Byeong Sub Kwak

Electrochemical performance of three shaped ZnO nanoparticles prepared in LiOH, NaOH and KOH alkaline solutions as anodic materials for Ni/Zn redox batteries

ZnO nanoparticles with three morphologies were synthesized by a hydrothermal route at 120 °C for 3 h in high alkaline aqueous solutions of LiOH, NaOH, and KOH. We analyzed them by X-ray diffraction (XRD), scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS), cyclic voltammetry (CV), Zeta potential measurement, and impedance. XRD and SEM showed that the obtained ZnO nanoparticles had high purity and perfect crystallinity, and the morphologies of the particles prepared in the LiOH, NaOH, and KOH solutions showed nanoplate, nanobead, and nanorod shapes, respectively. CV showed that the nanoplate ZnO-LiOH and nanorod ZnO-KOH have superior electrochemical activity to that of the other ZnO nanostructures. As electrode materials of Ni/Zn redox batteries, the nanoplate ZnO-LiOH showed a significantly improved cycle stability after the 30th cycle compared to that of ZnO-NaOH and conventional ZnO with a mean discharge capacity of 153 mA h g−1, a cell efficiency of 93%, and higher discharge voltages of 1.9. In addition, during the charging/discharging cycles, the growth of zinc dendrite clusters could be suppressed, which resulted in an improvement in the cycle stability of the Ni/nanoplate ZnO-LiOH redox cell.

Effect of oxidation states of Mn in Ca1−x Li x MnO3 on chemical-looping combustion reactions

We investigated the effect of the oxidation state of Mn in CaMnO3 perovskite particles to improve their oxygen transfer performance for chemical-looping combustion (CLC). Li was introduced in the Ca site of CaMnO3 to increase the Mn oxidation state. Ca1−xLixMnO3 particles were synthesized by the solid-state method, and the amount of Li added ranged from 0 to 0.015 mol. The structure of the synthesized Ca1−xLixMnO3 particles was examined using XRD, and all particles were confirmed to have a CaMnO3 perovskite structure. The shape and chemical properties of the prepared particles were characterized by using SEM and CH4-TPD. The binding energy and oxidation state of the different elements in the Ca1−xLixMnO3 particles were measured by XPS. When Li was added, the oxidation state of Mn in Ca1−xLixMnO3 was higher than that of Mn in CaMnO3. The oxygen transfer performance of the particles was determined by an isothermal H2-N2/air and CH4-CO2/air redox cycle at 850 °C, repeated ten times, using TGA. All particles showed an oxygen transfer capacity of about 8.0 to 9.0 wt%. Among them, Ca0.99Li0.01MnO3 particles had the best performance and the oxygen transfer capacity under H2-N2/air and CH4-CO2/air atmosphere was 8.47 and 8.75 wt%, respectively.

Facile synthesis of meso-porous ZnO nano-triangular prisms with enhanced photocatalytic activity

We report a simple route for the synthesis of ZnO nano-triangular prisms (ZnO-nt) with excellent photocatalytic activity. The activity of ZnO-nt was superior for the degradation of rhodamine-B (Rh-B) dye, when compared to ZnO-nanorods (ZnO-nr) and ZnO nano-particles (ZnO-np). The enhancement in photocatalytic activity of ZnO-nt is attributed to its special morphology, good surface area, high number of oxygen vacancies and suppression of electron–hole recombination. The results testify that ZnO-nt could act as a promising photocatalyst for the remediation of dye contaminated water under solar light irradiation.

Synthesis of Submicron Hexagonal Plate-Type SnS2 and Band Gap-Tuned Sn1−xTixS2 Materials and Their Hydrogen Production Abilities on Methanol/Water Photosplitting

SnS2 and Sn1−xTixS2 ( = 0, 0.1, 0.3, 0.5, and 0.7 mol) materials were designed using solvothermal method with the aim to enhance hydrogen production from water/methanol water photosplitting. Scanning electron microscopy revealed hexagonal plates with one side, 3.0 μm in length, in the SnS2 materials. Pure SnS2 showed absorption band edges of above 660 nm, and the absorption was shifted to low wavelengths with the insertion of Ti ions. The evolution of H2 from MeOH/H2O (1 : 1) photosplitting over SnS2 hexagonal plates in the photocatalytic liquid system was 0.016 mL h−1 g−1, and the evolutions were enhanced in Sn1−xTixS2. In particular, 0.049 mL h−1 g−1 of H2 gas was produced in Sn0.7Ti0.3S2 without electrolytes and it increased significantly to more than 90.6% (0.47 mL h−1 g−1 evolutions) at higher pH using 0.1 M of KOH. Based on the UV-visible absorption spectra, the high photocatalytic activity of Sn1−xTixS2 was attributed to the existence of an appropriate band-gap state that retarded recombination between the electrons and holes.

Effective Carbon Dioxide Photoreduction over Metals (Fe-, Co-, Ni-, and Cu-) Incorporated TiO2/Basalt Fiber Films

Mineralogical basalt fibers as a complementary adsorbent were introduced to improve the adsorption of CO2 over the surfaces of photocatalysts. TiO2 photocatalysts (M-TiO2) incorporated with 5.0 mol.% 3d-transition metals (Fe, Co, Ni, and Cu) were prepared using a solvothermal method and mixed with basalt fibers for applications to CO2 photoreduction. The resulting 5.0 mol.% M-TiO2 powders were characterized by X-ray diffraction, scanning electron microscopy, ultraviolet-visible spectroscopy, photoluminescence, Brunauer, Emmett, and Teller surface area, and CO2-temperature-programmed desorption. A paste composed of two materials was coated and fixed on a Pyrex plate by a thermal treatment. The 5.0 mol.% M-TiO2/basalt fiber films increased the adsorption of CO2 significantly, indicating superior photocatalytic behavior compared to pure TiO2 and basalt fiber films, and produced 158~360 μmol L−1 CH4 gases after an 8 h reaction. In particular, the best performance was observed over the 5.0 mol.% Co-TiO2/basalt fiber film. These results were attributed to the effective CO2 gas adsorption and inhibition of photogenerated electron-hole pair recombination.

Fe3O4–Ag2WO4: facile synthesis, characterization and visible light assisted photocatalytic activity

A Fe3O4–Ag2WO4 nano-composite was prepared through a sono-chemical route. Its physico-chemical properties were successfully characterized using XRD, BET, VSM, UV-vis-DRS, SEM, EDX, HR-TEM and PL techniques. The surface area of the Fe3O4–Ag2WO4 nano-composite was higher than those of pure Fe3O4 and Ag2WO4. The photocatalytic activity was tested for the degradation of a fast green (FG) dye under visible light irradiation. The Fe3O4–Ag2WO4 nano-composite showed good photocatalytic activity (81.11% of FG degradation within 120 min) when compared to Fe3O4, Ag2WO4, ZnO, and TiO2 (P25). The improved photocatalytic activity of the nano-composite was ascribed to efficient electron–hole separation. Fe3O4–Ag2WO4 was found to be a durable and stable photocatalyst during recycling experiments. A plausible mechanism for the electron–hole separation process on the heterojunction was also proposed.

Design of a Free-Ruthenium In2S3 Crystalline Photosensitized Solar Cell

A new type of sulfide-based, solid-state dye material that is sensitive to visible radiation was assessed as a potential replacement for commercial ruthenium complex dyes in a dye-sensitized solar cell (DSSC) assembly. The In2S3 crystals on the surface of the TiO2 bottom blocking layer were grown as a solid-state dye material. Scanning electron microscopy of In2S3 revealed a microsized, 3D-connected sheet-like shape, which was confirmed by X-ray diffraction to be a beta-structure. The efficiency of the dye-sensitized solar cells assembled with a layer grown with In2S3 increased with increasing In2S3 mole concentrations to 0.05 M (1.02%) but decreased at concentrations greater than 0.6~0.8%. This suggests that crystalline In2S3 acts as a dye sensitized to visible radiation, but the short-circuit current density is too low compared to the commercially available ruthenium dye. This suggests that In2S3 crystals did not grow densely but were bulk-grown with large pores, resulting in a smaller amount of In2S3 per unit area. Two IPCE curves were observed, which were assigned to TiO2 and In2S3, meaning that the TiO2 surfaces were covered completely with In2S3 crystals. The exposure of TiO2 eventually leads to a reaction with the electrolytes, resulting in lower quantum efficiency.

Ca(OH)2 nano-pods: investigation on the effect of solvent ratio on morphology and CO2 adsorption capacity

Ca(OH)2 nano-pods were synthesized through a precipitation method. Solvents such as ethanol/deionized water (DIW) and dimethylformamide (DMF)/deionized water (DIW) were used at different volume ratios to synthesize the samples. Various characterization techniques such as X-ray diffraction (XRD), filed emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and BET surface area analysis were employed to investigate the role of solvent on the crystallinity, morphology and surface area of Ca(OH)2. The solvent mixtures with a high volume of organic solvent (ethanol or DMF) acted as good capping agents to suppress the growth of Ca(OH)2 in the (1010) direction and induce anisotropic growth along the (0001) direction. A uniform pod like morphology was observed for the Ca(OH)2 sorbent synthesized using ethanol/DIW with a volume ratio of 78 ml/02 ml. Besides, the sorbents synthesized using ethanol/DIW showed good CO2 adsorption capacity and high surface area when compared to that of DMF/DIW.