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Magnetic Characterization of Co and Zn Nano Ferrites: Property Consideration for Oil-Spill Clean Up

Nano-magnetic particles are significantly finding applications in environmental remediation. This work studied the magnetic properties of Co and Zn nano-ferrites, CoFe2O4 and ZnFe2O4 respectively, with considerations for the properties that are desirable for oil spill cleanup. The ferrite nanoparticles were synthesized by the glycol-thermal method at 200 oC for 6 hours using a stirred pressure reactor. Single phase spinel crystal structures were obtained for both samples as determined by an X-ray diffraction (XRD) machine. The high-resolution transmission electron microscope and surface electron microscope images showed nano structures. The magnetic properties and magnetic hyperfine parameters were determined using a lakeshore vibrating sample magnetometer and 57 Fe Mössbauer spectroscopy. The XRD peaks were consistent with that of a single phase spinel structure. No impurity phase was detected. The nano-ferrites have crystallite sizes of about 10 nm and 17 nm for CoFe2O4 and ZnFe2O4. The nano almost-spherical nature of the samples was also confirmed from their morphology studies. The saturation magnetization of CoFe2O4 and ZnFe2O4 were about 50 emu/g and 30 emu/g respectively. They both exhibited superparamagnetic properties with a component of para-magnetism observed in ZnFe2O4 nanoferrites. CoFe2O4 nano-ferrites exhibited better desired property for oil spill cleanup. The ZnFe2O4 nano-ferrites showed tendencies of slow response to magnetic field, however, it is more Original Research Article Musa et al.; AJR2P, 5(3): 14-25, 2021; Article no.AJR2P.81418 15 environmentally friendly. The magnetic hyperfine parameters and the isomer shift values for CoFe2O4 and ZnFe2O4 showed strong internal magnetic fields and the obtained values are consistent with spinel structure. The oxidation state of Fe was observed to be Fe 3+ . Due to the low coercivity and high magnetization values, these particles could be considered as potential candidates for oil spill cleanup. These desirable magnetic properties will improve the recyclability of the nanoparticles using low applied magnetic fields.


INTRODUCTION
The emergence of nanotechnology has revolutionized innovations in almost all fields of endeavors such as in agriculture, pharmaceuticals, medicals, electronics, energy, environmental management, the oil and gas industry, aerospace and automobile [1]. It is an interdisciplinary technology that involves the application of materials with a size range of about 1-100 nm, known as nanomaterials [2]. The unique properties derived from nanomaterials found interesting applications compared to their bulk counterpart. This is primarily due to the nano-size effect which gives nanomaterials a high surface to volume ratio, increased number of surface atoms and enhanced reactivity [3]. Magnetic, optical, and electrical properties at the nano-scale behave very differently from the bulk samples. The understanding of these properties has enabled the deployment of nanotechnology in the oil and gas industries in areas such as exploration, drilling, logging, hydraulic fracturing, formation damage, enhanced oil recovery (EOR) and exsitu upgrading of oil shales/bitumen [2][3][4][5]. A more recent possible application of nanomaterials for environmental remediation is found in the oil and gas sector, which involves the use of amine functionalized or polymer coated magnetic nanoparticles for cleaning crude oil spills [3]. Therefore, there are enormous potentials in understanding the behavior of materials at nanoscale so that the properties can be tailored towards specific application.
The cubic spinel ferrites are one of such materials that are suitable for environmental remediation application. Ferrites belong to a special class of ceramic oxides which generally has a chemical formula of AB 2 O 4 . A represents one or more of the divalent transition metals, for example zinc (Zn), iron (Fe), magnesium (Mg), nickel (Ni), cobalt (Co), copper (Cu), or calcium (Ca) [6], and B represents solely iron (Fe). Generally, a cubic spinel structure consists of two interstitial sites known as tetrahedral A and octahedral B ions which contains 8 tetrahedral coordinated sites and 16 octahedral coordinated sites for divalent & trivalent metal cation occupancy, where A and B denote divalent and trivalent cations respectively [7][8]. For [10].
The current project synthesized single phase nanoparticles of CoFe 2 O 4 and ZnFe 2 O 4 ferrites using the glycol-thermal synthesis method. The selected elements such as Fe and Zn are environmentally friendly. However, Zn has low anisotropy compared to Co. Therefore, Co is selected because of its high anisotropy so as to improve the magnetization of the sample and help in making comparison. The S M can be empirically calculated from the law of approach to saturation magnetization given by [11]: The coercivity gives a measure of the anisotropy of a magnetic material [12]. This can be estimated by:

High saturation magnetization
where 1 C H and 2 C H are the negative and positive coercivity respectively along the magnetic field direction [13]. Soft superparamagnetic materials are expected to have insignificant or almost zero coercivity thereby quenching the propensity for high degree of agglomeration of the nanoparticles and the demand for high magnetic field. This implies that it will be easier to recycle the magnetic nanoparticles after every cycle of magnetization for reuse in the oil spoil cleaning process.
Micro-structural properties such as crystallite size D and lattice parameters a could significantly contribute to changes in structural and magnetic properties of materials [14][15][16]. The Scherrer's formula; It is often used Equation (3) to estimate D , where K is a constant associated with the shape factor with a value of about 0.94 for a spherical shape, λ is the X-ray diffraction wavelength, W is the full width at half maximum,  is the Bragg's angle and hkl are the Miller indices of a selected intensity peak [17].
Also, from Bragg's law of diffraction, one could establish that the lattice parameters a for a cubic crystal structure has a relationship with hkl as; (4)

MATERIALS AND METHODS
The materials and methodology are presented for the research work.

Glycol-Thermal Methods
The glycol-thermal method is a wet chemical method of synthesizing nanoparticles using a stoichiometric amount of the starting chemical materials in their metal chlorides or nitrates forms. These precursors are usually dissolved in de-ionized water. To precipitate the nitrates or the metal chloride from its solution, sodium hydroxide or ammonia potassium hydroxide is used respectively. The process is kept under continuous stirring condition while the pH value gradually increases until a desired value is reached. After about 40 minutes of continuous stirring, the solution is washed over a thin-pore filter using de-ionized water to remove the chlorides or nitrates. The resulting compound is put inside ethylene glycol and transferred to a pressure reactor for certain duration. A PARR 4843 stirred pressure reactor was used in the current work. The stirring conditions in the reaction chamber such as temperature, speed and pressure are recorded accordingly. about 20 minutes to achieve homogeneity of the mixture. Ammonium solution was carefully used to raise the pH to about 9, causing a precipitation of the metal chlorides. It was maintained under this condition for about 30 minutes. The chloride contained in the precipitate was washed off over a glass microfibre filter in a Büchner funnel using de-ionized water. Standard solution of silver nitrate was used to confirm the absence of chlorides in the filtrate by ensuring that the precipitate of silver chloride was not visible. The recovered precipitate was put in about 250 ml of ethylene glycol and reacted using a pressure reactor. The reaction occurred at a soak temperature of 200 ºC that was retained for 6 hours. The reactor was operated at a speed and pressure of about 300 rpm and 80 psi respectively. Similar process of washing the sample was followed after the reaction was completed. The recovered sample was dried for about 24 hours under an infra-red light to dry up any moisture, collected and stored in a vial.

X-ray diffraction
X-ray diffraction (XRD) is a technique used to investigate the structural characteristics of crystalline or amorphous materials. It can be applied to identify phases of crystalline structures in a sample. It provides detailed Information about the lattice fringes of crystalline samples often known as d spacing. Microstructural information such as crystal orientation, internal stress, crystallite size and shape can be derived from XRD data [18]. The principle of X-ray diffraction is such that when X-rays leave a crystal it causes the incident beam to interfere with each other. However, diffraction only occurs if Bragg's law is satisfied which is mathematically expressed as; where θ is the angle between the incident beam and the scattering plane, d is the interplaner distance, λ is the wavelength of the incident beam and n is an integer,. The lattice constant a of a cubic crystal has a relationship with the of a sample by the following equation; Therefore, substituting Bragg's law into the Equation (6) gives Equation (4). It is from the XRD high intensity peak (311) that the crystallite size of the nanoparticles is estimated using Equation (3).

High resolution transmission electron microscopy and scanning electron microscopy
The morphology and the physical microstructure of a sample can be studied using high resolution transmission electron microscopy (HRTEM). It is a technique used to study the images of a sample; carry out sample size investigation, shape and crystallographic structures in the micro or nano-nanometer range. The images are generated from electron beams interactions that are transmitted through the sample. The samples are prepared in a thin form for better results. The objective lens magnifies and focuses the images which then appear on an imaging screen which has charge-couple sensor that detects. A Joel-JEM-2100 HRTEM was used to investigate the samples studied in this. A very little sample quantity of about 0.0001 g was dispersed in ethanol and ultrasonically vibrated at 80Hz\40Watt for 3 minutes. A thin film of copper sample holder whose diameter is about 0.0005 mm was put into prepared sample and loaded in the HRTEM equipment for measuements.
Surface morphology of samples is better studied using high resolution scanning electron microscopy (HRSEM). The machine operates with similar principle as that of the HRTEM except that the electron beam is focused on the sample's surface. The strikes of the electron beam on the surface generate signals required for information about the elemental composition of the sample, also referred to as energy dispersive X-ray. A Zeiss ultra plus HRSEM model was used do sample surface scan in this work. The sample preparation performed by depositing very little quantity one side of a sticky carbon tape attached to a sample holder. The sample was coated with gold using a Polaron SC 500 sputter coater to avoid charging by the electron beam.

Magnetization measurements
A LakeShore vibrating sample magnetometer (VSM) was used to measure the magnetic properties. The sample was measured in a magnetic field of about 14 kOe. The VSM comprise a 735 VSM controller, a vibrating head mounted on a cryostat, a pair of pick-up coils at the faces of the poles of the electromagnet. It also contains a bipolar electromagnet power supply, a Gaussmeter for magnetic field measurements, a temperature controller, a chiller and a data card for data collection. The VSM operates on the Faraday's law of magnetic induction which is due to the induced voltage that is generated from the pair of pick-up coils. The voltage is proportional to the magnetization of the sample. According to Faraday's law; where dt d is the rate of change of magnetic flux and E is the induced voltage. This principle helps to measure the magnetic moment by using the AC signal produced from the vibrating magnetized sample. The degree of the magnetization of the sample bears a direct relationship with the amplitude of the induced signal.

57
Fe Mössbauer setup and measurements Fig. 1 shows a typical block diagram of a 57Fe Mössbauer spectroscopy deployed for the current work. It shows a Mössbauer drive unit (MDU), a CANBERRA amplifier, a pre-amplifier, a power supply, a detector and a multichannel analyzer (MCS). The device contains a 25 mCi 57 Co source that is sealed in a Rh matrix and attached to the transducer. The calibration was done at room temperature using an alpha-iron foil. A thin sample of about 0.2 g was sufficient for the measurement. The γ ray of energy intensity of 14.4 keV from the source interacts with the nucleus of the sample and the transmitted rays through the sample are detected by the detector. The signals are therefore amplified at high voltage (HV) and transmitted through a single channel analyser (SCA). They are further processed for data capture using a multi-channel analyzer (MCA). The data are interpreted on the computer screen as Mössbauer spectra. The collection of the data is carried out for a minimum of about 15 minutes. However, analytical Mössbauer recoil software is used for the data analysis.

RESULTS AND DISCUSSION
This section provides details on results and analysis of the magnetic nano ferrites material based on the structural studies, morphological and magnetization measurement.

X-ray Diffraction
The X-ray powder diffraction patterns for CoFe 2 O 4 and ZnFe 2 O 4 nano-ferrites are shown in Figs. 2 and 3 respectively. The XRD instrument used in the current work was equipped with a diffracted beam monochromator in the receiving optics equipped with a Cobalt anode at wavelengths λ 1 =1.7892 Å, and λ 2 =1.7931 Å. The calculated average wavelength λ was 1.7903 Å. The data were collected in a 2θ-range of 10° to 80° at an angular resolution of 0.02°. The results show that nano-ferrites samples are pure phases without impurities. All the peaks are well indexed with reference to the spinel structure (Joint Committee on Powder Diffraction Standards (JCPDS No. 022-1086) associated with the space group Fd3m [19]. The prominent 311 peak associated with spinel structure can be clearly seen. The broadness of the peaks, especially for the CoFe 2 O 4 nano-ferrites is indicative of the nano-particle nature of the sample. Therefore, the expectation is that the crystallite size of the CoFe 2 O 4 nano-ferrites should be smaller compared to that of ZnFe 2 O 4 nano-ferrites.
The crystal structure analysis were performed using the 311 peak to determine parameters such as the crystallite size D and the lattice parameters a. The D and a were calculated using equations 3 and 4 respectively, and the obtained values are presented in Table 3. As expected from the broadness of ZnFe 2 O 4 nano-ferrites XRD peaks, the crystallite size of the ZnFe 2 O 4 nanoparticles is bigger by about 8 nm compared to CoFe 2 O 4 nano-ferrites. This might not be unconnected to the ionic radii of Co and Zn, which are 125 pm and 137 pm respectively [20]. This implies that Zn with larger ionic radius could lead to expansion of the unit cell of ZnFe 2 O 4 nano-ferrites. The values of the lattice parameters, a, are consistent with reported values for ferrite spinel structure. No significant increase was, however, observed for a in both samples.

Morphological Studies
The shape of the nano-particles and their surface were examined using high resolution transmission electron microscopy and high resolution scanning electron microscopy (HRTEM and HRSEM) respectively. The morphologies of the samples are shown in Figs. 2 and 3.respectively The nano-ferrites sample of CoFe 2 O 4 shows a better crystalline structure with particles of almost spherical shape compared to ZnFe 2 O 4 nano-ferrites. The HRSEM images of both nano-ferrites measured at the same magnification of 100 nm reveal that the particle sizes of the ZnFe 2 O 4 nano-ferrites are bigger than that of CoFe 2 O 4 nano-ferrites. This supports the crystallite size estimation from the XRD data. The surface images also show the fairly spherical nature of the nano-particles.   Fig. 4 (a). The fit is shown in Fig.  4(b). The hysteresis loop show evidence of superparamagnetic behavior associated with smaller particle. The nano-ferrite had coercivity H C of about 0.03 kOe and saturation magnetization M S of about 50 emu/g. These features are desirous for practical application of nano-particles for oil spill cleanup. The implication is that a small magnet will be sufficient to magnetize and demagnetize the sample without significant remnant or residual magnetization in the sample. It also implies that during the recycling process of the nanoparticles, magnet with little magnetic field strength can help perform the cycle of magnetization with swift response to the magnetic field.  Similar analysis was performed on the ZnFe 2 O 4 nano-ferrites particles. The hysteresis loop is presented in Fig. 5(a). The magnetic loop appears to show mixed magnetic phases of super-paramagnetism and paramagnetism. The paramagnetic component can be observed from the steep shape of the hysteresis loop.

Magnetization Measurements
Coercivity, H C , of about 0.20 kOe and saturation magnetization M S of about 30 emu/g were obtained. The low magnetization value when compare to that obtained for CoFe 2 O 4 nanoferrites could be attributed to unpaired electrons in the valence shell of Zn atoms, thereby causing a poor magnetic response of the ZnFe 2 O 4 nanoferrites. Thus could also explain why there is a slight increase in the coercivity because, nonmagnetic atoms within a cluster of magnetic spins could increase coercivity [22]. Although, CoFe 2 O 4 nano-ferrites showed the most desirous properties for application, ZnFe 2 O 4 nano-ferrites is less toxic. Therefore, its response to the application and removal of magnetic field could be slow, it will be environmentally friendly.
The obtained parameters from the fit to the law of approach to saturation magnetization of Equation 1 are presented in Fig. 5(b). The significant change in the parameter for ZnFe 2 O 4 nanoferrites is indicative of a nonmagnetic inclusions arising from Zn atoms.

Fe
Mössbauer spectroscopy measurements for the CoFe 2 O 4 and ZnFe 2 O 4 nano-ferrites were performed at room temperature. α-Fe foil was used for the calibration at a constant acceleration. The Recoil Mössbauer analysis software was used for the fitting of the magnetic components using the Lorentzian site analysis.
Two sextets' δ 1 and δ 2 , were best suited to model the CoFe 2 O 4 nano-ferrites as shown in Fig. 6. The spectra shows a well ordered magnetic phase where sextets' δ 1 and δ 2 are associated with Fe 3+ ions at tetrahedral A-sites and octahedral B-sites of the spinel crystal structure [23]. Table 2 gives the values of the magnetic hyperfine field, H Bf , isomer shift, IS, quadrupole splitting, Δ EQ , and the fraction population, f, of Fe 3+ ions. The higher field of about 482 kOe is attributed to the B-sites and the lower field of about 445 kOe associated with the A-sites. The isomer shift values are in the appropriate range for Fe 3+ [24] and the hyperfine magnetic field are close to reported values [25]. Therefore, no evidence of Fe 2+ is observed.
However, the ZnFe 2 O 4 nano-ferrite was not a well ordered sextet compared to CoFe 2 O 4 nanoferrite, and it was best fitted with two sextets and two doublets as shown in Fig. 7. The results are shown in Table 3. The effect of the paramagnetic component can be seen to have a dropping effect and broadness of the spectra. The two sextets are associated with the coordination of Fe 3+ ions at tetrahedral A and octahedral B sites of the spinel crystal structure [26,27]. The doublets account for the paramagnetic component as was observed in the magnetic hysteresis loops. Again, the isomer shift values are in the appropriate range for Fe 3+ . Higher values of quadrupole splitting indicate that the ions are imbedded in non-cubic surroundings, while the relatively small values of the quadrupole splitting indicate that A and B sites have nearly cubic symmetry.  Fe Mössbauer spectroscopy measurement that the oxidation state of Fe was stable at Fe 3+ such stability in the oxidation state of Fe helps in reusability of the nanoparticles after every cycle of clean up. This is important because the magnetic properties of the nanoparticles can be fairly predicted not change significantly to affect its application around room temperature. We are also able to find that CoFe 2 O 4 has a low desirable coercivity property making it a better candidate for possible oil spill cleanup compared to the ZnFe 2 O 4 studied in the current work.

DISCLAIMER
The products used for this research are commonly and predominantly use products in our area of research and country. There is absolutely no conflict of interest between the authors and producers of the products because we do not intend to use these products as an avenue for any litigation but for the advancement of knowledge. Also, the research was not funded by the producing company rather it was funded by personal efforts of the authors.