One-pot synthesis of 3D flower-like heterostructured SnS₂/MoS₂ for enhanced supercapacitor behavior

Abstract

Novel three-dimensional flower-like heterostructured SnS₂/MoS₂ was produced via a one-step hydrothermal method. The full potential of the heterostructured SnS₂/MoS₂ material could be realized because of a strong synergetic effect, which was not only able to effectively weaken the agglomeration and restacking problems during the electrochemical reaction, but also able to ensure the high-rate and long-life. We found that SnS₂/MoS₂ had better electrochemical performance compared to MoS₂, due to the rapid electronic transport and volume change buffering of the formation of the SnS₂/MoS₂ heterostructure. The electrochemical tests showed that the SnS₂/MoS₂ electrode had a specific capacitance of 105.7 F g⁻¹ at a current density of 2.35 A g⁻¹ and it displayed good cyclic stability of 90.4% retention even after 1000 cycles, which indicated that the SnS₂/MoS₂ was an useful potential electrode material for the application of energy storage and deserves to be further investigated.

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1. Introduction

To meet the urgent needs for renewable and sustainable power sources, great attention has been focused on storage devices and energy conversion with ultra fast charge and discharge characteristics, such as supercapacitors.^1–3 Supercapacitors are a new energy storage device with high power density, fast charging/discharging rate, super-long cycle life, and excellent cycle stability.^4–7

Three-dimensional (3D) hierarchical structures, which are comprised of low dimensional nanosheet building block nanostructures, are promising as electrodes.^8–11 Because 3D hierarchical structures exhibit the advantages of the pristine building blocks and the improvement property of their secondary architecture. With regard to supercapacitor, the 3D hierarchical structures can facilitate the transportation of electrons and ions and accommodate the volume change of materials in electrochemical reaction process.^12–14

Molybdenum disulfide (MoS₂), as an important member of two-dimensional (2D) nanomaterials, is composed of three atom layers (S–Mo–S) stacking together via week van der Waals interaction. Up to now, 3D hierarchical structures MoS₂, such as flower-like nanostructures,¹⁵ hierarchically porous hollow spheres,^16–18 have been proved to be effective in improving their electrochemical properties. In general, self-assembled style, which minimize the energy of the reactive system in a spontaneous process, is the simplest synthetic route to obtain 3D hierarchical structures.¹⁹ However, the design and electric reactivity of 3D nano-heterostructures have rarely been studied. Kim et al. reported the MoS₂ nanostructures synthesized by a hydrothermal route and the specific capacitance of MoS₂ was 92.85 F g⁻¹ at a constant discharge current density of 0.5 mA cm⁻².²⁰ Ma and co-workers synthesized flower-like MoS₂ nanospheres through a hydrothermal method. However, the electrochemical tests showed that the maximum specific capacity was just about 122 F g⁻¹ at 1 A g⁻¹.²¹ Sun et al. demonstrated polyaniline/MoS₂ composites for high-performance supercapacitors and the specific capacitance of the pure MoS₂ electrodes was only 98 F g⁻¹ at 1 A g⁻¹.²² However, the rate performances and cycling stabilities of the electrode materials are still unsatisfactory. Ternary sulfides, such as nickel-cobalt sulfides, exhibit an electric conductivity that is much higher than those of single component sulfides. For example, Chen et al. reported that the hierarchical structured Ni_xCo_1−xS_1.097 electrodes exhibited a remarkable maximum specific capacitance of approximately five times higher than that of the CoS_1.097 precursors at a current density of 0.5 A g⁻¹.²³ Chen and colleagues proved that the specific capacitance of Ni@Ni₃S₂ was improved from 89 F g⁻¹ to 122 F g⁻¹ for Ni@Ni_1.4Co_1.6S₂ at a current density of 1 A g⁻¹ with a high loading level (20 mg cm⁻²). The as-assembled Ni@Ni_1.4Co_1.6S₂//AC showed better cycling stability and coulombic efficiency than Ni@Ni₃S₂//AC.²⁴ Guan and colleagues fabricated a 3D hierarchical nest-like Ni₃S₂@NiS with nanorods as building blocks, which was then used as template to prepare Ni₃S₂@Co₉S₈ and NiS@NiSe₂ electrodes. The specific capacities of Ni₃S₂@NiS, Ni₃S₂@Co₉S₈, and NiS@NiSe₂ electrodes were 2440, 6427, and 7717 F g⁻¹, respectively, at a current density of 0.5 A g⁻¹.²⁵ The results indicated that the synergistic effect of double metal ions could enhance the electrochemical performance.

Herein, we report the preparation of MoS₂ 3D hierarchical structures with grown SnS₂ by a simple one-pot approach without using any toxic chemicals. In this nanostructure, Sn ions can be readily embedded onto the flowerlike MoS₂ nanosheets through one-top hydrothermal technique. And three-dimensional flowerlike heterostructured SnS₂/MoS₂ nanosheets act as framework-like substrate to provide a path for ions diffusion. In the nanostructure, SnS₂ nanoplates residing in the flowerlike MoS₂ nanosheets to prevent the collapse of the MoS₂ nanosheets. Thereby, the synergistic effect of MoS₂ nanosheets and SnS₂ nanoplates is not only able to effectively weaken the agglomerating and restacking problems during the electrochemical reaction, but also able to ensure the high-rate and long-life. The structure, morphology and electrochemical performance of the electrode material were investigated. The as-prepared SnS₂/MoS₂ electrode exhibits a high capacitance of 105.4 F g⁻¹ at 2.35 A g⁻¹, and also shows excellent cycle stability. The results confirmed that the as-prepared heterostructured SnS₂/MoS₂ nanostructured electrode exhibits an enhanced electrochemical behavior.

2. Experimental section

2.1 Materials

Sodium molybdate, thiourea, and tin tetrachloride were of analytical grade and used without further purification. In a typical procedure, 2.4 mm Na₂MoO₄·2H₂O, 0.8 mm SnCl₄ and 9 mm (NH₂)₂CS were dissolved in 15 ml of deionized water and 5 ml of absolute ethanol. After stirring for 30 min, the solution was transferred into a 50 ml Teflon-lined stainless steel autoclave and sealed tightly and then heated at 210 °C for 22 h. After cooling naturally, the black precipitates were collected, washed by deionized water several times, and dried at 80 °C for 5 h in a vacuum oven. Finally, the hierarchical heterostructured SnS₂/MoS₂ were obtained. As a comparison, 3D flower-like MoS₂ was obtained, when 2.4 mm Na₂MoO₄·2H₂O and 4.8 mm (NH₂)₂CS were dissolved in 15 ml of deionized water and 5 ml of absolute ethanol with the other reaction conditions left unchanged.

2.2 Characterization

The crystal structure of the obtained SnS₂/MoS₂ were characterized by X-ray diffraction (XRD) on a PANalytical X' pert PRO instrument using Cu Kα radiation. The morphology of the sample was studied by transmission electron microscope (TEM) on a JEOLJEM-2100 instrument at an accelerating voltage of 80 kV and field-emission scanning electron microscope (FESEM) using a JEOL-JSM6701F instrument at an accelerating voltage of 5 kV. The qualitative information was obtained by X-ray Photoelectron Spectrometer (XPS, VG ESCALAB 210).

2.3 Electrochemical measurements

A typical three electrode test cell was used for capacitive performances of the as-prepared sample on a CHI660D (Chenhua, Shanghai, China) electrochemical working station. All of the measurements were carried out in a 1 M KCl aqueous electrolyte solution at room temperature. The working electrode was fabricated by mixing the as-prepared electroactive material, carbon black and poly(tetrafluoroethylene) at a weight ratio of 85 : 10 : 5 to form a homogeneous slurry (the total mass of the electrode material was 10 mg), which was pasted onto a piece of nickel foam current collector using a blade. Afterwards, the electrode was dried at 80 °C for 12 h. A saturated calomel electrode (SCE) and platinum sheet were used as the reference and counter electrodes, respectively. Cyclic voltammetry (CV) measurements were carried out in the potential range from −0.9 V to −0.3 V using different scan rates, which was varied from 2 to 20 mV s⁻¹. Galvanostatic charge–discharge (GCD) curves were recorded at different current densities within the potential range from −0.9 V to −0.3 V. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 10⁵ to 10⁻² Hz.

3. Results and discussion

The XRD pattern of the SnS₂/MoS₂ displays two kinds of diffraction peaks in Fig. 1, besides the diffraction peaks at 13.9° (002), 33.3° (100) and 57.5° (006) reflections assigned to the MoS₂ (JCPDS card no. 37-1492),²⁶ all additional ones are well-matched to the SnS₂ (JCPDS #23-0677)²⁷ indicating the presence of SnS₂ grown with MoS₂.

Fig. 1 XRD patterns of the flower-like MoS₂ and heterostructured SnS₂/MoS₂.

The low-magnification SEM image (Fig. 2a) demonstrates that the MoS₂ consists of a large quantity of uniform 3D flower-like nanostructures. The flower-like MoS₂ has diameters of about 600–800 nm. The high-magnification SEM image (Fig. 2b) reveals that the flower-like nanostructures are composed of intercrossed curved nanoflakes with a thickness of several nanometers. The morphology of the as-synthesized flowerlike MoS₂ was further characterized by TEM. As shown in Fig. 2c, TEM image confirmed the existence of flowerlike MoS₂ structures, which closely correlates with the results of the SEM measurement. Fig. 3a and b shows the SEM image of the as prepared flower-like SnS₂/MoS₂ heterostructure. The 3D architecture is helpful to increase the specific area. 3D flower-like heterostructured MoS₂/SnS₂ facilitates rapid electronic transport in electrode reactions. Furthermore, this structure also enhances the stability of electrochemical performance. Fig. 3c shows the TEM image of SnS₂/MoS₂ heterostructure. The image reveals a general trend with the sheets of SnS₂ homogeneously embedded in MoS₂, showing the layered platelets. The mapping analyses on the SnS₂/MoS₂ (Fig. 3d) reveal the presence of not only Mo and S but also Sn, which confirm the assumption that some SnS₂ may be formed in the interior of MoS₂. Inductively coupled plasma mass spectroscopy analysis reveals that the ratio of Mo : Sn was 3.4 : 1.

Fig. 2 (a and b) SEM and (c) TEM images of flower-like MoS₂.

Fig. 3 (a and b) SEM, (c) TEM images of flower-like heterostructured SnS₂/MoS₂ and (d) corresponding elemental mapping of Mo, Sn, and S.

The obtained SnS₂/MoS₂ were further characterized by XPS. The high-resolution XPS of Mo 3d exhibits three peaks in Fig. 4a. The peaks at 233.1 and 229.9 eV are Mo 3d_3/2 and Mo 3d_5/2 binding energies, respectively. These peaks can be attributed to the Mo ion in the +4 oxidation state. The peak 227.1 eV can be ascribed to the 2s binding energy of S in MoS₂. The high-resolution of S 2p spectrum showed main doublet located at binding energies of 162.1 and 163.2 eV in Fig. 4b, which can be assigned to the spin–orbit couple S 2p_3/2 and S 2p_1/2, respectively. These binding energies agrees well with the reported values for the MoS₂.^28,29 The two strong peaks at around 486.5 and 495 eV are displayed in Fig. 3c. These peaks can be attributed to Sn 3d_3/2 and 3d_5/2 respectively, which are consistent with the reference data of Sn⁴⁺ in SnS₂.³⁰

Fig. 4 XPS spectra of the flower-like heterostructured SnS₂/MoS₂. (a) High-resolution spectra for Mo 3d, (b) high-resolution spectra for S 2p and (c) high-resolution spectra for Sn 3d.

The electrochemical measurement results of the MoS₂ and SnS₂/MoS₂ electrodes were evaluated by cyclic voltammetry (CV). Fig. 5a shows the cyclic voltammograms curves of the SnS₂/MoS₂ electrode at various scan rates ranging from 2 to 20 mV s⁻¹ in a potential range of −0.9 V to −0.3 V. It can be observed that all the curves exhibit an approximately rectangular shape without any redox peaks which indicates a typical electrical double-layer capacitance feature with fast charging–discharging processes. In addition, the shapes of these CV curves do not significantly change with increasing scan rate from 2 to 20 mV s⁻¹, which reveals the ideal capacitive behavior and good charge collection as well as the facilitated diffusion of K⁺ in the SnS₂/MoS₂ electrode.³¹ Furthermore, the CV curve area increases with the scan rate, indicating that the rates of electric and proton transportation are rapid with respect to the scan rates. The normalized CV of MoS₂ nanoflowers at the scan rate of 10 mV s⁻¹ have also been measured and shown for comparison with SnS₂/MoS₂ in Fig. 5b. Obviously, the SnS₂/MoS₂ owns larger enclosed area than pure MoS₂, suggesting that the former has a larger areal capacitance. This is mainly due to the great contribution of the SnS₂/MoS₂, which prevents the collapse of the MoS₂ nanosheets. Thereby, the synergistic effect of MoS₂ nanosheets and SnS₂ nanoplates is not only able to effectively weaken the agglomerating and restacking problems, but also able to facilitate rapid electronic transport in electrode reactions.

Fig. 5 (a) CV curves of the SnS₂/MoS₂ electrodes at different scan rates, (b) normalized CV curves of the MoS₂ and SnS₂/MoS₂ electrodes at 10 mv s⁻¹, (c) galvanostatic charge–discharge curves of SnS₂/MoS₂ electrodes at different current density. (d) Specific capacitances of MoS₂ and SnS₂/MoS₂ electrodes a at different current density.

To further calculate the specific capacitance of the 3D flower-like heterostructured SnS₂/MoS₂ electrode, the charge/discharge measurements were performed between −0.9 V to −0.3 V at different current densities in 1 M KCl solutions as shown in Fig. 3c. The specific capacitance was calculated by the following equation:

where C_m (F g⁻¹) is the specific capacitance, I (A) is the discharge current, t (s) is the discharge time, ΔV (V) is the potential window, and m (g) is the mass of the active material.

According to the equation, the specific capacitances of the SnS₂/MoS₂ are 151.9, 127.4, 111.3, and 105.7 F g⁻¹ at 0.24, 0.59, 1.18, and 2.35 A g⁻¹, respectively (Fig. 5c). At low current densities, the inner active sites or the pores of the electrode can be fully accessed and diffused with cations; hence, high specific capacitance values are achieved. The charge/discharge behavior of MoS₂ had also been measured and showed in Fig. 5d. The capacitance of the electrode is calculated to be about 145.8, 125.1, 100.3 and 67.3 F g⁻¹ at 0.24, 0.59, 1.18, and 2.35 A g⁻¹, respectively. They own low capacitance (67.3 F g⁻¹ at 2.35 A g⁻¹) compared to SnS₂/MoS₂ electrode, which deliver an improved capacitance. The enlarged specific capacitance can be attributed to the synergistic effect of two-component heterostructured metal sulfides.

Fig. 6a shows Nyquist plots of the EIS data obtained for the SnS₂/MoS₂ and MoS₂ electrodes at open circuit potential over the frequency range 0.01–100 000 Hz in 1 M KCl electrolyte solutions. In low frequency area, the Warburg impedance (W), which results from the diffusive resistance of the electrolyte into the interior of the electrode and the ion diffusion into the electrode, is shown by the slope of the curve. The more vertical the curve is, the smaller Warburg impedance is. The slopes of the curve at low frequency area of SnS₂/MoS₂ electrode is more vertical compared to MoS₂, which demonstrates the decreasing of diffusive resistance between the electrode and the electrolyte. In the high frequency area, the semicircle corresponds to the charge-transfer resistance of the electrode and electrolyte interface.^32,33 Different from pure MoS₂, the semicircle is smaller in SnS₂/MoS₂ electrode, indicating that the resistance is significant lower. The bulk resistance of the electrochemical system can also be realized from the intersection of the curve at real part Z. From the plots, we can see SnS₂/MoS₂ electrode shows lower bulk resistance.

Fig. 6 (a) Nyquist plots of MoS₂ and SnS₂/MoS₂ electrodes in 1 M KCl, (b) cycling stability of the MoS₂ and SnS₂/MoS₂ at a current density of 2.35 A g⁻¹.

The cycling stability of the SnS₂/MoS₂ electrode was investigated by repeating the galvanostatic charge–discharge measurements ranging from −0.9 V to −0.3 V over 1000 cycles at the current density of 2.35 A g⁻¹, as shown in Fig. 6b. The specific capacitance gradually decreases with the cycle number, and the specific capacitance of this electrode still remains at 90.4% after 1000 cycles. Fig. 4b also shows the cycle characteristic of pure MoS₂ at a current density of 2.35 A g⁻¹ for up to 500 cycles. After that, it only retains 79% of the initial capacitance with a quite quick decrease. It is clear that the cycle stability of SnS₂/MoS₂ are greatly improved. The excellent electrochemical performance can be attributed to SnS₂/MoS₂ heterostructure, which forms an interconnected conducting network, and facilitates rapid electronic transport in electrode reactions.

4. Conclusions

In summary, we have demonstrated a one-step hydrothermal way to fabricate 3D flower-like heterostructured SnS₂/MoS₂, which had better electrochemical performance compared to the MoS₂. The as-prepared SnS₂/MoS₂ electrode exhibited a high capacitance of 105.4 F g⁻¹ at 2.35 A g⁻¹, and also showed excellent cycle stability. This capacitive behavior mainly resulted from the rapid electronic transport and volume change buffering of SnS₂/MoS₂ heterostructure during electrochemical measurement. Due to the excellent performance, we believe that the SnS₂/MoS₂ is a potential promising electrode material for the application of energy storage or conversion with fine electrochemical performance and deserved to be further investigated.

Conflict of interest

The authors declare no competing financial interest.

Acknowledgements

This work is supported by the Chinese Academy of Sciences and Technology Project (XBLZ-2011-013) and the Technologies R&D Program of Gansu Province (1104FKCA156).

References

1. M. Winter and R. J. Brodd, What Are Batteries, Fuel Cells, and Supercapacitors, Chem. Rev., 2004, 10, 4245–4270.
2. Y. Zhu, S. Murali, M. D. Stoller, K. J. Ganesh, W. Cai, P. J. Ferreira, A. Pirkle, R. M. Wallace, K. A. Cychosz, M. Thommes, D. Su, E. A. Stach and R. S. Ruoff, Carbon-Based Supercapacitors Produced by Activation of Graphene, Science, 2011, 332, 1537–1541.
3. J. M. Tarascon and M. Armand, Issues and Challenges Facing Rechargeable Lithium Batteries, Nature, 2001, 414, 359–367.
4. B. E. Conway, V. Birss and J. Wojtowicz, The Role and Utilization of Pseudocapacitance for Energy Storage by Supercapacitors, J. Power Sources, 1997, 66, 1–14.
5. D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P. L. Taberna and P. Simon, Ultrahigh-power Micrometre-sized Supercapacitors Based on Onion-like Carbon, Nat. Nanotechnol., 2010, 5, 651–654.
6. N. Mahmood, C. Zhang, H. Yin and Y. Hou, Graphene-based Nanocomposites for Energy Storage and Conversion in Lithium Batteries, Supercapacitors and Fuel Cells, J. Mater. Chem. A, 2014, 2, 15–32.
7. P. Simon, Y. Gogotsi and B. Dunn, Where Do Batteries End and Supercapacitors Begin, Science, 2014, 343, 1210–1211.
8. B. Li and Y. Wang, Facile Synthesis and Enhanced Photocatalytic Performance of Flower-like ZnO Hierarchical Microstructures, J. Phys. Chem. C, 2009, 114, 890–896.
9. J. N. Tiwari, R. N. Tiwari and K. S. Kim, Zero-dimensional, One-dimensional, Two-dimensional and Three-dimensional Nanostructured Materials for Advanced Electrochemical Energy Devices, Prog. Mater. Sci., 2012, 57, 724–803.
10. C. Hu, J. Guo and J. Wen, Hierarchical Nanostructure CuO with Peach Kernel-like Morphology as Anode Material for Lithium-Ion Batteries, Ionics, 2013, 19, 253–258.
11. J. Zhu, Z. Yin, D. Yang, T. Sun, H. Yu, H. E. Hoster, H. H. Hng, H. Zhang and Q. Yan, Hierarchical Hollow Spheres Composed of Ultrathin Fe₂O₃ Nanosheets for Lithium Storage and Photocatalytic Water Oxidation, Energy Environ. Sci., 2013, 6, 987–993.
12. J. Wang, G. Du, R. Zeng, B. Niu, Z. Chen, Z. Guo and S. Dou, Porous Co₃O₄ Nanoplatelets by Self-supported Formation as Electrode Material for Lithium-Ion Batteries, Electrochim. Acta, 2010, 55, 4805–4811.
13. M. H. Park, K. Kim, J. Kim and J. Cho, Flexible Dimensional Control of High-Capacity Li-Ion-Battery Anodes: From 0D Hollow to 3D Porous Germanium Nanoparticle Assemblies, Adv. Mater., 2010, 22, 415–418.
14. A. Esmanski and G. A. Ozin, Silicon Inverse-Opal-Based Macroporous Materials as Negative Electrodes for Lithium Ion Batteries, Adv. Funct. Mater., 2009, 19, 1999–2010.
15. K. J. Huang, L. Wang, Y. J. Liu, H. B. Wang, Y. M. Liu and L. L. Wang, Synthesis of polyaniline/2-dimensional graphene analog MoS₂ composites for high-performance supercapacitor, Electrochim. Acta, 2013, 109, 587–594.
16. Z. M. Wan, J. Shao, J. J. Yun, H. Y. Zheng, T. Gao, M. Shen, Q. T. Qu and H. H. Zheng, Core–Shell Structure of Hierarchical Quasi-Hollow MoS₂ Microspheres Encapsulated Porous Carbon as Stable Anode for Li-Ion Batteries, Small, 2014, 10, 4975–4981.
17. N. A. Dhas and K. S. Suslick, Sonochemical Preparation of Hollow Nanospheres and Hollow Nanocrystals, J. Am. Chem. Soc., 2005, 127, 2368–2369.
18. M. Wang, G. D. Li, H. Y. Xu, Y. T. Qian and J. Yang, Enhanced Lithium Storage Performances of Hierarchical Hollow MoS₂ Nanoparticles Assembled from Nanosheets, ACS Appl. Mater. Interfaces, 2013, 5, 1003–1008.
19. J. Ma, D. Lei, X. Duan, Q. Li, T. Wang, A. Cao, Y. Mao and W. Zheng, Designable Fabrication of Flower-like SnS₂ Aggregates with Excellent Performance in Lithium-Ion Batteries, RSC Adv., 2012, 2, 3615–3617.
20. K. Krishnamoorthy, G. K. Veerasubramani, S. Radhakrishnan and S. J. Kim, Supercapacitive properties of hydrothermally synthesized sphere like MoS₂ nanostructures, Mater. Res. Bull., 2014, 50, 499–502.
21. X. Zhou, B. Xu, Z. Lin, D. Shu and L. Ma, Hydrothermal Synthesis of Flower-Like MoS₂ Nanospheres for Electrochemical Supercapacitors, J. Nanosci. Nanotechnol., 2014, 14, 7250–7254.
22. B. Hu, X. Qin, A. M. Asiri, K. A. Alamry, A. O. AlYoubi and X. Sun, Synthesis of porous tubular C/MoS₂ nanocomposites and their application as a novel electrode material for supercapacitors with excellent cycling stability, Electrochim. Acta, 2013, 100, 24–28.
23. Y. Gao, S. Cui, L. Mi, W. Wei, F. Yang, Z. Zheng, H. Hou and W. Chen, Double Metal Ions Synergistic Effect in Hierarchical Metal Sulfide Microflowers for Enhanced Supercapacitor Performance, ACS Appl. Mater. Interfaces, 2015, 7, 4311–4319.
24. L. Mi, W. Wei, S. Huang, S. Cui, W. Zhang, H. Hou and W. Chen, Nest-like Ni@Ni_1.4Co_1.6S₂ Electrode for Flexible High-Performance Rolling Supercapacitors Device Design, J. Mater. Chem. A, 2015 10.1039/C5TA06265A.
25. W. Wei, L. Mi, Y. Gao, Z. Zheng, W. Chen and X. Guan, Partial Ion-Exchange of Nickel-Sulfide-Derived Electrodes for High Performance Supercapacitors, Chem. Mater., 2014, 26, 3418–3426.
26. H. Luo, C. Xu, D. Zou, L. Wang and T. Ying, Hydrothermal synthesis of hollow MoS₂ micro-spheres in ionic liquids/water binary emulsions, Mater. Lett., 2008, 62, 3558–3560.
27. M. K. Jana, H. B. Rajendra, A. J. Bhattacharyya and K. Biswas, Green ionothermal synthesis of hierarchical nanostructures of SnS₂ and their Li-ion storage properties, CrystEngComm, 2014, 16, 3994–4000.
28. W. J. Li, E. W. Shi, J. M. Ko, Z. Chen, H. Ogino and T. J. Fukuda, Hydrothermal synthesis of MoS₂ nanowires, Cryst. Growth Des., 2003, 250, 418–422.
29. H. Lin, X. Chen, H. Li, M. Yang and Y. Qi, Hydrothermal synthesis and characterization of MoS₂ nanorods, Mater. Lett., 2010, 64, 1748–1750.
30. Y. Zhang, J. Li, M. Zhang and D. D. Dionysiou, Size-tunable hydrothermal synthesis of SnS₂ nanocrystals with high performance in visible light-driven photocatalytic reduction of aqueous Cr(VI), Environ. Sci. Technol., 2011, 45, 9324–9331.
31. J. Li, Q. M. Yang and I. Zhitomirsky, Nickel foam-based manganese dioxide–carbon nanotube composite electrodes for electrochemical supercapacitors, J. Power Sources, 2008, 185, 1569–1574.
32. H. R. Oswald, J. R. Gunter and E. J. Dubler, Topotactic decomposition and crystal structure of white molybdenum trioxide-monohydrate: prediction of structure by topotaxy, Solid State Chem., 1975, 13, 330–338.
33. T. Nagasawa and K. Kobayashi, Paracrystalline Structure of Polymer-Crystal Lattice Distortion Induced by Electron Irradiation, J. Appl. Phys., 1970, 41, 4276–4284.

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