Aberration corrected
HRTEM
imaging of zeolitic nanocavities
Authors: Yoshida Kaname , Sasaki Yukichi , Kurata Hiroki 
Published: December 2016

Zeolites composed of alumino‐silicate frameworks have great potentials in various fields. Their covalent frameworks built from TO
4
‐tetrahedra (T: Si or Al) produce various kinds of nano‐cavities inside the crystals. These characteristic nanospaces play important roles in the various applications of zeolite. Non‐framework species such as counter cations and adsorbed molecules are located within nanocavities. Structural information of such non‐framework species is essential for understanding specific behavior of zeolites. High resolution imaging by an aberration corrected (scanning) transmission electron microscopy (AC‐(S)TEM) has been a powerful technique for structural analyses of various materials. However, structural parameters of non‐framework species within zeolitic nanocavities are too complex to analyze quantitatively with AC‐(S)TEM observations. As the first step of qualitative analysis for non‐framework species, it could be important to examine whether the nano‐cavities are empty, in order that it is necessary to evaluate the image contrast of the empty cavities. In this study, we consider the AC‐HRTEM image contrast of zeolitic nano‐cavities by experimental observations and multi‐slice image simulation depending on optical conditions and specimen thickness.

Commercially available high silica type MFI and FAU type zeolites with Si/Al ratios of 3760 and 1540 respectively were used in this study. In order to ignore the contribution of counter cations to AC‐HRTEM image, counter cations of these materials had been substituted originally to H
+
. Crystalline samples were crushed in an agate mortal with ethanol and collected on a gold coated TEM microgrid. The observations were performed on a JEOL JEM2200FS (accelerating voltage of 200kV) attached with a
C
S
corrector (CEOS CETCOR). In order to remove molecules adsorbed in zeolite, the specimens were held in a vacuum of microscopic column overnight prior to observation. The
C
S
values were controlled to be +15 µm or –15 µm. All HRTEM images were taken by a slow scan CCD camera (14 µm × 14 µm of 2048 × 2048 pixels, Gatan UltraScan 1000). The images were magnified by ~ 200 thousand times on the camera. Simulation of AC‐HRTEM images were executed on Total Resolution LLC MacTempas X based on a multi‐slice method. Comparing an experimental AC‐HRTEM image and a simulated through focus image series for various thickness of thin crystal, we find out an optimum focused area from one image. Pure silicalite composition was applied as a structural model for image simulations. So no counter cations were included in structural models for simulations.

Figure 1(a) shows structural models projected along [100] direction of the monoclinic MFI‐type zeolite with the unit cell dimensions of
a
= 1.9879 nm,
b
= 2.0107 nm,
c
= 1.3369 nm and β = 90.67°. The straight cavities are consisted of 10 membered ring surrounded by eight 5 membered rings and two 6 membered rings. As shown in Fig. 1(b), the amplitudes of exit wave function (EXWF) were increased at atomic column positions by electron channeling, in exchanged for reduction at neighboring area. Especially, amplitudes of EXWF at small channels go out of unity due to the adjacent atomic columns. The amplitude and phase of EXWF are intricately converted into the phase and amplitude on image plane, according to Lichte's diagram. For quantitative imaging of nanocavities as vacuum, it is necessary to make samples thinner to the utmost. Fig. 2 shows experimental AC‐HRTEM images taken under negative C
S
imaging (NCSI) and positive C
S
imaging (PCSI) conditions. Even with high resolution imaging in atomic scale, the contrast modulation of the area surrounding atomic columns is not so small. So the image contrasts of small zeolitic cavities swerve from the intensity of vacuum region.