CO2 methanation offers a pathway to produce a carbon-neutral methane fuel. Although a number of research efforts have been conducted on this topic, a greater understanding of the mechanism of the reaction, which is still under debate, is needed. Here, using in situ transmission electron microscopy, we provide direct insights into the dynamics of a metallic nickel catalyst supported on activated carbon during CO2 methanation. The keys to the high performance of the catalyst are the in situ formation and dynamic behavior of a Ni@NiO core@shell nanostructure. Based on the detailed electron microscopy investigation, the mechanism of such nanostructure formation during methanation is proposed. Our studies revealed that the deactivation of the catalyst is not due to the accumulation of carbon coke over nickel nanoparticles, but an increase in the size of the nickel nanoparticles that is responsible for the deactivation of the catalyst over time.Keywords: CO2 valorization; Hydrogenation; in situ TEM; Carbon catalysts; Core-shell nanoparticles; Microstructure.

Quantitative interpretation of transmission electron microscopy (TEM) data of crystalline specimens often requires the accurate knowledge of the local crystal orientation. A method is presented which exploits momentum-resolved scanning TEM (STEM) data to determine the local mistilt from a major zone axis. It is based on a geometric analysis of Kikuchi bands within a single diffraction pattern, yielding the center of the Laue circle. Whereas the approach is not limited to convergent illumination, it is here developed using unit-cell averaged diffraction patterns corresponding to high-resolution STEM settings. In simulation studies, an accuracy of approximately 0.1 mrad is found. The method is implemented in automated software and applied to crystallographic tilt and in-plane rotation mapping in two experimental cases. In particular, orientation maps of high-Mn steel and an epitaxially grown La0.7Sr0.3MnO3-SrTiO3 interface are presented. The results confirm the estimates of the simulation study and indicate that tilt mapping can be performed consistently over a wide field of view with diameters well above 100 nm at unit cell real space sampling.

Octahedral-shaped PtNi-alloy nanoparticlesare highlyactive oxygenreduction reaction catalysts for the cathode in proton exchange membranefuel cells. However, one major drawback in their application is theirlimited long-term morphological and compositional stability. Here,we present a detailed in situ electron microscopy characterizationof thermal annealing on octahedral-shaped PtNi catalysts as well ason doped octahedral PtNi-(Mo) and PtNi-(MoRh) catalysts. The evolutionof their morphology and composition was quantified during both exsitu and in situ experiments using energy dispersive X-ray spectroscopyin a scanning transmission electron microscope under a hydrogen atmosphereand in vacuum. Morphological changes upon heating, i.e., a gradualloss of the octahedral shape and a continuous rounding of the particles,were observed, as well as evidence for increased alloying. Furthermore,the evolution of the shape of the PtNi-(Mo) nanoparticles was quantifiedusing in situ experiments under hydrogen atmosphere in a transmissionelectron microscope. The shape change of the particles was quantifiedusing segmentation maps created by a neural network. It has been demonstratedthat morphological changes crucially depend on the composition andsurface doping: doping with Mo or Mo/Rh significantly stabilizes thestructure, allowing for persistence of a truncated octahedral shapeduring heat treatments.

Recently proposed bimetallic octahedral Pt-Ni electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes suffer from particle instabilities in the form of Ni corrosion and shape degradation. Advanced trimetallic Pt-based electrocatalysts have contributed to their catalytic performance and stability. In this work, we propose and analyse a novel quaternary octahedral (oh-)Pt nanoalloy concept with two distinct metals serving as stabilizing surface dopants. An efficient solvothermal one-pot strategy was developed for the preparation of shape-controlled oh-PtNi catalysts doped with Rh and Mo in its surface. The as-prepared quaternary octahedral PtNi(RhMo) catalysts showed exceptionally high ORR performance accompanied by improved activity and shape integrity after stability tests compared to previously reported bi- and tri-metallic systems. Synthesis, performance characteristics and degradation behaviour are investigated targeting deeper understanding for catalyst system improvement strategies. A number of different operando and on-line analysis techniques were employed to monitor the structural and elemental evolution, including identical location scanning transmission electron microscopy and energy dispersive X-ray analysis (IL-STEM-EDX), operando wide angle X-ray spectroscopy (WAXS), and on-line scanning flow cell inductively coupled plasma mass spectrometry (SFC-ICP-MS). Our studies show that doping PtNi octahedral catalysts with small amounts of Rh and Mo suppresses detrimental Pt diffusion and thus offers an attractive new family of shaped Pt alloy catalysts for deployment in PEMFC cathode layers.

Over the past decade, advances in the colloidal syntheses of octahedral-shaped Pt-Ni alloy nanocatalysts for use in fuel cell cathodes have raised our atomic-scale control of particle morphology and surface composition, which, in turn, helped raise their catalytic activity far above that of benchmark Pt catalysts. Future fuel cell deployment in heavy-duty vehicles caused the scientific priorities to shift from alloy particle activity to stability. Larger particles generally offer enhanced thermodynamic stability, yet synthetic approaches toward larger octahedral Pt-Ni alloy nanoparticles have remained elusive. In this study, we show how a simple manipulation of solvothermal synthesis reaction kinetics involving depressurization of the gas phase at different stages of the reaction allows tuning the size of the resulting octahedral nanocatalysts to previously unachieved scales. We then link the underlying mechanism of our approach to the classical "LaMer" model of nucleation and growth. We focus on large, annealed Mo-doped Pt-Ni octahedra and investigate their synthesis, post-synthesis treatments, and elemental distribution using advanced electron microscopy. We evaluate the electrocatalytic ORR performance and stability and succeed to obtain a deeper understanding of the enhanced stability of a new class of relatively large, active, and long-lived Mo-doped Pt-Ni octahedral catalysts for the cathode of PEMFCs.

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A variety of synthesis protocols for octahedral PtNi nanocatalysts have led to remarkable improvements in platinum mass and specific activities for the oxygen reduction reaction. Nevertheless, the values achieved are still only one tenth of the activity measured from Pt3Ni single-crystal (111) surfaces. These particles lose activity during potential cycling, primarily because of Ni leaching and subsequent loss of shape. Here, we present the syntheses and high catalytic oxygen reduction reaction activities of molybdenum-doped PtNi octahedral catalysts with different sizes (6-14 nm) and compositions. We show that the Mo-doped, Ni-rich, PtNi octahedral catalysts exhibit enhanced stability over their undoped counterpart. Scanning transmission electron microscopy with energy-dispersive X-ray analysis reveals the particular elemental distribution for the size and composition of the different catalysts. By combining high-resolution compositional analysis with electrochemical measurements and online inductively coupled plasma mass spectrometry, it was possible to correlate the size, morphology, and composition with the oxygen reduction reaction activities before and after accelerated stress tests. The octahedral catalysts show high electrochemical surface areas and increasing specific activity with increasing surface area of the (111) facets and Ni content, leading to high mass activities. These results demonstrate the advantages of increasing the (111) surface area and Ni content of PtNi nano-octahedral catalysts to improve the performance and stability for the oxygen reduction reaction.

The invention of silicon drift detectors has resulted in an unprecedented improvement in detection efficiency for energy-dispersive X-ray (EDX) spectroscopy in the scanning transmission electron microscope. The result is numerous beautiful atomic-scale maps, which provide insights into the internal structure of a variety of materials. However, the task still remains to understand exactly where the X-ray signal comes from and how accurately it can be quantified. Unfortunately, when crystals are aligned with a low-order zone axis parallel to the incident beam direction, as is necessary for atomic-resolution imaging, the electron beam channels. When the beam becomes localized in this way, the relationship between the concentration of a particular element and its spectroscopic X-ray signal is generally nonlinear. Here, we discuss the combined effect of both spatial integration and sample tilt for ameliorating the effects of channeling and improving the accuracy of EDX quantification. Both simulations and experimental results will be presented for a perovskite-based oxide interface. We examine how the scattering and spreading of the electron beam can lead to erroneous interpretation of interface compositions, and what approaches can be made to improve our understanding of the underlying atomic structure.

Computer simulations are used to assess the influence of a 20-nm-thick SiNx membrane on the quantification of atomic-resolution annular dark-field (ADF) scanning transmission electron microscopy images of Pt nanoparticles. The discussions include the effect of different nanoparticle/membrane arrangements, accelerating voltage, nanoparticle thickness and the presence of adjacent atomic columns on the accuracy with which the number of Pt atoms in each atom column can be counted. The results, which are based on the use of ADF scattering cross-sections, show that an accuracy of better than a single atom is attainable at 200 and 300 kV. At 80kV, the scattering in a typical SiNx membrane is sufficiently strong that the best possible atom counting accuracy is reduced to +/- 2 atoms. The implications of the work for quantitative studies of Pt nanoparticles imaged through SiNx membranes are discussed.

The combined development of silicon drift detectors (SDDs) and aberration correction in the scanning transmission electron microscope (STEM) has a significantly advanced the field of microanalysis [1,2]. In particular, the combination of improved detector efficiency and high current densities really opens up the possibilities of quantitative energy dispersive X-ray (EDX) spectroscopy. We can now routinely achieve beautiful EDX maps which reveal atomic-scale features within our materials. However, it still remains to understand exactly where the X-ray signals come from within the specimen and how accurately they can be quantified at these length scales.

In recent years, two branches of electron microscopy have seen major progresses in instrumentation. The introduction of fast cameras to scanning transmission electron microscopy (STEM) has given rise to momentum-resolved STEM where full diffraction patterns are recorded for each scan position. Real and reciprocal space information is thus simultaneously available in a four-dimensional dataset (4D STEM). In energy dispersive X-ray (EDX) spectroscopy, there is a combined improvement from silicon drift detectors, increasing detection efficiency, and aberration correction, increasing the available current density. The resulting higher counts in EDX data allows for atomic-scale maps on beam resilient samples.

Electron tomography has been widely applied to three-dimensional (3D) morphology characterization and chemical analysis at the nanoscale. A HAADF-EDS bimodal tomographic (HEBT) reconstruction technique has been developed to extract high resolution element-specific information. However, the reconstructed elemental maps cannot be directly converted to quantitative compositional information. In this work, we propose a quantification approach for obtaining elemental weight fraction maps from the HEBT reconstruction technique using the physical parameters extracted from a Monte Carlo code, MC X-ray. A similar quantification approach is proposed for the EDS-STEM tomographic reconstruction. The performance of the two quantitative reconstruction methods, using the simultaneous iterative reconstruction technique, are evaluated and compared for a simulated dataset of a two-dimensional phantom sample. The effects of the reconstruction parameters including the number of iterations and the weight of the HAADF signal are discussed. Finally, the two approaches are applied to an experimental dataset to show the 3D structure and quantitative elemental maps of a particle of flux melted metal organic framework glass.

A recently developed emulsion-templated assembly method promises the scalable, low-cost, and reproducible fabrication of hierarchical nanocrystal (NC) superstructures. These superstructures derive properties from the unique combination of choices of NC building blocks and superstructure morphology and therefore realize the concept of "artificial solids". To control the final properties of these superstructures, it is essential to control the assembly conditions that yield distinct architectural morphologies. Here, we explore the phase-space of experimental parameters describing the emulsion-templated assembly including temperature, interfacial tension, and NC polydispersity and demonstrate which conditions lead to the growth of the most crystalline NC superstructures or supercrystals. By using a combination of electron microscopy and small-angle X-ray scattering, we show that slower assembly kinetics, softer interfaces, and lower NC polydispersity contribute to the formation of supercrystals with grain sizes up to 600 nm, while reversing these trends yields glassy solids. These results provide a clear path to the realization of higher-quality supercrystals, necessary to many applications.

Semiconductor nanocrystals, or quantum dots (QDs), simultaneously benefit from inexpensive low-temperature solution processing and exciting photophysics, making them the ideal candidates for next-generation solar cells and photodetectors. While the working principles of these devices rely on light absorption, QDs intrinsically belong to the Rayleigh regime and display optical behavior limited to electric dipole resonances, resulting in low absorption efficiencies. Increasing the absorption efficiency of QDs, together with their electronic and excitonic coupling to enhance charge carrier mobility, is therefore of critical importance to enable practical applications. Here, we demonstrate a general and scalable approach to increase both light absorption and excitonic coupling of QDs by fabricating hierarchical metamaterials. We assemble QDs into crystalline supraparticles using an emulsion template and demonstrate that these colloidal supercrystals (SCs) exhibit extended resonant optical behavior resulting in an enhancement in absorption efficiency in the visible range of more than 2 orders of magnitude with respect to the case of dispersed QDs. This successful light trapping strategy is complemented by the enhanced excitonic coupling observed in ligand-exchanged SCs, experimentally demonstrated through ultrafast transient absorption spectroscopy and leading to the formation of a free biexciton system on sub-picosecond time scales. These results introduce a colloidal metamaterial designed by self-assembly from the bottom up, simultaneously featuring a combination of nanoscale and mesoscale properties leading to simultaneous photonic and excitonic coupling, therefore presenting the nanocrystal analogue of supramolecular structures.

Journal Article Pushing the Limits of Absolute Scale Energy Dispersive X-ray Quantification Get access Katherine E MacArthur, Katherine E MacArthur The Ernst Ruska Centre for Microscopy and Spectroscopy with Electrons and the Peter Grünberg Institute Forschungszentrum Jülich, Jülich, Germany Corresponding author: k.macarthur@fz-juelich.de Search for other works by this author on: Oxford Academic Google Scholar Sean M Collins, Sean M Collins Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, United Kingdom Search for other works by this author on: Oxford Academic Google Scholar Hamish G Brown, Hamish G Brown National Centre for Electron Microscopy, Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CaliforniaSchool of Physics and Astronomy, Monash University, Clayton, Australia Search for other works by this author on: Oxford Academic Google Scholar Scott D Findlay, Scott D Findlay School of Physics and Astronomy, Monash University, Clayton, Australia Search for other works by this author on: Oxford Academic Google Scholar Leslie J Allen Leslie J Allen School of Physics, University of Melbourne, Parkville, Australia Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 25, Issue S2, 1 August 2019, Pages 968–969, https://doi.org/10.1017/S1431927619005579 Published: 01 August 2019