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