Facile construction of graphene-like Ni₃S₂ nanosheets through the hydrothermally assisted sulfurization of nickel foam and their application as self-supported electrodes for supercapacitors

Abstract

A facile and low-cost approach has been developed for the fabrication of large-area nickel sulfide nanosheets via the hydrothermally assisted sulfurization of Ni foam. The obtained Ni₃S₂ nanosheets exhibit perfect supercapacitor performance, retaining almost 93.6% of the maximum capacitance after 3000 cycles. The strategy of self-sulfurization is promising for use in the construction of other nanoarchitectured sulfide (e.g., CuS, FeS and SnS) arrays for electrochemical applications.

---

The successful fabrication of graphene has stimulated new interests in ultrathin two-dimensional (2D) layer-structured materials, such as graphene-like MoS₂, WS₂, SnS₂, etc. Their excellent electronic features and high specific surface area are responsible for their extraordinary properties for energy storage, catalysis, sensing.^1–3 With the rapidly growing business of energy storage devices, supercapacitors have drawn much attention.^4–6 Metal sulfides (e.g. NiS_x, CuS_x, and CoS_x) have drawn the researchers’ attention as electrode materials for supercapacitors due to their interesting intrinsic properties. The family of nickel sulfides such as NiS and Ni₃S₂ has also been studied as electrode materials.^7–10 Generally, the material is mixed with a binder and a conductor, then coated on Ni foam and used as an electrode for electrochemical measurements. However, the binder and conductivity agent may hinder the contact between the active material and the electrolyte, leading to inadequate electrolyte penetration reducing the fast electron transport.

Many recent efforts have been made to synthesize diverse nanostructures of nickel sulfides, including hollow spheres, nanochains, nanowires, and nanorods for various applications.^8,11–13 However, the synthesis of graphene-like nickel sulfide nanosheets still requires further development for different applications. Besides, if the ultrathin 2D materials are vertically oriented on the conductor substrate, they can serve as an ideal material for energy storage devices, as catalyst support, or as efficient electroactive sites for electron emission due to a large surface area and the presence of sharp edges. For example, 2D layered materials are oriented along the current collectors in a conventional stacked geometry in energy storage devices. In that case, the electrolyte ions are often inhibited from penetrating deep inside the layers. This reduces the complete utilization of the electrochemical surface area of layered materials. A favorable in-plane design can effectively facilitate the electrolyte ions to interact with all the layers, leading to a full utilization of the high surface area offered by the layers.^14–17 However, aligning 2D materials vertically is more difficult due to the high aspect ratio of sheet-like structures with very thin thickness, which makes them unstable.^18,19 With this in mind, we have designed vertically aligned Ni₃S₂ ultrathin nanosheets facilely derived from self-sulfurized Ni foam and explored the application of this novel structure in supercapacitor devices.

Fig. 1 shows a schematic illustration of the preparation of graphene-like Ni₃S₂ nanosheets via a simple one-pot sulfurization process from Ni foam. The pretreated Ni foam was soaked in a thioacetamide (TAA) aqueous solution in an autoclave at 120 °C for 4 h. This simple method leads to the self-sulfurization of nickel foam to form sheet-like Ni₃S₂ on the surface of the Ni foam. There are some reports about the hydrothermal synthesis of nanomaterials on nickel foam in which the nickel foam acts as a substrate.^20–23 In this work, part of the nickel foam is the reagent that is sulfurized, which is different to most of the other reports. After the reaction, the weight of the nickel foam increased by 0.50 mg cm⁻², then the mass of Ni₃S₂ was calculated as 1.87 mg cm⁻². The morphology of the bare nickel foam and the self-sulfured Ni foam examined by SEM is shown in Fig. 2a–c. The submillimeter pores and nickel foam skeletons can be clearly seen in Fig. 2a, and the nickel grains of the skeletons are observable at a higher magnification (inset in Fig. 2a). Fig. 2b–e display the SEM images of the obtained supported Ni₃S₂ on the self-sulfurized Ni foam. At small magnification, it can be seen the Ni foam surface is uniformly covered with networks. A closer view of the film reveals that the Ni₃S₂ nanosheets formed by the self-sulfurization of Ni foam lie aslant or perpendicular to the Ni foam support, and are interconnected with each other forming a uniform nanostructure with a highly open and porous structure. Thus, most of the nanosheet surface is highly accessible by the electrolyte when used as an electrode for supercapacitors. Fig. S1† shows the section image of the prepared Ni₃S₂ nanosheets grown on Ni foam. From the figure, we can see the thickness of the Ni₃S₂ nanosheet is about 100 nm. Fig. 2f shows the XRD pattern of the Ni₃S₂ films on Ni foam. The three sharp peaks are indexed to the nickel substrate. The other minor peaks can be attributed to Ni₃S₂ (JCPDS 44-1418). No spare peaks are observed, indicating the absence of any impurities in the product and the uniform growth of Ni₃S₂ on the Ni foam surface.

Fig. 1 Schematic diagram for the preparation of Ni₃S₂ nanosheets on Ni foam.

Fig. 2 (a) SEM image of the Ni foam, inset, a closer view of the Ni foam; (b–e) SEM images of the Ni₃S₂ nanosheets sulfurized on Ni foam; (f) XRD pattern of Ni₃S₂ on Ni foam.

This ultrathin feature was further verified by the typical wrinkled structure of the nanosheets as observed from Fig. 3a and b. It must be noted that the thickness of as-synthesized Ni₃S₂ nanosheets is estimated to be only ca. 5 nm. A more revealing feature of the crystalline structure of the nanosheets was revealed by the HRTEM analysis for the rectangle area marked in Fig. 3a. The Ni₃S₂ nanosheets consist of graphene-like layers with an interlayer separation of 0.32 nm, which is larger than the interlayer distance (0.29 nm) for the (110) plane of bulk Ni₃S₂. EDX characterization of the Ni₃S₂ nanosheets on Ni foam (NSNF) is presented in Fig. 3d. The pattern shows the presence of Ni, S and the Ni arising from the nickel substrate.

Fig. 3 (a and b) TEM images of graphene-like Ni₃S₂ nanosheets; (c) HRTEM image of the rectangle area marked in (a); (d) the EDX pattern of the Ni₃S₂ nanosheets on Ni foam.

We directly applied the ultrathin Ni₃S₂ nanosheets supported on Ni foam as an electrode for supercapacitors in order to highlight the merits of this unique architecture. The electrochemical tests were carried out in a three-electrode configuration in 2 M KOH aqueous electrolyte. From Fig. S2,† NSNF display a bigger encircled area and longer charge–discharge times under the same testing conditions than Ni foam (NF), indicating excellent supercapacitor properties after the self-sulfurization of Ni foam. The electrochemical characterization of NSNF is shown in Fig. 4. Fig. 4a shows the CV results of the NSNF electrode at scan rates of 2–20 mV s⁻¹. The profiles clearly reveal pronounced redox peaks, indicating that the capacitance characteristics are mainly governed by Faradaic redox reactions, which are very different from those of conventional electric double-layer capacitors that usually produce a CV curve close to an ideal rectangular shape.²⁴ At a low scan rate of 2 mV s⁻¹, the redox couples at 0.45/0.29 (V) and 0.51/0.39 (V) correspond to the conversion between Ni–S/Ni–S–OH and Ni–S–OH/Ni–S–O. The reaction mechanism occurs as shown below, which has also been reported for other sulfide systems.^25,26

Ni₂S₃ + OH⁻ → Ni₂S₃OH + e⁻ (1)

Ni₂S₃OH + OH⁻ → Ni₂S₃O + H₂O + e⁻ (2)

Fig. 4 Electrochemical characterization of NSNF: (a) CV curves at different scan rates, (b) galvanostatic charge–discharge curves at different current densities, (c) the specific capacitance calculated from the discharge curves, and (d) cycling performance at a current density of 40 mA cm⁻².

Furthermore, it is obvious that the redox peak positions shift with the increasing scan rate from 2 to 20 mV s⁻¹. It may be that charge transfer kinetics is the limiting step of the reaction.^27,28 Fig. 4b presents the typical charge–discharge voltage vs. time plots of NSNF at various current densities. Symmetric triangular shapes with well-defined plateaus during the charge–discharge processes are observed, suggesting a good supercapacitive behavior. The corresponding supercapacitance at various current densities can be calculated based on the charge–discharge curves in Fig. 4b and the results are presented in Fig. 4c. Remarkably, the NSNF electrode displays capacitance values of 1.342, 1.125, 0.995, 0.851 and 0.663 F cm⁻² at current densities of 15, 25, 35, 50 and 70 mA cm⁻², respectively. The Ragone plot of NSNF is shown in Fig. S3.† At a power density of 47.11 kW kg⁻¹, the energy density of NSNF can reach 65.43 W h kg⁻¹ while at a lower power density of 4.55 kW kg⁻¹, the electrode delivers an energy density as high as 141.69 W h kg⁻¹. The cycle life of supercapacitors is a crucial parameter for practical applications. Fig. 4d shows the electrochemical stability of NSNF and NF at a current density of 15 mA cm⁻². After continuous cycling for 3000 cycles, 93.6% of the initial capacitance was still delivered. For comparison, thew same experiment was conducted with pure Ni foam at the same current density. About 5.6% of the initial capacitance was delivered after 3000 cycles. This also demonstrates that the capacitance contribution from the nickel foam is negligible after thousands of cycles due to the fast capacitance loss of the Ni foam. The main capacitance contribution for the NSNF electrode is attributed to the Ni₃S₂ sheets on the self-sulfurized Ni foam. This unique self-supported structure presents a robust adhesion to the substrate and offers fast ion/electron-transport access to the current collector providing large free space to accommodate strain variations.

In summary, the hydrothermally assisted sulfurization of Ni foam has been developed to fabricate uniform nickel sulfide nanosheets. The Ni₃S₂ nanosheets are easily derived from Ni foam. This strategy is facile, low-cost and suitable for large-scale production of supercapacitor electrodes. The as-formed Ni₃S₂ nanosheets are around 5 nm in thickness. In virtue of its unique structural features, this self-supported electrode exhibits high capacitance with excellent cycling stability, which suggests its potential application in supercapacitors. Meanwhile, it also demonstrates that the capacitance contribution from the nickel foam can be neglected after thousands of cycles due to the fast capacitance loss of the Ni foam. The strategy of self-sulfurization is a promising one for the construction of other nanoarchitectured sulfide arrays for electrochemical applications.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Grant no. 21003041, 21103046), Hunan Provincial Natural Science Foundation of China (Grant no. 10JJ1011, 11JJ7004), Specialized Research Fund for the Doctoral Program of Higher Education of China (20120161110016), Scholarship Award for Excellent Doctoral Student granted by Ministry of Education and the Hunan Provincial Innovation Foundation for Postgraduates (Grant no. CX2013B166).

Notes and references

1. J. W. Seo, Y. W. Jun, S. W. Park, H. Nah, T. Moon, B. Park, J. G. Kim, Y. J. Kim and J. Cheon, Angew. Chem., Int. Ed., 2007, 46, 8828.
2. F. Cao, R. X. Liu, L. Zhou, S. Y. Song, Y. Q. Lei, W. D. Shi, F. Y. Zhao and H. J. Zhang, J. Mater. Chem., 2010, 20, 1078.
3. Y. Wang, Q. S. Zhu, L. Tao and X. W. Su, J. Mater. Chem., 2011, 21, 9248.
4. C. C. Hu, M. J. Liu and K. H. Chang, J. Power Sources, 2007, 163, 1126.
5. T. Matsumura, K. Nakano, R. Kannano, N. Imanishi and Y. Takeda, J. Power Sources, 2007, 174, 632.
6. L. Zhang, H. B. Wu and X. W. Lou, Chem. Commun., 2012, 48, 6912.
7. W. M. Kriven, J. Am. Ceram. Soc., 1988, 71, 1021.
8. Y. Hu, J. F. Chen, W. M. Chen, X. H. Lin and X. L. Li, Adv. Mater., 2003, 15, 726.
9. T. Zhu, Z. Y. Wang, S. J. Ding, J. S. Chen and X. W. Lou, RSC Adv., 2011, 1, 397.
10. J. Q. Yang, X. C. Duan, Q. Qin and W. J. Zheng, J. Mater. Chem. A, 2013, 1, 7880.
11. W. Zhou, W. M. Chen, J. W. Nai, P. G. Yin, C. P. Chen and L. Guo, Adv. Funct. Mater., 2010, 20, 3678.
12. A. Ghezelbash, M. B. Sigman and B. A. Korgel, Nano Lett., 2004, 4, 537.
13. T. Zhu, H. B. Wu, Y. B. Wang, R. Xu and X. W. Lou, Adv. Energy Mater., 2012, 2, 1497.
14. J. P. Liu, J. Jiang, C. W. Cheng, H. X. Li, J. X. Zhang, H. Gong and H. J. Fan, Adv. Mater., 2011, 23, 2076.
15. C. Guan, J. P. Liu, C. W. Cheng, H. X. Li, X. L. Li, W. W. Zhou, H. Zhang and H. J. Fan, Energy Environ. Sci., 2011, 4, 4496.
16. C. C. Li, Q. H. Li, L. B. Chen and T. H. Wang, J. Mater. Chem., 2011, 21, 11867.
17. Y. G. Li, B. Tan and Y. Y. Wu, Nano Lett., 2008, 8, 265.
18. R. Tenne, Adv. Mater., 1995, 7, 965.
19. R. Tenne, Nat. Nanotechnol., 2006, 1, 103.
20. K. W. Qiu, H. L. Yan, D. Y. Zhang, Y. Lua, J. B. Cheng, W. Q. Zhao, C. L. Wang, Y. H. Zhang, X. M. Liu, C. W. Cheng and Y. S. Luo, Electrochim. Acta, 2014, 141, 248.
21. H. N. Zhang, Y. J. Chen, W. W. Wang, G. H. Zhang, M. Zhuo, H. M. Zhang, T. Yang, Q. H. Li and T. H. Wang, J. Mater. Chem. A, 2013, 1, 8593.
22. W. Y. Li, K. B. Xu, B. Li, J. Q. Sun, F. R. Jiang, Z. S. Yu, R. J. Zou, Z. G. Chen and J. Q. Hu, ChemElectroChem, 2014, 1, 1003.
23. K. W. Qiu, Y. Lua, D. Y. Zhang, J. B. Cheng, H. L. Yan, J. Y. Xu, X. M. Liu, J. K. Kim and Y. S. Luo, Nano Energy, 2015, 11, 687.
24. M. S. Wu, M. J. Wang and J. J. Jow, J. Power Sources, 2010, 195, 3950.
25. B. H. Qu, Y. J. Chen, M. Zhang, L. L. Hu, D. N. Lei, B. A. Lu, Q. H. Li, Y. G. Wang, L. B. Chen and T. H. Wang, Nanoscale, 2012, 4, 7810.
26. F. Tao, Y. Q. Zhao, G. Q. Zhang and H. L. Li, Electrochem. Commun., 2007, 9, 1282.
27. S. Chen, J. W. Zhu, X. D. Wu, Q. F. Han and X. Wang, ACS Nano, 2010, 4, 2822.
28. Y. J. Chen, B. H. Qu, L. L. Hu, Z. Xu, Q. H. Li and T. H. Wang, Nanoscale, 2013, 5, 9812.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45152f

---