Photoelectrochemical Degradation of Organic Compounds Coupled with Molecular Hydrogen Generation Using Electrochromic TiO2 Nanotube Arrays

Vertically aligned TiO₂ nanotube arrays (TNTs) were prepared by electrochemical anodization, and then cathodically polarized with dark blue coloration for the dual-functional photoelectrochemical water treatment of organic substrates degradation and accompanying H₂ generation. The resulting Blue-TNTs (inner diameter: ∼40 nm; length: ∼9 μm) showed negligible shift in X-ray diffraction pattern compared with the intact TNTs, but the X-ray photoelectron spectra indicated a partial reduction of Ti⁴⁺ to Ti³⁺ on the surface. The electrochemical analyses of Blue-TNTs revealed a marked enhancement in donor density and electrical conductivity by orders of magnitude. Degradations of test organic substrates on Blue-TNTs were compared with the intact TNTs in electrochemical (EC), photocatalytic (PC), and photoelectrochemical (PEC) conditions (potential bias: 1.64 V_NHE; λ > 320 nm). The degradation of 4-chlorophenol was greatly enhanced on Blue-TNTs particularly in PEC condition, whereas the PC activities of the Blue- and intact TNTs were similar. The potential bias of 1.64 V_NHE did not induce any noticeable activity in EC condition. Similar trends were observed for the degradation of humic acid and fulvic acid, where main working oxidants were found to be the surface hydroxyl radical as confirmed by hydroxyl radical probe and scavenger tests. H₂ generation coupled with the organic degradation was observed only in PEC condition, where the H₂ generation rate with Blue-TNTs was more than doubled from that of intact TNTs. Such superior PEC activity was not observed when a common TiO₂ nanoparticle film was used as a photoanode. The enhanced electric conductivity of Blue-TNTs coupled with a proper band bending in PEC configuration seemed to induce a highly synergic enhancement.