Grain-boundary segregation of 3d-transition metal solutes in bcc Fe: ab initio local-energy and d-electron behavior analysis.

We present ab initio calculations of grain-boundary (GB) segregation of the series of 3d transition-metal (TM) solutes in bcc Fe, taking advantage of the local-energy analysis. For [Formula: see text]11(3 3 2) and [Formula: see text]3(1 1 1) [Formula: see text] symmetrical tilt GBs, the segregation behaviors of 3d-TM solutes can be classified into three groups. The early TMs (Sc, Ti and V) are preferentially segregated to the looser sites of GBs antiferromagnetically, the late TMs (Co, Ni and Cu) are preferentially segregated to the tighter sites of GBs ferromagnetically, and the middle TMs (Cr and Mn) are segregated antiferromagnetically without fixed site preference. TMs at both ends of the 3d series show larger segregation-energy gains, while Mn shows a cusp at the center, which is similar to the ab initio interaction energies between the 3d-TM solutes and a screw-dislocation core in bcc Fe. By the local-energy analysis combined with the local densities of states, the segregation of the early TMs is mainly attributed to the stabilization of surrounding Fe atoms by the TM solute at the looser sites of GBs, and that of the late TMs is mainly attributed to the stabilization of the TM solute itself from bulk Fe to GB sites and the destabilization of Fe atoms around the TM solute in bulk Fe. The cusp at Mn is mainly caused by the destabilization of Fe atoms around the Mn solute in bulk Fe, due to nearly-localized high-spin d states of Mn, in contrast to substantial d-d hybridization for Mn in GBs. For each group of 3d-TM solutes, the effects on the magnetic and mechanical properties of Fe GBs are also analyzed by the d-electron behavior in common with the segregation mechanism.

Failure mode in first-principles computational tensile tests of grain boundaries: effects of a bulk-region size, dominant factors, and local-energy and local-stress analysis.

A first-principles computational tensile test (FPCTT) is a powerful tool to investigate intrinsic strength and failure processes of grain boundaries (GBs), according to atomic and electronic behaviors based on density-functional theory, while careful interpretation is required in comparison with experiments, because of ideal conditions used in FPCTTs. We observed serious effects of a bulk-region size in FPCTTs of the {0 0 1} [Formula: see text]5 GB in Al. For a GB supercell with enough thick bulk regions, the energy-strain curve shows spontaneous failure with catastrophic energy release just after the maximum stress point, which we name Type-A failure. For a GB supercell with thin bulk regions, the energy increases gradually even after the maximum stress and continuously becomes that of relaxed fracture surfaces, which we name Type-B failure, although the stress-strain curves are almost common until the maximum stress point in both the supercells. The peculiar failure of Type B occurs by the lack of accumulated strain energies for creating fracture surfaces even after the maximum stress point, because the accumulated strain energy is nearly proportional to the bulk-region size. We clarified that the failure mode in a FPCTT depends on the relationship among the three factors; the accumulated strain energy depending on the bulk-region size, the work of separation (the formation energy of fractured surfaces into a GB), and the maximum stress of the GB (the GB strength). We showed that the failure mode of previous FPCTTs of Al tilt GBs with segregated impurities can be reinterpreted from this viewpoint, by considering the changes of the work of separation and the GB strength by impurities. We should be aware of the distinction of the failure mode in FPCTTs, because experimentally Type-B failure does not occur except for special cases. Finally, we applied ab initio local-energy and local-stress analysis to the FPCTT of the {0 0 1} [Formula: see text]5 GB in Al, and discussed how to extract local energy-strain or energy-separation relations independent of the bulk-region size to be combined with meso- or macroscopic simulations.

Study of the Hydrate-Melt/Li4Ti5O12 Interphase by Scanning Electron Microscopy-Based Spectroscopy.

To develop safe and low-cost Li-ion batteries (LIBs), recently, an aqueous-based electrolyte so-called "hydrate-melt" (HDM) electrolyte is proposed. Li4Ti5O12 is a promising negative electrode material for a LIB with such a HDM electrolyte because of its unexpected reversible Li insertion and extraction properties without usually inevitable water reduction. The solid-electrolyte interphase formation is one of the reasons for this stable reaction, although a detailed analysis is not yet performed. Here, a Li4Ti5O12 electrode surface reacted in a HDM electrolyte is investigated by scanning electron microscopy-based analysis. Surface reaction products are clearly observed on the Li4Ti5O12 surface after the Li insertion reaction in a HDM electrolyte. Energy-dispersive X-ray spectroscopy and Auger electron spectroscopy indicated that the products do not contain any components originated from Li salts, whereas anion-derived passivation films seem to cover a bare surface below the products. Further, the surface products are identified as Li2O by the feature of Li-K-edge reflection electron energy-loss spectrum. The Li2O formation would be one of the key issues for stable Li insertion and extraction of a Li4Ti5O12 electrode in a HDM electrolyte.

Nanoscale controlled Li-insertion reaction induced by scanning electron-beam irradiation in a Li4Ti5O12 electrode material for lithium-ion batteries.

The development of a nanoscale battery reaction in an electrode material associated with in situ microscopic observation is significant to an understanding of the solid-state mechanism of a battery reaction. With a Li4Ti5O12 (LTO) crystal as the negative electrode of a Li-ion battery (LIB), we show that a nanoscale-controlled Li-insertion reaction can be produced by electron beam irradiation with scanning transmission electron microscopy (STEM). A selected area in a Li2O-coated thin LTO crystal was irradiated by the electron probe of STEM with a high beam intensity of 2.5 × 107 (electrons per nm2). Electron energy-loss spectroscopy (EELS) revealed that significant changes in the chemical feature occurred only in the high-dose irradiation area in the LTO specimen. The features of Li-K, Ti-L and O-K spectra in that area were completely equal to those of a Li7Ti5O12 (Li-LTO) phase, as an electrochemically Li-inserted LTO phase, in contrast to usual LTO-like spectra in the region surrounding the specimen. For a pristine LTO specimen without Li2O coating, no Li-insertion reaction was observed under the same irradiation conditions. The high-dose electron beam seems to induce the dissociation of Li2O, providing Li ions and electrons, and the rapid and directional growth of a Li-LTO phase along the electron beam in the LTO specimen, forming a nanoscale steep interface with the surrounding LTO phase. The present phenomenon is a new type of electron beam assisted chemical reaction in a solid state, and could have a large impact on the science and technology of battery materials.

Study of the interface between Na-rich and Li-rich phases in a Na-inserted spinel Li4Ti5O12 crystal for an electrode of a sodium-ion battery.

Spinel lithium titanate (LTO; Li4Ti5O12) is one of the promising materials for negative electrodes of sodium-ion batteries (SIBs). The stable charge-discharge performance of SIB cells using LTO electrodes depends on the reversible Na insertion-extraction mechanism of LTO, where the spinel lattice is expanded with Na insertion, and two phases, Na-inserted LTO (Na-LTO) and Li-inserted LTO (Li-LTO) phases, are generated. These phases are confirmed using X-ray diffraction (XRD), while the mechanism of the two-phase coexistence with different lattice volumes is yet unclear. Here, we investigate the detailed morphology of the coexisting Na-LTO and Li-LTO phases using in situ XRD measurements and high-resolution transmission electron microscopy (TEM) observation. Na-LTO (a = 8.74 Å) and Li-LTO (a = 8.36 Å) phases are confirmed in both the electrochemically formed Na-inserted LTO electrode and the single-crystalline LTO thin specimen. We observed that the Na-LTO/Li-LTO interface is parallel to the (001) plane, and contains an inevitable lattice mismatch along the interface, while the expansion of the Na-LTO phase can be partially relaxed normal to the interface. We observed that the Na-LTO/Li-LTO interface has interface layers of lattice disordering with a 1-2 nm width, relaxing the lattice mismatch, as opposed to results from the previous scanning TEM observation. How the different lattice volumes at the two-phase interface are relaxed should be the key issue in investigation of the mechanism of Na insertion and extraction in LTO electrodes.

Si segregation at Fe grain boundaries analyzed by ab initio local energy and local stress.

Using density-functional theory calculations combined with recent local-energy and local-stress schemes, we studied the effects of Si segregation on the structural, mechanical and magnetic properties of the Σ3(1 1 1) and Σ11(3 3 2) Fe GBs formed by rotation around the [1 1 0] axis. The segregation mechanism was analyzed by the local-energy decomposition of the segregation energy, where the segregation energy is expressed as a sum of the following four terms: the local-energy change of Si atoms from the isolated state in bulk Fe to the GB segregated state, the stabilization of replaced Fe atoms from the GB to the bulk, the local-energy change of neighboring Fe atoms from the pure GB to the segregated GB and the local-energy change of neighboring Fe atoms from the system of an isolated Si atom in the bulk Fe to the pure bulk Fe. The segregation energy and value of each term greatly depends on the segregation site and Si concentration. The segregation at interface Fe sites with higher local energies in the original GB configurations naturally leads to higher segregation-energy gains, while interface sites with lower local energies can lead to larger energy gains if stronger Si-Fe interactions occur locally in the final segregated configurations. The high Si concentration reduces the segregation-energy gain per Si atom due to the local-energy increases of Si atoms neighboring to each other or through the reduction in the number of stabilized Fe atoms per Si atom as observed in a Si dimer in bulk Fe. In the Si-segregated GBs, Si-Fe bonds enhance local Young's moduli and tend to suppress the interface weakening, while the GB adhesion is slightly reduced. And Fe atoms contacting Si atoms have reduced magnetic moments, due to Si-Fe sp-d hybridization interactions.

Ab initio local energy and local stress: application to tilt and twist grain boundaries in Cu and Al.

The energy-density and stress-density schemes (Shiihara et al 2010 Phys. Rev. B 81 075441) within the projector augmented wave (PAW) method based on the generalized gradient approximation (GGA) have been applied to tilt and twist grain boundaries (GBs) and single vacancies in Cu and Al. Local energy and local stress at GBs and defects are obtained by integrating the energy and stress densities in each local region by the Bader integration using a recent algorithm (Yu et al 2011 J. Chem. Phys. 134 064111) as well as by the layer-by-layer integration so as to settle the gauge-dependent problem in the kinetic terms. Results are compared with those by the fuzzy-Voronoi integration and by the embedded atom method (EAM). The features of local energy and local stress at GBs and vacancies depend on the bonding nature of each material. Valence electrons in Al mainly located in the interatomic regions show remarkable response to structural disorder as significant valence charge redistribution or bond reconstruction, often leading to long-range variations of charges, energies and stresses, quite differently from d electrons in Cu mainly located near nuclei. All these features can be well represented by our local energy and local stress. The EAM potential for Al does not reproduce correct local energy or local stress, while the EAM potential for Cu provides satisfactory results.

Electron microscopy study of gold nanoparticles deposited on transition metal oxides.

Many researchers have investigated the catalytic performance of gold nanoparticles (GNPs) supported on metal oxides for various catalytic reactions of industrial importance. These studies have consistently shown that the catalytic activity and selectivity depend on the size of GNPs, the kind of metal oxide supports, and the gold/metal oxide interface structure. Although researchers have proposed several structural models for the catalytically active sites and have identified the specific electronic structures of GNPs induced by the quantum effect, recent experimental and theoretical studies indicate that the perimeter around GNPs in contact with the metal oxide supports acts as an active site in many reactions. Thus, it is of immense importance to investigate the detailed structures of the perimeters and the contact interfaces of gold/metal oxide systems by using electron microscopy at an atomic scale. This Account describes our investigation, at the atomic scale using electron microscopy, of GNPs deposited on metal oxides. In particular, high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) are valuable tools to observe local atomic structures, as has been successfully demonstrated for various nanoparticles, surfaces, and material interfaces. TEM can be applied to real powder catalysts as received without making special specimens, in contrast to what is typically necessary to observe bulk materials. For precise structure analyses at an atomic scale, model catalysts prepared by using well-defined single-crystalline substrates are also adopted for TEM observations. Moreover, aberration-corrected TEM, which has high spatial resolution under 0.1 nm, is a promising tool to observe the interface structure between GNPs and metal oxide supports including oxygen atoms at the interfaces. The oxygen atoms in particular play an important role in the behavior of gold/metal oxide interfaces, because they may participate in catalytic reaction steps. Detailed information about the interfacial structures between GNPs and metal oxides provides valuable structure models for theoretical calculations which can elucidate the local electronic structure effective for activating a reactant molecule. Based on our observations with HRTEM and HAADF-STEM, we report the detailed structure of gold/metal oxide interfaces.

Ab initio study of symmetrical tilt grain boundaries in bcc Fe: structural units, magnetic moments, interfacial bonding, local energy and local stress.

We present first-principle calculations on symmetric tilt grain boundaries (GBs) in bcc Fe. Using density functional theory (DFT), we studied the structural, electronic and magnetic properties of Σ3(111) and Σ11(332) GBs formed by rotation around the [110] axis. The optimized structures, GB energies and GB excess free volumes are consistent with previous DFT and classical simulation studies. The GB configurations can be interpreted by the structural unit model as given by Nakashima and Takeuchi (2000 ISIJ 86 357). Both the GBs are composed of similar structural units of three- and five-membered rings with different densities at the interface according to the rotation angle. The interface atoms with larger atomic volumes reveal higher magnetic moments than the bulk value, while the interface atoms with shorter bond lengths have reduced magnetic moments in each GB. The charge density and local density of states reveal that the interface bonds with short bond lengths have more covalent nature, where minority-spin electrons play a dominant role as the typical nature of ferromagnetic Fe. In order to understand the structural stability of these GBs, we calculated the local energy and local stress for each atomic region using the scheme of Shiihara et al (2010 Phys. Rev. B 81 075441). In each GB, the interface atoms with larger atomic volumes and enhanced magnetic moments reveal larger local energy increase and tensile stress. The interface atoms constituting more covalent-like bonds with reduced magnetic moments have lower local energy increase, contributing to the stabilization, while compressive stress is generated at these atoms. The relative stability between the two GBs can be understood by the local energies at the structural units. The local energy and local stress analysis is a powerful tool to investigate the structural properties of GBs based on the behavior of valence electrons.

Generation of oxygen vacancies at a Au/TiO2 perimeter interface during CO oxidation detected by in situ electrical conductance measurement.

Although the importance of a Au/oxide perimeter interface has been recognized in the field of gold catalysis, the experimental evidence on the reaction occurring at the perimeter is still limited. In this research, we applied in situ electrical conductance measurement (in situ ECM) to measure the CO oxidation with O(2) over Au/TiO(2) catalyst powder and found that the oxygen vacancies are generated at the Au/TiO(2) perimeter interface during the CO oxidation. The present result supports the reaction model in which oxygen molecules are activated on such oxygen vacancies.

Theoretical study of atomic oxygen on gold surface by Hückel theory and DFT calculations.

It is fundamental to understand the behavior of atomic oxygen on gold surfaces so as to elucidate the mechanism of nano gold catalysts for low-temperature CO oxidation reactions since the atomic oxygen on gold system is an important intermediate involved in both the processes of O(2) dissociation and CO oxidation. We performed theoretical analysis of atomic oxygen adsorption on gold by using Hückel theory. It is found that formation of linear O-Au-O structure on Au surfaces greatly stabilizes the atomic oxygen adsorption due to stronger bond energy and bond order, which is confirmed subsequently by density functional theory (DFT) calculations. The linear O-Au-O structure may explain the surprising first order kinetics behavior of O(2) desorption from gold surfaces. This view of the linear O-Au-O structure as the natural adsorption status is quite different from the conventional view, which may lead to new understanding toward the reaction mechanism of low-temperature CO oxidation reaction on nano gold catalysts.

Study of surface reaction of spinel Li4Ti5O12 during the first lithium insertion and extraction processes using atomic force microscopy and analytical transmission electron microscopy.

Spinel lithium titanate (Li(4)Ti(5)O(12), LTO) is a promising anode material for a lithium ion battery because of its excellent properties such as high rate charge-discharge capability and life cycle stability, which were understood from the viewpoint of bulk properties such as small lattice volume changes by lithium insertion. However, the detailed surface reaction of lithium insertion and extraction has not yet been studied despite its importance to understand the mechanism of an electrochemical reaction. In this paper, we apply both atomic force microscopy (AFM) and transmission electron microscopy (TEM) to investigate the changes in the atomic and electronic structures of the Li(4)Ti(5)O(12) surface during the charge-discharged (lithium insertion and extraction) processes. The AFM observation revealed that irreversible structural changes of an atomically flat Li(4)Ti(5)O(12) surface occurs at the early stage of the first lithium insertion process, which induces the reduction of charge transfer resistance at the electrolyte/Li(4)Ti(5)O(12) interface. The TEM observation clarified that cubic rock-salt crystal layers with a half lattice size of the original spinel structure are epitaxially formed after the first charge-discharge cycle. Electron energy loss spectroscopy (EELS) observation revealed that the formed surface layer should be α-Li(2)TiO(3). Although the transformation of Li(4)Ti(5)O(12) to Li(7)Ti(5)O(12) is well-known as the lithium insertion reaction of the bulk phase, the generation of surface product layers should be inevitable in real charge-discharge processes and may play an effective role in the stable electrode performance as a solid-electrolyte interphase (SEI).

Visualizing gas molecules interacting with supported nanoparticulate catalysts at reaction conditions.

Understanding how molecules can restructure the surfaces of heterogeneous catalysts under reaction conditions requires methods that can visualize atoms in real space and time. We applied a newly developed aberration-corrected environmental transmission electron microscopy to show that adsorbed carbon monoxide (CO) molecules caused the {100} facets of a gold nanoparticle to reconstruct during CO oxidation at room temperature. The CO molecules adsorbed at the on-top sites of gold atoms in the reconstructed surface, and the energetic favorability of this reconstructed structure was confirmed by ab initio calculations and image simulations. This atomic-scale visualizing method can be applied to help elucidate reaction mechanisms in heterogeneous catalysis.

A theoretical study of CO adsorption on gold by Hückel theory and density functional theory calculations.

It is crucial to understand the nature of CO adsorption on gold so as to elucidate the mechanism of low-temperature CO oxidation on nanogold catalysts. We performed theoretical analysis of CO adsorption on gold by using Hückel theory and density functional theory (DFT) calculations. Hückel theory indicates that CO adsorption on gold is dominated by the electron distribution at the Au atom, which is greatly affected by neighboring Au atoms, coadsorbed or doping species. The increase of σ-bonding electrons should weaken the CO adsorption, while the increase of π-electrons should strengthen the adsorption. DFT calculations proved this prediction quantitatively for various systems, including CO adsorption on a Au(100)-hex surface with locally varying subsurface configurations and CO coadsorption with acceptor or donor species.

Enhanced electrocatalytic activity of Pt subnanoclusters on graphene nanosheet surface.

Graphene nanosheet (GNS) gives rise to an extraordinary modification to the properties of Pt cluster electrocatalysts supported on it. The Pt/GNS electrocatalyst revealed an unusually high activity for methanol oxidation reaction compared to Pt/carbon black catalyst. The Pt/GNS electrocatalyst also revealed quite a different characteristic for CO oxidation among the measured catalyst samples. It is found that Pt particles below 0.5 nm in size are formed on GNS, which would acquire the specific electronic structures of Pt, modifying its catalytic activities.

Infrared spectroscopic and theoretical studies on the formation of Au2NO- and Au(n)NO (n = 2-5) in solid argon.

Laser-ablated gold atoms have been codeposited at 4 K with nitric oxide in excess argon and the low temperature reactions of Au with NO in solid argon have been studied using infrared spectroscopy. The reaction products Au(2)NO(-), Au(2)NO, Au(3)NO, Au(4)NO, and Au(5)NO are formed in the present experiments and characterized on the basis of isotopic shifts, mixed isotope splitting patterns, stepwise annealing, the change in reagent concentration and laser energy, and comparison with theoretical predictions. Density functional theory calculations have been performed on these systems to identify possible reaction products. The agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these molecules based on the matrix infrared spectra. Plausible reaction pathways have been proposed for the formation of these molecules.

First-principles characterization of the anisotropy of theoretical strength and the stress-strain relation for a TiAl intermetallic compound.

We perform first-principles computational tensile and compressive tests (FPCTT and FPCCT) to investigate the intrinsic bonding and mechanical properties of a γ-TiAl intermetallic compound (L 1(0) structure) using a first-principles total energy method. We found that the stress-strain relations and the corresponding theoretical tensile strengths exhibit strong anisotropy in the [001], [100] and [110] crystalline directions, originating from the structural anisotropy of γ-TiAl. Thus, γ-TiAl is a representative intermetallic compound that includes three totally different stress-strain modes. We demonstrate that all the structure transitions in the FPCTT and FPCCT result from the breakage or formation of bonds, and this can be generalized to all the structural transitions. Furthermore, based on the calculations we qualitatively show that the Ti-Al bond should be stronger than the Ti-Ti bond in γ-TiAl. Our results provide a useful reference for understanding the intrinsic bonding and mechanical properties of γ-TiAl as a high-temperature structural material.

Ab initio study of EMIM-BF4 crystal interaction with a Li (100) surface as a model for ionic liquid/Li interfaces in Li-ion batteries.

We examined the atomic and electronic structures of an interface between a 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMIM-BF(4)) ionic-liquid crystal and a Li(100) surface by periodic density-functional calculations, as a model for a room-temperature ionic-liquid (RTIL) electrolyte/Li interface at a Li-ion battery electrode. Results are compared with our previous theoretical study of the EMIM-BF(4) molecular adsorption on Li surfaces [H. Valencia et al., Phys. Rev. B 78, 205402 (2008)]. For the EMIM-BF(4) crystal structure, the present projector augmented wave scheme with the generalized gradient approximation can reproduce rather correct intramolecular structures as well as satisfactory short-ranged intermolecular distances, while long-range intermolecular distances are overestimated due to the lack of correct description of long-range dispersive interactions. We constructed a coherent crystal/crystal interface model where four EMIM-BF(4) pairs are stacked on a p(4x3) Li (100) surface cell so as to simulate RTIL-layer deposition on a Li surface. We observed significant attraction of surface Li ions toward contacting BF(4)(-) anions, counterbalanced by electron transfer toward EMIM(+) cations near the interface, revealing the tendency of easy ionization of Li and Li(x)-BF(4) cluster formation, coupled with the reduction of EMIM(+). These features are similar to those observed in the EMIM-BF(4) molecular adsorption, while these have been proved to occur in the crystal-layer adsorption. We examined the adhesive energy, wetability, and detailed electronic structure at the crystal/crystal interface.

Reactions of gold atoms with nitrous oxide in excess argon: a matrix infrared spectroscopic and theoretical study.

Reactions of laser-ablated Au atoms with N(2)O molecules in excess argon have been investigated using matrix-isolation infrared spectroscopy and density functional theory calculation. On the basis of isotopic shifts, mixed isotopic splitting patterns, CCl(4)-doping experiments, and the comparison with theoretical calculations, the absorption at 2047.5 cm(-1) is assigned to the OAuNN(-) anion, and the absorption at 1512.3 cm(-1) is assigned to the AuNO(-) anion, respectively. No evidence is found for the formation of new neutral and cationic products in the present experiments. It is predicted that the OAuNN(-) anion is a linear singlet molecule and the AuNO(-) anion is a bent doublet molecule. The agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identifications of these species from the matrix infrared spectra. Plausible reaction pathways have also been discussed for the formation of these products.

CO adsorption on a LaNi5 hydrogen storage alloy surface: a theoretical investigation.

Density functional theory calculations are carried out to study CO adsorption on the (001) surface of a LaNi(5) hydrogen storage alloy. At low coverages, CO favors adsorption on Ni-Ni bridge sites. With an increase in CO coverage, the decrease in the adsorption energy is much larger for Ni-Ni-CO bridge adsorption than that for Ni-CO on-top adsorption. Thus, the latter sites in the relatively stable adsorption structure are preferentially utilized at high CO coverages. The nature of the bonding between CO and the LaNi(5) (001) surface is analyzed in detail.