Determination of adsorbate coverages by leed and XPS

Journal of Electron Spectroscopy and Related Phenomena, 20 (1980) 281-287 0 Elsevler Sclentlflc Pubhshmg Company, Amsterdam - Printed m the Netherlands

DETERMINATION OF ADSORBATE COVERAGES BY LEED AND XI’S

JAY B BENZIGER Department

and ROBERT

J MADIX

of Chemrcal Enganeenng,

(First received 11 September 1979,

Stanford

Unrversrty, Stanford,

CA 94305

(US A )

m final form 14 January 1980)

ABSTRACT Layers of carbon, oxygen, sulfur and potassmm adsorbed on an Fe(lOO) surface were stu&ed by LEED, AES and XPS When examined quantltatlvely by XPS, saturated c(2 X 2)CO(p), c( 2 x 2)C and p(1 X 1)O surface structures yielded the C/O ratios expected from surface coverages of 0 25, 0 5 and 1 0 monolayer, respectively When these results were normalized to the equivalent coverages of 1 0 monolayer, the relative XI’S crosssections obtained for S, 0, C and K were found to agree closely mth the results of theoretical calculations The results illustrate the use of these techniques for the quantltatlve determination of surface coverages

INTRODUCTION

Absolute coverage determmatlons for adlayers on surfaces has proved to be dlfflcult Low-energy electron diffraction (LEED) has often been used for adlayers that show well-defined LEED patterns [l-3] However, LEED 1s limited to coverages by specific adsorbates, and the results may be obscured due to island formation, adsorption mto the sub-surface re@on, or superposltlon of different phasedomams [4] . Auger electron spectroscopy (AES) can also be used to determine surface coverages [ 4,5], but because of problems m measunng Auger excltatlon cross-sections it has found greater use simply as a check for surface contammants X-ray photoelectron spectroscopy ls a technique well suited for coverage determlnatlons The photoelectron yield 1s given by the product of the number of emitters (adsorbed atoms), the photon flux, the photolomzatlon cross-section, and the collection efficiency. In a typical experunent, the photon flux and collection efflclencles are constants, and the photolomzatlon cross-se@lons for core-level electrons have been theoretically calculated, to a fau degree of accuracy [6] One cahbration standard should then he sufficient for coverage determmatlons for adlayers of different atoms. Welldefined adlayers of sulfur, carbon, oxygen, and potassmm on an Fe(lOO)

surface were prepared to test this hypothesis, and LEED and AES were combmed with XPS to verify surface structures and composltlons The relative photolonlxatlon cross-sections determined from these expernnents were then compared to theoretical values, and excellent agreement was found

EXPERIMENTAL

Experiments were carried out m a stamless-steel ultrahigh-vacuum chamber described elsewhere [7] A clean Fe(100) surface was prepared by argonsputtering and anneahng its LEED pattern 1s shown m Fig la AES and XPS showed a small residual carbon nnpurlty, which was found to correspond to -3% of a monolayer The photoelectrons were excited with an Mg Krr anode (1254 eV) and collected by a double-pass, cylmdncal-mn-ror analyzer The expernnental photoelectron yields were obtamed by mtegratlon of the area under the density-of-states curve, as Indicated m Fig 2 for the Fe( 2p3,2 ) transition Baselines for the adsorbates were estabhshed from the spectrum for a clean Fe(lOO) surface In choosmg a baseline for the Fe(2p,,, ) tranatlon, it was only necessary to take a value consistent across all the experiments, as the iron peak was used only for reference purposes Absolute

Fe(iOO)

(a)

(b) Fe(lOOkcI2x2)C

f

L

(4

IIIWU.,

W100)-p(1x1~o

id)

1..

Pe(IOC+pi2~21-i?

Fig 1 LEED patterns of adsorbates on Fe(lOO) ~(lXl)O,(d)p(2XW

(a) Clean surface, (b) c(2 X 2)C, (c)

283

FelEpl FOR

X-RAY

PHOTOELECTRON

EMISS1ON

Fe 11001 2P3,

690

700

BINDING

710

ENERGY

720

73(

(eV1

Fig 2 XPS of Fe( 2p) The shaded area represents the Fe( 2psn ) emission

coverages were determmed by scahng the ratio of the adsorbate photoelectron yield to the Fe(2p,,, ) yield agamst the ratlo for the calibration standard Bmdmg-energy scales are all referenced to the Fe( 2p3,2 ) transltlon at 707 0 eV (determined agamst an Au standard [ 81)

RESULTS

It has previously been observed that sulfur segregates to Fe(lOO) surfaces to form a well-ordered c(2 x 2) structure [ 7] LEED mtenaty analysis has shown this structure to result from sulfur atoms which occupy every other mterstltral site of four-fold symmetry on the Fe(lOO) surface [9] Saturation of the c(2 x 2) overlayer corresponds to one half of a monolayer coverage An Fe(lOO) - c(2 x 2)s surface was prepared by segregating sulfur to the Fe(lOO) surface, whereupon Auger spectroscopy mdlcated sulfur to be the only species adsorbed The S(2p) transltlon was used to determme the coverage The S( 2p)/Fe( 2p3,2 ) yield ratio for the Fe(100) - c( 2 x 2)s surface 1s Bven m Table 1, along with a scaled ratio for coverage to a full monolayer, calculated by assummg the prepared surface to be covered uniformly to 0 5 monolayer (Note that the total S(2p) emlsslon was used, as it was not possible to resolve the 2p,,, and 2p1,2 spin-states ) The decomposltlon of ethylene on Fe(lOO) has previously been shown to result m an ordered c(2 x 2) carbon overlayer [lo] In the present work, adsorption of C2 H4 at 200 K and subsequent heatmg to 400 K resulted in the evolution of H, from the surface and the formation of a ~(2 x 2)C overlayer. Contmued heating above 400 K caused the carbon to diffuse mto the crystal, as evrdenced by a decrease m the intensity of the half-order diffraction spots m the LEED pattern and a decrease m the C( Is) photoelectron emission [ 71. To form a saturated surface carbide without formmg a bulk carbide, the

284

crystal was exposed to Cz H4 at 200 K and heated to 400 K three times, after which no desorptlon of Hz was observed. LEED gave a sharp c(2 x 2) pattern, as shown m Fig 1, which was assumed to result from 0 5 monolayer carbon coverage The Auger spectrum for this surface mdlcated no other lmpuntles the C( ls)/Fe(2p,,2 ) ratio obtamed for this surface ISqven m Table 1 Adsorbed carbon monoxide provided a second cahbratlon-standard for carbon In the present mvestlgatlon, CO was adsorbed at temperatures between 150 and 600 K At low temperatures (< 350 K), no additional spots were seen m the LEED pattern, however, adsorption at > 400 K resulted m a well-developed c(2 x 2) pattern, m agreement mth the results of previous mvestlgatlons [lo, 111 Furthermore, If CO was adsorbed at 200 K and the sample was then heated to 450K, LEED gave a c(2 x 2) pattern Further heating of the sample to > 800 K resulted m desorptlon of CO and restoration of the clean surface. The XPS spectra for CO adsorbed on Fe( 100) at > 400 K showed that the bmdmg energy of C(ls) was the same as that for the surface carbide, and that of O(ls) the same as that for a surface oxide These results concur vvlth those of other mvestlgators who concluded that CO adsorption on Fe( 100) at > 400 K 1s dlssoclatlve [8,10,11] A LEED mtenslty analysis [11] has shown the c(2 x 2) structure to be the result of CO dlssoclatlvely adsorbed with the carbon and oxygen atoms m random occupation of every other mterstltral site of four-fold symmetry [ll] At saturation coverage, the c(2 x 2)C0 overlayer would correspond to 0 25 monolayer of carbon and of oxygen atoms To use the c(2 x 2)CO(/3) surface (this designation 1sused to indicate the dlssoclated state of CO when adsorbed on Fe( 100)) as a calibration standard, a saturated CO(p) surface was prepared by contmual adsorption of CO at 250 K and by heating to 500 K until no further change was observed m the mtenslty of either the half-order diffraction spots or both the C(ls) and 0( 1s) X-ray photoelectron spectra The photoelectron yield for carbon was consistent with the assumption of 0.25 monolayer TABLE 1 CALIBRATION

STANDARDS

FOR ADSORBATES

ON Fe( 100)

Transltion

LEED Pattern

Ratio of XPS yieldsa

Scaled ratlobfor one monolayer

S( 2P)/Fe( 2P3/2 ) C( WlFe(2P312 ) C( WFe( 2P3/2 ) Ws)/Fe(2P3/2) O( W/Fe( 2P3/2 ) K( 2P)lFe( 2P3/2 )

c(2 x 2)s c( 2 x 2)C

0 0 0 0 0 0

0 0 0 0 0 0

c$

; q~ow)

030 016 0075 085 020 030

a Rates determmed for the covered surface b Values scaled from the experimental ratios for fractIona coverages

060 032 030 085 080 120

285

carbon coverage, as shown by the results m Table 1 The surface thus prepared also provided a cahbratlon standard for oxygen (see Table 1) A second cahbratlon standard for oxygen was provided by a ~(1 x 1) oxygen overlayer formed on Fe(lOO) Exposure of the Fe(lOO) surface to oxygen while heating to 800 K resulted m a sharp p(1 x 1)0 LEED pattern, shown m Fig 1 Further exposure to oxygen while mamtammg the crystal at 800 K resulted m no change m the LEED pattern, m the Auger spectrum, or m the O(ls) XPS mtenslty Furthermore, no trace of Fe2+ or Fe3+ was mdlcated by XPS (however, exposure of the crystal to oxygen at 300K resulted m the formation of iron oxide, vvlth evidence from XPS for Fe3+) If the Fe(lOO) - p(1 x 1)0 surface was assumed to result from adsorption of one monolayer of oxygen, the cahbratlon standard shown m Table 1 was obtained other mvestlgators [ 12,131 have observed the formation of a p(1 x 1)0 overlayer dunng the early stages of oxldatlon, and LEED mtenslty analysis has shown it to result from adsorption of one monolayer of oxygen atoms which occupy every four-fold mterstltlal site on the Fe(lOO) surface [12] , this result 1s consistent with those obtamed here Potassium was deposited on the Fe(lOO) surface from a potassium-ion beam A potassmm-ion emitter (Spectra-Mat, Inc ) delivered a current of 1 PA to the Fe( 100) surface which was held at a potential of -50 V After 50s at 1pA (3 1 x 1Ol4 atoms), a p(2 x 2)K LEED pattern was obtained as shown m Fig 1 The Auger spectrum for this surface indicated very little contammatlon (less than 5% of a monolayer of carbon or oxygen) The K(2p) doublet was also clearly resolved, which m&cated CO contammatlon to be mmunal, as adsorption of CO causes the K(2p) doublet to be obscured [ 71 The p( 2 x 2)K LEED pattern was assumed to result from 0 25 monolayer of potassium adsorbed on the Fe(lOO) surface, on this basis, the cahbration standard was set as shown m Table 1

DISCUSSION

The correlation of LEED and XPS results presented here mdlcates the utlhty of XPS m determmmg coverages of adsorbates Table 1 clearly shows that the photoelectron yields obtamed for the Fe(lOO) -- ~(2 x 2)C and Fe( 100) - p(1 x 1)0 surfaces are consistent v&h the values obtamed for the Fe( 100) - ~(2 x 2)CO(/3) surface These results are m accordance with a lmear dependence of photoelectron yield on coverage, and m&cate that XPS may be used to dlstmgulsh LEED patterns which are the result of two dlfferent coverages A second important feature of these results 1s the agreement between the relative photolonlzatron cross-sections determined experunentally m this work and those predicted by theory (Table 2) The difference m all cases 1s < lo%, m&catmg that a single calibration standard could have been used with confidence to determine the coverage by any surface species

286 TABLE 2 PHOTOIONIZATLON RELATIVE TO C(ls)

CROSS-SECTIONS

Element

Theoreticala

Experlmentalb

C(ls) O( 1s) S(2P) K(~P)

100 2 85 1 74 4 04

10 27 19 38

a Ref 6 b This study

This conchuaon 1s important for adsorbates that do not form ordered surface structures, as results may be extrapolated from an adsorbate for which the coverage 1s known The msensltlvlty of core-level photolomzatlon crosssections to chemical environment and the ability of XPS to predict crosssections, therefore, make this a very useful technique m determmmg coverages by adsorbates It should be noted m addition that, m each case, the surface coverage assumed from the LEED patterns was quite close to that expected on the basis of XPS measurements Thus, m characterlzmg surface structures, considerable confidence can be placed m deductions of coverage from LEED structures, at least for the case of species adsorbed on non

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the support of the National Science Foundation through Grants NSF Eng 77-12964 under which this work was performed and NSF Eng 75-14191 with which the equipment was purchased

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