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J. Phys. Chem. C 2007, 111, 14992-14997
TiO2 Nanotube Arrays of 1000 µm Length by Anodization of Titanium Foil: Phenol Red Diffusion Maggie Paulose,† Haripriya E. Prakasam,† Oomman K. Varghese,‡ Lily Peng,§ Ketul C. Popat,| Gopal K. Mor,† Tejal A. Desai,§ and Craig A. Grimes*,‡ Department of Electrical Engineering and Department of Materials Science and Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, SentechBiomed Corporation, 200 InnoVation BouleVard, State College, PennsylVania 16803, Department of Physiology and DiVision of Bioengineering, UniVersity of California San Francisco, San Francisco, California 94158, and Department of Mechanical Engineering, School of Bioengineering, Colorado State UniVersity, Fort Collins, Colorado 80521 ReceiVed: July 5, 2007; In Final Form: August 3, 2007
We report for the first time fabrication of self-aligned hexagonally closed-packed titania nanotube arrays of over 1000 µm in length and aspect ratio ≈10 000 by potentiostatic anodization of titanium. We describe a process by which such thick nanotube array films can be transformed into self-standing, flat or cylindrical, mechanically robust, polycrystalline TiO2 membranes of precisely controlled nanoscale porosity. The selfstanding membranes are characterized using Brunauer-Emmett-Teller surface area measurements, glancing angle X-ray diffraction, and transmission electron microscopy. In initial application, such membranes are used to control the diffusion of phenol red.
Introduction Vertically oriented, highly ordered TiO2 nanotube arrays made by anodization of Ti thin or thick films are of increasing importance due to their impressive properties in a variety of applications including dye-sensitized solar cells,1-5 hydrogen generation by water photoelectrolysis,5-11 photocatalysis,12-15 and gas sensors;16-20 recent review papers on the general subject can be found.21,22 Biomedical use of TiO2 nanotube arrays as an adhesion and growth support platform for bone23 and stem24 cells have recently been reported, as well as their use in drug delivery25 and enhancing blood clotting for control of hemorrhage.26 Because properties are closely related to geometric surface areas, keen attention has been devoted in the recent past in synthesizing ultralong TiO2 nanotube arrays as well as controlling pore size and wall thickness.5,10,27,28 The two basic criteria for growth of the nanotube array are sustained oxidation of the metal and pore growth by chemical/field-assisted dissolution of the formed oxide27,29 with nanotube length determined by the dynamic equilibrium between growth and dissolution processes. The use of ethylene glycol as a solvent in electrochemical oxidation of titanium exhibits an extremely rapid titania nanotube growth rate of up to 15 µm/hr.27 As clearly seen from the first reports on the subject,10,20 nanotubes formed in ethylene glycol exhibit hexagonal, or nearly hexagonal, close packing. Recently, we reported on the complete, double-sided anodization of 0.25 mm titanium foil samples using an ethylene glycol + NH4F electrolyte resulting in two 360 µm long nanotube arrays separated by a thin barrier layer.27 The effective current efficiency for TiO2 formation was close to 100% after account* To whom correspondence should be addressed. E-mail: cgrimes@ engr.psu.edu. † The Pennsylvania State University. ‡ SentechBiomed Corporation. § University of California San Francisco. | Colorado State University.
ing for the porosity of the structure and the titanium dioxide dissolved during the formation of the nanotubular structure, indicating no side reactions during the anodization and negligible bulk chemical dissolution of formed TiO2 nanotube arrays. Reuse of the electrolyte solution did not result in nanotube formation but rather a uniform oxide layer several hundred nm thick; however, with the addition of NH4F and ethylene glycol the electrolyte was once again able to sustain nanotube array synthesis by anodization. This strongly suggests that depletion of H+ and F- species in the used solution renders it unable to produce sufficient local acidification at the pore bottom to limit the barrier layer thickness. Thus, the challenge in obtaining longer nanotubes is in obtaining optimum electrolyte composition; longer tubes are not necessarily obtained simply by increasing anodization duration. Achieving longer nanotube arrays from thicker Ti films requires increasing concentrations of NH4F and H2O. Herein we describe the complete anodization of a 1 mm thick Ti foil film sample, converting it into two backto-back self-aligned hexagonal-packed nanotube arrays each having lengths of no less than 1000 µm length. Prakasam and co-workers reported the separation of the titania nanotube array film from the underlying substrate;27 as the individual nanotube has the morphology like that of a test tube, a chemical-etching step was used to open the closed bottom end of the tube to yield a nanopipe or conduit open at both ends. In subsequent work, Schmuki reports, remarkably, photocatalytic activity in an amorphous titania membrane.30 While Prakasam did not suggest a filtration application,27 we note that what was not mentioned in the two papers27,30 was that the resulting membrane only remained flat when wet, dramatically cracking, fracturing, and curling into many small pieces upon air drying. Hence, a further aspect of this work is detailing the fabrication of mechanically robust, self-supporting and stable polycrystalline flat or cylindrical polycrystalline TiO2 nanotube array membranes. The curling of the membrane due to surface
10.1021/jp075258r CCC: $37.00 © 2007 American Chemical Society Published on Web 09/22/2007
TiO2 Nanotube Arrays by Anodization of Titanium Foil
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Figure 1. Ratio of wt % NH4F to vol % H2O for obtaining maximum growth rate in an ethylene glycol electrolyte. The horizontal bars show the range of wt% NH4F in which complete anodization of starting 0.25, 0.5, and 1 mm thick Ti foil can be achieved for a given concentration of water.
tensional forces during drying was a great problem, limiting the membranes to only “concept” demonstrations. The compressive stress developed at the barrier layer promotes membrane curling, which is particularly severe for thin membranes. We discovered that this could be solved, and stable flat membranes could be fabricated by critical point drying, a technique commonly used to dry biological specimens. After this step, we obtained flat, mechanically robust membranes consisting of strongly interconnected vertically aligned nanotubes having continuous channels (pores) from one side of the membrane to the other.30 An alternative but less successful drying route is by the repeated washing and drying of the membrane in lowsurface tension liquids like hexamethyldisilizane. We crystallized the membranes in anatase phase using low-temperature thermal annealing without damage to the membrane architecture. The photocatalytic polycrystalline TiO2 membranes enables the prospect of self-cleaning filters. Biofiltration membranes currently used for medical blood filtration therapies, such as hemeofiltration, hemeodialysis, and therapeutic plasmapheresis, are typically comprised of polymers; however, due to their wide pore size distribution their separation efficiency is significantly compromised.31 The majority of the membranes currently used for separation of submicron-sized particles in biomedical applications are prepared from polymers such as polysulfones, polyacrylonitrile, and polyamides.32-34 There are several bio-incompatibilities associated with these membranes, including sensitization to sterilization, complement activation due to the reactivity of polymeric membranes, and adhesion of various protein and immune components to membranes stimulating IL-1 and lysosomal enzyme release.35,36 Also, for many polymeric membranes pore size distribution variations can be as large as 30%; this is a matter of great importance because the leakage of just one virus or antibody or protein molecule through the membrane can compromise an entire system. Thus, development of well-controlled, stable, and uniform membranes capable of complete separation of viruses, proteins, or peptides is an important consideration for biofiltration application. It appears the novel TiO2 nanotube array membranes overcome some of the limitations of current polymeric biofiltration membrane technologies. In this work, we use phenol red as a model molecule for diffusion studies. Phenol red is a small water-soluble molecule about 354.4 D that is often used as a pH indicator in biological experiments because it changes from yellow to red over physiological pH ranges. It is usually added to tissue culture media to detect cell overgrowth or contamination because the
Figure 2. Cross-sectional view of self-standing titania membrane over 2 mm in thickness, mechanically fractured for imaging, achieved by anodizing both sides of a 1.0 mm thick Ti foil sample at 60 V for 216 h in 0.6 wt % NH4F and 3.5% water in ethylene glycol. The membrane consists of two, back-to-back nanotube arrays no less than 1000 µm in length.
metabolic waste products of cells or bacteria are acidic and will turn the media yellow. Because phenol red is excreted by the kidneys, it has in the past been used in kidney function tests to measure overall blood flow. In addition, it has also been shown that phenol red also acts as a weak agonist of the estrogen receptor,37 and it has been used in research studies as a differentiation factor to induce oogenesis.38 We have used phenol red as the model molecule for diffusion due to ease of detection by colorimetric techniques and its small size comparable to that of metabolic wastes in standard biofiltration applications. Experimental Section Prior to anodization, the Ti foils used in this study were ultrasonically cleaned with dilute micro-90 solution (International Products corporation, New Jersey). These were rinsed in deionized water and ethanol, then dried in nitrogen. Anodization was performed in a two-electrode configuration with titanium foil as the working electrode and platinum foil as the counter electrode under constant potential at room temperature (≈22 °C). A direct current power supply (Agilent E3612A) was used as the voltage source to drive the anodization, and a multimeter (Keithley 2000 model) was used to measure the resulting current. For ethylene glycol (EG) electrolytes, a maximum nanotube growth rate was observed at 60 V.27 We studied the anodization
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Figure 3. FESEM images of (a) top side of nanotube array film, (b) back side or barrier layer side of nanotube array film, and (c,d) cross sectional image of mechanically fractured nanotube array film showing its tubular nature.
Figure 4. FESEM image shows (a,b) bottom of the nanotube array partially opened by chemical etch at different levels of magnification and (c) fully opened array bottom.
of 0.25, 0.5, and 1.0 mm thick Ti foils in electrolytes containing different concentrations of NH4F and H2O in EG at 60 V. The optimum concentration of water for achieving the highest growth rates for different NH4F concentrations follows a general trend (black line) indicated in Figure 1. In the given range of NH4F and H2O concentrations, the anodic dissolution due to the increased wt percent of NH4F is compensated by the increase in H2O concentration and results in greater growth rates and corresponding longer nanotube lengths. The horizontal bars of Figure 1 show the range of H2O and NH4F concentrations for which complete anodization (utilization) of the Ti foil samples are achieved. From 0.1-0.5 wt % NH4F with 2% water, 0.25 mm foil samples could be completely anodized resulting in two 320-360 µm nanotube arrays separated by a thin barrier layer. Nanotubes of 360 µm length were obtained from a 0.25 mm Ti foil anodized in a solution containing 0.3 wt % NH4F and 2% H2O in EG for 96 h. Anodizing 0.5 mm foil in an identical electrolyte for 168 h (7 days), the maximum individual nanotube array length obtained was ∼380 µm, suggesting complete utilization of the active electrolyte species. In contrast complete
anodization of a 0.5 mm foil could be achieved in EG electrolytes containing 0.4-0.6% NH4F and 2.5% H2O. Anodizing a 0.5 mm foil at 60 V for 168 h in 0.4 wt % NH4F and 2.5% water in EG resulted in complete transformation of the Ti metal into two (oppositely oriented) titania nanotube arrays each of 538 µm length. A 1.0 mm thick Ti film could be completely anodized in EG electrolytes containing 0.6-0.7% NH4F and 3.5% H2O. As shown in Figure 2, a maximum individual nanotube array length of over 1000 µm was obtained upon anodizing 1.0 mm thick Ti foil at 60 V for 216 h (9 days!) in 0.6 wt % NH4F and 3.5% water in EG, forming a titania nanotube film over 2000 µm thick. Polycrystalline TiO2 Nanotube Array Membranes. The asanodized nanotube array samples were dipped in ethyl alcohol and subjected to ultrasonic agitation till the nanotube array film was separated from the underlying Ti substrate. The compressive stress at the barrier layer-metal interface facilitates detachment from the substrate. For membranes with only a thin residual metal layer remaining after anodization, selective chemical etching can be used for separation, while thin membranes (≈6 µm) were also separated from the underlying Ti substrate using electric field assisted stripping. Cylindrical membranes were made by complete anodization of (hollow) Ti tubing. Figure 3a is an illustrative field emission scanning electron microscopy (FESEM) top surface image of an ethylene glycol fabricated nanotube array film, while Figure 3b shows the back or barrier layer side. Figure 3c is a cross-sectional image of a mechanically fractured sample with Figure 3d showing the open tubes of a mechanically fractured sample. To open the closed end of the tubes, that is, remove the barrier layer as shown in Figure 3b, a dilute hydrofluoric acid/sulfuric acid solution was applied to the barrier layer side of the membrane, etching the oxide, then rinsed with ethyl alcohol. Figure 4 shows the opening of the backside layer; the acid rinse is repeated until the pores are completely opened as seen in Figure 4c. It was observed that the planar membranes, perfectly flat while wet, fractured and then curled significantly (often into
TiO2 Nanotube Arrays by Anodization of Titanium Foil
Figure 5. After ≈24 h of drying in air, the titania nanotube array membrane removed from the Ti foils samples in the foreground mechanically fracture into the shards seen in the upper right; the diameter of Petri dish is 90 mm.
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Figure 7. Membranes of significantly greater handling strength can be made by double-sided anodization of a region within a Ti metal frame. The inner light-colored window is the titania membrane, which is surrounded by Ti metal; the outer yellow film is protective tape.
Figure 8. Cylindrical titania nanotube membrane, immersed in alcohol, made by double-side anodization of a Ti pipe. Figure 6. A completely stable, mechanically robust nanotube array membrane after critical point drying. The 200 µm thick membrane of 120 pore diameter is approximately 2.5 cm × 4.5 cm.
small tight rings) due to surface tensional forces after they were removed from the liquid and dried in air (see Figure 5), posing a significant problem for use in filtering applications. However, the membrane flatness is preserved when dried in a critical point dryer (Bal-Tec CPD-030) with carbon dioxide; critical point drying is a well-known technique used to prepare biological specimens for SEM studies.26 Figure 6 shows a 4.5 cm × 2.5 cm 200 µm thick nanotube array membrane obtained after critical point drying. The planar dimensions of our membranes are currently limited by the capacity of the CO2 critical point drying instrument; in principle the technique can be readily used to fabricate much larger area membranes. Membranes 40 µm thick or thicker were found robust enough for easy handling, while membranes as thin as 4.4 µm have been successfully fabricated and (carefully) used. To promote facile handling of the membranes, we have made membranes within a Ti metal window, as seen in Figure 7. As shown in Figure 8, we note the fabrication of cylindrical nanotube array membranes by the complete anodization of hollow Ti tubing. Like their flat membrane counterparts, the cylindrical membranes fared best when dried via critical point drying. The as-fabricated membranes are amorphous. Although some applications such as normal solution filtering do not require crystalline membranes, crystallinity is essential when biocompatible, photocatalytic, or semiconducting properties are desired. Without disturbing their flatness, we crystallized the membranes via low-temperature annealing. The samples were readily crystallized in anatase phase (see Figure 9), by anneal in an oxygen environment at 280 °C for 1 h; glancing angle X-ray diffraction patterns (GAXRD) were recorded using a Scintag X2 diffractometer (Scintag Inc., CA). Figure 10 shows the
Figure 9. GAXRD spectra of 280 °C annealed nanotube array membrane, demonstrating anatase phase.
transmission electron microscopy (TEM) image of a crystallized nanotube obtained from a mechanically fractured membrane with the inset diffraction pattern confirming the presence of anatase. Figure 11 is a FESEM image of the surface of an annealed membrane (a thin layer of Au is on the top for imaging). Surface area measurements of the membranes were performed using a Micromeritics ASAP 2020. Dry TiO2 nanotube array membranes were evacuated to 2 mmHg pressure, and the physical adsorption of nitrogen gas was measured at 77.35 K. An adsorption isotherm was recorded as volume of gas adsorbed (cc/g at STP) versus relative pressure. The Brunauer-EmmettTeller (BET) equation was used to calculate surface area from the gas volume needed to form a monolayer on the sample surface. The BET surface area measurements show an average surface area of 38 m2/g, a value essentially constant for anodization voltages ranging from 40 to 60 V.
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Figure 13. Phenol red diffusion through titania nanotubular membrane of 120 nm pore size (standard deviation 10 nm), 200 µm thickness. Figure 10. TEM image of the nanotube from mechanically fractured titania membrane annealed at 280 °C; inset shows selected area diffraction pattern indicating anatase phase.
Figure 11. FESEM top surface image of annealed polycrystalline TiO2 membrane.
by measuring the absorbance of phenol red and converting that to concentration using a predetermined standard curve. The ratio of measured concentration (C) with original concentration (Co) was plotted against time to determine the diffusive transport through the membranes. Figure 13 shows release profile of phenol red diffusion through a titania nanotubular membrane; the phenol red diffusion is constrained with approximately 20% of phenol red diffusing in 5 h. The origin of the constrained diffusion through nanotubular membrane may be related to the phenomenon of single-file diffusion, involving particles that cannot pass each other due to constrained pathways. Single-file diffusion phenomena have been recently reported in one-dimensional zeolites and also in biological ion exchange.39-41 Another possible explanation of constrained diffusion could be the drag effects. When a colloidal particle suspended in a quiescent fluid is close to a flat wall, Stokes drag force acting on it is increased relative to that when far from the wall; therefore, the diffusion coefficient is smaller than that when from the wall. The increase of the drag force is attributed to the alteration of the hydrodynamic interaction between the particle and the fluid generated by the boundary condition imposed by the nearby wall. Using Fick’s law of diffusion, the diffusion coefficient for phenol red through titania nanotubular membranes was calculated to be 9 × 10-7 cm2/s. The Stokes-Einstein diffusivity for phenol red in aqueous solution is 2.84 × 10-6 cm2/s. This difference in the two values is due to the presence of the nanotubular membrane and its interaction with the diffusant. Conclusions
Figure 12. Experimental geometry for testing diffusion of phenol red through titania nanotube membrane.
Amorphous titania nanotubular membranes (60 V anodization, ethylene glycol) of 120 nm pore size (standard deviation 10 nm) and 200 µm thickness were tested for phenol red diffusion. Figure 12 illustrates the apparatus used for diffusion studies. The membrane was adhered with a cyanoacrylate adhesive to an aluminum frame as shown in the figure then sealed between the two diffusion chambers. Chamber A was filled with 2 mL of 500 µg/mL phenol red solution, and chamber B was filled with 2 mL of pure distilled H2O. The assembled setup was rotated at 4 rpm throughout the experiment to eliminate any boundary layer effects. Samples were collected from chamber B at specific time intervals. The concentration was calculated
We demonstrated the synthesis of TiO2 nanotube arrays 1000 µm in length with a free-standing nanotube array membrane thickness of over 2 mm by anodic oxidation of a 1.0 mm thick Ti film. Through the use of the described technique, such thick nanotube array films can be transformed into mechanically robust membranes with demonstrated application to phenol red diffusion. The stress developing at the oxide/metal interface during the anodization of the metal facilitates easy detachment of the thick membranes from an underlying Ti substrate when subjected to ultrasonic agitation. As the nanotube has a morphology similar to that of a test tube, a chemical-etching step was used to open the closed bottom end of the tube to yield a membrane film of nanoscale pores open at both ends. The curling of the membrane due to surface tensional forces during air drying, which was a significant problem, was solved by the use of critical point drying. After this step, we obtained flat membranes consisting of strongly interconnected vertically
TiO2 Nanotube Arrays by Anodization of Titanium Foil aligned nanotubes having continuous channels (pores) from one side of the membrane to the other. We crystallized the membranes in anatase phase using low-temperature thermal annealing without damage to the membrane architecture. We previously demonstrated that the polycrystalline nanotube arrays possess high-photocatalytic properties,16 promoting the decomposition of organic contaminants on the surface into volatile compounds and thereby self-cleaning. Hence, the crystalline membranes are promising for applications such as self-cleaning filters. Phenol red is a small water-soluble molecule about 354.4 D that is often used as a pH indicator in biological experiments. We have used phenol red as model molecule for diffusion due to ease of detection by colorimetric techniques and its small size comparable to that of metabolic wastes in standard biofiltration applications. Figure 13 shows the release profile of phenol red diffusion through titania nanotubular membranes; the phenol red diffusion is constrained with approximately 20% of phenol red diffusing in 5 h. The origin of the constrained diffusion through nanotubular membrane may be related to the phenomenon of single-file diffusion, involving particles that cannot pass each other due to constrained pathways. Acknowledgment. Support of this work by the Department of Energy under Grant DE-FG02-06ER15772 is gratefully acknowledged. The authors thank the reviewers for their helpful comments. References and Notes (1) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Letters 2006, 6, 215-218. (2) Zhu, K.; Neale, N. R.; Miedaner, A.; Frank, A. J. Nano Letters 2007, 7, 69-74. (3) Wang, H.; Yip, C. T.; Cheung, K. Y.; Djurisic, A. B.; Xie, M. H.; Leung, Y. H.; Chan, W. K. Appl. Phys. Lett. 2006, 89, 023508. (4) Leenheer, A. J.; Miedaner, A.; Curtis, C. J.; van Hest, M.; Ginley, D. S. J. Mater. Res. 2007, 22, 681-688. (5) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nanotechnology 2007, 18, 065707. (6) Uchida, S.; Chiba, R.; Tomiha, M.; Masaki, N.; Shirai, M. Electrochem. 2002, 70, 418-420. (7) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Letters 2005, 5, 191-195. (8) de Tacconi, N. R.; Chenthamarakshan, C. R.; Yogeeswaran, G.; Watcharenwong, A.; de Zoysa, R. S.; Basit, N. A.; Rajeshwar, K. J. Phys. Chem. B 2006, 110, 25347-25355. (9) Varghese, O. K.; Paulose, M.; Shankar, K.; Mor, G. K.; Grimes, C. A. J. Nanosci. Nanotechnol. 2005, 5, 1158-1165. (10) Paulose, M.; Shankar, K.; Yoriya, S.; Prakasam, H. E.; Varghese, O. K.; Mor, G. K.; Latempa, T. A.; Fitzgerald, A.; Grimes, C. A. J. Phys. Chem. B 2006, 110, 16179-16184. (11) Mor, G. K.; Prakasam, H. E.; Varghese, O. K.; Shankar, K.; Grimes, C. A. Nano Lett. 2007, 7, 2356-2364.
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