Unprecedented large solvent (H 2 O vs D 2 O) isotope effect in semiconductors photooxidation

Abstract

Glucose in water (L2O, L = H or D) at pH = 7 (phosphate buffer) is oxidized in presence of Bi2WO6 and light. An unusually large solvent isotope effects, kH2O =kD2O = 7:8 and 6.8, have been measured using solar light and solar simulator, respectively. These large values come from the contribution of the equilibrium L2O ⇄ L+ + OL− previous to the rate-limiting step (rls) and the kinetic one (proton transfer) involved at the transition state. The reaction is faster when [L+] increases; therefore, L+ species instead of OL− one participates in the photooxidation. The rls is the 1 e− reduction-adsorption of H+ on the Bi2WO6 surface (Bi2WO6/Bi2WO6-H, −0.6 V vs NHE). Subsequently, O2 reduction to L2O, as driving force, occurs at the catalyst conduction band (CB). Linear sweep voltammetry when Bi2WO6 is used as cathode shows two reduction processes: H+/H2 and O2/O2.−. The last one-electron reduction occurs at −0.2 V vs NHE and the first one at −0.4 V vs NHE. Both show solvent isotope effect, although the i0H2O = i0D2O are 2.4 and 1.9, respectively, quite smaller than the value obtained in the photooxidation due to the influence of the bias on the transition state symmetry in the first case and through water O2 reduction in the second. Cyclic voltammetry indicates that the reduced and adsorbed H (Bi2WO6-H2) is oxidized to Bi2WO6-H at approximately 0.2 V vs NHE. Positive conduction band potential of Bi2WO6 (+0.5 V vs NHE) establishes the difference with other semiconductors where oxidation (L2O/L+,OL.) instead of reduction has been proposed as rls in photooxidation.