Influence of excess S2- ion on the interfacial photocatalysis in superfine CdS system and band-potential matching

The influence of S2- ion and its concentration upon the photoinduced electron transfer and red-ox reaction of substrates on the surface of modified US colloid have been studied by means of ESR technique. The experimental results show that the existence of S2- ion with high concentration is advantageous to the interfacial photoreduction reaction and may restrain the photooxidation reaction. If the red-ox potential E-0 of the substrate is negative, photoreduction reaction is impossible to occur in the system with low concentration of S2- ion, but it can take place as the concentration of S2- ion is sufficient high; when E-0 of the substrate is positive, photooxidation reaction can be prohibited entirely under condition of high S2- ion concentration, even though it is able to occur while the concentration of S2- ion is low. Due to the strong interaction of S2- ion with the surface of US superfine particles, negative shift in flat potential E-fb of US leads to a change of gap position, and then to a variation in match relation between the red-ox potential E-0 of the substrate and the position of conducton band to valence band in semiconductor particles. According to Langmuir's adsorption isotherm model, a formula expressing the relation between the concentration c of modification compound and the negative shift A Efb in flat potential was derived: DeltaE(fb) = DeltaE(fb)(m) Kc/(1 + Kc), By using this formula, experiment facts may be explained rationally, the higher the concentration, the greater the amount of negative shift DeltaE(fb), and thus the more favourable to photoreduction reaction. Therefore, by choosing suitable modification substance and adjusting its concentration, the selection, direction as well as the mechanism of interfacial photoreaction can be controlled effectively.