Reliable Method for Determining the Oxidation State in Chromium Oxides

Textural, Microstructural and Chemical Characterization of Ferritic Stainless Steel Affected by the Gold Dust Defect

The "gold dust defect" (GDD) appears at the surface of ferritic stainless steels (FSS) and degrades their appearance. Previous research showed that this defect might be related to intergranular corrosion and that the addition of aluminium improves surface quality. However, the nature and origin of this defect are not properly understood yet. In this study, we performed detailed electron backscatter diffraction analyses and advanced monochromated electron energy-loss spectroscopy experiments combined with machine-learning analyses in order to extract a wealth of information on the GDD. Our results show that the GDD leads to strong textural, chemical, and microstructural heterogeneities. In particular, the surface of affected samples presents an α-fibre texture which is characteristic of poorly recrystallised FSS. It is associated with a specific microstructure in which elongated grains are separated from the matrix by cracks. The edges of the cracks are rich in chromium oxides and MnCr₂O₄ spinel. In addition, the surface of the affected samples presents a heterogeneous passive layer, in contrast with the surface of unaffected samples, which shows a thicker and continuous passive layer. The quality of the passive layer is improved with the addition of aluminium, explaining the better resistance to the GDD.

Degradation Chemistry and Kinetic Stabilization of Magnetic CrI3

The discovery of the intrinsic magnetic order in single-layer chromium trihalides (CrX₃, X = I, Br, and Cl) has drawn intensive interest due to their potential application in spintronic devices. However, the notorious environmental instability of this class of materials under ambient conditions renders their device fabrication and practical application extremely challenging. Here, we performed a systematic investigation of the degradation chemistry of chromium iodide (CrI₃), the most studied among CrX₃ families, via a joint spectroscopic and microscopic analysis of the structural and composition evolution of bulk and exfoliated nanoflakes in different environments. Unlike other air-sensitive 2D materials, CrI₃ undergoes a pseudo-first-order hydrolysis in the presence of pure water toward the formation of amorphous Cr(OH)₃ and hydrogen iodide (HI) with a rate constant of k_I = 0.63 day^-1 without light. In contrast, a faster pseudo-first-order surface oxidation of CrI₃ occurs in a pure O₂ environment, generating CrO₃ and I₂ with a large rate constant of k_Cr = 4.2 day^-1. Both hydrolysis and surface oxidation of CrI₃ can be accelerated via light irradiation, resulting in its ultrafast degradation in air. The new chemical insights obtained allow for the design of an effective stabilization strategy for CrI₃ with preserved optical and magnetic properties. The use of organic acid solvents (e.g., formic acid) as reversible capping agents ensures that CrI₃ nanoflakes remain stable beyond 1 month due to the effective suppression of both hydrolysis and oxidation of CrI₃.

Direct and Indirect Electron Transfer Routes of Chromium(VI) Reduction with Different Crystalline Ferric Oxyhydroxides in the Presence of Pyrogenic Carbon

Electron transfer mediated by iron minerals is considered as a critical redox step for the dynamics of pollutants in soil. Herein, we explored the reduction process of Cr(VI) with different crystalline ferric oxyhydroxides in the presence of pyrogenic carbon (biochar). Both low- and high-crystallinity ferric oxyhydroxides induced Cr(VI) immobilization mainly via the sorption process, with a limited reduction process. However, the Cr(VI) reduction immobilization was inspired by the copresence of biochar. Low-crystallinity ferric oxyhydroxide had an intense chemical combination with biochar and strong sorption for Cr(VI) via inner-sphere complexation, leading to the indirect electron transfer route for Cr(VI) reduction, that is, the electron first transferred from biochar to iron mineral through C-O-Fe binding and then to Cr(VI) with Fe(III)/Fe(II) transformation on ferric oxyhydroxides. With increasing crystallinity of ferric oxyhydroxides, the direct electron transfer between biochar and Cr(VI) became the main electron transfer avenue for Cr(VI) reduction. The indirect electron transfer was suppressed in the high-crystallinity ferric oxyhydroxides due to less sorption of Cr(VI), limited combination with biochar, and higher iron stability. This study demonstrates that electron transfer mechanisms involving iron minerals change with the mineral crystallization process, which would affect the geochemical process of contaminants with pyrogenic carbon.

WhatEELS. A python-based interactive software solution for ELNES analysis combining clustering and NLLS

The analysis of energy loss near edge structures in EELS is a powerful method for a precise characterization of elemental oxidation states and local atomic coordination with an outstanding lateral resolution, down to the atomic scale. Given the complexity and sizes of the EELS spectrum images datasets acquired by the state-of-the-art instrumentation, methods with low convergence times are usually preferred for spectral unmixing in quantitative analysis, such as multiple linear least squares fittings. Nevertheless, non-linear least squares fitting may be a superior choice for analysis in some cases, as it eliminates the need of calibrated reference spectra and provides information for each of the individual components included in the fitted model. To avoid some of the problems that the non-linear least squares algorithms may suffer dealing with mixed-composition samples and, thus, a model comprised by a large number of individual curves we proposed the combination of clustering analysis for segmentation and non-linear least squares fitting for spectral analysis. Clustering analysis is capable of a fast classification of pixels in smaller subsets divided by their spectral characteristics, and thus increases the control over the model parameters in separated regions of the samples, classified by their specific compositions. Furthermore, along with this manuscript we provide access to a self-contained and expandable modular software solution called WhatEELS. It was specifically designed to facilitate the combined use of clustering and NLLS, and includes a set of tools for white-lines analysis and elemental quantification. We successfully demonstrated its capabilities with a control sample of mesoporous cerium oxide doped with praseodymium and gadolinium, which posed challenging case-study given its spectral characteristics.

Direct Evidence of a Graded Magnetic Interface in Bimagnetic Core/Shell Nanoparticles Using Electron Magnetic Circular Dichroism (EMCD)

Interfaces play a crucial role in composite magnetic materials and particularly in bimagnetic core/shell nanoparticles. However, resolving the microscopic magnetic structure of these nanoparticles is rather complex. Here, we investigate the local magnetization of antiferromagnetic/ferrimagnetic FeO/Fe₃O₄ core/shell nanocubes by electron magnetic circular dichroism (EMCD). The electron energy-loss spectroscopy (EELS) compositional analysis of the samples shows the presence of an oxidation gradient at the interface between the FeO core and the Fe₃O₄ shell. The EMCD measurements show that the nanoparticles are composed of four different zones with distinct magnetic moment in a concentric, onion-type, structure. These magnetic areas correlate spatially with the oxidation and composition gradient with the magnetic moment being largest at the surface and decreasing toward the core. The results show that the combination of EELS compositional mapping and EMCD can provide very valuable information on the inner magnetic structure and its correlation to the microstructure of magnetic nanoparticles.

ZnCr₂O₄ Inclusions in ZnO Matrix Investigated by Probe-Corrected STEM-EELS

The ZnCr₂O₄/ZnO materials system has a wide range of potential applications, for example, as a photocatalytic material for waste-water treatment and gas sensing. In this study, probe-corrected high-resolution scanning transmission electron microscopy and geometric phase analysis were utilized to study the dislocation structure and strain distribution at the interface between zinc oxide (ZnO) and embedded zinc chromium oxide (ZnCr₂O₄) particles. Ball-milled and dry-pressed ZnO and chromium oxide (α-Cr₂O₃) powder formed ZnCr₂O₄ inclusions in ZnO with size ~400 nm, where the interface properties depended on the interface orientation. In particular, sharp interfaces were observed for ZnO [2113]/ZnCr₂O₄ [110] orientations, while ZnO [1210]/ZnCr₂O₄ [112] orientations revealed an interface over several atomic layers, with a high density of dislocations. Further, monochromated electron energy-loss spectroscopy was employed to map the optical band gap of ZnCr₂O₄ nanoparticles in the ZnO matrix and their interface, where the average band gap of ZnCr₂O₄ nanoparticles was measured to be 3.84 ± 0.03 eV, in contrast to 3.22 ± 0.01 eV for the ZnO matrix.

Probing the Chemistry of Adhesion between a 316L Substrate and Spin-on-Glass Coating

Hydrogen silsesquioxane ([HSiO_3/2] _n)-based "spin-on-glass" has been deposited on a 316L substrate and cured in Ar/H₂ gas atmosphere at 600 °C to form a continuous surface coating with submicrometer thickness. The coating functionality depends primarily on the adhesion to the substrate, which is largely affected by the chemical interaction at the interface between the coating and the substrate. We have investigated this interface by transmission electron microscopy and electron energy loss spectroscopy. The analysis identified a 5-10 nm thick interaction zone containing signals from O, Si, Cr, and Fe. Analysis of the energy loss near edge structure of the present elements identified predominantly signal from [SiO₄]^4- units together with Fe²⁺, Cr²⁺, and traces of Cr³⁺. High-resolution transmission electron microscopy images of the interface region confirm a crystalline Fe₂SiO₄ interfacial region. In agreement with computational thermodynamics, it is proposed that the spin-on-glass forms a chemically bonded silicate-rich interaction zone with the substrate. It was further suggested that this zone is composed of a corundum-type oxide at the substrate surface, followed by an olivine-structure intermediate phase and a spinel-type oxide in the outer regions of the interfacial zone.

Epitaxial growth of multiwall carbon nanotube from stainless steel substrate and effect on electrical conduction and field emission

The epitaxial growth of carbon nanotubes (CNTs) is an important subject of research. Recent attention has been paid to finding new strategies for the controlled growth of single-wall CNTs with a defined chirality. In addition, many potential applications require multiwall CNTs (MWCNTs) to grow vertically from the substrate and the interface property is crucial. Here, we report for the first time that MWCNTs can grow directly from the surface of a substrate by epitaxy, based on the experimental study of individual multiwall carbon nanotubes on a large-area stainless steel substrate, which is a very useful system for electrical and mechanical applications. In particular, evidence is given of the lattice matching between the MWCNT and the lattice of a hexagonal Cr₂O₃: (Fe, Mn) film formed on the surface of the substrate. Furthermore, a method is developed to increase the density of the MWCNTs; a mechanism of simultaneous top and bottom growth is proposed. The resultant significantly improved electrical transport and field emission properties are also presented, showing the Ohmic contact for electrical conduction and high performance in resisting the catastrophic cold-cathode vacuum breakdown of the CNTs.

A Complete Overhaul of the Electron Energy-Loss Spectroscopy and X-Ray Absorption Spectroscopy Database: eelsdb.eu

The electron energy-loss spectroscopy (EELS) and X-ray absorption spectroscopy (XAS) database has been completely rewritten, with an improved design, user interface, and a number of new tools. The database is accessible at https://eelsdb.eu/ and can now be used without registration. The submission process has been streamlined to encourage spectrum submissions and the new design gives greater emphasis on contributors' original work by highlighting their papers. With numerous new filters and a powerful search function, it is now simple to explore the database of several hundred EELS and XAS spectra. Interactive plots allow spectra to be overlaid, facilitating online comparison. An application-programming interface has been created, allowing external tools and software to easily access the information held within the database. In addition to the database itself, users can post and manage job adverts and read the latest news and events regarding the EELS and XAS communities. In accordance with the ongoing drive toward open access data increasingly demanded by funding bodies, the database will facilitate open access data sharing of EELS and XAS spectra.

Built-In Potential in Fe2O3-Cr2O3 Superlattices for Improved Photoexcited Carrier Separation

Hematite (α-Fe2 O3) is engineered to improve photoexcited electron-hole pair separation by synthesizing Fe2O3-Cr2O3 superlattices (SLs) with precise atomic control. The different surface terminations exhibited by Fe2O3 and Cr2O3 determine the hetero-junction interface structure and result in controllable, noncommutative band offset values. This controllable band alignment is harnessed to generate a built-in potential as large as 0.8 eV in Fe2 O3-Cr2O3 SLs.

Oxide Wizard: an EELS application to characterize the white lines of transition metal edges

Physicochemical properties of transition metal oxides are directly determined by the oxidation state of the metallic cations. To address the increasing need to accurately evaluate the oxidation states of transition metal oxide systems at the nanoscale, here we present "Oxide Wizard." This script for Digital Micrograph characterizes the energy-loss near-edge structure and the position of the transition metal edges in the electron energy-loss spectrum. These characteristics of the edges can be linked to the oxidation states of transition metals with high spatial resolution. The power of the script is demonstrated by mapping manganese oxidation states in Fe3O4/Mn3O4 core/shell nanoparticles with sub-nanometer resolution in real space.

2D atomic mapping of oxidation states in transition metal oxides by scanning transmission electron microscopy and electron energy-loss spectroscopy

Using a combination of high-angle annular dark-field scanning transmission electron microscopy and atomically resolved electron energy-loss spectroscopy in an aberration-corrected transmission electron microscope we demonstrate the possibility of 2D atom by atom valence mapping in the mixed valence compound Mn3O4. The Mn L(2,3) energy-loss near-edge structures from Mn2+ and Mn3+ cation sites are similar to those of MnO and Mn2O3 references. Comparison with simulations shows that even though a local interpretation is valid here, intermixing of the inelastic signal plays a significant role. This type of experiment should be applicable to challenging topics in materials science, such as the investigation of charge ordering or single atom column oxidation states in, e.g., dislocations.