Interaction of Water Molecules with the Î±-Fe2O3

Subscriber access provided by Iowa State University | Library

Article 2

3

Interaction of Water Molecules with the #-FeO(0001) Surface: A Combined Experimental and Computational Study Ludger Schöttner, Roman Ovcharenko, Alexei Nefedov, Elena Voloshina, Yuemin Wang, Joachim Sauer, and Christof Wöll J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 07 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Interaction of Water Molecules with the αFe2O3(0001) Surface: A Combined Experimental and Computational Study Ludger Schöttner,a, Roman Ovcharenko,b, Alexei Nefedov,a Elena Voloshina,b, §,* Yuemin Wang,a,* Joachim Sauer,b Christof Wölla,* aInstitute

of Functional Interfaces (IFG), Karlsruhe Institute of Technology (KIT), 76344

Eggenstein-Leopoldshafen, Germany bDepartment

of Chemistry, Humboldt-University of Berlin, 10099 Berlin, Germany

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 36

ABSTRACT The interaction of water with the basal plane (0001) of α-Fe2O3 (hematite) is a fundamental and challenging topic in the fields of surface science and earth science. Despite intensive investigations, many issues remain unclear especially due to the lack of direct spectroscopic evidence. Here, water adsorption on the pristine Fe-terminated α-Fe2O3(0001) surface was investigated by polarization-dependent infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy in conjunction with calculations from density functional theory. The combined results provide solid evidence that the interaction of water with α-Fe2O3(0001) is dominated by the heterolytic dissociation yielding an OwD species coordinated in atopconfiguration to surface Fe3+ and an OsD species resulting from the deuterium/hydrogen transfer to an adjacent substrate O2-. Both isolated hydroxyl groups do not feature any hydrogen bonding, while the intact water molecules were identified as minor species that are bound to surface Fe3+ ions and interact via a relatively strong H-bonding with substrate oxygen. Water adsorption on α-Fe2O3(0001) at 230 K and 200 K leads to the formation of water thin film including bilayers and multilayers, which are characterized by different types of intermolecular H-bonds.

1. INTRODUCTION The interaction of water with solid surfaces is a central issue in many scientific disciplines, and its investigation has attracted increasing interest.1-3 Especially the role of water on ferric oxides has drawn great attention, because Fe2O3 (particularly α-Fe2O3, hematite) has demonstrated a significant potential as a low-cost material in rich abundance for photoelectrochemical water splitting.4-10 Furthermore, the present study is also motivated by other specific aspects concerning corrosion processes, e.g., how the multifarious nature of iron containing oxides and oxyhydroxides is influenced by aqueous environments.11-13 It has


2

Page 3 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


been proposed that hematite in hydrated structures can strongly bind to metallic ions and enclose them.14-15 This ligand ability of surface hydroxyl groups for metal-ion complexations makes hematite suitable for water treatment applications against polluted media. However, research on the interaction between water and hematite remains still a big challenge due to the inherent complexity of iron oxide surfaces and their unique physical and chemical properties in general.16-20 Hematite is the thermodynamic most stable phase of all ferric oxides.21 The other important and naturally occurring polymorph γ-Fe2O3 (maghemite) is transformed to hematite when heated above 300 °C for example.22 The crystal structure of α-Fe2O3 is derived from corundum ( Figure 1a). It belongs to a trigonal crystal system with space group R3c and lattice constants of a = 504 pm and c =1387 pm.23 The structure of the unit cell is based on a hexagonal close-packed lattice with oxygen atoms, which is slightly distorted, so that only every sixth layer of oxygen lies on top of each other in [0001] direction ( Figure 1b). Only 2/3 of the octahedral holes are occupied by Fe3+ ions, which explains the symmetry reduction to the trigonal crystal system ( Figure 1c). The (0001) facet is the most frequently exposed surface in crystalline hematite minerals and therefore it is our primary target of investigation.24


3


Page 4 of 36

Figure 1. (a) Polyhedron model of the hematite bulk structure: each FeO6 octahedron shares a face with another in the layer above or below. (b) Schematic representation of various surface terminations of the clean α-Fe2O3(0001) surface. (c) Top view of the single-Fe termination of the Fe2O3(0001) surface. The yellow rhombus indicates the surface unit cell. (d) LEED pattern recorded at 90 eV for the pristine Fe-terminated α-Fe2O3(0001) singlecrystal surface. The reciprocal lattice vectors of the (1×1) structure are charted. Having a closer look at the atomic layer stacking sequence of bulk α-Fe2O3(0001), it is obvious that 3 different surface terminations can exist ( Figure 1b). A single Fe-termination (Fe-O3-Fe-R), a double Fe-termination (Fe-Fe-O3-R) and an oxygen termination (O3-Fe-Fe-R). Numerous theoretical studies have been described in literature with regards to the surface stability of hematite. All results predict the single Fetermination under oxygen poor conditions with strong surface relaxation, while the formation of the double Fe-termination is energetically unfavourable.25-29 Oxygen-rich conditions are expected to facilitate the formation of oxygen-terminated surfaces, but the situation is a bit more complicated in the latter case. Some preparation parameters can also evoke oxidation states higher than +3 for Fe-ions and result in versatile configurations of surface oxygen layers that may coexist with slab-built Fe-terminated structures.30-33 The O3-Fe-Fe-R termination has a polar character while the Fe-O3-Fe-R termination is ranked as a non-polar surface as a consequence of relaxation effects.34 Thevuthasan et al.35 have confirmed by Xray photoelectron diffraction that the Fe-terminated surface structure can be exclusively formed during thin film growth of α-Fe2O3(0001) with molecular beam epitaxy on Al2O3(0001). Density functional theory (DFT) calculations suggested that the initial water adsorption leads to a heterolytic dissociation on the Fe-terminated hematite (0001) surface, which is energetically preferred over a molecular adsorption.25-26,

29, 36

Consequently, the water

molecule splits into a HO- and a H+ species. As a Brønsted base, the hydroxyl-ion species (OwH) binds to a cationic Lewis center (Fe3+) on the surface via its oxygen atom, which is


4

Page 5 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


accompanied by transferring the conjugated proton to a nearby surface oxygen site (Os) according to the following mechanism: H2O + Os → OsH + OwH Ovcharenko et al.25 calculated adsorption energies of -104 kJ/mol and -95 kJ/mol for dissociated and molecular states, respectively. Yin et al.36 discussed the orientation of a neutrally charged water molecule that is bound via its oxygen atom to a surface Fe atom in a standing geometry with one H pointing to surface oxygen species. Nguyen et al.26 extended this model and claimed that also for dissociated water the existence of hydrogen bonds cannot be simply neglected, where the protonated surface oxygen OsH is also linked to OwH by a hydrogen bond. Trainor et al.29 found evidence for multiple coordinated environments of hydroxyl functional groups after water exposure on a Fe-terminated Fe2O3(0001) giving rise to a surface stoichiometry of (HO)3-Fe-H3O3-R, while a hydroxylated O-terminated surface features a stable (HO)3-Fe-Fe-R stoichiometry. The molecular dynamic simulations from Wasserman et al.37 revealed strong relaxation after hydroxylation of α-Fe2O3(0001), where the Fe atoms from the top layer are moving upwards and increase the distance to lower oxygen atoms approximately to bulk values. It was proposed that the four-fold coordinated surface Fe3+ carrying one hydroxyl-fragment with 1/3 of adjacent surface oxygen atoms being protonated, is the dominant species after water adsorption. No significant hydrogen bonding interaction was found at the hydroxylated surface. Kerisit38 predicted that single coordinated water molecules, double coordinated hydroxogroups (Fe2OH, bridging) and triple coordinated surface oxygen atoms have ability to form hydrogen bonds with other water molecules. Lützenkirchen et al.39 deduced from potentiometric measurements that only one hydroxyl group per iron atom can coordinate on the Fe-terminated hematite (0001) surface. Boily et al.40 found that protonated lattice oxygen species OsH of α-Fe2O3(0001) show some acidic properties and can be deprotonated (pKA~8), and the surface behaves charge-neutral in


5


Page 6 of 36

a pH range of 4-14. In comparative studies by X-ray reflection experiments, Catalano41 determined a lower magnitude of interfacial water ordering on the (0001) surface of α-Fe2O3 than on the isostructural corundum surface. In earlier experimental works of Kurtz et al.42 and Hendewerk et al.43, the stoichiometric α-Fe2O3(0001) was reported as an inert surface and only ice condensation was observed after water exposure. Junta-Rosso et al.44 reported in a multi-technique study that hematite did not undergo a direct transformation to goethite when exposed to water under ambient conditions. Instead, the hydroxylation and hydration process were restricted to monolayer dimensions on the surface. On the basis of near ambient pressure X-ray photoelectron spectroscopy (XPS) data, Yamamoto et al.45 proposed that only the hydroxylation occurred on the α-Fe2O3(0001) surface at low pressures, whereas the pressure increase led to a simultaneous increase in OH and H2O coverage. Despite the extensive experimental and theoretical studies, the interfacial water structure on hematite (0001) is not fully elucidated, and numerous controversial points remain. This is mainly due to the lack of direct and reliable spectroscopic evidence acquired in particular by infrared (IR) spectroscopy. In this paper, we report the first application of polarizationdependent infrared reflection absorption spectroscopy (IRRAS) to water adsorption on the αFe2O3(0001) surface. The systematic IRRAS experiments were carried out in conjunction with XPS over a coverage range from a full monolayer to multilayers. The interpretation of the IRRAS and XPS results was assisted by the state-of-the- art first-principles calculations. The combined experimental and theoretical data allow us to gain atomic level insights into the surface chemistry of water on α-Fe2O3(0001). 2. EXPERIMENTAL AND COMPUTATIONAL METHODS 2.1 Experimental methods. The IRRAS measurements were performed in an sophisticated ultrahigh vacuum (UHV) apparatus, which combines a state-of-the-art FTIR spectrometer (Bruker Vertex 80v) with a


6

Page 7 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


multi-chamber UHV system (Prevac) equipped with electron analyzer R4000 (Scienta Omicron) for photoelectron and Auger spectroscopies as well as with low energy electron diffraction (LEED) optics.46 The high resolution XPS measurements were carried out at the UHV endstation of the HESGM beamline of the synchrotron radiation facility BESSY II, which is a division of Helmholtz Zentrum Berlin (HZB).47 The acquisition of O1s XP spectra were carried out in normal emission geometry with the photon excitation energy of 580 eV. At this excitation energy the kinetic energy of the electrons is about 50 eV, therefore only the electrons emitted from the surface layer can reach the analyzer. The electron energy resolution was of ~0.4 eV. All experiments were carried out at a base pressure of 10-10 mbar. Prior to the actual sample transfer into the corresponding analysis chamber, the cryostat or/and cryogenic trap were firstly cooled down for several hours to prevent examined surface from contamination of undesired adsorbates. The α-Fe2O3(0001) single crystal (SurfaceNet) was mounted on a dedicated sample holder with e-beam heating. Temperature measurement was realized by a K-type thermocouple (NiCr-NiAl) that was placed between heating plate and rear-side of the crystal. The crystal surface was cleaned by several cycles of annealing at T = 850-950 K in a partial pressure of oxygen p(O2) = 10-5 mbar referring a literature procedure.45,

48

A certain minimum partial

pressure of oxygen is necessary to keep α-Fe2O3 stable at elevated temperatures.49-50 Annealing temperatures around 1050 K are not allowed, because they will lead to compositional changes on the crystal surfaces.51 It is clear, that oxidation parameters for thin film growth of α-Fe2O3(0001) cannot be simply adopted for the preparation of well-defined single-crystal structures, because the required temperatures are much higher in the former case.52-55 Ar-sputtering should be avoided on α-Fe2O3(0001), because it harms the surface and leads to partial reduction of the top layers.56-59 After ion sputtering the surface cannot be easily restored to a fully oxidized α-Fe2O3(0001), even the sample is annealed in oxygen at


7


Page 8 of 36

elevated temperatures.60 Such sputtering-induced surface restructuring was also observed for other metal oxide substrates (e.g. cerium oxide61-62). The preparation recipe for producing an O-terminated α-Fe2O3(0001) surface, well-defined in structure and composition, needs high oxygen pressures, which is beyond the scope of the present study. The LEED pattern shows an unreconstructed (11) structure for the clean surface of αFe2O3(0001) ( Figure 1d), which is characteristic for a Fe-terminated surface. Any reduced surface structures such as Fe3O4(111) and biphase-reconstructions of α-Fe2O3(0001) can be definitely excluded based on the present LEED data.63-64 The cleanliness and oxidation states of the sample were further monitored by XPS and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. As shown in the survey XPS scan (Figure S1, SI), the presence of any impurities (e.g. C, Na, Mg, K, etc.) can be definitely excluded. The Fe2p XP spectrum provide further evidence that only Fe3+ ions are present on the surface (see Figure S2, SI). The characteristic doublet in O K-edge NEXAFS spectra at ~530 eV (Figure S3, SI) is very well resolved. The value of the energy splitting and the relative intensities of the peaks in the doublet are functions of the O-Fe structure and observed in our experiments both the energy splitting (1.3 eV) and the peak intensity ratio of 1:1 are characteristic for hematite.65 The well-defined Fe3+-terminated α-Fe2O3(0001) surface is also evidenced by the IRRAS data using CO as a probe molecule which shows only one rather sharp band originating from CO bound to Fe3+ sites. The IRRAS data recorded for CO adsorption will be presented in a forthcoming publication. The polarization-dependent IRRAS measurements were conducted with p- and/or spolarized light components at an angle of incidence of 80° in respect to the surface normal. The resolution was set to 4 cm-1 and 2048 scans were added to the spectrum. Prior to each exposure, a spectrum of a clean sample was recorded as a background reference.


8

Page 9 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In this study, we used deuterated (heavy) water (D2O) instead of H2O for our experimental investigation to avoid misleading artifacts in the spectra from external influences. D2O (99.9 atom % D, Sigma-Aldrich) and D218O (99 atom% D, 75 atom%

18O,

Sigma-Aldrich) were purified by multiple freeze-thaw-pump cycles. Exposure to heavy water was carried out by backfilling the analysis chamber over a leak-valve. Doses are given in the units of Langmuir (1 L = 1.33∙10-6 mbars). 2.2 Computational methods. Spin-polarized DFT calculations based on plane-wave basis sets of 500 eV cutoff energy were performed with the Vienna ab initio simulation package (VASP).66-68 The PerdewBurke-Ernzerhof (PBE) exchange–correlation functional69 was employed. The electron-ion interaction was described within the projector augmented wave (PAW) method70 with Fe (3d, 4s), O (2s, 2p), and H (1s) states treated as valence states. The Brillouin-zone integration was performed on Γ-centered symmetry reduced Monkhorst-Pack meshes using a Gaussian smearing with σ = 0.05 eV, except for the calculation of total energies and densities of states (DOSs). For those calculations, the tetrahedron method with Blöchl corrections was employed.71 A 4 × 4 × 1 k-mesh was used in the case of ionic relaxations and 8 × 8 × 1 for single point calculations, respectively. The DFT+U scheme72-73 was adopted for the treatment of Fe 3d orbitals, with the parameter Ueff = U - J equal to 4 eV. The compensated polar (0001) hematite surface with a Fe-O3-Fe-R composition was modelled by symmetric slab. A (2×2) supercell in the lateral plane was adopted. The lattice constant in the lateral plane was set according to the optimized lattice constant of bulk hematite, a = 507 pm. The used supercell contains 21 atomic layers and a vacuum gap of approximately 2600 pm. The ions of the 13 middle inner layers were fixed at their bulk positions during the structural optimization procedure, whereas the positions (x, y, zcoordinates) of all other ions were fully relaxed until forces became smaller than 0.01 eV∙Å-1.


9


Page 10 of 36

In this work we considered a coverage of 1 ML water, corresponding to the adsorption of four water molecules per (2×2) supercell. All possible adsorption modes have been taken into account, namely: all molecules fully molecular (Figure 2a), mixed dissociative-molecular (Figure 2b-d), and fully dissociative (Figure 2e). Five representative examples are shown in Figure 2. The total number of configurations was 29. Dispersion interactions were considered by adding a 1/r6 atom-atom term as parameterized by Grimme (“D2” parameterization).74 In order to avoid truncation errors the Ewald summation for the dispersion term was performed.75

Figure 2. Representative examples of the considered nonequivalent adsorption configurations (top views): (a) 4 H2Omol; (b) H2Odis + 3 H2Omol; (c) 2 H2Odis + 2 H2Omol; (d) 3 H2Odis + 1 H2Omol; (e) 4 H2Odis. Red and blue spheres are addressed to oxygen and iron ions, respectively. The dark green and orange large spheres denote an oxygen of a water molecule and a hydroxyl group, respectively. The small light green spheres denote hydrogen atoms.

The core level binding energies were calculated within the initial-state approximation, where the valence electrons are kept frozen during photoionization. In order to simulate the XP spectra under realistic experimental conditions, the environmental dependent statistical analysis was performed. In the adopted model, the measured quantities (such as a Gibbs adsorption free energy) are averaged over an entire ensemble of symmetry-nonrelated or nonequivalent configurations 𝔾:

〈𝐺ads〉 = ∑𝑗 ∈ 𝔾𝐺𝑗ads ∙ 𝑓𝑗, 1

where 𝐺𝑗ads is calculated as 𝐺𝑗ads(𝑝,𝑇) = 𝑛[𝐺(𝑇,𝑛 ∙ H2O/Fe2O3) - 𝐺(𝑇,Fe2O3) - n ∙ 𝜇H2O(𝑝,𝑇)] with 𝑇, 𝑝, 𝜇H2O being the temperature of the system, the water vapor partial pressure and


10

Page 11 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


chemical potential, respectively. In thermal equilibrium, the ensemble has the following

[

1

distribution function: 𝑓𝑗 = 𝑍 ∙ 𝑁𝑗 ∙ exp ―

𝐺𝑗ads 𝑘𝑇

], where 𝑁

𝑗

is a number of symmetry-related

adsorption configurations for the j-th nonequivalent configuration. The corresponding

[

partition function Z takes the form: 𝑍 = ∑𝑗 ∈ 𝔾𝑁𝑗 ∙ exp ―

𝐺𝑗ads 𝑘𝑇

].

The vibrational frequency calculations were performed only at Γ-point. The frequencies were obtained in harmonic approximation by diagonalization of the Hessian matrix. The Hessian matrix was obtained by numerical differentiation within the central difference scheme, with displacement of 0.015 Å in both positive and negative directions. For comparison with experiment, the wavenumbers are scaled to account for systematic errors in the harmonic force constants and neglected anharmonicity effects. The scaling factor has been obtained from the ratio of the averaged symmetric and asymmetric isolated D2O stretching modes and the respective average of observed gas-phase results as described in the Supporting Information of ref. 76. 3. RESULTS AND DISCUSSION 3.1. D216O Adsorption on the Pristine Fe-Terminated α-Fe2O3(0001) Surface. The IRRAS study of water adsorption on monocrystalline oxide surfaces is hampered by the inherent experimental difficulties arising from the extremely low reflectivity in the infrared regime of dielectric substrates. In particular, the identification of hydroxyl species formed via the interaction of water with oxide surfaces represents a major challenge due to the very weak transition dipole moment (TDM) of O-H(D) vibrations. Here, we present a thorough polarization-dependent IRRAS investigation of D2O adsorption on the pristine Fe-terminated α-Fe2O3(0001) surface at different temperatures. We now turn our attention to the IRRAS data recorded for D216O. The spectra were recorded after saturating the room-temperature surface and are shown in


11


Page 12 of 36

Figure 3. We can clearly identify two signals at around 2720 cm-1 and 2700 cm-1. In accordance with previous work by Mirabella et al.77 we assign the peak at 2720 cm-1 to the 16O

wD

stretch vibration. The second feature at 2700 cm-1 is, again in accordance with

previous work76 assigned to the Df16OD vibration. Previously, also a third vibration has been observed around 2685 cm-1, which was assigned to hydroxy species where a proton has been transferred on top of a substrate oxygen (16OsD).77 Although we cannot unambiguously resolve a peak at this location there is a distinct shoulder at around this frequency. To achieve a thorough analysis of the data, we subjected the experimental results to a fitting procedure starting with Lorentzian peaks positioned at the frequencies reported in the previous works7677,

(see also Table 1). In the subsequent fitting process all half-widths were set to be equal

(one single fit parameter), and peak positions and peak heights were fit parameters. The results of the fitting process are shown in Table 1. Two main peaks are located at 2720 cm-1 and 2701 cm-1, and a third, weak peak at 2687 cm-1. It should be noted that the signal at 2687 cm-1 is just above the noise level, but inclusion of this OsD peak substantially improved the fit (see Figure 3).


12

Page 13 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. IRRAS data recorded at a saturating exposure of α-Fe2O3(0001) to D216O (10-7 mbar) at 300 K. The averaged data were deconvoluted by fitting the individual components with Lorentzian curves. The violet, green and blue lines illustrate OwD, DfOD, OsD species, respectively. The corresponding residuum is shown as an inset. The IR spectrum was recorded at a grazing incidence angle of 80° with p-polarized light. We now compare these experimental results to those obtained by the DFT calculations. According to the theoretical results, the molecularly adsorbed D216O features a non-H-bonded dangling OD group (Df16OD), which exhibits a typical IR band at 2697 cm-1, is in line with the value of 2701 cm-1 observed in IRRAS. With the exception of the 16OsD-vibration, for all other vibrations an excellent agreement is seen between experiment and theory, with deviations of 20 cm-1 or less (see Table 1). Only for the 16OsD-vibration the difference between the DFT results and the experiment, 42 cm-1, is so large that a direct, straightforward assignment is not possible. Interestingly, in the previous paper by Mirabella et al.77 on magnetite the DFT results were around 20 cm-1 for this vibration larger than in the experiment, whereas here, in case of hematite, the theoretical


13


Page 14 of 36

frequency is 42 cm-1 lower. At present, the reason for this substantial deviation only for the 16O

sD-vibration,

considering the good agreement for the other modes, is unknown.

Figure 4 displays the in-situ IRRAS data obtained after exposing the clean α-Fe2O3(0001) surface to D216O at indicated temperatures. The saturation adsorption of D2O at 300 K and 250 K leads to the formation of a full monolayer; three negative IR bands are resolved at 2720 cm-1, 2701 cm-1 and 2687 cm-1 in the p-polarized spectra, which are characteristic for O-D stretching modes. The temperature-dependent IRRAS data provide insights into the thermal stability of adsorbed species. These bands are stable for temperatures up to ~450 K and desorb completely only upon annealing to 650 K (see Figure S4, SI), indicating a strong interaction between water and α-Fe2O3(0001). The present IRRAS results reveal the presence of different types of hydroxyl species formed via the heterolytic dissociation of water, as further confirmed by XPS and DFT calculations presented below. A further comparison of the IRRAS results obtained for D2O adsorption at different temperatures reveals that the IR bands at 300 K have a smaller full width at half maximum (FWHM) than those at 250 K (Figure 4). This could be related to an enhanced ordering of adsorbed species from non-uniform distribution at lower temperatures to a more homogeneous molecular environment at higher temperatures.


14

Page 15 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. Temperature-dependent in-situ IRRAS spectra obtained for the α-Fe2O3(0001) surface exposed to a saturating dose of D216O (10-7 mbar) at indicated temperatures. All the spectra were recorded at a grazing incidence angle of 80° with p-polarized light.

As shown in Figure 4, after D2O exposure at 230 K, an additional IR band at 2730 cm-1 appears beside 2720 cm-1. These findings indicate the formation of D2O bilayer. The IR signal at 2730 cm-1 is typical for dangling mode of non-H-bonded OD groups of terminal D216O molecules (Df16ODad) in condensed phases.78 The weak band at 2720 cm-1 is associated with the dangling mode of dissociated water bound to surface Fe (16OwD). The clear absence of the dangling-modes at 2701 and 2687 cm-1 for Fe-bound water (Df16OD) and surface 16OsD species reveal that the top layer water molecules are interconnected via hydrogen bonding with surface D2O/OsD species. The establishment of a dimeric structure bridged by adlayer water, that is observable for saturating D216O exposures at 230 K, is underlined by the ability to form additional H-bonds (further details in section 3.3). This connectivity seems reasonable because of certain entropic and enthalpic effects. A further decrease in temperature to 200 K results in the formation of D2O multilayer on the α-Fe2O3(0001) substrate, as demonstrated by the emergence of a broad and predominant


15


Page 16 of 36

IR signal at ~2585 cm-1 in the p-polarized spectra (see Figure 4). The frequency of this band is characteristic for the formation of intermolecular H-bonds within the water network. In addition, the band at 2730 cm-1 becomes more intense, which is attributed to the non-Hbonded dangling OD groups located at the topmost layer of water 3D structures (Df16ODad). Simultaneously, the interfacial OD/D2O species are no longer visible in the IR-spectra due to the screening effects. It should be noted that the observation of water thin multilayers at 200 K is in line with the results from temperature programmed desorption (TPD), where the desorption of D2O multilayers on α-Fe2O3(0001) occurs between 175-220 K.43 In order to gain more insight into the interaction of water with the α-Fe2O3(0001) surface, we carried out high-resolution XPS measurements. The XPS investigation of water adsorption on monocrystalline Fe2O3 surfaces at low temperatures is severely hampered due to charge effects induced by the insulating properties of iron oxides with temperature reduction.79 The best XPS results for water monolayer adsorption were obtained near the socalled Morin-transition around 260 K, where the electrical conductivity related Seebeck coefficients undergoes a change due to anisotropic spin flipping.80 Figure 5 shows the experimental O1s XP spectra recorded before and after exposing the clean sample to the saturating dose of D2O at 10-9 mbar and at 260 K. The XPS data were deconvoluted via a peak-fit analysis using Gaussian curves after Shirley background subtraction. The peak FWHM for all Gaussian peaks was set to the value determined for the lattice oxygen O2- of the pristine substrate. The same FWHM value was used for all O1s peaks. As mentioned above, the low kinetic energy of the emitted electrons when using x-ray photons of 580 eV turns synchrotron-based XPS into an extremely surface sensitive technique. In fact, the adsorption of only one monolayer of D2O attenuates the O1s substrate signal by a factor of two, so that its intensity becomes comparable to the contribution of the adsorbate OD/D2O species. At full monolayer coverage of D2O on the α-Fe2O3(0001)


16

Page 17 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


surface, new O1s intensity was observed at higher binding energies. A good fit of these contributions was obtained by adding additional peaks at 533.4 and 531.6 eV (Figure 5), indicating the presence of at least two adsorbate species. In following previous reports in the literature, the peak at 533.4 eV was assigned to intact D2O molecules bound to surface Fe3+ sites while the peak at 531.6 eV is characteristic for hydroxyl groups.44-45, 81

Figure 5. O1s XP spectra of D2O on the α-Fe2O3(0001) surface: Experimental XP spectra obtained before and after exposing the clean sample to a saturating dose of D2O in 10-7 mbar and at 260 K. Experimental spectra (dotted lines) were fitted with Gaussian curves after Shirley background subtraction. The assignment is discussed in the main text.

It is known that XPS can provide quantitative information on the relative abundance of various surface compositions.82 The molar ratio between molecular D2O and reacted OD species was estimated with 0.63:1. Accordingly, water dissociation is the dominating process while intact water molecules are present on the surface as minority species. The Gaussian line fitting of the O1s signal for the clean α-Fe2O3(0001) surface reveals an asymmetric peak shape (Figure 5). This asymmetry may have different origins, such as shake-up satellites, stoichiometric deviated compositions, or residual pre-hydroxylated


17


Page 18 of 36

states.45, 83-85According to our peak-fit analysis, a small proportion of the surface active sites is already hydroxylated, presumably by dissociative of traces of water from present as residual gas in the UHV chamber.

Figure 6: The simulated O1s XP spectrum of H2O adsorbed on the α-Fe2O3(0001) surface at 295 K and 3∙10-5 mbar. The contributions from different oxygen species on the surface are separated. Each line represents a contribution from a single adsorption configuration with a height proportional to the corresponding probability.

We also simulated the XP spectrum for the model structures obtained from the DFT calculations. The computed O1s spectrum is presented in Figure 6. The discrete binding energies corresponding to single adsorption configurations were convoluted with Gaussians. The peak at 531.3 eV originates from the hydroxyl groups (OsH/OsD) formed by water dissociation and hydrogen transfer to substrate oxygen ions of α-Fe2O3(0001). The lines at 529.7 eV shifted to lower energies relative to the substrate (Ox) signal result from OwH/OwD species coordinated to Fe. Such an O1s signal with a binding energy lower than that of the O substrate line has not been previously reported in the


18

Page 19 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


literature, and the difference to the substrate O1s peak at 530.0 eV is too small to unambiguously resolve this species in the experimental data. Undissociated water molecules bound to surface Fe3+ sites give rise to the O1s peak at 532.1 eV. Overall, the agreement between experimental XPS data and theory is rather good. The deviations between experimental and calculated work function shifts or peak intensities for D2O species are explained by the approximations made in the theoretical treatments. First of all, only some of the possible adsorption configurations, where water molecules are directly bound to the hematite surface, were considered. For example, we have neglected adsorption structures containing dimer, trimer etc., where water molecules might be stabilized only by H-bonds with other water molecules and its residues already adsorbed on the surface. According to our calculations, this is a good assumption for 1 ML coverage and less. Nevertheless, for higher coverages the water adsorption is expected to be dominated by water-water interactions. A further source of inaccuracy in calculations is the initial-state approximation use for the calculations of XPS peak positions, which neglects all final-state effects during core-shell ionization. Although this approximation is considered the best oneelectron approach for insulating materials such as hematite and high photon energies used in XPS, the accurate theoretical treatment needs the many-electron relaxation effects being considered. 3.2. Isotope Substitution IRRAS Experiments and Band Assignments. We now turn our attention to the results recorded for the D218O isotopologue of water. The corresponding IRRAS data are shown in Figure 7. Again, we subjected the data to a fitting procedure (Figure S5, SI). For the initial guess we placed peaks at positions as for the D216O data but shifted by the expected (from the calculations) isotope shifts, which amount to 15-18 cm-1.


19


Page 20 of 36

Again, a very good agreement between IR experiment and DFT calculation is obtained. The results of the fitting process are listed in Table 1.

Figure 7. IRRAS data obtained after exposing the surface of α-Fe2O3(0001) to a saturating amount of D218O (bottom) in 10-7 mbar at 300 K. The corresponding spectrum for D216O (top) is shown in comparison within the O-D stretch vibration region. Both spectra were recorded at the grazing incidence angle of 80° with p-polarized light.

The isotopic labeling method with D216O/D218O is also capable to prescind the weak 16O

sD

band position and was previously applied by Mirabella et al.77 in water adsorption

studies on Fe3O4(111) thin films. For the heterolytic dissociation structure formed upon D2O adsorption on α-Fe2O3(0001), the OwD species is expected to show an expected isotopic shift in frequency when dosing D218O, whereas the vibration of OsD groups containing lattice oxygen (16O) should not be affected. The IRRAS data shown in Figure 7 was obtained after saturation adsorption of D218O on α-Fe2O3(0001) at 300 K. Upon exposure to isotopic labeled D218O at room temperature, three IR bands show up at 2702, 2687 and 2683 cm-1. Among them, the weak band at 2687 cm-1 does not exhibit any frequency shift compared to that for D216O, thus providing direct


20

Page 21 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectroscopic evidence that this band is ascribed to the hydroxyl group (16OsD) formed via proton transfer to surface O atoms. In contrast, the other two bands at 2702 and 2683 cm-1 exhibit clearly a redshift of 18 cm-1 with respect to those observed at 2720 and 2701 cm-1 for 16OD/D 16O, 2

revealing that these bands originate from the presence of D218O-related species

(18OD/D218O). The vibrational shift from the experiment is in good agreement with that of the theoretical prediction.

Figure 8. Top and side views of dissociative (a, b) and molecular (c, d, e) water adsorption structures on the pristine single Fe-terminated Fe2O3(0001) surface. Red and blue spheres represent hematite oxygen and iron ions, respectively. The water molecule is illustrated with green and light green spheres for oxygen and hydrogen, respectively. The hydroxyl group’s oxygen is marked with orange. The abbreviation index h shows a donating hydrogen-bond from the deuterium atom in the OD fragment of D2O to an adjacent hydrogen-bond acceptor, while the index f represents a free and terminal OD group in intact water molecules without H-bonding proportions.

To corroborate the band assignments and to gain deeper insights into the atomic structure of water on the α-Fe2O3(0001) surface, we performed the respective DFT calculations. In agreement with IRRAS, our calculations suggest that water prefers a heterolytic dissociation on the pristine single Fe-terminated α-Fe2O3(0001) surface yielding a hydroxyl species bound to surface Fe in atop configuration, involving an adjacent 3-fold coordinated surface oxygen atom that is protonated via H(D) transfer from water (Figure 8a-b). In addition, the molecularly adsorbed water could be present on the surface (Figure 8c-e) and interacts with a surface oxygen atom via an isolated H-bond.


21


Page 22 of 36

The calculated vibrational frequencies for the hydroxyl groups and molecularly adsorbed water are summarized in Table 1, where they are compared to experimental results from this work, as well as from previous reports on Fe3O4.76-77 Table 1. Summary of the calculated and experimental vibrational frequencies (𝜈 in cm-1) for monolayer 16OD/18OD and D216O/D218O species absorbed on the pristine Fe-terminated αFe2O3(0001) surface. For comparison, we also show the experimental results reported for D216O/D218O adsorption on Fe3O4(111). Adsorbed

α-Fe2O3(0001)

species

a Scaling

DFT a

Experimental

Fe3O4(111) Experimental (Ref. 77)

DFT b

300 K

300 K

250 K

Tet1

2723

2714

2729

16O

wD

2736

2720

18O

wD

2719

2702

16O

sD

2645

(2687)c

18O

sD

2630

2697 2680

2688

2705

2671

Df16OD

2697

2701

Df18OD

2681

2683

Dh16OD

2128

2250

Dh18OD

2115

factor 0.9956; b Ref. 77, Scaling factor 0.9935, see Ref. 76; c see text.

Whereas the

16O

wD

similar, the theoretical

frequencies computed for α-Fe2O3(0001) and Fe3O4(111) are very

16O

sD

frequencies are significantly lower (60 cm-1) for α-Fe2O3 than

for Fe3O4. This is explained by the different ionic bond strengths. At the Fe2O3(0001) surface the OsD group is coordinated to three Fe3+ ions which are sixfold coordinated. This adds up to an ionic bond strength of 3(+3/6) = 1.5 and implies a bond strength of 0.5 for the OD bond. At the Fe3O4(111) surface the OsD group is also coordinated to three six-fold coordinated Fe ions with a formal charge of +2.5 (the d-electrons are delocalized between the Fe ions). This adds up to 3(+2.5/6) = 1.25 and implies a bond strength of 0.75 for the OH bond. The stronger


22

Page 23 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bond results in a shorter bond with a larger force constant and a higher wavenumber as predicted by DFT. Only for one of the two types of hydroxyl species (OwD and OsD) created by heterolytic dissociation of D2O a clear assignment can be made. The predicted 16OwD/18OwD frequencies at 2736/2719 cm-1 fit to the observed bands at 2720/2702 cm-1. In contrast, for the second hydroxyl species resulting from this dissociation, OsD, the assignment of the 2687 cm-1 band suggested by experimental considerations is not supported by the predicted wavenumber at 2645 cm-1. The deviation (42 cm-1) is larger than for the other bands. Here is room for future computational studies, using, e.g., hybrid functionals86 and considering also higher water coverages. For molecular water, DfOD, there is good agreement between DFT calculations and experiment (Table 1). This assignment is further supported by the simultaneous observation of a broad and low-frequency IR feature centered at ~2250 cm-1 which is characteristic for the hydrogen bonding between intact D2O and the surface O atom, as discussed below in Section 3.3 in more detail. 3.3. Hydrogen Bonding and Adsorption Geometries. In this section we focus on a more detailed discussion of the hydrogen bonding between adsorbates (OD/D2O) with substrate and/or with adjacent adsorbate species. Hydrogen bonding in water-related systems is a rather complicated phenomenon. IRRAS is a powerful method to monitor the H-bond formation with an extremely high sensitivity.87 For the O-H∙∙∙O system five different classes have been postulated with different chemical and electrostatic surroundings that determine the individual strength of H-bonds.88 The O species involved in H-bonding can be further distinguished into hydrogen acceptor (HA) and hydrogen donator (HD). The polarization-dependent IRRAS data allow to provide detailed information about the adsorption geometries of adsorbates on dielectric substrates. The sign and intensity of IR


23


Page 24 of 36

bands vary strongly depending on the interaction between TDM with p-polarized (normal, Ep,n; tangential, Ep,t) and s-polarized (Es) components of the incident light.46 The absorbance bands excited by Es are always negative, while the IR bands excited by p-polarized light (Ep,n and Ep,t, showing always opposite signs) can be negative or positive depending on the incidence angle Θ and the refractive index n of the substrate (for details see Ref. 46). Figure 9 shows the p- and s-polarized IRRAS data obtained after D216O adsorption on the Feterminated α-Fe2O3(0001) surface at 300 K. Again, in the p-polarized spectra three negative bands at 2720, 2702 and 2687 cm-1 were resolved, which are assigned respectively to the 16O

wD,

Df16OD, and 16OsD species, as discussed in detail above. These bands appear at high-

frequency region with a narrow band shape, which are characteristic for vibrations of non-Hbonded O-D groups, as corroborated with the calculated structures (see Figure 8) and frequencies (Table 1).

Figure 9. Polarization-resolved IRRAS data obtained after exposing α-Fe2O3(0001) to a saturating amount of D2O in 10-7 mbar at 300 K. The spectra were recorded at a grazing incidence angle of 80° with p-polarized (black) and s-polarized (blue) light.


24

Page 25 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In addition, the negative sign in the p-polarized spectrum reveals that these bands are excited primarily via coupling to the normal component of the E vector Ep,n. Furthermore, these vibrations were not observed in the corresponding IRRAS data (Figure 9) recorded with s-polarized light (Es vector oriented parallel to the surface and perpendicular to the incident direction). In the latter case, only vibrations with a TDM oriented parallel to the surface can be excited. Overall, the present polarization-dependent IRRAS results support the calculated adsorption structures (see Figure 8), where both hydroxyl groups (OwD and OsD) and the non-H-bonded OD (DfOD) group in intact D2O adopt a slightly titled geometry, thus, possessing a major TDM oriented perpendicular to the surface. Importantly, both p- and spolarized IR spectra show a broad IR signal centered at ~2250 cm-1, which is largely redshifted with respect to those observed at 2700-2730 cm-1 (Figure 9). This broad lowfrequency IR band provides solid evidence for a relatively strong hydrogen bonding between the intact D2O molecule and a surface oxygen atom (Dh16OD), as illustrated in Figure 8d. Based on the calculated adsorption structure, the isolated H-bond adopts a tilted configuration with both TDM components perpendicular and parallel to the surface, which can be excited respectively by p- (Ep,n, negative sign) and s-polarized light, in line with the experimental observation (Figure 9). This assignment is further supported by the calculated frequencies of the H-bonded O-D stretch vibration using DFT for molecularly coordinated Dh16OD (2128 cm-1) and

16O

sD h

species (2182 cm-1), where the hydrogen-bonding interaction for both hydroxyl species (OsD and OwD) formed by the heterolytic dissociation of D2O can be ruled out based on the combined IRRAS and theoretical results. Interestingly, the two related hydroxyl groups resulting from D2O dissociation show an apparent intensity inconsistency in IRRAS: the 16OsD band at 2687 cm-1 is much weaker than


25


that of the

16O

18O

wD/

wD

Page 26 of 36

vibrations (2702/2720 cm-1, see Figure 7). On the basis of the

experimental and theoretical results discussed above, the adsorption geometry cannot be the main reason for the observation of OsD/OwD bands with rather different intensities. In addition, the charge density plays a crucial role in the TDM calculations.89-90 However, the electronic structures of the OwD and OsD groups formed on α-Fe2O3(0001) are not expected to show strong differences. We speculate that this intensity inconsistency is due to the diffusion of hydrogen (H/D) atoms into deeper layers. As a result, the concentration of surface OsD species could be much lower than that of the OwD groups. Note that the subsurface diffusion of hydrogen atoms has been observed in our previous work using HREELS for numerous metal oxide surfaces including ZnO(1010),91 O-ZnO,92 and TiO2(110)93. When exposing the pristine α-Fe2O3(0001) surface to a saturating amount of D216O at 230 K, we were able to prepare a bilayer-like surface structure. The corresponding p-polarized IRRAS spectrum (see Figure S6, SI) shows a characteristic negative band at 2730 cm-1 (Df16ODad), 2720 cm-1 (16OwD) and a broad feature around 2200 cm-1 originating from the formation of H-bonds. This finding suggests that strong hydrogen-bonding interactions occur between the two water layers, where the hydroxylated surface is stabilized by water molecules via H-bonding in a dimeric manner. The H-bonding interaction accounts for a large frequency red shift of the interfacial species towards ~2200 cm-1 and is attributed to Hdonating groups like protonated lattice oxygen lattice (16OsDh) and adlayer water (Dh16ODad). Finally, it is expected that the hydrogen-bonding interaction can also lead to chemical shifts in the O1s XP spectra due to the modification of electronic structures. To illustrate this, we have carried out DFT calculations of the O1s binding energies for various oxygen-containing species. Note that the modifications in binding energy may also include electrostatic shifts between adsorbate and substrate.


26

Page 27 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In the XPS calculations we make some approximation by fixing the binding energy of substrate O2- ion at 530.0 eV and give relative binding energy values for adsorbed OH/H2O species. It has been reported that increasing water coverage has some impact on the total charge states in oxide substrates and water adsorbates.94-95 Therefore it is more precise to characterize different species in relative binding energy shifts of the O1s level with respect to bulk oxygen. The calculated chemical shifts for different adsorbate structures including hydrogen-bonds are summarized in Table 2. Table 2. The calculated chemical shifts of several oxygen containing species from the configurations of hydrogen bonding with respect to the bulk oxygen ion.

Species Ox Ox,h O sH O sH h O wH O wH h HfOH HhOH

Binding energy shift / eV Theory Experiment 0.0 0.0 0.0 + 1.2 +1.6 + 0.8 - 0.3 +1.6 - 0.1 + 2.1 +3.4 + 1.9

Interestingly, the calculations yield chemical shifts of up to 0.4 eV only caused by H-bonding between monolayer species. However, due to the experimental occurrence of a relative broad water adsorbate signal in O1s region a straightforward distinction between H-bonded and not H-bonded single species cannot be obtained by fitting the XP spectrum. In this context, the IRRAS data are much better suited to provide direct information on the presence of Hbonding. 4. CONCLUSIONS Water adsorption on the pristine single Fe-terminated α-Fe2O3(0001) surface has been studied by polarization-dependent IRRAS in conjunction with XPS and DFT calculations. The results


27


Page 28 of 36

demonstrate consistently that the interaction of water with α-Fe2O3(0001) is mainly dominated by heterolytic dissociation yielding two distinct hydroxyl species. Non-dissociated water molecules were also identified as minor species. The isotope substitution experiments with D216O/D218O provide solid evidence for the presence of OwD surface hydroxyl groups coordinated to the surface Fe3+ ion, while the assignment of an additional band to OsD species created by proton transfer from D2O to an adjacent substrate O2- ion remains unclear. These two hydroxyl groups are characterized in the experimental IRRAS data by O-D stretch vibrations at 2720/2702 cm-1 (16OwD/18OwD) and 2687 cm-1 (16OD), respectively. On the basis of p- and s-polarized IRRAS data and DFT calculations, both hydroxyl groups adopt a slightly tilted adsorption geometry with respect to the surface normal without any H-bonding to the substrate oxygen, while the molecularly adsorbed water is bound to Fe3+ and interacts with a surface O2- via relatively strong H-bonding. The intact water species is characterized by a sharp dangling OD band at 2701 cm-1 (Df16OD) and a broad low-frequency IR signal centered at ~2250 cm-1 for a H-bonded OD group in water (Dh16OD). In addition, our DFT calculations show different chemical shifts of the O1s binding energies for various hydroxyl/water species (with and without H-bonding), in good agreement with the experimental XPS results. Water adsorption on the α-Fe2O3(0001) surface at saturation doses between 250 K and 230 K leads to the formation of the bilayer structure, in which the top layer water molecules are probably bound via strong hydrogen bonding to interfacial OD species, as supported by a broad low-lying negative signal at ~2200 cm-1 observed in the p-polarized IRRA spectrum (Figure S6, SI). Upon exposure of α-Fe2O3(0001) to water at 200 K, the p-polarized IRRA spectrum was dominated by a broad negative band at ~ 2585 cm-1, demonstrating the formation of intermolecular H-bonds within the water multilayers.


28

Page 29 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AUTHOR INFORMATION Corresponding Author *e-mail: [email protected] *e-mail: [email protected] *e-mail: [email protected]

ORCID Ludger Schöttner: 0000-0003-0855-6583 Roman Ovcharenko: 0000-0002-0432-4200 Alexei Nefedov: 0000-0003-2771-6386 Elena Voloshina: 0000-0002-1799-1125 Yuemin Wang: 0000-0002-9963-5473 Joachim Sauer: 0000-0001-6798-6212 Christof Wöll: 0000-0003-1078-3304 Present addresses §

Department of Physics, Shanghai University, 200444 Shanghai, P. R. China

Notes The authors declare no competing financial interest. These

authors contributed equally to this work.

ACKNOWLEDGMENT The Helmholtz-Research-School “Energy-related-catalysis” is gratefully acknowledged for providing financial support and the donation of a PhD-Scholarship to L.S. We acknowledge funding from the “Science and Technology of Nanosystems” Program (432202). The authors thank Helmholtz-Zentrum Berlin for the allocation of synchrotron radiation beamtime at BESSY II. The authors thank the German Research Foundation (DFG) for financial support


29


within

the

Collaborative

Research

Centre

(SFB)

1109

Page 30 of 36

and

the

North-German

Supercomputing Alliance (HLRN) for providing computer time. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Content: Additional XPS, NEXAFS, and polarization-resolved IRRAS analysis; a complete database of the computational results (PDF)


30

Page 31 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


REFERENCES 1. Thiel, P. A.; Madey, T. E., The interaction of water with solid surfaces: Fundamental aspects. Surf. Sci. Rep. 1987, 7, 211-385. 2. Henderson, M. A., The interaction of water with solid surfaces: Fundamental aspects revisited. Surf. Sci. Rep. 2002, 46, 1-308. 3. Hodgson, A.; Haq, S., Water adsorption and the wetting of metal surfaces. Surf. Sci. Rep. 2009, 64, 381-451. 4. Iandolo, B.; Hellman, A., The role of surface states in the oxygen evolution reaction on hematite. Angew. Chem. Int. Ed. 2014, 53, 13404-13408. 5. Tamirat, A. G.; Rick, J.; Dubale, A. A.; Su, W.-N.; Hwang, B.-J., Using hematite for photoelectrochemical water splitting: a review of current progress and challenges. Nanoscale Horiz. 2016, 1, 243-267. 6. Pan, H.; Meng, X.; Qin, G., Hydrogen generation by water splitting on hematite (0001) surfaces: first-principles calculations. Phys. Chem. Chem. Phys. 2014, 16, 2544225448. 7. Zhang, X.; Klaver, P.; van Santen, R.; van de Sanden, M. C. M.; Bieberle-Hütter, A., Oxygen evolution at hematite surfaces: The impact of structure and oxygen vacancies on lowering the overpotential. J. Phys. Chem. C 2016, 120, 18201-18208. 8. Yatom, N.; Neufeld, O.; Caspary Toroker, M., Toward settling the debate on the role of Fe2O3 surface states for water splitting. J. Phys. Chem. C 2015, 119, 24789-24795. 9. Khan, S. U. M.; Akikusa, J., Photoelectrochemical splitting of water at nanocrystalline n-Fe2O3 thin-film electrodes. J. Phys. Chem. B 1999, 103, 7184-7189. 10. Kay, A.; Cesar, I.; Grätzel, M., New benchmark for water photooxidation by nanostructured α-Fe2O3 films. J. Am. Chem. Soc. 2006, 128, 15714-15721. 11. McCafferty, E.; Zettlemoyer, A. C., Adsorption of water vapour on α-Fe2O3. Discuss. Faraday Soc. 1971, 52, 239-254. 12. Cornell, R. M.; Schwertmann, U., The iron oxides: structure, properties, reactions, occurrence, and uses. VCH: Weinheim ; New York, 1996; p xxxi, 573 p. 13. Zhang, W.-J.; Huo, C.-F.; Feng, G.; Li, Y.-W.; Wang, J.; Jiao, H., Dehydration of goethite to hematite from molecular dynamics simulation. J. Mol. Struct.: THEOCHEM 2010, 950, 20-26. 14. Mason, S. E.; Iceman, C. R.; Tanwar, K. S.; Trainor, T. P.; Chaka, A. M., Pb(II) adsorption on isostructural hydrated alumina and hematite (0001) surfaces: A DFT study. J. Phys. Chem. C 2009, 113, 2159-2170. 15. Cao, C.-Y.; Qu, J.; Yan, W.-S.; Zhu, J.-F.; Wu, Z.-Y.; Song, W.-G., Low-cost synthesis of flowerlike α-Fe2O3 nanostructures for heavy metal ion removal: Adsorption property and mechanism. Langmuir 2012, 28, 4573-4579. 16. Eggleston, C. M.; Hochella, M. F., The structure of hematite (001) surfaces by scanning tunneling microscopy - image interpretation, surface relaxation, and step structure. Am. Mineral. 1992, 77, 911-922. 17. Parkinson, G. S., Iron oxide surfaces. Surf. Sci. Rep. 2016, 71, 272-365. 18. Kuhlenbeck, H.; Shaikhutdinov, S.; Freund, H.-J., Well-ordered transition metal oxide layers in model catalysis – a series of case studies. Chem. Rev. 2013, 113, 3986-4034. 19. Henrich, V. E.; Cox, P. A., The surface science of metal oxides. Cambridge University Press: Cambridge ; New York, 1994; p xiv, 464 p. 20. Voloshina, E., Hematite, Its stable surface terminations and their reactivity toward water. In Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Elsevier: Amsterdam, Netherlands, 2018.


31


Page 32 of 36

21. Sakurai, S.; Namai, A.; Hashimoto, K.; Ohkoshi, S.-i., First observation of phase transformation of all four Fe2O3 phases (γ → ε → β → α-phase). J. Am. Chem. Soc. 2009, 131, 18299-18303. 22. Takei, H.; Chiba, S., Vacancy ordering in epitaxially-grown single crystals of γFe2O3. J. Phys. Soc. Jpn. 1966, 21, 1255-1263. 23. Blake, R.; Hessevick, R.; Zoltai, T.; Finger, L. W., Refinement of the hematite structure. Am. Mineral. 1966, 51, 123-129. 24. Hartman, P., The effect of surface relaxation on crystal habit: Cases of corundum (αAl2O3) and hematite (α-Fe2O3). J. Cryst. Growth 1989, 96, 667-672. 25. Ovcharenko, R.; Voloshina, E.; Sauer, J., Water adsorption and O-defect formation on Fe2O3(0001) surfaces. Phys. Chem. Chem. Phys. 2016, 18, 25560-25568. 26. Nguyen, M.-T.; Seriani, N.; Gebauer, R., Water adsorption and dissociation on αFe2O3(0001): PBE+U calculations. J. Chem. Phys. 2013, 138, 194709. 27. Wang, X. G.; Weiss, W.; Shaikhutdinov, S. K.; Ritter, M.; Petersen, M.; Wagner, F.; Schlogl, R.; Scheffler, M., The hematite (alpha-Fe2O3) (0001) surface: Evidence for domains of distinct chemistry. Phys. Rev. Lett. 1998, 81, 1038-1041. 28. Rohrbach, A.; Hafner, J.; Kresse, G., Ab initio study of the (0001) surfaces of hematite and chromia: Influence of strong electronic correlations. Phys. Rev. B 2004, 70, 125426. 29. Trainor, T. P.; Chaka, A. M.; Eng, P. J.; Newville, M.; Waychunas, G. A.; Catalano, J. G.; Brown Jr, G. E., Structure and reactivity of the hydrated hematite (0001) surface. Surf. Sci. 2004, 573, 204-224. 30. Bergermayer, W.; Schweiger, H.; Wimmer, E., Ab initio thermodynamics of oxide surfaces: O2 on Fe2O3(0001). Phys. Rev. B 2004, 69, 195409. 31. Barbier, A.; Stierle, A.; Kasper, N.; Guittet, M.-J.; Jupille, J., Surface termination of hematite at environmental oxygen pressures: Experimental surface phase diagram. Phys. Rev. B 2007, 75, 233406. 32. Lemire, C.; Bertarione, S.; Zecchina, A.; Scarano, D.; Chaka, A.; Shaikhutdinov, S.; Freund, H.-J., Ferryl (Fe=O) Termination of the Hematite α−Fe2O3(0001) Surface. Phys. Rev. Lett. 2005, 94, 166101. 33. Kiejna, A.; Pabisiak, T., Mixed Termination of hematite (α-Fe2O3)(0001) surface. J. Phys. Chem. C 2013, 117, 24339-24344. 34. Noguera, C., Polar oxide surfaces. J. Phys.: Condens. Matter 2000, 12, R367. 35. Thevuthasan, S.; Kim, Y.; Yi, S.; Chambers, S.; Morais, J.; Denecke, R.; Fadley, C.; Liu, P.; Kendelewicz, T.; Brown Jr, G., Surface structure of MBE-grown α-Fe2O3 (0001) by intermediate-energy X-ray photoelectron diffraction. Surf. Sci. 1999, 425, 276-286. 36. Yin, S.; Ma, X.; Ellis, D. E., Initial stages of H2O adsorption and hydroxylation of Fe-terminated α-Fe2O3(0001) surface. Surf. Sci. 2007, 601, 2426-2437. 37. Wasserman, E.; Rustad, J. R.; Felmy, A. R.; Hay, B. P.; Halley, J. W., Ewald methods for polarizable surfaces with application to hydroxylation and hydrogen bonding on the (012) and (001) surfaces of α-Fe2O3. Surf. Sci. 1997, 385, 217-239. 38. Kerisit, S., Water structure at hematite–water interfaces. Geochim. et Cosmochim. Acta 2011, 75, 2043-2061. 39. Lützenkirchen, J.; Preočanin, T.; Stipić, F.; Heberling, F.; Rosenqvist, J.; Kallay, N., Surface potential at the hematite (001) crystal plane in aqueous environments and the effects of prolonged aging in water. Geochim. et Cosmochim. Acta 2013, 120, 479-486. 40. Boily, J.-F.; Chatman, S.; Rosso, K. M., Inner-Helmholtz potential development at the hematite (α-Fe2O3) (001) surface. Geochim. et Cosmochim. Acta 2011, 75, 4113-4124. 41. Catalano, J. G., Weak interfacial water ordering on isostructural hematite and corundum (001) surfaces. Geochim. et Cosmochim. Acta 2011, 75, 2062-2071.


32

Page 33 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


42. Kurtz, R. L.; Henrich, V. E., Surface electronic structure and chemisorption on corundum transition-metal oxides: α-Fe2O3. Phys. Rev. B 1987, 36, 3413. 43. Hendewerk, M.; Salmeron, M.; Somorjai, G. A., Water adsorption on the (001) plane of Fe2O3: An XPS, UPS, Auger, and TPD study. Surf. Sci. 1986, 172, 544-556. 44. Junta-Rosso, J. L.; Hochella, M. F., The chemistry of hematite 001 surfaces. Geochim. et Cosmochim. Acta 1996, 60, 305-314. 45. Yamamoto, S., et al., Water adsorption on α-Fe2O3(0001) at near ambient conditions. J. Phys. Chem. C 2010, 114, 2256-2266. 46. Wang, Y.; Wöll, C., IR spectroscopic investigations of chemical and photochemical reactions on metal oxides: bridging the materials gap. Chem. Soc. Rev. 2017, 46, 1875-1932. 47. Nefedov, A.; Wöll, C., Advanced applications of NEXAFS spectroscopy for functionalized surfaces. In Surface Science Techniques, Springer: 2013; pp 277-303. 48. Lübbe, M.; Moritz, W., A LEED analysis of the clean surfaces of α-Fe2O3 (0001) and α-Cr2O3 (0001) bulk single crystals. J. Phys.: Condens. Matter 2009, 21, 134010. 49. Nguyen, M. T.; Seriani, N.; Gebauer, R., Defective α‐Fe2O3 (0001): An ab initio study. ChemPhysChem 2014, 15, 2930-2935. 50. Muan, A., Phase equilibria at high temperatures in oxide systems involving changes in oxidation states. Am. J. Sci. 1958, 256, 171-207. 51. Kurtz, R. L.; Henrich, V. E., Geometric structure of the α-Fe2O3(001) surface: A LEED and XPS study. Surf. Sci. 1983, 129, 345-354. 52. Ketteler, G.; Weiss, W.; Ranke, W.; Schlögl, R., Bulk and surface phases of iron oxides in an oxygen and water atmosphere at low pressure. Phys. Chem. Chem. Phys. 2001, 3, 1114-1122. 53. Shaikhutdinov, S. K.; Weiss, W., Oxygen pressure dependence of the α-Fe2O3(0001) surface structure. Surf. Sci. 1999, 432, 627-634. 54. Chambers, S. A.; Yi, S. I., Fe termination for α-Fe2O3(0001) as grown by oxygenplasma-assisted molecular beam epitaxy. Surf. Sci. 1999, 439, 785-791. 55. Liu, P.; Kendelewicz, T.; Brown, G. E.; Nelson, E. J.; Chambers, S. A., Reaction of water vapor with α-Al2O3(0001) and α-Fe2O3(0001) surfaces: synchrotron X-ray photoemission studies and thermodynamic calculations. Surf. Sci. 1998, 417, 53-65. 56. Lad, R. J.; Henrich, V. E., Structure of α-Fe2O3 single crystal surfaces following Ar+ ion bombardment and annealing in O2. Surf. Sci. 1988, 193, 81-93. 57. Condon, N. G.; Murray, P. W.; Leibsle, F. M.; Thornton, G.; Lennie, A. R.; Vaughan, D. J., Fe3O4(111) termination of α-Fe2O3(0001). Surf. Sci. 1994, 310, 609-613. 58. Paparazzo, E., XPS analysis of iron aluminum oxide systems. Appl. Surf. Sci. 1986, 25, 1-12. 59. Brundle, C. R.; Chuang, T. J.; Wandelt, K., Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron. Surf. Sci. 1977, 68, 459-468. 60. Tang, Y.; Qin, H.; Wu, K.; Guo, Q.; Guo, J., The reduction and oxidation of Fe2O3(0001) surface investigated by scanning tunneling microscopy. Surf. Sci. 2013, 609, 67-72. 61. Yang, C.; Yu, X.; Heißler, S.; Nefedov, A.; Colussi, S.; Llorca, J.; Trovarelli, A.; Wang, Y.; Wöll, C., Surface faceting and reconstruction of ceria nanoparticles. Angew. Chem. Int. Ed. 2017, 56, 375-379. 62. Yang, C.; Yu, X.; Heißler, S.; Weidler, P. G.; Nefedov, A.; Wang, Y.; Wöll, C.; Kropp, T.; Paier, J.; Sauer, J., O2 activation on ceria catalysts – the importance of substrate crystallographic orientation. Angew. Chem. Int. Ed. 2017, 56, 16399-16404. 63. Liu, S.; Wang, S.; Guo, J.; Guo, Q., Polarity and surface structural evolution of iron oxide films. RSC Adv. 2012, 2, 9938-9943.


33


Page 34 of 36

64. Camillone III, N.; Adib, K.; Fitts, J. P.; Rim, K. T.; Flynn, G. W.; Joyce, S. A.; Osgood, R. M., Surface termination dependence of the reactivity of single crystal hematite with CCl4. Surf. Sci. 2002, 511, 267-282. 65. Wu, Z.; Gota, S.; Jollet, F.; Pollak, M.; Gautier-Soyer, M.; Natoli, C., Characterization of iron oxides by x-ray absorption at the oxygen K edge using a full multiple-scattering approach. Phys. Rev. B 1997, 55, 2570. 66. Kresse, G.; Furthmüller, J., Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15-50. 67. Kresse, G.; Hafner, J., Norm-conserving and ultrasoft pseudopotentials for first-row and transition elements. J. Phys.: Condens. Matter 1994, 6, 8245. 68. Kresse, G.; Hafner, J., Ab initio molecular dynamics for liquid metals. Phys. Rev. B 1993, 47, 558. 69. Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865-3868. 70. Blöchl, P. E., Projector augmented-wave method. Phys. Rev. B 1994, 50, 1795317979. 71. Blöchl, P. E.; Jepsen, O.; Andersen, O. K., Improved tetrahedron method for Brillouin-zone integrations. Phys. Rev. B 1994, 49, 16223-16233. 72. Anisimov, V. I.; Aryasetiawan, F.; Lichtenstein, A. I., First-principles calculations of the electronic structure and spectra of strongly correlated systems: The LDA + U method. J. Phys.: Condens. Matter 1997, 9, 767. 73. Dudarev, S. L.; Botton, G. A.; Savrasov, S. Y.; Humphreys, C. J.; Sutton, A. P., Electron-energy-loss spectra and the structural stability of nickel oxide: An LSDA+U study. Phys. Rev. B 1998, 57, 1505-1509. 74. Grimme, S., Semiempirical GGA-type density functional constructed with a longrange dispersion correction. J. Comput. Chem. 2006, 27, 1787-1799. 75. Kerber, T.; Sierka, M.; Sauer, J., Application of semiempirical long-range dispersion corrections to periodic systems in density functional theory. J. Comput. Chem. 2008, 29, 2088-2097. 76. Dementyev, P.; Dostert, K.-H.; Ivars-Barceló, F.; O'Brien, C. P.; Mirabella, F.; Schauermann, S.; Li, X.; Paier, J.; Sauer, J.; Freund, H.-J., Water interaction with iron oxides. Angew. Chem. Int. Ed. 2015, 54, 13942-13946. 77. Mirabella, F.; Zaki, E.; Ivars-Barceló, F.; Li, X.; Paier, J.; Sauer, J.; Shaikhutdinov, S.; Freund, H.-J., Cooperative formation of long-range ordering in water ad-layers on Fe3O4(111) surfaces. Angew. Chem. Int. Ed. 2018, 57, 1409-1413. 78. Leist, U.; Ranke, W.; Al-Shamery, K., Water adsorption and growth of ice on epitaxial Fe3O4(111), FeO(111) and Fe2O3(biphase). Phys. Chem. Chem. Phys. 2003, 5, 2435-2441. 79. Morin, F. J., Electrical properties of αFe2O3 and αFe2O3 containing titanium. Phys. Rev. 1951, 83, 1005. 80. Patapis, S. K.; Alexopoulos, K., Thermoelectric power of hematite at the Morin transition. Phys. stat. sol. (a) 1984, 81, 57-61. 81. Clarke, N.; Hall, P., Adsorption of water vapor by iron oxides. 2. Water isotherms and X-ray photoelectron spectroscopy. Langmuir 1991, 7, 678-682. 82. Powell, C. J.; Jablonski, A., Progress in quantitative surface analysis by X-ray photoelectron spectroscopy: Current status and perspectives. J. Elect. Spectrosc. Rel. Phenomena 2010, 178, 331-346. 83. Chambers, S. A.; Joyce, S. A., Surface termination, composition and reconstruction of Fe3O4(001) and γ-Fe2O3(001). Surf. Sci. 1999, 420, 111-122.


34

Page 35 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


84. Cutting, R. S.; Muryn, C. A.; Vaughan, D. J.; Thornton, G., Substrate-termination and H2O-coverage dependent dissociation of H2O on Fe3O4(111). Surf. Sci. 2008, 602, 11551165. 85. Allen, G. C.; Curtis, M. T.; Hooper, A. J.; Tucker, P. M., X-Ray photoelectron spectroscopy of iron–oxygen systems. J. Chem. Soc., Dalton Trans. 1974, 1525-1530. 86. Paier, J., Hybrid density functionals applied to complex solid catalysts: successes, limitations, and prospects. Cat. Lett. 2016, 146, 861-885. 87. Kimmel, G. A.; Baer, M.; Petrik, N. G.; VandeVondele, J.; Rousseau, R.; Mundy, C. J., Polarization- and azimuth-resolved infrared spectroscopy of water on TiO2(110): Anisotropy and the hydrogen-bonding network. J. Phys. Chem. Lett. 2012, 3, 778-784. 88. Gilli, G.; Gilli, P., Towards an unified hydrogen-bond theory. J. Mol. Struct. 2000, 552, 1-15. 89. Person, W. B.; Newton, J. H., Dipole moment derivatives and infrared intensities. I. Polar tensors. J. Chem. Phys. 1974, 61, 1040-1049. 90. Werner, H. J.; Rosmus, P.; Reinsch, E. A., Molecular properties from MCSCF‐SCEP wave functions. I. Accurate dipole moment functions of OH, OH−, and OH+. J. Chem. Phys. 1983, 79, 905-916. 91. Wang, Y.; Meyer, B.; Yin, X.; Kunat, M.; Langenberg, D.; Traeger, F.; Birkner, A.; Wöll, C., Hydrogen induced metallicity on the ZnO(10-10) Surface. Phys. Rev. Lett. 2005, 95, 266104. 92. Qiu, H.; Meyer, B.; Wang, Y.; Wöll, C., Ionization energies of shallow donor states in ZnO created by reversible formation and depletion of H interstitials. Phys. Rev. Lett. 2008, 101, 236401. 93. Yin, X. L.; Calatayud, M.; Qiu, H.; Wang, Y.; Birkner, A.; Minot, C.; Wöll, C., Diffusion versus desorption: complex behavior of H atoms on an oxide surface. ChemPhysChem 2008, 9, 253-256. 94. Giordano, L.; Pacchioni, G.; Noguera, C.; Goniakowski, J., Spectroscopic evidences of charge transfer phenomena and stabilization of unusual phases at iron oxide monolayers grown on Pt(111). Top. Catal. 2013, 56, 1074-1081. 95. Fujimori, Y.; Zhao, X.; Shao, X.; Levchenko, S. V.; Nilius, N.; Sterrer, M.; Freund, H.-J., Interaction of water with the CaO(001) surface. J. Phys. Chem. C 2016, 120, 55655576.


35


Page 36 of 36

TOC GRAPHIC


36

Interaction of Water Molecules with the Î±-Fe2O3

Recommend Documents