Carbon Nanocables through a Facile

Direct Fabrication of Tellurium/Carbon Nanocables through a Facile Solution Route Weizhi Wang,*,† Lei Sun,‡ Zhen Fang,† Liyong Chen,§ and Zude Zhang*,§ College of Chemistry and Materials Science, Anhui Normal UniVersity, Wuhu 241000, P. R. China, Anhui Academy for EnVironment Science Research, Hefei 230061, P. R. China, and Department of Chemistry, UniVersity of Science and Technology of China, Hefei 230026, P. R. China

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ReceiVed June 26, 2008; ReVised Manuscript ReceiVed February 19, 2009

ABSTRACT: In this paper, a facile one-step solution route is designed for the fabrication of semiconductor nanocables with tellurium nanowire as core and carbon as shell (Te/C). The formation process of the Te/C nanocables was investigated. The results of the investigation and further experiments confirm that the shell thickness of the nanocables could be tailored through the controlling the experimental conditions. The as-synthesized Te/C nanocables have a functionalized shell and interesting optical properties. Introduction In recent years, there has been considerable interest in fabricating coaxial nanocables.1 Nanocables, as its name implies, are a new type of one-dimensional (1D) nanocomposite of nanowires (core) wrapped with one or more outer layers (shell). In light of their fascinating structures, nanocables are expected to have better and broader applications in nanotechnology compared with naked nanowires. Nanowires have a high surfaceto-volume ratio, which provides them with high chemical reactivity. Thus, nanowires are often very sensitive to air and moisture, which degrades their structure, morphology, properties, and performances in nanodevices.2 The core-shell configuration of nanocables can suppress the surface chemical reactivity of nanowires and so induces an enhanced stability and performance for nanodevices. Meanwhile, the generation of heterojunction between core and shell can provide nanocables new functions or superior properties compared to monocomponent nanomaterials.3 More importantly, because their intrinsic properties could be easily tuned by assembling different chemical composition of both core and shell, nanocables have been considered as an ideal nanomaterials to realize various tailor-made functions.4 Among the various compositions of nanocables, semiconductor-core nanocables have stimulated particular interest because they have demonstrated great applications in nanodevices such as cell separation, coaxial-gated transistors, and laser diodes.5 So far, many nanocables with semiconductor nanowires core such as Si/SiOx, SiC/SiO2/BNC, GaP/SiOx, ZnS/SiO2, and ZnS/ Zn have been fabricated by various high-temperature methods including laser ablation, chemical vapor deposition, thermal evaporation, and carbothermal reduction.1,6 Recently, several solution-based methods have also been developed to generate semiconductor/polymer, polymer/polymer, metal/polymer, and metal/carbon nanocables.7 Despite these advances, the synthesis of single-crystal semiconductor/carbon nanocables is few, especially by solution-based process.8 Single-crystal semiconductor nanowires, with their anisotropy, large surface area, and possible quantum confinement effect, exhibit distinct electric, optical, thermal, and chemical properties.9 Since the discovery * To whom correspondence should be addressed. E-mail: wangwz@ mail.ahnu.edu.cn (W. W. Z.); [email protected] (Z. Z. D.). † Anhui Normal University. ‡ Anhui Academy for Environment Science Research. § University of Science and Technology of China.

of fullerenes and carbon nantubes,10 carbon nanomaterials have gained great scientific and technological interest because of their novel properties associated with their novel structures.11 The combination of the two nanomaterials of single-crystal semiconductor and carbon to fabricate semiconductor/carbon heterostructures has been expected to be able to alter conducting, electronic, and mechanical properties of single-crystal semiconductor nanomaterials, which could provide more prospects and opportunities for new applications in a wide variety of areas.12 Therefore, the design of simple and efficient one-step solution-based routes to prepare single-crystal semiconductor/ carbon nanocables is very necessary. In this paper, we report a facile one-step solution route to fabricate single-crystalline tellurium/carbon (Te/C) nanocables. The reason that we select Te as the core of nanocables is both practically important and theoretically feasible for direct fabrication of Te/C nanocables via a solution route. First, as a valuable p-type elemental semiconductor, Te has many useful and interesting properties, such as a narrow band gap (∼0.35 eV),13 an effect of ultrafast electronic excitation on the A1 phonon frequency,14 photoconductivity,15 catalytic activity toward some reactions,16 strong piezoelectric effect,17 thermoelectricity,18 nonlinear optical responses,19 and a high reactivity with many chemicals to generate various functional materials such as Bi2Te3, CdTe, ZnTe, etc.20 Because of these properties, Te has been widely used in electronic and optical electronic devices.21 We expect that the fabrication of Te-core nanocables might enhance the performance of Te nanowires or introduce new types of applications in future nanodevices. Second, Te is an ideal candidate for fabricating 1D nanostructures as the core of nanocables because of its inherent crystal structure. The crystal structure of trigonal tellurium (t-Te) contains infinite 31 helical chains of covalently bound Te atoms along the c-axis. These chains are bound together to form hexagonal lattices through weak van der Waals interactions.22 Such a highly anisotropic structure provides Te with strong tendency to grow into 1D structure along the [001] direction without the assistance of any physical templates.23 Thus, it is feasible to fabricate 1D nanostructures of Te as the core of nanocables through an appropriate solution-phase oxidation-reduction reaction. We used dextran to react with sodium tellurite (Na2TeO3) to obtain Te. Dextran is a water-soluble polysaccharide composed of repeated monomeric glucose units with a predominance of R-1, 6-linkages.24 Recently, dextran is used as a soft template for the synthesis of metallic and metal oxide sponges,25 or as a

10.1021/cg800678m CCC: $40.75  2009 American Chemical Society Published on Web 04/16/2009

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crystal growth modifier for the controlled synthesis of silver microcrystals with various morphologies.24 In light of the presence of reductive aldehyde functional groups in dextran molecule, we used it as a reductive agent in our experiment. Compared to some inconvenience to deal with or harmful reductive agents such as N2H4 · H2O, the usage of dextran is safer and more environmentally friendly. More importantly, it has been reported that glucose solution heated in autoclaves to 160-180 °C,26 which is higher than the normal glycosidation temperature, could result in the aromatization and carbonization.27 We believe that dextran, as a polymer of glucose, can also carbonize to form a carbon shell of nanocables under similar conditions. Thus, we chose dextran not only as a reductive agent but also as a carbon source to form carbon shell for the fabrication of Te/C nanocables. On the basis of the above idea and strategy, we designed a surfactant-assisted, one-step solution route to fabricate Te/C nanocables by a mild hydrothermal process with Na2TeO3, dextran, and cetyltrimenthylammonium bromide (CTAB) as the starting materials. The experimental result is in good agreement with our expectations. Recently, it has been reported that Te/ Bi and Te/Bi2Te3 core/shell nanowires have enhanced thermoelectric properties,28 doped 1D Te nanomaterials have metallic character in the wide temperature range,29 and the presence of protective carbon shell can provide Ag/C nanocables with superior electrical conductivity compared to bare Ag nanowires.30 Therefore, we anticipate that the Te/C nanocables may find promising applications in electronic nanodevices. Experimental Section All the reagents were of analytical grade (Shanghai Chemical Reagents Co.) and were used as received without further purification. In a typical experimental process, 0.3 mmol of Na2TeO3 was dissolved in 40 mL of CTAB aqueous solution (7.5 mM) under magnetic stirring. Then, 0.8 g of dextran (MW 40000) was added to the solution under vigorous stirring until a clear solution was formed. The above solution was transferred into a Teflon-lined autoclave (55 mL capacity), which was sealed and heated in an oven at 180 °C for 24 h. After the autoclave was cooled to room temperature naturally, a large quantity of black floccules was collected through centrifugation. These black floccules were then cleaned by several cycles of centrifugation/washing/ redispersion in distill water and in absolute ethanol. Finally, the products were dried in a vacuum at 50 °C for 6 h. The products were characterized by powder X-ray diffraction (XRD) using a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphite monochromatized CuKR radiation (λ ) 1.541781 Å). Field emission scanning electron microscopy (FESEM) images were taken on a JEOL JSM-6700F scanning electron microscope. Transmission electron microscopy (TEM) images were taken with a Hitachi H-800 transmission electron microscope at an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) images and the corresponding selected area electron diffraction (SAED) pattern were recorded using a high-resolution transmission electron microscope operating at 200 kV (JEOL-2010). Raman spectrum was performed at room temperature with a LABRAM-HR confocal laser micro-Raman spectrometer with an argon-ion laser at an excitation wavelength of 514.5 nm. Fourier transform infrared (FTIR) spectrum was recorded on a Bruker EQUINOX55 FTIR spectrometer in the wave numbers of 500-4000 cm-1 at room temperature, with the sample in a KBr disk. X-ray photoelectron spectroscopy (XPS) was recorded on an ESCALAB 250 VG Lited X-ray photoelectron spectrometer equipped with a nonmonochromatic AlKR excitation source (hν ) 1486.6 eV). Absorption spectrum was collected on a UV-vis spectrophotometer (Shimadzu UV-240). Photoluminescence (PL) spectrum was recorded on a Fluorolog 3-TAU-P.

Results and Discussion Characterization of Te/C Nanocables. The phase of asprepared products was examined by powder XRD. Figure 1

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Figure 1. XRD pattern of the products obtained through the reaction of Na2TeO3 with dextran at 180 °C for 24 h.

shows a typical XRD pattern of the products. All the reflection peaks can be readily indexed to trigonal Te (t-Te) with calculated lattice constants of a ) 4.452 Å and c ) 5.892 Å; these values are consistent with the literature value of 4.457 and 5.927 Å (JCPDS Card File No. 36-1452). The XRD pattern shows that Te could be obtained through the reduction of Na2TeO3 by dextran. The morphology and structural character of the products are displayed through FESEM and TEM images in Figure 2. A lowmagnification FESEM image in Figure 2a shows a panoramic morphology and dimension of the as-prepared products. The products are completely wirelike morphology with the length reaching to around tens of micrometers. From a high-magnification FESEM image (Figure 2b), some products with a bare core head (pointed to by arrows) can be observed, which shows the character of core-shell structure of nanocables. This structural character can be observed more clearly through the TEM images of the products, as shown in Figure 2c-e. The TEM images reveal the distinct dark/light contrast along the axis direction, which indicates a different phase composition and core-shell structure of the products. The diameter of the inner dark core is between 20-40 nm and the average thickness of the outer light layer is 40 nm. These images reveal that the as-prepared products are nanocables with uniform core-shell structures. The structure of the core of nanocables was investigated by SAED and HRTEM in more detail. Figure 3a and the inset display the TEM image of an individual nanocable and the SAED pattern of the core. According to the SAED pattern, the core of the nanocable has a single-crystalline nature and can be indexed as the trigonal Te, which is in agreement with the result obtained from XRD. The HRTEM image (Figure 3b) taken from this nanocable shows three set of distinct lattice spacing of ca. 0.59, 0.39, and 0.32 nm, corresponding well to the (001), (100), and (101) planes of t-Te, respectively. Both the SAED pattern and the HRTEM image analyses demonstrate that the cores of the as-prepared nanocables are single-crystalline t-Te with the preferential growth along the [001] direction (c-axis of the crystal lattice) as shown in Figure 3a, which is consistent with the inherent helical chain of t-Te. The shell composition of the nanocables was examined by Raman spectroscopy and FTIR spectroscopy. The Raman spectrum shows the presence of the amorphous and graphite carbon in the products. The broad peak at 1371 cm-1 and the strong peak at 1581 cm-1 can be attributed to the carbon atoms

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Figure 2. FESEM and TEM images of the as-prepared nanocables: (a) general view of the nanocables; (b) magnified FESEM image of the products, showing the bare core head of nanocables (indicated by arrows); (c-e) TEM images of the nanocables.

Figure 3. (a) TEM image of an individual nanocable, inset shows the SAED pattern of the core of the nanocable; (b) HRTEM image of the nanocable.

vibrations of the disordered amorphous carbon and crystalline graphite, respectively (Figure 4a).31 The high background of the Raman spectrum should be attributed to the high photoluminescence properties of the Te nanowires core.32 The Raman

Figure 4. (a) Raman and (b) FTIR spectrum of the Te/C nanocables.

spectrum indicates that the outer layer of the nanocables is carbon resulted from the carbonization of dextran. The FTIR spectrum of the products reveals the presence of certain functional groups in the carbon shell (Figure 4b). The broad bands at around 3443, 2923, and 1453 cm-1 could be attributed to O-H bond vibrations and C-H bonding.33 The absorption bands at 1696 and 1613 cm-1 are attributed to the CdO and CdC vibrations, respectively, which also supports the concept of carbonization of dextran under hydrothermal condition similar to the carbonization of glucose.34 The surface functional groups and the composition of the nanocables could be further identified by XPS technique. Figure 5a is the high-resolution XPS spectrum of C 1s peak of the products, which shows the peak consists of three peaks at 284.68, 286.1, and 287.6 eV. The peak at 284.68 eV is assigned to the C-C bonds in the disordered carbon frameworks, while the peaks at 286.1 and 287.6 eV are attributed to the residue groups such as C-OH and -CdO.35 The wide-range XPS spectrum of the products shows two very strong peaks at 287.6 and 532.39 eV (Figure 5b), corresponding to the binding energies of C 1s and O 1s, respectively. Generally, the detecting

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Figure 5. (a) High-resolution XPS spectrum of the C 1s peak for the products; (b) wide-range XPS spectrum of the products; (c) XPS spectrum of the products after argon ion sputtering.

Figure 6. TEM images of the products, showing the formation process of Te/C nanocables. These samples were taken after reaction times of (a) 1, (b) 2, (c) 4, (d) 8, (e) 12, and (f) 16 h. The insets of a are the SAED patterns indicating that a-Te and t-Te nanoparticles coexisted in the initial products, and the insets in c-e are high-magnification TEM images of the products.

depth of XPS is about 10 nm,36 so the peak with the binding energy at ∼573 eV for Te 3d is almost undetectable because of the presence of carbon shell. Figure 5c is the XPS spectrum of the products after etching shell by argon ion sputtering. After sputtering, the peak of Te 3d can be observed more clearly. The XPS spectra confirm that the products are composed of inner Te core sheathed by outer carbon shell. The above analyses demonstrate that our strategy is successful for the fabrication of Te/C nanocables by the one-step, solutionphase process. Furthermore, the presence of many functional groups in the carbon shell provides the as-prepared Te/C nanocables potential application in biomedicine, biochemistry and diagnostics. Formation Process of Te/C Nanocables. To investigate the formation process of the as-prepared Te/C nanocables, we

examined the products obtained at different reaction stages by using TEM. Figure 6 shows a set of TEM images corresponding to the products obtained at different reaction times. After reaction for 1 h, gray precipitates were obtained. The TEM image reveals that the products are composed of a great number of particles and larger aggregates with sizes between 50 and 200 nm (Figure 6a). The SAED patterns of the products (insets of Figure 6a) indicate that these particles are a mixture of amorphous Te (a-Te) colloids and t-Te nanocrystals.37 When the reaction time was extended to 2 h, wirelike products could be clearly observed as shown in Figure 6b. The image indicates that these wires grew from the surface of the particles. After reaction for 4 h, the particles disappeared and the products were completely composed of nanowires with diameter of around 30 nm as shown in Figure 6c. From a high-magnification TEM


Figure 7. Schematic illustration of the formation process of Te/C nanocables, which can be rationally supported by TEM studies given in Figure 6.

Figure 8. TEM images of the products prepared from 0.3 mmol Na2SeO3 with various amount of dextran at 180 °C for 24 h: (a) 0.1, (b) 1.2, and (c) 1.5 g.

Figure 9. UV-vis spectra of the Te/C nanocables with different carbon shell thicknesses: (a) 10, (b) 40, (c) 120, and (d) 200 nm.

image of the nanowires (inset of Figure 6c), no obvious shell could be observed on the outside of these nanowires, which indicates that the carbonization of dextran did not take place to form the carbon shell. As the reaction time was extended to 8 h, thin shell with a thickness of around 5 nm began to appear on the outside of the nanowires (see inset of Figure 6d). The thickness of the carbon shell gradually increased with the further extension of the reaction time. When the reaction time reached 12 and 16 h, the thickness of the carbon shell was about 10


and 20 nm respectively, as shown in images e and f in Figure 6 (inset is a high-magnification TEM image of the products). In addition, on the basis of a contrasting experiment, we observed that the presence of CTAB is an important factor for the present synthesis of Te/C nanocables. When no CTAB was added into the reaction system, the products were nonuniform bundles and aggregations of 1D core-shell structures instead of uniform and separate nanocables. From our experimental observations and previous reports,38 we believe that the presence of CTAB can assist the growth of Te nanowires with uniform size and prevent the aggregation of these nanowires through adsorption onto their surfaces. The above investigation demonstrates that the formation process of the Te/C nanocables undergoes four main stages: (1) First, dextran reacts with Na2TeO3 to produce the mixture of a-Te and t-Te particles under hydrothermal condition. (2) Because of the higher free energy of a-Te relative to t-Te and the high anisotropic nature of t-Te, a-Te particles are dissolved and deposited on t-Te particles (served as seeds) to generate t-Te nanowires through a solid-solution-solid transformation.23 (3) The above transformation continues until all a-Te particles have been used up. Because the transformation process takes a short time, dextran is not fully carbonized to form the carbon shell. With the assistance of CTAB, only uniform and separate t-Te nanowires are obtained in this stage. (4) Further extension of the hydrothermal time, the carbonization of dextran takes place and leads to deposition of carbonaceous products on the surfaces of the Te nanowires to form a carbon shell, resulting in the formation of the Te/C nanocables. The above processes are described schematically in Figure 7. The whole formation process of Te/C nanocables takes place at the identical reaction conditions, and thus we can directly fabricate Te/C nanocables via the present one-step solution route. This is different from previous reported methods for the synthesis of Te/C nanocables, in which the formation of Te nanowires core and the carbonization of carbonaceous products to form carbon shell take place under different conditions.8c,32 Through the investigation of the formation process of Te/C nanocables, we know that the Te nanowires form first and then the carbonization of dextran takes place, leading to deposition of carbon on the surfaces of Te nanowires to form shell. This result suggests that we can adjust the shell thickness of the nanocables by changing the deposition time or deposition quantity of carbon. The deposition time of carbon can be adjusted by controlling the reaction time. Images d-f in Figure 6 confirm that Te/C nanocables with different shell thickness can be obtained by changing the reaction time. The deposition quantity of carbon can be adjusted by changing the concentration of dextran. Further experiments confirmed this opinion. Keeping other reaction conditions unchanged, the shell thickness of the nanocables is diverse with the alteration of dextran quality. When 0.1 g of dextran was used in reaction system, we obtained only Te nanowires (Figure 8a), because insufficient dextran was carbonized to form the carbon shell. As expected, the carbon shell of the nanocables became thick to about 120 nm when the quantity of dextran was increased to 1.2 g (Figure 8b). With a further increase in dextran to 1.5 g, the thickness of the carbon shell also increased to about 200 nm (Figure 8c). Optical Properties of Te/C Nanocables. As a p-type semiconductor, Te has interesting optical properties and is widely used in optical electronic devices. Therefore, we studied the optical properties of as-prepared Te/C nanocables. Figure 9 shows the UV-vis spectra of the Te/C nanocables with different thickness of carbon shell. When the thickness of the carbon

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Figure 10. (a) Photoluminescence excitation spectrum (λem ) 435 nm) and (b) emission spectrum (λex ) 365 nm) of the Te/C nanocables with the a shell thickness of 40 nm; (c) emission spectrum (λex ) 365 nm) of the Te/C nanocables with a shell thickness of 200 nm.

shell is 10 nm, the UV-vis spectrum of the Te/C nanocables shows a characteristic absorption peak at about 277 nm (4.47 eV), as shown in Figure 9a. According to the study of Isoma¨ki et al., this absorption peak is due to the direct transition from the valence band (p-bonding triplet) to the conduction band (pantibonding triplet) for Te.39 Compared to a broad absorption of bulk Te at around 300 nm, the absorption peak of the Te/C nanocables exhibits a blue shift. This result is similar to pure Te nanowires reported by Rao et al.,40 in which they pointed out that there is a blue shift of the absorption peak for Te nanowires with the diametrical decrease. When the shell thickness is 40 nm, a similar characteristic absorption peak can be also observed (Figure 9b). When the thickness of the carbon shell increases to 120 nm, there is only a broad absorption band between 230 and 350 nm on the UV-vis spectrum of the products that can be observed (Figure 9c). As the shell thickness further increases to 200 nm, no obvious absorption peak can be observed in the spectrum (Figure 9d). The UV-vis spectra indicate that the absorption of Te nanowires core is gradually weakened with the increase in shell thickness. The photoluminescence properties of the Te/C nanocables were investigated at room temperature. Figure 10a is the excitation spectrum of the Te/C nanocables with a shell thickness of 40 nm by the emission monitored at 435 nm. There are two excitation peaks in the excitation spectrum, namely, a weak one at 267 nm and a strong one centered at 365 nm. The emission spectrum of the sample shows two photoluminescence emission peaks (at 412 and 435 nm) in the blue-violet region (390-550 nm) with the excitation wavelength of 365 nm, as shown in Figure 10b. This result reveals that the Te/C nanocables with the shell thickness of 40 nm can give blue-violet emissions, which is similar to pure Te nanowires.38a We also measured the emission spectrum of the Te/C nanocables with a shell

thickness of 200 nm under the excitation wavelength of 365 nm. There is no distinct emission peak between 400 and 650 nm in the emission spectrum of the products (Figure 10c). The above results indicate that too thick a carbon shell will weaken the optical properties of the Te nanowire core. When the thickness of the carbon shell is thin, such as 40 nm, the Te/C nanocables can maintain the optical properties of pure Te nanowires. Because the appropriate thickness of carbon shell has little effect on the optical properties of Te nanowire core and can protect it from contamination and oxidation, the Te/C nanocables might find better applications in the fabrication of potential optoelectronic nanodevices. Conclusion In summary, we have designed a facile and environmentally friendly one-step solution route for fabrication of Te/C nanocables. Investigation of the formation process of the nanocables demonstrated that t-Te nanowires were obtained first through a solid-solution-solid transformation process with the assistance of CTAB, and then dextran gradually carbonized on the surfaces of the Te nanowires to form the Te/C nanocables. The results of the investigation and the practical experiments confirmed that the shell thickness of the Te/C nanocables could be easily tuned by controlling the reaction conditions. The as-prepared Te/C nanocables with appropriate shell thickness maintain the optical properties of Te nanowires core and have a functionalized carbon shell, which might find potential applications in nanodevices and biochemistry. This efficient approach might also be extended for the fabrication of other semiconductor/carbon or metal/carbon nanocables. Further work is under way. Acknowledgment. The National Natural Science Foundation of China, the College & University Foundation for Excellent


Young Talents (No. 2009SQRZ027ZD), the Scientific Research Starting Fund of Anhui Normal University for Doctor, and the Young College Teachers’ Research Fund Program of Anhui province (No. 2007jql060zd) are thanked for the financial support. The authors also thank Dr. Lijuan Jiao and Dr. Erhong Hao for helpful discussions.


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