Bon Heun Koo

Structural and magnetic properties of chemically synthesized Fe doped ZnO

We report on the synthesis of Fe-doped ZnO with nominal composition of Zn0.99Fe0.01O by using a coprecipitation method. X-ray diffraction and selective area electron diffraction studies reveal a single phase wurtzite crystal structure without any secondary phase. Field emission transmission electron microscopy measurements infer that Zn0.99Fe0.01O have nanorod-type microstructures. Magnetic hysteresis measurement performed at different temperatures show that Zn0.99Fe0.01O exhibits a weak ferromagnetic behavior at room temperature. A detailed investigation of the electronic and local structure using O K-, Fe L3,2 near edge x-ray absorption fine structure suggests that Fe is substituting Zn in ZnO matrix and is in Fe3+ state.

RAPID AND COST EFFECTIVE SYNTHESIS OF ZnO NANORODS USING MICROWAVE IRRADIATION TECHNIQUE

ZnO nanorods assembled in flower shaped morphology have been successfully synthesized using low power microwave irradiation in a very short duration. The diameter and length of the rods were within 150–190 nm (tip diameter ~15 nm) and 2 μm, respectively, with an aspect ratio of 20–22. The synthesized nanorods were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared microscopy (FT-IR), photoluminescence (PL) and magnetization measurements. The XRD and FT-IR results indicate that ZnO nanorods have the pure wurtzite structure with lattice parameters a and c of 3.254 and 5.197 A, respectively. The selected area electron diffraction (SAED) pattern reveals that the ZnO nanorods are single crystal in nature and grow along [001] plane. Room-temperature PL spectrum of the as-grown ZnO nanorods shows a near-band-edge (NBE) emission peak and defect induced emissions. Magnetization measurements indicate that ZnO nanorods exhibit room temperature ferromagnetism with remanent magnetization (Mr) and coercivity (Hc) about 2.92 × 10-4 (emu/g) and 29.75 Oe, respectively, which may be due to the presence of defects in the ZnO nanorods.

Morphological evolution between nanorods to nanosheets and room temperature ferromagnetism of Fe-doped ZnO nanostructures

In this work, undoped and Fe-doped single-crystalline ZnO nanostructures were successfully synthesized by a facile microwave irradiation method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) results showed that Fe-doped ZnO was comprised of a single phase nature with a hexagonal wurtzite structure up to 5% Fe doping, however, secondary phase ZnFe2O4 appeared upon further increasing the Fe dopant concentration. Field emission scanning electron microscopy (FESEM) and TEM micrographs suggested that the length and diameter of the undoped ZnO rods are about ∼2 μm and ∼200 nm, respectively. Interestingly, the morphology of ZnO changed from nanorods (1% Fe) with length ∼500 nm and diameter ∼50 nm to nanosheets (5% Fe) having thickness and lateral dimension of ∼30 nm and ∼400 nm, respectively. NEXAFS and EELS studies revealed the absence of metal clusters up to 5% and Fe is found to be in a mixed (Fe2+/Fe3+) valence state with Fe2+ as the dominant state. Optical studies depicted that the absorption peak of Fe-doped ZnO was blue-shifted as the concentration of Fe increases from 1 to 5%. However, for dopant concentration >5%, the absorption peak was found to be red-shifted with an additional absorption peak of ZnFe2O4. Also, the band gap energy decreased monotonically with the increase of Fe concentration from 1 to 5%, while increasing on further doping, band gap increased. Raman scattering spectra of Fe-doped ZnO revealed the lower frequency shift of Ehigh2 mode with doping. Magnetic studies showed that Fe doped ZnO exhibit room temperature ferromagnetism (RTFM) and the value of magnetization increased up to 5% doping and then decreased for 7 and 10% Fe-doped samples.

Morphological evolution of ZnO nanostructures and their aspect ratio-induced enhancement in photocatalytic properties

This work presented controllable growth of ZnO nanostructures with different aspect ratios by the microwave irradiation method and investigated the photocatalytic degradation of methyl red (MR). X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) measurements showed that all ZnO nanostructures were of a hexagonal phase structure. It was revealed by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) images that the morphology of ZnO can be effectively controlled as sheet-like, rod-like, brush-like, flower-like, prism-like, and pyramid-like only by changing the molar ratio (zinc acetate: KOH) and reaction time. With the increase of molar ratio and reaction time, modification in the E2(high) and E1(LO) Raman modes was observed. The energy band gap was found to be tuned by the aspect ratio of ZnO nanostructures. Photoluminescence spectroscopy revealed the low-intensity NBE emission and high and broad defect-related emission for high aspect ratio (14) nanorods. BET surface area porosity analysis confirmed the presence of a mesoporous network in all the nanostructures, showed high surface area and a uniform pore-size distribution for high aspect ratio nanorods. A terephthalic acid assay study confirmed the formation of hydroxyl radicals (OH) in MR dye solution treated with a ZnO nanostructures photocatalyst. The photodegradation of MR under UV light irradiation showed that ZnO nanorods with a high aspect ratio of ∼14 showed superior photodegradation (∼98% degradation of MR within 60 min) than that of the lower aspect ratio nanostructures. The apparent reaction rate constant for high aspect ratio (14) nanorods was higher than that of the lower aspect ratio nanostructures. The enhancement in photocatalytic performance could be due to the high surface area and enhanced charge separation and transfer efficiency of photoinduced charge carriers in the high aspect ratio nanorods.

Structural and Magnetic Properties of Co Doped CeO

Nanoparticles of Ce1-xCoxO2 diluted magnetic semiconductors have been prepared by co-precipitation method. Effect of Co substitution has been studied using X-ray diffraction (XRD), transmission electron microscopy (TEM) and DC magnetization measurements. XRD pattern demonstrate that all samples exhibit single phase polycrystalline behavior. The full width at half maximum (FWHM) calculated from XRD pattern has been found to decrease with increase in the Co contents, indicating that particle size decreases with increase in the Co concentration. TEM micrograph reflects the nano-crystalline behavior of all the samples and shows that doping of Co ions hinders growth of the particles. DC magnetization measurements performed at different temperature reveals that Co doped CeO2 exhibits room temperature ferromagnetism.

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We report studies on the magnetocaloric effect of samarium doped lanthanum manganites with different Sm-concentrations. Polycrystalline La0.7-xSmxCa0.30MnO3 (0≤x≤0.3) samples were prepared using the conventional solid-state reaction method with phase purity and structure confirmed using X-ray diffraction. Temperature dependent magnetization measurements and Arrott analysis reveal first order ferromagnetic transition in parent sample and second order ferromagnetic transition in doped sample with Curie temperature decreasing progressively with increasing Sm concentration from ~182 K for x = 0.05 to 109 K for x = 0.30. A large magnetic entropy change (~1.75 J kg-1 K-1 at 0.5 T) has been observed in La0.7Ca0.3MnO3 sample, and is greatly suppressed as a function of disorder caused by Sm doping. This is ascribed to the gradual loss of first-order ferromagnetic transition. This investigation suggests that La0.7-xSmxCa0.30MnO3 compounds can be used as a potential magnetic refrigerating material with wide range of temperature.

Nano-Particles

Abstract Plasma electrolytic oxidation (PEO) is carried out on 6061 Al-alloys in a weak alkaline electrolyte containing NaOH, Na 2 SiO 3 and NaCl. Centered on the correlation of composition and structure, analyses by means of X-ray diffration (XRD), scanning electron microscope (SEM) and energy dispersive spectrometry (EDS) are conducted on the specimens, which have been PEO-treated under hybrid voltages of different direct current (DC) values (140-280 V) with constant alternate current (AC) amplitude (200 V). Attention is paid to the composition, properties and growth mechanism of oxide layers formed with hybrid voltages. Moreover, the main effects of DC value are discussed. Ceramic layers with a double-layer structure which combines hard outer and soft inner layers are found to be consist of α-Al 2 O 3 , γ-Al 2 O 3 and mullite. With the DC values increasing, the growth of the ceramic layers tends to have increasingly obvious three-stage feature.

Study of A-Site Disorder Dependent Structural, Magnetic, and Magnetocaloric Properties in La0.7-xSmxCa0.3MnO3 Manganites

Abstract A convenient and scalable technique for the synthesis of rutile titanium dioxide (TiO2) nano-rods was presented by using bulk TiO2 powder, sodium hydroxide (NaOH) and distilled water as raw materials. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) studies indicate that the prepared sample is crystalline and free from any impurities, however, it has no distinct shape and possesses a huge degree of agglomeration, and the average crystal size is around 40 nm. After annealing the sample at 600 °C for 2 h, it is observed through FESEM that nano-rods are formed. And XRD analysis shows that the nano-rods are single crystalline with distinct and smooth surfaces in different sizes with average length of about 1 µm and diameter of about 80 nm. Further UV-visible spectroscopy and Raman studies were conducted for the prepared sample and the band gap of the final product is found to be 3.40 eV.

Effects of Hybrid Voltages on Oxide Formation on 6061 Al-alloys During Plasma Electrolytic Oxidation

The production level of foodgrains has become an issue of concern as it has shown a downward trend during the last decade. Since, there has been a drastic decrease in natural resources; it is through agriculture that we can visualize a self sustainable world. The growth in agriculture can be achieved only by increasing productivity through an effective use of modern technology as the land and water resources are limited. Nanobiotechnology provides the tool and technological platforms to advance agricultural productivity through genetic improvement of plants, delivery of genes and drug molecules, to specific sites at cellular levels. The interest is increasing with suitable techniques and sensors for precision in agriculture, natural resource management, early detection of pathogens and contaminants in food products and smart delivery systems for agrochemicals like fertilizers and pesticides. To achieve the goals of “nano-agriculture”, detailed investigation on the ability of nanoparticles to penetrate plant cell walls and work as smart treatment-delivery systems in plants, is needed. In this chapter, thorough studies and reliable information regarding the effects of nanomaterials on plant physiology and crop improvement at the organism level, are discussed.

Facile synthesis of single-crystalline rutile TiO2 nano-rods by solution method

In this paper, magnetic property and magnetocaloric effect (MCE) in perovskite manganites of the type La (0.75-X) Ce X Ca 0.25 MnO 3 (x = 0.0, 0.2, 0.3 and 0.5) synthesized by using the standard solid state reaction method have been reported. From the magnetic measurements as a function of temperature and applied magnetic field, we have observed that the Curie temperature (T C ) of the prepared samples strongly dependent on Ce content and was found to be 255, 213 and 150 K for x = 0.0, 0.2 and 0.3, respectively. A large magnetocaloric effect in vicinity of T C has been observed with a maximum magnetic entropy change (|ΔSM| max ) of 3.31 and 6.40 J/kgK at 1.5 and 4 T, respectively, for La 0.55 Ce 0.2 Ca 0.25 MnO 3 . In addition, relative cooling power (RCP) of the sample under the magnetic field variation of 1.5 T reaches 59 J/kg. These results suggest that La 0.55 Ce 0.2 Ca 0.25 MnO 3 compound could be a suitable candidate as working substance in magnetic refrigeration at 213 K.