Shape Evolution of Flag Types of Silver Nanostructures from Nanorod Seeds in PVP-Assisted DMF Solution

Shape evolution of novel flag types of silver nanostructures has been studied using a two-step reduction method. In the first step, pentagonal Ag nanorods were prepared by polyol reduction of AgNO3 in ethylene glycol (EG) in the presence of polyvinylpyrrolidone (PVP). In the second step, AgNO3 was reduced in N,N-dimethylformamide (DMF) in the presence of Ag nanorod-seeds and PVP. Then, trapezoid and triangular platelike structures having {111} facets were evolved from long side {100} facets of Ag nanorods. At higher AgNO3 concentrations, tetrahedral flags having {111} facets were evolved from equilateral triangular plates. The crystal structures of flag types of Ag nanostructures are determined, and their growth mechanisms are discussed on the basis of TEM, SEM, SAED, and TEM-EDS data. It was found that tetrahedral flags were grown either through the formation of two equilateral triangular plates connected on a nanorod or through the crystal growth of triangular pyramidal structures over triangular plates. The present results demonstrate that Ag nanorods can be used as new seeds for the unique flag types of Ag nanostructures.