Growth Mechanism of Self-Assembled TixWyO Nanotubes Fabricated by TiW Alloy Anodization

1Bachelor Program in Interdisciplinary Studies, National Yunlin University of Science and Technology, Yunlin, Taiwan 2Graduate School of Materials Science, National Yunlin University of Science and Technology, Yunlin, Taiwan 3Department of Electronic Engineering, National Yunlin University of Science and Technology, Yunlin, Taiwan 4Department of Materials Science and Engineering, National Chiao-Tung University, Hsinchu, Taiwan 5Graduate Institute of Electro-Optical Engineering and Department of Electronic and Computer Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan

In this study, we used an Al-coated TiW alloy layer on Si substrates to fabricate Ti x W y O nanotube arrays via a two-step anodization process. The first and second anodization processes were used to control the diameter and height of the Ti x W y O nanotubes, respectively. The pore size of AAO-assistant template and the percentage of tungsten in pre-deposited TiW alloys were both decisive conditions for the Ti x W y O arrays to form tubular nanostructures or just nanorods. Finally, the growth mechanism of Ti x W y O tubular nanostructures is identified as bottom-up by the observation of the tubular cap.

Experimental
To fabricate anodized Ti x W y O nanotubes, a TiW alloy film of 50 nm was deposited on a P-type (100) silicon substrates by a dual egun evaporation system. Moreover, different ratios of Ti to W for TiW alloy films were prepared to investigate the effects on the morphology of anodized Ti x W y O nanotubes. Then a 700 nm Al film was deposited by a thermal evaporation coater. The detail anodization process has been reported in previous studies. 34,37 In brief, a two-step anodization process was used on the Al-coated TiW alloy films to fabricate anodized Ti x W y O nanotubes, as shown in Figure 1a. 31,34 In order to control the morphology of anodized Ti x W y O nanotubes, the first anodization was carried out in a 0.3 M oxalic acid (H 2 C 2 O 4 ) electrolyte, applied by a bias voltage of 10 V to 80 V, and terminated at 1 mA current. Then the second anodization process was performed in the same condition with the bias changed from 40 V to 117 V, expecting that various heights of Ti x W y O nanotube arrays can be prepared. After that, in order to observe the tubular structure, a post-process of sonication vibration in a 0.1 wt% sodium hydroxide (NaOH) solution for 30 min was conducted to remove the cap on the top of Ti x W y O nanotubes. In order to remove AAO templates selectively, a wet chemical etching process was carried out for 20∼40 min at 60 • C in a mixed solution of 6 wt% phosphoric acid (H 3 PO 4 ) and 1.8 wt% chromic acid (H 2 Cr 2 O 4 ). Therefore, Ti x W y O nanotube arrays can be exhibited on the Si substrate after selective removal of AAO. This approach can produce selfaligned and height-controlled Ti x W y O nanotubes on a Si substrate.
The AAO current characteristics of all samples were recorded using a keithley 2400 source-meter. Surface morphology of the Ti x W y O nanostructure arrays was examined by a field-emission scanning electron microscope (FESEM, JSM-6500F) and a transmission electron microscope (TEM, JEM-2100F). Auger depth profile was performed by an ESCA PHI 1600 system with a monochromatic Al Ka source and a charge neutralizer.

Results and Discussion
The fabrication of Ti x W y O nanotubes.-The current as a function of time for Ti x W y O nanotubes in a two-step anodization process was recorded in Figure 1a, where the ratio of the TiW alloy is Ti:W = 1:1. Applying a voltage of 40 V in the first anodization process, the current plateau period shows the forming of porous AAO, and it ends up at a cliff point labeled as A in Figure 1a. At this moment of the first anodization, the anodic Ti x W y O nanostructure also starts forming by oxidizing TiW alloy, and its initial morphology are shown in Figures 1b and 1c for top-view and cross-section, respectively. Due to reaching the point A in Figure 1a imperceptibly and discrepantly with different thicknesses of Al layers, after the removal of AAO template, various morphologies such as nanodots, root-like nanostructures, residual Al metal, and unanodized TiW alloy are observed, as shown in Figure 1b. And without the removal of AAO template, the nanodot-like structures are forming inside the AAO pores, as Figure 1c shows. Continuously at the moment of the current downwards and terminated at 1 mA, labeled B in Figure 1a, a tubular nanostructure with a cap begins to grow, as shown in Figure 1d. In order to enhance the height of the Ti x W y O nanotubes, 34 a higher voltage of 100 V was introduced in the second anodization process for 1s and 10s, and the results were shown in Figures 1e and 1f, respectively. The extra-fast growth for Ti x W y O nanotubes up to 118 nm in height has been achieved completely within 1 s because of a strong electrical field. 37 Due to the dielectric breakdown effect, 1,45 no distinct enhancement in height was observed after 1 s anodization. However, the nanotube wall thickness is apparently enlarged from ∼8 nm to ∼11 nm based on the continuous anodization for 10s. The tubular nanostructure for Ti x W y O is also confirmed by a TEM image in Figure 2. Hence, the vertical Ti x W y O nanotube arrays on Si substrates were successfully constructed after the self-assembled two-step anodization process.   31,34 and Figure 4a shows that the diameter as a function of voltage is 0.55 nm/V. From previous studies and literatures, it has been reported that during the anodization process the penetration behavior for Ti 4+ and W 6+ ions is in the outer part of AAO cell walls. 11,34 Here, the tubular wall thickness of Ti x W y O nanostructures is also controlled by the thickness of the conductive outer part of AAO cell walls, 46 which is enlarged with an increase of voltages. Therefore, the 0.15 nm/V growth rate of the tubular wall thickness of Ti x W y O is observed when the first anodization bias is larger than 30 V, as shown in Figure 4a. Besides, due to the extension of the wall into AAO pores, the pore size less than 25 nm in diameter would be completely obstructed by  in Figure 4b. The electrical field strength of dielectric breakdown for TiO 2 and WO 3 is 0.65 GV/m and 0.885 GV/m, respectively. 11,34 Therefore, anodized WO 3 nanostructures show a shorter height than anodized TiO 2 nanostructures because larger electrical field strength is necessary for WO 3 growth. In addition, no evident growth is found as the electrical field strength is less than that of dielectric breakdown.

Component of the TiW layer.-
The Ti x W y O nanotubes fabricated through a different percentage of W in TiW alloys from 0%, 25%, 50%, 75% to 100% (pure Ti, Ti 3 W 1 , Ti 1 W 1 , Ti 1 W 3 and pure W, respectively) was carried out in a two-step anodization process at a 40-100 V bias in Figure 6. Solidly pillared structures for TiO 2 nanorods are constructed from pure Ti metal, where Ti 4+ ions (ionizing from Ti metal) migrate toward the TiO 2 channel and penetrate in the outer part of AAO cell walls simultaneously, so that it becomes nanorod structures, corresponding to previous reports. 34 Solid Ti x W y O nanorods are still acquired from the anodization on Ti 3 W 1 alloy. However, it is interesting that the tubular Ti x W y O nanostructures fabricated from the Ti 1 W 1 alloy are revealed. It is suggested that Ti 4+ and W 6+ ions migrating in the Ti x W y O channel anodized from Ti 1 W 1 alloy is terminated when the cap structures are accomplished and just pass through the outer part of AAO cell walls only, so that the tubular structures were formed. The tubular structures of the Ti x W y O and WO 3 are also observed from the anodization on Ti 1 W 3 alloy and pure W metal, respectively. Hence, tubular Ti x W y O nanostructures can be prepared via the anodization process on TiW alloys, where the percentage of W in TiW alloys have to be more than 50%. From the observation of SEM morphology, the similar diameter of 45 nm for the TiO 2 , WO 3 and Ti x W y O nanostructures is exhibited based on the limitation of the pore size of AAO. Moreover, ∼10.5 nm tubular wall thickness of Ti x W y O and WO 3 nanotubes, depending on the thickness of conductive outer part of AAO cell walls, is almost the same with each other at a fixed anodic voltage. Note that pillared WO 3 nanorods fabricated using the same AAO-assistant anodization technology have been reported, 11 whereas not for the WO 3 nanotubes until in this study. The major discrepancy in this two studies is the pore size of AAO, where the blocking effect occurs in the smaller pore size to be the nanorod stuctures, mentioned above.

Analysis of the composition profile.-
The Auger depth profiles, using Ar + sputtering as an etching source, were explored on the analyses of AAO-removal Ti x W y O nanotubes fabricated from Ti 1 W 1 alloy, as shown in Figure 7. The relatively stable atomic ratio of ∼45% for Ti element is recorded within 50s Ar + sputtering in the initial stage, whereas W element increases from 18% to 35%. This result may be attributed to an excellent migration capability for Ti 4+ ions during the anodization process, compared with W 6+ ions. A steady state is achieved with a ∼40% atomic ratio for both Ti and W elements after 100s Ar + sputtering. However, due to preferential Ar + ion bombardment, not only the Ti x W y O nanotube but also Ti 1 W 1 alloy layer in the valley contributes to the amount of Ti and W elements, resulting in high atomic ratio relative to O element of ∼18%. The oxygen-rich phenomenon in the initial stage could result from the absorbed oxygen, bounder water, or incorporated oxygen-containing ions on the  top of Ti x W y O nanotubes. 11,19 Besides, the Al element with ∼1.25% atomic concentration was always recorded in the Auger profile spectra during the Ar + sputtering process. The outer part of AAO cell walls is considered as a low ionic resistivity region due to physical defects, cation vacancies, anionic species, and bound water. 11,19 Therefore, the penetration behavior for Ti 4+ and W 6+ ions into the outer part of AAO cell walls during an anodization process is proposed to be the growth path consistent with other anodic oxides. 11,19,34 Hence, Figure 7. Auger depth profiles for Ti x W y O nanotubes fabricated in a twostep anodization process at a 40 V-100 V bias was recorded to examine various atomic concentrations, where Ar + sputtering is used as an etching source. The AAO template was removed completely before examining. The ratio for TiW alloys is Ti:W1:1. Figure 8a shows that 70% tube wall thickness is in the outer part of AAO cell walls and the other 30% is in AAO pores. In addition, based on the low quantity of 1.25% Al element, Al-O bonds in the outer part of AAO cell walls dissociate under the strong field. Then O 2− ions could participate continuously in the anodiztion process with  migrating Ti 4+ or W 6+ ions, and most of the Al 3+ ions were expelled in the electrolyte and a few ones are injected into the growth. 11,19,34 Growth mechanism of Ti x W y O nanotubes. -Figures 8b and 8c show the schematic diagram of Ti x W y O nanotubes growth mechanism. When the AAO barrier layer contacts the TiW alloy layer, a few part of underling TiW alloy is ionized, and Ti 4+ and W 6+ ions start to migrate outward to the AAO barrier layer in a strong electrical field more than 0.82 GV/m, dielectric breakdown strength. Meantime, O 2− ions migrate inward to the bottom. Then based on the reaction of Ti 4+ and W 6+ ions with O 2− ions, the root-like Ti x W y O nanostructures are constructed in the AAO barrier layer during the first anodization process, as shown in Figure 8b. After that, AAO barrier completely contacts TiW alloy layer and a void may be formed at the oxide/metal interface due to violent evolution of oxygen bubbles arising from tungsten metal in an oxidation reaction. The similar situation has been reported by Al-coated Au and Pt metals, ITO/glass and Si substrates. 1,3,29,47 Due to the interruption from voids, all of the Ti 4+ and W 6+ ions would migrate along the outer part of the AAO cell walls, indicating that a tubular structure with a cap is constructed shown in Figure 8c. In addition, according to the cap structure on the top of the Ti x W y O nanotubes, the growth point is proposed to be at the oxide/metal interface at the bottom rather than at the electrolyte/oxide interface on the top.

Conclusions
In summary, we have successfully fabricated the Ti x W y O nanotube arrays with an aid of AAO templates in a two-step anodization process. The extra-fast growth for Ti x W y O nanotubes is achieved completely within 1 s and is terminated based on the dielectric breakdown when the electric field is less than 0.82 GV/m. Due to 30% thickness of tubural wall inside the pores of AAO, the tubular structures are only synthesized when the pore size of AAO is larger than 25 nm. Furthermore, the tubular Ti x W y O nanostructures are formed after that the migration path for Ti 4+ and W 6+ ions is terminated by a void, and the W concentration in TiW alloys have to be more than 50%. In addition, based on the observation of cap structures on the top of the Ti x W y O nanotubes, the growth point is suggested to be at the Ti x W y O/TiW-alloy interface at the bottom. This study provides an alternative fabrication technology for preparing tubular nanostructures, of which the unique surface-area-to-volume ratio is useful for promoting the performance of such-made sensing devices.