributed to t-Se planes along the [001] direction. The crystalline nature of the sample was further confirmed by powder XRD measurements. The XRD pattern showed characteristic peaks for t-Te and t-Se planes (Figure S1 in the Supporting Information). Comprehensive conclusions about the mechanism of Se growth over Te templates can be achieved only through investigation of the theoretical aspects of the process. Rudimentary assumptions about the driving force behind growth of Se at the tips of Te nanowires can be made by considering the results of our experiments. The site of growth for an additional material on a given nanostructure decisively depends on factors such as the crystal symmetry of the structure, concentration, electrochemical characteristics of the reagents, and mobility of electrons through the 9196 DOI: 10.1021/la101455z
nanostructure.7 Anisotropic nanocrystals have higher reactivity at their tips, leading to the preferential deposition of a second material at these locations.9 Tips of tellurium nanorods also have been reported as preferential sites for growth of another species.7,10 It was observed in our experiments that the Se segments started to grow at both ends of Te nanowires simultaneously. This is evident from the similar size of Se segments during different intervals of time (Figure S2 in the Supporting Information). t-Se has a highly anisotropic crystal structure that gives it a natural tendency to undergo anisotropic growth in 1D morphology.6,11 Moreover, t-Se has a crystal structure nearly identical to that of t-Te.12 This can minimize the mismatch-induced strain during the growth process to a great extent. Thus, a plausible mechanism for the growth of Se segments can be assumed such that the natural tendency of anisotropic growth of Se, the identical crystal structures of the components involved, and the higher feasibility of heterogeneous growth at Te nanowire tips combined together provide the necessary driving force. Self-assembly of the hybrid nanowires to higher order superstructures was attempted by drying solutions of the sample on a copper grid. It was observed that the self-organization through lateral association of Se-Te-Se nanostructures is possible by the usage of adequate concentrations in suitable solvents along with appropriate temperature and duration of drying. When the dilute aqueous solutions of the nanostructure were dried slowly at room temperature, there was no significant ordering and assembly observed. However, drying the sample at higher temperature, that is, faster evaporation of solvent, leads to observation of parallel assembly of the Se-Te-Se nanowires in short-range order. It is believed that the high surface tension and the fast evaporation of water can increase the local concentration of nanowires leading to the lateral assembly. In the slow evaporation of water at room temperature, partially assembled structures (different from fast evaporation) were observed with higher concentration of the nanowires (Figure S9 in the Supporting Information). On the other hand, the treatment of the concentrated aqueous solution of the nanowires with isopropyl alcohol, which can effectively lower the surface tension, gives randomly oriented aggregation of the nanostructures. All these observations point to the conclusion that the high surface tension of water can drive the nanowires into a small volume so that the nanowires have more chance to interact (9) Kudera, S.; Carbone, L.; Casula, M. F.; Cingolani, R.; Falqui, A.; Snoeck, E.; Parak, W. J.; Manna, L. Nano Lett. 2005, 5, 445. Milliron, D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J. B.; Wang, L. W.; Alivisatos, A. P. Nature 2004, 430, 190. Mokari, T.; Rothenberg, E.; Popov, I.; Costi, R.; Banin, U. Science 2004, 304, 1787. (10) Lin, Z. H.; Chang, H. T. Langmuir 2008, 24, 365. (b) Lin, Z. H.; Lin, Y. W.; Lee, K. H.; Chang, H. T. J. Mater. Chem. 2008, 18, 2569. (11) Gates, B.; Mayers, B.; Cattle, B.; Xia, Y. N. Adv. Funct. Mater. 2002, 12, 219. (12) Mayers, B.; Xia, Y. N. J. Mater. Chem. 2002, 12, 1875.
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Figure 3. TEM images of assembled Se-Te-Se nanowires heat
treated at (a) 100 °C and (b) 140 °C. (c,d) “Nanotweezers” formed by selective melting of Se segments.
and assemble. The surface tension of water above a certain critical concentration can bring the nanowires within the range that van der Waals interactions become effective to assemble the nanowires. This inference is in agreement with the previously reported short-range ordering of Te nanorods at room temperature,13 where the assembly of nanorods is perceived as a thermodynamically favorable process influenced by factors such as surface tension, van der Waals forces, and capillary forces. Heat treatment of the assembled structures at elevated temperatures also was performed, which led to the observation of a fused architecture of Se-Te-Se nanowires in the shape of nanoscale tweezers. Figure 3a shows a typical structure of nanowires assembled in parallel ways when the Se-Te-Se nanowire solution was dried at 100 °C on a copper grid and the solvent evaporated. Figure 3b is a TEM image of nanowires dried at 140 °C. Since the melting temperature of Se (144 °C) is lower than that of Te (449.8 °C), Se segments of the nanowires can melt first and become a glue to fix (13) Liu, Z.; Hu, Z.; Liang, J.; Li, S.; Yang, Y.; Peng, S.; Qian, Y. Langmuir 2004, 20, 214.
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the assembled structures. This selective melting in nanowires is observed in Figure 3c and d. At 140 °C, Se parts of the nanowires melt and merge into a single head where the Te parts remain as separated legs resulting in a “nanotweezers” structure. This selective melting in hybrid nanomaterials is expected to be very useful to construct higher order nanoarchitectures. In Se-Te-Se nanowires, the Se segments act as a sacrificial part for assembly and fixation. Therefore, this type of hybrid nanomaterial can be a clue to design nanomaterials for nanoarchitecture engineering. In other words, the hybrid nanomaterials can be delineated as blocks and glues so that they can construct a variety of structures using different melting phenomena. In conclusion, we have synthesized a novel hybrid nanostructure of Te and Se with multisegmented nanowire morphology. The structure, composition, and growth pattern of the material were characterized. These hybrid nanowires form a parallel assembly of short-range order on drying from aqueous solutions of appropriate concentrations at suitable temperatures. The assembled structures can be fused to a “nanotweezers” structure by a selective melting of Se parts. The hybrid nanowires with a built-in block and glue functionality can be used as a building unit to construct desired nanoarchitectures. Further studies on the physical, chemical, and self-assembly related properties associated with this material are under progress. Acknowledgment. This work was financilly supported by the Converging Research Center Program (2009-0093712) and the Priority Research Center Program (2009-0093825) through the National Research Foundation of Korea (NRF) funded by the MEST. S.-H.K. is thankful for the financial support of the New and Renewable R&D Program (2009-T100102044) under the Korea Ministry of Knowledge and Economy (MKE). Supporting Information Available: XRD pattern, TEM images showing the time evolution of the synthetic process, EDS spectra, TEM image of Te nanowires used for the synthesis, and SEM images showing the assembly of SeTe-Se structures. This material is available free of charge via Internet at http://pubs.acs.org.
DOI: 10.1021/la101455z
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