Template-Free Electrodeposition of One-Dimensional Nanostructures of Tellurium Guangwei She,† Wensheng Shi,*,† Xiaohong Zhang,*,† Tailun Wong,‡ Yuan Cai,‡ and Ning Wang‡
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 663–666
Key Laboratory of Photochemical ConVersion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and Department of Physics and Institute of Nano Science and Technology, Hong Kong UniVersity of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ReceiVed August 27, 2008; ReVised Manuscript ReceiVed December 10, 2008
ABSTRACT: Without the aid of surfactants or catalysts, one-dimensional (1D) nanostructures of Te (nanowires with a small quantity of nanotubes and nanoribbons) have been synthesized directly from template-free electrodeposition (TFED) in an aqueous solution at low temperature. As observed by electron microscopy, the as-grown Te nanomaterials are single crystalline trigonal structure and contain few defects. The preferential growth of these 1D nanostructures along the [001] direction is attributed to the intrinsic anisotropic crystal structure of Te and the special growth condition of TFED. It is anticipated that TFED could be used as a versatile approach to synthesize various 1D materials which contain intrinsic highly anisotropic crystal structures. One-dimensional (1D) semiconductor nanostructures have attracted much attention owing to their excellent physical properties and potential applications in future nanodevices. In order to attain the potential offered by 1D nanostructures, one of the most important issues is how to synthesize 1D nanostructures in large quantities with a convenient method. As a facile route for the synthesis of nanostructures, electrodeposition has many advantages such as low-cost, environmentally friendly, high growth rate at a low temperature and better shape or size control.1 To generate 1D morphologies, templates were often used during the electrodeposition process (template-assisted electrodeposition, TAED).2 TAED has been widely used to prepare 1D nanostructures of many materials.3 By using a template, nanorods and nanotubes with uniform diameters as well as segmented nanorods can be readily grown in an array.4,5 It has been known that some materials attain 1D growth under certain conditions without using any template due to their intrinsic highly anisotropic crystal structures.6 For example, hexagonal ZnO nanorods or nanotubes7 and CuTe nanoribbons8 have been synthesized by template-free electrodeposition (TFED) based on the intrinsic anisotropic structures of these materials. It is therefore expected that under proper conditions, TFED could be employed as a versatile method for synthesizing various 1D materials which contain intrinsic highly anisotropic crystal structures. Trigonal tellurium (Te) has an anisotropic crystal structure consisting of helical chains.9 It has a tendency to form 1D nanostructures either through a solution or gas phase process.10 More important, Te is a p-type semiconductor with a narrow band gap (Eg ) 0.35 eV) and a lot of interesting physical properties, including photoconductive,11 piezoelectric,12 thermoelectric,13 nonlinear optical responses.14 Considering these novel properties, Te 1D nanostructures could be used as a multifunctional material in many electric and optoelectronic devices, such as self-developing holographic recording devices,15 radiative cooling devices,16 gas sensors,17 field-effect devices,18 and infrared acoustooptic deflectors.19 As the building blocks for Te-based nanodevices, 1D nanostructures of Te have been intensively studied in the past few years. 1D nanostructures of Te have been synthesized by the refluxing polyol method20 and hydrothermal method,21 the solutionphase method assisted with biomolecular,22 ultrasonic,23 and visible * Corresponding authors. (W.S.) Fax/Tel: +86-10-82543513. E-mail: shiws@ mail.ipc.ac.cn. (X.Z.) Tel: +86-10-82543510. E-mail:
[email protected]. † Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences. ‡ Hong Kong University of Science and Technology.
Figure 1. (a) A SEM image of the as-synthesized products. (b) An enlarged picture showing the morphology of the nanowires.
light,24 the microwave-assisted ionic liquid method,25 the thermal evaporation method,26 etc. Most of these methods involve complicated processes or those performed at a relatively high temperature. Considering the advantages and the applicability of the electrochemical method, TAED has also been used to fabricate Te nanowires with the anodic aluminum oxide (AAO) membranes as templates.27 In these processes, Te nanowires embedded in the nanochannels of the templates suffer various problems for further application. If the templates can be excluded from the electrodeposition process, it will be more convenience for the subsequent fabrication of nanodevices based on the 1D nanostructures of Te. Furthermore, the nonuse of templates will simplify the synthesis process and reduce the cost. In this communication, utilizing the
10.1021/cg800948w CCC: $40.75 2009 American Chemical Society Published on Web 12/24/2008
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Figure 2. The XRD patterns of the as-synthesized products. The vertical lines below indicate the diffractions from the standard pattern of trigonal Te powder (JCPDS No. 36-1452).
intrinsic highly anisotropic crystal structure of Te, its 1D nanostructures were successfully synthesized by TFED method without using any template or catalyst. The electrodeposition used for fabricating 1D nanostructures of Te was performed with a standard three-electrode system. A Pt electrode and a standard saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. A piece of ITO-coated glass (50Ω/0, 3 × 1 cm) was used as the working electrode. The ITO-coated glass was cleaned with a special cleaning agent (TFD 7, Franklab S.A.) and rinsed with distilled water before use. The electrolyte was prepared by dissolving TeO2 powder in 1 M KOH aqueous solution. The concentration of Te in the electrolyte was 0.01 M. The temperature of the electrolyte was kept at 85 °C during the electrodeposition process. A stationary potential of -1.3 V (vs SCE) was applied to the working electrode by an electrochemical analytical instrument (CHI 660C). After electrodeposition for 30 min, a thin film of dark gray electrodeposits was formed on the working electrode. The electrodeposits were
Communications scratched from the substrate, rinsed with distilled water, and dried for structural characterization. The morphology of the electrodeposits was first studied by scanning electron microscopy (SEM, Hitachi S-4300FEG microscope operated at 5 kV). Figure 1 shows the typical SEM images of the as-synthesized products. It was found that nanowires were predominant in the products. The diameters of the nanowires varied from 50 to 500 nm, and the length varied from several micrometers to several tens of micrometers. As shown in the high magnification SEM image in Figure 1b, the nanowires have uniform surface morphologies. Besides nanowires, a small quantity of nanotubes and nanoribbons was also found in the products. X-ray powder diffraction (XRD, Siemens D-500 diffractometer with Cu KR radiation) was carried out to characterize the crystal structure of the as-synthesized samples. All peaks in the XRD pattern (Figure 2) can be readily indexed by a single trigonal phase of Te (JCPDS No. 36-1452) with lattice constants a ) 4.457 Å and c ) 5.927 Å. These results indicated the electrodeposits mainly consisted of Te trigonal nanostructures. Further structural characterization of the Te nanostructures was carried out using transmission electron microscopy (TEM) (JEOL 2010, operated at 200 kV). Figure 3a shows the TEM image of an individual Te nanowire of about 50 nm in diameter and several micrometers in length. The selected-area electron diffraction (SAED) pattern of this nanowire is shown in the inset in Figure 3a which can be indexed by the trigonal Te structure along the [100] zone axis. From this SAED pattern, the growth direction of the nanowire is determined to be along the c axis. A high-resolution TEM (HRTEM) image taken from the edge area of this nanowire is shown in Figure 3b. The spacing between the adjacent lattice fringes is 0.59 nm, which agrees well with the d-spacing of (001) planes of the trigonal Te crystal. This result further confirms that the growth direction of Te nanowires is [001].
Figure 3. (a) TEM images of an individual Te nanowire. The inset is the SAED pattern of the Te nanowire; (b) HRTEM image of the Te nanowire.
Figure 4. (a) SEM and TEM images of an individual Te nanotube; (b) SAED pattern of the tube.
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Figure 5. (a) TEM image of a Te nanoribbon. The inset is the SAED pattern of the ribbon; (b) HRTEM image of the Te nanoribbon.
Figure 6. Structural model of trigonal Te crystal.
A small quantity of Te nanotubes has been found in the products. Figure 4a shows the SEM and TEM images of an individual Te nanotube, revealing that it consists of hollow hexagonal prisms, about 500 nm in diameter and 100 nm in wall thickness. Figure 4b is the SAED pattern recorded from the nanotube. Again, this diffraction pattern can be readily indexed as trigonal Te indicating that the nanotube is single crystalline Te and grows along the [001] direction. Besides nanowires and nanotubes, some nanoribbons were also found in the electrodeposits. Figure 5a is a typical TEM image of an individual Te nanoribbon. The width of the nanoribbon is about 1 µm and the length is up to several tens of micrometers. The SAED pattern is recorded with the electron beam parallel to the [100] zone axis of the trigonal Te. The HRTEM image (Figure 5b) taken from the nanoribbon illustrates that the nanoribbons is single crystalline and contains no defects. The observed interplane placing is about 0.59 nm, which corresponds to the distance between the (001) planes of trigonal Te. Since the (001) planes is perpendicular to the nanoribbon axes, the preferential growth direction is [001] which is same as that of Te nanowires and nanotubes. In order to understand the growth mechanism of the 1D nanostructures, it is necessary to consider the synthesis process and crystal structure of the Te. In the present synthesis process, TeO2 powders were dissolved in the alkaline solution and formed TeO32ions. Whereafter, the TeO32- ions were electrochemically reduced to Te under a negative potential. The whole process of the electrochemical synthesis of Te can be described by the following equations.
TeO2 + 2OH- f TeO23 + H2O
(1)
TeO23 + 3H2O + 4e f Te + 6OH
(2)
It is known that the structure of the trigonal Te crystal is composed of helical chains of Te atoms,27 which is schematically shown by the structural model in Figure 6. The atoms in the same chains are bonded by covalency, whereas the band between the chains are considered to be mixed interactions of weaker electronic and van der Waals forces.28 The helical chains are parallel to the c-axis and would lead to a strong anisotropic growth habit of the Te crystal along the c-axis, that is, the [001] direction.20 As a result,
the trigonal Te has a tendency to form 1D nanostructures along the [001] direction under proper conditions. Besides the intrinsic crystal structure of the Te, the exterior conditions, such as the surface state of the substrate and the local concentration of the nucleating agents, as well as electrodeposition parameters also have important influence on the final morphology of the products.21 It was found that the Te nanostructures did not firmly anchor to the ITO-coated glass to form a solid film in the present electrodeposition process. In contrast, the fluffy electrodeposits were loosely attached to the substrate. This result reveals that the variation of the growth conditions at different locations would influence the growth habit of the crystals. This may be why a small quantity of Te nanotubes and nanoribbons concomitantly grown with nanowires. The investigation on the relationship between the morphologies and growth conditions is still in progress. In summary, TFED was successfully employed to synthesize 1D single crystalline nanostructures of Te (nanowires with a small quantity of nanotubes and nanoribbons). The 1D growth of the Te nanostructures is ascribed to the intrinsic highly anisotropic property of the trigonal Te. Considering previous results in the 1D growth of ZnO and CuTe nanostructures by the TFED,7,8 it could be concluded that the present TFED technique should be a versatile approach for the synthesis of 1D nanostructures of other materials with highly anisotropic crystal structures.
Acknowledgment. We acknowledge the financial support from the Chinese Academy of Sciences “Hundred Talents Program”, the Knowledge Innovation Program of the Chinese Academy of Sciences, the National Basic Research Program of China (973 Program) (Grant No. 2006CB933000, 2007CB936001), National High-tech R&D Program of China (863 Program) (Grant No. 2006AA03Z302, 2007AA03Z300), National Natural Science Foundation of China (Grant No. 50772117,10874189), and Research Grants Council of Hong Kong (Project No. 603006).
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