Evolution of Silver Nanoparticles within an Aqueous Dispersion of Nanosized Zeolite Y: Mechanism and Applications

Stable aqueous dispersion of silver ion-exchanged nanozeolite Y was reacted with the three common dihydroxyphenols, hydroquinone, catechol, and resorcinol. With hydroquinone and catechol, there was rapid reduction of the intrazeolitic silver to form metallic silver with complete destruction of the zeolite framework. Resorcinol, the weakest reducing agent among the group, behaved differently. The formation of metallic silver was considerably slower, and the zeolite framework was mostly intact. This made it possible to examine the evolution of silver cluster formation with optical spectroscopy and transmission electron microscopy. In the first 135 min of reaction, extinction/fluorescence spectroscopy indicates the formation of Ag-n<4 and Ag-n>4 clusters. The smaller clusters disappear more rapidly with time. For the 1 h reduced sample, transmission electron microscopy showed uniform distribution of 1.4 nm Ag particles throughout the zeolite. After 2 h of reduction, the average size of the particles was 2.5 nm, and a fraction of these particles appeared on the zeolite surface. With further time of reduction (324 h), more of the intrazeolitic Ag migrated to the surface, and Ostwald ripening into larger nanoparticles (>3 nm) was observed at the zeolitesolution interface. We propose that the slow growth of the silver prevented the destruction of the zeolite framework. Two factors are considered important for the slow growth of silver with resorcinol. First, resorcinol is a weak reducing agent. Second, and more importantly, the intrazeolitic pH drops upon initial silver reduction, as measured by an acidochromic dye, which raises the reduction potential of resorcinol, thereby arresting further reduction and particle growth. By removing the resorcinol at any stage of the reduction, stable Ag nanoparticles on nanozeolite samples can be isolated. Such samples were investigated as SERS substrates as well as a heterogeneous catalyst. Optimal SERS enhancement as well as the optimal rate of reduction of nitrophenol by NaBH4 was observed with the sample obtained after reaction with resorcinol for 2 h.