Facile Fabrication of Nanoparticles in the Nanospace of Ultrathin TiO 2 -Gel Films: Composition, Morphology and Catalyti
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Facile Fabrication of Nanoparticles in the Nanospace of Ultrathin TiO2-Gel Films: Composition, Morphology and Catalytic Activity Junhui He and Toyoki Kunitake Frontier Research System The Institute of Physical and Chemical Research (RIKEN) 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ABSTRACT In this article, we present in-situ synthesis of nanoparticles of various compositions and morphologies in the nanospace of ultrathin TiO2-gel films using low temperature H2 and O2 plasmas as reductant and oxidant, respectively. Parameters that influence the formation of nanoparticles were investigated and discussed. Reversible chemical transformation was found to give monodisperse nanoparticles. Catalytic activities of Pd monometallic and Ag-core/Pd-shell bimetallic nanoparticles were tested for hydrogenation of methyl acrylate. The much enhanced catalytic activity is attributed mainly to a large ratio of the surface Pd atoms. INTRODUCTION Nanoparticles of metals and semiconductors are attracting much attention, as they possess promise in photonic, electronic, magnetic, and chemical applications [1-2]. Thin films consisting of such nanoparticles are especially interesting and important as new functional materials. Nanoparticles are usually synthesized in solutions by chemical, photochemical, radiolytic and hydrothermal reactions. Thus, effective immobilization of nanoparticles within the matrix or on the surface of an ultrathin film is required in most cases of practical applications, and becomes one of the major challenges in fabrication of functional thin films. Efforts have also been dedicated to in-situ preparation of nanoparticles in films of micrometer and sub-micrometer thicknesses. However, in those methods, nanoparticle loadings are often low and, in addition, care must be taken to avoid aggregation and precipitation of metal salts and coalescence of metallic particles. Recently we developed a new ion-exchange method for incorporation of metal ions into metal oxide thin films [3]. A variety of metal ions can be introduced, and the amount of metal ions incorporated is readily controlled by the amount of template and ion-exchange conditions. Incorporation of two or more metal species is also possible by simultaneous or sequential procedures. Such films are nanoporous, and may serve as a nanoreactor for in-situ synthesis of nanoparticles. This is in fact the case, as indicated by formation of noble metal nanoparticles, by interconversion between metal and oxide moieties, and by successful preparation of bimetallic nanoparticles in such ultrathin films. EXPERIMENTAL DETAILS A mixture of Mg(O-Et)2 (10 mM) and Ti(O-nBu)4 (100 mM) in 2-ethoxyethanol was used commonly for film assembly unless otherwise specified. Thin films were assembled layer-bylayer by immersing a substrate (e.g., cleaned quartz plate) in the precursor solution at room temperature for a given period of time, followed by rinsing with toluene to remove the physisorbed species, drying with N2 and hydrolysis in air. Usually 8 cycles of this procedure we
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