Colossal barocaloric effects in plastic crystals.

Refrigeration is of vital importance for modern society-for example, for food storage and air conditioning-and 25 to 30 per cent of the world's electricity is consumed for refrigeration1. Current refrigeration technology mostly involves the conventional vapour compression cycle, but the materials used in this technology are of growing environmental concern because of their large global warming potential2. As a promising alternative, refrigeration technologies based on solid-state caloric effects have been attracting attention in recent decades3-5. However, their application is restricted by the limited performance of current caloric materials, owing to small isothermal entropy changes and large driving magnetic fields. Here we report colossal barocaloric effects (CBCEs) (barocaloric effects are cooling effects of pressure-induced phase transitions) in a class of disordered solids called plastic crystals. The obtained entropy changes in a representative plastic crystal, neopentylglycol, are about 389 joules per kilogram per kelvin near room temperature. Pressure-dependent neutron scattering measurements reveal that CBCEs in plastic crystals can be attributed to the combination of extensive molecular orientational disorder, giant compressibility and highly anharmonic lattice dynamics of these materials. Our study establishes the microscopic mechanism of CBCEs in plastic crystals and paves the way to next-generation solid-state refrigeration technologies.

Surface Extraction Process During Initial Oxidation of Pt(111): Effect of Hydrophilic/Hydrophobic Cations in Alkaline Media.

The surface oxidation states of the metal electrodes affect the activity, selectivity, and stability of the electrocatalysts. Oxide formation and reduction on such electrodes must be comprehensively understood to achieve next-generation electrocatalysts with outstanding performance and stability. Herein, the initial electrochemical oxidation of Pt(111) in alkaline media containing hydrophilic and hydrophobic cations is investigated by X-ray crystal truncation rod (CTR) scattering, infrared (IR) spectroscopy, and nanoparticle-based surface-enhanced Raman spectroscopy (SERS). Structural determination using X-ray CTR revealed surface buckling and Pt extraction at the initial stage of surface oxidation, depending on the cationic species. Vibrational spectroscopy is performed to identify the potential- and cation-dependent formation of three oxide species (IR-active OHad, Raman-active OHad/Oad(H2O), and Raman-active Oad). Hydrophilic alkali metal cations (Li+) inhibit surface roughening via irreversible oxide formation. Hydrophilic Li+ can strongly stabilize IR-active OHad, hindering the extraction of Pt surface atoms. Interestingly, bulky hydrophobic cations such as tetramethylammonium (TMA+) cation also reduce the extent of irreversible oxidation despite the absence of IR-active OHad. Hydrophobic TMA+ inhibits the formation of Raman-active OHad/Oad(H2O) associated with Pt extraction. In contrast, the moderate hydrophilicity of K+ has no protective effect against irreversible oxidation. Moderate hydrophilicity enables the coadsorption of Raman-active OHad/Oad(H2O) and Raman-active Oad. The electrostatic repulsion between Raman-active OHad/Oad(H2O) and neighboring Raman-active Oad promotes Pt extraction. These results provide insights into controlling the surface structures of electrocatalysts using cationic species during the oxide formation and reduction processes.

High-throughput and high-resolution powder X-ray diffractometer consisting of six sets of 2D CdTe detectors with variable sample-to-detector distance and innovative automation system.

The demand for powder X-ray diffraction analysis continues to increase in a variety of scientific fields, as the excellent beam quality of high-brightness synchrotron light sources enables the acquisition of high-quality measurement data with high intensity and angular resolution. Synchrotron powder diffraction has enabled the rapid measurement of many samples and various in situ/operando experiments in nonambient sample environments. To meet the demands for even higher throughput measurements using high-energy X-rays at SPring-8, a high-throughput and high-resolution powder diffraction system has been developed. This system is combined with six sets of two-dimensional (2D) CdTe detectors for high-energy X-rays, and various automation systems, including a system for automatic switching among large sample environmental equipment, have been developed in the third experimental hutch of the insertion device beamline BL13XU at SPring-8. In this diffractometer system, high-brilliance and high-energy X-rays ranging from 16 to 72 keV are available. The powder diffraction data measured under ambient and various nonambient conditions can be analysed using Rietveld refinement and the pair distribution function. Using the 2D CdTe detectors with variable sample-to-detector distance, three types of scan modes have been established: standard, single-step and high-resolution. A major feature is the ability to measure a whole powder pattern with millisecond resolution. Equally important, this system can measure powder diffraction data with high Q exceeding 30 Å-1 within several tens of seconds. This capability is expected to contribute significantly to new research avenues using machine learning and artificial intelligence by utilizing the large amount of data obtained from high-throughput measurements.

Platinum-Group Metal High-Entropy Selenides for the Hydrogen Evolution Reaction.

The selenides of platinum-group metals (PGMs) are emerging as promising catalysts for diverse electrochemical reactions. To date, most studies have focused on single metal or bimetallic systems, whereas the preparation of a high-entropy (HE) selenide consisting of five or more PGM elements holds the promise to further enhance catalytic performance by introducing abundant active sites with various local coordination environments and electronic structures. Herein, we report for the first time the synthesis of PGM-based HE-Selenide (HE-Se) nanoparticles with a unique amorphous structure. The atomic metal-Se coordination and the presence of short-range order were thoroughly revealed. It is further shown that the amorphous HE-Se can be facilely transformed into a single-phase crystalline HE-Se with a cubic structure by thermal annealing. Catalytically, the amorphous HE-Se showed better acidic hydrogen evolution activity over monometallic PGM-based selenides and the crystalline counterpart, demonstrating the advantages of high-entropy configuration and amorphous structure. Our findings may pave the way toward the synthesis and property exploration of amorphous PGM-based selenides with tunable compositions.

Continuous-Flow Chemical Synthesis for Sub-2 nm Ultra-Multielement Alloy Nanoparticles Consisting of Group IV to XV Elements.

Multielement alloy nanoparticles have attracted much attention due to their attractive catalytic properties derived from the multiple interactions of adjacent multielement atoms. However, mixing multiple elements in ultrasmall nanoparticles from a wide range of elements on the periodic table is still challenging because the elements have different properties and miscibility. Herein, we developed a benchtop 4-way flow reactor for chemical synthesis of ultra-multielement alloy (UMEA) nanoparticles composed of d-block and p-block elements. BiCoCuFeGaInIrNiPdPtRhRuSbSnTi 15-element alloy nanoparticles composed of group IV to XV elements were synthesized by sequential injection of metal precursors using the reactor. This methodology realized the formation of UMEA nanoparticles at low temperature (66 °C), resulting in a 1.9 nm ultrasmall average particle size. The UMEA nanoparticles have high durability and activity for electrochemical alcohol oxidation reactions and high tolerance to CO poisoning. These results suggest that the multiple interactions of UMEA efficiently promote the multistep alcohol oxidation reaction.

Noble-Metal High-Entropy-Alloy Nanoparticles: Atomic-Level Insight into the Electronic Structure.

The compositional space of high-entropy-alloy nanoparticles (HEA NPs) significantly expands the diversity of the materials library. Every atom in HEA NPs has a different elemental coordination environment, which requires knowledge of the local electronic structure at an atomic level. However, such structure has not been disclosed experimentally or theoretically. We synthesized HEA NPs composed of all eight noble-metal-group elements (NM-HEA) for the first time. Their electronic structure was revealed by hard X-ray photoelectron spectroscopy and density function theory calculations with NP models. The NM-HEA NPs have a lower degeneracy in energy level compared with the monometallic NPs, which is a common feature of HEA NPs. The local density of states (LDOS) of every surface atom was first revealed. Some atoms of the same constituent element in HEA NPs have different LDOS profiles, whereas atoms of other elements have similar LDOS profiles. In other words, one atom in HEA loses its elemental identity and it may be possible to create an ideal LDOS by adjusting the neighboring atoms. The tendency of the electronic structure change was shown by supervised learning. The NM-HEA NPs showed 10.8-times higher intrinsic activity for hydrogen evolution reaction than commercial Pt/C, which is one of the best catalysts.

Continuous-Flow Reactor Synthesis for Homogeneous 1 nm-Sized Extremely Small High-Entropy Alloy Nanoparticles.

High-entropy alloy nanoparticles (HEA NPs) emerged as catalysts with superior performances that are not shown in monometallic catalysts. Although many kinds of synthesis techniques of HEA NPs have been developed recently, synthesizing HEA NPs with ultrasmall particle size and narrow size distribution remains challenging because most of the reported synthesis methods require high temperatures that accelerate particle growth. This work provides a new methodology for the fabrication of ultrasmall and homogeneous HEA NPs using a continuous-flow reactor with a liquid-phase reduction method. We successfully synthesized ultrasmall IrPdPtRhRu HEA NPs (1.32 ± 0.41 nm), theoretically each consisting of approximately 50 atoms. This average size is the smallest ever reported for HEA NPs. All five elements are homogeneously mixed at the atomic level in each particle. The obtained HEA NPs marked a significantly high hydrogen evolution reaction (HER) activity with a very small 6 mV overpotential at 10 mA/cm-2 in acid, which is one-third of the overpotential of commercial Pt/C. In addition, although mass production of HEA NPs is still difficult, this flow synthesis can provide high productivity with high reproducibility, which is more energy efficient and suitable for mass production. Therefore, this study reports the 1 nm-sized HEA NPs with remarkably high HER activity and establishes a platform for the production of ultrasmall and homogeneous HEA NPs.

Strain-Controlled Spin Transition in Heterostructured Metal-Organic Framework Thin Film.

Metal-organic framework (MOF) thin films have recently attracted much attention as a new platform for surface/interface research, where unconventional structural and physical properties emerge. Among the many MOFs as candidates for fabrication of thin films, Hofmann-type MOFs {Fe(pz)[M(CN)4]} [pz = pyrazine; M = Ni (Nipz), M = Pt (Ptpz)] are attractive, because they undergo spin transitions with concomitant structural changes. Here, we demonstrate the first example of a strain-controlled spin transition in heterostructured MOF thin films. The spin transition temperature of Ptpz can be controlled in the temperature range of 300-380 K by fabricating a nanometer-sized heterostructured thin film with a Nipz buffer layer, where the smaller lattice of Nipz causes epitaxial compressive strain to the Ptpz layer. The fabricated heterostructured thin film exhibited a remarkable increase in spin transition temperature with a dynamic structural transformation, confirmed by variable-temperature (VT) X-ray diffraction and VT Raman spectroscopy. By verifying interfacial strain in a heterostructured thin film, we can rationally control the characteristics of MOFs-not only spin transition but also various physical properties such as gas storage, catalysis, sensing, proton conductivity, and electrical properties, among others.

Rational Synthesis for a Noble Metal Carbide.

Transition metal carbides have attractive physical and chemical properties that are much different from their parent metals. Particularly, noble metal carbides are expected to be promising materials for a variety of applications, particularly as efficient catalysts. However, noble metal carbides have rarely been obtained because carbide phases do not appear in noble metal-carbon phase diagrams and a reasonable synthesis method to make noble metal carbides has not yet been established. Here, we propose a new synthesis method for noble metal carbides and describe the first synthesis of rhodium carbide using tetracyanoethylene (TCNE). The rhodium carbide was synthesized without extreme conditions, such as the very high temperature and/or pressure typically required in conventional carbide syntheses. Moreover, we investigated the electronic structure and catalytic activity for the hydrogen evolution reaction (HER). We found that rhodium carbide has much higher catalytic activity for HER than pure Rh. Our study provides a feasible strategy to create new metal carbides to help advance the field of materials science.

A middle energy-bandwidth X-ray monochromator for high-flux synchrotron diffraction: revisiting asymmetrically cut silicon crystals.

To supply the growing demand for high photon flux in synchrotron science including surface diffraction, a middle energy-bandwidth monochromator covering the 10-4 to 10-3 range has been adapted by applying an asymmetric diffraction geometry to a cryogenically cooled silicon 111 double-crystal monochromator used as a standard for the undulator source at SPring-8. The asymmetric geometry provides a great advantage with its ability to configure flux gains over a wide energy range by simply changing the asymmetry angle, while the angular divergence of the exit beam remains unchanged. A monolithic design with three faces has been employed, having one symmetrically cut and another two asymmetrically cut surfaces relative to the same atomic plane, maintaining cooling efficiency and the capability of quickly changing the reflection surface. With the asymmetric geometry, an X-ray flux greater than 1014 photons s-1 was available around 12 keV. A maximum gain of 2.5 was obtained relative to the standard symmetric condition.

Electric-Field-Driven Nanosecond Ferroelastic-Domain Switching Dynamics in Epitaxial Pb(Zr,Ti)O_{3} Film.

Epitaxial oxide ferroelectric films exhibit emerging phenomena arising from complex domain configurations even at pseudoequilibrium, including the creation of domain states unfavored in nature and abrupt piezoelectric coefficients around morphotropic phase boundaries. The nanometer-sized domain configurations and their domain switching dynamics under external stimuli are directly linked to the ultrafast manipulation of ferroelectric thin films; however, complex domain switching dynamics under homogeneous electric fields has not been fully explored, especially at the nanosecond timescale. This Letter reports the nanosecond dynamics of ferroelastic-domain switching from the 90° to 180° direction using time-resolved x-ray microdiffraction under homogeneous electric fields onto an epitaxial Pb(Zr_{0.35},Ti_{0.65})O_{3} film capacitor. It is found that the application of electric fields induces spatially heterogeneous domain switching processes via intermediate domain structures with rotated polarization vectors. In addition, the domain switching time is shown to be inversely proportional to the magnitude of the applied electric field, and electric fields higher than 480 kV/cm are found to complete the ferroelastic switching within nanoseconds.

Local Geometry and Electronic Properties of Nickel Nanoparticles Prepared via Thermal Decomposition of Ni-MOF-74.

Metal-organic frameworks (MOFs) provide highly selective catalytic activity because of their porous crystalline structure. There is particular interest in metal nanoparticle-MOF composites (MNP@MOF) that could take advantage of synergistic effects for enhanced catalytic properties. We present an investigation into the local geometry and electronic properties of thermally decomposed Ni-MOF-74 calcined at different temperatures and time durations. Pair distribution function analysis using high-energy X-ray diffraction reveals the formation of fcc-Ni nanoparticles with a mixture of MOF phase in samples heated at 623 K for 12 h. Elevating the calcination temperature and lengthening the time duration assisted complete precipitation of Ni nanoparticles in the MOF matrix. Local structures and valence states were investigated using X-ray absorption fine structure spectroscopy. Evidence of ligand-to-metal charge transfer and gradual reduction of Ni2+ is apparent for those samples heated above 623 K for 12 h. In addition, the Ni lattice was found to be slightly compressed as a result of surface stresses in the nanosized particles or surface ligand environment. Electronic structure investigation using hard X-ray photoelectron spectroscopy shows a significant narrowing of the valence band and a decrease in the d-band center (toward the Fermi level) when the heating temperature is increased, thus suggesting promising catalytic properties for NiNP@MOF composite.

Size effects on rhodium nanoparticles related to hydrogen-storage capability.

To unveil the origin of the hydrogen-storage properties of rhodium nanoparticles (Rh NPs), we investigated the electronic and crystal structures of the Rh NPs using various synchrotron based X-ray techniques. Electronic structure studies revealed that the hydrogen-storage capability of Rh NPs could be attributed to their more unoccupied d-DOSs than that of the bulk near the Fermi level. Crystal structure studies indicated that lattice distortion and mean-square displacement increase while coordination number decreases with decreasing particle size and the hydrogen-absorption capability of Rh NPs improves to a greater extent with increased structural disorder in the local structure than with that in the mean structure. The smallest Rh NPs, having the largest structural disorder/increased vacancy spaces and the smallest coordination number, exhibited excellent hydrogen-storage capacity. Finally, from the bond-orientational order analysis, we confirmed that the localized disordering is distributed more over the surface part than the core part and hydrogen can be trapped on the surface part of Rh NPs which increases with a decrease in NP diameter.

Disappearance of the Superionic Phase Transition in Sub-5 nm Silver Iodide Nanoparticles.

Bulk silver iodide (AgI) is known to show a phase transition from the poorly conducting ß/γ-phases into the superionic conducting α-phase at 147 °C. Its transition temperature decreases with decreasing the size of AgI, and the α-phase exists stably at 37 °C in AgI nanoparticles with a diameter of 6.3 nm. In this Letter, we investigated the atomic configuration, the phase transition behavior, and the ionic conductivity of AgI nanoparticles with a diameter of 3.0 nm. The combination of pair distribution function (PDF) analysis and reverse Monte Carlo (RMC) modeling based on high-energy X-ray diffraction (XRD) revealed for the first time that they formed the ß/γ-phases with atomic disorder. The results of extended X-ray absorption fine structure (EXAFS) analysis, differential scanning calorimetry (DSC), and AC impedance spectroscopy demonstrated that they did not exhibit the superionic phase transition and their ionic conductivity was lower than that of crystalline AgI. The disappearance of the superionic phase transition and low ionic conductivity in the very small AgI nanoparticles originates from their small size and disordered structure.

Change in the Crystallite Orientation of Poly(ethylene oxide)/Cellulose Nanofiber Composite Films.

The crystallite orientation and crystallographic domain structure of poly(ethylene oxide) (PEO) in cellulose nanofiber-incorporated (CNF-incorporated) PEO films developed for packaging materials were observed using wide-angle X-ray diffraction for different CNF filling ratios. When a CNF filling ratio of <10 wt % was used, the molecular chains in the PEO crystallite region were oriented in a direction perpendicular to the surface of the film; however, when the ratio was >50 wt %, the PEO molecular chains were oriented in a direction parallel to the surface of the film. The fiber axis of the CNFs became parallel to the surface of the PEO/CNF composite film when the filling ratio was >25 wt %. The change in the orientation of the PEO crystals occurred because increasing the amount of CNF in the composite films decreased the space in which the PEO could be crystallized. Furthermore, the hydrogen bonds between the PEO and the CNF may behave as crystallization nuclei for the PEO. Our results thus pave the way toward the development of packaging materials that are more impermeable to gases than the current materials.

Fabrication and Structural Characterization of an Ultrathin Film of a Two-Dimensional-Layered Metal-Organic Framework, {Fe(py)2[Ni(CN)4]} (py = pyridine).

We report the fabrication and characterization of the first example of a tetracyanonickelate-based two-dimensional-layered metal-organic framework, {Fe(py)2Ni(CN)4} (py = pyridine), thin film. To fabricate a nanometer-sized thin film, we utilized the layer-by-layer method, whereby a substrate was alternately soaked in solutions of the structural components. Surface X-ray studies revealed that the fabricated film was crystalline with well-controlled growth directions both parallel and perpendicular to the substrate. In addition, lattice parameter analysis indicated that the crystal system is found to be close to higher symmetry by being downsized to a thin film.

Guest-Induced Two-Way Structural Transformation in a Layered Metal-Organic Framework Thin Film.

Fabrication of thin films made of metal-organic frameworks (MOFs) has been intensively pursued for practical applications that use the structural response of MOFs. However, to date, only physisorption-induced structural response has been studied in these films. Chemisorption can be expected to provide a remarkable structural response because of the formation of bonds between guest molecules and reactive metal sites in host MOFs. Here, we report that chemisorption-induced two-way structural transformation in a nanometer-sized MOF thin film. We prepared a two-dimensional layered-type MOF Fe[Pt(CN)4] thin film using a step-by-step approach. Although the as-synthesized film showed poor crystallinity, the dehydrated form of this thin film had a highly oriented crystalline nature (Film-D) as confirmed by synchrotron X-ray diffraction (XRD). Surprisingly, under water and pyridine vapors, Film-D showed chemisorption-induced dynamic structural transformations to Fe(L)2[Pt(CN)4] thin films [L = H2O (Film-H), pyridine (Film-P)], where water and pyridine coordinated to the open Fe2+ site. Dynamic structural transformations were also confirmed by in situ XRD, sorption measurement, and infrared reflection absorption spectroscopy. This is the first report of chemisorption-induced dynamic structural response in a MOF thin film, and it provides useful insights, which would lead to future practical applications of MOFs utilizing chemisorption-induced structural responses.

Origin of the catalytic activity of face-centered-cubic ruthenium nanoparticles determined from an atomic-scale structure.

The 3-dimensional (3D) atomic-scale structure of newly discovered face-centered cubic (fcc) and conventional hexagonal close packed (hcp) type ruthenium (Ru) nanoparticles (NPs) of 2.2 to 5.4 nm diameter were studied using X-ray pair distribution function (PDF) analysis and reverse Monte Carlo (RMC) modeling. Atomic PDF based high-energy X-ray diffraction measurements show highly diffuse X-ray diffraction patterns for fcc- and hcp-type Ru NPs. We here report the atomic-scale structure of Ru NPs in terms of the total structure factor and Fourier-transformed PDF. It is found that the respective NPs have substantial structural disorder over short- to medium-range order atomic distances from the PDF analysis. The first-nearest-neighbor peak analyses show a significant size dependence for the fcc-type Ru NPs demonstrating the increase in the peak height due to an increase in the number density as a function of particle size. The bond angle and coordination number (CN) distribution for the RMC-simulated fcc- and hcp-type Ru NP models indicated inherited structural features from their bulk counterparts. The CN analysis of the whole NP and surface of each RMC model of Ru NPs show the low activation energy packing sites on the fcc-type Ru NP surface atoms. Finally, our newly defined order parameters for RMC simulated Ru NP models suggested that the enhancement of the CO oxidation activity of fcc-type NPs was due to a decrease in the close packing ordering that resulted from the increased NP size. These structural findings could be positively supported for synthesized low-cost and high performance nano-sized catalysts and have potential application in fuel-cell systems and organic synthesis.

Remarkable Lattice Shrinkage in Highly Oriented Crystalline Three-Dimensional Metal-Organic Framework Thin Films.

Highly oriented crystalline thin films of metal-organic frameworks (MOFs) have promising practical applications, such as in gas separation, catalysis, and sensing. We report on the successful fabrication of highly oriented crystalline thin films of three-dimensional porous MOFs, Fe(pz)[M(CN)4] (M = Ni, Pd; pz = pyrazine). Synchrotron X-ray diffraction studies reveal not only the highly oriented crystalline nature but also the remarkable shrunken structure of the thin films (∼3-7% volume shrinkage) compared with bulk samples. Furthermore, because of lattice shrinkage, these films exhibit large lattice expansions upon guest adsorption, in marked contrast to the almost unchanged lattice in the bulk samples.

Structural and thermal properties of ternary narrow-gap oxide semiconductor; wurtzite-derived ß-CuGaO2.

The crystal structure of the wurtzite-derived ß-CuGaO2 was refined by Rietveld analysis of high-resolution powder diffraction data obtained from synchrotron X-ray radiation. Its structural characteristics are discussed in comparison with the other I-III-VI2 and II-VI oxide semiconductors. The cation and oxygen tetrahedral distortions of the ß-CuGaO2 from an ideal wurtzite structure are small. The direct band-gap nature of the ß-CuGaO2, unlike ß-Ag(Ga,Al)O2, was explained by small cation and oxygen tetrahedral distortions. In terms of the thermal stability, the ß-CuGaO2 irreversibly transforms into delafossite α-CuGaO2 at >460 °C in an Ar atmosphere. The transformation enthalpy was approximately -32 kJ mol(-1), from differential scanning calorimetry. This value is close to the transformation enthalpy of CoO from the metastable zincblende form to the stable rock-salt form. The monovalent copper in ß-CuGaO2 was oxidized to divalent copper in an oxygen atmosphere and transformed into a mixture of CuGa2O4 spinel and CuO at temperatures >350 °C. These thermal properties indicate that ß-CuGaO2 is stable at ≤300 °C in both reducing and oxidizing atmospheres while in its metastable form. Consequently, this material could be of use in optoelectronic devices that do not exceed 300 °C.