PNNL

Abstract

Rational design and synthesis of materials, determination of environmental conditions at the time of mineral formation, and definition of mechanisms in biomineralizing systems all require an understanding of the effect of multiple factors on modulating crystallization pathways. In this study, we investigate the influence of temperature, additives, aging time, and addition of seed crystals on hematite (a-Fe2O3, Hm) crystallization from a two-line ferrihydrite (Fe2O3·xH2O, Fh) suspension. We also investigate the effect of temperature on the aging of spindle-shaped Hm mesocrystals, which we synthesize through the addition of sodium oxalate. The results show that Hm mesocrystals form in a temperature window of 45 °C - 130 °C. Higher temperatures favor formation of rhombohedral Hm, suggesting that classical monomer-by-monomer growth dominates, whereas crystallization by particle attachment (CPA) is dominant at low temperatures. Aging of mesocrystals in the original solution at 90 °C or 180 °C leads to a slow morphological evolution towards rhombohedral crystals due to Ostwald ripening. Finally, by utilizing rhombohedral Hm and rod-shaped goethite as substrates on which to assemble the mesocrystals, we demonstrate formation of complex heterogeneous structures