Journal of Nanomaterials & Molecular NanotechnologyISSN: 2324-8777

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Research Article, J Nanomater Mol Nanotechnol Vol: 3 Issue: 2

Synthesis and Characterization of Titania Nanotubes on Titanium alloy IMI 834 by Electrochemical Anodization Process

I.V. S. Yashwanth* and I. Gurrappa
Defence Metallurgical Research Laboratory, Kanchanbagh PO, Hyderabad, India
Corresponding author : Dr. Yashwanth I.V.S
Defence Metallurgical Research Laboratory, Kanchanbagh PO, Hyderabad, India
E-mail: [email protected]
Received: October 14, 2013 Accepted: March 06, 2014 Published: March 10, 2014
Citation: Yashwanth I.V. S, Gurrappa I (2014) Synthesis and Characterization of Titania Nanotubes on Titanium alloy IMI 834 by Electrochemical Anodization Process. J Nanomater Mol Nanotechnol 3:2. doi:10.4172/2324-8777.1000141

Abstract

Synthesis and Characterization of Titania Nanotubes on Titanium alloy IMI 834 by Electrochemical Anodization Process

Electrochemical anodization offers large surface area architecture with precisely controllable nanoscale features in the fabrication of highly ordered vertically oriented TiO2 nanotube arrays. Abundant research work has been accomplished on pure titanium and their alloys like Ti 64, TiNb, TiAl etc. Yet, no work has been proclaimed on Titanium alloy IMI 834. In the present investigation, the synthesis of highly ordered Titania nanotubes by electrochemical anodization of the alloy IMI 834 in different electrolytes was carried out. The synthesized nanotubes were characterized by Scanning Electron Microscope (SEM), Field Emission Scanning Electron Microscope (FESEM), Electron Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD) techniques. The results spectacles that the size and morphology rely on the electrolyte composition. Among the electrolytes, the combination of 0.5wt% HF and 1M phosphoric acid is more suitable in producing ordered Titania nanotubes on titanium alloy IMI 834.

Keywords:

Keywords

Titanium alloy; Electrochemical Anodization; Titania nanotubes

Introduction

Nanotubes are of great interest for a wide array of technological and biological applications, especially due to their high surface-tovolume ratios and size dependent properties [1]. The innovations of carbon nanotubes with their variety of interesting properties have stimulated the quest for the synthesis of nanotubular structures of other materials. One such widely acclaimed material that was developed through a simple process of electrochemical oxidation or anodization is the nanotubular array of titanium dioxide (TiO2); commonly called as titania nanotube arrays or TNAs, developed on metallic titanium substrates. Several recent studies have indicated that titania nanotubes have improved properties compared to any other form of titania for application in solar cells [2-4], gas sensors [5,6], photo catalysis [7,8], photo electrochemical hydrogen generation [9,10], orthopedic implants [11,12], etc. Titania nanotubes have been prepared by a variety of techniques including sol–gel method [13], electrophoretic deposition [14], hydrothermal [15] and anodization [16,17]. However, fabrication of oxide nanotube arrays by electrochemical anodization of the starting metal offers superior control over nanotube dimensions by optimization of various parameters such as pH, concentration and composition of the electrolyte, applied potential, time and temperature of anodization process. It is also possible to produce multi-layers of required material on a variety of materials and shapes. Further, electrochemical process is a simple and cost-effective surface modifications technique applicable directly to different industries. In particular, anodization has evolved to be an ideal approach to nano surface modification of titanium based alloys and hence became popular. Gong and co-workers [18] pioneered the synthesis of the first generation of vertically oriented titania nanotube arrays extending up to 500 nm length, by electrochemical oxidation of titanium in aqueous HF electrolyte. It is well known that the properties and performance of TiO2 are dependent partly on its crystallinity and morphology. Various applications of these nanotube arrays, such as dye sensitized solar cells (DSSC) and water photo electrolysis, demand high aspect ratio nanotubes [19,20]. Literature reports also suggest that photo-induced processes are strongly dependent on the fabrication conditions and morphology of the nanotubes [21,22]. The aligned porosity, crystalline and oriented nature of the nanotube arrays also make them attractive electron percolation pathways for vectorial charge transfer between interfaces [23]. Chen et al. has reported a power conversion efficiency of ~ 7% for TiO2 nanotube based dye sensitized solar cells (DSSC) at laboratory scale [24]. In order to enhance this efficiency further, a substantial improvement in dimensional and morphological features of the nanotubes during anodization process is essential. Alterations in nanotubular dimensions and spacing are an important attribute that enables the use of such nanostructures for a variety of applications. The nanotube diameter, length, spatial density, wall thickness, surface area, etc. are parameters that when altered can influence biological, optical and chemical properties of the surface. Subtle differences in the above parameters can influence the use of these tubular structures in applications as diverse as solar cells to biomedical implants.
Although all the valve metals find use in multitudes of applications, pure titanium has scored the highest in this list, with the technique of anodization being the most widely explored method of preparing nanotubes. This is clearly exemplified from the publication statistics on TiO2 nanotube layers formed by electrochemical oxidation. Apart from pure metallic titanium, the technique of anodization has also been extended to generating nanotubular features on various metallic binary alloys such as TiAl [25], TiNb [26], TiZr [27,28] and complex biomedical alloys such as Ti6Al7Nb [29] and Ti29Nb13Ta4.6Zr [30,31]. It was shown that the range of achievable diameters and lengths of TiO2-based nanotubes can be significantly expanded, if alloy is used as a substrate. For anodic nanotubes formed on TiZr alloys [32] the morphological character of the oxide nanotubes are between those of titanium oxide and zirconium oxide nanotubes.
Ti-6Al-2Sn-4Zr-2Mo (Ti 6242) and Ti-6Al-4V (Ti 64) are the most commonly used alloys in aero engines where the temperature reaches up to 300-450°C. New titanium alloys developed by changing alloy chemistry have made it possible to increase the aero engine temperature up to 600°C. The near alpha titanium alloy, IMI 834, developed recently, belongs to this category [33]. The mechanical properties of IMI 834 and other titanium alloys are dependent on variables such as alloy chemistry, manufacturing methods and environmental conditions during the service. Since, these variables greatly influence the microstructure, which inherently affects their properties. Extensive research on titanium alloy IMI 834 was carried out under simulate aero engine conditions by Gurrappa [34-38] and developed high performance and smart coating to increase its life during service [39-46].
Highly ordered vertically oriented TiO2 nanotube arrays fabricated by electrochemical anodization offers a large surface area architecture with precisely controllable nanoscale features [47]. As mentioned above, much research work has been carried out on pure titanium and their alloys like Ti 64, TiNb, TiAl etc. But no work has been reported on titanium alloy IMI 834. In the present paper, we have synthesized highly ordered Titania nanotubes by electrochemical anodization of IMI 834 in different electrolytes and their complete characteristics were examined using Scanning Electron Microscope (SEM), Field Emission Scanning Electron Microscope (FESEM), Electron Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD) techniques. Based on the results obtained, the best electrolyte to obtain highly ordered nanotubes on the alloy IMI 834 for variety applications was identified.

Experimental

The chemical composition of selected titanium alloy material IMI 834 is provided in Table 1. The titanium alloy was cut into the dimensions of 20 mm in diameter and 3mm in thickness from the alloy rods.
Table 1: Chemical composition of IMI 834 (all in wt %).
The cut samples were mechanically polished with different SiC papers to reach mirror-like finish and were ultrasonically cleaned in de-ionized water and subsequently in acetone for 15 minutes. The technique used to grow Titania nanotubes on the selected titanium alloy was by electrochemical anodization method. The anodization was performed using an Aplab Regulated DC Power Supply L3205 in different electrolytes as shown in Table 2. Platinum wire was used as cathode. The electrolyte mixture was stirred using magnetic stirrer during the anodization process. The distance between the two electrodes was fixed at 26 mm.
Table 2: Electrolytes used and time for electrochemical anodization of titanium alloy IMI 834.
A Potential of 20 V was applied and the time period varied between 45 to 120 minutes for the samples (Table 2). The experiments were carried out at room temperature. Anodization voltage was kept constant throughout the process. The electrolytic solution was changed after each process. The samples were then annealed at 500°C in a furnace (heating rate: 25°C/minute) for 3 hours and furnace cooled (cooling rate: 10°C/minute). The nanotubes obtained by anodization on IMI 834 were observed under Scanning Electron Microscope (SEM) and Field Emission Scanning Electron Microscope (FESEM). The compositions of nano Titania tubes were analyzed by Electron Dispersion Spectroscopy (EDS). The X-ray Diffraction technique (XRD) was used to determine the crystalline structure of the formed nanotubes.

Results

HF (0.5 wt %) Electrolyte
Figure 1 shows the surface morphology of anodized titanium alloy IMI 834 in 0.5wt% HF electrolyte for duration of 45 minutes. As can be seen, Titania nanotubes could not be formed on the surface of titanium alloy in HF electrolyte. It may be probably due to shorter duration of anodization process. The composition of the scale is analyzed by EDS as TiO2 (Figure 2).
Figure 1: Scanning Electron Micrograpgh of nanotubes grown on IMI 834 in 0.5%wt HF electrolyte.
Figure 2: EDS data of nanotubes grown on IMI 834 in 0.5%wt HF electrolyte.
CH3COOH (2M) + HF (0.5 wt %) Electrolyte
Figure 3 demonstrates the surface morphology of anodized titanium alloy IMI 834 in 0.5wt% HF and 2M acetic acid electrolyte for a period of 45 minutes. No initiation or formation of Titania nanotubes on the surface of IMI 834 is observed in the presence of acetic acid medium (Figure 3). It may be due to less corrosive nature of combination of HF and acetic acid to the selected titanium alloy to produce nanotubes and shorter duration of anodization process. The composition of the scale is analyzed by EDS as TiO2 (Figure 4).
Figure 3: Scanning Electron Micrograpgh of nanotubes grown on IMI 834 in 0.5%wt HF and 2M acetic acid electrolyte.
Figure 4: EDS data of nanotubes grown on IMI 834 in 2M CH3COOH and 0.5%wt HF electrolyte.
H3PO4 (1M) + HF (0.5 wt %) Electrolyte
The surface morphology of anodized titanium alloy IMI 834 in 0.5wt% HF and 1 M phosphoric acid for a period of 120 minutes is shown in Figure 5. Highly ordered titania nanotubes of about 100 nm were formed on the surface of titanium alloy. The composition of the scale is analyzed by EDS as TiO2 (Figure 6). It indicates that the fluoride and phosphate ions help to produce titania nanotubes on the alloy surface.
Figure 5: Field Emission Scanning Electron Microgragh of nanotubes grown on IMI 834 in 0.5%wt HF and 1M H3PO4 electrolyte.
Figure 6: EDS data of nanotubes grown on IMI 834 in 1M H3PO4 and 0.5%wt HF electrolyte.
H2SO4 (1M) + HF (0.5 wt %) Electrolyte
Figure 7 shows the surface morphology of anodized titanium alloy IMI 834 in 0.5wt% HF and 1 M sulphuric acid for duration of 120 minutes. As can be seen, an ordered titania nanotubes of about 100 nm were formed at the selected places on the surface titanium alloy. It indicates that the sulphate ions in presence of fluoride ions help to produce ordered nanotubes by reacting with selected phases in the titanium alloy. The composition of the tubes is confirmed by EDS (Figure 8).
Figure 7: Field Emission Scanning Electron Microgragh of nanotubes grown on IMI 834 in 0.5%wt HF and 1M H2SO4 electrolyte.
Figure 8: EDS data of nanotubes grown on IMI 834 in 1M H2SO4 and 0.5%wt HF electrolyte.
The XRD patters of Titania nanotubes formed on titanium alloy IMI 834 in 0.5% HF + 1M H3PO4 and 0.5% HF + 1M H2SO4 are presented in Figure 9 indicating the crystalline nature of formed nanotubes.
Figure 9: XRD patterns of Titania nanotubes formed on titanium alloy IMI 834 in 0.5% HF + 1M H3PO4 and 0.5% HF + 1M H2SO4 electrolytes.

Discussion

Anodization of titanium alloys
As mentioned earlier, the technique of anodization has been extended to generating nanotubess on various binary alloys such as TiNb, TiZr [26,27], intermetallic compounds like TiAl [25] and biomedical alloys [29-31] apart from pure titanium as surface modification of Ti and its alloys have emerged as a potential strategy in improving the biocompatibility of biomedical implants [48]. Several attempts have been made to improve the surface properties of titanium-based implants to enhance initial bone bonding. Different surface modification techniques such as sand-blasting, acid etching, plasma spraying, etc have been investigated [49-51]. However, as already mentioned, anodization has evolved to be a simple, costeffective and ideal approach to nano surface modification of titanium based alloys and hence has been used in the present study. The growth of nanotubes on various alloys increases the potential functionality of the tubes (e.g. incorporation of doping species in the nanostructure). Such nanotube layers can also be applied as surface coatings on various technical alloys. Using the same approach as for Ti, i.e., controlled anodization in dilute fluoride electrolytes, nanotube layers have been successfully grown on intermetallic compounds, binary alloys and complex biomedical alloys such as Ti-6A-l7Nb [52] and Ti-29Nb-13Ta-4.6Zr [53-55]. The alloy Ti29-Nb13-Ta4.6-Zr has been developed by Niinomi et al. for biomedical applications, in order to reduce the elastic modulus of titanium alloys to the level of living bone [56]. It was shown that the range of achievable diameters and lengths of TiO2- based nanotubes can be significantly expanded, if a binary Ti–Nb alloy, rather than pure Ti, is used as a substrate. For anodic nanotubes formed on TiZr alloys [55] the morphological character of the oxide nanotubes are between those of titanium oxide and zirconium oxide nanotubes. The nanotubes have a straight and smooth morphology with a diameter ranging from 15 to 470 nm and a length up to 21 μm depending on the anodization potential (i.e. they show a largely expanded structural flexibility compared to nanotubes formed on the individual elements). However, the nanotubes formed on the present titanium alloy, morphological difference could be observed due to more number of alloying elements present in the alloy.
Influence of electrolytes in anodization of Titanium alloy
Great improvements have been made in the modulation of the titania nanotube morphologies including the nanotube length, pore size, and nanotube ordering in different electrolytes by varying the anodization conditions on pure titanium. Electrolytes govern both the formation and dissolution rates of the oxide layers and subsequently have a significant effect on the morphology of the resultant titania nanotubular structures. To date, highly ordered TNAs with tunable pore size and nanotube length have been achieved in both aqueous and non-aqueous electrolytes, either fluoride containing or fluoride free [57].
In 1999, Zwilling and co-workers achieved self-organized porous TiO2 by anodizing a Ti-based alloy in an acidic, fluoridebased electrolyte [57]. In 2001, Gong and co-workers fabricated selforganized, highly uniform TiO2 nanotube arrays by anodizing Ti in an aqueous dilute hydrofluoric acid (HF) electrolyte [18]. Maximum nanotube lengths in this first synthesis generation were approximately 500 nm. Second generation nanotubes evolved, with typical lengths of ~ 7 μm through appropriate control of the electrolyte pH which helped to reduce the chemical dissolution of TiO2 during anodization [58,59]. The electrolyte pH should necessarily be high, but in the acidic range. Subsequently, the third-generation TiO2 nanotube arrays with lengths of up to approximately 1000 μm were fabricated using non-aqueous, polar organic electrolytes such as formamide, dimethylsulfoxide (DMSO), ethylene glycol (EG) or diethylene glycol (DEG) [60,61]. The fourth generation nanotubes would be produced in non-fluoride-based electrolytes and the work is in progress in author’s laboratory. Hence, attempts are ongoing to improve the nanotube array length for utilizing these structures for varied applications.
Although several reports are available on the surface modification of metallic alloys using anodisation technique, such surface modification has not been used for titanium alloys like IMI 834, 6242 etc in different electrolytes as mentioned earlier. The present results clearly indicating that the Titania nanotube formation is dependent on the electrolyte that is used for anodization process of titanium alloy. The results demonstrate that the nature of acid electrolyte plays an important role in synthesis of titania nanotubes. Etching and corrosion processes directly related to pH of the electrolyte containing fluoride ions. Apart from the pH of electrolyte solution, the nature of ions present in the acid electrolyte during formation of anodization reaction can induce local changes in the vicinity of nanotubes which have more profound influence on anodization process. The nanotubes could not be formed in HF as well as in association with acetic acid. While, ordered nanotubes formation is observed in HF and sulphuric acid medium at the selected phases. It indicates that sulphate ions are partially helpful in forming titania nanotubes. It is attributed to shorter duration of anodization process. But, highly ordered Titania nanotubes are formed on the titanium alloy in HF and phosphoric acid medium on the titanium alloy. The phosphate ions that are present in the electrolyte appeared to promote Titania nanotube formation. Further, the alloying elements and the phases present in the alloy also promoted Titania nanotubes formation. It is further evidenced by not growing the Titania nanotubes in the presence of 2M acetic (Figure 3) medium in the similar conditions. The results clearly indicate that the nature of acid plays an important role in nanotube formation. This is the first time to produce Titania nanotubes on titanium alloy IMI 834 in different electrolytes and reported. Hence the present investigation clearly shows that the nanotubes can be successfully produced on IMI 834 in the 0.5% HF and 1 M phosphoric acid and also 1M sulphuric acid media and can be used for a variety of applications. The model for initiation and formation of Titania nanotubes on the titanium alloy is explained in the next section.
Mechanistic model of Nanotube array formation
The key processes responsible for anodic formation of nanoporous Titania are fundamental to the formation of straight Titania nanotubes. The following are the major processes:
1. Oxide growth at the surface of Ti titanium alloy occurs due to interaction of titanium with O2- or OH- ions. After the formation of an initial oxide layer, these anions migrate through the oxide layer reaching the alloy / oxide interface where they react with the alloy.
2. Ti4+ ions migrate from the alloy at the alloy/oxide interface; Ti4+ cations will be ejected from the alloy / oxide interface under application of an electric field that moves towards the oxide/ electrolyte interface.
3. Field assisted dissolution of the oxide occurs at the oxide / electrolyte interface. Due to the applied electric field Ti-O bond undergoes polarization and is weakened promoting dissolution of the Ti4+ cations. These cations (Ti4+) dissolve into the electrolyte, and the free O2- anions migrate towards the alloy/oxide interface as mentioned in process 1, to interface with the alloy.
4. Chemical dissolution of the alloy, or oxide, by the acidic electrolyte also takes place during anodization. Chemical dissolution of Titania in HF, phosphoric and sulphuric acids electrolytes plays a key role in the formation of nanotubes.
The surface of titanium samples anodized at 20 V for different durations were reported [62]. As the anodization process begins, the initial oxide layer, formed due to interaction of the surface Ti4+ ions with oxygen ions (O2-) in the electrolyte, forms uniformly across the surface. The overall reaction for anodic oxidation of titanium can be represented as
2H2O = O2 + 4e + 4 H+
Ti+O2 = TiO2
In the initial stages of the anodization process field-assisted dissolution dominates chemical dissolution due to the relatively large electric field across the thin oxide layer. Small pits formed due to the localized dissolution of the oxide, represented by the following
TiO2 + 6F- + 4H+ = TiF62- + 2H2O
Then, these pits convert into bigger pores and the pore density increases [63]. After that, the pores spread uniformly over the surface. The pore growth occurs due to inward movement of the oxide layer at the pore bottom (barrier layer) due to the processes 1-3 mentioned above. The Ti4+ ions migrate from the alloy to the oxide/electrolyte interface dissolve in the HF electrolyte. The rate of oxide growth at the alloy/oxide interface and the rate of oxide dissolution at the porebottom/ electrolyte interface ultimately become equal, thereafter the thickness of the barrier layer remains unchanged although it moves further into the alloy making the pore deeper. Close examination of FESEM and SEM images shows the formation of the small pits in the inter-pore regions which eventually leads to pore separation and tube formation. The thickness of the tubular structure ceases to increase when the chemical dissolution rate of the oxide at the mouth of the tube (top surface) becomes equal to the rate of inward movement of the alloy/oxide boundary at the base of the tube. Higher anodization voltages increase the oxidation and field-assisted dissolution and hence a greater nanotube layer thickness can be achieved before equilibrating with the chemical dissolution.

Conclusion

1. Titania nanotubes were synthesized successfully by electrochemical anodization technique on titanium alloy IMI 834 for the first time by varying electrolyte compositions.
2. The results showed that synthesis parameters play a crucial role in nanotube arrays formation. Nanotube fabrication was found to be dependent on time and electrolyte composition.
3. A highly ordered Titania nanotube arrays were successfully fabricated on IMI 834 in 0.5 wt% HF + 1M H3PO4 and an ordered nanotubes in 0.5 wt% HF + 1M H2SO4 electrolytes and confirmed by advanced characterization techniques.
4. The formed Titania nanotubes on IMI 834 can be used successfully for a variety of applications.

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