Electrolyte-Driven Interfacial Redox Reconstruction in Magnetite Nanoparticles Revealed by In Situ Soft X-ray Absorption Spectroscopy

Magnetite nanoparticles (Fe3O4 NPs) exhibit dynamic interfacial redox chemistry in aqueous environments, which governs their stability and functionality in several applications. However, Fe3O4 electronic structure under aqueous conditions remains challenging to probe. Here, we combine in situ sXAS with DFT-based spectral analysis to resolve the interfacial electronic structure of commercial Fe3O4 NPs in different alkaline and acidic aqueous environments. Fe L3,2 and O K-edge spectra are acquired from nanoparticles suspended in tetramethylammonium hydroxide (TMAH, pH ∼ 12.5) and HNO3 (pH ∼ 1.5) using a custom liquid cell for sXAS. The same edges are measured on Fe3O4 NP films exposed to KOH (pH ∼ 12.5) and H2SO4 (pH ∼ 1.5), since stable NP dispersions cannot be maintained in these electrolytes. Comparison of TMAH with KOH and HNO3 with H2SO4 provides insight into electrolyte-specific effects within alkaline and acidic pH regimes. The complementary depth sensitivities of total electron yield (TEY) and fluorescence yield (FY) detection separate interfacial and bulk electronic contributions. The results reveal interfacial redox reconstruction characterized by Fe2+ stabilization in aqueous environments relative to the dry NP film. Under alkaline conditions, hydroxide interactions stabilize Fe2+ through Fe(OH)2-like species, whereas acidic environments promote proton-mediated dissolution and interfacial Fe2+ accumulation. Comparison of pH-matched electrolytes reveals ion-specific effects, including electrostatic screening by TMA+ and surface passivation by SO42–. These findings demonstrate the capability of liquid-cell sXAS to probe dynamic redox equilibria at nanoparticle–liquid interfaces.