Dongjin Byun

Photocatalytic TiO2 deposition by chemical vapor deposition

Dip-coating, spray-coating or spin-coating methods for crystalline thin film deposition require post-annealing process at high temperature. Since chemical vapor deposition (CVD) process is capable of depositing high-quality thin films without post-annealing process for crystallization, CVD method was employed for the deposition of TiO2 films on window glass substrates. Post-annealing at high temperature required for other deposition methods causes sodium ion diffusion into TiO2 film from window glass, resulting in the degradation of photocatalytic efficiency. Anatase-structured TiO2 thin films were deposited on window glass by CVD, and the photocatalytic dissociation rates of benzene with CVD-grown TiO2 under UV exposure were characterized. As the TiO2 film deposition temperature was increased, the (112)-preferred orientations were observed in the film. The (112)-preferred orientation of TiO2 thin film resulted in a columnar structure with a larger surface area for benzene dissociation. Obviously, benzene dissociation rate was maximum when the degree of the (112) preferential orientation was maximum. It is clear that the thin film TiO2 should be controlled to exhibit the preferred orientation for the optimum photocatalytic reaction rate. CVD method is an alternative for the deposition of photocatalytic TiO2.

Growth and properties of ZnO nanoblade and nanoflower prepared by ultrasonic pyrolysis

ZnO nanoblades and nanoflowers are synthesized using zinc acetate dihydrate Zn(CH3COO)2∙2H2O dissolved in distilled water by ultrasonic pyrolysis at 380–500°C. Thermogravimetry-differential scanning calorimetry, x-ray diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, Raman spectroscopy, and low-temperature photoluminescence (PL) were used to characterize the thermal properties, crystalline and optical features of the ZnO nanostructures. The results showed that at 400°C the formation of nanoblades resulted from the simultaneous precipitation and nucleation in zinc acetate precursor. At an elevated temperature of 450°C, decomposition was almost advanced and thus the size of nanopetal became smaller and aggregates became larger by as much as 60nm. The formation of aggregates is explained in terms of random nucleation model. Through PL measurement, nanoblade showed a strong near band-edge emission with negligible deep-level emission and free exciton ba...

Cu-doped ZnO-based p–n hetero-junction light emitting diode

Copper-doped p-ZnO thin films (Cu:ZnO) were grown on α-Al2O3(0 0 0 1) and 6H:SiC(0 0 0 1) single crystal substrates by plasma-assisted molecular beam epitaxy. A p–n hetero-junction with p-Cu:ZnO/n-6H:SiC was successfully fabricated and demonstrated as a greenish-blue light emitting diode (LED). The rectifying I–V curve along with the matching photoluminescence and electroluminescence emissions characterizes the fabricated p–n hetero-junction LED. The Cu cell temperature (TCu) and the post-deposition annealing environment greatly influence the Cu oxidation state, and hence the electrical conversion from n-type to p-type and carrier concentration in the films. The higher TCu and post-annealing in O-plasma were observed to be the favorable conditions for Cu2+ and hence the p-type nature of the films.

Photoluminescence of Ga-doped ZnO film grown on c‐Al2O3 (0001) by plasma-assisted molecular beam epitaxy

High quality gallium doped ZnO (Ga:ZnO) thin films were grown on c‐Al2O3(1000) by plasma-assisted molecular beam epitaxy, and Ga concentration NGa was controlled in the range of 1×1018–2.5×1020∕cm3 by adjusting∕changing the Ga cell temperature. From the low-temperature photoluminescence at 10K, the donor bound exciton I8 related to Ga impurity was clearly observed and confirmed by comparing the calculated activation energy of 16.8meV of the emission peak intensity with the known localization energy, 16.1meV. Observed asymmetric broadening with a long tail on the lower energy side in the photoluminescence (PL) emission line shape could be fitted by the Stark effect and the compensation ratio was approximately 14–17% at NGa⩾1×1020∕cm3. The measured broadening of photoluminescence PL emission is in good agreement with the total thermal broadening and potential fluctuations caused by random distribution of impurity at NGa lower than the Mott critical density.

One-Step Catalytic Synthesis of CuO/Cu2O in a Graphitized Porous C Matrix Derived from the Cu-Based Metal-Organic Framework for Li- and Na-Ion Batteries.

The hybrid composite electrode comprising CuO and Cu2O micronanoparticles in a highly graphitized porous C matrix (CuO/Cu2O-GPC) has a rational design and is a favorable approach to increasing the rate capability and reversible capacity of metal oxide negative materials for Li- and Na-ion batteries. CuO/Cu2O-GPC is synthesized through a Cu-based metal-organic framework via a one-step thermal transformation process. The electrochemical performances of the CuO/Cu2O-GPC negative electrode in Li- and Na-ion batteries are systematically studied and exhibit excellent capacities of 887.3 mAh g(-1) at 60 mA g(-1) after 200 cycles in a Li-ion battery and 302.9 mAh g(-1) at 50 mA g(-1) after 200 cycles in a Na-ion battery. The high electrochemical stability was obtained via the rational strategy, mainly owing to the synergy effect of the CuO and Cu2O micronanoparticles and highly graphitized porous C formed by catalytic graphitization of Cu nanoparticles. Owing to the simple one-step thermal transformation process and resulting high electrochemical performance, CuO/Cu2O-GPC is one of the prospective negative active materials for rechargeable Li- and Na-ion batteries.

Mechanochemical Synthesis of Li2MnO3 Shell/LiMO2 (M = Ni, Co, Mn) Core-Structured Nanocomposites for Lithium-Ion Batteries

Core/shell-like nanostructured xLi2MnO3·(1−x)LiMO2 (M = Ni, Co, Mn) composite cathode materials are successfully synthesized through a simple solid-state reaction using a mechanochemical ball-milling process. The LiMO2 core is designed to have a high-content of Ni, which increases the specific capacity. The detrimental surface effects arising from the high Ni-content are countered by the Li2MnO3 shell, which stabilizes the nanoparticles. The electrochemical performances and thermal stabilities of the synthesized nanocomposites are compared with those of bare LiMO2. In particular, the results of time-resolved X-ray diffraction (TR-XRD) analyses of xLi2MnO3·(1−x)LiMO2 nanocomposites as well as their differential scanning calorimetry (DSC) profiles demonstrate that the Li2MnO3 shell is effective in stabilizing the LiMO2 core at high temperatures, making the nanocomposites highly suitable from a safety viewpoint.

Structural Analysis on Photocatalytic Efficiency of TiO2 by Chemical Vapor Deposition

Structural analysis of anatase TiO2 films on window glass by chemical vapor deposition (CVD) was performed. Relationships between preferred orientation and photocatalytic activity of polycrystalline TiO2 films were investigated. Photocatalytic activity was affected by crystalline orientation that depended on the deposition temperature. -oriented TiO2 films were obtained at 360°C, while -oriented films were obtained at other temperatures. Photocatalytic activity of the -oriented film was greater than that of the -oriented film. The film with -preferred orientation exhibited a columnar structure resulting in a larger surface area for photocatalytic reaction than the films with -preferred orientation. Also, the film with -preferred orientation exhibited a facet consisting of s-faces {100} which showed the larger surface energy theoretically. It is clear that fabrication of the thin film TiO2 by CVD should be controlled to exhibit the -preferred orientation for the optimum photocatalytic efficiency.

A green recycling process designed for LiFePO4 cathode materials for Li-ion batteries

A green process route to recycle LiFePO4/C electrode materials is proposed in this work. First, a robust strategy to synthesize LiFePO4/C cathode materials from a precursor of a crystalline FePO4·2H2O phase (metastrengite I) is presented. In order to prepare crystalline FePO4·2H2O, a solution precipitation route is adapted, where the reaction conditions such as temperature and pH are precisely controlled. Among various heat treatment temperatures to calcine our prepared FePO4·2H2O with lithium sources, we find that LiFePO4/C cathode materials synthesized at 700 °C deliver a maximum discharge capacity of 168.51 mA h g−1 at 0.1 C (1 C rate = 170 mA h g−1) with a capacity retention of 99.36% after the 25th cycle at 1 C. Furthermore, commercially available LiFePO4 powders and recovered LiFePO4 electrode materials from spent batteries are both tested with our developed recycling process, where we decompose LiFePO4 powders/electrodes to prepare crystalline FePO4·2H2O, and then re-synthesize LiFePO4/C cathode materials. In both cases, our recycled LiFePO4/C exhibits a very comparable discharge capacity of ∼140 mA h g−1 at 1 C with a capacity retention of ∼99%.

Anti-fluorite Li6CoO4 as an alternative lithium source for lithium ion capacitors: an experimental and first principles study

As a promising hybrid energy storage system, lithium ion capacitors (LICs) have been intensively investigated regarding their practical use in various applications, ranging from portable electronics to grid support. The asymmetric LIC offers high-energy and high-power densities compared with conventional energy storage systems such as electrochemical double-layer capacitors (EDLCs) and lithium ion batteries (LIBs). To enable suitable operation of the LIC, the negative electrode should be pre-lithiated prior to cell operation, which is regarded as a key technology for developing self-sustainable LICs. In this work, we have demonstrated the potential use of Li6CoO4 as an alternative lithium source to metallic lithium. A large amount of Li+ can be electrochemically extracted from the structure incorporated into the positive electrode via a highly irreversible process. Most of the extracted Li+ is available for pre-lithiation of the negative electrode during the first charge. This intriguing electrochemical behaviour of Li6CoO4 is suitable for providing sufficient Li+ to the negative electrode. To obtain a fundamental understanding of this system, the electrochemical behaviour and structural stability of Li6CoO4 is thoroughly investigated by means of electrochemical experiments and theoretical validation based on first principles calculations.

Research on carbon-coated Li4Ti5O12 material for lithium ion batteries

Carbon-coated Li4Ti5O12 anode materials for the lithium ion battery were synthesized by using sucrose to improve the electrochemical properties of Li4Ti5O12, and the carbon content was then tested. X-ray diffraction (XRD) showed that the coating of carbon does not influence the formation of Li4Ti5O12. Transmission electron microscopy (TEM) and Raman spectroscopy confirmed that carbon content exists on the surface of Li4Ti5O12. Electronic conductivity measurement indicated that the electronic conductivity of the carbon-coated Li4Ti5O12 material was 3.8?10?4 ?S?cm?1, which is higher than that for the primary Li4Ti5O12 material (4.3?10?7 ?S?cm?1). CV results show that carbon-coated Li4Ti5O12 shows a larger diffusion coefficient. Charge and discharge tests show that rate capability and cycle performance were improved because of the carbon coating.