Kihyun Shin

Catalytic Characteristics of AgCu Bimetallic Nanoparticles in the Oxygen Reduction Reaction

Intensive research on oxygen reduction reaction (ORR) catalysts has been undertaken to find a Pt substitute or reduce the amount of Pt. Ag nanoparticles are potential Pt substitutes; however, the weak oxygen adsorption energy of Ag prompted investigation of other catalysts. Herein, we prepared AgCu bimetallic nanoparticle (NP) systems to improve the catalytic performance and compared the catalytic performance of Ag, Cu, AgCu (core-shell), and AgCu (alloy) NP systems as new catalyst by investigating the adsorption energy of oxygen and the activation energy of oxygen dissociation, which is known to be the rate-determining step of ORR. By analyzing HOMO-level isosurfaces of metal NPs and oxygen, we found that the adsorption sites and the oxygen adsorption energies varied with different configurations of NPs. We then plotted the oxygen adsorption energies against the energy barrier of oxygen dissociation to determine the catalytic performance. AgCu (alloy) and Cu NPs exhibited strong adsorption energies and low activation-energy barriers. However, the overly strong oxygen adsorption energy of Cu NPs hindered the ORR.

Synthesis of low-temperature-processable and highly conductive Ag ink by a simple ligand modification: the role of adsorption energy

Acetic acid (AA) has been employed to reduce the surface capping ligands of Ag nanoparticles (NPs) for the fabrication of low-temperature-processable and highly conductive Ag ink. The ligand reduction of the Ag NPs was achieved using a one-step method, in which oleylamine (OA)-capped Ag NPs were immersed in AA for different durations (1, 2, 3, 5 and 10 h). The weight of the total capping ligand was reduced from 12.1 wt% to 2.3 wt% by 10 h AA immersion. According to in situ transmission electron microscopy (TEM) and electrical resistivity, the ligand-reduced Ag NPs were cured at a much lower temperature (approximately 100 °C) and showed better electrical performance than OA-capped NPs under the same conditions. To investigate the reason for this enhancement of the electrical properties, we characterized the surface chemistry of the Ag NPs by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), which revealed that the surface capping ligand was exchanged from the OA to the acetate ion. In addition, the adsorption energy of the ligand was increased by the ligand exchange, which was studied using density functional theory (DFT) calculations. DFT was effective in explaining the adsorption of each ligand on Ag NPs and indicated that the ligand can be exchanged by AA immersion.

Density Functional Theory Studies of Oxygen Affinity of Small Au Nanoparticles

Through density functional theory calculations, to provide insight into the origins of the catalytic activity of Au nanoparticles (NPs) toward oxidation reactions, we have scrutinized the oxygen adsorption chemistry of 9 types of small unsupported Au NPs of around 1 nm in size (Au13, Au19, Au20, Au25, Au38, and Au55) looking at several factors (size, shape, and coordination number). We found that these NPs, except for the icosahedral Au13, do not strongly bind to O2 molecules. Energetically most feasible O2 adsorption that potentially provides high CO oxidation activity is observed in the icosahedral Au13, our smallest Au NP. In spite of the chemical inertness of bulk Au, the structural fluxionality of such very small Au NP enables strong O2 adsorption. Our results can support recent experimental findings that the exceptional catalytic activity of Au NPs comes from very small Au species consisting of around 10 atoms each.