ARTICLE pubs.acs.org/crystal
Complete Characterization of α-Hopeite Microparticles: An Ideal Nucleation Seed for Metal Organic Frameworks Dario Buso,†,‡,* Anita J. Hill,† Tobias Colson,§ Harold J. Whitfield,† Alessandro Patelli,|| Paolo Scopece,|| Cara M. Doherty,† and Paolo Falcaro†,* †
CSIRO, Materials Science and Engineering, Locked Bag 33, Clayton South MDC, VIC 3169, Australia Centre for Micro-Photonics & CUDOS, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, P.O. Box 218, Hawthorn, VIC 3122, Australia § School of Earth and Environment, The University of Western Australia, 35 Stirling Highway, Crawley, Perth, 6009 WA, Australia Associazione CIVEN, Via delle Industrie 5, 30175 Venezia, Italy
)
‡
bS Supporting Information ABSTRACT: This work reports on the structural and microstructural characterization of a new class of α-hopeite microparticles, which has recently been discovered as ideal seeding agents for the formation and functionalization of metal organic framework (MOF-5) crystals. The particles have been named desert rose microparticles (DRMs), as their morphology closely resembles that of the famous gypsum and Barite mineral. The DRMs form directly inside the MOF-5 precursor solution when a block copolymer surfactant, Pluronic F-127, is added in specific amounts. The particles formation is remarkably fast, and particles are observed to form within the first minute of reaction. The DRMs formation and growth has been monitored along a 3 h synthesis, until the first nuclei of MOF-5 start to appear on their surface. Electron microscopy, energy dispersive analysis, electron diffraction, FTIR, FTRaman, and BET give an all-around description of the chemical and morphological features that give the DRMs their remarkable MOF-seeding capacity.
’ INTRODUCTION A novel class of α-hopeite microparticles has recently been discovered to be the ideal nucleating agent for the fast formation of metal organic framework (MOF) crystals.1 Named desert rose microparticles (DRMs), they have demonstrated an unprecedented range of advantages for MOF production and MOF composite engineering, unmatched by any of the previously existing technologies. The DRMs not only trigger MOF formation in solution three times faster than the traditional solvothermal approach, but they have also proven to be an efficient tool to localize MOF growth on a variety of substrates regardless of their surface chemistry, and with no need for surface activation. In addition, the DRM approach offers for the first time the possibility to synthesize functional host guest MOF composites containing any sort of functional nanospecies compatible with the solvothermal synthesis conditions (presence of DMF or DEF at 95 C), in the form of either metal, semiconductor, or polymer nanoparticles. This latter point is not trivial, especially if we consider that, by using the DRMs to form MOF composites, the actual position of the functional nanospecies can be precisely controlled within each single MOF crystal, with no trace of the guest functionality on the outer surface of the MOF matrix, as recently observed for magnetic framework composites.2 This new method for the production of MOF “composites” achieves the synergy between the superior characteristics of the MOF host and the specific nanospecies functionality. From a practical point r 2011 American Chemical Society
of view, another remarkable advantage offered by the DRM approach is that the synthesis pathway of MOF composites remains the same regardless of the chemical nature of the functional species to be used; in fact, the use of DRMs eliminates the need to specifically customize or optimize the synthesis procedure according to the specific chemistry imposed by the guest species. Overall, the discovery of the DRMs represents a noticeable technological advantage for the production of functional MOF-based devices. The aim of the present work is to investigate the morphological, structural, and microstructural features of the DRMs which confer them such remarkable MOF nucleating characteristics. We present here an extended characterization of the DRMs by means of electron microscopy (SEM and TEM), energy dispersive analysis (EDS), electron diffraction, FTIR, FT-Raman, and BET, with an insight on the DRM formation dynamics within the MOF-5 precursor solution.
’ EXPERIMENTAL METHODS An initial mother batch solution of Zn nitrate (0.12 M) and terephthalic acid (0.025 M) was prepared in N,N-dimethylformamide Received: June 8, 2011 Revised: September 28, 2011 Published: September 28, 2011 5268
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Figure 1. SEM characterization of DRMs forming within the MOF-5 precursor solution. The images in the left column depict a DRM formed after 5, 15, 30, 60, 120, and 180 min at 95 C, in order from the top to the bottom. The scale bar for column one is 10 μm. The other columns of images refer to a magnified view of the previous images, with 2 μm and 500 nm scale bars, respectively. The right column reports the DRMs size distribution plots corresponding to each sample. (DMF). The reagents were fully dissolved by means of an ultrasonic bath for 15 min, until a transparent solution was obtained. Subsequently, Pluronic F-127 was added to the mother batch, according to a weight-tovolume ratio of 0.114/1 (g F127/mL mother batch), and the mixture was again sonicated until complete dissolution of the Pluronic F-127. All reagents except DMF were purchased from Aldrich and used with no further purification. DMF was supplied by Merck. Aliquots of the mother batch were then transferred into glass vials and sealed with
Teflon caps. The vials were then positioned in an MRC dry bath incubator (Thermoline Scientific) preset at 95 C. To monitor the DRMs formation and growth, aliquots of the precursor solution were taken at regular intervals within the initial 3 h of reaction, before formation of the MOF-5 crystals. The aliquot samples were extracted at 5, 15, 30, 60, 120, and 180 min after the reaction started. Prior to their characterization, the DRMs samples were filtered and washed with fresh DMF through a vacuum filter system using an alumina membrane with 5269
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Figure 2. Energy dispersive X-ray spectroscopy (EDS) of the DRMs formed after (a) 5 min and (b) 180 min at 95 C. 200 nm pores; the DRMs were then collected as a dry white powder and used in this form for further characterization. The DRMs need to be stored in a dry environment to preserve their exceptional heterogeneous MOF nucleation properties. The best condition found is to store them under a nitrogen atmosphere. The particle morphology was evaluated by means of SEM imaging using a Philips XL-30 FESEM with a mounted EDAX unit for linear EDS analysis. 2D EDS elemental maps were obtained using a VEGA TS 5130 LM (Tescan) SEM equipped with an EDAX Sapphire Si(Li) X-ray microanalysis detector. The particle size distribution has been obtained by measuring manually the diameter of each particle from SEM images at lower magnification (not shown) using ImageJ software, available free of charge at http://rsbweb.nih.gov/ij/. TEM was used for both imaging and electron diffraction analysis, using an analytical Jeol JEM 2010 transmission electron microscope fitted with a LaB6 filament. Linear FTIR spectroscopy was performed on dried DRM powder using a Bruker Alpha FTIR, by means of an attenuated total reflection (ATR) component. 2D chemical maps were obtained with a Bruker Hyperion 2000 microscope, coupled to a Vertex V80v FTIR spectrometer. Argon gas adsorption/desorption isotherms were obtained with a Micromeritics ASAP 2420 instrument at 77 K. The samples were degassed at 150 C for 16 h prior to measurement.
’ RESULTS AND DISCUSSION The entire formation and growth dynamics of the DRMs were precisely monitored by extracting aliquots from the reaction batch at fixed time intervals. This operation was conducted within the first 3 h of reaction, i.e. before the formation of macroscopic MOF-5 crystals. All the aliquots were sampled by SEM imaging. Figure 1 presents the SEM micrographs of the samples and gives an immediate picture of the DRMs formation and growth dynamics within the precursor solution at 95 C.
Figure 3. SEM and 2D EDS chemical mapping of two MOF-5 crystals showing a DRM attached on one of the crystals’ edges. The top left sample image was obtained with a backscattering detector (BSE) at 15 kV, while C, O, Si, P, and Zn element color mappings were obtained by EDS spatially resolved spectra.
Each row of the images refers to a specific aliquot, i.e. to a precise reaction time, and is accompanied with a size distribution plot that describes the average DRM size measured from each aliquot. Parts a c of Figure 1 demonstrate that 5 μm particles form already after 5 min within the precursor solution; at this initial stage the particles are spherical in shape and present an irregular surface morphology, with an average particle size of around 5 μm. In 15 min (Figure 1d f) the particles steadily grow up to 8 μm diameter, while maintaining similar surface morphology. At this stage of the reaction, a white cloudy suspension is observed to form within the hot solution; this is expected as the cloud, point for Pluronic F-127 sits at around 100 C,3 and its proximity with the reaction temperature triggers the formation of suspended surfactant aggregates. Optical microscopy indicates that the suspended surfactant clouds become preferential formation sites for the DRMs (Supporting Information Figure SI-1). The particles reach 13 μm diameter after 30 min (Figure 1g i), and a change in their surface morphology becomes evident, as the previously corrugated spongelike surface is now an arrangement of nanoflakes; higher SEM magnification reveals that each flake is a 20 nm thick platelet, connected to the others to form a desertrose-like surface. From this moment onward, the flakes further refine their shape through the whole reaction, while maintaining their desert-rose-like arrangement. In fact, particles extracted after 60, 120, and 180 min show a steady diameter growth up to 17, 20, and 23 μm, respectively, while presenting the same 5270
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Figure 4. IR spectroscopy of the DRMs. (a) FTIR and (b) Raman spectroscopy acquired on a dried powder of DRMs, in the 3700 450 cm 1 and 1300 200 cm 1 spectral ranges, respectively.
nanoflaked surface morphology (Figures 1j r). After 3 h of reaction (Figure 1p r) MOF-5 crystals begin to form directly on the microparticles flakes; this confirms the remarkable MOF nucleation properties already observed and extensively described in ref 1; this is the first direct observation of the formation of MOF-5 crystals on a heterogeneous nucleation agent, at around 3 h reaction time. The SEM images of Figure 1 testify the effect of Pluronic F-127 when added to a standard MOF-5 precursor solution, which is the first experiment of its kind; EDS analysis (Figure 2) gives a first insight into the microparticles chemical nature, revealing that the spherical particles are a combination of zinc, phosphorus, oxygen, and traces of carbon. Fingerprints of Kα and Lα emissions of Zn atoms are identified at 8.63 and 1.01 keV, respectively, the Kα emission of oxygen is recorded at 0.52 keV, and the carbon Kα is detected at 0.27 keV. An intense Kα emission from phosphorus is registered at 2.01 keV. The emission at 1.74 keV is that of the silicon substrate Kα. The chemical composition of the particles appears to be defined within the first minutes of reaction (Figure 2a) and remains mostly unvaried in the sample extracted after 180 min (Figure 2b). To evaluate the chemical difference between the composition of MOF-5 and the DRMs, a more extended 2D EDS analysis was also performed on a sample made of MOF-5 crystals in which a single DRM is observed to be attached on the outer surface of the framework (Figure 3). The sample was isolated from the reaction batch after 10 h at 95 C, and imaged using backscattered electrons (BSE, top-left frame of Figure 3). The BSE detector is coaxial to the incident electron beam close to the normal of the sample, while the EDS detector has a 45 take off angle, which explains the shade changes between the BSE image and the 2D chemical maps. The atomic element maps were obtained by filtering the X-ray emission to separately detect
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carbon, oxygen, silicon, phosphorus, and zinc. The representation gives an instantaneous picture of the spatial distribution of the species within the sample, highlighting that the DRMs contain all the phosphorus detected in the system and are less rich in carbon than the MOF-5 crystal; oxygen and zinc appear to be homogeneously distributed all through the sample. IR spectroscopy performed on dried DRMs gives a more comprehensive description of the microspheres, as the recorded signals are consistent with the vibration modes of zinc phosphates. In particular, the detected signals are in their majority associated with the chemical structure of α-hopeite, Zn3(PO4)2 3 4H2O. The FTIR ν1, ν3, and ν4 modes of the tetrahedron phosphate centers are clearly identified (Figure 4a), and the intense broad band in the 3600 3000 cm 1 region confirms the hydration state of the zinc phosphate compound. Signals from terephthalic acid inclusions can be detected in the 1600 1300 cm 1 range, while signals at 1250 and 2970 2860 cm 1 indicate the presence of residual solvent DMF in the DRMs structure. A comprehensive and detailed description of all the FTIR vibration modes identified in the plot of Figure 4a is reported in Table 1. Raman spectroscopy further confirms that the DRMs are microparticles of α-hopeite. The four peaks observed in the 1300 900 cm 1 range are attributable to stretching modes of the PO43 centers of α-hopeite;4,5 the ν1 mode at 995 cm 1 is the symmetrical stretching vibration, while the three peaks at 1130, 1050, and 950 cm 1 refer to the ν3 triple degeneracy vibration (stretching type) of phosphate tetrahedrons. Triple degeneracy vibrations (deformation type) ν4 are detected at 633, 595, and 550 cm 1. The formation of α-hopeite microparticles occurs as a consequence of the introduction of Pluronic F-127 in the traditional MOF-5 precursor solution. Commercial Pluronic F-127 is an abundant source of phosphorus in the form of ortho-phosphate centers, essential for the formation of α-hopeite minerals. The presence of phosphates in samples of commercial Pluronic F-127 was confirmed by 31P-NMR,1 while ion coupled plasma (ICP) mass spectroscopy revealed that the concentration of elemental phosphorus in commercial Pluronic F-127 is around 1 mg/g. Double-hydrophilic block copolymers have had a tremendous success since their introduction6 as mineralization agents and drivers in the formation of biological-like inorganic microstructures (biomimetics). Their molecules consist of one charged and hydrophilic block designed to interact strongly with the appropriate inorganic materials and surfaces and another hydrophilic block that does not interact (or only weakly and mainly promotes steric stabilization or solubility). To better understand the role played by Pluronic F-127 in the formation of DRMs, it is worth mentioning that the mineralization process of metal oxide structures can be promoted and morphologically driven by block copolymers, as the metal ions with a high degree of hydration, such as Ca2+ and Zn2+, show strong binding by the ethylene oxide (EO) units in solution.7,8 Such a tendency is especially manifest if these units are organized in long chains,7 as is the case for Pluronic F-127 molecules (polyethylene oxide polypropylene oxide polyethylene oxide). It has also been shown that surfactants influence the crystallization steps by selective adsorption on certain crystal planes and finally control the formation of the crystal phase.9,10 In the case of α-hopeite, surfactants can strongly enhance its growth along its Æ020æ crystalline plane, to form platelet-like microstructures11 similar 5271
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Table 1. FTIR Vibration Frequencies of DRMs in the 450 4000 cm band center (cm 1) 3000 3600
1
Wavenumber Range
chemical species ν modes of H2O
1656
ref
O H stretch in H2O
16 20
H O H bending in H2O
16, 20
O H in H2O
20
C H asymmetric stretch in CH3
21
2927
C H asymmetric stretch in CH
21
2866
C H symmetric stretch in CH3
21
1466 2966
ν modes of C H
1590
aromatic modes
aromatic CdC stretch
22, 23
1504 1590
ν modes of O C O
in plane C H bend in C6 ring C O asymmetric stretch
22, 23 24, 25
1436
C O symmetric stretch
24, 25
1404
C O symmetric stretch
24, 25
1390
C O symmetric stretch
24, 25
1301
O C O symmetric stretch
21, 24
C N stretch
21
P O stretch
16, , 26
P O stretch P O stretch
16 18 16 18
P O symmetric stretch
16, 26, 27
P O in P O Zn
16, 26
P O in P O Zn
16, 26
840
P O P asymmetric stretch
28
749
P O P symmetric stretch
26, 28
1254
ν modes of C N
1130
ν3 modes of (PO4)3
‑
1060 1012 980
ν1 modes of (PO4)3
952 923
ν modes of O P O
562
δ modes of O P O
O P O antisymmetric bend
19, 26, 27
650 600
ν4 modes of (PO4)3
P O P O
16, 27 19, 26, 27
576
P O
17, 27
535
P O
18
to those observed in the DRM samples. The precipitation of spherical DRMs is expected to be very fast, as the solubility product constant for α-hopeite is extremely low.12 Two-dimension FTIR maps (Figure 5a c) were also performed on a sample made of one MOF-5 crystal containing a DRM and an isolated DRM. Similar to the EDS maps, FTIR also spatially traces the chemical differences in the composition of MOF crystals and DRMs. The dotted lines in the sample optical image (Figure 5a) identify the target MOF-5 crystal (bottom left) and the isolated DRM (top edge). The chemical map of Figure 5b was obtained by integrating the transmitted IR signal along one of the main P O bonds’ vibration modes (980 cm 1, Figure 5b), and shows a maximum absorption corresponding to the DRMs in both the one sitting within the MOF-5 crystal and the isolated one. This confirms that all the phosphorus species are confined within the DRM even after the encapsulation process. The map of Figure 5c shows the absorption levels obtained by integrating the IR signal along the vibration frequency of the C H bending modes of the aromatic C6 rings, at 1504 cm 1 (see Table 1). This frequency is associated with the organic linkers of the MOF-5 structure, provided by the terephthalic acid molecules. The drop in absorbance in the sampling area which corresponds to the DRMs confirms the observations made using EDS maps, as well as further emphasizing the inorganic character of the DRMs. Samples for TEM analysis were prepared by lightly crushing crystalline microparticles with a few drops of ethanol followed by deposition on a holey carbon grid. Part of a flake was imaged
(Figure 5d), revealing a disordered arrangement of nanocrystals forming the microparticle’s flakes. One of the elongated crystals has been selected (Figure 5f), and the corresponding electron diffraction pattern has been recorded (Figure 5d). The electron microscope characterization gives a better insight on the particle microstructure; indexed with an orthogonal axis of 10.59 Å and 5.033 Å, the diffraction pattern of the selected area was identified to be in the Æ010æ zone axis orientation of crystalline α-hopeite, which is the principal microstructure observed in all the DRM samples. The pattern is a projection of the α-hopeite crystal structure which is orthorhombic with a space group of Pnma, as confirmed by the intensity and the relative position of the diffraction spots. The α-hopeite crystal has been reported13 with unit cell dimensions of a = 10.591 Å, b = 18.312 Å, and c = 5.0274 Å, which shows good agreement with those calculated from the electron diffraction pattern. The XRD pattern of DRM powder confirmed this analysis (see Supporting Information, Figure SI-2). The images of Figure 5e and f show a section of one of the nanoflakes, exposing the lamellar arrangements of the α-hopeite sheets, in accordance with the study reported by Li-Yiuan et al.11 The Zn atoms within the hopeite crystals are tetracoordinated with oxygen atoms to form continuous sheets of three- and four-membered rings of ZnO4 tetrahedra, which are connected by PO4 groups. One of the oxygen atoms of the PO4 links is not involved in the ring structure and represents the major link between the sheets.14 This information highlights a plausible driving force that explains the preferential formation of MOF-5 on the hopeite 5272
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Figure 5. Structural and microstructural aspects of the DRMs. Part a shows an optical microscope image of a MOF-5 crystal containing a DRM (dotted rectangle) together with an isolated DRM (dotted circle), both highlighted by the dotted lines. Parts b and c show the corresponding 2D FTIR chemical maps obtained by integrating the transmitted IR signal along the vibration frequencies of (b) P O bonds and (c) C H belonging to aromatic C6 rings. A comparison with the optical image of the sample gives an immediate representation of the spatial distribution of the chemical species, highlighting the different chemical natures of the MOF-5 and the DRMs. Part d shows the electron diffraction pattern of the DRMs flakes. The TEM images in parts e and f show the details of the DRM flakes. The pattern is consistent with diffraction from the orthorhombic lattice of crystalline α-hopeite. The scale bar in parts e and f is 100 nm.
phase; in fact, oxygen tetracoordinated zinc centers are the main backbone in both α-hopeite and MOF-5 crystals. The terephthalic acid molecules in solution, being the natural linker between ZnO2 centers, can therefore coordinate the exposed ZnO2 sites on the DRM surface and facilitate the formation of MOF-5 in the usual fashion, similarly to the mechanism reported by Hermes et al.15 The sorption isotherm in Figure 6a shows a distinct hysteresis at high partial pressures indicating the presence of large mesopores and macropores. This is confirmed with the pore distribution taken from the adsorption branch of the isotherm showing the presence of pores ranging from 20 to 100 nm (Figure 6b). BET calculations reveal that the DRMs show a significant degree of porosity; in fact, while dense α-hopeite crystal platelets usually have a BET surface of 0.6 m2 g 1,13 the surface area of the DRMs sits around 51 m2 g 1, remarkably 2 orders of magnitude higher. This aspect has been efficiently exploited in a series of experiments described in detail in ref 1 which show that the DRMs can accommodate relevant amounts of active nanospecies (metals, semiconductors, or polymers) within its core, while still maintaining their remarkable MOF nucleation properties. In this way they can be effectively used as carriers for the functional nanospecies inside the hosting framework, allowing for the fabrication of functional MOF composites with a finely controlled distribution of the nanospecies within each crystal. The porous arrangement of the hopeite flakes to form the final particle spherical shape is exposed in the SEM image in Figure 7, which depicts a cross section of a MOF-5 crystal that formed around a single DRM. The DRM hopeite lamellar flakes are consistently recognizable throughout the whole DRM section and give an immediate picture of the highly porous arrangement as suggested by the BET analysis.
Figure 6. BET surface area, pore volume, and pore size distribution of DRMs: (a) argon adsorption and desorption isotherms; (b) pore volume and pore size distribution evaluated from the adsorption branch of the isotherm.
Figure 7. SEM image of a sectioned MOF-5 crystal containing a single spherical DRM. The image depicts the arrangement of the α-hopeite flakes all throughout the particle core and illustrates the porous character of the DRMs.
’ CONCLUSIONS Inorganic desert rose microparticles (DRMs) are a new family of inorganic particles that was discovered recently and that show a remarkable tendency to nucleate MOF-5 crystals. 5273
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Crystal Growth & Design A comprehensive characterization of the DRMs morphological, structural, and microstructural characteristics is presented, which is of use to depict the key features that provide these objects with their remarkable MOF seeding capability. The particles, which are spherical agglomerates of α-hopeite flakes, grow within a traditional MOF-5 precursor solution when Pluronic F-127 is added, which has a double role of templating agent and phosphorus source that triggers the fast precipitation of zinc phosphate. The formation of DRMs is detected as soon as the solvothermal reaction starts. The evolution of the size and the growth of crystalline nanoflakes has been characterized using FESEM. The peculiar arrangement of the α-hopeite platelets in the DRM structure favors the chemical absorption of MOF-5 secondary building units, thus providing an excellent heterogeneous nucleation seed for the framework growth.
’ ASSOCIATED CONTENT
bS
Supporting Information. Optical image of DRMs after 3 h of growth and XRD pattern of 1 g of DRM powder. This material is available free of charge via the Internet at http://pubs. acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. *E-mail:
[email protected].
’ ACKNOWLEDGMENT The Authors acknowledge the travel funding provided by the International Synchrotron Access Program (ISAP) managed by the Australian Synchrotron. The ISAP is an initiative of the Australian Government being conducted as part of the National Collaborative Research Infrastructure Strategy. Part of this research was undertaken on the FTIR at the Australian Synchrotron, Victoria, Australia. The powder diffraction characterization was performed at the SAXS beamline of Elettra Synchrotron, Trieste, Italy. D.B., P.F., D.C. and A.J.H. acknowledge the CSIRO OCE Science Leader Scheme for support. The authors thank the CSIRO Computational and Simulation Sciences Transformational Capability Platform for financial support. D.B. acknowledges the Australian Research Council (ARC) for support through the APD Grant DP0988106. Dr. John Ward and Dr. Mark Greaves are thanked for their expert assistance with the SEM. Dr. Kate Nairn is acknowledged for her assistance in the acquisition of the Raman spectra of the DRMs.
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