X-ray Photoemission and Density Functional Theory Study of the Interaction of Water Vapor with the Fe3O4(001) Surface at Near-Ambient Conditions

The interaction of water with the Fe 3 O 4 (001) surface was investigated in a combined ambient pressure X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) study. The uptake of molecular water and hydroxyl species on the (001) surface of a natural magnetite sample at near-ambient conditions was quantified using O 1s spectra taken in the p(H 2 O) range from 10 –9 to 2 Torr. At low p(H 2 O) (≤10 –4 –10 –5 Torr) and room temperature, we found that water does not adsorb dissociatively on the surface, except on defect sites. In contrast, progressive dissociation into surface hydroxyl species was observed between 10 –4 and 10 –2 Torr p(H 2 O). The onset of hydroxylation coincides with the increasing presence of molecular water species on the surface, which demonstrates the key role played by cooperative interactions between adsorbed water molecules, leading to dissociation and surface hydroxylation. The measured O 1s chemical shifts of hydroxyl and molecular water species from both isotherm and isobar data are on average ∼1.2 eV and ∼3.3 eV, respectively, relative to lattice oxygen. The chemical shift of the hydroxyl species on magnetite(001) agrees with previously reported values for hydroxyl species on iron oxyhydroxides such as goethite (α-FeOOH). DFT calculations including an on-site Coulomb repulsion parameter (generalized gradient approximation (GGA) + U approach) predict O 1s surface core-level shifts (SCLS) at the clean (2 1/2 ×2 1/2 )R45° reconstructed Fe 3 O 4 (001) surface of up to ∼−1 eV depending on the specific bonding configuration of the surface O atoms. Hydroxyl groups formed by the dissociation of isolated water molecules at O vacancies have an SCLS value of ∼1.2 eV. With increasing coverage there is a transition toward partial dissociation on the (001) surface. The calculated SCLS for hydroxyl and adsorbed water are 1.2–1.9 and 2.6–3.0 eV, respectively, and compare very well with our experimental results. Final-state effects obtained within the Slater–Janak approach thus have the dominant contribution. In addition, the modest reduction of the work function (∼0.5 eV) predicted by DFT calculations for the mixed adsorption of dissociated and intact water molecules agrees well with work function changes measured experimentally. Finally, the similarity between isotherm and isobar data and the DFT calculations for the C-free Fe 3 O 4 (001) surface indicate that surface hydroxylation is indeed substrate induced and not catalyzed by the presence of adventitious carbonaceous species. Both theory and experiment show the importance of cooperative effects between adjacent water molecules in the dissociation reaction.