Crystal Growth Mechanisms and Morphological Control of the Prototypical Metal–Organic Framework MOF‐5 Revealed by Atomic Force Microscopy

Abstract

Crystal growth of the metal-organic framework MOF-5 was studied by atomic force microscopy (AFM) for the first time. Growth under low supersaturation conditions was found to occur by a two-dimensional or spiral crystal growth mechanism. Observation of developing nuclei during the former reveals growth occurs through a process of nucleation and spreading of metastable and stable sub-layers revealing that MOFs may be considered as dense phase structures in terms of crystal growth, even though they contain sub-layers consisting of ordered framework and disordered non-framework components. These results also support the notion this may be a general mechanism of surface crystal growth at low supersaturation applicable to crystalline nanoporous materials. The crystal growth mechanism at the atomistic level was also seen to vary as a function of the growth solution Zn/H(2)bdc ratio producing square terraces with steps parallel to the <100> direction or rhombus-shaped terraces with steps parallel to the <110> direction when the Zn/H(2)bdc ratio was >1 or about 1, respectively. The change in relative growth rates can be explained in terms of changes in the solution species concentrations and their influence on growth at different terrace growth sites. These results were successfully applied to the growth of as-synthesized cube-shaped crystals to increase expression of the {111} faces and to grow octahedral crystals of suitable quality to image using AFM. This modulator-free route to control the crystal morphology of MOF-5 crystals should be applicable to a wide variety of MOFs to achieve the desired morphological control for performance enhancement in applications.

Crystal growth of MOF-5 using secondary building units studied by in situ atomic force microscopy

“…19 Results from one such experiment, 6 ] followed a similar pattern to that observed previously. 19 The shape of the nuclei and terraces formed was almost perfectly square with edges parallel to the <100> direction, again following the same trends observed for MOF-5 grown from the α α α α1-type solutions. 19 The step advancement speed was approximately 0.021 ± 0.005 nm s .…”

“…19 The shape of the nuclei and terraces formed was almost perfectly square with edges parallel to the <100> direction, again following the same trends observed for MOF-5 grown from the α α α α1-type solutions. 19 The step advancement speed was approximately 0.021 ± 0.005 nm s . This sequence of heights was measured from more than 10 similarly developing nuclei growing on MOF-5 surfaces previously grown from a α α α α1-type solution across various experiments, but in no case were values larger than 1.3 nm ever observed.…”

“…Therefore, as was the case for the experiments performed in DMF, this nucleation behaviour follows that which has been reported previously for growth solutions prepared using zinc nitrate or zinc acetate salts. 19 These results indicate that a new growth step on the surface of MOF-5 does not develop by pathway (I) in Scheme 1 in which a bdc 2-/ Hbdc -species first adds to the crystal surface followed by addition of a complete Incomplete dissociation, for instance leaving the complete or partial inorganic Zn 4 O core, may be possible that would still be likely to increase the rates of formation of MOF-5 using the SBU precursor. However, such a species was not uniquely identifiable in this study.…”

“…This nucleation behaviour follows that which has been reported before for growth solutions prepared using zinc nitrate or zinc acetate salts. 18,19 An experiment (E -see Table 1) was carried out in which the solvent in the growth solutions (SE2-β β β β2 2 2 2-3, SE4-β β β β2 2 2 2-2) was predominantly acetone (97.6 % mol/ mol) in order to investigate whether the identity of the solvent would influence the mechanism of growth of the 2-D nuclei. [Zn 4 O(O 2 CC 6 H 5 ) 6 ] was crystallised from acetone so it was hypothesized that the likelihood of the [Zn 4 O(O 2 CC 6 H 5 ) 6 ] remaining intact during growth of MOF-5 was higher if acetone was used as the solvent.…”

“…[11][12][13][14] In situ AFM studies of differing resolution have been reported for a number of MOFs, [15][16][17][18][19] revealing valuable information concerning their crystal growth and the growth of nanoporous materials.…”