Effect of water content of ethylene glycol as electrolyte for synthesis of ordered titania nanotubes K.S. Raja, T. Gandhi, M. Misra * Metallurgical and Materials Engineering, Mail Stop 388, University of Nevada, Reno, NV 89557, United States Received 4 December 2006; received in revised form 13 December 2006; accepted 19 December 2006 Available online 28 December 2006 Abstract Anodization of Ti using fluoride containing polyhydric alcohols such as ethylene glycol or glycerol as electrolyte results in ordered arrays of TiO 2 nanotubes with a smooth surface and a very high aspect ratio. However, the reproducibility of the result is affected by many experimental parameters, notably the water content. In this investigation, anodizations of Ti foil in anhydrous ethylene glycol +0.2 wt% NH 4 F solution (EG solution) with 0–1.0 wt% water additions were carried out at 20 V for 45 min in a dry-argon filled con- trolled-atmosphere glove box. It was observed that a minimum amount of 0.18 wt% of water addition was required to form a well ordered TiO 2 nanotubular arrays. When the anhydrous EG solution was reused for third time, ordered arrays of nanotubes started to form. When the water addition to the EG solution was more than 0.5 wt%, formation of ridges was observed on the nanotubes. XPS results showed presence of un-anodized Ti element in the anhydrous condition and presence of organic and (NH 4 ) 2 TiF 6 type com- pounds in all the anodized samples in addition to the regular TiO 2 phase. The results underline the influence of water content and local pH condition to form the ordered nanotubular arrays. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Nanotubular TiO 2 ; Anodization; Ethylene glycol; Oxide film 1. Introduction Formation of ordered arrays of vertically oriented tita- nium dioxide nanotubes through a simple anodization pro- cess, first reported by Zwelling et al. [1], has been an attractive approach for many important engineering appli- cations. These potential applications include photoelectro- chemical hydrogen generation [2,3], solar cells [4], hydrogen storage [5], gas sensing [6], templates for growth of compound semiconductor nanowires for radiation sens- ing [7], substrate for high interfacial bond strength hydro- xyl apatite coating in implants, [8,9] biomedical applications [10] and as catalyst supports [11]. Synthesis of the TiO 2 nanotubes typically has been carried out in an acidified aqueous solution containing a fluoride salt. These nanotubes typically have diameters in the range of 60–150 nm and have lengths varying from 0.4 lmto 4 lm, depending upon the pH of the electrolyte [12]. The anodization potential and fluoride concentration determine the diameter. The length of the nanotubes is tailored by the solution pH and the anodization time. The nanotubes formed in the aqueous solution contained circumferential serrations or ridges. Presence of such ridges was explained based on the pH burst during anodization [13] or by repul- sion of accumulated charged vacancies [14]. Recently, Macak and Schmuki [15] reported formation of smooth and ridges-free nanotubes of much smaller diameter and very high aspect ratio using fluoride contain- ing high viscous organic elect rolytes such as glycerol and ethylene glycol. The diameter of the nanotubes varied from 40 nm to 60 nm at 20 V and anodization for 18 h resulted in a length of about 6lm. An improved ordering and smooth surface of the nanotube arrays formed in high vis- cous organic solutions could provide more interesting applications to these nanotubes such as optical wave 1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.12.024 * Corresponding author. Tel.: +1 775 784 1603; fax: +1 775 327 5059. E-mail address: Misra@unr.edu (M. Misra). www.elsevier.com/locate/elecom Electrochemistry Communications 9 (2007) 1069–1076 guides. Recently, Paulose et al. [16] reported 134 lm long nanotubular arrays by anodizing in various non-aqueous organic polar electrolytes such as ethylene glycol, dimethyl sulfoxide, formamide etc. When the TiO 2 nanotubes formed in the organic solutions were annealed at 500– 650 °C, the adsorbed carbon species during the anodization process modified the tubes as TiO 2Àx C x after annealing by internal diffusion of carbon into the lattice [17]. This pro- cess resulted in an improved photo-activity during photo- electrochemical generation of hydrogen, which was similar to the behavior of carbon modified TiO 2 prepared by flame pyrolysis [18] or a chemical vapor deposition method [19–21]. Because of the interesting morphology and the potential applications, the formation of smooth TiO 2 nanotubes using the protic non-aqueous organic media could be a process for further research. It was observed that the repro- ducibility of the formation of well ordered nanotubular arrays using a two-electrode configuration was affected by the experimental conditions, especially water content of the electrolyte. This aspect has not been addressed in the available reports mainly because a three electrode con- figuration with a 1 M KCl Luggin capillary Ag/AgCl refer- ence electrode was emp loyed in the majority of those studies, which could have introduced unspecified amount of water in the electrolyte. Further, anodization for a pro- longed periods of time >15 h in open-to-laboratory–air conditions could have resulted in absorption of the mois- ture leading to nanotube formation. When the anodization was carried out for a shorte r time in anhydrous polyh ydric alcohols (water content <0.03 wt %), as no external source of water could be introduced by using a conventional two- electrode configuration, the end product varied consider- ably. The formation of the ordered TiO 2 nanotubes with smooth surface (ripple free) was affected by the initial water content of the polyhydric alcohol, relative hum idity, expo- sure time of the electro lyte to the atmosphere and anodiza- tion duration. Therefore, initial water content of the electrolyte was considered to be an issue in getting repro- ducible results. Furthermore, Macak and Schmuki [11] reported 12 V as the limiting anodization potential in eth- ylene glycol; whereas, Paulose et al. [12] anodized at 60 V using anhyd rous ethylene glycol. These different observa- tions also could be related to the effect of the water content. In a very recent communication, Albu et al. [22] reported growth of 250 lm long TiO 2 nanotubes in ethylene glycol +0.2 M HF solution at 120 V. The formation of longer nanotubes at higher voltage without dielectric breakdown could be attributed to controlled fluoride and water con- tents. Addition of 0.2 M of 48% HF acted as a source of water. As the nanotubes formation was not uniform in the <300 ppm of water containing ethylene glycol in our preli- minary investigations, the major focus of this study was to understand the role of water content in the formation of ordered TiO 2 nanotubes and to estimate the minimum amount of water required for obtaining reproducible results. However, no attempt was made to experiment with totally water free electrolytes. It should be quickly pointed out that the anhydrous EG solution would have higher resistivity than the EG solutions containing water. In three-electrode configurations, the potential loss due to IR drop across the cell will be less as the reference electrode is placed closer to the Ti specimen. On the other hand, the IR drop will be more in the two-electrode configuration as the distance between the anode and cathode is relatively larger and can result in variations of the end products in the anhydrous solution. 2. Experimental Sixteen millimeter diameter discs were punched out from a commercial purity Ti foil (0.2 mm thick). After washing with soap water, tap water, distilled water and isopropyl alcohol, these discs were sonicated in acetone for 2 min and dried before loading on to a PEEK specimen holder. Anodization was carried out using a two-electrode config- uration. The electrolytes were: (a) 0.2 wt% NH 4 F dissolved in anhydrous ethylene glycol (Sigm a–Aldrich, initial water content <0.03 wt%) with 0, 0.05, 0.13, 0.18, 0.5 and 1.0 wt% water addition. The anode was the Ti disc with an exposed surface area of 0.7 cm 2 . A Pt flag with 7.5 cm 2 area acted as a cathode. The distance between the anode and the cathode was maintained at 1 cm. A DC power source (Agilent Technologies, USA, Model E 3649A) supplied the required potential, typically 20 V, for anodization. The anodization current was measured using a multimeter (Hewlett Packard, model 3468 A). The elec- trolyte has higher affinity to water. In order to minimize the atmospheric moisture adsorption by the electrolytes, all the anodization operations (including preparation of the electrolytes) were carried out inside a dry glove box. The atmosphere inside the glove box was controlled by purging with dry argon. The moisture and oxygen level was maintained at less than 10 ppm. This was ensured by burning a 25 W incandescent bulb with a pierced glass envelop (exposing the filament to the glove box atmo- sphere) for more than 2 h inside the glove box. Care was taken to ensure that no external water was added during the experiments . In effect, the salt, glass wares and the elec- trodes were vacuum dried before use. However, vacuum drying did not remove the water of crystallization of the salt. Therefor e, residual amount of water was always avail- able in the salt as well as in the as-received anhydrous EG. No extra distillation was carried out to remove the residual water from the chemicals. It was noted that such residual water content did not result in the formation of TiO 2 nano- tubes as described in the following sections. The anodiza- tion period was rest ricted to only 45 min, which was a typical time observed in a pH 2.0 aqueous fluoride solution for growth of $400 nm long TiO 2 nanotubes. Initially, the anodization runs were carried out by two different poten- tial application methods, viz, 1. potential stepping method (wherein the anodization potential of 20 V was applied in a 1070 K.S. Raja et al. / Electrochemistry Communications 9 (2007) 1069–1076 single step (without ramping) and the an odic current was recorded), and 2. potential sweeping method (where the potential was scanned from 0 V to 20 V at a rate of 0.1 V/s and the potential was kept constant at 20 V for 45 min). There was no significant difference in the results (morphology of the nanotubes) observed between these two potential application techniques. Almost similar results were observed for anodization of Ti and W in the aqueous solutions also in our laborat ory. Therefore, only potential stepping method was followed for all the experi- ments in this investigation. Three samples were anodized using the same solution. The anodized specimens were washed with acetone and ultrasonicated in isopropyl alco- hol for about 5 min. The dried samples were observed under a FESEM (S-4700, Hitachi) to record the surface morphology. Glancing angle X-Ray Diffraction (Philips- 12045 B/3 diffractometer, Cu target (k = 0.154 nm)) and X-ray photoelectron spectroscopic (XPS; surface science instruments, Al K a X-ray source) studies were carried out on a selected group of samples. 3. Results Fig. 1a shows the current transients recorded during anodization of Ti samples at 20 V in a freshly prepared eth- ylene glycol solution with addition of different amounts of water. This experiment is identified as the ‘first-run’. The initial current density recorded was the highest in anhy- drous ethylene glycol solution and it decreased with the addition of water. The behavior of the current transient in this solution was different from the transient observed in aqu eous acid fluoride solutions. The typical initial decay and rise before reaching a steady state value observed in the aqueous solutions was absent in the ethylene glycol (EG) + fluoride solutions (hereafter referred to as EG solu- tions). The anodic current continuously decayed as shown in the inset of the Fig. 1a. When the current transient was plotted in log–log scale, three different slopes of current decay could be observed, presumably denoting different stages of the anodic oxide layer formation. The initial shal- low (stage I) current decay of the anhydrous condition extended for a longer time as compared to the water added conditions. This indicated faster kinetics of a corrosion resistant barrier layer formation with water addition. The current transients were similar when the EG solutions con- tained more than 0.13 wt% water addition, where a third stage of cu rrent transient with a slightly shallower slope of current decay was observed. Typically the initial slope of the current decay was about À0.1 and the second stage of the current transient showed a slope of À0.9 to À1.2. The slope of the third stage of EG solutions with higher water contents was about À0.3. The EG solutions with 0–0.13 wt% water addition showed a slightly increas- ing stage III current transient (as against decaying tran- sient) after about 35 min of anodization. The current density at the end of 45 min of anodization at 20 V was about 1 mA/cm 2 in the case of anhydrous EG solution and 0.54–0.68 mA/cm 2 with addition of water. These trends were highly reproducible in the freshly prepared EG solutions inside the dry glove box. When the anodization was continued in the same solu- tion (that has been already used for anodization of one specimen) with a fresh Ti sample, a slightly different but distinguishable current trans ient behavior could be noted. This set of experiments is identified as ‘second run’. Fig. 1b shows the current transients recorded during the second run. When the anhydrous EG solution was reused for anodization, the length of the first stage of the current transient was similar to that of EG solutions with water addition. There was no significant difference in the initial and final current densities between the anhydrous and water containing EG solutions during the second run. The third run of the experiments (anodization of the third 0 1 2 3 4 5 6 7 8 9 10 Time, minutes Current density, mA/cm 2 no-water 0.13w t% water 0.18w t% water 0.5w t% water 1w t% water log Time log Current density I II III 01020 30 40 50 Fig. 1a. Current transients during anodization of Ti at 20 V in a freshly prepared ethylene glycol (EG) + 0.2 wt% NH 4 F solution with different amounts of water (First run). The inset shows the current transient in log– log scale. 0 1 2 3 4 5 6 Time, minutes Current density, mA/cm 2 No water sample 2 0.13% water sample 2 0.18% water sample 2 log Time log Current density 01020 30 40 50 Fig. 1b. Current transients during anodization of Ti at 20 V in ethylene glycol (EG) + 0.2 wt% NH 4 F solution with different amounts of water (Second run). Anodization was carried out in the already used solution of the experiment shown in Fig. 1a. The inset shows the current transient in log–log scale. K.S. Raja et al. / Electrochemistry Communications 9 (2007) 1069–1076 1071 specimen in the electrolyte which has been used for ano diz- ing two specimens) showed similar current transients irre- spective of the initial water content of the EG solution. Fig. 2a–f show the surface morphology of the TiO 2 formed during anodization in different conditions. The first anodization run in a fresh anhydrous EG solution resulted in an oxide layer of 150–200 nm thick. The surface con- tained irregular features and no ordered nanopores could be discernable (Fig. 2a). The second sample anodized in the same solution showed discrete nanoporous layer; how- ever there was no complete coverage of the anodized sur- face with the oxide layer as seen in Fig. 2b. Addition of 0.13 wt% water to the anhydrous EG solution resulted in formation of ordered nanoporous structure in the first run as shown in Fig. 2c. As observed from the figure, the formation of the nanotubes was not uniform throughout the anodized surface. The second run of anodization in the 0.13 wt% water content showed well ordered nanotubu- lar arrays (Fig. 2d). The length of the nanotubes was around 500–600 nm (not shown in the figure). When the water content was increased to 0.18 wt%, ordered na no- tubes could be observed in the first run itself as observed in Fig. 2e. There was no significant difference in the mor- phology of the nanotubular arrays between the first and second anodization runs with 0.18 wt% water content (Fig. 2 f). Overall, the results of the current transients and Fig. 2. FESEM images of the TiO 2 nanotubular arrays formed by anodization at 20 V for 45 min in anhydrous ethylene glycol +0.2 wt% NH 4 F solution with different water contents. The insets are the magnified views of the surfaces. 1072 K.S. Raja et al. / Electrochemistry Communications 9 (2007) 1069–1076 morphological observations showed that a minimum con- centration of about 0.18 wt% was required to form ordered nanotubular arrays in the ethylene glycol +0.2 wt% NH 4 F solution. Reusing the EG solution which was electrolyzed by previous anodization run resulted in a better nanotubu- lar morphology than the morphology observed using fresh EG solution with less than 0.18 wt% water addition. In fact, a third run of anodization in the initial anhydrous EG solution showed results similar to those observed with first run of 0.13 wt% water content. The above results clearly indicated that the extent of electrolysis of the EG solution played a significant role in the formation of nano- tubular TiO 2 arrays. Better reproducibility of the results with extended anodization period ($16 h) could be associ- ated with this feature in addition to the possible increase in the water content due to moisture absorption from the atmosphere. When the water addition was more than 0.18 wt%, there was no significant change in the morphol- ogy of the nanotubes. However, the nanotubes formed in higher water content (>0.5 wt%) showed ridges on the wall surface and the number of ridges increased with increase in the water content (not shown here). Figs. 3a, 3b and 3c show the high resolution Ti 2p, O 1 s and F 1 s XPS spectra of samples anodized in the EG solu- tion with and without water addition. Elemental Ti peak (453.9 eV) was observed in the sample anodized without water addition. The anodization was complete with the addition of 0.18 wt% water (no elemental Ti was observed). There is a possibility that the elemental Ti peak could have originated from the substrate because of the possible cracks in the oxide film. Whatever be the reason, the Ti 0 peak was observed only in the samples anodized in anhydrous EG solution indicating that the surface was not fully covered with an oxide layer. Whereas, specimens anodized in the EG solution with water addition did not show such Ti 0 peaks. The O 1 s spectra (Fig. 3b) showed a sharp peak at 530 eV associated with TiO 2 and a shoulder at 531.8 eV. This shoulder was more predominant in the sam- ple anodized without water addition. The additional shoul- der could be related to the presence of organic compo unds with CAOorC@O bonds, possibly (RCOO) 4 Ti type products. This aspect will be discussed later. Fig. 3c shows the F 1 s spectra. Two peaks were observed at 684.9 eV and 688.9, s. The peak at 684.8 eV could be related to Ti–F type compounds, possibly (NH 4 ) 2 TiF 6 . Nitrogen peak was also observed in the XPS spectra (not shown in figures), which supported the possibility of the above compound. Glancing angle XRD results poorly matched with the peaks of ammonium titanium hexafluoride. The second F 1 s peak at 688.9 eV could be assigned to (CF 2 ) x type compounds. The fluoride content of the anodized sample was more in the anhydrous (no water addition) condition than in the water added condition. Recently, Macak et al. [23] showed that the fluoride content could be minimiz ed by thermal annealing at 450 °C for 3 h. Ti-2p 0 500 1000 1500 2000 2500 3000 440445450455460465470475 Binding Energy, eV Counts no water 0.18% water 2p 3/2 2p 1/2 Ti 453.9 Fig. 3a. Ti 2p XPS high resolution spectrum of the surfaces anodized in 0 and 0.18 wt% water addition in EG + 0.2 wt% NH 4 F solution. The peak at 458.7 eV is associated with Ti 4+ ions. A small peak at 453.9 eV indicates presence of unoxidized Ti metal in the sample anodized in the EG solution without water addition. O 1s 0 500 1000 1500 2000 2500 3000 3500 4000 4500 520525530535540545 Binding Energy, eV Counts no water 0.18wt% Fig. 3b. O 1 s XPS high resolution spectrum of the surfaces anodized in 0 and 0.18 wt% water addition in EG + 0.2 wt% NH 4 F solution. The peak at 530 eV is attributed to TiO 2 . A wider shoulder starting at 531.8 eV could be related to CAOorC@O bonds. F 1s 3000 3500 4000 4500 5000 5500 675680685690695700 Binding Energy, eV Counts No wa te r 0.18wt% water Fig. 3c. F 1 s XPS high resolution spectrum of the surfaces anodized in 0 and 0.18 wt% water addition in EG + 0.2 wt% NH 4 F solution. The peak at 684.8 eV could be associated with Ti–F compounds. The second peak at around 688.3 eV could be related possibly to (CF 2 ) x compounds. K.S. Raja et al. / Electrochemistry Communications 9 (2007) 1069–1076 1073 4. Discussion The electrochemical oxidation of ethylene glycol in acidic and basic media has been investigated widely [24– 26]. In the aqueous solution, the oxidation of ethylene gly- col involves the following reaction steps: [25] Ethylene Glycol ! Glycolaldehyde ! Glyox al or Glycolic acid ! Glyox ylic Acid ! Oxalic acid ! Car bon dioxide: ð1Þ In acid media, predominant formation of glycolaldehyde (CH 2 OH–CHO) and glycolic acid (CH 2 OH–COOH) inter- mediates has been observed by in situ IR spectroscopy [25]. However, in water free conditions the oxidation mecha- nism could be different depending on the supporting elec- trolyte and the electrode surface. Addition of water, which is considered as a nucleophile, increases the reaction rate during electrochemical oxidation. Fig. 4, cyclic vol- tammograms of EG + 0.2 wt% NH 4 F solution carried out using two platinum electrodes at different water con- tents also supported this. The oxidation potentials de- creased with increase in the water content. The potentials were measured with reference to a Pt wire. Wieland et al. [25] observed a substantial amount of production of gly- colic acid during the oxidation of ethylene glycol (with 0.1 M HClO 4 supporting electrolyte) at 0.3 V vs. SCE. The increase in the anodic current above 0.8 V Pt in the case of 0.18 wt% water addition of this investigation could be associated with such oxidation process (Fig. 4). A signifi- cant anodic current was observed only above 1.5 V in the anhydrous condition. The cyclic voltammograms of EG solution with water addition revealed a minor cathodic cur- rent wave at around 0.5 V during the reverse scan. Such a cathodic wave was absent in the anhydrous EG solution. The cathodic peak could be attributed to desorption of the hydroxyl species from the Pt surface following the reaction: Pt=OH þ H þ þ e À ! Pt þ H 2 O ð2Þ Presence of such reduction wave indicated availability of H + ions closer to the electrode surface. The CV result in the anhydrous EG (no water addition) underlined the importance of the water addition to create local acidified condition. A critical concentration of H + ions has been considered a requirement for the formation of the TiO 2 nanotubes [14,15]. Similarly, availability of oxygen also is an important consideration for the formation of an anodic oxide layer. In the aqueous solutions, the hydroxyl ions are consid- ered source of the oxygen for oxide formation according to the reactions: H 2 O ! H þ þ OH À ð3aÞ 2OH À ! O 2À þ H 2 O ð3bÞ M ! M nþ þ ne À ð3cÞ M nþ þ 0:5nO 2À ! MO ð3dÞ Availability of the oxygen from organic solutions has been considered difficult because it is strongly bound to the car- bon atom by a double bond. Removal of the oxygen from carbon to react with the metal surface is considered unli- kely [27]. On the other hand, Ue et al. [28] observed organic anion acted as source of oxygen. These au thors anodized aluminum in anhydrous triethylmethylammonium hydro- gen maleate/c-butyrolactone solution and their surface analysis revealed maleate anion as the source of oxygen to form a composite aluminum oxide film. Tajima et al. [29] also observed an organic anion (HCONH À ) partici- pated in the formation of an organo-metallic type interme- diate which eventually resulted in an oxide film during anodization of aluminum in boric acid–formamide solution. In a salt-containing non-aqueous solution, passivation of metal has been considered to occur either by a re-precip- itated salt layer composed of dissolved metal cation and anion of the salt or by a monolayer of the oxidized species of the solvent molecule chemisorbed on to the surface [27]. For example, in acetonitrile/anhydrous HF media, nickel was passivated by a thick layer of NiF 2 [30]. Trace levels of water increased the passivity, especially in the acidified organic solutions [31]. Presence of water, in the acidified organic solutions has been considered to form an oxide film in addition to the salt layer. In this investigation, the focus was on the formation of nanotubular titanium dioxide arrays and not on the passivation of Ti by a salt layer or an oxide film mecha- nism. The important point that required to be clarified was, whether water addition was an absolute necessity -1.0E-03 0.0E+00 1.0E-03 2.0E-03 3.0E-03 4.0E-03 5.0E-03 -2024681012 Potential, V (Pt) Current, A no-water 0.13%H2O 0.18% H2O Fig. 4. Results of cyclic voltammetry (CV) conducted on two equal area platinum electrodes in ethylene glycol +0.2 wt% NH4F solutions with different water contents inside a dry argon filled glove box. The scan rate was 10 mV/sec. The inset shows the zoomed in view of the CV at lower potentials where a reduction peak was not observed in the anhydrous EG solution. 1074 K.S. Raja et al. / Electrochemistry Communications 9 (2007) 1069–1076 to form TiO 2 nanotubes or the oxidized products of eth- ylene glycol was sufficient to form TiO 2 . The analyses of the results of this investigation indicated that a minimum of 0.18 wt% water addition was required to form an ordered nanotubular TiO 2 arrays. Similar nanotubular formation could be obtained without water addition, if the anhydrous EG is electrolyzed at 20 V for >2 h using Ti anode (longer anodization times). Therefore, without water addition, the predominant oxide forming mecha- nism could be consumption of the air form ed film of Ti as the oxygen source according to the anodic reactions (4a)–(4c) and (4d): 2ðCH 2 OHÞ 2 þ TiO 2 ðair formed film Þ ! Ti=2ðCOOH À CH 2 OHÞþ4H þ þ 4e À ð4aÞ COOH À R !ðR À COOÞ À þ H þ þ e À ð4bÞ Ti þ 4ðR À COOÞ À !ðRCOOÞ 4 Ti þ 4e À ð4cÞ ðRCOOÞ 4 Ti ! TiO 2 þ CO 2 þ nH þ þ ne À ð4dÞ Presence of elemental Ti in the anhydrous condition (Fig. 3a implying incomplete reaction steps) and an extra shoulder in the O 1 s XPS spectra support the above reac- tion steps. More analyses should be carried out to unde r- stand the complete mechanism. Preliminary ex situ FTIR and GA-XRD analyses did not yield any useful informa- tion. More of in situ analytical experiments need to be performed. Formation of the ordered nanotubes during the ‘third run’ of anodization indicated the influ ence of lo- cal acidic conditions (formation of glycol ic acid). Presence of RCOO À , which was also a nucleophile like water [26], could facilitate oxidation reactions. Macak and Schmuki [15] illustrated the necessity of localized low pH condition for growth of longer nanotubes in the viscous organic solutions. When the pH of the bulk fluoride containing solution was low (for example in phosphoric acid, acetic acid, oxalic acid etc.), the dissolution rate of TiO 2 was re- ported to be high [32] and therefore could limit the steady state length of the nanotubes. In the EG solution, the walls of the nanotubes were considered to be in the pas- sive state. The concentration of the H + ions was higher only at the bottom of the nanotubes causing controlled dissolution. In this investigation, the dissolution was as- sumed to be electric field assisted, so that the negatively charged cation vacancies created by the dissolution moved with the field and reached the metal/oxide inter face. These cation vacancies were consumed at the metal/oxide inter- face to form new oxide lattices according to the point de- fect model [33]. Therefore, dissolution of the bottom of the nanotubes (barrier layer) resulted in movement of the metal/oxide interface into the metal substrate and pas- sive walls of the nanotubes manifested into vertical growth of the nanotubes. Almost similar results were obtained in the glycerol +0.2 wt% NH 4 F solutions. In this solution also 0.18 wt% water addition was found to give reproduc- ible result of formation of TiO 2 ordered nanotubular arrays. 5. Conclusions Anodizations of Ti foil in the anhydrous ethylene glycol +0.2 wt% NH 4 F solution (EG solution) with 0–1 wt% water additions were carried out at 20 V for 45 min in a dry-argon filled controlled atmosphere glove box. Bas ed on the exper- imental results, the following conclusions are drawn:  A minimum amount of 0.18 wt% of water addition was required to form a well ordered TiO 2 nanotubular arrays.  When the anhydrous EG solution was reused for third time, ordered arrays of nanotubes started to form.  Increase in the water content of the EG solution (>0.5 wt%) showed increased amount of ridges on the circumference of the nanotubes.  XPS results showed presence of un-anodized Ti element in the anhydrous condition and presence of organic and (NH 4 ) 2 TiF 6 type compounds in all the anodized samples in addition to the regular TiO 2 phase.  The results underline the influence of water content and local pH condition to form the ordered nanotubular arrays. Acknowledgement This work was supported by US Department of Energy through the co ntract No. DE-FC 52-98NV13492 and DE- FC-36-06G086066. References [1] V. Zwilling, M. Aucouturier, E. Darque-Ceretti, Electrochim. Acta 45 (1999) 921. [2] K.S. 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