Article pubs.acs.org/JPCC
Ultrathin SnS2 Nanoparticles on Graphene Nanosheets: Synthesis, Characterization, and Li-Ion Storage Applications Marappan Sathish,* Satoshi Mitani, Takaaki Tomai, and Itaru Honma* Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1, Katahira, Sendai 980-8577, Japan S Supporting Information *
ABSTRACT: Ultrathin SnS2 nanoparticle decorated graphene nanosheet (GNS) electrode materials with delaminated structure were prepared using stepwise chemical modification of graphene oxide (GO) nanosheets at very dilute conditions, followed by a hydrothermal treatment. The chemical modification of the graphene nanosheet surface with Sn ions enables the precipitation of ultrathin nanoparticles. The TEM analysis reveals the SnS2 nanoparticles are homogeneously distributed on the loosely packed graphene surface in such a way that the GNS restacking was hindered. X-ray photoelectron spectroscopic analysis reveals the bonding characteristics of the SnS2 on the GNS. The obtained nanocomposite exhibits a reversible capacity of 1002 mAh/g, which is significantly higher than its calculated theoretical capacity (584 mAh/g). Furthermore, its cycling performance is enhanced and after 50 cycles, and the charge capacity still remained 577 mAh/g, which is very close to its theoretical capacity. Due to the synergic effect, the Li-ion storage capacity observed for nanocomposites is much higher than its theoretical capacity. The ultrathin size (2 nm) and dimensional confinement of tin sulfide nanoparticles by the surrounding GNS limit the volume expansion upon lithium insertion, and the nanoporous structures serve as buffered spaces during charge/discharge and result in superior cyclic performances by facilitating the electrolyte to contact the entire nanocomposite materials and reduce lithium diffusion length in the nanocomposite.
■
INTRODUCTION Energy storage systems such as lithium ion (Li-ion) batteries need significant improvement to reach high power and high energy density for commercial applications in electric and hybrid vehicles. Currently, carbon-based materials are extensively used as an anode in Li-ion batteries owing to their unique physical and electrochemical properties, although they reveal low theoretical Li-ion storage capacity (372 mAh/g).1 Recently, various attempts have been made to explore alternative materials for succeeding high-capacity Li-ion battery applications.2−8 Tin compounds such as Sn, SnO2, SnS2, and Si nanostructures are promising alternative anode materials for reversible Li-ion storage due to their high theoretical gravimetric capacity, low cost, and eco-friendliness.9−17 The major drawback associated with these materials is high volume changes during charge−discharge cycling that result in rapid deterioration and low retention of capacity.18,19 However, attempts have been made to resolve this issue significantly by reducing the particle size that offers high surface to volume ratio, high surface area, enhanced electron transport, and reduced strain associated with the intercalation process.20−22 Thus, various synthetic strategies have been introduced for the preparation of SnO2 nanoparticles, nanosheets, and nanorods to achieve high Li-ion storage with improved cyclic performance.23−26 However, only a limited number of studies have been reported for SnS2 compared to SnO2, including the © 2012 American Chemical Society
preparation of SnS2 nanoparticles, two-dimensional nanoplates, nanosheets, and three-dimensional flowerlike for the Li-ion storage.27−32 SnS2 has a layered CdI2 crystalline-like structure composed of tin atoms sandwiched between two layers of hexagonal sulfur atoms, and the neighboring sulfur layers are connected with weak van der Waals forces. Thus, the Li ion can easily access the Sn atoms in the SnS2 structure, and the resulting volume change could be easily controlled without any structural damage. Besides, the preparation of an ultrathin SnS2 nanoparticle and its homogeneous dispersion on a highly conductive surface would greatly enhance their Li-ion storage capacity significantly with good cyclic performance. Graphene nanosheets (GNS), a two-dimensional honeycomb-like network of carbon atoms with superior electronic conductivity, remarkable structural flexibility, and high specific surface area, create significant attention in nanoscience and nanotechnology for their possible potential Li-ion batteries.33−37 A strong electronic coupling between negatively charged GNS and positively charged metal ions or metal oxides is expected, and it enhances the binding of metal ions or metal oxides on the GNS surface.38 The recent progress in the preparation of high surface area GNS with high conductivity offers GNS as better support Received: April 2, 2012 Revised: May 21, 2012 Published: May 21, 2012 12475
dx.doi.org/10.1021/jp303121n | J. Phys. Chem. C 2012, 116, 12475−12481
The Journal of Physical Chemistry C
Article
Then, L(+)-ascorpic acid was added in the second step to reduce the unutilized functional groups partly. Finally, the thiourea was added as a S2− source and the volume of the solution reduced to 30 mL for hydrothermal precipitation. In a typical preparation, graphene oxide dispersion (15 mL, ∼5 mg/ mL) was diluted to 600 mL using water and ultrasonicated for 30 min, and then 0.75 g of SnCl4·5H2O (Wako, Japan) and 0.8 g of L(+) ascorbic acid (Wako, Japan) was added and the resulting solution stirred for 30 min and ultrasonicated for 30 min. Finally, 0.35 g of thiourea (Sigma Aldrich) in 50 mL of water was added, and the solution was again stirred for 30 min vigorously and ultrasonicated for another 30 min. The resulting solution (∼650 mL) was subjected to rota-vapor, and the solvent was removed under reduced pressure at 80 °C until the final volume became 30 mL. Then, the residual solution was put into Teflon-lined stainless steel autoclave and heat treated at 180 °C for 15 h. The resulting nanocomposite powders were washed with ethanol and water and dried at 60 °C overnight. The as-prepared nanocomposite was further reduced using (i) hydrazine solution at 90 °C for 3 h or (ii) a H2−Ar gas mixture (5% H2) at 400 °C for 4 h. The as-prepared sample, hydrazine reduced sample, and H2−Ar reduced samples are denoted as SnS2/GNS, SnS2/GNS-RS, and SnS2/GNS-RG, respectively. Material Characterization. X-ray diffraction (XRD) patterns were collected on a RIGAKU (RINT2000 Tokyo, Japan) diffractometer using Ni-filtered Cu Kα radiation (λ = 1.5418 Å). Thermogravimetry (TG) experiments (SII, TG/ DTA 6300) were conducted in a temperature range of 25− 1000 °C and in an air atmosphere using ∼5−10 mg of the sample at the heating rate of 10 °C/min. The morphology of the nanocomposites was observed using a Hitachi-4800 fieldemission scanning electron microscope (FE-SEM). Scanning and high-resolution transmission electron micrographs (STEM and HR-TEM) were recorded with a JEOL JEM-2100F microscope, working at an accelerating voltage of 200 kV. Electrochemical Evaluation. The working electrodes were fabricated by mixing 85 wt % active material, 10 wt % conducting carbon black, and 5 wt % polytetrafluoroethylene (used as a binder, PTFE, Sigma Aldrich) and pressed on Ni mesh. The electrodes were dried in a vacuum oven at 120 °C overnight before transferring into an argon-filled glovebox. Conventional three electrode beaker cells were fabricated using lithium metal as the counter electrode and reference electrode and LiClO4 (1 M) in ethylene carbonate/diethyl carbonate (EC/DEC, 1:1 vol %) as the electrolyte. The electrochemical performances of the prepared electrodes were characterized by cyclic voltammetry (Solartron 1260, USA) and galvanostatic charge−discharge (HOKUTO DENKO, Japan) tests between 0.001 and 3 V vs Li/Li+.
for the dispersion of ultrathin SnS2 nanoparticles. Further, the GNS can also contribute to the Li-ion storage, and the nanoparticle decoration on their surface hinders the actual restacking of GNS that offers the advantage of an additional hidden surface available for Li-ion storage. Recently, Luo et al.39 and Chang et al.40 have investigated SnS2 nanoplates/sheets− GNS nanocomposites for Li-ion storage applications, and promising Li-ion storage was shown for the above nanocomposites. Thus, it is believed that the a few nanometer size (ultrathin) SnS2 nanoparticle decorations on GNS would greatly enhance the Li-ion storage capacity of GNS and SnS2 due to the synergic effect. It is worth mentioning here that no attempts have been made in this direction so far. Thus, the preparation and characterization of ultrathin SnS2 decorated GNS for Li-ion storage is highly warranted for both fundamental scientific understanding and applications points of view. Herein, we report the preparation of ultrathin SnS 2 nanoparticle decorated graphene nanosheets via a hydrothermal method using graphene oxide, SnCl4·5H2O, and thiourea as the starting materials. The as-prepared nanocomposite (SnS2/ GNS) was further reduced using hydrazine solution at 90 °C (SnS2/GNS-RS) or H2−Ar gas at 400 °C (SnS2/GNS-RG), and its electrochemical performance in lithium half-cells was evaluated using a conventional three-electrode setup with Li as counter and reference electrode in 1 M LiClO4 (1:1 EC:DEC) electrolyte. This novel nanocomposite electrode material is composed of micrometer-sized graphene sheets with a uniform distribution of ultrathin SnS 2 (∼2 nm) nanoparticles. Integrating the features of a large graphene surface area, an ultrathin SnS2 nanoparticle on highly conductive GNS support in the nanocomposite electrode materials demonstrates enhanced Li-ion storage with high capacity retention and good cyclic performance.
■
EXPERIMENTAL METHODS Graphene Oxide Preparation (GO). Graphene oxide dispersion (5 mg/mL) was prepared by a modified Hummers and Offeman’s method reported elsewhere.41,42 In a typical preparation, 0.5 g of graphite powder (Sigma Aldrich, 5−20 μm), 0.5 g of NaNO3, and 23 mL of H2SO4 were stirred together in an ice water bath. Then, 3 g of KMnO4 was slowly added. Once mixed, the solution was transferred to a 35 ± 5 °C water bath and stirred for about 1 h, forming a thick paste. An amount of 40 mL of water was added to the above paste, and the resulting solution was stirred for 30 min while the temperature was raised to 90 ± 5 °C. Finally, 100 mL of water containing 3 mL of H2O2 was added, and the color of the solution turned from dark brown to yellow. The warm solution was then filtered and washed with 200 mL of water. The filter cake was then dispersed in water by mechanical agitation. Lowspeed centrifugation was done at 1000 rpm for 5 min, and the large particles were removed completely from the precipitates. The supernatant then underwent two more high-speed centrifugation steps at 8000 rpm for 15 min to remove small GO pieces and water-soluble byproduct. The final sediment was redispersed in water with mechanical agitation and mild sonication, giving a solution of exfoliated GO. SnS2/GNS Preparation. The preparation of SnS2/GNS nanocomposites involes three steps: in the first step, Sn ions were anchored on graphene sheets through the functioanl group on the GO surface at very dilute condition (0.125 mg/ mL), where only the presence of GO monolayers was expected.
■
RESULTS AND DISCUSSION The preparation of SnS2/GNS nanocomposites involves three steps: in the first step, we anchor Sn ions on graphene sheets through the functional group on the GO surface at very dilute conditions (0.125 mg/mL), where only the presence of GO monolayers was expected. Then, L(+) ascorpic acid was added in the second step to reduce the unutilized functional groups partly. Finally, the thiourea was added as a S2− source, and the volume of the solution was reduced to 30 mL and subjected to hydrothermal conditions for SnS2 formation directly on the GNS surface. It is believed that stepwise chemical modification at very dilute GO condition has a vital role in the formation of ultrathin SnS2 nanoparticle on the GNS surface. No dilution 12476
dx.doi.org/10.1021/jp303121n | J. Phys. Chem. C 2012, 116, 12475−12481
The Journal of Physical Chemistry C
Article
and the absence of L(+)ascorpic acid result in the formation of SnS2 nanoplates and flowerlike morphology.32,43 The crystalline nature of the prepared SnS2/GNS, SnS2/GNS-RS, and SnS2/ GNS-RG nanocomposites was analyzed using powder X-ray diffraction analysis (XRD). As shown in Figure 1, the XRD
Figure 1. XRD pattern of (a) SnS2/GNS, (b) SnS2/GNS-RS, and (c) SnS2/GNS-RG nanocomposites.
pattern of SnS2/GNS (a) and SnS2/GNS-RS (b) shows the diffraction lines corresponding to the hexagonal crystalline structure with a calculated lattice parameter of a = 3.632 ± 0.003 Å and c = 5.887 ± 0.003 Å, and these values are in good concordance with the hexagonal structures (JCPDS: 23-677). However, a few additional lines (shown as*) indicate the presence of SnO2 impurities in both the samples, whereas the sample SnS2/GNS-RG shows diffraction lines corresponding to orthorhombic SnS with calculated lattice parameters of a = 4.285 ± 0.003, b = 11.17 ± 0.003, and c = 3.995 ± 0.003 (JCPDS: 39-354). No additional lines corresponding to SnO2 or SnS2 were observed. This clearly indicates that the SnO2 impurities were formed during the material preparation and are retained during the solution phase reduction of graphene oxide to graphene. The gas phase reduction at high temperature results in the complete reduction of SnS2 and the impurity SnO2 into SnS, in addition to reduction of graphene oxide to graphene. Further, there are no lines corresponding to the stacking of graphene layer observed, which indicates that the restacking of the GNS is completely prevented in the SnS2/ GNS nanocomposites. To investigate the morphology of the resulting SnS2/GNS nanocomposite, field emission scanning electron microscopy (FE-SEM) images were taken at different magnifications. Figures 2a and 2b shows the representative FE-SEM image of the as-prepared SnS2/GNS nanocomposite. It becomes clear that the homogeneous distribution of the ultrafine SnS2 nanoparticle with
