Oxidation of Iron: The Relation between Oxidation Kinetics and Oxide Electronic Structure

Roosendaal, S. J.; Vredenberg, A. M.; Habraken, F.H.P.M.

doi:10.1103/physrevlett.84.3366

Cited by 56 publications

(33 citation statements)

References 18 publications

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“…The sensitivity of the XAS signal to the oxidation state and the local environment of the absorbing atom is used to follow the evolution of the oxide's structure in a quantitative and site specific manner. The use of ultrathin layers (in contrast to other studies which used thick substrates [4,5,[7][8][9]) allows one to directly resolve the intrinsic composition of the growing oxide and disentangle the influence of the substrate.The experiment was performed in situ in a compact UHV deposition chamber (base pressure 1 10 ÿ9 mbar) mounted on the A1 beam line of the DORIS III storage ring (DESY, Hamburg). The Fe K-edge absorption signal was recorded by measure of the total electron yield and fluorescence yield.…”

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confidence: 99%

“…Several studies on iron oxidation at room temperature (RT) reported the possible presence of metallic Fe in the oxide layer [2,5,9], without being able to disentangle the influence of the thick Fe substrate on the origin of the signal. The use of ultrathin Fe layers allows us to address this question in a more quantitative manner.…”

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confidence: 99%

“…Studies performed on polycrystalline [4] as well as on single crystalline [5] surfaces have shown that the oxidation of iron can be understood in the framework of the coupled current mechanism introduced by Fromhold and Cook more than 50 years ago [6]. While those experiments explain very well the observed growth kinetics, the actual composition of the native oxide is still a matter of debate and is obviously highly dependent on preparation parameters like temperature and surface orientation ([7-11] and references therein).…”

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How Metallic Fe Controls the Composition of its Native Oxide

et al. 2008

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We have studied in situ the oxidation of ultrathin iron layers and monitored the chemical changes induced by subsequent deposition of Fe metal using hard x-ray absorption spectroscopy. The site sensitivity of the technique allows us to quantify the composition of the layer throughout the oxidation or deposition process. It is found that the thin native oxide incorporates a significant fraction of Fe atoms remaining in a metallic configuration even in the saturated state. Subsequent deposition of Fe leads to a complete reduction of the oxide that adopts an FeO-like structure containing Fe 2 sites only. DOI: 10.1103/PhysRevLett.101.056101 PACS numbers: 81.15.ÿz, 61.05.cj, 81.16.Pr In recent years, intensive research has been conducted on transition metal oxide compounds, thin films, and multilayers due to their potential use as building blocks in magnetic structures like spin valves and magnetic tunnel junctions (for a review, see [1] and references therein). Recently, metal -native-oxide multilayers (MNOM, produced by metal deposition and subsequent oxygen exposure) have been proposed as new compounds combining high magnetization and low conductivity [2]. In these systems, the buried oxide possesses magnetic properties that strongly deviate from any bulk iron oxide phase [2,3]. However, the influence of the surrounding metal on the structure of the oxide is not yet identified and prevents clear understanding of the observed magnetic properties of MNOM systems.The oxidation of iron itself has been extensively studied in the past decades. Studies performed on polycrystalline [4] as well as on single crystalline [5] surfaces have shown that the oxidation of iron can be understood in the framework of the coupled current mechanism introduced by Fromhold and Cook more than 50 years ago [6]. While those experiments explain very well the observed growth kinetics, the actual composition of the native oxide is still a matter of debate and is obviously highly dependent on preparation parameters like temperature and surface orientation ([7-11] and references therein). X-ray absorption spectroscopy at the L edges was recently used to characterize the extent of oxidation or reduction processes arising at the interface between Fe and other transition metal oxides (CoO and NiO) [12]. Remarkably, the interaction of Fe with its own oxide phases was never quantitatively studied before.In this Letter, we present a quantitative x-ray absorption spectroscopy (XAS) study of the oxidation of ultrathin iron layers and the changes induced by subsequent coverage with Fe. The sensitivity of the XAS signal to the oxidation state and the local environment of the absorbing atom is used to follow the evolution of the oxide's structure in a quantitative and site specific manner. The use of ultrathin layers (in contrast to other studies which used thick substrates [4,5,[7][8][9]) allows one to directly resolve the intrinsic composition of the growing oxide and disentangle the influence of the substrate.The experiment was performed in situ in...

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How Metallic Fe Controls the Composition of its Native Oxide

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“…Generally, one could interpret this finding as reflecting kinetic limitations to a continued growth, presumably due to slow diffusion of either O or Ru atoms through the formed film. 37,38 While this could slow the thickening of the oxide overlayer down beyond hitherto measured time scales, it could also be that the kinetics of the on-going catalytic reaction affect the surface populations in such a way, that no further net diffusion through the film occurs and the nanometer thin oxide overlayer represents in fact a kinetically limited steady-state in the given reactive envi- ronment. This would e.g.…”

Section: Kinetically Limited Film Thicknessmentioning

confidence: 99%

“…In fact, the classic example is a slow thickening of oxide films due to limitations in the diffusion of oxygen atoms from the surface to the oxidemetal interface, or in the diffusion of metal atoms from the interface to the surface. 37,38 Directly at the surface a similar bottleneck can be the penetration of oxygen, which might be significantly facilitated at steps and defects and thus adds to their relevance particularly for the formation of surface oxides also from a kinetic point of view. Still, although such kinetic barriers might slow the oxidation process down beyond noticeable time scales, in a pure oxygen gas phase the true thermodynamic ground state will eventually be reached.…”

Section: Implications For Oxidation Catalysismentioning

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Nanometer and Subnanometer Thin Oxide Films at Surfaces of Late Transition Metals

Reuter

2007

Nanocatalysis

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If metal surfaces are exposed to sufficiently oxygen-rich environments, oxides start to form. Important steps in this process are the dissociative adsorption of oxygen at the surface, the incorporation of O atoms into the surface, the formation of a thin oxidic overlayer and the growth of the once formed oxide film. For the oxidation process of late transition metals (TM), recent experimental and theoretical studies provided a quite intriguing atomic-scale insight: The formed initial oxidic overlayers are not merely few atomic-layer thin versions of the known bulk oxides, but can exhibit structural and electronic properties that are quite distinct to the surfaces of both the corresponding bulk metals and bulk oxides. If such nanometer or even sub-nanometer surface oxide films are stabilized in applications, new functionalities not scalable from the known bulk materials could correspondingly arise. This can be particularly important for oxidation catalysis, where technologically relevant gas phase conditions are typically quite oxygen-rich. In such environments surface oxides may even form naturally in the induction period, and actuate then the reactive steady-state behavior that has traditionally been ascribed to the metal substrates. Corresponding aspects are reviewed by focusing on recent progress in the modeling and understanding of the oxidation behavior of late TMs, using particularly the late 4d series from Ru to Ag to discuss trends.

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