Oxidation metallic nanoparticle composites

CNTs-nanoparticles composites have also been exploited for electrochemical sensing applications [17, 118, 119[. Incorporation of metal and oxide nanoparticles has been demonstrated to enhance the electrocatalytical efficiency. A wide range of particles have been used (Pt, Pd, Co, FeCo alloy, Co, Cu, Ag, Cu) and in some cases such CNT/nanoparticles have been combined together with charged polymers [17]. 

Dendrimers containing Pt " or Pt-metal nanoparticles are easily attached to Au and other surfaces by immersion in a dilute aqueous solution of the composite for 20 h, followed by careful rinsing and drying [59,129]. Therefore it is possible to use X-ray photoelectron spectroscopy (XPS) to determine the elemental composition and the oxidation states of Pt within dendrimers. For example, Pt(4f7/2) and Pt(4f5/2) peaks are present at 72.8 eV and 75.7 eV, respectively, prior to reduction, but after reduction they shift to 71.3 eV and 74.4 eV, respectively, which is consistent with the change in oxidation state from -i-2 to 0 (Fig. 13 a]. 

The polymer resulting from oxidation of 3,5-dimethyl aniline with palladium was also studied by transmission electron microscopy (Mallick et al. 2005). As it turned out, the polymer was formed in nanofibers. During oxidative polymerization, palladium ions were reduced and formed palladium metal. The generated metal was uniformly dispersed between the polymer nanofibers as nanoparticles of 2 mm size. So, Mallick et al. (2005) achieved a polymer- metal intimate composite material. This work should be juxtaposed to an observation by Newman and Blanchard (2006) that reaction between 4-aminophenol and hydrogen tetrachloroaurate leads to polyaniline (bearing hydroxyl groups) and metallic gold as nanoparticles. Such metal nanoparticles can well be of importance in the field of sensors, catalysis, and electronics with improved performance. 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

Willner and coworkers demonstrated three-dimensional networks of Au, Ag, and mixed composites of Au and Ag nanoparticles assembled on a conductive (indium-doped tin oxide) glass support by stepwise LbL assembly with A,A -bis(2-aminoethyl)-4,4 -bipyridinium as a redox-active cross-linker.8 37 The electrostatic attraction between the amino-bifunctional cross-linker and the citrate-protected metal particles led to the assembly of a multilayered composite nanoparticle network. The surface coverage of the metal nanoparticles and bipyridinium units associated with the Au nanoparticle assembly increased almost linearly upon the formation of the three-dimensional (3D) network. A coulometric analysis indicated an electroactive 3D nanoparticle array, implying that electron transport through the nanoparticles is feasible. A similar multilayered nanoparticle network was later used in a study on a sensor application by using bis-bipyridinium cyclophane as a cross-linker for Au nanoparticles and as a molecular receptor for rr-donor substrates.8... 

The deposition-precipitation method as proposed by Haruta et al. (1993) provides another way to synthesize composite materials with noble metal nanoparticles over metal-oxide supports. The synthesis of gold and platinum nanoparticles supported on various metal oxide substrates (such as Ti02, ZnO, Ce02, C03O4 and Fe203) (Bamwenda, Tsubota, Nakamura and Haruta 1995 Boccuzzi, Chiorino, Tsubota and Haruta 1996 Centeno, Carrizosa and Odriozola 2003 Moon, Lee, Park and Hong 2004 Zanella, Delannoy and Louis 2005 Li, Comotti and Schuth 2006) has been continually reported in the past one and a half decades. 

Fig. 14 Catalytic oxidation of benzyl alcohol in the presence of metal nanoparticles immobilized in thermosensitive core-shell microgels at different temperatures. At lower temperatures (T < 32°C) the microgel network is hydrophilic and swollen in water, whereas at high temperatures (T > 32°C), the network shrinks and becomes hydrophobic. Thus, microgel particles embedding the metal catalyst will move to the oil phase, which will be favorable for the uptake of hydrophobic benzyl alcohol into the metal-microgel composite. Therefore, the catalytic activity of the metal-microgel composites will be affected both by the volume transition and the polarity change of the microgel [29]...

Fig. 4) or just metal oxides (ZnO, CtO, Fig. 5). In the first case, the nanoparticles are more likely to be oxidized within their outer layer therefore they must have a core-shell structure (metallic core and metal oxide shell). The composition of some of the samples was confirmed by EPR and solid state NMR studies.

The fabrication of well-ordered arrays of well-defined nanoparticles or clusters is of fundamental and technological interest. As this is a difficult task, different techniques have been employed. An elegant approach would be to link well-defined building blocks in a chemically straightforward procedure yielding a monodisperse or a completely homogeneous material. We succeeded now to cross-link assembled nanosized metal-oxide-based clusters/composites - novel supramolecular entities - under one-pot conditions. 

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