Superlong Single-Crystal MetalâOrganic Framework Nanotubes

Subscriber access provided by UNIV OF LOUISIANA

Article

Super-Long Single-Crystal Metal-Organic Framework Nanotubes Lianli Zou, Chun-Chao Hou, Zheng Liu, Huan Pang, and Qiang Xu J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09092 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Journal of the American Chemical Society

Super-Long Single-Crystal Metal-Organic Framework Nanotubes Lianli Zou,†,‡ Chun-Chao Hou,§ Zheng Liu,‖ Huan Pang⏊ and Qiang Xu*,†,‡,§ 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, China

KEYWORDS: single-crystal, metal–organic framework nanotubes, carbon nanofibers, carbon nanotubes, oxygen reduction reaction

ABSTRACT: Nanotubes have attracted great attention. Here, we report the fabrication of the first single-crystal metalorganic framework (MOF) nanotubes. Super-long 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 (AMMRA). The synthesized MOF nanotubes can be used as a nano column for separation of large molecules. Carbonization of the Co-MOF nanotubes in 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 (DCDA) as a nitrogen and a secondary carbon source. The resulted hierarchical dendrites with carbon nanofiber trunks and carbon nanotube branches exhibit excellent electrocatalytic activity for oxygen reduction reaction (ORR), and exceptional applications in rechargeable Zn–air batteries. This work demonstrates a new strategy to fabricate MOF nanotubes and relative 1D nanostructures.

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 Recent years, extensive research efforts have focused on synthesis of MOFs in nanometers as they can have sizedependent 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 twodimensional (2D) single-crystal nano MOFs have been synthesized.19 The templating and surfactant-assistant approaches have been used to produce 1D MOF anisotropic structures, while they cannot afford singlecrystal 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. Up to date, there is no report for the synthesis of single-crystal MOF nanotubes. Kinetic control of crystallization has significant effect on crystal morphology and crystallinity,13,24,25 and a low reaction rate benefits the formation of an anisotropic nanostructure as well as single-crystals.26,27 The kinetic 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 is employed as a highly efficient nano column for the separation of large dye molecules, and a precursor for preparing carbon nanostructures via thermal transformation processes,

ACS Paragon Plus Environment

1

Journal of the American Chemical Society 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

which exhibit excellent catalytic activities for oxygen reduction reaction. 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 firstly dissolved in methanol solution (100 mL) under vigorous stirring. With a continue stirring, 50 mL methanol solution containing 1.5 mmol 2,5dihydroxyterephthalic acid was added dropwise, and the resulting mixture was further stirred for about 2 hours at room temperature (RT). The final precipitate, marked as Co-MOF-74-NP, was washed by methanol and followed by washing with water 2 times, and then dispersed in 50 mL water. The mixture was then transferred into a Teflon-lined autoclave, tightly capped, and placed in an oven at 175 °C for various hours (0.5, 1, 2, 3, 6 and 12 h). After cooling down 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, DMF, and 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. 20 mg of as-prepared Co-MOF-74-NTs and 0.4 g of DCDA were placed in two separate ceramic boats with DCDA at the upstream side of the furnace in argon atmosphere. With a heating ramp rate of 10 °C min-1, the samples were firstly heated at 400 °C for 3 h and then raised to 600, 700, 800, 900 and 1000 °C and maintained at these temperatures for another 4 h. After cooling down 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 (ϕ5 mm, 200 mm) with open ends was filled with Co-MOF-74-NT (10 mg) and was used as a chromatographic column. The aqueous solution of rhodamine B (0.01 mg mL-1, 3 mL) and mixture solution of methylene blue (0.03 mg mL-1)/rhodamine B (0.01 mg mL-1) (1:1, 3 mL) were passed through 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 condition. A platinum wire and Ag/AgCl (3 M NaCl) were used as the counter and reference electrode, respectively. A catalystloaded 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 + 0.958V), according to the calibration result in H2-saturated 0.1 M KOH electrolyte (Figure S1). A flow of N2 or O2 was maintained over the electrolyte (0.1 M KOH) during electrochemical measurements in order to ensure the N2, O2-saturated solution. The catalyst suspension was prepared by dispersing 2.5 mg of catalysts in 1 mL of

Page 2 of 12

solution containing 0.99 mL 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 N2- 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 were determined from the slopes of the linear lines according to the following K–L equation: 29,30

1 1 1 1 1 = + = + 𝐽 𝐽𝐿 𝐽𝐾 𝐵𝜔1/2 𝐽𝐾 𝐵 = 0.62𝑛𝐹𝐶0(𝐷0)2/3𝜐 ―1/6 𝐽𝐾 = 𝑛𝐹𝑘𝐶0 where 𝐽, 𝐽𝐿 and 𝐽𝐾 are the measured current density and diffusion-, 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. Zn-Air Battery Fabrication and Testing. The zinc-air battery was assembled with a Zn anode, 6 M KOH solution (mixed with 0.2 M zinc acetate for rechargeable Zn-air batteries), and an air cathode comprising a gasdiffusion layer, catalyst layer and a separator to prevent electrolyte leakage. The catalyst layer was prepared by dropping 40 uL 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 condition at room temperature.

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 c axis, a 1D tubular nanostructure composed of parallel multichannels is expected to form (Figure 1a). To achieve the anisotropic growth and a super-long single-crystal, an amorphous MOF-mediated recrystallization approach (AMMRA) has been developed. The reaction of cobalt acetate tetrahydrate (Co(C2H3O2)2•4H2O) with 2,5dihydroxyterephthalic acid (C8H6O6) in methanol solution at room temperature affords Co-MOF-74 nanoparticles (Co-MOF-74-NP) in amorphous phase (Figure S2, S3). Subsequently, the recrystallization of amorphous Co-MOF-74-NPs in water solution (pH = 7.15) at 175 °C produces super-long single-crystal Co-MOF-74


2

Page 3 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanotubes (Co-MOF-74-NT) as observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) (Figure 1).

Figure 1. (a) Scheme of synthesis of Co-MOF-74-NT and Co-MOF-74-MR. (b) Powder X-ray diffraction patters of the synthesized Co-MOF-74-NT, 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, (f) TEM image 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 Co-MOF-74-NT-filled column-chromatographic separation process for methylene blue and rhodamine B (1,6, C0-MOF-47-NT-filled column; 2, initial state of methylene blue and


3


Page 4 of 12

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 mixture solution before (left) and after (right) separation; 7, C0-MOF-47-NTfilled column after rhodamine B passing through, inset shows the color of rhodamine B before (left) and after (right) passing through the column).

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 super-long MOF nanotube in 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 CoMOF-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 nanometer, 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-74NP, -MR, and -MB (-MB prepared in solvent of DMF/ethanol/water = 1:1:1).32 The photographs of CoMOF-74 with different morphologies are shown in Figure S5. Powder X-ray diffraction (PXRD) pattern of Co-MOF74-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 the type I isotherms, similar to Co-MOF-74-MR and Co-MOF-74-MB, suggesting the formation of micro pores of these samples (Figure 1c, Figure 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 super-long, some of which are even larger than 30 µm (Figure S7, S8), corresponding to an aspect ratio larger than 400. The selected-area electron diffraction (SAED) pattern (Figure 1g) reveals that the nanotubes are single crystals. The regular diffraction spots could be indexed to the zone axis. The long axis of the nanotube is parallel to (00-3), indicating that the nanotube 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 59.3o, 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 individuals of 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 super long 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 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 of aggregation. Increasing the concentration of water to 40% results in the mergence of Co-MOF-74-NPs, showing a gel-like morphology. When the water concentration is increased to 60%, the SEM images display the Co-MOF74-NTs co-existing with numerous nanoparticles. With further increasing the water concentration to 80%, the super-long 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 it might have great effect on the morphology of MOFs.34 The super-long Co-MOF-74-NT are obtained at a pH value ranging from 6 to 10 (Figure S11, Table S1). However, MOF rods of ~60 µm in length and 56 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 understand 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


4

Page 5 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


h), the Co-MOF-74-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 the advantage of the low solubility of amorphous C0-MOF74-NPs in neutral water solvent, the diameter and length of Co-MOF-74-NT are precisely controlled. Column-Chromatographic Dye Separation. The assynthesized MOF nanotubes are employed as stationary phases in 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-74-NT-filled column. The methylene blue with a small molecule size, which can enter the channels of MOF nanotubes, are 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-74NTs are excellent porous materials for the separation of dye molecules or other relative mixtures owing to their well-defined 1D nanochannels. Practically, one Co-MOF74-NT is a nano column, and can be regarded as a nano device to filter or capture molecules in 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 nano fibers (CNF, ~70 nm in diameter and tens micrometers in length) embedded with Co nanoparticles (Co@CNF600-1000) (Figure S14). During the carbonization of Co-MOF-74-NTs, the co-existence of dicyandiamide (DCDA) as a secondary carbon and nitrogen source results in the growth of carbon nanotubes

(CNTs),43 by the catalysis of in-situ generated Co nanoparticles, on the CNFs formed from Co-MOF-74NTs, 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@CNT-NFT obtained at different temperatures have been studied by SEM and TEM measurements (Figure 2, S15-19). At 600 °C, the calcination of Co-MOF-74-NTs 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

848.8

0.68

Co-MOF-74-MR

911.9

0.69

Co-MOF-74-NP

878.7

0.77

Co-MOF-74-MB

1030.1

0.70

Co@CNF700

63.9

0.11

NCo@(CNT)-NF600

45.8

0.12

NCo@CNT-NF700

179.3

0.40

NCo@CNT-NF800

176.4

0.42

NCo@CNT-NF900

185.3

0.39

NCo@CNT-NF1000

151.9

0.38

NCo@CNT-NP700

93.8

0.22

NCo@CNT-MR700

119.5

0.23

NCo@CNT-MB700

95.4

0.21

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 3D hierarchical dendrite-like nanostructure. Representatively, the structure of NCo@CNT-NF700 is presented here in details. As shown in Figure 2, the Co@CNFs are wrapped by a lot of CNTs (~20 nm in diameter, hundreds nanometers to micrometers in length). TEM measurements (Figure S15) reveal that each CNT 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


5


Page 6 of 12

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, CoMOF-74-MR and Co-MOF-74-MB have also been carbonized with DCDA at 700 °C and their products are named as NCo@CNT-NP700, NCo@CNT-MR700 and NCo@CNT-MB700, respectively (Figure S21-23). SEM and TEM

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 of NCo@CNT-NF700. (i,j) TEM images of NCo@CNT-NF700.

measurements display that all the 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


6

Page 7 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Similar results are observed in other samples derived from different morphologies of MOFs (Figure S24). The Raman spectra (Figure 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 (Figure 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@CNTMB700 (95.4 m2 g−1) (Table 1). It is worth to mention that the pore volumes of NCo@CNT-NFT (T ≥ 700) are much larger than those of other carbons obtained from CoMOF-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 of CNTs, more CNTs in a

Figure 3. (a) Scheme of the catalytic process for ORR. (b) CV curves of NCo@CNT-NF700 in (solid line) O2- and (dash line) N2saturated 0.1 M KOH at 20 mV s−1. (c) LSV curves of the ay-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.

unit mass gives 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 (Figure 2e,h, Figure 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 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


7


are obtained in samples of NCo@CNT-NP700, 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 cyclic voltammetric (CV) measurements in N2-saturated and O2saturated 0.1 M KOH solutions (Figure S28-30). As shown in Figure 3b, no redox peak is observed for NCo@CNTNF700 in N2-saturated KOH solution, while a welldefined cathodic peak (0.88V vs. RHE; RHE = reversible hydrogen electrode) is observed in the case of O2saturated electrolyte, suggesting the excellent ORR performance. The ORR activity of the catalyst increases with increasing the pyrolysis temperature from 600 to 700 °C, but turns to decrease with a further elevated pyrolysis temperature. Among these samples, the NCo@CNTNF700 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@CNT-NF700 despite the similar dendrite-like structure of these two samples. This is attributed to the smaller diameter and 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 in larger size. The ORR activities of these catalysts are further studied by linear sweep voltammogram (LSV) polarization curves (Figure 3c,d, S31,32), 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@CNT-NF700, which is comparable to the best previously reported results of ORR catalysts derived from MOFs (Table S3), only shows 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 firstorder 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

Page 8 of 12

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 condition by using precursor MOFs in different morphologies or using different carbon templates (NCo@CNT-NP700, NCo@CNT-MR700, NCo@CNT-MB700, NCo-XC72, NCorGO), NCo@CNT-NF700 displays a much higher ORR activity, showing the advantage of using MOF nanotubes as a precursor for the catalysts with hierarchical dendritelike structures (Figure S32,34). 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@CNT-NF700, it shows excellent durability and methanol tolerance in alkaline solutions (Figure S37,38). 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 72000 s of continuous operation, whereas Pt/C catalysts suffer from a faster current loss of 34% over 60000 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 only a negative shift of 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


8

Page 9 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 Znair battery based on NCo@CNT-NF700 catalysts delivers an initial 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. CONCLUSION

Figure 4. (a) The schematic diagram of Zn-air battery. (b) Polarization curve and corresponding power density plot of 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 shows a value of 1.46 V. (d) Charge and discharge polarization curves of rechargeable Zn-air battery. (e) Discharge-charge cycling curves at 5 mA cm-2 for 400 cycles.

structure of NCo@CNT-NF700 is well retained after 3000 cycles of CV measurement (Figure S39). 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@CNT-NF700 possesses better longterm 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@CNTNF700 is as high as 220 mW cm-2 (Figure 4b), and the battery has an initial open-circuit potential (OCP) 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 condition, and comparable to the best electrocatalysts used for Znair 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

We have reported a novel and rational strategy to synthesize a 1D super-long 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 nano column 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, and superior durability and methanol tolerance to 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.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: MOF and catalyst syntheses; additional results of PXRD, BET, Raman, XPS, SEM and TEM; durability and stability results of catalysts.

AUTHOR INFORMATION Corresponding Author

*[email protected] Notes

The authors declare no competing financial interest.


9


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.

REFERENCES (1) Li, H.; Eddaoudi, M.; O'Keeffe, M.; Yaghi, O. M. Nature 1999, 402, 276-279. (2) Noro, S.; Kitagawa, S.; Kondo, M.; Seki, K. Angew. Chem. Int. Ed. 2000, 39, 2081-2084. (3) Rosi, N. L.; Kim, J.; Eddaoudi, M.; Chen, B.; O'Keeffe, M.; Yaghi, O. M. J. Am, Chem. Soc. 2005, 127, 1504-1518. (4) Zhu, Q.-L.; Xu, Q. Chem. Soc. Rev. 2014, 43, 5468-5512. (5) Cui, X.; Chen, K.; Xing, H.; Yang, Q.; Krishna, R.; Bao, Z.; Wu, H.; Zhou, W.; Dong, X.; Han, Y.; Li, B. Science 2016, 353, 141–144. (6) Krause, S.; Bon, V.; Senkovska, I.; Stoeck, U.; Wallacher, D.; Többens, D. M.; Zander, S.; Pillai, R. S.; Maurin, G.; Coudert, F. X.; Kaskel, S. Nature 2016, 532, 348−352. (7) Jagadeesh, R. V.; Murugesan, K.; Alshammari, A. S.; Neumann, H.; Pohl, M. M.; Radnik, J.; Beller, M. Science 2017, 358, 326−332. (8) Wang, H.; Zhu, Q.-L.; Zou, R.; Xu, Q. Chem 2017, 2, 52−80. (9) Yoon, J. W.; Chang, H.; Lee, S. J.; Hwang, Y. K.; Hong, D. Y.; Lee, S. K.; Lee, J. S.; Jang, S.; Yoon, T. U.; Kwac, K.; Jung, Y. Nat. Mater. 2017, 16, 526−531. (10) Chandler, B. D.; Enright, G. D.; Udachin, K. A.; Pawsey, S.; Ripmeester, J. A.; Cramb, D. T.; Shimizu, G. K. Nat. Mater. 2008, 7, 229−235. (11) Shen, K.; Zhang, L.; Chen, X.; Liu, L.; Zhang, D.; Han, Y.; Chen, J.; Long, J.; Luque, R.; Li, Y.; Chen, B. Science 2018, 359, 206–210. (12) Xu, X.; Zhang, Z.; Wang, X. Adv. Mater. 2015, 27, 5365–5371. (13) Rodenas, T.; Luz, I.; Prieto, G.; Seoane, B.; Miro, H.; Corma, A.; Kapteijn, F.; i Xamena, F. X. L.; Gascon, J. Nat. Mater. 2015, 14, 48–55. (14) Wang, S.; Fan, Y.; Teng, J.; Fan, Y. Z.; Jiang, J. J.; Wang, H. P.; Grützmacher, H.; Wang, D.; Su, C. Y. Small 2016, 12(41), 5702−5709. (15) Hu, P.; Zhuang, J.; Chou, L. Y.; Lee, H. K.; Ling, X. Y.; Chuang, Y. C.; Tsung, C. K. J. Am, Chem. Soc. 2014, 136, 10561−10564. (16) Jung, S.; Cho, W.; Lee, H. J.; Oh, M. Angew. Chem. Int. Ed. 2009, 48, 1459−1462. (17) Zhang, Z.; Chen, Y.; Xu, X.; Zhang, J.; Xiang, G.; He, W.; Wang, X. Angew. Chem. Int. Ed. 2014, 53, 429−433. (18) Kaminker, R.; Popovitz-Biro, R.; van der Boom, M. E. Angew. Chem. Int. Ed. 2011, 123, 3282−3284. (19) Zhao, M.; Wang, Y.; Ma, Q.; Huang, Y.; Zhang, X.; Ping, J.; Zhang, Z.; Lu, Q.; Yu, Y.; Xu, H.; Zhao, Y. Adv. Mater. 2015, 27(45), 7372–7378. (20) Zhang, W.; Wu, Z. Y.; Jiang, H.-L.; Yu, S.-H. J. Am. Chem. Soc. 2014, 136, 14385–14388. (21) Taylor, K. M. L.; Rieter, W. J.; Lin, W. J. Am, Chem. Soc. 2008, 130, 14358−14359. (22) Jahan, M.; Bao, Q.; Yang, J.; Loh, K. J. Am. Chem. Soc. 2010, 132, 14487–14495. (23) Pachfule, P.; Shinde, D.; Majumder, M.; Xu, Q. Nat. Chem. 2016, 8, 718–724. (24) Bosch, M.; Yuan, S.; Rutledge, W.; Zhou, H. C. Acc. Chem. Res. 2017, 50, 857–865. (25) Stavitski, E.; Goesten, M.; Juan-Alcañiz, J.; Martinez-Joaristi, A.; Serra-Crespo, P.; Petukhov, A. V.; Gascon, J.; Kapteijn, F. Angew. Chem. Int. Ed. 2011, 50, 9624−9628.

Page 10 of 12

(26) Jin, M.; He, G.; Zhang, H.; Zeng, J.; Xie, Z.; Xia, Y. Angew. Chem. Int. Ed. 2011, 50, 10560−10564. (27) Fischer, S.; Swabeck, J. K.; Alivisatos, A. P. J. Am, Chem. Soc. 2017, 139, 12325−12332. (28) Feng, D.; Wang, K.; Wei, Z.; Chen, Y. P.; Simon, C. M.; Arvapally, R. K.; Martin, R. L.; Bosch, M.; Liu, T. F.; Fordham, S.; Yuan, D. Nat. Commun. 2014, 5, 2573. (29) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 2001. (30) Zhou, R.; Zheng, Y.; Jaroniec, M.; Qiao, S.-Z. ACS Catal. 2016, 6, 4720−4728. (31) Dietzel, P. D. C.; Morita, Y.; Blom, R.; Fjellvåg, H. Angew. Chem. Int. Ed. 2005, 44, 6354–6358. (32) Caskey, S. R.; Wong-Foy, A. G.; Matzger, A. J. J. Am, Chem. Soc. 2008, 130, 10870−10871. (33) Otsubo, K.; Wakabayashi, Y.; Ohara, J.; Yamamoto, S.; Matsuzaki, H.; Okamoto, H.; Nitta, K.; Uruga, T.; Kitagawa, H. Nat. Mater. 2011, 10, 291−295. (34) Zhang, W.; Jiang, X.; Zhao, Y.; Carné-Sánchez, A.; Malgras, V.; Kim, J.; Kim, J. H.; Wang, S.; Liu, J.; Jiang, J.-S., Yamauchi, Y.; Hu, M. Chem. Sci. 2017, 8, 3538–3546. (35) Takenobu, T.; Takano, T.; Shiraishi, M.; Murakami, Y.; Ata, M.; Kataura, H.; Achiba, Y.; Iwasa, Y. Nat. Mater. 2003, 2, 683– 688. (36) Liu, B.; Shioyama, H.; Akita, T.; Xu, Q. J. Am. Chem. Soc. 2008, 130, 5390–5391. (37) Zhu, Q.-L.; Xia, W.; Akita, T.; Zou, R.; Xu, Q. Adv. Mater. 2016, 28, 6391–6398. (38) Dang, S.; Zhu, Q.-L.; Xu, Q. Nat. Rev. Mater. 2017, 3, 17075. (39) Hao, G. P.; Tang, C.; Zhang, E.; Zhai, P.; Yin, J.; Zhu, W.; Zhang, Q.; Kaskel, S. Adv. Mater. 2017, 29, 1702829. (40) Chaikittisilp, W.; Ariga, K.; Yamauchi, Y. J. Mater. Chem. A 2013, 1, 14-19. (41) Liu, J.; Zhu, D.; Guo, C.; Vasileff, A.; Qiao, S.-Z. Adv. Energy Mater. 2017, 7, 1700518. (42) Salunkhe, R. R.; Young, C.; Tang, J.; Takei, T.; Ide, Y.; Kobayashi, N.; Yamauchi, Y. Chem. Commun. 2016, 52, 4764– 4767. (43) Collins, P. G.; Arnold, M. S.; Avouris, P. Science 2001, 292, 706–709. (44) Futaba, D. N.; Hata, K.; Yamada, T.; Hiraoka, T.; Hayamizu, Y.; Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S. Nat. Mater. 2006, 5, 987–994. (45) Xia, W.; Zou, R.; An, L.; Xia, D.; Guo, S. Energy Environ. Sci. 2015, 8, 568–576. (46) Su, P.; Xiao, H.; Zhao, J.; Yao, Y.; Shao, Z.; Li, C.; Yang, Q. Chem. Sci. 2013, 4, 2941–2946. (47) Zhao, S.; Yin, H.; Du, L.; He, L.; Zhao, K.; Chang, L.; Yin, G.; Zhao, H.; Liu, S.; Tang, Z. ACS Nano 2014, 8, 12660–12668. (48) Zheng, Y.; Jiao, Y.; Zhu, Y.; Li, L. H.; Han, Y.; Chen, Y.; Du, A.; Jaroniec, M.; Qiao, S.-Z. Nat. Commun. 2014, 5, 3783. (49) Wang, X. X.; Cullen, D. A.; Pan, Y. T.; Hwang, S.; Wang, M.; Feng, Z.; Wang, J.; Engelhard, M. H.; Zhang, H.; He, Y.; Shao, Y. Adv. Mater. 2018, 30, 1706758. (50) Hou, Y.; Qiu, M.; Zhang, T.; Ma, J.; Liu, S.; Zhuang, X.; Yuan, C.; Feng, X. Adv. Mater. 2017, 30, 1604480. (51) Jiao, Y.; Zheng, Y.; Davey, K.; Qiao, S.-Z. Nat. Energy 2016, 1, 16130. (52) Chen, P.; Xiao, T. Y.; Qian, Y. H.; Li, S. S.; Yu, S.-H. Adv. Mater. 2013, 25, 3192–3196. (53) Zhang, P.; Sun, F.; Xiang, Z.; Shen, Z.; Yun, J.; Cao, D. Energy Environ. Sci. 2014, 7, 442−450. (54) Zhang, H.; Hwang, S.; Wang, M.; Feng, Z.; Karakalos, S.; Luo, L.; Qiao, Z.; Xie, X.; Wang, C.; Su, D.; Shao, Y. J. Am. Chem. Soc. 2017, 139, 14143–14149. (55) Qian, Y.; Khan, I. A.; Zhao, D. Small 2017, 13, 1701143.


10

Page 11 of 12 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(56) Young, C.; Wang, J.; Kim, J.; Sugahara, Y.; Henzie, J.; Yamauchi, Y. Chem. Mater. 2018, 30, 3379−3386. (57) Meng, C.; Ling, T.; Ma, T.-Y.; Wang, H.; Hu, Z.; Zhou, Y.; Mao, J.; Du, X.-W.; Jaroniec, M.; Qiao, S.-Z. Adv. Mater. 2017, 29, 1604607. (58) Tang, C.; Wang, B.; Wang, H.-F.; Zhang, Q. Adv. Mater. 2017, 29, 1703185.


11



Page 12 of 12

12

Superlong Single-Crystal MetalâOrganic Framework Nanotubes

Recommend Documents

Superlong Single-Crystal MetalâOrganic Framework Nanotubes