Formation of silver particles and periodicprecipitate layers in silicate glass induced by thermally assistedhydrogen permeation

Abstract

Nanoscale silver particles embedded in sodium silicate glass wereproduced by Na/Ag ion exchange and subsequent thermal treatment in ahydrogen atmosphere. Their structure and spatial distribution werestudied by conventional and high-resolution electron microscopy(HREM). Two different mechanisms of particle formation could beidentified: (i) reduction of ionic silver by hydrogen and formationof mostly defective particles (twinned) within a near-surfaceregion; and (ii) formation of single-crystalline particles in theinterior of the glass resulting from reduction by means ofpolyvalent iron ions. Electron microscopy investigation revealedthe completion of periodic layers of silver particles innear-surface regions with high silver concentration induced bythermally assisted hydrogen permeation. The self-organized periodiclayer formation may be explained in terms of Ostwald'ssupersaturation theory, assuming interdiffusion of two mobilespecies. Analysis of lattice plane spacings from HREM images ofsilver particles revealed the typical size-dependent latticecontraction. The extent of this, however, was found to be differentfor particles formed by hydrogen permeation and those formed byinteraction with polyvalent iron ions. These differences reflectdifferent influences of the surrounding glass matrix, probablyoriginating from the conditions of particle formation (thermal history).