Investigation of electrode surfaces by surface plasmon polariton spectroscopy

Investigation of electrode surfaces by surface plasmon polariton spectroscopy

Surface Science @ North-Holland 101 (1980) 99-108 Publishing Company INVESTIGATION OF ELECTRODE SURFACES BY SURFACE PLASMON POLARITON SPECTROSCOPY A...

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Surface Science @ North-Holland

101 (1980) 99-108 Publishing Company

INVESTIGATION OF ELECTRODE SURFACES BY SURFACE PLASMON POLARITON SPECTROSCOPY A. OTTO Physikalisches Institut III, Universitiit Diisseldorf, D-41Mx) Diisseldotf, Fed. Rep. Germany Received

22 October

1979

The article is no introduction to the physics of the surface plasmon polaritons (SPP), but reviews experiments on the influence of SPP dispersion by the potential changes in the double layer regime. by the crystal structure of the surface, by overlayers and by roughness and the use of the attenuated total reflection SPP resonance in Raman scattering from adsorbates.

1. Introduction Given the limited space allowed for this publication on one side and a couple of relevant review articles [l-4] on surface plasmons (SP) and surface plasmon polaritons (SPP) on the other side, it makes more sense to concentrate on new developments in SPP spectroscopy at electrode -electrolyte interfaces, than to write another general review. The newcomer to the field may learn about SP in ref. [I], about SPP in refs. [2,3], about SPP excitation by attenuated total reflection (ATR) in ref. [3], about roughness coupling to SPP in ref. [4], about field enhancement in the SPP-ATR resonance in ref. [5], and about ellipsometric SPP-ATR measurements and the influence of overlayers on SPP dispersion in ref. [6]. The ellipsometric SPP-ATR technique allows for a direct comparison of ellipsometry and SPP-ATR spectroscopy. In favorable cases, the latter may be more sensitive by about one order of magnitude (e.g. fig. 6 in ref. [16]). SPP-ATR spectroscopy seems to be better suited to study electronic surface anisotropy (see fig. 3) than ellipsometry. 2. Experimental investigations of electrodes by SPP-ATR spectroscopy SPP optical metals, The hence surfaces

are well defined excitations on metals in spectral ranges of high reflectivity, for instance for the noble metals, but not for transitions which have relatively low reflectivity. SPP dispersion is very sensitive to small changes at the interface the SPP-ATR spectroscopy is a good optical tool to study reactions at of electrodes, made out of metals with intrinsic high reflectance. 99

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A. Otto 1 Investigation of electrode surfaces by SPPS

2.1. Electrodes without overlayers Abel&s et al. [7] and Chao et al. [S] measured SPP dispersion on polycrystalline gold film electrodes in ZN H2SOJ and 1N HClO, electrolytes in the double layer region (at 0.2 to 1.4V SHE). In this voltage range, the only changes at the electrode are the charging of the double layer (DL) capacity and the possible specific adsorption of SO:- and ClO: anions. The anodic charging of the DL capacity corresponds on the metal side to a depletion of electronic charge within the screening depth of the electric field in the electrode and on the electrolyte side to electrostrictive compression of the Helmholtz DL and specific anion adsorption. For concentrated electrolytes, the change of the index of refraction of the electrolyte in the diffusion region near the DL may be neglected [S]. One may split the observed shift Ak of the wavevector of the SPP into Ak = Ak(meta1) Fig. 1 displays

+ Ak(DL) the experimental

results

[7]. The point

of zero charge

(pzc)

. HCIO,N 0 H,SO,N

Fig. 1. Imaginary part (Im) and real part (Re) of Ak versus surface electron H2S04 (open circles) and in IN HClO, solution (solid circles) at a wavelength ref. [7].

charge t, in 1N of 6093 A. After

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A. Otto I Investigation of electrode surfaces by SPPS

was determined [8] by finding that voltage, where the observed k vector of the SPP would agree with the value calculated from the optical constants of bulk Au. The charge on the Au electrode was evaluated by integration of the voltammogram, starting at the pzc. The results of fig. 1 were interpreted in the following way [7]: the imaginary part (Im) of Ak(DL) must be zero, because the DL is non absorbing. Hence Im Ak is only due to the depletion charge on the gold electrode and therefore independent from the anion species in the electrolyte. On the other hand, the real part (Re) of Ak has contributions from Ak(meta1) and Ak(DL). The observed differences must be due to differences in Re Ak(DL). In refs. [7,8] this is ascribed to the stronger tendency of the SOi- ion for specific adsorption with respect to the Cl04 ion. Kolb et al. measured the potential dependent SPP dispersion on polycrystalline silver and gold film electrodes [9, lo] and on silver (111) electrodes [ll] in 0.5M NaC104, A shift of about 0.02 eV per 1 V change of potential was observed, see fig. 2. In contrast to refs. [7,8], they attribute the observed shift only to the metal. The argument is as follows: On the anodic side from the pzc, electron density is depleted by retracting electrons from the surface, which is defined by the positive background of the atomic cores [ll]. This lowers the transitions from the silver metal d-band to the Fermi energy. This again will shift the SPP dispersion. However, electron density cannot be accumulated at the surface. According to Lang and Kohn [12], for cathodic potentials, the electron density extends now further into the direction towards the electrolyte, and no important shift of the SPP dispersion is expected by Kolb et al. The observed 5

I

I

I

I

I

I

I

I--’

o--

> \” 3

-5

-I

-

“a ro 0

x=475 -10

I > a m 0

“Ill

\

-15

-0.0

I

I -06

I

I -04

I

.I -02

I

‘1

I’I 0

I

I

-3

02

04

“SE’”

Fig. 2. Potential dependence of the shift in SPP dispersion for Ag(ll1) polycrystalline Ag (dashed line). I& = o(SPP)/c. After ref. [ll].

(solid line) and

102

A. Otto 1 Investigation of electrode surfaces by SPPS

differences in the shift of SPP dispersion on polycrystalline Ag films and Ag(l11) surfaces [ll] agree with the known difference AU of the pzc for polycrystalline and (111) silver surfaces [13]. SP’s with greater wavevector have smaller penetration depth into the electrode and should be influenced more strongly by the depletion of the electron charge. These SP are excited by roughness coupling. Indeed, Kolb and KGtz [14] found, that the reflectivity dip due to SP roughness coupling on polycrystalline silver was shifting at about -0.2 eV/V on the anodic side of the pzc (compared to about 0.02 eV/V for SPP, see fig. 2). The dispersion of SPP depends, apart from the effects discussed above, on the crystalline orientation of the electrode surface and on the azimuthal direction of the SPP wavevector, as demonstrated by Tadjeddine and Kolb [ll], see fig. 3. The dispersion curves on the (110) face are lower than on the (111) face, analogous to the influence of surface depletion of electronic charge. It may well be, that in first order of approximation, the transition layer from zero to bulk free electron density is broader on the more open (110) surface than on the close packed (111) surface. This would be similar to a depletion layer on the (110) surface. As already indicated by the electroreflectance measurements of Furtak and Lynch [15], the azimuthal anisotropy of the (110) surface induces anisotropic SPP dispersion. For e, perpendicular to the densely packed 30

2.8

26

18 102

1.04

106

108 k,

Fig. 3. Dispersion of the SPP on Ag(ll1) propagation of the SPP. After ref. [l I].

I IO

II2

I 14

‘%

and Ag(ll0)

surfaces.

e, denotes

the direction

of

A. Otto I Investigation of electrode surfaces by SPPS

103

ridges in [liO] direction, the dispersion curve is lower than for k(][liO]. This indicates, that one should also expect a lowering of surface plasmon dispersion for a high surface density of steps and adatoms. 2.2. Electrodes with adsorbates Chao et al. [16] have studied the oxidation of gold film electrodes by SPP spectroscopy. In the potential range of submonolayer oxygen coverage, they observed a shift of the SPP dispersion, but no additional damping, whereas appreciable damping was observed in the voltage range, where one to two monolayers of Au203 [17] are covering the electrode. By assuming reasonable values for the overlayer thickness, Chao et al. derive a dielectric constant Ef of the adsorbate-overlayer system (see fig. 4).

b

c

V (SHE)

d

a

d

0.8

10

12

14

b

c

1.6

1.8

2.0

V(SHE)

Fig. 4. Top: Voltammogram of a gold film in OSM HzS.04. Range a: chemisorption of oxygen; range b: lateral growth of Au203, starting at nucleation centers; range c: continuous overlayer of Au203. Bottop: Variation of the optical constants of the adsorbate, assuming an overlayer thickness of 3.7 A. After ref. 1161.

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A. Otto J Investigation of electrode swfaces by SPPS

The potential dependence of .sf is different for the anodic and cathodic sweep direction. At the potential, where the Au203 overlayer is reduced, they find a surprising increase in Re Er. In the opinion of the author, this may well be caused by the roughness of the gold electrode during and immediately after the overlayer reduction - and not by the overlayer.

3. The influence

of surface roughness

on SPP dispersion

It has been found experimentally that surface roughness changes the dispersion of SPP, decreasing their phase velocity and increasing their damping [18-201. For a statistical roughness of about 308, rms, there is already a considerable change of dispersion [19]. Kroger and Kretschmann [21] developed a second order scattering theory to explain these phenomena. They showed, that the surface roughness induced SPP damping could be understood by single scattering of the SPP into radiative modes and SPP’s with different directions of propagation. The shift of the SPP dispersion however is caused by multiple scattering. The Kroger-Kretschmann theory [21,22] assumes that the dielectric constant E(O) of the bulk metal is used also at the rough surface, independent of its local and atomic scale structure. As is evident from the SPP-ATR experiments on Ag electrodes (see section 2) the difference in atomic scale structure of the surface may cause considerable SPP dispersion shift. Therefore for rough surfaces, it is not easy to dissentangle experimentally the influence of multiple scattering from the changes in the electronic structure of the metal near the surface, caused by a change in the atomic scale surface structure. In other words scattering of the SPP by roughness is caused not only by scattering of the photonic part of the SPP (this is calculated in ref. [21]) but also by scattering of its electronic part.

4. The use of the SPP-ATR and overlayers

resonance

in Raman scattering

from adsorbates

It was first pointed out by Chen, Chen and Burstein [23] that the enhancement of the electric field at the metal surface in the SPP-ATR resonance [5] could be used to enhance the Raman signal from adsorbates, overlayers and also from phonons in the metal. Fig. 5 compares the expected Raman signal (for low frequency shifts) for a thin overlayer f for external reflectance (ER) and the ATR configuration. I’:> is the ratio E,(film)/E,(air) in ER and I’:: is the ratio El(film)lEz(prism) in ATR. EZ are the components of the incident electric field normal to the surface.

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A. Otto / Investigation of electrode surfaces by SPPS

250 N-

225-

,

,

,

ER

,

,

,

,

,

,

,

,

,

,

,

,

ATR

Fig. 5. Transfer functions 1I’.,,&f1* and IG,,l~rl* cos (I, for the ER configuration (superscript E) and the ATR configuration (supfrscript P) for a thin overlayer f on silver for A(incident)= A(scattered)=6471 A, and 530A thickness of the silver film. E(silver)= -19.6+iO.59 and .s(prism) = 2.25. After ref. [23].

The maximum I,, for ER is reached at rather grazing incidence, in agreement with the calculations and Raman experiments of Greenler and Slager [24]. For the ATR case, I,, peaks at the k-vector of the surface plasmon. GFZ is the ratio E,(air)/E,(film), Ge is the ratio E,(prism)/E,(film) for ATR, but this time for the scattered field. For ER, the maximum emission is reached at not so grazing an angle of emission, in agreement with ref. [24]. For the special case of fig. 5, for .sf = 1, an ATR enhancement of about 340 with respect to the optimum ER case is obtained [23]. However, one should notice, that for the z components of the field, the influence of Ef is important. In the opinion of the author, the sources of the Raman scattered field should be treated as a dielectric displacement field. In the case, that the Raman scattered light originates at the surface, but within the silver metal, one has to choose ~(silver) for Ef. In this case, the absolute Raman intensity is decreased by a factor of Ie(silver)]-2, compared to the case, where the same scatterer is outside the silver metal. For the parameters in fig. 5, this factor is 2.6 x 10m3(see the discussion below). For Raman scattering in a mixed ATR-ER configuration (SPP resonance with the incident light, but the scattered light collected on the side opposite to the prisms), in which there was no thin overlayer f, but liquid benzene adjacent to the silver film, Chen et al. [23] found an enhancement in Raman intensity (ATR to ER) of about 75. A similar experiment was reported by Benner et al. [25]. They observed Raman scattering from liquid benzene in

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A. Otto I Investigation of electrode surfaces by SPPS

the ATR configuration prism - benzene and prism - silver film - benzene, in both cases at the same angle given by the SPP resonance. The scattered light was collected through the benzene, an enhancement by the intermediate silver film and the SPP resonance of about 105 was found. Related work has been reported in ref. [26]. The first Raman SPP-ATR experiment from an absorbate involving the so-called giant Raman effect [27] was reported by Pettinger et al. [28] for the configuration in fig. 6. The dependence of the 1OlOcm~’ Raman line of adsorbed pyridine is given as function of the angle of incidence in fig. 6. The enhancement between the intensities at cy = 0 and the SPP resonance is surprisingly low. This may be partly due to the broadening of the resonance by surface roughness. In this context, the following comment may be of interest: An intensity versus (Y curve for the configuration in fig. 6 should be different for the sources of the scattered light above or below the silver surface. If one assumes a completely depolarizing Raman tensor, one has to expect contributions to the scattered field both from the laser field component parallel and perpendicular to the surface. The parallel components are the same above and below the surface, the normal components on the metal side of the surface is smaller by a factor E(electrolyte)/s(metal). For (Y = 0 the normal components are zero and the electric energy density of the incident laserfield is equal above and below the surface. However at the surface plasma resonance, the ratio of the energy density is different by a factor of

(

1+

I

01 0”

3o” Angle

60’ of

lncldence

9o” a

Fig. 6. Angle dependence of the Raman intensity at tOl0 cm-’ from pyridine adsorbed on silver after electrochemical activation. After ref. [28].

A. Otto I Investigation of electrode surfaces by SPPS

107

For the interface silver-aqueous electrolyte and the laser wavelength 5145 A, this amounts to 5.9. Additionally, there will be different scattered radiation patterns from the normal and parallel components of the Raman source term. In the context of the giant Raman effect [27], it would be interesting to know where the sources of scattered light are. For the image dipole models [29,30], they would be within the adsorbed molecules, that is, on the outside of the metal, whereas for the models involving photo-electron-hole pair interaction [31,32], they would be inside the metal. 5. Conclusion

For metals of high inherent reflectivity, the SPP-ATR resonance is very sensitive to changes of the electronic surface structure, to adsorbates and overlayers. The field enhancement in the resonance is useful for the Raman spectroscopy of adsorbates. Surface roughness complicates the analysis, but may well be studied on its own by SPP-ATR spectroscopy. However, the difficulty to extract microscopic, surface specific information from linear optical experiments is the same like in external reflectance spectroscopy and ellipsometry. 6. Outlook

Originally the author had planned to include a section on adsorbate SP interaction versus adsorbate electron-hole pair interaction. This problem is important in the understanding of the giant Raman effect (ref. [32] and references therein). Because of limited space, it could not be included in this paper, but will be published elsewhere. Acknowledgment

I thank D.M. Kolb, B. Pettinger and R.K. Chang for allowing me to use their results prior to publication. References [l] R.H. Ritchie, Surface Sci. 34 (1973) 1. (21 E. Burnstein, W.P. Chen. Y.C. Chen and A. Hartstein, .I. Vacuum Sci. Technol. 11 (1974) 1004. [3] A. Otto, in: Optical Properties of Solids; New Developments, Ed. B.O. Seraphin (NorthHolland, Amsterdam, 1976) (reprints available upon request). [4] H. Raether, Phys. Thin Films 9 (1977) 145.

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of electrode surfaces by SPPS

[5] A. Otto, in: Polaritons, Eds. E. Burstein and F. de Martini (Pergamon 1974) p. 117 (reprints available upon request). [6] F. Abelts, Surface Sci. 56 (1976) 237. [7] F. Abelts, T. Lopez-Rios and A. Tadjeddine, Solid State Commun. 16 (1975) X43. [g] F. Chao, M. Costa and A. Tadjeddine, J. Physique C5 (1977) 97. [9] R. Kiitz, D.M. Kolb and J.K. Sass, Surface Sci. 69 (1977) 359. [lo] D.M. Kolb, J. Physique C5 (1977) 167. [ll] A. Tadjeddine and D.M. Kolb, private communication. [12] N.D. Lang and W. Kohn, Phys. Rev. B7 (1973) 3541. [13] G. Valette and A. Hamelin, Electroanal. Chem. Interface Electrochem. 45 (1973) 301. [14] D.M. Kolb and R. Katz, Surface Sci. 64 (1977) 96. [15] T.E. Furtak and D.W. Lynch, Phys. Rev. Letters 35 (1075) 960. [16] F. Chao, M. Costa, A. Tadjeddine. F. Abelts, T. Lopez-Rios and M.L. Theye. J. Electroanal. Chem. 83 (1977) 65. [17] W. Schultze, Electrochim. Acta 17 (1972) 451. [lg] A.J. Braundmaier. Jr. and E.T. Arakawa, J. Phys. Chem. Solids 35 (lY74) Sl7. [19] J. Bodesheim and Z. Otto, Surface Sci. 45 (1974) 441, [20] D. Hornauer. H. Kapitza and H. Raether, J. Phys. D (Appl. Phys.) 7 (1974) LltXl. [21] E. Kroger and E. Kretschmann, Phys. Status Solidi (b) 76 (lY76) 515. [22] E. Kriiger and E. Kretschmann, Z. Physik 237 (1970) 1. [23] Y.J. Chen, W.P. Chen and E. Burstein, Phys. Rev. Letters 36 (1976) 1207. [24] R.G. Greenler and T.S. Slager, Spectrochim. Acta 29A (1973) 193. [25] R.E. Benner, R. Dornhaus and R.K. Chang, private communication. [26] M. Menetrier, R. Dupeyrat, Y. Levy and G. Imbert, Opt. Commun. 21 (1977) 162. [27] D.L. Jeanmaire and R.P. Van Duyne, J. Electroanal. Chem. X4 (1977) 1. [28] B. Pettinger, A. Tadjeddine and D.M. Kolb, Chem. Phys. Letters 66 (1979) 544. [29] F.W. King, R.P. Van Duyne and G.C. Schatz. J. Chem. Phys. 69 (197X) 4472. [30] S. Efrima and H. Metiu, Chem. Phys. Letters 60 (197X) 59. [31] E. Burstein, Y.J. Chen, C.Y. Chen, S. Lundquist and E. Tossatti, Solid State Commun. 20 (1979) 567. [32] J. Billmann, G. Kovacs and A. Otto, Surface Sci. 92 (19X0) 153; A. Otto, in: Proc. 6th Solid-Vacuum Interface Conf., Delft, 1980 (to be published in Appl. Surface Sci.).