Applying the statistical experimental method to evaluate the process conditions of TiO2 nanotube arrays by anodization method

Vertically oriented TiO2 nanotube arrays were successfully produced by the anodization technique in NH4F/H3PO4 electrolyte. The structure and morphology were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). It is found that TiO2 nanotube arrays annealed at 500 °C containing 100% anatase phase and entirely converted into rutile at 800 °C. The response surface methodology (RSM) and Box-Behnken design were applied to find the optimal factor conditions in production of TiO2 nanotube arrays. Based on the results in preliminary experiments, we selected anodization time, anodization voltage and NH4F concentration as the key factors to investigate their effects on responses. The regression models were built by fitting the experimental results with a second-order polynomial. By using the regression models, the optimal factor conditions were obtained as follows: anodization time of 300 min; anodization voltage of 15.39 V; NH4F concentration of 0.50 M. Corresponding to the optimal factor conditions, the predicted average length and diameter of nanotube array were 1429 nm and 33 nm, respectively. Confirmation experiments using the optimized conditions were performed: TiO2 nanotube arrays were obtained with an average tube length of 1420 nm and average tube diameter of 36 nm. The experimental results are in good agreement with the predicted results.