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Structural investigations of metal adsorbates on single crystal surfaces using reflectance spectroscopy
Surface Science @ North-Holland
101 (1980) 490-49X Publishing Company
STRUCTURAL INVESTIGATIONS OF METAL A~SORBA~S ON SINGLE CRYSTAL SURFACES USING REFLECTANCE SPECTROSCOPY D.M.
KOLB,
Fritz-Huber-lnstitut West Germany Received
R. KoTZ*
29 September
We determined crystal surfaces of linearly polarized adsorbate covered with respect to the such as H, Cu and with svstems where
and
D.L.
RATH
der Max-Planck-Gesellschaft.
Faradayweg
4-6, D-l(KXI Berlin 32,
1979
the relative reflectance change, AR/R. for various adsorbate< on single Pt and Ag as a function of coverage. The measurements were done with light at normal-incidence where any anisotropy in the optical response of surfaces is easily detected by rotating the electric vector of the probing light surface crystallographic orientation. The anisotropy in AR/R for adsorhatcs Pb on Pt(l10) and Pb on Ag(l10) is shown and discussed as well ah compared such effects arc not observed.
1. Introduction In situ structural investigations of adsorbates on electrode surfaces arc sparse. The highly structure sensitive electron diffraction techniques such as LEED or RHEED cannot be employed for in situ studies of the electrodeelectrolyte interface. Infrared spectroscopy, another powerful tool for structural investigations, also cannot be used in the presence of the bulk electrolyte because of the poor transmission properties of the commonly used electrolytes in the IR. Besides Raman spectroscopy which requires spectroscopy in the elaborate equipment for surface studies, reflectance visible and near UV range remains as one of the few techniques available for obtaining information on the structural properties of adsorbates in monolayer and submonolayer amounts on the electrode surface. In a predemonstrated that normal-incidence vious publication (11 we have reflectance spectroscopy with linearly polarized light is well suited to detect adsorbate induced anisotropy in the optical response of the electrode surface when the electric field vector of the light is rotated with respect to the crystallographic orientation of the surface. This method is straightforward
’ Present address: 44106. USA.
Chemistry
Department,
Case Western
490
Reserve
University,
Cleveland.
Ohio
D.M. Kolb et al. / Structural investigations
of metal adsorbates
491
[2] and does not demand further data reduction by the use of model calculations. The normal incidence condition guarantees that no complications will arise from a field component perpendicular to the surface due to non-local effects as may be found in non-normal incidence studies with p-polarized light [3-51. In the present communication we use this technique to study the optical response from various adsorbates on Pt and Ag single crystal surfaces, mainly (110) surfaces, as a function of coverage. This is done by measuring the relative reflectance change, AR/R, of the bare substrate surface upon adsorption or deposition of adsorbates such as H, Cu and Pb, the amount of which is controlled and governed by the electrode potential.
2. Experimental The optical arrangement for measuring relative reflectance changes has been described in a previous publication [6]. The optical measurements were made with linearly polarized light at near-normal incidence (cpl G 15”). The single crystal surfaces of Pt(llO), Ag(llO) and Ag(100) were prepared from cylindrical discs by standard metallurgic procedures with the surface quality checked by RHEED. The massive, cylindrical single crystal electrodes were glued onto microscope slides with Apiezon W [l], with the cylindrical surfaces masked which did not have the desired crystallographic orientation. Ag(ll1) was prepared by thin film evaporation onto mica [7]. Standard electrochemical equipment was used to control the potential of the working electrode. All potentials are measured and quoted with respect to the saturated calomel electrode (SCE). The electrolyte, usually 1N H2S04, 1M HClO, or 1M NaC104 with 1 mM CuS04 or Pb(ClO&, was made from p.a. grade chemicals and triply distilled water. The solutions were deaerated with purified nitrogen.
3. Results We will first consider the results obtained for Pt(llO), where we studied adsorption of hydrogen and oxygen (or oxide formation) and the deposition of Cu and Pb. From electrochemical measurements we know that hydrogen adsorbs on Pt(ll0) in acid solution up to one monolayer (approximately 150 PC cm-‘) within a narrow potential range from about -100 to -240 mV, where HZ evolution commences [8]. The spectral variation of AR/R for the complete monolayer of H on Pt(ll0) is shown in fig. la for the two main crystallographic directions. A strong anisotropy in AR/R is seen for A < 600 nm with the reflectance change for e ]I[liO] being roughly twice as large
492
D.M. Kolh et ul. 1 Structural
z
rj “0 -
200
inaestigations
of metal udsorhates
300
&OO
500
600
700
30@
200
500
600
700
Om-FFFr
200
Fig. I. Relative reflectance change. AR/R. as a function of wave length for Pt(ll0) due to formation of (a) a complete monolayer of hydrogen and (b) half a monolayer of lead. Normal incidence. The electric field vector is tither parallel or perpendicular to [I iOl.
as that for e _L[lit)]. This behaviour is also found for metal adsorbates such as Cu and Pb on Pt(ll0). The AR/R spectra for half a monolayer of Pb on Pt(l10) are shown in fig. lb as a typical example where again we find AR/R larger for e I][liO] than for e 1. [IiO]. In addition we find the anisotropy. expressed as the ratio of the AR/R measurements for both directions, to be a function of the wave length. In a number of publications it was shown [l. 9-l I] that the formation of a metallic monolayer very often occurs in several distinct steps. The adsorption isotherms, as derived from potential step measurements. are shown in fig. 2 for Cu and Pb on Pt(l10) together with their cyclic current-potential curves. The latter curves approach the derivative of the adsorption isotherms and hence reveal more clearly the structural steps involved. As reported previously [l], a complete monolayer of Cu on Pt(ll0) is formed in the underpotential region, the adsorption occurring in two well defined steps of approximately half a monolayer each. These two steps - seen as two desorption peaks in the cyclic current-potential curves (see insert in fig. 2a)-clearly indicate that we are deahng with two distinctly different ad-
D.M. Kolb et al. / Structural investigations
of metal adsorbates
493
0 -0.2
Pt(llO1
0
0.2
0.4
0.6
0
0.2
0.4
0.6
-Pb
06
-0 2
“SCE
’ ”
Fig. 2. Adsorption isotherms for (a) Cu on Pt(llO) in 1N HzSOd+ 1mM CuSOq and for (b) Pb on Pt(ll0) in 1M HC104+ 1rnM Pb(ClO&. The respective current-potential curves are shown as inserts.
sorption sites on Pt(ll0) for the Cu adatoms. The corresponding data for Pb on Pt(ll0) in 1M HClO, are reproduced in fig. 2b. Here some difficulties arise from the low hydrogen overvoltage on Pt. Deposition of lead which starts around +OS V in our case, can only be studied up to -0.24 V where hydrogen evolution starts which completely masks the Pb adsorption and desorption current [12]. However, we have determined the Pb adsorption isotherm up to about -0.2 V which is already reasonably close to the reversible Nernst potential of bulk Pb (U,, = -0.45 V). Assuming that no drastic changes in 8 occur in this small potential region we find a maximum underpotential coverage of only 0.6 for Pb on Pt(ll0). This assumption is also supported by measurements made with lower pH values. The coverage dependences of AR/R for Cu and Pb on Pt(ll0) are shown in figs. 3a and 4, respectively. The data for Cu on Pt(ll0) (fig. 3a) confirm earlier results where a marked change in the anisotropy around half
494
D.M. Kolh et al. I Structural investigations of metal adsorbates
08 -
Pt(ll0) -cu h =450 nm
0.6 a a
0
0.2
0.4
0.6
1.0
0 8
12
%” 5 X=300nm
Pt(llO)-0 4
: % “0 -7
-
3
**
al~liol
l
l
~l[liOl
0
0
’
’
-
.
’
‘7
-
2
1
0
A
0
04
0.8 8
1.2
1.6
Fig. 3. Relative reflectance change, AR/R, as a function of coverage for two different adsorbates on Pt(ll0): (a) Cu on Pt(ll0). 1N HzSOJ + 1mM CuS04; (b) oxygen on Pt(llO), in 1N HrS04. Normal incidence. Direction of electric field vector as indicated.
monolayer coverage was observed [l]. It is an interesting observation that a clear change in the optical response with coverage around 0 = 0.4 is observed only for e I [liO], but not for e I][liO]. For Pb on Pt(ll0) the anisotropy in AR/R is also quite strong, but .the AR/R values show the same 6dependence for both polarization directions (fig. 4).
D.M. Kolb et al. / Structural investigations of metal adsorbates
495
rY O8 2 a
06
“0 ’
OG
0
0.2
04 0Pb
0.6
0.8
0
0.2
0.4
0.6
0.8
ePb
Fig. 4. Relative reflectance change, AR/R, as a function of coverage for Pb on Pt(llO) for two different wave lengths, in 1M HC104 + 1mM Pb(ClO&. Normal incidence.
Oxygen adsorbed on Pt(ll0) is one of the few systems which show nearly no anisotropy [l]. The results for two different wave lengths are given in fig. 3b. At higher photon energies (h s 400 nm) absolutely no anisotropy in AR/R has been detected for the studied potential range from 0.6 to 1.2V, while for A > 400 nm a small but distinct anisotropy is discernable (see data for A = 450 nm in fig. 3b) at least at higher coverages (e.g. at potentials more positive than 0.9 V). It is interesting to note that AR/R is larger for e I [liO] than for e I][liO] in contrast to the findings for the metal deposits. It is also worth noting that the absence of an anisotropy for oxygen on Pt(ll0) is not due to a loss of single crystallinity as would occur with excessive potential cycling into the oxygen adsorption region. To prove this, the measurements with Cu were repeated after the oxygen adsorption experiments and the anisotropy was again observed. To contrast the results for Pt(hkl) where the adsorbates are believed to form epitaxial layers, we also investigated Pb on Ag(hkl) since for this system various experiments indicated the formation of a hexagonal close packed (hcp) structure for the monolayer, independent of the substrate surface symmetry [9,11,13]. The AR/R versus 19curves are shown in fig. 5 for Pb on Ag(100) and on Ag(llO). The wave lengths for observation of the coverage dependence were chosen such that electroreflectance effects from the substrate should only be minor [14]. Measurements were performed at 450 and 550nm both showing identical trends so that only the data for 450nm are displayed in the figures. For Pb on Ag(100) we find a linear correlation between AR/R and 8 over a wide range (0.2 Q 8 =S1.0). This indicates coverage independent optical constants for the adsorbate in this
49fJ
D.M.
Kolh et al. 1 Structural
inoestigations
of tnetal crd.wrhtrte.t
0 .
Puciioi .
l
-/ .
0
0.2
OL
0.6 @Pb
08
10
12
0
02
01,
06
08
10
12
1.L
8 Pb
Fig. 5. Relative reflectance change. AR/R, as a function of coverage for Pb (a) on Ag(ltHl) and (b) on Ag(l IO): A = 450 nm. Normal incidence. For Ag(l IO) the direction of the electric field vector is indicated in the figure. IM NaCIOd + ImM Pb(Cl0~)~.
region. Only for 0 < 0.2 a clear deviation from the straight line through the origin is observed. We assume that the sign change in AR/R at the lowest coverages is caused by the substrate electroreflectance effect. A more complex situation is found for Pb on Ag(llO) (fig. 5b). At low coverages (0 s 0.1) with e I [ IiO] the electroreflectance effect of Ag(l10) with a possible adsorbate induced enhancement [lS] is the main cause for the sign change in AR/R. Correction for this effect reveals that for 0 s 0. I there is hardly any response in AR/R with Pb deposition for both polarization directions. Further increase of 8. however, causes a clear anisotropy in AR/R to appear, the value for e 11 [lit)] being about twice as large as for e I [liO]. At half monolayer coverage the anisotropy starts to decrease until at 0 = 1 it has nearly completely disappeared. From electrochemical measurements we deduce that for Pb on Ag(l10) about 20% of the second layer is already deposited at underpotentials before bulk deposition commences [13]. In this range (0 3 1) the anisotropy remains relatively small (on the order of 10% or less).
4. Discussion As we have pointed out previously [l] the coverage dependence of AR/R can give some insight into the changes in the lateral interaction of the adatoms and hence into the structural changes of the adlayer. For constant
D.M. Kolb et al. I Structural investigations of metal adsorbates
wave length
in the linear
approximation
497
it is [l]:
AR/R - Of(&(e)) , where g*(e) is the coverage dependent dielectric function of the adsorbate. A few simple cases of this relation can be discussed in regard to our results, such as, for a &independent E1zwe find a straight line through the origin for AR/R(e). Bends and steps in the AR/R versus 8 curves at certain 8 values signal changes with coverage in the average optical properties of the adlayer. One of the most interesting examples is the formation of a Cu monolayer on Pt(ll0). The adsorption isotherm reveals that two energetically very distinct adsorption sites exist, the first half of the monolayer being more strongly bound to the substrate than the second half which completes the monolayer formation. This difference is clearly detected by optical measurements for e I [liO], but not seen at all with e I([liO]. In the latter case the optical properties seem coverage independent for the whole range of 0 s 8 s 1. We interpreted this finding by assuming a preferential deposition of Cu in the submonolayer range along [ITO] in the substrate grooves, with only every second row being occupied for /3 < 0.5 [l]. It has been reasoned from electrochemical measurements that metal adatoms with a diameter smaller than that of the substrate atoms form epitaxial layers at full coverage while adatoms with a larger diameter form hexagonal close packed (hcp) layers regardless of the substrate symmetry [9]. We therefore included Pb on Pt(ll0) and on Ag(ll0) in our investigation. In the first case the excess binding energy of Pb is much higher than in the second case. For Ag(hkl) it has been found that the maximum amount of Pb deposited at underpotentials is the same for all three low index faces and corresponds to a hcp structure [ll, 131. This finding is supported by our AR/R measurements. While a clear anisotropy for Pb on Ag(l10) is observed at medium coverages, this anisotropy disappears nearly completely as 8 approaches 1 as expected for a uniform hcp adlayer. This result is contrasted by that for Cu on Pt(ll0) where the anisotropy is still very pronounced at 8 = 1 as expected for an epitaxial adlayer on a surface of 2-fold symmetry. An even closer comparison of adatoms of various diameters on the same substrate is not yet possible, since neither Cu on Ag(l10) nor Pb on Pt(ll0) are suitable systems: in the first case no under-potential deposition is observed, in the second case full coverage cannot be reached. For Pb on Pt(ll0) we find in the accessible coverage range (0 s 8 s 0.6) a clear anisotropy in AR/R which, however, is wave length dependent. While no change in E2 with 13 is observed at 300 nm, it is at 450 nm indicating that different adsorption sites are occupied with 8 2 0.5. The data indicate that Pb shows no tendency to form a complete monolayer in the underpotential
498
D.M. Kolh et al. 1 Sfructural inoestigations
of metal
adsorhat~v
range, similar to Pb on Cu(hkl) [ln]. An epitaxial monolayer cannot be completed for steric reasons and obviously the adsorption energy for the adatoms at high symmetry adsorption sites which they occupy at low coverages, e.g. in a c(2 x 2) structure up to 0 = 0.5, is too high to favor a rearrangement at 0 > 0.5 to form a hcp layer. Another point worth noting is seen in fig. 5. At very low coverages, Pb deposition on Ag(hk1) has hardly any effect on AR/R. We attribute this to the presence of randomly adsorbed single Pb atoms (no lateral interaction) which in the case of Ag(100) is only on the order of a few per cent of a monolayer while in the case of Ag(ll0) it amounts to approximately 10%. Finally the very interesting fact that in many systems the anisotropy in AR/R is wave length dependent deserves mentioning. Hydrogen adsorption on Pt(ll0) demonstrates this wave length dependence quite clearly. A more detailed investigation of this effect should yield additional information on the electronic transition involved in a particular energy region.
Acknowledgement The financial support gratefully acknowledged.
by the Deutsche
Forschungsgemeinschaft
(DFG)
is
References [I] [2] [3] [4] [5] [6] [7]
[8] [9] [lo] [ll] [12] [13] [14] [15] [16]
D.M. Kolb. R. Katz and K. Yamamoto. Surface Sci. X7 (lY7Y) 20. T.E. Furtak and D.W. Lynch, Phys. Rev. Letters 35 (lY75) 960. P.J. Feibelman. Phys. Rev. B14 (1976) 762. T. Lopez-Rios. M. Decrescenzi and Y. Borensztein, Solid State Commun. 30 (lY7Y) 75.5. R. KGtz, D.M. Kolb and F. Forstmann, Surface Sci. 91 (1980) 4X9. D.M. Kolb and R. Kiitz. Surface Sci. 64 (lY77) 698. P.O. Nilsson and D.E. Eastman, Phys. Scripta 8 (1973) 11.3. K. Yamamoto, D.M. Kolb. R. Katz and G. Lehmpfuhl. J. Electroanal. Chem. Yh (lY7Y) 233. J.W. Schultze and D. Dickertmann, Surface Sci. 54 (1976) 4XY. H.O. Beckmann, H. Gerischer. D.M. Kolb and G. Lehmpfuhl, Symp. Faraday Sot. II (1977) 51. W.J. Lorenz, E. Schmidt, G. Staikov and H. Bort, Symp. Faraday Sot. 12 (1977) 14. R.R. Ad% M.D. Spasojevic and A.R. DespiC, Electrochim. Acta 24 (1979) 569. D. Dickertmann, F.D. Koppitz and J.W. Schultze, Electrochim. Acta 21 (1976) Yh7. D.M. Kolb and R. Kiitz, Surface Sci. 64 (1977) 96. D.M. Kolb, D. Leutloff and M. Przasnyski, Surface Sci. 47 (1975) 622. A. Bewick. J. JoviCeviE and B. Thomas, Symp. Faraday Sot. 12 (1077) 24.
101 (1980) 490-49X Publishing Company
STRUCTURAL INVESTIGATIONS OF METAL A~SORBA~S ON SINGLE CRYSTAL SURFACES USING REFLECTANCE SPECTROSCOPY D.M.
KOLB,
Fritz-Huber-lnstitut West Germany Received
R. KoTZ*
29 September
We determined crystal surfaces of linearly polarized adsorbate covered with respect to the such as H, Cu and with svstems where
and
D.L.
RATH
der Max-Planck-Gesellschaft.
Faradayweg
4-6, D-l(KXI Berlin 32,
1979
the relative reflectance change, AR/R. for various adsorbate< on single Pt and Ag as a function of coverage. The measurements were done with light at normal-incidence where any anisotropy in the optical response of surfaces is easily detected by rotating the electric vector of the probing light surface crystallographic orientation. The anisotropy in AR/R for adsorhatcs Pb on Pt(l10) and Pb on Ag(l10) is shown and discussed as well ah compared such effects arc not observed.
1. Introduction In situ structural investigations of adsorbates on electrode surfaces arc sparse. The highly structure sensitive electron diffraction techniques such as LEED or RHEED cannot be employed for in situ studies of the electrodeelectrolyte interface. Infrared spectroscopy, another powerful tool for structural investigations, also cannot be used in the presence of the bulk electrolyte because of the poor transmission properties of the commonly used electrolytes in the IR. Besides Raman spectroscopy which requires spectroscopy in the elaborate equipment for surface studies, reflectance visible and near UV range remains as one of the few techniques available for obtaining information on the structural properties of adsorbates in monolayer and submonolayer amounts on the electrode surface. In a predemonstrated that normal-incidence vious publication (11 we have reflectance spectroscopy with linearly polarized light is well suited to detect adsorbate induced anisotropy in the optical response of the electrode surface when the electric field vector of the light is rotated with respect to the crystallographic orientation of the surface. This method is straightforward
’ Present address: 44106. USA.
Chemistry
Department,
Case Western
490
Reserve
University,
Cleveland.
Ohio
D.M. Kolb et al. / Structural investigations
of metal adsorbates
491
[2] and does not demand further data reduction by the use of model calculations. The normal incidence condition guarantees that no complications will arise from a field component perpendicular to the surface due to non-local effects as may be found in non-normal incidence studies with p-polarized light [3-51. In the present communication we use this technique to study the optical response from various adsorbates on Pt and Ag single crystal surfaces, mainly (110) surfaces, as a function of coverage. This is done by measuring the relative reflectance change, AR/R, of the bare substrate surface upon adsorption or deposition of adsorbates such as H, Cu and Pb, the amount of which is controlled and governed by the electrode potential.
2. Experimental The optical arrangement for measuring relative reflectance changes has been described in a previous publication [6]. The optical measurements were made with linearly polarized light at near-normal incidence (cpl G 15”). The single crystal surfaces of Pt(llO), Ag(llO) and Ag(100) were prepared from cylindrical discs by standard metallurgic procedures with the surface quality checked by RHEED. The massive, cylindrical single crystal electrodes were glued onto microscope slides with Apiezon W [l], with the cylindrical surfaces masked which did not have the desired crystallographic orientation. Ag(ll1) was prepared by thin film evaporation onto mica [7]. Standard electrochemical equipment was used to control the potential of the working electrode. All potentials are measured and quoted with respect to the saturated calomel electrode (SCE). The electrolyte, usually 1N H2S04, 1M HClO, or 1M NaC104 with 1 mM CuS04 or Pb(ClO&, was made from p.a. grade chemicals and triply distilled water. The solutions were deaerated with purified nitrogen.
3. Results We will first consider the results obtained for Pt(llO), where we studied adsorption of hydrogen and oxygen (or oxide formation) and the deposition of Cu and Pb. From electrochemical measurements we know that hydrogen adsorbs on Pt(ll0) in acid solution up to one monolayer (approximately 150 PC cm-‘) within a narrow potential range from about -100 to -240 mV, where HZ evolution commences [8]. The spectral variation of AR/R for the complete monolayer of H on Pt(ll0) is shown in fig. la for the two main crystallographic directions. A strong anisotropy in AR/R is seen for A < 600 nm with the reflectance change for e ]I[liO] being roughly twice as large
492
D.M. Kolh et ul. 1 Structural
z
rj “0 -
200
inaestigations
of metal udsorhates
300
&OO
500
600
700
30@
200
500
600
700
Om-FFFr
200
Fig. I. Relative reflectance change. AR/R. as a function of wave length for Pt(ll0) due to formation of (a) a complete monolayer of hydrogen and (b) half a monolayer of lead. Normal incidence. The electric field vector is tither parallel or perpendicular to [I iOl.
as that for e _L[lit)]. This behaviour is also found for metal adsorbates such as Cu and Pb on Pt(ll0). The AR/R spectra for half a monolayer of Pb on Pt(l10) are shown in fig. lb as a typical example where again we find AR/R larger for e I][liO] than for e 1. [IiO]. In addition we find the anisotropy. expressed as the ratio of the AR/R measurements for both directions, to be a function of the wave length. In a number of publications it was shown [l. 9-l I] that the formation of a metallic monolayer very often occurs in several distinct steps. The adsorption isotherms, as derived from potential step measurements. are shown in fig. 2 for Cu and Pb on Pt(l10) together with their cyclic current-potential curves. The latter curves approach the derivative of the adsorption isotherms and hence reveal more clearly the structural steps involved. As reported previously [l], a complete monolayer of Cu on Pt(ll0) is formed in the underpotential region, the adsorption occurring in two well defined steps of approximately half a monolayer each. These two steps - seen as two desorption peaks in the cyclic current-potential curves (see insert in fig. 2a)-clearly indicate that we are deahng with two distinctly different ad-
D.M. Kolb et al. / Structural investigations
of metal adsorbates
493
0 -0.2
Pt(llO1
0
0.2
0.4
0.6
0
0.2
0.4
0.6
-Pb
06
-0 2
“SCE
’ ”
Fig. 2. Adsorption isotherms for (a) Cu on Pt(llO) in 1N HzSOd+ 1mM CuSOq and for (b) Pb on Pt(ll0) in 1M HC104+ 1rnM Pb(ClO&. The respective current-potential curves are shown as inserts.
sorption sites on Pt(ll0) for the Cu adatoms. The corresponding data for Pb on Pt(ll0) in 1M HClO, are reproduced in fig. 2b. Here some difficulties arise from the low hydrogen overvoltage on Pt. Deposition of lead which starts around +OS V in our case, can only be studied up to -0.24 V where hydrogen evolution starts which completely masks the Pb adsorption and desorption current [12]. However, we have determined the Pb adsorption isotherm up to about -0.2 V which is already reasonably close to the reversible Nernst potential of bulk Pb (U,, = -0.45 V). Assuming that no drastic changes in 8 occur in this small potential region we find a maximum underpotential coverage of only 0.6 for Pb on Pt(ll0). This assumption is also supported by measurements made with lower pH values. The coverage dependences of AR/R for Cu and Pb on Pt(ll0) are shown in figs. 3a and 4, respectively. The data for Cu on Pt(ll0) (fig. 3a) confirm earlier results where a marked change in the anisotropy around half
494
D.M. Kolh et al. I Structural investigations of metal adsorbates
08 -
Pt(ll0) -cu h =450 nm
0.6 a a
0
0.2
0.4
0.6
1.0
0 8
12
%” 5 X=300nm
Pt(llO)-0 4
: % “0 -7
-
3
**
al~liol
l
l
~l[liOl
0
0
’
’
-
.
’
‘7
-
2
1
0
A
0
04
0.8 8
1.2
1.6
Fig. 3. Relative reflectance change, AR/R, as a function of coverage for two different adsorbates on Pt(ll0): (a) Cu on Pt(ll0). 1N HzSOJ + 1mM CuS04; (b) oxygen on Pt(llO), in 1N HrS04. Normal incidence. Direction of electric field vector as indicated.
monolayer coverage was observed [l]. It is an interesting observation that a clear change in the optical response with coverage around 0 = 0.4 is observed only for e I [liO], but not for e I][liO]. For Pb on Pt(ll0) the anisotropy in AR/R is also quite strong, but .the AR/R values show the same 6dependence for both polarization directions (fig. 4).
D.M. Kolb et al. / Structural investigations of metal adsorbates
495
rY O8 2 a
06
“0 ’
OG
0
0.2
04 0Pb
0.6
0.8
0
0.2
0.4
0.6
0.8
ePb
Fig. 4. Relative reflectance change, AR/R, as a function of coverage for Pb on Pt(llO) for two different wave lengths, in 1M HC104 + 1mM Pb(ClO&. Normal incidence.
Oxygen adsorbed on Pt(ll0) is one of the few systems which show nearly no anisotropy [l]. The results for two different wave lengths are given in fig. 3b. At higher photon energies (h s 400 nm) absolutely no anisotropy in AR/R has been detected for the studied potential range from 0.6 to 1.2V, while for A > 400 nm a small but distinct anisotropy is discernable (see data for A = 450 nm in fig. 3b) at least at higher coverages (e.g. at potentials more positive than 0.9 V). It is interesting to note that AR/R is larger for e I [liO] than for e I][liO] in contrast to the findings for the metal deposits. It is also worth noting that the absence of an anisotropy for oxygen on Pt(ll0) is not due to a loss of single crystallinity as would occur with excessive potential cycling into the oxygen adsorption region. To prove this, the measurements with Cu were repeated after the oxygen adsorption experiments and the anisotropy was again observed. To contrast the results for Pt(hkl) where the adsorbates are believed to form epitaxial layers, we also investigated Pb on Ag(hkl) since for this system various experiments indicated the formation of a hexagonal close packed (hcp) structure for the monolayer, independent of the substrate surface symmetry [9,11,13]. The AR/R versus 19curves are shown in fig. 5 for Pb on Ag(100) and on Ag(llO). The wave lengths for observation of the coverage dependence were chosen such that electroreflectance effects from the substrate should only be minor [14]. Measurements were performed at 450 and 550nm both showing identical trends so that only the data for 450nm are displayed in the figures. For Pb on Ag(100) we find a linear correlation between AR/R and 8 over a wide range (0.2 Q 8 =S1.0). This indicates coverage independent optical constants for the adsorbate in this
49fJ
D.M.
Kolh et al. 1 Structural
inoestigations
of tnetal crd.wrhtrte.t
0 .
Puciioi .
l
-/ .
0
0.2
OL
0.6 @Pb
08
10
12
0
02
01,
06
08
10
12
1.L
8 Pb
Fig. 5. Relative reflectance change. AR/R, as a function of coverage for Pb (a) on Ag(ltHl) and (b) on Ag(l IO): A = 450 nm. Normal incidence. For Ag(l IO) the direction of the electric field vector is indicated in the figure. IM NaCIOd + ImM Pb(Cl0~)~.
region. Only for 0 < 0.2 a clear deviation from the straight line through the origin is observed. We assume that the sign change in AR/R at the lowest coverages is caused by the substrate electroreflectance effect. A more complex situation is found for Pb on Ag(llO) (fig. 5b). At low coverages (0 s 0.1) with e I [ IiO] the electroreflectance effect of Ag(l10) with a possible adsorbate induced enhancement [lS] is the main cause for the sign change in AR/R. Correction for this effect reveals that for 0 s 0. I there is hardly any response in AR/R with Pb deposition for both polarization directions. Further increase of 8. however, causes a clear anisotropy in AR/R to appear, the value for e 11 [lit)] being about twice as large as for e I [liO]. At half monolayer coverage the anisotropy starts to decrease until at 0 = 1 it has nearly completely disappeared. From electrochemical measurements we deduce that for Pb on Ag(l10) about 20% of the second layer is already deposited at underpotentials before bulk deposition commences [13]. In this range (0 3 1) the anisotropy remains relatively small (on the order of 10% or less).
4. Discussion As we have pointed out previously [l] the coverage dependence of AR/R can give some insight into the changes in the lateral interaction of the adatoms and hence into the structural changes of the adlayer. For constant
D.M. Kolb et al. I Structural investigations of metal adsorbates
wave length
in the linear
approximation
497
it is [l]:
AR/R - Of(&(e)) , where g*(e) is the coverage dependent dielectric function of the adsorbate. A few simple cases of this relation can be discussed in regard to our results, such as, for a &independent E1zwe find a straight line through the origin for AR/R(e). Bends and steps in the AR/R versus 8 curves at certain 8 values signal changes with coverage in the average optical properties of the adlayer. One of the most interesting examples is the formation of a Cu monolayer on Pt(ll0). The adsorption isotherm reveals that two energetically very distinct adsorption sites exist, the first half of the monolayer being more strongly bound to the substrate than the second half which completes the monolayer formation. This difference is clearly detected by optical measurements for e I [liO], but not seen at all with e I([liO]. In the latter case the optical properties seem coverage independent for the whole range of 0 s 8 s 1. We interpreted this finding by assuming a preferential deposition of Cu in the submonolayer range along [ITO] in the substrate grooves, with only every second row being occupied for /3 < 0.5 [l]. It has been reasoned from electrochemical measurements that metal adatoms with a diameter smaller than that of the substrate atoms form epitaxial layers at full coverage while adatoms with a larger diameter form hexagonal close packed (hcp) layers regardless of the substrate symmetry [9]. We therefore included Pb on Pt(ll0) and on Ag(ll0) in our investigation. In the first case the excess binding energy of Pb is much higher than in the second case. For Ag(hkl) it has been found that the maximum amount of Pb deposited at underpotentials is the same for all three low index faces and corresponds to a hcp structure [ll, 131. This finding is supported by our AR/R measurements. While a clear anisotropy for Pb on Ag(l10) is observed at medium coverages, this anisotropy disappears nearly completely as 8 approaches 1 as expected for a uniform hcp adlayer. This result is contrasted by that for Cu on Pt(ll0) where the anisotropy is still very pronounced at 8 = 1 as expected for an epitaxial adlayer on a surface of 2-fold symmetry. An even closer comparison of adatoms of various diameters on the same substrate is not yet possible, since neither Cu on Ag(l10) nor Pb on Pt(ll0) are suitable systems: in the first case no under-potential deposition is observed, in the second case full coverage cannot be reached. For Pb on Pt(ll0) we find in the accessible coverage range (0 s 8 s 0.6) a clear anisotropy in AR/R which, however, is wave length dependent. While no change in E2 with 13 is observed at 300 nm, it is at 450 nm indicating that different adsorption sites are occupied with 8 2 0.5. The data indicate that Pb shows no tendency to form a complete monolayer in the underpotential
498
D.M. Kolh et al. 1 Sfructural inoestigations
of metal
adsorhat~v
range, similar to Pb on Cu(hkl) [ln]. An epitaxial monolayer cannot be completed for steric reasons and obviously the adsorption energy for the adatoms at high symmetry adsorption sites which they occupy at low coverages, e.g. in a c(2 x 2) structure up to 0 = 0.5, is too high to favor a rearrangement at 0 > 0.5 to form a hcp layer. Another point worth noting is seen in fig. 5. At very low coverages, Pb deposition on Ag(hk1) has hardly any effect on AR/R. We attribute this to the presence of randomly adsorbed single Pb atoms (no lateral interaction) which in the case of Ag(100) is only on the order of a few per cent of a monolayer while in the case of Ag(ll0) it amounts to approximately 10%. Finally the very interesting fact that in many systems the anisotropy in AR/R is wave length dependent deserves mentioning. Hydrogen adsorption on Pt(ll0) demonstrates this wave length dependence quite clearly. A more detailed investigation of this effect should yield additional information on the electronic transition involved in a particular energy region.
Acknowledgement The financial support gratefully acknowledged.
by the Deutsche
Forschungsgemeinschaft
(DFG)
is
References [I] [2] [3] [4] [5] [6] [7]
[8] [9] [lo] [ll] [12] [13] [14] [15] [16]
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