Gold nanocrystals

Krug J T II, Wang G D, Emory S R and Nie S 1999 Efficient Raman enhancement and intermittent light emission observed in single gold nanocrystals J. Am. Chem. Soc. 121 9208-14... 

Figure C2.17.2. Transmission electron micrograph of a gold nanoneedle. Inverse micelle environments allow for a great deal of control not only over particle size, but also particle shape. In this example, gold nanocrystals were prepared using a photolytic method in surfactant-rich solutions the surfactant interacts strongly with areas of low curvature, thus continued growth can occur only at the sharjD tips of nanocrystals, leading to the fonnation of high-aspect-ratio nanostmctures [52].

An equally important challenge for nanocrystal assembly is the fonnation of specific nanocrystal arrangements in solution. By using complementary DNA strands as tethers, Mirkin et al [102, 103] fonned aggregates of gold nanocrystals with specific sizes Alivisatos et al also used DNA to stmcture semiconductor nanocrystal molecules, though in this case the molecules contained only a few nanocrystals placed controlled distances from each other [104, 105 and 106]. The potential applications of biomolecular teclmiques to this area of nanoscience are immense, and the opportunities have been reviewed in several recent publications [107, 108, 109 and 110]. 

Figure C2.17.5. Transmission electron micrograph of a field of anisotropic gold nanocrystals. In tliis example, a lower magnification image of gold nanocrystals reveals tlieir anisotropic shapes and faceted surfaces [36],...

Figure C2.17.13. A model calculation of the optical absorjDtion of gold nanocrystals. The fonnalism outlined in the text is used to calculate the absorjDtion cross section of bulk gold (solid curve) and of gold nanoparticles of 3 mn (long dashes), 2 mn (short dashes) and 1 mn (dots) radius. The bulk dielectric properties are obtained from a cubic spline fit to the data of [237]. The small blue shift and substantial broadening which result from the mean free path limitation are...

S. Link and M. A. El-Sayed, Shape and size dependence of radiative, non-radiative and photothermal properties of gold nanocrystals. Int. Rev. Phy. Chem. 19(3), 409-453 (2000). 

Cleveland, C.L., Landman, U., Schaaff, T.G., Shafigullin, M.N., Stephens, P.W. and Whetten, R.L. (1997) Structural Evolution of Smaller Gold Nanocrystals The Truncated Decahedral Motif Physical Review Letters, 79, 1873-1876. 

A similar route has been followed most recently by Weiss et al. who have fabricated a multi-island SE device from self-assembled ID gold nanocrystal chains [28]. They assembled thiol-stabilized 50 nm Au particles in a chainlike structure and used subsequently electron-beam lithography for electrode fabrication. Here it has to be... 

Arnold, R. J. Reilly, J. P. High resolution time of flight mass spectra of alkylthio-late-coated gold nanocrystals. J. Am. Chem. Soc. 1998,120,1528-1532. 

Saunders, A.E. and Korgel, B.A. (2004) Second virial coefficient measurements of dilute gold nanocrystal dispersions using small-angle x-ray scattering. Journal of Physical Chemistry B, 108 (43), 16732-16738. 

On the other hand, the inclusion of electrocatalytically active metal nanocrystals (graphite-supported palladium, platinum, and ruthenium or gold nanocrystals) allowed... 

Maye et al. studied gold nanoparticles supported on carbon black for ORR in both acidic and alkaline media.210 The gold nanoparticles were of a core-shell type where the particle consisted of a gold nanocrystal core of 1-6 nm in diameter that was surrounded by an organic monolayer shell.214 While the Au/C catalyst was found to be active for ORR, its activity was much lower than that of Pt/C, PtRu/C and AuPt/C. The electron transfer in 0.5 M H2S04 was reported as 2.9 0.2, indicating a mixed reduction pathway.210... 

Negishi Y, Nobusada K, Tsukuda T (2005) Glutathione-protected gold clusters revisited bridging the gap between gold(I)-thiolate complexes and thiolate-protected gold nanocrystals. J Am Chem Soc 127 5261-5270... 

Duan H, Nie S (2007) Etching colloidal gold nanocrystals with hyperbranched and multivalent polymers a new route to fluorescent and water-soluble atomic clusters. J Am Chem Soc 129 2412-2413... 

Also in 1980s Haruta recognized that supported gold nanocrystals can be highly effective catalysts for the oxidation of CO at very low temperatures (Fig. 4.6) and in particular at temperatures below 0°C. This is a surprisingly high activity and is not replicated by other metals. Haruta recognized that the method of preparation... 

In a recent review, Haruta has reassessed the mechanism of CO oxidation on the basis of the Bond and Thompson and Kung models, i.e. that the CO is activated by adsorption on Au° on the surface of the gold nanoparticles and that dioxygen is activated by the atoms at the periphery between the support and the gold nanocrystals (Fig. 4.7). The atoms at the periphery are proposed by Haruta to be cationic in nature, possibly Au(OH)s or Au(OH) formed by the presence of water vapor that is essential for the observed high activity catalysts. It is clear that the debate will continue for the immediate future. There are two reasons why finding an answer to the key question of the nature of the active site in gold catalysts for CO oxidation. The first is purely scientific... 

Bi, Y.P. Lu, G.X. 2008. Morphological controlled synthesis and catalytic activities of gold nanocrystals. Materials Letters, 62, 2696-2699. 

Zehner RW, Lopes WA, Morkved TL, Jaeger H, Sita LR. Selective decoration of a phase-separated diblock copolymer with thiol-passivated gold nanocrystals. Langmuir 1998 14 241-244. 

Huang MH, Kuo C-H (2005) Synthesis of Branched Gold Nanocrystals by a Seeding Growth Approach. Langmuir 21 2012-2016... 

Hupp JT, Schatz GC, Hao E, Bailey RC, Li S (2003) Synthesis and Optical Properties of Branched Gold Nanocrystals. Nano Lett 4 327-330... 

Scheme 3.16 Platonic gold nanocrystals of tetrahedral, cubic and icosahedral shape.

Large-scale icosahedral gold nanocrystals with good uniformity can be obtained in a one-step thermal process using PVP and HAuC14 as reactants in a reflux bath at 100 °C. Observation by shows that most of the nanoparticles have a projected quasi-spheroid shape of 220 40nm in size [104] (Figure 3.10). 

Figure 3.10 Icosahedral gold nanocrystals obtained from and PVP. Reprinted with permission from reference [104]. Copyright 2006 American Chemical Society.

Besides gold nanocrystals of spherical shape, elongated structures such as nanorods are the second most studied ones. Their first synthesis was reported in 2001233 and since then this synthetic procedure has been widely studied and improved. Nonetheless, the influence of various parameters such as the presence of seeds, the nature of the solvent, and the presence of surfactants on the reaction yield and the control of shape make the reproducibility of this synthesis still a challenging task.234-237... 

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