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1 Effect of fluoride concentration and water content on morphology of titania nanotubes in ethylene glycol solution M. Naghizadeh 1,a, S. Ghannadi 1,b, H. Abdizadeh 1,2,c, M.R. Golobostanfard 1,d 1 School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, P.O. Box: , Tehran, Iran 2 Center of Excellence for High Performance materials, University of Tehran, P.O. Box: , Tehran, Iran a m.naghizadeh@ut.ac.ir, b saber.ghannadi@ut.ac.ir, c abdizade@ut.ac.ir, d bostanfr@ut.ac.ir Keywords: Titania nanotube, Anodizing, Fluoride concentration, Water content. Abstract Titanium dioxide (TiO 2 ) nanotube arrays were prepared at room temperature by electrochemic alanodization of a pure titanium foil in electrolyte solutions containing ethylene glycol as a solvent and deionized water and ammonium fluoride as additives. Since the morphology and size of TiO 2 nanotubes play critical roles in determining their performance, the control of geometrical parameters of the nanotube arrays including length and inner diameter are of great importance. The present research, demonstrates the significant effects of fluoride concentration and water content in anodizing electrolyte on formation of nanotubes and their dimensions. Scanning electron microscope investigation shows that nanotube arrays are no longer formed in very low or very high concentration of ammonium fluoride. Also, increase in fluoride concentration causes increase in lengths and inner diameters of the nanotubes. Moreover, it is evident that the maximum nanotube growth rate was achieved in medium amount of water. In addition, it has been found that the nanotube inner diameter increases by adding more water to the solution. Introduction In the last decade, highly ordered TiO 2 nanotubes (NT)s have attracted great attentions due to their improved properties compared to any form of titanium dioxide for application in photocatalysis [1], hydrogen gas sensing [2], and dye sensitized solar cells [3]. TiO 2 nanotubes have been produced by different methods such as hydrothermal process [4], sol-gel [5], anodization method [6]. Anodization method is of more interest because it is a relatively simple and cost-effective process, while retaining uniform nanotube arrays [7]. For preparing TiO 2 NTs, most often, fluorine based compounds such as HF, KF, NaF, and NH 4 F have been used as electrolytes [8]. Aqueous [9] and nonaqueous organic solutions [10] are two main electrolyte solutions in anodization synthesis of titania NTs. The nanotubes anodized in nonaqueous organic electrolytes can be grown longer, while maintain smooth uniform tube architectures [11]. The morphology and dimensions of TiO 2 nanotubes play critical roles in determining their performance. Anodization voltage, ph, time, temperature, and electrolyte composition have fundamental influence on the nanotubes [12]. In the present work, we reported a detailed investigation on morphology and dimensions of tubes formed in ethylene glycol-based electrolyte containing various water contents and different NH 4 F concentrations. Experimental For investigating the effects of fluoride concentration, the electrolyte solutions contained ethylene glycol as a solvent with 2 vol% deionized water and different concentrations of NH 4 F (0.01, 0.1, 1, and 2 M) were considered. Also, ethylene glycol with 0.1 M NH 4 F and various water contents (0, 3, 5, and 10 vol%) were used as electrolytes for studying the effects of water content. Anodization process was performed in a two electrode bath with titanium foil (0.25 mm, 99.7% purity, Sigma- Aldrich)as the working electrode and copper foil as the counter electrode in constant potential of 40

2 V. Titanium foils were cleaned ultrasonically in deionized de water for 5 min followed by subsequent rinsing in ethanol for 1 min. The foils were dried with air stream. The distance ance between anode and cathode was 15 mm. All the experiments were performed in constant time of 45 min. The time dependent anodization current was recorded by a multimeter (GWINSTEK, GDM 396) during the anodization process. After each anodization, anodization the sample was rinsed with deionized water and dried with air stream. Finally, y, all the sample samples were calcined at 450 C for 1 h. Morphologies of resulting nanotubee arrays were characterized by field field emission scanning electron microscope (FESEM, Hitachi, S4160). Results and discussion Fig. 1 shows the FESEM images of anodized titanium in ethylene glycol with 0.1 molar ammonium fluoride and different water contents. As shown in Fig. 1, the nanotube length increases from 1.45 to 2.8 µm with increasing water content from 0 to 3 vol% (Fig. 1a and b),while increase in water content from 3 to 5, and 10 vol% (Fig. 1b-d) 1b causes decrease in length from 2.8 to 2.6, and 2.2 µm, µm respectively. Also, the insets show that the inner diameter of 33 to 65, 78, and 85 nm could be obtained for water content of 0 to 3, 5, and 10 vol%, respectively. Therefore, the inner diameter increases with increasing water content in electrolyte. As a rule, the key processes responsible for anodic formation of straight st titania nanotubes are [13]: (a) oxide growth at the surface of the metal occurs due to interaction of the metal with O2- or OHions. The overall reaction for anodic oxidation of titanium can be represented as: 2H2O O2+4e- + 4H+ (1) Ti+O2 TiO2 (2) (b) Metal ions (Ti4+) migrate from the metal at the metal/oxide interface and dissolve into the electrolyte under application n of an electric field: field Ti4++6F- TiF62(3) (c) Chemical dissolution of the oxide by the acidic electrolyte takes place according to the reaction: TiO2+6F-+4H+ TiF62-+2H2O (4) Fig. 1.FESEM images of titanium foils anodized at 40 V for 45 min in ethylene glycol-based based electrolyte electrolytes with 0.1 M NH4F, and (a) 0, (b) 3, (c) 5, and (d) 10 vol% deionized water. Insets are taken in higher magnification.

3 The chemical dissolution rate is determined by F - and H + (Eq. (4)). Adding more water to the solution increases H + ions in the electrolyte (Eq. (1)) and accelerates the chemical dissolution of the oxide and hence causes shorter nanotube lengths. It appears that water content more than 3 vol% results in nanotubes with shorter lengths. Water contributes oxygen to form anodic oxide film [13]. In very low water content the donation of oxygen, which comes from the water, is more difficult. Therefore, the oxide forms with reduced tendency. Thus, the maximum nanotube growth rate is achieved in medium amount of water. Moreover, accelerating chemical dissolution of the oxide causes nanotube with larger inner diameters due to addition of more water to the bath. Fig. 1 also shows that the morphology of TiO 2 nanotubes is affected by changing the water content. It can be seen that ripples among the side walls of the tubes decreases and the homogeneity of the nanotubes increases with increasing water content from 0 to 5 vol%. However, the homogeneity of the tubes decreases by adding more water to the bath from 5 to 10 vol%. Such sidewall inhomogeneity (ripples), is ascribed to the electric field perturbation, which break the balance of dissolution (Eq. (4)) and oxidation (Eq. (2)) during the anodization process [14]. Diffusion has the main effect on local acidification which could lead to a temporarily increased dissolution rate (Eq. (4)). The viscosity of the electrolyte decreases by addition of more water to the solution and it causes higher diffusivity (considering Stokes-Einstein relation). On the other hand, increase in the water content results in smaller current densities, which further increase the balance of dissolution and oxidation with a diffusion-controlled process during the anodization. Thus, smooth nanotubes are achieved at medium amount of water 5 vol%. Fig. 2 illustrates the current density versus time of titanium foils anodized at different electrolyte compositions. Fig. 2.Current density versus time plot for titanium foils anodized at 40 V in ethylene glycol-based electrolytes with (a) 0.1 M NH 4 F and various water contents, (b) 2 vol% water and different concentrations of NH 4 F. Inset is the first 80 seconds of anodization process in ethylene glycol-based electrolyte with 2 M NH 4 F and 2 vol% deionized water.

4 It is apparent from Fig. 2a that addition of more water to the solution leads to smaller current densities. Both chemical dissolution (Eq. (4))) and electrochemical oxidation rates (Eq. (2)) increase with increasing water content. However, However constant NH4F concentration centration causes faster increase in electrochemical oxidation rate (Eq. (2))) compared to chemical dissolution rate (Eq. (4)). Also, higher current densities are obtained with increasing the NH4F concentration (Fig. 2b). Increase in the NH4F concentration causes increase in the chemical and field-assisted field assisted dissolution rates (Eq. (3) and (4)), ), leading to higher diffusivity and ion ionic concentration in the electrolyte. Influence of NH4F concentration on the morphology and dimensions of resulting resulting nanotubes is shown in Fig. 3.. It can be seen that nanotubes cannot possibly formed at 0.01 and 2 M NH4F concentrations (Fig. 3a and d). ). This is generally due to the effects of fluoride:: (i) the ability to form water soluble TiF62- complexes according to Eq. 44, and (ii) the small ionic radios that makes them suitable for entering into the growing TiO2 lattice and to be transported through the oxide by the applied electric field (thus competing with O2- transport) [15]. Fig. 3. FESEM images of titanium foils anodized an at 40 V for 45 min in ethylene glycol-based based electrolyte electrolytes with 2 vol% deionized water, and (a) 0.01, 0. (b) 0.1, (c) 1, and (d) 2 M NH4F. Insets are cross cross-sectional views of resulting nanotubes. The ability of fluoride ion to form water soluble complexes reduces att very low concentration of NH4F and it appears to result in small pores at the surface of the oxide barrier layer. At very high concentration of NH4F, not only the ability of F- ion to form water soluble TiF62- complexes increases, but also the entrance of fluoride ion ions into TiO2 lattice and moving toward the oxide layer accelerates. Thus, almost no oxide formed at very high NH4F concentration, and hence no nanotube was synthesized. In addition, the insets in Fig.. 3b and c show that increase rease in fluoride concentration from 0.1 to 1 M causes increase in nanotube length from 2.3 to 3 µm. This may be due to the F- ions induce field assisted dissolution to enlarge large and deepen the pores. pores Moreover, the inner diameter of nanotubes increases from 54 to 65 nm with increase in NH4F concentration from 0.1 to 1 M,, which is due to the stronger etching effects caused by F-. Conclusions

5 Anodization of titanium foils in the ethylene glycol electrolyte solutions with different water contents and various NH 4 F concentrations were performed at 40 V for 45 min. The FESEM images indicate that at higher water contents (>3 vol%) the length of the tubes decreases, while at lower water contents (<3 vol%) tube length increases by adding more water to the electrolyte bath. In addition, nanotubes with larger inner diameter are produced by increasing water content. Moreover, it is found that smooth nanotubes are achieved at medium amount of water 5 vol%. Furthermore, no nanotubes are formed at very low (0.01 M) or very high (2 M) concentration of NH 4 F. Also, it is obvious that increase in NH 4 F concentration results in longer nanotubes with larger diameters. References [1] M. Adachi, Y. Murata, M. Harada, Y. Yoshikawa, Investigation of electrical transport in anodized single TiO 2 nanotubes, Chem. Lett. 29 (2000) [2] O.K. Varghese, D.W. Gong, M. Paulose, K.G. Ong, E.C. Dickey, C.A. Grimes, Extreme changes in the electrical resistance of titania nanotubes with hydrogen exposure, Adv. Mater. 15 (2003) [3] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Use of highly-ordered TiO 2 nanotube arrays in dye-sensitized solar cells, Nano. Lett. 6 (2006) [4] C.C. Tsai, H.S. Teng, Regulation of the physical characteristics of titania nanotube aggregates synthesized from hydrothermal treatment, Chem. Mater. 16 (2004) [5] T. Maiyalagan, B. Viswanathan, U.V. Varadarju, Fabrication and characterization of uniform TiO 2 nanotube arrays by sol gel template method, Bull. Mater. Sci. 29 (2006) [6] V. Zwilling, M. Aucouturier, E. Darque-Ceretti, Anodic oxidation of titanium and TA6V alloy in chromic media. An electrochemical approach, Electrochim. Acta. 45 (1999) [7] M. Paulose, K. Shankar, S. Yoriya, H.E. Prakasam, O.K. Varghese, G.K. Mor, T.A. Latempa, A. Fitzgerald, C.A. Grimes, Anodic Growth of Highly Ordered TiO 2 Nanotube Arrays to 134 μm in Length, J. Phys. Chem. B. 110 (2006) [8] Y. Alivov, M. Pandikunta, S. Nikishin, Z.Y. Fan, The anodization voltage influence on the properties of TiO 2 nanotubes grown by electrochemical oxidation, Nanotechnology, 20 (2009) [9] D. Gong, C.A. Grimes, O.K. Varghese, W.C. Hu, R.S. Singh, Z. Chen, E.C. Dickey, Titanium oxide nanotube arrays prepared by anodic oxidation, J. Mater. Res. 16 (2001) [10] H. Tsuchia, J.M. Macak, L. Taveira, E. Balaur, A. Ghicov, K. Sirotna, P. Schmuki, Selforganized TiO 2 nanotubes prepared in ammonium fluoride containing acetic acid electrolytes, Electrochem. Commun. 7 (2005) [11] M.R. Sturgeon, P. Lai, M.Z. Hu, A comparative study of anodized titania nanotube architectures in aqueous and nonaqueous solutions, J. Mater. Res. 26 (2011) [12] D.J. Yang, H.G. Kim, S.J. Cho, and W.Y. Choi, Thickness-conversion ratio from titanium to TiO2 nanotube fabricated by anodization method, Mat. Lett. 62 (2008) [13] G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, and C.A. Grimes, A review on highly ordered, vertically oriented TiO 2 nanotube arrays: Fabrication, material properties, and solar energy applications, Sol. Energy Mater. Sol. Cells, 90 (2006) [14] Y. Lai, J. Gong, C. Lin, Self-organized TiO 2 nanotube arrays with uniform platinum nanoparticles for highly efficient water splitting, Int. J. Hydrogen Energy. 37 (2012) [15] J.M. Macak, H. Tsuchiya, A. Ghicov, K. Yasuda, R. Hahn, S. Bauer, P. Schmuki, TiO 2 nanotubes: self-organized electrochemical formation, properties and applications, Curr. Opin. Solid State Mater. Sci. 11 (2007) 3-18.

Synthesis and characterization of titania nanotube arrays by electrochemical method for dye sensitized solar cells

Synthesis and characterization of titania nanotube arrays by electrochemical method for dye sensitized solar cells Available online at www.scholarsresearchlibrary.com Archives of Applied Science Research, 2013, 5 (5):28-32 (http://scholarsresearchlibrary.com/archive.html) ISSN 0975-508X CODEN (USA) AASRC9 Synthesis