Effect of NH4F concentration in electrolyte on the fabrication of TiO2 nanotube arrays prepared by anodisation

Highly-ordered TiO₂ nanotube arrays (TNTs) were prepared by anodisation in ethylene electrolytes containing 0.2-0.7 wt% NH₄F. The dimension, chemical composition and optical characterisation of prepared TNTs were determined by the field-emission scanning electron microscopy, X-ray photoelectron spectroscopy and photoluminescence spectroscopy, respectively. Both the length and inner diameter of the prepared TNTs were increased with increasing NH₄F concentration of electrolyte because of the faster electrochemical dissolution rate of TiO₂. However, dimension of TNTs approached stable lengths and inner diameters as the NH₄F concentration was higher than 0.3 wt%. The presence of Ti³⁺ state was observed for TNTs prepared with increasing NH₄F concentration because of the charge compensation of the substitution of fluoride ions for oxygen ions. TNTs prepared in the electrolytes containing higher NH₄F concentrations exhibited superior performance on the photocatalytic decomposition of Acid Red 4 possibly because more fluorides incorporated with TNTs to inhibit the recombination of electron-hole pairs.