An EXAFS study on the photo-assisted growth of silver nanoparticles on titanium dioxide thin-films and the identification of their photochromic states

Abstract

Anatase TiO₂ thin-films were formed on glass by a sol–gel dip-coating method and annealed at 500 °C. Ag nanoparticles were grown on the surface of TiO₂ by a photo-assisted process from AgNO₃ salt using either UVC – 254 nm or UVA – 365 nm light. The size, shape and coverage of the particles were assessed by scanning electron microscopy. Changes in surface plasmon properties were investigated by UV-visible spectroscopy. A greater level of spherical Ag nanoparticles grew on TiO₂ when using UVA light (365 nm); with particles 96 ± 33 nm wide on average and covering 29% of the surface. In the case of UVC light (254 nm), particles were 78 ± 14 nm wide on average and covered 13% of the surface. EXAFS measurements performed in situ of the Ag K-edge showed that the photo-assisted growth was more rapid when UVA light was used, leading to the full conversion of the AgNO₃ salt layer in ≈1900 seconds. When UVC light was used, ≈50% of the salt layer was converted in ≈6100 seconds. The inhibited growth under UVC conditions was attributed to the absorption of light by the Ag nanoparticles as they formed (as opposed to the semiconductor beneath). The films also displayed reversible photochromism. The change in phase from the coloured (metallic Ag) to the bleached state (oxidized Ag) was identified using EXAFS spectroscopy. By comparing the EXAFS pattern with simulated model structures, it was shown that the transition from cubic Ag to cubic Ag₂O was most likely, with an ≈70% conversion with 12 hours of white light irradiance. We believe that this is the first time the bleached form of silver in photochromic Ag–TiO₂ thin-films has been identified by a direct method. In addition, we believe that this is the first case in which the photo-assisted formation of Ag–TiO₂ has been monitored in situ under ambient temperature and pressure.