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Reactive properties of lead monolayers on copper and silver surfaces
Vacuum/volume 31/numbers 10-1 2/pages 705 to 709/1981 Printed m Great BNtam
0042-207X/81/100705-05S02 00/0 @ 1981 Pergamon Press Ltd
Reactive properties of lead monolayers on copper and silver surfaces D Chadwick,
A B C h r i s t = e a n d M A K a r o l e w s k i , Department of Chemical Engineering and Chemical
Technology, Imperml College, London, SW7 UK
The mteractton of oxygen wtth Pb overlayers on polycrystalhne Cu, Ag and Cu( 1 11) surfaces m the coverage range 0 05-1 monolayers has been studmd by X-ray photoelectron spectroscopy Oxygen reacts with the Pb overlayers to produce lead monoxide species for all three surfaces For Pb/Cu, the oxtdatton rate decreases hnearly wtth Pb coverage, whereas for Pb/Ag the oxMatton rate decreases sharply m the range Opb = 0 5--0 7 It tS concluded that the Pb overlayers on Ag react dffectly with oxygen from the gas phase and that the change m react/wty accompames a change m overlayer structure In contrast, tt m shown that for the Pb/Cu system with Opb < 1, the dominant oxtdatlon mechantsm Is probably a spdlover process m whtch an oxygen ad-spectes migrates from the exposed Cu surface to the Pb m/ands
1. Introduction There have been many studies of metalhc thin films on metal substrates covering a range of interest ~ For example, efforts to understand the electronic structure and surface properties of alloys have given rise to numerous studies of model blmetalhc systems in which one pure component is evaporated onto the other 2 It is equally important to investigate the chemical behavlour of thin metallic overlayers In addmon to the electronic and physical properUes, since this is of technological interest in the fields of bimetallic catalysis, corrosion, adhesion and lubrication However, it is only recently that the reactive propemes of ultrathin metal overlayers in a hmlted number of blmetalhc systems have been reported 3 -7 The growth of lead monolayers on copper and silver surfaces have been extensively studied with respect to the epltaxml relationships and surface melting behavmur s 14 In a prehmlnary communication we reported the anomalous oxldatmn behawour of lead overlayers on polycrystalhne copper surfaces 3 In view of this, it ~s of considerable interest to compare the reactive behawour of ultra-thin lead overlayers on a range of related metal substrates (e g Cu and Ag surfaces) The present paper described a study of the Interaction of oxygen with lead monolayers on polycrystalhne and (111) copper and polycrystalhne salver surfaces by X-ray photoelectron spectroscopy For all three surfaces, the oxldauon rates of the lead monolayers were found to be strongly dependent on lead coverage m the range 0 1 0 monolayers Explanations for this unusual behavlour are discussed and a comparison of the results for copper and silver surfaces is made For the Pb/Cu system, results from a study of hydrogen sulphide chemlsorpUon are also included, which
prowde additional insight into the oxldatmn behavlour of the lead monolayers
2. Experimental X-ray photoelectron spectra (XPS) were obtained wxth a Vacuum Generators ESCA-3 spectrometer using Mg K. ra&atlon XPS binding energies are referred to the clean copper 2p3/2 peak at 932 8 eV 15 Surfaces were cleaned prior to lead evaporation by cyclic ion bombardment and high temperature annealing XPS analysis of the clean surfaces before and after lead evaporation showed them to be free of contaminants with the exception of trace quantmes of carbon which gave a ls intensity of less than 0 1 °/,, of the Cu 2p3 2 or of the Ag 3d 5 2 intensities These values represent maximum contamination levels of approximately 0 05 monolayers respectively which was considered acceptable Lead overlayers were prepared by evaporation in the spectrometer of 4N purity lead from a tungsten-wire wound quartz evaporator onto the clean substrates at a constant, controlled rate of between 2 and 5 monolayers per hour The lead evaporatmn rate and coverage were calibrated using the X-ray Induced 93 eV kinetic energy lead Auger feature In an Auger signal intensity time plot (AS-T) AES studxes have shown that such a plot gwes a sharp 'knee' point at monolayer lead coverage 11 Although the 93 eV feature ~s very weak when photon induced and is on a high background c o m m o n in ESCA instruments, It provided a reliable and reproducible AS T plot For convemence, the Pb4f7 2/Cu3p peak area ratio or Pb4JT,z/Ag3ds,,2 ratxo was used to compare the lead coverage relative to the cahbrated 1 0 monolayer pomt Exposure of the surfaces to oxygen and 705
D Chadwick, A B Christie and M A Karolewskl
Reactivepropertiesof leadmonolayerson copperand sdversurfaces
hydrogen sulphide was carried out at r o o m t e m p e r a t u r e and gas pressures between 10 s and 10 . 6 torr
,~
~
.
{a)
3. Results and discussion 3.1. O x y g e n on Pb/polycrystalline and (1 l l ) C u . Prior to oxygen adsorption, the lead overlayers gave P b 4 f X-ray photoelectron spectra characteristic of clean metalhc lead I ~ for all coverages in the range 0 0 5 1 monolayers on polycrystalhne and ( l l l ) C u surfaces Exposure of the lead overlayers on b o t h copper surfaces to oxygen resulted m the a p p e a r a n c e of an O 1 s peak centred at 529 94-0 2 eV with a F W H M of 1 8 eV for s u b - m o n o l a y e r P b coverages, c o m p a r e d to values of 530 l + 0 1 e V and 1 9 eV respectively for the clean copper surfaces The O ls binding energies (b e ) a n d F W H M were found to be i n d e p e n d e n t of oxygen exposure After oxygen exposure the lead 4/ peaks developed a new feature arising from an oxidized lead species shifted 1 4 + 0 2 eW to higher binding energy c o m p a r e d to the metalhc peaks With increasing oxygen exposure this feature was observed to grow at the expense of the metallic peak, Figure 1(a) At no stage in the oxidation process was any o t h e r species observed which is consistent with the constancy of the O ls b e a n d F W H M O x i d a t i o n of polycrystalhne a n d single crystal lead surfaces results m the formation of o r t h o r h o m b l c P b O which has a 4 f c h e m l c a l shift of 1 0 eV c o m p a r e d to the clean metal l° 17 In view of the greater chemical shift on oxidation of the P b overlayers, the oxide structure c a n n o t be u n a m b i g u o u s l y identified However, it is likely that oxidation produces a lead m o n o x i d e species since the O 1 s/Pb4J7 2 (oxide c o m p o n e n t ) intensity ratio for one m o n o l a y e r P b coverage was found to be m good agreement with the 0 2 4 + 0 03 obtained for bulk lead 16 The intensities of the O ls peaks were found to be strongly d e p e n d e n t on oxygen exposure a n d P b coverage for b o t h polycrystalhne and ( l l l ) C u At low P b coverage, the surfaces took up oxygen as rapidly as the clean copper, but increasing the P b coverage results in a large decrease m the oxygen sticking coefficient on going from 0 05 1 m o n o l a y e r P b coverage This is s h o w n in Figure 2(a), where the O 1 s intensity for a c o n s t a n t 300L oxygen exposure is plotted against P b coverage for b o t h copper surfaces The O 1~ peak intensity measures the c o m b i n e d oxygen coverage of the P b overlayer a n d the exposed copper A measure of the rate of oxidation of the P b overlayers independently may be o b t a i n e d from the fraction of oxidized P b at a c o n s t a n t oxygen
Oxygen
/ / (Long/Ou5r;' _ ~ [ / - ' ~ exposure
0
I 0
\ \ X~-.--
I I I i 2 3 ev (a)
0 I 2 3 eV
i
(b)
Figure 1 Pb4Jv 2 spectra for oxidation of Pb overlayers at 0m,= 0 5 on (a) polycrystalhne Cu, (b) polycrystalhne Ag
706
--~
0 o
I
0
•o - % ~
05 Lead coverage,
~_
°
I
I monolayers
(b)
05
0
° ° .o
05
Lead coverage,
1
rnonoloyers
Figure 2. Oxidation of Pb overlayers on polyc rystalhne ( © ) and ( 111 ) ( • ) Cu with a constant oxygen exposure of 300 L (a) O Is intensity, (b) Pb oxide fractmn exposure F o r s u b - m o n o l a y e r coverages this is given by the ratio (Pb4J7 2 intensity, oxide c o m p o n e n t ) / ( t o t a l Pb4Jv 2 intensity) and the measured values are plotted against P b coverage in Figure 2(b) It is found that the oxidation rate of the P b overlayers varies essentially hnearly with P b coverage up to 1 m o n o l a y e r Since the ratio of O 1 s intensity to the Pb4fT. 2 intensity arising from the P b O is k n o w n (see above) it is possible to show that the rate of u p t a k e of oxygen by the exposed copper surfaces is not affected by the presence of the P b overlayers The structure of lead monolayers on copper surfaces has been extensively studied s 13, principally by Rhead and coworkersS 12 G r o w t h of the overlayers follows the SK m o d e forming dense two-dimensional structures at one m o n o l a y e r coverage in which the P b P b spacings are slightly different from bulk lead m order to faclhtate integral m a t c h i n g with the copper substrates Since Cu and P b exhibit neghglble m u t u a l solubility and P b AES signal intensities from a d s o r b e d P b monolayers on Cu remain c o n s t a n t over a wMe range of temperature, it can be concluded that surface alloy formation does not take place in this system s Only one ordered structure, p(4 x 4), corresponding to pseudo-(111) P b is observed on Cu(111 ) from approximately 0 3 1 m o n o l a y e r P b coverage O n the Cu(111) surface, therefore, the first m o n o l a y e r forms by growth and aggregation of closepacked, two-dimensional islands ° 13 The c o n t r i b u t i o n of different crystal orientations to the polycrystalhne surface is not known, but we may expect the formation of islands of closepacked P b at coverages a p p r o a c h i n g 1 monolayer, since this is observed for b o t h high and low index copper surfaces 1° 1= Two aspects of the novel oxidation b e h a v l o u r of the P b overlayers require explanation These are the dependence of the P b O fraction at 300 L oxygen exposure (and therefore oxidation rate) on P b coverage m the s u b - m o n o l a y e r range as shown in Figure 2, a n d the fact that close to 1 m o n o l a y e r coverage the P b overlayers are much less reactwe t h a n expected on the basis of bulk lead oxidation Since the t o p m o s t atomic layers m polycrystalhne and (111)Pb are completely oxld,zed at 700 and 1200 L exposure respecUvely 16, a P b O fraction at 300 L oxygen exposure in the range 0 25 0 4 would be expected for the P b overlayer at 1
D Chadwtck, A B Chnstte and M A Karolewskt Reactive properties of lead monolayers on copper and silver surfaces
m o n o l a y e r coverage, whereas the measured values are less t h a n 0 1 In fact, the m m a l sticking coeffiaents for oxygen on polycrystalhne a n d (111)Pb are 0 004 and 0 002 respectively, c o m p a r e d to 3 x 10 "~ for the complete P b m o n o l a y e r An e x p l a n a t m n of this latter b e h a w o u r m a y he in the two& m e n s l o n a l nature of the P b overlayers Bulk lead surfaces are k n o w n to oxidize by f o r m a t , o n of oxide nuclei at least 2 monolayers thick from the earhest stages of oxldatlon~ ~' ~s This is clearly not possible for the P b overlayers without considerable r e c o n s t r u c t m n for which there is no ewdence in our data Alternauvely the very low reactivity of the complete P b m o n o layer m a y arise from the slightly contracted P b P b spacing 13 The dependence of the overlayer oxidation rate on P b coverage may be explained m a n u m b e r of ways To be consistent with o u r data, a r a t e - d e t e r m m l n g step m the p r e d o m i n a n t oxidation m e c h a n i s m must be essentially a h n e a r functmn of P b coverage For example, the stacking coefficient of gas-phase oxygen on the P b overlayers may be a c o n t i n u o u s f u n c u o n of P b coverage This may arise m two ways Firstly, the oxygen m a y be a d s o r b e d to form oxide nuclm at island edge sites, resultmg in an oxidation rate p r o p o r t i o n a l to the fraction of edge a t o m s This would give rise to a dependence on the oxidation rate on 0pb~ 2, where 0 p b = P b coverage, rather than the observed linear dependence Alternatively. the physical or electromc structure of the overlayers m a y change with coverage While there appears to be a shght v a r m t l o n m P b b e with coverage, it seems unhkely that this would have such a drastic effect on the oxidation rate C o n t r a c t i o n of the P b P b spacing for which there is some evidence at m o n o l a y e r coverage ~ could conceivably affect the oxidation rate, but It is not reported to be a function of~sland area as would be required by o u r data If the sticking coefficient for direct a d s o r p t i o n of gas phase oxygen on the P b overlayer were i n d e p e n d e n t of P b coverage, it would be possible for o x l d a t m n to proceed by oxygen a d s o r p t m n on the exposed copper surface followed by surface diffusion of an oxygen ad-specms o n t o the P b islands, where P b O would be nucleated This 'splllover' process would be energetically favoured since the heat of f o r m a t m n of P b O is greater t h a n that o f C u 2 0 ~ 9 or the heat of a d s o r p t i o n of oxygen on Cu 2° The n u m b e r of oxygen ad-specms available for spillover would be related to the area of exposed copper surface (l--0pb) Thus, to a first approxJmaUon, the lead oxide fraction at a fixed oxygen exposure should be p r o p o r t i o n a l to ~1 - 0pb) This is m good agreement with the experimental data from b o t h Cu surfaces given m Figure 2(b) The m o b d e species may be oxygen a d - a t o m s since these are k n o w n to diffuse over large distances before being chemlsorbed on certain Cu surfaces 21 However, any m o b d e ad-specms would produce the same effect provided splllover were the rate h m m n g step In the P b oxidation
3.2, Hydrogen sulphide and oxygen on Pb/polyerystalhne and (I1 l ) c o p p e r . In contrast to the oxldatmn results, the P b 4 f peaks from the P b overlayers showed no new features after exposure to hydrogen sulphMe This is m agreement with results for bulk P b surfaces which do not sulphMe at r o o m t e m p e r a t u r e a7 H y d r o g e n sulphide was a d s o r b e d by the P b / C u c o m b i n a t i o n to an extent d e p e n d e n t on the P b coverage The ' s a t u r a t i o n ' sulphur 2p m t e n s m e s relative to the clean copper surfaces are a measure of the ' s a t u r a t i o n ' sulphur coverages, 0, The experimental values for the polycrystalhne surface are plotted against P b coverage, 0Vb, m Figure 3 a n d are found to be a good fit to the relationship 0, = (1 -- 0ph) Results o b t a i n e d from Pb/Cu(111 ) were found to be
~
05
4-t3
,>
g
0
O5
Leocl coveroge,
I
monolayers
Figure 3. Relative S 2p intensity after H2 S saturation of Pb Cu polycrystalhne surfaces as a function of Pb coxerage
consistent w~th this r e l a n o n s h l p Hence, there appears to be no significant P b sulphur interaction The sulphlded P b / C u surfaces were found to have a low sucking coefficmnt for oxygen similar to that for the Ore,= 1 surfaces This oxygen was taken up exclusively by P b to form the P b O specms It would a p p e a r that this low oxMatlon rate for Oph=l and sulphided P b / C u surfaces is that a p p r o p r i a t e to oxMatlon of the P b M a n d s with gas-phase oxygen directly a n d that the presence of sulphur blocks oxygen a d s o r p t m n sites on the exposed copper surface, thereby poisoning oxMaUon by the splllover process The above results o b t a i n e d from sulphlded surfaces and the h n e a r dependence of the o x , d a u o n rate of the overlayers on P b coverage for O,,b< 1 are strong evidence that for the Cu surfaces studmd, the d o m i n a n t oxidation m e c h a m s m is by splllover of an oxygen ad-specles from copper to lead 3.3. Oxygen on Pb/Ag. Prior to oxygen adsorption, the lead overlayer on polycrystalhne sliver gave P b 4 / X P E spectra characteristic of clean metalhc lead for all coverages m the range 0 05 1 5 monolayers Exposure of the P b overlayers to oxygen resulted m the a p p e a r a n c e of an O 1s peak and new Pb41 features shifted to higher b e relative to the clean, metalhc P b peaks Wtth increasing oxygen exposure the P b 4 l oxide c o m p o n e n t grew at the expense of the metalhc peak, Figure l(b) However, the Pb4/ chemical shift proved to be dependent on oxygen exposure and Pb coverage In addition, the oxidation rate of the P b overlayers was found to depend on P b coverage In order to examine dependence of the oxidation rate of the Pb overlayers on P b coverage is some detail, the O 1s intensity and P b oxide fraction were measured after a constant oxygen exposure of 150 L for several values of Opb At this oxygen exposure, there was a small oxygen adsorption on the clean Ag surface For the P b / A g surfaces, the O l s intensity at 150 L was observed to increase as P b coverage increased, level off and then fall to a low value at 0ph > 1 An idea of the oxldaUon rates of the P b overlayers were obtained, as m the Cu case, from the fraction of Pb existing as oxide after the c o n s t a n t oxygen exposure of 150 L The results are s h o w n m Figure 4 There a p p e a r to be two distract oxldaUon regimes according to P b coverage At low coverage. 0ph < a b o u t 0 5, the o x l d a h o n of the overlayers proceeds rapidly and is not strongly dependent on Pb coverage There is a fairly a b r u p t change in b e h a v l o u r at 0 p b = 0 5 0 7 after which oxidation proceeds at a slower rate, roughly i n d e p e n d e n t of 0p~, up to 2 monolayers In this latter regtme, the P b oxide fraction after 150 L oxygen exposure is a b o u t 0 2, which may be c o m p a r e d with the values of 0 2 and 0 13 expected for the t o p m o s t layer of 707
D ChadwIck,
A 6 Chr/st/e
and MA
Reactwe
Karolewskl
properties
of lead monolayers
are
/
I
consistent
and sliver
with
the
surfaces
reactive
behavlour
of the
overlayers on the polycrystallme surface The p(& x &)R30” structure 15dn open structure which would be expected to oxidize more rapldly than the close-packed twist structure Since the two structures co-exist to varying extents m the coverage range O,,=O 5 0 8 the shape of the oxldatlon rate curve, Figure 4, IS readdy understood However, further work IS necessary on the Ag( 111) surface before this mterpretatlon can be confirmed for the polycrystdlline surface
I
05
0
entirely
on copper
monolayers
Lead coverage,
Flgure 4 Oxldatlon of Pb overlayers on polycry\tallme oxygen expowre of 150 L Pb oxide frdctlon dgdm\t
Ag H Ith d conytdnt Pb coverage
dnd (1ll)Pb dt the \ame exposure The observed maxlmum m oxygen stlckmg coefficient 15 consistent with the relatively lower reactlvlty of Pb overlayers at coverages approachmg one monoldyer The existence of two oxiddtlon regimes IS also evident in the behdvlour of the Pb4j chemlcdl shift Imtldlly, at low oxygen exposure, t90 L for Oph5 0 5, the Pb4f peak from the oxide component was found to be broad dnd shlfted to higher be by 1 2 _+ 0 2 eV compared to the metalhc peaks, Figure 5 As oxygen exposure Increased, the oxide component of the Pb4f pedk became narrower and the chemical shift reduced to 0 8 + 0 1 eV after which this pedk contmued to mcrea\e m mtenslty The 0 1, b e for this oxide wds 529 0 k 0 3 eV At Pb coverages around H,,= I, the Pb4f component chemically chlfted by 0 8 eV was the only oxldlzed \pecles observed The 0 8 i 0 1 eV chemical shift 1s close to the value of 1 0 eV observed for oxlddtlon of bulk Pb which forms orthorhomblc PbO”’ I’ It seems reasonable to conclude thdt oxld‘ttlon of the Pb overlayers result\ m the form&Ion of d lend monoxide species However for low Pb coverages there 1s evidence m our data for d precursor oxide species with d shghtly gredter chemical shift LEED \tudles of the structure of Pb overlayers on Ag( 111) have been__ reported recentlyI lJ ‘* The first ordered structure, ~ polycrystalhne
~(~‘3 x ~‘3 )R30 , appears dl o,,, 10 2 dnd is complete at o,,, z 0 5 For II,,,> ‘ibout 0 5 n ‘twist’ structure develop5 conslstmg of mIsorIented close-packed domaIn I3 I’ These structural detdtls
3.4. Comparison of Pb/Cu and Pb/Ag. The results from Pb/Cu and Pb/Ag may be usefully compared The Pb/Ag system 1s essentially well-behaved m that each Pb overlayer structure appears to define an oxlddtion regime Wlthm these regimes, the reactlvtty of the overlayers dre not strongly dependent on Pb coverage In contrast, the behavlour of the Pb overlayers on Cu 1s clearly anomalous Since the reactlvlty of the close-packed Pb structure on the Ag surface appears to be approximately independent of coverage, the same would be expected of the close-packed structure known to form on Cu(ll1) In fact, the sulphldatlon results (3 2) demonstrated thdt this ts probably true for drrect reactlon with gas-phase oxygen The sharp contrast between the oxldatlon behavlour of the Pb/Cu and Pb/Ag systems 15 further support for the splllover mechanism m the Pb/Cu case The difference between the Influence ofCu and Ag substrates on the Pb overlayers presumably hes m the much greater affinity for oxygen of the former
4. Conclusions
It has been shown that the oxlddtion behavlour of Pb monolayers on polycrystalhne Cu and Ag surfaces and Cu( 111) 1s strongly dependent on Pb coverage m the range Up,,< 1 For Ag It 1s concluded that the Pb overlayers react with oxygen from the gas phdse directly and that each overlayer structure define\ dn oxlddtion regime In the Pb/Cu case, It 1s concluded that the dominant oxldatlon mechanism IS d spillover process in which an oxygen ad-species mtgrates from the exposed copper surface to the Pb Islands
Acknowledgement
k XI
0
I
2
ev
We thank the SRC and Esso Chemicals Ltd for the dwdrd of d CASE studentship to ABC and the SRC for the award of a studentship to MAK
References
x4
3 J
Flgure 5. Pb4/, 2 spectra for Ollh= 0 5 on Ag dt 90 L oxygen showmg resolution mto metdlllc dnd oxide componenta
708
expo\ure
1 E Bduer and H Poppd, Thrn So/id fr/m\, 12, 167 (1972) ’ P Helmdnn. H Neddermeyer and M Pessa, Phj\ Rev, B17, 427 (1978) ’ D Chddwlck and A B Chrlctle, Sur/uce SCI, 82, L293 (1979) ’ D Chadwtck and A B Chrlatle, ECOSS-3 Proceedmgs, Le Vu/e, /e\ Couche\ A41,1ce\, 201, 423 (1980) ’ W Palczewskd, 1 Slymerska, I RatdJczykowa and M LIpskI, Le VI&, Irr Couthr\ Mm-r\, 201, 407 (1980) ’ L Gonzales, R Mtranda and M Salmeron, Le VI&, Ire Couches fvf~nc+ 201, 229 (1980) ’ J W A Sachtler, M A Van Hove, J P Blberlan and G A Somoqal, Php Rrt Lrtr, 45, 1601 (1980) ” J Henrlon and G E Rhead, Surfuce Scr, 29, 20 (1972) 9 G E Rhead, J Vat SKI Technol, 13, 603 (1976) I” A Sepulvedd and G E Rhead, Surface Scr, 66, 436 (1977) ” C Argde and G E Rhead, J Phy~, C7, L261 (1974)
D Chadwick, A B Chrtst/e and M A Karolewskt Reactive properties of lead monolayers on copper and sdver surfaces 12 M G Barthes and G E Rhead, Sm[a~e S~t, 80, 421 (1979) 3 K J Rawhngs, M J Gibson and P J Dobson, J Ph) s, D I !, 2059 (1978) 14K Takayanagl and G Honjo, ECOSS-3 Proceedings, Le Vule, le~ Couches Minces, 201,267 (1980) ~5 G Johansson, J Hedman, A Berndtsson M Klasson and R Ndsson, J Electron Spe~tlos¢, 2, 295 (1973) l~, D Chadwick and A B Christie, J C ~ Farada~ I1, 76. 267 (1980)
t~ R W Joyner, K Klshl and M W Roberts, PJoc R Sot, A358, 223 (1977) Js R W Hewltt and N Wlnograd, Surla~e S~t, 78, 1 (1978) J9 Bull US Bureau Mines, 542 (1954) 20 I Toyosh~ma and G A Somorjal, Cutal~szs Ret, 19, 1(/5 (1979) 21 C S McKee, L V Renny and M W Roberts buJ/a~e S~t, 75, 92 (1978) 22 K Takayanagl, D M Kolb, K K a m b e and G Lehmpfuhl, Sin~ace ~'~1 100, 407 (1980)
709
0042-207X/81/100705-05S02 00/0 @ 1981 Pergamon Press Ltd
Reactive properties of lead monolayers on copper and silver surfaces D Chadwick,
A B C h r i s t = e a n d M A K a r o l e w s k i , Department of Chemical Engineering and Chemical
Technology, Imperml College, London, SW7 UK
The mteractton of oxygen wtth Pb overlayers on polycrystalhne Cu, Ag and Cu( 1 11) surfaces m the coverage range 0 05-1 monolayers has been studmd by X-ray photoelectron spectroscopy Oxygen reacts with the Pb overlayers to produce lead monoxide species for all three surfaces For Pb/Cu, the oxtdatton rate decreases hnearly wtth Pb coverage, whereas for Pb/Ag the oxMatton rate decreases sharply m the range Opb = 0 5--0 7 It tS concluded that the Pb overlayers on Ag react dffectly with oxygen from the gas phase and that the change m react/wty accompames a change m overlayer structure In contrast, tt m shown that for the Pb/Cu system with Opb < 1, the dominant oxtdatlon mechantsm Is probably a spdlover process m whtch an oxygen ad-spectes migrates from the exposed Cu surface to the Pb m/ands
1. Introduction There have been many studies of metalhc thin films on metal substrates covering a range of interest ~ For example, efforts to understand the electronic structure and surface properties of alloys have given rise to numerous studies of model blmetalhc systems in which one pure component is evaporated onto the other 2 It is equally important to investigate the chemical behavlour of thin metallic overlayers In addmon to the electronic and physical properUes, since this is of technological interest in the fields of bimetallic catalysis, corrosion, adhesion and lubrication However, it is only recently that the reactive propemes of ultrathin metal overlayers in a hmlted number of blmetalhc systems have been reported 3 -7 The growth of lead monolayers on copper and silver surfaces have been extensively studied with respect to the epltaxml relationships and surface melting behavmur s 14 In a prehmlnary communication we reported the anomalous oxldatmn behawour of lead overlayers on polycrystalhne copper surfaces 3 In view of this, it ~s of considerable interest to compare the reactive behawour of ultra-thin lead overlayers on a range of related metal substrates (e g Cu and Ag surfaces) The present paper described a study of the Interaction of oxygen with lead monolayers on polycrystalhne and (111) copper and polycrystalhne salver surfaces by X-ray photoelectron spectroscopy For all three surfaces, the oxldauon rates of the lead monolayers were found to be strongly dependent on lead coverage m the range 0 1 0 monolayers Explanations for this unusual behavlour are discussed and a comparison of the results for copper and silver surfaces is made For the Pb/Cu system, results from a study of hydrogen sulphide chemlsorpUon are also included, which
prowde additional insight into the oxldatmn behavlour of the lead monolayers
2. Experimental X-ray photoelectron spectra (XPS) were obtained wxth a Vacuum Generators ESCA-3 spectrometer using Mg K. ra&atlon XPS binding energies are referred to the clean copper 2p3/2 peak at 932 8 eV 15 Surfaces were cleaned prior to lead evaporation by cyclic ion bombardment and high temperature annealing XPS analysis of the clean surfaces before and after lead evaporation showed them to be free of contaminants with the exception of trace quantmes of carbon which gave a ls intensity of less than 0 1 °/,, of the Cu 2p3 2 or of the Ag 3d 5 2 intensities These values represent maximum contamination levels of approximately 0 05 monolayers respectively which was considered acceptable Lead overlayers were prepared by evaporation in the spectrometer of 4N purity lead from a tungsten-wire wound quartz evaporator onto the clean substrates at a constant, controlled rate of between 2 and 5 monolayers per hour The lead evaporatmn rate and coverage were calibrated using the X-ray Induced 93 eV kinetic energy lead Auger feature In an Auger signal intensity time plot (AS-T) AES studxes have shown that such a plot gwes a sharp 'knee' point at monolayer lead coverage 11 Although the 93 eV feature ~s very weak when photon induced and is on a high background c o m m o n in ESCA instruments, It provided a reliable and reproducible AS T plot For convemence, the Pb4f7 2/Cu3p peak area ratio or Pb4JT,z/Ag3ds,,2 ratxo was used to compare the lead coverage relative to the cahbrated 1 0 monolayer pomt Exposure of the surfaces to oxygen and 705
D Chadwick, A B Christie and M A Karolewskl
Reactivepropertiesof leadmonolayerson copperand sdversurfaces
hydrogen sulphide was carried out at r o o m t e m p e r a t u r e and gas pressures between 10 s and 10 . 6 torr
,~
~
.
{a)
3. Results and discussion 3.1. O x y g e n on Pb/polycrystalline and (1 l l ) C u . Prior to oxygen adsorption, the lead overlayers gave P b 4 f X-ray photoelectron spectra characteristic of clean metalhc lead I ~ for all coverages in the range 0 0 5 1 monolayers on polycrystalhne and ( l l l ) C u surfaces Exposure of the lead overlayers on b o t h copper surfaces to oxygen resulted m the a p p e a r a n c e of an O 1 s peak centred at 529 94-0 2 eV with a F W H M of 1 8 eV for s u b - m o n o l a y e r P b coverages, c o m p a r e d to values of 530 l + 0 1 e V and 1 9 eV respectively for the clean copper surfaces The O ls binding energies (b e ) a n d F W H M were found to be i n d e p e n d e n t of oxygen exposure After oxygen exposure the lead 4/ peaks developed a new feature arising from an oxidized lead species shifted 1 4 + 0 2 eW to higher binding energy c o m p a r e d to the metalhc peaks With increasing oxygen exposure this feature was observed to grow at the expense of the metallic peak, Figure 1(a) At no stage in the oxidation process was any o t h e r species observed which is consistent with the constancy of the O ls b e a n d F W H M O x i d a t i o n of polycrystalhne a n d single crystal lead surfaces results m the formation of o r t h o r h o m b l c P b O which has a 4 f c h e m l c a l shift of 1 0 eV c o m p a r e d to the clean metal l° 17 In view of the greater chemical shift on oxidation of the P b overlayers, the oxide structure c a n n o t be u n a m b i g u o u s l y identified However, it is likely that oxidation produces a lead m o n o x i d e species since the O 1 s/Pb4J7 2 (oxide c o m p o n e n t ) intensity ratio for one m o n o l a y e r P b coverage was found to be m good agreement with the 0 2 4 + 0 03 obtained for bulk lead 16 The intensities of the O ls peaks were found to be strongly d e p e n d e n t on oxygen exposure a n d P b coverage for b o t h polycrystalhne and ( l l l ) C u At low P b coverage, the surfaces took up oxygen as rapidly as the clean copper, but increasing the P b coverage results in a large decrease m the oxygen sticking coefficient on going from 0 05 1 m o n o l a y e r P b coverage This is s h o w n in Figure 2(a), where the O 1 s intensity for a c o n s t a n t 300L oxygen exposure is plotted against P b coverage for b o t h copper surfaces The O 1~ peak intensity measures the c o m b i n e d oxygen coverage of the P b overlayer a n d the exposed copper A measure of the rate of oxidation of the P b overlayers independently may be o b t a i n e d from the fraction of oxidized P b at a c o n s t a n t oxygen
Oxygen
/ / (Long/Ou5r;' _ ~ [ / - ' ~ exposure
0
I 0
\ \ X~-.--
I I I i 2 3 ev (a)
0 I 2 3 eV
i
(b)
Figure 1 Pb4Jv 2 spectra for oxidation of Pb overlayers at 0m,= 0 5 on (a) polycrystalhne Cu, (b) polycrystalhne Ag
706
--~
0 o
I
0
•o - % ~
05 Lead coverage,
~_
°
I
I monolayers
(b)
05
0
° ° .o
05
Lead coverage,
1
rnonoloyers
Figure 2. Oxidation of Pb overlayers on polyc rystalhne ( © ) and ( 111 ) ( • ) Cu with a constant oxygen exposure of 300 L (a) O Is intensity, (b) Pb oxide fractmn exposure F o r s u b - m o n o l a y e r coverages this is given by the ratio (Pb4J7 2 intensity, oxide c o m p o n e n t ) / ( t o t a l Pb4Jv 2 intensity) and the measured values are plotted against P b coverage in Figure 2(b) It is found that the oxidation rate of the P b overlayers varies essentially hnearly with P b coverage up to 1 m o n o l a y e r Since the ratio of O 1 s intensity to the Pb4fT. 2 intensity arising from the P b O is k n o w n (see above) it is possible to show that the rate of u p t a k e of oxygen by the exposed copper surfaces is not affected by the presence of the P b overlayers The structure of lead monolayers on copper surfaces has been extensively studied s 13, principally by Rhead and coworkersS 12 G r o w t h of the overlayers follows the SK m o d e forming dense two-dimensional structures at one m o n o l a y e r coverage in which the P b P b spacings are slightly different from bulk lead m order to faclhtate integral m a t c h i n g with the copper substrates Since Cu and P b exhibit neghglble m u t u a l solubility and P b AES signal intensities from a d s o r b e d P b monolayers on Cu remain c o n s t a n t over a wMe range of temperature, it can be concluded that surface alloy formation does not take place in this system s Only one ordered structure, p(4 x 4), corresponding to pseudo-(111) P b is observed on Cu(111 ) from approximately 0 3 1 m o n o l a y e r P b coverage O n the Cu(111) surface, therefore, the first m o n o l a y e r forms by growth and aggregation of closepacked, two-dimensional islands ° 13 The c o n t r i b u t i o n of different crystal orientations to the polycrystalhne surface is not known, but we may expect the formation of islands of closepacked P b at coverages a p p r o a c h i n g 1 monolayer, since this is observed for b o t h high and low index copper surfaces 1° 1= Two aspects of the novel oxidation b e h a v l o u r of the P b overlayers require explanation These are the dependence of the P b O fraction at 300 L oxygen exposure (and therefore oxidation rate) on P b coverage m the s u b - m o n o l a y e r range as shown in Figure 2, a n d the fact that close to 1 m o n o l a y e r coverage the P b overlayers are much less reactwe t h a n expected on the basis of bulk lead oxidation Since the t o p m o s t atomic layers m polycrystalhne and (111)Pb are completely oxld,zed at 700 and 1200 L exposure respecUvely 16, a P b O fraction at 300 L oxygen exposure in the range 0 25 0 4 would be expected for the P b overlayer at 1
D Chadwtck, A B Chnstte and M A Karolewskt Reactive properties of lead monolayers on copper and silver surfaces
m o n o l a y e r coverage, whereas the measured values are less t h a n 0 1 In fact, the m m a l sticking coeffiaents for oxygen on polycrystalhne a n d (111)Pb are 0 004 and 0 002 respectively, c o m p a r e d to 3 x 10 "~ for the complete P b m o n o l a y e r An e x p l a n a t m n of this latter b e h a w o u r m a y he in the two& m e n s l o n a l nature of the P b overlayers Bulk lead surfaces are k n o w n to oxidize by f o r m a t , o n of oxide nuclei at least 2 monolayers thick from the earhest stages of oxldatlon~ ~' ~s This is clearly not possible for the P b overlayers without considerable r e c o n s t r u c t m n for which there is no ewdence in our data Alternauvely the very low reactivity of the complete P b m o n o layer m a y arise from the slightly contracted P b P b spacing 13 The dependence of the overlayer oxidation rate on P b coverage may be explained m a n u m b e r of ways To be consistent with o u r data, a r a t e - d e t e r m m l n g step m the p r e d o m i n a n t oxidation m e c h a n i s m must be essentially a h n e a r functmn of P b coverage For example, the stacking coefficient of gas-phase oxygen on the P b overlayers may be a c o n t i n u o u s f u n c u o n of P b coverage This may arise m two ways Firstly, the oxygen m a y be a d s o r b e d to form oxide nuclm at island edge sites, resultmg in an oxidation rate p r o p o r t i o n a l to the fraction of edge a t o m s This would give rise to a dependence on the oxidation rate on 0pb~ 2, where 0 p b = P b coverage, rather than the observed linear dependence Alternatively. the physical or electromc structure of the overlayers m a y change with coverage While there appears to be a shght v a r m t l o n m P b b e with coverage, it seems unhkely that this would have such a drastic effect on the oxidation rate C o n t r a c t i o n of the P b P b spacing for which there is some evidence at m o n o l a y e r coverage ~ could conceivably affect the oxidation rate, but It is not reported to be a function of~sland area as would be required by o u r data If the sticking coefficient for direct a d s o r p t i o n of gas phase oxygen on the P b overlayer were i n d e p e n d e n t of P b coverage, it would be possible for o x l d a t m n to proceed by oxygen a d s o r p t m n on the exposed copper surface followed by surface diffusion of an oxygen ad-specms o n t o the P b islands, where P b O would be nucleated This 'splllover' process would be energetically favoured since the heat of f o r m a t m n of P b O is greater t h a n that o f C u 2 0 ~ 9 or the heat of a d s o r p t i o n of oxygen on Cu 2° The n u m b e r of oxygen ad-specms available for spillover would be related to the area of exposed copper surface (l--0pb) Thus, to a first approxJmaUon, the lead oxide fraction at a fixed oxygen exposure should be p r o p o r t i o n a l to ~1 - 0pb) This is m good agreement with the experimental data from b o t h Cu surfaces given m Figure 2(b) The m o b d e species may be oxygen a d - a t o m s since these are k n o w n to diffuse over large distances before being chemlsorbed on certain Cu surfaces 21 However, any m o b d e ad-specms would produce the same effect provided splllover were the rate h m m n g step In the P b oxidation
3.2, Hydrogen sulphide and oxygen on Pb/polyerystalhne and (I1 l ) c o p p e r . In contrast to the oxldatmn results, the P b 4 f peaks from the P b overlayers showed no new features after exposure to hydrogen sulphMe This is m agreement with results for bulk P b surfaces which do not sulphMe at r o o m t e m p e r a t u r e a7 H y d r o g e n sulphide was a d s o r b e d by the P b / C u c o m b i n a t i o n to an extent d e p e n d e n t on the P b coverage The ' s a t u r a t i o n ' sulphur 2p m t e n s m e s relative to the clean copper surfaces are a measure of the ' s a t u r a t i o n ' sulphur coverages, 0, The experimental values for the polycrystalhne surface are plotted against P b coverage, 0Vb, m Figure 3 a n d are found to be a good fit to the relationship 0, = (1 -- 0ph) Results o b t a i n e d from Pb/Cu(111 ) were found to be
~
05
4-t3
,>
g
0
O5
Leocl coveroge,
I
monolayers
Figure 3. Relative S 2p intensity after H2 S saturation of Pb Cu polycrystalhne surfaces as a function of Pb coxerage
consistent w~th this r e l a n o n s h l p Hence, there appears to be no significant P b sulphur interaction The sulphlded P b / C u surfaces were found to have a low sucking coefficmnt for oxygen similar to that for the Ore,= 1 surfaces This oxygen was taken up exclusively by P b to form the P b O specms It would a p p e a r that this low oxMatlon rate for Oph=l and sulphided P b / C u surfaces is that a p p r o p r i a t e to oxMatlon of the P b M a n d s with gas-phase oxygen directly a n d that the presence of sulphur blocks oxygen a d s o r p t m n sites on the exposed copper surface, thereby poisoning oxMaUon by the splllover process The above results o b t a i n e d from sulphlded surfaces and the h n e a r dependence of the o x , d a u o n rate of the overlayers on P b coverage for O,,b< 1 are strong evidence that for the Cu surfaces studmd, the d o m i n a n t oxidation m e c h a m s m is by splllover of an oxygen ad-specles from copper to lead 3.3. Oxygen on Pb/Ag. Prior to oxygen adsorption, the lead overlayer on polycrystalhne sliver gave P b 4 / X P E spectra characteristic of clean metalhc lead for all coverages m the range 0 05 1 5 monolayers Exposure of the P b overlayers to oxygen resulted m the a p p e a r a n c e of an O 1s peak and new Pb41 features shifted to higher b e relative to the clean, metalhc P b peaks Wtth increasing oxygen exposure the P b 4 l oxide c o m p o n e n t grew at the expense of the metalhc peak, Figure l(b) However, the Pb4/ chemical shift proved to be dependent on oxygen exposure and Pb coverage In addition, the oxidation rate of the P b overlayers was found to depend on P b coverage In order to examine dependence of the oxidation rate of the Pb overlayers on P b coverage is some detail, the O 1s intensity and P b oxide fraction were measured after a constant oxygen exposure of 150 L for several values of Opb At this oxygen exposure, there was a small oxygen adsorption on the clean Ag surface For the P b / A g surfaces, the O l s intensity at 150 L was observed to increase as P b coverage increased, level off and then fall to a low value at 0ph > 1 An idea of the oxldaUon rates of the P b overlayers were obtained, as m the Cu case, from the fraction of Pb existing as oxide after the c o n s t a n t oxygen exposure of 150 L The results are s h o w n m Figure 4 There a p p e a r to be two distract oxldaUon regimes according to P b coverage At low coverage. 0ph < a b o u t 0 5, the o x l d a h o n of the overlayers proceeds rapidly and is not strongly dependent on Pb coverage There is a fairly a b r u p t change in b e h a v l o u r at 0 p b = 0 5 0 7 after which oxidation proceeds at a slower rate, roughly i n d e p e n d e n t of 0p~, up to 2 monolayers In this latter regtme, the P b oxide fraction after 150 L oxygen exposure is a b o u t 0 2, which may be c o m p a r e d with the values of 0 2 and 0 13 expected for the t o p m o s t layer of 707
D ChadwIck,
A 6 Chr/st/e
and MA
Reactwe
Karolewskl
properties
of lead monolayers
are
/
I
consistent
and sliver
with
the
surfaces
reactive
behavlour
of the
overlayers on the polycrystallme surface The p(& x &)R30” structure 15dn open structure which would be expected to oxidize more rapldly than the close-packed twist structure Since the two structures co-exist to varying extents m the coverage range O,,=O 5 0 8 the shape of the oxldatlon rate curve, Figure 4, IS readdy understood However, further work IS necessary on the Ag( 111) surface before this mterpretatlon can be confirmed for the polycrystdlline surface
I
05
0
entirely
on copper
monolayers
Lead coverage,
Flgure 4 Oxldatlon of Pb overlayers on polycry\tallme oxygen expowre of 150 L Pb oxide frdctlon dgdm\t
Ag H Ith d conytdnt Pb coverage
dnd (1ll)Pb dt the \ame exposure The observed maxlmum m oxygen stlckmg coefficient 15 consistent with the relatively lower reactlvlty of Pb overlayers at coverages approachmg one monoldyer The existence of two oxiddtlon regimes IS also evident in the behdvlour of the Pb4j chemlcdl shift Imtldlly, at low oxygen exposure, t90 L for Oph5 0 5, the Pb4f peak from the oxide component was found to be broad dnd shlfted to higher be by 1 2 _+ 0 2 eV compared to the metalhc peaks, Figure 5 As oxygen exposure Increased, the oxide component of the Pb4f pedk became narrower and the chemical shift reduced to 0 8 + 0 1 eV after which this pedk contmued to mcrea\e m mtenslty The 0 1, b e for this oxide wds 529 0 k 0 3 eV At Pb coverages around H,,= I, the Pb4f component chemically chlfted by 0 8 eV was the only oxldlzed \pecles observed The 0 8 i 0 1 eV chemical shift 1s close to the value of 1 0 eV observed for oxlddtlon of bulk Pb which forms orthorhomblc PbO”’ I’ It seems reasonable to conclude thdt oxld‘ttlon of the Pb overlayers result\ m the form&Ion of d lend monoxide species However for low Pb coverages there 1s evidence m our data for d precursor oxide species with d shghtly gredter chemical shift LEED \tudles of the structure of Pb overlayers on Ag( 111) have been__ reported recentlyI lJ ‘* The first ordered structure, ~ polycrystalhne
~(~‘3 x ~‘3 )R30 , appears dl o,,, 10 2 dnd is complete at o,,, z 0 5 For II,,,> ‘ibout 0 5 n ‘twist’ structure develop5 conslstmg of mIsorIented close-packed domaIn I3 I’ These structural detdtls
3.4. Comparison of Pb/Cu and Pb/Ag. The results from Pb/Cu and Pb/Ag may be usefully compared The Pb/Ag system 1s essentially well-behaved m that each Pb overlayer structure appears to define an oxlddtion regime Wlthm these regimes, the reactlvtty of the overlayers dre not strongly dependent on Pb coverage In contrast, the behavlour of the Pb overlayers on Cu 1s clearly anomalous Since the reactlvlty of the close-packed Pb structure on the Ag surface appears to be approximately independent of coverage, the same would be expected of the close-packed structure known to form on Cu(ll1) In fact, the sulphldatlon results (3 2) demonstrated thdt this ts probably true for drrect reactlon with gas-phase oxygen The sharp contrast between the oxldatlon behavlour of the Pb/Cu and Pb/Ag systems 15 further support for the splllover mechanism m the Pb/Cu case The difference between the Influence ofCu and Ag substrates on the Pb overlayers presumably hes m the much greater affinity for oxygen of the former
4. Conclusions
It has been shown that the oxlddtion behavlour of Pb monolayers on polycrystalhne Cu and Ag surfaces and Cu( 111) 1s strongly dependent on Pb coverage m the range Up,,< 1 For Ag It 1s concluded that the Pb overlayers react with oxygen from the gas phdse directly and that each overlayer structure define\ dn oxlddtion regime In the Pb/Cu case, It 1s concluded that the dominant oxldatlon mechanism IS d spillover process in which an oxygen ad-species mtgrates from the exposed copper surface to the Pb Islands
Acknowledgement
k XI
0
I
2
ev
We thank the SRC and Esso Chemicals Ltd for the dwdrd of d CASE studentship to ABC and the SRC for the award of a studentship to MAK
References
x4
3 J
Flgure 5. Pb4/, 2 spectra for Ollh= 0 5 on Ag dt 90 L oxygen showmg resolution mto metdlllc dnd oxide componenta
708
expo\ure
1 E Bduer and H Poppd, Thrn So/id fr/m\, 12, 167 (1972) ’ P Helmdnn. H Neddermeyer and M Pessa, Phj\ Rev, B17, 427 (1978) ’ D Chddwlck and A B Chrlctle, Sur/uce SCI, 82, L293 (1979) ’ D Chadwtck and A B Chrlatle, ECOSS-3 Proceedmgs, Le Vu/e, /e\ Couche\ A41,1ce\, 201, 423 (1980) ’ W Palczewskd, 1 Slymerska, I RatdJczykowa and M LIpskI, Le VI&, Irr Couthr\ Mm-r\, 201, 407 (1980) ’ L Gonzales, R Mtranda and M Salmeron, Le VI&, Ire Couches fvf~nc+ 201, 229 (1980) ’ J W A Sachtler, M A Van Hove, J P Blberlan and G A Somoqal, Php Rrt Lrtr, 45, 1601 (1980) ” J Henrlon and G E Rhead, Surfuce Scr, 29, 20 (1972) 9 G E Rhead, J Vat SKI Technol, 13, 603 (1976) I” A Sepulvedd and G E Rhead, Surface Scr, 66, 436 (1977) ” C Argde and G E Rhead, J Phy~, C7, L261 (1974)
D Chadwick, A B Chrtst/e and M A Karolewskt Reactive properties of lead monolayers on copper and sdver surfaces 12 M G Barthes and G E Rhead, Sm[a~e S~t, 80, 421 (1979) 3 K J Rawhngs, M J Gibson and P J Dobson, J Ph) s, D I !, 2059 (1978) 14K Takayanagl and G Honjo, ECOSS-3 Proceedings, Le Vule, le~ Couches Minces, 201,267 (1980) ~5 G Johansson, J Hedman, A Berndtsson M Klasson and R Ndsson, J Electron Spe~tlos¢, 2, 295 (1973) l~, D Chadwick and A B Christie, J C ~ Farada~ I1, 76. 267 (1980)
t~ R W Joyner, K Klshl and M W Roberts, PJoc R Sot, A358, 223 (1977) Js R W Hewltt and N Wlnograd, Surla~e S~t, 78, 1 (1978) J9 Bull US Bureau Mines, 542 (1954) 20 I Toyosh~ma and G A Somorjal, Cutal~szs Ret, 19, 1(/5 (1979) 21 C S McKee, L V Renny and M W Roberts buJ/a~e S~t, 75, 92 (1978) 22 K Takayanagl, D M Kolb, K K a m b e and G Lehmpfuhl, Sin~ace ~'~1 100, 407 (1980)
709