Nano zero-valent iron impregnated on titanium dioxide nanotube array film for both oxidation and reduction of methyl orange

Here, we demonstrated that nano zero-valent iron (nZVI) impregnated onto self-organized TiO2 nanotube thin films exhibits both oxidation and reduction capacities in addition to the possible electron transfer from TiO2 to nZVI. The TiO2 nanotubes were synthesized by anodization of titanium foil in a two-electrode system. Amorphous TiO2 (amTiO2) nanotubes were annealed at 450 °C for 1 h to produce crystalline TiO2 (crTiO2) nanotubes. The nZVI particles were immobilized on the TiO2 array film by direct borohydride reduction. Field emission scanning electron microscopy (FE-SEM) analysis of the crystalline TiO2 nanotube with nZVI (nZVI/crTiO2) indicated that the nZVI particles with a mean particle diameter of 28.38 ± 11.81 nm were uniformly distributed onto entire crTiO2 nanotube surface with a mean pore diameter of 75.24 ± 17.66 nm and a mean length of 40.07 μm. Environmental applicability of our proposed nZVI/TiO2 nanotube thin films was tested for methyl orange (MO) degradation in the aqueous system with and without oxygen. Since oxygen could facilitate the nZVI oxidation and inhibit electron transfer from crTiO2 to nZVI surface, MO degradation by nZVI/crTiO2 in the presence of oxygen was significantly suppressed whereas nZVI/crTiO2 in the absence of oxygen enhanced MO degradation. MO degradation rate by each sample without oxygen were in following order: nZVI/crTiO2 (kobs = 0.311 min−1) > nZVI/amTiO2 (kobs = 0.164 min−1) > crTiO2 (kobs = 0.068 min−1). This result can be explained with a synergistic effect of the significant reduction by highly-dispersed nZVI particles on TiO2 nanotubes as well as the electron transfer from the conduction band of crTiO2 to the nZVI on the crTiO2 for the degradation of MO.

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► The nZVI were immobilized on TiO2 nanotube thin film by direct borohydride reduction.
► Highly-uniform nZVI distribution on the TiO2 film by FE-SEM image analysis.
► Reduction by nZVI and photocatalytic oxidation by TiO2.
► Electrons can be transferred from the conduction band of crystalline TiO2 to nZVI.