J . Phys. Chem. 1990, 94, 2091-2098
2091
type I1 spectrum resulted from direct interaction of the viologen with the Ag surface and the formation of a charge-transfer complex. In the present study, only weakly adsorbed counterions were used and the charge-transfer complex was always observed following anodization in the presence of viologen.
time-dependent and is observed most rapidly if the electrode is irradiated during the ORC. The experimental results provide strong evidence for a charge-transfer complex: (1) the SERS/ SERRS spectra are radically different from their solution counterparts, and (2) the concentration dependence of the SERRS intensities indicates that the surface species is chemisorbed. These resdults are in accord with previous studies by Lu et in which it was shown that type I1 spectra resulted in the presence of weakly adsorbed counterions when viologens were present during anodization. Strongly adsorbed counterions were found to prevent the formation of type I1 spectra. This would be expected if the
Acknowledgment. We thank Dr. Tianhong Lu for helpful discussions. Funding for this work was provided by the Office of Basic Energy Sciences, Division of Chemical Sciences, US. Department of Energy (DE-FG02-84ER13261). The authors are grateful for this financial support.
Anisotropic Nonlinear Optical Response from Silver Electrodes during Thin-Film Deposition Daniel A. KOOS,Victoria L. Shannon: and Geraldine L. Richmond* Department of Chemistry, University of Oregon, Eugene, Oregon 97403 (Received: September 6, 1988; In Final Form: August 31, 1989)
Rotational anisotropy in the optical second harmonic generation from Ag( 11 1) and Ag( 110) single-crystal electrodes is used to measure relative changes in the components of the nonlinear susceptibility of the interface during charging and adsorption processes. The application of a dc field and underpotential deposition of lead and thallium are studied. Some common features are noted for thallium adsorption on the two crystal faces, and the results are compared to a similar study on Cu(l11). The isotropic components show changes in both magnitude and phase that continue up to the formation of the first monolayer. The anisotropic tensor elements in both cases are relatively insensitive to the formation of the first adlayer. The magnitude of these anisotropic terms is enhanced upon the formation of the second adatom layer. The results are analyzed with a model that assumes a Langmuir adsorption isotherm and stepwise deposition. Probable mechanisms for the enhanced signals and reasons for the adsorption behavior are proposed.
I. Introduction Second harmonic generation (SHG) has been shown to be a useful technique for examining surface structure and studying adsorption behavior. Surface symmetry and ordering on singlecrystal surfaces of both metals and semiconductors have been observed in ultrahigh vacuum (UHV),'v2 under ambient conditions,' and in solution3 by exploiting the anisotropic polarization dependence of the second harmonic response. The technique has been used to monitor adsorption of ionic, molecular, and atomic species on surfaces4 Its sensitivity to specific electronic states has recently been dem~nstrated.~The continuing interest in this laboratory is in the development of S H G as an in situ probe of the electrode/electrolyte interface. Essential to this goal is the detailed investigation into the source of the nonlinear optical response and the effect of potential variation, adsorption, and deposition on this response. Over the past several years, these investigations have focused on examination of the overall S H response from smooth polycrystalline and single-crystal electrodes biased within the potentials of the double-layer charging region.6 More recently, this work has been extended to the study of rotational anisotropy in the SH fields as single-crystal electrodes are rotated within the electrochemical cell about the azimuthal axis3 If the anisotropic optical response can be used to correlate electrode surface structure with surface electronic properties, the technique will have a significant impact by providing valuable information about the relationship between these surface properties and electrochemical reactivity. This study seeks to examine in detail the anisotropic S H response from native Ag( 1 1 1) and Ag( 1 10) surfaces in solution and the effect of electrodeposited overlayers of thallium and lead on that response. Different geometries and polarization schemes allow
the monitoring of specific elements of the nonlinear susceptibility tensor while the electrode is capacitively charged and during the electrodeposition of the thin metal films. The results demonstrate the sensitivity of the technique to the applied voltage, submonolayer coverage of adsorbates, and changes in the optical properties of the interface during the formation of the thin metallic film. Silver electrodes have been chosen in these initial studies because high-quality, stable surfaces can be prepared, the properties of which have been well studied by other techniques. Underpotential deposition (UPD) is a convenient method for thin-film growth, offering precise control of coverages because it is a process governed by thermodynamic equilibrium.' Thallium and lead overlayers on silver electrodes have been the focus of numerous investigatiom8 Thallium UPD on silver is especially interesting (1) Tom, H. W. K.; Aumiller, G. D. Phys. Rev. 1986,833, 8818. (2) See, for example: Heinz, T. F.; Loy, M. M. T.; Thompson, W. A. Phys. Reu. Lett. 1985,54, 63. (3) (a) Shannon, V. L.; Koos, D. A,; Richmond, G. L. J . Chem. Phys. 1987,87,1440; J . Phys. Chem. 1987,91, 5548. (b) Shannon, V. L.; Koos, D. A,; Robinson, J. M.; Richmond, G. L. Chem. Phys. Lett. 1987,142,323. (c) Miragliotta, J.; Furtak, T. E. Phys. Reo. 1988,837, 1028. (d) Shannon, V. L.; Kms, D. A,; Kellar, S. A,; Huifang, P.; Richmond, G. L. J. Phys. Chem. 1989, 93, 6434. (e) Georgiadis, R.; Neff, G.; Richmond, G. L. To be published. (4)See, for a review: (a) Richmond, G. L.; Robinson, J. M.; Shannon, V. L. Second Harmonic Generation Studies of Interfacial Structure and Dynamics; Progress in Surface Science; Pergamon: New York, 1988; Vol. 28, p 1. (b) Shen, Y. R. Nature (London) 1989,337, 519. (5) Tom, H . W. K.; Aumiller, G. D. Enhanced Surface Second-Harmonic Sensitivity to Specific Electronic States: Study of 02fSi(lll)-7X 7. Preprint. (6) See, for example: (a) Richmond, G . L. Langmuir 1986,2, 132. (b) Richmond, G. L.; Rojhantalab, H. M.; Robinson, J. M.; Shannon, V. L. J . Opt. Soc. Am. 1987,84,228. (c) Rojhantalab, H . M.; Richmond, G. L. J . Phys. Chem. 1989,93, 3269. (7) See, for a review: Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11, pp 125-271.
'Current address: Tektronix, Inc., P.O. Box 500, M S 59-165, Beaverton, OR 97077.
0022-3654/90/2094-2091$02.50/0 , , I
0 1990 American Chemical Societv -
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The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
because the deposition takes place in four distinct stages. From the coulometric data, it has been suggested that the sequential formation of two intertwined p( d 3 X d 3 ) R 3 0 ° overlays are subsequently filled in to form a complete monolayer. A second monolayer is formed before bulk deposition takes place. 11. Theory While the nature of the SH response from metal surfaces has been a subject of investigation since the mid 1960s, an understanding of surface S H G at these interfaces is far from complete at this time.g Experimentally, many questions concerning surface SHG have been answered but the importance of band structure details and surface electronic potentials has not been fully addressed. For example, the recent observation of rotational anisotropy at the Cu/vacuum interface has documented the importance of interband transitions to the nonlinear response of metals.' However, a similar observation at the Ag/electrolyte interface with fundamental and SH field energies well below those required to excite interband transitions in bulk silverJa cannot be explained in terms of a free electron response within a jellium model. The complexity of the electrode/electrolyte interface introduces several possible sources for the nonlinear optical response in addition to those present at the metal/vacuum interface. The metal itself has a surface response due to both the breaking of the inversion symmetry and the large gradient in the electric field present at the boundary as well as a higher order bulk response. Adsorbates will have an intrinsic susceptibility, but often more importantly, the interaction of the adsorbate electronic states with the metallic surface potentials may significantly alter the SH efficiency of the interface. The application of an external bias will also add to the S H production through the coupling of the static electric field with the photon field. All of these nonlinear responses contribute to the production of radiation at 2w, with the phase relations between the contributions determining the overall observed intensity. As a starting point, the response of the metal surface is described in terms of a macroscopic theory based on crystal symmetry.l02'' The additional contributions that must be included in the presence of a dc field electric field and adsorbates are then discussed in terms of this theory. A. Symmetry Considerations. General expressions for the harmonic fields obtained upon reflection from the low-index faces of a cubic medium are given by Sipe et aI.'* The bulk contributions in the cases considered here have both anisotropic and isotropic components that appear in the expressions for the SH fields as the phenomenological constants { and y, respectively. The surface contribution contains dipolar terms due to the lack of inversion symmetry at the boundary in addition to higher order multipole terms that are generated by the discontinuity in the normal component of the electric field at the surface. Both effects are accounted for with an effective surface dipolar polarization, the form of which is governed by the symmetry of the surface through the surface susceptibility tensor. As recently noted by Guyot-Sionnest and Shen,13a,b a correct treatment of the surface (8) See, for example: (a) Takayanagi, K.; Kolb, D. M.; Kamba, K.; Lehmphuhl, G. Surf Sci. 1980,100, 407. (b) Rolland, A,; Bernadini, J.; Barthes-Labrousse, M. B.Surf. Sci. 1984,143,579. (c) Laguren-Davidson, L.; Lu, F.; Salaita, G. N.; Hubbard, A. T. Langmuir 1988,4, 224. (d) Samant, M. G.; Toney, M. F., Borges, G. L.; Blum, L.; Melroy. 0. R. Surf. Sci. 1988,193,L29;J . Phys. Chem. 1988,92, 220. (9) Recent attempts at modeling the nonlinear optical response of interfaces by the density functional approach have met with limited success in accurately describing the experimental results from metal surfaces. See: (a) Weber, M.; Liebsch, A. Phys. Reu. 1987,835,7411;1988,B36, 6411. (b) Murphy, R.; Yeganeh, M.; Song, K. J.; Plummer, E. W. Phys. Rec. Lett. 1989, 63,318. (IO) Tom, H. W. K . Ph.D. Dissertation, University of California, Berkeley, 1984. ( I 1 ) Tom, H. W. K.; Heinz, T. F.; Shen, Y. R. Phys. Reu. Lert. 1983,51, 1983. (12)Sipe, J. E.;Moss, D. J.; van Driel, H. M. Phys. Reu. 1987,B35, 1129. ( I 3) (a) Guyot-Sionnest, P.; Shen, Y. R. Phys. Rev. 1987,B35.4420;(b) Ibid, 1988,B38, 7985. (c) Sipe, J. E.; Mizrahi, V.; Stegeman, G. I. Phys. Rev. 1987,835,9091.
Koos et al. (a)
A
Y
A
X
s Figure 1. Diagrammatic presentation of the experimental crystal and beam geometries for p polarized radiation incident upon the (1 11) crystal face as viewed (a) from the side and (b) from the top, including the second atomic plane (cross-hatched region). The surface coordinates are labeled x y , z with the i direction along the [2iT] crystal direction. The beam coordinates are labeled s,k,z.
susceptibilities includes an additional nonlocal contribution that is properly considered a bulklike term because it depends only on the bulk dielectric constants. For cubic media, this bulk term appears in both the isotropic and anisotropic susceptibilities and can be considered an additional contriution to the bulk terms ( and y. The effective surface dipole polarization density can be written as
where the surface susceptibilities are in terms of beam coordinate geometries (s,k,z). The form of x ( ~in) surface coordinates (x,y,z) is obtained by assuming perfect termination of the bulk symmetry. The nonzero and equivalent tensor elements are determined by noting that the form of the tensor must remain invariant under symmetry operations that transform the materials into itself.I4 Transforming x(*)from surface to beam coordinates is then a straightforward application of the transformation law of a third rank tensor,
where aij is an element of the direction cosine matrix. In this manner, f i ( 2 w ) is given in terms of the surface susceptibility elements xijk(x,y,z).In the case of the (1 11) surface, which has C, (3m) symmetry, the nonzero elements are Xzzz3
xzxx
and
=
Xzyy.
xxxx
Xxrx
= -x,yx
= =
Xxxr -xxy,
= x,, = = -xyxy
Xyzy7
where the i direction is perpendicular to the surface and the mirror plane lies parallel to the 2 direction along the [ 2 i i ]crystal direction (Figure 1). The first three independent elements have a field component perpendicular to the surface while xxxxinvolves fields only in the plane of the surface. ~
~~
~
J. F.Physical Properties of Crystals: Their Representation by Tensors and Matrices; Claredon Press Oxford, 1985 ( 14) Nye,
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 2093
Anisotropic Optical Response from Ag Electrodes
TABLE I: Composition of the Parameters a , b , and c in Terms of the Surface and Bulk Susceptibility Elements“ [Ill)
3, a
XZ,,
.......................
Xzii
Xmi
* ..............
c ( 3 ) .....................................
b(3).....................................
3
........................
* ...................... * ...................... * ........ * ...................... * ........
c ( 3 ) ............................................. J4)
s
7
* .................. .* * ................
.....* ..... .....
x,
......t ...................... ......* ......................
...................................................................... ......... ..................* ...................... ...................... ..................................................................
b(2)........................................................................... b(4)...........................................................................
*
* ...................... * * * ...................... *
“ i = (x,y).
For all of the experimental data presented in this paper, the fundamental field is polarized parallel to the plane of incidence (p polarized) and the S H fields are resolved into their linear polarization with respect to that plane, parallel (p polarization) or perpendicular (s polarization). Having obtained expressions for the polarization induced in the medium at 2w, the SH fields generated can be calculated.12~15 The relevant expressions for the S H intensity in the ( 1 11) case are given as
p H ,0,: [ a + d 3 )cos (39)12
(3)
FHp,s a [bc3)sin (39)12
(4)
The subscripts p and s denote the fundamental and second harmonic beam polarizations, respectively. The linear and nonlinear dielectric constants of the material, as well as the appropriate Fresnel factors at w and 2w, are incorporated into the constants a, b(3),and d 3 ) .The susceptibilities contained in each of these constants are given in Table I. The term b(3)is linearly proportional to d3),where the constant of proportionality is determined by the dielectric constants of the media at w and 20 as well as the angle of incidence through the Fresnel coefficients. The azimuthal angle 9 is defined as the angle between the [2TT] direction and the component of the incident wave vector parallel to the surface (Figure I ) . It is apparent from eq 3 and Table I that xZrr,xZxx,xxZx,and y contained in the a term do not show any dependence on the azimuthal angle 9 and therefore are referred to as isotropic susceptibilities. Thus, two physical parameters that can be separated and monitored experimentally are the isotropic susceptibilities contained in a and the anisotropic susceptibilities contained in b(3. This can be accomplished by at an azimuthal angle of 9 = 30’. A following FH,p and FHp,s calculation of the Fresnel factor for the silver/water interface revealed that the coefficients preceding xxxxand xxZxare in phase ~ of phase with the coefficients for xzZz,xZxx,and and 0 . 5 2 out y. This is true at both 1Oo and 3 1 O, the two angles at which data was collected in this study. These phase factors will affect the SH patterns observed as the crystal is rotated about 9. Similarly, the nonvanishing surface tensor elements for the (1 10) face which has C2”(“2) symmetry are xrZz,xzxx,xryy,xxrx= x,,,, and xyzy= xyyr,where the directions 2 and are along the [ 1 IO] and [OOI] axes, respectively. The p and s polarized SH intensities are then given as
FH,, 0: [ a + d 2 )COS (29) + d4)COS (49)12 FHp,s a [ b ( 2 )sin (29)
+
sin (49)12
(5) (6)
respectively. The anisotropic and isotropic surface and bulk susceptibilities contained in the constants for the (1 10) face are also listed in Table I where it is shown that the terms b(4) and d4) are linearly proportional. There are two tensor elements contained in d2),xrxxand xzyy,which do not appear in b(2).For this reason an additional anisotropy may appear in the p polarized SH response that is not evidenced in the s polarized signal for the (1 IO) crystal. The angle a, in this case, is defined as the angle between (15) Mizrahi, V.; Sipe, J. E. J . Opt. SOC.Am. 1988, E5, 660.
the [I101 direction and the component of the incident wave vector parallel to the surface. The dielectric constants of the material and the appropriate Fresnel factors at w and 20 are incorporated into the constants a, b(2),b(4),c@),and d4).By orienting the (1 10) face at an azimuthal angle of 45O, a and b@) b(4)can be seprespectively. arately monitored by measuring FH,,and pH,,+, B. Direct Current Electric Field Effects. In addition to the surface and bulk contributions previously discussed, a contribution to the nonlinear polarizability of the material will be induced in the presence of an applied field through the third-order suscepThis contribution can be comparable to the sectibility #).I6 ond-order response when the dc field is large, as is the case for a metal electrode in solution held at a bias potential positive of the potential of zero charge (pzc). The S H fields can be viewed as arising from a sum frequency mixing process where two incident ac fields at frequency w are mixed with a dc field at frequency w = 0 to produce a reflected and transmitted wave at frequency 2w. x ( ~has ) the properties of a fourth-rank tensor so that the third-order source polarization has the form
+
(7) where E, = Edc. Making the appropriate transformations from , ~the / surface to beam coordinates, it can be shown that x ( ~ ) ~has same form as the second-order surface tensor X ( 2 ) j j k . This is a result of the applied field being parallel to the surface normal about which the sample is rotated. Because the field is localized at the surface, the x ( ~effect ) should mimic the dipolar surface response and an effective susceptibility can be written as where there is a one-to-one correspondence between the elements ) x ( ~ )Therefore, . the application of an applied field of x ( ~and would not be expected to change the form of the source polarization, and consequently, the symmetry observed in the S H rotational anisotropy from the interface should remain unchanged. ) x(~) The magnitude of the individual tensor elements in x ( ~and may differ appreciably, however, leading to a change in the observed SH patterns. C. Adsorbates. If all of the adsorption sites on the surface are equivalent and noninteracting, so that a Langmuir model for the adsorption is applicable, one expects a linear relationship between the change in the nonlinear susceptibility x ( ~and ) the surface coverage @I7
x ( ~=) A
+ B(6)
(9)
where A and B represent the substrate and adsorbate contributions, to the nonlinear susceptibility, respectively. 0 is the surface coverage of adatoms with respect to the silver substrate, normalized with respect to the saturation (monolayer) coverage. By including a factor of ei+ that accounts for the phase of E(2w) from the (16) (a) Corn, R. M.; Romagnoli, M.; Levenson, M. D.; Philpott, M. R. J . Chem. Phys. 1984,81, 4127; Chem. Phys. Lett. 1984, 106, 30. (b) Lee, C. H.; Chang, R. K.; Bloembergen, N. Phys. Rev. Lett. 1967, 18, 167. (c) Terhune, R. W.;Maker, P. D.; Savage, C. M. Phys. Reu. Lett. 1962,8.404. (17) Heinz, T. F. Ph.D. Dissertation, University of California, Berkeley, 1982.
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The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
adsorbate relative to the field due to the substrate, an expression for the observed intensities as a function of surface coverage can be written as I ( 2 0 ) = 11
+ (2B/A)B COS 4, + [ ( B / A ) ( e ) I 2 1
(10)
=A
+ B e , + c(e2- e , )
a
I
[
I
1 IOpA/cm2
g
ii
C
8
----
e
C
: ,
/ ,
---='
--
I
I
I
t' 5
as adjustable The data can be fit to eq IO with B / A and parameters. When the adsorption site changes or interactions between the adatoms change the optical response of the interface, an additional ) be included. We therefore write contribution to x ( ~must x(2)
Koos et al.
A
0
1
(11)
The observed intensity can then be written as a quadratic function of (c/4(e2- e!): I(2w) = II
+ (28,B/A)(cos
%)[cos 42
+ @$/A)
41)
+ (8,B/A)2+ 2(C/A)(82 -
cos (4, - 4211
+ [ ( C / 4 ( 0 2- @,)121 (12)
Again, the data can be fit by using the ratio of the contributions from the substrate and the adatoms, CIA, as a fitting parameter together with an additional phase factor 42. Subsequent deposition can be treated in an analogous manner. In this method, the sign of the phase angle is undetermined. Another assumption made is that the perturbation of the substrate contribution to x ( ~by) the adsorbate is negligible. The validity of this assumption is questionable in the present case where at higher coverages changes in the refractive index of the overlayer will effect substrate con) depend upon the field gradients. This may tributions to x ( ~that lead to errors in the calculated values of B / A , C I A , and the associated phase angles. The magnitude of this substrate/adatom interaction may be estimated by combining an actual measurement of the phase of the S H field1' with the method described here and is the object of ongoing studies in this laboratory. Any arbitrariness in assigning the points of abrupt change in 8 vs pH where steps are introduced in the adsorption isotherms will be resolved by additional phase measurements also. 111. Experimental Method The optical measurements are made with IO-ns pulses at 1.064 pm from a Nd:YAG laser operating at 10 Hz. The laser beam is passed through a half-wave plate and a polarizing beam splitter to select the beam intensity and the beam polarization impinging on the sample. The beam is condensed, collimated to a spot size of approximately 3 mm in diameter, and passed through visible filters before striking the electrode surface at an incident angle $ with respect to the surface normal. The specularly reflected light is passed through a set of infrared absorbing filters, a lens, and a polarizing beam splitter, before being focused into a monochromator equipped with a polarization scrambler. The output of the monochromator at 532 nm is detected by a cooled photomultiplier tube. The radiation of alternate polarization from the first polarizing beam splitter is similarly condensed and collimated before being used to generate a signal at 2 0 in a KDP crystal that is detected by a photodiode. The output of the photodiode serves as a reference against which the collected S H signals are normalized to eliminate the effects of fluctuations in the incident laser intensity. Gated electronics average the signal over 10 laser pulses. The averaged signals from the sample and the reference are stored by a computer that also controls rotation of the crystal via a stepper motor as well as the voltage applied by the potentiostat. The silver single crystals, either cut from a single crystal rod or obtained as discs (Monocrystals, Cleveland, OH), are oriented to within 1' by Laue back-reflection. The surfaces are mechanically polished to 1 wm (diamond paste) followed by electrochemical polishing in a cyanide bath that produces a highly specular surface. A significant amount of material is anodically removed, to a depth of approximately 3000 A, by this procedure. During the electrochemical polishing and all subsequent steps, the crystals are kept under an inert atmosphere (N, or Ar) or in purged solutions.
0
0.2
0.4
-E
0.6
0.8
vs Ag/AgCI
(volts1
1.0
Figure 2. Cyclic voltammograms for silver single-crystal electrodes in nonspecifically adsorbing electrolytes. (a) Ag( 1 1 1) in 0.25 M Na2S04, pH = 5.8. The cathodic ( K ) and anodic ( A ) currents were monitored at a sweep rate of 10 mV/s. (b) Ag(l10) in 0.25 M Na2S04,pH = 5.6, sweep rate = 1 mV/s. All potentials ( E ) are versus Ag/AgCl (saturated KCI).
The electrochemical cell is fashioned solely from Kel-F and Teflon and fitted with a glass window. The single crystal is mounted on a Kel-F shaft against an inlaid silver disk through which electrical contact to the back of the crystal is established. The face of the electrode, positioned 2 mm from the optical window, is sealed by a Kel-F mask designed to isolate the edges and back of the crystal from contact with the electrolyte. The electrode configuration employs semiinfinite linear diffusion conditions. A Pt wire surrounding the single crystal serves as a counter electrode. The reference electrode, a commercial Ag/ AgCl (saturated KCI) couple, is contained in a compartment separated from the working electrode by a ceramic junction. All potentials reported here are measured relative to this couple. Solutions are made from high-purity salts (Aesar, JohnsonMatthey) and high-purity water (Barnstead Nanopure 11, Sybron Corp.). All glass- and plasticware are soaked in a strong oxidant (Nochromix, Godax Laboratory) and rinsed repeatedly with water prior to use. The electrolyte is pumped through the optical cell from a bath of continuously purged (N, or Ar) solution at a slow rate (1-2 mL/min) with a peristaltic pump. By this method, optical measurements can be made while purging to eliminate trace amounts of 0,. The electrode bias voltage is controlled by a PAR 174 potentiostat equipped with a universal programmer and a digital coulometer. Currents generated in the potential sweep experiments are recorded on an x-y recorder. Two types of optical measurements are reported in this study. In one experiment, the variation in the SH intensity is recorded as the electrode rotates about the azimuthal axis while held at a fixed bias voltage. Several complete rotations through 360" are typically averaged to enhance the signal-to-noise ratio. In another set of experiments, the S H intensity is monitored at a fixed electrode angle while the applied voltage is varied at a set rate (10 mV/s). From this second type of data, the optical response as a function of charge or coverage can be obtained by integration of the voltammogram. IV. Results and Discussion A . Native Ag( 11I) and Ag( 110 ) Surfaces. Before examining the effects of adsorbates on the S H response, the results for a native silver surface in the absence of any specific adsorption or faradaic processes are considered. The criterion for a clean surface is the lack of any features in the cyclic voltammogram (CV) as the electrode potential is swept between the limits of electrode oxidation and solvent reduction. Figure 2 presents typical CVs for well-prepared Ag( 1 1 1) and Ag( 1 10) surfaces in nonspecifically adsorbing electrolytes. The region scanned includes the potential
Anisotropic Optical Response from Ag Electrodes
The Journal of Physical Chemistry, Vol. 94, No. 5, 1990 2095 Ag( 110)
11)
b 120
0 ANGLL
240
OF ROTXI'lOh
360
~ 1) (to la/d3)(< 1. Furthermore, the signal levels are enhanced by approximately 9 times those observed in the nonspecifically adsorbing electrolyte at the same potential. Separate determination of changes in the isotropic and anisotropic components of the surface susceptibility tensor is obtained by monitoring the SH intensity at a fixed angle of CP = 30°, where FH,~a la12 and FHp,s a lb(3)12.The results for the adsorption of thaflium are presented in Figure 9 as a function of coverage (8) and demonstrate the sensitivity of the isotropic and anisotropic terms to alterations in the electronic properties of the interfacial region with deposition. Fits to the data are obtained using eq 9-12 with the fitting parameters given in Table 111. Changes in the magnitude of the isotropic susceptibility a seen in Figure 9a reflect the complicated nature of the deposition process. The results can be modeled by assuming three distinct stages to the deposition process. From 8 = 0 to 0.87 (-0.35 to -0.54V in the CV), the isotropic signal is modified by the depositing thallium atoms = 128.8O. During the continuously with B / A = 1.12 and deposition peak labeled A,, an abrupt change is observed where further contribution from the adatoms can be modeled by setting C I A = -0.80 and +2 = 6 3 . 5 O . Once the first layer is complete, 0 = 1. I , as determined by the coulometry, the isotropic terms remain rather constant through the formation of the second adlayer. The magnitude of the anisotropic susceptibility @ (Figure 9b) is relatively unperturbed by the formation of the first monolayer (Ai-A3). This is followed by significant enhancement in the magnitude of 601, once the second monolayer begins to form and which continues until the second monolayer is 2 / 3 complete. It is interesting to note that the initial deposition of both lead and thallium on Ag( 11 1) introduces a phase shift between the a and d 3 )components of x ( ~ )Similar . changes in the rotational anisotropy of the silver/electrolyte interface were earlier observed
when the electrode was held in simple in simple aqueous electrolytes at a negative bias for extended periods of time (hours).3a It was found that the observed changes were concurrent with the deposition of trace amounts of impurities from the electrolyte. As little as 20 pC/cm2 of charge passed during anodic stripping of the deposit induced a similar phase effect. The precise identity of the contaminants, possibly organic in nature, is unknown but they have been eliminated in the studies reported here. The observation of an optical response at fraction1 monolayer (ML) coverages of contaminants similar to those observed for metal adsorbates at 8 5 1 M L argues in favor of the changes in FH corresponding to adsorbate-induced states in the metal surface. In a recent study of thallium deposition on Cu( 11 an abrupt change in the adsorption isotherm for the isotropic contribution ) noted at a coverage that corresponded to the formation to x ( ~was of an epitaxial overlayer, 0 = I / , . The lattice mismatch is not as great on the Ag( 11 1) surface, and the point at which an epitaxial overlayer will be reached on a Ag( 1 11) substrate occurs at a higher coverage. The peak in the CV labeled A,, during which abrupt changes in the isotropic components are observed, begins at 8 = 0.75 ML. A comparison of these results suggests that later stages in the formation of the first monolayer result in either buckled or incommensurate overlayer structures leading to abrupt changes in the isotropic components of the nonlinear optical response of the interface. 3. Ag(1 1 0 ) / T l + . The electrochemical deposition of thallium on Ag(ll0) is similar to that that takes place on the (1 11) face of silver. The voltammogram (not pictured) shows well-defined structure in the formation of the first monolayer, and further deposition occurs before formation of the bulk deposit. Figure 10 shows the response of the isotropic and anisotropic susceptibilities for Ag( 110) as a function of thallium coverage. Again, the solid lines are fits to the data from eq 9-12, and the fitting parameters are listed in Table 111. Similar to the Ag(1 1 l)/TI+ case, the first monolayer of adatoms shows a significant contribution to laI2with B / A = 0.94 and = 131.0°. The phase factor is also evidenced in the p polarized rotational scan (not shown). This change continues throughout the deposition of two monolayers. Also similar to the Ag( 1 1I)/Tl case is a large enhancement in the magnitude of the anisotropic response lb(2)+ b(4)12with C I A = 2.8 during the establishment of the second
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The Journal of Physical Chemistry, Vol. 94, No. 5, 1990
-, -
-I -5-
__-_
,
1)'.
ibl
-
1:
.,.
r
1
..
1
Figure 10. Second harmonic response at 0 = 45' for Ag( 110) immersed in 5 m M TllS04as a function of thallium coverage (e). (a) p polarized signal where FH a la1*. (b) s polarized signal where FH 0: 1b(2)+ b(4)12. The solid lines are fits to the data according to eq 12 (see text and table 111).
adlayer. The rotational scan of Ip,sat this coverage (not shown follows the (sin)2 form predicted by eq 6. The magnitude and phase of laI2from Ag( 1 IO) can be modeled by a constant contribution from the adatoms throughout the adsorption process. This is different from the deposition on Ag( 1 1 1) where two discrete steps in laI2 are noted during the completion of the first adlayer and the initial formation of the second adlayer. It is clear from the multiple peaks in the voltammetry that there are energetically different processes occurring during the formation of the first adlayer that are not reflected in the optical data. There are reasons to suspect the enhanced anisotropic signals seen for Ag( 11 1) and Ag( 110) at coverages greater than 0 = 1 result from the optical properties of the thallium thin film. An enhancement of the anisotropic nonlinear response would be expected if either w or 2w can couple to an eigenstate of the overlayer.22 Such resonance effects have been proposed to explain enhanced signals seen in the nonlinear reflectance of alkali metals adsorbed on Rh( 1 1 1)22 and in the linear reflectance of the metal/adsorbate/solution interface (Cu on Pt).23 These studies also indicated that bulk optical properties of the metallic adsorbate were achieved at a coverage of 2 ML. Bulk interband transitions are predicted by theory and observed experimentally for both lead and thallium in this wavelength region.24 The enhancement in the anisotropic component at 2 ML observed here is most likely due to such a resonance between w or 2w and bulk interband transitions of thallium that arise as the overlayer begins to develop (22) Tom, H. W.K.; Mate, C. M.; Zhu, X. D.; Crowell, J. E.; Heinz, T. F.; Somorjai, G. A,; Shen, Y . R. Phys. Reu. Lett. 1984, 52, 348; Surf. Sci. 1986, 172, 466. (23) Kolb, D. M.; Kotz, R. Surf. Sci. 1977, 64, 698. (24) (a) Myers, H. P. J . Phys. F Meral Physics 1978, 3, 1078. (b) Liljenvall, H. J.; Mathewson, A. G . ;Myers, H. P. Philos. Mug. 1970, 22, 243.
Koos et al. a three-dimensional structure. Another possible cause for the enhanced signal is the excitation of plasmon modes. Wavelength-dependent studies are underway in this laboratory to investigate these Recent theoretical workg has suggested an inverse relationship between work function changes and intensity in the second harmonic signal. For nearly all metal/adatom systems for which an underpotential shift is found, the work function of the substrate is greater than that of the adsorbate.' This supports the suggestion that the increase observed in the S H intensity at coverages approaching 2 ML be interpreted as the optical properties of the interface resembling those of the bulk adsorbate. The intensity of the isotropic SH signals exhibit a saturation effect at coverages exceeding 2 ML. The overall intensity decreases slightly, and the rotational anisotropy gradually decays. The fact that no further increase in S H intensity is observed at coverages exceeding two monolayers suggests that the nonlinear optical response arises from the first two atomic layers of the silver/thallium interface. At coverages estmated to be approximately 100 ML, the S H signals appear isotropic. V. Conclusions Important effects in the nonlinear optical properties of Ag( 1 11) and Ag( 1 10) electrodes have been recorded and studied as the interface was biased in simple electrolytes and during electrodeposition of foreign metals. Adsorption induces a phase shift in the fields contributing to the isotropic response. This effect is relatively independent of the specific adsorbate and leads to similar changes in the effective susceptibilities. Discrete steps in the isotropic contributions to the S H response are noted during monolayer formation of thallium on Ag( 11 1) that are not evident on the Ag( 1 10) surface. Comparison of this data to other recent findings suggests that these abrupt changes in the nonlinear reflectance may correlate to structural changes in the overlayer. The bulk contribution to the anisotropic components for silver at X = 1.06 pm is negligible compared to the surface dipolar contribution. The magnitude of this anisotropic response for both Ag( 1 11) and Ag( 1 10) is appreciably enhanced at thallium coverages exceeding a monolayer. This trend suggests the development of a 2D band structure in the thin film.
Acknowledgment. We express our appreciation to the reviewers for their considerate efforts and thoughtful insights. The assistance of Dr. Jeanne Robinson in all phases of this project was invaluable. Gratitude is also expressed to Dr. Rosina Georgiadis for her help with the Ag(ll0) measurements. The optical measurements were made at the University of Oregon Shared Laser Facility. Support for this work came from the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation (Grants NSF-8722798, NSF8451346). G.L.R. acknowledges financial support from the Alfred P. Sloan Foundation and the Camille and Henry Dreyfus Teacher Scholar Award. In addition, D.A.K. thanks the Electrochemical Society for support through a Summer Research Fellowship. Registry No. Ag, 7440-22-4; Pb, 7439-92- 1; TI, 7440-28-0; Na,S04, 7757-82-6; NaCIO,, 7601-89-0.
