Article pubs.acs.org/JPCC
Particular Transport Properties of NiFe2O4 Thin Films at High Temperatures Cecilia Solís,† Simona Somacescu,‡ Elena Palafox,† María Balaguer,†,§ and José M. Serra*,† †
Instituto de Tecnología Química, Universidad Politécnica de Valencia − Consejo Superior de Investigaciones Científicas, Av. Naranjos s/n, E-46022 Valencia, Spain ‡ “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Spl. Independentei 202, 060021 Bucharest, Romania § Forschungszentrum Jülich GmbH, Institute of Energy and Climate Research IEK-1, D−52425 Jülich, Germany S Supporting Information *
ABSTRACT: NiFe2O4 (NFO) thin films were deposited on quartz substrates by rf magnetron sputtering, and the influence of the deposition conditions on their physicchemical properties was studied. The films structure and the high temperature transport properties were analyzed as a function of the deposition temperature. The analysis of the total conductivity up to 800 °C in different pO2 containing atmospheres showed a distinct electronic behavior of the films with regard to the bulk NFO material. Indeed, the thin films exhibit p-type electronic conductivity, while the bulk material is known to be a prevailing ntype electronic conductor. This difference is ascribed to the dissimilar concentration of Ni3+ in the thin films, as revealed by XPS analysis at room temperature. The bulk material with a low concentration of Ni3+ (Ni3+/Ni2+ ratio of 0.20) shows the expected n-type electronic conduction via electron hopping between Fe3+−Fe2+. On the other hand, the NFO thin films annealed at 800 °C exhibit a Ni3+/Ni2+ ratio of 0.42 and show p-type conduction via hole hopping between Ni3+−Ni2+.
■
INTRODUCTION Nanostructured thin films of NiFe2O4 (NFO) are important candidates for magnetic components such as resonators, phase shifters, tunable signal filters, and, more recently, for spintronics applications.1−3 Along with their diverse applications, ferrites have properties of fundamental interest. These include their structure, which can be inverse, normal, or mixed spinel, and the dependence of their physical properties on size and synthesis techniques. Spinel ferrites have the general chemical 3+ 2+ 3+ 2− formula (M2+ 1−δFeδ )[Mδ Fe1−δ]O4 , where the divalent ions 2+ M can occupy either tetrahedral (A) or octahedral (B) sites or both. Bulk NiFe2O4 (NFO) is a well-known inverse spinel with all Ni2+ ions occupying only the B sites. However, NFO in nanocrystalline form has been reported to exhibit a mixed spinel structure with Ni2+ ions occupying both A and B sites.4 In addition, NFO has shown n-type or p-type electronic conductivity depending on the different synthesis techniques and crystal sizes. The n-type behavior has been attributed to the presence of Fe2+, which enables the electron hopping from Fe2+ to Fe3+. The p-type behavior has been attributed to the presence of Ni3+ and hole hopping from Ni3+ to Ni2+. The last can be assigned to the deficiency or excess in Ni, because a Ni excess corresponds to Fe3+ deficiency compensated with Ni3+ (p-type) while a Ni deficiency corresponds to a Fe 2+ compensation (n-type).4 This type of spinel is a potential candidate for solid oxide fuel cell component due to the tunability and the appropriate magnitude of the total conductivity at high temperature. Specifically, some doped nickel ferrites have been recently © 2014 American Chemical Society
studied as possible cathodes for SOFC applications due to their mixed ionic−electronic conduction; for example, NiFe1.5Co0.5O4 presents, at 700 °C in air, 0.24 S/cm and 9.6 × 10−4 S/cm of electronic and ionic conductivity, respectively.5 Because the ionic conductivity is very low as compared to the electronic transport and limits the oxygen transport, NFO was also proposed to be combined in a composite with a predominant ionic conducting material to work as dual-phase oxygen separation membrane.6,7 This work reports the fabrication of NFO spinel thin films by rf sputtering at different temperatures. The study transport properties at high temperature of thin films combined with XPS analysis make it possible to obtain deeper insight into electronic conduction mechanisms.
■
EXPERIMENTAL SECTION NiFe2O4 (NFO) powders were prepared by the sol−gel route (Pechini method),7 and the powder was calcined in air at 700 °C. One inch ceramic target was prepared by uniaxially pressing into pellets at 30 kN for 3 min and subsequently sintered in air at 1400 °C for 5 h. X-ray diffraction (XRD) of the sintered target confirmed the complete formation of the corresponding spinel structures.5 The thin films were deposited with a radio frequency (13.56 MHz) Pfeiffer Classic 250 deposition system. NiFe2O4 target was used for sputtering on a rotating substrate Received: July 11, 2014 Revised: October 1, 2014 Published: October 2, 2014 24266
dx.doi.org/10.1021/jp506938k | J. Phys. Chem. C 2014, 118, 24266−24273
The Journal of Physical Chemistry C
■
RESULTS AND DISCUSSION The XRD patterns of NFO thin films as-deposited at RT, 400, and 600 °C are shown in Figure 1a. It can be observed that, as
holder, with rf power of 25 W. The base pressure of the chamber before the deposition was 2 × 10−6 mbar or lower. The working pressure was 2.9 × 10−2 mbar with a mixed argon−oxygen gas ratio 10/2. The distance target−substrate was kept to 5 cm, and the substrate temperature was changed from room temperature (RT) to 600 °C. Different thicknesses were deposited and controlled by deposition time. All samples were deposited simultaneously on Si (100) and amorphous quartz substrates to perform different characterizations. The crystal structure of the films was patterned by XRD measurements carried out by a PANalytical Cubix fast diffractometer, using Cu Kα1,2 radiation and an X’Celerator detector in Bragg−Brentano geometry. The cell parameter (a) of the different NFO films was obtained by using Bragg’s law (eq 1) and the equation for a cubic system (eq 2): 2d sin θ = nλ
(1)
a = d h2 + k 2 + l 2
(2)
where d is the interplane distance, θ is the scattering angle, λ is 1.5406 Å of the Cu Kα1, n is the integer representing the order of the diffraction peak, and h, k, and l are the Miller indexes. Assuming that the crystal size (t) and the strain (ε) contributions to the line broadening are independent of each other and have a Cauchy profile, the average size can be obtained from the width of the XRD peaks after correcting the instrumental broadening and using the Williamson−Hall eq 3:8,9 0.9λ β ·cos θ = + 4·ε ·sin θ t
Article
Figure 1. XRD patterns of three different NFO films deposited at RT, 400, and 800 °C (a), and after annealing in air for 2 h at 800 °C (b), respectively. NFO reference patterns are also shown for comparison.
(3)
where β is the full width at half-maximum (fwhm) of the XRD peak. By plotting β cos θ versus sin θ, the strain is obtained from the slope and the crystal size from the intercept. The surface morphology of the films and the fracture cross sections were observed by ZEISS Ultra55 field emission scanning electron microscopy (FESEM). The thickness of the films was determined from the fracture cross-section FESEM images, preferably with samples deposited on Si. The typical thickness values range from 100 to 200 nm. The in-plane electrical conductivity of the films was measured by standard four-point DC technique on films deposited on quartz substrates by using silver wires and paste for contacting. Measurements were recorded as the temperature decreases from 800 to 400 °C at 1 °C/min at a constant atmosphere, in different oxygen partial pressures (pO2).10 The constant current was supplied by a programmable current source (Keithley 2601), and the voltage drop through the sample was detected by a multimeter (Keithley 3706). Surface analysis performed by X-ray photoelectron spectroscopy (XPS) was carried out on PHI Quantera equipment with a base pressure in the analysis chamber of 10−9 Torr. The X-ray source was monochromatic Al Kα radiation (1486.6 eV), and the overall energy resolution is estimated at 0.65 eV by the full width at half-maximum (fwhm) of the Au 4f7/2 photoelectron line (84 eV). The charging effect was minimized by using a dual beam (electrons and Ar ions) as neutralizer, while the spectra were calibrated using the C 1s line (BE = 284.8 eV) of the adsorbed hydrocarbon on the sample surface (C−C and/or (CH)n bondings). As this spectrum was recorded at the start and the end of each experiment, the energy calibration during experiments was reliable.
deposited, the film grown at RT looks amorphous due to the low energy amount provided in the deposition, which was not sufficient for the crystal formation. When the deposition temperature increases, to 400 °C or higher, the crystallization of the sample is observed, and diffraction peaks appear and can be assigned to those of the reported NFO cubic spinel (also depicted in the graph for comparison).5 With the increase of the deposition temperature, it can also be observed a slight narrowing of the peaks, as corresponds to bigger grains obtained at higher temperatures. When the thin films are subsequently annealed in air at 800 °C for 2 h (XRD patterns depicted in Figure 1b), the crystallization of the sample grown at RT is reached, and the NFO diffraction peaks can be observed. The films deposited at 400 and 600 °C do not change significantly their cell parameters as deduced from the same peak position before and after the annealing. However, there is a slight increase in the grain size with the annealing treatment as compared to the as-deposited samples, inferred from the slight narrowing of the peaks after the annealing treatment. From the XRD patterns, cell parameter a and grain size t and strain were calculated by using eqs 2 and 3, respectively. Regardless of the deposition temperature and the annealing procedure, the obtained cell parameters are around 8.28 Å, which matches previously reported values for NFO.11 The effect of the annealing on the grain size is highlighted in Figure 2a, where it is represented as a function of the deposition temperature, both as-deposited and after the annealing. The obtained strain is shown in Figure 2b. The lack of diffraction peaks of as-deposited NFO film grown at RT precluded the calculation of t. Figure 2a shows that as-deposited grain sizes 24267
dx.doi.org/10.1021/jp506938k | J. Phys. Chem. C 2014, 118, 24266−24273
The Journal of Physical Chemistry C
Article
Figure 2. Grain size t (a) and strain (b) of the different films deposited at different temperatures as deposited and after annealing for 2 h in air at 800 °C.
Figure 3. FESEM images of different NFO films deposited at RT (a), 400 °C (b), and 600 °C (c) and after annealing in air for 2 h at 800 °C (d−f), respectively, with corresponding cross-section images in the insets.
are almost identical, 56 nm (see “■” in Figure 2a). After the annealing, it can be seen that the sample deposited at 400 °C slightly increases its grains size up to 58 nm, while the grain size of the film deposited at 600 °C remains constant. This indicates that the sample deposited at 600 °C is grown at higher energy than that induced by the annealing process at 800 °C. Furthermore, the grain size of the samples after annealing slightly decreases when raising the deposition temperature, that is, from 60 nm of the sample deposited at RT to 56 nm of the one deposited at 600 °C. This can be due to the suppression of grain growth due to the increase in the number of available nucleation sites and the density of stacking faults and dislocations at high deposition temperatures.12−14 Furthermore, the strain of the films as deposited increases with the sintering temperature (Figure 2b), strain that decreases to a constant value, independent of the deposition temperature, after the annealing process. The corresponding surface morphology of the aforementioned NFO films as deposited at RT, 400, and 600 °C can be observed in Figure 3a−c, respectively. Analogous images after annealing in air at 800 °C for 2 h are depicted in Figure 3d−f for films deposited at RT, 400, and 600 °C, respectively, and corresponding cross-section images (insets). It can be observed that the film grown at RT presents some porosity, with small cracks spread along the surface of the sample. Those cracks are observed in the as-deposited material and increase in magnitude after the annealing. The morphology of this porosity
can be observed in the fracture cross-section shown in the inset of Figure 3d. The images of the films as deposited at 400 and 600 °C show high grain density that increases with temperature. After annealing, the sample grown at 600 °C presents some long cracks along the surface, ascribed to the very high density of this film and the very different temperature expansion coefficients (TEC) of the film as compared to the substrate (10.3 × 10−6 K−1 of NFO5 and 0.55 × 10−6 K−1 of quartz15). The very small crystal size of the samples, calculated from XRD patterns, makes very difficult a direct comparison of the grain sizes by employing the FESEM images. In any case, the small crystal size is confirmed and also the very small differences between the different samples are visible. Note that the cross-section images of the samples deposited at 400 and 600 °C were taken after 3 s Pt sputter deposition, and thus the grain size is more difficult to be calculated. Furthermore, these thickness differences correspond to distinct deposition time. The transport properties at high temperature of these films were studied by means of total conductivity measurements from 800 to 400 °C in different atmospheres. Figure 4 shows the temperature dependence of the total conductivity in oxygen after annealing NFO films deposited at RT, 400, and 600 °C, together with a NFO bulk sample for comparison. The thin films show higher conductivity in oxygen than the bulk material. This can be only partly ascribed to the very wellknown poor sinterability of NFO material, which gives rise to a low density bulk sample (see FESEM image in Supporting 24268
dx.doi.org/10.1021/jp506938k | J. Phys. Chem. C 2014, 118, 24266−24273
The Journal of Physical Chemistry C
Article
× 10−5 atm of O2. In this temperature regime, the oxygen exchange with the atmosphere is negligible, as can be deduced from the constant Ea values extracted from the straight experimental lines of Figure 5a. Consequently, the oxygen stoichiometry of the films should depend on the equilibrium state attained at the highest temperatures reached at different pO2. Thus, the observed increase in Ea at lower pO2 must be related to a progressive reduction in the oxygen content of the NFO when lowering the pO2. Such observation was previously reported for p-type Sr4Fe6O12+δ bulk16 and thin films17 and SrTi1−xFexO3−y ceramic oxides.18 In those cases, the increase in Ea was related to the progressive reduction of Fe3+ to Fe2+. One possible explanation is that this induces greater carrier localization, thereby increasing the energy barrier for polaron diffusion. Alternative explanations relate to the movement of the Fermi energy toward midgap and the consequent increase in the carrier ionization energy.17 Figure 6 depicts the total conductivity as a function of the pO2 of the films grown at the three studied deposition
Figure 4. Total conductivity of different NFO thin films and bulk material in oxygen as a function of the inverse temperature.
Information Figure S1). However, the higher conductivity of the thin films may be more likely associated with changes in the nature of the prevailing electronic charge carriers, which may stem from the variation of the electronic states of lattice cations, potential occurrence of cationic vacancies, and the partial exchange between them in crystallographic positions. The change in the nature of predominant charge carriers is also reflected in the significantly higher activation energy (Ea) observed for the thin films with regard to the bulk sample. Besides, the sample deposited at 400 °C shows the highest conductivity, as corresponds to the best microstructure among these films, that is, dense and with no cracks, as observed from FESEM images of Figure 3b and e. Figure 4 also shows that conductivity increases with temperature following an Arrhenius behavior, that is, σ(T) = (σ0)/T e−((Ea)/(K·T)), where K is the Boltzmann constant. Ea values are very similar for the three thin films. To analyze the transport mechanism of the thin films, Figure 5a shows σ·T as a function of 1000/T for the sample grown at
Figure 6. Total conductivity at 700 °C of different NFO thin films and bulk material as a function of the pO2.
temperatures and the bulk material. While the films show a 1/4 slope, as corresponds to a predominant p-type electronic conductor, the bulk material shows −1/4 slope typical of n-type electronic conductors. It has been reported that NFO shows ntype electronic conductivity due to the electron hopping between Fe3+−Fe2+, while the NFO p-type conductivity is ascribed to the hole hopping between Ni3+−Ni2+.4,11,19 The formation of the Ni3+ can be due to the creation of cation vacancies in the ferrite lattice, which forms the cation (Fe3+ + vacancy) complex that acts as p-type carriers. This results in the formation of Ni3+ species for the overall charge balancing and compensation.19 Thus, the Ni3+/Ni2+ ratio can determine the type of electronic transport of the NFO samples. Indeed, it has been reported that NFO nanoparticles with different electronic behavior were achieved just varying the pH of the starting solution of the synthesis, which makes it possible to tune the Ni3+ concentration in the oxide lattice.11 To analyze the cation oxidation state of the present thin films and the target material, X-ray photoelectron spectroscopy (XPS) measurements were carried out. XPS analysis was used to determine the oxidation states of the elements present on the surface and, after quantitative analysis, to find the element and the chemical state relative concentrations as well. After collecting survey XPS spectra (0− 1200 eV), the high-resolution photoelectron spectra of the most prominent XPS transitions (C 1s, O 1s, Ni 2p3/2, Fe 2p, Ni 3p, and Fe 3p) were recorded. The spectra were collected in
Figure 5. σ·T of the NFO film deposited at 400 °C under different pO2 atmospheres as a function of inverse temperature (a) and obtained Ea as a function of the pO2 (b).
400 °C and measured at different pO2. First, a clear conductivity decrease with the decrease of pO2 can be detected, as corresponds to a p-type electronic conductor. Moreover, Ea values can be directly calculated as the curves follow a single Arrhenius behavior. The extracted Ea values are represented in Figure 5b as a function of the pO2. Ea values are characteristic of the hopping of charge carriers. Ea values strongly increase with reducing pO2, ranging from ∼0.45 eV at 1 atm to ∼1.0 eV at 5 24269
dx.doi.org/10.1021/jp506938k | J. Phys. Chem. C 2014, 118, 24266−24273
The Journal of Physical Chemistry C
Article
Table 1. XPS Results: The Binding Energies (eV) and the Element Relative Concentrations atomic relative concentrations (atom %)
binding energy (eV) sample
C 1s
O 1s
Ni 2p3/2
Fe 2p3/2
Ni 3p
Fe 3p
O
Ni
Fe
TF, RT, as-received
284.8
67.0
55.1
65.8
11.6
22.6
284.8
710.6
67.6
55.0
49.8
17.2
33.0
TF, 400 °C, as-received
284.8
710.7
67.0
55.0
64.3
11.3
24.4
TF, 400 °C, 1 min Ar etching
284.8
710.6
67.2
55.2
51.9
15.5
32.6
TF, 600 °C, as-received
284.8
710.7
67.3
55.1
65.6
11.4
22.9
TF, 600 °C, 1 min Ar etching
284.8
854.4 855.8 861.2 852.6 854.4 855.9 861.5 854.3 855.7 861.1 852.6 854.3 855.8 861.3 854.4 855.8 861.3 852.9 854.3 855.8 861.2
710.8
TF, RT, 1 min Ar etching
529.8 531.5 532.8 530.0 531.5
710.6
67.5
55.3
59.3
13.7
27.0
529.8 531.5 532.9 530.1 531.5
529.9 531.5 533.0 530.0 531.4
Figure 7. O 1s XPS deconvoluted spectra of the “as-received” (surface) (a−c) and after 1 min Ar ion etching (subsurface/near surface region) (d−f) fNFO films deposited at RT, 400, and 600 °C (see Table 1 for the associated numerical values).
the “as-received state” and after Ar ion sputtering under mild conditions (1 keV, 1 min, (3 × 3) rastered area) to not disturb the surface/near surface chemical states. The characteristic BEs and the element relative concentration are summarized in Table 1. Besides the most prominent XPS transitions for Ni and Fe (2p3/2 photoelectron lines) the 3p minor lines at lower BEs (higher kinetic energies, KEs) (see Table 1) were also recorded. The aim was to improve the accuracy of BEs assignments as well as to check the homogeneity on the sampling depth because the large differences in BEs (and the corresponding KEs = 1486.6 − BEs) lead to different escape depths (detected volumes).
The oxygen chemistry of the three different thin films deposited at RT, 400, and 600 °C after annealing at 800 °C in air is shown in Figure 7, as-received (a−c) and after 1 min Ar etching (d−f). A close inspection of the spectra suggests differences in the oxygen chemistry mainly due to the components assigned to OH and H2O adsorbed on the surface, providing evidence that these surfaces were moisturesensitive (Figure 7a−f). The symmetric O 1s peak shape proves the cleanliness of the surfaces in contrast with our experimental spectra mainly in the “as-received” state (Figure 7a−c). Usually the component at ∼530.0 eV is assigned to the bound oxygen in the lattice, the component at ∼531.5 eV is ascribed to OH adsorbed groups 24270
dx.doi.org/10.1021/jp506938k | J. Phys. Chem. C 2014, 118, 24266−24273
The Journal of Physical Chemistry C
Article
Figure 8. Ni 2p3/2 XPS deconvoluted spectra of the “as-received” (surface) (a−c) and after 1 min Ar ion etching (subsurface/near surface region) (d−f) NFO films deposited at RT, 400, and 600 °C (see Table 1 for the associated numerical values).
on the surface, and the component at ∼533.0 eV is attributed to the presence of water. After “in situ” Ar etching under gentle conditions (1 min, 1 keV (3 × 3) mm2 scanned area), the adsorbed water is removed while the OH groups are still present, although in lower concentration. Following Grosvenor assignments for iron oxidation in air and water vapor,20 under OH curve, one can accommodate two contributions: one from the lattice OH− (leading to the formation of FeOOH on the top of the surface) and the other one from the adsorbed OH. Figure 8a−f and Table 1 show that Ni chemistry exhibits some peculiar characteristics on the surface (the detected volume ∼3λ = 3.0 nm, where λ is the inelastic mean free path) as compared to the near surface/subsurface region (∼5.0 nm, after 1 min Ar ion sputtering). Thus, in the “as-received” state, Ni is found as a mixture of 2+ and 3+ oxidation states with rather constant percentages (roughly 70% and 30%, respectively). After removing ∼2.0 nm under aforementioned moderate Ar ion etching to minimize the disturbing effect on the chemical species, a low percentage (
