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Adsorption bonds involving interfacial surface states
1. Introduction ¢ measurements of the electric field modulated reflectance (electroreflectance, ER) on single-crystal noble metal surfaces have been interpreted in terms of intrinsic surface states [l-4]. Since the experiments are performed in an electrochemical sample chamber. so as to exploit the large voltage gradient at the interface, this discovery was somewhat surprising. Normally intrinsic surface states are “quenched” under the slightest perturbation. This is frequently used as a test for the existence of surface states in UHV experiments [5]. Features in. for instance, UPS spectra which disappear upon exposure to low doses of an adsorbate are identified as surface states. In an electrochemical environment the surface is exposed to liquid Lvater and to the ions which are dissolved in the water (and kvhich make possible the large field at the interface). One might expect. therefore, that this would provide significant perturbation of the surface thus precluding the existence of intrinsic wrfact: states. In spite of the obvious complexity of the problem Ho. Harmon. and Liu predicted that the Ag( 110) surface states would survive under the field conditions at the interface with an electrolyte [6]. Their self-consistent slab calculation included all the effects of the electrochemical environment through a periodic “applied” potential which was allowed to “relax” under the iterative procedure. The result was a charged metal surface on which the surface state energies had shifted. The relevant state for our interests is the one at % in the surface Brillouin zone. This is an s-like unfilled state which lies 1.4 eV above the Fermi level on an uncharged surface. It shifts to 2.0 eV when the surface acquires a positive charge of 16.7 PC/cm’. This corresponds to an applied 0039-6028/X5/$03.30 ‘c: Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
voltage of about +0.4 V with respect to the zero-charge voltage. For Ag(ll0) -1.0 V versus the saturated calomel the zero-charge voltage is at about electrode (SCE). Although these calculations were initiated to help explain the major anisotropic feature in the electroreflectance of Ag [7] it \vas not until later that the experimental verification was complete [l]. Excellent correlation between subsequent calculations and experiments on Ag(100). Ag(l11) [2], Au(100). Au( 110). and Au(ll1) [4] have now been reported. In each case an applied voltage (or as we will say. bias voltage) dependent feature has been identified from at a photon energy correspondin g to that required to excite transitions the Fermi level to empty surface states. Evidently the interaction between water and the surface is sufficiently weak such that quenching of the intrinsic surface states in these cases is avoided. All of the experiments were performed in moderately concentrated solutions of NaF. This electrolyte, according to prevailing wisdom, does not directly adsorb on the electrode [8]. Recent electrochemical data have indicated. hou,ever. that even this electrolyte might participate to a slight degree in “specific adsorption” [9]. The latter term is used to describe the preferential interaction between an ion and the electrode beyond that expected by electrostatics alone. In contrast to F-. Cl- does adsorb on most metal surfaces by partialI> displacing water [lo]. In the material presented here \ve show direct in situ evidence of the interaction of Cl- with the surface in forming the “specific adsorption” bond. We have done this by monitoring the surface state electroreflectance feature under various conditions which are expected to modify the surface concentration of Cl-. 2. Experimental The experiments were performed in a conventional electroreflectance apparatus which is described in detail elsewhere [ 111. Linearly polarized light in a 1.2 nm bandwidth met the sample at near-normal incidence. The reflected light was focused onto a photomultiplier Lvhose gain was held constant. The sample was held in a “zero” nitrogen purged, three-electrode chamber which contained the solution, mixed from 4N grade chemicals and triply distilled water. The bias voltage as well as the modulating voltage were applied with a high-speed potentiostat. The modulated part of the signal. proportional to the change in reflectance, \vas detected with a lock-in amplifier. and the average value of the signal. proportional to the average reflectance. was detected uith a digital voltmeter. The division to generate JR/R u’s performed in an analog divider circuit. The single-crystal Ag(ll0) was metallographically polished through 0.05 pm abrasive and then chemically polished to remove the surface strain. The high quality of the surfaces produced by this technique has recently been docu-
mented with LEED measurements [12]. Weakly adsorbed atmospheric taminants are expected to be displaced by the water when the sample. treated. is introduced into the electrolyte.
3. Results
conthus
and discussion
Fig. 1 shows the surface state feature as a function of the bias voltage. These results compare favorably with those recorded earlier by the Berlin group [2]. Since the surface state is spatially distinct, existing in a region where the electrostatic potential is different from its value in the bulk metal, transitions between the bulk states and the surface state are sensitive to the electrostatic potential gradient and therefore to the magnitude of the applied voltage. The average value of the surface state energy is set by the average value of the bias voltage. The high-frequency modulation induces a periodic change in the transition energy thus contributing to structure in the spectrum of the reflectance change as the photon energy of the transition is reached. The feature in the electroreflectance shifts with the average value of the bias voltage as previously demonstrated [2] and as predicted by the theory [6]. This feature is observed only ahen the light is polarized along the (001) direction in the crystal surface. This comes from the optical selection rules involving transitions from p-like states to s-like states and is also predicted by the theory [h]. Fig. 2 shows the influence of Cl in the electrolyte at constant ionic strength (0.5 M). The coverage of Cl- is a function of the bulk concentration and the applied voltage. This quantity is difficult to evaluate. however it is likely that
a
-
-0.2 -
I
I 2.3
-0.3
I
PHOTON Fig.
1. Electroreflwtance
modulating shift
voltage
of the energy
wa\
I
I
I
ENERGY(eV)
of Ag( 110)
in 0.5
M NaF
0.1 VP_, at 180 Hz. The
of the unfilled
.
31
2.7
surface
state
at several
shift
with
values
of the major
the bias
voltage.
of the
feature
bias
voltage.
IS associated
with
The the
i? E. Furtak. U. Pahk / Adsorptron bonds
631
full coverage is reached at about - 0.1 V (SCE) and that the coverage decreases as the voltage is made more negative. At a bias voltage of -0.6 V the surface state feature is quenched progressively as the bulk concentration of Clincreases. This is caused by the formation of a chemical bond between the Clp-states and the Ag s-like surface state. It is likely that the bond involves the filling and downward shifting of the Ag surface state, thus making optical transitions between filled metal bulk states and the surface state impossible. We expect that the Cl- p-states are also hybridized with the Ag d-states in the adsorption bond [13]. There are many filled d-like surface states and surface resonances on Ag which would be candidates for this interaction [14]. The calculations have not yet been extended to include d-states in Ag and. at this point, electroreflectance features involving initially filled surface states have not been reported. In contrast to the case for a bias voltage of -0.6 V, increasing the bulk Clconcentration at -0.8 V has relatively little effect on the electroreflectance. Our first impression is that this is a simple matter of electrostatics. At -0.8 V fewer Cl- are able to adsorb by displacing the surface water. Therefore, as the
I
0.1
I
I
I
I
I
I
I
I
1
I
I
-
O-
V= -0.6
2.3
2.7
Volts
v=-0.8
3. I
2.3
Volts
2.7
3.1
PHOTON ENERGY (eV1 Fig. 2. Electroreflectance of Ag(ll0) in (0.5 - x)M voltage. A, x = 0; B, x = 0.001; C, x = 0.005; D, x -0.8 V (versus the SCE) we find little evidence concentration in the bulk, Cl- is adsorbed at -0.6
NaF+
.xM NaCl
for two values of the bias
= 0.01; E, x = 0.025. With the bias voltage at of adsorbed Cl- while, depending on the V.
concentration of Cl- in the bulk increases we realize little influence at the surface. Optical transitions from the Fermi level to the unfilled intrinsic surface state are still possible at -0.8 V with a Cl concentration of 0.025 M. This represents a bulk ratio of one Cl _ ion to 2200 water molecules. a reasonably high ratio. The x region unfilled surface state has recently been identified with inverse photoemission in UHV [15]. It is found on the free Ag(ll0) surface at an energy which corresponds in the calculation with a slightly positively charged surface. The feature associated with this state is very sensitive to adsorption. disappearing with a very small coverage of oxygen. We take this as further evidence that the quality of the surfaces investigated under electrochemical conditions is very high, and that. in the absence of preferential adsorption. these surfaces are very similar to those found under UHV conditions.
Acknowledgement
This work was supported
by the National
Science
Foundation
Note added in proof
In more recent experiments vve have exploited the optical polarization sensitivity of the surface state to emphasize structure in AR/R primarily associated with surface state transitions. These data show that the surface state is preserved, even in the presence of chloride adsorption. Contrary to our statement here the bonding most likely does not involve chlorine-p and Ag-surface state hybridization.
References [l] I1.M. Kolh, W. Bock. K.-M. Ho and S.H. Liu. Phyt. Rev. Letter\ 47 (1981) 1921. 121 W. Boeck and D.M. Kolh. Surface Sci. 118 (lY82) 613. [3] K.-M. Ho. C.-L. Fu. S.H. Liu. D.M. Kolh .md G. Piazra. .I. Elwtroanal. Chcm. 150 (19X3) 235. [4] S.H. LIU. C‘. H~nnen, C.N. Van Huogn, N.R. deTacconl rind K.-M. Ho. to hc pubhshed [S] W. Eherhardt. E.W. Plummcr. K. Horn and J. Er\klne. Phys. Reb. Letter\ 45 (19X0) 273. [6] K.-M. Ho. B.N. Harmon and S.H. LIU, Phy’. Rev. Letters 44 (19X0) 1531 [7] T.E. Furtak and D.W. Lynch. .I. Electroanal. Chem. 35 (1975) 960. [X] J. Bockris and I. Reddq. Modern Electrochemistry. Vol. 2 (Plenum. New York, 1971). [9] G. Valette. J. Electroanal. Chem. 122 (1981) 285; 13X (1982) 37. 101 C;. Valette. A. Ham&n and R. Parsons, Z. Physlk. Chem. (NF) 113 (197X) 71. 111 T.E. Furtak and U. Pahk. Phys. Rev.. to he published. 121 R.R. Adric, M.E. Hanson and E.B. Yeager, .I. Electrochem. Sot. 131 (1984) 1730. 131 E. Bartels and A. Goldmann. Solid State Commun. 44 (1982) 1419. 14) H.S. Greenside and D.R. Hamann, Phy\. Rev. B23 (19X1) 4X79. 151 B. RethI. R.R. Schllttler and H. Neff. Phy\. Re\. Letters 52 (19X4) 1X26.
B.V.
voltage of about +0.4 V with respect to the zero-charge voltage. For Ag(ll0) -1.0 V versus the saturated calomel the zero-charge voltage is at about electrode (SCE). Although these calculations were initiated to help explain the major anisotropic feature in the electroreflectance of Ag [7] it \vas not until later that the experimental verification was complete [l]. Excellent correlation between subsequent calculations and experiments on Ag(100). Ag(l11) [2], Au(100). Au( 110). and Au(ll1) [4] have now been reported. In each case an applied voltage (or as we will say. bias voltage) dependent feature has been identified from at a photon energy correspondin g to that required to excite transitions the Fermi level to empty surface states. Evidently the interaction between water and the surface is sufficiently weak such that quenching of the intrinsic surface states in these cases is avoided. All of the experiments were performed in moderately concentrated solutions of NaF. This electrolyte, according to prevailing wisdom, does not directly adsorb on the electrode [8]. Recent electrochemical data have indicated. hou,ever. that even this electrolyte might participate to a slight degree in “specific adsorption” [9]. The latter term is used to describe the preferential interaction between an ion and the electrode beyond that expected by electrostatics alone. In contrast to F-. Cl- does adsorb on most metal surfaces by partialI> displacing water [lo]. In the material presented here \ve show direct in situ evidence of the interaction of Cl- with the surface in forming the “specific adsorption” bond. We have done this by monitoring the surface state electroreflectance feature under various conditions which are expected to modify the surface concentration of Cl-. 2. Experimental The experiments were performed in a conventional electroreflectance apparatus which is described in detail elsewhere [ 111. Linearly polarized light in a 1.2 nm bandwidth met the sample at near-normal incidence. The reflected light was focused onto a photomultiplier Lvhose gain was held constant. The sample was held in a “zero” nitrogen purged, three-electrode chamber which contained the solution, mixed from 4N grade chemicals and triply distilled water. The bias voltage as well as the modulating voltage were applied with a high-speed potentiostat. The modulated part of the signal. proportional to the change in reflectance, \vas detected with a lock-in amplifier. and the average value of the signal. proportional to the average reflectance. was detected uith a digital voltmeter. The division to generate JR/R u’s performed in an analog divider circuit. The single-crystal Ag(ll0) was metallographically polished through 0.05 pm abrasive and then chemically polished to remove the surface strain. The high quality of the surfaces produced by this technique has recently been docu-
mented with LEED measurements [12]. Weakly adsorbed atmospheric taminants are expected to be displaced by the water when the sample. treated. is introduced into the electrolyte.
3. Results
conthus
and discussion
Fig. 1 shows the surface state feature as a function of the bias voltage. These results compare favorably with those recorded earlier by the Berlin group [2]. Since the surface state is spatially distinct, existing in a region where the electrostatic potential is different from its value in the bulk metal, transitions between the bulk states and the surface state are sensitive to the electrostatic potential gradient and therefore to the magnitude of the applied voltage. The average value of the surface state energy is set by the average value of the bias voltage. The high-frequency modulation induces a periodic change in the transition energy thus contributing to structure in the spectrum of the reflectance change as the photon energy of the transition is reached. The feature in the electroreflectance shifts with the average value of the bias voltage as previously demonstrated [2] and as predicted by the theory [6]. This feature is observed only ahen the light is polarized along the (001) direction in the crystal surface. This comes from the optical selection rules involving transitions from p-like states to s-like states and is also predicted by the theory [h]. Fig. 2 shows the influence of Cl in the electrolyte at constant ionic strength (0.5 M). The coverage of Cl- is a function of the bulk concentration and the applied voltage. This quantity is difficult to evaluate. however it is likely that
a
-
-0.2 -
I
I 2.3
-0.3
I
PHOTON Fig.
1. Electroreflwtance
modulating shift
voltage
of the energy
wa\
I
I
I
ENERGY(eV)
of Ag( 110)
in 0.5
M NaF
0.1 VP_, at 180 Hz. The
of the unfilled
.
31
2.7
surface
state
at several
shift
with
values
of the major
the bias
voltage.
of the
feature
bias
voltage.
IS associated
with
The the
i? E. Furtak. U. Pahk / Adsorptron bonds
631
full coverage is reached at about - 0.1 V (SCE) and that the coverage decreases as the voltage is made more negative. At a bias voltage of -0.6 V the surface state feature is quenched progressively as the bulk concentration of Clincreases. This is caused by the formation of a chemical bond between the Clp-states and the Ag s-like surface state. It is likely that the bond involves the filling and downward shifting of the Ag surface state, thus making optical transitions between filled metal bulk states and the surface state impossible. We expect that the Cl- p-states are also hybridized with the Ag d-states in the adsorption bond [13]. There are many filled d-like surface states and surface resonances on Ag which would be candidates for this interaction [14]. The calculations have not yet been extended to include d-states in Ag and. at this point, electroreflectance features involving initially filled surface states have not been reported. In contrast to the case for a bias voltage of -0.6 V, increasing the bulk Clconcentration at -0.8 V has relatively little effect on the electroreflectance. Our first impression is that this is a simple matter of electrostatics. At -0.8 V fewer Cl- are able to adsorb by displacing the surface water. Therefore, as the
I
0.1
I
I
I
I
I
I
I
I
1
I
I
-
O-
V= -0.6
2.3
2.7
Volts
v=-0.8
3. I
2.3
Volts
2.7
3.1
PHOTON ENERGY (eV1 Fig. 2. Electroreflectance of Ag(ll0) in (0.5 - x)M voltage. A, x = 0; B, x = 0.001; C, x = 0.005; D, x -0.8 V (versus the SCE) we find little evidence concentration in the bulk, Cl- is adsorbed at -0.6
NaF+
.xM NaCl
for two values of the bias
= 0.01; E, x = 0.025. With the bias voltage at of adsorbed Cl- while, depending on the V.
concentration of Cl- in the bulk increases we realize little influence at the surface. Optical transitions from the Fermi level to the unfilled intrinsic surface state are still possible at -0.8 V with a Cl concentration of 0.025 M. This represents a bulk ratio of one Cl _ ion to 2200 water molecules. a reasonably high ratio. The x region unfilled surface state has recently been identified with inverse photoemission in UHV [15]. It is found on the free Ag(ll0) surface at an energy which corresponds in the calculation with a slightly positively charged surface. The feature associated with this state is very sensitive to adsorption. disappearing with a very small coverage of oxygen. We take this as further evidence that the quality of the surfaces investigated under electrochemical conditions is very high, and that. in the absence of preferential adsorption. these surfaces are very similar to those found under UHV conditions.
Acknowledgement
This work was supported
by the National
Science
Foundation
Note added in proof
In more recent experiments vve have exploited the optical polarization sensitivity of the surface state to emphasize structure in AR/R primarily associated with surface state transitions. These data show that the surface state is preserved, even in the presence of chloride adsorption. Contrary to our statement here the bonding most likely does not involve chlorine-p and Ag-surface state hybridization.
References [l] I1.M. Kolh, W. Bock. K.-M. Ho and S.H. Liu. Phyt. Rev. Letter\ 47 (1981) 1921. 121 W. Boeck and D.M. Kolh. Surface Sci. 118 (lY82) 613. [3] K.-M. Ho. C.-L. Fu. S.H. Liu. D.M. Kolh .md G. Piazra. .I. Elwtroanal. Chcm. 150 (19X3) 235. [4] S.H. LIU. C‘. H~nnen, C.N. Van Huogn, N.R. deTacconl rind K.-M. Ho. to hc pubhshed [S] W. Eherhardt. E.W. Plummcr. K. Horn and J. Er\klne. Phys. Reb. Letter\ 45 (19X0) 273. [6] K.-M. Ho. B.N. Harmon and S.H. LIU, Phy’. Rev. Letters 44 (19X0) 1531 [7] T.E. Furtak and D.W. Lynch. .I. Electroanal. Chem. 35 (1975) 960. [X] J. Bockris and I. Reddq. Modern Electrochemistry. Vol. 2 (Plenum. New York, 1971). [9] G. Valette. J. Electroanal. Chem. 122 (1981) 285; 13X (1982) 37. 101 C;. Valette. A. Ham&n and R. Parsons, Z. Physlk. Chem. (NF) 113 (197X) 71. 111 T.E. Furtak and U. Pahk. Phys. Rev.. to he published. 121 R.R. Adric, M.E. Hanson and E.B. Yeager, .I. Electrochem. Sot. 131 (1984) 1730. 131 E. Bartels and A. Goldmann. Solid State Commun. 44 (1982) 1419. 14) H.S. Greenside and D.R. Hamann, Phy\. Rev. B23 (19X1) 4X79. 151 B. RethI. R.R. Schllttler and H. Neff. Phy\. Re\. Letters 52 (19X4) 1X26.