Surface Atomic Structure Research Articles

Quantifying fluxional behavior in catalytic CeO2 nanoparticles: toward thermodynamic insight into the stability of surface atomic structures

Quantifying fluxional behavior in catalytic CeO₂ nanoparticles: toward thermodynamic insight into the stability of surface atomic structures

Atomic Structure of the Cu2O(111) Surface: A Transmission Electron Microscopy and DFT + U Study

The atomic structures of Cu2O surfaces can significantly affect catalytic properties. By combining aberration‐corrected transmission electron microscopy (TEM) with density functional theory (DFT) calculations, it is shown that the Cu2O(111) surface stabilizes in a bulk‐truncated 1 × 1 structure with coordinatively unsaturated Cu atoms under low‐oxygen conditions. This differs from the known 1 × 1‐CuCUS structure with coordinatively unsaturated Cu atom vacancies.

Ab Initio modeling of Near-Edge EELS spectra for chemisorbed molecules

Electron energy loss spectroscopy (EELS) has recently been applied to probe chemisorbed molecules on metal nanostructures, but a fundamental understanding of the correlation between these spectra and the electronic structures of the adsorbates has been limited. We report here on the insights afforded by time-dependent density functional theory to decipher the energy loss near edge structure (ELNES) of EELS spectra associated with chemisorption. These first-principles calculations simulate the ELNES-EELS spectra for chemisorbed CO on various facets of Au and Pt. Computational predictions of key signatures such as the ‘red shift’ and reductions in the peak intensity of the 2π* and 6σ* peaks, as compared to free CO in the gas phase, are validated in comparison to experimentally collected EELS spectra. These signatures are revealed to arise from changes in the electronic structure in terms of unoccupied density of states associated with the chemisorption process. They are consistent with a Blyholder model that incorporates donation and back-donation of electrons. They are also characteristic of the chemisorption process, such as the choice of metal, site of adsorption and the coverage and distribution of adsorbates. Our simulations thus provide guidelines for the use of ELNES-EELS to characterize the atomic structure and adsorption property of nanostructured surfaces and facilitate the development of advanced nanomaterials for catalytic applications.

Interplay of Activity and Stability within Hydrous Cobalt-Iridium Oxide Oxygen Evolution Electrocatalysts

To further the development of proton-exchange membrane electrolyzers, oxygen evolution reaction (OER) electrocatalysts with high activity, extended durability, and lower costs are needed. We have explored iridium-nickel and iridium-cobalt two-dimensional nanoframes where the interaction of iridium with other metal alters the surface electronic structure and influences the activity and stabiity.1,2 In contrast to comparable structures with nickel, hydrous cobalt-iridium oxide two-dimensional nanoframes exhibit higher oxygen evolution activity and similar stability compared to commercial IrO2; however, the bimetallic catalyst undergoes different degradation processes compared to the monometallic catalyst. Bimetallic cobalt-iridium (Co-Ir) two-dimensional nanoframes are composed of interconnected Co-Ir alloy domains within an unsupported, carbon-free, porous nanostructure that allows three-dimensional molecular access to the catalytically active surface sites. After electrochemical conditioning within oxygen evolution potentials, the predominately bimetallic alloy surface is transformed to oxide/hydroxide surface. From rotating disk electrode measurements, hydrous Co-Ir oxide nanoframes provide 17 times higher OER mass activity and 16 times higher specific activity compared to those of commercial IrO2. The higher OER activities of hydrous CoIr nanoframes are attributed the interaction of Ir with Co within the surface and subsurface region that modifies the surface atomic and electronic structure. After accelerated durability testing, IrO2 has a lower specific activity and resulted in partial dissolution of Ir. In contrast, durability testing of hydrous Co-Ir oxide nanoframes resulted in an increase in specific activity, an increase in the relative contribution of surface iridium hydroxide groups, and a higher Ir dissolution rate. Understanding the differences between degradation processes invovled in bimetallic and monometallic catalysts furthers our ability to design high activity and stability acidic OER electrocatalysts. References Godínez-Salomón, F.; Albiter, L.; Mendoza-Cruz, R.; Rhodes, C.P. Bimetallic Two-dimensional Nanoframes: High Activity Acidic Bifunctional Oxygen Reduction and Evolution Electrocatalysts”, ACS Applied Energy Materials, 2020, 3, 2404-2421. http://dx.doi.org/10.1021/acsaem.9b02051 Godínez-Salomón, F.; Albiter, L.; Alia, S.M.; Pivovar, B.S.; Camacho-Forero, L.E.; Balbuena, P.B.; Mendoza-Cruz, R.; Arellano-Jimenez, M.J.; Rhodes, C.P. Self-Supported Hydrous Iridium-Nickel Oxide Two-dimensional Nanoframes for High Activity Oxygen Evolution Electrocatalysts, ACS Catalysis 2018, 8, 10498-10520. DOI: 10.1021/acscatal.8b02171

Atomic-Resolution Topographic Imaging of Crystal Surfaces

The surface of metal oxides is of technological importance and is extensively used as a substrate for various electronic and chemical applications. A real surface, however, is not a perfectly well-defined and clean surface, but rather contains a diverse class of atomistic defects. Here, we show the direct determination of the 3D surface atomic structure of SrTiO3 (001) including termination layers and atomistic defects such as vacancies, adatoms, ledges, kinks, and their complex combinations, by using depth sectioning of atomic-resolution annular dark-field scanning transmission electron microscopy (ADF STEM). To overcome the poor depth resolution in STEM, we statistically analyze the column by column depth profiles of ADF STEM images with a Bayesian framework fitting algorithm, and we achieve depth resolution at the entrance surface of ±0.9 Å for 1518 individual atomic columns. The present atomic-resolution 3D electron microscopy at the surface will provide fertile ground especially in surface science.

Single-atom level determination of 3-dimensional surface atomic structure via neural network-assisted atomic electron tomography

Functional properties of nanomaterials strongly depend on their surface atomic structures, but they often become largely different from their bulk structures, exhibiting surface reconstructions and relaxations. However, most of the surface characterization methods are either limited to 2D measurements or not reaching to true 3D atomic-scale resolution, and single-atom level determination of the 3D surface atomic structure for general 3D nanomaterials still remains elusive. Here we demonstrate the measurement of 3D atomic structure at 15 pm precision using a Pt nanoparticle as a model system. Aided by a deep learning-based missing data retrieval combined with atomic electron tomography, the surface atomic structure was reliably measured. We found that <100> and <111> facets contribute differently to the surface strain, resulting in anisotropic strain distribution as well as compressive support boundary effect. The capability of single-atom level surface characterization will not only deepen our understanding of the functional properties of nanomaterials but also open a new door for fine tailoring of their performance.

Photocatalytic Surface Restructuring in Individual Silver Nanoparticles.

Light absorption and scattering by metal nanoparticles can drive catalytic reactions at their surface via the generation of hot charge carriers, elevated temperatures, and focused electromagnetic fields. These photoinduced processes can substantially alter the shape, surface structure, and oxidation state of surface atoms of the nanoparticles and therefore significantly modify their catalytic properties. Information on such local structural and chemical change in plasmonic nanoparticles is however blurred in ensemble experiments, due to the typical large heterogeneity in sample size and shape distributions. Here, we use single-particle dark-field and Raman scattering spectroscopy to elucidate the reshaping and surface restructuring of individual silver nanodisks under plasmon excitation and during photocatalytic CO2 hydrogenation. We show that silver nanoparticles reshape significantly in inert N2 atmosphere, due to photothermal effects. Furthermore, by collecting the inelastic scattering during laser irradiation in a reducing gas environment, we observe intermittent light emission from silver clusters transiently formed at the nanoparticle surface. These clusters are likely to modify the photocatalytic activity of silver nanodisks and to enable detection of reaction products by enhancing their Raman signal. Our results highlight the dynamic nature of the catalytic surface of plasmonic silver nanoparticles and demonstrate the power of single-particle spectroscopic techniques to unveil their structure–activity relationship both in situ and in real time.

The Atomic Structure of the Se-Passivated CuInSe2

Through the first-principles calculations, we studied the atomic structure of Se-passivated CuInSe2. It is revealed that, the CuInSe2 can be passivated by selenium, under Se-rich condition, the structures with a full Se layer plus dimerized Se adatoms are favored. The formation energy of Se-terminated surface and the surface with Se overlayer are different, the formation energy of the surface with Se overlayer is more lower than the pure Se-terminated surface, that is to say, the surface with Se overlayer is more stable. On the other hand, the atomic structure of the Se-passivated CuInSe2 are also different from the atomic structure of the pure Se-terminated surface. Studying the structure of the Se-passivated CuInSe2 is very helpful for the high-efficiency solar cells.

Hydrogen evolution on different facets of δ1- MoN and δ3-MoN: Considering the adsorbed oxygen and hydroxyl by Surface Pourbaix diagrams

δ1-and δ3-MoN, the two most stable phases of molybdenum nitride, show a potential application in hydrogen evolution as their excellent corrosion resistance and high conductivity. However, we still lack the theoretical study about HER on their different surfaces. In order to simulate the realistic condition during heterogeneous catalysis, detailed atomic structure of each MoN surface (the ∗O and ∗OH) is determined by surface Pourbaix diagram. Further exploration of hydrogen evolution shows that the ∗O and ∗OH could change the catalytic site of one MoN surface, and weak the hydrogen adsorption ability. This is attributed to the downshift of Mo d-band center in top layer caused by the ∗O and ∗OH. And the hydrogen adsorption ability on catalytic sites of N and O atoms also follow the p-band center theory, respectively. It is interesting that several δ1-and δ3-MoN surfaces demonstrate comparable exchange current density, e.g., (101) and (001)N of δ1-MoN, and (110) of δ3-MoN with 1.76, 1.97 and 0.17 mA cm−2, respectively. This work is expected to contribute to the theoretical understanding of HER on different δ1-and δ3-MoN surfaces, and provide guidance for corresponding experiments on them.

Correlating 3D Surface Atomic Structure and Catalytic Activities of Pt Nanocrystals.

Active sites and catalytic activity of heterogeneous catalysts is determined by their surface atomic structures. However, probing the surface structure at an atomic resolution is difficult, especially for solution ensembles of catalytic nanocrystals, which consist of heterogeneous particles with irregular shapes and surfaces. Here, we constructed 3D maps of the coordination number (CN) and generalized CN () for individual surface atoms of sub-3 nm Pt nanocrystals. Our results reveal that the synthesized Pt nanocrystals are enclosed by islands of atoms with nonuniform shapes that lead to complex surface structures, including a high ratio of low-coordination surface atoms, reduced domain size of low-index facets, and various types of exposed high-index facets. 3D maps of are directly correlated to catalytic activities assigned to individual surface atoms with distinct local coordination structures, which explains the origin of high catalytic performance of small Pt nanocrystals in important reactions such as oxygen reduction reactions and CO electro-oxidation.

First-principles based theoretical calculations of atomic structures of hydroxyapatite surfaces and their charge states in contact with aqueous solutions.

Surface charge states of biomaterials are often important for the adsorption of cells, proteins, and foreign ions on their surfaces, which should be clarified at the atomic and electronic levels. First-principles calculations were performed to reveal thermodynamically stable surface atomic structures and their charge states in hydroxyapatite (HAp). Effects of aqueous environments on the surface stability were considered using an implicit solvation model. It was found that in an air atmosphere, stoichiometric {0001} and P-rich {101̄0} surfaces are energetically favorable, whereas in an aqueous solution, a Ca-rich {101̄0} surface is the most stable. This difference suggests that preferential surface structures strongly depend on chemical environments with and without aqueous solutions. Their surface potentials at zero charge were calculated to obtain the isoelectric points (pHPZC). pHPZC values for the {0001} surface and the Ca-rich {101̄0} surface were obtained to be 4.8 and 8.7, respectively. This indicates that in an aqueous solution at neutral pH, the {0001} and Ca-rich {101̄0} surfaces are negatively and positively charged, respectively. This trend agrees with experimental data from chromatography and zeta potential measurements. Our methodology based on first-principles calculations enables determining macroscopic charge states of HAp surfaces from atomic and electronic levels.

Rapid screening alloying elements for improved corrosion resistance on the Mg(0001) surface using first principles calculations.

The poor corrosion resistance of Mg alloys is a major challenge for their applications. The corrosion of Mg alloys is mainly controlled by the anodic dissolution of Mg and the cathodic hydrogen evolution reaction (HER), which is closely related to the stability and the hydrogen adsorption of the Mg surface. In this work, the effects of alloying elements (As, Ge, Cd, Zn, Ga, Al, and Y) on the stability and the hydrogen adsorption of a Mg(0001) surface have been studied based on first principles calculations. We have developed a horizontally integrated approach to evaluate their effects on corrosion resistance using parameters such as the surface energy, vacancy formation energy, Bader charge, electron density distribution, and the adsorption free energy of H atom at different adsorbed sites. We found that the doped atoms could significantly change the surface atomic structure and electron transfer on the Mg surface. These behaviors modified the energy required to detach the nearest neighbors of doped atoms from the Mg surface, the adsorption free energy of H atoms, and the stable adsorption sites of H atoms on the Mg surface, which regulate the corrosion resistance of Mg alloys. Interestingly, we found that Y doping on the Mg surface increased the corrosion resistance and our new method had tremendous potential in the rapid screening of alloying elements that could improve the stability of Mg alloys and inhibit the hydrogen evolution reaction.

Selecting ‘convenient observers’ to probe the atomic structure of CVD graphene on Ir(111) via photoelectron diffraction

CVD graphene grown on metallic substrates presents, in several cases, a long-range periodic structure due to a lattice mismatch between the graphene and the substrate. For instance, graphene grown on Ir(111), displays a corrugated supercell with distinct adsorption sites due to a variation of its local electronic structure. This type of surface reconstruction represents a challenging problem for a detailed atomic surface structure determination for experimental and theoretical techniques. In this work, we revisited the surface structure determination of graphene on Ir(111) by using the unique advantage of surface and chemical selectivity of synchrotron-based photoelectron diffraction. We take advantage of the Ir 4f photoemission surface state and use its diffraction signal as a probe to investigate the atomic arrangement of the graphene topping layer. We determine the average height and the overall corrugation of the graphene layer, which are respectively equal to 3.40 ± 0.11 Å and 0.45 ± 0.03 Å. Furthermore, we explore the graphene topography in the vicinity of its high-symmetry adsorption sites and show that the experimental data can be described by three reduced systems simplifying the moiré supercell multiple scattering analysis.

Atomic structure of the (111) surface of the antiferromagnetic 1/1 Au-Al-Tb approximant

The recently discovered antiferromagnetic 1/1 Au-Al-Tb approximant exhibits a whirling spin order oriented along the [111] direction. Here we report an investigation of the atomic structure of the (111) surface by scanning tunneling microscopy and density functional theory (DFT) calculations. Scanning tunneling microscopy reveals a Tb-terminated step-terrace morphology, with step heights appearing to minimize the number of broken bulk icosahedra. The atomic structure of the terraces is bias dependent, with Tb atoms imaged under positive bias and Au/Al atoms under negative bias. The surface is found to reconstruct, with the Au/Al atoms producing a linear row structure, an example of a surface reconstruction in a Tsai-type system. These observations are confirmed by DFT calculations.

Atomic Structure and Properties of SnO2 (100) and (101) Surfaces and (301) Steps in the (100) Surface

The atomic and electronic structures of SnO2 surfaces are closely related to their catalysis and gas detection properties. In this study, the atomic structure of SnO2 (100) and (101) surfaces has b...

Crystal Facet-Dependent CO2 Photoreduction over Porous ZnO Nanocatalysts.

Crystal facet engineering provides a promising approach to tailor the performance of catalysts because of the close relationship between the photocatalytic activity and the surface atomic and electronic structures. An in-depth understanding mechanism of crystal facet-dependent CO2 photoreduction is still an open question. Herein, two different types of porous ZnO nanocatalysts are used as model photocatalysts for the investigation, which are, respectively, with exposed {110} and {001} facets. The porous ZnO with an exposed {110} facet exhibits superior photocatalytic activity to the one with the {001} facet. Various influencing factors have been thoroughly studied both theoretically and/or experimentally, including light harvesting (i.e., band gap), reduction capability (potential of conduction band), crystallinity, CO2 adsorption ability, CO2 activation, and charge separation. The major influencing factors are eventually figured out based on the experimental and calculation results. The product selectivity and the influence of the hole scavenger can be explained too. Our work may pave a way for directing the future rational design of efficient photocatalysts for CO2 reduction.

Facile synthesis of channel-rich ultrathin palladium-silver nanosheets for highly efficient formic acid electrooxidation

The rational design and synthesis of two-dimensional (2D) materials with specific composition, morphology, and surface atomic structure is of great importance for the practical applications in electrocatalysis. Herein, we report a facile synthesis of channel-rich ultrathin palladium-silver nanosheets (PdAg NSs) by a simple wet-chemical strategy based on the self-assembly of docosyltrimethylammonium chloride (C22TAC). The nanosheets consist of 2D ultrathin nanosheets with many accessible channels and alloyed crystalline feature. The formation mechanism of PdAg NSs is systematically investigated via a series of control experiments, including reaction time, surfactant, Pd/Ag molar ratio, temperature, and reducing agent. It is demonstrated that C22TAC molecule with long alkyl chain and interplay between constituent metals is crucial for the anisotropic growth of 2D nanosheets. Such C22TAC-based synthetic strategy can also be extended to the synthesis of PdAg nanowires (PdAg NWs) and Pd mesoporous nanobowls (Pd NBs). Benefit from composition and structure advantages, the obtained PdAg NSs exhibit superior electrocatalytic performance toward formic acid oxidation reaction (FAOR) with mass activity of 987 mA mg−1 and specific activity of 50.1 A m−2 at 0.3 V, which are better than those of PdAg NWs and Pd NBs, even surpass commercial Pd black. We believe the synthetic strategy outlined here would open a new avenue for engineering of noble-metal nanocrystals for a broad range of applications.

The catalysis of acidic-atom Ti for Li2S5 and Li2S7 on a TiN (001) surface investigated by first principles calculations

The adsorption and degradation of long-chain lithium-sulfur compounds are more obvious on a transition metal nitride (e.g., TiN) surface. The adsorption strength and dissociation ability depend on the physical properties and chemical structure of the TiN surface atoms while reducing the shuttle effect. The shuttle effect of adsorbing Li2Sx and releasing "dead lithium and dead sulfur" is reduced by separating Li2Sx. The catalytic process depends on the properties of the bare surface atoms. The catalytic properties of the acidity and electronegativity of the exposed transition metal atoms on the surface were studied. The first-principles method was used to calculate the adsorption configuration and evolution structure of Li2S5 and Li2S7 on the TiN (001) surface, the adsorption energy, the configuration differential charge density, the charge transfer described by Bader analysis, and the density of states (DOS). The acidic atom Ti with a dangling bond obtains a common charge from the long-chain Li2Sx on the TiN (001) surface. The adsorbed Li2Sx reduces the potential energy to achieve stability under the effect of Ti atoms. The structure of Li2Sx tends to be gentle, resulting in cleavage of the Li-S bonds. The longer the Li2Sx chain is, the easier the bond is to break, even forming a short S chain. The catalytic performance is determined by the difficulty of obtaining the charge of the exposed transition metal from the long-chain Li2Sx on the surface. The Li2Sx is stably locked on the TiN surface due to the strong bond between the N and Li atoms. Long-chain Li2Sx decomposition is achieved by the bonding between Ti and S atoms. The catalytic process of the transition metal nitride TiN includes adsorption and decomposition.

Changes in the chemical compound and local atomic structure of ultrathin surface layers of Fe due to implantation of argon and oxygen ions

In this work, we consider the effect of irradiation in Ar+ → O+ and O+ → Ar+ sequences on changes in the chemical compound, type of chemical bond and the local atomic structure of ultrathin (~20 nm) iron surface layers. Investigation of the chemical compound and the type of chemical bond of the ion-modified surface was carried out by the XPS (X-ray photoelectron spectroscopy) and Auger electron spectroscopy. The study of changes in the local atomic structure was carried out by an XAFS-like method — electron energy loss fine structure (EELFS) spectroscopy. The parameters of the local atomic environment of oxygen and iron — partial interatomic distances, dispersion parameters, and coordination numbers — were obtained by analyzing atomic pair correlation functions. Analysis of experimental data showed that the two-stage irradiation of iron significantly changes the chemical compound and local atomic structure of the initial surface. This leads to formation of an oxide layer of greater depth than in the case of irradiation with only oxygen ions.

CuTe chains on Cu(111) by deposition of one-third of a monolayer of Te: Atomic and electronic structure

The surface atomic and electronic structure after deposition of 1/3 monolayer (ML) Te on Cu(111) was determined using a combination of low-energy electron diffraction (LEED), scanning tunneling microscopy and spectroscopy (STM/STS), angle-resolved single and two-photon photoelectron spectroscopy (ARPES /AR-2PPE) and density functional theory (DFT) calculations. Contrary to the current state in literature Te does not create a two-dimensional surface alloy but forms Cu$_2$Te$_2$ adsorbate chains in a $\left(2\sqrt{3} \times \sqrt{3}\right)\textrm{R30}^\circ$ superstructure. We establish this by a high-precision LEED-IV structural analysis with Pendry $R$ factor of $R = 0.099$ and corroborating DFT and STM results. The electronic structure of the surface phase is dominated by an anisotropic downward dispersing state at the Fermi energy $E_F$ and a more isotropic upward dispersing unoccupied state at $E-E_F = + 1.43\,\textrm{eV}$. Both states coexist with bulk states of the projected band structure and are therefore surface resonances.