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Chlorine adsorption on the low index surfaces of silver: Energetics and structures
Surface Science 0 North-Holland
85 (1979) 276-288 Publishing Company
CHLORINE ADSORPTION
ON THE LOW INDEX SURFACES OF SILVER:
ENERGETICS AND STRUCTURES
Yung-Yi TU * and J.M. BLAKELY Materials Science and Engineering New York 14853. USA
Received
20 February
and ‘the Materials Science Center, Cornell University, Ithaca,
1979
The interaction of Cl2 with Ag single crystal surfaces has been studied over a range of crystal temperature and gas pressure. Observations have been made for the (11 l), (100) and (110) surfaces. Measured adsorption isobars for Ag(100) were used to obtain isosteric heats of adsorption; values ranged from - -60 kcal/mole (of Clz) at low coverage to - -70 k&/mole near saturation. The structure formed by Cl2 adsorption on (100) is believed to be a simple overlayer. For (110) and (111) the values obtained for the heat of adsorption were - -55 kcaI/ mole. On Ag(ll1) an epitaxially oriented AgCl(111) is believed to form.
1. Introduction
The purpose of this study has been to measure the heat of adsorption of Cl* on silver surfaces and to obtain information about chlorine adlayer structures. A brief account of the results for Ag( 111) has previously been published [ 11. This paper is concerned mainly with the results for Ag(ll0) and Ag(lOO). Understanding the interaction of Clz with silver surfaces may be essential in explaining the efficiency of Ag catalysts for the oxidation of ethylene [2-51, the chlorination (or oxidation) mechanism 161, and the formation of epitaxial A&l layers on Ag [7,8] ; the adsorption step may also be important in understanding the process of latent image formation in silver halide emulsions and the kinetics of decomposition and reconstitution of silver halides in photochromic materials [9,10]. Moreover, the interaction of Cl2 with Ag surfaces involves one of the more attractive systems for theoretical study. A number of studies have previously been made on the Cl*-Ag system but several questions still remain. Rovida et al. [11,12] used CzH4C12 as the reacting gas and studied the structure and thermal stabilities of the “chlorine” adlayer on Ag while Zanazzi et al. [13] have performed LEED intensity calculations for Cl on
* Now at IBM Research
Laboratories,
San Jose, California, 276
USA.
Y.-Y. Tu, J.M. Blakely / Cl, adsorption on Ag
271
Ag( 100). Other recent publications have dealt with adsorption-desorption characteristics and structural aspects of Cl2 on Ag(ll1) [ 141 and Ag(331) [ 151. Our studies (ref. [l] and this paper) emphasize the thermodynamic aspects. To the authors’ knowledge, there are no published data on the heat of adsorption of CIZ on Ag. 2. Experimental The important details have been described previously in connection with the results on Ag( 111) [ 11. Ali experiments were done under dynamic flow conditions with the chlorine coverage being monitored by Auger electron spectroscopy (AES) performed at temperature with the help of a blanking resistive heating arrangement
[161. For photographing low energy electron diffraction (LEED) patterns a Kodak Wratten filter [No. 58 (green)] was placed in front of the view port; this helped to block the light from the electron gun and filament and the resistively heated specimen. A measure of the chlorine concentration was obtained from the Auger peakto-peak height ratio in derivative spectra (i.e., dA@‘)/ti versus I?) of chlorine (LZ3M2aMZ3 transition at -185 eV) to silver (M4sW transition at -355 eV). Experimentally, adsorption isotherms or isobars were measured and isosteres were constructed from these. Application of the Clausius-Clapeyron equation to the adsorption isosteres gives the modelindependent, isosteric heat of adsorption, 4, at chlorine coverage 0 according to [ 171
with R the gas constant, p the Cl2 partial pressure, and T the absolute temperature. As defined by this equation, q is a negative quantity. From the thermodynamic data on the formation of bulk AgCl from the reaction of Cl* gas with pure silver, it is clear that the compound should ultimately form over much of the pressure and temperature range of these experiments. However, due to limited solid state mobility we believe that equilibrium between vapor and bulk is not achieved and that for the purpose of analyzing our observations we may disregard the migration into the bulk and treat the problem as one of surface-vapor equilibrium only.
3. Results 3.1. Cl? on Ag(llI) The results for this surface have been published in a previous paper [l] and will be briefly reviewed here so that we may compare them with results for other surfaces.
278
Y.-Y.
Tu, J.M. Blakely / Cl2 adsorption on Ag
An adsorption isobar was obtained for the temperature range from room temperature to -600°C. The low coverage portion of this isobar when fitted to the Langmuir model gave a value of the heat q - -50 to - -60 kcal/mole. The formation of an epitaxial AgCl( 111 j layer on the chlorine saturated Ag( 111 j surface was inferred from LEED observations. The LEED pattern from the chlorine saturated Ag(ll1 j surface could be generated through double diffraction of the incident electron beam from the Ag(ll1 j and the AgCl(ll1 j two-dimensional nets. In addition the diffuse background intensity greatly increased after chlorine was adsorbed on Ag(ll1 j as would be expected for a compound such as AgCl with a low Debye temperature. 3.2. C/a on Ag(ll0) Adsorption isotherms for this surface (fig, 1) were obtained using a cylindrical mirror analyzer over the pressure range from low9 to 10e6 Torr and the temperature range from 458 to 560°C. To within the accuracy of the measurement the isotherms were reversible with pressure cycling. However, the experimental uncertainties in this particular set of measurements are rather large due to the loss of gain, upon Cl* adsorption, of the electron multiplier of the cylindrical mirror electron analyser. Isosteres constructed from these isotherms are shown in fig. 2 for
I
0.5-
0 *
_;_--
____f-
_;---,/--
A
0.4-
I’ .o ‘0 cr i .P I”
&’ R-
0
“,‘I
0’
H/H -c
2. .*’ 8 ./ ’ .
:
.H
.,I’ IO1’ 8 rg‘0 -!____;-“-------
I x$ 0.2: II a S ”
/’
0.3~rp t
I
d
_---
/Y--_
,o_=-
XV’
v 0
.3-
8
+-0 El-/
on
P0
0
o
0
_/--
P
; --k--B I D
---
f
I, II Ax, j I
0
v __--
I
:
0 _---
q
,’ I,’ 0.1 -: ,$
__-*
--
F---a
Ag(llO)+Cl2
isotherms
A 45tFx
0 405OC 0 516OC o 560°C
ed : 0
0
50
1 100
1 150
4 -_JL-_ 1 200 400
, 600
600
IO’ x P (Torr)
Fig. 1. Chlorine adsorption isotherms on Ag(ll0). Each isotherm is reversible with pressure cycling. The pressure ranged over about three orders of magnitude.
Y.-Y. Tu, J.M. Blakely / Cl2 adsorption on Ag
Ag(ll0) + Cl,
10-g
I
I
I
1.2
1.3
1.4
279
-
103 T("K)
Fig. 2. Adsorption isosteres for Cl2 on Ag( 110). These isosteres are constructed from fig. 1 at three different Cl/Ag ratios (Cl/Ag = 0.18, 0.3 and 0.35). The slope, d(ln p)/d(l/T) is proportional to the heat of adsorption.
different values of the Cl : Ag Auger peak ratio. The scatter in the data does not allow any definite conclusions about the variation of heat of adsorption with coverage. The average value from fig. 2 is -55 + 6 kcal/mole. 3.3. CIZ on Ag(lO0) A set of reversible adsorption isobars for this surface, taken with a retarding field analyzer, are shown in fig. 3. The CIZ partial pressures are represented by the quadrupole gas analyzer current outputs. Isosters constructed from these isobars are shown in fig. 4. The uncertainties in determining the precise Cl? partial pressure are indicated. The biggest error bar is associated with the lowest partial pressure, which was close to the detection limit of our instrument (Varian Gas Analyzer VGA-loo, model 978-1000). Using the Clausius-Clapeyron relation, the heat of adsorption of Cl* on Ag(100) at the various coverages can be obtained and is shown in fig. 5. The coverage, 8, appearing in this figure is calculated on the assumption that for both chlorine and silver the Auger peak-to-peak heights are linear functions of 8; 8 = 1 is
280
Y.-Y.
Tu, J.M. Blakely
/ Cl, adsorption
on Ag
r g100 0 0
‘;
vv
GAo~
q
A oA&
0
V
0
,
A qo
I
A
v
V
0 q
0
v
h
V 0 V
A 0
2
q
A
0
F i &
x 16”amp
0
0
A
050-
isobors
1.3 x IO~,~omp 3 x IO amp 6 x 16’3amp
A-7.5
A0 0 A0 0
cr,o75t
i? ii
AgUOO) + Cl,
ACp o
V
A
0
025A
0 o
A
V O0
0
V
400
A
0
I
I
300
200
100
I
400
500
0
0
1
p
I
I
I
600
700
600
T(‘C) Fig. 3. Adsorption isobars for Cla on Ag(100). The chlorine the quadrupole mass spectrometer current output. The actual mately. 1 0e9 to 10e6 Torr.
partial pressure is represented by pressure range covered is approxi-
Ag(l00) 0 I
_
0 2 03 04 05 0.6 0.7
0.9
0.6
+ Cl,
isosteres
lo-“-
E” a 0)
5
2 F
-12 _
IO
a 0 t
a” 2-J IO
-IS_
0
Io-‘;,o
1
I
,
I
1.1
1.2
1.3
1.4
IO3
*
1.5
Tt K) Fig. 4. Adsorption isosteres for Cl2 adsorption on Ag(100). The error in the partial pressure measurement is indicated. Isosteres are constructed at 0.1 intervals of the Cl/Ag ratio as indicated on the figure.
281
Y.-Y. Tu, J.M. Blakely / Cl2 adsorption on Ag
Ag(100)+ Cl2 G
z
E 1 s i
-5o-6O-
.g z
-70.
g 2
-0o-
r:+:/ ,,,, 0
01
0.2
0.3
0.4
, ,, 0.5
Coverage Fig. 5. The heat of adsorption data in fig. 4.
0.6
0.7
0.0
09
IO
8
versus coverage curve for Cl2 on Ag(100) calculated from the
taken to correspond to the saturation coverage. There is some inducation of an increase in the magnitude of the heat of adsorption with Increasing coverage but again the size of the experimental error does not allow a definite conclusion to be drawn. A room temperature LEED pattern from clean Ag(lOO) is shown in fig. 6a and that from the chlorine saturated Ag(lOO) surface is shown in fig. 6b. Some of the possible real space atomic arrangements corresponding to the c(2 X 2) LEED pattern are shown in fig. 7. Figs. 7a, 7c and 7d are simple chlorine overlayers with chlorine atoms at different symmetrical sites while fig. 7b is a reconstructed adlayer with the AgCl(lOO) surface structure.
4. Discussion Zanazzi et al. [13] have calculated the intensity variation with energy of LEED beams from the various structures of fig. 7 and have compared the results with those obtained experimentally. The comparison favored the simple overlayer model with Cl atoms in the fourfold symmetric sites of fig. 7a. It should be remembered, however, that Zanazzi et al. [ 131 (and Rovida et al.) used C2H,C12 as the reacting gas. It is therefore not clear whether or not chlorine was the only chemical species on the Ag surfaces [ 11,121. Unfortunately our experimental I-V curves (i.e., fig. 8) were not obtained for the same incidence angles as that used in Zanazzi’s calculations and therefore cannot be compared with their published theoretical curves.
Y.-Y. Tu, J.M. Blakely / Cl, adsorption on Ag
Fig. 6. Rc ,0*, cow xage.
temperature
LEED
pictures
at 78 eV: (a) clean Ag(100);
(b) at sat uraf ion
Cl2
Ho\ revel r, v/e believe from other observations that the Clz structure prc)du ted on Ag( 100) is of the simple overlayer type depicted in fig. 7a. The positic ens of the inte :gral ord er diffracted beams are the same as for clean Ag surface and the be; lfnS of 1type (4,1) are visible over the energy range from -30 to -200 eV. Th is i,Sin
Y.-Y. Tu, J.M. Slakely / Clz adsorption on Ag
(cl
283
(4
Fig. 7. Schematic models of possible c(2 X 2) structures. (a) Simple overlayer with Cl in fourfold symmetrical sites. (b) Ag-Cl mixed layer. (c) Overlayer with Cl in bridge sites (one of two domains). (d) Overlayer with Cl in top sites. Shaded circles: chlorine atoms. Open circles: substrate Ag atoms.
contrast to the behavior on Ag(lI1) where, in the energy range from -70 to 100 eV, the pattern is a combination (with double diffraction effects) of the patterns expected from Ag(ll1) [with lattice constant a0 = 4.09 A] and from AgCl(111) [with lattice constant 5.55 A]. At low energies, 570 eV, the diffraction pattern is as expected from AgCl(111) while above -100 eV the pattern is that of Ag(ll1) with a.strong diffuse background. The temperature dependence of the (4, $) type beams on the Cl2 covered Ag(lOO) surface is as shown in fig. 9; the intensity falls off more rapidly with temperature than the exponential type of dependence. As pointed out by Estrup [18] this type of temperature dependence is expected for a layer exhibiting structural disorder at high temperature and should be typical of simple overlayer adsorption [ 191. It is interesting to note that most LEED studies of c(2 X 2) structures on fcc(100) metal surfaces have led to the conclusion that simple overlayers were formed with the adsorbed atoms in the 4-fold coordinated sites. Examples are Se on Ag(lOO) [20], Na on Al(100) [21,22], Na on Ni(lOO) [23-261, 0 on Ni(100) 127-301. S on Ni(100) [25,28-341, Se on Ni(lOO) [28,29], and Te on Ni(100) [28,29,35]. Information obtained from these experiments on the heat of adsorption of Clz on Ag(lOO) is compared with other relevant data [36] for this system in fig. 10. As noted from the diagram the heat of adsorption is comparable in magnitude to the heat of formation of the compound AgCl. This is typical of dissociative adsorption;
Y.-Y. Tu, J.M. Blakely / Cl, adsorption on Ag
284
500
400 G-
c :
(001Beam
0
--o--
Ag(100)
-
Ag(100)
+ Cl2
100 V(eV)
4c (oil
beam
I 01
---o---
AgflOO)
b
Ag(lO0)
El (oil 3c
0
b
r C 0 H 20
IO
b
+ Cl,
Y.-Y. Tu, J.M. Blakely / Cl2 adsorption on Ag
285
V (eV)
Ag (100) + Cl,
q (
‘4 \ .? ‘? \ fI
1
(T
TT
z)
born
---*---
.
Cl
C&j)
bnam-
1
Fig. 8. Normalized I- V curves for room temperature Clz/Ag(lOO) tive position of each beam is shown. The incident electron beam normal and rotated - 1 O” about a [ 1 lo] type axis.
and clean Ag(100). The relawas at -5’ from the surface
286 Ag(lCO) + Cl, (‘i-&j A
AV
beam heatmg
A v V
v A
V A
V V A v
A
V
I
I
25 --cl
I
450
350
*
550
T (‘K) Fig. 9. Temperature
variation
of (i,
i) beam intensity
from Cl2 on Ag(lOO).
2 Cl (gas)
0
Cl2 (gas) Heat of Formation of AgCI
Heat of Adsorption
-50 Itcal/mole (-2.2 eV/atoml
Ag +
-60 Kcal/mole (-2.6 eV/atomJ
Adsol .bed
-70 kcal/mole (-3.0 eV/atom)
Fig. 10. Summary of relevant thermodynamic C12-Ag(lOO) system. The values correspond
data (from ref. [36] to 1 mole of Cl2.
and present
work)
for the
Y.-Y. Tu, J.M. Blakely / Cl, adsorption on Ag
281
Vacuum Vacuum
0
Level
Affinity -4.6 eV
-13.1 Density
0
Leve I
Level
eV
3P
-3.6
3s
-13.0
eV
eV
of
States Cl Atom
Ag Metal
Fig. 11. Same electronic structural information for
Ag
metal [ 371and atomic chlorine.
the observation of Goddard et al. [14] that only Cl atoms appeared in the flash desorption spectrum is at least consistent with our conclusions. From the data on fig. 10 we see that the binding energy per chlorine atom on a silver surface has a value in the range of 53 to 63 kcal/mole or 2.3 to 2.7 eV/atom. This value may be compared with available information on the electronic structure of Ag metal and of an isolated Cl atom as indicated in fig. 11. Theoretical studies of the binding in this system do not seem to have been performed although considerable information is available on the electronic structure of Ag and of Cl adsorbed on Ag [38].
5. Conclusion The interactions of Cl* with silver surfaces of orientations (1 lo), (11 l), and (100) have been studied in some detail. Under near equilibrium conditions the structure formed in the chlorine saturated Ag(lOO) is considered to be that of a simple chlorine overlayer. This is in contrast to the situation on Ag(ll1) where an epitaxially oriented AgCl(111) layer is believed to form. From measurements of adsorption isobars or isotherms the heat of adsorption is found to be _ -60 kcal/ mole, relatively insensitive to the silver surface orientation. The heat of adsorption is also found to be nearly constant throughout the range of chlorine coverage.
Acknowledgement The work has been supported by the National Science Foundation (Grant No. DMR76-23537) and by the Materials Science Center at Cornell University.
Y.-Y. Tu, J.M. Blakely / Cl2 adsorption on Ag
288
References [l] Y.-Y. Tu and J.M. Blakely, J. Vacuum Sci. Technol. 15 (1978) 563. [2] H.H. Voge and CR. Adams, Advan. Catalysis 17 (1967) 151. [3] P.A. Kilty, NC. Rol and W.M.H. Sachtler, in: Catalysis, Ed. J.W. Hightower (North-Holland, Amsterdam, 1973). [4] E.L. Force and A.T. Bell, J. Catalysis 40 (197.5) 356. [S] G. Rovida, F. Pratesi and E. Ferroni, J. Catalysis 41 (1976) 140. [6] C. Wagner, Z. Physik. Chem. B32 (1936) 447. [7] M. Lazzari, Electrochem. Metallorum 4 (1967) 453. [8] R. Wilken and E. Menzel, Z. Naturforsch. 28a (1973) 1914. [9] C.E.K. Mees and T.H. James, in: The Theory of the Photographic Process, 3rd ed. (MacMillan, New York, 1966). [lo] D.R. White, Ed., SPSE Symp. on Physics and Chemistry of the Silver Halide Crystal, Montreal, 1972. [ 1 l] G. Rovida, F. Pratesi, M. Maglietta and E. Ferroni, Japan. J. Appl. Phys., Suppl. 2, Part 2 (1974) 117. [ 121 G. Rovida and F. Pratesi, Surface Sci. 51 (1975) 270. [13] E. Zanazzi, F. Jona, D.W. Jepsen and P.M. Marcus, Phys. Rev. B14 (1976) 432. [14] P.J. Goddard and R.M. Lambert, Surface Sci. (1977) 180. [15] R.A. Marbrow and R.M. Lambert, Surface Sci. 71 (1978) 107. [ 161 Y.-Y. Tu and J.M. Blakely, Rev. Sci. Instr. 47 (1976) 1554. [ 171 A. Clark, The Theory of Adsorption and Catalysis (Academic Press, 1970). [ 181 P.J. Estrup, in: The Structure and Chemistry of Solid Surfaces, Ed. G.A. Somorjai (Wiley, New York, 1969) p. 19-1. [19] F. Delamare, Compt. Rend. (Paris) C275) (1972) 7.53. [20] A. Ignatiev, F. Jona, D.W. Jepsen and P.M. Marcus, Surface Sci. 40 (1973) 439. [21] M. Van Hove, S.Y. Tong and N. Stoner, Surface Sci. 54 (1976) 259. [22] B.A. Hutchins, T.N. Rhodin and J.E. Demuth, Surface Sci. 54 (1976) 419. [23] S. Andersson and J.B. Pendry, I. Phys. C6 (1973) 601. [24] S. Andersson and J.B. Pendry, J. Phys. C5 (1972) L41. [25] G.E. Laramore, C.B. Duke and N.O. Lipari, Phys. Rev. BlO (1974) 2246. [26] J.E. Demuth, D.W. Jepsen and P.M. Marcus, J. Phys. C8 (1975) L25. [27] S. Andersson and J.B. Pendry, Phys. Rev. Letters 31 (1973) 595. [28] P.M. Marcus, J.E. Demuth and D.W. Jepsen, Surface Sci. 53 (1975) 501. [29] J.E. Demuth, D.W. Jepsen and P.M. Marcus, Phys. Rev. Letters 31 (1973) 540. [30] C.B. Duke, N.O. Lipari and G.E. Laramore, J. Vacuum Sci. Technol. 11 (1974) 180. [ 3 1 ] J.E. Demuth, D.W. Jepsen and P.M. Marcus, Phys. Rev. Letters 32 (1974) 1182. [32] J.E. Demuth, D.W. Jepsen and P.M. Marcus, Surface Sci. 45 (1974) 733. [33] C.B. Duke, N.O. Lipari, G.E. Laramore and J.B. Theeten, Solid State Commun. 13 (1973) [34] [35] [36] [37] [38]
578. J.E. J.E. T.B. J.M. S.P.
Demuth, D.W. Jepsen Demuth, D.W. Jepsen Reed, in: Free Energy Burkstrand and G.G. Weeks and J.E. Rowe,
and P.M. Marcus, Solid State Commun. 13 (1973) 1311. and P.M. Marcus, J. Phys. C6 (1973) L307. of Formation of Binary Compounds (MIT Press, 1977). Tibbetts, Phys. Rev. B15 (1977) 5481. J. Vacuum Sci. Technol., to be published.
85 (1979) 276-288 Publishing Company
CHLORINE ADSORPTION
ON THE LOW INDEX SURFACES OF SILVER:
ENERGETICS AND STRUCTURES
Yung-Yi TU * and J.M. BLAKELY Materials Science and Engineering New York 14853. USA
Received
20 February
and ‘the Materials Science Center, Cornell University, Ithaca,
1979
The interaction of Cl2 with Ag single crystal surfaces has been studied over a range of crystal temperature and gas pressure. Observations have been made for the (11 l), (100) and (110) surfaces. Measured adsorption isobars for Ag(100) were used to obtain isosteric heats of adsorption; values ranged from - -60 kcal/mole (of Clz) at low coverage to - -70 k&/mole near saturation. The structure formed by Cl2 adsorption on (100) is believed to be a simple overlayer. For (110) and (111) the values obtained for the heat of adsorption were - -55 kcaI/ mole. On Ag(ll1) an epitaxially oriented AgCl(111) is believed to form.
1. Introduction
The purpose of this study has been to measure the heat of adsorption of Cl* on silver surfaces and to obtain information about chlorine adlayer structures. A brief account of the results for Ag( 111) has previously been published [ 11. This paper is concerned mainly with the results for Ag(ll0) and Ag(lOO). Understanding the interaction of Clz with silver surfaces may be essential in explaining the efficiency of Ag catalysts for the oxidation of ethylene [2-51, the chlorination (or oxidation) mechanism 161, and the formation of epitaxial A&l layers on Ag [7,8] ; the adsorption step may also be important in understanding the process of latent image formation in silver halide emulsions and the kinetics of decomposition and reconstitution of silver halides in photochromic materials [9,10]. Moreover, the interaction of Cl2 with Ag surfaces involves one of the more attractive systems for theoretical study. A number of studies have previously been made on the Cl*-Ag system but several questions still remain. Rovida et al. [11,12] used CzH4C12 as the reacting gas and studied the structure and thermal stabilities of the “chlorine” adlayer on Ag while Zanazzi et al. [13] have performed LEED intensity calculations for Cl on
* Now at IBM Research
Laboratories,
San Jose, California, 276
USA.
Y.-Y. Tu, J.M. Blakely / Cl, adsorption on Ag
271
Ag( 100). Other recent publications have dealt with adsorption-desorption characteristics and structural aspects of Cl2 on Ag(ll1) [ 141 and Ag(331) [ 151. Our studies (ref. [l] and this paper) emphasize the thermodynamic aspects. To the authors’ knowledge, there are no published data on the heat of adsorption of CIZ on Ag. 2. Experimental The important details have been described previously in connection with the results on Ag( 111) [ 11. Ali experiments were done under dynamic flow conditions with the chlorine coverage being monitored by Auger electron spectroscopy (AES) performed at temperature with the help of a blanking resistive heating arrangement
[161. For photographing low energy electron diffraction (LEED) patterns a Kodak Wratten filter [No. 58 (green)] was placed in front of the view port; this helped to block the light from the electron gun and filament and the resistively heated specimen. A measure of the chlorine concentration was obtained from the Auger peakto-peak height ratio in derivative spectra (i.e., dA@‘)/ti versus I?) of chlorine (LZ3M2aMZ3 transition at -185 eV) to silver (M4sW transition at -355 eV). Experimentally, adsorption isotherms or isobars were measured and isosteres were constructed from these. Application of the Clausius-Clapeyron equation to the adsorption isosteres gives the modelindependent, isosteric heat of adsorption, 4, at chlorine coverage 0 according to [ 171
with R the gas constant, p the Cl2 partial pressure, and T the absolute temperature. As defined by this equation, q is a negative quantity. From the thermodynamic data on the formation of bulk AgCl from the reaction of Cl* gas with pure silver, it is clear that the compound should ultimately form over much of the pressure and temperature range of these experiments. However, due to limited solid state mobility we believe that equilibrium between vapor and bulk is not achieved and that for the purpose of analyzing our observations we may disregard the migration into the bulk and treat the problem as one of surface-vapor equilibrium only.
3. Results 3.1. Cl? on Ag(llI) The results for this surface have been published in a previous paper [l] and will be briefly reviewed here so that we may compare them with results for other surfaces.
278
Y.-Y.
Tu, J.M. Blakely / Cl2 adsorption on Ag
An adsorption isobar was obtained for the temperature range from room temperature to -600°C. The low coverage portion of this isobar when fitted to the Langmuir model gave a value of the heat q - -50 to - -60 kcal/mole. The formation of an epitaxial AgCl( 111 j layer on the chlorine saturated Ag( 111 j surface was inferred from LEED observations. The LEED pattern from the chlorine saturated Ag(ll1 j surface could be generated through double diffraction of the incident electron beam from the Ag(ll1 j and the AgCl(ll1 j two-dimensional nets. In addition the diffuse background intensity greatly increased after chlorine was adsorbed on Ag(ll1 j as would be expected for a compound such as AgCl with a low Debye temperature. 3.2. C/a on Ag(ll0) Adsorption isotherms for this surface (fig, 1) were obtained using a cylindrical mirror analyzer over the pressure range from low9 to 10e6 Torr and the temperature range from 458 to 560°C. To within the accuracy of the measurement the isotherms were reversible with pressure cycling. However, the experimental uncertainties in this particular set of measurements are rather large due to the loss of gain, upon Cl* adsorption, of the electron multiplier of the cylindrical mirror electron analyser. Isosteres constructed from these isotherms are shown in fig. 2 for
I
0.5-
0 *
_;_--
____f-
_;---,/--
A
0.4-
I’ .o ‘0 cr i .P I”
&’ R-
0
“,‘I
0’
H/H -c
2. .*’ 8 ./ ’ .
:
.H
.,I’ IO1’ 8 rg‘0 -!____;-“-------
I x$ 0.2: II a S ”
/’
0.3~rp t
I
d
_---
/Y--_
,o_=-
XV’
v 0
.3-
8
+-0 El-/
on
P0
0
o
0
_/--
P
; --k--B I D
---
f
I, II Ax, j I
0
v __--
I
:
0 _---
q
,’ I,’ 0.1 -: ,$
__-*
--
F---a
Ag(llO)+Cl2
isotherms
A 45tFx
0 405OC 0 516OC o 560°C
ed : 0
0
50
1 100
1 150
4 -_JL-_ 1 200 400
, 600
600
IO’ x P (Torr)
Fig. 1. Chlorine adsorption isotherms on Ag(ll0). Each isotherm is reversible with pressure cycling. The pressure ranged over about three orders of magnitude.
Y.-Y. Tu, J.M. Blakely / Cl2 adsorption on Ag
Ag(ll0) + Cl,
10-g
I
I
I
1.2
1.3
1.4
279
-
103 T("K)
Fig. 2. Adsorption isosteres for Cl2 on Ag( 110). These isosteres are constructed from fig. 1 at three different Cl/Ag ratios (Cl/Ag = 0.18, 0.3 and 0.35). The slope, d(ln p)/d(l/T) is proportional to the heat of adsorption.
different values of the Cl : Ag Auger peak ratio. The scatter in the data does not allow any definite conclusions about the variation of heat of adsorption with coverage. The average value from fig. 2 is -55 + 6 kcal/mole. 3.3. CIZ on Ag(lO0) A set of reversible adsorption isobars for this surface, taken with a retarding field analyzer, are shown in fig. 3. The CIZ partial pressures are represented by the quadrupole gas analyzer current outputs. Isosters constructed from these isobars are shown in fig. 4. The uncertainties in determining the precise Cl? partial pressure are indicated. The biggest error bar is associated with the lowest partial pressure, which was close to the detection limit of our instrument (Varian Gas Analyzer VGA-loo, model 978-1000). Using the Clausius-Clapeyron relation, the heat of adsorption of Cl* on Ag(100) at the various coverages can be obtained and is shown in fig. 5. The coverage, 8, appearing in this figure is calculated on the assumption that for both chlorine and silver the Auger peak-to-peak heights are linear functions of 8; 8 = 1 is
280
Y.-Y.
Tu, J.M. Blakely
/ Cl, adsorption
on Ag
r g100 0 0
‘;
vv
GAo~
q
A oA&
0
V
0
,
A qo
I
A
v
V
0 q
0
v
h
V 0 V
A 0
2
q
A
0
F i &
x 16”amp
0
0
A
050-
isobors
1.3 x IO~,~omp 3 x IO amp 6 x 16’3amp
A-7.5
A0 0 A0 0
cr,o75t
i? ii
AgUOO) + Cl,
ACp o
V
A
0
025A
0 o
A
V O0
0
V
400
A
0
I
I
300
200
100
I
400
500
0
0
1
p
I
I
I
600
700
600
T(‘C) Fig. 3. Adsorption isobars for Cla on Ag(100). The chlorine the quadrupole mass spectrometer current output. The actual mately. 1 0e9 to 10e6 Torr.
partial pressure is represented by pressure range covered is approxi-
Ag(l00) 0 I
_
0 2 03 04 05 0.6 0.7
0.9
0.6
+ Cl,
isosteres
lo-“-
E” a 0)
5
2 F
-12 _
IO
a 0 t
a” 2-J IO
-IS_
0
Io-‘;,o
1
I
,
I
1.1
1.2
1.3
1.4
IO3
*
1.5
Tt K) Fig. 4. Adsorption isosteres for Cl2 adsorption on Ag(100). The error in the partial pressure measurement is indicated. Isosteres are constructed at 0.1 intervals of the Cl/Ag ratio as indicated on the figure.
281
Y.-Y. Tu, J.M. Blakely / Cl2 adsorption on Ag
Ag(100)+ Cl2 G
z
E 1 s i
-5o-6O-
.g z
-70.
g 2
-0o-
r:+:/ ,,,, 0
01
0.2
0.3
0.4
, ,, 0.5
Coverage Fig. 5. The heat of adsorption data in fig. 4.
0.6
0.7
0.0
09
IO
8
versus coverage curve for Cl2 on Ag(100) calculated from the
taken to correspond to the saturation coverage. There is some inducation of an increase in the magnitude of the heat of adsorption with Increasing coverage but again the size of the experimental error does not allow a definite conclusion to be drawn. A room temperature LEED pattern from clean Ag(lOO) is shown in fig. 6a and that from the chlorine saturated Ag(lOO) surface is shown in fig. 6b. Some of the possible real space atomic arrangements corresponding to the c(2 X 2) LEED pattern are shown in fig. 7. Figs. 7a, 7c and 7d are simple chlorine overlayers with chlorine atoms at different symmetrical sites while fig. 7b is a reconstructed adlayer with the AgCl(lOO) surface structure.
4. Discussion Zanazzi et al. [13] have calculated the intensity variation with energy of LEED beams from the various structures of fig. 7 and have compared the results with those obtained experimentally. The comparison favored the simple overlayer model with Cl atoms in the fourfold symmetric sites of fig. 7a. It should be remembered, however, that Zanazzi et al. [ 131 (and Rovida et al.) used C2H,C12 as the reacting gas. It is therefore not clear whether or not chlorine was the only chemical species on the Ag surfaces [ 11,121. Unfortunately our experimental I-V curves (i.e., fig. 8) were not obtained for the same incidence angles as that used in Zanazzi’s calculations and therefore cannot be compared with their published theoretical curves.
Y.-Y. Tu, J.M. Blakely / Cl, adsorption on Ag
Fig. 6. Rc ,0*, cow xage.
temperature
LEED
pictures
at 78 eV: (a) clean Ag(100);
(b) at sat uraf ion
Cl2
Ho\ revel r, v/e believe from other observations that the Clz structure prc)du ted on Ag( 100) is of the simple overlayer type depicted in fig. 7a. The positic ens of the inte :gral ord er diffracted beams are the same as for clean Ag surface and the be; lfnS of 1type (4,1) are visible over the energy range from -30 to -200 eV. Th is i,Sin
Y.-Y. Tu, J.M. Slakely / Clz adsorption on Ag
(cl
283
(4
Fig. 7. Schematic models of possible c(2 X 2) structures. (a) Simple overlayer with Cl in fourfold symmetrical sites. (b) Ag-Cl mixed layer. (c) Overlayer with Cl in bridge sites (one of two domains). (d) Overlayer with Cl in top sites. Shaded circles: chlorine atoms. Open circles: substrate Ag atoms.
contrast to the behavior on Ag(lI1) where, in the energy range from -70 to 100 eV, the pattern is a combination (with double diffraction effects) of the patterns expected from Ag(ll1) [with lattice constant a0 = 4.09 A] and from AgCl(111) [with lattice constant 5.55 A]. At low energies, 570 eV, the diffraction pattern is as expected from AgCl(111) while above -100 eV the pattern is that of Ag(ll1) with a.strong diffuse background. The temperature dependence of the (4, $) type beams on the Cl2 covered Ag(lOO) surface is as shown in fig. 9; the intensity falls off more rapidly with temperature than the exponential type of dependence. As pointed out by Estrup [18] this type of temperature dependence is expected for a layer exhibiting structural disorder at high temperature and should be typical of simple overlayer adsorption [ 191. It is interesting to note that most LEED studies of c(2 X 2) structures on fcc(100) metal surfaces have led to the conclusion that simple overlayers were formed with the adsorbed atoms in the 4-fold coordinated sites. Examples are Se on Ag(lOO) [20], Na on Al(100) [21,22], Na on Ni(lOO) [23-261, 0 on Ni(100) 127-301. S on Ni(100) [25,28-341, Se on Ni(lOO) [28,29], and Te on Ni(100) [28,29,35]. Information obtained from these experiments on the heat of adsorption of Clz on Ag(lOO) is compared with other relevant data [36] for this system in fig. 10. As noted from the diagram the heat of adsorption is comparable in magnitude to the heat of formation of the compound AgCl. This is typical of dissociative adsorption;
Y.-Y. Tu, J.M. Blakely / Cl, adsorption on Ag
284
500
400 G-
c :
(001Beam
0
--o--
Ag(100)
-
Ag(100)
+ Cl2
100 V(eV)
4c (oil
beam
I 01
---o---
AgflOO)
b
Ag(lO0)
El (oil 3c
0
b
r C 0 H 20
IO
b
+ Cl,
Y.-Y. Tu, J.M. Blakely / Cl2 adsorption on Ag
285
V (eV)
Ag (100) + Cl,
q (
‘4 \ .? ‘? \ fI
1
(T
TT
z)
born
---*---
.
Cl
C&j)
bnam-
1
Fig. 8. Normalized I- V curves for room temperature Clz/Ag(lOO) tive position of each beam is shown. The incident electron beam normal and rotated - 1 O” about a [ 1 lo] type axis.
and clean Ag(100). The relawas at -5’ from the surface
286 Ag(lCO) + Cl, (‘i-&j A
AV
beam heatmg
A v V
v A
V A
V V A v
A
V
I
I
25 --cl
I
450
350
*
550
T (‘K) Fig. 9. Temperature
variation
of (i,
i) beam intensity
from Cl2 on Ag(lOO).
2 Cl (gas)
0
Cl2 (gas) Heat of Formation of AgCI
Heat of Adsorption
-50 Itcal/mole (-2.2 eV/atoml
Ag +
-60 Kcal/mole (-2.6 eV/atomJ
Adsol .bed
-70 kcal/mole (-3.0 eV/atom)
Fig. 10. Summary of relevant thermodynamic C12-Ag(lOO) system. The values correspond
data (from ref. [36] to 1 mole of Cl2.
and present
work)
for the
Y.-Y. Tu, J.M. Blakely / Cl, adsorption on Ag
281
Vacuum Vacuum
0
Level
Affinity -4.6 eV
-13.1 Density
0
Leve I
Level
eV
3P
-3.6
3s
-13.0
eV
eV
of
States Cl Atom
Ag Metal
Fig. 11. Same electronic structural information for
Ag
metal [ 371and atomic chlorine.
the observation of Goddard et al. [14] that only Cl atoms appeared in the flash desorption spectrum is at least consistent with our conclusions. From the data on fig. 10 we see that the binding energy per chlorine atom on a silver surface has a value in the range of 53 to 63 kcal/mole or 2.3 to 2.7 eV/atom. This value may be compared with available information on the electronic structure of Ag metal and of an isolated Cl atom as indicated in fig. 11. Theoretical studies of the binding in this system do not seem to have been performed although considerable information is available on the electronic structure of Ag and of Cl adsorbed on Ag [38].
5. Conclusion The interactions of Cl* with silver surfaces of orientations (1 lo), (11 l), and (100) have been studied in some detail. Under near equilibrium conditions the structure formed in the chlorine saturated Ag(lOO) is considered to be that of a simple chlorine overlayer. This is in contrast to the situation on Ag(ll1) where an epitaxially oriented AgCl(111) layer is believed to form. From measurements of adsorption isobars or isotherms the heat of adsorption is found to be _ -60 kcal/ mole, relatively insensitive to the silver surface orientation. The heat of adsorption is also found to be nearly constant throughout the range of chlorine coverage.
Acknowledgement The work has been supported by the National Science Foundation (Grant No. DMR76-23537) and by the Materials Science Center at Cornell University.
Y.-Y. Tu, J.M. Blakely / Cl2 adsorption on Ag
288
References [l] Y.-Y. Tu and J.M. Blakely, J. Vacuum Sci. Technol. 15 (1978) 563. [2] H.H. Voge and CR. Adams, Advan. Catalysis 17 (1967) 151. [3] P.A. Kilty, NC. Rol and W.M.H. Sachtler, in: Catalysis, Ed. J.W. Hightower (North-Holland, Amsterdam, 1973). [4] E.L. Force and A.T. Bell, J. Catalysis 40 (197.5) 356. [S] G. Rovida, F. Pratesi and E. Ferroni, J. Catalysis 41 (1976) 140. [6] C. Wagner, Z. Physik. Chem. B32 (1936) 447. [7] M. Lazzari, Electrochem. Metallorum 4 (1967) 453. [8] R. Wilken and E. Menzel, Z. Naturforsch. 28a (1973) 1914. [9] C.E.K. Mees and T.H. James, in: The Theory of the Photographic Process, 3rd ed. (MacMillan, New York, 1966). [lo] D.R. White, Ed., SPSE Symp. on Physics and Chemistry of the Silver Halide Crystal, Montreal, 1972. [ 1 l] G. Rovida, F. Pratesi, M. Maglietta and E. Ferroni, Japan. J. Appl. Phys., Suppl. 2, Part 2 (1974) 117. [ 121 G. Rovida and F. Pratesi, Surface Sci. 51 (1975) 270. [13] E. Zanazzi, F. Jona, D.W. Jepsen and P.M. Marcus, Phys. Rev. B14 (1976) 432. [14] P.J. Goddard and R.M. Lambert, Surface Sci. (1977) 180. [15] R.A. Marbrow and R.M. Lambert, Surface Sci. 71 (1978) 107. [ 161 Y.-Y. Tu and J.M. Blakely, Rev. Sci. Instr. 47 (1976) 1554. [ 171 A. Clark, The Theory of Adsorption and Catalysis (Academic Press, 1970). [ 181 P.J. Estrup, in: The Structure and Chemistry of Solid Surfaces, Ed. G.A. Somorjai (Wiley, New York, 1969) p. 19-1. [19] F. Delamare, Compt. Rend. (Paris) C275) (1972) 7.53. [20] A. Ignatiev, F. Jona, D.W. Jepsen and P.M. Marcus, Surface Sci. 40 (1973) 439. [21] M. Van Hove, S.Y. Tong and N. Stoner, Surface Sci. 54 (1976) 259. [22] B.A. Hutchins, T.N. Rhodin and J.E. Demuth, Surface Sci. 54 (1976) 419. [23] S. Andersson and J.B. Pendry, I. Phys. C6 (1973) 601. [24] S. Andersson and J.B. Pendry, J. Phys. C5 (1972) L41. [25] G.E. Laramore, C.B. Duke and N.O. Lipari, Phys. Rev. BlO (1974) 2246. [26] J.E. Demuth, D.W. Jepsen and P.M. Marcus, J. Phys. C8 (1975) L25. [27] S. Andersson and J.B. Pendry, Phys. Rev. Letters 31 (1973) 595. [28] P.M. Marcus, J.E. Demuth and D.W. Jepsen, Surface Sci. 53 (1975) 501. [29] J.E. Demuth, D.W. Jepsen and P.M. Marcus, Phys. Rev. Letters 31 (1973) 540. [30] C.B. Duke, N.O. Lipari and G.E. Laramore, J. Vacuum Sci. Technol. 11 (1974) 180. [ 3 1 ] J.E. Demuth, D.W. Jepsen and P.M. Marcus, Phys. Rev. Letters 32 (1974) 1182. [32] J.E. Demuth, D.W. Jepsen and P.M. Marcus, Surface Sci. 45 (1974) 733. [33] C.B. Duke, N.O. Lipari, G.E. Laramore and J.B. Theeten, Solid State Commun. 13 (1973) [34] [35] [36] [37] [38]
578. J.E. J.E. T.B. J.M. S.P.
Demuth, D.W. Jepsen Demuth, D.W. Jepsen Reed, in: Free Energy Burkstrand and G.G. Weeks and J.E. Rowe,
and P.M. Marcus, Solid State Commun. 13 (1973) 1311. and P.M. Marcus, J. Phys. C6 (1973) L307. of Formation of Binary Compounds (MIT Press, 1977). Tibbetts, Phys. Rev. B15 (1977) 5481. J. Vacuum Sci. Technol., to be published.