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Water adsorption on bromine-covered copper single crystal surfaces
0042-207X/83$3.00 + .OO Pergamon Press Ltd
Vacuum/volume 33lnumbe.rs 1O-l Z/Pages 757 to 761 I 1983 Printed in Great Britain
Water adsorption on bromine-covered single crystal surfaces K Bange, R Diihl, D E Glider Farada yweg 46,
and J K Sass, Fritz-Haber-lnstitut D- 1000 Berlin 33, West Germany
copper
der Max-Planck-Gesellschaft,
The coadsorption of water with bromine on Cu(ll0) and Cu(l II) has been studied using UPS, LEED. TDS and A@. The results show that water is more strongly bound to the surface in the vicinity of the bromine than on the clean substrate. This behaviour is interpreted as surface hydration of the partially ionic bromine. lnformation about the dipole orientation of the solvation water is derived from the observed A@-values. The LEED studies indicate that island formation with long-range order of the mixed bromine-water layer occurs at fairly low bromine coverages. The experimental results are qualitatively similar on the close-packed (7 7 1) and the more open (110) surface. The relevance of these findings to the electrochemical phenomenon of specific anion adsorption on metal electrodes is briefly discussed.
1. Introduction
This work is par: of an extensive programme to apply surface science methods and concepts to the study of interracial electrochemical processes’. Our general motivation is the lack of microscopic concepts and surface-sensitive experimental techniques in traditional electrochemistry*. It has been suggested3, for example, that by coadsorbing water and ions on metal surfaces in ultra-high vacuum the composition and structure of the electric double layer formed at a metal-electrolyte interface can be simulated. Using this approach, it is possible to obtain detailed information at the molecular level about ion-solvent interactions at a metal electrode. We have chosen bromine as a coadsorbate because of the significance of halide ions in electrochemistry. These ions are known to adsorb directly onto the electrode, shedding part of their solvation shell in the process. In electrochemical terminology, this is called specific adsorption’, in contrast for example to the alkali ions which are believed to retain their full solvation shell in the double layer, just as in the bulk of an electrolyte solution. The ability of specifically adsorbed anions to influence the rate of electrochemical reactions* is certainly an important reason for obtaining a detailed understanding of halogen adsorption in the presence of water. Halogen adsorption, by itself, on single crystal metal surfaces has recently been reviewed5. Invariably, when the exposure is in molecular form, dissociative adsorption will take place, generally with a fairly large initial sticking coefficient. A few aspects of the adsorption behaviour are still somewhat puzzling, for example the high density at saturation coverage and the fairly constant dipole moment with coverage, in most cases’. The interest of surface scientists in the adsorption of water on clean metal surfaces has increased dramatically in recent years’. In general, it has been found that a low temperature (5 150 K)
water adsorbs molecularly with a sticking coefficient near unity, forms hydrogen-bonded networks and interacts only weakly with the substrate. At higher temperatures, the formation of fragmentation products such as hydroxyl groups or oxygen may be observed on some of the more reactive metals’. The influence of ionic coadsorbates on the adsorption behaviour of water has, with the exception of oxygen7’-9, not previously been reported in any detail”. In this communication, the results obtained for Cu( 110) are described fairly extensively; some selected findings for Cu(ll1) are given” to illustrate the qualitatively similar behaviour of both surfaces.
2. Experimentalprocedures The experiments were performed in two separate vacuum chambers: UPS and LEED studies were done in a commercial instrument (VG ADE!&400), TDS and A@measurements in a selfbuilt stainless steel system. Preliminary UPS data were obtained at normal emission using unpolarized He I and He II radiation, incident at 45“. Computercontrolled, resistive heating of the crystals was used for TDS. The Kelvin probe consisted of a stainless steel mesh coated with tin oxide. To minimize damage to the adsorbed H,O, the Varian 3-grid LEED optics were operated at Iow beam currents (-0.142 PA). Water was dosed from a glass microcapillary array. Comparative dosing from the vacuum chamber provided exposures in units of Langmuir (10m6 torr s). Molecular bromine was evolved from a solid AgBr electrochemical ~~11’~. The copper surfaces were prepared by mechanical polishing and electropolishing; they were cleaned in vacuum by argon ion bombardment at 500 K and subsequent annealing to 750 K. Cooling of the sample to _ 110 K was done by liquid nitrogen. Further experimental details will be given elsewhere’ 3. 757
K Bange. R Dtihl, D E Grider end J K Sass: Water adsorption on bromine-covered copper single crystal surfaces 3.
Results and discussion
3.1. Water adsorption. In this subsection, we want to briefly review our results for water adsorption on clean C~(ll0)‘~*‘~ and compare them to those obtained on the (111) surface. On both surfaces, the UPS results indicate that water is adsorbed molecularly from 110 K up to its desorption temperature. The appearance of two peaks in TDS at L 160 K and -175 K, indicating desorption from ice multilayers and from the more strongly bound first layer, is again very similar on the two faces. The hydrogen-bonded hexagon network for the first layer, deduced from the LEED observations on Cu(ll0) and other surfaces6*7~g*14, is also expected to form on Cu(l11). However, we have not yet been able to observe a reproducible LEED pattern on Cu(1ll), consistent with such structure. Unfortunately, because of its easy beam damage, investigations of H,O are considerably more difficult than for other adsorbates. A comparison of the work function decreases, associated with water adsorption on Cu(ll0) and Cu(lll), is shown in Figure 1.
observed on other metal substrate$, for example bromine on CU(NIO)~~. For Cu(l1 l), the adsorption of chlorine has been studied in some detailr6. A maximum work function change of 1200 mV was observed in this instance. The details of the work function increase with bromine exposure are different for both faces (cJFigure 4). Whereas the curve for Cu(1 1l)is essentially linear up to saturation, on Cu(l10) the linearity extends only to A@= 500 mV. Since another, independent means of determining the surface coverage was not available to us, we shall assume that it is proportional to the work function increase. Some justification for this assumption to hold even in the non-linear part of the A@ curve for Cu(ll0) comes from previous halogen adsorption studiess. For the adsorption of bromine on Cu(llO), a well defined sequence of LEED patterns is observed with increasing coverage. A schematic representation of the adsorbate induced spots is shown in Figure 2a. At small coverages, two extra spots along 0
0 xx 0
0
0
0
H,O/Cu T=llOK
x x )-
(xl
(xl
0
((XII x
x
((xl)
(xl
(xl
0
0
0 L
3
Hz0 exposure(L)
Figure 1. Work function decrease due to water adsorption on Cu( 110) and Cu(ll1). The initial dipole moments are 0.85 and 0.5 Debye, respectively.
The largest values of A@ are approximately - 1100 mV and - 1000 mV. The sign of the work function change is consistent with the concept that the majority of water molecules in the first layer arc oriented with the oxygen closer to the surface’, i.e. in the flip-up configuration according to Doering and Madey6. A significantly higher exposure is required on the (111) surface to attain the saturation value. This behaviour is also evident in the initial slopes of the two A@-curves. The apparent initial dipole moments, calculated from the slopes, are 0.85 and 0.5 Debye, respectively. The most likely explanation for these differences is that on the (111) face multilayer formation, which does not contribute to A@, occurs at fairly low coverages on top of first layer two-dimensional islands6*r3. Larger exposures are then required to complete the first layer which, when judged by the similar saturation values, seems to possess a quite comparable nature on both faces. 3.2. Bromine adsorption. Upon exposure of molecular bromine to the Cu(ll0) and Cu(l11) surfaces, and increase of the work function is observed. The saturation values are A@, ,o= 1000 mV and A@rrr= 800 mV. They are of a similar magnitude as those 758
ox
x0
xx
xx
ox
x0
(3x2) (a)
(b)
Figure 2. (a) Evolution of LEED patterns with increasing bromine coverage on Cu(ll0). The adsorbate-induced spots are indicated by crosses [(x) weak: ((x )) very weak]. (b) Adsorption model for Br/Cu( 110); with increasing bromine coverage progressive compression in the adlayer causes transition to a (3 x 2) configuration, with a coverage of 2/3 (see text).
(110) appear close to the centre of the substrate surface Brillouin zone. With increasing coverage, these spots move towards the l/3 and 2/3 positions, respectively, until eventually a full (3 x 2) pattern develops. This behaviour is indicative of progressive uniform compression of the adsorbate layer16*1’ along (110) and may be interpreted’s as shown in Figure 2b. At low coverages, the bromine atoms are located essentially in a c(2 x 2) configuration, with some compression along (110) already existing. With increasing coverage, the bromine atoms are moving closer and closer together until they have been translated by half a nearestneighbour distance along the troughs. In this arrangement, the
K 88nlJ8,
R Dtihl,
D E Grider
end J K Sass: Water adsorption on bromine-covered copper single crystal surfaces
adsorbate structure is then consistent with the observed (3 x 2) LEED pattern and corresponds to a saturation coverage of 213. Unfortunately, since the LEED studies and the A@measurements were not performed in the same vacuum system, we cannot directly correlate the two observations. It may be concluded, however, that the two extra spots (cfFigure 2a) are observed at coverages well below half a monolayer, indicating the formation of ordered bromine islands at an early stage. For the Cu(ll1) surface, we have not yet performed a systematic LEED investigation. 3.3. Codsorption of water with bromiw. The appearance of the three molecular orbitals lb,, 3at and lb, as peaks in the UPS spectrum is usually taken as evidence for the molecular adsotp tion of waters. The formation of hydroxyl species, for example, has been associated with the observation of only two peaks’ 3*1g. In the present study, there has been no indication from the UPS results that adsorbed bromine promotes the dissociation of H,O. A series of TDS measurements for water desorption from the Cu( 110) surface, covered with varying amounts of bromine, is shown in Figure 3. Compared to the clean surface, there are additional peaks at higher temperatures which indicate enhanced binding of the water in the vicinity of the adsorbed bromine. This behaviour may be interpreted as hydration of an ionic species on the surface’. By integrating the area under the bromine-induced peaks, the number of water molecules per bromine atom can be determined. For bromine coverages up to 5 l/3 of a monolayer, this number is approximately one. For higher coverages, it decreases rapidly and at 0.6 monolayers, for example, it has been reduced to 0.1. An interesting observation, which is clearly correlated with this behaviour, concerns the peak at -180 K.
I.5 L H,O/Br/Cu
For this desorption state to appear, patches which are free of bromine must exist on the surface (c/Section 3.1.). The intensity of this state is reduced with increasing bromine coverage, as might be expected, but most significantly vanishes for 0, > l/3. It appears therefore that for bromine coverages less than l/3, mixed bromine-water islands with a 1: 1 coverage ratio coexist in the first layer with hydrogen-bonded water on clean surface patches. When Os, z l/3, the mixed layer covers the whole surface. At still higher coverages of bromine, the number of adsorption sites for H,O in the vicinity of bromine is progressively reduced and apparently vanishes when saturation in the (3 x 2) configuration has been reached. An important question that remains to be answered concerns the structure of the mixed bromine-water layer. Here, the changes of the bromine-induced LEED patterns upon the addition of water provide important clues. Regardless of which of the four patterns, shown in Figure 2a, is initially present, a transformation to a sharp and intense (3 x 2) pattern occurs upon coadsorbing the water. This indicates first of all that long-range order is maintained in the mixed layers and that the island sixes must be fairly large in order to be seen in LEED. The observation of the (3 x 2) pattern also imposes some restrictions on the conceivable locations of bromine and water in the mixed layer unit cell, The arrangement in real space which seems most likely to us is shown in Figure 4. It requires that the island of bromine atoms expands uniformly along (110) to accommodate water molecules in
(I IO) Br
:
H2O
Br
Figure4. Arrangement of bromine and water in a composite surface layer, consistent with the observed (3 x 2) LEED pattern and the 1: 1 coverage ratio determine-d by TDS.
Ternparatun
T(K)
Figure 3. Thermal desorption spectra (heating rate: - 10 K s- ’ ) of water
from Cu( 1lo), covered with various amounts of preadsorbed bromine. The peak at - 180 K may be used as an indicator of bromine free patches on the surface. The higher temperature features are bromine-induced.
between the bromine atoms (cf Figure 2b). As required by the TDS results, there is a 1: 1 ratio of bromine to water and the surface is fully covered by this mixed layer at a bromine coverage of l/3. Information about the orientation of the water molecules in the bromine induced adsorption states may he obtained by work function measurements ” . In Figure 5a, the work function increase due to bromine adsorption on Cu(ll0) is shown as the reference curve (dashed line). The effect of adding water at different bromine coverages on AUIis given by the series of solid curves. The initial slopes of these curves are of particular interest, because they are expected to reflect the orientation of the water 759
K Bange, R Ddhl, D E Grider and J K Sass: Water adsorption on bromine-covered
I
2 I
# ,
3 1
copper single crystal surfaces
molecules to align along the field lines in a more perpendicular orientation.
.--------H,O/Br/t+~(lll)
:
4. con&lsions
(bl -IO00
Ho0 upaun
Hz0 -
Figure 5. (a) Work function change due to water adsorption on brominecovered Cu( 110). The A&values for bromine adsorption (- - -) provide the referena curve. (b) Same as (a), but for Cu(ll1).
bound to the bromine. In Figure 5a, these slopes are seen to vary substantially with bromine coverage, being much steeper than the clean surface at low coverages and decreasing for larger values of Oar. The corresponding curves for Cu( 111) are shown in Figure 5b. Although the effect of low bromine coverages on the initial slope of the AU+.,20curves is less pronounced on this face, the results are in general qualitatively similar to Cu(ll0). The perpendicular component of ~1 of the initial dipole moment per adsorbed water molecule may be calculated from these results with the Helmholtz-equation and the variation of this quantity on both crystal faces is shown in Figure 6. As indicated above, the value of ~1 is initially higher than on the clean surface and decreases to low values with higher bromine coverage. These findings would imply that the water molecules are more perpendicularly oriented for small values of 0a, with the oxygen end towards the metal surface. The reason for the variable orientation of the HI0 molecules is not immediately obvious. One possible origin is the complex variation of the electrostatic potential parallel to the surface which causes strong patch fields, particularly at the edges of islands, and might cause the water
It has been shown in this study that detailed information on the nature of the interaction between ionic species, wafer and a metal surface may be obtained in our model experiments. The more detailed results for Cu(ll0) may be summarized as follows. Bromine induces new adsorption states with a higher binding energy for water. Island formation of composite bromine-water layers occurs at fairly low bromine coverage. These layers exhibit long-range order, which has been observed for the first time. There is a 1: 1 ratio of bromine to water up to a bromine coverage of l/3 when the composite layer covers the whole surface. The orientation of the water molecules in $e surface hydration shells depends on the bromine coverage. For low values it is more perpendicular, with the oxygen pointing towards the surface. The tendency of bromine and water to form composite islands at bromine coverage below l/3 is of particular relevance to electrochemistry. Attempts to model the variation of the electrostatic potential in the interfacial region focus exclusively on the direction perpendicular to the surface2*4. However, if patches containing specifically adsorbed bromine may coexist with water islands on areas of the bare surface, averaging the potential along planes parallel to the surface can give quite a misleading impression. Instead, it would be necessary to consider in detail the potential variation along the surface and to incorporate the screening response of the bulk electrolyte to this heterogeneity of the potential in the double layer. A similar point can be made with regard to the variable orientation of the water molecules in the double layer. When trying to identify the various contributions to the total electrostatic potential across the interface, the effect arising from the permanent dipoles of the solvent molecules can probably not be accounted for by simply assuming two opposite, up or down, configurations of the water molecule2. By performing further experiments of the type reported here, it is hoped that more realistic models of the double layer may eventually be developed.
Acknowledgement Most valuable discussions with T E Madey are gratefully acknowledged. We thank D P Woodruff and R G Jones (cjref 18). The expert technical assistance of E Piltz and the generous help given by R Unwin must also be mentioned. This work has been supported by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 6.
H20/Br/Cu
References
I
0.2
!
I
04
06
Rotothm bromine coverage
8/19,.
Figure 6. Perpendicular component p.l of H,O initial dipole moment on Cu(ll0) and Cu(ll1) as a function of bromine coverage. The relative coverage ~/&,,,, has been obtained from the bromine-induced A@ values. 760
’ J K Sass, Vacuum, these proceedings. ’ J OX4 Bock& and A K N Reddy, Modern Electrochemistry. Plenum Press, New York (1970). 3 J K Sass, K Kretzschmar and S Holloway, Vacuum, 31,483 (1981). 4 W R Fawatt, J Chem Phys, 61,3842 (1974). 5 M Grunze and P A Dowben. Appl Surface Sci, 10, 209 (1982). 6 See, for example, D L Doering and T E Madey, Surface Sci, 123, 305 (1982). ‘I K Kretzschmar, J K Sass, A M Bradshaw and S Holloway, Sur/ace Sci, 115, 183 (1982). 8 G B Fisher and J L Gland, Sur$ace Sci, 94,446 (1980).
K Bsnge, R DBhL D E Glider and J K Sass: Water adsorption on bromine-covered copper single crystal surfaces ’ T E Madey and F P Netzer, Surjace Sci. 117,549 (1982).
” K Bange, D E Grider and J K !&us, Surjhce Sci, 126,437 (1983). ” A detailed auxunt of the Cu( 111) data is in preparation. ” B A Gottwald, Vakuum-Tecknik,22, 106 (1973). I3 K Bange, D E Grider, J K Sass and T E Madey, Swfaee Sci, submitted for publication. ” P A Thiel, F M Hoffmann and W H Weinberg, J Chem Phps, 75,5556 (1981).
” N V Richardson and J K Sass, Surfice Sci, 103,621 (1981). l6 P J Goddard and R M Lambert, Sur$xe Sci, 67, 180 (1977). ” R G Jones and D P Woodruff, Vacuum, 31,411 (1981). rsWe are very grateful to D P Woodruff and R G Jones for suggesting this interpretation. r9 C Bcnndorf. C Niibl, M Riisenberg and F Thieme, Surjace Sci, 111,87 (1981). ” D E Grider, K Bange and J K Sass, J Electrochem Sot, 130,246 (1983).
761
Vacuum/volume 33lnumbe.rs 1O-l Z/Pages 757 to 761 I 1983 Printed in Great Britain
Water adsorption on bromine-covered single crystal surfaces K Bange, R Diihl, D E Glider Farada yweg 46,
and J K Sass, Fritz-Haber-lnstitut D- 1000 Berlin 33, West Germany
copper
der Max-Planck-Gesellschaft,
The coadsorption of water with bromine on Cu(ll0) and Cu(l II) has been studied using UPS, LEED. TDS and A@. The results show that water is more strongly bound to the surface in the vicinity of the bromine than on the clean substrate. This behaviour is interpreted as surface hydration of the partially ionic bromine. lnformation about the dipole orientation of the solvation water is derived from the observed A@-values. The LEED studies indicate that island formation with long-range order of the mixed bromine-water layer occurs at fairly low bromine coverages. The experimental results are qualitatively similar on the close-packed (7 7 1) and the more open (110) surface. The relevance of these findings to the electrochemical phenomenon of specific anion adsorption on metal electrodes is briefly discussed.
1. Introduction
This work is par: of an extensive programme to apply surface science methods and concepts to the study of interracial electrochemical processes’. Our general motivation is the lack of microscopic concepts and surface-sensitive experimental techniques in traditional electrochemistry*. It has been suggested3, for example, that by coadsorbing water and ions on metal surfaces in ultra-high vacuum the composition and structure of the electric double layer formed at a metal-electrolyte interface can be simulated. Using this approach, it is possible to obtain detailed information at the molecular level about ion-solvent interactions at a metal electrode. We have chosen bromine as a coadsorbate because of the significance of halide ions in electrochemistry. These ions are known to adsorb directly onto the electrode, shedding part of their solvation shell in the process. In electrochemical terminology, this is called specific adsorption’, in contrast for example to the alkali ions which are believed to retain their full solvation shell in the double layer, just as in the bulk of an electrolyte solution. The ability of specifically adsorbed anions to influence the rate of electrochemical reactions* is certainly an important reason for obtaining a detailed understanding of halogen adsorption in the presence of water. Halogen adsorption, by itself, on single crystal metal surfaces has recently been reviewed5. Invariably, when the exposure is in molecular form, dissociative adsorption will take place, generally with a fairly large initial sticking coefficient. A few aspects of the adsorption behaviour are still somewhat puzzling, for example the high density at saturation coverage and the fairly constant dipole moment with coverage, in most cases’. The interest of surface scientists in the adsorption of water on clean metal surfaces has increased dramatically in recent years’. In general, it has been found that a low temperature (5 150 K)
water adsorbs molecularly with a sticking coefficient near unity, forms hydrogen-bonded networks and interacts only weakly with the substrate. At higher temperatures, the formation of fragmentation products such as hydroxyl groups or oxygen may be observed on some of the more reactive metals’. The influence of ionic coadsorbates on the adsorption behaviour of water has, with the exception of oxygen7’-9, not previously been reported in any detail”. In this communication, the results obtained for Cu( 110) are described fairly extensively; some selected findings for Cu(ll1) are given” to illustrate the qualitatively similar behaviour of both surfaces.
2. Experimentalprocedures The experiments were performed in two separate vacuum chambers: UPS and LEED studies were done in a commercial instrument (VG ADE!&400), TDS and A@measurements in a selfbuilt stainless steel system. Preliminary UPS data were obtained at normal emission using unpolarized He I and He II radiation, incident at 45“. Computercontrolled, resistive heating of the crystals was used for TDS. The Kelvin probe consisted of a stainless steel mesh coated with tin oxide. To minimize damage to the adsorbed H,O, the Varian 3-grid LEED optics were operated at Iow beam currents (-0.142 PA). Water was dosed from a glass microcapillary array. Comparative dosing from the vacuum chamber provided exposures in units of Langmuir (10m6 torr s). Molecular bromine was evolved from a solid AgBr electrochemical ~~11’~. The copper surfaces were prepared by mechanical polishing and electropolishing; they were cleaned in vacuum by argon ion bombardment at 500 K and subsequent annealing to 750 K. Cooling of the sample to _ 110 K was done by liquid nitrogen. Further experimental details will be given elsewhere’ 3. 757
K Bange. R Dtihl, D E Grider end J K Sass: Water adsorption on bromine-covered copper single crystal surfaces 3.
Results and discussion
3.1. Water adsorption. In this subsection, we want to briefly review our results for water adsorption on clean C~(ll0)‘~*‘~ and compare them to those obtained on the (111) surface. On both surfaces, the UPS results indicate that water is adsorbed molecularly from 110 K up to its desorption temperature. The appearance of two peaks in TDS at L 160 K and -175 K, indicating desorption from ice multilayers and from the more strongly bound first layer, is again very similar on the two faces. The hydrogen-bonded hexagon network for the first layer, deduced from the LEED observations on Cu(ll0) and other surfaces6*7~g*14, is also expected to form on Cu(l11). However, we have not yet been able to observe a reproducible LEED pattern on Cu(1ll), consistent with such structure. Unfortunately, because of its easy beam damage, investigations of H,O are considerably more difficult than for other adsorbates. A comparison of the work function decreases, associated with water adsorption on Cu(ll0) and Cu(lll), is shown in Figure 1.
observed on other metal substrate$, for example bromine on CU(NIO)~~. For Cu(l1 l), the adsorption of chlorine has been studied in some detailr6. A maximum work function change of 1200 mV was observed in this instance. The details of the work function increase with bromine exposure are different for both faces (cJFigure 4). Whereas the curve for Cu(1 1l)is essentially linear up to saturation, on Cu(l10) the linearity extends only to A@= 500 mV. Since another, independent means of determining the surface coverage was not available to us, we shall assume that it is proportional to the work function increase. Some justification for this assumption to hold even in the non-linear part of the A@ curve for Cu(ll0) comes from previous halogen adsorption studiess. For the adsorption of bromine on Cu(llO), a well defined sequence of LEED patterns is observed with increasing coverage. A schematic representation of the adsorbate induced spots is shown in Figure 2a. At small coverages, two extra spots along 0
0 xx 0
0
0
0
H,O/Cu T=llOK
x x )-
(xl
(xl
0
((XII x
x
((xl)
(xl
(xl
0
0
0 L
3
Hz0 exposure(L)
Figure 1. Work function decrease due to water adsorption on Cu( 110) and Cu(ll1). The initial dipole moments are 0.85 and 0.5 Debye, respectively.
The largest values of A@ are approximately - 1100 mV and - 1000 mV. The sign of the work function change is consistent with the concept that the majority of water molecules in the first layer arc oriented with the oxygen closer to the surface’, i.e. in the flip-up configuration according to Doering and Madey6. A significantly higher exposure is required on the (111) surface to attain the saturation value. This behaviour is also evident in the initial slopes of the two A@-curves. The apparent initial dipole moments, calculated from the slopes, are 0.85 and 0.5 Debye, respectively. The most likely explanation for these differences is that on the (111) face multilayer formation, which does not contribute to A@, occurs at fairly low coverages on top of first layer two-dimensional islands6*r3. Larger exposures are then required to complete the first layer which, when judged by the similar saturation values, seems to possess a quite comparable nature on both faces. 3.2. Bromine adsorption. Upon exposure of molecular bromine to the Cu(ll0) and Cu(l11) surfaces, and increase of the work function is observed. The saturation values are A@, ,o= 1000 mV and A@rrr= 800 mV. They are of a similar magnitude as those 758
ox
x0
xx
xx
ox
x0
(3x2) (a)
(b)
Figure 2. (a) Evolution of LEED patterns with increasing bromine coverage on Cu(ll0). The adsorbate-induced spots are indicated by crosses [(x) weak: ((x )) very weak]. (b) Adsorption model for Br/Cu( 110); with increasing bromine coverage progressive compression in the adlayer causes transition to a (3 x 2) configuration, with a coverage of 2/3 (see text).
(110) appear close to the centre of the substrate surface Brillouin zone. With increasing coverage, these spots move towards the l/3 and 2/3 positions, respectively, until eventually a full (3 x 2) pattern develops. This behaviour is indicative of progressive uniform compression of the adsorbate layer16*1’ along (110) and may be interpreted’s as shown in Figure 2b. At low coverages, the bromine atoms are located essentially in a c(2 x 2) configuration, with some compression along (110) already existing. With increasing coverage, the bromine atoms are moving closer and closer together until they have been translated by half a nearestneighbour distance along the troughs. In this arrangement, the
K 88nlJ8,
R Dtihl,
D E Grider
end J K Sass: Water adsorption on bromine-covered copper single crystal surfaces
adsorbate structure is then consistent with the observed (3 x 2) LEED pattern and corresponds to a saturation coverage of 213. Unfortunately, since the LEED studies and the A@measurements were not performed in the same vacuum system, we cannot directly correlate the two observations. It may be concluded, however, that the two extra spots (cfFigure 2a) are observed at coverages well below half a monolayer, indicating the formation of ordered bromine islands at an early stage. For the Cu(ll1) surface, we have not yet performed a systematic LEED investigation. 3.3. Codsorption of water with bromiw. The appearance of the three molecular orbitals lb,, 3at and lb, as peaks in the UPS spectrum is usually taken as evidence for the molecular adsotp tion of waters. The formation of hydroxyl species, for example, has been associated with the observation of only two peaks’ 3*1g. In the present study, there has been no indication from the UPS results that adsorbed bromine promotes the dissociation of H,O. A series of TDS measurements for water desorption from the Cu( 110) surface, covered with varying amounts of bromine, is shown in Figure 3. Compared to the clean surface, there are additional peaks at higher temperatures which indicate enhanced binding of the water in the vicinity of the adsorbed bromine. This behaviour may be interpreted as hydration of an ionic species on the surface’. By integrating the area under the bromine-induced peaks, the number of water molecules per bromine atom can be determined. For bromine coverages up to 5 l/3 of a monolayer, this number is approximately one. For higher coverages, it decreases rapidly and at 0.6 monolayers, for example, it has been reduced to 0.1. An interesting observation, which is clearly correlated with this behaviour, concerns the peak at -180 K.
I.5 L H,O/Br/Cu
For this desorption state to appear, patches which are free of bromine must exist on the surface (c/Section 3.1.). The intensity of this state is reduced with increasing bromine coverage, as might be expected, but most significantly vanishes for 0, > l/3. It appears therefore that for bromine coverages less than l/3, mixed bromine-water islands with a 1: 1 coverage ratio coexist in the first layer with hydrogen-bonded water on clean surface patches. When Os, z l/3, the mixed layer covers the whole surface. At still higher coverages of bromine, the number of adsorption sites for H,O in the vicinity of bromine is progressively reduced and apparently vanishes when saturation in the (3 x 2) configuration has been reached. An important question that remains to be answered concerns the structure of the mixed bromine-water layer. Here, the changes of the bromine-induced LEED patterns upon the addition of water provide important clues. Regardless of which of the four patterns, shown in Figure 2a, is initially present, a transformation to a sharp and intense (3 x 2) pattern occurs upon coadsorbing the water. This indicates first of all that long-range order is maintained in the mixed layers and that the island sixes must be fairly large in order to be seen in LEED. The observation of the (3 x 2) pattern also imposes some restrictions on the conceivable locations of bromine and water in the mixed layer unit cell, The arrangement in real space which seems most likely to us is shown in Figure 4. It requires that the island of bromine atoms expands uniformly along (110) to accommodate water molecules in
(I IO) Br
:
H2O
Br
Figure4. Arrangement of bromine and water in a composite surface layer, consistent with the observed (3 x 2) LEED pattern and the 1: 1 coverage ratio determine-d by TDS.
Ternparatun
T(K)
Figure 3. Thermal desorption spectra (heating rate: - 10 K s- ’ ) of water
from Cu( 1lo), covered with various amounts of preadsorbed bromine. The peak at - 180 K may be used as an indicator of bromine free patches on the surface. The higher temperature features are bromine-induced.
between the bromine atoms (cf Figure 2b). As required by the TDS results, there is a 1: 1 ratio of bromine to water and the surface is fully covered by this mixed layer at a bromine coverage of l/3. Information about the orientation of the water molecules in the bromine induced adsorption states may he obtained by work function measurements ” . In Figure 5a, the work function increase due to bromine adsorption on Cu(ll0) is shown as the reference curve (dashed line). The effect of adding water at different bromine coverages on AUIis given by the series of solid curves. The initial slopes of these curves are of particular interest, because they are expected to reflect the orientation of the water 759
K Bange, R Ddhl, D E Grider and J K Sass: Water adsorption on bromine-covered
I
2 I
# ,
3 1
copper single crystal surfaces
molecules to align along the field lines in a more perpendicular orientation.
.--------H,O/Br/t+~(lll)
:
4. con&lsions
(bl -IO00
Ho0 upaun
Hz0 -
Figure 5. (a) Work function change due to water adsorption on brominecovered Cu( 110). The A&values for bromine adsorption (- - -) provide the referena curve. (b) Same as (a), but for Cu(ll1).
bound to the bromine. In Figure 5a, these slopes are seen to vary substantially with bromine coverage, being much steeper than the clean surface at low coverages and decreasing for larger values of Oar. The corresponding curves for Cu( 111) are shown in Figure 5b. Although the effect of low bromine coverages on the initial slope of the AU+.,20curves is less pronounced on this face, the results are in general qualitatively similar to Cu(ll0). The perpendicular component of ~1 of the initial dipole moment per adsorbed water molecule may be calculated from these results with the Helmholtz-equation and the variation of this quantity on both crystal faces is shown in Figure 6. As indicated above, the value of ~1 is initially higher than on the clean surface and decreases to low values with higher bromine coverage. These findings would imply that the water molecules are more perpendicularly oriented for small values of 0a, with the oxygen end towards the metal surface. The reason for the variable orientation of the HI0 molecules is not immediately obvious. One possible origin is the complex variation of the electrostatic potential parallel to the surface which causes strong patch fields, particularly at the edges of islands, and might cause the water
It has been shown in this study that detailed information on the nature of the interaction between ionic species, wafer and a metal surface may be obtained in our model experiments. The more detailed results for Cu(ll0) may be summarized as follows. Bromine induces new adsorption states with a higher binding energy for water. Island formation of composite bromine-water layers occurs at fairly low bromine coverage. These layers exhibit long-range order, which has been observed for the first time. There is a 1: 1 ratio of bromine to water up to a bromine coverage of l/3 when the composite layer covers the whole surface. The orientation of the water molecules in $e surface hydration shells depends on the bromine coverage. For low values it is more perpendicular, with the oxygen pointing towards the surface. The tendency of bromine and water to form composite islands at bromine coverage below l/3 is of particular relevance to electrochemistry. Attempts to model the variation of the electrostatic potential in the interfacial region focus exclusively on the direction perpendicular to the surface2*4. However, if patches containing specifically adsorbed bromine may coexist with water islands on areas of the bare surface, averaging the potential along planes parallel to the surface can give quite a misleading impression. Instead, it would be necessary to consider in detail the potential variation along the surface and to incorporate the screening response of the bulk electrolyte to this heterogeneity of the potential in the double layer. A similar point can be made with regard to the variable orientation of the water molecules in the double layer. When trying to identify the various contributions to the total electrostatic potential across the interface, the effect arising from the permanent dipoles of the solvent molecules can probably not be accounted for by simply assuming two opposite, up or down, configurations of the water molecule2. By performing further experiments of the type reported here, it is hoped that more realistic models of the double layer may eventually be developed.
Acknowledgement Most valuable discussions with T E Madey are gratefully acknowledged. We thank D P Woodruff and R G Jones (cjref 18). The expert technical assistance of E Piltz and the generous help given by R Unwin must also be mentioned. This work has been supported by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 6.
H20/Br/Cu
References
I
0.2
!
I
04
06
Rotothm bromine coverage
8/19,.
Figure 6. Perpendicular component p.l of H,O initial dipole moment on Cu(ll0) and Cu(ll1) as a function of bromine coverage. The relative coverage ~/&,,,, has been obtained from the bromine-induced A@ values. 760
’ J K Sass, Vacuum, these proceedings. ’ J OX4 Bock& and A K N Reddy, Modern Electrochemistry. Plenum Press, New York (1970). 3 J K Sass, K Kretzschmar and S Holloway, Vacuum, 31,483 (1981). 4 W R Fawatt, J Chem Phys, 61,3842 (1974). 5 M Grunze and P A Dowben. Appl Surface Sci, 10, 209 (1982). 6 See, for example, D L Doering and T E Madey, Surface Sci, 123, 305 (1982). ‘I K Kretzschmar, J K Sass, A M Bradshaw and S Holloway, Sur/ace Sci, 115, 183 (1982). 8 G B Fisher and J L Gland, Sur$ace Sci, 94,446 (1980).
K Bsnge, R DBhL D E Glider and J K Sass: Water adsorption on bromine-covered copper single crystal surfaces ’ T E Madey and F P Netzer, Surjace Sci. 117,549 (1982).
” K Bange, D E Grider and J K !&us, Surjhce Sci, 126,437 (1983). ” A detailed auxunt of the Cu( 111) data is in preparation. ” B A Gottwald, Vakuum-Tecknik,22, 106 (1973). I3 K Bange, D E Grider, J K Sass and T E Madey, Swfaee Sci, submitted for publication. ” P A Thiel, F M Hoffmann and W H Weinberg, J Chem Phps, 75,5556 (1981).
” N V Richardson and J K Sass, Surfice Sci, 103,621 (1981). l6 P J Goddard and R M Lambert, Sur$xe Sci, 67, 180 (1977). ” R G Jones and D P Woodruff, Vacuum, 31,411 (1981). rsWe are very grateful to D P Woodruff and R G Jones for suggesting this interpretation. r9 C Bcnndorf. C Niibl, M Riisenberg and F Thieme, Surjace Sci, 111,87 (1981). ” D E Grider, K Bange and J K Sass, J Electrochem Sot, 130,246 (1983).
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