View PDF VersionPrevious ArticleNext Article
 
DOI: 10.1039/C5RA11960J (Paper) RSC Adv., 2015, 5, 69365-69370

NiMoO4@Ni(OH)2 core/shell nanorods supported on Ni foam for high-performance supercapacitors†

Ge Jiang, Mingyi Zhang*, Xueqing Li and Hong Gao*
Key Laboratory for Photonic and Electronic Bandgap Materials, Ministry of Education, School of Physics and Electronic Engineering, Harbin Normal University, Harbin 150025, P. R. China. E-mail: gaohong65cn@126.com; zhangmingyi@hrbnu.edu.cn

Received 21st June 2015 , Accepted 10th August 2015

First published on 10th August 2015

Abstract

Electrodes with rationally designed hybrid nanostructures can offer many opportunities for the enhanced performance in electrochemical energy storage. In this work, we demonstrate the design and fabrication of NiMoO4@Ni(OH)2 core/shell nanorods on nickel foam via a facile hydrothermal and electrodeposition process for supercapacitor applications. The novel nanoscale morphology has been proven to be responsible for their excellent capacitive performances. Ni(OH)2 nanosheets were uniformly wrapped on the surface of each NiMoO4 nanowire, which increased the capacitance of NiMoO4@Ni(OH)2 core/shell nanorods to a high areal capacitance of 7.43 F cm−2 at 4 mA cm−2. 72% of the initial capacity was retained after 1000 cycles at a current density of 8 mA cm−2. These results indicate that the NiMoO4@Ni(OH)2 core/shell nanorods could be a promising electrode material for high-performance electrochemical capacitors.

1. Introduction

Over recent years, significant attention has been paid to develop clean and highly efficient electrochemical energy storage devices because of the increasingly energy demand and concerns about environmental issues.1–3 Electrochemical capacitors, also called supercapacitors, offering transient but extremely high powers, are probably the most important next-generation energy storage device.4 To date, various materials, including carbonaceous materials, transition metal oxides, conducting polymers, and hybrid composites, have been widely studied as electrodes for supercapacitors.5–8 Among them, Ni(OH)2 is an especially attractive transition hydroxide for electrodes due to its layered structure, low cost, high theoretical capacitance, and well-defined electrochemical redox activity.9–11 The development of nanostructured materials provides a promising solution to enhance the capacitive performance because of their high surface area, short electron and ion transport pathways.12 In the last three years, several authors have reported the efficiency of Ni(OH)2 as a electrode materials for the pseudocapacitors (PCs). A number of published papers in previous years show an increasing interest in the search for the possible applications of Ni(OH)2 as a electrode materials. However, the poor intrinsic conductivity of these oxide materials still limits their performance.13

To overcome the problem above, most efforts have been focused on using highly conductive materials (such as graphene and carbon nanotubes) as the backbone to support Ni(OH)2 materials, significantly enhancing the rate capability of the active materials by shortening the distance of electron transport.14 In this case, Ni(OH)2 nanostructure is directly attached to the conductive substrate, the poor conductivity of the electroactive material is no longer a big concern. Obviously, making the capacitance performance and rate capability of Ni(OH)2 to meet the practical application requirements is still a challenge because of a bottleneck caused by the low surface area of those substrate materials. Therefore, to optimize the electrochemical properties of Ni(OH)2, increasing its surface area and providing a reliable electrical conductivity pathway become essential criteria in designing high-performance electrodes for Ni(OH)2-based electrochemical supercapacitors.

Three dimensional (3D) hybrid nanostructures with large surface area and short diffusion path for electrons and ions are promising electrode architectures for high-performance supercapacitors. In this work, the NiMoO4@Ni(OH)2 core/shell nanorods on Ni foam were rationally designed and successfully developed by growing Ni(OH)2 nanosheets on NiMoO4 nanorods. As promising pseudocapacitive electrode materials, the NiMoO4 may also function as active materials for charge storage and make contribution to the total capacitance of 3D hybrid system.15–17 Moreover, this unique nanostructure electrode for pseudocapacitors showed high performance and excellent rate capability. The crystalline NiMoO4 nanorods uniformly grown on Ni foam were used as the backbone to support and provide reliable electrical connection to the Ni(OH)2 nanosheets coatings with high surface areas accessible to electrolyte, enabling full utilization of the Ni(OH)2 and fast electronic and ionic conduction through the electrode.

2. Experimental

2.1 Materials synthesis

All the reagents used in experiment were analytical grade and used without further purification. The nickel nitrate and sodium molybdate were obtained from Tianjin Chemical Reagent Co. prior to the synthesis.

2.2 Synthesis of NiMoO4 nanorods

The Ni foam (length × diameter × thickness = 4 × 1.5 × 0.05 cm) was first treated with 3 M HCl solution to remove the possible oxide layer on the surface. After the acid treatment, the Ni foam was cleaned by sonication in ethanol and deionized water in sequence for 5 min each. The NiMoO4 nanorods were synthesized by using a simple hydrothermal method. In a typical procedure, 0.2 g of Ni(NO3)2·6H2O and 0.17 g of Na2MoO4·2H2O were dissolved into 20.0 mL of deionized water under constant magnetic stirring, then the transparent light green solution was obtained. The solution was transferred to a 25 mL Teflon-lined stainless steel autoclave. The pretreated Ni foam was put in and kept at 150 °C for 6 h. After cooled down, the Ni foam coated with light green products was rinsed with water and ethanol several times, and dried in an oven at 60 °C for 12 h. Finally, the Ni foam was annealed at 400 °C for 1 h in pure nitrogen.

2.3 Synthesis of NiMoO4@Ni(OH)2 core/shell nanorods

NiMoO4@Ni(OH)2 core/shell nanorods were obtained by depositing Ni(OH)2 nanosheets onto the surface of NiMoO4 nanorods by anodic electrodeposition. The electrodeposition was conducted in a 100 mL of solution containing 1 g of nickelnitrate at −1.0 V for 420 s at room temperature. Ni(OH)2 sheets were deposited on NiMoO4 nanorods and Ni foam under the same conditions for comparison.

2.4 Materials characterization

The morphology is observed by field-emission scanning electron microscopy (FE-SEM; S-4800, Hitachi, Japan) equipped. The crystalline structure is characterized by X-ray powder diffraction (XRD, D/max2600, Rigaku, Japan) and transmission electron microscopy (TEM; FEI, Tecnai TF20).

2.5 Electrochemical measurements

The electrochemical measurements were carried out in a three-electrode electrochemical cell containing 2 M KOH aqueous solution as the electrolyte. NiMoO4@Ni(OH)2 core/shell nanorods–Ni foam, Ni(OH)2 nanosheets–Ni foam and NiMoO4 nanorods–Ni foam were directly used as working electrodes. The area of the working electrodes immersed into the electrolyte was controlled to be about 1 cm2. The mass loading mass loading of the Ni(OH)2@NiMoO4 hybrid nanorods on Ni foam were around 1.5 mg cm−2. The electrochemical measurements were conducted with a EC-Lab electrochemical workstation. A standard calomel electrode (SCE) was used as the reference electrode and a Pt foil as the counter electrode, and all the experiments were done at ambient temperature. The cyclic voltammograms were acquired in a potential range between −0.2 V and 0.7 V at different scan rates, and the charge–discharge processes were performed by cycling the potential from 0 to 0.4 V at different current densities. EIS measurements were performed by applying an AC voltage with 5 mV amplitude in a frequency range from 0.01 Hz to 100 kHz. The cyclic stability was evaluated by cyclic voltammetry measurements at a current density of 8 mA cm−2 for over 1000 cycles. The areal capacitance was calculated according to following equation:
 
image file: c5ra11960j-t1.tif(1)
where I was the constant discharge current (A), t was discharge time (s), V was the potential window (V), S was the geometrical area (cm2) of the electrode.

3. Results and discussion

The morphologies of the NiMoO4 nanorods and the NiMoO4@Ni(OH)2 core/shell nanorods on nickel foam were investigated by SEM. Fig. 1a and S1a† shows that NiMoO4 nanorods are uniformly grown upon the whole nickel foam surface. The magnified image reveals that they have an average diameter of 200 nm and length up to around 4 μm (Fig. 1b and S1b†). Afterward, the NiMoO4 nanorods serve as the backbone can provide a vast of sites for the growing of Ni(OH)2 nanosheets, resulting in a high mass loading of the Ni(OH)2 nanosheets. Fig. 1c and d show the typical SEM images of the NiMoO4@Ni(OH)2 core/shell nanorods on the substrate large scale. Fig. 1c displays a low-magnification SEM image of NiMoO4@Ni(OH)2 core/shell nanorods after electrodeposition. It shows that the integration of Ni(OH)2 nanosheets into the NiMoO4 nanorods does not destroy the ordered structure. Fig. 1d shows the high-magnification SEM image. Obviously, no Ni(OH)2 is packed in the interspace of the nanorods, suggesting that the Ni(OH)2 nanosheets are preferentially deposited on the surface of NiMoO4 nanorods. Therefore, the uniform coverage of Ni(OH)2 nanosheets on the surface of individual NiMoO4 nanorods can be seen. Such a unique hierarchical nanostructure has open and free interspaces among themselves, which ensures that electrolytes are highly accessible to all the electrode materials and could improve the utilization rate of electrode materials.
image file: c5ra11960j-f1.tif
Fig. 1 Typical SEM image of the NiMoO4 nanorods on Ni foam at (a) low and (b) high magnifications; typical SEM image of the NiMoO4@Ni(OH)2 core/shell nanorods on Ni foam at (c) low and (d) high magnifications.

The phase structure of the as-prepared products on Ni foam was studied by the XRD. Fig. 2a exhibits the typical power XRD patterns of the NiMoO4 nanorods and NiMoO4@Ni(OH)2 core/shell nanorods supported on Ni foam. The three strong peaks in the XRD patterns are typical peaks coming from the Ni foam. Unfortunately, no new peaks are observed in the XRD pattern of NiMoO4 nanorods–Ni foam and the NiMoO4@Ni(OH)2 core/shell nanorods–Ni foam, indicating the poor crystallinity of them.18 In order to preclude the strong impact of Ni foam substrate on the XRD peak signals, the NiMoO4 nanorods are scratched from Ni foam for XRD analysis. As indicated in Fig. 2a, the observed major diffraction peaks at 2θ = 14.3, 23.9, 25.3, 28.7, 32.6, 41.2, 43.7 and 47.4° for the sample are well matched with the standard monoclinic NiMoO4 (JCPDS no. 33-0948). The highest intensity diffraction peak (220) at 28.7° is the typical peak of α-NiMoO4. While the presence of the Ni(OH)2 nanosheets was not confirmed by the XRD pattern, which may be due to there only being a tiny amount, and it is confirmed from the HRTEM images.


image file: c5ra11960j-f2.tif
Fig. 2 XRD patterns of (a) the NiMoO4 nanorods and the NiMoO4@Ni(OH)2 core/shell nanorods on Ni foam; (b) XRD patterns of the NiMoO4@Ni(OH)2 core/shell nanorods scratched from Ni foam.

Fig. 3a shows the typical TEM image of an individual NiMoO4 nanorod. The nanorods have a relative smooth surface, which is consistent with SEM observation. The corresponding high-resolution TEM image in Fig. 3b shows a fringe spacing of 0.28 nm, which corresponds well to that of the lattice space of (022) of NiMoO4. The selected-area electron diffraction (SAED) pattern (Fig. 3b inset) from a single nanorod clearly demonstrates the high crystallinity of the NiMoO4 nanorod. Fig. 3c shows the TEM images of NiMoO4@Ni(OH)2 core/shell nanorods. It is interesting to note that the Ni(OH)2 nanosheets are aligned with their planes more or less perpendicular to the NiMoO4 nanorod. It is also clear that the NiMoO4@Ni(OH)2 core/shell nanorods exhibit a core/shell morphology, with NiMoO4 nanorods (the dark-colored region) in the core and Ni(OH)2 nanosheets (the light-colored region) coats on the surface. This image further confirms that the entire surface of the NiMoO4 nanorod is covered by the Ni(OH)2 nanosheets. Fig. 3d shows a HRTEM image of the NiMoO4 nanorod. The interplanar spacing is calculated to be 0.27 nm, corresponding to the (100) lattice planes. In addition, the energy dispersive X-ray spectrometry (EDX) analysis was conducted to confirm the composition distribution of the product and structure properties. The spatial distribution of the compositional elements within the heterostructures is obtained by using TEM-EDX line scans along the nanorod's radial direction (marked by the red line in Fig. 3e). There are three signal peaks of Ni, Mo and O as shown. The TEM-EDX mapping clearly shows that the strongest signals for Ni, Mo and O were found in the backbone region, whereas only Ni and O signals were observed in the shell region, confirming the NiMoO4@Ni(OH)2 core/shell structure.


image file: c5ra11960j-f3.tif
Fig. 3 (a) TEM image of NiMoO4 nanorods; (b) HRTEM images of the NiMoO4 nanorods and the SAED patterns of NiMoO4 nanorods; (c) TEM image of NiMoO4@Ni(OH)2 core/shell nanorods; (d) HRTEM images of the NiMoO4@Ni(OH)2 core/shell nanorods; (e) the representative EDX line scanning spectroscopy for a single NiMoO4@Ni(OH)2 core/shell nanorods.

The electrochemical performance of the NiMoO4@Ni(OH)2 core/shell nanorods which were evaluated as binder-free electrodes for supercapacitors. Fig. 4a shows the typical cyclic voltammograms (CVs) of the NiMoO4@Ni(OH)2 hybrid electrodes, which were acquired in a potential range between −0.2 V and 0.7 V at various sweep rates, ranging from 5 to 60 mV s−1. All the CV curves were consisted of a pair of strong redox peaks, suggesting that the capacitance characteristics were mainly governed by Faradaic redox reactions. The Faradic reactions correspond to the redox peaks are as the following:

 
Ni(II) ↔Ni(III) + e− (2)


image file: c5ra11960j-f4.tif
Fig. 4 (a) CV curves of the NiMoO4@Ni(OH)2 core/shell nanorods electrode at various scan rates; (b) galvanostatic charge–discharge curves of the NiMoO4@Ni(OH)2 core/shell nanorods electrode at various current densities.

In other words, the electrochemical capacitance of Ni(OH)2@NiMoO4 is attributed to the quasi reversible electron transfer process that mainly involves the Ni2+/Ni3+ redox couple, and probably mediated by the OH− ions in the alkaline electrolyte.19 The Mo atoms do not involve any redox reactions, thereby, the redox behavior of Mo has no contribution to the measured capacitance. The main function of Mo element was to improve the conductivity of metal molybdates and then to achieve the enhanced electrochemical capacitance.20,21 The peak current increased linearly with the increment of the scan rate, suggesting that the kinetics of the interfacial Faradic redox reactions and the rates of electronic and ionic transportation were rapidly enough in the present scan rates.19,22 The shape of the CV curves is not significantly influenced by the increase of the scan rates, indicating the improved mass transportation and electron conduction in the host materials.23

The galvanostatic charge–discharge measurements were further performed in Fig. 4b in the voltage range between 0 to 0.4 V at various current densities. Consistent with the CV results, the nonlinearity of the curves indicates the existence of Faradaic processes. From the galvanostatic discharge curves, the areal capacitance was calculated by the eqn (1). Impressively, at a higher current density of 4, 8, 16, 24, 32, 40, 56 and 112 mA cm−1, the Ni foam supported NiMoO4@Ni(OH)2 core/shell nanorods showed high areal capacitance of 7.43, 5.63, 5.56, 5.16, 4.81, 4.57, 4.19 and 3.06 F cm−1, respectively, which further demonstrated the great rate capability of the Ni(OH)2@NiMoO4 core/shell nanorods. The areal capacitance gradually decreased with the increment of current density due to the incremental voltage drop and insufficient active material involved in redox reaction at a higher current density.19 However, the calculated capacitance was as high as 7.43 F cm−1, measured at the discharge current density of 4 mA cm−1. Such high areal capacitance at large current densities was much higher than those reported materials, such as nanoscale β-NiMoO4–CoMoO4·xH2O composites,24 Ni(OH)2@NiCoO4 hybrid composite,25 MnO2–NiO nanoflakes26 and NiCo2O4@MnO2 core/shell heterostructured nanowire arrays.27

Fig. 5a shows the CVs collected from NiMoO4@Ni(OH)2, NiMoO4, Ni(OH)2 and pure Ni foam electrodes, which were acquired in a potential range between −0.2 V and 0.7 V at a scan rate of 40 mV s−1. The response current of the pure Ni foam is quite weak, which is negligible in contrast with that of others. Compared with the pristine NiMoO4 and Ni(OH)2 electrode, the higher peak current values and the larger separation between leveled anodic and cathodic currents at the same scan rates for the hybrid array indicate its larger capacitances. As shown in Fig. 5b, galvanostatic charge–discharge measurements of NiMoO4@Ni(OH)2, NiMoO4 and Ni(OH)2 electrodes are further performed in the voltage range between 0 and 0.4 V at the current density of 8 mA cm−1. Evidently, the NiMoO4@Ni(OH)2 core/shell nanorods deliver higher capacitances than NiMoO4 nanorods and Ni(OH)2 nanosheets (Fig. 5c). Compared with the pristine NiMoO4 electrode, the NiMoO4@Ni(OH)2 hybrid electrodes have larger capacitance attributed to the additional pseudocapacitance contributed by the Ni(OH)2 shell, which can provide Ni2+ on the electrode surface. Compared with the pristine Ni(OH)2 electrode, larger areal capacitance of the hybrid electrodes with the reaction times can be attributed to the NiMoO4 core providing the open space, which allows ion easy diffusion and loads more shell material Ni(OH)2 nanosheets. And, the Ni(OH)2 nanosheets on NiMoO4 core with robust adhesion provide fast electron-transport access to the current collector, enhancing the rate capability. Moreover, each NiMoO4 nanorods can contribute to the better capacity of supercapacitors. In addition, the long-term cycle stability of the Ni(OH)2, the NiMoO4 and the Ni(OH)2@NiMoO4 hybrid electrodes were also investigated by repeating the chronopotentiometry (CP) tests at the current density of 8 mA cm−1 for 1000 cycles, as shown in Fig. 5d. As comparison, the areal capacitance of the pristine NiMoO4 electrode is about 2.20 F cm−2 after 1000 cycles (about 82% retention). For NiMoO4@Ni(OH)2 hybrid electrodes, the areal capacitance gradually decreased with the increase of the cycle number, and 72% of the initial areal capacitance remained after 1000 cycles. However, the capacitance loss occurred mainly during the first 500 cycles, which is due to the Ni(OH)2 with inferior stability. After this stage, the electrode shows a relatively good stability. The areal capacitance is about 4.05 F cm−2 after 1000 cycles was much higher than the pristine NiMoO4 electrode. The areal capacitance of the Ni(OH)2 electrodes only is about 33% retention after 1000 cycles. Moreover, the CP tests of the Ni(OH)2@NiMoO4 hybrid electrodes at the current density of 20, 30, 40 and 50 mA cm−1 for 1000 cycles, as shown in Fig. S2.† In addition, the direct growth of NiMoO4 nanowires on a conductive substrate could ensure good mechanical adhesion, and more importantly, good electrical connection with the conductive substrate that also serves as the current collector in such binder-free electrodes. After charging–discharging for 1000 cycles, the morphology and structure of the Ni(OH)2@NiMoO4 arrays can be largely retained with only slight aggregation observed (Fig. S3 and S4†).


image file: c5ra11960j-f5.tif
Fig. 5 (a) CV curves of the NiMoO4 nanorods electrode, the Ni(OH)2 nanosheets electrode, the pure Ni foam and the NiMoO4@Ni(OH)2 core/shell nanorods electrode at a scan rate of 40 mV s−1; (b) galvanostatic charge–discharge curves of the NiMoO4 nanorods electrode, the Ni(OH)2 nanosheets electrode and the NiMoO4@Ni(OH)2 core/shell nanorods electrode at a current density of 8 mA cm−2; (b) areal capacitances of the NiMoO4 nanorods electrode, Ni(OH)2 nanosheets electrode and NiMoO4@Ni(OH)2 core/shell nanorods electrode at various current densities; (d) the cycling performance of the NiMoO4 nanorods electrode, Ni(OH)2 nanosheets electrode and NiMoO4@Ni(OH)2 core/shell nanorods electrode at a current density of 8 mA cm−2.

For further elucidate the origin of excellent electrochemical performance of NiMoO4@Ni(OH)2 hybrid electrodes, electrochemical impedance spectrum (EIS) was carried out to reveal the reason. Fig. 6 shows the impedance Nyquist plots of the NiMoO4, the NiMoO4@Ni(OH)2 and the NiMoO4@Ni(OH)2 after 1000 GCD cycles hybrid electrodes. All the impedance spectra are almost the same, being compose of one semicircle component at high-frequency followed by a linear component at low-frequency.28 The internal resistance (Rb), which includes the ionic resistance of the electrolyte, the intrinsic resistance of the active materials and the contact resistance at the active material/current collector interface, can be obtained from the intercept of the plot with the real axis in the high-frequency region.29,30 The observed Rb value for these electrodes only is about 0.5 ohm (Fig. 6). The diameter of the semicircle corresponds to the charge-transfer resistance (Rct) caused by the Faradaic reactions and the double-layer capacitance (Cdl) on the grain surface.31 The NiMoO4@Ni(OH)2 hybrid electrodes exhibits a lower the diameter of the semicircle, suggesting a lower charge-transfer resistance (Rct). In low frequency area, the slope of the curve shows the Warburg impedance (ZW) which represents the electrolyte diffusion in the porous electrode and proton diffusion in the host material.32 The Ni(OH)2@NiMoO4 hybrid electrodes has a more ideal vertical line than Ni(OH)2 nanosheets, which demonstrates that it has the lower diffusion resistance.


image file: c5ra11960j-f6.tif
Fig. 6 Electrochemical impedance spectra (EIS) of the NiMoO4 nanorods electrode, the NiMoO4@Ni(OH)2 core/shell nanorods and the NiMoO4@Ni(OH)2 after 1000 GCD cycles hybrid electrodes.

4. Conclusions

In summary, we have demonstrated a facile and scalable strategy for the direct growth of NiMoO4@Ni(OH)2 core/shell nanorods on Ni foams via hydrothermal reaction and electrochemical deposition, and its electrochemical properties were investigated. Compared with the pure NiMoO4 nanorods and Ni(OH)2 nanosheets, the NiMoO4@Ni(OH)2 core/shell nanorods exhibited higher electrical capacity reflected by an extremely large capacitance, low resistance, high cycling stability and capacitance retention, and good rate capability. All these merits are important for the practical application in supercapacitors.

Acknowledgements

This work was partially supported by the Natural Science Foundation of China (no. 51472066 and 51402076), the Natural Science Foundation of Heilongjiang Province (ZD201112 and QC2014C056).

Notes and references

  1. G. F. Chen, Z. Q. Liu, J. M. Lin, N. Li and Y. Z. Su, J. Power Sources, 2015, 283, 484 CrossRef CAS PubMed.
  2. Q. F. Wang, J. Xu, X. F. Wang, B. Liu, X. J. Hou, G. Yu, P. Wang, D. Chen and G. Z. Shen, ChemElectroChem, 2013, 1, 1–7 CrossRef PubMed.
  3. Z. Q. Liu, K. Xiao, N. Li, Y. Z. Su, H. J. Wang and S. Chen, RSC Adv., 2013, 3, 4372 RSC.
  4. G. F. Chen, Y. Z. Su, P. Y. Kuang, Z. Q. Liu, D. Y. Chen, X. Wu, N. Li and S. Z. Qiao, Chem.–Eur. J., 2015, 21, 4614 CrossRef CAS PubMed.
  5. Y. M. Chen, J. H. Cai, Y. S. Huang, K. Y. Lee and D. S. Tsai, Nanotechnology, 2011, 22, 115706 CrossRef CAS PubMed.
  6. W. F. Wei, X. W. Cui, W. X. Chen and D. G. Ivey, Chem. Soc. Rev., 2011, 40, 1697 RSC.
  7. D. P. Cai, B. Liu, D. D. Wang, L. L. Wang, Y. Liu, H. Li, Y. R. Wang, Q. H. Li and T. H. Wang, J. Mater. Chem. A, 2014, 10, 1039 Search PubMed.
  8. X. Y. Cao, X. Xing, N. Zhang, H. Gao, M. Y. Zhang, Y. C. Shang and X. T. Zhang, J. Mater. Chem. A, 2015, 3, 3785 CAS.
  9. Y. Z. Su, K. Xiao, N. Li, Z. Q. Liu and S. Z. Qiao, J. Mater. Chem. A, 2014, 2, 13845 CAS.
  10. Z. H. Pu, Q. Liu, A. H. Qusti, A. M. Asiri, A. O. Al-Youbi and X. P. Sun, Electrochim. Acta, 2013, 109, 252 CrossRef CAS PubMed.
  11. J. Q. Tian, Z. C. Xing, Q. X. Chu, Q. Liu, A. M. Asiri, A. H. Qusti, A. O. Al-Youbi and X. P. Sun, CrystEngComm, 2013, 15, 8300 RSC.
  12. X. H. Xia, J. P. Tu, Y. Q. Zhang, Y. J. Mai, X. L. Wang, C. D. Gu and X. B. Zhao, J. Phys. Chem. C, 2011, 115, 22662 CAS.
  13. C. H. Tang, X. S. Yin and H. Gong, ACS Appl. Mater. Interfaces, 2013, 5, 10574 CAS.
  14. Y. X. Xu, X. Q. Huang, Z. Y. Lin, X. Zhong, Y. Huang and X. F. Duan, Nano Research, 2013, 6, 65–76 CrossRef CAS.
  15. B. Senthilkumar, K. Sankar, R. Selvan, M. Danielleb and M. Manickamb, RSC Adv., 2013, 3, 352 RSC.
  16. D. P. Cai, D. D. Wang, B. Liu, Y. R. Wang, Y. Liu, L. L. Wang, H. Li, H. Huang, Q. H. Li and T. R. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 12905 CAS.
  17. W. J. Ren, D. Guo, M. Zhuo, B. K. Guan, D. Zhang and Q. H. Li, RSC Adv, 2015, 5, 21881 RSC.
  18. D. P. Cai, B. Liu, D. D. Wang, Y. Liu, L. L. Wang, H. Li, Y. R. Wang, C. X. Wang, Q. H. Li and T. H. Wang, Electrochim. Acta, 2014, 125, 294 CrossRef CAS PubMed.
  19. D. Guo, P. Zhang, H. M. Zhang, X. Z. Yu, J. Zhu, Q. H. Li and T. H. Wang, J. Mater. Chem. A, 2013, 1, 9024 CAS.
  20. L. Q. Mai, F. Yang, Y. L. Zhao, X. Xu, L. Xu and Y. Z. Luo, Nat. Commun., 2011, 5, 11159 Search PubMed.
  21. X. Z. Yu, B. G. Lu and Z. Xu, Adv. Mater., 2013, 4, 148 Search PubMed.
  22. D. Ghosh, S. Giri and C. Kumai Das, Nanoscale, 2013, 5, 10428 RSC.
  23. M. C. Liu, L. B. Kong, C. Lu, X. J. Ma, X. M. Li, Y. C. Luo and L. Kang, J. Mater. Chem. A, 2013, 1, 1380 CAS.
  24. B. Senthilkumar, D. Meyrick, Y. S. Leec and K. Selvan, RSC Adv., 2013, 3, 16542 RSC.
  25. L. Huang, D. C. Chen, Y. Ding, Z. L. Wang, Z. Z. Zeng and M. L. Liu, ACS Appl. Mater. Interfaces, 2013, 5, 11159 CAS.
  26. J. P. Liu, J. Jiang, M. Bosmanc and H. J. Fan, J. Mater. Chem., 2012, 22, 2419 RSC.
  27. L. Yu, G. Q. Zhang, C. Z. Yuan and X. W. (David) Lou, Chem. Commun., 2013, 49, 137 RSC.
  28. D. P. Cai, D. D. Wang, B. Liu, L. L. Wang, Y. Liu, H. Li, Y. R. Wang, Q. H. Li and T. H. Wang, ACS Appl. Mater. Interfaces, 2014, 6, 5050–5055 CAS.
  29. L. B. Kong, J. W. Lang, M. Liu, Y. C. Luo and L. Kang, J. Power Sources, 2009, 194, 1194–1201 CrossRef CAS PubMed.
  30. Q. Zhang, Y. H. Deng, Z. H. Hu, Y. F. Liu, M. M. Yao and P. P. Liu, Phys. Chem. Chem. Phys., 2014, 16, 23451 RSC.
  31. B. Senthilkumar and R. Selvan, J. Colloid Interface Sci., 2014, 426, 280 CrossRef CAS PubMed.
  32. W. Tian, X. Wang, C. Y. Zhi, T. Y. Zhai, D. Q. Liua, C. Zhang, D. Golberg and Y. Bando, Nano Energy, 2013, 2, 754 CrossRef CAS PubMed.

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11960j
This journal is © The Royal Society of Chemistry 2015
Click here to see how this site uses Cookies. View our privacy policy here.