Superlong Single-Crystal Metal–Organic Framework Nanotubes

Oct 22, 2018 - Nanotubes have attracted great attention. Here, we report the fabrication of the first single-crystal metal–organic framework (MOF) n...
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Cite This: J. Am. Chem. Soc. 2018, 140, 15393−15401

Superlong Single-Crystal Metal−Organic Framework Nanotubes Lianli Zou,†,‡ Chun-Chao Hou,§ Zheng Liu,∥ Huan Pang,⊥ and Qiang Xu*,†,‡,§,⊥

J. Am. Chem. Soc. 2018.140:15393-15401. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/15/18. For personal use only.

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Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan ‡ Graduate School of Engineering, Kobe University, Nada Ku, Kobe, Hyogo 657-8501, Japan § AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ∥ Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya, Japan ⊥ School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225009, China S Supporting Information *

ABSTRACT: Nanotubes have attracted great attention. Here, we report the fabrication of the first single-crystal metal−organic framework (MOF) nanotubes. Superlong single-crystal cobalt−organic framework (Co-MOF) nanotubes, which have a diameter of ∼70 nm and length of 20−35 μm with parallel multichannels (window size: 1.1 nm), have been successfully synthesized via an amorphous MOF-mediated recrystallization approach. The synthesized MOF nanotubes can be used as a nanocolumn for separation of large molecules. Carbonization of the Co-MOF nanotubes in an argon atmosphere preserves the 1D morphology, affording long carbon nanofibers. A hierarchical architecture composed of carbon nanofibers wrapped by carbon nanotubes (20−30 nm in diameter and 200−300 nm in length) with cobalt nanoparticles on the top is formed by the carbonization of the Co-MOF nanotubes along with dicyandiamide as a nitrogen and a secondary carbon source. The resulting hierarchical dendrites with carbon nanofiber trunks and carbon nanotube branches exhibit excellent electrocatalytic activity for oxygen reduction reaction and exceptional applications in rechargeable Zn−air batteries. This work demonstrates a new strategy to fabricate MOF nanotubes and relative 1D nanostructures.

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INTRODUCTION Metal−organic frameworks (MOFs), a new class of porous crystalline materials assembled from metal ions/clusters and organic linkers, have been widely used in various fields such as catalysis and gas storage owing to their advantages such as extraordinarily high surface areas, ordered pore structures, and diverse compositions.1−8 The crystallinities and morphologies of MOFs are crucial to the specific properties and practical applications.9−12 In recent years, extensive research efforts have focused on the synthesis of MOFs in nanometers, as they can have size-dependent properties, which may expand their scope for numerous applications.13−15 The reported nano MOFs are usually grown as polycrystalline powders or amorphous products.16−18 For single-crystal nano MOFs with anisotropic structures, only a limited number of two-dimensional (2D) single-crystal nano MOFs have been synthesized.19 The templating and surfactantassistant approaches have been used to produce 1D MOF anisotropic structures, while they cannot afford single-crystal nano MOFs or nano MOFs with high aspect ratios.20−23 It is a big challenge to synthesize 1D MOF nanostructures with both single crystallinity and superlong morphology. To date, there is no report on the synthesis of single-crystal MOF nanotubes. Kinetic control of crystallization has a significant effect on crystal morphology and crystallinity,13,24,25 and a low reaction © 2018 American Chemical Society

rate benefits the formation of an anisotropic nanostructure as well as single crystals.26,27 The kinetics of the crystallization can be controlled by the use of different precursors, which have different solubilities in a specific solvent.24,28 Herein, we report an amorphous MOF-mediated recrystallization approach (AMMRA) to fabricate the first single-crystal Co-MOF nanotubes in a high aspect ratio. The reaction of cobalt acetate tetrahydrate and 2, 5-dihydroxyterephthalic acid in methanol solution at room temperature gives amorphous MOF-74 nanoparticles, which recrystallize into superlong single-crystal Co-MOF nanotubes in water at 175 °C. The obtained Co-MOF-74 nanotubes are employed as a highly efficient nanocolumn for the separation of large dye molecules and a precursor for preparing carbon nanostructures via thermal transformation processes, which exhibit excellent catalytic activities for oxygen reduction reaction.

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EXPERIMENTAL SECTION

Synthesis of Co-MOF-74-NP and Co-MOF-74-NT. In a typical process, cobalt(II) acetate tetrahydrate (4.0 mmol) was first dissolved in methanol solution (100 mL) under vigorous stirring. With continued stirring, 50 mL of methanol solution containing 1.5 mmol of 2, Received: August 25, 2018 Published: October 22, 2018 15393

DOI: 10.1021/jacs.8b09092 J. Am. Chem. Soc. 2018, 140, 15393−15401

Article

Journal of the American Chemical Society

Zn−Air Battery Fabrication and Testing. The zinc−air battery was assembled with a Zn anode, a 6 M KOH solution (mixed with 0.2 M zinc acetate for rechargeable Zn−air batteries), and an air cathode comprising a gas-diffusion layer, catalyst layer, and a separator to prevent electrolyte leakage. The catalyst layer was prepared by dropping 40 μL of ink of NCo@CNT-NF700 (5 mg mL−1) on a carbon paper (0.7 cm in diameter), giving a loading of 0.5 mg cm−2. The battery property was tested under air at room temperature.

5-dihydroxyterephthalic acid was added dropwise, and the resulting mixture was further stirred for about 2 h at room temperature (RT). The final precipitate, marked as Co-MOF-74-NP, was washed by methanol followed by washing with water two times and then dispersed in 50 mL of water. The mixture was then transferred into a Teflon-lined autoclave, tightly capped, and placed in an oven at 175 °C for various times (0.5, 1, 2, 3, 6, and 12 h). After cooling to RT, the yellow precipitate was collected and washed by water and methanol. The final product (marked as Co-MOF-74-NT) was dried at RT in a vacuum oven for further use. To investigate the effect of solvent on the formation of MOF nanotubes, the prepared Co-MOF-74-NP was dissolved in different solvents such as ethanol, methanol, dimethylformamide (DMF), and a water/methanol mixture solution (H2O/methanol = 2:8, 4:6, 6:4, and 8:2). Other parameters were the same as those for Co-MOF-74-NT. Synthesis of NCo@CNT-NFT Hierarchical Carbon Dendrites. A 20 mg amount of as-prepared Co-MOF-74-NTs and 0.4 g of dicyandiamide (DCDA) were placed in two separate ceramic boats with DCDA at the upstream side of the furnace in an argon atmosphere. With a heating ramp rate of 10 °C min−1, the samples were first heated at 400 °C for 3 h, then raised to 600, 700, 800, 900, and 1000 °C, and maintained at these temperatures for another 4 h. After cooling to RT naturally, the black products were collected and marked as NCo@ CNT-NFT (T is the pyrolysis temperature). Column-Chromatographic Dye Separation Process. The glass tube (⦶ 5 mm, 200 mm) with open ends was filled with Co-MOF-74NT (10 mg) and was used as a chromatographic column. The aqueous solution of rhodamine B (0.01 mg mL−1, 3 mL) and the mixture solution of methylene blue (0.03 mg mL−1)/rhodamine B (0.01 mg mL−1) (1:1, 3 mL) were passed through the chromatographic column. Electrochemical Measurements. The electrocatalytic measurements were carried out in a three-electrode cell using a rotating disk electrode (RRDE-3A) with a Solartron SI1287 workstation at ambient conditions. A platinum wire and Ag/AgCl (3 M NaCl) were used as the counter and reference electrode, respectively. A catalyst-loaded glassy carbon (GC) rotating disk electrode (5 mm in diameter) was used as working electrode. All potentials in this study refer to RHE (ERHE = EAg/AgCl + 0.958 V), according to the calibration result in H2-saturated 0.1 M KOH electrolyte (Figure S1). A flow of Ar or O2 was maintained over the electrolyte (0.1 M KOH) during electrochemical measurements in order to ensure the Ar, O2-saturated solution. The catalyst suspension was prepared by dispersing 2.5 mg of catalysts in 1 mL of solution containing 0.99 mL of a mixture solution of ethanol and H2O (1:1, v/v) and 10 μL of 5% Nafion solution, followed by ultrasonication for 30 min to form a homogeneous ink. Then a certain volume of catalyst suspension was pipetted onto the GC surface to give a 0.2 mg cm−2 loading for all samples except commercial 20% Pt/C (0.12 mg cm−2). The cyclic voltammetry (CV) profiles were obtained in Ar- or O2-saturated 0.1 M KOH solution with a scan rate of 20 mV s−1. Rotating disk electrode (RDE) tests were performed in O2-saturated 0.1 M KOH solution with a sweep rate of 10 mV s−1. All the electrochemical measurements were performed at room temperature. The electron transfer numbers of the oxygen reduction reaction (ORR) were determined from the slopes of the linear lines according to the following K−L equation:29,30

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RESULTS AND DISCUSSION Characterization of Co-MOF-74-NT. Cobalt MOF-74 (Co-MOF-74), having 1D honeycomb-like nanochannels (∼1.1 nm in diameter), is widely used in catalysis and gas sorption.31,32 If the Co-MOF-74 nanocrystal grows along the c axis, a 1D tubular nanostructure composed of parallel multichannels is expected to form (Figure 1a). To achieve the anisotropic growth and a superlong single crystal, an AMMRA has been developed. The reaction of cobalt acetate tetrahydrate (Co(C2H3O2)2·4H2O) with 2,5-dihydroxyterephthalic acid (C8H6O6) in methanol solution at room temperature affords Co-MOF-74 nanoparticles (Co-MOF-74-NP) in amorphous phase (Figures S2, S3). Subsequently, the recrystallization of amorphous Co-MOF-74-NPs in water solution (pH = 7.15) at 175 °C produces superlong single-crystal Co-MOF-74 nanotubes (Co-MOF-74-NT) as observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figure 1). In this approach, the amorphous Co-MOF-74-NPs slightly dissolve in water, serving as the raw material to provide ligands and metal ions and then recrystallizing into a superlong MOF nanotube in a single crystal. Compared with the direct use of metal salts (cobalt acetate tetrahydrate) and ligands (2,5-dihydroxyterephthalic acid) as raw materials, which leads to the formation of Co-MOF-74 rods in a micrometer scale with a low aspect ratio (Co-MOF-74-MR) (3−30 μm in length, 1−5 μm in diameter, Figure S3), the AMMRA exhibits a huge advantage of size control in nanometers, achieving a uniform morphology of Co-MOF-74 nanotubes with a high aspect ratio. The as-synthesized MOF nanotubes exhibit a superior dispersibility in a series of solutions such as methanol, ethanol, and water (Figure S4) in contrast to Co-MOF-74-NP, -MR, and -MB (-MB prepared in a solvent of DMF/ethanol/water = 1:1:1).32 The photographs of Co-MOF-74 with different morphologies are shown in Figure S5. The powder X-ray diffraction (PXRD) pattern of Co-MOF-74-NT matches well with the simulated XRD pattern of MOF-74. Unlike the amorphous Co-MOF-74-NP, which shows only two broad peaks with a very low intensity (Figure S3), the Co-MOF-74-NT displays sharp diffraction peaks with high intensities, evidencing its good crystallinity (Figure 1b). The unchanged PXRD pattern of Co-MOF-74-NT immersed in water over 6 months indicates its excellent stability in water (Figure S6). N2 adsorption−desorption measurements of the as-prepared Co-MOF-74-NTs exhibit type I isotherms, similar to Co-MOF-74-MR and Co-MOF-74MB, suggesting the formation of micropores of these samples (Figures 1c, S3). The BET surface area of Co-MOF-74-NT is 848.8 m2 g−1, comparable to those of Co-MOF-74-MR (911.9 m2 g−1) and Co-MOF-74-MB (1030.1 m2 g−1), further indicating the high crystallinity of the Co-MOF-74-NT (Table 1). Both low-magnification SEM and TEM images (Figure 1e,f) disclose the uniformity of MOF nanotubes. The average diameter of Co-MOF-74-NT is 66.8 ± 13.8 nm (Figure 1d), which is calculated from 100 nanotubes randomly selected from the SEM images. Furthermore, the nanotubes are superlong,

1 1 1 1 1 = + = + 1/2 J JL JK J Bω K

B = 0.62nFC0(D0)2/3 υ−1/6

JK = nFkC0 where J, JL, and JK are the measured current density and diffusion- and kinetic-limiting current densities, respectively; ω is the angular velocity of the disk (ω = 2πN, N is the rotation speed), n is the electron transfer number, F is the Faraday constant (F = 96485 C mol−1), C0 is the bulk concentration of O2 (C0 = 1.2 × 10−6 mol cm−3 in 0.1 M KOH), D0 is the diffusion coefficient of O2 (D0 = 1.9 × 10−5 cm2 s−1 in 0.1 M KOH), υ is the kinematic viscosity of the electrolyte (υ = 0.01 cm2 s−1 in 0.1 M KOH), and k is the electron transfer rate constant. 15394

DOI: 10.1021/jacs.8b09092 J. Am. Chem. Soc. 2018, 140, 15393−15401

Article

Journal of the American Chemical Society

Figure 1. (a) Scheme of synthesis of Co-MOF-74-NT and Co-MOF-74-MR. (b) Powder X-ray diffraction patterns of the synthesized Co-MOF-74NT and Co-MOF-74-MR and a comparison with simulated XRD patterns of Co-MOF-74. (c) N2 sorption isotherms at 77 K for Co-MOF-74-NT and Co-MOF-74-MR. (d) Diameter-distribution histogram of Co-MOF-74-NTs. (e) SEM and (f) TEM images of the Co-MOF-74-NT. (g) SAED pattern of the selected area in (f). (h, i) HRTEM images of two different nanotubes showing the lattice fringes of Co-MOF-74-NTs. (j) Photograph records for the Co-MOF-74-NT-filled column-chromatographic separation process for methylene blue and rhodamine B (1, 6, Co-MOF-74-NT-filled column; 2, initial state of methylene blue and rhodamine B mixture solution injected into the column; 3, 4, separation process; 5, complete separation of the two dyes with only methylene blue retained, inset shows the color of the mixture solution before (left) and after (right) separation; 7, Co-MOF-74-NTfilled column after rhodamine B passing through, inset shows the color of rhodamine B before (left) and after (right) passing through the column).

some of which are even larger than 30 μm (Figures S7, S8), corresponding to an aspect ratio larger than 400. The selectedarea electron diffraction (SAED) pattern (Figure 1g) reveals that

the nanotubes are single crystals. The regular diffraction spots could be indexed to the ⟨110⟩ zone axis. The long axis of the nanotube is parallel to (00−3), indicating that the nanotube 15395

DOI: 10.1021/jacs.8b09092 J. Am. Chem. Soc. 2018, 140, 15393−15401

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Journal of the American Chemical Society

pH value ranging from 6 to 10 (Figure S11, Table S1). However, MOF rods of ∼60 μm in length and 5−6 μm in diameter are observed at pH ≤ 5, and short MOF rods of 3−4 μm in length are obtained at pH = 11. This is caused by the dissolving of precursor Co-MOF-74-NPs at a very low or high pH value, which provides a much higher reactant concentration than the case of pH = 6−10, and thus resulting in the failure of morphology control. Further to understanding the growth process, the products obtained at different reaction times in water at 175 °C are analyzed (Figure S12). At the initial stage (0.5 h), the Co-MOF74-NPs merge together. After a reaction time of 1 h, a few MOF nanotubes appear at the surface of the merged MOF-74-NPs. As the reaction time reaches 3 h, the MOF nanotubes, mixed with only a small amount of nanoparticles, further grow along the long axis. With further increasing the reaction time to 6 h, the nanoparticles disappear. According to the SEM results of the samples obtained at the reaction time of 12 h at different reaction temperatures, uniform MOF nanotubes can be synthesized above 150 °C, whereas only gel-like products are obtained at 100 °C (Figure S13), indicating that the reaction temperature is also important for the formation of Co-MOF-74-NTs. The above results verify that the formation of single-crystal Co-MOF-74-NT is formed in a dissolving−recrystallization process. In methanol, ethanol, or DMF solutions, no 1D structure is observed because the Co-MOF-74-NPs cannot dissolve in these solvents. In water solvent with a pH value of 6∼11, the Co-MOF-74-NP shows a low solubility, which provides a proper concentration of metal ions and ligands for the growth of 1D single-crystal nanotubes. At pH < 5, Co-MOF-74-NPs are completely dissolved, leading to the formation of MOF rods in large sizes. Taking advantage of the low solubility of amorphous Co-MOF-74-NPs in neutral water solvent, the diameter and length of Co-MOF-74-NTs are precisely controlled. Column-Chromatographic Dye Separation. The as-synthesized MOF nanotubes are employed as stationary phases in a column for separation of large dye molecules in aqueous solution (Figure 1j). Methylene blue and rhodamine B can be successfully separated by passing them through the Co-MOF-74NT-filled column. The methylene blue, with a small molecule size, which can enter the channels of MOF nanotubes, is retained inside the MOF nanotubes, while the larger rhodamine B molecules are rapidly passed through the column along with aqueous stream, suggesting that the Co-MOF-74-NTs are excellent porous materials for the separation of dye molecules or other relative mixtures owing to their well-defined 1D nanochannels. Practically, one Co-MOF-74-NT is a nanocolumn and can be regarded as a nanodevice to filter or capture molecules of appropriate size.35 Characterization of MOF-Derived Carbon Dendrites. MOF-derived carbon materials, having excellent chemical and mechanical stabilities, adjustable pore structures, and various functionalities, have shown great potential applications in catalysis and energy storage and conversion.36−42 However, the severe calcination conditions usually result in a partial or complete collapse of their original morphologies. Fortunately, during the thermal transformation of Co-MOF-74-NTs to carbon, the 1D morphology with a high aspect ratio is retained. Carbonization of Co-MOF-74-NTs in an argon flow yields 1D carbon nanofibers (CNF, ∼70 nm in diameter and tens of micrometers in length) embedded with Co nanoparticles (Co@CNF600−1000) (Figure S14). During the carbonization of Co-MOF-74-NTs, the coexistence of DCDA as a secondary carbon and nitrogen source results in the growth of carbon nanotubes (CNTs),43 by the

Table 1. BET Surface Area (SBET), Pore Volume (Vp), and Average Pore Size (PA) of MOFs and MOF-Derived Catalysts sample

SBET (m2 g−1)

Vp (cm3 g−1)

Co-MOF-74-NT Co-MOF-74-MR Co-MOF-74-NP Co-MOF-74-MB Co@CNF700 NCo@(CNT)-NF600 NCo@CNT-NF700 NCo@CNT-NF800 NCo@CNT-NF900 NCo@CNT-NF1000 NCo@CNT-NP700 NCo@CNT-MR700 NCo@CNT-MB700

848.8 911.9 878.7 1030.1 63.9 45.8 179.3 176.4 185.3 151.9 93.8 119.5 95.4

0.68 0.69 0.77 0.70 0.11 0.12 0.40 0.42 0.39 0.38 0.22 0.23 0.21

grows along the [001] direction. In addition, high-resolution TEM (HRTEM) images of Co-MOF-74-NT exhibit clear lattice fringes (Figure 1h,i). The distances of 1.3 and 0.73 nm correspond to the interplanar spacings of {110} and {−330} planes, respectively, both of which are parallel to the long axis, further demonstrating the growth direction of [001]. The angle between the {−22−1} plane, showing a lattice spacing of 0.58 nm, and the {−330} plane is 59.3°, consistent with the proposed crystal structure of Co-MOF-74. These observations indicate that the multinanochannels are parallel to the long c axis of the MOF-74 nanotubes. In contrast to the MOF nanotube bundles33 in micrometer size, which are assembled by separate single walled tubules with van der Waals interactions or hydrogen bonds, the nanochannels in Co-MOF-74-NT share the wall with six neighboring channels, and a nanotube of Co-MOF-74-NT forms one single coordinating framework as a single crystal. Although various morphologies of MOF nanostructures have been reported, to the best of our knowledge, it is the first report of 1D single-crystal single-framework MOF nanotubes with superlong size and very high aspect ratio. To analyze the growth process of these nanotubes, we changed the synthesis conditions including the solvent, acidity/basicity, reaction time, and temperature. Experimental results indicate that the use of water in the reaction solvent is vital to synthesize Co-MOF-74-NT. Replacing water by DMF, ethanol, and methanol as the reaction solvent cannot lead to the formation of MOF nanotubes or fibrous/rod-like structures (Figure S9), suggesting that the water solvent is beneficial to the formation of the 1D anisotropic nanostructure of Co-MOF-74. As shown in the SEM and XRD results (Figure S10), both the length and the crystallinity of Co-MOF-74 nanotubes increase with the concentration of water in the reaction solvent. At a low water concentration (20%), the product is composed of Co-MOF-74-NPs with some aggregation. Increasing the concentration of water to 40% results in the merging of Co-MOF-74-NPs, showing a gellike morphology. When the water concentration is increased to 60%, the SEM images display the Co-MOF-74-NTs coexisting with numerous nanoparticles. With further increasing the water concentration to 80%, the superlong Co-MOF-74-NTs with almost no nanoparticles are obtained. The above results suggest that the amorphous Co-MOF-74-NPs can work as an effective precursor to recrystallize into a 1D nanostructure. The pH values of the original mixture solution have been studied, as they might have a great effect on the morphology of MOFs.34 The superlong Co-MOF-74-NTs are obtained at a 15396

DOI: 10.1021/jacs.8b09092 J. Am. Chem. Soc. 2018, 140, 15393−15401

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Journal of the American Chemical Society

Figure 2. (a) Scheme of synthesis of hierarchical carbon dendrites, (b) PXRD patterns, (c) N2 sorption isotherms at 77 K, and (d) Raman spectra of the 3D N-doped hierarchical carbon dendrites synthesized at different temperatures. (e, h) High-resolution XPS spectra of C 1s and N 1s of NCo@ CNT-NF700. (f, g) SEM images and (i, j) TEM images of NCo@CNT-NF700.

captures a Co nanoparticle on the top. The HRTEM image displays that the Co nanoparticles are coated by 6−10 layers of graphitic carbon with a lattice spacing of 0.35 nm, corresponding to the distance between the (002) planes of graphite.44 Additionally, the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and elemental mapping measurements verify that the hierarchical carbon dendrite features the typical morphology. The uniform distribution of nitrogen suggests the successful N-doping in the nanostructure. In place of DCDA, the 3D hierarchical structures are also prepared by using urea as a secondary carbon and a nitrogen source (Figure S20). For comparison, the samples of Co-MOF-74-NP, Co-MOF74-MR, and Co-MOF-74-MB have also been carbonized with DCDA at 700 °C, and their products are named NCo@CNTNP700, NCo@CNT-MR700, and NCo@CNT-MB700,

catalysis of in situ generated Co nanoparticles, on the CNFs formed from Co-MOF-74-NTs, giving a N-doped 3D hierarchical dendrite with carbon nanofiber trunks and carbon nanotube branches (NCo@CNT-NFT, where T is the pyrolysis temperature) (Figure 2a). The morphologies of NCo@CNTNFT obtained at different temperatures have been studied by SEM and TEM measurements (Figures 2, S15−19). At 600 °C, the calcination of Co-MOF-74-NTs with DCDA gives a 1D carbon nanofiber without formation of any CNTs (NCo@(CNT)NF600). Increasing the pyrolysis temperature to 700, 800, 900, and 1000 °C results in the formation of a 3D hierarchical dendrite-like nanostructure. Representatively, the structure of NCo@CNT-NF700 is presented here in detail. As shown in Figure 2, the Co@CNFs are wrapped by a lot of CNTs (∼20 nm in diameter, hundreds of nanometers to micrometers in length). TEM measurements (Figure S15) reveal that each CNT 15397

DOI: 10.1021/jacs.8b09092 J. Am. Chem. Soc. 2018, 140, 15393−15401

Article

Journal of the American Chemical Society

Figure 3. (a) Scheme of the catalytic process for ORR. (b) CV curves of NCo@CNT-NF700 in (solid line) O2- and (dashed line) Ar-saturated 0.1 M KOH at 20 mV s−1. (c) LSV curves of the as-synthesized catalysts with different morphologies and 20% Pt/C catalyst at 10 mV s−1 with a rotating speed of 1600 rpm. (d) LSV curves of NCo@CNT-NF700 at different rotation rates. (e) K−L plots of NCo@CNT-NF700 at various potentials. (f) LSV curves (1600 rmp) of NCo@CNT-NF700 before (black) and after (red) 3000 cycles.

N at ∼398.5 eV and pyrrolic N at ∼400.6 eV.20,50 It is obvious that the peak intensity of pyridinic N is much higher than that of pyrrolic N, suggesting the rich pyridinic nitrogen species in the samples. Similar results are obtained in samples of NCo@CNTNP700, NCo@CNT-MR700, and NCo@CNT-MB700, demonstrating the successful doping of N atoms. Electrocatalytic Activity of NCo@CNT-NFT for ORR. The as-prepared hierarchical carbon dendrites are employed as electrocatalysts for ORR. The electrocatalytic activities of these samples are first studied by CV measurements in Ar-saturated and O2-saturated 0.1 M KOH solutions (Figures S28−S30). As shown in Figure 3b, no redox peak is observed for NCo@CNTNF700 in Ar-saturated KOH solution, while a well-defined cathodic peak (0.88 V vs RHE; RHE = reversible hydrogen electrode) is observed in the case of O2-saturated electrolyte, suggesting the excellent ORR performance. The ORR activity of the catalyst increases with increasing the pyrolysis temperature from 600 °C to 700 °C, but decreases with a further elevated pyrolysis temperature. Among these samples, the NCo@CNT-NF700 shows the largest onset potential (EO) of 0.93 V and peak potential (EP) of 0.87 V, which is comparable to the commercial 20% Pt/C catalysts (EO = 0.94 V, EP = 0.88 V). In contrast, the 1D Co@CNFT synthesized at different pyrolysis temperatures without N-doping exhibits a poor ORR activity, suggesting that the N species play an important role in the ORR process, as they can introduce structural defects in carbon supports and provide additional active sites.51,52 Although NCo@CNT-NP700 and NCo@CNT-MB700 are also synthesized under the same carbonization condition with N doping, they exhibit lower catalytic activities than the dendrite-like NCo@CNT-MR700 and NCo@CNT-NF700, indicating the positive effects brought by this unique structure. However, the catalytic activity of NCo@CNT-MR700 is still lower than that of NCo@CNTNF700 despite the similar dendrite-like structure of these two samples. This is attributed to the smaller diameter and

respectively (Figures S21−S23). SEM and TEM measurements show that all three samples contain Co nanoparticles and carbon nanotubes. PXRD patterns of NCo@CNT-NFT (Figure 2b) reveal a broad characteristic diffraction peak at about 26°, corresponding to the (002) plane of graphitic carbon, and well-defined diffractions for the metallic Co (JCPDS No. 15-0806).45 Similar results are observed in other samples derived from different morphologies of MOFs (Figure S24). The Raman spectra (Figures 2d, S25) of these samples further verify the formation of graphitic carbon with a G-band at 1585 cm−1. The D-band centered at 1345 cm−1 indicates the presence of disordered or defect carbon.46,47 N2 sorption measurements show that the morphologies of MOFs have a significant effect on BET surface areas and pore structures of their derived carbons (Figures 2c, S26). The BET surface area of NCo@CNT-NF700 is 179.3 m2 g−1, which is higher than that of NCo@CNT-NP700 (93.8 m2 g−1), NCo@ CNT-MR700 (119.5 m2 g−1), and NCo@CNT-MB700 (95.4 m2 g−1) (Table 1). It is worth mentioning that the pore volumes of NCo@CNT-NFT (T ≥ 700) are much larger than those of other carbons obtained from Co-MOF-74 in different morphologies. This is benefited from the small diameter of NCo@CNT-NFT, which provides more sites for the growth of CNTs on the surface compared to the large bulk samples. Because the pore volume of these samples is proportional to the numerous CNTs, more CNTs in a unit mass give a higher pore volume. The above results suggest that the Co-MOF-74-NT is a good precursor to fabricate 3D hierarchical carbon nanostructures with a large porosity. The chemical status and elemental composition of NCo@CNT-NF700 are studied by X-ray photoelectron spectroscopic (XPS) analysis (Figures 2e,h, S27). The survey scan of NCo@CNT-NF700 reveals the presence of C, N, O, and Co. The high-resolution spectrum of C 1s can be divided into two peaks at 284.5 and 285.6 eV, corresponding to C−C and CN, respectively.48,49 The high-resolution N 1s spectrum indicates the presence of two types of nitrogen species, pyridinic 15398

DOI: 10.1021/jacs.8b09092 J. Am. Chem. Soc. 2018, 140, 15393−15401

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Journal of the American Chemical Society

well retained after 3000 cycles of CV measurement (Figure S39). A methanol crossover test shows that no noticeable disturbance of the current density is observed for NCo@CNT-NF700 after an addition of methanol into the electrolyte solution, whereas a significant decrease in the current density is observed in the case of 20% Pt/C catalyst. These results disclose that NCo@CNTNF700 possesses better long-term durability and stronger immunity toward methanol crossover than Pt/C, which make it highly promising as a Pt-free electrocatalyst for practical applications. Zn−Air Battery. To further evaluate the outstanding catalytic activity of these hierarchical carbon dendrites, a Zn−air battery is constructed with NCo@CNT-NF700 as the cathode electrode and Zn plate as the anode electrode (Figure 4a).58 The maximum power density of NCo@CNT-

interconnected structure of the NCo@CNT-NF700, which facilitates the electron transport during the catalytic process and provides more accessible active sites on the surface for ORR when comparing to NCo@CNT-MR700 with an isolated dendrite-like structure at larger size. The ORR activities of these catalysts are further studied by linear sweep voltammogram (LSV) polarization curves (Figures 3c,d, S31, S32), and the results are summarized in Table S2. In conformity with the CV results, NCo@CNT-NF700 exhibits the best ORR activity with a half-wave potential (E1/2) of 0.861 V and current density of 5.3 mA cm−2. The E1/2 of NCo@CNTNF700, which is comparable to the best previously reported results of ORR catalysts derived from MOFs (Table S3), is only 7 mV lower than that of commercial 20% Pt/C (0.868 V) (Figure 3c), suggesting the excellent electrocatalytic performance of the unique 3D hierarchical dendrite-like structure. The linearity of the Koutecky−Levich (K−L) plots (Figure 3e) indicates first-order reaction kinetics toward the concentration of dissolved oxygen and similar electron transfer number per oxygen molecule at different potentials.53 The electron transfer number (n) is calculated to be 3.87−3.92 at potentials varying from 0.4 to 0.7 V, which is similar to the commercial 20% Pt/C catalyst (Figure S33), evidencing an efficient 4e ORR pathway. Comparing to other catalysts fabricated under the same conditions by using precursor MOFs in different morphologies or using different carbon templates (NCo@CNT-NP700, NCo@ CNT-MR700, NCo@CNT-MB700, NCo-XC72, NCo-rGO), NCo@CNT-NF700 displays a much higher ORR activity, showing the advantage of using MOF nanotubes as a precursor for the catalysts with hierarchical dendrite-like structures (Figures S32, S34). The excellent electrocatalytic activities of NCo@CNT-NF700 might mainly be associated with the synergistic effects between the unique hierarchical dendrite-like nanostructures and chemical compositions.54,55 In this catalyst, the seamless connection between carbon nanofiber trunks and carbon nanotube branches, which forms a robust porous 3D structure, is beneficial to electron and mass transfer, and the N-doped carbon with a proper degree of graphitization, as observed from XRD, Raman, and TEM analysis, guarantees the good conductivity of the catalysts, both of which can enhance the electrocatalytic performance.56 Electrochemical impedance spectroscopy results reveal a smaller charge transfer resistance for NCo@CNT-NF700 compared with Pt/C (Figure S35). Moreover, the metallic Co nanoparticles embedded in the CNFs and located on the top of CNTs also play an important role in the excellent electrocatalytic activity.57 The removal of Co NPs in NCo@CNT-NF700 by acid leaching leads to an obvious decrease in ORR performance (Figure S36). Besides the remarkable ORR performance of NCo@CNTNF700, it shows excellent durability and methanol tolerance in alkaline solutions (Figures S37, S38). Both NCo@CNT-NF700 and 20% Pt/C catalysts are evaluated by chronoamperometric responses at 0.5 V with a rotation rate of 1600 rpm. The NCo@ CNT-NF700 displays a superior durability with 93% retention over 72 000 s of continuous operation, whereas Pt/C catalysts suffer from a faster current loss of 34% over 60 000 s. Moreover, the NCo@CNT-NF700 is further subjected to a CV test from 0.2 to 1.16 V for 3000 cycles, which shows a negligible performance loss. There is a negative shift of only 14 mV in the E1/2 value at 1600 rpm, and the current density at 0.2 V (5.3 mA cm−2) remains unchanged (Figure 3f), demonstrating the outstanding stability of this sample. TEM analysis confirms that the hierarchical dendrite-like structure of NCo@CNT-NF700 is

Figure 4. (a) Schematic diagram of a Zn−air battery. (b) Polarization curve and corresponding power density plot of the primary Zn−air battery using NCo@CNT-NF700 and Pt/C as the cathode catalyst. (c) Initial open-circuit potential of the Zn−air battery showing a value of 1.46 V. (d) Charge and discharge polarization curves of the rechargeable Zn−air battery. (e) Discharge−charge cycling curves at 5 mA cm−2 for 400 cycles.

NF700 is as high as 220 mW cm−2 (Figure 4b), and the battery has an initial open-circuit potential of 1.46 V (Figure 4c). This result is much higher than that of Pt/C catalyst (131 mW cm−2) tested under the same conditions and comparable to the best electrocatalysts used for a Zn−air battery (Table S4). Figure 4d illustrates the charge and discharge polarization curves of the rechargeable Zn−air battery. The NCo@CNT-NF700 shows a low voltage gap of 0.8 V at 50 mA cm−2, while the Pt/C displays a much lower performance with a voltage gap of 1.05 V. When cycled at a constant current density of 5 mA cm−2 in short cycle times (1200 s per charge−discharge period), the Zn−air battery based on NCo@CNT-NF700 catalysts delivers an initial 15399

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discharge potential of 1.18 V and a charge potential of 2.11 V (Figure 4e). After 400 cycles (about 133 h), the catalytic performance is unexpectedly increased, which maintains the initial discharge potential of 1.18 V and decreases the charge potential to 2.05 V, corresponding to a decrease in charge/discharge voltage gap by 0.06 V and demonstrating its excellent activity and stability.

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CONCLUSION We have reported a novel and rational strategy to synthesize a 1D superlong single-crystal MOF nanotube with multichannels via an amorphous MOF-mediated recrystallization approach without any surfactants and templates. The uniform, monodisperse morphology of MOF nanotubes can be used as a nanocolumn for the separation of large molecules such as methylene blue and rhodamine B in aqueous solution. These MOF nanotubes are excellent precursors for synthesizing 1D carbon nanofibers. Carbonization of the MOF nanotubes along with dicyandiamide as a nitrogen and a secondary carbon source produces 3D hierarchical carbon dendrites composed of carbon nanofiber trunks and carbon nanotube branches with N-doped and Co nanoparticles immobilized, which exhibit remarkable ORR catalytic activity, superior durability, and methanol tolerance compared with commercial 20% Pt/C catalyst. The Zn−air battery based on NCo@CNT-NF700 as a cathode exhibits an excellent performance with an initial open-circuit potential of 1.46 V and a maximum power density as high as 220 mW cm−2, which shows no degeneration in performance even after 400 cycles (133 h). The outstanding catalytic performance is attributed to the unique 3D hierarchical dendrite-like structures and chemical compositions. The synthetic strategy presented here can be extended to the preparation of other 1D single-crystal MOF nanostructures and opens up a new avenue for fabricating hierarchical carbon nanostructures with controllable morphology and composition for electrocatalysts.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b09092. MOF and catalyst syntheses; additional results of PXRD, BET, Raman, XPS, SEM, and TEM; durability and stability results of catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Huan Pang: 0000-0002-5319-0480 Qiang Xu: 0000-0001-5385-9650 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the reviewers for valuable suggestions, Dr. Takeyuki Uchida for microscopic measurements, and AIST and Kobe University for financial support. L.Z. thanks MEXT for the Japanese government scholarship.

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