Application of ultrasonic wave to clean the surface of the TiO2 nanotubes prepared by the electrochemical anodization
Highlights
► The ultrasonic wave was used to remove the precipitates from the surface of TiO2 nanotubes prepared by electrochemical anodization. ► The top surface of TiO2 nanotubes was cleaned effectively after 9 min of ultrasonic treatment (80W and 40 kHz) and the well-aligned and uniform TiO2 nanotubes appeared. ► When the treating time was extended to 40 min a small part of nanotubes broken and no nanotubes were left on the titanium base if the sonication further increased to 60 min.
Introduction
TiO2 nanotubes have attracted more and more attention due to their excellent electronic, photonic, catalytic, and gas-sensitive properties. It has a great potential in various applications such as photocatalysis [1], [2], solar energy cell [3], environmental purification [4], [5], and gas sensors [6]. TiO2 nanotubes have been successfully fabricated by templates based on nanoporous alumina [7], hydrothermal techniques [8], sol–gel transcription using organogelators [9], seed growth method [10] and electrochemical anodic oxidation. Among these techniques, electrochemical anodic oxidation is the simplest and likely the cheapest one. It is a self-assembling process, in which both the localized chemical dissolution and the field assisted oxidation and dissolution lead to the formation of orderly nanotubes [11], [12]. However, during the anodization process, high pH levels of the electrolyte [13] and the presence of ethylene glycol [14] can result in the deposit of unexpected precipitates on the top surface of the nanotubes, which hinders the filling of the nanotubes with other functional materials. Thus, it is necessary to develop a method to effectively remove the precipitates for gaining clean surface nanotubes, which facilitates the further modification of nanotubes arrays. Some researchers [14], [15], [16] reported that ultrasonic agitation could be used to remove the precipitates on the nanotube arrays. However, there is little information on the process parameters (such as treatment time and corresponding ultrasonic power) in the published papers.
In this study, ultrasonic wave was used for removing the precipitates from the surface of TiO2 nanotube arrays which were prepared by electrochemical anodization in a NH4F/Na2SO4/PEG400/H2O system. It should be noted that the whole ultrasonic treatment process was performed in the medium of water. Surface morphology and X-ray diffraction spectrum of the nanotubes treated by ultra wave at different times were compared. The purpose of this work is to investigate and optimize the application of ultrasonic wave in the surface cleaning of TiO2 nanotubes.
Section snippets
Material and methods
High purity of titanium (Ti) foils (99.6%) with 0.5 mm-thickness was used (BaoTi Co. Ltd., China) in this study. All the water was deionized water produced from an EPET-40TF system (EPET Co. Ltd., Nanjing, China).
Prior to the anodization, Ti foil was degreased by immersing it into aqua regia for 24 h, then rinsed with deionized water and air-dried. The electrochemical anodization was performed in a two-electrode system with the Ti foil as the anode and the platinum sheet as the counter electrode.
Results and discussion
FESEM images of the TiO2 nanotubes treated with ultrasonic wave at different times were displayed in Fig. 1(a–g). Fig. 1a and b show that the top surface of nanotubes without ultrasonic wave treatment was covered by debris because hydrous titanium oxides were generated as the precipitates at a high hydrolysis rate during the anodization process [12], [13]. Since the hydrous titanium oxides only physically deposits on the nanotube surface, after 9 min of ultrasonic wave treatment (80 W, 40 kHz) the
Conclusions
The ultrasonic wave was used to remove the precipitates from the surface of TiO2 nanotubes prepared by electrochemical anodization. Both of the FESEM and XRD images revealed that under the condition of 80 W and 40 kHz, the top surface of TiO2 nanotubes was cleaned effectively after 9 min of ultrasonic treatment (corresponding to 12 Wh) and the well-aligned and uniform TiO2 nanotubes appeared. When the treating time was extended to 40 min a small part of nanotubes broken and no nanotubes were left on
Acknowledgments
The authors gratefully acknowledge the financial supports from the National High Technology Research and Development Program of China (2008AA10Z307), President Research Fund of Xi’an Jiaotong University (08140016), and the Hong Kong Polytechnic University (GU-784).
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