Photophysics and photochemistry of colloidal CdS-TiO2 coupled semiconductors - photocatalytic oxidation of indole

The photophysical changes taking place by coupling of Cd(OH)(2)-coated Q-Cd S with colloidal TiO2 have been examined. The coupling of TiO2 causes the q uenching of the band gap emission of CdS and the red emission is not affect ed appreciably. For a typical 0.22 x 10(-3) mol dm(-3) of TiO2 the average emission lifetime of Cd(OH)(2)-coated CdS is reduced from 26.4 to 6.8 ns by trapping of the conduction band electron by TiO2. Cd(OH)(2)-coated Q-CdS d oes not sensitize the reaction of indole, however, the coupled CdS-TiO2 cat alyst photoinitiated this reaction efficiently. The photoactivity of the co upled semiconductor is suggested to increase due to chemical interaction be tween Cd(OH)(2)-coated Q-CdS and TiO2 which removes Cd(OH)(2) layer to form possibly [CdS-TiO2(OH)(2)]. The photogenerated hole in the coupled catalys t CdS(h(+)) forms an emissive exciplex with indole. The exciplex is long li ved and results in the decomposition of indole (phi = 0.18) to yield indigo with a quantum efficiency of 0.08. The concentration of both TiO2 and redo x couple controls the extent of charge separation in the photocatalyst. The application of coupled semiconductors as catalysts for photosynthetic work is suggested. (C) 2001 Elsevier Science B.V. All rights reserved.