Falk Muench

Magnetic refrigeration leads the current commercialization efforts of ambient caloric cooling technologies, is considered among its peers most promising in terms of anticipated energy efficiency gain, and allows for complete elimination of harmful coolants. By now, functional magnetocaloric components (so-called regenerators) based on Mn-substituted and hydrogenated LaFeSi alloys are commercially available. However, this alloy system exhibits magnetostriction, is susceptible to fracture, oxidation, and does not passivate well, rendering it prone to failure and corrosion, particularly when using water as favorable heat exchange medium. Demonstrating stable and extended operation of LaFeSi-based regenerators under realistic conditions with cost-sensitive measures thus constitutes a key milestone for derisking the materials system, paving a path toward reliable and maintenance-friendly magnetic cooling devices. Building upon a fundamental analysis of materials properties, process, and target specifications, we outline a 2-fold protection strategy, encompassing a highly conformal copper coating working in tandem with a tailored inhibitor system. The former is applied using an optimized electroless plating procedure, allowing us to evenly envelop complex regenerator architectures in a dense, nondefective, homogeneous, and ductile copper film of excellent interfacial quality. The latter addresses the corrosion characteristics of both coating and substrate in the application environment. In-device aging experiments prove the effectiveness of our multifaceted approach in maintaining the chemical, mechanical, and functional integrity of LaFeSi regenerators under continuous use.

Using a tunable electroless nanoplating reaction, different silver structures including two nanoplate-based structures were synthesized, characterized and tested as catalysts to improve the electrochemical reduction of CO₂ towards CO. Relative to a planar polycrystalline silver surface, the faradaic efficiency to CO was significantly improved, from 7% to 67% at −0.6 V vs RHE and from 51% to 97% at −1.0 V vs RHE, decreasing the parasitic evolution of hydrogen and formate. By comparing the catalytic performance of three intensively characterized silver structure types, namely high aspect ratio nanoplates, particulate nanoplate clusters, and interconnected grain-like particles, in-depth insights into various effects that influence the observed reactions are presented. In particular, at low potentials and high current densities, the catalytic performance is more related to the electrochemical surface area and local transport effects. The obtained results demonstrate the relevance of structural control in electrocatalysts and the special effects of nanoplate structures. Thus, our findings provide a useful groundwork for the practical design of electrocatalysts for CO₂ reduction.

Fe and/or Mn‐containing yellow ochre, red ochre, and umber earth pigments are omnipresent in 17th century paintings. Less common in the materials used in historical paintings of this period is the Fe and Mn‐rich earth pigment sienna. Different uses of historical pigments in one painting by Georg Flegel (1566–1638) and another version of the same painting but of disputed attribution were recently uncovered by means of macro‐x‐ray fluorescence (MA‐XRF) scanning and other non‐invasive analytical techniques. In this paper, an approach solely based upon the correlation of Fe and Mn MA‐XRF maps with the optical image of the painting is compared to the use of Mn/Fe correlation plots. The identification of clusters within a plot of the Fe counts vs. the Mn counts can aid to infer whether an area with a certain color matches with the use of the earth pigments found in the two paintings and to ultimately shed light on the different usage of these pigments. The analytical thresholds found in the Mn/Fe correlation plots allowed to identify clusters differing in composition, which matched an area of a certain color with the earth pigments used therein. This highlighted the differences and similarities between the two paintings, ultimately ascertaining the lower value of the painting of disputed attribution. The analysis of single‐pixel spectra allowed refining the interpretation of specific Mn/Fe correlation plots. The purpose of these data evaluation steps is presented and the limitations of the proposed methodology are also discussed.

Rhombohedral Ni-based nanotubes embedded in mica templates display triaxial magnetic anisotropy. This provides an additional degree of freedom for tuning the particular magnetic behavior exhibited by these nanostructures.

Invited for this month's cover picture is Falk Muench. The cover picture shows electrolessly plated nanostructures from the author′s research, clockwise and starting from the top: silver nanoparticles on glass, employed in plasmonic biosensing. Copper‐platinum nanotubes, fabricated to demonstrate the nanoscale conformality of the alloy plating reaction. Silver nanoplates, which spontaneously form during kinetically controlled, anisotropic growth. Stacked layers of porous silver nanosheets, which represent a dendritic version of the previous nanoplates. Read the full text of the Review at 10.1002/celc.202100285.

Electroless plating is commonly associated with metallizing macroscopic work pieces, but also represents a surprisingly powerful nanomanufacturing tool. This review details the technique's use for creating nanomaterials of arbitrary dimensionality, ranging from nanoparticles over nanotubes, nanowires, and ultrathin films to nanostructured lattices, focusing on the solution chemistry and mechanistic aspects. The synthesis of defined nanostructures serves as overarching perspective, which is enriched by drawing connections to and insights from related fields. Strategies for controlling the size, shape, crystallinity, porosity, composition, and arrangement of electrolessly plated nanostructures are outlined, including templating (which harnesses the method's exceptional conformality to guide and limit deposit formation), the complementary approach of selective growth, tailored seeding, and bath design. Being able to strike convincing compromises between material quality and ease of preparation, electroless nanoplating has a distinct potential to facilitate the application of metal nanomaterials.

Owing to their large surface area, continuous conduction paths, high activity and pronounced anisotropy, nanowires are pivotal for a wide range of applications, yet far from thermodynamic equilibrium. Their susceptibility towards degradation necessitates an in-depth understanding of the underlying failure mechanisms to ensure reliable performance under operating conditions. In this study, we present an in-depth analysis of the thermally triggered Plateau-Rayleigh-like morphological instabilities of electrodeposited, polycrystalline, 20-40 nm thin platinum nanowires using in situ transmission electron microscopy in a controlled temperature regime, ranging from 25 °C to 1100 °C. Nanowire disintegration is heavily governed by defects, while the initially present, frequent but small thickness variations do not play an important role, and are overridden later during reshaping. Changes of the exterior wire morphology are preceded by shifts in the internal nanostructure, including grain boundary straightening, grain growth, and the formation of faceted voids. Surprisingly, the nanowires segregate into two domain types, one being single-crystalline and essentially void-free, while the other preserves void-pinned grain boundaries. While the single-crystalline domains exhibit fast Pt transport, the void-containing domains are unexpectedly stable, accumulate platinum by surface diffusion, and act as nuclei for the subsequent nanowire splitting. This study highlights the vital role of defects in Plateau-Rayleigh-like thermal transformations, whose evolution not only accompanies, but guides the wire reshaping. Thus, defects represent strong parameters for controlling the nanowire decay and must be considered for devising accurate models and simulations.