Evidence for truncated octahedral structures in supported gold clusters

Extended x-ray absorption fine structure studies of the atomic structure of nanoparticles in different metallic matrices

“…These studies showed that the fcc structure of bulk Au was retained in the clusters, but with a contraction in the nearest neighbour distance of 0.05 -0.1 Å for Au 55 and up to 0.2 Å for Au 11 . This effect has also been observed in larger Au clusters prepared by wet chemical techniques, with diameters between 3 nm and 7 nm [55], as well as in vacuum deposited Au clusters, with diameters in the range 1 -6 nm [52,56]. Contractions of 0.1 Å in nearest neighbour distance have also been found in silica-supported (fcc) Pt clusters, at least for bare clusters [57]; coverage of the clusters by chemisorbed hydrogen, however, causes the nearest neighbour distance to revert to the bulk Pt value.…”

“…Such analysis, for example, is well described for supported Pt [48] and Au [52] particles, although in the case of the Au particles the fitted nearest neighbour distance was the same whether or not the third cumulant was used in the analysis. In our own work on embedded metal clusters, we have generally found that including a third cumulant into the EXAFS analysis does not alter the interatomic distance or improve the fit significantly.…”

Surface chemistry of catalysis by gold

“…Recently, in our own analysis involving copper and cobalt particles (343), we have obtained evidence that lattice contraction in very small particles can induce positive binding shifts due to hybridization as predicted by Mason (333). This situation is of particular importance to gold which has a strong tendency to undergo contraction in the surface layer (206,(344)(345)(346)(347)(348), hence the reconstructions of Au (111) and Au (100).…”

Nanocatalysis: hypothesis on the action mechanism of gold

“…6) Changes in some properties of Au clusters are thought to vary as the function of their sizes; which are assumed to be important in the nanocatalysis problem: a) the structural phase transition from FCC (bulk material) to the dodecahedral cluster (at R=25Å) and icosahedral cluster (at R=16Å) [38] is discussed; there are data that the latter transition is observed at a much larger size (not at 100 atoms, but at 550 atoms in the cluster) [39]. Note that the transition to the icosahedron was experimentally observed in Y-zeolites at R=10Å [41] and in 50-atomic Pt clusters on carbon [40]; b) there was a decrease in the bond lengths of Cr, Fe, Cu, Ag, Pd, Au, Pt metals at the size R<30Å [42][43][44][45][46][47][48][49][50][51][52][53]; so, at R=30Å the length of Au-Au bond is d=2.84Å, and at R=8Å it is d=2.72Å, whereas the bulk material has d=2.88Å [48,49,51]. c) such shortening of the bond lengths is characteristic for metal dimers (Fe, Mg) [54]; reduction of the Au-Au bond length also occurs in clusters; d) there was a reduction of the bond lengths in the metal particles on the substrates, like carbon, which are weakly interacting with the metal particles [52,55,56]; e) there are changes in other properties of metal clusters (electronic, magnetic, chemical) at the scale of few nanometers [57,64]; the most important of which is the modification of the electronic spectrum, namely, the displacement of d-band of Cu in its atomic spectrum with decreasing particle size [60]; the same effect was also observed for Pt and Pd [59].…”