Keumyoung Seo

Dynamic graphene filters for selective gas-water-oil separation

Selective filtration of gas, water, and liquid or gaseous oil is essential to prevent possible environmental pollution and machine/facility malfunction in oil-based industries. Novel materials and structures able to selectively and efficiently filter liquid and vapor in various types of solutions are therefore in continuous demand. Here, we investigate selective gas-water-oil filtration using three-dimensional graphene structures. The proposed approach is based on the adjustable wettability of three-dimensional graphene foams. Three such structures are developed in this study; the first allows gas, oil, and water to pass, the second blocks water only, and the third is exclusively permeable to gas. In addition, the ability of three-dimensional graphene structures with a self-assembled monolayer to selectively filter oil is demonstrated. This methodology has numerous potential practical applications as gas, water, and/or oil filtration is an essential component of many industries.

Tunable-white-light-emitting nanowire sources.

Tunable-white-light-emitting materials are developed by combining two single-crystal oxide nanowire materials-ZnO and SnO(2)-having different light emissions. The tuning of white-light emission from cool white to warm white is achieved for the first time by adjusting the growth sequence and growth time of the ZnO and SnO(2) nanowires. Combined ZnO:SnO(2) nanowire arrays yield a desired emission color from (0.30, 0.31) to (0.35, 0.37) and a white luminescence of approximately 100 cd m(-2), whose reproducibility can be controlled accurately. These results pave a new way to understand and generate a desired white-light emission, which is a key technology in large-area planar display devices, including flexible and/or transparent display devices.

Effect of nitrogen plasma on the surface of indium oxide nanowires

The change in the atomic nitrogen concentration on a semiconducting nanowires surface and the consequent changes in the electrical characteristics of a nanowire transistor were investigated by exposing In(2)O(3) nanowires to nitrogen (N(2)) plasma. After plasma was applied at N(2) flow rates of 20, 40, and 70 sccm with a fixed source power of 50 W, the In(2)O(3) nanowire transistor exhibited changes in the threshold voltage (V(th)), subthreshold slope (SS), and on-current (I(on)). In particular, after treatment at an N(2) flow rate of 40 sccm, V(th) shifted positively by ~2.3 V, the SS improved by ~0.24 V/dec, and I(on) increased by ~0.8 μA on average. The changes are attributed to the combination of nitrogen ions produced by the plasma with oxygen vacancies or indium interstitials on the nanowires. Optimization of the plasma treatment conditions is expected to yield desirable device characteristics by a simple, nondestructive process.

Wettability of graphene-laminated micropillar structures

The wetting control of graphene is of great interest for electronic, mechanical, architectural, and bionic applications. In this study, the wettability of graphene-laminated micropillar structures was manipulated by changing the height of graphene-laminated structures and employing the trichlorosilane (HDF-S)-based self-assembly monolayer. Graphene-laminated micropillar structures with HDF-S exhibited higher hydrophobicity (contact angle of 129.5°) than pristine graphene thin film (78.8°), pristine graphene-laminated micropillar structures (97.5°), and HDF-S self-assembled graphene thin film (98.5°). Wetting states of the graphene-laminated micropillar structure with HDF-S was also examined by using a urea solution, which flowed across the surface without leaving any residues.

Control of oxygen vacancy concentration in ZnO nanowires containing sulfur as a reducing agent

Light illumination influences the electrical characteristics and stability of oxide nanowire transistors. In this study, transistor characteristics of oxygen-vacancy-rich ZnO nanowires under illumination were investigated. In order to control the oxygen vacancies on the surface of ZnO, sulfur was used as a reducing agent during nanowire growth. Unlike pure nanowires, ZnO nanowires with sulfur as a reducing agent exhibited a dramatically enhanced green emission peak at around 520 nm in the photoluminescence spectrum, which is primarily generated under oxygen-deficient ambient conditions. The threshold voltage of a nanowire transistor using ZnO with sulfur showed no significant change under illumination. In contrast, the threshold voltage of pure ZnO shifted significantly in the negative direction under illumination. This phenomenon may arise from the fact that light illumination on the channel region of ZnO reduced with sulfur cannot generate additional oxygen vacancies on the nanowire surface because oxygen vacancies were created almost to the saturation point during nanowire growth.

Fabrication of controllable and stable In2O3 nanowire transistors using an octadecylphosphonic acid self-assembled monolayer

The controllability and stability of nanowire transistor characteristics are essential for the development of low-noise and fast-switching nano-electronic devices. In this study, the positive shift of threshold voltage and the improvement of interface quality on In2O3 nanowire transistors were simultaneously achieved by using octadecylphosphonic acid (OD-PA) self-assembly. Following the chemical bond of OD-PA molecules on the surface of In2O3 nanowires, the threshold voltage was positively shifted to 2.95 V, and the noise amplitude decreased to approximately 87.5%. The results suggest that an OD-PA self-assembled monolayer can be used to manipulate and stabilize the transistor characteristics of nanowire-based memory and display devices that require high-sensitivity, low-noise, and fast-response.

Nanowire-based ternary transistor by threshold-voltage manipulation

We report on a ternary device consisting of two nanowire channels that have different threshold voltage (Vth) values and show that three current stages can be produced. A microscale laser-beam shot was utilized to selectively anneal the nanowire channel area to be processed, and the amount of Vth shift could be controlled by adjusting the laser wavelength. Microscale laser annealing process could control Vth of the individual nanowire transistors while maintaining the other parameters the constant, such as the subthreshold slope, on–off current ratio, and mobility. This result could provide a potential for highly integrated and high-speed ternary circuits.

Hydrogen generation enhanced by nano-forest structures

In the ongoing attempts to alleviate current energy constraints, hydrogen gas, H2, is drawing much attention as an alternative energy storage medium to address the issues of limited energy resources as well as air pollution associated with conventional energy technologies. Here we report H2 generation by thermochemical water-splitting employing a hierarchical CeO2 nanostructure which allows for maximizing the reaction surface area and enhanced H2 generation at relatively low temperatures (800 °C or below). More importantly the nanostructure retains the surface area during H2 generation. The large surface area of the nanostructure was achieved by growing one-dimensional CeO2/SnO2 core–shell nanowires on a three-dimensional porous disk. Namely the three-dimensional core–shell “nanowire-forest” has increased H2 generation by 45.5% at 800 °C, in comparison to conventional CeO2 thin-film-coated disks used as a reference. Furthermore the total amount of H2 generated by the nanowire forest over one thermal cycle is greater than that of the previously reported values. We believe that such hierarchical CeO2 nanostructures offers a highly effective means for producing H2 gas as well as for other catalytic reactions.

Hydrogen production based on a photoactivated nanowire-forest

For several decades, the key challenge associated with thermochemical hydrogen generation has been the achievement of water splitting and catalyst regeneration at low temperatures while maintaining a reasonably high conversion efficiency over many cycles. Herein, we report low-temperature thermochemical hydrogen generation using hierarchically assembled iron oxide nanoarchitectures. Iron oxide nanoparticles conformally deposited onto a SnO2 nanowire forest allowed the splitting of water molecules and the production of hydrogen gas at temperatures of 400–800 °C, with a high specific gas-forming rate as high as ∼25000 μmol per g per cycle (250 min). More remarkably, deep-ultraviolet photoactivation enabled low-temperature (200 °C) catalyst regeneration and thereby multiple cycles of hydrogen production without any significant coalescence of the oxide nanoparticles nor substantial loss of the water-splitting efficiency. Hierarchically arranged iron oxide nanoarchitectures, in combination with photochemical catalyst regeneration, are promising for practical hydrogen generation by harvesting wasted thermal energy, even at temperatures below 500 °C.

Oxygen release from metal oxide for repeated hydrogen regeneration by proton irradiation with polyvinylpyrrolidone

In this study, we investigated the reduction of a 3D microporous NiOx structure, used as a metal oxide catalyst, by proton irradiation with polyvinylpyrrolidone (PVP) for hydrogen regeneration. In general, the reduction process for hydrogen regeneration requires high temperatures (1000–4000 °C) to release saturated oxygen from the metal oxide catalyst. Proton irradiation with PVP could regenerate abundant oxygen vacancies by releasing the oxygen attached to NiOx at room temperature. The 3D microporous NiOx structure provided the maximum hydrogen generation rate of ∼4.2 μmol min−1 g−1 with the total amount of generated hydrogen being ∼460 μmol g−1 even in the repetitive thermochemical cycle; these results are similar to the initial hydrogen generation data. Therefore, continuous regeneration of hydrogen from the oxygen-reduced 3D microporous NiOx structure was possible. It is expected that the high thermal energy, which is the major problem associated with hydrogen regeneration through the conventional heat treatment method, would be resolved in future using such a method.