Three-Dimensional Valency Mapping in Ceria Nanocrystals

Using electron tomography combined with electron energy loss spectroscopy (EELS), we are able to map the valency of the Ce ions in CeO_2–<i>x</i> nanocrystals in three dimensions. Our results show a clear facet-dependent reduction shell at the surface of ceria nanoparticles; {111} surface facets show a low surface reduction, whereas at {001} surface facets, the cerium ions are more likely to be reduced over a larger surface shell. Our generic tomographic technique allows a full 3D data cube to be reconstructed, containing an EELS spectrum in each voxel. This possibility enables a three-dimensional investigation of a plethora of material-specific physical properties such as valency, chemical composition, oxygen coordination, or bond lengths, triggering the synthesis of nanomaterials with improved properties

Do Binary Supracrystals Enhance the Crystal Stability?

We study the oxygen thermal stability of two binary systems. The larger particles are magnetic amorphous Co (7.2 nm) or Fe₃O₄ (7.5 nm) nanocrystals, whereas the smaller ones (3.7 nm) are Au nanocrystals. The nanocrystal ordering as well as the choice of the magnetic nanoparticles very much influence the stability of the binary system. A perfect crystalline structure is obtained with the Fe₃O₄/Au binary supracrystals. For the Co/Au binary system, oxidation of Co results in the chemical transformation from Co to CoO, where the size of the amorphous Co nanoparticles increases from 7.2 to 9.8 nm in diameter. During the volume expansion of the Co nanoparticles, Au nanoparticles within the binary assemblies coalesce and are at the origin of the instability of the binary nanoparticle supracrystals. On the other hand, for the Fe₃O₄/Au binary system, the oxidation of Fe₃O₄ to γ-Fe₂O₃ does not lead to a size change of the nanoparticles, which maintains the stability of the binary nanoparticle supracrystals. A similar behavior is observed for an AlB₂-type Co–Ag binary system: The crystalline structure is maintained, whereas in disordered assemblies, coalescence of Ag nanocrystals is observed

Atomic Structure of Quantum Gold Nanowires: Quantification of the Lattice Strain

Theoretical studies exist to compute the atomic arrangement in gold nanowires and the influence on their electronic behavior with decreasing diameter. Experimental studies, <i>e.g.</i>, by transmission electron microscopy, on chemically synthesized ultrafine wires are however lacking owing to the unavailability of suitable protocols for sample preparation and the stability of the wires under electron beam irradiation. In this work, we present an atomic scale structural investigation on quantum single crystalline gold nanowires of 2 nm diameter, chemically prepared on a carbon film grid. Using low dose aberration-corrected high resolution (S)TEM, we observe an inhomogeneous strain distribution in the crystal, largely concentrated at the twin boundaries and the surface along with the presence of facets and surface steps leading to a noncircular cross section of the wires. These structural aspects are critical inputs needed to determine their unique electronic character and their potential as a suitable catalyst material. Furthermore, electron-beam-induced structural changes at the atomic scale, having implications on their mechanical behavior and their suitability as interconnects, are discussed

Atomic Structure of Quantum Gold Nanowires: Quantification of the Lattice Strain

Theoretical studies exist to compute the atomic arrangement in gold nanowires and the influence on their electronic behavior with decreasing diameter. Experimental studies, <i>e.g.</i>, by transmission electron microscopy, on chemically synthesized ultrafine wires are however lacking owing to the unavailability of suitable protocols for sample preparation and the stability of the wires under electron beam irradiation. In this work, we present an atomic scale structural investigation on quantum single crystalline gold nanowires of 2 nm diameter, chemically prepared on a carbon film grid. Using low dose aberration-corrected high resolution (S)TEM, we observe an inhomogeneous strain distribution in the crystal, largely concentrated at the twin boundaries and the surface along with the presence of facets and surface steps leading to a noncircular cross section of the wires. These structural aspects are critical inputs needed to determine their unique electronic character and their potential as a suitable catalyst material. Furthermore, electron-beam-induced structural changes at the atomic scale, having implications on their mechanical behavior and their suitability as interconnects, are discussed

Monitoring Galvanic Replacement Through Three-Dimensional Morphological and Chemical Mapping

Galvanic replacement reactions on metal nanoparticles are often used for the preparation of hollow nanostructures with tunable porosity and chemical composition, leading to tailored optical and catalytic properties. However, the precise interplay between the three-dimensional (3D) morphology and chemical composition of nanostructures during galvanic replacement is not always well understood as the 3D chemical imaging of nanoscale materials is still challenging. It is especially far from straightforward to obtain detailed information from the inside of hollow nanostructures using electron microscopy techniques such as SEM or TEM. We demonstrate here that a combination of state-of-the-art EDX mapping with electron tomography results in the unambiguous determination of both morphology transformation and elemental composition of nanostructures in 3D, during galvanic replacement of Ag nanocubes. This work provides direct and unambiguous experimental evidence toward understanding the galvanic replacement reaction. In addition, the powerful approach presented here can be applied to a wide range of nanoscale transformation processes, which will undoubtedly guide the development of novel nanostructures

Crystal Structure of a Lightweight Borohydride from Submicrometer Crystallites by Precession Electron Diffraction

We demonstrate that precession electron diffraction at low-dose conditions can be successfully applied for structure analysis of extremely electron-beam-sensitive materials. Using LiBH₄ as a test material, complete structural information, including the location of the H atoms, was obtained from submicrometer-sized crystallites. This demonstrates for the first time that, where conventional transmission electron microscopy techniques fail, quantitative precession electron diffraction can provide structural information from submicrometer particles of such extremely electron-beam-sensitive materials as complex lightweight hydrides. We expect the precession electron diffraction technique to be a useful tool for nanoscale investigations of thermally unstable lightweight hydrogen-storage materials

Multiple Twinning As a Structure Directing Mechanism in Layered Rock-Salt-Type Oxides: NaMnO₂ Polymorphism, Redox Potentials, and Magnetism

New polymorphs of NaMnO₂ have been observed using transmission electron microscopy and synchrotron X-ray powder diffraction. Coherent twin planes confined to the (NaMnO₂) layers, parallel to the (101̅) crystallographic planes of the monoclinic layered rock-salt-type α-NaMnO₂ (O3) structure, form quasi-periodic modulated sequences, with the known α- and β-NaMnO₂ polymorphs as the two limiting cases. The energy difference between the polymorphic forms, estimated using a DFT-based structure relaxation, is on the scale of the typical thermal energies that results in a high degree of stacking disorder in these compounds. The results unveil the remarkable effect of the twin planes on both the magnetic and electrochemical properties. The polymorphism drives the magnetic ground state from a quasi-1D spin system for the geometrically frustrated α-polymorph through a two-leg spin ladder for the intermediate stacking sequence toward a quasi-2D magnet for the β-polymorph. A substantial increase of the equilibrium potential for Na deintercalation upon increasing the concentration of the twin planes is calculated, providing a possibility to tune the electrochemical potential of the layered rock-salt ABO₂ cathodes by engineering the materials with a controlled concentration of twins

Supracrystalline Colloidal Eggs: Epitaxial Growth and Freestanding Three-Dimensional Supracrystals in Nanoscaled Colloidosomes

The concept of template-confined chemical reactions allows the synthesis of complex molecules that would hardly be producible through conventional method. This idea was developed to produce high quality nanocrystals more than 20 years ago. However, template-mediated assembly of colloidal nanocrystals is still at an elementary level, not only because of the limited templates suitable for colloidal assemblies, but also because of the poor control over the assembly of nanocrystals within a confined space. Here, we report the design of a new system called “supracrystalline colloidal eggs” formed by controlled assembly of nanocrystals into complex colloidal supracrystals through superlattice-matched epitaxial overgrowth along the existing colloidosomes. Then, with this concept, we extend the supracrystalline growth to lattice-mismatched binary nanocrystal superlattices, in order to reach anisotropic superlattice growths, yielding freestanding binary nanocrystal supracrystals that could not be produced previously

Atomic Structure of Defects in Anion-Deficient Perovskite-Based Ferrites with a Crystallographic Shear Structure

Crystallographic shear (CS) planes provide a new structure-generation mechanism in the anion-deficient perovskites containing lone-pair cations. Pb₂Sr₂Bi₂Fe₆O₁₆, a new <i>n</i> = 6 representative of the A_<i>n</i>B_<i>n</i>O_{3<i>n</i>–2} homologous series of the perovskite-based ferrites with the CS structure, has been synthesized using the solid-state technique. The structure is built of perovskite blocks with a thickness of four FeO₆ octahedra spaced by double columns of FeO₅ edge-sharing distorted tetragonal pyramids, forming 1/2[110](101)_p CS planes (space group <i>Pnma</i>, <i>a</i> = 5.6690(2) Å, <i>b</i> = 3.9108(1) Å, <i>c</i> = 32.643(1) Å). Pb₂Sr₂Bi₂Fe₆O₁₆ features a wealth of microstructural phenomena caused by the flexibility of the CS planes due to the variable ratio and length of the constituting fragments with {101}_p and {001}_p orientation. This leads to the formation of “waves”, “hairpins”, “Γ-shaped” defects, and inclusions of the hitherto unknown layered anion-deficient perovskites Bi₂(Sr,Pb)­Fe₃O_8.5 and Bi₃(Sr,Pb)­Fe₄O_11.5. Using a combination of diffraction, imaging, and spectroscopic transmission electron microscopy techniques this complex microstructure was fully characterized, including direct determination of positions, chemical composition, and coordination number of individual atomic species. The complex defect structure makes these perovskites particularly similar to the CS structures in ReO₃-type oxides. The flexibility of the CS planes appears to be a specific feature of the Sr-based system, related to the geometric match between the SrO perovskite layers and the {100}_p segments of the CS planes

The Li₃Ru_<i>y</i>Nb_1–<i>y</i>O₄ (0 ≤ <i>y</i> ≤ 1) System: Structural Diversity and Li Insertion and Extraction Capabilities

Searching for novel high-capacity electrode materials combining cationic and anionic redox processes is an ever-growing activity within the field of Li-ion batteries. In this respect, we report on the exploration of the Li₃Ru_<i>y</i>Nb_1–<i>y</i>O₄ (0 ≤ <i>y</i> ≤ 1) system with an O/M ratio of 4 to maximize the number of oxygen lone pairs, responsible for the anionic redox. We show that this system presents a very rich crystal chemistry with the existence of four structural types, which derive from the rocksalt structure but differ in their cationic arrangement, creating either zigzag, helical, jagged chains or clusters. From an electrochemical standpoint, these compounds are active on reduction via a classical cationic insertion process. The oxidation process is more complex, because of the instability of the delithiated phase. Our results promote the use of the rich Li₃MO₄ family as a viable platform for a better understanding of the relationships between structure and anionic redox activity