Polyhedral to nearly spherical morphology transformation of silver microcrystals grown from vapor phase

Silver microcrystals of various shapes were grown from co-evaporation of Ag2O and SiO powders in the nominal evaporating temperature range (heat source temperature) from 800 to 1200°C under a reductive atmosphere. Pathways for the polyhedral to nearly spherical morphology transformation were established. At evaporating temperatures below 900°C, the polyhedral crystals are generally bounded by {111}-facets and {100}-facets. With increase in evaporation temperature facets of the {110}-, {210}- and {310}-families come forth successively, that make the Ag polyhedron microcrystals transform into nearly spherical microcrystals. A mechanism based on the recession of {100} or {110} steps induced by the solvent SiOx (x∼2.0), which modifies the growth kinetics, is proposed to explain the emergence of {210}- and {310}-facets at high temperatures. By evaporating at 1200°C, smooth-looking spherical Ag crystals with 6nm multi-layered steps on the {111} facets were obtained. These results are also of significant industry relevance when preparation of noble metal particles with high-index facets is of concern.