PNNL

The Science

The stability of nanoparticle suspensions is governed by a complex mixture of factors. Avoiding aggregation over time often involves modifying the surface of particles, which can affect the overall properties of the system. Researchers explored differences in the behavior of hematite nanoparticles with different facets exposed to water. They found that (104) hematite nanoparticles were highly stable and remained dispersed across multiple solution conditions, in contrast to (001) and (116) hematite. Calculations showed that water forms a tightly bound layer on the (104) surface, favoring particle dispersion over aggregation.

The Impact

Nanoparticle applications often rely on their high surface area to volume ratio. This requires that a large portion of the nanoparticle surface area remain accessible in solution, a challenge that usually requires surface modification to maintain particle stability and dispersity over time. This work demonstrates that controlling the faceting of nanoparticles can be an effective way to maintain particle dispersity without the introduction of surface modifying additives. 

Summary

Nanoparticle aggregation in solution controls both surface reactivity and the overall function of the materials. Complete dispersion usually requires adding sorbents that impart a net repulsive interaction between particles. Facet engineering offers an alternative approach to producing monodisperse suspensions simply based on facet-specific interactions with the solvent molecules. Researchers measured the dispersion/aggregation of three morphologies of hematite (α-Fe₂O₃) nanoparticles in water using ex situ electron microscopy and in situ small-angle x-ray scattering. The results showed a unique tendency of (104) hematite nanoparticles to maintain a monodisperse state across a wide range of solution conditions not observed in similarly sized (001)- and (116)-dominated particles. Density functional theory calculations reveal an inert, densely hydrogen-bonded first water layer on the (104) facet that favors interparticle dispersion. These data validate the notion that nanoparticle dispersions can be controlled through morphology for specific solvents, which may help in nanoparticle applications that rely on having highly accessible interfacial area in stable suspensions.