Synthesis of Fe3C@porous carbon nanorods via carbonizing Fe complexes for oxygen reduction reaction and Zn–air battery

Yijie Zhang , Yong Zhao , Muwei Ji , Han-ming Zhang , Minghui Zhang , Hang Zhao , Mengsi Cheng , Jiali Yu , Huichao Liu , Caizhen Zhu * and Jian Xu
Institute of Low-dimensional Materials Genome Initiative, College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen, 510086, China. E-mail: czzhu@szu.edu.cn

Received 27th November 2019 , Accepted 2nd January 2020

First published on 3rd January 2020


Fe-Based electrocatalysts on carbon substrates are considered suitable candidates for applications in Zn–air batteries due to their favorable ORR performance. Herein, unique Fe3C@N-doped porous carbon nanorods have been synthesized using a soft template method and carbonization. The material proved to have a porous hetero-structure, which is advantageous in providing effective electrocatalysis performance for oxygen reduction reaction, providing broad prospects for the development of non-precious metal electrocatalysts and metal–air batteries. This catalyst used for oxygen reduction reaction exhibits a half-wave of 0.83 V, which is close to that of the commercial Pt/C catalyst. Using the as-prepared Fe3C@N-doped porous carbon nanorods, the open circuit potential and power density of a Zn–air battery has been measured as 1.42 V and 126.4 mW cm−2, respectively, which are better than those of the commercial Pt/C. This study presents a novel strategy to prepare Fe3C@N-doped porous carbon nanorods, which can be used as highly effective oxygen reduction reaction catalysts for Zn–air batteries.


Introduction

Fe3C and its heterostructures are viewed as important potential electrocatalysts for oxygen reduction reaction (ORR) due to their low cost and high performance, and are expected to further applied in supercapacitors and metal–air batteries.1–4 Although numerous new electrocatalysts for ORR have been explored and the mechanisms are studied, the sluggish ORR is still a challenge because of the conflicts between the cost and performance of catalysts, which also limit the performance of metal–air batteries.5,6 For example, platinum-based catalysts are widely used catalysts.2,7–10 Nonetheless, their high cost and scarcity prevent them from large-scale commercial applications.1,8–12 Therefore, to explore effective and cheap catalysts to substitute the Pt-based or other noble metal-based catalysts is still a great challenge and opportunity.12–14 By considering the preparation of metal-based/N-doped carbon materials (M–N–C materials) in a facile way and the abundance of raw materials, it is completely possible to obtain and tailor highly active catalysts for ORR because the active sites can be enriched by adding transition metal–nitrogen–carbon-based materials.9,15–17 Among the metal-based/N-doped carbon materials, it is reported that Fe3C-based catalysts are prepared via facile ways and exhibit high activity for ORR. Particularly, Fe–N–C materials have been reported to possess similar properties for ORR catalysis to those of Pt/C, and even better than Pt/C.6,18,19 Jiang et al. reported that the synthesized Fe/Fe3C heterostructures in graphitic layers/carbon nanotubes exhibit high activity for ORR, and the half-wave potential of 0.83 V is slightly better than that of Pt/C under the same conditions.20 The half-wave potential of Fe3C/N, S-doped carbon was measured to be as high as 0.93 V and Lee et al. illustrated that the enriched Fe–Nx active sited promoted the efficiency of ORR.21 Further investigations suggest that the major active sites of Fe–N–C materials are nitrogen-bound iron, which play the leading catalytic role in ORR.22–24 Moreover, Fe–N–C materials exhibited high ORR activities in alkaline medium and the catalysis mechanism has been proved to be an approximate four-electron transfer process.25–27 The Fe3C-based heterostructure combined with N-doped carbon exhibited higher activity for ORR, therefore widening the scope of ORR catalysts and providing a way to explore cheap catalysts.28 Furthermore, owing to the facile preparation and tailoring, M–N–C materials are viewed as favorable materials for applications in metal–air batteries with high energy density and excellent stability.11,29,30

In this study, a series of Fe3C@N-doped porous carbon nanorods (FeCNRs) have been prepared by carbonizing Fe(acac)3/polydopamine nanorods at 700–800 °C, and the as-prepared catalyst obtained at 750 °C (FeCNRs-750) exhibits a high activity for ORR. The determined onset and half-wave potentials are close to those of Pt/C, and the limited current density is larger than that of Pt/C. The employment of the as-prepared FeCNR in a Zn–air battery proved that the Fe3C/N-doped carbon material is a suitable candidate for metal–air batteries in aqueous systems.

Results and discussion

As shown in Fig. 1a, FePNR was prepared by mixing dopamine hydrochloride, Fe(acac)3, Pluronic F127, and 1,3,5-trimethylbenzene (TMB), and used as a precursor for preparing FeCNRs. Fig. 1b shows the scanning electronic microscopy (SEM) images of dull-shape FePNR with a uniform size of 600 nm length and 60 nm diameter. Fig. 1c shows the infrared spectrum of FePNR and the peaks located at 1402, 1489, 1285, and 1618 cm−1 are assigned to C–H, C[double bond, length as m-dash]C, C–O and C[double bond, length as m-dash]O, respectively. The peaks at 649 and 515 cm−1 are attributed to the Fe–O bond, indicating that Fe3+ coordinated with organic ligands and formed Fe-based complexes. The peaks located in the range of 3000–3400 cm−1 are assigned to the stretching vibrations of N–H bonds. The broadened peaks reveal the coordination between Fe3+ and dopamine. The X-ray diffraction (XRD) spectrum of the as-prepared FePNR is shown in Fig. 1d, which illustrates the formation of amorphous FePNR. Fig. S1 shows the thermogravimetric analysis curve of the FePNR precursor material under nitrogen atmosphere. The mass loss of the sample ranges from room temperature to 1000 °C, which is mainly divided into the following three parts: (1) the mass loss is about 8.7% at the temperature lower than 130 °C, which is mainly due to the evaporation of water; (2) 23.8% of the mass is lost in the range of 130–400 °C, which is caused by the carbonization of residual 1,3,5-trimethylbenzene, the decomposition of organic functional groups in iron acetylacetonate and the decomposition of polydopamine; (3) 16.9% of the mass is lost in the range of 400–650 °C, which is attributed to considerable F127 decomposition. At the higher temperature, it is useful for Fe3C crystallization. In order to ensure the favourable performance of the material, it was necessary to control the carbonization temperature between 650 and 800 °C. In this study, 700, 750, and 800 °C were selected as the carbonization temperatures. The XRD spectra of the as-prepared FeCNRs are shown in Fig. 1d. The diffraction peaks of the series of FeCNRs are assigned to Fe3C (JCPDS #65-2412), which reveal the transformation of FePNRs into the Fe3C/carbon hetero-structure during heat-treatment. The diffraction peak at 26° was assigned to the graphite carbon (002) facet, clearly indicating the formation of graphite carbon layers. The XRD patterns of FeCNR-700, FeCNR-750, and FeCNR-800 are found to contain peaks ascribed to Fe3C and graphite carbon, but the graphite peak intensity in the patterns of FeCNR-750 and FeCNR-800 is higher, hinting the higher graphite carbon content. By taking FePNR as a precursor, a series of FeCNRs were obtained after heat treatment at 700–800 °C (Fig. 1e–g). As displayed in Fig. 1e–g, the morphology of FeCNR is well maintained at 700 °C, while the framework of FeCNR collapsed as the temperature was increased up to 800 °C. As presented in Fig. 1f, the morphology of FeCNR-750 remains the same after carbonization and the size of FeCNR-750 is 240 nm in length and 30 nm in diameter. The transmission electronic microscopy (TEM) images of FeCNRs carbonated at different temperatures (Fig. 1h–j) show Fe3C nanoparticles grown on nanorods. The HRTEM image of FeCNR-750 nanoparticles is displayed in Fig. 2a and confirms the formation of Fe3C. As presented in Fig. 2a, the lattice spacings are measured to 0.20 nm and 0.21 nm, which are assigned to the (031) and (211) facets of Fe3C, respectively. Moreover, Fig. S2 shows the HRTEM image of the as-prepared Fe3C/C and the hetero-structure is clearly displayed, confirming the reduction of Fe3+ into Fe3C and its growth on the C substrate. The element mapping of FeCNR-750 (Fig. 2c) shows elements C and N uniformly distributed on the nanorods, while Fe distribution was centred on the nanoparticles. The element content was measured by EDX and shown in Fig. S3, which also confirmed the formation of a hetero-structure. The ligands of the Fe-NPR samples pyrolyzed into carbon substrates, followed by the reduction of Fe3+ into Fe(0) to produce Fe3C nanoparticles. It is considered that the Fe3C nanoparticles form during the carbonization of ligands and then grow on the C substrate to form Fe3C/C heterostructures.
image file: c9qi01544b-f1.tif
Fig. 1 (a) Schematic of the preparation of FeCNR derived from FePNR. (b) SEM image of FePNR. (c) FTIR spectra of FePNR, Dopamine and Fe(acac)3. (d) XRD pattern of the FeCNRs at different temperatures and FePNR. The standard pattern (JCPDS #65-2412) of Fe3C is provided. The diffraction peak indicated by * is attributed to graphitic carbon (002). (e) SEM image of FeCNR-700. (f) SEM image of FeCNR-750. (g) SEM image of FeCNR-800. (h) TEM images of FeCNR-700. (i) TEM images of FeCNR-750. (j) TEM images of FeCNR-800.

image file: c9qi01544b-f2.tif
Fig. 2 (a) HRTEM images of FeCNR-750. (b) HADDF images of FeCNR-750. (c) The element mapping of FeCNR-750.

X-ray photoelectron spectroscopy (XPS) was used to study the chemical valence and bonding of FeCNRs. Fig. S4 shows the XPS maps of FeCNRs and the XPS peaks of N 1s and Fe 2p are located at 309.0 eV and 708.8 eV, respectively. The high-resolution Fe 2p spectrum of FeCNRs (Fig. 3a) is deconvoluted into peaks at around 721.8 eV and 708.1 eV, corresponding to Fe 2p1/2 and Fe 2p3/2, which are slightly higher than those of Fe(0) due to the lower electronegativity of Fe than that of C. Fig. 3a reveals that the chemical valence of Fe is close to 0 and hints the formation of Fe3C. This is in agreement with the reported value for Fe3C and further testifies the formation of Fe3C nanoparticles.31,32 The XPS peak of N (Fig. 3b) is deconvoluted into the peaks of graphite N (400.1 eV), pyrrolic N (397.2 eV), and pyridine N (398.8 eV). Fig. 3c illustrates that the pyrrolic N peak for FeCNR-800 is lower in intensity than that for FeCNR-700 and FeCNR-750, which is attributed to more defects being produced at higher temperatures. Fig. 3d shows the fitted the peak at 283.8 eV, which is assigned to C of Fe3C. All the XPS maps of the as-prepared samples show that the Fe complexes are transformed into Fe3C and the ligands of FePNRs are carbonized into N-doped carbon substrates at high temperatures. The XPS characterization of FeCNRs further confirms the presence of hetero-structures and active composition for electrocatalysis. Raman spectra of the as-prepared FeCNRs were collected to evaluate their graphitization structure (Fig. 4a). The Raman peaks for FeCNRs located at around 1343 cm−1 and 1595 cm−1 are assigned to D and G bands of porous carbon, respectively. The D band is associated with the stretching of hexagonal sp2 bonded carbon atoms in the graphite network, while the G band represents distorted sp3-hybrid defect sites on the carbon skeleton.33,34 It is reported that different carbonized defects resulted in different value of ID/IG and indicated various levels of activity on catalysis.30 In particular, the ID/IG value of FeCNR-800 is calculated as 0.938, which is higher than that of FeCNR-750 (0.928) and FeCNR-700 (0.925), indicating that the number of carbon defects increases as the carbonation temperature increases. Fig. 4b–d show the BET surface areas of the series of FeCNRs. The BET surface area of FeCNR-750 is measured as 345.9 m2 g−1, and the corresponding nitrogen adsorption isotherm indicates type IV adsorption.35,36 Significant hysteresis loops exist in the middle and high-pressure ranges in Fig. 4b–d. As shown in Fig. 4c, the surface area of FeCNR-750 is the largest and the pore size reaches 0.84 nm, which might be attributed to the catalysis of Fe during heat-treatment, thus providing more active sites for ORR.37,38


image file: c9qi01544b-f3.tif
Fig. 3 (a) XPS spectrum of Fe 2p for FeCNR-700, FeCNR-750 and FeCNR-800 (from top to bottom). (b) XPS spectrum of N 1s of FeCNR-700, FeCNR-750 and FeCNR-800 (from top to bottom); graphitic-N, pyrrolic-N and pyridinic-N species were fitted. (c) The concentration of pyridinic, graphitic, and pyrrolic N of as-prepared samples. (d) XPS spectrum of C 1s of FeCNR-700, FeCNR-750 and FeCNR-800.

image file: c9qi01544b-f4.tif
Fig. 4 (a) Raman spectra of FeCNR-700, and. (b–d) the N2 adsorption–desorption sIsotherms of FeCNR-700 (b), FeCNR-750 (c), FeCNR-800 (d) (there is the micropore size distributions of FeCNRs insert of (b)–(d), respectively).

The ORR activity and stability of FeCNRs are evaluated by the rotating disk electrode (RDE) in 0.1 mol L−1 KOH solution. The CV curve shows that the reduction peak of ORR on using FeCNR-750 is at 0.91 V with O2-statured KOH solution while the reduction peak cannot be observed in N2-statured KOH solution (Fig. 5a), hinting the high activity of FeCNR-750 for ORR. Fig. 5b displays the polarization curve of ORR on using FeCNRs and the onset potential (Eonset) and half wave potential (E1/2) of FeCNR-750 are measured as 0.96 V and 0.83 V, respectively. FeCNR-750 exhibits higher activity for ORR than FeCNR-700 and FeCNR-800, whose Eonset and E1/2 are measured as 0.95 V and 0.94 V and 0.80 V and 0.73 V, respectively (Fig. 5b). Furthermore, FeCNRs-750 exhibits higher activity for ORR than commercial Pt/C, whose Eonset and E1/2 are measured as 0.94 V and 0.81 V, respectively, while FeCNR-800 and FeCNR-700 show lower activity (Fig. 5b). The limited diffusion current density of FeCNR-750 is 5.10 mA cm−2, which is higher than that of Pt/C (4.39 mA cm−2), FeCNR-700 (4.41 mA cm−2) and FeCNR-800 (4.27 mA cm−2). The corresponding Tafel slopes are shown in Fig. 5c, and the Tafel slope of FeCNR-750 is calculated as 74.4 mV dec−1, which further confirms the higher activity of FeCNR-750 for ORR. In order to distinguish the activity of the composites, N-doped carbon was prepared and its electrocatalysis was studied. Fig. S5 and S6 show the activity comparison of N-doped carbon and FeCNR-750. Following the similar preparation process, N-doped carbon was prepared without adding Fe(acac)3 and its corresponding LSV curve was found to be at much lower current density than that of the FeCNR-750 (Fig. S5). Furthermore, the N-doped carbon obtained by removing Fe3C in H2SO4 solution was also found to exhibit poorer activity than FeCNR-750 (Fig. S6). These results clearly reveal that the catalytic activity mainly depends on the Fe3C nanoparticles and the high activity of FeCNR-750 is attributed to the synergetic effect of Fe3C nanoparticles and N-doped carbon substrate. The high activities of FeNCR-750 for ORR might be because of the synergetic effects of Fe3C nanoparticles and N-doped porous carbon. Different rotational speeds ranging from 400 to 2500 rpm were applied to further understand the reaction kinetics (Fig. 6a). The electron transfer number (n) of FeCNR-750 is calculated as 3.87 (Fig. 6b), which primarily indicates the 4e reduction process of ORR on using FeCNR-750. The oxidation current of H2O2 was measured ranging from 0.2 to 0.9 V by using a rotating ring-disk electrode (RRDE) and the yield of H2O2 was calculated as 12% (Fig. 6c and d). After 10[thin space (1/6-em)]000 cycles, FeCNR-750 shows good activity for ORR, illustrating that it is stable during ORR catalysis (Fig. S7). The methanol tolerance test of FeCNR-750 indicates higher capacity than that of Pt/C (Fig. S8). Compared with previously reported materials, FeCNR-750 also shows good catalytic performance (Fig. 5d).18,22,39–48


image file: c9qi01544b-f5.tif
Fig. 5 (a) Cyclic voltammetry curves of FeCNR-750 in 0.1 M KOH with O2-saturation and N2-saturation. (b) LSV curves of different catalysts. (c) Tafel curves of different catalysts. (d) Comparison of catalytic performance of FeCNR-750 and other references.18,22,39–48

image file: c9qi01544b-f6.tif
Fig. 6 (a) LSV curves of FeCNR-750 at various rotation speeds. (b) The K–L plots of FeCNR-750 at different potentials for the calculated number of electron transfer. (c) RRDE curves of FeCNR-750 and Pt/C. (d) Hydrogen peroxide yield (%H2O2) and number of electron transfer (n).

Based on its high activity for ORR, the as-prepared FeCNR-750 is loaded on carbon cloth and employed as an electrode material for a Zn–air battery. In our case, 6 mol L−1 KOH solution, zinc foil, and FeCNR-750 (or 20 wt% Pt/C) were used as an electrolyte, negative electrode, and a self-made positive air electrode, respectively. As depicted in Fig. 7a, the specific capacity of FeCNR-750 is 825.77 mA h gZn−1, which is close to that of commercial Pt/C (828.9 mA h gZn−1). Fig. 7b shows the polarization curves of FeCNR-750 and Pt/C, and the corresponding power density curve is obtained from the polarization curve. As shown in Fig. 7b, the maximum power density of FeCNR-750 is up to 126.4 mW cm−2, which is slightly lower than that of Pt/C (130.6 mW cm−2). Furthermore, it is much higher than that of the reported N-doped or N,S-co-doped porous carbon and close to the metal-doped porous carbon.11,44,45,47,48 These results are consistent with the ORR characterization and further confirm that Fe3C plays a more important role for ORR and Zn–air battery performance. The open circuit voltage of FeCNR-750 is 1.42 V, which is also close to that of Pt/C (1.43 V) (Fig. 7c). Furthermore, an LED panel was powered by Zn–air batteries connected in series with the FeCNR-750 catalyst as the air-cathode (Fig. S9). In constant discharge testing (from 0.05 mA cm−2 to 2 mA cm−2) of the Zn–air batteries, FeCNR-750 shows a highly stable and excellent discharge voltage platform (Fig. 7d). In contrast to other reports on Fe derivatives (Table S1), the performance of FeCNR-750 is higher, which hints that our catalyst exhibits good performance in Zn–air battery applications.


image file: c9qi01544b-f7.tif
Fig. 7 (a) Specific discharging capacity curves of FeCNR-750 and Pt/C. (b) Polarization curves and corresponding power density curves of FeCNR-750 and Pt/C. (c) Open circuit plots of FeCNR-750 and Pt/C. (d) Electrostatic discharge curves of FeCNR-750 and Pt/C.

Conclusions

In summary, we used a soft template method for synthesizing Fe3C nanoparticles encapsulated in nitrogen-doped porous carbon nanorods. The obtained FeCNR-750 had a short rod shape and exhibited excellent ORR performance (E1/2 = 0.83 v) relative to that of Pt/C (E1/2 = 0.81 v) and also had good long-term stability. The FeCNR-750 catalyst exhibited excellent discharge performance as the air-cathode electrode material for a Zn–air battery. The effective electrocatalytic performance attributing to the structural advantages provides broad prospects for non-precious metal based electrocatalysts and bring more possibilities for applications in metal–air batteries.

Experimental section

Chemicals and reagents

Dopamine hydrochloride (DA) and polyethylene-polypropylene glycol (Pluronic F127, Mn = 13[thin space (1/6-em)]000) were purchased from Shanghai Macklin Biochemical Co., Ltd, Shanghai, China. Iron(III) acetylacetonate (Fe(acac)3), 1,3,5-trimethylbenzene (TMB), ammonia and ethanol were obtained from Aladdin Industrial Co., CP, Shanghai, China.

Synthesis of FePNR

FePNR was prepared by mixing dopamine hydrochloride, Fe(acac)3, Pluronic F127, and 1,3,5-trimethylbenzene (TMB). Typically, acetylacetone iron was dissolved in the deionized water (10 mL) and ethanol (0.4 mL) to form the saturated solution. Then, dopamine hydrochloride (0.15 g) and block copolymer Pluronic F127 (0.1 g) were added and dissolved via ultrasonication at room temperature. Following this, saturated Fe(acac)3/TMB solution (0.4 mL) was added. After the solution was uniformly emulsified by a homogenizer, ammonia (28%, 0.55 mL) was added and stirred for another 2 hours at room temperature. After polymerization, the as-prepared Fe@PDA nanorods (FePNR) were collected by centrifugation and purified by washing with deionized water and ethanol. The obtained products were dried at 70 °C overnight.

Synthesis of FeCNR

FePNR was carbonized at 750 °C in Ar atmosphere for 2 hours with the heating rate of 2 °C min−1. The black powder was ground in the mortar to obtain FeCNR-750. In addition, FeCNR-800 and FeCNR-700 were prepared using a similar process at different carbonization temperatures of 800 °C and 700 °C, respectively.

Characterizations

The infrared spectrum of FePNR was recorded using a Fourier transform infrared spectrometer (IR Affinity-1). The thermogravimetric analysis curve of FePNR was measured using a thermogravimetric analyser (TGA Q-50). The test temperature was set from room temperature to 1000 °C with the heating rate of 10 °C min−1, and it was performed under nitrogen atmosphere. The nanostructure of the samples was investigated using a field emission scanning electron microscope (FESEM, JEOL 7500F). X-ray diffraction spectra (XRD, MiniFlex600, Rigaku, Japan) were recorded at an operating voltage and operating current of 40 kV and 15 mA, respectively, scan range of 10°–90°, and scan rate of 5° min−1. A transmission electron microscope (TEM, JEOL JEM-2100F) was used to observe the microstructure of the inorganic nanostructure. The specific surface area and pore size analyses in this study were performed using BELSORP-max. After vacuum drying the sample at 120 °C for 3 h, the approximate specific surface area of the sample was predicted and an appropriate amount of sample was weighed for nitrogen adsorption–desorption tests. X-ray photoelectron spectroscopy (XPS) was performed using a K-Alpha + electron spectrometer manufactured by Thermo Fisher Scientific, United Kingdom, equipped with Al-Kα as the radiation source. Raman spectra (Renishaw Invia Plus laser Raman spectrometer, Renishaw, UK) were recorded under 532 nm laser excitation to analyse the carbon structure.

Electrochemical measurements

A CHI 760E electrochemical workstation (CHI Instruments, Shanghai Chenhua Instrument Co, China), rotating disk electrode (RDE) system and ring rotating disk electrode (RRDE) system were used to perform the electrochemical measurements in this study. A platinum wire was used as the counter electrode and a saturated Ag/AgCl electrode was used as the reference electrode. Catalyst ink was composed of catalyst, ethanol, isopropanol and Nafion solution (perfluorosulfonic acid proton exchange membrane). Nafion solution was mainly used as a binder to adhere the catalyst film to the electrode surface. The preparation method is described as follows: 2 mg catalyst was uniformly dispersed in a mixture of 100 μL ethanol, 299 μL isopropyl and 0.8 μL Nafion solution (5 wt%). The test was performed in 0.1 mol L−1 KOH electrolyte. Before the test, the O2 saturated electrolyte was prepared by bubbling O2 for 30 minutes. To check the background current, the electrodes were tested in 0.1 mol L−1 N2-saturated KOH solution. The electron transfer number (n) was measured and calculated by using eqn (1):
 
N = 4IDN/(IR + IDN)(1)

The hydrogen peroxide yield (H2O2%) was evaluated from eqn (2):

 
H2O2% = 200IR/(IR + IDN)(2)
where IR, ID and N are the ring currents, disk currents and the current collection efficiency (0.37), respectively.

In this experiment, a self-made zinc–air battery was assembled and its electrochemical performance was evaluated. The air cathode is composed of a catalyst layer on the gas diffusion layer, and the catalyst loading is 1 mg cm−2. The zinc–air battery was assembled using a polished Zn plate having a thickness of 0.05 mm as an anode and 6 mol L−1 KOH solution as an electrolyte. The discharge polarization curve and open circuit potential of the zinc battery were obtained using an electrochemical workstation (CHI 760E), and the constant current discharge curve was recorded at a current density of 5 mA cm−2 on the test system.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by National Natural Science Foundation of China (51673117, 21805193, 51574166, 51602199), the Key R&D Programme of Guangdong Province (2019B010929002, 2019B010941001), the Science and Technology Innovation Commission of Shenzhen (JSGG20160226201833790, JCYJ20170818093832350, JCYJ20170818112409808, JSGG20170824112840518, JCYJ20180507184711069, JSGG20170824112840518).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9qi01544b

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