Hierarchically Porous Single Nanocrystals of ... - ACS Publications

Article Cite This: Chem. Mater. 2018, 30, 6458−6468

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Hierarchically Porous Single Nanocrystals of Bimetallic Metal− Organic Framework for Nanoreactors with Enhanced Conversion Jun Teng, Minqi Chen, Yanyu Xie, Dawei Wang,* Ji-Jun Jiang, Guangqin Li, Hai-Ping Wang, Yanan Fan, Zhang-Wen Wei, and Cheng-Yong Su* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China Chem. Mater. 2018.30:6458-6468. Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 11/09/18. For personal use only.

S Supporting Information *

ABSTRACT: Nanoreactors constructed by encapsulation of catalytic species in hierarchically porous metal−organic framework (HPMOF) nanostructures have emerged as a type of promising catalysts that deliver enhanced conversion and excellent selectivity/stability. However, the controlled synthesis of small HPMOF nanocrystals with tunable size and nanostructures remains challenging. Here, by coupling external ultrasonication with the inherent binding competitions between the metal ions of bimetallic Co/ZnZIF, we design and develop a new strategy, dynamic growth, for the facile synthesis of small (down to ca. 95 nm), single-crystalline and structure-/sizetunable HPMOF nanocrystals through a one-pot, additive-free procedure. Benefiting from the combined structure/size regulation, nanoreactors based on our small nanocrystals of meso-HP-Co/Zn-ZIF-50% are able to deliver exceptional conversion while keeping their excellent selectivity and stability. Insights gained from this work opens a new avenue for the development of complex/dynamic coordination materials in which the compositions, structures, properties, and functions can be designed/tuned through the coupling of their inherent binding competitions with external stimuli.

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INTRODUCTION Catalysts play essential roles in chemical/medicine production,1,2 energy conversion,3,4 and metabolic processes.5,6 Therefore, the development of advanced catalysts with excellent selectivity, conversion, and stability has been a perpetual pursuit in the field ranging from chemistry, pharmacy, materials science, to biology. As a new class of porous materials, metal−organic frameworks (MOFs) feature porous crystalline structure and exceptional structural/ compositional diversity,7 which make them particularly interesting and important in constructing advanced catalysts.8−10 One type of rapidly developing MOF-based catalysts, MOF nanoreactors, can be synthesized by controlled encapsulation of catalytic species such as enzymes11−13 or noble metal nanoparticles (NPs; e.g., Au, Pd, and Pt14−17) in nanoscale MOFs. Benefiting from the size-selective inclusion capability of MOF pores and physical confinement/isolation of catalytic species (against aggregation) in the porous framework, MOF nanoreactors are able to deliver extraordinary selectivity and chemical stability, which otherwise cannot be achieved by MOFs or catalytic species individually.18,19 These MOF nanoreactors, however, are usually subject to low conversion, due to the limited diffusion coefficient of molecules in the small MOF pores (typically micropores, < 2 nm).14,15 To significantly improve the conversion by accelerating the mass transfer, it is highly desirable, particularly from the point view of industrial applications, to introduce additional mesopores (2−50 nm) or even macropores (>50 © 2018 American Chemical Society

nm) to MOF architectures, that is, to construct hierarchically porous MOFs (HPMOFs).20−23 Preferably, the HPMOFs are single crystals and have the introduced meso-/macropores completely enclosed by their intact, thin MOF shells so as to perfectly preserve their intrinsic molecular-size selectivity. On the other hand, given the large size of most of the reported MOF nanoreactors (typically >500 nm20−22), downsizing HPMOFs and HPMOF nanoreactors (preferably to the sub100 nm scale) so as to substantially reduce the diffusion distance should also be an effective strategy to achieve enhanced conversion. Many strategies have been developed for the synthesis of HPMOFs, which can be typically categorized as templated or template-free strategies. The templated strategies introduce additional pores by selectively removing the templates (e.g., silica,21 Au NPs,20 MOF,22,24,25polystyrene,23 and CTAB26) that are previously encapsulated in MOFs. The template-free strategies, however, are a toolbox that includes various approaches, for instance, controlled etching,27−31 doublesolvent mediated overgrowth,32 sequential self-assembly,33 phase transformation,34 defect engineering,35−37 postsynthetic ligand exchange,38 competitive coordination,39 and linker labilization,40 to name just a few. Despite the great progress in the controlled synthesis of HPMOFs, the controlled Received: July 9, 2018 Revised: August 25, 2018 Published: August 28, 2018 6458

DOI: 10.1021/acs.chemmater.8b02884 Chem. Mater. 2018, 30, 6458−6468

Article

Chemistry of Materials synthesis of small HPMOF single nanocrystals remains challenging. It is unanticipated that the reported work focuses almost exclusively on the structure regulation (i.e., fine control over the size, shape, and/or spatial distribution) of the introduced meso-/macropores. The size regulation of HPMOFs has not been systematically investigated, even in the very rare cases in which small HPMOF crystals (500 nm) is probably due to the neglect/underestimation of the size effects on the properties/performances of HPMOFs, or the intrinsic limitation in the synthetic approaches. The size, however, may also have a critical effect that is even comparable to the hierarchical pores in HPMOF nanoreactor, as reduced HPMOF size should lead to further enhancement in nanoreactor conversion by reducing the diffusion distances. On the other hand, the controlled synthesis of small HPMOF nanocrystals itself is complicated and challenging, as it usually requires delicate balance/control over the HPMOF nucleation and growth process to (1) generate structure/composition/ stability heterogeneity within MOF nanocrystals for the formation of hierarchical pores,30,35,37,38,40 and meanwhile (2) achieve size regulation of HPMOF nanocrystals.42 Therefore, it would be of fundamental interest and practical importance to develop an effective synthetic strategy that allows both structure and size regulation to produce sub-100 nm HPMOF single nanocrystals. Benefiting from the combined structure/size regulation, nanoreactors based on such HPMOF nanocrystals should be able to achieve significantly enhanced conversion. Here, we propose a new strategy, dynamic growth, for the controlled synthesis of small HPMOF single crystals and facile regulation over their nanostructure and crystals size (Scheme 1). Our strategy is based on bimetallic MOF in which dynamic binding competitions exist between the two metal ions, because they bind to organic linkers with distinguishable binding rate (kinetics) and binding strength (thermodynamics). By further coupling the inherent binding competitions with external ultrasonication, the two metal ions function not only as the building units of HPMOFs but also as the structure and size modulator of HPMOF single nanocrystals. Our strategy thus enables the facile synthesis of small (down to ca. 95 nm), single-crystalline and structure-/size-tunable HPMOF nanocrystals through a one-pot, additive-free procedure. Such feature distinguishes our strategy from many of the reported strategies in which much larger HPMOF crystals are obtained or various additives like surfactant (e.g., PVP20,27,28 and CTAB32), modulator (e.g., maleic acid27), and/or etching agent (typically, acid24,29−31 or base21) are required for the synthesis of HPMOFs. We also note here that, although many mixed-component (or multivariate/solid-solution) MOFs have been synthesized in the last decades,43−48 the report of mixedmetal MOFs with core−shell-like nanostructure remains rare,49 not to mention the synergetic binding kinetics/thermodynamics coupling and its application in the controlled synthesis and size/structure regulation of HPMOF single nanocrystals. In our work, we further unambiguously verify the singlecrystalline structures of HPMOF nanocrystals with highresolution transmission electron microscope (HRTEM) that allows (ultra)low-dose imaging, rationalize the formation of HPMOF nanocrystals, and demonstrate the exceptional conversion of nanoreactors based on our sub-100 nm

Scheme 1. (a) Dynamic Growth Strategy for the Synthesis of HPMOF Single Nanocrystals; and (b) Synthetic Procedure for HPMOF Single Nanocrystals and NP@ HPMOF Nanoreactorsa

a

The scheme is not drawn to scale.

HPMOF nanocrystals. Our work not only provides an important new solution to the controlled synthesis of sub100 nm HPMOF single nanocrystals and HPMOF-based nanostructures, but also sheds light on the development of complex/functional metal−organic materials by virtue of coordination dynamics.

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EXPERIMENTAL SECTION

Materials. Cobalt nitrate hexahydrate (99.0%) was purchased from Adamas Reagent Co. Ltd. 2-Methylimidazole (98.0%), zinc nitrate hexahydrate (99.0%), and n-pentene (standard for GC, ≥99.5%) were purchased from Aladdin, China. cis-Cyclooctene (95.0%) and n-hexene (98.0%) were purchase from Alfa Aesar. Polyvinylpyrrolidone (PVP, Mw = 29 000) was purchased from SigmaAldrich. Chloroplatinic acid hexahydrate (H2PtCl6·6H2O, 99.0%) was purchased from Energy Chemical. All reagents were used as received unless otherwise noted. Synthesis of the Single Nanocrystals of Co/Zn-ZIF-75% with Introduced Macropores (Macro-HP-Co/Zn-ZIF-75%). In a typical experiment, Co(NO3)2·6H2O (0.820 g, 2.82 mmol) and Zn(NO3)2·6H2O (0.279 g, 0.94 mmol) were dissolved in methanol (15 mL) under ultrasonication for 10 min at room temperature. 2Methylimidazole (0.623 g, 7.6 mmol) was dissolved in methanol (15 mL) under ultrasonication for 5 min. After a homogeneous solution was formed, the methanolic solution of Co(NO3)2 and Zn(NO3)2 was slowly mixed with the 2-methylimidazole solution at 315 K under ultrasonication and gentle shake. The mixed solution was kept under ultrasonication for 7 h. Finally, the mixture was collected by centrifugation at 10 000 rpm for 3 min, washed with methanol at least four times, and dried at 80 °C under vacuum for 24 h. Unless otherwise noted, all of the sonication in our experiments was generated with a KQ-300DE ultrasonicator (Kunshan Ultrasonic 6459


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Chemistry of Materials

mL). The Zn(NO3)2·6H2O solution was mixed with Pt NPs solution (1.5 mL) under vigorous stirring, followed by the addition of the 2methylimidazole solution. The mixture solution then was allowed to react without stirring (under static condition) for 24 h. Finally, the product was separated from the dispersion by centrifugation and washed extensively with methanol (cf., Figure S25). The Pt loading was determined to be 1.0 wt % by ICP-AES. Catalytic Hydrogenation of Alkenes. The catalytic hydrogenation reactions of alkenes (1-pentene, 1-hexene, and cis-cyclooctene) were conducted in a flask under H2 atmosphere, using ethyl acetate as solvent.20 Prior to the hydrogenation reaction, the flask containing catalyst (1.03 × 10−3 mmol; in terms of Pt) was evacuated and purged with H2 four times. Ethyl acetate (5.0 mL) was injected into the flask with a degassed syringe. The mixture was sonicated for 20 min to afford a homogeneous suspension. Subsequently, alkene (0.1 mL) was added to the flask, and the mixture was sonicated for another 10 min. The catalytic hydrogenation was then allowed to proceed under static experimental condition (without external stirring) at 308 K for 24 h, and hydrogen was supplied from a balloon. When the reaction finished, the sample was separated by centrifugation and analyzed using a gas chromatograph (Agilent 6890N). For the cycling experiments, the used catalyst was separated by centrifugation, washed thoroughly with ethyl acetate, and dried under vacuum at 80 °C for the next hydrogenation reaction. Characterization. Powder X-ray diffraction (PXRD) patterns were recorded with a Bruker AXS D8 Advance diffractometer using Cu Kα radiation (λ = 1.5418 Å). Field emission scanning electron microscopy (FESEM) was performed on a Hitachi SU-8010 SEM. Transmission electron microscopy (TEM) was carried out with a FEI Tecnai G2 Spirit microscope at an accelerating voltage of 120 kV. High-resolution TEM (HRTEM) images, high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) images, selected-area electron diffraction (SAED) patterns, and energy dispersive X-ray spectrometry (EDX) elemental maps were obtained with a JEOL JEM-ARM200F TEM operated at 200 kV. HRTEM image that showed the lattice fringes of single meso-HP-Co/ Zn-ZIF-75% nanocrystal were acquired with an FEI Tecnai G2 F30 TEM equipped with Gatan K2 Summit direct-detection camera. Nitrogen adsorption−desorption isotherms were collected at 77 K with a Quantachrome Autosorb-iQ2-MP analyzer. Inductively coupled plasma (ICP) spectroscopy was conducted on a Thermo IRIS HR spectrometer. Thermogravimetric (TG) analysis was carried out on a Netzsch STA 449 F3 Jupiter simultaneous thermal analyzer, and the temperature was raised from 40 to 900 °C at a rate of 5 °C min−1.

Instrument Co., Ltd., China) that operated at 300 W (power) and 40 kHz (output frequency), and the bath temperature was set to 315 K. Synthesis of Macro-HP-Co/Zn-ZIF-50% Single Nanocrystals. In a typical experiment, Co(NO3)2·6H2O (0.552 g, 1.9 mmol) and Zn(NO3)2·6H2O (0.565 g, 1.9 mmol) were dissolved in methanol (15 mL) under ultrasonication for 10 min at room temperature. The following experimental steps were the same as those of macro-HPCo/Zn-ZIF-75% single nanocrystals. Synthesis of the Single Nanocrystals of Co/Zn-ZIF-75% and Co/Zn-ZIF-50% with Introduced Mesopores (Meso-HP-Co/ZnZIF-75% and Meso-HP-Co/Zn-ZIF-50%). The synthetic procedures for meso-HP-Co/Zn-ZIF-75% and meso-HP-Co/Zn-ZIF-50% single nanocrystals were almost identical to those for macro-HP-Co/ Zn-ZIF-75% and macro-HP-Co/Zn-ZIF-50%, respectively. The only difference in the procedure is the time for ultrasonication: 60 min for mesoporous single nanocrystals while 7 h for hollow single nanocrystals. Synthesis of Co/Zn-ZIF-x (x = 100%, 25%, and 0) Nanostructures. Except for the molar ratio of Co2+ (x), all other experimental conditions were kept the same as those of macro-HPCo/Zn-ZIF-75% single nanocrystals. Specifically, Co(NO3)2·6H2O (0.276 g, 0.94 mmol) and Zn(NO3)2·6H2O (0.846 g, 2.82 mmol) were used for Co/Zn-ZIF-25%; Co(NO3)2·6H2O (1.106 g, 3.8 mmol) or Zn(NO3)2·6H2O (1.128 g, 3.8 mmol) was used for Co/ZnZIF-100% and Co/Zn-ZIF-0, respectively. Synthesis of PVP-Stabilized Pt NPs.50 PVP-stabilized Pt NPs (3.1 nm in diameter) were prepared by refluxing the mixture of PVP (133 mg), methanol (180 mL), and aqueous solution of H2PtCl6 (6.0 mmol, 20 mL) in a flask (500 mL) for 3 h in air. After methanol was removed by rotary evaporation, Pt NPs in the remaining solution were precipitated with acetone and then collected by centrifugation at 8000 rpm for 3 min. The sample was washed with chloroform and hexane to remove excess PVP. Finally, Pt NPs were dispersed in methanol to form homogeneous solution (20 mL). Prior to further use, the solution was stirred for 15 min. Synthesis of Pt@Macro-HP-Co/Zn-ZIF-50% Nanoreactors. In a typical experiment, the as-prepared Pt NPs solution (100 uL) and 2methylimidazole (0.623 g, 7.6 mmol; dissolved in 15 mL of methanol) were mixed under ultrasonication to form a homogeneous solution. Co(NO3)2·6H2O (0.552 g, 1.9 mmol) and ZnF(NO3)2·6H2O (0.565 g, 1.9 mmol) were dissolved in methanol (15 mL) under ultrasonication for 10 min at room temperature. The methanolic solutions of Co(NO3)2·6H2O and Zn(NO3)2·6H2O then were slowly mixed with the Pt NPs/2-methylimidazole solution at 315 K under ultrasonication and gentle shake. The mixture solution was kept under ultrasonication for 7 h. Finally, the product was separated by centrifugation at 10 000 rpm for 3 min, washed with methanol at least four times, and dried in atmosphere for 24 h at 110 °C. The Pt content of this sample was 0.09 wt % (determined by ICP-AES). In this product, approximately one Pt NP was encapsulated in each macro-HP-Co/Zn-ZIF-50% nanocrystal (cf., Figure S23d). To get products with different Pt loading, the volume of Pt NP solution was adjusted to 1.5, 0.5, and 0.3 mL and 50 uL, respectively (Figure S24). Synthesis of Pt@Meso-HP-Co/Zn-ZIF-50% Nanoreactors. In a typical experiment, the as-prepared Pt NP solution (1 mL) was mixed with the methanolic solution (15 mL) of 2-methylimidazole (0.623 g, 7.6 mmol) under ultrasonication to give a homogeneous solution. Co(NO3)2·6H2O (0.552 g, 1.9 mmol) and Zn(NO3)2·6H2O (0.565 g, 1.9 mmol) were dissolved in methanol (15 mL) under ultrasonication for 10 min at room temperature. The methanolic solutions of the two metal salts then were mixed slowly with the Pt NPs/2-methylimidazole solution at 315 K under ultrasonication and gentle shaking. The mixture solution was then kept under ultrasonication for 60 min. Finally, the mixture was obtained after centrifugation at 10 000 rpm for 3 min, washed with methanol at least four times, and dried under vacuum for 24 h at 80 °C. The Pt loading was determined to be 0.92 wt % by ICP-AES. Synthesis of Core−Shell Pt@ZIF-8 Nanoreactors. In a typical experiment, Zn(NO3)2·6H2O (0.588 g, 2 mmol) and 2-methylimidazole (0.648 g, 8 mmol) were dissolved separately in methanol (40

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RESULTS AND DISCUSSION Synthesis of HP-Co/Zn-ZIF Single Nanocrystals. Our dynamic growth strategy is illustrated in Scheme 1 (see more information in Growth Mechanism of HP-Co/Zn-ZIF Single Nanocrystals). As a proof of concept, two metal ions, Co2+ and Zn2+, are selected to demonstrate our idea, because (1) they bind to organic linker of 2-methylimidazole (MIm) to form the isostructural sodalite-type bimetallic MOF of Co/Zn-ZIF (ZIF = zeolite imidazolate framework),22,51 and (2) dynamic binding competitions exist between the two metal ions. The dynamic binding competitions are a consequence of the binding rate/strength differences between the two ions: as compared to Zn2+, Co2+ binds faster52−54 but less strongly to MIm.22,55,56 Under the appropriate conditions, the inherent dynamic binding competitions can be regulated to induce a dynamic crystallization process that leads to the formation of a kinetically favorable Co-rich core region and a thermodynamically favorable Zn-rich shell region within solid Co/Zn-ZIF single nanocrystals (i.e., structure/composition/stability heterogeneity; Scheme 1a). When combined with external ultrasonication, a dynamic decomposition process can be 6460


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example).21,58 TEM images further reveal that meso- and macropores distribute exclusively within the core region of HPCo/Zn-ZIF nanocrystals and no meso-/macropores exist in the shell region (Figure 1b,c,e,f). PXRD patterns confirm that the HP-Co/Zn-ZIF nanocrystals synthesized under different experimental conditions exhibit the same sodalite-type structure (Figures S1 and S2), identical to that of purephase, single-crystalline monometallic ZIF-8 or ZIF-67.51,55 The HP-Co/Zn-ZIF-50% single nanocrystals also show similar structural features (Figures 2−4 and Figure S2). All of these

simultaneously generated to selectively decompose the lessstable Co-rich core region to introduce meso- or macropores exclusively within the interior of Co/Zn-ZIF single nanocrystals (the thin shell region is kept intact). Given the intrinsic micropores of intact Co/Zn-ZIF (pore and aperture size of 11.6 and 3.4 Å,57 respectively), hierarchically porous Co-ZnZIF (HP-Co-Zn-ZIF) single nanocrystals comprised of singlecrystalline microporous shell and meso-/macroporous interior can thus be synthesized in a straightforward way. It is worth noting that our dynamic growth strategy for the synthesis of HP-Co/Zn-ZIF can be readily achieved in a onepot, additive-free procedure (Scheme 1b). The nanostructures and size of HP-Co-Zn-ZIF single nanocrystals can be further regulated by tuning two critical parameters (the reaction time and Co2+/Zn2+ ratio) that play a key role in the dynamic crystallization and decomposition processes. Typically, by simply mixing Co2+ and Zn2+ with MIm under ultrasonication and varying the reaction time and Co2+/Zn2+ ratio (see details in the Experimental Section), single nanocrystals of bimetallic HP-Co/Zn-ZIF-x (x being the mole fraction of Co2+ in the methanolic solution of Co2+ and Zn2+) with distinctly porous structures can be readily obtained. Unlike many of the reported strategies in which various additives (for instance, templates, surfactants, modulators, and/or etching agents) are required for the synthetic process, our dynamic growth strategy requires no additive, which makes it a promising tool for the development of HPMOF materials. Moreover, in striking contrast to the common reaction and crystallization processes,22,49 under ultrasonication in our experiments, short reaction time results in many mesopores located in the core regions of HP-Co/Zn-ZIF-x single nanocrystals (denoted as meso-HP-Co/Zn-ZIF-x), while prolonged reaction time finally leads to sole macropore in the core region (denoted as macro-HP-Co/Zn-ZIF-x). These HPMOF single nanocrystals can be further employed to construct NP@HPMOF nanoreactors through controlled encapsulation14 of catalytic NPs during the dynamic growth process of HPMOFs (Scheme 1b; see details in the Experimental Section). Structures of HP-Co/Zn-ZIF Single Nanocrystals. FESEM images of the products show that HP-Co/Zn-ZIF nanocrystals possess the shape of pure-phase, monometallic nanocrystals of ZIF-8 (Zn-ZIF) or ZIF-67 (Co-ZIF), that is, a well-defined rhombic dodecahedron with smooth surface (see the SEM images of HP-Co/Zn-ZIF-75% in Figure 1a,d for an

Figure 2. Single-crystalline structure of HP-Co/Zn-ZIF nanocrystals. (a−d) SAED patterns of randomly selected single crystal of (a) mesoHP-Co/Zn-ZIF-75%, (b) macro-HP-Co/Zn-ZIF-75%, (c) meso-HPCo/Zn-ZIF-50%, and (d) macro-HP-Co/Zn-ZIF-50%. (e) HRTEM image of a meso-HP-Co/Zn-ZIF-75% single nanocrystal along the [111] zone axes. Regions within the red and blue squares in (e) are enlarged and showed in (f) and (g), respectively. (h) FFT of (e).

SEM, TEM, and PXRD results suggest that, despite the formation of hierarchically porous nanostructure, the HP-Co/ Zn-ZIF nanocrystals should preserve their single-crystalline sodalite-type structures. To unambiguously verify the single-crystalline nature of HPCo/Zn-ZIF nanocrystals, their single-crystal SAED patterns and HRTEM images are required (Figure 2). Although SAED and HRTEM have become the routine techniques for the local-structural determination of many nanomaterials, their application in MOFs has been limited to several typical ones, because MOFs, particular those with nanoscale dimensions and hierarchically porous structures, are electron beamsensitive and their structures can be damaged or decomposed within a few seconds under electron beam irradiation.59,60 One critical solution is to perform SAED and HRTEM under lowdose beam irradiation.61 Here, by elaborately adjusting the beam dose to its minimal level, we successfully obtained the SAED patterns of randomly selected meso-/macroporous HPCo/Zn-ZIF-75% and HP-Co/Zn-ZIF-50% nanocrystals (Figure 2a−d). All of the sharp SAED patterns can be consistently indexed according to the single-crystalline structure of ZIF-8, which has a cubic space group of I4̅ 3m and unit cell

Figure 1. HP-Co/Zn-ZIF-75% single nanocrystals. (a) SEM and (b,c) TEM images of meso-HP-Co/Zn-ZIF-75%. (d) SEM and (e,f) TEM images of macro-HP-Co/Zn-ZIF-75%. 6461


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Chemistry of Materials dimensions of 16.99 Å,57,62 and the d spacing of (011) planes in all of the SAED patterns is determined to be ∼1.2 nm. Both the SAED patterns and the d value are in excellent agreement with the theoretical model as well as experimental results of ZIF-8 reported in the literature,52,63 thus convincingly verifying the single-crystalline nature of HP-Co/Zn-ZIF nanocrystals. Moreover, by using the recently invented direct-detection electron-counting camera (Gatan K2 Summit) that allows for HRTEM imaging with ultralow electron beam dose,59,60 the single-crystalline structure of HP-Co/Zn-ZIF nanocrystals can be directly observed and confirmed. Figure 2e shows a typical HRTEM image of a meso-HP-Co/Zn-ZIF-75% single nanocrystal, in which the lattice fringes can be clearly observed throughout the nanocrystal (Figure 2f,g). These lattice fringes correspond to the (011) family of planes, as evidenced by their distance of ∼1.2 nm and fast Fourier transform (FFT) pattern in Figure 2h.52,59 Structure and Size Regulation of HP-Co/Zn-ZIF Single Nanocrystals. The hierarchical structure and size of HP-Co/ Zn-ZIF single nanocrystals can be readily regulated by two critical experimental parameters, that is, the mole fraction of Co2+ (x) and the reaction time (t). Typically, HP-Co/Zn-ZIF single nanocrystals with well-controlled meso-/macroporous core and intact microporous shell can only be obtained in a moderate mole fraction range of 90% > x > 40%, in which the macro-HP-Co/Zn-ZIF single nanocrystals turn out to be the final products (t ≥ 7 h), while the meso-HP-Co/Zn-ZIF nanocrystals are obtained in a shorter reaction time (2 min < t < 7 h) (Figures 3 and 4). We first demonstrate the role of the mole fraction of Co2+ in the structure and size regulation of HP-Co/Zn-ZIF single nanocrystals. When x ≥ 90%, Co/Zn-ZIF-x are partially decomposed into nanosheet, leaving size-reduced and shapeirregular nanostructures with a large number of mesopores distributing throughout the nanostructures, even on the surfaces (Figures 3a−c and S3). By prolonging the reaction time to 14 h, Co/Zn-ZIF-100% can be completely decomposed into nanosheets (Figure S4), which are verified by PXRD to be layered double hydroxides (LDHs) of Co (Figure S5).64,65 On the other hand, x ≤ 40% results in Co/ Zn-ZIF nanocrystals/NPs with no meso- or macropores (Figures 3j−o and S6). The PXRD patterns confirm that Co/Zn-ZIF-x (x ≤ 40%) possesses sodalite-type structure (Figure S2). Besides the changes in hierarchical structure, the average size of Co/Zn-ZIF-x nanostructures obtained after 7 h of reaction decreases monotonically from 480 to 210, 95, 54, and finally to 25 nm as x decreases from 100% to 75%, 50%, 25%, and to 0, respectively (Figure 3). Meanwhile, within the range of 90% > x > 40%, decreasing x leads to a reduction in the average macropore diameter and shell thickness of macroHP-Co/Zn-ZIF nanocrystals. For example, the average macropore size and shell thickness are ca. 120 and 45 nm, respectively, for macro-HP-Co/Zn-ZIF-75% nanocrystals, while they are ca. 55 and 20 nm, respectively, for macro-HPCo/Zn-ZIF-50% nanocrystals. We then illustrate the role of reaction time in the structure and size regulation of Co/Zn-ZIF nanocrystals. Taking Co/ Zn-ZIF-75% as the first example (Figure 4a−h), the initial product collected at an early reaction stage are Co/Zn-ZIF75% NPs with round corners (t = 2 min; ca. 130 nm in diameter; Figure 4a,e), which possess intrinsic micropores but no meso- or macropore. With prolonged reaction time (t = 15 min), a large number of small mesopores with an average

Figure 3. Structure and size regulation of Co/Zn-ZIF-x by tuning the mole fraction of Co2+. SEM (left column) and TEM images (middle and right columns) of Co/Zn-ZIF-x nanostructures collected after 7 h of reaction are shown here, where x is (a−c) 100%, (d−f) 75%, (g−i) 50%, (j−l) 25%, and (m−o) 0, respectively.

diameter of ca. 10 nm start to appear within the interior of Co/ Zn-ZIF-75%, accompanied by the growth of Co/Zn-ZIF-75% NPs into well-defined rhombic dodecahedrons with a smooth surface (ca. 175 nm in diameter; Figure 4b,f). The small mesopores further evolve into much larger mesopores (ca. 25 nm at t = 1 h; Figure 4c,g) and finally a single macropore (ca. 120 nm at t = 7 h; Figure 4d,h) in the inner region of each HPCo/Zn-ZIF-75% nanocrystal. Meanwhile, the average size of HP-Co/Zn-ZIF-75% nanocrystals increases to ca. 200 nm at 1 h and finally ca. 210 nm at 7 h, while the shape remains unchanged. The HP-Co/Zn-ZIF-50% nanocrystals show a similar time-dependent growth trend (Figure 4i−p). Much smaller HP-Co/Zn-ZIF-50% nanocrystals with similar mesoporous interior (crystal size of ca. 88 nm and mesopores size of ca. 20 nm; Figure 4k,o) can be obtained at a reaction time of 1 h and macroporous interior (crystal size of ca. 95 nm and single macropore size of ca. 55 nm; Figure 4l,p) at 7 h, respectively. The PXRD patterns of Co/Zn-ZIF-75% and Co/ Zn-ZIF-50% nanocrystals collected at various reaction times are almost identical to each other, and in excellent agreement with the PXRD patterns of ZIF-67 and ZIF-8 (Figures S1 and S2). Such results indicate that Co/Zn-ZIF nanocrystals 6462


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Figure 5. Evolution of element distribution in Co/Zn-ZIF-75% nanocrystals. The distribution of Co and Zn in Co/Zn-ZIF-75% collected at different reaction time is monitored with EDX: (a,b) 2 min, (c,d) 1 h, and (e,f) 7 h. (a,c,e) EDX area maps, and (b,d,f) EDX line scans. Greyscale images in (a−f) are the corresponding HAADF STEM images.

Figure 4. Structure and size regulation of HP-Co/Zn-ZIF by tuning reaction time. SEM and TEM images showing the typical growth stages of (a−h) Co/Zn-ZIF-75% and (i−p) Co/Zn-ZIF-50%. Panels a−d, i−l are SEM images, and panels e−h, m−p are TEM images. Scale bars are (a−d) 200 nm and (e−p) 100 nm, respectively.

in the core/inner region, (3) the meso-/macropores form exclusively in the Co-rich inner/core region, and (4) the proportion of Co to Zn decreases during the growth of HPCo/Zn-ZIF-75% nanocrystals (Table S2). The element distribution of Co and Zn in Co/Zn-ZIF-50% follows similar trends (Figure S15 and Table S2). However, in the case of Co/ Zn-ZIF-25% in which no meso- or macropores appear, Co and Zn distribute roughly homogeneously rather than in a core− shell-like fashion, and such element distribution almost does not change over growth time (Figure S16 and Table S2). These element distribution results, together with the results from SEM, TEM, EDX, PXRD, and N2 sorption, are in line with the dynamic growth process/mechanism we proposed earlier, and enable us to rationalize the controlled synthesis and structural/size regulation of HP-Co/Zn-ZIF nanocrystals with more details (vide infra). As we proposed, the overall dynamic growth of HP-Co/ZnZIF single nanocrystals is comprised of dynamic crystallization and decomposition processes that are attributed to the synergistic effects of the inherent binding rate/strength difference between two metal ions and the external influence exerted by ultrasonication. Specifically, at the early growth stage of HP-Co/Zn-ZIF nanocrystals, although both Co2+ and Zn2+ can bind to MIm to form small bimetallic Co/Zn-ZIF nuclei/NPs, a binding competition exists between Co2+ and Zn2+. The faster binding rate of Co2+ leads to the formation of a kinetically favorable Co-rich core. As the small Co/Zn-ZIF nuclei/NPs grow, the concentration of Co2+ in solution decreases continuously; to a certain extent, Zn2+ with stronger binding affinity starts to dominate the binding competition, leading to the formation of a thermodynamically favorable Znrich shell around each Co-rich core. Through such a competitive and dynamic crystallization process, Co/Zn-ZIF

preserve their sodalite-type structures during the reaction process, regardless of the formation of meso- or macropores under different conditions. The porosity of HP-Co/Zn-ZIF nanocrystals is further investigated by N2-sorption measurements (Figures S7−10). The N2 adsorption−desorption isotherms of HP-Co/Zn-ZIF50% and HP-Co/Zn-ZIF-75% collected at a reaction time of 1 h (mesoporous) and 7 h (macroporous) show a steep increase in N2 uptake at low relative pressure, confirming the existence of micropores in these HP-Co/Zn-ZIF nanocrystals. On the basis of N2-sorption isotherms, the micropore size of HP-Co/ Zn-ZIF nanocrystals is calculated to be ca. 1.0 nm, close to that of the intrinsic micropores of ZIF-67 or ZIF-8 (1.16 nm). The meso- and macropores in HP-Co/Zn-ZIF-50% and HP-Co/ Zn-ZIF-75% nanocrystals are evidenced by their hysteresis loops at moderate relative pressure (Figures S7 and S9) and high relative pressure (Figures S8 and S10), respectively. The sizes of the meso- and macropores are also in line with the results obtained from TEM images (Figures S7−S10 and Table S1). Moreover, the meso-/macropore volumes are unneglectable or even comparable to the micropore volumes (Table S1), further confirming the existence of meso-/macropores in the HP-Co/Zn-ZIF nanocrystals. Growth Mechanism of HP-Co/Zn-ZIF Single Nanocrystals. To prove the dynamic growth process/mechanism of HP-Co/Zn-ZIF single nanocrystals, their element distribution and nanostructures at typical growth stages are further probed with EDX and HAADF STEM (Figure 5). The area maps and line scans of Co/Zn-ZIF-75% collected at 2 min, 1 h, and 7 h clearly indicate that (1) regardless of the growth stages, similar core−shell-like distribution of Co and Zn exists in Co/Zn-ZIF75%, (2) Zn dominates the shell region while Co concentrates 6463


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The size dependence of Co/Zn-ZIF on the mole fraction of Co2+ (Figure 3) may also be a consequence of the inherent binding differences between Co2+ and Zn2+. Given that the synthetic conditions are identical for all Co/Zn-ZIF-x except the Co2+/Zn2+ ratios, the steadily decreasing size of Co/ZnZIF-x nanostructures suggests that the amount of Co/Zn-ZIFx nuclei gradually increases as x decreases from 100% (ZIF-67; 480 nm) to 0 (ZIF-8; 25 nm). The increasing amount Co/ZnZIF-x nuclei with respect to decreasing x is probably due to the gradually improved stability of Co/Zn-ZIF-x nuclei induced by the increasing fraction of Zn2+ (cf. Figures 5, S15, and S16), which inhibits the dissolution or recrystallization of metastable Co/Zn-ZIF-x nuclei during the crystallization process.67 We further verify the role of ultrasonication in the formation of HP-Co/Zn-ZIF nanocrystals. Control experiments under no ultrasonication but otherwise identical conditions reveal that, regardless of the mole fraction of Co2+ (x from 0 to 100%), no additional pores are formed in Co/Zn-ZIF-x (Figure S18). However, the distribution of Co and Zn is in line with that in the Co/Zn-ZIF-x nanocrystals synthesized under ultrasonication. That is, without ultrasonication, core−shell-like distribution of Co and Zn also exists in Co/Zn-ZIF-75% and Co/ZnZIF-50% nanocrystals (Figures S19 and S20), and, moreover, roughly homogeneous distribution of Co and Zn also exists in Co/Zn-ZIF-25% nanocrystals (Figure S21). These results indicate that (1) ultrasonication does not have a substantial impact on the distribution of Co and Zn in Co/Zn-ZIF nanocrystals, but (2) ultrasonication indeed induces the decomposition of the Co-rich core regions in Co/Zn-ZIF75% and Co/Zn-ZIF-50% nanocrystals to form hierarchically porous nanostructures; otherwise, the Co-rich core regions remain stable in methanol (without ultrasonication). We speculate that ultrasonication induces the decomposition of the less stable Co-rich core regions by generating local heat within the Co/Zn-ZIF nanocrystals.68 At an early growth stage (t = 2 min) of Co/Zn-ZIF-75% and Co/Zn-ZIF-50%, no additional pores are induced in the nanocrystals, probably due to the insufficient heat accumulated during that short time as well as the suppression of the core region decomposition by the high concentration of Co2+ and Zn2+ remaining in solution. With prolonged reaction time (t ≥ 15 min), the locally accumulated heat becomes sufficient to trigger the decomposition of the less stable Co-rich core regions into meso- and macroporous interior so that hierarchically porous nanocrystals of Co/ZnZIF-75% and Co/Zn-ZIF-50% are obtained. Construction of Pt@HP-Co/Zn-ZIF-50% Nanoreactors. To demonstrate the benefits of small HP-Co/Zn-ZIF nanocrystals, the nanocrystals of meso-HP-Co/Zn-ZIF-75% and meso-HP-Co/Zn-ZIF-50% (collected at reaction time of 1 h) are selected to build model nanoreactors of Pt@meso-HP-Co/ Zn-ZIF through the controlled encapsulation14 of Pt NPs (average diameter 3.1 nm; Figure S22). The Pt-catalyzed liquid-phase hydrogenation of alkenes is selected as the model reaction. The two nanoreactors of Pt@meso-HP-Co/Zn-ZIF-75% and Pt@meso-HP-Co/Zn-ZIF-50% are shown in Figure 6a−f. Their bright and dark field TEM images confirm that Pt NPs are completely embedded within meso-HP-Co/Zn-ZIF nanocrystals, predominantly located in the macro-/mesopores, and well separately from each. The HRTEM image in Figure S23 further verifies the encapsulation of Pt NPs, because the adjacent interlayer distance (0.139 nm) is in excellent agreement with the lattice spacing of Pt(220) planes.69 The

nanocrystals with core−shell-like distribution of metal ions can be obtained (Figure 5). It is worth noting that the size of core region depends strongly on the initial mole fraction of Co2+ in solution. Lower initial mole fraction of Co2+ disfavors the binding competition of Co2+ against Zn2+, thus leading to the formation of a smaller Co-rich core. In other words, the binding competition between Co2+ and Zn2+ can be modulated by the mole fraction of Co2+. Indeed, as observed in our experiments, the Co-rich core region, which can be subsequently decomposed to form meso- or macropores (see discussion below), of Co/Zn-ZIF nanocrystals decreases in size as the initial Co2+ concentration decreases from 75% to 50%, and even disappears at a low concentration to 25% (cf., Figures 3,5, S15, and S16). In the last case, Co2+ and Zn2+ actually cocrystallize in a roughly homogeneous way, implying an approximately even binding competition between Co2+ and Zn2+. The dynamic decomposition of Co/Zn-ZIF to form HP-Co/ Zn-ZIF nanocrystals is primarily attributed to the binding strength difference between Co2+ and Zn2+: Co2+ binds less strongly to MIm than does Zn2+.22,55,56 Accordingly, Co−N bonds are more labile than Zn−N bonds, which is consistent with the experimental observation that ZIF-67 (Co-ZIF) is chemically less stable than ZIF-8 (Zn-ZIF), and the chemical and/or thermal stability of bimetallic Co/Zn-ZIF can be improved by increasing the mole fraction of Zn2+ (Figure S17).55,66 Such thermodynamic difference gives rise to a stability difference between the Co-rich core region and the Zn-rich shell region (i.e., stability heterogeneity), which makes it possible to further convert Co/Zn-ZIF nanocrystals into HPCo/Zn-ZIF nanocrystals by a rationally controlled decomposition process. Strikingly, we find that ultrasonication is a convenient yet effective method for triggering and tuning the decomposition. The Co-rich core region is unstable against ultrasonication in methanol, and thus can be decomposed partially or even completely into Co/Zn layered double hydroxides (Co/Zn LDHs; cf., Figure S17 and related discussion therein). The decomposition of Co-rich core region probably starts in a random way, resulting in the formation of additional pores of different size distributing throughout the core region (Figure 4f,n). Moreover, the decomposition of Corich core region also has a dynamic nature, as it proceeds with ultrasonication time: short ultrasonication time leads to partial decomposition to form mesopores (e.g., 15 min ≤ t ≤ 1 h; Figure 4f,g,n,o), while prolonged ultrasonication time results in complete decomposition to give a single macropore (e.g., t = 7 h; Figure 4h,p). The Zn-rich shell, in contrast, is quite stable against ultrasonication, and thus is capable of perfectly preserving its single-crystalline microporous structure. In this regard, formation of Co-rich core and Zn-rich shell (i.e., with composition and stability heterogeneities) is the precondition for the growth of HP-Co/Zn-ZIF nanocrystals with intact MOF shell. Through such a selective and dynamic decomposition process, HP-Co/Zn-ZIF nanocrystals can be finally obtained. In the case of monometallic Co/Zn-ZIF-100% (Co-ZIF, i.e., ZIF-67) that have the worst stability, they can be decomposed partially to give mesopores throughout the nanocrystals, or even decomposed completely into Co LDH nanosheets (Figures 3a−c, S4, and S5). On the contrary, due to the high content of Zn2+ throughout their structures, Co/ Zn-ZIF-25% and monometallic Co/Zn-ZIF-0 (Zn-ZIF, i.e., ZIF-8) are so stable against ultrasonication that no additional pores form in their nanocrystals (Figure 3j−o). 6464


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investigated in this work, due to the limited conversion induced by their low Pt loading. Taking macro-Co/Zn-ZIF50% as an example, approximately one Pt NP can be encapsulated in the macropore of each nanocrystals to avoid the aggregation of adjacent Pt NPs under optimized condition (Figures 6g−i and S24; Pt loading 0.09 wt %). The preliminary results of the catalytic hydrogenation of 1-pentenes show conversion of 14% for Pt@macro-HP-Co/Zn-ZIF-50% and 55% for Pt@meso-HP-Co/Zn-ZIF-50% nanoreactors, respectively. Such distinct conversion difference is likely due to the large difference between their Pt loading. Although Pt@macroHP-Co/Zn-ZIF-50% nanoreactors may deliver relatively high catalytic efficiency if only specific catalytic activity is considered (Pt-loading 0.09% vs 0.92%, conversion 14% vs 55%), increasing their Pt loading should be avoided because it may lead to the aggregation of multiple Pt NPs in the macropore of macro-HP-Co/Zn-ZIF-50% nanocrystals. Performances of Pt@meso-Co/Zn-ZIF Nanoreactors. The nanoreactors of Pt@meso-HP-Co/Zn-ZIF-75% and Pt@ meso-HP-Co/Zn-ZIF-50% are utilized for the catalytic hydrogenation of alkenes of different molecular size (width × length, estimated with Multiwfn71): 1-pentene (1.8 × 6.7 Å), 1-hexene (1.9 × 7.8 Å), and cis-cyclooctene (3.8 × 5.4 Å). For comparison, core−shell Pt@ZIF-8 nanoreactors with diameter (ca. 95 nm) and Pt loading (1.0 wt %) similar to those of Pt@ meso-HP-Co/Zn-ZIF-50% but without meso- or macropores (Figure S25), and meso-HP-Co/Zn-ZIF-50% nanocrystals without Pt NPs are also tested under otherwise identical experimental conditions. As expected, both Pt@meso-HP-Co/ Zn-ZIF-75% and Pt@meso-HP-Co/Zn-ZIF-50% show much higher conversion than Pt@ZIF-8 nanoreactors (Figure 6j), thus confirming the benefit of introducing mesopores into Co/ Zn-ZIF nanocrystals. Of particular interest is that (1) Pt@ meso-HP-Co/Zn-ZIF-50% nanoreactors demonstrate exceptional conversion, and (2) substantial conversion differences exist between Pt@meso-HP-Co/Zn-ZIF-50% and Pt@mesoHP-Co/Zn-ZIF-75% nanoreactors, especially in the case of the hydrogenation of 1-hexene. The Pt@meso-HP-Co/Zn-ZIF50% nanoreactors deliver the highest conversion, exceeding that of Pt@ZIF-8 nanoreactors by as much as 64.7% for 1pentene (55.0% vs 33.4%) and 120.6% for 1-hexene (22.5% vs 10.2%). Such conversion is also superior to the typical core− shell Pt@ZIF-8 nanoreactors (e.g., 7.3% for 1-hexene14) and/ or Pt@meso-ZIF-8 nanoreactors (e.g., 44.4% for 1-pentene and 16.3% for 1-hexene20) reported in the literature. Considering that meso-HP-Co/Zn-ZIF-50% nanocrystals show no catalytic activity (Figure 6j) and the conversion rate of Pt-catalyzed alkene hydrogenation under static conditions depends solely on the molecular diffusion within MOFs,20 the exceptional conversion can be attributed to the accelerated diffusion of reactants (1-pentene and 1-hexene) facilitated by the small size, thin shell, and hierarchically porous structure of meso-HP-Co/Zn-ZIF-50% nanocrystals. Specifically, the small size and thin shell impart an enlarged surface area of Pt@mesoHP-Co/Zn-ZIF-50% nanoreactors (allowing more reactant molecules to diffuse simultaneously) and shortened diffusion distance of reactants within nanoreactors; meanwhile, the hierarchically porous interior of meso-HP-Co/Zn-ZIF-50% nanocrystals enable increased diffusion coefficient of reactants within the nanoreactors. In such a way, the overall diffusion of reactants inside meso-HP-Co/Zn-ZIF-50% nanocrystals and the access of reactants to Pt NPs can be significantly accelerated so that the conversion of reactants is substantially

Figure 6. Structure and performance of Pt@HP-Co/Zn-ZIF nanoreactors. TEM images of the nanoreactors of (a−c) Pt@meso-HPCo/Zn-ZIF-75%, (d−f) Pt@meso-HP-Co/Zn-ZIF-50%, and (g−i) Pt@macro-HP-Co/Zn-ZIF-50%. (a,d,g) Bright field TEM images, (b,c,e,f,h,i) HAADF STEM images. (j) Catalytic performance of Pt@ meso-HP-Co/Zn-ZIF-75% and Pt@meso-HP-Co/Zn-ZIF-50% nanoreactors for the hydrogenation of alkenes.

PXRD patterns before and after Pt NP encapsulation are almost identical to each other, suggesting that the NP encapsulation does not alter the sodalite-type crystalline structures of meso-HP-Co/Zn-ZIF nanocrystals. However, the peaks of Pt NPs are indiscernible in the PXRD pattern of the two nanoreactors, likely due to the small size of Pt NPs and their low loading in the nanoreactors (∼0.9 wt % for Pt@ meso-HP-Co/Zn-ZIF-75% and Pt@meso-HP-Co/Zn-ZIF50%; quantified with ICP).21,70 Benefiting from hierarchically porous nanostructures, the Pt@meso-HP-Co/Zn-ZIF-75% and Pt@meso-HP-Co/ZnZIF-50% nanoreactors are expected to (1) deliver enhanced conversion by facilitating the access of reactants to Pt NPs, (2) show excellent cycling stability by avoiding the aggregation of neighboring Pt NPs during the catalytic reactions, and (3) display extraordinary molecular-size selectivity endowed by the single-crystalline microporous MOF shell. We note here that, although macro-Co/Zn-ZIF-50% and macro-Co/Zn-ZIF-75% can also be used to build nanoreactors, they are not further 6465



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enhanced. Therefore, Pt@meso-HP-Co/Zn-ZIF-50% nanoreactors are able to deliver conversion superior to not only the Pt@ZIF-8 nanoreactors (with similar size but without meso- or macropores) but also the Pt@meso-HP-Co/Zn-ZIF75% nanoreactors (with greater size and shell thickness). Our results here thus clearly demonstrate that the conversion of HPMOF-based nanoreactors can be further enhanced significantly by combined structure/size regulation. The Pt@meso-HP-Co/Zn-ZIF-50% nanoreactors also show excellent molecular-size selectivity and cycling stability. Only 1-pentene and 1-hexene with size smaller than the aperture size of the intrinsic micropores of meso-HP-Co/Zn-ZIF-50% (3.4 Å) are allowed to access Pt NPs and be catalytically converted into alkanes (Figure 6j). In contrast, cis-cyclooctene that has a larger size cannot be converted. The distinct conversion difference between cis-cyclooctene and the other two alkenes also validates the complete encapsulation of Pt NPs inside meso-HP-Co/Zn-ZIF-50% nanocrystals (i.e., no Pt NPs on the outer surface of nanocrystals). Moreover, the conversion difference between 1-pentene and 1-hexene lies likely in their diffusivities. Briefly, smaller 1-pentene molecules should diffuse faster in MOF shells than 1-hexene and therefore achieve a higher conversion. The cycling stability of Pt@M-Co/Zn-ZIF50% nanoreactors is confirmed by the facts that the conversion of 1-pentene remains almost unchanged and the Pt@mesoHP-Co/Zn-ZIF-50% nanoreactors well preserve their crystallinity and mesoporous nanostructure after five consecutive runs of hydrogenation reactions (Figures S26 and S27).

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Dawei Wang: 0000-0003-3471-5744 Ji-Jun Jiang: 0000-0001-9483-6033 Guangqin Li: 0000-0002-1233-5591 Cheng-Yong Su: 0000-0003-3604-7858 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NNSF of China (21720102007, 21573291, and 21303273), NSF of Guangdong Province (2018A030313403, S2013030013474), STP Project of Guangzhou (201504010031), and Fundamental Research Funds for the Central Universities (14lgpy21). Dr. Xiaoxiao Cao (Gatan, Inc.), Prof. Wenxia Zhao, and Hao Zhang (IARC, Sun Yat-Sen University) are acknowledged for HRTEM imaging and helpful discussions.

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REFERENCES

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CONCLUSIONS We have demonstrated, for the first time, to the best of our knowledge, a facile dynamic growth strategy that allows the controlled synthesis and simultaneous structure/size regulation of small HP-Co/Zn-ZIF (with size down to ca. 95 nm). Our strategy combines the inherent binding competitions with external ultrasonication to achieve the dynamic generation of structure/composition/stability heterogeneity within Co/ZnZIF nanocrystals and subsequent conversion of such heterogeneity into hierarchical porous nanostructures. By building a nanoreactor based on our small nanocrystals of meso-HP-Co/Zn-ZIF-50%, we further demonstrate that the combined structure-size regulation of HPMOFs can indeed induce a substantial increase in their conversion. Our work should provide helpful insights for the development of MOFbased porous nanomaterials. Of particular interest in future research is to extend the present methodology to other types of MOFs so as to expand the family of HPMOFs and HPMOFbased nanomaterials, in which the metal ion pair should be delicately designed/testified for specific ligands to achieve dynamic binding competitions. Investigations along this line are in progress in our laboratory.

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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/acs.chemmater.8b02884. Additional information including PXRD patterns, SEM/ TEM/HRTEM/HAADF STEM/EDX images, N2 adsorption−desorption isotherms, TG/DTG curves, and tables (PDF) 6466


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