C Coaxial Nanocables through a Novel Solution

Mar 13, 2008 - Se/C nanocables were first obtained through the reduction of Na2SeO3 with glucose in the presence of cetyltrimethylammonium bromide ...
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J. Phys. Chem. C 2008, 112, 5352-5355

Fabrication of Se/C Coaxial Nanocables through a Novel Solution Process Xu Chun Song,*,† Yang Zhao,‡ Yi Fan Zheng,§ E. Yang,† Wen Qi Chen,† and Yan Qun Fang† Institute of Physical Chemistry, Department of Chemistry, Fujian Normal UniVersity, Fuzhou 350007, P. R. China, Department of Chemistry, Henan Normal UniVersity, Xinxiang 453007, P. R. China, and College of Chemical Engineering & Materials Science, Zhejiang UniVersity of Technology, Hangzhou, Zhejiang 310014, P. R. China ReceiVed: December 17, 2007; In Final Form: January 26, 2008

Se/C nanocables were first obtained through the reduction of Na2SeO3 with glucose in the presence of cetyltrimethylammonium bromide (CTAB) under hydrothermal conditions. In the process, glucose acts as a reducing agent and carbon source, and the final morphology of the product was determined by the CTAB concentration. The products are characterized in detail by multiform techniques: X-ray diffraction, energydispersive X-ray analysis, scanning electron microscopy, and transmission electron microscopy. The results show that the obtained products are coaxial nanocables with lengths of 2-6 µm, about 300-500 nm in diameter, and a surrounding sheath about 20-30 nm in thickness. It is of great importance and wide application that the obtained Se/C coaxial nanocables could be tailored freely by irradiation by an electron beam of transmission electron microscopy.

1. Introduction Nonmetallic selenium is known as an important trace element for humans because of its great nutritional effects in biology, for example, protecting cells against free radicals from oxygen metabolism and strengthening the functions of the immune system.1 Trigonal selenium (t-Se), an important elemental semiconductor, is widely used in photocells, photographic exposure meters, xerography, pressure sensors, and electrical rectifiers due to its high photoconductivity, excellent spectral sensitivity, and large piezoelectric, thermoelectric, and nonlinear responses.2-7 Nanoparticles exhibit significantly different properties relative to those of their corresponding bulk materials and, as such, are of interest for applications in catalysis, electronics, and optics.8-11 Recently, most efforts have been focused on preparing t-Se nanoparticles,12,13 nanowires,14-17 nanobelts,18 and nanotubes.2,3,19 As a new kind of nanostructure, coaxial nanocables are now attracting more and more interest since these composite nanoparticles are constructed of cores and shells of different chemical compositions, which endow us with the possibility to combine the advantages or distinctive properties of varied materials together and, especially, manipulate the surface functions to meet diverse application requirements.20 The study of coaxial nanocables has attracted much attention. Recently, coaxial nanocables with Ag cores (Ag/C and Ag/SiO2),21-24 silicon nanowires in graphitic B-C-N nanotubes,25 Te/C,26 and TiO2/SiO227 were described. A number of approaches, such as laser ablation, thermal evaporation, γ irradiation, and templating, have been developed to fabricate these kinds of coaxial 1D nanostructures.28-31 Coupled synthesis and encapsulation in one step is a good choice to form coaxial nanocables, which takes advantage of the reaction sequences and thus exhibits higher * To whom correspondence should be addressed. Phone: +86-59187441126. Fax: +86-591-83465376. E-mail: [email protected]. † Fujian Normal University. ‡ Henan Normal University. § Zhejiang University of Technology.

efficiency and facility. However, to date, this method has not been as well developed as postsynthesis methods and relative reports are rare. In this paper, we design a novel surfactantsassisted solution process to prepare selenium/carbon (Se/C) coaxial nanocables via one-step reduction and carbonization under mild hydrothermal conditions. It is the first report, to the best of our knowledge, for the synthesis of selenium/carbon coaxial nanocables using glucose as the reductant and carbon source in solution at low temperature. 2. Experimental Section 2.1. Sample Preparation. All chemicals were analyticalgrade reagents without further purification. The Se/C coaxial 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 Na2SeO3 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 150 °C for 12 h. After the reaction was completed, the autoclave was allowed to cool to room 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. The obtained black powders were collected for the following characterization. 2.2. 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), 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

10.1021/jp711814e CCC: $40.75 © 2008 American Chemical Society Published on Web 03/13/2008

Fabrication of Se/C Coaxial Nanocables

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Figure 3. EDS patterns of Se/C coaxial nanocables. Figure 1. XRD pattern of Se/C coaxial nanocables.

Figure 2. SEM images of Se/C coaxial nanocables.

200 kV. The energy-dispersive X-ray spectrometer was attached to the Hitachi S-4700 and PHILIPS CM200. 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 Se/C coaxial nanocables in the presence of CTAB under the hydrothermal method at 150 °C. All diffraction peaks can be indexed to be trigonal selenium with lattice parameters of a ) 4.363 Å and c ) 4.951 Å, which are in good agreement with the values in the literature (JCPDS card No. 06-0362, a ) 4.3662 Å and c ) 4.9536 Å). No other phases were detected in Figure 1, which indicates that Na2SeO3 would be completely changed into Se/C nanocables. The morphologies and microstructures of the as-prepared products were further surveyed by SEM and TEM. Figure 2 shows a typical SEM image of the as-prepared Se/C nanocables. From the SEM contrast it can be shown that the obtained products are 1-D nanostructures, and these nanocables have diameters ranging from 300 to 500 nm and lengths of 2-6 µm. Energy-dispersive X-ray analysis was employed to determine the composition of products and is shown in Figure 3. The EDS results confirm that the obtained nanocables are composed of inner selenium nanorods and outer carbonaceous layers. Figure 4a presents the low-magnification TEM image of the as-prepared Se/C nanocables. Although no obvious shell can be found in rod-like nanostructures in Figure 4a, it can be found clearly from the high-magnification image shown in Figure 4b that the contrast between the dark inner core and light sheath layer is about 20-30 nm in thickness along the axis direction (marked by dark arrow). It is very interesting to note that when the strengthened energy of the electron beam by minimizing the light spot of transmission electron microscopy was used to irradiate the obtained nanocables, the inner selenium core in

Figure 4. (a) TEM images of Se/C coaxial nanocables, (b) highmagnification image of Se/C coaxial nanocables, (c) TEM images of Se/C coaxial nanocables partially irradiated by the electron beam, and (d) TEM images of the whole Se/C coaxial nanocables irradiated by the electron beam.

the solid state was found to change into the gas phase and then released from nanocables. On the contrary, if we expanded the light spot to make the energy of the electron beam irradiate on the nanocable lower, the reaction was stopped correspondingly. Figure 4c shows the TEM images of the nanocable irradiated locally by the strong-energy electron beam. The results show that the partial selenium core disappeared and a hollow area was formed in the end. If the whole nanocable was irradiated by the strong-energy electron beam, it was found from Figure 4d that inner selenium cores disappeared thoroughly, the residual carbonaceous shells formed the nanotubles. EDS analysis was employed to determine the composition of the nanotubes. As shown in Figure 5, EDS clearly identify that the nanotubes are composed of C. The Cu peak comes from the Cu grid used for TEM measurement. Figure 6 shows a similar TEM image of the middle of the nanocable irradiated by the strong-energy electron beam. From the above discussions it could be concluded that the selenium core in the gas phase could release from the carbonaceous layers, which has good gas permeation. Generally, temperature is believed to have a great impact on the morphology of the final products. Considering this, analogous experiments at different temperatures have been carried out for comparison. Figure 7a shows the TEM images of Se/C coaxial nanocables synthesized at 130 °C, which is similar to that obtained at 150 °C (Figure 4a). It also could be found from Figure 7b that when the nanocable obtained at 130 °C was irradiated entirely by the strong-energy electron beam, the inner

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Figure 8. (a) TEM image of the sample obtained without CTAB, and (b) TEM images of sample irradiated by the electron beam.

SCHEME 1. Schematic Illustration of the Se/C Coaxial Nanocables Formation Processa Figure 5. EDS patterns of the residual carbon nanotubes.

a Steps: (A) formation of Se nanoparticles in hydrothermal condition; (B) selenium 1D nanostructured cores have formed under the assistance of CTAB molecules; (C) carbonization of glucose results in formation of Se/C coaxial nanocables.

Figure 6. TEM images of the middle of Se/C coaxial nanocables irradiated by the electron beam.

Figure 7. (a) TEM images of the Se/C coaxial nanocables synthesized at 130 °C. (b) TEM images of Se/C coaxial nanocables irradiated by the electron beam.

selenium cores disappeared and the residual carbonaceous shells collapsed correspondingly. It is shown that the strength of the carbonaceous shell obtained at 130 °C is lower than that obtained at 150 °C. On the other hand, the collapsed carbonaceous shells were not cracking further, which indicated that the carbonaceous shells had some flexibility. When the temperature was increased to 170 °C, the Se/C nanocables and a lot of carbonaceous spheres with diameters of 5 µm were obtained. The most probable reason would be that the glucose is carbonized with the temperature increasing. The influence of the concentration of glucose on the growth of Se/C nanocables was also investigated in the study. When 0.5 g of glucose was added into the systems, the products obtained at 160 °C under hydrothermal conditions are Se nanorods with diameters of 400 nm and lengths of several micrometers. No Se/C coaxial nanocables were found. When the content of glucose was increased to 4 g, much amorphous carbon was obtained besides the Se/C coaxial nanocables in the same reaction condition. Surfactants are useful in controlling the morphology of nanostructures because of their soft template effect, reproducibility, and simple maneuverability. Under our experimental conditions, the concentration of CTAB is important in determining the final morphology of the product. Figure 8a is the TEM

image of the sample obtained without CTAB, which shows that the product is mixed with many different morphologies including spheres, rods, and flowers. It is difficult to see whether the products have the core-shell structure or not. If the product was irradiated by the electron beam, a similar result was found as discussed above: the selenium core disappeared and hollow carbon spheres were formed (Figure 8b). As we know, the crystal structure of selenium is highly anisotropic,17 and formation of 1D nanostructures first needs the anisotropy during the growing process for the nanoparticles. In our experiments the presence of CTAB would help to enhance the anisotropy of selenium and cause formation of uniform 1-D structures. According to the results, the proposed formation mechanism of Se/C coaxial nanocables in this work could be illustrated as in Scheme 1. 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 Se/C coaxial nanocables should include two evolution stages. (1) In the reaction process, Na2SeO3 is reduced by the weak reducing agent of glucose under the hydrothermal condition, and the reaction can be described as follows

Na2SeO3 + 2CH2OH-(CHOH)4-CHO f Se + 2CH2OH-(CHOH)4-COONa + H2O As the CTAB molecule is easily adsorbed onto the surfaces of selenium particles, the relative growth rate at various crystal faces changes. As a result, the oriented growth of the 1D nanostructured is kept. (2) Carbonization of glucose and formation of an amorphous carbon layer on Se 1D nanostructured surface results in formation of Se/C coaxial nanocables. 4. Conclusion Se/C coaxial nanocables though a CTAB-assisted hydrothermal route were obtained in the experiments. This is the first report for the synthesis of Se/C nanocables using glucose as a reductant and carbon source in solution at low temperature. It

Fabrication of Se/C Coaxial Nanocables is interesting to note that the selenium core could be removed partially or entirely by irradiation of the electron beam. Accordingly, the Se/C coaxial nanocables could be tailored in this way. This is of great importance! On the other hand, the present Se/C coaxial nanocables could also have potential applications in nanodevices as electronic parts and sensors in the future. Acknowledgment. We wish to acknowledge the financial support from the Natural Science Foundation of Fujian Province (Nos. 2006J0153, E0610006). References and Notes (1) Ren, L.; Zhang, H.; Tan, P.; Chen, Y.; Zhang, Z.; Chang, Y.; Xu, J.; Yang, F.; Yu, D. J. Phys. Chem. B 2004, 108, 4627. (2) Ma, Y.; Qi, L.; Ma, J.; Cheng, H. AdV. Mater. 2004, 16, 1023. (3) Zhang, H.; Yang, D. R.; Ji, Y. J.; Ma, X. Y.; Xu, J.; Que, D. L. J. Phys. Chem. B 2004, 108, 1179. (4) Rajalakshmi, M.; Arora, A. K. Solid State Commun. 1999, 110, 75. (5) Marine, W.; Patrone, L.; Luk’yanohuk, B.; Sentis, M. Appl. Surf. Sci. 2000, 154, 345. (6) Poborchii, V. V.; Kolobov, A. V.; Tanaka, K. Appl. Phys. Lett. 1999, 74, 215. (7) Johnson, J. A.; Saboungi, M. L.; Thiyagarajan, P.; Csencsits, R.; Meisel, D. J. Phys. Chem. B 1999, 103, 59. (8) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (9) Eychmuller, A. J. Phys. Chem. B 2000, 104, 6514. (10) Mulvaney, P. Langmuir 1996, 12, 788. (11) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668.

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