Centre for Nanoporous Materials, School of Chemistry, The University of Manchester, Brunswick St., M13 9PL, Manchester, UK.
Chemistry. 2012 Nov 26;18(48):15406-15. doi: 10.1002/chem.201202261. Epub 2012 Oct 10.
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.
首次通过原子力显微镜（AFM）研究了金属有机骨架 MOF-5 的晶体生长。在低过饱和度条件下的生长被发现是通过二维或螺旋晶体生长机制发生的。在前者中观察到的发展核揭示了生长是通过亚稳和稳定亚层的成核和扩展过程发生的，这表明 MOF 可以被认为是晶体生长方面的密相结构，尽管它们包含由有序框架和无序非框架组成的亚层。这些结果也支持了这样的观点，即在低过饱和度下，这可能是适用于结晶纳米多孔材料的表面晶体生长的一般机制。在原子水平上的晶体生长机制也随着生长溶液 Zn/H(2)bdc 比例的变化而变化，当 Zn/H(2)bdc 比例大于 1 或约 1 时，分别产生与 <100> 方向平行的正方形平台和与 <110> 方向平行的菱形平台。相对生长速率的变化可以用溶液物种浓度的变化及其对不同平台生长点生长的影响来解释。这些结果成功地应用于合成的立方体形晶体的生长，以增加{111}面的表达，并生长出具有合适质量的八面体晶体，以便使用 AFM 成像。这种无调制器的方法来控制 MOF-5 晶体的晶体形态应该适用于各种 MOF，以实现所需的形态控制，从而提高应用中的性能。
