Growth of nordstrandite: effect of carboxylates on surface morphology

Abstract

We studied the effect of carboxylates on the crystallization of nordstrandite precipitated from ethylenediamine solutions by examining the nanometer scale step structure of the basal faces with atomic force microscopy (AFM). The crystals grown in the presence of carboxylates are more rounded in shape than those grown without added carboxylates, with irregular growth in the direction perpendicular to the basal faces and small protuberances from the crystals. AFM images show monolayer steps on the surfaces of the crystals. These steps are composed of straight segments down to a horizontal resolution of a few nanometers. A survey of the step segments showed that the steps formed in the presence of carboxylates have shorter segments. For the crystals grown in the control solutions, two-dimensional nucleation is evident in a few places on the surfaces, and concentric terraces are present in many cases. For the crystals grown in carboxylate solutions, two-dimensional nucleation had occurred more randomly and evenly on the surfaces, with smaller nuclei. These differences in step morphology are due to the adsorption of the carboxylates at the steps, which affected the step edge surface energy.