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Conductance measurement of thin oxide films on a platinum anode
Ektr?rhimica
Acm,
1977, Vol. 22, pp. 175-179
Pergamon Press. Printed in Great Britarn
CONDUCTANCE FILMS
MEASUREMENT ON A PLATINUM S.
Department
OF THIN ANODE
OXIDE
SH~ATA
of Chemistry, College of General Education, Tohoku University, Kawauchi, Sendai, Japan 980 (Final version received 27 May 1976)
AbstractZonductance of two types of oxides, which were galvanostatically formed on a smooth platinum anode with 200mA/cmZ at 45°C in 0.5 M HzSO.,, were measured with a new conductance Cell. It was found that two types of oxide films differ in conductance property. The conductivity of the one, a outermost monolayer oxide film, was equal to that of metalic Pt. The other, a multilayer oxide which grows under the monolayer oxide, showed a definite conductivity lower than that of the metal. The conductivity per monolayer of this oxide in the vertical direction of the film plane was 0.67 x 10’ Q-’ per unit real surface area. The specific conductance value of the multilayer oxide was estimated to be about l-2 x 10m3 R-’ cm-‘, assuming that the multilayer oxide is composed of Pt02 and that this monolayer is twice the thickness of a PtO monolayer.
INTRODUCTION was once believed that the growth of an oxide film on a pt anode is limited to a monolayer because of high electronic conductivity of the oxide. Recent electrochemical[l-12] and optical[lF161 measurements have shown that the quantity of the bound oxygen increases with increasing potential and exceeds that of the monolayer. It was found also that under severe anodic conditions two types of oxide form[17-251. The first one (called here oxide a) which is reduced in a broad potential range attains a limiting amount[l2,2&28]. In contrast to this type, the second one (called here oxide B), which is reduced in a narrow range of potential more negative than that of oxide, grows deeply adjacent to the metal at a rduch slower rate, forming a multilayer. For example, it grows to as many as about 60 layers of Pt02 by galvanostatic anodization with 100mA/cmZ for 28 h in 1 M HzS04[19]. The sustained growth of the oxide film requires the transport of material through the phase involved and across the interface. In general, the growth rate of the oxide may be controlled by either the transport rate of oxidizing particles (or metal) through boundaries between oxide and metal or oxide layer. The growth of oxide tl was explained in terms of the placeexchange[8,11,15,29-311 of adsorbed oxygen with underlying substrate metal atoms. On the other hand, the explanation for the growth of oxide fi is still inadequate. Oxide /?, if the growth is the boundary control like that of oxide CZ,should show the same growth rate with that of oxide a. There is, however, a very large difference between these growth rates. Therefore, it is expected that the growth of oxide /? is controlled ‘by the transport rate of the oxidizing particles through the oxide phase. When the oxidizing particles are ions, their transport rate may depend largely on the strength of the electric field in the oxide phase. A useful guide for the explanation of the anodic formation of oxide @ may be obtained from the conductance measurement of oxide layers.
It
EXPERIMENTAL
Electrodes
and electrolyte
Electrodes used for the conductance measurements were smooth platinum foils of 7 x 7mm and 0.025 mm in thickness. These were cut from the same large foil. Each of the electrodes was spot-welded to a platinum wire of 0.25 mm in diameter. The wire was sealed in soft glass tubing. Before sealing, the electrodes were treated successively with hot chromic acid and hot concentrated hydrochloric acid followed by washing with distilled water. The electrode was then heated to yellow glow in a gas flame. The electrolyte was OSM H,S04 solution which was made from triple distilled water and reagent grade sulfuric acid. Oxide film Electrodes were oxidized galvanostatically for a given time (lOs-210min) at 45 -I_ O.l”C with 2OOmA/cm* (all the current densities are expressed per geometric cm2 unless otherwise stated). The oxide films were analysed in 02-free solution at 25 + O.l”C by a galvanostatic stripping method with 50pA/cm2 and a potentiodynamic polarization method with a linear decreasing voltage at a rate of 2OmV/s. Voltammetric curves were recorded using an X-T or X-Y recorder. All the amounts of oxides were determined graphically with voltammetric curves corrected for the double layer charging and were expressed in charge per geometric cm’ unless otherwise stated. All potentials were referred to a hydrogen electrode in the same solution, although measured against a Hg, HgzSOd, 0.5 M H,SO, reference electrode. Apparatus
and conductance measurement
The contact of the test foil with a measuring circuit is one of the most critical parts of the experimental set up. The symmetrical cell design of Fig. 1 was devised to make the complete contact using mercury. The anodized electrode was dried in desiccated N2 gas at a room temperature for 2-3 h and then cut off from the spot-welded Pt wire. The foil was placed 175
176
S. SHIBATA
RESULTS
Formation and qeing
c-
Fig. 1. Conductance cell: A, sample foil; B, glass disk; C, polyacrylic resin plate; D, foam rubber cushion; E, glass tube; F, copper wire; G, mercury.
between flattened glass disk holders (B). During the cutting-off and placing of the foil in the test cell, extreme care was taken to avoid mechanical damage to the surface. A circular hole made at the center of each disk was 5 mm in diameter. A constant amount of purified mercury was poured into the cell. Copper wires, 2mm in diameter, were coated with insulating paint leaving an exposure of the end 5 mm in length so as to keep the area in contact with the mercury constant. Trouble caused by the formation of copper amalgam was avoided by using clean mercury in every run. When the applied voltage from the battery which was employed as power supply for the measuring circuit (a double bridge circuit) was zero, no residual voltage of the conductance cell was detected. This indicates that potentials which may generate at the interfaces between different phases were offset because of the symmetrical arrangement of these interface in the cell. The measurements were carried out in air thermostat&l at 25 + 0.2”C. A double-bridge circuit was used to measure the conductance. The circuit was connected through F with the cell. The change in resistance of 1 x 10m6 R could be measured without any diEculty. The total resistance value of the conductance cell mounting the foil without oxide was constant from foil to foil, giving the value of 5.3717 + 0.0001 mSL.
AND DISCUSSION
of oxides
Galvanostatic stripping curves for electrodes anodized for t 5 4 min exhibited only a plateau c(, which is due to reduction of oxide CI(Fig. 2). The amount of oxide D:increased gradually with the anodization time. When the anodization time exceeded 4min, a second sloping plataeu 8, which is due to the reduo tion of oxide j3, began to appear, and at the same time the increase in length of plateau D:stopped. The limiting amount of oxide D(was 2.9 mC/cm’. Plateau j? became horizontal during the anodization for times greater than about 12min and the length increased without limit as the anodization time was increased (Fig. 3). Progress of the growth of oxides a and fi were more clearly seen from E-i curves taken with the potentiodynamic polarization method. Some of curves are shown in Fig. 4. All show a distinct wave with a broad maximum at O.&O.5 V. This wave corresponds to the plateau c( of the galvanostatic stripping curve. The second peak corresponding to plateau B appeared when anodized for 2 > 4min. At the given scanning rate, peak J? lies in the same potential region as that of the first wave of hydrogen adsorption. The amount of oxide a did not. exceed a limiting value even if anodization time was prolonged. This limiting value was equal to that determined galvanostatically. A change in the state of the platinum oxide may occur after the electrode is removed from the electrochemical environment and dried and contacted with mercury to measure the conductance. To investigate this change, stripping curves were taken after ageing in the dry state for 3 h or after ageing under the same condition and then contacting with mercury for 5 min. These ageing and contacting times were equal to respective maximum ones spent actually during the conductance measurement. The stripping curves were
CH*RGE
Fig. 2. Cathodic stripping curves with 50 fi/cm’ for electrodes preanodized with 200 mA/cm’ at 45°C for 10s (a and a’) and 4 min (b and b’). Solid lines: immediately after anodization. Dotted lines: after ageing and contacting with Hg.
Conductance 1.6
_
1.2
-
of thin oxide films
measurement
w z w >
2 me/cm
>
-
w 0.4
B
0.0
-
CHARGE
Fig. 3. Cathodic stripping curves with 50 PA/cm’ for electrodes preanodized with 200 mA/cm’ at 45°C for 90min. Solid lines: immediately after anodization. Dotted lime: after ageing and contacting with Hg.
compared with the one taken immediately after the anodization. Typical curves of electrodes anodized for t g 4min are shown in Fig. 2. The rest potential in the O,-free solution dropped steeply after ageing and contacting with mercury, and the potential of plateau a for the electrode which was anodized for 10s shifted to more negative side by 40mV. No shift of the plateau potential, however, was observed when
the electrode was anodized for t > 4 min, except for the initial part of the plateau where a slight shift was observed. Figure 3 shows typical curves for the electrodes anodized for longer time, The potential shift of plateau a by ageing was similar to that of curve b’ in Fig. 2. Plateau B immediately after the anodization was almost horizontal. After ageing, the initial part rose slightly to the positive side and the final
-260_
A
-240-
-2oo-
B -I 60“E 0 > ri
(I -JZO_ :
._ -80
-
-40
-
O-
+40
I
I
I. 6
1.4
I. 2
E
I
I
I
I ..I
0.6
0. 6
0.4
I
I
0.2
0.0
(VVSNHE)
Fig. 4. Potentiodynamic E-i curves at 20mV/s for electrodes preanodized at 45°C for various times; 10 s (a), 1 min (b), 2 min (c), 4 min (d), 6 min (e), 12 min (fJ and 15 min (g). Broken line is the cyclic curve between 1.5 and 0.03 V.
178
S. SHIBATA Table 1. Change in total amount of oxides by ageing for 3 h and contacting with Hg for 5 min
Anodization time
(n-kin) 0.17 1.0 2.0 4.0 8.0 15.0 30.0 60.0 90.0 210.0
Before contacting with Hg
Immediately after
QIWWn’)
After ageing
Q&Wcm’I
1.53 1.91 2.21 2.71 3.39 4.85 7.95 11.51 14.78 22.78
After contacting with Hg
Q&Wcm2)
1.21 1.32 1.40 1.75 2.48 4.18 7.05 10.60 13.90 22.10
part fell slightly to negative side. However, no signifi-
cant change in length of plateau B was observed. The same results were obtained for all of the oxides formed under the condition of various anodization times. Any conversion of oxide a to oxide /3 or vice versa by ageing was not observed_ Total amounts of oxides a and @ before and after ageing are summarized in Table 1. Loss in amount of oxides by ageing in the dry state was almost constant for all the electrodes oxidized for t > 4min. Stripping curves in Figs 2 and 3 indicate that the lost oxide is the one that is reduced above 0.65 V. The saturation hydrogen coverage on the test electrode was 390~4C/cn?, giving the roughness factor of 1.86. The charge associated with a monolayer of chemisorbed oxygen (one oxygen atom to each surface platinum atom) was established to be 780 _~C/cm’ using the hydrogen coverage. This amount is close to the lost amount. In addition, the chemisorbed oxygen was reduced at 0.75 V and exhaustively at 0.65 V by galvanostatic cathodization with 50 pA/cm’. Consequently, it is concluded that the loss of oxide by ageing is virtually limited to the chemisorbed oxygen at the monolayer level. Any difference between stripping curves of the aged oxides before
AMOUNT
OF
OXIDE
1.18 1.37 1.58 1.92 2.58 4.05 7.10 10.75 13.90 21.88
Loss by ageing and contacting with Hg
Qr-QlnCmC/cm2) 0.35 0.54 0.64 0.79 0.81 0.80 0.85 0.82 0.88 0.90
and after contacting with mercury was not observed. Mercury is considered to be inactive for the aged oxides under the present condition. Conductance
Resistance data are plotted as a function of the total amount of oxide, after drying, in Fig. 5, taking the value for the reduced electrode as zero. No change in conductance was observed for all thd electrodes which were covered with oxide a only. When the total oxide exceeded 2.1 mC/crr?, the resistance began to increase. Since the limiting amount of oxide a was 2.1 mC/cm’ after drying the increase of the resistance is undoubtedly due to oxide j?. Although the nature of oxide a changed by ageing, as was shown in Fig. 2, this change is independent of the conductance property. Consequently, it is concluded that the conductance of the electrode decreases linearly with the amount of oxide p. The conductivity per monolayer of oxide in the vertical direction of the film plane is calculated to be 0.67 x lo5 R-’ per real cm’, taking the composition of oxide p as PtOJ35-J. The specific conductance value of oxide b could not be exactly determined, because the thickness of oxide ,!I is still unknown. However, it is satisfactory to say
tmclcm’i
Fig. 5. Plot of resistance us total amount of oxide.
Conductance
measurement of thin oxide films
that a monolayer thickness of PtO, is larger than that of PtO. The thickness of the PtO monolayer was estimated to be O&1.61& by an optical method[36]. This value is not unreasonable when compared with the ionic radii of platinum and oxygen, 0.5 and 1.4A. Approximating the thickness of the monolayer of oxide B to be twice as large as that of PtO, we can calculate the specific conductance value of oxide fi to be 1.1-2.1 x 10m3 a-’ cm-‘. When a current of 500 mA/real cm2 is passed through this oxide the ohmic drop in this phase is about 25&5OOV/cm. Although the strength of the electric field depends not only on the ohmic drop but on space charge distribution[37] in the oxide phase, the following condition may be fulfilled in oxide /?, e&z 4 kT where Y is distance between stable positions of an ion in the oxide and z is the charge number of the migrating particle. Under this condition, the low field approximation can be applied to the growth of oxide B. Then the flux i of migrating oxidizing particles is expressed a~[387
. S. W. Feldberg, C. G. Enke and C. E. Bricker, d. electro&em. Sot. 110, 826 (1963).
8. M. Visscher and M. A. V. Devanathan. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.
where v is frequency factor, D is the diffusion coefficient and a the activity of the particles in the oxide phase, respec$vely. Balej and Spalekc23-j showed that the growth rate of oxide p is approximated by this equation.
26. 27. 28. 29
Acknowledgement-Financial support by the Japan Chemical Research Foundation is gratefully acknowledged..
30. 31,
REFERENCES 1. W. B6ld and M. Breiter, Electrachim. Acta 5, 145 (1961). 2. F. G. Will and C. A. Knorr, Z. Eiektrochem. 64, 258 (1960). 3. H. A. Laitinen and C. G. Enke, J. elecctrochem.Sot. 107, 773 (1960). 4. M. Becker and M. Breiter, Z. Elekfrochem. 60, 1080 (1965). 5. H. Dietz and H. Gtihr, Electrochim. Acta 8, 343 (1963). 6. J. S. Mayell and H. Langer, J. electrochem. Sot. 111, 438 (1964).
179
32. 33 34 35 36 37 38,
J. electroan&. Chem. 8, 127 (1964). S. Gihnan. E[ectrochim. Acta 9. 1025 11964). D. Gilroy’and B. E. Conway, -Can. J: C&m. 46, 875 (1968). P. Stonehart. H. A. Kozlowska and B. E. Conwav..I Proc. Roy. S&z. Londuon A310, 541 (1969). T. Bieder and R. Woods, J. electronnal. Chem. 20, 73 (1969): .I. O’M. Bockris, M. Genshaw and A. K. N. Reddy, J. electroana!. Chem. 8. 406 11964). D. F. A. Koch, Natur; 2Q2, 387 (1964). A. K. N. Reddy, M. A. Genshaw and J. O’M. Bockris, J. &em. Phys. 48, 671 (1968). R. Greef. J. &em. Phvs. 51, 3148 (1969). A. D. ObruEeva. 2. Fiz. Khim. 26. 1448 119521. K. Nagel and G. Dietz, Electroch~m. Act; 4, i (1961). S. Shibata, Bull. &em. Sot. Japan, 34, 526 (1963). S. Shibata, Blrll. them. Sot. Japan, 40 696 (1967). A. Kozawa, J. elecfroanal. Chem. 8, 20 (1964). S. D. James, J. e&trochem. Sot. 116, 1681 (1969). J. Balej and 0. Spalek, C&n Czc& &em. Commun. 37, 499 (1972). S. Shibata and M. P. Sum&, Elcclrochirn. ncto 16, 1089 (1971). S. Shibata’and M. P. Sumino, Electrochim. Acta 17, 2215 (1972). J. S. Mayell and S. N. Langer, J. e[ectrochem. Sot. 111, 438 (1964). M. Fleischmann. I. R. Mansfield and W. F. K. WvnneJones, J. electroannl. Chem. 10, 511 (1965). K. Wiih and B. E. Conway, Anal. Chem. 224, 160 (1967): K. J. Vetter and J. W. Schultae, J. &ctr&nal. Chem. 34 141 (1972). B. E. Cobway and S. Gottesfeld, J. them. Sot. Faraday Trans. 1, 69, 1090 (1973). B. V. Tilak. B. E. Conwav and H. A. Kozlowska J. &ctroanul’Chem. 48, 1 (i973). W. Visscher and M. Bliilevens. J. electroan&. Chem. 47, 363 (1973). H. A. Kozlowska, B. E. Conway and W. B. A. Sharp, .I. elecrroanal. Chem. 43, 9 (1973). R. Thacher and J. P. Hoare, J. elecctroand. Chem. 30, 1 (1971). S. ‘Shib&, Electrochim. Acta 17, 395 (1972). J. L. Ord and F. C. Ho, J. electrochem. Sot. 118. 46 (1971). A. T. Fromhold, Jr. and J. Kruger, J. rlectrochem. Sot. 120, 722 (1973). L. Young, Anodic Oxide Films, p. 13. Academic Press, New York (1961).
Acm,
1977, Vol. 22, pp. 175-179
Pergamon Press. Printed in Great Britarn
CONDUCTANCE FILMS
MEASUREMENT ON A PLATINUM S.
Department
OF THIN ANODE
OXIDE
SH~ATA
of Chemistry, College of General Education, Tohoku University, Kawauchi, Sendai, Japan 980 (Final version received 27 May 1976)
AbstractZonductance of two types of oxides, which were galvanostatically formed on a smooth platinum anode with 200mA/cmZ at 45°C in 0.5 M HzSO.,, were measured with a new conductance Cell. It was found that two types of oxide films differ in conductance property. The conductivity of the one, a outermost monolayer oxide film, was equal to that of metalic Pt. The other, a multilayer oxide which grows under the monolayer oxide, showed a definite conductivity lower than that of the metal. The conductivity per monolayer of this oxide in the vertical direction of the film plane was 0.67 x 10’ Q-’ per unit real surface area. The specific conductance value of the multilayer oxide was estimated to be about l-2 x 10m3 R-’ cm-‘, assuming that the multilayer oxide is composed of Pt02 and that this monolayer is twice the thickness of a PtO monolayer.
INTRODUCTION was once believed that the growth of an oxide film on a pt anode is limited to a monolayer because of high electronic conductivity of the oxide. Recent electrochemical[l-12] and optical[lF161 measurements have shown that the quantity of the bound oxygen increases with increasing potential and exceeds that of the monolayer. It was found also that under severe anodic conditions two types of oxide form[17-251. The first one (called here oxide a) which is reduced in a broad potential range attains a limiting amount[l2,2&28]. In contrast to this type, the second one (called here oxide B), which is reduced in a narrow range of potential more negative than that of oxide, grows deeply adjacent to the metal at a rduch slower rate, forming a multilayer. For example, it grows to as many as about 60 layers of Pt02 by galvanostatic anodization with 100mA/cmZ for 28 h in 1 M HzS04[19]. The sustained growth of the oxide film requires the transport of material through the phase involved and across the interface. In general, the growth rate of the oxide may be controlled by either the transport rate of oxidizing particles (or metal) through boundaries between oxide and metal or oxide layer. The growth of oxide tl was explained in terms of the placeexchange[8,11,15,29-311 of adsorbed oxygen with underlying substrate metal atoms. On the other hand, the explanation for the growth of oxide fi is still inadequate. Oxide /?, if the growth is the boundary control like that of oxide CZ,should show the same growth rate with that of oxide a. There is, however, a very large difference between these growth rates. Therefore, it is expected that the growth of oxide /? is controlled ‘by the transport rate of the oxidizing particles through the oxide phase. When the oxidizing particles are ions, their transport rate may depend largely on the strength of the electric field in the oxide phase. A useful guide for the explanation of the anodic formation of oxide @ may be obtained from the conductance measurement of oxide layers.
It
EXPERIMENTAL
Electrodes
and electrolyte
Electrodes used for the conductance measurements were smooth platinum foils of 7 x 7mm and 0.025 mm in thickness. These were cut from the same large foil. Each of the electrodes was spot-welded to a platinum wire of 0.25 mm in diameter. The wire was sealed in soft glass tubing. Before sealing, the electrodes were treated successively with hot chromic acid and hot concentrated hydrochloric acid followed by washing with distilled water. The electrode was then heated to yellow glow in a gas flame. The electrolyte was OSM H,S04 solution which was made from triple distilled water and reagent grade sulfuric acid. Oxide film Electrodes were oxidized galvanostatically for a given time (lOs-210min) at 45 -I_ O.l”C with 2OOmA/cm* (all the current densities are expressed per geometric cm2 unless otherwise stated). The oxide films were analysed in 02-free solution at 25 + O.l”C by a galvanostatic stripping method with 50pA/cm2 and a potentiodynamic polarization method with a linear decreasing voltage at a rate of 2OmV/s. Voltammetric curves were recorded using an X-T or X-Y recorder. All the amounts of oxides were determined graphically with voltammetric curves corrected for the double layer charging and were expressed in charge per geometric cm’ unless otherwise stated. All potentials were referred to a hydrogen electrode in the same solution, although measured against a Hg, HgzSOd, 0.5 M H,SO, reference electrode. Apparatus
and conductance measurement
The contact of the test foil with a measuring circuit is one of the most critical parts of the experimental set up. The symmetrical cell design of Fig. 1 was devised to make the complete contact using mercury. The anodized electrode was dried in desiccated N2 gas at a room temperature for 2-3 h and then cut off from the spot-welded Pt wire. The foil was placed 175
176
S. SHIBATA
RESULTS
Formation and qeing
c-
Fig. 1. Conductance cell: A, sample foil; B, glass disk; C, polyacrylic resin plate; D, foam rubber cushion; E, glass tube; F, copper wire; G, mercury.
between flattened glass disk holders (B). During the cutting-off and placing of the foil in the test cell, extreme care was taken to avoid mechanical damage to the surface. A circular hole made at the center of each disk was 5 mm in diameter. A constant amount of purified mercury was poured into the cell. Copper wires, 2mm in diameter, were coated with insulating paint leaving an exposure of the end 5 mm in length so as to keep the area in contact with the mercury constant. Trouble caused by the formation of copper amalgam was avoided by using clean mercury in every run. When the applied voltage from the battery which was employed as power supply for the measuring circuit (a double bridge circuit) was zero, no residual voltage of the conductance cell was detected. This indicates that potentials which may generate at the interfaces between different phases were offset because of the symmetrical arrangement of these interface in the cell. The measurements were carried out in air thermostat&l at 25 + 0.2”C. A double-bridge circuit was used to measure the conductance. The circuit was connected through F with the cell. The change in resistance of 1 x 10m6 R could be measured without any diEculty. The total resistance value of the conductance cell mounting the foil without oxide was constant from foil to foil, giving the value of 5.3717 + 0.0001 mSL.
AND DISCUSSION
of oxides
Galvanostatic stripping curves for electrodes anodized for t 5 4 min exhibited only a plateau c(, which is due to reduction of oxide CI(Fig. 2). The amount of oxide D:increased gradually with the anodization time. When the anodization time exceeded 4min, a second sloping plataeu 8, which is due to the reduo tion of oxide j3, began to appear, and at the same time the increase in length of plateau D:stopped. The limiting amount of oxide D(was 2.9 mC/cm’. Plateau j? became horizontal during the anodization for times greater than about 12min and the length increased without limit as the anodization time was increased (Fig. 3). Progress of the growth of oxides a and fi were more clearly seen from E-i curves taken with the potentiodynamic polarization method. Some of curves are shown in Fig. 4. All show a distinct wave with a broad maximum at O.&O.5 V. This wave corresponds to the plateau c( of the galvanostatic stripping curve. The second peak corresponding to plateau B appeared when anodized for 2 > 4min. At the given scanning rate, peak J? lies in the same potential region as that of the first wave of hydrogen adsorption. The amount of oxide a did not. exceed a limiting value even if anodization time was prolonged. This limiting value was equal to that determined galvanostatically. A change in the state of the platinum oxide may occur after the electrode is removed from the electrochemical environment and dried and contacted with mercury to measure the conductance. To investigate this change, stripping curves were taken after ageing in the dry state for 3 h or after ageing under the same condition and then contacting with mercury for 5 min. These ageing and contacting times were equal to respective maximum ones spent actually during the conductance measurement. The stripping curves were
CH*RGE
Fig. 2. Cathodic stripping curves with 50 fi/cm’ for electrodes preanodized with 200 mA/cm’ at 45°C for 10s (a and a’) and 4 min (b and b’). Solid lines: immediately after anodization. Dotted lines: after ageing and contacting with Hg.
Conductance 1.6
_
1.2
-
of thin oxide films
measurement
w z w >
2 me/cm
>
-
w 0.4
B
0.0
-
CHARGE
Fig. 3. Cathodic stripping curves with 50 PA/cm’ for electrodes preanodized with 200 mA/cm’ at 45°C for 90min. Solid lines: immediately after anodization. Dotted lime: after ageing and contacting with Hg.
compared with the one taken immediately after the anodization. Typical curves of electrodes anodized for t g 4min are shown in Fig. 2. The rest potential in the O,-free solution dropped steeply after ageing and contacting with mercury, and the potential of plateau a for the electrode which was anodized for 10s shifted to more negative side by 40mV. No shift of the plateau potential, however, was observed when
the electrode was anodized for t > 4 min, except for the initial part of the plateau where a slight shift was observed. Figure 3 shows typical curves for the electrodes anodized for longer time, The potential shift of plateau a by ageing was similar to that of curve b’ in Fig. 2. Plateau B immediately after the anodization was almost horizontal. After ageing, the initial part rose slightly to the positive side and the final
-260_
A
-240-
-2oo-
B -I 60“E 0 > ri
(I -JZO_ :
._ -80
-
-40
-
O-
+40
I
I
I. 6
1.4
I. 2
E
I
I
I
I ..I
0.6
0. 6
0.4
I
I
0.2
0.0
(VVSNHE)
Fig. 4. Potentiodynamic E-i curves at 20mV/s for electrodes preanodized at 45°C for various times; 10 s (a), 1 min (b), 2 min (c), 4 min (d), 6 min (e), 12 min (fJ and 15 min (g). Broken line is the cyclic curve between 1.5 and 0.03 V.
178
S. SHIBATA Table 1. Change in total amount of oxides by ageing for 3 h and contacting with Hg for 5 min
Anodization time
(n-kin) 0.17 1.0 2.0 4.0 8.0 15.0 30.0 60.0 90.0 210.0
Before contacting with Hg
Immediately after
QIWWn’)
After ageing
Q&Wcm’I
1.53 1.91 2.21 2.71 3.39 4.85 7.95 11.51 14.78 22.78
After contacting with Hg
Q&Wcm2)
1.21 1.32 1.40 1.75 2.48 4.18 7.05 10.60 13.90 22.10
part fell slightly to negative side. However, no signifi-
cant change in length of plateau B was observed. The same results were obtained for all of the oxides formed under the condition of various anodization times. Any conversion of oxide a to oxide /3 or vice versa by ageing was not observed_ Total amounts of oxides a and @ before and after ageing are summarized in Table 1. Loss in amount of oxides by ageing in the dry state was almost constant for all the electrodes oxidized for t > 4min. Stripping curves in Figs 2 and 3 indicate that the lost oxide is the one that is reduced above 0.65 V. The saturation hydrogen coverage on the test electrode was 390~4C/cn?, giving the roughness factor of 1.86. The charge associated with a monolayer of chemisorbed oxygen (one oxygen atom to each surface platinum atom) was established to be 780 _~C/cm’ using the hydrogen coverage. This amount is close to the lost amount. In addition, the chemisorbed oxygen was reduced at 0.75 V and exhaustively at 0.65 V by galvanostatic cathodization with 50 pA/cm’. Consequently, it is concluded that the loss of oxide by ageing is virtually limited to the chemisorbed oxygen at the monolayer level. Any difference between stripping curves of the aged oxides before
AMOUNT
OF
OXIDE
1.18 1.37 1.58 1.92 2.58 4.05 7.10 10.75 13.90 21.88
Loss by ageing and contacting with Hg
Qr-QlnCmC/cm2) 0.35 0.54 0.64 0.79 0.81 0.80 0.85 0.82 0.88 0.90
and after contacting with mercury was not observed. Mercury is considered to be inactive for the aged oxides under the present condition. Conductance
Resistance data are plotted as a function of the total amount of oxide, after drying, in Fig. 5, taking the value for the reduced electrode as zero. No change in conductance was observed for all thd electrodes which were covered with oxide a only. When the total oxide exceeded 2.1 mC/crr?, the resistance began to increase. Since the limiting amount of oxide a was 2.1 mC/cm’ after drying the increase of the resistance is undoubtedly due to oxide j?. Although the nature of oxide a changed by ageing, as was shown in Fig. 2, this change is independent of the conductance property. Consequently, it is concluded that the conductance of the electrode decreases linearly with the amount of oxide p. The conductivity per monolayer of oxide in the vertical direction of the film plane is calculated to be 0.67 x lo5 R-’ per real cm’, taking the composition of oxide p as PtOJ35-J. The specific conductance value of oxide b could not be exactly determined, because the thickness of oxide ,!I is still unknown. However, it is satisfactory to say
tmclcm’i
Fig. 5. Plot of resistance us total amount of oxide.
Conductance
measurement of thin oxide films
that a monolayer thickness of PtO, is larger than that of PtO. The thickness of the PtO monolayer was estimated to be O&1.61& by an optical method[36]. This value is not unreasonable when compared with the ionic radii of platinum and oxygen, 0.5 and 1.4A. Approximating the thickness of the monolayer of oxide B to be twice as large as that of PtO, we can calculate the specific conductance value of oxide fi to be 1.1-2.1 x 10m3 a-’ cm-‘. When a current of 500 mA/real cm2 is passed through this oxide the ohmic drop in this phase is about 25&5OOV/cm. Although the strength of the electric field depends not only on the ohmic drop but on space charge distribution[37] in the oxide phase, the following condition may be fulfilled in oxide /?, e&z 4 kT where Y is distance between stable positions of an ion in the oxide and z is the charge number of the migrating particle. Under this condition, the low field approximation can be applied to the growth of oxide B. Then the flux i of migrating oxidizing particles is expressed a~[387
. S. W. Feldberg, C. G. Enke and C. E. Bricker, d. electro&em. Sot. 110, 826 (1963).
8. M. Visscher and M. A. V. Devanathan. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.
where v is frequency factor, D is the diffusion coefficient and a the activity of the particles in the oxide phase, respec$vely. Balej and Spalekc23-j showed that the growth rate of oxide p is approximated by this equation.
26. 27. 28. 29
Acknowledgement-Financial support by the Japan Chemical Research Foundation is gratefully acknowledged..
30. 31,
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