(Invited) Self-Doped TiO2 Nanotube Array for Photoelectrochemical Water Treatment: Effects of Cathodization Condition

Abstract

Electrochemical self-doping of TiO2 by cathodization in mild conditions has been evinced to enhance the photoelectrochemical (PEC) activities for water treatment and molecular hydrogen generation. In this study, the effects of doping conditions such as annealing, current density, electrolyte (pH, buffering intensity), and duration were systematically interrogated with respect to the physico-chemical properties and water treatment efficacy of the TiO2 nanotube array (TNA). The incident-photon-to-electron conversion efficiency (IPCE) and the rates of model organic compounds degradation were comparatively evaluated. The self-doping of crystalline (anatase) TNA reduced the band gap to 2.4 eV (blue TNA), while the same sequence for amorphous TNA even further narrowed the gap to 1.4 eV (black TNA). The level of passed charge during the cathodization showed marginal influence. Depth profiles of X-ray photoelectron spectroscopy indicated that the dopants (Ti3+) were exclusively located on surface for the blue TNA, whereas they penetrated into the bulk tube structure for the black TNA which was accompanied with quasi-permanent lattice distortion as confirmed by X-ray diffraction patterns. Therefore, the electrical conductivity in terms of donor density was found to be greater for the black TNA. The buffering intensity of the cathodization electrolyte switch the relative level of proton intercalation in comparison with oxygen vacancy formation, which was demonstrated by time-of-flight and dynamic secondary ion mass spectrometry analysis. Even though the black TNA showed superior double layer capacitance and dark electrochemical oxidation, the blue TNA marked greater IPCE and PEC water treatment efficiency. The findingss of this study would broaden the usage of TiO2 nanomaterials by tuning the physico-chemical characteristics depending on variable water treatment purposes (mineralization of organic compounds versus disinfection).