Accelerating the decomposition of KMnO₄ by photolysis and auto-catalysis: a green approach to synthesize a layered birnessite-type MnO₂ assembled hierarchical nanostructure

Abstract

In this work, we developed a facile, green and low-cost strategy for the controllable synthesis of birnessite-type MnO₂ (Bir-MnO₂) nanosheets assembled in a “desert rose”-like 3D hierarchical nanostructure and proposed a two-step growth route, namely photolysis and followed by auto-catalytic decomposition of KMnO₄. Bir-MnO₂ nanosheets were first obtained under the irradiation of a light source and acted as seeds for further decomposition of the excess MnO₄⁻ in the dark. The sizes of the Bir-MnO₂ 3D hierarchical nanostructures could be readily controlled by varying the concentration of KMnO₄, the light source and the pH value. Furthermore, Bir-MnO₂ nanosheets on CNTs were successfully fabricated utilizing the same strategy without the addition of any surfactants or organic templates. It is anticipated that the fabrication of Bir-MnO₂ based nanostructures and composites could be beneficial from this universal method.

---

Introduction

The design and fabrication of three-dimensional (3D) hierarchical nanostructures have been attracting much attention owing to their unique properties such as concerning the transfer of carriers inside the hierarchical nanoarchitectures and the diffusion of reactants in the interspace.^1–3 Those properties are known to be significantly affected by the tunnel size and shape of nanostructures.^4,5 However, the tailored synthesis of complex architectures with special size and shape is usually achieved at high temperature or by using organic solvents and templates, which obviously increase the production costs and technical complexity.⁶ Consequently, it is highly desirable to focus on the development of environmentally benign and low-cost synthetic methods that produce complex architectures to alleviate these problems.

As an important layered manganese oxide compound, birnessite-type MnO₂ (Bir-MnO₂) consists of coplanar layers of infinite two-dimensional sheets of edge-shared MnO₆ octahedra. Their formula are normally expressed as A_xMnO₂·yH₂O, where A is proton H⁺ or an alkali-metal cation such as K⁺, Na⁺ and Li⁺.^7,8 The cations and water molecules occupied in the interlayer space are exchangeable, which endows high mobility and little structural rearrangement. The distinctive layered structure provides abundant active sites for ion-exchange and molecular adsorption, which makes them very attractive in the applications of supercapacitor,^9,10 catalysis,^11–13 ion-exchangers,¹⁴ Li-battery,¹⁵ and so on.^16,17 Bir-MnO₂ with different morphologies^18–21 or loaded over different supporting matrix, for example, carbon nanotubes (CNTs),^22–24 graphene^25,26 and so forth,^27,28 have been proved to be effective in adjusting their chemical and physical properties for various applications. However, the rational design and green synthesis of 3D hierarchical Bir-MnO₂ and their composites with controllable size and morphology still remain as a significant challenge.

In this work, layered Bir-MnO₂ nanosheets assembled 3D hierarchical nanostructures were controllably synthesized through the direct decomposition of KMnO₄ with two steps at room temperature as illustrated in Scheme 1a. In the first step, Bir-MnO₂ nanosheets were obtained with the irradiation of lab light source, even sunlight. To the best of our knowledge, this is the first time to synthesize Bir-MnO₂ based on the well-known phenomenon that KMnO₄ undergoes decomposition when exposed to light. In the second step, residual KMnO₄ in the reacted solution was further decomposed in dark with the assistance of formed Bir-MnO₂ nanosheets by a so-called auto-catalytic effect, which leads to the formation of nanosheets assembled “desert rose”-like 3D hierarchical nanostructures. Apparently, this novel approach is straightforward and environmentally sustainability. More importantly, the above two steps could be utilized to conveniently prepare Bir-MnO₂ based composites. Herein, we also successfully loaded Bir-MnO₂ nanosheets over CNTs (Bir-MnO₂/CNTs), forming hierarchical nanostructure as illustrated in Scheme 1b.

Scheme 1 Schematic illustration of the fabrication of layered Bir-MnO₂ assembled hierarchical nanostructure (a) and Bir-MnO₂/CNTs hybrids (b).

Experimental

Preparation of layered Bir-MnO₂ assembled hierarchical nanostructure

The multiwalled CNTs with the diameter of 40–60 nm were purchased from Shenzhen Nanotech Nano Pon Co. Ltd (China). All the other chemical reagents are of analytical grade from Sinopharm Chemical Reagent Company and used as received without further purification.

Two steps were involved in the preparation of layered Bir-MnO₂ assembled hierarchical nanostructure. The first step is the photolysis of KMnO₄. In a typical preparation process, 0.079 g of KMnO₄ powders was completely dissolved in 20 mL of deionized water under magnetic stirring for about 10 min to form a dark purple solution. The as-obtained KMnO₄ solution subsequently was injected with a speed of 40 mL h⁻¹ controlled by a syringe into the Pyrex reaction cell containing 30 mL of deionized water in the irradiation of UV light (mercury lamp with the wavelength of 254 nm). The injection process was terminated in 30 min but the solution was still irradiated by UV light for 1.5–7.5 h. In other words, the total irradiation duration was 2.0–8.0 h. In the second step, the obtained suspension then was transferred in a beaker and placed in dark for a few more days. Finally, the products were collected via filtering and rinsing thoroughly with distilled water for several times to remove residual KMnO₄. The sample was then freeze-dried overnight for further use.

The photodecomposition reactions were carried out in a Pyrex reaction cell equipped with a circulating cooling water system to keep the whole irradiation process at 10 °C. The whole Pyrex reaction cell and the light source, a 32 W mercury lamp, were placed in a darkroom to avoid the influence of indoor light. In other comparative experiments, fluorescent lamp was used as the visible light source. The pH value of the KMnO₄ solution was adjusted by H₂SO₄.

Fabrication of Bir-MnO₂/CNTs hybrid nanostructure

Briefly, the CNTs were first refluxed in a 65% nitric acid at 140 °C for 6 h. Afterwards, 10 mg purified CNTs was dispersed in 30 mL of deionized water under an ultrasonic bath. The followed photodecomposition procedure was exactly similar with the preparation process of layered Bir-MnO₂. The whole irradiation process and statically aging procedure at room temperature were lasted for 2 h and 5 days, separately. The obtained products were harvested by filtration and washing fully with distilled water before freeze-drying overnight.

Characterizations

The X-ray diffraction pattern (XRD) measurements were performed on a Rigaku D/MAX2200V PC X-ray diffractometer employing a Cu Kα (40 kV, 40 mA) radiation and a secondary beam graphite monochromator from 10° to 90° at a step size of 0.020 (2θ). The morphologies and microstructures were characterized by transmission electron microscopic (TEM, JEM-200CX) and high resolution TEM (HRTEM, JEM-2100F), respectively. Energy-dispersive X-ray (EDX) analysis and elemental mapping were recorded to obtain the component of the samples by an Inca Energy 200 TEM system from Oxford instruments. The ultraviolet-visible absorption spectra were recorded on a double beam UV-vis spectrophotometer (760CRT). N₂ adsorption–desorption isotherms were obtained at 77 K on Autosorb iQ2-2MP-C (Quantachrome). The specific surface areas were determined using the Brunauer–Emmett–Teller (BET) equation. The pore volumes and average pore diameters were determined by the Barrett–Joyner–Halenda (BJH) method from the desorption branches of the isotherms.

Results and discussion

It is well-known that the decomposition of MnO₄⁻ in neutral solutions is a spontaneous reaction which can be described by the following reaction equations.²⁹

4MnO₄⁻ + 2H₂O → 4MnO₂ + 4OH⁻ + 3O₂ (1)

Nevertheless, this reaction was considered to be extremely slow.³⁰ Our experiments also proved this statement. No MnO₂ precipitate was observed when the KMnO₄ solution was aged in dark for more than 10 days. Nonetheless, the situation would change when light was applied to irradiate the KMnO₄ solution at room temperature. Black MnO₂ precipitates were obtained after two hours of irradiation. TEM images (Fig. 1a) showed that the MnO₂ nanostructures were composed of many curly and interlaced nanosheets with an average diameter of around 50 nm (Fig. 1a). Very interestingly, the nanosheets continued to grow up without the irradiation when the suspension was further aged in dark for a period of time. Fig. 1b depicted that the morphology became more complicated and formed to be “desert rose”-like 3D hierarchical nanostructures after 5 days of aging. The sizes of the nanostructures noticeably increased to about 200 nm. HRTEM measurement has been carried out to provide insights into the detailed morphologies and crystalline structures of the as-obtained Bir-MnO₂ at the atomic level. The lateral view in Fig. 1c confirmed the layered structure of the MnO₂ nanosheets with an interlayer spacing of 0.61 nm. Herein, the interlayer spacing is smaller than the interplanar space of representative birnessite (normally 0.7 nm) which is presumably due to the occurrence of dehydration under the electron beam bombardment.^31,32 The bending of the nanosheets indicate that the structure is flexible in nature. Fig. 1d reveals that the nanosheets are an assembly of many crystalline particles with sizes of about 5–10 nm. The clear lattice spacing of 0.21 and 0.24 nm correspond to the (−112) and (110) planes, respectively. It seems that the as-prepared Bir-MnO₂ crystal under irradiation have an anisotropic crystalline structure. The continuous growth of the polycrystalline is prone to form interconnected MnO₂ nanosheets orientated in many different directions, forming the “desert rose”-like 3D hierarchical nanostructures.

Fig. 1 TEM images of Bir-MnO₂ nanostructures obtained with different reaction durations: (a) 2 h of irradiation under UV light and (b) aging for further 5 days (Bir-MnO₂ assembled hierarchical nanostructure). (c and d) HRTEM images of Bir-MnO₂ assembled hierarchical nanostructure (e) XRD pattern of Bir-MnO₂ assembled hierarchical nanostructure (f) UV-vis absorption spectra and photograph (inset) of Bir-MnO₂ assembled hierarchical nanostructure at concentration of 320 mg L⁻¹.

The XRD pattern (Fig. 1e) evidenced that the characteristic peaks are readily indexable to K-birnessite-type layered MnO₂ (JCPDS card no. 80-1098, monoclinic, a = 5.15 Å, b = 2.84 Å, c = 7.17 Å), in which the diffraction peaks at 2θ of 12.5°, 25.2°, 37.3°, 63.3° correspond to the (001), (002), (−111), (−311) plane, respectively. The composition of Bir-MnO₂ was further evaluated by EDX spectroscopy, which verifies the atomic ratios of K/Mn/O being 0.15 : 1 : 3.2, suggesting a small amount of K⁺ ions in the interlayer to balance the net negative charge of the MnO₂ layer (Table S1†).³³

Benefited from the open and porous 3D structure, the as-prepared Bir-MnO₂ delivers a high BET specific surface area of 85.1 m² g⁻¹ and a total pore volume of 2.018 cm³ g⁻¹ with mesoporous texture by N₂ adsorption–desorption measurement at 77 K (Fig. S1 and Table S2†). Additionally, the Bir-MnO₂ was proved to be highly soluble in aqueous solution and exhibited Tyndall light scattering (the inset of Fig. 1f). Corresponding UV-vis adsorption spectrum (Fig. 1f) displays a broad absorption peak over a wavelength range of 250–700 nm centered at around 375 nm, which is a typical optical characteristic of the layered MnO₂ nanosheets in conformity with previous investigation^34–37 and corresponds to Mn⁴⁺ ← O₂⁻ charge transfer transitions.³⁸

To understand the formation route of Bir-MnO₂ nanosheets assembled hierarchical nanostructures, the decomposition process of KMnO₄ was detected by the evolution of UV-vis absorption spectra as shown in Fig. 2. The absorption peaks at 508, 526 and 544 nm assigned to the orbitally allowed transition ¹A₁ → ¹T₂ (t₁ → 2e), while the bands at about 310 and 345 nm are associated with a ¹A₁ → ¹T₂ (3t₂ → 2e) transition.³⁹ Firstly, it was worth noting that the concentration of KMnO₄ almost didn't change without the irradiation of light after 10 days of aging in dark, indicating that the direct decomposition of KMnO₄ is hard to happen due to the kinetically unfavorable reaction as mentioned above. Strikingly, the concentration of KMnO₄ continued to decrease in the irradiation of UV-vis light and the followed aging step in dark. Based on this observation, a conclusion could be drawn that light can accelerate the decomposition of the KMnO₄. Meanwhile, the formed Bir-MnO₂ nanosheets were significant to the successive decomposition of KMnO₄ in dark. It is also noticed that two hours of irradiation caused the decrease of concentration almost equal to that aged for one day, indicating that light is much more beneficial to the decomposition of KMnO₄ than the followed aging process in the dark. The importance of irradiation was further confirmed by the evolution of morphologies with the increase of irradiation duration. The sizes of the Bir-MnO₂ nanosheets increased to 80 nm and 100 nm with prolonging the irradiation duration up to 4 h (Fig. S2a†) and 8 h (Fig. S2b†). Above results demonstrated that the irradiation is beneficial to the decomposition of KMnO₄ and the concomitant formation of Bir-MnO₂ nanostructure.

Fig. 2 (a) UV-vis absorption spectra of KMnO₄ solutions before and after aging for 10 days in the dark. (b) The evolution of UV-vis absorption spectra of KMnO₄ solution at different reaction duration and the absorbance at 526 nm plotted against the treatment time (inset).

Based on the above observations, the formation of 3D “desert rose”-like nanostructured Bir-MnO₂ could be generalized into two steps as illustrated in Scheme 1a. The first step was carried out in the irradiation of UV/Vis light. Nuclear and growth of Bir-MnO₂ were involved in this stage, forming curly and interlaced nanosheets. In the second step, the formed Bir-MnO₂ nanosheets act as seeds for further decomposition of the excess MnO₄⁻ in the dark. The continued self-assembled and anisotropic growth of layered MnO₂ brought the formation of nanosheets assembled “desert rose”-like 3D hierarchical nanostructures. Obviously, the driving force involved in the two steps was different and could be addressed as following. It is well known that the decomposition of MnO₄⁻ in neutral solutions is thermodynamically favorable but dynamically impracticable. However, the rate of the decomposition of KMnO₄ could be accelerated by the decrease of activation energy. In the first step, it is understandable that the absorption of photons would make MnO₄⁻ excite from the ground state to the excited state, which is much easier to decompose. In other words, the activation energy for the decomposition of MnO₄⁻ decreases. As elucidated in Fig. 3, without irradiation, the activation energy for the decomposition of MnO₄⁻ is ΔE₁. In the irradiation, the initial energy state increased, thus the activation energy decreased to ΔE₂. In the aging process under dark ambient, MnO₄⁻ was at the ground state. However, it has been well demonstrated that MnO₂ was a good catalyst to decompose the KMnO₄, namely the auto-catalytic effect,⁴⁰ which means that the activation energy could also decreased from ΔE₁ to ΔE₃ by the new formed Bir-MnO₂.

Fig. 3 Schematic illustration of the change of activation energy during the reaction of KMnO₄ decomposition.

Based on the above understanding, it is feasible to obtain the key factors which affect the synthesis process then to controllably prepare the Bir-MnO₂ nanostructures. According to eqn (1), the factors that could change the reaction rate are the concentration of KMnO₄, the partial pressure of O₂ and the light sources. In fact, pH value is another important factor that affects the reaction kinetics when the decomposition of MnO₄⁻ is in acid solutions as demonstrated in eqn (2).³¹ Besides, temperature is of course a crucial factor, but it is out of the discussion in this work since the temperatures were kept at room temperature in all experiments. It is anticipated that the change of decomposition rate would have influences on the morphologies. For example, higher decomposition rate would boost the nuclear and growth process, inducing larger sizes of 3D hierarchical nanostructures.

4MnO₄⁻ + 4H⁺ → 4MnO₂ + 3O₂ + 2H₂O (2)

The influence of concentrations of KMnO₄ solution was first studied. The KMnO₄ solutions with the concentrations of 0.010, 0.005 and 0.001 M were directly irradiated by the UV-visible light. It was found out that almost no MnO₂ samples were obtained at the concentrations of 0.001 M, which could probably be ascribed to the low decomposition rate or the limited Bir-MnO₂ which was difficult to be collected. The morphologies of the obtained Bir-MnO₂ at the concentrations of 0.01 and 0.005 M were shown in Fig. 4. The “desert rose”-like Bir-MnO₂ were also obtained. Obviously, the size of the nanostructure decreased from 60 to 30 nm with the decrease of the concentration of KMnO₄ solution from 0.010 to 0.005 M, indicating that the decrease of decomposition rate decreases the particle size.

Fig. 4 TEM images of Bir-MnO₂ samples obtained at different KMnO₄ concentrations: (a) 0.01 M and (b) 0.005 M. Here, the KMnO₄ with special concentrations were directly irradiated under UV light instead of injection.

The influence of the pH values of the initial reaction system on the morphologies of the Bir-MnO₂ was further investigated. H₂SO₄ was used to adjust the pH of KMnO₄ solutions before injecting into the 30 mL aqueous solution. The TEM and SEM images in Fig. 5 clearly showed that nanosheets assembled 3D hierarchical nanostructures with uniform sizes were obtained at lower pH values. The average diameters of the nanostructures increased from around 200 to 500 nm and 2 μm when the pH value decreased from 5.0 to 3.0 and 1.0, respectively. Conspicuously, the change of particle size was much larger compared with that brought by the concentration of KMnO₄ solution, which is probably due to that the change of the pH value leads to prominent variation in the concentration of H⁺ than the concentration of the KMnO₄⁻. For example, the concentration of H⁺ changed about four orders of magnitude when the pH values changed from 5 to 1 in Fig. 5, while the concentration of KMnO₄ solution only changed less than one order of magnitude from 0.01 M to 0.005 M in Fig. 4.

Fig. 5 TEM and SEM images (inset) of Bir-MnO₂ assembled hierarchical nanostructure obtained at different pH values: (a and b) pH = 1, (c and d) pH = 3, (e and f) pH = 5.

The optical sources were also changed from UV light to visible light to investigate their effect on the product morphology. It was found that hierarchical Bir-MnO₂ can also be successfully fabricated and exhibited no substantial morphological alteration (Fig. S3a and b†). This result could be addressed on the basis of the UV-vis absorption spectra of KMnO₄ solutions. Fig. 2a indicates that KMnO₄ solution has two obvious absorption bands centered at 360 and 550 nm, which means that KMnO₄ could be excited and decomposed by both UV and visible light. This property makes the decomposition of KMnO₄ prone to achieve even under indoor light as shown in Fig. S3c and d.† All the results proved that the shapes of the hierarchical nanostructure Bir-MnO₂ almost didn't change in different optical sources, indicating that the synthetic method was scalable and convenient.

In general, the Bir-MnO₂ nanosheets assembled 3D hierarchical nanostructures could be easily obtained without the addition of any surfactants or organic templates, while the size of the nanostructures could be easily controlled from dozens of nanometers to several micrometers. More interestingly, we found that the two steps involved in the formation of Bir-MnO₂ could be used as a strategy to decorate Bir-MnO₂ over different supports, such as carbon materials, to form composites with hierarchical nanostructures. Here, we demonstrated the practicability of deposition of Bir-MnO₂ on CNTs (Bir-MnO₂/CNTs hybrids), which have been emerged as a promising candidate in the applications of supercapacitors, Li-batteries and etc.^41,42

The preparation process was quite similar with that of Bir-MnO₂ as illustrated in Scheme 1b. The MnO₄⁻ was reduced to form small Bir-MnO₂ particles on the CNTs surface in the irradiation of UV light. The suspension then was placed in dark for 5 days to allow the further deposition of Bir-MnO₂. Before the experiments, we studied the interaction between KMnO₄ and CNTs, since the reaction between KMnO₄ and CNTs was possible according to the literature.^43,44 CNTs were directly dispersed in the aqueous solution of KMnO₄ in dark at the temperature of 10 °C for a period of time. It was distinct that Bir-MnO₂ was difficult to decorate over CNTs in our experimental conditions.

Fig. 6a indicated that it was hard to find obvious hierarchical Bir-MnO₂ over CNTs after two hours of irradiation under UV light according to the SEM image. But the TEM image proved that small particles of Bir-MnO₂ could be decorated on CNTs. When the mixture was statically put in the dark for 5 days, MnO₂ nanosheets assembled hierarchical nanostructure uniformly decorated on the CNTs along the longitudinal axis of the nanotubes as shown in Fig. 6b. The elemental mapping images (Fig. 6c–f) further showed that both Mn and O elements were equably covered over the CNTs, confirming the uniformly decoration of Bir-MnO₂ over CNTs. XRD pattern (Fig. S4†) also confirmed the formation of Bir-MnO₂.⁴⁵

Fig. 6 (a) SEM and TEM images (inset) of MnO₂/CNTs hybrid nanostructures obtained after two hours of irradiation; (b) TEM image of MnO₂/CNTs hybrid nanostructures obtained after aging for 5 day in dark followed by the irradiation. (c–f) The elemental mapping images of the MnO₂/CNTs hybrid nanostructures.

Conclusions

We have put forward a facile, green and low-cost strategy for controllable synthesis of Bir-MnO₂ nanosheets assembled “desert rose”-like 3D hierarchical nanostructure. Bir-MnO₂ was achieved by the direct decomposition of KMnO₄ with the assistance of photolysis and autocatalysis effect at room temperature. Two steps were involved in the formation of 3D hierarchical nanostructure. The first step was carried out in the irradiation of UV/Vis light to form curly and interlaced nanosheets. The formed Bir-MnO₂ nanosheets then act as seeds for further decomposition of the excess MnO₄⁻ in the dark, facilitating the formation of nanosheets assembled “desert rose”-like 3D hierarchical nanostructures. The size of the nanostructures could be conveniently adjusted by changing the pH value, the concentration of MnO₄⁻ and the light source. Encouragingly, Bir-MnO₂ nanosheets on CNTs have further been successfully fabricated utilizing the same strategy without the addition of any surfactants or organic templates. We believe this is a universal method for the fabrication of Bir-MnO₂ based composites and diverse functional nanomaterials.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21303099), the National Basic Research Program of China (973 Program, 2014CB660803) and the Major Program of Shanghai Science and Technology Committee (14DZ1207400).

Notes and references

1. L. Huang, X. Zhao, L. Zhang, L. Shi, J. Zhang and D. Zhang, Nanoscale, 2015, 7, 2743–2749.
2. S. Cai, D. Zhang, L. Shi, J. Xu, L. Zhang, L. Huang, H. Li and J. Zhang, Nanoscale, 2014, 6, 7346–7353.
3. Y. S. Ding, X. F. Shen, S. Gomez, H. Luo, M. Aindow and S. L. Suib, Adv. Funct. Mater., 2006, 16, 549–555.
4. Q. Zhang, W. Li, C. Moran, J. Zeng, J. Chen, L.-P. Wen and Y. Xia, J. Am. Chem. Soc., 2010, 132, 11372–11378.
5. L. Zhang, T. Xu, X. Zhao and Y. Zhu, Appl. Catal., B, 2010, 98, 138–146.
6. D. Chen, L. Cao, F. Huang, P. Imperia, Y.-B. Cheng and R. A. Caruso, J. Am. Chem. Soc., 2010, 132, 4438–4444.
7. R. Z. Ma, Y. Bando, L. Q. Zhang and T. Sasaki, Adv. Mater., 2004, 16, 918–922.
8. R. Renuka and S. Ramamurthy, J. Power Sources, 2000, 87, 144–152.
9. W. Wei, X. Cui, W. Chen and D. G. Ivey, Chem. Soc. Rev., 2011, 40, 1697–1721.
10. L. Athouel, F. Moser, R. Dugas, O. Crosnier, D. Belanger and T. Brousse, J. Phys. Chem. C, 2008, 112, 7270–7277.
11. J. Hou, Y. Li, M. Mao, L. Ren and X. Zhao, ACS Appl. Mater. Interfaces, 2014, 6, 14981–14987.
12. T. Rennert, A. Pohlmeier and T. Mansfeldt, Environ. Sci. Technol., 2005, 39, 821–825.
13. E. Eren, M. Guney, B. Eren and H. Gumus, Appl. Catal., B, 2013, 132, 370–378.
14. L. Al-Attar, A. Dyer and R. Harjula, J. Mater. Chem., 2003, 13, 2963–2968.
15. L. Dang, C. Wei, H. Ma, Q. Lu and F. Gao, Nanoscale, 2015, 7, 8101–8109.
16. L. Wang and H. Cheng, Environ. Sci. Technol., 2015, 49, 3473–3481.
17. K. W. Nam, S. Kim, S. Lee, M. Salama, I. Shterenberg, Y. Gofer, J.-S. Kim, E. Yang, C. S. Park, J.-S. Kim, S.-S. Lee, W.-S. Chang, S.-G. Doo, Y. N. Jo, Y. Jung, D. Aurbach and J. W. Choi, Nano Lett., 2015, 15, 4071–4079.
18. M. Huang, Y. Zhang, F. Li, L. Zhang, R. S. Ruoff, Z. Wen and Q. Liu, Sci. Rep., 2014, 4, 3878.
19. Y. Sun, L. Wang, Y. Liu and Y. Ren, Small, 2015, 11, 300–305.
20. B. Ming, J. Li, F. Kang, G. Pang, Y. Zhang, L. Chen, J. Xu and X. Wang, J. Power Sources, 2012, 198, 428–431.
21. H. T. Zhu, J. Luo, H. X. Yang, J. K. Liang, G. H. Rao, J. B. Li and Z. M. Du, J. Phys. Chem. C, 2008, 112, 17089–17094.
22. H. Jiang, C. Li, T. Sun and J. Ma, Nanoscale, 2012, 4, 807–812.
23. S.-B. Ma, K.-Y. Ahn, E.-S. Lee, K.-H. Oh and K.-B. Kim, Carbon, 2007, 45, 375–382.
24. H. W. Chang, Y. R. Lu, J. L. Chen, C. L. Chen, J. F. Lee, J. M. Chen, Y. C. Tsai, C. M. Chang, P. H. Yeh, W. C. Chou, Y. H. Liou and C. L. Dong, Nanoscale, 2015, 7, 1725–1735.
25. J. Yan, Z. Fan, T. Wei, W. Qian, M. Zhang and F. Wei, Carbon, 2010, 48, 3825–3833.
26. L. Peng, X. Peng, B. Liu, C. Wu, Y. Xie and G. Yu, Nano Lett., 2013, 13, 2151–2157.
27. H. Jiang, C. Li, T. Sun and J. Ma, Chem. Commun., 2012, 48, 2606–2608.
28. J. Liu, J. Jiang, C. Cheng, H. Li, J. Zhang, H. Gong and H. J. Fan, Adv. Mater., 2011, 23, 2076–2081.
29. C. X. Guo, M. Wang, T. Chen, X. W. Lou and C. M. Li, Adv. Energy Mater., 2011, 1, 736–741.
30. A. H. Webster and J. Halpern, Trans. Faraday Soc., 1957, 53, 51–60.
31. D. Portehault, S. Cassaignon, N. Nassif, E. Baudrin and J. P. Jolivet, Angew. Chem., Int. Ed. Engl., 2008, 47, 6441–6444.
32. W. Xiao, D. Wang and X. W. Lou, J. Phys. Chem. C, 2010, 114, 1694–1700.
33. W. Xiao, J. S. Chen, Q. Lu and X. W. Lou, J. Phys. Chem. C, 2010, 114, 12048–12051.
34. K. Kai, Y. Yoshida, H. Kageyama, G. Saito, T. Ishigaki, Y. Furukawa and J. Kawamata, J. Am. Chem. Soc., 2008, 130, 15938–15943.
35. Y. Omomo, T. Sasaki, L. Wang and M. Watanabe, J. Am. Chem. Soc., 2003, 125, 3568–3575.
36. Y. Oaki and H. Imai, Angew. Chem., Int. Ed. Engl., 2007, 46, 4951–4955.
37. J. Zhu, Q. Li, W. Bi, L. Bai, X. Zhang, J. Zhou and Y. Xie, J. Mater. Chem. A, 2013, 1, 8154–8159.
38. M. L. Chacón-Patiño, C. Blanco-Tirado, J. P. Hinestroza and M. Y. Combariza, Green Chem., 2013, 15, 2920–2928.
39. S. L. Holt and C. J. Ballhausen, Theor. Chim. Acta, 1967, 7, 313–320.
40. F. Polzer, D. A. Kunz, J. Breu and M. Ballauff, Chem. Mater., 2010, 22, 2916–2922.
41. C. Jin, T. C. Nagaiah, W. Xia, B. Spliethoff, S. Wang, M. Bron, W. Schuhmann and M. Muhler, Nanoscale, 2010, 2, 981–987.
42. J. Yan, Z. Fan, T. Wei, J. Cheng, B. Shao, K. Wang, L. Song and M. Zhang, J. Power Sources, 2009, 194, 1202–1207.
43. X. Jin, W. Zhou, S. Zhang and G. Z. Chen, Small, 2007, 3, 1513–1517.
44. M. Liu, W. W. Tjiu, J. Pan, C. Zhang, W. Gao and T. Liu, Nanoscale, 2014, 6, 4233–4242.
45. H. Xia, M. Lai and L. Lu, J. Mater. Chem., 2010, 20, 6896–6902.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22668f

---