Effect of ionic strength on the kinetics of crystal growth by oriented aggregation

Oriented aggregation is an important, nonclassical crystal growth mechanism that occurs in a wide array of materials, often producing nanoparticles with unique, anisotropic, and symmetry-defying morphologies. The growth of acicular goethite (α-FeOOH) nanorods from ellipsoidal ferrihydrite (Fe ₅HO ₈·4H ₂O) nanoparticles was used as a model system to study the effects of sodium nitrate concentration on product crystal morphology and the kinetics of growth by oriented aggregation. Time-resolved sampling and transmission electron microscopy were employed with semi-automated image analysis to quantify the growth rate constant of oriented aggregation. Results are discussed in the context of Derjaguin and Landau, Verwey and Overbeek (DLVO) theory through the use of quantitative statistical comparisons and modeling of rate constants. Increasing the concentration of sodium nitrate results in an increasing second-order growth rate constant. The magnitude of increase depends on the primary particle size; however, the magnitude and trend in size dependence did not follow predictions based on DLVO theory.