KINETIC APPROACH TO THE PHOTOCURRENT TRANSIENTS IN WATER PHOTOELECTROLYSIS AT N-TIO2 ELECTRODES .2. ANALYSIS OF THE PHOTOCURRENT-TIME DEPENDENCE

The photocurrent-time dependence observed during experiments of water photo-oxidation at an n-TiO2 single crystal, as a function of the applied potential and illumination intensity, has been analyzed on the basis of a kinetic model previously proposed. In a basic medium, the model assumes as a first step the photogeneration of surface-bound OH∘s radicals from Ti coordinated OH− ions of basic character. OH∘s radicals further recombine on the TiO2 surface at a second step, leading to the formation of surface peroxo complexes (chemisorbed HO2− species). Finally, photogenerated hydrogen peroxide species become photo-oxidized and O2 evolves. At small bandbending the decay time constant, τ, characterizing the photocurrent transients is very sensitive to the applied potential, but does not depend on the illumination intensity. The photocurrent-time dependence is dominated by e− − h+ surface recombination, mainly via photogenerated OH∘s radicals. The pseudo first-order rate constant of this reaction shows a gaussian distribution around a main value k = k3nb exp (−Φs/kT), with k3 = 10−10 cm3 s−1, nb and Φs representing the bulk electron density and the bandbending. respectively. This behavior indicates the existence of a heterogeneous distribution of Ti—OH− surface sites mediating the process. In the high bandbending region τ is not sensitive to the applied external bias, but strongly depends on the light intensity. The transient is then due to the surface accumulation of positive charge at photogenerated OH∘s radicals, which produces a slow bandbending decrease and a concomitant diminution of the photocurrent. These effects are controlled by the rate, k2, of hydrogen peroxide formation, which is a second-order reaction in the surface concentration of photogenerated OH∘s radicals. A k2 ≈ 1014 cm2 s−1 can be estimated. © 1990, The Electrochemical Society, Inc. All rights reserved.