Hydrogen Adsorption and Site-Selective Reduction ... - ACS Publications

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Hydrogen Adsorption and Site-Selective Reduction of the Fe3O4(001) Surface: Insights From First Principles Narasimham Mulakaluri and Rossitza Pentcheva* Department of Earth and Environmental Sciences and Center of Nanoscience (CENS), University of Munich, Theresienstr. 41, 80333 Munich, Germany ABSTRACT: Density functional theory calculations including an onsite Hubbard term are used to explore hydrogen adsorption on the surface of Fe3O4(001). The adsorption energy exhibits a minimum for two hydrogen atoms per (√2 × √2)R45° surface unit cell and gets less favorable with increasing hydrogen coverage due to OH−OH repulsion. Terminations with two and four hydrogen atoms per surface unit cell are stable for moderate to high partial pressures of O and H. The strong tilt of the OH bond parallel to the surface facilitates hydrogen bonding to neighboring oxygen and hopping of the protons between surface oxygen sites. Furthermore, the formation of surface OH groups leads to a monotonic reduction of work function with increasing H coverage. The analysis of the electronic properties reveals selective switching of neighboring surface and subsurface Fe from Fe3+ to Fe2+ upon hydrogen adsorption. This provides a promising way to tune the catalytic activity of the Fe3O4(001) surface. metastable helium atom beam under external magnetic field. Previous DFT calculations16 have considered a single coverage of four hydrogen atoms per (√2 × √2)R45° unit cell. While these calculations were performed within the generalized gradient approximation (GGA), inclusion of correlation effects beyond GGA within GGA+U17 is crucial to correctly describe the electronic properties of the clean Fe3O4(001) surface.8,18 Here, we provide a systematic DFT GGA+U study of the adsorption of hydrogen on Fe3O4(001). We have varied the concentration of hydrogen atoms on the surface and identify different stable configurations (Section 3). To explain the energetic trends, we analyze the structural and electronic properties of the systems. In Section 4, a surface phase diagram is compiled within the framework of ab initio atomistic thermodynamics19 to identify the most stable terminations at a given temperature (T) and partial pressures (pH,pO) of oxygen and hydrogen. The type of charge and orbital order (CO−OO) of Fe3+ and Fe2+ at the FeB sites in the low-temperature bulk phase of Fe3O4 is a topic of extensive investigation.20−24 Previous DFT results have demonstrated that the presence of a surface and adsorbed molecules (e.g. water) influences significantly the CO−OO in the subsurface layers Fe3O4(001).8,18 In Section 5, we address how the charge and orbital order (CO−OO) of Fe3O4(001) and availability of Fe3+, Fe2+ is affected by H adsorption. In particular, while water adsorption leaves the surface layer exclusively Fe3+, a selective site reduction of surface Fe from Fe3+ to Fe2+ is identified through hydrogen adsorption.

1. INTRODUCTION Iron oxides are an important class of materials with versatile technological applications.1 Magnetite (Fe3O4) is used as a catalyst in ammonia synthesis,1 Fischer−Tropsch synthesis1 and in the high temperature water gas phase shift reaction.2 In all of these chemical processes, hydrogen interacts with the Fe3O4 surface. Hydrogen can be a rate limiting factor, e.g., in the synthesis of methanol on a carboxylated ZnO surface3 and can strongly influence structural relaxations of metal oxide surfaces.4 Because of the enormous economic impact of hydrogen as a reactant in catalytic processes,5 understanding of the bonding mechanism is of fundamental importance. Here we analyze how the structural and electronic properties of the Fe3O4(001) surface are altered upon hydrogen adsorption. Magnetite has an inverse spinel structure with a stacking of A- and B-layers in the [001] direction. The former contain tetrahedral iron (FeA) and the latter oxygen and octahedral iron (FeB). Both bulk truncations either with an A or a B layer are polar of type three according to the classification of Tasker.6 However, density functional theory (DFT) calculations7,8 have shown that the experimentally observed (√2 × √2)R45°reconstruction9−11 is achieved by a distorted bulk truncation with a B-layer. Evidence for this theoretically predicted termination is provided by quantitative X-ray diffraction,7 low energy electron diffraction (LEED)12 analysis and scanning tunneling microscopy (STM) measurements.13 According to a combined STM, DFT, and X-ray photoemission spectroscopy (XPS) study,14 upon hydrogen exposure the (√2 × √2)R45°-surface reconstruction is lifted and the surface is enriched with Fe2+. As the DFT calculations show, this is accompanied by an insulator-to-half-metal transition. An enhanced spin-polarization after hydrogen adsorption was measured by Kurahashi et al.15 using a spin-polarizable © 2012 American Chemical Society

Received: March 8, 2012 Revised: June 29, 2012 Published: June 29, 2012 16447

dx.doi.org/10.1021/jp302259d | J. Phys. Chem. C 2012, 116, 16447−16453

The Journal of Physical Chemistry C

Article

2. COMPUTATIONAL DETAILS Density functional theory calculations were performed using the FP-LAPW (full potential linear augmented plane wave) method in the WIEN2k25 implementation. The generalized gradient approximation (GGA)26 of the exchange-correlation potential is used. Previous systematic studies have shown that the PBE26 functional gives a good description of hydrogen bonding concerning equilibrium geometries with error bars of ±0.02 eV.27,28 We have considered the influence of electronic correlations within the LDA/GGA+U17 method applying U = 5 eV and J = 1 eV on the Fe 3d orbitals, similar to values used for MT bulk Fe3O4.22,23 The muffin-tin (MT) radii are RMT Fe = 1.90, RO MT = 1.10, and RH = 0.60 Bohr. A mixed augmented plane wave (APW+lo) and linear augmented plane wave (LAPW) basis set is used. Inside the MTs, the wave functions are expanded in spherical harmonics up to lwf max = 10. Nonspherical contributions to the electron density as well as potential are considered up to lpot max = 6. The energy cutoff for the plane wave representation pot and potential are Ewf max = 25 Ry and Emax = 196 Ry, respectively. For the integration in the Brillouin zone 16 k∥-points were used. A gas phase H2 molecule is calculated as reference system in a box of 8 × 8 × 8 Å using the same cutoff parameters as for the slab calculation. The Fe3O4(001) surface is modeled in the supercell geometry with slabs containing seven B-layers and six A layers. Laterally, a (√2 × √2)R45°unit cell is used. Hydrogen atoms are adsorbed on both sides of the inversion symmetric slab. The slab is separated in z-direction by 12 Å vacuum from its periodic images to avoid spurious interaction. The lateral lattice constant of the supercell is set to the GGA bulk lattice constant of 8.41 Å which is close to the experimental value of 8.39 Å. We have done full structural optimization of the various adsorbate configurations where the adsorbates along with the outer two AB-layers are allowed to relax, whereas the central three ABlayers are frozen to the bulk positions of the ions. The systems typically contain 100−130 atoms which results in a high numerical demand with matrix sizes for the diagonalization of up to 31 000 × 31 000. To reduce the computational cost and to search more efficiently for the most favorable adsorbate geometries, we have performed a structural optimization for some of the systems using the Vienna ab initio Simulation Package (VASP)29 with a default cutoff energy for the plane-wave basis and a force relaxation up to 0.01 eV/Å. Using these geometries, several further relaxation steps were done subsequently with the WIEN2k code.

Figure 1. Top views of the most stable surface terminations at different coverages of hydrogen atoms per (√2 × √2)R45°-unit cell. (a) 1H: single hydrogen atom adsorbed, all of the surface oxygen sites considered for adsorption are labeled (OS1 to OS5), (b) 2H: two hydrogen atoms adsorbed on the surface, (c) 4H: four hydrogen atoms adsorbed on the surface, and (d) 8H: eight hydrogen atoms adsorbed on the surface. Positions of oxygen, FeB, FeA, and H are marked by cyan, pink, orange, and white circles.

a single H atom adsorption and increase the coverage up to eight H atoms per (√2 × √2)R45° unit cell which corresponds to the full saturation of the surface oxygen sites. The adsorption energy Eads of H on the Fe3O4(001) surface in (eV/H) is calculated with respect to the energy of a gas phase H2 molecule (EH2) as follows: Eads =

⎞ 1 ⎛⎜ n EnH:Fe3O4 (001) − E Fe3O4 (001) − E H2⎟ ⎠ n⎝ 2

(1)

where n is the number of H atoms adsorbed. EnH:Fe3O4(001) and EFe3O4(001) are the total energies of the system with adsorbates and the clean surface, respectively. To analyze charge transfer and the mechanism of bond formation upon H adsorption on the Fe3O4(001) surface, we have plotted the electron density redistribution (Δρ), defined as follows: Δρ = ρnH:Fe O (001) − ρFe O (001) − ρnH

3. ADSORPTION PROPERTIES AND GEOMETRY AS A FUNCTION OF H COVERAGE Adsorption of hydrogen is considered at the distorted B-layer termination of Fe3O4(001), that was found to be stable over a broad range of oxygen partial pressures.7,8 It is characterized by FeB rows that relax perpendicularly to the [110] direction forming a wavy structure7,12 with two distinct pairs of neighboring oxygen atoms without tetrahedral iron neighbor: OS1 and OS2 at a shorter distance (dOS1−OS2 = 3.27 Å) where neighboring surface iron atom (FeSB) rows relax toward each other and OS3 and OS4 with a longer distance dOS3−OS4 of 3.45 Å)) when FeSB rows relax apart (see also ref 30). The surface oxygen atoms considered for hydrogen adsorption are displayed in Figure 1 and labeled as OS1 to OS5. In the following, we describe the adsorbate configurations, structural details, and electronic properties as a function of H coverage. We start with

3 4

3 4

(2)

where ρH:Fe3O4(001), ρFe3O4(001), and ρnH are the electron densities of the system with adsorbates, the clean Fe3O4(001) surface and H atoms, respectively. For the calculation of Δρ, the positions of the atoms in the reference systems correspond to the ones in the adsorbed system. 1H: For the adsorption of a single H atom, we consider several distinct surface oxygen sites as shown in Figure 1a. Our previous work on the H2O adsorption on Fe3O4(001) surface18,31 showed that the surface oxygen atoms with FeA neighbors (e.g., OS5 in Figure 1a) are energetically unfavorable for H adsorption. This is also confirmed here, e.g., the adsorption energy on top of OS5 is −0.1 eV which is ∼0.66− 0.75 eV less favorable than other sites (OS1 to OS4). We find the strongest preference for adsorption at OS3 (Eads = −0.85 eV) followed by OS1 (−0.81 eV), while OS2 and OS4 are slightly less 16448



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favorable (−0.76 eV). The differences in adsorption energy are associated with different charge and orbital order at the surface as we will discuss in Section 5. A top view of the most stable configuration with H adsorbed at OS3 (denoted in the following as 1H) is shown in Figure 1a.The OS3−H bond length is 0.97 Å (Table 1), which is Table 1. Structural Details and Eads of the Most Stable Adsorbate Configurations for Each Coverage: Bond Lengths (in Å) and Angles (in degrees), Adsorption Energy Eads (in eV/H atom) and Work Function Φ (in eV)a 1H 2H 4H 8H

S dFeS−1 B −O H

dOS−H

dOS···H

S ΘFeS−1 B −O −H

Eads

Φ

2.17 2.26 2.21/2.26 2.41

0.97 0.97 0.98 0.98

2.25 2.50 2.50 2.23

108.2 105.3 105.2/130.2 139.4/84.9

−0.85 −0.88 −0.63 −0.36

4.6 4.1 3.8 1.8

a

dOS...H is the hydrogen bond formed between the adsorbed hydrogen and a surface oxygen of the opposite row.

identical to the OH distance in a gas phase H2O molecule. The H atom strongly tilts from an initial on-top position resulting in an OH bond between OS4 and H nearly parallel to the surface. The tilt angle of OS3−H w.r.t. the subsurface iron atom (FeS−1 B ), S ΘFeS−1 , is 108.2°. This results in the formation of hydrogen B −O −H bond with an oxygen atom (OS4) of the opposite row, dH···OS4 = 2.25 Å. The hydrogen bond not only stabilizes this configuration but also facilitates the hopping of H atom between OS3 and OS4, although the slight preference for OS3 implies some asymmetry in occupation of the two sites. Proton hopping is observed in STM experiments as the switching of bright protrusions between the oxygen rows.14 In order to investigate further the mechanism of adsorption, we analyze the electronic properties of 1H. Figure 2a,b show the electron density redistribution upon adsorption. We observe strong charge accumulation along the OS3−H bond S and around FeS−1 B and the neighboring FeB site. The changes at S the latter two sites are related to the reduction of FeS−1 B and FeB from Fe3+ on the clean surface18 to Fe2+. This can be verified from the partial density of states (PDOS) of FeSB and FeS−1 B shown in Figure 2c. At the clean surface, the 3d band of FeSB in the majority spin channel is occupied, while the one in the minority spin channel is empty corresponding to Fe3+ state (red solid line). Upon H adsorption an additional peak appears just below EF in the minority spin channel, corresponding to the sixth d electron of the Fe2+ ion. This leads to an elongation of the in-plane FeSB−OS bonds from 1.98 Å at the clean surface to ∼2.06 Å. The total DOS (bottom panel Figure 2c) indicates that the band gap of ∼0.3 eV at the clean surface,8,18 is closed with the adsorption of a H atom due to the electronic states near the EF in the minority spin channel. As a consequence, the system becomes halfmetallic.14 The electron density redistribution reduces the Pauli repulsion33 between the adsorbate and the substrate. Furthermore, hybridization of H1s and O2p states32 leads to a broadening and splitting of electronic states in the energy range between −6 eV to −9 eV (top panel Figure 2c). Additionally, upon H adsorption the 2p occupation of OS3 within the MT sphere is slightly enhanced by +0.07e. Another effect of the interaction is that the top of the O2p band (solid black line) is shifted by 0.32 eV to lower energy compared to the one at the clean surface (black dotted line in top panel of Figure 2c).

Figure 2. Electron density redistribution upon adsorption of a single H atom on Fe 3 O4 (001), configuration 1H plotted in a plane perpendicular to the surface along the (a) [1−10] direction and (b) perpendicular to [001] in the top surface layer. Electron density accumulation (depletion) is shown in red (blue). The color coding of the ions corresponds to the one in Figure 1. (c) Total and projected density of states (DOS) of oxygen and hydrogen of the OS3−H (upper panel, solid black and gray line), as well as of surface oxygen at the clean surface (black dotted line); (central panels) PDOS of the 3d and FeSB (solid black line). For states and magnetic moments of FeS−1 B comparison, the PDOS of FeB on the clean Fe3O4(001) surface is shown with a solid red line.

2H: With the inclusion of a second hydrogen atom on the surface not only the adsorbate−substrate but also the adsorbate−adsorbate interaction becomes important. Several configurations were considered: Hydrogen adsorbed at OS2 and OS4, denoted as 2H (see Figure 1b), is most favorable with Eads of −0.88 eV. To donate a hydrogen bond both OS−H groups tilt strongly toward the opposite OS, thereby aligning S themselves nearly parallel to the surface with ΘFeS−1 = B −O −H 105.3° slightly lower than in 1H. A strong correlation between the Eads and the OSH−OSH distance is observed: the most favorable configuration corresponds to a maximized OH−OH distance of 5.95 Å, while Eads of a configuration with Hadsorbed at OS1 and OS2 with dOH−OH = 3.32 Å is reduced to −0.66 eV. Thus, the stability of 2H configuration is attributed primarily to the minimized electrostatic repulsion between the hydroxyl groups and to hydrogen bond formation between OS−H···OS. The changes in the electronic structure can be analyzed from the DOS plot shown in Figure 3. The top panel in Figure 3 shows that the O2p and the H1s bands of the surface OH group shift by ∼0.43 eV to lower energy compared to 1H, indicating a stronger hybridization in 2H. In contrast to 1H now FeSB remains Fe3+ while FeS−1 below the OH groups change from B Fe3+ to Fe2+ (we note that other adsorbate configurations, e.g., with H-adsorbed at OS1 and OS2 show reduction also of FeSB). 16449



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Figure 3. DOS of configuration 2H with two hydrogen atoms on the Fe3O4(001) surface. (top panel) projected DOS (PDOS) of O 2p states of surface oxygen with (OS2) and without (OS) an adsorbed H; beneath OS−H PDOS of 3d states and magnetic moments of FeS−1 B (second panel) and of FeSB (third panel). Black and red lines indicate the DOS before and after adsorption of the H atom. Note the change in valence state at FeS−1 B . Figure 4. (a) Electron density redistribution plotted in a plane perpendicular to the surface along the [1−10] direction for the adsorption of four H atoms on Fe3O4(001), configuration 4H. Electron density accumulation (depletion) is shown in red (blue). For the color coding of the ions see Figure 1. (b) total and projected DOS (PDOS): (upper panel) PDOS of oxygen and hydrogen of the OS−H (solid black and gray line), as well as of surface oxygen at the clean surface (black dotted line); (central panels) PDOS of the 3d states and S magnetic moments of FeS−1 B and FeB (solid black). For comparison, the PDOS of FeB on the clean Fe3O4(001) surface is shown with a solid red line.

S As a result the dFeS−1 is elongated to 2.26 Å (Table 1), B −O −H which is 0.1 Å longer than that in 1H, while the in-plane bond length between FeSB and OS remains similar to the one of the clean surface. The total DOS shows that 2H is also halfmetallic, as already observed for 1H. 4H: We now saturate all OS sites without subsurface FeA neighbors with H, leading to a coverage of four H atoms per (√2 × √2)R45° unit cell (4H) as shown in Figure 1c. In this configuration, the electrostatic repulsion between the neighboring OH groups leads to a reduction of Eads to −0.63 eV. Another consequence is that two OH groups (OS2H and OS3H S in Figure 1c) have a tilt angle (ΘFeS−1 ) increased to 130.2°, B −O −H S1 S4 while the other two, O H and O H in Figure 1c, have a tilt angle of 105.2°, similar to that in the 2H configuration. This enables hydrogen bond formation betweenOS−H···OS−H, where one OH is a donor and the other acts as an acceptor. S is elongated to 2.26/2.21 Å, while d S S The dFeS−1 FeB−O remains B −O 12,31 1.98 Å as at the clean surface compared to dFeB−O of 2.06 Å in bulk. This introduces buckling in the surface layer. The electron density redistribution upon adsorption shown in Figure 4, bears some similarities to Δρ of 1H (Figure 2), e.g., concerning the charge accumulation in the t2g orbitals of FeS−1 B , FeSB, and along the OS−H bond. Unlike the case of 2H, here both surface and subsurface FeB atoms are reduced. Upon H adsorption, FeS−1 is switched to Fe2+ (PDOS see Figure 4b), B whereas a neighboring FeSB is in an intermediate state Fe2+δ, which can be identified by a lower occupation of the sixth orbital in the minority spin channel and a magnetic moment of 3.92 μB. The total DOS shows an insulator-to-halfmetal transition as in the previous two cases. The hybridization of H1s and O2p states in the energy range between −9.5 eV to −8.5 eV results in stronger splitting and broadening compared to the 1H and 2H cases. Furthermore, the valence band edge of

the OS2p band shifts to lower energies with increasing H coverage by 0.32 (1H), 0.75 (2H) and 0.89 (4H) eV. 8H: Finally, we have considered a complete saturation of all OS sites resulting in eight H atoms per (√2 × √2)R45° unit cell (8H), shown in Figure 1d. The adsorption energy decreases to −0.36 eV due to strong adsorbate−adsorbate repulsion and adsorption at the unfavorable OS sites with FeA neighbors. The S tilt angle ΘFeS−1 for OS−H varies from 84° to 147°, with a B −O −H formation of hydrogen bond between OS4−H···OS3. Enrichment in Fe2+ is found not only in the surface and subsurface FeB layers, but also in the subsurface FeA layers again leading to the insulator-to-halfmetal transition.14 The presence of Fe2+ H to results in significant elongation of the FeB−O bonds dFeS−1 B −O S S 2.44 Å and dFeB−O to 2.07−2.15 Å. 3.1. Variation of Eads and Work Function with Hydrogen Coverage. The results discussed above are summarized in Figure 5a, where Eads as a function of H coverage is shown. A single H atom prefers to adsorb on an OS with no FeA neighbor and aligns itself parallel to the surface with an Eads of −0.85 eV. Similar values are obtained for Hadsorption, e.g., on a RuO2(110)34 within GGA. With the introduction of a second H atom, a configuration is stabilized where the distance between two OS−H groups is maximized to reduce repulsion of the OH groups. Hydrogen bond formation 16450



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Figure 5. (a) Adsorption energy (eV/H atom) and (b) work function (eV) as a function of the number of hydrogen atoms NH on the surface per (√2 × √2)R45° unit cell. For 1H and 2H different configurations were considered with varying adsorption energies, see text.

between OS−H and OS of the opposite row contributes to the total Eads of −0.88 eV, which is almost identical to that in the 1H configuration. This indicates that the OH−OH interaction at this distance (5.85 Å) is negligible. With increasing coverage the repulsion between OH groups is enhanced which leads to a reduction of Eads to −0.63 (4H) and, finally, −0.36 eV (8H). In 8H, the decrease in adsorption energy is also related to the unfavorable adsorption of H on OS with tetrahedral iron neighbor. A similar dependence of Eads on coverage was also reported on the ZnO(0001) surface,35 where it varies from −0.5 to +0.6 eV (within GGA). Figure 5b shows that Φ decreases with growing H coverage on the Fe3O4(001) surface. The clean surface has a work function Φ of 5.03 eV which reduces to 3.76 eV in 4H and further to 1.80 eV in 8H. The change in work function can be associated with the charge transfer between adsorbate and substrate36,37 and the dipole moment of the generated surface OH group. A similar trend is observed for the adsorption of molecular H2O on Fe3O4(001)38 with the strongest reduction for water adsorbed in an upright geometry.

The lower limit corresponds to the case where the bulk oxide would separate in metallic Fe and oxygen, while the upper limit refers to the energy of the O2 molecule, set as zero in the phase diagram. Similar ranges apply for the chemical potential of hydrogen. The limits chosen for the hydrogen chemical potential are consistent with calculations for H on ZnO(0001).35 Since the surface energy depends on two variables, μO and μH, the surface phase diagram for the H adsorption on Fe3O4(001) is three-dimensional. Figure 6 displays a two-

4. SURFACE PHASE DIAGRAM In order to determine the relative stability of the discussed terminations as a function of the hydrogen and oxygen pressure in the gas phase and at finite T, we combine DFT with concepts from thermodynamics in the framework of ab initio atomistic thermodynamics.19 The surface energy depends on the chemical potentials of the constituents μO, μFe, and μH: γ (T , p) =

Figure 6. Projection of the surface phase diagram of hydrogen adsorbed on Fe3O4 (001) showing the most stable configurations for given (μO,μH): B-layer with oxygen vacancies, B+VO (red), distorted B-layer (magenta), single hydrogen, (1H, black), two (2H, gray), four (4H, green), and eight (8H, cyan) hydrogen atoms Fe3O4(001).

1 slab (G Fe − NFeμFe − NOμO − NHμH ) 3O4 (001) 2A (3)

where G is the Gibbs free energy of the hydrogen terminated system and NO, NFe, and NH are the number of oxygen, iron, and hydrogen atoms in the system, respectively. The Gibbs free energy can be expressed by the total energy from the DFTcalculations.19 Under the condition that the particle reservoir of Fe and O atoms is in thermodynamic equilibrium with bulk Fe3O4, μO, and μFe satisfy:19

E Fe3O4 = 3μFe + 4μO

dimensional projection showing the most stable terminations for given μO and μH. At hydrogen poor to moderate chemical potentials, the phase diagram is dominated by the B-layer termination7,8 as well as by a B-layer termination with oxygen vacancies,8,18 at oxygen poor conditions. With increasing hydrogen pressures, there is a narrow area where 1H is stable, followed by a region where 2H becomes favorable. In this termination, hydrogen bonds are formed facilitating the hopping of hydrogen between OS of opposite rows as observed in STM experiments, performed at 10−7 mbar H2-pressure.14 A further increase in hydrogen pressure stabilizes the 4H termination. In the rich limit of the hydrogen chemical potential, there is a narrow region where the 8H termination is stable but experimentally these high pressures are difficult to achieve. Overall, the 2H and 4H terminations are stabilized in the pressure ranges accessible in UHV experiments.

(4)

where EFe3O4 is the total energy of the bulk magnetite. Substituting eq 4 in eq 3, we obtain a dependence only on the two gas phase components μO and μH. The chemical potential of oxygen varies between the following ranges: 1 ⎡ bulk 1 bulk ⎤ ⎣E Fe3O4 − 24E Fe ⎦ < μO < EO2 32 2

(5) 16451



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Figure 7. Side view of clean and H-adsorbed configurations with electron density integrated between −1.0 eV and EF. Nonvanishing electron density indicates occupation of the sixth d orbital at Fe-sites, corresponding to Fe2+. The color coding of the ions corresponds to the one in Figure 1, the frame colors correspond to the ones in the phase diagram in Figure 6.

while FeS−1 below OS H switches to Fe2+, as shown in Figure B 7d. We note that other (less favorable) adsorbate configurations for 2H, e.g., with H at OS2 and OS3, exhibit Fe2+ both in the surface an subsurface layer (not shown here). In the case of 4H, we find that FeSB are partially reduced to an intermediate valence state while all the FeS−1 are switched to B Fe2+, as shown in Figure 7e. The occupied sixth orbital in the surface layer has dxy character while in the subsurface layer dxz ± dyz is occupied. The enrichment of the surface in Fe2+ after exposure to H atoms is also observed in XPS measurements and in STM images14 as bright protrusions. The complete saturation of the surface with eight hydrogen atoms (Figure 7f) switches all of the surface and subsurface FeA and FeB atoms to Fe2+,14 followed by alternating Fe3+ (S-2), Fe2+ (S-3) in deeper layers. Overall, we observe that the Fe2+:Fe3+ ratio in the surface layers is enhanced with increasing H coverage. A similar change in valence state is reported, e.g., for H adsorption on CeO2(110) and (111) surfaces.40

5. INFLUENCE OF H ADSORPTION ON THE CHARGE AND ORBITAL ORDER OF FE3O4(001) In a previous study, it was found that the surface termination and water adsorption induce a unique charge and orbital ordering in the subsurface Fe3O4(001)18 layers, while the surface layer shows exclusively Fe3+. Here, we extend our study to the influence of hydrogen adsorption. To distinguish between Fe2+ and Fe3+ sites, the electron density is integrated between −1 eV and EF (see Figue 7). Nonvanishing electron density in this region corresponds to an occupied sixth 3d orbital at the Fe2+ sites. We note that differences in total occupation between Fe3+ and Fe2+ sites are small (0.2−0.4e within the MT sphere), but changes in the valence state can be more clearly identified in the PDOS (e.g., Figures 2−4) and the magnetic moments (3.54−3.75 μB for Fe2+ and 3.90−4.10 μB for Fe3+). As mentioned above, at the clean Fe3O4(001) surface (B-layer, Figue 7a) all FeSB are Fe3+ and charge ordering in the deeper layers leads to the opening a band gap of 0.3 eV,18,39 consistent with scanning tunneling spectroscopy measurements.39 With the adsorption of a single hydrogen atom at OS3 (1H), one neighboring FeSB switches to Fe2+ along with the FeS−1 B beneath the OS H group. As shown in Figure 7b, different t2g orbitals are occupied at both sites, dyz at FeSB and dxz at FeS−1 B . The orbital order in the deeper layers also differs from the one of the B-layer in Figure 7a. Besides the adsorption at OS3, we have plotted in Figure 7c another configuration with H adsorbed at OS4: The main difference observed is that now different FeSB and FeS−1 sites become Fe2+. This shows that B surface (and subsurface) Fe sites can be selectively switched from Fe3+ to Fe2+ upon hydrogen adsorption. Furthermore, the 0.1 eV difference in energy for adsorption at OS3 and OS4 can be related to the variation in CO−OO in the deeper layers of 1H(OS3) and 1H(OS4). With the adsorption of two hydrogens (2H) at OS4 and OS2, all of the surface FeSB remains in Fe3+ state

6. CONCLUSIONS In summary, we present a comprehensive DFT study of the adsorption of hydrogen on the Fe3O4(001) surface analyzing also the underlying electronic mechanisms. A single hydrogen atom prefers adsorption on a surface oxygen atom without a tetrahedral iron neighbor. Thereby, a neighboring FeSB and the subsurface FeBS−1 are reduced from Fe3+ to Fe2+. This termination is stable in a narrow region of the surface phase diagram. A surface with two hydrogen atoms adsorbed on (√2 × √2)R45° unit cell is stable at moderate H2 partial pressures. The stability of this termination is due to hydrogen bonding to OS and maximized OH−OH distance that reduces electrostatic repulsion. With increasing H-pressure a surface with four adsorbed hydrogen atoms per (√2 × √2)R45° unit cell becomes stable. The charge transfer from H via surface oxygen reduces the neighboring surface as well as subsurface iron ions 16452



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to Fe2+. Partial charge ordering in the deeper layers leads to an insulator-to-halfmetal transition upon H adsorption for all considered H coverages, which makes this surface interesting for spintronics applications. The DFT results demonstrate that, depending on the adsorption site and coverage of hydrogen, a unique charge and orbital order can be achieved with enhanced Fe2+ ratio in the surface layer. This provides means to tune the availability of ferrous and ferric ions and thereby the redox activity of theFe3O4(001) surface.41

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge discussions with Prof. Dr. Matthias Scheffler (FHI, Berlin), Prof. Dr. Ulrike Diebold, and Dr. Gareth Parkinson (TU Wien); and a grant for computational time (h0721) at the HLRBII supercomputer at the Leibniz Rechenzentrum. N.M. acknowledges a fellowship by the Fritz-Haber Institute, Berlin.

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