Potentiodynamic behaviour of mechanically polished titanium electrodes

0

POTENTIODYNAMIC POLISHED

00134486/84 s3.00+0.00 ,984. Pcrgamon Press Ltd.

BEHAVIOUR OF MECHANICALLY TITANIUM ELECTRODES*

0. R. C~MARA, C. P. DE PAULI~ and M. C. GIORDANO lnstituto de Investigaciones en Fisicoquimica de Cdrdoba (INFIQC), Departamanto de Fisicoquimica, Facultad de Ciencias Quimicas, Universidad National de Cdrdoba, Sucursal 16, Casilla de Correo 61,5016

C&doba, Argentina

(Receiaed

9 January

1984)

Abstract-The behaviour of titanium electrodes mechanically polished and/or anodically polarized at low positive potential in solutions at constant ionic strength between pH 0.3 and 11.0 is reported. The oxide electroformation potential on a mechanically polished electrode shows a complex dependence on the bulk solution pH. This dependence is similar to that obtained through acid-base titration with titanium as the indicating electrode. The formation of hydroxo-complexes on the spontaneously formed titanium oxide offers a possible explanation for the oxide electroformation potential dependence on pH. Anodic and cathodic wide current peaks are obtained between the potential of the hydrogen evolution and that of the massive oxide electroformation; the corresponding redox system becomes evident at pH 4.0 from the first potentiodynamic cycle.An interpretation of these processes involving the participation of non stoichiometric oxides and hydrogen ions is attempted.

INTRODUCTION analysis of oxide films composition[l] from titanium substrates under different experimental conditions have shown that TiO, Ti,03, Ti,O, and TiOz can be present in the film depending whether the oxidation process ischemical (by air or in H2S04, HCl, HN03, NaCl solutions) or electrochemical (anodic polarization at low or high potentials). Electron diffraction and transmission microscopy[l, 2] of electrochemically formed oxide films have shown that the TiOz film was either amorphous, anatase with some rutile or microcrystalline anatase depending on the cell voltage. Similar results were obtained with thin films (‘- 250 A) by AES and XPS[3]. Ellipsometric or reflectometric measurements on thin films[4] have shown that some physical properties (refractive index, dielectric constant, etc.) were similar to those of bulk titanium oxide whereas the differences in the electrochemical characteristics were mainly due to the film thickness. The anodic behaviour of titanium electrodes in concentrated chlorides at different pH was interpreted by a reaction formalism with an oxidation step on the base metal yielding adsorbed Ti(ITI), followed by parallel reactions given Ti(II1) and/or Ti(IV) in the solution[5,6]. This reaction scheme differs from that where a distribution of different oxide species in depth was proposed[3]. Results obtained by SIMS[7] demonstrated an island surface distribution of the different oxide species. Ti(III) and Ti(IV) species were also observed

The

* This paper is based on the communication No. B 33, presented at the 32nd ISE Meeting Dubrovnik/Cavtat, Yugoslavia, September 1981. t Departamento de Quimica, Facultad de Ciencias Agopecuarias, Universidad National de Cdrdoba.

by rde and rrde[S] techniques in fresh titanium surfaces. Electrochemical experiments, faradaic impedance and ellipsometric measurementsC9, lo] were interpreted through reversible reactions involving hydrogen ions in the reduction of Ti(IV) in the bulk of oxide phase. In the present work, the electrochemical response of titanium electrodes covered by a very thin titanium oxide film spontaneously formed in air or by potentiodynamic or potentiostaticanodic polarization up to 3.0 V (us see) in Na,SOI solutions at different pH was investigated. The cathodic potential limit comprises the potential region where hydrogen ions as well as titanium oxides are involved in the redox process.

EXPERIMENTAL A conventional one-compartment Pyrex glass electrolysis cell provided with a 45” side tube to locate the working electrode, was employed. A saturated calomel electrode placed into a glass jacket provided with a Luggin type capillary was used. The counterelectrode was a large Pt foil. Solutions were prepared from 0.5 M Na2S0, to maintain a constant ionic strength. Either HISO or NaOH was added to regulate solution pH (0.3 < pH & 11.0). Solutions were carefully deoxygenated with purified nitrogen (99.99 %) before use. The working electrodes were titanium cylinders (Alfa 99.9%) of either 6 or 8 mm diameter axially embedded in Teflon cylinders and cut with an angle which gave a perpendicular active surface for free gas evolution. The electrode pretreatment consisted of a mechanical polishing with diamond paste 6 to 12 pm SUSpended in ethylicalcohol and Al,03 1 to 0.05 /nn. This

1111

0. R. CAMARA, C. P. DE PALJL~AND M. C. GIORDANO

1112

method gave the most reproducible results[ll]. After polishing the titanium electrode was cleaned with acetone and dried in hot air before use. Potentiodynamic i/E profiles were obtained with different E/r perturbation programs. A Wenking LB 75M potent&tat and a wave generator LYP GF02 were used. Potentiodynamic profiles were recorded with a XY Omnigraphic 2000, Houston Instrument. Temperature was kept constant at 25 f 1°C. All potentials in the text are referred to the saturated calomel electrode (see).

RESULTS Conventional

potentiodynamic

profiles

at d$erent

pH

Figure 1 shows typical first cycle i/E profiles at 0.1 V s- ’ obtained with a mechanically polished titanium electrode at different pH. The potential sweep was started from the initial potential (Ei) at each pH towards the cathodic limit potential (El,,). Ei is the open circuit electrode potential of a mechanically polished electrode just immersed in the working solution. This potential turns more positive with the open . I clrcult tune. EA., was preset at different pH to avoid hydrogen bubbles on the electrode surface. In the positive going sweep at E more positive than E, an anodic current overshoot followed by a limiting current region (III,) of 0.40445 mAcm-* is observed. This potential region would be denominated III, in the next text. A small and constant current was recorded in this potential region in the subsequent sweep as already reported after massive titanium oxide electroformation[12]. The onset potential values (E,,,,) at the current region III, defined as the intersection of the tangent to the ascending branch of the i/E profile with the potential axis for the first cycle shows a complex dependence on pH (Fig. 1, right bottom). In the same This figure the Ei dependence on pH is also shown. plot exhibits three regions, namely at either pH ( 2.5 or pH > 9.0, the slope AE,,,,,/ApH is negative (AE/ApH u -0.1 V/pH) and in the 2.5 < pH c 9.0 independent on pH. E OnSerbecomes practically

-1.0

0

E/V

1.0

Acid-base titration curves performed with a mechanically polished titanium as the indicating electrode (Fig. 2) shows the complex pH/E behaviourdepicted in Fig. 1. There is a net difference in the titration characteristics between a mechanically polished electrode and a passivated titaniumelectrode that has been polarized up to 3.0 v. The changes of the i/E profile obtained after a potential holding at En,, during a certain time Z with a mechanically polished electrode are shown in Fig. 3. E Onlef shifted towards more negative values after the potential holding. This effect is pH dependent since E 0n9ft shift was significant in acidic solutions while in alkaline solutions E,,, at Z = 0 is almost coincident at Z=5min. with E,,,, The increase of El o from 0 to 2.0 V in a potential stepwise repetitive cydling produces the opposite effect [ 121,ie -%,,,,shifts to the positive side during cycling nd 1 El,,,(,_i, nd) and the i/E slope for the (E,,,,,, rising part of zone III, increased. A partial recovery of the oxidation charge in zone III, and the shift of E,,,,,

1

Fig. 2. Acid-base titration curves performed with a titanium electrode as the indicating electrode. o Mechanically polished titanium (spontaneous oxide) and stabilized in the same solution. l Potentiodynamic electroformed oxide up to 3.OV (US ser) at 0.1 V 5-l in pH 9.5.

- 1.0

’

E/v

Fig. 1. Potentiodynamic

i/E profiles obtained with mechanically polished electrodes at different pH. First cycle. Right bottom: initial potential (.I$) and onset potential (E,) dependence on pH.

Fig. 3. Modifications produced by the inclusion of a holding time (z) at E,., on the i/E profiles at different pH for a mechanically polished electrode.

Behaviour of titanium electrodes

towards the negative side after passivation of electrode with two potential sweep cycles (E,,, = l.OV) can be obtained in acidic solutions (pH = 2.0) by holding the potential at EL,c (Fig. 4). This effect is magnified with a polarized electrode at 1.0 V and kept at open circuit before the potential holding at EA.,. For a particular set of some experimental conditions a redox system which insinuates as a cathodic shoulder (II,) cu. - 0.8 V and a wide anodic current peak (II,) is observed (Fig. 5). The contributions of current peaks 11, and II, depend on pH, EL,=, El,+ and the number of repetitive cycles. The better defimtion of both current peaks II, and 11, was found at pH 4.0 already from the first cycle. In the same figure the i/E profile at pH 8.0 is

Fig. 4. Modifications produced on the i/E profile for a previously potentiodynamic passivated electrode (first and second cycle) by combining a time at open circuit (Z,) and a potential holding time at E,+, (2,).

Fig. 5. Differences between the

1113

shown. At this pH the potential in the zone III, is necessarily applied to obtain a definition of current peaks &II,. The pH value 4.0 was chosen for further study.

Results

obtained

at pH 4.0

Figure 6 shows the i/E profiles of titanium electrode run between - 1.6 and -0.35 V potential limits at different positive (or negative) and constant negative (or positive) potential sweep rates (v). EL,, was switched before massive oxide electroformation (zone III.,). A lineal dependence of current peak II, with “lit was obtained, while current peak II, varied linearly with u[l I]. The anodic charge (Qn) was ea. 0.7 mCcn_’ and independent of u while the cathodic charge (QH,) decreased as v increased. QIz varied linearly with u- 112. The Q,I,/Q,L ratio approached to unity at high L;.The peak potential (E,) for both peak II, and peak II, either remained constant or shifted in the positive direction as u increased. The variation of E, on u also considerably depends on the perturbation program applied to the electrode (El+., El,., only one cycle or repetitive cyciing, etc.). Both EPI, and E,,, were independent of u when a symmetric’triangular apotential perturbation was used. For E,, = - 1.4 and E,, = 0.0 V values a slope of 0.08 V de+- ’ u for E,,, was obtained from the E, us log v plot when the ne&ive potential going sweep rate was kept constant and different u were applied in the anodic potential sweep. The slope decreased to 0.012 V dec- ’ t’ when El,, = - 1.6 and E, (1= -0.35 V (Fig. 6). A new conjugated redox system {peaks I, and I,) at cu. - 1.4 V were observed at low u when El,, = - 1.6 V (Fig. 6). Peak I, disappeared at high U. Stirring of solution allowed the definition of this peak at high v (Fig. 7), while peak II, remained practically unaltered, &varied linearly with vi/‘, the slope of the EPl, us log u plot was - 0.07 Vdec- ’ and QL increased linearly with v- 112

first and the second potentiodynamic i/E profile at two pH showing redox processes li, and II,. (a) pH 4.0. (bj pH 8.0; first and second cycle.

1114

0.

R.

C~MARA,

C.

P.

DE

PAULX AND

M. C.

GIORDANO

0.8

Fig. 6. Dependence of the 11,-H, current peaks on potential sweep rate. (a) Constant positive potential sweep, (b) constant negative potential sweep. pH 4.0.

N

5 0.0 -

-3

E --_

o-

-0.2 -0.4 -1.5

-0.4 -

-10

-05

E/V

0

Fig. 8. Modifications observed on the potentiodynamic stabilized profiles at pH 4.0 by employing different Tat E,,, after electrode passivation. - 0.6 -

- 0% -

-1.5

-1.0

- 0.5

E/V

Fig. 7. Effect of solution stirring on the cathodic profile at different u. ~ Without stirring, --~-with stirring. pH = 4.0.

Intuence

of holding times at E,, e

Figure 8 shows potentiodynamic i/E profiles obtained with a potentiodynamically stabilized titanium electrode followed by different potential holdings at E +c = - 1.4 V. An anodic current increase was obtamed for Z -C 30 s while thecathodiccurrent remained almost constant. An inhibition for both peaks II, and II, and an increase of peak I, were obtained for Z

> 30 s. These effects were followed by a considerable increase in the hydrogen evolution current. Similar effects were observed starting with a mechanically polished electrode and including a potential holding for Z = 30 s at El,, between repetitive cycles (Fig. 9a). This produces as an accumulation of reduction products, but the i/E profile was similar to that obtained in Fig. 8 after a certain number of cycles. These effects were also encountered with a mechanical!y polished electrode by increasing Z (Fig. 9b). The shift of E,,, towards more negative values was also evident. The cathodic i/E profile was not appreciably changed by the potential holding at EL. c in the oxide formation potential region as only a small increase in peak II, was found. This effect is probably related to changes in the actual electrode area and not to the total amount of electroformed oxide. The greatest differences in the voltammogram are observed after a 2 h potential holding at En, 11= 3.0 V (Fig. 10). A i/E profile obtained with a mechanically polished electrode and with a potential holding at - 3.0 V for 20 min is included in the same figure. The

Behaviour

B

0.4

of titanium electrodes

1115

,

Fig. 9. Effect produced on the potcntiodynamic profiles by: (a) including a constant z between repetitive cycles and (b) different 2 at El,, on a mechanically polished electrode each time, pH = 4.0.

Fig. 10. Potentiodynamic after different electrode polished electrode with a min. -- - - Mechanically holding at

i/E profiles as pH 4.0 obtained pretreatments.-Mechanically potential holding at 3.0 V for 120 polished electrode with a potential -3.0 V for 20 min.

changes of this last pretreatment are gradually reverted by repetitive cycling between E,, e = - 1.4V and E* d = 0.0 V (Fig. 11). The recovery of a profile with peaks IL and 11,was independent of El, L1 for E,, L > - 0.5 V. For E,, ,z< -0.5 V, current peaks II, and Ik are not evident. A considerable recovery of anodic charge was found by repetitive cycling between El, c and - 0.55 V on an electrode previously electromechanically passivated as previously reported by combining potentiodynamic and ellipsometric experiments[13]. DISCUSSION The potentiodynamic i/E profiles obtained at difpH show a characteristic limiting current region

ferent

-0.6[ ’ -1.5

-l.o

-Q5

E/V

01,

Fig. 11. Recovery of the original potentiodynamic profile by repetitive potential cycling between - 1.4 and 0.0 V after the electrode has been held at - 3.0 V for 20 min. pH = 4.0.

from

EO”,,that corresponds to massive titanium oxide electroformation. The passivation in this potential region is readily found by stepwise increasing the anodic potential or after cycling on the EL.c-E,,., range. The oxide film electroformation on a mechamtally polished titanium electrode does not take place on a bare titanium surface since a chemically formed oxide film is always present on the metal surface[9, 14, 15-J.The thickness of the spontaneous film was calculated in 2%30A by ellipsomettic measurements[ 1l], with a growing rate of 25 A per volt under positive polarization at pH 4.0. Nevertheless, the acidbase response of the spontaneous film is different from that of the electroformed oxide film at low anodic

0.

1116

R. C..&ARA, C. P.

DE PAULI AND M. C. GIORDANO

potentials. It is known that crystalline titanium oxides[16] form hydroxo-complexes by chemisorption of water molecules. The hydroxyt groups on the metal oxide surface show amphoteric properties, they may either dissociate protons from bound water in the coordination sphere or add protons to hydroxide ions:

[M”+(OH).+, (H201,,,-II- +H+,

(1)

and CM”+ (OH), (H,O),]

+ H

l

+

CM”+ (OH),-,

W20)m.,l+.

(2)

From Fig. 1, a zero charge point (zpc) on the surface can be assigned around pH = 4.0 where an abrupt potential change is observed with the mechanically polished titanium. This coincides with the zpc of crystalline TiO, (between pH 4 and 6)[ 17, 181. At this pH the greatest titanium passivation is achieved. The electroformed thicker titanium oxide behaves as a reversible acid-base oxide electrode[l9]_ The gradual shift of E, towards more positive values at open circuit indicates a spontaneous passivation of the titanium electrode at all pH. Nevertheless, the response of the spontaneous film to a potential perturbation is slightly different from that of an electroformed film. This effect is clearly observed after holding the potentiai at EL, c. There is a shift of E,,,,, towards more negative values with both the spontaneous and the electroformed titanium oxide films, but the explanation given to both processes is rather different. A local change of pH at low pH is probably the best explanation for the Eons, shift obtained with the spontaneous film, since E,,,,, remains constant after potential holding in alkaline solutions. The decrease in thickness and the conductivity increase should be related to the mechanism of oxide electroformation as reported by ellipsometric experiments[13]. On the other hand, the oxide film electroformed at low pH is less stable than the film electroformed at ing is enough to when the oxide solutions (pH = gen ions into the

pH > 4.0. A slight mechanical polishrecover the original titanium surface has been electroformed in acidic 2.0). Probably the inclusion of hydro-

titanium oxide at low pH would give a rather porous and hydrated film more loosely bound than the film obtained at higher pH. Hydrogen ions participation in the redox couple 11,-H, is evident from the results obtained at pH 4.0. Processes given peaks H-11, are shown at the highest pH (8.0) after some titanium oxide has been electroformed and the local pH has been decreased considerably. These processes are related to the amphoteric properties of the titanium oxide film. An anodic current peak similar to peak II, obtained at pH 4.0 has been previously reported under steady state conditions[20, 211. This peak was explained as titanium dissolution at low pH and oxidation of low oxide states present in the spontaneously formed fllmE6, 8,221. The existence of peaks II. and II, in the experimental conditions used in this work would indicate that the corresponding redox processes should involve either some non stoichiometric oxide species or lattice imperfections present in the oxide film or at the film surface[9, IO, 231. Thus, an oxidation

process taking place in the oxide bulk can be inferred from the independence of i,t,. with v. The amount of non stoichiometric

oxides

obtained

within

the elec-

troformed oxide film depends on E,, c and E,, a and on

the solution pH. Peak II, depends on vi” indicating a diffusion control of the species participating in the redox process. The potential related to peak II, is close to the reduction potential of TiO, yielding either Ti203 or TiO(OH) depending on the TiOz film hvdrationll. 4.91. The diffusion coefficient value ob10-11 cmzs-I tained from the slope of iprt, z;s v liZwas while the value obtained from potentiostatic steps was film thickness of with a lo-l2 cm2 s-i of 100 A[24,25]. The order of magnitude of the diffusion coefficient is very close to that previously reported for hydrogen ions diffusion inside an oxide-hydroxide film and for atomic hydrogen diffusion in metals[26,27]. The rotating disc electrode experiments show that ip,,< does not change with w. This suggests that solid phase diffusion is the controlling process. On the basis of the possible formation of non stoichiometric oxides in the film surface when the thickness of the latter is estimated between 4 and 7 layers, the following reaction scheme can be advanced to account for the redox processes related to peaks

II,-II, :

TiO,+H,O*

+e $TiO,

1(OH) (H,O)

(3)

with 1.5 < n $ 2. The H30+ denotes a hydrated hydrogen ion. The new set of cathodic and anodic current peaks I, and I,, that appears after repetitive cycling between E, c < - 1.4 V and El, ~ = 0.0 V can, in principle, be assigned to hydrogen redox reactions on the oxide film surface influenced by the 1ocaI pH changes produced by the repetitive potential cycling. The influence of the local pH changes in determining surface oxide film structures and defining redox processes I and II is evident after potential holding at either E,, c or El, ~. At low cathodic E, and for short Z an increase of the conductivity of the oxide film is produced. As z increases there is an inhibition of the oxidation processes related to peaks II_ and II, probably due to the increase of local pH that incorporates hydroxide ions in the oxide lattice. The maks II_ and IL are evident after local and inside oxide pH re&oration-take place. The small charge involved in peaks II, and II, suggests that only a small fraction of the total titanium oxide film which is made of the spontaneous oxide plus an oxide electroformed at low positive potentials, contains non stoichiometric oxides which are oxidized to the more stable and non-reducible TiO, oxide. was sponsored by Acknowledgments-This research CONICET, SUBCYT, CONICOR and Universidad National de C&d&a of Argentina.

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