Rapid transformation from spherical nanoparticles, nanorods, cubes, or bipyramids to triangular prisms of silver with PVP, citrate, and H _2O _2

摘要

Rapid sphere-to-prism (STP) transformation of silver was studied in aqueous AgNO 3/NaBH 4/polyvinylpyrrolidone (PVP)/trisodium citrate (Na _3CA)/H _2O _2 solutions by monitoring time-dependent surface plasmon resonance (SPR) bands in the UV-vis region, by examining transmission electron microscopic (TEM) images, and by analyzing emitted gases during fast reaction. Roles of PVP, Na _3CA, and H _2O _2 were studied without addition of a reagent, with different timing of each reagent's addition, and with addition of H _2O _2 to mixtures of spheres and prisms. Results show that prisms can be prepared without addition of PVP, although it is useful to synthesize smaller monodispersed prisms. A new important role of citrate found in this study, besides a known role as a protecting agent of {111} facets of plates, is an assistive agent for shape-selective oxidative etching of Ag nanoparticles by H _2O _2. The covering of Ag nanoparticles with carboxylate groups is necessary to initiate rapid STP transformation by premixing citrate before H _2O _2 addition. Based on our data, rapid prism formation starts from the consumption of spherical Ag particles because of shape-selective oxidative etching by H _2O _2. Oxidative etching of spherical particles by H _2O _2 is faster than that of prisms. Therefore, spherical particles are selectively etched and dissolved, leaving only seeds of prisms to grow into triangular prisms. When pentagonal Ag nanorods and a mixture of cubes and bipyramids were used as sources of prisms, rod-to-prism (RTP), cube-to-prism (CTP), and bipyramid-to-prism (BTP) transformations were observed in Ag nanocrystals/NaBH _4/PVP/Na _3CA/H _2O _2 solutions. Shape-selective oxidative etching of rods was confirmed using flag-type Ag nanostructures consisting of a triangular plate and a side rod. These data provide useful information for the size-controlled synthesis of triangular Ag prisms, from various Ag nanostructures and using a chemical reduction method, having surface plasmon resonance (SPR) bands at a desired wavelength.