Effect of Ag loading and diameter of self-organized TiO ₂ nanotubes on their catalytic activity for oxygen reduction in 0.5 M H ₂ SO ₄

Self-organized titanium dioxide nanotubes were fabricated by electrochemical anodization in fluoride solution at room temperature. The influence of experimental parameters such as applied voltage, fluoride concentration, pH, additives, and bath temperature on the diameter of the fabricated titanium dioxide nanotubes was explored. The results revealed that the diameter of the nanotubes increased from 30 nm to 100 nm with increasing applied voltage from 5 V to 20 V and increased from 70 nm to 134 nm with increasing fluoride concentration from 0.1 M to 0.7 M at bath temperatures ranging from 20 °C to 80 °C. However, the diameter decreased from 100 nm to 45 nm with increasing pH from 1 to 7 and decreased from 100 nm to 87 nm with increasing glycerol concentration from 0 M to 5 M in the bath. All nanotubes fabricated by electrochemical anodization were in amorphous form, and they turned into anatase crystal after being annealed at 480 °C for 3 h. The annealed self-organized anatase nanotubes were treated with silver and palladium to increase their catalytic activity for electrochemical reduction of oxygen in a 0.5 M sulfuric acid solution saturated with oxygen at room temperature. It was found that the catalytic activity increased while the diameter of the nanotubes decreased. However, it increased with an increase in silver loading. The anatase nanotubes with a diameter of 65 nm treated with silver by immersion in 0.2 M silver nitrate and by illumination with ultra-violet light (λ= 254 nm) for 1 h exhibited the highest catalytic activity (with highest current density 242 μA/cm²).