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A Single-Crystal Open-Capsule Metal−Organic Framework Yong-Sheng Wei,† Mei Zhang,† Mitsunori Kitta,‡ Zheng Liu,§ Satoshi Horike,†,∥ and Qiang Xu*,†,‡

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†

AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), National Institute of Advanced Industrial Science and Technology (AIST), Sakyo-ku, Kyoto 606-8501, Japan ‡ Research Institute of Electrochemical Energy, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan § Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyamaku, Nagoya, Aichi 463-8560, Japan ∥ Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Institute for Advanced Study, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan S Supporting Information *

ABSTRACT: Micro-/nanocapsules have received substantial attention due to various potential applications for storage, catalysis, and drug delivery. However, their conventional enclosed non-/polycrystalline walls pose huge obstacles for rapid loading and mass diffusion. Here, we present a new single-crystal capsular-MOF with openings on the wall, which is carefully designed at the molecular level and constructed from a crystal-structure transformation. This rare open-capsule MOF can easily load the largest amounts of sulfur and iodine among known MOFs. In addition, derived from capsularMOF and melamine through pyrolysis−phosphidation, we fabricated a nitrogen-doped capsular carbon-based framework with iron−nickel phosphide nanoparticles immobilized on capsular carbons interconnected by plentiful carbon nanotubes. Benefiting from synergistic effects between the carbon framework and highly surface-exposed phosphide sites, the material exhibits efficient multifunctional electrocatalysis for oxygen evolution, hydrogen evolution, and oxygen reduction, achieving well-qualified assemblies of an overall water splitting (low potential of 1.59 V at 10 mA·cm−2) and a rechargeable Zn−air battery (high peak power density of 250 mW·cm−2 and excellent stability for 500 h), which afford remarkably practical prospects over previously known electrocatalysts.

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morphologies.18−21 It is a big challenge to rationally design and construct capsular MOF nanocrystals with openings on the wall, let alone present single crystallinity. Kinetic control of MOF crystal recrystallization (dissolution− regrowth) has a significant effect on crystal engineering and morphology.22,23 Especially when the structures of parent and shell crystals are well-matched, shell crystals will uniformly grow on outer surfaces of parent crystals, the cores of which simultaneously dissolve and diffuse out. In other words, this core-to-surface mass transfer will probably lead to the formation of single-crystal capsular structures and leave plentiful openings on the wall, although the shape-controlled regrowth of singlecrystal capsular MOFs from crystal-structure transformations are rarely achieved. Herein, we developed a self-templated strategy to successfully construct an iron−nickel-based single-crystal open capsularMOF, which is carefully designed and constructed from crystal-

INTRODUCTION Micro-/nanocapsules have gained considerable attention in various promising fields1 including storage/encapsulation,2−4 catalysis,5−7 and drug delivery8 by virtue of their high exposed surface, modifiable shell, and large accommodation space. However, to our knowledge, most of the capsules have enclosed structures, which would seriously hinder mass diffusion and exposure of inner surfaces.9 Thus, designing and constructing new capsules with openings is highly desired but rarely reported so far.10 In this context, metal−organic frameworks (MOFs), constructed by metal ions and organic ligands,11−14 have shown great potential in the synthesis of capsular crystals, which are generally prepared by prefabricating a core−shell intermediate with an inactive solid as inner template.15 Nevertheless, this method usually suffers from their complicated time-consuming processes and harsh conditions of template removal.16,17 Recently, self-templated strategies that do not require removal of the cores have been developed to prepare hollow MOF crystals, while most of them afford undesired polycrystalline hollows close-packed by small MOF crystals with enclosed © XXXX American Chemical Society

Received: March 4, 2019

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DOI: 10.1021/jacs.9b02417 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 1. (a) Schematic illustration for the formation of capsular-MOF from FeNi-MIL-88B. (b) TEM images of FeNi-MIL-88B. (c) SEM image of capsular-MOF. (d) HAADF-STEM image and elemental maps of capsular-MOF. (e) SAED pattern of the selected area of inserted capsular-MOF nanocrystal. Its regular diffraction spots can be indexed to the ⟨100⟩ zone axis. (f) N2 adsorption (solid) and desorption (empty) isotherms (77 K) of capsular-MOF, solid-MOF, and FeNi-MIL-88B. The inset shows their corresponding pore size distribution curves. (g, h) Single-crystal X-ray diffraction structures along the c-axis of Fe-MIL-88B (corresponding to FeNi-MIL-88B) and Fe-MIL-88B-tpy (corresponding to capsular-MOF), respectively. Hydrogen atoms and terminal ligands of trinuclear clusters are omitted for clearity. (i, j) Difference electron density maps for Fe-MIL-88B (corresponding to FeNi-MIL-88B) and Fe-MIL-88B-tpy (corresponding to capsular-MOF), respectively, showing the presence of tpy.

splitting (low potential of 1.59 V at 10 mA·cm−2) and a rechargeable Zn−air battery (high peak power density of 250 mW·cm−2 and excellent stability for 500 h). As far as we know, this is the first design and construction of an open-capsule nanostructure that offers excellent material encapsulation and multicatalytic performances with greatly potential applications in practice.

structure transformations at the molecular level. This rare opencapsule MOF can easily encapsulate sulfur and iodine with the highest capacities among known MOFs. Moreover, through pyrolysis−phosphidation with melamine, capsular nanocrystals can evolve into a nitrogen-doped carbon hollow (NCH) framework with iron−nickel phosphide (FeNiP) nanoparticles embedded in open capsular carbons interconnected by plentiful carbon nanotubes (FeNiP/NCH). Coupled with its superior electrocatalysis for oxygen evolution, hydrogen evolution, and oxygen reduction, we successfully use FeNiP/NCH as the only electrocatalyst to multifunctionally assemble an overall water

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RESULTS AND DISCUSSION Fabrication of Open Capsular-MOF Nanocrystals and Encapsulation of Sulfur and Iodine. With addition of ligand B

DOI: 10.1021/jacs.9b02417 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 2. (a) Schematic representation of experimental equipment for loading sulfur and iodine into capsular-MOF, respectively. (b, c) Photographs and SEM images of S@capsular-MOF (top) and I2@capsular-MOF (bottom). The original capsular morphology of MOF crystals was preserved, without any observable sulfur and iodine species on the crystal surface. (d) N2 adsorption (solid) and desorption (empty) isotherms of S@capsularMOF, I2@capsular-MOF, and pristine capsular-MOF. (e) TGA of S@capsular-MOF and I2@capsular-MOF. (f) Correlation between S and I2 uptake capacities and BET for capsular-MOF and the other reported MOFs (orange for I2 and violet for S).

SEM, and TEM shows that solid-MOF is isostructural to capsular-MOF (Figures S2, S8, S10, S12, and S13 and Table S1) but has a different uniform crystal external shape (fat hexagonal dipyramid) (Figures S2, S14, and S15), indicating that the capsular-MOF should grow from the parent crystal (FeNi-MIL88B). In particular, capsular-MOF and solid-MOF present identical Type-I N2 adsorption isotherms and pore size distributions (Figure 1f) with corresponding Brunauer− Emmett−Teller (BET) surface areas of 1157 and 1010 m2 g−1 and pore volumes of 0.728 and 0.558 cm3 g−1, respectively (Table S2). Notably, a remarkable desorption hysteresis loop was solely observed in capsular-MOF, sufficiently indicating the mesostructured voids caused by the capsular nanostructure.25 In contrast, guest-free FeNi-MIL-88B showed a type-II isotherm with a negligible uptake because its shrunken framework is hardly opened by nitrogen molecules.26,27 To reveal structural details for the growth mechanism of the open capsule, we carried out successive characterizations for time-dependent intermediates every 2 h during synthesis of capsular-MOF, including PXRD, BET surface areas, SEM, and TEM. PXRD analyses (Figure S16a) confirm that the crystals at the fourth hour possess mixed crystal structures of FeNi-MIL88B and capsular-MOF, indicating that the crystal of FeNi-MIL88B was partially transformed into capsular-MOF, which was verified by an obvious increase in BET surface area (Figure S16b and Table S3). Further increasing the reaction time resulted in

2,4,6-tris(4-pyridyl)pyridine (tpy), uniform capsular-MOF nanocrystals (elongated hexagonal dipyramid) can be facilely prepared by using nonhollow crystals of FeNi-MIL-88B24 as the template in hot N,N-dimethylformamide (DMF) (Figures 1a, S1, and S2). Scanning electron microscopy (SEM) and highangle annular dark field scanning transmission electron microscopy (HAADF-STEM) reveal, unlike the nonhollow nanocrystals of FeNi-MIL-88B (Figures 1b, S3, and S4), an extremely rare capsular morphology with openings on the wall (Figures 1c, S5, and S6). Energy-dispersive X-ray spectroscopy (EDS) maps show its uniform distribution of iron and nickel (Figures 1d and S7). Surprisingly, the selected area electron diffraction (SAED) pattern reveals that the open capsular-MOFs are single crystals (Figure 1e). Compared with FeNi-MIL-88B (P63/mmc, a = 12.8849(9) Å, c = 18.3064(2) Å, V = 2632.1(4) Å3, Figures S8 and S9), capsular-MOF shows a distinctly different powder X-ray diffraction (PXRD) pattern, which can be well indexed in the same space group by Pawley refinement (Figure S8). Its derived unit-cell parameters vary greatly (a = 16.8736(9) Å, c = 14.8709(7) Å, V = 3666.8(5) Å3, Figure S10 and Table S1), implying that tpy may successfully participate in constructing the capsular structure in combination with the results of infrared spectroscopy (IR) (Figures S11 and S12). For comparison, a conventional synthesis gives solid-MOF (nonhollow MOF) nanocrystals by directly using the same starting materials. A series of characterizations including PXRD, IR, C

DOI: 10.1021/jacs.9b02417 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 3. (a) Schematic illustration of the preparation process of FeNiP/NCH. (b) PXRD patterns, (c) SEM images, (d) TEM images, and (e) SAED of FeNiP/NCH. (f) HAADF-STEM image and elemental maps of FeNiP/NCH.

expected, the unit-cell parameters of Fe-MIL-88B-tpy and FeMIL-88B are almost identical to those obtained from capsularMOF and FeNi-MIL-88B with small differences of the axes (68 h) measurements demonstrate its good stability during HER (Figures S55 and S56). Moreover, the structure and chemical components of FeNiP/NCH can be preserved very well after HER tests (Figures S57 and S58). Considering the electrocatalytic activities of FeNiP/NCH for HER and OER with excellent stabilities, an electrolyzer was assembled using FeNiP/NCH as both cathode and anode for overall water splitting. As revealed in LSV (Figures 5b and S59), FeNiP/NCH exhibits outstanding splitting performance with a small cell voltage of 1.59 and 1.91 V to achieve 10 and 100 mA cm−2, respectively, superior to those of many reported non-noblemetal electrocatalysts (Table S12).61 In addition, the water electrolyzer durability was well confirmed by carrying out the reaction at different current densities from 10 to 100 mA cm−2 for over 40 h (Figures 5c and S60), proving the prominent stability of FeNiP/NCH catalyst for overall water splitting. Oxygen Reduction and Rechargeable Zn−Air Battery. The inherent high OER activity and excellent water electrolysis performance of FeNiP/NCH further prompted us to investigate its catalysis for oxygen reduction reaction (ORR) and even assemble it into a rechargeable Zn−air battery device to evaluate its practical application for energy conversion.39,62,63 As shown in the LSV curves (Figure 5d), FeNiP/NCH exhibits a higher positive half-wave potential (0.75 V vs RHE, reverse hydrogen electrode64) and larger diffusion-limiting current density (5.02 mA cm−2, close to that of Pt/C) than those of FeNiP/C (0.71 V, 3.22 mA cm−2) and FeNiP/NCS (0.73 V, 3.06 mA cm−2) at 0.6 V (Figures S61 and S67). Moreover, the structure and chemical components of FeNiP/NCH can also be preserved very well after ORR tests (Figures S57 and S58). In addition, the ECSAs of FeNiP/NCH catalyst in 0.1 M KOH were also investigated by Cdl (Figure S68). The Cdl of FeNiP/NCH catalyst (8.6 mF· cm−2) is much higher than those of the FeNiP/C (4.2 mF cm−2) and FeNiP/NCS (6.0 mF cm−2), indicating that the unique structure of FeNiP/NCH has a great influence on the electrochemically active surface areas. Actually, FeNiP/NCH can serve as one of the best ORR electrocatalysts among various metal phosphides, being comparable with other reported bi-/ trifunctional catalysts (Table S12). Considering the prominent catalytic activities of FeNiP/NCH for OER and ORR, a homemade Zn−air battery is assembled to further evaluate the performance under real operation conditions (Figure 5e). Entertainingly, this battery can work stably with an open-circuit voltage (OCV) of 1.48 V for more than 25 h (1.38 V for Pt/C) (Figure S69).65,66 Besides this, discharge polarization curve and peak power density of single battery clearly manifest that the FeNiP/NCH catalyst exhibits an extraordinary high power density (250 mW cm−2) that surpasses 131 mW cm−2 of Pt/Cbased battery and most of other recently reported state-of-the art electrode materials (Figure 5f and Table S13).67,68 The charge/ discharge potentials of the Zn−air battery based on FeNiP/ NCH were 1.89 and 1.23 V with a small voltage gap of 0.66 V when cycling at 5 mA cm−2, far better than the charge/discharge potentials of Pt/C (Figure 5g). More significantly, it displays remarkable stability with no obvious voltage gap change over 2100 cycles during 500 h when operating at different current densities. Obviously, FeNiP/NCH performs far better than Pt/ C including not only the peak power density of discharge but also the durability of operation. More impressively, two primary Zn−air batteries based on FeNiP/NCH connected in series can provide a working voltage of more than 2.95 V, which is expectedly sufficient to power the above water electrolyzer assembled by FeNiP/NCH (Figure S70).

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CONCLUSION

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EXPERIMENTAL SECTION

In summary, a new single-crystal capsular-MOF with openings on the wall is carefully designed at the molecular level and constructed from a rare crystal-structure transformation from a well-known three-dimensional MOF for the first time. Interestingly, the open-capsular MOF can be used as a nanoscale storage material for sulfur and iodine (cathodes for lithium batteries). Open-capsular MOF can also be upgraded to a capsular carbon-based network, which exhibits efficient multifunctional electrocatalysis for oxygen evolution, hydrogen evolution, and oxygen reduction, achieving well-qualified assemblies of an overall water splitting and a rechargeable Zn−air battery, which affords remarkably practical prospects over previously reported electrocatalysts. The open-capsule nanostructures have been demonstrated to be vastly beneficial for easily loading I2 and S into an MOF capsule and full exposure of the inner surfaces during catalysis with the carbon-based capsule, which could greatly endow microcapsules with better performance than traditional nonhollow nanomaterials including nanosized MOFs and sheetlike crystals. Considering the rich diversities of the reported ligands and MOFs with open metal sites,69,70 our strategy will be broadly applicable for design and construction of various novel MOF capsules. The synthetic strategies also point out a new way for preparing hierarchical carbon-based nanostructures with adjustable morphologies and compositions.

Synthesis of FeNi-MIL-88B. A mixture of iron(III) chloride hexahydrate (FeCl3·6H2O, 3.6 g, 13.3 mmol), nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, 2.0 g, 6.7 mmol), and 2-aminoterephthalic acid (H2bdc-NH2, 3.6 g, 20 mmol) was dissolved in DMF (200 mL) and sonicated for 20 min. Subsequently, aqueous sodium hydroxide solution (8 mL, 1 M) was dropwise added into the above solution under stirring. The resulted solution was sonicated for 40 min and then transferred into a 500 mL vial sealed with a screw cap. The mixture was heated at 100 °C for 12 h. After that, brown solid was collected by centrifuge and washed by DMF and methanol (MeOH) several times. The obtained sample was dried in the air at room temperature overnight (yield 4.2 g, ∼80%). Large single crystals for FeMIL-88B: a mixture of FeCl3·6H2O (2.7 g, 10 mmol), H2bdc-NH2 (1.8 g, 10 mmol), hydrofluoric acid (3 mL), and DMF (100 mL) was stirred for 30 min, transferred, and sealed into a 500 mL vial with a screw cap, which was heated at 100 °C for 3 days, finally giving orange crystals of Fe-MIL-88B ([Fe3(μ3-O)(bdc-NH2)3X(H2O)2], X = OH−/Cl−/F−). Synthesis of Capsular-MOF. A mixture of FeNi-MIL-88B (200 mg) and tpy (100 mg) was transferred into 10 mL of DMF using a 25 mL Teflon-lined autoclave, which was heated at 190 °C for 14 h. After being cooled to room temperature, the brown solid was collected by centrifuge and washed with DMF and MeOH several times. Finally, the sample was dried under vacuum at 100 °C for 4 h (yield 200 mg, ∼67%). Large single crystals for Fe-MIL-88B-tpy: a mixture of FeCl3· 6H2O (27 mg, 0.1 mmol), H2bdc-NH2 (18 mg, 10 mmol), tpy (10 mg, 0.033 mmol), hydrofluoric acid (10 μL), and DMF (1 mL) was stirred in a 9 mL vial for 10 min and then transferred and sealed into a 25 mL Teflon-lined autoclave, which was heated at 140 °C for 3 days, finally giving orange crystals of Fe-MIL-88B-tpy ([Fe 3 (μ 3 -O)(bdcNH2)3(tpy)]X, X = OH−/Cl−/F−). Synthesis of Solid-MOF. This compound was prepared by a procedure similar to that for capsular-MOF, except that FeNi-MIL-88B was replaced by a mixture of FeCl3·6H2O (108 mg, 0.4 mmol), Ni(NO3)2·6H2O (60 mg, 0.2 mmol), and H2bdc-NH2 (108 mg, 0.6 mmol). Brown solid was dried under vacuum at 120 °C for 4 h (yield 150 mg, ∼70%). Loading Sulfur into MOFs. Degassed MOF crystals (100 mg) were first mixed with sulfur (100, 200, and 300 mg) by hand-milling for H

DOI: 10.1021/jacs.9b02417 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society 30 min and then transferred into Teflon-lined autoclaves. These autoclaves were placed into an oven and heated at 135 °C for 12 h. After that, the obtained samples were collected and analyzed by TG. Iodine Vapor Adsorption. Degassed MOF crystals (50 mg) in a small beaker and iodine (1.5 g) were placed in a Teflon-lined autoclave and then transferred into an oven heated at 100 °C for 2.5 h. After that, the obtained samples were collected and analyzed by TG. Single-Crystal X-ray Diffraction and Structure Analysis. SCXRD measurements were carried out using a Rigaku AFC10 diffractometer with a Rigaku PILATUS 200 K system equipped with a MicroMax-007 HF/Varimax rotating-anode X-ray generator with confocal monochromated Mo Kα radiation. Crystal structures were solved by the direct method and refined by full matrix least-squares refinement using SHELXTL-2013 package. Fourier difference maps (F0 − Fc) were computed with the WinGX program system. Detailed structure refinement parameters and crystallographic data are given in Table S4. Fabrication of a Zn−Air Battery. The rechargeable Zn−air battery test was conducted in a home-built electrochemical cell where FeNiP/NCH catalysts were loaded on Ni foam (1 mg cm−2) as the air cathode and Zn plate as anode in 6.0 M KOH. For the rechargeable zinc−air batteries, 6.0 M KOH and 0.2 M ZnCl2 were employed as the electrolyte to ensure reversible Zn electrochemical reactions at the Zn plate. During discharge−charge cycling tests, the electrolyte was replenished once a day. Measurements were carried out at room temperature with a CHI 708EY electrochemical workstation and a Hokuto Denko HJ1001SD8C charge−discharge testing system.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b02417. Experimental procedures, SCXRD, PXRD, IR, TGA, XPS, and additional electrocatalysis measurements (PDF) X-ray data for Fe-MIL-88B (CIF) X-ray data for Fe-MIL-88B-tpy (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Mitsunori Kitta: 0000-0001-9800-7371 Zheng Liu: 0000-0001-9095-7647 Satoshi Horike: 0000-0001-8530-6364 Qiang Xu: 0000-0001-5385-9650 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the reviewers for valuable suggestions and AIST for financial support. We thank Ms. Nanae Shimanaka of KUIAS-iCeMS, Kyoto University, for single-crystal X-ray diffraction measurements.

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REFERENCES

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DOI: 10.1021/jacs.9b02417 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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DOI: 10.1021/jacs.9b02417 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX