1 Introduction

Cobalt spinel ferrite (CoFe2O4) is one of the crucial magnetic materials. CoFe2O4 is characterized by its electronic and magnetic properties, such as high coercivity, cubic magnetocrystalline anisotropy, high transition Curie temperature TC, chemical stability, magnetostriction, mechanical hardness, and electrical insulation [1,2,3,4,5]. CoFe2O4 has many applications, such as electronic devices, magnetic microwave devices, ferrofluids, heavy metal removal, Li ion batteries, and information storage devices [6, 7].

CoFe2O4 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 Co2+ and fcc close packing O2− ions, CoFe2O4 has an inverse spinel structure [8]. The magnetic features of CoFe2O4 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 CoFe2O4 ferrite's magnetic properties [9]. CoFe2O4 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 @ CoFe2O4 enhances the antibacterial activity of CoFe2O4. Many researchers studied the composite between the metal oxide and magnetic materials [16], while a few of them reported the antibacterial activity of the CoFe2O4/ZnO nanocomposite. Based on this fact, the preparation of the CoFe2O4/ZnO nanocomposites and the antibacterial activity of the composite will be examined.

In the present study, the nanoparticles CoFe2O4, ZnO, and their nanocomposite CoFe2O4/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.

2 Experimental Techniques

2.1 Materials Used

The chemicals used to prepare the investigated samples are of high purity. Ferric nitrate Fe(NO3)3.9H2O, cobalt nitrate Co(NO3)2.6H2O, zinc nitrate Zn(NO3)2 and citric acid C6H8O7 are brought from Sigma Aldrich Company.

2.2 Preparation of CoFe2O4 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.

2.3 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(NO3)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.

2.4 Preparation of CoFe2O4/ZnO Nanocomposite

The CoFe2O4/ZnO nanocomposite was synthesized by mixing 70% weight of CoFe2O4 to 30% weight of ZnO and then grinding for 2 h.

2.5 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 = 1.5418 Å), 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.

2.6 Antibacterial Activity

2.6.1 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.

2.6.2 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 × 105 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].

3 Results and Discussion

The XRD patterns of the investigated samples CoFe2O4, ZnO and CoFe2O4/ZnO nanoparticles are illustrated in Fig. 1. The XRD pattern of CoFe2O4 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 CoFe2O4 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:

Fig. 1
figure 1

XRD of CoFe2O4, ZnO, CoFe2O4/ZnO, ICDD card number 22-1086 and ICDD card 00-036-1451

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Table 1 Values of the lattice parameters, the unit cell volume (V), the crystallite size from XRD (L), the theoretical density (Dx) and the particle size (L) for the investigated samples
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$$\frac{1}{{d}^{2}}=({h}^{2}+ {k}^{2}+ {l}^{2} ) \frac{1}{{a}^{2}}$$
(1)

Figure 1(b) illustrates the XRD pattern of ZnO. The peak positions were indexed as (100), (002), (101), (102), (110), (103), (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.

$$\frac{1}{{d}^{2}}=[\frac{4}{3}({h}^{2}+{k}^{2}+hk)+{l}^{2}(\frac{a}{c})2]\frac{1}{{a}^{2}}$$
(2)

Figure 1(c) shows the XRD patterns of the nanocomposite CoFe2O4/ZnO. The average crystallite sizes of CoFe2O4, ZnO and CoFe2O4/ZnO were calculated according to the following well-known Scherrer formula: [21]

$$\mathrm{D}=\frac{0.9\;\lambda }{\beta cos\theta }$$
(3)

where λ is the wave length of the X-ray radiation (λ = 1.5406 Å), 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:

$$\mathrm{V=a}{3}\text{ (For cubic structure)}$$
(4)
$$\mathrm{V=a}{2}\text{c (For hexagonal structure)}$$
(5)

The theoretical densities were calculated using the following equation.

$${\mathrm{D}}_{\mathrm{x}}=\frac{ZM}{{N}_{A}V}$$
(6)

where Z (Z = 8 for CoFe2O4 and Z = 2 for ZnO) is the number of molecules per unit cell, M is the molecular weight, NA is the Avogadro’s number and V is the unit cell volume. The lattice parameter a is 8.373 Å to CoFe2O4 and increased to 8.385 Å for the CoFe2O4 / ZnO composite. The unit cell volume (V) increased from 586.96 Å3 for CoFe2O4 to 589.53 Å3 for the nanocomposite. According to Eq. (6), the theoretical density decreased with increasing the unit cell volume. So Dx decreased from 5.309 g/cm3 for CoFe2O4 to 5.286 g/cm3 for CoFe2O4 / ZnO nanocomposite.

HRTEM was used to study the morphology of the samples. Figure 2 illustrates the HRTEM images of CoFe2O4, ZnO and CoFe2O4/ZnO nanoparticles. The image of the CoFe2O4 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 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.

Fig. 2
figure 2

HRTEM images of a CoFe2O4, b  ZnO and c CoFe2O4/ZnO nanoparticles

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The morphology and structure of the samples were studied using FE-SEM. Figure 3 illustrates the FE-SEM images of the investigated samples. The shapes of CoFe2O4, ZnO, and CoFe2O4 /ZnO nanoparticles have a nearly spherical morphology. Figure 3a illustrates the spherical shapes of CoFe2O4, while the particles of ZnO have a flower shape, as illustrated in Fig. 3b. The morphology of the CoFe2O4/ZnO illustrated the presence of both CoFe2O4 and ZnO in the sample, as shown in Fig. 3c.

Fig. 3
figure 3

FESEM images of a CoFe2O4, b ZnO and c CoFe2O4/ZnO nanoparticles

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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: CoFe2O4, ZnO and CoFe2O4 / 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.

Fig. 4
figure 4

EDS curves of a CoFe2O4 b ZnO and c CoFe2O4 / ZnO

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Table 2 Illustrates the weight percentage (wt. %) and atomic percentage (at.%) of the O, Fe, Zn, and Co elements for the samples (a) CoFe2O4, (b) ZnO, and (c) CoFe2O4 / ZnO
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Fig. 5
figure 5

The elemental mapping of a CoFe2O4, b ZnO and c CoFe2O4 / ZnO samples

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Fig. 6
figure 6

XPS spectra of a CoFe2O4, b Co2p, c Fe2p, d O1s of CoFe2O4, e ZnO, f Zn2p of ZnO, g O1s of ZnO, and h CoFe2O4/ZnO

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Figure 6(a) illustrates the wide-scan XPS spectra of the investigated CoFe2O4 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 2p3/2 in CoFe2O4. The peak observed at BE at 781.57 eV is associated with Co2+ ions in tetrahedral sites [6]. The presence of a peak at BE 784.56 eV assures that some of the Co3+ ions are occupied in octahedral sites [24]. The two possibilities for the oxidation of Co2+ to Co3+ are that the oxidation of cobalt is compensated by a reduction of some Fe3+ to Fe2+ to make the lattice charge neutral, or the migration of Co2+ to tetrahedral sites. The low spin Co3+ atom is characterized by a much weaker satellite than that of high spin Co2+, due to the fact that the Co3+ orbital has unpaired valence electrons [24].

Figure 6(c) illustrates the spectrum of Fe ions in CoFe2O4 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 2p3/2 while the binding energies of Fe 2p1/2 were observed at 725.20 and 728.13 eV [6]. The doublets can be ascribed to Fe3+ ions in octahedral and tetrahedral sites, respectively. The Fe3+ ions in octahedral sites have the doublets of Fe 2p3/2 BE at 711.77 eV and Fe 2p1/2 BE at 725.20 eV, while the doublets of Fe 2p3/2 BE at 714.32 eV and Fe 2p1/2 BE at 728.13 eV are related to the Fe3+ ions in tetrahedral sites. From the integrated intensity of the fitted doublets, Fe3+ 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 Fe2+ in the CoFe2O4 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 Zn2p3/2 and Zn2p1/2 [27,28,29]. According to the area under the peaks, Zn2p3/2 and Zn2p1/2 present in the sample by a percentage ratio of 63% and 37% respectively. In Fig. 6(g), a broad peak at 530.59 eV is attributed to O 1s, which corresponds to O2− 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 CoFe2O4/ZnO is shown in Fig. 6(h). The spectra illustrated that the nanocomposite contains Co, Fe, Zn and O elements.

Figure 7 illustrates the hysteresis loop of the prepared nanoparticles and their nanocomposite at room temperature. It is clear that the CoFe2O4 nanoparticles revealed a large, saturated loop with an enhanced magnetization value and hard like behavior. The saturation and remanence magnetization values are listed in Table 3. The data assure that the positive magnetocrystalline anisotropy of the Co2+ ions is the main contribution to the very large coercive field values, as the highly crystalline cubic anisotropy is predominant here [32].

Fig. 7
figure 7

The M-H hysteresis loop of the prepared nanoparticles. The inset illustrates the loop opening for ZnO sample

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Table 3 The values of the saturation magnetization (Ms), remanence magnetization (Mr), the coercivity field (Hc), anisotropy constant (K), M-H loop area, and squareness ratio for CoFe2O4, ZnO, and CoFe2O4/ZnO samples
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The morphology of the crystals also makes it difficult to demagnetize and agrees well with the observed magnetic parameters despite the small values of the crystallite size. 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 constant was also calculated from [35] using Eq. (7) and listed in Table 3.

$$K=\frac{{H}_{C}\times {M}_{S}}{0.96}$$
(7)

where K is the magnetic anisotropy constant, Hc is the coercivity, and Ms 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 CoFe2O4. 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 CoFe2O4. 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%CoFe2O4 + 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].

Table 4 The antibacterial activity
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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.

4 Conclusion

A simple, fast, and effective citrate nitrate technique was successfully used for preparing CoFe2O4, ZnO, and their 70% CoFe2O4 + 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% CoFe2O4 + 30% ZnO. The crystallite size decreased for the nanocomposites compared to the parents, where L = 66.01 nm for CoFe2O4 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% CoFe2O4 + 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.