rapid anodic growth of tio2 and wo3 nanotubes in fluoride free electrolytes

Rapid anodic growth of TiO 2 and WO 3 nanotubes in fluoride free electrolytes R. Hahn, J.M. Macak, P. Schmuki * Department of Materials Science, WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, D-91058 Erlangen, Germany Received 17 November 2006; received in revised form 28 November 2006; accepted 30 November 2006 Available online 5 January 2007 Abstract In the present work we report on the formation of bundles of high aspect ratio TiO 2 nanotubes and WO 3 nanopores structures with very thin tube or pore walls using anodization under ‘‘high voltage’’ conditions in perchlorate or chloride containing electrolytes. The bundles of TiO 2 nanotubes consist of separated tubes with diameters in the range of approximately 20–40 nm and the WO 3 nanopores consist of pores with diameters in the range of 30–50 nm. Growth occurs locally at specific surface locations. Both the TiO 2 and the WO 3 structures can be grown up to several dozens of micrometers in length within few minutes. We suggest that the growth of these high aspect structures is initiated by localized anodic breakdown event, triggered by a sufficiently high applied anodic field. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Titanium dioxide; Tungsten trioxide; Nanotubes; Nanopores; Anodization 1. Introduction TiO 2 and WO 3 are technologically very important semi- conductive materials that provide a broad range of specific properties. These make the materials applicable in photo- catalysis [1], solar cells [2], photolysis [3] , sensing [4], and electrochromic devices [5,6]. As these materials become very important in nanotechnology for making very small functional devices with different purposes, it is of high sci- entific and technological interest to find different strategies, which allow production of nanostructures of these materi- als in a cheap, tunable and easily controllable manner. Classical approaches to produce for instance nanoporous or nanoparticulate TiO 2 layers include typically sol–gel or hydrothermal processes using alkoxides as a starting mate- rial [7]. Recently, self-organized TiO 2 nanotubes could be grown on Ti [8] using a relatively simple electrochemical approach, that is, anodization in an acidic electrolyte con- taining fluorides. Later, it was shown that fluoride contain- ing electrolytes could be used to grow tubular or porous oxides also on other valve metals such as Nb, Ta, Hf, Zr, and W [9–13]. In all these works, fluoride anions were used to establish conditions that mildly dissolve the anodic oxi- des while the anodic bias permanently provides new oxide growth. After establishing a steady state between oxide for- mation and dissolution, an equilibrium situation can be achieved leading to nanotubular or nanoporous oxide lay- ers. This process usually takes up to several hours. For instance, in case of Ti, it has been shown that the growth of nanotubes with different diameters and lengths up to aspect ratios of $2000 can be achieved [14–19]. Several important applications have already been found for these structures, such as high photoelectro- chemical performance under UV [20–22] and visible light illumination [23–25], hydrogen sensing [26], catalysis [27– 29], wettability control [30], electrochromism [31],and biological applications [32–34]. Recently, Masuda and coworkers presented a striking alternative approach show- ing first experimental findings [35] that using electrolytes 1388-2481/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.11.037 * Corresponding author. Tel.: +49 9131 852 7575; fax: +49 9131 852 7582. E-mail address: schmuki@ww.uni-erlangen.de (P. Schmuki). www.elsevier.com/locate/elecom Electrochemistry Communications 9 (2007) 947–952 containing perchlorate anions and using a set of specific anodization conditions, it is possible to form bundles of high aspect ratio TiO 2 nanotubes on Ti under very rapid growth conditions. These structures are intended to be used in dye-senstitized solar cells. In the present work, we investigate the anodization of Ti and W substrates in aqueous electrolytes with perchlorates or chlorides additions, to explore the general feasibility of this princi- ple and to gain some insight into the growth mechanism of this novel growth approach. 2. Experimental part Titanium and tungsten foils (0.1 mm, 99.6% purity, Advent Materials) were prior to electrochemical experi- ments degreased by sonication in acetone, isopropanol and metha nol, afterwards rinsed with deionized (DI) water and finally dried in nitrogen stream. The samples were pressed together with a Cu-plate contact against an O-ring in an electrochemical cell (1 cm 2 exposed to the electrolyte) and anodized at different potentials in the range of Fig. 1. SEM images of (a) TiO 2 nanotubes prepared in 0.1 M HClO 4 at 30 V for 60 s in the cross-sectional view; (b) WO 3 nanopores prepared in 0.1 M HClO 4 at 50 V for 60 s in the cross-sectional view; (c) TiO 2 nanotubes prepared in 0.3 M NaCl solution (buffered pH 4) at 40 V for 60 s in the cross- sectional view; (d) WO 3 nanopores prepared in 0.3 M NaCl (buffered pH 4) at 50 V for 60 s in the cross-sectional view. 948 R. Hahn et al. / Electrochemistry Communications 9 (2007) 947–952 10–100 V in aqueous electrolytes containing HClO 4 and NaClO 4 (0.01; 0.05; 0.1, 1 M). Anodization was carried out by stepping the potential to the desired value and hold- ing it at the final value for a given time (typically several minutes). For the electrochemical experiments, a high-volt- age potentiostat Jaissle IM P 88 and a conventional three- electrode configuration with a platinum gauze as a counter electrode and the Haber–Luggin capillary with Ag/AgCl (1 M KCl) as a reference electrode were used. All electro- lytes were prepared from reagent grade chemicals. Some experiments were conducted at lower temperatures using a Lauda RM6 thermostat with a cooling coil, which was directly immersed in the electrolyte solution. A scanning electron microscope Hitachi FE-SEM S4800 and a trans- mission electron microscope Phillips CM 30 T/STEM were employed for the morphological and structural character- ization of the formed layers. Energy dispersive X-ray ana- lyser (EDX) fitted to the SEM chamber was used for determining the composition. 3. Result s and discussion After some preliminary anodization experiments it was clear that using potential steps in perchlorate or chloride containing electrolytes, passivity breakdown conditions could be established – the latter being in line with extended Fig. 1 (continued ) R. Hahn et al. / Electrochemistry Communications 9 (2007) 947–952 949 work on ‘‘pitting corrosion’’ on Ti, see e.g., Ref. [36]. The result is that specific spots on the electrode surface become activated and very high current densities are observed. When stopping this process, after a few minutes, one can see (by eye) several white spots on the sample surface. Using a FE-SEM and zooming in on these locations, one can clearly observe nanotubular morphologies as shown in Fig. 1. Fig. 1a and b shows SEM images of bundles of closely packed TiO 2 nanotubes prepared in ClO À 4 and Cl À solutions. The tubes have an average diameter of 40 nm, a length of 30 lm, and a wall thickness of about 10 nm. Fig. 1c and d shows SEM images of nanoporous WO 3 prepared in ClO À 4 and Cl À containing electrolytes. In this case, bundles of WO 3 nanopore structures show an average pore diameter of 40 nm, and a structure length of 16 lm. Based on our direct observations and as confirmed by a time sequence of experiments, the tube growth in every case starts randomly on certain points on the surface, and the amount of these tubular bundles increases with anodiza- tion time until the whole surface is covered. In order to form these nanostructured materials, sufficiently ‘‘harsh’’ anodization conditions must be established to cause break- down events during the experiments. Specifically the elec- trolyte composition, temperature and applied potential must be such that the anodized metals undergo localized breakdown. For breakdown to occu r in the case of TiO 2 (or WO 3 ) in chloride containing electrolytes, typically potentials of several 10 V must be applied [37–39]. Further, it is very impor tant how these potentials are applied, either by sweeping or by stepping the voltage. This fact influences the formed oxide layers in terms of its density, porosity, and defects [40]. When we applied the potential by sweep- ing (‘‘mild anodization’’) to relatively high potentials (up to 80 V) only the formation of compact TiO 2 and WO 3 layers with thicknesses proportional to the app lied potentials could be observed. However, when the potential is stepped, a completely different situation occurs, i.e., breakdown events take place (as a result of the higher applied field strength) and as a side-effect, formation of the nanostruc- tures takes place. This very different behaviour is demon- strated in Fig. 2a. In one case, it shows the current–time Fig. 2. (a) Current transients of Ti sample anodized in 0.05 M HClO 4 at 30 V final potential recorded after a potential step and a potential sweep (with 1 V/s); (b) current transients of W sample anodized in 0.1 M HClO 4 at 50 V final potential after a potential step and a potential sweep (1 V/s); (c) current transients of Ti sample recorded after 20 V potential step in aqueous solutions containing different HClO 4 concentrations. Insets are typically SEM top views of the surface acquired under these conditions. 950 R. Hahn et al. / Electrochemistry Communications 9 (2007) 947–952 dependence recorded for Ti sample anodized in 0.05 M HClO 4 after applying the 30 V in one step and in the other case, after sweeping the potential to 30 V with 1 V/s. It is apparent that anodization occurs under very different cur- rent flow (the currents in the case of step anodization are more than 10 · higher) considering the localized nature of the events. The local dissolution currents are anticipated to be several decades higher (as expected for ‘‘pitting’’ cor- rosion [36]). For Ti as a substrate, the formation of TiO 2 nanotubes in ClO À 4 containing solution is possible over a broad range of the experimental conditions. Bundles of nanotubes can be observed between applied potentials of 15 and 60 V, in the ClO À 4 electrolyte with concentrations in the range between 0.01 and 3 M. This rapid process leads to forma- tion of flower-like bundles of nanotubes, with lengths of 3–50 lm within some minutes. Changes in the conditions do not affect the geometry of the individual tubes – rather they affect nucleation density and growth rate. The final surface coverage and amo unt of tubular bundles seem to be strongly dependent on the conditions used to induce the initial localized breakdown. Applying a high potential step leads to a higher number of initiation sites and thus to a rapid growth and coverage of the surface with nanotu- bular bundles. The same effect can be observed when using W as the substrate material instead of Ti (Fig. 2b). In the case of W, the formation of nanoporous WO 3 in ClO À 4 solution typically requires higher applied potentials (by steps), in the range between 50 and 80 V. Cooling the elec- trolytes (down to 4 °C) for both cases led to formation of similar nanostructures, but the growth rate could be decreased and in general it allowed us to use higher poten- tials, which in turn, increased the nucleation density and thus led to more homogeneous and denser coverage of the surfaces by the nanostructures. Fig. 2c presents the cur- rent transients of Ti samples recorded after a potential step to 20 V in HClO 4 solutions with different concentrations. It is evident that with higher concentrations, higher currents are recorded due to breakdown and nanotube formation. For very low HClO 4 , stable and reproducible breakdown conditions can hardly be establ ished, therefore no or only a few locations are present on the sample surface where successful nanotube growth was achieved. If the HClO 4 concentration is too high, the formed nanomaterial appears much more disordered; rough and porous struc- tures are formed instead of tubes. Therefore, not only ano- dic potential steps are required, but also the optimized concentration of the ions in the solutions as well. In order to gain more insight in these structures, we investigated them further by TEM and analyzed the com- position by EDX. Fig. 3a shows a TEM image of the TiO 2 bundles in detail and clearly demonstrates that these bundles consist of nanotubes with tube diameters of 20–40 nm and a wall thickness of up to 20 nm. X-ray dif- fraction measurements (XRD, not shown) revealed an amorphous structure of the as-formed TiO 2 nanotubes. Subsequent annealing at 450 °C for 3 h in air led to crystal- lization to anatase TiO 2 , which is of significance for, e.g., photoelectrochemical applications [1,2,21]. XRD of WO 3 nanoporous layers also revealed their amorphous struc ture. Fig. 3b shows examples of EDX spectra of the nanotubular TiO 2 and the nanoporous WO 3 structures. As can be seen for the Ti sample, there are four elements detected (Ti, O, Cl, Al). By evaluating these spectra, we found out that the approximate atomic ratio of Ti to O is 1 : 2 (matching TiO 2 ) and the Cl content in all samples is lower than 3% (from the traces of the electrolyte remaining in the tubes). The small Al peak stems from the screw of the sample holder Fig. 3. (a) TEM image of TiO 2 nanotubes; (b) EDX spectra of a nanotubular TiO 2 and a nanoporous WO 3 layer. R. Hahn et al. / Electrochemistry Communications 9 (2007) 947–952 951 used to anchor the sample. In the case of W, the ratio between W and O is 1 :3 (matching WO 3 ), but in this case, no chlorine peaks are detected. To summarize, the best and the most dense coverage can be achieved by using cooling , medium concentrations of perchlorates (0.1–0.5 M) and applying rather higher voltages (>40 V for Ti and >50 V for W) in one or just a few steps and stabilizing the pH with buffers. In contrast to well-ordered, self-organized nanotu- bular arrays grown in electrolytes containing fluorides [14– 20], the approach presented here is considerably more sto- chastic in nature. As for other localized corrosion processes initiation takes place at the ‘‘weakest’’ sites of an anodized surface. This leads to a less controlled growth and inhomo- geneous distributions not only in surface coverage but also in tube length at different locations on the surface. How- ever, as discus sed above, several measures can make the layers more uniform. Key to achieve homogeneous and dense packaging is triggering a higher number of initiation sites by a higher voltage. Other possibilities may be the introduction of predefined initiation sites or using pH-buf- fers to smoothen the pH-profile over the surface. At pres- ent, this breakdown approach does not seem to be limited to ClO À 4 or Cl À ions, but may be achieved by any ion that supports localized breakdown on anodized Ti or W surfaces. 4. Conclusions The results of the present work demonstrate that bun- dles of high aspect ratio TiO 2 nanotubes and WO 3 nanop- ores can successfully be formed using a fluoride free approach – the anodization in perchlorate or chloride con- taining electrolytes. The results clearly indicate that this method is capable of producing high aspect ratio nano- structures (several dozens of micrometers in length vs. sev- eral dozens of nanometers in diameter) in a very short time (some minutes). As the formation is induced by localized anodic breakdown and its progression, the grow th process is somewhat arbitrary. However, it is possible to tailor the conditions to achieve relatively homogeneous and dense- packed nano-bundles consisting of nanotubes or nanop- ores on the metallic surface. Acknowledgements The authors acknowledge DFG for financial support. Hans Rollig and Martin Kolacyak are acknowledged for valuable technical help. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37. [2] B. O’Regan, M. Gra ¨ tzel, Nature 353 (1991) 737. [3] M.A. Butler, R.D. Nashby, R.K. Quinn, Solid State Commun. 19 (1976) 1011. [4] O.K. Varghese, C.A. Grimes, J. Nanosci. Nanotech. 3 (2003) 277. [5] H. Tokudome, M. Miyauchi, Angew. Chem., Int. Ed. 44 (2005) 1974. [6] A.E. Aliev, H.W. Shin, Solid State Ion. 154 (2002) 425. [7] M. Gotic, M. Ivanda, A. Sekulic, S. Music, S. Popovic, A. Turkovic, K. Furic, Mater. Lett. 28 (1996) 225. [8] V. Zwilling, E. Darque-Ceretti, A. Boutry-Forveille, Electrochim. Acta 45 (1999) 921. [9] H. Tsuchiya, P. Schmuki, Electrochem. Commun. 6 (2004) 1131. [10] H. Tsuchiya, P. Schmuki, Electrochem. Commun. 7 (2005) 49. [11] I. Sieber, B. Kannan, P. Schmuki, Electrochem. Solid State 8 (2005) J10. [12] I. Sieber, H. Hildebrand, A. Friedrich, P. Schmuki, Electrochem. Commun. 7 (2005) 97. [13] H. Tsuchiya, J.M. Macak, I. Sieber, L. Taveira, A. Ghicov, K. Sirotna, P. Schmuki, Electrochem. Commun. 7 (2005) 295. [14] J.M. Macak, K. Sirotna, P. Schmuki, Electrochim. Acta 50 (2005) 3679. [15] J.M. Macak, H. Tsuchiya, P. Schmuki, Angew. Chem., Int. Ed. 44 (2005) 2100. [16] L. Taveira, J.M. Macak, H. Tsuchiya, L.F.P. Dick, P. Schmuki, J. Electrochem. Soc. 152 (2005) B405. [17] S. Albu, A. Ghicov, J.M. Macak, P. Schmuki, Phys. Status Solidi. (RRL) 1 (2007) R65. [18] H. Tsuchiya, J.M. Macak, L. Taveira, E. Balaur, A. Ghicov, K. Sirotna, P. Schmuki, Electrochem. Commun. 7 (2005) 576. [19] S. Bauer, S. Kleber, P. Schmuki, Electrochem. Commun. 8 (2006) 1321. [20] K.S. Raja, M. Misra, K. Paramguru, Electrochim. Acta 51 (2005) 154. [21] R. Beranek, H. Tsuchiya, T. Sugishima, J.M. Macak, L. Taveira, S. Fujimoto, H. Kisch, P. Schmuki, Appl. Phys. Lett. 87 (2005) 243114. [22] S. Berger, H.Tsuchiya.A. Ghicov, P. Schmuki, Appl. Phys. Lett. 88 (2006) 203119. [23] J.M. Macak, H. Tsuchiya, A. Ghicov, P. Schmuki, Electrochem. Commun. 7 (2005) 1138. [24] A. Ghicov, J.M. Macak, H. Tsuchiya, J. Kunze, V.Heaublein.L. Frey, P. Schmuki, Nanoletters 6 (2006) 1080. [25] J.M. Macak, A. Ghicov, R. Hahn, H. Tsuchiya, P. Schmuki, J. Mater. Res. 21 (2006) 2824. [26] O.K. Varghese, D. Gong, K.G. Ong, C.A. Grimes, Sensors Actuator. B93 (2003) 338. [27] J.M. Macak, P.J. Barczuk, H. Tsuchiya, M.Z. Nowakowska, A. Ghicov, M. Chojak, S. Bauer, S. Virtanen, P.J. Kulesza, P. Schmuki, Electrochem. Commun. 7 (2005) 1417. [28] P.J. Barczuk, H. Tsuchiya, J.M. Macak, P. Schmuki, D. Szymanska, O. Makowski, K. Miecznikowski, P.J. Kulesza, Electrochem. Solid State 9 (2006) E13. [29] Y. Lai, L. Sun, Y. Chen, H. Zhuang, Ch. Lin, J.W. Chin, J. Electrochem. Soc. 153 (2006) D123. [30] E. Balaur, J.M. Macak, H. Tsuchiya, P. Schmuki, J. Mater. Chem. 15 (2005) 4488. [31] A. Ghicov, H. Tsuchiya, R. Hahn, J.M. Macak, A.G. Munoz, Electrochem. Commun. 8 (2006) 528. [32] H. Tsuchiya, J.M. Macak, L. Muller, J. Kunze, F. Muller, S.P. Greil, S. Virtanen, P. Schmuki, J. Biomed. Mater. Res. 77A (2006) 534. [33] J.M. Macak, H. Tsuchiya, L. Taveira, A. Ghicov, P. Schmuki, J. Biomed. Mater. Res. 75A (2005) 928. [34] S. Oh, Ch. Daraio, L.H. Chen, T.R. Pisanic, R.R. Fin ˜ ones, S. Jim, J. Biomed. Mater. Res. 78A (2006) 97. [35] K. Nakayama, T. Kubo, A. Tsubokuira, Y. Nishikitani, H. Masuda, Abstract # 819, in: 208th ECS Meeting, Hawai, 2005. [36] S. Szklarska-Smialowska, Pitting Corrosion of Metals, NACE, Houston, 1986. [37] C.K. Dyer, J.S.L. Leach, J. Electrochem. Soc. 125 (1978) 1032. [38] N. Sato, Electrochim. Acta 16 (1971) 1683. [39] J.W. Schultze, M.M. Lohrengel, Electrochim. Acta 45 (2000) 2499. [40] T. Ohtsuka, T. Otsuki, Corros. Sci. 40 (1998) 951. 952 R. Hahn et al. / Electrochemistry Communications 9 (2007) 947–952 . Rapid anodic growth of TiO 2 and WO 3 nanotubes in fluoride free electrolytes R. Hahn, J.M. Macak, P. Schmuki * Department of Materials Science,. of 40 nm, a length of 30 lm, and a wall thickness of about 10 nm. Fig. 1c and d shows SEM images of nanoporous WO 3 prepared in ClO À 4 and Cl À containing