Photocatalytic growth and plasmonic properties of Ag nanoparticles on TiO₂ films

Abstract

Using nanoparticulate TiO 2 films, the photocatalytic growth of Ag nanoparticles (NPs) in the AgNO 3 aqueous solution has been studied in terms of reduction, nucleation, and coalescence. It was proved that Ag primary particles were formed in a growth time of ,1 s after the photocatalysis started. The growth dynamics was found to be critical for isotropic and anisotropic growth of Ag NPs, depending on the AgNO 3 concentration and surface properties of TiO 2 films. In the AgNO 3 solutions of #300 mg/L, the isotropic growth dominates the growth dynamic behavior, producing irregularly spherical Ag NPs. In the AgNO 3 solutions of $400 mg/L, the increased reduction rate promotes the formation of Ag nanoplates in the product. Ostwald ripening and oriented attachment were suggested to be the mechanisms dominating the isotropic and anisotropic growth, respectively. A photocatalytic growth model of Ag NPs was proposed by taking Ag atom and Ag 1 ion diffusion into consideration. The plasmonic properties of the Ag-TiO 2 films were studied in terms of extinction, surface enhanced Raman scattering, and fluorescence enhancement.

Photocatalytical decoration of thin titania coatings with silver nanostructures provides a robust and reproducible SERS signal

“…It is worthy to note that LSRP absorption bands observed here are narrower than in AgNSs/TiO 2 SERS substrates reported so far . This observation suggests that nanoplatforms designed by us exhibit small shape variation and size distribution of AgNSs compared with TiO 2 ‐based nanoplasmonic materials fabricated by a hydrothermal method, photocatalytic decoration of cicada and butterfly wings, or photocatalytic growth of Ag particles on 100‐nm TiO 2 films . In turn, the 0.1AgE/Ti‐a nanoplatform does not possess a LSPR peak in its extinction spectrum as displayed in Figure .…”

“…When TiO 2 coatings are covered by AgNSs, their electronic spectra exhibit the presence of an additional band in the region of 360–430 nm, except the sample 0.1AgE/Ti‐a. This band is attributed to local surface plasmon resonances (LSPRs) of AgNSs, and its position and shape depend on the size, shape, and distribution of metal particles as well as interactions between Ag and TiO 2 . The simplest explanation of LSPR positions considers dipole–dipole interactions in AgNSs, which become stronger when the size of the metal structures increases, and this fact is observed by a red shift of the LSPR maximum in the extinction spectrum.…”

“…Despite absorbance at approximately 400 nm, electronic spectra of 0.1AgW/Ti‐a and 1.0AgW/Ti‐a show the presence of weak absorbances at 570 and 621 nm, respectively (Figure ). This observation results from the presence of Ag polygonal flat objects and triangular plates generating the quadrupole and dipole resonances as observed also in the study of Li et al In turn, an increased extinction in the red and near‐infrared regions of high‐density silver nanoparticles (above 600 nm) is found in spectra of 1.0AgW/Ti‐b, 1.0AgE/Ti‐a, and 10AgE/Ti‐a nanoplatforms . SEM images of these samples indicate the formation of round and oval shapes mainly with the smallest interparticle distance of 20–30 nm that cover the titania coating with the highest coverage of 44–62% (Figure and Table ).…”

“…To date, the SERS activity of Ag–titania platforms were reported for substrates prepared by photocatalytic reduction of Ag + ions on TiO 2 films and nanoparticles, chemical reduction of Ag + ions on TiO 2 nanofibers and nanorods, magnetron sputtering of the silver on single‐crystalline TiO 2 nanosheet, and deposition of the metal on TiO 2 nanoarrays in the anodization process . Simple UV illumination of titania coatings in a solution of silver ions results in the formation of oval or polygonal shapes with diameters between 40 and 180 nm and that are distributed as single nanoparticles (with interparticle distance from few up to approximately 50 nm) as well as overlapped nanospheres or nanotriangles . Mills and coworkers showed that a nanoplatform of fish‐scales shape ( d > 40 nm, illuminated by 45 min) produced the SERS signal with relatively high reproducibility (relative standard deviation [RSD]: 24%), mechanical robustness, and long shelf life (over 1 year) .…”

“…They indicated that the size effect plays the main role in the SERS activity of these substrates. A similar study highlighted that electromagnetic enhancement of Raman signal on the surface of such nanoplatforms increases due to a high roughness of the AgNSs–TiO 2 films, which can be primarily controlled by the concentration of silver ions under a given UV illumination time . A critical concentration of metal ions was determined, below which the formation of irregular nanospheres is promoted, whereas nanoplates are produced in a solution of higher concentrations.…”