To facilitate the next generation of high-power white-light-emitting diodes (white LEDs), the discovery of more efficient red-emitting phosphor materials is essential. In this regard, the hardly ...explored compound class of nitridoaluminates affords a new material with superior luminescence properties. Doped with Eu(2+), SrLiAl3N4 emerged as a new high-performance narrow-band red-emitting phosphor material, which can efficiently be excited by GaN-based blue LEDs. Owing to the highly efficient red emission at λ(max) ~ 650 nm with a full-width at half-maximum of ~1,180 cm(-1) (~50 nm) that shows only very low thermal quenching (>95% relative to the quantum efficiency at 200 °C), a prototype phosphor-converted LED (pc-LED), employing SrLiAl3N4:Eu(2+) as the red-emitting component, already shows an increase of 14% in luminous efficacy compared with a commercially available high colour rendering index (CRI) LED, together with an excellent colour rendition (R(a)8 = 91, R9 = 57). Therefore, we predict great potential for industrial applications in high-power white pc-LEDs.
In the endeavor of discovering new noble metal–free electrocatalysts for the oxygen reduction reaction, noble metal–free multinary transition metal nanoparticle libraries are investigated. The ...complexity of such multiple principal element alloys provides access to a large variety of different elemental compositions, each with potentially unique properties. The strategy for efficient identification of novel electrocatalytically active systems comprises combinatorial co‐sputtering into an ionic liquid followed by potential‐assisted immobilization of the formed nanoparticles at a microelectrode which allows the evaluation of their intrinsic electrocatalytic activity in alkaline media. A surprisingly high intrinsic activity is found for the system Cr–Mn–Fe–Co–Ni, which is at least comparable to Pt under the same conditions, an unexpected result based on the typical properties of its constituents. Systematic removal of each element from the quinary alloy system yields a significant drop in activity for all quaternary alloys, indicating the importance of the synergistic combination of all five elements, likely due to formation of a single solid solution phase with altered properties which enables the limitations of the single elements to be overcome. Multinary transition metal alloys as a novel material class in electrocatalysis with basically unlimited possibilities for catalyst design, targeting the replacement of noble metal–based materials, are suggested.
The quinary high entropy alloy Cr–Mn–Fe–Co–Ni shows intrinsic oxygen reduction reaction activity which can compete with state‑of‑the‑art Pt nanoparticles. All subsystems show a significant drop in activity, proving the relevance of suitable interactions of specific elements forming new active sites within the formed single solid solution phase. Following this methodology, activity is not limited by the properties of the single contributing elements.
CaLiAl3N4:Eu2+ is an intriguing new narrow-band red-emitting phosphor material with potential for application in high-power phosphor-converted light-emitting diodes (pc-LEDs). With excitation by ...blue InGaN-based LEDs, the compound exhibits an emission maximum at 668 nm with a full width at half maximum of only 1333 cm–1 (∼60 nm). CaLiAl3N4:Eu2+ was synthesized from Ca, LiAlH4, LiN3, AlF3, and EuF3 in weld-shut Ta ampules, and the structure was solved and refined on the basis of single-crystal X-ray diffraction data. After isotypical crystallization with NaLi3SiO4, the compound forms a highly condensed framework of AlN4 and LiN4 tetrahedra I41/a (no. 88), Z = 16, a = 11.1600(16) Å, and c = 12.865(3) Å and can thus by classified as a nitridolithoaluminate. Both types of polyhedra are connected to each other by common edges and corners, yielding a high degree of condensation, κ = 1. The Ca site is positioned in the center of vierer ring channels along 001 and coordinated in a cuboidal manner by eight N atoms. To validate the presence of Li, transmission electron microscopy (TEM) investigations employing electron energy-loss spectroscopy (EELS) were carried out. Furthermore, to confirm the electrostatic bonding interactions and the chemical composition, lattice energy calculations Madelung part of lattice energy (MAPLE) have been performed.
Titanium dioxide nanowire (NW) arrays are incorporated in many devices for energy conversion, energy storage, and catalysis. A common approach to fabricate these NWs is based on hydrothermal ...synthesis strategies. A drawback of this low-temperature method is that the NWs have a high density of defects, such as stacking faults, dislocations, and oxygen vacancies. These defects compromise the performance of devices. Here, we report a postgrowth thermal annealing procedure to remove these lattice defects and propose a mechanism to explain the underlying changes in the structure of the NWs. A detailed transmission electron microscopy study including in situ observation at elevated temperatures reveals a two-stage process. Additional spectroscopic analyses and X-ray diffraction experiments clarify the underlying mechanisms. In an early, low-temperature stage, the as-grown mesocrystalline NW converts to a single crystal by the dehydration of surface-bound OH groups. At temperatures above 500 °C, condensation of oxygen vacancies takes place, which leads to the fabrication of NWs with internal voids. These voids are faceted and covered with Ti3+-rich amorphous TiO x .
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•Al graded Mg-Al thin films were synthesized via combinatorial magnetron sputtering.•Al graded Mg-Al thin films were analyzed via in-situ electrochemistry.•The Mg-Al solid solution ...corrosion resistance increases with the Al concentration.•Exceeding cAl ∼ 4 wt% causes a distinct anodic current density decrease.•Exceeding cAl ∼ 4 wt% causes a predominant cathodic partial reaction on the surface.
The effect of varying Al concentrations on the electrochemical corrosion resistance of binary Mg-Al solid solutions thin films under alkaline immersion conditions was investigated via a combination of in-situ flow-cell, scanning vibrating electrode technique and microscopy analysis. These spatially resolving characterization techniques are employed along the Al concentration gradient of the combinatorically grown thin films enabling efficient screening of the Al concentration dependent electrochemical corrosion behaviour. The analysis revealed an increasing corrosion resistance with increasing Al concentration, as a consequence of Al induced hydroxide reinforcement. Specifically, the addition of >4 wt.% Al decreases the corrosion current density in the range of 70–90 % compared to pure Mg.
Complex solid solution (CSS) nanoparticles were recently discovered as efficient electrocatalysts for a variety of reactions. As one of many advantages, they exhibit the potential to replace ...noble-metal catalysts with multinary combinations of transition metals because they offer formation of new unique and tailorable active sites of multiple elements located next to each other. This Perspective reports on the current state and on challenges of the (combinatorial) synthesis of multinary nanoparticles and advanced electron microscopy characterization techniques for revealing structure–activity correlations on an atomic scale. We discuss what distinguishes this material class from common catalysts to highlight their potential to act as electrocatalysts and rationalize their nontypical electrochemical behavior. We provide an overview about challenges in synthesis, characterization, and electrochemical evaluation and propose guidelines for future design of CSS catalysts to achieve further progress in this research field, which is still in its infancy.
The unique combination of atomic-scale composition measurements, employing atom probe tomography, atomic structure determination with picometer resolution by aberration-corrected scanning ...transmission electron microscopy, and atomistic simulations reveals site-specific linear segregation features at grain boundary facet junctions. More specific, an asymmetric line segregation along one particular type of facet junction core, instead of a homogeneous decoration of the facet planes, is observed. Molecular-statics calculations show that this segregation pattern is a consequence of the interplay between the asymmetric core structure and its corresponding local strain state. Our results contrast with the classical view of a homogeneous decoration of the facet planes and evidence a complex segregation patterning.
Photocorrosion imposes a fundamental limit to the longevity of devices that harvest energy from photons. As one of the best performing electrode materials for photoelectrochemical water oxidation ...reaction, BiVO4 undergoes photocorrosion with various postulated mechanisms under debate. We present time-resolved dissolution measurements to advance the mechanistic understanding, enabled by the recent development in illuminated scanning flow cell coupled to inductively coupled plasma mass spectrometry. The contact dissolution of predominantly V was distinguished from the stoichiometric photoelectrochemical dissolution of Bi and V. The citrate electrolyte was utilized to form soluble complexes with dissolved Bi and to act as hole scavengers that provide photocurrents at a wide range of potentials. The photoelectrochemical dissolution rates remain similar between 0.4 and 1.6 V vs reversible hydrogen electrode and become lower at the open circuit potential, 0.2 V. The time-resolved measurements support oxidation of Bi(III) by photogenerated holes as the main mechanism for photocorrosion.
Photocorrosion is one of the main challenges in photoelectrochemical water splitting. Traditional methods for degradation assessment, such as chronoamperometry or electrolyte/electrode post-analysis, ...provide limited information on the degradation mechanisms and kinetics. To address this issue, a new setup, which is based on a light source, an electrochemical cell, and an on-line inductively coupled plasma mass spectrometer (ICP-MS), has been developed. The first results on the photocorrosion of a commercial WO3 powder on Au are demonstrated in this work. It is shown that, in the absence of light, WO3 is stable in a wide potential range. On the other hand, in the presence of light, it dissolves proportionally to the anodic photocurrent. The latter is explained by the formation of aqueous tungsten complexes with the electrolyte that are thermodynamically more stable than WO3. As can be anticipated from these initial results, the novel method will enable the characterization of a wide range of photoelectrochemical materials, and thus eventually lead to the development of long-term stable devices.
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•New experimental setup for time-resolved photocorrosion analysis•In-situ dissolution analysis by ICP-MS coupled to a photoelectrochemical flow cell•Quantification of dissolved tungsten from WO3 during PEC water splitting•Dissolution of WO3 is proportional to the main reaction on the electrode.•WO3 cannot be considered stable in sulfuric acid during PEC water splitting.
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