Application of parallel factor analysis and x-ray photoelectron spectroscopy to the initial stages...

SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 27, 1037–1045 (1999)

Application of Parallel Factor Analysis andX-ray Photoelectron Spectroscopy to the InitialStages in Oxidation of AluminiumII. The O 1s Photoelectron Line

T. Do1;2* and N. S. McIntyre 2;3

1 Department of Materials and Mechanical Engineering, University of Western Ontario, London, Ontario, Canada N6A 5B92 Surface Science Western, University of Western Ontario, London, Ontario, Canada N6A 5B73 Department of Chemistry, University of Western Ontario, London Ontario Canada N6A 5B7

Three-way parallel factor analysis (PARAFAC) has been used to decompose several series of XPS O 1sspectra obtained in the study of oxidation of aluminium by water vapour. Reaction time, reactant pressureand binding energy in the XPS spectrum are the independent variables. Four factors have been extractedthat characterize the oxidation process of aluminium by the interaction of water vapour. Two of these factorsare associated with the formation of a lattice oxide, either on a metal surface or on a hydrided metal surface.The other two factors result from attachment of OH groups either to a metal surface or to the oxide. Allfactors extracted have a dual functionality: the oxide (or hydroxide) component and a water component, thelatter arising from water produced in each reaction. The PARAFAC analysis was essential in isolating theseparate effects of pressure and time on the oxidation process. Copyright 1999 John Wiley & Sons, Ltd.

KEYWORDS: aluminium oxide; oxidation; factor analysis; XPS

INTRODUCTION

The photoelectron line generated from oxygen atoms isdifficult to analyse and interpret in any chemical experi-ment using the XPS technique. This results from the widerange of energy states in which oxygen can be bound on asurface, e.g. as adsorbates, hydrates, hydroxides or latticeoxides. However, the information contained in this line-shape is important to any mechanistic study of metal oxi-dation and this has prompted a more systematic analysisin connection with our continuing studies of the oxidationof aluminium.

Most earlier XPS studies of aluminium oxidation1 – 3

have used the Al 2p lineshape to distinguish oxidicand metallic components. Although this information isvery useful for measuring the oxide growth kinetics,4 theamount of information from this lineshape concerning thechemical nature of the oxide has been limited. Recently,we have applied an advanced form of factor analysis, theparallel factor analysis (PARAFAC) method,5 to show theexistence of three separate components in the Al 2p spec-trum, which change during the early stages of the reactionof aluminium with water vapour as a function of pressureand exposure time. The present contribution represents theuse of the PARAFAC method to decompose several seriesof XPS O 1s spectra from the same study of the oxidationprocesses of aluminium surfaces with water.

As in our previous study,5 the use of the PARAFACmethod requires a concerted evaluation of the number of

* Correspondence to: T. Do, Surface Science Western, University ofWestern Ontario, London, Ontario, Canada N6A 5B7.E-mail: [email protected]

independent factors present in the data matrix. The factorsobtained from present analysis of the O 1s spectrum,combined with the information provided by the previousanalysis of the Al 2p spectrum obtained under parallelconditions, have allowed a more detailed model to be con-structed of the first stages of oxide growth following theinteraction of aluminium and water. This model suggestsimportant roles for both OH and H as precursors to theoxide formation and as participants in the oxide networkthat forms.

EXPERIMENTAL

Polycrystalline aluminium (99.999% purity) was suppliedby Alcan Aluminum Ltd., Kingston, Ontario. Specimenswere polished to a 0.05 µm Al2O3 finish, degreasedultrasonically, annealed in a vacuum (573 K, 30 min) andcleaned by ArC ion bombardment (3 keV, 10 min). Cleansurfaces then were exposed to water vapour in a separatecustom-designed preparation chamber attached to the XPSspectrometer at pressures ranging from 2.0 ð 10�6 to6.5ð10�4 Pa through six levels (levels 1–6 correspond to2.0ð10�6, 7.8ð10�6, 1.3ð10�5, 6.5ð10�5, 1.3ð10�4 and6.5ð 10�4 Pa) and exposure times of 1–60 min through15 levels (for every minute from 1 to 10 min, then for 15,20, 30, 45 and 60 min). After exposure to water vapourthe sample was transferred back into the XPS analyticalchamber and an XPS spectrum was measured at anoperating base pressure of 4.0ð10�7 Pa. All XPS spectrawere obtained using a Surface Science Laboratories SSX-100 spectrometer equipped with a monochromatic Al K˛(1486.6 eV) x-ray source; the binding energy scale was

CCC 0142–2421/99/121037–09 $17.50 Received 11 February 1999Copyright 1999 John Wiley & Sons, Ltd. Revised 26 May 1999; Accepted 20 July 1999

1038 T. DO AND N. S. MCINTYRE

calibrated to give an Au 4f7/2 photoelectron line positionat 83.98 eV. Figure 1 shows a representative series ofXPS O 1s spectra of clean aluminium surfaces exposed towater vapour at a pressure of 2.0ð 10�6 Pa. The centroidof the adventitious reference C 1s peaks was observed at286.20š 0.05 eV, which is in reasonable agreement withsome reported values in an earlier round-robin study.6 Inaddition, a value of 286.4š 0.3 eV is recommended foruse as a charge reference for aluminium oxide (V. Crist,XPS International, http://www.xpsdata.com/, personalcommunication). There was no charging effect observedfor all thin oxide films grown on aluminium surfaces inour exposure conditions described above. As evidenceof this, Fig. 2 shows the XPS Al 2p and O 1s spectracollected after exposure of a clean aluminium surface towater vapour at a pressure of 6.5ð 10�4 Pa for 20 min(pressure level 6 and time level 12) with and withoutthe use of an electron flood gun; the thickness of theoxide film is estimated as 1 nm. The pass energy ofthe hemispherical analyser was maintained at 50 eV,giving a constant energy resolution of 0.53 eV. Theuptake of oxygen on aluminium surfaces, as a result ofits interaction with water vapour at room temperature,was monitored by following the changes in the intensityof XPS Al 2p and O 1s photoelectron lines. A Shirleybackground subtraction was performed for all spectra asthe data preprocessing needed for the PARAFAC analysis.The XPS O 1s data matrix (dimensions of 94ð 15ð 6)for the three-way analysis is then built up from all O 1sspectra taken through studied ranges of exposure time andpressure. Similar to our previous work on XPS Al 2pdata,5 the PARAFAC analysis on O 1s data was carried outusing a program called ‘PMF3’ (three-way positive matrixfactorization7) with a weighted least-squares algorithmand non-negativity constraint.

Figure 1. Representative series of XPS O 1s spectra obtainedafter exposing clean aluminium surfaces to water vapour at apressure of 2.0 ð 10�6 Pa through 15 levels of exposure timefrom 1 to 60 min.

Figure 2. The effects of flood gun (FG) application on the XPSAl 2p (a) and XPS O 1s (b) spectra of a thin aluminium oxidefilm overlayer (thickness of ¾1 nm), which was formed afterexposure of clean aluminium surfaces to water vapour at apressure of 6.5ð 10�4 Pa for 20 min. The emission current in allFG-on cases was 50 µA.

RESULTS AND DISCUSSION

The applicability of the PARAFAC methodto the anal-ysis of the XPS data is basedon two assumptions:anXPS spectrumof an elementis a sum of severalpeaksrepresentingits chemicalshifts due to different bondingenvironmentsin theanalysedarea;andthefirst-principlesmodel of the XPS intensity can be extendedto a mul-tiplicative model, in which the effect of a set of sev-eral physical independentvariableson the intensity isdescribedas the productof individual effects from eachvariable.Thus,assumingthat the effectsof two indepen-dent physical variables,such as exposuretime (t) andpressure(p), on the photoelectronintensity (of the ithline) can be characterizedby two multiplication coeffi-cientsor factorsfti andfpi, the XPS intensity affectedsimultaneouslyby thesetwo variables,IZ,tp.E/, can beexpressedas

IZ,tp.E/ Dn∑iD1

IZ,i.E/ftifpi C etp.E/ .1/

wheren is thenumberof componentpeakspresentin thespectrum,IZ,i.E/ is the contributionof the ith componentpeak from elementZ at kinetic energy E, and etp.E/is the error term. By applying the PARAFAC techniqueto analyse a set of XPS data, one can obtain these

Surf. InterfaceAnal. 27, 1037–1045 (1999) Copyright 1999JohnWiley & Sons,Ltd.

PARALLEL FACTOR ANALYSIS AND Al OXIDATION 1039

quantities in Eqn (1) separately:IZ,i.E/, fti and fpi(as factor loadings) as a function of kinetic energyE(or equivalent binding energy, BE), exposure timet andpressurep, respectively. In the solution, these factorloadings are compiled into three matrices called BE mode,time mode and pressure mode. In general, there aretwo main steps in the PARAFAC analysis: determinationof factor number and interpretation of factors. In thefirst step, some additional analysis and calculations ortechnique may be needed to find a correct factor number,e.g. error and fit analysis were helpful to determine thefactor number for XPS Al 2p data.5 In this paper, theuse of curve-fitting technique combined with perturbationanalysis has been successful to refine the factor numberfor XPS O 1s data.

Determination of factor number

In the PARAFAC analysis the number of factors involvedin a data set must first be determined. Our previous papershowed that this could be accomplished by a plot of‘goodness of fit’ of the data vs. the number of factorsused in the calculation, which is shown in Fig. 3. Fromthis plot, the right factor number can be selected based onthe fact that as the factor number increases beyond the trueone, the data are fitted more and more by noise structureand therefore the fit will be improved only by a noticeablysmaller amount. Thus, a gradual change in fit should beobserved in the fit after the factor number has reached theright value. For the Al 2p data, obtained previously, a cleardiscontinuing ‘elbow’ in the curve signals the appropriatefactor number 3 (see Fig. 3). However, for the O 1sdata, also shown in Fig. 3, no ‘elbow’ is apparent andtherefore other statistical criteria must be used to discoverthe factor number for the O 1s data. The observed gradualchange in the fit value (Fig. 3) may be due to complexcorrelations between factors, which compete with the errorstructure in the O 1s data. Therefore, if the data werefitted by correlated factors, there would be only a smallimprovement in the fit as the factor number used in thecalculation increases.

Figure 3. Goodness of fit .�2/ as a function of the number offactors used. Plots are shown for the Al 2p5 and O 1s data sets.Goodness of fit was calculated as �2 D Q/N. The fit value Q isthe sum of error squares: Q D∑[eij.E/]2; N D 8460, which is thenumber of elements in the O 1s data matrix.

Another criterion that can be usedto find the correctfactor numberis the uniquenessof the solution for dif-ferent randomly initiated starting points in the iterativeprocedureof thePARAFAC calculations.Our calculationsshowedthat differentPARAFAC solutionswereobtainedfor differentrandomlyinitiatedstartingpointsfor a modelwith five factorsandhigher.Furtheranalysisof solutionsfor a factornumberof<5 hasshownthatthecorrectfactornumberis most likely to be 4. This numberis confirmedby a comparisonof thechangein factor loadingfor Al 2pand O 1s dataas a function of pressure.Factorloadingsin the pressuremodeobtainedfrom PARAFAC solutionsusing two-, three-and four-factor modelsfor O 1s dataareshownin Figs 4(a), 4(b) and4(c). Figure4(d) showsthefactorloadingsin thepressuremodefor theAl 2p datafor two factors,which representchemicalspeciesof alu-minium relatedwith oxygenatoms.FromFig. 4, only theO 1sfour-factormodelgivesloadingvaluesthataresimi-lar to thosefoundfor theAl 2p datain thepressuremode.The factor loadingsfor the time modeof the four-factormodel for O 1s dataareshownin Fig. 5(a); the relevantfactor loadingsfor the Al 2p dataareshownin Fig. 5(b).Another approachbasedon perturbationtheory that alsohasidentifiedfour factorsis describedin the Appendix.

In summary,four factorshavebeenfoundto character-ize theO 1sdata.Furtherrefinementandcomparisonwiththe literatureshowsthat thesefactorscanbe assignedtotwo kindsof ‘hydroxyl groups’:an ‘oxide associatedwithwater’ andan ‘oxide modifiedby water’ molecules.Thisis discussedbelow.

Refinementof componentpeaks

Figure6 showsthe factor loadingsin the BE modefromthe PARAFAC solution with the four-factor model forthe XPS O 1s data. From Fig. 6, all factors reveal amulti-peak structure,particularly so for factor 3. Eachfactorspectrumcanbethoughtof asrepresentingauniquechemicalprocessthatgivesriseto amulti-componentO 1sspectrum,i.e. there are severaldistinct oxygen speciesthatareboundto a surfaceasa resultof a singleprocess.Eachfactorspectrumthencanbedecomposedusingleast-squarescurve-fitting techniques.If two componentpeaksareusedto fit eachof the four factorspectra,theresultantpeak positions and shapesappearto have some strongconsistencies,which arepresentedin Fig. 7, i.e. a plot ofbinding energy vs. peak width from fitted peaksfor allfour factor spectra.This suggeststhat identicalor nearly-identicaloxygenchemicalstatesarepresentwithin morethanone factor. Resultsof curvefitting of factor spectraarepresentedin Table1. FromFig. 7, it canbeconcludedthat there are four common oxygen chemical statesorcomponentpeaks,which contributein pairsto eachof thefour factor spectra.The averagevaluesof binding energyandpeakwidth, i.e. full width at half-maximum(FWHM)of thesefour componentpeaks(all in eV) are: I(531.78,1.6), II(532.36, 1.6), III(532.95, 2.4) and IV(533.85, 2.1).A further rationalefor thesefour componentpeaksusinga procedurebasedon perturbationtechniqueis presentedin the Appendix.

In summary,the PARAFAC analysisof the XPS O 1sdata has shown that there are four componentpeakspresentin the XPS O 1s spectrumthat are associatedin pairs to the four factors (seeTable 1). The fact that

Copyright 1999JohnWiley & Sons,Ltd. Surf. InterfaceAnal. 27, 1037–1045 (1999)


Figure 4. Factor loadings in the pressure mode from thePARAFAC solution for O 1s data with: (a) two-factor model;(b) three-factor model; (c) four-factor model. For comparison,(d) shows factor 2 and factor 3 loadings in the pressure modefrom the PARAFAC solution for Al 2p data with the three-factormodel.5

thesecomponentpeaksare in associatedpairs indicatesthe complexity of the interactionof water vapour withaluminiumsurfaces,whichproducesfour chemicalspeciescontainingoxygenatomscompetitively involved in sev-eral chemicalprocesses.

Interpr etation of the factors

Unlike the PARAFAC solution for the Al 2p data,wheretwo factorsof its three-factormodel are (mostly) easily

Figure 5. Factor loadings in the time mode from the PARAFACsolution for: (a) O 1s data with the four-factor model; (b) Al 2pdata with the three-factor model.5

Figure 6. Factor loadings in the BE mode from the PARAFACsolution for O 1s data with the four-factor model.

definablesingle peaks,5 the PARAFAC solution for theO 1s datais rathermorecomplex,with four double-peakfactors that have commoncomponentpeaksdeterminedabove. The physical interpretationof each factor thendependson identificationof its two componentpeaksinthe BE modeandunderstandingof their variationsin thetime andpressuremode.Althoughsomecomponentpeakscan be identified from the literaturedata,the identity ofothersmay haveto be deducedas a result of argumentsinvolving the chemicalprocesses.



Figure 7. Binding energies (BE) and peak widths (FWHM) ofcomponent peaks obtained by two-peak fitting for four factorspectra in the BE mode from the PARAFAC solution for the O 1sdata (see Fig. 6).

Table 1. Binding energies(BE) and peak widths (full widthat half-maximum, FWHM) of componentpeaks inthe XPSO 1sspectrumobtainedby usingthe curve-fitting technique (seeFig. 7)

Low BE FWHM High BE FWHM Ratio of peak areasFactor (eV) (eV) (eV) (eV) (low BE/high BE)

Factor 1 532.30 (II) 1.65 534.02 (III) 2.06 2.1Factor 2 532.43 (II) 1.60 533.82 (III) 2.12 1.9Factor 3 531.78 (I) 1.63 533.71 (III) 2.13 1.6Factor 4 531.78 (I) 1.57 532.95 (IV) 2.40 1.9

Average valuesComponent peak I II III IVBE position (eV) 531.78 532.36 533.85 532.95

FWHM (eV) 1.60 1.63 2.10 2.40

Some literature binding energies of the O 1s photo-electron line for aluminium oxides and hydroxidesarepresentedin Table2. FromTable2, it is interestingto notethatthecentroidof theO 1speakfor aluminiummetaloxi-dizedby watervapouris observedashigh as532.7eV.15

However,whentwo or morepeaksareusedto fit theXPSO 1sspectrum,thecomponentpeakassignedto theoxidefrom oxidized aluminium by water vapour is found at abinding energy as low as 531.3 eV,16 which is close tothe reportedvaluesfor somepreparedaluminium oxides(seeTable 2). Basedon datapresentedin Table 2 and acomparisonof factor loadingsin the time modebetweenfactor 2 in the Al 2p PARAFAC solution [Fig. 5(b)] andfactor 3 in the O 1s PARAFAC solution [Fig. 5(a)], thecomponentpeakat a bindingenergy of 531.78 eV presentin factor 3 is likely to be due to oxygen in a structureconsistingonly of aluminium andoxygenatomsexhibit-ing the compositionof Al 2O3. Thus, the correspondenceof factor loading of thesetwo factors in the time modeconsistentlysuggeststhegrowth of anoxide overlayeronaluminium.Factor3 alsohasa componentpeakat a bind-ing energy of 533.85 eV. Basedon the data in Table 2,this could be interpretedas being due to water boundtothe structure.Both waterandthe latticeoxide appearin aconstantratio in factor 3.

The adsorptionof water on cleanaluminium surfacesat room temperaturehas beenshown to be completely

Table 2. Binding energiesof the XPS O 1sspectrum for alu-minium oxides, aluminium hydroxides and watermolecules

XPS O 1sMaterial (eV) Source

Sapphire 531.0 PHI Handbook8

.Al2O3/ 530.7 Wagner et al.9

531.3 Taylor10

531.7 Mullins and Averbach11

˛-Al2O3 531.5 Madey et al.12

530.62 Wagner et al.9

531.1 Tsuchida and Takahashi13

-Al2O3 531.2 Madey et al.12

530.85 Wagner et al.9


Boehmite 531.7 Taylor10

(AlOOH) 531.5 Mullins and Averbach11


Bayerite 531.3 Taylor10

Al(OH)3 531.15 Wagner et al.9


Gibbsite 531.33 Wagner et al.9

Al(OH)3 531.5 Mullins and Averbach11


Al metal 532.2 Szalkowski,14 peak centroidoxidized 532.7 Nylund and Olefjord,15 peak centroid

531.3 Strohmeier,16 two peaks fitted531.8 McCafferty and Wightman,17

four peaks fitted

H2O 534.5 Nefedov et al.,18 gaseous state532.9 Wagner et al.,19 solid state533.3 Szalkowski,14 vapour on Al(100), 100 K

dissociative.20 Basedon experimentalwork of Paul andHoffmann21 and semi-empiricalquantumcalculationsofZhukovskii et al.22 the hydroxyl groupsseemto be theprecursorspeciesin the oxidation process:H2O(ad)!OH(ad)C H(ad). As the H2O flux increases,this couldresultin anabundanceof surfacehydroxyl groups,whichis reflectedin theappearanceof factor1, with its principalO 1s binding energy of 532.30 eV. Based on manyexperimentalobservationsin Table 2 we have assignedthisspeciesto anOH group.This factoroccursat low timelevels when little or no oxidized aluminium is present;consequently,it is assignedto OH adsorbedonto themetallicaluminiumsurface.Factor2 with its primarypeakalsoat 532.43 eV appearsaftertheoxidehasnucleatedonthesurface;it is thereforealmostcertainlyassociatedwithan oxygenspeciesboundto the oxide structurethat hascompletelycoveredthe metal by the time levels wherefactor 2 is dominant.Thus we assignthis to hydroxidegroups associatedwith the oxide structure.Its bindingenergy, within experimentalerror, is fairly close to thatfor theOH groupin factor1. Fromthis, it is assumedthatboth OH groupsarechemisorbedspeciesthat experienceonly minor charge donation to their respectivesurfacesfrom the substrateon which they arelocated.

Figures8 and9 showreconstructedfactors,calculatedusingfactor loadingsfrom thePARAFAC solutionfor theO 1s spectrum,which was collectedafter exposureof aclean aluminium surfaceto water vapour at a pressure



Figure 8. Reconstructed factor 1 (top) and factor 2 (bottom)from the PARAFAC solution for the O 1s spectrum, whichwas collected after exposing clean aluminium surfaces to watervapour at a pressure of 1.3ð 10�5 for 30 min.

Figure 9. Reconstructed factor 3 (top) and factor 4 (bottom) fromthe PARAFAC solution for the same O 1s spectrum as in Fig. 8.

of 1.3ð 10�5 Pa for 30 min (pressurelevel 3 and timelevel 13). Figure 8 (top part) representsfactor 1 andits two componentpeaksdeterminedusing curve-fittingtechniques:the OH componentis at 532.30 eV and thewater componentat 533.85 eV; factor 2 (bottom part)also has the water componentat the sameBE, but itsmain componentis at a slightly different position at

532.43 eV. For factor 1, in the pressuremode[Fig. 4(c)],its loading increaseswith increasingpressurewhereasthe loading of factor 2 remains relatively unchanged.This suggeststhat factor 1 is pressuredependentandthat factor 2 is pressureindependent.From the kinetictheory, as the numberof water moleculesimpinging onaluminiumsurfacesincreaseswith anincreaseof pressure,the metal adsorbatesOH and H2O increaseaccordingly.The relatively low rate of conversionfrom OH(ad) tooxide formation leadsto the accumulationof an excessof OH(ad) on surfaces,which increasesas the pressureincreases.In the time mode[Fig. 5(a)], an increaseof theexposuretime resultsin a growth of an oxide overlayer;the numberof metal adsorptionsites thus decreasesandthe numberof OH groupsadsorbedto metal decreaseswith increasingexposuretime. This resultsin a decreasedfactor 1 loading in the time mode.

By contrast, the OH component in factor 2 canbe ascribedto OH groups adsorbedwithin the oxideoverlayer. The number of oxide sites available forchemisorptionis very large and increaseswith exposuretime,thereforenopressuredependenceis observedfor thisfactor [seeFig. 4(c)]. The fluctuationin factor 2 loadingin the first time levels (Fig. 5(a), i.e. from 1 to 5 minof exposuretime) may be due to the stability of oxideformation in that region wherethe OH adsorptionto themetalandoxidenucleationandgrowthareexpectedto becompetitive.

From Fig. 8, it is seenthat both high-binding-energycomponentsin factor1 andin factor2 appeareffectivelyatthebindingenergy of 533.85 eV, whichhasbeenidentifiedas oxygenatomsin water molecules.The associationoftwo componentpeaksin a factorindicatesthatthesepeaksarerelatedin onechemicalprocess,which is maintainedthroughtheentirerangeof time andpressurevariables.Infactor1 andfactor2, theOH groupsandwatermoleculescanbe relatedto eachotherby a recombinationreactionof OH groupsasdescribedby the reaction

2OH��⇀↽�� H2OCO .2/

Reaction (2) establishesthe equilibrium ratio of OHgroupsand H2O groupsto be 2.0. This is, in fact, veryclose to the ratio of peakareasof OH to H2O peaksinboth factor 1 and factor 2, which were measuredas 2.1and1.9, respectively.

Figure 9 shows factor 3 and factor 4 with their twofitted componentpeaks.Theseare two factors that havevery similar variations of factor loading in both timeand pressuremodesto thoseof factor 2 and factor 3 inthe PARAFAC solution for the Al 2p data. Factor 3 isassociatedwith theformationof oxide,asexplainedby theappearanceandgrowth of the peakat BE of 75.80 eV intheAl 2p spectrum.5 Factor3 growssteadilywith thetimelevel afteraninitial periodof oxidenucleation.Moreover,this factor is seento be pressureinvariant.This alsomayresult from the fact that conversionof OH(ad)! O2�

is a slow stepthat would only be influencedby pressurewhenthe supplyof OH(ad)wasvery small.Becausethishydrolysisstepappearsto requirelittle energy, thesupplyof OH groupsis almostalwayssufficient to keepfactor3pressureinvariant.

For factor 3 [shown in Fig. 9(a)] the higher bindingenergy peakassociatedwith themainpeakat 531.78 eV isat533.85 eV; this is identicalto thatpreviouslyascribedto



a water molecule associated with OH groups. In factor 3,the ratio of peak areas between component peaks (thelow BE to the high BE peak) is measured to be 1.6. Theoxidation process on aluminium surfaces could proceedvia a multi-step reaction summarized in the equations

AlCOH(ad)!��!AlOH

(chemisorbed state (cs)) .3/

2AlOH(cs)C3OH(ad)��! Al2O3

C2H2OC H(ad) .4/

These reactions (3) and (4) explain the association ofthe water XPS O 1s component peak in the formationof an aluminium oxide, as represented by factor 3 ofthe PARAFAC solution. These reactions also support theOH group as a precursor in the oxidation process andthe recombination reaction (reaction (2)) of adsorbed OHgroups on aluminium surfaces necessary for the oxideformation. From reaction (4), the ratio between numbersof oxygen atoms found in an oxide and in water moleculesis 1.5, which satisfactorily agrees with the ratio of peakareas (1.6) measured for corresponding component peaksin factor 3 [Fig. 9(a)].

Factor 4 is related to the occurrence of conditionsassociated with the formation of interfacial aluminiumhydrides in the Al 2p spectrum. Similar variations of factorloading in both time and pressure mode in the PARAFACsolution are observed for both Al 2p and O 1s data. TheAl 2p factor spectrum represents a process that results inthe simultaneous appearance of an aluminium hybride andan oxide species at binding energies of 72.44 and 75.43 eV,respectively.5 The O 1s factor 4 has a major componentat 531.78 eV, symptomatic of lattice oxide, and a highbinding energy shoulder at 532.95 eV, which is lower thanthat of water, as observed in the other factors. The ratio ofareas of component peaks in this factor, measured as 1.9,is also decidedly higher than that in factor 3. Factor 4 thenmay be characteristic of conditions where hydrogen mayplay a more important part of the oxide structure, becausethe presence of atomic hydrogen under these conditionsis signalled by the formation of aluminium hydride.5 Onepossible reaction could be a direct reaction of OH(ad)with an aluminium hydride to yield an oxide with somehydrogen still involved in the structure, which is stabilizedfurther by protons donated from the water, as describedby the equation

2AlHx C 3OH(ad)��! Al2HxO2 C H2OC .xC1/H(ad).5/

Equation (5) would give a value of 2.0 for the ratio ofoxide oxygen to aqueous oxygen. Further, the lower O 1sbinding energy of the water peak reflects a stronger asso-ciation of the water with the oxide than in the case offactor 3. Finally, the binding energy of the Al 2p line forthis particular process was found to be lower (75.44 eV)than for the more direct oxidation process (75.80 eV). Ifhydrogen is, in fact, involved in this particular oxide, itcould be expected to contribute to a fundamental modifi-cation of the oxide film character.

CONCLUSIONS

The three-way PARAFAC method combined with thecurve-fitting technique has been used to analyse a set ofXPS O 1s spectra obtained in an oxidation experiment inwhich clean aluminium surfaces were exposed to watervapour in ultrahigh vacuum conditions to varying extentsof time and pressure. Four factors were extracted from thePARAFAC analysis on the XPS O 1s data to character-ize the oxidation process occurring on aluminium surfacesduring water interaction. We have shown that four oxygenspecies are involved in the oxidation kinetics, which con-tribute four component peaks to an XPS O 1s spectrum:hydroxyl groups, oxide and oxide possibly modified withhydrogen and water.

The effects of reaction time and pressure, resolvedby the PARAFAC method, have shown that the surfacehydroxyl groups play a major role in the oxide formationand in its modification with growth. The formation of anoxide requires a reaction of adsorbed hydroxyl groups atthe metal surface. The production of these groups fromthe incoming flux of water molecules will decrease withgrowth of the oxide overlayer. The passivating role playedby the structure of that oxide remains to be explored. Evi-dence of the production of concentrations of OH groups onthe oxide surface supports the concept of active oxide sitesassisting the hydrolysis process. Further, it suggests thatpart of the atomic hydrogen produced also may remainassociated with the oxide lattice and could influence theoxide permeability.

Acknowledgements

The authors would like to thank Dr Pentti Paatero for kind permissionto use the ‘PMF3’ program, Professor Richard A. Harshman andMs Margaret E. Lundy for many invaluable discussions and Alcan forsupplying the aluminium samples used in this study.

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Chem. 1990; 64: 1825.

APPENDIX

A simple procedure based on perturbation theory has beenused to refine the component peaks involved in factorspectra in the BE mode. In this procedure, the fit value(sum of error squares) of the O 1s PARAFAC solutionwith the four-factor model is examined when a componentpeak in the BE mode is subject to a perturbation. Aperturbation (referred to further as a probe factor) simplyis a single Gaussian peak in the BE mode with knownpatterns in variation for factor loadings in the time andpressure modes. As the probe factor changes its positionin the BE mode (factor loadings in other modes remainunchanged), the fit value of the PARAFAC solution variesaccording to the degree of fit between the probe factor anda component peak in the O 1s spectrum; an improvement

Figure 10. Fit value Q as a function of BE position of the probefactor for three FWHM values selected by two-peak fitting ofFWHMs (see Fig. 7).

Figure 11. Fit value Q as a function of peak width (FWHM) of theprobe factor, which is located at a BE of 532.95 eV.

in the fit value shouldbe observedif a componentpeakis ‘intensified’ by the probe factor. Thus, by comparingthe fit values betweenthe perturbedand un-perturbedsolution one can verify the position and profile of thecomponentpeak in the BE mode. Figure 10 representsthe fit values,Q, as a function of position of the probefactor on the BE axis for threeFWHM values,which aresuggestedby curvefitting, i.e. 1.6, 2.1 and2.4 eV (Fig. 7and Table 1); for comparison,thosepoints obtainedbytwo-peakfitting from Fig. 7 (filled diamonds)and theQvalueof theunperturbedPARAFAC solution(dashedline)are also presented.From Fig. 10, the positions of fourcomponentpeaksdeterminedaboveby the curve-fittingtechniquecan be confirmed,i.e. at binding energies of531.78, 532.36, 532.95 and533.85 eV. Figure11representsthe Q value as a function of the FWHM of the BEprofile in the probefactor locatedat a binding energy of532.95eV. Theobservedlocal minimumsuggeststhattheoptimalpeakwidth for a componentpeakat this particularBE position is 2.4 eV; similar resultswere obtainedforthe other peak widths of 1.6 and 2.1 eV. In addition,Fig. 12 showsthe ‘intensifying’ effect in the BE modeof PARAFAC solutions for the two-, three- and four-factormodelscausedby theprobefactor(Figs12(a),12(b)and12(c) representunperturbedsolutionsandFigs 12(d),12(e) and 12(f) representperturbedsolutions),which anFWHM of 1.6 eV and is locatedat a binding energy of532.36 eV. Although the perturbationcausesa generalincreasein all factor loadingsfor the PARAFAC solutionwith the two- andthree-factormodels(of. Figs 12(a)and12(b) with Figs 12(d) and12(e)),an increasecanbe seen



Figure 12. Comparison of the BE modes between unperturbed (a,b,c) and perturbed (d,e,f) PARAFAC solutions for O 1s data with: atwo-factor model (a,d); a three-factor model (b,e); a four-factor model (c,f). The scales of factor loadings are the same in all parts of thefigure.

only for factor 1 and factor 2 in the PARAFAC solutionfor the four-factormodel [cf. Fig. 12(c) with Fig. 12(f)].This supportsa componentpeaklocatedarounda BE of

532.36eV (with FWHM of 1.6 eV), which contributestoboth factor 1 and factor 2. Similar resultswereobtainedfor othercomponentpeaks.


Application of parallel factor analysis and x-ray photoelectron spectroscopy to the initial stages...

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