pubs.acs.org/Langmuir © 2009 American Chemical Society
Transparent TiO2 Nanotube Electrodes via Thin Layer Anodization: Fabrication and Use in Electrochromic Devices S. Berger, A. Ghicov, Y.-C. Nah, and P. Schmuki* :: Department of Materials Science, WW4-LKO, University of Erlangen-Nurnberg, Martensstrasse 7, 91058 Erlangen, Germany Received February 4, 2009. Revised Manuscript Received March 7, 2009 In the present work, we describe an anodization process that is able to fully transform a thin Ti metal layer on a conductive glass into a TiO2 nanotubular array. Under optimized conditions, nanotube electrodes can be obtained that are completely transparent and defect-free and allow electrochromic switching. These electrochromic electrodes show remarkable properties and can be directly integrated into devices.
Introduction Anodic TiO2 nanotubes have attracted considerable scientific interest over the past few years. Since the first porous oxide layers were anodically grown on Ti metal in 1999,1 the anodic structures have been constantly improved and tailored toward a wide range of possible applications.2,3 The unique properties of TiO2 nanotubes are nowadays explored in many fields ranging from catalyst systems4-6 and sensors7 to biomedicine,8,9 solar energy conversion,10,11 and electrochromic devices.12-15 Such electrochromic devices are based on the switchability of the color of a material by an external voltage. This typically originates from the intercalation of protons or small ions causing additional electronic states in the band gap of materials such as WO3,16,17 TiO2,18,19 MoO3,20 and Nb2O5.21 The specific properties of TiO2 nanotube arrays fit well with electrochromic applications because *Corresponding author. Tel: +49-9131-852-7575. Fax: +49-9131-8527582. E-mail:
[email protected]. (1) Zwilling, V.; Aucounturier, E.; Darque-Ceretti, E. Electrochim. Acta 1999, 45, 921. (2) Macak, J. M.; Tsuchiya, H.; Ghicov, A.; Yasuda, K.; Hahn, R.; Bauer, S.; Schmuki, P. Curr. Opin. Solid State Mater. Sci. 2007, 11, 3. (3) Ghicov, A.; Schmuki, P. Chem. Commun., accepted, 2009. (4) Macak, J. M.; Barczuk, P. J.; Tsuchiya, H.; Nowakowska, M. Z.; Ghicov, A.; Chojak, A.; Bauer, S.; Virtanen, S.; Kulesza, P. J.; Schmuki, P. Electrochem. Commun. 2005, 7, 9. (5) Macak, J. M.; Zlamal, M.; Krysa, J.; Schmuki, P. Small 2007, 3, 300. (6) Macak, J. M.; Schmidt-Stein, F.; Schmuki, P. Electrochem. Commun. 2007, 9, 1783. (7) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Grimes, C. A. Sens. Actuators, B 2003, 93, 338. (8) Tsuchiya, H.; Macak, J. M.; Mueller, L.; Kunze, J.; Mueller, F.; Greil, P.; Virtanen, S.; Schmuki, P. J. Biomed. Mater. Res. 2006, 77, 534. (9) Park, J.; Bauer, S.; v. d. Mark, K.; Schmuki, P. Nano Lett. 2007, 7, 1686. (10) Hahn, R.; Stergiopoulus, T.; Macak, J. M.; Tsoukleris, D.; Kontos, A. G.; Albu, S. P.; Kim, D.; Ghicov, A.; Kunze, J.; Falaras, P.; Schmuki, P. Phys. Status Solidi 2007, 1, 135. (11) Jennings, J. R.; Ghicov, A.; Peter, L. M.; Schmuki, P.; Walker, A. B. J. Am. Chem. Soc. 2008, 130, 13364. (12) Ghicov, A.; Schmidt, B.; Kunze, J.; Schmuki, P. Chem. Phys. Lett. 2007, 433, 323. (13) Ghicov, A.; Tsuchiya, H.; Hahn, R.; Macak, J. M.; Munoz, A. G.; Schmuki, P. Electrochem. Commun. 2005, 8, 528. (14) Hahn, R.; Ghicov, A.; Tsuchiya, H.; Macak, J. M.; Munoz, A. G.; Schmuki, P. Phys. Status Solidi A 2007, 204, 1281. (15) Nah, Y.-C.; Ghicov, A.; Kim, D.; Berger, S.; Schmuki, P. J. Am. Chem. Soc. 2008, 130, 16154. (16) Deb, S. K. Philos. Mag. 1973, 21, 801. (17) Deb, S. K. Sol. Energy Mater. Sol. Cells 2008, 92, 245. (18) Seike, T.; Nagai J. Sol. Energy. Mater. 1991, 22, 107. (19) Oezer, N. Thin Solid Films 1992, 214, 17. (20) Hsu, C.-S.; Chan, C.-C.; Huang, H. T.; Peng, C.-H.; Hsu, W.-C. Thin Solid Films 2008, 516, 4839. (21) Reichmann, B.; Bard, A. J. J. Electrochem. Soc. 1980, 127, 241.
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the layers possess a high surface area at low thickness and the aligned tubular nature of the grown oxide is highly beneficial for transparency. However, until now nanoparticulate systems have mostly been used as an electrochromic insertion host for transparent electrochromic devices18,19,22,23 as a result of the ease of the fabrication process. While TiO2 nanoparticulate layers can be dispersed easily on conductive glass, until now the nanotubular layers had to be transferred to conductive glass by a lift off and attachment process in order to achieve transparent electrochromic devices.24 A more straightforward approach is to grow the nanotubular layer directly by the total anodization of a thin film of Ti metal deposited on a glass substrate. However, the anodization of thin films to obtain defect-free electrochromic layers is a challenge because of two key problems: (i) Usually, an initiation phase with disordered oxide growth occurs before self-ordering is established. In fact, under most anodization conditions the thin metal layer is completely consumed before ordering is achieved. (ii) In many electrolytes, anodization is accompanied by significant TiO2 losses due to competing chemical dissolution. Several approaches were explored to tackle these problems, such as low-temperature anodization25 or anodization in specific electrolytes.2,26 However, a remaining key problem was to stop the anodization process at the interface to the substrate to convert the Ti layer fully into oxide without disbonding and lift-off problems and, in particular, to avoid local shorts in the layer that affect the defect-free electrochromic switching of the layers. In the present work, we show an anodization process that allows us to fully convert a thin Ti layer on a conductive glass support into a highly ordered transparent TiO2 nanotubular array. It is shown that by using this process a fully operative, transparent and defect-free electrochromic host electrode can be obtained that can be integrated into a prototype electrochromic device.
Materials and Methods The samples used were 20 mm 20 mm ITO-coated glass substrates onto which a thin Ti layer was sputter deposited. (22) Vlachopoulos, N.; Liska, P.; Augustynsky, J.; Graetzel, M. J. Am. Chem. Soc. 1988, 110, 1216. (23) Hagfeldt, A.; Vlachopoulos, N.; Graetzel, M. J. Electrochem. Soc. 1994, 141, L82. (24) Ghicov, A.; Albu, S. P.; Macak, J. M.; Schmuki, P. Small 2008, 4(8), 1063. (25) Macak, J. M.; Tsuchiya, H.; Berger, S.; Bauer, S.; Fujimoto, S.; Schmuki, P. Chem. Phys. Lett. 2006, 421, 428. (26) Berger, S.; Macak, J. M.; Kunze, J.; Schmuki, P. Electrochem. Solid State Lett. 2008, 11, C37.
Published on Web 3/31/2009
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Letter The 1-μm-thick Ti metal layers were obtained from Nuricell Inc. (Korea) where DC sputtering was performed in an Ar atmosphere at 8 10-3 mbar (base pressure 6.6 10-7 mbar) with a deposition rate of 10 nm/min at 200 W and a substrate rotation speed of 4 rpm. Details on the sample preparation and anodization setup were given previously.26 The importance of using an optimally sputtered Ti layer is illustrated in the Supporting Information (S1-S5). Anodization treatments were carried out using a Jaissle IMP 88 high-voltage potentiostat and consisted of a voltage ramp from 0 V to the potential of 20 V with a sweep rate of 1 V/s, followed by holding the potential at 20 V for a certain time. High-purity glycerol containing 0.7 M NH4F and 2.5 vol % water was used as the electrolyte. The nanotubular layers on the glass supports were annealed at 450 °C for 3 h to transform the as-grown nanotubular array completely into an anatase structure.27-29 For lithium insertion and electrochromic measurements, the samples were pressed against an O-ring in an electrochemical cell with a built-in quarz glass window, Pt gauze as the counter electrode, and a Ag/AgCl electrode as the reference electrode. The electrolyte was 1 M LiClO4 (99% anhydrous, Chempur) in propylene carbonate (PC) (99.7%, anhydrous, Sigma-Aldrich). For the cycling voltammograms and the chronoamperometric measurements, an Autolab/PGSTAT30 setup was used. Optical transmission was measured using a USB 2000 white-light source with a wavelength of 600 nm and a Newport 1830-C digital optical power meter. The optical transmission of the transparent conductive glass was subtracted from the measured data to obtain the real transmittance through the TiO2 nanotubular layer.
Figure 1. Current voltage transient during anodization of a thin Ti layer on glass. Samples were taken out after III. The dashed line is the general transient found for continuative anodization. In the inset images, top views of the layers obtained at points (a) and (b) of the curve are shown.
Results and Discussion Figure 1 shows the current time characteristics during the anodization of the metallic Ti layer on the glass/ITO substrate. The electrolyte was glycerol containing 0.7 M NH4F and 2.5 vol % water. This composition of the electrolyte was selected after a series of preliminary experiments. In particular, it represents an optimum among achieving fast tube formation, entire layer anodization, and complete removal of the initiation layer (which, if present, blocks the electrolyte access to the upper part of the nanotubular layer26). The transient can be divided into four parts: (I) a current drop due to the formation of an initial oxide layer on the Ti metal surface; (II) a steady-state region assigned to the growth of a nanotubular layer; (III) a drop in the current density that can be assigned to reaching the Ti/ITO interface; (IV) an increase in the current density caused by breakdown events and the dissolution of the ITO layer, followed by a sharp drop due to complete consumption of all conductive material and reaching of the bare glass substrate (Supporting Information S6). A series of experiments showed that optimized metal conversion is achieved if anodization is stopped and the sample is removed from the electrolyte at the end of stage III (as indicated in Figure 1). In this case, regular nanotubular layers as shown in inset image (a) could be obtained. Extended anodization leads to the morphology as shown in inset (b) revealing a patchy surface of strongly etched and collapsed nanotubes. Upon extended anodization in region IV, deep flaws down to the glass substrate become apparent, and local disbonding occurs. In other words, by following the current transient during anodization, a highly reproducible indicator for terminating the anodization is provided by the end of region III (in point (a)). The quality and homogeneity of the TiO2 nanotubular array is also evident from optical images of the samples (27) Albu, S. P.; Ghicov, A.; Aldabergenova, S.; Drechsel, P.; LeClere, D.; Thompson, G. E.; Macak, J. M.; Schmuki, P. Adv. Mater. 2008, 20, 4135. (28) Macak, J. M.; Aldabergerova, S.; Ghicov, A.; Schmuki, P. Phys. Status Solidi A 2006, 203, R67. (29) Ghicov, A.; Tsuchiya, H.; Macak, J. M.; Schmuki, P. Phys. Status Solidi A 2006, 203, R28.
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Figure 2. SEM and optical microscope images of the TiO2 nanotubular layers on glass/ITO substrates prior to (a, b) and after (c, d, e) the annealing (3 h, 450 °C, air) and the related XRD spectra (f).
after the anodization process. It should be noted that the quality of the sputtered Ti layer is crucial in growing suitable nanotubular arrays (Supporting Information S1-S5). Figure 2a shows the circular anodized area with a diameter of approximately 1 cm for a sample anodized under optimized conditions (removed in point (a), see Figure 1). Here, the as-grown nanotubular layers are only partially transparent. Although the area along the border (O-ring) of the anodized sample is fully transparent, a large area of the sample is brownish in color and has poor transparency. This can be attributed to residues of metallic titanium still present underneath the nanotubular layer. To fully convert the layer to TiO2 and to achieve a defined crystalline structure, subsequent heat Langmuir 2009, 25(9), 4841–4844
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treatment was performed.24 In Figure 2f, XRD spectra of the asgrown and annealed TiO2 nanotubular layers on the glass/ITO substrate are shown. Clearly, the amorphous layer after the annealing treatment at 450° for 3 h in air is completely transformed into an anatase structure whereas all of the other peaks could be related to the remaining ITO coating and an amorphous shoulder could be related to the glass substrate. An optical microscope image of the annealed TiO2/ITO/glass electrode is shown in Figure 2c. The anodized area of the electrode is now fully transparent. The SEM images shown in Figure 2b,d,e indicate that the morphology of the TiO2 nanotubes does not significantly change during the annealing treatment. The vertically aligned TiO2 nanotubes remain intact and have a diameter of approximately 60 nm. The layer has an overall thickness of approximately 1.1 μm. These transparent nanotube electrodes were electrochemically characterized in view of electrochromic applications. Figure 3a shows a cyclic voltammogram of the nanotubular layer in 1 M LiClO4 electrolyte. The reductive and oxidative peaks can be easily identified at peak potentials of -1.5 and -0.7 V, respectively. These values are in line with former work on TiO2 nanotubular layers on Ti substrates.14 In the literature, the cathodic peak is ascribed to Ti4+ reduction coupled with Li+ intercalation, as shown in eq 1, whereas the anodic peak is assigned to the Li+ release.23 TiO2 þ xLi þ þ xe - TLix TiO2
ð1Þ
The absolute cathodic charge calculated from the voltammogram is 37.28 mC/cm2 whereas the absolute anodic charge is approximately 34.68 mC/cm2, resulting in a reversibility ratio (Qa/Qc) of 0.93. Additional information that can be gained out of the present cathodic and anodic charges is that the influence of the H+ uptake from the H2O traces in the nonaqueous electrolyte can be neglected because the difference in the charges is very small. Furthermore, experiments using the same electrolyte but replacing LiClO4 by NaClO4 did not show any coloration. Therefore, the switch in coloration can be attributed to Li+ intercalation rather than H+ effects. As reported in the literature, the Li+ intercalation into the TiO2 structure is accompanied by a coloration of the electrode;30 therefore, a change in the transmission is obtained. In Figure 3b, an optical transmission measurement on the TiO2 nanotubular layer upon a voltage step from 0 to -1.65 V and back to 0 V is shown. The response times (τ) of the nanotubular layers are defined in the image as the time passed until the coloration or decoloration reaches 90% of the maximum. For the coloration of the layer (i.e., minimum transmittance), τc results in approximately 18 s. The response time of the bleaching (τb) is approximately 9 s. This time constant may be attributed to the solid-state diffusion of Li+ to a depth of only a few nanometers.31,32 At 0 V, the TiO2 nanotubular layer is in the transparent state; quantitatively, after applying the cathodic potential, the transmittance of the layer changes by 80%. If the layer is again polarized anodically, then the transmittance changes back to the original value. The coloration efficiency (CE) of the nanotubular layer can be calculated using eq 2
Figure 3. (a) Cyclic voltammogram (5 mV/s) of a transparent TiO2 nanotubular electrode in 1 M LiClO4. Voltage scanned from 0 to -1.65 V vs Ag/AgCl. (b) Optical transmittance measurement of TiO2 nanotubular layers at 600 nm during cathodic (-1.65V) and anodic (0 V) pulsing. (c) Stability measurement of TiO2 nanotubular layers. Optical transmittance at 600 nm for 30 switching cycles (-1.65/0 V for 30 s).
ð2Þ
with the total charge passed (Q = 52 mC/cm2) calculated from current time response data and the transmittance in the bleached state (Tbleached = 99%) and in the colored state (Tcolored = 17%) taken from Figure 3b. The coloration efficiency calculated is approximately 15 cm2 C-1. This value is in good accordance with efficiencies measured for nanoparticulate oxide films.33-35 Another important requirement for an electrochromic electrode is its cycling stability. Figure 3c shows the optical transmittance over 30 cycles (0 to -1.65 V). It is clearly visible that the transmittance characteristic does not change and that after
(30) Cronemeyer, D. C. Phys. Rev. 1959, 113, 1222. (31) Pelouchova, H.; Janda, P.; Weber, J.; Kavan, L. J. Electroanal. Chem. 2004, 566, 73. (32) Lindstroem, H.; Soedergren, S.; Solbrand, A.; Rensmo, H.; Hjelm, J.; Hagfeldt, A.; Lindquist, S.-E. J. Phys. Chem. B 1997, 101, 7717.
(33) Granqvist, C. G. Handbook of Inorganic Electrochromic Materials; Elsevier: Amsterdam, 1995. (34) Dinha, N. N.; Oanha, N.Th.T.; Longa, P. D.; Bernardb, M. C.; Hugot-Le Goffb, A. Thin Solid Films 2003, 423, 70. (35) Bonh^ot, P.; Gogniat, E.; Graetzel, M.; Ashrit, P. V. Thin Solid Films 1999, 350, 269.
T
CEðλÞ ¼
Δoptical densityðλÞ log Tbleached colored ¼ ðcm2 C -1 Þ Q Q
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Figure 4. Schematic sketch and real images of the electrochromic device prototype built from the transparent nanotubular host electrode in the different coloration states upon anodic (bleached) and cathodic (colored) polarization.
30 cycles the same transmittance decrease/increase as in the first cycle can be observed. Finally, an electrochromic device prototype was built using the as-prepared TiO2/ITO/glass electrode, as schematically outlined in Figure 4. The device prototype was assembled using the TiO2 nanotube/ITO electrode as the electrochromic host and ITOcoated glass serving as the counter electrode. A sealing polymer was used to prevent leakage of the electrolyte and shortening of the device. The electrolyte, being 1 M LiCLO4 in propylene carbonate, was filled in through a hole in the covering glass, and voltage steps were applied between the top and bottom electrodes. In the lower part of Figure 4, the electrochromic device prototype is shown under anodic and cathodic polarization. On the left-hand side under anodic voltage, the bleached
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state is shown, and on the right-hand side, the colored state under cathodic polarization is depicted. It should be mentioned here that because of potential losses within the prototype cell a higher cathodic potential was needed to switch the color of the device. However, the prototype cell shows that the nanotubular electrochromic host electrodes can be easily adapted and incorporated into a “defect-free” electrochromic device. For a defect free color change, the quality of the presented anodization approach is of utmost importance because comparably small defects lead to nonuniform layer coloration (Supporting Information S7). In summary, the present work demonstrates the feasibility of direct anodic transformation of Ti thin layers on glass substrates into transparent TiO2 nanotubular arrays. The optimization of the anodization process and the ease of controlling the process by monitoring the current density during anodization is a basic advantage of this fabrication method. The potential use of these layers as an electrochromic host electrode is investigated by means of electrochemical and optical measurements. The obtained layers show remarkably good contrast behavior and a high cycling stability. The first experiments performed on a prototype electrochromic device indicate that these nanotubular TiO2/ITO/glass electrodes can be easily adapted to full electrochromic devices. Acknowledgment. We acknowledge Mr. Nam at Nuricell Ltd. for supplying the Ti-coated glass substrates and Anja Friedrich for the SEM investigations. DFG is acknowledged for the financial support of this work. Supporting Information Available: Information on different sputtered Ti layers and the importance of the optimized anodization treatment for the final device. Movie of the prototype electrochromic device. This material is available free of charge via the Internet at http://pubs.acs.org.
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