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Anodic formation of Ti nanorods with periodic length
Electrochemistry Communications 17 (2012) 14–17
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Anodic formation of Ti nanorods with periodic length Shanshan Huang a, c, Chengyun Ning a, c,⁎, Wanting Peng a, c, Hua Dong ⁎, b, c a b c
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China School of Bioscience and Bioengineering, South China University of Technology, Guangzhou 510006, China National engineering research center for tissue restoration and reconstruction, Guangzhou 510641, China
a r t i c l e
i n f o
Article history: Received 23 November 2011 Received in revised form 21 December 2011 Accepted 9 January 2012 Available online 16 January 2012
a b s t r a c t In this paper, one-dimensional homogeneous Ti nanorods are fabricated via selective corrosion of Ti substrate using electrochemical anodization technique. The nanorod length can be tuned periodically by tailoring the electrochemical conditions. A possible mechanism is proposed to explain the formation of Ti nanorods with periodic length. © 2012 Elsevier B.V. All rights reserved.
Keywords: Electrochemical anodization Ti nanorod Periodic length
1. Introduction One-dimensional metallic nanostructures such as nanorods, nanowires and nanotubes are of particular interest since they possess both the advantages of one-dimensional materials and of nanostructures, indicative of their unique optical, magnetic, electronic and catalytic properties [1–3]. Various metallic systems including Pd, Pt, Ag, Au, Ru and Cu etc. have been extensively investigated, and the relevant synthetic methods for these nanostructures have been explored in detail. In this paper, we demonstrate one-step synthesis of selforganized (in another word, template-free) homogeneous Ti nanorod array via selective corrosion of Ti substrate using electrochemical anodization technique. It is well known that potentiostatic and/or galvanostatic anodization of Ti in a fluoride-containing electrolyte leads to the formation of TiO2 nanotubes [4–7]. We herein prove that Ti nanorods can also be grown in a particular electrolyte containing oxalic acid and ammonium fluoride. Besides, we further illustrate that the nanorod length can be tuned periodically via tailoring the electrochemical conditions during titanium anodization. To the best of our knowledge, these results have never been reported previously.
2. Experimental 2.1. Surface treatment Ti foil (0.1 mm thick, 99.6% purity, Shenzhen, China) with the dimension of 3.5 × 3.5 cm was degreased by sonicating in acetone and ⁎ Corresponding authors. E-mail addresses: [email protected] (C. Ning), [email protected] (H. Dong). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.01.011
ethanol, treated with 1:1 (v/v) HF and HNO3 solution, followed by rinsing with deionized water and drying in a nitrogen stream. 2.2. Electrochemical anodization Electrochemical anodization was conducted in a two-electrode configuration, where Ti and Cu foils worked as the working and counter electrodes with the distance of 4.0 cm, and a mixture of 1.93 wt.% H2C2O4 and 1.45 wt.% NH4F was used as the electrolyte. Ti foils were contacted with a Cu plate and then pressed against an O-ring, leaving a dimension of 3 × 3 cm 2 exposed to the solution. Before electrochemical treatment, the Ti foil was placed in the fluoride-containing solution for 10 min. Anodization was performed by applying a constant current of 200 mA at room temperature via a DC power supply. The potential of Ti anode increased quickly from −1.02 V (vs. Ag/AgCl) to 0.06 V when the current was switched on, and then fluctuated between 0.02 V and 0.09 V until the end of the reaction (the voltage between anode and cathode was 1.00–1.05 V). The as-prepared specimens were thoroughly washed with deionized water. TiO2 nanotubes were fabricated using the same cell configuration, but with a constant voltage of 20 V for 25 min. To compare the difference in the composition of the oxidized nanorod and nanotube, an annealing process was carried out to oxidize nanorods and nanotubes. The temperature was increased from room temperature to 550 °C at a rate of 6 °C/min, and then maintained at 550 °C for 3 h. 2.3. Surface characterization FESEM (Nova Nano SEM 430) and AFM (Shimadzu, SPM-9600) were employed to characterize the morphology of nanorods. EPMA (Shimadzu, EPMA-1610) was used to compare the composition of
S. Huang et al. / Electrochemistry Communications 17 (2012) 14–17
nanorod, nanotube and foil. For characterization of annealed nanorod and nanotube, XRD was performed on Holland Pan Alytocal diffractometer and Raman spectrum was obtained on a LabRAM Aramis (HJY, France). 3. Results and discussion Fig. 1a and b shows scanning electron microscope (SEM) and atomic force microscope (AFM) images of the specimen obtained by galvanostatic anodization of Ti foil at room temperature. As can be seen, the self-organized nanostructure consists of an ordered and compact nanorod array with a uniform diameter of approximately 20 ± 4 nm and an average length of ca. 130 ± 10 nm. The aspect ratio and distribution density of the nanorods are calculated as 6.5 and 2.4 × 10 10/cm 2, respectively. It is evident that the nanostructure formed under galvanostatic mode is significantly different from that formed under potentiostatic mode using the same electrolyte which, as mentioned in the earlier literature, has a typical nanotube architecture (data not shown here). In addition to the discrepancy in top-view morphologies, the monolithic structure between nanorod array and underlying Ti substrate deviates from the double-layer structure comprising TiO2 nanotube layer and Ti substrate layer, implying the variation in composition between nanorods and nanotubes. In fact, although the peeling-off behavior is quite common for TiO2 nanotubes, we never observed the detachment of nanorod film from Ti substrate. To explain the phenomena, the electron probe micro-analysis (EPMA) was performed for the purpose of comparing the
Fig. 1. SEM (a) and AFM (b) images of nanorods. The sample was prepared by applying a constant current of 200 mA onto a Ti foil for 40 min.
15
composition of nanorod, nanotube and foil. Herein nanorod and nanotube were fabricated from foil via galvanostatic and potentiostatic anodization. EPMA results shown in Fig. 2a reveal that only Ti can be detected on the fleshly prepared nanorod surface. Similarly, Ti is the dominating element in the foil, and the presence of a trace amount of O can be attributed to the thin film of titanium oxide formed in the interface with air. In contrast, O and F are found in the nanotube except Ti, mainly due to the electrochemical formation of TiO2 and chemical dissolution of TiO2 in fluoride-containing electrolyte [5]. The resemblance of EPMA profiles between nanorod and Ti foil indicates that nanorods are made of Ti metal, rather than TiO2. We also carried out thermal annealing process and recorded the corresponding X-ray diffractometry (XRD) and Raman patterns. As seen from XRD curves in Fig. 2b, annealed nanorod and foil exhibit a characteristic diffraction peak of rutile TiO2, while nanotube shows complicated diffraction peaks including both rutile and anatase TiO2. Considering that only rutile TiO2 can be formed for the Ti substrate, it is reasonable to conceive that anatase TiO2 in nanotube is originated from the electrochemical oxidation of Ti during anodization. Raman spectrum (data not shown here) further prove the existence of rutile and anatase TiO2 in heat-treated nanotube as well as rutile TiO2 in heat-treated nanorod and foil, in good agreement with the XRD results. Thermal annealing behaviors of nanorod, nanotube and foil confirm, from another perspective, the composition of pure Ti in nanorod. A particularly interesting feature of Ti nanorods during galvanostatic anodization is that the nanorod length can be adjusted
Fig. 2. (a) EPMA profiles of nanorod, nanotube and foil; (b) XRD patterns of nanotube, nanorod and foil after heat-treatment at 550 °C. Note that nanorod and nanotube were not scratched off the Ti foil before they were subject to EPMA and XRD measurements.
16
S. Huang et al. / Electrochemistry Communications 17 (2012) 14–17
periodically by manipulation of the anodization time. Fig. 3a depicts the relative length of nanorods over the time period of 10–130 min. It is apparent that the average length of nanorods displays two periodic cycles: the first cycle starts at 20 min, reaches the vertex at 40 min and ends at 70 min, while the second cycle occurs in the range of 80–120 min with the peak appearing at 95 min. The longer nanorods at 130 min, in comparison to those at 120 min, implied the beginning of a new cycle. Fig. 3b–g lists the SEM images of nanorods obtained at the anodization time of 20, 30, 40, 60, 80, 95 min, from which the trend of short–long–short–long in nanorod length is vividly presented. However, the periodic change in nanorod length doesn't mean that the anodization of Ti is reversible. As a proof, Ti foil is etched thinner and thinner in the whole process. In addition to anodization time, other experimental conditions such as current amplitude, pH value of the electrolyte and the concentration of fluoride ion etc., also have important influences on the resulting nanorod morphology. For example, when the pH of electrolyte increases from 4.4 to around 7.0, nanoporous structure, instead of nanorod, is obtained after applying a current of 200 mA for 40 min, indicating the necessary role of H+ in the formation of nanorods. Besides, it is found that higher or lower concentration of F− cannot produce homogeneous and compact nanorods, suggesting that such nanorod structure can only be achieved within a limited range of F − concentration. In earlier studies on TiO2 nanotube formation, it is already established that TiO2 nanotubes grow as a result of a competition between electrochemical formation of TiO2 and chemical dissolution of TiO2 by fluoride ions. Actually, the formation of TiO2 nanotubes in neutral electrolyte was observed even if galvanostatic anodization was used. Based on the above analysis, a mechanism is proposed (Fig. 4), trying to explain the formation of Ti nanorod and its periodic length tuned by anodization time. In the first stage (Fig. 4a), Ti substrate is immersed in fluoride-containing acidic solution for 10 min before anodization. Due to the corrosivity of F − ions, the compact TiO2 barrier layer on the surface of Ti substrate suffers from pitting attack. Ti metal underneath the pitting sites is then exposed to the electrolyte and reacted with F − ions to form soluble, as shown in Eq. (1). In the second stage (Fig. 4b), when galvanostatic anodization starts, Ti is continuously oxidized into TiO2 (Eq. (2)). Because of the fast volume expansion during the electrooxidation of Ti, the newly-formed TiO2 layer is incompact compared to the original TiO2 layer formed in the air, and thus is quickly attacked and dissolved by acidic F − ions (Eq. (3)), resulting in the formation of Ti metal nanostructure instead of TiO2 nanostructure. þ
−
2−
Ti þ 4H þ 6F →TiF6 þ 2H2
ð1Þ
þ
−
Ti þ 2H2 O→TiO2 þ 4H þ 4e þ
−
2−
TiO2 þ 4H þ 6F →TiF6 þ 2H2 O:
ð2Þ ð3Þ
Since the available oxidization area in these pitting sites is much bigger, the pores formed in the first stage become larger as the galvanostatic anodization proceeds. H + ions produced in the electrochemical oxidation of Ti build a pH gradient between the open top and the bottom of the pores. As a consequence of local acidification, the dissolution rate of the substrate is greatly enhanced at the bottom, leading to the growth of nanorod. Meanwhile, self-organization takes place and an ordered structure is established via a natural selection process [6]. However, it should be noted that although the dissolution rate of Ti at the pore bottom is higher than that at the pore top in the beginning of the second stage, the ratio between the two dissolution rates at different positions changes with time. In the third stage (Fig. 4c), the dissolution rate of Ti at the pore bottom is restricted for the reason that the mass-transfer process of F − ions from the solution to the pore bottom is limited along with the increase in nanorod length. Reversely, the dissolution rate of Ti at the pore top is accelerated owing to the point effect (the electrical field intensity becomes higher at the nanorod tip, making it easier to be oxidized and dissolved). Therefore, the nanorods are shortened gradually. In the last stage (Fig. 4d), when the nanorod length declines to a certain extent, the dissolution rates of Ti at the pore top and bottom are restored again, and a new cycle starts. 4. Conclusion In summary, one-dimensional Ti nanorods are grown at room temperature via a simple and easy-going anodization method, which, in the authors' opinion, provides a novel way to fabricate template-free metal nanostructures. Nanoscale length control of Ti nanorod is feasible by tailoring the electrochemical parameters during anodization. Moreover, a hypothesis is proposed to interpret the mechanism, i.e., the periodic length of nanorod is mainly caused by the periodic oscillation of dissolution rate at the pore top and bottom. The Ti nanorod array with tunable length has potential applications in lots of fields, especially considering its directional electric conduction property and large surface area. Acknowledgements The authors gratefully acknowledge the financial support of National Basic Research Program of China (Grant No. 2012CB619100)
Fig. 3. (a) Relative length of nanorods fabricated by electrochemical anodization within the time period of 10–130 min. The average nanorod length obtained at 40 min is assumed as 100%, and the relative lengths of nanorods obtained at other times are defined as the ratio between the real length of nanorods at other times and that at 40 min. To describe the change in nanorod length in a vivid way, the top-view SEM images of nanorods obtained at 20 (b), 30 (c), 40 (d), 60 (e), 80 (f) and 95 min (g) are shown in this figure (b–g).
S. Huang et al. / Electrochemistry Communications 17 (2012) 14–17
17
Fig. 4. Schematic illustration of the Ti nanorod formation: (a) random pitting sites formed through chemical corrosion by F− ions; (b) growth of the pores resulting in a nanorod structure; (c) shortening of the nanorods; (d) restoration of the dissolution rates of Ti at the pore top and bottom, also beginning of a new cycle.
and the National Natural Science Foundation of China (Grant Nos. 51072057, 50872035, 21105029). References [1] A. Akimov, A. Mukherjee, C. Yu, D. Chang, A. Zibrov, P. Hemmer, H. Park, M. Lukin, Nature 450 (7168) (2007) 402–406. [2] N.R. Sieb, N. Wu, E. Majidi, R. Kukreja, N.R. Branda, B.D. Gates, ACS Nano 3 (6) (2009) 1365–1372.
[3] H.B.R. Lee, G.H. Gu, J.Y. Son, C.G. Park, H. Kim, Small 4 (12) (2008) 2247–2254. [4] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Letters 6 (2) (2006) 215–218. [5] J.M. Macak, H. Tsuchiya, L. Taveira, S. Aldabergerova, P. Schmuki, Angewandte Chemie International Edition 44 (45) (2005) 7463–7465. [6] K.S. Raja, M. Misra, K. Paramguru, Electrochimica Acta 51 (1) (2005) 154–165. [7] L.V. Taveira, J.M. Macak, K. Sirotna, L.F.P. Dick, P. Schmuki, Journal of the Electrochemical Society 153 (4) (2006) B137–B143.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Anodic formation of Ti nanorods with periodic length Shanshan Huang a, c, Chengyun Ning a, c,⁎, Wanting Peng a, c, Hua Dong ⁎, b, c a b c
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, China School of Bioscience and Bioengineering, South China University of Technology, Guangzhou 510006, China National engineering research center for tissue restoration and reconstruction, Guangzhou 510641, China
a r t i c l e
i n f o
Article history: Received 23 November 2011 Received in revised form 21 December 2011 Accepted 9 January 2012 Available online 16 January 2012
a b s t r a c t In this paper, one-dimensional homogeneous Ti nanorods are fabricated via selective corrosion of Ti substrate using electrochemical anodization technique. The nanorod length can be tuned periodically by tailoring the electrochemical conditions. A possible mechanism is proposed to explain the formation of Ti nanorods with periodic length. © 2012 Elsevier B.V. All rights reserved.
Keywords: Electrochemical anodization Ti nanorod Periodic length
1. Introduction One-dimensional metallic nanostructures such as nanorods, nanowires and nanotubes are of particular interest since they possess both the advantages of one-dimensional materials and of nanostructures, indicative of their unique optical, magnetic, electronic and catalytic properties [1–3]. Various metallic systems including Pd, Pt, Ag, Au, Ru and Cu etc. have been extensively investigated, and the relevant synthetic methods for these nanostructures have been explored in detail. In this paper, we demonstrate one-step synthesis of selforganized (in another word, template-free) homogeneous Ti nanorod array via selective corrosion of Ti substrate using electrochemical anodization technique. It is well known that potentiostatic and/or galvanostatic anodization of Ti in a fluoride-containing electrolyte leads to the formation of TiO2 nanotubes [4–7]. We herein prove that Ti nanorods can also be grown in a particular electrolyte containing oxalic acid and ammonium fluoride. Besides, we further illustrate that the nanorod length can be tuned periodically via tailoring the electrochemical conditions during titanium anodization. To the best of our knowledge, these results have never been reported previously.
2. Experimental 2.1. Surface treatment Ti foil (0.1 mm thick, 99.6% purity, Shenzhen, China) with the dimension of 3.5 × 3.5 cm was degreased by sonicating in acetone and ⁎ Corresponding authors. E-mail addresses: [email protected] (C. Ning), [email protected] (H. Dong). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.01.011
ethanol, treated with 1:1 (v/v) HF and HNO3 solution, followed by rinsing with deionized water and drying in a nitrogen stream. 2.2. Electrochemical anodization Electrochemical anodization was conducted in a two-electrode configuration, where Ti and Cu foils worked as the working and counter electrodes with the distance of 4.0 cm, and a mixture of 1.93 wt.% H2C2O4 and 1.45 wt.% NH4F was used as the electrolyte. Ti foils were contacted with a Cu plate and then pressed against an O-ring, leaving a dimension of 3 × 3 cm 2 exposed to the solution. Before electrochemical treatment, the Ti foil was placed in the fluoride-containing solution for 10 min. Anodization was performed by applying a constant current of 200 mA at room temperature via a DC power supply. The potential of Ti anode increased quickly from −1.02 V (vs. Ag/AgCl) to 0.06 V when the current was switched on, and then fluctuated between 0.02 V and 0.09 V until the end of the reaction (the voltage between anode and cathode was 1.00–1.05 V). The as-prepared specimens were thoroughly washed with deionized water. TiO2 nanotubes were fabricated using the same cell configuration, but with a constant voltage of 20 V for 25 min. To compare the difference in the composition of the oxidized nanorod and nanotube, an annealing process was carried out to oxidize nanorods and nanotubes. The temperature was increased from room temperature to 550 °C at a rate of 6 °C/min, and then maintained at 550 °C for 3 h. 2.3. Surface characterization FESEM (Nova Nano SEM 430) and AFM (Shimadzu, SPM-9600) were employed to characterize the morphology of nanorods. EPMA (Shimadzu, EPMA-1610) was used to compare the composition of
S. Huang et al. / Electrochemistry Communications 17 (2012) 14–17
nanorod, nanotube and foil. For characterization of annealed nanorod and nanotube, XRD was performed on Holland Pan Alytocal diffractometer and Raman spectrum was obtained on a LabRAM Aramis (HJY, France). 3. Results and discussion Fig. 1a and b shows scanning electron microscope (SEM) and atomic force microscope (AFM) images of the specimen obtained by galvanostatic anodization of Ti foil at room temperature. As can be seen, the self-organized nanostructure consists of an ordered and compact nanorod array with a uniform diameter of approximately 20 ± 4 nm and an average length of ca. 130 ± 10 nm. The aspect ratio and distribution density of the nanorods are calculated as 6.5 and 2.4 × 10 10/cm 2, respectively. It is evident that the nanostructure formed under galvanostatic mode is significantly different from that formed under potentiostatic mode using the same electrolyte which, as mentioned in the earlier literature, has a typical nanotube architecture (data not shown here). In addition to the discrepancy in top-view morphologies, the monolithic structure between nanorod array and underlying Ti substrate deviates from the double-layer structure comprising TiO2 nanotube layer and Ti substrate layer, implying the variation in composition between nanorods and nanotubes. In fact, although the peeling-off behavior is quite common for TiO2 nanotubes, we never observed the detachment of nanorod film from Ti substrate. To explain the phenomena, the electron probe micro-analysis (EPMA) was performed for the purpose of comparing the
Fig. 1. SEM (a) and AFM (b) images of nanorods. The sample was prepared by applying a constant current of 200 mA onto a Ti foil for 40 min.
15
composition of nanorod, nanotube and foil. Herein nanorod and nanotube were fabricated from foil via galvanostatic and potentiostatic anodization. EPMA results shown in Fig. 2a reveal that only Ti can be detected on the fleshly prepared nanorod surface. Similarly, Ti is the dominating element in the foil, and the presence of a trace amount of O can be attributed to the thin film of titanium oxide formed in the interface with air. In contrast, O and F are found in the nanotube except Ti, mainly due to the electrochemical formation of TiO2 and chemical dissolution of TiO2 in fluoride-containing electrolyte [5]. The resemblance of EPMA profiles between nanorod and Ti foil indicates that nanorods are made of Ti metal, rather than TiO2. We also carried out thermal annealing process and recorded the corresponding X-ray diffractometry (XRD) and Raman patterns. As seen from XRD curves in Fig. 2b, annealed nanorod and foil exhibit a characteristic diffraction peak of rutile TiO2, while nanotube shows complicated diffraction peaks including both rutile and anatase TiO2. Considering that only rutile TiO2 can be formed for the Ti substrate, it is reasonable to conceive that anatase TiO2 in nanotube is originated from the electrochemical oxidation of Ti during anodization. Raman spectrum (data not shown here) further prove the existence of rutile and anatase TiO2 in heat-treated nanotube as well as rutile TiO2 in heat-treated nanorod and foil, in good agreement with the XRD results. Thermal annealing behaviors of nanorod, nanotube and foil confirm, from another perspective, the composition of pure Ti in nanorod. A particularly interesting feature of Ti nanorods during galvanostatic anodization is that the nanorod length can be adjusted
Fig. 2. (a) EPMA profiles of nanorod, nanotube and foil; (b) XRD patterns of nanotube, nanorod and foil after heat-treatment at 550 °C. Note that nanorod and nanotube were not scratched off the Ti foil before they were subject to EPMA and XRD measurements.
16
S. Huang et al. / Electrochemistry Communications 17 (2012) 14–17
periodically by manipulation of the anodization time. Fig. 3a depicts the relative length of nanorods over the time period of 10–130 min. It is apparent that the average length of nanorods displays two periodic cycles: the first cycle starts at 20 min, reaches the vertex at 40 min and ends at 70 min, while the second cycle occurs in the range of 80–120 min with the peak appearing at 95 min. The longer nanorods at 130 min, in comparison to those at 120 min, implied the beginning of a new cycle. Fig. 3b–g lists the SEM images of nanorods obtained at the anodization time of 20, 30, 40, 60, 80, 95 min, from which the trend of short–long–short–long in nanorod length is vividly presented. However, the periodic change in nanorod length doesn't mean that the anodization of Ti is reversible. As a proof, Ti foil is etched thinner and thinner in the whole process. In addition to anodization time, other experimental conditions such as current amplitude, pH value of the electrolyte and the concentration of fluoride ion etc., also have important influences on the resulting nanorod morphology. For example, when the pH of electrolyte increases from 4.4 to around 7.0, nanoporous structure, instead of nanorod, is obtained after applying a current of 200 mA for 40 min, indicating the necessary role of H+ in the formation of nanorods. Besides, it is found that higher or lower concentration of F− cannot produce homogeneous and compact nanorods, suggesting that such nanorod structure can only be achieved within a limited range of F − concentration. In earlier studies on TiO2 nanotube formation, it is already established that TiO2 nanotubes grow as a result of a competition between electrochemical formation of TiO2 and chemical dissolution of TiO2 by fluoride ions. Actually, the formation of TiO2 nanotubes in neutral electrolyte was observed even if galvanostatic anodization was used. Based on the above analysis, a mechanism is proposed (Fig. 4), trying to explain the formation of Ti nanorod and its periodic length tuned by anodization time. In the first stage (Fig. 4a), Ti substrate is immersed in fluoride-containing acidic solution for 10 min before anodization. Due to the corrosivity of F − ions, the compact TiO2 barrier layer on the surface of Ti substrate suffers from pitting attack. Ti metal underneath the pitting sites is then exposed to the electrolyte and reacted with F − ions to form soluble, as shown in Eq. (1). In the second stage (Fig. 4b), when galvanostatic anodization starts, Ti is continuously oxidized into TiO2 (Eq. (2)). Because of the fast volume expansion during the electrooxidation of Ti, the newly-formed TiO2 layer is incompact compared to the original TiO2 layer formed in the air, and thus is quickly attacked and dissolved by acidic F − ions (Eq. (3)), resulting in the formation of Ti metal nanostructure instead of TiO2 nanostructure. þ
−
2−
Ti þ 4H þ 6F →TiF6 þ 2H2
ð1Þ
þ
−
Ti þ 2H2 O→TiO2 þ 4H þ 4e þ
−
2−
TiO2 þ 4H þ 6F →TiF6 þ 2H2 O:
ð2Þ ð3Þ
Since the available oxidization area in these pitting sites is much bigger, the pores formed in the first stage become larger as the galvanostatic anodization proceeds. H + ions produced in the electrochemical oxidation of Ti build a pH gradient between the open top and the bottom of the pores. As a consequence of local acidification, the dissolution rate of the substrate is greatly enhanced at the bottom, leading to the growth of nanorod. Meanwhile, self-organization takes place and an ordered structure is established via a natural selection process [6]. However, it should be noted that although the dissolution rate of Ti at the pore bottom is higher than that at the pore top in the beginning of the second stage, the ratio between the two dissolution rates at different positions changes with time. In the third stage (Fig. 4c), the dissolution rate of Ti at the pore bottom is restricted for the reason that the mass-transfer process of F − ions from the solution to the pore bottom is limited along with the increase in nanorod length. Reversely, the dissolution rate of Ti at the pore top is accelerated owing to the point effect (the electrical field intensity becomes higher at the nanorod tip, making it easier to be oxidized and dissolved). Therefore, the nanorods are shortened gradually. In the last stage (Fig. 4d), when the nanorod length declines to a certain extent, the dissolution rates of Ti at the pore top and bottom are restored again, and a new cycle starts. 4. Conclusion In summary, one-dimensional Ti nanorods are grown at room temperature via a simple and easy-going anodization method, which, in the authors' opinion, provides a novel way to fabricate template-free metal nanostructures. Nanoscale length control of Ti nanorod is feasible by tailoring the electrochemical parameters during anodization. Moreover, a hypothesis is proposed to interpret the mechanism, i.e., the periodic length of nanorod is mainly caused by the periodic oscillation of dissolution rate at the pore top and bottom. The Ti nanorod array with tunable length has potential applications in lots of fields, especially considering its directional electric conduction property and large surface area. Acknowledgements The authors gratefully acknowledge the financial support of National Basic Research Program of China (Grant No. 2012CB619100)
Fig. 3. (a) Relative length of nanorods fabricated by electrochemical anodization within the time period of 10–130 min. The average nanorod length obtained at 40 min is assumed as 100%, and the relative lengths of nanorods obtained at other times are defined as the ratio between the real length of nanorods at other times and that at 40 min. To describe the change in nanorod length in a vivid way, the top-view SEM images of nanorods obtained at 20 (b), 30 (c), 40 (d), 60 (e), 80 (f) and 95 min (g) are shown in this figure (b–g).
S. Huang et al. / Electrochemistry Communications 17 (2012) 14–17
17
Fig. 4. Schematic illustration of the Ti nanorod formation: (a) random pitting sites formed through chemical corrosion by F− ions; (b) growth of the pores resulting in a nanorod structure; (c) shortening of the nanorods; (d) restoration of the dissolution rates of Ti at the pore top and bottom, also beginning of a new cycle.
and the National Natural Science Foundation of China (Grant Nos. 51072057, 50872035, 21105029). References [1] A. Akimov, A. Mukherjee, C. Yu, D. Chang, A. Zibrov, P. Hemmer, H. Park, M. Lukin, Nature 450 (7168) (2007) 402–406. [2] N.R. Sieb, N. Wu, E. Majidi, R. Kukreja, N.R. Branda, B.D. Gates, ACS Nano 3 (6) (2009) 1365–1372.
[3] H.B.R. Lee, G.H. Gu, J.Y. Son, C.G. Park, H. Kim, Small 4 (12) (2008) 2247–2254. [4] G.K. Mor, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, Nano Letters 6 (2) (2006) 215–218. [5] J.M. Macak, H. Tsuchiya, L. Taveira, S. Aldabergerova, P. Schmuki, Angewandte Chemie International Edition 44 (45) (2005) 7463–7465. [6] K.S. Raja, M. Misra, K. Paramguru, Electrochimica Acta 51 (1) (2005) 154–165. [7] L.V. Taveira, J.M. Macak, K. Sirotna, L.F.P. Dick, P. Schmuki, Journal of the Electrochemical Society 153 (4) (2006) B137–B143.