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Hydrogen evolution reaction on anodic titanium oxide films
HYDROGEN
EVOLUTION REACTION ON ANODIC TITANIUM OXIDE FILMS* R. M. TORRESI,~. R. C~~ARA,C.
P.DE
PAULI
Instituto de Investigaciones en Fisicoquimica de Cbrdoba (INFIQC), Departamento de Fisicoquimica, Fat. de Ciencias Quimicas, Universidad National de Cbrdoba, Sue. 16, C.C. 61, 5016 Grdoba, Argentina and
M.C. lnstituto
de lnvestigaciones
GIORDANO
Fisicoquimicas Te&icas y Aplicadas (INIFTA), Plats, Sue. 4, C.C. 16, 1900 La Plata, Argentina
(Received 4 October 1986: in revised form 29 January
Universidad
National
de la
1987)
Abstract---Oxide titanium films were prepared using linear potential sweeps up to different limiting potential values between 0 and 60 V in 0.5 M H1S04. The electronic characteristics of these films were investigated through impedance measurements during their formation. The hydrogen evolution reaction (HER) in either HCIO, or H,S04 solutions in the pH range between 1 and 4 was studied on the different oxide films. The obtained experimental results could be explained assuming that the recombination of adsorbed hydrogen atoms is the rate determining step (rds), and that the Temkin type isotherm applies to the adsorbed intermediates. T’he log i us E plot after correction for diffusion control in the bulk gives a slope of 2.3 RT/F. The influence of the physicochemical properties of the oxide films on the HER has also been studied from the changes in the current-potential profiles. The oxide film thickness changes the HER overpotential, but the slope of the log i vs E relationship remains independent of it.
INTRODUCTION Metallic titanium and its oxides have been widely studied in the last years due to their recent applications in the industry, either as structure materials or as substrates for electrosynthesis, or for conversion reactions[l], as well as for use in solar energy[2]. This metal has a great resistance to corrosion because of the spontaneous formation of a thin oxide film on the surface. A thicker oxide film can be obtained by thermal[3,4] or electrochemical methods. The latter are mainly carried out through: (i) anodic current steps, (ii) anodic potential steps and (iii) a linear anodic potential sweep. In case (i) films of different morphological and structural characteristicsC5, 63 are obtained depending on the final anodic potentials. In case (ii) the results obtained show three potential regions where oxide films of different characteristics are obtained depending on whether electronic or thermal breakages[7-101 take place. In case (iii) stoichiometric films of homogeneous thickness[l l] are obtained at potentials lower than 10 V. The films formed at potentials higher than 50 V showed microfissures as they were observed by impedance and photocurrent measurements. A massive rupture joint with oxygen evolution is produced in the reverse sweep depending on the sweep rate[12].
Although several authors consider that these films show great stability under cathodic treatment, a decrease in their thickness occurs. This fact can be explained through the oxide dissolution[ 131. The HER takes place in the negative potential range on metallic titanium as substrate. Nevertheless, the results are not coincident with regard to the rate determining step, the adsorption isotherm followed and the presence or absence of an oxide film during the HER. High values in the Tafel slope have been explained in terms of a dual barrier model[1&17], while other authors have taken into account either the presence of an oxide film[18] or of oxy-hydroxy adsorbed species at the sub-monolayer level[19, 203. From these results no definite conclusions can be obtained regarding the effect of the oxide film on the HER. The aim of this paper is to investigate the HER on the electroformed oxide films of different thickness and electronic characteristics to get further insight into the influence of the oxide film properties on the hydrogen reaction.
EXPERIMENTAL Titanium
electrodes
The working electrode was made from a titanium Alpha rod (Koch-Light Ltd. 99.98 %) fixed in a Teflon holder; the exposed surface was a disc of 0.50 cm2 geometrical area. At the beginning of each electrochemical experiment the electrode was etched in a
l This paper is based in part on the poster 05200 presented at the 36th ISE Meeting, Salamanca, Spain, September 1985.
1291
R. M. TORRESI ef al.
1292
1:4:5 mixture of HF (48 %), HN03 (65 %) and water[21, 227 for 40 s and rinsed with purified water. The etching procedure causes the roughening of the surface. Taking as reference a roughness factor r = 1 for the electropolished electrode[22-241, we estimated the values for chemically polished electrodes from coulometric and capacity data to be 1.3[25], in good agreement with the results reported by Young[26]. The oxide tilms were obtained by applying anodic potential sweeps up to different final anodic potential (E, ) values in 0.5 M H$O, solutions in order to obtain different thicknesses. During the oxide electroformation the electrode was rotated (1000 rpm) to avoid oxygen bubble formation on the surface. Cell and electrolytes The electrolysis cell consisted of two Pyrex glass compartments of about 300 cm3. A platinum sheet of about 1Ocm’ geometric area was used as counter electrode. All solutions were prepared from a.r. chemicals and purified water (Milli Q system, Millipore Corp.). The electrolytes HC104-NaC104 and H2S04-Na2S04 were used in suitable concentrations to give a molar ionic force of 0.35 M. The solutions were deaerated by purified nitrogen previous to the electrochemical measurements. Saturated calomel or reversible hydrogen reference electrodes were used; however, all the measured potentials are hereafter referred to the standard hydrogen electrode (she).
The electronic equipment The experimental results of the oxide film electroformation and the HER were obtained by employing the voltammperometric technique with either simple or repetitive linear sweeps, current or potential pulses and rotating disc electrode (rde). The electronic equipment was the following: a potentiostat/galvanostat (EG&G PARC 173), a wave generator (EG&G PARC 175) and a X-Y recorder (Hewlett-Packard 7004B). In order to obtain capacity values of the electrode, some impedance measurements were performed under potentiostatic and potentiodynamic conditions with a 1 kHz and 0.7 mV signal superimposed onto the potential, and then, the phase shift was measured by means of a lock-in amplifier (EG&G PARC 124A) with a differential pre-amplifier (EG&G PARC 116).
RESULTS
Electroformation
AND
DISCUSSION
and characterization
of oxidefilms
The growth of thin oxide layers on titanium through electrochemical methods is known to take place irreversibly at more anodic values than the rest potential (E,,) according to the following reaction[27, 281: Ti + 2H,O
e
TiO,
+4H+
f4e.
E” = - 0.86 V.
Titanium is a typical valve metal and its electrochemical behaviour when the oxide is grown can be seen in Fig. 1, which shows simultaneous current (Fig. la) and capacity (Fig. 1b) measurements as a function of the potential under triangular perturbation in
E/
V
Fig. 1. Potentiodynamic oxide formation on titanium. (a) Current density-potential and (b) electrode capacity-potential profiles for different Ez values in repetitive cycles.
0.5 M H,SO+. The potentiodynamic sweep started at E reS1= -0.35 V in the positive direction and ended at increasing potentials (E/) up to 8 V. Detailed results of cathodic measurements at negative potentials are shown further on. It is observed that the anodic current reaches an almost steady value in each repetitive sweep according to the high-field mechanism of oxide film growth[29, 301. Nevertheless, the steady anodic current increases at potentials higher than 3.0 V due to the simultaneous electroformation of oxygen on the electrode. When this anodic potential sweep is reversed the current hastily decreases down to almost zero and the thickness of the oxide film remains constant during the cathodic and the next anodic sweep as long as the positive potential limit is not higher than the EJ value of the previous cycle. This behaviour is characteristic of valve metals. The overall charge involved in the anodic process is
Qtot = Qo, + Qdis + Q,,,
(1)
where Qo, corresponds to the charge involved in the oxygen evolution reaction, Q&i. corresponds to the metal dissolution to give ions in solution and Q, is the charge taking part in the oxide layer formation. According to the experimental conditions employed in this paper, Qdir can be considered negligible whereas Q, and Qo, are dependent on EY _ The increase of the oxide film thickness depends on the E, according to[9,31] Ad = u,E,,
(2)
where IX,, the anodization coefficient, depends on the different potentiodynamic formation conditions, the temperature and the electrolyte. The overall charge Q,,, varies linearly with Ef in the potential range between E,, and 3.0 V since Qo, is negligible and then Qtot = Q,,. Thus, the increase in film thickness can be stated as Ad = M Q,,lr
F a,,,
(3)
where M, 6,, and z are the molecular weight, the density and the number of exchanged electrons of the formed film, respectively. The M/z F 6,, ratio for the
Hydrogen
various crystalline structures of the titanium oxide has already been reported[31]. From equations (2) and (3) we obtain, (4)
Qo,=kE,,
where k depends on the anodization coefficient and the oxide film characteristics. The experimental dependence of Q,, on E, shows a linear variation up to 3.0 V, and from this value the simultaneous oxygen evolution produced positive deviations. Thus, the increase in thickness of the oxide film can be calculated from Q,,, only when E, < 3.0 V. The slope of the Ad us E, plot (Fig. 2) gives the anodization coefficient which decreases with the potential sweep rate assuming that a,,, remains constant. The cc, values obtained from these plots are shown in Table 1. The variation of the differential capacity with the potential in the anodic sweep (Fig. 1) is typical of the behaviour of a parallel plane capacitor with variable thickness values. During the reverse potential sweep it behaves as a surface film with a constant thickness whose characteristics correspond to those of a n-type semiconductor[32]. The thickness of the oxide film for E, > 3.0 V can be calculated from impedance measurements. In this case,
the anodic oxygen evolution reaction can be disregarded due to its high charge transference resistance, which is typical of this type of reaction on insulating oxide films. Therefore, the simple capacitor formula can be applied to the oxide films according to the following equation, C,’
V/V&
0.005 A 0.025 I 0.050
E F B
Fig. 2. Dependence of the thickness increase on the final anodic potential for different sweep rates.
Table 1. Characteristics of Ti02
E,(V)
d (nm)
a. (run V-‘)
(5)
which predicts a linear dependence of CL ’ with E, assuming that a, and E, do not change in the potential range studied. A plot of C; 1 us E, (Fig. 3) shows a linear dependence for different sweep rates up to ca E, = 10.0 V. In this way, Ad can be obtained from this type of measurements for E, > 3.0 V if 01,, E, and do are known. The values of E, and d, can be calculated from C/i us Ad plots in the E, c 3.0 V range. ln order to obtain the C,, value for this procedure it is necessary to know the Helmholtz layer differential capacity, C,, which is calculated further on. The do value was cn 1 nm, in good agreement with those obtained by another author[21], and the &,value so obtained depends on the potential sweep rate assuming that a,,, remains constant. Changes in the dielectric constant value with the electric field applied have already been reported by other authors[33-351. Thus, it is possible to obtain from equation (6) the oxide thickness formed in the potential range where the oxygen evolution reaction occurs. The electroformation of these oxide films which are thicker than about 11 nm was performed by potential sweeps up to E, = 60Vwithv=9Vs-‘.Thissweep rate was chosen because the electronic rupture of the film does not occur in the reverse sweep[l2]. After the oxide film was formed one cycle of potential sweep at
.
v(Vs_1)
+c,‘,
where C, is the experimental differential capacity is the differential capacity of the measured, C, Helmholtz layer at the oxide-electrolyte interface and C,, is the differential capacity of the oxide film. Taking into account that C,, = e’e,/d and that d = do + Ad, where co and e,are the dielectric constants of vacuum and the semiconductor oxide film, respectively, d, is the thickness of the oxide film present initially, the equation (5) can be rewritten as,
1
1 O-
= C,’
Ely:
oxide films
E’, (V)
E,
0’)
No (cm-‘)
Lo (nm)
0.005
1.0
0.005 0.005
3.0 9.0
3.90 12.50 37.00
4.10 4.10 4.10
68 68 68
-0.64 -0.32 -0.26
-0.30 -0.30 -0.26
1.46 x 10” 2.51 x 10’9 8.26 x 10”
0.81 1.98 3.42
0.025 0.025 0.025
1.0 3.0 9.0
3.60 11.30 34.00
3.75 3.15 3.15
57 57 57
-0.65 -0.28 -0.30
- 0.30 -0.28 -0.30
1.47 x 1020 2.17 x lOI9 3.22 x 10”
0.74 1.93 5.02
0.050 0.050 0.050
1.0 3.0 9.0
3.30 10.40 31.00
3.41 3.47 3.47
50 50 50
-0.50 -0.31 -0.30
- 0.30 -0.31 -0.30
1.34 x 1020 2.83 x lOI9 2.50 x 1O’a
0.73 1.27 5.33
10.30 40.00 85.yx)
1.90 1.90 1.90
30 30 30
-0.33 -0.30 - 0.30
- 0.30 -0.30 -0.30
8.90 x 10” 1.50 x 10’8 5.17 x 10”
2.19 5.33 9.08
9.0 9.0 9.0
5.0 20.0 45.0
(M/z F b,,) = 5.37 x lo-” d, = 1.0 nm.
cm’ C-t
[31].
1294
R. M. To~arst et al.
01
I 50
I
2.5
0
I
I 7.5
10.0
Ef / V
Fig. 3. Dependence of the rtiprocal electrode capacity on the final anodic potential for different sweep rates.
0.01 V s-r up to 8.0 V with a small sinusoidal superimposed wave was performed in order to determine its physico-chemical characteristics from capacity measurements. Figure 4 shows the i-E (upper part) and C-E (lower part) profiles for different oxide film thickness previously electroformed. A small current density is observed in this potential range, which means that no changes are produced on the film when the capacity measurements are performed. The C-E profile corresponds to the reverse potential sweep and shows that for E > 5.0 V the capacity of the oxide film is independent of the potential. This fact indicates an insulating behaviour for the oxide film, and then it is
possible to calculate the oxide thickness from the capacity data measured up to 5.0 V. Thus, the oxide dielectric constant for thicker films could be determined from electrode impedance measurements. The Q,,, results and their corresponding Ad values calculated from equation (3) are plotted us E, for this type of experiment and a linear behaviour up to E, = 20 V was observed, which means that in the potential range involved the anodic current for oxygen evolution reaction is negligible at these sweep rates. The a. value in the given experimental conditions was 1.9 nm V-l. On the other hand, in Fig. 5 can be seen that a plot of C&‘os E, gives a linear relationship up to E, = 50 V, which leads to suppose that tl,, and a,remain constant. With the above obtained a. value, the C& as Ad relationship was plotted in Fig. 5 for E, < 20 V. From the slope a value of a, = 30 was obtained. Also, in this case it is possible to calculate from the capacity data the oxide film thickness in the potential range where the oxygen evolution reaction is significant. The electronic characteristics of the grown oxide films up to different potentials were determined from the capacity-potential curves for potential values lower than 0.2 V. At these potentials, the electrode behaves as a n-type semiconductor following the Schottky-Mott equation modified for the potential drop in the Helmholts layer[36]. This equation can be written for T= 298K as,
c,= = c,z +
1.41 x 1020 %No
(E, + kT/e)
1.41 X 1020 E &ND
(7)
’
with the capacities in pF cm-2 and the potentials in volts. Noand E,,are the donor concentration and the flat band potential, respectively. From a SchottkyyMott plot for thin oxide layers of different thickness (Fig. 6) the donor concentration and the E’tb value can be obtained, where
1
Efb=
Efb+
We
-
1,41
%CND x 103,, c2
(8) H
Ef /
0 go .. . . .
I
I
v
25
50
1
I
1
3 L!L ---+
0
0.5
Q
b-
//
c
2
-
t
o-P------1
0
1
5
-I -
I
6
E/V Fig. 4. Potentiodynamic profiles of current density-potential (upper part) and electrode capacity-potential (lower part) performed on previously electroformed oxide films at different final anodic potentials: (a) 15, (b) 30 and (c) 45 V. (C-E profiles corresponding to the positive sweep are not shown.)
QO
20
40 Ad/nm
Fig. 5. Reciprocal capacity of passivated as a function of the final anodic potential in film thickness.
titanium electrode and of the increase
Hydrogen evolutionreactionon ancdic titaniumoxide The hydrogen
Fig.
6. Schottky-Mott
plot for different oxide thicknesses.
film
is the potential for CL ’ = 0. According to this equation, a linear E’rb us No relationship was obtained and the true E,, = - 0.30 V could be calculated from the extrapolation at the zero origin and the Helmholtz capacity, C, = 18 PF cme2, from the slope. Similar results were obtained for oxide films grown at different sweep rates. The donor impurity concentration (N,) could also be calculated from the slope of the Schottky-Mott plot (Fig. 6). For E, values higher than the exhausted potential a linear relationship for N, us d-’ is obtained[37]. In Fig. 7 these relations are shown for different oxide films. This behaviour could be taken as an indication that the oxide film thicknesses obtained from capacity data are reliable when the oxygen evolution reaction is significant. A summary of the physico-chemical properties of these oxide films are shown in Table 1. The Debye length (Lo) values,,which is the length of the potential drop in the space charge region of the semiconductor oxide film[32], is also included.
2.5
2.0_
I ” / vs-l . CL005
I
/ ! 7.5
Fig. 7. Donor concentration 1)sreciprocal square thickness for oxide films electroformed at different sweep rates.
evolution
1295 reaction
i-E profiles in HSO,/SO:solutions. The i-E profiles performed with an oxide film electrode of 79 nm thickness in a HSO; /SO:solution in the negative potential range are shown in Fig. 8. Each i-E profile corresponds to the first electrochemical experiment performed over the freshly formed oxide layer and the potential sweep started at E,, value. In the negative potential sweep the voltammograms show a cathodic current peak whose maximum, i,, depends on the sweep rate and on the hydrogen ion concentration in the solution. The small increase of the cathodic current between - 0.40 and - 1.00 V is assigned to the transformation processes of the oxide films which have been previously reported[3840]. HER from water[41] is known to occur at more negative potentials than the cathodic limit. Also in Fig. 8 the linear dependence of i, us vl” is shown with a slope that increases with the H’ ion concentration. These facts indicate that the reaction associated to the principal cathodic current peak is the reduction of H + ions to give molecular hydrogen, and that its overall rate is limited by the mass transport in the solution. The diffusion coefficient of the hydrogen ion in the solution, D H+, could be obtained from the slope of these plots. However, it should be pointed out that this value could be affected by some incomplete pseudo-ohmic compensation problems since the E, or E,,, shift with the sweep rate is higher than the expected one for a charge transference reaction. The D, value thus obtained is significantly lower than that reported in the literature[42]. In those experiments where the cathodic switch potential was extended, a pronounced hysteresis between the cathodic and anodic sweep at cn - 1.0 V as well as a higher Hz current formation from water were observed. This hysteresis phenomenon increases when the sweep rate is diminiShed and/or when the solution is stirred.
0,
.I.0 _
2.0-
3.0_ - 3.0 Fig. 8. Potentiodynamic i-E profiles for HER with an oxide film of 79 nm at several sweep rates. pH = 1.72, CHSO; = 0.163 M and Csoz - = 0.86 M. ip as a function of v _ ’ ” plot is shown inset for different pH. (0) pH = 1.72 and CHsoA= 0.091 M, (A) 2.18 and 0.080 M. ( n ) 2.44 and 0.048 M.’
1296
R. M. TORRESI~~
i-E profires in stirred HSO, /SO: and HCIO,/ClO~ solutions. In potential sweep experiquasi-stationary, conditions (v ments under = 0.01 V s-’ ) with rde a cathodic limiting current density (iL) appears at the same potential region where the cathodic current peak has been observed in the voltammograms (Fig. 9). It can be seen besides that a depolarization of about 0.18 V takes place at w = 530 rpm. The linear dependence of i, us w “’ is shown in Fig. 10 for HSO,/SO:(Fig. 1Oa) and HCIO,/ClO; (Fig. lob) solutions. The slope increases with the decrease of the pH in good agreement with Levi&s equation[43) for a controlled mass transport process for H+ ions in the solution. Taking into account the value of 2.8 x lo- ’ cm’ s-r with an iron rde in for D,+, Savitkin and Podobaev[42] weakly acidic sulphate solutions calculated a theoretical slope (Pth) for the i, us w112 relation using the concentration of H+ ions calculated from the pH
d.
values. They later compared the ratio between experimentally obtained slope (I’.,,) and Pth at a given value of pH with the ratio between the overall H + concentration { (cH* )pH + CHSO; } and that from the pH values only. A good agreement between them was observed. In the present paper the disagreement between these two ratios was about 5 % for pH between 1.49 and 3.38. This suggests that in the potential region of the limiting current, some of the H+ ions which are reduced are transported to the electrode as HSO, species which in turn dissociate into SO:and H+ ions in the solution layer near the surface. The value of the rate constant of the dissociation process is 1 x 10” cm3 mol-‘[44]. Thus, the diffusion of the H+ ion would become the slow step of the overall reaction. Assuming that the concentration of the H + ions for this process is determined by the H+ ions calculated from the pH values and from those coming from HSO, ions, DH+ = 2 x IO-’ cm2 s-r was obtained in HSO,/SO:solutions, and 2.5 x 10e5 cm* s-’ for HC104/C10; solutions. Similar values were obtained by other authors for nickel electrodes[45]. The reaction order with respect to the hydrogen ion species, nu*, can be calculated by means of the following equationC46, 473: log i = log i,,
- 1.5
- 0.5
-10
+nH*bg(l
-i/i‘).
A plot of log i us log (1 - i/iL) for different potentials is shown in Fig. 11. The nH+ value for both HSO, /SO:- and HCiO,/ClO, solutions was equal to 1. Once the reaction order is known, the rate of the charge transference process can be estimated independently of the diffusional steps from an analytical method as expressed in the following equation
a4
= ii,/i,-i, ‘m&IX
E/V
I
sq-
(10)
which is valid for nu’ = 1. For this system, the analytical method is used for calculating i,, (equation lo), because it shows a good correlation of the experimental values. A plot of
Fig. 9. Quasi-stationary i-E profiles for different cathodic switching potentiak on an electroformed oxide film of 79 nm (IV, = 4.0 x 10” cmM3). C,so; = 0.0145 M and Cso:= 0.152 M.
al l-sob-/
(9)
I
*a
-
I
I
b) UCIO~/CIOb-
Fig. 10. Cathodic limiting current density as a function of the root square rotation rate obtained with a 79 nm M, (b) pH = (a) oxide 6lm. (a) pH = (0) 2.49 and CH~; = 0.032 M, (A) 2.89 and 0.012 hi, (I) 3.38 and O.OO4
1.58, (A) 2.02 and (I)
2.42.
Hydrogen evolution reaction on anodic titanium oxide
1297
loY[i/Acti21 -2.2 5
-2ao
-1.75
-150
jF; I -
g I
1
l-O.6
- 2.50
- 2.75
- 3.00
3.25
log[i/Acti21
Fig. 11. Log (1 - i/i_,) DSlog i relation for HER on an oxide film of 79 nm at digerent potential values. (a) pH = 1.50, Cnso;= 3.22 x 1O-2 M and (b) pH = 2.42, Cclo;= 0.35 M.
us log Cu for both negative and positive potential sweeps, gEes a slope equal to 1 (Fig. 12). This fact supports the above result of nu+ = 1. On the other hand, since the slope for negative and positive potential sweeps are the same, the hysteresis phenomenon does not seem to produce changes in the kinetics for the electroreduction of H’ ions. As i,,, depends only on the charge transference process at the interphase, a plot of potential vs log i,, at sufficiently cathodic overpotentials should lead to the Tafel equation for the HER process (Fig. 13). The slope obtained was about 0.06 V dec-‘. Figure 14 shows that the same Tafel slopes are obtained for both solutions in the positive and negative potential sweeps. The E, ,z us log w “I plot leads to the same conclusions.
lois i,,,
Eflect of the oxide thickness and temperature on HER in HCiO,/ClO~ solutions. The variation of the oxide film thickness has apparently no influence on the magnitude of the cathodic limiting current density in experiments performed in quasi-stationary conditions at constant pH and w in HClO,/ClO, solutions, but, a shift of the current profile towards more negative potential values as d increases is observed (Fig. 15). However, the magnitude of the reactivation in the current between the negative and positive direction was practically not affected by the increase of d. On the other hand, the increase in the temperature produced a shift of the i-E profiles towards less negative potentials as well as an increase of i,at d, pH and w constants. ‘O¶c ‘IT.
lO9C”.
“so&7
EIV
. .
-l,zs -1.15
*
-1.05
A-1.10
b)
b)
E/V .-UO
::i/
Fig. 12. Log i,,
E/V
H
us log Cu+ relation for acidic sulphate and perchlorate electrolytes. d = 79 run. (a) Negative and (b) positive potential sweeps.
1298
R. M. TORRES et al.
log [i,,,/Ac&21 -2
-3
-4
r
-1
I
I
~-0.95
F
I
’
I
PH : 3.15 = 0.35M
_
-1.15
Fig 13. Tafel diagram for different pH values in acidic sulphate and perchlorate electrolytes. d = 79 nm. w = (0) 264, ( x ) 1132, (A) 2224 and (I) 4164 rpm.
loglimax/Acri$l -3
-4 1
-2
-1
I
I
I
m
The results obtained for the i,value as a function of w112 with film thickness between 3.3 and 79 nm and at different temperatures are shown in Fig. 16. As expected, the thickness of the oxide film does not significantly affect the limiting current density because the rate determining step of the reaction in this potential range is the Hf ion diffusion in the bulk of the solution. The diffusion coefficient of H+ ion depends on the following temperature according to the expression[48]
DH. = Do exp (-
AE+,/RT),
(11)
Do is a constant and AEg is the activation energy for the diffusion process of the species in solution. This equation predicts a linear dependence between log DHf and T-‘. A value of 2.35 kJ mol-’ for AEE, was obtained from the slope. The effect of the temperature on the charge transference process was investigated through the variation where
Fig. 14. Tafel diagram obtained with (a) negative and (b) positive potential sweeps. d = 79 nm. w = (a) 264, ( x ) 1132,
(A) 2224 and (I)
4164 rpm.
I
I
___
20
e&lO’B
.._._.
33 .
1.4- lo20 0 Y
p”s 2.37 C,.,,4’0.35M
I
1
-1.5
I
- 1.0 E/V
I
I
w
E
I
I
- 0.5
Fig. 15. Influence of the oxide film thickness on quasistationary i-E profiles for HER, v = 0.01 V s- ’ and w = 960 rpm.
Fig. 16. Influence ofthe oxide film thickness and temperature on the i,vs w”’ relation. d = (a) 3.3, (A) 7.0, (I ) 20.0 and ( x )
79.0 run.
1299
Hydrogen evolution reaction on anodic titanium oxide of the slope of the E us log i,, plots with the temperature for oxide films of different thickness (Fig. 17). A change in the Tafel slope and a shift of straight lines toward less negative potentials as T increased is observed. The value of i _xat a given potential depends on the trmperature according to[45] 1-X
= Bexp(-AE:/RT),
(12)
where B is a constant and AE: is the activation energy for the charge transference process. From the slope of log i_vs T-’ plot, AEZ for different potentials can be obtained. These values depend on the applied overpotential according to[35] AE:
= AEi--a
F(E-E’),
(13)
where AE !j corresponds to the activation energy at the overpotential zero and the constant a is related to the charge transference coefficient[49, SO]. In those systems with oxide surface films, it is difficult to determine AEZ because the equilibrium potential of the electrode is not known due to the fact that the potential of the system at open circuit would be a corrosion potential[39]. Nevertheless, Fig. 18 shows the AE$ values as a function of the potential for different thickness of the oxide film. It can be observed that the kinetics of the HER is not modified by a change in the thickness of the oxide film, and that the AE: for this system must be independent of the thickness. The latter conclusion is supported by the fact that the equilibrium potential of the oxide film is hardly dependent on its thickness. The E us log i,, plots (Fig. 19) obtained for different thickness did not show any changes in the values on the Tafel slope though a shift of the straight lines towards more negative potentials was observed as the thickness was increased. This result would indicate that the charge transference coefficient of the HER is independent of the thickness of the oxide film. Some studies about the oxygen evolution reaction on thin platinum and iridium anodic oxide film[Sl, 521 or with the ferro/ferricyanide system on titanium and niobium anodic oxide film[53] have shown an inverse
> \
- 1.15
273’KW 298-K 333’K
I--
t
-3
.
-2
linear behaviour between the activation energy and the overpotential and a direct linear behaviour between log iand film thickness, qualitatively in agreement with the results presented in this work. They explained these phenomena taking into account an electron transfer process through an oxide film which was correlated to the probability of the quantum mechanical tunnel transition. An explanation of our results based on their model would not be convenient due to complications in the HER considering that it is an inner sphere reaction involving some adsorbed intermediates. On the other hand, in the potential range where the HER takes place (negative with respect to the flat hand potential), the conduction band of the semiconductor bends down towards the Fermi level. In this case, the kinetics of the electron transference on a semiconductor should be like that of a metal and the i-E profiles would not depend on oxide film thickness, but these predictions are not in agreement with our results. The different steps reported for the HER[54,55] are Y= SH (Volmer
S+H&+e
C= H,
SH+H&+e
reaction),
(Heyrowski
HZf” (Tafel
SH+SH=
reaction),
reaction),
(14) (15) (16)
where S and SH stand for the active site and for the adsorbed hydrogen atom over the surface, respectively. The experimental results can be explained taking into account a first step of fast proton discharge (Volmer reaction) followed by the recombination reaction (Tafel reaction) which would be the rate determining step[56, 573 under Temkin adsorption conditions. From this mechanism, the following rate expression can be written[20]: i = -2
F Ku”+
exp(-
FEfRT),
(17)
where it is supposed a symmetry factor of 0.5 and K is a constant which includes the rate constant of the recombination step and the equilibrium constant of the charge transference step.
.
-1
d.20nm
0
1
tog(i,,,/Acm-2)
Fig. 17. Influence of the oxide film thickness and temperature on the Tafel diagram. w = (0) 264, (I) 1164. (A) 2502 and ( x ) 4164‘pm, Cao; = 0.35M and pH = 2.40.
700, (0)
R. M. ToRaFsiet
al.
explained taking into account the different hydrogen species ad/absorbed in the oxide films. As it was previously reported[58-611, the absorbed hydrogen in the oxide film changes its electronic properties, increasing the dielectric constant and the donor impurities concentration. All these modifications would promote a higher electrical conductivity of the oxide film. This phenomenon will be dealt with in another paper[62].
I - 1.5
CONCLUSIONS
50 - 1.0
- 0.5
E/V
Fig. 18. Dependence of the activation energy of the HER with the electrode potential for different oxide. film thicknesses.
Iog[i,,,/Acrii*
1
-4
I\
c,,,o,=
0.35t.A PHZ
2.40
_ -1.10 m
\ <
d/WT
.
3.3
.
m.0
.
79.0
_ -1.20
Fig. 19. Influence of the oxide film thickness on the Tafel
diagram. T = 298 K and w = 1000 rpm.
The proposed mechanism implies that the HER takes place at the interphase oxide/electrolytic solution and would explain the independence of the Tafel slope, na* and AE:on the film thickness, but it does not enable us to account for the diminution of the current as d increases. This seems to be not clear at all and must be further investigated. The values of the activation energy would indicate a mixed control of the reaction in the potential range studied. It can also be observed that the AE:values are higher than those obtained for the HER on mckel[45]. This fact suggests that the oxide film has a significant influence on the rate of HER increasing the activation energy with respect to other metals in which the HER takes place on an electrode completely free of any oxide film. In this sense the experimental results agree with the high overpotentials necessary for the reaction to take place in a significant magnitude. The observed hysteresis in the current profile of the HER can not be merely explained by a decrease in the thickness of the oxide film. In Fig. 14 it is possible to observe a depolarization of about 0.18 V between both sweeps. Based on the results of Fig. 19, this depolarization should correspond to a decrease in the thickness from 79 to 3 nm; however, this deduction is not in agreement with the characteristics of the oxide film obtained after only one potential cycle in the potential range of HER. The shift of the current profiles towards less negative potentials that occur during the positive sweep can be
From the experiments of eleetroformation of oxide layers on titanium through impedance measurements, it is possible to conclude that iq the potential range studied the increase in the thickness is a linear function of the anodic final potential. This effect is also observed at high potential values where the oxygen evolution reaction occurs simultaneously with the oxide electroformation. Assuming that the density of the oxide is independent of the potential sweep rate, a decrease of the anodization coefficient and the dielectric constant with the increase of u were observed. This means that the structural characteristics of the oxide films mainly depend on the way in which these films were electroformed. The results hitherto obtained in H,SO, and HClO,+ solutions in the pH range between 1 and 4 can be well described by a rate equation which may be derived from the mechanism consisting of two consecutive steps for HER: one involving the solvated hydrogen ion reduction and the other the recombination of the adsorbed hydrogen species, the latter being the rate determining step under Temkin-type adsorption isotherm conditions. In the potential range studied, the HER shows a mixed control but, the activation energy for the charge transference process is too high to be attributed to a mixed control only. This should be attributed to the influence of the oxide film on the HER. This influence is confirmed through the shift of the i-E profile towards more negative potentials with the increase of the film thickness. According to the results obtained, the changes in the HER seem to be produced by either the modifications in the thickness or by the donor impurity concentration in the oxide film. Nevertheless, the model derived for positive potentials with respect to E,, does not seem suitable to explain our results The depolarization observed in the i-E profiles can not be explained by a decrease in the thickness film. This fact could be explained by an increase in the oxide film conductivity. Acknowledgements-The authors wish to thank Dr V. A. Macagno for his interest in these investigations and for valuable advice and discussion. Financial support from the Consejo National de Investigaciones Cientificas y T&micas (CONICET)and the Consejo de Investigaciones Cientificas y Tecnologicas de Ia Provincia de Cordoba (CONICOR) is gratefully acknowledged. R.M.T thanks CONICET for the fellowship granted.
REEFERENCES 1. G. Bewer, H. Debrodt and H. Herbst, J. Met& (1982).
34, 37
Hydrogen
evolution
reaction
2. A. Fujishima and K. Honda, Nature 238, 37 (1972). 3. M. Dcchamps and P. Lehr, J. Less-Common Metals 56, 193 (1977). 4. M. Renaud and M. Rigaud, J. Less-Common Metals 32,
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33. J. L. Ord, J. electrochem. Sot. 127, 2682 (1980). 34. J. L. Ord, J. electrochem. Sot. 129, 2038 (1982). 35. J. L. Ord, M. A. Hopper and W. P. Wang, J. electrochem. Sot. 119, 439 (1972). 36. R. De Gryse, W. P. Gomez, F. Cardon and J. Vennik, J. electrochem. Sot. 122. 711 (1975). 37. V. A. Macagno and J.’ W. Schult~e, J. electroanal. Chem. II&o. 157 (19X41 38. R. M. Torresi, 0. R. Cbmara, C. P. De Pauli and M. C. Giordano, Anal. Asoc. Quim. Argentina, 74, 361 (1986). 39. 0. R. CXmara, C. P. De Pauli and M. C. Giordano, Electrochim. Acta 29, 1111 (1984). 40. 0. R. Gmara, Tesis Doctoral, Fat. de Ciencias Quimicas, Univ. National de Cordoba, Cordoba, Argentine, 1982. 41. S. Swathirajan and S. Bruckenstein, J. electroanal. Chem. 163, 77 (1984). 42. N. I. Savitkin and N. I. Podobaev, Soviet Electrochem. 10, 558 (1974). 43. V. G. Levich, Physicochemical Hydrodynamics. Prentice Hall, New York (1962). 44. E. F. Caldin, Fast Reactions in Solutions (Russian translation), Mir, Moscow (1966). 45. A. J. Arvia, R. C. Piatti and J. J. Podestl, Anal. Asoc. Quint. Argentina 57, 1 (1969). 46. T. Dickinson, R. Greef and Lord Wynne-Jones, Electrochim. Acta 14, 467 (1969). 47, A. C. Riddiford, in Advances in Electrochemistry and Electrochemical Engineering, (Edited by P. Delahay) Vol. I. Interscience, New York (1965). 48. D. M. MacArthur, J. electrochem. Sot. 117, 729 (1970). 49. J. W. Schultze and V. A. Macagno, Electrochim. Acta 31,
355 (1986). 50. C. Bartels. M. M. Lohrenxel and J. W. Schultze. Proc. Sym. Fundamental Aspects of Corrosion Proteciion by Surface Mod&z&ion (Edited by E. McCaITerty et al.).The 51.
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EVOLUTION REACTION ON ANODIC TITANIUM OXIDE FILMS* R. M. TORRESI,~. R. C~~ARA,C.
P.DE
PAULI
Instituto de Investigaciones en Fisicoquimica de Cbrdoba (INFIQC), Departamento de Fisicoquimica, Fat. de Ciencias Quimicas, Universidad National de Cbrdoba, Sue. 16, C.C. 61, 5016 Grdoba, Argentina and
M.C. lnstituto
de lnvestigaciones
GIORDANO
Fisicoquimicas Te&icas y Aplicadas (INIFTA), Plats, Sue. 4, C.C. 16, 1900 La Plata, Argentina
(Received 4 October 1986: in revised form 29 January
Universidad
National
de la
1987)
Abstract---Oxide titanium films were prepared using linear potential sweeps up to different limiting potential values between 0 and 60 V in 0.5 M H1S04. The electronic characteristics of these films were investigated through impedance measurements during their formation. The hydrogen evolution reaction (HER) in either HCIO, or H,S04 solutions in the pH range between 1 and 4 was studied on the different oxide films. The obtained experimental results could be explained assuming that the recombination of adsorbed hydrogen atoms is the rate determining step (rds), and that the Temkin type isotherm applies to the adsorbed intermediates. T’he log i us E plot after correction for diffusion control in the bulk gives a slope of 2.3 RT/F. The influence of the physicochemical properties of the oxide films on the HER has also been studied from the changes in the current-potential profiles. The oxide film thickness changes the HER overpotential, but the slope of the log i vs E relationship remains independent of it.
INTRODUCTION Metallic titanium and its oxides have been widely studied in the last years due to their recent applications in the industry, either as structure materials or as substrates for electrosynthesis, or for conversion reactions[l], as well as for use in solar energy[2]. This metal has a great resistance to corrosion because of the spontaneous formation of a thin oxide film on the surface. A thicker oxide film can be obtained by thermal[3,4] or electrochemical methods. The latter are mainly carried out through: (i) anodic current steps, (ii) anodic potential steps and (iii) a linear anodic potential sweep. In case (i) films of different morphological and structural characteristicsC5, 63 are obtained depending on the final anodic potentials. In case (ii) the results obtained show three potential regions where oxide films of different characteristics are obtained depending on whether electronic or thermal breakages[7-101 take place. In case (iii) stoichiometric films of homogeneous thickness[l l] are obtained at potentials lower than 10 V. The films formed at potentials higher than 50 V showed microfissures as they were observed by impedance and photocurrent measurements. A massive rupture joint with oxygen evolution is produced in the reverse sweep depending on the sweep rate[12].
Although several authors consider that these films show great stability under cathodic treatment, a decrease in their thickness occurs. This fact can be explained through the oxide dissolution[ 131. The HER takes place in the negative potential range on metallic titanium as substrate. Nevertheless, the results are not coincident with regard to the rate determining step, the adsorption isotherm followed and the presence or absence of an oxide film during the HER. High values in the Tafel slope have been explained in terms of a dual barrier model[1&17], while other authors have taken into account either the presence of an oxide film[18] or of oxy-hydroxy adsorbed species at the sub-monolayer level[19, 203. From these results no definite conclusions can be obtained regarding the effect of the oxide film on the HER. The aim of this paper is to investigate the HER on the electroformed oxide films of different thickness and electronic characteristics to get further insight into the influence of the oxide film properties on the hydrogen reaction.
EXPERIMENTAL Titanium
electrodes
The working electrode was made from a titanium Alpha rod (Koch-Light Ltd. 99.98 %) fixed in a Teflon holder; the exposed surface was a disc of 0.50 cm2 geometrical area. At the beginning of each electrochemical experiment the electrode was etched in a
l This paper is based in part on the poster 05200 presented at the 36th ISE Meeting, Salamanca, Spain, September 1985.
1291
R. M. TORRESI ef al.
1292
1:4:5 mixture of HF (48 %), HN03 (65 %) and water[21, 227 for 40 s and rinsed with purified water. The etching procedure causes the roughening of the surface. Taking as reference a roughness factor r = 1 for the electropolished electrode[22-241, we estimated the values for chemically polished electrodes from coulometric and capacity data to be 1.3[25], in good agreement with the results reported by Young[26]. The oxide tilms were obtained by applying anodic potential sweeps up to different final anodic potential (E, ) values in 0.5 M H$O, solutions in order to obtain different thicknesses. During the oxide electroformation the electrode was rotated (1000 rpm) to avoid oxygen bubble formation on the surface. Cell and electrolytes The electrolysis cell consisted of two Pyrex glass compartments of about 300 cm3. A platinum sheet of about 1Ocm’ geometric area was used as counter electrode. All solutions were prepared from a.r. chemicals and purified water (Milli Q system, Millipore Corp.). The electrolytes HC104-NaC104 and H2S04-Na2S04 were used in suitable concentrations to give a molar ionic force of 0.35 M. The solutions were deaerated by purified nitrogen previous to the electrochemical measurements. Saturated calomel or reversible hydrogen reference electrodes were used; however, all the measured potentials are hereafter referred to the standard hydrogen electrode (she).
The electronic equipment The experimental results of the oxide film electroformation and the HER were obtained by employing the voltammperometric technique with either simple or repetitive linear sweeps, current or potential pulses and rotating disc electrode (rde). The electronic equipment was the following: a potentiostat/galvanostat (EG&G PARC 173), a wave generator (EG&G PARC 175) and a X-Y recorder (Hewlett-Packard 7004B). In order to obtain capacity values of the electrode, some impedance measurements were performed under potentiostatic and potentiodynamic conditions with a 1 kHz and 0.7 mV signal superimposed onto the potential, and then, the phase shift was measured by means of a lock-in amplifier (EG&G PARC 124A) with a differential pre-amplifier (EG&G PARC 116).
RESULTS
Electroformation
AND
DISCUSSION
and characterization
of oxidefilms
The growth of thin oxide layers on titanium through electrochemical methods is known to take place irreversibly at more anodic values than the rest potential (E,,) according to the following reaction[27, 281: Ti + 2H,O
e
TiO,
+4H+
f4e.
E” = - 0.86 V.
Titanium is a typical valve metal and its electrochemical behaviour when the oxide is grown can be seen in Fig. 1, which shows simultaneous current (Fig. la) and capacity (Fig. 1b) measurements as a function of the potential under triangular perturbation in
E/
V
Fig. 1. Potentiodynamic oxide formation on titanium. (a) Current density-potential and (b) electrode capacity-potential profiles for different Ez values in repetitive cycles.
0.5 M H,SO+. The potentiodynamic sweep started at E reS1= -0.35 V in the positive direction and ended at increasing potentials (E/) up to 8 V. Detailed results of cathodic measurements at negative potentials are shown further on. It is observed that the anodic current reaches an almost steady value in each repetitive sweep according to the high-field mechanism of oxide film growth[29, 301. Nevertheless, the steady anodic current increases at potentials higher than 3.0 V due to the simultaneous electroformation of oxygen on the electrode. When this anodic potential sweep is reversed the current hastily decreases down to almost zero and the thickness of the oxide film remains constant during the cathodic and the next anodic sweep as long as the positive potential limit is not higher than the EJ value of the previous cycle. This behaviour is characteristic of valve metals. The overall charge involved in the anodic process is
Qtot = Qo, + Qdis + Q,,,
(1)
where Qo, corresponds to the charge involved in the oxygen evolution reaction, Q&i. corresponds to the metal dissolution to give ions in solution and Q, is the charge taking part in the oxide layer formation. According to the experimental conditions employed in this paper, Qdir can be considered negligible whereas Q, and Qo, are dependent on EY _ The increase of the oxide film thickness depends on the E, according to[9,31] Ad = u,E,,
(2)
where IX,, the anodization coefficient, depends on the different potentiodynamic formation conditions, the temperature and the electrolyte. The overall charge Q,,, varies linearly with Ef in the potential range between E,, and 3.0 V since Qo, is negligible and then Qtot = Q,,. Thus, the increase in film thickness can be stated as Ad = M Q,,lr
F a,,,
(3)
where M, 6,, and z are the molecular weight, the density and the number of exchanged electrons of the formed film, respectively. The M/z F 6,, ratio for the
Hydrogen
various crystalline structures of the titanium oxide has already been reported[31]. From equations (2) and (3) we obtain, (4)
Qo,=kE,,
where k depends on the anodization coefficient and the oxide film characteristics. The experimental dependence of Q,, on E, shows a linear variation up to 3.0 V, and from this value the simultaneous oxygen evolution produced positive deviations. Thus, the increase in thickness of the oxide film can be calculated from Q,,, only when E, < 3.0 V. The slope of the Ad us E, plot (Fig. 2) gives the anodization coefficient which decreases with the potential sweep rate assuming that a,,, remains constant. The cc, values obtained from these plots are shown in Table 1. The variation of the differential capacity with the potential in the anodic sweep (Fig. 1) is typical of the behaviour of a parallel plane capacitor with variable thickness values. During the reverse potential sweep it behaves as a surface film with a constant thickness whose characteristics correspond to those of a n-type semiconductor[32]. The thickness of the oxide film for E, > 3.0 V can be calculated from impedance measurements. In this case,
the anodic oxygen evolution reaction can be disregarded due to its high charge transference resistance, which is typical of this type of reaction on insulating oxide films. Therefore, the simple capacitor formula can be applied to the oxide films according to the following equation, C,’
V/V&
0.005 A 0.025 I 0.050
E F B
Fig. 2. Dependence of the thickness increase on the final anodic potential for different sweep rates.
Table 1. Characteristics of Ti02
E,(V)
d (nm)
a. (run V-‘)
(5)
which predicts a linear dependence of CL ’ with E, assuming that a, and E, do not change in the potential range studied. A plot of C; 1 us E, (Fig. 3) shows a linear dependence for different sweep rates up to ca E, = 10.0 V. In this way, Ad can be obtained from this type of measurements for E, > 3.0 V if 01,, E, and do are known. The values of E, and d, can be calculated from C/i us Ad plots in the E, c 3.0 V range. ln order to obtain the C,, value for this procedure it is necessary to know the Helmholtz layer differential capacity, C,, which is calculated further on. The do value was cn 1 nm, in good agreement with those obtained by another author[21], and the &,value so obtained depends on the potential sweep rate assuming that a,,, remains constant. Changes in the dielectric constant value with the electric field applied have already been reported by other authors[33-351. Thus, it is possible to obtain from equation (6) the oxide thickness formed in the potential range where the oxygen evolution reaction occurs. The electroformation of these oxide films which are thicker than about 11 nm was performed by potential sweeps up to E, = 60Vwithv=9Vs-‘.Thissweep rate was chosen because the electronic rupture of the film does not occur in the reverse sweep[l2]. After the oxide film was formed one cycle of potential sweep at
.
v(Vs_1)
+c,‘,
where C, is the experimental differential capacity is the differential capacity of the measured, C, Helmholtz layer at the oxide-electrolyte interface and C,, is the differential capacity of the oxide film. Taking into account that C,, = e’e,/d and that d = do + Ad, where co and e,are the dielectric constants of vacuum and the semiconductor oxide film, respectively, d, is the thickness of the oxide film present initially, the equation (5) can be rewritten as,
1
1 O-
= C,’
Ely:
oxide films
E’, (V)
E,
0’)
No (cm-‘)
Lo (nm)
0.005
1.0
0.005 0.005
3.0 9.0
3.90 12.50 37.00
4.10 4.10 4.10
68 68 68
-0.64 -0.32 -0.26
-0.30 -0.30 -0.26
1.46 x 10” 2.51 x 10’9 8.26 x 10”
0.81 1.98 3.42
0.025 0.025 0.025
1.0 3.0 9.0
3.60 11.30 34.00
3.75 3.15 3.15
57 57 57
-0.65 -0.28 -0.30
- 0.30 -0.28 -0.30
1.47 x 1020 2.17 x lOI9 3.22 x 10”
0.74 1.93 5.02
0.050 0.050 0.050
1.0 3.0 9.0
3.30 10.40 31.00
3.41 3.47 3.47
50 50 50
-0.50 -0.31 -0.30
- 0.30 -0.31 -0.30
1.34 x 1020 2.83 x lOI9 2.50 x 1O’a
0.73 1.27 5.33
10.30 40.00 85.yx)
1.90 1.90 1.90
30 30 30
-0.33 -0.30 - 0.30
- 0.30 -0.30 -0.30
8.90 x 10” 1.50 x 10’8 5.17 x 10”
2.19 5.33 9.08
9.0 9.0 9.0
5.0 20.0 45.0
(M/z F b,,) = 5.37 x lo-” d, = 1.0 nm.
cm’ C-t
[31].
1294
R. M. To~arst et al.
01
I 50
I
2.5
0
I
I 7.5
10.0
Ef / V
Fig. 3. Dependence of the rtiprocal electrode capacity on the final anodic potential for different sweep rates.
0.01 V s-r up to 8.0 V with a small sinusoidal superimposed wave was performed in order to determine its physico-chemical characteristics from capacity measurements. Figure 4 shows the i-E (upper part) and C-E (lower part) profiles for different oxide film thickness previously electroformed. A small current density is observed in this potential range, which means that no changes are produced on the film when the capacity measurements are performed. The C-E profile corresponds to the reverse potential sweep and shows that for E > 5.0 V the capacity of the oxide film is independent of the potential. This fact indicates an insulating behaviour for the oxide film, and then it is
possible to calculate the oxide thickness from the capacity data measured up to 5.0 V. Thus, the oxide dielectric constant for thicker films could be determined from electrode impedance measurements. The Q,,, results and their corresponding Ad values calculated from equation (3) are plotted us E, for this type of experiment and a linear behaviour up to E, = 20 V was observed, which means that in the potential range involved the anodic current for oxygen evolution reaction is negligible at these sweep rates. The a. value in the given experimental conditions was 1.9 nm V-l. On the other hand, in Fig. 5 can be seen that a plot of C&‘os E, gives a linear relationship up to E, = 50 V, which leads to suppose that tl,, and a,remain constant. With the above obtained a. value, the C& as Ad relationship was plotted in Fig. 5 for E, < 20 V. From the slope a value of a, = 30 was obtained. Also, in this case it is possible to calculate from the capacity data the oxide film thickness in the potential range where the oxygen evolution reaction is significant. The electronic characteristics of the grown oxide films up to different potentials were determined from the capacity-potential curves for potential values lower than 0.2 V. At these potentials, the electrode behaves as a n-type semiconductor following the Schottky-Mott equation modified for the potential drop in the Helmholts layer[36]. This equation can be written for T= 298K as,
c,= = c,z +
1.41 x 1020 %No
(E, + kT/e)
1.41 X 1020 E &ND
(7)
’
with the capacities in pF cm-2 and the potentials in volts. Noand E,,are the donor concentration and the flat band potential, respectively. From a SchottkyyMott plot for thin oxide layers of different thickness (Fig. 6) the donor concentration and the E’tb value can be obtained, where
1
Efb=
Efb+
We
-
1,41
%CND x 103,, c2
(8) H
Ef /
0 go .. . . .
I
I
v
25
50
1
I
1
3 L!L ---+
0
0.5
Q
b-
//
c
2
-
t
o-P------1
0
1
5
-I -
I
6
E/V Fig. 4. Potentiodynamic profiles of current density-potential (upper part) and electrode capacity-potential (lower part) performed on previously electroformed oxide films at different final anodic potentials: (a) 15, (b) 30 and (c) 45 V. (C-E profiles corresponding to the positive sweep are not shown.)
QO
20
40 Ad/nm
Fig. 5. Reciprocal capacity of passivated as a function of the final anodic potential in film thickness.
titanium electrode and of the increase
Hydrogen evolutionreactionon ancdic titaniumoxide The hydrogen
Fig.
6. Schottky-Mott
plot for different oxide thicknesses.
film
is the potential for CL ’ = 0. According to this equation, a linear E’rb us No relationship was obtained and the true E,, = - 0.30 V could be calculated from the extrapolation at the zero origin and the Helmholtz capacity, C, = 18 PF cme2, from the slope. Similar results were obtained for oxide films grown at different sweep rates. The donor impurity concentration (N,) could also be calculated from the slope of the Schottky-Mott plot (Fig. 6). For E, values higher than the exhausted potential a linear relationship for N, us d-’ is obtained[37]. In Fig. 7 these relations are shown for different oxide films. This behaviour could be taken as an indication that the oxide film thicknesses obtained from capacity data are reliable when the oxygen evolution reaction is significant. A summary of the physico-chemical properties of these oxide films are shown in Table 1. The Debye length (Lo) values,,which is the length of the potential drop in the space charge region of the semiconductor oxide film[32], is also included.
2.5
2.0_
I ” / vs-l . CL005
I
/ ! 7.5
Fig. 7. Donor concentration 1)sreciprocal square thickness for oxide films electroformed at different sweep rates.
evolution
1295 reaction
i-E profiles in HSO,/SO:solutions. The i-E profiles performed with an oxide film electrode of 79 nm thickness in a HSO; /SO:solution in the negative potential range are shown in Fig. 8. Each i-E profile corresponds to the first electrochemical experiment performed over the freshly formed oxide layer and the potential sweep started at E,, value. In the negative potential sweep the voltammograms show a cathodic current peak whose maximum, i,, depends on the sweep rate and on the hydrogen ion concentration in the solution. The small increase of the cathodic current between - 0.40 and - 1.00 V is assigned to the transformation processes of the oxide films which have been previously reported[3840]. HER from water[41] is known to occur at more negative potentials than the cathodic limit. Also in Fig. 8 the linear dependence of i, us vl” is shown with a slope that increases with the H’ ion concentration. These facts indicate that the reaction associated to the principal cathodic current peak is the reduction of H + ions to give molecular hydrogen, and that its overall rate is limited by the mass transport in the solution. The diffusion coefficient of the hydrogen ion in the solution, D H+, could be obtained from the slope of these plots. However, it should be pointed out that this value could be affected by some incomplete pseudo-ohmic compensation problems since the E, or E,,, shift with the sweep rate is higher than the expected one for a charge transference reaction. The D, value thus obtained is significantly lower than that reported in the literature[42]. In those experiments where the cathodic switch potential was extended, a pronounced hysteresis between the cathodic and anodic sweep at cn - 1.0 V as well as a higher Hz current formation from water were observed. This hysteresis phenomenon increases when the sweep rate is diminiShed and/or when the solution is stirred.
0,
.I.0 _
2.0-
3.0_ - 3.0 Fig. 8. Potentiodynamic i-E profiles for HER with an oxide film of 79 nm at several sweep rates. pH = 1.72, CHSO; = 0.163 M and Csoz - = 0.86 M. ip as a function of v _ ’ ” plot is shown inset for different pH. (0) pH = 1.72 and CHsoA= 0.091 M, (A) 2.18 and 0.080 M. ( n ) 2.44 and 0.048 M.’
1296
R. M. TORRESI~~
i-E profires in stirred HSO, /SO: and HCIO,/ClO~ solutions. In potential sweep experiquasi-stationary, conditions (v ments under = 0.01 V s-’ ) with rde a cathodic limiting current density (iL) appears at the same potential region where the cathodic current peak has been observed in the voltammograms (Fig. 9). It can be seen besides that a depolarization of about 0.18 V takes place at w = 530 rpm. The linear dependence of i, us w “’ is shown in Fig. 10 for HSO,/SO:(Fig. 1Oa) and HCIO,/ClO; (Fig. lob) solutions. The slope increases with the decrease of the pH in good agreement with Levi&s equation[43) for a controlled mass transport process for H+ ions in the solution. Taking into account the value of 2.8 x lo- ’ cm’ s-r with an iron rde in for D,+, Savitkin and Podobaev[42] weakly acidic sulphate solutions calculated a theoretical slope (Pth) for the i, us w112 relation using the concentration of H+ ions calculated from the pH
d.
values. They later compared the ratio between experimentally obtained slope (I’.,,) and Pth at a given value of pH with the ratio between the overall H + concentration { (cH* )pH + CHSO; } and that from the pH values only. A good agreement between them was observed. In the present paper the disagreement between these two ratios was about 5 % for pH between 1.49 and 3.38. This suggests that in the potential region of the limiting current, some of the H+ ions which are reduced are transported to the electrode as HSO, species which in turn dissociate into SO:and H+ ions in the solution layer near the surface. The value of the rate constant of the dissociation process is 1 x 10” cm3 mol-‘[44]. Thus, the diffusion of the H+ ion would become the slow step of the overall reaction. Assuming that the concentration of the H + ions for this process is determined by the H+ ions calculated from the pH values and from those coming from HSO, ions, DH+ = 2 x IO-’ cm2 s-r was obtained in HSO,/SO:solutions, and 2.5 x 10e5 cm* s-’ for HC104/C10; solutions. Similar values were obtained by other authors for nickel electrodes[45]. The reaction order with respect to the hydrogen ion species, nu*, can be calculated by means of the following equationC46, 473: log i = log i,,
- 1.5
- 0.5
-10
+nH*bg(l
-i/i‘).
A plot of log i us log (1 - i/iL) for different potentials is shown in Fig. 11. The nH+ value for both HSO, /SO:- and HCiO,/ClO, solutions was equal to 1. Once the reaction order is known, the rate of the charge transference process can be estimated independently of the diffusional steps from an analytical method as expressed in the following equation
a4
= ii,/i,-i, ‘m&IX
E/V
I
sq-
(10)
which is valid for nu’ = 1. For this system, the analytical method is used for calculating i,, (equation lo), because it shows a good correlation of the experimental values. A plot of
Fig. 9. Quasi-stationary i-E profiles for different cathodic switching potentiak on an electroformed oxide film of 79 nm (IV, = 4.0 x 10” cmM3). C,so; = 0.0145 M and Cso:= 0.152 M.
al l-sob-/
(9)
I
*a
-
I
I
b) UCIO~/CIOb-
Fig. 10. Cathodic limiting current density as a function of the root square rotation rate obtained with a 79 nm M, (b) pH = (a) oxide 6lm. (a) pH = (0) 2.49 and CH~; = 0.032 M, (A) 2.89 and 0.012 hi, (I) 3.38 and O.OO4
1.58, (A) 2.02 and (I)
2.42.
Hydrogen evolution reaction on anodic titanium oxide
1297
loY[i/Acti21 -2.2 5
-2ao
-1.75
-150
jF; I -
g I
1
l-O.6
- 2.50
- 2.75
- 3.00
3.25
log[i/Acti21
Fig. 11. Log (1 - i/i_,) DSlog i relation for HER on an oxide film of 79 nm at digerent potential values. (a) pH = 1.50, Cnso;= 3.22 x 1O-2 M and (b) pH = 2.42, Cclo;= 0.35 M.
us log Cu for both negative and positive potential sweeps, gEes a slope equal to 1 (Fig. 12). This fact supports the above result of nu+ = 1. On the other hand, since the slope for negative and positive potential sweeps are the same, the hysteresis phenomenon does not seem to produce changes in the kinetics for the electroreduction of H’ ions. As i,,, depends only on the charge transference process at the interphase, a plot of potential vs log i,, at sufficiently cathodic overpotentials should lead to the Tafel equation for the HER process (Fig. 13). The slope obtained was about 0.06 V dec-‘. Figure 14 shows that the same Tafel slopes are obtained for both solutions in the positive and negative potential sweeps. The E, ,z us log w “I plot leads to the same conclusions.
lois i,,,
Eflect of the oxide thickness and temperature on HER in HCiO,/ClO~ solutions. The variation of the oxide film thickness has apparently no influence on the magnitude of the cathodic limiting current density in experiments performed in quasi-stationary conditions at constant pH and w in HClO,/ClO, solutions, but, a shift of the current profile towards more negative potential values as d increases is observed (Fig. 15). However, the magnitude of the reactivation in the current between the negative and positive direction was practically not affected by the increase of d. On the other hand, the increase in the temperature produced a shift of the i-E profiles towards less negative potentials as well as an increase of i,at d, pH and w constants. ‘O¶c ‘IT.
lO9C”.
“so&7
EIV
. .
-l,zs -1.15
*
-1.05
A-1.10
b)
b)
E/V .-UO
::i/
Fig. 12. Log i,,
E/V
H
us log Cu+ relation for acidic sulphate and perchlorate electrolytes. d = 79 run. (a) Negative and (b) positive potential sweeps.
1298
R. M. TORRES et al.
log [i,,,/Ac&21 -2
-3
-4
r
-1
I
I
~-0.95
F
I
’
I
PH : 3.15 = 0.35M
_
-1.15
Fig 13. Tafel diagram for different pH values in acidic sulphate and perchlorate electrolytes. d = 79 nm. w = (0) 264, ( x ) 1132, (A) 2224 and (I) 4164 rpm.
loglimax/Acri$l -3
-4 1
-2
-1
I
I
I
m
The results obtained for the i,value as a function of w112 with film thickness between 3.3 and 79 nm and at different temperatures are shown in Fig. 16. As expected, the thickness of the oxide film does not significantly affect the limiting current density because the rate determining step of the reaction in this potential range is the Hf ion diffusion in the bulk of the solution. The diffusion coefficient of H+ ion depends on the following temperature according to the expression[48]
DH. = Do exp (-
AE+,/RT),
(11)
Do is a constant and AEg is the activation energy for the diffusion process of the species in solution. This equation predicts a linear dependence between log DHf and T-‘. A value of 2.35 kJ mol-’ for AEE, was obtained from the slope. The effect of the temperature on the charge transference process was investigated through the variation where
Fig. 14. Tafel diagram obtained with (a) negative and (b) positive potential sweeps. d = 79 nm. w = (a) 264, ( x ) 1132,
(A) 2224 and (I)
4164 rpm.
I
I
___
20
e&lO’B
.._._.
33 .
1.4- lo20 0 Y
p”s 2.37 C,.,,4’0.35M
I
1
-1.5
I
- 1.0 E/V
I
I
w
E
I
I
- 0.5
Fig. 15. Influence of the oxide film thickness on quasistationary i-E profiles for HER, v = 0.01 V s- ’ and w = 960 rpm.
Fig. 16. Influence ofthe oxide film thickness and temperature on the i,vs w”’ relation. d = (a) 3.3, (A) 7.0, (I ) 20.0 and ( x )
79.0 run.
1299
Hydrogen evolution reaction on anodic titanium oxide of the slope of the E us log i,, plots with the temperature for oxide films of different thickness (Fig. 17). A change in the Tafel slope and a shift of straight lines toward less negative potentials as T increased is observed. The value of i _xat a given potential depends on the trmperature according to[45] 1-X
= Bexp(-AE:/RT),
(12)
where B is a constant and AE: is the activation energy for the charge transference process. From the slope of log i_vs T-’ plot, AEZ for different potentials can be obtained. These values depend on the applied overpotential according to[35] AE:
= AEi--a
F(E-E’),
(13)
where AE !j corresponds to the activation energy at the overpotential zero and the constant a is related to the charge transference coefficient[49, SO]. In those systems with oxide surface films, it is difficult to determine AEZ because the equilibrium potential of the electrode is not known due to the fact that the potential of the system at open circuit would be a corrosion potential[39]. Nevertheless, Fig. 18 shows the AE$ values as a function of the potential for different thickness of the oxide film. It can be observed that the kinetics of the HER is not modified by a change in the thickness of the oxide film, and that the AE: for this system must be independent of the thickness. The latter conclusion is supported by the fact that the equilibrium potential of the oxide film is hardly dependent on its thickness. The E us log i,, plots (Fig. 19) obtained for different thickness did not show any changes in the values on the Tafel slope though a shift of the straight lines towards more negative potentials was observed as the thickness was increased. This result would indicate that the charge transference coefficient of the HER is independent of the thickness of the oxide film. Some studies about the oxygen evolution reaction on thin platinum and iridium anodic oxide film[Sl, 521 or with the ferro/ferricyanide system on titanium and niobium anodic oxide film[53] have shown an inverse
> \
- 1.15
273’KW 298-K 333’K
I--
t
-3
.
-2
linear behaviour between the activation energy and the overpotential and a direct linear behaviour between log iand film thickness, qualitatively in agreement with the results presented in this work. They explained these phenomena taking into account an electron transfer process through an oxide film which was correlated to the probability of the quantum mechanical tunnel transition. An explanation of our results based on their model would not be convenient due to complications in the HER considering that it is an inner sphere reaction involving some adsorbed intermediates. On the other hand, in the potential range where the HER takes place (negative with respect to the flat hand potential), the conduction band of the semiconductor bends down towards the Fermi level. In this case, the kinetics of the electron transference on a semiconductor should be like that of a metal and the i-E profiles would not depend on oxide film thickness, but these predictions are not in agreement with our results. The different steps reported for the HER[54,55] are Y= SH (Volmer
S+H&+e
C= H,
SH+H&+e
reaction),
(Heyrowski
HZf” (Tafel
SH+SH=
reaction),
reaction),
(14) (15) (16)
where S and SH stand for the active site and for the adsorbed hydrogen atom over the surface, respectively. The experimental results can be explained taking into account a first step of fast proton discharge (Volmer reaction) followed by the recombination reaction (Tafel reaction) which would be the rate determining step[56, 573 under Temkin adsorption conditions. From this mechanism, the following rate expression can be written[20]: i = -2
F Ku”+
exp(-
FEfRT),
(17)
where it is supposed a symmetry factor of 0.5 and K is a constant which includes the rate constant of the recombination step and the equilibrium constant of the charge transference step.
.
-1
d.20nm
0
1
tog(i,,,/Acm-2)
Fig. 17. Influence of the oxide film thickness and temperature on the Tafel diagram. w = (0) 264, (I) 1164. (A) 2502 and ( x ) 4164‘pm, Cao; = 0.35M and pH = 2.40.
700, (0)
R. M. ToRaFsiet
al.
explained taking into account the different hydrogen species ad/absorbed in the oxide films. As it was previously reported[58-611, the absorbed hydrogen in the oxide film changes its electronic properties, increasing the dielectric constant and the donor impurities concentration. All these modifications would promote a higher electrical conductivity of the oxide film. This phenomenon will be dealt with in another paper[62].
I - 1.5
CONCLUSIONS
50 - 1.0
- 0.5
E/V
Fig. 18. Dependence of the activation energy of the HER with the electrode potential for different oxide. film thicknesses.
Iog[i,,,/Acrii*
1
-4
I\
c,,,o,=
0.35t.A PHZ
2.40
_ -1.10 m
\ <
d/WT
.
3.3
.
m.0
.
79.0
_ -1.20
Fig. 19. Influence of the oxide film thickness on the Tafel
diagram. T = 298 K and w = 1000 rpm.
The proposed mechanism implies that the HER takes place at the interphase oxide/electrolytic solution and would explain the independence of the Tafel slope, na* and AE:on the film thickness, but it does not enable us to account for the diminution of the current as d increases. This seems to be not clear at all and must be further investigated. The values of the activation energy would indicate a mixed control of the reaction in the potential range studied. It can also be observed that the AE:values are higher than those obtained for the HER on mckel[45]. This fact suggests that the oxide film has a significant influence on the rate of HER increasing the activation energy with respect to other metals in which the HER takes place on an electrode completely free of any oxide film. In this sense the experimental results agree with the high overpotentials necessary for the reaction to take place in a significant magnitude. The observed hysteresis in the current profile of the HER can not be merely explained by a decrease in the thickness of the oxide film. In Fig. 14 it is possible to observe a depolarization of about 0.18 V between both sweeps. Based on the results of Fig. 19, this depolarization should correspond to a decrease in the thickness from 79 to 3 nm; however, this deduction is not in agreement with the characteristics of the oxide film obtained after only one potential cycle in the potential range of HER. The shift of the current profiles towards less negative potentials that occur during the positive sweep can be
From the experiments of eleetroformation of oxide layers on titanium through impedance measurements, it is possible to conclude that iq the potential range studied the increase in the thickness is a linear function of the anodic final potential. This effect is also observed at high potential values where the oxygen evolution reaction occurs simultaneously with the oxide electroformation. Assuming that the density of the oxide is independent of the potential sweep rate, a decrease of the anodization coefficient and the dielectric constant with the increase of u were observed. This means that the structural characteristics of the oxide films mainly depend on the way in which these films were electroformed. The results hitherto obtained in H,SO, and HClO,+ solutions in the pH range between 1 and 4 can be well described by a rate equation which may be derived from the mechanism consisting of two consecutive steps for HER: one involving the solvated hydrogen ion reduction and the other the recombination of the adsorbed hydrogen species, the latter being the rate determining step under Temkin-type adsorption isotherm conditions. In the potential range studied, the HER shows a mixed control but, the activation energy for the charge transference process is too high to be attributed to a mixed control only. This should be attributed to the influence of the oxide film on the HER. This influence is confirmed through the shift of the i-E profile towards more negative potentials with the increase of the film thickness. According to the results obtained, the changes in the HER seem to be produced by either the modifications in the thickness or by the donor impurity concentration in the oxide film. Nevertheless, the model derived for positive potentials with respect to E,, does not seem suitable to explain our results The depolarization observed in the i-E profiles can not be explained by a decrease in the thickness film. This fact could be explained by an increase in the oxide film conductivity. Acknowledgements-The authors wish to thank Dr V. A. Macagno for his interest in these investigations and for valuable advice and discussion. Financial support from the Consejo National de Investigaciones Cientificas y T&micas (CONICET)and the Consejo de Investigaciones Cientificas y Tecnologicas de Ia Provincia de Cordoba (CONICOR) is gratefully acknowledged. R.M.T thanks CONICET for the fellowship granted.
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