  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 Fe3O4 surfaces. Here, we describe a simple approach
to vary the pH through the partial pressure of CO2 (pCO2) 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 Fe3O4(001)-(rt2 x rt2)R45
surface begins to dissolve at a pH 4.0-3.9 (pCO2 = 0.8-1 bar) and, although it
is significantly roughened, the atomic-scale structure of the Fe3O4(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.
