Control of the stoichiometry in the deposition of cobalt oxides on SiO2

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 62È71 (1998)

Control of the Stoichiometry in the Deposition ofCobalt Oxides on SiO

2

V. M. Jime� nez, J. P. Espino� s and A. R. Gonza� lez-Elipe*Instituto de Ciencias de Materiales de Sevilla (CSIC-Univ. de Sevilla) and Dept. Qu•�mica Inorga� nica, Centro deInvestigaciones Cient•� Ðcas “Isla de la CartujaÏ, Ame� rico Vespucio, s/n. 41092 Sevilla, Spain

Both CoO and overlayers have been deposited on by evaporation from metallic Co and subsequentCo3O

4SiO

2oxidation with oxygen and a plasma of oxygen. The combined use of ion scattering spectroscopy and XPS showsthat both oxides grow in the form of small particles on the surface of Ion scattering spectroscopy also showsSiO

2.

that the surface of cobalt oxide exposed to a plasma of oxygen is enriched in oxygen ions with respect to thesurface of the cobalt oxide formed by exposure to oxygen. The Co 2p spectra corresponding to the depositsobtained by oxidation with are characteristic of CoO, while those corresponding to the deposits obtained afterO

2oxidation with a plasma are typical of Moreover, the ratios determined by XPS and factorCo3O

4. O

Co/Co

analysis indicate the formation of CoO stoichiometry in the former case and stoichiometry in the latter. ItCo3O

4has also been observed that no shift in either binding energy or modiÐed Auger parameter aº appears as a functionof coverage. This absence of shifts is interpreted as a consequence of the type of screening mechanism that domi-nates the relaxation of the photoholes in these oxides. 1998 John Wiley & Sons, Ltd.(

Surf. Interface Anal. 26, 62È71 (1998)

KEYWORDS: XPS; ISS ; CoO; interface e†ects ; cobalt oxideCo3O4 ;

INTRODUCTION

Metal oxides (MO) supported on another metal oxide(M@O) constitute the basis of many systems of greattechnological importance in Ðelds such as catalysts,photocatalysts, solar cells, etc. Metal/metal oxide inter-faces of a large variety of systems have been studied sys-tematically by surface science methods.1h4 On thecontrary, MO/M@O interfaces have not deserved somuch attention and only a few papers exist in the liter-ature on this subject.5h9

Cobalt oxide is a very interesting material widelyused as a catalyst for complete oxidation.10,11 Animportant issue in the studies by photoelectron spec-troscopy of this material has been the characterizationof the chemical state of cobalt,12h14 an element thatpresents two stable forms of oxide : CoO and Co3O4 .Related to this point, extensive work has been carriedout to study the oxidation of metallic cobalt.15h20 Inthese experiments, the exposure to oxygen of metalliccobalt results in the formation of a surface layer ofCoO. In contrast, the Co 2p XPS spectra of powderedcobalt oxide samples presenting the rock salt structureof bulk CoO do not resemble the spectrum of Co(II) inan oxidized layer of cobalt or in a CoO single crystal,but that of cobalt in or Thus, itLiCoO2 Co3O4 .13seems that the control and selection of the stoichi-

* Correspondence to : A. R. Gonza� lez-Elipe, Instituto de Cienciasde Materiales de Sevilla (CSIC-Univ. de Sevilla) and Dept. Qui�micaInorga� nica, Centro de Investigaciones Cienti� Ðcas “Isla de la CartujaÏ,Avda. Ame� rico Vespucio, s/n. 41092 Sevilla, Spain.

E-mail : agustin=cica.es.

ometry of the surface layers of cobalt oxide materials isnot straightforward and that additional information onthis system is required.

With the present paper, we want to contribute to thecharacterization of cobalt oxide materials and to theknowledge of the possible interface e†ects appearingwhen this compound is deposited on another oxide.Thus, the objectives of this research are the formation ofwell-deÐned CoO and materials, their character-Co3O4ization by XPS and a study of the deposition mecha-nism of these oxides on an support.SiO2

EXPERIMENT

Cobalt oxide was deposited on in an ultrahighSiO2vacuum (UHV) by evaporating Co in the presence of2 ] 10~6 Torr of In order to do that, a cobalt wireO2 .was spot-welded to a narrow Ta foil attached to a UHVfeedthrough, which was then resistively heated up to theevaporation temperature of Co. This source was care-fully outgassed before evaporation of Co, so that thebase pressure during the evaporation was \10~9 Torr.

Fully oxidized deposits of cobalt were prepared byexposure of the particles deposited in oxygen to anoxygen plasma. This treatment has been used previouslyto fully oxidize other oxides supported on such asSiO2 ,SnO and Conditions for plasma generation canSnO2 .7be found in this previous paper.7

The substrate was a thin Ðlm of ([200SiO2 SiO2 Ó)grown on an Si(111) wafer. It was degreased by rinsingin acetone and cleaned in situ by light bombardmentwith 500 eV ions provided by a Penning sourceO2`(AG10 from VG). This procedure ensures that the

CCC 0142È2421/98/010062È10 $17.50 Received 21 April 1997( 1998 John Wiley & Sons, Ltd. Accepted 18 September 1997

STOICHIOMETRY IN THE DEPOSITION OF COBALT OXIDES ON SiO2 63

sample is clean (the C 1s peak cannot be detected byXPS) and produces a stoichiometric surface.SiO2The XPS spectra at normal take-o† angle wererecorded for increasing coverages of cobalt oxides on aVG Escalab 210 spectrometer. Both Al Ka and Mg Kasources were used because a Co peak overlapsL3M23Vwith Co 2p signals when recorded with Al Ka andanother one overlaps with the O 1s signals if the spectraare acquired with Mg ka.21 The analyser was set in thepass energy constant mode at a value of 50 eV. Thebinding energy (BE) reference was taken at 103.4 eV forthe Si 2p level of the substrate. The sensitivity factorsprovided with the apparatus were used to quantify theresults. In the experiment, the amount of cobalt depos-ited is referred to as the Co/Si atomic ratio calculatedfrom the intensity of the corresponding XPS peaksmodiÐed by the sensitivity factors of the two elements.

Ion scattering spectra were taken on a Leybold-Heraeus LHS-10 spectrometer. A primary beam of He`of 1000 eV was used to excite the spectra, which wereobtained by setting the analyser in the retardation ratiomode with R\ 3. X-ray photoelectron spectra werealso recorded after each ion scattering spectrum tomonitor the coverage (the acquisition conditions werethe same as that for the VG ESCALAB 210).

THEORY

A set of Co 2p spectra obtained at di†erent stages of theexperiment of deposition of cobalt in the presence ofoxygen is shown in Fig. 1. The series is characterized bytwo broad main peaks separated by a spin-orbit split-ting of 15.6 eV and two strong associated satelliteslocated at the high BE side of the main peaks. It can beseen that neither the line shape nor the BE position ofany feature in the spectra change along the experiment,with the energy of the Co peak remaining Ðxed [email protected] eV. According to the literature,12h14 spectrasimilar to those in Fig. 1 are typical of cobalt in CoO.

As shown in Fig. 2, another series of spectra wereobtained for the experiment consisting of deposition ofcobalt and further exposure to a plasma of oxygen. Thespectra are characterized by the typical doublet separat-ed by 15.0 eV with low-intensity satellites. As in the pre-vious case, the BE of Co does not change with2p3@2coverage and remains Ðxed at 780.7 eV. According tothe literature,12h14 spectra similar to those in Fig. 2 aretypical of cobalt in a cobalt oxide with stoichi-Co3O4ometry and very similar to the spectra of mixed oxideswhere cobalt has an oxidation state of ] 3.12h14 We willassume that the plasma treatment induces the formationof a stoichiometry. This point will be provedCo3O4below by comparing the O/Co ratios for both depositedoxides.

For a complete characterization of the electronic andchemical state of the cobalt system, analysisoxide/SiO2of the O 1s and O KLL peaks of oxygen is necessary.Thus, for di†erent coverages of cobalt oxide, O 1sspectra were analysed by means of factor analysis(FA).22 This method was employed successfully by us ina previous paper to prove the presence of di†erentspecies of oxygen in powder samples of cobalt oxideswith di†erent textures.23 In previous papers on NiO24

and materials,23 we showed the advan-CoOÈCo3O4tages of FA in respect to conventional Ðtting withGaussianÈLorentzian bands for the analysis of XPSspectra. Here, FA has revealed that for the two cobaltoxides supported on there are only two principalSiO2components that reproduce the spectra : one for (i.e.OSioxygen pertaining to the phase) and another oneSiO2for (i.e. oxygen pertaining to the cobalt oxideOCophase). To obtain the true components, the target testtransformation procedure was performed using as targetvectors the GaussianÈLorentzian bands resulting fromthe Ðtting of the spectrum corresponding to the highestcoverage. The analysis program transformed thesebands into the “trueÏ components and the percentage ofevery component was then obtained for each spectrum.The di†erences between the target test vectors and the“trueÏ components are very small.

Figure 3 shows the results of FA for some selectedO 1s spectra of The Ðgure also shows the cal-CoO/SiO2 .culated components corresponding to oxygen bondedto Si and Co. These bands are very similar toGaussianÈLorentzian curves, not showing anomalousfeatures. The advantage of this FA method with respectto Ðtting is that it demonstrates that the same type ofbands are being used for reproducing all the spectra ofthe experimental series. Thus, the calculations show thatthe O 1s maxima of the bands corresponding to oxygenbonded to Co are located at a BE of 530.6 eV for CoO.Similar results were obtained for the Co3O4/SiO2system, although in this case the BE of the O 1s com-ponent pertaining to the cobalt oxide was 530.2 eV.These values agree with those previously reported in theliterature for these two forms of cobalt oxide.12h14

A way of conÐrming the stoichiometry of the twooxides in our experiment is to compare the ratiobetween the peaks of and Co in the two cases. ThisOCois possible by calculating the area of the O 1s com-ponent associated with the oxide of cobalt, which hasbeen determined by FA for CoO and As shownCo3O4 .in Fig. 4, a linear relationship exists between the per-centage of oxygen pertaining to Co and the percentageof Co for both systems. The slopes of the linear regres-sion of these points are 0.96 for CoO and 1.25 for

conÐrming the stoichiometry attributed pre-Co3O4 ,viously to both oxides.

Figure 5 shows the Co and O KLL AugerL3M23Vspectra for the highest coverage situation for CoO/SiO2and The Co spectra of bothCo3O4/SiO2 . L3M23Voxides are very broad, as for Auger transitions thatinvolve valence band levels. They are rather similar, anddo not exhibit sharp features to measure accurately thekinetic energy (KE) of the maximum of the peak. More-over, when the full series of spectra for di†erent cover-ages (not shown here) are scaled and superimposed, noshift of the whole band is observed for any of the twooxides. So, with respect to the degree of coverage, theCo Auger peaks behave in a similar way toL3M23Vthose of Co 2p. On the other hand, for both oxides theoriginal O KLL peak corresponding to develops aSiO2shoulder at higher KE than the main peak. This shoul-der grows as coverage increases until it becomes thedominant peak at the end of the deposition experiment.These high-coverage spectra are shown in Fig. 5. As inthe other photoelectron and Auger peaks, no shift in theposition of the component associated with the cobalt

( 1998 John Wiley & Sons, Ltd. SURFACE AND INTERFACE ANALYSIS, VOL. 26, 62È71 (1998)

64 V. M. JIMENEZ, J. P. ESPINOS AND A. R. GONZALEZ-ELIPE

Figure 1. Cobalt 2p spectra for increasing Co/Si ratios as determined by XPS for CoO/SiO2.

oxide overlayers could be found. However, from thesespectra it was possible to obtain the positions of themaxima of the O KLL spectra of the O bonded to Cofor the two oxides. These peaks were situated at KEvalues of 510.0 eV for CoO and 511.8 eV for Co3O4transitions).(KL23L23

Another point of interest was the growing mode ofthe two forms of cobalt oxide on Figure 6 showsSiO2 .the evolution of the normalized ISS Co signal as a func-tion of the Co/Si ratio determined from the measure-ment of the corresponding XPS peak areas corrected bytheir sensitivity factor. Normalization of the ISS signal

SURFACE AND INTERFACE ANALYSIS, VOL. 26, 62È71 (1998) ( 1998 John Wiley & Sons, Ltd.


Figure 2. Cobalt 2p spectra for increasing Co/Si ratios as determined by XPS for Co3O

4/SiO

2.

of Co has been made by dividing the intensity of thispeak in each spectrum of the series by the intensity ofthe spectrum obtained at the end of the experiment, i.e.when the surface of is totally covered by the cobaltSiO2oxide and no ISS signal due to Si is detected. When thenormalization is made in this way, the corrected Co ISSintensity is a direct measurement of the fraction of SiO2

surface covered by the cobalt oxide. Comparing bothcurves, it appears that CoO spreads better than Co3O4on the surface because, for the same Co/Si XPSSiO2ratio, the fraction of silica surface covered by CoO isalways higher than that obtained for Thus, theCo3O4 .increase of the intensity of the ISS peak of Co withdeposition dose is slower for than for CoO, andCo3O4



Figure 3. Oxygen 1s spectra of for the indicated Co/Si XPS ratios : experimental spectra ; (——) spectra calculated by FA;CoO/SiO2

(=)(– – –) spectra of the components. The Co/Si ratios for every spectrum are indicated on the right-hand side.

the surface of is covered completely by cobaltSiO2oxide only when the Co/Si XPS ratios are 0.45 and 0.20,respectively.

A careful analysis of these curves according to theideas of Bardi, to analyse by ISS the growing mecha-nisms of an overlayer on a substrate,25 reveals thatwhile CoO spreads relatively well on SiO2 , Co3O4tends to form aggregates that grow in thickness beforecompleting the Ðrst monolayer. A detailed theoreticalinterpretation of this analysis will be published else-

where. Here, it should be enough to mention that thegrowth in intensity of the ISS Co peak with respect tothe Co/Si XPS ratio, especially in the case of isCo3O4 ,slower than that previously found by us for andTiO2deposited on where a monolayer-by-mono-SnO2 SiO2 ,layer mechanism of growth was proved.5h8

As previously mentioned, the slower increase in ISSCo signals for than for CoO in Fig. 6 can beCo3O4attributed to the fact that particles are thickerCo3O4than those of CoO. However, it is also likely that an



Figure 4. Percentage of O bonded to Co vs. percentage of Co; Slopes of the lines are indicated on the(=) CoO/SiO2; (…) Co

3O

4/SiO

2.

figure.

enrichment of oxygen at the surface of the andCo3O4the corresponding shadowing e†ect of the Co atomsmay be an additional factor contributing to theobserved di†erence. Thus, Fig. 7 shows the ISS spec-trum of CoO deposited on up to an XPS ratio ofSiO20.54. The second spectrum in this Ðgure corresponds tothis sample after being exposed to a plasma of oxygenat 298 K. It is apparent from this Ðgure that the plasmatreatment induces a considerable decrease in the inten-sity of the Co peak. In this experiment it is very unlikelythat the CoO particles modify their shape at room tem-perature by the action of the plasma of oxygen. So, thechange in the relative intensities of the Co and Si peaksin Fig. 7 must be attributed to a change in the termina-tion of the surface of cobalt oxide particles, which, afterthe plasma treatment, must be enriched in oxygenatoms. This behaviour resembles that observed for the

couple previously studied in our labor-SnO2/SnOatory.6h8 In that case, pure was produced by oxi-SnO2dizing SnO with a plasma of oxygen, establishing thatthe di†erences in the evolution of the ISS Sn signals ofboth oxides were due to a shadowing e†ect of the Snatoms by the oxygen atoms at the topmost layer of theoxide. Figure 7 proves that the coupleCo3O4/CoObehaves in a similar way.

DISCUSSION

We have assumed in the previous section that layers ofCoO and can be prepared on silica with perfectCo3O4control of their stoichiometry. In conclusion withthese results a Ðrst point to evaluate is the growthmechanism of these two oxides on silica. According tocrystallographic data, the monolayer thickness for CoOand can be estimated as D4 and D8 C, i.e. theCo3O4unit cell thickness of each oxide. Considering that theattenuation of the XPS signal follows an exponentiallaw, and using the mean free paths andjCoD 26 Ó

the theoretical Co/Si ratios correspondingjSiD 36 Ó,26to a full coverage of the silica surface with only onemonolayer of each material are D0.12 and D0.27,respectively. This means that for these XPS ratios andwith a monolayer-to-monolayer growth mechanism, thenormalized Co ISS signal for each oxide should beunity. However, Fig. 6 shows that, for these XPS ratios,the normalized ISS Co signal is D0.81 for CoO andD0.77 for Thus, it can be concluded that bothCo3O4 .oxides begin to agglomerate before completing the Ðrstmonolayer. Under these conditions the cobalt oxide



Figure 5. Cobalt and O KLL Auger spectra corresponding to the highest coverage situations of andL3M

23V CoO/SiO

2Co

3O

4/SiO

2.

Figure 6. Evolution of the normalized ISS Co signal as a function of coverage as determined by XPS: (=) CoO/SiO2; (…) Co

3O

4/SiO

2.



Figure 7. Ion scattering spectra before and after plasma treatment for a Co/Si XPS ratio of 0.54.(=) (…)

deposits are likely to be formed by small crystallites ofdi†erent sizes.

The spectra in Fig. 2 show that the oxygen plasmatreatment oxidizes CoO into The di†erentCo3O4 .forms of cobalt oxide and metallic cobalt are well di†er-entiated by means of XPS. Thus, metallic cobalt is char-acterized by a narrow Co main peak at2p3@2BE\ 778.2 eV with no strong satellite structure, CoOpresents a broad main peak at 780.9 eV with a strongsatellite at its high BE side and presents a peak,Co3O4narrower than that of Co(II) in CoO, centred at 780.1eV with a weaker satellite structure.12,23 It must benoted that there is no spectrum reported in theCo2O3literature because of the low stability of this oxide. Ourspectra in Figs 1 and 2 fully agree with these previousdata in the literature and conÐrm that by our experi-mental procedure we can prepare selectively deposits ofeither CoO or Moreover, the stoichiometry ofCo3O4 .the deposits has been conÐrmed by the plots in Fig. 4,the slopes of the two lines being 0.96 and 1.25, in goodagreement with the theoretical Co/O ratios in these twooxides of cobalt.

In one-third of the cobalt ions are formallyCo3O4Co2` species. Early papers in the literature attributedsome small features in the Co 2p spectra to the contri-bution of these Co2` ions.27 Similar features have notbeen detected in our case, perhaps because we did nothave enough resolution in our experiment. In any case,it is expected that the spectral shape of Co2` ions in

is di†erent to that in CoO because of the dif-Co3O4ferent coordination, electronic state and CoÈO distanceof cobalt in these two oxides (tetrahedral coordinationand high spin, and octahedral coordination and lowspin, respectively). Other collective electronic pheno-mena could also contribute to the absence of a clear

structure in the main peak of the spectrum of Co3O4 .The resulting e†ect is that the overall line shape ofcobalt in this oxide is similar to that of Co3` species in

with some small di†erences in the satelliteLiCoO2 ,structure.13 It is also interesting that the Co BE is2p3@2lower in than in CoO. In principle, this seemsCo3O4contradictory with the fact that the majority cation in

is Co3`. Final-state e†ects (i.e. higher relaxationCo3O4energy of the photohole in this oxide) can be the reasonfor this behaviour. Unfortunately, the recorded Augerspectra of cobalt were too broad to accurately measuretheir KE (cf. Fig. 5) and, from the value of this param-eter, calculate the magnitude of these e†ects.29,30

Such an analysis is possible for the oxygen species.Table 1 summarises the values of some spectroscopic

Table 1. Summary of spectroscopic and electronic parametersobtained for CoO and Co

3O

4a

CoO Co3O

4

Co 2p3@2 (BE) 781.5 780.7

O 1s (BE) 530.6 530.2

O KL23

L23

(KE) 510.0 511.8

O KL1L

23(KE) 490.1 492.1

a0@ b 1040.6 1042.0

b0@ b 1020.7 1022.3

U2p2p

c 6.4 5.0

U2s2p

c 10.3 8.7

a All values in eV.and are modified Auger parameters for oxygen calculatedb a

0@ b

0@

from the O 1s photoemission peak and the andKL1L

23KL

23L

23Auger peaks, respectively.28

c Repulsion energies of the two holes in the final state of oxygenatoms after emission of and Auger electrons. Ener-KL

1L

23KL

23L

23gies calculated according to Moretti.28



parameters of oxygen obtained for thick layers of bothCoO and obtained at the end of the depositionCo3O4 ,experiments when no Si could be detected by XPS.These values calculated for high-coverage samplesshould be equivalent to those of bulk samples. Table 1also reports the modiÐed Auger parameters of oxygen

and and the holeÈhole repulsion energies (U) at(a0@ b0@)the oxygen atom after emission of orKL23L23 Kl1L23Auger electrons, calculated according to Moretti.28 Alsoincluded for a summary are the BE values of Co 2p3@2peaks. All the values are in general good agreementwith those reported in the literature.12h14

According to Wagner29 and Thomas30 the followingtwo expressions can be used to relate changes in thespectroscopic parameters with energetic magnitudesinvolved in the photoemission processes : a@\ 2*REand *BE\ *e [ *RE ; where RE is the relaxationenergy and e is a parameter related to the eigenvalue ofthe level undergoing photoemission. By comparisonwith the data in Table 1 for CoO and the fol-Co3O4 ,lowing di†erence can be calculated for the relaxationenergies of the photohole in the oxygen atoms of thesetwo oxides : eV. The*REO ls (Co3O4ÈCoO)\ ]0.7positive value obtained means that the photoholerelaxes more easily in than in CoO. FurtherCo3O4insight into the electronic state of the oxygen anions canbe gained, according to the Moretti,28 by analysis of thetwo Ðrst peaks of their Auger spectra, the derived Augerparameters (a@ and b@) and the repulsion energies of thetwo holes in the Ðnal state after Auger emission (U).The magnitudes of the U(2p2p) and U(3d3d) Coulombinteractions are important to calculate anti-ferromagnetic interactions of the O 2p and M 3d holes,3where M is a transition metal. The value of U is smallerin than in CoO, indicating that the two Ðnal-Co3O4state holes left after emission of the Auger electron arescreened more easily in the former oxide. This Ðnding,together with the higher value of RE reported above,are in agreement with the semiconductor27 andinsulator32 properties of and CoO, respectively.Co3O4This must also be the reason for the higher Co BE2p3@2in CoO than in Co3O4 .

Finally, another point to make is the constancy of theCo 2p BE and Co KE throughout the experi-L3M23Vment (cf. Figs 1 and 2). This fact contrasts with the pre-

vious results found in our laboratory where similarparameters for depositions of SnO, and onSnO2 TiO2do show a change as a function of coverage.5h9SiO2This di†erence has to be related to the type of deposi-tion mechanism found for those oxides where a layer-by-layer growth type was observed.5h9 Secondly, thedi†erent mechanism of screening of the photohole forthe late transition metals can be another reason for theobserved constancy in BE. According to Veal and Pau-likas,33 in the late transition metals this mechanism islocal, i.e. it occurs through the repopulating ofunhibridized 3d orbitals of the cation. A screeningmechanism of this type is quite insensitive to changes inthe polarizability of the oxygen ions, and therefore therelaxation energy becomes independent of any possibleinterface e†ect produced by unlike that observedSiO2 ,in those other systems with a layer-by-layer growthtype.5h9

CONCLUSIONS

Both CoO and have been evaporated onCo3O4 SiO2from a Co wire in an atmosphere of oxygen followed, inthe case of by oxidation with a plasma ofCo3O4 ,oxygen. Combined ISS/XPS showed that these oxidesgrow according to a multilayer mechanism when theyare deposited on although the CoO deposits tendSiO2 ,to spread better than those of The comparisonCo3O4 .of ISS spectra before and after the plasma treatmentindicates that the surface of is enriched inCo3O4oxygen with respect to CoO. Analysis of the Co 2p andO 1s spectra has demonstrated that the stoichiometry ofthe deposits corresponds to CoO and In addi-Co3O4 .tion, in agreement with Veal and Paulikas,33 the con-stancy in BE and a@, whatever the coverage, has beenrelated to the type of screening mechanism for the pho-toholes created in cobalt.

Acknowledgement

We thank the CICYT (prof. u. MAT94-1039-CO2-01 and MAT97-0689) for Ðnancial support.

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Control of the stoichiometry in the deposition of cobalt oxides on SiO2