Charge Transfer Tuned by the Surrounding Dielectrics in TiO₂-Ag Composite Arrays

TiO₂/Ag bilayer films sputtered onto a 2D polystyrene (PS) bead array in a magnetron sputtering system were found to form a nanocap-shaped nanostructure composed of a TiO₂-Ag composite on each PS bead, in which the Ag nanoparticles were trapped partially or fully in the TiO₂ matrix, depending on the TiO₂ thickness. X-ray Photoelectron Spectroscopy (XPS) results showed the opposite shifts of binding energy for Ti 2p and Ag 3d, indicating the transfer of electrons from metallic Ag to TiO₂ owing to the Ag-O-TiO₂ composite formation. UV-Vis absorption spectra showed the blue shifts of the surface plasma resonance peaks, and the maximum absorption peak intensity was obtained for TiO₂ at 30 nm. The surface-enhanced Raman scattering (SERS) peak intensity first increased and then decreased when the TiO₂ thickness changed. The observations of SERS, XPS, and UV-Vis absorption spectra were explained by the dependency of the charge-transfer process on TiO₂ thickness, which was ascribed to the changing dielectric properties in the metal/semiconductor system.