Donghyeuk Choi

Synthesis of noble metal/graphene nanocomposites without surfactants by one-step reduction of metal salt and graphene oxide.

We carried out hydrazine-free, surfactant-free synthesis of noble metal/graphene nanocomposites. The reduction of the noble metals and GO was carried out simultaneously in hot water using ascorbic acid as a reductant. In the noble metal/graphene nanocomposites of Pd, Pt, Au, and Ag nanoparticles, the GO and metal salts were reduced completely by this synthetic method. In addition, the Pd/graphene nanocomposites showed good catalytic activity in the Suzuki coupling reaction and could be reused many times without loss of catalytic activity.

Diameter-Controlled and Surface-Modified Sb 2 Se 3 Nanowires and Their Photodetector Performance

Due to its direct and narrow band gap, high chemical stability, and high Seebeck coefficient (1800 μVK−1), antimony selenide (Sb2Se3) has many potential applications, such as in photovoltaic devices, thermoelectric devices, and solar cells. However, research on the Sb2Se3 materials has been limited by its low electrical conductivity in bulk state. To overcome this challenge, we suggest two kinds of nano-structured materials, namely, the diameter-controlled Sb2Se3 nanowires and Ag2Se-decorated Sb2Se3 nanowires. The photocurrent response of diameter-controlled Sb2Se3, which depends on electrical conductivity of the material, increases non-linearly with the diameter of the nanowire. The photosensitivity factor (K = Ilight/Idark) of the intrinsic Sb2Se3 nanowire with diameter of 80–100 nm is highly improved (K = 75). Additionally, the measurement was conducted using a single nanowire under low source-drain voltage. The dark- and photocurrent of the Ag2Se-decorated Sb2Se3 nanowire further increased, as compared to that of the intrinsic Sb2Se3 nanowire, to approximately 50 and 7 times, respectively.

One-pot synthesis of Au@Pd/graphene nanostructures: electrocatalytic ethanol oxidation for direct alcohol fuel cells (DAFCs)

Surfactant-free bimetallic Au@Pd core-shell NPs on graphene sheets were fabricated from Au, Pd ions and graphene oxide by one-step reduction using ascorbic acid as an eco-friendly reducing agent. Au@Pd NPs with about 7 nm size are evenly distributed on the surface of graphene. The core/shell structures were confirmed with X-ray diffraction (XRD) and High-angle annual dark field scanning transmission microscopy (HAADF-STEM) mapping analysis. Prepared Au@Pd/graphene nanocomposites show an excellent electrocatalytic activity toward ethanol oxidation: If value that was the best for our catalysts is 11.6 A mg−1, which is about 5 times as high as that of the previous catalyst. The data resulted from extremely optimized bimetallic structure of a small Au core and a thin Pd shell. Pd atoms are constantly encouraged by adjacent Au atoms.

Solution-processed inverted solar cells using an inorganic bulk heterojunction of iron pyrite nanocrystals and cadmium selenide quantum dots with a polymeric hole-transport medium

Inverted solar cells were developed using a donor–acceptor (D–A) bulk heterojunction (BHJ) of iron pyrite nanocrystals (FeS2 NCs) and cadmium selenide quantum dots (CdSe QDs). The all-inorganic BHJ nanocomposites showed broad-range absorption and effective dynamics of charge dissociation and transfer through bi-continuous D–A interactions under a type II band-offset system. The developed solar cell, which contained an optimized ratio of FeS2–CdSe in the blend and a polymeric hole-transport layer between the photoactive layer and the anode electrode, showed a significantly enhanced photovoltaic efficiency. The long-term ambient stability of the inverted devices was better than that of conventional devices fabricated in this study, but was relatively lower than the air stability of other NC solar cells. Our findings suggest the potential of FeS2 NCs as an environmentally benign, earth-abundant material for solution-processed photovoltaic applications. The merits of an FeS2–CdSe D–A BHJ system with an inverted device architecture and its long-term stability are addressed.

Optical and electronic properties of type-II CdSe/CdS core–shell quantum dots

CdSe/CdS core–shell quantum dots (QDs) were synthesized using a facile method in aqueous phase. X-ray diffraction pattern, high-resolution transmission electron microscopy images, and energy dispersive spectroscopy profiles showed that stoichiometric CdSe/CdS QDs were formed. Temperature-dependent photoluminescence spectra showed that the activation energy of CdSe/CdS core–shell QDs was 15 meV. The potential profiles and interband transition energies of the strained type-II CdSe/CdS core–shell QDs were calculated. The calculated interband transition energies slightly decreased from 2.061 to 2.007 eV when the shell thickness increased from 10 to 17 A. The theoretical interband transition energy of 2.007 eV was in reasonable agreement with the photoluminescence excitonic transition energy of 1.98 eV.