Synthesis, Characterization, and Formation Mechanism of Copper

ABSTRACT: Copper sulfide-core/carbon-shell cables and spheres have been prepared by a simple hydrothermal method. The obtained CuS/C cables and ...
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Synthesis, Characterization, and Formation Mechanism of Copper Sulfide-Core/Carbon-Sheath Cables by a Simple Hydrothermal Route Guang-Yi Chen,† Bin Deng,‡ Guo-Bin Cai,‡ Wen-Fei Dong,† Wan-Xi Zhang,† and An-Wu Xu*,‡

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 7 2137–2143

College of Materials Science and Engineering, Jilin UniVersity, Changchun, 130025, China, and DiVision of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, UniVersity of Science and Technology of China, Hefei 230026, China ReceiVed October 22, 2007; ReVised Manuscript ReceiVed January 28, 2008

ABSTRACT: Copper sulfide-core/carbon-shell cables and spheres have been prepared by a simple hydrothermal method. The obtained CuS/C cables and spheres were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Raman, Fourier transform infrared spectroscopy, photoluminescence, and UV-vis-NIR spectrum analysis. The resulting CuS/C cables and spheres were obtained by a hydrothermal process in the presence of β-cyclodextrin (β-CD) acting as the ligand and the carbon source of the sheath. The influence of the reaction time, reaction temperature, different sulfur source, and different solution system on the final products was investigated in detail. The possible formation mechanism for these copper sulfide-core/ carbon-shell cables was also proposed. Moreover, by using hot hydrochloric acid to dissolve the CuS cores, amorphous carbon nanotubes and hollow nanospheres can be obtained. Introduction One-dimensional (1D) nanostructures have attracted considerable research attention owing to their unique properties and potential applications during the past few decades.1 As a result, various methods have been developed to produce different forms of 1D nanostructures, such as nanowires, nanotubes, and nanocables.2 Among them, fabrication of coaxial cables consisting of a core and sheath with multicomponent materials is quite significant because the formation of 1D heterostructures may lead to materials with unique properties and multiple functionalities not realized in single-component structures that are beneficial for their functions to be further improved. Some 1D core-shell heterostructures have already been synthesized.3 Because of their excellent physical and chemical properties, transition metal chalcogenides such as ZnS, CuS, Bi2S3, MoS2, and CdS have been widely studied.4 Among them, copper sulfides as an important semiconductor material have attracted increasing attention in recent years, owing to their variations in stoichiometric compositions, complex structures, nanocrystal morphologies, valence states, different unique properties, and potential applications in many fields.5 The stoichiometric composition of copper sulfides can vary in a wide range from CuS2 at the copper-deficient side to Cu2S at the copper-rich side, including CuS2, Cu7S4, Cu1.8S, Cu1.94S, Cu1.96S, and Cu2S.6 Copper sulfides with various compositions are important p-type semiconductors that exhibit nearly ideal solar control characteristics7 and fast-ion conduction at high temperatures.8 Copper sulfides can find applications in solar cell devices, photothermal conversion, coatings for microwave shields, and solar control.9 Covellite CuS shows metallic conductivity and transforms into a superconductor at 1.6 K10 and can be used as a cathode material in lithium rechargeable batteries.11 Recently, various morphologies of copper sulfides have been fabricated including nanoparticles, nanowires, nanovesicles, nanodisks, micrometerscale hierarchical tubular structures,12 and so on. Many synthetic * Corresponding author. Fax: 86-551-3600724. E-mail: [email protected]. † Jilin University. ‡ University of Science and Technology of China.

methods such as hydrothermal or solvothermal methods,13 thermolysis,14 microwave irradiation,15 chemical vapor deposition,16 and template-assisted methods17 have been developed to prepare copper sulfides. However, to the best of our knowledge, the large-scale synthesis of copper sulfide-core/ carbon-shell cables and spheres via a simple, mild, and effective hydrothermal route has still been limited up to now. Since the discovery of carbon nanotubes in 1991,18 much effort has been devoted to develop new synthetic strategies for preparing carbon nanoarchitectures, nanocomposites, and related materials,19 owing to their potential applications in many fields. Other materials can be filled into carbon nanocavities to form nanocomposites with improved functions based on capillary forces20 and the combination of an efficient nanotube production method.21 Carbon nanomaterials were prepared in a low yield by traditional methods, which include catalytic chemical vapor deposition, electric-arc discharge, and laser vaporization.22 Although some solution-based methods have been reported to synthesize carbon nanotubes and amorphous carbon materials,23 most of them are complicated and uncontrollable. In this paper, we demonstrate that CuS-core/C-shell cables and spheres can be synthesized on a large scale by a facile onepot hydrothermal route. In our synthetic process, Cu(NO3)2 acted as Cu-precursor, thiourea (Tu) acted as a S-source, and β-cyclodextrin acted as both the ligand and the C-source of sheath. By a hydrothermal treatment for a certain time, novel copper sulfide/carbon cables and spheres can be synthesized. The effect of the reaction time, temperature, sulfur sources, and different solution system on the final products was systematically investigated. The shape evolution process and the formation mechanism were also proposed. Experimental Section Chemical and Synthesis. All the reagents were analytical grade and used without any further purification. In a typical synthesis, 1 mmol of Cu(NO3)2 · 3H2O was dissolved in a mixed solution of 10 mL of water and 5 mL of anhydrous ethanol, and then 2 mmol of thiourea (Tu) and 0.5 g of β-cyclodextrin (β-CD) were added to the above solution under stirring. The resulting homogeneous solution was

10.1021/cg701043f CCC: $40.75  2008 American Chemical Society Published on Web 05/29/2008

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Figure 1. The XRD pattern of the obtained CuS/C cables under hydrothermal treatment at 180 °C for 12 h.

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Figure 2. SEM images of the CuS/C cables grown by hydrothermal treatment at 180 °C for 12 h. (a) At low magnification; (b) at high magnification.

transferred to and sealed in a Teflon-line stainless steel autoclave of 20 mL capacity. The autoclave was heated to 120-220 °C and maintained at this temperature for 2-24 h and then was allowed to cool to room temperature naturally. The resulting black precipitate was filtered, washed with distilled water and absolute alcohol several times, and then dried at 60 °C in air. Characterization. The XRD patterns of the sample were recorded on a Rigaku/Max-3A X-ray diffractometer with Cu KR radiation (λ ) 1.54056 Å). Transmission electron microscopic (TEM) images, highresolution transmission electron microscopic (HRTEM) images, and electron diffraction (ED) patterns were taken with a JEOL-2010 microscopic operating at 200 kV. Energy-dispersive X-ray spectroscopy (EDS) was attached to the JEOL-2010. Field emission scanning electron microscopic (FE-SEM) patterns were collected on a JEOL JSM-6330F apparatus, operating at 15 kV. The photoluminescence (PL) measurement was performed on a RF-5301PC spectrophotometer equipped with a 150 W xenon lamp as the excitation source. A UV-vis-NIR spectrophotometer (model Solid Spec-3700 series) was used to record the absorbance spectra of the samples. The Raman spectrum was recorded at room temperature on a Renishaw in Via Laser Micro-Raman spectrometer. Fourier transform infrared spectroscopy (FTIR) analysis was carried out using KBr disks in the region of 4000-400 cm-1 by using a Nicolet MAGNA-IR 750 FTIR spectrometer at ambient conditions.

Results and Discussion Phase and Morphology of the Obtained CuS/C Cables. The XRD pattern of the obtained CuS/C cables synthesized by hydrothermal treatment at 180 °C for 12 h is shown in Figure 1. All the diffraction peaks can be readily indexed to the hexagonal phase of CuS crystals with lattice parameters of a ) 3.788 Å and c ) 16.339 Å, consistent with the standard diffraction data (JCPDS Card File No. 06-0464). No other characteristic peaks were observed, indicating that the products were covellite CuS. The diffraction peaks are much broader than those of the standard diffraction pattern, indicating the formation of the crystallites with a small size. The morphology and size of the obtained product were investigated by scanning electron microscopy (SEM) measurements. The SEM images in Figure 2 indicate that the as-prepared products are mainly composed of CuS/C coaxial cables with the diameters of around 2 µm and lengths of tens of micrometers, and the thickness of the carbon sheath is about 300-400 nm. It has to be mentioned that some of the cables are flexible and tend to bend and fuse with others. A small portion of spherical particles can also be observed in the final product. The microstructures of the products were further studied with transmission electron microscopy (TEM), high resolution TEM (HRTEM), and electron diffraction (ED) measurements. Typical TEM images of as-prepared CuS/C cables are shown in Figure 3a,b. The different light and dark contrast indicates a different phase composition, thus confirming that the fibers are the

Figure 3. TEM images (a, b) and HRTEM image (c) of the products prepared at 180 °C for 12 h. The inset in c shows the SAED pattern.

core-shell structure. Further studies demonstrate that the dark contrast region is the CuS core and the light contrast region is the carbon shell. Although the entire cable is too large and thick for the HRTEM observation, we can obtain the HRTEM image from the cross section of an individual broken cable, as indicated in the white frame in Figure 3a. Figure 3c is the corresponding HRTEM image, displaying resolved lattice fringes of (102) planes (d102 ) 0.298 nm). The SAED pattern in Figure 3c shows that the core is a single crystal with a hexagonal phase corresponding to the covellite CuS. The EDS analysis of the obtained products shows that cables are composed of copper, sulfur, and carbon elements, as shown in Figure 4. The Raman spectrum analysis of the sample (Figure 5) displays three peaks appearing at 470, 1385, and 1588 cm-1; the peak at 470 cm-1 is attributed to CuS,24 and the other two broad peaks at 1385 and 1588 cm-1 are very similar to the spectrum of sp2 carbon-bonded amorphous carbon.25 All the above results further confirm that the asprepared samples are 1D cables with CuS as the core and amorphous carbon as the sheath.

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Figure 4. EDS spectrum of the as-prepared CuS/C nanocables.

Figure 5. Raman spectrum of the as-obtained CuS/C cables.

The Effect of the Experimental Conditions on the Final Products. The effect of the reaction conditions on the final products was systematically investigated. It is found that CuS/C cables can only be obtained under proper experimental conditions. Figure 6 shows the SEM images and XRD patterns of the products prepared under different reaction temperatures. The results show that the reaction temperature plays an important role in fabricating these CuS/C cables. When the temperature was decreased to 120 °C, only CuS fibers were obtained, as is clearly shown in Figure 6a. Larger SEM images (Figure 6b,c) show that these CuS rods are built by nanoflakes with a thickness of ca. 20 nm. The core of the CuS rod tends to dissolve forming the open end of the rod, as is clearly shown in Figure 6c. At this low temperature (120 °C), the β-cyclodextrin could not be decomposed to release carbon, and bare CuS rods formed. When the temperature increased to 150 °C, β-cyclodextrin began to be decomposed to release carbon to enwrap CuS to form this novel CuS/C cable structure as shown in Figure 6d, so increasing temperature is favorable for the decomposition of β-cyclodextrin and the formation of the cable structures. On the other hand, when the temperature was further increased to 220 °C, the cable structure almost disappeared; only spherical particles can be observed (Figure 6e), indicating that CuS/C cables have a tendency to be broken to form CuS/C core-shell spheres at a relatively high temperature. TEM measurements show that these spheres have a core-shell structure; the core is CuS and the shell is carbon, as is clearly shown in Figure 6f,g. It is clearly seen from XRD patterns shown in Figure 6h that all samples prepared under different reaction temperatures are the covellite CuS.

Figure 6. SEM, TEM images, and XRD patterns of products prepared for 12 h at different reaction temperatures. SEM images: (a-c) 120 °C; (d) 150 °C; (e) 220 °C. TEM images: (f, g) 220 °C. (h) XRD patterns of the samples obtained at (a) 120 °C; (b) 150 °C; (c) 220 °C.

Control experiments were also carried out at 180 °C for different reaction times, that is, 2, 4, 8, and 24 h. Figure 7a-d shows the corresponding SEM images of the obtained samples. When the reaction time was 2 h, the product was mainly spherical particles (Figure 7a). When the reaction time was increased, CuS/C cables began to form. From these figures, we can draw a conclusion that with the reaction time increasing in a certain range, the quantity and quality of these copper sulfide/ carbon cables were increased and improved, but just like the effect of the reaction temperature on the final products, during prolonged hydrothermal treatment, for example, 24 h, the cables were transformed into CuS@C core/shell spheres (Figure 7d). The optimal reaction time for the formation of CuS/C cable structures was found to be in the range of 8-14 h. The effect of different solution systems on the final products was also investigated. It was found that ethanol was not necessary in the system, and the CuS/C nanocables could also be prepared in pure water by hydrothermal treatment (Figure 8a), which further proved that the carbon sheath came from

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Figure 9. SEM image of products prepared in the absence of β-CD.

Figure 7. SEM images of products prepared at 180 °C for different reaction times: (a) 2 h; (b) 4 h; (c) 8 h; (d) 24 h.

Figure 10. The chemical structure of β-cyclodextrin (a) and the glucose units connecting manner (b).

Figure 8. SEM images of products prepared at 180 °C for 12 h in different solutions: (a) water; (b) ethanol.

β-cyclodextrin. However, the quality of CuS/C nanocables synthesized in pure water was not as good as that synthesized in the mixture of water and ethanol. Moreover, in a pure ethanol reaction system, almost no cable structure can be observed, as shown in Figure 8b. Other sulfur sources such as thioacetamide (TAA) were also used in our experiment; when TAA was used to replace thiourea, there were no cables observed in the products, indicating that copper-thiourea system is a crucial factor in the formation of a cable structure. Additional experiments were done in the absence of β-CD, while other reaction conditions were kept constant. It is found that obtained CuS products have sphere morphology comprised of nanoplates without the addition of β-CD (Figure 9), indicating the β-CD in this study acts as a directing agent to promote the preferential 1D growth of CuS/C core-shell nanocables. Formation Mechanism of the As-Obtained CuS/C Cables. It is well-known that the copper-thiourea system has already been used to prepare copper sulfides with various forms and compositions.14,26 Thiourea (Tu) can coordinate with copper(II) ion in a aqueous solution under ambient conditions.27,28 This reaction can be visually observed via a simple color change of the reaction solution. In our study, after Tu was introduced to the copper(II) salts solution, the color was changed from blue to green, indicating that Tucopper(II) was formed. The formed thiourea-copper(II)

Figure 11. FTIR of (a) the products prepared at 120 °C for 12 h and (b) pure β-CD.

complex ([Cu(Tu)n(H2O)x]2+) served as Cu2+ precursors. The following reactions may be expected as

Cu2++ nTu + xH2O S [Cu(Tu)n(H2O)x]2+

(1)

β-Cyclodextrin (β-CD), torus-shaped cyclic oligosaccharides consisting of seven 1,4-linked R-D-glucopyrannose units, is one of the well-known supramolecular compounds.29 Figure 10a is the chemical structure of β-CD. The glucose units are connected through glycosidic R-1,4 bonds, as illustrated in Figure 10b. The hydroxyls on β-CD molecules would strongly coordinate with heavy metal ions such as Cu2+ ions. After being introduced to the reaction mixture, the Cu2+ ions would be attracted by the β-CD to form the Cu-β-CD complex. Figure 11 shows the FTIR spectra of the products (Figure 11a) prepared at 120 °C for 12 h and the pure β-CD (Figure 11b),

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Scheme 1. Growth Mechanism for the CuS/C Coaxial Cable Structure and Carbon Tubes

respectively. The absence of the transmittance bands at 756 cm-1 and 946 cm-1 indicates that the free β-CD has almost disappeared in the final products by comparing Figure 11a,b. The dramatic decrease and the shift of peak positions at 706 cm-1 and 576 cm-1 can be interpreted as the “fixed” nature of the β-CD that prevents the skeletal vibration and pyranose ring vibration. The FTIR results confirm that there is strong interaction between Cu2+ ions and the hydroxyl to allow β-CD to attach to CuS crystals.30 Under hydrothermal treatment, the thiourea in the solution can be decomposed at a certain temperature to allow sulfur species to react with the Cu-β-CD complex. The possible reaction process can be described as follows:

[Cu(Tu)n(H2O)x]2+ f Cu2++ nTu + xH2O

(2)

Cu2+ + nβ-CD f [Cu(β-CD)n]2+

(3)

NH2CSNH2 + 2H2O f 2NH3 + CO2+H2S

(4)

[Cu(β-CD)n]2+ + H2S f CuS(β-CD)n + 2H+

(5)

When the reaction time was increased, the fiber-like covellite copper sulfide was formed, and the β-CD decomposed to release carbon to cover CuS fibers uniformly. So at the end of the reaction, the novel CuS/C coaxial cable structure can be successfully fabricated. The morphology and phase transformation in the present system could be described by Scheme 1. Optical Properties of the As-Obtained CuS/C Cable Structures. The optical properties of as-prepared CuS/C cable were characterized by UV-vis-NIR absorption spectra and photoluminescence spectra. Figure 12 represents the absorption spectrum of the products obtained at 180 °C for 12 h and dispersed in 1:1 water/ethanol. The spectrum shows that the sample strongly absorbs in the region of 300-500 nm. In addition, an absorption band extending into the near-IR region can be observed, which is the characteristic for covellite CuS materials.31 Figure 13 shows the room-temperature photoluminescence (PL) spectrum of the as-prepared CuS/C cables. Under the excitation wavelength of 370 nm, the sample shows a weak and broad emission peak at 465 nm. Some literature reported that there is no emission for CuS in the range of 400-800 nm,32 while some literature reported there is PL emission for CuS.33 Although PL emission mechanism of CuS is not clear at present, it can be proposed that different morphologies and microstructures of copper sulfide could be responsible for different phenomenon of PL.

Figure 12. UV-vis-NIR absorption spectrum of the product prepared at 180 °C for 12 h.

Figure 13. Room temperature PL spectrum of the product obtained at 180 °C for 12 h.

Carbon nanotubes and hollow carbon spheres can also be obtained from these copper sulfide/carbon cables and core-shell spheres by using hot hydrochloric acid to dissolve the CuS cores. The tubular structure of carbon is clearly shown in Figure 14a. Figure 14b is the corresponding HRTEM image, which demonstrates the amorphous nature of carbon nanotubes. No graphitic carbon layers were observed by high-resolution TEM measurements for the assynthesized samples. However, further thermal treatment of the as-obtained sample at 550 °C for 10 h under vacuum produced the graphitic structure. Hollow carbon spheres were obtained first by etching the CuS cores and then calcination at 550 °C for 10 h under vacuum, as shown in Figure 14c. The shell thickness of hollow spheres is ca. 100 nm, estimated from TEM image. The HRTEM image (Figure 14d) taken from the shell clearly shows a kind of graphitic carbon with a 0.33 nm layer-to-layer distance,34 with the graphene layers oriented nearly parallel to the interface. These results demonstrate that our novel methods for the fabrication of carbon nanotubes and hollow nanospheres are attractive because we can produce samples of both high purity and yield from readily available and inexpensive starting materials under mild conditions. These hollow carbon materials will find potential applications in catalyst-supports, nanocomposites, lithium batteries, electrode materials.35 Also, hollow

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(2)

(3)

(4)

Figure 14. TEM (a) and HRTEM (b) images of the amorphous carbon nanotubes obtained by etching the CuS cores with hot hydrochloric acid. (c) and (d) TEM and HRTEM images of hollow C spheres prepared by etching the CuS cores and then calcination at 550 °C for 10 h under vacuum.

carbon spheres have the unique feature of providing a protected core suitable for encapsulating a variety of guest materials. Conclusions In summary, we have successfully fabricated CuS-core/Cshell cables and spheres by a simple hydrothermal method. The morphology evolution process and various influencing factors have been systematically studied by means of XRD, TEM, Raman, FTIR, and so on. The formation mechanism was preliminarily studied. The optical properties of the products were also investigated in detail, and the UV-vis-NIR absorption spectra show the characteristic broadband for covellite CuS in the near-IR region. PL spectra of the products show a broad emission peak at 465 nm. Carbon nanotubes and hollow spheres can also be obtained from these copper sulfide/carbon cables and core-shell spheres by using hot hydrochloric acid to dissolve the CuS cores. Hollow carbon spheres have the unique feature of providing a protected core suitable for encapsulating a large variety of other functional materials. Coaxial nanocables consist of semiconductor cores and sheaths, and derived hollow carbon materials might find potential applications in related fields. CuS can be used as a cathode material of lithium batteries,11 and hollow carbon itself is an active material for additional Li+ ion storage.35a Therefore, our obtained CuS/C core-shell composites have great potential applications in lithium batteries. Further study on their applications in lithium batteries is ongoing. Acknowledgment. Support from the National Natural Science Foundation of China (20671096) and the special funding support from the Centurial Program of CAS is gratefully acknowledged.

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