Processes in solution and on the electrode surface monitored by IRS with metal film electrodes

ELECTROANALYTICAL CHEMISTRY AND INTERFACIAL ELECTROCHEMISTRY Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

327

PROCESSES IN S O L U T I O N A N D ON THE E L E C T R O D E SURFACE MONIT O R E D BY IRS W I T H METAL F I L M ELECTRODES

S. GOTTESFELD* AND M. A R I E L

Department of Chemisto~, Technion-lsrael Institute of Technology, Haifa (Israel) (Received 5th May 1971 ; in revised form 26th August 1971)

INTRODUCTION

Several investigators have employed a combination of electrochemical and IRS techniques at semi-transparent electrodes 1- 7 ; in these combined experiments the reflectance is monitored under conditions of total reflection from a three phase stratified medium, a glass-conducting film-solution, while the film is serving as the working electrode in an electrochemical cell. Figure 1 shows a schematic outline of a cell suitable for "Electro-IRS" (an abbreviated term, to be used henceforth). The electromagnetic wave, striking the electrode-solution interface at an angle exceeding the critical angle, penetrates the solution to the extent of the "penetration depth", 6, only. The effect of the solution on the wave is, therefore, limited to that of a very thin solution layer (less than one wave-length) adjacent to the film electrode. It follows that reflectance measurements of this kind, carried out in situ at a semi-transparent electrode, are potential sources of information regarding processes occurring at or near the electrode, with little or no interference from bulk solution components. A provision for multiple reflections (cf. Fig. 1) enhances sensitivity.

Silicon rubber gasket

Monochromatic beam

,t

Reference electrode

kN/V~--/~

Auxiliary electrode Tefloncell

~

~

Metalfilm electrode

~ V ~ .

=

To the detector

Total reflectance plate (glass) Glass prism Fig. 1. Schematic representation of unit for Electro-IRS measurements.

In the Electro-IRS experiment, modulation of the film potential is used to cause the changes in concentration of an electroactive light-absorbing species in the solution layer near the electrode surface. To date, the main purpose of the electrochemical investigations with the aid of IRS was essentially to learn from experimental * Present address : Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5.

J. Electroanal. Chem., 34 (1972)

328

S. GOTTESFELD, M. ARIEL

results in the form of A R / R o vs. potential, about the dependence of c~ = 0 on the electrode potential 5- s. However, there are some additional reasons for the reflectance to change with electrode potential, besides this composition change in the solution phase : (a) Electromodulation of the optical coefficients of the interface occurs both in a thin metal layer3 and in the double layer in the solution phase 9, as a result of surface charging. (b) Changes in the extent of surface coverage of the electrode, be it by adsorption of the electroactive species, of another species present in solution or by the formation of a surface oxide. The approximate expression correlating changes in reflectance with potential modulation and taking the above into account, is: AR/Ro

= - flN[esolbACx

=0 + ~ ~adidiACadi+ F(Aq)]

(1)

i

with

AR/;~ o - the relative change in rel:lectance a sensitivity factor, related to the optical parameters of the system the number of reflections the molar absorptivity index of the absorbing species in solution ~sol the penetration depth A c x = o - - the change in the concentration of the absorbing species in the monitored solution layer ACad i - - the concentration change of component i, occurring in the adsorbed film the molar absorptivity index of component i ~adi the thickness of the film formed by component i di F(Aq) - - the change in reflectance resulting from electromodulation of the optical parameters (due to injecting the charge Aq into the surface). For mono-molecular films, the use of surface concentrations is more convenient; eqn. (1) is transformed:

N

--

AR/Ro

= - flN[esol6 Acx=o + Z GadiAFi + F (Aq)]

(2)

i

The reflection absorbance, At, as defined by: Ar=loglo ( R o / R ) , is also useful in the description of reflectance changes; for low absorbances A r ~ - (1/2.3)(AR/Ro). The conclusion from the following series of Electro-IRS experiments can be represented, in fact, by eqn. (2) above. Successful IRS monitoring ofa faradaic reaction of a solution component is based on the recognition and correct subtraction of contributions to reflectance changes arising from sources other than the solution phase ; e.g. considerable potential dependent adsorption unless corrected for is apt to lead to misleading interpretation of IRS results. Although other combined methods (normal incidence) can generally be employed for monitoring concentration changes in the solution phase without such difficulties, IRS is still advantageous in some special cases e. 9. where an intermediate (in the solution phase) with a very small bulk concentration is being followed. On the other hand, whenever conditions are such that the absorbance measured is mainly due to light absorption in an adsorbed layer, valuable information about the latter may be obtained from IRS measurements. J. Electroanal. Chem., 34 (1972)

IRS WITH METAL FILM ELECTRODES

329

EXPERIMENTAL

The IRS cell A cell, similar in principle to that used by Hansen i, was employed (Fig. 1). The following modifications improved performance: 1. For experiments requiring an inert atmosphere a closed Teflon cell, fastened to the reflection plate with nylon screws and made water-tight by means of a silicon rubber gasket, was employed. 2. Exact adjustment of the point of incidence of the beam on the first prism is essential both for locating the optimum position (i.e. that in which maximum energy is reflected) and for achieving precision in replicate measurements ; this was effected by a finely threaded screw, placed in the cell base. The thickness of the plates employed was 2-2.5 mm, allowing for 5-6 reflections; the spectrophotometer beam, 10 mm wide on entering the cell, was further limited to 3 mm by an additional slit placed in front of the IRS accessory, The angle of incidence was 73° .

The film electrode A vacuum evaporated gold film, referred to below as "Gold LM"* served for most of the experiments. This film is relatively stable (in solutions that are not too acid) and withstands considerable current densities. Its resistance is approximately 20 f~ per square. The working range of the commercial films is limited to 420 nm by the absorbance of the "green glass" optically flat plates on which they are deposited. Another gold vacuum deposited on Pyrex plates with an intervening Bi20 3 layer 1l, was used in a few experiments.

Apparatus All spectra and measurements were taken with a Perkin-Elmer 350 UV VISNIR spectrophotometer; an identical IRS cell was placed in the reference beam (without the electrochemical system). For measurements at a fixed wave-length 100~ transmittance was adjusted with the complete system in place before any addition or generation of the absorbing electroactive substance had been made. The sensitivity was then increased (to 20~oT or 10~T full scale deviation). For work within a wide range of wave-lengths, significant base-line distortions, resulting from variations in the transmittance of the IRS system with wave-length, occur; these were compensated for using the compensator of the spectrophotometer. The electrical apparatus including a triangular wave signal generator, a potentiostat, and a current measuring unit, was constructed in this laboratory, employing operational amplifiers. A P t wire served as auxiliary electrode, while a commercial SCE micro-electrode with a ceramic tip served as reference electrode.

Reagents Triply-distilled water was used throughout. All reagents employed as supporting electrolytes were of highest grade, commercially available purity. The 4,7dimethyl-o-phenanthroline was supplied by G. Frederick Smith Chemical Co. * Supplied by Liberty Mirrors Div., Libbey-Owens-Ford Glass Company, Brackenridge, Pa. U.S_A. J. Electroanal. Chem., 34 (1972)

330

S. GOTTESFELD, M. ARIEL

I. THE CASE OF A SIMPLE FARADAIC REACTION--CORRECTIONS FOR THE FORMATION OF A SURFACE OXIDE FILM AND FOR ELECTROMODULATION

In the simplest case, when only one of the members of a reversible redox couple absorbs at the wave-length chosen for the IRS experiment, the Electro-IRS system may be put to test through the following equation, correlating changes in reflection absorbance with the electrode potential 1 : E = E ° + (0.059/n) log [(AA . . . .

(3)

- AA,)/AA,]

An earlier work 4, in which the "optical slope" for the oxidation of DMF [Fe n (4,7-dimethyl-o-phenanthroline)3] on gold LM film electrodes was determined, reported 0.052 V (as against the theoretical value of 0.059 V for a one electron exchange), using potential steps imposed on the working electrode in the range + 0.40 to + 0.94 V v s . SCE. In an attempt to reproduce this work it was found that at fixed potentials exceeding + 0.75 V, reflectance tended to drift towards lower values. In a similar experiment, carried out with the supporting electrolyte alone (1 N Na2SO4, pH 5) the behaviour shown in Fig. 2 was found. It is evident that as a result of the potential step, a rapid change in reflectance is obtained (electromodulation effect), accompanied by slow changes ; the latter become increasingly significant at sufficiently anodic potentials and are particularly marked on stepping back to + 0.40 V. These slow changes are the result of surface oxide film formation and of its slow reduction at + 0.40 V. R/%, 98

96 v 94

i/pAcm -2

400

92 90

620

, 400

765 i

v

~

8~0

970

ot

400 4,

16

8

O

,

,

8

Fig. 2. Current/time (lower) and reflectance/time (upper) curves, recorded simultaneously while potential steps (values designated on reflectance curve) were applied between a LM gold film electrode and a SCE. 1 N Na2SO4; 5 reflections; 2=512 nm.

The marked effect of surface oxide formation on IRS results with gold electrodes within the a.m. potential range has not been mentioned to date (only the caution not to exceed +0.94 V v s . SCE, so.as to prevent oxide formation, has been given4 ; relying on the data of Reid and Kruger 12, obtained in 1 M H2SO4, was probably not sufficiently accurate). If the formation of the hydrated surface oxide occurs according to ref. 13, i.e. : J. Electroanal. Chem., 34 (1972)

331

IRS WITH METAL FILM ELECTRODES

A u + H 2 0 ~ Au ... O H + H + + e it should commence at about + 0.65 V vs. SCE (cf. ref. 12), in neutral aqueous solutions. The effectiveness and sensitivity of IRS in sensing the presence of the first oxide layer is illustrated in Fig. 2; unfortunately, since the film electrodes are destroyed by excessive anodic polarization, IRS measurements cannot be used for the study of oxide formation. Current/potential curves and Electro-IRS measurements recorded within a wider potential range (in a closed, deaerated cell), proved the slow reflectance changes to be related to oxide formation. The current-potential curve shown in Fig. 3 exhibits both the increase in oxidation current in the vicinity of +0.7 V and the related reduction current peak (in the cathodic part of the cycle) ; the shape of the latter indicates the slow rate of oxide reduction at + 0.4 V, which in turn explains the slow increase in reflectance measured at this potential during the potential step experiment. Inspection of Fig. 4 shows the increasingly rapid stabilization of reflectance obtained at the more negative potential (0 V vs. SCE), due to the faster reduction of the surface oxide. Both gold and platinum have served as "inert electrodes" for faradaic reactions in the potential range in which the first oxide layer is formed ; in spite of this monolayer, currents resulting from the faradaic reaction of a component present in solution can be measured, provided the concentration of the latter is sufficient and the partial ///LA cm

6

o.b5

o.b~

o.;~

o.os E / V v s . SCE

Fig. 3. Current/potential curve for LM gold film in 1 N Na2SO 4. Rate of potential scan. 20 mV s - 1.

R/Olo ~i0

315

460

1

590

1

690

760

i

i

90

860

950

~

I

o

.

.

,jill ~Acm-2

8

88 86

82 8(

~

~

0

8 t

Fig. 4. Current/time (lower) and reflectance/time (upper) curves, recorded simultaneously while potential steps (values designated on reflectance curve) were applied between LM gold film electrode and a SCE. 1 N Na2SO4; 5 reflections ; 2 = 512 nm. J. Electroanal. Chem., 34 (1972)

332

S. GOTTESFELD, M. ARIEL

coverage of the electrode surface by the oxide does not inhibit the electrode reaction. Thus, the current/potential curves of the relatively concentrated solutions used for Electro-IRS (up to 10 -2 M) are practically unaffected by "residual currents" in the potential range of Au... OH formation. IRS measurements on the other hand may be affected emphatically : the formation of a monolayer of a surface-oxide may cause changes in reflectance equivalent to those caused by a 10-2 M change in concentration occurring in the solution phase. Continued potential step experiments carried out within the range of surface oxide formation did not lead to sufficiently reproducible results ; in addition, the high current density excursion occurring at the instant of applying the potential step rapidly destroys the gold film. On the other hand, the reflectance potential curve, obtained by effecting cyclic linear potential changes in the + 0.40- + 0.94 V range at the rate of 20 mV s- 1, is quite reproducible (surface roughening due to rapid potential changes R/°/o 100 ~

/ / ~ A cm -2

16

9£ 9(] 94 92 9C

0

88 86 84 82

16

8C 0.40

0.90

0.44

0.90

0.44 E/V vs. SCE

Fig. 5. Current/potential (lower) and reflectance/potential (upper) curves, recorded with potential scanning at rate of 20 mV s - 1.1 N Na2SO, ; 5 reflections ; 2 = 512 nm; new LM gold film. Reflectance curve serves as "blank" for results of Fig. 11. R/% loc 9E 9~ 94 92

8

9c 8~ 8c 84 8,c 0.44

0.90

0.44

0.90

0.44

i

0.90 0.44 E/V vs. SC E

Fig. 6. Current/potential (lower) and reflectance/potential (upper) curves, recorded with potential scanning at rate of 20 m V s - t. 1 N N a z S O , ; 5 reflections; 2 = 512 nm. LM gold film had been used in several previous expts, The reflectance carve serves as "blank" for the results of Fig. 8.

J. Electroanal. Chem., 34 (1972)

333

IRS WITH METAL FILM ELECTRODES

has been reported, while slow changes are believed to have little effect14). As a result of the irreversible reduction of the surface oxide, the correlation between reflectance and potential in the linear potential change experiment in l N NazSO4 is non-linear; it is, however, sufficiently reproducible to be used as "blank" to correct reflectance measurements obtained with an identical potential scan after th~ addition of DMF. Prolonged use with multiple potential recycling caused increasing reflectance changes, as well as increasing anodic currents in the "AuOH region" but usually only after several days of work with a single film ; see Figs. 5 and 6. In the absence of any change in surface coverage there is still the effect of electromodulation to be considered, if it has a significant contribution compared to that of the faradaic process. The correction for the electromodulation effect, including the dependence of the latter on wave-length, deserves further comment; the results of calculations using a free electron model and bulk gold optical properties proposed for its estimationa,agree with those obtained experimentally with gold-LM films. However, the fact that thin films prepared by different methods are apt to differ in their spectra, must not be disregarded : a gold film evaporated on Bi203 differs in colour from a commercial gold-LM film ; its reflectance drops sharply between 650 and 550 nm (the 5s--6d transition for gold), but rises slightly again between 550 and 450 nm; this explains the different electromodulation effect, including a reversed sign, observed for the BizO3-gold films in the region 550-450 nm (Fig. 7) (see ref. 3 for the relation between R vs. 2 and 1/R A R / A E vs. 2 curves). Summing up: due to changes in film properties related to film aging (Figs. 5 and 6), together with the difference in behaviour found in films with slightly differing spectra (Fig. 7), "blank" measurements must be taken immediately before the Electro-IRS experiment. The blank measurement must be made with the same film and within the same potential range as that intended for the main experiment, but with only the supporting electrolyte in the cell. When working with film electrodes, linear potential scans are preferable to potentiostatic step methods. Figure 8 shows current/potential and reflectance/potential curves simultaneously recorded in a 12.5 mM D M F solution at a gold-LM electrode ; after correcting (A R/R) / %

,

~

-5 -4

-3 -2 -1 0 +1 +2

I

I

I

I

400

500

600

700

2/nrn

Fig. 7. Reflectance modulation as function of wavelength, resulting from a 0.60 V potential shift, for two different gold films: ( 0 ) gold on Bi203, (Q) LM gold film. J. Electroanal. Chem.,

34 (1972)

334

S. GOTTESFELD, M. ARIEL

R/o/. i//zAcm 2

100' g8

80O

96 94

4OO

92 gO 88 86

400

84 82 80 0.44

8O0 0.90

0.44

0.90

0.44

0,90 E/V

vs,

0.44 SC E

Fig. 8. Current/potential and reflectance/potential curves, recorded with potential scanning at rate of 20 m V s-1, in a 12.5 m M D M F - 1 N N a 2 S O 4 soln. 5 reflections; 2 = 512 nm; used LM gold film. See " b l a n k " in Fig. 6.

for electromodulation and surface oxide formation (using the results shown in Fig. 6 and the principle of additivity of the AR/R contributions, eqn. (2)), the value obtained for the "optical slope" was still far removed from the theoretically predicted 0.059 V ; this discrepancy was at first considered to be only a result of the ohmic drop within the film. II. THE O H M I C D R O P I N FILM ELECTRODES

The resistance of an unused LM-gold film is of the order of 20 D per square; the current densities obtained in the a.m. DMF solutions were of the order of 1 mA cm-2 ; as a result, considerable ohmic drops were expected. Electrical contact to the film was made through silve r paint strips brushed on three sides of the rectangular electrode plate ; at a given externally imposed potential, the potential at any point on the film depends on the current density and the distance of the said point from the contact. An approximate correction carried out as suggested in ref. 4, by determining the average film resistance in a separate experiment using the same cell and employing a current step method, gave a corrected "optical slope" of 0.076 V when applied to the results of the experiment recorded in Fig. 8. To determine the sources of this persistently discrepant slope value, means of reducing the ohmic drop were sought. Contact for monitoring potential deviations Conducting silver paint - -

Solutionlevel

l---

and height of beam in o r d i n a r y measurement

1__

.

.

.

.

.

.

- -

=..~ .....................................

~

Prism

/. T

Solution level and beam height after reduction

Fig. 9. Measurement of ohmic potential drop in the film ; lowering the solution level--a means of minimizing

this potential drop. J. Electroanal. Chem., 34 (1972)

335

IRS WITH METAL FILM ELECTRODES

f~

^

/2

~'k

,,...^.

..77~..L ..... \ ; /

../

,j,,

;:;;

-_.k...... 4 . i ~ ~ 72

,..~, l.~d. -~/ ...~ ,s -i / ' ~ .... ? .............. r l.... ~...... , . . ,N~ /~ ....... ;¢~ ........ ............. \::/ I i

b

a

..... \ ....

,'~ ......

'/

• . . . . . . . . .

,

..\

J . , , :

N. .

.

.

.... ~ ' / \ \j

.

v

....

/-..

~ ' .

.

.

.

.

.

.

.

.

.

.

HII

....

.... 5" "~f c

Fig. 10. Inhomogeneity of the potential in the film, resulting from the ohmic drop: (a) 1.25 m M DMF, 1 N Na2SO4, scan rate:200 mV s -1, used film; (b) 12.5 m M DMF, 1 N Na2SO4, scan rate 20 mV s ' 1" new film; (c) 12.5 m M DMF, 1 N Na2SO4, scan rate 20 mV s -1, new film, lowered solution level. (1) current, (2) potential on the potentiostatically controlled electrical contact; (3) potentia'l at the point of the film farthest removed from the contact.

Figures 9 and 10 show the experimental set-up and results obtained in experiments to determine the degree to which an imposed potential is followed at a point on the film farthest removed from the electrical contact, using a separate silver paint strip contact. This additional contact came down to within 1 mm of the solution face, right in the center of the film. The ohmic drop in an over-used film (Fig. 10a) is considerable even with a D M F solution which is 10 times more dilute than that required for IRS. Completely distorted current/potential curves and reflectance/potential curves were recorded under such conditions, even though no appreciable visual change in the film was evident. New films allow scanning at the rate of 20 mV s- 1 in 12.5 m M solution, with potential differences not exceeding 50 mV between any two points on the film (Fig. 10B). Higher scanning rates are not advisable at these concentrations. Experiments in which the ohmic drop was even smaller were carried out with narrow strips of solution, designed to minimize the distance between any point on the film electrode and the potentiostatically controlled silver paint electrical contact. Solution strips of 4 mm, with the incident beam lowered and limited to this height, were used to record the curves shown in Fig. 10c. The area of the film wet by the solution (i.e. the actual electrode) is now practically equipotential (_+10 mV). Reflectance curves recorded with this improved set-up, and shown in Fig. 11, still exhibit an optical slope of 0.077 V for D M F oxidation ; the current/potential curves, recorded in the original 12.5 m M D M F solution and after its ten-fold dilution, also had the same slope when the narrow film strip was used. Current/potential curves recorded in the same solutions but at gold leaf electrodes exhibitedideally reversible J. Electroanal. Chem., 34 (1972)

S. GOTTESFELD, M. ARIEL

336 RI°Io

iliA cm -2

100'

400

98 96 94 92

400

90 88

800

86 84

1200

8780

o.~o

o.~o

o.~

o.~o

o.~ E/V vs. SC E

Fig. 11. Current/potential and reflectance/potential curves, recorded with potential scanning at rate of 20 m V s - 1 in a 12.5 m M D M F - 1 N Na2SO4 soln. 5 reflections; 2 = 512 n m ; only narrow strip of new L M gold film used as electrode. See "blank" in Fig. 5.

behaviour (Ep-Ep/2=0.056 V), showing that the irreversible behaviour at the gold film electrodes could not be ascribed to adsorbable contaminants present in the solution phase. The reproducible value of 0.077 V for the "optical slope" at a gold film, even in the absence of an ohmic drop, indicates that the rate of electron transfer in the DMF oxidation is probably slower than that at an ordinary gold electrode, possibly due to intensified DMF adsorption on the film surface, causing self inhibition. The presence of this adsorbed DMF layer is indicated by the marked residual absorbance measured at a potential value at which DMF should be completely oxidized; the adsorbed layer is apparently fully electroinactive and the amount adsorbed is not affected by potential change. A more complicated case of adsorption will be dealt with in section IV of this paper, The following general recommendations hold for successful measurement with film electrodes : (a) The films must not be over-used ; their gradual deterioration may be monitored by measuring the accompanying increase in film resistance which serves as a sensitive indicator of its condition. (b) Slow linear potential scans cause less deterioration than potential step scanning. (c) Using narrow solution strips (whenever energy considerations permit the necessary decrease in incident beam cross-area) circumvents the problem of applying approximate ohmic drop corrections. III. INTERMEDIATE MONITORING DURING AN ELECTROCHEMICAL REACTION W I T H STEPWISE ELECTRON REMOVAL

The ostensibly irreversible character of a current/potential curve often gives no immediate indication asto whether it is the result of the slow passage of two electrons or of the overlapping of two rapid, unresolved one electron transfers. In the latter case, the intermediate formed after the first step may also react homogeneously J. Electroanal. Chem., 34 (1972)

IRS WITH METALFILM ELECTRODES

337

with some solution component, or disproportionate. The usefulness of Electro-IRS for the detection of such intermediates was investigated. The reaction studied was the oxidation of N,N,N',N',-tetramethylbenzidine (TMB). Colourless T M B * is oxidized to a di-quinoidal cation (TMBOx), which absorbs strongly (maximum absorbance at 460 nm). The cation radical T M B +, obtained after one electron only is removed from TMB, is fairly stable in acetone-water solutions and has a life-time of a few minutes in neutral aqueous solutions. Its presence in solution gives rise to a fine structure ESR s p e c t r u m and to a greenish coloration (maximum absorbance at 690 nm); both decay simultaneously, proving the T M B + radical to be the absorbing species (and not the dimer, as in the case of otolidine oxidation16). The stability of T M B + in acid solution is severely limited: at p H 3 no optically measurable concentration can be prepared electrolytically, nor can the characteristic ESR signal be detected : the shape of the current/potential curve for the T M B oxidation obtained by cyclic voltammetry at p H 3 gives no indication of two separate electron transfer steps; its slope ( E p - Ep/2 -- 50 mV), however, shows that it is not the curve of a reversible two electron transfer reaction. R/%

i

i

i

i//zAcm-2

100

40 98 20 96 0 9,4 20 92 40 9O 60

88] 0.20

I

I

I

0.60

0.24

0.60

0.2.4 E / V vs. SC E

Fig. 12. Current/potential and reflectance/potentialcurves, recorded with potential scanning at rate of 20 mV s-1 in a 8 × 10-4 M TMB, pH 3.2 soln. 5 reflections; 2=690 nm (detection of TMB+).

Figure 12 shows the reflectance/potential curve (at 690 nm) and the current/ potential curve, simultaneously recorded at pH 3.2 in 8 × 10-4 M TMB solution. The concentration of TMB ÷ is seen to rise with the start of the oxidation process and to fall off again with increasing anodic potentials ; in the cathodic part of the cycle, it again rises at the start of the reduction process only to fall off again at sufficiently *--The TMB used was an Eastman Kodak product. J. ElectroanaL Chem., 34 (1972)

338 R/./o

S. GOTTESFELD, M. ARIEL i

i

~Acm

i

-2

lOO 40 9~

20

92! 0 88

20 84

40 80

60 76 0.20

I

I

I

0,60

0,24

0.60

~,24

E/Vvs. SCE

Fig. 13. Current/potential and reflectance/potential curves recorded with potential scanning at rate of 20 m V s -1 in a 8 x 10 - 4 M TMB, p H 3 . 2 soln. 5 reflections; 2 = 4 7 5 nm (detection of T M B O x ) .

cathodic potentials. Figure 13 shows IRS measurements taken at the wave-length of TMBOx absorption (TMBOx--the final oxidation product). In an attempt to fit the experimental curves, correlating reflection absorbance with potential, to theoretically computed curves, the following assumptions were made (for the case of an initial solution containing the reduced species only): E = E1 + (RT/F) In [(Cs)x=o/(CR)x= O]

(4)

E = E2 + (RT/F) In [(Co)z= o/(Cs)x=o]

(5)

(CR)x = 0 "4- (CS) x = 0 -~- (CO)x= 0 = (CR)b ulk

(6)

with: (CR)x=0, (CS)x=0, and (Co)x=o--the concentrations at the electrode surface, of the reduced, cation radical and oxidized species, respectively; (CR)bufk--the concentration of the reduced substance in solution; E1 and E2--the standard potentials of the consecutive, one electron, oxidation steps, correlated with the equilibrium constant (K = C2/CRCO) by" E1 - Ez = ( R T / F ) In K

(7)

(The same symbols have been employed by Hale17). For eqns. (4-6) to hold, both el.ectron transfers must be rapid, the rate of homogeneous reactions with the solvent molecules negligible, and the diffusion coefficients of all species involved must be equal (if TMB is adsorbed, the correlation between reflectance and potential developed below will continue to hold in one of two extreme cases :either when the adsorbed layer is completely electroinactive, or when the changes in the surface concentrations of O, S and R are also very rapid, obeying a set of equations similar to eqns. (4-6)). J. Electroanal. Chem., 34 (1972)

339

IRS WITH METAL FILM ELECTRODES (Cs)x =o/(CR) bulk i

0-6

0.5

0.4

0.3 •

#"

R-e~ S "S.o~O

\~

/

E1 E2

" ~ ' ~ Experi rnental curve

/

,,

~

°

oE2-E1 = 1 mV :-

;

3'o

omv

' (E- E,,,)/mV 60

Fig. 14. Stepwise electron removal: calcd, dependence On potential for the intermediate (TMB +) at x = 0. (...) Exptl. curve. E 2 - E 1 : (T) 60, (©) 1, ( 0 ) - 6 0 mY.

Curves computed to show the dependence of (Cs)~=o/(CR)bulk on potential are given in Fig. 14 for the following three cases: (1)

E2-EI=60mV

(2) E 2 - E , = (3)

(K=IO)

lmV(K=I.04)

E z - E I = - 6 0 mV ( K = 1 0 -1)

All three curves are symmetrical about Em= (El + E2)/2. Since in all three cases the concentration (Cs)x- o reaches a significant fraction of the concentration of R in the bulk solution, IRS monitoring of the intermediate S is possible even in situations where no direct electrochemical evidence of its presence is available, provided S absorbs sufficiently and the concentration (CR)bulk is high enough. To illustrate: for an absorptivity coefficient of 4 x 1041 mo1-1 cm- 1 and an optical path in solution, fiN6, of 10 -4 cm, a 10-3 M change in concentration of the absorbing species near the electrode will result in a 1 ~ change in reflectance; it follows that for a 10 -2 M solution of R, a i ~ change in reflectance due to the formation of S is obtainable even in cases where K is as low as 10-t (for resolution in the current/potential curves, values of K of the order of at least 50 are requiredt7). Inspection of Fig. 14 shows the sharpness of the curve maximum to be a function of the absolute value of (E2 - E 1); the larger this value, the flatter the peak. This correlation can be exploited in favorable cases to estimate K from experimental reflectance/ potential results, by curve fitting, as follows: in the narrow potential range, where the production of the intermediate causes a drop of the reflectance, - AR/R o is proportional to (Cs)x=o, and therefore.also to (Cs)x=0/(CR)b,lk (only S absorbs). The experimental reflectance peak potential (Fig. 12) corresponds to Era, as defined above, since according to Fig. 14, (Cs)~=o is maximum at E m for any value of K. In our case, the experimental curve of - AR/R o vs. E (Fig. 12), in the range OfEm _ 60 mV, (60 mV J. Electroanal. Chem., 34 (1972)

340

S. GOTTESFELD, M. ARIEL

r/-/. 100

.

.

.

.

.

\\\ 60

a/n m

Fig. 15. Transmittance spectra of aq. malachite green and aq. (pH 3) T M B O x solns.

on each side of the experimental reflectance peak potential) could be fairly well fitted to the calculated curve of (Cs)x=o/(CR)bulk in the same potential range for K ~ 1; the fitted curve is described by a broken line in Fig. 14 (flatter experimental reflectance peaks would have resulted if K were bigger or smaller than 1). Once stepwise electron removal was proved to be the reason for the "irreversible" character of the current peak, K can be estimated also from the i vs. E slope 17 ; the result for K corresponding to a slope (Ep-Ep/2) of 50 mV is in agreement with the a.m. value. While the absorbance of reaction products present in the bulk of the solution can be effectively monitored by simpler methods often leading to clear-cut conclusions, Electro-IRS excels over other combined electro-optical methods in cases where the concentration of the intermediate in the bulk solution or even in the diffusion layer is immeasurably small (due to low K or homogeneous reactions). IV. THE OXIDATION OF MALACHITE G R E E N - - A CASE WHERE REFLECTION ABSORBANCE IS DOMINATED BY THE CONTRIBUTION OF AN ABSORBED LAYER

For the 12.5 mM DMF solution used in this work (see also ref. 4), the reflectance decreased by 17 ~ when the absorbing species was added to the supporting electrolyte solution, while the residual decrease, ascribed to an adsorbed electroinactive DMF layer, was 7 % (see Fig. 11). The relative contribution of an adsorbed layer to IRS absorbance is much bigger in the following case of malachite green (MG) solutions. As a result, the reflectance changes during the electro-oxidation of MG are determined mainly by the behaviour of the absorbed layer in the potential range studied, although the current measured is due mainly to the oxidation of dissolved MG. Figure 16 shows a decrease of 4 4 ~ i n reflectance when the absorbing MG dye is added to the buffered solution, while the reflectance increase at 0.9 V, where the dye is known to be quantitatively oxidized 18, is only about 2 %. The fast quantitative oxidation of the dissolved MG dye at 0.9 V was confirmed for the electrode-solution system used, by performing a direct incidence sandwich. J. Electroanal. Chem., 34 (1972)

341

IRS WITH METAL FILM ELECTRODES

R/%

60f 5856

'

'

'

4 t i//~Acm-2

54 5~ 5O

020

L

o;o

02.

o;o

024

SCE

E/V vS.

Fig. 16. Current/potential and reflectance/potential curves, recorded with potential scanning at rate of 20 mV s - l ; 5 x 10 -4 M malachite green, pH 3.2; LM gold film; 5 reflections; 2=620 nm.

cell experiment with the same gold film and M G solution. In this case the drop of absorbance measured at 620 nm was fast and practically quantitative at 0.9 V. In the Electro-IRS measurement the second oxidation current peak (0.75-0.9 V) does correspond to the process of dissolved M G oxidation, but the adsorbed M G layer which causes a high reflection absorbance, is oxidized very slowly, and hence the very high residual absorbance at 0.9 V (Fig. 16). The curve correlating reflection absorbance with M G concentration in solution (Fig. 17) has two distinct regions : a sharp rise of A R up to 5 x 10- 5 M MG, followed by a linear portion, rising with a greatly reduced slope. This shape is the result of the superposition of absorbances due to th e adsorbed M G layer and the penetrated solution layer, respectively. At 5 x 10-4 M MG, the electrode surface is covered by at least one monolayer of MG. From eqn. (2), the respective contributions to absorbance of the solution and the surface layer may be estimated : if the absorptivity coefficients for dissolved and adsorbed M G are assumed to be the same, and the representative values of AF = 10- lO mol cm - 2 for monolayer formation, 6 = 10- 5 cm, and the M G concentration change of 5 x 10 .4 M are used, the contribution of the adsorbed layer to the relative reflectance change is found to exceed that of the solution phase twenty-fold. Due to the roughness factor of the film surface, which has been disregarded in the above calculation, the relative contribution of the adsorbed layer is even larger. It follows that even in cases of reversible adsorption, no significant increase in reflectance will be obtained, unless the M G concentration in the solution layer adjacent to the electrode falls to the low values corresponding to a coverage substantially smaller than 1 (Fig. 17). For MG, the latter condition is insufficient, since it adsorbs irreversibly : even after repeatedly flushing the cell with water, a residual reflection absorbance of AR----0.15 was measured. Figure 18 shows Electro-IRS curves recorded in a 100-fold diluted M G solution (5 x 10-6 M); in this cas e, the 10~o decrease in reflectance observed 15 min after the addition of the dye (a slow adsorption process from the dilute quiescent solution) is due exclusively to the adsorbed layer, the homogeneous solution being too dilute to affect reflectance. Oxidation at + 0.9 V slowly restores reflectance: about 30 s are required to achieve complete desorption (cf right-hand part of Fig. 18). In spite of J. Electroanal. Chem., 34 (1972)

S. GOTTESFELD, M. ARIEL

342 -4 r

/

//o

O.E /

,o

/

0.7

/

o/

/ o/ /

0.6

/ / /

0.5

jO f / /

0.4 /

0.3

/

/0

/ / 0

/ /

0.2

0.1 I

o

I

2

4

I

6

~

1'o

~'2

&

i

[MG] x 10~/M

Fig. 17. Reflection absorbance at 620 nm as function of malachite green concn. LM gold film; 5 reflections.

R/°/o ,

100~ 80

_.>_~__-f

"'

i/l~Acn~2

x... '

~l I

6c

! I 45mln

-~

4C

8 I~-- F~nai potential 0 . g 0 V

2C

12

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o.~4

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Fig. 18. Current/potential and reflectance/potential curves, recorded with potential scanning at rate of 20 mV s - l ; 5 x 10 -6 M malachite green, pH 3.2; LM gold film; 5 reflections; 2 = 6 2 0 nm.

its slowness, this electro-desorption process, followed optically, proves that an assumption of "electro-inactivity" does not necessarily hold in every case for these highly conjugated adsorbed molecules. IRS measurements carried out in a wide spectral range, together with prolonged "rest periods" at various imposed potentials, are summarized in Fig. 19 ; a marked decrease in reflectance at 475 nm starts at + 0.75 V, This decrease corresponds to the production of TMBOx (oxidized form of TMBlS). The spectra of dissolved MG and TMBOx are shown in Fig. 15. In a sandwich-cell experiment the spectrum of dissolved MG is completely replaced by that of TMBOx at 0.9 V, and at 0.2 V the solution becomes colorless when TMB Ox is reduced to TMB 18. Inspection of Fig. 19 proves that the process of oxidation on the electrode surface is more involved than that J. Electroanal. Chem., 34 (1972)

343

IRS WITH METAL FILM ELECTRODES

in solution ; on returning the potential to + 0.2 V, the spectrum of adsorbed MG, as recorded before oxidation, is not re-obtained. Possibly, as has been reported for the oxidation of the related compound dimethylaniline19, a coupling occurring on the electrode surface between oxidation products and reactants is at the root of the irreversibility observed. R/O/o lOO

80

60

40

20

I

I

425

450

f

475 50

~

I

I

I

I

I

I

525 5 5 0 5 7 5 6 0 0 650 700 ~/nm

Fig. 19 Reflectance spectrum changes in visible region during electro-oxidation of 8 x 10-s M malachite green (pH 3.2) soln. ; LM gold film; 5 reflections. (0) Base line; (O) MG addition, open circuit; (¥) open circuit, after "rest period'; (lI) 0.60; (El) 0.70; (V) 0.75; ([]) 0.85; (Q) 0.95;(@) 0.20 V.

Electro-IRS has been shown in this case to differ from regular spectrophotometric methods combined with an electrochemical process in that the spectrum and reflectance changes recorded reflect practically properties of the adsorbed layer only. If conclusions about reactants and products in the bulk of the solution are aimed at, this is not always the most suitable method; if, however, the behaviour of adsorbed layers is being investigated, an important potential use for Electro-IRS is manifested here. While the coulombic contribution of adsorbed molecules to a faradaic process is sometimes negligible ("electroinactivity") and in other cases it is small and indistinguishable from that of dissolved molecules, with this method it is possible to follow changes in surface concentration during the passage of considerable faradaic currents without interference from the reaction ; this may be true even if the absorption spectra of the adsorbed and dissolved molecules are identical. ACKNOWLEDGEMENTS

The authors wish to thank the U.S. National Bureau of Standards, Washington, for partial support of this work. Reproduction of this article, with the customary credit to the source, is permitted. This paper is part ofa D.Sc. Thesis presented by S.G. to the Senate of the Technion, I.I.T. J. l?lectroanal. Chem., 34 (1972)

344

S. GOTTESFELD, M. ARIEL

SUMMARY

The different contributions to reflectance changes during the modulation of the potential of a film electrode in an Electro-IRS experiment, are discussed. The electrosorption and desorption of the first surface oxide layer may have a significant effect on reflectance results, even when the current required for oxide production is small compared to the faradaic currents resulting from dissolved electroactive components. The correction for surface oxide formation and for the electromodulation effect is best performed using a "blank" experiment. Linear potential changes are used for both the"blank" and the main IRS experiment; the same metal film electrode, supporting electrolyte solution, and potential range are used in the "blank" experiment. The current densities resulting from electrolysis of 10-2 M solutions cause inhomogeneity of the potential in the metal film electrode. The iR drop in the film was measured with new and used films and it was found that when a narrow strip of a new film was used as an electrode, the potential inhomogeneity was practically eliminated, and thus the interpretation of "optical slope" experiments became easier. An Electro-IRS experiment was used for following changes in the concentration of a cation-radical intermediate at an electrode surface, during a stepwise electron removal process. The radical could be monitored by IRS in cases where the current-potential curve did not show two separate oxidation steps. The equilibrium constant for the formation of the radical from the fully oxidized and reduced forms could be estimated from the A R / R vs. E curve. The population of absorbing molecules in an adsorbed layer is, sometimes, much larger than their population in the solution region into which the evanescent wave penetrates. The result may be a predominant contribution of the adsorbed layer to IRS reflectance changes. This situation, demonstrated for malachite green oxidation, can be used for observing adsorbed layers during a faradaic process, without interference from the latter. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

W. N. HANSEN, T. KUWANA AND R. A. OSTERYOUNG, Anal Chem., 38 (1966) 1810. B. S. PONS, J. S. MATTSON, L. O. WINSTROM AND H. B. MARK, Anal. Chem., 39 (1967) 685. A. PROSTAKAND W. N. HANSEN, Phys. Rev., 160 (1967) 600. A. PROSTAK, H. B. MARK AND W. N. HANSEN, J. Phys. Chem., 72 (1968) 2576. N. WINOGRAD AND T. KUWANA, J. Amer. Chem. Soc., 92 (1970) 224. H. N. BLOUNT, N. WINOGRAD AND Z. KUWANA, J. Phys. Chem., 74 (1970) 3231. N. WINOGRAD AND T. KUWANA, Anal. Chem., 43 (1971) 252. N. WINOGRAD AND T. KUWANA, J. ElectroanaL Chem., 23 (1969) 333. M. STEDMAN,Chem. Phys. Lett., 2 (1968) 457. W. N. HANSEN, Surface Sci., 16 (1969) 205. A. E. ENNOS, Brit. J. App. Phys., 8 (1957) 113. W. D. REID AND J. KRUGER, Nature, 203 (1964) 402. H. A. LAITINENAND M. S. CHAO, J. Electrochem. Soc., 108 (1961) 726. T. BIEGLER, J. Electrochem. Sac., 116 (1969) 1131. B. E. CONWAY, E. GILEADI AND H. G. OSWIN, Can. J. Chem., 4l (1963) 2447. J. STROJEKAND T. KUWANA, Discuss. Faraday Soc., 45 (1968) 134. J. M. HALE, J. ElectroanaL Chem., 8 (1964) 181. Z. GALUS AND R. N. ADAMS, J. Amer. Chem. Soc., 86 (1964) 1666. Z. GALUS AND R. N. ADAMS, J. Amer. Chem. Soc., 84 (1962) 2061.

J. Electroanal. Chem., 34 (1972)