Hybrid Materials
Hybrid Crystals Comprising Metal–Organic Frameworks and Functional Particles: Synthesis and Applications Shaozhou Li* and Fengwei Huo*
Hybrid crystals containing encapsulated functional species exhibit promising novel physical and chemical properties. The realization of many properties critically depends on the selection of suitable functional species for incorporation, the rational control of the crystallinity of the host materials, and the manipulation of the distribution of the encapsulated species; only a few hybrid crystals achieve this. Here, a novel synthetic method enables the encapsulation of functional species within crystalline metal–organic frameworks (MOFs). Various kinds of single-crystalline MOFs with incorporated particles are presented. The encapsulated particles can be distributed in a controllable manner, and the hybrid crystals are applied to the heterogeneous catalysis of the reduction of nitroarenes. These findings suggest a general approach for the construction of MOF materials with potential applications; by combining species and MOFs with suitable functionalities, new properties—not possible by other means—may arise.
1. Introduction The incorporation of functional species such as inorganic particles into host crystals at the synthetic level is an attractive way to design new systems that possess novel properties— originating either from the collective interactions between the particles and host materials or from the combination of their pristine properties.[1–10] Previously reported structures of this type have exhibited technological implications in areas ranging from heterogeneous catalysis,[1] biomedical devices[2] to energy conversion and storage.[3] In many of the potential applications, it is essential to be able to control the S. Z. Li Institute of Advanced Materials (IAM) Nanjing University of Posts & Telecommunications 9 Wenyuan Road, Nanjing, China E-mail: [email protected] Prof. F. Huo School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue Singapore 639798, Singapore E-mail: [email protected] DOI: 10.1002/smll.201303564 small 2014, DOI: 10.1002/smll.201303564
shell crystallinity and the distribution of the incorporated particles.[1–4] The synthetic strategies for these systems generally involve either the nucleation and growth of crystals on a chemically modified surface of the incorporated particles[5] or epitaxial growth, which requires an appropriate choice of lattice-matched host materials.[6] When the former strategy is utilized, a non-uniform and polycrystalline shell usually forms due to the multiple nucleation sites on the chemically modified particle surface.[5,10] Although the epitaxialgrowth strategy can realize the formation of single-crystalline core–shell hybrid nanostructures, this strategy requires lattice-matching between the core and shell materials, and it is limited to the formation of single-core structures.[6a] This dilemma prevents the widespread practical applications of these hybrid materials. Metal-organic frameworks (MOFs) are a class of crystalline polymers with the permanent porosity. Due to their unique properties, which include high surface area, adaptable surface chemistry, tunable pore-size, and tunable structures, MOFs have a wide range of potential applications in the separation, capturing, storing, sensing, and catalysis of molecules.[11] The potential applications of MOFs can be developed further and extended by incorporating various functional species within the frameworks.[12] Recently, a controlled encapsulation method has been developed by
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Scheme 1. Schematic diagram for the encapsulation of the functional species, such as inorganic particles, into single-crystalline MOFs.
our group to incorporate species into the zeolitic imidazolate framework 8 (ZIF-8), a well-known MOF.[7] Herein, we demonstrate an improved method to form hybrid MOF single-crystals; this new method enables better control of the location of the incorporated particles, and a diverse set of MOFs can be used. In contrast with the MOF hybrid structures synthesized by the methods previously reported by others or by our group,[4,7–10,13] this strategy is not restricted by the synthetic route of the incorporated particles, and it is avoids the undesired deposition of particles on the outer surface of the MOFs. Unlike the other methods, which are usually restricted to one kind of MOF, this method is applicable to many MOFs. More importantly, the crystallinity of the MOF shells are not compromised by the spatial control of the incorporated particles. This approach allows a wide range of species and MOFs to be combined into the hybrid crystals, thus shedding light on the development of new functional hybrid materials.
MOF hybrid composites have indeed been produced by this method, i.e., AuNP@ZIF-7 and AuNP@ZIF-8 with Au NPs and PtNP@MIL-53 and PtNP@MIL-101_NH2 with Pt NPs (MIL stands for materials from the Institut Lavoisier). The ZIF-7 and ZIF-8 hybrid crystals exhibit rhombic dodecahedral shapes with edge-to-edge lengths of 200 nm and 1 µm, respectively. The MIL-53 and MIL-101_NH2 hybrid crystals are octahedral with crystal sizes of around 300 nm.
2.2. The Mechanistic Investigation on the Incorporation Process The AuNP@ZIF-8 hybrid crystal was employed as the model system to demonstrate the rational synthesis of the NP@MOF crystals. As has been pointed out, the nucleation kinetics of ZIF-8 is affected by the interactions between the polymers and the pre-nucleation clusters. Because the clusters formed
2. Results and Discussion 2.1. Synthesis of MOF Hybrid Crystals The encapsulation strategy is illustrated in Scheme 1. Briefly, nanoparticles (NPs) were coated with suitable polymers before being added to a growth solution containing pre-nucleation clusters, which were formed by the coordination of metal ions and organic ligands. The clusters were trapped and densificated on the polymer surface to activate the nucleation and growth of MOFs, resulting in the formation of the MOF hybrid crystals (denoted as NP@MOF). The encapsulation process greatly relied on the interplay between the clusters and polymers involved rather than the crystallographic structures or chemical properties of the encapsulated NPs. Therefore, this method was expected to be applicable to different kinds of core particles and to a diverse set of MOF crystals, regardless of their synthetic routes or intrinsic properties. As shown in Figure 1, various kinds of single-crystalline
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Figure 1. Transmission electron microscopy (TEM; a,b,d,e,g,h,j,k) and field-effect scanning electron microscopy (FESEM; c,f,i,l) images of the MOF hybrid crystals. a–c) AuNP@ZIF-7 crystals, d–f) AuNP@ZIF-8 crystals, g–i) PtNP@MIL-53 crystals, and j–l) PtNP@MIL-101_ NH2 crystals. b,e,h,k) Enlarged TEM images of a single NP@MOF crystal. The sizes of the incorporated Au- and PtNPs are 13 and 3 nm, respectively. The scale bars are 200 nm (a,h,k), 100 nm (b), 1 µm for (c,d,f,i,l); and 500 nm for (e,g,j). © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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by the complexation of zinc ions and 2-methylimidazole in the ZIF-8 growth solution are positively charged while the polymer-coated Au NPs have negative zeta-potentials (see Supporting Information (SI): Figure S2),[14] it is possible that—similar to the deposition of MOFs on a chemically modified flat surface[15]—electrostatic attractions between the polymer-modified NPs and the MOF clusters might be the one dominating factor for inducing the formation of the NP@ MOF crystals. To test this, Au NPs with different surface functionalizations, namely, 11-mercapto-1-undecanol (11-MPA), polyethylene glycol (PEG, weight-averaged molecular weight (Mw) in Daltons = 3350), and PEG (Mw = 40 000) were employed to synthesize AuNP@ZIF-8 crystals. All three coated NPs have exposed –OH groups and show similar zeta-potentials (SI: Figure S2). However, incorporation only occurred for the PEG-coated Au NPs (SI: Figure S3), indicating that having a negatively charge surface alone is not sufficient for ensuring the incorporation of NPs into the MOF crystals under this condition. On the other hand, the
nucleation kinetics of the crystals can be altered by the surface morphology of the polymers owing to the confinement effect.[16] The polymer chains can decrease the diffusivity of the solute or clusters within, thus promoting the nucleation and growth of crystals.[16b,17] It is interesting to note that when the NPs were coated by highly polymerized polymers such as PEG (Mw = 40 000), pluronic F-127 (Mw = 12 500), and polyvinylpyrrolidone (PVP, Mw = 29 000), the addition of these NPs into the growth solution leads to the formation of NP@MOF crystals (SI: Figure S3, S4). To further study the effect of the polymer on the encapsulation process, PVP molecules with different degrees of polymerization (Mw = 3500, 10 000, 29 000, 55 000, and 1 300 000) were used. Due to their similar structures and chemistry, it is expect that the weight difference (or chain length) can affect the thickness of the PVP shells formed on the NP surface. Dynamic light scattering (DLS) show that the hydrodynamic diameters (HD) of the Au NPs with different PVPs varied as the polymer weight varied (Figure 2a). Because all pristine
Figure 2. The encapsulation process is affected by the polymer shell on the particle surface. a) The HD measurements of the Au NPs coated with the different PVPs in methanol. K signifies ×1000. Inset shows the HD. b) The distribution of the NPs in the MOF can be tuned by the PVP concentration. The PVP concentration in the growth solutions were adjusted to 10−5 (sample 1), 5 × 10−4 (sample 2), 2 × 10−3 (sample 3), 10−3 (sample 4), and 2 × 10−2 (sample 5) wt%. Top right inset shows how the crystal size and the size of the NP core are defined. Other insets show the corresponding TEM images of a particle with the NP core indicated by a red circle. c,d) TEM images showing the spatial controllability of the distribution of the encapsulated Au NPs inside the MOF hybrid crystal. For these two samples, the Au NP solution was added at 0 (c) or 5 (d) min into reaction. The scale bars in (b) are all 500 nm. small 2014, DOI: 10.1002/smll.201303564
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of these clusters results in the formation of the amorphous precursor phase, and thirdly, the transformation of the amorphous phase to crystals is driven by chemical thermodynamics.[19a,20] Because the process is directed by the thermodynamic transition of the amorphous precursor phase to crystalline materials and because the physical and chemical properties of the encapsulated NPs have a minor effect on this transition, the synthesis of NP@ ZIF-8 crystals benefits from this unique crystallization behavior of ZIF-8. Without the lattice-matching constraint, which is a challenging requirement for the design and synthesis of hybrid crystals by the epitaxial method,[6a] the single-crystallline NP@MOF crystals can be synthesized easily. The obtained crystals are highly Figure 3. TEM images of the encapsulation results for PVP-coated Au NPs, where PVP of a) crystalline with a low defect density, Mw = 3500, b) 10 000, c) 29 000, d) 55 000, and e) 1 300 000 is used. The scale bars are and they exhibit good molecular-sieve all 1 µm. properties. This strategy also shows the merit Au NPs were synthesized at the same time, this difference of being able to control the positions of the encapsulated directly correlated with the thickness of the PVP shells. Ini- NPs inside the MOF crystals. The spatial distribution of the tially, the thickness of the PVP shells increased along with encapsulated NPs in the MOF crystals can be controlled by the increase in its weight. When the molecule becomes too adjusting the PVP concentration in the growth solution from large (Mw = 1 300 000), the decreased PVP solubility results 10−5 wt% (sample 1) to 2 × 10−2 wt% (sample 5). The size of in the formation of thin polymer shells. The thicker PVP the Au NP core (d) becomes larger as the PVP concentration shells lead to faster nucleation and growth of the AuNP@ increases (d increases from ∼220 nm to ∼1 µm from sample ZIF-8 crystals (SI: Figure S5, S6), while a thin PVP shell not 1 to sample 5. Figure 2b and 4.) Considering that the sizes only leads to a slower nucleation and growth rate, but it also of the AuNP@ZIF-8 crystals for these 5 samples were almost decreases the encapsulation quality of the NP in the hybrid the same (D = ∼1 µm for all samples), and that the amounts crystals (Figure 3). For example, when the PVP with Mw = of the added Au NPs were also the same, a larger d implies 3500 is used to coat the Au NPs, the coated NPs exhibits the a better spatial distribution of the NPs in the MOF hybrid smallest HD among the 5 different of PVP-coated NPs; when crystals. Further raising the PVP concentration results in they were employed for the synthesis of NP@MOF crys- the failure of the encapsulations (SI: Figure S8). In addition tals, the encapsulated Au NPs aggregated as indicated by a large surface plasmon resonance (SPR) band shift on the UV– vis absorption spectra (SI: Figure S7).[18] Moreover, ZIF-8 crystals without any NPs inside were also observed in the final products (Figure 3a). The formation of ZIF-8 crystal has been shown to occur via a non-classical nucleation and crystallization pathway.[19] Under certain circumstances, such behavior is general to many crystal systems.[20] Among them, the biomineralization of calcite and apatite in organisms has been well studied.[21] By mimicking the biomineralization process of calcite, many artificial calcium-based hybrid materials with different structures incorporated have been reported.[22] The growth of ZIF-8 exhibits many similarities with these biomineral materials. First, pre-nucleation clusters Figure 4. TEM images of the sample a) 1, b) 2, c) 3, d) 4, and e) 5, where the PVP concentration are involved. Second, the aggregation increases going from sample 1 to 5. The scale bars are 500 nm.
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Figure 5. TEM images of the AuNP@ZIF-8 crystals formed as a result of different addition times of the Au NP solution: a) 0, b) 10, c) 20, and d) 30 min.
to controlling the distribution of the encapsulated NPs, their positions can be tuned by varying the addition time of the NP solution (Figure 5). When the NP solution was added at the beginning of the reaction, clusters formed by the complexation of zinc ions with 2-methylimidazole and free Au NPs co-existed in the solution at the initial stage (stage I, SI: Figure S9). The Au NPs then acted as seeds to induce the nucleation and growth of ZIF-8 particles on their surface, and the free Au NPs preferred to bind to the AuNP@ZIF-8 particle surface (stage II, SI: Figure S9). The subsequent slow growth of ZIF-8 crystals leads to the formation of AuNPcentered crystals (stage III, Figure 2c and SI: Figure S9). If the NPs were added after the coordination reaction proceeded for a while (5 min for Figure 2d and SI: Figure S9c, 9d), only stage II and stage III occurred according to DLS, and a (ZIF-8 core)/(AuNP-rich transition layer)/(ZIF-8 shell) structure was formed (Figure 2d). The radius of this AuNPrich layer is controllable through the variation of the addition time of the NPs (Figure 5).
2.3. Applications of MOF Hybrid Crystals Through integration of the unique physical and chemical properties of NPs and MOFs, the new hybrid crystals have many promising applications. Since the formation process is independent of the properties of the pristine particles, it is possible to impart multi-functionalities to one MOF crystal by the incorporation of different kinds of NPs. The co-encapsulation of CdSe and Fe3O4 NPs into ZIF-8 crystals was demonstrated as a proof-of-concept experiment (Figure 6a,b). The synthesized product showed both fluorescent and magnetic properties. The light yellow product showed green florescence irradiation of UV light (365 nm). Simultaneously, small 2014, DOI: 10.1002/smll.201303564
thanks to the super-paramagnetic properties of the incorporated Fe3O4 NPs, the obtained product can be collected and re-dispersed through the manipulation of a magnetic field (Figure 6a). Furthermore, due to the high surface area, uniform pore size, and adoptable surface chemistry of MOFs,[23] the MOF hybrid crystals can be employed for selective heterogeneous catalysis. As a proof-of-concept experiment, the photoluminescence (PL) quenching experiment of the CdSe NPs encapsulated in ZIF-8 crystal was carried out. Figure 6c showed the quenching of CdSeNP@ZIF-8 crystals by different thiol species. The selective penetration of thiol molecules owing to the defined pore and cavity sizes of MOF shells was clearly presented. The PL quenching for the aliphatic thiol (2-mercaptoethanol) is very fast while the diffusion of the aromatic thiol (4-mercaptobenzoic acid) in the MOF is much slower, which leads to a retard quenching process. An even larger thiol, such as cyclohexanethiol, led to a remarkably slow quenching effect (about 5% emission drop in 2700 s). Additionally, the reduction of nitroarenes in aqueous solution catalyzed by gold NPs was carried out to demonstrate the molecular sieve effect of the MOF shells. The hydrogenation of aromatic nitro compounds to its corresponding amines is industrially important because functionalized anilines are essential intermediates for pharmaceuticals, polymers, herbicides and other fine chemicals.[24] Au NPs is a very efficient catalyst for the reduction of nitroarenes in aqueous medium at room temperature.[25] When p-nitrophenol was employed as the reactant, an absorption band centered at approximately 400 nm in the UV–vis spectra indicated the formation of p-nitrophenolate ions after the addition of NaBH4 as shown in the inset of Figure 6d. The intensity of this absorption band decreased quickly after the addition of 13-nm Au NPs, indicating the formation of p-aminophenol. However, when ZIF-8-coated Au NPs were employed as the catalyst, the reduction reaction is very slow due to the smaller pore size of ZIF-8 compared with p-nitrophenol and the intensity of the absorption band (∼400 nm) almost did not change even after the reaction took place for 90 min (Figure 6d). When smaller nitroanrenes such as nitrobenzene was employed as the reactant, the smaller size of nitrobenzene molecule make it possible to penetrate from the ZIF-8 shell and reduced by the catalysis of Au NPs inside, resulting in the decreasing of its absorption band (∼270 nm) (Figure 6e). This molecular selectivity of MOF structures combined with suitable catalysts encapsulated will enable the MOF hybrid material as potential candidates for novel heterogeneous catalysts.
3. Conclusion In summary, we have generated a method to incorporate a diverse set of particles into different crystalline MOFs. Control on the distributions and positions of the encapsulated NPs in the MOF crystals was investigated. The applications of these MOF hybrid crystals were demonstrated. By combining the particles and MOFs with suitable functionalities and controlling their formation architectures, this general approach enables the contruction of novel hybrid crystals
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Figure 6. Proof-of-concepts on the applications of the MOF hybrid crystals. a) The magnetic and florescence properties of the (CdSe+Fe3O4)NP@ZIF-8 crystals. Under magnetic force, the crystals accumulate on the wall of the vial; no florescence from the suspended solution. Insets are the photos of the crystal solution under lab light and under UV light (365 nm). The crystal solution shows green florescence. b) TEM images of the (CdSe+Fe3O4) NP@ZIF-8 crystals. The scale bars are 1 µm (left) and 200 nm (right). c) The time-resolved PL intensity at 541 nm before and after injection of the thiol solutions. An abrupt intensity drop is observed during the thiol solution injection due to the dilution of the CdSeNP@ZIF-8 solution. Different thiols show a distinct PL quenching process. Inset is the PL spectra of as prepared CdSeNP@ZIF-8 crystal solution (dark line) and the solutions after quenching by different thiols for 2700 s. d) The catalytic process for p-nitrophenol to p-aminophenol. With the ZIF-8 shell on the Au NP surface, the reaction is retarded and the absorption peak intensity of the reactant is almost constant in 90 min as compared with the reaction with 13-nm Au NPs without ZIF-8 shell(Inset). e) UV–vis spectra of the catalytic process for nitrobenze to aniline by AuNP@ZIF-8 in 90 min. In both d and e, the absorbance was measured at 0, 10, 20, 30, 40, 50, 60, and 90 min, respectively.
with advanced targeted properties, and it sheds light on the development of new functional materials.
4. Experimental Section Chemicals: Commercial reagents were purchased from Sigma-Aldrich (ACS grade) and used as received unless otherwise noted. Synthesis of PVP-Stabilized Au NPs: Au NPs were prepared by a method which involves the reduction of HAuCl4 using sodium citrate. In a typical procedure for the synthesis of 13-nm Au NPs, an aqueous solution of HAuCl4 (0.01%, 150 mL) was brought to a vigorous boil with rapid stirring in a round-bottom flask (250 mL) fitted with a reflux condenser. When the solution started to boil, an aqueous solution of trisodium citrate (1%, 4.5 mL) was added. The mixture was refluxed with stirring for another 20 min. The flask with a deep red suspension was then removed from the heat. After the Au NP solution was cooled down to room temperature, a solution of PVP (0.5 g, Mw = 55 000 without specifications) in water (20 mL) was added dropwisely into the Au NP solution with stirring, and the mixture was further stirred at room temperature for 24 h. The PVP-stabilized Au NPs were collected by centrifugation at 14 000 rpm for 30 min, washed by methanol three times, and
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finally dispersed in 175 mL of methanol or N,N-dimethylformamide (DMF). Synthesis of PVP-Stabilized Pt NPs: PVP-stabilized Pt NPs (3.3 nm) were prepared by refluxing a mixture of PVP (133 mg, Mw = 29 000), methanol (180 mL), and an aqueous solution of H2PtCl6 (6.0 mM, 20 mL) in a flask (500 mL) for 3 h under air. Methanol was removed by rotary evaporation. The NPs in the remaining solution were precipitated using acetone and then collected by centrifugation at 6000 rpm for 5 min. The sample was cleaned with chloroform and hexanes to remove excess free PVP and finally dispersed in 90 mL of DMF. Synthesis of PVP-Stabilized Fe3O4 NPs: A mixture of anhydrous FeCl3(0.324 g, 2.0 mmol), anhydrous CH3COONa (0.5 g, 6.0 mmol), and diethylene glycol (0.21 mol, 20.0 mL) was heated under stirring to form a clear solution. Then, the solution was poured into a stainless steel autoclave and maintained at 200 °C for 8 h. After cooling to room temperature, the black precipitate was separated by centrifugation (8500 rpm, 10 min) and washed with ethanol twice, then dried at 80 °C for 1 h. The obtained powder was redispersed into 250 mL of deionized (DI) water, and the pH value was tuned to 9 using a diluted NaOH solution. A 20-mL aliquot of 2 wt% PVP was added with stirring. The solution was stirred for 24 h before being precipitated with acetone. Water was removed by rotary evaporation. The sample was cleaned with chloroform
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and hexanes to remove the excess free PVP and finally dispersed in 250 mL of methanol. Synthesis of PVP-Stabilized CdSe NPs: Oleic-acid-coated CdSe NPs (4 nm) were synthesized using the hot-injection method. The as-synthesized NPs were precipitated with ethanol, collected after centrifugation at 6000 rpm, and washed 6 times with ethanol. The NPs were then re-dispersed in 20 mL of chloroform (0.5 mg/mL). A solution of PVP (250 mg, Mw = 55 000) in chloroform (10 mL) was then added. After the mixture was stirred for 24 h, the PVPstabilized NPs were precipitated with hexanes and collected by centrifugation at 6000 rpm for 5 min. The sample was cleaned with chloroform and hexanes to remove the excess free PVP. Finally, the PVP-stabilized NPs were re-dispersed into methanol (0.25 mg/mL). Encapsulation of Au NPs in ZIF-7: A 1-mL aliquot of a DMF solution of the Au NPs, a 5-mL aliquot of a DMF solution of benzeneimidazole (100 mM), and a 5-mL aliquot of a DMF solution of Zn(NO3)2·6H2O (50 mM) were mixed and then allowed to react at room temperature for 24 h without stirring. The product was collected by centrifugation, washed 4 times with ethanol, and vacuumdried overnight. Encapsulation of Au NPs in ZIF-8: A 1-mL aliquot of a MeOH solution of the Au NPs, a 5-mL aliquot of a MeOH solution of 2-methylimidazole (25 mM), and a 5-mL aliquot of a MeOH solution of Zn(NO3)2·6H2O (25 mM) were mixed and then allowed to react at room temperature for 24 h without stirring. The product was collected by centrifugation, washed 4 times with ethanol, and vacuum-dried overnight. Encapsulation of Pt NPs in MIL-53: FeCl3·6H2O (27 mg, 0.1 mmol) and 17 mg of terephthalic acid (0.1 mmol) was dissolved in 10 mL of DMF. A solution (0.5 mL) of PVP-coated Pt NP was added with stirring. The mixture was heated in a silicon oil bath at 120 °C for 8 h before allowing it to cool down to room temperature naturally. The product was washed 6 times with DMF and 3 times with methanol before any characterization. Encapsulation of Pt NPs in MIL-101_NH2: FeCl3·6H2O (27 mg, 0.1 mmol) and 18 mg of aminoterephthalic acid (0.1 mmol) was dissolved in 10 mL of DMF. A solution (0.5 mL) of PVP-coated Pt NP solution was added with stirring. The mixture was heated in a silicon oil bath at 120 °C for 8 h without disturbing before allowing it to cool down to room temperature naturally. The product was washed 6 times with DMF and 3 times with methanol before any characterization. Effect of PVP Concentration on the Particle Distributions in the Crystals: Au NPs were capped by PVP (Mw = 55 000) first. Then 1 mL of a MeOH solution of Au NP, 5 mL of a MeOH solution of 2-methylimidazole (25 mM), and 5 mL of a MeOH solution of Zn(NO3)2·6H2O (25 mM) were mixed. The PVP concentration in the solution was adjusted to 10−5 (sample 1), 5 × 10−4 (sample 2), 2 × 10−3 (sample 3), 10−3 (sample 4), 2 × 10−2 (sample 5), and 4 × 10−2 wt% (sample 6). The solutions were then allowed to react at room temperature for 24 h without stirring. The product was collected by centrifugation, washed several times with ethanol, and vacuum-dried over-night. Encapsulation of Fe3O4 NPs and CdSe NPs in ZIF-8: An aliquot of 0.5 mL of a MeOH solution of Fe3O4 NPs, 5 mL of the MeOH solution of 2-methylimidazole (25 mM), and 5 mL of a MeOH solution of Zn(NO3)2·6H2O (25 mM) were mixed and allowed to react at room temperature for 5 min without stirring; then 0.5 mL of a MeOH small 2014, DOI: 10.1002/smll.201303564
solution of CdSe was added. The solutions were then allowed to react at room temperature for 24 h without stirring. The product was collected by centrifugation, washed several times with ethanol, and vacuum-dried overnight. Fluorescence Quenching of CdSe NP@ZIF-8 with Various Thiols: Three kinds of thiols were used in the quenching experiment: 2-mercaptoethanol, 4-mercaptobenzoic acid, and cyclohexanethiol. A MeOH solution of CdSe NP@ZIF-8 (2 mL) was added in the cell. The time-resolved photoluminescence (PL) measurement was taken out for 5 min before 100-µL aliquots of different kinds of thiol solutions (100 mM in methanol) was injected. The quenching process was then recorded. UV–Vis Characterization of the Reduction of Nitroarenes: Two kinds of nitroarenes were used: p-nitrophenol and nitrobenzene. 1 mmols of nitroarene and 0.05 mol of NaBH4 were stirred in 10 mL of double-distilled water at room temperature under a N2 balloon for 15 min. Then an appropriate amount of catalysts (10 mg of PVP-coated Au NPs or 100 mg of Au NP@ZIF-8 powder) was added, and the contents were allowed to stir for the appropriate amount of time. A 200-µL solution was taken and diluted to 2 mL and centrifuged for UV–vis spectral characterizations at 0, 10, 20, 30, 40, 50, 60, and 90 min, respectively. Material Characterization: The surface morphologies of the NP@MOF composites were characterized using a JEOL-7600F field-emission scanning electron microscope (FESEM). The encapsulation process was characterized using a JEOL-2100F transmission electron microscope (TEM). The containment of the NPs in the MOFs was characterized by energy-dispersive X-ray spectroscopy (EDX). Powder X-ray diffraction (XRD) was taken at room temperature on a θ/2θ mode using a Bruker D8 Advance. The DLS and zeta-potential measurements were performed at 25 °C with a Malvern Nanosizer. The solutions were cleaned by passing solutions through 0.2-µm filters to remove the dust particles before mixing them into the scattering cell. The signal was collected at a beam angle of 173°. The ex-situ zeta-potential measurement was carried out by removing the solution of interest from the ZIF-8 reaction solution, and by injecting the resultant solution at time intervals into the cell for measurement. The in-situ DLS measurement on the process of Au NPs encapsulation in ZIF-8 crystals were taken under the following conditions: a) The control experiment on the addition of the NP solution at the beginning of the experiment. To slow down the nucleation process and obtain good signal for the DLS measurement, a diluted reactant solution was used. A 200-µL aliquot of a MeOH solution of the Au NPs, a 1-mL aliquot of a MeOH solution of 2-methylimidazole (5 mM), and a 1-mL aliquot of a MeOH solution of Zn(NO3)2·6H2O (5 mM) was mixed in the cell. Measurements began after 30 s, in order to allow the solution to first stabilize. b) The control experiment on the addition of the NP solution, after reaction for 5 min. A 1-mL aliquot of the MeOH solution of 2-methylimidazole (25 mM) and a 1-mL aliquot of the MeOH solution of Zn(NO3)2·6H2O (25 mM) was mixed in the cell. It was allowed to react for different time intervals before 200 µL of Au-NP MeOH solution was added. To minimize the effect of particle polydispersity on the measurement, DLS monitoring was carried out for the 5 min before the large NPs appeared in the solution. The sample for TEM demonstration was obtained with the addition of Au NPs after the reaction was allowed to continue for 20 min. To better visualize the Au NP core using TEM, the concentration of the Au NPs was
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increased fourfold for the synthesis of ZIF-8 (core)/Au NPs/ZIF-8 (shell) structures.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the start-up fund of Nanjing University of Posts& Telecommunications (NY213098) and National Science Foundation of China (GZ213054).
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[9] A. Aijaz, A. Karkamkar, Y. J. Choi, N. Tsumori, E. Ronnebro, T. Autrey, H. Shioyama, Q. Xu, J. Am. Chem. Soc. 2012, 134, 13926. [10] C. H. Kuo, Y. Tang, L. Y. Chou, B. T. Sneed, C. N. Brodsky, Z. P. Zhao, C. K. Tsung, J. Am. Chem. Soc. 2012, 134, 14345. [11] Special issue on metal–organic frameworks: Chem. Rev. 2012, 112, 673––1268. [12] A. Dhakshinamoorthy, H. Garcia, Chem. Soc. Rev. 2012, 41, 5262. [13] a) S. Hermes, M. K. Schroter, R. Schmid, L. Khodeir, M. Muhler, A. Tissler, R. W. Fischer, R. A. Fischer, Angew. Chem. Int. Ed. 2005, 44, 6237; b) C. Zlotea, R. Campesi, F. Cuevas, E. Leroy, P. Dibandjo, C. Volkringer, T. Loiseau, G. Ferey, M. Lotroche, J. Am. Chem. Soc. 2010, 132, 2991; c) H. L. Jiang, B. Liu, T. Akita, M. Haruta, H. Sakurai, Q. Xu, J. Am. Chem. Soc. 2009, 131, 11302; d) M. Meilikhov, K. Yusenko, D. Esken, S. Turner, G. Van Tendeloo, R. A. Fischer, Eur. J. Inorg. Chem. 2010, 2010, 3701; e) X. J. Gu, Z. H. Lu, H. L. Jiang, T. Akita, Q. Xu, J. Am. Chem. Soc. 2011, 133, 11822; f) R. Ameloot, M. B. J. Roeffaers, G. De Cremer, F. Vermoortele, J. Hofkens, B. F. Sels, D. E. De Vos, Adv. Mater. 2011, 23, 1788. [14] J. Cravillon, S. Munzer, S. J. Lohmeier, A. Feldhoff, K. Hubber, M. Wiebcke, Chem. Mater. 2009, 21, 1410. [15] a) S. Hermes, F. Schroder, R. Chelmowski, C. Woll, R. A. Fischer, J. Am. Chem. Soc. 2005, 127, 13744; b) E. Biemmi, C. Scherb, T. Bein, J. Am. Chem. Soc. 2007, 129, 8054. [16] a) Y. Diao, T. Harada, A. S. Myerson, T. A. Hatton, B. L. Trout, Nat. Mater. 2011, 10, 867; b) Y. Diao, M. E. Helgeson, A. S. Myerson, T. A. Hatton, P. S. Doyle, B. L. Trout, J. Am. Chem. Soc. 2011, 133, 3756. [17] M. K. Corbierre, N. S. Cameron, R. B. Lennox, Langmuir 2004, 20, 2867. [18] S. K. Ghosh, T. Pal, Chem. Rev. 2007, 107, 4797. [19] a) S. R. Venna, J. B. Jasinski, M. A. Carreon, J. Am. Chem. Soc. 2010, 132, 18030; b) J. Cravillon, C. A. Schroder, R. Nayuk, J. Gummel, K. Huber, M. Wiebcke, Angew. Chem. Int. Ed. 2011, 50, 8067. [20] a) J. Anwar, D. Zahn, Angew. Chem. Int. Ed. 2011, 50, 1996; b) T. H. Zhang, X. Y. Liu, Angew. Chem. Int. Ed. 2009, 48, 1308. [21] a) H. Colfen, Nat. Mater. 2010, 9, 960; b) H. Y. Li, H. L. Xin, D. A. Muller, L. A. Estroff, Science 2009, 326, 1244; c) Y. Y. Kim, K. Ganesan, P. C. Yang, A. N. Kulak, S. Borukhin, S. Pechook, L. Ribeiro, R. Kroger, S. J. Eichhorn, S. P. Armes, B. Pokroy, F. C. Meldrum, Nat. Mater. 2011, 10, 890; d) E. M. Pouget, P. H. H. Bomans, J. Goos, P. M. Frederik, G. de With, N. Sommerdijk, Science 2009, 323, 1455. [22] a) R. J. Park, F. C. Meldrum, Adv. Mater. 2002, 14, 1167; b) S. Ludwigs, U. Steiner, A. N. Kulak, R. Lam, F. C. Meldrum, Adv. Mater. 2006, 18, 2270; c) C. Li, L. M. Qi, Angew. Chem. Int. Ed. 2008, 47, 2388; d) H. Y. Li, L. A. Estroff, J. Am. Chem. Soc. 2007, 129, 5480. [23] a) G. Ferey, Chem. Soc. Rev. 2008, 37, 191; b) O. M. Yaghi, Q. W. Li, MRS Bull. 2009, 34, 682. [24] R. S. Dowing, P. J. Kunkeler, H. van Bekkum, Catal. Today 1997, 37, 121. [25] N. Pradhan, Colloids Surf. A 2002, 196, 247.
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Received: November 18, 2013 Published online:
small 2014, DOI: 10.1002/smll.201303564
Hybrid Crystals Comprising Metal–Organic Frameworks and Functional Particles: Synthesis and Applications Shaozhou Li* and Fengwei Huo*
Hybrid crystals containing encapsulated functional species exhibit promising novel physical and chemical properties. The realization of many properties critically depends on the selection of suitable functional species for incorporation, the rational control of the crystallinity of the host materials, and the manipulation of the distribution of the encapsulated species; only a few hybrid crystals achieve this. Here, a novel synthetic method enables the encapsulation of functional species within crystalline metal–organic frameworks (MOFs). Various kinds of single-crystalline MOFs with incorporated particles are presented. The encapsulated particles can be distributed in a controllable manner, and the hybrid crystals are applied to the heterogeneous catalysis of the reduction of nitroarenes. These findings suggest a general approach for the construction of MOF materials with potential applications; by combining species and MOFs with suitable functionalities, new properties—not possible by other means—may arise.
1. Introduction The incorporation of functional species such as inorganic particles into host crystals at the synthetic level is an attractive way to design new systems that possess novel properties— originating either from the collective interactions between the particles and host materials or from the combination of their pristine properties.[1–10] Previously reported structures of this type have exhibited technological implications in areas ranging from heterogeneous catalysis,[1] biomedical devices[2] to energy conversion and storage.[3] In many of the potential applications, it is essential to be able to control the S. Z. Li Institute of Advanced Materials (IAM) Nanjing University of Posts & Telecommunications 9 Wenyuan Road, Nanjing, China E-mail: [email protected] Prof. F. Huo School of Materials Science and Engineering Nanyang Technological University 50 Nanyang Avenue Singapore 639798, Singapore E-mail: [email protected] DOI: 10.1002/smll.201303564 small 2014, DOI: 10.1002/smll.201303564
shell crystallinity and the distribution of the incorporated particles.[1–4] The synthetic strategies for these systems generally involve either the nucleation and growth of crystals on a chemically modified surface of the incorporated particles[5] or epitaxial growth, which requires an appropriate choice of lattice-matched host materials.[6] When the former strategy is utilized, a non-uniform and polycrystalline shell usually forms due to the multiple nucleation sites on the chemically modified particle surface.[5,10] Although the epitaxialgrowth strategy can realize the formation of single-crystalline core–shell hybrid nanostructures, this strategy requires lattice-matching between the core and shell materials, and it is limited to the formation of single-core structures.[6a] This dilemma prevents the widespread practical applications of these hybrid materials. Metal-organic frameworks (MOFs) are a class of crystalline polymers with the permanent porosity. Due to their unique properties, which include high surface area, adaptable surface chemistry, tunable pore-size, and tunable structures, MOFs have a wide range of potential applications in the separation, capturing, storing, sensing, and catalysis of molecules.[11] The potential applications of MOFs can be developed further and extended by incorporating various functional species within the frameworks.[12] Recently, a controlled encapsulation method has been developed by
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Scheme 1. Schematic diagram for the encapsulation of the functional species, such as inorganic particles, into single-crystalline MOFs.
our group to incorporate species into the zeolitic imidazolate framework 8 (ZIF-8), a well-known MOF.[7] Herein, we demonstrate an improved method to form hybrid MOF single-crystals; this new method enables better control of the location of the incorporated particles, and a diverse set of MOFs can be used. In contrast with the MOF hybrid structures synthesized by the methods previously reported by others or by our group,[4,7–10,13] this strategy is not restricted by the synthetic route of the incorporated particles, and it is avoids the undesired deposition of particles on the outer surface of the MOFs. Unlike the other methods, which are usually restricted to one kind of MOF, this method is applicable to many MOFs. More importantly, the crystallinity of the MOF shells are not compromised by the spatial control of the incorporated particles. This approach allows a wide range of species and MOFs to be combined into the hybrid crystals, thus shedding light on the development of new functional hybrid materials.
MOF hybrid composites have indeed been produced by this method, i.e., AuNP@ZIF-7 and AuNP@ZIF-8 with Au NPs and PtNP@MIL-53 and PtNP@MIL-101_NH2 with Pt NPs (MIL stands for materials from the Institut Lavoisier). The ZIF-7 and ZIF-8 hybrid crystals exhibit rhombic dodecahedral shapes with edge-to-edge lengths of 200 nm and 1 µm, respectively. The MIL-53 and MIL-101_NH2 hybrid crystals are octahedral with crystal sizes of around 300 nm.
2.2. The Mechanistic Investigation on the Incorporation Process The AuNP@ZIF-8 hybrid crystal was employed as the model system to demonstrate the rational synthesis of the NP@MOF crystals. As has been pointed out, the nucleation kinetics of ZIF-8 is affected by the interactions between the polymers and the pre-nucleation clusters. Because the clusters formed
2. Results and Discussion 2.1. Synthesis of MOF Hybrid Crystals The encapsulation strategy is illustrated in Scheme 1. Briefly, nanoparticles (NPs) were coated with suitable polymers before being added to a growth solution containing pre-nucleation clusters, which were formed by the coordination of metal ions and organic ligands. The clusters were trapped and densificated on the polymer surface to activate the nucleation and growth of MOFs, resulting in the formation of the MOF hybrid crystals (denoted as NP@MOF). The encapsulation process greatly relied on the interplay between the clusters and polymers involved rather than the crystallographic structures or chemical properties of the encapsulated NPs. Therefore, this method was expected to be applicable to different kinds of core particles and to a diverse set of MOF crystals, regardless of their synthetic routes or intrinsic properties. As shown in Figure 1, various kinds of single-crystalline
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Figure 1. Transmission electron microscopy (TEM; a,b,d,e,g,h,j,k) and field-effect scanning electron microscopy (FESEM; c,f,i,l) images of the MOF hybrid crystals. a–c) AuNP@ZIF-7 crystals, d–f) AuNP@ZIF-8 crystals, g–i) PtNP@MIL-53 crystals, and j–l) PtNP@MIL-101_ NH2 crystals. b,e,h,k) Enlarged TEM images of a single NP@MOF crystal. The sizes of the incorporated Au- and PtNPs are 13 and 3 nm, respectively. The scale bars are 200 nm (a,h,k), 100 nm (b), 1 µm for (c,d,f,i,l); and 500 nm for (e,g,j). © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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MOF/Particle Hybrid Crystals
by the complexation of zinc ions and 2-methylimidazole in the ZIF-8 growth solution are positively charged while the polymer-coated Au NPs have negative zeta-potentials (see Supporting Information (SI): Figure S2),[14] it is possible that—similar to the deposition of MOFs on a chemically modified flat surface[15]—electrostatic attractions between the polymer-modified NPs and the MOF clusters might be the one dominating factor for inducing the formation of the NP@ MOF crystals. To test this, Au NPs with different surface functionalizations, namely, 11-mercapto-1-undecanol (11-MPA), polyethylene glycol (PEG, weight-averaged molecular weight (Mw) in Daltons = 3350), and PEG (Mw = 40 000) were employed to synthesize AuNP@ZIF-8 crystals. All three coated NPs have exposed –OH groups and show similar zeta-potentials (SI: Figure S2). However, incorporation only occurred for the PEG-coated Au NPs (SI: Figure S3), indicating that having a negatively charge surface alone is not sufficient for ensuring the incorporation of NPs into the MOF crystals under this condition. On the other hand, the
nucleation kinetics of the crystals can be altered by the surface morphology of the polymers owing to the confinement effect.[16] The polymer chains can decrease the diffusivity of the solute or clusters within, thus promoting the nucleation and growth of crystals.[16b,17] It is interesting to note that when the NPs were coated by highly polymerized polymers such as PEG (Mw = 40 000), pluronic F-127 (Mw = 12 500), and polyvinylpyrrolidone (PVP, Mw = 29 000), the addition of these NPs into the growth solution leads to the formation of NP@MOF crystals (SI: Figure S3, S4). To further study the effect of the polymer on the encapsulation process, PVP molecules with different degrees of polymerization (Mw = 3500, 10 000, 29 000, 55 000, and 1 300 000) were used. Due to their similar structures and chemistry, it is expect that the weight difference (or chain length) can affect the thickness of the PVP shells formed on the NP surface. Dynamic light scattering (DLS) show that the hydrodynamic diameters (HD) of the Au NPs with different PVPs varied as the polymer weight varied (Figure 2a). Because all pristine
Figure 2. The encapsulation process is affected by the polymer shell on the particle surface. a) The HD measurements of the Au NPs coated with the different PVPs in methanol. K signifies ×1000. Inset shows the HD. b) The distribution of the NPs in the MOF can be tuned by the PVP concentration. The PVP concentration in the growth solutions were adjusted to 10−5 (sample 1), 5 × 10−4 (sample 2), 2 × 10−3 (sample 3), 10−3 (sample 4), and 2 × 10−2 (sample 5) wt%. Top right inset shows how the crystal size and the size of the NP core are defined. Other insets show the corresponding TEM images of a particle with the NP core indicated by a red circle. c,d) TEM images showing the spatial controllability of the distribution of the encapsulated Au NPs inside the MOF hybrid crystal. For these two samples, the Au NP solution was added at 0 (c) or 5 (d) min into reaction. The scale bars in (b) are all 500 nm. small 2014, DOI: 10.1002/smll.201303564
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of these clusters results in the formation of the amorphous precursor phase, and thirdly, the transformation of the amorphous phase to crystals is driven by chemical thermodynamics.[19a,20] Because the process is directed by the thermodynamic transition of the amorphous precursor phase to crystalline materials and because the physical and chemical properties of the encapsulated NPs have a minor effect on this transition, the synthesis of NP@ ZIF-8 crystals benefits from this unique crystallization behavior of ZIF-8. Without the lattice-matching constraint, which is a challenging requirement for the design and synthesis of hybrid crystals by the epitaxial method,[6a] the single-crystallline NP@MOF crystals can be synthesized easily. The obtained crystals are highly Figure 3. TEM images of the encapsulation results for PVP-coated Au NPs, where PVP of a) crystalline with a low defect density, Mw = 3500, b) 10 000, c) 29 000, d) 55 000, and e) 1 300 000 is used. The scale bars are and they exhibit good molecular-sieve all 1 µm. properties. This strategy also shows the merit Au NPs were synthesized at the same time, this difference of being able to control the positions of the encapsulated directly correlated with the thickness of the PVP shells. Ini- NPs inside the MOF crystals. The spatial distribution of the tially, the thickness of the PVP shells increased along with encapsulated NPs in the MOF crystals can be controlled by the increase in its weight. When the molecule becomes too adjusting the PVP concentration in the growth solution from large (Mw = 1 300 000), the decreased PVP solubility results 10−5 wt% (sample 1) to 2 × 10−2 wt% (sample 5). The size of in the formation of thin polymer shells. The thicker PVP the Au NP core (d) becomes larger as the PVP concentration shells lead to faster nucleation and growth of the AuNP@ increases (d increases from ∼220 nm to ∼1 µm from sample ZIF-8 crystals (SI: Figure S5, S6), while a thin PVP shell not 1 to sample 5. Figure 2b and 4.) Considering that the sizes only leads to a slower nucleation and growth rate, but it also of the AuNP@ZIF-8 crystals for these 5 samples were almost decreases the encapsulation quality of the NP in the hybrid the same (D = ∼1 µm for all samples), and that the amounts crystals (Figure 3). For example, when the PVP with Mw = of the added Au NPs were also the same, a larger d implies 3500 is used to coat the Au NPs, the coated NPs exhibits the a better spatial distribution of the NPs in the MOF hybrid smallest HD among the 5 different of PVP-coated NPs; when crystals. Further raising the PVP concentration results in they were employed for the synthesis of NP@MOF crys- the failure of the encapsulations (SI: Figure S8). In addition tals, the encapsulated Au NPs aggregated as indicated by a large surface plasmon resonance (SPR) band shift on the UV– vis absorption spectra (SI: Figure S7).[18] Moreover, ZIF-8 crystals without any NPs inside were also observed in the final products (Figure 3a). The formation of ZIF-8 crystal has been shown to occur via a non-classical nucleation and crystallization pathway.[19] Under certain circumstances, such behavior is general to many crystal systems.[20] Among them, the biomineralization of calcite and apatite in organisms has been well studied.[21] By mimicking the biomineralization process of calcite, many artificial calcium-based hybrid materials with different structures incorporated have been reported.[22] The growth of ZIF-8 exhibits many similarities with these biomineral materials. First, pre-nucleation clusters Figure 4. TEM images of the sample a) 1, b) 2, c) 3, d) 4, and e) 5, where the PVP concentration are involved. Second, the aggregation increases going from sample 1 to 5. The scale bars are 500 nm.
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Figure 5. TEM images of the AuNP@ZIF-8 crystals formed as a result of different addition times of the Au NP solution: a) 0, b) 10, c) 20, and d) 30 min.
to controlling the distribution of the encapsulated NPs, their positions can be tuned by varying the addition time of the NP solution (Figure 5). When the NP solution was added at the beginning of the reaction, clusters formed by the complexation of zinc ions with 2-methylimidazole and free Au NPs co-existed in the solution at the initial stage (stage I, SI: Figure S9). The Au NPs then acted as seeds to induce the nucleation and growth of ZIF-8 particles on their surface, and the free Au NPs preferred to bind to the AuNP@ZIF-8 particle surface (stage II, SI: Figure S9). The subsequent slow growth of ZIF-8 crystals leads to the formation of AuNPcentered crystals (stage III, Figure 2c and SI: Figure S9). If the NPs were added after the coordination reaction proceeded for a while (5 min for Figure 2d and SI: Figure S9c, 9d), only stage II and stage III occurred according to DLS, and a (ZIF-8 core)/(AuNP-rich transition layer)/(ZIF-8 shell) structure was formed (Figure 2d). The radius of this AuNPrich layer is controllable through the variation of the addition time of the NPs (Figure 5).
2.3. Applications of MOF Hybrid Crystals Through integration of the unique physical and chemical properties of NPs and MOFs, the new hybrid crystals have many promising applications. Since the formation process is independent of the properties of the pristine particles, it is possible to impart multi-functionalities to one MOF crystal by the incorporation of different kinds of NPs. The co-encapsulation of CdSe and Fe3O4 NPs into ZIF-8 crystals was demonstrated as a proof-of-concept experiment (Figure 6a,b). The synthesized product showed both fluorescent and magnetic properties. The light yellow product showed green florescence irradiation of UV light (365 nm). Simultaneously, small 2014, DOI: 10.1002/smll.201303564
thanks to the super-paramagnetic properties of the incorporated Fe3O4 NPs, the obtained product can be collected and re-dispersed through the manipulation of a magnetic field (Figure 6a). Furthermore, due to the high surface area, uniform pore size, and adoptable surface chemistry of MOFs,[23] the MOF hybrid crystals can be employed for selective heterogeneous catalysis. As a proof-of-concept experiment, the photoluminescence (PL) quenching experiment of the CdSe NPs encapsulated in ZIF-8 crystal was carried out. Figure 6c showed the quenching of CdSeNP@ZIF-8 crystals by different thiol species. The selective penetration of thiol molecules owing to the defined pore and cavity sizes of MOF shells was clearly presented. The PL quenching for the aliphatic thiol (2-mercaptoethanol) is very fast while the diffusion of the aromatic thiol (4-mercaptobenzoic acid) in the MOF is much slower, which leads to a retard quenching process. An even larger thiol, such as cyclohexanethiol, led to a remarkably slow quenching effect (about 5% emission drop in 2700 s). Additionally, the reduction of nitroarenes in aqueous solution catalyzed by gold NPs was carried out to demonstrate the molecular sieve effect of the MOF shells. The hydrogenation of aromatic nitro compounds to its corresponding amines is industrially important because functionalized anilines are essential intermediates for pharmaceuticals, polymers, herbicides and other fine chemicals.[24] Au NPs is a very efficient catalyst for the reduction of nitroarenes in aqueous medium at room temperature.[25] When p-nitrophenol was employed as the reactant, an absorption band centered at approximately 400 nm in the UV–vis spectra indicated the formation of p-nitrophenolate ions after the addition of NaBH4 as shown in the inset of Figure 6d. The intensity of this absorption band decreased quickly after the addition of 13-nm Au NPs, indicating the formation of p-aminophenol. However, when ZIF-8-coated Au NPs were employed as the catalyst, the reduction reaction is very slow due to the smaller pore size of ZIF-8 compared with p-nitrophenol and the intensity of the absorption band (∼400 nm) almost did not change even after the reaction took place for 90 min (Figure 6d). When smaller nitroanrenes such as nitrobenzene was employed as the reactant, the smaller size of nitrobenzene molecule make it possible to penetrate from the ZIF-8 shell and reduced by the catalysis of Au NPs inside, resulting in the decreasing of its absorption band (∼270 nm) (Figure 6e). This molecular selectivity of MOF structures combined with suitable catalysts encapsulated will enable the MOF hybrid material as potential candidates for novel heterogeneous catalysts.
3. Conclusion In summary, we have generated a method to incorporate a diverse set of particles into different crystalline MOFs. Control on the distributions and positions of the encapsulated NPs in the MOF crystals was investigated. The applications of these MOF hybrid crystals were demonstrated. By combining the particles and MOFs with suitable functionalities and controlling their formation architectures, this general approach enables the contruction of novel hybrid crystals
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Figure 6. Proof-of-concepts on the applications of the MOF hybrid crystals. a) The magnetic and florescence properties of the (CdSe+Fe3O4)NP@ZIF-8 crystals. Under magnetic force, the crystals accumulate on the wall of the vial; no florescence from the suspended solution. Insets are the photos of the crystal solution under lab light and under UV light (365 nm). The crystal solution shows green florescence. b) TEM images of the (CdSe+Fe3O4) NP@ZIF-8 crystals. The scale bars are 1 µm (left) and 200 nm (right). c) The time-resolved PL intensity at 541 nm before and after injection of the thiol solutions. An abrupt intensity drop is observed during the thiol solution injection due to the dilution of the CdSeNP@ZIF-8 solution. Different thiols show a distinct PL quenching process. Inset is the PL spectra of as prepared CdSeNP@ZIF-8 crystal solution (dark line) and the solutions after quenching by different thiols for 2700 s. d) The catalytic process for p-nitrophenol to p-aminophenol. With the ZIF-8 shell on the Au NP surface, the reaction is retarded and the absorption peak intensity of the reactant is almost constant in 90 min as compared with the reaction with 13-nm Au NPs without ZIF-8 shell(Inset). e) UV–vis spectra of the catalytic process for nitrobenze to aniline by AuNP@ZIF-8 in 90 min. In both d and e, the absorbance was measured at 0, 10, 20, 30, 40, 50, 60, and 90 min, respectively.
with advanced targeted properties, and it sheds light on the development of new functional materials.
4. Experimental Section Chemicals: Commercial reagents were purchased from Sigma-Aldrich (ACS grade) and used as received unless otherwise noted. Synthesis of PVP-Stabilized Au NPs: Au NPs were prepared by a method which involves the reduction of HAuCl4 using sodium citrate. In a typical procedure for the synthesis of 13-nm Au NPs, an aqueous solution of HAuCl4 (0.01%, 150 mL) was brought to a vigorous boil with rapid stirring in a round-bottom flask (250 mL) fitted with a reflux condenser. When the solution started to boil, an aqueous solution of trisodium citrate (1%, 4.5 mL) was added. The mixture was refluxed with stirring for another 20 min. The flask with a deep red suspension was then removed from the heat. After the Au NP solution was cooled down to room temperature, a solution of PVP (0.5 g, Mw = 55 000 without specifications) in water (20 mL) was added dropwisely into the Au NP solution with stirring, and the mixture was further stirred at room temperature for 24 h. The PVP-stabilized Au NPs were collected by centrifugation at 14 000 rpm for 30 min, washed by methanol three times, and
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finally dispersed in 175 mL of methanol or N,N-dimethylformamide (DMF). Synthesis of PVP-Stabilized Pt NPs: PVP-stabilized Pt NPs (3.3 nm) were prepared by refluxing a mixture of PVP (133 mg, Mw = 29 000), methanol (180 mL), and an aqueous solution of H2PtCl6 (6.0 mM, 20 mL) in a flask (500 mL) for 3 h under air. Methanol was removed by rotary evaporation. The NPs in the remaining solution were precipitated using acetone and then collected by centrifugation at 6000 rpm for 5 min. The sample was cleaned with chloroform and hexanes to remove excess free PVP and finally dispersed in 90 mL of DMF. Synthesis of PVP-Stabilized Fe3O4 NPs: A mixture of anhydrous FeCl3(0.324 g, 2.0 mmol), anhydrous CH3COONa (0.5 g, 6.0 mmol), and diethylene glycol (0.21 mol, 20.0 mL) was heated under stirring to form a clear solution. Then, the solution was poured into a stainless steel autoclave and maintained at 200 °C for 8 h. After cooling to room temperature, the black precipitate was separated by centrifugation (8500 rpm, 10 min) and washed with ethanol twice, then dried at 80 °C for 1 h. The obtained powder was redispersed into 250 mL of deionized (DI) water, and the pH value was tuned to 9 using a diluted NaOH solution. A 20-mL aliquot of 2 wt% PVP was added with stirring. The solution was stirred for 24 h before being precipitated with acetone. Water was removed by rotary evaporation. The sample was cleaned with chloroform
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small 2014, DOI: 10.1002/smll.201303564
MOF/Particle Hybrid Crystals
and hexanes to remove the excess free PVP and finally dispersed in 250 mL of methanol. Synthesis of PVP-Stabilized CdSe NPs: Oleic-acid-coated CdSe NPs (4 nm) were synthesized using the hot-injection method. The as-synthesized NPs were precipitated with ethanol, collected after centrifugation at 6000 rpm, and washed 6 times with ethanol. The NPs were then re-dispersed in 20 mL of chloroform (0.5 mg/mL). A solution of PVP (250 mg, Mw = 55 000) in chloroform (10 mL) was then added. After the mixture was stirred for 24 h, the PVPstabilized NPs were precipitated with hexanes and collected by centrifugation at 6000 rpm for 5 min. The sample was cleaned with chloroform and hexanes to remove the excess free PVP. Finally, the PVP-stabilized NPs were re-dispersed into methanol (0.25 mg/mL). Encapsulation of Au NPs in ZIF-7: A 1-mL aliquot of a DMF solution of the Au NPs, a 5-mL aliquot of a DMF solution of benzeneimidazole (100 mM), and a 5-mL aliquot of a DMF solution of Zn(NO3)2·6H2O (50 mM) were mixed and then allowed to react at room temperature for 24 h without stirring. The product was collected by centrifugation, washed 4 times with ethanol, and vacuumdried overnight. Encapsulation of Au NPs in ZIF-8: A 1-mL aliquot of a MeOH solution of the Au NPs, a 5-mL aliquot of a MeOH solution of 2-methylimidazole (25 mM), and a 5-mL aliquot of a MeOH solution of Zn(NO3)2·6H2O (25 mM) were mixed and then allowed to react at room temperature for 24 h without stirring. The product was collected by centrifugation, washed 4 times with ethanol, and vacuum-dried overnight. Encapsulation of Pt NPs in MIL-53: FeCl3·6H2O (27 mg, 0.1 mmol) and 17 mg of terephthalic acid (0.1 mmol) was dissolved in 10 mL of DMF. A solution (0.5 mL) of PVP-coated Pt NP was added with stirring. The mixture was heated in a silicon oil bath at 120 °C for 8 h before allowing it to cool down to room temperature naturally. The product was washed 6 times with DMF and 3 times with methanol before any characterization. Encapsulation of Pt NPs in MIL-101_NH2: FeCl3·6H2O (27 mg, 0.1 mmol) and 18 mg of aminoterephthalic acid (0.1 mmol) was dissolved in 10 mL of DMF. A solution (0.5 mL) of PVP-coated Pt NP solution was added with stirring. The mixture was heated in a silicon oil bath at 120 °C for 8 h without disturbing before allowing it to cool down to room temperature naturally. The product was washed 6 times with DMF and 3 times with methanol before any characterization. Effect of PVP Concentration on the Particle Distributions in the Crystals: Au NPs were capped by PVP (Mw = 55 000) first. Then 1 mL of a MeOH solution of Au NP, 5 mL of a MeOH solution of 2-methylimidazole (25 mM), and 5 mL of a MeOH solution of Zn(NO3)2·6H2O (25 mM) were mixed. The PVP concentration in the solution was adjusted to 10−5 (sample 1), 5 × 10−4 (sample 2), 2 × 10−3 (sample 3), 10−3 (sample 4), 2 × 10−2 (sample 5), and 4 × 10−2 wt% (sample 6). The solutions were then allowed to react at room temperature for 24 h without stirring. The product was collected by centrifugation, washed several times with ethanol, and vacuum-dried over-night. Encapsulation of Fe3O4 NPs and CdSe NPs in ZIF-8: An aliquot of 0.5 mL of a MeOH solution of Fe3O4 NPs, 5 mL of the MeOH solution of 2-methylimidazole (25 mM), and 5 mL of a MeOH solution of Zn(NO3)2·6H2O (25 mM) were mixed and allowed to react at room temperature for 5 min without stirring; then 0.5 mL of a MeOH small 2014, DOI: 10.1002/smll.201303564
solution of CdSe was added. The solutions were then allowed to react at room temperature for 24 h without stirring. The product was collected by centrifugation, washed several times with ethanol, and vacuum-dried overnight. Fluorescence Quenching of CdSe NP@ZIF-8 with Various Thiols: Three kinds of thiols were used in the quenching experiment: 2-mercaptoethanol, 4-mercaptobenzoic acid, and cyclohexanethiol. A MeOH solution of CdSe NP@ZIF-8 (2 mL) was added in the cell. The time-resolved photoluminescence (PL) measurement was taken out for 5 min before 100-µL aliquots of different kinds of thiol solutions (100 mM in methanol) was injected. The quenching process was then recorded. UV–Vis Characterization of the Reduction of Nitroarenes: Two kinds of nitroarenes were used: p-nitrophenol and nitrobenzene. 1 mmols of nitroarene and 0.05 mol of NaBH4 were stirred in 10 mL of double-distilled water at room temperature under a N2 balloon for 15 min. Then an appropriate amount of catalysts (10 mg of PVP-coated Au NPs or 100 mg of Au NP@ZIF-8 powder) was added, and the contents were allowed to stir for the appropriate amount of time. A 200-µL solution was taken and diluted to 2 mL and centrifuged for UV–vis spectral characterizations at 0, 10, 20, 30, 40, 50, 60, and 90 min, respectively. Material Characterization: The surface morphologies of the NP@MOF composites were characterized using a JEOL-7600F field-emission scanning electron microscope (FESEM). The encapsulation process was characterized using a JEOL-2100F transmission electron microscope (TEM). The containment of the NPs in the MOFs was characterized by energy-dispersive X-ray spectroscopy (EDX). Powder X-ray diffraction (XRD) was taken at room temperature on a θ/2θ mode using a Bruker D8 Advance. The DLS and zeta-potential measurements were performed at 25 °C with a Malvern Nanosizer. The solutions were cleaned by passing solutions through 0.2-µm filters to remove the dust particles before mixing them into the scattering cell. The signal was collected at a beam angle of 173°. The ex-situ zeta-potential measurement was carried out by removing the solution of interest from the ZIF-8 reaction solution, and by injecting the resultant solution at time intervals into the cell for measurement. The in-situ DLS measurement on the process of Au NPs encapsulation in ZIF-8 crystals were taken under the following conditions: a) The control experiment on the addition of the NP solution at the beginning of the experiment. To slow down the nucleation process and obtain good signal for the DLS measurement, a diluted reactant solution was used. A 200-µL aliquot of a MeOH solution of the Au NPs, a 1-mL aliquot of a MeOH solution of 2-methylimidazole (5 mM), and a 1-mL aliquot of a MeOH solution of Zn(NO3)2·6H2O (5 mM) was mixed in the cell. Measurements began after 30 s, in order to allow the solution to first stabilize. b) The control experiment on the addition of the NP solution, after reaction for 5 min. A 1-mL aliquot of the MeOH solution of 2-methylimidazole (25 mM) and a 1-mL aliquot of the MeOH solution of Zn(NO3)2·6H2O (25 mM) was mixed in the cell. It was allowed to react for different time intervals before 200 µL of Au-NP MeOH solution was added. To minimize the effect of particle polydispersity on the measurement, DLS monitoring was carried out for the 5 min before the large NPs appeared in the solution. The sample for TEM demonstration was obtained with the addition of Au NPs after the reaction was allowed to continue for 20 min. To better visualize the Au NP core using TEM, the concentration of the Au NPs was
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S. Z. Li and F. Huo
increased fourfold for the synthesis of ZIF-8 (core)/Au NPs/ZIF-8 (shell) structures.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the start-up fund of Nanjing University of Posts& Telecommunications (NY213098) and National Science Foundation of China (GZ213054).
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Received: November 18, 2013 Published online:
small 2014, DOI: 10.1002/smll.201303564
