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Heterostructures with ZnSe Sheaths Coating on Carbon Submicrotubes: Preparation, Characterization, and Formation Mechanism Benxia Li, Yi Xie,* Yang Xu, Changzheng Wu, and Qingrui Zhao Department of Nanomaterial and Nanochemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, P. R. China ReceiVed: January 6, 2006; In Final Form: June 2, 2006
We designed a feasible one-step process to synthesize heterostructures with inorganic functional materials coating on carbon submicrotubes under a mild condition. The heterostructures of carbon submicrotubes with ZnSe sheaths were successfully synthesized through the polymerization-carbonization-coating process with glucose as both the carbon source and the reductive reagent and ammonia providing an alkaline environment and acting as a soft template. The compositions of the as-obtained product were confirmed by Raman spectroscopy and XRD measurement; the morphology and microstructure were studied by SEM, TEM, and HRTEM. Room-temperature photoluminescence (PL) measurement indicates the as-prepared tubular heterostructures have a sharp and well-resolved NBE emission centered at 436 nm besides the DL emission at 589 nm, which is possibly caused by the interface associated with the combination of carbon submicrotube and ZnSe nanocrystal. One of the advantages in this process is that glucose and ammonia play manifold roles in the formation of the submicroscaled tubular heterostructures. This suggests a new path for convenient synthesis of novel tubular heterostructures with inorganic functional materials attached on carbon tubes. Furthermore, this kind of tubular heterostructure may be an ideal system applied in the fabrication of submicroscaled optoelectronics devices, and investigations on its physical properties could extend the understanding of the structure-property relationships in solids, which are in progress.
1. Introduction Submicrotubes, a special type of one-dimensional hollow structure with sizes between nanoscale and microscale, have stimulated a great deal of interest owing to their unique characteristics: extraordinary mechanical, electrical, and adsorption properties.1-4 The synthesis and characterization of inorganic materials with tubular structures in the submicroscale, including carbon5-7 and other inorganic submicrotubes,8-10 have been actively pursued by many groups in the past decade because of their potential uses in various fields. It is a common understanding that combination of the distinctive properties of carbon tubes and other inorganic functional materials would meet broader application requirements, such as field emission displays, electronic devices, novel catalysts, and polymer or ceramic reinforcement.11,12 Thus, it is significant to assemble carbon tubes and other functional inorganic materials into heterostructures. While most of the current works on carbonrelated heterostructures have focused on semiconductor/carbon nanocables with semiconductor cores and carbon sheaths, such as Ga2O3-C, ZnS-C, and BN-C nanocables,13-15 tubular heterostructures consisting of carbon tubes inside and inorganic functional nanocrystals as the sheaths may possess the advantages of both C-related heterostructures and tubular nanostructures. In addition, harsh conditions or fussy manipulations, such as arc discharging or thermal evaporation, are always necessary for the synthesis of carbon-related heterostructures. Hence, it is necessary to find an efficient and facile method combining synthesis and coating in one step to form the tubular heterostructures with inorganic nanocrystals attached on carbon tubes. Glucose, a nontoxic, cheap, and water-soluble compound, has been used as the carbon source to synthesize colloidal carbon
spheres16 and Ag@C core/shell structured nanoparticles17 by its polymerization and carbonization at 160-180 °C. Enlightened by the multi-hydroxyl structure of glucose, we conceived that tubular carbonous frameworks with many hydroxy groups may be fabricated by the polymerization of glucose under appropriate conditions, and then these hydroxy groups restrict the reaction of inorganic ions on the surfaces of the tubular carbonous frameworks.18 Thus, it is possible to design a feasible one-step process to synthesize the heterostructures with inorganic functional materials coating on carbon tubes under mild conditions. As an example, preparation of tubular C/ZnSe heterostructures is investigated in the present study since ZnSe is an important II-VI wide band gap semiconductor (with a bandgap energy of 2.67 eV at room temperature). Herein novel heterostructures of carbon submicrotubes with ZnSe sheaths were successfully synthesized through a polymerizationcarbonization-coating process under mild one-pot hydrothermal condition, using glucose as both the carbon source and the reductive reagent with ammonia providing an alkaline environment and acting as a soft template. 2. Experimental Section Synthesis. All of the chemical reagents in this experiment were of analytical grade and used without further purification. Zinc chloride (ZnCl2), aqueous ammonia (25% NH3), selenium dioxide (SeO2), and glucose (C6H12O6) were purchased from Shanghai Chemical Reagents Co. A 20 mL amount of 0.2 M ZnCl2 aqueous solution was mixed with 20 mL of ammonia solution (25% NH3) under continuous stirring; then 8 mmol of glucose and 4 mmol SeO2 were added into the mixed solution. After 10 min of stirring, the resulting reaction mixture was transferred into a Teflon-lined stainless autoclave (50 mL
10.1021/jp0601062 CCC: $33.50 © 2006 American Chemical Society Published on Web 07/06/2006
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Figure 1. (a) Raman spectrum and (b) XRD pattern of the product obtained at 160 °C for 12 h.
capacity). The autoclave was sealed and maintained in an electric oven at 160 °C for different stages and then cooled to room temperature naturally. The brown precipitates were carefully collected and washed with distilled water and absolute ethanol several times and then dried in a vacuum at 50 °C for 4 h. Characterization. X-ray diffraction (XRD) analysis was performed using a Philip X’Pert PRO SUPER γA rotation anode with Ni-filtered Cu KR radiation (λ ) 1.54178 Å). Field emission scanning electron microscopic (FESEM) images were taken on a JEOL JSM-6700F SEM. The transmission electron microscopic (TEM) images and selected-area electron diffraction (SAED) patterns were recorded on a Hitachi model H-800 instrument at an accelerating voltage of 200 kV. The highresolution transmission electron microscopy (HRTEM) was performed on a JEOL-2010 high-resolution transmission electron microscope at an accelerating voltage of 200 kV. Bulk elemental analysis (from C-N-H combustion) was taken on an Elementar Vario EL-III elemental analyzer. Oxidation and reduction temperatures were 950 and 500 °C, respectively. Fourier transform infrared (FTIR) absorption spectra were obtained with a Shimadzu IR-440 spectrometer. The Raman spectrum was performed at room temperature with a LABRAM-HR Confocal Laser MicroRaman Spectrometer with an argon-ion laser at an excitation wavelength of 514.5 nm. Photoluminescence (PL) measurement was carried out on a Perkin-Elmer LS-55 luminescence spectrometer using a pulsed Xe lamp. 3. Results and Discussion
Figure 2. (a and b) FESEM and TEM images of the product obtained at 160 °C for 12 h, SAED pattern (inset of b) taken from the sheath on the submicrotube. (c) TEM image of individual bare submicrotubes found in the product, and the SAED pattern taken on the bare submicrotube. (d) HRTEM image of the C/ZnSe interface (left) and the ZnSe nanocrystal (right).
3.1. Compositions of the Product. The as-obtained product, studied by Raman spectroscopy, is confirmed as a composite material composed of amorphous carbon and ZnSe. In a typical Raman spectrum (Figure 1a) two broad vibrational peaks are located at 1371 and 1540 cm-1, the same as those of amorphous carbon.19 In detail, the peak at 1540 cm-1 corresponds to the Raman-active optical mode E2g of 2-dimensional graphite, which is closely related to the vibration in all sp2-bonded carbon atoms in a 2-dimensional hexagonal lattice; the peak at 1371 cm-1 is attributed to the Raman-inactive A1g vibration mode assigned to the vibrations of carbon atoms with dangling bonds in planar terminations of disordered graphite.20 In addition, the peak around 241 cm-1 is regarded as a weak peak centered at 200 cm-1 overlapping with a strong peak at 241 cm-1, corresponding to the transverse optical (TO) and longitudinal optical (LO) phonon modes of ZnSe,21,22 respectively. The carbon content in the product, calculated by bulk elemental analysis, exceeds 8.5 wt %. These results clearly demonstrate that the carbonization of glucose can be achieved by the hydrothermal process, and the composite material composed of C and ZnSe was obtained. The ZnSe component in the as-obtained C-ZnSe composite material was also characterized by XRD measurement, and the
XRD pattern is shown in Figure 1b. All diffraction peaks can be readily indexed as cubic ZnSe with lattice constant a ) 5.673 Å, in good agreement with the literature value (JCPDS Card No. 05-0522), and the broadening of the diffraction peaks indicates that the ZnSe crystals in the sample were nanosized. The carbon component in our product is amorphous and highly disordered, as indicated in the Raman spectrum, and thus no corresponding peaks can be observed in the XRD pattern. 3.2. Morphology and Microstructure of the Product. The FESEM and TEM images (Figure 2) of the typical product obtained at 160 °C for 12 h show that the product is composed of C/ZnSe submicrotubes with lengths up to several micrometers and these submicrotubes have rough surfaces with sphere-like ZnSe nanoparticles of 50-100 nm. An open end of a submicrotube shown in the inset of Figure 2a reveals that its outer diameter is approximately 500 nm and inner diameter about 350 nm. The TEM image (Figure 2b) reveals the virtual structure of the tubular configuration assembled by nanoparticles coating on a submicrotube. The SEAD pattern taken from the sheath of a submicrotube (inset of Figure 2b) displays its polycrystalline characteristic, and all the diffraction rings match the XRD peaks very well, suggesting that the sheath is composed of ZnSe nanocrystals. Individual bare tubes without ZnSe sheaths found
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SCHEME 1: Schematic Illustrations: (a) Formation of ZnSe Nanocrystals on the Carbon Submicrotubes and (b) Formation Process of the Tubular Carbon/ZnSe Heterostructures
in the product are confirmed to be amorphous (Figure 2c), in agreement with the amorphous carbon in the product characterized by Raman spectrum. On the basis of the SEM and TEM analyses, the as-obtained tubular submicrostructures are novel heterostructures consisting of amorphous carbon submicrotubes coated with ZnSe sheaths. The microstructure of the C/ZnSe heterostructure was further studied by HRTEM images (Figure 2d). The interface between the ZnSe sheath and amorphous carbon tube can be clearly observed, and the wall thickness of the amorphous carbon tube is about 20 nm (left image in Figure 2d). Moreover, it could be observed that the ZnSe sheaths were formed by many well-crystalline ZnSe nanocrystals with a size of about 10 nm, and the lattice spacing, 3.28 Å, is consistent with that of ZnSe (111) (right image in Figure 2d). 3.3. Possible Formation Mechanism. In our approach the formation process of ZnSe nanocrystals in the reaction system can be illustrated by a series of reactions as shown in Scheme 1a: first, SeO32- from the dissolution of SeO2 in the solution was reduced by glucose to Se; then, under alkaline conditions, Se dismutated into Se2- and SeO32-, and the later would react again as the Se source; finally, Se2- combined with Zn2+ from Zn(NH3)42+ to form ZnSe coating on the carbon submicrotubes.
Figure 3. (a) TEM image and XRD pattern of the 5 h product (peaks marked with # correspond to Se). (b) SEM image and XRD pattern of the 24 h product.
Formation of the intermediate Se was verified by stopping the reaction at 5 h (Figure 3a). The above evolution process could be confirmed by XRD characterization (Figures 1b, 3a, and 3b), which displays the gradual decrease in the diffractive intensity of Se reflections and the gradual increase in that of ZnSe reflections as prolonging the reaction time. It should be pointed out that SeO2 was specifically chosen as the Se2- source in order to slow formation of ZnSe nanocrystals, and thus it could be achieved that carbon tubes were formed before the nucleation and growth of ZnSe. The growth process and possible formation mechanism of the tubular heterostructure of carbon tubes coated with ZnSe sheaths, schematically illustrated in Scheme 1b, undergo the following four distinctive stages. (i) Glucose was polymerized via the dehydration and covalent bonding occurring among the OH or CHO groups to form one-dimensional polysaccharide frameworks under hydrothermal condition, and a large amount of NH3 molecules were simultaneously entrapped in the polysaccharide frameworks. Meanwhile, the reductive OH or CHO groups in glucose reduced SeO32- to Se. (ii) Subsequently, the polysaccharide frameworks were further carbonized to form the amorphous carbon tubes, and the NH3 molecules within were released. (iii) Numerous Zn(NH3)42+ ions were adsorbed onto the surfaces of carbon tubes due to their combinations with the residual hydroxyl groups, and then the carbon tubes could act as supports for the growth of ZnSe nanocrystals on their surfaces. (iv) Finally, the ZnSe nanocrystals on the carbon tubes grew and deposited continuously to form the tubular heterostructures with thicker and denser ZnSe sheaths. The proposed
Figure 4. FTIR spectrum of the product obtained at 160 °C for 12 h.
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Figure 5. SEM images and corresponding XRD patterns of the products prepared with different volumes of aqueous ammonia solution added: (a) 0, (b) 8, and (c) 40 mL. Note that the total volume of the solvent used was fixed at 40 mL (peaks marked with an asterisk (*) correspond to Zn(OH)2, and peaks marked with # correspond to Se).
mechanisms are fully supported by our experimental results. As indicated by the TEM image in Figure 3a (left), the amorphous carbon tubes have formed and been partially coated by ZnSe nanoparticles as the reaction proceeded for 5 h, suggesting that the tubular core/sheath heterostructures have primarily formed at this stage. Further extending the reaction time, ZnSe nanocrystals on carbon tubes became denser and the thickness of the ZnSe sheaths increased gradually. When the reaction proceeded for 24 h, the tubular heterostructures with thicker sheaths could be obtained (Figure 3b, left) and the outer diameters have evidently increased (∼300 nm) in comparison with that of the 12 h product. The experimental results indicate that the polymerization and carbonization were performed in a relatively short period of the reaction time (∼5 h), and the formation of ZnSe sheaths (coating process) took a longer period (12-24 h). In addition, the FTIR spectrum, shown in Figure 4, was used to identify the functional groups present in the final product. Peaks at 2920 and 1425 cm-1 were assigned to C-H bonding,23 suggesting the possible incorporation of hydrogen during growth of the product. The broad band at about 3394
cm-1 could be attributed to O-H bond vibrations from the residual hydroxy groups. 24 The peak at 1610 cm-1 could be assigned to the CdC stretching, 25 resulting from the carbonization of glucose. All of the above experimental results confirmed the proposed formation process and mechanism. In the present study we assumed that ammonia played crucial roles in the formation of the novel tubular heterostructures with ZnSe sheaths coating on carbon submicrotubes. To better understand the roles of ammonia in the reaction process, we investigated in detail the effect of the volume of the added aqueous ammonia solution (25%) on the purity and morphology of the products. The experimental results indicated that ammonia influenced the formation of both amorphous carbon tubes and ZnSe sheaths. On one hand, enough NH3 molecules act as a soft template by combining with the active OH groups in the polysaccharides to favor formation of carbon submicrotubes. On the other hand, ammonia provided an alkaline environment for the transformation from Se to Se2- and acted as a complex reagent for Zn(NH3)42+; thus, ZnSe sheaths formed by Se2- and Zn(NH3)42+ reacting on the surfaces of carbon tubes. As
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Figure 6. SEM images and XRD patterns of the products obtained at (a) 140 and (b) 200 °C for 12 h (peaks marked with an asterisk (*) correspond to Zn(OH)2, and peaks marked with # correspond to Se).
indicated in Figure 5a, when no ammonia was added in the reaction system with other experimental conditions unchanged, the product was composed of only microspheres with diameters of 0.5-2 µm and all the peaks in the corresponding XRD pattern could be indexed to hexagonal Se (JCPDS Card No. 01-0853), indicating that formation of neither carbon submicrotubes nor ZnSe nanoparticles could occur in the absence of ammonia solution. With 8 mL of aqueous ammonia solution added in the reaction system (the total volume of solvent was fixed at 40 mL), the obtained product was composed of the heterostructures mixed with some sphere-like particles (left in Figure 5b) and the corresponding XRD pattern (right in Figure 5b) indicates that a minor amount of Se exists in the product obtained after 12 h. However, when the aqueous ammonia solution (25%) was used alone as the solvent, the obtained product was composed of rodlike structures and irregular nanoparticles, as can be seen from Figure 5c, and the peaks in the XRD pattern of the product could be indexed to a mixture of ZnSe and Zn(OH)2.When no or a lower amount of aqueous ammonia solution was added to the solution, the reaction solution was not alkaline enough to transfer Se into Se2- completely and the residual Se atomic clusters congregated into Se microspheres in the final product, whereas when the amount of aqueous ammonia solution added to the reaction system was excessively high, a very high pH for the reaction solution resulted, favoring formation of Zn(OH)2; as a result, Zn(OH)2 impurity was contained in the product obtained with aqueous ammonia solution (25%) used alone as the solvent. The optimal amount of aqueous ammonia solution added in the synthesis system to form the tubular heterostructures with ZnSe sheaths is 20 mL when the total volume of solvent was fixed at 40 mL. The reaction temperature in the present reaction system was also found to be a factor that influenced the purity and morphology of the product. When the temperature was as low as 140 °C, the reaction produced tubular heterostructures mixed with some irregular nanoparticles (Figure 6a) and the corre-
sponding XRD pattern was indexed to the mixture of cubic ZnSe and hexagonal Se, indicating that the reaction proceeded at a very slow rate, and thus, a relatively longer reaction time was needed for the ZnSe nanoparticles on the carbon tubes. However, at high temperature adsorption of Zn(NH3)42+ on the surfaces of the carbon tubes might be much weakened and Zn(NH3)42+ was prone to hydrolyze into Zn(OH)2, resulting in the presence of Zn(OH)2 impurity in the product. As shown in Figure 6b, the XRD pattern of the product obtained at 200 °C indicates the product consists of ZnSe and minor amounts of Zn(OH)2, and the corresponding SEM images depict that the product is irregular particles with minor rodlike particles. Detailed experiments have revealed that reaction temperatures ranging from 160 to 180 °C are optimal for formation of uniform tubular heterostructures with ZnSe sheaths. 3.4 Optical Property. To shed light on the optical property of the as-prepared C/ZnSe heterostructures and their potential as photonic materials, a study of their luminescence spectrum was carried out. Figure 7 shows the room-temperature photoluminescence (PL) spectrum of the as-prepared C/ZnSe heterostructures for excitation at 350 nm. The PL spectrum consists of two characteristic emission peaks: the near band edge (NBE) emission at about 436 nm, usually associated with excitons, and the deep-level (DL) emissions at about 589 nm, usually associated with defects.26 The lower energy emission of about 589 nm from the ZnSe nanocrystals is attributed to self-activated luminescence, possibly due to donor deep acceptor pairs related to zinc vacancies and interstitial sites.27 However, the NBE emission peak at about 436 nm in the PL spectrum of the asprepared tubular C/ZnSe heterostructures is clearly observed and stronger than the DL emission at 589 nm. In addition, we remark that room-temperature NBE emissions of other ZnSe nanostructures are normally very weak and not even observed in some cases.28 The sharp and well-resolved NBE emissions from the present tubular C/ZnSe heterostructures are possibly caused by the proximity of the complexes to the surface and
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J. Phys. Chem. B, Vol. 110, No. 29, 2006 14191 Acknowledgment. This work was financially supported by the National Natural Science Foundation of China. References and Notes
Figure 7. Room-temperature PL spectrum of the as-prepared C/ZnSe heterostructures.
the interface associated with the combination of the carbon submicrotube and ZnSe nanocrystal.29 Thus, assembling carbon tubes and other functional inorganic materials into heterostructures is expected to increase the versatility and power of these building blocks in bottom-up-designed photonic and electronic devices on a microscale. 4. Conclusion Novel tubular heterostructures with ZnSe sheaths coating on amorphous carbon submicrotubes have been successfully synthesized based on the polymerization-carbonization-coating process under mild one-pot hydrothermal conditions. One of the advantages in this process is that glucose and ammonia play manifold roles in the formation of the submicroscaled tubular heterostructures. By adjusting the added volume of the aqueous ammonia solution and the reaction temperatures, the uniform tubular core/sheath heterostructures can be successfully synthesized. Compared with previous methods of preparing Crelated heterostructures,12-15 this concise one-step route has special advantages and suggests a new path for convenient synthesis of novel tubular heterostructures with inorganic functional materials coated on carbon tubes. Furthermore, the PL spectrum of the tubular C/ZnSe heterostructures indicates a strong and well-resolved NBE emission centered at 436 nm besides the DL emission at 589 nm, which is possibly caused by the interface associated with the combination of the carbon submicrotube and ZnSe nanocrystal. This kind of tubular heterostructures may be an ideal system applied in the fabrication of submicroscaled optoelectronics devices, and investigations on its physical properties could extend the understanding of the structure-property relationships in solids, which are in progress.
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