Kinetics of Water Oxidation at TiO2 Nanotube Arrays at Different pH Domains Investigated by Electrochemical and Light-ModulatedImpedance Spectroscopy

The electronic properties of a TiO2-nanotube (NT) array usedas a photoelectrode for water oxidation at neutral and basic pH werecharacterized by combining complementary measurement techniques:transient photocurrents, stationary photocurrent−voltage curves, photoelectrochemicalimpedance spectroscopy (PEIS), and intensity-modulatedphotocurrent spectroscopy (IMPS). Transient measurements point out theslow chemical modification of the TiO2 surface when going from dark to light,essentially around neutral pH. After this transient period, a new stationary stateof the TiO2 surface is established, allowing small amplitude perturbationtechniques (PEIS and IMPS) to be applied to obtain information on transferand recombination kinetics and on surface states contribution. The relevantinformation was obtained via theoretical models for the PEIS and IMPSresponses, involving physical parameters with values extracted by nonlinear least-squares fitting. The main conclusions takenfrom our experiments include the following: (i) Under ultraviolet light illumination, the surface chemistry of TiO2 was foundrelatively stable at basic pH but strongly modified (hydroxylation) at neutral pH. (ii) Hole transfer to solution species takes placepreferentially via the valence band. (iii) Recombination is mainly a surface process. (iv) Rate constants for charge transfer andrecombination were determined as a function of the applied potential in agreement with the stationary photocurrent−voltagecurve.