The ESR spin-trapping technique has been used to investigate the oxidation of N3- to N3• radical in CdS and ZnO suspensions using phenyl-tert-butylnitrone (PBN) as a spin trap. The initial growth rate of the ESR signal of the N3PBN adduct has been followed at different N3- concentrations. The effect of different added ionic species on the ESR signal intensity has also been tested. In CdS, cadmium ions enhanced the N3PBN signal, while several anions decreased it in the order Fe(CN)64- > I- > Br- > NO2- > HPO42- > Cl-. Somewhat analogous, but less pronounced, was the effect of some anions in ZnO. The effect of these anions cannot be generally explained as a competition for hole. The effects of HPO42- (which cannot be oxidized), Br-, and NO2- (whose redox potentials are very close to that of the N3-/N3• couple) are better explained as an adsorption competition. Likewise, the decrease in the N3PBN ESR signal as the pH increases is explained by the preferential interaction of OH- with the surface.
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Амаделли et al. (1989) studied this question.