Hierarchical NiCo₂O₄ nanowire arrays on Ni foam as an anode for lithium-ion batteries

Abstract

A facile two-step method is developed for large-scale growth of hierarchical mesoporous NiCo₂O₄ nanowire arrays on Ni foam with robust adhesion as a high performance electrode for lithium-ion batteries (LIBs). The as-prepared mesoporous NiCo₂O₄ nanowire arrays consist of numerous highly crystalline nanoparticles, the Ni foam supported NiCo₂O₄ nanowires promote fast electron and ion transport, and alleviate the volume change during the charge–discharge processes. When evaluated as a binder-free electrode for LIBs, the NiCo₂O₄ electrode exhibits a greatly enhanced charge capacity of 1520 mA h g⁻¹ at a current density of 100 mA g⁻¹. The mesoporous NiCo₂O₄ nanowire arrays exhibit great potential as an anode material for LIBs with the advantages of unique performance and facile preparation.

---

Introduction

As the global energy shortage and environmental problems are becoming more and more serious, it is urgent to find efficient and sustainable solutions to these issues. The investigation of high-performance, low-cost, and environmentally friendly energy storage systems has become a hot topic in recent years.^1–3 Among the various emerging energy storage systems, lithium ion batteries (LIBs) have attracted more and more attention and have been considered as the ideal candidate for green energy storage due to their high energy density, long lifespan and environmental benignity.^4–8 However, the electrode materials (mainly the conventional graphite anode) could not satisfy the needs of high-energy applications like electric vehicles.⁹ Therefore, it is essential to develop alternative anode materials with high capacity, long life and safety. One of the most promising routes toward the development of high performance batteries is the use of nanoscale materials to prepare the electrodes.^10,11

Recently, binary metal oxides have been regarded as a promising class of electrode materials for high-performance energy storage devices since they usually offers higher electrochemical activity and higher capacity than mono-metal oxides.^12–16 Among various binary metal oxides, NiCo₂O₄ has been considered as a very promising electrode material for LIBs and widely studied due to its better electrical conductivity, higher electrochemical activity and structural stability.^17–20 Until now, nanoparticles and porous nanoflakes of NiCo₂O₄ materials have been synthesized to be used as anode materials for LIBs, and the electrode was fabricated by mixing nanostructured NiCo₂O₄ with carbon and polymer additive/binder and compressing them into pellets.^19,21–23 In attempt to further improve the electrochemical performance of the NiCo₂O₄-based electrodes, the growth of electrode materials, directly on conductive substrates, especially 3D substrates has attracted more and more attention.^24–26 Such an additive/binder-free electrode architectures can avoid the “dead surface”, thus improve the interfacial contact between the current collector and the active materials, allowing for more efficient charge transfer and electrolyte penetration.^27,28 In such case, 3D hierarchical nickel foam is generally considered as a favorable current collector due to its huge supporting area and high electrical conductivity. More recently, Lou and co-workers reported a solution synthesis of mesoporous NiCo₂O₄ nanosheets on various conducive substrates with areal capacitance of 2.09 F cm⁻² at a current density of 8.5 mA cm⁻².¹² Tu et al. successfully grew hierarchical NiCo₂O₄@NiCo₂O₄ core/shell nanoflake arrays on nickel foam with a high areal specific capacitance of 1.55 F cm⁻² at 2 mA cm⁻² by a two-step solution-based method.²⁹ Luo et al. reported 3D hierarchical multi-villous NiCo₂O₄ nanocyclobenzene arrays on Ni foam with a high specific capacitance of 1545 F g⁻¹ (5 A g⁻¹) through a facile hydrothermal route and thermal decomposition.³⁰ However, to the best of our knowledge, there have been few reports to date on the preparation of NiCo₂O₄-based electrode on Ni foam for lithium-ion batteries, although some documents have already been reported about their application for supercapacitors.^31–33

Herein, we report a simple hydrothermal route followed by thermal treatment of precursor for the direct growth of NiCo₂O₄ nanowires with metasequoia-like framework on nickel foam. Impressively, the porous NiCo₂O₄ nanowire arrays (NWAs) exhibit a large specific area, which could provide excellent capability for fast ion and electron transfer. Directly grown electrode materials on nickel foam not only simplify the electrode processing but also avoid the use of polymer binder and conducting additives, allowing for more effectively charge and mass exchange. The good contact between NiCo₂O₄ NWAs and the current collector as well as the cross-linked network sustain a highly stable structure, ensuring a good cycle life. Due to the above-mentioned advantages, the as-prepared metasequoia-like NiCo₂O₄ NWAs exhibit high capacity, enhanced rate capability and improved cycling stability when applied as binders and additive-free anode materials for LIBs.

Experimental section

Synthesis of mesoporous NiCo₂O₄ nanowires arrays (NWAs) on Ni foam

All the reagents used in the experiment were analytical grade and used without further purification. In a typical experiment, a piece of Ni foam was carefully cleaned by sonication in 3 M HCl for 20 min to remove the NiO layer and then washed with absolute ethanol and deionized water for several times. In a typical synthesis, 1 mmol Ni(NO₃)₂·6H₂O and 2 mmol Co(NO₃)₂·6H₂O were dissolved into 70 ml of deionized water to form a clear pink solution, and then followed by addition of 3 mmol NH₄F and 5 mmol urea with vigorous magnetic stirring. After stirring for 1 h, the solution was transferred into the Teflon-lined autoclave, and then a piece of pretreated Ni foam was immersed into the reaction solution. Then, the back side of Ni foam was covered with polytetrafluoroethylene tape to inhibit the growth of nanostructures there. The autoclave was sealed and maintained at 120 °C for 12 h in an electric oven. After cooling to room temperature, the sample was collected and washed with deionized water and absolute ethanol several times and then dried at 60 °C for 10 h. Afterward, the samples were annealed at 400 °C for 2 h.

Materials characterization

The as-prepared samples were characterized by X-ray power diffraction (XRD, Rigaku-TTRIII, Cu Kα). The morphology and structure of the samples were analyzed by scanning electron microscopy (SEM, HELIOS NANOLAB 600i) and transmission electron microscopy (TEM, Titan G2 60-300, HOLLAND). X-ray photoelectron spectra (XPS) were recorded using a X-ray photoelectron spectrometer (ESCALAB 250Xi) with a monochromatic Al Kα X-ray source. Thermo-gravimetric analysis (TGA) was conducted with a heating rate of 5 °C min⁻¹ using a SDTQ600 in air. Nitrogen adsorption and desorption isotherms were obtained at 77 K using a Micromeritics ASAP 2020 system. Specific surface areas were determined according to the Brunauer–Emmett–Teller (BET) method. The pore size distribution (PSD) plot was recorded from the adsorption branch of the isotherm based on the non-local density functional theory (NLDFT) method.

Electrochemical measurements

The electrochemical properties of the samples were measured in CR 2025-type coin cells. For the coin-cell assembly, the samples were cut into round pieces with a diameter of 11 mm. The loading density of NiCo₂O₄ on the sample was about 2 mg cm⁻¹. The electrochemical performance of the as-prepared NiCo₂O₄ product was investigated by using it as a binder-free anode for lithium-ion batteries in coin-type half-cells. The assembly of the test cells was performed in an Ar-filled glove box where water and oxygen content was less than 1 ppm, respectively. A pure lithium foil was used as the counter working electrode, a piece of NiCo₂O₄ NWAs on Ni foam was used as working electrode, 1 M LiPF₆ in ethylene carbonate (EC)–dimethyl carbonate (DMC) (1 : 1 in volume) was used as the electrolyte, and a polypropylene (PP) film (Celgard 2400) was used as the separator. The galvanostatic charge–discharge tests were conducted on a LAND battery program-control test system at different current densities in the potential range of 0.01–3.0 V. Cyclic voltammetry (CV) test was performed on an electrochemical workstation (PARSTAT. MC) in the potential window of 0.01–3.0 V (Li/Li⁺) at a scanning rate of 0.5 mV s⁻¹.

Results and discussion

Structural and morphological characterization

Fig. 1a shows a Ni foam and the Ni foam deposited with NiCo₂O₄ NWAs before and after annealing process. Fig. 1b shows the fabrication process of NiCo₂O₄ NWAs on Ni foam. The fabrication included two steps. First, the Ni–Co hydroxide precursor was grown vertically on Ni foam through a hydrothermal condition, and the Ni foam changed from gray to light pink due to the coating of the precursor on the Ni foam. Next, the Ni–Co hydroxide precursor converted into NiCo₂O₄ NWAs via an annealing process, then the Ni foam changed from light pink to black indicating the successful preparation of NiCo₂O₄ NWAs.

Fig. 1 (a) Optical images of Ni foam, Ni–Co precursor, NiCo₂O₄ NWAs, respectively; (b) schematic image of the formation process of the NiCo₂O₄ NWAs.

The composition and phase purity of the as-prepared NiCo₂O₄ NWAs were examined by XRD (Fig. 2), and the results reveal that pure NiCo₂O₄ can be obtained under the 120 °C hydrothermal condition combining a following calcination process. As shown in Fig. 2, apart from three strong peaks belong to the Ni foam substrate, all the reflection peaks can be indexed to cubic NiCo₂O₄ with a spinel structure (JCPDF no. 20-0781).¹⁷ The highly broadened peaks indicate that the nanowire arrays are composed of NiCo₂O₄ crystalline with very small size. No additional peaks can be detected, implying that the NiCo₂O₄ has been formed without impurity.

Fig. 2 XRD pattern of the as-prepared NiCo₂O₄ NWAs on Ni foam.

Fig. 3 shows the thermogravimetric and differential thermal analysis (TG-DTA) curves of the Ni–Co hydroxide precursor after hydrothermal synthesis for 12 h. It can be seen that the Ni–Co hydroxide begins to decompose at 220 °C, and the weight loss below 220 °C is mainly attributed to the evaporation of physically and chemically adsorbed water and other small molecules. In the DTA curve, there is a strong heat absorption peak at around 330 °C, which is originated from the decomposition of Ni–Co hydroxide into NiCo₂O₄. The rapid weightlessness comes to an end at about 400 °C as shown in the TG curve. These results demonstrate that 400 °C is the reasonable annealing temperature for synthesis of high crystalline NiCo₂O₄ sample.

Fig. 3 TG and DTA curves for the precursor after hydrothermal synthesis for 12 h.

The specific surface area and pore size distribution of the hierarchical nanostructures were determined by BET N₂ adsorption–desorption analysis at 77 K. As shown in Fig. 4, the as-prepared sample exhibits a specific area of 67 m² g⁻¹ and a pore volume of 0.21 cm³ g⁻¹. The pore size distribution calculated from the adsorption isotherm using the BJH model (the inset Fig. 4), shows that most of the pores fall into the size range of 2 to 10 nm, and the average pore size is 6 nm. The formation of the mesopores could be due to the gas release during the decomposition of the Ni–Co precursor synthesized using the urea as capping agent.¹² Such a porous structure would effectively promote lithium storage, as it can greatly buffer the large volume change in the anode and enhance the diffusion of electrolyte to active materials during the repeated charge–discharge processes.^34,35

Fig. 4 N₂ adsorption and desorption isotherms of the as-prepared NiCo₂O₄ NWAs measured at 77 K and the inset was the corresponding pore distribution using the BJH method.

The morphology and microstructure of the as-prepared NiCo₂O₄ NWAs were investigated by scanning electron microscopy (SEM) at different magnification. As shown in Fig. 5a, it is obviously shown that the as-prepared NiCo₂O₄ NWAs with a uniform morphology were successfully grown on the Ni foam on a large scale. The magnified SEM image shown in Fig. 5b indicates that the sample composed of abundant unique 3D metasequoia-like nanowire arrays, which are up to 2 μm long with an average diameter of 50–80 nm and grow quasi-vertically on the Ni foam (Fig. 5c and d). This unique structure can be expected to help in fast electron transfer and lithium diffusion.³⁶

Fig. 5 (a–d) Typical SEM images of the NiCo₂O₄ NWAs on Ni foam at different magnifications.

Fig. 6 shows the transmission electron microscopy (TEM) image of the NiCo₂O₄ nanowire. As can be clearly seen, a single typical NiCo₂O₄ nanowire composed of interconnected nanoparticles with an average diameter of 5 nm and shows a mesoporous structure with the pore size ranging from 2–9 nm (Fig. 6a and b). The porosity in the nanowires can benefit Li-ion interaction and diffusion into the spinel lattice as well as electrolyte facile penetration into the anode material, thus enhancing the electrolyte/NiCo₂O₄ contact areas and shortening the Li-ion diffusion length in the nanowires. The HRTEM image shown in Fig. 6c reveals that the measured lattice fringe with inter-plane spacing of 0.203, 0.232 and 0.245 nm, correspond well to the (400), (222), (311) planes of cubic NiCo₂O₄, respectively. Fig. 6d shows the corresponding selected area electron diffraction (SAED) pattern of the NiCo₂O₄ NWAs. Apparently, the SAED pattern can be well indexed to the spinel polycrystalline structure.³⁷ Reflections corresponding to the (111), (220), (311), (400), (511), (440) planes confirm that the NiCo₂O₄ NWAs are composed of small single crystallines, which is in accordance with the XRD results (Fig. 2).

Fig. 6 (a and b) Typical TEM images of NiCo₂O₄ NWAs; (c) HRTEM image of the NiCo₂O₄ NWAs; (d) SAED pattern of the NiCo₂O₄ NWAs.

To further investigate the elemental composition and oxidation state of the as-prepared NiCo₂O₄ NWAs, X-ray photoelectron spectroscopy (XPS) measurements were carried out in the range of 0–1100 eV. Fig. 7a shows a typical full wide-scan XPS spectrum of the NiCo₂O₄ nanowires, where the existing characteristic peaks confirm the presence of Ni, Co, O as well as C from the reference for calibration and no other impurities. By using a Gaussian fitting method, the Ni 2p emission spectrum (Fig. 7b) can be best fitted with two spin–orbit doublets, characteristic of Ni²⁺ and Ni³⁺, and two shakeup satellites (indicated as “Sat.”).²¹ The Co 2p spectrum (Fig. 7c) was also fitted with two spin–orbit doublets, characteristic of Co²⁺ and Co³⁺ and two couple of shakeup satellites.³⁸ These results demonstrate that the chemical composition of NiCo₂O₄ nanowires contain Ni²⁺, Ni³⁺, Co²⁺ and Co³⁺, which are in good agreement with the results reported in the literature of NiCo₂O₄.^39,40 The high resolution spectrum for the O 1s region (Fig. 7d) shows three types of oxygen-containing species. Specially, the band centered on 529.5 eV is assigned to the metal–oxygen bond, whereas the band at 531.0 eV is associated with defects, contaminants, and a number of surface species including hydroxyls, chemisorbed oxygen, undercoordinated lattice oxygen, and species intrinsic to the surface of the spinel. The peaks at 532.9 eV can be attributed to the multiplicity of physi- and chemi-adsorbed water at or near the surface.⁴¹

Fig. 7 XPS spectra of NiCo₂O₄ nanowire arrays: (a) full-range survey and high-resolution (b) Ni 2p, (c) Co 2p, (d) O 1s spectra.

Electrochemical characteristics

To investigate the possible application of the as-grown NiCo₂O₄ NWAs in lithium-ion batteries, the samples were configured as CR2025 coin cells without using ancillary binders or conducting additives. The corresponding electrochemical performances of the NiCo₂O₄-based electrodes for lithium storage are shown in Fig. 8. Fig. 8a shows the first five consecutive cyclic voltammogram (CV) curves of the electrodes at a scanning rate of 0.5 mV s⁻¹ in the voltage window of 0.01–3.0 V. In the first cathodic scan, the intense peak at around 0.6 V can be corresponded to the reduction of the metallic cations to metallic Co and Ni. Also, in the subsequent four cycles, the reduction peak shifts to a higher potential at 0.88 V. Meanwhile, the two anodic peaks at around 1.5 and 2.3 V can be attributed to the oxidation of Ni⁰ to Ni²⁺ and Co⁰ to Co³⁺, respectively. On the basis of the CV results, the electrochemical reactions involved during the charge–discharge processes can be classified as follows:^21,42

NiCo₂O₄ + 8Li⁺ + 8e⁻¹ = Ni + 2Co + 4Li₂O (1)

Ni + Li₂O = NiO + 2Li⁺ + 2e⁻¹ (2)

Co + Li₂O = CoO + 2Li⁺ + 2e⁻¹ (3)

CoO + 1/3Li₂O = 1/3Co₃O₄ + 2/3Co₃O₄ + 2/3Li⁺ + 2/3e⁻¹ (4)

Fig. 8 (a) The first five CV curves of the NiCo₂O₄ NWAs at a scanning rate of 0.5 mV s⁻¹. (b) The 1st, 2nd, 3rd cycle discharge–charge curves of the NiCo₂O₄ NWAs at a current density of 100 mA g⁻¹ in the potential window of 0.01–3 V. (c) Cycling performance of the NiCo₂O₄ NWAs at a constant current density of 100 mA g⁻¹. (d) Rate capability of the NiCo₂O₄ NWAs at various current densities.

The discharge and charge curves for the first three at a current density of 100 mA g⁻¹ are shown in Fig. 8b. The initial discharge and charge capacities are 2025 and 1520 mA h g⁻¹, respectively, corresponding to a coulombic efficiency of 75.1%. The first cycle irreversible capacity loss could be due to the formation of solid electrolyte interphase (SEI) and the reduction of metal oxide to metal with Li₂O formation, which is commonly observed for a variety of metal oxides electrode.²⁴ The discharge potential plateau at around 1.0 V disappeared after the first cycle indicating the formation of a stable SEI. For the following discharge processes, the potential plateau shifts upward to near 1.1 V with a more sloping profile accompanied by a certain capacity loss.

In Fig. 8c, the cycling performance and coulombic efficiency of NiCo₂O₄ nanowires is presented in 0.01–3 V at a current density of 100 mA g⁻¹. It can be observed that the first charge–discharge capacities of NiCo₂O₄ electrodes are 1520/2025 mA h g⁻¹, respectively. The ultrahigh initial discharge capacity may be due to the larger surface area compared with other morphologies of the NiCo₂O₄ materials. And then, the specific capacity slowly decreased to 413 mA h g⁻¹ after 50 cycles. In addition, during the whole charge–discharge cycles, the coulombic efficiency has been maintained above 95% after the first cycle. Moreover, the irreversible capacity loss could be related to the diameter and the porosity of the nanowires. During the charge–discharge processes, the structure of the nanowires may be collapsed and destroyed, and the active materials shed from the Ni foam.²⁴ We believe that through the adjustment of the process to control the structural parameters of the nanowire arrays can realize the optimization of the electrochemical properties.^43–45 The further investigation is under way in our group.

To further investigate the rate performance of the NiCo₂O₄ nanowires, the rate performance was evaluated at various current densities ranging from 100 to 1000 mA g⁻¹, which are shown in Fig. 8d. It can be seen that the charge capacity slightly reduces from 1712 mA h g⁻¹ at a current density of 100 mA g⁻¹ to 1215, 797, 413 mA h g⁻¹ at current densities of 200, 500, 1000 mA g⁻¹, respectively. When the current density is returned to the initial value of 100 mA g⁻¹, the capacity recovers to 700 mA h g⁻¹. The capacities gradually fade at high discharge rates, which could be ascribed to the collapse of the as-prepared nanowires during the charge–discharge process at a high current density.

Conclusions

We have developed a fast and facile hydrothermal method for the growth of NiCo₂O₄ NWAs. The NiCo₂O₄ NWAs were directly grown on the Ni foam to form a binder free electrode for LIBs. The unique 3D metasequoia-like NiCo₂O₄ are mainly composed of porous nanowires, and the hierarchical structure exhibites large specific surface area. The architecture not only favors fast transportation of electrons and lithium ions, but also accommodates volume expansion/contraction during Li insertion/extraction processes. As an anode material for LIBs, the NiCo₂O₄ NWAs on Ni foam shows a high first reversible capacity of 1520 mA h g⁻¹ at a current density of 100 mA g⁻¹. More importantly, this facile method of fabricating NiCo₂O₄ nanowires can be easily generalized to grown other mesoporous metal oxides nanostructure on substrates for the fabrication of high performance energy storage devices.

Acknowledgements

We acknowledge financial support from the National Nature Science Foundation of China (Grant no. 51204209 and 51274240).

Notes and references

1. C. Liu, F. Li, L.-P. Ma and H.-M. Cheng, Adv. Mater., 2010, 22, E28–E62.
2. A. S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon and W. van Schalkwijk, Nat. Mater., 2005, 4, 366–377.
3. X. Y. Liu, Y. Q. Zhang, X. H. Xia, S. J. Shi, Y. Lu, X. L. Wang, C. D. Gu and J. P. Tu, J. Power Sources, 2013, 239, 157–163.
4. B. Liu, P. Soares, C. Checkles, Y. Zhao and G. Yu, Nano Lett., 2013, 13, 3414–3419.
5. S. Liu, S. Zhang, Y. Xing, S. Wang, R. Lin, X. Wei and L. He, Electrochim. Acta, 2014, 150, 75–82.
6. X. Hou, X. Wang, B. Liu, Q. Wang, T. Luo, D. Chen and G. Shen, Nanoscale, 2014, 6, 8858–8864.
7. B. Qu, L. Hu, Q. Li, Y. Wang, L. Chen and T. Wang, ACS Appl. Mater. Interfaces, 2013, 6, 731–736.
8. L. Yu, L. Zhang, H. B. Wu, G. Zhang and X. W. Lou, Energy Environ. Sci., 2013, 6, 2664–2671.
9. M. V. Reddy, G. V. Subba Rao and B. V. R. Chowdari, Chem. Rev., 2013, 113, 5364–5457.
10. S. G. Mohamed, T.-F. Hung, C.-J. Chen, C. K. Chen, S.-F. Hu, R.-S. Liu, K.-C. Wang, X.-K. Xing, H.-M. Liu, A.-S. Liu, M.-H. Hsieh and B.-J. Lee, RSC Adv., 2013, 3, 20143–20149.
11. N. Li, Z. Chen, W. Ren, F. Li and H.-M. Cheng, Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 17360–17365.
12. G. Zhang and X. W. Lou, Adv. Mater., 2013, 25, 976–979.
13. J. Bai, X. Li, G. Liu, Y. Qian and S. Xiong, Adv. Funct. Mater., 2014, 24, 3012–3020.
14. C. Yuan, J. Li, L. Hou, L. Zhang and X. Zhang, Part. Part. Syst. Charact., 2014, 31, 657–663.
15. J. Li, S. Xiong, X. Li and Y. Qian, Nanoscale, 2013, 5, 2045–2054.
16. W. Kang, Y. Tang, W. Li, Z. Li, X. Yang, J. Xu and C.-S. Lee, Nanoscale, 2014, 6, 6551–6556.
17. C. Yuan, J. Li, L. Hou, X. Zhang, L. Shen and X. W. Lou, Adv. Funct. Mater., 2012, 22, 4592–4597.
18. K. Xu, W. Li, Q. Liu, B. Li, X. Liu, L. An, Z. Chen, R. Zou and J. Hu, J. Mater. Chem. A, 2014, 2, 4795–4802.
19. H. S. Jadhav, R. S. Kalubarme, C.-N. Park, J. Kim and C.-J. Park, Nanoscale, 2014, 6, 10071–10076.
20. H. Jiang, J. Ma and C. Li, Chem. Commun., 2012, 48, 4465–4467.
21. J. Li, S. Xiong, Y. Liu, Z. Ju and Y. Qian, ACS Appl. Mater. Interfaces, 2013, 5, 981–988.
22. Y. Chen, M. Zhuo, J. Deng, Z. Xu, Q. Li and T. Wang, J. Mater. Chem. A, 2014, 2, 4449–4456.
23. A. K. Mondal, D. Su, S. Chen, X. Xie and G. Wang, ACS Appl. Mater. Interfaces, 2014, 6, 14827–14835.
24. B. Liu, J. Zhang, X. Wang, G. Chen, D. Chen, C. Zhou and G. Shen, Nano Lett., 2012, 12, 3005–3011.
25. L. Shen, Q. Che, H. Li and X. Zhang, Adv. Funct. Mater., 2014, 24, 2630–2637.
26. W. Zhou, D. Kong, X. Jia, C. Ding, C. Cheng and G. Wen, J. Mater. Chem. A, 2014, 2, 6310–6315.
27. F. Deng, L. Yu, M. Sun, T. Lin, G. Cheng, B. Lan and F. Ye, Electrochim. Acta, 2014, 133, 382–390.
28. H. Long, T. Shi, S. Jiang, S. Xi, R. Chen, S. Liu, G. Liao and Z. Tang, J. Mater. Chem. A, 2014, 2, 3741–3748.
29. X. Y. Liu, S. J. Shi, Q. Q. Xiong, L. Li, Y. J. Zhang, H. Tang, C. D. Gu, X. L. Wang and J. P. Tu, ACS Appl. Mater. Interfaces, 2013, 5, 8790–8795.
30. J. Cheng, Y. Lu, K. Qiu, D. Zhang, C. Wang, H. Yan, J. Xu, Y. Zhang, X. Liu and Y. Luo, CrystEngComm, 2014, 16, 9735–9742.
31. D. Cai, S. Xiao, D. Wang, B. Liu, L. Wang, Y. Liu, H. Li, Y. Wang, Q. Li and T. Wang, Electrochim. Acta, 2014, 142, 118–124.
32. D. Zhang, H. Yan, Y. Lu, K. Qiu, C. Wang, Y. Zhang, X. Liu, J. Luo and Y. Luo, Dalton Trans., 2014, 15887–15897.
33. K. Xu, X. Huang, Q. Liu, R. Zou, W. Li, X. Liu, S. Li, J. Yang and J. Hu, J. Mater. Chem. A, 2014, 2, 16731–16739.
34. N. Yan, L. Hu, Y. Li, Y. Wang, H. Zhong, X. Hu, X. Kong and Q. Chen, J. Phys. Chem. C, 2012, 116, 7227–7235.
35. C. Kim, M. Noh, M. Choi, J. Cho and B. Park, Chem. Mater., 2005, 17, 3297–3301.
36. H. Liu, D. Su, R. Zhou, B. Sun, G. Wang and S. Z. Qiao, Adv. Energy Mater., 2012, 2, 970–975.
37. Z.-Q. Liu, K. Xiao, Q.-Z. Xu, N. Li, Y.-Z. Su, H.-J. Wang and S. Chen, RSC Adv., 2013, 3, 4372–4380.
38. Z. Wang, Q. Sha, F. Zhang, J. Pu and W. Zhang, CrystEngComm, 2013, 15, 5928–5934.
39. W. Li, G. Li, J. Sun, R. Zou, K. Xu, Y. Sun, Z. Chen, J. Yang and J. Hu, Nanoscale, 2013, 5, 2901–2908.
40. G. Li, W. Li, K. Xu, R. Zou, Z. Chen and J. Hu, J. Mater. Chem. A, 2014, 2, 7738–7741.
41. J. Pu, J. Wang, X. Jin, F. Cui, E. Sheng and Z. Wang, Electrochim. Acta, 2013, 106, 226–234.
42. H. Guo, L. Liu, T. Li, W. Chen, J. Liu, Y. Guo and Y. Guo, Nanoscale, 2014, 6, 5491–5497.
43. W. Jiang, W. Zeng, Z. Ma, Y. Pan, J. Lin and C. Lu, RSC Adv., 2014, 4, 41281–41286.
44. J. X. Zhang, Z. S. Ma, J. J. Cheng, Y. Wang, C. Wu, Y. Pan and C. Lu, J. Electroanal. Chem., 2015, 738, 184–187.
45. W. X. Lei, Y. Pan, Y. C. Zhou, W. Zhou, M. L. Peng and Z. S. Ma, RSC Adv., 2014, 4, 3233–3237.

---