Metal Nanoparticles Covered with a MetalâOrganic Framework: From

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Metal Nanoparticles Covered with a Metal−Organic Framework: From One-Pot Synthetic Methods to Synergistic Energy Storage and Conversion Functions Hirokazu Kobayashi,*,†,‡ Yuko Mitsuka,§ and Hiroshi Kitagawa*,†,∥,⊥ †

Division of Chemistry, Graduate School of Science, Kyoto University, Kitashirakawa, Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Shoei Chemical Inc., 5-3 Aza-wakazakura Fujinoki-machi, Tosu-shi, Saga 841-0048, Japan ∥ Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan ⊥ INAMORI, Frontier Research Center, Kyushu University, Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan ‡

ABSTRACT: Hybrid materials composed of metal nanoparticles and metal−organic frameworks (MOFs) have attracted much attention in many applications, such as enhanced gas storage and catalytic, magnetic, and optical properties, because of the synergetic effects between the metal nanoparticles and MOFs. In this Forum Article, we describe our recent progress on novel synthetic methods to produce metal nanoparticles covered with a MOF (metal@MOF). We first present Pd@copper(II) 1,3,5-benzenetricarboxylate (HKUST-1) as a novel hydrogen-storage material. The HKUST-1 coating on Pd nanocrystals results in a remarkably enhanced hydrogenstorage capacity and speed in the Pd nanocrystals, originating from charge transfer from Pd nanocrystals to HKUST-1. Another material, Pd−Au@Zn(MeIM)2 (ZIF-8, where HMeIM = 2-methylimidazole), exhibits much different catalytic activity for alcohol oxidation compared with Pd−Au nanoparticles, indicating a design guideline for the development of composite catalysts with high selectivity. A composite material composed of Cu nanoparticles and Cr3F(H2O)2O{C6H3(CO2)3}2 (MIL-100-Cr) demonstrates higher catalytic activity for CO2 reduction into methanol than Cu/γAl2O3. We also present novel one-pot synthetic methods to produce composite materials including Pd/ZIF-8 and Ni@Ni2(dhtp) (MOF-74, where H4dhtp = 2,5-dihydroxyterephthalic acid). HKUST-1,11 and ZIF-8,12 using a solid grinding method followed by H2 reduction. The solution infiltration is reported as a convenient method to prepare nanoparticles immobilized in MOFs (Figure 1b). When the MOFs are soaked in the metal precursor solution, the precursors fill the MOF pores by capillary force, which is followed by H2 or NaBH4 reduction to obtain nanoparticles immobilized within the MOFs. This solution infiltration method has been employed for the synthesis of various nanoparticles/MOF composites such as Cu/MIL-101, Ag/HKUST-1,15,16 Au/MIL-101,23,24 Pd/MOF (MOF = MOF-5,43 HKUST-1,34 MIL-100,28 and MIL-10133), and Pt/MOF (MOF = MIL-10144−50), which were tested for their catalytic properties. Moreover, using this method, bimetallic nanoparticles such as Ni/Pt,31 Au/Ni,25 Au/Pd,26 and Pd/Cu29 have been successfully incorporated into MOFs. A chemical vapor deposition or chemical vapor infiltration (CVD or CVI) method has also been used to create nanoparticles/MOF composites.51−66 By this method, a precursor is loaded into porous MOFs in the vapor phase

1. INTRODUCTION Metal−organic frameworks (MOFs) or porous coordination polymers, consisting of metal ions connected by organic bridging ligands, have received much attention in a wide range of applications such as gas storage1−3 and separations,4,5 and optical, magnetic, and ion-conducting properties,6,7 owing to their high porosity and designability.8−10 Recently, multifunctional MOF composites, which include metal nanoparticles, have received increasing interest because of the possibility of enhancing gas storage and catalytic, magnetic, and optical properties, all of which have been attributed to the synergistic effects between the MOFs and metal nanoparticles. Various methods for synthesizing metal nanoparticle/MOF composites have been developed, and accordingly several types of composite structures have been reported that relate to the method of synthesis (Figure 1). The first attempts at loading a MOF with metal nanoparticles were performed by simply mixing the MOF and metal precursor via solid grinding11−13 or solution infiltration,14−50 and the metal nanoparticles/MOFs were fabricated by reducing the metal precursor. A solid grinding method is reported as a classical method to obtain metal nanoparticles deposited on a MOF (Figure 1a). Haruta and co-workers reported Au nanoparticles deposited on several porous MOFs, CPL-1,11 CPL-2,11 MIL-53(Al),11 MOF-5,11 © XXXX American Chemical Society

Special Issue: Metal-Organic Frameworks for Energy Applications Received: April 14, 2016

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DOI: 10.1021/acs.inorgchem.6b00911 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Representative structures of the composites consisted of metal nanoparticles and a MOF.

MOF copper(II) 1,3,5-benzenetricarboxylate (HKUST-1).71 Remarkably, the Pd nanocrystals covered by HKUST-1 have twice the storage capacity of the bare Pd nanocrystals. The composite material with a core/shell structure, where HKUST-1 is coated onto the surface of the Pd nanocrystals, was synthesized via a bottom-up approach. In a typical synthetic method, Pd nanocrystals with a flat surface were used as the core of the composite material.103 Pd nanocubes were first prepared in an aqueous solution by the reduction of Na2PdCl4 with ascorbic acid in the presence of bromide ions as the capping agents to form {100} facets, as reported previously. Following this, HKUST-1 coating onto the Pd nanocubes was performed by forming a suspension of Pd nanocubes in an ethanolic solution of Cu(NO3)2·3H2O and trimesic acid and stirring at room temperature for 48 h. To examine the structure of the composite material, the powder X-ray diffraction (PXRD) measurements with synchrotron radiation were performed at the BL02B2 beamline of SPring-8 with a wavelength of 1.000 Å. As shown in Figure 2a,

and is subsequently decomposed and/or reduced to obtain nanoparticles inside the MOF pores (Figure 1b); this method is advantageous because there is better control over the resultant nanoparticle size. Fischer and co-workers first employed MOF5 as a host framework and obtained Cu, Pd, and Au nanoparticles/MOF composites by using (η5-C5H5)Cu(PMe3), (η3-C3H5)Pd(η5-C5H5), and (CH3)Au(PMe3) as organometallic precursors, respectively.51 So far, various kinds of nanoparticles such as Pd-,51,53,59,61,62 Au-,51,52 Cu-,51,53 Mg-,55 Ni-,56,57 Pt-,63 and Ru/MOF64 composites have been obtained through the gas-phase method. Furthermore, bimetallic Ni/ Pd58 and Ru/Pt nanoparticles/MOFs66 were also successfully created by simultaneous gas-phase loading of the corresponding alloy precursors. Recently, nanoparticles/MOF materials have also been prepared through encapsulation of presynthesized nanoparticles by growing the MOF around them (Figure 1c).67−70 The functional cavities of the MOF remain unimpeded, and the composite materials can provide a synergistic function that is derived from both the nanoparticles and MOFs. The core/shell structure composed of a nanoparticle core and a MOF shell has received increasing attention for the development of gas storage71 and magnetic,72−85 optical,67,72,86−91 and catalytic92−102 materials. Because the MOFs work as molecular sieves, the core/shell composites are expected to be effective catalysts with high selectivity and activity, as well as efficient molecular sensing materials. In addition, if the MOF can act as a partition between nanoparticles, it is also expected to prevent the sintering of nanoparticles during the reaction, resulting in high durability. The interfacial region between MOFs and metal nanocrystals also plays an important role for combining the full potential functionalities of each component. In this Forum Article, we describe our recent progress in the development of metal nanoparticles covered with a MOF (metal@MOF) for hydrogen-storage or catalytic materials. Novel synthetic methods for the composite materials are also presented.

2. HYDROGEN STORAGE IN PD@HKUST-1 Hydrogen is an essential component in many industrial processes and also a potential clean energy source. As a hydrogen-storage material, the metal/MOF composite is a promising candidate. In particular, Pd/MOF composites have received much attention because Pd can absorb large amounts of hydrogen at ambient temperature and pressure. For example, Pd/SNU-3(Zn)14 and Pd/MIL-100(Al)28 have been reported to exhibit an enhanced hydrogen-storage uptake because of the spillover effect (the diffusion of H atoms dissociated on the Pd surface into the MOF), although the total hydrogen-storage capacity of the Pd did not change. In this section, we demonstrate the first example of altering the hydrogen-storage properties of Pd nanocrystals by coverage with the porous

Figure 2. (a) PXRD pattern and TEM images of (b) Pd nanocrystals and (c) a Pd@HKUST-1 composite material. The radiation wavelength was 1.000 Å. Reprinted with permission from ref 71. Copyright 2014 Nature Publishing Group. B


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Figure 4. PC isotherms of Pd nanocrystals and Pd@HKUST-1 at 303 K. Reprinted with permission from ref 71. Copyright 2014 Nature Publishing Group.

storage capability of Pd nanocrystals is doubly enhanced by the HKUST-1 covering. The hydrogen-storage capacity of Pd is known to correlate with the electronic states of Pd.104 For Pd@HKUST-1, the change in the electronic structure due to the HKUST-1 coating on the surface of Pd was examined by X-ray photoelectron spectroscopy (XPS) measurements. As shown in Figure 5, the

Figure 3. (a) HAADF-STEM image and (b−d) EDX mappings of Pd@HKUST-1: (b) Cu element; (c) Pd element; (d) an overlay of the Pd and Cu elements. Reprinted with permission from ref 71. Copyright 2014 Nature Publishing Group.

Pd and Cu STEM−EDX maps as the main constituent metals of Pd nanocubes and HKUST-1, respectively. An overlay of the Pd and Cu maps shown in Figure 3d revealed that the Cu elements of HKUST-1 are distributed around the surface of the Pd nanocubes. From these results, it is concluded that we successfully synthesized the composite material of Pd nanocrystals coated with HKUST-1. We then investigated the hydrogen-storage capacity of Pd@ HKUST-1 by measurement of the hydrogen pressure− composition (PC) isotherms at 303 K using an automatic PC isotherm apparatus (BELSORP-max, MicrotracBEL Corp.). Figure 4 shows the results of PC isotherms for Pd nanocrystals and Pd@HKUST-1. The logarithm of the hydrogen pressure is plotted against the hydrogen concentration (H/Pd). Both materials absorbed hydrogen with increasing hydrogen pressure, but the hydrogen concentration at 101.3 kPa was 0.5 H/Pd in bare Pd nanocubes, while the H/Pd in Pd@ HKUST-1 was 0.87. Pd@HKUST-1 absorbs nearly double the amount of hydrogen of bare Pd nanocubes. Taking into consideration that pure HKUST-1 adsorbs nearly zero H2 at 303 K (Figure 4, inset), the results indicate that the hydrogen-

Figure 5. XPS spectra of (a) Cu 2p and (b) Pd 3d for Pd nanocrystals, HKUST-1, and Pd@HKUST-1. Reprinted with permission from ref 71. Copyright 2014 Nature Publishing Group.

binding energy of Cu 2p originating from HKUST-1 is shifted to lower energy by coverage of the surface of the Pd nanocrystals. On the other hand, the binding energy of Pd 3d is shifted to higher energy. The XPS spectra of Pd@ HKUST-1 suggest that the electronic states of Pd@HKUST-1 are different from those of bare Pd nanocubes and pure HKUST-1 and that charge transfer occurs from the Pd nanocubes to HKUST-1 in the composite. In order to discuss the relationship between the significantly enhanced hydrogen-storage capacity and the electronic state of Pd@HKUST-1, we first examined the electronic states of Pd and palladium hydride (PdH). Figure 6 shows the calculated total densities of states (DOS) of pure Pd and PdH.104 As shown in Figure 6a, the DOS of Pd around the Fermi level EF is composed of the 5s band with a broad bandwidth and the 4d band with a narrow bandwidth. The total number of valenceC


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Figure 6. Electronic DOSs of Pd and PdH around the Fermi level. Reprinted with permission from ref 104. Copyright 1978 American Physical Society.

band electrons around EF is 10, the same as the number of valence electrons of a Pd atom. When hydrogen is absorbed inside the Pd lattice to form chemical bonds, Pd−H bonding and Pd−H antibonding states are formed between nonbonding states of Pd (Figure 6b). Then, the 4d conduction bands of Pd are filled up by the addition of a 1s electron of the H atom, and the energy level at EF apparently rises. Further hydrogen absorption in Pd causes destabilization of the electronic states of PdH because the 1s electron of the H atom begins to occupy the 5s band, which has a low DOS. Therefore, the hydrogen concentration of Pd strongly depends on the number of holes in the 4d band. In other words, it is possible to control the hydrogen concentration by means of alloying, which is based on the change in the number of Pd 4d band holes.105 For examples, Pd−Ag106−108 and Pd−Rh106,109 are classical hydrogen absorption alloys. In the Pd−Ag−H system, by the addition of Ag into Pd, the number of available 4d band holes decreases (Figure 7a) because Ag has one more electron than Pd. As a result, the hydrogen-storage capacity of Pd−Ag alloys is decreased with increasing Ag content.106−108 On the other hand, in the Pd−Rh−H system, the number of available 4d band holes increases because Rh has one less electron than Pd (Figure 7b). Therefore, the hydrogen-storage capacity of Pd− Rh alloys increases with increasing Rh content.106,109 Thus, the hydrogen-storage capacity of Pd-based hydrogen-storage alloys can be controlled on the basis of the band filling of Pd. In our case, the enhanced hydrogen-storage capacity of Pd nanocrystals in Pd@HKUST-1 originates from the increase in the available 4d band holes of Pd due to charge transfer from the Pd nanocubes to HKUST-1 in Pd@HKUST-1. The development of hydrogen-storage alloys has typically been based on alloying. These results demonstrate the first example to control the hydrogen-storage properties of metal nanocrystals by coating a MOF on the metal surface. These findings suggest that a MOF coating can alter not only the surface/bulk reactivity of the nanocrystals but also the physical properties, such as the magnetic or optical properties.

Figure 7. Electronic DOSs for (a) Pd−Ag112 and (b) Pd−Rh113 obtained by band-structure calculations. Reprinted with permission from refs 112 and 113. Copyright 1991 and 2006 American Physical Society.

catalysis, 92−102,110,111 and these materials often exhibit enhanced catalytic properties because of synergistic effects, especially when metal nanoparticles are encapsulated within the MOFs. Furthermore, tuning the pore size of the MOF can allow the MOF to act as a selective sieve for reactants, potentially providing high catalytic selectivity. An MOF can be used as a well-defined and tunable support for metal nanoparticles, which distinguishes MOFs from other traditional supports for heterogeneous catalysis. These catalytic studies can be mainly divided into two categories according to the type of reaction: liquid- and gas-phase reactions. In this section, we first introduce Pd−Au@Zn(MeIM)2 (ZIF-8, where HMeIM = 2methylimidazole) as a liquid-phase reaction catalyst, which was the first report of a bimetallic nanoparticles@MOF catalyst.92 Cu nanoparticles/Cr3F(H2O)2O{C6H3(CO2)3}2 (MIL-100Cr) is subsequently presented as an example of a gas-phase reaction catalyst. Pd−Au bimetallic nanoparticles with a mean diameter of 4.6 nm were synthesized in an aqueous solution by reducing HAuCl4 under H2 gas in the presence of Pd nanoparticles, as reported previously. The ZIF-8 coating on the surface of the bimetallic nanoparticles was performed by adding Zn(NO3)2· 6H2O and 2-methylimidazole as ZIF-8 precursors in the presence of the Pd−Au nanoparticles. The diffraction pattern of the composite material consisted of two kinds of Pd−Au and ZIF-8 diffractions. The positions of the Pd−Au diffraction peaks were located between those of pure Pd and Au. The lattice constant of the Pd−Au bimetallic nanoparticles estimated from the Rietveld refinement followed Vegard’s law. These results suggest that the obtained Pd−Au nanoparticles form solid−solution alloys in which Pd and Au are homogeneously mixed at the atomic level. Figure 8a shows

3. PD−AU@ZIF-8 FOR AEROBIC ALCOHOL OXIDATION AND CU/MIL-100 FOR CO2 REDUCTION The composite materials composed of metal nanoparticles and MOFs have been particularly developed for the field of D


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We also focused on Cu/MOF composites as gas-phase reaction catalysts for methanol synthesis from CO2 and hydrogen. MIL-100-Cr with superior thermal, water, and chemical stability was selected as the MOF. MIL-100-Cr was prepared by a previously reported hydrothermal synthesis.114 The composite materials were synthesized by thermal decomposition of copper acethylacetonate [Cu(acac)2] as a Cu precursor in the presence of MIL-100-Cr. In a typical synthesis, MIL-100-Cr and Cu(acac)2 were dispersed in an acetone solution, and the mixture was stirred at room temperature for 1 h. After impregnation, the solid was collected by centrifugation. The solid including MIL-100-Cr and Cu(acac)2 was then heated at 350 °C for 1 h under vacuum to produce a composite of MIL-100-Cr and Cu nanoparticles (Cu/MIL-100-Cr). A γ-Al2O3-supported Cu catalyst (Cu/γAl2O3) was also prepared by the same method to compare its catalytic activity with that of the Cu/MIL-100-Cr catalyst. As shown in Figure 9a, the diffraction pattern of the composite material is composed of overlapping patterns of

Figure 8. (a) HAADF-STEM image and (b−f) EDX mappings of Pd− Au@ZIF-8: (b) Zn element; (c) Pd element; (d) Au element; (e) an overlay of the Pd and Au elements, and (f) an overlay of the Pd, Au, and Zn elements. Reprinted with permission from ref 92. Copyright 2014 Wiley-VCH.

a HAADF-STEM image. Parts b−d of Figure 8 correspond to Zn, Pd, and Au STEM−EDX maps as the main constituent metals of ZIF-8, Pd, and Au, respectively. Figure 8b reveals that ZIF-8 has a hexagonal shape with an approximate size of 250 nm. From an overlay of the Pd and Au maps shown in Figure 8e, it was demonstrated that Pd and Au atoms randomly are distributed over the whole particle to form a solid−solution structure, which is consistent with the results of the X-ray diffraction (XRD) pattern. In addition, no surface solid− solution Pd−Au nanoparticles are observed, and the nanoparticles are perfectly covered with ZIF-8 (Figure 8f). In order to investigate the synergistic catalytic activity created by hybridization of the Pd−Au nanoparticles and ZIF-8, we tested the activity of the Pd−Au nanoparticles and Pd−Au@ ZIF-8 for aqueous alcohol oxidation. The catalytic oxidations of cyclopentanol to ketone for Pd−Au nanoparticles and Pd− Au@ZIF-8 were 82.2 and 1.9%, respectively, and the ZIF-8 coating on the surface of Pd−Au nanoparticles resulted in lower conversion. Considering that the ZIF-8 pore is a very hydrophobic because of the existence of the methyl group, poor conversion of Pd−Au@ZIF-8 may originate from the inhibition of access hydrophilic reductant onto Pd−Au catalysts. The decrease in the catalytic activity of Pd−Au nanoparticles by the ZIF-8 coating indicates a design guideline for the development of composite catalysts with high selectivity by choosing an appropriate MOF according to the target reaction.

Figure 9. (a) PXRD pattern and (b) N2 sorption isotherms at 77 K for MIL-100-Cr (black) and Cu/MIL-100-Cr (red). TEM images of (c) Cu/MIL-100-Cr and (d) Cu/γ-Al2O3.

diffraction from Cu and MIL-100-Cr. The N2 sorption isotherms indicated a decrease in the total volume for Cu/ MIL-100-Cr associated with the formation of Cu nanoparticles (Figure 9b). From the TEM images of Cu/MIL-100-Cr (Figure 9c), the mean diameter of Cu nanoparticles was estimated to be ca. 45 nm. On the other hand, the mean diameter of Cu nanoparticles in Cu/γ-Al2O3 was ca. 30 nm (Figure 9d). To investigate the catalytic activity of the composite material for CO2 reduction into methanol, we carried out activity tests using a fixed-bed flow reactor. The catalytic test was performed using a gas mixture of H2 (72%), CO2 (14%), and He (14%) with 140 mL/min at 220 °C. The produced methanol was analyzed by flame ionization gas chromatography. It is known that Cu shows almost no activity for the conversion of CO2 to methanol. In our case, Cu/γ-Al2O3 produced an extremely small amount of methanol (0.2 μmol/gcat·h). Interestingly, the Cu/MIL-100-Cr catalyst produced 2.0 μmol/gcat·h of methanol, and the catalytic activity of Cu/MIL-100-Cr was 10 times higher than that of Cu/γ-Al2O3. Considering that the mean E


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Figure 10. Representative synthesis methods for metal nanoparticles/MOF composites.

diameter of Cu nanoparticles in Cu/MIL-100-Cr is larger than that in Cu/γ-Al2O3, the significantly enhanced catalytic activity for the synthesis of methanol may originate from the synergistic effect at the interface between Cu nanoparticles and MIL-100Cr. The detailed mechanism is currently investigated.

4. ONE-STEP SYNTHETIC METHODS FOR METAL/MOF: PARTIAL THERMAL DECOMPOSITION OF MOF AND MICROWAVE (MW) SYNTHESIS The synthetic methods for metal nanoparticle/MOF composites are mainly classified into two types; one is hybridization with metal nanoparticles after MOFs were preprepared as templates (Figure 10a) and the other is hybridization with a MOF on preprepared metal nanoparticles (Figure 10b). The preparation methods reported to date for metal/MOF nanomaterials have required at least a two-step reaction, so the further development of facile hybridization methods of metal and MOFs is essential. In this section, we introduce two examples of one-step synthetic methods for metal nanoparticles/MOF including a MW irradiation technique (Figure 10c). As a first demonstration, we describe a one-pot synthesis of composites composed of Pd nanoparticles and ZIF-8 by the MW method. Zn(NO 3)2·6H2O, 2-methylimidazole, and palladium acethylacetonate [Pd(acac)2] were dissolved in N,N-dimethylformamide. The reaction solution was heated at 180 °C for 1 h under MW irradiation using Biotage Initiator+. As shown in Figure 11a, the XRD patterns reveal that the sample synthesized by the MW method (ZIF-8-MW) has the same structure as that of a commercially available ZIF-8, Basolite Z1200. Furthermore, the sample (Pd/ZIF-8-MW) synthesized by the same synthetic method as that of ZIF-8MW, except for the addition of Pd(acac)2, displays an XRD pattern composed of the diffraction peaks of ZIF-8 and Pd. IR spectra confirm that the absorption bands of ZIF-8-MW and Pd/ZIF-8-MW are consistent with those of ZIF-8 (Figure 11b). The IR spectrum of Pd/ZIF-8-MW also suggests that bare Pd without any protecting agent hybridizes with ZIF-8. N2

Figure 11. (a) XRD pattern, (b) IR spectra, and (c) N2 sorption isotherms at 77 K for ZIF-8 (black), ZIF-8-MW (blue), and Pd/ZIF8-MW (red).

sorption isotherms were measured at 77 K for ZIF-8, ZIF-8MW, and Pd/ZIF-8-MW, respectively (Figure 11c). Before the measurement of N2 gas sorption, each sample was activated at 150 °C for 12 h. For ZIF-8-MW and Pd/ZIF-8-MW, similar typical type I sorption behaviors originating from the microporosity were observed as expected for ZIF-8. The slight decrease in the total volume for Pd/ZIF-8-MW indicates hybridization with Pd nanoparticles. We further investigated the composite state of Pd/ZIF-8MW using microscopy. Figure 12a shows a bright-field STEM (BF-STEM) image of Pd/ZIF-8-MW. From Figure 12a, the mean diameter of Pd nanoparticles was estimated to be 14.5 ± F


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Figure 12. (a) HAADF-STEM image and (b−d) EDX mappings of Pd/ZIF-8-MW: (b) C element; (c) Zn element; (d) Pd element.

3.1 nm. Parts b−d of Figure 12 reveal that the Pd nanoparticles are distributed around ZIF-8. The MW-assisted synthesis is a low-cost, rapid, and scalable method. This MW technique is expected to be a powerful tool for the one-pot synthesis of various metal nanoparticles/MOF materials in the future. Finally, we introduce a method to generate nanoparticles in situ through partial thermal decomposition of a MOF as an example of a novel one-pot hybridization method for metal@ MOF (Figure 13).115 We used Ni-MOF-74, Ni2(dhtp) (H4dhtp

Figure 14. (a) PXRD patterns of Ni-MOF-74 (black line), 300-12h (green line), 350-12h (blue line), and 400-12h (red line). The radiation wavelength was 9.98 Å. (b) Rietveld analysis for 350-12h. Reprinted with permission from ref 115. Copyright 2015 Royal Society of Chemistry.

The lattice constants estimated from Rietveld refinement of the PXRD pattern for sample 350-12h (Figure 14b) were consistent with those of each component, MOF-74 and Ni nanoparticles.115 The crystal size of Ni was estimated to be ca. 5 nm. To investigate the composite state of Ni nanoparticles and MOF-74 in sample 350-12h, we performed measurements of HAADF-STEM and EDX analysis. As shown in Figure 15a, Ni nanoparticles were highly dispersed all over the MOF. The mean diameter was estimated to be ca. 4 nm, which is in agreement with the crystal size from the diffraction pattern. From the high-resolution TEM image, Ni-MOF-74 in sample 350-12h exhibited well-defined crystalline lattice fringes, and the lattice spacing was estimated to be 11 Å, which corresponds to the Ni-MOF-74 (2−10) lattice plane (Figure 15b). The mapping data shown in Figure 15d−f demonstrate that the Ni nanoparticles were distributed within Ni-MOF-74. Here, we explain a possible mechanism of Ni@MOF-74 via partial thermal decomposition of MOF-74 (Figure 16). For generation of the Ni nanoparticles within Ni-MOF-74, it seems that the redox activity of the 2,5-dihydroxyterephthalate (dhtp) ligand is essential for reducing Ni2+ to generate Ni0 nanoparticles. This is supported by previous reports showing that for quinine-based metal complexes there is significant electron transfer from the ligand to the metal ion.116 From this, it is reasonable to consider that, for Ni-MOF-74 undergoing partial thermal decomposition, the dhtp ligand reduces the Ni2+ centers, which is accompanied by cleavage of the Ni−O bond. This electron transfer would then generate a semiquinone-like radical, which would promote further reduction of

Figure 13. Scheme for the synthesis of Ni@MOF via partial thermal decomposition of a MOF. Reprinted with permission from ref 115. Copyright 2015 Royal Society of Chemistry.

= 2,5-dihydroxyterephthalic acid), with large, cylindrical, onedimensional pores (diameter of 11 Å) to create a hybrid material containing Ni nanoparticles. Ni-MOF-74 was heated at 250−350 °C under vacuum to obtain composites, Ni nanoparticles contained within MOF-74 (Ni@MOF-74). Figure 14a shows the PXRD patterns of Ni-MOF-74 heated at 300, 350, and 400 °C for 12 h under vacuum, respectively (300-12h, 350-12h, and 400-12h). The sample 300-12h shows a diffraction pattern identical with that of Ni-MOF-74. With increasing temperature, broad diffraction peaks originating from a face-centered-cubic (fcc) Ni lattice appeared in addition to the Ni-MOF-74 pattern (350-12h). The higher heat treatment temperature at 400 °C (400-12h) gave rise to complete decomposition of Ni-MOF-74, and only a fcc Ni pattern was observed. G


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reduction, and electrochemical reduction have been reported, but the synthesis of several nanometer-scale-sized Ni is still challenging owing to the tendency of Ni nanoparticles of this size to easily aggregate and oxidize. The new findings provide a facile synthetic method not only for highly dispersed Ni@MOF hybrid materials but also for several nanometer-scale-sized Ni. Actually, the size of the Ni nanoparticles is precisely controllable at the nanometer level by changing the conditions of the heat treatments, such as temperature and/or time.115 This approach is also expected as a useful method for the creation of various kinds of metal nanoparticle composites because a MOF-74 analogue, including Mg, Mn, Fe, Co, or Zn as the metal cation, exists. The further development of multifunctional materials is expected via this method of thermal decomposition of MOFs.

5. CONCLUSION Over the years, significant progress has been made in the combination of metal nanoparticles and MOF materials to form composite materials. In this Forum Article, we introduced novel synthetic methods for producing the composite materials as well as some applications, including hydrogen storage and catalysis. These types of hybrid materials are still in an early phase of development, so the origin of the enhanced properties associated with the synergistic effect between the nanoparticles and MOF and the mechanism of these interactions still remains unclear. In the future, by clarifying the origins or mechanisms through systematic studies, we expect that the targeted design of nanoparticles and MOF will be possible, leading to the creation of a variety of useful hybrid materials.

Figure 15. (a) TEM image of 350-12h with electron diffraction (inset) and (b) the high-magnification image. (c) HAADF-STEM image, (d) C−K STEM−EDX map, and (e) Ni−K STEM-EDX map of 350-12h. (f) Reconstructed overlay image of the maps shown in parts d and e. Reprinted with permission from ref 115. Copyright 2015 Royal Society of Chemistry.

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

2+

Ni and ultimately generate a more stable quinone (Figure 16). These multistep redox reactions are a reasonable mechanism for the generation of Ni0 nanoparticles in NiMOF-74 during partial thermal decomposition. Ni is a 3d transition metal and has been studied in various fields, especially for magnetic or catalytic applications, which require nanosized Ni. With respect to the synthesis of Ni nanoparticles, various methods such as sonochemical or thermal decomposition of organometallic precursors, chemical

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Core Research for Evolutional Science and Technology from JST and a Grant-in-Aid for Scientific Research (B) (Grant 15H03784) from JSPS. The

Figure 16. (a) Structure of MOF-74. (b) Possible mechanism of Ni@MOF-74 via partial thermal decomposition of MOF-74. H


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authors are grateful to our collaborators who have assisted with this work, especially Dr. Guangqin Li, Dr. Jared M. Taylor, Prof. Ryuichi Ikeda, Prof. Yoshiki Kubota, Dr. Kenichi Kato, Prof. Masaki Takata, Tomokazu Yamamoto, Dr. Shoichi Toh, Prof. Syo Matsumura, Megumi Mukoyoshi, Dr. Kohei Kusada, Dr. Mikihiro Hayashi, Prof. Teppei Yamada, Prof. Mitsuhiko Maesato, Christoph Rösler, Dr. Daniel Esken, Dr. Christian Wiktor, and Prof. Roland A. Fischer.

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