Kyeounghak Kim

Exsolution trends and co-segregation aspects of self-grown catalyst nanoparticles in perovskites

In perovskites, exsolution of transition metals has been proposed as a smart catalyst design for energy applications. Although there exist transition metals with superior catalytic activity, they are limited by their ability to exsolve under a reducing environment. When a doping element is present in the perovskite, it is often observed that the surface segregation of the doping element is changed by oxygen vacancies. However, the mechanism of co-segregation of doping element with oxygen vacancies is still an open question. Here we report trends in the exsolution of transition metal (Mn, Co, Ni and Fe) on the PrBaMn2O5+δ layered perovskite oxide related to the co-segregation energy. Transmission electron microscopic observations show that easily reducible cations (Mn, Co and Ni) are exsolved from the perovskite depending on the transition metal-perovskite reducibility. In addition, using density functional calculations we reveal that co-segregation of B-site dopant and oxygen vacancies plays a central role in the exsolution.

Concentration Effect of Reducing Agents on Green Synthesis of Gold Nanoparticles: Size, Morphology, and Growth Mechanism

Under various concentration conditions of reducing agents during the green synthesis of gold nanoparticles (AuNPs), we obtain the various geometry (morphology and size) of AuNPs that play a crucial role in their catalytic properties. Through both theoretical and experimental approaches, we studied the relationship between the concentration of reducing agent (caffeic acid) and the geometry of AuNPs. As the concentration of caffeic acid increases, the sizes of AuNPs were decreased due to the adsorption and stabilizing effect of oxidized caffeic acids (OXCAs). Thus, it turns out that optimal concentration exists for the desired geometry of AuNPs. Furthermore, we investigated the growth mechanism for the green synthesis of AuNPs. As the caffeic acid is added and adsorbed on the surface of AuNPs, the aggregation mechanism and surface free energy are changed and consequently resulted in the AuNPs of various geometry.

Precise control of defects in graphene using oxygen plasma

The authors report on a facile method for introducing defects in graphene in a controlled manner. Samples were mounted face down between supports, and exposed to oxygen plasma in a reactive ion etching (RIE) system. Defect density and the rate of defect formation in graphene were analyzed according to the oxygen flow rates and power conditions, using Raman spectroscopy. The mechanism of defect formation was systematically investigated via both experiment and density functional theory (DFT) calculation. Based on our DFT results, sp3 oxygen in the epoxide form would most likely be induced in pristine graphene after exposure to the oxygen plasma. Defect engineering through the fine tuning of the graphene disorder using a conventional RIE system has great potential for use in various graphene-based applications.

A virtual enterprise design method based on business process simulation

In order for a virtual enterprise (VE) to be successful, effective synchronisation of processes between the member enterprises is a major challenge. To overcome this challenge, it is important to select appropriate member enterprises at the establishment phase of a VE that can cope with the business goal and constraints. In this paper, we propose a simulation-based method to evaluate and select proper candidate enterprises from a repository. A broker enterprise after designing a business process provides corresponding business requirements (abstract) to the candidates in the repository, and those candidate enterprises that meet the business requirements respond to the broker. The broker enterprise then simulates the business process (global simulation) considering global performance of the VE, whose results become detailed requirements for the candidate enterprises. The participating enterprises adjust their business conditions using their local simulation to cope with the business constraints. If the candidate enterprise cannot cope with the business constraints, a new candidate is selected from the repository, and the business process is simulated again. This procedure continues until an appropriate candidate is identified, and a virtual enterprise is then established. To support the proposed VE design process, OMGs Model Driven Architecture and the web services technology are employed. Finally, the proposed methods are conceptually illustrated for an exemplary VE.

An autonomous land vehicle PRV III

PRV III (POSTECH Road Vehicle III) is the third generation outdoor intelligent vehicle. PRV III is controlled by a model reference based fuzzy controller along with a PID controller, taking both the nonlinear vehicle dynamics and the road conditions into consideration. The computer system used is composed of the C40 microprocessor based parallel architecture. The fitness based navigation algorithm which utilizes both a neural network and a conventional vision mode is encoded into our new computer system. A color vision mode based on the pyramid random model is added to adopt the unpaved cross country road environment. The laser range sensor and sonar sensors are used for the obstacle detection and avoidance module and also for the car following module.

Energy and dose dependence of proton-irradiation damage in graphene

Monolayer graphenes were irradiated with 5–15 MeV high-energy protons at various doses from 1 × 1016 to 3 × 1016 cm−2, and their characteristics were systematically investigated using micro-Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). As the energy and dose of the proton irradiation increased, the defects induced in the graphene layers also increased gradually. The average defect distances of 10 MeV proton-irradiated graphene decreased to 29 ± 5 nm at a dose of 3 × 1016 cm−2. The defect formation energies for various types of defects were compared by using density functional theory calculation. After proton irradiation, the results of micro-Raman scattering and XPS indicated p-doping effects due to adsorption of environmental molecules on the damaged graphene. Our results show a direct relationship between the defect formation of the graphene layers and the energy/dose of the proton irradiation.

An Autonomous Land Vehicle: Design Concept And Preliminary Road Test Results

This paper describes an autonomous land vehicle known as PRV I (Postech Road Vehicle I). It employs road following algorithms based on conventional computer vision techniques and on neural networks. These algorithms use CCD camera images as direct inputs, and outputs are steering angle and velocity of the vehicle.

A Simple Descriptor to Rapidly Screen CO Oxidation Activity on Rare-Earth Metal-Doped CeO2: From Experiment to First-Principles

Ceria (CeO2) is an attractive catalyst because of its unique properties, such as facile redoxability and high stability. Thus, many researchers have examined a wide range of catalytic reactions on ceria nanoparticles (NPs). Among those contributions are the reports of the dopant-dependent catalytic activity of ceria. On the other hand, there have been few mechanistic studies of the effects of a range of dopants on the chemical reactivity of ceria NPs. In this study, we examined the catalytic activities of pure and Pr, Nd, and Sm-doped CeO2 (PDC, NDC, and SDC, respectively) NPs on carbon monoxide (CO) oxidation. Density functional theory (DFT) calculations were also performed to elucidate the reaction mechanism on rare-earth (RE)-doped CeO2(111). The experimental results showed that the catalytic activities of CO oxidation were in the order of CeO2 > PDC > NDC > SDC. This is consistent with the DFT results, where the reaction is explained by the Mars-van Krevelen mechanism. On the basis of the theoretical interpretation of the experimental results, the ionic radius of the RE dopant can be used as a simple descriptor to predict the energy barrier at the rate-determining step, thereby predicting the entire reaction activity. Using the descriptor, a wide range of RE dopants on CeO2(111) were screened for CO oxidation. These results provide useful insights to unravel the CO oxidation activity on various oxide catalysts.

Cold welding of gold nanoparticles on mica substrate: Self-adjustment and enhanced diffusion

From the images of HR-TEM, FE-SEM, and AFM, the cold welding of gold nanoparticles (AuNPs) on a mica substrate is observed. The cold-welded gold nanoparticles of 25 nm diameters are found on the mica substrate in AFM measurement whereas the size of cold welding is limited to 10 nm for nanowires and 2~3 nm for nanofilms. Contrary to the nanowires requiring pressure, the AuNPs are able to rotate freely due to the attractive forces from the mica substrate and thus the cold welding goes along by adjusting lattice structures. The gold nanoparticles on the mica substrate are numerically modeled and whose physical characteristics are obtained by the molecular dynamic simulations of LAMMPS. The potential and kinetic energies of AuNPs on the mica substrate provide sufficient energy to overcome the diffusion barrier of gold atoms. After the cold welding, the regularity of lattice structure is maintained since the rotation of AuNPs is allowed due to the presence of mica substrate. It turns out that the growth of AuNPs can be controlled arbitrarily and the welded region is nearly perfect and provides the same crystal orientation and strength as the rest of the nanostructures.

Direct access to aggregation-free and small intermetallic nanoparticles in ordered, large-pore mesoporous carbon for an electrocatalyst

An intermetallic catalyst with ordered atomic arrays has a higher electrocatalytic activity than alloy, but the high temperature required for the formation makes the particles large, resulting in low mass activity. We report the simple synthesis of small Pt-based intermetallic nanoparticles on a carbon-based ordered mesoporous support by combining block copolymer-assisted evaporation-induced self-assembly and strong metal-support interaction (SMSI). Aluminosilicate in the mesostructured wall is an SMSI agent and charge transfer from Pt to the aluminosilicate suppresses the sintering of intermetallic nanoparticles. Intermetallic PtPb and Pt3Co on carbon-based mesoporous supports are synthesized, and their particle sizes are below 5 nm even at high loading. The PtPb catalyst shows 15 times higher mass activity for formic acid oxidation than Pt/C, and the Pt3Co catalyst shows 3.25 times higher mass activity for oxygen reduction than Pt/C. This procedure can be extended to synthesize various heterogenous catalysts that require high temperature for synthesis or to operate.