STUDIES OF ELECTRODE-ELECTROLYTE INTERFACES "IN SITU" BY MIRAGE DETECTION WITH POLARIZATION MODULATION

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STUDIES OF ELECTRODE-ELECTROLYTE

INTERFACES ”IN SITU” BY MIRAGE DETECTION WITH POLARIZATION MODULATION

J. Roger, D. Fournier, A. Boccara

To cite this version:

J. Roger, D. Fournier, A. Boccara. STUDIES OF ELECTRODE-ELECTROLYTE INTERFACES ”IN

SITU” BY MIRAGE DETECTION WITH POLARIZATION MODULATION. Journal de Physique

Colloques, 1983, 44 (C6), pp.C6-313-C6-316. �10.1051/jphyscol:1983650�. �jpa-00223209�

JOURNAL DE PHYSIQUE

Colloque

C6,

suppl6ment

au

nO1O, Tome 44, octobre 1983 page C6-

313

STUDIES OF ELECTRODE-ELECTROLYTE INTERFACES "IN s ITU" BY MI RAGE DETECTION WITH POLARIZATION MODULATION

J.P. Roger, D. Fournier and A.C. Boccara

k b o r a t o i r e dfOptique Physique, ESPCI, 10, rue VauqueZin, 7 5 2 3 1 Paris Cedex 0 5 , France

RESUME - En couplant la detection Mirage et une modulation de polarisation de la lumihre incidente, une sensihilite correspondant

1 un dizihme de

monocouche de Cu sur Pt

a

6th obtenue. On discute egalement des effets de gradient de concentration au voisinage de l'electrodt.

ABSTRACT - By coupling Mirage detection and polarization modulation of the incident light, a sensitivity corresponding to one tenth of a monolayer of Cu on Pt has been achieved. Concentration gradient effects in the vicinity of the electrode are discussed.

These last years, there has been a growing interest in the study of metal or semi- conductor-electrolyte interfaces by "in situ" photothermal methods 11-41. These techniques are less sensitive than ellipsometry or reflexion spectroscopies but they do not require any electrode preparations. In photothermal methods, the photoinduced periodic temperature change of the electrode surface has been measured either by a thermistor /I/ or a PZT detector /2/ attached to the back

of

the elec- trode, or by using a photoacoustic (PA) cell

1 3 ,

41, the window of which is constituted by the electrode itself.

Photothermal deflexion ("mirage effect") has been proved to be two orders of magni- tude more sensitive than PA when used at a solid-liquid interface

/ 5 / .

This

detection is thus particularly suitable for "in-situ" electrode-electrolyte inter- face studies. It has been recently applied to monitor the,electrochernical deposi- tion of a thin layer of metal or oxide on a metallic r~npolished electrode /6/

and to study the photocorrosion of semiconductors

/ 7 / .

Moreover, polarization effects in PA experiments have been used by NORDAL /8/ in the IR for surface studies.

The aim o f the present study is to demonstrate that

a

large enhancement of the sensitivity may be obtained by using a polarization modulation, rather than the classical intensity modulation of the excitation pump beam.

The experimental set-up shown on Figure 1, allows these two kinds of modulation.

The electrode is a sheet of platinium

(%

50 pm thick) stuck on a glass plate and is immersed in a

N R

SO solution containing ~ o - ~ M cu2+. During the experiment

2 4

dissolved oxygen is removed from the electrolyte by purging with nitrogen. The photothermal signal being not distrubed by the acoustic noise, purging is not stopped during the measurement.

For an intensity modulation, the electrode is illuminated by the argon laser beam at normal. incidence (dashed line). During an electrochemical deposition, the reflectivity of the electrode is varying and may be monitored by the photothermal signal. Indeed, this signal is proportional to [

¹

^-(R+AR)] 0 (where R is the reflectivity of the bare electrode, AR the reflectivity change induced by the electrochemical deposition, and 0 the incident flux).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983650

(36-314 JOURNAL DE PHYSIQUE

Fig. 1

Experimental set-up

(

---

)

Intensity modulation

(

...

⁾

Polarization modulation 1. He-Ne laser - 2. Position sensor - 3. Lock-in amplifier - 4. Electrode - 5. Electrolyte -

6.

Argon laser - 7. Chopper

-

8. Polarization modulator - 9. Glass plate compensator.

Let us point out that, for this electrochemical system, an underpocential deposi- tion (0 to-0.47

V )

of one monolayer is preceeding the thick deposition of copper during a cathodic sweep (Fig. 2A). The photothermal signal associated with the deposition and the dissolution of 10 layers of copper is presented in the lower part

of

Fig. 2A. The scnsitivity is rather good

(a

fraction of a monolayer), but the main disadvantage of this set-up is that the deposition is monitored on a weak signal (AR) superimposed on a large background (1-R) due to the bare elec- trode. Therefore, thc slow background signal drift reduces the ultimate scnsitivity.

In order to improve the stability of the measurement, we have ~ s e d an obliquely incident

(?. 7 0 " )

polarization modulated light (Fig.

1 ,

dotted line). An electro- optical modulator switches the polarization of the light from the plane of incidence ( / / ) to the perpendicular direction (I). In this case, the bare elec- trode photothermal signal i.s then proportional to [(I-K

) @

- (I-RI )O1]. By using a glass plate compensator set after the modulator,//it//is possible to cancel this signal by adjusting the ratio 0 ll/O1. Therefore, the signal associa- ted with the copper deposition is only proportional to

:

Fig. 2B illustrates the strong improvement o b ~ a i n e d by a low frequency polariza- tion modulation (38 Hz)

;

the noise indeed corresponds

L O

1/10 of surtace coverage and the drift is no more present.

A quantitativc comparison of the experimental results obtained both on polished and rough surfaces by ellipsomeLry and mirage detection is underway in collabora- tion wiLh Drs

CHAO

and COSTA

/ 9 / .

At last, we would like to mention the presence of a hudge transient beam

deflection related to the ionic concentration gradient which takes place during the

de~osition and the dissolution of the copper layer. Care must be taken to avoid the

influence of this cffect when recording the photothermal signal by a suitable

electronic filtering. Fig. 3 shows ttie experimental deflection associated with the

ionic concentration gradient. One can observe that this signal variation is in

close correlation with the currenc in the electrochemical cell. The DC ointing

stabilivy of our double beam experimental ser-up being of about 2.5 rd, one

I

10 L A Y E R S

Fig. 2

-- -

Potential programs (a), currents ( b ) , P T signals (c) for electroctiemicdl deposi- tions o f C u o n Pt rleclrode. IO-~MCU~', _{I N} P2SOC

A

-

Intensity modulation

-

Deposition and dissolution of 10 Layers of Cu.

B

-

Polarization modulation

-

Deposition and dissolution of one rno~lolayer of Cu.

JOURNAL DE PHYSIQUE

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F i g . 3

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/ I / FUJISHINA A . , YASUDA H . , HONDA K. a n d BARD A . J . , Anal.Chem.

52

( 1 9 8 0 ) 6 8 2 . / 2 / MALPAS R . E . a n d I3ARD A . J . , A n a l . Chem.

52

( 1 9 8 0 ) 1 0 9 .

/ 3 / NASUDA H . , FUJISIiIMA A. a n d HONDA K . , R u l l . Chem. S o c . J a p a n 53 ( 1 9 8 0 ) 1 5 4 2 . / 4 / SANDER U . , STKEHBLOW H.11. a n d DOHRMANN J . K . , J . I'hys. Chcm. 8 5 7 1 9 8 1 ) 4 4 7 . / 5 / FOURNIER D . , BOCCARA A.C. a n d BAUOZ J . , A p p l . O p t .

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S o c i e t y o f E l e c t r o c h e m i s t r y , J.yon, F r a n c e , ( 1 9 8 2 ) .

171

KOYCE B . S . H . , SANCIIEZ-SINENCIO F . , GOLDSTEIN R . , MURATORI R . , WILLIAMS R. a n d Y I M W . M . , J . E l e c t r o c h e m . S o c .

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