Atomic-scale studies of Fe3O4(001) and TiO2(110) surfaces following immersion in CO2-acidified water

TL;DR

This study introduces a novel UHV-compatible method to investigate mineral surfaces in water with controlled pH, revealing how Fe3O4 and TiO2 surfaces respond to acidic conditions at the atomic level.

Contribution

The paper presents a new experimental approach to study mineral-water interactions under ultra-high vacuum conditions with pH variation via CO2 pressure.

Findings

Fe3O4 dissolves at pH 3.9-4.0 with surface roughening

TiO2 remains unaffected by acidic water

Bicarbonate accumulation indicates Fe(II) extraction and complex formation

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 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.