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

Polymer molecules of polyvinyl alcohol (PVA) serve as a good Ag+ -> Ag-0 reducer as well as a template to produce Ag-0 nanocrystals of anisotropic shapes. Thin Ag - PVA films ( 50 - 100 mu m 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+ -> Ag-0 reaction and in-situ Ag-0 growth occur in support over the PVA molecules ( as per the local structure and Ag-0 - PVA interactions) of elongated surfaces. The Ag-0 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 rho = 9.69 g cm(-3). On heating in air, the well-known cubic structure develops, a = 0.4082 nm (rho = 10.53 g cm(-3)) after 150 degrees C, whereas a = 0.4071 nm (rho = 10.61 g cm(-3)) after 300 degrees C for 2 h. According to proposed models, it is a vacancy-stabilized allotrope, which evolves from multiply twinned particles.