Gold particles, icosahedral

The modern methods of high resolution can be applied to the study of small metallic particles. The most usefull technique is perhaps the projected potential images (1/7). Fig. 15 shows an image of a gold particle with icosahedral shape. Atomic resolution along the ill planes is observed. The continuity of the planes along the boundary is clearly seen. 

Very small gold particles can also be formed on magnesia and brucite (Mg(OH)2).55,83 Au/MgO made by deposition-precipitation contained particles smaller than 1 nm that were claimed to show icosahedral and fee cubo-octahedral structures,170 but this is hard to believe as the diameter of a single gold atom is already 0.29 nm, and lnm particles afflxed at the steps... 

Figure 24. Debye function analysis done on a colloid of 2-nm gold particles. Solid line is the fit using icosahedral and dodecahedral nanoparticles. Dotted vertical lines indicate FCC stracture peak positions. Essentially no FCC or cub-octahedral particles are present. The bottom plot shows the difference between calculation and observation. From Zanchet et al. (2000), used with permission of Wiley-VCH.

Mackay [9-73, 9-74] called attention to yet another limitation of the 230-space-group system. It covers only those helices that are compatible with the three-dimensional lattices. All other helices that are finite in one or two dimensions are excluded. Some important virus structures with icosahedral symmetry are among them. Also, there are very small particles of gold that do not have the usual face-centered cubic lattice of gold. They are actually icosahedral shells. The most stable configurations contain 55 or 147 atoms of gold. However, icosahedral symmetry is not treated in the International Tables, and crystals are only defined for infinite repetition. 

An alternative method of preparing cobalt nanopartides is based on the thermal decomposition of Co2(CO)g in the presence of a suitable surfactant [27]. A typical recipe involves the injection of a solution of Co2(CO)g in diphenylether (solution A) into the hot (200 °C) mixture of oleic acid and TOP or TBP, dissolved also in diphenylether (solution B). The mixture is then heated at 200 °C for 15-20 min. Decomposition of the cobalt carbonyl, under the conditions described above, results in Co nanopartides with a so-called multiply-twinned face-centered cubic (mt-fee) lattice (Figures 3.115b and (3.116)d). The multiply-twinned particles are composed of domains with a distorted fee lattice, this structure being similar to that observed in multiply-twinned, icosahedral gold particles [27]. Post-preparative size-selection... 

Zhang, C., Zhang, X, Han, B., Zhao,Y. and Li, W. (2008). Synthesis of icosahedral gold particles by a simple and mild route. Green Chemistry, 10,1094-1098. 

---