Supree Pinitsoontorn

Synthesis, characterization, and magnetic properties of monodisperse CeO2 nanospheres prepared by PVP-assisted hydrothermal method

Ferromagnetism was observed at room temperature in monodisperse CeO2 nanospheres synthesized by hydrothermal treatment of Ce(NO3)3·6H2O using polyvinylpyrrolidone as a surfactant. The structure and morphology of the products were characterized by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and field-emission scanning electron microscopy (FE-SEM). The optical properties of the nanospheres were determined using UV and visible spectroscopy and photoluminescence (PL). The valence states of Ce ions were also determined using X-ray absorption near edge spectroscopy. The XRD results indicated that the synthesized samples had a cubic structure with a crystallite size in the range of approximately 9 to 19 nm. FE-SEM micrographs showed that the samples had a spherical morphology with a particle size in the range of approximately 100 to 250 nm. The samples also showed a strong UV absorption and room temperature PL. The emission might be due to charge transfer transitions from the 4f band to the valence band of the oxide. The magnetic properties of the samples were studied using a vibrating sample magnetometer. The samples exhibited room temperature ferromagnetism with a small magnetization of approximately 0.0026 to 0.016 emu/g at 10 kOe. Our results indicate that oxygen vacancies could be involved in the ferromagnetic exchange, and the possible mechanism of formation was discussed based on the experimental results.

Structure and Magnetic Properties of Monodisperse Fe3+-doped CeO2 Nanospheres

This work reports the study concerning the structure and magnetic properties of undoped CeO2 and Fe-doped CeO2 (Ce1−xFexO2, 0.01 ≤ x ≤ 0.07) nanospheres with diameters of 100∼200 nm prepared by hydrothermal method using polyvinylpyrrolidone (PVP) as surfactant. The prepared samples were studied by using X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray absorption near-edge structure (XANES), and vibrating sample magnetometry (VSM). The XRD results showed that Fe-doped CeO2 was single-phased with a cubic structure, and with Fe3+ successfully substituting in Ce4+ sites. Raman spectra showed a redshift of F2g mode that caused by the Fe doping. The samples of both undoped CeO2 and Fe-doped CeO2 exhibited room temperature ferromagnetism, and the saturated magnetization (Ms) increased with increasing Fe content until x = 0.05, and then the samples displayed ferromagnetic loops as well as paramagnetic behavior. The roles of Ce3+ and Fe3+ spin electrons are discussed for the ferromagnetism in the Fe-doped CeO2.

Room-temperature ferromagnetism in Co-doped CeO2 nanospheres prepared by the polyvinylpyrrolidone-assisted hydrothermal method

Nanospheres of pure CeO2 and Co-doped CeO2 (Ce1−xCoxO2, 0.01 ≤ x ≤ 0.07) dilute magnetic oxide were prepared by hydrothermal treatment using cerium (III) nitrate, cobalt (III) nitrate, and polyvinylpyrrolidone (PVP) as a surfactant. The prepared samples were studied using x-ray diffraction (XRD), transmission electron microscopy, field-emission scanning electron microscopy (FE-SEM), and UV-Visible spectroscopy. The valence states of Ce and Co ions were determined by using x-ray absorption near edge spectroscopy. The magnetic properties of the samples were studied using vibrating sample magnetometry. The results from XRD indicated that the synthesized samples had a cubic structure without a change in the structure of CeO2 due to Co substitution. FE-SEM micrographs showed that the samples had a spherical morphology. The Co-doped CeO2 showed a red shift of the band gap energy that originates from defects caused by Co substitution. The samples of both CeO2 and Co-doped CeO2 exhibit room temperature ferromagne...

First-Principles Study of the Electronic Structure and Thermoelectric Properties of Al-Doped ZnO

ZnO materials doped with elements such as Al, Ga, etc. are of great interest for high-temperature thermoelectric applications. In this work, the effects of Al doping on the electronic structure and thermoelectric properties of the ZnO system are presented. The energy band structure and density of states of Al-doped ZnO were investigated using the projector-augmented plane wave pseudopotential method within the local density approximation. The calculated energy band structure was then used in combination with the Boltzmann transport equation to calculate the thermoelectric parameters of Al-doped ZnO. The electronic structure calculation showed that the position of the Fermi level of the doped sample was shifted to a higher energy level compared with the undoped material. The conduction band near the Fermi energy was a combination of hybridized Zn sp-orbitals and Al s-orbital. The calculated thermoelectric properties were compared with the experimental results, showing some agreement. For the Al-doped ZnO system, the Seebeck coefficient was shown to be negative and its absolute value increased with temperature. The electrical conductivity and electronic thermal conductivity followed the trend of the experimental results.

Synthesis and thermoelectric properties of Ca3Co4O9 prepared by a simple thermal hydro-decomposition method

Thermoelectric Ca3Co4O9 powders were synthesized by a simple thermal hydro-decomposition method, which is novel, simple and cost effective for preparing such materials. The stoichiometric ratio of the acetate salts was mixed in de-ionized water and heated at 1073 K to obtain a single phase of Ca3Co4O9, which was confirmed by TG-DTA, XRD and chemical analysis. The electron micrograph showed plate-like particles with a diameter of ∼1 µm. The hot-pressed powders show the electrical resistivity of 11.6 mΩcm, the Seebeck coefficient of 200 µV/K, and the thermal conductivity of 1.2 Wm−1K−1 at 880 K, which corresponds to a dimensionless figure-of-merit ZT of 0.23.

Reducing the ordering temperature of CoPt nanoparticles by B additive

We reported the effect of boron addition on magnetic properties and structure of CoPt nanoparticles prepared by a polyol method. The magnetic property measurement showed that the CoPt-B sample exhibited a much larger coercivity compared to the sample without B additive at the same annealing temperature. Transmission electron microscopy and energy dispersive X-ray spectroscopy revealed that the average particle size was about 2 nm for the as-synthesized sample with the ratio of Co and Pt close to 1:1. After annealing, the particle sizes increased but the composition was maintained. The phase transformation of the nanoparticles versus temperature was investigated using a combination of X-ray diffraction and in-situ X-ray absorption analysis. It was shown that the phase transition temperature at which the nanoparticles change from the disordered A1 phase to the ordered L10 phase occurs at temperature of 600 °C. We concluded that boron additives could reduce the ordering temperature of CoPt of about 100 °C.

Thermoelectric properties of Al-doped ZnO： experiment and simulation

Advancement in doping other elements, such as Ce, Dy, Ni, Sb, In and Ga in ZnO[1], have stimulated great interest for high-temperature thermoelectric application. In this work, the effects of Al-doping in a ZnO system on the electronic structure and thermoelectric properties are presented, by experiment and calculation. Nanosized powders of Zn1−x Al x O (x = 0,0.01, 0.02, 0.03 and 0.06) were synthesized by hydrothermal method. From XRD results, all samples contain ZnO as the main phase and ZnAl2O4 (spinel phase) peaks were visible when Al additive concentrations were just 6 at%. The shape of the samples changed and the particle size decreased with increasing Al concentration. Seebeck coefficients, on the other hand, did not vary significantly. They were negative and the absolute values increased with temperature. However, the electrical resistivity decreased significantly for higher Al content. The electronic structure calculations were carried out using the open-source software package ABINIT[2], which is based on DFT. The energy band gap, density of states of Al-doped ZnO were investigated using PAW pseudopotential method within the LDA + U. The calculated density of states was then used in combination with the Boltzmann transport equation[3] to calculate the thermoelectric parameters of Al-doped ZnO. The electronic band structures showed that the position of the Fermi level of the doped sample was shifted upwards in comparison to the undoped one. After doping Al in ZnO, the energy band gap was decreased, Seebeck coefficient and electrical conductivity were increased. Finally, the calculated results were compared with the experimental results. The good agreement of thermoelectric properties between the calculation and the experimental results were obtained.

Effects of Ge substitution on the structural and physical properties of CuFeO2 delafossite oxide

Delafossite CuFe1?xGexO2 (0.0 ? x ? 0.1) semiconductors were synthesized by solid-state reaction. The effects of Ge concentration on microstructural, optical, magnetic and electrical properties were investigated. X-ray diffraction (XRD) analysis results reveal the delafossite structure of all the samples. The lattice spacing of CuFe1?xGexO2 decreased with increasing substitution of Ge at the Fe site. The optical properties measured at room temperature by UV?visible spectroscopy showed an absorption peak at 283 nm (4.38 eV). The corresponding direct optical band gap was found to decrease with increasing Ge content (from 3.69 eV for x = 0 to 3.61 eV for x = 0.10), exhibiting transparency in the visible region. The magnetic hysteresis loops measured at room temperature showed that the Ge-doped CuFeO2 samples exhibit ferromagnetic behavior. The Curie temperature suggests that ferromagnetism originates from CuFe1?xGexO2 matrices. The substitution of Fe3+ by Ge4+ produces a mixed effect on the magnetic properties of CuFeO2 delafossite oxide. The resistivity of CuFe0.99Ge0.01O2 was observed to be ?0.1 ??cm at room temperature.

Structural, Magnetic Properties and Dye-Sensitized Solar Cells Application of Pure and La Doped BiFeO3 Powders Prepared by Sol-Gel

Multifunctional materials Bi1-xLaxFeO3 (x = 0.05, 0.10, 0.15, 0.20) powders with and without annealing (700°C) were prepared by sol-gel method to improve the magnetic and optical properties. The particle size, chemical structures, and chemical compositions of the powders were investigated by bright-field TEM, XRD, SADP, and XRF. The bright-field TEM results show the particle size of the prepared Bi1-xLaxFeO3 powders without annealing (around 200 nm) is less than that of the annealed Bi1-xLaxFeO3 powders (more than 500 nm). The XRD results of the powders without annealing show the formation of rhombohedral structure of BiFeO3 with Bi24Fe2O39 as impurity phase in all samples, whereas the prepared powders annealed at 700°C have a higher phase purity. The M – H hysteresis loop of the powders with annealing and without annealing were measured by vibrating sample magnetometer at room temperature up to the field of 10 kOe. The results demonstrate that the powders with higher purity phase and larger particle size were larger saturation magnetization. The saturation magnetizations were increased by increasing the La doping content. The optical absorption edge result can be considered energy band gap (∼2.0–2.2 eV) of all prepared powders. The prepared powders were used as photoelectrode in dye-sensitized solar cell applications. The current density versus voltage characteristics of solar cell were measured under illumination of simulated sunlight coming from a solar simulator with the radiant power of 100 mW/cm2 to obtain photocurrent, photovoltage, and overall photoconversion efficiency of solar cells. The power conversion, photocurrent density and photovoltage depending on purity, particle size with and without annealing powders using as photoelectrode in dye sensitized solar cells are discussed.

Effect of boron addition on the structure and magnetic properties of CoPt nanoparticles

The effect of B addition on CoPt nanoparticles was investigated. The CoPt-B nanoparticles were synthesized by means of the polyol process. Transmission electron microscopy has shown that the as-synthesized particles have a spherical morphology with average size about 2–3 nm. The X-ray absorption spectroscopy and the X-ray diffraction technique showed the effect of B concentration on phase transformation. The addition of B at up to 60% promoted the formation of the L10 phase when the nanoparticles were subjected to annealing at 600 °C. If the B content is higher than 60%, the phase transition is suppressed. The evidence of B addition on the structure of CoPt nanoparticles was further supported by the magnetic measurements. The results show that the coercivity of the annealed CoPt-B nanoparticles was enhanced by the B additions from 20% to 60%, with the maximum coercivity of 12 000 Oe for the CoPt-40%B sample.