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Surface Structural Evolution in Iron Oxide Thin Films
Abstract
Ordered iron oxide ultrathin films were fabricated on a single-crystal Mo(110) substrate under ultrahigh vacuum conditions by either depositing Fe in ambient oxygen or oxidizing preprepared Fe(110) films. The surface structure and electronic structure of the iron oxide films were investigated by various surface analytical techniques. The results indicate surface structural transformations from metastable FeO(111) and O-terminated Fe(2)O(3)(0001) to Fe(3)O(4)(111) films, respectively. The former depends strongly on the oxygen pressure and substrate temperature, and the latter relies mostly upon the annealing temperature. Our experimental observations are helpful in understanding the mechanisms of surface structural evolution in iron oxides. The model surfaces of Fe-oxide films, particularly O-terminated surfaces, can be used for further investigation in chemical reactions (e.g., in catalysis).
... Hematite (0 0 1) surface is coherent with magnetite (1 1 1) and wustite (1 1 1) surfaces. It has been found that the surface phase transformations are sensitive to the temperature and pressure of O 2 [25]. The reduced surface phases (magnetite (1 1 1) and wustite (1 1 1)) should be formed and sustained from high-temperature processes where they are thermodynamically more stable than hematite. ...
... The O-rich termination (B) was predicted to be thermodynamically more stable than the stoichiometric termination using GGA (Generalized Gradient Approximation) [39,31] and was confirmed by the structure characterizations [25,28,38]. However, a reverse stability order was proposed when considering the strongly-correlated effects [21]. ...
... The XPS spectrum across an area of 2×0.2 mm 2 confirms that all Fe ions in the nanosheets are Fe 3+ , with no trace of Fe 2+ ( Fig. 1(d)). [26,27] As shown in Fig. 1(e), the Raman spectrum of ε-Fe 2 O 3 nanosheets has four unique Raman peaks between 100 cm −1 and 200 cm −1 as hallmarks to verify the ε-Fe 2 O 3 phase, which do not exist in γ-Fe 2 O 3 or α-Fe 2 O 3 phase. [23,28] The Raman mapping result of a typical nanosheet indicates the pure ε-Fe 2 O 3 phase in the whole sample ( Fig. 1(f)). ...
Two-dimensional (2D) magnet/superconductor heterostructures can promote the design of artificial materials for exploring 2D physics and device applications by exotic proximity effects. However, plagued by the low Curie temperature and instability in air, it is hard to realize practical applications for the reported layered magnetic materials at present. In this paper, we developed a space-confined chemical vapor deposition method to synthesize ultrathin air-stable ε-Fe 2 O 3 nanosheets with Curie temperature above 350 K. The ε-Fe 2 O 3 /NbSe 2 heterojunction was constructed to study the magnetic proximity effect on the superconductivity of the NbSe 2 multilayer. The electrical transport results show that the subtle proximity effect can modulate the interfacial spin-orbit interaction while undegrading the superconducting critical parameters. Our work paves the way to construct 2D heterojunctions with ultrathin nonlayered materials and layered van der Waals (vdW) materials for exploring new physical phenomena.
... The results of the fit procedure for the Fe 2p lines (Fig. S7(e) and Fig. 3(b)) show the slight intensity increase of the Fe 3+ component at E − E F ≈ −710 eV with increase of the Ni concentration (x = 0.25 → x = 0.75), indicating the change of the charge state of Fe ions with variation of x. The increase of the charge state of Fe in Fe 1−x Ni x PS 3 for x = 0.50 and x = 0.75 is also reflected in the increase of the intensity of the satellite peak at E − E F ≈ −713.5 eV.[56][57][58] This can be considered as a result of the band gap widening when going from FePS 3 to NiPS 3 (Fig. 1(a-e)). ...
The electronic structure of the alloyed transition-metal phosphorus trichalcogenide van der Waals Fe1–xNixPS3 compounds is studied using X-ray absorption spectroscopy and resonant photoelectron spectroscopy combined with intensive density functional theory calculations. Our systematic spectroscopic and theoretical data demonstrate the strong localization of the Fe- and Ni-ions-derived electronic states that leads to the description of the spectroscopic data as belonging simultaneously to Mott–Hubbard and charge-transfer insulators. These findings reveal Fe1–xNixPS3 as unique layered compounds with dual character of the insulating state, pointing to the importance of these results for the description and understanding of the functionality of this class of materials in different applications.
... Xue et al. [57] (110), après une exposition à l'oxygène de 1200 L, les diagrammes LEED montrent une symétrie hexagonale identifiée comme α-Fe 2 O 3 (0001) ordonné. Après un recuit sans oxygène, le motif de Fe 3 O 4 (111) apparaît, lié à une transformation de structure surfacique (TSS). ...
Le but de cette recherche est de comprendre le rôle de la résistance à l'amorçage de la corrosion des hétérogénéités chimiques et structurelles créées dans la passivation par les mécanismes d'oxydation/passivation des surfaces d'alliage contenant du Cr. Ceci sera réalisé en utilisant un système UHV permettant la surveillance de la croissance de l'oxyde pendant et après l'exposition à l'oxygène par XPS et STM, et permettant aussi le transfert direct (sans exposition à l'air) à une boîte à gants remplie d'argon équipée d'une cellule électrochimique pour contrôler précisement de l'exposition aux électrolytes aqueux agressifs. Les environnements aqueux corrosifs avec des conditions agressives contrôlées (pH, potentiel électrochimique, concentration en chlorure) peuvent résulter des modifications de surface (chimiques et nanostructurales) qui sont observées par XPS et STM. La croissance du film passif et l'enrichissement en chrome seront analysés in situ par XPS et par STM afin de comprendre l'effet des paramètres d'oxydation sur le comportement local.
... The growth of 2D FeO layer have also been studied on a variety of metal substrates, such as Au(111) [20], Pd(111) [19], Pt(111) [21][22][23], and Ag(111) [24]. Yet, to understand the behavior of supported 2D FeO layer, it is essential to study in-situ their structural dynamics under reaction conditions [7,[25][26][27], which has been limited especially for 2D FeO on Au(111) [28][29][30][31]. ...
Understanding the dynamic changes of catalytically active nanostructures (NSs) under reaction
conditions is a pivotal challenge in catalysis research, which has been intensively studied on
metal catalysts, but less on oxide NSs. Here, we synthesized two-dimensional (2D) FeO NSs
and thin flms on Au(111) and studied their oxidation process in O2 from ultrahigh vacuum to
near-ambient-pressure (NAP) conditions, using the combination of in-situ NAP scanning tunneling microscopy and x-ray photoelectron spectroscopy. Our studies revealed atomic details on the transition process from the FeO bilayer to the FeO2 tri-layer on Au(111). Further, we found FeO2 NSs and thin layers are metastable on Au(111) and would undergo a three-dimensional phase change upon further thermal treatments. Our study has thus provided insight on the structural dynamics of 2D iron oxide under reaction conditions and enabled further understanding on the design of the oxide-metal interface.
... According to previous studies, it was found that iron oxidation is always accompanied by the formation of a multiphase oxide layer [41,42]. The phase composition of the oxide layer may include several oxides: wustite (Fe x O, where x = 0.836-0.954), ...
The article presents a detailed study and characterization of the oxide layers on the surface of iron particles of various sizes. Ten iron samples with a size range from a few nm to 50 µm were studied in detail using SEM, TEM, XRD, and TGA analysis. The composition of the multiphase oxide layers on the powder surface was investigated. The main components of the oxide layer were FeO, Fe3O4, and Fe2O3. By the obtained data, a model for the calculation of a multiphase oxide layer thickness on the surface of iron particles was proposed. The proposed model was validated and can be used for the characterization and certification of micro– and nanoscale iron particles.
... Fig. 8d shows the core-level XPS spectrum of Fe 2p, the main peaks are observed at the binding energy of 725.0 and 711.1 eV can be ascribed to Fe 2p 1/2 and Fe 2p 3/2 respectively. The shoulder peaks at 733.1 and 719.1 eV are shown in Fig. 8d that are evidence to Fe 3+ [141][142][143]. ...
Ferrites have been widely known as excellent photocatalysts thanks to their excellent properties such as relatively narrow bandgaps, chemical, and thermal stability, excellent magnetic property, and low-cost. They have recently been explored for their visible-light activity for various photocatalysis applications. In this review, the classification of ferrites according to their crystalline structure is discussed. Several ferrites synthesis approaches have been reviewed including the citric acid-assisted sol-gel method, hydrothermal methods, coprecipitation, and solid-state reactions. Besides, optical, structural, electronic, and morphological properties of ferrites were characterized using several physical and analytical methods are highlighted. A comprehensive survey on their photocatalytic applications with more emphasis on their novelties is briefly reviewed. These applications not only include the degradation of organic and inorganic pollutants and bacterial disinfection, but also on hydrogen production via water splitting, and CO2 photoreduction. In this review, we summarize the latest developments in designing and developing highly efficient ferrite photocatalysts. We conclude with an overview and some observations on challenges and future directions in the exploration as a photocatalyst of ferrite-based materials.
... Moreover, recent X-ray photoelectron spectroscopy (XPS) analyses carried out on the fracture surfaces evidenced the segregation of Cr in both ductile and brittle (quasi-cleavage) fields [28]. To shed more light on these aspects, the fracture behaviour of a Cr martensitic steel prepared in two different conditions, (1) as-quenched with cooling rate of 3600 • C/min and (2) quenched and annealed at 700 • C for 10 h, has been investigated through Charpy tests. ...
The fracture surfaces of a 10.5 wt.% Cr martensitic stainless steel broken in Charpy tests have been investigated through X-ray photoelectron spectroscopy (XPS). The specimens have been examined in two different conditions: as-quenched and heat treated for 10 h at 700 °C. The trends of Fe/Cr ratio vs. test temperature are similar to the sigmoidal curves of absorbed energy and, after both ductile and quasi-cleavage brittle fractures, such ratio is always significantly lower than the nominal value of the steel chemical composition. Cr segregation does not occur on a macroscopic scale but takes place in microscopic zones which represent weaker spots in the steel matrix and a preferred path for moving cracks. Small area (diameter 300 µm) XPS measurements evidenced a higher density of such microscopic zones in the inner part of probes; this is explained by the different diffusion length of Cr atoms in the external and inner parts during quenching from austenitic field which has been calculated through FEM simulations. No significant differences of Cr concentration were observed in fracture surfaces of probes with and without heat treatment. The results highlight how Cr segregation plays a role not only in the intergranular mode of fracture but also in the quasi-cleavage and ductile ones.
... The passivation of nanoparticles consists of applying various protective layers on their surface. Most often, the protective layer is an oxide film and its thickness and structure are determined by the origin and dispersion of the metal [15,16]. Long-term storage of nanoparticles leads to a number of processes, including an increase of the oxide layer thickness and modification of the surface structure. ...
Oxidation of cobalt, nickel, molybdenum and tungsten nanoparticles under prolonged exposure in the air was studied. Nanoparticles were obtained by chemical dispersion with the following reduction temperatures: 200–400°C for cobalt; 200–400°C for nickel; 700–800°C for molybdenum and 750–850°C for tungsten. Scanning electron microscopy, transmission electron microscopy, XRD and thermogravimetric analysis were used for nanoparticles characterisation. The average size of nanoparticles, specific surface area and the coherent scattering area were determined. The method for oxide film thickness calculation on the surface of the nanoparticles was proposed and confirmed its adequacy. Based on experimental data, the growth law of the oxide film on the studied nanoparticles was determined. The obtained results can be used in the certification of nanoparticles and evaluation of a highly dispersed metal powders shelf life.
... Based on this idea, a chemical sensor can be constructed. While iron oxides, due to their poor conductivity and instability under ambient conditions, are poor choices for application in chemical sensors [108], SWCNTs are highly conductive and chemically inert [109]. In addition, nanomaterials are desirable choices for chemical sensing as their high surface to bulk ratio maximizes their interactions with the analyte [48]. ...
The focus of this review is an introduction to chemiresistive chemical sensors. The general concept of chemical sensors is briefly introduced, followed by different architectures of chemiresistive sensors and relevant materials. For several of the most common systems, the fabrication of the active materials used in such sensors and their properties are discussed. Furthermore, the sensing mechanism, advantages, and limitations of each group of chemiresistive sensors are briefly elaborated. Compared to electrochemical sensors, chemiresistive sensors have the key advantage of a simpler geometry, eliminating the need for a reference electrode. The performance of bulk chemiresistors can be improved upon by using freestanding ultra-thin films (nanomaterials) or field effect geometries. Both of those concepts have also been combined in a gateless geometry, where charge transport though a percolation network of nanomaterials is modulated via adsorbate doping.
... This was explained as follows: during annealing at 200 °C , electrons and cations are transported to the surface of the sample; the Fe 3+ ions present near the surface are reduced to Fe 2+ . In addition, Xue et al. [17] reported that Fe 2 O 3 can be transformed to Fe 3 O 4 after annealing at 800 K without ambient oxygen. Therefore, regarding the electronic properties of the surface overlayer after the Up1 scan to 339 °C , the outermost region contained accumulated electrons, which did not lead to emission. ...
We report the activation energy, ΔEa, for the quantum yield in thermally assisted photoelectron emission (TAPE) under 210-nm-wavelength light irradiation, and the associated X-ray photoelectron spectroscopy (XPS) results. Samples were cleaned only in acetone and scratched in air, water, methanol, ethanol, acetone, benzene, and cyclohexane. Glow curves, describing the temperature dependence of photoelectron emission (PE) quantum yield (emitted electrons/photon), Y, were obtained. A simple method of determining ΔEa using Y, called YGC, at seven temperatures up to 353 °C, for the same Y glow curve, was proposed. The ΔEa obtained using this method was almost the same as that obtained from Y for seven stationary temperatures (YST). For scratched samples, the TAPE was measured over two cycles of temperature increase and subsequent decrease (Up1, Down1 and Up2, Down2 scans) in the 25–339 °C range, and ΔEa was obtained from YGC. The Arrhenius plot was approximated by a straight line, although a convex swelling peak appeared in the Up1 scan. ΔEaUp1 was in the 0.212–0.035 eV range, depending on the environment in which scratching was performed; ΔEaUp1 for water was much higher than that for acetone. This was explained in terms of the mode of the acid–base interaction between the liquid molecules and the hydroxyl group of Fe–OH. The values of ΔEaDown1, ΔEaUp2, and ΔEaDown2 were in the 0.038–0.012 eV range. The total count of electrons emitted during the Up1 and Up2 scans was found to decrease with increasing ΔEaUp1 and ΔEaUp2, respectively. ΔEaUp2 was found to increase with increasing presence of the FeO component in the analyzed Fe oxides. The convex swelling peak was attributed to the removal of carbon materials from the scratched surface and the effect of the increased electron density of the surface hydroxyl group of FeOH under the light irradiation.
... Fly ash (Bhardwaj et al., 2009), which is a by-product of coal-burning, and transition metal oxides, which are components of fly ash, and noble metals such as gold (Au), palladium (Pd), and platinum (Pt) (Aboud et al., 2011;Lim et al., 2012;Lim and Wilcox, 2013;Sasmaz et al., 2009;Sasmaz and Wilcox, 2008) have shown to have significant catalytic activity toward Hg adsorption and oxidation (Dunham et al., 2003;Ghorishi et al., 2005;Sasmaz and Wilcox, 2008). Mercury removal using α-Fe 2 O 3 has also been suggested by several researchers both experimentally (Barbier et al., 2007;Becker et al., 1996;Chambers and Yi, 1999;Condon et al., 1998;Eggleston and Hochella, 1992;Eggleston et al., 2003;Goniakowski et al., 2008;Henderson, 2002;Henderson et al., 1998;Hendewerk et al., 1986;Jew et al., 2015;Kim et al., 2004;Kong et al., 2011;Krutz and Henrich, 1983;Kurtz and Henrich, 1987;Lemire et al., 2005;Liu et al., 1998;Shaikhutdinov and Weiss, 1999;Tanwar et al., 2007;Thevuthasan et al., 1999;Trainor et al., 2004;Wang et al., 1998;Waychunas, 2012;Weiss and Ranke, 2002;Wu et al., 2006;Xue et al., 2011;Yamamoto et al., 2010) and theoretically (Bergermayer et al., 2004;Blanchard et al., 2012;Bulgakov and Sadykov, 1996;Goniakowski et al., 2008;Guo and Barnard, 2012;Guo et al., 2011;Jin et al., 2007;Jones et al., 2000;Jung et al., 2015;Kerisit and Rosso, 2006;Lo et al., 2007;Martin et al., 2009;Mason et al., 2009;Nguyen et al., 2014;Rohrbach et al., 2004;Rustad et al., 1999;Wang et al., 1998;Wasserman et al., 1997). A prior experimental study carried out by Jew et al. (2015) revealed that α-Fe 2 O 3 contained in fly ash has the potential to physically adsorb Hg in its oxidized form. ...
One of the biggest environmental concerns caused by coal-fired power plants is the emission of mercury (Hg), which is toxic metal. To control the emission of Hg from coal-derived flue gas, it is important to understand the behavior, speciation of Hg as well as the interaction between Hg and solid materials in the flue gas stream. In this study, atomic-scale theoretical investigations using density functional theory (DFT) were carried out in conjunction with lab-scale experimental studies to investigate the adsorption behavior of Hg on hematite (?-Fe2O3).
According to the DFT simulation, the adsorption energy calculation proposes that Hg physisorbs to the ?-Fe2O3(0001) surface with an adsorption energy of −0.278 eV, and the subsequent Bader charge analysis confirms that Hg is slightly oxidized. In addition, Cl introduced to the Hg-adsorbed surface strengthens the Hg stability on the ?-Fe2O3(0001) surface as evidenced by a shortened Hg-surface equilibrium distance. The PDOS analysis also suggests that Cl enhances the chemical bonding between the surface and the adsorbate, thereby increasing the adsorption strength. In summary,?-Fe2O3 has the ability to adsorb and oxidize Hg, and this reactivity is enhanced in the presence of Cl.
For the lab-scale experiments, three types of ?-Fe2O3 nanoparticles were prepared using the precursors Fe(NO3)3, Fe(ClO4)3, and FeCl3, respectively. The particle shapes varied from diamond to irregular stepped and sub-rounded, and particle size ranged from 20 to 500 nm depending on the precursor used. The nanoparticles had the highest surface area (84.5 m2/g) due to their highly stepped surface morphology. Packed-bed reactor Hg exposure experiments resulted in this nanoparticles adsorbing more than 300 ?g Hg/g. The Hg LIII-edge extended x-ray absorption fine structure spectroscopy also indicated that HgCl2 physisorbed onto the ?-Fe2O3 nanoparticles.
... Cappus et al. found that a FeO(111) film is grown on Fe(110) at temperatures of 600 K [19]. Xue et al. studied the influence of temperature on the oxide structure [20]. These studies provide an understanding that oxidation at different temperatures and pressures may lead to the formation of different oxide phases. ...
The initial oxidation of Fe(110) in oxygen gas at 400 °C beyond initial adsorbate structures has been studied using X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, low-energy electron diffraction, and scanning tunneling microscopy (STM). Formation of several ordered phases of surface oxides is observed at oxygen coverages between approximately 2.3 and 3.5 oxygen atoms/Fe(110) surface atom. Initially, a FeO(111)-like film is formed with a parallelogram-shaped moiré pattern. It has two mirror domains that are formed symmetrically around the growth direction of a zigzag-shaped adsorbate structure. With increased local oxygen coverage, the moiré structure transforms into a ball-shaped form. Both these moiré structures have equal atomic stacking at the surface and equal apparent height in STM, suggesting oxygen ions diffusing into the film upon oxidation and that the oxide growth takes place at the iron–iron oxide interface. The FeO(111)-like film turns into a Fe3O4(111)-like film with a triangular bistable surface termination as the oxidation proceeds further. The FeO(111)-like film growth proceeds according to the Frank–van der Merwe mechanism while the Fe3O4(111)-like film grows according to the Stranski–Krastanov mechanism.
... Fig. 2(d) shows the XPS spectrum of Fe 2p, the main peaks are observed at 725.0 and 711.1 eV, which can be assigned to Fe 2p 1/2 and Fe 2p 3/2 respectively. The shoulder peaks at 733.1 and 719.1 eV originating from shakeup process of 2p 1/2 and 2p 3/2 are marked as Sat. in Fig. 2(d), which are evidence to Fe 3+ [23][24][25]. The XPS results presented that Zn and Fe elements existed in the synthesized samples were in the forms of Zn 2+ and Fe 3+ , respectively, which corresponding to the characterization of the above XRD pattern. ...
... Cappus et al. found that a FeO(111) film is grown on Fe(110) at temperatures of 600 K [19]. Xue et al. studied the influence of temperature on the oxide structure [20]. These studies provide an understanding that oxidation at different temperatures and pressures may lead to the formation of different oxide phases. ...
A well-ordered surface oxide grown on Fe(110) has been studied using scanning tunneling microscopy (STM), low energy electron diffraction, low energy electron microscopy, and core level photoelectron spectroscopy. The iron oxide film exhibits wide terraces and is formed after exposure to 100–1000 L at 1 × 10− 6 mbar O2 and 400 °C. Two domains, mirror symmetric in the Fe(110)-lattice mirror symmetry planes but otherwise equal, are observed. The surface oxide forms a relatively large coincidence surface unit cell (16.1 Å × 26.5 Å). Imaging by STM reveals a strong bias dependence in the apparent height within the surface unit cell. The oxygen terminating atomic layer has a hexagonal atomic structure, FeO(111)-like, with the atomic spacing of 3.2 Å, that is expanded by ~ 6.3% relative to bulk FeO(111).
... FeO always grows with (1 1 1) orientation on (quasi-)hexagonal substrates as Pt (1 1 1), Ru(0 0 0 1) [11,12] or Mo(1 1 0) [27] although the lattice mismatch varies significantly for these substrates. The FeO(1 1 1) films are undulated and exhibit a Moiré pattern due to the interaction with the substrate. ...
Iron oxide monolayers are grown on Ag(0 0 1) via reactive molecular beam epitaxy (metal deposition in oxygen atmosphere). The monolayer shows FeO stoichiometry as concluded from x-ray photoemission spectra. Both low energy electron diffraction as well as scanning tunneling microscopy demonstrate that the FeO layer has a quasi-hexagonal (1 1 1) structure although deposited on a surface with square symmetry. Compared to bulk values, the FeO(1 1 1) monolayer is unidirectionally expanded by 3.4% in [Formula: see text] directions while bulk values are maintained in [Formula: see text] directions. In [Formula: see text] directions, this lattice mismatch between FeO(1 1 1) monolayer and Ag(0 0 1) causes a commensurate undulation of the FeO monolayer where 18 atomic rows of the FeO(1 1 1) monolayer match 17 atomic rows of the Ag(0 0 1) substrate. In [Formula: see text] directions, however, the FeO(1 1 1) monolayer has an incommensurate structure.
Understanding the dynamics of organic thin film formation is crucial to quality control in organic electronics and smart coatings. We have studied the nucleation and growth of the reduced and the oxidized states of phenyl-capped aniline tetramer (PCAT) deposited on hematite(1000) surfaces by physical vapor deposition. The fully reduced PCAT molecules form two-dimensional islands on the surface while the fully oxidized molecules form three-dimensional islands. Through scaled island size distribution, it was found that the critical island sizes, i, for the reduced and oxidized molecules are i=4 and i=5 respectively. From the dependence of the island density on substrate temperature, the activation energies for the diffusion of the molecules away from the critical cluster were calculated to be 1.30 eV and 0.55 eV, respectively. At low temperatures, the reduced and the oxidized PCAT molecules form compact islands on the surface. At higher temperatures, the reduced islands become dendritic while the oxidized islands become slightly dendritic. The attempt frequencies for surface diffusion of the reduced and the oxidized islands were estimated to be about 5 × 10^25 s-1 and 5 × 10^11 s-1, respectively. The former value is in line with the high degree of surface wetting by the reduced PCAT while the latter value shows the higher degree of intermolecular interaction in the fully oxidized PCAT and the low degree of its interaction with the iron oxide surface. Interconversion between oxidized and reduced islands through exposure to a reducing environment, and its impact on island morphology was examined. We also found that the presence of Fe2+ defects on the hematite surface did not impact the nucleation and growth of the molecular islands, likely due a discrepancy in timescale. This study elucidates the interactions between an oligoaniline-based molecular switch (PCAT) and hematite surfaces as a function of molecular oxidation state, with applications in molecular electronics, chemical sensors, and smart coatings.
The hollow ZnO/ZnFe2O4 microspheres with heterogeneous structure are synthesized by direct pyrolysis of metal-organic frameworks. The as-prepared ZnO/ZnFe2O4 microspheres have well-defined spherical morphology with ∼1.5 μm in diameter and multiple porous shells constructed by interpenetrated ZnO and ZnFe2O4 heterogeneous nanoparticles. Interestingly, the hollow ZnO/ZnFe2O4 microspheres based gas sensors show interestingly temperature-dependent n-p-n type conductivity transition in detecting low-concentration VOC gases including ethanol, acetone, toluene and benzene. This interestingly n-p-n transition phenomenon is mainly ascribed to the trade-off of highly separated electron-hole pairs originated from the staggered type-II band alignment of in-shell ZnO-ZnFe2O4 hetero-interfaces, which is modulated by thermally-dependent ionization reaction of surface-absorbed oxygen molecules and extra electron injection due to surface reaction of reductive VOCs during gas-sensing process. This work presents a facile route to construct hollow nanostructures with heterogeneous feature and provides a new insight into sensing mechanism, exhibiting the potential application of ZnO/ZnFe2O4 microspheres in developing highly sensitive and selective gas-sensing materials in detecting low-concentration VOCs.
Redox-active organic compounds have been studied as corrosion inhibitors for steel. Even though it is clear that chemical interactions at the organic-metal oxide interface are behind this inhibitive process, the detailed mechanism is not yet fully understood. Using phenyl-capped aniline tetramer (PCAT), we have elucidated the interactions at the interface with iron oxide. We demonstrate partial reduction of fully oxidized PCAT and partial oxidation of fully reduced PCAT upon interaction with iron oxide. X-ray photoelectron spectroscopy reveals the appearance of charged nitrogen structures in PCAT deposited on hematite. Iron oxide films in contact with reduced PCAT show a higher conductance due to introduction of defects, resulting in n-doping. In contrast, the iron oxide film in contact with oxidized PCAT show a lower conductance, indicating defects in the film are removed via oxidation, thus reducing the doping level. This is consistent with accepted models for corrosion inhibition, in which PCAT assists in the formation of a passive oxide film. These results are indicative of interfacial charge transfer between PCAT and iron oxide. The extent of the charge transfer and the direction of redox processes depend on the oxidation state of the molecules, enabling construction of redox-active devices, including sensors and switches.
A facile and efficient approach was employed to synthesize CuFe2O4/rGO (reduced graphene oxide) nanocomposites. The morphology, crystal structure and properties of the prepared CuFe2O4/rGO nanocomposites were characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction and thermo-gravimetric analysis. The CuFe2O4/rGO nanocomposites were applied as adsorbents to study their adsorption performance for Congo red. The adsorption capacity and recyclability, adsorption dynamics and adsorption models were investigated. The results show that the CuFe2O4/rGO nanocomposites are efficient and recyclable adsorbents.
We have studied the growth of Fe3O4 (111) epitaxial films on Al2O3 (001) substrates using a pulsed laser deposition / thermal reduction cycle using an {\alpha}-Fe2O3 target. While direct deposition onto the Al2O3 (001) substrates results in an {\alpha}-Fe2O3 epilayer, deposition on the Fe3O4 (111) surface results in a {\gamma}-Fe2O3 epilayer. The kinetics of the transitions between Fe2O3 and Fe3O4 were studied by measuring the time constants of the transitions. The transition from {\alpha}-Fe2O3 to Fe3O4 via thermal reduction turns out to be very slow, due to the high activation energy. Despite the significant grain boundaries due to the mismatch between the unit cells of the film and the substrate, the Fe3O4 (111) films grown from deposition/thermal reduction show high crystallinity.
The growth and electronic structure of Ag on polar MgO(111) thin films with {100} facets were in situ investigated by low energy electron diffraction and photoemission spectroscopies. The Ag epitaxially grows on faceted MgO(111) surface as Ag(111) films due to the large surface energy of the MgO(111) face in spite of the existence of {100} facets, compared with a three-dimensional growth of Ag on MgO(100) films. Based on the photoemission spectroscopies studies, the shifts of the Ag3d core level line at the initial coverage are mainly associated with size effects rather than chemical interaction. The results are helpful to the understanding of the growth mechanisms and electronic structure of metals on polar oxide surfaces.
The reversible transformations of thin magnetite (Fe3O4) and hematite (α-
Fe2O3) films grown on Pt(111) and Ag(111) single crystals as support have been
investigated by a combined low energy electron microscopy (LEEM) and low-energy
electron diffraction (LEED) study. The conversions were driven by oxidation, annealing in
ultrahigh vacuum (UHV), or Fe deposition with subsequent annealing. As expected, the
oxidation of a Fe3O4
film yielded an α-Fe2O3 structure. Unexpectedly, the annealing in UHV
also led to a transformation from Fe3O4 into α-Fe2O3, but only if Pt(111) was used as
substrate. In contrast, on a Ag(111) substrate the inverse reaction, a slow transformation
from α-Fe2O3 into Fe3O4, was observed, as expected for oxygen desorption. Fe deposition on
α-Fe2O3 and subsequent annealing in UHV transformed the film into Fe3O4. As the most
probable explanation we propose that the UHV conversion on Pt(111) supports proceeds by
Fe cation diffusion through the film and Fe atom dissolution in the substrate, decreasing the
Fe concentration within the iron oxide film. This process is not possible for a Ag(111)
substrate. The interconversions, which were best observable in mixed films containing
domains of both oxides, occurred by growth of one domain type with well-defined boundaries and growth rates.
In this article, an iterative method for estimating the size distribution of non-interacting polydisperse spherical particles from small-angle scattering data is presented. It utilizes the iterative addition of relevant contributions to an instantaneous size distribution, as obtained from the fractional difference between the experimental data and the simulated profile. An inverse relation between scattering vector and real space is assumed. This method does not demand the consideration of any basis function set together with an imposed constraint such as a Lagrange multiplier, nor does it depend on the Titchmarsh transform. It is demonstrated that the method works quite well in extracting several forms of distribution. The robustness of the present method is examined through the successful retrieval of several forms of distribution, namely monomodal, bimodal, trimodal, triangular and bitriangular distributions. Finally, the method has also been employed to extract the particle size distribution from experimental small-angle X-ray scattering data obtained from colloidal dispersions of silica.
Polar surfaces of metal oxides have attracted considerable attention in fundamental science and technological applications because of their peculiar stabilization mechanisms and unusual adsorption and catalytic properties. In this study, various ordered iron oxide films, including FeO(1 1 1), Fe3O4(1 1 1) and Fe2O3(0 0 0 1), were successfully used as the buffer layers for the growth of polar NiO(1 1 1) films with several nanometer thickness. The results indicated that the iron oxide buffer layers reduced the lattice mismatch between NiO(1 1 1) and Mo(1 1 0) substrate, and also decreased the interfacial energy by means of the interfacial reaction between NiO and iron oxide surfaces, benefiting the initial nucleation and the growth of polar NiO(1 1 1) films. This study will be essential for understanding the physical and chemical properties of polar surfaces.
We report the XPS characterization of a thermally evaporated iron thin film (6 nm) deposited on an
Si/SiO2/Al2O3 substrate using Al Ka x-rays. An XPS survey spectrum, Fe 2p and O 1s narrow
scans, and a valence band scan are shown.
Herein we show characterization of an Fe thin film on Al2O3 after thermal annealing under H2
using Al Ka x-rays. The XPS survey spectrum, Fe 2p and O 1s narrow scans, and valence band
regions are presented. The survey spectrum shows aluminum signals due to exposure of the
underlying Al2O3 film during Fe nanoparticle formation.
Many stick insects and mantophasmids possess tarsal 'heel pads' (euplantulae) covered by arrays of conical, micrometre-sized hairs (acanthae). These pads are used mainly under compression; they respond to load with increasing shear resistance, and show negligible adhesion. Reflected-light microscopy in stick insects (Carausius morosus) revealed that the contact area of 'heel pads' changes with normal load on three hierarchical levels. First, loading brought larger areas of the convex pads into contact. Second, loading increased the density of acanthae in contact. Third, higher loads changed the shape of individual hair contacts gradually from circular (tip contact) to elongated (side contact). The resulting increase in real contact area can explain the load dependence of friction, indicating a constant shear stress between acanthae and substrate. As the euplantula contact area is negligible for small loads (similar to hard materials), but increases sharply with load (resembling soft materials), these pads show high friction coefficients despite little adhesion. This property appears essential for the pads' use in locomotion. Several morphological characteristics of hairy friction pads are in apparent contrast to hairy pads used for adhesion, highlighting key adaptations for both pad types. Our results are relevant for the design of fibrillar structures with high friction coefficients but small adhesion.
The exceptionally adhesive foot of the gecko remains clean in dirty environments by shedding contaminants with each step. Synthetic gecko-inspired adhesives have achieved similar attachment strengths to the gecko on smooth surfaces, but the process of contact self-cleaning has yet to be effectively demonstrated. Here, we present the first gecko-inspired adhesive that has matched both the attachment strength and the contact self-cleaning performance of the gecko's foot on a smooth surface. Contact self-cleaning experiments were performed with three different sizes of mushroom-shaped elastomer microfibres and five different sizes of spherical silica contaminants. Using a load-drag-unload dry contact cleaning process similar to the loads acting on the gecko foot during locomotion, our fully contaminated synthetic gecko adhesives could recover lost adhesion at a rate comparable to that of the gecko. We observed that the relative size of contaminants to the characteristic size of the microfibres in the synthetic adhesive strongly determined how and to what degree the adhesive recovered from contamination. Our approximate model and experimental results show that the dominant mechanism of contact self-cleaning is particle rolling during the drag process. Embedding of particles between adjacent fibres was observed for particles with diameter smaller than the fibre tips, and further studied as a temporary cleaning mechanism. By incorporating contact self-cleaning capabilities, real-world applications of synthetic gecko adhesives, such as reusable tapes, clothing closures and medical adhesives, would become feasible.
a b s t r a c t Hydrolysis across tiny spray droplet allows a facile one step synthesis of interesting sub-micrometric structures owing to the large available surface area unlike bulk hydrolysis. In the present work, it has been demonstrated that titania precursor concentration plays a significant role in effecting morphological transformation during spray hydrolysis. While hollow microspheres are formed primarily at low precur-sor concentration, fractal like grains, having two levels of hierarchy, result at high precursor concentra-tion. Mesoscopic structure of these spray hydrolyzed grains has been investigated by ultra small-angle neutron scattering, small-angle X-ray scattering and scanning electron microscopy. Thermal evolution of initial amorphous phase of titania into crystalline rutile phase, through intermediate anatase and brookite phases, is followed by high temperature X-ray diffraction. A plausible mechanism has been elu-cidated for the observed morphological transition with variation of precursor concentration. Ó 2013 Elsevier B.V. All rights reserved.
We present a first-principles investigation of the structural, electronic, and magnetic properties of ultrathin Fe2O3(0001) films on a polar MgO(111) substrate. The results imply that the heterointerface is atomically abrupt with oxidelike stacking for film thicknesses between ∼1.5 and 8.5 Å. The Fe-Fe bilayer (nominal separation of 0.59 Å in Fe2O3) at the interface collapses into an “Fe2” monolayer. Both electronic polarization and structural relaxations effectively screen the dipole field of the polar interface system. The structural relaxations—consisting of interpenetration, separation, and merger of Fe and oxygen planes—are particularly drastic in the three- and four-bilayers-thick films, giving rise to barrierless movement of oxygen towards the surface and the formation of an “Fe2|FeO3” layer structure not seen in hematite. Comparisons to calculations of unsupported polar Fe2O3(0001) slabs demonstrate that these unusual changes in stacking sequence and electronic structure are associated with the polar nature of this oxide heterointerface.
The electron work function (EWF) is an important parameter of a semiconductor. The understanding of the correlation between the EWF and surface morphology is of much significance for revealing related photoelectric mechanisms. In this study, the surface of indium tin oxide (ITO) was treated by chemical corrosion or absorption of copper phthalocyanine molecules, and their changes in EWF were systematically investigated using scanning Kelvin probe. The decrease of the EWF with the increase of surface roughness was found. Based on a microcapacitor model, the correlation between the EWF and surface microstructures was built up, which was well consistent with the experimental results. These data are of help for improving the photoelectric behaviors of ITO-based devices by adjusting surface/interface structures.
By using scanning tunneling microscopy (STM), we study the structure of the (0001) surface of hematite (α-Fe2O3) pre.pared by ultra-high vacuum treatment. The surface is reduced into a Fe3O4(111) phase by Ar ion sputtering followed by annealing in vacuum, while it is oxidized to a honeycomb superstructure by annealing in O2. High-resolution STM images reveal that the O-terminated FeO(111), Fe- and O-terminated Fe2O3(0001) domains coexist with each other in the superstructure. Unlike the reduction of Fe2O3(0001) whose depth increases with repeated annealing in vacuum, the oxidation of Fe3O4(111) occurs only partially on the top layers by annealing in O2.
The surface and electronic structure of silver grown on well defined FeO(111), Fe3O4(111) and Fe2O3(0001) films have been in situ studied using various surface analytical techniques. For Ag grown on FeO(111) or Fe2O3(0001) surfaces, the silver Mie plasmon resonance observed at about 3.8 eV at initial coverage indicates three dimensional growth mode of silver. Contrarily, no Mie plasmon peak detected for Ag on Fe3O4(111) at initial coverage of Ag suggests a wetting behavior. The main mechanism of Ag growth mode on iron oxide films and the interfacial interaction are discussed. The interfacial information is useful for noble metal–iron oxides system in chemical reactions, e.g. in photocatalysis and biology.
Based on the vital effect of the interfacial behaviors on tunneling magnetoresistance in Fe/MgO/Fe junctions, the thickness-dependent electronic structure of iron on MgO(1 1 1) films with {1 0 0} facets was investigated. The results illustrated that the chemical interaction at the interface of Fe and MgO films was rather weak. Instead, a upward band bending of MgO with 1.2 eV was obviously observed with the increase of Fe thickness. The particle size effect was responsible for these shifts of core levels owing to the three-dimensional growth of Fe. These results were further testified by the data from the lattice vibration in Fe–MgO system.
Magnetically recyclable ZnFe2O4/ZnO nanocomposites immobilized on different content of graphene with favorable photocatalytic activity under solar light irradiation were successfully prepared on the basis of an ultrasound aided solution method. The molar ratio of ZnFe2O4 to ZnO and the content of graphene could be controlled by adjusting the amount of zinc salts and graphene oxide dispersions. The most excellent photocatalytic activity under solar light irradiation was displayed when the molar ratio of ZnFe2O4 to ZnO was 0.1 and the weight ratio of graphene to ZnFe2O4/ZnO was 0.04. Furthermore, the presence of magnetical ZnFe2O4 will facilitate the recycling process of photocatalyst nanoparticles.
Various ordered Fe oxide films were prepared in an ultrahigh vacuum system, and their surface and electronic structure were studied in situ by low-energy electron diffraction and photoelectron spectroscopy. We obtain Fe2O3(0001) films with oxygen termination (O–Fe2O3(0001)), a polar surface, via FeO(111) films. The O–Fe2O3(0001) films can be transformed to a non-polar surface of iron-terminated Fe2O3(0001) (Fe–Fe2O3(0001)) films. Fe3O4(111) films are formed after annealing O–Fe2O3(0001) films. A co-existed phase of Fe–Fe2O3(0001) and Fe3O4(111) is observed via annealing Fe–Fe2O3(0001) films. These Fe oxide films with different surface structures and properties can be designedly used as model plates for further studies in surface physics and chemistry.
In many fields the determination of electronic structures of a solid material is a prerequisite in order to investigate its physical/chemical properties as well as related applications. The effect of surface structures and ambient environment on the electronic behavior is of both fundamental and practical significance. In this study, the electron work function (EWF) of Al–Mg alloys is investigated using a scanning Kelvin probe. The results show that the EWF decreases with the increase of surface smoothness, whereas surface oxidation layers would result in the increase of the EWF. Furthermore EWF is strongly dependent on the relative humidity, especially when the relative humidity is higher than 70%, implying that considerable care should be takenon such dependence in order to gain a meaningful parameter for the characterization of surface behavior.
We apply a suite of analytical tools to characterize materials created in the production of microfabricated thin layer chromatography plates. Techniques used include X-ray photoelectron spectroscopy (XPS), valence band spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS) in both positive and negative ion modes, Rutherford backscattering spectroscopy (RBS), and helium ion microscopy. Materials characterized include: the Si(100) substrate with native oxide: Si/SiO2, alumina (35 nm) deposited as a diffusion barrier on the Si/SiO2: Si/SiO2/Al2O3, iron (6 nm) thermally evaporated on the Al2O3: Si/SiO2/Al2O3/Fe, the iron film annealed in H2 to make Fe catalyst nanoparticles: Si/SiO2/Al2O3/Fe(NP), and carbon nanotubes (CNTs) grown from the Fe nanoparticles: Si/SiO2/Al2O3/Fe(NP)/CNT. The Fe films and nanoparticles appear in an oxidized state. Some of the analyses of the CNTs/CNT forests appear to be unique: (i) the CNT forest appears to exhibit an interesting ‘channeling’ phenomenon by RBS, (ii) we observe an odd–even effect in the SIMS spectra of Cn - species for n = 1 – 6, with the n ≥ 6 ions showing a steady decrease in intensity, and (iii) valence band characterization of CNTs using X-radiation is reported. Initial analysis of the CNT forest by XPS shows that it is 100 at.% carbon. After one year, only ca. 0.25 at.% oxygen is observed. The information obtained from the combination of the different analytical tools provides a more complete understanding of our materials than a single technique, which is analogous to the story of ‘The Blind Men and the Elephant’. The raw XPS and ToF-SIMS spectra from this study will be submitted to Surface Science Spectra for archiving.
The Fe3O4(111)/graphene/Ni(111) trilayer is proposed to be used as an ideal spin-filtering sandwich where the half-metallic properties of magnetite are used. Thin magnetite layers on graphene/Ni(111) were prepared via successive oxidation of a thin iron layer predeposited on graphene/Ni(111) and the formed system was investigated by means of low-energy electron diffraction and photoelectron spectroscopy. The electronic structure and structural quality of the graphene film sandwiched between two ferromagnetic layers remain unchanged upon magnetite formation as confirmed by experimental data.
Preparation of the HMF/graphene/FM junction.
(© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Polar surfaces of compound materials have been the subject of intense activity in the past. Electrostatic arguments, based on a model of rigid charges, predict that they should have an infinite surface energy, because of the presence of a macroscopic dipole, and thus should never be observed. Moreover, peculiar electronic surface states may lie in the gap of the oxide and induce enhanced basic or acid character at surface atoms, with important implications on reactivity. Finally, surface reconstructions that are sometimes consequences of the polar instability can be used as nanostructured substrates to grow artificial structures with predetermined conformations. Polar nano-objects raise a number of very new questions related to the relevance of electrostatic interactions and the role of dimensionality (2D or 3D) in driving the polar instability. As compared to semi-infinite surfaces, additional mechanisms of polarity compensation exist at the nanoscale, involving complete changes of structures, strong lattice relaxations, inhomogeneous charge redistributions, among others.
The reactive growth of ultrathin Fe oxide films on Ru(0001) has been studied and characterized using low-energy electron microscopy, diffraction, and laterally resolved spectroscopies. Under exposure to molecular oxygen at 900 K, we observed the growth of a bicomponent film composed of micrometer-sized flat triangular Fe3O4 (magnetite) islands on a FeO (wüstite) wetting layer. Subsequent oxidation using NO2 at 600 K resulted in the chemical transformation of the initially grown film to a Fe2O3 composition but still in bicomponent form. The triangular magnetite islands evolve to γ-Fe2O3 (maghemite), and the surrounding layer is converted to α-Fe2O3 (hematite). The evolution of both members of the bicomponent iron oxide films, wüstite to hematite and magnetite to maghemite, can be understood by considering that both are topotactic transformations occurring by the diffusion of iron in octahedral sites to react with oxygen on the film’s surface.
The oxidation of Fe(111) was studied using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ion scattering spectroscopy (ISS) and scanning tunnelling microscopy (STM). Oxidation of the crystal was found to be a very fast process, even at 200 K, and the Auger O signal saturation level is reached within ~ 50 Ã 10â 6 mbar s. Annealing the oxidised surface at 773 K causes a significant decline in apparent surface oxygen concentration and produces a clear (6 Ã 6) LEED pattern, whereas after oxidation at ambient temperature no pattern was observed. STM results indicate that the oxygen signal was reduced due to the nucleation of large, but sparsely distributed oxide islands, leaving mainly the smooth (6 Ã 6) structure between the islands. The reactivity of the (6 Ã 6) layer towards methanol was investigated using temperature programmed desorption (TPD), which showed mai
The surface morphology of in situ oxidized 200 Å thick Fe(110) films grown on Al2O3(11−20) substrates using a Mo(110) buffer layer was investigated by low energy electron diffraction and scanning tunneling microscopy (STM). The epitaxially grown Fe(110) films were oxidized to magnetite, Fe3O4(111), in an oxygen atmosphere. For a low oxygen exposure of 6 L at room temperature a c(2×2)-O-reconstruction and for an intermediate oxygen exposure of 100 L followed by annealing at 600 K the formation of FeO(111) was observed. After high oxygen exposures (⩾1500 L) and annealing at 800–1000 K the formation of Fe3O4(111) films of high crystalline quality has been achieved. Atomically resolved STM images of the Fe3O4(111) surface show a hexagonal symmetry with a 6 Å periodicity. The system was identified as a well-ordered epitaxial Fe3O4(111) layer on the Fe(110)/Mo(110)/Al2O3(11−20) system.
The surface and interface structure as well as the electronic properties of thin epitaxial Fe3O4(111) films prepared by in situ oxidation of thin Fe(110) films grown on Al2O3(112¯0) substrates using a Mo(110) buffer layer were investigated by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), transmission electron microscopy (TEM), and spin-polarized angle-resolved photoemission spectroscopy (SPARPES). The annealing of Fe(110) films at 700 °C in an O2 atmosphere leads to the formation of epitaxial Fe3O4(111) films. Atomically resolved STM images of the Fe3O4(111) surface show a hexagonal symmetry with 6 Å periodicity. Well-controlled interface properties at the Fe3O4(111)/Fe(110) and Fe(110)/Mo(110) interfaces were confirmed by TEM. A high spin polarization value of about -(60±5)% was found near the Fermi energy EF at room temperature by means of SPARPES with a photon energy of hν=21.2eV. The electronic structure and spin polarization are compared to the corresponding values recently found on epitaxial Fe3O4(111) films grown on W(110) single-crystal substrates.
In situ prepared Fe3O4(100) thin films were studied by means of scanning tunneling microscopy (STM) and spin-polarized photoelectron spectroscopy (SP-PES). The atomically resolved (√2×√2)R45° wavelike surface atomic structure observed by STM is explained based on density functional theory (DFT) and ab initio atomistic thermodynamics calculations as a laterally distorted surface layer containing octahedral iron and oxygen, referred to as a modified B layer. The work-function value of the Fe3O4(100) surface extracted from the cutoff of the photoelectron spectra is in good agreement with that predicted from DFT. On the Fe3O4(100) surface both the SP-PES measurements and the DFT results show a strong reduction of the spin polarization at the Fermi level (EF) compared to the bulk density of states. The nature of the states in the majority band gap of the Fe3O4 surface layer is analyzed.
By alternate deposition of Mg and exposure of O2, layer-by-layer growth, polar MgO(111) ultrathin films with Mg-terminated or O-terminated surfaces have been successfully fabricated on Mo(110) substrate. The surface geometric structure and electronic structures of the polar MgO(111) films were investigated using surface analysis techniques including low-energy electron diffraction and photoelectron emission and electron energy loss spectroscopies. The results indicate that the O-terminated surface is of an insulating character, while for Mg-terminated surface, a prominent new surface state at 2-3 eV and appreciable density of states near Fermi level have been observed. The polar oxide films provide ideal model surfaces for further investigation of support-particle system.
Magnetite (Fe3O4), an archetypal transition-metal oxide, has been used for thousands of years, from lodestones in primitive compasses to a candidate material for magnetoelectronic devices. In 1939, Verwey found that bulk magnetite undergoes a transition at TV approximately 120 K from a high-temperature 'bad metal' conducting phase to a low-temperature insulating phase. He suggested that high-temperature conduction is through the fluctuating and correlated valences of the octahedral iron atoms, and that the transition is the onset of charge ordering on cooling. The Verwey transition mechanism and the question of charge ordering remain highly controversial. Here, we show that magnetite nanocrystals and single-crystal thin films exhibit an electrically driven phase transition below the Verwey temperature. The signature of this transition is the onset of sharp conductance switching in high electric fields, hysteretic in voltage. We demonstrate that this transition is not due to local heating, but instead is due to the breakdown of the correlated insulating state when driven out of equilibrium by electrical bias. We anticipate that further studies of this newly observed transition and its low-temperature conducting phase will shed light on how charge ordering and vibrational degrees of freedom determine the ground state of this important compound.
Iron oxides occur ubiquitously in environmental, geological, planetary, and technological settings. They exist in a rich variety
of structures and hydration states. They are commonly fine-grained (nanophase) and poorly crystalline. This review summarizes
recently measured thermodynamic data on their formation and surface energies. These data are essential for calculating the
thermodynamic stability fields of the various iron oxide and oxyhydroxide phases and understanding their occurrence in natural
and anthropogenic environments. The competition between surface enthalpy and the energetics of phase transformation leads
to the general conclusion that polymorphs metastable as micrometer-sized or larger crystals can often be thermodynamically
stabilized at the nanoscale. Such size-driven crossovers in stability help to explain patterns of occurrence of different
iron oxides in nature.
The adsorption of Au and Pd atoms on two nanostructured titania monolayers grown on the Pt(111) surface is investigated via a computational approach. These phases present compact regions (zig-zag-like stripes) with titanium atoms at the oxide-metal interface and oxygen in the top-most overlayer, sometimes intercalated by point defects, i.e. holes exposing the bare metal support, and give rise to very regular patterns extending for large distances. A Pd atom experiences a rather flat energy landscape on the compact regions whereas it is strongly bound to the defects which act as nucleation centers, whence the interest of these substrates as nanotemplates for the growth of metal clusters. The interaction of a Au atom with these phases is peculiarly different: a charge transfer from the underlying Pt(111) support occurs so that Au gets negatively charged and strongly interacts with a titanium atom extracted from the interface in the compact regions, whereas it penetrates less easily than Pd into the defective holes due to its larger size. These results are discussed as paradigmatic examples of the interaction of metals with polar ultrathin films of oxides grown on metal supports, a novel and promising field in materials science.
A magnetite (Fe3O4) single crystal (111) surface has been studied at various oxygen-iron surface stoichiometries. The stoichiometry was modified by controlling the in-situ sample anneal conditions. We have found the conditions that lead to the formation of an oxygen-rich surface that forms a quasi-hexagonal superstructure with a 42A periodicity. The superstructure is highly regular and was observed by both LEED and STM. The superstructure consists of three regions, two of which have identical atomic scale structures with a periodicity of 2.8A, and a third having a periodicity that is about 10% larger (3.1A). The subtle difference in the atomic periodicities between the three areas results from the modulation of intrinsic strain developed along the surface. The superstructure results from electronic effects rather than being a mosaic of different iron oxide terminations. The onset of the superstructure is sensitive to the surface stoichiometry. From our results we could estimate the critical density of defects leading to the disappearance of the superstructure. We have modelled the experimental results and calculated the electron density using a DFT algorithm. The model clearly shows the development of strain along the surface.
The surface and interface structure as well as the electronic properties of thin epitaxial Fe3O4(III) films prepared by in situ oxidation of thin Fe(110) films grown on Al2O3(1120) substrates using a Mo(110) buffer layer were investigated by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), transmission electron microscopy (TEM), and spin-polarized angle-resolved photoemission spectroscopy (SPARPES). The annealing of Fe(110) films at 700 °C in an O2 atmosphere leads to the formation of epitaxial Fe 3O4(111) films. Atomically resolved STM images of the Fe3O4(111) surface show a hexagonal symmetry with 6 Å periodicity. Well-controlled interface properties at the Fe 3O4(111)/Fe(110) and Fe(110)/Mo(110) interfaces were confirmed by TEM. A high spin polarization value of about -(60 ± 5) % was found near the Fermi energy EF at room temperature by means of SPARPES with a photon energy of h ν = 21.2 eV. The electronic structure and spin polarization are compared to the corresponding values recently found on epitaxial Fe3O4(111) films grown on W(110) single-crystal substrates.
Pure-phase, single-crystalline epitaxial films of α-Fe2O3(0001) and Fe3O4(001) have been grown on Al2O3(0001) and MgO(001) substrates, respectively, using oxygen-plasma-assisted molecular beam epitaxy. We discuss the growth conditions required to synthesize these phases, as well as the associated characterization by means of reflection high-energy electron diffraction, low-energy electron diffraction, and X-ray photoelectron spectroscopy and diffraction. The selective growth of these phases depends critically on the choice of substrate, the iron and oxygen fluxes, and the substrate temperature. MgO(001) and Al2O3(0001) were chosen as substrates for the growth of Fe3O4(001) and α-Fe2O3(0001), respectively, because of good lattice and crystal symmetry matching. The growth of α-Fe2O3 is achieved using a low iron-to-oxygen flux ratio compared with that used to grow Fe3O4. Fe3O4 must be grown at the relatively low substrate temperature of 250°C on MgO(001) to avoid interface reaction and Mg outdiffusion. The α-Fe2O3 film surface is unreconstructed whereas the Fe3O4 surface exhibits a (√2 × √2)R45° reconstruction. Application of a simple electron counting rule to the Fe3O4(001) surface suggests that the reconstruction is due to an ordered array of tetrahedral Fe vacancies.
A (112̄3) surface of α-Fe2O3 (haematite) was prepared by Ar+-ion sputtering and annealing in ultrahigh vacuum at 1123K. Examination of this surface by low-energy electron diffraction (LEED) reveals a complex diffraction pattern from which we identify two distinct reciprocal unit cells. The first is identical to that observed for a (111) bulk termination of Fe3O4 (magnetite); the second corresponds to a bulk termination of the (112̄3) plane. Images obtained by scanning tunnelling microscopy (STM) from the prepared α-Fe2O3(112̄3) surface show terraces of close-packed features. These features are separated by 6.0±0.5Å and, together with step heights of 4.8±0.5Å which separate these terraces, are consistent with previously reported STM images of Fe3O4(111) surfaces. Features corresponding to a bulk termination of α-Fe2O3(112̄3) were not observed. We propose that Fe3O4(111) nucleates on the reduced α-Fe2O3(112̄3) substrate, with the [11̄00] direction of α-Fe2O3 parallel to the [1̄10] direction of Fe3O4.This epitaxial relationship is favoured by substrate oxygen planes parallel to α-Fe2O3(112̄3), and by close-packed oxygen planes parallel to Fe3O4(111).
High-resolution electron energy-loss spectroscopy (HREELS), low-energy electron diffraction, and X-ray photoelectron spectroscopy have been used to study clean 825 K-preannealed α-Fe2O3-1 × 1 (haematite) surfaces, an α-Fe2O3-(0001)-1 × 1 surface reconstructed with Fe3O4(111)-1 × 1 and to study Cu deposited on room-temperature surfaces of those. Three pronounced losses, at 47.5, 55.5 and 78.0 meV, of the surface phonons for the clean α-Fe2O3(0001) were observed. By deposition of copper, CuO vibrational features observed by HREELS indicate formation of a Cu(I) state for the very low coverages. Increased submonoloayer amounts of Cu result in clustering of the copper, leading for both the α-Fe2O3(0001)-1 × 1 and the reconstructed composite substrate surfaces to Cu(111) epitaxial growth.
Selective growth of well-defined α-Fe2O3 structures was achieved on a Au(111) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Although oxidation of Fe particles on Au(111) with molecular O2 at room temperature forms FeO, Fe2O3 is prepared by oxidation of Fe particles on Au(111) with NO2 at an elevated temperature, as verified by XPS, based on the binding energy (BE) value of the Fe 2p3/2 (710.9 eV) peak and atomic ratio of O and Fe 1.5:1. STM images reveal that Fe2O3 adopted ordered three-dimensional structures on Au(111). Although the general morphology of the Fe2O3 structures on Au(111) depends on the coverage, varying from nanoparticles at low coverage to islands at high coverage, all of these Fe2O3 structures have nearly identical heights of 5−6 Å at all coverages. The surface structures of the Fe2O3 are all consistent with an O-terminated α-Fe2O3(0001), showing a hexagonal unit cell with a lattice constant of 3 Å in atomically resolved STM images.
Thin FeO(111) buffer layers prepared on Mo(110) substrate were used to grow ordered ZnO films under ultrahigh vacuum condition, and were in situ characterized by various surface analytical techniques. A chemical interaction between Zn (or ZnO) and FeO(111) can effectively lower the interfacial energy, which is in favor of an epitaxial growth of ZnO on FeO layers. Compared with the MgO(111) buffer layer used for the growth of ZnO(0001) on sapphire (0001) surface, the FeO(111) thin films might be a better one because it is more thermally stable. Our experimental results provide constructive information on the growth mechanism of ZnO-based materials, which is helpful for further understanding the growth mechanism of related oxide materials.
Valence-band photoemission spectra have been measured from cleaved stoichiometric iron oxide single crystals using synchrotron radiation. The total photoelectron yield is observed to increase dramatically at photon energies above the Fe 3p→3d excitation threshold, and the resonant onset correlates directly with the oxidation state of the Fe cations. Features in the total yield profiles are interpreted in terms of atomic multiplets. The phenomenon of resonant photoemission is used to distinguish the Fe 3d-derived valence states from the overlapping unhybridized O 2p states in each of the oxides. By use of cleaved single crystals, the subtle differences in the valence-band photoemission features associated with ferrous (Fe2+) and ferric (Fe3+) cations have been determined. The measured energy distribution of the photoemission final states in single crystal α-Fe2O3 are in good agreement with recent measurements and configuration-interaction cluster calculations by Fujimori et al., which take into account ligand-to-metal charge transfer. Constant-initial-state spectra measured across the 3p→3d threshold in each oxide indicate that most of the photoemission intensity near the top of the valence band is derived from hybridized cation-3d–ligand-2p states, whereas the emission at higher binding energies results from primarily 3dn-1 final states. On this basis, each of the iron oxides is classified as a charge-transfer rather than a Mott-Hubbard insulator.
The growth of iron-oxide films on Pt(111) prepared by iron deposition and subsequent oxidation was studied by scanning tunneling microscopy (STM) and high-resolution low-energy electron diffraction (LEED). Despite a 10% lattice mismatch to the substrate, an epitaxial growth of well-ordered films is observed. The oxide starts to grow layer by layer in a (111) orientation of the metastable cubic FeO structure up to a thickness of about 2.2 monolayers (ML). The completion of the second and third FeO layer depends on the precise oxidation temperature, and at coverages of approximately 2 ML three-dimensional Fe3O4(111) islands start to grow. The FeO(111) layers consist of hexagonal close-packed iron-oxygen bilayers that are laterally expanded when compared to bulk FeO and slightly rotated against the platinum substrate. They all exhibit oxygen-terminated unreconstructed (1×1) surface structures. With increasing coverage several structural film changes occur, and four coincidence structures with slightly different lateral lattice constants and rotation misfit angles against the platinum substrate are formed. In the submonolayer regime an FeO(111) bilayer with a lattice constant of 3.11 Å and rotated by 1.3° against the platinum substrate is observed. Upon completion of the first layer the film gets compressed leading to a lattice constant of 3.09 Å and a rotation misfit angle of 0.6°. Between 1.5 and 2 ML a coincidence structure rotated by 30° against the platinum substrate forms, and at 2 ML a nonrotated coincidence structure with a lattice constant of 3.15 Å evolves. All these coincidence structures exhibit large periodicities between approximately 22 and 38 Å that are visible in the STM images up to the third FeO layer surface. The LEED patterns exhibit characteristic multiple scattering satellite spots. The different coincidence structures reflect lowest-total-energy arrangements, balancing the contributions of substrate-overlayer interface energies and elastic energies within the strained oxide overlayer for each coverage.
The epitaxial growth of iron oxide films on Pt(111) substrates was investigated by scanning tunneling microscopy and low-energy electron diffraction. The film growth was accomplished by repeated cycles of iron deposition and subsequent oxidation at p(O2)=10-6mbar. For oxidation temperatures of 870 K second and third FeO(111) layers grow layer by layer, whereas for oxidation temperatures of 1000 K only one FeO(111) monolayer is formed. On top of the FeO(111) films a homogeneous nucleation of Fe3O4(111) islands takes place, resulting in a Stranski-Krastanov growth for iron oxides on Pt(111). The islands grow in the Fe3O4 bulk structure laterally much faster than vertically, forming flat platelets with heights up to 100 Å and hexagonal and triangular basal planes 1000–5000 Å in diameter. The islands only expose low index {1¯11} and {21¯1¯} facet planes, and their growth can be described by an Ostwald ripening mechanism that takes place during each oxidation cycle. Eventually the islands coalesce and form smooth Fe3O4(111) films at least 150 Å thick. The atomic and mesoscopic surface roughness of these films depends on the growth temperature, where the latter ranges between 40 and 100 Å on a length scale of 1 μm. By a high-pressure oxidation at p(O2)=10-1mbar the Fe3O4(111) films were transformed into well-ordered α-Fe2O3(0001) films with similar surface morphologies. In all oxide phases formed the hexagonal oxygen (111) planes are aligned to the Pt(111) substrate surface lattice. The film growth is discussed in terms of surface and interfacial energies, oxidation and growth kinetics, as well as thermodynamic stability ranges of the different oxide phases.
It has recently been shown that well-ordered Fe3O4(111) films can be prepared epitaxially on clean Pt(111) surfaces; various techniques have indicated that these multilayer films are chemically identical to bulk single crystals. We have studied the electronic structure of such an ordered Fe3O4(111) film using angle-resolved photoemission in conjunction with synchrotron radiation. The valence-band structure along the Gamma(L) symmetry line and the resonant emission enhancement across the Fe 3p-->3d excitation threshold have been examined in detail both above (at 300 K) and below (at 90 K) the Verwey transition temperature (similar to 120K) for magnetite. The observed band dispersion agrees reasonably well with band-structure calculations for the high-temperature phase, particularly near the Fermi level, suggesting that Fe3O4, should be treated with band theory. Subtle differences in the valence-band structure are observed between the two temperatures, which may be attributed to a structural change and/or a charge ordering associated with the Verwey transition. The resonant behavior shows, however, no temperature dependence, indicating that resonant photoemission in Fe3O4 remains a localized process and is not influenced by the Verwey transition.
Whenever a compound crystal is cut normal to a randomly chosen direction, there is an overwhelming probability that the resulting surface corresponds to a polar termination and is highly unstable. Indeed, polar oxide surfaces are subject to complex stabilization processes that ultimately determine their physical and chemical properties. However, owing to recent advances in their preparation under controlled conditions and to improvements in the experimental techniques for their characterization, an impressive variety of structures have been investigated in the last few years. Recent progress in the fabrication of oxide nano-objects, which have been largely stimulated by a growing demand for new materials for applications ranging from micro-electronics to heterogeneous catalysis, also offer interesting examples of exotic polar structures. At odds with polar orientations of macroscopic samples, some smaller size polar nano-structures turn out to be perfectly stable. Others are subject to unusual processes of stabilization, which are absent or not effective in their extended counterparts. In this context, a thorough and comprehensive reflexion on the role that polarity plays at oxide surfaces, interfaces and in nano-objects seems timely.This review includes a first section which presents the theoretical concepts at the root of the polar electrostatic instability and its compensation and introduces a rigorous definition of polar terminations that encompasses previous theoretical treatments; a second section devoted to a summary of all experimental and theoretical results obtained since the first review paper by Noguera (2000 J. Phys.: Condens. Matter 12 R367); and finally a discussion section focusing on the relative strength of the stabilization mechanisms, with special emphasis on ternary compound surfaces and on polarity effects in ultra-thin films.
The valence band photoemission spectra of epitaxially grown film and bulk single crystal of magnetite were measured by the angle-resolved ultraviolet photoemission spectroscopy (ARUPS) at 300 K and 120 K. Four main features of the electron photoemission were observed at about –1.0 eV, –3.5 eV, –5.0 eV and –6.5 eV below the chemical potential; they are rather strongly dependant on structure and preparation of surface. The three higher-energy features showed significant energy dispersion. The ARUPS spectra were compared to the accessible band structure calculations. Special attention was paid to the leading edge of the emission at –1.0 eV of the low-temperature spectrum where an insulating gap should be observed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
The so-called Biphase termination on α-Fe2O3 has been widely accepted to be a structure with a ∼40 Å unit supercell composed of coexisting islands of Fe1−xO and α-Fe2O3. Based on thermodynamic arguments and experimental evidence, including transmission electron diffraction, imaging, magnetic and spectroscopic information, it is found that the previously proposed structure model is inaccurate. The actual Biphase structure is instead a layered structure related to the reduction of α-Fe2O3 to Fe3O4. A model for the Biphase termination is proposed which does not contain islands of Fe1−xO but instead consists of bulk α-Fe2O3 and a Fe3O4-derived overlayer. The proposed model is consistent with all current and previously reported experimental findings.
The surface structure formed on epitaxial Fe3O4(111) magnetite films grown onto Pt(111) was re-examined by a full dynamical low energy electron diffraction (LEED) intensity analysis. Prior to the LEED measurements the films were investigated with scanning tunneling microscopy regarding their surface defect concentrations and the possible coexistence of different surface terminations. After a final oxidation at 1000 K in 10−6 mbar oxygen partial pressure one defined surface structure is formed, and for films with low surface defect concentrations the best fit structure reveals a Pendry R-factor of 0.20 based on a data set with a total energy range of 1300 eV. It corresponds to an unreconstructed bulk termination of Fe3O4(111), which exposes 1/4 monolayer of iron atoms over a hexagonal close-packed oxygen layer underneath. The outermost iron plane is relaxed inward towards the underlying oxygen plane by 41±7% of the corresponding bulk spacing, followed by strong relaxations of the next three interlayer spacings. The same surface termination with slightly different relaxations was obtained in an earlier analysis, which corresponded to a local R-factor minimum in parameter space [W. Weiss et al., Phys. Rev. Lett. 71 (1993) 1848]. The energetics of the Fe3O4(111) surface structure is discussed considering the mixed iono-covalent bond character in this oxide.
Fe3O4 nanoparticles and thin films were prepared on the Au(1 1 1) surface and characterized using X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Fe3O4 was formed by annealing α-Fe2O3(0 0 0 1) structures on Au(1 1 1) at 750 K in ultrahigh vacuum (UHV) for 60 min. Transformation of the α-Fe2O3(0 0 0 1) structures into Fe3O4 nanoparticles and thin films was supported by XPS. STM images show that during the growth procedure used, Fe3O4 initially appears as nanoparticles at low coverages, and forms thin films at ∼2 monolayer equivalents (MLE) of iron. Two types of ordered superstructures were observed on the Fe3O4 particles with periodicities of ∼50 and ∼42 Å, respectively. As the Fe3O4 particles form more continuous films, the ∼50 Å feature was the predominant superstructure observed. The Fe3O4 structures at all coverages show a hexagonal unit cell with a ∼3 Å periodicity in the atomically resolved STM images.
We report on a systematic analysis of x-ray photoelectron spectroscopy (XPS) core- and valence-level spectra of clean and well-characterized iron oxide films, i.e., α-Fe2O3, γ-Fe2O3, Fe3-δO4, and Fe3O4. All iron oxide films were prepared epitaxially by NO2-assisted molecular-beam epitaxy on single crystalline MgO(100) and α-Al2O3(0001) substrates. The phase and stoichiometry of the films were controlled precisely by adjusting the NO2 pressure during growth. The XPS spectrum of each oxide clearly showed satellite structures. These satellite structures were simulated using a cluster-model calculation, which could well reproduce the observed structures by considering the systematic changes in both the Fe 3d to O 2p hybridization and the d-d electron-correlation energy. The small difference in the satellite structures between α-Fe2O3 and γ-Fe2O3 resulted mainly from changes in the Fe-O hybridization parameters, suggesting an increased covalency in γ-Fe2O3 compared to α-Fe2O3. With increasing reduct
We introduce an interfactant for zirconium oxide heteroepitaxy and propose that dispersed, epitaxial films of materials which are hard to crystallize are accessible by growing them on top of an ultrathin, polar FeO(111) film.
Epitaxial films of different iron oxide phases and of potassium iron oxide were grown onto Pt(111) substrates and used for studying structure–reactivity correlations. The film morphologies and their atomic surface structures were characterized by scanning tunneling microscopy and low energy electron diffraction including multiple scattering calculations. The adsorption of water, ethylbenzene, and styrene was investigated by temperature programmed desorption and photoelectron spectroscopy. A dissociative chemisorption of water and a molecular chemisorption of ethylbenzene and styrene is observed on all oxides that expose metal cations in their topmost layers, whereas purely oxygen-terminated FeO(111) monolayer films are chemically inert and only physisorption occurs. Regarding the technical styrene synthesis reaction, which is performed over iron oxide based catalysts, we find a decreasing chemisorption strength of the reaction product molecule styrene, if compared to ethylbenzene, when going from Fe3O4(111) over α-Fe2O3(0001) to KFexOy(111). Extrapolation of the adsorbate coverages to the technical styrene synthesis reaction conditions using the Langmuir isotherm for coadsorption suggests an increasing catalytic activity along the same direction. This result agrees with previous kinetic experiments performed at elevated gas pressures over the model systems studied here and over polycrystalline iron oxide catalyst samples. It indicates that the iron oxide surface chemistry does not change across the pressure gap and that the model systems simulate technical styrene synthesis catalysts in a realistic way.
Metal-oxide based catalysts are used for many important synthesis reactions in the chemical industry. A better understanding of the catalyst operation can be achieved by studying elemantary reaction steps on well-defined model catalyst systems. For the dehydrogenation of ethylbenzene to styrene in the presence of steam both unpromoted and potassium promoted iron-oxide catalysts are active. Here we review the work done over unpromoted single-crystalline FeO(111), Fe3O4(111) and a-Fe2O3(0001) films grown epitaxially on Pt(111) substrates. Their geometric and electronic surface structures were characterized by STM, LEED, electron microscopy and electron spectroscopic techniques. In an integrative approach, the interaction of water, ethylbenzene and styrene with these films was investigated mainly by thermal desorption and photoelectron emission spectroscopy. The adsorption-desorption energetics and kinetics depend on the oxide surface terminations and are correlated to the electronic structures and acid-base properties of the corresponding oxide phases, which reveal insight into the nature of the active sites and into the catalytic function of semiconducting oxides in general. Catalytic studies, using a batch reactor arrangement at high gas pressures and post reaction surface analysis, showed that only a-Fe2O3(0001) containing surface defects is catalytically active, whereas Fe3O4(111) is always inactive. This can be related to the elementary adsorption and desorption properties observed in ultrahigh vacuum, which indicates that the surface chemical properties of the iron-oxide films do not change significantly across the "pressure-gap". A model is proposed according to which the active site involves a regular acidic surface sites and a defect site next to it. The results on metal-oxide surface chemistry also have implications for other fields, such as environmental science, biophysics and chemical sensor
Thermodynamic stability ranges of different iron oxides were calculated as a function of the ambient oxygen or water gas phase pressure (p⩽1 bar) and temperature by use of the computer program EquiTherm. The phase diagram for Fe–H2O is almost completely determined by the O2 pressure due to the H2O dissociation equilibrium. The formation of epitaxially grown iron oxide films on platinum and ruthenium substrates agrees very well with the calculated phase diagrams. Thin films exhibit the advantage over single crystals that bulk diffusion has only limited influence on the establishment of equilibrium phases. Near the phase boundary Fe3O4–Fe2O3, surface structures are observed consisting of biphase ordered domains of FeO(111) on
both oxides. They are formed due to kinetic effects in the course of the oxidation to hematite or reduction to magnetite, respectively. Annealing a Fe3O4(111) film in 5 × 10−5
mbar oxygen at 920–1000 K results in a new γ-Fe2O3(111)-like
intermediate surface phase during the oxidation to α-Fe2O3(0001). A model is suggested for the growth
of iron oxides and for redox processes involving iron oxides. The formation of several equilibrium surface phases is discussed.
Scanning tunneling microscopy and low energy electron diffraction have been used to study the alpha-Fe2O3\(0001\) surface in an ultrahigh vacuum. Our results show that this surface can be stabilized by coexisting alpha-Fe2O3\(0001\) and FeO(111) phases, with each phase existing in atomically well-ordered islands of mesoscopic dimensions. Furthermore, the islands themselves are arranged to form a superlattice. The formation of this superlattice can be explained in terms of the lattice mismatch between two types of oxygen sublattices.
The test of the validity of the Fromhold-Cook theory of metal oxidation for the O2/Fe system in the tunnel regime is impeded by the growth of a passivating Fe2+/Fe3+ double layer at T</=150 degrees C. However, during an intermediate anneal step at 200 degrees C, the Fe3+ is reduced to Fe2+. The oxidation rate of this annealed layer is in agreement with the Fromhold-Cook theory. At room temperature, the ionic current is rate limiting. For T>150 degrees C, the thermionic emission of electrons is rate limiting for oxygen coverages larger than 13x10(15) atoms/cm(2).
The interaction of zinc and faceted MgO(111) thin films prepared on a Mo(110) substrate was investigated in situ by using various surface analysis techniques, including X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, high-resolution electron energy loss spectroscopy, and low-energy electron diffraction. The results revealed that three-dimensional Zn islands exist on the faceted MgO(111) films and that no chemical interaction takes place at the interface at room temperature. Initially, deposited Zn is stable at temperatures below 400 K and diffuses into MgO at temperatures above 425 K. A portion of Zn is oxidized at approximately 10 (-6) mbar O 2 at room temperature. An interfacial phase of Zn x Mg 1- x O was formed after Zn was exposed to approximately 10 (-6) mbar O 2 at temperatures >or=500 K. The faceted structure on the MgO(111) surface is of a disadvantage for the epitaxial growth of ZnO films.
- D J Vaughan
- Surf
- X Sci Deng
- J Lee
- C Matranga
- G Barcaro
- A Fortunelli
- G M Granozzi
- D V Vyalikh
Vaughan, D. J. Surf. Sci. 2000, 445, 11.
(26) Deng, X.; Lee, J.; Matranga, C. Surf. Sci. 2010, 604, 627.
(27) Barcaro, G.; Fortunelli, A.; Granozzi, G. Phys. Chem. Chem. Phys. 2008,
10, 1876.
(28) Goniakowski, J.; Finocchi, F.; Noguera, C. Rep. Prog. Phys. 2008, 71,
016501.
(29) Fonin, M.; Pentcheva, R.; Dedkov, Yu. S.; Sperlich, M.; Vyalikh, D. V.;
- M Scheffler
- U Rudiger
- G Guntherodt
- M Ritter
- W Weiss
- M Xue
- Q Guo
- K Wu
- J Guo
- Langmuir
Scheffler, M.; Rudiger, U.; Guntherodt, G. Phys. Rev. B 2005, 72, 104436.
(30) Ritter, M.; Weiss, W. Surf. Sci. 1999, 432, 81.
(31) Xue, M.; Guo, Q.; Wu, K.; Guo, J. Langmuir 2008, 24, 8760.
(32) Ketteler, G.; Ranke, W.; Schl€ ogl, R. Phys. Chem. Chem. Phys. 2004, 6, 205.
- R A Felows
- A R Lennie
- H Raza
- C L Pang
- G Thornton
- D Vaughan
- X Deng
- J Lee
- C Matranga
Felows, R. A.; Lennie, A. R.; Raza, H.; Pang, C. L.; Thornton, G.;
Vaughan, D. J. Surf. Sci. 2000, 445, 11.
(26) Deng, X.; Lee, J.; Matranga, C. Surf. Sci. 2010, 604, 627.
- G Barcaro
- A Fortunelli
- G Granozzi
- J Goniakowski
- F Finocchi
- C Noguera
Barcaro, G.; Fortunelli, A.; Granozzi, G. Phys. Chem. Chem. Phys. 2008,
10, 1876.
(28) Goniakowski, J.; Finocchi, F.; Noguera, C. Rep. Prog. Phys. 2008, 71,
016501.
- M Fonin
- R Pentcheva
- Yu S Dedkov
- M Sperlich
- D V Vyalikh
- M Scheffler
- U Rudiger
- G Guntherodt
- M Ritter
- W Weiss
- M Xue
- Q Guo
- K Wu
- Guo
Fonin, M.; Pentcheva, R.; Dedkov, Yu. S.; Sperlich, M.; Vyalikh, D. V.;
Scheffler, M.; Rudiger, U.; Guntherodt, G. Phys. Rev. B 2005, 72, 104436.
(30) Ritter, M.; Weiss, W. Surf. Sci. 1999, 432, 81.
(31) Xue, M.; Guo, Q.; Wu, K.; Guo, J. Langmuir 2008, 24, 8760.