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Growth and properties of Cu on Ni(100)
Surface Science 152/153 (1985) 247-253 North-Holland, Amsterdam
247
GROWTH AND PROPERTIES
OF Cu ON Ni(100)
A. NILSSON
** and D. CHADWICK
*, M.A. MORRIS
Department of Chemical Engineering and Chemicaf Technologv, Imperial C&&e, UK Received
London S N/7 23 Y,
2 April 1984
The growth of Cu on Ni(100) at room temperature has been investigated by XPS and is found to be layer-by-Iayer up to three monotayers coverage. Cu 2p,,,, binding energy increased with Cu coverage to a plateau at 6,” = 0.3 and increased further to the Cu bulk value with the growth of the second layer. Chemisorption of CO on Cu/Ni(lCO) surfaces has been investigated at room temperature. The saturation CO uptake decreased linearly with Cu coverage up to I&., = 0.3 after which there was a more gradual linear decrease up to one monolayer Cu coverage when the surface did not take up CO to a measurable extent. This behaviour is interpreted in terms of a transition from a c(2 X 2) type Cu structure to (I X 1).
1. Introduction There have been many studies of metallic thin films on metal substrates covering a range of interest. For example, these systems have been investigated in order to aid understanding of the electronic structure and surface properties of bimetallic alloys. In addition to electronic and physical properties, it is important to establish the chemical behaviour of thin metallic overlayers, particularly in the sub-monolayer region, since this is of technological interest in the fields of bimetallic catalysis, corrosion, adhesion and lubrication. From the point of view of catalysis, an investigation of the reactive behaviour of adsorbed metal overlayers in the sub-monolayer region offers the possibility of discriminating between electronic and geometric effects. However, it is only recently that the reactive properties of such model systems have been studied [l-5]. Cu-Ni alloys have been the topic of many investigations with particular reference to the surface composition and its influence on adsorption and reactivity [6,7]. It is generally accepted that annealed Cu-Ni alloys are strongly
* Present address: Institute of Physics, University of Uppsala. ** Postal address: Department of Chemistry, Imperial College,
S-75121 London,
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Uppsala, UK.
Sweden.
24X
A. Ntl.tson et al. /
Growth
urtd properties
of Cu on Ni(lO0)
enriched in copper even at low bulk compositions [8]. The properties of overlayers on Ni would appear, therefore, to be of considerable interest. In paper we report an XPS study of the growth and adsorptive behaviour to of Cu overlayers on Ni(lOO) at room temperature with emphasis on important sub-monolayer region.
Cu this CO the
2. Experimental Experiments were performed under UHV conditions in a VG ESCALAB Mk II and a VG ESCA-3 although the results presented here were obtained using the former instrument. XPS data were collected at emission angles of 77” and 10” with respect to the surface using unmonochromatised Al Koc radiation and at normal emission using monochromatised radiation. The spectra were analysed using a VG 1000 data system. XPS binding energies were calibrated with respect to Ag 3d,,,, = 368.4 eV. Two Ni(lOO) crystals were studied. Both were cleaned by cyclic Ar ion bombardment and annealing and in one case by annealing in oxygen. The surfaces were judged to be clean when the C Is to Ni 2p,,,, ratio was better than 1 : 1000. The orientation of the crystal used in the ESCALAB experiments was checked by LEED. Cu was evaporated onto the Ni(lOO) surface at room temperature from a Ta tube heated by electron bombardment which produced a constant Cu flux. During evaporation the base pressure rose to about 10 ‘) mbar. Carbon contamination was monitored by XPS throughout and did not increase significantly after each deposition cycle. Each data point was obtained from a single Cu deposition cycle and the crystal was cleaned and annealed prior to subsequent deposition.
3. Results 3. I. Intensity
unulysis
The intensities of the Cu 2p,,, and Ni 2~,,/~ peaks were monitored as a function of Cu deposition time using an emission angle of 77’. The intensities were the measured peak areas determined using the VG 1000 data system with a non-linear background subtraction routine. The results are presented in fig. la. The Cu 2p i,2 peak intensity rises linearly with deposition time reaching a sharp change in slope at approximately 3 min after which the peak intensity continues to increase linearly though at a slower rate until a second break is observed at approximately 6 min deposition time. Subsequently. the Cu 2p,,., peak intensity increases linearly up to 9 min deposition time. In parallel with these changes the Ni 2~,,~ intensity decreased with increasing Cu deposition
A. Nilsson et al. / Growth and properties of Cu on Ni(100)
249
time showing a break in slope at 3 min and possibly a second at 6 min. The intensity variations in fig. la are consistent with layer-by-layer growth of the Cu overlayer up to 3 monolayers in agreement with previous AES work [9,10]. There is no evidence for surface alloy formation and this conclusion is further supported by the fact that the attenuation of the Ni 2p,,, peak after 3 min deposition time is consistent with the presence of a Cu monolayer. Montano et al. [lo] reported surface alloy formation only for substrate temperatures above 300°C. The results of an AES study to be reported later [ll] showed evidence for the formation of crystallites at coverages above 3 monolayers in agreement with the previous AES work [9,10]. The growth of the Cu overlayer was studied further at grazing emission. The measured Cu 2p 3,2 peak intensities are plotted against the Ni 2p,,, intensities in fig. 1 b. A straight line is obtained up to an exceptionally sharp change in slope which corresponds to completion of the first monolayer and occurs after 3 min deposition time as in fig. la. The finite Ni 2p,,, intensity which remains after completion of the first monolayer contains a contribution from the crystal edge due to the low emission angle used. The sharp break point in fig. 1b was used to define one monolayer Cu coverage and the Cu Zp,,,/Ni 2p,,, intensity ratios were used to obtain the Cu coverage in subsequent adsorption experiments.
a
I _I
(, , , /, , , ,jqy,, 12 3 L 5 6 7 DOSE TIME,MINS.
8
9
,j
?40120 100 80 60 40 20 IO NI 21;L2PEAY INTENSITY,(ARBUNITS)
Fig. 1. (a) Cu 2p,,, and Ni 2p,,, intensities against Cu deposition time for Cu on Ni(lOO). (b) CU 2P 3,2 intensity against Ni 2p,,, intensity determined at grazing emission for various Cu coveragea on Ni(100); the sharp break marks completion of the first monolayer.
250
A. Nilsson et al. /
3.2. Coverage dependence
Growth
of Cu
and properties
on Ni(100)
of XPS binding energies
The coverage dependence of the Cu 2~,,~ binding energy (BE) and halfwidth are shown in fig. 2. The data were obtained from a large number of deposition runs using grazing emission and 77” and from a single run using monochromatic radiation. The solid line has been drawn through the data obtained using the monochromator. The Cu 2p,,, BE increased with coverage up to 0.3 monolayer followed by a plateau and increased further with the addition of the second layer reaching the bulk copper value at high coverages. In contrast, the halfwidth remained constant below 0.3 monolayers after which it rose to a constant value above 2 monolayers. Small increases in Ni 2p,,, BE and reduction in halfwidths were detected but were not measured accurately. Further experiments are in progress. 3.3. Cnrhon monoxide
udsorption
CO adsorption on the Cu covered Ni(lOO) surface at room temperature was studied for a range of Cu coverages in the sub-monolayer range. The C Is peak intensities following exposure of the surfaces to a CO dose of 1000 L (at 10Ph mbar) are shown in fig. 3. This dose was sufficient to saturate the surface in all cases. Two regimes are apparent in fig. 3. At low Cu coverages the saturation CO uptake falls linearly with increasing Cu coverage up to just over 0.3 monolayers where a dramatic change in behaviour occurs. In this lower regime
Fig. 2. Coverage dependence of Cu 2p,,, (0) and monochromatic radiation (0).
BE and linewidths
determined
using unmonochromatised
A. Nilsson et al. / Growth and properties of Cu on Ni(lOO)
251
the slope of the line is such that it would intercept the Cu coverage axis at 0.5 monolayers. Assuming a c(2 X 2) structure for the CO [12], it can be concluded that below 0.3 monolayer Cu coverage, each adsorbed Cu removes one CO adsorption site. Above 0.4 monolayer Cu coverage the CO saturation uptake again decreases linearly with increasing Cu coverage, but more slowly. At one monolayer Cu coverage and above the surface no longer takes up CO. In this regime, assuming a c(2 X 2) structure for the CO, the slope suggests that 3.5 adsorbed Cu atoms are required to remove one CO adsorption site. Montano et al. [lo] reported that only 0.1 monolayer of Cu was required to suppress CO adsorption on Ni(lOO) at room temperature in sharp contrast to the results presented here. However, we obtained very similar results to those in fig. 3 for the second Ni(lOO) crystal in a VG ESCA-3 using a 6 L CO dose (at 5 x 10m9 mbar) and the 0 1s intensity to measure adsorbed CO concentration. 4. Discussion A most interesting aspect of the present influence of Cu coverage on CO adsorption
I
Fig. 3. Dependence
I
I
of CO saturation
I
I
uptake
I
results is the sharp change in the (fig. 3). It is well known that CO
I
I
at room temperature
I
on Cu coverage
on Ni(lOO).
on Ni(lOO) forms a c(2 x 2) structure [12] with CO in the on-top position [13,14]. The coverage may be increased beyond 8,.,, = 0.5 forming compression structures below room temperature. It is assumed, therefore, that in the present experiments performed at room temperature CO exists in the ~(2 X 2) structure and at saturation 0,.,, = 0.5. Previous work on Cu/Ru(OOOl) found that CO adsorption decreased rapidly for very small Cu coverages which was attributed to suppression of compression structures [5]. In the present case such a sharp fall off in CO adsorption at very low Cu coverage was not observed. In the regime below e,.,, = 0.3, fig. 3. each adsorbed Cu atom removes one CO adsorption site. This can be explained easily by a simple site blocking mechanism in which the adsorbed Cu forms a structure which locally at least is c(2 x 2) with Cu in the on-top position or 4-fold hollow site. the latter being more likely. This structure would reach completion at 0,,, = 0.5 when CO would no longer absorb. However, this structure does not reach completion since above B,.,, = 0.330.4 a new regime exists in which 3.5 Cu atoms are required to remove one CO adsorption site assuming that the CO structure remains ~(2 x 2) as Cu coverage increases. Since at higher coverages the Cu structure is (1 X 1) [9,10]. the dramatic change in the influence of adsorbed Cu on CO adsorption must mark the onset of the (1 x 1) structure. The change in slope in fig. 3 occurs at f3,,, = 0.31 and up to this coverage the Cu appears to be in a c(2 x 2) type structure. leaving 0.38 monolayer of Ni surface unoccupied. Assuming the sticking probability for adsorbed Cu in the (1 X 1) structure formed on the free Ni surface is identical to that for Cu forming a (1 X 1) structure by incorporation into the c(2 x 2) type structure, then for every 2 C’u atoms in the (1 X 1) structure on the free Ni surface. 1.6 c’u atoms will be incorporated into the ~(2 x 2) type structure. Only the (1 x 1) structure forming on the free Ni surface removes CO adsorption sites, which occurs at a rate of 2 Cu atoms per CO site. Consequently, the decrease in CO saturation uptake with Cu coverage in the higher regime should be at the rate of 3.6 adsorbed Cu atoms per CO adsorption site removed. This is in excellent agreement with the observed slope which is 3.5 Cu atoms per CO adsorption site removed. plot in fig. la shows no evidence for a change The Cu 2p,,, intensity/time in sticking probability which might be associated with a structural transition in the sub-monolayer region. Differences in sticking probability arising from such structure changes are usually very small and would be difficult to detect in an XPS intensity/time plot. Previous LEED investigations of Cu on Ni(lOO) [9] have reported only the (1 x 1) structure. However. the coverage dependencies of the Cu 2p,,, BE and linewidth observed in the present study, fig. 2. are consistent with the CO adsorption behaviour. There is a transition in both BE and linewidth dependence on Cu coverage at 8,.,, = 0.3, the coverage where CO adsorption is observed to change. Below e,.,, = 0.3 the Cu 2p3,, linewidth does not vary suggesting a constant local environment for the adsorbed Cu while the
A. Nilsson et al. / Growth and properiies of Cu on Ni(lOO)
253
Cu 2p,,, BE rises as has been observed for other metal overlayer systems [15]. Between Oc, = 0.3 and 0,” = 1, where the (1 X 1) structure is forming, the Cu 2P 3,2 BE remains constant while the linewidth increases smoothly from 19~”= 0.3 up to 8,” = 2 presumably due to the gradual evolution of a Cu band structure. In the above discussion it has been assumed that the adsorbed CO forms a c(2 x 2) structure irrespective of Cu coverage below one monolayer. Further experiments are in progress to elucidate this system more fully.
5. Conclusions It has been shown that growth of Cu on Ni(lOO) occurs layer-by-layer up to 3 monolayers. CO does not adsorb on Cu/Ni(lOO) above one monolayer Cu coverage. The dependence of CO adsorption, Cu 2p,,, BE and Cu 2p,,, linewidth on Cu coverage in the sub-monolayer region indicate a structural transition at 13,” = 0.3-0.4 which is suggested to be from a c(2 x 2) type structure to (1 x 1) and that the decrease in CO adsorption occurs by a simple site blocking mechanism.
Acknowledgements We thank the SERC (UK) for financial the loan of a Ni(lOO) crystal.
support
and Dr. P.R. Webber
References [l] [2] [3] [4] [5] [6] [7] (81 [9] [lo] [ll] [12] [13] [14] [15]
D. Chadwick, A.B. Christie and M.A. Karolewski, Vacuum 31 (1981) 705. D. Chadwick and M.A. Karolewski, Phys. Scripta T4 (1983) 103. W.A. Sachtler, J.P. Biberian and G.A. Somorjai, Surface Sci. 110 (1981) 43. K. Christmann, G. Ertl and H. Shim& J. Catalysis 61 (1980) 412. J.C. Vickerman, K. Christmann and G. Ertl, J. Catalysis 71 (1981) 175. K.Y. Yu, D.T. Ling and W.E. Spicer, J. Catalysis 44 (1976) 373. D.T. Ling and W.E. Spicer, Surface Sci. 94 (1980) 403. P.R. Webber, C.E. Rojas, P.J. Dobson and D. Chadwick, Surface Sci. 105 (1981) 20. A. Chambers and D.C. Jackson, Phil. Mag. 31 (1975) 1357. P.A. Montano, P.P. Vaishnava and E. Pooling, Surface Sci. 130 (1983) 191. A. Nilsson, M.A. Morris and D. Chadwick, to be published. J.C. Tracy, J. Chem. Phys. 56 (1972) 2736. S. Andersson, Solid State Commun. 27 (1977) 75. S. Andersson and J.B. Pendry, Phys. Rev. Letters 43 (1979) 363. M.A. Karolewski, PhD Thesis, University of London (1983).
for
247
GROWTH AND PROPERTIES
OF Cu ON Ni(100)
A. NILSSON
** and D. CHADWICK
*, M.A. MORRIS
Department of Chemical Engineering and Chemicaf Technologv, Imperial C&&e, UK Received
London S N/7 23 Y,
2 April 1984
The growth of Cu on Ni(100) at room temperature has been investigated by XPS and is found to be layer-by-Iayer up to three monotayers coverage. Cu 2p,,,, binding energy increased with Cu coverage to a plateau at 6,” = 0.3 and increased further to the Cu bulk value with the growth of the second layer. Chemisorption of CO on Cu/Ni(lCO) surfaces has been investigated at room temperature. The saturation CO uptake decreased linearly with Cu coverage up to I&., = 0.3 after which there was a more gradual linear decrease up to one monolayer Cu coverage when the surface did not take up CO to a measurable extent. This behaviour is interpreted in terms of a transition from a c(2 X 2) type Cu structure to (I X 1).
1. Introduction There have been many studies of metallic thin films on metal substrates covering a range of interest. For example, these systems have been investigated in order to aid understanding of the electronic structure and surface properties of bimetallic alloys. In addition to electronic and physical properties, it is important to establish the chemical behaviour of thin metallic overlayers, particularly in the sub-monolayer region, since this is of technological interest in the fields of bimetallic catalysis, corrosion, adhesion and lubrication. From the point of view of catalysis, an investigation of the reactive behaviour of adsorbed metal overlayers in the sub-monolayer region offers the possibility of discriminating between electronic and geometric effects. However, it is only recently that the reactive properties of such model systems have been studied [l-5]. Cu-Ni alloys have been the topic of many investigations with particular reference to the surface composition and its influence on adsorption and reactivity [6,7]. It is generally accepted that annealed Cu-Ni alloys are strongly
* Present address: Institute of Physics, University of Uppsala. ** Postal address: Department of Chemistry, Imperial College,
S-75121 London,
0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Uppsala, UK.
Sweden.
24X
A. Ntl.tson et al. /
Growth
urtd properties
of Cu on Ni(lO0)
enriched in copper even at low bulk compositions [8]. The properties of overlayers on Ni would appear, therefore, to be of considerable interest. In paper we report an XPS study of the growth and adsorptive behaviour to of Cu overlayers on Ni(lOO) at room temperature with emphasis on important sub-monolayer region.
Cu this CO the
2. Experimental Experiments were performed under UHV conditions in a VG ESCALAB Mk II and a VG ESCA-3 although the results presented here were obtained using the former instrument. XPS data were collected at emission angles of 77” and 10” with respect to the surface using unmonochromatised Al Koc radiation and at normal emission using monochromatised radiation. The spectra were analysed using a VG 1000 data system. XPS binding energies were calibrated with respect to Ag 3d,,,, = 368.4 eV. Two Ni(lOO) crystals were studied. Both were cleaned by cyclic Ar ion bombardment and annealing and in one case by annealing in oxygen. The surfaces were judged to be clean when the C Is to Ni 2p,,,, ratio was better than 1 : 1000. The orientation of the crystal used in the ESCALAB experiments was checked by LEED. Cu was evaporated onto the Ni(lOO) surface at room temperature from a Ta tube heated by electron bombardment which produced a constant Cu flux. During evaporation the base pressure rose to about 10 ‘) mbar. Carbon contamination was monitored by XPS throughout and did not increase significantly after each deposition cycle. Each data point was obtained from a single Cu deposition cycle and the crystal was cleaned and annealed prior to subsequent deposition.
3. Results 3. I. Intensity
unulysis
The intensities of the Cu 2p,,, and Ni 2~,,/~ peaks were monitored as a function of Cu deposition time using an emission angle of 77’. The intensities were the measured peak areas determined using the VG 1000 data system with a non-linear background subtraction routine. The results are presented in fig. la. The Cu 2p i,2 peak intensity rises linearly with deposition time reaching a sharp change in slope at approximately 3 min after which the peak intensity continues to increase linearly though at a slower rate until a second break is observed at approximately 6 min deposition time. Subsequently. the Cu 2p,,., peak intensity increases linearly up to 9 min deposition time. In parallel with these changes the Ni 2~,,~ intensity decreased with increasing Cu deposition
A. Nilsson et al. / Growth and properties of Cu on Ni(100)
249
time showing a break in slope at 3 min and possibly a second at 6 min. The intensity variations in fig. la are consistent with layer-by-layer growth of the Cu overlayer up to 3 monolayers in agreement with previous AES work [9,10]. There is no evidence for surface alloy formation and this conclusion is further supported by the fact that the attenuation of the Ni 2p,,, peak after 3 min deposition time is consistent with the presence of a Cu monolayer. Montano et al. [lo] reported surface alloy formation only for substrate temperatures above 300°C. The results of an AES study to be reported later [ll] showed evidence for the formation of crystallites at coverages above 3 monolayers in agreement with the previous AES work [9,10]. The growth of the Cu overlayer was studied further at grazing emission. The measured Cu 2p 3,2 peak intensities are plotted against the Ni 2p,,, intensities in fig. 1 b. A straight line is obtained up to an exceptionally sharp change in slope which corresponds to completion of the first monolayer and occurs after 3 min deposition time as in fig. la. The finite Ni 2p,,, intensity which remains after completion of the first monolayer contains a contribution from the crystal edge due to the low emission angle used. The sharp break point in fig. 1b was used to define one monolayer Cu coverage and the Cu Zp,,,/Ni 2p,,, intensity ratios were used to obtain the Cu coverage in subsequent adsorption experiments.
a
I _I
(, , , /, , , ,jqy,, 12 3 L 5 6 7 DOSE TIME,MINS.
8
9
,j
?40120 100 80 60 40 20 IO NI 21;L2PEAY INTENSITY,(ARBUNITS)
Fig. 1. (a) Cu 2p,,, and Ni 2p,,, intensities against Cu deposition time for Cu on Ni(lOO). (b) CU 2P 3,2 intensity against Ni 2p,,, intensity determined at grazing emission for various Cu coveragea on Ni(100); the sharp break marks completion of the first monolayer.
250
A. Nilsson et al. /
3.2. Coverage dependence
Growth
of Cu
and properties
on Ni(100)
of XPS binding energies
The coverage dependence of the Cu 2~,,~ binding energy (BE) and halfwidth are shown in fig. 2. The data were obtained from a large number of deposition runs using grazing emission and 77” and from a single run using monochromatic radiation. The solid line has been drawn through the data obtained using the monochromator. The Cu 2p,,, BE increased with coverage up to 0.3 monolayer followed by a plateau and increased further with the addition of the second layer reaching the bulk copper value at high coverages. In contrast, the halfwidth remained constant below 0.3 monolayers after which it rose to a constant value above 2 monolayers. Small increases in Ni 2p,,, BE and reduction in halfwidths were detected but were not measured accurately. Further experiments are in progress. 3.3. Cnrhon monoxide
udsorption
CO adsorption on the Cu covered Ni(lOO) surface at room temperature was studied for a range of Cu coverages in the sub-monolayer range. The C Is peak intensities following exposure of the surfaces to a CO dose of 1000 L (at 10Ph mbar) are shown in fig. 3. This dose was sufficient to saturate the surface in all cases. Two regimes are apparent in fig. 3. At low Cu coverages the saturation CO uptake falls linearly with increasing Cu coverage up to just over 0.3 monolayers where a dramatic change in behaviour occurs. In this lower regime
Fig. 2. Coverage dependence of Cu 2p,,, (0) and monochromatic radiation (0).
BE and linewidths
determined
using unmonochromatised
A. Nilsson et al. / Growth and properties of Cu on Ni(lOO)
251
the slope of the line is such that it would intercept the Cu coverage axis at 0.5 monolayers. Assuming a c(2 X 2) structure for the CO [12], it can be concluded that below 0.3 monolayer Cu coverage, each adsorbed Cu removes one CO adsorption site. Above 0.4 monolayer Cu coverage the CO saturation uptake again decreases linearly with increasing Cu coverage, but more slowly. At one monolayer Cu coverage and above the surface no longer takes up CO. In this regime, assuming a c(2 X 2) structure for the CO, the slope suggests that 3.5 adsorbed Cu atoms are required to remove one CO adsorption site. Montano et al. [lo] reported that only 0.1 monolayer of Cu was required to suppress CO adsorption on Ni(lOO) at room temperature in sharp contrast to the results presented here. However, we obtained very similar results to those in fig. 3 for the second Ni(lOO) crystal in a VG ESCA-3 using a 6 L CO dose (at 5 x 10m9 mbar) and the 0 1s intensity to measure adsorbed CO concentration. 4. Discussion A most interesting aspect of the present influence of Cu coverage on CO adsorption
I
Fig. 3. Dependence
I
I
of CO saturation
I
I
uptake
I
results is the sharp change in the (fig. 3). It is well known that CO
I
I
at room temperature
I
on Cu coverage
on Ni(lOO).
on Ni(lOO) forms a c(2 x 2) structure [12] with CO in the on-top position [13,14]. The coverage may be increased beyond 8,.,, = 0.5 forming compression structures below room temperature. It is assumed, therefore, that in the present experiments performed at room temperature CO exists in the ~(2 X 2) structure and at saturation 0,.,, = 0.5. Previous work on Cu/Ru(OOOl) found that CO adsorption decreased rapidly for very small Cu coverages which was attributed to suppression of compression structures [5]. In the present case such a sharp fall off in CO adsorption at very low Cu coverage was not observed. In the regime below e,.,, = 0.3, fig. 3. each adsorbed Cu atom removes one CO adsorption site. This can be explained easily by a simple site blocking mechanism in which the adsorbed Cu forms a structure which locally at least is c(2 x 2) with Cu in the on-top position or 4-fold hollow site. the latter being more likely. This structure would reach completion at 0,,, = 0.5 when CO would no longer absorb. However, this structure does not reach completion since above B,.,, = 0.330.4 a new regime exists in which 3.5 Cu atoms are required to remove one CO adsorption site assuming that the CO structure remains ~(2 x 2) as Cu coverage increases. Since at higher coverages the Cu structure is (1 X 1) [9,10]. the dramatic change in the influence of adsorbed Cu on CO adsorption must mark the onset of the (1 x 1) structure. The change in slope in fig. 3 occurs at f3,,, = 0.31 and up to this coverage the Cu appears to be in a c(2 x 2) type structure. leaving 0.38 monolayer of Ni surface unoccupied. Assuming the sticking probability for adsorbed Cu in the (1 X 1) structure formed on the free Ni surface is identical to that for Cu forming a (1 X 1) structure by incorporation into the c(2 x 2) type structure, then for every 2 C’u atoms in the (1 X 1) structure on the free Ni surface. 1.6 c’u atoms will be incorporated into the ~(2 x 2) type structure. Only the (1 x 1) structure forming on the free Ni surface removes CO adsorption sites, which occurs at a rate of 2 Cu atoms per CO site. Consequently, the decrease in CO saturation uptake with Cu coverage in the higher regime should be at the rate of 3.6 adsorbed Cu atoms per CO adsorption site removed. This is in excellent agreement with the observed slope which is 3.5 Cu atoms per CO adsorption site removed. plot in fig. la shows no evidence for a change The Cu 2p,,, intensity/time in sticking probability which might be associated with a structural transition in the sub-monolayer region. Differences in sticking probability arising from such structure changes are usually very small and would be difficult to detect in an XPS intensity/time plot. Previous LEED investigations of Cu on Ni(lOO) [9] have reported only the (1 x 1) structure. However. the coverage dependencies of the Cu 2p,,, BE and linewidth observed in the present study, fig. 2. are consistent with the CO adsorption behaviour. There is a transition in both BE and linewidth dependence on Cu coverage at 8,.,, = 0.3, the coverage where CO adsorption is observed to change. Below e,.,, = 0.3 the Cu 2p3,, linewidth does not vary suggesting a constant local environment for the adsorbed Cu while the
A. Nilsson et al. / Growth and properiies of Cu on Ni(lOO)
253
Cu 2p,,, BE rises as has been observed for other metal overlayer systems [15]. Between Oc, = 0.3 and 0,” = 1, where the (1 X 1) structure is forming, the Cu 2P 3,2 BE remains constant while the linewidth increases smoothly from 19~”= 0.3 up to 8,” = 2 presumably due to the gradual evolution of a Cu band structure. In the above discussion it has been assumed that the adsorbed CO forms a c(2 x 2) structure irrespective of Cu coverage below one monolayer. Further experiments are in progress to elucidate this system more fully.
5. Conclusions It has been shown that growth of Cu on Ni(lOO) occurs layer-by-layer up to 3 monolayers. CO does not adsorb on Cu/Ni(lOO) above one monolayer Cu coverage. The dependence of CO adsorption, Cu 2p,,, BE and Cu 2p,,, linewidth on Cu coverage in the sub-monolayer region indicate a structural transition at 13,” = 0.3-0.4 which is suggested to be from a c(2 x 2) type structure to (1 x 1) and that the decrease in CO adsorption occurs by a simple site blocking mechanism.
Acknowledgements We thank the SERC (UK) for financial the loan of a Ni(lOO) crystal.
support
and Dr. P.R. Webber
References [l] [2] [3] [4] [5] [6] [7] (81 [9] [lo] [ll] [12] [13] [14] [15]
D. Chadwick, A.B. Christie and M.A. Karolewski, Vacuum 31 (1981) 705. D. Chadwick and M.A. Karolewski, Phys. Scripta T4 (1983) 103. W.A. Sachtler, J.P. Biberian and G.A. Somorjai, Surface Sci. 110 (1981) 43. K. Christmann, G. Ertl and H. Shim& J. Catalysis 61 (1980) 412. J.C. Vickerman, K. Christmann and G. Ertl, J. Catalysis 71 (1981) 175. K.Y. Yu, D.T. Ling and W.E. Spicer, J. Catalysis 44 (1976) 373. D.T. Ling and W.E. Spicer, Surface Sci. 94 (1980) 403. P.R. Webber, C.E. Rojas, P.J. Dobson and D. Chadwick, Surface Sci. 105 (1981) 20. A. Chambers and D.C. Jackson, Phil. Mag. 31 (1975) 1357. P.A. Montano, P.P. Vaishnava and E. Pooling, Surface Sci. 130 (1983) 191. A. Nilsson, M.A. Morris and D. Chadwick, to be published. J.C. Tracy, J. Chem. Phys. 56 (1972) 2736. S. Andersson, Solid State Commun. 27 (1977) 75. S. Andersson and J.B. Pendry, Phys. Rev. Letters 43 (1979) 363. M.A. Karolewski, PhD Thesis, University of London (1983).
for