Kang Pyo So

Monitoring multiwalled carbon nanotube exposure in carbon nanotube research facility.

With the increased production and widespread use of multiwalled carbon nanotubes (MWCNTs), human and environmental exposure to MWCNTs is inevitably increasing. Therefore, this study monitored the possible exposure to MWCNT release in a carbon nanotube research laboratory. To estimate the potential exposure of researchers and evaluate the improvement of the workplace environment after the implementation of protective control measures, personal and area monitoring were conducted in an MWCNT research facility where the researchers handled unrefined materials. The number, composition, and aspect ratio of MWCNTs were measured using scanning transmission electron microscopy with an energy-dispersive x-ray analyzer. The gravimetric concentrations of total dust before any control measures ranged from 0.21 to 0.43 mg/m3, then decreased to a nondetectable level after implementing the control measures. The number of MWCNTs in the samples obtained from the MWCNT blending laboratory ranged from 172.9 to 193.6 MWCNTs/cc before the control measures, and decreased to 0.018–0.05 MWCNTs/cc after the protective improvements. The real-time monitoring of aerosol particles provided a signature of the MWCNTs released from the blending equipment in laboratory C. In particular, the number size response of an aerodynamic particle sizer with a relatively high concentration in the range of 2 to 3 μ m in aerodynamic diameter revealed the evidence of MWCNT exposure. The black carbon mass concentration also increased significantly during the MWCNT release process. Therefore, the present study suggests that the conventional industrial hygiene measures can significantly reduce exposure to airborne MWCNTs and other particulate materials in a nano research facility.

Probing graphene grain boundaries with optical microscopy

Grain boundaries in graphene are formed by the joining of islands during the initial growth stage, and these boundaries govern transport properties and related device performance. Although information on the atomic rearrangement at graphene grain boundaries can be obtained using transmission electron microscopy and scanning tunnelling microscopy, large-scale information regarding the distribution of graphene grain boundaries is not easily accessible. Here we use optical microscopy to observe the grain boundaries of large-area graphene (grown on copper foil) directly, without transfer of the graphene. This imaging technique was realized by selectively oxidizing the underlying copper foil through graphene grain boundaries functionalized with O and OH radicals generated by ultraviolet irradiation under moisture-rich ambient conditions: selective diffusion of oxygen radicals through OH-functionalized defect sites was demonstrated by density functional calculations. The sheet resistance of large-area graphene decreased as the graphene grain sizes increased, but no strong correlation with the grain size of the copper was revealed, in contrast to a previous report. Furthermore, the influence of graphene grain boundaries on crack propagation (initialized by bending) and termination was clearly visualized using our technique. Our approach can be used as a simple protocol for evaluating the grain boundaries of other two-dimensional layered structures, such as boron nitride and exfoliated clays.

TAILORING THE CHARACTERISTICS OF GRAPHITE OXIDES BY DIFFERENT OXIDATION TIMES

Graphite oxide was synthesized using various oxidation times and characterized by its physical and chemical properties. The degree of oxidation of the graphite oxide was systematically controlled via oxidation time up to 24h. Three phases of interlayer distances were identified by x-ray diffraction: pristine graphite (3.4A), intermediate (4A) and fully expanded graphite oxide (6A) phases. These phases were distinguished by an atomic ratio of O/C, which occurred from the different compositions of epoxide, carboxyl and hydroxyl groups. The band gap of the graphite oxides was also tuned via the oxidation time, resulting in direct band gap engineering from 1.7 to 2.4eV and strong correlation with the atomic ratio of O/C. (Some figures in this article are in colour only in the electronic version)

High-rate aluminium yolk-shell nanoparticle anode for Li-ion battery with long cycle life and ultrahigh capacity

Alloy-type anodes such as silicon and tin are gaining popularity in rechargeable Li-ion batteries, but their rate/cycling capabilities should be improved. Here by making yolk-shell nanocomposite of aluminium core (30 nm in diameter) and TiO2 shell (∼3 nm in thickness), with a tunable interspace, we achieve 10 C charge/discharge rate with reversible capacity exceeding 650 mAh g−1 after 500 cycles, with a 3 mg cm−2 loading. At 1 C, the capacity is approximately 1,200 mAh g−1 after 500 cycles. Our one-pot synthesis route is simple and industrially scalable. This result may reverse the lagging status of aluminium among high-theoretical-capacity anodes.

Synthesis and systematic characterization of functionalized graphene sheets generated by thermal exfoliation at low temperature

We describe the low-temperature thermal exfoliation of graphite oxide to obtain functionalized graphene sheets (FGSs). Graphite oxide, which is highly oxidized graphite produced by a modified Brodie method, is further exfoliated by a simple heat treatment at 270‐275 ◦ C under ambient Ar. The FGSs that are generated have fewer defects and less oxygen content than in commercial graphene sheets (GSs) prepared at high temperatures (>900 ◦ C). X-ray photoelectron spectroscopy demonstrates a clear π-plasmon peak in the FGSs of the type that is seen in precursor graphite, but not in commercial GSs. Thus, our FGSs exhibit high 2D crystallinity and minimal defects. (Some figures in this article are in colour only in the electronic version)

Hydrolysis-induced immobilization of Pt(acac)2 on polyimide-based carbon nanofiber mat and formation of Pt nanoparticles

Electrospun polyimide (PI)-based carbon nanofibers have recently garnered much interest due to their high conductivity and high mechanical strength. Promising applications include electrodes for supercapacitors, filters, sensors, and fuel cells. Here, we demonstrate that Pt nanoparticles can be loaded on the surface of PI nanofibers via an immobilization process induced by hydrolysis. The uniform distribution and sizes of Pt nanoparticles were controlled further by carbonization. Pt(acac)2 dissolved in acetone was impregnated on the hydrolyzed PI nanofibers. Pt ions were localized exclusively on the surface of PI nanofibers by precise control of the hydrolysis process. Our X-ray photoelectron spectroscopy results show that Pt ions in Pt(acac)2 molecules (40%) are immobilized on the hydrolyzed PI surface while some of them (60%) bind to O in the carboxylic group to form a PtO structure, and then are fully decomposed into Pt nanoparticles during carbonization. Using density functional calculations, we show that the binding of Pt(acac)2 on hydrolyzed PI is strong with a binding energy of −4.3 eV, which originates mostly from Pt–O binding and π-stacking between (acac) and PAA, confirming experimental observations of robust formation of Pt nanoparticles on hydrolyzed PI. The cyclic voltammetric test demonstrates that our robust carbon nanofiber mat can be utilized for fuel cell electrodes.

Metal-nanotube composites as radiation resistant materials

The improvement of radiation resistance in nanocomposite materials is investigated by means of classical reactive molecular dynamics simulations. In particular, we study the influence of carbon nanotubes (CNTs) in an Ni matrix on the trapping and possible outgassing of He. When CNTs are defect-free, He atoms diffuse alongside CNT walls and, although there is He accumulation at the metal-CNT interface, no He trespassing of the CNT wall is observed, which is consistent with the lack of permeability of a perfect graphene sheet. However, when vacancies are introduced to mimic radiation-induced defects, He atoms penetrate CNTs, which play the role of nano-chimneys, allowing He atoms to escape the damaged zone and reduce bubble formation in the matrix. Consequently, composites made of CNTs inside metals are likely to display improved radiation resistance, particularly when radiation damage is related to swelling and He-induced embrittlement.

Intragranular Dispersion of Carbon Nanotubes Comprehensively Improves Aluminum Alloys

Abstract The room‐temperature tensile strength, toughness, and high‐temperature creep strength of 2000, 6000, and 7000 series aluminum alloys can be improved significantly by dispersing up to 1 wt% carbon nanotubes (CNTs) into the alloys without sacrificing tensile ductility, electrical conductivity, or thermal conductivity. CNTs act like forest dislocations, except mobile dislocations cannot annihilate with them. Dislocations cannot climb over 1D CNTs unlike 0D dispersoids/precipitates. Also, unlike 2D grain boundaries, even if some debonding happens along 1D CNT/alloy interface, it will be less damaging because fracture intrinsically favors 2D percolating flaws. Good intragranular dispersion of these 1D strengtheners is critical for comprehensive enhancement of composite properties, which entails change of wetting properties and encapsulation of CNTs inside Al grains via surface diffusion‐driven cold welding. In situ transmission electron microscopy demonstrates liquid‐like envelopment of CNTs into Al nanoparticles by cold welding.