Min Gyu Kim

Critical Enhancements of MRI Contrast and Hyperthermic Effects by Dopant-Controlled Magnetic Nanoparticles†

Doped up: The incorporation of Zn(2+) dopants in tetrahedral sites leads to the successful magnetism tuning of spinel metal ferrite nanoparticles (see picture). (Zn(0.4)Mn(0.6))Fe(2)O(4) nanoparticles exhibit the highest magnetization value among the metal ferrite nanoparticles. Such high magnetism results in the largest MRI contrast effects (r2=860 mm(-1) s(-1)) reported to date and also huge hyperthermic effects.

Washing Effect of a LiNi0.83Co0.15Al0.02O2 Cathode in Water

The formation of LiOH and Li 2 CO 3 impurities on high Ni content LiNi 0 . 8 3 Co 0 . 1 5 Al 0 . 0 2 O 2 powders due to H 2 O and CO 2 absorption from the air can be reduced without structural degradation by washing in water. Although the as-synthesized sample had a moisture content of 570 ppm immediately after firing, this level increased rapidly to 1270 ppm in air with a relative humidity of 50%. However, its content was decreased to 210 ppm after washing twice in water, followed by heat-treatment at 700°C. It is believed that this improvement was due to the decreased level of Li 2 CO 3 and LiOH impurities on the particles. This was highlighted by the decreasing swelling of the Li-ion cell at 90°C, and the thickness of the cell containing the washed samples was decreased by 50% compared with the bare sample.

Nanoparticle iron-phosphate anode material for Li-ion battery

Nanoparticle crystalline iron phosphates (FePO4∙2H2O and FePO4) were synthesized using a (CTAB) surfactant as an anode material for Li rechargeable batteries. The electrochemical properties of the nanoparticle iron phosphates were characterized with a voltage window of 2.4–0 V. A variscite orthorhombic FePO4∙2H2O showed a large initial charge capacity of 609mAh∕g. On the other hand, a tridymite triclinic FePO4 exhibited excellent cyclability: the capacity retention up to 30 cycles was ∼80%, from 485 to 375mAh∕g. The iron phosphate anodes exhibited the highest reported capacity, while the cathode LiFePO4 has an ideal capacity of 170mAh∕g.

Structural Characterization of the Surface-Modified Li x Ni0.9Co0.1O2 Cathode Materials by MPO4 Coating (M = Al , Ce, SrH, and Fe) for Li-Ion Cells

Structural characterization of surface-modified Li x Ni 0.9 Co 0.1 O 2 cathodes (x = 0.3 and 0.15) using an MPO 4 coaling (M = Al, Ce, SrH, and Fe) were investigated for their potential applications to Li-ion cells. MPO 4 nanoparticles that were precipitated from metal nitrate and (NH 4 ) 2 HPO 4 in water at pH = 10 were coated on the cathodes via mixing and heat treatment at 700°C. The CePO 4 and SrHPO 4 -coated Li 0.3 Ni 0.9 Co 0.1 O 2 cathodes heat treated at 300°C were mainly made up of the rock-salt phase (Fm3m), while AlPO 4 and FePO 4 -coated cathodes showed disordered [Li 1-x (Ni,Co) x ] 3b [(Ni,Co) x ] 3a O 2 -type hexagonal structure (R3m) with a cation mixing. However, when the x value decreased from 0.3 to 0.15, bare and coated cathodes which had a spinel (Fd3m) or hexagonal structure (R3m) at x = 0.3 were transformed into a NiO-type rock-salt structure. AlPO 4 -coated sample exhibited lowest degree of oxygen generation after 300°C annealing at x = 0.15, indicating the highest thermal stability among the bare and coated cathodes.

Ge nitride formation in N-doped amorphous Ge2Sb2Te5

The chemical state of N in N-doped amorphous Ge2Sb2Te5 (a-GST) samples with 0–14.3Nat.% doping concentrations was investigated by high-resolution x-ray photoelectron spectroscopy (HRXPS) and Ge K-edge x-ray absorption spectroscopy (XAS). HRXPS showed negligible change in the Te 4d and Sb 4d core-level spectra. In the Ge 3d core-level spectra, a Ge nitride (GeNx) peak developed at the binding energy of 30.2eV and increased in intensity as the N-doping concentration increased. Generation of GeNx was confirmed by the Ge K-edge absorption spectra. These results indicate that the N atoms bonded with the Ge atoms to form GeNx, rather than bonding with the Te or Sb atoms. It has been suggested that the formation of Ge nitride results in increased resistance and phase-change temperature.

Effect of Capping Agents in Tin Nanoparticles on Electrochemical Cycling

Tin particles that were prepared using three different capping agents, hydrobenzamide, citrate, and polyvinyl pyrrolidone (PVP) exhibited different particle sizes. The hydrobenzamide-capped Sn had the smallest particle size (∼50) nm) and uniform distribution while the citrate and PVP-capped Sn had particle sizes of ∼100 and ∼300 nm. respectively. with severe particle aggregation. However, there was no SnO 2 or SnO detected on the particle surfaces. The cycling results using coin-type half cells confirmed that the hydrobenzamide-capped Sn had the highest charge capacity of 994 mAh/g between 1.5 and 0 V and the best capacity retention. In contrast, the citrate and PVP-capped Sn showed severe capacity decay. Further analysis using cycled electrodes showed that the hydrobenzamide-capped Sn showed the least particle agglomeration and growth, compared with the others. From Fourier transform magnitude (FT) of Sn L m -edge energy dispersive X-ray analyst spectra, these facts could be supported by the strong coordination formed as a result of chemical bonding between the nitrogen of the hydrobenzamide capping agent effectively inhibiting the particle growth during cycling.

Investigation of phase transition of Ge2Sb2Te5 and N-incorporated Ge2Sb2Te5 films using x-ray absorption spectroscopy

For this study, the phase-change materials Ge2Sb2Te5 and N-doped Ge2Sb2Te5 films were investigated using x-ray absorption near-edge structure and extended x-ray absorption fine structure. During the phase transition, change in electronic structure is observed by the shift of the absorption edge energy, i.e., structural coordination of Ge–Te changes from tetrahedral to octahedral coordination, of which the interatomic distances are 3.12 and 2.83A, respectively. In addition, nitrogen incorporation into the film led to a p-p orbital hybridization and a different crystallization behavior. The hybridization caused by the formation of a Ge–N bond was related to suppression of the phase transition.

Effect of Anisotropic Volume Change in Tin Phosphate Nanoparticle Anode Material with Mesocellular Foam Structure on Capacity Retention

Tin phosphate nanoparticles within microcellular foams were prepared using a nonionic triblock copolymer surfactant P123. The annealed sample at 500°C showed the particle size of 50-200 nm, and the size of the mesocelluar foam was ranged from 4 to 20 nm. Due to the irregular porewall thickness and pore size of the annealed sample, the pore wall structure had completely collapsed after first cycle. As the number of cycles increased, metallic tin clusters grew in the lithium phosphate matrix, and uniformly dispersed tetragonal tin nanoparticles with a particle size of 3 nm were observed after 100 cycles. This indicated that tin clusters decomposed from tin phosphate expanded and contracted reversibly in the matrix without particle aggregation. This was well supported by the electrochemical data, and the capacity increased to from 285 to 520 mAh/g with no capacity fading.

Phase Transition of Bare and Coated Li x CoO2 (x = 0.4 and 0.24) at 300 ° C

The phase transitions of the bare and AlPO 4 -coated delithiated Li x CoO 2 (x = 0.4 and 0.24) according to the coating concentration (thickness) after heat-treatment at 300°C was investigated using nuclear magnetic resonance and X-ray absorption spectra. It was found that the bare and coated Li x CoO 2 predominantly decomposed into spinel Li x Co 2 O 4 or Co 3 O 4 phases depending on the charging voltage. As the charging voltage was increased from 4.3 to 4.6 V, the bare and 1 wt % AlPO 4 -coated Li x CoO 2 decomposed into the Li x Co 2 O 4 phase while the 2.4 wt % AlPO 4 -coated sample decomposed into the Co 3 O 4 phase. The improvement in the thermal stability of the 2.4 wt % AlPO 4 -coated Li x CoO 2 , compared to the bare and 1 wt % AlPO 4 -coated samples, could be explained by the dominant local formation of the Co 3 O 4 phase over the Li x Co 2 O 4 phase.

A Ternary Ni46Co40Fe14 Nanoalloy-Based Oxygen Electrocatalyst for Highly Efficient Rechargeable Zinc-Air Batteries

Replacing noble-metal-based oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts is the key to developing efficient Zn-air batteries (ZABs). Here, a homogeneous ternary Ni46 Co40 Fe14 nanoalloy with a size distribution of 30-60 nm dispersed in a carbon matrix (denoted as C@NCF-900) as a highly efficient bifunctional electrocatalyst produced via supercritical reaction and subsequent heat treatment at 900 °C is reported. Among all the transition-metal-based electrocatalysts, the C@NCF-900 exhibits the highest ORR performance in terms of half-wave potential (0.93 V) in 0.1 m KOH. Moreover, C@NCF-900 exhibits negligible activity decay after 10 000 voltage cycles with minor reduction (0.006 V). In ZABs, C@NCF-900 outperforms the mixture of Pt/C 20 wt% and IrO2 , cycled over 100 h under 58% depth of discharge condition. Furthermore, density functional theory (DFT) calculations and in situ X-ray absorption spectroscopy strongly support the active sites and site-selective reaction as a plausible ORR/OER mechanism of C@NCF-900.