Understanding the Formation Mechanism of Metal Nanocrystal@ MOF

Article pubs.acs.org/cm

Understanding the Formation Mechanism of Metal Nanocrystal@MOF-74 Hybrids Ignacio Luz,§,‡ Anna Loiudice,†,‡ Daniel T. Sun,† Wendy L. Queen,† and Raffaella Buonsanti*,† §

Department of Materials Science, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States Department of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne, Sion 1950, Switzerland

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S Supporting Information *

ABSTRACT: We report a one-step synthetic strategy to encapsulate copper nanocrystals (Cu NCs) in a carboxylatebased metal−organic framework (MOF-74) to construct Cu@ Cu-MOF-74 hybrids. One of the hurdles to realizing metal NC@MOF hybrids with uniform morphology and with a homogeneous NC spatial distribution by chemical transformation is to precisely match the metal dissolution rate and the MOF crystallization rate. Our study defines the parameter space which controls this kinetic balance. Such an understanding and the generality of our approach is crucial to develop rules of design for NC@MOF hybrids, which represent an attractive class of materials for several applications.

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INTRODUCTION Colloidal nanocrystals (NCs) are atomically defined building blocks whose properties can be finely tuned through their size, shape, and composition.1−5 The assembly of colloidal NCs with domains of different chemical nature (i.e., polymers, glasses, and carbon-based materials) show considerable promise to satisfy the demand for complexity of several applications ranging from gas storage to energy storage to catalysis.6−14 The functionalities of such hybrid materials are dictated not only by the intrinsic properties of the single units but also by the morphology of the hybrids, the reciprocal spatial distribution of their constituents and the interfaces between them.6−14 Therefore, it is crucial to develop synthetic approaches to control these features. Hybrids comprising NCs and metal−organic frameworks (MOFs) are emerging as a new class of multicomponent materials with interesting catalytic, gas adsorption, and optical properties.11−31 In particular, synergistic effects have been observed in core@shell metal NC@MOF hybrids in hydrogen storage and in a variety of organic catalytic reactions.11−14 Three main approaches are commonly employed for encapsulation of metal NCs into MOFs. The first method involves the wet- or gas-impregnation of presynthesized MOFs with metal precursors, which are subsequently converted into the NCs.15−20 Here, small and naked NCs are embedded into the frameworks; however, the control of size, shape, and composition of the NCs themselves is highly constrained by the preformed frameworks.15−20 The second approach includes the encapsulation of presynthesized NCs in MOFs by addition of the MOF ligands and precursors.15−22 The use of presynthesized NCs enables a much better control and broader tunability of size, shape, and composition.21,22 Heterogeneous and homogeneous NC seeding facilitates the device integration of © XXXX American Chemical Society

MOFs and NC@MOF by enabling their patterning on various substrates.20,23 In the third approach, preformed NCs are chemically converted into the MOF by addition of the MOF ligands only.15−20 Solvothermal transformation of metal oxides has enabled the assembly of two- and three-dimensional MOF architectures on various substrates, such as on polymers used for separation applications.24−29 Despite the demonstrated potential of chemical conversion, the synthesis of metal NC@ MOFs by such approach still remains an underexplored area compared to the oxide counterpart.12,30−32 To the best of our knowledge, the only example of an in situ transformation of metal NCs into hybrids is the conversion of PVP(polyvinylpyrrolidone)-functionalized NiPt into [email protected] However, the presence of PVP, a surfactant commonly used to stabilize the NCs in the MOF growth medium, impedes a pristine interface between NCs and MOF, which is important to fully exploit synergistic interactions between the two systems.11−14,21,33 Herein, we report a one-step synthetic strategy to encapsulate ligand-free (naked) Cu NCs into a carboxylatebased MOF (Cu-MOF-74) for the formation of Cu@Cu-MOF74 hybrids, which have not been reported so far. The system selection was dictated by the following. With regards to the choice of the Cu as metal: (1) Cu NCs are active catalysts for many high temperature gas phase reactions, such as methane production and methanol synthesis from CO2, as well as electrochemical CO2 reduction.34−36 (2) To date, only a few examples of Cu NCs embedded in framework materials have been reported and these are grown by chemical vapor Received: March 2, 2016 Revised: April 20, 2016

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DOI: 10.1021/acs.chemmater.6b00880 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials infiltration.35−37 (3) The as-synthesized Cu@MOF-177, Cu@ MOF-5, and Cu/ZnO@MOF-5 have shown decent activity as catalysts for methanol synthesis.37−39 With regards to the choice of the framework material, M-MOF-74 (alternatively known as M2(dobdc), CPO-27-M, or M2(dhtp), where M = Mg, Mn, Fe, Co, Ni, Cu, or Zn) constitutes an extensive framework family with extreme chemical versatility and thermal and chemical stability, allowing for a wide variety of applications ranging from gas storage and separations to ion conductivity and catalysis.40−57 The M-MOF-74 structure type consists of one-dimensional metal oxide chains that are connected by 2,5-dioxido-1,4-benzene-dicarboxylate (dobdc4−) ligands to form a hexagonal array of one-dimensional channels.51 Upon heating the material under dynamic vacuum a solvent molecule is freed from each metal, leaving behind open-coordination sites that point into the channel, a structural feature shown to induce selectivity in the adsorption of small guest molecules and also to facilitate chemical reactions on the framework surface.43,44 It is hypothesized that the creation of metal NC@M-MOF-74 hybrids can promote synergistic effects as the framework can provide size and shape selectivity of guest molecules, concentrate guest molecules around the catalysts, and also promote the activation of the guest molecules, allowing the MOF to play a direct role in the catalytic process and/or enhance the catalytic behavior of the metallic NCs.11−14,58 Despite the intrinsic potential of NC@MOF-74 hybrids, only a few examples have been reported so far in the literature.12,59 In one example, Ni@Ni-MOF-74 hybrids were obtained by a one step procedure wherein Ni-MOF-74 was treated at high temperature (350 °C for 12 h) to form aggregated, polydispersed Ni NCs within the framework.59 One of the hurdles to realizing metal NC@MOF hybrids with uniform morphology and with a homogeneous NC spatial distribution by chemical transformation is to precisely match the metal dissolution and the MOF crystallization rate. Our study defines the parameter space that provides control of the NC dissolution rate. By explaining the role played by the solvent polarity, the reaction temperature, and the reactant concentration, we contribute to build new knowledge toward a more rational approach to the synthesis of metal NC@MOF hybrids which contrasts with traditional trial and error methods. Cu@Cu-MOF-199 (alternatively known as HKUST-1, CuBTC, and Cu3(BTC)2)60 hybrids were synthesized to highlight the generality of our method. The access to a broader compositional library of metal NC@MOF is foreseen considering the variety of metal NCs attainable by colloidal chemistry and the generality of the chemistry described herein.61 Finally, we demonstrate the applicability of the developed metal NC-to-MOF conversion to grow the hybrids on different conductive supports, thus providing an effective strategy for their future integration into devices.

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free conditions using a standard Schlenk line setup. In a typical synthesis, TOA (10 mL) was heated at 130 °C inside a 50 mL threeneck flask for 30 min under a flow of N2 to remove water and dissolved O2. After cooling to room temperature, 1 mmol of CuOAc and 0.5 mmol of TDPA were added with vigorous stirring. The solution was flushed with N2, heated to 180 °C with a rate of 10 °C/ min, maintained there for 30 min, heated to 270 °C with a rate of 10 °C/min, and then held there for an additional 30 min. The purplish red colloidal solution was cooled to room temperature naturally after removing the heating mantle. The 50 nm Cu NCs were prepared by slightly modifying the procedure reported by Yang et al.63 A stock solution of copper was prepared by dissolving 0.1 g of CuCl and 0.2 g of OLA in 2 mL of squalene on a hot plate at 200 °C in the nitrogen-filled glovebox. 19 mL of OLA and 1 mL of TOP were mixed in a 50 mL three-neck flask under nitrogen. The flask was then heated to 330 °C with a rate of 10 °C/min. Next, 2 mL of Cu stock solution was quickly injected into the hot flask. The reaction was held at 330 °C for 3 min and then cooled down using a water bath. After synthesis, the NCs were precipitated by addition of ethanol through centrifugation at 4000 rpm for 15 min. After three washing cycles, the resulting capped Cu NCs were redispersed in the required amount of hexane to obtain a 0.3 M Cu solution (determined by thermogravimetric analysis) and kept under N2 in the glovebox. Ligand Stripping. Naked Cu NCs were obtained by ligand exchange with Me3OBF4.61 One milliliter of the 0.3 M solution of capped Cu NCs in hexane was added to a solution of 50 mg of Me3OBF4 in 1 mL of CH3CN and was vigorously stirred for 1 min in a nitrogen-filled glovebox. One milliliter of toluene was added to the mixture, and the naked particles were precipitated through centrifugation at 4000 rpm for 15 min. Subsequently, the product was washed two times by redispersion in 0.5 mL of DMF and precipitation with 2 mL of toluene. The required amount of DMF was added to the naked Cu NCs to prepare a 0.3 M solution. Synthesis of Cu@Cu-MOF-74. The general procedure to prepare Cu@Cu-MOF-74 hybrids consisted of adding a specific amount of H4dobdc depending on the selected molar ratio R (R = [ligand]/[Cu], ranging from 0.2 to 3) dissolved in 0.5 mL of DMF into a mixture containing 50 μL of the aforementioned 0.3 M solution of naked Cu NCs (Table S1). The resulting solution was then diluted with EtOH and/or DMF to prepare 5 mL solutions with various EtOH/DMF volume ratios (L, ranging from 0 to 10). All the syntheses were carried out in 8 mL scintillation vials inside a N2 purged globe box at a temperature (T) equal to 60 °C with vigorous stirring. As described in the Article, different conditions (in atmosphere or N2-filled glovebox) and reaction times (t) were explored (t ranging from 0 to 12 h). Reactions were halted at the desirable conversion time by addition of 2 mL of MeOH and the resulting solids were recollected by centrifugation. The precipitates were redispersed and reprecipitated three times, each time by adding 2 mL of MeOH. Synthesis of Cu@Cu-MOF-199. Cu@Cu-MOF-199 hybrids were obtained with R = 2 in 0.5 mL of DMF and 50 μL of the aforementioned 0.3 M solution of naked Cu NCs. The solutions were again diluted with EtOH and DMF to prepare a 5 mL solution with a volume ratio L = 1. The syntheses were carried out in 8 mL scintillation vials under inert conditions inside a globe box or in atmosphere at 60 °C with vigorous stirring for 5 min. The remaining steps were the same as for Cu@Cu-MOF-74. Synthesis of Cu@Cu-MOF-74 on Substrate. Ten microliters of a 0.3 M hexane solution of Cu NCs were drop-casted on a 1 × 1 cm2 carbon paper. As an alternative method to drop-casting, the 1 × 1 cm2 carbon paper can also be soaked overnight in 1 mL of the same NC solution for a more uniform substrate coverage. Fifty nanometer Cu NCs were used for this experiment to facilitate imaging by scanning electron microscopy. The substrate was then immersed in 1 mL (EtOH/DMF = 1:1) containing different amount of ligands (ranging from 0.02 to 2 mg). Optimal amount of ligands was found to be 1 mg (see Figure 9). The reaction was carried in air, at room temperature and for 12 h. The substrate was then washed with methanol to eliminate unreacted ligands.

EXPERIMENTAL SECTION

Materials. Copper (I) acetate (CuOAc, 97%), copper (I) chloride (CuCl, 99.999%), 2,5-dihydroxyterephthalic acid (H4dobdc, 98%), 1,3,5-benzenetricarboxylic acid (H3btc, 98%), trioctylamine (TOA, 98%), oleylamine (OLA,70%), trioctylphosphine (TOP, 90%), tetradecylphosphonic acid (TDPA, 97%), and trimethyloxonium tetrafluoroborate (Me3OBF4) were all purchased from Sigma-Aldrich and used as received. All solvents used were of analytical grade, anhydrous, and purchased from Sigma-Aldrich. Synthesis of Cu NCs. The 8 nm Cu NCs were prepared by decomposition of CuOAc in TOA in the presence of TDPA following the procedure of Hung et al.62 The synthesis was carried out under airB


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Chemistry of Materials Transmission Electron Microscopy. TEM images were recorded on an Analytical JEOL-2100F FETEM using a beam energy of 200 kV, equipped with a Gatan camera. Samples were drop-casted on a copper TEM grid (Ted Pella, Inc.) prior to imaging. Size statistics was performed using the software ImageJ and counting at least 50 particles per sample. Scanning Electron Microscopy. SEM images were acquired on a FEI Quanta 200 FEG Analytical Scanning Electron Microscope using a beam energy of 5 kV. Samples were drop-casted on a silicon substrate prior to imaging. Powder X-ray Diffraction. The XRD pattern reported in the Article was acquired on a Bruker D8 Discover diffractometer with a Cu Kα source equipped with a Lynxeye one-dimensional detector. The data reported in the ESI were collected with a Bruker D8 Discover with a Cu Kα source equipped with a Vantec-500 areal detector. Powder samples were measured. X-ray Photoelectron Spectroscopy. XPS was performed using a monochromatized Al Kα source (hν = 1486.6 eV), operated at 225 W, on a Kratos Axis Ultra DLD with a pass energy for narrow scan spectra of 20 eV, corresponding to an instrument resolution of approximately 600 meV. Survey spectra were collected with a pass energy of 80 eV. Unless otherwise noted, measurements were performed at a takeoff angle of 0° relative to the surface normal. Spectral fitting was performed using Casa XPS analysis software. Spectral positions were corrected by shifting the primary C 1s core level position to 284.8 eV, and curves were fitted with quasi-Voigt lines following Shirley background subtraction. Samples were deposited on conductive Si substrates. Measurements were taken on three different areas of each sample. N2 Sorption Isotherms. Nitrogen uptake was measured at 77 K using a Micromeretics Tristar II system. Samples were outgassed at 150 °C overnight for approximately 16 h. Surface area measurements were calculated using the Brunauer−Emmett−Teller (BET) method.

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RESULTS Influence of the Reaction Conditions. Cu@Cu-MOF-74 hybrids were synthesized via partial conversion of naked Cu NCs into Cu-MOF-74 in the presence of H4dobdc in a solvent mixture of DMF and EtOH. The influence of several reaction parameters, such as reaction time, reactant concentration, and solvent identity, on the formation of the hybrids was systematically investigated; the results are described below. The conditions necessary to obtain porous and crystalline Cu@ Cu-MOF-74 hybrids with optimal features, which are a uniform morphology and a homogeneous distribution of embedded Cu NCs, were identified. These conditions were finally determined to be EtOH/DMF volume ratio (L) equal to 1, ligand-to-Cu molar ratio (R) equal to 2, reaction temperature (T) equal to 60 °C, and reaction time (t) equal to 40 min. Effect of the Reaction Time. Figures 1 and S1−S5 illustrate the typical temporal evolution of the growth of Cu@Cu-MOF74 starting from naked Cu NCs of 8 nm in diameter (Figure 1a,b). TEM images capture the conversion of the NCs into the MOF matrix at the nanoscale, while SEM pictures show the morphology change of the hybrids at the macroscale. At the earliest growth stage (Figure 1c,d) the Cu NCs were surrounded by a lower contrast matrix. This matrix corresponded to a nonporous and poorly crystalline material (Figures S3−S5) and is referred to as Cu-CP in the rest of the Article. Subsequently, the Cu-CP continued to grow, and after 30 min it converted into Cu-MOF-74, with no residual Cu-CP phase (Figures 4, 6, and S1−S5), while the Cu NCs became smaller (Figure 1e,f). At the final stage the Cu NCs were dissolved below a size detectable by TEM (Figure 1g,h). SEM shows that the morphology of the hybrids evolved from 50 nm short rods at t = 10 min to hyper-branched rods, to sheaf-like

Figure 1. Typical temporal evolution of the hybrids during the growth. SEM (left panels) and TEM (right panels) of Cu@Cu-MOF-74 hybrids at different reaction times: (a,b) t = 0 min; (c,d) t = 10 min; (e,f) t = 40 min; (g,h) t = 60 min. For this series, the following reaction conditions were used: L = 1, R = 2, T = 60 °C in ambient atmosphere.

rods, and eventually to spherulitic-like structures (Figures 1c,e,g and S2a,c). The formation of elongated crystals is consistent with the one-dimensional channels of the MOF-74 structure type.49 Figure 2 illustrates the temporal evolution of the NC size distribution at different reaction stages. The initial Cu NCs uniformly dissolve during the conversion, and this results in a narrow size distribution of the NCs in the final material. Effect of the Ligand-to-Cu Molar Ratio. The influence of R on the nanoscale and macroscale features of the Cu@Cu-MOF74 hybrids was systematically investigated. Figure 3 shows SEM and TEM images of the Cu@Cu-MOF-74 hybrids obtained for R values ranging from 0.2 to 2 and a reaction time of 40 min. The reaction with R = 0.2 (Figure 3a,b) was not sufficient to drive the conversion, and instead only a thin layer of an amorphous matrix formed around ∼6 nm Cu NCs. For R = 0.5 (Figures 3c,d and S2−S5) Cu-CP needles decorated with ∼4− C


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Figure 2. Temporal evolution of the NC size distribution over time: (a) size histograms from TEM images in Figure 1 and (b) average size versus reaction time.

5 nm NCs formed. While reactions with R = 1 begin to render the formation of Cu-MOF-74 with (Figures 3e,f and S3) ∼4−5 nm NCs encapsulated in the framework, the NCs were not uniformly distributed in the MOF matrix. Additional reactions with R = 2 (Figure 3g,h) gave rise to an optimal sample, yielding porous, crystalline Cu@Cu-MOF-74 hybrids with 2−3 nm Cu NCs uniformly distributed throughout the spheruliticshaped hybrids (corresponding XRD and adsorption data are reported in Figures 5 and 7, respectively). While similar results were obtained for R = 3, for R bigger than 3 the rate of NC conversion to Cu-MOF-74 became much faster and full dissolution of the NCs occurred within 10 min. Effect of the Solvent. The influence of the solvent mixture was evaluated by starting from a fixed amount of Cu NCs at L ranging from pure DMF (L = 0) to almost pure EtOH (L = 10) with the other reaction parameters set to their optimal values. The results are shown in Figures 4 and S6−S7. In pure DMF, total dissolution of the NCs and unidentified amorphous, hollow structures were obtained (Figure S6), suggesting that EtOH is needed to form the hybrids. Slower conversion kinetics were observed as L increased. Figure 4 shows that for L = 0.1 at 40 min the NCs were fully converted into the CuMOF-74 (Figure 4a,b and S7). For L = 1 nicely dispersed 2−3 nm Cu NCs were encapsulated into the Cu-MOF-74 matrix, and the hybrids assumed a uniform spherulitic-like morphology (Figure 4c,d with corresponding XRD and adsorption data in Figures 5 and 7, respectively). EtOH, used as the main solvent, lended to a further decrease of the reaction kinetics. At 40 min 4−5 nm Cu NCs were embedded in two-dimensional platelets, around 100 nm in size (Figure 4e,f). Up to 3 h were needed for full conversion. It should be noted that the platelet phase is not Cu-MOF-74 and has not been identified yet due to the low reaction yield under the aforementioned conditions.

Figure 3. Dependence of the hybrid growth on the Cu-to-ligand molar ratio. SEM (left column) and TEM (right column) of Cu@Cu-MOF74 hybrids synthesized at different R: 0.2 (a,b); 0.5 (c,d); 1 (e,f); 2 (g,h). For this series, the following reaction conditions were used: L = 1, t = 40 min, T = 60 °C, in ambient atmosphere.

Effect of the Reaction Temperature. The effect of this parameter on the resulting hybrids was investigated under the optimal conditions (R = 2, L = 1, t = 40 min), which were selected due to the formation of a highly porous Cu-MOF-74 material that contains homogeneously spaced NCs. Based on the results of these experiments (Figure S8) 60 °C was chosen as the optimal reaction temperature. At room temperature, only a very thin layer of amorphous matrix was observed. Above 60 °C the increased rate of the reaction kinetics did not allow a fine control of the hybrid formation. Characterization of Cu@Cu-MOF-74 Hybrids. X-ray Diffraction Analysis (XRD). The powder XRD pattern of the initial Cu NCs matched well the fcc structure of Cu (Figure S9). Figure 5 shows a typical XRD pattern of Cu@Cu-MOF-74 hybrids synthesized in the optimal reaction conditions (L = 1, R = 2, T = 60 °C, and t = 40 min). The peak positions and intensities are indicative of the Cu-MOF-74 structure in these hybrids.49 The presence of the 2−3 nm Cu NCs was not detected in the XRD pattern of the hybrids, even when using D


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The results are shown in Figure 6. The presence of Cu2+ in all the samples is evidenced by the 2p3/2 peak component centered around 934 eV and the two satellites peaks at higher binding energies.64,65 The 2p3/2 peak component centered around 932 eV is usually associated with reduced Cu, which can be Cu0 or Cu1+. While it was not possible to distinguish between the two even by analysis of the Auger peaks, considering the decreased intensity as a function of reaction time, we are confident that the signal at 932 eV is attributed to the NCs. It is noted that XPS is a surface sensitive technique. Ninety-five percent of the signal is collected from the first 3 nm, considering that the electron inelastic mean free path for copper is around 1 nm at these kinetic energies. While this analysis is not representative of the bulk, XPS still allows us to follow the overall reduction kinetics with the reduced Cu decreasing with the increase of the reaction time, which is consistent with NC dissolution evidenced by the TEM results. At t = 60 min the TEM indicates a full conversion to the MOF; this is further supported by the significant reduction in the amount of reduced Cu0 from 38% to 5%. Gas-Sorption Properties. To exploit the synergistic properties of the NCs and MOFs, the MOF needs to retain porosity so that the NCs remain accessible to guest species. To probe porosity, N2 isotherms were measured for both Cu-MOF-74 obtained from the complete conversion of Cu NCs and the optimized Cu@Cu-MOF-74 hybrid. The adsorption and

Figure 4. Dependence of the hybrid growth on the EtOH/DMF volume ratio. SEM (left column) and TEM (right column) of Cu@ Cu-MOF-74 hybrids synthesized at different L: 0.1 (a,b); 1 (c,d); 10 (e,f). For this series, the following reaction conditions were used: R = 2, t = 40 min, T = 60 °C, in ambient atmosphere.

Figure 5. Structural analysis of Cu@Cu-MOF-74 hybrids. Typical XRD pattern of Cu@C-MOF-74 hybrids synthesized in the optimal conditions (L = 1, R = 2, T = 60 °C, and t = 40 min).

synchrotron radiation (Figure S10), due to the peak broadening and weak signal of the NCs compared to the MOF. This observation was consistent with other works reporting about 2−3 nm metal NCs encapsulated in MOFs.13,14,30 X-ray Photoelectron Spectroscopy (XPS). To confirm the presence of metallic Cu NCs embedded in the MOF framework, the composition of the hybrids was further assessed by XPS. XPS allows one to semiquantitatively distinguish between the Cu in the NCs and the Cu in the MOF, as they are expected to be in the oxidation states 0 and +2, respectively.

Figure 6. Compositional variation of the hybrids with the reaction time. Typical XPS spectra of Cu@Cu-MOF-74 hybrids at different t: (a) t = 10 min; (b) t = 30 min; (c) t = 40 min; (d) t = 60 min. The Cu0 percentage ratio (from total Cu) is indicated. For this series, reaction conditions were set to L = 1, R = 2, T = 60 °C. TEM images corresponding to these samples are shown in Figure 1. E


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uniform size and morphology of the as-prepared MOF combined with a homogeneous spatial distribution of the encapsulated NCs; these essential properties are expected to influence the functionality of NC@MOF hybrids in various applications. Unique to our approach is the use of naked Cu NCs. The absence of the native ligands on the surface promotes faster dissolution kinetics compared to other literature reports.12 In our study, optimal NC@MOF hybrids, with uniformly distributed Cu NCs, were synthesized in as little as 30 min. Instead, 24 h were required to obtain the same hybrids when using ligand-capped 8 nm Cu NCs (Figure S11). As another example, the reaction time needed for the synthesis of Pt−Ni@ Ni-MOF-74 hybrids from PVP-coated NiPt NPs in DMF was 12 h.12 The faster NC-to-MOF conversion kinetics when naked NCs are employed are consistent with a recent study on MOF formation by interconversion of metal oxide nanocrystals, wherein the NC etching rate of the metal oxide was proven to be critical for optimizing the MOF synthesis.67 The progressive dissolution of the Cu NCs was evidenced by their reduced size in the TEM images taken over time (Figures 1 and 2), the disappearance of Bragg reflections of Cu in the XRD pattern (Figure 5), and a decrease in the reduced Cu0 content in the XPS spectra (Figure 6). Oxidative etching is the most plausible mechanism leading to the dissolution of the Cu NCs:

desorption of N2 (up to 1 bar at 77 K) in Figure 7 confirms high BET surface areas of 985 and 394 m2/g, respectively. The decreased surface area of the hybrid compared to the bare CuMOF-74 is consistent with the actual encapsulation of the NC in the framework material in agreement with previous work.14,21,59,66 The sharp uptake of N2 at lower relative pressures indicates a standard type I isotherm, which is characteristic of microporous materials and consistent with the structural properties of Cu-MOF-74 and previous surface area measurements (937 m2/g).40 We further did not observe any additional uptake at high relative pressures, which implies the absence of macropores generated by packing of the NCs. This further supports NC incorporation into the framework, an observation that is consistent with the aforementioned TEM images (Figure 1). Thermal Stability. As it is well-known that MOF-74 is a widely used framework material due to its excellent chemical robustness and thermal stability.40−57 Based on the profiles of TGA analysis (Figure S5), the framework decomposition starts at about 350 °C for the Cu@Cu-MOF-74, which is in good agreement with the as-prepared pure Cu-MOF-74.40

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DISCUSSION

We have developed a dissolution/precipitation approach to encapsulate naked NCs, synthesized by means of colloidal chemistry, in a carboxylate-based MOF. As a proof-of-concept, we have focused on Cu@Cu-MOF-74 hybrids, which were yet to be reported. The proposed synthetic strategy is based on the oxidative etching of naked colloidal Cu NCs, and we demonstrate that the reaction kinetics can be controlled through a rational choice of reaction conditions, which were systematically screened. Understanding how variations in these conditions affect product formation is essential to define the design rules toward controlling composition, topology, and morphology of these novel systems. Formation Mechanism of Cu@Cu-MOF-74 Hybrids. Scheme 1 describes the two main processes involved in the formation of the Cu@Cu-MOF-74 hybrids: oxidative etching of the Cu NCs (dissolution reaction) and heterogeneous nucleation and growth of the framework material (precipitation reaction). A careful balance between the kinetics of these two processes is needed to succeed in the synthesis of hybrids with both

Cu(0) + O2 + 4H+ ⇒ Cu 2 + + 2H 2O

In agreement with previous reports, it is hypothesized that the O2 and a mildly acidic environment generated by the dissociation of H4dobdc participate in the dissolution process of the NC seeds, likely through the formation of a copper oxide shell.12,32,68 This mechanism is consistent with the fact that Cu NC conversion into Cu@Cu-MOF-74 does not proceed when reactions are carried out in an inert atmosphere (Figures S12 and 13).12,32,68 To obtain NC@MOF hybrids, both thermodynamic and kinetic requirements need to be satisfied. First, heterogeneous nucleation of the MOF on the NC seeds must be favored over homogeneous nucleation of the MOF in solution. Second, the growth rate of the MOF has to be properly modulated to trap the NCs inside the framework and obtain core@shell hybrids. When the first but not the second requirement is satisfied, a different configuration of the hybrids might form (i.e., NCs decorating the MOF surface, as shown in Figure 3d). According to Classical Nucleation Theory, to favor heterogeneous over homogeneous nucleation, the monomer concentration must remain below the supersaturation level.69 To be in such a regime, an exceedingly fast dissolution of the NCs must be avoided. Furthermore, we observe that if the dissolution is too fast (T > 100 °C, R > 3 or L = 0.1) uncontrollable and full conversion of the Cu NCs into Cu-MOF-74 occurs (A). However, when the oxidative etching of the NCs is considerably slower (inert atmosphere, T < 60 °C or R ≤ 0.5), and thus the monomer flux in solution is extremely low, hybrids comprising a nonporous and poorly crystalline coordination polymer (Cu-CP) form instead of Cu-MOF-74 (C). It is interesting to note that when ligand-capped 8 nm Cu NCs were used as nucleation seeds, a mixture of Cu-CP and Cu-MOF-74 was observed at t = 40 min (Figure S11). Eventually, the Cu@Cu-CP hybrids transformed into Cu@CuMOF-74 after 24 h. This slower conversion rate is consistent with a slower dissolution rate of the NCs due to the presence of

Figure 7. Adsorption properties of the hybrids. N2 adsorption (■)/desorption (□) isotherms at 77 K of Cu@Cu-MOF-74 hybrids (L = 1, R = 2, T = 60 °C, t = 40 min) and pure Cu-MOF-74 obtained by complete conversion using R = 10 and L = 1. F


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Scheme 1. Proposed Formation Mechanism of the Cu@Cu-MOF-74 Hybrids (Top) and Impact of NC Dissolution Rate on the Growth of the Same (Bottom)

during a reaction with the monomer supply rate being one of these.70 Quantification of the monomer concentration would be desirable; however, the dissolution/precipitation mechanism impedes such a quantitative analysis. As soon as the Cu2+ ions are released in solution, they react with dobdc4− to form CuMOF74 (or Cu-CP depending on the reaction conditions). The washing solution appears completely clear as an indication that all the Cu2+ ions are converted into the framework. Furthermore, our findings suggest that at low monomer supply rate, the crystallization of the Cu-MOF-74 does not occur,

capping ligands. In the ideal reaction conditions (B), after heterogeneous nucleation occurs, the dissolving Cu NCs become trapped in the growing framework material (Figure 1). During the earlier growth stages (t < 10 min, Figures 1c,d and S3−S5), Cu@Cu-CP hybrids first formed, which subsequently transformed into Cu-MOF-74 at longer reaction times (t > 20 min, Figures 1e−h and S2). From a thermodynamic standpoint, phase transitions are governed by changes in the chemical potential in the reaction medium.70 Different parameters modulate the energy landscape G


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Chemistry of Materials instead Cu-CP forms. While a detailed investigation and discussion are out of the scope of the present work, we speculate that Cu-CP is a reaction intermediate of Cu-MOF-74. Role of the Solvent System. A 1:1 volume ratio of DMF and EtOH was found to be a suitable medium for the growth of the hybrids. While discussing the role of the solvent, it is helpful to bear in mind that the naked Cu NCs are soluble in DMF, so a minimum amount of this solvent is needed to hinder aggregation of the seeds. However, the dissolution kinetics in almost pure DMF (L = 0.1) are too fast and prevent a homogeneous distribution of encapsulated NCs (Figures 4a,b). We rationalize this observation with a greater extent of H4dobdc deprotonation in DMF, which possesses a higher dielectric constant than EtOH (36.7 vs 24.5). As a reminder, deprotonation of the ligands accounts also for the slightly acidic environment, which is responsible for NC dissolution. By slowing down the dissociation of the linkers, the addition of ethanol can be used to decrease the NC dissolution rate. In the presence of benzyl alcohol, which has an even lower dielectric constant (13) compared to EtOH, the reaction kinetics became exceedingly slow; the conversion of the Cu NCs to the final Cu@Cu-MOF-74 product required more than 24 h (data not shown). It is interesting to compare our results with the study by Liu et al. where PVP-coated metallic NCs were encapsulated in Cu-MOF-199 by using a sacrificial shell of Cu2O as the metal-ion source.32 In this work, the lower polarity of the solvent was key to impede the homogeneous nucleation of the Cu-MOF-199, thus preventing the hybrid formation. This points to another important factor, which is the activation energy barrier for nucleation of the framework itself.32 It is thought that the ease of nucleation of Cu-MOF-199 compared to Cu-MOF-74 requires much slower dissolution of the NCs. In our study, homogeneous nucleation was never observed for any of the screened reaction conditions. Role of the Ligands. In our synthetic scheme, the ligands serve a double purpose. In addition to functioning as a framework strut in the MOF structure, their deprotonation contributes to the oxidative etching of the Cu NCs. According to the Cu-MOF-74 molecular formula [Cu2(dobdc)], R = 0.5 should correspond to the minimum ratio needed for the complete conversion of Cu NCs into MOF. Yet, for this ratio, Cu-CP formed (Figure S2−S5). Furthermore, the dissolution/ precipitation kinetics were not well matched, and NCs were not trapped within the Cu-CP, rather they decorated the surface of the rod-like Cu-CP structures (Figure 3c,d). For R = 1, while Cu-MOF-74 crystallized, the NCs were not homogeneously distributed within the matrix (Figure 3e,f). This might be a consequence of an inhomogeneous dissolution of the NCs. Also, we cannot exclude an interaction between the deprotonated ligands and the naked NC surface. While difficult to probe, such type of interaction may help to disperse the NCs in the reaction medium, thus aiding a homogeneous dissolution and uniform spatial distribution within the final hybrids. The possibility of such a surface coordination was also suggested by the experiments with ligand-capped NCs (Figure S11). As a consequence of these convoluted effects, an excess of ligand is required (R = 2) to obtain Cu@Cu-MOF-74 spherulitic crystallites. Higher R, which lead to a higher monomer flux, lend to an uncontrollable and complete transformation of the NCs into the MOF structure.

Figure 8. Generality of the approach. Typical TEM image (a), typical XRD (b), N2 adsorption (■)/desorption (□) isotherms (c) at 77 K of Cu@Cu-MOF-199 hybrids.

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CONCLUSIONS A NC-seeded growth approach has been developed to obtain a novel example of metal NC@MOF, wherein naked Cu NCs are transformed into a Cu-MOF-74 shell, which encapsulates them during the growth to form Cu@Cu-MOF-74 hybrids. Detailed structural and compositional data have been provided to demonstrate that each hybrid nanostructure is composed of 2− 3 nm Cu NCs that are homogeneously distributed throughout the porous crystalline framework. Uniform size distribution of the Cu NC seeds obtained by means of colloidal chemistry was key to the monodispersity of the NCs in the final materials. The formation mechanism of the Cu@Cu-MOF-74 proceeds through oxidative etching of the naked Cu NCs. The NC dissolution rate, which controls the monomer (Cu2+ metal ions) flux for the MOF growth, needs to be properly adjusted to satisfy thermodynamic and kinetic requirements for the formation of uniform hybrids. The role of reaction time, reaction temperature, ligand concentration, and solvent was elucidated, and these parameters were tuned to achieve such an optimal condition. While this work focuses on Cu@Cu-MOF74 hybrids, other carboxylic acid based ligands can be used to H


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Chemistry of Materials Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Most of this work was supported by Laboratory Directed Research and Development (LDRD) funding from Berkeley Lab, provided by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. Work at the Molecular Foundry and the Advanced Light Source was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02- 05CH11231. The authors thank Dr. Christine Beavers of the Advanced Light Source for her assistance in obtaining powder X-ray diffraction data on BL 12.2.2. W.L.Q. acknowledges support from the Swiss National Science Foundation under grant number PYAPP2_160581. A.L. and R.B. acknowledge the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

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Figure 9. SEM images of Cu@Cu-MOF-74 hybrids obtained by chemical conversion of 50 nm Cu spheres deposited on carbon paper obtained for different amount of ligand: (a) 0.02 mg, (b) 0.1 mg, and (c) 1 mg of H4dobdc in 1 mL of solvent (EtOH/DMF = 1:1).

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obtain different MOF shells. Figures 8 and S14 show Cu@CuMOF-199 as one example. To disclose the potentiality of the Cu@Cu-MOF-74 hybrids for device integration, we have demonstrated that the seeding approach reported here works on different substrates commonly used in electrocatalysis, such as glassy carbon and carbon paper (Figures 9 and S15). Charge transport studies and structural stability will be objects of future studies. Nowadays, a huge compositional library of NCs is accessible by colloidal chemistry and the general applicability of the ligand stripping approach employed in this work has been previously demonstrated.61 Therefore, the metal NC-to-MOF conversion described herein will be easily extended to NCs with different sizes and shapes or to more complex system, such as bimetallic NCs. Furthermore, the access to a pristine interface between inorganic NCs and MOFs will pave the way toward exploring new synergistic effects at the interface, especially in catalysis or gas adsorption where surface interactions dictate the material properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b00880. Details of experimental observations and characterization, including XRD, TGA, FTIR, SEM, and TEM (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: raffaella.buonsanti@epfl.ch. Author Contributions ‡

These authors contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. I


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Understanding the Formation Mechanism of Metal Nanocrystal@ MOF

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