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Ultrasonic-induced growth of crystalline tellurium nanorods and related branched structures
ARTICLE IN PRESS
Journal of Crystal Growth 295 (2006) 69–74 www.elsevier.com/locate/jcrysgro
Ultrasonic-induced growth of crystalline tellurium nanorods and related branched structures Wei Zhua,b, Wenzhong Wanga,, Haolan Xua,b, Lin Zhoua,b, Lisha Zhanga, Jianlin Shia a
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China b Graduate School of the Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, PR China Received 7 June 2006; received in revised form 22 July 2006; accepted 25 July 2006 Communicated by P. Rudolph
Abstract An ultrasonic route for the preparation of tellurium nanorods in aqueous solution is established at room temperature. The as-obtained nanorods are characterized to be single crystalline with [0 0 1] growth orientation, and have 30–60 nm in diameter and 200–300 nm in length. Some branched architectures, consisting of several nanorods, are also found in the products. In the synthesis, tellurium nitrate powder is utilized as the tellurium source. UV-vis spectra are recorded at ordinal stages accompanying with the prolongation of the sonochemical reaction. Based on the anisotropic growth determined by the intrinsic linear crystallographic structure of trigonal tellurium, the shaping mechanism of the nanorods is investigated. And the formation of the branched structures is suggested to be the result of multi-nuclei growth in monomer colloid. r 2006 Elsevier B.V. All rights reserved. PACS: 81.05.Cy; 81.10.Dn; 81.20.Ka Keywords: A1. Crystal morphology; A1. Nanostructures; A2. Growth from solutions; A2. Single crystal growth
1. Introduction For the growth of inorganic nanocrystals (NCs) in liquid solutions, the dependence of kinetic shape control on the intrinsic crystallographic structure of the target material is always an important research subject due to the affinitive relationship between the growth kinetics and surface energy [1]. As a response to anisotropic crystal structure, asymmetric morphology is generally of preference resulting from the different growth rate of the facets. Some relevant progressions, based on ‘‘Ostwald ripening’’ or ‘‘oriented attachment’’ mechanism, have been reported. For example, Peng et al. [2] presented the controllable preparation of spherical and rod-like CdSe NCs by manipulation of the monomer concentration. Weller et al. reported the formation of high-quality single crystalline ZnO nanorods based Corresponding author. Tel.: +86 21 5241 5295; fax: +86 21 5241 3122.
E-mail address: [email protected] (W. Wang). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.07.023
on oriented attachment of preformed quasi-spherical ZnO nanoparticles [3]. Thus, rigorous exploration about the impact of the crystal structure on the NC evolution and the detailed realization process is technologically meaningful, which would enhance the comprehension about the growth behavior of NC in solution system and be helpful for the following property investigation. Herein, we establish an ultrasonic approach for the synthesis of tellurium nanorods, where the shaping mechanism is investigated based on the anisotropic crystallographic nature. Elemental tellurium (Te) with trigonal (t-) phase is an important material in applications as photoconductivity, non-linear optical responses, piezoelectricity and thermoelectricity [4,5]. It is characteristic to be an intrinsic 31 helical-chain structure. So far, a wealth of chemical methods have been developed for the synthesis of tellurium nanostructures that mainly focuses on one-dimensional (1D) shapes, such as wires, rods, tubes, and belts. For example, Rao et al. reported controlled synthesis of Te
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nanorods, nanowires, nanobelts and related structures by the disproportionation of NaHTe in different solvent systems [6]. Xia et al. have successfully prepared uniform Te nanowires and nanotubes through the reduction of H6TeO6 by N2H4 H2O or EG in refluxing process [7,8]. And Qian’s group produced a series of 1D Te nanostructures including nanowires, nanobelts and nanotubes via hydrothermal synthesis [9–12]. In the above studies, the intrinsic linear crystallographic structure of t-Te acts as the crucial factor that induces the 1D growth. It is demonstrated that, when tellurium grows from solution, the growth tends to occur along c-axis for the stronger covalent bonds than the Van-der-Waals forces among the chains. Up to now, further exploration to simple synthetic route for preparing 1D Te nanostructures is still an interesting subject. Sonochemistry, as a significant technique, is currently found to be facile and efficient for preparing nanostructured inorganic materials at room temperature and under atmosphere pressure. In an irradiated liquid, the implosive collapse of micron-sized bubbles during acoustic cavitation results in localized hotspots [13,14]. With observed temperatures of 5000 K, pressures of 300 atm, and cooling rates in excess of 109 K/s [15,16], these extreme conditions under ultrasonic irradiation are responsible for a variety of chemical and physical effects. In previous research, some inorganic nanomaterials with novel morphologies, such as highly porous Mo2C powders [17], hollow spheres of MoS2 [18], CeOHCO3 nanorods [19], elemental selenium nanotubes and nanowires [20,21], and nanobelts of PbS [22], have been successfully synthesized by sonochemical approaches. For elemental tellurium, recently, Zhu et al. [23] presented an ultrasonic-assisted solution-phase approach for the fabrication of tellurium bundles of nanowhiskers. In their report, however, the ultrasound was only used to facilitate the crystallization of precursor suspension that containing amorphous tellurium nanoparticles. The manifestation that the sonication serves as the driving force for the reaction was not involved. Herein, we develop a one-step sonochemical process to the synthesis of 1D nanocrystalline Te rods. Ultrasonic treatment is directly placed on the solution, resulting in the formation of precursor sol. Ultraviolet-visible (UV-vis) spectra at ordinal stages are recorded to confirm the reaction process. The formation mechanism of the final nanorods is studied based on the linear crystallographic nature of t-Te. And the nanorod branched structures in the product, resulting from the multi-nuclei evolution from monomer colloid, are also investigated. 2. Experimental procedure 2.1. Materials preparation All reagents were purchased from the Shanghai Chemical Company and used without further purification, including condensed nitric acid (HNO3, AR), tellurium
(Te, 99.999%), sodium hydroxide (NaOH, AR), D-glucose (C6H12O6 H2O, AR), and polyethylene glycol (PEG-400, CP). The tellurium nitrate (Te–N–O) powder, which is used as the Te source in the sonochemical reaction, was prepared from the oxidation of bulk gray tellurium in condensed HNO3. One gram bulk tellurium was added to 7 mL HNO3 at room temperature. Under agitation, a white suspension formed after about 10 h. Then the suspension was filtered and evaporated at 40 1C in air, resulting in the white Te–N–O powder. In a typical sonochemical synthesis, 0.08 g Te–N–O, 1 g C6H12O6 H2O and 5 mL PEG-400 were added into 50 mL of 0.2 M NaOH aqueous solution. Under agitation, a pellucid solution was obtained. Then the solution was placed in a laboratory ultrasonic bath (160 W and 59 KHz, KUDOS SK3300HP, Shanghai) for 2 h treatment. With the prolonging of sonication, the colorless solution gradually turned yellow. Subsequently, the resultant yellow sol was kept in darkness for 24 h to allow the growth of Te NCs. Then the dark precipitate was collected by centrifugation, washed with de-ionized water and absolute ethanol for several times, and dried in air. 2.2. Characterization UV-vis spectra are measured using a UV-2300 spectrophotometer (Techcomp, China), and the samples are taken from the solution at different sonication time, including 20, 40, 60, and 80 min. The composition and phase structure were investigated by the use of wide X-ray powder diffraction (XRD) that were recorded with a Rigaku Rotaflex diffractometer equipped with a rotating anode and using Cu Ka radiation. The transmission electron microscope (TEM) images and selected area electron diffraction (SAED) as well as high-resolution electron microscope (HRTEM) images were obtained on a JEOL JEM-2100F field emission electron microscope. 3. Results and discussion 3.1. The formation of Te NCs The phase and composition of the products is first examined by the use of X-ray diffraction. Te–N–O powder is utilized as the Te source in the sonochemical preparation of 1D Te nanostructures. Fig. 1 illustrates the XRD patterns of both the precursor Te–N–O powder and the asprepared tellurium NCs. In Fig. 1a, all the diffraction peaks are in well accordance with the corresponding literature data of Te–N–O (JCPDS 01-0669). Fig. 1b displays a typical XRD pattern of the final products Te. As represented in the micrograph, the diffraction peaks can be readily indexed to trigonal tellurium, with lattice constants a ¼ b ¼ 4.458 A˚ and c ¼ 5.927 A˚ (JCPDS 36-1452). That no impurity such as Te–N–O is detected confirms the high purity of the product.
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along their longitudinal axis, it can be speculated that the boundary is formed by the intersection of the same plane of the connected nanorod branches. 3.3. Growth mechanism
Fig. 1. XRD patterns of (a) Te–N–O powder; (b) tellurium nanorods obtained in the sonochemical synthesis.
3.2. The microstructures of Te nanostructures The morphologies and microstructures of as-prepared Te nanostructures were investigated by TEM and HRTEM. Fig. 2a and b display the panoramic view of the sample with different magnifications. The product consists mainly of nanorods with diameters in range of 30–60 nm and length 200–300 nm. An individual nanorod is represented in Fig. 2c. It has a uniform diameter of 30 nm and length of about 250 nm. The inserted SAED pattern (the zone axis [0 1 0]) reveals that the nanorod is single crystalline with [0 0 1] orientation along the longitudinal axis. In the HRTEM image (Fig. 2d), average lattice spacing around 0.586 nm, corresponding to the (0 0 3) planes of trigonal tellurium, is clearly displayed. Since the (0 0 3) crystal planes are approximately vertical to the long axis, it further confirms that the tellurium nanorods grow predominantly along the [0 0 1] direction. In the product, some intriguing nanorod branched structures are also found. They are similar to the structures reported by CNR Rao’s group, where Te nanobelt branches and junctions were prepared via a hydrothermal process [6]. Fig. 2e shows a typical example of the branched structure of Te. It consists of six nanorod branches. The representative SAED pattern (inset of Fig. 2e) shows that the branch is single crystalline with [0 0 1] growth direction, which is identical to the nanorods as mentioned above. The HRTEM image in Fig. 2f, corresponding to the connected part marked with white circle in Fig. 2e, displays an evident and straight grain boundary in the connection. This boundary denies the supposition that the structure results from the simple aggregation of Te nanorods. Further, the measurements of angles demonstrate that the boundary is exactly consilient to the angle bisector of these two branches, with ca. 36.51 from each longitudinal axis. Since both the branches have the same [0 0 1] growth orientation
The ultrasonic treatment serves as the driving force for the reaction in the solution that resulting in the precursor sol. Under ultrasonic irradiation, the resultant localized hotspots would induce the reduction of Te source by Dglucose, leading to the production of a large quantity of Te molecules. Amorphous Te colloids, with the highest free energy among all the phases of tellurium, are of preference resulting from the rapid aggregation of the molecules. Further sonication also facilitates the formation of trigonal nuclei (seeds) through a homogeneous nucleation process [21]. With the help of the stabilizer PEG-400, a homogeneous sol is achieved instead of sedimentation. Correspondingly, we have measured the changes of the reactant solution in optical properties accompanying with the prolongation of the sonication time. Fig. 3 shows the ultraviolet-visible (UV-vis) absorption spectra of the solution recorded at different intervals, where the original solution before sonication acts as the reference. As revealed in the spectra, evident absorption peaks in the region of 300–400 nm appear. And along with the extension of the time, the peak is obviously red-shifted and the intensity increases, implying the occurrence of the reaction and the size change of the colloids [24]. When the sol containing both amorphous colloids and tTe seeds is aged in darkness, subsequently, the colloids would dissolve by inches into the solution due to their higher free energies. Then the as-generated Te atoms stack on the initial seeds to allow the nanorod growth in trigonal form for its lower free energy as compared to amorphous phase [7,25]. During the Ostwald ripening process, the 1D morphology is determined, as shown in Fig. 4a, by the intrinsic anisotropy of the building blocks, responding to the extended and helical chains of Te atoms in trigonal phase. When Te is crystallized from solution system, the growth tends to occur along longitudinal orientation for the stronger covalent bonds than the Van-der-Waals forces among the chains. To investigate the growth process of 1D Te in the ultrasonic-induced approach, Fig. 4b and c reveal an intermediate sample obtained by aging the precursor sol for 12 h. With the co-existence of some colloids, the middle outlooks of Te nanostructures in the evolution process are represented with different scope. Fig. 4b captures a growing unit (marked by the arrow), which is belt-like and displays the helical twisting characteristic. This structure is enantiomorphous to the unique helical-chain structure of t-Te and somewhat similar to the nanobelts reported by Qian [12]. In the solution, however, the abundance of the dangling bonds on the surface of the intermediate makes the structure more active than the mature Te nanorods. With the prolonging of aging time,
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Fig. 2. TEM micrographs of a typical Te sample prepared by ultrasonic-induced method and kept for 24 h after sonication. (a) Panoramic view at low magnification; (b) panoramic view at higher magnification; (c) an individual nanorod, the inset is the corresponding SAED pattern; (d) HRTEM image of the individual nanorod; (e) TEM micrograph of a representative nanorod branched structure, the inset is representative SAED pattern of a branch; (f) HRTEM image of the connected area between two branches.
the subsequent growth from the dangling sites along the [0 0 1] orientation lead to further morphology evolution that resulting in stable nanorods. Fig. 4c shows a larger scope of the intermediate morphology where more quasinanorods are in upgrowth. Compared with the single one in Fig. 4b (marked by the arrow), they are more straight and evolutional to rod-like structure. Since most of the
quasi-nanorods in the image evolve from the central colloid (marked with white circle in Fig. 4c) and towards different directions, the rudiment status of multi-branched structure is also demonstrated. Very recently, branched architectures are realized beneficial to improve structural complexity and induce novel functionality [26]. The relevant exploration on the
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synthetic strategy and formation mechanism attracts increasing attention [27–31]. For elemental Te, Rao’s group reported the synthesis of the feather-like structure consisted of Te nanorods by a self-seeding solution process [6], where the mechanism based on defect-based growth was proposed. It was suggested that, in the growth process of Te nanorods, the defects on the shaping nanorods could
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act as new nuclei giving rise to another branch growth. However, their experimental conditions, where the formation of abundant a-Te in the solution resulted in rapid precipitation leading to defected rod-like structure, are different from ours where a relative homogeneous sol was obtained. As depicted in Fig. 4c, we suggest that the nanorod branched structures in the ultrasonic-induced synthesis result from the multi-nuclei growth in monomer colloid. In the sonochemical synthesis, ultrasonic treatment would result in the production of amorphous Te colloids and t-Te nuclei. While the isolated nuclei in the sol evolve to individual nanorod, every nuclei sticking on the surface of each colloid could act as a new seed and growth site for the nanorod branch at the expense of surrounding colloids. Due to the same origination, the vicinal nanorods adhere to the central colloid and branched structure forms. Through the re-organization and self-modified arrangement of Te atoms in the junctions, nanorod branched structures rather than simple physical aggregations of the nanorods are obtained as a whole. A schematic growth mechanism for the Te nanorod and nanorod branched structure is illustrated in Fig. 5. 4. Conclusions
Fig. 3. UV-vis absorbance spectra, taken from the reactant solution at different time during the sonication process.
In summary, crystalline tellurium nanorods and nanorod branched structures are successfully prepared at room
Fig. 4. (a) A schematic illustration showing the crystal structure of t-Te. (b, c) TEM micrographs with different extensions of a sample obtained by aging the sol for 12 h after sonication.
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Fig. 5. Schematic illustration of the shaping process of individual Te nanorod and nanorod branched structure in the ultrasonic-induced synthesis.
temperature via an ultrasonic-induced process in aqueous solution. Polyethylene glycol acts as stabilizer and plays an important role for the preparation of homogeneous precursor sol. The natural helical-chain structure of t-Te acts as the dominant factor resulting in 1D growth. Owing to the high reactivity of Te to a wealth of chemicals, these structures are of interest as templates for fabricating nanocables and heterojunctions of other functional materials. Acknowledgements Financial support from Chinese Academy of Sciences and Shanghai Institute of Ceramics under the program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) is gratefully acknowledged. References [1] Y.D. Yin, A.P. Alivisatos, Nature 437 (2005) 664. [2] X.G. Peng, L. Manna, W.D. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos, Nature 404 (2000) 59.
[3] C. Pacholski, A. Kornowski, H. Weller, Angew. Chem. Int. Ed. 41 (2002) 1189. [4] A.A. Kudryavstev, The Chemistry and Technology of Selenium and Tellurium, Collet’s Ltd., London, 1974. [5] P. Tangney, S. Fahy, Phys. Rev. B 65 (2002) 054302. [6] U.K. Gautam, C.N.R. Rao, J. Mater. Chem. 14 (2004) 2530. [7] B. Mayers, Y. Xia, J. Mater. Chem. 12 (2002) 1875. [8] B. Mayers, Y. Xia, Adv. Mater. 14 (2002) 279. [9] L.Q. Xu, Y.W. Ding, G.C. Xi, W.Q. Zhang, Y.Y. Peng, W.C. Yu, Y.T. Qian, Chem. Lett. 33 (2004) 592. [10] Z.P. Liu, Z.K. Hu, J.B. Liang, S. Li, Y. Yang, S. Peng, Y.T. Qian, Langmuir 20 (2004) 214. [11] G.C. Xi, Y.Y. Peng, W.C. Yu, Y.T. Qian, Cryst. Growth Des. 5 (2005) 325. [12] M.S. Mo, J.H. Zeng, X.M. Liu, W.C. Yu, S.Y. Zhang, Y.T. Qian, Adv. Mater. 14 (2002) 1658. [13] L.A. Crum, T.J. Mason, J.L. Reisse, K.S. Suslick, Sonochemistry and Sonoluminescence, Kluwer Academic Publishers, Dordrecht, 1999. [14] K.S. Suslick, Science 247 (1990) 1439. [15] E.B. Flint, K.S. Suslick, Science 253 (1991) 1397. [16] W.B. McNamara, Y.T. Didenko, K.S. Suslick, Nature 401 (1999) 772. [17] T. Hyeon, M. Fang, K.S. Suslick, J. Am. Chem. Soc. 118 (1996) 5492. [18] I. Uzcanga, I. Bezverkhyy, P. Afanasiev, C. Scott, M. Vrinat, Chem. Mater. 17 (2005) 3575. [19] R.J. Qi, Y.J. Zhu, G.F. Cheng, Y.H. Huang, Nanotechnology 16 (2005) 2502. [20] X. Li, Y. Li, S. Li, W. Zhou, H. Chu, W. Chen, I.L. Li, Z. Tang, Cryst. Growth Des. 5 (2005) 911. [21] B. Gates, B. Mayers, A. Grossman, Y. Xia, Adv. Mater. 14 (2002) 1749. [22] S.M. Zhou, X.H. Zhang, X.M. Meng, X. Fan, S.T. Lee, S.K. Wu, J. Solid State Chem. 178 (2005) 399. [23] B. Zhou, J.R. Zhang, L. Zhao, J.M. Zhu, J.J. Zhu, Ultrason. Sonochem. 13 (2006) 352. [24] J.J. Li, Y.A. Wang, W.Z. Guo, J.C. Keay, T.D. Mishima, M.B. Johnson, X.G. Peng, J. Am. Chem. Soc. 125 (2003) 12567. [25] R.B. Zheng, W.L. Cheng, E.K. Wang, S.J. Dong, Chem. Phys. Lett. 395 (2004) 302. [26] D.L. Wang, C.M. Lieber, Nature Mater. 2 (2003) 355. [27] D.L. Wang, F. Qian, C. Yang, Z.H. Zhong, C.M. Lieber, Nano Lett. 4 (2004) 871. [28] K.A. Dick, K. Deppert, M.W. Larsson, T. Martensson, W. Seifert, L.R. Wallenberg, L. Samuelson, Nature Mater. 3 (2004) 380. [29] Y. Cheng, Y.S. Wang, D.Q. Chen, F.J. Bao, J. Phys. Chem. B 109 (2005) 794. [30] C. Ribeiro, E.J.H. Lee, E. Longo, E.R. Leite, Chem. Phys. Chem. 6 (2005) 690. [31] X.B. Cao, Y. Xie, L.Y. Li, J. Solid State Chem. 177 (2004) 202.
Journal of Crystal Growth 295 (2006) 69–74 www.elsevier.com/locate/jcrysgro
Ultrasonic-induced growth of crystalline tellurium nanorods and related branched structures Wei Zhua,b, Wenzhong Wanga,, Haolan Xua,b, Lin Zhoua,b, Lisha Zhanga, Jianlin Shia a
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, PR China b Graduate School of the Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, PR China Received 7 June 2006; received in revised form 22 July 2006; accepted 25 July 2006 Communicated by P. Rudolph
Abstract An ultrasonic route for the preparation of tellurium nanorods in aqueous solution is established at room temperature. The as-obtained nanorods are characterized to be single crystalline with [0 0 1] growth orientation, and have 30–60 nm in diameter and 200–300 nm in length. Some branched architectures, consisting of several nanorods, are also found in the products. In the synthesis, tellurium nitrate powder is utilized as the tellurium source. UV-vis spectra are recorded at ordinal stages accompanying with the prolongation of the sonochemical reaction. Based on the anisotropic growth determined by the intrinsic linear crystallographic structure of trigonal tellurium, the shaping mechanism of the nanorods is investigated. And the formation of the branched structures is suggested to be the result of multi-nuclei growth in monomer colloid. r 2006 Elsevier B.V. All rights reserved. PACS: 81.05.Cy; 81.10.Dn; 81.20.Ka Keywords: A1. Crystal morphology; A1. Nanostructures; A2. Growth from solutions; A2. Single crystal growth
1. Introduction For the growth of inorganic nanocrystals (NCs) in liquid solutions, the dependence of kinetic shape control on the intrinsic crystallographic structure of the target material is always an important research subject due to the affinitive relationship between the growth kinetics and surface energy [1]. As a response to anisotropic crystal structure, asymmetric morphology is generally of preference resulting from the different growth rate of the facets. Some relevant progressions, based on ‘‘Ostwald ripening’’ or ‘‘oriented attachment’’ mechanism, have been reported. For example, Peng et al. [2] presented the controllable preparation of spherical and rod-like CdSe NCs by manipulation of the monomer concentration. Weller et al. reported the formation of high-quality single crystalline ZnO nanorods based Corresponding author. Tel.: +86 21 5241 5295; fax: +86 21 5241 3122.
E-mail address: [email protected] (W. Wang). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.07.023
on oriented attachment of preformed quasi-spherical ZnO nanoparticles [3]. Thus, rigorous exploration about the impact of the crystal structure on the NC evolution and the detailed realization process is technologically meaningful, which would enhance the comprehension about the growth behavior of NC in solution system and be helpful for the following property investigation. Herein, we establish an ultrasonic approach for the synthesis of tellurium nanorods, where the shaping mechanism is investigated based on the anisotropic crystallographic nature. Elemental tellurium (Te) with trigonal (t-) phase is an important material in applications as photoconductivity, non-linear optical responses, piezoelectricity and thermoelectricity [4,5]. It is characteristic to be an intrinsic 31 helical-chain structure. So far, a wealth of chemical methods have been developed for the synthesis of tellurium nanostructures that mainly focuses on one-dimensional (1D) shapes, such as wires, rods, tubes, and belts. For example, Rao et al. reported controlled synthesis of Te
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nanorods, nanowires, nanobelts and related structures by the disproportionation of NaHTe in different solvent systems [6]. Xia et al. have successfully prepared uniform Te nanowires and nanotubes through the reduction of H6TeO6 by N2H4 H2O or EG in refluxing process [7,8]. And Qian’s group produced a series of 1D Te nanostructures including nanowires, nanobelts and nanotubes via hydrothermal synthesis [9–12]. In the above studies, the intrinsic linear crystallographic structure of t-Te acts as the crucial factor that induces the 1D growth. It is demonstrated that, when tellurium grows from solution, the growth tends to occur along c-axis for the stronger covalent bonds than the Van-der-Waals forces among the chains. Up to now, further exploration to simple synthetic route for preparing 1D Te nanostructures is still an interesting subject. Sonochemistry, as a significant technique, is currently found to be facile and efficient for preparing nanostructured inorganic materials at room temperature and under atmosphere pressure. In an irradiated liquid, the implosive collapse of micron-sized bubbles during acoustic cavitation results in localized hotspots [13,14]. With observed temperatures of 5000 K, pressures of 300 atm, and cooling rates in excess of 109 K/s [15,16], these extreme conditions under ultrasonic irradiation are responsible for a variety of chemical and physical effects. In previous research, some inorganic nanomaterials with novel morphologies, such as highly porous Mo2C powders [17], hollow spheres of MoS2 [18], CeOHCO3 nanorods [19], elemental selenium nanotubes and nanowires [20,21], and nanobelts of PbS [22], have been successfully synthesized by sonochemical approaches. For elemental tellurium, recently, Zhu et al. [23] presented an ultrasonic-assisted solution-phase approach for the fabrication of tellurium bundles of nanowhiskers. In their report, however, the ultrasound was only used to facilitate the crystallization of precursor suspension that containing amorphous tellurium nanoparticles. The manifestation that the sonication serves as the driving force for the reaction was not involved. Herein, we develop a one-step sonochemical process to the synthesis of 1D nanocrystalline Te rods. Ultrasonic treatment is directly placed on the solution, resulting in the formation of precursor sol. Ultraviolet-visible (UV-vis) spectra at ordinal stages are recorded to confirm the reaction process. The formation mechanism of the final nanorods is studied based on the linear crystallographic nature of t-Te. And the nanorod branched structures in the product, resulting from the multi-nuclei evolution from monomer colloid, are also investigated. 2. Experimental procedure 2.1. Materials preparation All reagents were purchased from the Shanghai Chemical Company and used without further purification, including condensed nitric acid (HNO3, AR), tellurium
(Te, 99.999%), sodium hydroxide (NaOH, AR), D-glucose (C6H12O6 H2O, AR), and polyethylene glycol (PEG-400, CP). The tellurium nitrate (Te–N–O) powder, which is used as the Te source in the sonochemical reaction, was prepared from the oxidation of bulk gray tellurium in condensed HNO3. One gram bulk tellurium was added to 7 mL HNO3 at room temperature. Under agitation, a white suspension formed after about 10 h. Then the suspension was filtered and evaporated at 40 1C in air, resulting in the white Te–N–O powder. In a typical sonochemical synthesis, 0.08 g Te–N–O, 1 g C6H12O6 H2O and 5 mL PEG-400 were added into 50 mL of 0.2 M NaOH aqueous solution. Under agitation, a pellucid solution was obtained. Then the solution was placed in a laboratory ultrasonic bath (160 W and 59 KHz, KUDOS SK3300HP, Shanghai) for 2 h treatment. With the prolonging of sonication, the colorless solution gradually turned yellow. Subsequently, the resultant yellow sol was kept in darkness for 24 h to allow the growth of Te NCs. Then the dark precipitate was collected by centrifugation, washed with de-ionized water and absolute ethanol for several times, and dried in air. 2.2. Characterization UV-vis spectra are measured using a UV-2300 spectrophotometer (Techcomp, China), and the samples are taken from the solution at different sonication time, including 20, 40, 60, and 80 min. The composition and phase structure were investigated by the use of wide X-ray powder diffraction (XRD) that were recorded with a Rigaku Rotaflex diffractometer equipped with a rotating anode and using Cu Ka radiation. The transmission electron microscope (TEM) images and selected area electron diffraction (SAED) as well as high-resolution electron microscope (HRTEM) images were obtained on a JEOL JEM-2100F field emission electron microscope. 3. Results and discussion 3.1. The formation of Te NCs The phase and composition of the products is first examined by the use of X-ray diffraction. Te–N–O powder is utilized as the Te source in the sonochemical preparation of 1D Te nanostructures. Fig. 1 illustrates the XRD patterns of both the precursor Te–N–O powder and the asprepared tellurium NCs. In Fig. 1a, all the diffraction peaks are in well accordance with the corresponding literature data of Te–N–O (JCPDS 01-0669). Fig. 1b displays a typical XRD pattern of the final products Te. As represented in the micrograph, the diffraction peaks can be readily indexed to trigonal tellurium, with lattice constants a ¼ b ¼ 4.458 A˚ and c ¼ 5.927 A˚ (JCPDS 36-1452). That no impurity such as Te–N–O is detected confirms the high purity of the product.
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along their longitudinal axis, it can be speculated that the boundary is formed by the intersection of the same plane of the connected nanorod branches. 3.3. Growth mechanism
Fig. 1. XRD patterns of (a) Te–N–O powder; (b) tellurium nanorods obtained in the sonochemical synthesis.
3.2. The microstructures of Te nanostructures The morphologies and microstructures of as-prepared Te nanostructures were investigated by TEM and HRTEM. Fig. 2a and b display the panoramic view of the sample with different magnifications. The product consists mainly of nanorods with diameters in range of 30–60 nm and length 200–300 nm. An individual nanorod is represented in Fig. 2c. It has a uniform diameter of 30 nm and length of about 250 nm. The inserted SAED pattern (the zone axis [0 1 0]) reveals that the nanorod is single crystalline with [0 0 1] orientation along the longitudinal axis. In the HRTEM image (Fig. 2d), average lattice spacing around 0.586 nm, corresponding to the (0 0 3) planes of trigonal tellurium, is clearly displayed. Since the (0 0 3) crystal planes are approximately vertical to the long axis, it further confirms that the tellurium nanorods grow predominantly along the [0 0 1] direction. In the product, some intriguing nanorod branched structures are also found. They are similar to the structures reported by CNR Rao’s group, where Te nanobelt branches and junctions were prepared via a hydrothermal process [6]. Fig. 2e shows a typical example of the branched structure of Te. It consists of six nanorod branches. The representative SAED pattern (inset of Fig. 2e) shows that the branch is single crystalline with [0 0 1] growth direction, which is identical to the nanorods as mentioned above. The HRTEM image in Fig. 2f, corresponding to the connected part marked with white circle in Fig. 2e, displays an evident and straight grain boundary in the connection. This boundary denies the supposition that the structure results from the simple aggregation of Te nanorods. Further, the measurements of angles demonstrate that the boundary is exactly consilient to the angle bisector of these two branches, with ca. 36.51 from each longitudinal axis. Since both the branches have the same [0 0 1] growth orientation
The ultrasonic treatment serves as the driving force for the reaction in the solution that resulting in the precursor sol. Under ultrasonic irradiation, the resultant localized hotspots would induce the reduction of Te source by Dglucose, leading to the production of a large quantity of Te molecules. Amorphous Te colloids, with the highest free energy among all the phases of tellurium, are of preference resulting from the rapid aggregation of the molecules. Further sonication also facilitates the formation of trigonal nuclei (seeds) through a homogeneous nucleation process [21]. With the help of the stabilizer PEG-400, a homogeneous sol is achieved instead of sedimentation. Correspondingly, we have measured the changes of the reactant solution in optical properties accompanying with the prolongation of the sonication time. Fig. 3 shows the ultraviolet-visible (UV-vis) absorption spectra of the solution recorded at different intervals, where the original solution before sonication acts as the reference. As revealed in the spectra, evident absorption peaks in the region of 300–400 nm appear. And along with the extension of the time, the peak is obviously red-shifted and the intensity increases, implying the occurrence of the reaction and the size change of the colloids [24]. When the sol containing both amorphous colloids and tTe seeds is aged in darkness, subsequently, the colloids would dissolve by inches into the solution due to their higher free energies. Then the as-generated Te atoms stack on the initial seeds to allow the nanorod growth in trigonal form for its lower free energy as compared to amorphous phase [7,25]. During the Ostwald ripening process, the 1D morphology is determined, as shown in Fig. 4a, by the intrinsic anisotropy of the building blocks, responding to the extended and helical chains of Te atoms in trigonal phase. When Te is crystallized from solution system, the growth tends to occur along longitudinal orientation for the stronger covalent bonds than the Van-der-Waals forces among the chains. To investigate the growth process of 1D Te in the ultrasonic-induced approach, Fig. 4b and c reveal an intermediate sample obtained by aging the precursor sol for 12 h. With the co-existence of some colloids, the middle outlooks of Te nanostructures in the evolution process are represented with different scope. Fig. 4b captures a growing unit (marked by the arrow), which is belt-like and displays the helical twisting characteristic. This structure is enantiomorphous to the unique helical-chain structure of t-Te and somewhat similar to the nanobelts reported by Qian [12]. In the solution, however, the abundance of the dangling bonds on the surface of the intermediate makes the structure more active than the mature Te nanorods. With the prolonging of aging time,
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Fig. 2. TEM micrographs of a typical Te sample prepared by ultrasonic-induced method and kept for 24 h after sonication. (a) Panoramic view at low magnification; (b) panoramic view at higher magnification; (c) an individual nanorod, the inset is the corresponding SAED pattern; (d) HRTEM image of the individual nanorod; (e) TEM micrograph of a representative nanorod branched structure, the inset is representative SAED pattern of a branch; (f) HRTEM image of the connected area between two branches.
the subsequent growth from the dangling sites along the [0 0 1] orientation lead to further morphology evolution that resulting in stable nanorods. Fig. 4c shows a larger scope of the intermediate morphology where more quasinanorods are in upgrowth. Compared with the single one in Fig. 4b (marked by the arrow), they are more straight and evolutional to rod-like structure. Since most of the
quasi-nanorods in the image evolve from the central colloid (marked with white circle in Fig. 4c) and towards different directions, the rudiment status of multi-branched structure is also demonstrated. Very recently, branched architectures are realized beneficial to improve structural complexity and induce novel functionality [26]. The relevant exploration on the
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synthetic strategy and formation mechanism attracts increasing attention [27–31]. For elemental Te, Rao’s group reported the synthesis of the feather-like structure consisted of Te nanorods by a self-seeding solution process [6], where the mechanism based on defect-based growth was proposed. It was suggested that, in the growth process of Te nanorods, the defects on the shaping nanorods could
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act as new nuclei giving rise to another branch growth. However, their experimental conditions, where the formation of abundant a-Te in the solution resulted in rapid precipitation leading to defected rod-like structure, are different from ours where a relative homogeneous sol was obtained. As depicted in Fig. 4c, we suggest that the nanorod branched structures in the ultrasonic-induced synthesis result from the multi-nuclei growth in monomer colloid. In the sonochemical synthesis, ultrasonic treatment would result in the production of amorphous Te colloids and t-Te nuclei. While the isolated nuclei in the sol evolve to individual nanorod, every nuclei sticking on the surface of each colloid could act as a new seed and growth site for the nanorod branch at the expense of surrounding colloids. Due to the same origination, the vicinal nanorods adhere to the central colloid and branched structure forms. Through the re-organization and self-modified arrangement of Te atoms in the junctions, nanorod branched structures rather than simple physical aggregations of the nanorods are obtained as a whole. A schematic growth mechanism for the Te nanorod and nanorod branched structure is illustrated in Fig. 5. 4. Conclusions
Fig. 3. UV-vis absorbance spectra, taken from the reactant solution at different time during the sonication process.
In summary, crystalline tellurium nanorods and nanorod branched structures are successfully prepared at room
Fig. 4. (a) A schematic illustration showing the crystal structure of t-Te. (b, c) TEM micrographs with different extensions of a sample obtained by aging the sol for 12 h after sonication.
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Fig. 5. Schematic illustration of the shaping process of individual Te nanorod and nanorod branched structure in the ultrasonic-induced synthesis.
temperature via an ultrasonic-induced process in aqueous solution. Polyethylene glycol acts as stabilizer and plays an important role for the preparation of homogeneous precursor sol. The natural helical-chain structure of t-Te acts as the dominant factor resulting in 1D growth. Owing to the high reactivity of Te to a wealth of chemicals, these structures are of interest as templates for fabricating nanocables and heterojunctions of other functional materials. Acknowledgements Financial support from Chinese Academy of Sciences and Shanghai Institute of Ceramics under the program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program) is gratefully acknowledged. References [1] Y.D. Yin, A.P. Alivisatos, Nature 437 (2005) 664. [2] X.G. Peng, L. Manna, W.D. Yang, J. Wickham, E. Scher, A. Kadavanich, A.P. Alivisatos, Nature 404 (2000) 59.
[3] C. Pacholski, A. Kornowski, H. Weller, Angew. Chem. Int. Ed. 41 (2002) 1189. [4] A.A. Kudryavstev, The Chemistry and Technology of Selenium and Tellurium, Collet’s Ltd., London, 1974. [5] P. Tangney, S. Fahy, Phys. Rev. B 65 (2002) 054302. [6] U.K. Gautam, C.N.R. Rao, J. Mater. Chem. 14 (2004) 2530. [7] B. Mayers, Y. Xia, J. Mater. Chem. 12 (2002) 1875. [8] B. Mayers, Y. Xia, Adv. Mater. 14 (2002) 279. [9] L.Q. Xu, Y.W. Ding, G.C. Xi, W.Q. Zhang, Y.Y. Peng, W.C. Yu, Y.T. Qian, Chem. Lett. 33 (2004) 592. [10] Z.P. Liu, Z.K. Hu, J.B. Liang, S. Li, Y. Yang, S. Peng, Y.T. Qian, Langmuir 20 (2004) 214. [11] G.C. Xi, Y.Y. Peng, W.C. Yu, Y.T. Qian, Cryst. Growth Des. 5 (2005) 325. [12] M.S. Mo, J.H. Zeng, X.M. Liu, W.C. Yu, S.Y. Zhang, Y.T. Qian, Adv. Mater. 14 (2002) 1658. [13] L.A. Crum, T.J. Mason, J.L. Reisse, K.S. Suslick, Sonochemistry and Sonoluminescence, Kluwer Academic Publishers, Dordrecht, 1999. [14] K.S. Suslick, Science 247 (1990) 1439. [15] E.B. Flint, K.S. Suslick, Science 253 (1991) 1397. [16] W.B. McNamara, Y.T. Didenko, K.S. Suslick, Nature 401 (1999) 772. [17] T. Hyeon, M. Fang, K.S. Suslick, J. Am. Chem. Soc. 118 (1996) 5492. [18] I. Uzcanga, I. Bezverkhyy, P. Afanasiev, C. Scott, M. Vrinat, Chem. Mater. 17 (2005) 3575. [19] R.J. Qi, Y.J. Zhu, G.F. Cheng, Y.H. Huang, Nanotechnology 16 (2005) 2502. [20] X. Li, Y. Li, S. Li, W. Zhou, H. Chu, W. Chen, I.L. Li, Z. Tang, Cryst. Growth Des. 5 (2005) 911. [21] B. Gates, B. Mayers, A. Grossman, Y. Xia, Adv. Mater. 14 (2002) 1749. [22] S.M. Zhou, X.H. Zhang, X.M. Meng, X. Fan, S.T. Lee, S.K. Wu, J. Solid State Chem. 178 (2005) 399. [23] B. Zhou, J.R. Zhang, L. Zhao, J.M. Zhu, J.J. Zhu, Ultrason. Sonochem. 13 (2006) 352. [24] J.J. Li, Y.A. Wang, W.Z. Guo, J.C. Keay, T.D. Mishima, M.B. Johnson, X.G. Peng, J. Am. Chem. Soc. 125 (2003) 12567. [25] R.B. Zheng, W.L. Cheng, E.K. Wang, S.J. Dong, Chem. Phys. Lett. 395 (2004) 302. [26] D.L. Wang, C.M. Lieber, Nature Mater. 2 (2003) 355. [27] D.L. Wang, F. Qian, C. Yang, Z.H. Zhong, C.M. Lieber, Nano Lett. 4 (2004) 871. [28] K.A. Dick, K. Deppert, M.W. Larsson, T. Martensson, W. Seifert, L.R. Wallenberg, L. Samuelson, Nature Mater. 3 (2004) 380. [29] Y. Cheng, Y.S. Wang, D.Q. Chen, F.J. Bao, J. Phys. Chem. B 109 (2005) 794. [30] C. Ribeiro, E.J.H. Lee, E. Longo, E.R. Leite, Chem. Phys. Chem. 6 (2005) 690. [31] X.B. Cao, Y. Xie, L.Y. Li, J. Solid State Chem. 177 (2004) 202.