Quinary, Senary, and Septenary High Entropy Alloy Nanoparticle Catalysts from Core@Shell Nanoparticles and the Significance of Intraparticle Heterogeneity

A catalyst family of high-entropy alloy atomic layers with square atomic arrangements comprising iron- and platinum-group metals

We report a catalyst family of high-entropy alloy (HEA) atomic layers having three elements from iron-group metals (IGMs) and two elements from platinum-group metals (PGMs). Ten distinct quinary compositions of IGM-PGM-HEA with precisely controlled square atomic arrangements are used to explore their impact on hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR). The PtRuFeCoNi atomic layers perform enhanced catalytic activity and durability toward HER and HOR when benchmarked against the other IGM-PGM-HEA and commercial Pt/C catalysts. Operando synchrotron x-ray absorption spectroscopy and density functional theory simulations confirm the cocktail effect arising from the multielement composition. This effect optimizes hydrogen-adsorption free energy and contributes to the remarkable catalytic activity observed in PtRuFeCoNi. In situ electron microscopy captures the phase transformation of metastable PtRuFeCoNi during the annealing process. They transform from random atomic mixing (25°C), to ordered L10 (300°C) and L12 (400°C) intermetallic, and finally phase-separated states (500°C).

Disentangling Competitive and Synergistic Chemical Reactivities During the Seeded Growth of High-Entropy Alloys on High-Entropy Metal Sulfide Nanoparticles

The seeded growth of one type of nanoparticle on the surface of another is foundational to synthesizing many multifunctional nanostructures. High-entropy nanoparticles that randomly incorporate five or more elements offer enhanced properties due to synergistic interactions. Incorporating high-entropy nanoparticles into seeded growth platforms is essential for merging their unique properties with the functional enhancements that arise from particle-particle interactions. However, the complex compositions of high-entropy materials complicate the seeded growth process due to competing particle growth and chemical reactivity pathways. Here, we design and synthesize a 36-member nanoparticle library to identify and disentangle these competitive interactions, ultimately defining chemical characteristics that underpin the seeded growth of high-entropy alloys on high-entropy metal sulfide nanoparticles. As a model system, we focus on (Cu,Zn,Co,In,Ga)S-SnPdPtRhIr, which combines a high-entropy metal sulfide semiconductor with a high-entropy alloy catalyst. We study the seeded growth of all possible pairwise combinations of Sn, Pd, Pt, Rh, Ir, and SnPdPtRhIr on the metal sulfides Cu1.8S, ZnS, Co9S8, CuInS2, CuGaS2, and (Cu,Zn,Co,In,Ga)S, which have comparable morphologies and sizes. Through these studies, we uncover unexpected chemical reactivities, including cation exchange, redox reactions, and diffusion. Reaction temperature, threshold reduction potentials, metal/sulfide chemical reactivity, and the relative strengths of the various bonds that could be formed during particle growth emerge as the primary factors that underpin seeded growth. Finally, we disentangle these competitive and synergistic chemical reactivities to generate a reactivity map that provides practical guidelines for achieving seeded growth in compositionally complex systems.

Unraveling Reactivity Origin of Oxygen Reduction at High-Entropy Alloy Electrocatalysts with a Computational and Data-Driven Approach

High-entropy alloys (HEAs), characterized as compositionally complex solid solutions with five or more metal elements, have emerged as a novel class of catalytic materials with unique attributes. Because of the remarkable diversity of multielement sites or site ensembles stabilized by configurational entropy, human exploration of the multidimensional design space of HEAs presents a formidable challenge, necessitating an efficient, computational and data-driven strategy over traditional trial-and-error experimentation or physics-based modeling. Leveraging deep learning interatomic potentials for large-scale molecular simulations and pretrained machine learning models of surface reactivity, our approach effectively rationalizes the enhanced activity of a previously synthesized PdCuPtNiCo HEA nanoparticle system for electrochemical oxygen reduction, as corroborated by experimental observations. We contend that this framework deepens our fundamental understanding of the surface reactivity of high-entropy materials and fosters the accelerated development and synthesis of monodisperse HEA nanoparticles as a versatile material platform for catalyzing sustainable chemical and energy transformations.

Synthetic Roadmap to a Large Library of Colloidal High-Entropy Rare Earth Oxyhalide Nanoparticles Containing up to Thirteen Metals

Nanoparticles of high-entropy materials that incorporate five or more elements randomized on a crystalline lattice often exhibit synergistic properties that can be influenced by both the identity and number of elements combined. These considerations are especially important for structurally and compositionally complex materials such as multimetal multianion compounds, where cation and anion mixing can influence properties in competitive and contradictory ways. Here, we demonstrate the synthesis of a large library of colloidal high-entropy rare earth oxyhalide (REOX) nanoparticles. We begin with the synthesis of (LaCePrNdSmEuGdDyHoErYbScY)OCl, which homogeneously incorporates 13 distinct rare earth elements. Through time point studies, we find that (LaNdSmGdDy)OCl, a 5-metal analogue, forms through in situ generation of compositionally segregated core@shell@shell intermediates that convert to homogeneously mixed products through apparent core-shell interdiffusion. Assuming that all possible combinations of 5 through 13 rare earth metals are synthetically accessible, we propose the existence of a 7099-member REOCl nanoparticle library, of which we synthesize and characterize 40 distinct members. We experimentally validate the incorporation of a large number of rare earth elements using energy dispersive X-ray spectra, despite closely spaced and overlapping X-ray energy lines, using several fingerprint matching strategies to uniquely correlate experimental and simulated spectra. We confirm homogeneous mixing by analyzing elemental distributions in high-entropy nanoparticles versus physical mixtures of their constituent compounds. Finally, we characterize the band gaps of the 5- and 13-metal REOCl nanoparticles and find a significantly narrowed band gap, relative to the constituent REOCl phases, in (LaCePrNdSmEuGdDyHoErYbScY)OCl but not in (LaNdSmGdDy)OCl.

High-entropy alloy electrocatalysts go to (sub-)nanoscale

Alloying has proven power to upgrade metallic electrocatalysts, while the traditional alloys encounter limitation for optimizing electronic structures of surface metallic sites in a continuous manner. High-entropy alloys (HEAs) overcome this limitation by manageably tuning the adsorption/desorption energies of reaction intermediates. Recently, the marriage of nanotechnology and HEAs has made considerable progresses for renewable energy technologies, showing two important trends of size diminishment and multidimensionality. This review is dedicated to summarizing recent advances of HEAs that are rationally designed for energy electrocatalysis. We first explain the advantages of HEAs as electrocatalysts from three aspects: high entropy, nanometer, and multidimension. Then, several structural regulation methods are proposed to promote the electrocatalysis of HEAs, involving the thermodynamically nonequilibrium synthesis, regulating the (sub-)nanosize and anisotropic morphologies, as well as engineering the atomic ordering. The general relationship between the electronic structures and electrocatalytic properties of HEAs is further discussed. Finally, we outline remaining challenges of this field, aiming to inspire more sophisticated HEA-based nanocatalysts.

Explosive Leidenfrost-Droplet-Mediated Synthesis of Monodispersed High-Entropy-Alloy Nanoparticles for Electrocatalysis

High-entropy-alloy nanoparticles (HEA NPs) exhibit promising potential in various catalytic applications, yet a robust synthesis strategy has been elusive. Here, we introduce a straightforward and universal method, involving the microexplosion of Leidenfrost droplets housing carbon black and metal salt precursors, to fabricate PtRhPdIrRu HEA NPs with a size of ∼2.3 nm. The accumulated pressure within the Leidenfrost droplet triggers an intense explosion within milliseconds, propelling the carbon support and metal salt rapidly into the hot solvent through explosive force. The exceptionally quick temperature rise ensures the coreduction of metal salts, and the dilute local concentration of metal ions limits the final size of the HEA NPs. Additionally, the explosion process can be fine-tuned by selecting different solvents, enabling the harvesting of diverse HEA NPs with superior electrocatalytic activity for alcohol electrooxidation and hydrogen electrocatalysis compared to commercial Pt (Pd) unitary catalysts.

Synthesis of metallic high-entropy alloy nanoparticles

The theoretically infinite compositional space of high-entropy alloys (HEAs) and their novel properties and applications have attracted significant attention from a broader research community. The successful synthesis of high-quality single-phase HEA nanoparticles represents a crucial step in fully unlocking the potential of this new class of materials to drive innovations. This review analyzes the various methods reported in the literature to identify their commonalities and dissimilarities, which allows categorizing these methods into five general strategies. Physical minimization of HEA metals into HEA nanoparticles through cryo-milling represents the typical top-down strategy. The counter bottom-up strategy requires the simultaneous generation and precipitation of metal atoms of different elements on growing nanoparticles. Depending on the metal atom generation process, there are four synthesis strategies: vaporization of metals, burst reduction of metal precursors, thermal shock-induced reduction of metal precursors, and solvothermal reduction of metal precursors. Comparisons among the methods within each strategy, along with discussions, provide insights and guidance for achieving the robust synthesis of HEA nanoparticles.

Nanoscale Design for High Entropy Alloy Electrocatalysts

Due to their distinctive physical and chemical characteristics, high entropy alloys (HEAs), a class of alloys comprising multiple elements, have garnered a lot of attention. It is demonstrated recently that HEA electrocatalysts increase the activity and stability of several processes. In this paper, the most recent developments in HEA electrocatalysts research are reviewed, and the performance of HEAs in catalyzing key reactions in water electrolysis and fuel cells is summarized. In addition, the design strategies for HEA electrocatalysts optimization is introduced, which include component selection, size optimization, morphology control, structural engineering, crystal phase regulation, and theoretical prediction, which can guide component selection and structural design of HEA electrocatalysts.

Synthesis of high-entropy germanides and investigation of their formation process

High-entropy alloys and compounds have emerged as an attractive research area in part because of their distinctive solid-solution structure and multi-element compositions that provide near-limitless tailorability. A diverse array of reports describing high-entropy compounds, including carbides, nitrides, sulfides, oxides, fluorides, silicides, and borides, has resulted. Strikingly, exploration of high-entropy germanides (HEGs) has remained relatively limited. In this study, we present a detailed investigation into the synthesis of HEGs, specifically AuAgCuPdPtGe and FeCoNiCrVGe, via a rapid thermal annealing. The structural, compositional, and morphological characteristics of the synthesized HEGs were assessed using laboratory X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Complementing these post-synthesis analyses, we interrogated the formation and growth mechanisms using in situ heating XRD and TEM and determined that HEG formation involved initial decomposition of germanane (GeNSs) during the annealing, followed by gradual grain growth via atom diffusion at temperatures below 600 °C, and finally a rapid grain growth process at elevated temperatures.

Two Birds with One Stone: Contemporaneously Enhancing OER Catalytic Activity and Stability for Dual-Phase Medium-Entropy Metal Sulfides

Transition metal-based sulfides exhibit remarkable potential as electrocatalysts for oxygen evolution reaction (OER) due to the unique intrinsic structure and physicochemical characteristics. Nevertheless, currently available sulfide catalysts based on transition metals face a bottleneck in large-scale commercial applications owing to their unsatisfactory stability. Here, the first fabrication of (FeCoNiMn2 )S2 dual-phase medium-entropy metal sulfide (dp-MEMS) is successfully achieved, which demonstrated the expected optimization of stability in the OER process. Benefiting from the "cell wall" -like structure and the synergistic effect in medium-entropy systems, (FeCoNiMn2 )S2 dp-MEMS delivers an exceptionally low overpotential of 169 and 232 mV at current densities of 10 and 100 mA cm-2 , respectively. The enhancement mechanism of catalytic activity and stability is further validated by density functional theory (DFT) calculations. Additionally, the rechargeable Zn-air batteries integrated with FeCoNiMn2 )S2 dp-MEMS exhibit remarkable performance outperforming the commercial catalyst (Pt/C+RuO2 ). This work demonstrates that the dual-phase medium-entropy metal sulfide-based catalysts have the potential to provide a greater application value for OER and related energy conversion systems.

Physical and chemical parameters determining the formation of gold-sp metal (Al, Ga, In, and Pb) nanoalloys

Alloying is a key step towards the fabrication of advanced and unique nanomaterials demanded by the next generation of nanotechnology solutions. In particular, the alloys of Au with the sp-metals are expected to have several appealing plasmonic and electronic properties for a wide range of applications in optics, catalysis, nanomedicine, sensing and quantum devices. However, little is known about the thermodynamic and synthetic factors leading to the successful alloying of Au and sp-metals at the nanoscale. In this work, Au-M nanoalloys, with M = Al, Ga, In, or Pb, have been synthesized by a green and single step laser ablation in liquid (LAL) approach in two environments (pure ethanol and anhydrous acetone). To delve deeper into the key parameters leading to successful alloying under the typical operating conditions of LAL, a multiparametric analysis was performed considering the mixing enthalpy from DFT calculations and other alloying descriptors such as the Hume-Rothery parameters. The results showed that the dominant factors for alloying change dramatically with the oxidative ability of the synthesis environment. In this way, the tendency of the four sp metals to alloy with gold was accurately predicted (R2 > 0.99) using only two and three parameters in anhydrous and non-anhydrous environments, respectively. These results are important to produce nanoalloys using LAL and other physical methods because they contribute to the understanding of factors leading to element mixing at the nanoscale under real synthetic conditions, which is crucial for guiding the realization of next-generation multifunctional metallic nanostructures.

Colloidal Nanoparticles of High Entropy Materials: Capabilities, Challenges, and Opportunities in Synthesis and Characterization

Materials referred to as "high entropy" contain a large number of elements randomly distributed on the lattice sites of a crystalline solid, such that a high configurational entropy is presumed to contribute significantly to their formation and stability. High temperatures are typically required to achieve entropy stabilization, which can make it challenging to synthesize colloidal nanoparticles of high entropy materials. Nonetheless, strategies are emerging for the synthesis of colloidal high entropy nanoparticles, which are of interest for their synergistic properties and unique catalytic functions that arise from the large number of constituent elements and their interactions. In this Perspective, we highlight the classes of materials that have been made as colloidal high entropy nanoparticles as well as insights into the synthetic methods and the pathways by which they form. We then discuss the concept of "high entropy" within the context of colloidal materials synthesized at much lower temperatures than are typically required for entropy to drive their formation. Next, we identify and address challenges and opportunities in the field of high entropy nanoparticle synthesis. We emphasize aspects of materials characterization that are especially important to consider for nanoparticles of high entropy materials, including powder X-ray diffraction and elemental mapping with scanning transmission electron microscopy, which are among the most commonly used techniques in laboratory settings. Finally, we share perspectives on emerging opportunities and future directions involving colloidal nanoparticles of high entropy materials, with an emphasis on synthesis, characterization, and fundamental knowledge that is needed for anticipated advances in key application areas.

Boosting Bifunctional Catalysis by Integrating Active Faceted Intermetallic Nanocrystals and Strained Pt-Ir Functional Shells

PtIr-based nanostructures are fascinating materials for application in bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalysis. However, the fabrication of PtIr nanocatalysts with clear geometric features and structural configurations, which are crucial for enhancing the bifunctionality, remains challenging. Herein, PtCo@PtIr nanoparticles are precisely designed and fabricated with a quasi-octahedral PtCo nanocrystal as a highly atomically ordered core and an ultrathin PtIr atomic layer as a compressively strained shell. Owing to their geometric and core-shell features, the PtCo@PtIr nanoparticles deliver approximately six and eight times higher mass and specific activities, respectively, as an ORR catalyst than a commercial Pt/C catalyst. The half-wave potential of PtCo@PtIr exhibits a negligible decrease by 9 mV after 10 000 cycles, indicating extraordinary ORR durability because of the ordered arrangement of Pt and Co atoms. When evaluated using the ORR-OER dual reaction upon the introduction of Ir, PtCo@PtIr exhibits a small ORR-OER overpotential gap of 679 mV, demonstrating its great potential as a bifunctional electrocatalyst for fabricating fuel cells. The findings pave the way for designing precise intermetallic core-shell nanocrystals as highly functional catalysts.

Resolving Atomic-Scale Structure and Chemical Coordination in High-Entropy Alloy Electrocatalysts for Structure-Function Relationship Elucidation

The recent breakthrough in confining five or more atomic species in nanocatalysts, referred to as high-entropy alloy nanocatalysts (HEAs), has revealed the possibilities of multielemental interactions that can surpass the limitations of binary and ternary electrocatalysts. The wide range of potential surface configurations in HEAs, however, presents a significant challenge in resolving active structural motifs, preventing the establishment of structure-function relationships for rational catalyst design and optimization. We present a methodology for creating sub-5 nm HEAs using an aqueous-based peptide-directed route. Using a combination of pair distribution function and X-ray absorption spectroscopy, HEA structure models are constructed from reverse Monte Carlo modeling of experimental data sets and showcase a clear peptide-induced influence on atomic-structure and chemical miscibility. Coordination analysis of our structure models facilitated the construction of structure-function correlations applied to electrochemical methanol oxidation reactions, revealing the complex interplay between multiple metals that leads to improved catalytic properties. Our results showcase a viable strategy for elucidating structure-function relationships in HEAs, prospectively providing a pathway for future materials design.

Hydrogenated borophene enabled synthesis of multielement intermetallic catalysts

Supported metal catalysts often suffer from rapid degradation under harsh conditions due to material failure and weak metal-support interaction. Here we propose using reductive hydrogenated borophene to in-situ synthesize Pt/B/C catalysts with small sizes (~2.5 nm), high-density dispersion (up to 80 wt%Pt), and promising stability, originating from forming Pt-B bond which are theoretically ~5× stronger than Pt-C. Based on the Pt/B/C module, a series (~18 kinds) of carbon supported binary, ternary, quaternary, and quinary Pt intermetallic compound nanocatalysts with sub-4 nm size are synthesized. Thanks to the stable intermetallics and strong metal-support interaction, annealing at 1000 °C does not cause those nanoparticles sintering. They also show much improved activity and stability in electrocatalytic oxygen reduction reaction. Therefore, by introducing the boron chemistry, the hydrogenated borophene derived multielement catalysts enable the synergy of small size, high loading, stable anchoring, and flexible compositions, thus demonstrating high versatility toward efficient and durable catalysis.

Hollow-Carbon Confinement Annealing: A New Synthetic Approach to Make High-Entropy Solid-Solution and Intermetallic Nanoparticles

High-entropy alloy (HEA) nanoparticles (NPs) have been emerging with superior compositional tunability and multielemental synergy, presenting a unique platform for material discovery and performance optimization. Here we report a synthetic approach utilizing hollow-carbon confinement in the ordinary furnace annealing to achieve the nonequilibrium HEA-NPs such as Pt0.45Fe0.18Co0.12Ni0.15Mn0.10 with uniform size ∼5.9 nm. The facile temperature control allows us not only to reveal the detailed reaction pathway through ex situ characterization but also to tailor the HEA-NP structure from the crystalline solid solution to intermetallic. The preconfinement of metal precursors is the key to ensure the uniform distribution of metal nanoparticles with confined volume, which is essential to prevent the thermodynamically favored phase separation even during the ordinary furnace annealing. Besides, the synthesized HEA-NPs exhibit remarkable activity and stability in oxygen reduction catalysis. The demonstrated synthetic approach may significantly expand the scope of HEA-NPs with uncharted composition and performance.

Quinary RuRhPdPtAu high-entropy alloy as an efficient electrocatalyst for the hydrogen evolution reaction

Quinary RuRhPdPtAu high-entropy alloy nanoparticles (HEA-NPs) were prepared for the first time from a deep eutectic solvent by an electrochemical method. Owing to the benefits of high entropy and abundant surface active sites, the RuRhPdPtAu HEA-NPs exhibit outstanding electrocatalytic performance for the hydrogen evolution reaction.

Thermally Induced Domain Migration and Interfacial Restructuring in Cation Exchanged ZnS-Cu1.8S Heterostructured Nanorods

Partial cation exchange reactions can be used to rationally design and synthesize heterostructured nanoparticles that are useful targets for applications in photocatalysis, nanophotonics, thermoelectrics, and medicine. Such reactions introduce intraparticle frameworks that define the spatial arrangements of different materials within a heterostructured nanoparticle, as well as the orientations and locations of their interfaces. Here, we show that upon heating to temperatures relevant to their synthesis and applications, the ZnS regions and Cu1.8S/ZnS interfaces of heterostructured ZnS-Cu1.8S nanorods migrate and restructure. We first use partial cation exchange reactions to synthesize a library of seven distinct samples containing various patches, bands, and tips of ZnS embedded within Cu1.8S nanorods. Upon annealing in solution or in air, ex situ TEM analysis shows evidence that the ZnS domains migrate in different ways, depending upon their sizes and locations. Using differential scanning calorimetry, we correlate the threshold temperature for ZnS migration to the superionic transition temperature of Cu1.8S, which facilitates rapid diffusion throughout the nanorods. We then use in situ thermal TEM to study the evolution of individual ZnS-Cu1.8S nanorods upon heating. We find that ZnS domain migration occurs through a ripening process that minimizes small patches with higher-energy interfaces in favor of larger bands and tips having lower-energy interfaces, as well as through restructuring of higher-energy Cu1.8S/ZnS interfaces. Notably, Cu1.8S nanorods containing multiple patches of ZnS thermally transform into ZnS-Cu1.8S heterostructured nanorods having ZnS tips and/or central bands, which provides mechanistic insights into how these commonly observed products form during synthesis.

Synthetic Strategies toward High Entropy Materials: Atoms-to-Lattices for Maximum Disorder

High-entropy materials are a nascent class of materials that exploit a high configurational entropy to stabilize multiple elements in a single crystal lattice and to yield unique physical properties for applications in energy storage, catalysis, and thermoelectric energy conversion. Initially, the synthesis of these materials was conducted by approaches requiring high temperatures and long synthetic time scales. However, successful homogeneous mixing of elements at the atomic level within the lattice remains challenging, especially for the synthesis of nanomaterials. The use of atom-up synthetic approaches to build crystal lattices atom by atom, rather than the top-down alteration of extant crystalline lattices, could lead to faster, lower-temperature, and more sustainable approaches to obtaining high entropy materials. In this Perspective, we discuss some of these state-of-the-art atom-up synthetic approaches to high entropy materials and contrast them with more traditional approaches.

Temporal Evolution of Morphology, Composition, and Structure in the Formation of Colloidal High-Entropy Intermetallic Nanoparticles

Morphology-controlled nanoparticles of high entropy intermetallic compounds are quickly becoming high-value targets for catalysis. Their ordered structures with multiple distinct crystallographic sites, coupled with the "cocktail effect" that emerges from randomly mixing a large number of elements, yield catalytic active sites capable of achieving advanced catalytic functions. Despite this growing interest, little is known about the pathways by which high entropy intermetallic nanoparticles form and grow in solution. As a result, controlling their morphology remains challenging. Here, we use the high entropy intermetallic compound (Pd,Rh,Ir,Pt)Sn, which adopts a NiAs-related crystal structure, as a model system for understanding how nanoparticle morphology, composition, and structure evolve during synthesis in solution using a slow-injection reaction. By performing a time-point study, we establish the initial formation of palladium-rich cube-like Pd-Sn seeds onto which the other metals deposit over time, concomitant with continued incorporation of tin. For (Pd,Rh,Ir,Pt)Sn, growth occurs on the corners, resulting in a sample having a mixture of flower-like and cube-like morphologies. We then synthesize and characterize a library of 14 distinct intermetallic nanoparticle systems that comprise all possible binary, ternary, and quaternary constituents of (Pd,Rh,Ir,Pt)Sn. From these studies, we correlated compositions, morphologies, and growth pathways with the constituent elements and their competitive reactivities, ultimately mapping out a framework that rationalizes the key features of the high entropy (Pd,Rh,Ir,Pt)Sn intermetallic nanoparticles based on those of their simpler constituents. We then validated these design guidelines by applying them to the synthesis of a morphologically pure variant of flowerlike (Pd,Rh,Ir,Pt)Sn particles as well as a series of (Pd,Rh,Ir,Pt)Sn particles with tunable morphologies based on control of composition.

Facile synthesis of high-entropy alloy nanoparticles on germanane, Ge nanoparticles and wafers

The unique solid-solution structure and multi-element compositions of high-entropy alloy nanoparticles (HEA NPs) have garnered substantial attention. Various methods have been developed to prepare a diverse array of HEA NPs using different substrates for support and stabilization. In this study, we present a facile surface-mediated reduction method to prepare HEA NPs (AuAgCuPdPt) decorated germanane (HEA NPs@GeNSs), and employ X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) to characterize their structure, composition, and morphology. Subsequently, we demonstrate that the HEA NPs can be liberated from the surfaces of GeNSs as freestanding systems via straightforward exposure to UV light. We also explore germanium nanoparticles (GeNPs) as an alternative substrate for HEA NP formation/production, given their similarity to germanane and their Ge-H surface. Finally, we extend our investigation to bulk Ge wafers and demonstrate successful deposition of HEA NPs.

Programmable Synthesis of High-Entropy Nanoalloys for Efficient Ethanol Oxidation Reaction

Controllable synthesis of nanoscale high-entropy alloys (HEAs) with specific morphologies and tunable compositions is crucial for exploring advanced catalysts. The present strategies either have great difficulties to tailor the morphology of nanoscale HEAs or suffer from narrow elemental distributions and insufficient generality. To overcome the limitations of these strategies, here we report a robust template-directed synthesis to programmatically fabricate nanoscale HEAs with controllable compositions and structures via independently controlling the morphology and composition of HEA. As a proof of concept, 12 kinds of nanoscale HEAs with controllable morphologies of zero-dimension (0D) nanoparticles, 1D nanowires, 2D ultrathin nanorings (UNRs), 3D nanodendrites, and vast elemental compositions combining five or more of Pd/Pt/Ag/Cu/Fe/Co/Ni/Pb/Bi/Sn/Sb/Ge are synthesized. Moreover, the as-prepared HEA-PdPtCuPbBiUNRs/C demonstrates the state-of-the-art electrocatalytic performance for the ethanol oxidation reaction, with 25.6- and 16.3-fold improvements in mass activity, relative to commercial Pd/C and Pt/C catalysts, respectively, as well as greatly enhanced durability. This work provides a myriad of nanoscale HEAs and a general synthetic strategy, which are expected to have broad impacts for the fields of catalysis, sensing, biomedicine, and even beyond.

Practical Considerations for Simulating the Plasmonic Properties of Metal Nanoparticles

Simulating the plasmonic properties of colloidally derived metal nanoparticles with accuracy to their experimentally observed measurements is challenging due to the many structural and compositional parameters that influence their scattering and absorption properties. Correlation between single nanoparticle scattering measurements and simulated spectra emphasize these strong structural and compositional relationships, providing insight into the design of plasmonic nanoparticles. This Perspective builds from this history to highlight how the structural features of models used in simulation methods such as those based on the Finite-Difference Time-Domain (FDTD) method and Discrete Dipole Approximation (DDA) are of critical consideration for correlation with experiment and ultimately prediction of new nanoparticle properties. High-level characterizations such as electron tomography are discussed as ways to advance the accuracy of models used in such simulations, allowing the plasmonic properties of structurally complex nanoparticles to be better understood. However, we also note that the field is far from bringing experiment and simulation into agreement for plasmonic nanoparticles with complex compositions, reflecting analytical challenges that inhibit accurate model generation. Potential directions for addressing these challenges are also presented.

Pulsed Light Synthesis of High Entropy Nanocatalysts with Enhanced Catalytic Activity and Prolonged Stability for Oxygen Evolution Reaction

The ability to synthesize compositionally complex nanostructures rapidly is a key to high-throughput functional materials discovery. In addition to being time-consuming, a majority of conventional materials synthesis processes closely follow thermodynamics equilibria, which limit the discovery of new classes of metastable phases such as high entropy oxides (HEO). Herein, a photonic flash synthesis of HEO nanoparticles at timescales of milliseconds is demonstrated. By leveraging the abrupt heating and cooling cycles induced by a high-power-density xenon pulsed light, mixed transition metal salt precursors undergo rapid chemical transformations. Hence, nanoparticles form within milliseconds with a strong affinity to bind to the carbon substrate. Oxygen evolution reaction (OER) activity measurements of the synthesized nanoparticles demonstrate two orders of magnitude prolonged stability at high current densities, without noticeable decay in performance, compared to commercial IrO₂ catalyst. This superior catalytic activity originates from the synergistic effect of different alloying elements mixed at a high entropic state. It is found that Cr addition influences surface activity the most by promoting higher oxidation states, favoring optimal interaction with OER intermediates. The proposed high-throughput method opens new pathways toward developing next-generation functional materials for various electronics, sensing, and environmental applications, in addition to renewable energy conversion.

Chemical Insights into the Formation of Colloidal High Entropy Alloy Nanoparticles

Nanoparticles of high entropy alloys (HEAs) have distinct properties that result from their high surface-to-volume ratios coupled with synergistic interactions among their five or more constituent elements, which are randomly distributed throughout a crystalline lattice. Methods to synthesize HEA nanoparticles are emerging, including solution approaches that yield colloidal products. However, the complex multielement compositions of HEA nanoparticles make it challenging to identify and understand their reaction chemistry and the pathways by which they form, which hinders their rational synthesis. Here, we demonstrate the synthesis and elucidate the reaction pathways of seven colloidal HEA nanoparticle systems that contain various combinations of noble metals (Pd, Pt, Rh, Ir), 3d transition metals (Ni, Fe, Co), and a p-block element (Sn). The nanoparticles were synthesized by slowly injecting a solution containing all five constituent metal salts into oleylamine and octadecene at 275 °C. Using NiPdPtRhIr as a lead system, we confirmed the homogeneous colocalization of all five elements and achieved tunable compositions by varying their ratios. We also observed heterogeneities, including Pd-rich regions, in a subpopulation of the NiPdPtRhIr sample. Halting the reaction at early time points and characterizing the isolated products revealed a time-dependent composition evolution from Pd-rich NiPd seeds to the final NiPdPtRhIr HEA. Similar reactions applied to FePdPtRhIr, CoPdPtRhIr, NiFePdPtIr, and NiFeCoPdPt, with modified conditions to most efficiently incorporate all five elements into each HEA, also revealed similar Pd-rich seeds with system-dependent differences in the rates and sequences of element uptake into the nanoparticles. When moving to SnPdPtRhIr and NiSnPdPtIr, the time-dependent formation pathway was more consistent with simultaneous coreduction rather than through formation of reactive seeds. These studies reveal important similarities and differences among the pathways by which different colloidal HEA nanoparticles form using the same synthetic method, as well as establish generality. The results provide guidelines for incorporating a range of different elements into HEA nanoparticles, ultimately providing fundamental knowledge about how to define and optimize synthetic protocols, expand into different HEA nanoparticle systems, and achieve high phase purity.

Facet-Controlled Synthesis of Platinum-Group-Metal Quaternary Alloys: The Case of Nanocubes and {100} Facets

We report a robust method for the facet-controlled synthesis of nanocrystals with an ultrathin shell made of a nearly equimolar RuRhPdPt quaternary alloy. Our strategy involves the use of well-defined Rh cubic seeds, halide-free precursors, and a method for precisely controlling the reaction kinetics of different precursors. In the setting of dropwise addition, the precursors with different reactivities can be reduced at about the same pace for the generation of an alloy with a uniform and well-controlled composition. The core-shell nanocubes show greatly enhanced activity toward ethanol oxidation when benchmarked against Pd and Pt counterparts. Combining in situ and ex situ electron microscopy studies, we also demonstrate that the core-shell nanocubes possess good thermal and electrochemical stability in terms of both geometrical shape and elemental composition, with their cubic shape and alloy composition retained when annealing at 500 °C or after long-term electrochemical cycling. It is expected that the synthetic approach can be further extended to fabricate multimetallic catalysts with enhanced activities toward a variety of thermal and electrochemical reactions.

Designing strategies and enhancing mechanism for multicomponent high-entropy catalysts

High-entropy materials (HEMs) are new-fashioned functional materials in the field of catalysis owing to their large designing space, tunable electronic structure, interesting "cocktail effect", and entropy stabilization effect. Many effective strategies have been developed to design advanced catalysts for various important reactions. Herein, we firstly review effective strategies developed so far for optimizing HEM-based catalysts and the underlying mechanism revealed by both theoretical simulations and experimental aspects. In light of this overview, we subsequently present some perspectives about the development of HEM-based catalysts and provide some serviceable guidelines and/or inspiration for further studying multicomponent catalysts.