Three-dimensional imaging of dislocation dynamics during the hydriding phase transformation

Balancing Pd-H Interactions: Thiolate-Protected Palladium Nanoclusters for Robust and Rapid Hydrogen Gas Sensing

The transition toward hydrogen gas (H2) as an eco-friendly and renewable energy source necessitates advanced safety technologies, particularly robust sensors for H2 leak detection and concentration monitoring. Although palladium (Pd)-based materials are preferred for their strong H2 affinity, intense palladium-hydrogen (Pd-H) interactions lead to phase transitions to palladium hydride (PdHx), compromising sensors' durability and detection speeds after multiple uses. In response, this study introduces a high-performance H2 sensor designed from thiolate-protected Pd nanoclusters (Pd8SR16), which leverages the synergistic effect between the metal and protective ligands to form an intermediate palladium-hydrogen-sulfur (Pd-H-S) state during H2 adsorption. Striking a balance, it preserves Pd-H binding affinity while preventing excessive interaction, thus lowering the energy required for H2 desorption. The dynamic adsorption-dissociation-recombination-desorption process is efficiently and highly reversible with Pd8SR16, ensuring robust and rapid H2 sensing at parts per million (ppm). The Pd8SR16-based sensor demonstrates exceptional stability (50 cycles; 0.11% standard deviation in response), prompt response/recovery (t90 = 0.95 s/6 s), low limit of detection (LoD, 1 ppm), and ambient temperature operability, ranking it among the most sensitive Pd-based H2 sensors. Furthermore, a multifunctional prototype demonstrates the practicality of real-world gas sensing using ligand-protected metal nanoclusters.

Coherent Phase Change in Interstitial Solutions: A Hierarchy of Instabilities

Metal hydrides or lithium ion battery electrodes can take the form of interstitial solid solutions with a miscibility gap. This work discusses theory approaches for locating, in temperature-composition space, coherent phase transformations during the charging/discharging of such systems and for identifying the associated transformation mechanisms. The focus is on the simplest scenario, where instabilities derive from the thermodynamics of the bulk phase alone, considering strain energy as the foremost consequence of coherency and admitting for stress relaxation at free surfaces. The extension of the approach to include capillarity is demonstrated by an example. The analysis rests on constrained equilibrium phase diagrams that are informed by geometry- and dimensionality-specific mechanical boundary conditions and on elastic instabilities-again geometry-specific-as implied by the theory of open-system elasticity. It is demonstrated that some scenarios afford the analysis of chemical stability to be based entirely on a linear stability analysis of the mechanical equilibrium, which provides closed-form solutions in a straightforward manner. Attention is on the impact of the system geometry (infinitely extended or of finite size) and on the chemical (closed or open system) and mechanical (incoherent or coherent) boundary conditions. Transformation mechanism maps are suggested for documenting the findings. The maps reveal a hierarchy of instabilities, which depend strongly on each of the above characteristics. Specifically, realistic, finite-sized systems differ qualitatively from idealized systems of infinite extension. Among the transformation mechanisms exposed by the analysis are a uniform switchover to the other phase when the open system reaches its chemical spinodal, practical coherent nucleation, as well as chemo-elastically coupled spontaneous buckling modes, which may take the form of either, single-phase or dual-phase states.

Revealing the Substrate Constraint Effect on the Thermodynamic Behaviour of the Pd-H through Capacitive-Based Hydrogen-Sorption Measurement

Mechanical constraints imposed on the Pd-H system can induce significant strain upon hydrogenation-induced expansion, potentially leading to changes in the thermodynamic behavior, such as the phase-transition pressure. However, the investigation of the constraint effect is often tricky due to the lack of simple experimental techniques for measuring hydrogenation-induced expansion. In this study, a capacitive-based measurement system is developed to monitor hydrogenation-induced areal expansion, which allows us to control and evaluate the magnitude of the substrate constraint. By using the measurement technique, the influence of substrate constraint intensity on the thermodynamic behavior of the Pd-H system is investigated. Through experiments with different constraint intensities, it is found that the diffefrence in the constraint intensity minimally affects the phase-transition pressure when the Pd-H system allows the release of constraint stress through plastic deformation. These experiments can improve the understanding of the substrate constraint behaviours of Pd-H systems allowing plastic deformation while demonstrating the potential of capacitive-based measurement systems to study the mechanical-thermodynamic coupling of M-H systems.

Impact of palladium/palladium hydride conversion on electrochemical CO2 reduction via in-situ transmission electron microscopy and diffraction

Electrochemical conversion of CO2 offers a sustainable route for producing fuels and chemicals. Pd-based catalysts are effective for converting CO2 into formate at low overpotentials and CO/H2 at high overpotentials, while undergoing poorly understood morphology and phase structure transformations under reaction conditions that impact performance. Herein, in-situ liquid-phase transmission electron microscopy and select area diffraction measurements are applied to track the morphology and Pd/PdHx phase interconversion under reaction conditions as a function of electrode potential. These studies identify the degradation mechanisms, including poisoning and physical structure changes, occurring in PdHx/Pd electrodes. Constant potential density functional theory calculations are used to probe the reaction mechanisms occurring on the PdHx structures observed under reaction conditions. Microkinetic modeling reveals that the intercalation of *H into Pd is essential for formate production. However, the change in electrochemical CO2 conversion selectivity away from formate and towards CO/H2 at increasing overpotentials is due to electrode potential dependent changes in the reaction energetics and not a consequence of morphology or phase structure changes.

Probing three-dimensional mesoscopic interfacial structures in a single view using multibeam X-ray coherent surface scattering and holography imaging

Visualizing surface-supported and buried planar mesoscale structures, such as nanoelectronics, ultrathin-film quantum dots, photovoltaics, and heterogeneous catalysts, often requires high-resolution X-ray imaging and scattering. Here, we discovered that multibeam scattering in grazing-incident reflection geometry is sensitive to three-dimensional (3D) structures in a single view, which is difficult in conventional scattering or imaging approaches. We developed a 3D finite-element-based multibeam-scattering analysis to decode the heterogeneous electric-field distribution and to faithfully reproduce the complex scattering and surface features. This approach further leads to the demonstration of hard-X-ray Lloyd's mirror interference of scattering waves, resembling dark-field, high-contrast surface holography under the grazing-angle scattering conditions. A first-principles calculation of the single-view holographic images resolves the surface patterns' 3D morphology with nanometer resolutions, which is critical for ultrafine nanocircuit metrology. The holographic method and simulations pave the way for single-shot structural characterization for visualizing irreversible and morphology-transforming physical and chemical processes in situ or operando.

Porous Pd-Sn Alloy Nanotube-Based Chemiresistor for Highly Stable and Sensitive H2 Detection

While H₂ is indispensable as a green fuel source, it is highly flammable and explosive. Because it is difficult to detect due to its lack of odor and color, a solution for proper monitoring of H₂ leakage is essential to ensure safe handling. To this end, we have successfully fabricated hollow Pd-Sn alloy nanotubes (NTs) with a Brunauer-Emmett-Teller surface area of 223.0 m²/g through electrospinning and a subsequent etching method, which is the first demonstration of synthesizing Pd-based hollow alloy nanofibers with ultrafine grain sizes. We found that the alloying of Pd with Sn could effectively prevent degradation of the sensing performance upon the α-β phase transition during hydrogen detection. Besides, the highly porous structure with smaller nanograins offered more exposed active sites and higher gas accessibility to bulk materials. The resultant Pd-Sn NTs exhibited excellent sensitivity toward H₂ (0.00005-3%). Notably, the limit of detection of 0.0001% is an outstanding achievement on H₂ sensing among state-of-the-art H₂ sensors. Moreover, when exposed to a high concentration of H₂ (3%), Pd-Sn NTs showed excellent cycling stability with a standard deviation of 0.07% and a sensitivity of 9.27%. These obtained sensing results indicate that Pd-Sn NTs can be used as a highly sensitive and stable H₂ gas sensor at room temperature (25 °C).

X-ray Diffraction Imaging of Deformations in Thin Films and Nano-Objects

The quantification and localization of elastic strains and defects in crystals are necessary to control and predict the functioning of materials. The X-ray imaging of strains has made very impressive progress in recent years. On the one hand, progress in optical elements for focusing X-rays now makes it possible to carry out X-ray diffraction mapping with a resolution in the 50–100 nm range, while lensless imaging techniques reach a typical resolution of 5–10 nm. This continuous evolution is also a consequence of the development of new two-dimensional detectors with hybrid pixels whose dynamics, reading speed and low noise level have revolutionized measurement strategies. In addition, a new accelerator ring concept (HMBA network: hybrid multi-bend achromat lattice) is allowing a very significant increase (a factor of 100) in the brilliance and coherent flux of synchrotron radiation facilities, thanks to the reduction in the horizontal size of the source. This review is intended as a progress report in a rapidly evolving field. The next ten years should allow the emergence of three-dimensional imaging methods of strains that are fast enough to follow, in situ, the evolution of a material under stress or during a transition. Handling massive amounts of data will not be the least of the challenges.

Simple Method of Including Density Variation in Quantitative Continuum Phase-Change Models

We present a simple, quantitative, and thermodynamically self-consistent method of capturing density and pressure variation in continuum phase-change models. The formalism shows how the local state of homogenous dilation may be entirely given by species concentration in an Eulerian formulation. A hyperelastic contribution to the thermodynamic potential generalizes the lattice constraint while permitting composition, temperature, and phase-dependent specific volumes. We compare the results of models implementing this paradigm to those with the lattice constraint by examining the composition and size-dependent equilibrium of a Ni-Cu nanoparticle in its melt and free dendritic growth.

Five-second STEM dislocation tomography for 300 nm thick specimen assisted by deep-learning-based noise filtering

Scanning transmission electron microscopy (STEM) is suitable for visualizing the inside of a relatively thick specimen than the conventional transmission electron microscopy, whose resolution is limited by the chromatic aberration of image forming lenses, and thus, the STEM mode has been employed frequently for computed electron tomography based three-dimensional (3D) structural characterization and combined with analytical methods such as annular dark field imaging or spectroscopies. However, the image quality of STEM is severely suffered by noise or artifacts especially when rapid imaging, in the order of millisecond per frame or faster, is pursued. Here we demonstrate a deep-learning-assisted rapid STEM tomography, which visualizes 3D dislocation arrangement only within five-second acquisition of all the tilt-series images even in a 300 nm thick steel specimen. The developed method offers a new platform for various in situ or operando 3D microanalyses in which dealing with relatively thick specimens or covering media like liquid cells are required.

Sub-pixel high-resolution imaging of high-energy x-rays inspired by sub-wavelength optical imaging

We have developed and demonstrated an image super-resolution method-XR-UNLOC: X-Ray UNsupervised particle LOCalization-for hard x-rays measured with fast-frame-rate detectors that is an adaptation of the principle of photo-activated localization microscopy (PALM) and stochastic optical reconstruction microscopy (STORM), which enabled biological fluorescence imaging at sub-optical-wavelength scales. We demonstrate the approach on experimental coherent Bragg diffraction data measured with 52 keV x-rays from a nanocrystalline sample. From this sample, we resolve the fine fringe detail of a high-energy x-ray Bragg coherent diffraction pattern to an upsampling factor of 16 of the native pixel pitch of 30 μm of a charge-integrating fastCCD detector. This was accomplished by analysis of individual photon locations in a series of "nearly-dark" instances of the diffraction pattern that each contain only a handful of photons. Central to our approach was the adaptation of the UNLOC photon fitting routine for PALM/STORM to the hard x-ray regime to handle much smaller point spread functions, which required a different statistical test for photon detection and for sub-pixel localization. A comparison to a photon-localization strategy used in the x-ray community ("droplet analysis") showed that XR-UNLOC provides significant improvement in super-resolution. We also developed a metric by which to estimate the limit of reliable upsampling with XR-UNLOC under a given set of experimental conditions in terms of the signal-to-noise ratio of a photon detection event and the size of the point spread function for guiding future x-ray experiments in many disciplines where detector pixelation limits must be overcome.

Single alloy nanoparticle x-ray imaging during a catalytic reaction

The imaging of active nanoparticles represents a milestone in decoding heterogeneous catalysts’ dynamics. We report the facet-resolved, surface strain state of a single PtRh alloy nanoparticle on SrTiO3 determined by coherent x-ray diffraction imaging under catalytic reaction conditions. Density functional theory calculations allow us to correlate the facet surface strain state to its reaction environment–dependent chemical composition. We find that the initially Pt-terminated nanoparticle surface gets Rh-enriched under CO oxidation reaction conditions. The local composition is facet orientation dependent, and the Rh enrichment is nonreversible under subsequent CO reduction. Tracking facet-resolved strain and composition under operando conditions is crucial for a rational design of more efficient heterogeneous catalysts with tailored activity, selectivity, and lifetime.

Twin boundary migration in an individual platinum nanocrystal during catalytic CO oxidation

At the nanoscale, elastic strain and crystal defects largely influence the properties and functionalities of materials. The ability to predict the structural evolution of catalytic nanocrystals during the reaction is of primary importance for catalyst design. However, to date, imaging and characterising the structure of defects inside a nanocrystal in three-dimensions and in situ during reaction has remained a challenge. We report here an unusual twin boundary migration process in a single platinum nanoparticle during CO oxidation using Bragg coherent diffraction imaging as the characterisation tool. Density functional theory calculations show that twin migration can be correlated with the relative change in the interfacial energies of the free surfaces exposed to CO. The x-ray technique also reveals particle reshaping during the reaction. In situ and non-invasive structural characterisation of defects during reaction opens new avenues for understanding defect behaviour in confined crystals and paves the way for strain and defect engineering.

Structure of a seeded palladium nanoparticle and its dynamics during the hydride phase transformation

Palladium absorbs large volumetric quantities of hydrogen at room temperature and ambient pressure, making the palladium hydride system a promising candidate for hydrogen storage. Here, we use Bragg coherent diffraction imaging to map the strain associated with defects in three dimensions before and during the hydride phase transformation of an individual octahedral palladium nanoparticle, synthesized using a seed-mediated approach. The displacement distribution imaging unveils the location of the seed nanoparticle in the final nanocrystal. By comparing our experimental results with a finite-element model, we verify that the seed nanoparticle causes a characteristic displacement distribution of the larger nanocrystal. During the hydrogen exposure, the hydride phase is predominantly formed on one tip of the octahedra, where there is a high number of lower coordinated Pd atoms. Our experimental and theoretical results provide an unambiguous method for future structure optimization of seed-mediated nanoparticle growth and in the design of palladium-based hydrogen storage systems.

Facet-Dependent Strain Determination in Electrochemically Synthetized Platinum Model Catalytic Nanoparticles

Studying model nanoparticles is one approach to better understand the structural evolution of a catalyst during reactions. These nanoparticles feature well-defined faceting, offering the possibility to extract structural information as a function of facet orientation and compare it to theoretical simulations. Using Bragg Coherent X-ray Diffraction Imaging, the uniformity of electrochemically synthesized model catalysts is studied, here high-index faceted tetrahexahedral (THH) platinum nanoparticles at ambient conditions. 3D images of an individual nanoparticle are obtained, assessing not only its shape but also the specific components of the displacement and strain fields both at the surface of the nanocrystal and inside. The study reveals structural diversity of shapes and defects, and shows that the THH platinum nanoparticles present strain build-up close to facets and edges. A facet recognition algorithm is further applied to the imaged nanoparticles and provides facet-dependent structural information for all measured nanoparticles. In the context of strain engineering for model catalysts, this study provides insight into the shape-controlled synthesis of platinum nanoparticles with high-index facets.

Bragg Coherent Diffraction Imaging for In Situ Studies in Electrocatalysis

Electrocatalysis is at the heart of a broad range of physicochemical applications that play an important role in the present and future of a sustainable economy. Among the myriad of different electrocatalysts used in this field, nanomaterials are of ubiquitous importance. An increased surface area/volume ratio compared to bulk makes nanoscale catalysts the preferred choice to perform electrocatalytic reactions. Bragg coherent diffraction imaging (BCDI) was introduced in 2006 and since has been applied to obtain 3D images of crystalline nanomaterials. BCDI provides information about the displacement field, which is directly related to strain. Lattice strain in the catalysts impacts their electronic configuration and, consequently, their binding energy with reaction intermediates. Even though there have been significant improvements since its birth, the fact that the experiments can only be performed at synchrotron facilities and its relatively low resolution to date (∼10 nm spatial resolution) have prevented the popularization of this technique. Herein, we will briefly describe the fundamentals of the technique, including the electrocatalysis relevant information that we can extract from it. Subsequently, we review some of the computational experiments that complement the BCDI data for enhanced information extraction and improved understanding of the underlying nanoscale electrocatalytic processes. We next highlight success stories of BCDI applied to different electrochemical systems and in heterogeneous catalysis to show how the technique can contribute to future studies in electrocatalysis. Finally, we outline current challenges in spatiotemporal resolution limits of BCDI and provide our perspectives on recent developments in synchrotron facilities as well as the role of machine learning and artificial intelligence in addressing them.

High-Throughput 3D Ensemble Characterization of Individual Core-Shell Nanoparticles with X-ray Free Electron Laser Single-Particle Imaging

The structures as building blocks for designing functional nanomaterials have fueled the development of versatile nanoprobes to understand local structures of noncrystalline specimens. Progress in analyzing structures of individual specimens with atomic scale accuracy has been notable recently. In most cases, however, only a limited number of specimens are inspected lacking statistics to represent the systems with structural inhomogeneity. Here, by employing single-particle imaging with X-ray free electron lasers and algorithms for multiple-model 3D imaging, we succeeded in investigating several thousand specimens in a couple of hours and identified intrinsic heterogeneities with 3D structures. Quantitative analysis has unveiled 3D morphology, facet indices, and elastic strain. The 3D elastic energy distribution is further corroborated by molecular dynamics simulations to gain mechanical insight at the atomic level. This work establishes a route to high-throughput characterization of individual specimens in large ensembles, hence overcoming statistical deficiency while providing quantitative information at the nanoscale.

On Compton scattering as a source of background in coherent diffraction imaging experiments

Compton scattering is generally neglected in diffraction experiments because the incoherent radiation it generates does not give rise to interference effects and therefore is negligible at Bragg peaks. However, as the scattering volume is reduced, the difference between the Rayleigh (coherent) and Compton (incoherent) contributions at Bragg peaks diminishes and the incoherent part may become substantial. The consequences can be significant for coherent diffraction imaging at high scattering angles: the incoherent radiation produces background that smears out the secondary interference fringes, affecting thus the achievable resolution of the technique. Here, a criterion that relates the object shape and the resolution is introduced. The Compton contribution for several object shapes is quantified, and it is shown that the maximum achievable resolution along different directions has a strong dependence on the crystal shape and size.

Three-dimensional strain dynamics govern the hysteresis in heterogeneous catalysis

Understanding catalysts strain dynamic behaviours is crucial for the development of cost-effective, efficient, stable and long-lasting catalysts. Here, we reveal in situ three-dimensional strain evolution of single gold nanocrystals during a catalytic CO oxidation reaction under operando conditions with coherent X-ray diffractive imaging. We report direct observation of anisotropic strain dynamics at the nanoscale, where identically crystallographically-oriented facets are qualitatively differently affected by strain leading to preferential active sites formation. Interestingly, the single nanoparticle elastic energy landscape, which we map with attojoule precision, depends on heating versus cooling cycles. The hysteresis observed at the single particle level is following the normal/inverse hysteresis loops of the catalytic performances. This approach opens a powerful avenue for studying, at the single particle level, catalytic nanomaterials and deactivation processes under operando conditions that will enable profound insights into nanoscale catalytic mechanisms.

Combining Laue diffraction with Bragg coherent diffraction imaging at 34-ID-C

Measurement modalities in Bragg coherent diffraction imaging (BCDI) rely on finding a signal from a single nanoscale crystal object which satisfies the Bragg condition among a large number of arbitrarily oriented nanocrystals. However, even when the signal from a single Bragg reflection with (hkl) Miller indices is found, the crystallographic axes on the retrieved three-dimensional (3D) image of the crystal remain unknown, and thus localizing in reciprocal space other Bragg reflections becomes time-consuming or requires good knowledge of the orientation of the crystal. Here, the commissioning of a movable double-bounce Si (111) monochromator at the 34-ID-C endstation of the Advanced Photon Source is reported, which aims at delivering multi-reflection BCDI as a standard tool in a single beamline instrument. The new instrument enables, through rapid switching from monochromatic to broadband (pink) beam, the use of Laue diffraction to determine crystal orientation. With a proper orientation matrix determined for the lattice, one can measure coherent diffraction patterns near multiple Bragg peaks, thus providing sufficient information to image the full strain tensor in 3D. The design, concept of operation, the developed procedures for indexing Laue patterns, and automated measuring of Bragg coherent diffraction data from multiple reflections of the same nanocrystal are discussed.

Rapid defect characterization: The efficiency of diffraction contrast-scanning transmission electron microscopy

Defect information is critical for both fundamental research and industrial analysis of metals and semiconductors. Diffraction contrast is the basis for defect imaging using either X-ray or electron microscopy. Taking the advantage of high resolution in electron microscopy techniques, here we evaluate the efficiency for diffraction contrast imaging based on scanning transmission electron microscopy. The working principle and application are demonstrated using the typical semiconductor material silicon as an example. The efficiency is improved at least an order of magnitude compared with conventional electron microscopy method.

Design of Nanomaterials for Hydrogen Storage

The interaction of hydrogen with solids and the mechanisms of hydride formation experience significant changes in nanomaterials due to a number of structural features. This review aims at illustrating the design principles that have recently inspired the development of new nanomaterials for hydrogen storage. After a general discussion about the influence of nanomaterials’ microstructure on their hydrogen sorption properties, several scientific cases and hot topics are illustrated surveying various classes of materials. These include bulk-like nanomaterials processed by mechanochemical routes, thin films and multilayers, nano-objects with composite architectures such as core–shell or composite nanoparticles, and nanoparticles on porous or graphene-like supports. Finally, selected examples of recent in situ studies of metal–hydride transformation mechanisms using microscopy and spectroscopy techniques are highlighted.

Electronic and plasmonic phenomena at nonstoichiometric grain boundaries in metallic SrNbO3

Grain boundaries could exhibit exceptional electronic structure and exotic properties, which are determined by a local atomic configuration and stoichiometry that differs from the bulk. However, optical and plasmonic properties at the grain boundaries in metallic oxides have rarely been discussed before. Here, we show that non-stoichiometric grain boundaries in the newly discovered metallic SrNbO₃ photocatalyst show exotic electronic, optical and plasmonic phenomena in comparison to bulk. Aberration-corrected scanning transmission electron microscopy and first-principles calculations reveal that a Nb-rich grain boundary exhibits an increased carrier concentration with quasi-1D metallic conductivity, and newly induced electronic states contributing to the broad energy range of optical absorption. More importantly, dielectric function calculations reveal extended and enhanced plasmonic excitations compared with bulk SrNbO₃. Our results show that non-stoichiometric grain boundaries might be utilized to control the electronic and plasmonic properties in oxide photocatalysis.

Palladium/cobalt nanowires with improved hydrogen sensing stability at ultra-low temperatures

The metallic dopants in palladium (Pd) sensing materials enable modification of the d-band center of Pd, which is expected to tune the α-β phase transitions of the PdH_x intermediate, thus improve the sensing stability to hydrogen. Here, the boosted hydrogen-sensing stability at ultra-low temperatures has been achieved with palladium/cobalt nanowires (PdCo NWs) as the sensing material. The various Co contents in PdCo NWs were modulated via AAO-template-confined electrodeposition. The temperature-dependent sensing evaluations were performed in 0.1-3 v/v% hydrogen. Such sensors integrated with PdCo NWs are able to stably detect hydrogen as low as 0.1 v/v%, even when the temperature is lowered to 273 K. In addition, the critical temperatures of "reverse sensing behavior" of the PdCo NWs (Pd₈₂Co₁₈: T_c = 194 K; Pd₆₃Co₃₇: T_c = 180 K; Pd₃₃Co₆₇: T_c = 184 K) are observed much lower than that of pristine Pd NWs (T_c = 287 K). Specifically, the Pd₆₃Co₃₇ NWs (∼37 at% Co content) sensor shows outstanding stability of sensing hydrogen against α-β phase transitions within the wide temperature range of 180-388 K, which is attributed to both the electronic interactions between Pd and Co and the lattice compression strain caused by Co dopants. Moreover, the "reverse sensing behavior" of the PdCo NWs is explicitly interpreted using the α-β phase transition model.

Defect Dynamics at a Single Pt Nanoparticle during Catalytic Oxidation

Defects can affect all aspects of a material by altering its electronic properties and controlling its chemical reactivity. At defect sites, preferential adsorption of reactants and/or formation of chemical species at active sites are observed in heterogeneous catalysis. Understanding the structural response at defect sites during catalytic reactions provides a unique opportunity to exploit defect control of nanoparticle-based catalysts. However, it remains difficult to characterize the strain and defect evolution for a single nanocrystal catalyst in situ. Here, we report Bragg coherent X-ray diffraction imaging of defect dynamics in an individual Pt nanoparticle during catalytic methane oxidation. We observed that the initially tensile strained regions of the crystal became seed points for the development of further strain and subsequent disappearance of diffraction density during oxidation reactions. Our detailed understanding of the catalytically induced deformation at the defect sites and observed reversibility during the relevant steps of the catalytic oxidation process provide important insights of defect control and engineering of heterogeneous catalysts.

CMOS-compatible plasmonic hydrogen sensors with a detection limit of 40 ppm

Sensing of leakage at an early stage is crucial for the safe utilization of hydrogen. Optical hydrogen sensors eliminate the potential hazard of ignition caused by electrical sparks but achieve a detection limit far higher than their electrical counterparts so far. To essentially improve the performance of optical hydrogen sensors in terms of detection limit, we demonstrate in this work a plasmonic hydrogen sensor based on aluminum-palladium (Al-Pd) hybrid nanorods. Arranged into high-density regular arrays, the hybrid nanorods are capable of sensing hydrogen at a concentration down to 40 ppm, i.e., one thousandth of the lower flammability limit of hydrogen in air. Different sensing behaviors are found for two sensor configurations, where Pd-Al nanorods provide larger spectral shift and Al-Pd ones exhibit shorter response time. In addition, the plasmonic hydrogen sensors here utilize exclusively CMOS-compatible materials, holding the potential for real-world, large-scale applications.

Rationally Designed PdAuCu Ternary Alloy Nanoparticles for Intrinsically Deactivation-Resistant Ultrafast Plasmonic Hydrogen Sensing

Hydrogen sensors are a prerequisite for the implementation of a hydrogen economy due to the high flammability of hydrogen-air mixtures. They are to comply with the increasingly stringent requirements set by stakeholders, such as the automotive industry and manufacturers of hydrogen safety systems, where sensor deactivation is a severe but widely unaddressed problem. In response, we report intrinsically deactivation-resistant nanoplasmonic hydrogen sensors enabled by a rationally designed ternary PdAuCu alloy nanomaterial, which combines the identified best intrinsic attributes of the constituent binary Pd alloys. This way, we achieve extraordinary hydrogen sensing metrics in synthetic air and poisoning gas background, simulating real application conditions. Specifically, we find a detection limit in the low ppm range, hysteresis-free response over 5 orders of magnitude hydrogen pressure, subsecond response time at room temperature, long-term stability, and, as the key, excellent resistance to deactivating species like carbon monoxide, notably without application of any protective coatings. This constitutes an important step forward for optical hydrogen sensor technology, as it enables application under demanding conditions and provides a blueprint for further material and performance optimization by combining and concerting intrinsic material assets in multicomponent nanoparticles. In a wider context, our findings highlight the potential of rational materials design through alloying of multiple elements for gas sensor development, as well as the potential of engineered metal alloy nanoparticles in nanoplasmonics and catalysis.

Facets and vertices regulate hydrogen uptake and release in palladium nanocrystals

Crystal facets, vertices and edges govern the energy landscape of metal surfaces and thus the chemical interactions on the surface^1,2. The facile absorption and desorption of hydrogen at a palladium surface provides a useful platform for defining how metal-solute interactions impact properties relevant to energy storage, catalysis and sensing^3-5. Recent advances in in operando and in situ techniques have enabled the phase transitions of single palladium nanocrystals to be temporally and spatially tracked during hydrogen absorption^6-11. We demonstrate herein that in situ X-ray diffraction can be used to track both hydrogen absorption and desorption in palladium nanocrystals. This ensemble measurement enabled us to delineate distinctive absorption and desorption mechanisms for nanocrystals containing exclusively (111) or (100) facets. We show that the rate of hydrogen absorption is higher for those nanocrystals containing a higher number of vertices, consistent with hydrogen absorption occurring quickly after β-phase nucleation at lattice-strained vertices^9,10. Tracking hydrogen desorption revealed initial desorption rates to be nearly tenfold faster for samples with (100) facets, presumably due to the faster recombination of surface hydrogen atoms. These results inspired us to make nanocrystals with a high number of vertices and (100) facets, which were found to accommodate fast hydrogen uptake and release.

Oxidation induced strain and defects in magnetite crystals

Oxidation of magnetite (Fe₃O₄) has broad implications in geochemistry, environmental science and materials science. Spatially resolving strain fields and defect evolution during oxidation of magnetite provides further insight into its reaction mechanisms. Here we show that the morphology and internal strain distributions within individual nano-sized (~400 nm) magnetite crystals can be visualized using Bragg coherent diffractive imaging (BCDI). Oxidative dissolution in acidic solutions leads to increases in the magnitude and heterogeneity of internal strains. This heterogeneous strain likely results from lattice distortion caused by Fe(II) diffusion that leads to the observed domains of increasing compressive and tensile strains. In contrast, strain evolution is less pronounced during magnetite oxidation at elevated temperature in air. These results demonstrate that oxidative dissolution of magnetite can induce a rich array of strain and defect structures, which could be an important factor that contributes to the high reactivity observed on magnetite particles in aqueous environment.

Mapping of individual dislocations with dark-field X-ray microscopy

This article presents an X-ray microscopy approach for mapping deeply embedded dislocations in three dimensions using a monochromatic beam with a low divergence. Magnified images are acquired by inserting an X-ray objective lens in the diffracted beam. The strain fields close to the core of dislocations give rise to scattering at angles where weak beam conditions are obtained. Analytical expressions are derived for the image contrast. While the use of the objective implies an integration over two directions in reciprocal space, scanning an aperture in the back focal plane of the microscope allows a reciprocal-space resolution of ΔQ/Q < 5 × 10−5 in all directions, ultimately enabling high-precision mapping of lattice strain and tilt. The approach is demonstrated on three types of samples: a multi-scale study of a large diamond crystal in transmission, magnified section topography on a 140 µm-thick SrTiO3 sample and a reflection study of misfit dislocations in a 120 nm-thick BiFeO3 film epitaxially grown on a thick substrate. With optimal contrast, the half-widths at half-maximum of the dislocation lines are 200 nm.

Nanostructured Metal Hydrides for Hydrogen Storage

Knowledge and foundational understanding of phenomena associated with the behavior of materials at the nanoscale is one of the key scientific challenges toward a sustainable energy future. Size reduction from bulk to the nanoscale leads to a variety of exciting and anomalous phenomena due to enhanced surface-to-volume ratio, reduced transport length, and tunable nanointerfaces. Nanostructured metal hydrides are an important class of materials with significant potential for energy storage applications. Hydrogen storage in nanoscale metal hydrides has been recognized as a potentially transformative technology, and the field is now growing steadily due to the ability to tune the material properties more independently and drastically compared to those of their bulk counterparts. The numerous advantages of nanostructured metal hydrides compared to bulk include improved reversibility, altered heats of hydrogen absorption/desorption, nanointerfacial reaction pathways with faster rates, and new surface states capable of activating chemical bonds. This review aims to summarize the progress to date in the area of nanostructured metal hydrides and intends to understand and explain the underpinnings of the innovative concepts and strategies developed over the past decade to tune the thermodynamics and kinetics of hydrogen storage reactions. These recent achievements have the potential to propel further the prospects of tuning the hydride properties at nanoscale, with several promising directions and strategies that could lead to the next generation of solid-state materials for hydrogen storage applications.

Nucleation-Controlled Plasticity of Metallic Nanowires and Nanoparticles

Nanowires and nanoparticles are envisioned as important elements of future technology and devices, owing to their unique mechanical properties. Metallic nanowires and nanoparticles demonstrate outstanding size-dependent strength since their deformation is dislocation nucleation-controlled. In this context, the recent experimental and computational studies of nucleation-controlled plasticity are reviewed. The underlying microstructural mechanisms that govern the strength of nanowires and the origin of their stochastic nature are also discussed. Nanoparticles, in which the stress state under compression is nonuniform, exhibit a shape-dependent strength. Perspectives on improved methods to study nucleation-controlled plasticity are discussed, as well the insights gained for microstructural-based design of mechanical properties at the nanoscale.

Visualizing Facet-Dependent Hydrogenation Dynamics in Individual Palladium Nanoparticles

Surface faceting in nanoparticles can profoundly impact the rate and selectivity of chemical transformations. However, the precise role of surface termination can be challenging to elucidate because many measurements are performed on ensembles of particles and do not have sufficient spatial resolution to observe reactions at the single and subparticle level. Here, we investigate solute intercalation in individual palladium hydride nanoparticles with distinct surface terminations. Using a combination of diffraction, electron energy loss spectroscopy, and dark-field contrast in an environmental transmission electron microscope (TEM), we compare the thermodynamics and directly visualize the kinetics of 40-70 nm {100}-terminated cubes and {111}-terminated octahedra with approximately 2 nm spatial resolution. Despite their distinct surface terminations, both particle morphologies nucleate the new phase at the tips of the particle. However, whereas the hydrogenated phase-front must rotate from [111] to [100] to propagate in cubes, the phase-front can propagate along the [100], [11̅0], and [111] directions in octahedra. Once the phase-front is established, the interface propagates linearly with time and is rate-limited by surface-to-subsurface diffusion and/or the atomic rearrangements needed to accommodate lattice strain. Following nucleation, both particle morphologies take approximately the same time to reach equilibrium, hydrogenating at similar pressures and without equilibrium phase coexistence. Our results highlight the importance of low-coordination number sites and strain, more so than surface faceting, in governing solute-driven reactions.

Angle calculations using two detector circles and wavelength

A `2D + λ' mode using two detector angles and wavelength is added to the modes of operation of a six-circle diffractometer, i.e. `4S + 2D' diffractometer [You (1999). J. Appl. Cryst. 32, 614–623]. In synchrotron sources, the X-ray energy, and hence the wavelength, can be a degree of freedom for angle calculations. For diffractometers with two detector circles, therefore, the variable X-ray energy can be the third degree of freedom, sufficient for accessing reciprocal space without rotating the sample. This mode is useful for limited sample rotations or X-ray beams focused smaller than the diffractometer circle of confusion.

In-situ visualization of solute-driven phase coexistence within individual nanorods

Nanorods are promising components of energy and information storage devices that rely on solute-driven phase transformations, due to their large surface-to-volume ratio and ability to accommodate strain. Here we investigate the hydrogen-induced phase transition in individual penta-twinned palladium nanorods of varying aspect ratios with ~3 nm spatial resolution to understand the correlation between nanorod structure and thermodynamics. We find that the hydrogenated phase preferentially nucleates at the rod tips, progressing along the length of the nanorods with increasing hydrogen pressure. While nucleation pressure is nearly constant for all lengths, the number of phase boundaries is length-dependent, with stable phase coexistence always occurring for rods longer than 55 nm. Moreover, such coexistence occurs within individual crystallites of the nanorods and is accompanied by defect formation, as supported by in situ electron microscopy and elastic energy calculations. These results highlight the effect of particle shape and dimension on thermodynamics, informing nanorod design for improved device cyclability.

Compared to thin films and other geometries, nanorods can exhibit particularly high performance in solute-intercalation-based energy and information storage devices. Here, the authors use in situ electron microscopy and spectroscopy to study the hydrogenation of palladium nanorods, revealing relationships between nanorod structure and device cyclability and capacity.

Dynamic diffraction artefacts in Bragg coherent diffractive imaging

The article presents a theoretical study on dynamic artefacts in the reconstruction of Bragg coherent diffractive imaging using an iterative phase retrieval algorithm.

This article reports a theoretical study on the reconstruction artefacts in Bragg coherent diffractive imaging caused by dynamical diffraction effects. It is shown that, unlike the absorption and refraction effects that can be corrected after reconstruction, dynamical diffraction effects have profound impacts on both the amplitude and the phase of the reconstructed complex object, causing strong artefacts. At the dynamical diffraction limit, the reconstructed shape is no longer correct, as a result of the strong extinction effect. Simulations for hemispherical particles of different sizes show the type, magnitude and extent of the dynamical diffraction artefacts, as well as the conditions under which they are negligible.

Imaging the Hydrogen Absorption Dynamics of Individual Grains in Polycrystalline Palladium Thin Films in 3D

Defects such as dislocations and grain boundaries often control the properties of polycrystalline materials. In nanocrystalline materials, investigating this structure-function relationship while preserving the sample remains challenging because of the short length scales and buried interfaces involved. Here we use Bragg coherent diffractive imaging to investigate the role of structural inhomogeneity on the hydriding phase transformation dynamics of individual Pd grains in polycrystalline films in three-dimensional detail. In contrast to previous reports on single- and polycrystalline nanoparticles, we observe no evidence of a hydrogen-rich surface layer and consequently no size dependence in the hydriding phase transformation pressure over a 125-325 nm size range. We do observe interesting grain boundary dynamics, including reversible rotations of grain lattices while the material remains in the hydrogen-poor phase. The mobility of the grain boundaries, combined with the lack of a hydrogen-rich surface layer, suggests that the grain boundaries are acting as fast diffusion sites for the hydrogen atoms. Such hydrogen-enhanced plasticity in the hydrogen-poor phase provides insight into the switch from the size-dependent behavior of single-crystal nanoparticles to the lower transformation pressures of polycrystalline materials and may play a role in hydrogen embrittlement.

The self-healing of defects induced by the hydriding phase transformation in palladium nanoparticles

Nanosizing can dramatically alter material properties by enhancing surface thermodynamic contributions, shortening diffusion lengths, and increasing the number of catalytically active sites per unit volume. These mechanisms have been used to explain the improved properties of catalysts, battery materials, plasmonic materials, etc. Here we show that Pd nanoparticles also have the ability to self-heal defects in their crystal structures. Using Bragg coherent diffractive imaging, we image dislocations nucleated deep in a Pd nanoparticle during the forward hydriding phase transformation that heal during the reverse transformation, despite the region surrounding the dislocations remaining in the hydrogen-poor phase. We show that defective Pd nanoparticles exhibit sloped isotherms, indicating that defects act as additional barriers to the phase transformation. Our results resolve the formation and healing of structural defects during phase transformations at the single nanoparticle level and offer an additional perspective as to how and why nanoparticles differ from their bulk counterparts.

Nanoscale materials commonly have improved properties over their bulk counterparts. Here, the authors use Bragg coherent diffractive imaging to reveal that Pd nanoparticles can self-heal crystallographic defects induced during the hydriding phase transformation, making them more resistant to strain-induced damage.

3D Imaging of a Dislocation Loop at the Onset of Plasticity in an Indented Nanocrystal

Structural quality and stability of nanocrystals are fundamental problems that bear important consequences for the performances of small-scale devices. Indeed, at the nanoscale, their functional properties are largely influenced by elastic strain and depend critically on the presence of crystal defects. It is thus of prime importance to be able to monitor, by noninvasive means, the stability of the microstructure of nano-objects against external stimuli such as mechanical load. Here we demonstrate the potential of Bragg coherent diffraction imaging for such measurements, by imaging in 3D the evolution of the microstructure of a nanocrystal exposed to in situ mechanical loading. Not only could we observe the evolution of the internal strain field after successive loadings, but we also evidenced a transient microstructure hosting a stable dislocation loop. The latter is fully characterized from its characteristic displacement field. The mechanical behavior of this small crystal is clearly at odds with what happens in bulk materials where many dislocations interact. Moreover, this original in situ experiment opens interesting possibilities for the investigation of plastic deformation at the nanoscale.

Grain boundary mediated hydriding phase transformations in individual polycrystalline metal nanoparticles

Grain boundaries separate crystallites in solids and influence material properties, as widely documented for bulk materials. In nanomaterials, however, investigations of grain boundaries are very challenging and just beginning. Here, we report the systematic mapping of the role of grain boundaries in the hydrogenation phase transformation in individual Pd nanoparticles. Employing multichannel single-particle plasmonic nanospectroscopy, we observe large variation in particle-specific hydride-formation pressure, which is absent in hydride decomposition. Transmission Kikuchi diffraction suggests direct correlation between length and type of grain boundaries and hydride-formation pressure. This correlation is consistent with tensile lattice strain induced by hydrogen localized near grain boundaries as the dominant factor controlling the phase transition during hydrogen absorption. In contrast, such correlation is absent for hydride decomposition, suggesting a different phase-transition pathway. In a wider context, our experimental setup represents a powerful platform to unravel microstructure-function correlations at the individual-nanoparticle level.

Identifying Defects with Guided Algorithms in Bragg Coherent Diffractive Imaging

Crystallographic defects such as dislocations can significantly alter material properties and functionality. However, imaging these imperfections during operation remains challenging due to the short length scales involved and the reactive environments of interest. Bragg coherent diffractive imaging (BCDI) has emerged as a powerful tool capable of identifying dislocations, twin domains, and other defects in 3D detail with nanometer spatial resolution within nanocrystals and grains in reactive environments. However, BCDI relies on phase retrieval algorithms that can fail to accurately reconstruct the defect network. Here, we use numerical simulations to explore different guided phase retrieval algorithms for imaging defective crystals using BCDI. We explore different defect types, defect densities, Bragg peaks, and guided algorithm fitness metrics as a function of signal-to-noise ratio. Based on these results, we offer a general prescription for phasing of defective crystals with no a priori knowledge.

Bragg Coherent Diffractive Imaging of Zinc Oxide Acoustic Phonons at Picosecond Timescales

Mesoscale thermal transport is of fundamental interest and practical importance in materials such as thermoelectrics. Coherent lattice vibrations (acoustic phonons) govern thermal transport in crystalline solids and are affected by the shape, size, and defect density in nanoscale materials. The advent of hard x-ray free electron lasers (XFELs) capable of producing ultrafast x-ray pulses has significantly impacted the understanding of acoustic phonons by enabling their direct study with x-rays. However, previous studies have reported ensemble-averaged results that cannot distinguish the impact of mesoscale heterogeneity on the phonon dynamics. Here we use Bragg coherent diffractive imaging (BCDI) to resolve the 4D evolution of the acoustic phonons in a single zinc oxide rod with a spatial resolution of 50 nm and a temporal resolution of 25 picoseconds. We observe homogeneous (lattice breathing/rotation) and inhomogeneous (shear) acoustic phonon modes, which are compared to finite element simulations. We investigate the possibility of changing phonon dynamics by altering the crystal through acid etching. We find that the acid heterogeneously dissolves the crystal volume, which will significantly impact the phonon dynamics. In general, our results represent the first step towards understanding the effect of structural properties at the individual crystal level on phonon dynamics.