Controllable Charge Transfer in Ag-TiO₂ Composite Structure for SERS Application

Abstract

The nanocaps array of TiO₂/Ag bilayer with different Ag thicknesses and co-sputtering TiO₂-Ag monolayer with different TiO₂ contents were fabricated on a two-dimensional colloidal array substrate for the investigation of Surface enhanced Raman scattering (SERS) properties. For the TiO₂/Ag bilayer, when the Ag thickness increased, SERS intensity decreased. Meanwhile, a significant enhancement was observed when the sublayer Ag was 10 nm compared to the pure Ag monolayer, which was ascribed to the metal-semiconductor synergistic effect that electromagnetic mechanism (EM) provided by roughness surface and charge-transfer (CT) enhancement mechanism from TiO₂-Ag composite components. In comparison to the TiO₂/Ag bilayer, the co-sputtered TiO₂-Ag monolayer decreased the aggregation of Ag particles and led to the formation of small Ag particles, which showed that TiO₂ could effectively inhibit the aggregation and growth of Ag nanoparticles.

Keywords: SERS; TiO2-Ag nanocap array; magnetron sputtering; metal-semiconductor composite.

Figure 1

Schematic diagram of the preparation process for the nanocap arrays of the TiO₂/Ag bilayer and the TiO₂-Ag monolayer.

Figure 2

SEM images of bilayer TiO₂ (10 nm)/Ag t nm (from (a) to (d), t = 10 nm, 20 nm, 30 nm, 40 nm) on the PS template for different thicknesses of the Ag layer. The illustration is the cross-section of the sample.

Figure 3

SERS spectra of (a) PS/TiO₂ (10 nm) /Ag (10–40 nm) bilayer; (b) PS/Ag (10–40 nm) monolayer resembled on 200 nm PS template; (c) UV-Vis absorption spectra of PS/TiO₂ (10 nm)/Ag (10–40 nm) bilayer; (d) UV-Vis absorption spectra of PS/Ag (10–40 nm).

Figure 4

Column statistics of relative peak intensity and charge transfer for (a) I₁₃₉₁/I₁₀₇₂ of the PS/TiO₂ (10 nm)/Ag (10–40 nm) bilayer and (b) I₁₃₈₉/I₁₀₇₁ of the pure PS/Ag (10–40 nm) monolayer.

Figure 5

(a–c) SEM images and (d) SERS spectra of TiO₂-Ag (20 nm) with different TiO₂ atomic percent contents: (a) 10%; (b) 15%; (c) 25%.

Figure 6

TEM and HRTEM images of (a,c) the TiO₂ (10 nm)/Ag (10 nm) bilayer, and (b,d) the co-sputtered TiO₂-Ag (20 nm) monolayer.

Figure 7

Reproducibility test for SERS spectra of the TiO₂ (10 nm)/Ag (10 nm) bilayer.