Surface Effects on Regularities of Electron Transfer in CdS and CdS/Cu x S Colloids as Studied by Photoluminescence Quenching

CdS colloids of different sizes were prepared using two different methods: either with an excess of S2- ions or with an excess of Cd2+ ions. The shape of the luminescence spectra and regularities of luminescence quenching have been studied for both colloids. It has been found that positively charged electron acceptor molecules (bication of methyl viologen) effectively quench the luminescence of the colloids with the S2- ions excess and practically do not quench the luminescence of the colloids with the Cd2+ ions excess. Vice versa, negatively charged electron acceptor molecules (anions of heteropolyacids) effectively quench the luminescence of the colloids with excessive Cd2+ ions and do not quench the luminescence of the colloids with excessive S2- ions. The observed phenomenon can be explained easily by taking into account the necessity of adsorption of the quencher molecules on the surface of semiconductor particles in the electron-transfer process. It has also been observed that for both types of colloids the efficiency of the luminescence quenching is related to the luminescence wavelength. The regularities of the luminescence quenching have been studied for the CdS colloids of different sizes coated with Cu x S as well. It was observed that the addition of copper ions increases the efficiency of luminescence quenching by methyl viologen (MV2+) and tungstophosphoric heteropolyacid (PW12). Moreover, the shape of the luminescence spectra of these coated colloids depends on the excitation wavelength, the increase of the excitation wavelength resulting in the shift of luminescence spectra to the “red” region.