Amorphous metal nanoparticles

Ultrasonic irradiation of solutions containing volatile organometaUic compounds such as Fe(CO)j, Ni(CO), and Co(CO)3NO produced porous, coral-like aggregates of amorphous metal nanoparticles [25]. A classic example is the sonication of Fe(CO)j in decane at 0 C under Ar, which yielded a black powder. The material was >96% iron, with a small amount of residual carbon and oxygen present from the solvent and CO ligands. Bimetallic alloy particles have also been prepared in this way. Sonication of FeCCO) and Co(CO)3NO leads to Fe-Co alloy particles [26]. Nanostructured MoS can be synthesized by the sonication of Mo(CO)g with elemental sulphur in 1,2,3,5-tetramethylbenzene under Ar [27]. Metal nitrides are prepared by the sonication of metal carbonyls under a mixture ofNH3andH2atO°C [28]. 

The acidic conditions of standard SBA-15 synthesis [35] cause the precipitation of metal nanoparticles without silica encapsulation, or the formation of amorphous silica due to the presence of the polymer used for nanoparticle synthesis. Therefore, the SBA-15 framework was synthesized under neutral condition using sodium fluoride as a hydrolysis catalyst and tetramethylorthosilicate (TMOS) as the silica precursor. Pt particles with different sizes were dispersed in the aqueous template polymer solution sodium fluoride and TMOS were added to the reaction mixture. The slurry aged at 313 K for a day, followed by an additional day at 373 K. Pt(X)/SBA-15-NE (X = 1.7, 2.9, 3.6, and 7.1nm) catalysts were obtained by ex-situ calcination (see Section 3.2). TEM images of the ordered... 

Among various methods to synthesize nanometer-sized particles [1-3], the liquid-phase reduction method as the novel synthesis method of metallic nanoparticles is one of the easiest procedures, since nanoparticles can be directly obtained from various precursor compounds soluble in a solvent [4], It has been reported that the synthesis of Ni nanoparticles with a diameter from 5 to lOnm and an amorphous-like structure by using this method and the promotion effect of Zn addition to Ni nanoparticles on the catalytic activity for 1-octene hydrogenation [4]. However, unsupported particles were found rather unstable because of its high surface activity to cause tremendous aggregation [5]. In order to solve this problem, their selective deposition onto support particles, such as metal oxides, has been investigated, and also their catalytic activities have been studied. 

Palladium metallic clusters have been prepared at room temperature by sonochemical reduction of Pd(OAc)2 and a surfactant, myristyltrimethylammonium bromide, in THE or MeOH [160[. It is noteworthy that nanosized amorphous Pd is obtained in THE, but in a crystalline form in MeOH. In this solvent, and in higher homologous alcohols, sonolysis of tetrachloropalladate(II) leads to Pd nanoclusters in which carbon atoms, formed by complete decomposition of the solvent, can diffuse. This results in an interstitial solid having the formula PdQ (0 < x < 0.15) [161]. Noble metal nanoparticles of Au, Pd, and Ag are obtained by sonicating aqueous solutions of the corresponding salts in the presence of a surfactant, which largely stabilise the naked col-... 

As a result of CNT synthesis, catalyst metal nanoparticles (iron, cobalt, nickel) together with amorphous carbon and fullerenes are unavoidably present in the CNT soot. 

The best known heterogeneous catalysts are oxide-supported Ru, Rh, and Ni, and Ru exhibits the highest selectivity. Marked support effects are observed and Ti02 is usually found to be the best support material. Pd on zirconia and Ni on zirconia are particularly effective catalysts when prepared using amorphous Pd-Zr, Ni-Zr, and Ni-containing multicomponent alloys by controlled oxidation-reduction treatment240-242 or generated under reaction conditions.243-245 Stabilized metal nanoparticles of uniform dispersion embedded into the oxide matrix are the... 

Among the supports that have been used in the preparation of supported transition metal nanoparticles are carbon, silica, alumina, titanium dioxide, and polymeric supports [57], and the most frequently used support is alumina [56], These supports normally produce an effect on the catalytic activity of the metallic nanoparticles supported on the amorphous material [60], In Chapter 3, different methods for the preparation of metallic catalysts supported on amorphous solids were described [61-71],... 

SWCNTs exhibit unique physical and chemical properties that make them very attractive candidates for the production of new materials. Carbon nanotubes are made by wrapping up single sheets of graphite, known as graphene, upon themselves to form hollow, straw-like structures. Traditionally, SWCNTs have been prepared by electric arc-discharge, laser ablation and chemical vapor deposition (CVD) methods these techniques produce significant quantities of impurities, such as amorphous and graphitic forms of carbon and encapsulated catalytic metal nanoparticles. 

Another example of the stable nonequilibricity of the catalytically active components is the subnormal melting temperature of metal nanoparticles in the course of amorphous carbon graphitization they catalyze (see Figure 4.12). The process follows the stepwise reaction... 

Figure 4.12 Micrograph (A) and a schematic (B) of catalytic graphitization of amorphous carbon in the presence of metal nanoparticles. The top arrow indicates the streamline of the fluidized particle [4]. (Courtesy of K. I. Zamaraev )...

The similar thermodynamic analysis along with the modem experimen tal data argue for the possibility of fluidization of small metal nanoparticles due to the formation of the oversaturated carbon solution not only during the graphitization of amorphous carbon or CO disproportionation but also during pyrolysis of low hydrocarbons by stepwise reaction... 

Xu and coworkers have demonstrated that even non-noble metals can exhibit very high catalytic activity for the AB hydrolysis reaction [119]. They found that with in situ synthesized amorphous Fe nanoparticles, the hydrolysis of AB (0.16 M aqueous AB solution, mol Fe/mol AB = 0.12) is complete within only 8 min. The as-prepared Fe-catalyst was reused up to 20 times with no obvious loss of activity. It was concluded... 

The structure of the as-deposited film composites was investigated using transmission (TEM) and scanning (SEM) electron microscopes. TEM confirmed granular structure of the films with dimensions of metallic nanoparticles of 2-10 nm embedded into amorphous alumina [3-5]. 

Another approach to deposit conducting polymers can be achieved by photochemical polymerization of the monomer precursors. This procedure provides a means by which different composites (metals and/or various alloy materials with or without biomolecules) can be deposited from an electrolyte onto a non-conducting surface. Such a procedure was optimized and applied for polymerization of pyrrole in the presence of metal nanoparticles [61]. Photopolymerized films containing metals analyzed by environmental scanning electron microscopy (SEM) appeared to be typical of amorphous polypyrrole in which bright Ag particles were found on the surface (Fig. 7.6). 

Several strategies were appUed to produce samples for TEM and kinetic studies [8, 21], but only one route is presented here (Fig. 15.3). Noble metal nanoparticles were grown via metal evaporation on a crystalline soluble substrate (e.g., NaCl(OOl)), leading to an epitaxial growth of particles with regular shape and well-developed low-Miller index facets (Fig. 15.3). Thereafter, the metal particles were embedded in a thin (25 nm) amorphous oxide fdm, before the metal-oxide system was lifted off the substrate via flotation in water [8, 18, 20, 31]. 

A different approach was taken by Kumar and associates [61]. Fie also embedded metals in polymers, but used as his precursor the polymer and not the monomer. In his first study a composite material containing amorphous Cu nanoparticles and nanocrystalline CU2O embedded in polyanUine matrices was prepared by a sonochemical method. These composite materials were obtained from the soni-cation of copper (II) acetate when aniline or 1% v/v aniline-water was used as the solvent. Mechanisms for the formation of these products are proposed and discussed. The physical and thermal properties of the as-prepared composite materials are presented. A band gap of 2.61 eV is estimated from optical measurements for the as-prepared CU2O in polyaniline. 

Whereas the majority of experimental works has been focused on silica-, glass-or alumina-embedded noble metal nanoparticles, or aqueous colloidal solutions, a few ones have dealt with other kinds of matrices, either amorphous (BaO [177], BaTiOj [164, 167], Bi.()., [178], Nb.O, [179], TiO. [180, 181], ZrO. .. [167]) or crystalline (BaTiOj [164, 182, 183], BiT) [184], LiNbOj [185], SrTiOj [172], ZnO... [186]). A direct comparison of the nonlinear properties from one matrix to another is difficult to carry out, since all other parameters should be kept constant while tuning the wavelength as to match the SPR maximum. 

Holzinger, D. and Kickelbick, G. (2003) Preparation of amorphous metal-oxide-core polymer-shell nanoparticles via a microemulsion-based sol-gel approach. Chem. Mater., 15, 4944— 4948. 

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