Received: 23 Dec 2018
Accepted: 15 Jan 2019
Published online: 16 Jan 2019
Yejing Li, Xuefeng Wang, Zhaoxiang Wang and Liquan Chen
Key Laboratory for Renewable Energy, Chinese Academy of Sciences, Beijing Key Laboratory for New Energy Materials and Devices, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, China
Nanostructure is attractive and has been proved superior in numerous applications due to the unique physical and chemical properties of the nano-materials. However, facile preparation of the nanostructured materials remains challenging; much effort is still essential to obtain materials with designed morphology. As a semiconductor, SnO2 has been found a variety of applications such as solar cells and sensors, and has been extensively investigated as an anode material for lithium (Li)-ion batteries. Herein, we present a one-step and eco-friendly method to synthesize SnO2 nanorods without any templates or additives. On the basis of its structural and morphologic evolutions probed by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), SnO2 nanoparticles are proposed to firstly form from its precursor Na2SnO3·3H2O and then assembled to nanorods with increased hydrothermal reaction time. When used as an active material for sodium (Na)-ion batteries, the as-prepared SnO2 nanorods show a high Na-storage capacity and initial coulombic efficiency as well as good cycling stability. Our findings shed light on the preparation of nanostructured materials and contribute to developing highperformance Na-ion batteries.
Nanostructured materials have shown many great advantages in various applications, owing to their distinctive physical and chemical properties.1-3 A variety of nanostructures have been developed, including nanoparticles,4, 5 nanorods,6, 7 nanotubes8, 9 and nanobelts,10 nanosheets,11 and hollow spheres,12 etc. Among these nanostructures, one-dimensional (1D) nanorods demonstrate excellent performances in electronic and photonic devices due to their specific properties along the axial or in the radial directions.13, 14 However, templates or additives usually have to be employed during the preparation,15 making the fabrication complicated and the commercialization difficult. Therefore, it is essential to develop new facile synthesis methods. As a semiconductor, SnO2 has been widely applied in solar cells,16 gas sensors,17 catalysts,18 microwave absorption,19-23 etc.. It is also a promising anode material for lithium-ion (Li-ion) batteries due to its high Li-storage capacity (780 mAh g-1), low-toxicity and high abundance.24 Its electrochemical performances in sodium (Na)-ion batteries, however, are still far from satisfactory due to its low electric conductivity and large volume changes during cycling25, 26. Nanostructures are expected to mitigate these issues by shortening the diffusion lengths of the ions and alleviating the volume change during cycling.27-29 Li and the coworkers30 recently synthesized a hierarchical structure SnO2/carbon composite via a hydrothermal reaction, which shows a high initial charge capacity of 633 mAh g-1. By means of the freezedrying method, Wang and the coworkers31 succeeded to anchor the SnO2 nanocrystals on the sulfur/nitrogen co-doped graphene. Due to the bonding effect between the Sn2+ and the functionalized graphene, it displays an excellent rate performance for both the Li and Na ion batteries. Different from the SnO2 nanocrystals, Wang and coworkers32 constructed a 1D SnO2 nanorods/3D graphene aerogel composite through a novel chemical reduction-induced self-assembly process. With this unique structure, the composite exhibits a good storage performance and excellent rate performance. Herein, we report a one-pot method for facile preparation of SnO2 nanorods without any templates or additives. On the basis of the characterization on the structural and morphologic evolution, a formation mechanism of the SnO2 nanorods was proposed. The high reversible Na-storage capacity and cycling stability proved the superiority of the SnO2 nanorods as an anode material for Na-ion batteries. These findings shed light on the preparation of nanostructured materials and contribute to the development of Na-ion batteries.
Results and discussions
Fig. 1 Evolution of (a) XRD patterns and (b) Raman spectra of the SnO2 with different hydrothermal reaction duration in the hybrid solvent. The SnO2 obtained in deionized water (S(H2O)-12) is shown for comparison.
Fig. 1a shows the XRD patterns of the SnO2 nanorods with different hydrothermal reaction time in the hybrid solvents (the corresponding samples are denoted as S-xH; xH stands for the reaction time). Phasepure SnO2 is obtained and all the diffraction peaks can be indexed to the tetragonal SnO2 (JCPDS No. 41-1445). The broad peaks of S-0.5H at about 26.6°, 33.9° and 51.7° (2θ) demonstrate its low crystallinity. The increase of the reaction time from 0.5 h to 12 h results in enhanced crystallinity of SnO2. Therefore, intensive diffraction peaks with a narrow peak width are observed in the XRD pattern of S-12H. The structural evolution of the SnO2 nanorods is further characterized by Raman spectroscopy (Fig. 1b). The region of 400-900 cm-1 is usually used to determine the evolution of the crystallinity of SnO2.33 The broad peak at about 576 cm-1 is ascribed to the nanostructured SnO2. As the reaction time increases to 2 h, new peaks appear at about 628 and 772 cm-1, referred to the A and B modes of 1g 2g SnO2, respectively, corresponding to the stretching mode of Sn-O bonds.34 These two peaks grow with increased reaction time and become dominant in S-12H. In this process, the intensity of the broad peak at 576 cm-1 fades. These indicate that the crystallinity of the SnO2 nanorods is enhanced with increased reaction time, consistent with the above XRD results.
Fig. 2 SEM images of SnO2 nanorods prepared in water/ethanol hybrid media for different hydrothermal reaction times: a) 0.5 h, b) 1 h, c) 2 h, d) 12 h
and side view (inset). The morphologies of the SnO2 prepared with deionized water solvent for a hydrothermal treatment time of e) 2 h, f) 12 h are
shown for comparison (inset for its magnified image).
The evolution of the morphology of the SnO2 nanorods is characterized by SEM (Fig. 2). The SnO2 is firstly formed as nanoparticles on the Na2SnO3 slabs (Fig. 2a) and becomes angular after 1 h hydrothermal treatment (Fig. 2b). As the reaction time increases to 2h, the shape of the nanorods becomes clear and the rods are well aligned. As the reaction time further increases to 12 h, only the nanorods of about 200 nm in length can be observed; they are regularly arranged to form an array (Fig. 2d). However, nano-spheres of about 200 nm in diameter are obtained when sole deionized water is used as the solvent, with some nanorods on the surface of the nano-spheres (Figs. 2e and f).
Fig. 3 The (a) TEM (inset for SAED pattern) and (b) HRTEM (inset for the magnified) images of the SnO2 nanorods (sample S-12H).
Transmission electron microscopy (TEM) was employed to investigate the structure of the SnO2 nanorods. As shown in Figure 3a, the as-prepared nanorods are uniform in size. The dotted ring-like selected area electron diffraction (SAED) pattern (inset in Fig. 3a) indicates the polycrystalline nature of SnO2. The diffraction rings from the inner to the outer are indexed to the (110), (101), (200), (211) and (310) planes of SnO2. Fig. 3b shows the HRTEM image of the SnO2 nanorods. The spacing of the two adjacent lattice fringes is measured to be about 3.32 Å, corresponding to the (110) facet of SnO2. Therefore, the direction of growth of the nanorods is supposed to be along the  axis of SnO2, a characteristic of its anisotropic structures.13, 35
Fig. 4 The schematic formation mechanism of the SnO2 nanorods in the hybrid solvent.
A combination of the XRD, Raman, SEM, and TEM results suggests a possible formation mechanism of SnO2 nanorods as schematically illustrated in Fig. 4. As the slab-like Na2SnO3·3H2O, a source for SnO2 is gradually dissolved in the hybrid solvent, and SnO2 is nucleated on the undissolved slab. The reaction can be expressed as36
SnO32- + 3H2O → Sn(OH)62- (1)
Sn(OH)62- → SnO2+ 2H2O + 2OH- (2)
The SnO2 nanoparticles are then formed and assembled into nanorods along the  axis due to the higher surface energy of the (001) plane than that of the (110) plane.35 When the reaction time is increased to 12 h, all the nanoparticles are assembled into nanorods. The properties of the solvent for the hydrothermal reaction are critical for the morphology of SnO2. As Fig. 2 shows, although phase- 2 pure SnO2 can be obtained in H2O (Fig. 1a), the SnO2 nanoparticles aggregate to form spheres with some nanorods on the surface (Fig. 2f). On the other hand, the SnO2 obtained from the pure C2H5OH as the sole solvent has very poor crystallinity and irregular particle morphology.
Uniform nanorods can only be produced in the hybrid solvent of H2O and C2H5OH. Therefore, the presence of C2H5OH in the hybrid solvent is essential as a structure-directing agent for the formation of nanorods and as a tailor of reaction rate for preventing the aggregation of the SnO2 nanoparticles.36
The as-prepared SnO2 nanorods were used as the anode material for Na-ion batteries and cycled between 0.01 and 3.0 V vs. Na+/Na at 10 mA g-1 (Fig. 5). The discharge slope between 0.30 V and 0.50 V is ascribed to the reduction of SnO2 to Sn while the plateau at about 0.20 V is referred to the formation of the Na Sn alloys (02.28, 40 The discharge and charge capacities of the SnO2 nanorod are 622.2 and 218.4 mAh g-1, respectively, and the columbic efficiency is about 35.11 % in the first cycle. The low initial coulombic efficiency is attributed to the irreversible reduction of SnO2 to Sn (the theoretical coulombic efficiency for the reactions, SnO + 4Na+ + 4e- → Sn + 2Na O, Sn + 2 2 3.75Na+ + 3.75e- ↔ Na Sn, is only 49.6 %) and the electrolyte 3.75 decomposition.27, 29, 41 In the subsequent cycling, the coulombic efficiency
increases to 95 %. The reversible capacity is stable and maintains at 300 mAh g-1 after 90 cycles (Fig. 5b), better than previously reported nanostructured SnO2.38
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