Site-selective overgrowth of Ag nanocrystals by Polyvinylpyrrolidone-mediated atom deposition by Ostwald ripening

Abstract

The growth modulation of metal nanocrystals (NCs) by Ostwald ripening (OR) involves control of the relocation of matter by diffusional mass transfer from the dissolution of small nanocrystals (SNCs) towards large nanocrystals whose surface energy is lower. A partial control of the crystallinity by oxidative etching allows a balanced bath of Polyvinylpyrrolidone (PVP) protective Ag-nanocubes and right-bipyramids as models of crystal growth. PVP controls the growth pathways via the interplay of two mechanisms brought about by the dual role of PVP as mild reductant and {100}-facets-blocking-ligand. The growth patterns depend on the reduction kinetics of silver (Vdep) which increases with PVP concentration and thermodynamically by lowering the surface free energy (γ) of the Ag(100)-surfaces. Low PVP concentration results in a thermodynamically-controlled conformal growth of the {100}-crystal-facets as the adatoms migrate to the facets of lowest γ. At higher PVP concentrations Vdep is higher and a site-selective growth is kinetically performed by the deposition of Ag atoms and partially dissolved SNCs (pdSNCs) on the corner/edges. Asymmetric growth patterns are induced when a different number of pdSNCs were deposited. The atom supply during the OR was nonhomogeneous and slower than the Vdep of the Ag salt precursor, hence promoting crystal symmetry breakings.