Optical Properties of Au Nanoparticles Included in Mesoporous TiO<sub>2</sub> Thin Films: A Dual Experimental and Modeling Study

Gold nanoparticles (NP) were synthesized inside ordered mesoporous TiO<sub>2</sub> thin films (MTTF) by stepwise reduction of AuCl<sub>4</sub><sup>–</sup> with NaBH<sub>4</sub>. This leads to an optical material (Au@TiO<sub>2</sub>) of interest for plasmonic applications. The films (pure titania or gold-titania nanocomposites) were thoroughly characterized by UV–visible and ellipsometry spectroscopies. The dielectric function of the MTTF, considered as the dielectric environment in which the NP are embedded, was acquired by ellipsometry and rationalized by the asymmetric Bruggeman model as an effective medium formed by the mixture of dense TiO<sub>2</sub> and air. Nanocomposite Au@TiO<sub>2</sub> systems present an isotropic dispersion of Au NP in the 5–8 nm range. The UV–visible spectra obtained with a low nanoparticle filling fraction of the pore volume (<i>f</i><sub>NP</sub> < 2%) are accurately reproduced by both Maxwell–Garnett (MG) and Mie theories. Accurate and coincident values of <i>f</i><sub>NP</sub> and NP size are obtained by this method. The dielectric function of Au NP used in this work was studied in detail; in particular, the interface damping parameter related to the NP/MTTF interface was determined by comparison with TEM microscopy. The potential of the ellipsometry technique to determine the material plasmonic response, and its correspondence with the UV–visible spectra, are discussed. This spectroscopy technique opens the possibility to study the plasmon response of the material to changes in the environment due to the presence of vapors, and other in situ experiments, as well as to provide nanostructural information of metallic nanoparticles (NP size, interparticle distance, number of NP) with well-defined spatial localization in a multilayered system.