The adsorption of submonolayer coverages of water and hydroxyl molecules on hematite (0001) surface is investigated using density functional theory with Hubbard correction U (DFT+U). The effect of adsorption on the structural, energetic, and electronic properties of both iron and oxygen terminated hematite surfaces is examined. The influence of the van der Waals interactions on the adsorption binding energy and geometry is also considered. It is found that tilted orientations of molecules are energetically more favored than planar ones, because the hydrogen bond stabilizes molecules on the surface. Bonding of H₂O is more than twice weaker than that of OH. For both molecules adsorption on the iron-rich termination is much stronger than on the oxygen-terminated surface. The differences in bonding properties of water and hydroxyl molecules to the hematite surfaces are explained by different character of the charge transfer in the molecule–oxide system.

Ultrathin transition metal oxide films exhibit unique physical and chemical properties not observed for the corresponding bulk oxides. These properties, originating mainly from the limited thickness and the interaction with the support, make those films similar to other supported 2D materials with bulk counterparts, such as transition metal dichalcogenides. Ultrathin iron oxide (FeO) films, for example, were shown to exhibit unique electronic, catalytic and magnetic properties that depend on the type of the used support. Ag(111) has always been considered a promising substrate for FeO growth, as it has the same surface symmetry, only ~5% lattice mismatch, is considered to be weakly-interacting and relatively resistant to oxidation. The reports on the growth and structure of ultrathin FeO films on Ag(111) are scarce and often contradictory to each other. We attempted to shed more light on this system by growing the films using different preparation procedures and studying their structure using scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and X-ray photoelectron spectroscopy (XPS). We observed the formation of a previously unreported Moiré superstructure with 45 Å periodicity, as well as other reconstructed and reconstruction-free surface species. The experimental results obtained by us and other groups indicate that the structure of FeO films on this particular support critically depends on the films’ preparation conditions. We also performed density functional theory (DFT) calculations on the structure and properties of a conceptual reconstruction-free FeO film on Ag(111). The results indicate that such a film, if successfully grown, should exhibit tunable thickness-dependent properties, being substrate-influenced in the monolayer regime and free-standing-FeO-like when in the bilayer form.

This is the first of two papers dealing with the adsorption of Au and formation of Au_n nanostructures (n = 1–4) on hematite (0001) surface and adsorption of CO thereon. The stoichiometric Fe-terminated (0001) surface of hematite was investigated using density functional theory in the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) form with Hubbard correction U, accounting for strong electron correlations (PBE+U). The structural, energetic, and electronic properties of the systems studied were examined for vertical and flattened configurations of Au_n nanostructures adsorbed on the hematite surfaces. The flattened ones, which can be viewed as bilayer-like structures, were found energetically more favored than vertical ones. For both classes of structures the adsorption binding energy increases with the number of Au atoms in a structure. The adsorption of Au_n induces charge rearrangement at the Au_n/oxide contact which is reflected in work function changes. In most considered cases Au_n adsorption increases the work function. A detailed analysis of the bonding electron charge is presented and the corresponding electron charge rearrangements at the contacts were quantified by a Bader charge analyses. The interaction of a CO molecule with the Au_n nanostructures supported on α-Fe₂O₃ (0001) and the oxide support was studied. It is found that the CO adsorption binding to the hematite supported Au_n structures is more than twice as strong as to the bare hematite surface. Analysis of the Bader charges on the atoms showed that in each case CO binds to the most positively charged (cationic) atom of the Au_n structure. Changes in the electronic structure of the Au_n species and of the oxide support, and their consequences for the interactions with CO, are discussed.

The adsorption of small Au_n (n = 1–4) nanostructures on oxygen terminated α-Fe₂O₃(0001) surface was investigated using density functional theory in the generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) form with Hubbard correction U, accounting for strong electron correlations (PBE+U). The structural, energetic, and electronic properties were examined for two classes of the adsorbed Au_n nanostructures with vertical and flattened configurations. Similarly to the Fe-terminated α-Fe₂O₃(0001) surface considered in Part I, the flattened configurations were found energetically more favored than vertical ones. The binding of Au_n to the O-terminated surface is much stronger than to the Fe-termination. The adsorption bonding energy of Au_n and the work function of the Au_n/α-Fe₂O₃(0001) systems decrease with the increased number of Au atoms in a structure. All of the adsorbed Au_n structures are positively charged. The bonding of CO molecules to the Au_n structures is distinctly stronger than on the Fe-terminated surface; however, it is weaker than the binding to the bare O-terminated surface. The CO molecule binds to the Au_n/α-Fe₂O₃(0001) system through a peripheral Au atom partly detached from the Au_n structure. The results of this work indicate that the most energetically favored sites for adsorption of a CO molecule on the Au_n/α-Fe₂O₃(0001) systems are atoms in the Au^0.5+ oxidation state.

We review results of density functional theory calculations of the adsorption of single gold atoms and formation of sub-nanometer Au _n structures ( n = 2 to 5) on most stable iron oxide surfaces: hematite (0001), and magnetite (111) and (001). Structural, energetic, and electronic properties of Au _n structures on both Fe- and O-rich oxide terminations are discussed. Different chemical character of the two oxide terminations is reflected in distinctly stronger binding of gold at the oxygen- than at the iron-terminated surface, and in different changes of the adsorption binding energy with the size of the Au _n cluster. On the iron-terminated oxide surface the binding energy increases whereas on the oxygen-rich termination it decreases with the number of Au atoms in the structure. Upon CO adsorption on magnetite surface all Au _n structures have a net positive charge and CO binds to the most cationic Au atom of a cluster. Interactions of Au _n and CO with magnetite (111) show many similarities with those on hematite (0001) surface. The influence of the substrate relaxation effects on adsorption energy is also discussed.