Infra red spectroscopy of the electrode-electrolyte interphase

Surface Science 101 (1980) 131-138 @ North-Holland

Publishing

Company

INFRA RED SPECTROSCOPY ELECTRODE-ELECTROLYTE Alan

OF THE INTERPHASE

BEWICK and Keiji KUNIMATSU* of Chemistry, University of Southampton,

Deparfment Received

7 November

Southampton

SO!, 5NH,

UK

1979

A simple specular reflectance method is described for obtaining the infra red spectra of species in the electrode/electrolyte solution interphase. The method is shown to be sufficiently sensitive to enable aqueous electrolytes to be used. Results are presented indicating the existence of discrete water structures, resembling small clusters of water molecules, in the inner region of the double layer at platinum and electrodes. Preliminary data are also given for adsorbed hydrogen on a platinum electrode.

1. Introduction Electrochemists are borrowing increasingly from the techniques recently developed in other areas of surface science in order to try to obtain vital data on the electronic and molecular structure of the electrode/electrolyte interphase. This trend is clear from the papers in the present volume and from those presented at other meetings [l]. A fundamental limitation of most of these techniques is the requirement that the surface should be in a vacuum during the measurement. This seriously curtails their range of useful application to electrochemical problems. In situ spectroscopic methods have also been developed: reflectance spectroscopy [2-41 using uv-visible radiation is yielding valuable information on the electronic properties of the interface and it is also an established tool of great power for the determination of mechanism and kinetics in complex processes involving nonadsorbed intermediates, but it lacks specificity towards molecular structure; more recently Raman spectroscopy has been used to identify adsorbed molecular species [5,6] and non-adsorbed reaction intermediates [7] but difficulties with instrumentation and interpretation are hindering the full exploitation of the method. We have recently reported [8] the successful development of the simple technique. modulated specular reflectance spectroscopy, into the true vibrational infra red region; spectra were obtained l

On leave from:

Research

Institute

for Catalysis. 131

Hokkaido

University,

Sapporo,

Japan

19

A. Bewick, K. Kunirmtsu

! IR spectroscopy of elecrrodu-electmlyte

interphaw

both from aqueous and non-aqueous systems. Previous attempts 19. IO] to apply IR spectroscopy to species at the electrode surface made use of internal reflectance at germanium electrodes; little useful information was obtained and the method has not been developed.

2. Experimental The method is entirely analogous to modulated specular reflectance For measurements with aqueous spectroscopy in the uv-visible region. systems the electrode was pushed to within a few microns of the window which was a flat Si or Si02 plate. Typical time constants of about 3 X 10 ’ were observed for charging of the double layer in solutions of 1M coc~cc~~tration. For most of the measurements. parallel polarised radiation was employed since the perpendicularly polarised component is effectively inactive at the metal surface and only vibrational modes producing dipole changes with a component perpendicular to the electrode surface will be observable from adsorbed species.

3. Results and discussion

Spectra obtained from a platinum electrode in IM H,SO, with modulation of the potential in two different regions are shown in fig. 1: modulation between 0.4 and 0.75 V is entirely within the double layer region and between 0.5 and 0.05 V it spans from the double layer region to a potential at which the electrode is substantially covered with adsorbed hydrogen. The spectrum from the double layer region is relatively small, Hat and featureless, the sign and magnitude being consistent with the data for the uv-visible region [ 1l] which have been ascribed predominantly, for this angle of incidence, to the electroreflectance effect. The spectrum for the hydrogen region is very different: it rises to a broad maximum which is an order of magnitude larger than the signal in the double layer region and it has superimposed upon it a number of smaller peaks. The sign of the effect corresponds to an increase in the amount of radiation reflected when the electrode is covered with adsorbed hydrogen. This is the same sign as that observed between 300 and 1000 nm for strongly adsorbed hydrogen [l 11. Measurements were made at a number of fixed wavelengths while sweeping the potential linearly between 0.43 and 0.05 V in order to try to observe separately the affects of the different kinds of adsorbed hydrogen. A small amplitude, square-wave modulation was superimposed on the linear sweep

A. Bewick, K. Kunimatsu

I IR spectroscopy of electrode-electrolyte

interphase

133

/ j-

I

I

4

5 X/P

Fig.

I. Spectrum

from PtilM

HzSO+

Modulation

at 8.5 Hz between

the stated

potentials.

so that the differential optical coefficient R-’ dR/dE could be obtained as a function of potential. An example is given in fig. 2 together with the corresponding current/potential linear sweep voltammogram; this latter shows the adequate time response of the thin layer cell configuration. The major feature is the large peak in R-I dR/dE, which correlates with the current peak for the formation of strongly bound hydrogen, and its sign corresponds to increased reflectance with increasing coverage, 0, by hydrogen i.e. R-‘dR/dO is positive. There is, however, a smaller peak of opposite sign which correlates with the current peak for weakly bound hydrogen. There is, therefore, an absorbance from weakly bound hydrogen which contrasts markedly with the increased reflectance from the strongly bound. These two components of opposite sign were found over the complete range of wavelength from 3.86 to 5.3 pm and we conclude that there is a broad absorbance from the weakly adsorbed hydrogen in this region although the sensitivity of the method does not enable us to determine its shape. This contrasts with the rather sharp peak at 1.73 pm reported by Pliskin and Eischens [16] for adsorption from the gas phase. In the solution

134

A. Bewick.

Fig. 2. Potential &SO,

K. Kuninrutsu

/ IR spectroscopy

dependence of the di!ferenti;~l

across the hydrogen adsorption

superimposed currertt/pt,tentiai

of electrode-electrolyte

ahsorbancc.

R

‘(dR/dE).

irlterphuse

:I! 3.M pm

for Pt/lM

region from 0.335 to 0.05 V. Sweep speed I5 mV s

square wave modulation voit~tnlm~~rgram without

of + 10 mV at S.5

f-17.

Da?hcd line:

’ with

the cornxponding

rn(~d~il~lti(~Il.

phase, considerable interaction between adsorbed hydrogen and the water is to be expected and this could lead to a much broader peak. The magnitude of the reflectivity increase for strongly bound hydrogen is reasonably consistent with that found at shorter wavelengths [I I]. A sample calculation using refractive index data for platinum from the literature [17] (rj = 5 and k = -20 at 4 pm) and values for strongly adsorbed hydrogen based upon the hehaviour at shorter wavelengths (n = 5 and k = -30) gave changes AR/R of about +3x IW’.

.7.2. Changes in the water structure In current theories of the double layer the structure of the water in the inner region. and in particular the role of small clusters of water molecules. is given particular prominence [12]. It is interesting to note, therefore, that modulation of the electrode potential in the aqueous systems we have investigated invariably gives rise to large changes of optical absorption in the wavelength range c~~rresponding to O-H vibrations. 2.7 to 3.1 pm. Figs. 3 and 4 show data for the modulation of the Pt/lM HzSOj interface over two ranges of potential. one in the double layer region and the other into hydrogen adsorption. Spectra for the Au/O.SM NaF interface are given in figs. 5 and 6 for modulation from near the point of zero charge (E,,,, = 0.25 V) to large positive and large negative potentials respectively. In all

A. Bewick.

K. Kunimatsu 1.5

z

J IR spectroscopy

I

of electrode-electrolyte

interphase

135

I

q 2.76

1.0

L
2.72 0.5

i C

d

3.0

2.6

3. 4

x/CL

Fig. 3. Spectrum 0.8 V at X.5 Hz.

from

Pt!lM

H$SOr in the O-H

absorption

region.

Modulation

from

0.1 to

1

I

l-

B . 5 ro

0 -

.5 -

0

I 2.6

I 3.0

3.4

X/P

Fig. 3. Spectrum 0.6 V at 85 Hz.

from Pt/lM

Hz!504 in the O-H

absorption

region.

Modulation

from 0.05 to

cases the presence of several sharp bands within the O-H region is strikingly apparent. These bands appear to be superimposed on a broad absorbance background. In the case of gold, the sign of the change corresponds to increased absorbance at +0.7 V and at -0.5 V as compared to that at 0 V. whereas for platinum, there is increased absorbance at the more negative potential in each experiment. The origin of these absorbance changes appears to lie in the changing structure of the water at the electrode surface; variation in the nature of the anion, e.g. NaC104 instead of NaF, produced only minor changes in the

136

A. Bewick, K. Kunimatsu

1 Ii7 spectroscopy

5

of electrode-electrolyte interphuse

I

I

2.79 2.74

(‘; i !

2.86 #I 3.01

E

h

:;1~

a

/ \\ -

v

0

fi;.li

I 3.0

I 2.5

0

X’P

Fig. 5. Spectrum from AulO.5M NaF in the O-H absorption +0,7 V at X.5 Hz.

0

region. M~~dul~ltion from 0 to

f

I

2.5

3.0 x/r-

Fig.

0. Spectrum

from

Au/O.SM

NaF

in

the O-H absorption

region. Modulation

from 0 to

-0.5 V at 8.5 Hr.

spectra. Large changes in the electrical field strength at the electrode surface would be expected to modify the total integrated intensity of the broad absorption band for water. This effect could explain the broad background the sign of the effect is correct. increased absorption on the spectra; absorption at higher field strengths (in the case of Pt, the field will be higher at 0.6 V than at 0.05 V; a large part of the potential drop is in the hydrogen

A. Bewick, K. Kunimatsu

/ IR specfroscopy of electrode-electrolyte

interphase

137

layer [13]). The sharp absorption peaks are particularly interesting. Measurements on bulk water have led to the identification of a number of absorbance bands in this wavelength region [14] but these are always very broad and their exact origin is in doubt, i.e. single vibrations, combination and the energies are possibly modified by Fermi bands, overtones, resonance. On the other hand, sharp bands have been observed for small clusters of water molecules using matrix methods [15]. These bands were observed at 2.70, 2.82, 2.85, 2.95 and 3.00p.m and they were assigned respectively to free OH in a molecule with the other OH hydrogen bonded, the hydrogen bond in the dimer, the hydrogen bond in the trimer, the hydrogen bond in the tetramer and the hydrogen bond in the polymer. These absorption maxima coincide very closely with those observed in the present work. Our tentative conclusions are that near the potential of zero charge, the bulk water structure persists up to a position very close to the electrode surface; at high surface field strengths orientation and electrostriction disrupts the polymeric structure and favours the formaton of small clusters. Further experiments are required to confirm this and measurements on HDO should be particularly useful in establishing the origin of the various sharp bands.

4. Conclusion The data presented show that very useful IR spectra from the electrode/electrolyte interphase can be obtained by a simple external reflectance method and the analysis of these should lead to information of great value in many areas of current interest.

References [l] Topics in Surface Chemistry, Proc. Intern. Symp., Bad-Neuenahr. West Germany, 1977 (Plenum, New York, 1978). [2] J.D.E. McIntyre, in: Optical Properties of Solids; New Developments, Ed. B.O. Seraphin (North-Holland, Amsterdam, 1976). [3] T. Kuwana, Ber. Bunsenges. Physik. Chem. 77 (1973) 85X. [4] A. Bewick, J.M. Mellor and S.B. Pans, Electrochim. Acta 25 (1980) 931. [5] P.J. Hendra, M. Fleischmann and A.J. McQuillan, JCS Chem. Commun. (1973) 80. [6] M. Fleischmann, P.J. Hendra, R. Cooney and E. Reid, J. Am. Chem. Sot. 99 (1977) 2002. [7] R.P. Van Duyne and D.L. Jeanmarie, J. Am. Chem. Sot. 98 (1976) 4034. [8] A. Bewick, K. Kunimatsu and B.S. Pans, Electrochim. Acta 25 (1980) 465. [9] H.B. Mark and B.S. Pans, Anal. Chem. 3X (1966) 119. [lo] A.H. Reed and E. Yeager, Electrochim. Acta 15 (1970) 1345. (111 A. Bewick and A.M. Tuxford, J. Electroanal. Chem. 47 (1973) 255.

13x

A. Bewick. K. Kunimalsu

1 IR .spectrmcopy

of elecfrode-elecrml~re

interphrrse

[12] B.B. Damaskin and A.N. Frumkin, Electrochim. Acta 10 (1973) 173: R. Parsons. Electroanal. Chem. SO (lY75) 220. [I31 B.B. Damaskin, O.A. Petrii and V.V. Batraker. Adsorption of Organic Compound\ on Electrodes (Plenum. New York. 1971) ch. 0. 1141 R.E. Verrall, in: Water. A Comprehensive Treatise. Vol. 2. Ed. F. Franks (Plenum, New York. 1972) ch. 5. [IS] W.A.P. Luck, in: Water. A Comprehensive Treatise. Vol. 2. Ed. F. Franks (Plenum. New York, 1972) ch. 4. [lb] W.A. Pliskin and R.P. E&hens, Z. Physik. Chern. 24 (I~JhO) I I, [ 171 A.P. Lenham and D.M. Trenherne. J. Opt. Sot. Am. 56 (1966) 1137