Construction of Unconventional Hexapod-like Tellurium Nanostructure

May 6, 2009 - Unconventional hexapod-like tellurium nanostructures have been successfully constructed through an easy solution-based approach by recry...
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Construction of Unconventional Hexapod-like Tellurium Nanostructure with Morphology-Dependent Photoluminescence Property Jian-Min Shen,*,† Jiang-Ying Li,‡,§ Yuan Chen,† and Zhen Huang‡ School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, Singapore, Department of Chemistry, UniVersity of Science and Technology of China, China, and Institute of Materials Chemistry, School of Materials Science Engineering, Tongji UniVersity, China ReceiVed: March 11, 2009; ReVised Manuscript ReceiVed: April 17, 2009

Unconventional hexapod-like tellurium nanostructures have been successfully constructed through an easy solution-based approach by recrystallizing Te powder in a mixture of ethylenediamine and hydrazine hydrate at 110 °C for 72 h. The hexapod-like nanostructure is composed of six cross-linked branches with length of about 1 µm. Each branch consists of two perpendicularly intersected nanoribbons with a constant thickness of 12 nm and tapered width from ∼200 nm at root to less than 20 nm at tip. Prominent photoluminescence emission centered at around 428 nm in blue-violet region under excitation wavelength of 365 nm has been found to be closely associated with the unique morphology of the product. A possible solid-solution-solid mechanism has been proposed on the basis of experimental facts to elucidate the formation this unconventional nanostructure. The influence of reaction temperature and solution property on the morphology and structure of the final product has also been discussed. Introduction In the past decade, extensive efforts have been devoted to the synthesis of different materials at nanoscale, a domain wherein the size and shape are of great importance in determining the properties of materials.1 Materials with unique morphologies and structures often bring about new application fields and emergence of nanodevices with promising functions.2,3 To date, the methodology in shape and morphology control in synthesizing one-dimensional (1D) nanomaterials has been intensively studied. Various 1D nanostructured materials, including nanowires, nanobelts, and nanotubes, have been successfully realized through novel approaches such as anisotropic crystal growth4-7 and template-directed growth.8,9 To fulfill the requirements of constructing nanoscale devices in the future, three-dimensional (3D) nanostructures with complicated configurations and unique properties have been proved to be a superior candidate to their 1D counterparts.10 Since the first report of tetrapod CdSe nanocrystals,11 some special 3D nanoarchitectures, such as dart-, comb-, windmill-, and tetrapod-like structures have been synthesized in II-VI group semiconductors (CdS, CdTe, ZnO, ZnS, ZnSe).12-19 Very recently, another type of hexapod-like structure with all its branches extending along six equivalent 〈100〉 directions has been realized in WO3-δ, PbSe, Fe3O4/C nanowire networks20-23 and branched MnO, PbS/PbSe nanocrystals.24-26 To the best of our knowledge, the unconventional hexapod-like nanostructure either originates in the intrinsic high symmetry of crystal lattices or results from the assembly of subunits through oriented attachment. Construction of functional nanomaterials with complicated 3D architectures through a more effective and convenient pathway still remains a challenge in material science. * Corresponding author. Tel: +65-83507182. Fax: +65-67947553. E-mail: [email protected], [email protected]. † Nanyang Technological University. ‡ University of Science and Technology of China. § Tongji University.

Figure 1. XRD pattern of the final products.

Having a combination of many important properties and being a promising candidate in fabricating new nanodevices, tellurium has attracted plenty of research interest in recent years. To date, not only various 1D Te nanostructures, such as tubes, rods, wires, and belts27-43 but also some planar branched structures44,45 have been successfully realized in solution phase. In this work, we demonstrate an easy wet chemistry approach to the synthesis of unconventional 3D hexapod-like Te nanostructures. Experimental Section In a typical experiment, 1 mmol of Te powder (AR) was first put into 20 mL of ethylenediamine (en) (AR), which was then mixed with 8 mL of hydrazine hydrate (∼85%) and 10 mL of distilled water. The mixture was magnetically stirred for 10 min and transferred into a Teflon-lined autoclave of 50 mL capacity, which was kept at 110 °C for 72 h and naturally cooled down. The resulting dark solid raw products were centrifuged at 2000

10.1021/jp902171x CCC: $40.75  2009 American Chemical Society Published on Web 05/06/2009

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Figure 2. (a) FESEM image of a hexapod microstructure in the raw product. (b) FESEM and (c) TEM study of a branch. (d) TEM image of a hexapod-like nanostructure viewed along one of its branches and (e) the corresponding SAED pattern with a tetrad rotation axis (marked with “[”) obtained at its center; one group of the diffraction spots has been indexed.

Figure 3. (a) TEM image of a hexapod-like nanostructure. (b) Detailed HRTEM study on the vertical nanoribbon; (inset) SAED pattern. (c) Crystal structure of the horizontal nanoribbon; (inset) SAED pattern. (d) HRTEM image of the whole branch; (inset) SAED pattern, in which “V”, “H”, and “VH” prefixes means that the diffraction spot is from the vertical, horizontal nanoribbon, and both of the two above, respectively.

rpm to collect the precipitation, which was washed with distilled water and absolute ethanol three times and finally dried in a vacuum at 70 °C for 8 h. X-ray powder diffraction (XRD) pattern, recorded with a Philips X′ Pert PRO SUPER XRD diffractometer (λ ) 0.154 187 4 nm), was used to study the phase and composition of the final product. The morphology and structure of the product

were studied by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), respectively. FESEM examinations were carried out on a JEOL JSM-6700F instrument working at 10 kV. TEM, high-resolution TEM (HRTEM) images, selected area electron diffraction (SAED) patterns, and energy dispersive X-ray spectroscopy (EDS) analysis were used to examine the microstructures and

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Figure 4. (a) HRTEM image of the transition section between two horizontal nanoribbons; the disordered segment is highlighted with white line. (b) Perspective view of the t-Te crystal structure.

elemental composition of the final products on a JEOL-2010 instrument with an accelerating voltage of 200 kV. UV-vis spectra were recorded on a Varian Cary 5000 UV-vis-NIR spectrophotometer at room temperature. Photoluminescence measurements were conducted on a Jobin-Yvon Nanolog-3 spectrofluorometer instrument at room temperature. Results and Discussion Powder X-ray diffraction (XRD) was used to examine the crystallinity and phase purity of the as-prepared product. As shown in Figure 1, all reflections can be readily indexed to pure trigonal phase [space group: P3121 (152)] of Te with lattice constants a ) 0.4461 nm and c ) 0.5880 nm, which are close to the literature value of a ) 0.4458 nm and c ) 0.5927 nm (JCPDS Card, No. 36-1452). Compared with the results of previous reports31-38 and its rod-like bulk precursor (Figure S1b, Supporting Information), an extraordinarily strong (003) diffraction peak indicates that the [001] crystalline direction of the sample is not completely parallel to the sample support, which further suggests that the product is not only composed of simple 1D Te nanostructures. In the pattern, no peaks induced by impurities can be observed, which indicates the high purity of the product. The morphology and structure of the product were studied by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), respectively. A typical FESEM image of the raw product in Figure 2a reveals some particles and an unconventional hexapod-like nanostructure, indicating a great morphological and structural variety from its bulk precursor (Figure S1a, Supporting Information). Each branch of the nanostructure is composed of two tapered ribbonlike structures intersecting with each other. The branch is about 1 µm in length with a 200 nm wide root connecting to the center and a tip less than 20 nm. Detailed inspection in Figure 2b reveals that one branch of the nanostructure is built of two almost perpendicularly intersected nanoribbons and the thickness of the vertical one (∼12 nm) remains constant along its growth direction. Shown in Figure 2c is the corresponding TEM image of a branch, in which an obvious line in the middle section of the structure with darker contrast is consistent with the vertical nanoribbon of the nanostructure. Figure 2d is a TEM study imaged along one branch and clearly reveals the other two pairs

of perpendicularly linked branches of the hexapod-like nanostructure. In Figure 2e, a selected area electron diffraction (SAED) pattern is obtained by focusing the incident electron beam at the center of the hexapod-like nanostructure, a position marked with an arrow in Figure 2d. The diffraction spots correspond to four sets of (100), (101), (102), and (003) crystalline planes of trigonal tellurium (t-Te) along the same [010] zone axis with a 4-fold rotation axis, marked with a “[”, at its center. According to the rules of crystallography, no 4-fold symmetry exists inside a trigonal system. Thus the 4-fold symmetric SAED pattern can only originate from the configuration of the four crystalline branches of the hexapod-like nanostructure. Further structural information of the product comes from HRTEM study. As shown in Figure 3a, the investigation was taken on one branch of a typical hexapod microstructure. An HRTEM image in Figure 3b clearly resolves the (001) and (110) atomic planes of the vertical nanoribbon at the tip of the branch with interplanar spacing of 0.59 and 0.22 nm, respectively. The inset ED pattern can be indexed to the diffraction pattern along the [110j] zone axis of t-Te, which confirms that the growth of the vertical nanoribbon occurs along the [001] crystallographic direction. The HRTEM image obtained from the horizontal nanoribbon of the same branch is shown in Figure 3c, which also clearly displays the fringe spacing that agrees well with the separation among the (001), (100), and (101) planes of t-Te, respectively. Many atomic steps appear periodically at the edge of the nanoribbon and explain the gradual decrease in width along the branch length. The inset ED pattern recorded with the electron beam along the [010] zone axis focused on the horizontal nanoribbon also demonstrates a preferential growth direction along the [001] direction. As the HRTEM investigation was conducted on the whole branch, the lattice fringes of both the vertical and the horizontal nanoribbons can be discerned in Figure 3d. The corresponding ED pattern is composed of two sets of diffraction spots sharing one common (003) diffraction spot, from which we can conclude that the branch is made up of two cross-linked crystalline nanoribbons growing along the same [001] direction. The elemental analysis result of the final product (Figure S2, Supporting Information) probed by energy dispersive X-ray spectrum (EDS) indicates that the hexapodlike nanostructure is composed of pure Te and is consistent with

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Figure 5. TEM images of the products from reactions carried out for (a) 12 h (inset, SAED pattern), (b) 24 h (inset, SAED pattern), and (c) 48 h; one branch of a hexapod-like nanostructure is highlighted with a white line. TEM images of the products obtained at (d) 100 °C and (e) 130 °C, respectively.

SCHEME 1: Schematic Illustration of the Formation Mechanism of Various Te Nanostructuresa

a (a) Te powder is reduced and chelated to form [Te(en)2]n2n- intermediates in solution. (b) When the temperature is below 100 °C, Te crystal seeds grow along (001) direction and (c) finally produces 1D nanowires. (d) At 110 °C, the intermediates can assemble along six equivalent 〈001〉 directions from the center and (e) develop into a hexapod-like nanostructure. (f) As the temperature exceeds 130 °C, some small crystal seeds can interact with larger ones and (g) produce hierarchical nanostructure.

the XRD examination result. In the course of the HRTEM and EDS characterization, the hexapod-like Te nanostructure keeps its morphology and crystal structure unchanged, exhibiting a high stability under the irradiation of a high-energy electron beam. In previous literature, there usually exist twinned crystal planes dividing the nanocrystals into branched structures with clear and

fixed angles according to intrinsic crystal symmetries.11,13-19,44 However, in electronic microscopy examination of our product (Figure 2a,d), an unusual smooth transition joint section between two perpendicularly intersecting nanoribbons can be obviously discerned. HRTEM study (Figure 4a) at one of the joint sections (circle in Figure 2d) clearly resolves the (101) crystal planes of the two adjacent horizontal nanoribbons growing along different

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Figure 6. (a) UV-vis spectrum of the hexapod-like Te nanostructure. (b) Excitation spectrum of the product at an emission wavelength of 435 nm. (c) Photoluminescence spectrum recorded with an excitation wavelength of 365 nm. All measurements were carried out at room temperature.

directions and some disordered fillings between them. The formation mechanism of the disordered areas inside the transition section in the hexapod-like nanostructure can be understood because the crystal lattice at the joint sides has been distorted to such an extent that twinned crystal planes alone are incapable of relaxing crystal strains and stabilizing the Te nanostructure. Thus, the unique joint section is crucial in the formation of this unconventional cross-like nanostructure with a t-Te crystal lattice, which consists of atom-scale helix chains bound together through van der Waals interactions (Figure 4b). Xia et al. took advantage of the t-Te crystal structure to realize various 1D nanostructures in solution phase and proposed a solid-solution-solid mechanism.27,28 On the formation of various branched Te nanostructures, Kotov et al. suggested that the aqueous environment was advantageous to the assembly of t-Te 1D nanoscale building blocks into branched structures,44 while Rao et al. emphasized the importance of pressure in the solvothermal reaction process.45 Nevertheless, formation of cross-linked 3D nanostructures with t-Te crystal lattice in solution phase is rather unexpected. We believe that the construction of the unconventional hexapod-like Te nanostructures can be explained by a similar solid-solution-solid transformation process, in which both the aqueous environment and proper pressure are important. The results have been

evidenced by TEM images taken from products of a series of supplementary experiments at different stages. In a typical reaction, Te powder was slowly reduced and dissolved into solution as Te2-, which could be chelated by en to form some metastable polynuclear intermediates, e.g. [Te(en)2]n2n-.46 As the solution was gradually saturated, a TEM study in Figure a shows that the precipitation is mainly composed of amorphous Te colloids, which aggregate together to form clusters with diameters of ∼20 nm for their relatively high surface energy. The amorphous Te clusters could transform into polycrystalline t-Te crystal seeds through a hydro/solvothermal recrystallization process (Figure 5b).32-39,45,46 Under the morphological effect of chelant, repelling force among [Te(en)2]n2n- polynuclear intermediates, and the high pressure of the solvothermal environment, the t-Te crystal seeds could develop with the involvement of subsequent Te feedstock along six equivalent 〈100〉 directions and form perpendicularly intersected Te nanoribbons (Figure 5c). Meanwhile, some amorphous Te clusters preferentially deposit between the two nanoribbons, a position with relatively higher crystalline strain at the surface of the t-Te crystal seeds, and form a smooth transition section with some disordered segments. In this solid-solution-solid transformation process, we have found that the formation of the unconventional hexapod-like

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Figure 7. (a) FESEM image of the product after grinding and (b) its photoluminescence spectrum under excitation wavelength of 365 nm at room temperature.

nanostructures is not only closely associated with the anisotropic growth of t-Te along the [001] direction, but also with the reaction parameters, including temperature and solvent composition. When the reaction temperature is below 100 °C, not only does the system pressure decrease but the transportation of Te feedstock is mainly confined to the (001) plane of the crystal seeds because of low kinetic energy and high solution viscosity as well. Thus, the product is mainly composed of thin t-Te nanowires (Figure 5d). At higher temperature (>130 °C), the formation of 3D hexapod-like nanostructure is not that uniform. Some smaller crystal seeds even obtain enough energy to interact with larger ones and develop into more complex hierarchical architectures (Figure 5e). The whole process is schematically illustrated in Scheme 1. In addition to the reaction temperature, the solvent composition is another crucial factor determining the morphology and structure of the final product. Many studies have proved that chemicals can direct the evolution of anisotropic semiconductor nanocrystals.2,31-40,45,46 In our case, the chelant, en, plays a key role on the formation process of the 3D hexapod Te nanostructure, because the formation of [Te(en)2]n2n- intermediate and its further assembly cannot be realized without it. In the reaction, hydrazine hydrate not only helps reduce Te powder but also introduces proper viscosity and pressure when it decomposes at high temperature. The volume of hydrazine hydrate should be no less than 6 mL, or the system pressure is not high enough to generate nanoribbons.45 However, the amount of hydrazine hydrate must not exceed 10 mL to prevent possible risks in the reaction process. The optical properties of the hexapod-like Te nanostructures have been studied, and the results are shown in Figure 6. A UV-vis spectrum in Figure 6a exhibits two weak and broadened peaks centered at around 271 nm (4.58 eV) and 572 nm (2.17 eV). Previous literature indicated two characteristic absorption peaks of Te at 2.21 and 4.60 eV, which can be assigned to a forbidden direct transition from the valence band (p-nonbonding triplet) to the conduction band (p-antibonding triplet) and an allowed direct transition from the valence band (p-bonding triplet) to the conduction band (p-antibonding triplet), respectively.47,48 Although the obtained UV-vis spectrum is similar to the reported results of nanowires and thin films,37,48 the origin of the absorption peak at 576 nm still remains unclear at present.45 Room-temperature photoluminescence spectroscopy of the asproduced hexapod-like Te nanostructures was also studied.

Figure 6b shows the excitation spectrum under the wavelength of 455 nm, in which two prominent peaks at 262 and 368 nm can be discerned, respectively. In the blue-violet region (Figure 6c), a strong emission peak at 428 nm can be observed under an excitation wavelength of 365 nm, which is similar to that of the Te nanowires35,37,40,49 and nanotubes.38 In previous reports, belt-like Te nanostructures several hundreds of nanomters wide and a few nanometers thick failed to possesses luminescent properties because of their relatively large sizes and ordinary structures.37 Our unusual result may be associated with the unique hexapod-like structure, since no obvious luminescent peaks can be discerned from a ground sample in which all crosslinked nanostructures were destroyed (Figure 7). Thus, we speculate that the nanoribbons with perpendicularly intersecting configurations have more chances to expose their thin edges (∼12 nm wide), which is probably responsible for the production of photoluminescence in the blue-violet region. Conclusions In conclusion, we have successfully constructed unconventional hexapod-like tellurium nanostructures in solution phase through a possible solid-solution-solid transformation approach. The as-prepared nanostructure is composed of branches along six equivalent 〈100〉 directions, while each branch consists of two perpendicularly intersected nanoribbons with tapered morphology. Our study not only focuses on the evolution of material structure, but also demonstrates an example of its influence on physical properties. Under excitation wavelength of 365 nm, the final product exhibits an unusual morphologydependent photoluminescence emission at around 428 nm in the blue-violet region. A possible solid-solution-solid mechanism has been proposed to elucidate the formation of the unique nanostructure on the basis of our observation results. This simple process is believed to be also applicable for the morphology control of other inorganic nanomaterials, and the product can be integrated into potential optoelectronic nanodevices in the future. Acknowledgment. This work was supported by Nanyang Technological University (AcRF Grant RG38/06); Defense Science & Technology Agency, Singapore (MINDEF-NTU-JPP/ 08/03); 973 Project of China (Grant 2005CB623601); and Foundation of Visiting Ph. D. Candidates from Tongji University.

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