Visualizing Hydrogen Nanobubbles Generation at Nanoscale Zero-valent Iron/Water Interfaces

Abstract

Nanobubbles (NBs) possess unique interfacial and physicochemical properties; however, their formation in reactive metal-water systems still lacks convincing in-liquid, nanoscale-resolved evidence. Here we visualize hydrogen nanobubbles (HNBs) generation during anaerobic aqueous corrosion of nanoscale zero-valent iron (nZVI) using a light-based scattering, synchrotron imaging and interfacial microscopy. Laser Tyndall scattering and nanoparticle tracking analysis reveal a population of rapidly moving nanoscale scatterers in 0.22 μm-filtered nZVI-H2O reaction filtrates; these signals are negligible in ethanol controls and markedly diminished after freeze-vacuum degassing, supporting a gaseous origin for the dispersible entities. To localize these scatterers at reactive interfaces, in-liquid scanning transmission X-ray microscopy at the O K-edge resolves discrete circular, high-transmission voidlike features adjacent to nZVI aggregates in water but not in ethanol, providing nanoscale visualization of interfacial gas domains. Time-resolved liquid-phase scanning electron microscopy and atomic force microscopy further show that these low-density/low-stiffness domains emerge rapidly and accumulate at the Iron/Water interface. Finally, cyclic voltammetry and electron paramagnetic resonance spin trapping link gas-domain formation to sustained hydrogen evolution with transient hydrogen-associated intermediates during aqueous corrosion. Together, these results establish aqueous nZVI corrosion as an intrinsic generator of nanobubbles, expanding iron reactivity from classical solid-liquid pathways to coupled gas-liquid-solid interfacial processes with implications for groundwater remediation.