effect of water content of ethylene glycol as electrolyte for synthesis

Effect of water content of ethylene glycol as electrolyte for synthesisof ordered titania nanotubes Metallurgical and Materials Engineering, Mail Stop 388, University of Nevada, Reno, NV

Effect of water content of ethylene glycol as electrolyte for synthesis

of ordered titania nanotubes

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 TiO2nanotubes 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% NH4F 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 TiO2 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 (NH4)2TiF6type com-pounds in all the anodized samples in addition to the regular TiO2phase 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 TiO2 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 lm to

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 electrolytes 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

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 TiO2 nanotubes

formed in the organic solutions were annealed at 500–

650°C, the adsorbed carbon species during the anodization

process modified the tubes as TiO2xCxafter annealing by

internal diffusion of carbon into the lattice [17] This

pro-cess resulted in an improved activity during

photo-electrochemical generation of hydrogen, which was

similar to the behavior of carbon modified TiO2prepared

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 TiO2 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 employed 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 shorter time in anhydrous polyhydric

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 TiO2nanotubes with

smooth surface (ripple free) was affected by the initial water

content of the polyhydric alcohol, relative humidity,

expo-sure time of the electrolyte to the atmosphere and

anodiza-tion duraanodiza-tion 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 anhydrous 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 TiO2nanotubes 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 TiO2 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% NH4F dissolved

in anhydrous ethylene glycol (Sigma–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 cm2 A Pt flag with 7.5 cm2 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 Therefore, 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 TiO2 nano-tubes as described in the following sections The anodiza-tion period was restricted to only 45 min, which was a typical time observed in a pH 2.0 aqueous fluoride solution for growth of400 nm long TiO2nanotubes Initially, the anodization runs were carried out by two different poten-tial application methods, viz, 1 potenpoten-tial stepping method (wherein the anodization potential of 20 V was applied in a

single step (without ramping) and the anodic 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 laboratory 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 Ka 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 aqueous 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 theFig 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 current transient with a slightly shallower slope

of current decay was observed Typically the initial slope of the current decay was about0.1 and the second stage of the current transient showed a slope of 0.9 to

1.2 The slope of the third stage of EG solutions with higher water contents was about 0.3 The EG solutions with 0–0.13 wt% water addition showed a slightly increas-ing stage III current transient (as against decayincreas-ing 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/cm2 in the case of anhydrous EG solution and 0.54–0.68 mA/cm2 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 anodizasolu-tion of one specimen) with a fresh Ti sample, a slightly different but distinguishable current transient 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

2 no-water

0.13w t% water 0.18w t% water 0.5w t% water 1w t% water

log Time

I II III

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

No water sample 2 0.13% water sample 2 0.18% water sample 2

log Tim e

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.

specimen in the electrolyte which has been used for

anodiz-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 TiO2

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 inFig 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 nano-tubes could be observed in the first run itself as observed

inFig 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 2f) 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.

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% NH4F

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 TiO2 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 3cshow 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 addisolu-tion 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 Ti0peak 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 Ti0

peaks The O 1 s spectra (Fig 3b) showed a sharp peak

at 530 eV associated with TiO2 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 compounds with CAO or C@O bonds, possibly (RCOO)4Ti type products This aspect will be discussed later.Fig 3cshows 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 (NH4)2TiF6 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 (CF2)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 minimized by thermal annealing at 450°C for 3 h

Ti-2p

0

500

1000

1500

2000

2500

3000

440 445 450 455 460 465 470

475

Binding Energy, eV

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 Ti4+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

520 525

530 535

540 545

Binding Energy, eV

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 C AO or C@O bonds.

F 1s

3000 3500 4000 4500 5000 5500

675 680

685 690

695 700

Binding Energy, eV

No water 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.

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

! Glyoxal or Glycolic acid

! Glyoxylic Acid ! Oxalic acid

In acid media, predominant formation of glycolaldehyde

(CH2OH–CHO) and glycolic acid (CH2OH–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% NH4F 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 HClO4 supporting electrolyte) at 0.3 V vs SCE

The increase in the anodic current above 0.8 VPtin 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:

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 TiO2 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:

Availability of the oxygen from organic solutions has been considered difficult because it is strongly bound to the car-bon atom by a double car-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 authors 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) 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 NiF2[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

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.

to form TiO2nanotubes or the oxidized products of

eth-ylene glycol was sufficient to form TiO2 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 TiO2 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 formed film of

Ti as the oxygen source according to the anodic reactions

(4a)–(4c) and (4d):

2ðCH2OHÞ2þ TiO2ðair formed filmÞ

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

under-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 influence of

lo-cal acidic conditions (formation of glycolic acid) Presence

of RCOO, 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 TiO2was

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 interface 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% NH4F solutions In this solution

also 0.18 wt% water addition was found to give

reproduc-ible result of formation of TiO2 ordered nanotubular

arrays

5 Conclusions Anodizations of Ti foil in the anhydrous ethylene glycol +0.2 wt% NH4F 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 Based 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 TiO2 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 (NH4)2TiF6type compounds in all the anodized samples

in addition to the regular TiO2phase

 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 contract No FC 52-98NV13492 and DE-FC-36-06G086066

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