C Nanocables and

CRYSTAL GROWTH & DESIGN

Fabrication and Characterization of Te/C Nanocables and Carbonaceous Nanotubes

2009 VOL. 9, NO. 1 344–347

Xu Chun Song,*,† Yang Zhao,‡ E Yang,† Yi Fan Zheng,§ Li Zhong Chen,† and Fan Mei Fu† Institute of Physical Chemistry, Department of Chemistry, Fujian Normal UniVersity, Fuzhou 350007, People’s Republic of China, Department of Chemistry, Henan Normal UniVersity, Xinxiang 453007, People’s Republic of China, and College of Chemical Engineering & Materials Science, Zhejiang UniVersity of Technology, Hangzhou 310014, People’s Republic of China ReceiVed May 15, 2008; ReVised Manuscript ReceiVed September 27, 2008

ABSTRACT: In this Article, a green chemical synthetic route was developed to synthesize Te/C nanocables by using glucose as reducing agent and carbon source, Na2TeO3 as a tellurium source, and surfactant cetyltrimethylammonium bromide (CTAB) as a structure-directing agent. The products are characterized in detail by multiform techniques: X-ray diffraction, energy-dispersive X-ray analysis, scanning electron microscopy, and transmission electron microscopy. The formation mechanisms of Te/C nanocables are tentatively proposed. By chemical etching of Te/C nanocables with different morphologies, various carbonaceous nanotubes can be obtained at room temperature.

1. Introduction Recently, trigonal tellurium (t-Te) and related materials have attracted more and more attention. Elemental tellurium is a narrow band gap semiconductor (∼0.35 eV) that displays interesting physical properties, including nonlinear optical responses, photoconductivity, and thermoelectric properties.1 The properties make it useful as an infrared photoconductive detector, piezoelectronic device, and thermoelectronic material.2-4 As those properties within the nanometer regime might be associated with their morphologies, they might act as highly functionalized materials.5,6 Several reports on physical or chemical Te syntheses have recently been published. Qian7 and co-workers reported solution-phase approaches to the synthesis of tellurium nanotubes. Te nanobelts and nanotubes were synthesized by the in situ disproportionation of Na2TeO3 in an aqueous ammonia system under hydrothermal conditions at 180 °C.8 Zhu9 et al. synthesized Te nanowires with a diameter ranging from 20 to 100 nm via a microwave-assisted ionic liquid method. Low-temperature green chemical synthesis in aqueous solution is highly desirable because it represents an environmentally benign and user-friendly approach.10-14 Recently, Qian1 et al. reported a facile green chemical route for the synthesis of highly crystalline Te nanowires by using ascorbic acid as a reducing agent, Na2TeO3 as tellurium source, and cationic surfactant cetyltrimethylammonium bromide (CTAB) as a structure-directing agent. The advantage of this process is that large-scale ultrathin Te nanowires with an average diameter of 7 nm can be prepared at a low temperature of 90 °C, and no harmful or volatile chemicals are used. Nanocables, a new kind of 1-D nanocomposite of nanowires (core) wrapped with one or more outer layers (shell), have emerged recently and attracted much intensive investigation. A number of approaches, such as laser ablation, thermal evaporation, γ irradiation, and templating, have been developed to fabricate these kinds of 1-D nanocomposites.15-18 Recently, many types of nanocables such as ZnO/ZnS,19 Si/SiO2,20 * Corresponding author. Tel.: +86-591-87441126. Fax: +86-591-83465376. E-mail: [email protected]. † Fujian Normal University. ‡ Henan Normal University. § Zhejiang University of Technology.

Figure 1. XRD pattern of Te/C nanocables.

SiC/SiO2,21,22 Ag/C,23-25 Ag/SiO2,26 TiO2/SiO2,27 Se/C,28 and Si/SiO2/C29 have been prepared by various methods. Coupled synthesis and encapsulation in one step is a good choice to form nanocables, which takes advantage of the reaction sequences and thus exhibits higher efficiency and facility. However, up to now, this method has not been as well developed as the post synthesis methods, and relative reports are rare. In this Article, we design a novel surfactants-assisted solution process to prepare tellurium/carbon (Te/C) nanocables via one-step reduction and carbonization under mild hydrothermal conditions. Carbonaceous nanotubes were fabricated through etching Te/C nanocables with a solution of HCl and H2O2 at room temperature.

2. Experimental Section Fabrication of Te/C Nanocables. All of the chemicals were analytic grade reagents without further purification. The Te/C nanocables were synthesized under hydrothermal conditions. Experimental details were as follows: CTAB (1 g) was dissolved in 25 mL of distilled water, and glucose (2 g) was added to it with vigorous stirring. When the solution clarified, 10 mL of aqueous solution containing 0.1 g of Na2TeO3 was added slowly to the above solution under continuous stirring. The final solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and filled with distilled water to 90% of the total volume. The autoclave was sealed and maintained at 170 °C for 12 h. After the reaction was completed, the autoclave was allowed to cool to room

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Te/C Nanocables and Carbonaceous Nanotubes

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Figure 2. (a) SEM and (b) TEM images of Te/C nanocables.

Figure 3. EDS patterns of Te/C nanocables. temperature naturally. The solid black precipitate was filtered, washed several times with distilled water and anhydrous ethanol to remove impurities, and then dried at 60 °C in air. Fabrication of Carbonaceous Nanotubes. A total of 0.1 g of Te/C nanocables was dispersed into 25 mL of aqueous solution containing 3 mL of hydrochloric acid (36.5 wt %) and 5 mL of H2O2 (30 wt %). The solution was kept for 2 h, after which it was filtered to isolate the precipitate that contained carbonaceous nanotubes. The precipitates were washed by distilled water and anhydrous ethanol several times. The products were then dried under vacuum at 60 °C for further characterization. Characterization. The X-ray powder diffraction (XRD) patterns of the samples were performed on a Thermo ARL SCINTAG X’TRA X-ray diffractometer with Cu KR irradiation (λ ) 1.54056 Å), with the operation voltage and current maintained at 45 kV and 40 mA, respectively. Scanning electron microscopic (SEM) images were obtained with a Hitachi S-4700 operated at an accelerating voltage of 15.0 kV. Transmission electron microscopic (TEM) images were performed on a PHILIPS CM200 microscope with an accelerating voltage of 200 kV. Energy-dispersive X-ray spectroscopy (EDS) is attached to the Hitachi S-4700.

3. Results and Discussion The phase and purity of the products were examined by XRD. Figure 1 displays the XRD patterns of the as-prepared Te/C nanocables in the presence of CTAB under hydrothermal method at 170 °C. All of the diffraction peaks can be indexed to be trigonal phase (space group: P3121(152), with infinite, helical chains of tellurium atoms packed parallel to each other along the c-axis) of tellurium with lattice parameters of a ) 4.461 Å and c ) 5.922 Å, which are in good agreement with the values in the literature (JCPDS card no. 36-1452, a ) 4.458 Å and c ) 5.927 Å). No other phases were detected in Figure 1, which indicate that high purity of Te/C nanocables was obtained by the present synthetic method. The morphologies and microstructures of the as-prepared products were further surveyed by SEM and TEM images. Figure 2a shows a typical SEM image of the as-prepared

Te/C nanocables. These nanocables have diameters ranging from 200 to 300 nm and lengths of several micrometers. The TEM image in Figure 2b shows that the products are a composite comprised of a smooth core about 50-80 nm in diameter and a surrounding sheath about 60 nm in thickness. It can be found very clearly from Figure 2b that there is contrast between the dark inner core and light sheath layer along the axis direction. The energy-dispersive X-ray (EDS) analysis was employed to determine the composition of products and was shown in Figure 3. The EDS results confirm that the obtained nanocables are composed of inner tellurium nanowires and outer carbonaceous layers. Generally, temperature is believed to have a great impact on the morphology of final products. We have carried out analogous experiments at different temperatures for comparison. The results revealed that there were no Te/C nanocables formed at 150 °C (Figure 4a). When the temperature was at 150 °C, the products were Te nanowires with 50-80 nm in width and several micrometers in length. The most probable reason would be that glucose could not be carbonized at low temperature, and only Na2TeO3 was deoxidized to Te in the reaction. Considering this, the nanocables were not formed in the products. Figure 4b shows the TEM images of Te/C nanocables synthesized at 190 °C. It reveals the Te/C nanocables with core about 20 nm in diameter and a surrounding sheath about 80-100 nm in thickness. As compared to the Te/C nanocables prepared at 170 °C, it could be found that the diameter for the core decreased significantly, and the thickness for the carbon sheath increased at 190 °C. It is interesting to note that the dual-core nanocables were also formed under this condition (marked by an arrow in Figure 4b). This may be that the diameters of Te nanowires at initial formation were small and two nanowires could be arrayed closely. When glucose was carbonized during the reaction and the carbon shell was formed in the surface of Te nanowires, the two closed nanowires would make the carbon shell unite together, and the dual-core cable was formed accordingly. CTAB has been widely used in preparing nanowires, because it has a tendency to form an elongated rodlike micellar structure that possibly assists in rod formation, as well as stabilizing the rods. Under our experimental conditions, the concentration of CTAB is important in determining the final morphology of the product. Figure 5a and b shows the TEM images of the samples obtained without CTAB at 170 and 150 °C individually. It can be seen that the products obtained at 170 °C were nanocables with crude Te core, and Te nanowires with crude surface were obtained at 150 °C. As we know, the formations of 1D nanostructure first need the anisotropy during the growing

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Figure 4. TEM images of products obtained at different temperatures: (a) 150 °C and (b) 190 °C.

Figure 5. TEM and SEM images of the sample obtained without CTAB at different temperatures: (a) 170 °C, (b) 150 °C.

Figure 6. TEM images of carbonaceous nanotubes obtained from various Te/C nanocables: (a) at 170 °C, (b) at 190 °C, (c) dual-bore, and (d) high-magnification image.

process for the nanoparticles. The selective interaction of the capping molecules on the facets of the first-formed nanoparticles is crucial to the anisotropic growth of nanostructures. In our experiments, the presence of CTAB would help to enhance the anisotropy of tellurium and cause the formation of a smooth nanowire. According to the results, a possible formation mechanism of Te/C nanocables has been proposed. It is known that glucose is a typical soft reducer, and the glucose solution in autoclaves

at high temperature would lead to aromatization and carbonization. The formation process of Te/C nanocables should includetwo evolution stages: (1) In the reaction process, Na2TeO3 is reduced by the weak reducing agent of glucose under the hydrothermal condition. The CTAB molecule is easily adsorbed onto the surfaces of tellurium particles and thus changes the relative growth rate at various crystal faces. As a result, the oriented growth of the 1D nanostructured is maintained. (2) The carbonization of glucose and the formation of an amorphous

Te/C Nanocables and Carbonaceous Nanotubes

carbon layer on Te nanowires surface result in the formation of Te/C nanocables. By chemical etching, large numbers of amorphous carbonaceous nanotubes can be obtained from Te/C nanocables. As we know, the apexes of nanocables are quite perfectly closed; it could be concluded that carbonaceous layers of nanocables have good permeation. Just because of this, H+ ions and H2O2 molecules can easily be diffused into nanocables. During the diffusion and dissolution process, the Te core would be completely consumed. Because this etching is operated at room temperature, these carbonaceous nanotubes keep the original frame of the Te/C nanocables. Figure 6 shows the TEM image of carbonaceous nanotubes. These nanotubes keep the initial size and the shape of nanocables well. Carbonaceous nanotubes with 200-300 nm in diameter and about 60 nm in wall thickness can be obtained from the as-prepared Te/C nanocables at 170 °C (Figure 6a). Carbonaceous nanotubes with 20 nm in inner diameter shown in Figure 6b were fabricated through etching as-prepared Te/C nanocables at 190 °C. Figure 6c presents the low-magnification TEM image of the dual-bore nanotubes. In Figure 6d, the TEM image further displays these kinds of tubular nanostructures more clearly. 1D tubular nanostructures can be used as nanocargo and nanoreactors because of their inner cavity. Although various kinds of nanotubes have been prepared, such as inorganic nanotubes and organic nanotubes, the fabrication of novel dual-bore nanotubes is still desirable.

4. Conclusion In summary, via a low-temperature, green chemical route, large numbers of Te/C nanocables were synthesized using glucose as reducing agent and carbon source, and CTAB as a structure-directing agent. By removing the Te core, we obtained carbonaceous nanotubes with different morphologies. These carbonaceous nanotubes kept the same lengths of the original Te/C nanocables and sizes of the shell. Acknowledgment. We wish to acknowledge the financial support from the National Natural Science Foundation of China (no. 20873020, 50843030).

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