Facet-Dependent Dispersion and Aggregation Behavior of Aqueous Hematite Nanoparticles

The aggregation state of nanoparticles in solution plays a commanding role over their function. In technological applications, particularly in aqueous media, particle dispersion often requires additive sorbents to impart a net repulsive interaction. However, facet engineering of nanocrystals holds promise for achieving clean monodisperse suspensions simply based on facet-specific interaction with solvent molecules. Here, we systematically studied the dispersion/aggregation behavior of three sets of hematite (Fe₂O₃) nanoparticles in various aqueous solutions using ex situ electron microscopy and in situ small angle X-ray scattering (SAXS). Comparison of particles dominated by (104), (001), and (116) facets revealed a unique tendency of (104) hematite nanoparticles to maintain a monodisperse state across a wide range of pH, electrolyte type and concentration, temperature, and aging time. Density functional theory (DFT) calculations reveal how this behavior arises from a relatively inert, densely hydrogen-bonded first water layer unique to the (104) facet that favors interparticle dispersion by saturating interaction sites on the surface. Our finding validates the notion that nanoparticle dispersion can be controlled through expression of specific facets for specific solvents, and thus may help in the development of various nanoparticle applications that rely on their interfacial area to be highly accessible in robustly stable suspensions.