Facet-specific reactivity of hematite nanocrystals during Fe(II)-catalyzed recrystallization

Abstract

Hematite (α-Fe2O3) surfaces represent one of the most common redox-active interfaces in the environment, playing an important role in the biogeochemical cycling of metal elements in the subsurface. Fe(II)-catalyzed recrystallization of iron (hydr)oxides is a fundamental process of iron cycling in earth's surface environments, but the proposed Fe(II)aq-Fe(III) electron transfer and Fe atom exchange mechanism of recrystallization remains poorly understood at the atomic level. Here, two different structurally well-defined single crystals of hematite were employed to react with Fe(II)aq at neutral pH. Compared with hematite nanoplates (HNPs) exposed with {001} facets, hematite nanocubes (HNCs) exposed with {012} facets displayed higher adsorption capacity for Fe(II)aq and underwent more Fe atom exchange, indicating the Fe(II)-catalyzed recrystallization process relied on exposed facets of hematite. Hematite recrystallization caused suppression on Morin transition and increase in crystallinity for both HNCs and HNPs, with the extent depending on surface structure. Density functional theory calculations revealed that the facet-specific differences were directly linked with the density of surface Fe atoms and their coordination conditions. The findings highlight the important roles of surface structure of iron (hydr)oxides in the Fe(II)-catalyzed recrystallization, and provide fundamental data to iron atom exchange, suggesting that the surface structure sensitivity should be considered to estimate the reactivity at the mineral-liquid interface.