1 Introduction

Nowadays, magnetic nanoparticles are used in ferrofluids, nuclear magnetic resonance (NMR) imaging, biomolecule detectors, catalyst, bearings and dampers, drug delivery, pollutant removal or toxicity mitigation, hyperthermia, high-density recording device, and permanent magnet applications [1,2,3,4,5]. Among the spinel ferrites, cobalt ferrite has received considerable attention due to its moderate saturation magnetization, high coercivity, high Curie temperature, excellent chemical stability, biocompatibility, and very high cubic magnetocrystalline anisotropy [6,7,8,9].

Magnetic properties, purity, particle size distribution, morphology, and particle size, are the key factors for the considered application influenced by the method and conditions of synthesis. Based on this, different synthesis methods are being employed [10,11,12]. Cobalt ferrite nanoparticles can be synthesized with different methods such as microemulsion [13], co-precipitation [14], electrochemical [15], thermal decomposition [16], sol–gel [17], hydrothermal [18], sol–gel hydrothermal [19]. Among these methods, hydrothermal is a cost-effective and convenient method that yielded high purity, narrow particle size, and controlled morphology products. Besides, surfactants and chelating agents, such as CTAB, PVA, citric acid (CA), PVP, are used as growth control agents that inhabited particle growth or caused particular morphologies in the hydrothermal synthesis [20, 21]. The effects of citric acid [19, 22], PVA [23], and CTAB [24] addition on hydrothermal synthesis of cobalt ferrite nanoparticles were investigated earlier.

Citric acid concentration is an effective factor on the final properties of products synthesized via the CA-assisted hydrothermal method. In this regard, Jiang et al. [25] reported the effects of CA concentration on the α-NaYF4 particle size. Di et al. [26] prepared TbPO4 nanocrystals by the hydrothermal route and revealed the effect of CA: Tb ratio on morphology and particle size. Cho et al. [27] showed that the CA concentration influenced ZnO nanoparticles’ morphologies. Zahraei et al. [28] showed that the CA concentration affects the purity of the synthesized Mn0.6Zn0.4Fe2O4 nanoparticles. Behdadfar et al. [29] synthesized Fe3O4 nanoparticles, and results showed a certain amount of CA was required to obtain single phase. Saini et al. [30] prepared different phases of VO2 by varying the vanadium to the CA molar ratio. Soo-Min Park and Chunghee Nam [31] synthesized different morphologies of WO3 nanoparticles using different CA concentrations. However, up to our knowledge, there is no report that investigated the effect of the CA concentration on the hydrothermal synthesis of cobalt ferrite nanoparticles.

In this present work, cobalt ferrite nanoparticles were synthesized via a citric acid-assisted hydrothermal method, and the impact and role of citric acid to metal ion molar ratio, as a criterion of citric acid concentration, was investigated and discussed based on XRD, FE-SEM, FTIR, and VSM results.

2 Experimental section

2.1 Materials

Starting materials were iron (III) nitrate-nonahydrate (Fe (NO3)3.9H2O, Merck No. 103883), cobalt (II) nitrate-hexahydrate (Co(NO3)2.6H2O, Merck No. 102536), sodium hydroxide (NaOH, Merck No. 106482), and citric acid monohydrate (C6H8O7.H2O, Merck No. 100242). These reagents were used without any further purification.

2.2 Preparation of cobalt ferrite nanopowder

Cobalt ferrite nanopowder was prepared as follows: first, appropriate amounts of cobalt nitrate and iron nitrate (molar ratio 1:2) separately were dissolved in deionized water. Subsequently, appropriate amounts of citric acid (molar ratio of CA to metal ion 0.5, 1, and 1.5) were dissolved in deionized water and separately were added to cobalt and iron nitrates solutions and stirred for 2 h. After that, the cobalt-containing solution was added to the iron-containing solution and stirred for 3 h. In the next step, the pH value of the solution was adjusted to 12 by dropwise adding the sodium hydroxide solution. Then the obtained suspension was transferred into a 100-ml Teflon sailed autoclave and heated to 180 ℃ for 9 h. After being cooled to room temperature, the obtained precipitate was collected by centrifuge and external magnet and washed with deionized water several times until reaching neutral pH. Finally, the obtained black powder was dried at 110 ℃ for 3 h. By comparison, a sample was also synthesized without CA (C/M ratio0). The synthesized samples at different CA to metal ion molar ratios 0, 0.5, 1, 1.5 were named as CM0, CM0.5, CM1, and CM1.5, respectively.

2.3 Characterization

Phase analysis of samples was performed by the X-ray diffractometer (XRD, Rigaku Ultima IV) using Cu Kα radiation and graphite monochromator. The mean crystallite size (MCS) of nanoparticles was calculated by using the Scherrer equation:

$$D = \frac{{k\lambda }}{{\beta {\text{cos}}\theta }}$$
(1)

where β is the width of the diffraction peak at half-maximum intensity, λ is the X-ray wavelength, θ is diffraction angle, and k is a constant [32]. The morphology of nanoparticles was investigated by field emission scanning electron microscopy (FE-SEM, Tescan MiraII). Before the investigation, the powders were dispersed in the ethanol media by ultrasonication and dried on a slide. The size of one hundred particles in the FE-SEM micrographs were determined by the ImageJ software, and the particle size distribution graphs were drawn.

Magnetic properties of the samples were measured with the vibrating sample magnetometer (VSM, Meghnatis Daghigh Kavir, MDK6) at room temperature and the maximum applied field of ± 10 kOe. IR spectra were recorded by Fourier transform infrared spectrometer (FTIR, Shimadzu 8500) in the region 400–4000 cm−1.

3 Results and discussion

The XRD results of synthesized samples are shown in Fig. 1 (CM0, CM0.5, CM1, and CM1.5 related to the C/M ratio of 0, 0.5, 1, and 1, respectively). The obtained diffraction peaks are well matched with standard patterns of cubic CoFe2O4 (JCDPS card no. 22-1086). The diffraction peaks located in 2θ = 18.2°, 30.0°, 35.4°, 37.0°, 43.0°, 53.4°, 56.9°, 62.5°, 65.7°, 70.9°, 74.0°, 75.0°, and 78.9° are related to (111), (220), (311), (222), (400), (422), (511), (440), (531), (620), (533), (622), and (444) crystallographic planes of CF’s cubic spinel structure. Also, in the presence of citric acid, no impurity phase peaks were detected. Nevertheless, in the absence of citric acid, hematite (α-Fe2O3, JCPDS card no. 33-0664) phase trace has been detected. In addition, cobalt ferrite’s diffraction peaks became sharper and narrower, which indicates the increase in crystallinity and crystallite size.

Fig. 1
figure 1

XRD patterns of synthesized samples

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Hematite and goethite impurities were observed in other researches, and the presence of these phases is attributed to the difference in hydrolysis rates of iron and cobalt cations [18]. During the hydrothermal process, first, the hydroxide and oxyhydroxide species are formed. In the next step, these species are converted to the oxide phase. Researches show that the hydrolysis constant of Fe3+ is higher than that of Co2+, which means that the tendency of Fe3+ for hydroxide formation is higher than that of Co2+ [33]. This higher tendency during the reaction can lead to forming iron hydroxide-rich areas that are eventually converted into hematite. Also, one of the purposes and benefits of adding citric acid is to control the hydrolysis rate [34].

The average crystallite sizes were calculated from the most intense peak (311) by the Scherrer equation. The obtained results are presented in Table 1. The mean crystallite size of samples in C/M 0.5, 1, and 1.5 is 7.8, 4.7, and 7.3 nm, respectively. The mean crystallite size of CM1 is smaller than the other samples. The results confirm that different concentrations of citric acid have different effects and roles.

Table 1 Mean crystallite size of synthesized cobalt ferrite nanoparticle
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The proposed mechanism for citric acid-assisted hydrothermal included three main steps: (i) citrate-metal ion complex formation, (ii) decomposition of citrate-metal ion complexes and conversion to the metal hydroxide phases, (iii) conversion of metal hydroxide to the desired phase during the hydrothermal procedure [35, 36]. However, dissolution–crystallization is considered the primary mechanism of the hydrothermal process. The hydrothermal synthesis method consists of three main steps: (i) hydration (Fe(OH)3, Fe(OH)4ˉ, Co(OH)2, Co(OH)3ˉ formation), (ii) dehydration, (iii) nucleation and growth of oxide phase [37, 38]. The FTIR analysis was performed to ensure citric-metal complexes formed during the precursor preparation process. The IR spectra of the precursors (suspension obtained after the addition of sodium hydroxide to cobalt, iron, and citrate acid solution, before the hydrothermal treatment) are shown in Fig. 2. The absorption peaks at 1361 and 1591 cm−1 attributed to the asymmetric and symmetric stretching vibrations of carboxyl ions (COOˉ) in the metal carboxylates, respectively, which confirmed the citrate-metal ion complex formation. Also, the absorption band at 622, 840, and 1155 cm−1 corresponded to goethite (α-FeOOH), nitrate ion, and lepidocrocite (γ-FeOOH), respectively [39, 40].

Fig. 2
figure 2

FTIR spectra of precursors at C/M ratio (a) 0.5 (b) 1

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The FTIR analysis results demonstrated that the prepared precursors were contained citrate-metal complexes and iron oxyhydroxide. As mentioned above, the citrate-metal ion complexes were decomposed, and metal ions were released when the temperature increases during the hydrothermal treatment. The released metal ions form metal hydroxides in the alkali medium. Therefore, when the citrate-metal ion complexes formed, hydrothermal synthesis steps were postponed and occurred at a higher temperature. One of these steps is nucleation.

Citric acid has three carboxylic acid groups in its structure. These groups can be bonded to the metal ion in different geometries, such as monodentate bonding, bidentate chelating bond, and bidentate bridging, to a form citrate-metal ion complex. Different bond geometries give rise to complexes with different geometries; for instance, 35 conformers have been identified for the zinc-citrate complex. In addition, different complex geometries have different global energy conformation and stability [41]. Different stability means, the more stable complex needed more energy to decompose. So it can be assumed that the formed complexes with C/M ratio 1 might have different geometries and higher stability than those formed at 0.5 and 1.5 ratios. Complexes with higher stability decomposed at higher temperatures; thus, nucleation has occurred at the higher temperatures.

Based on the classical theory of nucleation and growth, nucleation rate (dN/dt) can be expressed as:

$$\frac{{dN}}{{dt}} = A\exp \left[ { - \frac{{16\pi V_{m}^{2} \gamma ^{3} }}{{3k_{B}^{3} T^{3} N_{A}^{2} (\ln S)^{2} }}} \right]$$
(2)

N, NA, T, kB, A, S, Vm, and γ are the number of nuclei, Avogadro’s number, temperature, the Boltzmann constant, the pre-exponential factor, supersaturation, and surface free energy per unit area. Therefore, the nucleation rate increases exponentially when the temperature increases [42]. Stable citrate-metal ion complex formation caused that nucleation occurred at higher temperatures, and the nucleation rate increased. Increasing the nucleation rate increases the number of produced nuclei and thus decreases the size of the final crystallites.

The FESEM micrographs of the synthesized cobalt ferrite nanoparticles are presented in Fig. 3. It can be seen that as-synthesized particles had the nanometer size, uniform, narrow particle size distribution, and irregular shape. Fine particles stuck together and formed agglomerates. Furthermore, the addition of CA dramatically influenced the particle size, and the particles became finer. The sample obtained at C/M ratio 1 exhibited a smaller particle size than synthesized samples at 0.5 and 1.5. Therefore, the C/M ratio affected not only crystallite size but also particle size. Also, it seems that the synthesized particles at C/M ratio 1 show a greater tendency to agglomerate. Besides, some nanorod shape particles are also observed in the micrograph of sample CM0, as shown in Fig. 3a. These nanorods can be attributed to hematite impurities in which its trace was detected in the XRD pattern. Furthermore, hematite nanorods synthesis using the hydrothermal method was reported [43, 44].

Fig. 3
figure 3

FESEM micrograph of the samples (a) CM0, (b) CM0.5, (c) CM1, (d) CM1.5

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The magnetic hysteresis loops (M-H) of the synthesized nanoparticles measured at room temperature are shown in Fig. 4. The maximum magnetization (Mmax), romance magnetization (Mr), and coercivity (Hc) values obtained from magnetic hysteresis loops are listed in Table 2. The Mmax values were decreased from 50.3 to 40 emu/g with increasing the C/M ratio from 0.5 to 1 and then increased to 48.3 emu/g for C/M ratio 1.5. These values are lower compared to the sample prepared without CA (58.4 emu/g) and saturation magnetization value of bulk cobalt ferrite 80 emu/g [45]. Generally, cation distribution, impurity, and particle size are effective factors in the ferrite nanoparticle’s magnetization. However, similar trends were observed in Mmax, MCS, and particle size values. Therefore, the decrease in Mmax values could be due to the particle size decreasing and the existence of spin canting layer. The disordered spin orientation or spin-canted layer exists on the surface of magnetic nanoparticles. These disordered spins lead to a decrease in the magnetization of magnetic particles. As the size of the magnetic nanoparticles decreases, the surface to volume ratio increases, and so the effect of the spin-canted layer on the total magnetization of particle increases [46].

Fig. 4
figure 4

VSM results of the samples

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Table 2 Magnetic properties and mean crystallite size of the samples
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The coercivity values have been changed from 236 to 4 Oe and then to 261 Oe by the variation of the C/M ratio from 0.5 to 1, and 1.5, respectively. Results revealed that the coercivity of nanoparticles was decreased with particle size decreasing. Generally, coercivity increases with decreasing particle size until reaching a maximum value at the single-domain critical diameter of particles. Then coercivity decreases until reaching zero in the superparamagnetic region. In the single-domain state, with the decrease in the particle size, the effect of thermal agitation becomes more effective. Therefore, the coercivity decreases with the decrease in particle size. The single-domain critical diameter for cobalt ferrite was reported to be 40–70 nm, and related coercivity is 4650 Oe [47,48,49]. It can be concluded that synthesized particles are single domain, and coercivity values were decreased because of particle size decreases.

For comparison purposes, obtained results in the present research and other researches on the synthesis of cobalt ferrite by various methods are presented in Table 3. Compared to superparamagnetic nanoparticles synthesized by other methods, CF nanoparticles synthesized in C/M ratio 1 have the smallest MCS, while its Mmax is acceptable. By a small and simple change in the concentration of citric acid, a range of properties of cobalt ferrite nanoparticles can be obtained.

Table 3 A summary of the results obtained for cobalt ferrite nanoparticles from this research and other researches
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4 Conclusion

In this research, the role of citric acid concentration in the hydrothermal synthesis of cobalt ferrite nanoparticles was investigated. The citric acid to the metal ion ratio (C/M) was considered as a criterion of citric acid concentration. The XRD, FE-SEM, and VSM results revealed that the mean crystallite size (MCS), particle size, and magnetic properties of synthesized nanoparticles fluctuated by the variation of the C/M ratio. The X-ray diffraction analysis exhibited that single-phase cobalt ferrite was obtained in the presence of citric acid. Besides, the crystallite size first has decreased and then increased by increasing the concentration of citric acid according to the Scherrer equation. The FE-SEM micrographs demonstrated uniform particle size distribution and irregular morphology of synthesized cobalt ferrite nanoparticles, and the citric acid concentration affects the nanoparticles’ size. The FTIR spectrums of precursors revealed citrate-metal ion complexes were formed during the precursor preparation.

Moreover, cobalt ferrite nanoparticles synthesized with C/M = 0.5 exhibited Mmax value of 50 emu/g and MCS of 7.8 nm. The Mmax (40 emu/g) and MCS (4 nm) values decreased when the C/M ratio increased to 1. Further, increasing C/M to 1.5 increased Mmax (48.2 emu/g) and MCS (7.3 nm). Therefore, similar trends in particle size, MCS, and Mmax values were observed. It can be concluded that Mmax fluctuations are due to particle size variations. These changes can be due to the formation of complexes with different geometry and stability in different citric acid concentrations, which affected nucleation temperature. Therefore, the particle size, crystal size, and magnetic properties are affected by the variation of the nucleation temperature and the number of created nuclei. As a result, cobalt ferrite nanoparticles with desired magnetic properties and particle size can be synthesized by adjusting the concentration of citric acid.