Subpicosecond studies of primary photochemical events of CdS particles with surface modified by various capping agents

Q-CdS particles with surface modified by thiophenol (PhS) or 2-mercaptobenzoxazole (MBO) were prepared and their time-resolved transient absorption spectra were measured in acetonitrile in subpicosecond to picosecond time domain in order to investigate the influences of capping agents on primary photochemical events such as electron and hole trapping processes and decay kinetics of the trapped charge carriers on capped semiconductor Q-particles. In addition, time-resolved transient absorption spectra of aqueous CdS colloidal solutions containing sodium hexametaphosphate (HMP) as a stabilizer against flocculation were also measured in the same time domain and compared to obtain knowledge about the trap sites for electrons photo-produced on capped CdS particles in acetonitrile. The shape and the decay profile of transient absorption spectra obtained by subpicosecond laser flash photolysis of PhS-CdS and MBO-CdS in acetonitrile are markedly different from those of non-capped CdS particles in acetonitrile obtained previously using picosecond laser flash photolysis by Kamat et al. The results suggest that capping agents such as PhS acid MBO influence significantly hole trapping process and decay dynamics of the trapped charge carriers on the capped CdS. Furthermore, it seems that the trap site of the electrons generated by laser excitation of PhS-CdS or MBO-CdS in acetonitrile is mainly at the interface of particle-solution and the possibility for the production of solvated electrons is low because the shapes and decay kinetics of the transient absorption spectra of PhS-CdS or MBO-CdS in acetonitrile are clearly different from those HMP-CdS in water in which the photo-production of the hydrated electrons is known. (C) 1998 Elsevier Science S.A. All rights reserved.