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Study on the formation micromechanism of TiO2 nanotubes on pure titanium and the role of fluoride ions in electrolyte solutions
Thin Solid Films 519 (2011) 5150–5155
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Study on the formation micromechanism of TiO2 nanotubes on pure titanium and the role of fluoride ions in electrolyte solutions Y.Q. Liang a, Z.D. Cui a, S.L. Zhu a,b, X.J. Yang a,b,⁎ a b
School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China Tianjin Key Laboratory of Composite and Functional Materials, Tianjin, 300072, China
a r t i c l e
i n f o
Available online 15 January 2011 Keywords: Pure titanium Nanotubes Growth mechanism Drilling factor
a b s t r a c t The present paper reports the mechanism of nanotube growth and the effect of NH4F on the nanotube formation on a pure titanium substrate. A set of experiments was performed under different polarization times (ranging from 10 min to 4 h) and different NH4F concentrations of anodization. The mechanism of nanotube growth can be explained as follows. The nanoporous structure firstly formed on the pure titanium after a threshold time (0.5 h–1 h). Then the nanopores converted into self-organized nanotubes. The “shut down” tubes were found in the cross-sectional SEM images of the nanotubes due to the insufficient fluoride ion concentration. Furthermore, a possible physical model was also reported to explain the function of fluoride ions in the formation of nanotubes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Self-organized nanotubes produced by anodic oxidization have attracted much attention in the past decade. The method can control the titania down to nanoscale dimensions. Due to the unique properties, the titania nanotubes are expected to be used in various fields, such as sensing, solar cells, water photolysis, fuel cells, molecular filtration and tissue engineering [1–10]. In recent work, it has been shown that highly ordered high-aspect-ratio TiO2 nanotubes with single or double wall, oriented perpendicular to the substrate can be grown on titanium alloys in neutral buffers of NH4F solution or acid electrolyte of HF solution [4,11–13]. However, the effect of fluoride ions was not illustrated in detail. Moreover, some researchers suggested that the thickness of the nanotube layer substantially increases with the anodization time during the full anodizaiton process [14]. Nevertheless, the mechanism of variations of layer thickness and bottom view of nanotubes during the first anodization stage (10 min–60 min) have not yet been studied in detail. This stage is directly related to the formation of nanotubes. Accordingly, in the present study, the self-organized nanotubes by anodizing in water/glycerol mixtures containing different concentrations of NH4F were prepared. And a new viewpoint on the growth mechanism was proposed. Furthermore, the effect of NH4F on the dimension of nanotubes was discussed. Last but not the least, a possible physical model was also proposed to explain the function of F− ions in the formation of nanotubes.
Pure titanium (11 mm × 9 mm × 0.6 mm) samples were degreased in ethanol by ultrasonic, and then rinsed with deionized (DI) water and finally dried in air. Potentiostatic anodizations were carried out in a two-electrode electrochemical cell at room temperature by a highvoltage potentiostat (GPR-60300 PC). Pure titanium samples served as a working electrode, and platinum plate as a counter electrode. Samples were anodized with water/glycerol (1:1 vol.%) mixtures containing different NH4F concentrations under a potential of 30 V. The electrolytes used in this work were made of reagent grade chemicals. After potentiostatic anodization, the samples were rinsed with DI water and dried in air. The structural and morphological characters of the samples were tested using a scanning electron microscope (SEM; FE-SEM S4800; Hitachi, Tokyo, Japan) equipped with Energy-Dispersion X-ray Spectroscopy (EDS). The cross-section images were taken from mechanically bent samples, where a lift-off of the porous layer occurred.
⁎ Corresponding author. School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China. Tel.: + 86 22 27405602. E-mail address: [email protected] (X.J. Yang). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.075
3. Results and discussion In order to illustrate the formation mechanism of nanotubes, the interface between nanotubes and substrate should be investigated in detail. Fig. 1 shows the bottom SEM images taken from the samples anodized at different times (a—10 min, b—30 min, c—1 h, d—1.5 h, e—2 h, and f—2.5 h). It was observed that irregular protrusions were formed on the bottom of the nanoporous layer in Fig. 1(a) and (b). During this stage, the nanoporous oxide layer was firstly formed on the substrate [15]. After 1 h of anodization, some regular (round or oval) bottoms at local areas are exhibited in Fig. 1c, indicating the formation of initial nanotubes.
Y.Q. Liang et al. / Thin Solid Films 519 (2011) 5150–5155
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Fig. 1. SEM bottom images of different stages of the TiO2 nanotube formation. Anodization stopped after (a) 10 min, (b) 30 min, (c) 1 h, (d) 1.5 h, (e) 2 h and (f) 2.5 h at 30 V in water–glycerol electrolyte (1:1 vol%) containing 0.27 M NH4F.
However, some irregular protrusions still existed at other locations (inner area marked with white arrows). It implied that the porous oxide structure started to convert into a nanotube structure after 1 h. With the oxidization time further increasing, these regular mounds are apparent on almost the entire surface (as shown in Fig. 1d). A whole self-organized nanotube layer was fully evolved after 2 h oxidization (as shown in Fig. 1e and f). The formation of nanotube arrays in fluoride containing electrolytes is due to the following three simultaneously occurring processes: (i) field assisted oxidation of Ti metal to form titanium dioxide; (ii) field assisted dissolution of Ti metal ions in the electrolyte; and (iii) chemical dissolution of Ti and TiO2 due to etching by fluoride ions [16–18]. Fig. 2a shows the anodization time dependence of the tubular layer thickness (the thicknesses were measured using the SEM crosssectional images). With the time increasing, the layer thickness firstly increased from 10 min to 30 min, and then decreased at 1 h. Then a steeply increased state with a maximum thickness about 1.13 μm was observed when the time reached 3 h. With the anodization time further increasing, the layer thickness decreased. The first decrease of the oxide layer thickness at 1 h indicates that the top nanoporous layer dissolved after 0.5 h [19], accompanied by the conversion from a porous structure to a nanotube structure. It indicates that the threshold time to form a nanotube structure in pure titanium is from 0.5 h to 1 h in anodization. The second decrease of layer thickness at 4 h reveals that, after prolonged anodization, the TiO2 nanotube dissolved due to the formation of a soluble hexafluorotitanium complex [TiF6]2− [12]. Fig. 2b shows the top and bottom diameter evolution with the increasing
anodization time. It is obvious that top and bottom diameters of nanotubes have no significant dependence on the anodization time. Particularly, a slight increase of the top diameter at 4 h can be observed. The shape of the nanotubes is conical because the inner diameters slightly decrease with depth. After prolonged anodization (all together 4 h), the oxide formation at the metal interface cannot keep up with the dissolution of TiO2 nanotubes on the surface in the electrolyte containing F− [20]. Hence, the tube length decreases but the tube diameter increases. Fig. 3a shows the nanotube length L plotted against the concentration of NH4F. The oxide layer thickness firstly sharply increases from 0.85 μm to the maximum 1.29 μm by increasing the concentration from 0.027 M to 0.067 M, and then decreases with further increasing the fluoride ion concentration. The higher fluoride ion concentration results in chemical dissolution of oxide nanotubes, according to reactions (1), (2) and (3) [21]: 2H2 O→O2 + 4e
−
þ
ð1Þ
+ 4H
Ti + O2 →TiO2 TiO2 + 6F
−
ð2Þ þ
+ 4H →½TiF6
2−
+ 2H2 O
ð3Þ
Actually, it can be assumed that a fluoride ion concentration of 0.067 M is the critical value to form the longest tubes according to Fig. 3a. Fig. 3b records the I–t curves under different fluoride ion concentrations ((a) 0.027 M, (b) 0.067, (c) 0.202 M, (d) 0.337 M, (e)
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Fig. 2. (a) Oxide layer thickness evolution as a function of oxidization time; the insets show the cross-sectional images of initial stage. (b) Top diameter and bottom diameter evolution of nanotubes as a function of oxidization time (all the samples were prepared at the same conditions as Fig. 1).
Fig. 3. (a) Oxide layer thickness evolution of nanotubes as a function of NH4F concentration (the layer was grown on the Ti substrate by its anodization in water/glycerol (1:1 vol. %) mixtures at 30 V for 3 h), (b) current transients recorded during anodization at different NH4F concentrations ((a) 0.027 M, (b) 0.067, (c) 0.202 M, (d) 0.337 M, (e) 0.472 M, and (f) 0.607 M).
0.472 M, and (f) 0.607 M). It can be seen that the current firstly increases up to the peak followed by an initial exponential decay. The inset in Fig. 3b indicates that the peak current increases with the fluoride ion concentration. Subsequently, the current rises again. After extended constant potential anodization, the current then drops to a steady value gradually. Similarly, the constant current also increases with the fluoride ion concentration. In the initial stage, the formation of a TiO2 compact layer in the titanium substrate results in the current exponential decay. The following slight increase of the current can be ascribed to the breakdown of the compact layer. After this period, the nanoporous structure is evolved gradually. Thereby, the current gets to a steady value. While fluoride ion concentration is lower than 0.067 M, as shown in Fig. 3a, fluoride ion is not enough to dissolve the nanotube mouth but is mainly used to drill pore at the interface between tubes and substrate. In contrast, when fluoride ion concentration is higher than 0.067 M, the redundant fluoride ions might participate in the chemical dissolution of nanotubes, which results in the decrease of layer thickness. From the I–t curve in Fig. 3b, the higher fluoride ion leads to the higher current density on the titanium substrate, which cannot only facilitate the nanotube formation but also accelerate dissolution of the tube mouth. However, the dissolution effect is more significant. Therefore, the layer thickness decreases with fluoride ion concentration. Fig. 4a shows the elemental distribution of the cross-section of nanotubes. It can be seen that, at the bottom of nanotubes, the oxygen concentration suddenly increases and that of the titanium decreases.
This phenomenon further confirms that the reaction (2) exists in the interface between the nanotube layer and the pure titanium substrate. Additionally, the element of fluoride shows a sudden increase and then keeping constant fluctuation along the nanotube depth. The peak indicated by a black arrow in Fig. 4a reveals that fluoride ion concentration is at the highest value at the bottom of nanotubes. It could be explained as follows: F− can diffuse into the tube bottom, while [TiF6]2− formed in the (3) reaction should diffuse out of the tubes simultaneously. However, the small ions (F−) is apt to diffuse faster than complex [TiF6]2−, which leads to the accumulation of the fluoride element at the tube bottom. This indicates that fluoride ions play a key factor in pore drilling and result in faster movement of the Ti/TiO2 interface into the Ti metal. Besides, fluoride ions can also lead to the chemical dissolution of the oxide at the nanotube mouth. Therefore, it can be concluded that fluoride ion concentration determines the tube length to some extent [6]. Meanwhile, it is worth to notice that some nanotubes stop growing in Fig. 4b (marked by circle 1). Similar results were also observed in the nanotubes prepared on β-type Ti–Nb–Ta–Zr alloy [16]. Some researchers suggested that tubes would survive and continue growing if keeping a sufficient acidification of the pore tip and higher currents in tubes [11]. Tubes under a critical current value would shut down. However, the results in the present work show that the fluoride ion concentration is also attributed to the “dead” nanotubes. The fluoride content on different locations (at the bottom of nanotubes, see circle 1 and 2 in Fig. 4b) was measured by EDS in Fig. 5. After repeated
Y.Q. Liang et al. / Thin Solid Films 519 (2011) 5150–5155
Fig. 4. (a) Elemental distribution of the cross-section of nanotubes, (b) cross-sectional SEM image of nanotubes (circle 1 represents the dead tube, and circle 2 represents the normal one). Both (a) and (b) are prepared in water/glycerol (1:1 vol%) mixtures containing 0.27 M NH4F at 30 V for 3 h.
measurements, the results show that the fluoride content in the bottom of “dead” nanotubes (Fig. 5b) is smaller than that in the bottom of normal tubes (Fig. 5a). Therefore, another possible reason which results in the “shut down” of tubes can be concluded as follows: the insufficient fluoride ion concentration in some certain locations would not continue drilling pores. In order to evaluate the effect of fluoride ion concentration on the amount of oxide dissolution, the dissolution model was constructed (see Fig. 6a). In this model, the conical outlines and the remaining blank parts represent the as-formed nanotube and the oxide dissolution, respectively. The amount of oxide dissolution is calculated according to the following two assumptions: (1) dissolution at the tube mouth is negligible; and (2) the nanotubes can be considered as regular conical-shape. The amount of oxide dissolution (Mdis) is expressed as follows (the deduction can be seen in the appendix), Mdis = ρ Vdis = ρd πd f 2 f = L R2 −
h i 1 2 2 2 2 R2 R2 −R − R1 −R3 : 3ðR2−R1Þ
However, Fig. 3a shows that the layer thickness decreases due to the dissolution of the tube mouth. Therefore, the effect of dissolution should be taken into account. A nanotube length of 1.29 μm (prepared in water/glycerol (1:1 vol.%) mixtures containing 0.067 M NH4F at 30 V) can be supposed as the initial length, and the
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Fig. 5. EDS spectrum of the nanotube bottom. (a) normal nanotubes, and (b) “dead” nanotubes.
subtractions between this value and all other lengths (Lmax − L) are taken as the length of the dissolved layer (see Fig. 6b). The total amount of oxide dissolution (Mtotal) is expressed as, h i 2 Mtotal = ρdVtotal = ρ πf + ðLmax –LÞπR2 = ρdπdF 2
F = f + ðLmax –LÞR2 where Vdis, Vtotal, Lmax, L and ρ are the volume of oxide dissolution regardless of the dissolution of the tube mouth, the total volume including the dissolution of the tube mouth, the nanotube length of 1.29 μm, the length of as-formed nanotubes and the density of titania. Two variables of F and f as the drilling factors to characterize the dissolution behavior of the anodic titanium oxide are defined (f just indicates the dissolution part during the drilling pores, while F includes not only dissolution during the drilling pores but also the dissolution of layer thickness). Fig. 7 gives the drilling factors of F and f in dependence of the concentration of NH4F. These two line with each other at the first two points, as the supposed first point with a tube length of 0.85 μm is not dissolved due to the lower fluoride ion concentration, while the second point with a length of 1.29 μm is taken as the initial length. It is clearly observed from Fig. 7 that the amount of oxide dissolution does not increase with the NH4F concentration. There is contradiction that the layer thickness decreases with increasing of fluoride ion concentration (NH4F concentration 0.067 M–0.607 M) due to the tube mouth dissolution
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Fig. 6. Schematic representation of oxide dissolution model. (a) Dissolution at the tube mouth is negligible, and (b) dissolution at the tube mouth is taken into consideration; the upper column drawn by broken line represents the dissolved layer, and the middle inset indicates the cross-sectional image of a cone-shape nanotube (R1 and R3 represent the outer and inner semidiameters of tube mouth, R2 and R4 represent the outer and inner one of the tube bottom).
(Fig. 3a), while the amount of oxide dissolution does not increase correspondingly but decreases to the minimum value and then increases (Fig. 7). Besides, although the tube mouth dissolution is taken into consideration and F is higher than f , two factors show similar variation trend except for the third point, which is the maximum value in the F line. It can be deduced that the tube mouth dissolution is negligible compared with the oxide dissolution during drilling pores. However, it is noticeable that the dissolved layer of nanotubes (Dlayer = F − f) increases with the NH4F concentration, as shown in the inset of Fig. 7. In any case, a steady-state situation between tube formation at the Ti/TiO2 interface and chemical dissolution of the tube mouth is established.
4. Conclusions In the present work, the nanotube growth mechanism and the effect of NH4F on the morphology of nanotubes on a pure titanium substrate were reported. A novel point was proposed that the threshold time to form a nanotube structure for pure titanium is from 0.5 h to 1 h. At the same time, a decrease of layer thickness due to electrical dissolution can be observed. After the threshold time, the nanopores begin to convert into self-organized nanotubes. The “shut
Fig. 7. Drilling factors (F and f) of nanotubes as a function of NH4F concentration for anodization in water/glycerol (1:1vol. %) mixtures at 30 V for 3 h; the inset shows the relationship between the dissolved layer of nanotubes (Dlayer = F−f) and NH4F concentration.
down” tubes were found in the cross-sectional SEM images of nanotubes. This phenomenon should result from insufficient fluoride ion concentration in these locations, which will not maintain the proper chemical environment to drill pores. Additionally, a possible physical model of oxide dissolution to evaluate the effect of fluoride ions on the formation of a nanotube was also reported. The results show that the tube mouth dissolution is negligible compared with the oxide dissolution during drilling pores. However, the dissolved layer of nanotubes (Dlayer = F − f) increases with the NH4F concentration.
Acknowledgements This work was supported by the Key Project in the Science and Technology Pillar Program of Tianjin (09ZCKFGX29100) and the Natural Science Foundation of Tianjin (08JCYBJC08900). The infra-structural supports from the Tianjin University are also acknowledged.
Appendix
R4 =
R2dR3 R1
H=
R2dL R2−R1
Y.Q. Liang et al. / Thin Solid Films 519 (2011) 5150–5155
V1 =
1 1 2 2 R2dL πR2 H = πR2 3 3 R2−R1
Mtotal = ρdVtotal = ρdπdF
V2 =
1 1 2 2 R2dL πR4 H = πR4 3 3 R2−R1
References
π R2dL 2 2 R2 −R4 Va = V1 −V2 = 3 R2−R1 V3 =
1 1 2 2 R1dL πR1 ðH−LÞ = πR1 3 3 R2−R1
V4 =
1 1 2 2 R1dL πR3 ðH−LÞ = πR3 3 3 R2−R1
Vb = V3 −V4 =
π R1dL 2 2 R1 −R3 3 R2−R1
Vtube = Va −Vb =
h i π L 2 2 2 2 R2 R2 −R4 −R1 R1 −R3 3 R2−R1
2 2 Vdis = πR2 L−Vtube = πL R2 −
2 f = L R2 L−
h i 1 2 2 2 3 R2 R2 −R4 −R1 R1 −R3 3ðR2−R1Þ
h i 1 2 2 2 2 R2 R2 −R4 −R1 R1 −R3 3ðR2−R1Þ
Mdis = ρ dVdis = ρdπdf 2
2
Vtotal = Vdis + ðLmax −LÞπR2 = πf + ðLmax −LÞπR2 = πdF 2
F = f + ðLmax −LÞR2
= πdf
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[1] R. Tenne, C.N.R. Rao, Philos. Trans. R. Soc. London Ser. A (2004) 20998 362/1823. [2] E. Balaur, J.M. Macak, H. Tsuchiya, P. Schmuki, J. Mater. Chem. 15 (42) (2005) 4488. [3] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Lett. 5 (1) (2005) 191. [4] R. Beranek, H. Tsuchiya, T. Sugishima, J.M. Macak, L. Taveira, S. Fujimoto, H. Kisch, P. Schmuki, Appl. Phys. Lett., 87/24, 2005. [5] M. Paulose, K. Shankar, O.K. Varghese, G.K. Mor, B. Hardin, C.A. Grimes, Nanotechnology 17 (5) (2006) 1446. [6] K. Shankar, G.K. Mor, H.E. Prakasam, S. Yoriya, M. Paulose, O.K. Varghese, C.A. Grimes, Nanotechnology 18 (6) (2007) 065707. [7] A. Ghicov, B. Schmidt, J. Kunze, P. Schmuki, Chem. Phys. Lett. 433 (4–6) (2007) 323. [8] Y.Q. Wang, J. Tao, L. Wang, P.T. He, T. Wang, Trans. Nonferrous Met. Soc. China 18 (3) (2008) 631. [9] F. Lindberg, J. Heinrichs, F. Ericson, P. Thomsen, H. Engqvist, Biomaterials 29 (23) (2008) 3317. [10] J. Kunze, L. Müllerr, J.M. Macak, P. Greil, P. Schmuki, F.A. Müller, Electrochim. Acta 53 (23) (2008) 6995. [11] J.M. Macak, H. Tsuchiya, P. Schmuki, Angew. Chem. Int. Edit. 44 (14) (2005) 2100. [12] J.M. Macak, K. Sirotna, P. Schmuki, Electrochim. Acta 50 (18) (2005) 3679. [13] S.P. Albu, A. Ghicov, S. Aldabergenova, P. Drechsel, D. LeClere, G.E. Thompson, J.M. Macak, P. Schmuki, Adv. Mater. 20 (21) (2008) 4135. [14] J.M. Macak, H. Hildebrand, U. Marten-Jahns, P. Schmuki, J. Electroanal. Chem. 621 (2) (2008) 254. [15] Y.Q. Liang, X.J. Yang, Z.D. Cui, S.L. Zhu, Curr. Nanosci. 6 (3) (2010) 256. [16] V.P. Parkhutik, V.I. Shershulsky, J. Phys. D Appl. Phys. 25 (8) (1992) 1258. [17] I. Sieber, H. Hildebrand, A. Friedrich, P. Schmuki, Electrochem. Commun. 7 (1) (2005) 97. [18] H. Tsuchiya, P. Schmuki, Electrochem. Commun. 7 (1) (2005) 49. [19] H. Tsuchiya, J.M. Macak, A. Ghicov, Y.C. Tang, S. Fujimoto, M. Niinomi, T. Noda, P. Schmuki, Electrochim. Acta 52 (1) (2006) 94. [20] R. Beranek, H. Hildebrand, P. Schmuki, Electrochem. Solid St. 6 (3) (2003) B12. [21] Q.Y. Cai, M. Paulose, O.K. Varghese, C.A. Grimes, J. Mater. Res. 20 (1) (2005) 230.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Study on the formation micromechanism of TiO2 nanotubes on pure titanium and the role of fluoride ions in electrolyte solutions Y.Q. Liang a, Z.D. Cui a, S.L. Zhu a,b, X.J. Yang a,b,⁎ a b
School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China Tianjin Key Laboratory of Composite and Functional Materials, Tianjin, 300072, China
a r t i c l e
i n f o
Available online 15 January 2011 Keywords: Pure titanium Nanotubes Growth mechanism Drilling factor
a b s t r a c t The present paper reports the mechanism of nanotube growth and the effect of NH4F on the nanotube formation on a pure titanium substrate. A set of experiments was performed under different polarization times (ranging from 10 min to 4 h) and different NH4F concentrations of anodization. The mechanism of nanotube growth can be explained as follows. The nanoporous structure firstly formed on the pure titanium after a threshold time (0.5 h–1 h). Then the nanopores converted into self-organized nanotubes. The “shut down” tubes were found in the cross-sectional SEM images of the nanotubes due to the insufficient fluoride ion concentration. Furthermore, a possible physical model was also reported to explain the function of fluoride ions in the formation of nanotubes. © 2011 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiments
Self-organized nanotubes produced by anodic oxidization have attracted much attention in the past decade. The method can control the titania down to nanoscale dimensions. Due to the unique properties, the titania nanotubes are expected to be used in various fields, such as sensing, solar cells, water photolysis, fuel cells, molecular filtration and tissue engineering [1–10]. In recent work, it has been shown that highly ordered high-aspect-ratio TiO2 nanotubes with single or double wall, oriented perpendicular to the substrate can be grown on titanium alloys in neutral buffers of NH4F solution or acid electrolyte of HF solution [4,11–13]. However, the effect of fluoride ions was not illustrated in detail. Moreover, some researchers suggested that the thickness of the nanotube layer substantially increases with the anodization time during the full anodizaiton process [14]. Nevertheless, the mechanism of variations of layer thickness and bottom view of nanotubes during the first anodization stage (10 min–60 min) have not yet been studied in detail. This stage is directly related to the formation of nanotubes. Accordingly, in the present study, the self-organized nanotubes by anodizing in water/glycerol mixtures containing different concentrations of NH4F were prepared. And a new viewpoint on the growth mechanism was proposed. Furthermore, the effect of NH4F on the dimension of nanotubes was discussed. Last but not the least, a possible physical model was also proposed to explain the function of F− ions in the formation of nanotubes.
Pure titanium (11 mm × 9 mm × 0.6 mm) samples were degreased in ethanol by ultrasonic, and then rinsed with deionized (DI) water and finally dried in air. Potentiostatic anodizations were carried out in a two-electrode electrochemical cell at room temperature by a highvoltage potentiostat (GPR-60300 PC). Pure titanium samples served as a working electrode, and platinum plate as a counter electrode. Samples were anodized with water/glycerol (1:1 vol.%) mixtures containing different NH4F concentrations under a potential of 30 V. The electrolytes used in this work were made of reagent grade chemicals. After potentiostatic anodization, the samples were rinsed with DI water and dried in air. The structural and morphological characters of the samples were tested using a scanning electron microscope (SEM; FE-SEM S4800; Hitachi, Tokyo, Japan) equipped with Energy-Dispersion X-ray Spectroscopy (EDS). The cross-section images were taken from mechanically bent samples, where a lift-off of the porous layer occurred.
⁎ Corresponding author. School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China. Tel.: + 86 22 27405602. E-mail address: [email protected] (X.J. Yang). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.075
3. Results and discussion In order to illustrate the formation mechanism of nanotubes, the interface between nanotubes and substrate should be investigated in detail. Fig. 1 shows the bottom SEM images taken from the samples anodized at different times (a—10 min, b—30 min, c—1 h, d—1.5 h, e—2 h, and f—2.5 h). It was observed that irregular protrusions were formed on the bottom of the nanoporous layer in Fig. 1(a) and (b). During this stage, the nanoporous oxide layer was firstly formed on the substrate [15]. After 1 h of anodization, some regular (round or oval) bottoms at local areas are exhibited in Fig. 1c, indicating the formation of initial nanotubes.
Y.Q. Liang et al. / Thin Solid Films 519 (2011) 5150–5155
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Fig. 1. SEM bottom images of different stages of the TiO2 nanotube formation. Anodization stopped after (a) 10 min, (b) 30 min, (c) 1 h, (d) 1.5 h, (e) 2 h and (f) 2.5 h at 30 V in water–glycerol electrolyte (1:1 vol%) containing 0.27 M NH4F.
However, some irregular protrusions still existed at other locations (inner area marked with white arrows). It implied that the porous oxide structure started to convert into a nanotube structure after 1 h. With the oxidization time further increasing, these regular mounds are apparent on almost the entire surface (as shown in Fig. 1d). A whole self-organized nanotube layer was fully evolved after 2 h oxidization (as shown in Fig. 1e and f). The formation of nanotube arrays in fluoride containing electrolytes is due to the following three simultaneously occurring processes: (i) field assisted oxidation of Ti metal to form titanium dioxide; (ii) field assisted dissolution of Ti metal ions in the electrolyte; and (iii) chemical dissolution of Ti and TiO2 due to etching by fluoride ions [16–18]. Fig. 2a shows the anodization time dependence of the tubular layer thickness (the thicknesses were measured using the SEM crosssectional images). With the time increasing, the layer thickness firstly increased from 10 min to 30 min, and then decreased at 1 h. Then a steeply increased state with a maximum thickness about 1.13 μm was observed when the time reached 3 h. With the anodization time further increasing, the layer thickness decreased. The first decrease of the oxide layer thickness at 1 h indicates that the top nanoporous layer dissolved after 0.5 h [19], accompanied by the conversion from a porous structure to a nanotube structure. It indicates that the threshold time to form a nanotube structure in pure titanium is from 0.5 h to 1 h in anodization. The second decrease of layer thickness at 4 h reveals that, after prolonged anodization, the TiO2 nanotube dissolved due to the formation of a soluble hexafluorotitanium complex [TiF6]2− [12]. Fig. 2b shows the top and bottom diameter evolution with the increasing
anodization time. It is obvious that top and bottom diameters of nanotubes have no significant dependence on the anodization time. Particularly, a slight increase of the top diameter at 4 h can be observed. The shape of the nanotubes is conical because the inner diameters slightly decrease with depth. After prolonged anodization (all together 4 h), the oxide formation at the metal interface cannot keep up with the dissolution of TiO2 nanotubes on the surface in the electrolyte containing F− [20]. Hence, the tube length decreases but the tube diameter increases. Fig. 3a shows the nanotube length L plotted against the concentration of NH4F. The oxide layer thickness firstly sharply increases from 0.85 μm to the maximum 1.29 μm by increasing the concentration from 0.027 M to 0.067 M, and then decreases with further increasing the fluoride ion concentration. The higher fluoride ion concentration results in chemical dissolution of oxide nanotubes, according to reactions (1), (2) and (3) [21]: 2H2 O→O2 + 4e
−
þ
ð1Þ
+ 4H
Ti + O2 →TiO2 TiO2 + 6F
−
ð2Þ þ
+ 4H →½TiF6
2−
+ 2H2 O
ð3Þ
Actually, it can be assumed that a fluoride ion concentration of 0.067 M is the critical value to form the longest tubes according to Fig. 3a. Fig. 3b records the I–t curves under different fluoride ion concentrations ((a) 0.027 M, (b) 0.067, (c) 0.202 M, (d) 0.337 M, (e)
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Fig. 2. (a) Oxide layer thickness evolution as a function of oxidization time; the insets show the cross-sectional images of initial stage. (b) Top diameter and bottom diameter evolution of nanotubes as a function of oxidization time (all the samples were prepared at the same conditions as Fig. 1).
Fig. 3. (a) Oxide layer thickness evolution of nanotubes as a function of NH4F concentration (the layer was grown on the Ti substrate by its anodization in water/glycerol (1:1 vol. %) mixtures at 30 V for 3 h), (b) current transients recorded during anodization at different NH4F concentrations ((a) 0.027 M, (b) 0.067, (c) 0.202 M, (d) 0.337 M, (e) 0.472 M, and (f) 0.607 M).
0.472 M, and (f) 0.607 M). It can be seen that the current firstly increases up to the peak followed by an initial exponential decay. The inset in Fig. 3b indicates that the peak current increases with the fluoride ion concentration. Subsequently, the current rises again. After extended constant potential anodization, the current then drops to a steady value gradually. Similarly, the constant current also increases with the fluoride ion concentration. In the initial stage, the formation of a TiO2 compact layer in the titanium substrate results in the current exponential decay. The following slight increase of the current can be ascribed to the breakdown of the compact layer. After this period, the nanoporous structure is evolved gradually. Thereby, the current gets to a steady value. While fluoride ion concentration is lower than 0.067 M, as shown in Fig. 3a, fluoride ion is not enough to dissolve the nanotube mouth but is mainly used to drill pore at the interface between tubes and substrate. In contrast, when fluoride ion concentration is higher than 0.067 M, the redundant fluoride ions might participate in the chemical dissolution of nanotubes, which results in the decrease of layer thickness. From the I–t curve in Fig. 3b, the higher fluoride ion leads to the higher current density on the titanium substrate, which cannot only facilitate the nanotube formation but also accelerate dissolution of the tube mouth. However, the dissolution effect is more significant. Therefore, the layer thickness decreases with fluoride ion concentration. Fig. 4a shows the elemental distribution of the cross-section of nanotubes. It can be seen that, at the bottom of nanotubes, the oxygen concentration suddenly increases and that of the titanium decreases.
This phenomenon further confirms that the reaction (2) exists in the interface between the nanotube layer and the pure titanium substrate. Additionally, the element of fluoride shows a sudden increase and then keeping constant fluctuation along the nanotube depth. The peak indicated by a black arrow in Fig. 4a reveals that fluoride ion concentration is at the highest value at the bottom of nanotubes. It could be explained as follows: F− can diffuse into the tube bottom, while [TiF6]2− formed in the (3) reaction should diffuse out of the tubes simultaneously. However, the small ions (F−) is apt to diffuse faster than complex [TiF6]2−, which leads to the accumulation of the fluoride element at the tube bottom. This indicates that fluoride ions play a key factor in pore drilling and result in faster movement of the Ti/TiO2 interface into the Ti metal. Besides, fluoride ions can also lead to the chemical dissolution of the oxide at the nanotube mouth. Therefore, it can be concluded that fluoride ion concentration determines the tube length to some extent [6]. Meanwhile, it is worth to notice that some nanotubes stop growing in Fig. 4b (marked by circle 1). Similar results were also observed in the nanotubes prepared on β-type Ti–Nb–Ta–Zr alloy [16]. Some researchers suggested that tubes would survive and continue growing if keeping a sufficient acidification of the pore tip and higher currents in tubes [11]. Tubes under a critical current value would shut down. However, the results in the present work show that the fluoride ion concentration is also attributed to the “dead” nanotubes. The fluoride content on different locations (at the bottom of nanotubes, see circle 1 and 2 in Fig. 4b) was measured by EDS in Fig. 5. After repeated
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Fig. 4. (a) Elemental distribution of the cross-section of nanotubes, (b) cross-sectional SEM image of nanotubes (circle 1 represents the dead tube, and circle 2 represents the normal one). Both (a) and (b) are prepared in water/glycerol (1:1 vol%) mixtures containing 0.27 M NH4F at 30 V for 3 h.
measurements, the results show that the fluoride content in the bottom of “dead” nanotubes (Fig. 5b) is smaller than that in the bottom of normal tubes (Fig. 5a). Therefore, another possible reason which results in the “shut down” of tubes can be concluded as follows: the insufficient fluoride ion concentration in some certain locations would not continue drilling pores. In order to evaluate the effect of fluoride ion concentration on the amount of oxide dissolution, the dissolution model was constructed (see Fig. 6a). In this model, the conical outlines and the remaining blank parts represent the as-formed nanotube and the oxide dissolution, respectively. The amount of oxide dissolution is calculated according to the following two assumptions: (1) dissolution at the tube mouth is negligible; and (2) the nanotubes can be considered as regular conical-shape. The amount of oxide dissolution (Mdis) is expressed as follows (the deduction can be seen in the appendix), Mdis = ρ Vdis = ρd πd f 2 f = L R2 −
h i 1 2 2 2 2 R2 R2 −R − R1 −R3 : 3ðR2−R1Þ
However, Fig. 3a shows that the layer thickness decreases due to the dissolution of the tube mouth. Therefore, the effect of dissolution should be taken into account. A nanotube length of 1.29 μm (prepared in water/glycerol (1:1 vol.%) mixtures containing 0.067 M NH4F at 30 V) can be supposed as the initial length, and the
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Fig. 5. EDS spectrum of the nanotube bottom. (a) normal nanotubes, and (b) “dead” nanotubes.
subtractions between this value and all other lengths (Lmax − L) are taken as the length of the dissolved layer (see Fig. 6b). The total amount of oxide dissolution (Mtotal) is expressed as, h i 2 Mtotal = ρdVtotal = ρ πf + ðLmax –LÞπR2 = ρdπdF 2
F = f + ðLmax –LÞR2 where Vdis, Vtotal, Lmax, L and ρ are the volume of oxide dissolution regardless of the dissolution of the tube mouth, the total volume including the dissolution of the tube mouth, the nanotube length of 1.29 μm, the length of as-formed nanotubes and the density of titania. Two variables of F and f as the drilling factors to characterize the dissolution behavior of the anodic titanium oxide are defined (f just indicates the dissolution part during the drilling pores, while F includes not only dissolution during the drilling pores but also the dissolution of layer thickness). Fig. 7 gives the drilling factors of F and f in dependence of the concentration of NH4F. These two line with each other at the first two points, as the supposed first point with a tube length of 0.85 μm is not dissolved due to the lower fluoride ion concentration, while the second point with a length of 1.29 μm is taken as the initial length. It is clearly observed from Fig. 7 that the amount of oxide dissolution does not increase with the NH4F concentration. There is contradiction that the layer thickness decreases with increasing of fluoride ion concentration (NH4F concentration 0.067 M–0.607 M) due to the tube mouth dissolution
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Fig. 6. Schematic representation of oxide dissolution model. (a) Dissolution at the tube mouth is negligible, and (b) dissolution at the tube mouth is taken into consideration; the upper column drawn by broken line represents the dissolved layer, and the middle inset indicates the cross-sectional image of a cone-shape nanotube (R1 and R3 represent the outer and inner semidiameters of tube mouth, R2 and R4 represent the outer and inner one of the tube bottom).
(Fig. 3a), while the amount of oxide dissolution does not increase correspondingly but decreases to the minimum value and then increases (Fig. 7). Besides, although the tube mouth dissolution is taken into consideration and F is higher than f , two factors show similar variation trend except for the third point, which is the maximum value in the F line. It can be deduced that the tube mouth dissolution is negligible compared with the oxide dissolution during drilling pores. However, it is noticeable that the dissolved layer of nanotubes (Dlayer = F − f) increases with the NH4F concentration, as shown in the inset of Fig. 7. In any case, a steady-state situation between tube formation at the Ti/TiO2 interface and chemical dissolution of the tube mouth is established.
4. Conclusions In the present work, the nanotube growth mechanism and the effect of NH4F on the morphology of nanotubes on a pure titanium substrate were reported. A novel point was proposed that the threshold time to form a nanotube structure for pure titanium is from 0.5 h to 1 h. At the same time, a decrease of layer thickness due to electrical dissolution can be observed. After the threshold time, the nanopores begin to convert into self-organized nanotubes. The “shut
Fig. 7. Drilling factors (F and f) of nanotubes as a function of NH4F concentration for anodization in water/glycerol (1:1vol. %) mixtures at 30 V for 3 h; the inset shows the relationship between the dissolved layer of nanotubes (Dlayer = F−f) and NH4F concentration.
down” tubes were found in the cross-sectional SEM images of nanotubes. This phenomenon should result from insufficient fluoride ion concentration in these locations, which will not maintain the proper chemical environment to drill pores. Additionally, a possible physical model of oxide dissolution to evaluate the effect of fluoride ions on the formation of a nanotube was also reported. The results show that the tube mouth dissolution is negligible compared with the oxide dissolution during drilling pores. However, the dissolved layer of nanotubes (Dlayer = F − f) increases with the NH4F concentration.
Acknowledgements This work was supported by the Key Project in the Science and Technology Pillar Program of Tianjin (09ZCKFGX29100) and the Natural Science Foundation of Tianjin (08JCYBJC08900). The infra-structural supports from the Tianjin University are also acknowledged.
Appendix
R4 =
R2dR3 R1
H=
R2dL R2−R1
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V1 =
1 1 2 2 R2dL πR2 H = πR2 3 3 R2−R1
Mtotal = ρdVtotal = ρdπdF
V2 =
1 1 2 2 R2dL πR4 H = πR4 3 3 R2−R1
References
π R2dL 2 2 R2 −R4 Va = V1 −V2 = 3 R2−R1 V3 =
1 1 2 2 R1dL πR1 ðH−LÞ = πR1 3 3 R2−R1
V4 =
1 1 2 2 R1dL πR3 ðH−LÞ = πR3 3 3 R2−R1
Vb = V3 −V4 =
π R1dL 2 2 R1 −R3 3 R2−R1
Vtube = Va −Vb =
h i π L 2 2 2 2 R2 R2 −R4 −R1 R1 −R3 3 R2−R1
2 2 Vdis = πR2 L−Vtube = πL R2 −
2 f = L R2 L−
h i 1 2 2 2 3 R2 R2 −R4 −R1 R1 −R3 3ðR2−R1Þ
h i 1 2 2 2 2 R2 R2 −R4 −R1 R1 −R3 3ðR2−R1Þ
Mdis = ρ dVdis = ρdπdf 2
2
Vtotal = Vdis + ðLmax −LÞπR2 = πf + ðLmax −LÞπR2 = πdF 2
F = f + ðLmax −LÞR2
= πdf
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