Joon T. Park

White light-emitting diodes of GaN-based Sr2SiO4:Eu and the luminescent properties

We have synthesized a Eu2+-activated Sr2SiO4 yellow phosphor and investigated an attempt to develop white light-emitting diodes (LEDs) by combining it with a GaN blue LED chip. Two distinct emission bands from the GaN-based LED and the Sr2SiO4:Eu phosphor are clearly observed at 400 nm and at around 550 nm, respectively. These two emission bands combine to give a spectrum that appears white to the naked eye. Our results showed that GaN (400-nm chip)-based Sr2SiO4:Eu exhibits a better luminous efficiency than that of the industrially available product InGaN (460-nm chip)-based YAG:Ce.

Room-temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film

Porous tungsten oxide films were deposited onto a sensor substrate with a Si bulk-micromachined hotplate, by drop-coating isopropyl alcohol solution of highly crystalline tungsten oxide (WO2.72) nanorods with average 75nm length and 4nm diameter. The temperature-dependent gas sensing characteristics of the films have been investigated over the mild temperature range from 20to250°C. While the sensing responses for ammonia vapor showed increase in electrical conductivity at temperatures above 150°C as expected for n-type metal oxide sensors, they exhibited the opposite behavior of unusual conductivity decrease below 100°C. Superb sensing ability of the sensors at room temperature in conjunction with their anomalous conductivity behavior might be attributed to unique nanostructural features of very thin, nonstoichiometric WO2.72.

Single‐Crystalline Hollow Face‐Centered‐Cubic Cobalt Nanoparticles from Solid Face‐Centered‐Cubic Cobalt Oxide Nanoparticles

Hollow nanoparticles are of great interest because of their applications in catalysis, nanoelectronics, photonics, drug delivery system, nanoreactors, lubrication, and chemical storage. Various known hollow spheres include those composed of carbon, polymers, metals, and inorganic materials. Diverse synthetic methods have been developed to prepare these hollow nanoparticles, such as removal of the templating core, galvanic replacement, and through the Kirkendall effect. Cobalt exhibits hexagonal closed-packed (hcp Co) and face-centered cubic (fcc Co) structures in the bulk, and a metastable cubic structure labeled e-Co in the nanometer range. Cobalt nanostructures have been widely studied because of their potential applications, mainly in ultrahighdensity magnetic storage. However, reports on hollow cobalt nanoparticles are very limited thus far, although they are interesting materials in terms of their unusual magnetic domains and quantum properties. We herein report that fcc Co hollow nanoparallelepipeds have been prepared by thermolysis of fcc CoO solid nanoparallelepipeds in oleylamine (C18H35NH2). The fcc CoO solid nanoparallelepipeds, surprisingly, are reduced by the oleylamine surfactant to form fcc Co hollow nanoparallelepipeds. This new phenomenon could signify an important methodology to produce constituent metal (M) hollow nanoparticles from metal oxide (MO) solid nanoparticles. In our previous work, we reported phaseand sizecontrolled syntheses of hexagonal and cubic CoO nanocrystals (hcp CoO and fcc CoO). The fcc CoO solid nanoparallelepipeds in oleylamine undergoes reduction at high temperatures (270–290 8C) to transform into hollow nanoparallelepipeds composed of cubic metallic Co (fcc Co). Figure 1 shows the evolution of the morphology of fcc Co

Designed Synthesis of Well-Defined Pd@Pt Core-Shell Nanoparticles with Controlled Shell Thickness as Efficient Oxygen Reduction Electrocatalysts

Improving the electrocatalytic activity and durability of Pt-based catalysts with low Pt content toward the oxygen reduction reaction (ORR) is one of the main challenges in advancing the performance of polymer electrolyte membrane fuel cells (PEMFCs). Herein, a designed synthesis of well-defined Pd@Pt core-shell nanoparticles (NPs) with a controlled Pt shell thickness of 0.4-1.2 nm by a facile wet chemical method and their electrocatalytic performances for ORR as a function of shell thickness are reported. Pd@Pt NPs with predetermined structural parameters were prepared by in situ heteroepitaxial growth of Pt on as-synthesized 6 nm Pd NPs without any sacrificial layers and intermediate workup processes, and thus the synthetic procedure for the production of Pd@Pt NPs with well-defined sizes and shell thicknesses is greatly simplified. The Pt shell thickness could be precisely controlled by adjusting the molar ratio of Pt to Pd. The ORR performance of the Pd@Pt NPs strongly depended on the thickness of their Pt shells. The Pd@Pt NPs with 0.94 nm Pt shells exhibited enhanced specific activity and higher durability compared to other Pd@Pt NPs and commercial Pt/C catalysts. Testing Pd@Pt NPs with 0.94 nm Pt shells in a membrane electrode assembly revealed a single-cell performance comparable with that of the Pt/C catalyst despite their lower Pt content, that is the present NP catalysts can facilitate low-cost and high-efficient applications of PEMFCs.

Phosphidation of Li4Ti5O12 nanoparticles and their electrochemical and biocompatible superiority for lithium rechargeable batteries

Phosphidated-Li(4)Ti(5)O(12) shows high capacity with a significantly enhanced kinetics opening new possibilities for ultra-fast charge/discharge of lithium rechargeable batteries. The in vitro cytotoxicity test proves its fabulous cell viability, indicating that the toxicity problem of nanoparticles can be also solved by phosphidation.

Remarkably efficient photocurrent generation based on a [60]fullerene-triosmium cluster/Zn-porphyrin/boron-dipyrrin triad SAM.

A new artificial photosynthetic triad array, a [60]fullerene-triosmium cluster/zinc-porphyrin/boron-dipyrrin complex (1, Os(3)C(60)/ZnP/Bodipy), has been prepared by decarbonylation of Os(3)(CO)(8)(CN(CH(2))(3)Si(OEt)(3))(mu(3)-eta(2):eta(2):eta(2)-C(60)) (6) with Me(3)NO/MeCN and subsequent reaction with the isocyanide ligand CNZnP/Bodipy (5) containing zinc porphyrin (ZnP) and boron dipyrrin (Bodipy) moieties. Triad 1 has been characterized by various spectroscopic methods (MS, NMR, IR, UV/Vis, photoluminescence, and transient absorption spectroscopy). The electrochemical properties of 1 in chlorobenzene (CB) have been examined by cyclic voltammetry; the general feature of the cyclic voltammogram of 1 is nine reversible one-electron redox couples, that is, the sum of those of 5 and 6. DFT has been applied to study the molecular and electronic structures of 1. On the basis of fluorescence-lifetime measurements and transient absorption spectroscopic data, 1 undergoes an efficient energy transfer from Bodipy to ZnP and a fast electron transfer from ZnP to C(60); the detailed kinetics involved in both events have been elucidated. The SAM of triad 1 (1/ITO; ITO=indium-tin oxide) has been prepared by immersion of an ITO electrode in a CB solution of 1 and diazabicyclo-octane (2:1 equiv), and characterized by UV/Vis absorption spectroscopy, water contact angle, X-ray photoelectron spectroscopy, and cyclic voltammetry. The photoelectrochemical properties of 1/ITO have been investigated by a standard three-electrode system in the presence of an ascorbic acid sacrificial electron donor. The quantum yield of the photoelectrochemical cell has been estimated to be 29 % based on the number of photons absorbed by the chromophores. Our triad 1 is unique when compared to previously reported photoinduced electron-transfer arrays, in that C(60) is linked by pi bonding with little perturbation of the C(60) electron delocalization.

Composition-Controlled PtCo Alloy Nanocubes with Tuned Electrocatalytic Activity for Oxygen Reduction

Modification of the electronic structure and lattice contraction of Pt alloy nanocatalysts through control over their morphology and composition has been a crucial issue for improving their electrocatalytic oxygen reduction reaction (ORR) activity. In the present work, we synthesized PtCo alloy nanocubes with controlled compositions (Pt(x)Co NCs, x = 2, 3, 5, 7, and 9) by regulating the ratio of surfactants and the amount of Co precursor to elucidate the effect of the composition of nanocatalysts on their ORR activity. Pt(x)Co NCs had a Pt-skin structure after electrochemical treatment. The electrocatalysis experiments revealed a strong correlation between ORR activity and Co composition. Pt₃Co NCs exhibited the best ORR performance among the various Pt(x)Co NCs. From density functional theory calculations, a typical volcano-type relationship was established between ORR activity and oxygen binding energy (E(OB)) on NC surfaces, which showed that Pt₃Co NCs had the optimal E(OB) to achieve the maximum ORR activity. X-ray photoelectron spectroscopy and X-ray diffraction measurements demonstrated that the electronic structure and lattice contraction of the Pt(x)Co NCs could be tuned by controlling the composition of NCs, which are highly correlated with the trends of E(OB) change.

Shape‐Controlled Synthesis of Pt3Co Nanocrystals with High Electrocatalytic Activity toward Oxygen Reduction

Polymer electrolyte membrane fuel cells (PEMFCs), a promising alternative energy system, have been intensively explored as a power source for electric vehicles or appliances. Since their performance is limited by the slow reduction kinetics of the oxygen reduction reaction (ORR) at the cathode, improving the ORR activity is key to developing efficient PEMFCs. For this reason, the synthesis of Pt alloy nanoparticles with controlled compositions and morphologies has been extensively exploited for the last decade. Previous studies revealed that the ORR activities of Pt–M (where M is Co, Ni, Fe, Mn, Cr, Cu, and Pd) alloys are superior to those of pure Pt catalysts. In addition, their ORR kinetics can be controlled by tuning their morphologies because the enhancement of the ORR activity depends highly on the surface atom arrangements of the catalyst particles. For example, Pt3Mn nanocubes dominantly bound by {100} planes exhibited ORR activity that was three-times higher than that of the commercial Pt/C catalyst in H2SO4 media. [11] However, investigations of catalysts based on the Pt3Ni octahedra and truncated octahedra in HClO4 have shown that the catalytic activity is higher on the Pt3Ni {111} planes than that on the {100} planes because the trend of binding the sulfate anions on the {111} planes is not established in this media. In addition, Stamenkovic et al. demonstrated that the catalytic activity is determined by the low coverage of OHad, and that the ORR on Pt3Ni {111} planes is the highest among the low-index planes. The Pt–Co alloy nanoparticles have drawn particular interest as ORR electrocatalysts due to their enhanced catalytic activities, which can be attributed to the modified electronic structure of Pt, the tuned strength of the oxygen– metal bonding, and the low toxicity of dissolved Co ions against the polymer electrolyte. Although it might be possible to modulate the catalytic properties of Pt–Co nanoparticles by controlling their morphologies, the shapecontrolled synthesis of Pt–Co nanoparticles and their morphology-dependent ORR activities have rarely been reported. Recently, we have demonstrated that the Pt9Co alloy nanocubes bound by {100} planes show a fourfold improvement of catalytic activity toward ORR over the commercial Pt/C catalyst in an H2SO4 solution. [18] Herein, we describe the synthesis of Pt3Co nanocubes (NCs) and nanotruncated octahedra (NTO), the surfaces of which are dominated by {100} and {111}/ ACHTUNGTRENNUNG{100} planes, respectively. The selective synthesis of Pt3Co NCs and NTO was achieved by controlling the reaction time. The prepared Pt3Co NTO exhibited significantly enhanced ORR activity compared to the Pt3Co NCs and commercial Pt/C catalyst. In a typical synthesis of Pt3Co nanocrystals, Pt ACHTUNGTRENNUNG(acac)2 (acac=acetylacetonate) and Co2(CO)8, in a molar ratio of 2:1 were employed as precursors, and oleylamine (OAm) and oleic acid (OA) were used as surfactants. The Pt3Co NTO and Pt3Co NCs were selectively obtained as black– brown nanocrystals by heat treatment from 120 to 200 8C for 30 min, with a temperature increase rate of 2–3 8C min , and incubation at 200 8C for 30 min for Pt3Co NTO and 90 min for Pt3Co NCs under identical experimental conditions (see the Experimental Section). The resultant Pt3Co nanocrystals were readily dispersed in organic solvents, such as toluene and hexane. The transmission electron microscopy (TEM) images of the as-synthesized Pt3Co NCs and NTO, with an average size of 8 nm, indicate the selective formation of NCs and NTO with good uniformity (Figure 1 a and 1 c, respectively). Figure 1 b and 1 d shows high-resolution TEM (HRTEM) images of a single Pt3Co NC and NTO, respectively.

Tetraglyme-mediated synthesis of Pd nanoparticles for dehydrogenation of ammonia borane

Palladium nanoparticles (PdNPs) were conveniently prepared in tetraglyme (TG) solution using a variety of palladium precursors. At 140 °C, TG promoted Pd(3)(OAc)(6) to produce irregular shaped PdNPs with an average size of 4 nm. When these PdNPs were re-dispersed in TG and used for the dehydrogenation of ammonia borane (AB) at 85 °C, remarkably enhanced catalytic performance was achieved to release 2.3 equiv. of H(2) in 1 h.

Hollow Sn–SnO2 Nanocrystal/Graphite Composites and Their Lithium Storage Properties

Hollow spheres have been constructed by applying the Kirkendall effect to Sn nanocrystals. This not only accommodates the detrimental volume expansion but also reduces the Li(+) transport distance enabling homogeneous Li-Sn alloying. Hollow Sn-SnO2 nanocrystals show a significantly enhanced cyclic performance compared to Sn nanocrystal alone due to its typical structure with hollow core. Sn-SnO2/graphite nanocomposites obtained by the chemical reduction and oxidation of Sn nanocrystals onto graphite displayed very stable cyclic performance thanks to the role of graphite as an aggregation preventer as well as an electronic conductor.