pubs.acs.org/Langmuir © 2009 American Chemical Society
Self-Assembled TiO2 Nanotube Arrays by Anodization of Titanium in Diethylene Glycol: Approach to Extended Pore Widening Sorachon Yoriya and Craig A. Grimes* Department of Materials Science and Engineering, Department of Electrical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received June 4, 2009. Revised Manuscript Received September 18, 2009 We report on the formation of titanium dioxide nanotube arrays having the largest known pore size, approximately 350 nm diameter. The nanotube arrays are synthesized by Ti foil anodization in a diethylene glycol electrolyte containing low (0.5-2%) concentrations of hydrofluoric acid. The large pore size nanotube arrays are achieved with extended anodization durations of approximately 120 h, with the anodization duration showing a more significant effect on pore diameter than the anodization voltage. It appears that the combined effects of hydrofluoric acid content and anodization duration determine the lateral etching rate of the nanotubes, leading to the larger pore size nanotubes.
Introduction TiO2 nanotube arrays fabricated by electrochemical anodization of titanium have been used for a variety of applications including hydrogen production by water photoelectrolysis,1-3 photocatalytic reduction of CO2 to hydrocarbon fuels,4 gas sensing,5-7 dye-sensitized solar cells,8-11 and drug delivery.12-16 The rate of drug elution from the nanotubes is dependent upon nanotube length, nanotube pore size, and the polarity of the eluting molecules relative to that of TiO2.12,15,16 Larger pore sizes result in faster rates of elution, longer nanotubes result in elution periods of extended duration, while molecules having the same polarity as TiO2 are eluted faster than molecules having opposite polarity.16 Control of nanotube pore size is also desirable for application in polymer-based heterojunction solar cells where it is necessary to uniformly fill the nanotube arrays with a hole transporting polymer for maximum device performance; larger pore sizes *Corresponding author. E-mail: cgrimes.engr.psu.edu. (1) Mor, G. K.; Paulose, M.; Shankar, K.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191. (2) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (3) Varghese, O. K.; Paulose, M.; Shankar, K.; Mor, G. K.; Grimes, C. A. J. Nanosci. Nanotechnol. 2005, 5, 1158. (4) Varghese, O. K.; Paulose, M.; LaTempa, T. J.; Grimes, C. A. Nano Lett. 2009, 9, 731. (5) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. Adv. Mater. 2003, 15, 624. (6) Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Grimes, C. A. Sens. Actuators B: Chem. 2003, 93, 338. (7) Mor, G. K.; Varghese, O. K.; Paulose, M.; Grimes, C. A. Sens. Lett. 2003, 1, 42. (8) O0 Regan, B.; Gratzel, M. Nature 1991, 353, 737. (9) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nanotechnology 2007, 18, 065707. (10) Shankar, K.; Bandara, J.; Paulose, M.; Wietasch, H.; Varghese, O. K.; Mor, G. K.; LaTempa, T. J.; Grimes, C. A. Nano Lett. 2008, 8, 1654. (11) Zhu, K.; Neale, N.; Miedaner, R. A.; Frank, A. J. Nano Lett. 2007, 7, 69. (12) Popat, K. C.; Eltgroth, M.; LaTempa, T. J.; Grimes, C. A.; Desai, T. A. Small 2007, 3, 1878. (13) Peng, L.; Mendelsohn, A. D.; LaTempa, T. J.; Yoriya, S.; Grimes, C. A.; Desai, T. A. Nano Lett. 2009, 9, 1932. (14) Peng, L.; Eltgroth, M. L.; LaTempa, T. J.; Grimes, C. A.; Desai, T. A. Biomaterials 2009, 30, 1268. (15) Popat, K. C.; Leoni, L.; Grimes, C. A.; Desai, T. A. Biomaterials 2007, 28, 3188. (16) Popat, K. C.; Eltgroth, M.; LaTempa, T. J.; Grimes, C. A. Biomaterials 2007, 28, 4880.
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facilitate intercalation of the polymers into the nanotubes, thus improving interfacial contact between the TiO2 and p-type polymers for superior exciton dissociation.17,18 Fabrication of TiO2 nanotube arrays via electrochemical anodization has been extensively developed through several synthesis generations.19-22 In 2001, Gong and co-workers first reported fabrication of TiO2 nanotube arrays, with a nanotube length limited to approximately 500 nm, by anodization of Ti foils in an aqueous electrolyte containing hydrofluoric acid (HF).23 The ability to grow significantly longer nanotubes was based on the use of nonaqueous anodization electrolytes.23,24 To date, TiO2 nanotube arrays of up to 1 mm length have been achieved using ethylene glycol electrolytes.23 The electrolyte composition and pH, as well as anodization voltage, are important factors in determining the resulting nanotube length. In comparison to the relatively mature techniques developed to control nanotube array length, from 100 nm to 1 mm (a factor of 10 000), a much more limited variation of pore size has been achieved. Pore formation is controlled through competition between metal oxidation and oxide dissolution, with these variables in turn determined by anodization parameters including electrolyte properties.25-27 In water-containing electrolytes, the rate of water dissociation during anodization has been found to be a key factor in determining the porosity of the resulting anodic film,27 with (17) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Appl. Phys. Lett. 2007, 91, 152111. (18) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Varghese, O. K.; Grimes, C. A. Langmuir 2007, 23, 12445. (19) Mor, G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes, C. A. Sol. Energy Mater. 2006, 90, 2011. (20) Grimes, C. A.; Mor. G. K. TiO2 Nanotube Arrays: Synthesis, Properties, and Applications; Springer: New York, 2009. (21) Prakasam, H. E.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 7235. (22) Yoriya, S.; Prakasam, H. E.; Varghese, O. K.; Shankar, K.; Paulose, M.; Mor, G. K.; LaTempa, T. J.; Grimes, C. A. Sens. Lett. 2006, 4, 334. (23) Paulose, M.; Shankar, K.; Yoriya, S.; Prakasam, H. E.; Varghese, O. K.; Mor, G. K.; LaTempa, T. J.; Fitzgerald, A.; Grimes, C. A. J. Phys. Chem. B 2006, 110, 16179. (24) Yoriya, S.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 13770. (25) Su, Z.; Zhou, W. Adv. Mater. Commun. 2008, 20, 3663. (26) Macak, J. M.; Hildebrand, H.; Marten-Jahns, U.; Schmuki, P. J. Electroanal. Chem. 2008, 621, 254. (27) Su, Z.; Zhou, W. J. Mater. Chem. 2009, 19, 2301.
Published on Web 10/09/2009
DOI: 10.1021/la9020146
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Figure 1. Length of TiO2 nanotube arrays obtained from anodization of Ti foil sample in DEG-1%HF electrolyte at 60 V for different durations, and corresponding current density during anodization.
nanotube pore size primarily controlled by the anodization potential.28,29 For example, our earlier work on dimethyl sulfoxide/HF anodization electrolytes showed that the nanotube pore size increased, respectively, from 50 to 150 nm as the anodization potential increased from 20 to 60 V.23,24 Similarly in diethylene glycol (DEG)/HF electrolytes, larger anodization voltages resulted in larger nanotube pore sizes, 140 nm at 40 V to 190 nm at 100 V.30 We note that with the 100 V anodization potential, the pore size of the 7.5 μm long nanotube is considerably larger at the top of the tube than that at the bottom (190 nm versus 50 nm);30 a voltage drop proportional to the length of the tube results in larger pores at the top and smaller pores at the bottom.32 In 2008 we reported the largest nanotube array pore size of 190 nm, obtained using a DEG/HF electrolyte.30 Herein we show extension of the pore diameter up to 350 nm, an increase of 84%, by manipulation of anodization parameters including fluoride concentration and anodization time. We investigate pore size dependence on electrolyte acid content, anodization duration, and anodization voltage. Our ability to achieve such large nanotube pore sizes should help to facilitate their use in applications including drug delivery and heterojunction solar cells where nanotube pore size, in addition to nanotube length, are critical device parameters.
Experimental Section Titanium foil samples, 250 μm 1 cm 2.5 cm (99.7%, SigmaAldrich) were cleaned with soap, acetone, and iso-propanol before anodization. A two-electrode cell was used for anodization, with titanium foil as the anode, 1 cm 1.5 cm immersed in the electrolyte, and platinum foil as the cathode. The anodization electrolytes were prepared by mixing DEG (99.7%, SigmaAldrich) with HF (48% solution, Merck). All experiments were carried out at room temperature, approximately 22 °C. The anodization current was monitored using a Keithley (model 2000) digital multimeter interfaced with a computer. After anodization, the nanotube array samples were rinsed with isopropanol and dried with nitrogen gas. The tube morphology, including nanotube length and pore size, were determined using a (28) O0 Sullivan, J. P.; Wood, G. C. Proc. R. Soc. London, A 1970, 317, 511. (29) Jessensky, O.; Muller, F.; Gosele, U. Appl. Phys. Lett. 1998, 72, 1173. (30) Yoriya, S.; Mor, G. K.; Sharma, S.; Grimes, C. A. J. Mater. Chem. 2008, 18, 3332. (31) Shankar, K.; Mor, G. K.; Fitzgerald, A.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 21. (32) Mor, G. K.; Varghese, O. K.; Paulose, M.; Mukherjee, N.; Grimes, C. A. J. Mater. Res. 2003, 18, 2588.
418 DOI: 10.1021/la9020146
Figure 2. Pore sizes of TiO2 nanotube arrays as a function of (a) voltage (DEG electrolyte containing 2%HF, 48 h), (b) HF concentration (DEG electrolyte, 60 V, 120 h), and (c) voltage and HF concentration (120 h anodization time).
field-emission scanning electron microscope (FESEM, JEOL JSM-6300).
Results and Discussion The current density of a titanium foil anodized in DEG-1% HF electrolyte at 60 V, and variation of the resulting tube length as a function of anodization duration, is shown in Figure 1. During the first 40 h, tube growth dominates the kinetics of the anodization process with the current density increasing along with tube length. A nanotube growth rate of þ0.2 μm h-1 is obtained for the first 20 h, and þ0.4 μm h-1 is obtained for 20-40 h anodization durations. Electrolyte conductivity increases with anodization duration since more titanium ions are dissolved into the electrolyte, in turn increasing the nanotube growth rate.24 However, as the nanotube length increases, the current density is incrementally reduced with longer tube lengths, limiting the diffusion of ionic species to the metal/oxide interface. After reaching a maximum value, the current gradually decreases as chemical dissolution begins to dominate the anodization kinetics; as a result, the tube length decreases beyond 40 h with a negative growth rate of -0.1 μm h-1. Shorter tubes with longer Langmuir 2010, 26(1), 417–420
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Figure 3. FESEM images of TiO2 nanotube arrays fabricated in DEG electrolyte containing 1.0%HF, at 60 V for 120 h: (a,b) top surface using different degrees of magnification; (c) back-side bottom layer of the tubes with the sample mechanically fractured for imaging, showing one region still with its bottom plug, e.g., like the bottom of a laboratory test tube, and another section open; (d) cross-sectional view of the nanotubes.
anodization times indicate that the oxide dissolution rate is faster than the oxidation growth rate. Current density fluctuations are believed to be due to the changes in oxide thickness, the electrolyte conductivity, and potential drop across the electrolyte/oxide interface during anodization.30 The details of the time-dependent anodization current behavior of the DEG sample in Figure 1 are noticeably different from those observed in aqueous19 and organic electrolytes such as dimethyl sulfoxide24 and ethylene glycol.21 However, the same fundamental anodic reactions are generally described by the localized dissolution model,9,19,32 with the current behavior explained on the basis of diffusion-limited anodic growth at the pore bottom.21,27,33 The movement of ionic species, i.e., O2- and OH-, passing through the anodic film in each electrolyte medium is believed to govern the growth process, resulting in variations of the current-time behavior that in turn lead to the unique tube morphologies achieved using the different electrolytes. Figure 2a shows the variation in pore diameter of TiO2 nanotubes synthesized by Ti anodization in a DEG-2%HF electrolyte for 48 h with an anodization voltage ranging from 40 to 100 V; it is no surprise that the pore diameter increases with voltage. Under optimized conditions (1% HF, 60 V, and 120 h anodization duration), a 350 nm pore is obtained (see Figure 2b). The 350 nm pore is the largest TiO2 nanotube array pore diameter reported.20 Pore size tends to increase with voltage but only with the proper combination of fluoride concentration and anodization time. For example, Ti anodization using DEG-1% HF at 100 V resulted in the Ti foil completely dissolving into the electrolyte after ∼80 h. Ti anodization using DEG-2% HF at 100 V resulted in a pore size of approximately 200 nm for anodization durations of 40-60 h. Anodization in DEG-2% HF for 120 h resulted in 120 nm pores, and that in DEG-1%HF for 144 h resulted in 100 nm pores; for both synthesis conditions, (33) Lee, W.; Ji, R.; Gosele, U.; Nielsch, K. Nat. Mater. 2006, 5, 741.
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Figure 4. (a) Cross sectional drawing of a nanotube. (b) Enlarged image of the selected area of Figure 3c showing bottom pores with a constant wall thickness in enhanced color.
many of the nanotubes had broken walls due to extensive oxide etching. The combined effect of voltage and HF concentration on the resulting nanotube pore size, for 120 h anodization, is shown in Figure 2c. Figure 3 presents FESEM images of the 350 nm pore size nanotube arrays, with (a,b) showing top surface images, (c) the bottom of the tubes with the open pore region being due to mechanical fracture, and (d) a cross-sectional image. FESEM studies on the time-dependent evolution of the nanotube array structure reveal that the Ti surface is initially dominated by fibers that thin with time during 0-70 h anodizations. These fibers disappeared after ∼120 h anodization leaving behind the openpore nanotube array geometry as presented in Figure 3a,b. The 120 h duration is the stage at which the outermost fibrous layer is completely dissolved, and the oxide wall of nanotubes underneath starts being etched; the largest pore diameter nanotubes are achieved, without covering debris. Removal of the nanoscale fibers is an important advantage offered by the higher conductivity electrolytes, owing to the longer anodization durations, with chemical dissolution dominating the reaction. The conductivity of an as-prepared 1%HF-DEG electrolyte is 1.5 μS/cm; after anodizing the 3 cm2 Ti sample, 60 V for 120 h, the conductivity is approximately 120 μS/cm. This complete removal of the debris associated with neutral pH anodization of Ti is similar to that observed for dimethyl sulfoxide23,24 and formamide electrolytes.31 Since 120 h anodization durations are not particularly convenient for device fabrication, further studies will investigate DOI: 10.1021/la9020146
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the use of electrolytes having an initially higher conductivity to reduce this time. Figure 3c is an image of the bottom of the 350nm pore TiO2 nanotube array, mechanically fractured for viewing. At the bottom, the interconnected pores exhibit irregular shapes and size variation, hence the resulting tube formation is loosely packed and randomly arranged.34 As proposed by a number of anodic oxidation models, the pore diameter is proportional to the anodization voltage.28,32,35,36 As depicted in Figure 4a, the anodization voltage directly controls the thickness of the barrier oxide layer (d) and nanotube wall (w).32 The wall thickness of each pore, as highlighted in Figure 4b, is approximately the same size, 170-180 nm. The volume expansion associated with transformation of titanium to titania plays an important role in determining the self-organized nanotube array geometry.28,29 Hexagonal close packing of the nanotubes, commonly observed using ethylene glycol electrolytes, have yet to be observed using the DEG electrolyte. The self-organized hexagonal pattern arrangement is driven by repulsive forces between the pores arising from mechanical stresses associated with the titanium-to-titania volume expansion, dependent upon anodization (34) Yasuda, K.; Schmuki, P. Electrochim. Acta 2007, 52, 4053. (35) Masuda, H.; Yosuya, M.; Ishida, M. Jpn. Appl. Phys. 1998, 37, 1090. (36) Han, C. Y.; Willing, G. A.; Xiao, Z.; Wang, H. H. Langmuir 2007, 23, 1564.
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voltage and electrolyte used, during the oxide formation at the metal/oxide interface.28,29
Conclusions We have successfully synthesized self-organized TiO2 nanotube arrays with 350 nm pore diameters, the largest known value, by anodization of titanium in an HF containing DEG electrolyte. Large nanotube pores are achieved in the electrolyte containing 1% HF (60 V anodization potential) for an anodization duration of 120 h. We believe nanotube arrays of such large pore size should find utility in applications including drug delivery, where the drug is eluted from the nanotube array, and heterojunction solar cells where p-type polymers are intercalated into the nanotube arrays. Future research will focus on achieving large pore nanotube arrays with shorter anodization durations through control of the electrolyte conductivity. Acknowledgment. Support of this work by the Department of Energy under Grant DE-FG36-08GO18074 is gratefully acknowledged. S.Y. gratefully acknowledges support under the Royal Thai Government Scholarship provided by the National Metal and Materials Technology Center (MTEC), the National Science and Technology Development Agency (NSTDA), Thailand.
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