Incipient adsorption of water and hydroxyl on hematite (0001) surface

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.


Introduction
During the last three decades or so, adsorption of water on iron oxide surfaces has attracted substantial research interest [1][2][3][4][5][6][7]. This is due to its important role played in the initial stages of reaction processes occurring at the oxide surfaces in an ambient environment leading to corrosion and reduction of iron oxides [7], and in photoelectrochemical water splitting devices for solar energy conversion [8]. Majority of these studies have focused on the interaction of water with the hematite (α-Fe 2 O 3 ) (0001) surface and have revealed many interesting features occurring upon adsorption of water and hydroxyl molecules thereon. One of the basic questions from the beginning was about molecular or dissociative character of water adsorption. The earliest experimental studies [1,2,4] pointed to the conclusion that water does not bind strongly to the α-Fe 2 O 3 (0001) surface and that water dissociation is not favored under ultra-high vacuum (UHV) condition. X-ray photoelectron spectroscopy (XPS) studies by Liu et al [6] suggested a threshold for hydroxylation at a pressure of 10 −4 Torr. Below the threshold, only a small amount of dissociated water was found, mostly at surface defect sites. Surface x-ray scattering studies of the hydrated iron and oxygen terminated surfaces of hematite, combined with density functional theory (DFT) calculations performed by Trainor et al [9] showed that water weakly associates at the surface and rather dissociates, thus the surface is hydroxylated. Yamamoto et al [10] studied water adsorption on iron-terminated α-Fe 2 O 3 (0001) surface at near ambient conditions and showed that adsorption of water molecules is preceded by surface hydroxylation, which is initiated at very low relative humidity, and molecular water starts to adsorb at higher relative humidity. On the theory side DFT studies of Yin et al of the initial stages of adsorption and hydroxylation of water on the perfect [11] and defective [12] Feterminated surfaces of hematite showed that even at perfect surface dissociation is strongly favored over molecular adsorption at low coverage. They were followed by several DFT+U studies. Souvi et al [13] found that first water molecule is adsorbed dissociatively, while second and third water adsorb associatively. Calculations of low-coverage water adsorption and dissociation on various surface terminations ofα-Fe 2 O 3 (0001) performed by Nguyen et al [14] revealed very low energy barriers (0.06-0.3 eV) for water dissociation. Their later work [15] OPEN ACCESS RECEIVED 14 February 2019 Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
showed that upon contact with water the surface can easily be covered with O atoms and/or OH groups. Low energy barriers for water dissociation on iron-rich hematite surface were confirmed by a combined ab initio molecular dynamics and experimental studies [16]. Calculation of the isolated water molecule on an Fe-and O-terminated (0001) surfaces yielded the adsorption energy of merely 0.49eV for the O-terminated surface [17]. Stability of two hydroxylated, iron and oxygen terminated surfaces has been studied using surface-specific DFT+U approach [18]. The effects of OH adsorption at different coverages on the structure of Fe-terminated α-Fe 2 O 3 (0001) surface was also studied [19]. Calculations of Negreiros et al [20], including the van der Waals correction, of the interaction of very low coverage of water with this surface, showed that a negatively charged surface reduces the energy barrier and enhances substantially the water binding and inhibits water dissociation. A similar theoretical framework was recently applied to study water adsorption on defective Fe-terminated hematite surfaces [21], and different terminations of hematite [22].
All those calculations concluded that water dissociation is favored over molecular adsorption at low coverage. This seem to contradict experimental observation of molecular water adsorption on the partially covered hematite surface under UHV condition [1,2,6]. However, the existing DFT simulations do not show a spontaneous H 2 O dissociation at the low coverages considered. Thus, the picture they convey is that initially water adsorbs intact and subsequently it dissociates.
Interaction of water and/or hydroxyl with hematite surfaces has been usually studied for only one and relatively high coverage, and adsorption on the Fe-terminated surface. A quite a large scatter in the calculated adsorption energies is seen due to the use of partially constrained or fully relaxed slab [23]. It is also likely that iron-and oxygen-rich terminations of α-Fe 2 O 3 (0001) surface coexist [24]. All this calls for a more systematic comparison of the initial H 2 O and OH adsorption on the two terminations. Therefore, in this work we apply DFT+U to investigate in detail the influence of very low-coverages of H 2 O and OH molecules on surface geometry and composition, electronic structure, and energetics and stability of these two terminations of hematite (0001) surface. We also examine the influence of the correction for the long-range van der Waals interactions on the structure and properties of the two terminations upon adsorption of H 2 O and OH.

Computational details
The calculations were performed within the spin-polarized density functional theory as implemented in the Vienna ab initio simulation package (VASP) [25,26]. The electron ion-core interactions were described by the potentials generated by the projector augmented wave (PAW) method [27,28] with Fe 3d 7 4s 1 , O 2s 2 2p 4 , and H 1s 1 states, treated as valence states. A plane waves basis with cut-off energy of 500eV and the conjugate gradient algorithm were applied to determine the electronic ground state. The Perdew-Burke-Ernzerhof (PBE) version [29] of the generalized gradient approximation (GGA) to exchange and correlation energy functional was used. To account for the on-site Coulomb repulsion of the Fe 3d electrons the Hubbard correction term U was included using the rotationally invariant approach of Dudarev et al [30]. Following our previous studies [31] the effective parameter = -= U U J 4.0 eV eff was adopted. The integrations over the Brillouin zone were performed using the k-point sampling method with a 6×6×1 Monkhorst-Pack [32] grid for the calculations using a 1×1 surface unit cell, and 3×3×1 mesh for 2×2 cell. A Gaussian broadening of the Fermi surface of 0.1eV was applied to improve the convergence of the solutions. This PBE+U calculation of antiferromagnetic bulk yields the following lattice parameters of the hexagonal unit cell, a=5.072 Å and c=13.892 Å [33,34]. In order to examine an influence of the van der Waals (vdW) interactions on the water and hydroxyl adsorption the empirical D3 functional proposed by Grimme and co-workers [35] has been used. With vdW D3 correction included, a=5.057 Å, and c=13.840 Å.
Iron Fe1 terminated (Fe-O 3 -Fe-) and oxygen O 3 -terminated (O 3 -Fe-Fe-) (0001) surfaces of α-Fe 2 O 3 were simulated by symmetric slabs consisting of 18 (12 Fe and 6 O) and 16 (10 Fe and 6 O) atomic layers, respectively, with a symmetry plane in the middle of the Fe bilayer [31], separated from their periodic images by a vacuum region of 23 Å. The positions of all atoms were optimized until the forces on atoms were smaller than 0.05 eV/Å. The molecules were adsorbed on both sides of the slab, and the adsorption binding energy was calculated from the expression = - where a minus sign yields positive E ad , so larger adsorption energy means more favored binding. E X/hem is the total energy of the substrate slab covered with adsorbed molecule species, X, E hem represents the energy of the relaxed bare oxide surface, E X is the energy of a free molecule, and n is the number of molecules per surface cell.

Results and discussion
In adsorption studies we adopted the convention in which a single molecule adsorbed in a 1×1 surface cell corresponds to 1/3 monolayer (ML) coverage [2,38], because there are three oxygen atoms in an oxygen layer in 1×1 surface cell. Thus, using a 2×2 surface cell in which up to four molecules can be placed in four equivalent adsorption sites allows to study molecular coverages varying from 1/12 ML (cf figure 1) to 1/3 ML. Note that even at the highest coverage of 1/3 ML, the molecules are laterally separated from each other by ≈5 Å, which excludes formation of the ice structure in case of water adsorption. The deposited molecule was originally placed above one of the surface sites and the initial distance between topmost surface atom and molecule's oxygen atom (O m ) was at least 2.3 Å. In order to determine preferred adsorption geometries, differently oriented water and hydroxyl molecules: vertical, flat, or tilted were used as starting configurations, in several possible adsorption sites.

Fe1-terminated surface
On the Fe1-termination we considered adsorbate positions above the atom of the surface Fe layer, and above the threefold coordinated subsurface Fe atoms, labeled respectively as A, and B, C, in figure 1. The position above, and in the vicinity of the surface iron atom (site A), appeared to be the most stable for both molecules. The other two surface sites, B and C, are much less stable.

H 2 O adsorption
Geometry. Upon adsorption on the Fe1-terminated α-Fe 2 O 3 (0001) surface, regardless of the initial configuration, after structure optimization the adsorbed water molecule prefers tilted orientation, with one of its hydrogens closer to the surface than the other one ( figure 1(a)). With the coverage increased up to 1/3 ML the orientation and geometries of adsorbed water molecules remained unchanged. As can be seen from figure 1(a [14], with small differences resulting from different computational parameters applied. At coverages smaller than 1/3 ML, the presence of water induces some changes in the geometry of the topmost surface Fe layer. Some of the Fe atoms which are not saturated with H 2 O (one for 1/12 and 1/4 ML, and two for 1/6 ML) are substantially lowered and immersed, up to 0.7 Å, just below the average position of first oxygen layer. This uncovers electronegative oxygen ions and leads to an increase of surface dipole moment, and the work function, which can be seen in figure 2(b). These significant changes in the local structure of the remaining regions of the clean surface may make the adsorption of further water molecules in these modified surface (Fe) adsorption sites less favorable.
Energetics. A single H 2 O molecule (1/12 ML coverage) binds relatively strong to the Fe1-terminated (0001) surface, with E ad =0.97 eV. With the coverage increased to 1/6 ML the adsorption energy is only slightly increased (by 0.016 eV) which is connected to the above mentioned changes in the surface local structure. However, as is seen from figure 2, with a further increase of H 2 O coverage the calculated adsorption energy significantly decreases, to 0.88 eV for 1/3 ML (for other numerical values see SI). This E ad value agrees well with recent PBE+U results (0.75 eV [14]; 0.82eV [22]) for molecularly adsorbed water. Figure 2(a) also shows that the account for vdW interactions enhances E ad values by nearly same amount of 0.2 eV, at all water coverages. The lowest coverage (1/12 ML) value, E ad =1.19 eV, agrees well with PBE+U+D3 result, 1.26 eV [20]. The overall relative adsorption energy variation between the different coverages is rather small which indicates that the molecule-surface interaction is dominant over the molecule-molecule interaction. The lowering of the adsorption energy with increasing coverage and shortening of the hydrogen bonds with the substrate oxygen atoms, support the picture of adsorption of molecular H 2 O, which precedes the surface hydroxylation that occurs above the pressure threshold [6].
Charge transfer. A qualitative information about charge transfer on the surface is provided by the work function change (ΔΦ), with respect to the clean surface value. Compared to that of the clean Fe1-terminated surface (4.35 eV, PBE+U; 4.51 eV, PBE+U+D3) at small H 2 O coverages the work function increases by 0.30 eV for 1/12 ML, and 0.43 eV for 1/6 ML (figure 2). At 1/4 ML water coverage the work function is almost unchanged (within 0.09 eV) but decreases by −0.20 eV for 1/3 ML (see SI). Calculations using the vdW functional give the same initial trend of ΔΦ change which is reduced by 0.05-0.13 eV; for higher H 2 O coverages work function is lower than that of the clean Fe1-termination. These ΔΦ changes correlate well with the electron charge transfer in the adsorbate-oxide system. The calculated Bader charges on surface atoms are small. For the three lowest coverages, each H 2 O molecule loses −0.018e, −0.025e, and −0.006e, respectively, while at 1/3 ML it becomes slightly cationic by gaining 0.003e. The surface Fe atom loses even more charge than the water molecule (−0.04-−0.05e). The majority of the charge (0.04-0.05e) is transferred to the surface oxygen atoms which form bonds with H atoms. The amount of charge transfer upon water adsorption resulting from PBE+U +D3 calculations is nearly the same as that calculated within PBE+U.
Electronic structure. Figure 3 presents the calculated partial density of states (PDOS) for molecular adsorption on the Fe1-terminated surface resulting from PBE+U. The electronic structure is slightly modified by additional peaks from H 2 O which appear in the energy region below the main conduction band minimum

OH adsorption
Geometry. On the Fe1-terminated surface, the adsorbed hydroxyl group binds with the topmost surface Fe atom through its oxygen and forms a bond of 1.84 Å, tilted with respect to the surface plane ( figure 1(b)). The hydrogen atom points out from the surface, even though the initial geometry was with H pointing to the surface. For all coverages considered, the O-H bond length is shortened to 0.97 Å, compared to its length (0.99 Å) in a gas phase. The Fe-O m bond length shrinks a little bit with increasing coverage, from 1.84 Å to 1.81 Å for 1/ 3 ML. The OH configuration on the Fe1-termination excludes bond formation between H and surface atoms. It is worth noting that the hydrogen atom has no preferred position with respect to the surface. It means that there are other similar OH configurations with degenerated energies, which suggests that OH can rotate around the O m atom of the adsorbed molecule pinned over the surface Fe atom. The geometries of adsorbed OH on the Fe1termination obtained from PBE+U+D3 calculations are nearly the same as those resulting from PBE+U (cf. SI).
Energetics. The binding of OH to the Fe1-terminated surface is twice as strong as that of H 2 O molecule. For the smallest OH coverage (1/12 ML) the adsorption energy is 2.28 eV. Thus, our results support the conclusion of previous studies [9, 10] that hydroxylation of hematite surface is energetically favored. E ad decreases by 0.09 eV in the range of OH coverages considered [figure 4(a)] to 2.19 eV for 1/3 ML. This agrees well with recent PBE+U result (2.31 eV [19]). Adsorption energies calculated by using PBE+U+D3, i.e. with the contribution of vdW interactions included, are larger by 0.14 eV, but their relative variation for the considered coverage range is the same. Similar to water adsorption this indicates that the OH-surface interaction dominates over the OH-OH interaction in adsorbed layer.
Charge transfer. The calculated work function changes induced by OH adsorption are significantly larger than upon H 2 O adsorption. DF amounts to 1.52 eV for 1/12 ML, and it increases gradually with coverage up to 2.69 eV for 1/3 ML ( figure 4(b)). This results from a larger charge transfer between adsorbed OH and the hematite surface atoms which is by an order of magnitude larger than in case of H 2 O adsorption. An analysis of Bader charges shows that at the lowest coverage the hydroxyl O atom loses −0.37e. With increasing coverage the loss increases to −0.46e for 1/3 ML. Also the Fe and O atoms of the topmost oxide layer lose some charge (up to −0.11e and −0.08e, respectively) by transferring electrons to subsurface layers. The charges on atoms calculated using PBE+U+D3 do not differ from those resulting from PBE+U calculations.
Electronic structure. In contrast to H 2 O, adsorption of OH substantially modifies the electronic structure of the Fe1-termination. As can be seen in figure 3, in the presence of adsorbed OH, even at the smallest OH coverage, the Fe 3d and O 2p bands are shifted by about 0.5 eV to higher energies, and the surface becomes metallic. The metallic character of this termination is further enhanced by the OH states present just below the Fermi energy, which hybridize with O surf 2p and Fe 3d states. Figure 3 shows that compared to water adsorption, there is much larger asymmetry in the density of majority and minority spin states which results in the magnetization of adsorbed OH molecules substantially enhanced. The magnetic moment on the hydrogen atom is 0.01 μ B , while on the hydroxyl oxygen it rises from 0.16 μ B at 1/12 ML coverage, to 0.21 μ B at 1/3 ML. A similar value of the magnetic moment on the hydroxyl oxygen, which is much larger than in the water molecular state, was also noticed in [14]. The magnetic moments calculated using the vdW corrected functional are unaltered with respect to those obtained from PBE+U.

O3-terminated surface
On this surface the most stable position both for H 2 O and OH molecule is site A, in a deep hollow left by the surface Fe atom removed from the Fe-termination during formation of the O3 termination ( figure 5). Other, less stable sites, are the threefold coordinated hollow sites, above Fe atoms of the second and first subsurface Fe layer.

H 2 O adsorption
Geometry. On the O3-terminated surface a water molecule binds flat-like over the surface hollow site A with both its hydrogens pointed towards the surface ( figure 5). The molecule is shifted by 0.48 Å from the hollow   Energetics. The PBE+U calculated adsorption energy shows very weak coverage dependence. For the lowest H 2 O coverage (1/12 ML) E ad amounts to 0.67 eV and decreases with the increasing coverage to 0.61 eV at 1/ 3 ML ( figure 6(a)). This is distinctly higher than that reported by other PBE+U calculations (0.51 eV [14]; 0.49eV [17], and 0.41 eV [22]). With vdW corrections included, the calculated E ad is 0.3-0.5eV higher and its decreasing trend with the coverage is more distinct. Nevertheless, H 2 O binds 30% weaker to the O3-terminated surface than to the Fe1-termination. The much weaker bonding of H 2 O to the O3-termination allows to distinguish between different terminations of the hematite (0001) surface which may be useful for the interpretation of experimental measurements on these surfaces.
Charge transfer. The presence of adsorbed H 2 O molecules reduces the surface dipole moment, and consequently the work function is lowered compared with the clean O3-termination values (8.40 eV, PBE+U; 8.42 eV, PBE+U+D3). Even for the smallest coverage the work function is significantly reduced (−0.24 eV) and the reduction is larger with growing H 2 O coverage, reaching −0.78 eV for 1/3 ML ( figure 6(b)). The work function changes obtained from the calculations accounting for vdW interactions are very similar (within 5%). This is quantitatively different from H 2 O adsorption on the Fe1-termination, where even a small coverage of H 2 O leads to an increase of the work function. The different work function change upon water adsorption can be understood by comparing very different orientation of the dipolar H 2 O molecules adsorbed on the two terminations ( figure 1(a) and 5(a)) which contribute differently to surface dipole moment. Calculated Bader charges show that the amount of charge transferred from a single H 2 O molecule (−0.38e) adsorbed on the O3termination is more than twice as large as that on the Fe1-terminated surface. Most of the electrons are transferred to the surface oxygen layer of the oxide, but also atoms of deeper subsurface layers gain some small charge (up to 0.04e). The charge transfer does not depend much on the H 2 O coverage and is almost the same for all coverages. The charges on the atoms obtained from PBE+U+D3 are almost the same as those calculated without the vdW correction included.
Electronic structure. As can be seen from the PDOS plotted in figure 7, water adsorption does not affect much the surface electronic structure of the O3 termination. The presence of water opens a narrow energy gap, just below the Fermi energy, which widens with increasing H 2 O coverage. Additional peaks from adsorbed water molecules appear also at energies far below the Fermi level. Increasing of H 2 O coverage affects mostly unoccupied surface O 2p states above the Fermi level ( figure 7). In contrast to the Fe1-terminated surface, adsorption of H 2 O on the O3-termination enhances significantly the magnetic moment on the oxygen atom of the molecule (0.36 μ B ) and leads to a significant magnetization of water, 0.41 μ B , which was also noticed in [14]. The magnetization is almost independent of H 2 O coverage and is the same when calculated with PBE+U or PBE+U+D3.

OH adsorption
Geometry. On the O3-termination of the α-Fe 2 O 3 (0001) surface the OH molecule binds with the surface by forming O m -O surf and H-O surf bonds, in a flat-like geometry with respect to the surface ( figure 5(b)). The O m atom binds with two neighboring surface oxygens at a distance of 2.08 Å and 2.12 Å, while the H-O surf bond length is 1.65 Å, so the OH molecule lies closer to the surface than H 2 O. These geometries do not change much with increasing coverage and do not alter when calculated within PBE+U+D3.
Energetics. The adsorption binding of OH is almost 0.5 eV stronger than that of H 2 O. At the lowest coverage (1/12 ML) it amounts to 1.09 eV. Similarly as for H 2 O, with increasing coverage of OH to 1/3 ML, E ad decreases only by about 0.05 eV ( figure 8(a)). PBE+U+D3 calculations predict 0.2-0.5 eV larger adsorption energies and a more pronounced coverage dependence. Again, OH binds much weaker (by 1 eV) to the O3-than to the Fe1termination of hematite (0001).
Charge transfer. The work function of the O3-terminated surface decreases with increasing amount of adsorbed OH. At the smallest coverage (1/12 ML) the work function change is −0.16 eV and increases up to −0.65 eV at 1/3 ML ( figure 8(b)). The vdW calculations yielded slightly smaller values (by 0.03-0.11 eV) of ΔΦ. The calculated Bader charges on atoms are smaller than in the case of H 2 O adsorption. The OH molecule loses up to 0.21e at the highest coverage (1/3 ML). The electron charge transfer calculated within PBE+U+D3 is almost the same.
Electronic structure. The changes in the surface electronic structure induced by the presence of OH on the O3-termination can be seen in the PDOS plots displayed in figure 7. Similarly as for H 2 O, at higher coverages adsorption of OH leads to an opening of the energy gap just below the Fermi energy. Additional peaks from adsorbed hydroxyl are present, however, at 2 eV closer to the Fermi level. With increasing coverage, the OH states dominate in the unoccupied minority states. Interestingly, the magnetic moment on the oxygen atom of the adsorbed hydroxyl is greatly increased to 0.98 μ B and is the largest of all systems considered in this work. On the surface oxygen atoms it does not exceed 0.17 μ B and is almost independent of coverage both when calculated with or without vdW correction.

Surface stability
The stability of different bare terminations of the α-Fe 2 O 3 (0001) surface as a function of oxygen partial pressure was considered based on the thermodynamic approach [39] in several previous works (see for instance [14,18,21,31]). The stoichiometric Fe1-termination is the most stable one in the available range of the chemical potential of oxygen. The clean O3-terminated surface is unstable. In order to determine the effect of adsorption on stability of surface terminations we consider the relative variation of the Gibbs free energy, ΔG, with respect to the energy of the bare hematite surface termination. Upon adsorption of molecule X, X=H 2 O or OH, ΔG can be approximately expressed as where E X hem , E hem , and n have the same meaning as in equation (1), while μ X =E X is standing for the chemical potential of the molecule. Hence, the surface energy change upon adsorption of molecule can be written as where A is the surface cell area. The variations of surface energy of the two hematite (0001) terminations versus H 2 O or OH coverage are plotted in figure 9. As is seen, the presence of adsorbed water or hydroxyl molecules distinctly lowers the surface energy and thus enhances the stability of the surface. The variation of Δγ with coverage is nearly linear. The lowering of γ is stronger for adsorption on the Fe1 than on the O3 termination and stronger for PBE+U+D3 than for PBE+U. At 1/3 ML of H 2 O the surface energy of the Fe1 termination is lowered by about 40 meV/Å 2 (50 meV/Å 2 ) as calculated with PBE+U (PBE+U+D3). On the O3-terminated surface the variation of Δγ upon H 2 O adsorption is 30% weaker. Upon OH adsorption the variation of Δγ is nearly 2.5 times stronger than that due to H 2 O. At the OH coverage of 1/3 ML, Δγ of Fe1 termination is −99 meV/Å 2 , and is nearly the same independent of whether calculated using PBE+U or PBE+U+D3. Added to the clean Fe1-termination value (67 meV/Å 2 [31,40]) it gives for the surface energy −32 meV/Å 2 , which is in very good agreement with the PBE+U result calculated in [18]. On the O3 termination the variation of Δγ upon OH adsorption is weaker compared to that on the Fe1-terminated surface, which reflects a weaker binding of OH to this surface.

Summary and conclusion
We have presented results of investigation of submonolayer coverages of water and hydroxyl on iron, and oxygen terminated α-Fe 2 O 3 (0001) surfaces. On the Fe1-terminated surface, both water and hydroxyl molecules prefer adsorption on the topmost Fe ion through molecule oxygen, in a tilted geometry. In the case of H 2 O adsorption, an additional hydrogen bond with surface oxygen is formed, which stabilizes the molecule on the surface. Such a configuration may be considered as an initial stage for dissociation of an adsorbed water molecule. For all coverages studied, the calculated adsorption energy of hydroxyl is more than twice as large as that of water. On the Fe1-terminated surface the work function change increases with increasing amount of adsorbed hydroxyls. This result may be helpful in analysis of experimental data to distinguish different termination domains. Adsorption of OH substantially modifies the electronic structure of the Fe1-termination and makes it metallic. On the oxygen termination, both water and hydroxyl bind weaker than on the Fe1terminated surface. However, the binding of hydroxyl is much stronger than water. Both molecules prefer formation of O and H bonds with surface oxygens, and consequently their geometry is rather flat. At higher coverages, adsorption of H 2 O and OH leads to an opening of the narrow energy gap around the Fermi energy. The adsorption of H 2 O and OH lowers the surface energy of the α-Fe 2 O 3 (0001) surfaces. Our results show that taking into account van der Waals interactions affects mainly the adsorption binding and surface energies, and to lesser extent work functions, while the geometry of the adsorbate-oxide system and their electronic and magnetic structures are almost unaffected.