The effects of pH and adsorbed hydrolysed metal ions on the photodissolution of colloidal cadmium sulphide

Abstract

Colloides J PhotochemistryThe quantum yields for the photodissolution of colloidal CdS in Ludox stabilized aerated solutions, have been measured as a function of pH. It has been found that the quantum yield decreases with increasing pH. Electrophoresis measurements indicate that this decrease is due to the formation of Cd2+ hydrolysis products on the surface of the colloid as the pH is increased. Support for this conclusion is obtained from experiments in which hydrolysed Co", Nil' and Zn" species were deliberately deposited onto CdS colloids. The reactions involved in the photocorrosion of CdS are discussed in light of the present results and with respect to mechanims proposed in the literature. IntroductionThe thermal-and photo-oxidation of metal sulphides have received considerable attention over many years because of their relevance to mineral flotation [l]. The products of oxidation are various: elemental sulphur, metal hydroxides, thiosulphates (SzOi-), polythionates (S,O;-, x = 2 -6), sulphate (SO: -) and volatile SO2. The amount and type of oxidation product depends on the conditions such as pH, oxygen pressure, temperature, presence of catalytic agents and duration of the oxidation reaction.In more recent years the photo-oxidation of metal sulphides has come under quite intense study because of the possibility of utilizing some colloidal metal sulphides as photocatalysts for the production of H2 from water [2]. CdS in particular has been extensively investigated because its catalytic activity can be initiated using visible light, an important advantage for the photochemical conversion of solar energy [3].Although a good deal of understanding of the photooxidation of CdS has been attained, there remain some aspects that are unclear. The purpose of the present study was to provide additional information on the photo-degradation of CdS as a function of pH and in the presence of hydrolysable metal ions in order to explain some of the observed photocatalytic behaviour of CdS colloids. To this end we have investigated CdS particles in 3 systems (i) CdS/Ludox solutions, (ii) CdS in water (iii) systems (i) and (ii) with added hydrolysable metal ions. The two metal ions Ni" and Co" were investigated in order to test their potential as scavengers for photoproduced holes in CdS. Both ions have been shown to catalyse H2Oz/OZ formation from H 2 0 [4,5].

An efficient method to enhance the stability of sulphide semiconductor photocatalysts: a case study of N-doped ZnS

“…The main chemical reactions for the photocorrosion of ZnS in aqueous solution are as follows: 41,42 ZnS + 2h + + O 2 -Zn 2+ + SO 4…”