Fabrication and photocatalysis of nanotubular C-doped TiO2 arrays: Impact of annealing atmosphere on the degradation efficiency of methyl orange

One-dimensional TiO2 nanotube arrays produced by the anodization of Ti foil at 60 V in an ethylene glycol bath containing ammonium fluoride and 1 wt% H2O holds a promise as a photocatalyst for degradation of methyl orange. However, as-anodized TiO2 nanotubes are amorphous in nature, imposing constraints on the potent use of this nanostructure. To address this issue, the as-anodized TiO2 nanotubes were annealed in inert (argon), reducing (nitrogen), or oxidizing (oxygen) atmosphere at 500 °C for 4 h. The amorphous TiO2 nanotubes were successfully transform into anatase phase after annealing. In this study, the TiO2 nanotubes annealed in argon exhibited the highest degradation rate of methyl orange (MO) solution under ultraviolet irradiation among the samples. The degradation rate was approximately 98% after 5 h, which may be ascribed to the large amount of oxygen vacancies and defects (carbon) within the Ar–TiO2 sample that simultaneously increased the degradation rate of MO. Oxygen vacancies and defects could be favorable in capturing photoinduced e− during the photocatalytic reaction such that the recombination of e− and h+ could be effectively inhibited.