Atomic Scale View Research Articles

Atomic-Scale View of Water Chemistry on Nanostructured Iron Oxide Films.

The interaction of water with solid surfaces is crucial for a wide range of disciplines, including catalysis, environmental science, corrosion, geology, and biology. In this study, we present a combined experimental and theoretical investigation that elucidates the interaction of water with a model iron oxide surface under near ambient conditions (i.e., room temperature and water vapor in the mbar range). Our findings reveal that surface hydroxylation can be controlled at the nanoscale by the local properties of the oxide film, such as local rumpling and electrostatic potential. The iron oxide surface presents alternating hydrophilic and hydrophobic domains, creating after water exposure a hexagonal pattern with a pitch of approximately 3 nm, where the highly hydroxylated regions act as nucleation centers for nanoconfined water molecule clusters.

Atomic-scale view of the photoinduced structural transition to form sp3-like bonded order phase in graphite

Photoexcitation of solids often induces structural phase transitions between different ordered phases, some of which are unprecedented and thermodynamically inaccessible. The phenomenon, known as photoinduced structural phase transition (PSPT), is of significant interest to the technological progress of advanced materials processing and the fundamental understanding of material physics. Here, we applied scanning tunnelling microscopy (STM) to directly characterise the primary processes of the PSPT in graphite to form a sp3-like carbon nano-phase called diaphite. The primary challenge was to provide microscopic views of the graphite-to-diaphite transition. On an atomic scale, STM imaging of the photoexcited surface revealed the nucleation and proliferation processes of the diaphite phase; these were governed by the formation of sp3-like interlayer bonds. The growth mode of the diaphite phase depends strongly on the photon energy of excitation laser light. Different dynamical pathways were proposed to explain the formation of a sp3-like interlayer bonding. Potential mechanisms for photon-energy-dependent growth were examined based on the experimental and calculated results. The present results provide insight towards realising optical control of sp2-to-sp3 conversions and the organisation of nanoscale structures in graphene-related materials.

Atomic-scale friction of MoS2/Fe and MoO3/Fe interfaces: A first-principles investigation

Although it is a common phenomenon that MoS2 is oxidized during the service process, the role of its oxidation product MoO3 in the friction interface is rarely reported. Here, the adhesive work, sliding potential energy, and friction force of MoS2(001)/Fe(110) and MoO3(001)/Fe(110) interfaces were calculated via first-principles method. Furthermore, the relationship between charge density difference, potential barrier, and friction force was analyzed. The results show that the interfacial sliding potential barrier and friction force increased when MoS2 was oxidized to MoO3, but the adhesion with Fe substrate improved. The greater charge density difference may be responsible for the bigger friction force and adhesion between MoO3(001)/Fe(110) interface, which provides an explanation for the oxidation effect on tribological behaviors of MoS2 from an atomic-scale view.

Exfoliated 2D Layered and Nonlayered Metal Phosphorous Trichalcogenides Nanosheets as Promising Electrocatalysts for CO2 Reduction.

Two-dimensional (2D) materials catalysts provide an atomic-scale view on a fascinating arena for understanding the mechanism of electrocatalytic carbon dioxide reduction (CO2 ECR). Here, we successfully exfoliated both layered and nonlayered ultra-thin metal phosphorous trichalcogenides (MPCh3 ) nanosheets via wet grinding exfoliation (WGE), and systematically investigated the mechanism of MPCh3 as catalysts for CO2 ECR. Unlike the layered CoPS3 and NiPS3 nanosheets, the active Sn atoms tend to be exposed on the surfaces of nonlayered SnPS3 nanosheets. Correspondingly, the nonlayered SnPS3 nanosheets exhibit clearly improved catalytic activity, showing formic acid selectivity up to 31.6 % with -7.51 mA cm-2 at -0.65 V vs. RHE. The enhanced catalytic performance can be attributed to the formation of HCOO* via the first proton-electron pair addition on the SnPS3 surface. These results provide a new avenue to understand the novel CO2 ECR mechanism of Sn-based and MPCh3 -based catalysts.

Elastic Strain Relaxation of Phase Boundary of α' Nanoscale Phase Mediated via the Point Defects Loop under Normal Strain.

Irradiation-induced point defects and applied stress affect the concentration distribution and morphology evolution of the nanophase in Fe-Cr based alloys; the aggregation of point defects and the nanoscale precipitates can intensify the hardness and embrittlement of the alloy. The influence of normal strain on the coevolution of point defects and the Cr-enriched α' nanophase are studied in Fe-35 at.% Cr alloy by utilizing the multi-phase-field simulation. The clustering of point defects and the splitting of nanoscale particles are clearly presented under normal strain. The defects loop formed at the α/α' phase interface relaxes the coherent strain between the α/α' phases, reducing the elongation of the Cr-enriched α' phase under the normal strains. Furthermore, the point defects enhance the concentration clustering of the α' phase, and this is more obvious under the compressive strain at high temperature. The larger normal strain can induce the splitting of an α' nanoparticle with the nonequilibrium concentration in the early precipitation stage. The clustering and migration of point defects provide the diffusion channels of Cr atoms to accelerate the phase separation. The interaction of point defect with the solution atom clusters under normal strain provides an atomic scale view on the microstructure evolution under external stress.

Precious metals can be green

In this chapter, a general view of metallic alloys nanoparticles from the atomic-scale view using advanced transmission electron microscopy techniques such as aberration-corrected electron microscopy (Cs-corrected) and high-resolution electron microscopy, is covered. It is well established that catalytic activity of metal nanoparticles is increased by roughness at the atomic scale. Indeed a flat smooth surface will be less active than its stepped counterpart. Metal nanoparticles have in most cases facets with different Miller indexes. However, the control of the shape and crystal structure is not easy to achieve. The possibility of passivizing the nanoparticle surface with organic molecules is a way to control the nanoparticle shape. Using separation techniques such as electrophoresis, it is possible to obtain a batch of particles with the same mass in average and hence the same number of atoms. Passivized particles still have enough free sites to allow catalytic activity. Then, it is possible by using aberration-corrected electron microscopy to obtain the structure and surface structure of nanoparticles. In this work, we will report those methods and demonstrate its application for mono- and multimetallic clusters. The surface of the metallic nanoparticles plays a crucial role in the catalytic activity and interaction between elements in the surface and in the core of the core-shell structures. In this way, the analysis of chemical composition and chemical bonding needs to be obtained using analytic techniques such as X-ray energy-dispersive spectroscopy and electron energy-loss spectroscopy. Herein, we also present quantitative analysis of particles with few atoms by atomic counting using a statistical method applied in Cs-corrected high-angle annular dark-field scanning transmission electron microscopy images.

Atomic Scale Investigation of the CuPc-MAPbX3 Interface and the Effect of Non-Stoichiometric Perovskite Films on Interfacial Structures.

Metal halide perovskites (MHPs) have become a major topic of research in thin film photovoltaics due to their advantageous optoelectronic properties. These devices typically have the MHP absorber layer sandwiched between two charge selective layers (CSLs). The interfaces between the perovskite layer and these CSLs are potential areas of higher charge recombination. Understanding the nature of these interfaces is key for device improvement. Additionally, non-stoichiometric perovskite films are expected to strongly impact the interfacial properties. In this study, the interface between CH3NH3PbI3 (MAPbI3) and copper phthalocyanine (CuPc), a hole transport layer (HTL), is studied at the atomic scale. We use scanning tunneling microscopy (STM) combined with density functional theory (DFT) predictions to show that CuPc deposited on MAPbX3 (X = I,Br) forms a self-assembled layer consistent with the α-polymorph of CuPc. Additionally, STM images show a distinctly different adsorption orientation for CuPc on non-perovskite domains of the thin film samples. These findings highlight the effect of non-stoichiometric films on the relative orientation at the MHP/HTL interface, which may affect interfacial charge transport in a device. Our work provides an atomic scale view of the MHP/CuPc interface and underscores the importance of understanding interfacial structures and the effect that the film stoichiometry can have on interfacial properties.

Chemical dynamics from the gas‐phase to surfaces

AbstractThe field of gas‐phase chemical dynamics has developed superb experimental methods to probe the detailed outcome of gas‐phase chemical reactions. These experiments inspired and benchmarked first principles dynamics simulations giving access to an atomic scale picture of the motions that underlie these reactions. This fruitful interplay of experiment and theory is the essence of a dynamical approach perfected on gas‐phase reactions, the culmination of which is a standard model of chemical reactivity involving classical trajectories or quantum wave packets moving on a Born–Oppenheimer potential energy surface. Extending the dynamical approach to chemical reactions at surfaces presents challenges of complexity not found in gas‐phase study as reactive processes often involve multiple steps, such as inelastic molecule‐surface scattering and dissipation, leading to adsorption and subsequent thermal desorption and or bond breaking and making. This paper reviews progress toward understanding the elementary processes involved in surface chemistry using the dynamical approach.Key points The fruitful interplay between chemistry and physics leads to an atomic scale view of reactions taking places at catalytically active surfaces. Improved observations of chemical reactions taking place in complex environments drive the development of new approaches to theoretical chemistry. Complex reaction networks from real catalysts are boiled down to their elementary steps and examined from first principles.

Prevalence of oxygen defects in an in-plane anisotropic transition metal dichalcogenide

Atomic scale defects in semiconductors enable their technological applications and realization of novel quantum states. Using scanning tunneling microscopy and spectroscopy complemented by ab-initio calculations we determine the nature of defects in the anisotropic van der Waals layered semiconductor ReS$_2$. We demonstrate the in-plane anisotropy of the lattice by directly visualizing chains of rhenium atoms forming diamond-shaped clusters. Using scanning tunneling spectroscopy we measure the semiconducting gap in the density of states. We reveal the presence of lattice defects and by comparison of their topographic and spectroscopic signatures with ab initio calculations we determine their origin as oxygen atoms absorbed at lattice point defect sites. These results provide an atomic-scale view into the semiconducting transition metal dichalcogenides, paving the way toward understanding and engineering their properties.

Formation and Modulation of Nanotubular Assemblies of Oligourea Foldamers in Aqueous Conditions using Alcohol Additives.

There is considerable interest in the rational design of controllable, bioinspired supramolecular systems as a potential means to create new biocompatible and functional materials able to mimic and build upon the characteristics of natural biopolymers. Here, the alcohol-controlled aqueous self-assembly of an amphiphilic helical oligourea foldamer (artificial folded oligomer) into a diverse array of tubular fibril architectures is reported. Electron microscopy studies provide details of the morphological evolution of the foldamer nanostructures from protofibrils to fibers, with high resolution X-ray crystal structures providing an atomic-scale view of these assemblies, and solution studies indicating the assembly and morphology to be affected by alcohol polarity and concentration. Overall, the results reported here highlight oligourea foldamers as suitable building blocks for the formation of a diverse range of tubular morphologies in a controllable manner.

Atomic-Scale View into the Degradation of Ir-Ru Alloys during Anodic Oxygen Evolution

Hydrogen is likely to play a key role as central energy carrier in the world’s transition towards a more sustainable future. Electricity from renewable sources can be used to power water electrolyzers to generate carbon-neutral hydrogen [1]. ‘Green’ hydrogen can then, for example, be used for the synthesis of carbon-neutral fuels, in the transportation sector to power H2 fuel-cell fitted vehicles, as a carbon-neutral agent for the direct reduction of metals from ores, or to re-generate electricity in time of high demand. However, the technological upscaling of proton exchange membrane water electrolysis (PEM) devices is currently hindered by the low reactivity and instability of most catalyzing materials for the anodic oxygen evolution reaction (OER). The development of electrocatalysts with superior performance requires a detailed understanding of how their surfaces evolve during OER, ideally at the atomic scale [2,3]. The identification of the role of individual atoms in different chemical and structural environments in the near-surface region of the catalyst is crucial to understand the electrochemical performance and degradation processes.Here, we provide atomic scale insights into the degradation mechanism of an Ir-Ru model alloy during electrochemical oxygen evolution. Ir-Ru alloys are promising candidates for PEM electrolysis combining the stability of Ir and high activity of Ru towards OER. The changes in electronic state of the catalysts induced by OER are studied by X-ray photoelectron spectroscopy (XPS), while an in-situ activity-stability analysis was performed online using an inductively coupled plasma mass spectrometer. Atom probe tomography (APT) was used to reveal the structure and composition of the first few atomic layers of the catalyst in three dimensions.APT reveals strong compositional differences between intragranular and grain boundary regions in the pristine alloy, resulting from significant Ru segregation to crystalline defects during the deposition process [4,5]. Significant Ru dissolution occurs during the OER, as revealed by online dissolution measurements, resulting in a few surface layers enriched in Ir, consistent with XPS results. APT further shows that the entire near-surface region affected by the oxidation exhibits signs of Ru dissolution. In addition, oxides formed in Ru-rich regions across the catalyst´s surface, i.e. in the proximity of grain boundaries, generally exhibit a higher oxygen content, compared to oxides on top of intergranular regions. Our data suggests that the defect structure of the material governs the enrichment of Ru in grain boundaries with consecutive preferential oxidation of these regions. This preferential oxidation potentially contributes to an accelerated, localized formation of volatile RuO4 species during the OER, leading to an enhanced Ru leaching along defect regions, eventually resulting in the deterioration of the overall reactivity of the material.Finally, using the new insights gained by this study, we discuss potential design strategy improvements of Ir-Ru alloys, to engineers an OER catalyst with a higher performance and longer durability. With this work we stress the need to investigate processes in catalytic materials with a combination of independent methods and at the near-atomic scale. Only the combination of insights enables the establishing of reliable structure-function relationships of electrocatalytic materials.

Watching Water, Sodium, and Chloride Passing through a Graphitic Pore

In this issue of Matter, Malmir and colleagues use sophisticated methods to give an atomic-scale view of saltwater permeation through a graphitic pore with high salt rejection. Na+ and Cl− traverse the pore at different rates, leading to an electrostatic potential across the membrane.

Structural transition dynamics of the formation of warm dense gold: From an atomic scale view

With the highly optimized embedded-atom-method (EAM) potential and electron-phonon coupling factor obtained from experimental data, the dynamics of the formation of warm dense gold and the nuclear response of gold foils upon intense laser excitation were investigated using two-temperature molecular dynamics simulations. Considering laser energy densities ranging from 0.18 to 1.17 MJ/kg, we provide a microscopic picture of the formation of warm dense gold. A threshold (0.19 MJ/kg) for the laser energy density was determined, identifying two different melting mechanisms. For an energy density below 0.19 MJ/kg, the melting of the foil is controlled by the propagation of melt fronts from external surfaces, which results in heterogeneous melting on the time scale of hundreds of picoseconds. For an energy density above 0.19 MJ/kg, homogeneous nucleation and growth of liquid regions inside the foil play the leading role, and homogeneous melting occurs with several picoseconds. Compared with previous simulations and experimental measurements, the evaluated different threshold value indicates that the improvement in the electron heat capacity for the two-temperature model by including the kinetic information of electrons may predict better laser-matter interactions under such extreme non-equilibrium conditions.

An atomic-scale view of cyclocarbon synthesis.

Combining organic synthesis with microscopy addresses long-standing bottlenecks in cyclocarbon production

Atomic-Scale View of the Oxidation and Reduction of Supported Ultrathin FeO Islands.

By means of scanning tunneling microscopy (STM) measurements, we studied in situ the oxidation and reduction of FeO bilayer islands on Au(111) by oxygen (O2) and hydrogen (H2), respectively. The FeO islands respond very dynamically toward O2, with the coordinatively unsaturated ferrous (CUF) sites at the island edges being essential for O2 dissociation and O atom incorporation. An STM movie obtained during oxidation reveals how further O2 molecules can dissociate after the consumption of all initially existing CUF sites through the formation of new CUF sites. In contrast, we found that H2 molecules only dissociate when vibrationally excited through the ion gauge and only at the basal plane of FeO islands, implying that the CUF sites are not relevant for H2 dissociation. Our STM results reveal how excess O atoms are incorporated and released in O2 and H2 and thus shed light onto the stability of inverse catalysts during a catalyzed reaction.

Atomic-scale view of stability and degradation of single-crystal MAPbBr3 surfaces

MAPbBr3 (MA: CH3NH3+) undergoes surface degradation under dark and vacuum conditions that leads to the formation of surface PbBr2 species.

A near atomic-scale view at the composition of amyloid-beta fibrils by atom probe tomography

Amyloid-beta (Ab) proteins play an important role in a number of neurodegenerative diseases. Ab is found in senile plaques in brains of Alzeimer’s disease patients. The 42 residues of the monomer form dimers which stack to fibrils gaining several micrometers in length. Using Ab fibrils with 13C and 15N marker substitution, we developed an innovative approach to obtain insights to structural and chemical information of the protein. We deposited the modified protein fibrils to pre-sharped aluminium needles with >100-nm apex diameters and, using the position-sensitive mass-to-charge spectrometry technique of atom probe tomography, we acquired the chemically-resolved three dimensional information for every detected ion evaporated in small fragments from the protein. We also discuss the influence of experimental parameters such as pulse energy and pulse frequency of the used Laser beam which lead to differences in the size of the gained fragments, developing the capability of localising metal atom within Ab plaques.

Atomistic Interface Dynamics in Sn-Catalyzed Growth of Wurtzite and Zinc-Blende ZnO Nanowires.

Unraveling the phase selection mechanisms of semiconductor nanowires (NWs) is critical for the applications in future advanced nanodevices. In this study, the atomistic vapor-solid-liquid growth processes of Sn-catalyzed wurtzite (WZ) and zinc blende (ZB) ZnO are directly revealed based on the in situ transmission electron microscopy. The growth kinetics of WZ and ZB crystal phases in ZnO appear markedly different in terms of the NW-droplet interface, whereas the nucleation site as determined by the contact angle ϕ between the seed particle and the NW is found to be crucial for tuning the NW structure through combined experimental and theoretical investigations. These results offer an atomic-scale view into the dynamic growth process of ZnO NW, which has implications for the phase-controllable synthesis of II-VI compounds and heterostructures with tunable band structures.

Thiol Treatment Creates Selective Palladium Catalysts for Semihydrogenation of Internal Alkynes

Summary Surface and interfacial engineering of heterogeneous metal catalysts is effective and critical for optimizing selective hydrogenation for fine chemicals. By using thiol-treated ultrathin Pd nanosheets as a model catalyst, we demonstrate the development of stable, efficient, and selective Pd catalysts for semihydrogenation of internal alkynes. In the hydrogenation of 1-phenyl-1-propyne, the thiol-treated Pd nanosheets exhibited excellent catalytic selectivity (>97%) toward the semihydrogenation product (1-phenyl-1-propene). The catalyst was highly stable and showed no obvious decay in either activity or selectivity for over ten cycles. Systematic studies demonstrated that a unique Pd-sulfide/thiolate interface created by the thiol treatment was crucial to the semihydrogenation. The high catalytic selectivity and activity benefited from the combined steric and electronic effects that inhibited the deeper hydrogenation of C=C bonds. More importantly, this thiol treatment strategy is applicable to creating highly active and selective practical catalysts from commercial Pd/C catalysts for semihydrogenation of internal alkynes.

In Situ Acoustic Diagnostics of Particle-Binder Interactions in Battery Electrodes

Summary The high phase-transformation strain developed upon intercalation in the host particles of a composite battery electrode affects the polymeric binder network mechanically, deteriorating the electrode cycling performance. Here, electrochemical quartz crystal microbalance with dissipation monitoring (EQCM-D) is used to demonstrate a new strain-accommodation mechanism, in high-strain NaFePO 4 /PVdF electrodes, via relaxation of the binder network surrounding the intercalation particles. Complete mechanical degradation of the polymer network occurs during long-term cycling of NaFePO 4 electrodes in aqueous solutions (hard and tough behavior). In contrast, in aprotic solutions, a softened binder easily accommodates the high transformation strain, ensuring excellent electrode cycling performance (soft and tough behavior). Quantification of the high-frequency viscoelastic properties of an operating composite electrode linked to the binder's fracture toughness ensures fast and facile screening of the optimal polymeric binder/electrolyte solution combinations. This methodology should be extremely important for optimization of cycling performance of Li-Si anodes undergoing huge volume changes during cycling.