liquid(or solid) interfaces

222

Applied Surface Science 48/49 (1991) 222-226 North-Holland

Kretschmann ATR-IR spectroscopy of investigating metal/liquid(or solid) interfaces A. H a t t a *, Y. S u z u k i , T. W a d a y a m a

a n d W. S u ~ t a k a Department of Materials Science, Faculty of Engineering, Tohoku University. Sendal 980, Japan Received 13 August 1990: accepted for publication 3 October 1990

In the Kretschmann ATR configuration, a short-range strong electric field is generated on the surface of a thin metal film by the collective resonance of free electrons upon incidence of infrared light. This strong field enhances the intensity of infrared absorption from species present in the immediate neighborhood of the metal surface. In a few cases, infrared absorption of adsorbed species is enhanced through the coupling of vibration modes with charge-transfer excitation. Consequently. the Kretschmann ATR configuralion is feasible for the in-situ observation of infrared spectra of species on metal surfaces without significant interference of signals from bulk species.

I. Introduction

2. Experimental

ln-situ observation of vibrational spectra of species at m e t a l / l i q u i d a n d m e t a l / s o l i d interfaces provides us with information crucial for a better u n d e r s t a n d i n g of various p h e n o m e n a at elect r o d e / e l e c t r o l y t e (solid or liquid) interfaces, m e t a l - a d h e s i v e bonding, behavior of lubricant molecules on metal surfaces, growth process of a thin film on a metal substrate a n d so forth. K r e t s c h m a n n A T R - I R spectroscopy is a potential technique for the in-situ observation of these interfaces. P r o m i n e n t features of this technique are: (1) Very short-range e n h a n c e m e n t of infrared absorption. This enables us to observe surface species without interference of bulk species. (2) Since the infrared b e a m does not travel t h r o u g h the liquid or solid laygr, a wide variety of liquids or solids can be used, even if they are strongly absorbing. These features are particularly desirable for the direct observation of m e t a l / c o n d e n s e d - p h a s e interfaces at the molecular level.

The a p p a r a t u s and set-up for the A T R measurement were reported elsewhere [1]. T h i n metal films were vacuum-evaporated on the reflecting plane of a G e (or Si) hemicylindrical prism. The thickness of the film was estimated from the frequency change of a quartz oscillator installed in the vacuum cell. T h i n films of organic c o m p o u n d s were formed from their acetone solutions d r o p p e d o n the metal surface.

* To whom correspondence should be addressed.

3. Results and discussion Fig. 1 shows a n A T R spectrum of a p-nitrobenzoic acid ( P N B A ) thin film formed on a continuous Ag film of 20 n m in thickness. W h e n s-polarized light was used, no feature was recorded regardless of the incidence angle, a n d the o b t a i n e d s p e c t r u m is not shown in the figure. The top spectrum in this figure was recorded as the external specular reflection m e a s u r e m e n t of the same sample. A striking difference is seen between these two spectra. T h e external reflection spectrum is in good agreement with the conventional

0169-4332/91/$03.50 cj 1991 - Elsevier Science Publishers B.V. (North-Holland)

A. Hatta et aL / Kretschmann A TR-IR ~pectroscopy of muestigatmg M ~ L(or S) interfaces

Z 0

0 uJ .J wuJ ¢¢

ATR

I 1800

I

I

I 1500

I

I

t 1200

W A V E N U M B E R / c m -1 Fig. 1. Infrared spectra of PNBA films (thickness: 12 nm) formed on a Ag film; p-polarized light was incident at an angle of 70 ° (ATR) or 80 ° (reflection). Prism: Ge. absorption spectrum of PNBA. The A T R spectrum, on the other hand, is attributable to oriented p-nitrobenzoate ions, which probably resulted from a reaction of PNBA with silver oxide on the metal surface. N o band of PNBA molecules was recorded in the A T R measurement. This means most w, obably that only the species in the immediate neighborhood of the metal surface gives enhanced absorption in the A T R method. The intensity of the bands of the A T R spectrum increases gradually as the angle of incidence increases. The Kretschmann A T R configuration [2] is employed for the excitation of the surface plasma polariton (SPP) in the visible and near-ultraviolet region. Since the strong electromagnetic field of the SPP extends to a distance an SPP wavelength long, a long-range enhancement is obtained. Furthermore, SPP is excited upon incidence of ppolarized light at an incidence angle slightly larger than the critical angle for total reflection, and the strength of the generated field should show a very sharp incidence angle dependence [3]. N o n e of these features are present in the results obtained

223

in the present work. Hence, the observed enhancement of infrared absorption cannot be attributed to the excitation of SPP. When the metal film is less than 10 nm in thickness and consists of fine particles, the infrared absorption of an overlayer on the metal increases with thickness [4]. Fig. 2 shows the intensity increase of the carbonyl stretching band of a polycyanoacrylate overlayer with thickness. In this figure, the intensity obtained with the conventional A T R method is also shown. As can be seen in this figure, the intensity increases steeply in the presence of the Ag film until the polymer layer reaches a thickness of 5 rim, and continues to grow thereafter at the same rate as in the normal A T R spectrum. The non-linear intensity increase may be explained by assuming a superposition of two different mechanisms, one being a short-range effect due to the presence of the Ag film and the other a long-range electromagnetic field of the evanescent wave. These two observations show that the shortrange enhancement of infrared absorption takes place not only for a continuous film but also on the island films of Ag. We therefore calculated the electric field strength generated within Ag films of various thicknesses using a three-layered model. Because our films have island-like deposits or are

20

A

~

x

~o Z

o

1C /

0 ,,n ,~

~

A

o-

f I 5

/ /-

/

/0

5

3'0

10

Polymer

'

~'o

Thickness/nm

Fig. 2. Absorption intensity of the C--O stretching band of polycyanoacrylate film as a function of thickness: (o) observed intensity with a Ag film of 5 nm thickness; (z,) intensity observed without Ag film.

A. Hatta et al. / Kretschmann A TR-IR s7.ctroscopy of investigating M ~ L(or S) interfaces

224

10 o

5* O x

(D

6

o

7.," uJ

2

Ez

' ,

I ~1 30

Incident

I

° I

I 60

I

l

1

90

Angle (0)/deg

Fig. 3. Intensity change of absorption bands of the p-nitrobenzoate ion at 1350 cm - I ( o ) and of Ag(1)-mercaptobenzothiazole at 1035 c m - i (zx) with angle of incidence. Incident IR light: p-polarized. Solid line: computed electric field strength induced upon incidence of p-polarized light; E~, and E..: electric field amplitude parallel and normal to the prism base. respectively; Eo: amplitude of the incident light; thickness of Ag film: 20 rim.

extremely bumpy, we assumed a homogeneous layer of A g - a i r composite instead of a homogeneous Ag layer. The dielectric constant of the composite layer was estimated from the Bruggeman effective-medium approximation. The calculated electric fields parallel to the prism base are in good qualitative agreement with the observed absorption intensity [5]. An example of the calculation is shown in fig. 3. The solid line in the figure shows the normalized electric field strength within a A g - a i r composite generated parallel to the prism base upon incidence of p-polarized infrared light having a wavenumber of 1350 cm -I. Open circles in the figure are observed intensities of the band at 1350 c m - i of the p-nitrobenzoate ion [5], and triangles are those of the band at 1035 c m - i of the Ag(I)mercaptobenzothiazole complex formed at a Ag/water interface [6]. These absorption bands were recorded with p-polarized infrared light. Qualitative agreement between the calculated and

observed values is seen in this figure. The calculated strength, however, is much weaker than that of the incident infrared light and cannot explain the observed absorption enhancement. We therefore hypothesize the generation of an additional short-range strong field, whose strength is proportional to the parallel field within the composite film. That the electric field oscillating parallel to the surface of the solid substrate is responsible for the enhanced absorption was confirmed by a transmission absorption measurement. In the transmission measurement, the incident infrared light produces an oscillating electric field parallel to the substrate surface and should give rise to the enhancement of infrared absorption. Fig. 4 shows absorption spectra of PNBA overlayers on a Ag film of 5.3 nm in thickness on a Ge plate. Infrared absorption bands of PNBA, the dominant component of the overlayer, can barely be seen only in the spectrum B of an organic layer of 10 nm in thickness. Strongly enhanced absorption of the p-nitrobenzoate ion, a minor component of the layer, is clearly seen even if the thickness of the PNBA film is only 5 nm (spectrum A). This indi-

Z O el r~O ¢/) =O

y

1 I

1800

I

1600

I

I

I

! 400

WAVENUMBER/cm

I

1200 -1

Fig. 4. Infrared absorption spectra of PNBA films formed on a thin Ag film. Absorption bands of PNBA are designated with asterisks.

225

A. Hatta et al. / Kretschmann A TR-IR spectroscopy of tm,esttgatmg M / L(or S) interfaces

cates the absorption of the benzoate ion, the species present in the immediate neighborhood of the metal surface, is enhanced by the short-range field induced by the parallel electric field. Mechanisms which may give rise to the shortrange absorption enhancement proportional to the field within the composite film are as follows. (1) A chemical effect: When a charge-transfer complex is formed on the metal surface upon adsorption, its totally symmetric vibrations may remarkably be enhanced as a result of the charge transfer associated with the vibration [7]. (2) A strong electric field in voids: Several workers have predicted the generation of a strong electric field in voids of a metal film upon incidence of an electromagnetic wave [8,91. (3) Collective resonance of electrons in metal islands and bumps [10]: A chain of fine metal particles as well as a chain of bumps generates a strong electric field due to the collective resonance of free electrons upon incidence of infrared light, provided the incident electric field is parallel to the chain. The chemical effect should take place only on the species adsorbed directly on the solid surface. The results obtained using polycyanoacrylate film rule out this mechanism, because the enhancement extends to the molecule present at a distance of 5 nm from the metal surface. It is also unlikely that the enhancement stems from the strong electric field in voids. We observed enhanced infrared absorption of a cyanide ion adsorbed on a Ag electrode, [11]. The behavior of the ion was in perfect agreement with those observed for the same ion adsorbed on a flat Ag electrode surface [12]. Cyanide ions trapped in voids should behave in a different manner from those on the flat metal surface, because the potential and pH in the voids should be different from those on the open surface. As mentioned already, the surface of continuous Ag films was full of bumps, and the collective electron resonance may be excited in the bumps by the oscillating electric field parallel to the base of the prism. Moreover, the reflectivity of the prism decreased in the infrared region as a result of metal deposition, and the decrease was proportional to the magnitude of enhancement [5]. This suggests that the collective resonance is generated at the cost of the incident infrared energy. The

-

t/

2050

'i

21i50

2050

WAVENIJMBER/cm

2150 -I

Fig. 5. Spectra of species on a Ag electrode in an aqueous solution of K A g ( C N ) , as a function of electrode potential. Electrode potential: (A) -0+5, (B) -0.6, (C) -0.8. (D) - I.O. (E) - 1.2, (F) - 1.3 V (versus A g / A g C I ) . BG: background.

collective resonance should produce a strong electric field in hollows of the bumpy metal surface, and species present in the hollows should show enhanced infrared absorption. When the metal film is composed of islands, a strong field is generated between the islands, and the species present between islands should show enhanced absorption. In Kretschmann ATR infrared spectroscopy, therefore, species present between metal bumps and islands produce enhanced infrared absorption. The absorption enhancement of such a species is probably the origin of the observed short-range enhancement. An example of the in-situ observation of a metal/liquid interface is shown in fig. 5. This figure shows the spectra of surface species on a Ag electrode in an aqueous solution containing 0.1M K 2 S O 4 and 2.5 × 10-aM KAg(CN) 2. A feature at around 2100 c m - I is attributed from its frequency to the adsorbed cyanide ion which was formed in the dissociation of the complex ion of Ag(CN)~

226

A. Hatta et al. / h'retschmann A T R - I R .~pe('trt)scop;" of tm'e~tigato~g ,~t / l.(or S) t/~r(,rfates

The decrease in intensity of this feature upon cathodic potential shift can be ascribed to a decrease in the quantity of adsorbed ions, because the cyanide ion is charged negatively. T h e feature at a b o u t 2140 cm-~ is ascribable to the complex ion because the complex ion shows its antisymmetric C N stretching vibration at 2135 c m - ~ in aqueous solutions [13]. The intensity of this feature increases sharply when the electrode potential approaches a value of - 0 . 5 V (versus A g / A g C I ) . This increase corresponds to the formation of the complex ion oll the Ag electrode, because the equilibrium potential of formation is - 0 . 5 0 6 V (versus A g / A g C I ) . The feature at 2140 cm-1 remains invariant in peak frequency and in intensity if the potential is lower than - 0 . 8 V. This exceptional b e h a v i o r needs to be discussed. The complex ion dissociates at about - 0 . 5 V a n d the resultant Ag a t o m s deposit o n t o the electrode. N u m e r o u s b u m p s exist on the surface of the Ag electrode film and the depositing Ag atoms p r o b a b l y form voids, in which undissociated complex ions may be trapped. The trapped ions should be insensitive to changes in electrode potential a n d in p H at the electrode surface. As already mentioned, a strong field is p r o b a b l y generated in voids, so the signals of species in the voids may be enhanced. W e deduce therefore the feature at 2140 cm-~ a p p e a r i n g at highly negative potentials is arising from the trapped Ag(CN)2 ion. Most of the absorption e n h a n c e m e n t in the K r e t s c h m a n n configuration can be explained by the strong localized electric field arising from the collective electron resonance. However, the charge-transfer e n h a n c e m e n t of infrared absorption takes place in particular systems [14,15]. In the K r e t s c h m a n n A T R configuration, therefore, infrared absorption of species present in the immediate neighborhood of the metal surface are e n h a n c e d by the localized strong electric field

a n d / o r the vibronic coupling of charge transfer. Consequently, this technique is feasible for the in-situ observation of m e t a l / l i q u i d a n d m e t a l / solid interfaces, t h o u g h the applicable metal is limited to free-electron metals such as Ag, Au, Cu and some others. It should be worthy to note that when the metal film is very thin, the field of the evanescent wave has a substantial strength, and the signals from bulk species may overlap those of surface species as shown in the polycyanoa c r y l a t e - A g system. The use of a rather thick ( 2 0 - 2 5 nm) film is desirable for the obserw~tion of species on metal surfaces.

References [1] A. Haua, T. Ohshima and W. Su;./taka, Appl. Phys. A 29 09821 71. [2] E. Kretschmann. Z. Phys. 241 (1971) 313. [3] H. Ueba and S. lchimura, Surf. Sci. 118 (1982) L273. [4] A. Hatla, N. Suzuki. Y. Suzuki and W. SuStaka. Appl. Surf. Sci. 37 (1989) 299. [5] Y. Suzuki. M. Osawa, A. Hatta and W. Su;dtaka, Appl. Surf. Sci. 33/34 (1988) 875. [6] A. Hana, Y. Chiba and W. Su;2taka, Surf. Sci. 158 (1985) 616. [7] J.P. Dcvlin and K. Consani, J. Phys. Chem. 85 (1981) 2597. [8] D.J. Bergman and A. Nilzan, ('hem. Phys. Lett. 88 (1982) 409. [9] Il. Chew and M. Kcrkcr. J. Opt. Soc. Am. B 2 (1985) 1025. [10] T. Yamaguchi, Oyo Buturi 44 (1975) 64. [11] A. Hatta. Y. Sasaki and W. SuStaka. ]. Electroanal. Chem. 215 (1986) 93. [12] K. Kunimat~u. H. Seki and W.G. Golden. Chem. Phys. Left. 108 (1984) 195. i13] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed. (Wilcy-lntcr';ciencc. New York, 1986) p. 276. [14] T. Wadayama. Y. Momota, A. Hatta and W. Su;2taka. J. Electroanal. Chem. 289 (1990) 29. [15] T. Wadayama. T. Sakurai. S. lchikawa and W. Su.2taka, Surf. Sci. 198 (19881 L359.