Transition from five-fold symmetric to twinned FCC gold particles by thermally induced growth

Stable gold clusters around 1 nm in size have been prepared by evaporation of the metal into an organosilicon polymer solution. A controlled particle growth of the entrapped gold particles was achieved by annealing in helium at similar to 410 degrees C. Above 460 degrees C the growth rate is dramati cally increased, concurrent with the onset of pyrolysis of polysilazane to form a porous solid. X-ray diffraction and simulation calculations using De bye functions of model clusters of different sizes and both crystallographi c and noncrystallographic symmetry were used to characterize these particle s. The exclusive presence of noncrystallographic (decahedral and icosahedra l) multiply twinned particles (MTPs) in the as-prepared material is confirm ed by EXAFS. The high precision of the XRD experiments and theoretical data fits gives detailed insight into the thermally induced growth process, dur ing which the MTPs progressively disappear and are replaced by singly twinn ed fee particles of larger Size. This transition occurs in the range simila r to 2-3 nm. This analysis requires the introduction of a size-dependent co ntraction for the Au-Au distance in the order of 1-3% compared to bulk gold . This is discussed in the context of theoretical predictions.