Rebekka Klemmt

Multimetallic nanoparticles (NPs) offer access to a vast compositional and architectural design space, yet controlled formation of core-shell and multi-shell architectures rapidly becomes unpredictable as compositional complexity increases. As a result, architectural outcomes are often regarded as system-specific and governed primarily by kinetic effects. Here, we identify a unifying thermodynamic principle that governs shell formation in multimetallic NPs based on binary mixing enthalpies and phase behaviour. By systematically establishing enthalpy-driven rules in binary NPs and extending them to higher-order compositions, we show that elemental segregation and the composition and sequence of multiple shells in NPs containing up to five elements are predictable despite their combinatorial complexity, even when synthesised in a simple one-pot wet-chemical reaction. These results demonstrate that thermodynamic interactions maintain predictive power as compositional complexity increases and provide a general framework for rationalising architectural design in multimetallic NPs.

Our knowledge of crystals rests on precisely measured structural models, which are lacking for amorphous matter. Without long-range order, the available experimental evidence matches competing structural models, including paracrystalline and random network models, limiting our understanding of formation mechanisms and properties. This impedes discovery of new amorphous materials, including high-density sp³-rich carbons, among the hardest known. We show that the pair-angle distribution function (PADF) overcomes these limitations, applying it to a non-crystalline sp³-rich carbon produced by compression of fullerenes at room temperature. The PADF reveals that this phase contains nanostructures with distinct short-range (4 ^th –5 ^th neighbours, < 5 Å) and medium-range (5–10 Å) order. The nanostructure planes and angles relate to diamond, suggesting coordinated diamond-like tetrahedra too disordered to be nanodiamonds. The characterisation of nanoscale disorder via the PADF thus offers insight into the nature of structural order in amorphous materials and the formation pathways of diamond-like materials.

The bending and orientation of layers, along with any potential pore structures in disordered carbons, are crucial for their performance in many applications. Therefore, it is essential to characterize them. Traditionally, detection and analysis of fringes in high-resolution transmission electron microscopy (HR-TEM) images are used for characterization. However, fringes are affected by instrumental artifacts, such as spectral modulation by the microscope's contrast transfer function and multiple overlapping structures in projection, which can lead to misinterpretation. Here, we propose to visualize and further analyze the nature of curved, graphite-like stacked layers in disordered carbons by examining changes in Bragg disc orientation in scanning electron nanobeam diffraction (SEND) patterns. The new approach uses non-negative matrix factorization (NMF) to reduce the complexity of the SEND patterns. It enables the detection and characterization of curved layers over a larger field of view than is readily achievable with individual HR-TEM images, thereby enhancing the statistical relevance of the methodology. Moreover, the approach yields structural information about curved layers of multiple intertwined and overlapping regions. It allows separation of contributions to overlapping layers of different pores and enables independent analysis of their curvatures. The potential of the new approach is demonstrated by analyzing simulated and experimentally measured SEND data from disordered carbons.

High‐entropy alloy (HEA) nanoparticles (NPs) have been subject to increasing interest as prospective catalyst materials. However, highly controllable synthesis methods for HEA NPs remain scarce. To address this issue, we present a systematic study of the synthesis parameter space in a simple colloidal synthesis approach where IrPdPtRuRh NPs were synthesised in triethylene glycol. We show that ultra‐small (1–3 nm), single‐phase HEA NPs can be synthesised through a hot‐injection synthesis approach. In contrast, multiple phases are observed when a one‐pot synthesis is carried out. We also show that the particle sizes and morphologies are affected by the choice of metal precursor counterion, and further investigate the dependence of metal reduction on the reaction temperature. Additionally, we present the synthesis of mixed noble/non‐noble‐metal IrPdPtRuNi and IrPdPtRuCo HEA NPs. By carrying out reduction kinetics experiments, we relate phase separation and compositional gradients of some of the synthesised samples to large differences in the reduction rates of the reacting metals. The individual metal reduction rates are found to vary depending on which other elements are present during particle formation. These results emphasise the importance of understanding the chemical landscape during reaction for the controlled synthesis of HEA NPs.

We present a medium‐throughput synthesis approach for High Entropy Alloy (HEA) noble metal nanoparticles on conductive supports via electrodeposition. The presented method utilizes aqueous electrolyte solutions, exploiting high overpotentials to achieve mass transport‐controlled deposition. This ensures electrodeposition independent from individual equilibrium potentials of the different elements. Hydrogen evolution which interferes with electrodeposition is suppressed by operating in a pulsed mode at a mildly acidic pH. Applying the approach to the Au–Ir–Pt–Pd–Rh–Ru composition space, this study demonstrates that the developed method is fast, adaptable, and enables compositional control while maintaining a homogeneous element distribution. The mechanism of HEA nanoparticle synthesis is further investigated by examining material‐specific seed formation and diffusion phenomena. The results indicate that together with the electrolyte composition, seed formation as well as the diffusion of metal precursors in the aqueous phase govern the average composition of the synthesized HEA nanoparticles, while the formation enthalpies of element pairs explain the atomic‐scale segregation observed.

High-Entropy Alloy (HEA) nanoparticles have been subject to increasing interest as prospective catalyst materials. The potential of HEAs stem from their multi-component nature, allowing the tailoring of catalytic properties by controlling their structure and composition. However, highly controllable synthesis methods of HEA nanoparticles remain scarce. To address this issue, we present a systematic study of the synthesis parameter space in a simple colloidal synthesis approach where RhIrPtPdRu nanoparticles were synthesised in triethylene glycol. We show that ultra-small (1-3 nm), single-phase HEA nanoparticles can be easily synthesised through a hot-injection synthesis approach. In contrast, multiple phases are observed when a one-pot synthesis is carried out. We also show that the particle size and morphologies relate to the choice of metal precursor counterion, and further investigate the dependence of metal reduction on the reaction temperature. Additionally, we present the synthesis of mixed noble/non-noble-metal IrPdPtRuNi and IrPdPtRuCo HEA nanoparticles. By carrying out reduction kinetics experiments, we relate phase separation and compositional gradients of some of the synthesised samples to the large differences in the reduction rates of the reacting metals. The individual metal reduction rates are found to vary depending on which other elements are present during particle formation. This emphasises the importance of understanding the chemical landscape during reaction for the controlled synthesis of HEA nanoparticles.