The direct growth of highly dispersed CoO nanoparticles on mesoporous carbon as a high-performance electrocatalyst for the oxygen reduction reaction

Abstract

Cobalt monoxide (CoO) nanoparticles (NPs) and mesoporous carbon (MC) with large specific surface area were combined as a novel nanocomposite (CoO/MC) using a hydrothermal method to reveal outstanding electrocatalytic activity in the oxygen reduction reaction (ORR). The addition of polyvinylpyrrolidone (PVP) as the surfactant during the hydrothermal process is beneficial for the high dispersion of CoO NPs on the surface of MC. Among the as-acquired products, the CoO/MC nanocomposite prepared with 1.5 g PVP as the surfactant (CoO/MC-1.5) exhibits much better catalytic activity for the ORR with a more positive onset potential, a highly efficient four-electron transfer pathway and a larger current density than the others. Furthermore, the CoO/MC-1.5 nanocomposite demonstrates outstanding durability based on current–time chronoamperometric tests, which significantly prevails over a commercial Pt/C catalyst. The eminent catalytic activities of the CoO/MC-1.5 nanocomposite should be a result of the synergistic effect of the highly dispersed CoO nanoparticles and the ordered mesostructures with large specific surface area, which are advantageous for increasing the exposure of the active sites and promoting fast transfer of the reactants and products.

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Introduction

Nowadays, the oxygen reduction reaction (ORR) has been one of the concerns of electrochemistry due to the persistent challenges in a series of energy applications containing fuel cells and metal–air batteries.¹ Highly efficient electrocatalysts are desirable due to the slow kinetics of the ORR.² Pt-based metals have been considered as the most active catalysts for the ORR, but they are subject to high cost and poor stability.³ The ORR catalysts with high activity need to lower the overpotential and endure the erosive operating conditions. Nevertheless, the catalysts that have been reported in the literature are still far from satisfying the demands of high catalytic activity, supernal stability and low cost.⁴ Therefore, there are various challenges to develop more efficient electrocatalysts for the ORR by adopting different strategies.

To date, theoretical and experimental results revealed that transition metal oxides are an important class with promising catalytic activity and reliability for the ORR in the alkaline solution.⁵ Furthermore, carbon-based nanomaterials are promising matrix for manufacturing hybrid and composite materials, in terms of their high electrical conductivity, large surface area and structural flexibility.⁶ For example, FeO_x nanoparticles (NPs) embedded in porous carbon synthesized using the chemical vapour deposition method have been reported as an ORR electrocatalyst.⁷ Yusuke Yamauchi and co-workers have reported nanoporous nickel oxide flakes coated with graphene oxide sheets to improve the ORR performance.⁸ Ketjen black carbon supported amorphous manganese oxides nanowires have been investigated as a highly efficient electrocatalyst for the ORR.⁹ Moreover, Dai and his co-workers have studied Co₃O₄ NPs attached on graphene as a synergistic catalyst for the ORR,¹⁰ as well as the CoO nanocrystals coupled with carbon nanotubes.¹¹ In comparison with other carbon materials (e.g., carbon nanotube, graphene, carbon aerogel, and carbon black),¹² mesoporous carbon (MC) synthesized via a nanocasting method has drawn a lot of attention in the field of energy storage and conversion, in terms of its high specific surface area, outstanding electrical conductivity and adjustable mesopore sizes.¹³ Considering its excellent properties, MC can be used as the outstanding matrix for various transition metal oxides as electrocatalysts for the ORR. MC with a large surface area is beneficial for the high dispersion of active nanoparticles, thus resulting in the enhanced exposure of accessible catalytic sites. Thus, MC-supported catalytic materials can show a significantly enhanced activity for the ORR. We have studied spinel CoFe₂O₄ nanoparticles grown on ordered mesoporous carbon in the electrocatalytic ORR.¹⁴ To the best of our knowledge, no more related reports on MC as the support utilized in the ORR have been reported in the previous studies.

In this study, we demonstrated the direct growth of CoO NPs on the surface of MC (CoO/MC) via a facile hydrothermal approach with different contents of polyvinylpyrrolidone (PVP) as the surfactant. The MC utilized as the matrix was synthesized through a traditional nanocasting method with ordered mesoporous silica rod as a hard template.¹⁵ The CoO/MC nanocomposite prepared using 1.5 g PVP as the surfactant demonstrates excellent electrocatalytic activity for the ORR, which was studied using a conventional three electrode equipment in alkaline solution. The CoO/MC-1.5 nanocomposite mainly favours a direct four electron reaction path for the ORR and owns a high current density contrasted to commercial Pt/C. Furthermore, the CoO/MC-1.5 nanocomposite reveals the exceeding long-term reliability for the ORR to commercial Pt/C.

Experimental

Materials synthesis

MC was prepared via a nanocasting method with ordered mesoporous silica (MS) as a hard template, which was prepared in advance according to our previously reported procedure.^14,16 The nanocomposites were synthesized as follows. Typically, MC (0.06 g) was immersed into a mixture of 40 mL distilled water and 20 mL EtOH under ultrasonication for 120 min. Then, Co(NO₃)₂·6H₂O (0.004 mol) was dissolved in the abovementioned suspending solution under vigorous stirring, followed by the addition of different contents of polyvinylpyrrolidone (PVP) as a surfactant (0, 0.5, 1, or 1.5 g). Subsequently, aqueous ammonia (NH₄OH, 6 mL), utilized as the precipitator, was added dropwise under vigorous stirring. After stirring for 60 min, the suspension was transferred into a Teflon-lined stainless autoclave and heated at 180 °C for 12 h. The obtained sediment was collected, centrifuged, washed with ethanol and distilled water and dried in vacuum. Then, the obtained powders were annealed at 400 °C under a N₂ atmosphere for 4 h. The final products were labelled as CoO/MC-X (X = 0, 0.5, 1, or 1.5), in which X stands for the content of PVP used in grams.

Structural characterization

X-ray diffraction (XRD, D8 ADVANCE, Cu Kα radiation, 40 kV, 200 mA) was used to determine the phase and crystal structure of the catalysts. The morphology and microstructure of the acquired catalysts were observed by scanning electron microscopy (SEM, JEOL JSM-6700) and transmission electron microscopy (TEM, JEM-2100F). X-ray photoelectron spectroscopy (XPS, ESCALAB 250 X-ray photoelectron spectrometer, Al K_α radiation) was employed to analyze the surface of the obtained samples. Nitrogen adsorption/desorption measurements (Micromeritics ASAP 2010, −196 °C) were utilized to analyze the surface area and pore size of the catalysts. All the catalysts were dried at 200 °C overnight in vacuum before their measurement. We used the Brunauer–Emmett–Teller (BET) method and Barrett–Joyner–Halenda (BJH) method to calculate the surface area and pore size distributions, respectively.

Electrochemical measurements

The electrochemical activities of the catalysts were tested using a PINE instrument with traditional three electrode equipment in 0.1 M KOH solution employed as the electrolyte. The reference and counter electrodes correspond to the saturated calomel electrode (SCE) and Pt flake electrode, respectively. The working electrode corresponds to glassy carbon electrode (GCE) with a geometric surface area of 0.196 cm². The geometric surface area of the GC disk and Pt ring of the rotating ring-disk electrode (RRDE) were 0.2475 cm² and 0.1866 cm², respectively. The catalyst inks were prepared by mixing the active sample (5 mg), Nafion solution (5 wt%, 30 μL), distilled water (500 μL) and EtOH (500 μL), followed by ultrasonication for about 60 min to form a homogeneous ink. Subsequently, 20 μL of ink was pipetted on a GCE and dried at 50 °C, which was employed for further electrochemical test. The ink of commercial 20 wt% Pt/C (Johnson Matthey) was prepared via the same method for comparison.

For the ORR test, the electrolyte (0.1 M KOH) was purged with high-purity O₂ gas to ensure O₂ saturation. The cyclic voltammetry (CV) measurements were carried out at a sweep rate of 50 mV s⁻¹. The working electrode was cycled at least fifty times ahead of assembling the data. Linear sweep voltammetry (LSV) tests were investigated at a sweep rate of 10 mV s⁻¹, with the electrode rotating at 400, 625, 900, 1225, 1600, and 2025 rpm. Based on the LSV data, we acquired the current densities and normalized the results according to the geometric surface area. We analyze the kinetic parameters of the ORR test using the Koutecky–Levich equation (eqn. (1)):

where j, j_k and j_d represent the measured disk current density, the kinetic and diffusion limiting current densities, respectively; ω represents the electrode rotation speed; B can be analyzed using the following equation (eqn (2)):

B = 0.2nFC_O2D_O2^2/3ν^−1/6 (2)

where n represents the transferred electrons number, F represents Faraday constant (96 485 C mol⁻¹), D_O2 represents the diffusion coefficient of oxygen (D_O2 = 1.86 × 10⁻⁵ cm² s⁻¹), ν represents the kinetic viscosity of solution (ν = 0.01 cm⁻² s⁻¹) and C_O2 represents the bulk concentration of O₂ dissolved in the electrolyte (C_O2 = 1.21 × 10⁻⁶ mol cm⁻³).

For the (RRDE) test, we held the ring potential at 0.2 V (vs. SCE) at a sweep rate of 10 mV s⁻¹. We calculated the percentage of H₂O₂ and the transferred electrons number during the ORR procedure using the following equations (eqn (3) and (4)):

where I_d represents the disk current, I_r represents the ring current, and N represents the current collection efficiency of the Pt ring (N = 0.37).

Results and discussion

The morphology and microstructure of MS and its replica MC were surveyed using scanning electron microscopy (SEM). The SEM image of the parent MS indicates that it is a macroporous skeleton, including interconnected mesoporous silica rods (Fig. 1a). SEM indicated that MC possesses fully interconnected, homogeneous and sponge-like macroporosity (Fig. 1b), which is the positive replica of the parent MS at the micrometer level. Both MS and MC have inerratic branched rods as the building blocks concatenating end-to-end with each other, which fabricate the rigid macro-porous skeletons.

Fig. 1 The SEM images of (a) MS and (b) MC.

The CoO NPs were formed on the surface of MC via a facile hydrothermal treatment, followed by pyrolysis at 400 °C. The resulting CoO/MC nanocomposites were further characterized by the X-ray diffraction (XRD) measurements shown in Fig. 2. Based on the standard PDF card (PDF#43-1004), all the characteristic peaks of the samples can be well indexed as the CoO phase without any collateral peaks. In addition, the content of PVP has no significant effect on the formation of the CoO phase. Furthermore, the content of MC in the CoO/MC-1.5 nanocomposite was about 6.8 wt%, determined by thermogravimetric analysis.

Fig. 2 The X-ray diffraction patterns of the CoO/MC nanocomposites.

The transmission electron microscopy (TEM) image of MC demonstrates the triumphant synthesis of MC (Fig. 3a) with the safeguard of regular ordered hexagonal nanowires arrays via a nanocasting method (inset of Fig. 3a).^15,17 MC with a unique microstructure of ordered mesochannels and large surface area can be employed as the catalytic matrix, which is beneficial for not only dispersing active sites, but also promoting fast transfer of the reactants and resultants. As shown in Fig. 3b, the CoO NPs severely agglomerated in CoO/MC-0 nanocomposite in the absence of PVP as the surfactant. In the presence of PVP as the surfactant (Fig. 3c), the CoO nanoparticles in the CoO/MC-1.5 nanocomposite were well dispersed on MC, which still maintained the ordered mesoporous structures. The histogram of the CoO particle size distribution in CoO/MC-1.5 nanocomposite was in the range of 8–18 nm with a medial particle size of 12.2 nm (Fig. 3d). The MC matrix with large surface area may supply a large number of active sites (e.g. oxygen-containing groups) for the germinating CoO NPs, which possess smaller sizes. The high dispersion and uniformity of CoO NPs on the MC are beneficial for increasing the density of the ORR active sites. Furthermore, the diffusion kinetic of reactants, ions, electrons, and products can be improved in the CoO/MC nanocomposites ascribed to the ordered mesoporous channels. Accordingly, the electrocatalytic ORR performance of the resulting composite will be improved.

Fig. 3 The TEM images of (a) pure MC, (b) CoO/MC-0 and (c) CoO/MC-1.5; the particle size distribution of CoO nanoparticles in the CoO/MC-1.5 nanocomposite (d); the inset of (a) represents the high-magnification TEM images of the selected area.

The cation oxidation state and surface chemical composition of the CoO/MC nanocomposites were inquired by XPS. As demonstrated in Fig. 4a, the XPS survey spectra of both CoO/MC-0 and CoO/MC-1.5 nanocomposites display the peaks for Co 2p, O 1s and C 1s. The de-convolutions of the Co 2p band in the CoO/MC-0 and CoO/MC-1.5 nanocomposites show four peaks (Fig. 4b). The two peaks of 780.5 and 796.3 eV correspond to Co 2p_3/2 and Co 2p_1/2, respectively.¹⁸ The binding energies at around 786.8 and 803.5 eV are two satellite peaks, which are shake-up type peaks of Co at high binding energies. The primary peaks and satellite peaks for Co 2p_3/2 and Co 2p_1/2 reveal the presence of Co²⁺ cations in the CoO/MC nanocomposites. This result is consistent with those obtained from the XRD patterns showing the existence of the CoO phase in the nanocomposites. Moreover, there exhibits no shift in the Co 2p peaks whether PVP was used as the surfactant or not through a comparison of Co 2p spectra of the CoO/MC-0 and CoO/MC-1.5 nanocomposites, which implies that both Co cations have the same chemical surroundings. The usage of PVP only increases the dispersion of CoO NPs on the MC, which does not change the chemical surroundings of the Co cations in the final samples. The C 1s XPS spectra (Fig. 4c) were fitted based on the three carbon bonding conditions: 285.0 eV (C–C), 287.0 eV (C–O–C and C–O–H), and 288.7 eV (O–C=O).¹⁹ The O 1s XPS spectra (Fig. 4d) show a strong distinct M–O–M peak at 530.3 eV, indicating most of the oxygen atoms are in the lattice. Another peak for the O–C=O bond at 531.7 eV, corresponding to the oxygen functional groups on the surface of the MC are due to an incomplete reduction or the moisture adsorbed onto the MC under ambient conditions.²⁰ Furthermore, it can be suggested the existence of strong coupling between the CoO nanoparticles and carbon matrix in the CoO/MC nanocomposites based on previous reports.^14,21

Fig. 4 (a) Full XPS spectra, (b) high-resolution Co 2p XPS spectra, (c) high-resolution C 1s XPS spectra, and (d) high-resolution O 1s XPS spectra of the CoO/MC nanocomposites.

To investigate the porosity of pure MC and the CoO/MC nanocomposites, N₂ sorption measurements were performed at −196 °C. As shown in Fig. 5a and b, the N₂ adsorption–desorption isotherms obtained for both the CoO/MC-0 and CoO/MC-1.5 nanocomposites reveal that they are mesoporous materials, with BET specific surface areas of 250.9 m² g⁻¹ and 290.5 m² g⁻¹, respectively. When compared to that found for pure MC, the specific surface area of the CoO/MC nanocomposites decreased significantly.¹⁶ The N₂ sorption isotherm obtained for MC is a typical IV isotherm with a BET specific surface area of 1103 m² g⁻¹ (Fig. S1a†). The pore size distribution curve for MC exhibits a defined peak at 3.84 nm (Fig. S1b†). The MC with high specific surface area as the support for CoO nanoparticles can help increase the accessible surface area. As we can see, there are two capillary condensation steps in the P/P₀ range of 0.4–0.6 and 0.8–1.0, which imply the existence of dual mesoporosity in both the CoO/MC-0 and CoO/MC-1.5 nanocomposites, respectively. The former was due to the ordered mesoporous channels of the MC support and the latter was ascribed to the large mesopores derived from the accumulation of CoO nanoparticles grown on the MC.²² Fig. 5c and d display the pore size distribution curves obtained for CoO/MC. For CoO/MC-0, the two well-defined peaks, located at 3.38 nm and 11.3 nm, are reflected by two hysteresis loops in the isotherm. However, the peaks for CoO/MC-1.5 are located at 3.38 nm and 9.25 nm and the peak at 9.25 nm is gentle. As shown in the TEM image, the CoO NPs agglomerated without using PVP as the surfactant, thus decreasing the accessible surface area and affecting the pore size distribution of the hybrid catalysts. With PVP as the surfactant, the CoO NPs were symmetrically distributed on the MC, which still maintained the ordered mesoporous channels in the CoO/MC-1.5 nanocomposite. Furthermore, the diffusion kinetic of reactants, ions, electrons, and products can be significantly improved for the CoO/MC nanocomposites by the unique mesoporous structure and large specific surface, and thus, the catalytic activity of the nanocomposites can be increased.

Fig. 5 The N₂ adsorption–desorption isotherms of (a) CoO/MC-0 and (b) CoO/MC-1.5, and the pore size distribution curves of (c) CoO/MC-0 (c) and (d) CoO/MC-1.5.

The electrochemical characterization of the CoO/MC nanocomposites was conducted using traditional three-electrode systems in 0.1 M KOH solution; pure CoO, MC and Pt/C were also tested as a comparison under the same conditions. Fig. 6a exhibits the LSVs in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm. The onset potentials for all the CoO/MC nanocomposites (−0.13 to −0.16 V) significantly precede those obtained for pure CoO (−0.23 V) and MC (−0.29 V). This significant increase in activity found for the nanocomposite can be ascribed to the synergistic effect of both constituents. Furthermore, the onset potential of the CoO/MC-1.5 nanocomposite (−0.13 V) was superior to those obtained for CoO/MC-0 (−0.16 V), CoO/MC-0.5 (−0.16 V) and CoO/MC-1 (−0.14 V), which is due to the high dispersion of nanoparticles in the nanocomposite that can provide more active sites for the ORR. To analyze the kinetic properties of the ORR on the as-prepared electrocatalysts, Tafel slopes were obtained from the linear plots of the LSVs (Fig. S2†). The CoO/MC-1.5 nanocomposite exhibits a lower Tafel slope (95 mV dec⁻¹) than CoO/MC-0 (101 mV dec⁻¹), implying enhanced ORR kinetics due to the high dispersion of CoO nanoparticles on the MC. The CoO/MC-1.5 nanocomposite shows a more supernal limit current density (5.96 mA cm⁻²) than the other catalysts at −1.0 V (Fig. 6b), and the commercial Pt/C catalyst (5.52 mA cm⁻²). Moreover, the limiting current density of the CoO/MC-1.5 nanocomposite was also higher than those for CoO/N-CNT,¹¹ Fe₃O₄/N-graphene,²³ NiO/CNT,²⁴ MnO₂/CNT,²⁵ Co₃O₄/graphene,²⁶ and quasi oxygen-deficient indium tin oxide nanoparticles.²⁷ It can be concluded that the CoO/MC-1.5 nanocomposite prepared using 1.5 g PVP as the surfactant during the hydrothermal process has optimal activity for the ORR among the as-acquired catalysts, probably due to having the highest dispersion of CoO nanoparticles in the nanocomposite. Fig. 6c demonstrates the CV curves for the CoO/MC-1.5 nanocomposite in N₂ and O₂-saturated 0.1 M KOH solution. In N₂-saturated solution, no redox peak can be found for the CoO/MC-1.5 nanocomposite. However, it exhibits an obvious reduction peak at −0.26 V in O₂-saturated solution, which could hint that O₂ is reduced on the surface of the CoO/MC-1.5 nanocomposite. Fig. 6d demonstrates the LSVs at different rotation rates for the ORR catalyzed by the CoO/MC-1.5 nanocomposite of which the diffusion limiting currents increase gradually as the rotation rate was increased. Moreover, the RDE data were collected by the LSVs utilizing the K–L equation. The corresponding K–L plots for the CoO/MC-1.5 nanocomposite are shown in Fig. 6e, which were obtained at −0.5, −0.6, −0.7 and −0.8 V. The parallel and linear K–L plots can reveal the first-order dependence of the kinetics of the ORR on the CoO/MC-1.5 nanocomposite. The calculated results indicate an electron transfer number of ∼4.0. Fig. 6f summarizes the number of transferred electrons (n) for CoO/MC-0, CoO/MC-1.5 and commercial Pt/C, which demonstrate the favorable selectivity for the 4-electron reaction pathway of the CoO/MC-1.5 nanocomposite during the ORR. Furthermore, our catalyst shows preferable or considerable performance to the previous related report.²⁸

Fig. 6 (a) The LSVs of the CoO/MC nanocomposites, CoO, MC and commercial Pt/C in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm; (b) the limit current density of the CoO/MC nanocomposites and commercial Pt/C in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm; (c) the CV curves of the CoO/MC-1.5 nanocomposite in N₂ and O₂-saturated solution; and (d) the LSVs for the ORR using the CoO/MC-1.5 nanocomposite in O₂-saturated 0.1 M KOH solution at different rotation rates; (e) the K–L plots based on the ORR curves of the CoO/MC-1.5 nanocomposite, and (f) the number of transferred electrons (n) of CoO/MC-0, CoO/MC-1.5 and commercial Pt/C.

To further verify the electron transfer pathway during the ORR, rotating ring-disk electrode (RRDE) equipment was employed, with which we could accurately calculate the amount of H₂O₂ generated at the disk electrode. The LSVs for CoO/MC-0, CoO/MC-1.5 and Pt/C (Fig. 7a) were collected in O₂-saturated 0.1 M KOH solution using a ring potential of 0.2 V (vs. SCE). The CoO/MC-1.5 nanocomposite shows the positive onset potential and supernal current density than those of the CoO/MC-0 nanocomposite. The formation of peroxide species (HO₂⁻) and the number of transferred electrons (n) according to the corresponding RRDE data during the ORR procedure are shown in Fig. 7b. The calculated HO₂⁻ yield of the CoO/MC-1.5 nanocomposite and Pt/C were 1.47–1.69% and 0.91–2.51%, respectively, during the potential range from −0.8 to −0.4 V (vs. SCE). The calculated n values obtained for the CoO/MC-1.5 nanocomposite and Pt/C were 3.96–3.97 and 3.95–3.98. The ultra-low HO₂⁻ production and n value very close to 4 found for the CoO/MC-1.5 nanocomposite indicate its high activity for the ORR and high selectivity for the 4-electron reaction pathway during the ORR process.

Fig. 7 (a) The LSVs on the RRDE for the CoO/MC nanocomposites and commercial Pt/C in O₂-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm. The ring potential was held at 0.2 V (vs. SCE); (b) the calculated number of transferred electrons (n) and the determined peroxide percentage over various potentials based on the corresponding RRDE data in (a).

The stability of the catalysts is one of the important factors for the development of fuel cells or metal–air batteries. The durability of the CoO/MC nanocomposites and Pt/C were tested using chronoamperometry in O₂-saturated 0.1 M KOH solution, respectively. As indicated in Fig. 8, the current density of the ORR for CoO/MC-1.5 decreased by about 8.6% at a constant potential of −0.6 V (vs. SCE) after 20 000 s of sustained operation, while the current density of the ORR for CoO/MC-0 decreased by 29.1% after 20 000 s. Furthermore, the worst case was Pt/C, the current density for which decreased by about 39.7%. Using PVP as the surfactant, the CoO NPs were symmetrically distributed on the surface of MC, which prevents the detachment and aggregation of the CoO NPs over the catalytic procedure and thus enhance the electrode cyclic stability. These results indicate the CoO/MC-1.5 nanocomposite has significant long-term durability in alkaline media during the ORR.

Fig. 8 The current–time (i–t) chronoamperometric responses for the ORR on the CoO/MC nanocomposites and commercial Pt/C in O₂-saturated 0.1 M KOH solution at −0.6 V and a rotating speed of 1600 rpm.

Conclusions

In summary, we have synthesized highly dispersed CoO NPs directly grown on the surface of mesoporous carbon (MC) via a facile hydrothermal method. With 1.5 g PVP as the surfactant, the resulting CoO/MC-1.5 nanocomposite shows optimal electrocatalytic activity for the ORR among the various CoO/MC catalysts prepared with different contents of PVP. The CoO/MC-1.5 nanocomposite displays a 4-electron transfer path in the ORR process, in terms of the K–L plots and RRDE data. Moreover, the CoO/MC-1.5 nanocomposite exhibits the supernal durability for the ORR, which exceeds that found for Pt/C. The excellent electrocatalytic activity of the CoO/MC-1.5 nanocomposite was primarily attributed to the synergistic effect of the uniform dispersed CoO NPs and ordered mesoporous carbon matrix with high specific surface areas.

Acknowledgements

The authors would like to thank the financial support provided by the National Natural Science Foundation of China (No. 51502327, 21307145), Key Project for Young Researcher of State Key Laboratory of High Performance Ceramics and Superfine Microstructure, One Hundred Talent Plan of Chinese Academy of Sciences, the Youth Science and Technology Talents “Sail” Program (No. 15YF1413800) and International Cooperation Program (15520720400) of Shanghai Municipal Science and Technology Commission, and the Research Grant (No. 14DZ2261203) obtained from the Shanghai Government.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Additional Fig. S1 and S2 mentioned in the text. See DOI: 10.1039/c6ra14394f

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