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

Exceptional Layered Ordering of Cobalt and Iron in Perovskites

Exceptional Layered Ordering of Cobalt and Iron in Perovskite

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

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

Epitaxy-Enabled Vapor–Liquid–Solid Growth of Tin-Doped Indium Oxide Nanowires with Controlled Orientations

Controlling the morphology of nanowires in bottom-up synthesis and assembling them on planar substrates is of tremendous importance for device applications in electronics, photonics, sensing and energy conversion. To date, however, there remain challenges in reliably achieving these goals of orientation-controlled nanowire synthesis and assembly. Here we report that growth of planar, vertical and randomly oriented tin-doped indium oxide (ITO) nanowires can be realized on yttria-stabilized zirconia (YSZ) substrates via the epitaxy-assisted vapor–liquid–solid (VLS) mechanism, by simply regulating the growth conditions, in particular the growth temperature. This robust control on nanowire orientation is facilitated by the small lattice mismatch of 1.6% between ITO and YSZ. Further control of the orientation, symmetry and shape of the nanowires can be achieved by using YSZ substrates with (110) and (111), in addition to (100) surfaces. Based on these insights, we succeed in growing regular arrays of planar ITO nanowires from patterned catalyst nanoparticles. Overall, our discovery of unprecedented orientation control in ITO nanowires advances the general VLS synthesis, providing a robust epitaxy-based approach toward rational synthesis of nanowires

Ultrasmall CoO(OH)_<i>x</i> Nanoparticles As a Highly Efficient “True” Cocatalyst in Porous Photoanodes for Water Splitting

The coupling of light absorbers to cocatalysts with well-designed optical and catalytic properties is of fundamental importance for the development of efficient photoelectrocatalytic devices for solar-driven water splitting. We achieved an effective loading of visible-light-active porous hybrid photoanodes for water photooxidation with ultrasmall (∼1–2 nm), highly disordered CoO­(OH)_<i>x</i> nanoparticles using a two-step impregnation method. Under visible light (λ > 420 nm) irradiation, the resulting photoanodes significantly outperformed photoanodes loaded with conventional cobalt-based cocatalyst (Co-Pi) comprising larger nanoparticles (∼5 nm) in terms of both Faradaic efficiency of oxygen evolution (by the factor of 2) and performance stability under long-term irradiation. A combination of STEM, XAS, cyclic voltammetry, and photoelectrochemical techniques was used to elucidate the advantages of using ultrasmall CoO­(OH)_<i>x</i> nanoparticles as cocatalysts. Specifically, due to the high transparency of ultrasmall CoO­(OH)_<i>x</i> nanoparticles in the visible range, higher loading of porous photoanodes with cobalt catalytic sites can be achieved, while the photocurrent losses due to parasitic light absorption by the cocatalyst are minimized. Notably, a significant enhancement in stability of ultrasmall CoO­(OH)_<i>x</i> nanoparticles in borate electrolytes as compared to phosphate electrolytes has been observed. EXAFS data recorded before and after photoelectrocatalysis indicated that the effect of the electrolyte on the stability can be explained by the difference in structural ordering dictated by different interaction of the electrolyte anions with cobalt ions, as corroborated by DFT calculations. This study highlights the strong impact of structural and optical properties of cocatalysts as well as the strong influence of the electrolyte composition on the activity and stability of photoelectrocatalytic systems comprising transition metal oxide electrocatalysts

Toward Deep Blue Nano Hope Diamonds: Heavily Boron-Doped Diamond Nanoparticles

The production of boron-doped diamond nanoparticles enables the application of this material for a broad range of fields, such as electrochemistry, thermal management, and fundamental superconductivity research. Here we present the production of highly boron-doped diamond nanoparticles using boron-doped CVD diamond films as a starting material. In a multistep milling process followed by purification and surface oxidation we obtained diamond nanoparticles of 10–60 nm with a boron content of approximately 2.3 × 10²¹ cm^–3. Aberration-corrected HRTEM reveals the presence of defects within individual diamond grains, as well as a very thin nondiamond carbon layer at the particle surface. The boron K-edge electron energy-loss near-edge fine structure demonstrates that the B atoms are tetrahedrally embedded into the diamond lattice. The boron-doped diamond nanoparticles have been used to nucleate growth of a boron-doped diamond film by CVD that does not contain an insulating seeding layer

Evidence for Metal–Support Interactions in Au Modified TiO_<i>x</i>/SBA-15 Materials Prepared by Photodeposition

Gold nanoparticles have been efficiently photodeposited onto titanate-loaded SBA-15 (Ti­(<i>x</i>)/SBA-15) with different titania coordination. Transmission electron microscopy shows that relatively large Au nanoparticles are photodeposited on the outer surface of the Ti­(<i>x</i>)/SBA-15 materials and that TiO_<i>x</i> tends to form agglomerates in close proximity to the Au nanoparticles, often forming core–shell Au/TiO_<i>x</i> structures. This behavior resembles typical processes observed due to strong-metal support interactions. In the presence of gold, the formation of hydrogen on Ti­(<i>x</i>)/SBA-15 during the photodeposition process and the performance in the hydroxylation of terephthalic acid is greatly enhanced. The activity of the Au/Ti­(<i>x</i>)/SBA-15 materials is found to depend on the TiO_<i>x</i> loading, increasing with a larger amount of initially isolated TiO₄ tetrahedra. Samples with initially clustered TiO_<i>x</i> species show lower photocatalytic activities. When isolated zinc oxide (ZnO_<i>x</i>) species are present on Ti­(<i>x</i>)/SBA-15, gold nanoparticles are smaller and well dispersed within the pores. Agglomeration of TiO_<i>x</i> species and the formation of Au/TiO_<i>x</i> structures is negligible. The dispersion of gold and the formation of Au/TiO_<i>x</i> in the SBA-15 matrix seem to depend on the mobility of the TiO_<i>x</i> species. The mobility is determined by the initial degree of agglomeration of TiO_<i>x</i>. Effective hydrogen evolution requires Au/TiO_<i>x</i> core–shell composites as in Au/Ti­(<i>x</i>)/SBA-15, whereas hydroxylation of terephthalic acid can also be performed with Au/ZnO_<i>x</i>/TiO_<i>x</i>/SBA-15 materials. However, isolated TiO_<i>x</i> species have to be grafted onto the support prior to the zinc oxide species, providing strong evidence for the necessity of Ti–O–Si bridges for high photocatalytic activity in terephthalic acid hydroxylation

Pyramid-Shaped Wurtzite CdSe Nanocrystals with Inverted Polarity

We report on pyramid-shaped wurtzite cadmium selenide (CdSe) nanocrystals (NCs), synthesized by hot injection in the presence of chloride ions as shape-directing agents, exhibiting reversed crystal polarity compared to former reports. Advanced transmission electron microscopy (TEM) techniques (image-corrected high-resolution TEM with exit wave reconstruction and probe-corrected high-angle annular dark field-scanning TEM) unequivocally indicate that the triangular base of the pyramids is the polar (0001̅) facet and their apex points toward the [0001] direction. Density functional theory calculations, based on a simple model of binding of Cl^– ions to surface Cd atoms, support the experimentally evident higher thermodynamic stability of the (0001̅) facet over the (0001) one conferred by Cl^– ions. The relative stability of the two polar facets of wurtzite CdSe is reversed compared to previous experimental and computational studies on Cd chalcogenide NCs, in which no Cl-based chemicals were deliberately used in the synthesis or no Cl^– ions were considered in the binding models. Self-assembly of these pyramids in a peculiar clover-like geometry, triggered by the addition of oleic acid, suggests that the basal (polar) facet has a density and perhaps type of ligands significantly different from the other three facets, since the pyramids interact with each other exclusively <i>via</i> their lateral facets. A superstructure, however with no long-range order, is observed for clovers with their (0001̅) facets roughly facing each other. The CdSe pyramids were also exploited as seeds for CdS pods growth, and the peculiar shape of the derived branched nanostructures clearly arises from the inverted polarity of the seeds