Ho-Gi Kim’s research while affiliated with Korea Advanced Institute of Science and Technology and other places

Highly ordered TiO2 nanotube arrays were prepared by anodic oxidation of Ti foil in an application to dye-sensitized solar cells (DSCs). A fullerene derivative called PC61BM was used as a material for the surface modification of TiO2 nanotube arrays to improve the power conversion efficiency of DSCs Although open circuit voltages (Voc) were slightly decreased by PC61BM interlayer, short circuit current densities (Jsc) were increased and thus the power conversion efficiencies were improved. EIS (Electrochemical Impedance Spectroscopy) results showed superior properties for PC61BM-coated samples.

SnO2 hollow nanofibers (HNFs) with diameters of 300–500nm were fabricated via electrospinning of templating nanofibers and RF-sputtering of SnO2 thin overlayers (15–20nm thick and comparable grain size) followed by heat-treatment at 450°C. Gas sensors using these HNFs exhibited n-type gas sensing characteristics and an enhanced gas response to 2ppm NO2 (Rgas/Rair=81.4) as compared to that (Rgas/Rair=19.9) of planar SnO2 thin films. The enhanced response to NO2 is attributed to the greater accessible active area and the greater space charge modulation depth associated with the hollow thin walled structures.

This work presents a simple and versatile route to produce macroporous p-type metal oxide thin films. Two-dimensional arrays of p-type NiO films with a hollow hemisphere structure were fabricated by colloidal templating and RF-sputtering followed by a subsequent heat treatment. The diameter and shell thickness of the NiO hemisphere were 800nm and 20nm, respectively. X-ray diffraction and high-resolution transmission electron microscopy analysis indicate that the pure NiO phase with grain size of 10nm was obtained at calcination temperatures that exceeded 450°C. Close-packed arrays of hollow NiO hemispheres were found to exhibit p-type gas sensing properties against (CO, H2, C3H8, CH4, NO2, and C2H5OH), leading to significantly enhanced responses to C2H5OH (Rgas/Rair=5.0 at 200ppm).

Surface modification of Li[Ni0.35Co0.3Mn0.35]O2 as a cathode material of lithium–ion batteries was carried out by hydrothermal treatment using lithium lanthanum titanate ([Li,La]TiO3). The modified surfaces were analyzed by morphology observation using transmission electron microscopy and by element investigation using X-ray photoelectron spectroscopy. It was thereupon found that the [Li,La]TiO3-coated layer formed by the surface modification played a definitive role in suppressing the solid electrolyte interface during repeated charge and discharge cycles. In addition, the thermal stability was enhanced by coated layer, resulting in an increase of the onset temperature to occur an exothermic reaction during thermal runaway. KeywordsSurface coating–Lithium lanthanum titanate–Cathode–Lithium–ion batteries

A sample of LiMn2O4 spinel oxide was surface-modified with lithium lanthanum titanate ([Li,La]TiO3), which was developed as a lithium ionic conductor, by means of hydrothermal processing and subsequent heat treatment at 400°C. The surface coating layers were analyzed by morphology observation using a transmission electron microscopy. Energy-dispersive spectrometry and X-ray photoelectron spectroscopy were used for element investigation. The surface modification effects on rate capability during cycling and capacity retention for the LiMn2O4 spinel oxide were confirmed. Then Mn dissolution during storage at elevated temperatures of the pristine, coated sample was characterized. The Mn dissolution characterization was based on the idea that Mn dissolution is one of the most significant reasons for capacity loss for LiMn2O4 spinel oxide, and this phenomenon is especially severe at elevated temperatures. Our experimental results indicate that the surface-modified sample shows much a better initial capacity and rate capability compared with the pristine sample. The [Li,La]TiO3 coating effectively enhances the structural stability of LiMn2O4 at elevated temperatures, most likely because the [Li,La]TiO3-modifying layers play a definitive role in suppressing Mn dissolution in the electrolyte during storage.

Highly ordered TiO2 nanotube arrays fabricated by anodization are very attractive to dye-sensitized solar cells (DSCs) due to their superior charge percolation and slower charge recombination. However, the efficiency of TiO2-nanotube-based DSCs is 6.89%, which is still lower than that of TiO2-nanoparticle-based DSCs. We have suggested the transplanting the highly ordered TiO2 nanotube arrays to FTO glass to improve the performance of TiO2-nanotube-based DSCs. DSCs based on transplanted TiO2 nanotube arrays and TiO2 nanoparticles were fabricated by same process and materials to exclude the unexpected factors. In TiO2 thickness of ca. 15 μm, the efficiency of 2.91% in front-side illuminated DSCs based on TiO2 nanotube arrays was higher than those in back-side illuminated DSCs based on TiO2 nanotube arrays and in front-side illuminated DSCs based on TiO2 nanoparticle. Front-side illuminated DSCs based on TiO2 nanotube arrays having various thicknesses were successfully fabricated. The efficiency in DSCs having 20.0 μm thick TiO2 nanotube arrays was improved to 5.36% by TiCl4 treatment.

Significantly reduced leakage current characteristics of the Bi1.5Zn1.0Nb1.5O7 (BZN) gate dielectric for producing high-performance ZnO-thin film transistors (TFTs) were achieved by an addition of MgO (30 atom %). The overall TFT parameters using MgO-BZN gate insulator against those that used pure BZN dielectric were enhanced remarkably. The diphasic MgO-BZN composite oxide structure was confirmed by an analysis of the spectroscopically detected bandedge electronic structures. The bandgap energy of MgO-BZN was identical to that of BZN at similar to 3.3 eV, but the Fermi energy level was shifted to 1.2 eV from 0.6 eV for BZN against the valence bandedge.

Thin platinum layer was used for the modification of the interface state between (Ba,Sr)TiO3 (BST) thin films and RuO2 bottom electrodes. Pt/RuO2 hybrid bottom electrodes were fabricated with various platinum deposition temperatures by a conventional dc magnetron sputtering method. Although the surface morphology and XRD patterns of BST thin films were not changed, the electrical properties of BST films deposited on Pt/RuO2 hybrid electrodes were improved compared to the films on RuO2 electrodes. Dielectric constant (εr) and leakage current density of BST thin films on Pt/RuO2 hybrid electrodes prepared at the platinum deposition temperature of 400°C were 498 and 8.6 × 10−8 A/cm2 at 1.5V, respectively.

The facile synthesis, by electrospinning of highly conductive RuO2 fiber mats as the conducting core of electrochemical capacitors, is reported. Electrospun RuO2 fiber mats, calcined at 400 °C, exhibited highly porous morphologies composed of crystalline RuO2 nano-particles with an average particle size of 10 nm. The highly conducting (σ = 288 S cm−1) fiber mats were used as the basis of a hybrid electrochemical capacitor in which the fiber mats served as the conducting core for electrochemically deposited hydrate RuO2·nH2O overlayers. These electrochemical capacitors exhibited superior characteristics: a high specific capacitance of 886.9 F g−1 (based on the mass of the hydrous RuO2 coating layer) at scan rate of 10 mV s−1 and high rate capability with a capacity loss of only 30% from 10 to 2000 mV s−1. This is attributed to the superior electron transport pathways provided by the fiber mat core to the upper ionically conducting hydrous RuO2 coating layer.

This paper provides the properties of TiO_2 nanotube arrays which are fabricated by anodic oxidation of Ti metal. Highly ordered TiO_2 nanotube arrays could be obtained by anodic oxidation of Ti foil in 0.3\;wt{\cdot}%NH_4F contained ethylene glycol solution at 30^{\circ}C. The length, pore size, wall thickness, tube diameter etc. of TiO_2 nanotube arrays were analyzed by field emission scanning electron microscopy. Their crystal properties were studied by field emission transmission electron microscopy and X-ray photoelectron spectroscopy.