Visualizing hydrogen nanobubble generation at nanoscale zero-valent iron/water interfaces

Abstract

Nanobubbles (NBs) possess unique interfacial and physicochemical properties, yet convincing in-liquid, nanoscale-resolved evidence of their formation in reactive metal–water systems remains lacking. Here, we visualize hydrogen nanobubble (HNB) generation during the anaerobic aqueous corrosion of nanoscale zero-valent iron (nZVI) using optical scattering methods, synchrotron imaging and interfacial microscopy. Laser Tyndall scattering and nanoparticle tracking analyses reveal fast-moving nanoscale scattering centers in 0.22 μm filtered nZVI–H₂O reaction filtrates; these signals are negligible in ethanol controls and are strongly suppressed by freeze–vacuum degassing, supporting a gaseous origin. In-liquid scanning transmission X-ray microscopy at the O K-edge resolves discrete high-transmission void-like features adjacent to nZVI aggregates in water but not in ethanol, directly visualizing interfacial gas domains. Liquid-phase scanning electron microscopy and atomic force microscopy show that these low-density, low-stiffness domains emerge rapidly and accumulate at the iron/water interface. Cyclic voltammetry and electron paramagnetic resonance spin trapping further link gas-domain formation to sustained hydrogen evolution and short-lived hydrogen-associated intermediates. These results establish aqueous nZVI corrosion as an intrinsic generator of HNBs and extend iron corrosion from a classical solid–liquid pathway to coupled gas–liquid–solid interfacial processes, which have implications for groundwater remediation.