Synthesis and electrochemical performance of P2-Na0.67AlxCo1-xO2 (0.0 ≤ × ≤ 0.5) nanopowders for sodium-ion capacitors

Ionics - Layered P2-Na0.67Al x Co 1-x O2 (0.0 ≤ × ≤ 0.5) nanopowders were prepared by solution combustion process. These nanopowders were...     

Novel NdCo<Subscript>5</Subscript> nanoflakes and nanoparticles produced by surfactant-assisted high-energy ball milling

High-energy ball milling has been shown to be a promising method for the fabrication of rare earth—transition metal nanopowders. In this work, NdCo₅ nanoflakes and nanoparticles have been produced by a two-stage high-energy ball milling (HEBM), by first using wet HEBM to prepare precursor nanocrystalline powders followed by surfactant-assisted HEBM. NdCo₅ flakes have a thickness below 150 nm and an aspect ratio as high as 10²–10³; the nanoparticles have an average size of 7 nm. Both the nanoparticles and nano-flakes exhibited high coercivities at low temperatures, with values at 50 K of 3 and 3.7 kOe, respectively. The high values of coercivity can be attributed to the large surface anisotropy of nanoparticles that leads to an effective uniaxial-type of behavior in contrast to the planar anisotropy of the bulk samples. Angle-dependent magnetization measurements at different temperatures were used to determine the spin reorientation transitions in the nanopowders and nanoparticles. The nanoparticles showed spin reorientation temperatures, T _SR1 = 276 and T _SR2 = 237 K which are lower when compared with the values of 290 and 245 K, respectively for bulk.     

Preparation of highly efficient Al-doped ZnO photocatalyst by combustion synthesis

Novel Al-doped ZnO (AZO) photocatalysts with different Al concentrations (0.5–6.0 mol%) were prepared through a facile combustion method and followed by calcination at 500 °C for 3 h. The obtained nanopowders were characterized by powder X-ray diffraction (XRD), scanning electron microscope (SEM) combined with EDX, transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FTIR), UV–vis spectroscopy and photoluminescence spectroscopy. The XRD patterns of AZO nanopowders were assigned to wurtzite structure of ZnO with the smallest crystallite size about 11 nm consistent with the results from TEM. The doping of Al in ZnO crystal structure successfully suppressed the growth of ZnO nanoparticles confirmed by XRD patterns. The absorption spectra analysis showed that the optical band gap energy (E_g) for the AZO nanopowders were in the range of 3.12–3.21 eV and decreased with increasing of Al dopant. The photocatalytic activities of the samples were evaluated by photocatalytic degradation of methyl orange under visible light (λ ≥ 420 nm) and sunlight irradiation. The results showed that the AZO photocatalyst doped with 4.0 mol% Al exhibited five times enhanced photocatalytic activity compared to pure ZnO. The enhanced photocatalytic activity could be attributed to extended visible light absorption, inhibition of the electron–hole pair's recombination and enhanced adsorptivity of MO dye molecule on the surface of AZO nanopowders.     

Improvement of photocatalytic behavior of chemical-vapor-synthesized TiO2 nanopowders by post-heat treatment

Anatase-type TiO₂ nanopowders less than 10 nm in average diameter were synthesized by a chemical vapor synthesis method. The TiO₂ nanopowders showed very poor photocatalytic properties, in spite of their large surface area. With subsequent heat treatment of the TiO₂ powders, their photocatalytic properties determined by measuring the degradation of 2-propanol were improved at temperatures up to 600 °C and then diminished along with formation of a rutile phase. This improvement in the photocatalytic properties of TiO₂ nanopowders was attributed to both a morphology change and a change in the electronic surface characteristics of TiO₂ particles during heat treatment.     

Synthesis and characterization of magnetite nanopowders

We have synthesized the iron oxide nanoparticles using the newly developed mechanical ultrasonication method with the FeSO₄ · 7H₂O. We have also investigated the crystallographic structural properties, morphology, and magnetic properties of the nanopowders. According to the high resolution X-ray diffraction result, the as-synthesized iron oxide nanoparticles were magnetite (Fe₃O₄). The particle size of the magnetite nanoparticles was about 6 nm confirmed by transmission electron microscopy image. The particle shape was almost a sphere confirmed by scanning electron microscopy image. The coercivity and saturation magnetization of the as-synthesized iron oxide nanopowders were 114 Oe, and 3.7 emu/g, respectively.     

Antibacterial properties of Ag-doped hydroxyapatite layers prepared by PLD method

Thin hydroxyapatite (HA), silver-doped HA and silver layers were prepared using a pulsed laser deposition method. Doped layers were ablated from silver/HA targets. Amorphous and crystalline films of silver concentrations of 0.06 at.%, 1.2 at.%, 4.4 at.%, 8.3 at.% and 13.7 at.% were synthesized. Topology was studied using scanning electron microscopy and atomic force microscopy. Contact angle and zeta potential measurements were conducted to determine the wettability, surface free energy and electric surface properties. In vivo measurement (using Escherichia coli cells) of antibacterial properties of the HA, silver-doped HA and silver layers was carried out. The best antibacterial results were achieved for silver-doped HA layers of silver concentration higher than 1.2 at.%.     

Perovskite Pb(Sc1/2Nb1/2)O3 nanopowders synthesized by surfactant-modulated precipitation

A novel surfactant-modulated precipitation method is proposed to produce pure perovskite Pb(Sc_1/2Nb_1/2)O₃ (PSN) nanopowders at a low temperature of 700 °C. The samples were characterized by XRD, SEM, TEM, and HRTEM, respectively. The results indicate that the powders exhibit a granular shape of 80 nm in diameter. It is believed that the nanosize of as-prepared PSN powders is dependent on the well-dispersed performance, which arouse from the combined effects of both precipitant TMAH and surfactant CTAB. The mechanism of the formation of the pure perovskite PSN nanopowders is discussed in detail. Our method is expected to create a new, low-cost route to produce the promising nanopowders of multi-component perovskite systems.     

Luminescence study of alumina nanopowders prepared by various methods

Mixed nanopowders of transition alumina prepared by combustion synthesis and phase pure ultra-porous α-alumina by oxidation method were investigated using low temperature time-resolved cathodoluminescence and photoluminescence spectroscopy under VUV-XUV excitation. In all samples along with the 7.6 eV emission of self-trapped excitons of α-alumina, luminescence bands due to F, F⁺ centres with maxima at 3 and 3.8 eV and other UV–visible luminescence bands of intrinsic and extrinsic origin with varying intensity depending on sample preparation method and thermal treatment were studied. In alumina nanopowders the excitonic excitation peak at ∼9.1 eV near fundamental absorption edge is shifted to the higher energies by 0.15 eV in comparison with the same feature in single crystals. The nanostructure of alumina is responsible for this shift.     

In situ fabrication and thermoelectric properties of PbTe–polyaniline composite nanostructures

PbTe–polyaniline (PANi) composite nanopowders were in situ fabricated via an interfacial polymerization method at room temperature (~293 K). The phase structure, composition, and morphology of the powders were characterized by X-ray powder diffraction, infrared spectroscopy, transmission electron microscopy (TEM), and high-resolution TEM, respectively. The results show that the composite nanopowders consist of PbTe nanoparticles, PANi/PbTe core–shell nanostructure, and PbTe/PANi/PbTe three-layer sphere-like nanostructures. Formation mechanism of the PbTe–PANi composite nanostructures was proposed. The thermoelectric properties of the composite powders after being cold pressed into pellets were measured from 293 to 373 K. As the temperature increases from 293 to 373 K, the Seebeck coefficient of the composite decreases from 626 to 578 μV K⁻¹ and the electrical conductivity increases from 1.9 to 2.2 S m⁻¹.     

Increase of the blue upconversion emission in YAG:Tm3+ nanopowders by codoping with Yb3+ ions

The YAG nanopowders were prepared by a co-precipitation method using nitrate and ammonium hydrogen carbonate as raw materials. To obtain homogenous precipitate, reverse-strike (adding salt solutions to the precipitant solution) technique was adopted. Therefore, single (Tm³⁺) and codoped (Tm³⁺–Yb³⁺) YAG nanopowders with a size between 40–90 nm have been obtained.Blue upconversion emission at around 480 nm has been found in YAG: Tm³⁺ nanopowders under excitation to the ³H₄ level of Tm³⁺ at around 800 nm. However, this upconversion emission in nanopowders codoped with Tm³⁺–Yb³⁺ ions is increased by a factor of about 10. The analysis of the temporal evolution of the involved levels and the dependence of the upconversion intensity on the pump power at 800 nm allowed to distinguish the upconversion mechanism. In YAG: Tm³⁺ nanopowders the upconversion mechanism is due to excited state absorption processes. However, in the codoped samples, Yb³⁺ ions acts as the sensitizers; in consequence, the blue upconversion is strongly increased.     

Ni-GDC microtubular cermets

Ni-GDC cermet tubes (~ 3 mm inside diameter, 0.3–0.4 mm wall thickness and ～ 100 mm in length) with different amounts of porosity have been prepared by sintering of NiO-Ce_0.9Gd_0.1O_1.95 (NiO-GDC) nanopowders and further reduction. The NiO-GDC nanopowders were synthesized by chemical route and the different pore density was obtained by modifying the processing conditions (calcining and adjustment of carbonous remains).Ni-GDC cermets were obtained from the sintered NiO-GDC composites using reduction treatment at 700 °C for 3 h in reducing atmosphere (5 vol.% H₂–95 vol.% Ar). After the reduction process, tubes with 25–32% porosity were obtained. Gas permeability and Hg porosimetry studies were used to evaluate permeability, pore distribution and pore density. Electrical conductivity studies of the cermets were also performed.     

Structural characteristics and chemical bonding states with temperature in barium titanate nanopowders prepared by using the solvothermal method

We report the structural characteristics and the chemical bonding states of nano-sized BaTiO₃ powders before and after having been heat treatment. We prepare BaTiO₃ nanopowders by using the solvothermal method at different reaction temperatures. We anneal the prepared powders at 400 and 600 °C, respectively, for 1 h. We examine the structures of the prepared powders by using the Rietveld analysis, and the chemical bonding states of the ions by using a fitting program with an assumption that the measured spectra are Gaussian. Then, we study the morphology of the nanopowders, and measure the ferroelectric properties with frequency and reaction temperature. We show that the BaTiO₃ nanopowders prepared by using the solvothermal method at lower reaction temperature exhibit the tetragonality and are useful for the electronic device applications.     

Intrinsic and impurity luminescence of rare earth ions doped KYF4 nanophosphors

The KYF₄ nanopowders, non-doped and doped with Ce³⁺ or Tb³⁺, having well-crystallized, unaggregated, monodisperse (±15%) nanoparticles with the cubic (the size in the range from ∼15 to ∼30 nm) or hexagonal (from ∼30 to ∼50 nm) crystal structure have been successfully synthesized by microwave-hydrothermal treatment of as-precipitated gels. In KYF₄ hexagonal nanopowders an intense STE-type luminescence at ∼4.4 eV was observed which is not quenched at room temperature. In contrast to single crystals or cubic nanopowders, in KYF₄ hexagonal nanopowders doped with Ce³⁺ or Tb³⁺, a rather efficient energy transfer is observed from the host to Ce³⁺ or Tb³⁺ ions, respectively, because of overlapping the emission spectrum of STE-type luminescence and the spectrum of efficient absorption on 4f-5d transitions in Ce³⁺ or Tb³⁺.     

One-pot fabrication and enhanced thermoelectric properties of poly(3,4-ethylenedioxythiophene)-Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> nanocomposites

Bi₂S₃ nanotubes and de-doped poly(3,4-ethylenedioxythiophene) (PEDOT) composite nanopowders were synchronously synthesized by a one-pot self-assembly method. The powders were characterized by X-ray powder diffraction, infrared spectroscopy, and transmission electron microscopy, respectively. Thermoelectric properties of the Bi₂S₃–PEDOT composite nanopowders with different Bi₂S₃ contents after being cold pressed into pellets were measured at room temperature. The sample with 36.1 wt% Bi₂S₃ has a highest power factor of 2.3 μWm⁻¹K⁻², which is higher than that of both pure PEDOT (0.445 μWm⁻¹K⁻²) and Bi₂S₃ (1.94 μWm⁻¹K⁻²).     

Critical behavior of the heat capacity of Ag-doped manganites

The critical behavior of the heat capacity of silver-doped manganites has been studied. The laws of variation in the universal critical parameters near the phase transition point have been established. It has been shown that the universality class of the critical behavior of the heat capacity for the La_1−x Ag _y MnO₃ (y≤x) system does not depend on the silver concentration or the synthesis conditions.     

The luminescence of ZnO ceramics

The luminescence properties of ZnO ceramics with grains 100–5000 nm sintered by different techniques from nanopowders were studied. The luminescence decay times were compared with that obtained for ZnO single crystal. The temperature dependence of non-exponential decay of defect luminescence (2.0–2.6 eV) was measured in wide time, intensity and temperature range. The luminescence decay kinetic at T ≤ 20 K shows the decay close to I(t) ～ t^?1 dependence. At temperature region 50–250 K the decay kinetics is more complicate since the TSL was observed in this temperature region. It is shown that the luminescence properties of NP and ceramics strongly depend on defect distribution on grains surface and the volume/surface ratio determine the luminescence decay in ZnO nanostructures and ceramics.     

Far-infrared spectra of dysprosium doped yttrium aluminum garnet nanopowder

The solution combustion synthesis was used to prepare nanopowders of yttrium aluminum garnet (YAG) and YAG doped with dysprosium ions, Dy³⁺, (YAG:Dy). The morphology, specific surface area, texture, and optical properties of the prepared materials were studied by the means of scanning electron microscopy (SEM), nitrogen adsorption method, and far-infrared spectroscopy at room temperature in the spectral region between 80 and 600 cm⁻¹. It was established that all the examined samples were microporous. The Maxwell–Garnet formula was used to model dielectric function of YAG and YAG:Dy nanopowders as mixtures of homogenous spherical inclusions in air.     

Identification of F+ centers in hafnia and zirconia nanopowders

Luminescence properties of undoped hafnia and zirconia nanopowders prepared by solution combustion synthesis were investigated under photo- and electron-beam excitation in 10–400 K temperature range. Along with the main luminescence band revealed in investigated materials at low temperatures at 4.2–4.3 eV and ascribed to the emission of self-trapped excitons, there are luminescence bands due to defects and impurities introduced during sample preparation. At room temperature the latter emissions dominate in the luminescence spectra as the intrinsic self-trapped exciton emission is quenched. Analysis of decay kinetics of defect centers allowed identification of F⁺ centers emission at 2.8 eV with lifetimes ∼3–6 ns in hafnia and zirconia under intra-center excitation.     

Study of energy transfer from capping agents to intrinsic vacancies/defects in passivated ZnS nanoparticles

The study of energy transfer mechanism from different capping agents to intrinsic luminescent vacancy centres of zinc sulphide (ZnS) has been reported in the present work. Nanoparticles of capped and uncapped ZnS are prepared by co-precipitation reaction. These nanoparticles are sterically stabilized using organic polymers—poly vinyl pyrrolidone, 2-mercaptoethanol and thioglycerol. Monodispersed nanoparticles were observed under TEM for both capped and uncapped ZnS nanopowders. However, for uncapped ZnS nanopowders, tendency for formation of nanorod like structure exists. Size of ZnS crystallites was calculated from X-ray diffraction pattern. The primary crystallite size estimated from X-ray diffraction pattern is 1.95–2.20 nm for capped nanostructures and 2.2 nm for uncapped nanostructures. FTIR spectra were conducted to confirm capping. Zeta potential measurements have been done to check the stability of dispersed nanoparticles. Band gap measurement was done by UV–visible spectrophotometer. Excitation and emission spectra are also performed in order to compare optical properties in various samples. Increase in emission intensity and band gap has been observed by adding different capping agents in comparison to uncapped ZnS nanoparticles. The results show that in capped ZnS nanoparticles the mechanism of energy transfer from capping layer to photoluminescent vacancy centres is more pronounced.     

Intrinsic and extrinsic luminescence of nanosize transition alumina powders

Luminescence spectroscopy in the VUV-visible range under electron-beam excitation and synchrotron radiation was applied to investigate electronic properties of alumina nanopowders, which were prepared using the combustion synthesis method. By varying reaction and post treatment conditions we were able to prepare phase pure samples and powders with mixtures of α- and γ-phases mainly. In addition to the well-known 7.6 eV luminescence of STE of α-alumina, all samples possessed complex emission bands in UV range (3–5 eV) which originate from intrinsic excitonic emissions and extrinsic electronic excitations.