Cobalt Ferrite Nanorods Synthesized with a Facile “Green” Method in a Magnetic Field

We report a new facile method for the synthesis of prolate cobalt ferrite nanoparticles without additional stabilizers, which involves a co-precipitation reaction of Fe3+ and Co2+ ions in a static magnetic field. The magnetic field is demonstrated to be a key factor for the 1D growth of cobalt ferrite nanocrystals in the synthesis. Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy are applied to characterize the morphology and structure of the obtained nanoparticles. According to TEM, they represent nanorods with a mean length of 25 nm and a diameter of 3.4 nm that have a monocrystalline structure with characteristic plane spacing of 2.9 Å. XRD and Raman spectroscopy confirm the spinel CoFe2O4 structure of the nanorods. After aging, the synthesized nanorods exhibit maximum saturation magnetization and coercivity equal to 30 emu/g and 0.3 kOe, respectively. Thus, the suggested method is a simple and “green” way to prepare CoFe2O4 nanorods with high aspect ratios and pronounced magnetic properties, which are important for various practical applications, including biomedicine, energy storage, and the preparation of anisotropic magnetic nanocomposites.


Introduction
In recent years, high attention has been attracted to the synthesis and investigation of magnetic nanomaterials [1][2][3], which can be used in many areas, such as the preparation of magnetic nanocomposites and gels [4,5], soft robotics [6], energy storage devices [7], green energy production [8], and various biomedical applications [9,10], including hyperthermia [11], magnetic resonance imaging [12], the development of magnetically responsive industrial systems [13], and so forth.For such nanomaterials, anisotropic (cylindrical, plate-like, etc.) magnetic nanoparticles (NPs) are of high interest [14][15][16] because of their enhanced magnetic properties [17], anisotropy of magnetism, a larger area of the locally induced magnetic field in comparison to nanospheres, etc. [10], as well as their ability to impart anisotropy to nanocomposite materials.
Various methods for synthesis of elongated magnetic NPs have been described, including solvothermal [18], hydrothermal [19], sol-gel [20] or co-precipitation [21] reactions, ultrasound treatment [22], synthesis in the presence of polymers [23] or surfactants [24][25][26], modification of the crystal structure of pre-synthesized rod-like particles [12], etc.In most of these approaches, either a template or a stabilizer is used to provide the growth of NPs in one direction.After synthesis, the surface of NPs usually remains covered with the stabilizer, which may complicate its further modification with organic or inorganic compounds.
Recently, the use of a magnetic field as a template for the 1D growth of magnetic NPs was proposed [27][28][29][30][31][32][33].For instance, a facile and "green" method for the synthesis of elongated magnetite (Fe 3 O 4 ) nanoparticles was elaborated, which consists of the coprecipitation of Fe 3+ and Fe 2+ ions in a magnetic field [30,31].This method allowed obtaining cylindrical Fe 3 O 4 NPs of different lengths of up to 150 nm and magnetizations of ca.29 emu/g.A growth mechanism was proposed, which includes the appearance of small spherical "seed" nanoparticles at the first reaction stage, which then self-assemble into a rod in the external magnetic field and fuse to form a cylindrical NP [32].The magnetic field was further applied for the synthesis of several 1D nanomaterials, mostly nanowires [33].
Most of the approaches for anisotropic NP synthesis described above are rather well developed for iron oxides, such as magnetite or maghemite.It seems reasonable to apply this method to the preparation of other kinds of magnetic NPs; for instance, cobalt ferrite (CoFe 2 O 4 ) NPs, which are of high interest for practical applications due to high coercivity [34] and chemical stability [35].Current methods of the synthesis of anisotropic cobalt ferrite NPs are rather complex and involve additional compounds [36][37][38].For instance, 25 nm × 120 nm CoFe 2 O 4 nanorods were synthesized by a hydrothermal reaction at 130 • C in the presence of cetyltrimethylammonium bromide [38].Further studies reported the hydrothermal synthesis of cobalt ferrite nanorods doped with Gd 3+ ions [39] or Pr 3+ ions [40] in the absence of surfactants.Micrometer-sized cobalt ferrite rods were obtained by thermal decomposition at 400-700 • C of a CoFe 2 (C 2 O 4 ) 3 precursor prepared by a solvothermal reaction [41].There is only one work in which small (several nm) isotropic cobalt ferrite NPs were assembled into rod-like microaggregates during the thermal decomposition of iron(III) and cobalt(II) acetylacetonates in oleic acid, oleylamine, and a benzyl ether at 200 • C under the gradient magnetic field [42].However, those microaggregates did not represent single crystals but were composed of individual spherical NPs.
In this article, for the first time, we apply the synthesis in a magnetic field to prepare single-crystal cobalt ferrite nanorods.We evidence that a static magnetic field of 0.4 T is sufficient to obtain single-crystal NPs with a length of 25 nm and a diameter of 3.4 nm, which exhibit stronger superparamagnetic properties than the corresponding isotropic NPs.One can expect that such magnetic-field-assisted 1D growth may be further applied to other magnetic metal oxides as well.

Synthesis of NPs
Synthesis of NPs was performed by a precipitation reaction of Fe 3+ and Co 2+ ions in an alkaline solution.The solution of ions with a Fe 3+ :Co 2+ molar ratio of 2:1 was prepared by dissolving 2 M FeCl 3 and 1 M Co(NO 3 ) 2 in water at magnetic stirring.A total of 2 mL of this solution was put into a reaction vessel kept at 70 • C [43] in the presence or absence of a static magnetic field of 0.4 T created by a permanent NdFeB magnet.Then, 2.5 mL of 6.5 M NaOH was added to the reaction medium, and the reaction was allowed to proceed for 4 h.At the end of the reaction, pH was ca.6.3, which was measured with a MettlerToledo SevenMulti pH meter(Columbus, OH, USA).To increase the magnetic properties, the solution of NPs was aged at 80 • C. The aging time varied from 1 to 250 h.The final product was separated from the liquid by magnetic decantation and washed with distilled water.The purification was repeated 3 times.

Transmission Electron Microscopy
For TEM and high-resolution (HR) TEM measurements, the samples were diluted 10 times with distilled water and after that sonicated for 40 min in pulse mode (5 s of pulses followed by 5 s of rest) with a Sonics VCX 500 ultrasonicator (Newtown, CT, USA) to break the aggregates of NPs.Then, 10 µL of the NP solution was placed onto a 140 mesh Formvarcoated copper grid and air-dried for 3 min at 25 • C. TEM images were obtained using a JEM 2100 F/Cs (Jeol, Tokyo, Japan) operated at 200 kV and equipped with a UHR pole tip as well as a spherical aberration corrector (CEOS, Heidelberg, Germany) and an EEL spectrometer (Gatan, Munich, Germany).The details of the measurements are described elsewhere [44].The electron micrographs were processed by ImageJ software version 1.54i in order to obtain distances between the crystal planes and to plot the histograms with the NP size distribution [45].

X-ray Diffraction
The crystal structure of the samples was determined by X-ray diffraction using a Bruker AXS D8 DISCOVER setup with a Cu Kα (wavelength λ = 0.15418 nm) radiation at θ-2θ geometry at room temperature.For measurements, samples were prepared by drying the NP solution on a monocrystalline Si (111) wafer.

Raman Spectroscopy
The crystal structure and phase composition of the NPs were investigated using a micro-Raman spectrometer LabRam HR800 (Horiba Jobin Yvon, Villeneuve d'Ascq, France) with an ×100 magnification objective (numerical aperture of 0.9).Details of the experimental procedures are described elsewhere [46].Measurements were conducted at room temperature in the air environment.A He-Ne laser with a 632.8 nm wavelength was used to excite Raman scattering.The irradiation power density on the sample was continuously decreased until no further changes were observed in the spectra obtained.We found that a power of approximately 0.5 mW and a laser spot diameter of about 10 µm were sufficient to avoid structural changes or phase degradation in the films.The spectra were recorded in the range of 100-800 cm −1 , and in our measurement conditions, the total acquisition time to obtain a spectrum with a good signal-to-noise ratio was several hours.

X-ray Photoelectron Spectroscopy
The chemical state of the elements in NPs was analyzed by X-ray photoelectron spectroscopy (XPS) on an Axis Ultra DLD spectrometer (Kratos Analytical, Manchester, UK) with the monochromatic Al Kα X-ray source (1486.7 eV, 150 W) under ultra-high vacuum conditions (10 −9 mbar).Pass energies of 160 and 40 eV were used, respectively, for survey spectra and high-resolution scans.The powder samples were fixed on a holder using non-conductive double-sided adhesive tape.The Kratos charge neutralizer system was used, and the spectra were charge referenced to give the lattice oxygen component in the O1s spectra a binding energy of 530.1 eV, which is typical for iron oxides [47].This led to the binding energy of the C1s peak, which is characteristic of adventitious carbon (about 285.0 eV), which confirmed the reliability of the charge reference procedure.

Magnetometry
The dependencies of magnetization M vs. the applied field strength H of the NPs were measured with a vibrating sample magnetometer, LakeShore 7407 (Westerville, OH, USA) (VSM), at 300 K.The strength of the applied magnetic field varied from −16 to 16 kOe.The samples were dried at room temperature and demagnetized before the measurements.The mean values of saturation magnetization M s and coercive force H c were deduced from 3 different measurements.

Results and Discussion
In this study, we have synthesized NPs by the co-precipitation of Fe 3+ and Co 2+ ions in an alkaline medium.Under a magnetic field of 0.4 T, the elongated rod-like NPs were obtained (Figure 1A).According to TEM data, their mean length <L> and diameter <d rod > are equal to 25 and 3.4 nm, respectively (Figure 2A,B), which corresponds to a rather high aspect ratio of ca. 7. The nanorods co-exist with some spheres indicated by arrows in Figure 1A.The diameter of spheres is ca.4.7 nm, which is close to that of the nanorods (Figure 2B,C).Previously, the co-existence of spherical NPs with nanorods, grown in the magnetic field, was reported for systems containing magnetite Fe 3 O 4 [30,32].
kOe.The samples were dried at room temperature and demagnetized before the meas urements.The mean values of saturation magnetization Ms and coercive force Hc wer deduced from 3 different measurements.

Results and Discussion
In this study, we have synthesized NPs by the co-precipitation of Fe 3+ and Co 2+ ion in an alkaline medium.Under a magnetic field of 0.4 T, the elongated rod-like NPs wer obtained (Figure 1A).According to TEM data, their mean length <L> and diameter <drod are equal to 25 and 3.4 nm, respectively (Figure 2A,B), which corresponds to a rather high aspect ratio of ca. 7. The nanorods co-exist with some spheres indicated by arrows in Fig ure 1A.The diameter of spheres is ca.4.7 nm, which is close to that of the nanorods (Figur 2B,C).Previously, the co-existence of spherical NPs with nanorods, grown in the magneti field, was reported for systems containing magnetite Fe3O4 [30,32].kOe.The samples were dried at room temperature and demagnetized before the measurements.The mean values of saturation magnetization Ms and coercive force Hc were deduced from 3 different measurements.

Results and Discussion
In this study, we have synthesized NPs by the co-precipitation of Fe 3+ and Co 2+ ions in an alkaline medium.Under a magnetic field of 0.4 T, the elongated rod-like NPs were obtained (Figure 1A).According to TEM data, their mean length <L> and diameter <drod> are equal to 25 and 3.4 nm, respectively (Figure 2A,B), which corresponds to a rather high aspect ratio of ca. 7. The nanorods co-exist with some spheres indicated by arrows in Fig- ure 1A.The diameter of spheres is ca.4.7 nm, which is close to that of the nanorods (Figure 2B,C).Previously, the co-existence of spherical NPs with nanorods, grown in the magnetic field, was reported for systems containing magnetite Fe3O4 [30,32].The presence of a magnetic field is a principal factor in obtaining the nanorods.Indeed, under the same conditions but in the absence of a magnetic field, there are only isotropic spherical NPs (Figure 1B) with mean diameters of <dsph> = 5.6 nm (Figure 2D).This is consistent with the literature data, where the size of the prepared spherical NPs in the sub-10 nm range by the co-precipitation of Fe 3+ and Co 2+ was reported to be dependent on the reaction conditions, e.g., temperature [43,48].
The crystal structure of the synthesized NPs was investigated by TEM.It was shown that the nanorods obtained under a magnetic field are single crystalline (Figure 3A), and the {220} crystallographic planes of cobalt ferrite [49] with a characteristic plane spacing of 2.9 Å can be identified at the micrograph.Isotropic NPs co-existing with nanorods are also single crystals, and {311} planes with spacings of 2.5 Å [50] are seen (Figure 3B).The energy dispersive X-ray (EDX) spectrum shows that the ratio of Fe to Co atoms in the synthesized NPs is close to 2 (Figure 3C), which corresponds to that in the CoFe2O4 phase (C and Cu peaks at the EDX spectrum arise due to the substrate used in TEM measurements).Thus, HR TEM results show that the co-precipitation of Fe 3+ and Co 2+ ions in the magnetic field results in obtaining cobalt ferrite single nanocrystals.Note that NPs synthesized in the absence of a magnetic field are also single crystalline (Figure 3D).The presence of a magnetic field is a principal factor in obtaining the nanorods.Indeed, under the same conditions but in the absence of a magnetic field, there are only isotropic spherical NPs (Figure 1B) with mean diameters of <d sph > = 5.6 nm (Figure 2D).This is consistent with the literature data, where the size of the prepared spherical NPs in the sub-10 nm range by the co-precipitation of Fe 3+ and Co 2+ was reported to be dependent on the reaction conditions, e.g., temperature [43,48].
The crystal structure of the synthesized NPs was investigated by TEM.It was shown that the nanorods obtained under a magnetic field are single crystalline (Figure 3A), and the {220} crystallographic planes of cobalt ferrite [49] with a characteristic plane spacing of 2.9 Å can be identified at the micrograph.Isotropic NPs co-existing with nanorods are also single crystals, and {311} planes with spacings of 2.5 Å [50] are seen (Figure 3B).The energy dispersive X-ray (EDX) spectrum shows that the ratio of Fe to Co atoms in the synthesized NPs is close to 2 (Figure 3C), which corresponds to that in the CoFe 2 O 4 phase (C and Cu peaks at the EDX spectrum arise due to the substrate used in TEM measurements).Thus, HR TEM results show that the co-precipitation of Fe 3+ and Co 2+ ions in the magnetic field results in obtaining cobalt ferrite single nanocrystals.Note that NPs synthesized in the absence of a magnetic field are also single crystalline (Figure 3D).The presence of a magnetic field is a principal factor in obtaining the nanorods.Indeed, under the same conditions but in the absence of a magnetic field, there are only isotropic spherical NPs (Figure 1B) with mean diameters of <dsph> = 5.6 nm (Figure 2D).This is consistent with the literature data, where the size of the prepared spherical NPs in the sub-10 nm range by the co-precipitation of Fe 3+ and Co 2+ was reported to be dependent on the reaction conditions, e.g., temperature [43,48].
The crystal structure of the synthesized NPs was investigated by TEM.It was shown that the nanorods obtained under a magnetic field are single crystalline (Figure 3A), and the {220} crystallographic planes of cobalt ferrite [49] with a characteristic plane spacing of 2.9 Å can be identified at the micrograph.Isotropic NPs co-existing with nanorods are also single crystals, and {311} planes with spacings of 2.5 Å [50] are seen (Figure 3B).The energy dispersive X-ray (EDX) spectrum shows that the ratio of Fe to Co atoms in the synthesized NPs is close to 2 (Figure 3C), which corresponds to that in the CoFe2O4 phase (C and Cu peaks at the EDX spectrum arise due to the substrate used in TEM measurements).Thus, HR TEM results show that the co-precipitation of Fe 3+ and Co 2+ ions in the magnetic field results in obtaining cobalt ferrite single nanocrystals.Note that NPs synthesized in the absence of a magnetic field are also single crystalline (Figure 3D).In order to identify the phase composition of the synthesized NPs, XRD (Figure 4) and Raman spectroscopy (Figure 5) were employed.The XRD pattern of the NPs synthesized in the magnetic field (Figure 4b) is consistent with a powder diffraction pattern of a Co3-xFexO4 spinel structure (Figure 4c), which was calculated using Profex 5.2.7 software [51] based on a crystal structure from Ref. [52] with a lattice constant of a = 8.36 Å.This lattice constant confirms the composition with x ≈ 2 (for pure CoFe2O4, a = 8.381 Å, for pure Co2FeO4, a = 8.242 Å) [52], proving the formation of the CoFe2O4 inverse spinel.Isotropic particles prepared without a magnetic field produce a much weaker pattern with broader lines (Figure 4a), which is due to the small size of NPs.However, the most intensive (311) spinel reflection at 2θ ≈ 35.5° is also detected.A slight shift of the diffraction peak to lower angles is observed for the spheres (synthesized without magnetic field) compared to the rods (obtained in magnetic field).This may be due to a change in composition [52] or to structural strains in the samples [53].We believe that the shifts of XRD peaks are due to the minor changes in the elemental composition of the crystals, which is confirmed by XPS data (presented below) showing that cobalt ferrite spheres have a slightly higher content of iron than nanorods (note that the EDX data in Figure 3 show the same trend-the atomic ratio Fe:Co is 2.06 for rods and 2.31 for spheres).A higher amount of iron increases the lattice constant, so XRD peaks shift to lower 2θ values.In order to identify the phase composition of the synthesized NPs, XRD (Figure 4) and Raman spectroscopy (Figure 5) were employed.The XRD pattern of the NPs synthesized in the magnetic field (Figure 4b) is consistent with a powder diffraction pattern of a Co 3−x Fe x O 4 spinel structure (Figure 4c), which was calculated using Profex 5.2.7 software [51] based on a crystal structure from Ref. [52] with a lattice constant of a = 8.36 Å.This lattice constant confirms the composition with x ≈ 2 (for pure CoFe 2 O 4 , a = 8.381 Å, for pure Co 2 FeO 4 , a = 8.242 Å) [52], proving the formation of the CoFe 2 O 4 inverse spinel.Isotropic particles prepared without a magnetic field produce a much weaker pattern with broader lines (Figure 4a), which is due to the small size of NPs.However, the most intensive (311) spinel reflection at 2θ ≈ 35.5 • is also detected.A slight shift of the diffraction peak to lower angles is observed for the spheres (synthesized without magnetic field) compared to the rods (obtained in magnetic field).This may be due to a change in composition [52] or to structural strains in the samples [53].We believe that the shifts of XRD peaks are due to the minor changes in the elemental composition of the crystals, which is confirmed by XPS data (presented below) showing that cobalt ferrite spheres have a slightly higher content of iron than nanorods (note that the EDX data in Figure 3 show the same trend-the atomic ratio Fe:Co is 2.06 for rods and 2.31 for spheres).A higher amount of iron increases the lattice constant, so XRD peaks shift to lower 2θ values.The Raman spectrum of rod-like NPs (Figure 5a) is in perfect agreement with that reported for CoFe2O4 nanoparticles in the literature [54] (Figure 5b).The peaks at 643 and 690 cm −1 correspond to the highest frequency A1g mode split into two due to cation inversion [55,56].These modes involve the symmetric stretching of an oxygen atom with respect to a metal ion in a tetrahedral void.The modes below 600 cm −1 involve symmetrical or asymmetrical bending of metal-oxygen bonding in octahedral sites.They have T2g (567, 481, and 181 cm −1 ) or Eg (306 cm −1 ) symmetries.The spectrum of NPs synthesized in the absence of a magnetic field has broader lines, which is due to the small NP size, but the main bands are visible and coincide with the CoFe2O4 structure.Therefore, XRD and Raman spectroscopy confirm that the NPs synthesized both in the presence and absence of a magnetic field have a spinel CoFe2O4 structure.
The high-resolution XPS spectra of NPs are presented in Figure 6.Overlapping with Co LMM and Fe LMM Auger lines complicates the analysis of the Fe 2p and, especially Co2p spectra.Both the Fe2p (Figure 6A) and Co2p (Figure 6B) spectra are virtually the same for the rods and spheres.The Fe2p spectra (Figure 6A) show a doublet of the Fe2p3/2 and Fe2p1/2 lines at binding energies of 711.1 and 724.6 eV with a series of shake-up satellites separated for about 8 and 18 eV from the main lines.Both the position and the satellite structure of the Fe2p spectra are typical for Fe 3+ species in iron oxides [47,57].The synthetic component proposed for Fe 3+ species in [58] fits the Fe2p spectra of NPs.The Co 2p XPS spectra of the samples demonstrate well-pronounced shake-up satellites shifted for about  The Raman spectrum of rod-like NPs (Figure 5a) is in perfect agreement with tha reported for CoFe2O4 nanoparticles in the literature [54] (Figure 5b).The peaks at 643 and 690 cm −1 correspond to the highest frequency A1g mode split into two due to cation inver sion [55,56].These modes involve the symmetric stretching of an oxygen atom with re spect to a metal ion in a tetrahedral void.The modes below 600 cm −1 involve symmetrica or asymmetrical bending of metal-oxygen bonding in octahedral sites.They have T2g (567, 481, and 181 cm −1 ) or Eg (306 cm −1 ) symmetries.The spectrum of NPs synthesized in the absence of a magnetic field has broader lines, which is due to the small NP size, bu the main bands are visible and coincide with the CoFe2O4 structure.Therefore, XRD and Raman spectroscopy confirm that the NPs synthesized both in the presence and absenc of a magnetic field have a spinel CoFe2O4 structure.
The high-resolution XPS spectra of NPs are presented in Figure 6.Overlapping with Co LMM and Fe LMM Auger lines complicates the analysis of the Fe 2p and, especially Co2p spectra.Both the Fe2p (Figure 6A) and Co2p (Figure 6B) spectra are virtually th same for the rods and spheres.The Fe2p spectra (Figure 6A) show a doublet of the Fe2p3 and Fe2p1/2 lines at binding energies of 711.1 and 724.6 eV with a series of shake-up satel lites separated for about 8 and 18 eV from the main lines.Both the position and the satellit structure of the Fe2p spectra are typical for Fe 3+ species in iron oxides [47,57].The syntheti component proposed for Fe 3+ species in [58] fits the Fe2p spectra of NPs.The Co 2p XP spectra of the samples demonstrate well-pronounced shake-up satellites shifted for abou The Raman spectrum of rod-like NPs (Figure 5a) is in perfect agreement with that reported for CoFe 2 O 4 nanoparticles in the literature [54] (Figure 5b).The peaks at 643 and 690 cm −1 correspond to the highest frequency A1g mode split into two due to cation inversion [55,56].These modes involve the symmetric stretching of an oxygen atom with respect to a metal ion in a tetrahedral void.The modes below 600 cm −1 involve symmetrical or asymmetrical bending of metal-oxygen bonding in octahedral sites.They have T2g (567, 481, and 181 cm −1 ) or Eg (306 cm −1 ) symmetries.The spectrum of NPs synthesized in the absence of a magnetic field has broader lines, which is due to the small NP size, but the main bands are visible and coincide with the CoFe 2 O 4 structure.Therefore, XRD and Raman spectroscopy confirm that the NPs synthesized both in the presence and absence of a magnetic field have a spinel CoFe 2 O 4 structure.
The high-resolution XPS spectra of NPs are presented in Figure 6.Overlapping with Co LMM and Fe LMM Auger lines complicates the analysis of the Fe 2p and, especially Co2p spectra.Both the Fe2p (Figure 6A) and Co2p (Figure 6B) spectra are virtually the same for the rods and spheres.The Fe2p spectra (Figure 6A) show a doublet of the Fe2p 3/2 and Fe2p 1/2 lines at binding energies of 711.1 and 724.6 eV with a series of shake-up satellites separated for about 8 and 18 eV from the main lines.Both the position and the satellite structure of the Fe2p spectra are typical for Fe 3+ species in iron oxides [47,57].The synthetic component proposed for Fe 3+ species in [58] fits the Fe2p spectra of NPs.The Co 2p XPS spectra of the samples demonstrate well-pronounced shake-up satellites shifted for about 6.5 eV to higher binding energies from the main Co2p 3/2 and Co2p 1/2 lines located at 780.5 and 796.3 eV.These spectra are typical for Co 2+ species in cobalt oxides [59].The O1s spectra (Figure 6C), along with the lattice oxygen peak at 530.1 eV, contain two additional peaks.The peak at 531.5 eV can be attributed to surface hydroxide and carbonate species, while the peak at 533.5 eV can be assigned to single O−C bonds in adventitious carbon on the surface.Such oxygen spectra are typical for the ex situ prepared metal oxide samples.Summarizing, the XPS data confirm that both samples mainly contain Fe 3+ and Co 2+ species coordinated with lattice oxygen.
Nanomaterials 2024, 14, x FOR PEER REVIEW 8 of 14 and 796.3 eV.These spectra are typical for Co 2+ species in cobalt oxides [59].The O1s spectra (Figure 6C), along with the lattice oxygen peak at 530.1 eV, contain two additional peaks.The peak at 531.5 eV can be attributed to surface hydroxide and carbonate species, while the peak at 533.5 eV can be assigned to single O−C bonds in adventitious carbon on the surface.Such oxygen spectra are typical for the ex situ prepared metal oxide samples.Summarizing, the XPS data confirm that both samples mainly contain Fe 3+ and Co 2+ species coordinated with lattice oxygen.These results suggest a mechanism for the anisotropic growth of rod-like cobalt ferrite NPs in the magnetic field (Figure 7), which is similar to the mechanism proposed earlier for the magnetic-field-assisted synthesis of rod-like magnetite [32].At the first stage of the co-precipitation reaction, small isotropic "seed" NPs are formed.At this stage, most of the OH -ions of alkali are consumed for the formation of cobalt ferrite according to the following reaction: This induces a drop in pH from an initial value of ~14 to ~6.3, which was measured for the syntheses carried out in the present work.CoFe2O4 has an isoelectric point of ca.7 [60]; therefore, at pH 6.3, the "seed" NPs are only slightly positively charged.Thus, electrostatic repulsion between the "seeds" is not strong enough to prevent self-assembly into columnar structures due to magnetization obtained in the external magnetic field [61].At the latter reaction stages, the "seeds", which are assembled together, fuse into a singlecrystalline rod.The growth of rods by aggregation and the re-crystallization of primary particles in the magnetic field was previously described for micrometer-sized rod-like magnetite particles synthesized by co-precipitation [28].This resembles the general features of the nanoparticles growth [62].In the absence of a magnetic field, the "seeds" do not fuse into rods but into some larger isotropic NPs (Figure 7).These results suggest a mechanism for the anisotropic growth of rod-like cobalt ferrite NPs in the magnetic field (Figure 7), which is similar to the mechanism proposed earlier for the magnetic-field-assisted synthesis of rod-like magnetite [32].At the first stage of the co-precipitation reaction, small isotropic "seed" NPs are formed.At this stage, most of the OH − ions of alkali are consumed for the formation of cobalt ferrite according to the following reaction: Nanomaterials 2024, 14, x FOR PEER REVIEW 9 of 14 The magnetic properties of synthesized cobalt ferrite NPs were studied with magnetometry.In Figure 8a, the hysteresis curves, obtained from the VSM measurements at room temperature (300 K), are depicted.The values of saturation magnetization Ms and coercivity Hc determined from the curves (Figure 8b) are presented in Table 1.In Figure 8a, one can see that the size of the hysteresis loop is much larger for nanorods.As a consequence, the sample containing nanorods demonstrates significantly higher values of Ms and Hc (Figure 8b), which indicates that nanorods possess stronger magnetic properties than isotropic NPs.The rise of saturation magnetization and coercivity with the size of monocrystalline CoFe2O4 NPs was previously described in the literature [63].It was explained by increasing the size of magnetic domains where the atomic spins are aligned This induces a drop in pH from an initial value of ~14 to ~6.3, which was measured for the syntheses carried out in the present work.CoFe 2 O 4 has an isoelectric point of ca.7 [60]; therefore, at pH 6.3, the "seed" NPs are only slightly positively charged.Thus, electrostatic repulsion between the "seeds" is not strong enough to prevent self-assembly into columnar structures due to magnetization obtained in the external magnetic field [61].At the latter reaction stages, the "seeds", which are assembled together, fuse into a single-crystalline rod.The growth of rods by aggregation and the re-crystallization of primary particles in the magnetic field was previously described for micrometer-sized rod-like magnetite particles synthesized by co-precipitation [28].This resembles the general features of the nanoparticles' growth [62].In the absence of a magnetic field, the "seeds" do not fuse into rods but into some larger isotropic NPs (Figure 7).
The magnetic properties of synthesized cobalt ferrite NPs were studied with magnetometry.In Figure 8a, the hysteresis curves, obtained from the VSM measurements at room temperature (300 K), are depicted.The values of saturation magnetization M s and coercivity H c determined from the curves (Figure 8b) are presented in Table 1.In Figure 8a, one can see that the size of the hysteresis loop is much larger for nanorods.As a consequence, the sample containing nanorods demonstrates significantly higher values of M s and H c (Figure 8b), which indicates that nanorods possess stronger magnetic properties than isotropic NPs.The rise of saturation magnetization and coercivity with the size of monocrystalline CoFe 2 O 4 NPs was previously described in the literature [63].It was explained by increasing the size of magnetic domains where the atomic spins are aligned along the direction of the applied magnetic field.As a result, the maximum magnetization of the particles and the strength of the reverse external field required to demagnetize them increased [63].
Obtained nanoparticles are single crystals (Figure 3A), and they have slightly lower but comparable magnetic properties compared to larger polycrystalline particles [35] and bulk CoFe 2 O 4 [64].An increase in the nanocrystal size induces a rise of saturation magnetization, reaching 80 emu/g for 50 nm NPs and not increasing further up to the bulk.The coercive force shows a non-linear dependence on the NP size.A maximum of 1.2 kOe is reached for 25 nm NPs, and it is lower for both smaller (as in this work, where it equals 0.6 kOe) and larger NPs.For the bulk, the coercive force is only 0.06 kOe.The magnetic properties of synthesized cobalt ferrite NPs were studied with magnetometry.In Figure 8a, the hysteresis curves, obtained from the VSM measurements at room temperature (300 K), are depicted.The values of saturation magnetization Ms and coercivity Hc determined from the curves (Figure 8b) are presented in Table 1.In Figure 8a, one can see that the size of the hysteresis loop is much larger for nanorods.As a consequence, the sample containing nanorods demonstrates significantly higher values of Ms and Hc (Figure 8b), which indicates that nanorods possess stronger magnetic properties than isotropic NPs.The rise of saturation magnetization and coercivity with the size of monocrystalline CoFe2O4 NPs was previously described in the literature [63].It was explained by increasing the size of magnetic domains where the atomic spins are aligned along the direction of the applied magnetic field.As a result, the maximum magnetization of the particles and the strength of the reverse external field required to demagnetize them increased [63].Obtained nanoparticles are single crystals (Figure 3A), and they have slightly lower but comparable magnetic properties compared to larger polycrystalline particles [35] and bulk CoFe2O4 [64].An increase in the nanocrystal size induces a rise of saturation magnetization, reaching 80 emu/g for 50 nm NPs and not increasing further up to the bulk.The coercive force shows a non-linear dependence on the NP size.A maximum of 1.2 kOe is reached for 25 nm NPs, and it is lower for both smaller (as in this work, where it equals 0.6 kOe) and larger NPs.For the bulk, the coercive force is only 0.06 kOe.
The solution of synthesized nanorods was aged at 80 °C to increase the magnetic properties [65].The hysteresis loop of the nanorods after aging for 1 h demonstrates a 3 times higher value of saturation magnetization Ms, than nanorods before aging (Figure 8a, Table 1).The increase in Ms upon aging can be attributed to the increased size of the crystallite, as was suggested by F. Huixia and co-authors [65] for aged isotropic CoFe2O4 NPs.Unlike Ms, the value of coercivity Hc of the nanorods before and after aging remains almost the same (Table 1).Probably, the temperature of aging is not high enough to additionally adjust the aligned atomic spins inside the rod-like particles.Longer aging up to 250 h (Figure 9) does not affect the Ms and Hc values (see Supplementary Information for the corresponding hysteresis loops).Therefore, 1 h of aging is enough to obtain anisotropic CoFe2O4 NPs with maximum saturation magnetization Ms and coercivity Hc, which are equal to 30 ± 1 emu/g and 330 ± 15 Oe, respectively.The solution of synthesized nanorods was aged at 80 • C to increase the magnetic properties [65].The hysteresis loop of the nanorods after aging for 1 h demonstrates a 3 times higher value of saturation magnetization M s , than nanorods before aging (Figure 8a, Table 1).The increase in M s upon aging can be attributed to the increased size of the crystallite, as was suggested by F. Huixia and co-authors [65] for aged isotropic CoFe 2 O 4 NPs.Unlike M s , the value of coercivity H c of the nanorods before and after aging remains almost the same (Table 1).Probably, the temperature of aging is not high enough to additionally adjust the aligned atomic spins inside the rod-like particles.Longer aging up to 250 h (Figure 9) does not affect the M s and H c values (see Supplementary Information for the corresponding hysteresis loops).Therefore, 1 h of aging is enough to obtain anisotropic CoFe 2 O 4 NPs with maximum saturation magnetization M s and coercivity H c , which are equal to 30 ± 1 emu/g and 330 ± 15 Oe, respectively.

Conclusions
In this paper, it is shown, for the first time, that a static magnetic field of mode strength can be used as a template for the anisotropic growth of cobalt ferrite NPs, wh is one of the most attractive nanomaterials in many applications.Nanorods with an asp

Figure 1 .
Figure 1.TEM micrographs of NPs synthesized by the co-precipitation of Fe 3+ and Co 2+ ions at 70 °C under a magnetic field of 0.4 T (A) and in the absence of a magnetic field (B).Arrows point ou several spheres co-existing with nanorods.

Figure 1 .
Figure 1.TEM micrographs of NPs synthesized by the co-precipitation of Fe 3+ and Co 2+ ions at 70 • C under a magnetic field of 0.4 T (A) and in the absence of a magnetic field (B).Arrows point out several spheres co-existing with nanorods.

Figure 1 .
Figure 1.TEM micrographs of NPs synthesized by the co-precipitation of Fe 3+ and Co 2+ ions at 70 °C under a magnetic field of 0.4 T (A) and in the absence of a magnetic field (B).Arrows point out several spheres co-existing with nanorods.

Figure 2 .
Figure 2. Size distribution histograms of NPs obtained from TEM micrographs: length (A) and diameter (B) of nanorods synthesized under magnetic field; diameter of spheres co-existing with nanorods (C); diameter of NPs synthesized in the absence of magnetic field (D).The numbers indicated in the diagrams are relative to the total number of particles analyzed (which was between 100 and 200).

Figure 2 .
Figure 2. Size distribution histograms of NPs obtained from TEM micrographs: length (A) and diameter (B) of nanorods synthesized under magnetic field; diameter of spheres co-existing with nanorods (C); diameter of NPs synthesized in the absence of magnetic field (D).The numbers indicated in the diagrams are relative to the total number of particles analyzed (which was between 100 and 200).

Figure 2 .
Figure 2. Size distribution histograms of NPs obtained from TEM micrographs: length (A) and diameter (B) of nanorods synthesized under magnetic field; diameter of spheres co-existing with nanorods (C); diameter of NPs synthesized in the absence of magnetic field (D).The numbers indicated in the diagrams are relative to the total number of particles analyzed (which was between 100 and 200).

Figure 3 .
Figure 3. (A,B) HR TEM micrographs of NPs synthesized under a magnetic field of 0.4 T: (A)nanorod; (B)-sphere; (C) energy-dispersive X-ray (EDX) spectrum for the sample synthesized under a magnetic field; (D) HR TEM micrograph of an NP synthesized in the absence of a magnetic field; (E) energy-dispersive X-ray (EDX) spectrum for the sample synthesized in the absence of a magnetic field.In the HR TEM pictures, crystallographic planes and plane spacing distances of cobalt ferrite are identified.

Figure 3 .
Figure 3. (A,B) HR TEM micrographs of NPs synthesized under a magnetic field of 0.4 T: (A)-nanorod; (B)-sphere; (C) energy-dispersive X-ray (EDX) spectrum for the sample synthesized under a magnetic field; (D) HR TEM micrograph of an NP synthesized in the absence of a magnetic field; (E) energy-dispersive X-ray (EDX) spectrum for the sample synthesized in the absence of a magnetic field.In the HR TEM pictures, crystallographic planes and plane spacing distances of cobalt ferrite are identified.

Figure 4 .
Figure 4. XRD patterns of CoFe2O4 NPs synthesized in the absence (a) and presence (b) of a magnetic field and calculated for Co3-xFexO4 powder (c).

Figure 5 .
Figure 5. Raman spectra for cobalt ferrite NPs synthesized in the presence (a) and absence (c) of a magnetic field; Raman spectrum for CoFe2O4 nanoparticles [54] (b); Reproduced with permission from [54], ACS Publications, 2009.

Figure 4 . 1 Figure 4 .
Figure 4. XRD patterns of CoFe 2 O 4 NPs synthesized in the absence (a) and presence (b) of a magnetic field and calculated for Co 3−x Fe x O 4 powder (c).

Figure 7 .
Figure 7. Schematic representation of the growth mechanism of cobalt ferrite nanorods in the presence and absence of a magnetic field.

Figure 7 .
Figure 7. Schematic representation of the growth mechanism of cobalt ferrite nanorods in the presence and absence of a magnetic field.

Figure 7 .
Figure 7. Schematic representation of the growth mechanism of cobalt ferrite nanorods in the presence and absence of a magnetic field.

Figure 8 .
Figure 8. Magnetic hysteresis loops of isotropic cobalt ferrite NPs (violet) and anisotropic cobalt ferrite nanorods before (red) and after (black) aging for 1 h at 80 • C (a); the enlargement of the loops at low strength of the applied magnetic field (b).

Figure 9 .
Figure 9. Dependences of saturation magnetization Ms and coercivity Hc of cobalt ferrite nanor on aging time at 80 °C.

Figure 9 .
Figure 9. Dependences of saturation magnetization M s and coercivity H c of cobalt ferrite nanorods on aging time at 80 • C.

Table 1 .
Values of saturation magnetization M s and coercivity H c anisotropic cobalt ferrite nanorods before and after 1 h of aging at 80 • C in comparison with isotropic cobalt ferrite NPs.

Table 1 .
Values of saturation magnetization Ms and coercivity Hc anisotropic cobalt ferrite nanorods before and after 1 h of aging at 80 °C in comparison with isotropic cobalt ferrite NPs.