Facile growth of silver crystals with greatly varied morphologies by PEO-PPO-PEO tri-block copolymers

Abstract

Silver crystals with various novel nano-scale morphologies were synthesized from the reduction of silver nitrate (AgNO₃) by ascorbic acid (AsA) in aqueous PEO-PPO-PEO tri-block copolymer (F127) solutions. The presence of only high-concentration AsA at the very initial stage of reduction acts as a key factor in forming distinct branched quasi-spherical Ag nanocrystals where coral-like and houseleek-like morphologies can be kinetically controlled via subsequent addition of F127. When AsA and F127 were mixed before the reduction, three-dimensional dendritic Ag nanocrystals with highly ordered dendrites were largely obtained. In contrast to these branched crystals prepared under low F127 concentration, the face-centered cubic (FCC) packed micelles at high F127 concentration, which provided orderly reaction spaces for AgNO₃ and AsA reagents, dominated the growth of triangular and hexagonal prisms. The great variety in morphology is directly correlated with not only the concentration ratio between these reactants but the F127 induced kinetic mechanisms. Moreover, the greatly branched Ag nanocrystals exhibit significant surface-enhanced Raman scattering (SERS) for facilely, rapidly and effectively determining malachite green oxalate (MG) in aqueous solution. The present results provide valuable data not only for fundamental understanding of the directing effect of both F127 and AsA on the synthesis of Ag nanocrystals, but for advanced design and practical manufacturing of metals enhanced SERS sensors.