Solvothermally synthesized graphene nanosheets supporting spinel NiFe₂O₄ nanoparticles as an efficient electrocatalyst for the oxygen reduction reaction

Abstract

The production of efficient and low-cost electrocatalysts for the oxygen reduction reaction (ORR) is one of the key issues for the extensive commercialization of fuel cells. In this paper, we describe a facile one-pot hydrothermal synthesis route to in situ grow spinel NiFe₂O₄ nanoparticles onto the graphene nanosheets which were produced in advance by a scalable solvothermal reduction of chloromethane and metallic potassium. The resultant NiFe₂O₄/graphene nanohybrid exhibits superior electrocatalytic activity for the ORR to pure graphene nanosheets and unsupported NiFe₂O₄ nanoparticles, which mainly favours a desirable direct 4e⁻ reaction pathway during the ORR process. Meanwhile, the NiFe₂O₄/graphene nanohybrid exhibits the outstanding long-term stability for the ORR, outperforming the commercial 20 wt% Pt/C based on the current–time chronoamperometric test. The excellent catalytic activity and stability of NiFe₂O₄/graphene nanohybrid are ascribed to the strong coupling and synergistic effect between NiFe₂O₄ nanoparticles and graphene nanosheets.

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Introduction

With the continuous depletion of fossil fuels and the increasing requirement of energy, the research of clean and efficient energy sources has attracted worldwide attention.¹ Consequently, fuel cells have given rise to special interest among the whole world with low or zero emissions from operation. Electrocatalytic oxygen reduction reaction (ORR) plays a crucial role in the development of fuel cells.² However, the obtuse kinetics of the ORR precludes the performance of fuel cells.³ As we know, Pt and its alloys are the most reliable electrocatalysts for the ORR.⁴ Nevertheless, the high-cost and exiguity of Pt hamper its extensive commercial production and application as the electrocatalysts for the ORR. Therefore, it is expected that low-cost and high-performance electrocatalytic materials can be prepared to replace the expensive Pt-based electrocatalysts at present.

Recently, mixed valence oxides of transition metals with a spinel structure (AB₂O₄) are emerging as promising electrocatalysts for the ORR considering their low-cost, considerable activity, high stability as well as environmental friendliness.⁵ To our knowledge, cobaltite spinel oxides M_xCo_3−xO₄ (M = Cu, Mn) have been reported as outstanding electrocatalysts for the ORR.^5b,6 Because spinel oxides are semi-conducting, they are usually attached to or supported on a conducting surface (e.g. porous carbon, graphene) to ensure fast electron transport. Pu et al. has reported that spinel ZnCo₂O₄/N-doped carbon nanotube composite prepared by solvothermal method exhibited efficient activities for the ORR.⁷ Liu et al. studied the use of CoFe₂O₄/biocarbon nanohybrid as the excellent electrocatalyst for the ORR.⁸ Compared with other carbon materials, graphene, which has a two-dimensional carbon nanostructure, has been widely investigated due to its unique characteristics, such as chemical stability, high electrical conductivity, and large surface area.⁹ Based on these encouraging characteristics, graphene can be considered as an excellent substrate for spinel oxides. The CoFe₂O₄/graphene composite has been synthesised by the hydrothermal method, which shows promising electrocatalyst activity for the ORR. Moreover, Lee et al. has synthesized a mesoporous NiCo₂O₄ nanoplatelet–graphene hybrid as a highly active catalyst for the ORR by a one-pot precipitation reaction and hydrothermal process. Considering relatively high cost of Co elements in spinel oxides showing the electrocatalytic activity for the ORR, some cheaper spinel oxides such as NiFe₂O₄ can be further investigated as the electrocatalysts for the ORR. To the best of our knowledge, the synthesis of NiFe₂O₄ and its nanocomposites have been reported in the fields of photocatalyst,¹⁰ microwave absorbers,¹¹ electrochemical sensor,¹² supercapacitor¹³ and Li ion batteries.¹⁴ However, the related studies on its electrocatalytic activity for the ORR are seldom reported.

In this work, we demonstrate the synthesis of NiFe₂O₄/graphene nanohybrid by a hydrothermal strategy. The graphene nanosheets used as the supports were in advance prepared by a facile, scalable solvothermal reduction of carbon chloride (CCl₄) by potassium (K). The electrocatalytic activities of NiFe₂O₄/graphene nanohybrid for the ORR in alkaline solution were investigated by rotating disk (RDE) voltammetry, rotating ring-disk electrode (RRDE) voltammetry and cyclic voltammetry (CV) methods. Based on the strong coupling between NiFe₂O₄ nanoparticles and graphene nanosheets, the as-synthesized NiFe₂O₄/graphene nanohybrid shows high ORR performance in alkaline 0.1 M KOH solution, but also superior durability to commercial Pt/C electrocatalyst.

Experimental section

Preparation of graphene

Different from the traditional preparation method of graphene (chemical exfoliation^9c and chemical vapour deposition¹⁵), the graphene nanosheets used as the supports are synthesized by a solvothermal reduction method.¹⁶ Typically, 5 mL of tetrachloromethane (CCl₄) was placed into an 80 mL Teflon-lined stainless steel autoclave, and then 1 g of metallic potassium (K) was rapidly added to the autoclave in a plastic glove box purged with Ar gas. The autoclave sealed in the glove box was heated at 180 °C in an oven for 6 h, and then naturally cooled down to room temperature. The resultant product was dispersed in a mixed solution of ethanol and distilled water (1 : 1 vol%) under magnetic stirring for 2 h to remove the residual CCl₄. The black graphene product was obtained by filtering, washing with ethanol and water several times, and then drying at 80 °C for 12 h.

Preparation of NiFe₂O₄/graphene nanohybrid

The NiFe₂O₄/graphene nanohybrid was prepared by a conventional hydrothermal method. In a typical synthesis procedure, 0.04 g of graphene was added to 40 mL of H₂O and 20 mL of EtOH and subjected to ultrasonication for 120 min to form a suspending solution. Then, 0.582 g of Ni(NO₃)₂·6H₂O, 1.616 g of Fe(NO₃)₃·9H₂O (the molar ratio of Ni/Fe = 1/2) and 0.4 g of polyvinylpyrrolidone (PVP) were dissolved under magnetic stirring. Subsequently, 3 g of CO(NH₂)₂ was added under vigorously stirring until it was completely dissolved. The mixed solution was transferred into a Teflon-lined stainless autoclave and heated to 180 °C for 12 h. After the black product was collected by filtering, washed with ethanol and deionized water, vacuum dried, it was then treated at 500 °C in nitrogen for 4 h to remove the residual PVP molecules in the composite. The pure NiFe₂O₄ nanoparticles were prepared using the same procedure without adding graphene.

Characterization

X-ray powder diffraction (XRD) patterns were collected using a diffractometer (D8 ADVANCE) with Cu Kα radiation (40 kV and 200 mA). Transmission electron microscopy (TEM) images were acquired using a JEM-2100F transmission electron microscope. To confirm the amount of graphene in NiFe₂O₄/graphene, thermogravimetric analysis (TG) was performed on a thermal analysis instrument (STA449C) with a heating rate of 10 °C min⁻¹ in an air flow and the temperature ranging from 30 to 800 °C. Raman spectra were recorded on a DXR Raman Microscope (Thermal Scientific Co., USA) with 532 nm excitation length.

Electrode preparation and electrochemical measurements

The electrochemical activities of the materials were carried out on a PINE instrument using a conventional three electrode system with 0.1 M KOH aqueous solution via rotating disk electrode (RDE) voltammetry, rotating ring-disk electrode (RRDE) voltammetry and cyclic voltammetry (CV) for the ORR. The Ag/AgCl electrode and Pt flake electrode were used as the reference and counter electrodes, respectively. Glassy carbon electrode (GCE, diameter = 5 mm) was used as the working electrode. The RRDE electrode consisted of a GC disk (0.2475 cm² of geometric surface area), surrounded by a Pt ring (0.1866 cm² of geometric surface area). 5 mg of active sample was dispersed in 1 mL 1 : 1 (v/v) water/ethanol mixed solution which contains 30 μL of Nafion solution (5 wt%) by sonication for about 2 h to form a homogeneous ink finally. Then, 20 μL of ink was pipetted on a GCE, yielding a catalyst level of 0.64 mg cm⁻². The electrode with the catalyst was dried at 50 °C, which was used as the working electrode for further electrochemical measurements. The commercial 20 wt% Pt/C (Johnson Matthey) was used for comparison.

For the ORR test, the electrolyte was purged with high-purity O₂ gas to ensure O₂ saturation. The CV measurement was performed in by sweeping the potential from 0 V cathodically to −0.9 V at a scan rate of 50 mV s⁻¹. The LSV tests were carried out by sweeping the potential from 0 V cathodically to −0.9 V at a scan rate of 10 mV s⁻¹, with the electrode rotating at 400, 625, 900, 1225, 1600, 2025 rpm. The kinetic parameters of the ORR tests could be determined by the Koutecky–Levich equation (eqn (1)):

where j, j_k and j_d were corresponded to the measured disk current density, the kinetic and diffusion limiting current densities, respectively; ω is the electrode rotation speed; B can be analyzed on the following equation (eqn (2)):in which n is the transferred electrons number of the ORR pathway, F is the Faraday constant (96 485 C mol⁻¹), D_O2 is the diffusion coefficient of oxygen (D_O2 = 1.86 × 10⁻⁵ cm² s⁻¹), ν is the kinetic viscosity of solution (ν = 0.01 cm⁻² s⁻¹), C_O2 is the bulk concentration of O₂ dissolved in electrolyte (C_O2 = 1.21 × 10⁻⁶ mol cm⁻³).

For the RRDE test, the ring potential was held at 0.2 V (vs. Ag/AgCl) at a scanning rate of 10 mV s⁻¹. The percentage of H₂O₂ and the transferred electrons number during the ORR process can be calculated via the following equations (eqn (3) and (4)):

in which I_d is the disk current, I_r is the ring current, and N is the current collection efficiency of the Pt ring (N = 0.37).

Results and discussion

TG measurement was performed to confirm the content of graphene in the NiFe₂O₄/graphene nanohybrid. Fig. 1 shows the TG curves of NiFe₂O₄/graphene powder measured in an air flow from 30 to 800 °C. It can be clearly observed that there is a weight decrease of about 1.2 wt% from 30 to 210 °C, which is ascribed to the desorption of physically absorbed water as well as the decomposition of the hydroxide compound in the precursor. The weight loss about 6.0 wt% from 450 to 660 °C corresponds to the weight loss due to the combustion of graphene among the NiFe₂O₄/graphene.

Fig. 1 TG curve of NiFe₂O₄/graphene in an air flow from 30 to 800 °C.

The XRD profiles of the NiFe₂O₄ and NiFe₂O₄/graphene are shown in Fig. 2. All the characteristic peaks can be well indexed as cubic spinel phase (PDF#54-0964) with no collateral peaks. The characteristic peaks occurring at 2θ of 18.4°, 30.4°, 35.8°, 43.4°, 53.9°, 57.4°, 63.0° and 74.6° correspond to the crystal planes of (111), (220), (311), (400), (422), (511), (440) and (533), respectively. Furthermore, the intense and sharp peaks indicate that both pure NiFe₂O₄ nanoparticles and NiFe₂O₄/graphene nanohybrid are well crystallized. No addition peaks ascribed to the graphene nanosheets were detected because of very low content of graphene in the composite.

Fig. 2 XRD patterns of pure NiFe₂O₄ and NiFe₂O₄/graphene nanohybrid.

The morphology and microstructure of the as-synthesized graphene, NiFe₂O₄ and NiFe₂O₄/graphene nanohybrid were investigated by transmission electron microscopy (TEM). Fig. 3a and Fig. S1† show the TEM and high-resolution TEM (HR-TEM) images of the as-resulting graphene nanosheets prepared by the solvothermal reduction procedure, indicating the porous textures with the number of lower than ten layers and the lattice spacing is calculated to be ca. 0.37 nm. Fig. 3b shows the TEM and particle size distribution images of the unsupported NiFe₂O₄ synthesized by the hydrothermal method. As we can see, the particle size of NiFe₂O₄ nanoparticles is in the range from 20 to 40 nm, with the average particle size of 29 nm. By the one-pot hydrothermal treatment, the NiFe₂O₄ nanoparticles in situ grew on graphene nanosheets successfully, as confirmed by TEM. As shown in Fig. 3c–d, the TEM images of NiFe₂O₄/graphene nanohybrid with different magnifications indicate that the NiFe₂O₄ nanoparticles were well dispersed on the graphene nanosheets. The particle size of NiFe₂O₄ distributed on the graphene sheets is in the range from 25 to 50 nm (insert in Fig. 3c), with the average particle size of 36 nm, which is a little larger than that of pure NiFe₂O₄. In addition, HR-TEM can provide more information about the crystal structure of the NiFe₂O₄ nanoparticles growing on the graphene sheets. The measured d spacing of 0.35 nm is labelled as the lattice spacing of the (220) plane of NiFe₂O₄ (Fig. 3e). Selected area electron diffraction (SAED) patterns of NiFe₂O₄ particle on the graphene sheets are demonstrated in Fig. 3f. The five most-distinct concentric diffraction rings from the centre correspond to the crystal planes of (220), (311), (400), (511) and (440), which agree well with the results acquired from the XRD pattern (Fig. 2).

Fig. 3 TEM images of (a) pure graphene, (b) unsupported NiFe₂O₄ particles, and (c and d) TEM images of NiFe₂O₄/graphene nanohybrid with different magnifications. High-resolution TEM image of a NiFe₂O₄ nanoparticles in situ grown on graphene nanosheets (e); selected area electron diffraction (SAED) pattern of NiFe₂O₄/graphene (f); inset of (b): particle size distribution of pure NiFe₂O₄ nanoparticles; inset of (c): particle size distribution of NiFe₂O₄ nanoparticles in NiFe₂O₄/graphene nanohybrid.

Raman spectroscopy provides more information about the structural property of the as-prepared NiFe₂O₄/graphene nanohybrid. As displayed in Fig. 4a, for the as-synthesized graphene, the characteristic D and G bands of carbon materials locate at around 1335 and 1574 cm⁻¹, respectively. The D band is associated with disordered samples or graphene edges, while the G band is the result of the first-order scattering of the E_2g mode of sp² carbon domains.¹⁷ Another prominent feature of graphene is the second-order two phonon mode 2D band at about 2667 cm⁻¹ as shown in the spectrum of graphene, which corresponds to the graphitic graphene and is sensitive to the layers of graphene.¹⁸ After in situ growth of NiFe₂O₄ nanoparticles on the graphene sheets, the D band shifts to 1329 cm⁻¹ and the G band shifts to 1600 cm⁻¹ for the resulting NiFe₂O₄/graphene nanohybrid as shown in Fig. 4a. In addition, the intensity of 2D band for the NiFe₂O₄/graphene strongly weakens compared to that for the pure graphene, indicating a decrease in the layer number of the graphene nanosheets. The shifts of D and G bands and the decrease of 2D for the graphene hybridized with NiFe₂O₄ nanoparticles are possibly ascribed to the strong interaction of NiFe₂O₄ and graphene surface.

Fig. 4 (a) Raman spectroscopy of graphene and NiFe₂O₄/graphene and (b) Raman spectroscopy of NiFe₂O₄/graphene nanohybrid.

Fig. 4b shows the Raman spectroscopy of NiFe₂O₄/graphene nanohybrid in the region of NiFe₂O₄. All peaks are assigned as normal spinel structure. The A_1g mode is due to symmetric stretching of oxygen atoms along Fe–O (and Ni–O) bonds in the tetrahedral coordination. E_g is due to symmetric bending of oxygen with respect to the metal ion and F_2g(3) is caused by asymmetric bending of oxygen. F_2g(2) is due to asymmetric stretching of Fe (Ni) and O. F_2g(2) and F_2g(3) correspond to the vibrations of octahedral group. F_2g(1) is due to translational movement of the tetrahedron (metal ion at tetrahedral site together with four oxygen atoms). Compared to those for pure NiFe₂O₄, there is a slight displacement of metal atoms in modes in A_1g, E_g and F_2g(3).¹⁹ It evidently shows the existence of the coupling via some chemical bonds between NiFe₂O₄ nanoparticles and graphene nanosheets, which forms during the hydrothermal process and subsequent pyrolysis.

The ORR activity of NiFe₂O₄/graphene nanohybrid was measured using a three-electrode electrochemical station by CV and RDE (Fig. 5). In order to understand the electrocatalytic performance of NiFe₂O₄/graphene nanohybrid during the ORR process, the ORR activities of pure NiFe₂O₄, graphene and commercial Pt/C are also included for comparison.

Fig. 5 (a) CV curves of NiFe₂O₄/graphene nanohybrid in N₂ and O₂-saturated 0.1 M KOH solution; (b) LSVs of pure graphene, NiFe₂O₄, NiFe₂O₄/graphene nanohybrid and commercial Pt/C in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm; (c) LSVs for the ORR of NiFe₂O₄/graphene nanohybrid at different rotation rates; Koutecky–Levich plots of NiFe₂O₄/graphene nanohybrid (d), graphene (e) and NiFe₂O₄ (f) based on the LSVs in O₂-saturated 0.1 M KOH solution at different rotation speeds.

Fig. 5a demonstrates the CV curves of NiFe₂O₄/graphene nanohybrid in N₂ and O₂-saturated 0.1 M KOH solution. The NiFe₂O₄/graphene shows no redox peak in potential range from −0.9 to 0 V (vs. Ag/AgCl) in N₂-saturated 0.1 M KOH solution. However, an evident reduction peak corresponding to the ORR at −0.36 V (vs. Ag/AgCl) can be observed in O₂-saturated solution, indicating the occurrence of the ORR on the surface of NiFe₂O₄/graphene nanohybrid. Fig. 5b exhibits the LSVs of pure graphene, NiFe₂O₄, NiFe₂O₄/graphene nanohybrid and Pt/C in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm. The NiFe₂O₄/graphene nanohybrid shows the higher diffusion-limiting current density and more positive onset potential than pure NiFe₂O₄ nanoparticles and graphene nanosheets, although pure NiFe₂O₄ nanoparticles possess smaller uniform particle sizes. Due to the high electric conductivity of the graphene, the hybridization of NiFe₂O₄ nanoparticles with graphene nanosheets can endow the resulting nanohybrid with high electric conductivity, which is favourable for increasing the ORR activity. Meanwhile, the two-dimensional structure of graphene allows the easy access of O₂ from both sides to the actives sites for the ORR.²⁰ The ORR activity of NiFe₂O₄/graphene nanohybrid is enhanced compared to those of pure graphene and NiFe₂O₄, also suggesting the synergistic effect of two components in the composite possibly ascribed to the strong coupling between NiFe₂O₄ and graphene. The analysis of the Tafel plots for the samples in lower overpotential region are shown in Fig. S2.† The NiFe₂O₄/graphene nanohybrid shows a lower Tafel slope (98 mV dec⁻¹) than pure NiFe₂O₄ (108 mV dec⁻¹) and graphene (115 mV dec⁻¹), which can indicate the enhanced ORR kinetics after in situ growth of NiFe₂O₄ nanoparticles on the graphene sheets. The LSVs for the ORR of pure graphene, NiFe₂O₄ nanoparticles, and NiFe₂O₄/graphene nanohybrid at different rotation rates are shown in Fig. S3–4,† and Fig. 5c, respectively. As we can see, with the rotation rates increasing, the diffusion limiting currents are improved. Fig. 5d demonstrates the corresponding Koutecky–Levich plots obtained from the inverse current density (j⁻¹) as a function of the inverse of the square root of the rotation rate (ω^−1/2) for NiFe₂O₄/graphene at −0.5, −0.6, −0.7 and −0.8 V (vs. Ag/AgCl), respectively. The plots are parallel and linear, which can indicate the first-order dependence of the kinetics of the ORR of NiFe₂O₄/graphene surface. The transferred electrons number n calculated from the B-factor is in the range of 3.9–4.0, suggesting that NiFe₂O₄/graphene favours a desirable 4e⁻ reduction reaction process to obtain maximum energy capacity.²¹ In comparison, the transferred electron number n of the as-synthesized graphene ranges from 3.6 to 3.7 (Fig. 5e) and pure NiFe₂O₄ has n value ranging from 2.1 to 3.1 (Fig. 5f) calculated based on the Koutecky–Levich plots derived from their LSV curves (Fig. S3–S4†), indicating that they demonstrate the lower selectivity for the direct 4e⁻ transfer pathway than the NiFe₂O₄/graphene nanohybrid. These results indicate that the integration of NiFe₂O₄ and graphene could not only significantly improve the electrocatalyst activity of NiFe₂O₄/graphene but also obtain the desirable 4e⁻ ORR catalytic pathway.²²

In order to further confirm the ORR catalytic activities and pathways of the NiFe₂O₄/graphene nanohybrid, the electrocatalytic activities of NiFe₂O₄/graphene and Pt/C (Fig. 6a) were further investigated by rotating ring-disk electrode (RRDE) in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm with the ring potential at 0.2 V (vs. Ag/AgCl). The NiFe₂O₄/graphene nanohybrid shows high current density compared to Pt/C. The yield of peroxide species (HO₂⁻) and electrons transferred number (n) based on the corresponding RRDE data during the ORR procedure were shown Fig. 6b. The measured HO₂⁻ yield of NiFe₂O₄/graphene nanohybrid and Pt/C is 4.94–15.6% and 7.39–8.48%, respectively, over the potential range from −0.9 to −0.5 V (vs. Ag/AgCl). The calculated electrons transferred numbers for NiFe₂O₄/graphene nanohybrid and Pt/C are 3.68–3.90 and 3.83–3.86, respectively. The low HO₂⁻ production and large n values of NiFe₂O₄/graphene nanohybrid indicate the desirable 4e⁻ transfer pathway for the ORR. These results are well consistent with those obtained by RDE measurements.

Fig. 6 (a) LSVs on the RRDE for NiFe₂O₄/graphene nanohybrid and commercial Pt/C in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm (the ring potential: 0.2 V (vs. Ag/AgCl)). (b) Calculated electron transfer number n and HO₂⁻ percentage at various potentials based on the corresponding RRDE data in (a).

Another main challenge for fuel cell applications is the long-term durability of the catalysts. The stabilities of NiFe₂O₄/graphene and commercial Pt/C for the ORR were examined using the chronoamperometric method in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm. As shown in Fig. 7, the ORR current density of NiFe₂O₄/graphene decreases about 17% at a constant potential of −0.6 V (vs. Ag/AgCl) over 20 000 s of continuous operation, while the ORR density current of commercial Pt/C decreases by 30% after 20 000 s. These results reveal that the resulting NiFe₂O₄/graphene nanohybrid is quite stable for the ORR. The strong coupling between NiFe₂O₄ and graphene can prevents detachment and aggregation of NiFe₂O₄ during the ORR process, which improves the electrode cyclic stability.

Fig. 7 Current–time (i–t) chronoamperometric response for NiFe₂O₄/graphene nanohybrid and Pt/C at −0.6 V (vs. Ag/AgCl) in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm.

Conclusions

In summary, we have reported a facile in situ growth strategy for preparing the NiFe₂O₄/graphene nanohybrid electrocatalysts via a one-pot hydrothermal method. The graphene nanosheets used as the supports were in advance prepared by the solvothermal reduction procedure. The resultant NiFe₂O₄/graphene nanohybrid has the particle size ranging from 25 to 50 nm with the average particle size of 36 nm. Raman analysis confirmed the existence of the strong coupling between NiFe₂O₄ nanoparticles and graphene nanosheets. The results of the electrochemical measurements indicate that the NiFe₂O₄/graphene possess the promising activity for the ORR. It demonstrates high activity for the ORR with a four-electron reaction pathway while pure NiFe₂O₄ nanoparticles and graphene nanosheets promote the ORR pathway with a low efficient two-electron pathway. At the same time, it can be found that the NiFe₂O₄/graphene is quite stable during the ORR process, which outperforms the commercial Pt/C electrocatalyst. The excellent catalytic activity and stability of NiFe₂O₄/graphene nanohybrid are ascribed to the strong coupling and synergistic effect between NiFe₂O₄ and graphene. As a result, the NiFe₂O₄/graphene nanohybrid could be considered as a potential electrocatalyst for fuel cells.

Acknowledgements

The authors thank the financial support from Shanghai Institute of Ceramics, the One Hundred Talent Plan of Chinese Academy of Sciences, National Natural Science Foundation of China (no. 21307145), the Youth Science and Technology Talents “Sail” Program of Shanghai Municipal Science and Technology Commission (no. 15YF1413800), and the Research Grant (no. 14DZ2261200) from Shanghai Government.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional figures mentioned in the main text. See DOI: 10.1039/c5ra08368k

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