Ultrathin two-dimensional cobalt–organic framework nanosheets for high-performance electrocatalytic oxygen evolution

Yuxia Xu a, Bing Li a, Shasha Zheng a, Ping Wu a, Jingyi Zhan a, Huaiguo Xue a, Qiang Xu ab and Huan Pang *a
aSchool of Chemistry and Chemical Engineering, Institute for Innovative Materials and Energy, Yangzhou University, Yangzhou, 225009, Jiangsu, P. R. China. E-mail: huanpangchem@hotmail.com; panghuan@yzu.edu.cn
bAIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), Yoshida, Sakyo-ku, Kyoto 606-8501, Japan

Received 5th April 2018 , Accepted 13th June 2018

First published on 13th June 2018

Ultrathin two-dimensional cobalt–organic framework (Co–MOF) nanosheets [Co2(OH)2BDC, BDC = 1,4-benzenedicarboxylate] with good catalytic activity for the oxygen evolution reaction were synthesized by a simple surfactant-assisted hydrothermal method. The resulting material shows a low overpotential of 263 mV at 10 mA cm−2 and a Tafel slope of 74 mV dec−1 in 1.0 M KOH. The morphology and properties of the material before and after electrocatalysis did not change significantly over a long period of time and it maintained electrochemical stability for 12[thin space (1/6-em)]000 s. The unsaturated CoII active center on the surface of the ultrathin two-dimensional cobalt–organic framework nanosheets enhances their oxygen evolution reaction performance. These results are significantly superior to most of the previously reported transition metal–organic framework or metal oxide electrocatalysts. We believe that the ultrathin two-dimensional metal–organic framework nanosheets synthesized by this simple process will be widely used in future energy conversion devices.


Since 2004, ultrathin two-dimensional (2D) nanomaterials have attracted widespread attention due to the discovery of graphene stripping from graphite by a mechanical cracking method.1,2 As a class of emerging nanomaterials, ultrathin 2D nanomaterials have been extensively studied by researchers in recent years.3–7 Ultrathin 2D metal–organic framework (MOF) nanosheets, which represent a class of attractive chemical and structural materials, are usually prepared in powder form.8 While some MOF nanosheets exhibit outstanding properties in sensing9 and separation,10 there are still many other applications that need to be explored, such as catalysis,11 electronics12 and energy storage.13 Therefore, the development of a simple, feasible preparation method to explore the synthesis of ultrathin 2D MOF nanosheets is significant for their performance, function and application.

In recent years, oxygen evolution reaction (OER) electrocatalysis has drawn much attention because it plays a prime role in practical applications of water splitting, fuel cells, and metal–air batteries.14–16 Many transition metals have OER activity and are largely insoluble in alkaline electrolytes, because of which efficient, alternative homogeneous and heterogeneous electrocatalysts have become a subject of intense focus.17,18 MOFs are a new type of crystalline and microporous material formed by coordination bonds between organic ligands and metal atoms.19–21 Due to their inherent characteristics, including large surface area, unique porosity, and tailorable functionality, MOFs with various types of catalytic sites can provide the inherent advantages of both homogeneous and heterogeneous catalysts.22–24 Although MOFs have abundant metal sites, few of them have, however, been directly used for electrocatalysis due to their poor conductivity.19,25–27 Recent reports have revealed that Ni-, Fe- and Co-based MOFs are catalytically active for the OER,19,25,28–34 while the catalytic performances of MOFs with large sizes need significant improvements.

In this work, we report ultrathin 2D Co–MOF nanosheets synthesized from a mixed solution of Co2+ and benzenedicarboxylic acid (BDC) by a one-pot hydrothermal method. The obtained product exhibits excellent electrocatalytic activity towards the OER, demonstrating a low overpotential of ∼263 mV at 10 mA cm−2 under alkaline conditions and a small Tafel slope of 74 mV dec−1 and maintaining electrochemical stability for 12[thin space (1/6-em)]000 s. Due to their ultrathin thickness and their high accessibility of the active sites exposed on the surface, 2D Co–MOF nanosheets exhibit outstanding electrocatalytic performance, outperforming most of the previously reported MOF-based catalysts, with excellent electron transfer and rapid mass transport. This work will open a new avenue for the use of ultrathin 2D MOF nanomaterials as promising electrocatalysts in advanced energy storage and conversion devices, such as rechargeable metal–air batteries.


Materials and reagents

All chemicals, Co(NO3)2·6H2O, p-phthalic acid (PTA, 99%), N,N-dimethylformamide (DMF), platinum carbon black (Pt/C), and ethanol (C2H5OH), were purchased from Shanghai Sinopharm Chemical Reagent and used without further treatment or purification. All aqueous solutions were prepared with high-purity de-ionized water (DI-water, resistance 18 MΩ cm−1).

Preparation of 2 nm ultrathin 2D Co–MOF nanosheets

The 2 nm ultrathin 2D Co–MOF nanosheets were synthesized by a surfactant-assisted solvothermal route. Typically, a solution of Co(NO3)2·6H2O (0.859 mmol, 0.250 g) in 20 mL deionized water was added to a solution of p-phthalic acid (PTA) (0.4295 mmol, 0.0714 g) in 40 mL DMF/C2H5OH (1[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v) and mixed to obtain a stable solution, and then 0.25 g polyvinylpyrrolidone (PVP) was added to the solution. The resulting solution was then transferred into a 100 mL Teflon-lined high-pressure autoclave. After sealing, the autoclave was heated at 80 °C for 60 h, and then naturally cooled to room temperature. Finally, the obtained samples were washed using deionized water and ethanol to remove impurities and dried in air naturally.

Preparation of micro–nano Co–MOFs

The temperature was changed to 180 °C during the experiment, and the other conditions were the same as those for the preparation of 2 nm ultrathin 2D Co–MOF nanosheets without any change.

Preparation of bulk Co–MOFs

The amount of reagent (Co(NO3)2·6H2O (1.717 mmol, 0.5 g), PTA (0.859 mmol, 0.143 g), and PVP (1.0 g)) was changed, and the other conditions were the same as those for the preparation of 2 nm ultrathin 2D Co–MOF nanosheets without any change.


The products were tested by X-ray diffraction (XRD) on a Bruker D8 Advanced X-ray Diffractometer (Cu-Kα radiation: λ = 0.15406 nm) for their phase analysis. A scanning electron microscope (SEM, Zeiss_Supra55) was used for observing the morphology of the samples at an acceleration voltage of 5.0 kV. High-resolution transmission electron microscopy (HRTEM) images, selected area electron diffraction (SAED) images, and energy dispersive X-ray spectroscopy mapping were captured on a Tecnai G2 F30 transmission electron microscope at an acceleration voltage of 300 kV. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific ESCALAB 250 apparatus. In addition, Fourier Transform Infrared Spectroscopy (FTIR) measurement was performed on a TENSOR27. Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) were tested on an Autosorb IQ3. All electrochemical measurements were carried out by using a CHI 760E instrument.

Fabrication of working electrodes

To fabricate working electrodes, Co–MOF (4 mg) and Nafion solution (5 wt%, 120 μL) were mixed with 666 μL of water solution and 334 μL of ethanol solution, followed by ultrasonication for 1 h. 5 μL of the catalyst dispersion was deposited on the surface of a glassy carbon electrode (GCE, a loading density of 0.25 mg cm−2) and dried naturally at room temperature.

Electrochemical measurement and calculation

The electrochemical water oxidation experiments were carried out on an electrochemical working station (CHI 760E, Shanghai Chenhua). A conventional three-electrode system was implemented for all the electrochemical measurements by utilizing a GCE (diameter: 3.0 mm) as the working electrode, Pt wire as the counter electrode, and a Hg–HgO electrode as the reference electrode in 1.0 M KOH electrolyte. According to the Nernst equation (ERHE = EHg/HgO + 0.059 × pH + 0.098), the current density was normalized to the geometrical surface area and the measured potential vs. Hg/HgO was converted to a reversible hydrogen electrode (RHE). Prior to the electrochemical test, we first passed N2 gas for half an hour in the 1.0 M KOH to ensure the O2/H2O equilibrium at 1.23 V vs. RHE. Cyclic voltammograms (CVs) were obtained at a scan rate of 50 mV s−1, and the working electrodes were scanned several times until stabilization before CV data were collected. The linear sweep voltammograms (LSVs) were obtained with a scan rate of 5 mV s−1. The working electrodes were scanned several times until the signals were stabilized, and then LSV data were collected, corrected for the iR contribution within the battery. CV and LSV were applied for confirming the electrocatalytic properties of the working electrodes.

Results and discussion

The Co–MOF (Co2(OH)2BDC, BDC = 1,4-benzenedicarboxylate) was synthesized via a one-pot hydrothermal method (Fig. 1) using cobalt nitrate hexahydrate and terephthalic acid as the metal source and organic ligand, respectively. PVP was used as an anionic surfactant to promote the vertical growth of Co–MOF nanosheets. Moreover, ultrathin Co–MOF nanosheets were synthesized by controlling the reaction temperature and the amount of reactants. The specific synthetic method is displayed in the experimental part of the ESI.
image file: c8ta03128b-f1.tif
Fig. 1 Schematic illustration of the synthesis of ultrathin two-dimensional cobalt–organic framework (Co–MOF) nanosheet catalysts.

The morphologies and microstructural features of the as-synthesized samples are examined by SEM, transmission electron microscopy (TEM), and atomic force microscopy (AFM). A typical high-magnification SEM image in Fig. 2a reveals the morphology of ultrathin nanosheets. The low-magnification SEM images in Fig. S1 show relatively separate 2D Co–MOF nanosheets but partially stacked. For comparison, we also prepared micro–nano Co–MOFs and bulk Co–MOFs. The corresponding SEM images of micro–nano Co–MOFs and bulk Co–MOFs are shown in Fig. S2a, b and c, d, respectively. Both share the same crystal phase and a similar 2D shape, but they are irregular, with a very large layered sheet structure. Fig. 2b and c show high-resolution TEM (HRTEM) images of the as-synthesized ultrathin 2D Co–MOF nanosheets. Notably, the obvious lattice fringes of ultrathin 2D Co–MOF nanosheets are not observed in the HRTEM image, in accordance with other observations of previously reported Co–MOFs,19 and the SAED pattern is shown in the upper-right inset of Fig. 2c. All of them further confirm that the obtained nanosheet is an ultrathin structure, which is also consistent with the thickness of the nanosheet in the AFM test later. The obvious lattice fringes are not observed under HRTEM, which may be related to the damage of the light beam under high-voltage. In order to further reveal the crystal structure of ultrathin 2D Co–MOF nanosheets, we tried a low-voltage TEM (120 kV), as shown in Fig. S3. The interplanar spacings of the prepared samples are about 0.290 nm and 0.278 nm, representing the (202) and (−402) lattice planes of Co–MOFs, respectively, which is consistent with the crystal information of Co–MOFs.36 The interrelated energy dispersive X-ray spectroscopy (EDX) elemental mapping of the sample further proves the existence of C, Co and O elements (Fig. 2d). Additionally, the EDX spectra also demonstrate the presence of these elements in the sample (Fig. S4). The thickness of a single ultrathin 2D Co–MOF nanosheet is further determined via AFM testing, as shown in Fig. S5. The thickness is ∼2 nm in the inset of Fig. S5.

image file: c8ta03128b-f2.tif
Fig. 2 (a) SEM image of ultrathin 2D Co–MOF nanosheets. (b, c) HRTEM images of ultrathin 2D Co–MOF nanosheets. (c) Inset: SAED pattern. (d) EDX elemental mapping images of C, Co, and O in the nanosheets.

Moreover, Fig. 3a and b also reveal their structural features. Fig. 3a shows that the ultrathin 2D Co–MOF nanosheets have three metal coordination layers, and the theoretical thickness of a single sheet is 1.941 nm, which corresponds to the AFM. On the surface of ultrathin nanosheets, the Co centers are partially five-coordinated due to edge growth constraints, which can reversibly bind solvent/reactant molecules. Obviously, the ultrathin structure of the Co–MOF is expected to produce rich coordination sites on the surface. Moreover, the crystal structure of ultrathin 2D Co–MOF nanosheets is shown in Fig. 3b, in which some pseudo-octahedra are formed in the edge region. These unsaturated tetragonal pyramidal metal centers exposed on the surface would be used as the active sites in the electrocatalytic reaction. In summary, the above results further demonstrate the ultrathin 2D lamellar structure of the material and the exposed active site of the metal atom, all of which satisfy the ideal structure of the MOF-based catalysts.

image file: c8ta03128b-f3.tif
Fig. 3 (a) Molecular structure and (b) crystal structure schematic diagrams of ultrathin 2D Co–MOF nanosheets.

The phase and crystal structure of the as-synthesized Co–MOF are analyzed by using their XRD patterns (Fig. S6). The XRD results are in agreement with previously reported Co2(OH)2BDC.35,36 The strong and sharp characteristics of these peaks suggest the high crystallinity of the samples. The curves also indicate that these three samples are the same substances. To confirm the chemical composition of the obtained samples, the XPS spectra are given in Fig. 4 and S7. Fig. S7 shows the Co 3s, Co 3p, Co 2p, O 1s, O 2s, O KLL, and C 1s peaks in the samples. The high-resolution XPS spectra of Co 2p are displayed in Fig. 4a, and the peak at 797.48 eV (796.88 eV, 796.48 eV) is from Co 2p1/2, while the peak at 781.18 eV (780.58 eV, 779.88 eV) is caused by Co 2p3/2. There are Co2+ and Co3+ ions in the Co 2p spectra of Co–MOFs.20 However, the chemical shift in the direction of high binding energy indicates that some of the CoII in the ultrathin 2D Co–MOF nanosheets is oxidized to CoIII, which is very important for their catalytic performance.19,37,38 In addition, there are two shake-up type peaks of Co near Co 2p1/2 and Co 2p3/2. The O 1s peaks of the three samples are presented in Fig. 4b, and they can be deconvoluted into O[double bond, length as m-dash]C–O (approximately 531.5 eV) and C–O (approximately 533.0 eV) species.

image file: c8ta03128b-f4.tif
Fig. 4 (a) The Co 2p spectrum and (b) O 1s spectrum of the Co–MOF materials.

Meanwhile, a comparison among the FT-IR spectra of the three different Co–MOFs demonstrates that they have the same molecular structure (Fig. S8). The compound exhibits a sharp peak at approximately 3600 cm−1, which is attributed to the vibrational peak of –OH. The two distinct absorption peaks approximately at 1360 and 1580 cm−1 refer to the symmetric and antisymmetric stretching vibrations of the carboxylate groups from the benzenedicarboxylate anion (BDC2−) in the framework, respectively. The three peaks near 750 cm−2 indicate the presence of a benzene ring in the resulting sample.

The electrocatalytic activity of the different samples for the OER is determined in N2-saturated 1.0 M KOH electrolyte in a standard three-electrode system (Fig. S9 and S10). The LSV curves of the electrodes obtained at a slow scan rate of 5 mV s−1 are shown in Fig. 5a and 6. Notably, ultrathin 2D Co–MOF nanosheets show the earliest onset potential at 1.411 V vs. RHE (defined as the potential at a current density of 0.1 mA cm−2) in comparison to all other electrodes (Eonset for micro–nano Co–MOFs and bulk Co–MOFs is 1.416 and 1.422 V, respectively) (Fig. 6a). In addition, a working potential of 10 mA cm−2 is an important parameter for OER performance evaluation. The ultrathin 2D Co–MOF nanosheet electrode delivers a very low overpotential of 263 mV at 10 mA cm−2, which is obviously smaller than that of micro–nano Co–MOF (287 mV) and bulk Co–MOF (326 mV) (Fig. 6b), and even better than that of commercial RuO2 (360 mV) (Fig. 5a). The high OER performance obtained from the ultrathin 2D Co–MOF nanosheet electrode is prominent in comparison with other cobalt-based catalysts (Table S1). The corresponding Tafel graphs of the electrocatalysts are constructed according to the polarization curves (Fig. 5b). A smaller Tafel slope means that the OER rate increases rapidly with an increase in potential.38 The Tafel slope for the ultrathin 2D Co–MOF nanosheet catalyst is lower (74 mV dec−1) than those of micro–nano Co–MOFs (94 mV dec−1), bulk Co–MOFs (122 mV dec−1) and RuO2 (118 mV dec−1), signifying the superior reaction kinetics of the ultrathin 2D Co–MOF nanosheet-based catalyst for the OER. In addition, the reaction mechanism of cobalt-based materials is discussed.23,39–41 In the OER, cobalt-based catalysts have three intermediate steps, namely:

CoOOH + OH ↔ CoO(OH)2 + e(1)
CoO(OH)2 + 2OH ↔ CoOO2 + 2H2O + 2e(2)
CoOO2 + OH → CoOOH + O2 + e(3)
Overall: 4OH ↔ 2H2O + O2 + 4e(4)

image file: c8ta03128b-f5.tif
Fig. 5 (a) LSV curves for the OER in N2-saturated 1.0 M KOH electrolyte at 5 mV s−1 and (b) Tafel slopes of ultrathin 2D Co–MOF nanosheets, micro–nano Co–MOFs, bulk Co–MOFs and RuO2. (c) Durability test for the ultrathin 2D Co–MOF nanosheets for 1000 cycles and (d) chronoamperometric testing of ultrathin 2D Co–MOF nanosheets for 12[thin space (1/6-em)]000 s at a static overpotential of 263 mV in N2-saturated 1.0 M KOH electrolyte.

image file: c8ta03128b-f6.tif
Fig. 6 Magnified polarization curves of ultrathin 2D Co–MOF nanosheet, micro–nano Co–MOF and bulk Co–MOF materials. Comparative potentials at current densities of (a) 0.1 and (b) 10 mA cm−2.

Steps (1) and (2) are highly reversible. Step (3) is fast and irreversible and determines the rate of the whole process, which is a potential limiting step; the catalyst is mainly used to promote the kinetic reaction of step (3). In the process of the anode OER, CoO6 inside the MOF is oxidized to CoO6/CoOOH as the active center, thus promoting the oxidation of OH to molecular oxygen.23,42,43 In addition, the activity of cobalt-based catalysts is further enhanced by introducing ultrathin nanosheets into cobalt-deficient electrons. However, we still need to explore the true mechanism of the MOF system by further experiments/calculations. Moreover, the turnover frequency (TOF) is another important parameter for evaluating the electrocatalytic activity. Assuming that all the CoII and RuO2 sites are involved in the OER, the TOF calculated for ultrathin 2D Co–MOF nanosheets at an overpotential of 300 mV is 0.27 s−1 (calculation details in the ESI), much higher than the OER TOF values of commercial RuO2 (0.028 s−1). The TOF results show that ultrathin 2D Co–MOF nanosheets have excellent electrocatalytic activity. Table S2 also summarizes a comparison of the different MOFs with recently reported OER electrocatalysts.

The electrochemical double-layer capacitance (Cdl) is used to represent a reasonable parameter of the electrochemical surface area. Fig. S11 shows that ultrathin 2D Co–MOF nanosheets have a high Cdl value (20.36 mF cm−2), approximately 1.1 and 3.89 times those of micro–nano Co–MOFs (18.78 mF cm−2) and bulk Co–MOFs (5.28 mF cm−2), respectively. The larger the Cdl is, the greater the roughness of the corresponding electrode is, and the more the active sites are on it, which further indicates that the ultrathin 2D Co–MOF nanosheet material contributes more to the progress of the catalytic reaction. The durability of the tested samples in the OER is crucial to electrocatalysts in future energy conversion and storage devices. Fig. 5c shows the polarization curves obtained before and after the 1000th cycle at a scan rate of 5 mV s−1. After the 1000th cycle, the ultrathin 2D Co–MOF nanosheet-modified electrode reveals slight degradation of the current density compared with the initial cycle, which merely needs a 19 mV increase in η to reach a current density of 10 mA cm−2. Furthermore, we also conducted a potentiostatic test in 1.0 M KOH electrolyte (Fig. 5d). The it curve shows that the catalytic current density exhibits a slight decline after 12[thin space (1/6-em)]000 s of continuous measurement, which could be caused by the catalyst mass loss from the working electrode during the long-term OER test.19 After 12[thin space (1/6-em)]000 s of stability, negligible changes in the PXRD, SEM and XPS were observed (Fig. S12, S13 and S15). However, ultrathin 2D Co–MOF nanosheets completely collapsed after 20 h of OER testing (Fig. S12, S14 and S16), indicating that their electrocatalytic activity was derived from their open framework structure. Despite the collapse, amorphous materials also have a certain catalytic activity. To determine the surface area and pore sizes of ultrathin 2D Co–MOF nanosheets, N2 adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution tests were carried out (Fig. S17). The tests show that the BET specific surface area of the material is 260 m2 g−1, and its pore size distribution is approximately 2 nm. A high specific surface area and porous structure lead to more active sites, so the material shows better OER performance.

Over the past few years, researchers have tested their OER performance on many Co/Fe/Ni/Cu–MOF systems.16,18–20 For example, some researchers obtained UTSA-16 and Ti3C2Tx–CoBDC, and these kinds of materials exhibited good OER performance.19,20 Du et al.44 prepared a novel 2D NiPc–MOF with good stability through a bottom-up approach. In addition, bimetallic MOFs exhibit better electrocatalytic properties than monometallic MOFs.14,18,23 Through rapid electrochemical deposition, a highly active and stable binder-free Fe/Ni-BTC thin film was obtained by Wang et al.18 Recently, NiFe–MOF, NiCo–UMOFN nanosheets have also been used by researchers to study OER performance.14,23 The 2D Co–MOF nanosheets obtained by us are relatively simple and show lower overpotential than previously reported UTSA-16 and Ti3C2Tx–CoBDC. In addition, the sheet thickness is relatively thin compared to previously reported bimetallic MOFs, and the reaction mechanism is similar to that of nickel-based catalysts. However, MOFs are applied to the conductive substrate using a binder (i.e. Nafion). The use of these electrically insulative and inactive adhesives reduces the conductivity of the electrode and may partially block the pores and active sites of the MOF particles leading to a reduction in electrocatalytic performance, and the stability of the materials may need further optimization.

However, the reason why ultrathin 2D Co–MOF nanosheets have excellent electrocatalytic activity is that an ultrathin flaky Co–MOF leads to Co-deficient electrons (Fig. 3a). Its electron cloud density is reduced, forming coordinatively unsaturated Co–metal active sites on the exposed surface, which promote the OER performance of the material.14,45 Especially on the surface of 2D Co–MOF nanosheets, many accessible active sites are exposed, rather than trapped in their pores or channels, which promotes more interaction between the active site and the substrate molecules, leading to the improvement of their properties in electrocatalysis.8,46 In addition, high-valence Co has a better OER performance in the catalytic reaction, while the CoII of ultrathin 2D Co–MOF nanosheets is partially oxidized to a higher valence state (as obtained from Fig. 4a), thereby improving their catalytic activity. The porous structure of ultrathin 2D Co–MOF nanosheets provides highly accessible CoII active sites and short diffusion pathways (as obtained from Fig. S17b) to allow fast mass transport and excellent electron transfer, thus facilitating the kinetic response.

To prove the electron transport ability of Co–MOF, electrochemical impedance spectroscopy (EIS) of Co–MOF synthesized under different conditions was performed with a glassy carbon electrode (GCE), as shown in Fig. S18. The ultrathin 2D Co–MOF nanosheets provide a charge transfer resistance less than those of other Co–MOF samples, indicating that the charge transfer of ultrathin 2D Co–MOF nanosheets is faster during the electrochemical OER and they have a high electrical conductivity. Moreover, the electrode has a high electrical conductivity, which is conducive to reducing the overpotential for oxygen evolution, thereby improving the hydrogen production efficiency.


In summary, this work demonstrates a one-step hydrothermal method to fabricate ultrathin 2D Co–MOF nanosheets, which serve as high-performance catalysts for the OER. The ultrathin 2D Co–MOF nanosheets show a low onset potential and good catalytic durability in alkaline solution, better than most Co3O4 and MOF-based electrocatalysts. However, the stability of MOFs needs further improvement. In the future, we will try to improve the stability of the material by combining MOFs with some carbon materials or conductive polymers. The excellent electrocatalytic properties are mainly ascribed to the porous structure of ultrathin nanosheets, rapid ion transport, and the coordination of unsaturated CoII active sites on the surface. These results provide an in-depth understanding for the future design and synthesis of new MOF electrocatalysts and will pave the way for the development of energy materials in various areas such as fuel cells, metal–air batteries, water splitting devices and other critical renewable energy systems.

Conflicts of interest

There are no conflicts to declare.


This work was supported by the National Natural Science Foundation of China (NSFC-21671170, 21673203, and 21201010), the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP), the Program for New Century Excellent Talents of the University in China (NCET-13-0645), the Six Talent Plan (2015-XCL-030), and the Qinglan Project. We also acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions and the technical support we received at the Testing Center of Yangzhou University.


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Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta03128b

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