An ellipsometric investigation of adsorbed layers on platinum electrodes at high anodic potentials

329

Electroanalytical Chemistry and Interracial Electrochemistry Elsevier Sequoia S.A., L a u s a n n e - Printed in The Netherlands

AN E L L I P S O M E T R I C I N V E S T I G A T I O N OF A D S O R B E D LAYERS ON PLATINUM ELECTRODES AT HIGH ANODIC POTENTIALS

R O G E R P A R S O N S * AND W. H. M. VISSCHER**

Department of Physical Chemistry, Tile University, Bristol, BS8 fTS (England) (Received 7th December 1971)

INTRODUCTION

Controversy continues concerning the nature of the oxygen film formed electrolytically on platinum electrodes, in particular as to whether this is limited to the amount which could be accommodated in a monolayer. Some new coulometric results were presented recently by Biegler and Woods I who also summarized the literature relevant to this limiting coverage. The evidence they presented was in favour of the formation of a monolayer*** and this has since been supported by a revised method for the estimation of the surface area of platinum electrodes 2. Since ellipsometric methods for the determination of film thickness in principle do not depend on the area of the electrode it appeared that such measurements could lead to confirmation of the interpretation given by Biegler and Woods. Related work by Woods 3 has indicated also a limiting surface coverage on platinum electrodes in acetate solutions. Under anodic polarisation in these solutions the Kolbe reaction occurs and there is also controversy about the nature of the adsorbed layer4~ In particular the problem of the inhibition of oxygen evolution and its replacement by the Kolbe reaction is not fully understood although the presence of an absorbed dipolar layer of acetate radicals has been proposed to account for this phenomenon. Most of the evidence for the nature of the layer under these conditions has come from electrical measurements which are not specific4. Hence the use of an independent method such as ellipsometry should provide evidence for the nature of the surface film. EXPERIMENTAL

An ellipsometer for measurements on a horizontal sample was contructed in the workshops of this department. The divided circles for the polariser, analyser and quarter-wave plate can be read with the aid of verniers to 1', and 0.5' can be estimated. The polariser and analyser are Glan-Taylor prisms supplied by Crystal Optics, Chicago. The retardation plate was mica sheet and was placed between the polarizer * To whom correspondence and reprint requests should be addressed ** Permanent address: Laboratorium voor Elektrochemie, Technische Hogeschool Eindhoven, Postbus 513, Eindhoven, The Netherlands. *** See also ref. 17.

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and sample. The light source was a Mazda 125 watt mercury vapour lamp in a watercooled holder. The light was collimated using a diaphragm, lens and pinhole before the polarizer. The light intensity was measured with a photomultiplier (EM1 type 6256B) powered with a Brandenburg E H T supply (type PM. 2500/R) and recorded using a Servoscribe recorder. An interference filter (Barr and Stroud, 2 = 546 nm) was placed immediately in front of the photomultiplier window. Calibration of the instrument was carried out according to the procedure described by McCrackin et al.5. Most of the experiments were carried out in a trapezium cell of fused "suprasil" quartz with angle of incidence ~b= 68° 55', the light beam being perpendicular to the cell windows. The platinum electrode was a flat sheet (3.5 x 2 cm) on the bottom of the cell, held down by a U-shaped piece of glass which rested on the edges of the Pt sheet parallel to the light beam. The working electrode potential was controlled with a Chemical Electronics (valve type) potentiostat with respect to a saturated calomel electrode in a compartment connected by a bridge to the optical cell. The Pt auxiliary electrode was in the optical cell. Potentials are referred to the potential of a hydrogen electrode in the working solution and are not corrected for I R drop. A few experiments were carried out with a different arrangement in which the optical windows were fixed at the end of nylon tubes attached directly to the arms of the ellipsometer. The ends of these tubes could be submerged in the electrolyte near the working electrode. In this way the angle of incidence could be varied with the beam remaining perpendicular to the optical windows. In order to keep the cell sufficiently small the optical windows were small (0.5 cm diameter) and the angle of incidence was limited to less than (60° ) that used with the trapezium cell. The sensitivity was therefore better with the latter. Before each run the electrode was pretreated outside the cell by alternate anodic and cathodic charging in 0.2 M H2SO4. Measurements were made at room temperature. After setting up the cell, ellipsometer readings were taken at steady potentials starting from a point close to the hydrogen potential. The potential was held for 15 min before readings were taken. At potentials about 1.8-1.9 V gas bubbles begin to be formed and at higher potentials they are copiously evolved. This seriously interferes with the ellipsometric measurements. Attempts to deal with this problem by measuring after a number of potential pulses were not successful since bubble formation could not be prevented. Flushing the electrolyte across the electrode surface was also unsatisfactory. The method used by Biegler and Woods 1 was therefore adapted for the ellipsometric measurements. The platinum was anodized at a high potential for a fixed time (15 rain) and then the potential was lowered to a value (1.5 V) at which it can be assumed that no reduction of the film occurs. While the electrode was held at this potential the gas bubbles were removed by passing a stream of N 2 along the surface. All the measurements reported here were made with an angle of incidence of 68 ° 55'. The refractive index of the aqueous solutions was measured with an Abb6 refractometer using light from a sodium lamp and corrected to 546 nm by comparison with the refractive index of water. Some coulometric measurements were also made (in Eindhoven) following the method of Biegler and Woods1. The experiments were carried out in a three-compartment cell using a Pt-hydrogen electrode in the same solution as a reference electrode. The working electrode was a Pt foil, 1.8 cm z in area, which was pretreated by heating to redness in a flame. It was then washed in chromic-sulphuric acid followed by twice J. Electroanal. Chem., 36 (1972)

331

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distilled water. Once in the cell the electrode was activated by sweeping the potential from 1.5 V to 0.05 V (vs. the hydrogen electrode) several times. Measurements were made by oxidising the electrode for 15 min under potentiostatic (Wenking) control. A cathodic sweep of 111 mV s 1 was then applied and the voltammogram displayed on a Philips X - Y recorder. For all potentials greater than 1.5 V the potential was first lowered to 1.5 V while oxygen-free nitrogen was passed through the solution to remove all oxygen ; the cathodic sweep was then applied. The true surface area was estimated from the hydrogen peaks in the voltammogram assuming 6 that 1 cm 2 ofa monolayer corresponds to 210 #C, i.e. relative to an ideal 100 plane. The observed roughness factor was 1.17 in 1 M sulphuric acid, 1.07 in 0.5 M and 1.03 in 2 M acetate solution. The oxygen reduction peak gave the oxygen coverage Qo. RESULTS

Figure 1A shows a typical result for the variation of O and A with potential for a platinum electrode in 1 M H2SO4. To a first approximation the change in A from its value on a film-free surface is proportional to the thickness of the film. Thus the Apotential plot may be compared with the coulometric results obtained by Biegler and Woods 1 or with the similar results obtained in the present work and shown in Fig. lB. The general features are in agreement: oxide film formation begins at about 0.8 V, ,

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R. PARSONS, W. H. M. VISSCHER

then growth is approximately linear up to an inflection point at about 1.5 1.6 V ; finally there is an approximately constant thickness above 2.0 V. In the optical measurements the linear section between the inflection and the limiting value is less welldefined than in the coulometric work. Variation of the potential at which oxygen is removed and at which the optical measurements are made on films formed at higher potentials does not cause significant changes in these results. The coulometric results lead to a limiting value of Qo/2QHof 2.68 for a 15 min oxidation time compared with the value of 2.66 found by Biegler and Woods 1 for oxidation times from 100 to 5100 s (this ratio is based on the same assumption as in their work; their more recent work 2 starts from slightly different assumptions about QH)- At the lower potentials these results appear to agree with the more detailed results of Ord and H o 7, which were obtained much more rapidly than the present results. It is not clear from their work whether they observed the inflection at 1.5-1.6 V, since their measurements are limited to lower potentials than shown in Fig. 1. In addition their technique for obtaining results at 1.5 V and above is so completely different from that used here that comparison may not be valid, in view of the time dependence reported by Biegler and Woods 1. Similar experiments were made in acetate buffer solution of unit salt/acid ratio. Results for the 2 M solution are shown in Fig. 2. The optical results in Fig. 2A have the same general shape as the results in sulphuric acid, both the inflection and the

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ELLIPSOMETRY OF Pt AT HIGH POTENTIALS

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limiting coverage being observed. However there is evidence for the formation of a film at lower potentials, possibly of adsorbed acetate, while the main region of film growth is delayed and starts only above 1.0 V. This initial difference is not seen in the coulometric results in Fig. 2A, but it does seem to occur in both coulometric and optical results at 0.5 M. The coulometric results agree with those of Woods 3 and the optical data in yielding a limiting value above 2.0 V. However, Woods did not study the behaviour at lower potentials very intensively and it is not possible to judge from his data whether an inflection is observed. Figure 2 indicates that there is an inflection in acetate solutions similar to that in sulphuric acid. The results in 0.5 M acetate also show these features; the value of Qo/2QH in this solution after a 15 rain oxidation is 2.93 which is somewhat higher than the value of 2.50 found by Woods. The value found here for the 2 M solution after a 15 min oxidation is 2.91. After reduction of the oxide film it was generally found that the value of ~ in the double layer region returned to its original value, while there was a tendency for A to increase by about 0.1 ° above its original value. It is possible that this effect is due to the removal of impurities from the electrode surface by oxidation or alternatively to the reorganization of the surface during the oxidation as described by Biegler 8. At present it does not seem possible to distinguish between these possibilities. DISCUSSION

F r o m the results of measurements in the double layer region the refractive index of platinum was calculated assuming a system of the two bulk phases only. For all solutions studied it was found to be n = 1.967 4-0.01 - (4.46 4-0.01)i. This value was then used to calculate the properties of the oxide film. If the observed value of A is plotted against ~ (Fig. 3) then the points for platinum in sulphuric acid fall on a single straight line within the experimental accuracy. This suggests that these results may be represented by a single refractive index and that there is no change in the composition of the oxide film in the range investigated. This conclusion differs from that of Ord and H o 7 who find a change in film properties at 1.1 V. This difference may be due to the different technique used in the formation of the films or to the smaller number of readings taken in this region in the present ,

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R. PARSONS, W. H. M. VISSCHER

work which could lead to the apparent absence of the small region below 1.1 V observed by Ord and Ho. If, following previous work (see also Schultze 1°) it is assumed that the film is PtO and that the density of this material is 14.9 g cm-3, then the thickness of the film can be calculated from the coulometric data. With these values of the film thickness the optical data can be used to obtain the optical constants of the film. Two computer programmes were used : the first calculates A and ~ for assumed values of the optical constants of the system and the thickness. The second follows the method of McCrackin 5 as set out by Mowat and Muller'1; the film thickness is calculated from the experimental A and ~ values for assumed values of the optical constants. In either case a set of thickness and ~ = n - i k values is obtained. With the assumption that the film has the density of PtO the refractive index of the film is found to be 3.05-1.35i at 546 nm in satisfactory agreement with the value of 3.0-1.5i obtained previously 9. With these assumptions the limiting plateau above 2 V would correspond to a layer just over 0.8 nm thick. Since it is a limiting value it seems reasonable to assume that the coverage is approximately uniform at this thickness. The assumption that the layer has the same density as bulk PtO is of course a very rough one. However, it seems unlikely that the film can be much thinner than 0.8 nm as this would lead to values of both real and imaginary parts of the refractive index which were higher than seems reasonable ( c f Fig. 5 of ref. 12). A thinner layer would also have a density which would seem to be unreasonably high. Thus these results seem to conflict with the suggestion of Biegler and Woods I that the amount of oxygen present on the platinum surface can be accommodated in a monolayer. Since the covalent radii of platinum and oxygen are 0.139 and 0.074 nm respectively, a plane monolayer would have a thickness of not more than 0.42 nm if it is assumed that only the first layer of Pt atoms have modified optical properties as a result of the adsorption. This value is not essentially modified if it is assumed that the surface layer is completely ionic since the ionic radii of Pt 2+ and O 2- are 0.052 and 0.135 nm respectively. In fact 13 the P t - O distance in PtO is about 0.202 nm so that the thickness of a plane of O atoms on top of a plane of Pt atoms must be close to 0.4 nm. Although such a structure is an oversimplification the dimensions of the real structure are unlikely to be largely different from this. Recent measurements of adsorption of oxygen from the gas phase made by Akhtar and Tompkins 14 suggests that two oxygen atoms per original surface Pt atom are active in surface reactions. At low temperatures (195K) "titration" experiments showed that one H atom was replaced by one O atom, but at higher temperatures (343K) larger amounts of O were taken up by the Pt film. Of this oxygen, an amount corresponding to two O per H was readily reducible by hydrogen while the remainder was not and was considered to be incorporated into the Pt lattice. These experiments provide clear evidence for a limiting surface layer comparable to that observed by Biegler and Woods I and in the present experiments as well as for the "dermasorption" of oxygen. The latter would not be observed in the cathodic reduction of the surface layer. Although Akhtar and Tompkins 14 used the formation of PtO 2 on the surface layer as a working hypothesis, it seems more probable that their results, as well as those of Biegler and Woods I and the present results, can be best interpreted in terms of a larger reactivity of oxygen in the first two layers. Oxygen which penetrates further into the lattice of Pt is considerably less reactive. It remains doubtful whether these J. Electroanal. Chem., 36 (1972)

ELLIPSOMETRY OF Pt AT HIGH POTENTIALS

335

two layers should be described as chemisorbed oxygen or as a phase oxide. The crystal structure of PtO is not particularly certain, but it appears 13 to have a bimolecular tetragonal unit cell with ao = 3.04 A and c o = 5.34 A. It thus seems possible that the single layer might consist ofa PtO structure with the C axis perpendicular to the surface. This would then essentially consist of alternate planes: P t - O - P t - O / s o l u t i o n . The kink in ~he coulometric and A-potential curves at 1.5-1.6 V occurs at approximately one O atom per Pt atom and this suggests that up to this point the layer may be considered more as a chemisorbed layer. The tracer experiments of Rozental and Veselovskii 15 show that the oxygen layer formed under these conditions does not contribute to oxygen evolved, whereas oxygen layers formed above 1.5 V do. This suggests that the oxygen laid down at the lower potentials is "buried" at the higher potentials, in agreement with the model described above. On the other hand these two layers of oxygen seem to be optically equivalent since there is no break in the O-A plot at 1.5-1.6 V. In acetate solutions the most striking difference in behaviour appears in the 0 - A plot of Fig. 3. The non-linear nature of this plot indicates clearly that the optical properties of the film are changing. This can be interpreted either using a continuously varying reflective index or by approximating the curves in Fig. 3 by two straight lines in terms of a two-layer model. These calculations were carried out making the assumption that the thickness of the layer was proportional to the charge required to reduce it and that its density was the same as in the HzSO 4 solution. In the 2 M acetate buffer, the first procedure using McCrackin's method leads to a value of h = 2 . 5 - i k with k varying from 3.2 at 1.03 V to 2.0 at the limiting plateau. In 0.5 M solution n drops from 3.3 to 2.9 and k from 1.9 to 1.1. On the other hand with a two-layer model in the 2 M solution, the first layer has fi = 2.5 - 2 . 5 i up to 0.4 nm thick at 1.28 V, then formation of the second layer with ~ = 2.5 - 1.8i. In the 0.5 M solution the first layer has ~ = 3.3 - 1.9i up to 0.3 nm thick at 1.25 V and then formation of the second layer with ~ =2.8 - 0.8i. These results are clear evidence for the presence of acetate residues adsorbed on the electrode in spite of the fact that there is no evidence for their presence in the cathodic reduction of the films formed in these solutions 3. The detailed interpretation of the results is at present open to question owing to the necessity for assumptions about multiple films and their thickness. However there seems little doubt that the presence of acetate in the films causes a drop in the imaginary part of the refractive index. It seems likely that this is related to a reduced ability of the film to transfer electrons and hence to the inhibition of the oxygen evolution reaction. In contrast to the O-A plots, the plots of A against potential show a remarkable similarity to those obtained in H z S O 4. They are also closely similar to the chargepotential plots. Hence it seems probable that the structure of the oxide layer is relatively little affected by the presence of acetate. This would be consistent with the "substituted oxide layer" model of acetate adsorption proposed by Vijh and Conway 16. The acetate radical can thus be considered as replacing the water which is probably adsorbed on the oxide layer in the H z S O 4 but which is not detected by the optical experiments. ACKNOWLEDGEMENT

We are pleased to thank the British Council for a maintenance grant to J. Electroanal. Chem., 36 (1972)

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R. PARSONS~ W. H. M. VISSCHER

W.H.M.V. and for an apparatus grant, and the Technische Hogeschool, Eindhoven for partial support to W.H.M.V. SUMMARY

Ellipsometric measurements have been made of the films formed at high potentials on Pt in H2SO 4 and CH3COOH + CH3COONa solutions. They confirm the coulometric results of Biegler and Woods that there is a limiting amount of adsorbed material at potentials above 2V (vs. RHE). The optical properties of the film formed in H 2 S O 4 appear to be constant throughout the growth of the film whereas those of the film formed in acetate buffer vary during its growth. This demonstrates that acetate is incorporated in the film. This incorporation causes a lowering of the imaginary part of the refractive index which may be related to the inhibiting properties of the film. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

T. BIEGLERAND R. WOODS, J. Electroanal. Chem., 20 (1969) 73. T. BIEGLER, D. A. J. RAND AND R. WOODS, J. Electroanal. Chem., 29 (1971) 269. R. WOODS, J. Electroanal. Chem., 21 (1969) 457. A. K. V i m AND B. E. CONWAY, Chem. Rev., 67 (1967) 623. F. L. MCCRACKIN, E. PASSAGLIA, R. R. STROMBERG AND H. STEINBERG, J. Res. Natl. Bur. Stand., 67A (1963) 363. S. GILMAN in A. J. BARD (Ed.), Electroanalytical Chemistry, Vol. 2, Dekker, New York, 1967 R. 111. J. L. ORD AND F. C. Ho, J. Electrochem. Soc., 118 (1971) 46. T. BIEGLER, J. Electrochem. Soc., 116 (1969) 1131. W. H. M. VISSCHER, Opt&, 26 (1967) 402. J . W . SCHULTZE, Z. Phys. Chem., N.F., 73 (1970) 29. J. R. MOWAT AND R. H. MULLER, Reflection of Liyht from Fihn-Covered Surfaces, U C R C Report No. 17128 (1967). M. A. BARRETTAND R. PARSONS, Syrup. Faraday Soc., 4 (1970) 72. W. J. MOORE AND L. PAULING, J. Amer. Chem. Soc., 63 (1941) 1392. M. AKHTAR AND F. C. TOMPKINS, Trans. Faraday Soc., 67 (1971 ) 2461. K. I. ROZENTALAND V. I. VESELOVSKn, Dokl. Akad. Nauk SSSR, 111 (1956) 637. A. K. V i m AND B. E. CONWAY, Z. Anal. Chem., 224 (1967) 160. G. F. VOLODIN AND Y. M. TYURIN, Dvoinoi Sloi i Adsorbtsiya na Tvevdykh Elektrodakh. 11, Tartu, 1970, p. 124.

J. Electroanal. Chem., 36 (1972)