SIZE-DRIVEN STRUCTURAL CHANGES IN HEMATITE NANOPARTICLES

Navrotsky and her coworkers have shown that Fe oxide nanoparticles can exhibit size-dependent crossovers in stability (e.g., hematite versus goethite). To further elucidate the changes in structure and reactivity of nanoparticulate Fe oxides, we are probing the fundamental properties and structural variation of hematite (α-Fe₂O₃) as a function of size within the nanometer range, along with sorption and dissolution behavior.

Five hematite nanoparticle samples were synthesized from solution with average axes or diameter: 3.6, 8.6, 41, 66, and 72nm . Particles were characterized for phase, degree of crystallinity, morphology, and surface area using electron diffraction, XRD, TEM, and BET surface area analysis. Amounts of incorporated and surficial water were investigated by thermogravimentric and differential thermal analysis (TGA and DTA, respectively). All of the samples were highly crystalline hematite. BET and TEM surface area measurements agreed well and ranged from 16m²/g to 327m²/g. Total hydration loss, as measured by TGA, was >1% by mass only in the particles smaller than 10nm and was identified in all samples as dominantly surficial water loss over a wide temperature range. DTA results suggest moderate, consistent recrystallization and surface dehydration of all particle sizes.

In an effort to describe mechanistic control of size-dependent chemical reactivity, samples were analyzed for structural differences as a function of size via extended X-ray absorption fine structure analysis. An increase in Fe-O coordination relative to Fe-Fe coordination was observed trending from ‘bulk’ hematite particles (>10nm) through the 8.6nm to the smallest, 3.6nm, particles. Models for surface versus core structure as a function of particle diameter are discussed. Our ongoing research focuses on how not only size but also morphology (spheroidal versus rhombohedral versus pseudohexagonal) may affect reactivity.