Core-shell nanostructures and nanocomposites of Ag@TiO₂: effect of capping agent and shell thickness on the optical properties

Abstract

Two different shell-forming reagents viz. titanium isopropoxide and titanium hydroxyacylate, have been employed to obtain core-shell nanostructures of Ag@TiO₂. However, nanocomposites were formed when the shell-forming agent, titanium isopropoxide, was added before breaking the micelles. Titanium hydroxyacylate has been used for the first time as a shell-forming agent which resulted in uniform core-shell structures of Ag@TiO₂ with core diameter ranging from 10 to 40 nm and a shell thickness of 10-50 nm. The low rate of hydrolysis of titanium hydroxyacylate than titanium isopropoxide (used in other methods) appears to be responsible for the uniform shell thickness. The presence of capping agent (2-mercaptoethanol) disrupts the formation of a uniform shell structure of Ag@TiO₂. HRTEM, IR, and XPS studies of Ag@TiO₂ synthesized using capping agent show the formation of Ag2S coated with an amorphous layer of TiO₂. A red shift of 25 and 10 nm was observed in the surface plasmon band of silver for Ag@TiO₂ core-shell structures (compared with that of silver nanoparticles) synthesized using titanium hydroxyacylate and titanium isopropoxide, respectively. The presence of capping agent (2-mercaptoethanol) masks the surface plasmon peak. Photoluminescence studies show an increase in the emission intensity for the core-shell structures when compared to that of TiO₂ nanoparticles.