Oleylamine surface functionalized FeCo_yFe_2−yO₄ (0.0 ⩽ y ⩽ 1.0) nanoparticles

2.1 Materials and instrumentations

All the chemicals, Cobalt(III) acetylacetonate (99.99% trace metals), Iron(II) acetylacetonate, Iron(III) (99.95% trace metals basis) and Oleylamine (technical grade, 70%) were received from Merck and used as received.

To investigate the nature of the chemical bonds formed in the final products, Perkin Elmer BX FT-IR infrared spectrometer was used in the range of 4000–400 cm⁻¹.

The crystalline structure of FeCo_yFe_2−yO₄@OAm NCs was detected by X-ray diffraction measurements (XRD) of Rigaku D/Max—IIIC with Cu Kα, in the 2θ range of 20–70°.

Scanning Electron Microscopy (SEM) analysis was performed using FEI XL40 Sirion FEG Digital Scanning Microscope. Samples were coated with gold at 10 mA for 2 min prior to SEM analysis.

Additionally the surface morphology of the FeCo_yFe_2−yO₄@OAm NCs was done by Transmission electron microscopy (TEM) analysis using a FEI Tecnai G2 Sphera microscope. A drop of diluted sample in alcohol was dripped on the TEM grid.

For TG analysis, Perkin Elmer Instruments model, STA 6000 instrument was used to get the information of thermal stability of FeCo_yFe_2−yO₄@OAm NCs. It was recorded under nitrogen gas atmosphere in the temperature range of 30–750 °C (10 °C/min).

Vibrating sample magnetometer (LDJ Electronics Inc. Model 9600) was used for VSM measurements and the magnetic characterization of FeCo_yFe_2−yO₄@OAm NCs was carried out at room temperature in an external field up to 15 kOe.

The 25 mCi ⁵⁷Co (Rh matrix) radiation source in transmission geometry was used to record the Mössbauer spectra of FeCo_yFe_2−yO₄@OAm NCs at room temperature. Moreover, Wissel velocity drive was applied and α-Fe was used to calibrate the speed scale which was performed with laser interferometry. Well-known Win-Normos least squares fitting programs were used to get the Mössbauer spectra and were fitted by the least square method. The quality of this data fitting was investigated by the χ²-test.

2.2 Procedure

For typical synthesis of FeCo_yFe_2−yO₄@OAm NCs (0.0 ⩽ y ⩽ 1.0) nanocomposites, stoichiometric amounts of metal acetylacetonates (Fe(acac)₃, Fe(acac)₂ and Co(acac)₃) were dissolved slowly with 15 ml OAm into a glass flask of three neck. Then solution was refluxed at two different temperatures, first at 110 °C for 60 min and then second was done at 300 °C for 1 h under Ar gas. Then dark brown products were taken out by simple permanent magnet. After that it was washed by solvent like ethanol three times and then dried at 80 °C for 4 h (Scheme 1).

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Scheme 1

Schematic representation of synthesis.

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3.1 Infrared analysis

FT-IR spectrum of FeCo_yFe_2−yO₄@OAm NCs (0.0 ⩽ y ⩽ 1.0) is presented in Fig. 1 which confirmed that 1380–1529 cm⁻¹ bands are due to the presence of NH₂ bending mode of OAm, and similarly the bands around 2836 and 2932 cm⁻¹ are for methyl stretching (Xu et al., 2009; Niasari et al., 2009) and the peaks at 711 cm⁻¹ for C–C and 1005 cm⁻¹ belong to the C–N bending vibration (Özkaya et al., 2009). Moreover, a main broad metal-oxygen typical stretching modes are also observed between 520 and 570 cm⁻¹ which attributed the formation of spinel ferrites (Özkaya et al., 2009). Thus, based on FT-IR results, we deduce that FeCo_yFe_2−yO₄@OAm NCs are successfully capped by oleylamine.

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Figure 1

(a) FT-IR spectra of FeCo_yFe_2−yO₄@OAm (0.0 ⩽ y ⩽ 1.0) NCs and (b) FT-IR spectra of FeCo_0.4Fe_1.6O₄@OAm (y = 0.4).

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3.2 XRD analysis

XRD powder patterns with Rietveld analysis for FeCo_yFe_2−yO₄@OAm NCs (0.0 ⩽ y ⩽ 1.0) NCs presented in Fig. 2 confirmed that all products have cubic spinel structure consistent with Fe₃O₄ ICDD card no. 19-629. The low crystallinity of all products revealed the nanoscale product formation for all substitutions (Ünal et al., 2010). Rietveld analyses were done by using the following hkl values (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) (Ünal et al., 2010). The average crystallite sizes (t) were calculated based on 3 1 1 peak using Scherrer’s equation.

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Figure 2

XRD powder patterns with Rietveld analysis for FeCo_yFe_2−yO₄@OAm (0.0 ⩽ y ⩽ 1.0) NCs.

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The Co content and refined structural parameters for FeCo_yFe_2−yO₄@OAm NCs (0.0 ⩽ y ⩽ 1.0) NCs with space group Fd-3m are tabulated in Table 1.

3.3 VSM analysis

The room temperature M-H hysteresis curves of FeCo_yFe_2−yO₄@OAm NCs (0.0 ⩽ y ⩽ 1.0) NCs are given in Fig. 3. The saturation magnetization of all products was determined by intersection of the hysteresis curve with 1/H² axis as 1/H² approaches to zero. These magnetizations are not the saturation (ss) values of powder samples. σ_s for each sample was estimated by Stoner-Wohlfarth (S-W) model by extrapolating s vs 1/H² plot to 1/H² approaches zero (Sanchez-De Jesus et al., 2014; Kojima and Wohlfarth, 1982). The S-W model applies for non-interacting, single domain structures. The plot of magnetization as a function of 1/H² is given for each y = 0.0 and CoyFe1₋yFe₂O₄@OAm nanocomposite sample in Fig. 4. It can be seen in Fig. 4, the estimated σ_s magnitudes vary between minimum of 36.8 emu/g and maximum of 61.3 emu/g. Magnetization decreases with increasing Co³⁺ content at different magnitudes. The saturation magnetization of the sample having minimum Co³⁺ concentration (i.e. y = 0.2) is significantly lower than that of the pure Fe₃O₄ NPs which is nearly 76.5 emu/g (Lee et al., 1998). This substantial decrease in Ms can be due to the adsorption of Oleylamine molecules to the surface of Fe₃O₄ NPs through oxygen ions which have an important role in super exchange interaction between Fe³⁺ and Fe²⁺ ions. This weakens interlayer super exchange interaction (J_AB) Fe³⁺ - O²⁻- Fe²⁺ which gives rise to a decrease in Ms. When Co³⁺ ions were substituted to the Fe₃O₄, Ms continued to decrease up to Co³⁺ content of y = 0.4. It was reported that Co³⁺ ions prefer to replace Fe²⁺ ions on octahedral side up to some concentration (Ahmed et al., 2010). The smaller ionic radii of the Co³⁺ ions (0.78 Å) disturb the J_AB exchange interaction diminishing the magnetization of B-sublattice (i.e., M_B). As a result, the net magnetization of Fe₃O₄ NPs decreases. Further increase of Co³⁺ concentration to y = 0.6 increased Ms from 36.8 emu/g to 61.3 emu/g which is a consequence of a decrease in M_A due to the replacement of the substituted ions with Fe²⁺ ions on tetrahedral side. Our Mössbauer analysis also confirmed that Co³⁺ ions replace the Fe²⁺ ions on tetrahedral sites when y ⩾ 0.6.

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Figure 3

Plot of specific magnetization as function of 1/H² of FeCo_yFe_2−yO₄@OAm (0.0 ⩽ y ⩽ 1.0) NCs.

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Figure 4

The room temperature M-H hysteresis curves of FeCo_yFe_2−yO₄@OAm (0.0 ⩽ y ⩽ 1.0) NCs.

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The coercive field of FeCo_yFe_2−yO₄@OAm NCs (0.0 ⩽ y ⩽ 1.0) NCs increases significantly when Co³⁺ concentration becomes y = 0.4. In general, this may happen when shape anisotropy or magnetocrystalline anisotropy of the particles is improved. All the samples have been prepared through the same preparation method and under the same conditions. Therefore, it is reasonable to think that shape anisotropy of the particles nearly the same. On the other hand, it is well known that CoFe₂O₄ has the largest positive anisotropy due to the strong spin-orbit coupling at Co³⁺ sites. Hence, substitution of Co³⁺ ions in magnetite increases the magnetocrystalline anisotropy of the FeCo_yFe_2−yO₄@OAm NCs (0.0 ⩽ y ⩽ 1.0) NCs, and thus, coercive field increases (Zhu et al., 2013). However, the presence of excess amount of Co³⁺ ions (i.e. y ⩾ 0.6) in magnetite lattice decreases the coercivity again, and it was calculated at Table 2.

We also performed theoretical (Langevin) fit studies for superparamagnetic data. The equations for Langevin fit studies were given in the previous reports of our research group (Baykal et al., 2015a, 2015b). The σ_s, statistical median (dm), the geometric standard deviation (Δ) are the fit parameters and they are given with average magnetic particle diameter (D_mag) in Table 2.

3.4 TG analysis

The content of organic OAm in FeCo_yFe_2−yO₄@OAm NCs (0.0 ⩽ y ⩽ 1.0) NCs was determined by TGA and three selected compositions of y = 0.2, y = 0.6 and 1.0 were taken for the TGA analysis. As shown in Fig. 5, the weight loss from RT to 150 °C is due to the loss of absorbed residual water for nanocomposites. Up to the temperature of 500 °C TG curves of all three composition showed the degradation of organic backbone of Oleylamine groups grafted to the FeCo_yFe_2−yO₄ surface and above this temperature, the weight loss for Oleylamine is almost zero which confirmed the presence of only inorganic content of FeCo_yFe_2−yO₄ (0.0 ⩽ y ⩽ 1.0) NPs (Kurtan et al., 2016). Thus the percent of organic content in three selective compositions of y = 0.2, y = 0.6, and y = 1.0 was estimated as ∼21.60%, ∼25.28%, and ∼30.51% and the percentage of inorganic content ∼78.40, ∼74.72, ∼88 and ∼69.49 belonged to FeCo_yFe_2−yO₄@OAm NCs respectively. D_mag were also specified as 14.25 nm and 15.37 of 13.67 nm for y = 0.2 y = 0.6 and y = 1.0 respectively. These value are in accordance with the crystallite sizes 11.7 nm, 10.7 nm and 7.3 nm obtained from XRD analysis respectively. Therefore the TG thickness was calculated as 2.55 nm, 4.67 nm and 6.37 nm for y = 0.2 y = 0.6 and y = 0.8 FeCo_yFe_2−yO₄@OAm NCs respectively.

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Figure 5

TG analysis of FeCo_yFe_2−yO₄@OAm (0.0 ⩽ y ⩽ 1.0) NCs for y = 0.2, y = 0.6 and y = 1.0 substitutions.

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3.5 SEM analysis

SEM micrographs with their related EDX spectra and elemental maps of FeCo_0.2Fe_1.8O₄@OAm and FeCo_0.8Fe_1.2O₄@OAm nanocomposites are presented in Fig. 6a and b respectively. Uniformity in size and shape of all the compositions of FeCo_yFe_2−yO₄@OAm NCs (0.0 ⩽ y ⩽ 1.0) NCs was obtained by the polyol route and can be observed in SEM images. FeCo_yFe_2−yO₄@OAm NCs (0.0 ⩽ y ⩽ 1.0) NCs consisted of regular shaped spherical nanoparticles having small agglomeration. The SEM analysis was done with 100 nm resolution view of particles which showed smaller crystallites size and the particles are very close to each other. EDX analysis and electro mapping were also performed to characterize elements of nanocomposites and it can be seen in that nanocomposite consists of elements such as C, N, O, Fe, Co (in both EDX and elemental maps), Fig. 6a and b also shows the representative X-ray elemental maps of all elements (C, N, O, Fe, Co) detected in energy dispersive X-ray spectroscopy for FeCo_0.2Fe_1.8O₄@OAm and FeCo_0.8Fe_1.2O₄@OAm NCs. The maps indicate fairly homogeneous elemental distributions which suggest all products are uniform.

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Figure 6

SEM micrographs with its EDX spectra of (a) FeCo_0.2Fe_1.8O₄@OAm and (b) FeCo_0.8Fe_1.2O₄@OAm NCs.

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Figure 6

SEM micrographs with its EDX spectra of (a) FeCo_0.2Fe_1.8O₄@OAm and (b) FeCo_0.8Fe_1.2O₄@OAm NCs.

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3.6 TEM analysis

TEM micrographs and related particle size distribution diagrams of FeCo_0.2Fe_1.8O₄@OAm and FeCo_0.6Fe_1.4O₄@OAm NCs are given in Fig. 7a and b respectively. Both TEM images confirmed the spherical morphology of both products. The particle sizes of FeCo_0.2Fe_1.8O₄@OAm and FeCo_0.6Fe_1.4O₄@OAm NCs were calculated as 12.2 ± 02 and 11.0 ± 0.3 nm respectively. These results are in good agreement with the crystallite sizes (from XRD) of same products.

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Figure 7

TEM micrographs of (a) FeCo_0.2Fe_1.8O₄@OAm and (b) FeCo_0.6Fe_1.4O₄@OAm NCs along with their particle size distribution diagrams.

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3.7 Cation distribution

The cation distribution in spinel ferrite can be obtained from the analysis of X-ray diffraction pattern. In the present work, the Bertaut method (Amir et al., 2015) is used to determine the cation distribution. Ferrites are ferrimagnetic oxides with their magnetic cations forming two sublattices, namely the tetrahedral (A) and the octahedral [B] crystallographic sites.

It can be noticed in Table 3, that Fe²⁺ ions preferentially occupy tetrahedral A-site whereas Fe³⁺ ions only occupy octahedral B-site. Co³⁺ ions initially up to y = 0.4, only occupy octahedral B-site, as the Co³⁺ ions increased for y > 0.4 and it also occupies tetrahedra A-site by some fraction.

3.8 Mössbauer studies

The 57Fe Mössbauer spectra of FeCo_yFe_2−yO₄@OAm (0.0 ⩽ y ⩽ 1.0) NCs were done with “The 25mCi 57Co (Rh matrix) radiation source, shown in Fig. 8. The fitted spectra in Fig. 8 were used to calculate the related Mössbauer results are depicted in Table 4. The spectra were fitted using four sextets, A for the tetrahedral sites and B, B₁ and B₂ for the octahedral positions. Using the fitting computer program the B-site pattern has been fitted with three sextets. These three sextets belong to the Co³⁺ ions majorly at A-site nearest and Co³⁺ ions are nearest neighbors of the Fe at B-site.

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Figure 8

Room temperature Mössbauer spectra of FeCo_yFe_2−yO₄@OAm (0.0 ⩽ y ⩽ 1.0) NCs.

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The spectra for the sample of FeCo_yFe_2−yO₄@OAm (0.0 ⩽ y ⩽ 1.0) NCs with y = 0 showed two paramagnetic central doublets due to Fe³⁺ ions at A–site, where one of the sextets arises due to its smaller hyperfine field of the Zeeman pattern of to Fe³⁺ ions at tetrahedral (A) and the second sextet corresponds to the Fe³⁺ ions at octahedral (B) site which existed due to its larger hyperfine field of the Zeeman pattern (Amir et al., 2015; Greenwood and Gibb, 1971; Baldha and Kulkarni, 1984). The presence of paramagnetic phase C in the Mössbauer results was assumed due to the magnetically ordered spins behavior of few nearest neighbors of a fraction of Fe ions (Dobson et al., 1970; Ok et al., 1990).

The Mössbauer spectra of FeCo_0.6Fe_1.4O₄@OAm, FeCo_0.8Fe_1.2O₄@OAm and FeCoFe₂O₄@OAm NCs exhibit the additional presence of a quadrupole-split doublet. A fraction of Fe ions consist of few nearest neighbors possessing magnetically ordered spins are responsible for the existence of the paramagnetic phase in the Mössbauer spectra (Dobson et al., 1970). The largest hyperfine with bigger isomer shift is characteristic of Fe³⁺ ions in the octahedral B-site but the lower values of both parameters are due to Fe³⁺ ions in the tetrahedral A site (Widatallah et al., 2013; Siddique and Butt, 2010; Jadhav et al., 2015). This is because of the presence of magnetic ion (Co³⁺) at B-site (Jadhav et al., 2015). In most of the magnetic materials such as magnetite and ferrites, hyperfine magnetic field at B-site is generally considered to be larger than hyperfine magnetic field of that A-site and this corresponds to the dipolar field. This hyperfine behavior of A-site and B-site is due to deviation from covalent nature and cubic symmetry of tetrahedral bonds (Widatallah et al., 2013; Siddique and Butt, 2010; Jadhav et al., 2015; Lakshman et al., 2006).

According to Table 4, as the concentration of Co³⁺ ions increases, the hyperfine field values at B- and A- sites slowly decrease which can be explained by Neel’s super exchange interaction model (Neel, 1948). Neel’s model is based on the inter sublattice exchange interactions. From the Mössbauer spectra, it can be said that both A and B sites are occupied by Fe³⁺ and Co³⁺ ions, and mostly: ${\text{Fe}}_{\text{A}}^{3+}-\text{O}-{\text{Fe}}_{\text{B}}^{3+}$ , ${\text{Fe}}_{\text{A}}^{3+}-\text{O}-{\text{Co}}_{\text{B}}^{3+}$ , ${\text{Fe}}_{\text{B}}^{3+}-\text{O}-{\text{Co}}_{\text{A}}^{3+}$ interactions can be taken under consideration, as the interaction of AA or BB assumed to be very weak and can be neglected. Therefore only strong interaction can originate the net magnetic field. As the concentration of larger number Co³⁺ ions of lower magnetic moment of 5.4 μ_B increased, Fe³⁺ ions of higher magnetic moment of 5.92 μ_B replaced by the Co³⁺ ions and the number of magnetic linkages in ${\text{Fe}}_{\text{A}}^{3+}-\text{O}-{\text{Fe}}_{\text{B}}^{3+}$ , ${\text{Fe}}_{\text{A}}^{3+}-\text{O}-{\text{Co}}_{\text{B}}^{3+}$ and ${\text{Fe}}_{\text{B}}^{3+}-\text{O}-{\text{Co}}_{\text{A}}^{3+}$ decrease and consequently Fe³⁺ nuclei experience a fraction of reduction in the magnetic field at both the sub lattices.

The chemical isomer shifting (I.S) normally takes place due to the change in differing chemical environments and nuclear radius and depends on s-electron density and the shielding effect of p, d and f electrons. As it can be seen in Mössbauer spectra, the isomer shift at B sites is larger than that of A-sites, due to the higher covalence which resulted in the larger overlapping of Fe³⁺–O²⁻ ions at B-sites as compared to A-site (Lakshman et al., 2006). The observed and calculated ranges of isomer shift of the A and B magnetic pattern are 0.2863–0.5152 mm s⁻¹ at room temperature. It is well known that in the magnetically ordered phase, the valence of Fe should be mainly distinguished by the isomer shift (0.6–1.7 mm s⁻¹ for Fe²⁺, 0.05–0.5 mm s⁻¹ for Fe³⁺ and −0.15 to 0.05 mm s⁻¹ for Fe⁴⁺) (Hodges et al., 2000). Thus the isomer shift values attribute to Fe³⁺ charge state behavior in A and B sites, and are specific characteristics of the high spin Fe³⁺ charge state. The high values of isomer shift of B₂ of FeCo_yFe_2−yO₄@OAm (0.0 ⩽ y ⩽ 1.0) NCs for y = 0.4 may signal the presence of Fe²⁺ ions at B sites. The Mössbauer study suggested that isomer shift values do not change as the concentration of Co³⁺ ions varies, which explained that there is no specific change in s-electron charge distribution around the Fe³⁺ nucleus at A or B sites depending on concentrations of Co³⁺ ions.

In Table 4, Q.S value was calculated for all the compositions of FeCo_yFe_2−yO₄@OAm (0.0 ⩽ y ⩽ 1.0) NCs. This value gives the information regarding the local distortions and symmetry of crystal lattice. The electric field gradient (EFG) of varying magnitude, sign, direction and symmetry causes due to the nonspherical distribution of 3d electrons of the cations and effective charge on the neighboring ions (Ata- Allah et al., 2000). Siddique et al. also reported that the ionic radii play a large role than their charges in the local symmetry of EFG (Siddique and Butt, 2010). The values of Q.S for observed components with respect to composition of FeCo_yFe_2−yO₄@OAm (0.0 ⩽ y ⩽ 1.0) NCs are negligible and change randomly with addition y value.