Effect of Sr-doping on electronic and thermal properties of Pr2-xSrxFeCrO6 (0≤x≤1) oxide materials synthesized by using sol-gel technique

ABSTRACTThe thermoelectric properties of a new type of Pr2-xSrxFeCrO6 double perovskite were investigated at higher temperature after its sol-gel synthesis. The XRD results validate the single-phase orthorhombic structure, and the crystallite sizes meet the morphological measurements. XPS examination, which formed the defect sites in these oxides, confirmed the varied oxidation states of constituents. The temperature-dependent electrical conductivity (σ) was poor in the original Pr2FeCrO6 composition, but after substituting Sr on the Pr-site, a significant rise in σ with two semiconductors and one metal transition was observed. A positive Seebeck coefficient confirmed the presence of a p-type charge carrier in the entire composition, and the charge transport mechanism was driven by the SPH model. Thermal conductivity increases in all doped samples, while it decreases in pristine compounds over the broad analyzed temperature range. Thermal expansion coefficient increased after doping in oxygen-deficient compound. The PrSrFeCrO6 compound had the maximum ZT (0.105), which was 3.9 times higher than that of the pristine compound

Template synthesis and characterization of carbon nanomaterials from ferrocene crystals

Filamentous ribbon-like structures of highly disordered carbon of thickness 10-100 nm built from merged individual carbon nanofibers were synthesised by chemical vapour deposition from saturated ferrocene-benzene solution at 950 K. The materials obtained were characterized by electron microscopy, x-ray and electron diffraction, Raman spectroscopy and a possible growth mechanism for their formation was proposed and discussed. The synthesis demonstrates the possibility of a template growth of carbon nanomaterials and supports the vapour-solid-solid growth model of carbon materials because the catalysing metal particles are solid under the experimental conditions. Due to the large number of structural defects, filamentous structure, submicrometer thickness and low intraparticle diffusion of the nanomaterials, they can find application in catalysis as catalyst supports and sorbents. © 2014 Elsevier B.V

Composition and Electronic Structure of La₂O₃/CNFs@C Core-Shell Nanoparticles with Variable Oxygen Content

La2O3 nanoparticles stabilized on carbon nanoflake (CNF) matrix were synthesized and graphitized to produce core-shell structures La2O3/CNFs@C. Further oxidation of these structures by nitric acid vapors for 1, 3 or 6 h was performed, and surface-oxidized particles La2O3/CNFs@C_x (x = 1, 3, 6) were produced. Bulk and surface compositions of La2O3/CNFs@C and La2O3/CNFs@C_x were investigated by thermogravimetric analysis and X-ray photoelectron spectroscopy. With increasing the duration of oxidation, the oxygen and La2O3 content in the La2O3/CNFs@C_x samples increased. The electronic structures of samples were assessed by electron paramagnetic resonance. Two paramagnetic centers were associated with unpaired localized and mobile electrons and were registered in all samples. The correlation between bulk and surface compositions of the samples and their electronic structures was investigated for the first time. The impact of the ratio between sp2- and sp3-hybridized C atoms, the number and nature of oxygen-containing groups on the surface and the presence and proportion of coordinated La atoms on the EPR spectra was demonstrated

Electrochemical Fabrication of Inverse Opals of Silver with Cyanide-Free Electrolytes

Silver inverse opals were prepared electrochemically using non-toxic water–ethanol rhodanic and sulfite electrolytes for silver plating. Electrochemical crystallization of silver occurred in the pores of opal matrices made of 350 nm polystyrene microspheres. Samples with the most uniform structure and lowest percentage of surface admixtures were obtained from rhodanic electrolytes. Reflectance optical spectra of the silver inverse opals with a high surface periodicity showed various plasmon excitation modes in a visible spectral range making the silver opals attractive for plasmonics

MoS2 nanosheets decorated Ni3S2@MoS2 coaxial nanofibers: Constructing an ideal heterostructure for enhanced Na-ion storage

The performance of sodium ion batteries (SIBs) is mainly determined by the electrochemical activity and kinetic feature of electrode materials. High performance relies largely on the scrupulous design of nano-architectures and smart hybridization of bespoke active materials. It is fundamentally important for establishing a relationship between the structure/chemistry of these materials and their properties. Herein, we developed a novel synergistic Ni3S2-MoS2 core-shell nanofiber superstructure on 3D Ni/graphene foam by a one-step PVP-assisted hydrothermal reaction. Such hierarchical nanofibers can provide the homogeneous atomic heterointerface with porous hierarchical structure, resulting in the maximization of synergistic interaction. This unique structure results in very high specific capacity and rate capability as well as extremely long-term cycle stability. As anode electrode of SIBs, it exhibits a very high reversible specific capacity of 568 mAh g−1 at a current density of 200 mA g−1 with excellent rate capability (283 mAh g−1 at 5 A g−1), and the specific capacity can be well-maintained to 207 mAh g-1 at 5 A g−1 even after 400 cycles. The strategy developed in our study can open a new way to prepare other layered-material-based hybrid superstructure for next-generation energy storage devices.MOE (Min. of Education, S’pore)Accepted versio

Graphene Nanoflake- and Carbon Nanotube-Supported Iron–Potassium 3D-Catalysts for Hydrocarbon Synthesis from Syngas

Transformation of carbon oxides into valuable feedstocks is an important challenge nowadays. Carbon oxide hydrogenation to hydrocarbons over iron-based catalysts is one of the possible ways for this transformation to occur. Carbon supports effectively increase the dispersion of such catalysts but possess a very low bulk density, and their powders can be toxic. In this study, spark plasma sintering was used to synthesize new bulk and dense potassium promoted iron-based catalysts, supported on N-doped carbon nanomaterials, for hydrocarbon synthesis from syngas. The sintered catalysts showed high activity of up to 223 μmolCO/gFe/s at 300–340 °C and a selectivity to C5+ fraction of ~70% with a high portion of olefins. The promising catalyst performance was ascribed to the high dispersity of iron carbide particles, potassium promotion of iron carbide formation and stabilization of the active sites with nitrogen-based functionalities. As a result, a bulk N-doped carbon-supported iron catalyst with 3D structure was prepared, for the first time, by a fast method, and demonstrated high activity and selectivity in hydrocarbon synthesis. The proposed technique can be used to produce well-shaped carbon-supported catalysts for syngas conversion

N-doping and oxidation of carbon nanotubes and jellyfish-like graphene nanoflakes through the prism of Raman spectroscopy

Present work demonstrates the differences between the structure and surface transformations of carbon nanotubes (CNTs) and graphene nanoflakes with small particle size (GNFs) under oxidation and N-doping using Raman spectra processing. Jellyfish-like GNFs are rather new material that differs from the commonly known graphene nanoflakes by rounded edges and small size of particles (<50 nm). This material is promising in electrochemistry due to its high specific capacitance. It was found that, despite the graphene nature of both CNTs and GNFs, their Raman spectra change differently under oxidation and N-doping because of the different growth mechanisms of CNTs and GNFs, strong curvature of carbon layers in CNTs, and a large number of edge carbon atoms in GNFs.Accepted versionThe authors thank Dr. O.Y. Isaikina for Raman spectroscopy study and Dr. A.V. Egorov for his help in TEM investigations. This work was supported by the Russian Science Foundation under grant No. 18-13- 00217. The authors acknowledge support from “Nanochemistry and Nanomaterials” MSU Equipment Center acting under Lomonosov Moscow State University Program of Development

New Composite Contrast Agents Based on Ln and Graphene Matrix for Multi-Energy Computed Tomography

The subject of the current research study is aimed at the development of novel types of contrast agents (CAs) for multi-energy computed tomography (CT) based on Ln&ndash;graphene composites, which include Ln (Ln = La, Nd, and Gd) nanoparticles with a size of 2&ndash;3 nm, acting as key contrasting elements, and graphene nanoflakes (GNFs) acting as the matrix. The synthesis and surface modifications of the GNFs and the properties of the new CAs are presented herein. The samples have had their characteristics determined using X-ray photoelectron spectroscopy, X-Ray diffraction, transmission electron microscopy, thermogravimetric analysis, and Raman spectroscopy. Multi-energy CT images of the La-, Nd-, and Gd-based CAs demonstrating their visualization and discriminative properties, as well as the possibility of a quantitative analysis, are presented

Enhanced Pseudocapacitive Performance of α‑MnO₂ by Cation Preinsertion

Although the theoretical capacitance of MnO₂ is 1370 F g^–1 based on the Mn³⁺/Mn⁴⁺ redox couple, most of the reported capacitances in literature are far below the theoretical value even when the material goes to nanoscale. To understand this discrepancy, in this work, the electrochemical behavior and charge storage mechanism of K⁺-inserted α-MnO₂ (or K_<i>x</i>MnO₂) nanorod arrays in broad potential windows are investigated. It is found that electrochemical behavior of K_<i>x</i>MnO₂ is highly dependent on the potential window. During cyclic voltammetry cycling in a broad potential window, K⁺ ions can be replaced by Na⁺ ions, which determines the pseudocapacitance of the electrode. The K⁺ or Na⁺ ions cannot be fully extracted when the upper cutoff potential is less than 1 V vs Ag/AgCl, which retards the release of full capacitance. As the cyclic voltammetry potential window is extended to 0–1.2 V, enhanced specific capacitance can be obtained with the emerging of new redox peaks. In contrast, the K⁺-free α-MnO₂ nanorod arrays show no redox peaks in the same potential window together with much lower specific capacitance. This work provides new insights on understanding the charge storage mechanism of MnO₂ and new strategy to further improve the specific capacitance of MnO₂-based electrodes