Surface Atomic Structure Research Articles

Step-by-step silicon carbide graphitisation process study in terms of time and temperature parameters

This work investigates the temperature and time as key parameters for graphene formation on the silicon carbide surface during the high thermal decomposition process. Measurements were performed using various experimental techniques under ultra-high vacuum conditions. The graphitisation process was divided into various stages, after which the surface chemical composition and atomic structures were analysed in detail. It has been shown that despite the known theory of graphitisation mechanism and initial condition for occurrence of this process, the application of different temperatures and heating times affect the quality and quantity of formed graphene layers. Applying a temperature too low or annealing the sample for a too short time led to an inefficient silicon sublimation process. On the other hand, too high temperature during flashing modifies the visibility of surface structures, which may be crucial for other investigations and potential applications of such systems.

PtCu-a-SnO2 interface engineering on PtCu-SnO2 aerogels for ethanol oxidation electrocatalysis

Designing catalysts with high activity and stability to boost their ethanol oxidation reaction (EOR) performance is one of the central targets in the development of direct ethanol fuel cells (DEFCs). However, how to design the EOR catalysts in a rational way is still challenging, needing more efforts put in. Herein, we have synthesized a catalyst of PtCu-SnO2 aerogel via ingeniously introducing Cu into Pt-SnO2 system, resulting in the formation of a main phase of alloy (PtCu)-amorphous oxide (a-SnO2) interface. Benefiting from the optimized structures, the PtCu-SnO2 aerogel displays excellent acidic EOR performance with 4.5 times promotion in activity compared to pure Pt aerogel. In-depth investigations through electrochemical in-situ Fourier transform infrared spectroscopy and density functional theory calculations have shown that the absorbed OH on a-SnO2 surface together with the greatly-modified electronic structure of the alloy surface atoms can facilitate the C1 pathway and the oxidation of *CO intermediates for ethanol oxidation reaction. The resulting affinity of surface OH and the modification of reaction energies both in turn contribute to the improvement of EOR performance with high activity, stability, and anti-poisoning ability, proving interfacial engineering an effective strategy for the rational design of EOR catalyst.

Probing the Structure of the Electrochemical Interface Using in-Situ Surface X-Ray Scattering Techniques

Surface X-ray Scattering (SXS) or Surface X-ray Diffraction (SXRD) using synchrotron radiation is a powerful tool for the study of single crystal metal electrodes in the electrochemical environment. The single crystal surfaces have well defined adsorption sites and this enables fundamental links to be made between the surface atomic structure and the electrochemical response as has been illustrated, for example, in electrocatalysis [1]. As the development of electrochemical surface science progresses, there is a growing need for a comprehensive understanding of the atomic sturcture of electrochemical interface, both of the solid electrode and in the liquid electrolyte. This is because changes in the atomic structure across the interface induced by changes in the electric field can affect the system’s reactivity and stability [2]. These processes can be subtle and do not always involve the direct transfer of charge between electrode and electrolyte. Examples include metal surface relaxation and surface reconstruction, as well as charging of the double layer, which leads to rearrangement of the electrolyte side of the interface [3]. To further extend the fundamental understanding of the electrochemical interface structure, we have been developing resonant surface x-ray diffraction (RSXRD), a technique which combines diffraction with spectroscopy, to investigate the charge distribution in the double layer region. Combining experimental data and first-principles calculations, in-situ RSXRD was used to establish the charge distribution both at the Pt(111) electrode surface and at the Cu(001)-c(2x2)-Br electrode surface [5,6].In this talk results of recent studies of single crystal metal electrodes to probe the liquid side of the interface, particularly the role of cations in the electrochemical double layer, will be presented. The results include a systematic study of the low index Ag(hkl) surfaces to explore the role of the surface atomic geometry in determining the electrolyte layering and an exploration of the role that Cs+ cations play in forming the double layer structures on single crystal Au and Pt electrodes.[1] Y. Gründer and C. A. Lucas, Surface X-ray Diffraction Studies of Single Crystal Electrocatalysts, Nano Energy, 29, 378 (2016)[1] O. M. Magnussen and A. Groß, Toward an Atomic-Scale Understanding of Electrochemical Interface Structure and Dynamics. Journal of the American Chemical Society, 141, 4777 (2019)[2] Y. Gründer and C.A. Lucas, Potential-induced structural deformation at electrode surfaces. Current Opinion in Electrochemistry, 19, 168 (2020)[3] Y. Soldo-Olivier et al., Unraveling the Charge Distribution at the Metal-Electrolyte Interface Coupling in Situ Surface Resonant X-ray Diffraction with Ab Initio Calculations, ACS Catalysis, 12, 2375 (2022)[4] Y. Gründer et al., Charge Reorganization at the Adsorbate covered Electrode Surface probed through in-situ resonant X-ray Diffraction Combined with Ab-initio Modelling, Journal of Physical Chemistry C, 126, 4612 (2022)

(Invited) In Situ x-Ray Diffraction Studies of the Atomic Structure and Charge Distribution at the Electrochemical Interface

The presence of specifically adsorbed anions can significantly affect the electrochemical reactivity of a metal electrode which is of major interest for galvanic deposition, etching, corrosion and electrocatalysis. In-situ surface x-ray diffraction has enabled an atomic/molecular-level understanding of the interface under reactive conditions, including its potential and time dependence, to be developed. While information about the atomic structure of the electrode surface in electrochemical in-situ cells has been widely investigated, insight into the charge distribution and the structure of the electrolyte at the interface is still lacking. Advances in these directions offer possibilities in elucidating atomic scale models of the electrochemical interface and thus will help to establish structure-stability-reactivity relationships.A fundamental understanding of the nature of the charge transfer, especially the influence of the applied potential and the screening by the electrolyte, is a major goal in electrochemistry to better understand electrochemical processes and charge transfer during adsorption and deposition. [1]Thus, combining x-ray spectroscopy and x-ray diffraction to gain site specific information about the charge distribution at buried interfaces is a promising tool. [2,3]Examples of how the use of surface x-ray scattering techniques can help to characterize electrochemical interfaces in-situ in order to link, structure, reactivity and stability will be presented. [4-5] Advances in these directions offer possibilities in elucidating atomic scale models of the electrochemical interface and thus will help to establish structure-stability-reactivity relationships and to understand growth kinetics and electrochemical phase formation.

Effect of surface functionalization on the quantum capacitance of M2CF2 (M = Ti, V, Cr, Zr, Nb, Mo) as supercapacitor electrodes

The surface charge storage capacity, atomic structures, and quantum capacitance of M2CF2 (M=Ti, V, Cr, Zr, Nb, Mo) are investigated using density functional theory (DFT). The influence of different surface functionalization methods on the capacitance performance of M2CF2 is also examined, including the introduction of vacancy defects (C-, F-, M-vacancy) and nonmetal doping (N, O, and S). The findings show that both defects and doping reduce the quantum capacitance of V2CF2, Cr2CF2, and Mo2CF2, but they have a negligible impact on the capacitance behavior of Ti2CF2, Zr2CF2 and Nb2CF2. Furthermore, the pristine Cr2CF2 system exhibits relatively high quantum capacitance and surface charge storage capacity under both positive and negative bias. However, the presence of vacancies and dopants significantly decreases its capacitance. Among the structures studied, pristine and nonmetal-doped V2CF2 demonstrate the highest quantum capacitance values (1302.02–1517.56 μF/cm2), suggesting its potential as an effective positive electrode material. Overall, the findings of this study are expected to boost the development of high-capacitance electrodes in the future.

Atomic-Level Observation of Potential-Dependent Variations at the Surface of an Oxide Catalyst during Oxygen Evolution Reaction.

Understanding the intricate details of the surface atomic structure and composition of catalysts during the oxygen evolution reaction (OER) is crucial for developing catalysts with high stability in water electrolyzers. While many notable studies highlight surface amorphization and reconstruction, systematic analytical tracing of the catalyst surface as a function of overpotential remains elusive. Heteroepitaxial (001) films of chemically stable and lattice-oxygen-inactive LaCoO3 are thus utilized as a model catalyst to demonstrate a series of atomic-resolution observations of the film surface at different anodic potentials. The first key finding is that atoms at the surface are fairly dynamic even at lower overpotentials. Angstrom-scale atomic displacements within the perovskite framework are identified below a certain potential level. Another noteworthy feature is that amorphization (or paracrystallization) with no long-range order is finally induced at higher overpotentials. In particular, surface analyses consistently support that the oxidation of lattice oxygen is coupled with amorphous phase formation at the high potentials. Theoretical calculations also reveal an upward shift of oxygen 2p states toward the Fermi level, indicating enhanced lattice oxygen activation when atom displacement occurs more extensively. This study emphasizes that the degradation behavior of OER catalysts can distinctively vary depending on the overpotential level.

Unveiling Atomic-Scaled Local Chemical Order of High-Entropy Intermetallic Catalyst for Alkyl-Substitution-Dependent Alkyne Semihydrogenation.

High-entropy intermetallic (HEI) nanocrystals, composed of multiple elements with an ordered structure, are of immense interest in heterogeneous catalysis due to their unique geometric and electronic structures and the cocktail effect. Despite tremendous efforts dedicated to regulating the metal composition and structures with advanced synthetic methodologies to improve the performance, the surface structure, and local chemical order of HEI and their correlation with activity at the atomic level remain obscure yet challenging. Herein, by determining the three-dimensional (3D) atomic structure of quinary PdFeCoNiCu (PdM) HEI using atomic-resolution electron tomography, we reveal that the local chemical order of HEI regulates the surface electronic structures, which further mediates the alkyl-substitution-dependent alkyne semihydrogenation. The 3D structures of HEI PdM nanocrystals feature an ordered (intermetallic) core enclosed by a disordered (solid-solution) shell rather than an ordered surface. The lattice mismatch between the core and shell results in apparent near-surface distortion. The chemical order of the intermetallic core increases with annealing temperature, driving the electron redistribution between Pd and M at the surface, but the surface geometrical (chemically disordered) configurations and compositions are essentially unchanged. We investigate the catalytic performance of HEI PdM with different local chemical orders toward semihydrogenation across a broad range of alkynes, finding that the electron density of surface Pd and the hindrance effect of alkyl substitutions on alkynes are two key factors regulating selective semihydrogenation. We anticipate that these findings on surface atomic structure will clarify the controversy regarding the geometric and/or electronic effects of HEI catalysts and inspire future studies on tuning local chemical order and surface engineering toward enhanced catalysts.

Tuning the active plane and crystallinity of GaN microcrystals for high-performance supercapacitors through potassium nitrate-mediated synthesis

The surface atomic structure and crystallinity have an important effect on the electrochemical energy storage of electrode materials, in addition to the surface chemistry and textural properties. We report here for the first time that the surface atomic structure and crystallinity of GaN, a renowned electrode material for energy storage, can be tuned by controlling the annealing time via potassium nitrate-mediated synthesis. The underlying mechanism for GaN microcrystals with enhanced intensity ratios of I(002)/(100) and I(101)/(100) manifesting excellent rate performance has been revealed by theoretical computations. The energy storage mechanism and electrode kinetics of the GaN electrodes have been clarified. In addition, the GaN microcrystals-based symmetric supercapacitors empowered by 52 wt% H3PO4 can deliver an output voltage of 1.5 V and volumetric specific energy of 11.6 and 40.2 W h L−1 at a specific power of 392.2 W L−1 when operating at −60 and 60 °C, respectively, with electrode material on a commercial loading level.

Effects of chain-chain interaction on the configuration of short-chain alkanethiol self-assembled monolayers on a metal surface.

Surface-specific sum frequency generation vibrational spectroscopy is applied to study the molecular configuration of short-chain n-alkanethiol self-assembled monolayers (SAMs with n = 2-6) on the Au surface. For monolayers with n≥ 3, the alkanethiols are upright-oriented, with the CH3 tilt angle varying between ∼33° and ∼46° in clear even-odd dependency. The ethanethiol monolayer (n = 2) is, however, found to exhibit a distinct lying-down configuration with a larger methyl tilt angle (67°-79°) and a smaller CH2 tilt angle (56°-68°). Such a unique configurational transition from n = 2 to n≥ 3 discloses the steric effect owing to chain-chain interaction among neighboring molecules. Through density functional theory calculations, the transition is further confirmed to be energetically favorable for thiols on a defective reconstructed Au(111) surface but not on the pristine one. Our study highlights the roles of the chain-chain interaction and the substrate surface atomic structure when organizing SAMs, offering a strategic pathway for exploiting their applications.

Influence of Nb substrate morphology and atomic structure on Sn nucleation and early Nb3Sn growth

The enhanced accelerating performance of Nb3Sn-coated Nb superconducting radio frequency (SRF) cavities is severely limited by quenching sites at material defects formed during the Nb3Sn film growth procedure. In this work, we aimed to understand the complex surface-mediated interactions that drive initial Nb3Sn formation during the Sn vapor deposition procedure used to fabricate Nb3Sn coated SRF cavities. Nb substrates were modified, coated with Sn, and then characterized using an ultra-high vacuum (UHV) chamber equipped with metal deposition and in situ surface analysis capabilities. The atomic structure of the Nb oxide surface was modified to assess how the Nb surface defect density and crystallographic orientation impact the size of nucleated Sn islands and relative Nb3Sn growth rates. Formed Nb3Sn/Nb surfaces were visualized using ex situ scanning electron microscopy (SEM) and post-deposition annealing revealed Sn desorption and Nb3Sn degradation pathways. Finally, we showed that the Sn deposition conditions can be modified to overcome the growth barriers that are imposed by the Nb morphology. This study provides a unique bridge between fundamental growth studies in pristine conditions with observed phenomena of Nb3Sn grown on realistic cavity surfaces.

A highly-stable and low-energy-barrier photoelectrocatalyst (NiFe-LDH@Ti-MOF) for water splitting at high current densities via breaking the atomic structure symmetry

There is a flurry of interest in enhancing the stability and efficiency of photoelectrocatalysts in water splitting at high current densities. In the present study, nickel–iron layered double hydroxide (NiFe-LDH) was decorated with MIP-177-LT (Ti-metal organic framework (MOF)) to fabricate a heteroatom electrocatalyst (NiFe-LDH@Ti-MOF). The overpotentials of the oxygen evolution reaction (OER) turned out to be very low: 199 and 232 mV at current densities of 200 and 600 mA cm−2, in that order. Then, current densities of 1,000 mA cm−2 were applied to the modified electrode over a long period of 800 h, and this generated a higher stability than that of the commercial IrO2. Further, its mass activity proved ∼2 and 7 times as high as those of IrO2 and NiFe-LDH, in that order. Furthermore, for water splitting, the produced photoelectrocatalyst obtained a current density of 1,000 mA cm−2 using only 1.6 V, and this performance remained changeless for 800 h, which can be considered the most superb performance ever reported. The experimental characterization revealed that during the process of doping Ti-MOF on NiFe-LDH, symmetry breaking takes place in the atomic structure of the LDH surface, which increases the active sites in the photoelectrocatalyst. The presence of oxygen in the Ti-MOF active sites improves the efficiency of OER at high current densities and also contributes to the higher stability of multiple heteroatomic interfaces for water splitting. The propounded strategy enables the generation of oxygen and hydrogen from water splitting at high current densities on an industrial scale by changing the symmetry of the atomic structure of heteroatomic photocatalysts.

Sulfidation-Induced Surface Local Electronic and Atomic Structures in a Silver Catalyst Enables Silicon Photocathode for Selective and Efficient Photoelectrochemical CO2 Reduction.

Converting CO2 to value-added chemicals through a photoelectrochemical (PEC) system is a creative approach toward renewable energy utilization and storage. However, the rational design of appropriate catalysts while being effectively integrated with semiconductor photoelectrodes remains a considerable challenge for achieving single-carbon products with high efficiency. Herein, we demonstrate a novel sulfidation-induced strategy for in situ grown sulfide-derived Ag nanowires on a Si photocathode (denoted as SD-Ag/Si) based on the standard crystalline Si solar cells. Such an exquisite design of the SD-Ag/Si photocathode not only provides a large electrochemically active surface area but also endows abundant active sites of Ag2S/Ag interfaces and high-index Ag facets for PEC CO production. The optimized SD-Ag/Si photocathode displays an ideal CO Faradic efficiency of 95.2% and an onset potential of +0.26 V versus the reversible hydrogen electrode, ascribed to the sulfidation-induced synergistic effect of the surface atomic arrangement and electronic structure in Ag catalysts that promote charge transfer, facilitate CO2 adsorption and activation, and suppress hydrogen evolution reaction. This sulfidation-induced strategy represents a scalable approach for designing high-performance catalysts for electrochemical and PEC devices with efficient CO2 utilization.

Oxygen Vacancies Boost High‐Energy {100} Faceted TiO2 for Efficient Photocatalytic Degradation of Rhodamine B

AbstractPhotocatalytic reaction takes place on the surface of the catalyst, the crystal plane engineering which can adjust and optimize the surface atomic ordering and electronic energy band structure has been widely studied, considering the microchemical environment changes caused by defects on different surfaces, the combination of crystal plane engineering and defect engineering is expected to become a promising strategy to greatly improve the performance of photocatalysis. In this paper, we introduce 2,2’‐Bipyridyl to induce oxygen vacancy in the synthesis of highly exposed {100} faceted TiO2 nanorods, and obtain oxygen‐rich TiO2 nanomaterials. Benefit from the synergistic effect of high‐energy crystal plane and oxygen vacancy, 96.8 % Rhodamine B (10 mg ⋅ L−1) is degraded after irradiation of 30 min in T100‐VO photocatalytic system. This study provides a new strategy and inspiration for low‐cost control of semiconductor oxides and the design of efficient photocatalysts for environmental purification.

Pd nanocubes enclosed by {100} facets for activating electroless Cu deposition on liquid crystal polymer substrates with strong adhesion strength

In this study, we investigate the catalytic performance of palladium (Pd) nanocatalysts with different surface atomic structures for activating electroless copper (Cu) deposition on liquid crystal polymer (LCP) substrates. These nanocatalysts include Pd nanocubes featuring {100} facets of square atomic arrangement as their predominant surface structure and Pd spherical nanoparticles characterized by random atomic arrangements, synthesized using an aqueous method. Our electrochemical measurements reveal that the induction time for the growth of Cu films is significantly shorter when utilizing Pd nanocubes compared to Pd nanospheres. This shorter induction time signifies a quicker initiation of Cu growth, primarily due to the presence of Pd {100} facets. Furthermore, we also develop a surface modification approach involving a crosslinking reaction that utilizes the highly branched polyethylenimine and linear polyvinylpyrrolidone on the Pd nanocubes. This method not only facilitates the adsorption of Pd nanocubes onto the LCP substrates but also achieves a robust peel adhesion strength (630 gf/cm) of the resulting Cu films, exceeding the industrial adhesion goal of 500 gf/cm. Moreover, in-depth synchrotron X-ray photoelectron spectroscopy analysis provides valuable insights into the depth-dependent chemical changes within the Cu films and proves the removal of Cu oxides after the acid treatment.

How a mineral that’s always wet gets wetter

Potassium-rich feldspar is a hydrophilic mineral that accelerates ice nucleation in the atmosphere. For the first time, the atomic surface structure has been observed.

Atomistic Investigation of Grain Boundary Fracture in Alumina.

Grain boundary (GB) fracture is a major mechanism of material failure in polycrystalline ceramics. However, the intricate atomic arrangements of GBs have impeded our understanding of the atomistic mechanisms of these processes. In this study, we investigated the atomic-scale crack propagation behavior of an α-Al2O3 ∑13 grain boundary, using a combination of in situ transmission electron microscopy (TEM) and scanning TEM. The atomic-scale fracture path along the GB core was directly determined by the observation of the atomic structures of the fractured surfaces, which is consistent with density functional theory calculations. We found that the GB fracture can be attributed to the weaker local bonds and a smaller number of bonds along the fracture path. Our findings provide atomistic insights into the mechanisms of crack propagation along GBs, offering significant implications for GB engineering and the toughening of ceramics.

Heteroatom surface engineering with distinct doping ways and valence state regulation: Boosting electrocatalysis performance of Ni-doped MoS2/rGO host for excellent lithium sulfur batteries

Sulfur host materials are crucial to the electrochemical performances of lithium sulfur batteries (LSBs). Two-dimensional (2D) nanomaterials with close atomic thickness and large specific surface area are considered as excellent sulfur hosts in lithium sulfur batteries due to their extraordinary physical properties and special structures, which play a key role in improving the redox reaction kinetics of sulfur/Li2S. The heteroatom doping, which is usually applied in surface engineering, can not only adjust surface coordination environment but also form surface functional groups. Herein, the surface structures and properties of 2D Ni-doped MoS2/rGO composite are regulated via two surface engineering strategies. One is that nickel atoms are doped into 2D MoS2 nanosheets by two distinct hydrothermal techniques, which would result in notable differences of surface heteroatom content, and another is that the valence state of surface heteroatom nickel is changed by the thermal reduction technology. The content and valence state of surface heteroatom nickel can significantly influence the electrocatalysis activities of host materials during the redox reaction processes of sulfur/Li2S, which would substantially alter the electrochemical performances of LSBs. When the S@3%Ni-MoS2/rGO-IIRe is used as sulfur host, the lithium‑sulfur cell has an initial discharge capacity of 936.4 mA h g−1 and a low capacity attenuation of only 0.027 % each cycle with the high capacity of 819.7 mA h g−1 after 450 cycles at 1C, exhibiting an excellent coulombic efficiency and outstanding cycle stability. This study would provide some inspiration for improving the electrochemical performance of advanced lithium‑sulfur batteries through the rational regulation of surface atomic structures and electronic states of sulfur hosts.

Real-time observation of liquid-gallium ordering on epitaxially-grown GaN(0001) by X-ray scattering measurements

X-ray crystal truncation rod (CTR) measurements are commonly used to analyze surface or interfacial atomic structures quantitatively. However, it is difficult to expand the measurement range to determine three-dimensional atomic structures in real time. We developed a method for CTR measurements that involves the simultaneous intensity measurements of X-rays and electron diffraction under molecular beam epitaxial conditions. Using the proposed method, we observed the formation of liquid gallium ordering on the GaN(0001) surface and determined the ordered structure within each layer.

The topological soliton in Peierls semimetal Sb

Sb is a three-dimensional Peierls insulator. The Peierls instability gives rise to doubling of the translational period along the [111] direction and alternating van der Waals and covalent bonding between (111) atomic planes. At the (111) surface of Sb, the Peierls condition is violated, which in theory can give rise to properties differing from the bulk. The atomic and electronic structure of the (111) surface of Sb have been simulated by density functional theory calculations. We have considered the two possible (111) surfaces, containing van der Waals dangling bonds or containing covalent dangling bonds. In the models, the surfaces are infinite and the structure is defect free. Structural optimization of the model containing covalent dangling bonds results in strong deformation, which is well described by a topological soliton within the Su–Schrieffer–Heeger model centered about 25 Å below the surface. The electronic states associated with the soliton see an increase in the density of states (DOS) at the Fermi level by around an order of magnitude at the soliton center. Scanning tunneling microscopy and spectroscopy (STM/STS) measurements reveal two distinct surface regions, indicating that there are different surface regions cleaving van der Waals and covalent bonds. The DFT is in good agreement with the STM/STS experiments.

Formation of grain boundaries in nanocrystalline SiC ceramics examined by powder diffraction supported by MD simulations

Atomic structure of nanocrystalline SiC powder sintered under high-pressure high-temperature conditions was examined with use of powder diffraction technique supported by molecular dynamics simulation. Shape and atomic structure of grain surfaces was identified based on real space PDF analysis of experimental diffraction data and theoretical calculations performed for atomic models of nanograins relaxed by MD simulation. It is shown that in presence of about 5 % of excess carbon in the initial 11 nm size SiC powder HPHT sintering leads to formation of (100) and (110) grain boundaries formed by single C planes and (111) boundaries formed by double C planes. Under pressure 3 GPa and 1600°C sphere-like grains tend to transform to dodecahedrons terminated by single C atomic planes. Under 8 GPa the grains adopt the shape of cube-octahedrons terminated by (100) single C planes and (111) planes terminated by single and double C planes.