Co x Fe 3−x O 4 nanoparticles (x = 0.4 to x = 2.5) and thin films (x = 0.9 to x = 2.2) are analyzed by Raman, absorption, and photoluminescence spectroscopy to link structural and optical properties to different cobalt to iron (Co/Fe) ratios. Raman spectroscopy shows that with decreasing Co content, the crystal structure changes from a predominantly normal cubic spinel phase to a mixed inverse spinel phase. This finding is supported by absorption spectroscopy that points out that inter valence charge transfer (IVCT) processes between octahedrally coordinated Co 2+ and Fe 3+ cations become more prominent with increasing Fe content. Independent of the Co/Fe ratio, Co x Fe 3−x O 4 nanoparticles show a broad photoluminescence (PL) band with a maximum at around 510 nm. Time-resolved photoluminescence spectroscopy shows subnanosecond lifetimes and temperatureresolved photoluminescence experiments reveal that the green PL increases with decreasing temperature (300 to 10 K) while showing no temperature-dependent shift in energy. It is proposed that this green PL originates from OH-groups on the particles' surface.
Thin-film continuous composition spreads of Fe−Co− O were fabricated by reactive cosputtering from elemental Fe and Co targets in reactive Ar/O 2 atmosphere using deposition temperatures ranging from 300 to 700 °C. Fused silica and platinized Si/SiO 2 strips were used as substrates. Ti and Ta were investigated as adhesion layer for Pt and the fabrication of the Fe−Co−O films. The thin-film composition spreads were characterized by high-throughput electrondispersive X-ray spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, scanning electron microscopy, and optical transmission spectroscopy. The Fe-content ranged from 28 to 72 at. %. The spinel phases Fe 2 CoO 4 and FeCo 2 O 4 could be synthesized and stabilized at all deposition temperatures with a continuous variation in spinel composition in between. The dependence of the film surface microstructure on the deposition temperature and the composition was mapped. Moreover, the band gap values, ranging from 2.41 eV for FeCo 2 O 4 to 2.74 eV for Fe 2 CoO 4 , show a continuous variation with the composition.
In this study, we propose the use of nondestructive, depth-resolved, element-specific characterization using grazing exit X-ray absorption near-edge structure spectroscopy (GE-XANES) to investigate the corrosion process in compositionally complex alloys (CCAs). By combining grazing exit X-ray fluorescence spectroscopy (GE-XRF) geometry and a pnCCD detector, we provide a scanning-free, nondestructive, depth-resolved analysis in a sub-micrometer depth range, which is especially relevant for layered materials, such as corroded CCAs. Our setup allows for spatial and energy-resolved measurements and directly extracts the desired fluorescence line, free from scattering events and other overlapping lines. We demonstrate the potential of our approach on a compositionally complex CrCoNi alloy and a layered reference sample with known composition and specific layer thickness. Our findings indicate that this new GE-XANES approach has exciting opportunities for studying surface catalysis and corrosion processes in real-world materials.
Polyelemental material systems, specifically high‐entropy alloys, promise unprecedented properties. Due to almost unlimited combinatorial possibilities, their exploration and exploitation is hard. This challenge is addressed by co‐sputtering combined with shadow masking to produce a multitude of microscale combinatorial libraries in one deposition process. These thin‐film composition spreads on the microscale cover unprecedented compositional ranges of high‐entropy alloy systems and enable high‐throughput characterization of thousands of compositions for electrocatalytic energy conversion reactions using nanoscale scanning electrochemical cell microscopy. The exemplary exploration of the composition space of two high‐entropy alloy systems provides electrocatalytic activity maps for hydrogen evolution and oxygen evolution as well as oxygen reduction reactions. Activity optima in the system Ru–Rh–Pd–Ir–Pt are identified, and active noble‐metal lean compositions in the system Co–Ni–Mo–Pd–Pt are discovered. This illustrates that the proposed microlibraries are a holistic discovery platform to master the multidimensionality challenge of polyelemental systems.
The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adem.202201050.
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