Optimizing the Shell Thickness of Ag@TiO₂ Nanostructures by a Simple Top-Down Method to Engineer Effective SERS Substrates and Photocatalysts

In this article, we discuss a simple method to prepare core-shell Ag@TiO₂ nanoparticles (NPs) with an optimized shell thickness to engineer plasmonic photocatalysts and surface-enhanced Raman scattering (SERS) substrates. Variation in the shell (TiO₂) thickness was analyzed by an acid-etching method, and the deterioration of the shell was traced by monitoring the extinction spectra of both colloidal and solid-supported Ag@TiO₂ NPs. Attainment of the optimum shell thickness was confirmed by noticing the simultaneous appearance of the LSPR absorption band (at 450 nm) of core silver nanostructures (<i>d</i> = ∼10 nm) and the scattering signature of the shell (TiO₂) in the extinction spectrum of Ag@TiO₂ NPs. This study showed that the optimum thickness of TiO₂ is ∼2 nm, which allowed LSPR excitation by visible light. The observed blue shift of the LSPR peak, compared to the unetched Ag@TiO₂ NPs, with etching time indicated the size reduction of the NPs. Ag@TiO₂ with the optimum thickness exhibited a reaction rate five times faster than that of unetched Ag@TiO₂ under visible light irradiation. Ag@TiO₂ NPs exhibited higher photocatalytic activity under visible light irradiation than under UV light. Furthermore, Ag@TiO₂ NPs with the optimized thickness exhibited significantly higher SERS activity than the unetched Ag@TiO₂ NPs. The elevated photocatalytic and SERS activities exhibited by engineered Ag@TiO₂ NPs reveal the effectiveness of the etching process in creating a plasmonic effect in core(plasmonic)-shell (semiconductor) nanostructures.