Surface structure of magnetite (111) under oxidizing and reducing conditions

Structural and magnetic properties of the Fe[Formula: see text]O[Formula: see text] (110) surface

We have studied the (110) surface of Fe[Formula: see text]O[Formula: see text] single crystals by means of X-ray Photoemission Electron Microscopy (PEEM) and Low-Energy Electron Microscopy (LEEM) to characterize its structural and magnetic properties. After sputtering and annealing a well defined surface was achieved. This preparation method resulted in a one-dimensional reconstruction formed by rows aligned in the [001] direction. By acquiring X-ray magnetic circular dichroism PEEM images at various azimuthal angles, the vector magnetization map of the (110) surface was obtained. Domains were observed with their magnetization aligned along the two [Formula: see text] type bulk easy axes which are in the (110) surface plane, featuring 180[Formula: see text], 109[Formula: see text], and 71[Formula: see text] domain walls. The domain walls are of Néel type. Using the sum rules we estimated an iron spin and orbital magnetic moment of 3.4 [Formula: see text] and 0.6 [Formula: see text] respectively for the reconstructed surface. At the oxygen K edge we observe dichroic contrast of close to 1%, which is reversed relative of the contrast detected from octahedral iron in the L[Formula: see text] edge.

A multi-molecular beam/infrared reflection absorption spectroscopy apparatus for probing mechanisms and kinetics of heterogeneously catalyzed reaction from ultrahigh vacuum to near-ambient pressure conditions

A novel multi-molecular beam/infrared reflection absorption spectroscopy (IRAS) apparatus is described, which was constructed for studying mechanisms and kinetics of heterogeneously catalyzed reactions following a rigorous surface science approach in the pressure range from ultrahigh vacuum (UHV, 1 × 10-10 mbar) to near-ambient pressure (NAP, 1000 mbar) conditions. The apparatus comprises a preparation chamber equipped with standard surface science tools required for the preparation and characterization of model heterogeneous catalysts and two reaction chambers operating at different pressure ranges: in UHV and in the variable pressure range up to NAP conditions. The UHV reaction chamber contains two effusive molecular beams (flux up to 1.1 × 1015 molecules cm-2 s-1), a quadrupole mass spectrometer, a Fourier-Transform (FT) IRA spectrometer, and a molecular beam monitor for beam aligning. This combination of the methods allows us to independently dose different reactants on the surface in a highly controlled way while simultaneously monitoring the evolution of gaseous products by QMS and recording the evolution of the surface species by FT-IRAS. The second reaction chamber operating in the variable pressure range is equipped with polarization-modulation-IRAS and three gas dosers and is designed as a small reactor, which can be operated in a continuous flow mode. The sample prepared under well-controlled UHV conditions can be in situ transferred between all chambers, thus allowing for investigations of structure-reactivity relationships over model surfaces. In this contribution, we provide a detailed description of the apparatus and the test measurements of the different crucial parts of the apparatus in the variable pressure range.

Unveiling the role of inorganic nanoparticles in Earth's biochemical evolution through electron transfer dynamics

This article explores the intricate interplay between inorganic nanoparticles and Earth's biochemical history, with a focus on their electron transfer properties. It reveals how iron oxide and sulfide nanoparticles, as examples of inorganic nanoparticles, exhibit oxidoreductase activity similar to proteins. Termed "life fossil oxidoreductases," these inorganic enzymes influence redox reactions, detoxification processes, and nutrient cycling in early Earth environments. By emphasizing the structural configuration of nanoparticles and their electron conformation, including oxygen defects and metal vacancies, especially electron hopping, the article provides a foundation for understanding inorganic enzyme mechanisms. This approach, rooted in physics, underscores that life's origin and evolution are governed by electron transfer principles within the framework of chemical equilibrium. Today, these nanoparticles serve as vital biocatalysts in natural ecosystems, participating in critical reactions for ecosystem health. The research highlights their enduring impact on Earth's history, shaping ecosystems and interacting with protein metal centers through shared electron transfer dynamics, offering insights into early life processes and adaptations.

Stability and Speciation of Hydrated Magnetite {111} Surfaces from Ab Initio Simulations with Relevance for Geochemical Redox Processes

Magnetite is a common mixed Fe(II,III) iron oxide in mineral deposits and the product of (anaerobic) iron corrosion. In various Earth systems, magnetite surfaces participate in surface-mediated redox reactions. The reactivity and redox properties of the magnetite surface depend on the surface speciation, which varies with environmental conditions. In this study, Kohn-Sham density functional theory (DFT + U method) was used to examine the stability and speciation of the prevalent magnetite crystal face {111} in a wide range of pH and Eh conditions. The simulations reveal that the oxidation state and speciation of the surface depend strongly on imposed redox conditions and, in general, may differ from those of the bulk state. Corresponding predominant phase diagrams for the surface speciation and structure were calculated from first principles. Furthermore, classical molecular dynamics simulations were conducted investigating the mobility of water near the magnetite surface. The obtained knowledge of the surface structure and oxidation state of iron is essential for modeling retention of redox-sensitive nuclides.

Oxidation-State Dynamics and Emerging Patterns in Magnetite

Magnetite is an important mineral with many interesting applications related to its magnetic, electrical, and thermal properties. Typically studied by electronic structure calculations, these methods are unable to capture the complex ion dynamics at relevant temperatures, time, and length scales. We present a hybrid Monte Carlo/molecular dynamics (MC/MD) method based on iron oxidation-state swapping for accurate atomistic modeling of bulk magnetite, magnetite surfaces, and nanoparticles that captures the complex ionic dynamics. By comparing the oxidation-state patterns with those obtained from density functional theory, we confirmed the accuracy of our approach. Lattice distortions leading to the stabilization of excess charges and a critical surface thickness at which the oxidation states transition from ordered to disordered were observed. This simple yet efficient approach paves the way for elucidating aspects of oxidation-state ordering of inverse spinel structures in general and battery materials in particular.

Adsorption of oleic acid on magnetite facets

The microscopic understanding of the atomic structure and interaction at carboxylic acid/oxide interfaces is an important step towards tailoring the mechanical properties of nanocomposite materials assembled from metal oxide nanoparticles functionalized by organic molecules. We have studied the adsorption of oleic acid (C₁₇H₃₃COOH) on the most prominent magnetite (001) and (111) crystal facets at room temperature using low energy electron diffraction, surface X-ray diffraction and infrared vibrational spectroscopy complemented with molecular dynamics simulations used to infer specific hydrogen bonding motifs between oleic acid and oleate. Our experimental and theoretical results give evidence that oleic acid adsorbs dissociatively on both facets at lower coverages. At higher coverages, the more pronounced molecular adsorption causes hydrogen bond formation between the carboxylic groups, leading to a more upright orientation of the molecules on the (111) facet in conjunction with the formation of a denser layer, as compared to the (001) facet. This is evidenced by the C=O double bond infrared line shape, in depth molecular dynamics bond angle orientation and hydrogen bond analysis, as well as X-ray reflectivity layer electron density profile determination. Such a higher density can explain the higher mechanical strength of nanocomposite materials based on magnetite nanoparticles with larger (111) facets.