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Second harmonic generation studies of anionic adsorption on polycrystalline and single crystal silver surfaces
Volume
110, number
6
CHEMICAL
SECOND HARMONIC GENERATION ON POLYCRYSTALLINE
PHYSICS
19 October
LETTERS
1984
STUDIES OF ANIONIC ADSORPTION
AND SINGLE CRYSTAL
SILVER SURFACES
G .L. RICHMOND Department
of Chemistry.
Bryn hfawr College.
Bryn hfawr, Pennsylvania
19OIO.
USA
Received 14 July 1984; in final form 22 August 1984 Adsorption of anions on single crystal and polycrystalline silver surfaces has been monitored with optical second harmonic generation. Potentials investigated were restricted to the range between solvent reduction and the onset of metal osidation. Excess charge density on the metal surface is concluded to be a major factor in the voltage-dependent production of the harmonic light. The results demonstrate the potential of this method to measure ionic adsorption quantitatively at the interface.
1. Introduction
P”‘S(2w)
Recent use of second harmonic generation (SHG) has demonstrated its sensitivity in detecting low levels of silver complexes [l-3] and organic adsorbates [4,5] on silver surfaces in electrochemical systems. Earlier studies from this laboratory [l] demonstrated the detectability of monolayer amounts of relatively soluble Ag2S04 on silver electrodes but also noted voltage-dependent SHG prior to the chemical formation of Ag$304. Originating at voltages near the potential of zero charge (PZC) of silver [6], this signal grew steadily until the onset of oxidation and was attributed to anionic adsorption. This Letter reports the results of the first detailed studies aimed at using SHG to monitor quantitatively the adsorption of anions on metal surfaces. The interaction of a wide variety of ions with polycrystalline and single crystal silver surfaces has been investigated. In contrast to surfaceenhanced Raman scattering (SERS) studies which have also detected adsorbed anions [7], these measurements were performed on silver surfaces which were free of electrochemical roughening. The attractive feature of SHG as a surface probe is its intrinsic sensitivity to the interfacial region [3]. The intensity of the second harmonic light generated at the junction between two centrosymmetric media is proportional to the square oft& non-linear polarizability [8] Pn1s(20), where
Contributions to the effective non-linear susceptibility tensor Xc21 include the electric-quadrupole and magnetic-dipole terms on the metal side of the interface and the non-linear molecular polarizability term for the noncentrosymmetric adsorbate in contact with the metal on the solution side of the interface. The sensitivity to the interfacial region arises from the symmetry requirement that this second-order process is restricted to media lacking inversion symmetry [9] _ An additional mechanism for optical SHG is possible at an electrochemical interface. Lee et al. [lo] first observed a strong SHG dependence on applied electric field, Edc, for silicon and silver electrodes and attributed it to a third-order process where the general expression for the non-linear polarization of the source is given by
0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Pnls(20)
= X(‘l(20)
:E(o)
*E(o)
= X(3k??dc-E(W)-,?(0)
-
_
It was proposed that with fields as large as lo8 V/cm present at the electrochemical interface, this effect should dominate over the second-order contributions to the polarizability [11,12] _In fact, simple comparative calculations indicate that the magnitude of the third-order effect is at least as large as the magueticdipole and electric-quadrupole terms in the secondorder polarizabllity. Although third-order processes are ahowed in centrosymmetric media,
571
Volunle 110, number 6
CHEMICAL PHYSICS
lution falls to a very low value beyond the dimensions of the electrical double layer. For a perfectly conducting smooth surface, the dc field is proportional to the excess charge density residing on the metallic side of the interface. Recent experimental studies by Corn et al. [ 131 on silver thin films and previous theoretical studies [ 141 suggest that this metal charge density, gm, is responsible for the observation of SHG at surfaces where 1(2w) a qz, _ The results reported here indicate a close correspondence between the voltage dependence of the SHG response and adsorption of anions on the metal and furthermore support the proposal that the charge density on the metal surface plays an important role in detecting ionic adsorption.
2. Experimental
procedure
The silver electrodes were immersed in a quartz electrochemical cell equipped with a platinum counter electrode and a saturated calomel reference electrode (SCE). All voltages quoted are relative to the SCE. Potentials investigated were restricted to the range betwcen solvent reduction and the onset of metal oxidation. Using a 5 mV/s ramp, the scans were initiated near --I .3 V and terminated at -0.4 to -0.2 V depending upon the electrolyte being studied. The solutions wcrc prepared with doubly distilled, deionized water and were purged with nitrogen prior to study. The K?SO, and KC104 solutions were adjusted to a pH of 5 with H2S04 and HC104 respectively. The optical instrumentation used was similar to that described previously [1,15]. The 1 .OG pm laser pulses of cross-sectional area 0.5 cm2 were incident on the silver electrode surfaces at an angle of 45”. To avoid laser damage [2,3] tlic power was maintained at 5-6 mJ/pulse. The reproducibility of the signal over 20-30 rcpctitive scans was within an experimental crfor of approximately 10%. The Ag(ll1) and Ag(ll0) single crystals were oricntcd and cut within 1/2O of the bulk axis. Both these surfaces and the polycrystalline silver were mechanically polished to a level of 0.05 pm alumina grit. Sligltt chemical etching with a dilute NH,01_1:lI~O~ mixture followed the polishing. After
LEi-IXRS
19
October
1984
thorough rinsing, the metals were immersed in the electrochemical cell and potentiostated at -1.3 V prior to the scans.
3. Results and discussion Fig. 1 shows the results obtained from a polycrystalline silver electrode polarized in a variety of aqueous electrolytes. For aX solutions, the dominant feature in the voltage-dependent signal is a sharp rise in harmonic intensity near -1 .O V. This signal approaches a constant value as the electrode potential is scanned to more positive values. Prior to this rise, a small background signal is observed which shows a slight decrease with increased positive bias. Quadratic power dependence and 532 nm spectral purity has been verified for the detected light over the voltage range. The relative adsorptivlty of anions to silver is well documented [6] and follows the trend of I- > Br> Cl- > SO:- a ClQz. This sequence is reflected in the SH voltage profiles where the more strongly adsorbing solutions result in a greater response at more negative potentials. More importantly, the potential at which this SH rise occurs correlates closely with the reported TZC values for silver in these electrolytes [6,16,17]. As the positive charge density on the metal increases from zero at the PZC, anion adsorption should show a corresponding increase. This behavior is clearly reflected in the SH profiles and supports the
I)
KCIOd
J -I
,
b)
KCI
.
.
.
,
.
.
.
,
.
*
.
1
-1.2 ’
-0!8
-0!4 V&.,,,(volts)
Pig. 1. SHG front polycrystnllinc silver OSa function of applied potential. The concentration of KC10 was 0.05 hi. The hlidc clcctrolytcs
WUICat a concentration
of 0.1 M.
Volume 110, number 6
a)
CHEMCAL
SHG method to monitor adsorption at potentials where the charge density at the interface is only slightly increased from its zero value. Further details of these studies wilI appear in a later publication [22] _ With increased positive bias on the electrode, the SH intensity reaches a constant level indicating saturation. This saturation is attributed to the formation of a monolayer of coverage_ For the more strongly adsorbing halide ions the saturation intensities are relatively similar yet follow the trend I” < Br- < Cl- _ The relative magnitudes of the SH response from the various anions offers insight into the contributions to the non-linear polarizability at the interface. If tlte harmonic generation were dominated by the third-order effect, the SH signal at maximum adsorption should reflect the relative magnitudes of the charge density of these ions at monolayer coverages. For the halide ions this charge density varies from 96 PC/cm:! for I- to 144 &/cm2 for CI- in agreement with the trend measured here. An opposite and stronger ordering would be expected if the intrinsic non-linear polarizability of the halide ion was dominant. For sulfate and perchlorate ions, the constancy in intensity is not as evident yet the signal is comparable in magnitude to +Jle halide ions. Intensity comparisons of these ions with the specifically adsorbed halide ions is complicated. Perchlorate ions, which do not specifically adsorb 116,191, are separated from tfle metal surface by both a hydration sheath and the inner plane of dipole oriented water molecules adjacent of the surface. Sulfate ions show slight evidence of specific adsorption on polycrystalline silver [21]. The ability to detect these ions which are not in direct contact with
Ad1 10)
l-2 -X 5 d 2
:T,d,,
,
‘TO IT
b) Ag(111) ; ‘!z cl z d*,, OJ, ,
-1.5
- 1.0
-0.5
0
V,qW,c,(volts) Pig. 2. SHG from silver single crystals biased in 0.1 N KBr.
ear&x conclusion [l ,I 51 that the SIG in this preoxidative region is reiated to anion adsorption. Identical studies were performed with Ag(l11) and Ag(l10) crystal faces which have significantly different adsorptive properties. The PZC values for the Ag(ll0) face are known to occur at potentials 250-300 mV more negative than the A&l 1) face f 18-21 f . This dlfference in adsorptivity appears in the SH profdes for all of the eIectrolytes studied. A representative spectrum for KBr is shown in fig. 2. The rise in harmonic intensity for Ag(ll0) occurs at potentials 290 mV more negative than that of Ag(l11). Table 1 compares reported PZC values for these surfaces with the potential at whicfl tfle SH intensity indicates a change in the electrode interface. The remarkable agreement between these values demonstrates the sensitivity of the Table 1 Con~p~rison of silver PZC wlucs with the onset of anion adsorption .-_
SO;CIOF aIkI-
as measured by SHG a) A&Ill)
A&l 10)
Ag(poly) SHC(V) b)
p=xV
-0.95 -0.90 -0.98 -1.05 -1.25
-0.94 -0.95 -0.93 -1.15 -1.3
c) e) g) 6) 6)
19 October 1984
PlfYSICS LETTERS
WC
PZC
SHG
PZC
-1.05 -1.05 -1.15 -1.24 -1.33
-1 .o d) -0.99 fJ -1.05 I’)
-0.75 -0.74 -0.8i -0.95 -1.18
-0.72 d, -0.735 d) -0.83 ‘0
a) All solutions studies wcrc 0.1 M csccpt for KC104 which \V;IS 0.05 M. b) Experimental uncertainty = *30 mV_ f) Ref. f19f. g)Ref. 161. d) Ref. /2t]. cl Ref. f16]. c) Ref. [17]. I~) Dan from ref. [ 201 for 0.04 hi.
573
Volume 110, number 6
CHEMICAL PHYSICS LETTERS
the meta strongly suggests that SHG originates from the metallic portion of the interface. Since the charge density on the metal surface is always equal to the charge density on the solution side of the interface, the nleasurement of the former is independent of ion location. A similar conclusion for perchlorate ions on silver thin fiims was recently reported [ 133. Additional experiments were performed to compare the concentration dependence of the SH profiles with the predicted behavior_ For specifically adsorbed anions, the PZC shifts to more negative potentials with added ion concentration 1121. Metal charge density~ profiles for ions*which do not specifically adsorb should show no concentration dependence. For all of the halides, the SH adsorption profiles shift to more negative potentials with increased halide concentration. Fig. 3 depicts the results for KBr over the concentration ranges of 10-1-10-” M. K-$X& was used in these studies to maintain a constant ionic strength although similar observations were made without added K2S01 _ No variation in the profnes was observed for either K$SO, or KCIO, over a similar concentration range. The reported shifts for K,SO, 1211 are less than the experimental uncertainty of these SH measurements. The results described above give strong support to the claim that the SH intensity generated at the silveraqueous interface reflects the adsorption of anions to the metal surface. Furthermore there is evidence that
-1.2
-0.8
-0.4
I:& 3. SHG from polycrystalline silver as a function of applied voltage in the folllowing concentrations of KBr: 0.1 hl ---; 1.3 X IO’ M . . . . 5 X IO+ hl -_ Tiie O.Ol~!+l-.-.-; solutions were adjusted to 2 constant ionic strength of 0.1 M with i(zSO+.
19 October
1964
Fig. 4. (I - 10)~” as a function of applied potential for 0.1 M KC1 l ; 0.1 hf KBr 4; and 0.1 hi KI =_ lo refers to the value of the SII intensity at the minimum (PZC) of the intensity profile. (Data from fig. 1.)
the charge density on the electrode surface is an important factor in the voltage-dependent SH response. Assuming that the potential dependence of the optical constants in the second-order polar&ability is small, a plot of the square root of the intensity versus applied voltage should correspond to charge density curves measured by other methods. Fig. 4 contains the data for the halide ions on polycrystalline silver where Io refers to the SH background signal measured at the apparent PZC in the intensity profiles. With (I,, - 1;))lj2 corresponding to qrn at monolayer coverage, the magnitude and growth in the charge density measured by SHG is similar to identical studies using e&psometry [23], differential capacitance [16,20] and thin layer voltammetry [24] _Furthermore, with a Frumkin isotherm analysis [6] of the concentration dependence data of fig. 3, a free energy of adsorption can be determined. By approximating the fractional coverage 0 as I(1 - lo)/(lsat - I,)] If3 a free energy of adsorption for KBr at -0.8 V is estimated as --I 13 kJ/m. The reported value for KBr at slightly lower charge densities is -114 kJ/m [6] _ When the electrode is polarized to voltages more negative than the PZC, SH intensity is observed which is small relative to the region of anion adsorption. Extensive studies of this region have been performed with respect to cationic adsorption,pH, surface preparation and electrochemical roughening and will be described in a later publication [25] _Surface roughness was found to have the most dramatic effect on the intensity levels at these negative potentials. This was first
Volume 110. number 6
evident when silver samples were prepared with a rougher surface polish, but was subsequently confirmed by electrochemical oxidation and reduction of a few monolayers of the silver surface. This surface roughening results in a large SH intensity at the initiation of the scan which decreased to a minimum near -0.8 V for polycrystalline silver. Consequently the potential for the initiation for the anion adsorption was obscured. Corn et al. [13] have concurrently used SH generated surface polaritons to study the adsorption of perchlorate ions on evaporated silver thin films with a dominant Ag(ll1) crystal structure. Their results, and the earlier studies by Lee [lo] show that this strong signal at negative potentials reaches a minimum near -0.7 V. Similar “parabolic’* intensity curves were obtained for Ag(ll1) in this work when the crystal surface was altered by either mechanical or slight electrochemical roughening [25] _ Since charge density at the metal surface reflects the ionic structure of the double layer, its measurement has itnportance in both fundamental and applied science. Measurements of these values are notoriously difficult for metals. The results of these studies provide encouraging evidence that SHG may be a useful probe of the electrochemical interface. Experiments in this direction are currently in progress in this laboratory.
Acknowiedgement Financial
support
19 October
CHEMICAL PHYSICS LETTERS
from Research
Corporation
and
the National Science Foundation (CDP 8018969) is gratefully acknowledged. Discussions with J. Varimbi and R. Corn are also appreciated_
1984
References [ 11 G.L. Richmond, Chem. Phys. Letters 106 (1984) 26. [2] D.V. hiurphy, KU. von Raben.T_T. Chen, J.F. Owen and RR. Chang, Surface Sci. 124 (1983) 529. [ 31 C.K. Chen. T.F. Heinz. D. Ricard and Y.R. Shen. Phys. Rev. B27 (1983)
1965. and references therein.
[4] T-F. Heinz. C.K. Chen, D. Ricard and Y-R. Shen, Chem. Phys. Letters 83 (1981) 180. [S ] C.K. Chen. T.R. Heinz, D. Ricard and Y.R. Shcn, Phys. Rev. Letters 46 (1981) 1010. (61 D. Larkin. K.L. Gyer. J-T. Hupp and hi-J_ \\‘eaver, J. Elcctroanal. Chem. 138 (1982) 401. [7] B. Pettinger. M.R. Philpott and J.C. Gordon II, J. Chem. Phys. 74 (1981) 934; hf. Fleischmann. P-J. Hcndra, I.R. Hill and h1.E. Pemble, J. Electroanal. Chem. 117 (1981) 243. [81 J.A. Arnmstron8. N. Bloembergen, J. Ducuing and P.S. Pershan. Phys. Rev. 127 (1962) 1918. [9] N. Bloembergen, R.K. Chang. S.S. Jha and C.H. Lee, Phys. Rev. 174 (1968) 813; 178 (1969) 1528 E. [lo] C.H. Lee, RX. Chang and N. Bloembegen. Phys. Rev.
Letters 18 (1967) 167. C.C. Wang, Phys. Rev. 178 (1969)
1457. F. Brown and M. hlatsuoka. Phys. Rev. 185 (1969) 985. (131 R.N. Corn, M. Romagnoli, hi.D. Levenson and h1.R. Philpott, Chem. Phys. Letters 106 (1984) 30. (141 I.E. Sipe. V.C.Y. So, hl. Fukui and G-1. Stegeman. Phys.
t:::
Rev. B21 (1980) 4389.
I161 E.S. Sevant’yanov, hLN. Ter-Akopyan and V.K. Chubarova, Soviet Eleetrochem. 14 (1978) 243. (171 D-1. Leikis. DOW. Akad. Nauk. SSSR 135 (1960) 1429. [l81 I.A. Bagotskaya. B.B. Darnaskin and M.D. Levi. J. Electroanal. Chem. 115 (1980) 189, and references therein. 1191 G. Valette, J. Electroanal. Chem. 122 (1981) 285.
WI
G. Valette, A. Hamelin and R. Parsons, 2. Physik. Chem. NF 113 (1978) 71.
l211 A. Hamelm and G. Valettc. Compt. Rend. Acad. Sci.
(Paris) 279 (1974) 295; G. Valette and A. Ham&n. J. Electroanal Chem. 131 (1982) 299. [22] G-L. Richmond, in preparation. [331 W. Paik, M.A. Genshaw and J. O’M. Bock&, J. Phys. Chem. 74 (1970) 4266. r241 V.E. Schmidt and S. Stucki. Ber. Bunsenges. Physik.
PI
Chem. 77 (1973) 913. G.L. Richmond, Chem. Phys. Letters, submitted for publication.
575
110, number
6
CHEMICAL
SECOND HARMONIC GENERATION ON POLYCRYSTALLINE
PHYSICS
19 October
LETTERS
1984
STUDIES OF ANIONIC ADSORPTION
AND SINGLE CRYSTAL
SILVER SURFACES
G .L. RICHMOND Department
of Chemistry.
Bryn hfawr College.
Bryn hfawr, Pennsylvania
19OIO.
USA
Received 14 July 1984; in final form 22 August 1984 Adsorption of anions on single crystal and polycrystalline silver surfaces has been monitored with optical second harmonic generation. Potentials investigated were restricted to the range between solvent reduction and the onset of metal osidation. Excess charge density on the metal surface is concluded to be a major factor in the voltage-dependent production of the harmonic light. The results demonstrate the potential of this method to measure ionic adsorption quantitatively at the interface.
1. Introduction
P”‘S(2w)
Recent use of second harmonic generation (SHG) has demonstrated its sensitivity in detecting low levels of silver complexes [l-3] and organic adsorbates [4,5] on silver surfaces in electrochemical systems. Earlier studies from this laboratory [l] demonstrated the detectability of monolayer amounts of relatively soluble Ag2S04 on silver electrodes but also noted voltage-dependent SHG prior to the chemical formation of Ag$304. Originating at voltages near the potential of zero charge (PZC) of silver [6], this signal grew steadily until the onset of oxidation and was attributed to anionic adsorption. This Letter reports the results of the first detailed studies aimed at using SHG to monitor quantitatively the adsorption of anions on metal surfaces. The interaction of a wide variety of ions with polycrystalline and single crystal silver surfaces has been investigated. In contrast to surfaceenhanced Raman scattering (SERS) studies which have also detected adsorbed anions [7], these measurements were performed on silver surfaces which were free of electrochemical roughening. The attractive feature of SHG as a surface probe is its intrinsic sensitivity to the interfacial region [3]. The intensity of the second harmonic light generated at the junction between two centrosymmetric media is proportional to the square oft& non-linear polarizability [8] Pn1s(20), where
Contributions to the effective non-linear susceptibility tensor Xc21 include the electric-quadrupole and magnetic-dipole terms on the metal side of the interface and the non-linear molecular polarizability term for the noncentrosymmetric adsorbate in contact with the metal on the solution side of the interface. The sensitivity to the interfacial region arises from the symmetry requirement that this second-order process is restricted to media lacking inversion symmetry [9] _ An additional mechanism for optical SHG is possible at an electrochemical interface. Lee et al. [lo] first observed a strong SHG dependence on applied electric field, Edc, for silicon and silver electrodes and attributed it to a third-order process where the general expression for the non-linear polarization of the source is given by
0 009-2614/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
Pnls(20)
= X(‘l(20)
:E(o)
*E(o)
= X(3k??dc-E(W)-,?(0)
-
_
It was proposed that with fields as large as lo8 V/cm present at the electrochemical interface, this effect should dominate over the second-order contributions to the polarizability [11,12] _In fact, simple comparative calculations indicate that the magnitude of the third-order effect is at least as large as the magueticdipole and electric-quadrupole terms in the secondorder polarizabllity. Although third-order processes are ahowed in centrosymmetric media,
571
Volunle 110, number 6
CHEMICAL PHYSICS
lution falls to a very low value beyond the dimensions of the electrical double layer. For a perfectly conducting smooth surface, the dc field is proportional to the excess charge density residing on the metallic side of the interface. Recent experimental studies by Corn et al. [ 131 on silver thin films and previous theoretical studies [ 141 suggest that this metal charge density, gm, is responsible for the observation of SHG at surfaces where 1(2w) a qz, _ The results reported here indicate a close correspondence between the voltage dependence of the SHG response and adsorption of anions on the metal and furthermore support the proposal that the charge density on the metal surface plays an important role in detecting ionic adsorption.
2. Experimental
procedure
The silver electrodes were immersed in a quartz electrochemical cell equipped with a platinum counter electrode and a saturated calomel reference electrode (SCE). All voltages quoted are relative to the SCE. Potentials investigated were restricted to the range betwcen solvent reduction and the onset of metal oxidation. Using a 5 mV/s ramp, the scans were initiated near --I .3 V and terminated at -0.4 to -0.2 V depending upon the electrolyte being studied. The solutions wcrc prepared with doubly distilled, deionized water and were purged with nitrogen prior to study. The K?SO, and KC104 solutions were adjusted to a pH of 5 with H2S04 and HC104 respectively. The optical instrumentation used was similar to that described previously [1,15]. The 1 .OG pm laser pulses of cross-sectional area 0.5 cm2 were incident on the silver electrode surfaces at an angle of 45”. To avoid laser damage [2,3] tlic power was maintained at 5-6 mJ/pulse. The reproducibility of the signal over 20-30 rcpctitive scans was within an experimental crfor of approximately 10%. The Ag(ll1) and Ag(ll0) single crystals were oricntcd and cut within 1/2O of the bulk axis. Both these surfaces and the polycrystalline silver were mechanically polished to a level of 0.05 pm alumina grit. Sligltt chemical etching with a dilute NH,01_1:lI~O~ mixture followed the polishing. After
LEi-IXRS
19
October
1984
thorough rinsing, the metals were immersed in the electrochemical cell and potentiostated at -1.3 V prior to the scans.
3. Results and discussion Fig. 1 shows the results obtained from a polycrystalline silver electrode polarized in a variety of aqueous electrolytes. For aX solutions, the dominant feature in the voltage-dependent signal is a sharp rise in harmonic intensity near -1 .O V. This signal approaches a constant value as the electrode potential is scanned to more positive values. Prior to this rise, a small background signal is observed which shows a slight decrease with increased positive bias. Quadratic power dependence and 532 nm spectral purity has been verified for the detected light over the voltage range. The relative adsorptivlty of anions to silver is well documented [6] and follows the trend of I- > Br> Cl- > SO:- a ClQz. This sequence is reflected in the SH voltage profiles where the more strongly adsorbing solutions result in a greater response at more negative potentials. More importantly, the potential at which this SH rise occurs correlates closely with the reported TZC values for silver in these electrolytes [6,16,17]. As the positive charge density on the metal increases from zero at the PZC, anion adsorption should show a corresponding increase. This behavior is clearly reflected in the SH profiles and supports the
I)
KCIOd
J -I
,
b)
KCI
.
.
.
,
.
.
.
,
.
*
.
1
-1.2 ’
-0!8
-0!4 V&.,,,(volts)
Pig. 1. SHG front polycrystnllinc silver OSa function of applied potential. The concentration of KC10 was 0.05 hi. The hlidc clcctrolytcs
WUICat a concentration
of 0.1 M.
Volume 110, number 6
a)
CHEMCAL
SHG method to monitor adsorption at potentials where the charge density at the interface is only slightly increased from its zero value. Further details of these studies wilI appear in a later publication [22] _ With increased positive bias on the electrode, the SH intensity reaches a constant level indicating saturation. This saturation is attributed to the formation of a monolayer of coverage_ For the more strongly adsorbing halide ions the saturation intensities are relatively similar yet follow the trend I” < Br- < Cl- _ The relative magnitudes of the SH response from the various anions offers insight into the contributions to the non-linear polarizability at the interface. If tlte harmonic generation were dominated by the third-order effect, the SH signal at maximum adsorption should reflect the relative magnitudes of the charge density of these ions at monolayer coverages. For the halide ions this charge density varies from 96 PC/cm:! for I- to 144 &/cm2 for CI- in agreement with the trend measured here. An opposite and stronger ordering would be expected if the intrinsic non-linear polarizability of the halide ion was dominant. For sulfate and perchlorate ions, the constancy in intensity is not as evident yet the signal is comparable in magnitude to +Jle halide ions. Intensity comparisons of these ions with the specifically adsorbed halide ions is complicated. Perchlorate ions, which do not specifically adsorb 116,191, are separated from tfle metal surface by both a hydration sheath and the inner plane of dipole oriented water molecules adjacent of the surface. Sulfate ions show slight evidence of specific adsorption on polycrystalline silver [21]. The ability to detect these ions which are not in direct contact with
Ad1 10)
l-2 -X 5 d 2
:T,d,,
,
‘TO IT
b) Ag(111) ; ‘!z cl z d*,, OJ, ,
-1.5
- 1.0
-0.5
0
V,qW,c,(volts) Pig. 2. SHG from silver single crystals biased in 0.1 N KBr.
ear&x conclusion [l ,I 51 that the SIG in this preoxidative region is reiated to anion adsorption. Identical studies were performed with Ag(l11) and Ag(l10) crystal faces which have significantly different adsorptive properties. The PZC values for the Ag(ll0) face are known to occur at potentials 250-300 mV more negative than the A&l 1) face f 18-21 f . This dlfference in adsorptivity appears in the SH profdes for all of the eIectrolytes studied. A representative spectrum for KBr is shown in fig. 2. The rise in harmonic intensity for Ag(ll0) occurs at potentials 290 mV more negative than that of Ag(l11). Table 1 compares reported PZC values for these surfaces with the potential at whicfl tfle SH intensity indicates a change in the electrode interface. The remarkable agreement between these values demonstrates the sensitivity of the Table 1 Con~p~rison of silver PZC wlucs with the onset of anion adsorption .-_
SO;CIOF aIkI-
as measured by SHG a) A&Ill)
A&l 10)
Ag(poly) SHC(V) b)
p=xV
-0.95 -0.90 -0.98 -1.05 -1.25
-0.94 -0.95 -0.93 -1.15 -1.3
c) e) g) 6) 6)
19 October 1984
PlfYSICS LETTERS
WC
PZC
SHG
PZC
-1.05 -1.05 -1.15 -1.24 -1.33
-1 .o d) -0.99 fJ -1.05 I’)
-0.75 -0.74 -0.8i -0.95 -1.18
-0.72 d, -0.735 d) -0.83 ‘0
a) All solutions studies wcrc 0.1 M csccpt for KC104 which \V;IS 0.05 M. b) Experimental uncertainty = *30 mV_ f) Ref. f19f. g)Ref. 161. d) Ref. /2t]. cl Ref. f16]. c) Ref. [17]. I~) Dan from ref. [ 201 for 0.04 hi.
573
Volume 110, number 6
CHEMICAL PHYSICS LETTERS
the meta strongly suggests that SHG originates from the metallic portion of the interface. Since the charge density on the metal surface is always equal to the charge density on the solution side of the interface, the nleasurement of the former is independent of ion location. A similar conclusion for perchlorate ions on silver thin fiims was recently reported [ 133. Additional experiments were performed to compare the concentration dependence of the SH profiles with the predicted behavior_ For specifically adsorbed anions, the PZC shifts to more negative potentials with added ion concentration 1121. Metal charge density~ profiles for ions*which do not specifically adsorb should show no concentration dependence. For all of the halides, the SH adsorption profiles shift to more negative potentials with increased halide concentration. Fig. 3 depicts the results for KBr over the concentration ranges of 10-1-10-” M. K-$X& was used in these studies to maintain a constant ionic strength although similar observations were made without added K2S01 _ No variation in the profnes was observed for either K$SO, or KCIO, over a similar concentration range. The reported shifts for K,SO, 1211 are less than the experimental uncertainty of these SH measurements. The results described above give strong support to the claim that the SH intensity generated at the silveraqueous interface reflects the adsorption of anions to the metal surface. Furthermore there is evidence that
-1.2
-0.8
-0.4
I:& 3. SHG from polycrystalline silver as a function of applied voltage in the folllowing concentrations of KBr: 0.1 hl ---; 1.3 X IO’ M . . . . 5 X IO+ hl -_ Tiie O.Ol~!+l-.-.-; solutions were adjusted to 2 constant ionic strength of 0.1 M with i(zSO+.
19 October
1964
Fig. 4. (I - 10)~” as a function of applied potential for 0.1 M KC1 l ; 0.1 hf KBr 4; and 0.1 hi KI =_ lo refers to the value of the SII intensity at the minimum (PZC) of the intensity profile. (Data from fig. 1.)
the charge density on the electrode surface is an important factor in the voltage-dependent SH response. Assuming that the potential dependence of the optical constants in the second-order polar&ability is small, a plot of the square root of the intensity versus applied voltage should correspond to charge density curves measured by other methods. Fig. 4 contains the data for the halide ions on polycrystalline silver where Io refers to the SH background signal measured at the apparent PZC in the intensity profiles. With (I,, - 1;))lj2 corresponding to qrn at monolayer coverage, the magnitude and growth in the charge density measured by SHG is similar to identical studies using e&psometry [23], differential capacitance [16,20] and thin layer voltammetry [24] _Furthermore, with a Frumkin isotherm analysis [6] of the concentration dependence data of fig. 3, a free energy of adsorption can be determined. By approximating the fractional coverage 0 as I(1 - lo)/(lsat - I,)] If3 a free energy of adsorption for KBr at -0.8 V is estimated as --I 13 kJ/m. The reported value for KBr at slightly lower charge densities is -114 kJ/m [6] _ When the electrode is polarized to voltages more negative than the PZC, SH intensity is observed which is small relative to the region of anion adsorption. Extensive studies of this region have been performed with respect to cationic adsorption,pH, surface preparation and electrochemical roughening and will be described in a later publication [25] _Surface roughness was found to have the most dramatic effect on the intensity levels at these negative potentials. This was first
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evident when silver samples were prepared with a rougher surface polish, but was subsequently confirmed by electrochemical oxidation and reduction of a few monolayers of the silver surface. This surface roughening results in a large SH intensity at the initiation of the scan which decreased to a minimum near -0.8 V for polycrystalline silver. Consequently the potential for the initiation for the anion adsorption was obscured. Corn et al. [13] have concurrently used SH generated surface polaritons to study the adsorption of perchlorate ions on evaporated silver thin films with a dominant Ag(ll1) crystal structure. Their results, and the earlier studies by Lee [lo] show that this strong signal at negative potentials reaches a minimum near -0.7 V. Similar “parabolic’* intensity curves were obtained for Ag(ll1) in this work when the crystal surface was altered by either mechanical or slight electrochemical roughening [25] _ Since charge density at the metal surface reflects the ionic structure of the double layer, its measurement has itnportance in both fundamental and applied science. Measurements of these values are notoriously difficult for metals. The results of these studies provide encouraging evidence that SHG may be a useful probe of the electrochemical interface. Experiments in this direction are currently in progress in this laboratory.
Acknowiedgement Financial
support
19 October
CHEMICAL PHYSICS LETTERS
from Research
Corporation
and
the National Science Foundation (CDP 8018969) is gratefully acknowledged. Discussions with J. Varimbi and R. Corn are also appreciated_
1984
References [ 11 G.L. Richmond, Chem. Phys. Letters 106 (1984) 26. [2] D.V. hiurphy, KU. von Raben.T_T. Chen, J.F. Owen and RR. Chang, Surface Sci. 124 (1983) 529. [ 31 C.K. Chen. T.F. Heinz. D. Ricard and Y.R. Shen. Phys. Rev. B27 (1983)
1965. and references therein.
[4] T-F. Heinz. C.K. Chen, D. Ricard and Y-R. Shen, Chem. Phys. Letters 83 (1981) 180. [S ] C.K. Chen. T.R. Heinz, D. Ricard and Y.R. Shcn, Phys. Rev. Letters 46 (1981) 1010. (61 D. Larkin. K.L. Gyer. J-T. Hupp and hi-J_ \\‘eaver, J. Elcctroanal. Chem. 138 (1982) 401. [7] B. Pettinger. M.R. Philpott and J.C. Gordon II, J. Chem. Phys. 74 (1981) 934; hf. Fleischmann. P-J. Hcndra, I.R. Hill and h1.E. Pemble, J. Electroanal. Chem. 117 (1981) 243. [81 J.A. Arnmstron8. N. Bloembergen, J. Ducuing and P.S. Pershan. Phys. Rev. 127 (1962) 1918. [9] N. Bloembergen, R.K. Chang. S.S. Jha and C.H. Lee, Phys. Rev. 174 (1968) 813; 178 (1969) 1528 E. [lo] C.H. Lee, RX. Chang and N. Bloembegen. Phys. Rev.
Letters 18 (1967) 167. C.C. Wang, Phys. Rev. 178 (1969)
1457. F. Brown and M. hlatsuoka. Phys. Rev. 185 (1969) 985. (131 R.N. Corn, M. Romagnoli, hi.D. Levenson and h1.R. Philpott, Chem. Phys. Letters 106 (1984) 30. (141 I.E. Sipe. V.C.Y. So, hl. Fukui and G-1. Stegeman. Phys.
t:::
Rev. B21 (1980) 4389.
I161 E.S. Sevant’yanov, hLN. Ter-Akopyan and V.K. Chubarova, Soviet Eleetrochem. 14 (1978) 243. (171 D-1. Leikis. DOW. Akad. Nauk. SSSR 135 (1960) 1429. [l81 I.A. Bagotskaya. B.B. Darnaskin and M.D. Levi. J. Electroanal. Chem. 115 (1980) 189, and references therein. 1191 G. Valette, J. Electroanal. Chem. 122 (1981) 285.
WI
G. Valette, A. Hamelin and R. Parsons, 2. Physik. Chem. NF 113 (1978) 71.
l211 A. Hamelm and G. Valettc. Compt. Rend. Acad. Sci.
(Paris) 279 (1974) 295; G. Valette and A. Ham&n. J. Electroanal Chem. 131 (1982) 299. [22] G-L. Richmond, in preparation. [331 W. Paik, M.A. Genshaw and J. O’M. Bock&, J. Phys. Chem. 74 (1970) 4266. r241 V.E. Schmidt and S. Stucki. Ber. Bunsenges. Physik.
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Chem. 77 (1973) 913. G.L. Richmond, Chem. Phys. Letters, submitted for publication.
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