Simultaneous Reduction of Metal Ions by Multiple Reducing Agents Initiates the Asymmetric Growth of Metallic Nanocrystals

摘要

Thermodynamically unfavorable metallic nanocrystals can be prepared only by the growth of the nanocrystals under kinetically controlled experimental conditions. The common technique to drive the growth of metallic nanocrystals under kinetic control is to adjust the rate of the generation of metal atoms to be slower than the rate of deposition of such atoms onto the surface of nano crystal nuclei, which form in the first step of the nanoparticle synthesis. The kinetically controlled growth leads to the formation of seeds with crystal defects, which are needed for the growth of anisotropic nanocrystals such as silver nanodisks (AgNDs). The simultaneous multiple asymmetric reduction technique (SMART) is introduced here to successfully prepare AgNDs of controllable sizes and on a large scale within a few seconds. The SMART is simply based on the simultaneons reduction of silver ions with a strong reducing agent such as borohydride (redox potential of 1.24 V) and a weak reducing agent such as L-ascorbic acid (redox potential of 0.35 V) in the presence of a polyvinylpyrrolidone capping agent. The random formation and deposition of silver atoms by the two different reducing agents generated stacking faults in the growing nanocrystal. The hexagonal close-packed {111} layers of silver atoms were then deposited on the surface of the growing nanocrystal containing stacked faults along the [111] plane. This initiated asymmetric growth necessary for the formation of platelike seeds with planar twin defects, which is required for the formation of anisotropic AgNDs.