Applied Surface Science 254 (2008) 6972–6975

Electronic structure studies of the spinel CoFe2O4 by X-ray photoelectronspectroscopy

Zhongpo Zhou a, Yue Zhang a, Ziyu Wang a, Wei Wei a, Wufeng Tang a, Jing Shi a,b, Rui Xiong a,c,*a Department of Physics and Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education, Wuhan University, Wuhan 430072, Chinab International Center for Materials Physics, Shen Yang 110015, Chinac Hubei Key Laboratory on Organic and Polymeric Opto-electronic Materials, Wuhan 430072, China

A R T I C L E I N F O

Article history:

Received 20 December 2007

Received in revised form 6 April 2008

Accepted 1 May 2008

Available online 8 May 2008

Keywords:

CoFe2O4

Electronic structure

XPS

Chemical state

A B S T R A C T

The spinel CoFe2O4 has been synthesized by combustion reaction technique. X-ray photoelectron

spectroscopy shows that samples are near-stoichiometric, and that the specimen surface both in the

powder and bulk sample is most typically represented by the formula (Co0.4Fe0.6)[Co0.6Fe1.4]O4, where

cations in parentheses occupy tetrahedral sites and those within square brackets in octahedral sites. The

results demonstrate that most of the iron ions are trivalent, but some Fe2+ may be present in the powder

sample. The Co 2p3/2 peak in powder sample composed three peaks with relative intensity of 45%, 40%

and 15%, attributes to Co2+ in octahedral sites, tetrahedral sites and Co3+ in octahedral sites. The O 1s

spectrum of the bulk sample is composed of two peaks: the main lattice peak at 529.90 eV, and a

component at 531.53 eV, which is believed to be intrinsic to the sample surface. However, the vanishing

of the O 1s shoulder peak of the powder specimen shows significant signs of decomposition.

� 2008 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Applied Surface Science

journal homepage: www.elsevier .com/locate/apsusc

1. Introduction

The cobalt ferrite CoFe2O4 is a very important magneticmaterials, which has covered a wide range of applicationsincluding electronic devices, ferrofluids, magnetic delivery micro-wave devices and high density information storage due to itswealth of magnetic and electronic properties, such as cubicmagnetocrystalline anisotropy, high coercivity, moderate satura-tion magnetization, high Curie temperature TC, photomagnetism,magnetostriction, high chemical stability, wear resistance andelectrical insulation, etc. [1–12].

In structure, the spinel cobalt ferrite CoFe2O4 crystallizes in aface-centered cubic structure with a large unit cell containing eightformula units. There are two kinds of lattices for cation occupancy,A and B sites have tetrahedral and octahedral coordination,respectively. In the normal spinel structure Co is a divalent atom,occupying tetrahedral A sites, while Fe is a trivalent atom, sittingon the octahedral B sites. When A sites being Fe3+ ions, while B sitesequally populated by Co2+ and Fe3+ ions, the spinel structure isreferred to as the inverse kind [13,14]. Commonly, the CoFe2O4

material is considered to be mostly an inverse spinel compoundwith most divalent Co ions occupying octahedral sites [7,15,16]. It

* Corresponding author.

E-mail address: xiongrui@whu.edu.cn (R. Xiong).

0169-4332/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2008.05.067

means that the Co and Fe cations distribute at both sites. Since theFeA

3+–FeB3+superexchange interaction is normally different from

the CoA2+–FeB

3+ interaction, variation of the cation distributionover the A and B sites in the spinel leads to different magneticproperties of these oxides even though the chemical compositionof the compound does not change [14].

The spinel cobalt ferrite CoFe2O4 materials have been synthe-sized by different methods [17–19]. In most cases, variation of themagnetic properties was obtained due to the different distributionof the Co and Fe cations over the A and B sites. Thus, investigationson the distribution of the Co and Fe cations over the A and B sites inthe spinel cobalt ferrite CoFe2O4 are important [4,7,13,14].

Quantitative X-ray photoelectron spectroscopy (XPS) gives notonly the chemical composition, but also information on thechemical bonding and chemical state. This will help us tounderstand the distribution of the Co and Fe cations in the spinelcobalt ferrite CoFe2O4. Thus, in this paper, homogeneous CoFe2O4

powder and bulk were synthesized by combustion technique, andthe XPS was taken to study on the valence of the elements andelectronic configuration.

2. Experimental

CoFe2O4 powder and bulk samples in our experiment weresynthesized by combustion technique. In brief description,analytical reagent cobalt (II) nitrate hexahydrate, iron (III) nitrate

Fig. 2. Wide-Scan XPS spectra of the CoFe2O4, bulk (curve 1) and powder (curve 2).

Z. Zhou et al. / Applied Surface Science 254 (2008) 6972–6975 6973

nonahydrate and urea were used. Stoichiometric amounts of thecobalt and iron nitrates were weighed out under dry conditions,intimately mixed in a widemouth vitreous silica basin and heatedon a hot blanket inside a fume cupboard, under ventilation. With arise in temperature, melting occurred and a dark liquid wasproduced. Soon after the thickened liquid began frothing, ignitiontook place, leading to rapid increase that propagated in swiftripples to walls of the basin. The reaction produced dry, very fragilefoam, which transformed into powder [19]. The powders wereground in an agate mortar and pestle to a fine powder, and then,some powders were cold pressed into disks with diameter of15 mm and thickness of about 1.4 mm at room temperature undera pressure of 12 MPa. In the final, the compacted powders and thedisks were directly put into silica boats separately, and sintered at1200 8C for 4 h in argon atmosphere.

X-ray diffraction (XRD) patterns were recorded on Bruker D8ADVANCE powder diffractometer using Cu Ka radiation. Theworking voltage V, current I and time constant t were 40 kV, 40 mAand 0.2 s, respectively. X-ray Photoelectron Spectroscopy studieswere performed using a KRATOS XSAM-800 ESCA/SIMS/ISS spec-trometer with monochromatic Mg Ka (1253.6 eV) radiation, thebinding energies of samples has been calibrated by taking thecarbon 1s peak as reference (285.0 eV).

3. Results and discussion

Fig. 1 is the X-ray powder diffraction patterns of the CoFe2O4

powder and bulk. It shows that the powder and bulk samples allhave a single spinel phase and all peaks could be indexed accordingto the standard card of the spinel cobalt ferrite CoFe2O4 [JCPPS cardNo. 22-1086].

Fig. 2 gives the wide-scan XPS spectra of the CoFe2O4 bulk(curve 1) and powder (curve 2) samples in the binding energy of 0–1000 eV. As demonstrated in Fig. 2, the crystals contain Fe, Co, andO elements, and no other impurity element was detected in thespectrum up to 1000 eV except carbon. The carbon on the surfacesof the specimens is probably due to contamination caused byhandling or pumping oil, since the samples have been undergone a1200 8C high-temperature calcinations procedure.

Stoichiometric information can be obtained from core photo-emission intensity data. The element composition can bequantified by use of X-ray photoelectron intensity values (In)and appropriate sensitivity factors (Sn): rn = In/Sn. SFe, SCo and SO

values of 3.8, 4.5 and 0.68 were found by procedures similar to thatof Ref. [20]. Because the iron, cobalt and oxygen X-ray photoelec-tron peaks overlap significantly, the spectrum must first be

Fig. 1. XRD patterns of the CoFe2O4, (a) bulk and (b) powder.

integrated to obtain the N(E) vs. X-ray photoelectron spectrum andthe individual components separated to obtain the integrated XPSintensity for each element independently [21]. Then, rCo:r-

Fe:rO = 1:1.8:4.1 and 1:1.9:3.7 to bulk sample and powder samplewere obtained separately.

Regions in which detailed core spectra were collected includingthe 2p photoelectron regions of the metallic constituents of theoxides, as well as the oxygen 1s core photoelectron regions. Thehigh-resolution narrow-scan XPS spectra of Fe 2p, Co 2p, and O 1speaks of the CoFe2O4 specimen are shown in Figs. 3–5, respectively.

Fig. 3 shows the Fe 2p core-electron spectrum of CoFe2O4

powder and bulk samples. The peak shape for the two samples issimilar, and has an asymmetric shape. The spectrum apparentlyreveals the presence of two nonequivalent bonds of Fe ions inCoFe2O4 compounds, which is consistent with that there are twokinds of lattice sites for Fe ions occupancy in CoFe2O4 compounds.We attempt to resolve the data into two components to representthese two sites. For the CoFe2O4 powder, it yields Fe 2p3/2 bindingenergies of 710.65 and 713.26 eV, and Fe 2p1/2 binding energies of724.23 and 725.36 eV. For the CoFe2O4 bulk, it yields Fe 2p3/2

binding energies of 710.48 and 713.00 eV, and Fe 2p1/2 bindingenergies of 723.60 and 725.70 eV. The doublets in powder and bulksamples can be ascribed to Fe3+ ions in octahedral sites and Fe3+

Fig. 3. Fe 2p XPS for bulk (a) and powder (b) samples. (The arrow indicates the

approximate position of the satellite characteristic of octahedral Fe2+.)

Fig. 4. Co 2p3/2 XPS for bulk (a) and powder (b) specimens. (The arrow indicates the

approximate position of the satellite characteristic of octahedral Co2+.)

Z. Zhou et al. / Applied Surface Science 254 (2008) 6972–69756974

ions in tetrahedral sites, respectively. In CoFe2O4 powder, thedoublets of Fe 2p3/2 binding energies at 710.65 eV and Fe 2p1/2

binding energies at 724.23 eV are due to the contributions fromFe3+ ions in octahedral sites, while the doublets of Fe 2p3/2 bindingenergies at 713.26 eV and Fe 2p1/2 binding energies at 725.36 eVare due to the contributions from Fe3+ ions in tetrahedral sites. InCoFe2O4 bulk, the doublets of Fe 2p3/2 binding energies at710.48 eV and Fe 2p1/2 binding energies at 723.60 eV are due tothe contributions from Fe3+ ions in octahedral sites, while thedoublets of Fe 2p3/2 binding energies at 713.00 eV and Fe 2p1/2

binding energies at 725.70 eV are due to the contributions fromFe3+ ions in tetrahedral sites. The relative contributions to theoverall intensity of Fe3+ ions in octahedral sites and tetrahedralsites are 70% and 30% for both the powder and the bulk sample,which means that the Fe3+ ions occupy 60% tetrahedral sites and70% octahedral sites in both the powder and the bulk sample. Theresult is consistent with that was reported by Nakagomi et al. [22],in which Co2+ was found to occupy both the tetrahedral andoctahedral sites in spinel CoxFe3–xO4 prepared by combustionreaction.

The 2p3/2 to 2p1/2 separation and satellite structure are useful incharacterizing the iron chemical environment. The iron 2pspectrum of the powder sample is different from that of the bulksamples, and shows developed satellite structure characteristic of

Fig. 5. The XPS spectrum of O 1s peak for the CoFe2O4 surface, bulk (a) and powder

(b).

high spin octahedral cations, as that exhibited in the Fe2+ metalmonoxides [23]. Thus, some Fe2+ may be present in the powdersample. This spectrum of the powder sample also broadens,although in this case the broadening of the iron is predominately tothe higher binding energy side of the 2p transitions.

The XPS spectrum of Co 2p3/2 in CoFe2O4 shown in Fig. 4 alsoexhibits asymmetric. The quantitative peak fitting procedure forCo 2p3/2 is rather complicated owing to various physics effectsincluding core and valence band interactions. Inspection of themeasured Co 2p3/2 peak of the bulk sample (a) shows that itcomposes of two main doublets with peak position at 779.82 and781.40 eV, and with relative contributions to the overall Cointensity of 60% and 40%, respectively. The two peaks with bindingenergy of 779.82 and 781.40 eV are ascribed to Co2+ ions inoctahedral sites and Co2+ ions in tetrahedral sites, respectively.However, the Co 2p3/2 peak in powder sample (b) composes ofthree peaks positioned at 779.70, 781.00 and 782.80 eV, withrelative contributions to the overall Co intensity of 45%, 40% and15%, ascribes to Co2+ in octahedral sites, tetrahedral sites and Co3+

in octahedral sites.Compared with spectrum of the powder, the spectrum of the

octahedral Co2+ cations in bulk sample has a very intense,characteristic satellite at �786 eV, which is some 4–6 eV higherin binding energy than the Co 2p3/2 signal. Therefore, majority highspin Co2+ cations occupy octahedral sites in the CoFe2O4 spinellattice [21,15]. The intense satellite structure found at the highbinding energy side of the Co 2p3/2 and Co 2p1/2 (not shown)transition is believed to be a direct consequence of the bandstructure associated with octahedral Co2+ in the oxide lattice,which allows for admixture of oxygen 2p character and leads twopossible final states in the photoemission process:

2p63d7 þ hn!2p53d7 þ e� or 2p53d8Vþ e�;

where V represents an electron vacancy in the 2p band of theneighboring O2� lattice anions.

Because of the extreme sensitivity of the charge-transferprocess to the overlap between adjacent Co2+ and O2� orbital,the satellite to main peak intensity is highly dependent upon thegeometry and defect nature of the compound [21].

The possibilities of the oxidation of Co2+ to Co3+ is not entirelyimpossible if the oxidation of cobalt is compensated by a numberof iron cations reduced from Fe3+ to Fe2+, or the migration of Co2+ totetrahedral sites. The low spin Co3+ atom gives rise to much weakersatellite feature than do high spin Co2+, owing to the presence ofunpaired valence electrons in the Co3+ orbital [15]. The exact formof this defect-cobalt structure cannot be unequivocally determinedwith XPS. However, the cobalt 2p spectrum corresponding to thepowder sample has no significant difference compared with thebulk specimen, although the spectral features broaden slightly,occurring at the higher binding energy side of the 2p peak in theintensity of the 781.00 eV peak.

Fig. 5 shows the core-level spectrum of O 1s in CoFe2O4. Themain peak of the bulk specimen (a) has a binding energy of529.90 eV, which has been observed for lattice O2� at approxi-mately this point in a number of rocksalt and spinel 3d metaloxides, including CoO, and Co3O4 [16]. A second higher-bindingenergy peak is found at 531.53 eV, and perhaps a third peak at533.00 eV. The third peak is only present in about 7.7% of the totalO 1s intensity and may simply be an artifact of fitting asymmetricXPS peaks with symmetric Gaussian functions [21]. Establishing aunique assignment for the 531.53 eV O 1s peak is not straightfor-ward. There are several possible defects and contaminants with acomparable XP binding energy and similar peaks in O 1s spectrataken on cobalt and iron containing oxides, those results may befrom ambient adsorption, under-coordinated lattice oxygen,

Table 1The analysis results of Co 2p, Fe 2p and O 1s XPS spectra for the CoFe2O4 powder and

bulk samples

Sample Spectrum BE(eV) Assignment Atomic percentage (%)

Powder Co 2p3/2 779.70 Octahedral Co2+ 45

781.00 Tetrahedral Co2+ 40

782.80 Octahedral Co3+ 15

Fe 2p3/2 710.65 Octahedral Fe3+ 70

713.26 Tetrahedral Fe3+ 30

O 1s 530.17 CoFe2O4 89

532.29 CoFe2O4 11

Bulk Co 2p3/2 779.82 Octahedral Co2+ 60

780.40 Tetrahedral Co2+ 40

Fe 2p3/2 710.48 Octahedral Fe3+ 70

713.00 Tetrahedral Fe3+ 30

O 1s 529.90 CoFe2O4 60

531.53 CoFe2O4 33

533.00 Artifact 7

Z. Zhou et al. / Applied Surface Science 254 (2008) 6972–6975 6975

Co2O3/Fe2O3 surface phases, or species intrinsic to the surface ofthe spinel. Thus, while it is not possible to rule out low levels ofcontaminants or defect species entirely, a fair percentage of oxygenintensity in the CoFe2O4 O 1s spectrum is believed to berepresentative of surface oxide intrinsic to the CoFe2O4 spinellattice.

In Fig. 5(b), the main peak is at 530.17 eV, and decreases slightlycompared with that in Fig. 5(a), and the O 1s shoulder vanisheswhich shows significant signs of oxygen depletion. The reductionof oxygen concentration may come from the oxidation of Co2+ toCo3+, or may be caused by calcination procedure for the loosecompaction. Thermodynamic considerations have to be made incases of high temperature and low oxygen partial pressure, aspresent in the surfaces when the spinel was formed. An attemptmay be made to attribute this to the relative mobility of the ionsconcerned via tetrahedral and octahedral lattice sites, with therelative preferences for the two sites affecting rates of diffusion.The relative ion mobility may be deduced to be: Fe3+ > Fe2+ > Co2+.The mobility can be used to explain the enhanced surfaceconcentration of cobalt and depletion of iron. The 532.29 eV peakis assigned an artifact also.

Thus, the sample can be represented by (Co0.4Fe0.6)[Co0.6-

Fe1.4]O4 approximately, where cations in parentheses locate intetrahedral sites and those within square brackets in octahedralsites. Table 1 summarizes the analysis results of the Co 2p, Fe 2pand O 1s XPS spectra for the CoFe2O4 powder and bulk samples.

4. Conclusion

In conclusion, powder and bulk CoFe2O4 samples have beenfabricated by combustion reaction and analyzed by XPS technique.The specimens are found to be near-stoichiometric, althoughsubsequent calcinations tend to reduce oxygen concentration. The

results show that the Fe3+ cations occupy 60% tetrahedral sites and70% octahedral sites both in the bulk and powder specimens, butsome Fe2+ may be present in the powder sample. The cobalt cationsare predominantly present as Co2+ cations. For the bulk sample,60% Co2+ cations located in octahedral sites, and 40% in tetrahedralsites. The Co 2p3/2 peak in powder sample consisted three peakswith relative contributions to the overall Co intensity of 45%, 40%and 15%, ascribes to Co2+ in octahedral sites, tetrahedral sites andCo3+ in octahedral sites. The O 1s spectrum of the bulk sample iscomposed of two peaks: one at 520.90 eV, similar to that was foundfor lattice O2� in the monoxides, and a second at 531.53 eV, whichwe believe to be characteristic of the surface spinel. Furthermore,the O 1s peak of the powder specimen show significant signs ofdecomposition for depletion of oxygen.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (Grant No. 10674105, No. 10474074 and No.10534030) and 973 Program (2007CB607501).

References

[1] A. Lisfi, M. Williams, J. Appl. Phys. 93 (2003) 8143.[2] H.-T. Jeng, G.Y. Guo, J. Magn. Magn. Mater. 239 (2002) 88.[3] J.C. Slonczewski, Phys. Rev. 110 (1958) 1341.[4] C.N. Chinnasamy, B. Jeyadevan, K. Shinoda, K. Tohji, D.J. Djayaprawira, M. Taka-

hashi, R. Justin Joseyphus, A. Narayanasamy, Appl. Phys. Lett. 83 (2003) 2862.[5] C.N. Chinnasamy, B. Jeyadevan, O. Perales-Perez, K. Shinoda, K. Tohji, A. Kasuya,

IEEE Trans. Magn. 38 (2002) 2640.[6] S. Ammar, A. Helfen, N. Jouini, F. Fievet, I. Rosenman, F. Villain, P. Moliniee, M.

Danot, J. Mater. Chem. 11 (2001) 186.[7] A.K. Giri, E.M. Kirkpatrick, P. Moongkhamklang, S.A. Majetichc, V.G. Harris, Appl.

Phys. Lett. 80 (2002) 2341.[8] H. Zheng, J. Wang, S.E. Lofland, Z. Ma, L. Mohaddes-Ardabili, T. Zhao, L. Salamanca-

Riba, S.R. Shinde, S.B. Ogale, F. Bai, D. Viehland, Y. Jia, D.G. Schlom, M. Wuttig, A.Roytburd, R. Ramesh, Science 303 (2004) 661.

[9] R.V. Chopdekar, Y. Suzuki, Appl. Phys. Lett. 89 (2006) 182506.[10] H. Zheng, F. Straub, Q. Zhan, P.-L. Yang, W.-K. Hsieh, F. Zavaliche, Y.-H. Chu, U.

Dahmen, R. Ramesh, Adv. Mater. 18 (2006) 2747.[11] S.D Bhame, P.A. Joya, J. Appl. Phys. 100 (2006) 113911.[12] I.H. Gul, A.Z. Abbasi, F. Amin, M. Anis-ur-Rehman, A. Maqsood, J. Magn. Magn.

Mater. 311 (2007) 494.[13] Z. Szotek, W.M. Temmerman, D. Kodderitzsch, A. Svane, L. Petit, H. Winter, Phys.

Rev. B 74 (2006) 174431.[14] C. Liu, B. Zou, A.J. Rondinone, Z. John Zhang, J. Am. Chem. Soc. 122 (2000) 6263.[15] G.C. Allen, K.R. Hallam, Appl. Surf. Sci. 93 (1996) 25.[16] V.M. Jimenez, A. Fernandez, J.P. Espinos, A.R. Gonzadez-Elipe, J. Elec. Spec. Relat.

Phenomena 71 (1995) 61.[17] R.T. Olsson, G. Salazar-Alvarez, M.S. Hedenqvist, Ulf.W Gedde, F. Lindberg, S.J.

Savage, Chem. Mater. 17 (2005) 5109.[18] J.H. Yin, J. Ding, B.H. Liu, J.B. Yi, X.S. Miao, J.S. Chen, J. Appl. Phys. 101 (2007)

09K509.[19] A. Franco Jr., E.C. de Oliveira Lima, M.A. Novak, P.R. Wells Jr., J. Magn. Magn. Mater.

308 (2007) 198.[20] W. Gang, Materials Characterization and Application, Chemical Industry Press,

Beijing, 2002, p. 461.[21] J.-G. Kim, D.L. Pugmire, D. Battaglia, M.A. Langell, Appl. Surf. Sci. 165 (2000) 70.[22] F. Nakagomi, S.W. da Silva, V.K. Garg, A.C. Oliveira, P.C. Morais, A. Franco Junior,

E.C.D. Lima, J. Appl. Phys. 101 (2007) 09M514.[23] P.C.J Graat, M.A.J. Somers, Appl. Surf. Sci. 100/101 (1996) 36.