Effects of TiO₂ nanotube array dimension and annealing temperature on the Acid Red 4 degradation in aqueous solution by photocatalytic process

Degradation of Acid Red 4 (AR4) by photocatalytic processes with TiO₂ nanotube arrays (TNTs) of various dimensions was carried out in this study. TNTs was fabricated by the anodization of titanium foil in electrolyte containing fluoride. The dimension and the induced photocurrent of TNTs were determined by a field-emission scanning electron microscopy and a potentialstat/galvanostat, respectively. The well-defined and highly-ordered TNTs were formed at the anodization voltage ranging from 10 to 40 V in water/glycerol solution (20:80 wt.%) containing 0.5 wt.% NH₄F. TNTs annealed at 600°C was found to induce the highest photocurrent and to exhibit the preeminent performance of AR4 degradation. The apparent first-ordered reaction rate constant for AR4 degradation was roughly linearly dependent on the induced photocurrent of TNTs, despite the dimension of nanotubes. The depths of incident UV light penetration through the nanotubes and of AR4 diffusion inside nanotubes restricted the degradation of AR4.