Copper substitution–induced internal charge redistribution drives manganese(IV) surface passivation and growth suppression in spinel hausmannite (Mn3O4) nanoparticles

Abstract

Understanding how cation substitution redistributes charge and reconfigures surface chemistry in spinel oxides is central to controlling growth and stability of nanoparticles. Here we investigate Cu incorporation into hausmannite (Mn₃O₄) and show that substitution suppresses crystallite coarsening, stabilizing ultrasmall (<10 nm) nanoparticles that assemble into nanoclusters upon drying. Although Cu increases colloidal stability in suspension, drying induces charge redistribution that renders the nanoparticles non-redispersible. Structural and spectroscopic analyses indicate that Cu occupies both tetrahedral and octahedral sites while preserving the spinel framework. Periodic spin-polarized density functional theory plus Hubbard U identifies tetrahedral Cu as the preferred bulk motif, while mixed and octahedral configurations produce stronger local distortion and Mn-centered charge/spin redistribution, providing an atomistic basis consistent with suppressed coherent coarsening. The combined evidence favors internal Mn valence redistribution over vacancy-dominated compensation, although minor oxygen-vacancy contributions cannot be excluded. Depth-profiling X-ray photoelectron spectroscopy, interpreted together with sputtering-stability controls and bulk-averaged spectroscopy, is consistent with partial Mn(IV)-like enrichment at the pristine outermost surface and more extensive passivation after moderate Cu substitution; the inferred coverage depends on the assumptions of the geometric model. This passivated surface state alters reaction pathways during aging: under oxic, near-neutral conditions, pristine Mn₃O₄ undergoes redox-driven oxidative hydration and transforms to manganite, whereas Mn(IV)-passivated Cu-substituted nanoparticles largely retain the spinel phase. At extreme pH, however, proton-promoted dissolution yields amorphous Mn(IV)O₂-like products under acidic conditions, and hydroxide-mediated hydrolysis/dissolution and reprecipitation yield layered Na-birnessite under alkaline conditions. Thus, passivation selectively suppresses redox-driven transformation without eliminating acid/base reactivity. Together, these results link substitution-driven internal charge redistribution to Mn(IV)-rich surface passivation, growth suppression, and selective redox stabilization in Mn₃O₄ nanoparticles.