• Jinschek, Joerg R.
• Kollmannsberger, Kathrin L.
• Banerjee, Pritam
• Fischer, Roland A.

Abstract

Zeolitic imidazolate frameworks 8 (ZIF 8) are a sub class of metal organic frameworks (MOF), hierarchically porous materials, where i n ZIF 8 zinc (Zn) atoms are linked to 2 methylimidazolate linkers. ZIF 8 has a cubic crystal structure (space group I43m, lattice parameter = 16.99Å). This chemically stable crystalline structure with large pores of 11.6Å is well suited for the adsorption of both guest aggregates and gas molecules. This ability opens a wide field of application,including catalysis as well as gas storage and separation. However, this also depends on our ability to characterize such potentially complex structures with atom l evel accuracy. <br/><br/>In general, high resolution scanning and transmission electron microscopy (HR S/TEM) exhibits such capability, but by using high energy electrons. Using an electron beam as a probe when assessing the structure of beam sensitive and softmatt er, can be invasive. Therefore, we need to investigate and understand when and how the studied original structure might be altered unintentionally in these atomic scale experiments. This knowledge will give us control over all electron sample interactions in our experiment and allows us to minimize (or at least mitigate) unintended structural changes. Here, we study the stability of ZIF 8 in HR S/TEM atomic scale imaging experiments, specifically the critical total electron dose tolerated. In the literatur e, values of total dose are reported, however not all affecting parameters have been considered. <br/><br/>One method of assessing structural changes in a crystalline sample, e.g. caused by exposure to high energy electrons, is by quantifying the drop in the intens ity of specific Bragg spots in electron diffraction (ED) pattern. Here, we have examined the dose rate dependence of the stability of ZIF 8 MOF nanoparticles (average size ~ 160 nm) supported on amorphous carbon with varying dose rate of 0. 33 0.5, 1 , 2 an d 4 eA 2 s, respectively. Time series of ED pattern have been recorded in FEI Tecnai G2 microscope (LaB 6 , 200 keV) with exposure time of 1 sec until all the Bragg diffraction spots have completely faded out. The bright field TEM image of the ZIF 8 nanoparticles and the corresponding ED pattern, taken with a dose rate of 1 eA 2 s, are shown in fig. 1a and 1b, respectively. The radial average of the intensity of the Bragg rings (shown in fig. 1b) are plotted in fig. 1c. The time dependence of the relat ive intensity of the {431} Bragg spot, corresponding to an interplanar spacing of 3.3 Å between Zn triplets in the unit cell along [111] direction and an indication of resolving these Zn triplets in the HR S/TEM imaging, is plotted in fig. 1d. We use this approach to estimate the effect of dose rate on the critical dose tolerance limit in our ZIF 8 material. At last, we also examined the effect of graphene as support on increasing the critical dose tolerance limit.

Topics

• nanoparticle
• porous
• impedance spectroscopy
• pore
• amorphous
• Carbon
• experiment
• electron diffraction
• zinc
• transmission electron microscopy
• size-exclusion chromatography
• space group
• elemental analysis