Fabrication Of TiO₂ Photoanode With A Nanotube Structure By Anodization Under Ultrasonic Stirring

TiO₂ nanotube arrays, as one of the ideal photoanode materials for dye-sensitized solar cells (DSSC), possess a large specific surface area, strong absorption ability and efficient electron transfer path and could be fabricated easily. For a photoanode absorbing dyes, a nanotube structure with higher length/diameter ratio and pure tube surface is propitious to dye absorption, and thus could enhance the photoelectric transformation efficiency of DSSC. As a result, goal of the paper is how to obtain a nanotube structure with longer length and a pure nanotube surface.TiO₂ nanotube arrays were fabricated in magnetic stirring or ultrasound stirring electrolyte contained 0.25wt% NH₄F, 2.5vol% water and the ethylene glycol at the range of 20℃⁶0℃for 0.5⁸ hours by anodization. And then the as-prepared TiO₂ on specimens was annealed at 450℃for 3 hours. The surface morphologies of TiO₂ nanotube arrays were observed by field-emission scanning electron microscopy (FESEM). The phase structures of the oxidation products on the specimen before or after heat treatment were identified by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The amount of defects in TiO₂ nanotube arrays was analyzed by photoluminescence (PL) experiment. And electrochemical impedance spectroscopy (EIS) of TiO₂ nanotube arrays was measured by using an electrochemical workstation.The main conclusions for the experiment are listed as below: The TiO₂ nanotube arrays prepared at 40℃by ultrasound stirring show a longer length and pure nanotube surface morphology, which is benefit to the photoanode application of DSSC. The TiO₂ nanotube arrays prepared at 60℃by ultrasound stirring has a non-tube structure on the surface, while the specimen anodized by magnetic stirring at the same temperature shows a severe corrosion surface. The length of TiO₂ nanotube arrays prepared at room temperature shows a tendency of first increasing and then decreasing with the increase in time, which is related to the chemical dissolution during the process of anodization. The phase of TiO₂ nanotubes changes from amorphous to anatase after annealing at 450℃for 3 hours. The result of PL experiment shows that there is a less defect in prepared TiO₂ nanotube arrays. By the EIS analysis, an equivalent circuit R(CR(R(QR)(CR))) is used to explain the electrochemical behavior of TiO₂ nanotube arrays with a non-tube layer on top. It can be concluded that the charge transfer resistance at the electrode/electrolyte interface controls the electrochemical process of TiO₂ nanotubes...