Effects of various applied voltages on physical properties of TiO2 nanotubes by anodization method
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Three steps anodization process is used to synthesize highly ordered and uniform multilayered titanium oxide (TiO2) nanotubes and effect of different anodization voltages are studied on their physical properties such as structural, morphological and optical. The crystalized structure of the synthesized tubes is investigated by X-ray diffractometer analysis. To study the morphology of the tubes, field emission scanning electron microscopy is used, which showed that the wall thicknesses and the diameters of the tubes are affected by the different anodization voltages. Moreover, optical studies performed by diffuse reflection spectra suggested that band gap of the TiO2 nanotubes are also changed by applying different anodization voltages. In this study using physical investigations, an optimum anodization voltage is obtained to synthesize the uniform crystalized TiO2 nanotubes with suitable diameter, wall thickness and optical properties.
KeywordsAnodization TiO2 XRD SEM
Among various oxide semiconductor materials, Titanium dioxide (TiO2) is attracted much attention due to its band gap, which is about 3 eV. Its wide band gap causes photo-activity effect in the UV range, good mechanical strength, non-toxicity, low cost and long-term photostability [1, 2, 3, 4]. Different nanostructures of TiO2 such as nanotubes , nanofibers , nanowires  and nanorods  have been investigated for its wide application in gas sensors [9, 10], biomedical application , hydrogen generation , battery electrode  and dye-sensitized solar cells [14, 15, 16, 17] so far. TiO2 nanoparticles were used in dye-sensitized solar cells as photoanode in 1991 by O’Regan and Gratzel . Different approaches were suggested so far to improve dye cells efficiency like replacing TiO2 nanoparticles with nanotubes since one dimensional TiO2 nanotube arrays provide direct path for electrons and improves the electron transport velocity and also cause reduction of the charge recombination [18, 19]. TiO2 nanotubes have been prepared by various methods such as sol–gel [20, 21, 22], liquid-phase deposition , hydrothermal [24, 25, 26] and electrochemical anodization processes [27, 28]. Amongst them, the electrochemical anodization of Titanium foil is producing compacted and oriented arrays and is known to be one of the most simple and low cost processes. On the other hand, one of the advantages of this process is morphological control of the TiO2 nanotubes by changing anodization conditions like voltage and time of anodization, temperature and concentration composition of the electrolyte [29, 30, 31]. Since the morphological properties of the TiO2 nanotubes such as diameter and wall thickness of tubes and arrays’ length are so effective on their optical application hence it is important to synthesize the nanotubes by morphological controlled methods. At first Masuda and Fukuda suggested anodization process to grow alumina (Al2O3) nanostructures in 1995  and then Zwilling et al. promoted this methods for synthesizing TiO2 nano porous on Ti metal in electrolyte containing HF  and at last the TiO2 nanotubes which had acceptable arrays’ length were anodized by Schmuki et al. [34, 35]. However, the nanotubes, which are synthesized by anodization are not top open channel and the suggested solution is two step anodization process to produce highly ordered and open channel arrays of TiO2 nanotube [36, 37]. In this method, the first anodized foils were ultra sonicated to remove the nanotubes and later second anodization steps can be performed. Therefore, the three step electrochemical anodization is known to be more concerned due to the possibility of producing vertically oriented and controllable dimension nanotube arrays. In this work we focus on the influence of the anodizing voltage on the morphological properties of the TiO2 nanotubes grown by three steps anodization process. Furthermore, correlation between structural, morphological and optical properties are studied using X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM) and diffuse reflection spectra (DRS).
Top open TiO2 nanotubes are grown through a three step anodization process. Firstly the titanium foils (0.25 thickness, 99.9% pure Sigma Aldrich) were cut and they were polished by ultra sonication in ethanol, acetone and deionized water for 20 min, respectively, to remove a surface contamination and later were dried by N2 stream. For the electrochemical anodizing setup, the two electrodes of a platinum foil (as cathode) and a titanium foil (as anode) were connected to the DC power supply in electrolyte solution. The distance between the anode and the cathode was adjusted about 2 cm. The electrolyte solution contained ethylene glycol (C2H6O2), 0.3 wt% ammonium fluoride salt (NH4F) and 2 v% deionized water (DI). Electrochemical setup and the anodizing parameters such as process temperature, concentration of electrolyte and anodizing time were kept same for all of the processes and different voltages of 35, 45 and 55 V were applied to study the effect of the applied voltage on the TiO2 grown samples. The first step of the electrochemical process was anodizing Ti foil for 1 h. After that the TiO2 nanotubes grown on the substrate were detached by ultra sonication in methanol to prepare hexagonal patterns on the Ti surface. Presence of this pattern is essential to grow compact and top open arrays at the next step. For the second step, the prepared Ti substrate was re-anodized at the same condition for 3 h and then ultrasonicated in methanol. This is followed by annealing the samples w in air ambient at 450 °C for 1 h with a heating ramp of 2.5 °C min−1. In the last step the annealed samples were anodized for 1 h and dried by N2 stream. The high resolution FESEM and XRD Cu Kα radiation (λ = 1.5418 Å) are used to observe morphological and structural properties of the TiO2 nanotube films. Moreover, DRS were also obtained in 200–800 nm wavelength range to calculate the band gap energy of the samples.
Results and discussion
Diameter, wall thickness, porosity and roughness factor calculated from SEM images for the samples anodized at 35, 45 and 55 V
Wall thickness (nm)
Roughness factor (μm)−1
In this paper the effective approach, three step anodization process, proposed to develop highly ordered crystallized and top open channel TiO2 nanotubes. The pore diameter, porosity and wall thickness of the TiO2 nanotubes could be tuned by changing the anodization voltages. Moreover, it was observed that the anodization voltage has significant effect on band gap values. The optimum anodization voltage was found to be 55 V in our experiments using (2 v%) DI water and (0.3 wt%) NH4F solution to grow compact TiO2 nanotubes with 67% porosity, which can be used as photoanode in dye-sensitized solar cells.
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