Self-limited Growth of an Oxyhydroxide Phase at the Fe3O4(001) Surface in Liquid and Ambient Pressure Water

TL;DR

This study investigates the stability and growth of oxyhydroxide phases on Fe3O4(001) surfaces in aqueous environments, revealing self-limited hydroxide layer formation that passivates the surface.

Contribution

It introduces a new experimental setup for liquid dosing in UHV conditions and uncovers the self-limited growth mechanism of oxyhydroxide phases on Fe3O4(001).

Findings

Surface reverts to (1x1) termination after hydroxylation

Oxyhydroxide phase saturates at ~40% coverage

Surface passivates due to saturation with H

Abstract

Atomic-scale investigations of metal oxide surfaces exposed to aqueous environments are vital to understand degradation phenomena (e.g. dissolution and corrosion) as well as the performance of these materials in applications. Here, we utilize a new experimental setup for the UHV-compatible dosing of liquids to explore the stability of the Fe3O4(001)-c(2x2) surface following exposure to liquid and ambient pressure water. X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED) data show that extensive hydroxylation causes the surface to revert to a bulk-like (1x1) termination. However, scanning tunnelling microscopy (STM) images reveal a more complex situation, with the slow growth of an oxyhydroxide phase, which ultimately saturates at approximately 40% coverage. We conclude that the new material contains OH groups from dissociated water coordinated to Fe cations extracted from subsurface layers, and that the surface passivates once the surface oxygen lattice is saturated with H because no further dissociation can take place. The resemblance of the STM images to those acquired in previous electrochemical STM (EC-STM) studies lead us to believe a similar structure exists at the solid-electrolyte interface during immersion at pH 7.