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ACS Appl Mater Interfaces
ACS Appl Mater Interfaces
am
aamick
ACS Applied Materials & Interfaces
1944-8244
1944-8252
American Chemical Society

39167683
10.1021/acsami.4c05542
Research Article
Electronic and Structural Properties of Thin Iron Oxide Films on CeO2
https://orcid.org/0000-0002-5762-1030
Piliai Lesia †#
https://orcid.org/0000-0003-3763-5766
Castro-Latorre Pablo ‡#
https://orcid.org/0009-0000-6734-2724
Pchálek František †
https://orcid.org/0009-0005-2442-0661
Oveysipoor Shiva †
https://orcid.org/0000-0002-6983-4068
Kosto Yuliia †§
https://orcid.org/0000-0003-2929-4148
Khalakhan Ivan †
https://orcid.org/0000-0003-2909-9422
Skála Tomáš †
https://orcid.org/0000-0002-5242-5567
Neyman Konstantin M. ‡∥
https://orcid.org/0000-0002-3139-6189
Alemany Pere ‡
https://orcid.org/0000-0001-8382-7027
Vorochta Michael †
https://orcid.org/0000-0003-2585-5542
Bruix Albert *‡
https://orcid.org/0000-0002-1567-6930
Matvija Peter *†
https://orcid.org/0000-0001-6808-7809
Matolínová Iva †
† Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, Prague 8 180 00, Czech Republic
‡ Departament de Ciència de Materials i Química Física and Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Barcelona 08028, Spain
§ Applied Physics and Semiconductor Spectroscopy, Brandenburg University of Technology Cottbus-Senftenberg, Konrad-Zuse-Strasse 1, Cottbus 03046, Germany
∥ ICREA (Institució Catalana de Recerca i Estudis Avançats), Barcelona 08010, Spain
* E-mail: abruix@ub.edu.
* E-mail: peter.matvija@mff.cuni.cz.
21 08 2024
04 09 2024
16 35 4685846871
04 04 2024
22 07 2024
17 06 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by/4.0/ Permits the broadest form of re-use including for commercial purposes, provided that author attribution and integrity are maintained (https://creativecommons.org/licenses/by/4.0/).

Modification of CeO2 (ceria) with 3d transition metals, particularly iron, has been proven to significantly enhance its catalytic efficiency in oxidation or combustion reactions. Although this phenomenon is widely reported, the nature of the iron–ceria interaction responsible for this improvement remains debated. To address this issue, we prepared well-defined model FeOx/CeO2(111) catalytic systems and studied their structure and interfacial electronic properties using photoelectron spectroscopy, scanning tunneling microscopy, and low-energy electron diffraction, coupled with density functional theory (DFT) calculations. Our results show that under ultrahigh vacuum conditions, Fe deposition leads to the formation of small FeOx clusters on the ceria surface. Subsequent annealing results in the growth of large amorphous FeOx particles and a 2D FeOx layer. Annealing in an oxygen-rich atmosphere further oxidizes iron up to the Fe3+ state and improves the crystallinity of both the 2D layer and the 3D particles. Our DFT calculations indicate that the 2D FeOx layer interacts strongly with the ceria surface, exhibiting structural corrugations and transferred electrons between Fe2+/Fe3+ and Ce4+/Ce3+ redox pairs. The novel 2D FeOx/CeO2(111) phase may explain the enhancement of the catalytic properties of CeO2 by iron. Moreover, the corrugated 2D FeOx layer can serve as a template for the ordered nucleation of other catalytically active metals, in which the redox properties of the 2D FeOx/CeO2(111) system are exploited to modulate the charge of the supported metals.

ceria
CeO2
iron oxide
2D layer
catalysis
STM
XPS
DFT
AgÃ¨ncia de GestiÃ³ d''Ajuts Universitaris i de Recerca 10.13039/501100003030 2021SGR00286 ''la Caixa'' Foundation 10.13039/100010434 NA Red EspaÃ±ola de SupercomputaciÃ³n NA NA European Cooperation in Science and Technology 10.13039/501100000921 CA21101 GrantovÃ¡ Agentura CeskÃ© Republiky 10.13039/501100001824 20-13573S Central European Research Infrastructure Consortium 10.13039/100020931 20217192 European Cooperation in Science and Technology 10.13039/501100000921 CA18234 Central European Research Infrastructure Consortium 10.13039/100020931 20212210 Central European Research Infrastructure Consortium 10.13039/100020931 20207146 Central European Research Infrastructure Consortium 10.13039/100020931 20207106 Ministerio de Ciencia, InnovaciÃ³n y Universidades 10.13039/100014440 RYC2021-032281-I Ministerio de Ciencia, InnovaciÃ³n y Universidades 10.13039/100014440 PRE2019-088979 Ministerio de Ciencia, InnovaciÃ³n y Universidades 10.13039/100014440 PID2022-140120OA-I00 Ministerio de Ciencia, InnovaciÃ³n y Universidades 10.13039/100014440 PID2021-128217NB-I00 Ministerio de Ciencia, InnovaciÃ³n y Universidades 10.13039/100014440 MDM-2017-0767 Ministerio de Ciencia, InnovaciÃ³n y Universidades 10.13039/100014440 CEX2021-001202-M document-id-old-9am4c05542
document-id-new-14am4c05542
ccc-price
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pmc1 Introduction

Over the past decade, cerium dioxide (ceria, CeO2) has remained a leading material for energy storage, electrocatalysis, and heterogeneous catalytic applications.1 Benefiting from the unique redox potential of the Ce4+–Ce3+ couple and the facile formation of oxygen vacancies (VO), ceria is able to provide an excellent support for noble metals which are known to exhibit high catalytic performance.2 Although noble metal-based catalysts show sufficient activity over a wide range of temperatures, their scale-up implementation is limited by their scarcity and high cost. Therefore, the development of an active and inexpensive catalyst is highly desirable.

Modification of CeO2 with transition metals (TMs) such as iron aims to enhance its catalytic performance while keeping the price of the resulting catalysts low.3 Recent studies have shown that the addition of iron to ceria significantly improves its performance in CO oxidation, NO oxidation and soot combustion.3−5 Morphologically, this can be attributed to an increased specific surface area and a higher number of open active sites.4,6 Electronically, it has been suggested that the promotion of Ce-based oxides with Fe species facilitates electron transfer between the mixed Fe2+–Ce4+ and Fe3+–Ce3+ sites, which had been identified as active centers in various catalytic processes.6,7 Furthermore, iron-modified ceria shows not only improved redox properties but also promotes the dispersion of supported metal nanoparticles and enhances the interaction of metal species with ceria supports.8

While the improved activity of Fe–Ce mixed oxide catalysts has been widely reported, the nature of the iron–ceria interaction remains controversial. First, many studies have shown that the introduction of redox-active cations into the CeO2 lattice gives rise to the formation of homogeneous solid solutions.8,9 Specifically, the presence of Fe dopant leads to strong structural distortions, resulting in a hematite-like mixed oxide10 or ceria-like solid solutions9 with a higher density of oxygen vacancies at the surface. However, findings presented by Polychronopoulou11 and others12,13 demonstrate that iron has a poor solubility in the ceria fluorite lattice compared to the rest of binary TM-doped (TM = Cu, Co, Ni, Zn) ceria systems. Second, it has been documented that the doping of low valence cations such as Fe3+ and Fe2+ in CeO2 can facilitate the formation of surface oxygen vacancies.4,7,14 The increased occurrence of oxygen vacancies in the Ce–Fe mixed oxide system can be attributed to the combined redox behavior of cerium (Ce4+/Ce3+) and iron (Fe3+/Fe2+) cations.14 However, Li et al. reported that the distribution of Fe3+ over Ce4+ sites leads to the absence of Ce3+ ions and consequently results in a low oxygen vacancy concentration.9 The absence of Ce3+ ions is also supported by theoretical calculations, which show that Fe adsorption on the stoichiometric CeO2(111) surface suppresses the formation of oxygen vacancies in the ceria lattice.15 These findings underscore the necessity for a better understanding of the Fe–CeO2 interaction.

In contrast to the majority of previous research, which has primarily focused on complex powder catalysts investigating parameters such as iron content16 and the distribution of iron ions17 in Fe-doped CeO2 structures, the present study examines more precisely defined single-crystalline systems. Despite not having exactly the same characteristics as technical catalysts based on combinations of ceria and Fe, our model systems exhibit many of the surface sites defining the catalytic properties of mixed FeOx–CeO2 systems: bare CeO2(111) facets, CeO2(111) step edges, FeOx 3D clusters, the FeOx 2D layer with edges, and the FeOx–CeO2 interface. We investigate the electronic and chemical states of these features using synchrotron radiation photoelectron spectroscopy (SRPES), resonant photoelectron spectroscopy (RPES) and conventional X-ray photoelectron spectroscopy (XPS) techniques. We explore the thermally induced morphological and structural changes of FeOx/CeO2(111), particularly the formation of a thin FeOx layer over the ceria surface, through scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) techniques. To complement and further rationalize these experimental methods, we model the FeOx/CeO2(111) interface using density functional theory (DFT) calculations, focusing on the structure of the FeOx thin film and the interaction between Fe2+/Fe3+ and Ce4+/Ce3+ redox pairs. Building upon our previous work on metal nanoparticles supported by CeO2(111) thin films,18−20 this multimodal approach not only allows us to gain valuable insights into the interactions within the FeOx–CeO2 system but also provides a deeper understanding of the structure–property relationship at the nanoscale.

2 Experimental Section

2.1 Sample Preparation

Well-ordered CeO2(111) thin films were prepared on Cu(111) and Pt(111) single crystals by the physical vapor deposition (PVD) technique. The Cu(111) (MaTecK GmbH, 99.999% purity) and Pt(111) (MaTecK GmbH, 99.99% purity) samples were cleaned by Ar+ sputtering at 300 K and annealing (750 and 950 K, respectively) until no traces of carbon or any other contaminants were found in the subsequent XPS analysis. The CeO2 thin films were grown by PVD of metallic Ce (Goodfellow GmbH, 99.9%) using an electron-beam evaporator (Tectra GmbH) in 5 × 10–7 mbar of oxygen at the deposition rate of about 0.1 monolayer (ML)/min, where 1 ML(CeO2(111)) corresponds to a 0.31 nm thick layer or about 7.9 × 1014 cm–2 of O–Ce–O groups. The substrate temperature during the CeO2(111) film growth was kept within a range of 523–723 K, depending on the desired density of step edges on the surface.21 The temperature was monitored by a K-type thermocouple attached to the back of the crystal. The thickness of the prepared layer was determined from the attenuation of the Cu 2p3/2 peak or the Pt 4f7/2 peak and was about 2.5 nm (8 ML of CeO2(111)). The surface structure of the CeO2 layer was confirmed by LEED. Fe (Goodfellow GmbH, 99.99%) was deposited by PVD from an electron-beam evaporator from an Fe rod (2 mm in diameter) onto the CeO2(111) surface. The deposition was carried out either stepwise (in the study of the growth mechanism) or in one deposition step (in the thermal stability study). The sample was grounded during the deposition. The nominal Fe thickness in Fe/CeO2(111) systems was determined from the attenuation of the Cu 2p3/2 signal from the Cu(111) substrate or the Pt 4f7/2 signal from the Pt(111) substrate. The thickness varied within the 0.3–2 ML range. Here, 1 ML = 1.72 × 1015 cm–2, which corresponds to a surface density of Fe atoms in the most stable (110) crystal plane in the Fe bcc crystal. In the case of STM measurements of the CeO2(111) surface covered exclusively by the 2D FeO layer, the amount of deposited Fe atoms was determined by considering the fraction of the surface covered by the FeO layer and its measured lattice parameter aFeO = 0.31 nm.

2.2 Synchrotron Radiation Photoelectron Spectroscopy

High-resolution SRPES and resonant photoemission spectroscopy (RPES) experiments were carried out at the Materials Science Beamline (MSB) at the Elettra synchrotron light facility in Trieste, Italy. The UHV end-station of MSB with base pressure 2 × 10–10 mbar was equipped with a multichannel electron energy analyzer (Specs Phoibos 150), a nonmonochromatized Mg Kα X-ray source (1253.6 eV), LEED optics, a sputter gun (Ar+), and a gas inlet system for O2. During the experiment, two electron-beam evaporators for the deposition of Ce and Fe metals were used.

Spectra acquisition utilizing SRPES was performed for the Ce 4d, Fe 3p, C 1s, and O 1s core levels with photon energies of 180 eV (Ce 4d and Fe 3p), 410 eV (C 1s) and 650 eV (O 1s) to keep the electron kinetic energy around 100 eV with a total resolution of about 0.5 eV. RPES analysis was done by collecting the valence band (VB) spectra at 115, 121.4, and 124.8 eV photon energies. Ce3+ valence states resonate strongly at a photon energy of 121.4 resulting in a distinctive peak at about 1.4 eV. On the other hand, Ce4+ valence states reach their maximum at 124.8 eV, with a peak at about 4.0 eV. By subtracting the off-resonant spectra (115 eV) from the resonant spectra, it is possible to determine the features of D(Ce3+) and D(Ce4+) as shown in Supporting Information. The resonant enhancement ratio (RER) which is equal to D(Ce3+)/D(Ce4+) ratio, provides information about reduction state of the surface cerium cations.18 In addition to collecting the SRPES spectra, XPS spectra of O 1s, Ce 3d, Fe 2p, and Cu 2p3/2 core levels were acquired with a total resolution of 1 eV. All spectra were recorded at normal emission (SRPES and RPES) and 20° off normal (XPS). The binding energies in the spectra obtained with synchrotron radiation were calibrated with respect to the Fermi level (EF) measured on a clean gold foil. All PES data were processed using the KolXPD fitting software and normalized to the incident photon intensity, as determined by a flux monitor. The obtained Fe 3p and Fe 2p spectra were fitted after subtraction of a composite background which consisted of a baseline spectrum acquired before Fe deposition and the Shirley background. The results of XPS Fe 2p spectra fitting for metallic Fe, Fe2O, FeO and Fe2O3 are reported in Table S1.

2.3 Scanning Tunneling Microscopy

STM experiments were performed in a UHV system (base pressure 1 × 10–10 mbar) at Charles University, Prague, Czech Republic. The system is equipped with a scanning probe microscope (Specs SPM Aarhus 150 NAP), a photoelectron spectrometer consisting of a hemispherical electron energy analyzer with a 1D line detector (Specs Phoibos 150 1D-DLD) and a monochromatized Al Kα X-ray source (μ-FOCUS 600 equipped with XR 50 MF), LEED, and a quadrupole mass spectrometer (Pfeiffer PrismaPlus).

The FeOx/CeO2(111) samples were prepared in situ using two e-beam evaporators and examined by multiple methods without breaking the UHV. The chemical composition and thickness of the prepared FeOx/CeO2 layers for STM analysis were probed by UHV XPS. STM imaging was performed at 300 K using a combined STM/AFM Specs KolibriSensor.22 The pressure in the STM chamber during the UHV measurements was below 5 × 10–10 mbar. The microscope was operated in the constant current mode with current set points in the 6–15 pA range and sample voltages in the 2–4 V range. Nanonis electronics controlled the scanning process.

2.4 Computational Details

2.4.1 Computational Methods

DFT calculations were carried out using the PW91 functional23 as implemented in the Vienna ab initio simulation package (VASP).24−26 The projector augmented wave method (PAW) was used to describe the interaction between fixed core electrons and explicitly described valence electrons. The PAW potentials describe 2s and 2p electrons explicitly for O, 3d and 4s electrons for Fe and 5s, 5p, 6s, 5d and 4f electrons for Ce. All structures were optimized with convergence criteria of 5 × 10–6 eV for total energies and 0.05 eV Å–1 for the forces acting on the atoms.

To properly describe electron localization on 3d states of Fe atoms and 4f states of Ce atoms, the GGA + U approach was employed to introduce energy penalties on partial occupations of Fe 3d and Ce 4f spin–orbitals. In line with previous studies, a U value of 5 eV was used for 3d states of Fe27 and 4 eV was used for 4f states of Ce.28 To describe electronic states involving different numbers of electrons transferred to the CeO2 support, we used the strategy described in our previous work.100

To account for van der Waals interactions when evaluating the stability of the different FeOx/CeO2(111) models considered, we have used the D2 correction of Grimme.29,30 Since there are no default parameters for the PW91+U approach used here (nor for plain PW91), we have used those parametrized for the similar PBE functional. We used C6 and R0 values of 0.7 J·nm6/mol and 1.342 Å for O,29 10.8 J·nm6/mol and 1.562 Å for Fe,29 and 20.0 J·nm6/mol and 1.860 Å for Ce.31,32

2.4.2 Structural Models

Two different structural models were used to describe the interface between the 2D FeO thin film and the CeO2(111) support (Figure 1). The inherent mismatch between the 2D FeO and CeO2(111) lattices (with the PW91+U optimized lattice parameters of 3.35 and 5.48 Å, respectively) was solved by combining supercell lattices of FeO and CeO2(111) of different dimensions and adapting the lattice parameter of the ceria substrate to make commensurate supercell models of the interface. The first, smaller, model corresponds to a 2 × 2 supercell of the FeO monolayer over a supercell of the CeO2(111) surface (Figure 1a). This results in a small compressive strain of the ceria surface of just 0.16%. Given the easily tractable size of the supercell (with 4 FeO units and 3 CeO2 surface units), this model is ideal for systematically evaluating different positions of the FeO ML with respect to the ceria surface (e.g., with a Fe atom located on top of either Ce, O or hollow sites of CeO2(111)) and different electronic states (i.e., with varying spin orientations and number of electrons transferred from FeO to the ceria substrate). The second, larger, model consists of a 6 × 6 supercell of FeO on a 5 × 5 supercell of CeO2(111) support (Figure 1b). Here, the ceria lattice parameter was stretched by 3.63%. This larger model is a better representation of the experimentally observed supercell and should better reproduce the coexistence of various Fe–substrate locations and the resulting long-range corrugation. In both models, 10.0 Å of vacuum were included in the z direction, and the CeO2(111) surface was described by 3 O–Ce–O trilayers, where only the bottom trilayer was kept fixed during structural relaxations.

Figure 1 Structural models of the FeO/CeO2(111) system. FeO(2 × 2)/CeO2() (a) and FeO(6 × 6)/CeO2(5 × 5) (b) supercells. Blue, red, beige and orange circles correspond to Fe, O atoms in FeO and Ce, O atoms in CeO2, respectively. Dashed lines indicate the supercell boundaries. In (b), regions with different locations of Fe atoms with respect to the ceria surface are indicated.

3 Results

The study is structured into three distinct segments. Initially, we present two sets of SRPES experiments examining the incremental deposition of Fe onto the CeO2(111) surface at 300 K, as well as the stepwise annealing of the FeOx/CeO2(111) surface up to 700 K. These experiments provide valuable insights into the stability of the FeOx layer and the intricate chemical interactions occurring between the FeOx and CeO2 layers. Subsequently, we discuss the structure of the FeOx layers and their chemical transformations characterized by in situ STM, LEED, and XPS. Finally, the results of the FeOx/CeO2(111) DFT calculations are discussed to delve deeper into the observed structures and their associated electronic structures and energetics.

3.1 Deposition of Fe on a Stoichiometric CeO2(111) Surface

Figure 2a presents Fe 3p SRPES spectra measured during stepwise Fe deposition on the CeO2(111) surface at 300 K in UHV (pressure ∼1 × 10–9 mbar). The obtained spectra were fitted with two unresolved doublets with a spin–orbital splitting of less than 1 eV. According to the literature, the doublet with the main peak position at about 55.7 eV and additional peak arising due to multiplet splitting at about 58 eV can be attributed to the formation of Fe3+ states.33,34 The doublet with the main peak at about 54.2 eV originates from the Fe2+ species.33,34 As can be seen from Figure 2b, both states gradually grew upon increasing the amount of Fe. In the case of 0.3–0.4 ML Fe/CeO2 surfaces, the concentrations of Fe2+ and Fe3+ were similar, whereas starting from the 0.4 ML coverage, Fe2+ states became dominant.

Figure 2 (a) Fe 3p SRPES spectra obtained during stepwise deposition of Fe onto the CeO2(111) surface at 300 K in UHV; (b) integrated intensities of Fe 3p spectral components; (c) the evolution of the RER shown as a function of Fe coverage.

The corresponding Fe 2p core level spectra obtained by conventional XPS (Figure S1) also confirmed the formation of Fe2+ and Fe3+ cations. Since it is not easy to use the peak position of Fe 2p3/2 alone to distinguish between different oxidation states of Fe, they are usually detected from satellite features.35 In particular, FeO, consisting of Fe2+ ions, typically contains a prominent satellite feature at around ∼715 eV (6 eV above the main peak),36 while the broad satellite centered at ∼718 eV (8 eV above the main peak) is a characteristic of the Fe3+ state.35 During the stepwise deposition of Fe, the Fe 2p spectra revealed a minor satellite at approximately ∼718 eV and a progressively increasing peak at around 715 eV.37 These spectral features indicate a successive growth of Fe2+ and Fe3+ ion content on the CeO2 surface with each Fe deposition step. From the steeper growth of the Fe2+ peak, it can be concluded that Fe3+ is likely to be accommodated at the ceria interface while Fe2+ ions are formed in the upper layers. The formation of Fe2+,3+ states has been reported for Fe submonolayers on Al2O3,38 TiO239 and ZnO40 supports, as well as for Fe single atoms on CeO241 nanoparticles. These observations are also consistent with the general picture of the behavior of metals on oxide supports.42

Deposition of Fe onto the stoichiometric CeO2(111) surface at 300 K caused an immediate reduction of Ce4+ ions. The facile conversion of Ce4+ into Ce3+ is evident from the RER displayed in Figure 2c (for the corresponding VB spectra see Figure S2). Before the deposition of Fe, the RER was approximately 0.02, indicating the presence of a negligible amount of Ce3+ cations due to intrinsic defects in the surface region of the well-ordered CeO2(111) film.43 At the lowest Fe coverage (0.3 ML), the Ce3+ state started to appear and grew slowly with each addition of iron. Up to 1 ML coverage, iron uniformly reduced the ceria surface, increasing the contribution of Ce3+ steeply up to the value of 2.95. After exceeding the Fe coverage of 1.3 ML, the RER started to decrease, indicating that the Ce3+ content has reached saturation. The observed reduction of Ce4+ on the ceria surface can be explained by the charge transfer from the iron clusters to the ceria substrate15 and the migration of O atoms from the ceria surface to the supported Fe clusters. The charge transfer and concomitant reduction of Ce4+ to Ce3+ occurs via the formation of Fe–O bonds, created at the interface between the FeOx clusters and the ceria surface and as a result of the O migration (reverse spillover) from the ceria surface to the FeOx clusters.41 This reverse spillover, which involves the formation of O vacancies and has been observed for other ceria-supported metal particles,44 is the only mechanism that can explain the oxidation of noninterface Fe2+ and Fe3+ cations, which would repel each other in the absence of Fe–O–Fe motifs. Once all the surface oxygen is covered with a layer of FeOx or the ceria surface is saturated with O vacancies, further charge transfer is hindered.

To observe the surface morphology, a 0.7 ML Fe/CeO2(111) surface was characterized by STM. Figure 3a,b shows STM images obtained from the clean CeO2(111) and Fe/CeO2(111) surfaces, respectively. The thin film of CeO2(111) exhibited a well-ordered structure with a continuous layer terminated by atomically flat terraces, in agreement with our previous studies.21 The LEED pattern obtained from this film confirmed its crystallinity. After about 0.7 ML of Fe was deposited on the surface at 300 K in UHV, small circular islands appeared (Figure 3b). These islands were from about 0.2 nm up to 0.6 nm in height, up to 3 nm in diameter and were uniformly distributed across the surface. LEED observation shows almost complete disappearance of the diffraction spots, indicating that the ceria surface was covered by a disordered layer of clusters. These circular entities were interpreted as nonstoichiometric FeOx clusters by PES. The formation and random distribution of the FeOx clusters indicates that Fe does not preferentially agglomerate at ceria steps. This is further supported by our experiments with a lower 0.2 ML coverage of FeOx (see Figure S6). This behavior can be attributed to the high oxophilicity of iron atoms.45,46 Due to their high affinity for oxygen and presumably low mobility at 300 K, Fe atoms most likely tend to localize on O sites on the stoichiometric CeO2(111) terraces, where they experience high diffusion barriers.46 Therefore, their growth is dominated by the formation of irregular nanoparticles over the entire surface of the substrate rather than the preferential nucleation on the surface defects.

Figure 3 STM images of the clean CeO2(111) surface (a) and as-deposited 0.7 ML Fe/CeO2 surface (b). Parameters of the STM images: 80 × 80 nm2, Us = 3.5 V, It ≈ 10 pA. LEED patterns of the CeO2(111) surface without/with Fe taken with a beam energy of 60 eV are shown in the top right insets. Red lines mark the positions of the height profiles shown in the bottom right insets.

3.2 Oriented FeO Layer Formation on CeO2(111)

In the next step, the 0.7 ML Fe/CeO2(111) system was stepwisely annealed in UHV (pressure ∼1 × 10–9 mbar) up to 700 K. The evolution of the Fe 3p spectra is shown in Figure 4a. We noted that the total spectral intensity within the Fe 3p region remained relatively stable up to 400 K. However, a gradual decrease becomes evident as temperature increases above this point (Figure 4b). An analysis of the Ce/Fe ratio, derived from the Ce 4d and Fe 3p spectral areas divided by corresponding photoionization cross sections as a function of annealing temperature, also revealed a significant decrease in the intensity of the iron signal compared to cerium. These observations strongly suggest changes in the morphology of the deposited iron. As shown in STM images with a higher surface coverage (1–2 ML) in Figure 5, this is partially due to the formation of large FeOx particles on the surface. Specifically, the growth of these larger FeOx clusters is expected to expose a greater portion of the substrate, thereby contributing to the increase in the Ce/Fe ratio, as shown in Figure 4c. In addition to the surface changes of the FeOx layer, this ratio can also be increased by thermally induced diffusion of Fe ions into the bulk of ceria.12 Furthermore, the elevated temperature promoted the oxidation of Fe ions to a higher valence state. The calculated relative intensities of Fe2+ and Fe3+ components presented in Figure 4b show that the dominant Fe2+ contribution began to drop already at 400 K, and upon reaching a temperature of 700 K, its intensity decreased more than 2-fold in favor of the Fe3+ state.

Figure 4 (a) Fe 3p SRPES spectra acquired from the 0.7 ML of Fe on the CeO2(111)/Cu(111) system in UHV at different temperatures; (b) integrated intensities of the Fe 3p spectral components, (c) Ce/Fe surface atomic ratio and (d) RER as a function of temperature.

Figure 5 STM images of the CeO2(111) surface upon deposition of Fe and subsequent annealing in UHV and oxygen. Top: ∼1 ML of Fe was deposited on the CeO2(111) surface at 300 K and annealed at 400 K (a), and 600 K (b) in UHV. Bottom: 2 ML of Fe was deposited on the CeO2(111) surface at 600 K in 1 × 10–8 mbar of O2 (c) and annealed at 600 K for 20 min in 1 × 10–8 mbar of O2 (d). All images have the size of 80 × 80 nm2 and were obtained with Ubias ≈ 3 V and It ≈ 10 pA. Red lines mark positions of the height profiles shown in bottom right insets.

Simultaneously with the oxidation of Fe, a gradual decrease in the amount of Ce3+ ions was observed (Figures 4d and S3). The fact that the spectral signal for Fe3+ and Ce4+ states increased during UHV annealing can be explained by the oxygen diffusion from deeper ceria layers to the surface, typically observed within the temperature range of 470–600 K.47 This diffusion process together with direct adsorption of oxygen from a residual atmosphere led to the replenishment of surface oxygen vacancies, which had initially formed due to the Fe–CeO2 interaction, and to further oxidation of iron.48 The accumulation of additional oxygen on the surface, leading to the change in the surface potential, can also explain an observed slight shift of the Fe 3p spectral components to a lower binding energy.

The STM images obtained after annealing of ∼1 ML of Fe on the CeO2(111) surface at 400 and 600 K in UHV (pressure ∼1 × 10–9 mbar) are shown in Figure 5a,b, respectively. Upon annealing at 400 K, the size and distribution of FeOx clusters did not change significantly from the ones observed in Figure 3b. However, at 600 K, STM images revealed the appearance of significantly larger FeOx particles up to 1.5 nm in height and up to 8 nm in apparent lateral size. Additionally, a hexagonal structure covering the rest of the ceria surface emerged. Based on its orientation and visible parameters of the lattice, the structure was interpreted as a moiré pattern, which in STM images arises as a result of electronic or morphological interference between two crystal lattices with the same symmetry but different periodicity.49,50 In this case, we observe the overlap of an oriented 2D FeOx layer and the CeO2(111) substrate. To the best of our knowledge, the growth of the thin FeOx layer on the CeO2(111) substrate has not been reported to date. Nonetheless, the amount of oriented iron oxide is relatively low when the Fe/CeO2(111) system undergoes annealing in UHV. This is attributed to a substantial portion of iron being incorporated into large amorphous particles, resulting in the above-mentioned lowering of the Fe 3p SRPES signal.

To enhance the formation of the 2D oriented iron oxide overlayer and simultaneously investigate its correlation with the presence of FeOx particles, we deposited 2 ML of Fe onto the CeO2(111) substrate at 600 K in 1 × 10–8 mbar of O2. The resulting surface topography can be seen in Figure 5c. Under these conditions, the formation of round FeOx clusters with heights up to 4 nm and apparent lateral sizes up to 20 nm was observed. The rest of the ceria surface was uniformly covered by the 2D FeOx layer, the arrangement of which was substantially improved by elevated oxygen pressure. The deposited layer contains a relatively low number of defects, concentrated mostly in the vicinity of CeO2 and FeOx step edges. After prolonged annealing in O2 (Figure 5d), the clusters transition from round hemispherical shapes to faceted pyramidal structures, indicating a structural shift from amorphous to crystalline. In contrast to the striking oxidation-induced morphological changes of the large FeOx clusters, the 2D layer remains unchanged. This indicates that the 2D FeOx layer is formed relatively quickly after the deposition at 600 K, while remaining Fe atoms can diffuse more freely on the FeOx-passivated surface and agglomerate into large clusters.

To further study the structure of the 2D FeOx layer, unaffected by the amorphous FeOx clusters, we deposited 0.7 ML of Fe onto the CeO2 at 300 K in UHV and annealed the sample at 700 K in 1 × 10–8 mbar of O2. This procedure resulted in the surface being completely covered by the moiré superstructure with a relatively high degree of order (Figure 6a), enabling us to determine the structure of the FeOx layer. Specifically, having calibrated the STM data by the uncovered CeO2 patches (Figure 6c,d), we determined that the elementary moiré cell is rotated by approximately 30° with respect to the CeO2 lattice and has a periodicity almost four times greater than the CeO2 lattice. Based on the lattice constants and the relative orientation of the FeOx layer, it can be determined that the FeOx unit cell is rotated by ±6° with respect to the CeO2 unit cell and that the lattice constant of the FeOx layer is = (0.31 ± 0.01) nm. The atomic periodicity of 0.31 nm corresponds to those obtained for FeO/Pt(111) (0.311 nm)49 and FeO/Ru(0001) (0.308 nm)51 and is only slightly higher than that expected from the theoretical calculations of FeO.52

Figure 6 Structure of the 2D FeOx layer. (a) STM image of the moiré superstructure on the FeOx/CeO2(111) surface after annealing at 700 K in 1 × 10–8 mbar of O2. Surface area 70 × 70 nm2, Ubias ≈ 3 V, It ≈ 10 pA. (b) LEED image obtained from the same surface. Electron energy: 50 eV. The CeO2(111), FeOx and the moiré superstructure elementary cells are marked by blue, green and red parallelograms, respectively. A diffraction spot corresponding to a minority surface structure with the FeOx structure aligned with the main direction of CeO2(111) is marked by a yellow arrow. (c) Two simultaneously captured STM images showing the structure of the CeO2 lattice and the moiré superstructure on the FeOx layer. The top image represents the height channel of the constant current STM image, covering a surface area of 30 × 30 nm2, with Ubias set at 1.6 V and It ≈ 10 pA. The bottom image displays the frequency shift channel as measured by a Kolibri sensor.22 (d) A composite image comprising parts of (c) illustrating the relative orientation of the two structures. Blue and red lines mark the main directions of the ceria and the superstructure lattices, respectively. Relative angles between the structures are marked in (b) and (d).

The validity of the measured FeO lattice parameters was also corroborated by the LEED analysis (Figure 6b). Bright, hexagonally arranged spots, marked by a blue parallelogram, correspond to the CeO2(111)–(1 × 1) structure.21 Satellite diffraction peaks observed in the vicinity of the main ceria diffraction spots arose due to the superposition of two metal-oxide lattices and multiple elastic scattering of diffracting electrons on both gratings,53 resembling the superstructures reported for FeO(111)/Pt(111) and FeO(111)/Au(111) surfaces.49,54 By comparing the distances of the diffraction spots from the zeroth series with the distances of the (1 × 1) spots of the CeO2 substrate, we obtained the lattice constants of the deposited layer (see green parallelograms) and the moiré superstructure (see a red parallelogram) to be = (0.31 ± 0.01) nm and amoiré = (1.55 ± 0.04) nm, respectively. Furthermore, the angle between the moiré superstructure and the CeO2 lattice was determined to be (30 ± 1)°, corresponding to the (6 ± 1)° angle between the main direction of the (1 × 1) structure and the FeO structure.53 Both of these values agree very well with the values measured from the STM images further indicating that the 2D FeO-like phase is indeed formed on the surface of CeO2. Additionally, the presented LEED image features an extra set of spots marked by a yellow arrow. These spots are located in the main crystallographic directions of the CeO2(111)–(1 × 1) structure but correspond to the lattice constant of about 0.31 nm. We assign these spots to a minority FeO-like structure which is aligned with the ceria substrate. The substrate-aligned minority superstructures were also observed in our STM images (see Figure S4).

The structural properties of the FeO/CeO2(111) interface were further elucidated by means of the DFT+U calculations described in the computational details section. For the smaller FeO(2 × 2)/CeO2(√3 × √3) structural model (Figure 1a), we have evaluated three different orientations of the FeO ML corresponding to one of the 4 Fe atoms of the FeO supercell placed on top of an O atom, a Ce atom, or a hollow site of the ceria substrate, respectively. We note that these alternative arrangements involve different interactions between the OFeO and the ceria surface, which also affects the overall stability of each structure. The most stable structure found for this model is illustrated in Figures 1a and 7a, exhibiting both an Fe atom and an OFeO atom on top of two different Ce atoms of the ceria surface. The different interactions of Fe and O atoms with the ceria surface lead to a corrugation of ∼0.7 Å, with the Fe and OFeO atoms found at a 3.51 and 2.84 Å distance above the underlying Ce atoms, respectively. The calculated adhesion energy Eadh between the FeO monolayer and the ceria surface is −0.47 eV per FeO unit (calculated as Eadh = E(FeO/CeO2) – E(FeO) – E(CeO2)). This energy gain is larger than the −0.36 eV energy difference between 2D FeO and the most stable (monoclinic/halite) phase of bulk FeO, which indicates that formation of the FeO/CeO2 heterostructure is more thermodynamically favorable than agglomerated 3D particles of FeO. A large fraction (−0.20 eV) of this adhesion energy (−0.47 eV) corresponds to van der Waals interactions.

Figure 7 Height profile of the FeO monolayer supported on CeO2 for the (a) FeO(2 × 2)/CeO2(√3 × √3) and (b) FeO(6 × 6)/CeO2(5 × 5) models. Upper and lower panels correspond to top and side views, respectively. The Fe (big circles) and O (small circles) atoms of the FeO monolayer are colored depending on their distance to the CeO2 support according to the color bar. Distances are calculated with respect to the outermost O atoms of the bare CeO2(111) surface. The white circles indicate Ce (big circles) and O (small circles) atoms of the CeO2 surface.

The optimized structure of the larger FeO(6 × 6)/CeO2(5 × 5) model is illustrated in Figure 7b and exhibits a more substantial corrugation of ∼2.3 Å. The lowest-lying region of the FeO ML corresponds to that with Fe atoms bonding to O atoms of the ceria surface, with Fe– distances of ∼1.9–2.2 Å (similar to the 1.93 Å Fe–O distances in the free-standing FeO ML). These bonds are formed for several Fe atoms near top positions of atoms and for one Fe atom on the hollow site between 3 atoms. In the middle-lying region, the OFeO atoms neighboring the Fe atom are also close to Ce atoms of the ceria surface, forming OFeO–Ce bond distances between 2.6 to 2.7 Å, close to those of the Ce–O bonds in the clean ceria surface (2.43 Å). These Fe– and OFeO–Ce distances reflect a strong interaction between the FeO ML and the ceria surface. Indeed, for the FeO(6 × 6)/CeO2(5 × 5) model, Eadh is of −0.45 eV per FeO unit, further confirming the high stability of the FeO/CeO2 interface. The highest-lying regions of the FeO ML correspond, in turn, to Fe atoms on top of Ce atoms of the ceria surface, with Fe–Ce distances of 4.34 Å. Thus, an apparent repulsion between Fe and Ce cations emerges from both FeO(6 × 6)/CeO2(5 × 5) and FeO(2 × 2)/CeO2(√3 × √3) models, although the larger supercell can accommodate more pronounced corrugations leading to significantly longer Fe–Ce distances.

We also compare the stability of the FeO/CeO2(111) system to that of Fe-doped CeO2(111), which has been proposed as a relevant component for catalysts based on Fe and CeO2.4,6 To do so, we compare the adhesion energy of FeO to CeO2(111) to the energy of the Fe-doping reaction using the same 2D FeO reference and a 2 × 2 supercell of the CeO2(√3 × √3) surface: FeO + (x/2)O2 Ce36O72 → FeCe35O71+x + CeO2, where FeCe35O71+x corresponds to the Fe-doped CeO2(2√3 × 2√3) surface (see Figure S7) and x takes values of 1, 0, or −1 for the Fe-doped surface when fully oxidized, with one O vacancy, or with two O vacancies, respectively. FeO corresponds to the 2D FeO monolayer, CeO2 corresponds to bulk ceria, and Ce36O72 is the slab model of the CeO2(2√3 × 2√3) supercell. Edop is thus calculated as Edop = E(FeCe35O71+x) + E(CeO2) – E(FeO) – E(Ce36O72) – (x/2)E(O2). The calculated Edop values are 0.66, 1.09, and 2.66 eV per Fe atom, for the Fe-doped CeO2(111) system with zero, one, and two vacancies, respectively. Considering the calculated Eadh (−0.47 eV), the ceria supported FeO monolayer is at least 1.13 eV more stable than Fe-doped CeO2(111), which explains why Fe dissolution into the ceria lattice is very limited.

The STM and LEED investigation was also accompanied by in situ XPS measurements aiming to study the surface chemistry of the resulting FeOx/CeO2 systems. Two notable sets of Fe 2p spectra are presented in Figure 8 and corresponding Ce 3d spectra in Figure S5. The top-left spectrum in Figure 8 corresponds to the surface covered by relatively small FeOx clusters with a high surface-to-volume ratio, as depicted in Figure 3b. Upon deposition of Fe in UHV, this system initially contained the metallic Fe0 and other oxidized states of iron. We note that Feδ+ probably corresponds to just partially oxidized Fe atoms, which have donated electrons to the ceria surface but without transforming into an oxidized phase of Fe. Similar δ+ atomic charges have been detected for other ceria-supported transition metals.55,56 It is worth noting that we observed a gradual decrease of Fe0 and Feδ+ and a simultaneous increase of Fe2+ and Fe3+ components during the spectrum acquisition in Figure 8a (acquisition time was around 1 h). Subsequently, upon finishing LEED and STM experiments (after 24 h), the Fe0 and Feδ+ components disappeared completely, as shown in Figure 8c, while the FeOx on the surface remained in the form of small clusters. The spontaneous, albeit slow, oxidation of Fe0 and Feδ+ species in UHV is an indication of O migration from the ceria surface to the supported FeOx particles. After annealing this surface at 700 K in 1 × 10–8 mbar of O2, the FeOx rearranged and formed the well-ordered 2D FeOx layer, as shown in Figure 6, with about a 5:1 Fe3+/Fe2+ ratio (Figure 8e).

Figure 8 In situ XPS Fe 2p core level spectra showing the evolution of FeOx/CeO2 (111) samples with 0.7 ML Fe coverage (left) and 2 ML Fe coverage (right) after Fe deposition (a, b), STM measurement (c, d) and annealing in 1 × 10 –8 mbar of O2 (e, f). The spectra were acquired with Al Kα radiation (1486.6 eV) and normalized to the same maximum height. Relative intensities of Fe3+, Fe2+, Feδ+ (δ ϵ (0, 2)) and Fe0 spectral components determined from the peak areas are specified in the columns.

The Fe 2p spectrum in Figure 8b corresponds to the surface featuring large FeOx clusters, as illustrated in Figure 5c. Similar to the surface with small clusters, the XPS spectrum contains higher oxidation states of Fe as well as states attributed to Feδ+ and Fe0, despite iron being deposited onto the surface in the 1 × 10–8 mbar of O2. Obviously, the oxygen supplied during the deposition at 600 K was not sufficient to fully oxidize the iron deposit, resulting in the presence of Fe0 species. This metallic iron is presumably located within noninterfacial regions (i.e., bulk and/or surface positions of the upper layers) of the large clusters. After the following STM and LEED measurements (Figure 8d), the average Fe oxidation state on the surface remained almost unchanged, indicating that the interface FeOx region or the FeOx layer on the surface of large clusters impedes further oxygen diffusion into the clusters at 300 K. Complete oxidation of Fe was only achieved after an additional 20 min annealing in 1 × 10–8 mbar of O2 at 600 K, as shown in the bottom-right spectra of Figure 8. At this stage, the Fe3+/Fe2+ ratio is about 2.5:1, indicating that even after prolonged oxygen exposure at elevated temperatures, Fe atoms within FeOx clusters are, on average, less oxidized than those within the 2D FeOx layer.

In the SRPES experiment presented in Figure 4 we gradually annealed the sample in UHV. The Fe3+/Fe2+ ratio measured from the Fe 3p spectra stabilized at about 3:1, aligning more closely with the scenario observed with large FeOx clusters, as determined by XPS. The Fe0 component in this set of spectra was not observed from the beginning, indicating rapid oxidation of rather small clusters immediately after deposition. Similar to the surface depicted in Figure 5a,b, the small clusters annealed under relatively low oxygen pressure may have coalesced into larger clusters, leaving the CeO2 surface only partially covered by the 2D FeOx layer. The coalescence of small FeOx particles is further evidenced by the gradual decrease in the total spectral intensity of the Fe 3p peak (Figure 4b). Furthermore, disparities in the SRPES and XPS spectra are attributable to the higher surface sensitivity of SRPES (180 eV vs 1487 eV energy of the primary radiation results in information depths of 2 vs 5 nm, respectively). Consequently, the SPRES spectra are more indicative of the Fe3+/Fe2+ ratio in the topmost surface layer of the FeOx clusters, whereas XPS provides a representation of the ratio across entire clusters.

A dominant Fe2+ oxidation state of Fe atoms in the 2D layer is expected based on its structural parameters corresponding to the FeO phase, however, a dominant Fe3+ state (up to 5:1 ratio) is observed from our spectroscopy experiments. We address this discrepancy by means of DFT calculations. The Bader atomic charges, projected density of states (pDOS), and magnetic moments derived from the DFT calculations for the different models allow us to further characterize the electronic structure of the FeO/CeO2(111) interface. We have focused on evaluating the charge distribution and comparing the stability between electronic states differing in the spin-alignment of the Fe cations and on the number of transferred electrons from the FeO ML to the underlying ceria substrate. As illustrated in Figure 9, this redox process leads to the oxidation of one Fe2+ cation to Fe3+ and the concomitant reduction of one Ce4+ cation to Ce3+. The formal charges of the Fe and Ce cations can be inferred from their Bader charges and magnetic moments. Upon oxidizing Fe2+ to Fe3+, the (positive) Bader charge increases from 1.4 |e| to 1.8 |e| and the absolute magnetic moment increases from ∼3.7 to ∼4.1 μB. This indicates that the donated electron is that with the opposite spin orientation than the Fe cation, consistently with Hund’s rule and as illustrated in the Fe 3d pDOS in Figure 9a,c. In turn, Ce3+ centers exhibit a lower (positive) Bader charge than Ce4+ and a characteristic magnetic moment close to ∼0.9 μB due to the occupation of a Ce 4f spin–orbital.

Figure 9 Electronic structure of Fe atoms in the FeO/CeO2(111) system for different electronic states without (a and b) or with (c and d) transferred electrons from Fe 3d to Ce 4f orbitals. The density of states projected in the 3d states of Fe are shown in (a) and (c), and schemes illustrating formal Fe and Ce charges and the electronic configuration of the Fe 3d states are shown in (b) and (d).

For the most stable structure of the smaller model, we have identified three different electronic states with zero, one, or two electrons transferred from FeO to CeO2(111), represented in Figure 10a–c. We note that as for the free-standing FeO monolayer, the most stable electronic states for the FeO/CeO2(111) system exhibit an antiferromagnetic ordering of the Fe cations, independently of them being Fe2+ or Fe3+. The most stable electronic state found for the FeO(2 × 2)/CeO2(√3 × √3) model contains only one Fe3+ cation (one transferred electron from FeO to CeO2). The states with two or zero transferred electrons are 0.56 and 0.40 eV (0.14 and 0.1 eV per FeO unit) less stable, respectively. The most stable state with one transferred electron corresponds to a 1:3 Fe3+/Fe2+ ratio and 1/3 of outermost Ce atoms reduced to Ce3+. The concentration of Fe3+ is therefore lower than that measured for the FeO on CeO2 system annealed at high temperatures.

Figure 10 Different electronic states differing in the number of transferred electrons (TE) from FeO to CeO2 in the FeO(2 × 2)/CeO2() (a, b, and c) and FeO(5 × 5)/CeO2(6 × 6) (d) models. Blue, white, beige, black and orange circles correspond to Fe, OFeO, Ce4+, Ce3+ and Oceria atoms, respectively. The labels above Fe atoms indicate their oxidation state (2+ or 3+).

For the larger FeO(6 × 6)/CeO2(5 × 5) model, we have only sampled a single electronic state due to the large computational cost of every structural relaxation. This single optimization has converged to an electronic state with 6 Fe3+ cations, corresponding to a 1:5 ratio between Fe3+ and Fe2+ cations, where 6 of the 25 outermost Ce cations are reduced to Ce3+. The distribution of the Fe2+, Fe3+, Ce3+, and Ce4+ cations is illustrated in Figure 10d. We have not exhaustively sampled all possible electronic states differing in the number and position of the Ce3+ cations formed, but for the larger model, Ce3+ centers generally form in the vicinity of Fe3+ centers.

The distribution of Fe oxidation states for both FeO/CeO2(111) models indicates that the FeO monolayer is only partially oxidized by the ceria surface in the absence of O2. To evaluate how exposure to O2 would affect this distribution, we have carried out calculations with one or two additional O atoms on the FeO(2 × 2)/CeO2() model (see Figure 11). The most stable adsorption sites for O on this system correspond to hollow sites between three Fe cations and right on top of a Ce atom of the ceria surface. O atoms adsorbed on these positions bond to the 3 Fe cations and to the Ce below, with Ce–O distances of 2.3 Å. The system with one adsorbed O atom (Figure 11a) has 2 Fe3+ and 2 Fe2+ cations, indicating that this adsorption reoxidized one Ce3+ to Ce4+ and one Fe2+ to Fe3+, therefore also leading to the oxidation of the Ce3+ center that had formed upon interaction with FeO back to Ce4+. Upon adsorption of the second O atom (Figure 11b), the two remaining Fe2+ cations are oxidized to Fe3+. The adsorption energy Eads(O) (calculated as Eads(O) = E[O-surface] – E[surface] – 0.5E[O2]) of the first and second O atoms is −0.72 and −1.26 eV, respectively, which indicates that the oxidation of FeO (Fe4O4) to Fe2O3 (Fe4O6) is thermodynamically favorable. Here, we just added O atoms in three-Fe-coordinated sites and relaxed the structure. A more exhaustive search for stable structures of the Fe2O3 monolayer is likely to result in even more favorable oxidation of the ceria-supported 2D FeO.

Figure 11 Oxidation of the FeO(2 × 2)/CeO2() model. Structure and Fe oxidation state for the adsorption of one (a) and two (b) O atoms on FeO(2 × 2)/CeO2(), leading to Fe4O5 and Fe4O6 stoichiometries, respectively, of the FeOx monolayer. White circles indicate Ce (big circle) and O (small circle) atoms of the CeO2 support, while Fe (big colored circle) and O (small colored circle) are colored to indicate their vertical distance to the bare CeO2 support according the color bar on the right-hand side.

The activity toward oxidation reactions of catalysts based on Fe and CeO2 has been linked to the effect of Fe dopants on the oxygen storage capacity of ceria.5,6 Since the oxygen storage capacity of ceria-based systems depends on their reducibility, their formation energy Ef(Ovac) is often used as a descriptor for activity.57,58 Therefore, to establish differences between Fe-doped CeO2(111) and the FeOx/CeO2(111) systems, we have calculated Ef(Ovac) for their corresponding models (Figures S7 and S8). Ef(Ovac) is calculated as Ef(Ovac) = E(X–Ovac) – E(X) – 0.5E(O2), where E(X–Ovac) and E(X) are the energies of the considered systems with and without the formed vacancy, respectively.

We start from the most oxidized states (i.e., Fe4O6/Ce9O18 for FeOx/CeO2(111) and FeCe35O72 for Fe-doped CeO2(111)). For FeOx/CeO2(111), the first, second, and third O vacancies have Ef(Ovac) values of 1.26, 0.72, and 2.99 eV, respectively. For Fe-doped CeO2(111), the first and second O vacancies result in Ef(Ovac) of 0.43 and 1.56 eV, respectively. These results indicate that it is not significantly more favorable to form vacancies in Fe-doped CeO2(111) than on the 2D FeOx/CeO2(111) system, and that both systems have a rich redox chemistry with readily available (i.e., low Ef(Ovac)) O atoms.

4 Discussion

As mentioned above, PES and STM results indicate that nonstoichiometric FeOx clusters are formed as a result of iron–ceria interaction. The process can be described by the reaction yFe + xCeO2 → xCeO(2–y) + yFeOx, where x = 0···1.5 and y = 0···0.5. Interestingly, the molar enthalpy change (ΔHf298) for the full redox reaction Fe + 2CeO2 → Ce2O3 + FeO involving bulk oxides is 109.2 kJ/mol,59 suggesting that the process is thermodynamically unfavorable at 300 K. However, it is thermodynamically more favorable to partially reduce the CeO2(111) surface than bulk ceria,58 and to partially oxidize Fe particles than bulk Fe. A similar phenomenon has also been reported for other systems, such as Ni/CeO2 or Co/CeO2, where the formation of CoO60 and NiO61 clusters was observed. The oxidation of supported metal particles by lattice O atoms of CeO2(111) is typically enabled by reverse spillover mechanism, which has been reported for even more noble metals such as Pt.44 The spontaneous oxidation of ceria-supported 3D Fe particles in UHV thus confirms that a reserve spillover mechanism is enabled in the FeOx/CeO2(111) interface.

We have shown that Fe atoms, which form small clusters upon deposition, are readily oxidized to varying degrees. Conversely, bigger FeOx clusters only oxidize after prolonged annealing in O2 at 600 K. The slower oxidation kinetics observed in the bulk or upper layers of large FeOx clusters, even at elevated temperatures, suggests relatively lower oxygen mobility within the superficial or interfacial iron oxide, which likely encapsulates and passivates metallic cores of the Fe clusters.

In a 2D configuration, the (111) planes of both CeO2 ( = 0.382 nm)62 and FeO ( = 0.31 nm)49 surfaces exhibit hexagonal symmetry, indicating their potential for epitaxial arrangement. Although significant lattice mismatches, such as that between the FeO and CeO2(111) (exceeding 20%), often prevent epitaxial growth, that is not always the case. Instead of just the lattice mismatch, the growth of one oxide on another is also governed by the stability of the oxide–oxide interface and the process kinetics.63 The modification of the surface free energy Δγ induced by a film formation on a substrate can be written as Δγ = γf + γi – γs, where γf and γs are surface free energies of a film and a substrate, respectively, while γi is the interface energy. If Δγ < 0, the formation of a wetting layer is energetically favored, while Δγ > 0 would lead to islanding or clustering. Taking γf as that of O-terminated FeO(111) (1.3 J/m2),64 γs as that of O-terminated CeO2(111) (0.7 J/m2),65 and the γi of −0.7 J/m2 (−0.47 eV/FeO unit) calculated by DFT in this work for the FeO/CeO2(111) interface, results in Δγ = −0.1 J/m2, indicating a preference for FeO film formation on ceria. This is in line with the slight preference to form the FeO/CeO2(111) interface rather than bulk FeO derived solely from our DFT calculations. In addition to being thermodynamically favorable, the formation of the FeO films on CeO2(111) may be dominated also by other factors such as the high degree of reduction of the ceria surface after the deposition of Fe or the different formation and oxidation kinetics of 2D films and 3D FeOx nanostructures.

Nevertheless, our DFT calculations showed that the interaction of the 2D FeOx layer with ceria is strengthened by the structural corrugation of the 2D FeOx layer (which can be observed experimentally by STM), by van der Waals interactions, by the further oxidation of the 2D FeOx layer upon incorporation of additional O atoms, or by the transfer of electrons from the 2D FeO layer to the ceria substrate. The latter is analogous to the electronic metal–support interactions widely reported for ceria-supported metals.56,66

5 Conclusions

This model catalyst study offers valuable insights into the intricate behavior of iron in real Fe/CeO2 catalysts.3,4,6,7 The presented findings shed light on the growth dynamics and interaction between iron and ceria during thermal treatment under UHV conditions and in the presence of O2. Our data demonstrate that iron readily undergoes oxidation upon deposition, giving rise to small FeOx clusters on the ceria substrate at 300 K. Upon annealing in UHV, some clusters grow into larger particles, while others disperse and form a thin FeOx layer. Annealing in an oxygen-rich environment promotes the dispersion of iron on the ceria surface and enhances the ordering of the resulting 2D FeOx thin film, as validated by STM and LEED. By means of DFT calculations, we showed that this 2D surface layer is thermodynamically more stable than bulk FeO or Fe-doped CeO2(111) due to (i) the formation of bonds between the Fe atoms and the O atoms of the ceria lattice, (ii) van der Waals interactions between the FeOx layer and the CeO2(111) surface, (iii) the structural corrugation of the FeO monolayer, (iv) the transfer of electrons from Fe to ceria, and (v) the additional oxidation by adsorbed oxygen.

In addition, we have proposed a method for producing stable, well-ordered epitaxial FeOx films on a ceria support. Similar ultrathin FeO films grown on much more expensive noble metal substrates have demonstrated exceptional catalytic activity in CO oxidation reactions.67 Thanks to the structural corrugation of the 2D FeOx layer, this system could also serve as an excellent template for supporting ordered arrays of noble-metal nanoparticles.68,69 Lastly, the capacity of the 2D FeO film to donate electrons suggests that the 2D FeO/CeO2(111) system can be used as a reducing support for catalytically active metal particles, enabling the synthesis of catalytic materials with negatively charged metal centers.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.4c05542.Additional Fe 2p spectra fitting parameters, Fe 2p core-level spectra, valence band spectra, STM images, in situ Ce 3d core-level spectra, and DFT supporting calculations (PDF)

Supplementary Material

am4c05542_si_001.pdf

Author Contributions

# L.P. and P.C.-L. contributed equally to the work and are considered cofirst authors.

The authors declare no competing financial interest.

Acknowledgments

The work was financially supported by the Czech Science Foundation, project No. 20-13573S. The authors acknowledge CERIC–ERIC Consortium for access to experimental facilities at the LRI SPL-MSB (Proposal numbers: 20212210, 20207106, 20217192, 20207146). Authors acknowledge financial support from the Spanish/FEDER Agencia Estatal de Investigación of the Ministerio de Ciencia, Innovación y Universidades (grants PRE2019-088979 (for P.C.L.), and PID2021-128217NB-I00, PID2022-140120OA-I00, MDM-2017-0767, CEX2021-001202-M, and RYC2021-032281-I (for A.B.), as part of the Next Generation EU/Plan de Recuperación Transformación y Resiliencia -PRTR-), Generalitat de Catalunya/AGAUR (grant 2021SGR00286), and the Junior Leader Program from “La Caixa Foundation” (for A.B.). This study was also supported by the COST Actions CA18234 and CA21101. We are also thankful for the computational resources provided by the Red Española de Supercomputación. P.C.L. acknowledges Prof. Henrik Grönbeck for helpful discussions.
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