Coordination Polymers

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Article Cite This: Acc. Chem. Res. 2017, 50, 2684-2692

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Transformation of Metal−Organic Frameworks/Coordination Polymers into Functional Nanostructured Materials: Experimental Approaches Based on Mechanistic Insights Kyung Joo Lee, Jae Hwa Lee, Sungeun Jeoung, and Hoi Ri Moon* Department of Chemistry, School of Natural Science, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea CONSPECTUS: Nanostructured materials such as porous metal oxides, metal nanoparticles, porous carbons, and their composites have been intensively studied due to their applications, including energy conversion and storage devices, catalysis, and gas storage. Appropriate precursors and synthetic methods are chosen for synthesizing the target materials. About a decade ago, metal−organic frameworks (MOFs) and coordination polymers (CPs) emerged as new precursors for these nanomaterials because they contain both organic and inorganic species that can play parallel roles as both a template and a precursor under given circumstances. Thermal conversions of MOFs offer a promising toolbox for synthesizing functional nanomaterials that are difficult to obtain using conventional methods. Although understanding the conversion mechanism is important for designing MOF precursors for the synthesis of nanomaterials with desired physicochemical properties, comprehensive discussions revealing the transformation mechanism remain insufficient. This Account reviews the utilization of MOFs/CPs as precursors and their transformation into functional nanomaterials with a special emphasis on understanding the relationship between the intrinsic nature of the parent MOFs and the daughter nanomaterials while discussing various experimental approaches based on mechanistic insights. We discuss nanomaterials categorized by materials such as metal-based nanomaterials and porous carbons. For metal-based nanomaterials transformed from MOFs, the nature of metal ions in the MOF scaffolds affects the physicochemical properties of the resultant materials including the phase, composite, and morphology of nanomaterials. Organic ligands are also involved in the in situ chemical reactions with metal species during thermal conversion. We describe these conversion mechanisms by classifying the phase of metal components in the resultant materials. Along with the metal species, carbon is a major element in MOFs, and thus, the appropriate choice of precursor MOFs and heat treatment can be expected to yield carbon-based nanomaterials. We address the relationship between the nature of the parent MOF and the porosity of the daughter carbon materiala controversial issue in the synthesis of porous carbons. Based on an understanding of the mechanism of MOF conversion, morphologically or compositionally advanced materials are synthesized by adopting appropriate MOF precursors and thermolysis conditions. Despite the progressive understanding of conversion phenomena of MOFs/CPs, this research field still has rooms to be explored and developed, ultimately in order to precisely control the properties of resultant nanomaterials. In this sense, we should pay more attention to the mechanism investigations of MOF conversion. We believe this Account will facilitate a deeper understanding of MOF/CP conversion routes and will accelerate further development in this field.

1. INTRODUCTION Nanostructured materials such as porous metal oxides, assembled metal nanoparticles (NPs), porous carbons, and their composites have been intensively studied due to a variety of applications including energy conversion and storage devices, catalysis, and gas storage.1 In order to synthesize the target materials, the appropriate precursors and synthetic methods are adopted. For instance, soft- and hard-templating routes are commonly used to develop pores by incorporating organic and inorganic precursors for porous carbon and metal oxides, respectively.2,3 To prepare composite materials of carbon and metal species, sequential and/or separate synthesis are required. As a new type of a precursor for these nanomaterials, metal− © 2017 American Chemical Society

organic frameworks (MOFs) deserve attention because they contain both organic and inorganic species, which can play parallel roles as both a template and a precursor in parallel under appropriate conditions.4 MOFs are well-ordered crystalline solids composed of coordination bonds between metal ions and organic ligands. Thus far, most studies on MOFs have been focused on synthesizing new MOFs and exploring their applications such as gas sorption and separation, catalysis, sensing, and electronic and optoelectronic devices.5,6 In contrast, this Account aims to Received: May 24, 2017 Published: October 9, 2017 2684

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already been extensively discussed in the literature,14 our major focus will be carboxylate-based MOFs. Applications of nanomaterials have also been intensively studied,15 but utilization of the resultant nanomaterials is beyond the scope of this review, and readers can refer to other excellent review papers.4,8,16 Here, we will discuss converted nanostructured materials as categorized by their main materials such as metalbased nanomaterials and porous carbons.

review the utilization of MOFs as precursors for transformation into functional nanomaterials rather than the MOFs themselves. In MOF scaffolds, the organic and inorganic components are periodically arranged at a certain distance, which enables their conversion reaction to produce materials with well-defined structures and chemical compositions even in the solid state. Owing to the countless types of metal ions/ clusters and organic ligands that comprise MOFs, the coordination strength between those components and the thermal stability of the framework can vary extensively. This implies that various chemical reactions can occur according to the precursor MOFs and conversion condition, thus providing inexhaustible potential for synthesizing diverse functional nanomaterials. Over the past decade, the thermal conversion of MOFs and applications of the resulting materials have been increasingly studied, particularly for electrocatalysis. The results have proved that MOFs offer a promising toolbox for synthesizing advanced nanomaterials that are not easily obtainable using conventional methods. Many relevant reviews have been published by groups of Xu, Gascon, and Yamauchi.4,7−9 However, comprehensive discussions revealing the conversion mechanism remain insufficient, although understanding this mechanism is critical for the rational design of the precursor MOFs and the formation of nanomaterials with desired physicochemical properties. Therefore, in this review, we intend to focus on research that can help readers to understand the relationship between the intrinsic nature of the parent MOF and the daughter nanomaterial, as well as the experimental approaches based on mechanistic insights. In this Account, coordination polymers (CPs) will be included in the scope of the review. According to the IUPAC classification,10 CPs represent a conceptually larger family of materials than MOFs; CPs are defined as “a coordination compound with repeating coordination entities extending in 1, 2, or 3 dimensions,” and MOFs as a “coordination network with organic ligands containing potential voids.” Even though MOFs are distinguished from other coordination compounds by their high dimensionality and porosity, those factors are not necessary requirements for the targeted functionalization of nanomaterials, according to previous results in the literature. Therefore, we will extend the materials of interest from MOFs to CPs if they impart new insights into this field of study. Pioneering studies on MOF conversion were conducted by the Morsali group, who synthesized various nanosized metal oxides by calcining CPs in air, which were collectively discussed in a review article in 2012.11 However, because of rapid, severe reactions between CPs and O2 such as carbon combustion and metal oxidation, the products were not sufficiently well controlled to generate nanomaterials. Thus, recent studies on converting MOFs and CPs have mostly focused on thermolysis under an inert atmosphere. Performing the conversion reaction under these conditions enables emphasizing and revealing the role of the organic ligands on the phase and structural transition during the decomposition of MOFs, which will be discussed in this Account. Meanwhile, several intriguing solution-based conversions of MOFs have been reported,12,13 but those wetchemical synthetic routes will be excluded from this Account, which deals solely solid-state conversion. Furthermore, Prussian blue analogue (PBA), a representative cyanide-based coordination compound, has been widely used as a precursor, usually to produce hollow metal-based nanomaterials.14 However, since the conversion of PBAs and its underlying mechanism have

2. MOFs TO METAL-BASED NANOMATERIALS To examine the thermal stability of MOFs, thermogravimetric analysis (TGA) is routinely performed under an inert atmosphere. Upon heating, MOFs mostly lose solvent molecules first, and then their entire structures thermally decompose at higher temperatures (principally >400 °C). In a TGA trace, this decomposition is indicated by an abrupt weight loss, which is usually attributed to organic ligands. Tens of percent by weight remain after the thermal decomposition is completed. In this section, our main area of interest is the remaining compounds during and after the destruction of MOFs, especially metal-based materials. Das et al. proposed that the reduction potentials of metal ions in MOFs are the standard factor to explain and predict the resultant materials (Figure 1).17 Metal ions with a standard reduction potential of

Figure 1. Effect of the reduction potentials of the metal ions in MOFs on the formation of metal/metal oxide species. Reproduced with permission from ref 17. Copyright 2012 Royal Society of Chemistry.

−0.27 V or higher generate the metal phase after pyrolysis under an inert atmosphere, while those with a reduction potential of less than −0.27 V generate the metal oxide phase. Although highly significant, this threshold would be not generally applicable because it does not consider chemical reactions with organic substances. Organic ligands for MOFs mostly consist of C, N, and O, but the compositions can be extended to include other elements such as S, P, and Se to produce metal chalcogenides and pnictides. Therefore, understanding the mechanism of MOF conversion is not a simple issue, and thus, designing tailor-made MOFs to synthesize target nanomaterials is very challenging. 2.1. Nanostructured Metal Oxides

In 2010, Hu and Zhang were the first to propose a mechanism for the decomposition of MOF-5 ([Zn4O(BDC2−)3]n; BDC = terephthalate).18 As shown in Figure 2, three types of cleavages, a, b, and c, are plausible below 500 °C, and as evidenced by the temperature-programmed mass spectrum (MS) the combination of breaking at positions (a + c) and b resulted in ZnO and CO2 with benzene-derived carbon species, which then formed amorphous carbon. Therefore, the product of the thermolysis of MOF-5 was a composite of ZnO nanocrystals covered with amorphous carbon. Based on an understanding of this MOF-5 decomposition mechanism, Yang et al. synthesized porous carbon-coated ZnO quantum dots (∼3.5 nm) via a controlled pyrolysis.19 During this process, hydrocarbons, which can evolve from the cleavage of benzene rings, were not detected. 2685

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Figure 2. Schematic view of the decomposition mechanism of MOF-5. Reproduced with permission from ref 18. Copyright 2010 American Chemical Society.

each temperature during heating up to 500 °C support this conversion mechanism (Figure 3b−d). Specifically, at 350 °C, organic vesicles were distributed over the material, which indicates the generation and confinement of large amounts of organic moieties, and at the end of heat treatment at 500 °C, we obtained hierarchical nanoporous MgO (np-MgO). The pyrolysis−gas chromatography/mass spectrometry (pyro-GC/ MS) results demonstrated how the organic substances were generated during thermolysis (Figure 3e). Many new organic species with high boiling points evolved in each temperature range. This process indicated that organic fragments that had decomposed from Mg-aph-MOF first reacted to generate new organic moieties, which were then confined in the solid until the temperature was sufficiently high to evaporate them. This approach also successfully extended to Ce-aph-MOF to prepare np-CeO2. We were also able to synthesize nanoporous manganese oxides via the thermal conversion of a Mn-based MOF by introducing another aliphatic ligand.21 Furthermore, we could control the oxidation states of the manganese oxides (i.e., MnO, Mn3O4, Mn5O8, and Mn2O3) by optimizing the sequential conversion reactions during annealing in N2 and calcination with atmospheric oxygen. Importantly, the resultant MnO, Mn3O4, and Mn5O8 exhibited almost identical textural properties including their morphology, surface areas, pore volumes, and the size of nanocrystals composing the nanoporous frameworks. Thus, these manganese oxides served as a model system for studying the effect of their oxidation state on catalysis. A conversion process that retains the morphology of the precursor is called pseudomorphic conversion. The pseudomorphic conversion of MOFs enabled controlling the macroscopic morphologies of metal oxide nanomaterials. To investigate this control strategy in a comparative study, we prepared two different MOFs composed of the same building blocks, Co2+ and BDC ligands, having plate- or rod-like morphologies (p-MOF and r-MOF, respectively, Figure 4).22 Accordingly, plate- and rod-like Co3O4 nanomaterials (p-Co3O4 and r-Co3O4, respectively) prepared via pseudomorphic conversion retained the morphology of the parent MOF. In other words, the resultant nanostructured metal oxides were composed of the Co3O4 NPs with the similar diameter, but those constructed different secondary or tertiary architectures to generate plate and rod morphology, respectively. In addition, this pseudomorphic conversion of MOFs facilitates the fabrication of more diverse morphologies, including cubic, octahedral, spinel-like, hexagonal, polyhedral, and so forth.23 Conversion reactions that transform mixed-metal coordination compounds into multimetal oxide nanomaterials have

Paying attention to the robustness of the aromatic carboxylate, we conversely employed aliphatic carboxylate ligands.20 These ligands may construct less thermally stable aliphatic-ligand-based MOFs (aph-MOFs), and thus, a different thermal conversion route can be expected. As schematically illustrated in Figure 3a, during the thermolysis of a Mg-aphMOF under N2 flow, before the structure was completely destroyed, labile adipate ligands were first transformed into organic moieties to act as porogens (i.e., pore formers), which subsequently evaporated at higher temperatures. Ex situ transmission electron microscope (TEM) images recorded at

Figure 3. (a) Schematic view of the thermally induced conversion process of aph-MOF and (b−d) ex situ TEM images taken at various temperatures during heating. (e) TGA and pyro-GC/MS result of Mgaph-MOF. Reproduced with permission from ref 20. Copyright 2013 American Chemical Society. 2686

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containing Li and Si was converted into Li-based ceramic, Li4SiO4 with a coral-like morphology, which would be very difficult to achieve using other methodologies.26 2.2. Metal/Metal Carbides Nanoparticles

In order to generate the metallic phase via the thermolysis of MOFs, the aforementioned reduction potentials of metal ions constructing MOFs should be considered.17 Meanwhile, carbon is an active reducing agent at high temperature, and in the thermolysis of MOFs, metal ions are reduced by the carbon species derived from the decomposition of organic ligands. This carbothermic reduction is also a very important process for obtaining the metallic phase. Therefore, the Gibbs free energies (ΔG) of the reactions for reducing the metal oxide and oxidizing carbon play a key role in determining whether the carbothermic reduction is favorable.7 For instance, the ΔG for the reduction of ZnO by carbon is negative as the temperature exceeds ca. 900 °C, and thus, ZnO carbothermically reduces to metallic Zn, leading to metal/carbon composites. Likewise, Cu/ C, Co/C, and Ni/C nanostructured composites have been readily prepared by thermolysis of the corresponding metalbased MOFs under an inert atmosphere. During thermolysis under optimized conditions, the organic substance tended to prevent the metal nanocrystals from aggregating, thus homogeneously dispersing the metal NPs within the carbon matrix. On the other side, the carbon properties in M/C such as graphiticity and porosity are affected by the type of metal species in MOFs owing to diverse chemical reactions with metal NPs. The detailed interpretations will be described in section 3. Furthermore, by employing mixed-metal CPs, heterometallic alloys have been easily synthesized. As shown in Figure 6, Deng et al. converted (Co,Ni)EDTA (EDTA4− =

Figure 4. Scheme of the pseudomorphic conversion of Co-MOFs and the corresponding scanning electron microscope (SEM) and TEM images. Reproduced with permission from ref 22. Copyright 2014 Royal Society of Chemistry.

emerged as a new synthetic approach in MOF-mediated synthesis. Particularly, achieving a highly homogeneous dispersion of each component remains a formidable challenge. In MOFs, metal species are distributed uniformly in the framework, and thus, multimetallic solid solutions can be obtained in a facile manner. Yang and co-workers synthesized transition-metal-substituted CeO2 NPs with a diameter of 3 nm by pyrolyzing bimetallic Schiff base complexes as precursors.24 We also exploited the pyrolysis of bimetallic CPs with adipate introduced as an aliphatic ligand to prepare nanoporous structures consisting of nanocrystalline frameworks of transition-metal−ceria solid solutions (np-TMxCe1−xO2−δ; TM = Ni, Co, Fe, and Mn, Figure 5).25 Furthermore, the composition of MOF-derived metal oxide nanomaterials can be further tuned by extending the range of components for the ligands as well as the metal species. For example, a Si-centered tetracarboxylic acid ligand that was capable of charge-balancing four Li+ ions was demonstrated. This newly designed MOF

Figure 6. (a, b) HRTEM images of CoNi@NC, showing the graphene shells and encapsulated metal NPs. (c) Schematic illustration of the CoNi@NC structure. Reproduced with permission from ref 27. Copyright 2015 WILEY-VCH.

ethylenediaminetetraacetate) into CoNi nanoalloys (4−7 nm) encapsulated in ultrathin graphene layers using a simple heat treatment.27 As evidenced by high-resolution TEM (HRTEM) images, the graphene shells on the CoNi NPs mostly consisted of one to two layers. These composites could be used to demonstrate the role of carbon-encapsulation for metal NPs in electrocatalysis. In addition to using mixed metal-based CPs themselves as precursors, the inclusion of foreign metal NPs into the pores of MOFs is an effective way to form mixed metal alloys. The Kitagawa group synthesized a body-centered cubic (bcc) CuPd nanoalloy via the thermal decomposition of a Pd NPs@ HKUST-1 ([Cu3(BTC3−)2]n; BTC = 1,3,5-benzenetricarboxylate) material as a precursor in H2 gas (Figure 7).28 In situ temperature-dependent X-ray diffraction (XRD) patterns

Figure 5. Scheme of the synthesis and nanoporous structure of npTMxCe1−xO2−δ solid solution. Reproduced with permission from ref 25. Copyright 2017 American Chemical Society. 2687

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reported the thermal conversion of HKUST-1 with Mo-based polyoxometalates (POMs).30 After a postetching step to eliminate Cu NPs, mesoporous MoCx nano-octahedrons could be obtained. Likewise, understanding the physicochemical properties of metal ions as well as chemical reactions with carbon species that occurred during thermolysis enables selecting the proper MOF precursors and designing the conversion conditions for synthesizing the target materials. 2.3. Metal Chalcogenides and Pnictides

Via a MOF conversion reaction, we easily extend the possible compositions of metal-based nanomaterials to metal chalcogenides or pnictides such as sulfides, selenides, phosphides, and so forth. The conversion system must include the corresponding elements such as the chalcogen and pnictogen, which can be introduced at the beginning of thermolysis as additives or sequentially reacted with the first conversion products.31 Fan and her co-workers synthesized CoSe2 microspheres via a onestep conversion by using Co-based MOF with Se powder.32 In the case of the stepwise reaction, the second reaction corresponds to a phase transition such as sulfidation, selenylation, or phosphidation on the metal or metal oxide nanomaterials. For instance, by the thermolysis of Ni2EDTA, we first prepared Ni NPs encapsulated in a mesoporous graphitic carbon shell (Ni@mesoG), which we subsequently conducted the phosphidation with NaH2PO2, yielding Ni2P@ mesoG (Figure 9).33 Since the Ni NPs were well covered by the graphitic carbon shell, their size almost did not change, even after a postsynthetic modification.

Figure 7. Scheme of the formation of bcc CuPd nanoalloy by the thermolysis of Pd@HKUST-1 under H2 gas. Reproduced with permission from ref 28. Copyright 2014 Royal Society of Chemistry.

demonstrated that during the thermal conversion, the lattice parameter of the Pd NPs within the MOF expanded due to hydride formation, and the structure was later converted into a face-centered cubic (fcc) CuPd phase. Further heating the fcc CuPd nanoalloy to 673 K transformed it into a bcc CuPd. Some kinds of transition metal ions in MOFs are converted to metal carbide nanomaterials rather than the metallic phase by thermolysis under an inert atmosphere. Transition metal carbides have attracted attention as promising electrocatalysts as alternatives to precious noble-metal-based catalysts. Binary phase diagrams can be used to identify the thermodynamically favored phase among the metal carbon alloys. As shown in Figure 8a, the iron carbide phase is more stable than metallic iron at both the conversion temperature and room temperature. Gascon and co-workers synthesized highly dispersed iron carbide NPs embedded in a porous carbon by pyrolyzing an Febased MOF ([Fe(BTC)]n, Figure 8b).29 Similarly, Lou et al.

Figure 9. Scheme (top left), HRTEM (top right) and energydispersive X-ray spectrometry mapping (bottom) of the Ni2P NPs entrapped in mesoporous graphene via a post-transformation process. Reproduced with permission from ref 33. Copyright 2017 Royal Society of Chemistry.

In addition to these two approaches, the targeted chalcogen or pnictogen elements can be directly incorporated into organic ligands to construct a MOF to be used as a single precursor. As shown in Figure 10, we synthesized a CP precursor with a phosphorus-containing ligand, tris(2-carboxyethyl)phosphine. Simply via thermolysis under an inert atmosphere, we could obtain metal phosphide NPs embedded into porous P-doped carbon (PPC) such as Ni12P5@PPC and Co2P@PPC (our unpublished results). Yang et al. also reported NiS particles embedded in carbon codoped with N and S (NiS@N/S−C), which were synthesized by directly converting a Ni(II) complex with vanillic thiosemicarbazone containing N and S (C9H11N3O2S).34 Figure 8. (a) Binary phase diagram of iron and carbon. (b) Direct pyrolysis of an Fe-based MOF and impregnation with a carbon source (furfuryl alcohol) followed by pyrolysis. Reproduced with permission from ref 29. Copyright 2015 Nature Publishing.

3. POROUS CARBONS Along with metal species, carbon is a major element in MOFs, and thus, the appropriate choice of precursor MOFs and heat 2688

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resultant carbon have been developed that utilized the gas products generated during thermolysis. Kitagawa et al. synthesized foam-like porous carbon by carbonization of a Zn-based porous CP, [Zn(5-NO2-isophthalate)(4,4′-bipyridyl)]n, which includes a nitro group in the ligands.36 The nitro groups led to fast NO2 gas evolution kinetics during thermal conversion, which afforded a foam-like microstructure to the porous carbon material. Unlike metallic Zn NPs, metals such as Ni, Co and Fe cannot be evaporated due to their high boiling points. Thus, they must be eliminated by etching with acid to afford porous carbon. Meanwhile, those can act as catalysts for graphitization, which will lead to the formation of graphitic porous carbon after a postetching process. Recently, our group converted a [Ni2(EDTA)] coordination complex into graphitic and mesoporous carbon (Figure 12).37 During the thermal conversion, Ni(II) ions were transformed into 4 nm-sized Ni NPs embedding in the carbon matrix. Owing to the catalytic function of Ni NPs, C−C bond cleavage occurred in the organic species, and sp2 carbons formed on the Ni NPs. As shown in Figure 12, the harsh thermal treatment at 1000 °C induced severe agglomeration of the Ni metals on the carbon matrix, and the subsequent etching step eventually yielded mesoporous graphene (3D mesoG). To date, many efforts have been devoted to investigating factors affecting the properties of the carbon materials converted from MOFs. Several papers mentioned that the porosity of the parent MOFs facilitates the generation of porous structures of the resultant carbon from Zn-based MOFs.35,38 Meanwhile, Kim et al. reported the thermal conversion of different kinds of Zn-based MOFs into porous carbon materials.39 They demonstrated a linear relationship between the Zn/C ratio of the parent MOFs and the surface area of the resultant carbons. To comprehensively compare these studies, we organized the reported results from various references, including our own, to verify the correlations between the surface area of parent MOFs and daughter carbons (Figure 13a) and between the Zn/C ratio of parent MOFs and the surface area of carbons (Figure 13b). We classified the types of parent MOFs, as differentiated by symbols (△ for MOF-5, ▽ for MOF-74 ([Zn2(dioxidoterephthalate)]n), and □ for ZnCPazo-1 ([Zn(fumarate)(4-(phenylazo)pyridine)2(H2O)]n),○ for others), and the corresponding literatures, as differentiated by color (red,19 green,38 purple,39 and blue40). As being noticed, there are no apparent trends in both graphs corresponding to the kind of MOFs and no relationship appears between the results of each study with the results of previous reports, but meaningful trends are only observed in each literature. That is because many variables such as the framework structure and its stability, ligands (as carbon source), Zn ions/clusters, porosity, and synthetic conditions comprehensively play a role in determining the porosity of the resultant carbon materials. Thus, attributing the porosity to a single factor in this transformation would be an oversimplification. Instead, the plots demonstrate that carbon porosity highly depends on the conversion conditions, as indicated in Figure 13a and b, wherein even when converted the same MOF, the surface area of carbon dramatically varies with the synthetic conditions. This means that the physicochemical properties of parent MOFs do not directly transfer to the daughter carbons, but understanding the conversion mechanism enables controlling the porosity when utilizing parent MOFs. In this context, we reported the

Figure 10. Structure of Ni12P5@PPC and Co2P@PPC synthesized by directly pyrolyzing P-containing CPs (unpublished results).

treatment can be expected to yield carbon-based nanomaterials. One way to generate nanoporous carbon is by removing metal species from composite materials, which are obtained during thermolysis. In this regard, Zn-based MOFs are proper precursor system for this approach. Yang et al. synthesized highly microporous carbon by carbonizing MOF-5 (Figure 11a).19 They confirmed that the ZnO species formed within

Figure 11. (a) Schematic view of the structural change of MOF-5 heat-treated to yield porous carbon. Reproduced from ref 19. Copyright 2012 American Chemical Society. (b) Formation of the foam-like porous carbon by the NO2 gas evolution during carbonization. Reproduced with permission from ref 36. Copyright 2015 WILEY-VCH.

carbon matrix reduced to metallic Zn NPs during the thermal conversion of MOF-5 by a carbothermic reduction process at a temperature above 750 °C. Subsequently, metallic Zn vaporized at over 900 °C under an inert atmosphere due to its low boiling point (908 °C), and only the porous carbon remained. As another representative Zn-based MOF, ZIF-8 ([Zn(2-methylimidazolate)2]n) has been often utilized as well.35 Further exploiting the conversion mechanism of Zn-based MOFs, some novel approaches for contributing porosity to the 2689

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Figure 12. Scheme and TEM images of the thermal conversion of [Ni2(EDTA)] into 3D mesoG. Reproduced with permission from ref 37. Copyright 2015 Royal Society of Chemistry.

eliminating Zn porogens with 10 °C/min ramping rate to 1000 °C for 1 h afforded the micropore-dominant carbon, while a slower ramping rate, a lower heating temperature and a longer retention time (5 °C/min, 750 °C for 6 h) induced a larger volume proportion of mesopores. In MOF thermolysis, heteroatoms (N, P, and/or S) in organic ligands cannot be prevented from being included in the transformed carbon, easily producing functional heteroatomdoped porous carbon, which has attracted considerable attention due to its high electrocatalytic activity. For instance, porous N-doped carbon was synthesized using IRMOF-3 composed of NH2-functionalized BDC ligands.41 Moreover, Fu et al. synthesized P/N-co-doped porous carbon via thermolysis of UiO-66-NH2 functionalized with P-containing precursors through a postsynthetic modification.42

4. CONCLUDING REMARKS AND FUTURE OUTLOOK Over the past decade, the various conversion routes of MOFs/ CPs have demonstrated the facile and novel synthesis of metalbased nanomaterials, porous carbons, and their composites. Focusing on the transformation mechanism, we summarized the research results that reveal the relationship between states of precursor MOFs and properties of resulting materials. First, the intrinsic nature of metal ions in MOFs is key in determining the phase of nanomaterials (including metal, metal oxide, metal sulfide, and metal phosphide), and then, organic ligands and synthetic conditions are regarded as the tools that extend the boundary of possible composition and morphology of resulting materials. On the other side, porous carbons can be obtained by eliminating metal parts with the two approaches, the metal evaporation during thermolysis and the postetching process. The graphiticity and porosity of porous carbon can be also tuned by the appropriate choice of synthetic conditions. In this context, the transformation of MOFs has been a significant synthetic method to afford unique functional nanomaterials.

Figure 13. (a) Correlation between the surface area of precursor MOFs and the surface area of the thermally converted carbon products. (b) Correlation between the Zn/C ratio of precursor MOFs and the surface area of the thermally converted carbon products. (red,19 green,38 purple,39 and blue markers40). (c) Schematic illustration of porosity-tuned porous N-doped carbons (PNCs) converted from a nonporous Zn-based CP under different thermolysis conditions. Reproduced with permission from ref 40. Copyright 2016 Royal Society of Chemistry.

conversion of a nonporous Zn-based CP (ZnCPazo-1) to hierarchically porous carbon with different porosity by controlling the thermolytic conditions such as ramping rate, reaction temperature, and retention time (Figure 13c).40 In this research, since metallic Zn species can act as porogens, we controlled their degree of agglomeration by varying the thermal conversion conditions, thereby tuning the porosity. Rapidly 2690

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Jae Hwa Lee is currently taking a combined M.S-Ph.D. program at UNIST. Sungeun Jeoung is a combined M.S-Ph.D. student at UNIST. Hoi Ri Moon received her Ph.D. degree in chemistry from Seoul National University in 2007, and worked at the Lawrence Berkeley National Laboratory from 2008 to 2009 as a postdoctoral fellow in Molecular Foundry. She then joined the faculty of UNIST where she is currently an associated professor of Department of Chemistry.

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ACKNOWLEDGMENTS We acknowledge financial support from the National Research Foundation (NRF) Grant funded by the Korean Government (MSIP) (No. 2016R1A5A1009405 and 2016R1D1A1B03933809). J.H.L. acknowledges the Global Ph.D. Fellowship (NRF-2013H1A2A1033501).

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Figure 14. Scheme of the recyclable redox-zincothermic reaction during the conversion of a Zn-based MOF and GeO2 to the Ge/ carbon composite. Reproduced with permission from ref 43. Copyright 2016 American Chemical Society.

usefulness of metallic Ge in battery systems has been well demonstrated, but pure-phase NPs are not easily synthesized by H2 gas or carbothermic reduction. Thus, we utilized metallic Zn, which evolved in situ during the thermolysis of a Zn-MOF, to reduce GeO2 particles. Interestingly, ZnO is also produced by the zincothermic reduction, but it was simultaneously reduced to metallic Zn via carbothermic reduction, which can then be used as the reducing agent. Thus, the repeated occurrence of the zincothermic and carbothermic reduction reactions promotes a recyclable redox-metallothermic reaction. After GeO2 is completely reduced, metallic Zn spontaneously vaporizes to yield Ge and porous carbon composites. This facile method can be successfully extended to other metal oxides including In2O3, Bi2O3, and SnO. Likewise, the knowledge of the conversion mechanism leads us to maximize the advantages of MOF conversion by designing systems that are more advanced in terms of composition and morphology. Despite the rapidly grown research progress, the MOF conversion still has challenging issues that should be solved. Especially, it lacks the precise control of the final properties of MOF derivatives, which allows customization of functional nanomaterials in practical applications. Therefore, future work should pay more attention to the mechanism investigations. Ultimately, by designing tailor-made MOFs, we will be able to synthesize target functional nanomaterials that overcome remaining obstacles in practice application system.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hoi Ri Moon: 0000-0002-6967-894X Notes

The authors declare no competing financial interest. Biographies Kyung Joo Lee received her Ph.D. degree at UNIST in 2017. 2691

DOI: 10.1021/acs.accounts.7b00259 Acc. Chem. Res. 2017, 50, 2684−2692

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DOI: 10.1021/acs.accounts.7b00259 Acc. Chem. Res. 2017, 50, 2684−2692