Photoreduction of silver ions in a colloidal titanium dioxide suspension

The photoreduction of Ag+ ions at the surface of colloidal TiO2 nanoparticles has been investigated. Irradiation of an aqueous solution containing TiO2 and Ag+ ions with near-UV light brings about the growth of a sharp absorption band of colloidal silver in absorption spectrum. The photoreduction of Ag+ ions by TiO2 is optically sensitized by a simple cyanine dye, 3,3′-disulfopropyl-5,5′-dichloro-thiacyanine sodium salt (TC). TC molecules are adsorbed at the surface of TiO2 nanoparticles in the form of the J-aggregate (JA). Visible-light irradiation of the solution containing TiO2, Ag+ ions, and TC causes the growth of a broad absorption band of the colloidal silver and the decrease of whole absorption bands of TC. Experimental results have been interpreted in terms of electronic energy levels of the colloidal silver and the TC aggregate, as well as energy bands of TiO2. In the sensitized photoreduction, TiO2 could play the role of a medium to facilitate efficient electron transfer from the excited TC aggregate to Ag+ ions adsorbed at the surface.