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Optimizing Anodization Conditions for the Growth of Titania Nanotubes on Curved Surfaces Karan Gulati,† Abel Santos,† David Findlay,‡ and Dusan Losic*,† †

School of Chemical Engineering and ‡Discipline of Orthopaedics & Trauma, University of Adelaide, Adelaide, SA, Australia S Supporting Information *

ABSTRACT: Titania nanotubes (TNTs), fabricated by electrochemical anodization due to their outstanding properties, have been widely explored for solar cells, catalysis, electronics, drug delivery, biosensing, and medical implants. Rational design of the anodization conditions is the key to obtaining high quality TNTs that are well aligned and strongly adherent onto the underlying titanium substrate. With the development of many anodization procedures on a substrate with various shapes and sizes, catering to various applications, the mechanical stability of anodic layers is often neglected. Here we consider the factors that lead to unstable and poorly adherent nanotube arrays produced upon anodization of curved titanium surfaces. The role of electrolyte aging, water content, voltage/time of anodization, and the substrate dimensions were investigated for optimization of the fabrication of nanotubes on curved surfaces such as Ti wires. Finally, the most optimal fabrication procedure and anodization parameters are presented that yield high-quality nanotubes, which are stable and well-adherent on the underlying substrate.

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INTRODUCTION Nanoporous and nanotubular structures like titania nanotubes (TNTs), fabricated by self-ordering electrochemical anodization, have gained much popularity, especially in the field of solar cells, catalysis, electronics, drug delivery, biosensing, and medical implants.1,2 For example, improved biocompatibility, suitable biomechanics, ease of functionalization, and the ability to load/release drugs locally have made TNTs an excellent choice for modifying the surface of conventional bone implants composed of titanium and its alloys.3−5 Moreover anodization is a cost-effective industrial process that permits easy control over the dimensions of the TNTs, which in turn can be used to modulate immune responses, prevent bacterial infections of implants, enhance bone−implant bonding, and enable focal and controlled release of loaded drugs into body tissues.6,7 Many reports in in vitro and in vivo settings have established that the above-mentioned technical features of TNTs are well suited for the bone implant industry.6 Most studies on TNTs are focused on the growth of TNT layers on planar surfaces, such as flat titanium foils. Although flat surfaces are easier to manage, bone implants, for example for fracture fixation or joint replacement, typically have far more varied/complex surfaces, with curves, edges, and grooves, as seen in bone screws and plates. Also, alternative metal implants, such as catheters, bone fixation pins/wires, and dental implants, for which the incorporation of TNT coatings is suggested, possess curved surfaces. Recently several studies of fabrication of TNTs on complex curved surfaces, such as wires, meshes, and hollow tubes, have been reported, but the mechanical stability of these layers, which contained many cracks, was not satisfactory.8−14 Thus, the optimization of the © 2015 American Chemical Society

anodization procedure to achieve well-adherent and mechanically stable TNT arrays on a curved surface with minimum cracks remains challenging. Stability is critical for applications like bone implants, where loss of the surface layer as a result of surface instabilities, cracks, and delamination of nanotube structures might release toxic nanoparticulates. For instance, recent in vivo studies with porcine models have demonstrated that the shedding of particle debris released from inorganic layers grown on the surface of implants can lead to inflammation of surrounding tissues and subsequent rejection of the implant.15 Moreover, load-bearing conditions for applications such as fractures demand a well-adherent nanosurface that will stay bound to the implant during the course of its lifetime. Similarly for other TNTs applications, including solar cells and electro-optical devices, stable anodic films that are welladherent to underlying titanium substrates are desired.16 In fact, for any suggested application of TNTs like drug-releasing implants, solar cells, or sensing devices, besides being able to control the dimensions and surface characteristics, the ability to fabricate mechanically stable long-lasting TNTs is essential for their performance and long-term functions. While previous studies have focused on optimizing the fabrication methodology to achieve appropriately adherent TNTs onto the underlying substrate, they mainly consider planar surfaces, i.e., in the form of flat foils.17−19 The role of water content, electrolyte conductivity, suitable surface smoothness of bare Ti Received: April 8, 2015 Revised: June 21, 2015 Published: June 22, 2015 16033

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Scheme 1. Scheme Showing the Various Electrolyte Changes and the Nanotube Characteristics Compared between Anodization Performed with (a,b) Fresh Electrolyte (No Aging Performed) and, (b,c) Appropriately Aged Electrolytea

Briefly, in a controlled anodization experiment, aged electrolyte represents a water- and fluoride-deficient system, whereby the lowered conductivity and reduced oxide formation/dissolution leads to thicker barrier oxide films. a

and ammonium fluoride (NH4F) were obtained from Sigma− Aldrich (Sydney, Australia). High purity water Option Q− Purelabs (Australia) (18.2 MΩ) was used for preparation of all solutions throughout this study. Electropolishing. Ti wires were annealed at 500 °C for 2 h followed by sonication in acetone and ethanol. Electropolishing was carried out at 25 V for 5 min using perchloric acid containing electrolyte (with butanol and ethanol, P:B:E = 1:6:9) at subzero temperature using a computer-controlled voltage/time program (Labview) with National Instruments power supply. Polishing was followed by thorough sonication in acetone and ethanol. Preparation of TNTs and Aging of Electrolyte. All anodizations were performed in a special electrochemical cell, which permitted only a fixed length/area of the Ti flat foil or Ti wire to be exposed to the ethylene glycol electrolyte [with 0.3% NH4F (w/v) and 1−3% deionized water (v/v)]. Moreover, the anodization cell was sealed to avoid any absorption of environmental moisture by the hygroscopic electrolyte. Clean Ti flat foil was used as the counter electrode for all anodizations maintained at 25 °C. Voltage/time and current were varied and continuously monitored using Labview (National Instruments). The term “aging” is used to represent the anodization of flat Ti foil (∼dummy titanium) prior to anodization of Ti wires. Mechanically polished Ti foil in a specially designed holder was used as an anode to perform “aging” of the electrolyte. “Time of aging” represents the “time of anodizing the flat Ti foil at 75 V”; e.g., “fresh electrolyte” means “no anodization performed”, and “1 h aged electrolyte” means “electrolyte used to anodize Ti flat foil at 75 V for 1 h”. Later the anodization of clean and electropolished Ti wires was performed using different electrolytes, which were aged for various time periods. For optimizing the fabrication of TNTs on Ti wires various influencing factors were investigated: water content (1−3% water v/v), age of electrolyte (fresh to >30 h), time of anodization (15 s to over 3 h), voltage of anodization (60−120 V), and substrate dimensions (varied diameter of Ti wires: 0.50 and 0.80 mm). Characterization of Electrolyte Composition and Conductivity. For conductivity measurements, a fixed volume of different aged electrolytes was analyzed at 25 °C using a sensION5 Conductivity Meter (Hach). The electrolytes were

substrate, aging of electrolyte, and electrolyte composition has been investigated as factors in yielding high-quality and mechanically stable TNTs on Ti. 17−21 However, such parameters have been explored for anodization of flat Ti substrates but not for complex substrate geometries like that of a Ti wire, which still remains challenging. Studies with TNT fabrication on curved surfaces indicate the presence of unavoidable cracks or pits in the anodic films.8,9,14 The curved nature of the substrate presents more challenges that reduce the mechanical stability of the resulting anodic film, which includes the radial growth of TNTs and the generation of internal stresses as a result of the uneven distribution of the electric field across the surface of the substrate. Another factor that might compromise the mechanical integrity is the volume expansion of oxide film on the curved substrate. Despite the recent advances in TNT fabrication by the anodization process, more research is required toward optimizing the fabrication of high-quality and mechanically robust TNTs on curved substrates. Such investigations can lead to better integration of this nanomaterial into areas like medical implants and optoelectronics. In this context, we present here a systematic study on the optimization of various anodization parameters that contribute toward obtaining mechanically stable and adherent TNTs films on the curved surface of Ti wires. The study explores first the influence of electrolyte aging, which is recognized as one of the key parameters linked with the quality and the adherence of the fabricated anodic film. Fine-tuning of the electrolyte via aging is critical as it is related to the growth rate and the formation of an oxide barrier layer and its thickness, which consequently have an influence on the layer integrity and adherence on the underlying Ti substrate. Other important anodization parameters such as water content, voltage/time of anodization, and substrate dimensions were also studied toward optimizing the anodization of curved surfaces. The proposed study of different anodization conditions and their impact on TNT structure and their properties is outlined in Scheme 1.

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EXPERIMENTAL SECTION Materials and Chemicals. High purity titanium wires (diameter 0.50 mm and 0.80 mm) and titanium flat foil (0.25 mm thick) were supplied from Nilaco (Japan). Ethylene glycol 16034

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Figure 1. Stability of anodic films for anodization performed in (a,b) fresh and (c,d) aged (10 h) electrolytes. Anodization was performed at 75 V for 10 min using ethylene glycol/NH4F electrolyte with 1% (v/v) water.

covered and sealed during all anodizations, sample changes, and also while measuring conductivity. Furthermore, to quantify the Ti ion concentration in the electrolyte as a result of anodizations, ICP-MS (inductively coupled plasma mass spectrometry) of the electrolytes was performed. Briefly, the settings were calibrated using various Ti solution standards from 10 to 500 ppb in pure ethylene glycol with 2% (v/v) ultrahigh purity HNO3, followed by measurement of various ages of electrolytes. Structural Characterization. Structural morphology of the fabricated TNT/Ti wires was characterized using a field emission scanning electron microscope (SEM) (FEI Quanta 450). The samples were mounted on a holder with doublesided conductive tape and coated with a layer of platinum 5 nm thick. Images with a range of scan sizes at normal incidence and at a 30° angle were acquired from the top/bottom surfaces and cross sections. SEM images were subsequently analyzed by ImageJ (public domain program developed at the RSB of the NIH).

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RESULTS AND DISCUSSION Fresh vs Aged Electrolyte. When electropolished Ti wires were anodized (at 75 V for 10 min) using two different electrolytes, fresh and aged electrolyte, the prepared TNTs structures (TNTs-F and TNTs-A, respectively) exhibited considerably different features. The low- and high-resolution SEM images presented in Figure 1a−d show the anodized Ti wires prepared by fresh and aged (10 h) electrolyte solution. The SEM images confirm the structural instability of the TNTsF showing numerous cracks and significant delamination of the TNT layer. On the contrary, a well-adherent and stable anodic TNT layer was obtained for aged electrolyte (TNTs-A), as shown in Figure 1c,d. These results clearly indicate the significance of aging the electrolyte prior to the anodization process, which is equally important for both planar and curved surfaces. It is interesting that most published studies on TNTs using this method do not describe the aging condition of the electrolyte, which obviously could have a significant impact on

Figure 2. Variation in the properties of the anodization electrolyte upon aging: (a) Ti ion concentration and (b) conductivity. Age of electrolyte represents number of hours Ti flat foil was anodized in a 1% water (v/v) electrolyte at 75 V.

the quality of fabricated TNT structures. We initially observed this problem on a planar Ti surface, and based on these initial observations, further in-depth analysis and the effect of aging on anodic film stability was performed focused on curved surface 16035

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Figure 3. Effect of fresh and aged electrolyte on the stability of the anodic film for short-term anodization monitored by SEM imaging of the surface and the current density changes during different anodization times: (a−c) 15 s, (d−f) 30 s, and (g−i) 60 s. Anodization was performed at 75 V. All scales represent 500 μm.

anodization electrolyte affects the chemical composition of electrolyte, mainly the Ti ion concentration in the form of [TiF6], which is known to be critical for the anodization process. However, the water content is also known to influence the anodization process; however, water evaporation/uptake from the environment was not considered in these studies, and no control measurements were established in order to manage this factor. We performed a systematic study to monitor changes in Ti ion concentration and conductivity of the electrolyte by aging, which are summarized in Figure 2a,b. These results confirm that by electrolyte aging the Ti ion concentration of the electrolyte increases significantly (∼32 ppm for 2 h to ∼283 ppm for 16 h). At the same time the conductivity is slightly reduced with aging, for, e.g. ∼475 μS/ cm, for the fresh electrolyte to ∼431 μS/cm for ∼15 h aged. This behavior can be explained by the following reactions that occur during the anodization process. The complexation of Ti and F ions into the electrolyte upon aging, coupled with consumption of water from the ethylene glycol based electrolyte, reduces the conductivity in accordance with the following equations

(Ti wire) to enable better understanding of the process and optimize the anodization performance. The mechanism of nanotube or nanopore formation during anodization has been well researched and applied.1,16,17 Three main processes have been identified that play a key role in fabrication of TNTs during anodization of the Ti substrate: (a) formation of an initial oxide layer, (b) field-driven oxide dissolution at the oxide/electrolyte interface, and (c) chemical (electrolyte-assisted) dissolution of oxide at the metal/oxide interface.1 The last step of chemical dissolution of TiO2 in the acidic electrolyte reduces the thickness of the barrier layer (BL), which continues the ongoing process of field-driven oxidation and dissolution.17 This dissolution rate further depends on the F− and H+ (from water) concentration of the electrolyte, which is expected to change over time as the anodization is performed. The electrolyte aging, as mentioned in previous studies, suggests the anodization of dummy titanium to age the electrolyte before beginning the fabrication of nanotubes.17,19,22 These reports point out that aging aids in fabrication of well-ordered nanotubes, as compared to fresh electrolyte. The explanation was that the aging of the 16036

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Figure 4. SEM images of the Ti surface anodized at 75 V for 15 s using two different electrolytes: (a−c) fresh (no aging performed) and (d−f) aged electrolyte (10 h aged). Short time of anodization reveals highly cracked surfaces, which readily peeled off when TNTs were anodized using fresh electrolyte and stable, well-adherent, and crack-free TNTs anodized using aged electrolyte.

Figure 5. Comparing dimensions of nanotubes fabricated using fresh and aged (10 h) electrolytes. Anodization was performed using 1% (v/v) water at 75 V for 10 min.

Ti + 2H 2O → TiO2 + 4H+ + 4e−

TiO2 + 6HF → [TiF6]2 − + 2H 2O + 2H+

(1)

16037

(2)

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Figure 6. Anodization of 0.5 mm Ti wire (at 75 V for 60 min) using various ages of electrolyte. (a,b) Current density vs time (showing 1st 120 s only, which signify the variation in current−density reduction pattern) for different aged electrolytes and (c) plot showing increasing teq (time to reach equilibrium) with the age of electrolyte.

prevents water evaporation from electrolyte or moisture intake by the hygroscopic electrolyte. Since our system avoided moisture absorption, the water content continuously reduced, and hence the conductivity was decreased. These data clearly demonstrate the changes in both the chemical composition (Ti ion and water content) and the conductivity of the anodizing electrolyte, which in turn can significantly influence the nature and properties of the anodic films fabricated. To further link the mechanical stability of the TNT anodic layer with the changes in electrolyte composition as a result of the aging process, we investigated the growth of TNTs performed using short anodization times (15, 30, and 60 s) with fresh and 10 h aged electrolyte. A short duration of time was chosen because previous studies indicate that the very early and critical stage of TNT growth (which involves initial oxide growth and nanotube formation) determines the stability of a fully grown TNT layer and its growth rate. In this study, early anodization was carefully monitored by SEM imaging to confirm changes in the anodic surface of the film associated with the current density profile during this process. To this end,

Table 1. Variation of Electrolyte Characteristics, Anodization Profile, TNT Dimensions, and Stability of Anodic Film for Anodization (75 V, 10 min) Performed Using Fresh and 10 h Aged Electrolyte anodization (75 V, 10 min)

fresh electrolyte

10 h old electrolyte

Ti ion conc. of electrolyte (ppm) conductivity of electrolyte (μS/cm) time to reach equilibrium teq (s) TNT length (μm) TNT diameter (nm) TNT wall thickness (nm) averaged crack width (μm) stability of anodic film

0 475 ± 5 12 ± 3 5.0 ± 0.2 48.5 ± 1 14.7 ± 4 1.5 ± 0.3 ↓

163 ± 12 458 ± 5 45 ± 5 4.7 ± 0.15 42.7 ± 1 26.6 ± 3 0.80 ± 0.1 ↑

During anodization water is required for oxide formation and also for dissolution of the oxide, and as a result its concentration is reduced with each anodization step. To investigate the influence of water content in this experiment, a controlled and sealed electrochemical cell was used, which 16038

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Figure 7. SEM images showcasing the variation of stability with anodization performed using various water contents of electrolyte at different voltages. All anodizations were performed in 10 h aged electrolyte for a time of 10 min each. (a,b) 75 V using 1% water electrolyte, (c,d) 75 V using 3% water electrolyte, (e,f) 100 V using 1% water electrolyte, and (g,h) 100 V using 3% water electrolyte. Water content in electrolyte is expressed in % v/v. Scale bars represents 200 μm for a, c, e, g and 1 μm for the insets (b, d, f, h).

resulted in a well-adherent TNT layer onto the underlying substrate of Ti wire and hence is a critical parameter for the formation of TNTs. The current−density plots corresponding to the early anodization results presented in Figure 3 also confirmed differences in current density and the time to reach equilibrium stage (teq) between fresh and aged electrolyte. From these graphs, we can observe that for a given anodization time the current density is higher with the aged electrolyte as compared with fresh electrolyte. This phenomenon is directly linked to the electrochemical equilibrium established through the oxide barrier layer at the base of the nanotube. In other words, the thicker the barrier layer, the higher the current density. This would denote that the aging of the anodization electrolyte leads to nanotubes with a thicker oxide barrier layer, and thus the resulting film presents a more compact structure (mechanically stable). TNTs produced in a fresh electrolyte (TNTs-F), however, result in the formation of a thinner oxide barrier layer and a fragile oxide film with poor mechanical properties, which is verified by SEM imaging [Figure 3a,d,g]. Figure 4 presents the high-resolution SEM images showing the typical surface of Ti wires at 15 s of anodization, compared between fresh and aged electrolyte. Clearly it can be established that for F-TNTs the anodic film shows compromised structures after only 15 s of anodization [Figure 4a−c]. Conversely, for ATNTs very stable and crack-free anodic film was observed with well-formed nanoporous structures similar to those observed on a flat Ti surface [Figure 4d−f]. Also, a comparison of the close up of the anodic film for 15 s anodization [Figure 4c,f] shows that pore initiation has begun for both the electrolyte systems; however, the structural stability is much compromised

the anodization process was terminated at very short time intervals, when TNT formation is beginning. During the anodization stage, the current equilibrium is achieved, and close SEM examination of the resulting TNT layer makes it possible to elucidate the structural integrity of the fully grown anodic film. This also informs whether time plays any role when it comes to comparing surface stability between anodizations performed using fresh and aged electrolyte. All parameters for this study (voltage, time, temperature, volume of electrolyte, size of Ti wire) were kept constant to evaluate the role of aging. Anodization was carried out at 75 V for 15, 30, and 60 s for electropolished Ti wires (diameter 0.50 mm). SEM images and the corresponding current−density plots for the short-time anodization compared between fresh and aged (10 h) electrolyte are presented in Figure 3a−i. For the fresh electrolyte (TNTs-F), it can be seen that the anodic film is unstable and leads to delamination. Clearly, at 15 and 30 s anodization using fresh electrolyte, the anodic film presents many surface instabilities, and as a result fractured/delaminated fragments are easily visible. For 60 s of anodization using fresh electrolyte, the entire anodic film is totally delaminated from the underlying Ti wire substrate, and its fragments can be seen on the carbon tape used to mount the samples for SEM [Figure 3g]. On the basis of these observations, we can conclude that fresh electrolyte is not suitable for preparation of TNT layers and is not recommended for the preparation of TNTs on a curved surface. Conversely, for anodizations performed using aged electrolyte (TNTs-A), stable and well-attached anodic films are seen, during all these early stages of anodization, which are critical for the fabrication of mechanically stable anodic films. Therefore, it is evident that the aged electrolyte 16039

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Figure 8. SEM images showing the dependence of stability of anodic film on Ti wire on the time of anodization. All anodizations performed with 10 h aged electrolyte (1% v/v water) at 75 V. Each scale represents 50 μm [for insets: (g) and (i) scale bar is 1 μm].

influences the reaction kinetics of the anodization (as shown in Scheme 1). As a result, the aged electrolyte reduces the dissolution of TiO2, and hence it takes longer to achieve the self-ordering equilibrium state. This can also be linked with the barrier layer (BL) thickness at the bottom of the TNTs. Faster kinetics of TiO2 formation and dissolution (in fresh electrolyte) forms a thin BL, which leads to unstable anodic films (Scheme 1). Conversely, for the aged electrolyte, a thicker BL is obtained, which represents a more compact structure and hence improved stability of the resulting anodic film. By comparing growth rate, wall thickness, and hence the barrier layer between F-TNTs and A-TNTs, we can relate the nanotube features with the stability of the anodic film. For this purpose, TNTs were fabricated using fresh and 10 h aged electrolyte, at 75 V for 10 min, similar to Figure 1. The SEM images presented in Figure 5 compare structural features of the

for F-TNTs. Also the higher initial current density for aged electrolyte [Figure 3c] resulted in wider pores for 15 s anodization, as shown in Figure 4c,f. The approximate diameters of these pores were ∼23 nm for F-TNTs and ∼34 nm for A-TNTs. Interestingly, while pores were wider for aged electrolyte, the time taken to reach a stable low current value (∼equilibrium stage) is much longer in comparison with the fresh electrolyte system. These differences in the current behavior are explained by the aging-dependent increase in [TiF6]− concentration and the decrease in the water content (in a controlled system like ours), which in turn reduce the conductivity of the electrolyte [Figure 2b]. In fact, the aged electrolyte system behaves like a very low water content electrolyte.23,24 This reduction in both the available F-ions (as the formed TiO2 is dissolved as TiF6− complexes) and water content (water used in both oxide formation and dissolution) 16040

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enriched electrolyte with high conductivity caused increased growth rates. On the contrary, for A-TNTs, a high initial current density and delayed time to reach equilibrium (teq) resulted in wider initial pores for a 15 s anodization; however, for long-term anodization (10 min), once the teq has passed the reduced growth rate phase starts. Moreover in Figure 5c and 5f we can clearly see the differences in the wall thickness, with ATNTs possessing thicker walls (thickness ∼26 nm) as compared to F-TNTs (thickness ∼14 nm). For TNT fabrication wall thickness can be directly related to the barrier layer (BL) thickness, so based on SEM images provided for the close up of the nanotube, it can be assumed that A-TNTs possess thicker BL as compared to F-TNTs. Moreover, a thicker BL means better contact between nanotubes and the underlying substrate and hence better stability of the anodic film. This claim is well supported by reports, which confirm that for TNT fabrication the oxide layer at the oxide/electrolyte interface is less dense, and the layer at the metal−oxide interface is a dense and stable oxide.1 Note that our anodization setup does not allow any water intake from the environment, and as a result, with increased number of anodizations, water content is significantly lowered. Such a system almost resembles a nonaqueous electrolyte, and hence an alternate explanation for the above-mentioned features could be the voltage drop effects at the substrate electrode, as described below Ueffective = Uapplied − iR

where R = resistivity of the electrolyte and i = current.22,25−27 This voltage drop associated with thicker BL results in a reduced overpotential for TiO2 formation and hence reduces the growth rate of the TNTs.22 Lowered growth rates combined with reduced dissolution of the oxide result in well-adherent stable TNT layers. Though the growth rate (for aged electrolyte) is lower as compared to the fresh electrolyte, the more time it takes to reach the steady current state, the more improved stability of anodic films is observed (Figure 3). For the fresh electrolyte, an exact opposite mechanism can be assumed: appropriate water content and F-ions in the electrolyte (high conductivity) result in very high growth rates due to increased oxide formation and dissolution, and since the reaction rate is higher for such systems, it takes less time to attain the equilibrium state (teq). Moreover for fresh electrolyte other factors like higher PBR (Pilling−Bedworth ratio: volume of oxide formed/metal consumed) and higher oxide formation efficiency, obtained for higher growth rates, can increase the higher compressive stress at the barrier layer and metal interface, which can in turn result in a fragile anodic layer and delamination.1,28 Such stress-related factors resulting in anodic film instability are discussed in more detail below. In order to further explore the significance of electrolyte aging, the effect of various ages of electrolytes was investigated during the early stage of the anodization process. Figure 6a−c shows the current−density/time graphs observed for the first 120 s of anodization for various ages of the electrolyte from fresh (no aging performed) to over 30 h old electrolyte. Please note that anodization was performed at 75 V for 60 min to identify teq (which correlates with very low value of current density) for each aged electrolyte; however, Figure 6a,b only shows the first 120 s of anodization to easily visualize the variation in current density reduction patterns for each electrolyte. It clearly shows that for each age of electrolyte the current density drops differently after initiation of the

Figure 9. Growth rates for the anodization of 0.5 and 0.8 mm diameter Ti wires. All wires were electropolished prior to anodization at 75 V using a 1% water (v/v) electrolyte aged for 10 h. Dependence of time of anodization on: (a) diameter of TNTs, (b) length of TNTs, and (c) the cracks on the anodic films.

fabricated TNTs between the two anodization electrolytes. The average values of length/diameter of the fabricated nanotubes were observed to be ∼5.0 μm/48.5 nm for F-TNTs and 4.7 μm/42.7 nm for A-TNTs [Figure 5a−f]. Besides the higher initial current density for A-TNTs, we observed a slightly lower growth rate for the nanotubes in comparison with F-TNTs (for a 10 min anodization). Moreover, as described above, for a 15 s anodization, we obtained the initial pore diameter of ∼23 nm for F-TNTs and ∼34 nm for A-TNTs. Comparing these two time periods (15 s and 10 min anodization, Figures 4 and 5), clearly at a very early stage of anodization (15 s) large pores were obtained for A-TNTs, and later for long-term anodization (10 min) we observed a relatively reduced growth rate, in comparison with the corresponding time values for F-TNTs. This observation can be attributed to the high TiO2 formation/ dissolution rates for a fresh electrolyte system, whereby upon quickly attaining the equilibrium stage the water-/fluoride16041

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Figure 10. SEM images depicting the most favorable conditions for obtaining good quality TNTs that are well adherent with reduced surface cracks. (a) Commercially available Ti wire (diameter 0.50 mm), (b) electropolished Ti wire, and (c−g) anodized Ti wire showing details of cracks and nanotube structures. Anodization was performed using 10 h aged electrolyte containing 1% (v/v) water at 75 V for 10 min. Besides the presence of cracks in the anodic film (arrows), the TNTs are well-adherent. All unmarked scales (a−d) represent 100 μm.

Table 2. Summary of Various Anodization Factors Studied and the Final Optimized Value for Each to Fabricate Stable Anodic Films on Curved Surfaces factors investigated

details

range

optimized value

electrochemical setup aging of electrolyte water content of electrolyte anodization voltage time of anodization substrate dimensions

closed setup to avoid water evaporation or intake from environment influences Ti ion concentration and conductivity of electrolyte % v/v in the electrolyte, controls growth rate controls growth rate and hence the expansion factor to control nanotube dimensions smaller cracks and higher growth rates for large diameter of Ti wires

fresh to >30 h 1−3% (v/v) 60−120 V 10−180 min 0.50−0.80 mm

water controlled system >10 h old 1% 75 V 10−40 min 0.80 mm

dependent changes in the conductivity of electrolyte and the potential difference. Potential difference (PD): (a) at metal− oxide interface, (b) across oxide, (c) at oxide−electrolyte interface, and (d) across the electrolyte; sums up to the total applied anodization voltage.17 As the conductivity reduces with aging of electrolyte (evident from Figure 2), the PD across the electrolyte increases. If we assume that PD at the interfaces remains unchanged, the aging causes a reduction in the PD across the oxide layer.17 This in turn reduces the formation of TiF62− complexes and hence impedes the formation of TiO2 [as per eq 2]. This is in agreement with the Le Chatelier’s

anodization process. For a fresh electrolyte the current density drops to a very low value (60 min, severe etching of the formed TNTs was visible. This results in formation of thin fibers or grasslike structures.1 For longer anodization times, etching of the tube top occurs, which results in inhomogeneity of the top structures. The thinning of the nanotube walls caused the formation of grasslike morphologies which are also observed on planar Ti surfaces. Furthermore, such structures represent poor stability as the grass-like structures (Figure 8e−j) collapse and often bundle together to counteract the capillary actions upon drying, thereby generating internal stress in the oxide layer, which leads to wide cracks in the resulting oxide film. These results clearly confirm the importance of optimizing the time of the anodization process to control cracks and the stability of the anodic film on Ti wire. Curve Dimensions. A prerequisite for fabricating wellordered TNTs is the smooth surface of the substrate, to prevent the difference in the electric field distribution.20 This is achieved by surface smoothening by mechanical and electropolishing. Moreover, the instability of the anodic films has been demonstrated to depend on the surface topography, with the presence of defects greatly affecting the yield quality.20 In the anodization process, the electric field is directly perpendicular to the substrate surface, which in turn dominates the perpendicular growth of oxide structures of nanopores or nanotubes. The growth and volume expansion from metal to oxide on planar surface presents fewer mechanical stressed conditions and competition among the tubes/pores to grow. Therefore, cracks are usually not seen while anodizing flat substrates such as Ti foils. However, for curved surfaces like that of a Ti wire, where the electric field and the self-ordered growth of nanotubes follow the same principle as for planar surface, but with considerably higher mechanical stress due to volume expansion and limited space for TNT growth, the results are unavoidable cracks as observed on Ti wires. Moreover, the instability of the anodic film that leads to swelling and delamination can also be attributed to the presence of so-called “weak spots”, as described by Proost et al.18,29 There is the possibility that such spots in the anodic film could allow electrolyte penetration, which can result in unstable and fragile anodic layers.18 Furthermore, “internal stress development” during anodic oxidation can also lead to stability

equilibrium principle: as the electrolyte already contains high concentrations of TiF62−, the forward-reaction kinetics as per eq 2 must reduce. This “adjustment” to stay in the chemical equilibrium causes delay in reaching the “anodization equilibrium” which determines the self-ordering regime of TNT formation. As a result we obtain the characteristic transient curves for aged electrolyte systems, whereby in comparison with the fresh electrolyte system, they take longer (higher value of teq) to reach the steady state which is represented by a low and almost stable current−density phase. Furthermore, this delay yields anodic films with improved stability. Various changes in the electrolyte and the characteristics of the TNTs produced are compared between anodizations performed using fresh and 10 h aged electrolytes in Table 1. Furthermore, although, with slight differences in terms of current density values, growth rate and nanotube diameter were observed, a similar trend of anodization (i.e., current density transient) was observed when flat Ti foil substrate was anodized using fresh and appropriately aged electrolyte (10 h). A representative comparison in that respect is presented in the Supporting Information (Figure S1). Water Content and Anodization Voltage. The presence of water in the anodization electrolyte plays a very significant role in providing oxygen for the formation of TiO2 required for growth of nanotubes.23,24 In order to determine the effect of water content on obtaining high-quality well-adhering TNTs, two water contents of the electrolytes were investigated: 1% and 3% (v/v). Electropolished 0.50 mm diameter Ti wires were anodized in two separate electrolytes (both aged 10 h) containing 1% and 3% water (v/v), respectively. The anodization was carried out using 75 and 100 V for 10 min, to also simultaneously study the effect of voltage on the stability of the anodic film. Since from previous studies on planar Ti it is known that the higher water content induced the faster growth rates of TNTs, the same was expected for Ti wire. The comparative SEM images of the TNT layer prepared by different water contents (1% and 3%) and different voltages (75−100 V) using 10 h aged electrolyte are presented in Figure 7. The figure shows that increasing the water content from 1% to 3% (v/v) resulted in the peeling of the anodic layer. For 1% water, the anodic film can be seen well adherent with less surface cracks (Figure 7a,b); however for 3% water the anodic film is seen to be severely damaged and very prone to delamination (Figure 7c,d, g,h). A similar trend for stability can be confirmed by comparing the anodization performed using different voltages; clearly higher voltages (Figure 7e−h) promote surface cracks and lead to anodic film instability. Figure 7g which represents anodization performed at 100 V (for 10 min) using 3% water electrolyte can be seen with large cracks and delaminated anodic film, exposing the underlying Ti substrate. These observations can be linked directly with higher growth rates (both due to high water content and high anodization voltage), which enhance the expansion factor (and the PBR) and lead to severely damaged anodic film. These results correlate directly with previous findings by Albu et al. using flat Ti substrates, who concluded that water content of electrolyte and voltage of anodization greatly influence the expansion factor of TNTs.21 On the basis of these observations, a minimum of 10 h aged electrolyte was selected for anodization at 75 V, as the optimum condition to anodize polished Ti wires. Furthermore, water content below 1% was also studied; however, it yielded very low growth rates. 16043

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Our current investigation confirmed how aging of electrolyte along with other factors (water content, voltage/time of anodization, and substrate dimensions) contributes toward achieving stable anodic films with highly ordered nanotube structures. Therefore, a rationally designed anodization strategy can enable easy integration of TNT layers on complex geometries and thereby into the applications ranging from medical implants to electro/optical devices. More research in elucidating the contributors to the instability of the fabricated anodic film is required especially on a curved surface, to improve the anodization process and production of TNTs for different applications.

issues. Studies have shown that metal oxidation causes a relative increase in volume, which is governed by the Pilling−Bedworth ratio (PBR).18,30 Our investigation aims to identify the most optimal methodology to minimize the instability of anodic films on curved surfaces. Also, besides the growth-induced internal stresses, extrinsic factors such as field-induced stress can also cause stability issues in the resultant anodic film.31 When similar time/voltage of anodization was compared between 0.50 and 0.80 mm diameter Ti wires, more information relating to the presence of cracks was found. The results showing change in length/diameter of TNTs and the resulting cracks for two different substrates at various times are compared graphically in Figure 9a−c. These results confirm wider cracks in the anodic films for 0.50 mm diameter Ti wire in comparison to 0.80 mm Ti wire. A possible explanation here is the large diameter (for 0.8 mm wire) allowing for a large tangential flat area, over which the alignment of TNTs was very similar to planar surfaces, and as a result the competition among adjacent TNTs during the anodization procedure is reduced, which maintained the structural integrity of the TNT layer (similar to anodization of flat Ti foils). Furthermore, for smaller substrate surfaces with curvature (like that of 0.5 mm wire in comparison with 0.8 mm wire) the electric field concentration can be assumed to be very dense and also the tube initiation/self-ordering to be more congested. This causes increased stress development for growth and volume expansion of TNTs, and the competition of space can result in collapse/ bundling of tubes to minimize surface energy. This in turn can result in wider cracks, which worsen the mechanical stability and adhesion of the resulting oxide film. When compared with anodization performed on a flat Ti substrate (as presented in Figure S2, Supporting Information), clearly the difference in growth rate between the curved surface (Ti wire) and flat surface (Ti flat foil) is established, for the same anodization conditions. This again can be attributed to slowed growth rates of the anodic film (in the case of anodizing wires) due to stress effects and competitive growth between adjacent nanotubes during their growth. Optimized Anodization Conditions. To summarize the various factors taken into consideration in the study we present the most appropriate conditions toward obtaining stable anodic films on titanium wires. Figure 10 shows the SEM images depicting the various stages of anodization. The commercial Ti wire (diameter 0.50 mm) as seen in Figure 10a shows very rough surface features which can affect the quality of the anodized film and required polishing.20,32 Upon electropolishing, the smooth Ti surface is obtained as shown in Figure 10b. Later the anodization was performed under optimized conditions in 10 h old electrolyte containing 1% (v/v) water, at 75 V for 10 min. This yielded TNTs on Ti wire [Figure 10c−g] with a very stable and organized structure. The TNTs with ∼4.7 μm length and ∼43 nm diameter were obtained, which can be further controlled using various parameters, as discussed earlier. Following this simple stepby-step optimized anodization procedure yields high-quality and well-adherent TNTs, which can also be confirmed visually, for instance by monitoring the current changes during the anodization process, as presented above. The various parameters investigated to optimize the anodization procedure are also presented in Table 2. It shows the range of anodization factors investigated that influence the stability of anodic film on substrate, finally arriving at the most appropriate step-by-step procedure.

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CONCLUSIONS The influence of various anodization parameters in obtaining stable anodic films with highly ordered TNTs on the curved surface of Ti wire was systematically explored. The effect of aging the electrolyte was systematically investigated to identify the key changes that occur in the resulting TNT films as well as the current profile of the anodization. Clear evidence was obtained for the role of electrolyte aging (∼10 h) and achieving mechanically robust and stable anodic films. Moreover, other anodization parameters, including water content, voltage/time of anodization, and substrate size, were also studied to conclude the most optimum anodization conditions [1% v/v water electrolyte, at 75 V for 10−40 min, and for reduced cracks increased diameter of the wire substrate] that yield high-quality nanotubes that are well-adherent onto the underlying substrate. Such advances can promote integration of TNTs into a wide range of applications, such as medical implants and electrooptic devices, where the peeling-off or instability of anodic films can result in undesirable effects.

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ASSOCIATED CONTENT

S Supporting Information *

Comparative analysis between TNTs grown on flat and wirelike Ti substrates in terms of anodization profile, nanotube growth rate, and nanotube diameter. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03383.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 8 8013 4648. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of ARC DP 120101680, FT 110100711, DE14010054, and The University of Adelaide. Also acknowledged is the use of characterization facility at the Adelaide Microscopy, The University of Adelaide. ICPMS analysis was performed by Aoife McFadden at The Adelaide Microscopy.

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ABBREVIATIONS Ti, titanium; TNTs, titania nanotubes; TNTs-F, titania nanotubes fabricated using fresh electrolyte; TNTs-A, titania nanotubes fabricated using aged electrolyte; BL, barrier layer; teq, time to reach equilibrium stage during anodization; PBR, Pilling−Bedworth ratio; time of anodization, (tanod) 16044

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(20) Fan, M.; La Mantia, F. Effect of Surface Topography on the Anodization of Titanium. Electrochem. Commun. 2013, 37, 91−95. (21) Albu, S. P.; Schmuki, P. Influence of Anodization Parameters on the Expansion Factor of TiO2 Nanotubes. Electrochim. Acta 2013, 91, 90−95. (22) Lee, K.; Kim, J.; Kim, H.; Lee, Y.; Tak, Y.; Kim, D.; Schmuki, P. Effect of Electrolyte Conductivity on the Formation of a Nanotubular TiO2 Photoanode for a Dye-Sensitized Solar Cell. J. Korean Phys. Soc. 2009, 54, 1027−1031. (23) Valota, A.; LeClere, D. J.; Skeldon, P.; Curioni, M.; Hashimoto, T.; Berger, S.; Kunze, J.; Schmuki, P.; Thompson, G. E. Influence of Water Content on Nanotubular Anodic Titania Formed in Fluoride/ Glycerol Electrolytes. Electrochim. Acta 2009, 54, 4321−4327. (24) Wei, W.; Berger, S.; Hauser, C.; Meyer, K.; Yang, M.; Schmuki, P. Transition of TiO2 Nanotubes to Nanopores for Electrolytes with Very Low Water Contents. Electrochem. Commun. 2010, 12, 1184− 1186. (25) LeClere, D. J.; Velota, A.; Skeldon, P.; Thompson, G. E.; Berger, S.; Kunze, J.; Schmuki, P.; Habazaki, H.; Nagata, S. Tracer Investigation of Pore Formation in Anodic Titania. J. Electrochem. Soc. 2008, 155, C487−C494. (26) Valota, A.; LeClere, D. J.; Hashimoto, T.; Skeldon, P.; Thompson, G. E.; Berger, S.; Kunze, J.; Schmuki, P. The Efficiency of Nanotube Formation on Titanium Anodized Under Voltage and Current Control in Fluoride/Glycerol Electrolyte. Nanotechnology 2008, 19, 355701. (27) Macak, J. M.; Schmuki, P. Anodic Growth of Self-Organized Anodic TiO2 Nanotubes in Viscous Electrolytes. Electrochim. Acta 2006, 52, 1258−1264. (28) Hebert, K. R.; Albu, S. P.; Paramasivam, I.; Schmuki, P. Morphological Instability Leading to Formation of Porous Anodic Oxide Films. Nat. Mater. 2012, 11, 162−166. (29) Di Quarto, F.; Doblhofer, K.; Gerischer, H. Instability of Anodically Formed TiO2 Layers. Electrochim. Acta 1978, 23, 195−201. (30) Pilling, N. B.; Bedworth, R. E. The Oxidation of Metals at High Temperatures. J. Inst. Met. 1923, 29, 529−591. (31) Archibald, L. C. Internal Stresses Formed During the Anodic Oxidation of Titanium. Electrochim. Acta 1977, 22, 657−659. (32) Lu, K.; Tian, Z.; Geldmeier, J. A. Polishing Effect on Anodic Titania Nanotube Formation. Electrochim. Acta 2011, 56, 6014−6020.

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