Distorted f.c.c. arrangement of gold nanoclusters: a model of spherical particles with microstrains and stacking faults

The structures of two samples of gold nanoclusters supported on silica were studied by X-ray powder diffraction (XRD) and X-ray absorption spectroscopy. The data relative to both techniques were analysed by an approach involving simulation based on structural models and fitting. The XRD model concerned a distorted f.c.c. (face-centred cubic) arrangement, with microstrains and parallel stacking faults in approximately spherical particles; as an alternative possibility, a linear combination of ordered f.c.c. and noncrystalline (decahedral and icosahedral) particles was also taken into account. Both approaches gave calculated patterns closely resembling the experimental data. X-ray absorption spectra were fitted on the basis of f.c.c. and noncrystalline arrangements. The best results were obtained by the f.c.c. motif, while a simulation consisting in the superposition of f.c.c. and noncrystalline components in the relative amounts determined by XRD analysis gave a poor agreement with the experimental data. It was concluded that the good XRD fitting obtained by linear combination of lognormal size-distributed f.c.c. cuboctahedral, decahedral and icosahedral contributions was a result of the flexibility of the basis set of functions, but that the complementary analysis of X-ray absorption data did not confirm the presence of a noteworthy fraction of noncrystalline particles.