Metal-Organic Frameworks for Nanoarchitectures: Nanoparticle, Composite, Core-Shell, Hierarchical, and Hollow Structures

Chapter 2.2

Metal-Organic Frameworks for Nanoarchitectures: Nanoparticle, Composite, Core-Shell, Hierarchical, and Hollow Structures Nazmul Abedin Khan*, Zubair Hasan†, Imteaz Ahmed*, Sung Hwa Jhung* ⁎

Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Daegu, Republic of Korea, †Department of Mathematical and Physical Sciences, East West University, Dhaka, Bangladesh

1. Introduction 1.1  Metal-Organic Frameworks Metal-organic frameworks (MOFs), a fascinating class of crystalline porous frameworks, are usually prepared by coordinating an organic linker with an inorganic node (metal or metal oxide cluster) [1, 2]. The flexibility in choosing the inorganic nodes and organic linkers to construct MOFs has opened the opportunity to synthesize an enormous number of novel frameworks. Because of their easy synthesis methods; exclusive characteristics such as large surface area, high pore volume, and open metal sites (OMSs); and potential applications in various fields, MOFs have attracted much attention both in academia and industry. MOFs have numerous advantages compared to inorganic porous materials such as zeolites or metal phosphate-type materials [3]. Importantly, most metals can participate in MOF synthesis [4], in contrast to inorganic porous materials, which are based on a few metals such as Al and Si (and sometimes P is incorporated even though P is nonmetallic component). Inorganic materials usually require a “structure directing agent” (SDA) or template for structural formation; on the other hand, the solvent is the only templating molecule for the formation of MOF structures [1, 4]. Structural diversity is another of the most important Advanced Supramolecular Nanoarchitectonics. https://doi.org/10.1016/B978-0-12-813341-5.00002-4 © 2019 Elsevier Inc. All rights reserved.

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advantages of MOFs [5]. MOFs can be extended to several analogous (isomorphous or isostructural) frameworks that are synthesized from different metallic components and identical linkers. A series of MOFs named isoreticular MOFs [5, 6] have also been obtained by coordinating a series of linkers (with different lengths) with identical metallic components, wherein the pore sizes/shapes of the corresponding MOFs depend on the length of the linkers. Moreover, MOFs can be easily modified to impart various functionalities to both the linkers and the metal centers [5, 7–9]. MOFs have been applied in various potential applications, including the adsorption/storage of carbon dioxide [10], hydrogen [11], and methane [12]; separation of chemicals [13, 14]; adsorption of vapors [13, 15]; catalysis [16, 17]; magnetism [18]; drug delivery/biomedicine [19]; and luminescence [20]. MOFs are interesting materials because of the easy functionalization/modification of their pore surfaces, which leads to efficient adsorption/catalysis of particular guest molecules having specific functional groups. Moreover, a variety of central metals, OMSs, functionalized linkers, and loaded active components of MOFs have been employed successfully for various potential applications.

1.2  Nanoarchitectonics and Nanoarchitectured Metal-Organic Frameworks “Nanoarchitectonics” is a novel and advanced concept to generate nanoscale functional materials for various potential applications [21–24]. It refers to a conceptual linkage between nanotechnology and methodology to arrange nanoscale structural units in a specific configuration. As stated by Ariga et al., the aim of nanoarchitectonics is to design and prepare new functionalities via dynamic harmonization of atomic/molecular level controlled manipulation, chemical fabrication, and self-organization [21, 22]. In supramolecular chemistry, nanoarchitectonics deals with (i) the creation of nanoscale objects, and (ii) their organization (harmonized assembly) and modification to generate functional materials. So far, nanoarchitectonics has been well explained for the creation/ organization of various nanoarchitectured materials, including hybrid carbons [22, 25, 26], MOFs and related materials [21, 25, 27], biomaterials [21], and mesoporous metal oxides [24, 26, 28]. In material chemistry, nanoarchitecture deals with atom/molecular-level design, precise placement of connecting units, rational assembly of components, and controlled growth direction to prepare nanostructured materials. In this chapter, nanoarchitectured MOFs will be discussed as one of the subconcepts of nanoarchitectonics.

1.3  Aim of This Study Recently, hierarchical, hollow, and defective pore architectures of MOFs provided new insights into MOF-based studies/applications. Porous MOFs are used as a host matrix to accommodate various functional components within

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the porous structure for composite formation. In this chapter, the synthesis of MOFs with various structural features, including bimetallic MOFs, hierarchical/hollow pore architectures, and defects in coordination, will be discussed systematically. Moreover, various techniques to synthesize nanosized MOFs and MOF-based composites will be addressed.

2. Discussion 2.1  Metal-Organic Frameworks Composed of Different Central Metals and Bridging Linkers MOFs are three-dimensional porous networks, where the pore architecture is regular in order and interconnected by the metal (or metal cluster) and the bridging linker. By choosing the constructing units (metal center and linker), the internal architecture of the MOF can be tuned to the intended configuration. MOFs having similar crystal structures can be constructed via the assembly of either different metal cations or different bridging linkers; these are also known as analogous MOFs [5]. For example, MOF-74 having a one-dimensional channel (~1.1 nm) is composed of metal ions and a 2,5-dihydroxybenzenedicarboxylate (DOBDC) linker [5]. A variety of metallic components (Co, Mn, Zn, Ni, Mg, and Fe) can be accommodated with this linker to form similar crystalline structures as MOF-74, even though the chemical behavior of the MOFs differs from one another. The central metal and the nature of the metal-O bond control the formation rate (nucleation and crystal growth) [29], inner pore architecture, and porosity, as well as the overall behavior of the MOF-74s. Zhou and coworkers studied hydrogen adsorption over analogous MOF-74s [30]. The adsorption capacity and the metal-hydrogen bond strength followed the order Ni>Co>Mg>Mn>Zn; this trend was explained by the ionic radii of the metal cations, where Ni had the smallest radius. On the other hand, CO2 was preferentially adsorbed onto the Mg-based MOF because of the increased ionic character of the Mg-O bond and the low density of Mg within the MOF-74 structure. MOFs with different topologies and crystalline structures can also be constructed from similar building units with the addition of various organic SDAs [31]. Eddaoudi and coworkers reported the synthesis of three different zeolite-like MOFs (ZMOFs) with three different topologies from In3+ and 4,5-imidazoledicarboxylate [31, 32]. The use of imidazole as the SDA resulted in sod-ZMOF-1, in which each six-coordinated In3+ was heterochelated with two linker anions and coordinated with nitrogen from two other linkers. rho-ZMOF-1 was produced with the addition of 1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidine as the SDA; each In3+ was eight-coordinated and heterochelated with four linker anions. Moreover, 1,2-diaminocyclohexane (applied as the SDA) assisted the formation of the med-topology of ZMOF-1, in which eight-coordinated In3+ was linked with four nitrogen atoms and four oxygen atoms of four separate linkers.

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The coordination of Zr4+ ions and benzenedicarboxylate, biphenyldicarboxylate, or triphenyldicarboxylate linkers resulted in the UiO-66, UiO-67, and UiO-68 frameworks, respectively [33]. Depending on the size and shape of the bridging linkers, the pore architecture, chemical behavior, and application of the analogous MOFs differed from one another notably. The cages of UiO-66 were interconnected by triangular windows (diameter 0.6 nm), and the calculated Langmuir surface area was ~1100 m2 g−1. The UiO-67 and UiO-68 MOFs constructed with longer bridging linkers had comparatively larger free spaces within the pore architecture, and therefore, their surface areas were ~3000 and ~4200 m2 g−1 and their window diameters were 0.8 nm and 1.0 nm, respectively. Deng et al. reported a strategy to expand the pore aperture of MOF-74(Zn) by varying the length of the linkers [34]. The systematic expansion of the phenylene ring (linker) from one to eleven afforded a series of isoreticular MOF-74 noninterpenetrating structures, with the pore apertures and Langmuir surface areas varying from 1.4 to 9.8 nm and from 1600 to 9880 m2 g−1, respectively. The ultrahigh porosities and large pore apertures of the synthesized MOFs demonstrated a new size regime for the inclusion of large molecules such as vitamin B12 (largest dimension of 2.7 nm), an inorganic spherical cluster (MOP-18, diameter of 3.4 nm), myoglobin (globular protein with the largest dimension of 4.4 nm), and a green fluorescent protein (GFP, with largest diameter of 4.5 nm). MOFs were also reported to be synthesized from precursors containing mixed metals or mixed linkers and used for various potential applications [35– 39]. Hillman et al. synthesized CoZn-ZIF-8 MOFs (linker: 2-methylimidazolate) with different Co/Zn ratios via a microwave-assisted one-step method [36]. The mixed metal CoZn-ZIF-8 exhibited similar crystallinity and topology to the parent ZIF-8 for any Co/Zn ratio. Elemental mapping and X-ray absorption spectroscopy confirmed the uniform distribution of Zn and Co metal centers within the MOF framework. The mixed-metal CoZn-ZIF-8 (Co/Zn~1) was successfully used as a membrane to separate a propylene/propane binary gas mixture with an average separation factor of around 120.2, whereas a Zn-ZIF-8-based membrane resulted in an average separation factor of only around 62.9. The higher selectivity for the gas separation was explained by the smaller effective aperture of CoZn-ZIF-8, which was confirmed by the blueshift of the Co-N band in the IR spectrum. Moreover, the authors used two linkers, 2-methylimidazolate (for ZIF-8) and benzimidazolate (for ZIF-7), together to cosynthesize the mixed-linker MOF ZIF-7-8. Wang et  al. demonstrated a one-pot synthesis of mixed-metal MOF-74 containing 2, 4, 6, 8, or 10 different metal centers coordinated with the linker in one framework structure [40]. Metals Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, and Cd were added by choice to the reaction precursor of MOF-74(Zn) and heated hydrothermally to synthesize the desired mixed-metal MOF-74s. Elemental mapping revealed the uniform presence of all metals within the framework of the crystalline particles. Cohen and coworkers reported the synthesis of bifunctional UiO-66-(Br)(NH2) using two different linkers together [41]. In a typical reaction mixture, 2-aminobenzenedicarboxylate

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and 2-bromobenzenedicarboxylate were mixed with ZrCl4 in DMF for solvothermal synthesis. The synthesized UiO-66-(Br)(NH2) MOF possessed the same crystalline structure and thermal stability as UiO-66. NMR spectroscopy and electrospray-ionization mass spectrometry confirmed the successful coordination of both ligands within the UiO-66-(Br)(NH2) framework. The central metals or linkers of MOFs can also be exchanged via post-­ synthetic modification to incorporate other metals or linkers within the parent MOF frameworks [42–44]. Cohen and coworkers modified the pore architecture of ZIF-71 (composed of Zn2+ and 4,5-dichloroimidazolate, rho topology) via post-synthetic exchange of the central metal and bridging linkers [44]. The Zn-containing MOF was treated with Mn-acetylacetonate in methanol for 24 h at 55°C, and they observed that ∼12% of the tetrahedral Zn2+ centers were replaced with Mn2+ ions. In addition, the linker (4,5-dichloroimidazolate) was subjected to replacement with 4-bromoimidazolate via a similar post-synthetic exchange (treatment in methanol for 3 days at 55°C). Interestingly, the modified ZIF-71(Zn/Mn) or ZIF-71(Cl2/Br) was further allowed to exchange the linker and metal, respectively, in a similar manner (see Fig. 1). The crystallinity,

FIG. 1  Step-by-step postsynthetic exchange of metal ion and linkers of ZIF-71(Zn). (Reproduced with permission from H.H. Fei, J.F. Cahill, K.A. Prather, S.M. Cohen. Tandem postsynthetic metal ion and ligand exchange in zeolitic imidazolate frameworks. Inorg. Chem. 52 (2013) 4011–4016. Copyright © 2013, American Chemical Society).

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t­ opology, porosity, and stability of ZIF-71(Zn/Mn)-(Cl2/Br) and ZIF-71(Cl2/Br)(Zn/Mn) were well maintained after the stepwise exchange of the metal center and bridging linker. Similarly, UiO-66(Zr) could be converted into UiO-66(Zr/ Ti) via a post-synthetic cation exchange procedure, and the exchanged cations resulted in increased interaction with a microporous polymer to produce a microporous membrane [43]. The bimetallic MOF-based polymeric membrane showed 153% CO2 permeability compared to the UiO-66-based membrane.

2.2  Defects in Metal-Organic Frameworks Ordered MOFs have coordination-based assembly and directional bonding. Perturbed coordination during the synthesis may cause defects or disorder in the framework. Recently, defects in the crystal lattice have been studied as a powerful strategy to overcome the drawbacks of the parent MOFs in various applications such as adsorption [45–49] and catalysis [45, 46, 48, 50]. A number of approaches have been suggested to introduce defects in the crystal lattice of MOFs, including (i) using an acid modulator to synthesize missing-linker MOFs [51]; (ii) acid etching of MOFs [52]; (iii) fast crystallization to synthesize missing-linker MOFs [53]; and (iv) using mixed linkers to restrict proper coordination [54]. The defective MOFs usually contain hierarchical or hollow pore structures within the framework, which allow the access and diffusion of large molecules for efficient adsorption or catalysis, whereas the ordered-MOFs fail. Kitagawa and coworkers used synthetic modulators (such as acetic acid) to directly influence the coordination as well as to control the crystal growth of the Cu2(ndc)2(dabco)n framework (ndc: napthalenedicarboxylate; dabco: 1,4-­diazabicyclo[2.2.2]octane) (see Section 2.4 for modulation) [55]. The copperacetate (metal-modulator) interaction terminated the proper coordination (metal-linker) and resulted in defects in the MOF framework/porosity. Shearer et al. studied the defect chemistry of UiO-66 extensively through the addition of various acidic modulators (such as acetic acid, formic acid, and fluoroacetic acid) [51]. Based on the concentration and/or the acidity of the modulator, they demonstrated that the competitive acetate-carboxylate interactions resulted in both missing linker and missing cluster defects. They also stated that missing cluster defects were the predominant defects, and that the defects can be tuned by altering the modulator type and concentration. Vermoortele et  al. also reported the synthesis of defective crystals of UiO-66 following a modulation approach [56]. During the synthesis, the modulator (trifluoroacetate) coordinated to the metal centers and partially substituted the benzenedicarboxylate linkers, resulting in the restriction of metal-linker coordination. Missing-linker UiO-66 crystals with more open pores and a large number of unsaturated metal sites were obtained via the removal of trifluoroacetate groups during the thermal activation process. The defective crystals were used as efficient catalysts for several Lewis-acid catalytic reactions, including the Meerwein reduction of 4-tert-butylcyclohexanone with isopropanol. The defective crystals showed

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~93% conversion, whereas the ordered MOF exhibited only 7% conversion. Park et  al. synthesized a defective Zn-based MOF (known as MOF-5, Znbenzenedicarboxylate) by using the mixed linkers benzenedicarboxylate and 1,3,5-tris(4-carboxyphenyl)benzene [57]. The obtained octahedral framework was exceptionally functionalized with dangling carboxylates and remarkably differed from cubic MOF-5 in its affinity toward Pd2+ ions, demonstrating its role as an efficient catalyst for the phenylation of naphthalene. Li and coworkers demonstrated that the synthesis of Al-benzenedicarboxylate (MIL-53(Al)) proceeded through an initial assembly of the central metal and the linkers into MOF nanoparticles [58]. Fast crystallization kinetics resulted in the aggregation of the particles and continued coordination of the aggregated clusters into large-sized MOF particles. Disruption at the secondary crystallization stage led to defective coordination and eventually yielded metal-organic gels.

2.3  Construction of Metal-Organic Frameworks in a Layer-by-Layer Fashion Three-dimensional MOFs were also reported to be constructed by an unconventional integration fashion, i.e. the “layer-by-layer growth” approach [21, 27, 59, 60]. A noncoordinating bond between adjacent two-dimensional layers (synthesized via coordination of the metal and linker) binds them to generate a threedimensional framework. For example, the three-dimensional MOF NAFS-1 was constructed via a similar layer-by-layer growth approach using 5,10,15,20tetrakis-(4-carboxlatophenyl)-porphyrin-cobalt(III) chloride (CoTCPP) as a linker [27]. Initially, the coordination of CoTCPP, pyridine (py), and CuCl2 resulted in a two-dimensional array of CoTCPP-py-Cu sheets (thickness of 1.235 nm). The sheets were arranged in a layer-by-layer fashion along the caxis with an interlayer distance of 0.938 nm. As shown in Fig. 2, the interlayer stacking was achieved by attractive π-π interactions between the pyridine units coordinating axially to Cu2+. Haldar et  al. reported a fluorescent MOF with color-sensing properties when amine guests were trapped in the nanospaces [60]. The three bridging units, Zn2+ ions, 2,6-naphthalenedicarboxylate linkers, and 1,10-phenanthroline chelators, were assembled together in a one-dimensional fashion (zigzag chain). Further extension to a three-dimensional porous framework was achieved by chelator-linker interactions between adjacent layers. The six-coordinated Zn2+ ions were chelated to two carboxylate groups of two linkers, and two coordination sites of Zn2+ were occupied by the chelator. The porous framework was very efficient in recognizing aromatic amines via characteristic turn-on emission. The variable emission with different amines was attributed to the charge transfer phenomenon of the amines and 1,10-phenanthroline.

2.4  Metal-Organic Frameworks Nanoparticles The size of the MOFs, especially at the nanoscale level, and their topology can play a vital role in tuning and optimizing their properties. Akin to other

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FIG. 2  Proposed crystalline structure and π-π interactions between adjacent Cu2+-coordinated pyridine molecules of NAFS-1. (Adopted from R. Makiura, S. Motoyama, Y. Umemura, H. Yamanaka, O. Sakata, H. Kitagawa. Surface nano-architecture of a metal-organic framework. Nat. Mater. 9 (2010) 565–571).

n­ anomaterials, nanosized MOFs with a high density of exposed active sites have exhibited enhanced performance in gas adsorption/storage [61–63], catalysis [64, 65] (including electrocatalysis [66]), bio-imaging [67, 68], drug delivery [67], and so on. The particle size of an MOF primarily depends on the preparation method such as ultrasound-aided synthesis. Additionally, other parameters such as the reaction temperature and heating rate can influence the size of the MOFs by changing the rates of nucleation and crystal growth. Moreover, various strategies have been adopted to prepare nano-MOFs, such as coordination modulation [55, 69], sonochemical synthesis [70–73], microwave-assisted synthesis [72, 73], microemulsion synthesis [74], solvent- or additive-assisted synthesis [75], spray-drying synthesis [76], and template methods [77]. Table 1 shows the particle sizes of various MOFs synthesized by conventional heating, microwave, and ultrasonic irradiation methods [29, 71, 73, 78–90]. In the coordination modulation method, a modulator, which is generally a monodentate ligand (with a functional group similar to that of the linkers), is added to the synthesis mixture containing multidentate ligands. The addition of the monodentate ligand can modulate the coordination equilibrium via competition with the usual multidentates (bridging linkers of the MOF) to form coordination bonds with the metal nodes. Therefore, a modulator may both promote and inhibit crystal growth. During the promotion of crystal growth, depending on the concentration, the modulator can control the nucleation, which results in the formation of MOF particles of various sizes. On the other hand, to inhibit crystal growth, the modulator can act as a capping agent that terminates the extension of the network structure by coordination with the metal nodes. Thus, a capping agent can control the growth of the crystal by preventing further coordination via blocking the binding sites. The modulation mechanism was well explained by Guo et al. for the synthesis of a nanoscale dysprosium-based MOF (Dy-BTC) in the presence of sodium

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TABLE 1  Particle Sizes of Various MOFs Synthesized With Conventional, Microwave or Ultrasound Heating Methods MOF

Conventional Heating a

Microwave Irradiation

Ultrasonic Irradiation

a

MOF-74(Co)

>15 μm

5–7 μm

Co-, Mn-NDC

50–200 μm

5–20 μm

Cu-BTC

10–30 μm

0.5–1 μm

10–40 nm

a

[79]

a

2–4 μm

200–400 nm

Cu-BTC

~20 μm

~10 μm

MOF-5

500 μm

[80] 200–400 nm

20–25 μm a

>10 μm

2–5 μm a

5–25 μm

Tb-BTC

>20 μm

a

1–2 μm

a

MOF-177

0.5–1 mm

ZIF-8

250 μm

MIL-88A

250 nm

MOF-5

5–25 μm a

[83]

a

25–250 μm

[81] [82]

a

MIL-53(Fe)

MIL-53(Fe)

[29] [78]

MIL-53(Cr)

Cu-BTC

Ref.

a

a

400–800 nm

[84]

2–6 μm

[71] a

0.5–1 μm

500–800 nm

[73]

15–50 μm

5–20 μm

[85]

700 nm

[86]

100 nm

[87]

900 μm

[88]

20 nm

a

MIL-53(Al)

3–5 μm

0.5–1 μm

[80]

MIL-101(Cr)

100 nm

50 nm

[89]

MIL-47

2–5 μma

400–800 nma

[80]

MIL-101(Cr)

~500 nma

40–80 nm

[90]

a

The size of the particles were calculated from SEM images.

acetate as a capping agent [91]. The size of the Dy-BTC was adjusted from 60 μm to 71 nm by controlling the amount of sodium acetate added to the reactant mixture. The acid-base environment of the reaction mixture also governed the reaction kinetics and modulation process. The basicity of the sodium acetate greatly enhanced the deprotonation of the linkers for fast nucleation/crystallization. Fig. 3 shows the coordination modulation mechanism with pH adjustment for the synthesis of nanosized MOFs. Fischer et al. reported the use of a monocarboxylic acid capping agent (p-perfluoro-ethyl-benzoic acid, PFMBC) for the synthesis of MOF-5 [92]. Here, the modulator PFMBC coordinated with the Zn species, inhibiting the usual coordination process for further crystal growth. A general solvothermal synthesis might produce MOF-5 particles of ~350 nm,

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Lower pH

Metal ion +

Slow deprotonation

Slow nucleation

Crystal growth Bulk crystal Without CA

Fast deprotonation

Fast nucleation

Microsized crystal

Higher pH

= Capping agent CA

With CA Nano crystal

FIG. 3  Coordination modulation mechanism with pH adjustment for the synthesis of nano-sized MOFs. (Reproduced with permission from H.L. Guo, Y.Z. Zhu, S. Wang, S.Q. Su, L. Zhou, H.J. Zhang. Combining coordination modulation with acid-base adjustment for the control over size of metal-organic frameworks. Chem. Mater. 24 (2012) 444–450. Copyright © 2014, American Chemical Society).

whereas the addition of PFMBC to the linker (at a ratio of 2:1) reduced the particle size to 100 nm. Kitagawa et al. used n-dodecanoic acid as a modulator for the synthesis of Cu-BTC (Cu-benzenetricarboxylate) under microwave irradiation [69]. Here, copper acetate was used as a metal source to connect with the linker. After the reaction, a poorly crystalline material was produced, whereas the addition of n-dodecanoic acid resulted in the formation of welldefined cubic crystals of varying sizes (1 to 20 nm, depending on the ratio of modulator to linker). Here, the modulator acted as a capping agent as well as a promoter that competed for coordination with the metal and reduced the rate of nucleation to grow well-defined crystals. Liu et al. reported that the use of sodium acetate (in a 1:1 ratio with the linker) as a modulator during the synthesis of Cu-BTC reduced the particle size from 20 μm to 600 mm [93]. Both sodium acetate and sodium formate could modulate the coordination equilibrium and reduce the size of Cu-BTC significantly. However, the use of sodium formate as a modulator produced much smaller Cu-BTC particles (less than 100 nm in size) compared to those obtained with sodium acetate (600 nm). This finding revealed that a small-sized modulator favored the production of nanosized MOFs. So far, the coordination modulation method has also been used to reduce the size of various MOF particles, including MIL-101(Cr) [94], NH2-MIL-53(Al) [95], ZIF-8 [96], and UiO-66 [97]. Recently, microwave heating has been used for the synthesis of MOFs and other coordination polymers [72]. In the conventional solvothermal method, thermal energy produced by an external heating source is transferred to the r­ eaction

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FIG. 4  (A) The alignment of dipole moment of reactant molecules with altering the electric field of microwave irradiation; (B) acoustic cavitation (generation/growth, and collapse of bubbles) of ultrasound-assisted synthesis which generates high pressures, extreme local heat and short lifetimes. (Adopted from N.A. Khan, S.H. Jhung. Synthesis of metal-organic frameworks (MOFs) with microwave or ultrasound: rapid reaction, phase-selectivity, and size reduction. Coord. Chem. Rev. 285 (2015) 11–23).

vessel to heat the solution. In the case of microwave-assisted synthesis, electromagnetic waves provide instantaneous heat to the precursors (including the solvent) of the reaction mixture, and crystal formation starts at the hot spots (see Fig. 4A). This phenomenon results in faster nucleation, which ultimately produces smaller-sized particles. Earlier studies on the microwave-assisted synthesis of typical MOFs, MIL-100(Cr) and MIL-101(Cr) (Cr-benzenetricarboxylate and Cr-benzenedicarboxylate, respectively) were reported by Jhung et  al. [90, 98] To prepare MIL-101(Cr) by conventional heating, 4 days were required to achieve 45% crystal yield, while it took only 4 h via microwave heating (44% crystal yield). Besides, the microwave-assisted synthesis resulted in the formation of 40- to 90-nm-sized particles, which were much smaller compared to the particles prepared by conventional heating. Moreover, compared to conventionally synthesized MIL-101(Cr), microwave synthesized MIL-101(Cr) exhibited competitive performance in terms of adsorption capacity and rapid adsorption kinetics for the adsorption of benzene. Li et al. successfully synthesized nanosized MOF-808 (Zr-benzenetricarboxylate) within 5 min using a 400-W household microwave [99]. The reaction was conducted in a boiling flask rather than a Teflon-lined autoclave. The prepared particles exhibited octahedral geometry with an average particle size of 150–200 nm, which was significantly smaller compared to those synthesized under conventional heating. The prepared MOFs were tested for the adsorptive removal of arsenic. MOF-808 synthesized by the microwave-assisted method showed better performance for the removal of arsenic compared to the conventionally synthesized MOF-808. The excellent chemical stability and reusability (five cycles) also suggested the effectiveness of nanosized MOF-808 in liquid-phase adsorption. Additionally, microwaves were successfully used to reduce the particle sizes of well-known MOFs [72] such as Cu-BTC, MOF-74, MIL-53, and ZIF-8, and used in various potential applications (see Table 1).

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Ultrasonic irradiation is a nonconventional technique that has attracted a great deal of research interest in chemistry, especially in the synthesis of nanomaterials. In this process, a powerful ultrasonic wave (20 KHz–1 MHz) is applied to the reactant molecules, usually through an ultrasonic probe dipped in the reaction mixture [100]. In the case of liquids, the ultrasound induces the generation, growth, and collapse of bubbles (see Fig. 4B). This phenomenon is also known as acoustic cavitation, which produces considerable local heat as well as high pressure [101]. The temporary and localized hot spots generated by acoustic cavitation possess several exceptional characteristics, such as high temperatures of around ~5000 K, rapid heating/cooling rates (<1010 K s−1), and high pressures of ~1000 atm. Due to such unique physical properties, ultrasonic irradiation can generate “super conditions” that can initiate chemical reactions in solution to produce nanomaterials. Recently, ultrasonic irradiation has drawn considerable attention for the synthesis of MOFs because of the very fast and facile synthesis [72]. These extreme conditions can also enhance the formation of nanosized MOF structures (see Table 1), mostly via an increase in nucleation [61, 102]. Li et al. reported that ultrasound-assisted synthesis produced Cu-BTC particles with sizes ranging from 10 to 40 nm within 5 min, which were much smaller than the particles (10–30 μm) prepared via conventional heating [79]. Khan et al. reported the preparation of nanosized Cu-BTC within a short reaction time of 1 min using ultrasonic irradiation [81]. This study revealed that ultrasound is the most effective way to produce nanosized Cu-BTC particles of 200 nm in size, which are 100 and 50 times smaller in size than those obtained with conventional heating (20 μm) and microwave irradiation (10 μm), respectively. The trend of producing small-sized MOF particles was further confirmed in the synthesis MIL-53(Fe) (Fe-benzenedicarboxylate) by Haque et  al. [73] and Gordon et al. [84] In both cases, ultrasound was found to be very effective for producing nanosized MOFs. The beneficial effect of ultrasonic irradiation in preparing the small ZIF-8 was also reported [86]; ultrasound-assisted synthesis produced 700 nm ZIF-8, which was around 400 times smaller compared to the particles (250 μm) synthesized via conventional heating. Xu et  al. reported the synthesis of nanosized MIL-100(Fe) (Febenzenetricarboxylate) particles using glycol for the first time [75]. During the synthesis, glycol acted as a cosolvent and was adsorbed on the surface of MOF, controlling the crystal growth and morphology of the MOF. The average particle sizes of the MOFs prepared by this method were 40–50 nm. The synthesized MIL-100(Fe) exhibited excellent photocatalytic activity for the oxidation of benzene to phenol. MIL-100(Fe) synthesized by the glycol-assisted method exhibited comparatively better catalytic conversion and selectivity compared to bulk MIL-100(Fe), and good reusability up to three consecutive cycles. Ranft et  al. reported a facile solvothermal method to prepare nanosized CuBTC and IRMOF-3 (Zn-aminobenzenedicarboxylate) particles using auxiliary additives under mild conditions [103]. The crystal growth of the synthesized

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MOFs was controlled by regulating the reactant concentration and type of stabilizer. The size of Cu-BTC prepared at room temperature was ~30 nm, and the particle size varied with the amount of polyacrylic acid (PAA). The most suitable ratio to achieve a high production yield (65%–70%) with a narrow size distribution was found to be 1:3 (H3BTC to PAA). For IRMOF-3, a combination of the polymer polyvinylpyrrolidone (PVP) and cetyltrimethylammonium bromide (CTAB) surfactant was used to control the particle size. Use of either the polymer or the surfactants did not lead to such control in nanoparticle synthesis. Using PVP-CTAB, IRMOF-3 with an average particle size of 36 nm was produced, and the use of a larger amount of PVP-CTAB reduced the particle size to below 30 nm. Zheng et  al. prepared nanosized ZIFs using an ionic liquid-based ­microemulsion (H2O/TX-100/BmimPF6; BmimPF6 denotes 1-butyl-3-­ methylimidazolium hexafluorophosphate) [104]. The dispersed phase of a microemulsion is assumed to be numerous “nanoreactors” that can control the growth of the particles. In the syntheses of ZIF-8 and ZIF-67, the use of such a microemulsion produced ZIF particles with an average size of ~2 nm. Moreover, this system was applied successfully to prepare nanosized Cu-BTC. The authors demonstrated that this environmentally-friendly process can be considered as a potential method to prepare nanosized MOFs because of the use of a green solvent (ionic liquid), lower energy consumption, and simple de-emulsification method.

2.5  Metal-Organic Framework Composites A composite is a physical mixture of two or more components in which the identity of the components retains intact in any desirable task. Generally, there is a dominant component that stands for a matrix material (continuous phase). The other component that forms a composite with the matrix material is denoted as the functional species or discontinuous phase. MOFs have been extensively reported to form composites with other components and used in various potential applications [105–114]. In MOF composites, MOFs are utilized both as functional components embedded into a variety of matrix materials and the matrices to provide confined spaces for the functional components. Several methods, including solution impregnation, chemical vapor deposition (CVD), solid grinding, and double-solvent-aided loading, have been reported for preparing MOF composites [108, 109, 114]. Although the definitions of and the differences between a composite (active species/MOFs) and a core shell (active components@MOFs) are not very clear, the terminologies used in this chapter are based mainly on those used by the authors of the original documents. In this study, MOF composites will be represented in “x/y” fashion, where x denotes the functional component and y denotes the matrix material (or continuous phase). Moreover, “x@y” represents the core-shell composites that will be discussed in the next section.

164 SECTION | 2  Nanoarchitectonics of Nanostructured Materials

2.5.1  Metal-Organic Frameworks as Functional Components The active sites of MOFs (central metals or functional linkers) are very efficient in various applications such as adsorption/separation, storage, and catalysis; however, their applicability is limited by their relatively low thermal, chemical, or mechanical stabilities and poor processability (in the preparation of target products such as granules). To overcome this difficulty, researchers have investigated the incorporation of MOFs into various organic or inorganic porous matrices such as polymers, silica, and alumina [114]. For example, MOF/polymer composites were widely studied to have functional MOF moieties within the organic polymer. Among the synthesis techniques, solution blending is the simplest and most effective to embed MOF crystals within the polymers. As polymers usually dissolve in organic solvents, this technique has been widely used to synthesize MOF/polymer composites. Usually, sonication or mechanical stirring is applied to a solution mixture containing the MOFs and dissolved polymer to obtain the maximum homogeneity of the MOFs within the solution. Finally, evaporation of the solvent results in the formation of an MOF/polymer composite. Successful MOF/polymer composites that have been synthesized following this procedure include NH2-MIL-53(Al)/matrimid [115, 116], CuTPA/PVA [117], Cu-BTC/PMMA [118], and ZIF-90/6FDA-DAM [119]. Sabetghadam et  al. prepared NH2-MIL-53(Al)/matrimid and NH2-MIL53(Al)/6FDA-DAM (6FDA: 4,4′-(hexafluoroisopropylidene) diphthalic anhydride; DAM: 2,4,6-trimethyl-m-phenylenediamine) polymer-based composites in tetrahydrofuran (THF) solvent [120]. The composite membranes were applied in CO2 separation from an equimolar mixture of CO2 and CH4. The CO2 permeability was enhanced significantly by the incorporation of NH2-MIL53(Al) nanoparticles, with constant selectivity. Notably, when using the 6FDADAM polymer, the CO2 permeability increased up to 85% upon the addition of NH2-MIL-53(Al) nanoparticles. Moreover, the composite membrane had a CO2 permeability of 660 Barrer with a separation factor of 28, giving rise to a membrane performance very close to the Robeson limit of 2008. Wang et al. synthesized a polyaniline polymer (pANI) matrix in the presence of NH2-UiO-66 [121]. NH2-UiO-66 powder was dispersed in the pANI precursor and mixed well with sonication before the in situ growth of the pANI. The NH2-UiO-66/ pANI composite was used as an electrochemical sensor for the detection of Cd2+ ions, with an acceptable linearity in the concentration range of 0.5–600 μg L−1. The detection mechanism was explained by the chelation between the Cd2+ ions and the amino groups of the composite. In situ growth of MOFs is another widely used method to impart the functionalities of MOFs within the matrices [114, 122, 123]. The matrices are dispersed into a solution containing the MOF precursors and heated following similar conditions as those for the MOF synthesis. Crystal growth of the MOFs occurs on the internal or external surface of the matrices. Carbon [105, 124], graphene [125, 126], CNTs [127, 128], alumina [129], and silica [130] are widely used as matrices for the in situ growth of MOF composites. Ahmed

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et al. reported the solvothermal synthesis of Cu-BTC in the presence of silica microspheres [131]. Cu-BTC nanocrystals were produced and attached firmly to silica particles with adjustable porosity. The synthesized Cu-BTC/silica composite was used as a column material and applied to efficiently separate toluene/ethylbenzene/styrene and toluene/o-xylene/thiophene within 1.5 min. Qian et al. demonstrated the synthesis of Cu-BTC in the presence of “hierarchical porous carbon monoliths” (HCMs) [132]. A Cu-BTC/HCM composite exhibiting the structural characteristics of both Cu-BTC and the HCM was applied in the adsorption of CO2. The composite adsorbed 22.7 cm3 cm−3 of CO2, which was nearly twice (based on the unit volume of the adsorbent) the amount adsorbed by the HCM. The breakthrough curve of the composite for the separation of CO2 from N2 (16% CO2 in N2 as feedstock) is shown in Fig. 5. The Cu-BTC/ HCM composite showed highly preferential uptake of CO2 compared to N2, with a separation factor of ~100, demonstrating the composite as a competitive adsorbent for CO2 capture. A Co-MOF ([Co2(4-ptz)2-(bpp)(N3)2]n) was also synthesized hydrothermally in the presence of macroporous carbon by Zhang et al. [124] These composite materials exhibited excellent electrocatalytic ability for the reduction of nitrobenzene and the oxidation of hydrazine, with better stability and higher sensitivity compared to the individual components. Wu and coworkers reported the in situ hydrothermal synthesis of MIL-88(Fe) in the presence of graphene oxide (GnO) [133]. The resulting MIL-88(Fe)/GnO composite was applied as a catalyst in organic dye degradation. The composite demonstrated very fast methylene blue and rhodamine B degradation, within 20 and 30 min, respectively, compared to that of the individual components. A composite of Ni-benzenedicarboxylate and CNT was reported by Wen et al. and showed excellent performance in electrochemistry [128]. The composite e­ xhibited high

FIG. 5  (A) Cu-BTC/HCM composite; (B) breakthrough curve of Cu-BTC/HCM using a stream of 16% (v/v) CO2 in N2 at 25°C. (Reproduced with permission from D. Qian, C. Lei, G.P. Hao, W.C. Li, A.H. Lu. Synthesis of hierarchical porous carbon monoliths with incorporated metal-organic frameworks for enhancing volumetric based CO2 capture capability. ACS Appl. Mater. Interfaces 4 (2012) 6125–6132. Copyright © 2012, American Chemical society).

166 SECTION | 2  Nanoarchitectonics of Nanostructured Materials

specific capacitance of 1765 F g−1 for a supercapacitor (at a current density of 0.5 A g−1). The authors explained the high efficiency by a synergistic effect of the specific MOF structure and the CNT’s conductivity. In situ growth of various matrices in the presence of the MOF was also reported for synthesizing MOF composites [114, 131, 134]. For example, Huo et  al. prepared ZIF-8/ polystyrene microcapsules via the immobilization of MOF nanoparticles into a polystyrene precursor [134]. The authors demonstrated that the composites were very effective for encapsulating and retaining dye molecules, because of the combination of a narrow-pore MOF and a hierarchically structured polymer (with excellent interfaces).

2.5.2  Metal-Organic Frameworks as Matrix Materials MOFs have received greater attention as matrix materials for composite formation compared to other porous materials. The tunable pore size/shape and adequate pore openings of MOFs allow the precursors of the active species to be impregnated and react inside the framework. Moreover, the internal surface of the MOFs can be modified to adsorb/bind the guest molecules for further chemical reactions. So far, MOF matrices have been very successful to compose with functional components such as ionic liquids (ILs) [135], metals [106, 112], metal oxides [112], carbon [105], quantum dots (QDs) [111], polyoxometalates (POMs) [114], polymers [108], and biomolecules [106]. The attachment of the functional components within the MOF framework has mainly been achieved via solution impregnation, double-solvent, sol-gel, CVD, and solid grinding methods. In situ synthesis methods can be classified as ship-in-bottle (SIB) and bottle-around-ship (BAS) synthesis, where the matrix MOF accommodates the functional component (via synthesis within the MOF), or the MOF is synthesized (in the presence of its pre-synthesized counterpart), respectively. Table 2 shows various synthesis methods for metal/MOF composites and their applications [136–150]. An SIB synthesis of TiO2 within an MOF framework was reported by Chang et  al. [151], where a titanium butoxide precursor was introduced into MIL-101(Cr) and the TiO2/MIL-101(Cr) composite was obtained under hydrothermal conditions. The mesoporous TiO2/MIL-101(Cr) not only exhibited excellent adsorption capacity, but also played an important role in the TiO2assisted catalytic degradation of methyl orange with ultra-high efficiency. CuCl2 was incorporated into MIL-47 via solution impregnation, and the impregnated Cu2+ was autoreduced by the central metal ion (V3+) of MIL-47 (Fig. 6) [152]. The reduced Cu+ in the composite effectively bound benzothiophene from a liquid fuel. The remarkable adsorption/removal of benzothiophene from the fuel was explained by the efficient π-complexation [153] between the Cu+ sites and the benzothiophene molecule. Graphite oxide/MOF (GO/MOF) is one of the well-studied MOF-based composites that has been reported widely for various applications [110]. Bandosz et al. demonstrated several results for synthesizing MOFs in the presence of GO (via a

TABLE 2  Various Synthesis Methods to Prepare Metal/MOF Composites MOF

Other Component

Application

Ref.

MIL-101(Cr)

Pd

Synthesis of methyl isobutyl ketone

[136]

Au

Aerobic oxidation of alcohols

[137]

Ag

Conversion of CO2

[138]

Au

Synthesis of propargylamines

[139]

Pd

Hydrogenation of styrene

[140]

Pt

Cinnamaldehyde hydrogenation

[141]

ZIF-8

Au

Sensor sensitization

[142]

MIL-101(Cr)

Pt

Chromium reduction

[143]

UiO-66

Pd

Suzuki coupling reaction

[144]

ZIF-8

Ni

Hydrolysis of ammonia borane

[145]

MIL-101(Cr)

Ru

Phenol hydrogenation

[146]

MOF-5

Pd

Hydrogenation of cyclooctene

[147]

Ru

Oxidation of benzyl alcohol

[148]

ZIF-8

Au

CO oxidation

[149]

MOF-5

Au

Aerobic oxidation of alcohols

[150]

Solution impregnation IRMOF-3 UiO-66

Double solvent approach

CVD

Solid grinding

Metal-Organic Frameworks for Nanoarchitectures  Chapter | 2.2  167

Method

168 SECTION | 2  Nanoarchitectonics of Nanostructured Materials

FIG. 6  (A) П-complex formation of Cu+ and benzothiophene molecule; (B) adsorption isotherms of virgin and CuCl/MIL-47 composite for the adsorption of benzothiophene from liquid fuel. (Adopted from N.A. Khan, S.H. Jhung. Remarkable adsorption capacity of CuCl2-loaded porous vanadium benzenedicarboxylate for benzothiophene. Angew. Chem. Int. Ed. 51 (2012) 1198–1201).

BAS method) and applied them for the efficient adsorptive removal of H2S, NO2, NH3, etc. As shown in Fig. 7, the GO layers were separated to a certain extent and arranged between the cage planes of the MOFs structure. A GO/MIL-101(Cr) composite was reported by Ahmed et al. to effectively adsorb/remove nitrogencontaining organic compounds from liquid fuel [154]. GO/MIL-101(Cr) (up to a certain amount of GO) resulted in a higher surface area than that of MIL-101(Cr).

FIG. 7  Schematic representation of coordination directions between various MOFs and GO layers. (Adopted from I. Ahmed, S.H. Jhung. Composites of metal-organic frameworks: preparation and application in adsorption. Mater. Today 17 (2014) 136–146).

Metal-Organic Frameworks for Nanoarchitectures  Chapter | 2.2  169

The authors explained this phenomenon by the creation of an extra space between the MIL-101(Cr) cages due to the presence of GO layers. The composite material with a higher surface area adsorbed indole and quinoline preferentially compared to the virgin MIL-101(Cr). Keggin-type POMs (PMo12O40, SiMo12O40, and PW12O40) were immobilized in the cages of a copper based-MOF under solvothermal conditions following a BAS technique [155]. The well-dispersed POMs (active components) within the ordered structure of rht-MOF-1 were applied to the catalytic oxidation of alkylbenzene and the adsorption of dyes (crystal violet and rhodamine B) from water. The composite showed superior catalytic and adsorptive performance compared to the pristine MOF and POMs. Furthermore, the POMs (usually unstable in liquid-phase operations) were stable within the MOF framework during the adsorption/catalysis and regeneration processes. Usually, impregnation is a classical and effective method to impart functional components into MOFs; however, it is difficult to confirm the position of the loaded guest species within the MOFs. The precursors attach to both the inner and outer surfaces of the MOF, and in most cases, aggregation of the guest components occurs (especially on the outer surface). Researchers have established a double-solvent method to overcome these types of difficulties, in which the guest components are precisely encapsulated within the pores of the MOFs without aggregation. For example, an aqueous solution (volume not larger than the pore volume of the MOF) of any guest precursor can be fully incorporated to a hydrophilic MOF (dispersed in a low-boiling organic/hydrophobic solvent) via hydrophilic interactions and capillary force, while the organic solvent is eventually removed by evaporation. Recently, the double-solvent method has been applied extensively to impart various functionalities by loading the active components precisely inside the pores of the MOF [107]. For example, metal nanoparticles have been widely incorporated into MOFs via this unique technique. Fig. 8 shows a scheme for incorporating bimetallic AuNi nanoparticles inside the MIL-101(Cr) cages [156]. NiCl2 and HAuCl4 were introduced into the pores of MIL-101(Cr) using the double-solvent method; strong reduction with a high-concentration NaBH4 solution resulted in highly dispersed metal nanoparticles. On the other hand, the reduction of the precursor was not complete when a low-concentration NaBH4 solution was used; therefore, some of the precursors diffused out of the cavities, resulting in aggregated nanoparticles on the outer surface of MIL-101(Cr).

2.5.3  Metal-Organic Framework/Metal-Organic Framework Composites MOF/MOF composites were also reported to provide mixed frameworks in which different MOFs form a composite with each other as the matrix and functional component. Choi et  al. demonstrated the incorporation of CuBTC nanocrystals within the pores of MOF-5 via a BAS method and applied it for methane adsorption (Fig. 9) [157]. Compared to the individual MOFs, the r­esulting material adsorbed methane very efficiently at room temperature and 80 bar. Hasan et al. reported the synthesis of MOFs such as MIL-100(Fe)

170 SECTION | 2  Nanoarchitectonics of Nanostructured Materials

FIG. 8  Schematic representation for the incorporation of AuNi nanoparticles within MIL-101(Cr) matrix via double solvent strategy. (Reproduced with permission from Q.L. Zhu, J. Li, Q. Xu. Immobilizing metal nanoparticles to metal-organic frameworks with size and location control for optimizing catalytic performance. J. Am. Chem. Soc. 135 (2013) 10210–10213. Copyright © 2013, American Chemical Society).

and Cu-BTC in the presence of pre-synthesized nonporous (Cu2(pyz)2(SO4) (H2O)2)n MOF (denoted as CP) by a similar BAS method [158]. The CP/MOFs composites, having Cu+ sites (originating from CP) within the framework, were very efficient for the removal of benzothiophene and dibenzothiophene from liquid fuels. The authors demonstrated that the nonporous CP with Cu+ sites could bind benzothiophene or dibenzothiophene via π-complex formation within the porous framework. Bhadra et al. dispersed ZIF-8 crystals into a Ti-containing MIL-125-NH2 (Ti-aminobenzenedicarboxylate) precursor and prepared a ZIF-8/MIL-125-NH2 composite via hydrothermal synthesis [159]. TiO2 inside a nitrogen-rich carbon was prepared from the composite material via pyrolysis in a nitrogen environment. ZIF-8 enhanced the proportion of carbon with high porosity and resulted in good distribution of the active TiO2 particles within the carbon framework. The mesoporous TiO2/C derived from the MOF composite was used as an effective catalyst in the oxidative removal of dibenzothiophene from liquid fuel. A MIL-88B/ZIF-8 composite (MIL88B: Fe-benzenedicarboxylate) was prepared by a similar approach, in which iron-based MIL-88B nanorods were incorporated into the ZIF-8 matrix [160]. Pyrolysis of the composite produced nitrogen-doped carbon nanotubes embedded with Fe3C nanocrystals. The resulting Fe3C/CNTs exhibited remarkable catalytic activity for the oxygen reduction reaction.

2.5.4  Core-Shell Synthesis of Metal-Organic Framework Composites Core-shell structures are composed of an inner core of fine particles encapsulated by, for example, a porous framework. The pores of the shell materials

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Nanocrystalline HKUST-1 (nHKUST-1)

Mixing nHKUST-1 with the MOF-5 synthesis solution

Crystal growth for nHKUST-1MOF-5

(A)

(nHKUST-1) Embedding

(B)

nHKUST-1MOF-5

MOF-5

FIG. 9  (A) Schematic methods for the BAS synthesis of MOF-5 crystals in the presence of CuBTC crystals; (B) structural representation of Cu-BTC/MOF-5 composite. (Reproduced with permission from K.M. Choi, J.H. Park, J.K. Kang. Nanocrystalline MOFs embedded in the crystals of other MOFs and their multifunctional performance for molecular encapsulation and energy-carrier storage. Chem. Mater. 27 (2015) 5088–5093. Copyright © 2015, American Chemical Society).

a­ ccommodate the active guest materials (cores) mainly via SIB and BAS synthesis methods. MOFs are outstanding for encapsulating one or multiple cores within a shell or multiple shells of the MOF matrix. Moreover, a yolk-shell structure consisting of a core within a void/hollow structure of MOFs has been reported [161]. Recently, MOFs as encapsulating shell materials have received great interest because of (i) the abundance and diversity of structures, which ensure easy selection of the best-matching MOF; (ii) the confinement of the guest particles within the pores, which provides shape selectivity; and (iii) the possible interactions between the functionalized linkers and guest particles [161]. So far, functionalities such as metals [162, 163], metal oxides [164, 165], POMs [166], QDs [167], silica [168], and even MOFs [157, 169] have been incorporated into the pores of MOFs via core-shell synthesis. Khan et al. demonstrated the synthesis of acidic ILs as the core within the MIL-101(Cr) cavities following an SIB technique [170]. The IL@MIL-101(Cr) core-shell composite adsorbed a higher amount of benzothiophene compared

172 SECTION | 2  Nanoarchitectonics of Nanostructured Materials

FIG.  10  Controlled encapsulation of nanoparticles of various sizes, shape and fashions within ZIF-8 shell. (Adopted from G. Lu, S. Li, Z. Guo, O.K. Farha, B.G. Hauser, X. Qi, Y. Wang, X. Wang, S. Han, X. Liu, J.S. DuChene, H. Zhang, Q. Zhang, X. Chen, J. Ma, S.C.J. Loo, D. Wei, Y. Yang, J.T. Hupp, F. Huo. Imparting functionality to a metal-organic framework material by controlled nanoparticle encapsulation. Nat. Chem. 4 (2012) 310-316).

to the virgin MIL-101(Cr). The authors compared the SIB technique with other incorporation techniques, including solution impregnation, in terms of the stability of the ILs within the pores of MIL-101(Cr). They demonstrated that only the SIB-synthesized composite was stable up to several runs for liquid-phase adsorption/regeneration. Lu et al. reported the core-shell incorporation of metal nanoparticles (Pt, Au) into a ZIF-8 framework via a BAS method [163]. The surfaces of the synthesized nanoparticles (having various compositions, sizes, and shapes) were modified with PVP and encapsulated in various fashions within the ZIF-8 framework (see Fig. 10). The authors demonstrated that the spatial distribution of the incorporated nanoparticles could also be controlled by the selection of a suitable addition sequence during the ZIF-8 synthesis. Depending on the addition sequence and the orientation of the nanoparticles, the nanoparticle@ZIF-8 core-shell structures had efficient catalytic, optical, and magnetic properties. Thiol-functionalized magnetic Fe3O4 nanoparticles were incorporated into Cu-BTC by Ke et  al. and applied in wastewater treatment [164]. The Fe3O4@Cu-BTC magnetic structures showed remarkable adsorption selectivity for Hg2+ and Pb2+, resulting in distribution coefficient (Kd) values of 5.98×104 and 1.23×104 mL g−1, respectively. Zeng et al. reported a Co3O4@ MOF-5 core-shell nanoreactor with a void space between the active Co3O4 core and the octahedral MOF-5 shell [171]. The MOF was synthesized hydrothermally in the presence of pre-synthesized Co3O4 core nanoparticles. The authors demonstrated that Co3O4@MOF-5 served as a nanoreactor to accommodate a

Metal-Organic Frameworks for Nanoarchitectures  Chapter | 2.2  173

sulfate radical-based advanced oxidation process in the void space. The active Co3O4 and the void space (nanoreactor) around Co3O4 allowed rapid diffusion of the reactant molecules to the Co3O4 sites and made the material very efficient for the removal of 4-chlorophenol in the presence of peroxymonosulfate. The degradation of 4-chlorophenol reached 100% within an hour, compared to only 59% under the same conditions with bare Co3O4 particles. Miao et al. reported the magnetic core-shell-shell structure of the Fe3O4@ P4VP@MIL-100(Fe)-type (P4VP: poly(4-vinylpyridine)) catalyst and applied it to the selective aerobic oxidation of various alcohols [165]. Initially, the Fe3O4@P4VP core was obtained via the polymerization of P4VP on the presynthesized Fe3O4 microsphere surface. Subsequently, the MIL-100(Fe) shell was introduced through the BAS technique. The authors demonstrated that during the MIL-100(Fe) synthesis, the inner shell of the P4VP polymer played a significant role in the absorption of Fe3+ species for the synthesis of the MOF shell and in protecting the magnetic Fe3O4 core from aggregation/destruction. The activity of the catalyst remained intact (without significant leaching of Fe3O4) even after ten cycles, confirming the efficiency of the core-shell strategy for nanoparticle incorporation. Fig. 11 shows the synthesis of ZIF-8@ZIF-67 core-shell structures by applying a seed-mediated growth technique following the BAS method [172]. The core sizes and shell thicknesses could be adjusted simply by applying different sizes of ZIF-8 seeds and changing the Co2+/Zn2+ feeding ratios. Thermal treatment of the materials resulted in hybrid carbons (composed of nitrogenrich carbon as the core and graphitic carbon as the shell). The hybrid carbon showed excellent electrochemical properties, with a specific capacitance of 270 F g−1 at a current density of 2 A g−1. CdSe/CdS/ZnS QDs were prepared and incorporated into the porous structure of MOF-5 [167]. The synergistic effect of the ordered porosity of MOF-5 and the luminescent QDs were very effective as optical sensors for the discrimination of small molecules (by harnessing the emission of the QDs). Sorribas et al. reported ordered micro-mesoporous silica@ZIF-8 core-shell structures in which a hydrophobic microporous ZIF-8 shell controlled the access of guest molecules to the hydrophilic silica mesopores [168]. By varying the cycles of ZIF-8 growth, the microporous MOF layer could be changed. In other words, the mesoporous silica cores could be embedded in ZIF-8 with different shell thicknesses. Song et al. reported the synthesis of a Cu-BTC shell in the presence of a Keggin-type POM (as a core); synergistic stabilization of both the POM and the MOF was also demonstrated [166]. The POM@Cu-BTC resulted in a large increase in the catalytic turnover rate of the POMs (for air-based oxidation reactions) and was employed as a detoxification catalyst for the conversion of various sulfur compounds (including H2S) to S8 using only air. CVD of metal nanocrystals in MOFs was also studied extensively to prepare metal@MOF core-shell structures. In a typical procedure, the shell MOF is exposed to the vapors of the organometallic precursor, and then, either hydrogen

174 SECTION | 2  Nanoarchitectonics of Nanostructured Materials

+

Solvothermal method 100 ºC 12h

Zn2+ Methylimidazolc

ZIF-8 seed With Co2+ & methylimidazole 100 ºC 12h

Seed-mediated growth

800 ºC 3h N2

Nitrogen-doped Carbon @Graphitic Carbon

ZIF-8 @ ZIF-67

FIG.  11  Synthetic scheme for the BAS synthesis of core-shell ZIF-8@ZIF-67 crystals, and ­nitrogen-doped hybrid carbon. (Reproduced with permission from J. Tang, R.R. Salunkhe, J. Liu, N.L. Torad, M. Imura, S. Furukawa, Y. Yamauchi. Thermal conversion of core-shell metal-organic frameworks: a new method for selectively functionalized nanoporous hybrid carbon. J. Am. Chem. Soc. 137 (2015) 1572–1580. Copyright © 2015, American Chemical Society).

or high temperature is used to reduce the metallic species to metal nanoparticles. Fischer and coworkers incorporated Cu, Pd, and Au into MOF-5 via CVD, and constructed core-shell metal@MOF structures [147]. Later, a series of studies were conducted following a CVD technique for the preparation of coreshell-type metal@MOF structures. The widely studied MOFs in this strategy are ZIF-8, ZIF-90, MOF-5, and MOF-177, while the metallic precursors are (η5-C5H5)Cu(PMe3), (CH3)Au(PMe3), and (η3-C3H5)Pd-(η5-C5H5) for the incorporation of Cu, Au, and Pd nanoparticles, respectively [149, 150, 156, 173]. Au nanoparticles were incorporated into a ZIF-8 shell via CVD of the Au(CO)Cl precursor (and further reduction by hydrogen treatment) [174]. The resulting particles were very small and matched the cavity size of the shell. Finally, the Au@ZIF-8 material was used as an efficient catalyst for the aerobic oxidation of benzyl alcohol. Huang and coworkers reported the synthesis of Pd nanoparticles within the porous UiO-66 shell via a solution impregnation method [175]. The Pd precursor was reduced to metallic Pd via hydrogen treatment, and the Pd@UiO-66 was further treated with polydimethylsiloxane (PDMS). Modification of Pd@ UiO-66 by a thin hydrophobic PDMS layer made the surface of the material hydrophobic and led to enhanced catalytic affinity for hydrophobic substrates.

Metal-Organic Frameworks for Nanoarchitectures  Chapter | 2.2  175

Thus, Pd@UiO-66@PDMS showed significantly enhanced activity compared to pristine UiO-66 or Pd/UiO-66 in various catalytic reactions, including the catalytic reduction of 4-nitrophenol and nitrobenzene.

2.6  Hierarchical and Hollow Structures 2.6.1  Hierarchically-Structured Metal-Organic Frameworks Mesoporous MOFs were mainly synthesized by ligand extension strategies; however, this strategy is restricted by the synthesis conditions, limited mesopore diameter, possible interpenetration, and structural collapse upon the removal of the solvent. Therefore, alternative techniques were reported for tuning the microporous MOFs into hierarchically-structured MOFs. Recently, hierarchicallystructured MOFs attracted great interest for applications in catalysis, separation, and energy storage [77, 176–180]. The most common techniques to introduce hierarchical pores into MOFs include (i) perturbation-assisted synthesis [176, 181, 182] and (ii) template-directed synthesis [178, 180, 183]. Additionally, linker labilization [184] and “ionic liquid/supercritical CO2 emulsion” [77] processes were reported. Perturbation-assisted syntheses were performed under strong stirring, which limited the nucleation of MOFs kinetically and created mesopores among the MOF crystals [182]. In case of template-assisted synthesis, the widely used templates include surfactants, block polymers, and metal-organic assemblies (MOA). Usually, the templates form domains within the pores of the MOFs, and removal of the domains via chemical or thermal treatments generates free spaces or large cavities within the framework. Yue and coworkers demonstrated a perturbation-assisted approach to synthesize MOF-74(Zn) with hierarchical porosity [182]. The reaction mixture of zinc acetate, DOBDC, and N,N-dimethylformamide (DMF) was stirred for various reaction times at room temperature. Hierarchically-structured MOF-74(Zn) was obtained within 15 min of reaction time, and the mesoporosity increased with an increase in stirring time (see Fig. 12). According to the pore size distribution analysis, the micropore volume of the materials decreased with increasing reaction time, and the mesopores could be enlarged up to 15 nm (in the case of the 4 h reaction). Based on this observation, the authors revealed that the mesoporosity of the MOF was obtained at the expense of the micropore and that the polarity of the solvent played a crucial role in controlling the crystal growth and mesoporosity. The suggested mechanism includes redissolution and reagglomeration of the MOF particles in the acetate/DMF solvent system. The hierarchical MOF-74(Zn) was then employed for the adsorption of large dye molecules such as Brilliant Blue R-250 (dimensions of ~1.8×2.2 nm; larger than the microporous cavity of the MOF). The MOF adsorbed 17.9% of the dye, while microporous MOF-74(Zn) adsorbed only 2.7%. Perturbation-assisted syntheses of IRMOF-3, Cu-benzenedicarboxylate, and Cu-benzenetetracarboxylate were also reported by the same group [181], wherein a nanofusion mechanism was applied for the creation of hierarchical structures. In these cases, under strong

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FIG. 12  (A) Schematic representation of the synthesis of hierarchical MOF-74(Zn) crystals; (B) N2 adsorption isotherms; and (C) pore size distributions of the MOFs obtained at different reaction times (t). (Reproduced with permission from Y.F. Yue, Z.A. Qiao, P.F. Fulvio, A.J. Binder, C.C. Tian, J.H. Chen, K.M. Nelson, X. Zhu, S. Dai. Template-free synthesis of hierarchical porous metal-organic frameworks. J. Am. Chem. Soc. 135 (2013) 9572–9575. Copyright © 2013, American Chemical Society).

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stirring conditions, coordination of the reactants was partially prevented, leading to the formation of crystal building units, which ultimately assembled to form hierarchically structured MOFs. Moreover, the presence of large-sized dye molecules in the reaction mixture prevented their coordination/growth, and therefore, hierarchical structures were obtained. Surfactant templating is the most widely used and traditional technique to prepare hierarchical MOFs [180]. Hierarchical Cu-BTC was the very first example of a templated meso-MOF, as reported by Qui et al. [185] The MOF was synthesized in the presence of a cationic surfactant (CTAB) in the reaction mixture, and hierarchical micro- and mesopores were obtained after the removal of the surfactant via solvent extraction. The mesopore volumes increased with an increase in the CTAB/Cu2+ molar ratio, and the nitrogen adsorption isotherm profiles of the MOFs were between type I and type IV. It was also stated that the porosity of the MOFs could be controlled by the appropriate selection of templating materials and variation of the surfactant/metal molar ratio. Pahm and coworkers used a nonionic triblock copolymer (F127, EO97PO69EO97) as a supramolecular template for the preparation of hierarchically-porous Cu-BTC and Cu-BTB (BTB=1,3,5-tris[4carboxyphenyl]benzene) [186]. Acetic acid was employed as a coordination and deprotonation modulator that prevented fast crystallization. Higher crystallinity and hierarchically porous MOFs with uniform mesostructures were achieved by increasing the acetic acid/Cu2+ molar ratio. Huang and coworkers reported an in situ self-assembly template method to synthesize stable MOFs, in which an acid sensitive and/or soluble MOA was also obtained (see Fig. 13) [178]. The removal of the chemically less-stable MOA (by acid washing) resulted in mesoporosity as well as hierarchical structures in the MOFs. A variety of sacrificial MOAs (including MOF-5, ZIF-8, and metal-organic polyhedra) were synthesized in the early stage of the reaction and employed to fabricate hierarchical structures of UiO-66, MIL-101(Cr), ZIF-8, and DUT-5. Hierarchical UiO-66 having mesopore sizes of 4–12 nm was employed to adsorb large molecules such as organic dye molecules (Direct Blue 86 or DB 86, 0.4×1.2×1.4 nm in size), a metal-organic polyhedron (MOP-OH, 4×4×4 nm in size), and biological protein molecules (bovine serum

FIG. 13  Schematic procedure of the synthesis of hierarchical MOF via self-assembly template method. (Adopted from H.L. Huang, J.R. Li, K.K. Wang, T.T. Han, M.M. Tong, L.S. Li, Y.B. Xie, Q.Y. Yang, D.H. Liu, C.L. Zhong. An in situ self-assembly template strategy for the preparation of ­hierarchical-pore metal-organic frameworks. Nat. Commun. 6 (2015) 8847).

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albumin, BSA, 14×4×4 nm in size) from aqueous solution. Microporous UiO-66 could not adsorb DB 86 dye molecules, but UiO-66 with about 4 nm mesopores effectively adsorbed this large molecule. It was also observed that UiO-66 with 4 nm mesopores could hardly adsorb MOP-OH and BSA molecules, but UiO-66 with 12 nm mesopores could. Yuan et al. reported a linker labilization technique to create mesoporosity within a Zr-based microporous MOF (PCN-160, Zr-azobenzenedicarboxylate) [184]. Linker labilization is nothing but the reverse of the linker installation process. The authors demonstrated the replacement of the terminal –OH/H2O of the coordinatively unsaturated metal cluster (Zr6O4(OH)8(H2O)4) by the carboxylates of the “pro-labile-linker” (see Fig. 14). Hierarchical PCN-160 was obtained by the labilization of the linker (splitting into two detachable monocarboxylates) under acidic conditions (AcOH/DMF). The maximum pore size of the hierarchical PCN-160 was found to vary from 1.5 to 18 nm by precise control of the acid concentration and pro-labile-linker content.

FIG. 14  Schematic illustration of the construction of PCN-160 framework. (A) linker installation; (B) linker labilization; and (C) formation of hierarchical structure by linker labilization. (Adopted from S. Yuan, L. Zou, J.-S. Qin, J. Li, L. Huang, L. Feng, X. Wang, M. Bosch, A. Alsalme, T. Cagin, H.-C. Zhou. Construction of hierarchically porous metal-organic frameworks through linker labilization. Nat. Commun. 8 (2017) 15356).

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2.6.2  Hollow-Structured Metal-Organic Frameworks Hollow-structured MOFs [187] are very fascinating because of their potential applications in adsorption [188, 189], catalysis [171, 190, 191], drug delivery [192], and similar areas. The construction mechanism of hollow MOF particles is critical due to their controlled formation procedure. So far, a number of synthesis methods have been reported for obtaining hollow MOF particles, including the self-templated strategy [193, 194], template-assisted synthesis [195, 196], “emulsion-based interfacial assembly,” [197, 198] and spray-drying technique [76]. Self-templated strategies are much more convenient than the other synthetic methods as they are commonly one-step methods, and do not require any chemical or thermal operations to remove the templates. However, it is still challenging to optimize the appropriate reaction conditions for the synthesis of hollow-structured MOFs. Moreover, the transformation mechanisms from kinetically unstable intermediates to thermodynamically stable hollow structures are not well understood. Oh and coworkers demonstrated a one-step synthesis of hollow-structured Zn-benzenetricarboxylate (Zn-BTC) under solvothermal conditions without the addition of an external template [193]. As shown in Fig. 15, the authors proposed a self-templated multistep mechanism, where the kinetically- favored spherical microparticles of Zn-BTCs were formed as selftemplates at the first stage. Then, simultaneous formation of new crystalline Zn-BTC particles and dissolution of the spherical particles occurred. Finally, thermodynamically stable hollow Zn-BTC particles were obtained via complete dissolution of the spherical particles. A similar template-free strategy was also reported by Huo et al. for the synthesis of an iron-based hollow MOF composed of Fe3+ and ferrocenedicarboxylate [194]. The reaction time, temperature, and molar ratio of the reactants controlled the formation of the hollow spherical particles. Based on transmission electron microscopy, scanning electron microscopy, and powder Xray diffraction) analyses, the authors reported that the hollow structures were produced by the dissolution of the initially formed spherical particles following an Ostwald ripening mechanism. A template-assisted synthesis of hollow ZIF-8 was reported by Lee et  al. [199], wherein polystyrene (diameter of 0.87 nm, with a carboxylate terminal) spherical polymer was used as a solid amorphous template. In the synthesis procedure, the polymeric template was added to the reaction precursors containing Zn2+ ions and 2-methylimidazolate. The carboxylates on the surfaces of the polystyrene polymer interacted with metallic species (Zn2+) and initiated the crystallization of ZIF-8 on the surface of the template spheres following a core-shell strategy (polystyrene@ZIF-8). Finally, hollow ZIF-8 microspheres were obtained by treating polystyrene@ZIF-8 with DMF, which selectively removed the polystyrene polymer. MOF@MOF core-shell structures have been reported to form a hollow-structured shell after etching/removal of the core MOF (used as a hard template) [200–202]. For example, Yang and coworkers reported the seed-mediated (ZIF-67) synthesis of core-shell ZIF-67@ZIF-8 [202]. A subsequent methanol-based solvothermal treatment resulted in hollow

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FIG. 15  Schematic representation of the synthesis of hollow Zn-BTCs via self-templated procedure. The length of scale bars in the figure is 1 μm. (Reproduced with permission from I. Lee, S. Choi, H.J. Lee, M. Oh. Hollow metal-organic framework microparticles assembled via a self-templated formation mechanism. Cryst. Growth Des. 15 (2015) 5169–5173. Copyright © 2015, American Chemical Society).

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ZIF-8 crystals, along with some interlaced Co-ZIF within the hollow cavity. The Pd-incorporated hollow structures were successfully employed as effective shape-selective catalysts for some important reactions, including the catalytic hydrogenation of acetylene. An emulsion-based interfacial reaction method was reported by Yang et al. for the synthesis of hollow-structured ZIF-8 [198]. First, a “water-in-oil” emulsion was prepared with Zn2+ containing aqueous droplets and octanol (as the oil phase). Then, an organic solution containing 2-methylimidazole was added to the emulsion to initiate the formation of hollow ZIF-8 crystals via an interfacial reaction between the organic phase and the aqueous droplets. The shell thickness of the hollow ZIF-8 could be tuned by controlling the precursor amount or by varying the reaction time. Liquid-phase catalytic hydrogenation reactions of olefins (1-hexene, trans-stilbene, and tetraphenylethylene) were studied using Pd-incorporated hollow ZIF-8s to take advantage of structure of the hollow ZIF-8s. Shape-selective catalysis was suggested, considering both the shell thicknesses of ZIF-8s and the molecular sizes of the olefins.

3.  Conclusion and Future Perspectives MOFs, which are synthesized by the coordination of metal ions with organic linkers, have recently emerged as a new class of highly crystalline porous materials. The amenability to design inorganic and organic units as well as tunable pores makes them promising materials for various applications. The characteristics of MOFs, such as atomic/molecular-level design, rational assembly of components, and controlled growth direction, facilitate the precise placement of connecting units within the pore architectures. However, it remains a challenge to synthesize/modify MOFs with precise pore shapes/sizes, particle sizes, and functionalities for specific applications. MOFs with various structures, including hollow or hierarchical pore architectures, show various potential applications. Moreover, nanosized MOFs have attracted great interest for various applications. Controlled integration of functional materials and MOFs has led to the development of new multifunctional hybrid/composite materials that demonstrate superior properties through the synergetic effects of the individual components. Additionally, MOFs are fascinating for the encapsulation of well-defined nanoparticles and have attracted great interest for a variety of applications. Moreover, encapsulating various functional moieties (such as metals, metal oxides, POMs, and QDs) within porous MOFs to construct coreshell nanostructures can impart additional functionalities to the MOFs for better reactivity and durability. Based on previous achievements, some ideas for the possible future development of MOF-based research are suggested, as follows: ●

MOF composites are obtained extensively with a variety of MOFs and other components. The components can be selected carefully considering their applications in industrial conditions, which are usually different from laboratoryscale conditions. For example, IL/MOF or POM/MOF composites may

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●

●

●

●

●

show high selectivity/efficiency for gas separation or organic-phase operations. However, the existence of a trace amount of water or moisture (common in real processes) may cause leaching of the ILs or POMs, and more importantly, may affect the crystallinity of water-sensitive MOFs (such as MOF-5, MOF-177, and Cu-BTC). Layer-structured MOFs or MOF sheets are seldom studied (excluding membranes for separations), even though these types of materials are more structurally flexible and very promising for various technological devices. Therefore, the synthesis of layered-MOFs might be an interesting topic for future MOF-based research. Ultrasonic irradiation is one of the best methods to synthesize MOF nanoparticles with significantly reduced size, especially under ambient reaction conditions. However, most of the MOFs are usually synthesized at high temperature and pressure (in a sealed reactor), and therefore, ultrasound-assisted synthesis under these conditions is not common. Sonication within a sealed reactor (to ensure high pressure) may produce various MOFs nanoparticles that cannot be synthesized by ultrasound under ambient conditions. Regeneration of MOF-based composites is another challenge for future research. Simple impregnation, solid grinding, and physical mixing processes have been extensively studied to prepare composites for various lab-scale applications. However, these techniques cannot prevent the leaching or wash-out of components (that are loosely bound to the matrix surface) during the application and regeneration processes. Therefore, we suggest the use of more efficient techniques such as SIB, BAS, or CVD to trap the active components within the matrix framework. So far, with the techniques applied to construct hierarchical MOFs, it is still a challenge to provide uniform pore diameter and ordered mesopores within the framework. Good distribution of mesopores may allow maximum access of larger molecules for efficient applications in adsorption and catalysis. The size and shape of hollow-structured MOFs generated via acid etching or soft-templating approaches are not well controlled. Using crystalline templates with specific size/shape may lead to the creation of the desired/welldefined hollow structures.

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