Methyl orange as a probe of the semiconductor–electrolyte interfaces in CdS suspensions

Abstract

Cadmium sulphide (CdS) powder dispersions have been used to sensitise the photoreduction of methyl orange (D^–) at pH < 7 in aqueous solution. A number of electron donors were added (PVA, TEOA, nitrate, tartrate, cysteine, citrate, acetate and EDTA), but at pH 4.4 only EDTA enhanced appreciably the rate of methyl orange photoreduction. A steady-state equation was developed which successfully described the observed variation in initial rate of methyl orange photoreduction as a function of the concentrations of methyl orange, photons, EDTA and O₂(when present). From this work it was shown that the rate constant for hole (h⁺) scavenging by EDTA was ca. 19000 times greater than that for the recombination reaction between h⁺ and a conductance band electron (e^–). In addition, the rate constant for the reduction of O₂ by a conductance band electron was ca. 40–50 times smaller than that for the reduction of methyl orange by e^–. Corrosion studies indicated that photoreduction of methyl orange to a hydrazine derivative (D^3–) sensitised by CdS, was accompanied by anodic corrosion of the semiconductor even in the presence of EDTA. In this work the role of EDTA (at pH 4.4) appeared to be as a mediator of the reaction [graphic omitted] and not as an ‘ideal’ sacrificial electron donor. Reasons for this are discussed.