Showing papers by "Seungho Cho published in 2013"

Highly Efficient and Stable Cadmium Chalcogenide Quantum Dot/ZnO Nanowires for Photoelectrochemical Hydrogen Generation

Abstract: Although cadmium chalcogenide quantum dot-sensitized photoanode can utilize the whole visible region of the solar spectrum, its poor photochemical stability owing to hole-induced anodic corrosion remains a major problem for the application in photoelectrochemical hydrogen generation systems. Here, modification with IrOx·nH2O, a well-known water-oxidation catalyst substantially improves the photochemical stability of the quantum dot-sensitized photoanode. Moreover, it induces an increased photocurrent and a cathodic shift of the onset potential. This is the first example that an oxygen-evolution catalyst is employed on a quantum dot-sensitized electrode system, and it shows 13.9 mA cm–2 (at 0.6 V) and −0.277 V vs the reversible hydrogen electrode (RHE), which are the highest photocurrent density and the lowest onset potential attained with a ZnO-based electrode, respectively. An average hydrogen evolution rate of 240 μmol h–1 cm–2 at 0.6 V vs RHE has been achieved on a IrOx·nH2O modified electrode, with al...

Anion-Doped Mixed Metal Oxide Nanostructures Derived from Layered Double Hydroxide as Visible Light Photocatalysts

Abstract: Mixed metal oxide (MMO) nanostructures co-doped uniformly by carbon and nitrogen are synthesized for the first time by annealing a terephthalate-intercalated layered double hydroxide (LDH) under ammonia gas flow. The interlayer gallery of LDH allows effective access of NH3 and the carbon source to its crystal lattice for a uniform nitrogen and carbon doping. Such co-doped MMO exhibit significantly red-shifted absorption spectra to visible light region relative to pure MMO. Photoelectrochemical water oxidation and incident-photon-to-current-conversion efficiency of LDH-derived photocatalysts demonstrate that all the visible light absorption caused by the anion doping contributes to the photocatalytic activity over the entire absorbed wavelength range of <610 nm. Density functional theory calculations of electronic structures are performed to elucidate the possibility of bandgap narrowing upon nitrogen and carbon co-doping on MMO structures.

Highly Fluorescent and Stable Quantum Dot-Polymer-Layered Double Hydroxide Composites

Abstract: We report a designed strategy for a synthesis of highly luminescent and photostable composites by incorporating quantum dots (QDs) into layered double hydroxide (LDH) matrices without deterioration of a photoluminescence (PL) efficiency of the fluorophores during the entire processes of composite formations. The QDs synthesized in an organic solvent are encapsulated by polymers, poly(maleic acid-alt-octadecene) to transfer them into water without altering the initial surface ligands. The polymer-encapsulated QDs with negative zeta potentials (−29.5 ± 2.2 mV) were electrostatically assembled with positively charged (24.9 ± 0.6 mV) LDH nanosheets to form QD-polymer-LDH composites (PL quantum yield: 74.1%). QD-polymer-LDH composite films are fabricated by a drop-casting of the solution on substrates. The PL properties of the films preserve those of the organic QD solutions. We also demonstrate that the formation of the QD-polymer-LDH composites affords enhanced photostabilities through multiple protections o...

Strategy for synthesizing quantum dot-layered double hydroxide nanocomposites and their enhanced photoluminescence and photostability.

Abstract: Layered double hydroxide-quantum dot (LDH-QD) composites are synthesized via a room temperature LDH formation reaction in the presence of QDs. InP/ZnS (core/shell) QD, a heavy metal free QD, is used as a model constituent. Interactions between QDs (with negative zeta potentials), decorated with dihydrolipoic acids, and inherently positively charged metal hydroxide layers of LDH during the LDH formations are induced to form the LDH-QD composites. The formation of the LDH-QD composites affords significantly enhanced photoluminescence quantum yields and thermal- and photostabilities compared to their QD counterparts. In addition, the fluorescence from the solid LDH-QD composite preserved the initial optical properties of the QD colloid solution without noticeable deteriorations such as red-shift or deep trap emission. Based on their advantageous optical properties, we also demonstrate the pseudo white light emitting diode, down-converted by the LDH-QD composites.

The reason for an upper limit to the height of spinnable carbon nanotube forests

Abstract: The reason for the upper limit on the height of spinnable carbon nanotube (CNT) forests was studied. To analyze the differences between CNT forests with different heights, we synthesized CNT forests using different growth times (3, 6, 9, 12, 15, and 60 min). The height of the CNT forests increased from 260 μm at 3 min to 1.7 mm at 60 min, and the spinnability decreased sharply after 9 min of growth, where a wavy morphology first appeared. Raman analysis of the CNT forest grown for 9 min showed that the intensity ratio of G-band to D-band at the upper region was 1.50 and that near the bottom was 1.14. We also found that the reaction termination process affected the spinnability of the CNT forests. Depending on the termination process, both spinnable and non-spinnable CNT forests could be selectively synthesized, because of the different morphologies in their lower regions. The results suggested that any wavy morphology produced due to a disturbance in growth conditions causes a loss of spinnability.

Facile fabrication of two-dimensional inorganic nanostructures and their conjugation to nanocrystals

Abstract: Nanocomposites of two-dimensional (2D) inorganic nanosheets and inorganic nanocrystals are fabricated. Freestanding atomically flat γ-AlOOH nanosheets (thickness <1 nm) are synthesized from a one-pot hydrothermal reaction. The freestanding and binder-free film composed of the γ-AlOOH nanosheets is fabricated by sedimentation. Because they have positive zeta potentials in the pH range below ca. 9.3, the γ-AlOOH nanosheets can function as positively charged 2D inorganic matrices in a broad pH range. By solution phase (pH 7.0) mixing of the γ-AlOOH nanosheets (zeta potential: 30.7 ± 0.8 mV) and inorganic nanocrystals with negative surface charge, including Au nanoparticles, Au nanorods, CdSe quantum dots, CdSe/CdS/ZnS quantum dots and CdSe nanorods, the nanocomposites are self-assembled via electrostatic interactions. Negatively charged inorganic nanostructures with a wide range of chemical compositions, shapes, sizes, surface ligands and adsorbates can be used as building blocks for γ-AlOOH nanocomposites. Adsorption densities of inorganic nanocrystals on the nanocomposites can be controlled by varying concentrations of nanocrystal solutions. Nanocomposite films containing alternating layers of γ-AlOOH and nanocrystals are obtained by a simple drop casting method.

Highly Efficient and Stable Cadmium Chalcogenide Quantum Dot/ZnO Nanowires for Photoelectrochemical Hydrogen Generation.

Abstract: The oxygen-evolution catalyst IrOx·nH2O is used to modify a CdSe/CdS quantum dot cosensitized ZnO nanowire photoanode in a photoelectrochemical hydrogen generation system.

Synthesis of quantum dot/polymer/layered-structure ceramic composite

Abstract: The present invention relates to a quantum dot and a preparation method therefor, and more specifically, to a novel quantum dot composite having high surface stability, and a preparation method therefor. The quantum dot composite according to the present invention constitutes a layered-structure ceramic composite in which the layered-structure ceramic comprises a polymer-quantum dot composite between the layers thereof.