Photoelectrochemical properties of "increasingly dark" TiO2 nanotube arrays

"Black" TiO2 nanotube arrays (TNTAs) of different darkness are formed by thermal reduction of anodic TiO2 nanotube layers in H-2 atmosphere at different temperatures (300-500 degrees C), and their (photo-)electrochemical properties are studied by IPCE spectroscopy, transient photocurrent measurements, IMPS, PETS, CV, and Mott-Schottky analysis. Hydrogenation of the TNTAs leads to the enhancement of their IPCE (by up to 1.5 times), despite increased recombination losses at anodic potentials. Reduced TNTAs demonstrate a "metallic" behavior by a characteristic frequency response of their impedance and phase angle, reduced charge transfer resistance at anodic bias, increased double layer capacitance, and remarkably increased donor densities (up to 7.25 x 10(23) cm(-3)). The observed alterations in the properties of TNTAs after hydrogenation are ascribed to the formation of defects and TiHx. (C) 2020 Published by Elsevier B.V.