Shape and Size Controlled Synthesis of MOF Nanocrystals with the

Article pubs.acs.org/Langmuir

Shape and Size Controlled Synthesis of MOF Nanocrystals with the Assistance of Ionic Liquid Mircoemulsions Wenting Shang,†,‡ Xinchen Kang,† Hui Ning,† Jianling Zhang,† Xiaogang Zhang,‡ Zhonghua Wu,§ Guang Mo,§ Xueqing Xing,§ and Buxing Han*,† †

CAS Key Laboratory of Colloid and Interface and Thermodynamics, Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, China ‡ Department of Chemistry, Renmin University of China, Beijing 100872, China § Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: In this work, the La-metal−organic frameworks (La-MOFs) were synthesized using lanthanum(III) nitrate and 1,3,5-benzenetricarboxylic acid (BTC) in H2O-in-1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6), bmimPF6-inwater, and the bicontinuous microemulsions stabilized by surfactant TX-100. The MOFs prepared were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), thermal gravimetric analysis (TGA), and FT-IR methods, and the microstructures of the microemulsions in the H2O/bmimPF6/TX-100 system were studied by small-angle X-ray scattering (SXAS) technique. It was shown that the dispersed droplets in the water-inbmimPF6, bicontinuous and bmimPF6-in-water microemulsions were spherical, lamellar, and cylindrical, respectively. The shapes of the La-MOFs synthesized were similar to that of the droplets in the corresponding microemulsions. This indicated that the morphology of MOFs could be controlled by the microstructures of the microemulsions. On the basis of the systematic experimental results, the mechanism for controlling the morphology of the MOFs was proposed.

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CO2-in-IL,21−23 and IL-in-IL24 microemulsions. The microemulsions have some unusual properties, and their applications in several fields have been studied, including chemical reactions,25,26 material synthesis,27,28 etc. The microstructures of the microemulsions have been characterized by various techniques, such as transmission electron microscopy (TEM),16 small-angle neutron scattering,17 small-angle X-ray scattering(SAXS),23,29 UV−visible spectroscopy,30 and dynamic light scattering.31 Metal−organic frameworks (MOFs) are a class of crystalline materials that consist of metal ions and organic ligands linked together by coordination bonds. In recent years, design and synthesis of MOFs have attracted much attention because of their potential applications in gas storage and separation,32,33

INTRODUCTION A microemulsion is a thermodynamically stable dispersion of two immiscible solvents stabilized by surfactants.1−3 Owing to the capacity to host a variety of polar and nonpolar species simultaneously, microemulsions have been widely applied in extraction,4 chemical reaction,5 and material synthesis.5−8 Ionic liquids (ILs) are organic salts that have low melting points (e.g., below 100 °C). They have many unique properties, such as negligible vapor pressure, wide liquid-temperature range, excellent solvents for both organic and inorganic substances, and their functions are designable.9 Applications of ILs in different fields have been studied extensively in recent years, including material synthesis,10 reactions,11 adsorption of acid gases,12 extraction and fractionation.13 IL microemulsions, in which at least one component is IL, have attracted much attention in recent years.14,15 Various IL microemulsions have been prepared, including IL-in-oil and oilin-IL,16−18 IL-in-water and water-in-IL,19,20 IL-in-CO2 and © XXXX American Chemical Society

Received: August 1, 2013 Revised: September 12, 2013

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catalysis,34 molecular detection,35 etc. With organic linkers, it is possible to fine-tune the shape, size, and functionality of the cavities and the internal surfaces. Different methods have been used to synthesize MOFs, including solvothermal, microwave, emulsions, ultrasonic method, etc.36−38 MOF crystals with various morphologies, such as hollow particles,39 nanocubes,40 nanorods,41 nanosheets,42 core−shell,43 and mesoporous44 structures have been fabricated. Controlled synthesis of MOFs is still a very interesting topic,39−48 although this has been studied extensively. It is wellknown that the microstructures of IL-in-water, water-in-IL, and bicontinuous microemulsions are very different, which provides excellent opportunities for controlled synthesis of MOFs with different sizes and morphologies. La-MOFs have great potential for applications and have been widely studied in recent years.49 Herein, we carried out the first systematic work to study the effects of subregions and microstructures of IL microemulsions on the size and morphology of the MOFs synthesized. A series of La-MOFs formed from lanthanum(III) nitrate and 1,3,5benzenetricarboxylic acid (BTC) were prepared in various H2O/bmimPF6/TX-100 microemulsions. The microstructures of the microemulsions were characterized by SAXS method. It was demonstrated that the size and morphology of the MOFs could be controlled by changing the microstructures of the microemulsions.

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Figure 1. Phase diagram of the H2O/bmimPF6/TX-100 ternary system at 25 °C.19 A, B, and C are water-in-bmimPF6, bicontinuous, and bmimPF6-in-water microemulsion subregions, respectively. a−i are the compositions of the microemulsions by which the MOFs are prepared.

region had three types of subregions, water-in-bmimPF6, bicontinuous, and bmimPF6-in-water microemulsions, which are marked as A, B, and C, respectively in Figure 1. In this work, we synthesized La-MOFs in different subregions, and the effects of the microstructures on the morphology of the MOFs were studied. The microemulsions selected are marked in Figure 1 as a−i. SAXS Study of the Water/bmimPF6/TX-100 Microemulsions. In this work, we studied the microstructures of the water-in-bmimPF6, bicontinuous, and bmimPF6-in-water microemulsions by SAXS technique, and the results are presented in Figure 2. The SAXS planar data were processed to by wolfram methematica8.0 one-dimension curve, and the distance distribution function p(r) was fitted by Igor23 and gnom.51 Figure 2A shows the SAXS curves of the microemulsions a−c marked in Figure 1. In this subregion, the water-in-IL microemulsions are formed; that is, water droplets are dispersed in IL continuous phase. The data in the small-angle region (q < 1) were used to obtain the size of the dispersed water droplets, and the results are given as the inset of the figure (the r−p(r) chart). The distance distribution curves are symmetric, suggesting that the dispersed water droplets are spherical.52,53 Additionally, in the region of q = 1.5 to 2.0 diffraction peaks can be observed, which is originated from the aggregation of the IL in the continuous phase.54,55 Figure 2B shows the SAXS curves of microemulsions d−f marked in Figure 1, which are in the bicontinuous region, i.e., IL and water droplets are dispersed in each other. The sizes of the droplets obtained from the SAXS data in the small-angle region (q < 1) are shown as the inset of the figure (the r−p(r) chart). For each size distribution curve, p(r) increases linearly with r at beginning. When r > T, the linearly decreases slowly, which is due to the interface loss occurred among monodisperse micelles, indicating that the droplets in the microemulsions are lamellar.56 Similarly, there are diffraction peaks in the region of q = 1.5 to 2.5, suggesting the aggregation of IL in the continuous phase. Figure 2C shows the SAXS curves of the microemulsions g−i marked in Figure 1. In the IL-in-water microemulsions, IL droplets disperse in water continuous phase. The inset (the r−p(r) chart) is the size distribution

EXPERIMENTAL SECTION

Materials. The IL 1-butyl-3-methylimidazolium hexafluorophosphate (bmimPF6) with a purity of >99% was provided by Lanzhou Greenchem ILS, LICP, CAS, China. It was dried under vacuum at 70 °C until the mass was not changed with drying time. The surfactant Triton X-100 was purchased from Alfa Aesar. It was also dried in an oven at 40 °C under vacuum for 24 h before use. La (NO3)3·6H2O was purchased from Sinopharm Chemical Reagent Co., Ltd. BTC was purchased from TCI (Shanghai) Co., Ltd. Preparation of the La-MOFs. In the experiment, the microemulsion of desired composition containing water, bmimPF6, and Triton X-100 was prepared using the method reported previously.19 La (NO3)3·6H2O (0.2 mmol) and BTC (0.2 mmol) were added into the microemulsion (5 g). The system was stirred for 24 h at 25 °C. Then, the product was collected by centrifugation and washed with ethanol and acetone solution (v/v = 1:1) three times (3 × 50 mL) to remove the surfactant and bmimPF6. Then, the products were dried in a vacuum oven at 70 °C for 10 h. Characterization. The SAXS experiments were conducted at Beamline 1W2A at the Beijing Synchrotron Radiation Facility.50 The wavelength was 1.54 Å, and the sample-to-detector distance was 1.596 m. The data were collected using a CCD detector (MAR) with a maximum resolution of 3450 × 3450 pixels. The morphologies were characterized by SEM (HITACHI S-4800) and TEM (JeoL-1011) operated at 100 kV. The thermal gravimetric analysis (TGA) was carried out on a PerkinElmer TGA instrument (TA Q50) with a heating rate of 10 °C/min in nitrogen atmosphere. Powder XRD analysis was performed on the X-ray diffractometer (model D/ MAX2500, Rigaka) with Cu Kα radiation. FT-IR spectra were obtained using a Bruker Tensor 27 spectrometer, and the samples were prepared by the KBr pellets method. N2 adsorption/desorption isotherms were determined by a Quadrasorb SI-MP system. Prior to measurement, the sample was degassed at 130 °C for 20 h. Brunauer− Emmett−Teller (BET) method was used to get surface area.

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RESULTS AND DISCUSSION Phase Diagram of the Water/bmimPF6/TX-100 System. Our previous work indicated that the water/bmimPF6/ TX-100 system could form microemulsions, and the detailed phase diagram is depicted in Figure 1.19 The single phase B

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It can be known from Figure 3g−i that the MOFs prepared in the IL-in-water microemulsions of different compositions have needle structures. It can be known from g, h, and i that the diameters of the MOFs particles decrease from the g to i, and the order is consistent with that of the droplets sizes in the microemulsions (Dc in the inset of Figure 2C). It can be known from the results in Figures 2 and 3 that the size and morphology of the MOFs depend strongly on the microstructures of the microemulsions. Figure 4 illustrates the powder XRD patterns of the MOFs. Figure 4a−i corresponds to the XRD patterns of the MOFs prepared using the microemulsions marked as a−i in Figure 1, respectively. Figure 4j is simulated XRD pattern using single crystal X-ray data of La(BTC)(H2O)6 (CCDC 290771).58 It can be known from the XRD patterns that all samples are crystalline. The patterns of all the MOFs were similar, indicating that they have similar crystal structures although their sizes and shapes are different. In addition, the XRD patterns are nearly the same as the La-MOF reported by other researchers.59 As an example, the La-MOF shown in Figure 3c, which was prepared in microemulsion b marked in Figure 1, was further characterized by TGA and FT-IR techniques. Figure 5 shows the TGA curve of the MOF. The curve exhibits two major stages of rapid weight loss in the temperature range from 30 to 800 °C, corresponding to the loss of the six water molecules (21.25%) from 30 to 120 °C and the organic ligand (40.26%) respectively. The result is in agreement with the theoretical weight loss of assumed structure La(1,3,5-BTC)(H2O)6 MOF (23.79% and 40.31%), showing that the La-MOF has been formed. Figure S1 in the Supporting Information gives the FT-IR spectrum of the MOF shown in Figure 3c. The MOF has strong absorption bands at 1556 and 1617 cm−1, which can be attributed to the asymmetric stretching vibration and the stretching vibration of the carboxylic acid group, respectively.59 The wide absorption band at about 3409 cm−1 is the typical vibration of water molecules. The characteristic peaks of the protonated carboxyl groups (1730−1680 cm−1) are not observed in the IR spectrum, indicating that the BTC ligand has been completely deprotonated in the reaction. This indicated that the carboxylate groups of the BTC are coordinated to La (III) ions. We also characterized the MOF by the N2 adsorption−desorption method, and the adsorption− desorption isotherm curve is presented in Figure S2 of the revised manuscript. The BET surface area of the MOF calculated was 28.5 m2/g. We also determined the surface areas of the MOFs in Figure 3e and i, and the values were 27.8 and 30.1 m2/g, respectively. Such low surface area can be attributed to the closed porosity.60 Mechanism of Controlling the Morphology. As discussed above, the dispersed droplets in the water-inbmimPF6 microemulsion, bicontinuous microemulsion, and bmimPF6-in-water microemulsion are spherical, lamellar, and cylindrical, respectively (Figure 2). The shape of a La-MOF is similar to the droplets in the corresponding microemulsion, but the size of the MOF is much larger than that of the droplets in the microemulsion. On the basis of the experimental results, the mechanism for controlling the morphology is discussed using the formation of MOF i in Figure 3 as the example, which is prepared in microemulsion i with cylindrical droplets. Figure 6 shows the schematic for the formation of needle-like MOFs in bmimPF6-in-water microemulsion i. In the micro-

Figure 2. SAXS results of water-in-bmimPF6 (A), bicontinuous (B), and bmimPF6-in-water (C) microemulsions.

curves obtained from the data in the small-angle region (q < 1). Each size distribution curve has the characteristics that as r > Dc, the curve is linear, which indicated that the IL droplets in the microemulsions are cylinders.57 The figure also shows that the Dc decreased from microemulsions g to i, indicating that the one-dimensional structure became more obvious from g to i. The curves in Figure 2C show weak peaks in the high-q region due to the interaction between the micelles. La-MOFs Formed in the Microemulsions. Figure 3 gives the SEM images of the MOFs prepared in the microemulsions of in different subregions shown in Figure 1. The images in Figure 3a−i correspond to the MOFs prepared using the microemulsions marked as a−i in Figure 1, respectively. Figure 3a−c shows that, in the water-in-IL microemuslions, spherical particles are formed. It is observed that the size of the La-MOF particles changes with the order of 3a < 3b < 3c. The order is the same as that of the water droplets in the microemulsions (inserted Figure 2A). The La-MOFs synthesized in the bicontinuous microemulsions have lamellar structures (Figure 3d−f), which is consistent with shape of the dispersed droplets. C

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Figure 3. The SEM images of the MOFs prepared. a−i correspond to the MOFs prepared using the microemulsions a−i marked in Figure 1, respectively.

Figure 4. XRD patterns of the MOFs prepared. Patterns a−i correspond to the MOFs prepared using the microemulsions a−i marked in Figure 1, respectively; j is simulated XRD pattern from single crystal X-ray data.58

Figure 5. TGA curve of the La-MOF prepared microemulsion b marked in Figure 1

MOF crystal nuclei are formed, which grew into the larger spherical and lamellar particles, respectively.

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emulsion, the La (III) salt dissolves in the water continuous phase, and organic BTC dissolves in the cylindrical IL droplets. At the beginning, MOF nuclei are formed by coordination reaction of the La (III) salt and BTC at the water/IL interface. The MOF nuclei are needle-like because the cylindrical IL droplets are dispersed in water continuous phase. The needlelike nuclei, which act as the seeds of the MOFs, grew gradually into the larger needle-like particles. In this process, the shape of the IL droplets controls the morphology of the MOF nuclei formed at the beginning, which determine the shape of the final particles. Similarly, the droplets in water-in-bmimPF6 microemulsion and bicontinuous microemulsion are spherical and lamellar, respectively. Therefore, the spherical and lamellar

CONCLUSION Study of the microstructures of IL microemulsions and controlled synthesis of MOFs are both very interesting. In this work, we have found that the dispersed droplets in the water-in-bmimPF6, bicontinuous, and bmimPF6-in-water microemulsions in the H2O/bmimPF6/TX-100 system have the shapes of spherical, lamellar, and cylindrical, respectively. The La-MOFs synthesized in these microemulsions follow the shapes of the dispersed droplets in corresponding microemulsions. In other words, the morphology of MOFs can be easily controlled by changing the microstructures of the microemulsions. The mechanism for controlling the morpholD

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Figure 6. Proposed mechanism for the formation of the needle-like MOFs in the bmimPF6-in-water microemulsion. The La (III) salt and BTC dissolve in the water continuous phase and the cylindrical IL droplets, respectively (1); the needle-like MOF crystal nucleus are formed in the beginning (2); the nucleus grow gradually into the larger needle-like particles (3); the final crystal and its TEM image (4). Carbon Dioxide Microemulsions: An Environment for Hydrophiles Including Proteins. Science 1996, 271, 624−626. (5) Holmberg, K. Organic Reactions in Microemulsions. Eur. J. Org. Chem. 2007, 2007, 731−742. (6) Ganguli, A. K.; Ganguly, A.; Vaidya, S. Microemulsion-Based Synthesis of Nanocrystalline Materials. Chem. Soc. Rev. 2010, 39, 474− 485. (7) Fathi, H.; Kelly, J. P.; Vasquez, V. R.; Graeve, O. A. Ionic Concentration Effects on Reverse Micelle Size and Stability: Implications for the Synthesis of Nanoparticles. Langmuir 2012, 28, 9267−9274. (8) Eastoe, J.; Hollamby, M. J.; Hudson, L. Recent Advances in Nanoparticle Synthesis with Reversed Micelles. Adv. Colloid Interface Sci. 2006, 128, 5−15. (9) Rogers, R. D.; Seddon, K. R. Ionic Liquids–Solvents of the Future? Science 2003, 302, 792−793. (10) Taubert, A.; Li, Z. H. Inorganic Materials from Ionic Liquids. Dalton Trans. 2007, 723−727. (11) Hallett, J. P.; Welton, T. Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis. 2. Chem. Rev. 2011, 111, 3508− 3576. (12) Wu, W. Z.; Han, B. X.; Gao, H. X.; Liu, Z. M.; Jiang, T.; Huang, J. Desulfurization of Flue Gas: SO2 Absorption by an Ionic Liquid. Angew. Chem., Int. Ed. 2004, 43, 2415−2417. (13) Sun, X. Q.; Luo, H. M.; Dai, S. Ionic Liquids-Based Extraction: A Promising Strategy for the Advanced Nuclear Fuel Cycle. Chem. Rev. 2011, 112 (4), 2100−2128. (14) Greaves, T. L.; Drummond, C. J. Solvent Nanostructure, the Solvophobic Effect and Amphiphile Self-Assembly in Ionic Liquids. Chem. Soc. Rev. 2013, 42, 1096−1120. (15) Qiu, Z. M.; Texter, J. Ionic Liquids in Microemulsions. Curr. Opin. Colloid Interface Sci. 2008, 13, 252−262. (16) Gao, H. X.; Li, J. C.; Han, B. X.; Chen, W. N.; Zhang, J. L.; Zhang, R.; Yan, D. D. Microemulsions with Ionic Liquid Polar Domains. Phys. Chem. Chem. Phys. 2004, 6, 2914−2916. (17) Eastoe, J.; Gold, S.; Rogers, S. E.; Paul, A.; Welton, T.; Heenan, R. K.; Grillo, I. Ionic Liquid-in-Oil Microemulsions. J. Am. Chem. Soc. 2005, 127, 7302−7303. (18) Li, N.; Gao, Y. A.; Zheng, L. Q.; Zhang, J.; Yu, L.; Li, X. W. Studies on the Micropolarities of BmimBF4/TX-100/Toluene Ionic Liquid Microemulsions and Their Behaviors Characterized by UVVisible Spectroscopy. Langmuir 2007, 23, 1091−1097. (19) Gao, Y. A.; Han, S. B.; Han, B. X.; Li, G. Z.; Shen, D.; Li, Z. H.; Du, J. M.; Hou, W. G.; Zhang, G. Y. TX-100/Water/1-Butyl-3methylimidazolium Hexafluorophosphate Microemulsions. Langmuir 2005, 21, 5681−5684.

ogy of the MOFs is that, in the beginning, the MOF crystal nuclei with a shape similar to the dispersed droplets in the microemulsions are formed and the nuclei grow into the larger particles of similar shape. Therefore, the microstructure of the miscroemulsions determines the final morphology of the MOFs. We believe that this method can be also used for controlled synthesis of other MOFs with different compositions and morphologies.

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ASSOCIATED CONTENT

S Supporting Information *

FT-IR spectrum of the La(1,3,5-BTC)(H2O)6 MOF (Figure S1). N2 adsorption-desorption isotherm of the La(1,3,5BTC)(H2O)6 MOF (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+)86-10-62559373. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (21133009, U1232203, 21173238, 21021003, and 11079041), Ministry of Science and Technology of China (2009CB930802), and Chinese Academy of Sciences (KJCX2.YW.H16).

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