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Te/Carbon and Se/Carbon Nanocables: Size-Controlled in Situ Hydrothermal Synthesis and Applications in Preparing Metal M/Carbon Nanocables (M ) Tellurides and Selenides) Guangcheng Xi,*,†,‡ Chao Wang,† Xing Wang,† Yitai Qian,‡ and Haiqing Xiao† National Nanomaterials Inspection and Research Center, Chinese Academy of Inspection and Quarantine, Beijing 100025, People’s Republic of China, and Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, People’s Republic of China ReceiVed: August 10, 2007; In Final Form: September 19, 2007
In this paper, Te/carbon nanocables with tunable diameters and lengths have been synthesized via a simple in situ hydrothermal reduction-carbonization route by using Na2TeO3 as the Te resource and vitamin C as the reduction agent and carbon source. At the same time, this synthetic route can be used to prepare Se/ carbon nanocables when Na2SeO3 takes the place of Na2TeO3. Using these presynthesized Te (or Se)/carbon nanocables as self-templates, M/carbon nanocables (M ) metal tellurides and selenides) could be conveniently synthesized via reacting these Te (or Se)/carbon nanocables and metal salts under mild hydrothermal conditions. A reduction-carbonization growth mechanism for the present Te (or Se) nanocables and a template-assisted oriented attachment mechanism for the tellurides (or selenides)/carbon nanocables were proposed, respectively. In addition, these as-synthesized nanocables have one layer of hydrophilic, organic-group-loaded surfaces. The functional groups loaded on the carbonaceous sheaths have high chemical activity, which could be noble metal ions (such as Pd2+, Ag+, and Au3+) reduced to elements.
1. Introduction With the rapid extension of the applications of nanomaterials, great importance has been recognized to develop functional nanomaterials integrated with multiple functions in a single particle;1 core-shell structures are a conventional protocol to realize this aim. The isolation of the core from the surroundings can be used to create objects with properties fundamentally different from those of the bare nanocrystal. For example, the coating may be used to passivate the core chemically to modify its optical properties, or to create nanoscale objects with specific electrical functionality.1a Therefore, developing simple and effect synthetic routes to core-shell nanomaterials is very significant. Nanowires with sheaths, which are known as nanocables, are an important kind of practical one-dimensional nanostructure. Via rational selection of wire and sheath from different materials, the functions of the nanocables can be further improved. Since the first coaxial core-shell structure, C-BN-C nanocable, was synthesized in 1997,2 different forms of multiplayer nanocables have been produced by high-temperature synthetic methods, such as SiC/SiO2/C/BN,3 Si/SiOx,4 and Si/SiO2/C.5 The structures have been constructed in many current electronic devices on the basis of their unique optical, electronic, and structural characteristics generated from the interfaces of two materials. Recently, a low-temperature solution-phase method has been developed to synthesize various nanocables. For example, CdSe/ poly(vinyl acetate),6 polypyrrole/poly(methyl methacrylate),7 SnO2/Fe2O3,8 Au/polyelectrolyte,9 Ag/poly(vinyl alcohol),10 and Ag/SiO211 nanocables have been synthesized in solution at relative low temperature. The nanocables prepared via the lowtemperature solution-phase methods often have hydrophilic, organic-group-loaded surfaces, which would greatly enhance * To whom correspondence should be addressed. Phone: +86-01085757002. Fax: +86-010-85772625. E-mail:
[email protected]. † Chinese Academy of Inspection and Quarantine. ‡ University of Science and Technology of China.
Figure 1. XRD pattern of as-synthesized nanocables: (a) Te/carbon; (b) CdTe/carbon; (c) PbTe/carbon; (d) Se/carbon; (e) CdSe/carbon; (f) PbSe/carbon.
their applications in solution. More recently, studies have demonstrated that the hydrothermal method is an effective and simple method to synthesize several core-shell nanomaterials. For example, Ag/C,12 Te/C,13 Se/C,14 and oxides/C15 core-shell nanostructures have been synthesized under hydrothermal conditions. Metal telluride 16 and selenide17 nanocrystals are very important semiconductor materials that have been studied extensively. One-dimensional telluride18 and selenide19 nanocrystalline materials, including nanorods, nanotubes, and nanowires, have been intensively studied because of their unique properties, which are derived from their low dimensionality and unique quantum properties. Due to their large specific surface and natural properties, telluride and selenide nanocrystals will
10.1021/jp0764539 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/08/2008
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Figure 2. Shape and microstructure images of as-synthesized Te/carbon nanocables. (a) Low-magnification FSEM image; (b) high-magnification FSEM image; (c) TEM image; (d) HRTEM image (inset: corresponding SAED pattern).
be generally hydrolyzed and oxidized when kept in air for a long time. Coating telluride and selenide nanocrystals with a layer of compact material can effectively protect them against attack from water and oxygen in air. Although the modification and stabilization of CdTe and CdSe nanowires with SiO2 or low molecular weight organic species were reported,20 it is still a big challenge to generally and effectively fabricate long metal telluride and selenide nanowires with a layer of sheath. In this paper, we report a general and reproducible route to synthesize coaxial metal telluride (selenide)/carbon nanocables via an in situ reaction of metal ions and Te (Se)/carbon nanocables under hydrothermal conditions at 160 °C. These metal telluride (selenide)/carbon nanocables have hydrophilic, organic-grouploaded surfaces, which indicate their potential applications in biochemistry, biological sensors, and biomaterials. To the best of our knowledge, the synthesis of telluride (selenide)/carbon nanocables has not been reported. 2. Experimental Section Preparation of Te/Carbon Nanocables. For the synthesis of Te/carbon nanocables, 1.0 g of ascorbic acid (vitamin C, C6H8O6), 0.15 g of sodium tellurite (Na2TeO3), and 0.1 g of sodium dodecylbenzenesufonate (SDBS) were added to 40 mL of distilled water under vigorous stirring, forming a white suspension. The white suspension was transferred into a Teflonlined stainless steel autoclave of 60-mL capacity. The autoclave was sealed and maintained at 120 °C for 24 h, and then heated at 180 °C for 6 h. Finally, the autoclave was cooled to room temperature naturally. The resulting black product was retrieved by centrifugation and washed several times with distilled water and absolute ethanol, and then dried in a vacuum at 50 °C for 4 h. Preparation of Telluride/Carbon Nanocables. For the synthesis of telluride/carbon nanocables, the preproduced Te/ carbon nanocables and a moderate amount of metal salts (such as Cd(Ac)2 and Pb(Ac)2‚3H2O) were dispersed in 30 mL of distilled water to form a dark suspension. Then, 10 mL of N2H4‚ H2O was added to the dark suspension. The final solution was transferred into a Teflon-lined stainless steel autoclave of 60mL capacity. The autoclave was sealed and maintained at 160 °C for 24 h. After that, the autoclave was allowed to cool to room
Figure 3. FSEM, TEM, and HRTEM images of as-synthesized Te/ carbon nanocables prepared under different conditions: (a, b) 0.15 g of Na2TeO3, 160 °C for 30 h; (c, d) 0.15 g of Na2TeO3, 200 °C for 30 h; (e, f) 0.05 g of Na2TeO3, 120 °C for 24 h, and then 180 °C for 6 h; (g, h) 0.4 g of Na2TeO3, 120 °C for 24 h, and then 180 °C for 6 h.
temperature naturally. The black products were filtered off and washed several times with distilled water and absolute ethanol, and then dried in a vacuum at 50 °C for 4 h. Preparation of Se/Carbon Nanocables. The synthetic route to Se/carbon nanocables is similar to that of Te/carbon nanocables. In a typical procedure, 0.2 g of Na2SeO3, 0.1 g of SDBS, and 1 g of ascorbic acid were added to 40 mL of distilled water under vigorous stirring, forming a colorless solution. This colorless solution rapidly transformed to a yellow solution, followed by an immediate formation of a brick-red suspension. The obtained brick-red suspension was transferred into a Teflonlined stainless steel autoclave of 60-mL capacity. The autoclave was sealed and maintained at 120 °C for 24 h, and then heated at 180 °C for 6 h. Finally, the autoclave was cooled to room temperature naturally. The resulting black product was retrieved by centrifugation and washed several times with distilled water and absolute ethanol, and then dried in a vacuum at 50 °C for 4 h.
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Figure 4. XPS spectrum (a) and FTIR spectrum (b) of as-synthesized Te/carbon nanocables.
Figure 5. (a, b) Typical FSEM and TEM images of Pd/carbon/Te nanostructures. (c) EDS spectrum of Pd/carbon/Te nanostructures.
Preparation of Selenide/Carbon Nanocables. The synthetic route to selenide/carbon nanocables is similar to that of telluride/ carbon nanocables. In a typical procedure, the preproduced Te/ carbon nanocables and a moderate amount of metal salts (such as Cd(Ac)2 and Pb(Ac)2‚3H2O) were dispersed in 30 mL of distilled water to form a dark suspension. Then, 5 mL of N2H4‚ H2O was added to the dark suspension. The final solution was transferred into a Teflon-lined stainless steel autoclave of 60mL capacity. The autoclave was sealed and maintained at 160 °C for 20 h. After that, the autoclave was allowed to cool to room temperature naturally. The black products were filtered off and washed several times with distilled water and absolute ethanol, and then dried in a vacuum at 50 °C for 4 h. Characterization. X-ray powder diffraction (XRD) patterns of the products were recorded on a Rigaku (Japan) D/max-γA X-ray diffractometer equipped with graphite monochromatized Cu KR1 radiation (λ ) 1.541 78 Å). Fourier transform infrared (FTIR) absorption was recorded from a Magna IR-750FT spectrometer. The Raman spectra were produced at room temperature with a LABRAM-HR Confocal Laser MicroRaman spectrometer. The emission scanning electron microscope (SEM) image of the products were examined by a field-emission
Figure 6. TEM and HRTEM images of as-synthesized CdTe/carbon nanocables (a, b) and PbTe/carbon nanocables (c, d). Insets in (b) and (d): corresponding SAED patterns recorded from one single CdTe/ carbon and PbTe/carbon nanocables, respectively.
scanning electron microscope (JEOL-6300F). The transmission electron microscope images, selected area electron diffraction (SAED) patterns, and high-resolution transmission electron microscope (HRTEM) images were recorded on a JEOL 2010 microscope. X-ray photoelectron spectra (XPS) were carried out on a VGESCALAB MKII X-ray photoelectron spectrometer, using nonmonochromatized Mg KR X-ray as the excitation source. 3. Results and Discussion The powder X-ray diffraction patterns (XRD) of the assynthesized nanocable samples are shown in Figure 1. All diffraction peaks can be assigned to the respective phases without indication of other crystalline byproducts. In detail, patterns a-c can be assigned to hexagonal Te (JCPDS No. 361452), cubic zinc blende CdTe (JCPDS No. 15-0770), and face centered cubic PbTe (JCPDS No. 78-1905), respectively. Patterns d and f can be assigned to hexagonal Se (JCPDS No. 73-0465) and cubic PbSe (JCPDS No. 78-1903). In pattern e, the characteristic zinc blende planes of 111, 220, 311, 400, and
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Figure 7. TEM image (a), EDS spectrum (b), and XRD pattern (c) of Te nanowires prepared at 120 °C for 24 h.
Figure 8. Schematic illustration of formation process of Te/carbon nanocables.
331 located at 25.38, 42.06, and 49.76° in the 2θ range of 2080° for CdSe (JCPDS No. 19-191) have been observed. At the same time, there is a weak peak representing wurtzite facet (marked in parentheses) observed in the XRD pattern, which indicates that the cubic CdSe contains a small quantity of hexagonal phase component. Figure 2a,b presents typical field-emission scanning electron microscopic (FESEM) images of the Te/carbon nanocables, which show that the product is composed of a large quantity of wirelike nanostructures that are usually 40 nm in diameter and tens of micrometers in length. Figure 2c shows a lowmagnification TEM image of the product. The evident contrast between the inner core and outer shell suggest the core-shell
Xi et al. nanostructure. The thickness of the shell layer is about 15-17 nm, while the diameter of the core is about 7-20 nm. Figure 2d shows an SAED pattern and an HRTEM image of a Te core, which indicates that the Te nanowires grow along the [001] direction. Several reaction parameters, such as reaction temperature and concentration, have obvious effects on the final shapes of the as-synthesized Te/carbon nanocables. On one hand, keeping other conditions unchanged, Te/cross-lined carbon nanocables were obtained when the reaction mixture was hydrothermally treated at 160 °C during the whole reaction process (Figure 3a,b). The thickness of the carbonaceous layer is about 18 nm, while the diameter of the Te core is about 30 nm. Increasing the reaction temperature to 200 °C, thicker Te/carbon nanocables could be obtained (Figure 3c,d). The thickness of the shell layers is about 20-30 nm, while the diameter of the Te cores is about 50-60 nm. On the other hand, keeping other parameters unchanged, Te/carbon nanocables with ultrathin Te nanowire cores were prepared when only 0.05 g of Na2TeO3 was added to the reaction system (Figure 3e,f). The diameters of the Te nanowire cores are only about 3-4 nm, while the thickness of the carbonaceous sheaths is about 4-8 nm. When 0.4 g of Na2TeO3 was added to the reaction system, Te/carbon nanocables also could be prepared. The nanocables generally are tens of micrometers in length and 100 nm in diameter, while the thickness of carbonaceous layers is about 20 nm (Figure 3g,h). XPS and FTIR were used to analyze the components and chemical bonding at the surface of the nanocables. For the assynthesized Te/carbon nanocables, the typical XPS survey spectrum (Figure 4a) only shows the peaks of C and O, illustrating that all of the Te nanowires in the product are buried within the carbonaceous sheaths. The FTIR spectrum of the Te/ carbon nanocables is shown in Figure 4b. The bands at 1708 and 1615 cm-1 are attributed to CdO and CdC vibrations, respectively. The bands in the range of 1000-1300 cm-1 could be attributed to the C-OH stretching and OH bending vibrations, which suggest the existence of a large amount of hydroxy groups. The OH and CHO groups are covalently bonded to the carbonaceous shells, improving the hydrophilicity and stability of the core-shell nanostructures. Colloidal suspensions of the Te/carbon nanocables were stable for 2 weeks. Furthermore, the organic-group-loaded surfaces would make surface modification easier. On the basis of the above analyses, it can be conclude that the surfaces of the as-synthesized Te/carbon nanocables are highly functionalized. To further prove the chemical activities of the functional groups loaded on the carbonaceous sheaths, a series of chemical experiments were carried out. When presynthesized Te/carbon nanocables and noble metal salts (such as AgNO3 and PbCl2) were added to water at 80 °C, noble metal nanoparticles interspersed on the carbonaceous sheaths were obtained, which demonstrated that the organic functional groups on the carbonaceous sheaths are strongly reductive. For example, Figure 5a,b shows typical FSEM and TEM images of Pd/carbon/ Te nanostructures. These Pd nanoparticles are 5-8 nm in diameter and are randomly interspersed on the carbonaceous sheaths. Energy dispersive spectrometric (EDS) analysis demonstrated the components of the sample (Figure 5c). These assynthesized noble metal/carbon/Te nanomaterials may be have potential applications in catalytic technology. When these presynthesized Te/carbon nanocables reacted with metal salts under hydrothermal conditions, metal telluride/carbon nanocables could be synthesized. Figure 6a shows a representative low-magnification TEM image of the as-synthesized CdTe/
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Figure 9. Low-magnification TEM image (a) and HRTEM image (b) of polycrystalline CdTe nanowires prepared at 160 °C for 10 h.
Figure 10. Schematic illustration of formation process of telluride/ carbon nanocables: (a) Te/carbon nanocable template; (b) formation of polycrystalline telluride nanowire encapsulated in carbon sheet; (c) formation of single-crystalline telluride nanowire encapsulated in carbon sheet (namely telluride/carbon nanocable) via oriented attachment and recrystallization of the nanoparticles.
carbon nanocables. It can be seen that the morphology of the CdTe/carbon nanocables is similar to that of the Te/carbon nanocables. The HRTEM image in Figure 6b shows the singlecrystal nature of the CdTe core. The lattice spacing of 0.37 nm between adjacent lattice planes in the image corresponds to the distance between two (111) crystal planes, confirming [111] as the preferred growth direction for the cubic phase CdTe nanowire. When Pb2+ ion replaced Cd2+ ion, PbTe/carbon nanocables could be prepared (Figure 6c). The HRTEM image of a single PbTe core (Figure 3d) exhibits clear fringes along the wire, indicating that the PbTe core is highly crystalline and not free of dislocation. The corresponding SAED pattern (inset in Figure 6d) can be indexed to the [001] zone of a face centered cubic PbTe. From the SAED and HRTEM analyses, clearly, the cubic phase PbTe nanowires grow along the [010] direction. These results suggest that the present method may be used as a general method to synthesize various metal telluride/carbon nanocables. Furthermore, the outer carbonaceous sheaths effectively protect the inner telluride nanowires against attack from moisture and oxygen. The HRTEM image demonstrates that no oxide layers were formed on the surfaces of the CdTe or PbTe nanowires after they were placed in air for 2 months. EDS was used to analyze the composition of the assynthesized telluride/carbon nanocables. Figures SI-1a and SI1b (Supporting Information) show the EDS spectra taken from the CdTe/carbon nanocables and the PbTe/carbon nanocables, respectively. From the two spectra, it can be seen that Te was completely transformed to CdTe and PbTe. We carefully studied the formation mechanism of the Te/ carbon nanocables because of the importance in the followed
synthesis of telluride/carbon nanocables. For the present synthetic route, ascorbic acid is used as the reducing agent and carbon source to synthesize Te/carbon nanocables. In the initial step, H+ resulted from the ionization of ascorbic acid reacts with TeO32- to generate white TeO2, which is not dissolved in water at room temperature, so a white suspension is formed. When the reaction mixture was heated to 120 °C, the initially generated TeO2 solids gradually dissolved in the solution with a slow rate. Since ascorbic acid is a weak reducing agent,21 it would slowly reduce Te4+ to Te in the solution. With the assistance of the cationic surfactant SDBS,22 the newly formed Te atoms would gradually evolve into nanowires. Figure 7a shows a TEM image of the sample produced under hydrothermal conditions for 24 h at 120 °C, which clearly shows that the sample is composed of pure nanowires with an average diameter of 9 nm. The corresponding EDS spectrum (Figure 7b) and XRD pattern (Figure 7c) demonstrate that this sample is hexagonal Te crystals. Subsequently, when the reaction temperature reaches 180 °C, polymerization and carbonization reaction of ascorbic acid will take place, which results in the formation of the carbonaceous sheath coating on the surfaces of the preformed Te nanowires. The whole formation process of the Te/carbon nanocables is illustrated in Figure 8. The chemical reactions involved in this process are as follows:
TeO32- + 2H+ f TeO2 + H2O
(1)
C6H8O6 + TeO2 f C6H4O6 + Te + 2H2O
(2)
C6H8O6(C6H4O6) f carbon(amorphous) + H2O
(3)
Although the exact formation mechanism of the telluride/ carbon nanocables is still unknown at this moment, we believe the templating effect of the presynthesized Te/carbon nanocables is important. Due to the porous characteristic of the carbonaceous sheaths,12c,13 metal ions and N2H4 molecules would diffuse into the carbonaceous nanotubes via capillary action.23 Because the standard electrode potentials of Cd2+/Cd, Pb2+/ Pb, and N2/N2H4 are -0.4030, -0.1262, and -1.15 V, respectively, the Cd2+ and Pb2+ ions would be reduced to Cd and Pb atoms by N2H4. The fresh Cd and Pb atoms have high chemical activities, which make them ease to react with Te nanowires to generate telluride nanoparticles under alkaline hydrothermal conditions. To investigate the intermediate growth steps in the nanoparticle to nanowire transition process, the CdTe/carbon samples generated in the early stages were examined. The TEM image (Figure 9a) of the sample obtained
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2Cd2+ + N2H4 + 4OH- f 2Cd + N2 + 4H2O
(4)
Cd + Te f CdTe
(5)
2Pb2+ + N2H4 + 4OH- f 2Pb + N2 + 4H2O
(6)
Pb + Te f PbTe
(7)
The synthetic routes to Se/carbon and selenide/carbon nanocables are similar to those of Te/carbon and telluride/carbon nanocables. Figure 11a shows a typical FSEM image of the as-prepared Se/carbon nanocables. From this image, it can be seen that the nanocables are generally 150 nm in diameter and several micrometers in length. The TEM image (Figure 11b) shows that the thickness of the carbonaceous sheaths is about 30 nm and the diameter of the inner Se nanowires is about 90 nm. Furthermore, the nanocables are very straight and have smooth surfaces. Figure 11c,d and Figure 11e,f show the TEM images of the as-synthesized CdSe/carbon and PbSe/carbon nanocables, respectively, which have shapes similar to that of the Se/carbon nanocables. Unfortunately, due to their relativey larger volumes, an electron beam could not penetrate these nanocables. Therefore, we did not get clear HRTEM images of them. Figure SI-2 shows the EDS spectra of the selenide/carbon nanocables, which clearly demonstrate their composition. 4. Conclusions
Figure 11. FSEM image (a) and TEM image (b) of Se/carbon nanocables. Low-magnification TEM image (c) and high-magnification TEM image of CdSe/carbon nanocables (d). Low-magnification TEM image (e) and high-magnification TEM image (f) of PbSe/carbon nanocables.
by hydrothermal treating for 12 h indicates that the cores of the nanocables are composed of polycrystalline CdTe nanowires. The corresponding HRTEM image (Figure 9b) further demonstrates the polycrystalline nature of the nanowires (three single CdTe nanocrystals are marked with white rims). Because the nucleation and growth of telluride nanocrystals occurred in the carbonaceous nanotubes, that is, to grew in a confined space (one-dimensional space), the space-confined nucleation and crystallization obviously benefit the formation of telluride nanowires. As a rational result, polycrystalline CdTe nanowires formed in the early stage of the growth process. Recently, Kotov et al. reported that CdTe single-crystalline nanowires could be formed via an oriented attachment and recrystallization mechanism in solution.18a The driving force of nanoparticle selforganization was attributed to the strong dipole-dipole interaction among the CdTe nanoparticles. In the present experiments, the intermediary CdTe polycrystalline nanowires were confined inside the carbonaceous sheaths, which strengthened the dipoledipole interaction. At the same time, the present hydrothermal environment would accelerate the recrystallization process. As a result, increasing the hydrothermal treatment time, the polycrystalline CdTe nanowires gradually developed into singlecrystaline nanowires. Similar growth mechanisms also have been reported in other systems.24 This formation process of the telluride/carbon nanocables may be illustrated in Figure 10. Of course, the exact formation mechanism should be further
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