(472b) Science of Shape-Controlled Synthesis of Metallic Nanoparticles

The hydrophilic polymer, polyvinylpyrrolidone (PVP) is ubiquitously used in the synthesis of shape-controlled colloidal transition metal (Ag, Au, Pd, Pt, etc.) nanoparticles (NPs). Nearly all hypotheses regarding the role of PVP rely on observations from parametric studies of the impact of PVP-related variables on the final shape of the as-synthesized nanoparticles. The dominant hypothesis in the literature is the preferential binding of PVP to Ag(100) facets compared to Ag(111) facets; however, there is no unified agreement on the exact shape control mechanism in Ag NPs growth as both Ag cubes and octahedron are synthesized in the presence of PVP. We experimentally measured adsorption isotherms of PVP on different-shaped Ag NPs to determine the thermodynamics of PVP adsorption to Ag(100) and Ag(111) surfaces and found the preferential binding of PVP is not responsible for thermodynamic shape control in Ag nanoparticles synthesis.

Rather, the addition of Cl^- during synthesis is critical for for the formation of Ag nanocubes. In the absence of Cl-, truncated octahedron are formed in a polyol synthesis, which has been confirmed by recent theoretical investigastions of Wulff constructions on PVP-covered Ag surfaces. We examined the role of Cl- in the synthesis of Ag nanocubes utilizing various ex-situ characterization techniques such as scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray photoemission spectroscopy (XPS) to probe the growth mechanisms of Ag nanocubes. The initially formed nanostructures during the early stages of synthesis were AgCl nanocubes which were subsequently etched by incoming Ag⁺ flux to form metallic Ag nanocubes containing a half monolayer of Cl^- on the surface. Variation of Cl^- concentration alone at constant H⁺ and PVP concentration led to modification of the nanoparticle shape. As the concentration of Cl^- decreased, truncated cubes, cuboctahedron and truncated octahedron formed, which confirms the importance of Cl^- for Ag NPs shape control.

A number of syntheses exploit the observation that the addition of foreign metal ions influences the shape of the final nanostructure. We focused on the ability of Cu2+ to impact the transformation of Ag cubes to octahedron. The conversion is initiated using Ag cube as seeds in the presence of Cu²⁺ to convert cube to a final octahedron product. The evolution of shape from truncated cube, cuboctahedron, truncated octahedron and octahedron was captured by ex-situ microscopy analysis. The existence of Cu²⁺ on the Ag octahedron surface was confirmed by x-ray photoemission spectroscopy (XPS) and the bulk content by inductively coupled plasmon optical emission spectroscopy. Control experiments demonstrate shape transformation only occurs with the addition of Cu²⁺. Substitution of Cu²⁺ with other metal ions (Ni²⁺ and Co²⁺) with a lower reduction potential than Ag⁺ led to a similar transformation of Ag cubes to octahedron. We hypothesize foreign metal ions adsorb to the (111) facets on the truncated cubes by under potential deposition (UPD), which favors the deposition of incoming Ag on the (100) facets. Shape control can be achieved by the addition of foreign ions during synthesis; the mechanism of the shape control is complex. To the contrary, it does appear organic additives, such as PVP are responsible for shape control in Ag nanocube synthesis. The origin of shape control may be more kinetic in origin than thermodynamic in nature, a concept that has recently infiltrated crystallization theory.