A Model for Nucleation When Nuclei Are Nonstoichiometric: Understanding the Precipitation of Iron Oxyhydroxide Nanoparticles

Despite years of study, quantitative models for the nucleation and growth of metal oxyhydroxide nanoparticles from aqueous solution have remained elusive. The problem is complicated by surface adsorption, which causes the stoichiometry of the nucleus to differ from that of the bulk precipitate and causes the surface tension of the precipitate-water interface to depend upon solution chemistry. Here we present a variation of classical nucleation theory that can accommodate surface adsorption, and apply it to understand the nucleation of β-FeOOH (akaganeite) nanoparticles from aqueous FeCl3 solutions. We use small-angle X-ray scattering (SAXS) to quantify nucleation rates over a range of concentrations (5–200 mM FeCl3) and temperatures (47–80 °C), then apply our model to estimate the critical nucleus size and surface tension at each condition. The surface tension varies from 0.07 J/m2 in 200 mM solutions to 0.16 J/m2 in 5 mM solutions. This behavior indicates that the nuclei contain an excess of Cl– and H+ r...