Universal route to fabricate facile and flexible micro-supercapacitors with gold-coated silver electrodes

Abstract

Currently, microsupercapacitors (MSCs) are considered as a promising strategy to solve the energy problems of micro-scale devices. Due to complex fabrication processes and high costs, innovative techniques need to be developed to micro-fabricate and integrate MSCs on chips. In this paper, for the first time, a facile, cost-effective and universal route to fabricate flexible Ag@Au MSCs was proposed. This technique combines a surface ion-exchange (SMIE) process with immersion plating. The commercially-available polyimide film was first treated with SMIE, followed by immersion plating. Via this method, the pristine polyimide is finally turned into gold coated silver electrodes. The whole fabrication is cleanroom-free and environmentally friendly. With this novel patterning and slicing method, a new route for simple and scalable fabrication of MSCs may fulfill a wide variety of applications. As demonstrations, two types of MSCs have also been fabricated. Coated with MWNTs, MSCs were examined in an electrolyte. For sandwiched MSCs, high cycling properties enable MSCs to work at a scan rate up to 10 V s⁻¹. The maximum phase angle is above −75°. For planar MSCs, the scan rate is easily over 100 V s⁻¹. The mechanical flexible testing indicated that the SMIE Ag@Au electrodes are sufficiently robust.

---

Introduction

Fabrication of sensors, actuators and power sources directly on polymeric substrates can achieve low-cost sensor systems.¹ Micro-supercapacitors (MSCs) are considered as the most promising tools to power these sensor systems since they possess many advantages like high energy/power density, high flexibility and low weight.² However, it is difficult for the fabrication process of MSCs to be compatible with traditional and printed electronic processes on flexible substrates.^3,4 Furthermore, the deposition and patterning of high-quality electrically conductive pathways on flexible systems is one of bottlenecks to commercial manufacture.^5–7 In flexible MSCs fabrication, the metallization process, which is applied to fabricate metal current collectors (MCCs), is vital since it directly influences the MSC performance greatly.⁸ Although a few novel MSCs without specific MCCs have been reported,^9–12 MCCs are indispensable because they play an important role to guide and quickly direct charges to the active materials. Therefore, it is still a big challenge to develop a new low-cost eco-friendly MCCs technique.¹³

Generally, both dry and wet methods are used for the fabrications of MCCs. Dry methods, like physical vapor deposition (PVD) and chemical vapor deposition (CVD) are always involved in various expensive facilities.^14–16 On the other hand, harmful reagents like chromium acid in wet methods like electro- or electroless plating, are inevitable, which is not friendly to environment.^17,18 As for the metal film deposition, unlike traditional dry and wet methods aforementioned, surface modification and ion-exchange technique (SMIE) is a novel low-cost low-temperature technique which can in situ grow various metal films from the internal of polymer.^19,20 The internal growth has strong interfacial interaction and thus leads to the superior adhesion between the polymer and the metal.²¹ In most reported MSCs, gold was chosen for MCCs to protect the electrodes from the deterioration of the electrolyte and enhance the performance of the device applications.^8,22–24 However, the usage of pure gold MCCs would limit the wide applications due to its high cost and rarely available. Au is accepted as a standard electrode material in the micro-electronics industry. To reduce the amount of Au, silver coated with gold, Ag@Au core–shell structure, is proposed as a promising material.²⁵ For example, Ag@Au nanoparticles used in biosensors and surface-enhanced Raman spectroscopy (SERS) can offer the advantages such as excellent electrical and optical properties and extra stability as well.²⁶ However, to our knowledge, there are still no reported MSCs based on Ag@Au MCCs.

In this paper, we present a strategy for production of Ag@Au MCCs for MSCs by combination of SMIE Ag-core and immersion plated Au-shell innovatively. Ag@Au structure, consisting of Ag wrapped by a thin gold layer, can protect Ag layer from harsh environment.²⁵ The excellent performance of the developed Ag@Au MCCs is demonstrated by the potential applications. With carbon nanotubes as active materials, the two-electrode MSCs exhibit high scan rate capability up to 10 V s⁻¹ and high characteristic frequency (3 Hz). And the maximum capacitance reaches up to 5.7 mF cm⁻² at 10 mV s⁻¹ scan rate. More important, the electrodes can stand 2500 CV cycles in 500 mV s⁻¹ with 3% capacitance loss. Also, we demonstrated planar on-chip MSCs with long lifespan and high scan rate over 100 V s⁻¹. MSCs with fast scan rate can be used in AC-line filtering.

Experimental

Materials and reagents

Commercial available ∼50 μm pyromellitic dianhydride–oxydianiline (PMDA–ODA) polyimide films were purchased from Qian Feng Insulating Material Company (Shanghai, China). KOH, NH₃·H₂O, AgNO₃ (99.8%) and polyvinyl alcohol (PVA) purchased from Aladdin Industrial Corporation (Shanghai, China) were used without further purification. HAuCl₄ was purchased from Sinopharm Chemical Reagent. MWNT and the dispersion were supplied by BoYu High Tech Com., China.

Ag film fabrication via SMIE technique

The Ag metallization on PI comprises the surface hydrolysis of PI film in alkali solution resulting in the cleavage of the imide rings and the formation of carboxylic acid groups, the loading of metal ions through the ion exchange of the carboxyl group with the metal cations in the inorganic metal salt aqueous solution, and H₂O₂ reduction to generate the surface metallized PI films.

The flexible PI films were bonded to the glass substrate and cleaned with acetone, ethanol, and DI water carefully. Then the films were immersed in 4 M KOH aqueous solution for 90 min at room temperature. After that, the treated films were washed with large amount of ethanol and DI water subsequently.

KOH-treated PI films were then soaked in 0.02 M Ag(NH₃)₂⁺ solution for 10 min and further dipped into in 0.02 M H₂O₂ for 1–2 min until the shiny Ag films were obtained.

Ag@Au film fabrication via self-reduction

In order to enhance the stability of SMIE Ag films, the self-reduction process had been employed to coat the silver surface with gold, denoted as Ag@Au in this paper. The reaction mechanism is as below (3Ag = Au)

Au³⁺ + 3Ag → Au + 3Ag⁺

The SMIE obtained Ag/PI samples were dipped into 0.01 M chlorauric acid solution until the shiny Ag film turned into silver gray. By this process, Ag@Au films were obtained.

Characterization methods and tools

The microstructures of SMIE-MSCs were investigated by means of scanning electron microscopy (SEM, Hitachi S4800). Electrical measurements were carried out on Hall effect measurement system (RH2035). The electrochemical properties of MSCs were investigated with cyclic voltammetry (CV), chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS) measurements, using CHI660D electrochemical workstation. The electrochemical impedance spectroscopy (EIS) was taken in the frequency range from 100 kHz to 0.01 Hz with potential amplitude of 5 mV. The mechanical bending test was undergone on the mechanical stirring machine (Shanghai Lushuo JB50-D, China).

MSCs assembly and testing

As active material, 90 wt% MWNTs and 10 wt% MWNTs dispersion agents were mixed in DI water and sonicated for 1 hour. The metallized PI films with Ag@Au electrodes were dipped into the MWNT solution and dried. The same steps were repeated several times. At last, Ag@Au/MWCNT electrodes were achieved. The area of each electrode is 4 cm².

For the two-electrodes MSCs assemble, the process is as below:

For electrolyte preparation, 8.5 g NaNO₃ is dissolved into 100 ml DI water. Then 1 M NaNO₃ electrolyte was obtained. The flexible Ag@Au/MWCNT electrodes were fixed on glass slides and immersed into NaNO₃ electrolyte. In testing, the distance of two electrodes was kept at 3 cm.

For the all-solid-state MSCs assemble, the process is as below:

MWNTs were dip-coated on a 1.5 cm × 0.5 cm Ag@Au MCC. After dried in oven, the MCCs were carefully cut in two electrodes with continuous PI substrate.

For solid electrolyte preparation, 5 g PVA, 50 ml DI water and 5 g NaNO₃ powder were mixed. The solution was stirred at 85 °C until the solution became a clear gel. Next, 0.1 ml gel solution was dropped carefully on Ag@Au MSCs. After vaporizing the excess water, all-solid-state SMIE-MSCs were obtained.

In the bending testing, the blade of machine could hit the free-end of MSCs every cycle and cause the bending. Upon hitting the middle part of electrode, the deflection angle is ∼60°, corresponding to the original MSCs plane. The bending rates, determined by the rotation speed of machine, can be adjusted from 120 rpm to 300 rpm. The square resistance is measured during testing.

Results and discussion

In SMIE experiment, the PI hydrolysis can generate the cleavage of imide rings which form the carboxylic acid groups. Before Ag films grow on polyimide of the surface of PI, the Ag ions load the carboxyl groups through ion exchange. The metal ions are finally reduced to form the metal films. Au film is immersion plated from chlorauric acid solution to enhance the stability of MSCs (Fig. 1a). In Fig. 1b–d, a piece of pristine PI film is turned into Ag@Au MCC, which can be folded and rolled arbitrarily. The structure of the Ag@Au MCC is schematically illustrated in Fig. 1e. With inkjet printer and photosensitive dry film, the SMIE products can be patterned and tailored in various sizes and styles for different purposes (Fig. 1f).

Fig. 1 (a) Schematic illustration of fabrication processes of flexible Ag@Au MCCs. (b) Photographs of PI film without treatment. (c and d) Photographs of the folded and rolled Ag@Au MCCs. (e) Schematic showing the structure of the SMIE Ag@Au MSCs. (f) Photograph of various kinds of SMIE devices. (g) SEM images of the Ag MCCs and Ag MCCs surface before (left) and after (right) tape peeling testing.

Interdigital electrodes of various sizes, spiral electrodes and two-sided electrodes are achieved. All these electrodes can be fabricated into an array structure. It is obvious that the novel process is flexible and general.

Fig. 1g is the SEM graph showing Ag film surfaces before and after adhesive tape peeling testing. The white spots in the right side picture are the residuals from adhesive tape. No obvious surface variations can be observed, suggesting that the peeling damage is negligible in turn also implying the superior adhesion strength between the metal layer and the substrate.

Fig. 1a illustrates the main processes of SMIE technique. After immersion plated in chlorauric acid solution, Au ions in the solution are reduced by Ag atoms and form a tight Au protection layer onto the silver surface. Fig. 2a and b show the top-view SEM images of SMIE Ag film before and after this treatment process. The HAuCl₄ plating time determines the resistance of MCCs. Before soaking, the Ag film resistance is about 0.17 Ω cm⁻². The resistance will rise to 0.35 Ω cm⁻² after 25 s immersion in HAuCl₄ (Fig. S1†). The fine conductivity of Ag@Au MCC indicates that this film has good continuity and uniformity. EDS mapping images of Ag@Au samples are shown in Fig. S2† the mass ratio of Au and Ag are 9% and 80%, respectively.

Fig. 2 (a and b) SEM images of the Ag MCCs before (left) and after (right) Au coating. (c) CV curves of Ag film and Ag@Au MCC 5 V s⁻¹. (d) Nyquist plot for different electrodes. The inset shows the high frequency regime.

The electrical performance of Ag film and Ag@Au MCCs is compared to verify the importance of Au immersion layer. CV testing has been conducted to investigate the respective stability of the Ag@Au MCCs and Ag films. For example, two-electrode configuration with two identical Ag@Au MCCs was employed. The space between two electrodes is about ∼3 cm with the electrodes area ∼4 cm². 1 M NaNO₃ aqueous solution was used as the electrolyte. Fig. 2c and d show the CV curves of Ag@Au MCCs and Ag films under different scan rates. It is found that the former has better symmetric shape CV curves than the latter. In addition, compared to orange curve (for Ag film), no excessive or spurious peaks were found in blue curve (for Ag@Au MCC), indicating that the introduction of Au layer could effectively depress the undesired redox reactions which might degrade or harm the stabilization of current collector slowly. Thus, although SMIE-Ag is highly conductive and appropriate to be electrodes, the introduction of Au is able to further enhance the MCC performance due to the stability of Au.

Due to the fast ion transport at electrode–electrolyte interface, no semicircles in EIS plots can be found in both Ag@Au MCC, and Ag film curves (Fig. 2).^4,6,10,27 For both MCCs, lower internal resistances can be achieved. In addition, it is worth noting that the Nyquist plots of Ag@Au MCCs is quite close to imaginary axis and exhibits larger slope at low frequency, which is a typical characteristic of the pronounced capacitive behavior.^16,28–30 All above tests demonstrate an improved electrochemical behavior of Ag@Au MCCs by introduction of Au layer.

To demonstrate the electrochemical performance of Ag@Au MCCs in a MSC, MWNTs are used as the active material. Raman spectra and SEM image of MWNTs are shown in Fig. S3 and S4.† The properties of MSCs were studied. As shown in Fig. 3a and b, the CV scan rates of MWNTs-loaded Ag@Au range from 100 mV s⁻¹ to 10 V s⁻¹ in 1 M NaNO₃ liquid electrolyte, and the potential window is fixed at 0–0.8 V (Fig. S5a and b†). At low scan rates such as 100 mV s⁻¹, the CV curve is almost ideal rectangular, suggesting near-perfect formation of electrical double layer. Even at high scan rate up to 1 V s⁻¹, the CV curve still remains a pretty rectangle shape, suggesting good capacitive behavior. Fig. S6† shows the dependence of the discharge current upon the scan rate clearly. The linear range of the discharge current versus the scan rate is up to 1 V s⁻¹ (Fig. S6†).

Fig. 3 (a and b) CV curves for MSCs at 100 mV s⁻¹ to 10 V s⁻¹. (c) Galvanostatic charge/discharge curves at 0.25–3 mA cm⁻². (d) Nyquist plot for different electrodes. The inset shows the high frequency regime. (e) Negative phase versus frequency plot for Ag@Au MCC and MWNT Ag@Au MSC where the −45° shows the characteristic frequency (f) cycling stability at 500 mV s⁻¹.

In Fig. S7,† the galvanostatic charge/discharge testing was performed from 0.125 up to 3.75 mA cm⁻². Triangle curves of a series become well symmetrically, indicating that the good electrical double layer capacitance can be achieved, consistent with the CV results. No obvious IR drop can be seen at the beginning of the increasing current. When the currents density increases large enough, up to 2 mA cm⁻² in Fig. S7b,† the IR drop become visible but still remains a reasonable level. As shown in Fig. 3d. EIS curve of MWNTs-loaded Ag@Au MSC plot is quite close to imaginary axis, which is a typical characteristic of the fine capacitive behavior. Due to the active material, the orange line becomes the blue line which is characteristic of better capacitive behavior. In EIS testing, the phase angle as a function of the frequency is also primary index for MWNTs-loaded Ag@Au MSC and Ag@Au MCC (Fig. 3e). MWNTs-loaded Ag@Au MSC (yellow curve) shares the similar properties of Ag@Au MCCs (pink curve). MWNTs as active materials increase the phase angle, which represents the contribution to the capacitive behavior. In Fig. 3e, the maximum phase angle for MWNTs Ag@Au MSC (in pink color) is over −75°, which represents the fine capacitive response.^28,31 The characteristic frequency f₀ is about 3 Hz. The calculated time constant is ∼0.33 s, which is comparable to that of other MSCs.^5,27

The stability is also an important parameter to evaluate the electrochemical performance of capacitors. The CV measurement was done to investigate the cycling stability of Ag@Au MCCs. More than 97% of the initial capacitance is still maintained after 2500 CV cycles (Fig. 3f). The inset demonstrates the MSC working under different cycles. The CV shapes of 1^st, 1000^th, 2500^th match very well without any obvious recession. Even after 2500 cycles at high scan rate of 500 mV s⁻¹, the CV curves change is still negligible. Therefore, these results clearly demonstrate that the developed Ag@Au MSCs have the superior cycling stability.

The area capacitances calculated from the current density and the scan rate are plotted in Fig. S8.† The area capacitances decrease with the increase of the scan rate and the charging current. The maximum capacitances achieved are respectively around 5.7 and 4.6 mF cm⁻², which is higher than that of many reported MSC whose capacitances range from 0.08 to 3.5 mF cm⁻².^10,16,31 When current densities and scan rates rise to 3 mA cm⁻² and 10 V s⁻¹, the capacitances reduce to 2.6 and 0.7 mF cm⁻² correspondingly.

Many different MSC configurations can be realized via the proposed Ag@Au MCCs readily (Fig. 1f). For the demonstration of Ag@Au MSC planar device, a simple planar MSC is fabricated by blade cutting. Blade cutting process is used to form the narrow grooves on PI substrate and realize the electrical isolation between the electrode pairs (Fig. 4a). Since the PI substrate is mechanically robust and Ag@Au layer is thin and highly ductile, it is conventional to cut off the metal layer while keep the flexible PI substrate continuous. The whole Ag@Au MCC can be divided into two electrodes of the clear and sharp edges separated by a 50 μm wide groove (Fig. 4b) which is the minimal achievable size for on-chip MSCs based on printed electronic technology. PI substrate is exposed and no metal residues are found which may result electric leakage. As shown in Fig. 4c, the plot still keeps an obvious rectangular shape, when the scan rate is up to 5 V s⁻¹. The maximum achievable scan rate is over 100 V s⁻¹ demonstrating the ultrafast charge/discharge rate (Fig. 4d). The linear range of the discharge current versus the scan rate is up to 20 V s⁻¹ (Fig. 4e). In the galvanostatic charge/discharge testing, the maximum current density can reach 5 mA cm⁻², which is comparable to other reported MSCs of sandwich structure.^32–36

Fig. 4 (a) Schematic showing the structure of strip-type MSCs. (b) Optical image of the structure of strip-type MSCs. (c and d) CV curves for MSCs at 1 V s⁻¹ to 100 V s⁻¹. (e) A plot of the discharge current as a function of the scan rate. (f) Galvanostatic charge/discharge curves at 1.1–5 mA cm⁻². (g) Cycling stability of Ag and Ag@Au MSCs at 20 V s⁻¹. (h) Hitting rate and R/R₀ versus hitting number in 2000 hittings.

The planar MSCs stability based on Ag and Ag@Au MCCs is compared in Fig. 4g with CV scan rate at 20 V s⁻¹. Ag@Au MSC remains 96.5% of the initial capacitance while Ag MSC remains 85.4%, demonstrating that Au remarkably enhances the stability of electrode.

For the wearable application of MSCs, the MCCs have to be equipped strong mechanical strength. It actually means the as-prepared devices need to perform not only high flexibility but also good durability. A practical flexible chip can resist a large amount of mechanical force.³⁶ To simulate a severe testing condition, a mixer machine was used to examine the MCCs. The turning mixer machine blade can cause the bending state on MCCs with variant hitting rates from low speed to high speed. 2000 bending cycles were completed within 500 s. The calculated R/R₀, plotted in Fig. 4h, rises to 108% after 2000 bending cycles which is a small value. The MCCs survived even at ultrahigh 300 rpm, demonstrating the strong mechanical strength.

Conclusion

In summary, we developed a novel process to fabricate low-cost Ag@Au electrodes for MSCs. The introduction of gold can maintain the high conductivity and achieve the high flexibility as well. Au layer is proved to increase the performances of Ag MCCs. MSCs with SMIE Ag@Au MCCs also show good performance with high scan rate capability up to 10 V s⁻¹ and phase angle above −75°. At scan rate of 10 mV s⁻¹, the capacitance of these MSCs can reach 5 mF cm⁻². The developed technique is proven to be facile and general for MSCs since it can offer some advantages like room temperature, batch fabrication, low cost and clean-room free.

Acknowledgements

This work was supported by Projects of Science and Technology Commission of Shanghai Municipality (Grant No. 15JC1401800). We would like to acknowledge support from National Natural Science Foundation of China under grant No. 11504111, 61574060, and Projects of Science and Technology Commission of Shanghai Municipality Grant No. 15JC1401800. This work is also sponsored by the State Key Laboratory of Transducer Technology, with grant No. SKT1504. The authors thank Mr Xiaoyu Zheng for picture design.

Notes and references

1. M. Beidaghi and Y. Gogotsi, Energy Environ. Sci., 2014, 7, 867.
2. P. Yang and W. Mai, Nano Energy, 2014, 8, 274.
3. B. Yao, L. Yuan, X. Xiao, J. Zhang, Y. Qi, J. Zhou, J. Zhou, B. Hu and W. Chen, Nano Energy, 2013, 2, 1071.
4. Z.-K. Wu, Z. Lin, L. Li, B. Song, K.-S. Moon, S.-L. Bai and C.-P. Wong, Nano Energy, 2014, 10, 222.
5. M. F. El-Kady, V. Strong, S. Dubin and R. B. Kaner, Science, 2012, 335, 1326.
6. M. F. El-Kady and R. B. Kaner, Nat. Commun., 2013, 4, 1475.
7. Y. Wang, Y. Shi, C. X. Zhao, J. I. Wong, X. W. Sun and H. Y. Yang, Nanotechnology, 2014, 25, 094010.
8. T. M. Dinh, K. Armstrong, D. Guay and D. Pech, J. Mater. Chem. A, 2014, 2, 7170.
9. W. Gao, N. Singh, L. Song, Z. Liu, A. L. M. Reddy, L. Ci, R. Vajtai, Q. Zhang, B. Wei and P. M. Ajayan, Nat. Nanotechnol., 2011, 6, 496.
10. B. Hsia, M. S. Kim, M. Vincent, C. Carraro and R. Maboudian, Carbon, 2013, 57, 395.
11. L. Cao, S. Yang, W. Gao, Z. Liu, Y. Gong, L. Ma, G. Shi, S. Lei, Y. Zhang, S. Zhang, R. Vajtai and P. M. Ajayan, Small, 2013, 9, 2905.
12. F. Wen, C. Hao, J. Xiang, L. Wang, H. Hou, Z. Su, W. Hu and Z. Liu, Carbon, 2014, 75, 236.
13. Q. Jiang, N. Kurra and H. N. Alshareef, Adv. Funct. Mater., 2015, 25, 4976.
14. B. Hsia, J. Marschewski, S. Wang, J. B. In, C. Carraro, D. Poulikakos, C. P. Grigoropoulos and R. Maboudian, Nanotechnology, 2014, 25, 055401.
15. W. Si, C. Yan, Y. Chen, S. Oswald, L. Han and O. G. Schmidt, Energy Environ. Sci., 2013, 6, 3218.
16. Z.-S. Wu, K. Parvez, X. Feng and K. Muellen, J. Mater. Chem. A, 2014, 2, 8288.
17. A. Garcia, T. Berthelot, P. Viel, J. Polesel-Maris and S. Palacin, ACS Appl. Mater. Interfaces, 2010, 2, 3043.
18. A. Garcia, J. Polesel-Maris, P. Viel, S. Palacin and T. Berthelot, Adv. Funct. Mater., 2011, 21, 2096.
19. K. Akamatsu, H. Shinkai, S. Ikeda, S. Adachi, H. Nawafune and S. Tomita, J. Am. Chem. Soc., 2005, 127, 7980.
20. S. K. Lim, K. J. Chung, Y.-H. Kim, C. K. Kim and C. S. Yoon, J. Colloid Interface Sci., 2004, 273, 517.
21. Y. Yu, J. Zhang, X. Wu and Z. Zhu, J. Mater. Chem. A, 2015, 3, 21009.
22. W.-W. Liu, Y.-Q. Feng, X.-B. Yan, J.-T. Chen and Q.-J. Xue, Adv. Funct. Mater., 2013, 23, 4111.
23. Z.-S. Wu, K. Parvez, A. Winter, H. Vieker, X. Liu, S. Han, A. Turchanin, X. Feng and K. Muellen, Adv. Mater., 2014, 26, 4552.
24. N. Kurra, Q. Jiang and H. N. Alshareef, Nano Energy, 2015, 16, 1.
25. A. Gutés, R. Maboudian and C. Carraro, Langmuir, 2012, 28, 17846.
26. H. Liu, T. Liu, L. Zhang, L. Han, C. Gao and Y. Yin, Adv. Funct. Mater., 2015, 25, 5435.
27. S. Wang, B. Hsia, C. Carraro and R. Maboudian, J. Mater. Chem. A, 2014, 2, 7997.
28. M. Beidaghi and C. Wang, Adv. Funct. Mater., 2012, 22, 4501.
29. D. Pech, M. Brunet, H. Durou, P. Huang, V. Mochalin, Y. Gogotsi, P.-L. Taberna and P. Simon, Nat. Nanotechnol., 2010, 5, 651.
30. X. Xiao, T. Li, P. Yang, Y. Gao, H. Jin, W. Ni, W. Zhan, X. Zhang, Y. Cao, J. Zhong, L. Gong, W.-C. Yen, W. Mai, J. Chen, K. Huo, Y.-L. Chueh, Z. L. Wang and J. Zhou, ACS Nano, 2012, 6, 9200.
31. Z. S. Wu, K. Parvez, X. L. Feng and K. Mullen, Nat. Commun., 2013, 4, 3487.
32. X. Li, T. Zhao, Q. Chen, P. Li, K. Wang, M. Zhong, J. Wei, D. Wu, B. Wei and H. Zhu, Phys. Chem. Chem. Phys., 2013, 15, 17752.
33. Z. Su, C. Yang, B. Xie, Z. Lin, Z. Zhang, J. Liu, B. Li, F. Kang and C. P. Wong, Energy Environ. Sci., 2014, 7, 2652.
34. Q. Wang, J. Xu, X. Wang, B. Liu, X. Hou, G. Yu, P. Wang, D. Chen and G. Shen, ChemElectroChem, 2014, 1, 559.
35. Y. Xu, Z. Lin, X. Huang, Y. Liu, Y. Huang and X. Duan, ACS Nano, 2013, 7, 4042.
36. K. Mun Sek, B. Hsia, C. Carraro and R. Maboudian, Carbon, 2014, 74, 163.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13432g

---