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

Abstract

Vertically aligned TiO 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 H 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 Ti to Ti 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; 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 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. H generation coupled with the organic degradation was observed only in PEC condition, where the H generation rate with Blue-TNTs was more than doubled from that of intact TNTs. Such superior PEC activity was not observed when a common TiO 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.

Coupling copper and hydrogenated TiO2 to bare TiO2 structures for improved photocatalytic performance

“…Two peaks at binding energies of 464.1 and 458.4 eV, corresponding to Ti 2p 1/2 and Ti 2p 3/2 , respectively, were observed in the XPS spectrum of bare TiO 2 , indicating the presence of Ti 4+ . In contrast, these peaks were shifted to lower binding energies (463.7 and 458.0 eV for Ti 2p 1/2 and Ti 2p 3/2 , respectively) in the case of HT and unused CuHTT samples, which was ascribed to the formation of Ti 3+ . Unlike in the spectra of unused CuHTT‐0.1, the two lower binding energy peaks at 463.7 and 458.0 eV (Ti 2p 1/2 and Ti 2p 3/2 , respectively) in the spectra of photocatalytically used CuHTT‐0.1 re‐shifted to higher binding energies (464.1 and 458.4 eV for Ti 2p 1/2 and Ti 2p 3/2 , respectively), indicating that Ti 3+ species were oxidized to Ti 4+ during the photocatalytic reaction.…”

Semiconductor Electrode Materials Applied in Photoelectrocatalytic Wastewater Treatment—an Overview

“…metal removal/recovery [99][100][101][102]; and, simultaneous generation of electricity and hydrogen [12,[103][104][105].…”

“…The semiconductor materials that are applied in photoelectrocatalytic treatment of wastewater include photoanodes (n-type) and photocathodes (p-type) if oxidation or reduction is the main process at the interface [84]. Thus, photoelectrocatalysis is developed in such wastewater treatment field as: organic compounds oxidation [92,93]; inorganic ions reduction [94,95]; disinfection [96][97][98]; metal removal/recovery [99][100][101][102]; and, simultaneous generation of electricity and hydrogen [12,[103][104][105].…”

Effect of Electrolyte Pretreatment on the Formation of TiO2 Nanotubes: An Ignored yet Non‐negligible Factor

“…Anodic titania nanotubes have been used in various applications, such as solar cells,‐ photocatalysts, electrochromic devices and biosensors . The performance of functional devices incorporating anodic titania nanotubes is closely related to the geometry of the nanotubes .…”