In spite of the absence of significant segregation of either cobalt oxide or iron oxide, core level photoemission binding energy shifts tend to indicate that the surface is significantly different from the bulk for CoFe2O4(111) thin films grown on Al2O3(0001). CoFe2O4(111) thin films show a surface-to-bulk core level shift in both the Co 2p and Fe 2p core level photoemission spectra. Surface weighted components in the core level photoemission spectra of both Co 2p3/2 and Fe 2p3/2 can be distinguished from the bulk components, by angle-resolved x-ray photoemission spectroscopy, for CoFe2O4(111) thin films. The surface termination of CoFe2O4(111) contains both Co and Fe with no evidence of strong preferential surface termination of either an iron or cobalt oxide, except for CoFe2O4(111) in the thin film limit. With extensive annealing above room temperature, the cobalt oxide component of very thin CoFe2O4(111) films, grown on Al2O3 (0001), will lose oxygen.
CoFe2O4 favors an inverse spinel structure in which tetrahedral (Td) sites are occupied by Fe3+ and octahedral (Oh) sites are occupied by both Fe3+ and Co2+ cations.1–11 CoFe2O4 is stable, hard, and readily fabricated with magnetic properties that include high magnetic anisotropy, high coercivity, high Curie point, and moderate saturation magnetization.12–19 The nominally single-component ferrimagnetic CoFe2O4/Al2O3 thin films can exhibit exchange bias.20
CoFe2O4 has attracted attention for a wide range of potential applications such as in magnetic recording media, gas sensors, and catalysts.12,21–23 An understanding of the structure and surface properties of the cobalt ferrite thin films is also important in corrosion control in boiling water reactors. In boiling water reactors, cobalt ions, released as a corrosion product, become Co-60 after neutron activation. The cobalt species may become deposited on out-of-core surfaces.24–28 It is believed that the soluble Co-60 is absorbed into iron oxides resulting in the formation of a tenacious and more thermodynamically stable cobalt ferrite layer, under the boiling water reactor conditions.24,25,29–32 The result is a possible corrosion layer and embrittlement of the stainless steel pipe and valve surfaces in boiling water reactors.24,25,27,31
It is typical that the surface of a thin film material, especially oxides, is chemically different from the bulk because of the different chemical environment of the surface atoms compared to that of the bulk.32,33 The surface of an oxide can differ from bulk as a result of surface segregation, a different oxide at the surface (including oxygen loss), hydroxy adsorption, and a surface enthalpy and lower coordination that differs from the bulk, effects that are all intertwined. The dissimilar chemical environment at the surface as well as differences in the screening of the photohole during the photoemission process may result32 in a significant surface-to-bulk core level shift in core level photoemission.33–35 The dielectric properties of the surface of an oxide may also differ significantly from the bulk leading to a different screening of the photohole in the photoemission process perturbing the photoemission final state.32 The characterization of the surface of CoFe2O4, by x-ray photoemission, presents an even more significant challenge because the tetrahedral (Td) sites are occupied by Fe3+ and the octahedral (Oh) sites are occupied by both Fe3+ and Co2+ cations.
There have been multiple efforts to characterize the inverse spinel CoFe2O4, using x-ray photoemission spectroscopy (XPS), and the most common interpretation of the XPS spectra is to ascribe the Td and Oh sites to different Fe and Co 2p3/2 core level features.1–11 In addition to the assignment of the Td and Oh interstitial sites to different Co and Fe 2p3/2 core level XPS features, there is also a satellite feature in the core level XPS spectra typically ascribed to a photoemission two hole bound state.
This analysis where the different Fe and Co 2p3/2 core level features are assigned to the Td and Oh sites ignores the obvious complexities that, as noted above, can occur at the surface of an oxide. These complications at the surface of CoFe2O4 may be more significant than simply noting that the XPS spectra are affected by tetrahedral (Td) sites occupied by Fe3+ and octahedral (Oh) sites occupied by both Fe3+ and Co2+ cations.
Here, we investigate the possibility that there are both surface and bulk weighted components in the Co and Fe 2p3/2 core level spectra of CoFe2O4 thin films, of various thicknesses, grown on Al2O3.
Epitaxial CoFe2O4 (111) thin films of thicknesses 5.5, 1.7, and 1.2 nm were grown on Al2O3 (0001) substrates using pulsed laser deposition, as described elsewhere.20 X-ray photoemission spectra were acquired using a SPECS x-ray Mg Kα anode (hv = 1253.6 eV) source and VG100AX hemispherical analyzer. All the angle-resolved x-ray photoemission spectroscopy (ARXPS) measurements were carried out at room temperature in an ultrahigh vacuum, as a function of emission angle, with respect to the surface normal (0°). The limited electron mean free path ensures that by changing the photoelectron emission angle, the angle-resolved XPS can be used to distinguish components in the XPS spectra that have greater surface weight, i.e., larger emission angles with respect to the surface normal of the sample result in core level photoemission spectra with greater surface weights. The XPS measurement at higher emission angles provides a more surface sensitive characterization of a sample.33–38 The kinetic energy of the photoelectrons coming out of Fe and Co core levels in this work is below 600 eV, and the photoelectrons with this kinetic energy have an inelastic mean free path of about 1 nm.39,40
The XPS hemispherical analyzer has an acceptance angle of ±10°. At each emission angle, the binding energies of the various XPS core level components are referenced to the adventitious C 1s contamination position to eliminate variations in binding energy due to surface charging.
III. THE SURFACE-TO-BULK CORE LEVEL SHIFTS IN CoFe2O4 THIN FILMS
The ARXPS spectra of the Co 2p3/2 core level spectra for CoFe2O4 thin films of thicknesses 5.5 and 1.7 nm grown on Al2O3 are shown in Figs. 1(a) and 1(b), respectively. For the 5.5 nm thick CoFe2O4 thin film, the Co 2p3/2 XPS core level spectra contain three components: P1 at a binding energy of 779.4 eV, P2 at a binding energy of 781.4 eV, and S (satellite) at a binding energy of 786.0 eV. For the 1.7 nm thick CoFe2O4 thin film, these three Co 2p3/2 core level features, P1, P2, and S, have binding energies of 779.6, 781.6, and 785.8 eV, respectively. For both the 5.5 and the 1.7 nm thick CoFe2O4 thin films, the binding energies of the Co 2p3/2 P1, P2, and S core level components measured here are somewhat smaller than the CoFe2O4 film Co 2p3/2 core level component binding energies of 779.8, 781.9, and 785.9 eV reported elsewhere.2 Given that the measured binding energies depend somewhat on film thickness, the binding energies of the Co 2p3/2 P1, P2, and S core level components for both the 5.5 and the 1.7 nm thick CoFe2O4(111) thin films are in general agreement with the CoFe2O4(111) Co 2p3/2 core level component binding energies reported previously.1,2,20
Similarly, the ARXPS spectra of Fe 2p3/2 core level spectra for the CoFe2O4(111) films of thicknesses 5.5 nm and 1.7 nm are depicted in Figs. 1(c) and 1(d), respectively. Unlike cobalt, the Fe 2p3/2 core level spectra have two components P1 and P2. For the 5.5 nm thick CoFe2O4 film, the Fe 2p3/2 XPS core level spectra contain two components: P1 at 709.8 eV and P2 at 711.8 eV. For the 1.7 nm CoFe2O4 film, these two Fe 2p3/2 core level features, P1 and P2, are at the binding energies of 710.0 and 712.0 eV, respectively. The binding energies of the components of Fe core levels in both 5.5 and 1.7 nm CoFe2O4(111) films are significantly smaller than Fe core level binding energies of 710.7 and 713.7 eV reported previously.2 The binding energies of the component peaks of both the Co 2p3/2 and Fe 2p3/2 core levels slightly increased for thinner samples, indicating that the thinner films may be more dielectric or the dielectric substrate affects the measured core level photoemission binding energies.
The principal issue here has less to do with the actual core level binding energies but rather whether the P1 and P2 Fe and Co 2p3/2 core level components in the x-ray core level photoemission are the result of the different Td and Oh sites in CoFe2O4, as ascribed elsewhere1–11 or if these different 2p3/2 core level components are the result of the surface or surface oxide that is distinct from the bulk in terms, as has been suggested elsewhere.20 The ARXPS spectra of 2p3/2 core levels for both Co and Fe, from both the 5.5 and 1.7 nm thick CoFe2O4(111) films, were carried out at seven different emission angles, as shown in Fig. 1.
The surface sensitivity of the various cobalt and iron 2p3/2 core level components is established by plotting the ratio of the P1 and P2 component intensities (shown in Fig. 1), as a function of emission angle, for 1.7 and 5.5 nm thick CoFe2O4(111) films, as shown in Fig. 2(a) (for Co) and Fig. 2(b) (for Fe). As illustrated in Fig. 2(a), the value of the P1/P2 intensity ratio for cobalt decreases at higher angles, indicating that the Co 2p3/2 P2 component is more representative of the surface than the P1 component. This is consistent with the slight shift in the Co 2p3/2 core level to high binding energies with increasing emission angle [Fig. 1(b)]. As noted above, the concept that the surface oxidation and surface properties of CoFe2O4 (111) thin films may result in a surface oxide of Co that differs from the bulk has been previously proposed;20 nonetheless, here the analysis has the P2 component of the Co 2p3/2 core level photoemission spectra more representative of the cobalt oxide at the surface and P1 component more representative of the bulk.
For Fe 2p3/2 core levels, from the P1/P2 component intensity ratio as illustrated in Fig. 2(b), the P1 component, of the Fe 2p3/2 core level photoemission spectra, is found to be more representative of the surface. The assignment of the P1 component as having more surface weight is possible because the intensity ratio P1/P2 increases with increasing emission angle [Fig. 2(b)]. Obviously, then the Fe 2p3/2 core level photoemission P2 component is more indicative of the bulk iron oxide environment. We note that the greater surface of the P1 component of the Fe 2p3/2 core level is consistent with a shift of the Fe 2p3/2 envelope to smaller binding energies with increasing emission angle. This small core level shift with emission angle is more clear than is the case for Co and is independent of any fitting routine. The deconvolution of the Fe 2p3/2 XPS core level feature into P1 and P2 components, as shown here, is very similar to the 2p3/2 XPS core level component breakdown done for x-ray photoemission spectra from CoFe2O4(111) thin films reported elsewhere.1–11,20 Rather than simply assigning these 2p3/2 core level components to the Td and Oh sites of CoFe2O4, here we find that one component is favored at the surface.
The surface-to-bulk core level shifts in both 1.7 and 5.5 nm thick CoFe2O4 samples, for both Fe and Co 2p3/2 core levels, are found to be 2 eV but we recognize the surface oxides may differ from the bulk. Although here we ascribe the individual P2 component of Co and P1 component of Fe as surface weighted components, this does not exclude the prior assignment of the Fe 2p3/2 XPS core level components to the Td and Oh sites in CoFe2O4. If the components of the Fe 2p3/2 XPS core level are the result of the Td and Oh sites in CoFe2O4, then the Td and Oh sites cannot be uniformly occupied at the surface and near surface region. Although such an explanation is difficult to reconcile with the ideal structure of CoFe2O4, it is not entirely excluded by the data. Assigning the 2p3/2 XPS core level components to Td and Oh sites means that the surface is rich in iron in the octahedral sites and cobalt in the tetrahedral sites. Just the same, it is important to note that Co3+ and Co2+ generally have similar Co 2p3/2 binding energies. Neither the Co 2p3/2 binding energy of Co3+ [binding energies 779.9 eV (Ref. 41) and 780.1 eV (Ref. 42)] or Co2+ [binding energies 780.0 eV (Ref. 41) and 780.1 eV (Ref. 43)] resembles the P2 Co 2p3/2 component with a binding energy of 781.4–781.9 eV measured here but rather the binding energy of the P1 Co 2p3/2 component of CoFe2O4 (779.4–779.8 eV as noted above). This makes it difficult to assign the Co 2p3/2 XPS core level components to the Td and Oh sites.
Surface segregation also does not explain these data. When plotting the ratio of the Co and Fe 2p3/2 core level XPS intensities, there is a very little variation with changes in the emission angle. The Co to Fe ratio for the 5.5 nm film of CoFe2O4 is found to be 0.59, which is close to the expected value of 0.50. Figure 3 shows the ratio of the sum of surface and bulk 2p3/2 core level components for Co relative to Fe, for 1.2, 1.7, and 5.5 nm thick CoFe2O4(111) thin films. Surface segregation of either cobalt or iron oxide does not appear to be significant, as this should result in a variation in the 2p3/2 core intensities for Co relative to Fe, with increasing emission angle.32
IV. REDOX OF CoFe2O4(111) ULTRATHIN FILMS IN VACUUM
In ultrathin CoFe2O4(111) thin films, it has been observed that reduction of the cobalt to a suboxide is possible and leads to a low binding energy Co 2p3/2 core level photoemission spectra component.20 The evolution of this low binding energy Co 2p3/2 core level photoemission spectra component through annealing of a 1.2 nm thick CoFe2O4(111) thin film is shown in Fig. 4(a). The Co 2p3/2 core level photoemission spectra from 5.5 and 1.7 nm thick CoFe2O4(111) thin films contain three components, which clearly differs from the four Co 2p3/2 core level components observed for the 1.2 nm thick film. The component peaks P1, P2, and S of the 2p3/2 core level, from 1.2 nm thick CoFe2O4, were observed at binding energies of 780.1, 782.1, and 786.6 eV, respectively. An additional Co 2p3/2 core level photoemission component, at a binding energy of 778 eV, was also observed, on the lower binding energy side of the main Co 2p3/2 core level feature, after extensive annealing [Fig. 4(a)]. The x-ray photoemission spectra taken after three different annealing stages of the 1.2 nm thick CoFe2O4(111) thin film in vacuum are plotted in Fig. 4(a). The CoFe2O4 (111) thin film was annealed at 200 and 285 °C, respectively, for about 5 h, then annealed at 303 °C for about 3.5 h. The evolution of Co 2p3/2 core level is evident in the appearance of a small shoulder on the lower binding energy side and a slight broadening of the main 2p3/2 peak. This low binding energy Co 2p3/2 component implies that there exists another cobalt species represented by P0 in Fig. 4(b).
The result is that the shape of the Co 2p3/2 core level photoemission envelope changes, as shown in Fig. 4(a), with annealing of the 1.2 nm thick CoFe2O4(111) thin film. In contrast, there exists no additional feature appears in the Fe 2p3/2 core level photoemission spectra, with the annealing of a 1.2 nm thick CoFe2O4(111) thin film. The absence of a corresponding additional component at lower binding energies in the Fe 2p3/2 core level photoemission spectra for 1.2 nm thick CoFe2O4(111) thin films may be associated with the observations that 1.2 nm thick CoFe2O4(111) thin films are cobalt rich compared to thicker CoFe2O4(111) thin films, as seen in Fig. 3.
The observed core level binding energy position of the P0 Co 2p3/2 core level photoemission spectral component at a very low binding energy (778.0 eV) is at a binding energy closely observed to be the binding energy of Co 2p3/2 of Co metal [778.0 (Ref. 41) to 778.2 eV (Ref. 43)] so represents a highly reduced oxide of cobalt. This suggests that extensive annealing of a 1.2 nm thick CoFe2O4(111) thin film, at the higher temperature of 285–303 °C, may cause loss of some oxygen from the surface. Some oxygen loss from the surface, after the annealing at 303 °C, is expected and a 1.2 nm thick CoFe2O4(111) thin film does not have a large volume to replace lost surface oxygen. The observation of P0 segregation/enrichment at the surface (see the supplementary material44 for surface enrichment of reduced cobalt, as indicated in Fig. S5) suggests that there may be some relation between the P0 component and oxygen vacancies at the surface.
The surface-to-bulk core level shifts in the binding energy of Co and Fe core levels are observed in 1.2 nm thick CoFe2O4(111) film, illustrated in Fig. 4(b), which are similar to those observed in 5.5 and 1.7 nm thick CoFe2O4(111) films, which means that the P1 component remains representative of a surface (bulk) weighted species and while the P2 component is bulk (surface) of Fe (Co) 2p3/2 core levels. This surface-to-bulk core level shift appears to be little disturbed by the presence of the P0 component.
A large intrinsic exchange bias was reported20 for these nominally single-component CoFe2O4 thin films, and an interfacial mechanism, i.e., the emergence of a third layer containing antiferromagnetic CoO between the CoFe2O4 thin film and the sapphire substrate, was proposed20 to explain the pinning of the magnetic moments of CoFe2O4 to produce the observed exchange bias. However, the mechanisms for such an interfacial reconstruction remain elusive. The observation of P0 component in the 1.2 nm sample after annealing but also indicates that the assumed CoO could actually exist by drawing oxygen to the surface while creating oxygen vacancies at the interface between the CoFe2O4 thin film and the sapphire substrate. This could also occur in those CoFe2O4 thin film samples grown with a relatively low oxygen pressure (10 m Torr).20
V. SUMMARY AND CONCLUSIONS
We have found no compelling evidence of either Co or Fe surface segregation in CoFe2O4(111), yet the surface differs substantially from the bulk, perhaps as a result of preferential surface oxide formation. The traditional assignment of the different XPS Co 2p3/2 and Fe 2p3/2 core level components of CoFe2O4 to the Td and Oh sites cannot be reconciled to the ARXPS results. This is particularly true of the XPS Co 2p3/2 core level. Cobalt oxide at the interfaces of CoFe2O4(111) appears to be subject to reduction.
This research was supported by the National Science Foundation (NSF) through the EPSCoR RII Track-1: Emergent Quantum Materials and Technologies (EQUATE), Award No. OIA-2044049. The authors would like to acknowledge the seminal role Scott Chambers has played in our understanding of oxides through XPS, and P.A.D. acknowledges many fruitful and educational discussions with Scott Chambers.
The data that support the findings of this study are available from the corresponding author upon reasonable request.