Surface Ostwald-ripening and evaporation of gold beaded films on sapphire

The kinetics of morphological evolutions of gold nanoparticles on alumina, resulting from evaporation and surface Ostwald-ripening coarsening, have been investigated by means of Auger electron spectroscopy. When the fraction of the covered area is small, the kinetics of evaporation can be related to the desorption of adatoms. In the temperature range 943–1043 K we obtained the evaporation flux J(m^-2s^-1)=4.8×10²⁷exp-196±9 (kJ/mol)/RT]. The experimental activation energy of evaporation of gold from a sapphire surface, Q_evap=196±9 kJ/mol, is lower than the tabulated value of enthalpy of sublimation of gold, ΔH_subl=368 kJ/mol. At lower temperatures, in the range 623–778 K, Ostwald-ripening experiments, carried out on nanosized clusters, yield the mass transfer surface diffusion coefficients of gold on alumina, D_s(m²/s)=2.6×10^-14exp-58±9 (kJ/mol)/RT]. These results, providing information on the evolution of granular gold films such as those used in catalysts or sensors, are compared to previous data on similar systems. PACS 68.47.Jn; 68.43.Jk; 68.55.-a