Dopant-Induced Diffusion Processes at MetalâOxide Interfaces

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Dopant-Induced Diffusion Processes at Metal-Oxide Interfaces Studied for Fe-and Cr-Doped MgO/Mo(001) Model Systems Stefania Benedetti, Niklas Nilius, Sergio Valeri, Sergio Tosoni, Elisa Albanese, and Gianfranco Pacchioni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04182 • Publication Date (Web): 01 Jun 2016 Downloaded from http://pubs.acs.org on June 3, 2016

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The Journal of Physical Chemistry

Dopant-Induced Diffusion Processes at Metal-Oxide Interfaces Studied for Feand Cr-Doped MgO/Mo(001) Model Systems Stefania Benedetti,1 Niklas Nilius,2,* Sergio Valeri,1 Sergio Tosoni,3 Elisa Albanese,3 Gianfranco Pacchioni 3 1

CNR, Istituto Nanoscienze, Via G. Campi 213/a, 41125 Modena, Italy

2

Carl von Ossietzky Universität, Institut für Physik, D-26111 Oldenburg, Germany

3

Dipartimento di Scienza dei Materiali, Università di Milano-Bicocca, via Cozzi 53, 20125 Milano, Italy

Abstract: X-ray photoelectron spectroscopy reveals spontaneous atom diffusion from a Mo(001) support into a MgO thin film doped with transition-metal ions. The amount of interfacial mixing depends on the nature of the dopants and is considerably larger for Fe than for Cr impurities. DFT calculations find the reason for Mo diffusion in the ability of the dopants to change oxidation state. Cr exclusively occurs as 3+ ion in the rocksalt lattice, whereby the charge mismatch to native Mg2+ ions is compensated for by Mg vacancies. Iron, on the other hand, switches its thermodynamically preferred configuration from 3+ to 2+ with increasing temperature. As a result, Mo atoms from the support move into the Mg vacancies upon sample annealing and get oxidized via charge transfer into the Fe3+ species. Our study unravels a new charge-compensation scheme in doped oxides that proceeds via chemical intermixing at a metal-oxide interface. The mechanism may rationalize the often observed inactivity of doped oxides in charge-transfer reactions.

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INTRODUCTION Doping has proven to be a versatile means to tailor the physical and chemical properties of oxides.1,2 The insertion of aliovalent dopants, i.e. with a charge state different from the host ions, perturbs the electron balance and produces excess charges in the material. High- (Cr, Mo, Fe) and low-valence ions (Li, Na, K) substituting cationic species hereby release electrons and holes to the lattice, respectively.3 The opposite effect is revealed for anion doping, where impurities with higher (N) and lower valence (F) as compared to oxygen introduce hole- and electron-states in the oxide band-structure.4,5 Dopants therefore influence the electronic, optical and chemical response of the host oxide. They weaken internal lattice bonds,6 enable reinforced interactions with adsorbates7,8 and help activating and dissociating molecular species;9 processes that often result in an enhanced reactivity of the material. Moreover, the dopants energy-levels typically reside inside the oxide band gap, which increases the absorption for visible light and imprints a p- or n-type conductance behavior on the material.4,10,11 Despite these benefits, the practical use of doped oxides is often limited due to strong and unpredictable neutralization effects of the dopants.12,13 In experiments, charge centers develop in much lower quantities as expected from the dopant concentration and are often unable to cycle electrons in a redox-like manner.3,14 The reason is found in a structural and/or electronic reorganization of the oxide lattice in response to the dopantinduced charge imbalance.15,16 While the insertion of O vacancies removes electron deficiencies, canceling the impact of electron-poor dopants,17 cationic defects are able to trap the extra electrons released by highvalence dopants. In both cases, the excess charges inserted via doping are neutralized and the anticipated charge-transfer characteristic of the oxide vanishes. The energetically-driven incentive to compensate for excess charges has, however, much wider implications for oxide materials. In fact, not only native defects but also impurity ions are able to remove the charge imbalance due to doping. Charge compensation can be realized deliberately by adding co-dopants with opposite polarity, e.g. Li ions to Cr-doped MgO.18 While the two dopant-species indeed stabilize each other, the desired redox character of the oxide disappears. Another, more distant example concerns the spontaneous hydroxylation of polar oxides, where negative surface charges get removed by attaching protons from heterolytic water splitting.19 A charge-driven adsorption behavior is also found for larger molecules that preferentially bind to electron-rich or electron-poor surface regions depending on their electronegativity.20 Largely unexplored is the effect of self-doping at metal-oxide interfaces, where the chemical composition of the oxide changes due to atom/ion diffusion from the adjacent metal. This stimulated diffusion opens an alternative pathway to remove excess charges from the system at little energy cost. In our study, we demonstrate the effect of charge-driven chemical mixing for a MgO thin film grown on a Mo(001) support. The initial charge imbalance is adjusted by doping the oxide layer with either Cr or Fe ions. While Fe impurities induce massive Mo diffusion from the support into the MgO, no chemical mixing is observed in the presence of Cr. On the basis of DFT calculations, we connect this unintuitive result to different pathways to reach charge equilibrium in the doped oxide. Our observations are of general importance, as they unravel a yet unknown charge-compensation mechanism occurring at metal-oxide interfaces. Although the phenomenon is 2 ACS Paragon Plus Environment

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discussed for metal-supported thin films here, it also affects the chemical composition of supported metal catalysts as used in heterogeneous catalysis. Moreover, it provides a possible explanation for the limited reactivity of oxide catalysts, despite a presuming large number of charge centers introduced via doping.

EXPERIMENT AND THEORY All experiments have been performed in an ultrahigh-vacuum setup (base pressure: 10-10 mbar), equipped with a hemispherical electron analyzer and a non-monochromatized Al source for photoelectron spectroscopy (XPS). The oxide films were grown by room-temperature Mg deposition at 4 Å/min rate onto a sputtered and annealed Mo(001) single crystal in 5×10-7 mbar oxygen atmosphere.21 Different local O2 pressures were adjusted by a nozzle, and will be denoted as oxygen-poor and oxygen-rich preparation conditions for O:Mg ratios of 50 and 150, respectively. Fe or Cr atoms from two thermal evaporators were added to the growth process at 0.02-0.7 Å/min deposition rate. The films were converted to a crystalline state by vacuumannealing at 1050 K, resulting in a sharp square pattern in low-energy electron diffraction.22 The film thickness was set to 30 ML in all experiments. The chemical composition, especially the dopant concentration, was revealed from XP spectra, quantitatively analyzed via Shirley background subtraction and Gaussian/Doniach-Sunjic fitting. The electronic structure of doped MgO was computed with CRYSTAL14, using the PBE0 functional at full spin polarization.23 The system was described with the 8-411(d) basis set for O, 8-511(d) for Mg, 86-411 (d41) for Cr and Fe and the HAYWSC-311(d31) set for Mo.24 A Monkhorst-Pack grid at ten special k-points was used to sample the reciprocal space, setting the SCF convergence threshold to 10-6 Hartree. The bulk MgO lattice was described with a 4×4×3 supercell, containing 48 O and 48 Mg ions (Fig. 1). Two dopants (either Fe, Cr or Mo) were introduced at Mg lattice sites with maximum mutual distance at the following coordinates: M1 (0, 0, 0), M2 (-0.5, -0.5, 0.33). To ensure charge neutrality in presence of two 3+ dopants, one Mg vacancy was included in the cell at (0.25, 0.25, -0.33), while a second (0.25, -0.50, 0.33) and third defect (0.00, -0.25, 0.00) was added for two Mo ions in the +4 / +5 oxidation state, respectively. The atomic positions in the cell were allowed to relax for each dopant configuration.

Fig. 1 Mg48-xO48 supercell as used in the calculations. The cell contains two dopants and one compensating Mg vacancy; the O and Mg ions are depicted in red and grey, respectively.



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RESULTS AND DISCUSSION

X-ray Photoelectron Spectroscopy Before addressing the properties of doped oxides, we present XP spectra of pristine, 30 ML-thick MgO films grown on Mo(001). Besides intense Mg2p and O1s core level peaks, appearing at the reported binding energies (BE) for bulk MgO,25 complex Mo3d spectra with at least four maxima are detected (Fig. 2a). The lowenergy peaks at 227.8 and 231.0 eV represent the Mo3d 5/2 and 3/2 doublet of metallic Mo from the support. As these peaks dominate the spectra for as-grown films, the development of a MoOx interface oxide can be neglected after reactive Mg deposition. Upon annealing to 1050 K, two additional Mo3d-related maxima emerge at 234 and 237 eV, i.e. at BEs that are higher than the literature values for Mo6+ ions.26,27 The peaks gain intensity at grazing emission, suggesting that Mo species from the support have entered the oxide film and accumulated in a near-surface region. Their concentration in the film has been determined to 0.15 at% by means of an exponential attenuation model. The Moδ+ signature strongly attenuates if the MgO film is doped with 3-12 at% Cr (Fig. 2b). In fact, MoOx species can only be detected at very high doping concentrations (33 at% Cr); however we doubt that MgO is still present in the original rocksalt structure in this case. The Cr impurities themselves show up as an intense Cr2p doublet at 578/588 eV (Fig. 3a). This value perfectly matches the BE of Cr3+ ions in the MgO matrix, as discussed in an earlier study.22 Deviations from the 3+ charge state are only found at very high Cr concentration, when a faint shoulder, characteristic for Cr6+ species, appears next to the main peak.26 We assign this spectral feature to a Cr-rich surface oxide that decomposes at high temperature. For details of the structure and chemical properties of MgOCr films, the reader is referred to our previous paper.22

Fig. 2: Mo3d peaks measured at grazing emission for (a) bare Mo(001) and non-doped MgO, (b) Cr- and (c) Fe-doped MgO films annealed to 1050 K. Spectra denoted with ‘O-poor’ and ‘O-rich’ refer to MgO films prepared with an O:Mg ratio of 50 and 150, respectively. In samples denoted with ‘buffer’, the doped MgO was separated from the Mo support by 10 ML of bare oxide.

An entirely different picture arises for MgO films doped with 3 at% Fe and subsequently annealed to 1050 K. In contrast to Cr, the Fe dopants strongly enhance Mo segregation from the support, as reflected by pronounced Mo3d level peaks (Fig. 2c). Both, intensity and energy of the Mo levels depend on the effective O2 pressure during oxidation. At oxygen-poor conditions, a relatively weak Mo3d doublet appears at 233 eV BE for the 5/2 component, which doubles in intensity and redshifts to 232.5 eV for preparations in oxygen excess, 4 ACS Paragon Plus Environment

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i.e. at an O:Mg ratio of 150. Although these BE values are 1.0-1.5 eV lower than for the Moδ+ species in pristine MgO, they lie inside the energy range that is typically assigned to Mo6+ species.27 The associated Mo concertation has been determined to 0.1 and 1.3 at% for MgOFe films prepared at oxygen-poor and oxygenrich conditions, respectively. Especially in the latter case, the value is considerably larger than for pristine (0.15 at%) and Cr-doped films (0.05 at%), suggesting a promoting effect of the Fe impurities on the Mo diffusion. To investigate the importance of a direct metal-oxide interface for Mo exchange, we have prepared samples in which the doped MgO was spatially separated from the metal support by a 10 ML thick oxide buffer (Fig. 2b,c). Surprisingly, the amount of embedded Mo was only slightly reduced for MgOFe and unchanged for MgOCr, implying that the driving forces for Mo diffusion are long-ranged with respect to the thickness of the buffer layer.

Fig.3: (a) Cr2p spectra taken for as-grown MgOCr films at increasing doping level. (b) Fe2p spectra for as-grown (black) and annealed (red) MgOFe films at 3 at% doping level. Deconvolution of the low-energy Fe2p peak in MgOFe prepared at oxygen-rich conditions, (c) before and (d) after annealing. Green, pink and black curves represent the Fe3+ and Fe2+ contribution as well as their shake-up satellite, respectively. Apart from the topmost, dashed curve in (a), all spectra have been acquired at normal emission.

To complete the picture, we have measured the spectral signature of Fe in the MgO films (Fig. 3b). In contrast to Cr that exclusively occurs as 3+ ion (Fig. 3a), the oxidation state of Fe varies with the oxygen pressure and annealing temperature. In as-grown films prepared at O-rich conditions, the Fe2p core-level peaks can be de-convoluted into two doublets centered at 709 and 712 eV for the low-energy component, respectively. These positions are indicative for Fe2+ and Fe3+ ions, a conclusion that is corroborated by a characteristic satellite structure next to the main peaks. For an in-depth analysis, tabulated peak positions and intensity ratios of Fe2+ (as in FeO), Fe3+ (as in Fe2O3) and respective shake-up satellites have been considered, while surface peaks are excluded as we assume a homogenous Fe distribution in the oxide.28 For as-grown films prepared at oxygen-rich conditions, a Fe3+/Fe2+ intensity ratio of 0.62 is determined, i.e. 40% of the dopants are in the 3+ oxidation state. This ratio decreases to 0.45 in oxygen-poor samples, where only 30% of the Fe ions are in the 3+ charge state. A more drastic change is induced by annealing the MgOFe samples to 1050 K. Independent of the preparation history, the Fe3+ ions completely disappear from the spectra and only the Fe2+ component remains detectable (Fig. 3d). To summarize the experimental results, Fe ions dispersed in MgO 5 ACS Paragon Plus Environment


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thin films were found to initiate strong atom diffusion from the Mo support, while hardly any interface mixing occurs in the presence of Cr dopants. In the following, DFT calculations are employed to link the effect of Mo incorporation to the mechanism of charge compensation in the doped oxide.

DFT Calculations We start our analysis with a search for the thermodynamically preferred charge state of Cr, Fe and Mo ions, located on Mg substitutional sites of a bulk MgO lattice (Fig.1). In response to the oxide crystal field, the valence d-levels of all three dopants split into a low-lying t2g and a high-lying eg manifold. Whereas Cr2+ and Fe2+ prefer a high-spin, quintuplet state, the low-spin, triplet solution is favorable for Mo2+. Higher oxidation states are adopted in presence of Mg vacancies in the lattice that take up the excess electrons.3 Each Mg defect hereby generates a deficiency of two electrons and therefore compensates for either two 3+ dopants or an impurity-pair in the 2+/4+ state. Even higher charge states require additional vacancies to be inserted into MgO. The free-energy for stabilizing the various defect structures at a given temperature and oxygen pressure is calculated with the ab-initio atomistic thermodynamics approach developed by Reuter and Scheffler (see supporting materials).29 The results are summarized in the stability diagrams calculated at two specific temperatures for Fe2+/Fe3+, Cr2+/Cr3+ and Mo2+/ Mo3+/Mo4+ ion configurations (Fig. 4). Apparently, Cr3+ is the only stable charge configuration in MgO at all considered temperature and pressure conditions. Iron, on the other hand, is incorporated as 3+ ion at 300 K, but gets reduced to 2+ at 1000 K and below 10-4 mbar of oxygen. The Mo phase diagrams depicts that Mo4+ is the most stable species at low temperature, while 4+/3+ reduction takes place in 60 mbar of oxygen and 1000 K.

Fig. 4. Fe2+/Fe3+, Cr2+/Cr3+ and Mo2+/ Mo3+/Mo4+ stability diagrams plotted as a function of the oxygen chemical potential at (a) T= 300 K and (b) 1000 K.

Apart from perturbing the MgO charge balance, the dopants induce moderate lattice strain, as their radius differs from the one of a substituted Mg2+ ion. The structural distortion is analyzed with supercells that contain either two D2+ (D = Cr,Mo,Fe) or two D3+ dopants plus an Mg vacancy (Fig. 1). Our approach reproduces well the experimental lattice parameter of bulk MgO (4.21 Å). Almost identical numbers are obtained for the doped oxide, pointing to an internal compensation mechanism for the dopant-induced lattice strain. For example, Cr2+ gives rise to a small lattice expansion that disappears however for Cr3+ as the effects of Cr-Mg 6 ACS Paragon Plus Environment

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substitution (expansion) and Mg-vacancy generation (reduction) cancel out. Also, Fe2+ substitution produces a small lattice expansion that vanishes again for Fe3+ due to the inserted Mg vacancy. Only Mo-Mg exchange leads to a net-increase of the MgO lattice parameter (see supporting material). Also, the distortion of the first coordination sphere around the dopants has been examined. For Cr and Fe ions, the distance to adjacent Mg2+ species slightly increases; an effect that becomes larger for Mo+3 but vanishes for Mo4+ due to its small radius. The octahedral O2- arrangement of the dopants is altered by the JahnTeller effect. For Cr2+ ions, four Cr-O distances elongate to 2.15-2.21 Å (compared to 2.10 Å in pure MgO), while the other two contract to 2.07 Å. For Cr3+, one Cr-O bond shrinks to 1.91 Å and the remaining five stay at 2.03 Å. A similar behavior is found for Fe2+ and Fe3+, although changes in the bond lengths are smaller than for Cr. Finally, Mo2+ ions lead to increased equatorial (2.22 Å) and axial (2.16 Å) Mo-O bond lengths; while Mo3+ and Mo4+ impurities break the symmetry of the rocksalt lattice with five elongated and one contracted Mo-O distance. A summary of the dopant-induced lattice distortions is available in the supporting material. In general, structural changes induced by transition-metal dopants in the MgO lattice are rather small and likely irrelevant to for the observed phenomenon of Mo incorporation.

Interpretation Based on our experimental and theoretical results, we propose the following model for the interplay between Cr and Fe doping and Mo incorporation into the MgO films. We start our discussion with Cr that does not promote Mo diffusion. According to XP spectra and DFT stability diagrams, Cr dopants exclusively occur in the 3+ charge state in the rocksalt lattice. The charge mismatch with substituted Mg2+ ions is accounted for by Mg vacancies, the formation of which requires only 1 eV-energy in presence of two Cr dopants as compared to 8 eV in pristine MgO.3 Although no experimental evidence for the existence of Mg vacancies is given here, multiple indications were acquired in earlier works. Photoluminescence measurements on MgOCr crystals revealed a strong emission line at ∼700 nm, related to an actually dipole-forbidden transition within the d-manifold of the Cr ions.30 The emission becomes visible, because Mg vacancies break the octahedral symmetry of Cr3+ ions in the rocksalt lattice and soften the dipole-selection rules. The accumulation of Mg vacancies in the MgOCr surface has also been observed directly in STM measurements.31 The compensation mechanism via Mg vacancies loses importance in the interface region of the oxide film, where the Mo(001) is available as quasi infinite reservoir for the electrons arising from Cr2+/Cr3+ oxidation. In this zone, excess electrons from the doped MgO flow directly to the support, producing an electric field whose negative side points towards the metal. The development of such interface fields has been demonstrated for MgOCr and CaO films on Mo(001),14,22 using the field-induced shifts of the oxide states as internal measure. The electric field at the metal/oxide interface now results in a diffusion barrier for the Moδ+ species that would have to move against the potential gradient. Moreover, Mo cannot be oxidized in the MgO film, as low-lying states to trap the excess electrons are absent in the stoichiometric lattice. Since neutral Mo is highly unfavorable in rocksalt MgO, Mo diffusion across the metal/oxide interface is blocked, in agreement with the experiment results. Structural aspects, on the other hand, would favor Mo incorporation into MgO, 7 ACS Paragon Plus Environment


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as the concomitant lattice expansion helps releasing the misfit strain induced by the Mo support (Table S2, Supporting Material). However the energy gain (1% mismatch reduction at 5% Mo doping level) seems insufficient to override the costs for transporting cationic Mo species against the electric field. Note that strong Mo diffusion has been observed upon high-temperature annealing of a CaO/Mo(001) film. The partial MoCa substitution in that case leads to a drastic reduction of the CaO lattice parameter, which results in a better lattice match with the Mo support and promotes the diffusion process.32 The situation for Fe-doped MgO seems comparable to the MgOCr case at first glance. Also Fe favors a 3+ oxidation state in the rocksalt lattice and initiates a vacancy-driven compensation scheme at low temperature. However, the preference for a 3+ charge state is smaller than for Cr, as suggested by experiments performed at varying O2 pressures. Fe3+ ions are abundant only at oxygen-rich conditions, at which the development of O vacancies is thermodynamically unfavorable while compensating Mg defects are easily inserted into the lattice. The equilibrium between Mg and O vacancies shifts towards the latter in oxygen-poor samples. As a result, the excess electrons arising from Fe3+ formation are only poorly stabilized by Mg vacancies there and more dopants adopt a 2+ state that does not require internal charge compensation. This trend becomes evident from the observed decrease of the Fe3+/Fe2+ ratio from 0.62 to 0.45 when going from oxygen-rich to oxygen-poor preparations, respectively.

Fig. 5. Schemes illustrating the interplay between dopants, cationic vacancies and Mo diffusion in bare and doped MgO. Charge-transfer processes within the film are depicted by red arrows. Filling of Mg vacancies with Mo ions from the support is only possible in presence of reducible Fe3+ ions.

The comparable stability of Fe2+ and Fe3+ ions opens however a new route for charge compensation at elevated temperature. Upon annealing, Mo species from the support may move into the lattice by filling up the Mg vacancies, being introduced to compensate for the Fe3+ ions in the as-grown films. As neutral Mo is energetically unfavorable in rocksalt MgO, it becomes a strong reducing agent for the Fe3+. As a consequence, Mo0 oxidizes to energetically favorable Mo3+ and Mo4+ ions while Fe3+ gets reduced to Fe2+ (Fig. 5). This process is thermodynamically feasible, as the Fe2+ ions are stable in the oxide lattice at 1000 K (Fig. 4). When lowering the temperature again, the Fe dopants remain in the 2+ charge state, as re-oxidation is impossible in absence of Mg vacancies. Also Mo reduction is inhibited, because the lowest unoccupied Mo3+ state is higher in energy than the highest occupied Fe2+ level. This has been shown by an unrestricted Hartree-Fock calculation that finds a Fe3+/Fe3+/Mo2+ dopant configuration, plus one Mg vacancy for the charge neutrality, 8 ACS Paragon Plus Environment

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to be 1.67 eV higher in energy than a Fe2+/Fe3+/Mo3+ structure. Evidently, Mo2+→Fe3+charge transfer is possible while the revers process is not. An analogous calculation performed at the DFT level did not converge due to the intrinsic instability of the Fe3+/Fe3+/Mo2+ solution. The above scenario explains our main experimental results, i.e. the Mo incorporation into annealed MgOFe films and the exclusive appearance of Fe2+ ions. It also accounts for the close connection between the O2 pressure during MgO growth and the amount of incorporated Mo. Remind that Fe3+ ions and their compensating Mg defects preferably develop at oxygen-rich conditions, while Fe2+ species are mainly formed at low O2 pressure. Both, Mg vacancies and Fe3+ ions are now required for Mo diffusion into the MgO lattice. Whereas the vacancies offer potential binding sites for the Mo species, the Fe3+ ions enable their oxidation to energetically stable entities. Hereby, four Fe3+ produce two Mg vacancies, but can oxidize only one incorporated Mo atom to a 4+ ion. Therefore, not all defects can be filled with Mo. Our experimental data indicate a Moδ+/Fe3+ ratio slightly below one in oxygen-rich samples, suggesting that Mo oxidation may proceed not only via Fe3+ reduction and probably involves the Mo support. A complete analysis of the charge flow inside the oxide film cannot be given here due to uncertainties of the Moδ+ oxidation state in the MgO film. Although tabulated XPS data suggest a formal 6+ state for the Mo ions in MgOFe, full Mo oxidation is unlikely due to the limited reduction strength of the Fe3+ dopants. Moreover, the experimental BE of Mo impurities in pristine MgO points to an oxidation state even beyond +6, in conflict with the laws of chemistry. We therefore doubt that literature values for bulk Mo compounds are suitable guides to determine the oxidation state of single Mo ions in the MgO matrix and have calculated Mo3d BEs for differently charged Mo ions from the respective Kohn-Sham eigenvalues (see supporting information). For a given oxidation state, we find larger shifts of the Mo3d BE for matrix-embedded Mo than for charge-equivalent bulk-compounds, e.g. for Mo6+ species in MoO3. Moreover, computed shifts are smaller for a 3+/4+ as compared to a 4+/6+ charge switch, regardless of the matrix. The results call for a reinterpretation of the experimental Mo3d spectra, in which a BE of 232.5 eV as detected in MgOFe is assigned to Mo3+, while Mo ions in the non-doped films (234 eV BE) are likely in the 4+ state. The new picture also reduces the discrepancy between the number of Fe dopants and embedded Mo ions in MgO thin films.

CONCLUSIONS The charge imbalance inherent to doped oxides initiates spontaneous atom diffusion from a Mo(001) support into a MgO thin film. The amount of Mo incorporation was found to depend on the redox-properties of the dopants. While Cr only appears in the 3+ oxidation state in MgO, Fe takes a 2+ or 3+ charge depending on the oxygen chemical potential, i.e. on the easiness to insert compensating Mg vacancies. The multivalent nature of Fe enables the insertion of Mo into the MgO lattice at strongly reducing conditions. The impurities occupy cationic vacancies and get oxidized by reducing the Fe3+ to Fe2+ species. A similar scheme is unavailable for Cr that cannot exchange electrons with Mo in the rocksalt lattice. Our study has several implications for the doping of oxide materials. It unravels a yet unknown charge-compensation mechanism in doped oxides that proceeds via chemical intermixing with an adjacent metal, either in form of a bulk support or of na9 ACS Paragon Plus Environment


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noparticles. The associated composition change might explain the low reactivity that is sometimes revealed for doped oxides. On the other hand, the stimulated diffusion opens an interesting path to tailor redox properties of oxides. As demonstrated here, MgO films containing only Fe2+ ions can be prepared, although Fe3+ is the energetically favored species in the rocksalt lattice. The electron-rich Fe2+ ions introduce a strong donor character to the oxide, in contrast to Fe3+ and Cr3+ ions that are electrically inactive. Engineering the charge state of impurities via co-doping might therefore improve the chemical response of oxide catalysts in chargetransfer reactions.

SUPPORTING INFORMATION for electronic-structure calculations of all considered impurity ions, a compilation of dopant-induced structural changes in MgO and calculated core-level binding energies for differentially charged Mo ions.

■ AUTHOR INFORMATION Corresponding Author: [email protected], phone: 49-441-798-3152 Notes: The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS: Support from the COST Action CM1104 and FIRB project RBAP115AYN is gratefully acknowledged. N.N. is grateful for financial support via the DFG grant ‘Exploring photocatalytic processes at the atomic scale’.

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TOC:



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Figure_1 120x125mm (96 x 96 DPI)

ACS Paragon Plus Environment

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Figure_2 135x53mm (300 x 300 DPI)



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Figure_3 130x53mm (300 x 300 DPI)


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Figure_4 132x95mm (250 x 250 DPI)



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Figure_5 128x43mm (300 x 300 DPI)


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TOC Graphic 85x38mm (300 x 300 DPI)


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