Dae Hoon Park

Soft-chemical exfoliation route to layered cobalt oxide monolayers and its application for film deposition and nanoparticle synthesis.

A colloidal suspension of exfoliated, layered cobalt oxide nanosheets has been synthesized through the intercalation of quaternary tetramethylammonium ions into protonated lithium cobalt oxide. According to atomic force microscopy, exfoliated nanosheets of layered cobalt oxide show a plateau-like height profile with nanometer-level height, underscoring the formation of unilamellar 2D nanosheets. The exfoliation of layered cobalt oxide was cross-confirmed by X-ray diffraction, UV/Vis spectroscopy, and transmission electron microscopy. The maintenance of the hexagonal in-plane structure of the cobalt oxide lattice after the exfoliation process was evidenced by selected-area electron diffraction and Co K-edge X-ray absorption near-edge structure analysis. The zeta-potential measurements clearly demonstrated the negative surface charge of cobalt oxide nanosheets. Adopting the nanosheets of layered cobalt oxide as a precursor, we were able to prepare the monodisperse CoO nanocrystals with a particle size of approximately 10 nm as well as the heterolayered film composed of cobalt oxide monolayer and polycation.

Direct Soft‐Chemical Synthesis of Chalcogen‐Doped Manganese Oxide 1D Nanostructures: Influence of Chalcogen Doping on Electrode Performance

We have developed a direct nonhydrothermal route to nanostructured chalcogen-doped manganese oxides; K(x)MnO(2)Q(y) (Q = S, Se, and Te). According to combinative diffraction and microscopic analyses, the S- and Se-doped manganese oxides exhibit 1D nanowire-type morphology with layered delta-MnO(2)- and alpha-MnO(2)-structures, respectively, whereas the Te-doped compound consists of 3D nanospheres that are amorphous according to X-ray diffraction. X-ray absorption and X-ray photoelectron spectroscopy analyses clearly demonstrate that the doped chalcogen ions exist in the form of hexavalent chalcogenate clusters mainly on the sample surface or grain boundary. According to electrochemical and ex situ X-ray absorption spectroscopy investigations, the Se-doped manganate nanowires show higher structural stability and better electrode performance with excellent rate characteristics compared to the S-/Te-doped and undoped manganate nanostructures. This is attributed to the presence of chemically stable SeO4(2-) species, leading to enhanced stability of the manganate lattice through the prevention of structural deformation during cycling and/or to the improvement of Li(+) ion transport through the maintenance of intercrystallite voids. Based on the present experimental findings, we are able to conclude that the present one-pot soft-chemical route with chalcogen dopants can provide a simple method not only to economically synthesize 1D nanostructured manganese oxides but also to finely control their electrode performance, crystal structure and morphology, and lattice stability.