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Ellipsometry-LEED study of oxygen adsorption and the carbon monoxide- oxygen interaction on Ag(110)
Surface Science 0 North-Holland
77 (1978) 1-13 Publishing Company
ELLIPSOMETRY-LEED CARBON MONOXIDE-
STUDY OF OXYGEN ADSORPTION OXYGEN INTERACTION
AND THE
ON Ag(ll0)
H. ALBERS, W.J.J. VAN DER WAL, O.L.J. GIJZEMAN and G.A. BOOTSMA Van ‘t Hoff Laboratory, Received
University of Utrecht, Padualaan
8, Utrecht,
The Netherlands
25 April 1978
The adsorption of oxygen on Ag(ll0) has been studied by ellipsometry and LEED. Oxygen pressures varied between lop6 and 5 X 10e5 Torr and the crystal temperature between room temperature and 200°C. The change in the ellipsometric parameter A was found to be proportional to the oxygen coverage derived from (n X 1) superstructures in LEED. Initial sticking coefficients are about 5 X 10W4 and the maximum coverage is 0.5 0 atom/Ag atom. The reaction of CO with adsorbed oxygen was studied between room temperature and 200°C at CO pressures between 10h7 and 4 X 106 Torr. The decrease in oxygen coverage is slow at high oxygen coverages and accelerates at lower coverages. Two distinct rate constants may be derived from a simple model, each proportional to the CO pressure. LEED evidence points to considerable freedom of motion for O-atoms along the troughs of the Ag(l10) face. Above 200°C oxygen desorption takes place. Ellipsometric measurements indicate that at these temperatures either a surface rearrangement takes place, or that the bonding character of oxygen to the substrate changes. Isosteric heats of adsorption were estimated for crystal tenperatures between 240 and 340°C.
1. Introduction
The adsorption of oxygen on silver is an example of a structure-sensitive reaction, i.e. both the adsorbed amount and the adsorption kinetics strongly depend on the crystallographic orientation of the surface. Because of the catalytic interest, especially in the oxidation of ethylene, the nature and reactivity of the adsorbed oxygen has been the subject of many studies and speculations [ 11. Rovida et al. have studied the chemisorption of oxygen on Ag(l11) [2] and Ag(ll0) [3,4], by using mainly LEED-AES and thermal desorption. At oxygen pressures from 10v3 to 10-l Torr and crystal temperatures up to 250°C they observed formation of a (2 X 1) structure in a few minutes, accompanied by an increase in work function (A@ = 0.8 eV). A (3 X 1) structure, and in some cases also a (4 X 1) structure, was observed by thermal degradation of the (2 X 1) structure. Rovida et al. [3] propose a structural model for Ag(1 lo)-2 X 1-O with silver and oxygen atoms in a mixed adsorption layer, a two-dimensional silver oxide with a coverage of 0 = 1 (1 0 atom/surface Ag atom). In contrast with this rigid struc1
2
H. Alhers et al. /Bllipsometry-LEED
study of oxygen adsorption
ture the (3 X 1) and (4 X I) structures were supposed to represent unreconstructed layers, which disorder around 15O”C, a temperature significantly below the desorption temperature (>ZOO”C). Engelhardt and Menzel [5] have investigated the adsorption of oxygen on Ag(ll0). (111) and (100) surfaces by LEED. AES and work function measurements at oxygen pressures up to 10e5 Torr. For Ag(ll1) very small and nonreproducible effects were observed and for Ag(lOO) disordered adsorption with work function changes differing in sign depending on the temperature. The (1 10) face showed only an increase in work function upon adsorption and the formation of a consecutive series of (I2 X 1) superstructures with n running from 7 to 2. The sticking coefficient, determined from A@, was found to drop sharply with coverage and not to depend strongly on the surface temperature (initial value at saturation with the room temperature se = 3 X 10-3). At room temperature, (2 X 1) structure was achieved after an exposure 22.5 X lo4 Langmuir (L), with A@ = 0.85 eV. In contrast with Rovida et al. Engelhardt and Menzel prefer to interpret the (2 X 1) structure by a model which assumes 0 atoms to be adsorbed in the troughs between two row atoms and in bridge position on top of two atoms in the bottom of the trough, as proposed by Heiland et al. on the basis of ion scattering spectrometry [6]. According to this model. 0 = 0.5 for the (2 X 1) structure, and in general 0 = l/t7 for the (n X 1) structures. In a recent preliminary account of a LEED intensity analysis of the Ag(1 IO)-2 X 1-O structure, prepared at about 200°C and 10m3 Torr, Zanazzi et al. [7] also indicate a preference for the model proposed by Heiland et al. [6], but a definite conclusion cannot yet be drawn. In a previous paper [8] we have presented results obtained with ellipsometry in conjunction with LEED and AES for oxygen adsorption and the carbon monoxide-oxygen interaction on Ag( 1 1 1). On a well-annealed (1 11) surface the amount of adsorbed oxygen appeared to be very small and to increase with increasing damage introduced by ion bombardment. Ellipsometry may be used for monitoring in situ the oxygen coverage at pressures exceeding lop5 Torr and also appeared to be suitable for the evaluation of the extent of damage produced by ion bombardment [9]. In this paper results for the chemisorption of oxygen and the interaction of adsorbed oxygen with carbon monoxide on Ag(ll0) are presented and compared with those obtained for the (111) face.
2. Experimental The experiments were performed in the UHV system provided with LEED and ellipsometry described in ref. [9]. The procedures were essentially as described previously [8,9]. A disc-shaped crystal. spark-cut to within lo of the (110) orientation from a 6N purity silver rod (Metals Research Ltd.) was ground, electro-lappolished and mounted in the sample holder. Cleaning was achieved by sputtering (room temperature, 15-30 min, 500 eV
H. Albers et al. / Elli~sometr~-BEEP
study of oxygen adsorption
3
argon ions, 1 PA/cm*) and annealing to 500°C. To avoid unintentional interaction with CO the precautions described in ref. [8] were taken, i.e. continuous renewal of oxygen, exclusion of hot filaments as far as possible, renewal of titanium film. The changes in the ellipsometric angles, 6A = n - A and S$ = 7 - jt (At $: clean surface), were mostly determined with off-null irradiance measuren~ents at an angle of incidence (p, = 71’ and wavelength h = 632.8 nm. For comparison with the literature, changes in work function due to adsorption were measured with the diffraction equipment as suggested by Lander and Morrison [ 101 and applied by MacRae [ 1 l] and by the constructor of our vacuum system [ 121. The work function was derived from that voltage, between electron gun cathode and sample, at which first order diffraction beams leave the crystal nearly parallel to the surface. By applying a positive voltage on the first grid of the analyser these diffraction beams became visible on the screen at normal incidence of the primary beam [12]. A spot photometer was focussed on the point of appearance of a diffracted beam and by automatically sweeping the voltage between cathode and sample an intensity versus voltage graph was recorded. The work function change upon adsorption was derived from the shift of this curve along the voltage axis.
3. Results and discussion 3.1. Cali~r~ti~~i~ of the oxygen coverage The occurrence of the consecutive series of (n X 1) superstructures offers the possibility of calibrating the changes in the ellipsometric parameters. Fig. 1 shows the changes in 9, and A observed upon exposure to oxygen at pressures up to 4 X 10e5 Torr, with the crystal at room temperature. The (3 X 1) and (2 X 1) structures could be observed visually and the (5 X 1) structure was established with the aid of a microdensitometer. AS the work function results compare reasonably well with those of Engelhardt and Menzel [S], the existence of higher order patterns was not further investigated. in our case, at exposures of about 6000 L the (2 X 1) structure is clearly visible, in agreement with ref. [6j. At higher exposures the elhpsometric parameter &A becomes virtually constant. Engelhardt and Menzel I.51 reported a continuous increase in work function up to 2.5 X lo4 L and remarked that it was difficult to obtain the (2 X 1) structure fully developed. Assuming that a (n X 1) structure corresponds to an oxygen coverage 6 = l/n, within experinlental accuracy, linear relationships are obtained (fig. 2): 0 = (0.92 C 0.05) 6A ,
(1)
0 = (0.58 ? 0.05) A$.
(2)
4
H. Albers et al. / E~~i~so~~try-~EE~
study of oxygen adso~~t~o~
6A ideg
A@ IeV)
--
-4
;O_ 0
0.5-
__---
1
.L
/)
/#/’
2
I
f
1
/
I 0
I 2000
I
I LOO0
+J exposure
Fig. 1. Changes in work function (o) and A (0) for various room temperature. The dotted line is taken from ref. [5].
exposures
30000* (L)
of oxygen
to Ag( 110) at
113
I I
A@
6A 6 (deg)
leVl
0.5 -
0
1
oc 2
-
$ coverage
(8)
Fig. 2. Work function changes (A@) and ellipsometric parameter changes (6A) as a function of oxygen coverage on Ag(l10) as derived from LEED patterns. For the (3 X 1) structure the error bar indicates the range of 6A for which this structure was visible, for the (2 X 1) structure the error in SA observed
upon
saturation.
H. Albers et al. / Ellipsometry-LEED study of oxygen adsorption
5
It may be remarked that from combined ellipsometric and AES measurements for oxygen on Cu( 111) [ 131 and Cu( 110) [ 141 linear relationships between 6A and 0 were obtained up to the saturation coverages (6 = 0.5). In this paper such a relationship will also be assumed (eq. (1)). The change in the other ellipsometric parameter appeared to be smaller. 3.2. Kinetics of oxygen adsorption The rate of oxygen adsorption was studied at crystal temperatures between 294 and 47.5 K and pressures in the range 10P6-5 X lo-’ Torr. The gas was always admitted at room temperature. At all temperatures the initial rate of adsorption was proportional to the gas pressure, as illustrated in fig. 3. Initial sticking coefficients, derived from these data are given in table 1. As can be seen, the sticking coefficient decreases with increasing temperature, s 0: exp(E/RT) with E = 0.6 + 0.1 kcal/mol. The adsorption curves can be reasonably described by an equation of the type de/dt = b(B,,,
- 0))
(3)
with emax independent of temperature in the range studied and equal to about 0.5 as judged from the (2 X 1) LEED pattern. Fig. 4 shows the fit of the integrated form of eq. (3) to some experimentally observed curves.
d(6Al dt (10-3deg,s)
Fig. 3. The initial rate of oxygen temperature, (A) at 469 K.
adsorption (d(dA)/dt) versus oxygen
pressure:
(0) room
6
H. Aibers et al. / ElIipsornefry-LARD
study of oxygen ~d~or~~~~ll
Table 1 initial sticking coefficients for oxygen on Ag(1 i(l)
TW So x 104
294 5.9
362 4.9
314 4.7
381 4.4
426 4.8
469 3.9
475 4.1 --.
Our findings differ in some respects from those obtained by Engelhardt and Menzel [S]. At room temperature the sticking coefficient is about 6 times lower and it shows less variation with temperature (0.6 kcal/mol versus 2.0 kcal/mol). Also the shape of the adsorption curves seems to be different~ since the previous authors used a different, and a more complicated, expression for the dependence of the sticking coefficient on coverage. At low coverages, up to 8 N 0.4 B,,,, the two expressions can be made to match within experimental error. At higher coverages, however, discrepancies appear. These may be traced back to the experimental differences shown in fig. 1 and discussed in section 3.1. It should be remarked that eq. (3) may be derived for dissociative adsorption with large attractive interactions between adsorbed atoms (cf. ref. [I 51).
-In
---EE-
t
154
Fig. 4. Test of eq. (3) at oxygen pressures of 6 X lo- 5 Torr (n), 2.4 x lo-’ low6 Torr (a) (room temperature).
Torr (a) and 3.5 X
I
H. Albers et al. / Ellipsometry-LEED study of oxygen adsorption
3.3. Reaction with CO The reaction of adsorbed oxygen with gaseous CO was studied in the temperature range 294-475 K and at CO pressures between lop7 and 4 X 10e6 Torr. Complicated reaction kinetics were found in all cases as illustrated in fig. 5. The decrease in oxygen coverage is slow in the first stage of the reaction and accelerates at later times, in agreement with results obtained by Engelhardt et al. [5]. This behaviour could be due to attractive (effective) interactions between adsorbed oxygen atoms, which facilitate the removal of “lone oxygen atoms” at lower coverages. Since LEED patterns taken during the CO reaction show disorder mainly in the direction along the troughs on the (110) face (section 3.4) a simple one-dimensional model for this reaction may be devised. Let Nrr be the number of occupied nearest neighbour site pairs and N,, the number of pairs of sites that are occupied by one atom. If the pairwise interaction energy is w,M the total number of sites, N the total number of adsorbed atoms and f3 =N/M, the most probable distribution of Nor is, assuming thermodynamic equilibrium [ 161: (4) /3= [l-40(1
-tI){l
2N= 2N,r +N,,
= - k2N;,
(5)
.
(6)
One may now take the rate equation tifdt
,
-exp(-o/kT))]“*
-
2k,NTL
to be
,
(7)
where a different rate constant has been used for the removal of different types of oxygen atoms. Eqs. (4)-(7) cannot be solved analytically except for o = 0. The
6AIdeg)
0.6 -
0 0
1 100
200
t Fig. 5. Change in 6A as function of time after admission 3.7 X lop7 Torr; (b) 10K6 Torr; (c) 1.9 X lop6 Torr.
(set)
of CO at room temperature:
(a) pco
=
H. Albers et al. / Ellipsornetry-LEED
study of oxygen adsorption
t lsec) eq. (8) to experimentally observed reaction (o) 294 K, pco = 8 X 10d7 Torr, kl = 2 X 10e3
culated: IO-‘Torr,
k, =6.4
solution
X 10e4
curves. S-l,
Drawn
K = 5;
(0)
lines have been cal475 K, pco = 3 X
SC’, E(= 10.
is then given by: .--.. .-!-_ (K ~ 1) 80 + (K - K@o + 8,) eXp(2
0 = fjO where
K =
k2fj
(8)
’
k2/kl and O,, the oxygen coverage at zero time.
Eq. (8) shows qualitatively the observed behaviour with time (see fig. 6), although the tail of the curve falls off too slowly, which may be due to the neglect d(&A) dt (10-2deg /sl
F&S. 7. T]lc
pressure
initial
at room
(A) and “midway”slopes
temperature.
PC0 ilO-DTorrb
(0) of curves
as shown
in fig. 5 as function
of to
H. Albers et al. / Ellipsometry-LEED study of oxygen adsorption
9
t lsec) Fig. 8. Adsorption curves for mixed Oz/CO exposures at room temperature. The right hand side shows the desorption reaction from the partially saturated surface (CO pressure -lop6 Torr).
of interatomic interactions. Since their inclusion would introduce an unwarranted amount of computation and too many parameters, an exact fit has not been attempted. Experimentally one observes that the initial and “midway” slopes of the curves are proportional to the CO pressure (fig. 7). In terms of our model these slopes are given by -2kr and -kz/2(kz - k,) respectively. Taking kt = k:pco we obtain for room temperature ky = 0.3 X lo4 s-l Torr-’ and k(: = 2.7 X lo4 s-l Torr-‘, which implies that the reaction is faster if at least one neighbour is missing. It may be remarked that these numbers are different from those used in fig. 6, since in that case the line was constrained to go through the experimental points instead of having the correct midway slope. Consequently the values of ky and ki can only be considered to be order of magnitude estimates. For the temperature dependencies of the experimental slopes, kp a exp(E/RT), we get E = t 0.8 + 0.1 kcal/mol for the initial process and E = -2.1 f 0.4 kcal/mol for the later reaction. This indicates a nonactivated reaction on a pair of occupied sites and an activated reaction if the site pair is only half occupied. As can be expected, an exposure to CO/O* mixtures results in a lower steady state coverage (fig. 8). The form of the CO reaction curve with decreasing amount of adsorbed oxygen is in qualitative agreement with eq. (8), which shows that for Be + 0 the kinetics change into a simple exponential. In summary, the overall agreement can be considered satisfactory, taking into account the approximate nature of the description of the CO reaction. 3.4. LEED experiments Upon introducing a low but constant pressure of CO to a fully developed (2 X 1) structure at room temperature this structure slowly transformed into that of the
T?g. 9. LEED patterns obtained during reaction with CO (-50 eV): (a), (b), (c) at room temperature after 0, 36 and 62 min respectively; (d), (e), (t) at 393 K after 50, 131 and 204 min respectively. The CO pressure was of order 10e8 Torr in both cases.
clean Ag(1 IO) surface. At first streaks in the h direction developed, followed by a much lesser degree of disorder in the k direction. No clearly defined (3 X I) pattern could be seen (fig. 9 a-c). The same experiment at 120°C revealed only increasing disorder in the h direction with increasing CO exposure and additionally showed the characteristic splitting of the (1/2k) superspots into a cIearly defined (3 X 1) structure, cf. refs. [ I’?,1 S]. This is illustrated in fig. 9 d-f. At temperatures above 200°C oxygen desorption takes place, which also proceeds via increasing disorder in the h direction. This could be observed visually via the disappearance of the (2 X 1) structure which was formed up to 270°C. At higher temperatures, desorption was too fast to allow LEED patterns to be taken after evacuation of the sample chamber. The foregoing results indicate that, at least above 120°C. the mobility of the adsorbed oxygen atoms along the troughs of the silver (1 IO) face is high enough for rearrangement to the (3 X 1) structure. 3.5. oxygen
adsorption is0 therms
Adsorption isot~~erms were detcrlllined at six temperatures between 570 and 610 K and at oxygen pressures between 10d6 and IO-3 Torr. The coverage was recorded via the cllipsometric parameter 6A. Results are shown in fig. IO. The saturation value of sA at 542 K is -0.65”, which is significantly higher than the value of 0.45’ below 200°C. However. a clear (2 X 1) structure could still be seen. This implies that either surface reconstruction has taken place with a higher oxygen coverage, or that the oxygen-silver bond has changed, with a coilcoi~ii~allt
H. Albers et al. / Ellipsometry-LEED study of oxygen adswtio@
11
change in poiarisability of the adlayer, leading to a different relationship between B and 6~3. Assuming the same reIation between SA and 6 to hold at all temperatures, isosteric heats of adsorption, AHis<>, can be obtained. Results are plotted in fig. 11.
607
Fig. 1 I.
Isosterb heat of adsorption
expressed as 68.
(Af&)
of oxygen
on A&l IO> as
functionOF coverage,
12
1% Alhers et al. / Ellipsometry-LEED
study of oxygen adsorption
It can be seen that the values increase about linearly with increasing coverage: c\Hiso (kcal/mol) = 33 + 33 &A, or with eq. (I): nisi,, = 33 + 35 8. Similar results have been reported by Engelhardt and Menzel [5,19]. The increase in nisi,, with coverage again indicates the existence of attractive interactions between oxygen atorns. Using the model proposed by Guggenheim [ 161, one gets for the pairwise interaction energy a value of 17 kcal/mol. which is a rather high value. It thus seems likely that a more complicated model must be used, which includes other interactions besides those between nearest neighbours. Alternatively, of course, if the relation between 6A and 0 changes with the temperature, no meaningful conclusions can be drawn from fig. 10 as it stands.
4. Conclusions
and comparison
with Ag(ll1)
(i) In contrast with Ag( 111) oxygen is readily adsorbed on Ag( 110) with initial sticking coefficients of about 5 X 10m4 between room temperature and 200°C and a saturation coverage tImax = 0.5. For a well-annealed Ag( 11 1) surface the corresponding values were 3 X lo-’ and emax = 0.03. (ii) The reaction of oxygen adsorbed on Ag( 1 IO) with CO proceeds initially slowly at higher oxygen coverages and accelerates at lower coverages. This can be rationalized by assuming the removal of both “row 0 atoms” and “terminal 0 atoms” with different rate constants. On a damaged Ag(l 11) surface the amount of adsorbed oxygen decreases exponentially with time with a time constant of 8 X lo4 sP1 (Torr CO))’ at room temperature. For low coverages on Ag( 110). an exponential decrease is also observed, time constant 5.4 X lo4 s-’ (Torr CO))‘. Evidently for the terminal 0 atoms on Ag(ll0) and the 0 atoms adsorbed on damaged Ag(l1 1) the reaction probability (number of oxygen atoms removed per hitting CO molecule) is comparable.
[ 11 I:or a recent review, cf. H. Albers, Thesis, Univ. of Utrecht (1978) ch. 2. [ 21 G. Rovida, I’. Pratesi, M. Maglietta and E. Ferroni, Surface Sci. 43 (1974) 230. [ 31 G. Rovida and F. Pratesi. Surface Sci. 52 (1975) 542. [4] G. Rovida. J. Phys. Chem. 80 (1976) 150. [5] 1l.A. Ikgelhardt and D. Menzcl. Surface Sci. 57 (1976) 591; I1.A. Engelhardt, A.M. Bradshaw and D. Menzel, Surface Sci. 40 (1973) 410; H.A. Engelhardt, Thesis, Technische Univ., Miinchen (1975). (61 W. Heiland, I;. Iberl, E. Taglauer and D. Menzel, Surface Sci. 53 (1975) 383. [7] E. Zanazzi, M. Maglietta, U. Bardi, F. Jona, D.W. Jepsen and P.M. Marcus, in: Proc. 7th Intern, Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces (Vienna, 1977) p. 2447. 181 H. Albers, W.J.J. van der Wal and C.A. Bootsma, Surface Sci. 6X (1977) 47. [91 Il. Albcrs, J.M.M. Droog and G.A. Bootsma, Surface Sci. 64 (1977) 1. [ 101 J.J. Lander and J. Morrison, 1. Appl. Phys. 34 (1963) 3517.
H. Albers et al. / Ellipsometry-LEED
study of oxygen adsorption
13
[ 1 l] A.U. MacRae, Surface Sci. 1 (1964) 3 19. [ 121 L. Bakker, Thesis, Technical Univ., Delft (1972). [ 131 F.H.P.M. Habraken, E.P. Kieffer and G.A. Bootsma, in: Proc. 7th Intern. Vacuum Cow. and 3rd Intern. Conf. on Solid Surfaces (Vienna, 1977) p. 877. [14] F.H.P.M. Habraken and G.A. Bootsma, Ned. Tijdschrift voor Vacuum techniek 16 (1978) 142. [15] D.A. King and M.G. Wells, Proc. Roy. Sot. (London) A339 (1974) 245. [ 161 T.L. Hill, Introduction to Statistical Thermodynamics (Addison-Wesley, London, 1960) ch. 14. [ 17) J.E. Houston and R.L. Park, Surface Sci. 21 (1970) 209. [18] G. Ertl and J. Kiippers, Surface Sci. 21 (1970) 61. [19] D. Menzel, in: The Physical Basis for Heterogeneous Catalysis, Eds. E. Drauglis and R.J. Jaffee (Plenum, New York, 1975) p. 437.
77 (1978) 1-13 Publishing Company
ELLIPSOMETRY-LEED CARBON MONOXIDE-
STUDY OF OXYGEN ADSORPTION OXYGEN INTERACTION
AND THE
ON Ag(ll0)
H. ALBERS, W.J.J. VAN DER WAL, O.L.J. GIJZEMAN and G.A. BOOTSMA Van ‘t Hoff Laboratory, Received
University of Utrecht, Padualaan
8, Utrecht,
The Netherlands
25 April 1978
The adsorption of oxygen on Ag(ll0) has been studied by ellipsometry and LEED. Oxygen pressures varied between lop6 and 5 X 10e5 Torr and the crystal temperature between room temperature and 200°C. The change in the ellipsometric parameter A was found to be proportional to the oxygen coverage derived from (n X 1) superstructures in LEED. Initial sticking coefficients are about 5 X 10W4 and the maximum coverage is 0.5 0 atom/Ag atom. The reaction of CO with adsorbed oxygen was studied between room temperature and 200°C at CO pressures between 10h7 and 4 X 106 Torr. The decrease in oxygen coverage is slow at high oxygen coverages and accelerates at lower coverages. Two distinct rate constants may be derived from a simple model, each proportional to the CO pressure. LEED evidence points to considerable freedom of motion for O-atoms along the troughs of the Ag(l10) face. Above 200°C oxygen desorption takes place. Ellipsometric measurements indicate that at these temperatures either a surface rearrangement takes place, or that the bonding character of oxygen to the substrate changes. Isosteric heats of adsorption were estimated for crystal tenperatures between 240 and 340°C.
1. Introduction
The adsorption of oxygen on silver is an example of a structure-sensitive reaction, i.e. both the adsorbed amount and the adsorption kinetics strongly depend on the crystallographic orientation of the surface. Because of the catalytic interest, especially in the oxidation of ethylene, the nature and reactivity of the adsorbed oxygen has been the subject of many studies and speculations [ 11. Rovida et al. have studied the chemisorption of oxygen on Ag(l11) [2] and Ag(ll0) [3,4], by using mainly LEED-AES and thermal desorption. At oxygen pressures from 10v3 to 10-l Torr and crystal temperatures up to 250°C they observed formation of a (2 X 1) structure in a few minutes, accompanied by an increase in work function (A@ = 0.8 eV). A (3 X 1) structure, and in some cases also a (4 X 1) structure, was observed by thermal degradation of the (2 X 1) structure. Rovida et al. [3] propose a structural model for Ag(1 lo)-2 X 1-O with silver and oxygen atoms in a mixed adsorption layer, a two-dimensional silver oxide with a coverage of 0 = 1 (1 0 atom/surface Ag atom). In contrast with this rigid struc1
2
H. Alhers et al. /Bllipsometry-LEED
study of oxygen adsorption
ture the (3 X 1) and (4 X I) structures were supposed to represent unreconstructed layers, which disorder around 15O”C, a temperature significantly below the desorption temperature (>ZOO”C). Engelhardt and Menzel [5] have investigated the adsorption of oxygen on Ag(ll0). (111) and (100) surfaces by LEED. AES and work function measurements at oxygen pressures up to 10e5 Torr. For Ag(ll1) very small and nonreproducible effects were observed and for Ag(lOO) disordered adsorption with work function changes differing in sign depending on the temperature. The (1 10) face showed only an increase in work function upon adsorption and the formation of a consecutive series of (I2 X 1) superstructures with n running from 7 to 2. The sticking coefficient, determined from A@, was found to drop sharply with coverage and not to depend strongly on the surface temperature (initial value at saturation with the room temperature se = 3 X 10-3). At room temperature, (2 X 1) structure was achieved after an exposure 22.5 X lo4 Langmuir (L), with A@ = 0.85 eV. In contrast with Rovida et al. Engelhardt and Menzel prefer to interpret the (2 X 1) structure by a model which assumes 0 atoms to be adsorbed in the troughs between two row atoms and in bridge position on top of two atoms in the bottom of the trough, as proposed by Heiland et al. on the basis of ion scattering spectrometry [6]. According to this model. 0 = 0.5 for the (2 X 1) structure, and in general 0 = l/t7 for the (n X 1) structures. In a recent preliminary account of a LEED intensity analysis of the Ag(1 IO)-2 X 1-O structure, prepared at about 200°C and 10m3 Torr, Zanazzi et al. [7] also indicate a preference for the model proposed by Heiland et al. [6], but a definite conclusion cannot yet be drawn. In a previous paper [8] we have presented results obtained with ellipsometry in conjunction with LEED and AES for oxygen adsorption and the carbon monoxide-oxygen interaction on Ag( 1 1 1). On a well-annealed (1 11) surface the amount of adsorbed oxygen appeared to be very small and to increase with increasing damage introduced by ion bombardment. Ellipsometry may be used for monitoring in situ the oxygen coverage at pressures exceeding lop5 Torr and also appeared to be suitable for the evaluation of the extent of damage produced by ion bombardment [9]. In this paper results for the chemisorption of oxygen and the interaction of adsorbed oxygen with carbon monoxide on Ag(ll0) are presented and compared with those obtained for the (111) face.
2. Experimental The experiments were performed in the UHV system provided with LEED and ellipsometry described in ref. [9]. The procedures were essentially as described previously [8,9]. A disc-shaped crystal. spark-cut to within lo of the (110) orientation from a 6N purity silver rod (Metals Research Ltd.) was ground, electro-lappolished and mounted in the sample holder. Cleaning was achieved by sputtering (room temperature, 15-30 min, 500 eV
H. Albers et al. / Elli~sometr~-BEEP
study of oxygen adsorption
3
argon ions, 1 PA/cm*) and annealing to 500°C. To avoid unintentional interaction with CO the precautions described in ref. [8] were taken, i.e. continuous renewal of oxygen, exclusion of hot filaments as far as possible, renewal of titanium film. The changes in the ellipsometric angles, 6A = n - A and S$ = 7 - jt (At $: clean surface), were mostly determined with off-null irradiance measuren~ents at an angle of incidence (p, = 71’ and wavelength h = 632.8 nm. For comparison with the literature, changes in work function due to adsorption were measured with the diffraction equipment as suggested by Lander and Morrison [ 101 and applied by MacRae [ 1 l] and by the constructor of our vacuum system [ 121. The work function was derived from that voltage, between electron gun cathode and sample, at which first order diffraction beams leave the crystal nearly parallel to the surface. By applying a positive voltage on the first grid of the analyser these diffraction beams became visible on the screen at normal incidence of the primary beam [12]. A spot photometer was focussed on the point of appearance of a diffracted beam and by automatically sweeping the voltage between cathode and sample an intensity versus voltage graph was recorded. The work function change upon adsorption was derived from the shift of this curve along the voltage axis.
3. Results and discussion 3.1. Cali~r~ti~~i~ of the oxygen coverage The occurrence of the consecutive series of (n X 1) superstructures offers the possibility of calibrating the changes in the ellipsometric parameters. Fig. 1 shows the changes in 9, and A observed upon exposure to oxygen at pressures up to 4 X 10e5 Torr, with the crystal at room temperature. The (3 X 1) and (2 X 1) structures could be observed visually and the (5 X 1) structure was established with the aid of a microdensitometer. AS the work function results compare reasonably well with those of Engelhardt and Menzel [S], the existence of higher order patterns was not further investigated. in our case, at exposures of about 6000 L the (2 X 1) structure is clearly visible, in agreement with ref. [6j. At higher exposures the elhpsometric parameter &A becomes virtually constant. Engelhardt and Menzel I.51 reported a continuous increase in work function up to 2.5 X lo4 L and remarked that it was difficult to obtain the (2 X 1) structure fully developed. Assuming that a (n X 1) structure corresponds to an oxygen coverage 6 = l/n, within experinlental accuracy, linear relationships are obtained (fig. 2): 0 = (0.92 C 0.05) 6A ,
(1)
0 = (0.58 ? 0.05) A$.
(2)
4
H. Albers et al. / E~~i~so~~try-~EE~
study of oxygen adso~~t~o~
6A ideg
A@ IeV)
--
-4
;O_ 0
0.5-
__---
1
.L
/)
/#/’
2
I
f
1
/
I 0
I 2000
I
I LOO0
+J exposure
Fig. 1. Changes in work function (o) and A (0) for various room temperature. The dotted line is taken from ref. [5].
exposures
30000* (L)
of oxygen
to Ag( 110) at
113
I I
A@
6A 6 (deg)
leVl
0.5 -
0
1
oc 2
-
$ coverage
(8)
Fig. 2. Work function changes (A@) and ellipsometric parameter changes (6A) as a function of oxygen coverage on Ag(l10) as derived from LEED patterns. For the (3 X 1) structure the error bar indicates the range of 6A for which this structure was visible, for the (2 X 1) structure the error in SA observed
upon
saturation.
H. Albers et al. / Ellipsometry-LEED study of oxygen adsorption
5
It may be remarked that from combined ellipsometric and AES measurements for oxygen on Cu( 111) [ 131 and Cu( 110) [ 141 linear relationships between 6A and 0 were obtained up to the saturation coverages (6 = 0.5). In this paper such a relationship will also be assumed (eq. (1)). The change in the other ellipsometric parameter appeared to be smaller. 3.2. Kinetics of oxygen adsorption The rate of oxygen adsorption was studied at crystal temperatures between 294 and 47.5 K and pressures in the range 10P6-5 X lo-’ Torr. The gas was always admitted at room temperature. At all temperatures the initial rate of adsorption was proportional to the gas pressure, as illustrated in fig. 3. Initial sticking coefficients, derived from these data are given in table 1. As can be seen, the sticking coefficient decreases with increasing temperature, s 0: exp(E/RT) with E = 0.6 + 0.1 kcal/mol. The adsorption curves can be reasonably described by an equation of the type de/dt = b(B,,,
- 0))
(3)
with emax independent of temperature in the range studied and equal to about 0.5 as judged from the (2 X 1) LEED pattern. Fig. 4 shows the fit of the integrated form of eq. (3) to some experimentally observed curves.
d(6Al dt (10-3deg,s)
Fig. 3. The initial rate of oxygen temperature, (A) at 469 K.
adsorption (d(dA)/dt) versus oxygen
pressure:
(0) room
6
H. Aibers et al. / ElIipsornefry-LARD
study of oxygen ~d~or~~~~ll
Table 1 initial sticking coefficients for oxygen on Ag(1 i(l)
TW So x 104
294 5.9
362 4.9
314 4.7
381 4.4
426 4.8
469 3.9
475 4.1 --.
Our findings differ in some respects from those obtained by Engelhardt and Menzel [S]. At room temperature the sticking coefficient is about 6 times lower and it shows less variation with temperature (0.6 kcal/mol versus 2.0 kcal/mol). Also the shape of the adsorption curves seems to be different~ since the previous authors used a different, and a more complicated, expression for the dependence of the sticking coefficient on coverage. At low coverages, up to 8 N 0.4 B,,,, the two expressions can be made to match within experimental error. At higher coverages, however, discrepancies appear. These may be traced back to the experimental differences shown in fig. 1 and discussed in section 3.1. It should be remarked that eq. (3) may be derived for dissociative adsorption with large attractive interactions between adsorbed atoms (cf. ref. [I 51).
-In
---EE-
t
154
Fig. 4. Test of eq. (3) at oxygen pressures of 6 X lo- 5 Torr (n), 2.4 x lo-’ low6 Torr (a) (room temperature).
Torr (a) and 3.5 X
I
H. Albers et al. / Ellipsometry-LEED study of oxygen adsorption
3.3. Reaction with CO The reaction of adsorbed oxygen with gaseous CO was studied in the temperature range 294-475 K and at CO pressures between lop7 and 4 X 10e6 Torr. Complicated reaction kinetics were found in all cases as illustrated in fig. 5. The decrease in oxygen coverage is slow in the first stage of the reaction and accelerates at later times, in agreement with results obtained by Engelhardt et al. [5]. This behaviour could be due to attractive (effective) interactions between adsorbed oxygen atoms, which facilitate the removal of “lone oxygen atoms” at lower coverages. Since LEED patterns taken during the CO reaction show disorder mainly in the direction along the troughs on the (110) face (section 3.4) a simple one-dimensional model for this reaction may be devised. Let Nrr be the number of occupied nearest neighbour site pairs and N,, the number of pairs of sites that are occupied by one atom. If the pairwise interaction energy is w,M the total number of sites, N the total number of adsorbed atoms and f3 =N/M, the most probable distribution of Nor is, assuming thermodynamic equilibrium [ 161: (4) /3= [l-40(1
-tI){l
2N= 2N,r +N,,
= - k2N;,
(5)
.
(6)
One may now take the rate equation tifdt
,
-exp(-o/kT))]“*
-
2k,NTL
to be
,
(7)
where a different rate constant has been used for the removal of different types of oxygen atoms. Eqs. (4)-(7) cannot be solved analytically except for o = 0. The
6AIdeg)
0.6 -
0 0
1 100
200
t Fig. 5. Change in 6A as function of time after admission 3.7 X lop7 Torr; (b) 10K6 Torr; (c) 1.9 X lop6 Torr.
(set)
of CO at room temperature:
(a) pco
=
H. Albers et al. / Ellipsornetry-LEED
study of oxygen adsorption
t lsec) eq. (8) to experimentally observed reaction (o) 294 K, pco = 8 X 10d7 Torr, kl = 2 X 10e3
culated: IO-‘Torr,
k, =6.4
solution
X 10e4
curves. S-l,
Drawn
K = 5;
(0)
lines have been cal475 K, pco = 3 X
SC’, E(= 10.
is then given by: .--.. .-!-_ (K ~ 1) 80 + (K - K@o + 8,) eXp(2
0 = fjO where
K =
k2fj
(8)
’
k2/kl and O,, the oxygen coverage at zero time.
Eq. (8) shows qualitatively the observed behaviour with time (see fig. 6), although the tail of the curve falls off too slowly, which may be due to the neglect d(&A) dt (10-2deg /sl
F&S. 7. T]lc
pressure
initial
at room
(A) and “midway”slopes
temperature.
PC0 ilO-DTorrb
(0) of curves
as shown
in fig. 5 as function
of to
H. Albers et al. / Ellipsometry-LEED study of oxygen adsorption
9
t lsec) Fig. 8. Adsorption curves for mixed Oz/CO exposures at room temperature. The right hand side shows the desorption reaction from the partially saturated surface (CO pressure -lop6 Torr).
of interatomic interactions. Since their inclusion would introduce an unwarranted amount of computation and too many parameters, an exact fit has not been attempted. Experimentally one observes that the initial and “midway” slopes of the curves are proportional to the CO pressure (fig. 7). In terms of our model these slopes are given by -2kr and -kz/2(kz - k,) respectively. Taking kt = k:pco we obtain for room temperature ky = 0.3 X lo4 s-l Torr-’ and k(: = 2.7 X lo4 s-l Torr-‘, which implies that the reaction is faster if at least one neighbour is missing. It may be remarked that these numbers are different from those used in fig. 6, since in that case the line was constrained to go through the experimental points instead of having the correct midway slope. Consequently the values of ky and ki can only be considered to be order of magnitude estimates. For the temperature dependencies of the experimental slopes, kp a exp(E/RT), we get E = t 0.8 + 0.1 kcal/mol for the initial process and E = -2.1 f 0.4 kcal/mol for the later reaction. This indicates a nonactivated reaction on a pair of occupied sites and an activated reaction if the site pair is only half occupied. As can be expected, an exposure to CO/O* mixtures results in a lower steady state coverage (fig. 8). The form of the CO reaction curve with decreasing amount of adsorbed oxygen is in qualitative agreement with eq. (8), which shows that for Be + 0 the kinetics change into a simple exponential. In summary, the overall agreement can be considered satisfactory, taking into account the approximate nature of the description of the CO reaction. 3.4. LEED experiments Upon introducing a low but constant pressure of CO to a fully developed (2 X 1) structure at room temperature this structure slowly transformed into that of the
T?g. 9. LEED patterns obtained during reaction with CO (-50 eV): (a), (b), (c) at room temperature after 0, 36 and 62 min respectively; (d), (e), (t) at 393 K after 50, 131 and 204 min respectively. The CO pressure was of order 10e8 Torr in both cases.
clean Ag(1 IO) surface. At first streaks in the h direction developed, followed by a much lesser degree of disorder in the k direction. No clearly defined (3 X I) pattern could be seen (fig. 9 a-c). The same experiment at 120°C revealed only increasing disorder in the h direction with increasing CO exposure and additionally showed the characteristic splitting of the (1/2k) superspots into a cIearly defined (3 X 1) structure, cf. refs. [ I’?,1 S]. This is illustrated in fig. 9 d-f. At temperatures above 200°C oxygen desorption takes place, which also proceeds via increasing disorder in the h direction. This could be observed visually via the disappearance of the (2 X 1) structure which was formed up to 270°C. At higher temperatures, desorption was too fast to allow LEED patterns to be taken after evacuation of the sample chamber. The foregoing results indicate that, at least above 120°C. the mobility of the adsorbed oxygen atoms along the troughs of the silver (1 IO) face is high enough for rearrangement to the (3 X 1) structure. 3.5. oxygen
adsorption is0 therms
Adsorption isot~~erms were detcrlllined at six temperatures between 570 and 610 K and at oxygen pressures between 10d6 and IO-3 Torr. The coverage was recorded via the cllipsometric parameter 6A. Results are shown in fig. IO. The saturation value of sA at 542 K is -0.65”, which is significantly higher than the value of 0.45’ below 200°C. However. a clear (2 X 1) structure could still be seen. This implies that either surface reconstruction has taken place with a higher oxygen coverage, or that the oxygen-silver bond has changed, with a coilcoi~ii~allt
H. Albers et al. / Ellipsometry-LEED study of oxygen adswtio@
11
change in poiarisability of the adlayer, leading to a different relationship between B and 6~3. Assuming the same reIation between SA and 6 to hold at all temperatures, isosteric heats of adsorption, AHis<>, can be obtained. Results are plotted in fig. 11.
607
Fig. 1 I.
Isosterb heat of adsorption
expressed as 68.
(Af&)
of oxygen
on A&l IO> as
functionOF coverage,
12
1% Alhers et al. / Ellipsometry-LEED
study of oxygen adsorption
It can be seen that the values increase about linearly with increasing coverage: c\Hiso (kcal/mol) = 33 + 33 &A, or with eq. (I): nisi,, = 33 + 35 8. Similar results have been reported by Engelhardt and Menzel [5,19]. The increase in nisi,, with coverage again indicates the existence of attractive interactions between oxygen atorns. Using the model proposed by Guggenheim [ 161, one gets for the pairwise interaction energy a value of 17 kcal/mol. which is a rather high value. It thus seems likely that a more complicated model must be used, which includes other interactions besides those between nearest neighbours. Alternatively, of course, if the relation between 6A and 0 changes with the temperature, no meaningful conclusions can be drawn from fig. 10 as it stands.
4. Conclusions
and comparison
with Ag(ll1)
(i) In contrast with Ag( 111) oxygen is readily adsorbed on Ag( 110) with initial sticking coefficients of about 5 X 10m4 between room temperature and 200°C and a saturation coverage tImax = 0.5. For a well-annealed Ag( 11 1) surface the corresponding values were 3 X lo-’ and emax = 0.03. (ii) The reaction of oxygen adsorbed on Ag( 1 IO) with CO proceeds initially slowly at higher oxygen coverages and accelerates at lower coverages. This can be rationalized by assuming the removal of both “row 0 atoms” and “terminal 0 atoms” with different rate constants. On a damaged Ag(l 11) surface the amount of adsorbed oxygen decreases exponentially with time with a time constant of 8 X lo4 sP1 (Torr CO))’ at room temperature. For low coverages on Ag( 110). an exponential decrease is also observed, time constant 5.4 X lo4 s-’ (Torr CO))‘. Evidently for the terminal 0 atoms on Ag(ll0) and the 0 atoms adsorbed on damaged Ag(l1 1) the reaction probability (number of oxygen atoms removed per hitting CO molecule) is comparable.
[ 11 I:or a recent review, cf. H. Albers, Thesis, Univ. of Utrecht (1978) ch. 2. [ 21 G. Rovida, I’. Pratesi, M. Maglietta and E. Ferroni, Surface Sci. 43 (1974) 230. [ 31 G. Rovida and F. Pratesi. Surface Sci. 52 (1975) 542. [4] G. Rovida. J. Phys. Chem. 80 (1976) 150. [5] 1l.A. Ikgelhardt and D. Menzcl. Surface Sci. 57 (1976) 591; I1.A. Engelhardt, A.M. Bradshaw and D. Menzel, Surface Sci. 40 (1973) 410; H.A. Engelhardt, Thesis, Technische Univ., Miinchen (1975). (61 W. Heiland, I;. Iberl, E. Taglauer and D. Menzel, Surface Sci. 53 (1975) 383. [7] E. Zanazzi, M. Maglietta, U. Bardi, F. Jona, D.W. Jepsen and P.M. Marcus, in: Proc. 7th Intern, Vacuum Congr. and 3rd Intern. Conf. on Solid Surfaces (Vienna, 1977) p. 2447. 181 H. Albers, W.J.J. van der Wal and C.A. Bootsma, Surface Sci. 6X (1977) 47. [91 Il. Albcrs, J.M.M. Droog and G.A. Bootsma, Surface Sci. 64 (1977) 1. [ 101 J.J. Lander and J. Morrison, 1. Appl. Phys. 34 (1963) 3517.
H. Albers et al. / Ellipsometry-LEED
study of oxygen adsorption
13
[ 1 l] A.U. MacRae, Surface Sci. 1 (1964) 3 19. [ 121 L. Bakker, Thesis, Technical Univ., Delft (1972). [ 131 F.H.P.M. Habraken, E.P. Kieffer and G.A. Bootsma, in: Proc. 7th Intern. Vacuum Cow. and 3rd Intern. Conf. on Solid Surfaces (Vienna, 1977) p. 877. [14] F.H.P.M. Habraken and G.A. Bootsma, Ned. Tijdschrift voor Vacuum techniek 16 (1978) 142. [15] D.A. King and M.G. Wells, Proc. Roy. Sot. (London) A339 (1974) 245. [ 161 T.L. Hill, Introduction to Statistical Thermodynamics (Addison-Wesley, London, 1960) ch. 14. [ 17) J.E. Houston and R.L. Park, Surface Sci. 21 (1970) 209. [18] G. Ertl and J. Kiippers, Surface Sci. 21 (1970) 61. [19] D. Menzel, in: The Physical Basis for Heterogeneous Catalysis, Eds. E. Drauglis and R.J. Jaffee (Plenum, New York, 1975) p. 437.