chapter 7 ti/tio electrode prepared by anodizing method...

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CHAPTER 7

Ti/TiO2 ELECTRODE PREPARED BY ANODIZING METHOD

7.1 Introduction

The properties of metal oxide surfaces play an important role in the kinetics of

electrochemical reactions on passive electrodes. The desire to enhance the electrocatalytic

properties of passive electrode surfaces motivates research into developing understanding

of the crystallographic and the phase formation of the oxide crystal of the films. The

fixation of TiO2 onto a substrate is very important and some methods to fix the TiO2

particles have been developed. Many deposition methods have been employed in the

literature to fix the oxide film on the substrate such as thermal oxidation of titanium,

electron beam evaporation, chemical vapor deposition, plasma enhanced chemical vapor

deposition, reactive sputtering, sol-gel, spray pyrolysis etc have been reported to prepare

nanometer-sized particle powder or thin films on titanium substrate [1]. Among such

deposition methods anodization represents a simple and low cost method to synthesis

TiO2 by electrochemical oxidation on the surface of a metallic Ti substrate. Previously,

production of anodized titania has usually been associated with a decoration process,

where colours are generated by an interference effect caused by the thickness of the film.

Cavities and pores allow passage of ions through the oxide film [1]. Recently, Paulose

and co-workers have prepared TiO2 by maximizing the electrochemical oxidation and

controlling the chemical dissolution by reducing the amount of water in their electrolytes

[2]. A number of theories, based on field-enhanced dissolution [3, 4] and localised

acidification at the base of the pores which increases chemical dissolution [5, 6] have

been proposed to explain aspects of the growth mechanism leading to the growth of

particles. The approach of anodic oxidation to form porous titanium oxide films of

144

controllable pore size, good uniformity and conformability over large areas at low cost

constitutes a great challenge.

Anodic oxidation of titanium metal in acidic solution by applying electric current

between a Ti electrode and a counter electrode is an effective process to prepare a TiO2

film. The thickness and morphology of such TiO2 films can be easily controlled by

regulating electrical potential/current or chemical composition of electrolyte solution.

The high voltage anodization process has been intensively studied and applied to prepare

the crystallized TiO2 films without any thermal treatment [7, 5]. However, these anodized

TiO2 films usually have a detached hole structure at a scale of micrometers, resulting

from a violent electrolyzing oxidation reaction at a high anode-cathode voltage (usually

above 160v) far beyond a sparking discharge voltage (about 100v) [8, 9]. This present

study aims at developing a low voltage anodization process to prepare some well-

crystallized Ti/TiO2 films with a smaller pore size. We had carried out a low cost route

for the preparation of Ti/TiO2 electrodes by low voltage anodizing method. Aodization of

titanium substrate at different current density, temperature and time was carried out as per

details given in Table: 3.2 in chapter 3. The prepared electrodes were subjected to surface

morphology studies and phase study of TiO2 on titanium substrate by Scanning Electron

Microscopy (SEM) and X-Ray Diffraction (XRD). Based on these studies, the electrode

prepared at 60°C, current density 10mA/dm2 with 30 minutes time was chosen for further

corrosion studies and cyclic voltammetric studies. For the preparative electrolysis

titanium mesh was used to prepare Ti/TiO2 electrode by the same procedure. The aim of

this work is to provide further insight into the structure of anodized TiO2 films, using

information provided by SEM and XRD.

145

7.2 Surface morphology by SEM Analysis

SEM Micrographs for TiO2 films prepared at different temperatures, current

densities and time are given in Fig: 7.1 A&B. The figure shows the micrographs of

Ti/TiO2 electrodes by anodizing method at different current densities, temperature and

time. It is observed that specimen ‘b’ is similar to the pretreated Ti (Fig: 7.1 A, a) at room

temperature and at 10 mA/cm2 that the TiO2 film was not formed properly. By increasing

the temperature but with same current density and time interval the formation of TiO2

with a round pattern on the surface of the specimen can be detected (Fig: 7.1 A&B, c, d, h).

This is probably due to increase in temperature and the formation of secondary clusters

from the agglomeration of primary particles was observed. SEM micrograph (Fig: 7.1 B, f)

shows that TiO2 film was formed with a multiporous structure on titanium substrate. The

size of the particles was found to in the range of 113-261nm. These results demonstrate

that the specimen ‘f’ which was prepared at 600C 10 mA/cm2 with the time interval of 30

minutes as anodization time, produced much uniform TiO2 film than other micrographs,

which means the growth of TiO2 film can be well controlled by temperature and time.

These findings are in agreement with others research (10). It is generally believed that a

heterogeneous electrocatalytic reaction in aqueous solution mainly occurs on the interface

of the solid catalysts and dissolved compound molecules. Furthermore electrocatalytic

reactivity of a well-crystallized TiO2 film mostly depends on its mass-transfer,

conducting efficiency and on the solid-liquid contacting area. Some research has

confirmed that the increase of TiO2 film thickness does not always lead to a

corresponding enhancement of electro activity once a TiO2 film reaches a certain

thickness (10). At 600C nanocrystalline porous TiO2 film formed. The primary particle

size of TiO2 film is calculated using Scherrer equation [11] from XRD peak position and

146

full width half maximum (FWHM). The crystallite size of the thin films can be

determined from broadening of corresponding XRD peaks by Scherer equation. The

particle size of the film TiO2 obtained from SEM studies and the nature of the film are

tabulated in Table: 7.1.

Table: 7.1 Characterization of surface layers of TiO2 film prepared by anodization

Temparature (°C)

Current Density

(mA/cm2)

Time (Minutes)

Surface Thickness

(nm) Crystallinity

30 10 30 - No particle

40 10 30 - Amorphous

50 10 30 - Crystalline




70 10 30 172,202,254 Crystalline

Porous TiO2 can be formed at higher potentials [12, 13] due to the breakdown of

the oxide. (Fig: 7.1 B, e, f, g) show tubular structures of TiO2 on titanium substrate but

may not be well defined tubes. (Fig: 7.1 B, e) where the condition of the preparation was

at 600C, 10mA/cm2 (45 minutes) and more of tubular structures formed, by reducing the

anodization time (Fig: 7.1 B, f). But at the same temperature and current, tubular

structure reduced and by increasing the temperature (Fig: 7.1 B, g) agglomeration of TiO2

particles had taken place and thereby increasing the particle size.

147

(a) (b)

(c) (d)

Fig: 7.1 A SEM Micrographs for a) Titanium substrate and of TiO2 films on

titanium prepared by anodization at 10 mA/cm2 for 45 minutes at

b) 300C, c) 400C, d) 500C

148

(e) (f)

(g) (h)

Fig: 7.1 B SEM Micrographs of TiO2 films on titanium prepared by anodization at

10 mA/cm2 for 45 minutes at e) 600C, f) 600C at 10 mA/cm2 for

30 minutes, at 12 mA/cm2 for 45 minutes at g) 600C, h) 700C

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7.3 Microstructure analysis of Ti/TiO2 by X-Ray diffraction

XRD data has taken for the sample at different temperatures and at different

current densities and time by anodizing method are given in Fig: 7.2. The particle size

calculated using Debye-Scherer equation is tabulated in Table: 7.2. XRD results show

that the prepared TiO2 electrode is in good agreement with standard rutile type.

From the XRD data the microstructure of the electrode explains the effect of

temperature, current density and anodization time. Tang et al [14] measured the conductivity of

TiO2 films in both anatase and rutile phases, prepared by sputtering, and showed that anatase

films are more conductive than rutile films, irrespective of the preparation condition. As the

temperature increased keeping the current density the same and at less anodizing time (Fig: 7.2

B e) shows more of rutile phase formation. During the anodic oxidation of titanium, because of

quick cooling effects of the electrolyte a certain amount of amorphous TiO2 may be formed in

the film except for the formation of rutile TiO2. Increase in current density has led to the

formation of more of rutile TiO2. Besides the characteristics different peaks corresponding to

titanium substrate also appeared in the patterns because the film is thin and porous [15].

Tianium and rutile TiO2 are represented as T, R in Fig: 7.2.

To determine the crystallinity of the TiO2 films, the XRD patterns of the TiO2

thin films anodized at different current density, temperature and anodization time are

compared and analyzed. At low temperature and current density crystallization was less

but as the temperature increased both anatase and rutile phases appeared on the film

which indicates the formation of fine crystallites in the film (Fig: 7.2 A. b, c). As the

temperature further increased the phase transition of anatase as well as amorphous and

rutile been observed [Fig: 7.2 (A. d), (B. e, f, g].

150

f)

g) Ti

RA

TiR

Ti

20 40 60 80

Intens

ity (a

.u.)

2Θ (deg)

e)

R

R

RRA

TiR Ti

B

A

Fig: 7.2-A X-Ray diffractograms of TiO2 films prepared by anodization at 10

mA/cm2 for 45 minutes at a) 300C, b) 400C, c) 500C, d) 600C

Fig: 7.2-B. X-Ray diffractograms of TiO2 films prepared by anodization at 10 mA/cm2 at

e) 600Cfor 30minutes, And at 12 mA/cm2 for 45 minutes at f) 600C, g) 700C

20 40 60 80

Intens

ity (a

.u.)

2Θ (deg)

a)

b)

c)

A

d)

Ti

ARTiTiR

Ti

151

The process of formation of anodic oxide appears rather complex, yet a general

and simplified model of the chemistry involved is required to understand the process. The

outer anodic layer (partly exposed to the electrolyte) has an excess of hydroxyl ions

compared to the inner layer [16], and is considered to be Ti(OH)4. The inner layer, where

the de-hydroxylation of the film (water releasing) has occurred, is represented as TiO2. In

reality there is likely to be a concentration gradient across the film, which can be written

as TiO2.xH2O, to represent the inner (dry) and outer (hydrated) anodic oxide. The

reactions which occur at the anode are:

(i) oxidation of the metal which releases Ti4+ ions and electrons,

2Ti 2Ti4+ + 8e ¯ (Eq. 1)

(ii) combination of Ti4+ ions with OH¯ and O2– species provided by the water.

Equations (2) and (3) given below account for the hydrated anodic layer and the

oxide layer.

Further oxide is produced when the hydrated anodic layer releases water by a

condensation reaction, Eq. (4):

Ti4+ + 4OH¯ Ti(OH)4 (Eq. 2)

Ti4+ + 2O2- TiO2 (Eq. 3)

Ti(OH)4 TiO2 + 2H2O (Eq. 4)

At the cathode there is hydrogen evolution,

8H + + 8e¯ 4H2 (Eq. 5)

152

By summing the equations from (1) to (5), the overall process of oxide formation is

given:

Ti + + 2H2O TiO2 + 2H2 (Eq. 6)

The generation of oxygen at the anode, Eq. (7), has been previously reported as a

side reaction during the growth of barrier layer [17] and nano-tubular anodic titania [18]

2H2O O2 + 4e¯ + 4H+ (Eq. 7)

By anodization method the deposition of titanium dioxide on the titanium

substrate contains mostly of rutile phase of TiO2. But at very low temperature [Fig: 7.2 A. a]

more rutile and one peak corresponding to anatase phase appeared and the TiO2 film

appeared to be of having amorphous TiO2. As the temperature increased from 30 to 400C,

amorphous to rutile phase transfer occurred. The same results were observed in the XRD

pattern of the electrodes prepared at 400C, 500C and 600C at same current density and the

same anodization time. (Fig: 7.2. A: b, c, d).

The electrode prepared at 600C (Fig: 7.2 B. e) at the same current density but

reducing the anodization time more of rutile phase peaks and few characteristic peak of

anatase phase are also obtained (2θ = 25, 38, 45). This infers that by reducing the

anodization time phase transformation of anatase from amorphous occurred. But

increasing the current density and temperature (Fig: 7.2. B. f, g) show only rutile phase

and this may be due to anatase phase being unstable there might be the transformation of

anatase phase to rutile. Size of the crystal grain of anatase and rutile TiO2 in the film

catalysts calculated using Scherrer equation is tabulated in Table: 7.2.

153

Table: 7.2 Size of the crystal grain of anatase and rutile TiO2 in the film catalysts.

Preparation conditions Anatase Rutile

Current density

(mA/dm2)

Temperature (°C)

Time

(mts) 2θ (ο)

Full Width at Half

Maximum

(FWHM)

Grain Size (nm)

2θ (ο)

Full Width at Half

Maximum

(FWHM)

Grain Size (nm)

10 30 45 39.36 0.4297 185 69.89 0.6337 126

10 40 45 - - - 38.41 0.4349 183

10 50 45 - - - 37.87 0.4149 191

10 60 45 - - - 38.24 0.6167 129

10 60 30 39.79 0.2906 273 27,28 0.2628 302

12 60 45 - - - 38.01 0.4468 178

12 70 45 - - - 38.10 0.5726 138

Rutile phase is the most stable phase of titanium dioxide. On heating anatase, it

transforms irreversibly into rutile. The temperature at which the transformation takes

place varies from 400 to 1,0000C depending on the concentration of impurities in the

crystals or annealing atmosphere [19]. But an anatase phase of titanium dioxide has

higher electrocatalytic activity than rutile phase and the charge carrier in anatase thin film

[20] has higher mobility than that of rutile phase.

154

7.4 Corrosion studies

7.4.1 Tafel polarization technique

The Tafel polarization curves obtained for the electrode Ti/TiO2 prepared by

anodization method at 10 mAcm2 for 30 minutes at 60°C and titanium substrate are given

in Fig: 7.3, 7.4 and also the passivation graphs for the same are given in Fig: 7.5 and 7.6.

The various parameters calculated from Tafel technique are given in Table: 7.3.

Corrosion of the coating is influenced by several parameters, the most important being

thickness, porosity, presence of self-healing pores and high thickness, tend to increase the

corrosion resistance of the coatings. When TiO2 coating by anodizing titanium substrate

exposed in 0.1M H2SO4 shows corrosion resistance compared to the uncoated but the

difference is not much.

A thin and dense layer of TiO2 is always present on the surface of titanium, acting

as an anticorrosive film. The thickness of the film is proportional to the applied voltage

and the rate of corrosion declines as the applied voltage increases; however, gas escapes

and titanium becomes heated when the applied voltage is excessive, increasing the

corrosion rate [21]. The corrosion potential (Ecorr) and corrosion rate for the coated and

uncoated were calculated from Tafel polarization curves (Fig: 7.3, 7.4) and are reported

in Table: 7.3. The corrosion potential of the coated one had the negative shift, confirming

the cathodic protective nature of the coating.

155

Table: 7.3 Results obtained from Tafel polarization techniques

System studied

Passivation Range

(mA)

Ecorr

(mv vs SCE) Ip (A) CR (mmpy)

Ti/TiO2 (-0.6061) to (+0.3329) -0.731 -2.6374 11.858

Titanium (0.0955) to (+0.9943) -0.556 -2.8173 1.3

Fig: 7.3 Tafel polarization plot of Ti/TiO2 electrode in 0.1M H2SO4 prepared by

anodization at 10 mA/cm2, 600C for 30 minutes

Fig: 7.4 Tafel polarization plot of titanium substrate 0.1M H2SO4

156

Fig: 7.5 Passivation plot of Ti/TiO2 electrode in 0.1M H2SO4 prepared by


Fig: 7.6 Passivation plot of titanium substrate in 0.1M H2SO4

157

7.4.2 Electrochemical impedance spectroscopic studies

The AC- impedance curves obtained for the electrode Ti/TiO2 and titanium

substrate are given in Fig: 7.7, 7.8 and the various parameters calculated from this

technique are given in Table: 7.4.

Table: 7.4 Results obtained from electrochemical impedance spectroscopy techniques

System Studied Phase Angle (θ) Rct (Ω) RF(Ω) Icorr (mA/cm2)

Ti/TiO2 15 001 0.200 13.1852

Titanium 14 1.1 - 11.85

The charge transfer resistance, (Rct), The film resistance (Rf) corrosion current

(Icorr) corrosion rate (mmpy) and phase angle for electrode and titanium substrate in 0.1M

H2SO4 were calculated from Nyquist plots (Fig: 7.7, 7.8) are reported in Table: 7.4. The

results indicate that the corrosion resistance of the TiO2 film on titanium substrate had

been improved to small extent after anodizing. This slight improvement towards

corrosion resistance may be due to to the number of larger pores in the film and therefore

the rate of corrosion improved slightly and also as the particle size increases which will

lead to decreasing of electron transition and rate of electrochemical reactions and

adhesion of coating will decrease TiO2 particle size grows. In the electrode subjected for

the corrosion studies, the particle size calculated from XRD data using Scherrer equation

is in the range of 126-302 nm. When thickness of coating grows up and also brittle ability

of the TiO2 particle coating will increase. Stress may be increased on the coating by

158

applying high voltage during anodization process which proceeds with cracks and defects

in final coating. Therefore presence of defects and cracks in coating will increase the

corrosion rate of the coating [22] and also corrosion rate can be counteracted by the

presence of the greater porosity and roughness [23-25].

From the Table: 7.4 it is found that the phase angle value obtained from the Fig:

7.7 is 15 and the range of values for phase angle from 40 to70 indicates that the

impedance has a significant contribution in the corrosion resistance properties of the

electrode TiO2 [26] compared to the uncoated titanium substrate (phase angle = 14)

shows slight improvement alone towards corrosion resistance. A decrease of the

maximum absolute value of phase angle of the electrode is attributed to the decrease of

the corrosion resistance of the metal [27-31].

Fig: 7.7 AC-Impedance spectroscopic plot of Ti/TiO2 electrode in 0.1M H2SO4

prepared by anodization at 10 mA/cm2, 600C for 30 minutes

159

Fig: 7.8 AC-Impedance plot of titanium substrate in 0.1M H2SO4

7.5 Cyclic voltammetric analysis of Ti/TiO2 in H2SO4

Cyclic voltammetric experiments were carried out using Ti/TiO2 electrode prepared by

anodization of titanium substrate at 10 mA/cm2, 600C for 30 minutes. Fig: 7.9 present the cyclic

voltammetric behavior of this electrode in 1.0 M H2SO4. Basically any anodizing process will

lead to a very thin film of the respective metal oxide. Hence the formed film on titanium is very

thin film of TiO2 on Ti. A redox oxide thin film contains uniformly distributed oxidizing and

reducing species on a planar electrode surface. In may be assumed to be compared of a number

of such uniform monolayers. Each layer the total concentration of redox species may be

assumed to be constant. The number of such layers would be a few to 20000. Hence the total

concentration of redox species in the complete film is also a constant. Ti/TiO2 electrode,

prepared by anodizing process, resulted in a very thin film which consists of few monolayers of

oxide of titanium. The concentration of reducing species present in these monolayers is very

low. Because of this, there was not much redox behavior noticed in cyclic voltammetry.

However, even with this low concentration of these species (Ti4+), cathodic and anodic peaks

were visible, which are coupled with hydrogen evolution.

160

-1.0E-04

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

5.0E-04

6.0E-04

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

Cur

rent

(A)

Potential (V)

Fig: 7.9 Cyclic Voltammogram of Ti/TiO2 electrode in 1.0 M H2SO4 prepared by


7.5.1 Cyclic voltammetric analysis of Ti/TiO2 in H2SO4 with fumaric acid

On adding fumaric acid to the electrode, a marginal increase in cathodic current

was noticed from the cyclic voltammograms presented in (Fig: 7.10 A – C). However, the

anodic peak current also marginally increased corresponding to the cathodic peak. The

approximate peak potential difference, which was noticed from (Fig: 7.10.C) shows that

the catalytic process is a reversible one. Building up higher thickness of the film may

yield a well defined peak, which could prove the reversibility of the electrode.

From the cyclic voltammetric studies, it is clear that the surface redox process

occurring in these electrodes are reversible. But because of the very thin film formed well

defined anodic and cathodic peaks are not formed. This is also proved by very low yield

efficiency in the preparative scale electrolysis.

161

-2.0E-04

-1.0E-04

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

5.0E-04

6.0E-04

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

Cur

rent

(A)

Potential (V)

Fig: 7.10.A Cyclic voltammogram of Ti/TiO2 electrode in 1.0 M H2SO4 with 0.1mM

fumaric acid prepared by anodization at 10 mA/cm2, 600C for 30 minutes

-1.0E-04

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

5.0E-04

6.0E-04

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

Cur

rent

(A)

Potential (V)

Fig: 7.10.B Cyclic voltammogram of Ti/TiO2 electrode in 1.0 M H2SO4 with 0.2mM


162

-2.0E-04

-1.0E-04

0.0E+00

1.0E-04

2.0E-04

3.0E-04

4.0E-04

5.0E-04

6.0E-04

-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0

Cur

rent

(A)

Potential (V)

Fig: 7.10.C Cyclic voltammogram of Ti/TiO2 electrode in 1.0 M H2SO4 with 0.5mM


7.6 Preparatory electrolysis

The results of the preparative electrolysis for the reduction of fumaric to succinic

acid are summarized in Table: 7.5. The concentration of sulfuric acid is 1M. Fumaric acid

gets reduced to succinic acid at low current densities. The maximum percentage yield of

succinic acid obtained is 15% at 4.0 A/dm2 at 650C. At 1-5 A/dm2 the percentage yield is

very low but at 650C the yield is 15%. The electrolyte temperature has been varied from

45-750C. The attempt to get more yields by increasing the bath temperature also ended up

in getting only 15 % (Table: 7.5, Expt 9). Low yield was obtained using this electrode for

the reduction of fumaric acid to succinic acid.

163

The reason for low yield of succinic acid from fumaric acid using Ti/TiO2

prepared by anodizing method the TiO2 film resistance was low which was confirmed by

Tafel and AC-impedance spectroscopic studies. And the phase formation of TiO2 on

titanium was in rutile form which has less electrical conductivity than anatase phase.

Table: 7.5 Galvanostatic reduction of fumaric acid at a Ti/TiO2 electrode

Conditions: Amount of Fumaric acid taken : 10g

Electrolyte : 1M H2SO4

Anode : Stainless steel

Expt. No Current Density (A/dm2)

Temperature (0C)

Weight of Succinic Acid

(g)

Yield Efficiency (%)

1 1 55 0.17 1.7

2 2 55 0.25 2.5

3 3 55 0.69 6.9

4 4 55 0.89 8.9

5 5 55 0.86 8.6

6 4 45 0.15 1.5

7 4 55 0.89 8.9

8 4 65 1.50 15.0

9 4 75 1.50 15.0

164

7.7 Product analysis

The product succinic acid was isolated as it is given in chapter 4. The separated

crystals were subjected to the lH-NMR spectrum of the solid isolated in D2O. NMR

studies showed the result with a single sharp peak at 2.56 d corresponding to the four

protons of two methylene groups confirming the formation of succinic acid as the

exclusive product from the electro reduction of fumaric acid using Ti/TiO2.

7.8 Conclusion

The Ti/TiO2 electrode prepared by anodizing method at 600C, current density at

10 mA/cm2 and time 30 minutes, was uniform, having good porous structure and good

adhesion. Microstructure and morphology studies revealed that the electrodes having

crystalline TiO2 structure and resembles the rutile phase with (200) phase of TiO2.

Micrographs revealed the uniform crystallites spread over the entire surface with

minimum surface cracks.

The corrosion resistance of the Ti/TiO2 electrode is fairly good compared to Titanium

metal and this may be due to the uniform crystallite TiO2 present over the titanium metal.

However the electrodes have very low catalytic activity which was confirmed from the

cyclic voltammetric studies. It is clear from this study that the surface redox process

occurring in these electrodes are reversible. But because of the very thin film formed well

defined anodic and cathodic peaks are not formed. This is also proved by very low yield

efficiency in the preparative scale electrolysis.

165

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chapter 7 ti/tio electrode prepared by anodizing method...

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