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

• Published in: Journal of physical chemistry. C Vol. 119; no. 45; pp. 25548 - 25558
• Main Authors: Cachet, H, Sutter, E. M. M
• Format: Journal Article
• Language: English
• Published: American Chemical Society 12.11.2015
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/acs.jpcc.5b06103

The electronic properties of a TiO2-nanotube (NT) array used as a photoelectrode for water oxidation at neutral and basic pH were characterized by combining complementary measurement techniques: transient photocurrents, stationary photocurrent–voltage curves, photo-electrochemical impedance spectroscopy (PEIS), and intensity-modulated photocurrent spectroscopy (IMPS). Transient measurements point out the slow chemical modification of the TiO2 surface when going from dark to light, essentially around neutral pH. After this transient period, a new stationary state of the TiO2 surface is established, allowing small amplitude perturbation techniques (PEIS and IMPS) to be applied to obtain information on transfer and recombination kinetics and on surface states contribution. The relevant information was obtained via theoretical models for the PEIS and IMPS responses, involving physical parameters with values extracted by nonlinear least-squares fitting. The main conclusions taken from our experiments include the following: (i) Under ultraviolet light illumination, the surface chemistry of TiO2 was found relatively stable at basic pH but strongly modified (hydroxylation) at neutral pH. (ii) Hole transfer to solution species takes place preferentially via the valence band. (iii) Recombination is mainly a surface process. (iv) Rate constants for charge transfer and recombination were determined as a function of the applied potential in agreement with the stationary photocurrent–voltage curve.