Optical Hot Spot Generation by the Plasmonic Coupling of Au Nanoparticles in the Nanospaces of Mesoporous Titanium(IV) Oxide.

Abstract

An in situ reduction technique consisting of chemisorption of 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) and subsequent reaction with HAuCl4 has been developed for depositing Au nanoparticles (NPs) uniformly in the depth direction of a mesoporous TiO2 nanocrystalline film (Au/TMCTS/mp-TiO2). The TMCTS monolayer is further converted into silicon oxide by heating in the air (Au/SiOx/mp-TiO2). In the absorption spectra of Au/SiOx/mp-TiO2 prepared at varying HAuCl4 concentrations (C), the localized surface plasmon resonance (LSPR) band of Au NPs significantly broadens C ≈ 1.22 mM at 546 nm to be split into two peaks around 500 and 700 nm at C ≥ 2.43 mM, whereas such a phenomenon is not observed for the usual Au NP-loaded TiO2 particles. Three-dimensional-finite difference time domain simulations showed that the unique optical property of Au/SiOx/mp-TiO2 stems from the effective LSPR coupling of very close Au NPs and partial fusions in the nanospaces of mp-TiO2. Further, the optical hot spots in Au/TMCTS/mp-TiO2 as well as Au/SiOx/mp-TiO2 generate an intense local electric field giving increase to a great enhancement of the absorption in the infrared spectrum of the TMCTS monolayer on mp-TiO2.