Phenol degradation on electrochemically self-doped TiO2 nanotubes via indirect oxidation

Recently, electrochemically self-doped TiO2 nanotubes (R-TiO2 NTs) have emerged as promising anodes in the electrochemical oxidation process for wastewater treatment, owing to their high degradation capability of organic compounds. However, the degradation mechanism of organic compounds on R-TiO2 NTs has not been well demonstrated, and therefore, industrial practices have been limited. This study aimed to elucidate the oxidation mechanism of organic compounds on R-TiO2 NTs by investigating phenol degradation. The performance of phenol degradation on R-TiO2 NTs was comparable to that on the boron-doped diamond (BDD) electrode that is well-known as an excellent anode for phenol degradation. In particular, phenol degradation on R-TiO2 NTs was completely achieved at the electrode potential of 5.0 V while negligible degradation was found at the electrode potential of 2.0 V. In addition, no peaks associated with phenol oxidation in cyclic voltammograms were noticed on R-TiO2 NTs at the potential range between 1.0–2.5 V. Considering direct oxidation is a fundamental factor for phenol degradation at low electrode potential, it contributes less to phenol degradation on R-TiO2 NTs. Therefore, phenol degradation on R-TiO2 NTs is mostly owing to indirect oxidation mediated by hydroxyl radicals.