Enhanced Photoelectrocatalytic Degradation of Methylene Blue on Smooth TiO2 Nanotube Array and Its Impedance Analysis

The smooth titania nanotube array fabricated via two-step anodization in F--containing ethylene glycol electrolytes plus a post-calcination has been developed as a photoelectrocatalytic (PEC) application on degradation of methylene blue (MB). It was found that the MB degradation kinetics meets the law of first-order reaction, and the rate constants of MB degradation were 0.147 h(-1) for photocatalytic (PC) process and 0.853 h(-1) for PEC process, a 5.8-fold increase under the condition of open-circuit and zero applied potential, respectively. Electrochemical impedance spectroscopy was used to analyze the enhanced degradation phenomenon in PEC as compared to PC on such electrodes, and the results revealed that the enhanced degradation efficiency of PEC process should be ascribed to the systematic change of capacitance and resistance and thus leading to a dramatic decrease of impedance on the interface of semiconductor under combination of illumination and bias potential. Further investigation revealed that the crystal structure of TiO2 nanotube array and the width of its space charge layer, which is mainly determined by the thickness of the nanotube wall as well as the potential applied across the wall of the nanotube, might be critical factors for the enhancing degradation efficiency due to its effective charge separation. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3089312] All rights reserved.