Surface Atoms Research Articles

Crystal-Phase Engineering in Heterogeneous Catalysis.

The performance of a chemical reaction is critically dependent on the electronic and/or geometric structures of a material in heterogeneous catalysis. Over the past century, the Sabatier principle has already provided a conceptual framework for optimal catalyst design by adjusting the electronic structure of the catalytic material via a change in composition. Beyond composition, it is essential to recognize that the geometric atomic structures of a catalyst, encompassing terraces, edges, steps, kinks, and corners, have a substantial impact on the activity and selectivity of a chemical reaction. Crystal-phase engineering has the capacity to bring about substantial alterations in the electronic and geometric configurations of a catalyst, enabling control over coordination numbers, morphological features, and the arrangement of surface atoms. Modulating the crystallographic phase is therefore an important strategy for improving the stability, activity, and selectivity of catalytic materials. Nonetheless, a complete understanding of how the performance depends on the crystal phase of a catalyst remains elusive, primarily due to the absence of a molecular-level view of active sites across various crystal phases. In this review, we primarily focus on assessing the dependence of catalytic performance on crystal phases to elucidate the challenges and complexities inherent in heterogeneous catalysis, ultimately aiming for improved catalyst design.

Time‐Resolved Photoemission Electron Microscopy on a ZnO Surface Using an Extreme Ultraviolet Attosecond Pulse Pair

AbstractElectrons photoemitted by extreme ultraviolet attosecond pulses derive spatially from the first few atomic surface layers and energetically from the valence band and highest atomic orbitals. As a result, it is possible to probe the emission dynamics from a narrow 2D region in the presence of optical fields, as well as obtain elemental specific information. However, combining this with spatially‐resolved imaging is a long‐standing challenge because of the large inherent spectral width of attosecond pulses, as well as the difficulty of making them at high repetition rates. Here, this work demonstrates an attosecond interferometry experiment on a zinc oxide (ZnO) surface using spatially and energetically resolved photoelectrons. Photoemission electron microscopy is combined with near‐infrared pump ‐ extreme ultraviolet probe laser spectroscopy and the instantaneous phase of an infrared field is resolved with high spatial resolution. Results show how the core level states with low binding energy of ZnO are well suited to perform spatially resolved attosecond interferometry experiments. A distinct phase shift of the attosecond beat signal is observed across the laser focus which is attributed to wavefront differences between the pump and the probe fields at the surface. This work demonstrates a clear pathway for attosecond interferometry with high spatial resolution at atomic scale surface regions opening up for a detailed understanding of nanometric light‐matter interaction.

Atomically Thin Decoration Layers for Robust Orientation Control of 2D Transition Metal Dichalcogenides

Abstract2D semiconducting transition metal dichalcogenides (TMDs) are emerging as promising candidates in the pursuit of advancing semiconductor technology. One major challenge for integrating 2D TMD materials into practical applications is developing an epitaxial technique with robust reproducibility for single‐oriented growth and thus single‐crystal growth. Here, the growth of single‐orientated MoS2 on c‐plane sapphire with atomically thin Fe2O3 decoration layers under various growth conditions is demonstrated. The statistical data highlight robust reproducibility, achieving a single orientation ratio of up to 99%. Density functional theory calculations suggest that MoS2 favors a 0° alignment () on the Fe2O3 (0001) surface. This preference ensures single‐oriented growth, even on mirror‐reflected exposed surfaces which typically lead to antiparallel domains. Subsequent optical and electrical analyses confirm the uniformity and undoped nature of MoS2 on Fe2O3‐decorated sapphire, showing its quality is comparable to MoS2 grown on bared sapphires. The results underscore the potential of Fe2O3‐decorated sapphire as an effective substrate for the consistent and high‐quality epitaxial growth of 2D TMDs, illuminating the pathway to epitaxial control of 2D TMD orientation through strategic modulation of crystalline atomic surfaces.

Proton Shuttle-Assisted Surface Reconstruction toward Nonpolar Facets-Terminated Zinc-Blende CdSe/CdS Core/Shell Quantum Dots.

Surface reconstruction can rearrange the surface atoms of a crystal without the need of growth processes and has the potential to synthesize crystals with novel morphologies and facets that cannot be obtained through regular synthesis. However, little is known about the molecular mechanisms of the surface reconstruction process. Here, utilizing surface reconstruction, we report the synthesis of nonpolar facets (110) facets)-terminated dodecahedral zinc-blende CdSe/CdS core/shell quantum dots. The morphology transformation is achieved by first fully exchanging the cadmium carboxylate ligand with oleylamine and then undergoing surface reconstruction. The surface reconstruction-induced morphology transformation is confirmed by transmission electron microscopy and absorption spectroscopy. Details of kinetic experiments and simulation results demonstrated that successful surface reconstruction must be assisted by a proton shuttle. Except for the first report on zinc-blende quantum dots terminated with (110) facets, the surface reconstruction aided by the proton shuttle offers valuable insights for devising methods to regulate the properties of nanocrystals.

Atomic Three-Dimensional Investigations of Pd Nanocatalysts for Acetylene Semi-hydrogenation.

Deciphering the three-dimensional (3D) insight into nanocatalyst surfaces at the atomic level is crucial to understanding catalytic reaction mechanisms and developing high-performance catalysts. Nevertheless, better understanding the inherent insufficiency of a long-range ordered lattice in nanocatalysts is a big challenge. In this work, we report the local structure of Pd nanocatalysts, which is beneficial for demonstrating the shape-structure-adsorption relationship in acetylene hydrogenation. The 5.27 nm spherical Pd catalyst (Pdsph) shows an ethylene selectivity of 88% at complete acetylene conversion, which is much higher than those of the Pd octahedron and Pd cube and superior to other reported monometallic Pd nanocatalysts so far. By virtue of the local structure revelation combined with the atomic pair distribution function (PDF) and reverse Monte Carlo (RMC) simulation, the atomic surface distribution of the unique compressed strain of Pd-Pd pairs in Pdsph was revealed. Density functional theory calculations verified the obvious weakening of the ethylene adsorption energy on account of the surface strain of Pdsph. It is the main factor to avoid the over-hydrogenation of acetylene. The present work, entailing shape-induced surface strain manipulation and atomic 3D insight, opens a new path to understand and optimize chemical activity and selectivity in the heterogeneous catalysis process.

Mitigating Surface Defects in Tin-Based Perovskite Films with α-Tocopherol for Enhanced Photovoltaic Performance.

Surface defects in tin-based perovskite films disrupt the periodic arrangement of atoms in crystals, making surface atoms more susceptible to interactions with water and oxygen molecules in the surrounding environment. The diffusion of oxygen ions into the perovskite interior leads to the formation of severe bulk defects, which compromises the performance of tin-based perovskite solar cells (PSCs). As a result, surface defects are recognized as the primary source of degradation and require special attention. In this study, α-Tocopherol (also known as vitamin E) into tin-based perovskite films is introduced. Experimental results show that because of its larger volume, α-Tocopherol does not enter the perovskite lattice. Instead, it forms van der Waals and hydrogen bond interactions with the formamidine ion (FA+) and the [SnI6]4- octahedron at the perovskite terminals. Through α-Tocopherol passivation, both surface and interior oxidation of the perovskite are significantly suppressed as α-Tocopherol firmly embeds itself on the perovskite surface. Density functional theory analysis confirms the inhibition of I─Sn antisite defects (ISn) and Sn interstitial defects (Sni), which possess deep trap states within the bandgap. Ultimately, it is demonstrated that α-Tocopherol enhances the power conversion efficiency (PCE) from 9.19% to 13.14% and prolongs the lifetime of tin-based PSCs to over 50 days.

Unveiling siliborophene's potential as an anode material for advancing Li ion batteries: DFT and ab-initio molecular dynamics

A favorable anodic candidate with significant potential is currently in high demand for Lithium-ion batteries. The χ3 borophene, a two-dimensional material, shows potential as an anode in Li-ion batteries. However, its original structure requires improvement to enhance its Li diffusivity and specific capacity. This research investigates the modification of χ3 borophene by silicon atoms, resulting in a new structure called siliborophene, using density functional theory. Ab-initio molecular dynamics simulations confirm the stability of siliborophene structure at room temperature. The Siχ3 structure hosts adsorption sites for the Li atom, and after exploring various positions, the most favorable configuration exhibits an adsorption energy of −1.33 eV. The siliborophene surface can adsorbed up to 13 Li atoms but according to the further analyses the results of open-circuit voltage suggest that the siliborophene structure can store 10 Li atoms, so Li10Siχ3 structure is considered as the structure with maximum adsorption capacity. The interaction between the surface and Li atoms is stronger in this configuration, with −1.10 eV adsorption energy which yields a maximum specific capacity of 1291 mAh/g. Analysis of the partial density of states and band structure reveals metallic characteristics of the surface during lithiation. Furthermore, using the CI-NEB approach, the barrier activation energy for Li diffusion was calculated, showing exceptionally low values at 0.32 eV, enabling rapid Li diffusion on the surface. These findings indicate that siliborophene holds promise as a potential anode material for lithium-ion batteries.

Effects of lattice orientation and defect degree on Si/Al solid interfacial structure and thermal resistance

ABSTRACT The effect of variation in the Si/Al interface structure on the thermal properties is explored with non-equilibrium molecular dynamics method in present work, and two distinct approaches are employed to manipulate the interface structure. The first is to change the arrangement of the atoms in the Al region by changing the orientation of the axis. The second one is to dig out several layers of atoms near the Al interface, so that the interface appears as a groove-like defect with different depths. Both methods exhibit a direct impact on atomic surface density. The high atomic surface density indicates a more compact atomic arrangement inside the structure, which improves the heat transport. In addition, the internal mechanism of thermal resistance change is probed from the aspects of Grain Boundary (GB) energy, vibrational density of states (VDOS) and the overlap of phonon spectrums parameter. It is found that structures with high atomic surface density correspond to smaller GB energies and larger overlap of phonon spectral parameters. Furthermore, by comparing the two ways of changing the interface structure, it is found that changing the degree of interface defects has a larger effect on the interfacial thermal resistance.

Quantum and classical molecular dynamics for H atom scattering from graphene.

This work presents systematic comparisons between classical molecular dynamics (cMD) and quantum dynamics (QD) simulations of 15-dimensional and 75-dimensional models in their description of H atom scattering from graphene. We use an experimentally validated full-dimensional neural network potential energy surface of a hydrogen atom interacting with a large cell of graphene containing 24 carbon atoms. For quantum dynamics simulations, we apply Monte Carlo canonical polyadic decomposition to transform the original potential energy surface (PES) into a sum of products form and use the multi-layer multi-configuration time-dependent Hartree method to simulate the quantum scattering of a hydrogen or deuterium atom with an initial kinetic energy of 1.96 or 0.96 eV and an incident angle of 0°, i.e., perpendicular to the graphene surface. The cMD and QD initial conditions have been carefully chosen in order to be as close as possible. Our results show little differences between cMD and QD simulations when the incident energy of the H atom is equal to 1.96 eV. However, a large difference in sticking probability is observed when the incident energy of the H atom is equal to 0.96 eV, indicating the predominance of quantum effects. To the best of our knowledge, our work provides the first benchmark of quantum against classical simulations for a system of this size with a realistic PES. Additionally, new projectors are implemented in the Heidelberg multi-configuration time-dependent Hartree package for the calculation of the atom scattering energy transfer distribution as a function of outgoing angles.

Hydrogen embrittlement of iron nanowires: investigating size and orientation dependence on loading behaviour

ABSTRACT With the ever-increasing use and applications of nanowires it has never been more imperative to understand how environmental interactions modify their unique mechanical properties and loading behaviour. While experimental research has shown that atomic hydrogen degrades mechanical properties through hydrogen embrittlement, results are limited and often do not directly quantify the hydrogen concentration or consider small diameter nanowires. In this study, we have used molecular dynamics simulations to the study the effect of atomic hydrogen on iron nanowires with various orientations and diameters. Results demonstrate that with increasing hydrogen concentration there is a clear reduction in the elastic modulus and yield stress as compared to the hydrogen free case for all diameters and orientations considered. In addition, this reduction in mechanical properties appears to exhibit a size dependence, with larger reductions being found in nanowires with larger cross-sectional diameters. We suggest that smaller diameter nanowires, with a higher ratio of surface to bulk atoms, are more influenced by free surface atoms than lattice distortions from atomic hydrogen. As this ratio of surface to bulk atoms is decreased, the larger diameter nanowires become less affected by free surfaces and more susceptible to the effect of atomic hydrogen.

Reentrant phenomenon in the decorated ising model

Based on the transverse-field Ising model, this paper utilizes the theory of correlated effective fields to explore the ferromagnetic phase transition properties of nanoislands with decorated lattice. The findings suggest that the unique phase transition properties of the system are predominantly determined by the surface atoms, while the transverse field and decorating exchange interaction impact the specific details of the phase transition properties. Additionally, the magnetization of each lattice follows a quantitative relationship, in which the magnetization of the decorated lattice is weaker and that of the central lattice is stronger.

Information hidden behind a single peak in the C 1s spectrum of graphene on Ir(111)

The energy resolution that can be achieved in x-ray photoelectron spectroscopy experiments allows to disentangle the contribution arising from the presence of a large variety of surface atoms in non-equivalent configurations which manifests itself not only with the appearance of different spectral components, but also as unusual lineshape.In the present work, we show that the fit of the C 1s core level spectrum of graphene grown on Ir(111) realized using 200 peaks based on ab initio calculations, accounting for the non-equivalent C atoms in the 10×10 moiré cell, does not improve the fit quality with respect to the use of a single component. On the contrary, the quantitative fit quality can be drastically increased by introducing a dependency of the Lorentzian width on the distance between C and Ir first-layer atoms. This result is associated to the different electronic properties, and in particular to the different density of states of the σ and π bands, of C atoms sitting on TOP (hills) or FCC (valleys) regions of graphene which affect the lifetimes of the core-holes generated during the photoemission process.

Effect of morphology and their oxygen vacancies of nanostructured CeO2 catalyst for carboxymethylation of biomass-derived alcohols

An effective and highly selective protocol has been realized for the synthesis of asymmetric organic carbonates using dimethyl carbonate (DMC) as a reactant and solvent. Herein, the performance of CeO2 nanostructures with different morphology was explored in the carbonate interchange reaction (CIR) of alcohols to synthesize asymmetric organic carbonates. The CeO2 nano-catalyst with rod morphology having the highest oxygen vacancy enabled remarkable enhancement in the conversion, to our knowledge this is superior to most of the reported heterogeneous catalytic materials. The CeO2-r (r-rods) is used as low as <0.8 mol% with respect to the limiting reagent (alcohol). The CeO2 characterization data revealed the intriguing properties such as exposed active sites, defect density, coordination state of surface atoms, and reducibility of the catalytic materials are the contributing factors in realizing the highest catalytic activity. The CeO2 was recovered by a simple centrifugation and used in more than four recycles with consistent catalytic activity. The work conducted herein intensifies the utilization of bio-alcohols and CO2 derived DMC for the sustainable synthesis of organic carbonates that have immense applications in various sectors.

Nanoporous ALD-modified oxygen-deficient NiO flakes as anodes for lithium-ion batteries

The flake-like nature of materials possessing continuous porous skeletons makes them naturally suited for energy storage and other applications. Nickel oxide (NiO) stands out as a highly attractive anode material for lithium-ion batteries due to its relatively high theoretical specific capacity. Nevertheless, the sluggish kinetics of its reactions and significant volume variations during cycling curtail its rate capability and electrochemical stability. In this work, we successfully fabricate nanoporous oxygen-deficient NiO@Al2O3 flakes by a multi-step process involving magnetron sputtering deposition, subsequent thermal oxidation treatment, and a particle atomic layer deposition surface coating technique. The resultant nanoporous oxygen-deficient NiO@Al2O3 flakes demonstrate the potential to establish an intricate network that facilitates efficient charge transfer, rapid ion diffusion, and mechanical robustness throughout electrochemical processes. Particularly noteworthy is the role of the ultrathin Al2O3 coating, which greatly bolsters the cycle life. When employed as lithium storage materials, the nanoporous NiO@Al2O3 flakes exhibit remarkable attributes, including high specific capacity (720 mAh g−1 at 25 mA g−1), superior rate capability (300 mAh g−1 at 5000 mA g−1) and sustained cycling ability, showcasing a reversible capacity of 413 mAh g−1 after 500 cycles at 150 mA g−1 (equivalent to a duration exceeding 4 months). Moreover, the distinctive morphologies and compositions observed after extended cycles definitively corroborate the efficacy of the nanoporous flake architecture. Finally, we present a full-cell configuration comprising a NiO@Al2O3 flakes anode paired with a LiNi0.8Mn0.1Co0.1O2 cathode, which notably exhibits remarkable cycle stability, retaining 84.2 % of its capacity after 100 cycles at 200 mA g−1. This effective and versatile strategy might hold promise for extension to alternative anode and cathode materials, potentially enabling enhanced specific capacity and rendering them suitable for energy storage applications.

Oxygen adsorption in Ag(m)@X(n) (X=Pd, Pt) core-shell nanoalloys (m+n=55), structural and energetic properties: A DFT study

The interaction between surface atoms and adsorbates is a major element of the reactivity and stability of the catalyst. Previous experimental research has indicated potential associations between oxygen adsorption and Ag surfaces. The principal objective of this study is to identify the atomic oxygen adsorption effect on the structural families and energies of Pt13Ag42 and Pd13Ag42 core-shell nanoalloys using the Gupta and DFT approaches. Results showed that Pt and Pd atoms were located in the inner sites of the nanoalloys, and Ag atoms were found on the surface. The findings also indicated a positive relationship between the Ag–O bond number and energetic stability in the oxygen adsorption of the Pt13Ag42 and Pd13Ag42 nanoalloys. Our general results are that chiral Dh is the most suitable configuration for oxygen adsorption due to DFT relaxations, and the energy gains of similar configurations in both nanoalloys are almost equal.

Hardware, methodology and applications of 2+D backscatter Mössbauer spectroscopy with simultaneous x-ray and γ-ray detection

A unique method is presented for the acquisition and analysis of 57Fe backscatter Mössbauer spectra with simultaneous detection of the resonant 14.4 keV γ-rays and the characteristic 6.4 keV x-rays, using a custom-built multi-parameter analyser constructed on the basis of commercial analogue to digital converters and high-speed digital latches. The system allows for the simultaneous registration of Doppler-modulation velocities and photon energies, with up to 4096 and 8192 digital channels respectively. This arrangement is in contrast to most related systems, which detect at a single narrow energy window per detector. Samples of arbitrary atomic structure, morphology and surface topography can be studied without altering the setup or the analysis procedure, provided that the samples are at least micrometre sized. The hardware and software that are used to acquire data with minimal dead time are described and the custom and self-contained methods for post-measurement energy discrimination, background correction and velocity-axis folding are discussed. The data are fit using a general Hamiltonian model for the nuclear energy levels of 57Fe and a quantum mechanical description of the angular momentum coupling is utilised, with consideration of the crystalline and chemical disorder of the sample under examination. Three examples of distinct magnetic systems, with thicknesses ranging from m to 6 mm, that were studied using this method are presented, these are: an amorphous CoFeB-based ribbon with ultra-soft coercivity for high-frequency applications, magnetically hard Nd-Fe-B thick films on Si substrates, examined in both as-deposited and annealed states, and a sample from the nickel-rich iron meteorite NWA 6259 that contains the atomically ordered, elevated coercivity, phase of FeNi, tetrataenite. The wide applicability and usefulness of this method is thus demonstrated on three distinct sample morphologies that required little to no surface preparation prior to examination.

A novel insight of interaction mechanism of carboxymethyl cellulose with talc surface: A combined molecular dynamic simulation and DFT investigation

Separation of talc from sulfide minerals through adding depressor in flotation field is very necessary, while the interaction of depressor on the surface of talc still needs to be further explained. Here, a typical depressor, carboxymethyl cellulose (CMC), interacted with talc surface was deeply investigated from atomic structure and surface hydration though molecular dynamic simulation (MDs) and density functional theory (DFT) calculation. MDs was applied on investigating adsorption behaviors of CMC on basal and edge plane of talc in aqueous solutions, and DFT was used for studying the interacted orbitals and charge transfer. It was found that adsorption of CMC on surface of edge plane was stronger than that on the surface of basal plane due to the enhanced interaction of polar group on the edge plane and the dragged water molecules. Interactions existed between H atoms in CMC and O atoms in edge plane, between O atoms in CMC and Si, Mg atoms in edge plane. The strong interacted water molecules dragged CMC close to the surface, thus to strengthen the binding of CMC and edge plane of talc. Furthermore, the hydration shell on the basal and edge plane of talc was investigated for explaining the phenomenon. Therefore, our work gave a new insight into the microscopic interaction between talc and CMC, thus to guide the actual application.

A single response to reducing gases by NiO-TiO2 heterojunction nanocrystals

Utilizing p-n composite material to fabricate metal-oxide semiconductors (MOSs) gas sensors is a promising strategy to achieve ultrahigh selectivity detection of reducing gases, such as H2 and CO. This work utilized a facile doping method to synthesize p-NiO/n-xTiO2 nanoparticles with p-n heterojunction. The p-NiO/n-1.0TiO2 nanoparticles annealed at 550 °C exhibited ultrahigh selectivity with an n-type gas-sensitive response to H2 at 300 °C, eliminating the cross-sensitivity phenomenon. This gas selectivity to H2 and CO highly depends on the Ni:Ti molar ratios and operating temperature. Under repeated ventilation conditions, the response values of p-NiO/n-1.0TiO2 sensor to H2 and CO remained unchanged, confirming its excellent stability. Multiple gas experiments show that the p-NiO/n-1.0TiO2 sensor can be utilized to selectively detect any of H2, CH4, NO2, and SO2 in the presence of CO. Furthermore, the study of H2 and CO adsorption behavior on the NiO-TiO2 revealed that H2 is more easily adsorbed on the NiO-TiO2 heterojunction surface than CO. Introduction of H2 caused electron transfer between the gas molecule and surface atoms of the heterojunction, while the electronic structure of the system remained unchanged after CO adsorption. This study provides an effective method to improve the selectivity of MOS-based gas sensors.

First principles study on the adsorption of lead ions by black phosphorus in aqueous solution

In this paper, the adsorption behavior of intrinsic black phosphorus (BP) and BP doped with five atoms of O, Si, S, Cu and Zn on Pb[Formula: see text] in aqueous solution was investigated by using the first principle of nature. The formation energy was calculated to be the smallest for Si-doped BP at 2.608[Formula: see text]eV, indicating the best structural stability. The adsorption energy was calculated and showed that the doped BP is more capable of adsorbing Pb[Formula: see text] than the intrinsic BP, with the S-doped BP having the strongest adsorption capacity of 0.151[Formula: see text]eV. The density of states was calculated and showed that the bandgaps of the doped BP are all smaller than the 1.3[Formula: see text]eV of the intrinsic BP, indicating that the doping can improve electrical conductivity of the adsorption system. The charge transfer was calculated and showed that S doping, O doping, Cu doping and Zn doping enhanced the ionic bonding between BP surface atoms and Pb[Formula: see text], while Si doping weakened the ionic bonding. The charge transfer of S doping was the largest at 1.371[Formula: see text]e, indicating that S-doped BP had the strongest ability to adsorb Pb[Formula: see text].

High‐Quality van der Waals Epitaxial CsPbBr3 Film Grown on Monolayer Graphene Covered TiO2 for High‐Performance Solar Cells

Two‐dimensional materials have been widely used to tune the growth and energy‐level alignment of perovskites. However, their incomplete passivation and chaotic usage amounts are not conducive to the preparation of high‐quality perovskite films. Herein, we succeeded in obtaining higher‐quality CsPbBr3 films by introducing large‐area monolayer graphene as a stable physical overlay on top of TiO2 substrates. Benefiting from the inert and atomic smooth graphene surface, the CsPbBr3 film grown on top by the van der Waal epitaxy has higher crystallinity, improved (100) orientation, and an average domain size of up to 1.22 μm. Meanwhile, a strong downward band bending is observed at the graphene/perovskite interface, improving the electron extraction to the electron transport layers (ETL). As a result, perovskite film grown on graphene has lower photoluminescence (PL) intensity, shorter carrier lifetime, and fewer defects. Finally, a photovoltaic device based on epitaxy CsPbBr3 film is fabricated, exhibiting power conversion efficiency (PCE) of up to 10.64% and stability over 2000 h in the air.