Superstructures on Epitaxial Fe3O4(111) Films: Biphase Formation

Subscriber access provided by TULANE UNIVERSITY

C: Surfaces, Interfaces, Porous Materials, and Catalysis 3

4

Superstructures on Epitaxial FeO (111) Films: Biphase Formation Versus the Degree of Reduction Nika Spiridis, Kinga Freindl, Joanna Wojas, Natalia Kwiatek, Ewa Madej, Dorota Wilgocka-#l#zak, Piotr Dró#d#, Tomasz #l#zak, and Józef Korecki J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11400 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on February 5, 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 40 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

Superstructures on Epitaxial Fe3O4 (111) Films: Biphase Formation versus the Degree of Reduction Nika Spiridis1*, Kinga Freindl1, Joanna Wojas1, Natalia Kwiatek1, Ewa Madej1, Dorota Wilgocka-Ślęzak1, Piotr Dróżdż2, Tomasz Ślęzak2 and Józef Korecki1,2.

1

Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences,

Niezapominajek 8, 30-239 Kraków, POLAND. 2

AGH University of Science and Technology, Faculty of Physics and Applied Computer

Science, Mickiewicza 30, 30-259 Kraków, POLAND.

* Corresponding author: [email protected]

1

ACS Paragon Plus Environment

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

Abstract: We analyzed the formation of biphase superstructures on the (111)-oriented epitaxial magnetite films on Pt(111) as a function of controlled film stoichiometry. The stoichiometry of the films in the several nanometer thickness range was changed by UHV in situ deposition of a few monolayers of metallic iron onto the stoichiometric film, followed by annealing. The samples were characterized in situ. The surface structure was determined using low-energy electron diffraction and scanning tunneling microscopy, whereas the phase composition and electronic structure were verified using X-ray photoemission spectroscopy and conversion electron Mössbauer spectroscopy (CEMS) with isotopic 57Fe probe layers. As a function of the added-Fe (ad-Fe) dose and annealing temperature, we identified four types of superstructures that were homogenously distributed over the entire surface, and we associated them with an increasing degree of surface reduction. We have proposed a coherent atomic-scale model of the observed superstructures that explains them in terms of the modifications of the two outermost Fe atomic layers. CEMS also allowed us to follow in-depth changes accompanying the biphase occurrence. An excess of ad-Fe, which stabilizes a given surface superstructure, migrates towards the substrates, partially dissolves in Pt and partially forms an interfacial layer with FeO stoichiometry.

2


Page 2 of 40

Page 3 of 40 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


Introduction Nanocrystals of magnetite (Fe3O4) have attracted vast attention due to their biomedical1,2,3,4,5,6 and catalytic applications7,8. In parallel, single crystalline magnetite surfaces are often used in model studies on the applications of iron oxides in catalysis9,10, for example, in fuel-cell related studies of the reactivity of alcohols with iron oxides11 or as the supports for highly dispersed metal or oxide catalysts12,13. Magnetite is also an attractive material for spintronic applications because this oxide is predicted to be half-metallic at room temperature and multiferroic at low temperatures14,15. In all these aspects, the surface structure and its variations have a strong impact on the unique physical and chemical properties. At room temperature, magnetite crystallizes into the cubic inverse spinel structure (227 space group - Fd3m) with a lattice constant of 8.396 Å16. The unit cell contains 56 atoms, and therefore, even the low-index surfaces can have many distinct atomic-layer terminations. The structure becomes even more complex at low temperatures due to the Verwey transition that occurs at 124 K17; however, this issue is not within the scope of the present paper. Experimental studies reveal that the conditions for forming the magnetite surface have a strong impact on its structure. This concerns both the surface of epitaxial layers as well as single crystals, for which the surface preparation process (usually ion bombardment) can lead to changes in the surface stoichiometry (oxygen is sputtered easier than iron)18,19,20. On the (111)-oriented surface, which is one of the most stable and the most common natural growth facets16, various surface inhomogeneities form particularly easily, and most of them have a strong tendency to order in quasi-hexagonal structures with a periodicity of a few nanometers that are termed in the literature as a "biphase"21. Simultaneously, it is known that even a small 3



fraction of biphase surface superstructures seriously influences the adsorption and reaction mechanisms because the reactivity of the Fe3O4 (111) surface is highly sensitive to surface reconstruction10,11,22,23. To the best of our knowledge, no Fe3O4(111) surfaces with homogeneous biphase structures have been reported thus far, and consequently, there are no structural biphase characterizations using quantitative diffraction methods that are area-averaging techniques. Furthermore, a coherent model covering the atomic structures of existing biphases is lacking, which is a serious weakness in all model studies of iron oxide surfaces. In stoichiometric magnetite, the oxygen sublattice is a close-packed fcc-structure in which iron occupies the tetrahedral (Fetet) and octahedral (Feoct) interstitials. Fig. 1 shows the magnetite unit cell in the hexagonal setting (166-space group). The lattice constants are a=5.936 Å and c=14.532 Å24. Along the c-axis, which is parallel to a direction of the cubic cell, one mutually finds two types of Fe layers (the so-called kagome layer or “a dense iron layer”25 and the mix-trigonal layer) separated by close-packed oxygen layers (O1 and O2) that exhibit slight buckling. The average in-plane distance between the oxygen ions is approximately 3 Å. The kagome layer (Feoct1) is a geometric monolayer formed by the atoms, whose smallest in-plane distance is approximately 3 Å (same as in the oxygen planes), but the site occupancy is only ¾ compared to the oxygen monolayers (MLs). The mix-trigonal layer is composed of three hexagonal low-density geometric monolayers, with an Fe-occupancy ¼ and the nearest neighbor in-plane distances approximately 6 Å. The central monolayer (Feoct2) contains the octahedrally coordinated iron cations, and the other (Fetet1 and Fetet2), tetrahedral iron cations. The stacking sequence of these atomic planes along [111] in the magnetite structure can be written as O2/¼ Fetet2/¼ Feoct2/¼ Fetet1/O1/¾ Feoct1. These six sequential geometric layers form a 4


Page 4 of 40

Page 5 of 40 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


stoichiometric neutral slab and define a 4.85 Å thick physical monolayer (pML) of Fe3O4(111). The bulk structure is fully reproduced by three physical monolayers that are translated in the basal plane.

Figure 1. Magnetite unit cell in the hexagonal setting and the details of the atom arrangement in the sublayers. Consequently, from the bulk structure point of view, six surface terminations are possible for the (111)-oriented surface. Each bulk-terminated Fe3O4(111) surface is polar and of type-III according to Tasker’s classification26. According to theoretical considerations, the most stable over a wide range of the oxygen pressures is the Fetet1 termination, but the Feoct2 termination has a comparable surface energy under oxygen-poor conditions.27,28,29,30,31,32,33,34. Quantitative low-energy electron diffraction (LEED) studies of epitaxial Fe3O4(111) films with a so-called “regular” surface unambiguously indicate the Fetet1-terminated surface that exposes a ¼ 5



monolayer of Fe ions over a slightly distorted hexagonal close-packed O1 oxygen layer35,36,37,38. Accordingly, the regular surface39 refers to an unreconstructed bulk-terminated surface of Fe3O4(111) with the (1x1) hexagonal surface unit cell that has a lattice constant of 5.936 Å. Scanning tunneling microscope (STM) studies of the stoichiometric (111) magnetite surface support the finding that the regular Fetet1 termination is the most stable one under typical ultrahigh vacuum environment conditions9,39,40,41,42, whereas the Feoct2 termination is more likely to form if samples are annealed in oxygen-poor conditions42. On the other hand, areas with complex structures have been reported for magnetite (111) surfaces, depending on the preparation conditions. In particular, several different biphase structures were noted, most of them on reduced surfaces21,22,32,33,34,39,43. Only a few papers40,44 report a biphase structure that was formed as a result of oxidation. The atomic-level explanation for the biphase nanostructures remains somewhat controversial. They are interpreted as an ordered arrangement of surface magnetite regions with different terminations (Fetet1 Feoct1, O1) and/or non-magnetite stoichiometry (Fe1-xO-like), in which the octahedral Fe ions form a hexagonal densely packed monolayer on the oxygen layer: on the O241,44, or on the O1 or O2 layers39. Some authors presume that particular biphase nanostructures represent an electronic rather than simply a structural effect40. Different biphase nanostructures often coexist, and no precisely defined conditions that favor the formation of a given superstructure type have been identified. In the present paper, we analyze biphase formation on (111)-oriented epitaxial magnetite films on Pt(111) as a function of controlled film stoichiometry. The stoichiometry was changed by the UHV in situ deposition of metallic iron onto the stoichiometric film, followed by annealing. The

6


Page 6 of 40

Page 7 of 40 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


samples were characterized in situ. The surface structure was determined using LEED and STM, whereas the phase composition and electronic structure were verified using X-ray photoemission spectroscopy (XPS) and conversion electron Mössbauer spectroscopy (CEMS) with isotopic 57Fe probe layers. CEMS also allowed us to follow in-depth changes accompanying the biphase occurrence. As a function of the added-Fe (ad-Fe) dose and annealing temperature, we identified four types of superstructures that were homogenously distributed over the entire surface, and we associated them with an increasing degree of surface reduction. We have proposed a coherent atomic-scale model of the observed superstructures. The model assumes the O1 layer is the outermost oxygen layer. On this basis, three types of subareas in various proportions make up one of three different biphase structures. The first subarea corresponds to magnetite with Fetet1 termination. For the two other, the resulting stoichiometry of the terminating double layer is FeO, however, with different atomic structures: (i) a densely packed hexagonal monolayer of octahedrally coordinated Fe atoms located over an O1 layer and (ii) a densely packed hexagonal monolayer of tetrahedrally coordinated Fe atoms located over an O1 layer, which is supported by ab initio calculations of Fe adsorption onto the regularly terminated magnetite surface45. Generally, the superstructures observed in STM and LEED are moiré coincidence structures from the outermost layers, which, with increasing ad-Fe dose, stepwise change their composition from the regular termination, through the three biphase structures, to the final two FeO-like layers with slightly differing lattice constants, i.e., to a strained interface and a coincident surface structure.

Experimental section

7



The magnetite films on Pt(111) were grown in a multipurpose UHV system with a base pressure of 1 × 10−10 mbar. The system is equipped with a home built molecular beam epitaxy (MBE) facility system for the deposition of several metals (BeO crucibles heated with wraparound tungsten coils), including

56

Fe and

57

Fe isotopes, and a 4-grid optics (OCI Vacuum

Microengineering Inc) for LEED and Auger electron spectroscopy (AES). In separate chambers of the same system, STM (Burleigh Instruments) and a home-built CEMS spectrometer were used for in situ sample analysis. In the CEMS spectrometer, a small active-area 100 mCi 57Cο γ-source is placed outside the UHV inside a stainless steel tube that penetrates the chamber close to the sample and is capped by a beryllium window that is transparent to the γ-rays. The sample can be irradiated at any angle from zero to 90◦ with respect to the surface normal. A standard constant acceleration drive system and a compact electromechanical transducer (Palacky University, Olomouc) are used to move the Mössbauer source. A large opening (25 mm in diameter) channeltron (Dr. Sjuts Optotechnik GmbH) is used to detect the conversion electrons resulting from the resonant absorption of the γ-radiation in the sample. To avoid a parasitic signal from the sample holder, a movable shutter is used to limit the electrons to those generated in the sample. The spectrometer is calibrated with a standard metallic 57Fe absorber kept in the UHV system and movable to the sample position. The CEMS spectra were numerically fit using Voigt lines (convolution of the Lorentzian and Gaussian)46, which allows for consistent implementation of the hyperfine parameter distribution that is an inherent feature of low-dimensional systems. Throughout this paper, the isomer shift values are given with respect to metallic α-Fe. Complementary X-ray photoemission spectroscopy (XPS) measurements were performed in a second UHV multichamber system (PREVAC) that also allows for MBE, LEED/AES and STM 8


Page 8 of 40

Page 9 of 40 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


(RHK). The core-level photoelectron spectra were measured using non-monochromatized Al Kα radiation (hν = 1486.6 eV) and a hemispherical analyzer (Scienta). Binding energies are referenced to the Pt 4f signal, which was set to 79.9 eV. Universal sample holders and a cooling/heating system (Prevac) were used for all characterization methods. The Pt(111) crystal temperature could be varied between 100 K and 1500 K, as measured using a K-type thermocouple pressed against the crystal. The holders are transferrable between the UHV systems in a vacuum “suitcase” under a vacuum lower than 1×10-8 mbar. The Pt(111) substrate was cleaned by a standard procedure of annealing in an oxygen atmosphere (1x10-7 mbar, 10 min, 800 K), Ar+ bombardment (1,2 kV, 10-40 min) and flashing (1200 K) until a sharp (1×1)-Pt(111) LEED pattern was observed, and only traces of impurities were visible in the AES. In parallel, an extremely weak resonant

57

Fe signal (corresponding to approximately

10% of a 57Fe monolayer) was detectable in the CEMS spectra for the clean substrate subjected to numerous preparations using 57Fe. The deposition of iron, used for both reactive growth of the magnetite films and changing their stoichiometry, was controlled by a quartz thickness monitor with an accuracy greater than 0.1 Å. A typical deposition rate of 2.5 Å/min for magnetite growth was reduced to below 1 Å/min for better control of the low ad-Fe doses. For estimation of the magnetite layer thickness, the following equivalence was assumed based on atomic densities of alfa-bcc iron and magnetite: 1 Å of deposited metallic Fe provides 2.1 Å of Fe3O4. Throughout this paper, the amount of deposited iron is given as the equivalence of the densely packed Fe monolayers (dp-Fe ML) with

9



the atomic density of the close-packed oxygen (111)-layers in magnetite (1.30×1015 at./cm2). The nominal thickness of 1 dp-Fe ML is 1.52 Å.

Results and discussion A. Magnetite films with regular termination Fe3O4(111) films with regular termination were grown on a Pt(111) substrate kept at 550 K by reactive deposition of Fe at an O2 partial pressure of 8×10-6 mbar. After deposition, the samples were UHV annealed at 870 K for 15 min. As we showed previously using CEMS, which probes the entire film thickness47, this preparation method resulted in magnetite films that present bulklike features, except for one interfacial monolayer at the Pt(111) substrate and low-coordinated surface atoms. Additionally, for thicknesses below 20 Å, the electronic and magnetic properties are masked by superparamagnetic size effects; therefore, for the present studies, we used films in the thickness range of 20 to 100 Å, typically 50 Å.

10


Page 10 of 40

Page 11 of 40 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 2. Structural, chemical and electronic characterization of stoichiometric 57Fe3O4(111) film on Pt(111): (a) CEMS spectrum (for explanation of the color code of components refer to the text); (b) LEED pattern (electron energy 63 eV) and (c) I-V curves for selected spots; (d) atomically resolved 25×25 nm2 STM image (sample bias Vs=0.785 V, tunneling current 0.32 nA); (e) vertical profile along the scan line marked in (f) and (f) topographic large area 400×400 nm2 STM image. An exemplary CEMS spectrum of an 57Fe3O4 film (50 Å, i.e., approximately 10 pMLs) is shown in Fig. 2a, and the best-fit hyperfine parameters are summarized in Supporting Information, Table S1. The spectrum was measured in an efficiency-optimized geometry, i.e., with the γ-rays at 36° to the surface. For the discussion below, it is useful to note that an ideal 10 pML thick

11



Fe3O4(111) film contains 15 dp-Fe MLs, i.e., 1 dp-Fe ML would contribute to 6.5% of the spectral intensity. The spectrum was fitted with four magnetic components (labeled A, B, C and D), distinct in their isomer shifts (IS) and one low-intensity single line (L). The two most intense components, red and green, with IS characteristics of bulk magnetite, correspond to the tetrahedral (A) and octahedral (B) sites, respectively. The hyperfine magnetic field values (Bhf) of these sites are only slightly lower and broader compared to the bulk, which reflects either size effects or effects of lower coordination at the surface/interface48,49. These two components constitute 89±1 % of the total spectral intensity, which corresponds to nearly 9 pMLs. The B/A intensity ratio of 1.64±0.10 means a small excess of Fe3+ ions compared to stoichiometric magnetite50, interpreted as either nonstoichiometry (Fe3-δO4 with δ≈0.02) or frozen electron hopping at some octahedral sites related to a lowered dimensionality51,52. The remaining two sites have markedly different hyperfine patterns. The C-site (yellow subspectrum in Fig. 2a), with a 7% contribution to the spectral intensity, is characterized by Bhf=35.0±0.1 T, which is considerably lower than for magnetite, and intermediate IS between those of the A and B magnetite sites. The most notable characteristic for the C-sites is a significant quadrupole interaction parameter ε = 0.13±0.01 mm/s, compared to a negligible ε value for the cubic magnetite structure. This quadrupole interaction signifies a lower local symmetry for atoms at this site, which locates them either at the interface with Pt or on the surface, as discussed further. Site D (gray subspectrum in Fig. 2a), with 4.8±1% of the spectral intensity, contributes to the central part of the spectrum with a broadly distributed low-Bhf component. We attribute this component to low coordinated surface sites (e.g., at steps) or point defects. Finally, a single line L (black contour, spectral intensity below 0.5% is from dilute 57Fe impurities in the Pt substrate, accumulated during the numerous preparation cycles. Concerning the orientation of spontaneous magnetization, from a CEMS measurement, in which the sample was irradiated along the normal, we unambiguously found that 12


Page 12 of 40

Page 13 of 40 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 film is magnetized in-plane, similar to ultrathin Fe3O4(111) films on Ru(0001)53. This remains in strong contrast to Fe3O4(001) films on Mg(001), which have always shown a perpendicular magnetization component in a remanent state54,55. The film exhibited a (1×1) hexagonal LEED pattern (Fig. 2b) characteristic of regular Fe3O4(111) surfaces. LEED-spot intensity vs. energy curves (I–V curves) acquired at room temperature are shown in Fig. 2c. The I-V curves are almost identical to those reported in Ref. [38] (note a different indexing of the LEED spots) for the sample that was UHV-flashed at 900 K, which means that the film has only one structural domain and regular termination. This is confirmed by our atomically resolved STM images (compare Fig. 2d), which reveal a hexagonal lattice of lowprofile protrusions with a 6-Å periodicity. Similar structures, reported in several STM studies of the (111)-oriented magnetite surface, were assigned to Fe atoms of the ¼ Fetet1 surface39, 40, 41. The film is flat and continuous, and its growth reflects the surface morphology (atomic steps) of the Pt substrate. A line profile (Fig. 2e) through an STM large scan of the 50-Å magnetite films (Fig. 2f) reveals a 2.3-Å Pt-substrate step and 4.9-Å (1 pML) up and down structures of the topmost magnetite layer.

B. Biphase superstructures Stoichiometric Fe3O4(111)-(1×1) films with regular surfaces were the starting materials to follow the controlled formation of biphase superstructures as a function of the degree of surface reduction. Surface reduction was implemented by room-temperature deposition of a defined amount, dFe, of metallic iron (0.1 to 4 dp-Fe MLs), followed by a few minutes (typically 20 min) of UHV annealing. Our general observation was that starting from the lowest coverage, Fe is 13



homogenously adsorbed in the form of small grains, whereas annealing at moderate temperatures results in the formation of 3-dimensional islands that also homogenously cover the surface. Annealing at higher temperatures (depending on the film thickness and Fe-coverage) is necessary to induce the onset of biphase formation and incorporation of Fe into the film structure. Below, we present in detail the process of biphase formation for a 50-Å magnetite film with different doses of ad-Fe annealed at 870 K for 20 min.

Figure 3. Characterization of a reduced magnetite surface, obtained by adding 0.25 dp-Fe MLs: (a,b) LEED patterns at electron energy 63 eV and 30 eV, respectively; (c,d) large area 400×400 nm2 STM images at sample bias Vs = 0.63 V and Vs = - 1.20 V, respectively; (e) 50×50 nm2

14


Page 14 of 40

Page 15 of 40 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


STM image at Vs = 0.43 V that shows the coexistence of nonreconstructed (upper) and biphase “A” (lower) areas and (f) 14×14 nm2 image that shows the atomic details of biphase “A” with alpha and beta type subareas marked by pink/dotted and blue/solid circles, respectively. LEED patterns for the reduced magnetite surface, obtained by adding 0.25 dp-Fe MLs, are presented in Fig. 3a,b. The patterns taken at 63 eV (Fig. 3a), with characteristic satellites around the (2,0)-class spots, are typical for Fe3O4(111) surfaces with biphase superstructures41,43,44. However, the LEED pattern taken at 30 eV (Fig. 3b) reveals the superstructure not only for the second zone spots but also for the (1,0)-class reflexes. Corresponding large-scale STM images (Fig. 3c,d) show that a biphase for this amount of ad-Fe covers approximately 40% (±10%) of the surface. Due to the relatively large biphase areas, all low-intensity satellites around the magnetite reflexes were visible, that was not obvious in earlier studies of reduced Fe3O4 (111) single-crystal surfaces41, for which the surface contribution of the biphase was estimated to be 10%. The visibility of the biphase in the STM images strongly depends on the sample bias (compare Figs. 3c and d). From the images taken at a negative sample bias, Vs, it is clear that the biphase areas are attached to the Pt steps. Small-area STM scans in Figs. 3e,f show the atomic details of the coexisting surface structures. The area without reconstruction (upper part of Fig. 3e) is characterized by a 6-Å periodicity; however, the surface corrugations are less uniform than for the regular termination (compare Fig. 2d). In this biphase superstructure (we will call it “biphase A”), one can recognize two types of subareas marked with color circles in Fig. 3f. The first subarea type (termed type alpha, pink/dotted circle in Fig. 3f) has the atomic periodicity of the nonreconstructed areas (6 Å) and a similar contrast dependence on the sample bias. These subareas have a lateral dimension of approximately 20 Å and they form a hexagonal network with a periodicity of approximately 55 Å. The second subarea type (type beta, blue/solid circles 15



in Fig. 3f) is usually interpreted in the literature as being FeO-like39,41,43. It has a 3-Å atomic periodicity with rather uniform corrugations. Six beta subareas surround each of the alpha subareas. To the best of our knowledge, biphase A has not been observed before in STM measurements. The coexistence of biphase A with nonreconstructed areas proves that it is formed on the same oxygen atomic base layer as the regular termination, i.e., on the O1 layer. Taking into account the surface contribution of the biphase areas and noting that 0.25 dp-Fe nominally corresponds to one Fe atom per surface unit cell of Fe3O4(111), we conclude that, despite the 870 K annealing, the majority of the Fe atoms remained in the top-most atomic layer. On the other hand, for the smallest ad-Fe dose (0.1 dp-Fe ML), we did not observe any changes in either LEED or STM, which means that such a small amount of Fe can be accumulated in the magnetite layer without forming a biphase structure. STM images for increasing ad-Fe doses are shown in Fig. 4. When dFe increased to 0.6 dp-Fe MLs, the superstructure covers almost the entire surface. We observed a differentiation of the beta subareas, as shown in Figs. 4a,b. Three of the areas remained more or less unchanged, whereas the other three (termed gamma type, green circles in Fig. 4b) displayed additional modulation, making their periodicity similar to the Feoct1 layer. This biphase superstructure is termed “B”. With a further increase in the ad-Fe dose, the alpha subareas shrink, and for the 50-Å film, they practically vanish when dFe = 2 dp-Fe MLs. Simultaneously, we observed that for a given annealing temperature, the Fe amount for limiting the presence of the alpha subareas depends on

16


Page 16 of 40

Page 17 of 40 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 film thickness. For example, for a 30-Å (6 pML) film, the alpha subareas were absent for an Fe dose as small as dFe = 1.3 MLs.

Figure 4. STM images of surface superstructures that appear with increasing doses of ad-Fe (increasing degree of reduction): (a,b) biphase “B”, dFe = 0.6 dp-Fe MLs, 100×100 nm2 and 15×15 nm2 (Vs=-0.3 V, I=0. 32 nA), respectively, insert 10x10 nm2 (Vs =-1.20 V, I=1,65 nA); (c,d) biphase “C”, dFe = 2 dp-Fe MLs, 100×100 nm2 and 12×12 nm2 (Vs=-0.70 V, I= 1.1 nA), respectively; (e,f) superstructure “D”, dFe = 2.6 dp-Fe ML, 100×100 nm2 and 15×15 nm2 (Vs=0.85 V, I=0.9 nA), respectively. Pink/dotted, blue and green circles mark alpha, beta and gamma subareas, respectively.

17



The disappearance of the 6-Å periodicity, characteristic of the (1×1)-Fe3O4(111) surface, is also reflected in the LEED patterns. The intensity of the (01) spots decreased with increasing Fe dose, and above dFe = 2 dp-Fe MLs, these spots are no longer present, as shown in Fig. 5a. The vanishing of the alpha areas led to the next superstructure type, termed biphase C (Figs. 4 c,d). Biphase C can be understood as the coexistence of beta and gamma subareas that are paired and form a unit cell of the superstructure.

Figure 5. LEED pattern of biphase “C” (a) and its simple model (b) as a moiré superposition of the top-most hexagonal (red) and subsurface kagome (black) Fe layers. Blue (beta subarea) and green (gamma subarea) circles mark two types of atomic coincidence between the surface and subsurface Fe atoms. We propose that for both beta and gamma subareas, the outermost oxygen layer is the O1 layer, whereas the outermost iron layer is the densely packed hexagonal monolayer, which means that the external Fe-O double layer is FeO-like in terms of its stoichiometry. The difference in the atomic structure is the coordination of the iron atoms. For the gamma subareas, iron occupies sites corresponding to tetrahedral coordination, i.e., hollow sites in the O1 layer, directly above the atoms in the Feoct1 layer. Because the occupation of the kagome-Feoct1 layer is 3/4 and the Feoct1 sites belong to the first coordination shell of the surface-most Fe atoms, the atomic arrangement of the subsurface ¾ Feoct1 layer is reflected in an additional (kagome-like)

18


Page 18 of 40

Page 19 of 40 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


modulation observed in the STM images of the gamma subareas. The stacking sequence of surface layers in the gamma subareas can be written as 1 dp-Fetet1/O1/¾ Feoct1/O2 ... Strong support for the proposed structure is given by Pabisiak and Kiejna45, who studied the adsorption of Fe onto hematite (0001) and magnetite (111) surfaces using density functional theory. They showed that, for the regular Fe3O4(111) termination (i.e., ¼ Fetet1), a more favorable adsorption site than the Feoct2, which corresponds to the (111)-continuation of the bulk structure, is the hollow site of the O1 layer, above the Feoct1 atom. In our model for the beta subareas, the surface-most Fe atoms occupy octahedral positions (Feoct2). These sites are not affected by the occupation of the Feoct1 layer; hence, additional modulation in the STM is absent. The stacking sequence of surface layers in the beta subareas can be written as 1 dp-Feoct2/O1/¾ Feoct1/O2…. A simple visualization of the model for biphase C is shown in Fig. 5b. Within a simplification that takes into account only the arrangement of the Fe atoms, biphase type C can be understood as a moiré pattern of the top-most hexagonal and subsurface kagome Fe layers. If the hexagonal layer is strained and its atomic spacing is a factor of 18/17 larger than the spacing in the kagome layer, the periodicity of the resulting moiré superstructure is approximately 54 Å. Additionally, in the unit cell of the moiré superstructure, two types of atomic coincidences between the surface and subsurface Fe atoms can be distinguished (as marked with circles) that correspond to the beta and gamma subareas of the biphase structure. For an even higher Fe dose, biphase C gradually transforms into the last observed superstructure, type D. A small area of D is seen in the upper part of the STM image for 2 dp-Fe MLs of ad-Fe (Fig. 4c), and superstructure D covers the entire surface in the STM image for 2.6 dp-Fe MLs, Fig. 4e. A small-scale STM image in Fig. 4f shows that this ultimate superstructure presents a 19



typical simple moiré pattern of two densely packed hexagonal lattices with a slightly different periodicity, approximately 3 Å. Compared to biphase C, additional modification is subjected to the subsurface Feoct1 layer, which becomes a fully occupied densely packed hexagonal monolayer. The stacking sequence of surface layers for superstructure D can be written as 1 dpFeoct2/O1/1 dp-Feoct1 /O2/… Interestingly, the periodicities of the superstructures B, C and D are identical in the limit of the experimental error of the LEED analysis. On the other hand in the STM images, local inhomogeneities up to ten percent are observed. The 50 Å magnetite films are not capable to fully incorporate 4 dp-Fe MLs that was deposited at room temperature and annealed at 870 K, and an excess of Fe forms 3-dimensional islands on the biphase surface, as discussed in the next sections.

C. Surface versus volume structural changes induced by ad-Fe When iron was deposited on the magnetite film surface at room temperature, its incorporation into the film strongly depended on the Fe dose, film thickness and annealing temperature, TA. Fig. 6 presents the evolution of the surface morphology for a 50 Å Fe3O4(111) film with 2 Å (1.3 dp-Fe MLs) of ad-Fe as a function of TA. Below 550 K, ad-Fe stays on the surface in the form of a granular, homogenously distributed adsorbate. The Fe grains that appeared connected to each other at room temperature (Fig. 6a) were subjected to minor agglomeration and became individually distinguishable after mild annealing at 500 K (Fig. 6b). With TA increasing to 650 K, the adsorbate morphology entirely changed. The surface became decorated with large, 3dimensional islands, and the area between the islands exposed type C and D superstructures (Fig. 6c). The islands are elongated and have a flat (110)-oriented top. Similar Fe islands were 20


Page 20 of 40

Page 21 of 40 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


observed on Mo(110)56. Obviously, the surface layer is quite enriched in Fe. A further increase of TA led to dissolution of the islands, which finally disappeared after annealing at TA=750 K (Fig. 6d). Simultaneously, as precisely seen in LEED, with increasing TA, the surface became dominated by biphase B, which witnesses a decrease in the Fe concentration in the surface magnetite layer. Whereas for dFe= 2 Å (1.3 dp-Fe MLs), the islands vanished after annealing at TA=750 K, for dFe=6 Å (4 dp-Fe MLs), they partially remained even as TA was increased to 870 K and the annealing time was extended by a factor of three. However, to finally “dissolve” the islands, it was enough to raise TA by only 30 K to 900 K, which clearly shows that the incorporation of Fe into the magnetite film is a thermally activated process. A further increase of TA, which might lead to higher Fe incorporation and, possibly, structural film transformation, was limited by dewetting of the magnetite film, whose onset was found at TA=930 K.

Figure 6. Evolution of the surface morphology for a 50 Å Fe3O4(111) film with 1.3 dp-Fe MLs of ad-Fe as a function of the annealing temperature TA: (a) as deposited at room temperature, (b) TA=500 K, (c) TA=650 K, and (d) TA=750 K. 21



We noticed that the process of island formation and their dissolution was independent of whether Fe was deposited in one or in several successive doses. Therefore, we were able to follow the indepth Fe transport in a cumulative manner for a single magnetite film over a wide range of ad-Fe doses. To enhance the thermal stability of the film and to prevent its dewetting, the magnetite thickness was increased to 100 Å. Ad-57Fe was deposited in six doses, 4 Å (2.6 dp-Fe MLs) each, totaling 24 Å (15.8 dp-Fe MLs). After each dose, the sample was annealed in situ at 870 K, cooled down and characterized by LEED, STM, AES and XPS. After the 2nd, 4th and 6th dose, the sample was UHV transferred to another system for the CEMS measurements.

Figure 7. Fe 2p XPS spectra of a 100-Å Fe3O4(111) film (from bottom to top) : stoichiometric as-prepared film, with a total of 4 Å (2.6 dp-Fe MLs) and 24 Å (15.8 dp-Fe MLs) of ad-Fe after annealing at 870 K (all collected with the electron take-off angle of 90o measured from the surface), and for 24 Å of ad-Fe at the electron take-off angle of 15o. LEED and STM measurements showed biphases B and C at dFe= 4 Å (2.6 dp-Fe MLs), biphases C and D after a cumulative dFe= 8 Å (5.3 dp-Fe MLs) and only biphase D after a cumulative dFe= 12 Å (7.9 dp-Fe MLs). At this stage, (0,1) spots of magnetite were absent, and the LEED pattern exhibited only (0,2) spots with the characteristic moiré superstructure. Surprisingly, after the next 22


Page 22 of 40

Page 23 of 40 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


Fe doses, the (0,1) spots reappeared, and their intensity increased with the cumulative Fe dose. However, contrary to the initial doses, the relative intensities of the (0,1) and (0,2) spots varied depending on their position on the sample. The changes in the LEED pattern across the sample indicate the coexistence of different surface structures, which was indeed observed on a micrometer scale in STM images. The sequence of the surface structures appearing with increasing Fe doses suggests that iron might not stay in the near-surface layers. The comparative use of spectroscopic methods with different probing depths (XPS, AES and CEMS) shed more light on this problem. The Fe 2p core-level photoemission spectra are shown in Fig. 7. For the take-off angle of 90◦ (measured from the surface), the probing depth was comparable to the film thickness. The as-prepared 100 Å magnetite films gave a typical magnetite XPS spectrum, in which the overlapping of equally intense satellites from Fe2+ and Fe3+ results in an unresolved structure between the two spin– orbit, 2p3/2 and 2p1/2, components57. Upon adding Fe followed by annealing, the 2p3/2 and 2p1/2 maxima were shifted to lower binding energies, and satellite peaks at approximately 716 eV and 729 eV, characteristic of the Fe2+ valence state, appeared. However, the excess Fe2+ ions are not localized in the surface layers because in the XPS at a take-off angle of 15°, i.e., when the probing depth is reduced by a factor of four, the Fe2+ satellite is strongly suppressed. Apparently, the high contribution of the Fe2+ signal has its origin in layers closer to the Pt/magnetite interface. These observations are compared with the CEMS results shown in Fig. 8a. With an increasing dose of ad-Fe, from 8 Å (5.3 dp-Fe MLs) through 16 Å (10.5 dp-Fe MLs) to 24 Å (15.8 dp-Fe MLs) for the spectra from top to bottom, respectively, the most distinct change in the spectra is the increasing intensity of the central spectrum part. Quantitatively, the stoichiometric magnetite components dominate the spectra, however, they are accompanied by subspectra that are not 23



specific to the stoichiometric iron compounds. For the film without ad-Fe, the amount of nonmagnetite spectral components is limited to an equivalent of 1 dp-Fe ML (compare Section A), and their total intensity increases fairly linearly with the ad-Fe dose, as shown in Fig. 8b.

24


Page 24 of 40

Page 25 of 40 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 8. (a) CEMS spectra of a 100 Å Fe3O4(111) film with a total of 8 Å, 16 Å and 24 Å (from top to bottom, respectively) after annealing at 870 K (for explanation of the component color code refer to the text). (b) contribution (in equivalent of dp-Fe ML) of the spectral components, as colored in (a), as a function of the total amount of Fe. Remarkably, the absolute amount of the magnetite components remains constant, which may suggest that magnetite constitutes the core of the film and that additional Fe species are located at the film boundaries. The mechanism that would lead to such a phase separation is the migration of excess iron from the surface to the film-Pt interface. On the other hand, according to the proposed model, biphase superstructures require an increased surface content of Fe; hence, a certain amount of ad-Fe should remain at the surface. We interpret the spectral components that appear with increasing amounts of ad-Fe accordingly. The weakly magnetic component with a large IS>0.7 mm/s (marked blue in Figs. 8a,b), absent in the spectrum of the as-prepared film, unambiguously evidences the Fe2+ species, which, by XPS analysis, are located in deep layers. The single line (marked black in Figs. 8a,b) is attributed to Fe atoms dissolved in the Pt substrate. For comparison, Genuzio et al. observed that UHV annealing of an iron oxide film on Pt(111) led to the transformation of Fe3O4 into α-Fe2O358,59. These authors proposed the interpretation that an excess of iron in magnetite compared to hematite was dissolved in the Pt substrate, which requires the diffusion of Fe cations across the film. On the other hand, Dieckmann and Schmalzried60 showed that in magnetite with an excess of iron cations, which is our case, the mobility of interstitial iron is exceptionally fast that explains the easy transport of Fe toward the interface. A similar process is evidenced for our films, and the isomer shift of iron diluted in Pt (0.34 mm/s61) reasonably agrees with the average IS=0.30 ±0.01 for the single line component. The final intensity of the single line component, equivalent to over five dp-Fe MLs, indicates that 25



iron is diluted in Pt over a depth of several micrometer because otherwise, a more complex spectrum (possibly due to a ferromagnetic FePt alloy) would be expected. Such a deep penetration of the Fe atoms occurs after the highest annealing temperature and requires a laborious process to restore a clean surface to the Pt(111) substrate. Finally, all spectra contain a distinct magnetic component (marked yellow in Figs. 8a,b) characterized by a significant quadrupole interaction parameter |ε|= 0.13±0.02, negative for the regular termination and generally positive, though occasionally negative, depending on the surface superstructure. The isomer shift and hyperfine magnetic field in the ranges of 0.35 - 0.56 mm/s and 30 - 35 T, respectively, combined with the large electric field gradient, are not specific for either magnetite or any other Fe-oxide compound. Moreover, the spectral intensity of this component roughly corresponds to integer numbers of dp-Fe MLs increasing from one to three with the amount of ad-Fe. Logically, we attribute this component to the surface layer, which was modified in the process of reduction. Such a spectral interpretation is applicable for many magnetite samples, with thicknesses varying between 20 Å and 100 Å, subjected to deposition of ad-Fe and the formation of surface superstructures. The CEMS measurements revealed the actual composition of the film, whose nominal average stoichiometry after the total Fe dose was Fe1.1O. According to CEMS, in the final sample, iron occurred as phases of Fe3O4 and FeO stoichiometry or in metallic form (dissolved in Pt) in 64%, 23% or 13%, respectively. This gives an average stoichiometry of Fe0.82O, which is considerably more oxygen rich than nominal. Evidently, the sample was oxidized during the complex preparation and characterization cycles and exposed to oxygen-containing residual atmosphere (water and carbon oxides). The amount of oxygen additionally incorporated into the sample that is necessary to explain the final composition is equivalent to approximately 10 oxygen 26


Page 26 of 40

Page 27 of 40 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


monolayers, to be compared to more than 104 Langmuir expositions to residual gases. This is certainly sufficient to explain the apparently surprising effect of oxidation under UHV conditions, especially considering that oxidation of magnetite film using water adsorption is the effective way for the preparation of epitaxial hematite (0001) on Pt(111) under UHV conditions62. The above discussion is based on an arbitrary location of iron species contributing to different spectral components. In the next section, we present an additional analysis of the magnetite doping with ad-Fe based on the CEMS analysis using

56

Fe and

57

Fe probe layers, which sheds

more light on the depth distribution of the film composition.

D. CEMS analysis of the ad-Fe/Fe3O4(111) films with isotopic 57Fe probe layers To study the incorporation of ad-Fe and the formation of surface superstructures upon annealing, different samples with isotopic Fe probes were prepared. For a 50 Å

56

Fe3O4 film with 4 Å (2.6 dp-Fe MLs) of ad-57Fe, more than half (59%) of the as-

deposited ad-Fe, which shows up in STM as a granular adsorbate (compare Fig. 6a), appears in a Mössbauer spectrum (Fig. 9a) as having a metallic character (purple subspectrum). This is clear from the metallic-Fe-like IS = 0.05±0.03 mm/s, although the hyperfine magnetic field, peaked at 36 T, and is clear evidence of the surface character of low-coordinated Fe atoms with strongly enhanced magnetic moments. The remaining iron, contributing to a green component, is of oxidecharacter (IS=0.64±0.09 mm/s), and a broadly distributed hyperfine magnetic field not exceeding 45 T are representative of defect octahedral sites, similar to those found at the Fe3O4(001) surface49. The described distribution of the CEMS components means a simple layer morphology

27



of the ad-Fe adsorbate, with one monolayer either in direct contact or embedded in a surface magnetite layer and the remaining metallic-Fe on top of it. As described above, ad-Fe penetrates the film interior by intensive annealing. However, there is a combination of ad-Fe dose and annealing temperature where 3-dimensional islands are formed on the surface, as evidenced by STM (compare Fig. 6c). A clear morphological separation of the islands from the magnetite substrate indicates their chemical distinctness, which was confirmed by CEMS. To enhance surface sensitivity, we used films with isotopic probe layers. It should be noted that due to an intermixing between 57Fe and 56Fe, the information contained in the spectra, although strongly enhanced, is not restricted to the nominal position of the probe layer, especially after annealing at higher temperatures.

28


Page 28 of 40

Page 29 of 40 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 9. CEMS spectra with probe layers deposited using

57

Fe in magnetite films composed

otherwise of 57Fe: (a) a 50 Å 56Fe3O4 film with metallic 2.6 dp-57Fe MLs deposited on top; (b) a 40 Å

56

Fe3O4 film with a 10 Å 57Fe3O4 probe top-layer and two doses of ad-Fe (4 Å of

56

Fe

annealed at 750 K and 4 Å of 57Fe annealed at 650 K); (c) same as in (b) but with the 4 Å 57Fe dose additionally annealed at 750 K. A 40 Å 56Fe3O4 film with a 10 Å 57Fe3O4 probe top layer was able to fully incorporate 4 Å of adFe (56Fe) as a result of annealing at 750 K. Simultaneously, a biphase structure appeared in the LEED. However, the subsequent 4 Å of 57Fe remained at the surface, at least partially. Figs. 9b,c show the Mössbauer spectra after the second 4 Å Fe dose and annealing at 650 K and 750 K, respectively. After the first annealing, LEED displays a simple six-fold symmetry, typical for an FeO-like surface, with broad spots, which do not allow for the identification of the biphase type. Simultaneously, in CEMS, an atypical feature of the magnetite-like components in Fig. 9b is the large intensity ratio of the octahedral to tetrahedral Fe3+ abundance, which is 2.8 (compared to ~2.0 for magnetite). This can be simply explained by decomposition of the octahedral spectra component into a regular sharp component (green in Fig. 9b) and a component with a smaller hyperfine magnetic field and its broader distribution (a darker shade of green in Fig. 9b). The surface enrichment in octahedral Fe2+ ions, represented by the darker green component, is then an effect of partial oxidation of the ad-Fe. Moreover, a significant amount of ad-Fe remains metallic, as represented by the purple component. In comparison to the as-deposited state of ad-Fe, after annealing the hyperfine parameters of metallic iron become bulk-like, which is consistent with the better distinguishable grains of the Fe adsorbate (compare Fig. 6b). The two remaining magnetic subspectra originate from the surface layer of the magnetite film. We attribute the sharp 29



orange component to well-defined Fe surface sites of octahedral symmetry and a valence state of approximately 2+. Its hyperfine parameters are close to the yellow surface component in Fig. 8, except for the negligible quadrupole splitting. Evidently, the ad-Fe adsorbate restores the translational symmetry. The gray component represents the irregular sites with the lowest coordination from the structural and electronic points of view. In the bulk, an IS=0.24 mm/s found for the gray component would unambiguously indicate the Fe3+ state, but in low dimensional systems, the bulk systematics may be not applicable. This is because the IS value for a 3dn4sx configuration results from a combination of the direct contribution of the s-electrons (with negative correlation between the occupation x and IS) and indirect (due to a screening effect by d-electrons) contribution of the d-electrons (with positive correlation between the occupation n and IS). The charge transfer in the surface magnetite layer32 and a complicated relation between the electronic structure and the Mössbauer isomer shift63 make precise assignment of the valence states of surface atoms rather difficult. After the second annealing (Fig. 9c), a clear biphase structure appears in the LEED on the areas exposed as a result of the large island formation (compare Fig. 6c), and the corresponding Mössbauer spectrum changes. First, the proper intensity ratio of the magnetite components was restored, which is at least partially due to the

57

Fe in-depth diffusion. For the same reason, the

intensity of the metallic Fe component decreased. The most interesting result is a qualitative change in the surface component, which is now marked in yellow. For the exposed biphase surface, the large quadrupole splitting is restored. The isomer shift, IS > 0.8 mm/s, and ε = 0.21(5) mm/s signify a surface superstructure containing Fe2+ ions. The blue component has a similar assignment as discussed above (compare Figs. 8a,b), i.e., we attribute it to Fe-rich deep oxide layers. 30


Page 30 of 40

Page 31 of 40 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


Conclusions Combined analysis of Fe-enriched Fe3O4 epitaxial films on Pt(111) using surface sensitive methods and CEMS that probes the entire film volume gives a coherent picture of the surface and volume modification of the film structure. We showed that surface enrichment with deposition of metallic iron, followed by annealing, is a simple and clear method to tune the superstructures (biphases) characteristic of the Fe3O4(111) surface. An alternative way of controlling the biphase superstructure on magnetite films by an annealing at oxygen deficient conditions seems to be less effective and may lead to an unexpected oxidation instead the expected surface reduction58. Our method could be also applied for single crystal surfaces, as an alternative way to cycles of ion sputtering and UHV annealing41. We were able to systematize the superstructures as a function of the increasing surface-Fe content and give a simple phenomenological explanation of the STM images based on the moiré coincidence structure of the two outermost Fe atomic layers. A first principle justification of the proposed models is a challenge for theory. On the other hand, our CEMS measurements revealed the mechanism of the volume enrichment of the films in Fe. An excess of ad-Fe, which stabilizes a given surface superstructure, migrates towards the substrate, partially dissolves in Pt and partially forms an interfacial layer with FeO stoichiometry. The present research opens a path of controlled adsorption studies both of gaseous and solid deposits at specific sites of precisely characterized substrates that are distinct by various chemical, atomic and electronic surface structures. Acknowledgment

31



This work was conducted under Grant No. 2016/21/B/ST3/00861 funded by the National Science Centre, Poland, and partially supported by the statutory research funds of ICSC PAS and AGH UST (task No. 11.11.220.01/6) within subsidy of the Ministry of Science and Higher Education, Poland. Supporting Information Supporting Information Available: The best-fit hyperfine parameters of CEMS spectra in Figs. 2a, 8a and 9. This material is available free of charge via the Internet at http://pubs.acs.org. References

(1) Qiao, L.; Fu, Z.; Li, J.; Ghosen, J.; Zeng, M.; Stebbins, J.; Prasad, P. N.; Swihart, M. T. Standardizing Size- and Shape-Controlled Synthesis of Monodisperse Magnetite (Fe3O4) Nanocrystals by Identifying and Exploiting Effects of Organic Impurities. ACS Nano 2017, 11 , 6370–6381. (2) Kim, J.; Kim, H. S.; Lee, N.; Kim, T.; Kim, H.; Yu, T.; Song, I. C.; Moon, W. K.; Hyeon, T. Multifunctional Uniform Nanoparticles Composed of a Magnetite Nanocrystal Core and a Mesoporous Silica Shell for Magnetic Resonance and Fluorescence Imaging and for Drug Delivery. Angew. Chemie Int. Ed. 2008, 47, 8438–8441. (3) Salas, G.; Casado, C.; Teran, F. J.; Miranda, R.; Serna, C. J.; Morales, M. P. Controlled Synthesis of Uniform Magnetite Nanocrystals with High-Quality Properties for Biomedical Applications. J. Mater. Chem. 2012, 22, 21065. (4) Hu, F. Q.; Wei, L.; Zhou, Z.; Ran, Y. L.; Li, Z.; Gao, M. Y. Preparation of Biocompatible Magnetite Nanocrystals for In Vivo Magnetic Resonance Detection of Cancer. Adv. Mater. 2006, 18, 2553–2556. 32


Page 32 of 40

Page 33 of 40 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


(5) Revia, R. A.; Zhang, M. Magnetite Nanoparticles for Cancer Diagnosis, Treatment, and Treatment Monitoring: Recent Advances. Mater. Today 2016, 19, 157–168. (6) Song, H.-T.; Choi, J.; Huh, Y.-M.; Kim, S.; Jun, Y.; Suh, J.-S.; Cheon, J. Surface Modulation of Magnetic Nanocrystals in the Development of Highly Efficient Magnetic Resonance Probes for Intracellular Labeling. J. Am. Chem. Soc. 2005, 127, 9992–9993. (7) Gawande, M. B.; Branco, P. S.; Varma, R. S. Nano-Magnetite (Fe3O4) as a Support for Recyclable Catalysts in the Development of Sustainable Methodologies. Chem. Soc. Rev. 2013, 42, 3371. (8) Karami, H. Heavy Metal Removal from Water by Magnetite Nanorods. Chem. Eng. J. 2013, 219, 209–216. (9) Weiss, W.; Ranke, W. Surface Chemistry and Catalysis on Well-Defined Epitaxial Iron-Oxide Layers. Prog. Surf. Sci. 2002, 70, 1–151. (10) 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 (11) Li, Z.; Potapenko, D. V.; Rim, K. T.; Flytzani-Stephanopoulos, M.; Flynn, G. W.; Osgood, R. M.; Wen, X.-D.; Batista, E. R. Reactions of Deuterated Methanol (CD3OD) on Fe3O4(111). J. Phys. Chem. C 2015, 119, 1113–1120. (12) Freund, H.-J.; Pacchioni, G. Oxide Ultra-Thin Films on Metals: New Materials for the Design of Supported Metal Catalysts. Chem. Soc. Rev. 2008, 37, 2224–2242. (13) Parkinson, G. S. Iron Oxide Surfaces. Surf. Sci. Rep. 2016, 71, 272–365. (14) Moussy, J.-B. From Epitaxial Growth of Ferrite Thin Films to Spin-Polarized Tunnelling. J. Phys. D. Appl. Phys. 2013, 46, 143001.

33



(15) Parkinson, G. S.; Diebold, U.; Tang, J.; Malkinski, L. Tailoring the Interface Properties of Magnetite for Spintronics. In Advanced Magnetic Materials; Malkinski, L., Ed.; InTech, 2012; pp 61–88. (16) Cornell, R. M.; Schwertmann, U. The Iron Oxides; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, FRG, 2003. (17) Walz, F. The Verwey Transition - a Topical Review. J. Phys. Condens. Matter 2002, 14, R285–R340. (18) Parkinson, G. S.; Novotný, Z.; Jacobson, P.; Schmid, M.; Diebold, U. A Metastable Fe(A) Termination at the Fe3O4(001) Surface. Surf. Sci. 2011, 605, 1–4. (19) Ruiz-Gómez, S.; Serrano, A.; Carabias, I.; García, M. A.; Hernando, A.; Mascaraque, A.;

Pérez, L.; González Barrio, M. A.; Rodríguez de la Fuente, O. Formation of a Magnetite/Hematite Epitaxial Bilayer Generated with Low Energy Ion Bombardment. Appl. Phys. Lett. 2017, 110, 093103. (20) Choudhury, T.; Saied, S. O.; Sullivan, J. L.; Abbot, A. M. Reduction Of Oxides Of Iron, Cobalt, Titanium And Niobium By Low-Energy Ion-Bombardment. J. Phys. D: Appl. Phys. 1989, 22, 1185-1195. (21) Condon, N. G.; Leibsle, F. M.; Lennie, A. R.; Murray, P. W.; Vaughan, D. J.; Thornton, G. Biphase Ordering of Iron Oxide Surfaces. Phys. Rev. Lett. 1995, 75, 1961–1964. (22) Rim, K. T.; Fitts, J. P.; Müller, T.; Adib, K.; Camillone, N.; Osgood, R. M.; Joyce, S. A.; Flynn, G. W. CCl4 Chemistry on the Reduced Selvedge of a α-Fe2O3(0001) Surface: A Scanning Tunneling Microscopy Study. Surf. Sci. 2003, 541, 59–75.

34


Page 34 of 40

Page 35 of 40 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


(23) Rim, K. T.; Eom, D.; Chan, S.; Flytzani-Stephanopoulos, M.; Flynn, G. W.; Wen, X.-D.;

Batista, E. R. Scanning Tunneling Microscopy and Theoretical Study of Water Adsorption on Fe3O4  : Implications for Catalysis. J. Am. Chem. Soc. 2012, 134, 18979–18985. (24) Wright, J. P.; Bell, A. M. T.; Attfield, J. P. Variable Temperature Powder Neutron Diffraction Study of the Verwey Transition in Magnetite Fe3O4. Solid State Sci. 2000, 2, 747–753. (25) Ahdjoudj, J.; Martinsky, C.; Minot, C.; Van Hove, M. A.; Somorjai, G. A. Theoretical Study of the Termination of the Fe3O4(111) Surface. Surf. Sci. 1999, 443, 133–153. (26) Tasker, P. W. The Stability of Ionic Crystal Surfaces. J. Phys. C Solid State Phys. 1979, 12, 4977–4984. (27) Zhu, L.; Yao, K. L.; Liu, Z. L. First-Principles Study of the Polar (111) Surface of Fe3O4. Phys. Rev. B 2006, 74, 035409. (28) Grillo, M. E.; Finnis, M. W.; Ranke, W. Surface Structure and Water Adsorption on Fe3O4(111): Spin-Density Functional Theory and on-Site Coulomb Interactions. Phys. Rev. B 2008, 77, 075407. (29) Yu, X.; Huo, C.-F.; Li, Y.-W.; Wang, J.; Jiao, H. Fe3O4 Surface Electronic Structures and Stability from GGA+U. Surf. Sci. 2012, 606, 872–879. (30) Kiejna, A.; Ossowski, T.; Pabisiak, T. Surface Properties of the Clean and Au/Pd Covered Fe3O4(111): DFT and DFT+U Study. Phys. Rev. B - Condens. Matter Mater. Phys. 2012, 85, 1– 11. (31) Santos-Carballal, D.; Roldan, A.; Grau-Crespo, R.; de Leeuw, N. H. A DFT Study of the Structures, Stabilities and Redox Behaviour of the Major Surfaces of Magnetite Fe3O4. Phys. Chem. Chem. Phys. 2014, 16, 21082–21097.

35



(32) Noh, J.; Osman, O. I.; Aziz, S. G.; Winget, P.; Brédas, J.-L. Magnetite Fe3O4(111) Surfaces:

Impact of Defects on Structure, Stability, and Electronic Properties. Chem. Mater. 2015, 27, 5856–5867. (33) Li, X.; Paier, J. Adsorption of Water on the Fe3O4(111) Surface: Structures, Stabilities, and Vibrational Properties Studied by Density Functional Theory. J. Phys. Chem. C 2016, 120, 1056– 1065. (34) Lewandowski, M.; Groot, I. M. N. N.; Qin, Z.-H.; Ossowski, T.; Pabisiak, T.; Kiejna, A.; Pavlovska, A.; Shaikhutdinov, S.; Freund, H.-J.; Bauer, E. Nanoscale Patterns on Polar Oxide Surfaces. Chem. Mater. 2016, 28, 7433–7443. (35) Weiss, W.; Barbieri, A.; Van Hove, M. A.; Somorjai, G. Surface Structure Determination of an Oxide Film Grown on a Foreign Substrate: Fe3O4 Multilayer on Pt(111) Identified by Low Energy Electron Diffraction. Phys. Rev. Lett. 1993, 71, 1848–1852. (36) Barbieri, A.; Weiss, W.; Van Hove, M. A.; Somorjai, G. A. Magnetite Fe3O4(111): surface structure by LEED crystallography and energetics, Surf. Sci. 1994, 302, 259–279. (37) Ritter, M.; Weiss, W. Fe3O4(111) Surface Structure Determined by LEED Crystallography. Surf. Sci. 1999, 432, 81–94. (38) Sala, A.; Marchetto, H.; Qin, Z.-H.; Shaikhutdinov, S.; Schmidt, T.; Freund, H.-J. Defects and Inhomogeneities in Fe3O4(111) Thin Film Growth on Pt(111). Phys. Rev. B 2012, 86, 155430. (39) Shaikhutdinov, S. K.; Ritter, M.; Wang, X.; Over, H.; Weiss, W. Defect Structures on Epitaxial Fe3O4(111) Films. Phys. Rev. B 1999, 60, 11062–11069. (40) Berdunov, N.; Murphy, S.; Mariotto, G.; Shvets, I. V. Room Temperature Study of a StrainInduced Electronic Superstructure on a Magnetite (111) Surface. Phys. Rev. B 2004, 70, 085404. 36


Page 36 of 40

Page 37 of 40 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


(41) Paul, M.; Sing, M.; Claessen, R.; Schrupp, D.; Brabers, V. Thermodynamic Stability and Atomic and Electronic Structure of Reduced Fe3O4(111) Single-Crystal Surfaces. Phys. Rev. B 2007, 76, 075412. (42) Shimizu, T. K.; Jung, J.; Kato, H. S.; Kim, Y.; Kawai, M. Termination and Verwey Transition of the (111) Surface of Magnetite Studied by Scanning Tunneling Microscopy and First-Principles Calculations. Phys. Rev. B 2010, 81, 235429. (43) Condon, N. G.; Leibsle, F. M.; Parker, T.; Lennie, A. R.; Vaughan, D. J.; Thornton, G. Biphase Ordering on Fe3O4(111). Phys. Rev. B 1997, 55, 15885–15894. (44) Berdunov, N.; Mariotto, G.; Murphy, S.; Balakrishnan, K.; Shvets, I. V. Self-Assembly of Iron Nanoclusters on the Fe3O4(111) Superstructured Surface. Phys. Rev. B 2005, 71, 113406. (45) Pabisiak, T.; Kiejna, A. Fe Adsorption on Hematite (α-Fe2O3) (0001) and Magnetite (Fe3O4) (111) Surfaces. J. Chem. Phys. 2014, 141, 134707. (46) Rancourt, D. G.; Ping, J. Y. Voigt-Based Methods for Arbitrary-Shape Static Hyperfine Parameter Distributions in Mössbauer Spectroscopy. Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms 1991, 58, 85–97.

(47) Spiridis, N.; Wilgocka-Ślęzak, D.; Freindl, K.; Figarska, B.; Giela, T.; Młyńczak, E.; Strzelczyk, B.; Zając, M.; Korecki, J. Growth and Electronic and Magnetic Structure of Iron Oxide Films on Pt(111). Phys. Rev. B 2012, 85, 075436. (48) Korecki, J.; Handke, B.; Spiridis, N.; Ślęzak, T.; Flis-Kabulska, I.; Haber, J. Size Effects in Epitaxial Films of Magnetite. Thin Solid Films 2002, 412, 14–23 (49) Spiridis, N.; Handke, B.; Slezak, T.; Barbasz, J.; Zajac, M.; Haber, J.; Korecki, J. Surface Structure of Epitaxial Magnetite Fe3O4 (001) Films: In Situ STM and CEMS Studies. J. Phys. Chem. B 2004, 108, 14356–14361. 37



(50) Gorski, C. A.; Scherer, M. M. Determination of Nanoparticulate Magnetite Stoichiometry by Mossbauer Spectroscopy, Acidic Dissolution, and Powder X-Ray Diffraction: A Critical Review. Am. Mineral. 2010, 95, 1017–1026. (51) Łodziana, Z. Surface Verwey Transition in Magnetite. Phys. Rev. Lett. 2007, 99, 206402. (52) Shvets, I. V.; Mariotto, G.; Jordan, K.; Berdunov, N.; Kantor, R.; Murphy, S. Long-Range Charge Order on the Fe3O4(001) Surface. Phys. Rev. B 2004, 70, 155406. (53) Lewandowski, M.; Miłosz, Z.; Michalak, N.; Ranecki, R.; Sveklo, I.; Kurant, Z.; Maziewski, A.; Mielcarek, S.; Luciński, T.; Jurga, S. Room Temperature Magnetism of Few-NanometersThick Fe3O4(111) Films on Pt(111) and Ru(0001) Studied in Ambient Conditions. Thin Solid Films 2015, 591, 285–288. (54) Voogt, F. C.; Palstra, T. T. M.; Niesen, L.; Rogojanu, O. C.; James, M. A.; Hibma, T. Superparamagnetic Behavior of Structural Domains in Epitaxial Ultrathin Magnetite Films. Phys. Rev. B 1998, 57, R8107–R8110. (55) Zając, M.; Freindl, K.; Ślęzak, T.; Ślęzak, M.; Spiridis, N.; Wilgocka-Ślęzak, D.; Korecki, J.; Zaja̧ c, M.; Freindl, K.; Ślȩzak, T.; et al. Electronic and Magnetic Properties of Ultra-Thin Epitaxial Magnetite Films on MgO(001). Thin Solid Films 2011, 519, 5588–5595. (56) Hertel, R.; Fruchart, O.; Cherifi, S.; Jubert, P.-O.; Heun, S.; Locatelli, A.; Kirschner, J. Three-Dimensional Magnetic-Flux-Closure Patterns in Mesoscopic Fe Islands. Phys. Rev. B 2005, 72, 214409. (57) Moussy, J.-B. From Epitaxial Growth of Ferrite Thin Films to Spin-Polarized Tunnelling. J. Phys. D. Appl. Phys. 2013, 46, 143001.

38


Page 38 of 40

Page 39 of 40 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


(58) Genuzio, F.; Sala, A.; Schmidt, T.; Menzel, D.; Freund, H.-J. Interconversion of α-Fe2O3 and

Fe3O4 Thin Films: Mechanisms, Morphology, and Evidence for Unexpected Substrate Participation. J. Phys. Chem. C 2014, 118, 29068–29076. (59) Genuzio, F.; Sala, A.; Schmidt, T.; Menzel, D.; Freund, H.-J. Phase Transformations in Thin Iron Oxide Films: Spectromicroscopic Study of Velocity and Shape of the Reaction Fronts. Surf. Sci. 2016, 648, 177–187. (60 ) Dieckmann R.; Schmalzried, H. Defects and Cation Diffusion in Magnetite (II). Ber. Bun. Phys. Chem. 1977, 81, 414 – 419. (61) Qaim S. M. Mössbauer Effect of

57

Fe in Various Hosts: Isomer Shifts of the 14.4 keV

Gamma Line of 57Fe in Different Metallic Lattices. Proc. Phys. Soc. 1967, 90, 1065-1076. (62) Kaya, S.; Ogasawara, H.; Nilsson, A. Determination of the Surface Electronic Structure of Fe3O4 (111) by Soft X-Ray Spectroscopy. Catal. Today 2015, 240, 184–189. (63) Neese, F. Prediction and Interpretation of the

57

Fe Isomer Shift in Mössbauer Spectra by

Density Functional Theory. Inorganica Chim. Acta 2002, 337, 181–192.

39



TOC Graphic:

40


Page 40 of 40

Superstructures on Epitaxial Fe3O4(111) Films: Biphase Formation

Recommend Documents