Influence of Isomorphous Substituting Cobalt Ions on the Crystal

Publication Date (Web): August 23, 2013. Copyright © 2013 American Chemical Society ... Web page: ... Crystal Growth & Design 2014 14 (9), 4553-4561...
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Influence of Isomorphous Substituting Cobalt Ions on the Crystal Growth of the MOF‑5 Framework Determined by Atomic Force Microscopy of Growing Core−Shell Crystals Pablo Cubillas, Michael W. Anderson, and Martin P. Attfield* Centre for Nanoporous Materials, School of Chemistry, The University of Manchester, Brunswick Street, Manchester, M13 9PL, United Kingdom S Supporting Information *

ABSTRACT: Atomic force microscopy is used to conduct the first detailed nanoscopic study on the crystal growth of a complex mixed metal/metal−organic framework based on the MOF-5 framework topology. Shells of isomorphously substituted Co/Zn-MOF-5 and MOF-5 were epitaxially grown on MOF-5 core crystals at room temperature and low supersaturation to produce complex core−shell−shell structures with a hierarchal mixed metal nature involving mixing at the atomic level in the isomorphously substituted Co/ZnMOF-5 shell and at the nanometer level through segregation of the Co/Zn-MOF-5 and MOF-5 layers. The presence of cobalt in the growth solutions was found to retard the overall rate of surface growth in comparison to a cobalt-free growth solution and stop growth entirely for a growth solution containing a Zn/Co ratio = 0.6. The presence of cobalt in the growth solutions was also found to affect the relative rates of terrace spreading in different crystallographic directions compared to cobalt-free growth with spreading in the ⟨110⟩ directions decreasing relative to the rate along the ⟨100⟩ directions. The work provides new understanding of the crystal growth of complex mixed metal/metal−organic frameworks and a methodology to prepare these complex forms in a more controlled manner.

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INTRODUCTION Mixed-component metal−organic frameworks (MC-MOFs) are MOFs which contain isomorphously substituted metal ions,1 organic linkers,1 or both these components2 in some region of the crystalline particle. MC-MOFs form an emerging subgroup of the MOF family, as they allow access to an even wider range of MOFs with designed pore sizes, compositions, chemical functionality, crystal forms, and properties that may often be considerably different to that of the parent unsubstituted MOFs.1 This area has recently been extensively reviewed by Burrows.1 One such MOF from which a variety of MC-MOFs have been derived is the archetypal permanently porous MOF, MOF-5 [Zn4O(bdc)3, bdc = 1,4-benzenedicarboxylate].3 The crystal structure of MOF-5 consists of Zn4O units connected by bdc linkers to form a cubic network (space group Fm3m ̅ ), as shown in Figure 1. The alternation of the carboxylate linkers along each unit cell axis results in a unit cell length of a = 25.669 Å. MC-MOFs derived from MOF-5 have been reported through formation of mixed organic linker systems in which the linkers are substituted randomly within the crystallites or are segregated in certain parts of the crystallite through formation of core−shell and core−shell−shell structures.4−6 These mixed organic linker systems display interesting adsorption properties; for example, a MC-MOF-5 material containing three different © XXXX American Chemical Society

organic linkers exhibits a 400% better selective adsorption of CO2 over CO than the parent MOF-5 material.4 Additionally, mixed metal MC-MOFs have been formed for the MOF-5 system. Nickel has been successfully isomorphously substituted into MOF-5 with a maximum 25% occupancy of the framework metal sites.7,8 The resultant Ni/Zn-MOF-5 has an enhanced hydrostability compared to the parent MOF-5.7 Cobalt has also been isomorphously substituted into MOF-5 to a similar extent as for the nickel and the resultant Co/Zn-MOF-5 materials exhibit an enhanced ability to adsorb H2, CH4, and CO2 compared to the parent MOF-5.9 Development of synthetic routes to produce MC-MOFs with specific compositional or crystal properties is a necessity for enhanced application, or producing new applications, of MOFs.10,11 Gaining a fundamental understanding of the crystallization of MC-MOFs at the nanoscale will aid the design of such synthetic routes,12 but currently little has been reported in this area other than the actual syntheses and postsynthesis structural characterization of such MC-MOFs.1 In a previous study, we reported the use of in situ atomic force microscopy (AFM) to conduct the first detailed Received: July 3, 2013 Revised: August 21, 2013

A

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In situ growth experiments were performed by introducing a seed crystal covered slide in a temperature-controlled open fluid cell (Biocell, JPK Instruments AG), which was then attached to the AFM (Nanowizard II, JPK Instruments AG). Only crystals with facets approximately parallel to the glass slide surface were scanned to minimize artifacts derived from the nonlinearity of the piezo displacement on the z axis. The crystals were kept in the mother solution all the time prior to introduction of the growth solutions. Growth solutions were always exchanged at least three times prior to the onset of imaging to prevent any dilution effects and contamination from the previous solution. The growth solutions passed over the seed crystals can be divided in two groups, namely, standard growth solutions and cobalt-containing growth solutions. Standard growth solutions were prepared following the procedure of Tranchemontagne et al.15 A solution of Zn(CH3COO)2·2H2O (Sigma) and N,N′-dimethylformamide (DMF) (AlfaAesar) was added to a solution of H2bdc, triethylamine (Fluka), and DMF. These solutions are referred to as standard because their composition was known to produce new crystal growth on MOF-5 seed crystals.13 Cobalt-containing growth solutions were prepared by dissolving Co(NO3)2·6H2O (AlfaAesar) and Zn(NO3)2·4H2O in a given amount of DEF. The molar ratios of all the growth solutions used in this work are given in Table 1. All AFM experiments were conducted in contact mode using silicon rectangular-shaped cantilevers with a radius of curvature of the tip < 10 nm and a force constant of 0.15 N m−1 (CSC17/no Al supplied by MikroMasch). All experiments were carried out at a load close to zero nN, as it was found that increasing the load above 5 nN severely influenced the experimental results either by inhibiting crystal growth or by inducing dissolution through etch pit formation. Zoomed-out scans were performed in all experiments around previously scanned areas to verify that the load did not have any effect on the observed results. No effect of prior scanning was observed on these areas in all the experiments reported here. Image analysis was carried out using the JPK Image Processing software (JPK Instruments AG). Height images were flattened by first applying a first order line by line fit and then a plane fit to further correct for any tilt. Height analysis of step heights was carried out in about a dozen images, using the histogram tool to sample a larger area around the step. Deflection images were left untreated. Terrace spreading rates were determined either by measuring the half length of a spreading terrace produced from a 2D nuclei along the ⟨100⟩ direction or by using a screw dislocation as a reference point for terrace measurements. Measurements were done over a time span of ∼22 min, utilizing the first 12 images recorded after injection of a new growth solution thus minimizing any effect of decreasing supersaturation due to nutrient depletion. Some seed-crystal-covered glass slides were also exposed to the solutions listed in experiment A (see Table 1) externally to the AFM but in a manner similar to the in situ AFM growth experiment. The cobalt-containing solution SA-2 was replenished with fresh solution once during the soaking of the crystals with this solution in these experiments. Scanning electron micrographs were taken using a FEI-Quanta environmental scanning electron microscope (SEM). Chemical analyses were performed by means of an EDAX Genesis energy dispersive X-ray spectrometry system attached to the SEM. Energy

Figure 1. The structure of MOF-5 viewed along the [100] direction. The unit cell of a = 25.67 Å is represented by the green dashed line. The structure is represented in stick and polyhedral modes: blue tetrahedra = Zn, red sticks = bdc linker, H atoms are omitted for clarity.

nanoscopic study on the crystal growth of MOF-5 and provided insights on its crystal growth mechanisms, two-dimensional (2D) nucleation, and the effect of solution composition in controlling the rate of growth of different facets.13 In this work, we use a similar approach to report the first study of the epitaxial crystal growth of shells of the mixed metal MC-MOF, Co/Zn-MOF-5, and MOF-5 on MOF-5 core crystals. The work allows the effect of the Co2+ ions on the crystal growth of the MOF-5 type frameworks to be determined directly and demonstrates the possibility of forming complex MC-MOFs of core−shell−shell structure in which the mixed metal nature of the product coexists at the atomic level in the isomorphously substituted Co/Zn-MOF-5 shell and at the nanometer level through segregation of the Co/Zn-MOF-5 and MOF-5 layers in the product crystals. The study of the crystal surface of the Co/Zn-MOF-5 surface also provides information concerning the extended defects in this material.

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EXPERIMENTAL SECTION

The MOF-5 seed crystals used in the AFM experiments were synthesized on glass slides following the procedure of Rowsell.14 Zn(NO3)2·4H2O (Merck) was mixed in solid form with a given amount of H2bdc (Sigma Aldrich), and then N,N′-diethylformamide (DEF) (TCI) was added as the solvent. The molar composition of the synthesis solution was 1 Zn:0.33 H2bdc:111 DEF. Glass slides were introduced into a vial containing the reagent solution. The vial was then capped and placed in an oven at 90 °C for 48 h.

Table 1. Summary of the Chemical Composition of the Growth Solutions Utilized in This Work experiment

solution

A

SA-1 SA-2 SA-3 SB SC SD

B C D

Zn(OAc)2·2H2O (mol)

Zn(NO3)2·4H2O (mol)

0.0149 0.0896 0.0149 0.0422 0.0308 0.020

H2bdc (mol)

triethylamine (mol)

DMF (mol)

0.0012 0.0045 0.0012 0.0016 0.0016 0.0016

0.058

11.62

0.058

11.62

B

DEF (mol)

Co(NO3)2·6H2O (mol)

11.62

0.0528

11.62 11.62 11.62

0.0102 0.0222 0.0334

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Figure 2. AFM deflection images of a {100} face captured at certain times during experiment A showing the formation of (a) square terraces of MOF-5 in solution SA-1, (b) rounded terraces of Co/Zn-MOF-5 15 min after the introduction of solution SA-2, closed loops from growth spirals of Co/Zn-MOF-5 (c) 100 and (d) 1160 min after introduction of solution SA-2 and (e) square nuclei of MOF-5 formed on the spiral of Co/Zn-MOF5 shown in (d) in solution SA-3. Insets in (c) and (e) show cross-sectional analyses of 1.3 nm high steps taken along the white lines in the corresponding images that are shown in terms of the MOF-5 framework in (f). The structure is represented in stick and polyhedral modes: blue tetrahedra = Zn, red balls = O, black balls = C. H atoms are omitted for clarity. dispersive X-ray spectroscopy analyses were performed on the crystals grown during the in situ AFM growth experiments and externally to the AFM. Some of the crystals studied were fractured with a scalpel blade before analysis to expose the core of the crystals. Powder X-ray diffraction patterns were obtained using a Philips X’pert diffractometer. Samples were ground prior to mounting into the sample holder. Data were collected in the 2θ range of 3−60° using Cu Kα radiation, with a wavelength of 1.54 Å. Powder X-ray data was taken on samples of the seed crystals to determine their phase purity and in all cases pure MOF-5 was synthesized.

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still be seen in Figure 2b. Nucleation stopped after a few minutes and was followed by terrace spreading and the formation of growth spirals. An example of the latter is seen in Figure 2c, where two pairs of dislocations generate closed looped terraces. The shape of these terraces is slightly more square than those observed in Figure 2b, with the more rectilinear directions being parallel to the ⟨110⟩ directions. This terrace shape resembles that observed when MOF-5 crystals were grown under growth solutions with a Zn/H2bdc ratio ≈ 1 or at high supersaturation conditions.13 Height analysis performed on the Co/Zn-MOF-5 terraces shows a monolayer step height of 1.3 ± 0.1 nm (see Figure 2c) that is identical to the observed height of the extended terraces in MOF-5 and corresponds to the d200 crystal spacing (1.28 nm) of the structure, as shown in Figure 2f.13 The terrace spreading rate had decreased by about an order of magnitude 1160 min after injecting the cobalt-rich growing solution (SA-2), indicating a significant decrease in supersaturation too close to equilibrium conditions. However, the shape of the terraces was still fairly circular, as shown in Figure 2d. This indicates that the terrace shape is relatively unaffected by supersaturation, which is contrary to what was observed for the growth of MOF-5 crystals with a growth solution of Zn/H2bdc ratio > 1 for which the terrace shape evolved from pure circles at high supersaturation to almost perfect squares at low supersaturation.13 The last growth solution introduced into the fluid cell in experiment A was a standard growth solution (SA-3, Table 1). This solution promoted the formation of multiple MOF-5 2D nuclei on the Co/Zn-MOF-5 terraces that displayed the square terrace morphology associated with growth of MOF-5 from growth solutions with a Zn/H2bdc ratio > 1 (see Figure 2e).13

RESULTS AND DISCUSSION

The MOF-5 seed crystals prepared possess a cubic morphology bound by {100} facets, as has been verified by means of electron diffraction.16 Figure 2 shows AFM deflection images taken from experiment A in Table 1. This experiment was started by introducing a standard growth solution (SA-1, Table 1) with a Zn/H2BDC > 1 over the MOF-5 seed crystals to induce MOF-5 growth. This growth solution induced new crystal growth via a birth and spread mechanism that gave rise to well-defined square terraces with edges parallel to the ⟨100⟩ directions, as shown in Figure 2a. The observed growth was similar to that reported previously.13 After 135 min of growth, a cobalt-containing growth solution (SA-2, Table 1) was introduced into the fluid cell. This solution immediately led to a higher rate of nucleation and terrace spreading, as compared to rates induced by solution SA-1. The new Co/ZnMOF-5 nuclei and terraces have an almost perfectly rounded shape, and nucleation occurred repeatedly over specific points on the surface, giving rise to terraces several monolayers in height, as seen in Figure 2b. The orientation of the underlying MOF-5 terraces with edges parallel to the ⟨100⟩ directions can C

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The height analysis on the newly formed nuclei and terraces revealed a monolayer step height of 1.3 ± 0.1 nm (see Figure 2e) in perfect agreement with that previously reported for MOF-5.13 This experiment was finally stopped after approximately 10 new MOF-5 layers were grown, by removal of the growth solution. The newly formed core(MOF-5)−shell(Co/ZnMOF-5)−shell(MOF-5) crystals were allowed to dry and then they were observed by SEM. In addition, samples of core(MOF-5)−shell(Co/Zn-MOF-5)−shell(MOF-5) crystals formed externally to the AFM but, following a similar procedure to experiment A, were observed by SEM. Figure 3a and Figure S1 (Supporting Information) show the micrographs of the crystals that still retain their overall cubic crystal habit. The cracks observed on the crystals in Figure S1 (Supporting Information) appeared after drying as has been observed previously for the other MOFs such as HKUST-1.17 Energy dispersive X-ray spectroscopy analyses were performed on several of these crystals to prove cobalt had been incorporated in the crystal. Figure 3 (panels b and c) shows the results of the spot analyses done on the crystal shown in Figure 3a at the points marked by an “x”. The spectrum obtained from analysis at the crystal surface, point “1x” (Figure 3b), indicates the presence of cobalt, while the spectrum obtained from the core of the crystal, point “2x” (Figure 3c) indicates the absence of cobalt. As the crystals were treated with cobalt-free solutions via eight washes (four with solution SA-3 and four with pure DEF), it is assumed that the cobalt identified in the EDAX spectrum is isomorphously substituted framework cobalt. Any nonframework cobalt species would have been washed out of the pores of the framework due to the relatively large amount of cobalt-free solution passed over the crystals and the weak binding between the charge neutral MOF-5 framework and the charged nonframework species within its pore volume. Three room temperature sample washes with DMF were found to be sufficient to remove nonframework cobalt species from solvothermally prepared Co/ZnMOF-5.9 The absence of cobalt in the core of the crystals also supports the assertion that it is contained in a Co/Zn-MOF-5 shell containing isomorphously substituted framework cobalt introduced during the formation of the framework of the shell and not by ion-exchange of the framework zinc8 that would cause the cobalt to be dispersed throughout the crystal. A Zn/ Co ratio = 24(6) was derived from multiple surface EDAX analyses of these core−shell−shell crystals. This ratio is lower than the previously reported values for Co/Zn-MOF-59, which may result from the drastically different synthesis conditions used to prepare the Co/Zn-MOF-59 and the fact that the penetration depth of the electron beam is >1 μm so the total crystal volume probed would include the top MOF-5 shell formed at the end of the experiment, the Co/Zn-MOF-5 shell, and a large volume of the underlying core MOF-5 crystal. Attempts to identify the presence of the Co/Zn-MOF-5 shell by powder X-ray diffraction were inconclusive, most probably due to the low concentration of cobalt in the Co/Zn-MOF-5 shell and the small thickness of the shell. Three additional growth experiments were performed using the cobalt-containing growth solutions B, C, and D listed in Table 1 to establish the influence of the Zn/Co ratio on the surface growth rates and surface topography. In these experiments, the H2bdc and total metal (Co and Zn) concentrations were the same, but the Zn/Co ratio was varied from (B) 4.1 to (C) 1.4 to (D) 0.6. The concentrations of

Figure 3. (a) SEM micrograph, (b) the EDAX analysis spectrum at the point “1x” in (a) at the crystal surface and (c) the EDAX analysis spectrum at the point “2x” in (a) at the crystal core of a core(MOF5)−shell(Co/Zn-MOF-5)−shell(MOF-5) crystal. The EDAX analysis spectra show the relative intensities of the Co and Zn signals. The crystal shown was extracted from a sample treated under the conditions of experiment A external to the AFM.

H2bdc and the metal ions in these experiments were less than half that used in experiment A to assess whether the initial supersatuation had any effect on the surface growth. Figure 4 shows AFM deflection images from experiments B and C that reveal the formation of almost rounded terraces with the same orientation as those shown from experiment A, solution SA-2 (see Figure 2, panels b−d). The measured terrace spreading rates in these two experiments decreased by approximately 50% from ≈1 nm s−1 (experiment B) to ≈0.5 nm s−1 (experiment C). Both rates are slower than the terrace spreading rate of ≈1.9 nm s−1, observed for the growth of MOF-5 from a cobaltfree growth solution of similar metal and H2bdc concentration in DEF (solution SL-1, experiment L reported in Cubillas et D

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Figure 5. A schematic diagram showing the relationship between the different growth rates along the ⟨100⟩ and ⟨110⟩ directions represented by the size of the blue and red arrows, respectively, and how they are related to the observed terrace morphologies in the absence (left) and presence (right) of cobalt in the growth solutions.

Figure 4. AFM deflection images of a {100} face of Co/Zn-MOF-5 taken (a) 70 min after introducing solution SB in experiment B and (b) 120 min after introduction of solution SC in experiment C.

the growth solution, both growth rates decrease but that along the ⟨110⟩ directions decreases more relative to the rate along the ⟨100⟩ directions, as represented in Figure 5b. This means that nucleation at the numerous kink sites along the step edge parallel to the ⟨110⟩ directions now become the rate determining step. Changes in terrace morphology in other systems have been attributed to changing the supersaturation conditions,19−22 but in the presented experimental results this does not appear to be the cause (compare Figure 2 panel d to panels b and c and Figure 4 to Figure 2, panels b−d). This means the most likely explanation behind the change in growth rates is that the presence of cobalt in the growth solutions induces atomistic changes in the crystal growth mechanism related to differences in the number and type of solution species and their potential to inhibit growth at the step edge or kink sites. This inhibition is most likely to arise from a large reduction in the rate of inclusion of zinc species as simple, or as part of a more complex, solution species at the different types of site on the different step edges. The formation of metastable intermediate structures with differing degrees of stability at the different types of growth sites may also be involved in the observed overall surface growth process. A similar disturbance of the standard route of inclusion of zinc species into growing terraces on the {100} facets and the related change in surface terrace morphology was observed to occur from cobalt-free growth solutions with a Zn/H2bdc ratio ≈ 1.13 Unfortunately, our poor current knowledge concerning the solution speciation in metal−organic framework synthesis solutions and the true atomic step edge structure restricts further explanation of this effect. These in situ AFM studies do highlight the importance of the zinc concentration in identifying particular mechanisms for the crystal growth of the MOF-5 framework. The presence of cobalt in the growth solutions also retards the rate of terrace growth with greater inhibition occurring as the Zn/Co ratio of the growth solution decreases. The higher the cobalt concentration, the more probable that the cobalt species will interfere with the normal mechanism of inclusion of zinc into the framework during crystal growth and so further retard the rate of terrace growth. However, the previous reports on the isomorphous substitution of Co2+ or Ni2+ ions into the MOF-5 framework report formation of [CoxZn4‑xO(bdc)3] (0 ≤ x ≤ 1) and [NixZn4‑xO(bdc)3] (0 ≤ x ≤ 1) from reagent mixtures with a Zn/Co ratio = 19 and Zn/Ni ratio = 0.17,8 respectively. This appears to conflict with the results presented in this work for which terrace spreading is completely stopped when the Zn/Co ratio was 0.6. This difference in behavior is likely to be related to the drastically different conditions under

al.13). Experiment D resulted in no observed growth on the MOF-5 seed crystals. Experiment D was repeated three times in order to corroborate the absence of any growth under these conditions and in all cases, no terrace spreading or nucleation was observed. These results indicate that the Co2+ ions have an important effect on changing the surface terrace morphology from being perfectly square terraces in cobalt-free growth solutions to round/severely truncated square terraces when Co2+ ions are present in the growth solutions down to concentrations of less than ∼20% relative to that of zinc. In addition, the lower supersaturation of the growth solutions SB and SC, in comparison to solution SA-2, appears not to have an effect on the observed change in terrace morphology, which supports the results from experiment A that the observed terrace morphology changes are not instigated by changes in supersaturation. The results also show that the rates of terrace spreading decrease as the Zn/Co ratio in the growth solution drops to the point at which growth is completely inhibited when there is more cobalt in the growth solution than zinc. These results demonstrate that mixed metal core−shell−shell crystals based upon the MOF-5 framework may be constructed at room temperature with some control of the composition of the shells and, by working with low supersaturated solutions, some control over the thickness of the deposited shells. The complex core−shell−shell structure has a hierarchal mixed metal nature with mixing at the atomic level in the isomorphously substituted Co/Zn-MOF-5 shell and at the nanometer level through segregation of the Co/Zn-MOF-5 and MOF-5 layers. The presence of cobalt in the growth solutions is also found to affect the relative rates of terrace spreading in different crystallographic directions and the overall rate of surface growth. The presence of cobalt in the growth solutions where there is a Zn/Co ratio > 1 causes a change in the shape of the nuclei and terraces that evolve from being perfect squares with edges parallel to the ⟨100⟩ directions in the absence of cobalt to becoming almost circular/severely truncated squares with the more pronounced rectilinear part of the steps being parallel to the ⟨110⟩ directions. The change in terrace shape is due to the modification of the relative growth rates along the ⟨100⟩ and ⟨110⟩ directions, as shown schematically in Figure 5. To form the square terrace morphology of MOF-5 shown in Figure 5a, the rate of growth along the ⟨110⟩ directions is much faster than along ⟨100⟩, and in this case step advancement is limited by the nucleation at the edge sites or the sparse kink sites on the step edges parallel to the ⟨100⟩ directions, if one assumes growth via a simple Kossel model.18 When cobalt is present in E

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to the surface are highlighted in red and black. Two plausible Burgers vectors can be envisioned that will produce a singlestep dislocation step of 1.3 nm on the {100} planes by only breaking the carboxylate oxygen−metal bonds. In the first case, the Burgers vector 1/2⟨100⟩ has a length of 1.3 nm and is oriented along the ⟨100⟩ directions perpendicular to the {100} surfaces. A planar orientation of the bdc groups around this dislocation is not possible once this dislocation forms, as shown for the bdc groups encircled in Figure 6c. This dislocation can only be formed if the connecting bdc molecules around the defect adopt a nonplanar conformation. Such connectivity around the dislocation may be achieved as the adoption of such conformations is not too energetically demanding as revealed by the presence of nonplanar bdc linkers in the crystal structure of Sc2(bdc)324 and the spectroscopic measurement of the ability of the phenyl rings in the bdc linker to flip at relatively low temperatures.25 The second possibility for the single-step spiral is that the Burgers vector is 1/2⟨110⟩, which is parallel to the ⟨110⟩ directions as shown in Figure 6d and would have a component of height 1.3 nm perpendicular to the {100} planes. In this case, the Burgers vector length would be 1.83 nm but the resulting displacement would result in perfect connectivity of the structure around the dislocation, thus reducing the stress in the defect-containing material. The 1/2⟨110⟩ Burgers vector is a common screw dislocation found in face-centered cubic materials.26 Isotropic continuum elastic theory shows that dislocations with the shortest Burgers vector length tend to be formed preferentially,26 but in this case both vectors would seem to be plausible sources for the spiral dislocations at the core of these observed defects.

which the crystal growth of the Co/Zn-MOF-5 material is occurring and the related differences in solution species, thermodynamic stabilities, and rates of the kinetic processes involved. It is interesting to note that a Co-MOF-5 [Co4O(bdc)3] has been synthesized directly by reacting preformed [Co8(μ4-O)2(μ1, 2-OOCCMe3)12] clusters,23 that contain preassembled Co4O cores, in the presence of H2bdc in DEF. It is presumably the preassembly of these clusters that allows formation of the Co-MOF-5 [Co4O(bdc)3] that cannot currently be synthesized from simple Co2+ salt precursors. As reported above, spiral growth was observed during the course of the growth of Co/Zn-MOF-5. This type of growth has also previously been observed for MOF-5.13 Individual, closed-loop, and composite growth spirals were observed in this work. Examples of single-step spirals can be seen in Figure 6a,

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CONCLUSIONS This work demonstrates the potential to epitaxially grow complex mixed metal core−shell−shell MOF-5-type framework crystals at room temperature under low supersaturation conditions with some control over the composition and the thickness of the deposited shells. The work also reveals the influence of the additional metal ion type, cobalt, on the relative rates of terrace spreading in different crystallographic directions and the overall rate of surface growth for Co/Zn-MOF-5 in comparison to MOF-5. This methodology may potentially be expanded to form, and understand, complex crystal forms of other MOF systems for new or improved functionality and application.

Figure 6. (a) AFM deflection image showing the several single-step growth spirals of Co/Zn-MOF-5 and an associated cross-sectional analysis of a 1.3 nm high spiral step. (b) Simplified MOF-5 structure highlighting the different bdc linker orientations with different colors and showing the position of a {100} face (dotted line). (c) Simplified diagram showing the crystal structure formed around a growth spiral with a Burgers vector 1/2⟨100⟩ perpendicular to a {100} face. (d) Simplified diagram showing the crystal structure formed around a growth spiral with a Burgers vector 1/2⟨110⟩ parallel to a ⟨110⟩ direction. The presence of connectivity around the dislocation involving nonplanar bdc linkers is shown by the blue and green bdc linkers highlighted with the red ellipses in (c).

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ASSOCIATED CONTENT

S Supporting Information *

An additional SEM micrograph and EDAX analysis spectrum of core(MOF-5)−shell(Co/Zn-MOF-5)−shell(MOF-5) crystals extracted after the in situ AFM experiment A is provided. This material is available free of charge via the Internet at http://pubs.acs.org.

which shows an AFM deflection image taken during experiment A, solution SA-2. The step height of these single-step growth spirals is 1.3 nm, which corresponds to the d200 crystal spacing (see Figure 6a). The fact that the step emanating from the dislocation is only half the unit cell raises questions as to the nature of the spiral defect at the core of this type of crystal growth in the material. MOF-5 is composed of layers for which metal clusters linked by planar bdc molecules alternate orientations along the x, y, and z axes every 1.3 nm. This is the reason why the unit cell length of MOF-5 is 2.56 nm. Figure 6b shows a cross section of the MOF-5 structure where the alternating orientations of the bdc linkers parallel to a {100} face are highlighted in blue and green, and those perpendicular

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AUTHOR INFORMATION

Corresponding Author

*E-mail: m.attfi[email protected]. Tel: +44-161-306-4467. Fax: +44-161-275-4598. Web page: http://www.chemistry. manchester.ac.uk/people/staff/profile/?ea=M.Attfield. Notes

The authors declare no competing financial interest. F

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ACKNOWLEDGMENTS We acknowledge the Leverhulme Trust for funding. REFERENCES

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dx.doi.org/10.1021/cg401001f | Cryst. Growth Des. XXXX, XXX, XXX−XXX