Microstructure and magnetoresistance of heterogeneous gold-rich Ag₃Au_1.1Te₂

L. Kienle, V. Duppel, G. Lembke, B. Mogwitz, J. Janek, M.v. Kreutzbruck and A. Simon, Solid State Sci., in print (2010), available online

Dispersions of very small non-magnetic metal particles or inclusions in a non-magnetic semiconductor matrix are well known to produce unusually large and linear magnetoresistance effects. So far these materials were limited to the binary silver-rich chalcogenides Ag₂Se and Ag₂Te. In this contribution Ag₃AuTe₂ was selected as a first candidate for a ternary matrix material, thus offering enhanced capabilities for the generation of heterogeneous microstructure and spatially varying composition on the nanoscale. In gold-rich Ag₃Au_1.1Te₂ two kinds of inhomogeneities are present, namely Au deposits with a size on the micron scale and an inhomogeneous distribution of Au and Ag within the matrix. The matrix consists of micron-sized grains with the structure type of Ag₃AuTe₂ as studied by electron microscopy. Like the binary silver chalcogenide phases, the material also shows a large and linear magnetoresistance effect. The transversal magnetoresistance effect was measured between 20 K and 270 K in magnetic fields up to B = 5 T. The results are discussed on the basis of existing models for a large and linear positive MR effect.

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Janus-Faced SiO2: Activation and Passivation in the Electrode System Platinum/Yttria-Stabilized Zirconia
Eva Mutoro, Nils Baumann, and Jürgen Janek, J. Phys. Chem. Lett. 1 (2010) 2322

Small amounts of impurities can strongly impact the electrode kinetics of electrochemical cells and devices, leading to an improved or declined performance. This study shows that one material, silica, can influence the electrochemical behavior of the basic solid-state electrode system Pt( O2 )/YSZ ( yttria-stabilized zirconia) in opposite directions, depending on its local distribution in the system. Compared to a silica-free electrode as the reference, SiO₂ located at the interface Pt/YSZ caused a reduced performance, while SiO₂ on the surface of the Pt electrode caused a small enhancement. The passivation can be explained by blocking of active three-phase boundary sites ( tpb, Pt/YSZ/O2) , while silica surface doping is hypothesized to influence the oxygen adsorption/desorption and surface diffusion.

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Mesoporous TiO2: Comparison of Classical Sol Gel and Nanoparticle Based Photoelectrodes for the Water Splitting Reaction
Pascal Hartmann, Doh-Kwon Lee, Bernd M. Smarsly and Jürgen Janek , ACS nano, in print (2010), available online,

Digital Object Identifier :: 10.1021/nn1004765

This paper describes a systematic comparison of the photoelectrochemical properties of mesoporous TiO₂ films prepared by the two most prevalent templating methods: The use of preformed, crystalline nanoparticles is generally considered advantageous compared to the usage of molecular precursors such as TiCl₄, since the latter requires a separate heat treatment at elevated temperature to induce crystallization. However, our photoelectrochemical experiments clearly show that sol gel derived mesoporous TiO₂ films cause an about 10 times higher efficiency for the water splitting reaction than their counterparts obtained from crystalline TiO₂ nanoparticles. This result indicates that for electrochemical applications the performance of nanoparticle-based metal oxide films might suffer from insufficient electronic connectivity.

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