THE DECOMPOSITION RATE OF HYDROGEN-PEROXIDE TO OXYGEN ON ILLUMINATED SEMICONDUCTOR ELECTRODES

A kinetic approach to the photocurrent transient observed at a semicon ductor-aqueous electrolyte system was made. It has been concluded that the chemical properties of hydrogen peroxide in the electrolyte used cause the transient phenomena. If the hydrogen peroxide generated as a n intermediate of the oxidation product of water keeps adsorbing on th e electrode surface for a long time probably due to a lack of overpote ntial and/or due to its instability in the solution used, it has time to decompose to oxygen on the electrode surf ace and the resultant oxy gen can disturb the charge transfer from the electrode to the solution , causing the photocurrent transient. The current transient can be cla ssified into two types of time dependence: I(t) is-proportional-to exp (- t/tau) and I(t) is-proportional-to 1/t. The former is observed at pH approximately 11.0 where the charge exchange between the hydrogen p eroxide and the reactant in the solution dominates: H2O2,ad + OH(sol)- -->(KEX) HO2- + H2O, or equivalently H2O2,ad + OH-(sol) + h -->(KEX) HO2,ad + H2O. The rate constant k(EX) can be expressed by an exponent ial function of an applied external potential and ranges from k(EX) = 5.1 x 10(-18) cm3/s at - 0.4 V vs. SCE to 4.2 x 10(-17) cm3/s at 0.1 V vs. SCE. On the contrary, the latter is observable at pH 9 approximat ely 10 where the decomposition of H2O2 to O2 or of HO2 to O2 is predo minant: 2H2O2,ad -->(kDEC) O2,ad + 2H2O, or 2HO2,ad --> (kDEC)O2,ad H2O. The rate constant k(DEC) was estimated to be 2.5 x 10(-15) cm2/s and 7.1 X 10(-16) cm2/s at pH 10.0 and pH 9.0, respectively.