Photophysics and photocatalytic properties of Ag+-activated sandwich Q-CdS–TiO2

The photophysical and photocatalytic behavior of sandwich Cd(OH)2-coated Q-CdS–Ag2S and the mixture of sandwich colloidal semiconductors consisting of Ag+-containing Q-CdS–TiO2 and Ag+-doped TiO2–Cd(OH)2-coated Q-CdS have been examined. The addition of Ag+ to Cd(OH)2-coated Q-CdS causes the quenching of bandgap emission of CdS and induces simultaneously the red emission. For a typical [Ag+] (2.0×10−5 mol dm−3) the average lifetime (〈τ〉) of Cd(OH)2-coated Q-CdS decreases from 19.97 to 15.99 ns whereas at 650 nm it increases from 7.72 to 14.68 ns. Unlike stoichiometric Q-CdS–Ag2S system these particles do not sensitize the reaction of indole–O2 redox couple. Coupling of Ag+-containing Cd(OH)2-coated Q-CdS with TiO2 as well as of Ag+-containing TiO2 with Cd(OH)2-coated Q-CdS quenches the 462 nm emission of CdS completely and produces a new emission band at around 650 nm. Quenching of emission fits to a Perrin model. In both the cases the doping of Ag+ modifies the TiO2 phase. At high [Ag+] it produces a three component composite catalyst, CdS–TiO2–Ag2S. The photogenerated hole CdS (h+) in the Ag+-activated composite CdS–TiO2 catalyst forms an emissive exciplex with indole and results in its decomposition to yield indigo. The photoinduced charge separation in this system was twofold higher compared to the undoped composite photocatalyst (CdS–TiO2). This catalytic action of Ag+ is understood in terms of the positive redox potential of the Ag+/Ag couple, which serves to intercept the conduction band electron and reduces the e−–h+ recombination.