Photo‐Degradation and Fluorescence of Colloidal‐Cadmium Sulfide in Aqueous Solution

Colloidal cadmium sulfide deposited on 13 nm silicon dioxide particles shows a weak fluorescence with a maximum at 620 – 660 nm (depending on the sample). Oxygen does not quench the fluorescence. All anions exert a quenching effect at concentrations below 10−3 M, S2-, Br−, and I− being the most effective ones. Tl+, Ag+, Pb2+, and Cu2+ strongly quench the fluorescence at concentrations below 10−4 M. Nitrobenzene quenches at higher concentrations in the 10−3 M range. Quenching is attributed to adsorbed ions or molecules. In the case of the anions, a mechanism is proposed in which positive hole-anion pairs are formed at the surface of the colloidal particles before neutralization by electrons takes place. In the case of the cations and of nitrobenzene, quenching is explained by electron capture. Colloidal CdS doped with less than 1% Cu2+ or Ag+ showed a much stronger fluorescence than undoped samples.—Colloidal cadmium sulfide is degraded by light only in the presence of oxygen the quantum yield in aerated solution being 0.04. The action of oxygen is mainly attributed to a reaction with intermediates of the sulfide oxidation by positive holes. In the presence of Tl+ or Pb2+ ions at some 10−5 M, the photo-degradation is drastically promoted up to a quantum yield of 0.24. Cu2+ and Ag+, however, retard the degradation. Both effects are understood in terms of electron scavenging by the adsorbed metal ions and reoxidation of the reduced metal by oxygen or by positive holes. Sodium sulfide protects cadmium sulfide from degradation. In the absence of air, small amounts of hydrogen are formed in Na2S containing solution.