PHOTOCHEMISTRY OF SEMICONDUCTOR PARTICLES .3. EFFECTS OF SURFACE-CHARGE ON REDUCTION RATE OF METHYL-ORANGE PHOTOSENSITIZED BY ZNS SOLS

The photoreduction of methyl orange sensitized by ZnS sols was studied. The reaction is diffusion controlled under the experimental conditions without stirring. A steady state equation is proposed which successfully describes the kinetic process. It can be deduced from the kinetic analysis that the reduction rate is first order with respect to the concentration of methyl orange if the diffusion effect is eliminated. The effects of the surface charge of the ZnS particles on the reaction kinetics were investigated. A positive surface charge formed by adsorbed cations, such as methylviologen (MV(2+)), Cd2+ and Cu2+, favours photoreduction, whereas a negative surface charge formed by adsorbed anions (e.g. S2-) decreases the reaction rate. This is due to the influence of the surface charge on the electron transfer from the ZnS particle to the methyl orange molecule. In addition, the pH dependence of the reaction rate was studied. The optimum pH for the photoreduction is pH 4.0.