Surface Atomic Structure and Growth Mechanism of {1 0 0}‐Faceted Perovskite Oxide Nanocubes

The highly sensitive and selective properties of monodisperse faceted nanocrystals inherently stem from the atomic and electronic structures on the faceted surfaces. For elemental nanocrystals, the atomic structure on the surfaces is merely determined by the geometric shape itself. However, for compound materials such as alloys and complex oxides, atomic details on the faceted surfaces need to be studied on the atomic level. Here, we demonstrate that the surface atomic structure of faceted nanocrystals of complex oxides, {1 0 0}-faceted strontium titanate zirconate nanocubes, can be unambiguously resolved by aberration-corrected scanning transmission electron microscopy. The resolved surface atomic details reveal a layerwise growth process of the nanocubes, thereby allowing an in-depth understanding of the growth mechanism.

Monodisperse Pt nanoparticles with atomic structures that span the cluster to crystal transition have recently been synthesized in electrostatically stabilized, aqueous-based suspensions. In the present study, the anionic charge from the stabilizing SnCl(2) sheath adsorbed on the surface of these particles is used for the first time to assemble Pt directly onto porous carbon supports via electrostatic assembly. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveals that these assemblies have substantially higher Pt-C dispersions than obtained from precipitation methods commonly used for commercial electrocatalyst systems. Energy dispersive spectroscopy (EDS) and inductively coupled plasma-mass spectrometry (ICP-MS) are used to determine that loadings of 10-30% by weight Pt (particle packing fractions from 0.05 to 0.25) are obtained through a single electrostatic application of these particles on Vulcan carbon, depending on particle size. The highest average oxygen reduction reaction (ORR) mass activity obtained using this approach is 90.4 A/g(Pt) at 0.9 V vs RHE in 0.1 M perchloric acid is with 1-2 nm particles that exhibit a transitional atomic structure. This activity compares to an average value of 74.0 A/g(Pt) obtained from densely packed electrostatic layer-by-layer (LbL) assemblies of unsupported particles and 36.7 A/g(Pt) commercial Vulcan electrocatalyst from Tanaka Kikinzoku Kogyo (TKK). Enhanced activity is observed with electrostatic assembly of any particle size on Vulcan relative to unsupported or commercial electrocatalyst with comparable durability. Such enhanced activity is attributed to improved reactant accessibility to the catalyst surface due to the increase in particle dispersion. An extinction coefficient of 7.41 m(2)/g at 352 nm is obtained across the entire cluster to crystal transition from 20 atom clusters to 2.9 nm single crystal nanoparticles, indicating that observed variation in ORR activity with particle size may be associated primarily with changes in atomic surface structure as opposed to the metallic character of the nanoparticles as assessed by UV-vis spectroscopy.

Three-Dimensional Atomic Structure of Grain Boundaries Resolved by Atomic-Resolution Electron Tomography

Construction of Synergistic Co and Cu Diatomic Sites for Enhanced Higher Alcohol Synthesis

Various properties of ceramics can be significantly influenced by the presence of grain boundaries. The influence on the properties is closely related to the grain‐boundary atomic structures. As different grain boundaries have different atomic structure, different grain boundaries have different influence on the properties. It is difficult to examine the atomic structure and properties of individual grain boundaries in ceramics. In order to understand the atomic–structure–property relationships, well‐defined single grain boundaries should be characterized. In the present paper, we review our recent results on the investigations of atomic structures and electrical properties of ZnO single grain boundaries. The relationships between the atomic structures and the electrical properties were investigated using ZnO bicrystals, whose grain‐boundary orientation relationship and grain‐boundary planes can be arbitrarily controlled. The discussion focuses on the microscopic origin of nonlinear current–voltage ( I–V ) characteristics across ZnO grain boundaries. High‐resolution transmission electron microscopy (HRTEM) observations and lattice‐statics calculations revealed the atomic structures of the undoped ZnO [0001] Σ7 and Σ49 grain boundaries, enabling a comparison between coincidence site lattice (CSL) boundaries with small and large periodicity. These grain boundaries contained the common structural units (SUs) featuring atoms with coordination numbers that are unusual in ZnO. The Σ49 boundary was found to have characteristic arrangement of the SUs, where two kinds of the SUs are alternatively formed. It is considered that the characteristic arrangement was formed to effectively relax the local strain in the vicinity of the boundary. Such a relaxation of local strain is considered to be one of dominant factors to determine the SU arrangements along grain boundaries. I–V measurements of the undoped ZnO bicrystals showed linear I–V characteristics. Although the coordination and bond lengths of atoms in the grain boundaries differ from those in the bulk crystal, this does apparently not generate deep unoccupied states in the band gap. This indicates that atomic structures of undoped ZnO grain boundaries are not responsible for the nonlinear I–V characteristics of ZnO ceramics. On the other hand, the nonlinear I–V characteristic appeared when doping the boundaries with Pr. High‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) image of Pr‐doped boundaries revealed that Pr segregates to specific atomic columns, substituting Zn at the boundary. However, the Pr itself was not the direct origin of the nonlinear I–V characteristics, as the Pr existed in the three‐plus state and would not produce acceptor states in the boundary. First‐principles calculations revealed that Pr doping instead promotes the formations of acceptor‐like native defects, such as Zn vacancies. We believe that such acceptor‐like native defects are microscopic origin of the nonlinear I–V characteristics. Investigations of various types of grain boundaries in the Pr and Co‐codoped ZnO bicrystals indicated that the amounts of Pr segregation and the nonlinear I–V characteristics significantly depend on the grain‐boundary orientation relationship. Larger amount of Pr segregation and, as a result, higher nonlinearity in I–V characteristics was obtained for incoherent boundaries. This indicates that Pr doping to incoherent boundaries is one of the guidelines to design the single grain boundaries with highly nonlinear I–V characteristics. Finally, a Pr and Co‐codoped bicrystal with an incoherent boundary was fabricated to demonstrate a highly nonlinear I–V characteristic. This result indicates that ZnO single‐grain‐boundary varistors can be designed by controlling grain‐boundary atomic structures and fabrication processes. Summarizing, our work firstly enabled us to gain a deeper understanding for the atomic structure of ZnO grain boundaries. Secondly, we obtained important insight into the origin of nonlinear I–V characteristics across the ZnO grain boundaries. And, finally, based on these results, we demonstrated the potential of this knowledge for designing and fabricating ZnO single‐grain‐boundary varistors.

Physical properties of ceramics are often changed by presence of grain boundaries (GBs). One possible cause of the change is the grain boundary (GB) itself, because the GB atomic structure is different from that of the bulk. Another possible cause would be dopant segregation at the GBs. Thus, it is important to understand GB atomic structure and dopant segregation at the GBs at the atomic level. Here, we are going to present our studies on atomic structure and dopant (Pr) segregation at ZnO [0001] tilt GBs. We fabricated undoped and Pr-doped [0001] tilt GBs within ZnO bicrystals [1]. Atomic structure of the undoped ZnO GB was observed by high-resolution transmission electron microscopy (HRTEM), and that of the Pr-doped ZnO GB was observed using high-angle annular dark-field scanning TEM (HAADF-STEM). Atomistic calculations were performed for both of the undoped and the Pr-doped ZnO GBs to obtain further information in detail. The calculation results were compared with the HRTEM and the HAADF-STEM images.

ZnCl2 Enabled Synthesis of Highly Crystalline and Emissive Carbon Dots with Exceptional Capability to Generate O2⋅–

Developing novel catalysts with high efficiency and selectivity is critical for enabling future clean energy conversion technologies. Interfaces in catalyst systems have long been considered the most critical factor in controlling catalytic reaction mechanisms. Interfaces include not only the catalyst surface but also interfaces within catalyst particles and those formed by constructing heterogeneous catalysts. The atomic and electronic structures of catalytic surfaces govern the kinetics of binding and release of reactant molecules from surface atoms. Interfaces within catalysts are introduced to enhance the intrinsic activity and stability of the catalyst by tuning the surface atomic and chemical structures. Examples include interfaces between the core and shell, twin or domain boundaries, or phase boundaries within single catalyst particles. In supported catalyst nanoparticles (NPs), the interface between the metallic NP and support serves as a critical tuning factor for enhancing catalytic activity. Surface electronic structure can be indirectly tuned and catalytically active sites can be increased through the use of supporting oxides. Tuning interfaces in catalyst systems has been identified as an important strategy in the design of novel catalysts. However, the governing principle of how interfaces contribute to catalyst behavior, especially in terms of interactions with intermediates and their stability during electrochemical operation, are largely unknown. This is mainly due to the evolving nature of such interfaces. Small changes in the structural and chemical configuration of these interfaces may result in altering the catalytic performance. These interfacial arrangements evolve continuously during synthesis, processing, use, and even static operation. A technique that can probe the local atomic and electronic interfacial structures with high precision while monitoring the dynamic interfacial behavior in situ is essential for elucidating the role of interfaces and providing deeper insight for fine-tuning and optimizing catalyst properties. Scanning transmission electron microscopy (STEM) has long been a primary characterization technique used for studying nanomaterials because of its exceptional imaging resolution and simultaneous chemical analysis. Over the past decade, advances in STEM, that is, the commercialization of both aberration correctors and monochromators, have significantly improved the spatial and energy resolution. Imaging atomic structures with subangstrom resolution and identifying chemical species with single-atom sensitivity are now routine for STEM. These advancements have greatly benefitted catalytic research. For example, the roles of lattice strain and surface elemental distribution and their effect on catalytic stability and reactivity have been well documented in bimetallic catalysts. In addition, three-dimensional atomic structures revealed by STEM tomography have been integrated in theoretical modeling for predictive catalyst NP design. Recent developments in stable electronic and mechanical devices have opened opportunities to monitor the evolution of catalysts in operando under synthesis and reaction conditions; high-speed direct electron detectors have achieved sub-millisecond time resolutions and allow for rapid structural and chemical changes to be captured. Investigations of catalysts using these latest microscopy techniques have provided new insights into atomic-level catalytic mechanisms. Further integration of new microscopy methods is expected to provide multidimensional descriptions of interfaces under relevant synthesis and reaction conditions. In this Account, we discuss recent insights on understanding catalyst activity, selectivity, and stability using advanced STEM techniques, with an emphasis on how critical interfaces dictate the performance of precious metal-based heterogeneous catalysts. The role of extended interfacial structures, including those between core and shell, between separate phases and twinned grains, between the catalyst surface and gas, and between metal and support are discussed. We also provide an outlook on how emerging electron microscopy techniques, such as vibrational spectroscopy and electron ptychography, will impact future catalysis research.

The surface atomic structure and chemical state of Pt is consequential in a variety of surface-intensive devices. Herein we present the direct interrelationship between the growth scheme of Pt films, the resulting atomic and electronic structure of Pt species, and the consequent activity for methanol electro-oxidation in Pt/TiO(2) nanotube hybrid electrodes. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) measurements were performed to relate the observed electrocatalytic activity to the oxidation state and the atomic structure of the deposited Pt species. The atomic structure as well as the oxidation state of the deposited Pt was found to depend on the pretreatment of the TiO(2) nanotube surfaces with electrodeposited Cu. Pt growth through Cu replacement increases Pt dispersion, and a separation of surface Pt atoms beyond a threshold distance from the TiO(2) substrate renders them metallic, rather than cationic. The increased dispersion and the metallic character of Pt results in strongly enhanced electrocatalytic activity toward methanol oxidation. This study points to a general phenomenon whereby the growth scheme and the substrate-to-surface-Pt distance dictates the chemical state of the surface Pt atoms, and thereby, the performance of Pt-based surface-intensive devices.

We present a theory which permits for the first time a detailed analysis of the dependence of the absorption spectrum on atomic structure and cluster size. Thus, we determine the development of the collective excitations in small clusters and show that their broadening depends sensitively on the atomic structure, in particular at the surface. Results for ${\mathrm{Hg}}_{\mathit{n}}^{+}$ clusters show that the plasmon energy is close to its jellium value in the case of spherical-like structures, but is in general between ${\mathrm{\ensuremath{\omega}}}_{\mathit{p}}$/ \ensuremath{\surd}3 and ${\mathrm{\ensuremath{\omega}}}_{\mathit{p}}$/ \ensuremath{\surd}2 for compact clusters. A particular success of our theory is the identification of the excitations contributing to the absorption peaks.

ZnGeP2 (ZGP) crystals have attracted tremendous attention for their applications as frequency conversion devices. Nevertheless, the existence of native point defects, including at the surface and in the bulk, lowers their laser-induced damage threshold by increasing their absorption and forming starting points of the damage, limiting their applications. Here, native point defects in a ZGP crystal are fully studied by the combination of high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and optical measurements. The atomic structures of the native point defects of the Zn vacancy, P vacancy, and Ge-Zn antisite were directly obtained through an HAADF-STEM, and proved by photoluminescence (PL) spectra at 77 K. The carrier dynamics of these defects are further studied by ultrafast pump-probe spectroscopy, and the decay lifetimes of 180.49, 346.73, and 322.82 ps are attributed to the donor Vp+ → valence band maximum (VBM) recombination, donor GeZn+ → VBM recombination, and donor–acceptor pair recombination of Vp+ → VZn-, respectively, which further confirms the assignment of the electron transitions. The diagrams for the energy bands and excited electron dynamics are established based on these ultrahigh spatial and temporal results. Our work is helpful for understanding the interaction mechanism between a ZGP crystal and ultrafast laser, doing good to the ZGP crystal growth and device fabrication.

In order for Li-ion batteries to mature to a level useful for integration into the current or future energy infrastructure, basic problems such as cyclability, cost and rate capability must be overcome. LiNi0.5Mn1.5O4 (LNM), a spinel cathode material, has the advantage of being both cost-effective and a high-rate capable material, but it is plagued with cyclability problems. In the LNM system the main contributor to cycling degradation is the high operating voltage which leads to solid-electrolyte interphase (SEI) formation. We find that excess-Mn doping of this material (LiNi0.5-XMn1.5+XO4where x=0.05) leads to increased cyclability through natural passivation [1]. To understand the exact role that excess Mn plays in the passivation of this cathode material, it is crucial to determine the surface’s atomic structure. This is because the surface structure determines how reactive the cathode will be with the electrolyte during oxidation and reduction cycles.In order to understand how excess-Mn LNM reacts with the electrolyte, it is critical to understand the different phases that form in this system. In this regard, aberration-corrected high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) was used to identify the surface and bulk structures in the excess-Mn LNM system. A HAADF STEM image of LiNi0.45Mn1.55O4along the [110] zone axis is shown in Figure 1. The image has been deconvoluted for clarity. This confirms the spinel structure (blue) and shows good agreement with STEM simulations in the bulk. Near the surface however, other phases are observed which include the rock-salt phase (green) and an unexpected phase defined here as “ring-type” (red). The rock-salt structure is expected from x-ray diffraction (XRD) results but the ring-type phase; so-called because of the characteristic rings that are formed within the first few atomic surface layers, is not. All three phases are observed near the surface, however only the spinel is found within the bulk of the particles. HAADF STEM enables a detailed characterization of these phases and has led to an important understanding of the cycling degradation mechanisms in the excess-Mn LNM system. In turn, this work enables us to develop a well-suited cathode material for future energy storage that will potentially spur the evolution of the future sustainable energy landscape.

The palladium (Pd)-catalyzed Suzuki reaction is widely applied in the pharmaceutical industry, where constructing highly active and low-cost Pd sites are impendent. Here, we report the fabrication ...

We present the first-principles total-energy electronic-structure calculations that provide a firm theoretical framework to consider atomic and electronic structures of alumina surfaces. Exploring detailed atomic structures and electron states of stable and metastable surfaces of three important planes, C plane [the (0001) surface], R plane [the $(1\overline{1}02)$ surface], and A plane [the $(11\overline{2}0)$ surface], of $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$, we find that the stoichiometric surface of the C plane has the lowest surface energy, followed by the stoichiometric surfaces of the R plane and then the A plane, irrespective of the chemical potentials of constituting elements. Detailed atomic structures for stable and metastable surfaces of each plane have been obtained, which is imperative for atom-scale clarification of reactions on the $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$ surface. The electron states of each surface have been calculated in detail. The obtained surface energy bands offer a possibility to identify atomic structures of $\ensuremath{\alpha}{\text{-Al}}_{2}{\text{O}}_{3}$ surface by spectroscopic measurements. It is found that the ionic interaction between Al and O and the covalent interaction among Al atoms or O atoms are both important to determine the surface atomic structures.

In the past few decades, tremendous advances have been made in electrocatalysis due to the rational design of electrocatalysts at the nanoscale level. Further development requires engineering electrocatalysts at the atomic level, which is a grand challenge. Here, the recent advances in cation exchange strategy, which is a powerful tool for fine-tuning atomic structure of electrocatalysts via surface faceting, heteroatom doping, defects formation, and strain modulation, are the main focus. Proper atomic structure engineering effectively adjusts the electronic structure, and thus enhances the electronic conductivity and facilitates the adsorption/desorption of reaction intermediates. By virtue, the cation exchange strategy greatly boosts the intrinsic and apparent activities of electrocatalysts and shows a great potential toward design of new energy conversion devices, such as water splitting devices and metal-air batteries. It is believed that cation exchange offers new insights and opportunities for the rational design of a new generation of electrocatalysts.