Structural, Surface, Magnetic Study and Application of Nanoparticles CoFe2O4, ZnO and its Nanocomposite

Nanoparticles of CoFe2O4 and ZnO were synthesized using the citrate auto-combustion method. A nanocomposite CoFe2O4/ZnO (70:30) was prepared. XRD verified that the samples were synthesized in nanoscale with crystallite sizes of 66.01, 12.48, and 17.47, respectively. The HRTEM image of CoFe2O4 illustrates the cubic structure. FESEM showed that the shape of CoFe2O4, ZnO, and CoFe2O4/ZnO nanoparticles is nearly spherical in morphology. EDAX measurements illustrated that the samples were obtained with nominal compositions similar to their chemical formula. XPS spectra of the investigated CoFe2O4 confirmed the presence of Co2+ and Co3+ ions, as well as Fe3+ and Fe2+ ions. The saturation magnetization increased for CoFe2O4/ZnO than that of the parent CoFe2O4 due to strong ferromagnetic coupling. Antibacterial activity was recorded for the nanocomposite despite its absence from the two parents.

CoFe 2 O 4 has a cubic structure with eight formula units in each unit cell.There are two sites that the cations can occupy: the tetrahedral [A] and octahedral [B] sites.With octahedral Co 2+ and fcc close packing O 2− ions, CoFe 2 O 4 has an inverse spinel structure [8].The magnetic features of CoFe 2 O 4 originate from the presence of antiparallel sublattices which produce a superexchange interaction through the oxygen ions, producing the ferrimagnetic behavior [8].The preparation methods of ferrites affect the shape and size of the prepared nanoparticles, which influence the CoFe 2 O 4 ferrite's magnetic properties [9].CoFe 2 O 4 was synthesized using a variety of techniques, including sol-gel [10], reverse micelles [11], forced hydrolysis in a polyol medium [12], and citrate auto combustion [13].The preparation of nanoparticles by the citrate combustion method is characterized by a fast, simple, and easy technique [14].
Based on ZnO, this is a semiconductor material with unique characteristics such as nontoxicity, a bandgap of 3.3 eV at room temperature, good chemical, photosensitivity, and thermal stability [15].The photocatalysis performance and antibacterial activity depend on the movement of photogenerated electrons and holes to the surface.This transfer causes pollutants to degrade and has weak antibacterial activity [16,17].The preparation of composites from metal oxide and magnetic nanoparticles leads to an increase in their chemical and physical behaviors [18].The addition of ZnO @ CoFe 2 O 4 enhances the antibacterial activity of CoFe 2 O 4 .Many researchers studied the composite between the metal oxide and magnetic materials [16], while a few of them reported the antibacterial activity of the CoFe 2 O 4 /ZnO nanocomposite.Based on this fact, the preparation of the CoFe 2 O 4 /ZnO nanocomposites and the antibacterial activity of the composite will be examined.
In the present study, the nanoparticles CoFe 2 O 4 , ZnO, and their nanocomposite CoFe 2 O 4 /ZnO were synthesized via the simple, fast citrate combustion technique.The structure and morphology of the samples were studied using XRD, FESEM, HRTEM, XPS, SAED, EDS and mapping.The magnetic properties of the samples were studied using VSM.The antibacterial activity of the investigated samples was also examined.

Materials Used
The chemicals used to prepare the investigated samples are of high purity.Ferric nitrate Fe(NO 3 ) 3 .9H

Preparation of CoFe 2 O 4 Nanoparticles
The cobalt spinel ferrite was synthesized by the citrate nitrate auto combustion method [19].The initial ingredients, cobalt nitrate (1 mol) and iron nitrate (1 mol) were mixed with citric acid (2 mol) in stoichiometric ratios in an aqueous solution.The ratio between the metal nitrates and citric acid is 1:1.The solution's pH was adjusted to 7 by adding ammonia droplets as a fuel.To create a fluffy powder, the sample was thoroughly combined and heated on a hot plate.Then the powder was calcined at 600 °C for 4 h.

Synthesis of Zinc Oxide ZnO
ZnO was prepared by the citrate nitrate combustion method.The zinc nitrate (1 mol) and citric acid (1 mol) were mixed in stoichiometric ratios and dissolved in distilled water.The Zn(NO 3 ) 2 to citric acid ratio is 1:1.The ammonia solution adjusted the solution's pH to 7. The sample was placed on a hot plate and heated to create a fluffy powder.The sample was calcined at 550 °C for 3 h.

Preparation of CoFe 2 O 4 /ZnO Nanocomposite
The CoFe 2 O 4 /ZnO nanocomposite was synthesized by mixing 70% weight of CoFe 2 O 4 to 30% weight of ZnO and then grinding for 2 h.

Characterization and Measurement of Samples
The structure and morphology of the prepared samples were investigated by X-ray diffraction (XRD, Bruker Advance D8 diffractometer, λ = 1.5418Å), high-resolution transmission electron microscope (HRTEM, JEOL-2100), elemental mapping and energy-dispersive X-ray analysis (EDAX) using a scanning electron microscope (FESEM, model Quanta 250), and X-ray photoelectron spectroscopy (XPS) analysis via K-ALPHA (Thermo Fisher Scientific, USA).A vibrating sample magnetometer (VSM; 9600-1 LDJ, USA) was used to study the magnetic properties of the samples at room temperature.

Antimicrobial Assay
The antimicrobial activity was examined using the agar well diffusion technique for all investigated nanoparticles and their nanocomposite.Using nutritional agar medium, all the compounds were evaluated in vitro for their antibacterial activity against gram positive bacteria like Staphylococcus aureus and Streptococcus mutans as well as gram negative bacteria like Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumonia.Standard drugs for gram positive and gram negative bacteria are ampicillin and gentamicin, respectively.This test uses a 15 mg/ml concentration against both bacterial strains.

Method of Testing
Twenty ml of the sterilized media was added to each sterilized petri dish, and they were left to harden at room temperature.A 1.5 × 10 5 CFU mL −1 microbial suspension was created in sterile saline, and its turbidity was adjusted to an OD of 0.13 using a spectrophotometer at 625 nm.A sterile cotton swab should ideally be saturated on the dried agar surface and left to dry for 15 min with the lid on within 15 min of adjusting the turbidity of the inoculum suspension.Using a sterile borer, 6 mm-diameter wells were created in the solidified material.With the aid of a micropipette, 100 μL of the tested compound's solution were added to each well.In order to test for antibacterial activity, the plates were incubated at 37 °C for 24 h.Zones of inhibition were quantified in mm.scale, and the tests were triple-blinded [20].

Results and Discussion
The XRD patterns of the investigated samples CoFe 2 O 4 , ZnO and CoFe 2 O 4 /ZnO nanoparticles are illustrated in Fig. 1.The XRD pattern of CoFe 2 O 4 nanoparticles is presented in Fig. 1a in which diffraction peaks located at 2θ = 30.16º,35.51 º, 43.15º, 57.04º and 62.65º are attributed to the (220), (311), ( 400) and (440) planes, respectively.The XRD data of spinel ferrite CoFe 2 O 4 was indexed with ICDD card no.022-1086 as illustrated in Fig. 1a.Co ferrite sample was prepared in a single phase cubic structure with space group Fd3m.The lattice parameters were calculated and listed in Table 1 according to the following relation for cubic structure:  200), (112), ( 201), (004) and (202) planes of ZnO (ICDD card 036-1451).ZnO was synthesized in single phase hexagonal structure with space group P63mc.According to Eq. ( 2), the lattice parameters were calculated on the basis of the hexagonal symmetry.
(2) (3) D = 0.9 cos where λ is the wave length of the X-ray radiation (λ = 1.5406Å), D denoted the average crystallite size, β is the corrected width at half maximum intensity of the powder pattern peak and θ refers to the Bragg angle.The values of the crystallite size of the investigated samples are reported in Table 1.The unit cell volumes were calculated according to the following equations:   6), the theoretical density decreased with increasing the unit cell volume.So D x decreased from 5.309 g/cm 3 for CoFe 2 O 4 to 5.286 g/ cm 3 for CoFe 2 O 4 / ZnO nanocomposite.
HRTEM was used to study the morphology of the samples.Figure 2 1 and indicated that the investigated samples were prepared in nano scale.The agglomeration of the particles appears due to the magnetostatic interaction and the absence of surfactant or coating [22,23].The inset illustrates the selected-area electron diffraction (SAED) consisting of concentric distinguished rings which assured that the samples were synthesized in polycrystalline nature with very good crystallinity despite the small size.
The morphology and structure of the samples were studied using FE-SEM.Figure 3  To assure that the samples were synthesized in pure form, the energy-dispersive X-ray analysis (EDAX) measurement was performed.Figure 4 shows the chemical composition of the investigated samples: CoFe 2 O 4 , ZnO and CoFe 2 O 4 / ZnO.The spectra illustrate strong, intense peaks of Fe, Co, Zn and O which reveal their presence in the investigated samples.Table 2 shows the values of weight percentage (wt.%)and atomic percentage (at.%)calculated experimentally from EDAX and theoretically from the chemical formula.The EDAX maps of the investigated samples are illustrated in Fig. 5 which shows the homogenous distribution of iron, cobalt, oxygen and zinc throughout the samples.
Figure 6(a) illustrates the wide-scan XPS spectra of the investigated CoFe 2 O 4 nanoparticles.The sample contains Co, Fe and O elements without any other impurity elements except carbon.The carbon is present on the surface of the sample due to contamination caused by handling [6]. Figure 6(b-d) show the high-resolution narrow-scan XPS spectra of the Co 2p, Fe 2p, and O 1s peaks, respectively.
Figure 6(b) illustrates the XPS spectrum of Co 2p 3/2 in CoFe 2 O 4 .The peak observed at BE at 781.57eV is associated with Co 2+ ions in tetrahedral sites [6].The presence of a peak at BE 784.56 eV assures that some of the Co 3+ ions are occupied in octahedral sites [24].The two possibilities for the oxidation of Co 2+ to Co 3+ are that the oxidation of cobalt is compensated by a reduction of some Fe 3+ to Fe 2+ to make the lattice charge neutral, or the migration of Co 2+ to tetrahedral sites.The low spin Co 3+ atom is characterized by a much weaker satellite than that of high spin Co 2+ , due to the fact that the Co 3+ orbital has unpaired valence electrons [24].
Figure 6(c) illustrates the spectrum of Fe ions in CoFe 2 O 4 nanoparticles which reveals the presence of two kinds of Fe bonds in the cobalt ferrite sample, referring to the octahedral and tetrahedral sites.The peaks at binding energies of 711.77 and 714.32 eV correspond to Fe 2p 3/2 while the binding energies of Fe 2p 1/2 were observed at 725.20 and 728.13 eV [6].The doublets can be ascribed to Fe 3+ ions in octahedral and tetrahedral sites, respectively.The Fe 3+ ions in octahedral sites have the doublets of Fe 2p 3/2 BE at 711.77 eV and Fe 2p 1/2 BE at 725.20 eV, while the doublets of Fe 2p 3/2 BE at 714.32 eV and Fe 2p 1/2 BE at 728.13 eV are related to the Fe 3+ ions in tetrahedral sites.From the integrated intensity of the fitted doublets, Fe 3+ ions contribute about 65% in octahedral sites and about 35% in tetrahedral sites.The satellite structure gives useful information about the iron chemical environment.The binding energy at 717.88 eV corresponds to the satellite peak, [25] which indicates the presence of some Fe 2+ in the CoFe 2 O 4 sample [26].
The O 1s XPS spectrum is illustrated in Fig. 6(d).The spectrum has three peaks at binding energies of 530.3, 531.9, and 534.1 eV.The main peak at 531.9 eV is a result of the contribution of the ferrite crystal lattice oxygen [25].
Figure 6(e) illustrates the XPS survey spectra for ZnO nanoparticles.The photoelectron peaks corresponding to Zn and O were observed.Figure 6(f) showed two strong peaks at binding energies of 1022.01 and 1044.99 eV, which are related to Zn2p 3/2 and Zn2p 1/2 [27][28][29].According to the area under the peaks, Zn2p 3/2 and Zn2p 1/2 present in the sample by a percentage ratio of 63% and 37% respectively.In    3. The data assure that the positive magnetocrystalline anisotropy of the Co 2+ ions is the main contribution to the very large coercive field values, as the highly crystalline cubic anisotropy is predominant here [32].
The morphology of the crystals also makes it difficult to demagnetize and agrees well with the observed magnetic The obtained values of coercivity oriented us to recommend these ferrites for hard magnets.It was well known that ZnO is distinguished by its paramagnetic nature as reported by many researchers [33].After being synthesized in nanoscale, some authors guaranteed the weak ferromagnetic nature at very small size [34].
The ferromagnetic nature of our prepared ZnO nanoparticles could originate from the following: i. redistribution of Zn ions among tetrahedral and octahedral sites.ii. the existence of some oxygen vacancies resulting from small particle size.iii.The appearance of some secondary phases due to impurities.The latter is the least probable reason as XRD analysis was proven to be single phase.For the investigated nanocomposite, the results were improved in terms of the increase in saturation magnetization as a consequence of the ferromagnetic coupling between the ferrimagnetic lattice of Co ferrite and the ferromagnetic ZnO one.The anisotropy Fig. 7 The M-H hysteresis loop of the prepared nanoparticles.The inset illustrates the loop opening for ZnO sample constant was also calculated from [35] using Eq. ( 7) and listed in Table 3.
where K is the magnetic anisotropy constant, H c is the coercivity, and M s is the saturation magnetization.
The value of the coercive field increased for the nanocomposite compared to the nanoferrite itself, this could be attributed to the decrease in crystallite size after adding the ZnO to the CoFe 2 O 4 .Our results agree with the explanation reported by Neetu Dhanda et al. [36] The anisotropy constant for the nanocomposite is the largest one as the ZnO acted as pinning centers between the ferrimagnetic domains of the CoFe 2 O 4 .Herein, ZnO nanocrystals make it difficult to demagnetize the nanocomposite itself, where it impedes the spins of the ferrite to be reoriented with the external magnetic field direction.The values of the squareness ratio assured the strong coupling between the two phases with hard like nature for the ferrite nanocrystal as well as the nanocomposite and soft ZnO characters.The abovementioned parameters for the nanocomposite make it suitable for use in spintronic devices.
Table 4 depicts the antibacterial activity of the prepared nanoparticles and the nanocomposite.It is obvious that the parent nanoparticles the nanoferrite, nano ZnO didn't reveal any inhibition zones.While their nanocomposite revealed an inhibitory zone against all tested strains.This means that 70%CoFe 2 O 4 + 30%ZnO had better antibacterial activity.This was directly correlated with concentration used.The (15mg/ ml) wasn't suitable content for both parents.On the other hand, lower concentrations of both in their nanocomposite form were good bactericidal agents for the examined strains.The observed antibacterial activity for the nanocomposite is related directly to its smallest crystallite size which enhanced the surface area.Consequently, this is the direct reason of the large catalytic activity at the surface and the antagonistic behavior for the used pathogen.The exact antimicrobial mechanism may need further experiments.In small size of crystals, better contact with the bacterial cell wall membrane is also a reason, thereby reactive oxygen species (ROS) are produced, resulting in lipid peroxidation, protein oxidation, as well as DNA damage.Accordingly, the death of bacteria is the final result [37].
Additionally, the broken chemical bonds at the surface of the nanoferrite results in the generation of metal ions from nano ferrites.Those positively charged ions are electrostatically attracted to the membrane of the negatively charged bacterial cell.Subsequently, they bond ensuing in DNA damage.Another plausible cause could be the shape and larger surface-to-volume ratio of nanocomposite, which is fortunate for better interaction with the bacterial cell membrane.Future work will be directed towards the determination of minimum inhibitory concentration (MIC) and the possible use of these nanocomposites in coating medical devices.

Conclusion
A simple, fast, and effective citrate nitrate technique was successfully used for preparing CoFe 2 O 4 , ZnO, and their 70% CoFe 2 O 4 + 30%ZnO nanocomposite.Nanocrystals were characterized using a diversity of practices.A single phase was obtained and identified for the two parent compounds, while two separate phases are identified in the nanocomposite with the ratio 70% CoFe 2 O 4 + 30% ZnO.The crystallite size decreased for the nanocomposites compared to the parents, where L = 66.01 nm for CoFe 2 O 4 and L = 17.47 nm for the nanocomposite.The nanoferrites are observed to possess a cubic geometric shape, while the ZnO is hexagonal and their nanocomposite groups the two different shapes and morphologies.The particle sizes of the investigated samples ranged from 15 to 51 nm.The experimental and theoretical atomic percentages (at.%) of the elements are close to each other.The elemental mapping of the elements shows the homogenous distribution of Fe, Co, O, and Zn throughout the samples.The nanoferrite is a typical ferrimagnet, while ZnO nanoparticles have weak ferromagnetic properties.The nanocomposite revealed better magnetic properties due to the size and shape of both parents.The magnetization reached 58.9 emu/g, while the anisotropy was 87,719 emu Oe/ g for the 70% CoFe 2 O 4 + 30% ZnO nanocomposite.From the results of antibacterial studies, the nanocomposite was recommended as an antimicrobial coating for medical devices in hospitals, diagnostic centers, and ambulatory surgical centers industries.

Fig. 2
Fig. 2 HRTEM images of a CoFe 2 O 4 , b ZnO and c CoFe 2 O 4 /ZnO nanoparticles illustrates the HRTEM images of CoFe 2 O 4 , (4) V = a3 (For cubic structure) (5) V = a2c (For hexagonal structure) (6) D x = ZM N A V ZnO and CoFe 2 O 4 /ZnO nanoparticles.The image of the CoFe 2 O 4 shows the cubic structure of the sample which confirms the data obtained from XRD.The ZnO particles at the nanoscale are clearly seen as hexagonal platelets in Fig. 2(b) and in the nanocomposite also their morphology is kept unchanged.The particle size values were tabulated in Table

Fig. 6 (
Fig.6(g), a broad peak at 530.59 eV is attributed to O 1s, which corresponds to O 2− in the hexagonal structure of ZnO nanoparticles[30,31].By fitting the broad peak of O 1s, two Gaussian peaks appeared at two binding energies, 530.61 and 532.27 eV.The XPS survey spectrum for the nanocomposite CoFe 2 O 4 /ZnO is shown in Fig.6(h).The spectra illustrated that the nanocomposite contains Co, Fe, Zn and O elements.Figure7illustrates the hysteresis loop of the prepared nanoparticles and their nanocomposite at room temperature.It is

Figure 7
Fig.6(g), a broad peak at 530.59 eV is attributed to O 1s, which corresponds to O 2− in the hexagonal structure of ZnO nanoparticles[30,31].By fitting the broad peak of O 1s, two Gaussian peaks appeared at two binding energies, 530.61 and 532.27 eV.The XPS survey spectrum for the nanocomposite CoFe 2 O 4 /ZnO is shown in Fig.6(h).The spectra illustrated that the nanocomposite contains Co, Fe, Zn and O elements.Figure7illustrates the hysteresis loop of the prepared nanoparticles and their nanocomposite at room temperature.It is

Fig. 5
Fig. 5 The elemental mapping of a CoFe 2 O 4 , b ZnO and c CoFe 2 O 4 / ZnO samples

Author
Contribution M.M. Arman put the idea on paper.S.I.El-Dek prepared a ZnO sample, and M.M. Arman prepared CoFe 2 O 4 and their nanocomposite.S.I.El-Dek and M.M. Arman were sharing the methodology, experiments and revising the final form of the article.M.M. Arman was discussing and writing the results of the structure (XRD, XPS) and the microscopy (FESEM, the elemental mapping, EDS, HRTEM).S.I.El-Dek was discussing and writing the results of the magnetic properties (VSM) and the antibacterial activity.Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB).

Table 1
Values

Table 2
Illustrates the weight percentage (wt.%) and atomic percentage (at.%) of the O, Fe, Zn, and Co elements for the samples (a) CoFe 2 O 4 ,

Table 3
The values of the saturation magnetization (M s ), remanence magnetization (M r ), the coercivity field (H c ), anisotropy constant (K), M-H loop area, and squareness ratio for CoFe 2 O 4 , ZnO, and CoFe 2 O 4 /ZnO samples

Table 4
The antibacterial activity Types of antibacterial Standard CoFe 2 O 4 ZnO CoFe 2 O 4 /ZnO