Au. nanoparticles

Hornyak G L and Martin C R 1997 Optical properties of a family of Au-nanoparticle containing alumina... 

The cospreading of hexanethiol-capped gold particles with the cross-linking agent 4,4 -thiobisbenzenethiol (TBBT) has been reported as a means of producing cross-linked monolayers of hexanethiol-capped Au nanoparticles [125]. In the presence of TBBT, the... 

Herein we briefly mention historical aspects on preparation of monometallic or bimetallic nanoparticles as science. In 1857, Faraday prepared dispersion solution of Au colloids by chemical reduction of aqueous solution of Au(III) ions with phosphorous [6]. One hundred and thirty-one years later, in 1988, Thomas confirmed that the colloids were composed of Au nanoparticles with 3-30 nm in particle size by means of electron microscope [7]. In 1941, Rampino and Nord prepared colloidal dispersion of Pd by reduction with hydrogen, protected the colloids by addition of synthetic pol5mer like polyvinylalcohol, applied to the catalysts for the first time [8-10]. In 1951, Turkevich et al. [11] reported an important paper on preparation method of Au nanoparticles. They prepared aqueous dispersions of Au nanoparticles by reducing Au(III) with phosphorous or carbon monoxide (CO), and characterized the nanoparticles by electron microscopy. They also prepared Au nanoparticles with quite narrow... 

Our first attempt of a successive reduction method was utilized to PVP-protected Au/Pd bimetallic nanoparticles [125]. An alcohol reduction of Pd ions in the presence of Au nanoparticles did not provide the bimetallic nanoparticles but the mixtures of distinct Au and Pd monometallic nanoparticles, while an alcohol reduction of Au ions in the presence of Pd nanoparticles can provide AuPd bimetallic nanoparticles. Unexpectedly, these bimetallic nanoparticles did not have a core/shell structure, which was obtained from a simultaneous reduction of the corresponding two metal ions. This difference in the structure may be derived from the redox potentials of Pd and Au ions. When Au ions are added in the solution of enough small Pd nanoparticles, some Pd atoms on the particles reduce the Au ions to Au atoms. The oxidized Pd ions are then reduced again by an alcohol to deposit on the particles. This process may form with the particles a cluster-in-cluster structure, and does not produce Pd-core/ Au-shell bimetallic nanoparticles. On the other hand, the formation of PVP-protected Pd-core/Ni-shell bimetallic nanoparticles proceeded by a successive alcohol reduction [126]. 

Peng et al. [150] prepared AgAu nanoalloys via three different procedures by using laser-induced heating (i) mixture of Au nanoparticles and Ag(I) ions irradiated by a 532 nm laser, (ii) mixture of Au and Ag nanoparticles irradiated by a 532 nm laser, and (iii) mixture of Au and Ag nanoparticles irradiated by a 355 nm laser. In procedures (ii), nanoalloys with a sintered structure were obtained. The morphology of the obtained nanoalloys depended not only on the laser wavelength but also on the concentration of nanoparticles in the initial mixture. Large-scale interlinked networks were observed upon laser irradiation when the total concentration of Ag and Au nanoparticles in the mixture increased. 

It is noteworthy that the HRTEM cannot distinguish core and shell even by combining X-ray or electron diffraction techniques for some small nanoparticles. If the shell epitaxially grows on the core in the case of two kinds of metals with same crystal type and little difference of lattice constant, the precise structure of the bimetallic nanoparticles cannot be well characterized by the present technique. Hodak et al. [153] investigated Au-core/Ag-shell or Ag-core/Au-shell bimetallic nanoparticles. They confirmed that Au shell forms on Ag core by the epitaxial growth. In the TEM observations, the core/shell structures of Ag/Au nanoparticles are not clear even in the HRTEM images in this case (Figure 7). 

The environment (e.g. the substrate) of the nanoparticles is a critical experimental parameter, which should be inert with respect to the nanoparticles. In the case of gold the native Si02 covered Si(l 0 0) seems to be an environment without any influence on the valence band of Au nanoparticles. The chemical and catalytic properties which are probably strongly correlated with the electronic structures of different systems, give another possibility to use and check the size dependent properties of nanoparticles. 

Molecular-dynamics simulations also showed that spherical gold clusters is stable in the form of FCC crystal structure in a size range of = 13-555 [191]. This is more likely a key factor in developing extremely high catalytic activity on reducible Ti02 as a support material. Thus, it controls the electronic structure of Au nanoparticles (e.g. band gap and BE shift of Au 4f7/2 band) and thereby the catalytic activity. 

We infer the importance of the FeO c/Au interface which occurs for films as well as nanoparticles. However, since we observe FeO c/Au nanoparticles/Si02/Si(l 0 0) to be the most active of all samples we advance the hypothesis of the occurrence of a strong electronic effect at the FeOJ nanoparticle interface coupling through the FeO layer thus producing the high catalytic activity. Au promotion was also experienced in the catalytic activity of Ti02 overlayers [207]. 

Figure 17. SEM image of single 20 nm and 30 nm-diameter Au nanoparticles asembled from solution and bridging the two adjacent nanoelectrode junctions. (Reprinted with permission from Ref [32], 2005, Wiley-VCH.)...

Figure 14. Au nanoparticle on polyaniline support with twin boundary (F. Klasovsky, P. Claus, unpublished results, 2006).

In both cases, the Au nanoparticles behave as molecular crystals in respect that they can be dissolved, precipitated, and redispersed in solvents without change in properties. The first method is based on a reduction process carried out in an inverse micelle system. The second synthetic route involves vaporization of a metal under vacuum and co-deposition of the atoms with the vapors of a solvent on the walls of a reactor cooled to liquid nitrogen temperature (77 K). Nucleation and growth of the nanoparticles take place during the warm-up stage. This procedure is known as the solvated metal atom dispersion (SMAD) method. 

First, polyhedral-shaped Au nanoparticles (Figure 6A) are synthesized by sodium borohydride reduction of AUCI3 dissolved in toluene in the presence of DDAB (step 1). 

Figure 5. Synthetic steps for preparation of monodispersed Au nanoparticles by the inverse micelle method and digestive ripening.

Figure 8. TEM micrographs representing the Au nanoparticles at different stages of the synthetic process (SMAD system). (Reprinted with permission from Ref [29], 2002, American Chemical Society.)...

Figure 9. TEM micrographs of nanocrystal superlattices of Au nanoparticles prepared by the inverse micelle method and digestive ripening, (a) and (b) low-magnification images (c (f) regularly-shaped nanocrystal superlattices (g) magnified image of a superlattice edge. Note the perfect arrangement of the Au nanoparticles. (Reprinted with permission from Ref. [30], 2003, American Chemical Society.)...

Figure 24. SEM micrograph of siloxane nanowires obtained by the reflux of octadecylsilane with Au nanoparticles in presence of water.

To study the nucleation and growth of Au nanoclusters in silica within the above theoretical frame, we implanted fused silica slides with 190keV-energy Au ions, at room temperature and current densities lower than 2 pA/cm, to reduce sample heating [49,50]. The implantation conditions were chosen to have, after annealing, a subsurface buried layer of Au nanoparticle precipitation of about... 

In 2001, we developed a simple and quite useful method to manipulate the size of Au nanoparticles by using the heat-treatment of small Au nanoparticles [9,10], which is far from the conventional techniques. The 1-dodecanethiol-protected Au nanoparticles (C12S-AU) of 1.5 0.2 nm in size synthesized by the Brust s two-phase (toluene/water) reaction procedure [3] were heat treated at 150-250 °C at the heating rate of 2°Cmin and held for 30min. This heat treatment of as-synthesized C12S-AU nanoparticles... 

Figure 2. TEM images of C12S-AU nanoparticles after the heat treatment of 1.5nm C12S-AU nanoparticles at (a) 150, (b) 190, and (c) 230 °C and (d) CigS-Au nanoparticles heat treated at 250 °C. (Reprinted from Ref. [10], 2003, American Chemical Society.)...

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