A new allotropic structure of silver nanocrystals nucleated and grown over planar polymer molecules

Abstract

Polymer molecules of polyvinyl alcohol (PVA) serve as a good Ag⁺ → Ag⁰ reducer as well as a template to produce Ag⁰ nanocrystals of anisotropic shapes. Thin Ag–PVA films (50–100 µm thick) were prepared by carrying out the reaction in aqueous PVA (under heating conditions) and then spin casting the sample in this shape. A topotactic Ag⁺ → Ag⁰ reaction and in-situ Ag⁰ growth occur in support over the PVA molecules (as per the local structure and Ag⁰–PVA interactions) of elongated surfaces. The Ag⁰ state is confirmed with 3d _5/2 and 3d _3/2 XPS signals of 368.37 and 374.13 eV binding energies, respectively. As analyzed with X-ray diffraction, it is a new allotrope with an orthorhombic structure, lattice parameters a = 0.8535, b = 0.8410 nm and c = 0.4119 nm, and density ρ = 9.69 g cm⁻³. On heating in air, the well-known cubic structure develops, a = 0.4082 nm (ρ = 10.53 g cm⁻³) after 150°C, whereas a = 0.4071 nm (ρ = 10.61 g cm⁻³) after 300°C for 2 h. According to proposed models, it is a vacancy-stabilized allotrope, which evolves from multiply twinned particles.

Acknowledgement

This work has been supported (SR) in part by the AvH foundation, Germany.