Observations of layer growth in synthetic zeolites by field emission scanning electron microscopy

Field emission scanning electron microscopy (FE-SEM) has been used to image the crystal growth-induced surface features of synthetic zeolites. The capability of FE-SEM to image layers of nanometer scale thickness in zeolites A, X, and Silicalite has been demonstrated by comparing atomic force microscope (AFM) and FE-SEM images of their surface topography. Layers on the {100} faces of zeolite A, {111} faces of zeolite X, and {100} and {010} faces of Silicalite with AFM measured thickness of ∼1.2, ∼1.5, ∼1-2, and ∼1-2 nm, respectively were successfully imaged by FE-SEM using crystals coated with thin Cr films. Also, FE-SEM could discern layers on the uncoated crystals but the resulting images were characterized by much poorer topographic contrast. In addition, layers on the {110} zeolite A surfaces hitherto not reported have been imaged by FE-SEM. The presence of growth terraces on the {110} faces has been related to the high ratio of the rate of surface nucleation to the rate of lateral propagation of layers occurring on these surfaces near the end of crystallization. The FE-SEM images of layers on the basal planes of cylindrical zeolite L crystals have been also acquired and also discussed in context with the previously hypothesized spiral growth mechanism on these surfaces.