Structure, Optical, and Magnetic Properties of Magnetite Nanoparticles Doped with Zinc and Lanthanum and Prepared in Oxygen and Nitrogen Atmosphere

A comparative study of structural and magnetic properties for the Zn2+ and La3+ ions co-doped magnetite nanoparticles of the general formula [(1-x)Fe3O4:Zn:xLa, where x = 0.0, 0.1, 0.15, 0.2, 0.25, and 0.3] was prepared via co-precipitation method under oxygen and nitrogen atmosphere. The stoichiometric ratios of the elements in the given general formula were proved and confirmed by both Inductively coupled plasma (ICP) measurements and atomic absorption data. Various analytical and characterization techniques were used to analyze the prepared nanoparticles. X-ray diffraction (XRD) confirmed the formation of one single phase of nanoparticles components with average crystalline size ranging from 6.0 nm to 9.0 nm at oxygen atmosphere and from 6.0 nm to 8.0 nm at nitrogen atmosphere. These size values were calculated by Debye-Scherrer equation. The morphology of Zn2+ and La3+ ions co-doped magnetite nanoparticles was studied using field-emission scanning electron microscope (FE-SEM) and high-resolution transmission electron microscope (HR-TEM). Optical properties were demonstrated by UV-visible-NIR spectrophotometer. The high absorbance values of ZnLa(x)Fe(3-x)O4 were found to be 80–97% and 80–99% in the visible wavelength range of 400–800 nm for the prepared samples in the presence of O2 and N2 gases at room temperature, respectively; which proved the lower values of energy bandgap in both conditions. The lowering of energy bandgap of 1.89 eV for nanoparticles prepared in O2 gas and 1.78 eV for those prepared in N2 gas in most samples may be attributed to incorporation of Zn cation. Magnetic properties of the given samples, obtained from vibrating sample magnetometer (VSM), showed that most of these samples exhibited almost superparamagnetic behavior. These magnetic values (32.87 emu/g and 24.79 emu/g) infer super saturation magnetization of the given samples prepared in O2 and N2 gases at room temperature, respectively. Examination of the magnetic properties revealed decrease in saturation magnetization with increasing La ion concentrations incorporation up to x= 0.3.


Introduction
Iron oxide nanoparticles (IONPs) include many attributes of high magnetic saturation, stability, biocompatibility, and interactive functions at the surface [1]. The surface of IONPs could be adjusted by organic materials or inorganic materials, such as polymers, biomolecules, silica, metals, and others, that improve from its properties [2]. The synthesis of magnetic IONPs has been greatly developed not only for its fundamental scientific interest but also for its many technological applications, such as targeted drug delivery, magnetic resonance imaging (MRI), magnetic hyperthermia and thermoablation, bioseparation, and biosensing [3,4]. Besides, bioapplications based on magnetic nanoparticles offer unique advantages over other materials such as being inexpensive to produce, physically and chemically stable, biocompatible, and environmentally safe [5]. IONPs have a large surface area and can be engineered to provide a large number of functional groups for cross-linking to tumor-targeting ligands such as monoclonal antibodies, peptides, or small molecules for diagnostic imaging or the delivery of therapeutic agents [6].
In the last few decades, the synthesis of magnetite (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ) using hematite (α-Fe 2 O 3 ) as a starting material has been intensively pursued for both their fundamental scientific interests and technological applications [7]. Magnetite (Fe 3 O 4 ) has the face-centered cubic spinel structure, based on 32 O 2ions, and close-packed along the direction [8]. Fe 3 O 4 differs from most other iron oxides in that it contains both divalent and trivalent iron. Fe 3 O 4 has a cubic inverse spinel structure that consists of a cubic close-packed array of oxide ions, where all of the Fe 2+ ions occupy half of the octahedral sites and the Fe 3+ ions are split evenly across the remaining octahedral sites and the tetrahedral sites. In stoichiometric magnetite Fe 2+ /Fe 3+ = 1/2, and the divalent irons may be partly or fully replaced by other divalent ions (Co, Mn, Zn, etc.). Thus, Fe 3 O 4 can be both an n-and p-type semiconductor; while it has the lowest resistivity among iron oxides due to its small bandgap (0.1 eV) [9].
A variety of synthetic methods such as co-precipitation [10], thermal decomposition [11], hydrothermal and solvothermal syntheses, sol-gel synthesis [12], microemulsion, ultrasound irradiation, and biological synthesis have been applied to produce magnetic IONPs [13]. As the most conventional method, the co-precipitation method consists of mixing ferric and ferrous ions in a 1:2 molar ratio in very basic solutions at room temperature or at elevated temperature. The reaction mechanism can be simplified as The ferric nanoparticles were coated with sheets of ferrous to produce magnetite nanoparticles [14]. After many research works, it has been proved that the magnetite nanoparticles can be improved further by doping [15]. The synthesis of magnetite in the presence of divalent and trivalent cations of transition metals (viz V 3+ , Zn 2+ , Cr 3+ , Mn 2+ , Co 2+ , Ni 2+ , and Li 2+ ) can modify electrical, magnetic, structural, optical, and saturation magnetization properties of iron oxide; these properties are helpful in understanding the mechanism of formation of rust in alloy elements and also help to better understand the factors which influence magnetism change because of impurities [12,14].
In this study, we describe the careful investigation of Zn 2+ and La 3+ ions co-doped magnetite nanostructures upon their change in phase structure, crystallite sizes, space lattice parameters, and crystal morphologies. Moreover, the improvement in the optical properties of the prepared samples using UV-Vis-NIR was studied. The ultravioletvisible spectroscopy was carried out in order to inspect optical reflectivity and also clarify the bandgap energy characteristics of the produced materials. The magnetic properties of the prepared materials were determined by means of the magnetic susceptibility and vibrating sample magnetometer (VSM). , and hexahydrate ferric chloride (0.2 M) was added in 2:1:1 ratio and the same volumes were taken from each solution (50 mL). After 10 min of constant stirring, NaOH was added drop-wise to the mixture with stirring until the suspension became homogenous and the pH of the solution reached 11; that was in the presence of oxygen atmosphere (O 2 ) once and then repeated again in the presence of nitrogen atmosphere, where (N 2 ) gas flow (with 99.9% of purity) continued for 20 min. The particles were washed with warmed distilled water several times [16]. Then the reaction was kept at 85-90°C for 4 h. Afterwards, zinc doped magnetite nanoparticles were magnetically collected with the formula ZnFe 3 O 4 .

Materials and reagents
Then, the aqueous solutions of tetrahydrate ferrous chloride, zinc acetate dihydrate, and hexahydrate ferric chloride were mixed by 50 mL of the aqueous solution of lanthanum chloride 7 · H 2 O for different ratios (0.1, 0.15, 0.2, 0.25, and 0.3 M). Also after 10 min of constant stirring, NaOH was added drop-wise to the mixture with stirring until the suspension became homogenous and the pH of the solution reached 11. The reactions occured in the presence of oxygen and then in a nitrogen atmosphere. The particles were washed with warmed distilled water several times. The reaction was kept at 85-90°C for 4 h. Then Zn-La doped   Figure 1, which were obtained by following Scheme 1 of precipitation.

Analytical characterization: chemical compositions
Different techniques were used to ensure that the phase in the nanocomposites was continued to (1-x)Fe 3 O 4 :Zn:xLa and to confirm the formation of a single-phase structure. Table 1 shows that the main components of the sample are iron, zinc, and oxygen, which describes the presence of iron oxide and zinc oxide. The spectrum in Figure 2 contained three peaks which refer to Fe, Zn, and O; there are no other impurities identified. Tables 2, 3, and 4 show the atomic absorption and ICP rates for the three elements, iron, zinc, and lanthanum used in the preparation of Zn-La magnetite nanoparticles.

Characterization
X-ray diffraction (XRD) patterns of the resulting products were characterized by a Brucker D8-advanced X-ray powder diffractometer with Cu Kα radiation (λ = 1.5406Å). The crystallite sizes of the produced Zn 2+ and La 3+ ions codoped magnetite nanostructures for the most intense peak   (3 1 1) plane were determined from the X-ray diffraction data using the Debye-Scherrer equation as follows [17]: where d RX is the crystallite size, k = 0.9 is a correction factor to account for particle shapes, β is the full width at half maximum (FWHM) of the most intense diffraction peak (3 1 1) plane, λ is the wavelength of Cu target = 1.5406Å, and θ is the Bragg angle. The micrographs of produced samples were examined by direct observation via FE-SEM model JEOL instrument (Japan) model JSM-7800F.  The UV-VIS-NIR absorption spectrum was recorded by a double-beam UV-VIS-NIR spectrophotometer. Diffuse reflectance measurements were done on the Jasco-V-570 spectrophotometer (Japan) fitted with integrating sphere reflectance unit.
The magnetic properties were accomplished of produced hexagonal ferrite samples at room temperature using a 7400-1 VSM (Lake Shore Cryotronics, Ltd, USA) in a maximum applied field of 20 kOe. From the obtained hysteresis behavior, the saturation magnetization (M s ), remanence magnetization (M r ), and coercivity (H c ) were determined.  It can be concluded, obviously, that the La 3+ ion-doped led to a decrease in the crystallite size of the cubic magnetite (Fe 3 O 4 ) phase. The crystallite size of Zn 2+ and La 3+ ions co-doped F 3 O 4 nanopowders was estimated from the most intense peak (3 1 1) from XRD data based on Debye-Scherrer formula. However, at the nitrogen atmosphere, the crystallite size of cubic magnetite (Fe 3 O 4 ) was 8.0 nm, which was smaller than at the oxygen atmosphere (9.8 nm). The crystallite size was found to be decreasing from 9.8 nm to 6.6 nm for the increasing La 3+ doping from as low dopant concentration as 0.00 to as high as 0.30 molar ratio, respectively as well as crystallite size using nitrogen atmosphere decreasing from 8.0 nm to 3.9 nm as listed in Tables 5 and 6.

XRD analysis
The lattice parameter of the samples was calculated using the following equation: where a is the lattice parameter, d is the inter-planar distance, and h, k, and l are the Miller indices. The values of the lattice parameter were listed in Tables 5 and 6. The lattice parameter was increased with mounting La content of (1-x)Fe 3 O 4 :Zn:xLa nanoparticles because of a higher ionic radius of La 3+ ions (2.74Å) in comparison to the Fe 3+ ions (1.72Å) [18]. It was found that the volume of the unit cell increases with increasing La content which may be due to increasing the lattice parameter and values of the volume of the unit cell are tabulated in Tables 5 and 6. The X-ray theoretical density was estimated using the equation where ρ x is the X-ray theoretical density, A is the sum of the atomic weights of all the atoms in the unit cell, V is the volume of the unit cell, and N A is the Avogadro's number [19]. The X-ray theoretical density diminishes with rising La content of (  Figures 5 and 6 do not allow us to see the microstructures of Zn 2+ and La 3+ ions co-doped magnetite (Fe 3 O 4 ) nanoparticles. The HR-TEM analysis will be useful for these two frustrating problems that arise for quantifying the microstructure. For the O 2 atmosphere, the majority of the particles present cubic morphology, characteristic of magnetite, as well as in the N 2 atmosphere the particles present the pseudocubic-shaped morphology, also characteristic of magnetite. The irregular geometry of Zn 2+ and La 3+ ions co-doped magnetite (Fe 3 O 4 ) nanoparticles O 2 and N 2 atmosphere nanoparticles reflects their low crystallinity  showed by XRD results. Therefore, the chemical precipitation of Zn 2+ and La 3+ ions co-doped and mixed Fe 2+ /Fe 3+ solution allowed to obtain particles with diameters significantly smaller than the chemical precipitation of only Fe 2+ solution. The decrease in diameter can be directly related to higher surface areas and more reactive systems of Zn 2+ and La 3+ ions co-doped Fe 3 O 4 nanoparticles. FE-SEM revealed that at the oxygen atmosphere, La ions led to the formation of aggregates that are polydispersed and have many shapes as plates and rods as shown in Figure 5(c). When La ions increased, cracks were formed as shown in Figure 5(e), that is due to the separation of La ions from the magnetite nanoparticles, and also it led to a decrease of magnetism. HR-TEM clarified accurately the separation of La ions Figure 5(f). In the case of nitrogen, magnetite has very tiny particles and high dispersity as shown in Figures 6(a) and 6(b) which have no La ions, but when La ions were added, aggregates were formed in the shape of rods as shown in Figures 6(c) and 6(d). The increase of La ions led to the formation of conglomerates papers which appear in SEM as shown in Figure 6(e), but appear objectively in HR-TEM as separated rods due to separation of La ions from magnetite as shown in Figure 6(f).

Optical properties (UV-Vis-NIR spectroscopy characteristics)
The optical properties measurements were taken over the wavelength range 200-1,200 nm using integrating sphere unit of the spectrophotometer. Optical absorbance and diffuse reflectance spectra of Zn 2+ and La 3+ ions co-doped magnetite (Fe 3 O 4 ) nanopowders via co-precipitation route under O 2 and N 2 atmosphere at room temperature curves are shown in Figures 7 and 8. Apparently, the optical reflectance of the nanopowders was highly dependent on La 3+ ions content and under O 2 and N 2 atmosphere route as shown in Figures 7 and 8. However, the absorbance spectra for the O 2 atmosphere in the IR region were smaller than the N 2 atmosphere absorbance spectra. The as-deposited sample exhibited the highest optical reflectivity of around 12-14% and 10-13% for O 2 and N 2 atmosphere, respectively, in the infrared (IR) and visible regions. Further increasing in La 3+ ions doping was found to change the average reflectivity of around 45-28% in the IR and visible regions, respectively.
There was a definite drop in reflectance as the La 3+ ions doped increased. Furthermore, the results revealed that the recorded absorbance was a characteristic peak of Fe and La ions.
In the limiting case of an infinite powder sample, thickness and sample holders have no influence on the value of reflectance (R). In this case, the Kubelka-Munk equation at any wavelength becomes where F (R∞) is the so-called remission or Kubelka-Munk function. In the parabolic band structure, the bandgap Eg, and absorption coefficient α, of a direct bandgap semiconductor are related through the well-known equation where α is the linear absorption coefficient of the material, hν is the photon energy, and A is proportionality constant. When the material scatters in a perfectly diffuse manner (or when it is illuminated at 60± incidence), the K-M absorption coefficient K becomes equal to 2α (K = 2α). In this case, considering the K-M scattering coefficient S as constant with respect to wavelength, and using the remission function in (6) we obtain the expression Therefore, obtaining F (R∞) from (6) Tables 7 and 8. Room temperature magnetization measurements showed that there is a decrease in the saturation magnetization (M s ), with increasing the La 3+ molar ratios from 0.1 to 0.3, from 18.813 emu/g to 3.915 emu/g, respectively in oxygen atmosphere. Meanwhile, in nitrogen atmosphere saturation magnetization (M s ) was decreasing from 16.723 emu/g to 3.623 emu/g with increasing the La molar ratios from 0.1 to 0.3, respectively. Furthermore, increasing the doping Zn 2+ and La 3+ ions does not allow the atomic diffusion into each particle to change its chemical homogeneity and induced partial densification affecting the strength   which enhanced inter-sub-lattice A−B super-exchange interaction between the magnetic ions of the sub-lattices,    The magnetic moment per molecule (μ B ) was obtained as summarized in Tables 7 and 8 using the empirical formula where M wt is molecular weight, M s is saturation magnetization, and 5585 is a magnetic factor. It is observed that as La 3+ content increases the μ B values go to decrease. This might be attributed to that La 3+ ions occupied the A-site and Fe 3+ ions B-site and consequently resulting in an increase of magnetic moment of B-site as well as an increase in the value of the magnetic moment of A-site. Low squareness ratio (M r /M s ) and coercivity (H c ) values reflect the superparamagnetic nature of samples at different La 3+ values. As a result, the M r /M s ratio for interacting superparamagnetic particles is approximately 0.10, that is, the sample loses its magnetization when the applied magnetic field is removed. As mentioned in Tables 7 and 8, all of the samples possessed a M r /M s ratio of less than 0.10.

Conclusion
Different techniques were used to ensure the phase in the nanocomposites and to confirm the formation of a singlephase structure. The stoichiometric ratios of the elements in the general formula of and to (1-x)Fe 3 O 4 :Zn:xLa were proved and confirmed by EDX, ICP, and atomic absorption data. Characterization techniques have been used to analyze and characterize the prepared nanoparticles. XRD confirms the formation of one single phase of nanoparticles components with average crystalline size ranging from 6.0 nm to 9.0 nm at oxygen atmosphere and 6.0 nm to 8.0 nm at the nitrogen atmosphere. The morphology of Zn 2+ and La 3+ ions co-doped magnetite nanoparticles was studied using FE-SEM and HR-TEM. Optical properties were demonstrated by UV-visible-NIR spectrophotometer. Magnetic properties of the given samples were obtained from VSM.