Atomic Motifs Research Articles

Quaternary Mixed Oxides of Non-Noble Metals with Enhanced Stability during the Oxygen Evolution Reaction.

Robust electrocatalysts required to drive the oxygen evolution reaction (OER) during water electrolysis are still a missing component toward the path for sustainable hydrogen production. Here a new family of OER active quaternary mixed-oxides based on X-Sn-Mo-Sb (X = Mn, Fe, Co, or Ni) is reported. These nonstoichiometric mixed oxides form a rutile-type crystal structure with a random atomic motif and diverse oxidation states, leading to the formation of cation vacancies and local disorder. The successful incorporation of all cations into a rutile structure was achieved using oxidizing agents that facilitates the formation of Sb5+ required to form the characteristic octahedral coordination in rutile. The mixed oxides exhibit enhanced stability in both acidic and alkaline environments under anodic potentials with no changes in their crystal structure after extensive electrochemical stress. The improved stability of these mixed oxides highlights their potential application as scaffolds to host and stabilize OER active metals.

Electrochemical CO2 Activation and Valorization on Metallic Copper and Carbon‐Embedded N‐Coordinated Single Metal MNC Catalysts

AbstractThe electrochemical reductive valorization of CO2, referred to as the CO2RR, is an emerging approach for the conversion of CO2‐containing feeds into valuable carbonaceous fuels and chemicals, with potential contributions to carbon capture and use (CCU) for reducing greenhouse gas emissions. Copper surfaces and graphene‐embedded, N‐coordinated single metal atom (MNC) catalysts exhibit distinctive reactivity, attracting attention as efficient electrocatalysts for CO2RR. This review offers a comparative analysis of CO2RR on copper surfaces and MNC catalysts, highlighting their unique characteristics in terms of CO2 activation, C1/C2(+) product formation, and the competing hydrogen evolution pathway. The assessment underscores the significance of understanding structure–activity relationships to optimize catalyst design for efficient and selective CO2RR. Examining detailed reaction mechanisms and structure‐selectivity patterns, the analysis explores recent insights into changes in the chemical catalyst states, atomic motif rearrangements, and fractal agglomeration, providing essential kinetic information from advanced in/ex situ microscopy/spectroscopy techniques. At the end, this review addresses future challenges and solutions related to today's disconnect between our current molecular understanding of structure–activity‐selectivity relations in CO2RR and the relevant factors controlling the performance of CO2 electrolyzers over longer times, with larger electrode sizes, and at higher current densities.

Electrochemical CO2 Activation and Valorization on Metallic Copper and Carbon-Embedded N-Coordinated Single Metal MNC Catalysts.

The electrochemical reductive valorization of CO2, referred to as the CO2RR, is an emerging approach for the conversion of CO2-containing feeds into valuable carbonaceous fuels and chemicals, with potential contributions to carbon capture and use (CCU) for reducing greenhouse gas emissions. Copper surfaces and graphene-embedded, N-coordinated single metal atom (MNC) catalysts exhibit distinctive reactivity, attracting attention as efficient electrocatalysts for CO2RR. This review offers a comparative analysis of CO2RR on copper surfaces and MNC catalysts, highlighting their unique characteristics in terms of CO2 activation, C1/C2(+) product formation, and the competing hydrogen evolution pathway. The assessment underscores the significance of understanding structure-activity relationships to optimize catalyst design for efficient and selective CO2RR. Examining detailed reaction mechanisms and structure-selectivity patterns, the analysis explores recent insights into changes in the chemical catalyst states, atomic motif rearrangements, and fractal agglomeration, providing essential kinetic information from advanced in/ex situ microscopy/spectroscopy techniques. At the end, this review addresses future challenges and solutions related to today's disconnect between our current molecular understanding of structure-activity-selectivity relations in CO2RR and the relevant factors controlling the performance of CO2 electrolyzers over longer times, with larger electrode sizes, and at higher current densities.

Hidden structures: a driving factor to achieve low thermal conductivity and high thermoelectric performance.

The long-range periodic atomic arrangement or the lack thereof in solids typically dictates the magnitude and temperature dependence of their lattice thermal conductivity (κlat). Compared to crystalline materials, glasses exhibit a much-suppressed κlat across all temperatures as the phonon mean free path reaches parity with the interatomic distances therein. While the occurrence of such glass-like thermal transport in crystalline solids captivates the scientific community with its fundamental inquiry, it also holds the potential for profoundly impacting the field of thermoelectric energy conversion. Therefore, efficient manipulation of thermal transport and comprehension of the microscopic mechanisms dictating phonon scattering in crystalline solids are paramount. As quantized lattice vibrations (i.e., phonons) drive κlat, atomistic insights into the chemical bonding characteristics are crucial to have informed knowledge about their origins. Recently, it has been observed that within the highly symmetric 'averaged' crystal structures, often there are hidden locally asymmetric atomic motifs (within a few Å), which exert far-reaching influence on phonon transport. Phenomena such as local atomic off-centering, atomic rattling or tunneling, liquid-like atomic motion, site splitting, local ordering, etc., which arise within a few Å scales, are generally found to drastically disrupt the passage of heat carrying phonons. Despite their profound implication(s) for phonon dynamics, they are often overlooked by traditional crystallographic techniques. In this review, we provide a brief overview of the fundamental aspects of heat transport and explore the status quo of innately low thermally conductive crystalline solids, wherein the phonon dynamics is majorly governed by local structural phenomena. We also discuss advanced techniques capable of characterizing the crystal structure at the sub-atomic level. Subsequently, we delve into the emergent new ideas with examples linked to local crystal structure and lattice dynamics. While discussing the implications of the local structure for thermal conductivity, we provide the state-of-the-art examples of high-performance thermoelectric materials. Finally, we offer our viewpoint on the experimental and theoretical challenges, potential new paths, and the integration of novel strategies with material synthesis to achieve low κlat and realize high thermoelectric performance in crystalline solids via local structure designing.

Engineering Interfacial Pt─O─Ti Site at Atomic Step Defect for Efficient Hydrogen Evolution Catalysis.

The hydrogen evolution reaction (HER) activity of defect-stabilized low-Pt-loading catalysts is closely related with defect type in support materials, while the knowledge about the effect of higher-dimensional defects on the property and activity of trapped Pt atomic species is scarce. Herein, small size (5-10nm) TiO2 nanoparticles with abundant surface step defects (one kind of line defect) are used to direct the uniform anchoring of Pt atomic clusters (Pt-ACs) via Pt─O─Ti linkage. The as-made low-Pt catalysts (Pt-ACs/S-TiO2-NP) exhibit exceptional HER intrinsic activity due to the unique step-site Pi-O-Ti species, in which the mass activity and turnover frequency are as high as 21.46AmgPt -1 and 21.69s-1 at the overpotential of 50mV, both far beyond those of benchmark Pt/C catalysts and other Pt-ACs/TiO2 samples with less step sites. Spectroscopic measurements and theoretical calculations reveal that the step-defect-located Pt─O─Ti sites can simultaneously induce the charge transfer from TiO2 substrate to the trapped Pt-ACs and the downshift of d-band center, which helps the proton reduction to H* intermediates and the following hydrogen desorption process, thus improving the HER. The work provides a deep insight on the interactions between high-dimensional defect and well-dispersed atomic metal motifs for superior HER catalysis.

Atomic motifs control chemical properties of grain boundaries in Fe alloy

Atomic motifs control chemical properties of grain boundaries in Fe alloy

Remote epitaxial interaction through graphene.

The concept of remote epitaxy involves a two-dimensional van der Waals layer covering the substrate surface, which still enable adatoms to follow the atomic motif of the underlying substrate. The mode of growth must be carefully defined as defects, e.g., pinholes, in two-dimensional materials can allow direct epitaxy from the substrate, which, in combination with lateral epitaxial overgrowth, could also form an epilayer. Here, we show several unique cases that can only be observed for remote epitaxy, distinguishable from other two-dimensional material-based epitaxy mechanisms. We first grow BaTiO3 on patterned graphene to establish a condition for minimizing epitaxial lateral overgrowth. By observing entire nanometer-scale nuclei grown aligned to the substrate on pinhole-free graphene confirmed by high-resolution scanning transmission electron microscopy, we visually confirm that remote epitaxy is operative at the atomic scale. Macroscopically, we also show variations in the density of GaN microcrystal arrays that depend on the ionicity of substrates and the number of graphene layers.

Novel two-dimensional square-AlAs: Direct semiconductor coupled with high electron-hole mobility ratio toward sunlight-driven water-splitting

The design of new monolayered atomic motifs always attracts enormous attention due to their interesting properties induced by unique crystal phases and promising catalytic applications in low dimensional devices. Herein, we propose a novel square-AlAs monolayer based on ab initio density functional theory calculations, whose good stability is confirmed from the perspectives of energy, dynamics, thermodynamics, and mechanics. Electronic structure calculations reveal that the newly predicted square-AlAs monolayer is a direct-gap semiconductor with a bandgap of 1.39 eV, which is sensitive to in-plane strain, meanwhile the strain induced changes in bandgap allow advantageous modulations of optical absorption in visible-light region. Unfortunately, the unfavorable hydrogen evolution reaction (HER) band edge levels make square-AlAs not suitable for photocatalytic water-splitting in spite of the appropriate bandgap, high electron-hole mobility ratio and strong visible light absorption. However, a favorable effect caused by strain engineering is revealed by the fact that the square-AlAs can meet the requirement of photocatalytic water-splitting under the suitable biaxial loading. Particularly, compared to pure AlAs, the Gibbs free energy of hydrogen adsorption indicate that Boron-doping can effectively decrease the HER overpotential. Moreover, the external potential provided by photoexcited electrons for oxygen evolution reaction (OER) is quite positive, which can serve as a free and pollution-free driving force to drive OER to proceed spontaneously for pure AlAs or reduce the OER overpotential to a very low value (0.15 V) for Boron-doped AlAs, overmatching widely used IrO2 catalyst.

Improving fatigue property of Zr–Ni–Al metallic glass by tailoring chemical composition with high structure stability

Improving the tolerance to fatigue damage for bulk metallic glass (BMG) is of vital importance for its industrial applications during long-term mechanical service. In this work, the composition dependence of structure stability against thermal and mechanical stimuli are studied with Zr–Ni–Al BMGs from the perspective of atomic topological packing and chemical affinity. Large atomic size disparity and strong interactions between component elements facilitate the stability of glass structure, which impart BMGs with relatively higher glass transition temperature and yield strength. Fatigue failure of BMG initiates from the accumulation of inelastic shear transformation events on the sample surface under cyclic loading. The fatigue endurance limit of Zr–Ni–Al BMG is effectively improved by tailoring the chemical composition with increased structure stability, which is characterized by high mismatch entropy and mixing enthalpy. Aluminum plays a role in enhancing the fatigue tolerance of Zr–Ni–Al BMGs, which presumably arises from the covalent-like bond character of aluminum that promotes the formation of rigid atomic packing motifs with great resistance to shear rearrangement, and consequently mitigates the fatigue crack initiation process.

A group of ductile metallic glasses prepared by modifying local structure of icosahedral quasicrystals

Inspired by research into the association between icosahedral local orders and the plasticity of metallic glasses (MGs), beryllium (Be) is added to the icosahedral quasi-crystal forming alloy Zr40Ti40Ni20. In this way, bulk metallic glasses (BMGs) with favorable compressive plasticity are fabricated. Therein, the icosahedral quasi-crystalline phase is the main competing phase of amorphous phases and icosahedral local orders are the main local atomic motifs in amorphous phases. The alloys of (Zr40Ti40Ni20)76Be24 and (Zr40Ti40Ni20)72Be28 with their greater plastic strain capacity show similar characteristics to highly plastic amorphous systems: The serrated flow of compression curves always follows a near-exponential distribution. The primary and secondary shear bands intersect each other, bifurcate, and bend. Typical vein patterns are densely distributed on the fracture surfaces. The relaxation enthalpy of four MGs is linearly correlated with the plastic strain, that is, the greater the relaxation enthalpy, the larger the plastic strain.

Searching for iron nanoparticles with a general-purpose Gaussian approximation potential

We present a general-purpose machine learning Gaussian approximation potential (GAP) for iron that is applicable to all bulk crystal structures found experimentally under diverse thermodynamic conditions, as well as surfaces and nanoparticles (NPs). By studying its phase diagram, we show that our GAP remains stable at extreme conditions, including those found in the Earth's core. The new GAP is particularly accurate for the description of NPs. We use it to identify new low-energy NPs, whose stability is verified by performing density functional theory calculations on the GAP structures. Many of these NPs are lower in energy than those previously available in the literature up to $N_\text{atoms}=100$. We further extend the convex hull of available stable structures to $N_\text{atoms}=200$. For these NPs, we study characteristic surface atomic motifs using data clustering and low-dimensional embedding techniques. With a few exceptions, e.g., at magic numbers $N_\text{atoms}=59$, $65$, $76$ and $78$, we find that iron tends to form irregularly shaped NPs without a dominant surface character or characteristic atomic motif, and no reminiscence of crystalline features. We hypothesize that the observed disorder stems from an intricate balance and competition between the stable bulk motif formation, with bcc structure, and the stable surface motif formation, with fcc structure. We expect these results to improve our understanding of the fundamental properties and structure of low-dimensional forms of iron, and to facilitate future work in the field of iron-based catalysis.

Atomic motifs govern the decoration of grain boundaries by interstitial solutes

Grain boundaries, the two-dimensional defects between differently oriented crystals, tend to preferentially attract solutes for segregation. Solute segregation has a significant effect on the mechanical and transport properties of materials. At the atomic level, however, the interplay of structure and composition of grain boundaries remains elusive, especially with respect to light interstitial solutes like B and C. Here, we use Fe alloyed with B and C to exploit the strong interdependence of interface structure and chemistry via charge-density imaging and atom probe tomography methods. Direct imaging and quantifying of light interstitial solutes at grain boundaries provide insight into decoration tendencies governed by atomic motifs. We find that even a change in the inclination of the grain boundary plane with identical misorientation impacts grain boundary composition and atomic arrangement. Thus, it is the smallest structural hierarchical level, the atomic motifs, that controls the most important chemical properties of the grain boundaries. This insight not only closes a missing link between the structure and chemical composition of such defects but also enables the targeted design and passivation of the chemical state of grain boundaries to free them from their role as entry gates for corrosion, hydrogen embrittlement, or mechanical failure.

Fluctuations in local shear-fault energy produce unique and dominating strengthening in metastable complex concentrated alloys

Local chemical short-range ordering (SRO) and spatial fluctuations of planar fault energy are important features of multi-element and metastable complex concentrated alloys (CCAs). Arising from them, dislocations in such alloys are distinctively wavy in both static and migrating conditions; yet, such effects on strength have remained unknown. In this work, molecular dynamics simulations are used to show that the wavy configurations of dislocations and their jumpy motion in a prototypic CCA of NiCoCr are due to the local fluctuations of the energy of SRO shear-faulting that accompanies dislocation motion, with the dislocation getting pinned at sites of hard atomic motifs (HAMs) associated with high local shear-fault energies. Unlike the global averaged shear-fault energy which in general will subdue on successive dislocation passes, the local fluctuations in the fault energy always remain in a CCA, thus offering a strength contribution that is unique in such alloys. Analysis of the magnitude of this form of dislocation resistance shows that this is dominating over contributions due to elastic misfit of alloying elements and is in good agreement with strengths predicted from molecular dynamics simulations and experiments. This work has unfolded the physical basis of strength in CCAs, which is important for the development of these alloys into useful structural materials.

Nanostructured intermetallics: from rational synthesis to energy electrocatalysis

Intermetallics are a large family of structurally ordered alloys that combines a metal element with other metal/metalloid elements with a clearly defined stoichiometric ratio. Intermetallics possess abundant crystal structures and atomic packing motifs, giving rise to a great variety of electronic configurations and surface adsorption properties. The wide electronic and geometric diversity makes intermetallics a highly promising population for discovering advanced materials for various catalytic applications. This review presents recent advances in the reaction synthesis of intermetallic materials at the nanoscale and their energy-related electrocatalytic applications. Initially, we introduce general principles for the formation of stable intermetallic structures. Subsequently, we elaborate on common synthetic strategies of nanostructured intermetallics, such as thermal annealing, wet-chemical methods, metallothermic reduction, and template-directed synthesis. Furthermore, we discuss the wide employment of these intermetallic nanocatalysts in many different kinds of electrocatalytic applications, as well as highlight the theoretical and experimental evidence for establishing a reasonable relationship between atomic arrangement and catalytic activity. Finally, we propose some perspectives for future developments of intermetallic preparation and catalytic applications.

Quasicrystal stability in complex aluminum alloys: Insights from first principles models on structural motifs in crystalline approximants

Al81Cu7Fe4Cr8 is an alloy that readily forms quasicrystalline phases but understanding the complex atomic arrangements in the structure of such quasicrystals is challenging. For the Al-Cu-Fe-Cr system, the structures of crystalline approximants have been identified. These exhibit atomic motifs similar to those for the quasicrystalline configurations. We employ here a comprehensive methodology to analyze statistically generated ground states of orthorhombic approximants using density functional theory to shed light on the role of elemental variations of such complex alloys. Our findings show that the stability of QC approximants has a strong dependence on the specific locations of Cu and Cr atoms. The geometry of the atomic motifs from structural optimization suggests that the icosahedral phase is more favorable than the decagonal phase. The formation energy difference between these is small which indicates that the two quasicrystalline phases may coexist.

Exploiting layered double hydroxide with modulated atomic motifs enables enhanced peroxydisulfate activation

Exploring advanced Fenton-like catalysts with modulated local structure and grasping how the atomic motifs govern the activity and stability is a major challenge. Herein, a series of nickel–iron layered double hydroxides (NiFe-LDH) with regulated local atomic structure were presented via introducing Fe2+ to partially substitute the Ni2+-site, thereby constructing Fe-O-Fe motifs. These regulated Fe2+ containing NiFe-LDHs (Fe2+-NiFe-LDH) demonstrated enhanced catalytic performance on peroxydisulfate (PDS) activation compared with pristine NiFe-LDH. Remarkable degradation efficiency was achieved: about 92.5 % of Orange II (30 mg·L−1) could be degraded within 60 min. Meanwhile, other simulated wastewater of tetracycline, bisphenol A, and ciprofloxacin were capable of achieving efficient removal as well. Detailed characterizations elucidated that the substitution of Fe2+ resulted in the formation of Fe-O-Fe couplings, which functioned as novel motifs and accounted for the expressively promoted intrinsic catalytic performance. Simultaneously, the incorporated Fe-O-Fe motifs can improve electrochemical properties, thereby accelerating the electron transfer between catalysts and oxidants, further clarifying the exceptionally enhanced catalytic activity. Furthermore, density functional theory (DFT) calculations corroborated that Fe2+ substitution could modulate surface electronic state, give rise to strengthened binding energy, and intensify electron transfer for PDS activation. Moreover, other Fe-O-M (M represents Co, Cu, Mn) motifs were also constructed within NiFe-LDH structure, with Co exhibiting the superior catalytic performance. This work conclusively proved the significance of tailoring the local atomic structure for constructing high-efficiency catalysts, and predicted to stimulate the rational design of LDHs-based catalysts.

Learning motifs and their hierarchies in atomic resolution microscopy.

Characterizing materials to atomic resolution and first-principles structure-property prediction are two pillars for accelerating functional materials discovery. However, we are still lacking a rapid, noise-robust framework to extract multilevel atomic structural motifs from complex materials to complement, inform, and guide our first-principles models. Here, we present a machine learning framework that rapidly extracts a hierarchy of complex structural motifs from atomically resolved images. We demonstrate how such motif hierarchies can rapidly reconstruct specimens with various defects. ing complex specimens with simplified motifs enabled us to discover a previously unidentified structure in a Mo─V─Te─Nb polyoxometalate (POM) and quantify the relative disorder in a twisted bilayer MoS2. In addition, these motif hierarchies provide statistically grounded clues about the favored and frustrated pathways during self-assembly. The motifs and their hierarchies in our framework coarse-grain disorder in a manner that allows us to understand a much broader range of multiscale samples with functional imperfections and nontrivial topological phases.

Exploiting Layered Double Hydroxide with Modulated Atomic Motifs Enables Enhanced Peroxydisulfate Activation

Exploiting Layered Double Hydroxide with Modulated Atomic Motifs Enables Enhanced Peroxydisulfate Activation

Three-dimensional atomic packing in amorphous solids with liquid-like structure.

Liquids and solids are two fundamental states of matter. However, our understanding of their three-dimensional atomic structure is mostly based on physical models. Here we use atomic electron tomography to experimentally determine the three-dimensional atomic positions of monatomic amorphous solids, namely a Ta thin film and two Pd nanoparticles. We observe that pentagonal bipyramids are the most abundant atomic motifs in these amorphous materials. Instead of forming icosahedra, the majority of pentagonal bipyramids arrange into pentagonal bipyramid networks with medium-range order. Molecular dynamics simulations further reveal that pentagonal bipyramid networks are prevalent in monatomic metallic liquids, which rapidly grow in size and form more icosahedra during the quench from the liquid to the glass state. These results expand our understanding of the atomic structures of amorphous solids and will encourage future studies on amorphous-crystalline phase and glass transitions in non-crystalline materials with three-dimensional atomic resolution.

δ-SnS: An Emerging Bidirectional Auxetic Direct Semiconductor with Desirable Carrier Mobility and High-Performance Catalytic Behavior toward the Water-Splitting Reaction.

We propose a novel two-dimensional SnS allotrope (monolayer δ-SnS) based on an auxetic δ-phosphorene configuration using first-principles calculations. This monolayer appears to have outstanding stability as revealed by its energetic, kinetic, thermodynamic, and mechanic calculations, and it can withstand temperatures as high as 900 K. Monolayer δ-SnS is a wide direct-bandgap (2.354 eV) semiconductor, and its electron mobility is as high as ∼1.25 × 103 cm2 V-1 s -1, higher than that of monolayer KTlO (∼450 cm2 V-1 s-1) and MoS2 (∼200 cm2 V-1 s-1). Optical absorption spectra, reaching up to the order of ∼105 cm-1, are obviously excellent in the visible-light region, suggesting efficient harvesting of solar radiation. Because of its unique atomic motif, monolayer δ-SnS presents an unusual bidirectional auxetic effect: a high negative in-plane Poisson's ratio (-0.048 and -0.068), which is larger than those for many recently reported two-dimensional auxetic materials, e.g., black phosphorene (-0.027), borophene (-0.04), and monolayer penta-B2N4 (-0.02). The bandgap and band edge can be substantially manipulated under strain to meet the requirement of the water-splitting reaction. Particularly, when pH = 7, suitable band-edge alignments and small overpotentials of the photocatalytic OER (oxygen evolution reaction) and HER (hydrogen evolution reaction) appear, endowing monolayer δ-SnS with great potential as an efficient visible-light-driven bifunctional photocatalyst for water splitting.