Photophysical and photocatalytic behaviours of Cd(OH)2-coated Q-CdS in the presence of tryptophan

Coating of Cd(OH)(2) on Q-CdS particles enhances their photostability, luminescing efficiency and emission lifetime. The presence of tryptophan quenches the bandgap emission of CdS and reduces its emission lifetime. For a typical 2x10(-4) mol dm(-3) of tryptophan the average emission lifetime of CdS is reduced from 22.6 to 8.5 ns. For this process a quenching rate constant of about 4 x 10(11) dm(3) mol(-1) s(-1) has been evaluated. The red emission is not influenced appreciably. Cd(OH)(2)-coated Q-CdS sensitised photooxidation of tryptophan has been examined in the presence of oxygen by irradiating with visible light. The photogenerated hole on the particle is intercepted by the bulk substrate (phi(-tryp)=0.22) to produce 5-hydroxytryptophan (phi(5-OHtryp)=0.08) as one of the main products. Shallowly trapped hole has been assigned to participate in the oxidation via hydrogen bonding interaction involving the surface of the particle and the substrate. Emission experiments indicate a difference in nature of reactive hole photogenerated on stoichiometric and Cd(OH)(2)-coated Q-CdS particles. A mechanism of this reaction has been proposed. (C) 2000 Elsevier Science S.A. All rights reserved.