Applicationsof SurfaceScience5 (1980)92—97© North-HollandPublishingCompany

SURFACE SEGREGATION IN Ni096Sb0~

GeorgeD. PARKSPhillipsPetroleumCompany,Researchand Development,Bartlesville,Oklahoma74004,USA

Received7 May 1979

An alloy of compositionNi096Sb004hasbeenstudiedusingX-ray photoelectronspectros-copy (XPS) andAugerelectronspectroscopy(AES). Annealingin vacuumat725°Cgavea stable,reproducibleantimony enrichedsurface. Surface layerswere removedby Ar+ sputteringandtop monolayer compositionwas calculatedfrom changesin Auger signal intensities.The firstmonolayerof the annealedalloy was foundto have an antimony atom fraction of about0.5.This suggeststhat an orderedmonolayerof “NiSb” maybe formedat thealloy surface.Exper-imental photoionizationcrosssectionsfor Ni and Sb werealso determined.

Dreiling andSchaffer [1] haveshownthat solid solutionsof antimonyin nickelappearto beimportantin thepassivationof nickel depositson crackingcatalystsbyantimonycontainingcompounds.Their calculationsof surfacecompositionof nick-el-antimonysolid solutionspredicta surfacelayerof antimonyover a secondlayerof purenickel. Thesecalculationsassumea regularsolution,andthereforemaynotbe appropriatefor the nickel antimonysystemwherelargevolume effectshavebeenobserved [21and surfaceorderingmay occur [3]. LatanisionandOpperhauser[4]have shown that antimonyand tin both segregateat nickel grain boundariesunderconditionswhich producehydrogenembrittlement,althoughno attemptwasmadeto measuresurfaceconcentrationsquantitatively.This studywasundertakento de-•termine theextentof segregationat thealloy—vacuuminterface.

An alloy of compositionNi096Sb004was preparedandstudiedby Augerspec-troscopy(AES) and X-ray photoelectronspectroscopy(XPS). The alloy wasmadeby melting a mixtureof highpuritypowdersunderhelium.A I X 1 X 0.1cm samplewas polished to a mirror finish and mounted on a samplestage which couldbeheatedto 1000°C by electronbombardmentof a piece of molybdenumfoil behindthe sample.Temperaturewasmeasuredusinga chromel—alumelthermocoupleheldagainst the front surfaceof the alloy. All datawere takenon a Model 548 PhysicalElectronicsIndustries surface analysissystemoperatedunder computercontrol.Auger datawere obtainedwith a 3 kV electronbeamandMgK~radiationwasused

for XPS.After initial removalof impuritiesby argonion sputtering,a clean, stableanti-

G.D. Parks/ Surfacesegregationin Ni0 96Sb004 93

— I I I I I I I I I

0 L~~Xl ixoKINETIC ENERGY (EV)

Fig. I. Augerspectrumof Ni096Sb004heatedto 725°Cin vacuum.

mony enrichedsurfacecouldbe createdby heatingthe sampleto 725°Cfor a fewminutes. The Auger spectrumof the sample following such a treatment(fig. 1)showedonly slight amountsof surfacecontamination.The surfacewas quite inerttowardadsorptionandshowedvery little increasein carbonor oxygensignalsafterseveral daysin thevacuumsystem(p 1 X I 0~Torr). To determinetheextentofsurfacesegregation,the annealedsamplewas sputteredwith a defocusedbeamofI kV argonions (Ar pressure= 5 X l0—~Torr) for five secondintervals;after everysputteringinterval Auger spectra of the nickel and antimonyregions were takenand the datawere storedby the computer.Fig. 2 showsthe depth profile for thesampleusing Auger peak-to-peakheights.The antimonysignal dropsdramaticallywhile the nickel peaksincreasein intensity,with the Ni MMM line increasingmorethanthe Ni LMM line.

The data were analyzedusin~the following model to determinethe extentofsurfacesegregation[5], whereI~is the intensityof an Augerpeakfrom elementAat energyE:

~ (I)

K containsincident electron current, instrumentaldetectionfunction, atomic

94 G.D. Parks/Surfacesegregationin Ni0 96Sb004

35

30

25

0000vvvvv— VVV~

~2O o N: ~ (61EV)V N:U44(8148EV)

~ SB~N(1494EV)o~150~o~

10 0

0

05 0~

0000

00 0 000 0000 00 000 DO 00 00 000 00 000 0 0’

I I ~ I I I I0 1 2 14 5 6 7SPLITTER TIME (SIINLIrES)

Fig. 2. Depthprofile for Ni0 96Sb004.

density (all assumedto be constant),and the backscatteringfactor (assumedto beunity for all layers).The variableXf~is the atom fraction of A in layer i, d is the

averageinterlayerspacing,XE is the electron mean free path in the alloy at energyE, and B is the analyzeracceptanceangle.Forthe alloy with no surfacesegregation

~ (2)

whereX~is the atom fractionof A in the bulk alloy. Assumingthat only thecom-position of thetop monolayeris affectedby segregation,we havefor the alloy withsurfacesegregation

~ _X~+x~]. (3)

Experimentally,we haveassumedthat the surfaceof the sputteredalloy showsno segregation(i.e., no selectivesputteringtakesplace).Thisassumptionis supportedby a simple analysisof the sputteredalloy usingtabulatedAugersensitivity factors

G.D. Parks/ Surfacesegregationin Ni0 96Sh0.04 95

[6] which indicatesa compositionof Ni095Sb005.Givenan experimentallydeter-mined valueof R, the ratio of the intensityofa givenAugerline from A on thean-nealedsampleto that on the sputteredsample,the atom fraction of A in thefirstmonolayerof theannealedalloy is

(4)

Therefore,by measuringthe increase(or decrease)in individual peak intensitiesupon sputtering,thecompositionof thefirst monolayercanbedeterminedindepen-dently for eachpeakmeasured.The increasein theNiMMM/NiLMM intensityratiowhich occursupon sputteringcanalso be usedto computefirst monolayercompo-sition; the use of this ratio makesthe solution less dependenton the alloy’s bulkcomposition.

Thecuberoot of the atomic volumeof polycrystallinenickel (0.22nm) wasusedas the averagelayer spacing.First monolayercompositionwascalculatedusingtwosets of meanfree path values. The first set,X~,was determinedusing theoreticalparameterstabulatedby Penn [7] and a value for the NiMMM line (Penn’svaluesare not valid below 200 eV) from a “universalcurve”plot by Palmberg[8]. A sec-ond set, X5, was determinedusing an empirical equationpublished by SeahandDench [9]. Table 1 gives line intensity ratios of annealedto sputteredsamplesde-terminedfrom bothpeak-to-peakheightsand integratedpeakareas.Mean free pathvaluesare also given alongwith calculatedvaluesof X1

51~,theconcentrationof anti-mony in the first monolayer.The dataidentified with “NiMMM/NiLMM” give val-uesofX~calculatedfrom the changein the intensity ratio of the NiMMM line totheNiLMM line upon scattering.

Table 1Alloy R valuesandcorrespondingfirst monolayerSb concentrations

Line R x~p—p heights(peakareas) a) b)

NiMMM 0.756 0.49 0.45(0.747) (0.51) (0.46)

NiLMM 0.866 0.54 0.57(0.822) (0.71) (0.74)

SbMNN 5.19 0.50 0.58(5.78) (0.56) (0.66)

NiMMM/NiLMM 0.873 0.44 0.36(0.909) (0.36) (0.28)

a) 0.41 nm, 1.01 nm and0.66 nm. b) = 0.37 nm, 1.25 nm and0.92 nm.

96 G.D. Parks / Surfacesegregationin Ni0 96Sb004

With the exceptionof the peakareasfrom the NiLMM line, data are centeredaround a first monolayercompositionof aboutNi05 Sb05. The inconsistentdatafor theNiLMM peakareasmaybe due to theproblemsin settingalinearbackgroundfor the peakareadetermination,The integrationlimits were set beforetheexperi-ment was begun,anda changein the shapeor the slope of the backgroundduringsputteringcould haveartificially affectedthe integratedarea.Peak-to-peakheightsin thederivativespectrumare affectedlessby suchchanges.

XPS peakpositions and areasfor sputteredandannealedsamplesare listed intable2, along with relative valuesof the experimentallymeasuredphotoionizationcrosssections.Peak areaswere numericallyintegratedafter subtractionof a linearbackgrounddrawnto include themain peakandany“satellite” lines athigherbind-ing energies.Crosssectionswere determinedusingPenn’smeanfree path data [7],assumingthat the Ni096Sb004 is coveredby a monolayerof Ni05 Sb0,5. Ni 2p~was arbitrarily assigneda crosssection of 13.92 for comparisonwith Scofield’sval-ues [101. Data for nickel show good internalagreementwith the calculatedcrosssections.Calculatedantimonycrosssectionsdo notagreeso closely,and differ withthe measuredvaluesby as much as 33% relative to the nickel values. It shouldbenotedthat the validity of thesemeasuredcrosssectionsis dependentuponthe cor-rectnessof our interpretationof the Auger sputteringdata.

Differencesin atomic radii andsurfacefree energies(~GNi= 61.7kJ X mol~.

Table 2XPSdatafor annealedandsputteredsamples

Line Binding energya) Cross-section

(Intensity)b)

Sputtered Annealed Measured Calculated

Ni2p3,2 852.6 852.4 13.92 13.92(100) (100)

Ni3s NM c) 110.4 0.768 0.753(3.88)

Ni3p 66.5 66.3 1.97 2.06(10.9) (9.85)

Sb3d512 528.0 527.8 19.22 16.13(5.0) (22.7)

Sb4d 32.7 32.3 4.45 2.98(1.0) (3.4)

CIs 283.9 ND c)(2.0)

a) Cu2p3,2 = 932.5, Cu3p3,2= 75.0. b) Ni2p312 = 100.c) NM = not measured,ND = notdetected.

GD. Parks/ Surfacesegregationin Ni0 96Sb0~ 97

~Gsb = 24.6 kJ X moI~[11]) both tend to causeantimonyto segregateto thesurface of nickel. Our data indicate that antimony segregatesto the surfaceandreachesa saturationlevel of aboutone halfmonolayer.It is possiblethat the strongbondingbetweennickel andantimony(iXFJ~.= 32 kJ X mol~(12))leadsto a stablesurface nickel antimonide type compoundso that no moreantimonysegregates.Guttmann [3] has predictedthat metals capableof forming compoundswithB8(NiAs) structuresare likely to show bidimensionalcompoundformation at sur-faces. OverburyandSomorjai [13] observedsimilar behaviorfor thegold—tin sys-tem where first monolayercompositionsclose to Au0 5Sn05were foundfor threedifferent bulk phases.The Cu—SnandNi(lOO)-chalcogensystemshavebeenshownto form ordered overlayers at maximum segregation[14,15]. All of thesesystemsform bulk phaseswith B8 structure.The half monolayersaturationcoverageonnickel-antimonysolid solution suggeststhat this systemmay also form an orderedoverlayerstructure,although LEED experimentswill be neededto fully describethesurfacestructure.

References

[1] M.J. Dreiing and A.M. Schaffer, J. Catalysis56 (1979) 130.[21 J. Hudis, M.L. Perlman and R.E. Watson, Am. Inst. Phys. Conf. Proc. 18 (1973) 267.

[31M. Guttmann, SurfaceSd. 53 (1975)213.[41R.M. Latanision and H. Opperhauser Jr., Met. Trans. 5 (1974)483.[5] J.M. McDavid and S.C. Fain, Surface Sci. 52 (1975) 161.[61L.E. Davis, N.C. MacDonald, P.W. Palmberg, G.E. Riach and RE. Weber, Handbook of

Auger Spectroscopy(PhysicalElectronicsIndustries, Eden Prairie, Minn., 1976).[7] DR. Penn, J. Electron Spectry. 9 (1976) 29.[8] P.W. Palmberg, Anal. Chem. 45 (1973)549A.[91M.P. SeahandW.A. Dench,Natl. PhysicsLab. Report, Chem. 82 (April, 1978).

[101 J.H. Scofield, J. Electron Spectry. 8(1976)129.[11] S.H. Overbury, P.A. Bertrand and GA. Somorjai, Chem. Rev. 75 (1975)547.[12] R. Hultgren, PD. Desai, D.T. Hawkins, M. Gleiser and K.K. Kelley, SelectedValuesof the

Thermodynamic Properties of Binary Alloys (Am. Soc. for Metals, Metals Park, Ohio,1973).

[13] S.H. Overbury and G.A. Somorjai, J. Chem. Phys. 66 (1977) 3181.[14] J. Erlwein and S. Hofmann, Surface Sci. 68 (1977) 71.[15] J.E. Demuth, D.W. Jepsenand P.W. Marcus, Phys. Rev. Letters 31(1973)540.