Local chemical order (LCO) is a key descriptor linking composition, atomic arrangement, and function in high-entropy alloys (HEAs) yet remains difficult to quantify. This perspective highlights how X-ray absorption spectroscopy (XAS) provides element-specific, quantitative insight into LCO in complex alloys. We outline practical considerations for XAS data collection and fitting, including the width of spectra range, multitemperature analysis, and X-ray absorption edge choice for 5d elements. We then highlight a coordination-number-based framework for LCO analysis and use model HEAs to show how single-atom and near-single-atom motifs naturally emerge as component number increases, bridging HEAs and single-atom alloys. Finally, we identify priorities, including standardized protocols, uncertainty quantification, expanded operando and time-resolved XAS, and integration with complementary characterization and modeling.

Multielemental nanoalloys have shown significant promise in applications like catalysis, due to the structural features that arise from their complex structure. A key feature of particular interest is local chemical order (LCO) at the scale of a single neighboring bond length. LCO has proven challenging and inconclusive to identify and assess in these materials, particularly in samples containing elements with similar atomic numbers. Herein, we apply a methodology combining experimental X-ray absorption spectroscopy and computational simulations, allowing for the reliable verification and quantification of LCO in a five-element high-entropy alloy (HEA-5) on an element-specific basis. The analysis identifies the Ru-Ir bonding pair as a significant component of LCO, which correlates with HEA-5's established high catalytic performance in ammonia decomposition. The methodology is further applied to a complex 15-element HEA sample, where consistent LCO trends are observed. These results support an element-specific approach for investigating LCO, structural analysis, and the catalytic design of high-entropy nanoalloys.

Surface modification of TiO 2 with single‐atom catalysts (SACs) is an effective strategy for enhancing photocatalytic efficiency. However, thorough characterization of SACs at the atomic scale remains challenging. X‐ray absorption spectroscopy (XAS) offers unique advantages for the in‐depth analysis of TiO 2 ‐supported SACs. By employing XAS, the local atomic structure, oxidation state, and electronic properties of the SACs, as well as the underlying photocatalytic mechanism, can be revealed. Herein, we present a short review on the application of XAS in studying TiO 2 ‐supported SACs. We first elucidate the key role of XAS in simultaneously probing the structure and electronic properties of monometallic SACs across different periods. Next, we discuss XAS studies of bimetallic SACs from the perspective of each constituent element and highlight the element‐specific capabilities of XAS for analyzing multi‐element SACs. Finally, we demonstrate how in situ XAS can effectively monitor structural and electronic property changes in SACs under real photocatalytic reaction conditions. Overall, this review highlights the unique advantages of XAS in achieving a more comprehensive understanding of the structure−property relationships in SACs, ultimately aiding the rational design of future photocatalysts. Additionally, we provide practical suggestions for utilizing XAS more efficiently in the analysis of various SAC systems.

The detailed characterization of AuAg alloy nanoclusters is essential to guide the discovery of species ideal for applications in various fields including catalysis and biomedicine. This work presents structural analysis of the Au104Ag40(PET)60 species through X-ray absorption spectroscopy (XAS). First, XAS fitting is utilized to model the distribution of Au and Ag atoms within the structure. Our proposed model assigns Ag atoms to the vertex sites of the second shell of the metal core, as well as the outermost staple sites. This distribution reveals Au104Ag40(PET)60 to be a Ag single-atom alloy. The proposed model shows outstanding agreement with the coordination number values derived from XAS. XAS near-edge analysis is employed to investigate the alloy bonding interactions between Au and Ag. Substantial d-electron transfer from Au to Ag is observed in this sample, beyond the magnitude of previously studied AuAg NCs. This work enhances the understanding of the structure-property relationship of AuAg alloy NCs, offering insights which can be applied to other large NCs and even NPs. These insights will in turn aid the discovery of new materials for use in various applications.

Medium/High-entropy alloys (MEAs/HEAs) have attracted much attention during the past two decades and have been studied extensively owing to their excellent physical and mechanical properties. These materials form simple lattice structures and thermodynamically favored single-phase solutions. Despite having a single-phase, the local structure of MEAs/HEAs still contain some degree of order. Recently, short-range order (SRO) has been studied to better understand the local structure of MEAs/HEAs and how this order impacts their properties. Efforts to characterize SRO in high-entropy alloys have included non-imaging methods such as X-ray diffraction and X-ray absorption spectroscopy, as well as imaging methods such as transmission electron microscopy-based techniques. In this perspective, structural studies using non-imaging and imaging techniques to investigate SRO in MEAs/HEAs are discussed. Moreover, the impact of SRO on the physical and mechanical properties of MEAs/HEAs is also covered.

Carbon-supported nanocomposites are attracting particular attention as high-performance, low-cost electrocatalysts for electrochemical water splitting. These are mostly prepared by pyrolysis and hydrothermal procedures that are time-consuming (from hours to days) and typically difficult to produce a nonequilibrium phase. Herein, for the first time ever, we exploit magnetic induction heating-quenching for ultrafast production of carbon-FeNi spinel oxide nanocomposites (within seconds), which exhibit an unprecedentedly high performance towards oxygen evolution reaction (OER), with an ultralow overpotential of only +260 mV to reach the high current density of 100 mA cm -2 . Experimental and theoretical studies show that the rapid heating and quenching process (ca. 10 3 K s -1 ) impedes the Ni and Fe phase segregation and produces a Cl-rich surface, both contributing to the remarkable catalytic activity. Results from this study highlight the unique advantage of ultrafast heating/quenching in the structural engineering of functional nanocomposites to achieve high electrocatalytic performance towards important electrochemical reactions.

Atomically dispersed catalysts show great promise, but their design is challenging. A library of catalysts spanning 37 elements was created to uncover unified principles for catalyst design.

Nanoscale multi-principal element intermetallics (MPEIs) may provide a broad and tunable compositional space of active, high-surface area materials with potential applications such as catalysis and magnetics. However, MPEI nanoparticles are challenging to fabricate because of the tendency of the particles to grow/agglomerate or phase-separated during annealing. Here, we demonstrate a disorder-to-order phase transition approach that enables the synthesis of ultrasmall (4 to 5 nm) and stable MPEI nanoparticles (up to eight elements). We apply just 5 min of Joule heating to promote the phase transition of the nanoparticles into L10 intermetallic structure, which is then preserved by rapidly cooling. This disorder-to-order transition results in phase-stable nanoscale MPEIs with compositions (e.g., PtPdAuFeCoNiCuSn), which have not been previously attained by traditional synthetic methods. This synthesis strategy offers a new paradigm for developing previously unexplored MPEI nanoparticles by accessing a nanoscale-size regime and novel compositions with potentially broad applications.

The application of Pt alloy catalysts for oxygen reduction reactions (ORRs) in proton-exchange membrane fuel cells is severely impeded by base metal leaching, since the produced metal ions can resu...

The ability to alloy different elements is critical for property tuning and materials discovery. However, general alloying at the nanoscale remains extremely challenging due to strong immiscibility and easy oxidation, particularly for early transition metals that are highly reactive. Here, we report nanoscale alloying using a high-temperature- and high-entropy-based strategy (T*Delta S-mix) to significantly expand the possible alloys and include early transition metals, While high-temperature synthesis favors alloy formation and metal reduction, the high-entropy compositional design is critical to further extending the alloying to strongly repelling combinations (e.g., Au-W) and easily oxidized elements (e.g., Zr). In particular, we explicitly characterized a record 15-element nanoalloy, which showed a solid-solution structure featuring localized strain and lattice distortions as a result of extreme mixing. Our study significantly broadens available compositions of nanoalloys and provides clear guidelines by utilizing the less-explored entropic chemistry.

High-entropy alloy nanoparticles (HEA NPs) are an area of interest due to their interesting structural features, large compositional space, and high potential for catalytic applications. While many HEA NPs have been successfully synthesized and characterized, little is known about the impact of composition on structural features. This work utilizes element-specific X-ray spectroscopy techniques to compare 5- and 15-element HEA NPs with shared elements, aiming to understand the impact that adding additional elements to an HEA has on its structure and properties. Here, we report significant differences between HEA-5 and HEA-15. In particular, the 3d elements show different oxidation behavior associated with different bonding properties. Varied bonding properties are also observed in the 4d and 5d elements. These results provide an understanding of the impact composition plays on the HEA structure, which will serve to aid in the fine-tuning of structure and properties of HEAs through adjusting their composition.

Platinum nanocatalysts have shown high performance in a wide variety of electrocatalytic reactions, making them the focus of many studies. High quality characterization results are vital for the proper understanding of a catalyst's structure and properties, allowing for further discoveries to be made. X-ray absorption spectroscopy is a powerful characterization tool, permitting both the local structure and electronic properties to be determined for a sample of interest. This can be taken further by collecting insitu measurements, which offer the unique advantage of characterization results collected in real-time while the catalytic reaction is taking place. This review summarizes recent studies which utilized insitu X-ray absorption spectroscopy to characterize platinum nanocatalysts, highlighting the important structural parameters and electronic properties determined under the real catalytic reaction conditions. Specific examples of catalysts for use in the oxygen reduction reaction, chlorine evolution reaction, and carbon monoxide oxidation are discussed in detail. Future prospects for work in this field are also highlighted and discussed.

ABSTRACT Photoacoustic spectroscopy using pulsed laser excitation is being developed by a number of research groups as one of the most promising methods for studying speciation of actinides in solution at environmentally relevant concentrations. We present details of a number of hardware and software techniques we have implemented which, once fully developed, we believe will improve the sensitivity of the method. Our approach is based on more extensive waveform analysis. While most signal processing techniques extract the analytical signal from only a small portion of the acoustic waveform produced in the detector by an absorption event, we describe two methods that use more of the waveform. Other methods of minimizing noise sources using both hardware and software are also described.

Tuning the performance of nanoparticle (NP) catalysts by controlling the NP surface strain has evolved as an important strategy to optimize NP catalysis in many energy conversion reactions. Here, we present our new study on using an eigenforce model to predict and experiments to verify the strain-induced catalysis enhancement of the oxygen reduction reaction (ORR) in the presence of L10-CoMPt NPs (M = Mn, Fe, Ni, Cu, Ni). The eigenforce model allowed us to predict anisotropic (that is, two-dimensional) strain levels on distorted Pt(111) surfaces. Experimentally, by preparing a series of 5 nm L10-CoMPt NPs, we could push the ORR catalytic activity of these NPs toward the optimum region of the theoretical two-dimensional volcano plot predicted for L10-CoMPt. The best ORR catalyst in the alloy NP series we studied is L10-CoNiPt, which has a mass activity of 3.1 A/mgPt and a specific activity of 9.3 mA/cm2 at room temperature with only 15.9% loss of mass activity after 30 000 cycles at 60 °C in 0.1 M HClO4.

Metal-nitrogen-carbon (MNC) nanocomposites have been hailed as promising and efficient electrocatalysts toward oxygen reduction reaction (ORR), due to the formation of MNx coordination moieties. However, MNC hybrids are mostly prepared by pyrolysis of organic precursors along with select metal salts, where part of the MNx sites are inevitably buried in the carbon matrix. This limited accessibility compromises the electrocatalytic performance. Herein, we describe a wet-impregnation procedure by facile thermal refluxing, whereby palladium is atomically dispersed and enriched onto the surface of hollow, nitrogen-doped carbon cages (HNC) forming Pd-N coordination bonds. The obtained Pd-HNC nanocomposites exhibit an ORR activity in alkaline media markedly higher than that of metallic Pd nanoparticles, and the best sample even outperforms commercial Pt/C and relevant Pd-based catalysts reported in the literature. The results suggest that atomic dispersion and surface enrichment of palladium in a carbon matrix may serve as an effective strategy in the fabrication of high-performance ORR electrocatalysts.

Oxygen reduction reaction (ORR) plays an important role in dictating the performance of various electrochemical energy technologies. As platinum nanoparticles have served as the catalysts of choice towards ORR, minimizing the cost of the catalysts by diminishing the platinum nanoparticle size has become a critical route to advancing the technological development. Herein, first-principle calculations show that carbon-supported Pt9 clusters represent the threshold domain size, and the ORR activity can be significantly improved by doping of adjacent cobalt atoms. This is confirmed experimentally, where platinum and cobalt are dispersed in nitrogen-doped carbon nanowires in varied forms, single atoms, few-atom clusters, and nanoparticles, depending on the initial feeds. The sample consisting primarily of Pt2~7 clusters doped with atomic Co species exhibits the best mass activity among the series, with a current density of 4.16 A mgPt−1 at +0.85 V vs. RHE that is almost 50 times higher than that of commercial Pt/C.

ABSTRACTThe speciation of UO22+ and UO22+/ TBP mixtures has been investigated in solution and intercalated with the reference smectite clay SAz-1 using x-ray absorption, Raman, andluminescence spectroscopies. Neither aquated UO22+ nor its TBP complex undergoes any detectable changes in uranium oxidation state on intercalation. Further, at the pH values employed in this work, there is no evidence for hydrolysis of the uranium species to generate dimeric or higher order uranium oligomers. However, we do find indications that the structures of the solution complexes are altered on intercalation, particularly for the UO22+TBP system and for more dilute UO22+/aqueous systems. In addition, several lines of evidence suggest that, at the loading levels used in this study, the uranyl species is interacting with two or more spectroscopically distinguishable sites on SAz-1.

Multi-elemental alloy nanoparticles (MEA-NPs) hold great promise for catalyst discovery in a virtually unlimited compositional space. However, rational and controllable synthesize of these intrinsically complex structures remains a challenge. Here, we report the computationally aided, entropy-driven design and synthesis of highly efficient and durable catalyst MEA-NPs. The computational strategy includes prescreening of millions of compositions, prediction of alloy formation by density functional theory calculations, and examination of structural stability by a hybrid Monte Carlo and molecular dynamics method. Selected compositions can be efficiently and rapidly synthesized at high temperature (e.g., 1500 K, 0.5 s) with excellent thermal stability. We applied these MEA-NPs for catalytic NH3 decomposition and observed outstanding performance due to the synergistic effect of multi-elemental mixing, their small size, and the alloy phase. We anticipate that the computationally aided rational design and rapid synthesis of MEA-NPs are broadly applicable for various catalytic reactions and will accelerate material discovery.

Aromaticity and antiaromaticity, as defined by Hückel’s rule, are key ideas in organic chemistry, and are both exemplified in biphenylene 1 – 3 —a molecule that consists of two benzene rings joined by a four-membered ring at its core. Biphenylene analogues in which one of the benzene rings has been replaced by a different (4 n + 2) π-electron system have so far been associated only with organic compounds 4 , 5 . In addition, efforts to prepare a zirconabiphenylene compound resulted in the isolation of a bis(alkyne) zirconocene complex instead 6 . Here we report the synthesis and characterization of, to our knowledge, the first 2-metallabiphenylene compounds. Single-crystal X-ray diffraction studies reveal that these complexes have nearly planar, 11-membered metallatricycles with metrical parameters that compare well with those reported for biphenylene. Nuclear magnetic resonance spectroscopy, in addition to nucleus-independent chemical shift calculations, provides evidence that these complexes contain an antiaromatic cyclobutadiene ring and an aromatic benzene ring. Furthermore, spectroscopic evidence, Kohn–Sham molecular orbital compositions and natural bond orbital calculations suggest covalency and delocalization of the uranium f 2 electrons with the carbon-containing ligand.

Removal of chloride from Cp*Ir(glycinato)Cl in noncoordinating solvents with Ag[PF6] or Tl[PF6] leads to the formation of a closed octametallic loop of cations. The same loop also sequesters a number of PF6 - counter anions. This is in contrast with reports that piano-stool complexes with amino acids form only trimetallic [Cp*Ir(aminoacidato)]3 3+ moieties upon creating the cation. Cp*Ir(glycinato)Cl also forms a trimetallic compound as well as a octametallic compound, and the octametallic vs trimetallic formation appears to be dependent on the anion. The synthesis and characterization of the octametallic complex, as well as some monometallic and trimetallic compounds, are reported, including the X-ray crystal structures.