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ATR STUDIES OF WATER ON EMERSED ELECTRODES
J. Gordon
To cite this version:
J. Gordon. ATR STUDIES OF WATER ON EMERSED ELECTRODES. Journal de Physique Col-
loques, 1983, 44 (C10), pp.C10-171-C10-174. �10.1051/jphyscol:19831034�. �jpa-00223491�
JOURNAL DE PHYSIQUE
Colloque CIO, supplément au n°12, Tome W, décembre 1983 page C10-171
ATR STUDIES OF WATER ON EMERSED ELECTRODES
J.G. Gordon II
IBM Research Laboratory, 5600 Cottle Road, San Jose, CA 95193, U.S.A.
Résumé - En utilisant la spectroscopie de plasmon de surface, nous avons observe' que le film d eau sur une électrode d'argent émergée a une épaisseur d'au plus 4nm.
L'épaisseur est fonction de l'humidité relative et, à faible humidité, du potentiel.
Abstract - We have used surface plasmon spectroscopy to determine that the water on an emersed silver electrode is at most 4 nm thick. The thickness depends on the re- lative humidity and, at low humidity, on the emersion potential.
Recently Hansen and his collaborators have presented evidence that under con- trolled conditions an electrode can be re- moved from solution with no change in the surface charge or electrode potential.1 2
It appears that the electrode is removed with at least the compact part of the dou- ble layer intact.3 * If the emersed elec- trode were representative of the electrode interface in solution, then this would be a very useful technique for studying interfacial structure. The still unanswered question is "How closely does the surface layer of an emersed elec- trode resemble the interphase of an im- mersed electrode?" In answering this question, an essential piece of informa- tion is the thickness of the water layer removed with the electrode.
EXPERIMENTAL
The essential features of the ATR-surface plasmon technique are illus- trated in Figure 1. Polarized light is shined onto the base of a prism coated with a thin silver film. For p-polarized light, the reflectivity is a minimum at a particular angle of incidence as shown in the bottom half of Figure 1. At this an-
gle, a surface plasma oscillation is ex- Angle of Incidence cited at the silver air interface. The
coupling angle is affected by the optical Fig. 1. Top: Internal reflection config- constants and thickness of a film at this uration. Bottom: Reflectivity versus an- interface and can be used to quantitative- gle of incidence.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831034
ATR Cell
JOURNAL DE PHYSIQUE
Reflection Cell
Fig. 2. Small volume attenuated total re- flection cell. Dashed lines are channels for electrolyte and electrodes.
ly determine these parameters.' For transparent materials of known refractive index, the shift in the minimum is a di- rect measure of the thickness.
The electrodes were silver films, about 50 nm thick, deposited on sapphire or LASES glass prisms. Two different cells were used. One, illustrated in Fig- ure 2, was a small volume cell machined from a Plexiglasm block. The electrode was sealed to this cell with an '0'-ring and the electrolyte and counter and refer- ence electrodes were brought in through plastic HPLC fittings. Emersion was ac- complished by injecting a bubble of argon into the central compartment. This iso- lated the film, but left the reference and counter electrodes immersed in a common solution. The other type of cell is il- lustrated in Figure 3. In this one, the prism with the silver film on its base was suspended in the center of a polished cy- lindrical beaker. Only glass, Kel-FTM or gold plated copper contact the electro- lyte. The liquid level was adjusted to immerse the bottom half of the prism. Em- ersion was accomplished by draining the electrolyte from the cell. The two cells yielded comparable results.
Fin. 3. Large attenuated total re- flection cell.
The experimental procedure was a fol- lows. The electrode was first cleaned electrochemically by polarizing it at -1.3V vs SCE for fifteen to twenty minutes and then changing the solution. This pro- cedure was repeated until the ATR spectrum and the cyclic voltammogram between -1.3 and +0.2V no longer changed. The dielec- tric constant of the silver film always became more metallic, closer to bulk metal, after the pretreatment. The elec- trode was polarized at a chosen potential and then emersed with the potential still applied. The ATR spectrum was recorded.
Then dry argon was blown through the cell for 15 minutes to dry the electrode and the ATR spectrum recorded again. This was the bare silver reference state. The cell was refilled and the procedure repeated for a different potential or electrolyte composition. The optical constants and thickness of the silver film were deter- mined from a least squares fit of Fresnel's equations to the reflectivity curves from the argon purged The overlayer thickness was calculated either by fitting the reflectivity curve or from the shift of the angle of the minimum us-
ing a linear approximation valid for small shifts.
RESULTS AND DISCUSSION
The results for an electrode emersed from .1 M NaF solution into water satu- rated argon are given in Table I. The wa- ter layer is approximately 3.8(+.4) nm thick. The thickness is independent of electrolyte concentration and electrode potential between
+.
12 and -1.28 V vs SCE.TABLE I. Water thickness (nm) high relative humidity Wavelength
(nm) 600 650 700 750 800 800 850 900 950 1000 Average
Potential
Measurements were made at many wave- lengths to obtain an indication of the ac- curacy of the technique and to test the validity of the assumption that blowing argon through the cell produced a dry electrode. In the absence of a direct measurement, of course, we cannot be cer- tain that the argon purged electrode has no residual film. But, if there were a substantial film on this reference sur- face, one would expect a systematic vari- ation in the apparent water thickness with wavelength. This is not observed. Any residual film is probably at most one mon- olayer and has a negligible effect on the thickness determination for the wet elec- trode. Model calculations confirm that the NaF residue from evaporation of 4 nm of electrolyte is optically undetectable, as is the ionic diffuse layer.
These results suggest that indeed em- ersion removes at most 4 nm of electrolyte and support Hansen's claim that the elec- trode does not carry a thick (hundreds of nm's) electrolyte layer with it. It is
possible that even less is removed, since, in this experiment, we probably do not measure the thickness of the water removed with the electrode, but rather, the equi- librium thickness under the atmospheric conditions (relative humidity) in the ATR cell. The water layer varies with the hu- midity, being thinner at lower humidity.
Some typical results are listed in Table 11. The thicknesses we measure are compa- rable to those reported by Rice, Phipps and Tremoureux.' They used a quartz mi- crobalance to measure isotherms for the adsorption of water on various metals in air and report, for example, that about 11.5 monolayers or 3.6 nm of water is ad- sorbed on gold at 80% R.H. and 2 5 O ~ .
TABLE 11. Water thickness (nm) low relative humidity Wavelength
500 550 600 650 700 750 800 Average
Potential +.12V -.38V -1.28V
.30 1.4 2.2 .27 1.3 2.4 .25 1.2 2.5 .24 1.2 2.5 .24 1.2 2.5 .22 1.2 2.5 .26 1.2 2.5
A surprising observation is that, at low humidity, the water thickness is strongly dependent on the potential of em- ersion. For the experiment reported in Table 2, it varies from 0.26 nm to 2.5 nm corresponding to from 0.84 to 8.1 monolay
-
ers of water, respectively (assuming a monolayer thickness of 0.31 nm). Unfor- tunately, while the relative humidity was constant from emersion to emersion in this experiment, we do not know its value.
Clearly the more negative the poten- tial, the more water is adsorbed, implying that the heat of adsorption of water in- creases in the same way. The two things strongly affected by potential are the electric field at the surface and the ion- ic composition of the interface. It is possible that the electric field orients the water molecules at the surface.and hence changes the water-water bonding.
This effect might be expected to extend some few nanometers into the solution.
C10-174 JOURNAL DE PHYSIQUE
A l t e r n a t i v e l y , t h e r e a r e more p o s i t i v e bye l e n g t h , a few nanometers. I t i s puz- i o n s n e a r t h e s u r f a c e a t n e g a t i v e poten- z l i n g why t h e s t r o n g l y s o l v a ~ a d f l u o r i d e t i a l s and t h e presence of t h e s e s t r o n g l y i o n would n o t have t h e same e f f e c t . s o l v a t e d ions may a l t e r t h e a c t i v i t y of C l e a r l y , a s y s t e m a t i c s t u d y of a d s o r p t i o n water i n t h e i r v i c i n i t y l e a d i n g t o s t r o n g - a s a f u n c t i o n of e l e c t r o l y t e composition e r a d s o r p t i o n . This e f f e c t would c l e a r l y and c o n c e n t r a t i o n and e l e c t r o d e p o t e n t i a l extend t o a d i s t a n c e comparable t o t h e De- i s needed t o s o r t o u t t h e s e c o n t r i b u t i o n s .
REFERENCES.
[ I ] W . N . Hansen, C . L . Wang, and T. W . Humphreys, J . E l e c t r o a n a l . Chem 90 (1978) 137.
[ 2 ] N . Hansen, C . L . Wang, and T. W . Humphreys, J . E l e c t r o a n a l . Chem.
93 (1978) 87.
[ 3 ] N. Hansen and D . M. Kolb, J . E l e c t r o a n a l . Chem.
100
(1979) 493 [ 4 ] D . M . Kolb and W. N . Hansen, Surface Science2
(1979) 205[ 5 ] J. G . Gordon 11, SPIE Proceedings 276 (1981) 96
[ 6 ] I . Pockrand, J . D . Swalen, J . G . Gordon I 1 and M . R . P h i l p o t t , S u r f a c e Science
74 (1977) 237
[ 7 ]
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Pockrand, J . D . Swalen, J. G . Gordon and M . R . P h i l p o t t , J . Chem. Phys.70 (1979) 3401
[ 8 ] W. R i c e , P. B . Phips, and R . Tremoureux, J . Electrochem.Soc.
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(1979) 1460 and127
(1980) 563[ 9 ] P.B.P. Phipps and D . W . Rice, i n " ~ o r r o s i o n Chemistry," ACS Symposium S e r i e s , 89
(1979) G. R . Brubaker and P. B . P. Phipps, Eds. pp 236-259.
