Hyun Gil Cha

Fabrication of hollow metal oxide nanocrystals by etching cuprous oxide with metal(II) ions: approach to the essential driving force

Hollow metal oxide nanocrystals were prepared by etching cuprous oxide with metal ions and were applied as photoelectrodes. As a hard template, polyvinylpyrrolidone stabilized cuprous oxide (PVP-Cu2O) and non-stabilized cuprous oxide (nPVP-Cu2O) were synthesized by a precipitation method. Hollow iron oxide and cobalt oxide nanocrystals with a truncated octahedral morphology were fabricated by an etching reaction with transition metal(II) ions (Fe(2+) or Co(2+)). In the etching reaction process, a cationic exchange reaction occurs between the divalent metal ion and Cu(+) due to the higher Lewis acidity. Facet selective etching of cuprous oxide has been observed during the ionic exchange reaction of Cu(+) and O(2-) ions in PVP-Cu2O complexes with transition metal(II) ions (Fe(2+) or Co(2+)) at the surface of a (110) facet. Amorphous states of hollow metal oxide products were annealed to form α-Fe2O3 (hematite) and Co3O4 and their crystal structure was examined with X-ray diffraction and HR-TEM. The optical absorption behavior of semiconductor nanocrystals was measured with UV-vis spectroscopy to define band gap energy. The hollow hematite structure has a 2.08 eV band gap and Co3O4 (Co(II,III) oxide) has a 1.80 eV indirect band gap. Using these hollow nanocrystals, a metal oxide monolayer film was fabricated with a secondary growth approach and was studied for its photocatalytic properties.

Vertical cobalt dendrite array films: electrochemical deposition and characterization, glucose oxidation and magnetic properties

Vertically and laterally oriented cobalt dendrite films with or without dendritic structures were synthesized by cathodic electrodeposition under different experimental conditions. The morphology of Co deposits was varied significantly depending on deposition conditions such as applied potential, precursor concentration and especially pH value. Co dendritic crystal growth along the [110] direction is preferred. The possible growth mechanism is discussed by supposing differing concentration areas. Results of glucose electrooxidation demonstrate that vertical Co dendrite films are promising materials as carbohydrate sensors with high sensitivity and fast response. In addition, magnetic measurements on cobalt samples under parallel and perpendicular direction of the applied magnetic field show that they exhibit a ferromagnetic behavior with different saturation magnetizations and coercivities.

Study on synthesis and magnetic properties of Nd–Fe–B alloy via reduction–diffusion process

Synthesis of amorphous Nd–Fe–B nanoparticles under an aqueous borohydride solution and the magnetic behavior of the amorphous structure after mild heat treatment with CaH2 have been studied. The ultra-fine Nd–Fe–B nanoparticles were first synthesized from reduction of FeCl2 and NdCl3 using NaBH4 as a reducing agent in aqueous solution. The as-prepared precipitates, which were composed of Fe–B phase and Nd oxide phase, were then mixed with calcium hydride. The Nd oxide in the precipitates was reduced by CaH2 and then Nd diffused into the Fe–B phase via reduction–diffusion process. After the as-obtained Nd–Fe–B material was washed with acetic acid, its magnetic property was measured showing its ferromagnetic behavior.

One-dimensional ferromagnetic dendritic iron wire array growth by facile electrochemical deposition

One-dimensional ferromagnetic iron dendritic wire array film is prepared by facile electrodeposition. The space hindrance effect caused by neighbouring crystals resists the free growth directions parallel to the substrate, which is considered as a possible growth mechanism of one-dimensional morphology. Dendritic iron wire can be transformed into α-Fe(2)O(3) without destroying the dendritic morphology by thermal oxidation.

Synthesis and characterization of crystalline FeCo nanoparticles.

The monodispersed FeCo nanoparticles were synthesized with borohydride as a reducing agent by coprecipitation method in aqueous solution. The composition and the size of FeCo nanoparticle was controlled by fitting the molar ratio of starting material, the reaction time and the falling rate. To prepare the crystallized nanoparticles, the amorphous FeCo nanoparticles were annealed at the different temperatures for 1 hr, respectively. The size of the crystalline FeCo nanoparticles was controlled in order to maintain the uniform size of 20 nm. The as-annealed FeCo alloy could get the highest Ms value of the powder which has been ever synthesized by coprecipitation or other chemical method. The size- and shape-controlled crystalline FeCo nanoparticles can be applied for spring magnet in order to get exchange coupling effect.

Photocatalysis: progress using manganese-doped hematite nanocrystals

Manganese-doped hematite nanocrystals were synthesized by hydrothermal method and study on the activity of photocatalytic properties by different Mn ratio. From the crystal structure and optical study, the oxidation state of the manganese-doped hematite crystals was modulated by doping. The activity of samples (M1–M3) was tested by photodegradation of methyl orange dye in the presence of manganese-doped hematite as photocatalyst.

Hierarchical NiO hollow microspheres: electrochemical and magnetic properties

The interesting electrochemical performances and magnetic properties of hierarchical NiO hollow microspheres were investigated. A rational formation mechanism of NiO hollow spheres from Ni(OH)2 was proposed on the basis of agglomerated-mediated growth.

Kinetics of Decolorization of Spironaphthooxazine-Doped Photochromic Polymer Films

Photochromic polymeric films were prepared by doping photochromic dye spironaphthooxazine into polymer resin gels such as polyurethane, vinyl copolymer, and copolymer of vinyl and nitorocellulose at different concentrations. All of the composite films show normal photochromism. The kinetics of the photochromism/decoloration in the films were quantified by fitting biexponential equations to their photochromic decay curves after irradiation. It was observed that the decoration process is faster in vinyl copolymer than that in the copolymer of vinyl and nitrocellulose and is the slowest in the case of polyurethane. The decoloration mechanisms of spironaphthooxazine in those polymeric matrixes have been discussed.

PREPARATION AND CHARACTERIZATION OF Ag(CORE)/SiO2(SHELL) NANOPARTICLES

Ag nanoparticles were synthesized by using thermal decomposition of Ag+–oleate complex. The mean size of Ag nanoparticle was confirmed as 10 nm with transmission electron microscopy (TEM). The crystal structure and optical property of Ag nanoparticles were studied with X-ray powder diffraction (XRD) and UV–vis spectroscopy, respectively. The Ag nanoparticles were used as seeds for Ag(core)/SiO2(shell) nanocomposite particles. TEM images of Ag(core)/SiO2(shell) nanocomposite particles showed that silver core was coated with 15 nm thickness of SiO2 shell. The thickness of SiO2 shells could be conveniently controlled by changing the concentration of sol–gel precursor or reaction time. The composition of Ag(core)/SiO2(shell) nanocomposite particle was investigated with energy-dispersive X-ray (EDX) spectrometer.

Enhanced photoluminescence of single crystalline ZnO nanotubes in ZnAl2O4 shell

Epitaxial growth of ZnO/ZnAl2O4 (core/shell) nanotubes from Al2O3 coated ZnO nanorods were synthesized using a three-step reaction. In the first step we synthesized ZnO nanorod on Si(100) wafers using chemical vapor deposition, followed by Al2O3 deposition on synthesized ZnO nanorod with the help of atomic-layer deposition. Finally in the last step, the resultant sample (i.e.Al2O3 deposited ZnO nanorod) was calcined at 500 °C for 1 h. Hexagonally well-faceted ZnO nanorod arrays were aligned to Si(100) wafers without metal-catalyst at 500 °C for 1 h. Thereafter it was transformed to spinel-type ZnAl2O4 nanotubes, which still have ZnO nanotube as a core, by interfacial solid-state reaction in Al2O3 coated ZnO nanorods. XRD and Raman analysis demonstrated that single crystallites of both nanotubes co-exist. The photoluminescence spectra show that ZnO nanorod, Al2O3 coated ZnO nanorod, ZnO/ZnAl2O4 (core/shell) nanotube indicated different luminescence bands with different intensity at He–Cd laser excitation (325 nm line).