Understanding the growth mechanism of Al-bearing hematite nanoparticles via two-dimensional oriented attachment

Abstract

Hematite occurs ubiquitously in nature and is important for controlling the migration and conversion of various nutrients and pollutants. The morphology of hematite is a key factor affecting its reactivity. Naturally occurring hematite usually contains an amount of aluminum (Al) impurities. Al has a significant effect on the morphology of hematite. However, the details of Al-bearing hematite formation and the role of Al in the morphological evolution remain poorly understood. In this work, a series of Al-bearing hematite were synthesized using hydrothermal methods. Their growth process was followed by examining the intermediate products harvested at different intervals of reaction time using the XRD and electron microscopy. DFT calculations were used to investigate the effect of Al on the stability of hematite {001} and {110} crystal facets. The results indicate that the formation of Al-bearing hematite follows two stages: the initial nucleation of hematite nuclei and the subsequent ripening of nuclei into nanoplates. The nucleation process involves the formation of ferrihydrite, followed by the amorphous nanoparticles undergoing phase transition into more thermodynamically stable hematite nuclei. In the ripening stage, hematite nuclei aggregate into single-crystalline hematite particles through oriented attachment and then crystallize into nanoplates. The surface energy of the {110} crystal facet is higher than the {001} crystal facet at high concentrations of Al, which promotes oriented attachment of {110} facets between hematite nuclei. The proposed mechanism in this article not only provides deep insight into the formation of naturally occurring hematite but also a path for the efficient fabrication of environmentally friendly iron oxide materials.