Synthesis and Control of Silver Aggregates in Ion-Exchanged Silicate Glass by Thermal Annealing and Gamma Irradiation

Abstract

Samples of commercial silicate glass have been subjected to ion exchange by silver ions. The ion exchange was performed at 320 °C for periods from two minutes to one hour, in a molten mixture of AgNO₃ and NaNO₃ with a molar ratio of 1:99, 5:95 and 10:90. The ion exchange process was followed by different treatments: thermal annealing, gamma irradiation and their combined role in order to initiate the synthesis and control of silver aggregates in the surface of the glass matrix. UV-Visible absorption spectrometry results indicated that various states of silver existing in these glasses depend on heat treatment conditions. The silver ions (Ag⁺) exist in almost all conditions, neutral silver atoms (Ag⁰) exist only in samples subjected to heat treatment in the range of 250–450 °C, neutral silver aggregates (Ag⁰) produced by thermal annealing at 550 °C were responsible for the absorption bands observed from 305, 350 and 450 nm, respectively. The effect of gamma irradiation in doses from 10 to 100 kGy and thermal annealing on glass samples was also investigated. The main modification induced by gamma rays on the structure of silicate glass was the creation of colour centres, Non-Bridging Oxygen Hole Centres (NBOHCs) and trapped electrons. The (NBOHCs) defects caused the absorption of light. The Ag⁺ ions trapped electrons to form neutral silver Ag⁰. The first step of silver aggregation was observed, following the irradiation by gamma rays, as well as after thermal annealing. After annealing at 550 °C, silver atoms spread out over glass surface to form silver aggregates. An absorption band at 430 nm was observed characterizing the Surface Plasmon Resonance (SPR) of silver aggregates. The calculated average radius increases from 0.9 to 1.35 nm as the annealing time increased from 10 to 490 min. The average radius of nanoparticles varied as a function of absorbed dose. Unexpectedly, 10 kGy was found to be the optimally absorbed dose corresponding to the maximum of the nanoparticle’s average radius. The average radius of nanoparticles was decreased at a higher dose.