Atomic-Scale Studies of Fe3 O4 (001) and TiO2 (110) Surfaces Following Immersion in CO2 -Acidified Water

Abstract

Difficulties associated with the integration of liquids into a UHV environment make surface-science style studies of mineral dissolution particularly challenging. Recently, we developed a novel experimental setup for the UHV-compatible dosing of ultrapure liquid water and studied its interaction with TiO₂ and Fe₃ O₄ surfaces. Herein, we describe a simple approach to vary the pH through the partial pressure of CO₂ ( $p_{C{O}_2}$ ) in the surrounding vacuum chamber and use this to study how these surfaces react to an acidic solution. The TiO₂ (110) surface is unaffected by the acidic solution, except for a small amount of carbonaceous contamination. The Fe₃ O₄ (001)-( $\sqrt{2}$ × $\sqrt{2}$ )R45° surface begins to dissolve at a pH 4.0-3.9 ( $p_{C{O}_2}$ =0.8-1 bar) and, although it is significantly roughened, the atomic-scale structure of the Fe₃ O₄ (001) surface layer remains visible in scanning tunneling microscopy (STM) images. X-ray photoelectron spectroscopy (XPS) reveals that the surface is chemically reduced and contains a significant accumulation of bicarbonate (HCO₃ ^- ) species. These observations are consistent with Fe(II) being extracted by bicarbonate ions, leading to dissolved iron bicarbonate complexes (Fe(HCO₃ )₂ ), which precipitate onto the surface when the water evaporates.

Keywords: CO2; TiO2; dissolution; magnetite; water drop.