Influences of proton-dissociation equilibrium of capping agents on the photo-chemical events of the colloidal solutions containing the thiol-capped cadmium sulfide particles

The primary photo-chemical events of the aqueous colloidal solutions containing surface-modified CdS particles were investigated by two different methods, of which one employed subpicosecond spectroscopy to measure the transient absorption due to photo-generated electrons in the primary photo-chemical events, and the other stationary irradiation to measure their photo-stability. The pH dependence of transient absorption due to photo-generated electrons (trapped and hydrated electrons) in the colloidal solutions of thioglycerol (TG)-capped CdS (TG-CdS) and mercaptoacetate (MA)-capped CdS (MA-CdS) is similar to that of their photo-stabilities under the stationary irradiation, respectively. However, there was large difference in the pH dependence between TG-CdS and MA-CdS. Namely, the absorption intensity and the photo-stability decrease with lowering pH of the solutions for MA-CdS in the pH range lower than 7, where the negative charge of a carboxyl group of MA is neutralized by addition of proton. However, absorption intensity and photo-stability were hardly influenced by pH of the solution in whole pH range in the case of TG-CdS. These results suggest that the negative charge of the capping agents acts advantageously on the hole-trapping process in the primary photo-chemical events of the surface-modified CdS particles and their photo-stability.