Shaodong Cheng

Quantification of the boron speciation in alkali borosilicate glasses by electron energy loss spectroscopy.

Transmission electron microscopy and related analytical techniques have been widely used to study the microstructure of different materials. However, few research works have been performed in the field of glasses, possibly due to the electron-beam irradiation damage. In this paper, we have developed a method based on electron energy loss spectroscopy (EELS) data acquisition and analyses, which enables determination of the boron speciation in a series of ternary alkali borosilicate glasses with constant molar ratios. A script for the fast acquisition of EELS has been designed, from which the fraction of BO4 tetrahedra can be obtained by fitting the experimental data with linear combinations of the reference spectra. The BO4 fractions (N4) obtained by EELS are consistent with those from 11B MAS NMR spectra, suggesting that EELS can be an alternative and convenient way to determine the N4 fraction in glasses. In addition, the boron speciation of a CeO2 doped potassium borosilicate glass has been analyzed by using the time-resolved EELS spectra. The results clearly demonstrate that the BO4 to BO3 transformation induced by the electron beam irradiation can be efficiently suppressed by doping CeO2 to the borosilicate glasses.

A general soft-enveloping strategy in the templating synthesis of mesoporous metal nanostructures

Metal species have a relatively high mobility inside mesoporous silica; thus, it is difficult to introduce the metal precursors into silica mesopores and suppress the migration of metal species during a reduction process. Therefore, until now, the controlled growth of metal nanocrystals in a confined space, i.e., mesoporous channels, has been very challenging. Here, by using a soft-enveloping reaction at the interfaces of the solid, liquid, and solution phases, we successfully control the growth of metallic nanocrystals inside a mesoporous silica template. Diverse monodispersed nanostructures with well-defined sizes and shapes, including Ag nanowires, 3D mesoporous Au, AuAg alloys, Pt networks, and Au nanoparticle superlattices are successfully obtained. The 3D mesoporous AuAg networks exhibit enhanced catalytic activities in an electrochemical methanol oxidation reaction. The current soft-enveloping synthetic strategy offers a robust approach to synthesize diverse mesoporous metal nanostructures that can be utilized in catalysis, optics, and biomedicine applications.Metal species are highly mobile within mesoporous silica, making it difficult to template growth of metallic nanocrystals inside the channels. Here, the authors introduce a solid-liquid-solution interfacial strategy to suppress migration of the metal species, achieving control over a variety of mesostructured nanomaterials.

Atomistic understanding of the origin of high oxygen reduction electrocatalytic activity of cuboctahedral Pt3Co–Pt core–shell nanoparticles

PtM-based core–shell nanoparticles are a new class of active and stable nanocatalysts for promoting oxygen reduction reaction (ORR); however, the understanding of their high electrocatalytic performance for ORR at the atomistic level is still a great challenge. Herein, we report the synthesis of highly ordered and homogeneous truncated cuboctahedral Pt3Co–Pt core–shell nanoparticles (cs-Pt3Co). By combining atomic resolution electron microscopy, X-ray photoelectron spectroscopy, extensive first-principles calculations, and many other characterization techniques, we conclude that the cs-Pt3Co nanoparticles are composed of a complete or nearly complete Pt monolayer skin, followed by a secondary shell containing 5–6 layers with ~78 at% of Pt, in a Pt3Co configuration, and finally a Co-rich core with 64 at% of Pt. Only this particular structure is consistent with the very high electrocatalytic activity of cs-Pt3Co nanoparticles for ORR, which is about 6 times higher than commercial 30%-Pt/Vulcan and 5 times more active than non-faceted (spherical) alloy Pt3Co nanoparticles. Our study gives an important insight into the atomistic design and understanding of advanced bimetallic nanoparticles for ORR catalysis and other important industrial catalytic applications.

Investigation of the oxidation states of Cu additive in colored borosilicate glasses by electron energy loss spectroscopy

Three optically transparent colorful (red, green, and blue) glasses were synthesized by the sol-gel method. Nano-sized precipitates were found in scanning electron microscopy images. The precipitates were analyzed by transmission electron microscopy (TEM) and high resolution TEM. The measured lattice parameters of these precipitates were found to fit the metallic copper in red glass but deviate from single valenced Cu oxides in green and blue glasses. The chemistry of these nano-sized particles was confirmed by electron energy loss spectroscopy (EELS). By fitting the EELS spectra obtained from the precipitates with the linear combination of reference spectra from Cu reference compounds, the oxidation states of Cu in the precipitates have been derived. First principle calculations suggested that the Cu nano-particles, which are in the similar oxidation states as our measurement, would show green color in the visible light range.

Enhanced magnetic properties in epitaxial self-assembled vertically aligned nanocomposite (Pr0.5Ba0.5MnO3)0.5:(CeO2)0.5 thin films

Epitaxial self-assembled vertically aligned nanocomposite (VAN) Pr0.5Ba0.5MnO3:CeO2 (PBMO:CeO2) and pure PBMO films were fabricated on (001) (La,Sr)(Al,Ta)O3 substrates by pulsed laser deposition. Besides confirmation of the in-plane and out-of-plane orientations using X-ray diffraction, transmission electron microscopy study has revealed the columnar structure of PBMO:CeO2, with column width around 10–20 nm; furthermore, energy dispersive X-ray spectroscopy has revealed distinct phase separation between PBMO and CeO2. The introduction of CeO2 does not change the crystal quality of PBMO, and both films exhibit good crystalline quality. However, the vertical compressive strain induced by CeO2 partially relaxes the in-plane and out-of-plane strain of PBMO relative to the pure film. The magnetization of the VAN thin film is enhanced and almost two times higher than that of the pure film. Moreover, the relaxed strain and the insulating CeO2 nanopillar act as an energy barrier to induce the larger resistivity and enhanced magneto-resistance. All of these phenomena indicate that the VAN structure is an effective method to tune the strain states in thin films and obtain desired physical properties.

B-site ordering and strain-induced phase transition in double-perovskite La 2 NiMnO 6 films

The magnetic and electrical properties of complex oxide thin films are closely related to the phase stability and cation ordering, which demands that we understand the process-structure-property relationships microscopically in functional materials research. Here we study multiferroic thin films of double-perovskite La2NiMnO6 epitaxially grown on SrTiO3, KTaO3, LaAlO3 and DyScO3 substrates by pulsed laser deposition. The effect of epitaxial strains imposed by the substrate on the microstructural properties of La2NiMnO6 has been systematically investigated by means of advanced electron microscopy. It is found that La2NiMnO6 films under tensile strain exhibit a monoclinic structure, while under compressive strain the crystal structure of La2NiMnO6 films is rhombohedral. In addition, by optimizing the film deposition conditions a long-range ordering of B-site cations in La2NiMnO6 films has been obtained in both monoclinic and rhombohedral phases. Our results not only provide a strategy for tailoring phase stability by strain engineering, but also shed light on tuning B-site ordering by controlling film growth temperature in double-perovskite La2NiMnO6 films.

Structural transition induced enhancement of magnetization and magnetoresistance in epitaxial (Pr0.5Ba0.5MnO3)1−x:(CeO2)x vertically aligned thin films

Epitaxial (Pr0.5Ba0.5MnO3)1−x:(CeO2)x (PBMO:CeO2, x = 0, 10%, 20%, 35%, and 50% stands for the molar ratio of the CeO2 secondary phase) vertically aligned nanocomposite (VAN) thin films were successfully fabricated on single crystalline (001) (La,Sr)(Al,Ta)O3 substrates by pulsed laser deposition. With increasing x, a structural transition induced by the vertical strain from the CeO2 secondary phase was observed at 20% ≤ x ≤ 35% by reciprocal space mapping. Whats more, it is not only the enhancement of magnetoresistance that was observed but also the increasing magnetization. The maximum magnetization and magnetoresistance were achieved at x = 35%, which are almost 4.7 times (at 20 K) and 1.6 times (at 110 K) as high as those at x = 0%, respectively. Such great enhancement of magnetization can be attributed to the vertical strain induced structural transition. Our research indicates that the structural transition induced by the introduction of secondary phase CeO2 plays an important role in improving the magnetic and transport properties of VAN thin films.

Formation of Ruddlesden–Popper Faults and Their Effect on the Magnetic Properties in Pr0.5Sr0.5CoO3 Thin Films

Epitaxial Pr0.5Sr0.5CoO3 thin films have been grown on single-crystalline (La0.289Sr0.712)(Al0.633Ta0.356)O3(001) substrates by the pulsed laser deposition technique. The magnetic properties and microstructure of these films are investigated. It is found that Ruddlesden-Popper faults (RP faults) can be introduced in the films by changing the laser repetition rate. The segregation of Pr at the RP faults is characterized by atomic-resolution chemical mapping. The formation of the RP faults not only contributes to the epitaxial strain relaxation but also significantly decreases the ferromagnetic long-range order of the films, resulting in lower magnetizations than those of the fault-free films. Our results provide a strategy for tuning the magnetic properties of cobalt-based perovskite films by modifying the microstructure through the film growth process.

Microstructure and Electrical Conductivity of (Y, Sr)CoO3-δ Thin Films Tuned by the Film-Growth Temperature

Perovskite-based rare earth cobaltates have been the topic of extensive research since they possess fascinating electrical and magnetic properties [1, 2]. YxSr1−xCoO3−δ (YSCO) compounds with roomtemperature ferromagnetic properties were reported. In particular, the YSCO compound with x≈0.25 exhibits the highest Curie temperature (Tc≈335 K) among the perovskite-based cobaltates and the properties of YSCO could be modulated by its chemical composition [3]. In contrast to the considerable studies on the YSCO bulk materials, investigations on the microstructure and the properties of the YSCO thin films are limited. In this work, we report a detailed study on the microstructural and electrical properties of the YxSr1−xCoO3−δ (x=0.25) films, which were prepared at different growth temperatures on the (La0.289Sr0.712)(Al0.633Ta0.356)O3 (LSAT) substrates.

Understanding Phonon Scattering by Nanoprecipitates in Potassium-Doped Lead Chalcogenides

We present a comprehensive experimental and theoretical study of phonon scattering by nanoprecipitates in potassium-doped PbTe, PbSe, and PbS. We highlight the role of the precipitate size distribution measured by microscopy, whose tuning allows for thermal conductivities lower than the limit achievable with a single size. The correlation between the size distribution and the contributions to thermal conductivity from phonons in different frequency ranges provides a physical basis to the experimentally measured thermal conductivities, and a criterion to estimate the lowest achievable thermal conductivity. The results have clear implications for efficiency enhancements in nanostructured bulk thermoelectrics.