PHOTOELECTROCHEMICAL REDUCTION OF TETRAZOLIUM SALTS TO FORMAZANS ON SURFACES OF SEMICONDUCTOR POWDERS IN ALCOHOL-SOLUTIONS

The formation of colored formazans from the reduction of colorless tet razolium salts was found to occur on the surfaces of photoexcited TiO2 and ZnO powders in ethanol solutions. The yields of the conversions o f 2,3,5-triphenyltetrazolium chloride to 1,3,5-triphenylformazan and o f 2,5-diphenyl-3-(l-naphthyl)tetrazolium chloride to 1,3-diphenyl-5-(l -naphthyl)formazan were 41.6% and 40.0%, respectively, after a 25-min UV irradiation in the TiO2 suspension, whereas they were 55.8% and 35. 2%, respectively, after a 60-min UV irradiation in the ZnO suspension. These reactions proceeded via photoelectrochemical reduction by the e lectrons from the conduction bands of the semiconductors, but were que nched in the presence of oxygen or Ag+ ions.