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SnO2-Based Nanomaterials: Synthesis and Application in Lithium-Ion Batteries and Supercapacitors
Tin dioxide (SnO2) is an important n-type wide-bandgap semiconductor, and SnO2-based nanostructures are presenting themselves as one of the most important classes due to their various tunable physicochemical properties. In this paper, we firstly outline the syntheses of phase-pure SnO2 hierarchical structures with different morphologies such as nanorods, nanosheets, and nanospheres, as well as their modifications by doping and compositing with other materials. Then, we reviewed the design of SnO2-based nanostructures with improved performance in the areas of lithium-ion batteries (LIBs) and supercapacitors.
As one of the most important classes of materials, metal oxide semiconductor nanomaterials present themselves in various areas of science and technology, due to their shape- and size-dependent physical and chemical properties [1, 2]. Among various metal oxide nanomaterials, SnO2 has become the foremost one, because of its wide applications in lithium batteries [3–6], supercapacitors [7–10], gas sensors [11, 12], and catalysis [13, 14]. Recently, reports on the applications of SnO2 nanostructures mainly depend on their morphologies and structural features. Thus, considerable efforts have been devoted to synthesizing SnO2 nanostructures with different morphologies, such as nanorods [15–17], nanowires [18–20], nanotubes , nanosheets [2, 21], and 3D nanospheres self-assembled from these low-dimensional nanostructures via interactions such as van der Waals forces, hydrogen, and covalent bonding [3, 22–24].
Various methods have been adopted for the preparation of nanoscaled SnO2 nanostructures, such as hydrothermal route and template method. However, because the performance enhancement to meet the increasing requirements is still to be a challenge, then many researchers have established various ways to improve the performance of SnO2-based materials, including doping [25, 26], synthesis of stannate nanomaterials , and constructing the heterojunctions . In this respect, development of suitable synthetic strategies has become crucial to achieve the desired properties.
In this review, we outline the synthetic strategies of phase-pure SnO2 hierarchical structures and the approaches to enhance the performance. The applications of hierarchical SnO2-based nanostructures in lithium-ion batteries and supercapacitors are also reviewed. By focusing on the hierarchical SnO2-based nanostructures, we hope to provide a better understanding on their physicochemical properties and the design principles when used in energy conversion and energy storage and further explore the new possibilities to advance the future research.
2. SnO2-Based Nanomaterials
2.1. Phase-Pure SnO2 Nanostructures
Many researchers have devoted their efforts to manipulate the structures and morphologies of SnO2 in order to improve the performances and widen their applications. Two kinds of synthesis strategies have generally been explored as follows.
(1) Hydrothermal Method. Hydrothermal method has been paid much attention, due to its simplicity, low cost, high efficiency, and convenient manipulation combined with flexible control over the sizes and morphologies of the resulting nanostructures [9, 11, 17, 23, 24], in which aqueous solution is used as the reaction medium, to create a high temperature and high pressure reaction environment by heating the reaction vessel to a certain temperature. In 2004, Cheng et al.  synthesized single-crystalline SnO2 nanorods with diameter of about 5 nm and length of about 20 nm. Furthermore, many researchers developed the method and synthesized different SnO2 architectures by adjusting the precursors and experimental conditions [2, 11, 17]. For example, SnO2 hollow microspheres composed of SnO2 nanoparticles have been synthesized . Recently, Liu et al.  developed a facile approach to fabricate hierarchical SnO2 nanosheets, using SnCl2·2H2O as tin source and sodium citrate as controlling agents. The ultrathin nanosheets with a thickness of about 20 nm are shown in Figures 1(a) and 1(b), which corresponds to the TEM image of Figure 1(c). The HRTEM image (Figure 1(d)) exhibits the well-defined lattice fringes combined with the SAED pattern. Their growth mechanism is generally summarized in Figure 2 [2, 30]. First of all, SnO2 nanocrystals were formed due to the hydrolysis of SnCl2, whereas in the second step, the grown small SnO2 nanoparticles are further assembled with each other to form nanosheets because of the “oriented attachment” growth process. Subsequently, the fast oriental attachment of the SnO2 nanoparticles results in the formation of SnO2 nanosheets. Moreover, the new formed particles would spontaneously “land” on the as-formed sheets and further grow to another sheet, which led to the formation of flower-like SnO2 architectures.
(2) Template Method. Template-based methods offer many advantages, including simplicity, low cost, and narrow size distribution . However, there are some limitations. For example, the post treatment process of the templates always damages the nanoarchitectures, and it is difficult to remove the template and limit the ability to produce large-scaled nanomaterials. Generally, templates such as silica nanorods , MoO3 nanorods , and carbon nanotubes  can be removed through calcination at high temperature (e.g., to remove carbon or polystyrene spheres), or chemical dissolution (e.g., use of hydrofluoric acid to remove silica templates), which may result in collapse of some fraction of the hollow structures [34, 35]. Therefore, it is highly desirable to develop new strategies for synthesizing hollow SnO2 nanostructures. Zhang et al.  developed a reactive-template method to fabricate porous SnO2 nanotubes using MnO2 nanorods as the sacrificial template. The overall synthesis procedure is illustrated in Figure 3, which is based on the redox chemistry between reductive Sn2+ and oxidative MnO2 in an acidic environment. Chemical reactions for the formation of SnO2 nanotubes included in a basic aqueous solution are shown in (1)–(3):
When the MnO2 nanorods are completely dissolved because of reduction, then the Sn(OH)4 nanotubes with a hollow interior are eventually formed. Finally, porous SnO2 nanotubes were obtained by annealing the product at 500°C for 2 h ((4)-(5)):
The morphologies of the MnO2 nanorods and the as-prepared SnO2 nanotubes (Figures 4(a), 4(b), and 4(c)) exhibit their 1D structure. The SnO2 nanotubes show a rough surface because the tube walls are composed of many nanoparticles with a size in the range of 5–15 nm (Figures 4(d) and 4(e)). The hollow porous structure of the nanotubes is also visible, as indicated by the dashed lines in Figures 4(d) and 4(e). The pore-size distribution (inset of Figure 4(f)) calculated using the Barrett-Joyner-Halenda (BJH) method for both the adsorption and desorption branches of the isotherm indicates that most of the pores have a diameter in the range of 2–6 nm. Such porous structure leads to a Brunauer-Emmett-Teller (BET) specific surface area of 66.1 m2/g.
Wang et al.  fabricated SnO2 nanorods that consisted of SnO2 hollow microspheres via the soft template relying on the use of (CH2)6N4 and demonstrated that it is a nontoxic, water-soluble method to prepare the hollow structure under the hydrothermal treatment.
(3) Other Synthesis Methods. Besides the above-mentioned methods, there are many other approaches for the fabrication of SnO2-based nanostructures. Spray pyrolysis is a process for preparing particles or films by forming droplets from a precursor solution and then evaporating and decomposing them in a reactor. This process has proven to be quite useful for the preparation of various nanostructure and composite particles, as is shown in Figure 5, with many reports onto the effect of the main variables on particle formation . Hong et al.  prepared Pd-loaded double-shelled SnO2 yolk-shell spheres by one-step spray pyrolysis. Patil et al.  synthesized high-purity nanostructured SnO2 powders through spray pyrolysis. Ju et al.  reported the use of spray pyrolysis to produce SnO2 powders with uniform morphology and narrow size distribution. In addition, others method are also used to synthesize SnO2-based nanostructures. Yan et al.  synthesized the hierarchical SnO2 hollow spheres by two layers of tetragonal prism nanorod arrays, formed on the surface of self-generated NO bubbles in the aqueous solution. This method is promising in the design of the hollow structures without further heat treatment. Dai et al.  synthesized SnO2 nanowires, sandwiched nanoribbons, and nanotubes by high temperature thermal oxide method. Chen et al.  reported that SnO2 nanorod arrays were synthesized in a ternary solvent system comprising acetic acid, ethanol, and water using SnCl4 as the Sn source and NaBr as the additive. The growth of SnO2 crystals was carefully controlled in the mixed solvents, leading to the exclusively heterogeneous nucleation on a substrate and finally the mesocrystalline nanorod arrays were obtained. Most of the above methods are illustrated in Table 1.
2.2. Doped Hierarchical SnO2 Nanostructures
Doping of oxide nanomaterials is a general approach to tailor their electrical and optical properties. Thus, many efforts have been carried out to improve the performance of the materials by doping other elements. For example, Yin and Guo  prepared Fe-doped SnO2 gas sensor for CO detection, and the response value of the composite material to 2000 ppm CO was raised 13 times than that of pure SnO2. Turgut et al.  synthesized Mo/F double doped SnO2 films and obtained the best electrical and optical properties. In relation to SnO2 nanostructures, doping with element Zn has been paid more attention by several groups [14, 44–46]. As previous reports [2, 44], Sn(IV) would form ions in highly alkaline solution and then undergo decomposition to obtain SnO2 nuclei. After introduction of Zn2+ into the reaction mixture, different morphologies of Zn-doped SnO2 nanostructures can be formed. Jia et al.  synthesized Zn-doped SnO2 hierarchical architectures assembled by nanocones via a solvothermal approach. Li et al.  reported that Zn-doped SnO2 nanostructures are composed of dense SnO2 nanowires with growth orientation along the direction. Huang et al.  modified the morphologies and properties of the flower-like single-crystalline SnO2 nanorods by Zn doping in a facile hydrothermal synthesis route without any complex or toxic organic reagents. After Zn doping, the morphology changed from nanorods with a constant diameter (Figures 6(a) and 6(b)) to needle-like nanorods (Figures 6(c) and 6(d)). The needle-like nanorods are of smooth surfaces and rectangular cross section with a diameter of 30–50 nm and a length up to several hundred nanometers. The lattice spacing of crystallographic planes marked in the image (Figure 6(f)) has been measured to be 0.35 nm and 0.27 nm, respectively, corresponding to the (110) and (101) planes of rutile SnO2. In addition, doping of other elements, such as Co and Fe, was also synthesized by a chemical route using polyvinyl alcohol as surfactant . Moreover, Wang et al.  have reported the synthesis of hierarchical SnO2 nanoflowers using NaF as the morphology-controlling agent and SnCl2·2H2O as the tin source. This resulted in the simultaneous Sn2+ self-doping of SnO2 nanostructures and led to the formation of tunable oxygen vacancies bandgap states and the corresponding shifting in the semiconductor Fermi levels and further extended absorption in the visible spectral range.
2.3. SnO2-Based Nanocomposites
Recently, many researchers have reported the fabrication of sensitized semiconductor by noble metal particles or the formation of semiconductor heterojunction, in which the performance can be greatly improved. Chung et al.  synthesized AuZnO core-shell structure for gaseous formaldehyde sensing at room temperature. The sensor response of the AuZnO core-shell structure was enhanced to 10.57 from 1.91 of the pure ZnO. Ju et al.  prepared NiO/ZnO PN heterojunction TEA gas sensor and the response is much higher than that of pure ZnO nanosheet sensor and other reported oxide chemiresistive gas sensors. Herein, SnO2-based nanocomposites have been actively pursued in order to improve their performance in gas sensors [50, 51], dye-sensitized solar cells , and so on. Li et al.  prepared the composites of SnO2 nanocrystal/graphene-nanosheets on the basis of the reduction of graphene oxide (GO) by Sn2+ ion. The morphologies of SnO2/graphene-nanosheets composites were changed with different ratios of Sn2+ and GO. Li et al.  synthesized WO3-SnO2 hollow nanospheres by hydrothermal process with a diameter and thickness of about 550 nm and 30 nm, respectively. The influence of the Na2SnO3 concentrations on the morphologies of nanocomposites was investigated due to its alkaline nature Na2SnO3 which may control the hydrolysis degree of Na2WO4, as shown in Figure 7. When the reaction was carried out with 0.2 mmol Na2SnO3, a small number of nanospheres coexisted with irregular aggregates of nanoparticles (Figure 7(a)). As the amount of Na2SnO3 was increased to 0.5 mmol, the obtained product consisted of large nanospheres (Figure 7(b)). However, particles aggregated without an orderly shape were collected when the amount of Na2SnO3 increased to 1 mmol, as shown in Figure 7(c).
Besides, the composites of SnO2 nanostructures with other materials such as Fe2O3, CuO, and ZnO have also been reported. Liu et al.  demonstrated the growth of Fe2O3SnO2 nanoparticle decorated graphene flexible films. Choi et al.  prepared CuO-loaded SnO2 hollow spheres by ultrasonic spray pyrolysis. Moreover, ZnO-SnO2 nanocomposites have been also investigated in some reports [55, 56].
2.4. Stannate Nanomaterials
Stannate nanomaterials like ZnSnO3 [57, 58], Zn2SnO4 [59, 60], and CdSnO3  have also attracted much more attention for higher reversible capacities, low cost, easy preparation, and especially various morphologies [58–61]. Wang et al.  synthesized highly porous CdSnO3 nanoparticles using citric acid (Figure 8) and applied it as an anode material for rechargeable LIBs. It can be seen that there was a sharp contrast between the bright cavities and dark edges, which further confirmed the formation of the porous structure (Figures 8(a), 8(b), and 8(c)). For the CdSnO3 nanoparticles, the HRTEM images showed lattice fringes spacing of ca. 0.1996 nm, corresponding to the (024) planes of rhombohedral-phase CdSnO3 (Figure 8(d)).
Zn2SnO4 have also drawn much attention as anode materials . Zhao et al.  fabricated monodispersed hollow Zn2SnO4 boxes by the simple coprecipitation and alkali etching way. The hollow boxes exhibit an electrochemical performance with high capacity and good cycling stability than the solid cubes and those reported. Wang et al.  prepared flower-like Zn2SnO4 composites through a green hydrothermal synthesis, in which the flower-like Zn2SnO4 structures are composed of several 1D Zn2SnO4 nanorods. These structures generally consist of several sharp tips branching out in three dimensions with large surface area. Duan et al.  fabricated amorphous ZnSnO3 hollow nanoboxes for the first time in a large scale by a facile alkaline solution etching method.
3. Applications of SnO2-Based Nanomaterials
3.1. Lithium-Ion Batteries
In order to address both energy and power demands, there is an urgent need to develop clean energy sources systems. Herein, LIBs have attracted widespread attention because of their high energy density, high power, smooth discharge, and light weight as well as being environment friendly . The electrode’s material is one of the key components for perfecting LIBs. It plays a crucial role in establishing the overall properties of the battery. SnO2 has been demonstrated to be one of the most promising anode materials for high performance LIBs [33, 64–66], due to its high theoretical specific storage capacity (782 mAh/g), compared with the commercially used graphite (372 mAh/g) [67, 68]. The mechanism of LIBs for SnO2 is based on the alloying/dealloying processes, which are the intrinsic driving force for the electrochemical activity [68, 69]:
However, the lithiation/delithiation process often induces the large volume expansion and further causes pulverization. This will lead to severe internal strain, cracking, and blocking of the electrical contact pathways in the electrodes, which result in their rapid deterioration and low retention of the electrical capacity [70, 71]. To overcome the above problems, many efforts have been carried out to optimize the structures, compositions, and morphologies of SnO2-based materials, such as 0D nanoparticles [64, 72], 1D nanowires , 2D nanosheets , and the doping [74, 75] or composition  with other materials.
Wu et al.  reported that hierarchical SnO2 nanostructures that consist of SnO2 nanosheets exhibit superior reversible capacities (discharge capacity of 516 mAh/g) and cyclic capacity retention (80%) after 50 cycles at a current rate of 400 mA/g, which is much higher than that of commercial SnO2 nanoparticles (286 mA/g, 48% retention). This can be ascribed to its stable porous structure. The high porosity, short transport paths of SnO2 nanosheets, and the interconnections between the individual building blocks of such hierarchical structures endow those promising candidates as anode materials for LIBs. Wang et al.  fabricated N-doped G-SnO2 sandwich papers. The sandwich structure not only assures solid contact between the SnO2 particle and the graphene layer, but also facilitates high electrode conductivity and renders the elastomeric space needed to accommodate the volume changes of SnO2. When this material is used in LIBs, it exhibits a very large capacity, high rate capability, and excellent cycling stability. All the excellent electrochemical performances of this material with respect to commercial SnO2 nanoparticles can be attributed to the structural features that provide a large number of surface defects induced onto the graphene by N-doping, excellent electronic conductivity, short transportation length for both lithium ions and electrons, and enough elastomeric space to accommodate volume changes upon Li insertion/extraction. Lin et al.  synthesized a composite made from graphene nanoribbons (GNRs) and SnO2 nanoparticles used as the anode material for LIBs. The synthesis route of the GNRs/SnO2 composite is illustrated in Figure 9. First, the GNRs were obtained using K/Na alloy to unzip the MWCNTs. Then, SnCl2 and 2-pyrrolidinone were added into the GNRs with ultrasonication for reducing Sn2+ to Sn0. Lastly, the Sn NPs were oxidized overnight using ultrasonication in air. The CV curves of the initial three cycles show the reversibility of the composite electrode charge/discharge process, as shown in Figure 10(a). The composite, as an anode material for LIBs, exhibits reversible capacities of over 1520 and 1130 mAh/g for the first discharge and charge, respectively, which is more than the theoretical capacity of SnO2 (Figure 10(b)). The reversible capacity retains ~825 mAh/g at a current density of 100 mA/g with a Coulombic efficiency of 98% after 50 cycles (Figure 10(c)). Furthermore, the composite shows good power performance with a reversible capacity of ~580 mAh/g at the current density of 2 A/g, as shown in Figure 10(d). The high capacity and good power performance and retention can be attributed to uniformly distributed SnO2 NPs along the high-aspect-ratio GNRs. The GNRs act as conductive additives that buffer the volume changes of SnO2 during cycling. This work provides a starting point for exploring the composites made from GNRs and other transition metal oxides for lithium storage applications.
Besides, Yang et al.  synthesized SnO2/graphene nanosheet nanocomposite as an anode material for LIBs. The SnO2-In2O3/GNS nanocomposite exhibits obvious enhancement electrochemical performance in terms of lithium storage capacity (962 mAh/g at 60 mA/g rate), initial Coulombic efficiency (57.2%), cycling stability (60.8% capacity retention after 50 cycles), and rate capability (393.25 mAh/g at 600 mA/g rate after 25 cycles) compared to SnO2/GNS and pure SnO2-In2O3 electrode.
Supercapacitors, also called electrochemical capacitors, have been known for over than fifty years and considered as one of the most promising energy storage devices for a wide range of uninterruptible power supplies and civilian and military applications in electric vehicles [80–82]. In contrast to conventional capacitors and LIBs, supercapacitors possess much higher energy density and also exhibit higher specific power. Up to now, there are mainly three kinds of electrode materials for supercapacitors which are as follow: metal oxides, carbon, and conducting polymers [83–86]. Due to low cost and environmental compatibility consideration, metal oxide candidates with good capacitive values have attracted much attention .
SnO2-based supercapacitors have been paid significant attention due to their high electrochemical capacitor (EC) and chemical stability [88, 89]. Yan et al.  synthesized SnO2/MnO2 composite for the application of supercapacitor electrodes. The prepared process was shown in Figure 11. This nanostructure has several advantages: a thin layer of MnO2 would enable a fast, reversible faradic reaction and would provide a short ion diffusion path; SnO2 nanowires, with high conductivity, would provide a direct path for the electrons transport; and SnO2 nanowires would create channels for the effective transport of electrolyte. Based on the above, it exhibited a specific capacitance (based on MnO2) as high as 637 F g−1 at a scan rate of 2 mVs−1 (800 F g−1 at a current density of 1 A g−1) in 1 M Na2SO4 aqueous solution (Figure 12(a)). Temperature is an important influence factor on supercapacitor cells. It is valuable to evaluate the capacitive behavior of SnO2/MnO2 composites at various temperatures. Figure 12(b) shows the specific capacitance obtained at different temperatures and scan rates. It can be seen obviously that the specific capacitance and rate capability increase with the increase of temperature. This is attributed to the decrease of effective internal resistance with increasing temperature. The energy density and power density measured at 50 A g−1 are 35.4 W h kg−1 and 25 kW kg−1, respectively, demonstrating a good rate capability. After that, the SnO2/MnO2 composite electrode shows an excellent long-term cyclic stability (less than 1.2% decrease of the specific capacitance is observed after 2000 CV cycles), as shown in Figure 12(c). Li et al.  fabricated Fe3O4SnO2 core-shell nanorods. The hybrid nanorod film displayed well-defined electrochemical features in Na2SO4 aqueous electrolyte, charging/discharging within seconds and with much higher areal capacitance (7.013 mF cm−2 at 0.20 mA cm−2) than pristine Fe3O4 nanorod film. In addition, the PANI/SnO2 composite electrode exhibited specific capacitance of 173 F g−1 at a scan rate of 25 mV s−1 [8, 91]. Bao et al.  designed Zn2SnO4/MnO2 core/shell nanocable-carbon microfiber hybrid composites for high-performance supercapacitor electrodes. The hybrid composite exhibited excellent rate capability with specific energy of 36.8 Wh/kg and specific power of 32 kW/kg at current density of 40 A/g, respectively, as well as good long-term cycling stability (only 1.2% loss of its initial specific capacitance after 1000 cycles).
Moreover, compared with other materials, graphene has been used as the electrode of electrochemical supercapacitors (ESCs), due to its good capacitive performance, superior conductivity, large surface-to-volume ratio, and suitable pore size distribution . As we know, carbon and metal oxide materials are two hotspots as electrode materials for electrochemical supercapacitors (ESCs), in which energy can be stored due to the formation of an electrical double layer at the interface of the electrode. If integrating the above two kinds of materials into the electrodes of ESCs, their capacitive performance will be greatly enhanced because most of the metal oxide can contribute pseudo-capacitance to the total capacitance apart from the double-layer capacitance from carbon materials [94–96]. Thus, combined SnO2 with graphene for the supercapacitors can obviously enhance the performance of capacitance. Li et al.  synthesized SnO2/graphene (SnO2/G) nanocomposites by a facile solvent-based synthesis route based on the oxidation-reduction reaction. The electrochemical performance of SnO2/graphene showed an excellent specific capacitance of 363.3 F g−1, which was five-fold higher than that of the as-synthesized graphene (68.4 F g−1). El-Deen et al.  fabricated graphene/SnO2 nanocomposite which exhibited high specific capacitance (323 F g−1), excellent cycling stability, very good salt removal efficiency (83%), and distinct electrosorptive capacity of 1.49 mg g−1.
In this paper, we discussed the synthesis of phase-pure SnO2 hierarchical structures with different morphologies including nanoparticles, nanorods, nanosheets, nanosphere, and the porous and hollow structures. We also reviewed their modifications by doping and compositing with other materials and synthesis of stannate nanomaterials. Reaction parameters such as the chemical state of the tin precursors (Sn(IV) versus Sn(II) salts), concentration, additives, and solvents play an important role in adjusting their morphologies. SnO2-based nanostructures, such as anode materials, demonstrate superior cycle performance of lithium storage by doping. In the supercapacitors, the prepared SnO2-based nanostructures provide fast ion and electron transfer, which led to a prominent supercapacitor performance. Therefore, SnO2-based nanostructures with a proper design can possess advanced physical and chemical properties, which are vital for a variety of energy and environment applications.
Although significant progress has been made in the synthesis of SnO2-based nanostructures, further efforts are still required to understand the mechanism of doping and nanocomposites better, which are still unclear but crucial for the design of SnO2-based nanostructures in enhancement of their lithium storage, supercapacitors, and energy conversion performance. After the introduction of components with different chemical compositions, SnO2-based nanomaterials got wide applications. However, it is still a challenge in the large-scale synthesis of SnO2 nanocrystals with more specific facets exposed. We hope that the present paper will further expand the applications of SnO2-based nanostructures to meet the environment- and energy-related demands.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank the University of Jinan (UJN) for the support on new staff, and the project was supported by the Taishan Scholar (no. TSHW20120210), the National Natural Science Foundation of China (Grant no. 11304120), and the Encouragement Foundation for Excellent Middle-aged and Young Scientist of Shandong Province (Grant no. BS2012CL005).
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