Fabrication, Geometry, and Mechanical Properties of Highly Ordered

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J. Phys. Chem. C 2009, 113, 7107–7113

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Fabrication, Geometry, and Mechanical Properties of Highly Ordered TiO2 Nanotubular Arrays Xinhu Tang and Dongyang Li* Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2V4 ReceiVed: January 12, 2009; ReVised Manuscript ReceiVed: February 27, 2009

Highly ordered TiO2 nanotubular arrays have attracted increasing interest because of their exceptional physical properties and wide range of existing and potential applications. Techniques for fabrication and characterization of TiO2 nanotubular arrays have been reported extensively; however, the mechanical behavior of TiO2 nanotubular arrays has not been systematically investigated, which could be critical for their applications. In this study, we synthesized a series of highly ordered TiO2 nanotubular array films from a few to hundreds of microns in thickness by changing the anodic voltage or reaction time during anodization. The as-prepared nanotubular arrays were examined carefully using field emission scanning electron microscopy, and their mechanical properties were evaluated using a micromechanical probe and a microtribometer. It was demonstrated that the diameter and wall thickness of TiO2 nanotubes were almost independent of the anodic voltage and anodization time, whereas thicker nanotubular array films or longer nanotubes could be fabricated at higher voltage or with longer anodization time. The micromechanical probing tests demonstrated that the apparent Young’s modulus, η value, and hardness decreased as the thickness of the nanotubular array films increased due to the densification and collapse of longer nanotubes under external force. The resistance of the nanotubular array films to scratch was evaluated by performing the microscratch tests with in situ measurement of the contact electrical resistance. To determine their resistance to sliding wear, sliding wear tests were performed in different environments. Compared to wear in air, the wear loss in water significantly decreased. The pH value of water slightly affected the wear loss of TiO2 nanotubular arrays; the results showed that the wear loss of TiO2 nanotubular arrays decreased with increasing pH from 4, through 7, to 10. 1. Introduction Highly ordered and vertically oriented TiO2 nanotubular arrays have received significant attention in the past decade due to their existing and potential applications, for example, purification of environmental pollutants,1-4 hydrogen generation,5-9 photovoltaic cells,10-14 and sensing.15-18 The TiO2 nanotubular arrays have also found many potential biomedical applications, for example, used as a bond scale and supporting platform for bone19,20 and stem cells,21 local delivery of antibiotics off-implant at the site of implantation,22 and the control of hemorrhage by forming significantly stronger clots at reduced clotting times.23 It should be indicated that, wherever TiO2 nanotubular arrays are used, their mechanical properties must be taken into account. For instance, the mechanical properties of TiO2 nanotubes or porous TiO2 on Ti-based implants are particularly important for bone-implant interfaces, where long-term in vivo structural stability is crucial.24 Bone resorption may occur due to the mismatch in mechanical properties (i.e., elastic modulus) between bone and the implant surface layer, resulting in implant loosening and eventual failure.25 Furthermore, the wear resistance is another important mechanical property of implants since fretting wear or oscillation at bone-implant interfaces generates wear debris, jeopardizing the stability of the prostheses.26 Regarding applications of TiO2 nanotubes in photocatalysis and sensing, the mechanical properties of TiO2 nanotubular arrays affect their operation, function, and service life. Thus, it is important to evaluate the mechanical properties and related wear * Corresponding author. Phone: +1-780-492-6750. Fax: +1-780-4922881. E-mail: [email protected].

behavior of TiO2 nanotubular arrays to establish the relationship between their microstructural features and corresponding mechanical behavior. However, only very limited information can be found in the literature regarding the microstructure-mechanical property relationship for TiO2 nanotubular arrays. Crawford et al.24 investigated the mechanical behavior of TiO2 nanotubular arrays with their thickness less than 1 µm using a nanoindenter and noticed that the measured elastic modulus was higher for thinner films, ascribed to the increasing influence from the titanium substrate. As the authors indicated, because of the significant substrate effect on the very thin TiO2 nanotubular arrays, the obtained results were somewhat unconvincing. To date, the thickness of TiO2 nanotubular arrays has been successfully increased up to 1000 µm,27 and the geometric structure of TiO2 nanotubular arrays can also be tailored to achieve improved performance for specific applications.5,10,28-31 Thus, it is possible to evaluate the mechanical behavior of TiO2 nanotubular arrays with respect to their dimensions more precisely. In this work, we fabricated a series of highly ordered and vertically oriented TiO2 nanotubular arrays by changing the anodic voltage or anodization time. The as-prepared arrays were examined using a field emission scanning electron microscope (FE-SEM). Effects of anodic voltage and oxidization time on the microstructure and mechanical properties of the fabricated TiO2 nanotubular arrays were investigated using various techniques, including microindentation, microscratch with in situ monitoring of the contact electrical resistance (CER), and sliding wear tests in air and water at different pH values. Efforts were

10.1021/jp900311d CCC: $40.75  2009 American Chemical Society Published on Web 04/06/2009

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Figure 1. Schematic diagram of the pin-on-disk mode of the UMT.

made to elucidate the mechanism responsible for variations in mechanical properties with respect to the geometric features of TiO2 nanotubular arrays. 2. Experimental Details 2.1. Fabrication. Using a process similar to that we previously used,5 we synthesized highly ordered TiO2 nanotubular arrays in an organic-based electrolyte by potentiostatic anodization. During this process, a titanium foil with a thickness of 2.0 mm (99.2%, Sigma-Aldrich) was connected to a metal wire and then mounted using epoxy resin with an exposure area of 15 mm × 15 mm. Prior to anodization, the mounted titanium foil was ground with silicon carbide papers (up to 1200 grit) and finally polished using 0.05 µm alumina powder. The polished foil was then cleaned with acetone, ethanol, and DI water, successively, in an ultrasonic cleaner. Anodization was conducted in a two-electrode electrochemical cell with a platinum foil (12 mm × 12 mm) used as the counter electrode in an ethylene glycol (99+%, enzyme-grade, Fisher Scientific) electrolyte containing 0.25 wt % ammonium fluoride (99.3%, ACS reagent, Fisher Scientific) and 2.0 vol % water at room temperature. A direct current power supply (1715A, B&K Precision Corporation, Yorba Linda, CA) was used as the voltage source to drive reactions involved in the anodization process. All as-prepared TiO2 nanotubular array films were rinsed using DI water and then dried in a nitrogen flow. 2.2. Characterization. FE-SEM observations were carried out under a JEOL JSM6301FXV scanning electron microscope with a field emission electron source running at 5 kV and a JEOL field emission JAMP 9500F at 25 kV. The microhardness of as-fabricated nanotubular arrays was determined using a micromechanical probe (Fischer Technology Ltd., Winsor, CT) under a small load of 25 mN. Each reported hardness value was an average of at least five measurements. Young’s modulus and the ratio (η) of the elastic work (We) to the total work (Wtot), which is a measure of the contribution of elasticity to the deformation,32 were determined from the load-unload curves recorded during microindentation. The resistance of TiO2 nanotubular arrays to scratch was evaluated using a Universal Micro-Tribometer (UMT) (Center for Tribology, Inc., Campbell, CA). During the tests, a tungsten carbide tip scratched a surface under a normal load that was increased from 0 to a designed level within 60 s. The critical load at TiO2 nanotubular array films’ failure during scratch tests was determined by monitoring the variation in CER between the tip and titanium substrate.33 By modifying the setup of the UMT and operation parameters, the UMT could work in a “pinon-disk” mode to perform a sliding wear test, as illustrated in Figure 1. The pin was a silicon nitride ball with its diameter equal to 4 mm. The diameter of the wear track was set to 5

Figure 2. FE-SEM images of TiO2 nanotubular arrays fabricated at 60 V for 16 h in an ammonium fluoride/ethylene glycol solution: (A) top view, (B) bottom view, (C) cross-sectional view, and (D) magnified view of (C).

mm. The wear resistance of nanotubular array films was evaluated under a normal load of 25 mN for 10 min at a sliding speed of 2 mm/s in air and water at different pH levels. The width of the wear track was measured using a calibrated microscope system (Media Cybernetics, Inc., Bethesda, MD). The wear volume loss (mm3) was calculated on the basis of the track width and the diameter of the ball tip. 3. Results and Discussion The TiO2 nanotubular arrays fabricated at 60 V for 1, 2, 4, 8, 12, 16, and 24 h were examined using FE-SEM. Illustrative top view, bottom view, and cross-sectional view of a representative nanotubular array film are presented in Figure 2. As shown, the highly ordered and vertically oriented nanotubular array film has its thickness equal to 98 µm with a roughness factor (the real surface area per unit nominal surface area) of 3400 (tubular packing, 138 nm inner diameter, 10 nm wall thickness) and a length-to-width aspect ratio of 710 (the tube length ) 98 µm; the average tube inner diameter ) 138 nm). The individual nanotube is very smooth, and no obvious differences in the diameter and wall thickness were observed under FE-SEM among all fabricated TiO2 nanotubes, implying that these two structural parameters are independent of the anodization time. Through analyzing the FE-SEM images of the cross-sectional view, the curve of the thickness of the TiO2 nanotubular array film versus anodization time was determined, which is illustrated in Figure 3. It is demonstrated that the growth curve of nanotubular arrays at 60 V could be split into two regions according to the growth rate characteristics. During the first 8 h, the thickness of the TiO2 nanotubular array film increased linearly with the anodization time. The growth rate of the nanotubular arrays was determined as 6.3 ( 0.2 µm/h by fitting the growth curve during the first 8 h. The growth rate then decreased gradually, for example, to 1.6 ( 0.7 µm/h (average) when the anodization time was about 20 h (see Figure 3). It has been well-established that the formation of TiO2 nanotubular arrays in a fluoride-containing electrolyte is a result of three simultaneous processes: (1) field-assisted oxidation of

Highly Ordered TiO2 Nanotubular Arrays

Figure 3. Growth curve of TiO2 nanotubular arrays fabricated at 60 V in the ammonium fluoride/ethylene glycol electrolyte.

titanium to form titanium dioxide, (2) field-assisted dissolution of titanium ions in the electrolyte, and (3) chemical dissolution of titanium and TiO2 nanotubes through etching by fluoride ions.24,34 The final thickness of nanotubular array film is dominated largely by the dynamic equilibrium between the growth and dissolution processes. As shown in Figure 3, within the first 8 h, the diffusion of fluoride anions (F-) through the short nanotubes, which etched the titanium substrate to develop TiO2 nanotubular arrays, was relatively stable. However, as the TiO2 nanotubules grew longer, the diffusion of fluoride anions became difficult and needed longer time to reach the titanium substrate to produce nanotubes. As a result, the growth rate of nanotubular arrays decreased. Meanwhile, when the nanotubular array film became thicker, the transfer of water molecules necessary to form titanium oxides also became difficult, thus retarding the growth of the nanotubes. However, further studies are needed to fully clarify the mechanism for the variation in growth rate of highly ordered TiO2 nanotubular arrays. Mechanical properties of various TiO2 nanotubular arrays were evaluated using a micromechanical probe for the information on their local hardness and elastic behavior. The elastic behavior was characterized using the ratio of elastic deformation energy (We) to the total deformation energy (Wtot).32 It is known that, when the indentation depth is smaller than 10% of the thickness of a film, the influence of the substrate on the measurement of mechanical properties of the film could be neglected. In our case, a light load of 25 mN was used for microindentation tests, which resulted in indentation depths smaller than 10% of the corresponding thickness of TiO2 nanotubular array films so that the above requirement was basically met for all fabricated TiO2 nanotubular arrays. Load-depth curves and apparent Young’s modulus, elastic behavior (We/Wtot), hardness, and maximum depth (hmax) versus the anodization time determined from the microindentation tests are illustrated in Figure 4A,B, respectively. As shown, hmax increased with increasing the anodization time (corresponding to a thicker nanotubular array film). This result indicates that the thicker nanotubular arrays are weaker than their thin counterparts. Consequently, the apparent Young’s modulus, We/Wtot, and hardness all decrease, as shown in Figure 4B. The apparent Young’s modulus decreased from 29.8 to 11.7 GPa when the thickness of TiO2 nanotubular array films increased from 18 to 99 µm. The hardness and We/Wtot also

J. Phys. Chem. C, Vol. 113, No. 17, 2009 7109 sharply decreased from 42.6 to 5.9 HV and from 11.4 to 3.1%, respectively, as the anodization time was increased from 1 to 4 h. After the initial 4 h, the hardness decreased to 2.4 and the ratio of We/Wtot reached a stable value of around 3%. Checking the load-unload curves of different TiO2 nanotubular arrays shown in Figure 4A, one may notice that the loading part in all load-depth curves is slightly “turning up”, resulting from an increase in the resistance to indentation. This increase in the resistance to indentation could be attributed to the densification of the substance when the nanotubes collapse under the indentation force. Besides, as illustrated in Figure 4A, the ratio of We/Wtot is very small for all load-depth curves, which indicates that the overall elastic behavior of TiO2 nanotubular arrays is very minor. In other words, these nanotubular arrays are brittle. The anodic voltage is another factor affecting the formation of TiO2 nanotubular arrays. For comparison, we fabricated a series of TiO2 nanotubular arrays at 10, 20, 40, and 60 V for 8 h. The resulting TiO2 nanotubular array films were examined with FE-SEM, and their mechanical properties were evaluated using the micromechanical probe. No obvious effects of anodic voltage were observed on the diameter and wall thickness of the nanotubes. The growth curve of TiO2 nanotubular arrays, for example, thickness versus anodic voltage, is illustrated in Figure 5A. The apparent mechanical properties of TiO2 nanotubular arrays determined from load-depth curves of indentation are presented in Figure 5B. As shown in Figure 5A, the growth of TiO2 nanotubular arrays is accelerated as the anodic voltage is increased, which is understandable since higher voltages speed up the ionization or oxidation rate of titanium foil, accelerating the formation of TiO2 nanotubular arrays. Figure 5B shows that, as the anodic voltage is increased, the apparent Young’s modulus, We/Wtot, and hardness all decrease, similar to the changes shown in Figure 4B. It should be pointed out that, because TiO2 nanotubular array films fabricated at the lowest voltage (10 V) are very thin, the measurement of mechanical properties could be more or less influenced by the substrate, and the obtained results may somewhat deviate from the real values. However, the general trend is consistent with that of thicker samples fabricated at 60 V (see Figure 4). Since the anodic voltage and anodization time did not show significant influences on the wall thickness and diameter of nanotubes, all fabricated TiO2 nanotubular array films had the same geometrical features, except the thickness of the nanotubular film or the length of the nanotubes. The variations in mechanical properties with respect to the anodic voltage and anodization time, which influence the thickness of TiO2 nanotubular array film, could, thus, be ascribed to the densification and buckling of nanotubes under the indentation force rather than possible changes in their intrinsic mechanical behavior. During microindentation, the region of the nanotubes right under the indenter was subjected to densification due to the collapse of the nanotubes, in which buckling of the nanotubes might play a role. Similar to columns, when nanotubes are subjected to an axial compressive load, they could fail, beginning with buckling, followed by subsequent fracture or catastrophic collapse. The critical load at failure for a clamped simple support column is expressed as follows:35

Pcr )

2.04π2EI L2

(1)

where Pcr is the critical load, E is the modulus of elasticity, I is the moment of inertia of the cross section about the axis of

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Figure 4. Load-depth curves (A) and calculated mechanical properties (B) of a series of TiO2 nanotubular arrays fabricated at 60 V with various anodization durations.

Figure 5. Growth curve (A) and corresponding mechanical properties (B) of TiO2 nanotubular arrays synthesized at different anodic voltages for 8 h.

bending, and L is the length of the column. Because the only difference among these nanotubes under study is their length, E and I of all fabricated nanotubular array films could be assumed to be the same. Thus, Pcr is inversely proportional to L2. The longer the column, the smaller the critical load. It must be indicated, however, that each nanotube was constrained by neighboring nanotubes, which were deformed in a similar manner. Therefore, eq 1 is only used to qualitatively explain the poorer mechanical behavior of longer nanotubes rather than quantitatively describe the failure process. When a compressive force is applied to TiO2 nanotubular arrays, the arrays are crushed down, resulting in densification.36 As the nanotubes experience further compression, the collapse of individual nanotubes and densification of the nanotube structure may lead to an increase in the apparent Young’s modulus,37 with higher resistance to indentation (corresponding to the “turning up” of the indentation curve), as illustrated in Figure 4A. In summary, the thicker TiO2 nanotubular array films, fabricated with higher anodic voltage or longer anodization time, are easier to fail during microindentation, thus exhibiting weaker apparent mechanical properties.

Microscratch tests were performed to evaluate the resistance of the TiO2 nanotubular arrays to scratch damage. During the scratch tests, a tungsten carbide tip scratched a TiO2 nanotubular array film under a linearly increasing load with in situ monitoring variations in CER. Since titanium dioxide is a semiconductor, TiO2 nanotubular array film has a higher electrical resistance than the titanium substrate. When the TiO2 nanotubular array film is broken during scratching, the scratch tip touches the metal substrate, resulting in a sharp drop of CER. The critical normal load corresponding to the drop of CER reflects the minimum scratch load that causes failure of the TiO2 nanotubular array film. Two representative microscratch curves are illustrated in Figure 6A,B. Figure 6A shows a very thin TiO2 nanotubular array film fabricated at 10 V for 8 h, which was damaged immediately when the tip touched it. As the load force increased to 1.9 N, the film completely failed. Figure 6B shows another TiO2 nanotubular array film prepared at 60 V for 1 h, which showed a higher critical load of 3.4 N. Critical loads for TiO2 nanotubular array films fabricated under various conditions are given in Table 1.

Highly Ordered TiO2 Nanotubular Arrays

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Figure 7. Wear of TiO2 nanotubular array films in various environments: representative wear tracks generated by wear tests in air (a), in neutral water (b), and determined volume losses (c).

TABLE 1: Critical Loads for Nanotubular Arrays Fabricated under Various Conditions no.

Figure 6. Microscratch curves of TiO2 nanotubular array films fabricated under various conditions: (A) at 10 V for 8 h and (B) at 60 V for 1 h.

It was demonstrated that the critical loads largely depended on the thickness rather than the apparent hardness or Young’s modulus of TiO2 nanotubular arrays, as shown in Table 1. The thicker a TiO2 nanotubular array film, the higher is its resistance to scratch. It must be indicated, on the basis of the results of microindentation and scratch tests, that the higher scratch resistance of a thicker TiO2 nanotubular array film does not mean that the thicker film is stronger; it just makes the scratch tip less easy to penetrate the damaged film to reach the substrate. We also performed sliding wear tests to evaluate the resistance of TiO2 nanotubular array films to wear using a modified UMT, as shown in Figure 1. Since the TiO2 nanotubular arrays have a variety of applications in different environments, such as photocatalysts, antibacteria coatings, sensors, and so forth, we evaluated the sliding wear resistance of TiO2 nanotubular arrays fabricated at 60 V for 24 h as a model film in various environments: air and water at pH 4, 7, and 10. Typical wear tracks of TiO2 nanotubular array films tested in air and in neutral water are illustrated in Figure 7a,b, respectively. The wear tracks are smooth with clear edges. The volume losses can be determined on the basis of the dimensions of the wear track on the TiO2 nanotubular array films. Results of the tests are presented in Figure 7c. As shown in Figure 7c, the volume losses caused by sliding wear in water at various pH levels were lower than that in air and decreased as pH increased from 4 to 7 and 10. This is

1 2 3 4 5 6 7 8

voltage (V)

time (h)

thickness (µm)

critical load (N)

60 60 60 60 10 20 40 60

1 2 4 24 8 8 8 8

18 25 39 112 0.6 5 15 57

3.4 6.8 8.0 16.5 1.9 2.0 3.6 10.6

ascribed to the lubrication effect of water, which could reduce friction, demonstrated by the decrease in frictional force with respect to the pH values observed during sliding wear tests, as shown in Figure 8. The changes of the frictional force may well explain the variations in volume loss of TiO2 nanotubular array films during sliding wear in different environments. Compared to those observed during the sliding wear tests in air, the frictional forces observed during wear tests in water were significantly decreased,

Figure 8. Variations in frictional force during sliding wear tests in air (a) and in water at pH 4 (b), pH 7 (c), and pH 10 (d).

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(2)

example, 10 V. Microindentation tests demonstrated that the apparent Young’s modulus, We/Wtot, and hardness decreased as the TiO2 nanotubular array film became thicker. The TiO2 nanotubular arrays showed smaller wear loss in water than in air due to the lubrication effect. The resistance of TiO2 nanotubular arrays to sliding wear in acidic water was lower than that in alkaline and neutral water due to the change from attractive interaction to repulsion between the Si3N4 ball tip and the TiO2 nanotubular array film as the pH value was increased from pH 4, through pH 7, to pH 10.

(3)

Acknowledgment. This research was sponsored by the Natural Science and Engineering Research Council of Canada (NSERC). We would also like to thank Deloro Stellite Inc., and Dynetek Industries Ltd. for their support to this project.

resulting in smaller mechanical forces to cause wear of the TiO2 nanotubular array film. As for the small differences in frictional force caused by sliding wear in water at various pH levels, the mechanism could be complicated. As known, TiO2 is amphoteric, whose surface can be hydrolyzed into ≡TiOH in water. Two relevant equations may occur.

In acidic conditions: pK1

≡TiOH + H+ 798 ≡TiOH+ 2

In alkaline conditions: pK2

≡TiOH + OH- 798 ≡TiO- + H2O

References and Notes +

In acidic conditions, ≡TiOH reacts with H , forming a positively charged surface, whereas it will carry a negative charge in alkaline conditions, according to eq 3.38 A specific pH value at which the net surface charge is zero is defined as the isoelectric point (pHiep). The pHiep of TiO2 in water has been widely reported to be in the range of 5-7.39,40 Therefore, at pH 4, the surface of TiO2 is positively charged, whereas it is negatively charged at pH 10. Bullard and Cima41 measured the force-distance curves for the interaction between a silica sphere and TiO2 in aqueous solutions over a wide range of pH values using atomic force microscopy (AFM) and found that the interaction changed from attraction to repulsion as the pH value of the aqueous solution increased, attributed to the change in sign of the electrical double layer of the TiO2 surface. MacDonald et al.42 studied the interaction between TiO2 and the AFM tip (Si3N4) in solutions with various pH values. They observed that, at pH 4.5, there was clear evidence of an attractive interaction between TiO2 and the Si3N4 tip, whereas this did not occur at pH 8.5. Therefore, in the present study, the higher frictional force at pH 4 in water could be ascribed to the attractive interaction between the silicon nitride ball tip and the TiO2 nanotubular array film, whereas the frictional force decreased at pH 7 and pH 10, resulting from the change in the interaction from attraction to repulsion. The increased frictional force could enhance the wear attack. However, more work is needed to fully explain the mechanism for the effect of pH value on the wear of TiO2 nanotubular arrays, though the present discussion is supported by the above-mentioned studies reported in literature. 4. Conclusions A series of TiO2 nanotubular array films with different thicknesses were fabricated using different anodic voltages and anodization times in an organic electrolyte. It was shown that the diameter and wall thickness of the nanotubes were independent of the anodic voltage and anodization time. However, thicker TiO2 nanotubular array films (i.e., longer nanotubes) were fabricated at higher anodic voltage or longer anodization duration. The growth of TiO2 nanotubular arrays at 60 V within the first 8 h was fast and linear with time, showing a growth rate of 6.3 ( 0.2 µm/h, which decreased gradually to 1.6 ( 0.7 µm/h after around 20 h of anodization. Compared to the anodization time, the anodic voltage played a more important role in the formation of TiO2 nanotubular arrays. The growth of TiO2 nanotubular arrays was accelerated with increasing anodic voltage, and smooth TiO2 nanotubes could not be made if the voltage was too low, for

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