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Journal of Nanomaterials

Volume 2013 (2013), Article ID 157494, 9 pages

http://dx.doi.org/10.1155/2013/157494

## The Microstructure and Capacitance Characterizations of Anodic Titanium Based Alloy Oxide Nanotube

^{1}Department of Electrical Engineering, Texas A&M University, College Station, TX 77843-3128, USA^{2}Department of Engineering Technology, Texas A&M University, College Station, TX 77843-3367, USA^{3}Department of Energy Engineering, National United University, Miaoli 36003, Taiwan

Received 15 April 2013; Accepted 18 June 2013

Academic Editor: Anukorn Phuruangrat

Copyright © 2013 Po Chun Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This paper presents a simple anodization process to fabricate ordered nanotubes (NTs) of titanium and its alloys (Ti-Mo and Ti-Ta). TiO_{2}, MoO_{3}, and Ta_{2}O_{5} are high dielectric constant materials for ultracapacitor application. The anodic titanium oxide contains a compact layer on the NT film and a barrier layer under the NT film. However, the microstructure of oxide films formed by anodic Ti-Mo and Ti-Ta alloys contains six layers, including a continuous compact layer, a continuous partial porous layer, a porous layer, a net layer, an ordering NT film, and an ordering compact barrier layer. There are extra layers, which are a partial porous layer and a porous layer, not presented on the TiO_{2} NT film. In this paper, we fabricated very high surface area ordered nanotubes from Ti and its alloys. Based on the differences of alloys elements and compositions, we investigated and calculated the specific capacitance of these alloys oxide nanotubes.

#### 1. Introduction

The demands for energy storage and energy generation are increasing rapidly with the global energy crisis. Ultracapacitor is a technology for energy storage with advantages of low cost and high efficiency. Barium titanate (BaTiO_{3}), which exhibits a very high dielectric constant, is a good material for ultracapacitor fabrication [1–4]. However, the processes of producing BaTiO_{3}, such as hydrothermal treatment [5, 6], metal-organic process [7], alkoxide hydrolysis [8, 9], RF sputtering [10], and sol-gel process [11], have been reported and they are very complex and costly. Titanium dioxide (TiO_{2}) can be formed nanotube by one-step anodizing process compared with the complex processes fabricating BaTiO_{3}. However, the dielectric constant of TiO_{2} is not as high as BaTiO_{3}, but TiO_{2} nanotube could be an ideal dielectric template due to its high surface area. A typical TiO_{2} nanotube fabrication can be achieved by anodization [12], and the ordered channel array of anodic titanium oxide nanotubes is able to serve as multiple parallel dielectric layers for the ultracapacitor.

On the other hand, metals (Al [13], Hf [14], Nb [15], Ta [16], W [17], and V [18]) and alloys (Ti-Mo [19], Ti-W [20], Ti-Nb [21], Ti-V [22], Ti-Zr [23], Ti-Ta [24], and Ti-Al [25]) have been reported that they can also be formed high surface area of nanoporous oxide film. WO_{3}, Ta_{2}O_{5}, and TaTiO_{3}, which have higher dielectric constants than TiO_{2}, of 1000 [26], 110 [27], and 200 [28], are the alternate dielectric materials for ultracapacitor. Unfortunately, they cannot form nanotubes structures as good as TiO_{2} nanotubes. Thus, in this paper, we used a simple process of anodization to fabricate TiO_{2}, TiO_{2}-MoO_{3}, and TiO_{2}-Ta_{2}O_{5} nanotubes. Their high dielectric constants and large surface areas are very useful materials to build ultracapacitors. Based on the nanotube structural properties, such as diameter, porosity, and length, we also investigated the specific capacitances of the different titanium alloys.

#### 2. Experimental Procedure

An ordered channel array of anodic titanium and titanium alloy oxides was fabricated by anodizing Ti, Ti-10Ta (90 wt.% Ti + 10 wt.% Ta), Ti-20Ta (80 wt.% Ti + 20 wt.% Ta), and Ti-10Mo (90 wt.% Ti + 10 wt.% Mo) alloys. The metal substrates were first put through electropolishing (EP). The EP electrolyte included 5 vol.% perchloric acid (HClO_{4}), 53 vol.% ethylene glycol monobutylether (HOCH_{2}CH_{2}OC_{4}H_{9}), and 42 vol.% methanol (CH_{3}OH). EP processes of Ti and Ti alloys were conducted at 15°C under 52 V for 1 minute and 28 V for 13 minutes with platinum as a counter electrode at a constant stirring rate of 200 rpm. After EP, the samples were etched in 5 vol.% HF for 5 min to form an additional thin anodic film on the metal substrates. TiO_{2}, TiO_{2}-Ta_{2}O_{5}, and TiO_{2}-MoO_{3} nanotubes were anodized in an electrolyte of 0.5 wt.% ammonium fluoride (NH_{4}F, 99.9%) and 2 wt.% H_{2}O in ethylene glycol (C_{2}H_{4}(OH)_{2}) solvent at a constant voltage of 60 V for 2 hours. After anodic films were formed by anodization, the films were then annealed in an air furnace at 450°C for 1 hour for crystallization. The surface morphologies of the anodic oxides were observed by using a scanning electron microscope (SEM, FEI Quanta 600). The alloy oxide nanotubes compositions can be analyzed by Energy Dispersive Spectrometer (EDS) (Oxford).

Cyclic voltammetry (CV) performances were evaluated by an electrochemical analyzer (CH Instruments, Model 600B, USA) using a standard three-electrode cell system with platinum as a counter electrode and silver-silver chloride electrode (Ag/AgCl) as a reference electrode in 0.5 M H_{2}SO_{4} solution at room temperature. The CV scan rate was set as 20 mV/s in a potential range of 0 V to 0.9 V (Ag/AgCl).

#### 3. Results and Discussion

Figure 1 presents the SEM images of long-range ordered nanochannel TiO_{2} NT structures formed by anodizing pure Ti foil: (a) an unwanted film covered on TiO_{2} NT, (b) partial unwanted film removed, (c) all unwanted films removed and the top view of TiO_{2} NT, (d) side view of TiO_{2} NT, (e) bottom view of TiO_{2} NT, and (f) a barrier layer under the TiO_{2} NT. TiO_{2} NT feature a pore diameter ~120 nm, pore density ~8 × 10^{9} pores/cm^{2}, and wall thickness ~25 nm; the length of the NT can be controlled from several *μ*ms to hundred *μ*ms with different types of the electrolytes (e.g., NH_{4}F) and the anodization times at a constant applied voltage (e.g., 60 V).

Immersing titanium in electrolyte causes complex reactions with 16 forms of Ti ions and oxides [29]. The Pourbaix diagram is useful to simplify the complex reactions [30]. Based on the Pourbaix diagram of Ti (Figure 2(a)), TiO^{2+} ion is a favorite formation when pH value is lower than 2.3 and voltage is higher than −0.2 V (SHE) at 25°C. TiO^{2+} can further react with H_{2}O to from Ti(OH)_{4} which is anodic titanium oxide. Similarly, Ta Pourbaix diagram (Figure 2(b)) shows that TiO^{2+} is formed and converted to Ta_{2}O_{5} under the condition of pH < 5.1 and applying voltage >−1.2 V (SHE) at room temperature. Also Mo Pourbaix diagram (Figure 2(c)) implies that Mo^{3+} can be produced and form MoO_{3} in the condition of pH being below 4.2 and voltage being higher than −0.35 V (SHE) at 25°C. However, anodic TiO_{2}, MoO_{3}, and Ta_{2}O_{5} can be formed in the neutral pH value electrolyte when it contains halogen element in it.

Anodization of titanium forms close-packed and vertical-aligned nanotubes in a nonaqueous organic polar electrolyte with F^{−} ions and minimizing water content. These electrochemical processes can be described as follows [31–34]:

During anodization, there are oxidation reactions at the interface between metal and electrolyte. Ti^{4+} is formed and the water in the electrolyte is decomposed, reactions (1) and (2). TiO_{2} is then formed between the metal and the electrolyte through ion migration, reactions (3) and (4). F^{−} ions etch the TiO_{2} forming [TiF_{6}]^{2−} and then combine with the H_{2}O to form , reactions (5) and (6). Because the F^{−} ions are doped in the TiO_{2} but do not form a compound, reaction (6) can be rewritten as (7). Finally, reacts with 2H^{+} to form TiO_{2} nanotubes, reaction (8).

Based on reactions (1)–(8), anodization of Ta can be described as Also, anodization of Mo can be described as

Figure 3 shows SEM images of TiO_{2}-Ta_{2}O_{5} nanotubes structure from anodizing Ti-10Ta alloy. There was a compact layer on the top of nanotubes in Figure 3(a). A continuous porous layer and grain boundary under the compact layer are observed in Figure 3(b). Figure 3(c) shows a porous film is covering the compact layer and following a net structure (Figure 3(d)) is covering the gap between ordered TiO_{2}-Ta_{2}O_{5} nanotubes (Figure 3(e)). There were extra continuous porous layers and net structures which were not presented on pure TiO_{2} nanotubes. The compact layer, continuous porous layer, and net structure were removed by 5 wt.% of 1 *μ*m Al_{2}O_{3} powders in ethanol solvent assisted by ultrasonic vibration. Similar to Ti-10Ta alloy, Figure 4 shows SEM images of TiO_{2}-Ta_{2}O_{5} nanotube by anodizing Ti-20Ta alloy. Figure 4(a) shows a net film on the NT top, Figure 4(b) without a net film on the NT top, Figure 4(c) a barrier layer on the NT bottom, and Figure 4(d) partial barrier layer under the NT.

For the Ti-10Mo alloy, Figure 5(a) shows partially removed continuous porous layer on the net structure, larger pores on the top of TiO_{2}-MoO_{3} nanotubes (Figure 5(b)), smaller pores (Figure 5(c)), and barrier layer (Figure 5(d)) on the bottom side. According to Figures 3–5, Figure 6 is a schematic diagram of anodic Ti alloy oxide structure with compact layer, continuous porous layer, net structure, and ordered nanotubes on the alloys surfaces.

Figure 7 shows a schematic structure and geometry of the Ti alloy oxide nanotube. Larger open pores are on the top (Figure 7(a)), smaller closed pores and a barrier layer in a hexagonal pattern are on the bottom side (Figure 7(b)), tube inner surface area (Figure 7(c)), and outer surface area (Figure 7(d)). Denoting and , and and are the radius and pores width of the top and bottom pores, respective, is the thickness of the outer barrier layer, and and is the inner height and total length of the nanotube. We have + = + = , and total length of nanotube is . Thus, the volume of a single alloy oxide can be calculated by where and can be obtained by

Based on the SEM images in Figures 3, 4, and 5, , , , and were 60 nm, 25 nm, 80 nm, and 40 nm, respectively. For two hours anodization process, 20 *μ*m length of Ti alloy oxide nanotubes () could be formed on the Ti alloy surface. Thus, was 0.4 *μ*m^{3}, and was 0.12 *μ*m^{3}, and the volume of a single alloy oxide nanotube () was 0.28 *μ*m^{3}. The TiO_{2} nanotubes density has been recently reported by Chen et al. [29] such that there are 4,510,548,978 nanotubes per cm^{2}. Therefore, the total volume of Ti alloy nanotubes was cm^{3} in 1 cm^{2} sample area. Moreover, it has also been reported that nanotube surface area is greatly increased when *μ*m, cm^{2}, cm^{2} and *μ*m, cm^{2}, cm^{2}. Figure 8 furthermore accumulated anodic Ti and Ti alloy NT inner and outer surface areas increased with film thickness increased based on 1 cm^{2} substrate. Hence, the extremely high surface area is able to provide more chances for electrochemical reactions.

According to the Pourbaix diagrams in Figure 2, anodizing Ti, Ti-20Ta, Ti-10Ta, and Ti-10Mo can form anodic oxide films of these Ti alloys. Therefore, the following alloy anodic oxide films densities are able to be calculated based on the TiO_{2}, Ta_{2}O_{5}, and MoO_{3} densities of 4.2 g/cm^{3}, 8.2 g/cm^{3}, and 4.7 g/cm^{3}, respectively. According to EDS results in Table 1, Ti, Ti-20Ta, Ti-10Ta, and Ti-10Mo formed 100% TiO_{2}, 83.9% TiO_{2} + 16.1% Ta_{2}O_{5}, 91.3% TiO_{2} + 8.7% Ta_{2}O_{5}, and 92.8% TiO_{2} + 7.2% MoO_{3}. The densities of 100% TiO_{2}, 83.9% TiO_{2} + 16.1% Ta_{2}O_{5}, 91.3% TiO_{2} + 8.7% Ta_{2}O_{5}, and 92.8% TiO_{2} + 7.2% MoO_{3} were 4.23 g/cm^{3}, 4.84 g/cm^{3}, 4.54 g/cm^{3}, and 4.24 g/cm^{3}, respectively. Therefore, the mass of nanotubes films 1 cm^{2} sample for each alloy was listed in Table 2 being 5.32 mg/cm^{2}, 6.09 mg/cm^{2}, 5.72 mg/cm^{2}, and 5.34 mg/cm^{2}.

Cyclic voltammograms (CV) are used to characterize the capacitors behavior of the alloy oxide nanotubes. Figure 9 shows capacitance performance evaluations for the Ti alloy anodic oxide nanotubes by cyclic voltammograms. It is clear that Ti alloy oxide nanotubes had larger area of CV curve than pure TiO_{2} nanotube did. It means that Ti alloys oxide nanotubes had larger capacitances than pure TiO_{2} nanotubes. Besides, more Ta_{2}O_{5} content can significantly enhance the capacitor performance by comparing two Ti-Ta alloys curves with different compositions. Moreover, Ti-10Mo alloy oxide nanotubes CV curve shows a symmetrical shape which indicates that the revisable redox reaction of Mo^{2+}/Mo^{3+} was helpful to improve the capacitor performance. The specific capacitance () can be measured by voltage step, current step, or voltage ramp methods and evaluated by the equations of and [35], where is applied voltage and is the quantity of charge on the electrode (which can be evaluated from the area of the CV curve). Table 2 shows the specific capacitance based on 1 cm^{2} sample area and 20 *μ*m film thickness of pure Ti, Ti-20Ta, Ti-10Ta, and Ti-10Mo oxide nanotubes films which are 13.7 F/g, 26.1 F/g, 23.3 F/g, and 21.4 F/g, respectively. The specific capacitances of Ti alloys oxide nanotubes films were higher than that of TiO_{2}-B nanowires/MWCNTs hybrid supercapacitor with specific capacitance of 17.7 F/g [36].

#### 4. Conclusions

In summary, we fabricated ultracapacitors based on the working electrode made of highly ordered anodic TiO_{2}, Ta_{2}O_{5}, and MoO_{3} nanotubes directly formed on pure Ti, Ti-20Ta, Ti-10Ta, and Ti-10Mo substrates. The ordered alloys oxide nanotubes structure has a volume of cm^{3} in 1 cm^{2} sample area with nanotube density of tubes/cm^{2}. The mass of pure Ti and Ti alloys oxide nanotubes films with 1 cm^{2} sample size and 20 *μ*m film thickness can be calculated as 5.32 mg (TiO_{2} nanotubes), 6.09 mg (Ti-20Ta oxide nanotubes), 5.72 mg (Ti-10Ta oxide nanotubes), and 5.34 mg (Ti-10Mo oxide nanotubes), respectively. Furthermore, Ti alloy anodic oxide nanotubes films with 1 cm^{2} surface and 20 *μ*m thickness have an inner surface area of 241.0 cm^{2} and outer surface area of 480.4 cm^{2}. Thus, such large surface area of dielectric oxides caused very high specific capacitances. The specific capacitance can further be enhanced by reacting with barium nitrate (Ba(NO_{3})_{2}) [37] or barium hydroxide (Ba(OH)_{2}) [38] to form a very high dielectric constant BaTiO_{3} film, increasing nanotubes length by longer anodization process, and increasing nanotubes surface area by coating TiO_{2} nanoparticles on the nanotubes surface [39].

#### Acknowledgment

This study was partially supported by a Grant from the National Science Council, Taiwan (102-3113-P-042A-005-).

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