ARTICLE pubs.acs.org/JPCC
Synthesis of SnO2 Hierarchical Structures Assembled from Nanosheets and Their Lithium Storage Properties Hao Bin Wu,†,‡,§ Jun Song Chen,† Xiong Wen (David) Lou,*,†,§ and Huey Hoon Hng‡,§ †
School of Chemical and Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, 637457 Singapore School of Materials Science and Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore § Energy Research Institute @ NTU, Nanyang Technological University, 50 Nanyang Drive, 637553 Singapore ‡
ABSTRACT: In this work, we have developed a facile hydrothermal method to synthesize various 3D hierarchical structures assembled from 2D SnO2 nanosheets. The samples were thoroughly characterized by FESEM/TEM/XRD/BET techniques. Through in-depth investigation of experimental conditions, it is discovered that the oxidation process is crucial for the formation of the nanosheet structure. The electrochemical properties of the sample were subsequently studied by cyclic voltammetry and chargedischarge cycling. When compared to SnO2 nanoparticles from a commercial source, the result shows that the asprepared hierarchical SnO2 nanostructure exhibits much better lithium storage properties with higher reversible capacities and improved cyclic capacity retention for extended cycling.
’ INTRODUCTION Synthesis of nanocrystals with controllable shapes and structures is a fundamental challenge in nanoscience and nanotechnology, for realizing and enhancing the shape- and structure-dependent properties in widespread fields.15 In recent years, there has been growing interest in constructing three-dimensional (3D) hierarchical structures with low dimensional nanoscale building blocks.610 Ordered 3D hierarchical structures could exhibit the advantages of the pristine building blocks, and more importantly, also possess new physicochemical properties arising from their secondary architecture. As an important metal oxide, SnO2 draws considerable attention because of its widespread applications in chemical sensors,11,12 lithium-ion batteries,1315 photocatalysts,16,17 etc. Numerous efforts have been made to fabricate SnO2 nanocrystals with various shapes, including 0D (nanoparticles18,19), 1D (nanobelts,11 nanowires,20,21 and nanorods22,23), and 2D (diskettes24,25 and nanosheets2628) nanocrystals. Furthermore, these low dimensional nanocrystals could be grown or self-assembled into 3D hierarchical structures. For example, the preparation of hollow octahedral structures via the 2D oriented attachment of SnO2 nanoparticles has been demonstrated by Zeng’s group.29 Ordered mesoporous SnO2 with high thermal stability has also been fabricated by Niederberger and co-workers, which could effectively prevent the aggregation of SnO2 nanocrystals.30 However, most of the reported 3D hierarchical structures are based on 0D (nanoparticles into hollow/porous structures) and 1D (nanorod arrays) building blocks.2931 Thus, there is strong motivation to develop novel SnO2 hierarchical structures with 2D build ing blocks such as nanosheets (NSs), which have been widely realized on many other materials such as TiO2 and iron oxides.7,10 Although there have been several reports on SnO2 NSs,2527,32 and some of them are readily agglomerated into assemblies owing to their structural feature, constructing 2D r 2011 American Chemical Society
SnO2 NSs into ordered 3D hierarchical structures still remains as a great challenge.33 Herein, we report a facile hydrothermal method for synthesis of SnO2 hierarchical structures (HSs) assembled from nanosheets. The SnO2 HSs with controllable morphologies are expected to exhibit attractive properties of 2D nanomaterials, as well as additional benefits from the secondary hierarchical structures. Specifically, it is found that these SnO2 HSs exhibit enhanced lithium storage capabilities because of the short transport length of SnO2 NSs, high porosity between NSs, and enhanced interconnection between individual building blocks. Such SnO2 HSs could serve as a promising candidate as anode materials of the next generation lithium-ion batteries.
’ EXPERIMENTAL SECTION Materials Synthesis. In a typical synthesis, urea (99100.5%, Sigma-Aldrich) was dissolved in a mixture of 35 mL of deionized (DI) water and 15 mL of ethanol, followed by the addition of tin(II) chloride dihydrate (SnCl2 3 2H2O, 98%, Sigma-Aldrich) to reach a concentration of 1020 mM. The mixture was gently shaken to form a turbid mixture. After that, a certain amount of 6 M NaOH solution was slowly added to the mixture while shaking the mixture gently. Finally, the mixture was transferred to a 60 mL Teflon lined autoclave and then heated in an oven at 180200 °C for 18 h. The product was collected by centrifugation and washed several times with DI water. The washed raw products were redispersed in water. After 5 min, large gray sediments were removed. Final products were again harvested by centrifugation and dried at 60 °C overnight. The morphologies Received: August 24, 2011 Revised: November 2, 2011 Published: November 10, 2011 24605
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Table 1. Major Experimental Conditions for Different Samples sample
[Sn2+] (mM)
urea (g)
6 M NaOH (mL)
temperature (°C)
HS-A
10
0.2
1.0
200
HS-B HS-C
10 15
0.2 0.1
1.0 1.0
180 200
HS-D
20
0.1
1.0
200
HS-E
20
0.1
0.8
200
HS-F
20
0
1.0
200
and structures of SnO2 HSs can be controlled by varying the amounts of chemicals (SnCl2 3 2H2O, urea, and NaOH solution) and reaction temperature, keeping other parameters unchanged. Detailed experimental conditions are given in Table 1. Materials Characterization. The morphologies and structures of the products were characterized with field-emission scanning electron microscopy (FESEM; JEOL, JSM-7600F, 5 kV) and transmission electron microscopy (TEM; JEOL, JEM-2010, 200 kV). The crystallographic information was collected by powder X-ray diffraction (XRD; Bruker, D8 Advance X-ray diffractometer, Cu Kα radiation, λ = 1.5406 Å). The nitrogen adsorptiondesorption isotherm was measured using a Micromeritics ASAP 2020 sorptometer. Electrochemical Measurements. The working electrode was prepared with active material (HS-A, commercial SnO2 nanoparticles), conductive agent (carbon black, SuperP-Li), and polymer binder [poly(vinylidene difluoride), PVDF, Aldrich] in a weight ratio of around 70:20:10. Half-cells were assembled using two-electrode Swagelok cells in an Ar-filled glovebox, with lithium foil as both the counter and reference electrodes. The electrolyte was a solution of 1.0 M LiPF6 in ethylene carbonate and diethyl carbonate (w/w = 1:1). Cyclic voltammetry was carried out between 2.5 and 0.01 V at a scan rate of 0.5 mV s1 using an electrochemical workstation (CHI 660C). The charge/ discharge measurements were performed in a voltage window of 0.011.2 V at different current densities using a NEWARE battery tester.
’ RESULTS AND DISCUSSION Our SnO2 HSs were synthesized via a template-free hydrothermal method. A representative sample was prepared according to the first condition listed in Table 1, denoted as HS-A. The crystallographic structure of the as-synthesized sample was first characterized by XRD (Figure 1a). All the identified peaks can be indexed to SnO2 with a tetragonal rutile structure (JCPDF card no. 411445; space group: P42/mnm; ao = 4.738 Å, co = 3.1865 Å). The sample is highly crystalline, and the broadening of the peaks indicates the nanoscale crystallites of the as-synthesized products. The morphology and structure of HS-A were further investigated by FESEM (Figure 1bd) and TEM (Figure 1e). The sample consists of spherical particles at a low magnification, with sizes ranging from 1.5 to 3 μm (Figure 1b). A typical FESEM image of one spherical assembly (Figure 1c) reveals that the particles are constructed from NSs, and numerous exposed NSs can be clearly observed from the hierarchical spheres. Figure 1d displays a high magnification FESEM image showing the surface of the hierarchical spheres. Most of the NSs are arranged perpendicularly and pointing out from the center. The thickness of the NSs is found to be around 35 nm estimated from the
Figure 1. (a) XRD pattern, (bd) FESEM images, (e) TEM image (inset: SAED pattern), and (f) HRTEM image of HS-A.
FESEM image. A TEM image of a flat lying NS is shown in Figure 1e. The size of the NS is around 500 nm, consistent with the observation from the FESEM images. One set of bright diffraction patterns can be identified from the selected-area electron diffraction (SAED) pattern (inset of Figure 1e) taken from the same flat lying NS, with some additional dim spots that could be ascribed to contamination of small crystals or overlapping of other NSs. The result suggests that the NSs in hierarchical spheres are likely to be single-crystal. On the basis of the indexed SAED pattern, the observation is along the [111] projection. Lattice fringes with a spacing of 0.33 nm can be seen from the HRTEM image, corresponding to the (110) planes of SnO2. Considering the crystallographic structure of SnO2, it might be deduced that the nominal exposed top and bottom planes of these NSs should be (111) facets although the NSs appear porous from the TEM image, which is in agreement with that observed in a previous report.27 The effects of experimental parameters on the morphology and structure of the SnO2 HSs were also investigated. Several SnO2 HSs samples with different morphologies were prepared according to the conditions listed in Table 1, and their FESEM images are presented in Figure 2. Flower-like HS-B was prepared at a lower temperature (180 °C), with larger exposed NSs of size around 1 μm (Figure 2a,b). Increasing the concentration of Sn2+ to 15 mM together with 0.1 g of urea results in urchin-like HS-C (Figure 2c,d). The structure in the HS-C sample is more compactly assembled, showing triangular exposed NSs with size of about 300 nm. Flower-like HS-D with loosely assembled NSs of size around 500 nm is obtained by further increasing the concentration of Sn2+ to 20 mM (Figure 2e,f). Thus, with a small 24606
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method, and the pore-size distributions (PSD) are calculated using the BarrettJoynerHalenda (BJH) method from the adsorption branches as shown in the insets. All of the four isotherms can be approximately classified as type IV isotherms with a type H3 hysteresis loop, indicating the presence of slit-like pores in the materials, which are formed by the aggregation of sheet- or plate-like particles.34 The highly mesoporous structures are suggested by the prominent hysteresis loops at the relative pressure of 0.45 to 1.0. Interestingly, the mesopores in all the four samples could be generally divided into two groups by their sizes according to the PSD curves: smaller pores of diameter less than 3 nm and larger pores with broad size distribution. The BET surface areas of these four samples were found to be 37.4 m2 g1, 33.7 m2 g1, 25.6 m2 g1, and 26.5 m2 g1, respectively. The relatively low specific surface area could be partly attributed to the high density of SnO2 (∼6.9 g cm3), as well as the relatively large thickness of the NSs that are the building blocks of the hierarchical structures. The SnO2 HSs are prepared via hydrolysis and oxidation of tin(II) ions in a basic aqueous solution. The chemical reactions taken place could be described as follows:25,32 þOH
ð1Þ
SnðOHÞ2 f SnO þ H2 O
ð2Þ
SnO þ
1 O2 f SnO2 2
SnðOHÞ2 þ
Figure 2. FESEM images of (a,b) HS-B with larger exposed NSs, (c,d) urchin-like HS-C, (e,f) flower-like HS-D, (g) HS-E with small assemblies, and (h) feather-like HS-F.
amount of urea, the higher concentration of Sn2+ results in better-defined NS morphology. However, higher concentration of Sn2+ might also lead to the formation of additional impurity, which will be further discussed shortly. All of the above three samples still remain a spherical morphology and similar particle sizes of the hierarchical assemblies to that of HS-A except for HS-B, which is slightly bigger because of the larger exposed NSs. Reducing the amount of 6 M NaOH solution to 0.8 mL, while keeping other parameters the same to those of HS-D, leads to the formation of small and less uniform assemblies denoted as HS-E (Figure 2g). However, in the absence of urea, only feather-like NSs are found in the product, which are randomly assembled into large aggregates (Figure 2h). This observation suggests that urea is crucial for the synthesis of ordered hierarchical structures. Although the exact mechanism of how urea tailors the morphology is not clear at this moment, it is suggested that the ammonia formed and gradually increasing basicity caused by the decomposition of urea would facilitate the assembling of pristine NSs and lead to the growth of hierarchical structures. Four representative samples, namely, HS-A, HS-B, HS-D, and HS-E, were further investigated by nitrogen adsorption and desorption isotherms at 77 K (Figure 3). The specific surface areas are measured by the BrunauerEmmettTeller (BET)
þOH
SnCl2 s f SnClðOHÞ s f SnðOHÞ2
ð3Þ
1 O2 þ H2 O f SnðOHÞ4 f SnO2 þ 2H2 O 2 ð4Þ
The solution becomes turbid shortly after the addition of SnCl2, indicating the rapid hydrolysis of Sn2+ ions. After hydrothermal treatment, gray sediments could be found in some of the assynthesized products, and they can be easily separated by sedimentation. These gray particles are large plate-like SnO with a size of around 10 μm, and their crystal structure is confirmed by XRD analysis (Figure 4a). The formation of SnO particles is attributed to the incompletion of the oxidation process that could possibly happen on both SnO and Sn(OH)2 (eqs 3 and 4). When a high Sn(II) concentration (e.g., 20 mM) is used, the yield of SnO2 HSs does not increase significantly. Instead, a large amount of SnO particles coexists in the products, which is hardly observed in the product obtained with a low Sn(II) concentration (e.g., 10 mM). In some cases, the SnO particles cannot be completely removed, forming an undesirable impurity in our SnO2 HSs (Figure 4b). The yield of SnO2 is probably limited by the amount of O2 dissolved in H2O. Thus, the oxidation process could be promoted by introducing more O2 into the system or preoxidizing Sn(II) in an open system. These ideas were verified by synthesizing the SnO2 HSs in autoclaves with a much larger capacity (e.g., 20 mL of solution in 50 mL autoclave) and stirring the mixture overnight before the hydrothermal reaction. However, both of these two approaches result in the diminishment of the NS structure (Figure 4c,d). Therefore, it is likely that a mild oxidation process during the crystallization under hydrothermal condition is crucial for the formation of the NS structure. Moreover, considering the plate-like SnO particles obtained in the present system and the use of Sn(II) salts for the synthesis of other 2D nanostructured SnO2,2527,32 it is postulated that the 24607
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Figure 3. N2 adsorptiondesorption isotherms at 77 K (insets: pore size distributions calculated by the BJH method from the adsorption branch): (a) HS-A, (b) HS-B, (c) HS-D, and (d) HS-E.
The lithium storage mechanism of SnO2 can be described by the following two reactions:
Figure 4. (a) FESEM image (inset: XRD pattern) of gray sediments separated from HS-E and (b) XRD pattern of HS-E after twice sedimentation. FESEM images of samples (c) synthesized in an autoclave with large volume and (d) stirred overnight before the hydrothermal reaction.
hydrolysis of Sn(II) could result in some sheet-like intermediates, and the formation of SnO2 NSs is based on the oxidation of these sheet-like intermediates as structure-directing precursors. If a fast oxidation process is carried out, or the Sn(II) is oxidized to Sn(IV) before the formation of sheet-like intermediates, no NSs-based nanostructures would be formed. Thus, the diminishment of NSs in the samples (Figure 4c,d) could be explained. SnO2 is considered as one of the promising anode materials for the next generation lithium-ion batteries,3539 owing to its high theoretical specific lithium storage capacity of 790 mA h g1.
SnO2 þ 4Liþ þ 4e f Sn þ 2Li2 O
ð5Þ
Sn þ xLiþ þ xe T Lix Sn
ð6Þ
0 e x e 4:4
It is generally accepted that the reversible lithium storage capability of SnO2 is essentially attributed to the formation of LiSn alloys (eq 6). One major challenge of Sn-based anode materials is the pulverization of electrodes due to the large volume change during the alloying/dealloying processes, leading to the poor cycling performance. The unique structure of our SnO2 HSs could partly overcome such a problem in virtue of the high porosity and short diffusion length. Thus, a series of electrochemical measurements were performed on the representative HS-A to study the lithium storage properties. The cyclic voltammograms (CV) of SnO2 HS-A are shown in Figure 5a, which are generally in agreement with that observed in previous reports on SnO2 anode materials.35,36,38 In general, the features are less pronounced in these CVs except the strong cathodic reduction in the range of 00.5 V corresponding to the LiSn alloying (eq 6). The intensity of the anodic peak at 0.8 V increases significantly in the second cycle and stays the same after five cycles, suggesting an activation process and the high reversibility of the alloying/dealloying reaction. Another reduction peak at about 1.7 V could be attributed to the conversion of SnO2 to metallic Sn (eq 5), which is usually considered to be irreversible. Interestingly, a corresponding oxidation band is observed at around 1.8 V. Both peaks do not disappear after the first cycle. The reduction peak shifts to 1.3 V in the following cycles, while the oxidation peak also shifts to a lower voltage. This observation might suggest partial reversibility of the reaction by eq 5, which is also observed for SnO2/C composites.36,38 24608
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Figure 5. (a) Cyclic voltammograms of HS-A for the first, second, and fifth cycles. (b) Chargedischarge profiles of HS-A at a current rate of 400 mA g1 for the first, second, and fifth cycles. (c) Cycling performance of HS-A (I) and commercial SnO2 nanoparticles (II) at a current rate of 400 mA g1. (d) Cycling performance of HS-A at different current rates.
Figure 5b depicts the chargedischarge voltage profiles in the range of 0.011.2 V at a current density of 400 mA g1. Two poorly defined plateaus can be identified from the discharge curve of the first cycle. The first plateau at ∼0.7 V could be attributed to the formation of Sn from SnO2, and it becomes indiscernible in the following cycles. The second one at ∼0.2 V probably corresponds to the formation of LixSn alloy. Only one plateau at ∼0.6 V is observed from the charge curves, ascribed to the dealloying process. The sample delivers a high initial discharge capacity of 1600 mA h g1 with low Coulombic efficiency upon charging to 1.2 V, which is common in SnO2-based anode materials. During the first discharge, SnO2 is first irreversibly reduced to metallic Sn as described by eq 5. When the anode is further discharged at a low potential range, the electrolyte starts to decompose and form a solid electrolyte interface (SEI) layer on the surface of electrode materials. These two irreversible processes cause additional consumption of charge, resulting in a very high initial discharge capacity and large initial capacity loss. Figure 5c shows the cycling performance of HS-A and one commercial SnO2 sample consisting of nanoparticles (1050 nm) with a voltage window of 0.011.2 V at a constant current density of 400 mA g1 . Our SnO 2 HS-A exhibits a high discharge capacity of 516 mA h g1 after 50 cycles that retains 80% of the capacity in the second cycle. Both of the capacity and retention are much higher than that of the SnO2 nanoparticles (286 mA h g1 and 48%, respectively). It is suggested that such hierarchical structure could provide more interconnection between the building blocks and a more stable porous structure due to the effective prevention of dense aggregation of NSs. During the charge/discharge processes, the pores and void space between NSs could effectively accommodate the large volume expansion/contraction, while the hierarchical assemblies
would probably better maintain the integrity of the electrode. Moreover, the NS building blocks with a large contact area with the electrolyte facilitate the Li+ insertion/deinsertion into/from the active materials. Thus, the excellent lithium storage properties of our SnO2 HSs could be attributed to the unique hierarchical structure and the nanoscale thickness of the nanosheet building blocks. The cycling performance at various current rates is also evaluated at the same voltage window as shown in Figure 5d. The specific capacity slightly decreases as the current density increases, and it still shows a discharge capacity of above 400 mA h g1 at a current density of 800 mA g1, which is still higher than the theoretical capacity of graphite (372 mA h g1). This observation suggests excellent rate capability of these SnO2 HSs.
’ CONCLUSIONS In summary, we have successfully synthesized various SnO2 hierarchical structures assembled from nanosheets by a facile hydrothermal method. These 3D hierarchical structures provide several advantages for lithium storage, including a short diffusion length for lithium ions determined by the small thickness of nanosheets, better interconnection between building blocks, and high porosity for efficient transport of lithium ions. As a result, these SnO2 hierarchical structures can retain a high reversible capacity of 516 mA h g1 at a current density of 400 mA h g1 after 50 cycles. It is believed that such SnO2 hierarchical structures are also promising in other applications such as gas sensors and photocatalysis. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 24609
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