Electronic Reducibility Scales with Intergranular Interface Area in Consolidated In₂O₃ Nanoparticles Powders

Interfaces between nanoparticles of reducible metal oxides play a critical role for stoichiometry changes and associated self-doping effects. We explored the susceptibility of consolidated In₂O₃ nanoparticle ensembles exhibiting enhanced concentrations of intergranular interfaces toward vacuum annealing induced lattice oxygen depletion. Dielectric loss effects observed for nonstoichiometric In₂O_3–<i>x</i> nanoparticles inside the cavity of an Electron Paramagnetic Resonance (EPR) spectrometer system were used to determine trends in oxygen deficiency and n-type doping level for differently consolidated nanoparticle powders. Moreover, interfacial electron transfer from the In₂O_3–<i>x</i> nanoparticles to O₂ was utilized to evaluate the abundance of paramagnetic O₂^δ− adsorbates as a function of different levels of nanoparticle consolidation. Both particle aggregation inside aqueous nanoparticle dispersions, which is driven by capillary forces, and mechanical powder compaction were employed for the adjustment of intergranular interface area. For the first time, we observed a clear correlation between reducibility of In₂O_3–<i>x</i> nanoparticles achieved by vacuum annealing and the amount of intergranular interface area. This study clearly underlines the multiple role of intergranular interfaces. Inside ensembles of semiconducting oxide nanoparticles, they not only provide diffusion paths for charge carriers, but also offer a handle to adjust the n-type doping level via heat treatment in vacuum or other reducing gas atmospheres

Microwave-Assisted Ge_1–<i>x</i>Sn_<i>x</i> Nanowire Synthesis: Precursor Species and Growth Regimes

This study illustrates the different stages of Ge_1–<i>x</i>Sn_<i>x</i> nanowire formation with high Sn content in solution and also the molecular precursors involved in the synthesis. We can identify homometallic Ge­(II) as well as heterometallic Ge­(II) and Sn­(II) containing imido cubane derivatives being involved in the growth process. Two different scenarios are described for the initiation of the nanowire growth: a random seeding and a prenucleation step. Both scenarios can lead to constant diameter growth under continuous replacement of tin being consumed for the crystal formation from the Sn growth promoter. Once the growth medium is depleted from the Sn containing molecular species, the Sn growth seed is consumed resulting in diameter shrinkage. Most interestingly, the tin content increases with diminishing nanowire diameter from 10.7% to 28.4% at the very tip (270 to 10 nm). Similar results are obtained in Raman studies along a nanowire with shrinking diameter, while the Raman shift remains constant along nanowires of similar diameter. The nanowires are investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), and μ-Raman spectroscopy

Pushing the Composition Limit of Anisotropic Ge_1–<i>x</i>Sn_<i>x</i> Nanostructures and Determination of Their Thermal Stability

Ge_1–<i>x</i>Sn_<i>x</i> nanorods (NRs) with a nominal Sn content of 28% have been prepared by a modified microwave-based approach at very low temperature (140 °C) with Sn as growth promoter. The observation of a Sn-enriched region at the nucleation site of NRs and the presence of the low-temperature α-Sn phase even at elevated temperatures support a template-assisted formation mechanism. The behavior of two distinct Ge_1–<i>x</i>Sn_<i>x</i> compositions with a high Sn content of 17% and 28% upon thermal treatment has been studied and reveals segregation events occurring at elevated temperatures, but also demonstrates the temperature window of thermal stability. <i>In situ</i> transmission electron microscopy investigations revealed a diffusion of metallic Sn clusters through the Ge_1–<i>x</i>Sn_<i>x</i> NRs at temperatures where the material composition changes drastically. These results are important for the explanation of distinct composition changes in Ge_1–<i>x</i>Sn_<i>x</i> and the observation of solid diffusion combined with dissolution and redeposition of Ge_1–<i>y</i>Sn_<i>y</i> (<i>x</i> > <i>y</i>) exhibiting a reduced Sn content. Absence of metallic Sn results in increased temperature stability by ∼70 °C for Ge_0.72Sn_0.28 NRs and ∼60 °C for Ge_0.83Sn_0.17 nanowires (NWs). In addition, a composition-dependent direct bandgap of the Ge_1–<i>x</i>Sn_<i>x</i> NRs and NWs with different composition is illustrated using Tauc plots

Mechanism of Rare Earth Incorporation and Crystal Growth of Rare Earth Containing Type‑I Clathrates

Type-I clathrates possess extremely low thermal conductivities, a property that makes them promising materials for thermoelectric applications. The incorporation of cerium into one such clathrate has recently been shown to lead to a drastic enhancement of the thermopower, another property determining the thermoelectric efficiency. Here we explore the mechanism of the incorporation of rare earth elements into type-I clathrates. Our investigation of the crystal growth and the composition of the phase Ba_8–<i>x</i>RE_<i>x</i>TM_<i>y</i>Si_{46–<i>y</i>} (RE = rare earth element; TM = Au, Pd, Pt) reveals that the RE content <i>x</i> is mainly governed by two factors, the free cage space and the electron balance

Porphyrin Metalation at the MgO Nanocube/Toluene Interface

Molecular insights into porphyrin adsorption on nanostructured metal oxide surfaces and associated ion exchange reactions are key to the development of functional hybrids for energy conversion, sensing, and light emission devices. Here we investigated the adsorption of tetraphenyl-porphyrin (2HTPP) from toluene solution on two types of MgO powder. We compare MgO nanocubes with an average size <i>d</i> < 10 nm and MgO cubes with 10 nm ≤ <i>d</i> ≤ 1000 nm. Using molecular spectroscopy techniques such as UV/vis transmission and diffuse reflectance (DR), photoluminescence (PL), and diffuse reflectance infrared Fourier-transform (DRIFT) spectroscopy in combination with structural characterization techniques (powder X-ray diffraction and transmission electron microscopy, TEM), we identified a new room temperature metalation reaction that converts 2HTPP into magnesium tetraphenyl-porphyrin (MgTPP). Mg²⁺ uptake from the MgO nanocube surfaces and the concomitant protonation of the oxide surface level off at a concentration that corresponds to roughly one monolayer equivalent adsorbed on the MgO nanocubes. Larger MgO cubes, in contrast, show suppressed exchange, and only traces of MgTPP can be detected by photoluminescence

Adsorption, Ordering, and Metalation of Porphyrins on MgO Nanocube Surfaces: The Directional Role of Carboxylic Anchoring Groups

The understanding of porphyrin adsorption on oxide nanoparticles including knowledge about coverages and adsorbate geometries is a prerequisite for the improvement and optimization of hybrid materials. The combination of molecular spectroscopies with small-angle X-ray scattering provides molecular insights into porphyrin adsorption on MgO nanocube dispersions in organic solvents. In particular, we address the influence of terminal carboxyl groups on the adsorption of free base porphyrins, on their chemical binding, on the metalation reaction as well as on the coverage and orientation of adsorbate molecules. We compare the free base form 5,10,15,20-tetraphenyl-21,23<i>H</i>-porphyrin (2HTPP) with the carboxyl-functionalized 5,10,15,20-tetrakis­(4-carboxyphenyl)-21,23<i>H</i>-porphyrin (2HTCPP) and show that without carboxylic anchoring groups the free base form metalates on the nanocube surface and adopts a flat-lying adsorbate geometry. The saturation limit for flat-lying adsorption on nanocubes with an average edge length of 6 nm corresponds to 90 ± 14 molecules per particle. This limit is surpassed when 2HTCPP molecules attach via their terminal carboxyl groups to the surface. The resulting upright adsorption geometry suppresses self-metalation, on the one hand, and allows for much higher porphyrin coverages, on the other (at porphyrin concentrations in the stock solution of 2 × 10^–2 mol·L^–1). UV–vis diffuse reflectance results are perfectly consistent with conclusions from SAXS data analysis. The experiments reveal concentration dependent 2HTCPP coverages in the range between 0.4 to 1.9 molecules nm^–2 which correspond to the formation of a shell of upright standing porphyrin molecules around the MgO nanocubes. In contrast, after adsorption and metalation of nonfunctionalized 2HTPP the resulting porphyrin shells are in the range of a tenth of a nanometer and thus too thin to be captured by SAXS measurements. Related insights advance our opportunities to prepare well-defined nanohybrids containing highly organized porphyrin films

Straightforward Solvothermal Synthesis toward Phase Pure Li₂CoPO₄F

Li₂CoPO₄F, a promising high potential cathode material, has been synthesized for the first time via a one-pot solvothermal synthesis route. The characterization with respect to its crystal structure and electrochemical performance in a lithium half-cell is reported. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction studies reveal a strong influence of the solvent on the purity of the obtained crystalline phases and the particle morphology with preferred crystal growth orientations. The electrochemical tests of carbon coated materials demonstrate excellent characteristics in terms of high capacity

Setting Directions: Anisotropy in Hierarchically Organized Porous Silica

Structural hierarchy, porosity, and isotropy/anisotropy are highly relevant factors for mechanical properties and thereby the functionality of porous materials. However, even though anisotropic and hierarchically organized, porous materials are well known in nature, such as bone or wood, producing the synthetic counterparts in the laboratory is difficult. We report for the first time a straightforward combination of sol–gel processing and shear-induced alignment to create hierarchical silica monoliths exhibiting anisotropy on the levels of both, meso- and macropores. The resulting material consists of an anisotropic macroporous network of struts comprising 2D hexagonally organized cylindrical mesopores. While the anisotropy of the mesopores is an inherent feature of the pores formed by liquid crystal templating, the anisotropy of the macropores is induced by shearing of the network. Scanning electron microscopy and small-angle X-ray scattering show that the majority of network forming struts is oriented towards the shearing direction; a quantitative analysis of scattering data confirms that roughly 40% of the strut volume exhibits a preferred orientation. The anisotropy of the material’s macroporosity is also reflected in its mechanical properties; i.e., the Young’s modulus differs by nearly a factor of 2 between the directions of shear application and perpendicular to it. Unexpectedly, the adsorption-induced strain of the material exhibits little to no anisotropy

Pt–B System Revisited: Pt₂B, a New Structure Type of Binary Borides. Ternary WAl₁₂-Type Derivative Borides

On the basis of a detailed study applying X-ray single-crystal and powder diffraction, differential scanning calorimetry, and scanning electron microscopy analysis, it was possible to resolve existing uncertainties in the Pt-rich section (≥65 atom % Pt) of the binary Pt–B phase diagram above 600 °C. The formation of a unique structure has been observed for Pt₂B [X-ray single-crystal data: space group <i>C</i>2/<i>m</i>, <i>a</i> = 1.62717(11) nm, <i>b</i> = 0.32788(2) nm, <i>c</i> = 0.44200(3) nm, β = 104.401(4)°, <i>R</i>_F2 = 0.030]. Within the homogeneity range of “Pt₃B”, X-ray powder diffraction phase analysis prompted two structural modifications as a function of temperature. The crystal structure of “<i>h</i>T-Pt₃B” complies with the hitherto reported structure of anti-MoS₂ [space group <i>P</i>6₃/<i>mmc</i>, <i>a</i> = 0.279377(2) nm, <i>c</i> = 1.04895(1) nm, <i>R</i>_F = 0.075, <i>R</i>_I = 0.090]. The structure of the new “lT-Pt₃B” is still unknown. The formation of previously reported Pt_∼4B has not been confirmed from binary samples. Exploration of the Pt-rich section of the Pt–Cu–B system at 600 °C revealed a new ternary compound, Pt₁₂CuB_6–<i>y</i> [X-ray single-crystal data: space group <i>Im</i>3̅, <i>a</i> = 0.75790(2) nm, <i>y</i> = 3, <i>R</i>_F2 = 0.0129], which exhibits the filled WAl₁₂-type structure accommodating boron in the interstitial trigonal-prismatic site 12<i>e</i>. The isotypic platinum–aluminum–boride was synthesized and studied. The solubility of copper in binary platinum borides has been found to attain ∼7 atom % Cu for Pt₂B but to be insignificant for “lT-Pt₃B”. The architecture of the new Pt₂B structure combines puckered layers of boron-filled and empty [Pt₆] octahedra (anti-CaCl₂-type fragment) alternating along the <i>x</i> axis with a double layer of boron-semifilled [Pt₆] trigonal prisms interbedded with a layer of empty tetrahedra and tetragonal pyramids (B-deficient α-TlI fragment). Assuming boron vacancies ordering (space group <i>R</i>3), the Pt₁₂CuB_6–<i>y</i> structure exhibits serpentine-like columns of edge-connected boron-filled [Pt₆] trigonal prisms running infinitely along the <i>z</i> axis and embedding the icosahedrally coordinated Cu atom. Pt₂B, (Pt_1–<i>y</i>Cu_<i>y</i>)₂B (<i>y</i> = 0.045), and Pt₁₂CuB_6–<i>y</i> (<i>y</i> = 3) behave metallically, as revealed by temperature-dependent electrical resistivity measurements

Dislocations Accelerate Oxygen Ion Diffusion in La_0.8Sr_0.2MnO₃ Epitaxial Thin Films

Revealing whether dislocations accelerate oxygen ion transport is important for providing abilities in tuning the ionic conductivity of ceramic materials. In this study, we report how dislocations affect oxygen ion diffusion in Sr-doped LaMnO₃ (LSM), a model perovskite oxide that serves in energy conversion technologies. LSM epitaxial thin films with thicknesses ranging from 10 nm to more than 100 nm were prepared by pulsed laser deposition on single-crystal LaAlO₃ and SrTiO₃ substrates. The lattice mismatch between the film and substrates induces compressive or tensile in-plane strain in the LSM layers. This lattice strain is partially reduced by dislocations, especially in the LSM films on LaAlO₃. Oxygen isotope exchange measured by secondary ion mass spectrometry revealed the existence of at least two very different diffusion coefficients in the LSM films on LaAlO₃. The diffusion profiles can be quantitatively explained by the existence of fast oxygen ion diffusion along threading dislocations that is faster by up to 3 orders of magnitude compared to that in LSM bulk