Author's personal copy - Central Michigan University · Author's personal copy. Graphical Abstract...

RESEARCH ARTICLE

Investigation of the formation mechanism of titanium oxidenanotubes and its electrochemical evaluation

U. H. Shah1 • Z. Rahman2 • K. M. Deen3 • H. Asgar1 • I. Shabib1,2 •

W. Haider1,2

Received: 24 March 2017 / Accepted: 14 June 2017 / Published online: 26 June 2017! Springer Science+Business Media B.V. 2017

Abstract The electrochemical synthesis of titanium diox-ide nanotubes (TNTs) on titanium alloys depends on the

electrolyte composition and process parameters. To gain

insight of titanium oxidation on commercially pure tita-nium grade 2 (cpTi-2), the effects of individual electrolyte

species have been investigated by cyclic voltammetry. At

constant 60 V potential, different nanotubular structureswere obtained by varying time. Under synergistic action of

anionic species and electric field with increase in anodizing

time, the field enhanced dissolution become dominant asvalidated from the increase in the inner tubes diameter and

decrease in wall thickness. On the other hand, the disso-

lution of inter-tubular region was also observed whichresulted in the separation of nanotubes at higher anodizing

time. The electrochemical performance of anodized sam-

ples has been evaluated in 1 wt% NaCl. The lower corro-sion current density and passive current density registered

by 30 min anodized sample corresponded to the limited

ingress of ionic species at the Ti/TNTs interface comparedto 60, 90, 120 and 150 min samples which contained large

inter-tubular spacing. The electrochemical impedance

spectroscopy (EIS) analysis was in support to potentiody-namic results confirming the diffusion-controlled reactions

at the Ti/TNTs interface.

& W. [email protected]

1 School of Engineering and Technology, Central MichiganUniversity, Mount Pleasant, MI 48859, USA

2 Science of Advanced Materials, Central Michigan University,Mount Pleasant, MI 48859, USA

3 Department of Materials Engineering, University of BritishColumbia, Vancouver, BC V6T 1Z4, Canada

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J Appl Electrochem (2017) 47:1147–1159

DOI 10.1007/s10800-017-1102-1

Author's personal copy

Graphical Abstract

Keywords Electrochemistry ! Titanium oxide nanotubes !Cyclic voltammetry ! Potentiodynamic polarization !Impedance spectroscopy

1 Introduction

With the advancement in the society, nanomaterials have

gained significant importance owing to its promisingstructural, dimensional and catalytic features, suitability for

gas sensing, solar cells, biomedical, battery materials,

photocatalysis and fuel cell applications [1–6]. Amongvarious nanostructures, TiO2 gained more popularity due to

its unique properties like uni-dimensional growth pattern,

effective band gap due to quantum confinement and largesurface area for most catalytic reactions [7, 8]. The struc-

tural integrity of TiO2 under ambient conditions would be

dependent on its surface morphology, texture, crystallinityand corrosion resistance. For example, the improved cor-

rosion resistance and osteo-integration of Ti bio-implants

would require surface modification at the nanoscale level.The formation of TiO2 nano tubes through electrochemical

anodization could be an effective method to achieve better

surface characteristics. These properties can be tuned byvarying the processing parameters. Recently, Momeni et al.

and co-workers have reported interesting developments in

the field of photocatalysis. Their group have loaded theTiO2 nanoparticles with PbO, WO3, Cu, Co, Cr & Fe and

have shown remarkable increase in the aforementioned

properties [9–13]. A review by Chen and Mao and refer-ences cited therein shows the numerous applications of

TiO2 nanotubes, e.g. photocatalysis, electrochromic devi-

ces, energy storage, drug delivery systems and biomedicalimplants [14].

Electrochemical testing techniques are based on the

interaction between chemical changes and electrical energy

(flow of electrons and ions). Various electrochemicaltesting methods like galvanodynamic, potentiodynamic,

impedance spectroscopy and cyclic voltammetry (CV) can

be used to gain the insights of the chemical process. Smithet al. [15] used potentiostatic curves to describe the tita-

nium dioxide nanotubes formation mechanism. Similarly,

Ghicov et al. [16] used the current transient curves toexplain the titanium oxide nanotubes growth process in

phosphate-based electrolyte. Timmerman evaluated the

titanium oxide nanotubes loaded with metal nanoparticlesfor the electrochemical detection of the pharmaceutical and

personal care products [17]. Nicholson [18] measured the

electrode potential of cadmium in 1 M sodium sulphateusing CV. Deen [19] reported the faradic and non-faradic

capacitance of the electrode using CV. Siham [20]

observed the complexes formed as a result of chargetransfer between phenacetin and tetra-cyano-ethylene using

CV.

The basic requirement of any material to be used in acertain application is that it should withstand the corrosion

attack. It should not lose its integrity to a point that renders

the material inappropriate for desirable performance.Therefore, it is always important to evaluate the corrosion

behaviour of implant material under simulated conditions

ex situ. Zhang studied the corrosion behaviour of titaniumoxide nanotube using impedance spectroscopy and sug-

gested that it has adequate electrochemical behaviour for

use as a dental implant [21]. Campanelli evaluated theelectrochemical response of nanotubes grown on Ti-6Al-

4V and Ti-6Al-7Nb alloys in Ringer solution through

potentiodynamic polarization scans and impedance spec-troscopy measurements [22].

1148 J Appl Electrochem (2017) 47:1147–1159

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In this work, we investigated the electrochemical

response of individual electrolyte species on the titaniumoxidation. Keeping in view the importance of TiO2 nanos-

tructures and widely accepted theories about growth

mechanism in the existing literature, we have tried toexplain the influence of individual electrolyte species on the

formation of titanium dioxide nanotubes on cpTi-2 based on

the experimental observations. After anodization at constantapplied potential in the optimized electrolyte, the corrosion

behaviour of anodized samples was investigated by usingpotentiodynamic polarization scans (PPS) and electro-

chemical impedance spectroscopy (EIS). The finding of this

work explains the oxidation mechanism of titanium partic-ularly in accordance with the classical dissolution theory. In

addition, post-electrochemical tests explained the corrosion

behaviour of titanium dioxide nanostructures. The CVresults supported the existing theories of titanium oxidation

and the corrosion behaviour of anodized titanium explained

the electrochemical stability of titanium nanostructures in asaline solution. This could be beneficial for the researchers

to optimize the use of titanium dioxide for functional

applications like floating solar cells or biomedical implants.

2 Experimentation

2.1 Materials

Commercially pure titanium grade 2 (C 0.08%, N 0.03%, O

0.25%, Fe 0.2%, H 0.01%, Others 0.4%& balance Ti), cpTi-

2 was used in this study. The analytical grade chemicals, i.e.ethylene glycol (C2H6O2), ammonium di-hydrogen phos-

phate (NH4H2PO4), ammonium fluoride (NH4F), glycerol

(C3H8O3), sodium chloride (NaCl), acetone (CH3COCH3)and ethanol (C2H5OH), were purchased from Fisher Sci-

entific, US and used as received without further purification.

2.2 Sample preparation

To fabricate TNTs, cpTi-2 circular disks of 5 mm thicknessand 16 mm diameter were used. The surfaces were

sequentially polished on 180–1200 grit size silicon carbide

sand papers followed by degreasing with acetone in asonication bath for 5 min. The specimens were then

washed in acetone, deionized water and ethanol followed

by drying in hot air. Schematic presentation of samplepreparation and experimentation procedure is shown in

Fig. 1.

2.3 Electrolyte species

The chemical composition of each electrolyte containingvarious species is given in Table 1. We studied the

influence of each species (present in the final composition)by their sequential addition in the electrolyte. These elec-

trolytes are designated as SET 1, SET 2, SET 3, SET 4, SET

5 and SET 6. SET 6 is designated as optimized compositionbased on the experiment evidence and formation of nan-

otubes on the cpTi specimen surface as explained else-

where [23]. The optimized composition of the finalelectrolyte used for the formation of TNTs was composed

of ethylene glycol, deionized water, glycerol, ammonium

di-hydrogen phosphate and ammonium fluoride as given inTable 1 (SET 6).

2.4 Anodization

Two electrode cell setups were used for electrochemical

anodization connected with a DC power source (SorensenSGA 100X50C-0AAA). The cell contained graphite rod

cathode and cpTi-2 disks as anode (working electrode),

respectively. The anode to cathode surface area ratio (1:3)was kept constant in all the experiments. The distance

between the working and graphite electrode was main-

tained constant (*2 cm) in all the experiments. The

Fig. 1 Schematic presentation of sample preparation and experimen-tation procedure

Table 1 Chemical composition of electrolytes containing variousspecies to evaluate electrochemical behaviour by using cyclicvoltammetry

Name C2H6O2 NH4H2PO4 NH4F DI H2O C3H8O3

SET 1 92 mL – – – –

SET 2 92 mL – – 8 mL –

SET 3 92 mL – – 8 mL 0.5 mL

SET 4 92 mL 0.1 M – 8 mL 0.5 mL

SET 5 92 mL – 0.3 M 8 mL 0.5 mL

SET 6 92 mL 0.1 M 0.3 M 8 mL 0.5 mL

J Appl Electrochem (2017) 47:1147–1159 1149

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electrolyte used for cpTi-2 anodization is given in Table 1

(SET 6). Anodization was done at room temperature and atconstant 60 V potential for 30, 60, 90, 120 and 150 min on

separate samples. The pH of the solution was measured

before and after the anodization using Jenway 3510 pHmeter. Fresh electrolyte was used each time for anodiza-

tion. After anodizing treatment, the samples were washed

with ethanol and deionized water ultrasonically for 5 minto remove any surface debris. These anodized samples are

designated as 30, 60, 90, 120 and 150 min in the followingdiscussion.

2.5 Structural and morphological characterization

The surface morphology and approximate elemental com-

position of each sample were examined using scanningelectron microscopy (FEI Nova 200 nanolab SEM/FIB)

coupled with energy-dispersive spectroscopy (EDS)

detector. The images were analysed for the measurement ofsurface density, porosity and dimension of nanotubes by

using Image J software provided by (National Institute of

Health, US). The material structure of anodized sampleswas characterized using XRD (Cuka radiation) Rigaku

Miniflex-II X-ray diffractometer before and after heat

treatment (thermally annealed at 400 "C for 2 h in air).

2.6 Electrochemical evaluation

Gamry reference 3000 potentiostat connected with a three-

electrode cell setup was used for electrochemical mea-

surements. The graphite counter, saturated calomel elec-trode (SCE) reference (?240 mV vs. SHE) and anodized

sample as working electrode were used in the cell. The

electrochemical tests were conducted at room temperaturein a stationary electrolyte. After setting up the cell and

addition of electrolyte, prior to each electrochemical test,

1 h initial delay was exercised to stabilize the open circuitpotential (OCP). In order to ensure the reproducibility,

each experiment was repeated three times.

To estimate the role of individual species in the devel-opment of TNTs, the cyclic voltammetry (CV) scans were

obtained in a high potential range (0–10 V vs. SCE) at a

sweep rate of 20 mV/s. Based on the results, morphologicaland structural characteristics, the mechanism of nanotube

formation and growth is proposed. After anodizing in the

optimized electrolytes (SET 6) for different time durations,the electrochemical performance of samples was examined

by potentiodynamic polarization (PD) and electrochemical

impedance spectroscopy (EIS). The electrolyte used was1 wt% NaCl solution prepared in deionized water (20 lS/cm). The potentiodynamic polarization was performed

within -0.5 to ?2 V (versus OCP) potential range at1 mV/s scan rate. The EIS spectra were obtained at 10 mV

AC potential perturbation in the frequency range of

100 kHz–10 mHz. All the experiments were conducted atroom temperature and no appreciable change in the elec-

trolyte pH was observed before and after the tests.

3 Results and discussions

3.1 Role of electrolyte species

To evaluate the electrochemical response of electrolytespecies on the formation of TNTs, the cyclic voltammetry

scans of as prepared cpTi-2 specimen in each electrolytespecies were obtained, as shown in Fig. 2. The scans were

initiated from the OCP in the anodic direction with a sweep

rate of 20 mV/s. In SET 1, the cpTi-2 provided very lowcurrent and large overpotential of 1.823 V followed by an

increase in current showing two broad anodic peaks at

5.362 and 7.232 V. These current peaks could be attributedto the conversion and degradation of ethylene glycol (EG)

at the surface of titanium. It has been reported that EG may

oxidize into glycol-aldehyde, glycolic acid, glyoxylic acid,oxalic acid and formic acid followed by decomposition into

carbon dioxide and water [24, 25]. Another possible reason

is the high IR drop because of low ionic conductivity of theethylene glycol. In the electrolyte containing ethylene

glycol and deionized water (SET 2), the cpTi-2 presented

very low current without any observable current peakwhich could be attributed to the enhanced stability of EG in

the presence of water [26, 27]. This behaviour could be due

to the formation of complex molecules with water, andstrong hydrogen bonding between EG and water molecules

Fig. 2 Cyclic voltammogram of cpTi-2 in the electrolytes as given inTable 1; Scan rate; 20 mV/s, Curves 1–6 corresponds to SET 1–SET6, respectively. (Samples were polarized from 0 to 10 V w.r.t SCE inanodic and cathodic direction)


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[28]. Addition of 0.5 vol% glycerol (GL) in EG?H2O

(SET 3) solution provided even larger overpotential andlowered the current response. The very small current

response in the reverse scan further confirmed the

enhanced stability of organic solvents even in the presenceof 8 vol% water. In other words, it could be evaluated that

water stability window was broadened (limited oxygen

evolution reaction) in the presence of organic solvents (EGand GL) which may probably develop strong bonding with

EG?GL. The addition of 0.1 M Ammonium Di-hydrogenPhosphate (ADP) into EG?H2O?GL (SET 4) solvent

markedly increase the current with increased in potential,

as shown in Fig. 2. The ADP could dissociate into di-hy-drogen phosphate (H2PO4

-) and ammonium (NH4?) ions

in the presence of water. In SET 4, the pH was dropped to

4.88 ± 0.03 validating the formation of di-hydrogenphosphate ions [29]. The sharp anodic current peak

observed at 2.289 V versus SCE could be attributed to the

oxidation of TiO2 to TiO22? (Ti4? to Ti6?) at the surface of

cpTi-2 (reaction 1).

TiO2 ! TiO2þ2 þ 2e# ð1Þ

The presence of inherent passive film at the surface ofcpTi-2 could be validated by the thermodynamic stability

of TiO2 species at a given EOCP (-0.312 mV vs. SCE) and

pH. At potential 5.614 V versus SCE, a second anodic peakwas observed which could be associated with reaction of

H2PO4- with cpTi-2 surface species (TiO2

2?). The post-

peak decrease in current at large potential could be relatedwith the diffusion or to the reaction overpotential in the

presence of surface oxide film. At higher potential, the

relatively large current would promote the dissolution ofoxide film and this could be realized by the reaction of

oxidized surface species with the available anionic species

in the electrolyte, i.e. OH-, H2PO4-, HPO4

2-, PO43- etc.

As reported by Bereznitski [30] it could be due to the

formation of titanium phosphate complexes such as Ti2-O(PO4)2H2O, Ti2(PO4)3, TiO.PO4, Ti(PO4)3, Ti2O3(H2-

PO4).2H2O, Ti(HPO4).2H2O, Ti(H2PO4).(PO4)2H2O,

Ti(HPO4)2.H2O, Ti5(PO4)3, Ti2O(PO4)2.2H2O. The addi-

tion of ammonium fluoride (AF) in EG?H2O?GL (SET 5)would dissociate into F- and NH4

?. The observable cur-

rent peak during anodic scan was in consistent with the

reaction 1 independent of the nature of anionic species(H2PO4

- or F-) present in the electrolyte. The constant

current obtained in EG?H2O?GL?AF (Fig. 2) could be

affiliated with the limited oxidation of F- (reaction 2) ionsat the surface of cpTi-2 followed by adsorption at some

active sites (reaction 2). The presence of fluoride in the

TNTs was confirmed in the EDX analysis as shown inTable 2. The high anodic potential would also promote the

migration of anionic species particularly F- ions towards

the surface and hence the dissolution of TiO2 may also

occurs according to reaction 3.

TiO2þ2 þ 2F# ! TiO2:2ðFÞads ð2Þ

TiO2 þ 6F# þ 4Hþ ! TiO2þ2 þ 2e# ð3Þ

The slight increase in pH (5.47 ± 0.04) after cyclicscans was in support to the field-enhanced dissolution

reaction by complexation (formation of soluble [TiF6]2-

species). Here it could be compared that the nature ofanionic species significantly affects the growth and disso-

lution pattern of the oxide film. In our final optimized

electrolyte composition (SET 6) containing both ADP(0.1 M) and AF (0.3 M) in (EG?H2O?GL), very low

anodic current was observed with a characteristic anodic

peak of TiO2 oxidation as discussed above (reaction 1).The anodic current peak was shifted to relatively lower

potential (1.899 V vs. SCE) compared to peak potential

observed in case of other anionic species during forwardscan as discussed above. Furthermore, the low current

which preceded this current peak could be related to thelocalized attack of inherent TiO2 film by the synergistic

action of H2PO4- and F- ions. This shows the accelerated

dissolution tendency of cpTi-2 in electrolyte containingNH4F only (SET 5) but addition of NH4H2PO4 (SET 6) was

beneficial to maintain balance between dissolution and

growth of nanotubes. Under controlled conditions, theinherent oxide film would be oxidized with increase in

potential prior to small anodic peak observed in CV scan

which may be due to the oxidation of TiO2 into TiO2þ2

species (reaction 1). During anodization, the transport of

anionic species towards the surface could be acceleratedunder the applied electric field. The post-peak situation

could be associated with the balance of uniform growth and

localized dissolution imposed by the synergistic effects ofF- and H2PO4

- ions. The combined action of H2PO4- and

F- ionic species in the electrolyte at high potential could

oxidize (growth) and generate water soluble complexes.The relatively low and constant current observed before

and after the anodic peak in case of SET 6 could also be

associated with the complexation of anionic species with

Table 2 EDS Data showing the elemental composition (atomicpercent) at nanotubular surfaces

Element 30 min 60 min 90 min 120 min 150 min

C 9.13 9.76 9.14 8.79 8.79

O 37.48 35.72 35.79 37.75 37.40

F 9.83 9.10 9.33 9.51 10.72

Si 0.39 0.57 – – –

P 0.26 0.30 0.36 0.33 0.34

Ti 42.91 44.55 45.39 43.62 42.67


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the surface and/or limited ionic mobility in the organic

electrolyte towards or away from the electrode surface[23]. It is therefore predicted that balance in the growth and

dissolution tendency of TNTs is strongly dependant on the

amount of water and concentration of ionic species in theelectrolyte. The nature and concentration of the anionic

species could be adjusted to alter the growth pattern and

morphology of the TNTs. In other words, under highapplied field, the individual anionic species and amount of

water would either promote dissolution or growth of the

TNTs depending upon their competitive synergistic action

in the formation of final morphology.

3.2 Structural and morphological characterization

Based on the electrochemical results and optimization of

the electrolyte as cited elsewhere [23], the electrolyte

composition used as (SET 6) is considered as optimized. Inthis electrolyte, the cpTi-2 specimens were anodized at

constant DC (60 V) potential. The immersion time was

Fig. 3 SEM images showing the microstructural features of TNTs formed at various anodizing time in SET 6 (C2H6O2 = 92 mL,NH4H2PO4 = 0.1 M, NH4F = 0.3 M, DI H2O = 8 mL and C3H8O3 = 0.5 mL) a 30 min, b 60 min c 90 min, d 120 min, e 150 min


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varied from 30 to 150 min to examine any change in the

morphology of TNTs. It was observed that increase inanodizing time at constant applied potential could signifi-

cantly influence the morphology of nanotubes. Figure 3

represents the morphology of nanotubes at variousanodizing periods. It was determined that the inner diam-

eter increases with the increase in anodizing time which

could be associated with the preferential dissolution ofnanotubes walls both from interior and exterior. This could

be further confirmed from the subsequent decrease in tubeswall thickness. The presence of upper (inherent) oxide film

could also be observed at the surface of nanotubes after

30 min anodization (Fig. 3). This feature indicated thelocalized dissolution of existing oxide film (TiO2) and

initiation of nanotubes formation underneath this upper

oxide film due to the ingress of anionic species at the Ti/TiO2 interface. Furthermore, the balance in the field-as-

sisted growth and dissolution are very much essential for

the formation of uniform nanotubes at the surface of cpTi-2. With increase in anodizing time, the dissolution of

nanotubes can be observed from the increase in the inner

tube diameter and decrease in wall thickness. The quanti-tative representation of variation in the inner tube diameter,

wall thickness, % porosity and surface density is shown in

Fig. 4a, b. After 30 min anodization, the inner diameterand wall thickness was found to be 94 ± 8.5 and

33.8 ± 1.3 nm, respectively. It was observed that inner

tube diameter increased rapidly from 100.4 ± 5.5 (after60 min) to 161.8 ± 5.4 nm after 150 min anodization. It is

also possible to get the surface porosity and roughness

factor using length, diameter and thickness of the nan-otubes by using following relation [31].

P ¼ 1# ½ 2pt tþ dð Þffiffiffi3

pd þ 2tð Þ2

(;

where ‘P’ is the porosity factor, ‘t’ is the nanotube wall

thickness and ‘d’ corresponds to the inner diameter of the

nanotubes. The % porosity could be calculated by multi-plying ‘P’ with 100 as shown in Fig. 4b. It was observed

that with increase in anodizing time the dissolution of

nanotubes could be enhanced which resulted in the increaseof diameter and % porosity. Similarly, the decrease in wall

thickness (from 33.8 ± 1.3 nm after 30 min to

7.6 ± 2.1 nm after 150 min anodization) and decrease insurface density (from 38 to 22 tubes/lm2) could also be

associated with dissolution of nanotubes simultaneously

from both inside and inter-tubular regions (outer wallspace). From the trends shown in Fig. 4, it was evaluated

that with increase in anodizing time, the localized attack of

F- ions by formation of soluble complexes in the elec-trolyte could also initiate dissolution at the inter-tubular

region.

To further confirm this behaviour, the cross sectional

images of anodized samples were obtained, as shown in

Fig. 5. It was evident that the nanotubes were unidirec-tional and oriented normal to the surface. The increase in

the tubes length was found to be directly related with the

anodizing time. The lengths of the nanotubes achievedafter 30, 60, 90, 120 and 150 min were 1.28 ± 0.23,

1.97 ± 0.19, 2.56 ± 0.16, 3.20 ± 0.48 and

4.60 ± 0.42 lm, respectively. It was observed that thenanotubes were well stacked and connected with each other

after 30 min anodizing containing porous oxide film at the

top surface. The presence of dimple structure at thetube/substrate interface and striations over the surface of

nanotubes could be associated with the inward growth

(from top to bottom) of nanotubes. The well-defined nan-otubes were connected with each other at less anodizing

time, i.e. 30, 60 and 90 min (Fig. 3 and 5). On the otherhand, the long anodizing time could also increase the dis-

solution of inter-tubular regions as evident in Fig. 3d, e.

Fig. 4 a, b Change in diameter, wall thickness, porosity and surfacedensity at constant potential and by changing anodizing time


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XRD was used to determine the phases and crystallinity.XRD patterns are shown in Fig. 6. The XRD peaks of

anodized samples were matched exactly with the titanium

reference pattern in Crystallography Open Database (COD# 1512547). It was observed that the nanotubes obtained

via electrochemical anodization are amorphous in nature.

After heat treatment, the peaks characteristic peaks ofanatase (COD # 9015929) and rutile (COD # 9009083)

were detected. The formation of more oxidized and regular

phases were observed after heat treatment in air. With theincrease in the anodization time, slight increase in the

intensity anatase peaks were observed.

3.3 Electrochemical behaviour of TNTs

The electrochemical behaviour of TNTs obtained aftervarious anodizing periods was determined in deaerated 1%

NaCl solution as shown in Fig. 7. At the surface of Ti, the

Fig. 5 The cross sectional images of as formed TNTs after a 30 min, b 60 min, c 90 min, d 120 min, e 150 min, of anodization and f Thevariation in the length of nanotubes as function of anodizing time is also provided for comparison


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reduction of water (reaction 4) could either actively pro-

duce hydrogen gas or the reaction rate could be limited bythe effective adsorption of hydrogen (ad-atoms) at the

surface prior to evolution [32]. For hydrogen reduction

reaction when the single reacting electron is involved;n = 1 with a symmetry factor (b = 0.5), the transfer

coefficient (a) and cathodic Tafel slope (bc) correspond

typically to 0.5 and *0.18-0.2 V/dec, respectively[33, 34]. The kinetics parameters determined from poten-

tiodynamic polarization are given in Table 3. Independent

to the dimensional features of nanotubes formed at variousanodizing time, the ‘bc’ values were close to

the *0.12 ± 0.02 V/dec. This validated the possibility of

water reduction over the TNTs. The relatively more

negative corrosion potential (Ecorr), large corrosion current

density (icorr) and shift in potential (Ecorr - Eocp) after

anodizing could be associated with the adsorption ofintermediate species at the TNTs surface or to the hydrogen

evolution [35, 36]. Almost double icorr values at 60, 90, 120

and 150 min compared to 30 min anodized samples couldbe associated with the active redox kinetics of water

reduction (reaction 4).

H2Oþ e# ! H2 þ OH# ð4Þ

The negative shift in potential (Ecorr - Eocp\ 0) after

cathodic scan during potentiodynamic polarization ofanodized sample could also be related with the adsorption

and/or catalytic reduction of water. The mechanical dete-

rioration of nanotubes due to hydrogen evolution cannot beneglected in case of extremely negative potential shift

during cathodic polarization. This could expose the fresh

‘Ti’ surface to the electrolyte and could shift Ecorr to morenegative values as given in Table 3. The potential shifts of

-0.22, -0.23 and -0.16 V were observed for 30, 120 and

150 min anodized samples, respectively. The smallerlength of nanotubes achieved after 30 min anodizing and

relatively large inter-tubular gap (lower tubes density)

observed after 120 and 150 min anodizing. Thermody-namically, the slight increase in Eocp values from

-0.39 ± 0.02 to -0.30 ± 0.01 V represented by each

anodized sample could be related with the existence ofstable anodized film. The very negative Ecorr (-0.61 V)

and about two times lower icorr (0.48 lA/cm2) of 30 min

sample than other anodized samples could be associatedwith the existence of small length nanotubes with minimal

inter-tubular region. The passive current density (ip) of

30 min anodized sample was also small (15.58 lA/cm2)compared to the current density observed for other

Fig. 6 XRD patterns of a as anodized and b as anodized and heat treated

Fig. 7 Potentiodynamic polarization scans of anodized samples andthe current density are presented based on geometrical surface area.(Here time corresponds to anodization time and samples werepolarized from -0.5 to 2.0 V w.r.t OCP at 1 mV/s scan rate)


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anodized samples. The approximately two times larger

anodic Tafel slopes (ba) compared to bc values of anodized

samples also represented the higher resistance to oxidationand/or anodic dissolution of surface films. This behaviour

could be attributed to the higher stability of oxide nan-

otubes which may have negligible inter-tubular gap after30 min anodizing. Whereas on the other hand with increase

in anodizing time, the preferential dissolution of top sur-face oxide film and field-assisted uniform dissolution of

nanotube walls from both (inner and outer) sides could

increase the inter-tubular gap. This could enhance theingress of (OH-, Cl-) ions towards the (Ti/TNTs) interface

which may oxidize and/or hydrolyse at the Ti/TNTs

interface according to reactions (5, 6, 7 and 8). The pos-sibility of these reactions to occur at the interface cannot be

neglected even in the presence of TNTs because of rela-

tively large inter-tubular separation, and this behaviourcould be validated from relatively higher ip values

observed in case of 120 and 150 min anodized samples

(26.98 and 30.11 lA/cm2, respectively).

4OH# ! 2H2Oþ 4e# ð5Þ

Ti4þ þ 4OH# ! TiO2:2H2O ð6Þ

Ti4þ þ 4Cl# ! ½TiCl4(aq ð7Þ

Ti2þ2 þ 2Cl# ! TiO2Cl2 ð8Þ

To further confirm the electrochemical kinetic behaviour

of anodized samples in saline solution, the EIS analysis (atDC 0 V vs. Eocp) was carried out by exerting AC amplitude

of 10 mVrms (Fig. 8). The EIS spectra of each anodized

samples were modelled and simulated with equivalentelectrical circuit (EEC) to extract quantitative kinetic

information. Impedance response of 30, 60, 90, 120 and

150 min anodized samples could be associated with thesurface charge transfer (electrolyte/TNTs interface) and

mass transfer-controlled interfacial kinetic reactions within

the inter-tubular gap (TNTs/Ti interface). The best fit of theexperimental spectrum with the simulated EEC model was

achieved by iterative method and the results displayed may

Table 3 Electrochemical kinetic parameters evaluated from the potentiodynamic polarization scans

Time Eocp (V)SCE Ecorr (V)SCE Ecorr - Eocp (V) ba (V/dec) bc (V/dec) icorr (lA/cm2) ip (lA/cm

2)

30 min -0.39 -0.61 -0.22 0.25 0.10 0.48 15.86

60 min -0.34 -0.48 -0.14 0.22 0.12 0.85 25.22

90 min -0.332 -0.44 -0.10 0.23 0.13 0.78 22.25

120 min -0.32 -0.55 -0.23 0.24 0.12 0.87 26.98

150 min -0.30 -0.46 -0.16 0.23 0.13 0.85 30.11

Fig. 8 Nyquist plot of anodized samples obtained in 1 wt% NaClsolution. (Here time corresponds to anodization time and an ACpotential signal of 10 mV(rms) was applied at OCP within frequency100 kHz–10 mHz range)

Fig. 9 Equivalent electrical circuits (EEC) after a 30 min, b 60, 90and 120 min, c 150 min of anodization


123


contain ±0.04 X cm2 residual error in the reported values.

EEC model 1 (Fig. 9a) was used for 30 min sample’s EISspectrum and provided useful information regarding the

faradaic charge transport via charge transfer resistance

(Rct)and surface distribution of charge represented by theconstant phase element related with the double layer (Udl),

and nanotubular oxide film (Uf) and passive film resistance

(Rf). With the charge relaxation on non-homogenous sur-faces (like TNTs), the capacitance is replaced by the

constant phase element Udl and relaxation coefficient (n). Itis evident from the experimental results and has been

discussed above that with increase in anodizing time from

60 to 120 min the dissolution of inter-tubular region hasbeen enhanced. The ingress of ionic species through this

inter-tubular space towards the TNTs/Ti interface has been

simulated with the Warburg diffusion coefficient (W1),interfacial charge transfer resistance (Rin) and interfacial

charge relaxation phase element (Uin) in the fitting process

of data (Fig. 9a). The difference in EEC model 2 andmodel 3 (Fig. 9b, c) is the presence of additional Warburg

diffusion coefficient (W2). In case of 150 min sample, the

third time constant simulated by EEC model fitting of EISspectrum could be associated with the occurrence of

electrochemical redox reactions at the bottom of nanotubes

(at the interface of TNT/Ti). The larger inter-tubular gap asobserved in case of 150 min anodized samples which could

definitely provide lower Rin depicting higher activity for

water decomposition (reaction 4) followed by formation offilm at the substrate (reaction 7 and 8). The formation of

this thin oxide film (TiO2Cl2) at the interfacial area could

limit further reduction of water. The charge transferthrough this film would be diffusion controlled which

could be modelled by W2. The quantitative EIS data of

each sample after model fitting are provided in Table 4 andthe values were in confirmation with the potentiodynamic

results. The very large film resistance Rf

(73.6 9 1012 X cm2) registered by 30 min anodized sam-ple corresponded to large thickness of nanotubes, which

were well connected without having any inter-tubular gap.

The increase in W1 from 0.564 to 18.23 mS s0.5/cm2 by60–150 min anodized samples also suggested the ingress

of ionic species towards the interface (TNTs/Ti), which

could be directly related with the increase of inter-tubulargap (or decrease in tube surface density) due to increase in

anodizing time. The relatively larger interfacial resistance

(Rin) provided by 60, 90 and 120 min compared to 150 minwas in support to this behaviour. However, very low

(299.9 X cm2) resistance (Rin) provided by 150 min sam-

ple could be associated with the rapid redox reactionsfollowed by the depletion of redox species at the interfacial

region (within the inter-tubular gap). Moreover, in case of

150 min sample, the W2 (1.79 mS s0.5/cm2) correspondedto the mass transport-controlled reactions at the TNTs/TiT

able

4Kinetic

param

etersobtained

aftersimulatingofexperim

entalEIS

spectrawithEEC

models

RsX

cm2

UdllS

sn 1=cm

2n1

RctX

cm2

UflSsn 1=cm

2n2

RfX

cm2

W1mss0

.5/cm

2UinlS

sn 1=cm

2n 3

RinX

cm2

W2mss0

.5/cm

2

30min

28.52

42.43

0.89

6584

278.2

0.67

73.6

9101

2–

––

––

60min

19.75

15.0

0.84

6.92

40.20

0.91

1172

0.56

0.84

13133

–

90min

21.78

44.74

0.72

19.57

65.05

0.87

498

0.68

4.94

15396

–

120min

7.33

17.33

0.83

15.82

71.6

0.89

230.3

1.74

3.86

111700

–

150min

15.69

104.8

0.65

16.25

71.11

0.94

1301

18.23

48.049

10-6

0.93

299.9

1.79


123


interface and this was in support with the relatively large

icorr and ip values as calculated from potentiodynamicpolarization scans.

4 Conclusions

TNTs were formed on cpTi-2 in the organic solutioncontaining phosphates and fluorides ions by electrochemi-

cal anodization method. Effect of each electrolyte com-

ponent was studied using cyclic voltammetry. Thesynergetic action of H2PO4

- and F- species in the presence

of water in organic electrolyte could form water solublecomplexes. The controlled activities of species (like

phosphates and fluorides) in the electrolyte can be used to

tune the nanotubular morphology. The nanotubes ofdiameter ranging from *90 to *160 nm as a function of

anodizing time were produced. The maximum 4.6 lm long

TNTs were formed after anodizing for 150 min underoptimized conditions. From potentiodynamic scans, the

more negative Ecorr and increase in passive current density

(ipass) with increase in anodizing time could be attributed tothe accelerated water reduction capability of TNTs and

oxidation of ‘Ti’ substrate in the inter-tubular region,

respectively. The analyses of impedance spectra also val-idated the transport of ionic species within the inter-tubular

region of nanotubes produced after extended anodizing

periods. This behaviour was confirmed from the increase inthe W1 value from 0.564 to 18.23 mS s0.5/cm2. Moreover,

the relatively small resistance (Rin) (299.9 X cm2) and

existence of W2 (1.79 mS s 0.5/cm2) in the EEC modelused to analyse impedance spectrum of 150 min sample

corresponded to the occurrence of accelerated redox reac-

tions and depletion of ionic species within the inter-tubularregion.

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