Fabrication of Vertically Oriented TiO2 Nanotube Arrays Using

Aug 23, 2007 - using an electrolyte of dimethyl sulfoxide (DMSO) containing either hydrofluoric acid (HF), potassium fluoride. (KF), or ammonium fluor...
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13770

J. Phys. Chem. C 2007, 111, 13770-13776

Fabrication of Vertically Oriented TiO2 Nanotube Arrays Using Dimethyl Sulfoxide Electrolytes Sorachon Yoriya,† Maggie Paulose,‡ Oomman K. Varghese,‡ Gopal K. Mor,† and Craig A. Grimes*,† Department of Materials Science and Engineering, Department of Electrical Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802, and SentechBiomed Corporation, 200 InnoVation BouleVard, State College, PennsylVania 16803 ReceiVed: June 15, 2007; In Final Form: July 19, 2007

We detail anodic oxidation variables affecting the fabrication of vertically oriented TiO2 nanotube arrays using an electrolyte of dimethyl sulfoxide (DMSO) containing either hydrofluoric acid (HF), potassium fluoride (KF), or ammonium fluoride (NH4F). Various anodization variables including F- ion concentration, voltage, anodization time, water content, and previous use of the electrolyte can be combined to achieve nanotube arrays with length and morphology relevant to required applications. Using an anodization potential of 60 V with an electrolyte of 2% HF in DMSO, 70 h duration, nanotubes are achieved having a length of 101 µm, inner diameter 150 nm, and wall thickness 15 nm for a calculated geometric area of 3475. The weak adhesion of the DMSO fabricated nanotubes to the underlying oxide barrier layer and low tube-to-tube adhesion facilitates their separation for applications where dispersed nanotubes are desired. We examine the photoelectrochemical properties of 45 µm long nanotube arrays, crystallized by annealing at 580 °C for 6 h in oxygen gas, tested under UV (320-400 nm) and solar simulated light (AM 1.5) illumination.

Introduction Vertically oriented TiO2 nanotube arrays have demonstrated their utility in applications including room-temperature hydrogen gas sensing,1-4 generation of hydrogen by water photoelectrolysis,5-7 photocatalyst,8 tissue engineering scaffolds,9 and heterojunction solar cells.10-14 The performance of such devices is determined by the geometrical parameters of the nanotube arrays including shape, wall thickness, pore size, and length. The large surface area and excellent charge-transfer properties of the TiO2 nanotubes remarkably enhances their properties. Gong and co-workers initiated this research field, reporting 500 nm length nanotube arrays made using an aqueous HF based electrolyte.15,16 Subsequently, the pH of F- ion containing electrolytes was controlled to form nanotubes up to a few micrometers in length.17 This was followed by use of nonaqueous polar organic electrolytes, for example, dimethyl formamide, formamide, ethylene glycol, and dimethyl sulfoxide containing fluoride (F-) ions,18-22 where the length (up to 1000 µm), wall thickness (5 to 35 nm), and pore size (10 to 150 nm) of the nanotubes can be varied over a large range. We note that tube properties can vary dramatically with the electrolyte chemistry in which they were formed. For example, close-packed hexagonal tubes are formed when using an ethylene glycol electrolyte;18 removal of the tube “plug” by an acid rinse enables fabrication of free-standing, self-supportive membranes that can be used for biofiltration.21,22 In contrast, tubes formed from dimethyl sulfoxide (DMSO) electrolytes have weak tube-to-tube binding, as well as weak adhesion to the underlying barrier layer. Hence, the DMSO synthesized tubes can be easily dispersed enabling their use, for example, in * Corresponding author. E-mail: cgrimes.engr.psu.edu. † The Pennsylvania State University. ‡ SentechBiomed Corporation.

enhancing blood clotting to control a hemorrhage.23 DMSO, as well as dimethylformamide, DMF, are protophilic aprotic solvents behaving quite differently than ethylene glycol (EG), water, formamide, and N-methylformamide (NMF). Ethylene glycol, formamide, and NMF are amphiprotic exhibiting both protophilic and protogenic character. Protic solvents, including water, methanol, and EG, have a free proton enabling them to donate hydrogen bonds. In contrast, DMSO does not have a proton of its own and is very attracted to one, hence strongly protophilic. We note that we have been able to obtain nanotube arrays using dimethylformamide; however, a detailed study appears not yet to have been reported. Herein, we report for the first time a detailed investigation on anodic oxidation of titanium in DMSO organic electrolyte containing F- ions. The effect of various anodization parameters including voltage, duration of anodization, and F- ion concentration on nanotube array growth and formation are discussed. The surface morphology is discussed in detail, in particular, with respect to the variation of nanotube length, presence of precipitate on the top surface, and adhesion of the nanotubes to the underlying titanium substrate. Experimental Section Titanium foils (250 µm thick, purity ∼ 99.7%, SigmaAldrich) were ultrasonically cleaned in acetone and ethanol prior to anodization. These foils were anodized at different direct current (DC) voltages in a two-electrode electrochemical cell in an electrolyte containing DMSO (99.6%, Aldrich) and hydrofluoric acid (HF, 48% aqueous solution, JT Baker). The anodization voltages were varied from 10 to 70 V; HF concentration varied from 1 to 6%, and duration of anodization varied from 20 to 90 h. In order to effectively template the surface, some samples were pre-anodized in a 0.5% HF aqueous solution at 20 V before anodizing in 2.0% HF-DMSO solution

10.1021/jp074655z CCC: $37.00 © 2007 American Chemical Society Published on Web 08/23/2007

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Figure 1. FESEM images of a TiO2 nanotube array sample grown from a 2.0% HF-DMSO electrolyte for 70 h at (a,c,e) 40 V and (b,d,f) 60 V. Top surface images are seen in a,b; cross sectional images are in c,d; and tube bottom view images, removed from the underlying barrier layer, are seen in e,f.

at 40 V. The effect of water content incorporated in 2.0% HFDMSO solution was studied. In all cases, a platinum foil was used as the counter electrode. The distance between the two electrodes was maintained at 1 cm. All experiments were conducted at about 20 °C. The anodization current was monitored using a Keithley (model 2000) digital multimeter interfaced with a computer. After anodization, samples were washed with deionized (D.I.) water and then ultrasonicated to remove surface debris (precipitate falling out of solution). Nanotube array morphology was investigated by use of a field emission scanning electron microscope (FESEM, JEOL model JSM-6300). For photoelectrochemical measurements, crystallized samples were used obtained by annealing as-anodized amorphous titania nanotubes in oxygen ambient at 580 °C for 6 h. Sample photoresponse was measured electrochemically in a standard three-electrode configuration with TiO2 nanotubes (on Ti foil) as the working electrode, platinum foil as a counter electrode, and saturated Ag/AgCl as a reference electrode with 1.0 M KOH solution as the electrolyte. A scanning potentiostat (CH Instruments, model CHI 600B) was used to measure dark and illuminated current at a scan rate of 10 mV/s. Samples were tested under two different types of illumination: (i) solar simulated light equivalent to AM 1.5 light at 100 mW/cm2 (intensity set using NREL calibrated solar cell) and (ii) UV

illumination (320-400 nm) of 100 mW/cm2 (thermopile detector, Spectra Physics) from metal halide lamp (EXFO lite). Results and Discussion Figure 1 shows FESEM images of samples prepared in the 2.0% HF-DMSO solution anodized at 40 V (images a,c,e) and 60 V (images b,d,f) for 70 h. The surface of the as-anodized nanotube samples are covered with unwanted debris, precipitate from solution that can be removed by sonication. The vertical alignment of the nanotubes and general morphology are independent of the voltage used, with the nanotube arrays separated from the underlying titanium foil by a dense barrier layer. Voltage affects pore size, wall thickness, and length of the nanotubes as seen in Table 1. At 40 V, the wall thickness was found to be in the range of 8 to 17 nm, a variation due to fluctuations in dissolution of the titanium metal. An average wall thickness of ∼15 nm was found to grow to approximately 30 nm with crystallization; as a result, the inner diameter of the annealed nanotubes reduced from ≈ 120 to 105 nm. The packing of the nanotubes appears to be a slight deviation from an ideal hexagonal close packing arrangement. Figure 2 is a cross-sectional view of a nanotube array obtained from a 3.0% HF-DMSO electrolyte at 60 V. Figure 3a-c shows the effect of HF concentration on the wall thickness of

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Figure 2. Cross-sectional FESEM image of a TiO2 nanotube array sample grown from a 3.0% HF-DMSO electrolyte at 60 V for 70 h.

TABLE 1: Effect of Anodization Voltage on the Morphology of the Resulting TiO2 Nanotube Arrays, for a 70 h 2% HF-DMSO Anodization

pore size (nm) bottom size (nm) wall thickness (nm) length (µm) calculated geometrical surface area10

40 V anodized sample

60 V anodized sample

∼ 120 ∼ 250 ∼ 15 (8-17 nm variation) 45 (after sonication) ∼ 1800

∼ 150 ∼ 400 ∼ 50 93 ∼ 3200

the resulting nanotubes, with higher HF concentrations resulting in thinner walls. As seen in Figure 3b for 4.0% HF, the wall thickness is so thin that the tubes are essentially transparent to the electron beam of the FESEM. As seen in Figure 4, anodization parameters including voltage, HF concentration, electrolyte water content, and anodization time play key roles in determining the length of the resulting nanotubes. In Figure 4a, the length of nanotubes formed on samples with and without a pre-anodization treatment, in effect templating of the Ti foil surface, is seen to increase with anodization voltage. A maximum length is achieved at 60 V; a dramatically decreased length is achieved at 70 V, and at 80 V, the Ti foil sample dissolved into the electrolyte after 24 h. Titanium foils that were pre-anodized in 0.5% HF in water at 20 V for 20 min followed by anodization in 2.0% HF containing DMSO electrolyte showed superior growth rates. The maximum length obtained was 101 µm upon anodizing a pre-anodized sample in 2.0% HF-DMSO at 60 V for 70 h. To increase the nanotube length, it is necessary to both reduce the chemical dissolution of the oxide at the pore mouth while maintaining active growth at the bottom of the pore. DMSO, being protophilic, accepts a proton from HF reducing its activity. The presence of DMSO modifies the space charge region in the pores thereby avoiding lateral etching, leading to a steady pore growth and low chemical etching of the nanotube walls. Figure 4b shows the effects of HF concentration on the length of the nanotubes. A 2.0% HF in DMSO electrolyte yielded the maximum nanotube length for 40 and 60 V. HF concentrations higher than 2.0% appear to result in relatively faster etching of the TiO2 and therefore in shorter tubes having thinner walls. At HF concentrations below 2%, the nanotube growth rate is so small as to dominate the tube formation behavior, extending anodization durations required to achieve the same tube lengths.

Figure 3. Effect of HF concentration on wall thickness of TiO2 nanotube arrays anodized at 60 V for 70 h in a DMSO electrolyte containing (a) 2% HF, (b) 4% HF, and (c) 6% HF.

While addition of water to the electrolyte ensured the field assisted etching of the Ti foil at the pore bottom (eq 1, discussion below), water also assists chemical dissolution (eq 2) to open the pore mouth (thinner nanotube walls) and dissolve the end of the nanotube. Figure 4c summarizes the length of the nanotube arrays achieved as a function of water content in 2.0% HF-DMSO electrolytes anodized at 40 V for 40 h, showing a decrease in tube length with increasing water content. We note that addition of water to the electrolyte greatly improved adhesion between the nanotubes and the underlying oxide barrier layer. Figure 4d indicates the lengths achieved at 40 V, in electrolytes containing 0%, 3%, and 5% water content, as a function of anodization duration. A duration of 70 h yielded tubes of maximum length. We note that the anodization duration not only affects the tube length but also the top-surface features of nanotube arrays. An anodization time less than 40 h usually forms tubes clogged with surface debris (precipitate from solution). Longer anodization times result in more titanium dissolution in the electrolyte, leading to higher electrolyte conductivity that helps to prevent precipitate, or debris, occurring

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Figure 4. Variation of TiO2 nanotube array length as a function of (a) applied voltage for a 70 h anodization using 2% HF-DMSO electrolyte with and without a pre-anodization step to template the surface, (b) HF concentration in DMSO for 40 and 60 V 70 h anodization, (c) variation of nanotube length obtained from a 2.0% HF-DMSO electrolyte containing different deionized water concentrations (40 V, 40 h), and (d) duration for a 40 V anodization in 2.0% HF-DMSO electrolytes with 0%, 3%, and 5% deionized water content.

on the top surface of the nanotubes. Figure 5 depicts changes observed in the morphology of the as-anodized nanotubes upon adding water to the electrolyte; pore clogging disappears at a water content of ≈ 5%. In common with the fabrication of ZnO nanostructures using electrodeposition in DMSO as reported by Choi,24,25 the use of previously used electrolytes affects the length and pore size of the TiO2 nanotubes achieved by anodization. The most important advantage offered by the use of used electrolytes was greatly improved adhesion between the nanotubes and the underlying oxide barrier layer. The conductivity of 2.0% HF-DMSO used for anodization of one titanium sample, 40 V for 70 h, is approximately 100.6 µS/cm in comparison to 8.76 µS/cm for a fresh electrolyte. The electrolyte conductivity increases with longer anodization durations because of more titanium ions dissolving in the electrolyte, reaching 160.1 µS/cm for a 140 h duration. Compared with fresh electrolytes, previously used electrolytes usually yielded nanotubes of shorter length. For example, a fresh 2.0% HF-DMSO electrolyte resulted in 53 µm long nanotubes for a 40 V 24 h anodization; then, using the same solution again for a subsequent 40 V 70 h anodization gave nanotubes 50 µm in length. We note that the higher conductivity of used electrolytes is useful in the initiation of nanotube growth; therefore, we observed faster initial rates of nanotube formation in used electrolytes. Also, nanotubes obtained from used electrolytes do not suffer from debris, having open, unclogged pores without the need for surface cleaning by sonication. This is probably because of a slightly higher chemical etching rate dominating

the reaction. As shown in Figure 6, tubes grown from used electrolytes also have relatively thicker walls, approximately 100 nm, are much rougher, and have pore sizes of about 7090 nm compared with 120 nm for nanotubes obtained from fresh electrolytes. Previously, the effect of five different cationic species on the formation of TiO2 nanotube arrays by anodization of titanium in formamide-water mixtures containing fluoride ions was reported, with the cation choice found to be a key parameter influencing both the nanotube growth rate and the resulting nanotube length.20 We find HF can be replaced, on a generally equivalent basis, by potassium fluoride and ammonium fluoride (NH4F), to yield nanotube arrays several micrometers in length; however, NH4F has quite limited solubility in DMSO. On using 0.19% of F- in a NH4F(aqueous)-DMSO electrolyte, tube lengths of about 18 µm were obtained on anodization at 40 V for 40 h. Real Time Current Response during Anodization. The fundamental formation of nanotube arrays in fluoride containing electrolytes is, we believe, the result of three simultaneously occurring processes: (1) field assisted oxidation of Ti metal to form titanium dioxide, (2) field assisted dissolution of Ti metal ions in the electrolyte, and (3) chemical dissolution of Ti and TiO2 due to etching by fluoride ions. The same three processes govern the anodic formation of several other self-organized nanoporous metal oxides.26-30 The formation of nanotubes in an aqueous HF containing electrolyte is

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Figure 5. FESEM images of TiO2 nanotube arrays, at different levels of magnification, obtained from 2.0% HF-DMSO solutions containing (a) 1%, (b) 2%, (c) 3%, (d) 5%, (e) 10%, and (f) 12% H2O (40 V, 40 h).

Figure 6. FESEM images of an illustrative TiO2 nanotube array sample grown (40 V, 70 h) from a previously used electrolyte (2.0% HF-DMSO, 40V, 24 h). Note high degree of surface roughness.

described by a localized dissolution model,16,20,21 with an oxide layer initially formed by

Ti + 2H2O f TiO2 + 4H+

(1)

The pore formation occurs as a result of the localized chemical dissolution of the oxide by F- according to the reaction

TiO2 + 6F- + 4H+ f TiF62- + 2H2O

(2)

This leads to a higher field at the bottom of the pore that drives further oxidation and field assisted dissolution where Ti ions come out of the metal and dissolve in solution. The metallic region between the pores undergoes a similar transition leading to tube formation. For organic electrolytes, the fundamental growth mechanism is generally the same; however, the bond structure of the solvents greatly affects the (lateral) tube etching and hence the resulting morphology, for example, hexagonal closed packed tubes or considerable tube-to-tube spacing.1,14

Figure 7 shows typical anodization current behavior during anodic oxidation of titanium (both sides of the foil sample are being anodized simultaneously) at 40 V in 2.0% HF-DMSO and 4.0% HF-DMSO electrolytes. The anodization current density is less than 2 mA/cm2, considerably smaller than the anodization currents seen for any other aqueous or polar organic electrolytes that can initially be several tens of milliamperes per square centimeter. The slight increase in anodization current in the 4% HF curve (Figure 7) after about 10 h indicates relatively stronger chemical etching compared with that observed in 2.0% HF-DMSO and hence comparatively shorter nanotube lengths (see Figure 4b). We do not understand the fluctuations seen in the current density, other than to say they are due to a combination of changes in oxide thickness, electrolyte conductivity, and resulting change in potential drop across the TiO2electrolyte interface during anodization. Fresh DMSO electrolytes have low conductivities that increase with anodization duration because of chemical and field assisted dissolution of

Vertically Oriented TiO2 Nanotube Arrays

Figure 7. Current density versus time behavior during anodization of a Ti foil sample at 40 V using a DMSO electrolyte containing 2.0% HF and 4.0% HF.

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Figure 9. Variation of photocurrent density as a function of measured potential (vs Ag/AgCl) for an illustrative crystallized nanotube array sample fabricated in 2.0% HF-DMSO at 40 V for 70 h, under two different illuminations: (a) UV radiation (320-400 nm) and (b) AM 1.5. The light intensity from each source was set to 100 mW/cm2.

Photoelectrochemical Properties.

Figure 8. Current density versus time behavior during anodization of a Ti foil sample in a 2.0% HF-DMSO electrolyte at 20, 40, and 60 V.

titanium into the electrolyte. Hence, initially, the applied potential drop occurs primarily across the electrolyte between the two electrodes. As the electrolyte conductivity increases with anodization duration, the drop in the applied DC potential shifts from the electrolyte to the oxide layer, increasing the rate of oxide formation, resulting in the first “hump” seen in the current density-time curves of Figure 7. A local minimum in the current density is obtained with formation of an oxide layer upon the exposed Ti surface. Then, because of nonuniform chemical etching of the oxide layer, that is, pitting, the anodization current increases until a somewhat uniform oxide layer begins to form. Thereafter, the current decreases until an equilibrium is reached between the rate of field assisted oxide growth, the field assisted oxide dissolution, and the chemical etching. The chemical dissolution is very low in DMSO versus aqueous electrolytes. Therefore, we expect and observe higher tube lengths from DMSO electrolytes with specific details dependent upon a combination of factors including anodization duration, voltage, HF concentration, and electrolyte condition (fresh or previously used for an anodization). Figure 8 shows the current-time behavior as a function of anodization potential for a DMSO electrolyte of fixed F- ion concentration. The 60 V curve generally shows an increase in current over the first 10 h, presumably because of etching or pitting of any resultant oxide layer. Higher potential enhances the titanium extraction process and simultaneously facilitates the TiO2 layer formation, yielding longer nanotube arrays (Figure 4a).

Longer length tubes having open pores show superior photoelectrochemical properties. As-anodized nanotube arrays obtained from 2%HF-DMSO electrolyte at 40 V for 70 h were cleaned with dilute HF solution and then crystallized at 580 °C for 6 h in oxygen ambient. The resulting 45 µm long nanotubes had open pores with no covering surface debris. Figure 9 shows the I-V characteristics of such a sample under UV light (320400 nm) and AM 1.5 solar simulated light, both illuminations with an intensity 100 mW/cm2. The observed dark currents for both illuminations are negligible. The measured photocurrents of the DMSO originating nanotube are considerably less than their formamide and ethylene glycol counterparts because of, we believe, the relatively poor connection between the nanotubes and the underlying barrier layer through which electrons travel to the back metal contact (underlying Ti substrate). Conclusions We consider the fabrication of TiO2 nanotubes by constantvoltage anodization of titanium foil samples in DMSO electrolytes. Various anodization variables including HF concentration, voltage, anodization time, water content, and previous use of the electrolyte can be combined to achieve nanotube arrays with length and morphology relevant to required applications. Addition of water to the DMSO electrolyte removed precipitate debris from the top surface of the nanotube arrays but reduced nanotube length. Similarly, used electrolytes of higher conductivity, that is, electrolytes previously used to anodize samples, yielded shorter length tubes with open pores. TiO2 nanotube arrays up to 101 µm in length, aspect ratio ≈ 560, using HFDMSO electrolyte were fabricated. The weak adhesion of the DMSO fabricated nanotubes to the underlying oxide barrier layer and low tube-to-tube adhesion facilitates their separation for applications where dispersed nanotube are desired.23 However, the weak adhesion limits their utility in applications including water photoelectrolysis, dye sensitized solar cells, and gas sensors.1,14 Adhesion to the Ti substrate was weakest when the tubes were fabricated in new (un-used) DMSO; the used electrolyte and the water-containing electrolytes significantly improved this adhesion. Photoelectrochemical studies show reasonable but smaller photocurrent values than previous reports on nanotube arrays fabricated in formamide or ethylene glycol based electrolytes because of, we

13776 J. Phys. Chem. C, Vol. 111, No. 37, 2007 believe, the poor adhesion and hence poor electrical contact between the tubes and the underlying barrier layer. Acknowledgment. Support of this work by the Department of Energy under Grant DE-FG02-06ER15772 is gratefully acknowledged. S.Y. gratefully acknowledges support under the Royal Thai Government Scholarship provided by the National Metal and Materials Technology Center, the National Science and Technology Development Agency, Thailand. The authors thank the Referees for their helpful comments and suggestions. References and Notes (1) Grimes, C. A. J. Mater. Chem. 2007, 17, 1451-1457. (2) Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A.; Ong, K. G. Nanotechnology 2006, 17, 398-402. Varghese, O. K.; Yang, X.; Kendig, J.; Paulose, M.; Zeng, K.; Palmer, C.; Ong, K. G.; Grimes, C. A. Sens. Lett. 2006, 4, 120-128. (3) Varghese, O. K.; Mor, G. K.; Grimes, C. A.; Paulose, M.; Mukherjee, N. J. Nanosci. Nanotech. 2004, 4, 733-737. (4) Mor, G. K.; Varghese, O. K.; Paulose, M.; Grimes, C. A. Sens. Lett. 2003, 1, 42-46. Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Grimes, C. A. Sens. Actuators, B 2003, 93, 338-344. Varghese, O. K.; Gong, D.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. AdV. Mater. 2003, 15, 624-627. (5) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5, 191-195. (6) Paulose, M.; Mor, G. K.; Varghese, O. K.; Shankar, K.; Grimes, C. A. J. Photochem. Photobiol. A 2006, 178, 8-15. (7) Varghese, O. K.; Paulose, M.; Shankar, K.; Mor, G. K.; Grimes, C. A. J. Nanosci. Nanotech. 2005, 5, 1158-1165. (8) Albu, S. P.; Ghicov, A.; Macak, J. M.; Hahn, R.; Schmuki, P. Nano Lett. 2007, 7, 1286-1289. (9) Popat, K. C.; Leoni, L.; Grimes, C. A.; Desai, T. A. Biomaterials 2007, 28, 3188-3197. (10) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nanotechnology 2007, 18, 065707; 11 pages. (11) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215-218.

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