This paper reports the findings of our efforts toward gaining a more complete understanding and utilization of galvanic replacement reactions involving manganese oxide with noble metals. It was revealed that the site of metal deposition is significantly affected by the variable oxidation state of manganese oxide. The use of carbon-encapsulated MnO nanoparticles as a reaction template led to metal growth specifically on the outermost surfaces of the carbon shells rather than on the MnO cores, which allowed for the selective decoration of the external surfaces of hollow carbon nanospheres with catalytic nanocrystals of various noble metals, including Pt, Pd, Rh, and Ir. By rearranging the sequence between carbon-shell coating and galvanic replacement processes, the deposited metal nanocrystals could be placed on the interior surfaces of hollow carbon nanospheres and, moreover, separately on the internal and the external surfaces, which may enable the respective control of the catalytic functionalities of each specific surface.

We report a simple one-step electrodeposition of triangular Pd rod nanostructures on clean Au substrates without additives. Scanning electron microscopy, transmission electron microscopy, and electrochemical techniques were utilized to characterize the structural features of the triangular Pd rod nanostructures. The regulation of the electrodeposition rate by optimizing the electrolyte concentration and applied potential was critical for the anisotropic growth of Pd in the vertical direction. The triangular Pd rod structures exhibited electrocatalytic activities for oxygen reduction and methanol oxidation reactions. These surfaces could be effectively utilized as reproducible surface-enhanced Raman scattering (SERS) active substrates to produce stable SERS signals under electrochemical systems. A simple preparation of well-defined triangular Pd rod structures would allow new opportunities in various areas utilizing Pd-based nanostructured surfaces.

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A novel reverse microemulsion strategy was developed to asymmetrically encapsulate metal-oxide nanoparticles in silica by exploiting the self-catalytic growth of aminosilane-containing silica at a single surface site. This strategy produced various colloidal Janus nanoparticles, including Au/Fe3O4@asy-SiO2, which were converted to an Au-containing silica nanosphere, Au@con-SiO2, by reductive Fe3O4 dissolution. The use of Au@con-SiO2 as a metal-growing nanoreactor allowed the templated synthesis of various noble-metal nanocrystals, including a hollow dendritic Pt nanoshell which exhibits significantly better electrocatalytic activities for the oxygen reduction reaction than commercial Pt/C catalysts.

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