AFM and HRSEM Invesitigation of Zeolite A Crystal Growth. Part 1: In the Absence of Organic Additives

The crystallization of zeolite A by the verified synthesis method was studied by means of high-resolution scanning electron microscopy (HRSEM) and atomic force microscopy (AFM). These methods show an evolution in the growth mode of zeolite A from an adhesive type at the beginning of the synthesis (high supersaturation) to birth-and-spread growth at the end of the synthesis (low supersaturation). Additionally, HRSEM provides direct proof on the formation of zeolite A crystals at the surface of the amorphous gel and the aggregation of crystals at early stages of synthesis, which leads to intergrowth formation. For the first time, high-resolution AFM images were taken on the {110} and {111} faces of zeolite A. Growth on {110} faces takes place by a birth-and-spread mechanism, and the shape of the terraces is rectangular, with growth along the ⟨100⟩ directions being twice as fast as in the ⟨110⟩ directions. Growth on the {111} faces also takes place by a birth-and-spread mechanism via triangular-shaped terraces with edges parallel to ⟨110⟩ directions. Possible surface terminations for both faces are discussed, and crystal habit and surface topography are modeled by Monte Carlo simulations.