Influence of annealing temperature on microstructure and lithium storage performance of self-templated Cu_xCo_3−xO₄ hollow microspheres

Abstract

Spinel Cu_xCo_3−xO₄ (x ≤ 0.30) hollow microspheres have been readily prepared via a self-templated solvothermal reaction followed by a thermal annealing step. Scanning electron microscopy and transmission electron microscopy images show that the as-prepared hollow microspheres possess an average diameter of ∼450 nm and a compact, thin, polycrystalline shell with an average thickness of ∼50 nm. When used as an anode material for lithium ion batteries, Cu_xCo_3−xO₄ hollow microspheres exhibit a high lithium storage capacity, which is strongly dependent on the annealing temperature of the Cu_xCo_3−xO₄ intermediate. As a result, Cu_xCo_3−xO₄ hollow microspheres annealed at 400 °C deliver a reversible discharge specific capacity as high as 1187 mA h g⁻¹ at a current density of 100 mA g⁻¹ after 50 cycles. Such superior lithium storage performance is derived from the particular microstructure of the shell, which is composed of highly close-connected fine nanoparticles, as well as the resultant better electronic conductivity.

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1. Introduction

Ternary transition metal oxides have attracted extensive attention for use as anode materials in lithium ion batteries (LIBs) owing to the stabilization of active phases and the synergistic interactions between two different metal oxides during the lithiation/delithiation process.^1–9 For example, nanosized CuCo₂O₄ shows high reversible specific capacities due to the complementarity and synergy occurring between two distinct components Cu²⁺ and Co²⁺ during the Li⁺ charge–discharge cycling process.^10–12 However, some knotty issues still exist, such as the large volume expansions and the rapid declining of reversible capacities. Large volume changes appear during the Li⁺ insertion and extraction process, which results in pulverizations of the electrodes and detachment of the active materials from the current collector. Therefore, considerable efforts have been devoted to design novel nanostructures so as to improve cycling stabilities and rate performances of the anodes.

Owing to the large surface areas, sophisticated interior architectures, and controlled shapes, hollow nanostructures have drawn increasing attentions.^13–19 The large surface areas of hollow nanostructures can raise more active sites to enable better access to lithium ions as a result of the increased electrode–electrolyte contact areas.²⁰ The hollow interior provides additional free volumes to alleviate the structural strain/stress during the repeated Li⁺ insertion/extraction processes, which leads to the improved cycling stabilities.^21,22 With that, numerous efforts have been made to produce a variety of hollow spheres, such as Zn_xFe_3−xO₄,²³ MnCo₂O₄,²⁴ Co_xMn_2−xO₄,²⁵ NiCo₂O₄,²⁶ CdMo₂O₄,²⁷ ZnMn₂O₄ (ref. 28) and so on. However, hollow-structured CuCo₂O₄ or Cu_xCo_3−xO₄ have never been reported even though various nanostructured CuCo₂O₄ or Cu_xCo_3−xO₄ have been studied as the anode materials for LIBs,^29–31 e.g., nanoparticles,^11,32,33 nanosheets,³⁴ nanowires,^30,31,35 porous nanocubes,²⁹ porous polyhedrons,³⁶ mesoporous nanostructures,¹² etc.

Herein, spinel Cu_xCo_3−xO₄ (x ≤ 0.30) hollow microspheres have been readily synthesized through a simple self-templated solvothermal method followed by a thermal annealing step. When used as anode materials in LIBs, the as-prepared Cu_xCo_3−xO₄ hollow microspheres exhibit the superior lithium storage capacities and the excellent cyclabilities. Effects of annealing temperature on microstructures and electrochemical properties of Cu_xCo_3−xO₄ hollow microspheres have been investigated in detail.

2. Experimental

2.1 Synthesis

All the reagents were of analytical grade. In a typical synthesis, 51 mg of Cu(NO₃)₂·3H₂O and 245 mg of Co(NO₃)₂·6H₂O were dissolved in 66.6 ml of isopropanol. Then, 13.4 ml of glycerol was added to the above solution under stirring to form a clear solution. Subsequently, the resultant solution was transferred into a 100 ml Teflon-lined stainless steel autoclave and kept at 180 °C for 6 h. After cooled to room temperature, the pink intermediate products, which were collected by centrifugation and washing with ethanol for several times, were dried at 60 °C for 12 h. Lastly, the intermediate products were annealed in air at 300, 400, 500 or 600 °C (denoted as CCO-300, CCO-400, CCO-500 and CCO-600, respectively) for 2 h with a temperature ramp rate of 1 °C min⁻¹.

2.2 Characterizations

X-ray diffraction (XRD) patterns were collected using a Maxima 7000 diffractometer (Shimadzu, Japan) with a Cu Kα radiation (40 kV, 100 mA, λ = 1.5418 Å). Scanning electron microscope (SEM) images and the energy-dispersive X-ray spectroscopy (EDAX) were measured by a Phillips XL 30 FEG SEM (Phillips, Eindhoven, Netherlands). Transmission electron microscope (TEM) was performed on a JEM-2100 (Japan). Nitrogen adsorption–desorption measurements were conducted on ASAP 2020 (Micrometrics, USA). Thermogravimetric analysis (TGA) was tested using a METTLER SF/1382 under a flow of air with a temperature ramp of 10 °C min⁻¹.

2.3 Electrochemical measurements

The slurry of working electrode was fabricated by mixing the active materials, carbon black (Super-P-Li) and sodium carboxymethyl cellulose (CMC) at a weight ratio of 70 : 20 : 10 into distilled water, which was uniformly casted onto copper foils. After drying under vacuum at 70 °C overnight, the film was cut into circular electrodes with an area of 1.96 cm². The average loading amount of active materials was kept at ca. 0.49 mg cm⁻² in the electrodes. The electrolyte was formed by dissolving 1 M LiPF₆ in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1 : 1 in volume). Coin-type cells were assembled in an argon filled glove box (LS800S, Dellix, China) for electrochemical measurements. The galvanostatic charging–discharging measurements were carried out on a battery tester (Land CT2001A, China) within a voltage range of 0.01–3.0 V at various current densities. Cyclic voltammetry (CV) measurements were performed on an electrochemical workstation (CHI 660B, Shanghai, China). Electrochemical impedance measurements (EIS) were taken on CHI 660D (Shanghai, China) under a frequency range from 0.1 Hz to 1.0 × 10⁵ Hz.

3. Results and discussion

The XRD patterns of Cu_xCo_3−xO₄ hollow microspheres were shown in Fig. 1. The distinct diffraction peaks, centered at 31.3°, 36.9°, 38.6°, 44.9°, 59.4° and 65.3°, could be indexed to (220), (311), (222), (400), (511) and (440) crystal planes of spinel Cu_xCo_3−xO₄ phase (JCPDS card no. 25-0270). No diffraction peaks from other phases were detected in the whole patterns, except CCO-600 exhibited a small amount of CuO as a secondary phase,³⁷ which indicated the easy formation of phase pure Cu_xCo_3−xO₄ below 600 °C. In addition, the intensities of diffraction peaks gradually increased with an increase of the annealing temperatures. According to the Scherrer's equation,¹² the average crystallite sizes of all the samples were estimated and listed in Table 1. Obviously, CCO-600 possessed the largest crystallite size of 29.2 nm while grain sizes of the other samples kept in the same order of magnitude. But, CCO-400 had a slightly smaller value of 15.2 nm.

Fig. 1 XRD patterns of CCO-300, CCO-400, CCO-500, and CCO-600.

Table 1 Compositions, grain sizes, specific surface areas, mean pore sizes and pore volumes of CCO-300, CCO-400, CCO-500, and CCO-600

Sample no.	CuxCo3−xO4 (x ≤ 0.30)				Mean grain size (nm)	Specific surface area (m^2 g^−1)	Mean pore size (nm)	Pore volume (cm^3 g^−1)
	Cu	Co	O	x
CCO-300	2.89	36.18	60.93	0.22	17.0	40.37	8.87	0.12
CCO-400	3.51	46.70	49.79	0.21	15.2	30.40	10.33	0.11
CCO-500	3.38	39.86	56.76	0.23	17.6	9.44	7.84	0.04
CCO-600	3.48	33.78	62.74	0.28	29.2	8.70	5.89	0.03

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Morphologies and compositions of the as-prepared Cu_xCo_3−xO₄ hollow microspheres were characterized by TEM, SEM and EDAX. The typical SEM image of CCO-400 in Fig. 2a proved the spherical structure with an average diameter of ∼450 nm, and the rough surface which was composed of fine nanoparticles. It was also found that a fraction of microspheres got inter-connected or partially merged together, which might favor the electronic conductivities among microspheres in the electrodes. EDAX analysis of CCO-400 in Fig. 2b manifested the existence of Cu, Co and O elements. The x values of all the samples were similar and less than 0.3 (Table 1).^38–40 The slightly higher x value of CCO-600 should be due to the impurity of CuO detected by XRD. The representative TEM images of the as-prepared Cu_xCo_3−xO₄ hollow microspheres in Fig. 2c–f illustrated a pronounced contrast between the dark edges and the pale internals, which provided convincing evidences of the hollow interior structure. All the shells of the as-prepared Cu_xCo_3−xO₄ hollow microspheres showed a similar average thickness of ∼50 nm but consisted of close-packed nanoparticles with completely different sizes. By observing the edges of the microspheres, CCO-300 (Fig. 2c) and CCO-400 (Fig. 2d) displayed quite fine nanoparticles, whose sizes were only ∼15 nm. When the annealing temperature rose to 500 °C, the particle sizes on the edges grew up to ∼25 nm, slightly larger than the calculated crystallite size of 17.6 nm by XRD, which should result from the size difference between primary and secondary particles. If the annealing temperature was further raised to 600 °C, the particle size would dramatically increase to 23–60 nm (Fig. 2f for CCO-600), which was comparable to the XRD result. In the meanwhile, lots of voids (white or light regions) were discovered in CCO-500 and CCO-600. By comparison, the fine nanoparticles in the shells of CCO-300 and CCO-400 dispersed more uniformly, and more importantly, packed more closely, which would be very beneficial to improve the electronic conductivity during the repeated lithiation/delithiation process.

Fig. 2 (a) SEM image and (b) EDAX spectrum of CCO-400; TEM images of (c) CCO-300, (d) CCO-400, (e) CCO-500, and (f) CCO-600.

Nitrogen sorption measurements were conducted to quantitatively investigate the specific surface area and porosity of the as-prepared Cu_xCo_3−xO₄ hollow microspheres. Fig. 3 indicated that the adsorption–desorption isotherms of all the samples were type IV. As summarized by Table 1, the specific surface areas of CCO-300 and CCO-400 were much larger than that of CCO-500 and CCO-600. Meanwhile, the specific surface areas kept a decreasing trend with raising the annealing temperatures owing to the shrinkage of the intermediates during the thermal annealing step. The isotherms of CCO-300 and CCO-400 showed distinct hysteresis loops at a high relative pressure, indicating existences of the mesoporous structures and the relatively large pore volumes (Table 1), which derived from plenty of the inter-microsphere voids between the adjacent dense shells, while the isotherms of CCO-500 and CCO-600 displayed little hysteresis loop, indicating extremely less pore volumes (Table 1) due to that the loose shells of the microspheres formed at the relatively high annealing temperatures could hardly hold the voids.

Fig. 3 N₂ adsorption–desorption isotherms of (a) CCO-300, (b) CCO-400, (c) CCO-500, and (d) CCO-600 (all the insets showed the corresponding pore size distributions).

The TG/DTG curves of the Cu–Co glycerate microspheres were recorded from room temperature to 800 °C under air flow with a temperature ramp of 10 °C min⁻¹. Fig. 4 exhibited two distinct weight loss steps. The first 3.52% weight loss below 150 °C was ascribed to evaporation of moisture and solvents such as isopropanol. The following large weight loss of 39.68% occurs between 150 °C and 304 °C, which was attributed to the decomposition of the Cu–Co glycerate intermediates into Cu_xCo_3−xO₄. Above 304 °C, there was no obvious weight loss step, which hinted that a completed conversion of the Cu–Co glycerate intermediates to Cu_xCo_3−xO₄ at 300 °C for 2 h with a temperature ramp rate of 1 °C min⁻¹ was attainable.

Fig. 4 Thermal analysis of Cu–Co glycerate microspheres under air flow with a temperature ramp of 10 °C min⁻¹.

According to the analyses above, microstructures of the as-prepared Cu_xCo_3−xO₄ hollow microspheres strongly depended on the annealing temperature of the intermediate. In a typical hydrothermal process, Cu²⁺ and Co²⁺ reacted with glycerol to form the immediate products, solid Cu–Co glycerate microspheres (not shown here).⁴¹ Subsequently, the hollow microspheres were easily obtained by a nonequilibrium thermal annealing step.²⁶ With the increase of the annealing temperature, the grains in the shells would aggregate and grow up due to the Ostwald ripening mechanism,^42,43 which in turn accounted for the influence of annealing temperature on microstructures of Cu_xCo_3−xO₄ hollow microspheres.

The CV curves for the electrode of CCO-400 were acquired by the reversible deintercalation and intercalation of Li-ions (Fig. 5). In the first cathodic cycle, a prominent reduction peak at 0.5–0.8 V was attributed to the formation of solid electrolyte interphase (SEI) films, accompanied by the reductions of Cu_xCo_3−xO₄ into metallic Cu and Co embedded in a Li₂O matrix.^34,44 During the anodic scans, the CV curve exhibited a broad oxidation peak at 2.25 V, which was ascribed to the conversion reactions of Cu atom to CuO and Co atom to CoO. This phenomenon suggested that Cu and Co embedded in Li₂O matrix generated CuO and CoO instead of Cu_xCo_3−xO₄ after charging, which was consistent with the previous reports.^11,12,29,34 In the following cycles, it could be observed that the reduction peak moved to about 1.0 V, which was different from the irreversible electrochemical reaction during the first discharge cycle. The CV curves illustrated that the electrode of CCO-400 would show good stability and cyclability during the continuous cycling. Combined with the CV analyses, the entire electrochemical process of the as-prepared Cu_xCo_3−xO₄ hollow microspheres could be assumed as follows:^10,12,45

Cu_xCo_3−xO₄ + 8Li⁺ + 8e⁻ → xCu + (3 − x)Co + 4Li₂O (1)

Cu + Li₂O ↔ CuO + 2Li⁺ + 2e⁻ (2)

2Co + 2Li₂O ↔ 2CoO + 4Li⁺ + 4e⁻ (3)

Fig. 5 Cyclic voltammetry curves for the electrode of CCO-400 in the potential window between 0.005 and 3 V at a rate of 0.1 mV s⁻¹.

The profiles of voltage vs. capacity were analyzed to learn about the electrochemical performance. The 1^st, 2^nd, 20^th and 50^th cycles for the electrode of CCO-400 at a constant current density of 0.1C (1C = 1000 mA g⁻¹ = 0.49 mA cm⁻²) were shown in Fig. 6a. The electrode of CCO-400 displayed a wide discharge voltage plateau at ∼1.2 V in the first cycle, and then the discharge voltage plateau moved to ∼1.4 V in the subsequent cycles. The electrode of CCO-400 delivered the initial discharge and charge capacity of 980 and 950 mA h g⁻¹, respectively. The relatively low discharge voltage plateau and capacity in the first cycle, might be closely related to the formation of SEI films and the decomposition of electrolyte.^21,46

Fig. 6 (a) Voltage vs. capacity profiles of CCO-400 at a current density of 0.1C; (b) discharge capacities at current density of 0.1C and the coulombic efficiency of the CCO-400; (c) rate capability at current densities of 0.1, 0.2, 0.5, 1 and 2C (1C = 1 A g⁻¹); (d) discharge capacities of CCO-400 at a current density of 5 A g⁻¹. SEM images for the electrodes of CCO-400: (e) before cycling; (f) after 50 cycles at 0.1C.

In order to identify lithium storage properties, the electrode of Cu_xCo_3−xO₄ hollow microspheres were measured at 0.1C for 50 cycles as an anode material for LIBs (Fig. 6b). It was found that the electrode of CCO-400 had delivered the highest specific capacity for the initial 50 cycles. Surprisingly, the specific capacity of CCO-400 gradually increased to 1196 mA h g⁻¹ after 50 cycles, and such increasing trend occurred for all the electrodes. These interesting phenomena could be explained as follows. On one hand, the hollow nanostructure could provide large specific surface areas to accommodate more active sites as a result of the increased electrode–electrolyte contact areas. Furthermore, the hollow interior feature possessed additional free volumes to alleviate volume changes associated with repeated Li⁺ insertion/extraction processes.^42,43 On the other hand, the reversible decomposition of the electrolyte to form the SEI films and extra lithium ion adsorption/desorption on the SEI films also resulted in the high experimental lithium storage capacity during cycling.^42,45,47,48 Meanwhile, the stable coulombic efficiencies of CCO-400 approaching 100% were observed for cycles from 3 to 50 (Fig. 6b). Such very high coulombic efficiency was attributed to the thorough delithiation process and the resultant minimal irreversible process due to the unique hollow structure of Cu_xCo_3−xO₄ microspheres.^20,42,49

The rate capability was investigated at various current densities ranging from 0.1C to 2C (Fig. 6c). It was clearly observed that the rate capacities for the electrode of CCO-400 remained overwhelmingly superior at various current densities. At the current densities of 0.1, 0.2, 0.5, 1 and 2C, the final specific discharge capacities of CCO-400 kept at 1061, 1053, 982, 878 and 761 mA h g⁻¹ respectively. When the current density was recovered to 0.1C, the discharge capacity also returned to 1173 mA h g⁻¹. In comparison, the rate capacities of CCO-300, CCO-500 and CCO-600 were much lower than that of CCO-400.

To further explore the cyclability at a high current density, the electrode of CCO-400 was measured at a current density of 5C (Fig. 6d). It delivered the initial discharge capacity of 520 mA h g⁻¹, while the value rapidly decreased to 395 mA h g⁻¹ at the 17^th cycle. After that, the capacity quickly increased to 470 mA h g⁻¹ at the 27^th cycle. Again, the capacity decreased to 382 mA h g⁻¹ at the 55^th cycle, and then increased to 452 mA h g⁻¹ at the 84^th cycle. Later, a long decreasing trend maintained to 176^th cycle with a capacity of 267 mA h g⁻¹. From the 177^th cycles on, the capacity showed a relatively mild variation trend with two distinct increasing tendencies. After 550 cycles, the electrode of CCO-400 still remained an impressive discharge capacity of 350 mA h g⁻¹, which was comparable to the theoretical maximum of graphite (372 mA h g⁻¹).

To examine if Cu_xCo_3−xO₄ hollow microspheres were fragile during processing into electrodes, the anodes of CCO-400 were directly viewed by SEM. The spherical morphologies of CCO-400 before cycling were clearly observed in Fig. 6e. The microspheres kept intact even after 50 cycles (Fig. 6f), which meant that the as-prepared Cu_xCo_3−xO₄ hollow microspheres were quite robust and would not break off during processing into electrodes and during cycling.

To unveil the charge transfer mechanisms of the as-prepared Cu_xCo_3−xO₄ hollow microspheres, electrochemical impedance measurements (EIS) were carried out. As observed from the Nyquist plots (Fig. 7), the typical sloping lines in the low frequency region were assigned to the Li⁺ ion diffusion impedance in the electrodes (R_s), while the depressed semicircles in the high to medium frequency regions, corresponded to the charge-transfer impendence between the electrode–electrolyte interface (R_ct). Based on the Nyquist plots, the values of R_ct for CCO-300, CCO-400, CCO-500 and CCO-600 were determined to be 186, 165, 211, 248 Ω, respectively. Therefore, the electrode of CCO-400 exhibited the smallest charge transfer resistance, suggesting the most efficient charge transfer in LIBs. In combination with the previous results of characterizations, it was concluded that the excellent lithium storage performance of CCO-400 was derived from the most efficient charge transfer as a result of the more compact shell and the larger specific surface area than CCO-500 and CCO-600, as well as the higher crystallinity than CCO-300. All in all, the annealing temperature played a considerably critical role in the microstructure and lithium storage performance of the self-templated Cu_xCo_3−xO₄ hollow microspheres.

Fig. 7 EIS for the electrodes of CCO-300, CCO-400, CCO-500, CCO-600 at the open circuit voltage.

4. Conclusions

In summary, Cu_xCo_3−xO₄ hollow microspheres had been readily synthesized through a simple solvothermal strategy after the annealing process. The as-prepared hollow microspheres exhibited an average diameter of ∼450 nm and a compact, thin, polycrystalline shell with an average thickness of ∼50 nm. As an anode material for LIBs, Cu_xCo_3−xO₄ hollow microspheres annealed at 400 °C delivered a specific discharge capacity of ∼1181 mA h g⁻¹ after 50 cycles at a current density at 100 mA g⁻¹. Even cycled at a high discharge capacity of 5 A g⁻¹, it still retained an impressive discharge capacity of 350 mA h g⁻¹ after 550 cycles. Such superior lithium storage performance should benefit from the compact shell with relatively large specific surface area and the relatively high crystallinity, which resulted in the excellent electronic conductivity.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51402242), and the Cultural Program for Young Talents of Science and Technology in Innovating New Products from CQ CSTC (CSTC2013KJRC-QNRC50001).

References

1. A. La Rosa-Toro, R. Berenguer, C. Quijada, F. Montilla, E. Morallón and J. L. Vázquez, J. Phys. Chem. B, 2006, 110, 24021–24029.
2. U. Chellam, Z. P. Xu and H. C. Zeng, Chem. Mater., 2000, 12, 650–658.
3. M. Wojciechowska, M. Zieliński, A. Malczewska, W. Przystajko and M. Pietrowski, Appl. Catal., A, 2006, 298, 225–231.
4. G. Zhang, L. Yu, H. B. Wu, H. E. Hoster and X. W. Lou, Adv. Mater., 2012, 24, 4609–4613.
5. M. Li, Y.-X. Yin, C. Li, F. Zhang, L.-J. Wan, S. Xu and D. G. Evans, Chem. Commun., 2012, 48, 410–412.
6. D. Gu, C.-J. Jia, C. Weidenthaler, H.-J. Bongard, B. Spliethoff, W. Schmidt and F. Schüth, J. Am. Chem. Soc., 2015, 137, 11407–11418.
7. G. Yang, H. Cui, G. Yang and C. Wang, ACS Nano, 2014, 8, 4474–4487.
8. S. Jin, G. Yang, H. Song, H. Cui and C. Wang, ACS Appl. Mater. Interfaces, 2015, 7, 24932–24943.
9. G. Yang, H. Song, H. Cui and C. Wang, Nano Energy, 2014, 5, 9–19.
10. M. V. Reddy, C. Yu, F. Jiahuan, K. P. Loh and B. V. R. Chowdari, RSC Adv., 2012, 2, 9619–9625.
11. Y. Sharma, N. Sharma, G. V. S. Rao and B. V. R. Chowdari, J. Power Sources, 2007, 173, 495–501.
12. S. Sun, Z. Wen, J. Jin, Y. Cui and Y. Lu, Microporous Mesoporous Mater., 2013, 169, 242–247.
13. X. Lai, J. Li, B. A. Korgel, Z. Dong, Z. Li, F. Su, J. Du and D. Wang, Angew. Chem., Int. Ed., 2011, 50, 2738–2741.
14. X. W. Lou, L. A. Archer and Z. Yang, Adv. Mater., 2008, 20, 3987–4019.
15. F. Caruso, R. A. Caruso and H. Möhwald, Science, 1998, 282, 1111–1114.
16. S.-W. Kim, M. Kim, W. Y. Lee and T. Hyeon, J. Am. Chem. Soc., 2002, 124, 7642–7643.
17. Y. Sun, B. Mayers and Y. Xia, Adv. Mater., 2003, 15, 641–646.
18. Z. Wang, L. Zhou and X. W. Lou, Adv. Mater., 2012, 24, 1903–1911.
19. L. Shen, H. Song, G. Yang and C. Wang, ACS Appl. Mater. Interfaces, 2015, 7, 11063–11068.
20. S. Xu, C. M. Hessel, H. Ren, R. Yu, Q. Jin, M. Yang, H. Zhao and D. Wang, Energy Environ. Sci., 2014, 7, 632–637.
21. A. S. Arico, P. Bruce, B. Scrosati, J.-M. Tarascon and W. van Schalkwijk, Nat. Mater., 2005, 4, 366–377.
22. J. Wang, N. Yang, H. Tang, Z. Dong, Q. Jin, M. Yang, D. Kisailus, H. Zhao, Z. Tang and D. Wang, Angew. Chem., Int. Ed., 2013, 52, 6417–6420.
23. Z. Yang, Z. Li, Y. Yang and Z. J. Xu, ACS Appl. Mater. Interfaces, 2014, 6, 21911–21915.
24. J. Li, J. Wang, X. Liang, Z. Zhang, H. Liu, Y. Qian and S. Xiong, ACS Appl. Mater. Interfaces, 2014, 6, 24–30.
25. G. Zhang and X. W. Lou, Angew. Chem., Int. Ed., 2014, 53, 9041–9044.
26. L. Shen, L. Yu, X.-Y. Yu, X. Zhang and X. W. Lou, Angew. Chem., Int. Ed., 2015, 54, 1868–1872.
27. L. Zhou, W. Wang, H. Xu and S. Sun, Cryst. Growth Des., 2008, 8, 3595–3601.
28. L. Zhou, H. B. Wu, T. Zhu and X. W. Lou, J. Mater. Chem., 2012, 22, 827–829.
29. W. Kang, Y. Tang, W. Li, Z. Li, X. Yang, J. Xu and C.-S. Lee, Nanoscale, 2014, 6, 6551–6556.
30. Q. Wang, J. Xu, X. Wang, B. Liu, X. Hou, G. Yu, P. Wang, D. Chen and G. Shen, ChemElectroChem, 2014, 1, 559–564.
31. K. Zhang, W. Zeng, G. Zhang, S. Hou, F. Wang, T. Wang and H. Duan, RSC Adv., 2015, 5, 69636–69641.
32. Y. Liu, L.-J. Cao, C.-W. Cao, M. Wang, K.-L. Leung, S.-S. Zeng, T. F. Hung, C. Y. Chung and Z.-G. Lu, Chem. Commun., 2014, 50, 14635–14638.
33. C. Cheng, F. Chen, J. Wang, G. Lai and H. Yi, J. Electron. Mater., 2015, 45, 553–556.
34. S. Liu, S. Zhang, Y. Xing, S. Wang, R. Lin, X. Wei and L. He, Electrochim. Acta, 2014, 150, 75–82.
35. A. Pendashteh, S. E. Moosavifard, M. S. Rahmanifar, Y. Wang, M. F. El-Kady, R. B. Kaner and M. F. Mousavi, Chem. Mater., 2015, 27, 3919–3926.
36. J. Ma, H. Wang, X. Yang, Y. Chai and R. Yuan, J. Mater. Chem. A, 2015, 3, 12038–12043.
37. D. V. Cesar, C. A. Peréz, M. Schmal and V. M. M. Salim, Appl. Surf. Sci., 2000, 157, 159–166.
38. R. N. Singh, J. P. Pandey, N. K. Singh, B. Lal, P. Chartier and J. F. Koenig, Electrochim. Acta, 2000, 45, 1911–1919.
39. T.-C. Wen and H.-M. Kang, Electrochim. Acta, 1998, 43, 1729–1745.
40. I. Nikolov, R. Darkaoui, E. Zhecheva, R. Stoyanova, N. Dimitrov and T. Vitanov, J. Electroanal. Chem., 1997, 429, 157–168.
41. J. Zhao, Y. Zou, X. Zou, T. Bai, Y. Liu, R. Gao, D. Wang and G.-D. Li, Nanoscale, 2014, 6, 7255–7262.
42. C. Yuan, J. Li, L. Hou, L. Zhang and X. Zhang, Part. Part. Syst. Charact., 2014, 31, 657–663.
43. X. W. Lou, Y. Wang, C. Yuan, J. Y. Lee and L. A. Archer, Adv. Mater., 2006, 18, 2325–2329.
44. Q. Su, W. Yuan, L. Yao, Y. Wu, J. Zhang and G. Du, Mater. Res. Bull., 2015, 72, 43–49.
45. F. Zheng, D. Zhu and Q. Chen, ACS Appl. Mater. Interfaces, 2014, 6, 9256–9264.
46. Y. Su, S. Li, D. Wu, F. Zhang, H. Liang, P. Gao, C. Cheng and X. Feng, ACS Nano, 2012, 6, 8349–8356.
47. C. Peng, B. Chen, Y. Qin, S. Yang, C. Li, Y. Zuo, S. Liu and J. Yang, ACS Nano, 2012, 6, 1074–1081.
48. M. Kundu, C. C. A. Ng, D. Y. Petrovykh and L. Liu, Chem. Commun., 2013, 49, 8459–8461.
49. R. Zhang, Y. He and L. Xu, J. Mater. Chem. A, 2014, 2, 17979–17985.

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