Influence of Structure Parameters and Crystalline Phase on the Photocatalytic Activity of TiO₂ Nanotube Arrays

Titanium oxide nanotube arrays (TiO₂–NTAs) with different diameters and lengths are prepared by anodization of titanium foils in a water/ethylene glycol solution (5:95 V/V) containing 0.3 wt% NH₄F. The effects of the diameters, lengths and crystalline phases of the NTAs on the photocatalytic (PC) activity are systematically evaluated. Larger pore diameter results in higher PC activity. The PC activity increases initially and then decreases with lengths for TiO₂–NTAs and the optimal length that yields the highest PC activity is observed to be 6.2 μm. The crystalline phase and corresponding PC activity depend on the calcination temperature and their relationship is also investigated. The amorphous-to-anatase and anatase-to-rutile phase transitions initially occur at 300 and 500 °C, respectively. The PC activity of TiO₂–NTAs initially increases with calcination temperature from 250 to 500 °C and then decreases at higher calcination temperature. The enhanced PC activity observed from the samples annealed at 250-450 °C is attributed to the better anatase crystalline structure at higher calcination temperature. The highest PC activity with regard to photodecomposition of methyl orange is observed from TiO₂–NTAs calcined at 500 °C, which coincides with the anatse-to-rutile phase transformation. The synergistic effect of the anatase TiO₂–NTAs and rutile barrier layers facilitate interfacial electron transfer consequently enhancing the PC activity. Further elevation of the calcination temperatures to 550 and 600 °C exhibits diminished PC activity because the NTs become shorter due to conversion of the bottom of anatase NTs into rutile film.