Verified Syntheses of Zeolitic Materials

2nd Revised Edition

Characterization of zeolites by SEM

Koos Jansen
DeIft University of Technology, Laboratory of Organic Chemistry and Catalysis
2628 BL, DeIft The Netherlands

1. Introduction

In the scope of this contribution the reader is referred to the regular textbooks for technical information on the Scanning Electron Microscope (SEM).The resolution of SEM in the world of microscopy clearly bridges Optical Microscopy (OM) with Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) as is depicted in Figure 1.

The size of the zeolites that can be studied with SEM is thus between 20 nm and 20 μm.

Regarding zeolite sample preparation, two approaches are possible: for the ultimate picture and for fast exploration.

Click here for Figure 1

Click here for Figure 2

2. Approach for the ultimate picture

Start with a metal holder of which the surface is polished. The background is then smooth and even and does not interfere with the contours of the crystals.

Bring a hair from your head in contact with a glue in a way that you get individual small drops of glue on this hair. Touch the sample holder with this hair and a row of very tiny spots of glue are now present on the holder. Either powder or, preferably, crystal(lites) can be fixed on this row of glue spots, thus representing one type of zeolites. More rows with the same or another zeolite material can be arranged this way on one sample holder. Next, the samples need to be coated via a sputter technique with a conduction layer of preferably noble metal. This layer must be thin so that details of the sample are not blurred. Thus, in the case of an Edwards sputtering chamber, one minute for gold at the proper setting is enough.

In case the crystals are very small (<0.1 μm) or flat oriented on the support, it is difficult to make a picture of the crystallite with enough contrast to the background, actually the support. To observe the crystal clearly it is convenient to use an additional prismatic shaped support that is mounted on the primary support as shown in Figure 2. Not only the form, but also the aspect ratio of crystals, can be well studied. In particular, the crystal size and aspect ratio of crystals are determined with accuracy only when a crystal face is oriented perpendicular to the beam. With the additional support, the crystal can be oriented either perpendicular to the beam, which means rotation around the lateral axis as indicated in Figure 2, or almost parallel to the beam. With the rotation around the axial axis b, shown in Figure 2, at least three crystal faces perpendicular to each other can be studied on one crystal.

Regular sample tables of SEM apparatus only rotate thirty degrees in the lateral orientation.

3. Approaches for fast exploration

Start with a metal holder on which a preshaped, double-sided, sticky, conducting layer is applied. Divide the sticky area in, for instance, six segments. Dust each segment with a sample and remove loose material. Sputter for one or two minutes. Pictures can now be made.In all cases the samples need to be dry and/or free of volatile material in the high vacuum chamber.The information that can be drawn from SEM pictures of zeolites is given in Table 1.

Table I. Subjects and details that can be observed.

Subjects Details
Crystal form Type of zeolite
  Aspect ratio
  Influence on crystal growth, for example, inhibition of crystal
    faces that indicate the presence of an unknown factor
  Crystal size distribution phenomena, for example, aggregation /
    twinning / intergrowth
  Indication of single crystals
External surface Relative roughness
  Secondary nucleation effects
Purity of the phase Other zeolite types
  Amorphous material
Unknown species New material?