Jeunghee Park

Growth model of bamboo-shaped carbon nanotubes by thermal chemical vapor deposition

Vertically aligned carbon nanotubes were grown on iron-deposited silicon oxide substrate by thermal chemical vapor deposition of acetylene. The carbon nanotubes have no encapsulated iron particles at the closed tip and a bamboo structure in which the curvature of compartment layers is directed to the tip. A base growth model is suggested for the bamboo-shaped carbon nanotubes grown under our experimental conditions.

Temperature effect on the growth of carbon nanotubes using thermal chemical vapor deposition

Abstract Vertically aligned carbon nanotubes (CNTs) are grown on iron-deposited silicon oxide substrates by thermal chemical vapor deposition (CVD) of acetylene gas at the temperature range 750–950°C. As the growth temperature increases from 750°C to 950°C, the growth rate increases by four times and the average diameter also increases from 30 nm to 130 nm while the density decreases by a factor of about two. The relative amount of crystalline graphitic sheets increases progressively with the growth temperature and a higher degree of crystalline perfection can be achieved at 950°C. This result demonstrates that the growth rate, diameter, density, and crystallinity of CNT can be controlled with the growth temperature.

Catalyst effect on carbon nanotubes synthesized by thermal chemical vapor deposition

Abstract The catalyst effect on the synthesis of carbon nanotubes (CNTs) using thermal chemical vapor deposition (CVD) was investigated. The respective growth rate of CNTs shows that the performance of catalysts is in the order of nickel ( Ni )> cobalt ( Co )> iron (Fe). The average diameter of CNTs follows the sequence of Fe, Co, and Ni catalysts. The structure of CNTs reveals almost same morphology regardless of catalyst but the crystallinity of CNTs is largely dependent on catalyst. The crystallinity of CNTs synthesized from Fe catalyst is higher than that from Ni or Co catalyst. The results indicate that the growth rate, the diameter, and the crystallinity can be manipulated by the selection of the catalyst.

Reversible Halide Exchange Reaction of Organometal Trihalide Perovskite Colloidal Nanocrystals for Full-Range Band Gap Tuning

In recent years, methylammonium lead halide (MAPbX3, where X = Cl, Br, and I) perovskites have attracted tremendous interest caused by their outstanding photovoltaic performance. Mixed halides have been frequently used as the active layer of solar cells, as a result of their superior physical properties as compared to those of traditionally used pure iodide. Herein, we report a remarkable finding of reversible halide-exchange reactions of MAPbX3, which facilitates the synthesis of a series of mixed halide perovskites. We synthesized MAPbBr3 plate-type nanocrystals (NCs) as a starting material by a novel solution reaction using octylamine as the capping ligand. The synthesis of MAPbBr(3-x)Clx and MAPbBr(3-x)Ix NCs was achieved by the halide exchange reaction of MAPbBr3 with MACl and MAI, respectively, in an isopropyl alcohol solution, demonstrating full-range band gap tuning over a wide range (1.6-3 eV). Moreover, photodetectors were fabricated using these composition-tuned NCs; a strong correlation was observed between the photocurrent and photoluminescence decay time. Among the two mixed halide perovskite series, those with I-rich composition (x = 2), where a sole tetragonal phase exists without the incorporation of a cubic phase, exhibited the highest photoconversion efficiency. To understand the composition-dependent photoconversion efficiency, first-principles density-functional theory calculations were carried out, which predicted many plausible configurations for cubic and tetragonal phase mixed halides.

CoSe2 and NiSe2 Nanocrystals as Superior Bifunctional Catalysts for Electrochemical and Photoelectrochemical Water Splitting

Catalysts for oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) are central to key renewable energy technologies, including fuel cells and water splitting. Despite tremendous effort, the development of low-cost electrode catalysts with high activity remains a great challenge. In this study, we report the synthesis of CoSe2 and NiSe2 nanocrystals (NCs) as excellent bifunctional catalysts for simultaneous generation of H2 and O2 in water-splitting reactions. NiSe2 NCs exhibit superior electrocatalytic efficiency in OER, with a Tafel slope (b) of 38 mV dec(-1) (in 1 M KOH), and HER, with b = 44 mV dec(-1) (in 0.5 M H2SO4). In comparison, CoSe2 NCs are less efficient for OER (b = 50 mV dec(-1)), but more efficient for HER (b = 40 mV dec(-1)). It was found that CoSe2 NCs contained more metallic metal ions than NiSe2, which could be responsible for their improved performance in HER. Robust evidence for surface oxidation suggests that the surface oxide layers are the actual active sites for OER, and that CoSe2 (or NiSe2) under the surface act as good conductive layers. The higher catalytic activity of NiSe2 is attributed to their oxide layers being more active than those of CoSe2. Furthermore, we fabricated a Si-based photoanode by depositing NiSe2 NCs onto an n-type Si nanowire array, which showed efficient photoelectrochemical water oxidation with a low onset potential (0.7 V versus reversible hydrogen electrode) and high durability. The remarkable catalytic activity, low cost, and scalability of NiSe2 make it a promising candidate for practical water-splitting solar cells.

Strained gallium nitride nanowires

Gallium nitride nanowires were synthesized on silicon substrates by chemical vapor deposition using the reaction of gallium and gallium nitride mixture with ammonia. Iron nanoparticles were used as catalysts. The diameter of nanowires is uniform as 25 nm and the lengths are 20–40 μm. The nanowires have single crystalline wurtzite structure with a few stacking faults. A careful examination into x-ray diffraction and Raman scattering data revealed that the separations of the neighboring lattice planes along the growth direction are shorter than those of bulk gallium nitride. The nanowires would experience biaxial compressive stresses in the inward radial direction and the induced tensile uniaxial stresses in the growth direction. The shifts of the band gap due to the stresses have been estimated using the experimental data, showing that the reduction of the band gap due to the tensile stresses can occur more significantly than the increase due to the compressive stresses. The temperature-dependent photolumi...

Composition-Tuned ZnO−CdSSe Core−Shell Nanowire Arrays

Vertically aligned ZnO--CdSSe core-shell nanocable arrays were synthesized with a controlled composition and shell thickness (10-50 nm) by the chemical vapor deposition on the pregrown ZnO nanowire arrays. They consisted of a composition-tuned single-crystalline wurtzite structure CdS1-xSex (x=0, 0.5, and 1) shell whose [0001] direction was aligned along the [0001] wire axis of the wurtzite ZnO core. The analysis of structural and optical properties shows the formation of Zn containing alloy in the interface region between the ZnO core and shell, which can facilitate the growth of single-crystalline shell layers by reducing both the lattice mismatch and the number of defect sites. In contrast, the TiO2 (rutile) nanowire array can form the polycrystalline shell under the same condition. The photoelectrochemical cell using the ZnO--CdS photoelectrode exhibits a higher photocurrent and hydrogen generation rate than that using the TiO2-CdS one. We suggest that the formation of the CdZnSSe intermediate layers contributes to the higher photoelectrochemical cell performance of the ZnO--CdSSe nanocables.

Synthesis of bamboo-shaped multiwalled carbon nanotubes using thermal chemical vapor deposition

The vertically aligned multiwalled carbon nanotubes (CNT) are synthesized in high density on a large area of cobalt deposited silicon oxide substrate, by thermal chemical vapor deposition using C2H2 gas, at 950°C. The diameter of CNTs is distributed in the range of 80–120 nm and the length is about 20 μm. High-resolution transmission electron microscopy analysis reveals that the multiwalled CNTs have the crystalline graphite sheets and the bamboo structure that the tube consists of hollow compartments separated with graphite layers. A possible growth mechanism is suggested to explain the structure of CNTs. The emission current density is 1.1 mA cm−2 at 4.5 V μm−1, showing the Fowler–Nordheim behavior.

Temperature-dependent growth of carbon nanotubes by pyrolysis of ferrocene and acetylene in the range between 700 and 1000 °C

Abstract Aligned carbon nanotubes were grown by pyrolysis of ferrocene and acetylene in the temperature range 700–1000 °C. The average diameter is constantly 20 nm for all growth temperatures. As the temperature increases, the growth rate increases by 60 times. The length reaches up to 3 mm at 1000 °C. These long carbon nanotubes exhibit a cylindrical structure. The relative amount of crystalline graphitic sheets increases significantly with the growth temperature. The Arrhenius plot yields the activation energy 35±3 kcal/mol, which is close to the diffusion energy of carbon in bulk γ-Fe. We suggest that the bulk diffusion of carbons would play an important role in the growth of cylindrical structured carbon nanotubes.

Dissociation of methanol and ethanol activated by a chemical reaction or by light

When energized sufficiently either vibrationally or electronically, ROH (where R is methyl or ethyl) can dissociate to form H atoms and RO radicals. We have determined the translational energy release (〈ETr 〉=0.82Eavl ) and angular distribution (β=−0.60±0.03) from the laser induced fluorescence spectra of H atoms produced in the 193 nm photodissociation of CD3OH. We have also determined that the quantum yield for producing H from CD3OH is 0.86±0.10. In contrast, the reaction of O(1D)+CH4 which produces vibrationally excited CH3OH, has a quantum yield for producing H atoms of roughly 0.25 with only 22% of the available energy released as translation. We conclude that although the total available energy is the same in both cases, the dissociation of photoexcited methanol is prompt whereas the dissociation of chemically activated methanol shows some degree of internal vibrational equilibration.