Self-limited growth of an oxyhydroxide phase at the Fe3O4(001) surface in liquid and ambient pressure water

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 Fe 3 O 4 (001)-(√2 × √2)R45°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 (1 × 1) 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.

Atomic‐Scale Studies of Fe3O4(001) and TiO2(110) Surfaces Following Immersion in CO2‐Acidified Water

“…[30] After the interaction with ultrapure liquid water, the O 1s spectrum shows an additional component at 531.6 eV (a fit for this peak is reported in Ref. [10]), which is assigned to hydroxyl groups due to dissociative water adsorption. [10] Molecular water would be expected at 533 eV, but it is not observed.…”

“…We recently interpreted these chains as representative of an Fe-(oxy)hydroxide phase, formed when two or more O water H groups from dissociated water coordinate tetrahedrally coordinated Fe tet atoms extracted from the subsurface. [10] The surface Fe rows along the < 110 > directions remain visible in the surrounding surface, but all have almost the same apparent height due to extensive hydroxylation of the surface oxygen lattice. The necessary H is the counterpart of the OH resulting from dissociative water adsorption.…”

“…40 %: when no sites are available to accommodate the H atoms, no water dissociation is possible. [10] Figure 1c,d shows the Fe 3 O 4 (001) surface imaged following exposure to acidic water drops performed with a p CO 2 = 20 mbar (pH � 4.8) and p CO 2 = 1 bar (pH � 3.9), respectively. The former surface (c) exhibits bright chains consistent with the growth of an iron(oxy)hydroxide phase, as observed at pH 7, but with the coverage of the bright chains decreased to � 30 % of the surface.…”

“…[10]), which is assigned to hydroxyl groups due to dissociative water adsorption. [10] Molecular water would be expected at 533 eV, but it is not observed. As the pH of the water drop decreases to 4.8, the surface oxygen signal decreases in intensity and a new contribution appears at 532.2 eV, which is in the region typical for CÀ O bonds.…”

“…[7,9] Fe 3 O 4 (001), on the other hand, responds to liquid water exposure forming a new surface phase, which we interpreted as an Fe oxy-hydroxide layer based on scanning tunnelling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) results. [10] In this paper, we vary the pH of the water drop in a controlled way through the dissolution of CO 2 in the water drop during immersion. Specifically, we show that the Fe 3 O 4 (001) surface changes with decreasing pH, and begins to dissolve at pH � 4.…”