Reduced Graphene Oxide-Mediated Growth of Uniform Tin-Core/Carbon-Sheath Coaxial Nanocables with Enhanced Lithium Ion Storage Properties

Graphene oxide (GO) is believed to contain a large number of defects in comparison with pure graphene.1–4 In many cases, these defects have a negative influence on the pristine properties of graphene,5–7 while in certain cases GO is more attractive than graphene itself.8, 9 For instance, the combination of metal (oxide) nanoparticles with GO is much more efficient than with high quality graphene and homogeneous decoration of GO or reduced GO (RGO) with tiny metal (oxide) particles is therefore easy to achieve.10–14 These nanoparticle-decorated GO and/or RGO hybrids have attracted great attention recently owing to, first, their promising properties for catalysis,14, 15 lithium-ion batteries (LIBs),11, 13, 16, 17 supercapacitors,18 chemical and biosensors,19 and optoelectronic devices20 and, second, their unique structures, which can be used to further construct multifunctional nanomaterials with hierarchical structures.12, 21 In particular, the growth of one-dimensional nanostructures on GO or RGO results in fascinating 1D/2D hybridizations.22–25 In this regard, carbon nanotubes (CNTs) have been successfully fabricated recently on the surface of metal oxide–decorated RGO using a chemical vapor deposition (CVD) method, resulting in RGO-CNT hybrids with promising properties as electrodes in supercapacitors,21 LIBs,26, 27 photocatalysis,28 and optoelectronic devices.25, 27, 29

Tin (Sn) nanostructures have recently received considerable attention as anode materials for LIBs as a result of their high theoretical specific capacity (992 mAh g⁻¹).30, 31 However, the high capacity has never been realized in practice because of the pulverization-induced capacity fading that is caused by huge specific-volume changes during the insertion and extraction of lithium ions.30 Carbon has therefore been selected as a buffer layer to encapsulate Sn nanostructures, which show significantly improved electrochemical properties compared to bare Sn nanomaterials.32–40 Also, different encapsulation and composition strategies have been reported to synthesize tin-carbon composites mostly from zero-dimensional Sn nanoparticles35–38, 41 or low-aspect-ratio Sn rods.32, 42, 43 However, the highly efficient, large-scale fabrication of Sn/C nanowires is a challenge owing to the very low melting point of metallic tin, although Sn/C nanowires with high aspect ratio are much more attractive for practical applications. Very recently, Deng and Lee prepared micrometer-scale SnO₂/C spheres by hydrothermal reaction of SnCl₄ and glucose. Sn@CNT nanowires were successfully grown on the surface of these spheres.32 However, in that case, most of the SnO₂ particles embedded in the sphere could not be used, and actually a mixture of tin-containing carbon mesospheres, carbon nanotubes with completely or partially filled tin interiors, and carbon-coated pear-shaped tin nanoparticles was obtained.

In this Communication, we report for the first time a new strategy to grow tin-core/carbon-sheath coaxial nanocables directly integrated onto the RGO surface by a RGO-mediated procedure (Figure1a). The as-synthesized nanocables not only exhibit uniform diameter and high aspect ratio but are also capable of being easily tailored into other interesting one-dimensional nanomaterials (Figure 1b). Most importantly, the nanocables exhibit excellent lithium storage performances, as revealed by electrochemical evaluation.

Figure 1. a) Schematic of procedures for the synthesis of RGO-supported Sn@C nanocables: (1) SnO₂ nanoparticle-decorated RGO (RGO-SnO₂) hybrids are first synthesized by a concurrent hydrolysis and reduction process; (2) RGO-Sn@C nanocables are grown from the RGO-SnO₂ hybrids by CVD. b) Schematic of experimental procedures showing the transformation from the nanocables to carbon nanotubes with one open end and SnO₂ nanotubes. c) XRD patterns of (1) GO, (2) RGO-SnO₂, and (3) the final nanocable product.

The synthesis of RGO-supported tin-core/carbon-sheath (RGO-Sn@C) nanocables consists of two main steps as outlined in Figure 1a. Typically, tin oxide (SnO₂) nanoparticles were first decorated on RGO nanosheets by a hydrolysis process, during which GO was reduced to graphene. Then, the obtained RGO-SnO₂ hybrids were heated under a gas mixture of C₂H₂ and Ar at 600 °C for the required time. Detailed synthetic procedures are provided in the Experimental Section. X-ray diffraction (XRD) experiments were first carried out to reveal the composition and structure changes of GO, RGO-SnO₂, and RGO-Sn@C nanocables during the two steps. As shown in Figure 1c, the disappearance of the characteristic diffraction peak of GO at ca. 11° demonstrated that GO was reduced during the first step, which was in agreement with the Fourier transform infrared (FTIR) results (Figure S1, Supporting Information). The broad diffraction peaks for the RGO-SnO₂ indicated the small crystal size of the tetragonal rutile SnO₂ phase, which was consistent with the transmission electron microscopy (TEM) results (Figure S2). Furthermore, there were no distinct diffraction peaks for RGO nanosheets owing to its low content. In the case of RGO-Sn@C nanocables, all intense peaks were well indexed to β-Sn (JCPDS No. 04–0673) and no SnO₂ crystalline phase was found, implying that nearly all the SnO₂ nanoparticles were reduced during the second step. The sharp diffraction peaks indicated good crystallinity of the tin phase.

The unique morphology of as-synthesized RGO-Sn@C nanocables was characterized by field emission scanning electron microscopy (FE-SEM; Figure2a, Figure S3) and FE-TEM (Figure 2b). High-density, high-aspect-ratio tin-core/carbon-sheath coaxial nanocables with a uniform diameter of ca. 100 nm and a length of several micrometers were observed protruding from the RGO surface. Interestingly, in many cases the carbon sheath was not fully filled with Sn, as demonstrated by the enlarged TEM image in Figure 2b. This may be attributed to the volume shrinkage of the metallic tin during the cooling down process. Such a vacancy structure inside the carbon sheath is especially attractive when it is used as an electrode material in LIBs, since it is expected to be beneficial for accommodating the huge volume change during the lithium insertion/extraction process. High-resolution TEM (HRTEM) investigation (Figure 2c) disclosed that the carbon sheath of Sn@C nanocables was approximately 5 nm thick and made up of staggered, shortened graphene-like sheets. Detailed HRTEM and selected area electron diffraction (SAED) studies confirmed the presence of single-crystalline metallic tin. As exhibited in Figure 2d, the tin and carbon element mapping for a single nanocable further validated the tin-core/carbon-sheath coaxial structure of the nanocables. The tin content in the nanocable samples was determined to be 61.0 wt% by thermogravimetric analysis (TGA) after the product had been calcined at 1000 °C in air (see Supporting Information, Figure S4). In addition, it should be noted that the here-synthesized heterogeneous nanocables could be easily tailored into other interesting one-dimensional nanomaterials (Figure 1b), such as carbon nanotubes with one open end and SnO₂ nanotubes, by acid etching and calcination in air, respectively. As exhibited in Figure S5 (Supporting Information), both the carbon nanotubes and the SnO₂ nanotubes perfectly mimic the geometric morphology of one-dimensional nanocables. Combined with their unique structures, such as the asymmetric ends of carbon nanotubes and polycrystalline porous walls of SnO₂ nanotubes, these nanomaterials deserve further investigation for applications in many fields, including optoelectronics, energy storage, sensors, and catalysis.

Figure 2. SEM (a) and TEM (b,c) images of the as-synthesized nanocables. The lower part of (b) is an enlarged image of the area marked by the box in the upper part with a schematic diagram of the morphology of an individual nanocable inserted. The insets in (c) show the HRTEM image and corresponding SAED pattern. d) Scanning TEM (STEM) and element mapping images of an individual nanocable.

To understand the growth mechanism of these nanocables, time-dependent experiments were carried out. As shown in Figure3, the samples collected at three different stages of CVD present totally different morphologies. At the preliminary stage of CVD, the metallic tin obtained from reduction of SnO₂ at 600 °C with the assistance of acetylene could easily coalesce to form liquid tin droplets on the RGO surface (Figure 3a). Meanwhile, carbon decomposed from the acetylene gas was deposited on the surface of the tin droplets and then dissolved into the metallic tin. After the saturated solubility of carbon in the tin had been reached, carbon deposition started, forming a shell surrounding the liquid tin particles. Afterwards, in the presence of an additional carbon source, the carbon shell tended to grow into tubular structures on the liquid-state tin catalyst, owing to its anisotropic properties.44 At the same time, capillary forces would draw additional molten tin into the nanotubes,45 filling the interiors of these nanotubes. As a result, a pear-shaped Sn@C heterogeneous structure (Figure 3b) was formed as an intermediate before further growing into the final cable structure (Figure 3c). During the subsequent cooling process, some vacancies were produced as a result of the volume shrinkage of the metallic tin on going from the liquid to the solid state (Figure 3d). In comparison with glucose-derived carbon/SnO₂ spheres, where the Sn supply was limited for a “base growth” model,32 the high-density loading of SnO₂ nanoparticles with fine grain size onto RGO sheets in our case could guarantee a continuous supply of liquid tin to induce growth of high-aspect-ratio nanocables (Figure 3e). Furthermore, the uniform decoration of RGO sheets with SnO₂ nanoparticles afforded nearly the same local conditions of nucleation and growth for each nanocable, which might be responsible for their uniform diameter and controlled distribution on the RGO (Figure 3e).

Figure 3. a–c) SEM images of samples collected at different CVD growth times: a) 0.5 h, b) 1 h, and c) 2 h. d) SEM (left) and TEM (right) images showing the vacancies at the bottom or the top of the nanocables. e) A suggested model for synthesis of RGO-Sn@C nanocables from RGO-SnO₂ hybrids by a CVD process.

The electrochemical performance of the RGO-Sn@C nanocables was evaluated by galvanostatic charge/discharge cycling at a current density of 100 mA g⁻¹. For comparison, we also tested a control sample, the RGO-supported Sn composite (RGO-Sn) (Figure S6), which was prepared by thermal reduction of RGO-SnO₂ at a temperature of 700 °C in the absence of acetylene. The first discharge and charge steps (Figure S7) delivered a specific capacity of 1351 mAh g⁻¹ for the nanocables and 1572 mAh g⁻¹ for the RGO-Sn, with initial charge/discharge efficiencies of 66.9% and 64.8%, respectively. Figure4a shows the cycling performance of these two electrodes and the theoretical specific capacity of graphite. The RGO-Sn@C nanocables had specific capacities higher than 760 mAh g⁻¹ in the initial 10 cycles and higher than 630 mAh g⁻¹ after the 50th cycle, which is approximately 69% higher than the theoretical specific capacity of commercial graphite and much better than the corresponding values for the RGO-Sn control sample. This result is also superior to those for Sn@C composite nanomaterials reported previously.32–34 The rate capability of the electrodes from RGO-Sn@C nanocables and the RGO-Sn were further investigated as shown in Figure 4b. Compared with the RGO-Sn and other reported Sn@C composites, the nanocable-based electrode consistently exhibited much higher specific capacities at a series of measured discharge/charge current rates. The improved electrochemical performances of the RGO-Sn@C nanocables can be attributed to their unique morphology and structure, which promise the following advantages. First, the carbon sheath of the nanocables can function as a physical matrix or barrier, which protects the tin core against pulverization and simultaneously prevents the tin cores of different nanocables from coalescing into bulk during discharge/charge processes. Second, the one-dimensional nature of the carbon sheaths in the nanocables, combined with the interconnectivity of these discrete nanocables through the underlying RGO matrix, facilitates fast electron transfer as an electrode material. Third, in contrast to zero-dimensional nanomaterials or bulk materials, the high-aspect-ratio nanocable-integrated electrodes are endowed with high porosity due to their intrinsic mechanical properties, which not only affords increased electrode–electrolyte contact area and fast transport of lithium ions, but also easily accommodates the volume change of tin during discharge/charge processes. Finally, the presence of inherent vacancies inside the carbon sheaths of the nanocables is another plus for alleviating the problems resulting from the large volume change of tin.

Figure 4. a) Cycling performances of RGO-Sn@C nanocables and RGO-Sn electrodes at a rate of 50 mA g⁻¹. b) Rate performances of RGO-Sn@C nanocables and RGO-Sn electrodes at the rates of 100, 200, 400, 800, and 1600 mA g⁻¹.

In summary, for the first time, large-scale tin-core/carbon-sheath coaxial nanocables with uniform diameter and high aspect ratio have been successfully synthesized in a controlled fashion by a simple CVD process using RGO-based hybrid material as an efficient platform. The electrochemical evaluation revealed that the as-synthesized nanocables exhibit high reversible specific capacities and remarkable high-rate capabilities. These outstanding lithium storage properties are attributed to the unique morphology and structure of the nanocables, which exhibit many properties required for high-performance electrodes in LIBs. With these nanocables as starting materials, the successful synthesis of other one-dimensional nanomaterials, including interesting carbon nanotubes with one open end and tin oxide nanotubes, further highlights the versatile nature of the here-constructed heterogeneous nanocables, which will definitely extend their applications.

Experimental Section

Material synthesis: GO was synthesized from natural graphite (300 μm, Qingdao Graphite Company) by a modified Hummers method. RGO-SnO₂ hybrids were prepared by a controllable hydrolysis process of tin salts in a GO-containing ethylene glycol (EG) and water solution.46 In a typical experiment, the as-synthesized GO (150 mg) was first suspended in 100 mL of a mixture of EG and water (water content: 10 mL) in a round-bottom flask, and then ultrasonically treated for 1 h, forming a brown dispersion. Then 0.5 g of SnCl₂·2H₂O (dissolved in 10 mL of EG) was added to the above dispersion. After magnetic stirring for 20 min, the solution was heated to 120 °C and refluxed for 2 h with constant stirring under atmospheric pressure. After filtration and desiccation, the resultant black solid product was placed in a horizontal quartz tubular reactor and heated to 600 °C at 10 °C min⁻¹ in Ar (99.999%) atmosphere with a flow rate of 200 sccm. Subsequently, C₂H₂ and Ar gas (volume ratio 1:19) were introduced in turn with a flow rate of 200 sccm and kept in the reactor for 2 h. Finally the system was cooled down to room temperature under Ar atmosphere. To investigate the growth mechanism of the RGO-Sn@C nanocables, samples were collected after CVD growth under the same conditions for 0.5 h and 1 h, respectively. For comparison, the RGO-supported Sn composite (RGO-Sn) was prepared by thermal reduction of the RGO-SnO₂ hybrids at a temperature of 700 °C in the absence of acetylene.

To prepare carbon nanotubes with one open end, the RGO-Sn@C nanocables were treated in 2 M HCl to etch away the metallic Sn core overnight. In the case of preparation of SnO₂ nanotubes, the nanocables were calcined in air at 550 °C to completely remove the carbon sheaths and underlying RGO matrices, and at the same time, the molten metallic Sn was in situ oxidized and transformed into SnO₂ nanotubes.

Material characterization: X-ray diffraction (XRD) with Cu Kα radiation (Rigaku D/max-2500B2+/PCX system) was used to determine the phase composition and the crystallinity. The morphology and microstructure of the samples were investigated by FE-SEM (Hitachi S4800), FE-TEM (FEI Tecnai G2 20 ST), and TGA (PerkinElmer, Diamond TG) measurements. FTIR spectra were recorded by a PerkinElmer Spectrum One FTIR spectrometer.

Electrochemical cycling tests were performed with coin-type half-cells (2032 size). The working electrode was made from the RGO-Sn@C nanocables as active material, super P carbon black, and poly(vinylidene fluoride) (PVDF) binder in a weight ratio of 80:10:10 on a copper foil. The electrolyte was 1 M LiPF₆ with 1:1 ethylene carbonate/diethylene carbonate (EC/DEC), and the counter and reference electrodes were made from lithium foils. The cycle-life of the cells was tested at different rates within a fixed voltage window of 2.5 V–5 mV. The rate capability was evaluated by varying the discharge/charge rate from 100 mA g⁻¹ to 1600 mA g⁻¹. All the capacities of RGO-Sn@C nanocables here were calculated based on the total weight of carbon (including RGO- and acetylene-derived carbon) and tin.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the authors.

Acknowledgements

Financial support from the National Natural Science Foundation of China (Grant nos. 20973044, 21173057), the Ministry of Science and Technology of China (nos. 2009AA03Z328, 2009DPA41220, and 2012CB933403), the Chinese Academy of Sciences (No. KJCX2-YW-H21), and the Guangdong-CAS strategic cooperation Program (2009B091300007) is acknowledged.