Large surface area titanium oxide nanotube arrays anodized in KH<inf>2</inf>PO<inf>4</inf>/NH<inf>4</inf>F/citric acid electrolytes by multi step voltage method

Abstract

We present titanium oxide nanotube arrays for solar cell, gas sensor, ultra filter, and biomaterials. To make titanium oxide nanotubes, potassium phosphate monobasic (KH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) 1M aqueous electrolytes containing fluorine 0.15M and citric acid 0.2M were firstly prepared and 99.7% pure titanium was anodized. Titanium oxide nanotube arrays were fabricated at anodization potential range from 20 V to 28 V and 4.64 pH. Citric acid was very useful to control the pH of the 1M KH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> PO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> water electrolyte solution within 3 to 5 [1]. Nanotube length of 2 mum was independent on anodization time at 20 V for anodization time from 5 to 23 hrs which was observed by FESEM. Pore diameter and length of titanium oxide nanotube were increased with anodization voltage of 20 to 28 V. The pore diameter and length of titanium oxide nanotube, as shown in Fig. 1 and 3, were 100 to 150 nm and 2.0 to 3.0 mum, respectively. At higher anodization voltages than 25 V, porous layer was made and titanium oxide nanotube could not be made. In previous work, it has been found that there were undesired thin oxide layer blocking the top of titanium oxide nanotubes which could be wiped out by increasing anodization time or etching in solution with higher HF concentration [2, 3]. We could wipe out surface blocking layer by increasing anodization potential with the voltage steps reached to higher voltage than primary anodization voltage, which was named as "multi step voltage method (MSVM)" [1]. The MSVM makes nanotube enlarge in comparison with the typical method having one holding anodization voltage. Fig. 2 shows the current transient curve depending on anodization time in both cases one holding voltage and MSVM. In MSVM, current was steeply increased when higher voltages were stepwise applied. Following eight step voltages, dramatically increased current and additional growth of titanium oxide nanotube arrays were observed. Fig. 3 shows FESEM images of titanium oxide nanotube arrays which were made by one holding voltage and MSVM. The final voltage of 28 V was same, but the length of two cases was very different. Anodization voltage of 28 V was too high to obtain titanium oxide nanotube arrays by typical method having one holding voltage. In MSVM, we could stepwise increase the voltage to 28V, which offered a driving force to corrode the titanium with fluorine ion in electrolytes. We could obtain the clean-top titanium oxide nanotube arrays having length of 3.0 mum and diameter of 150 nm by MSVM at 30 degC for 8hrs. Anatase phase of titanium oxide nanotube arrays was observed after lhr annealing at 500degC by X-ray diffraction patterns. The titanium oxide nanotubes having a very large surface area are very attractive for the battery, gas sensor, photocatalytic applications, and biomaterials [4-6].