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

摘要

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 3d5/2 and 3d3/2 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℃, whereas a = 0.4071 nm (ρ = 10.61 g cm-3) after 300℃ for 2 h. According to proposed models, it is a vacancy-stabilized allotrope, which evolves from multiply twinned particles.