A controllable one-pot hydrothermal synthesis of spherical cobalt ferrite nanoparticles: synthesis, characterization, and optical properties

Abstract

We herein report the controllable synthesis of spherical cobalt ferrite nanoparticles with average crystallite size in the range of 3.6–12.9 nm using a facile, eco-friendly, hydrothermal method. The hydrothermal treatment was carried out by utilizing cobalt nitrate, ferric nitrate, and ammonium hydroxide in the presence and absence of Arabic gum as a surfactant agent. The purity and crystallinity of the products were tuned by varying reaction conditions such as reaction time (0.5–8 h), reaction temperature (120–180 °C), percentage of ethylene glycol (0–100% (v/v)), pH (8–9.6), and amount of Arabic gum (0–2 g). We characterized the prepared products using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), Fourier transform infrared spectroscopy (FT-IR), high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy analysis (EDS), selected area electron diffraction (SAED) patterns, and UV-visible diffuse reflectance spectra (DRS). The optimal hydrothermal treatment was performed at 180 °C and pH 9.6 for 4 h in aqueous media. The results also revealed that the as-prepared spinel cobalt ferrite nanoparticles have an estimated optical band gap energy in the range of ca. 1.6–1.9 eV, indicating the semiconducting characteristics of the products.

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1. Introduction

Spinel ferrite nanoparticles have recently attracted significant attention from researchers owing to their wide range of applications in different fields like catalysts, refractory materials, microwave absorbing materials, biomedical materials, high-frequency magnetic materials, electrical devices, antibacterial materials, gas sensors, water decontaminants, and energy materials.^1–8 Spinel ferrites have the general formula of (M)^A(Fe₂)^BO₄, where M stands for divalent metal cations such as Co, Ni, Mg, Mn, Cu, Zn, Cd, etc. Their properties are attributed to the cation distribution because of the existence of altered distribution of cations in A (tetrahedral site (T_d)) and B (octahedral site (O_h)) sites.⁵ The spinel ferrite structures depend mainly on the cation distribution, so they are categorized into three types: normal, inverse, and mixed spinels.^9–11 Because of its excellent mechanical hardness, high chemical stability, and distinctive electrical, magnetic, and optical properties, CoFe₂O₄ has received a lot of attention among ferrites. Cobalt ferrite has thus been applied for various applications including catalysis, microwave devices, data storage, drug delivery, magnetic resonance imaging, ferrofluid recording media, gas sensors, water treatment, and lithium-ion rechargeable batteries.^2,12–16

Cobalt ferrite nanoparticles have been synthesized using various synthesis methods such as mechanical milling, laser ablation, co-precipitation, plasma synthesis, hydrothermal, auto-combustion, microwave, sol–gel, and microemulsion.^14,17,18 Choice of the synthetic route and the applied reaction conditions have a great impact on the characteristics of the produced CoFe₂O₄ nanoparticles such as morphology, crystallinity, surface area, as well as chemical and physical properties.^17,18 Among the synthesis routes, hydrothermal synthesis is a controllable, facile, eco-friendly, and energy-efficient route for the synthesis of pure CoFe₂O₄ nanoparticles with tunable particle size.^14,19–21 However, a significant number of the published reports claimed that CoFe₂O₄ NPs were prepared in a two-step process; where the hydrothermally prepared precursors were subsequently thermally treated to produce CoFe₂O₄ NPs. In this light, Bastianello and his colleagues prepared CoFe₂O₄ NPs via a hydrothermal method at 135 °C for 72 h and investigated the effect of the calcination at different temperatures on the electronic conduction properties.²² Anila et al. prepared cobalt ferrite nanoparticles using a hydrothermal technique at 180 °C for 12 h, and then the final products were obtained by annealing at 550 °C. Nassar et al. synthesized cobalt ferrite nanoparticles via a hydrothermal technique by hydrothermal preparation of nanocomposites of cobalt and iron carbonates followed by their thermal decomposition.¹⁴ On the other hand, Zhijun et al. prepared cobalt ferrite nanoparticles by rapid mixing metal cations with sodium borohydride as a reducing agent and simultaneous reduction in a colloid mill at 6000 rpm for 2 min, then the produced slurry was transferred to an autoclave at 120 °C for 12 h.²³ Besides, Guangbin and coworkers hydrothermally synthesized CoFe₂O₄ NPs at 130 °C for 15 h in the presence of cetyltrimethylammonium bromide (CTAB) as a surfactant.²⁴ Varma et al. have studied the preparation of CoFe₂O₄ NPs by solid state, citrate precursor, and polymerized complex methods.²⁵ However, to the best of our knowledge, we have not noticed a detailed investigation about tuning the morphology, phase purity, and particle size of cobalt ferrite nanoparticles via a one-pot hydrothermal method without subsequent heat treatment. This prompted us to explore extensively the preparation of CoFe₂O₄ NPs by developing a one-pot, time-saving hydrothermal route without further annealing or using complex equipment. Moreover, exploring the growth mechanism of the prepared cobalt ferrite particles is crucial for practical use.

Herein, in the present work, we have developed a one-pot, facile, and eco-friendly method for the synthesis of cobalt ferrite nanoparticles by hydrothermal treatment of ammonical solutions of cobalt and iron nitrates in 4 h without subsequent heat treatment or complex equipment. Various factors influencing the hydrothermal process such as reaction temperature, reaction time, ethylene glycol : water ratio, pH, and the amount of Arabic gum have been investigated. Additionally, we have explored the growth mechanism of the prepared cobalt ferrite particles. We have characterized the morphology, composition, crystallite size, and band gap of the products by using XRD, FE-SEM, HR-TEM, SAED, EDS, FT-IR spectra, and UV-visible diffuse reflectance (DRS).

2. Experimental

2.1 Materials and reagents

Ferric nitrate (Fe(NO₃)₃·9H₂O) and ethylene glycol (EG) were purchased from Sd Fine-Chem. Limited (SDFCL), India. Cobalt nitrate (Co(NO₃)₂·6H₂O) was bought from Alpha Chemika, India. El-Nasr Pharmaceutical Chemicals company (ADWIC), Egypt supplied us with an ammonia solution (25%), ethanol (70%), and sodium hydroxide pellets. Arabic gum (AG) was obtained from LOBA Chemie, India. All of the materials and reagents were utilized as received with no further purification.

2.2 Preparation of cobalt ferrite nanoparticles

Cobalt ferrite nanoparticles were synthesized by a hydrothermal method. This was performed by dissolving ferric nitrate (6.4 g, 15.84 mmol, 2 eq.) and cobalt nitrate (2.305 g, 7.92 mmol, 1 eq.), separately, in 40 mL EG, then both solutions were mixed with stirring. The produced solution was heated at 70 °C. To the heated and stirring solution, an aqueous ammonia solution (3 M, 50 mL) was added dropwise. Afterward, the brown reaction blend was allowed to stir for 30 min at 70 °C then transferred to a Teflon-lined autoclave. The autoclave was then sealed and placed in an oven set up at 180 °C for 4 h. After the autoclave was allowed to get the ambient temperature naturally, the black precipitate was separated and washed with bi-distilled water and ethanol (70%) until the pH value of the washing became neutral. The separated precipitate was dried at 100 °C for ca. 5 h in an electric oven. For comparison, a sample was prepared using similar conditions except for the hydrothermal treatment step. The produced sample was denoted as CF. On the other hand, we studied the effects of several parameters on the hydrothermal experiment such as (a) reaction time (0.5, 4, and 8 h); (b) temperature (180, 150, and 120 °C); (c) the amount of EG (100%, 75%, 50%, 25%, 0% (v/v)); (d) the pH value by using ammonia solution (9.6, 9, and 8); and (e) the amount of AG (0.5, 1, and 2 g). The hydrothermally prepared samples were referred to as listed in Table 1. The preparation method is outlined in Scheme 1. The proposed reaction mechanism for the formation of cobalt ferrite nanoparticles is presented in Scheme 2.

Table 1 Symbols and D_XRD values of the hydrothermally prepared samples

Sample	Factor under study	DXRD (nm)	Sample	Factor under study	DXRD (nm)
CF-0.5	0.5 h	3.6	CF-EG25	25% EG	8.5
CF-4	4 h	7.8	CF-W	Water	12.9
CF-8	8 h	8.5	CF-W-9.6	3 M, 50 mL	12.9
CF-180	180 °C	7.8	CF-W-9	3 M, 36 mL	12.7
CF-150	150 °C	6.1	CF-W-8	3 M, 30 mL	12.3
CF-120	120 °C	4.5	CF-AG-0	Without AG	12.9
CF-EG100	100% EG	7.8	CF-AG-0.5	0.5 g AG	12.4
CF-EG75	75% EG	8.1	CF-AG-1	1 g AG	12.1
CF-EG50	50% EG	8.4	CF-AG-2	2 g AG	11.1

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Scheme 1 Preparation of cobalt ferrite nanoparticles by hydrothermal method.

Scheme 2 Proposed reaction mechanism of cobalt ferrite nanoparticles formation.

2.3 Characterization of cobalt ferrite nanoparticles

The crystallographic studies of CoFe₂O₄ NPs were obtained using an X-ray powder diffractometer (Bruker Co. D8 Advanced) with Cu Kα radiation source (λ = 1.5406 Å), a generator voltage of 40 kV and 40 mA generator current (Central Metallurgical R&D Institute Cairo, Egypt). The powder sample was scanned over a 2θ range of 10–80° at a scanning rate of 0.02°/0.4 s. Fourier transform infrared (FT-IR) spectra were obtained using (Thermo Fisher, Nicolet IS10) spectrometer (Mansoura University, Faculty of Science, Mansoura, Egypt). The FT-IR spectra of the prepared samples were recorded at room temperature in the range from 4000 to 400 cm⁻¹ to characterize the bond formation and functional groups of the samples under investigation. The morphology characteristics of cobalt ferrite nanoparticles and their texture were examined by HR-TEM analysis and SAED patterns. The high-resolution transmission electron microscopy (HR-TEM) as well as the corresponding selected area electron diffraction (SAED) were performed by a (JEOL, JEM-2100) electron microscope equipped with a high-resolution gun, operated at an acceleration voltage of 200 kV (Nanotech Center, Dreamland, Cairo, Egypt). For the HR-TEM images, aqueous suspensions of the samples were ultrasonicated for 15 min then the samples were deposited on a carbon film supported on a copper grid. The morphology and the energy-dispersive X-ray spectroscopy (EDS) analysis for prepared samples were also investigated using field Emission Scanning Electron Microscope (FE-SEM) (Thermo Scientific, Quattro S, Field Emission Gun (FEG)) (Nanotechnology Research Center, British University, Cairo, Egypt). The UV-vis diffuse reflectance spectra of prepared products were performed in the range of 200–1200 nm by using a UV-vis spectrophotometer (Jasco, Model v670) connected to an integral sphere (Jasco, Model ISN-723; Benha University, Egypt).

3. Results and discussion

3.1 Synthesis and characterization of CoFe₂O₄ nanostructures

3.1.1. Synthesis of CoFe₂O₄ nanostructures. CoFe₂O₄ nanoparticles were synthesized using a facile, one-pot, hydrothermal method. This was performed by hydrothermal treatment of inexpensive materials such as iron nitrate, cobalt nitrate, and ammonia solution for a relatively short time without subsequent thermal treatment. The products of the hydrothermal treatment were tuned by studying various experimental conditions affecting the hydrothermal reactions like the reaction time, temperature, amount of EG, pH value, and amount of AG. The optimization process of the hydrothermal treatment was mainly studied using the XRD technique, as will be discussed shortly. The crystallite sizes (D_XRD) of the produced CoFe₂O₄ products of the optimization process are tabulated in Table 1. Several other techniques such as FE-SEM, TEM, FT-IR, UV-vis, and DRS were used to characterize the products.

3.1.2. Effect of reaction time. We have investigated the influence of the reaction time of the hydrothermal treatment on the preparation process of cobalt ferrite nanoparticles. The XRD patterns of the products generated at various hydrothermal treatment periods are shown in Fig. 1. The distinctive Bragg diffraction peaks of all the hydrothermal treatment samples are well indexed at 2θ values of 18.2° (111), 30.08° (220), 35.4° (311), 43.05° (400), 53.4° (422), 56.9° (511), 62.5° (440), and 74.01° (533). These peaks correspond to database reference pattern JCPDS card no. 022-1086 (ref. 26) with a cubic spinel structure of cobalt ferrite, space group of Fd3m (227), and lattice parameters of a = b = c = 8.39 Å, a = β = γ = 90. We have not detected peaks from other crystalline impurities. As depicted in Fig. 1, increasing the hydrothermal reaction time from 0.5 to 8 h brought about enhancing the sharpness and narrowness of the XRD reflection peaks of the products. It can be concluded that the longer the hydrothermal period of the reactions, the more the crystallinity of the products. The average crystallite sizes (D_XRD) of the prepared samples were estimated by using the Debye–Scherrer formula (1).^11,27,28

D_XRD = Kλ/β cos θ (1)

where D_XRD stands for crystallite size (nm), K is a constant related to the shape of the crystal (=0.89), β is the full width of the diffraction line at the half-maximum intensity (FWHM) in radians, λ is the X-ray wavelength (λ = 1.5406 Å for Cu Kα), and θ refers to Bragg's angle of diffraction. It was observed that the lowest average crystallite size was found to be 3.6 nm for CF-0.5 sample while the largest average crystallite size was 8.5 nm for CF-8 sample. However, we have chosen the optimum hydrothermal reaction time to be 4 h since it gave CF-4 product with good crystallinity. Besides, this hydrothermal reaction time (4 h) saved time and energy as well as there is no significant difference between CF-4 and CF-8. The growth and nucleation of CoFe₂O₄ particles of can be attributed to the Ostwald ripening process. The Ostwald ripening process is the dissolution of small size particles which are redeposited into large size particles by passing time. This process may occur because smaller particles have higher surface energy and total Gibbs energy than larger particles, which are energetically undesirable.^29–34 Notably, our results are consistent with those reported in the literature.³¹ For instance, Cabuil et al. found that the mean size of the prepared CoFe₂O₄ NPs by the hydrothermal method was slightly larger than the mean diameter of the particles obtained with the usual co-precipitation method, which is probably due to the Ostwald ripening process.³¹ Yang et al. prepared hollow anatase TiO₂ nanospheres via Ostwald ripening under hydrothermal conditions. They demonstrated the crystallinity of the products increased gradually with increasing the reaction time from 2 to 100 h.³⁴ Medeiros et al. concluded that the growth of crystallites of CoFe₂O₄ NPs by the hydrothermal method was enhanced by increasing the reaction time from 2 to 6 h.³²

Fig. 1 XRD patterns of the prepared sample using co-precipitation method (CF), and the hydrothermal products: CF-0.5, CF-4, and CF-8, prepared at different reaction times: 0.5, 4, and 8 h, respectively.

For comparison, we carried out the reaction of cobalt nitrate and iron nitrate in the presence of ammonium hydroxide using the usual co-precipitation method at 70 °C. However, the co-precipitation reaction did not produce pure cobalt ferrite nanoparticles and produced a semi-amorphous cubic cobalt ferrite (CF) with JCPDS card no. 022-1086 (ref. 26) along with other phases such as hematite with JCPDS card no. 089-0598,³⁵ as shown in Fig. 1. From the research that has been carried out, it is possible to conclude that the co-precipitation method was insufficient to give a pure cobalt ferrite phase while the hydrothermal procedure produced the desired CoFe₂O₄ product in single straightforward step without the need for a post thermal treatment.

3.1.3. Effect of temperature. The influence of the reaction temperature (120–180 °C) on the hydrothermal reaction of interest was studied. The XRD patterns of the products are displayed in Fig. 2. As shown in Fig. 2, the temperature of the hydrothermal process has a remarkable effect on the crystallinity and purity of the products. The data revealed that 180 °C reaction temperature produced pure cobalt ferrite product with an average crystallite size of 7.8 nm while decreasing the temperature influenced negatively the crystallinity and purity of the CoFe₂O₄ product. Lower reaction temperatures; 150 and 120 °C, generated CoFe₂O₄ products; CF-150 and CF-120, with lower crystallite sizes of 6.1 and 4.5 nm, respectively, along with other phases. Among these impurities was hematite (JCPDS card no. 15-0615)³⁶ and the corresponding diffraction peaks appeared at 2θ values of 31.9° (009), 33.9° (109), 38.8° (209), and 40.3° (316). Thus, the optimum reaction temperature for this hydrothermal treatment was selected to be 180 °C for the subsequent reactions. The high purity of the hydrothermally prepared cobalt ferrite product is probably owing to the precursors might be more reactive at higher temperatures and the lattice rearrangement was facilitated.²⁹ The obtained results are in good agreement with those reported by Abdulwahab et al. who reported on the synthesis of monodispersed iron cobalt oxide (Fe₂CoO₄) and iron manganese oxide (Mn_0.43Fe_2.57O₄) NPs using bimetallic pivalate clusters of [Fe₂CoO(O₂CtBu)₆(HO₂CtBu)₃], [Co₄Fe₂O₂(O₂CtBu)₁₀(MeCN)₂], and [Fe₂MnO(O₂CtBu)₆(HO₂CtBu)₃], respectively, as single source precursors.²⁹

Fig. 2 XRD patterns of the hydrothermal products: CF-120, CF-150, and CF-180, prepared at different reaction temperatures: 120, 150, and 180 °C, respectively.

3.1.4. Effect of solvent. The effect of solvent on the hydrothermal preparation of cobalt ferrite nanoparticles was also investigated. This was performed by carrying out the hydrothermal reactions in different solvents: ethylene glycol (EG), water (W), and mixtures of them (% EG (v/v)). The obtained XRD patterns are presented in Fig. 3. The XRD results revealed that the intensity of the diffraction peaks gradually enhanced as the ratio of ethylene glycol was reduced. The crystallite size of the products increased from 7.8 nm (for 100% ethylene glycol) to 12.9 nm (for 100% water). Based on these results, it could be concluded that the decrease of the crystallinity in the cobalt ferrite product when ethylene glycol (100%) was applied as a pure solvent might be attributed to the issue that ethylene glycol served as a capping agent during the growth of the product nuclei. Besides, ethylene glycol might form complexes with the metal ions of the utilized metal precursors which might restrict also the growth rate and formation of large crystallite-size products.³⁷ Additionally, the viscosity of water is lower than that of ethylene glycol. Consequently, the mobility of ions in water is higher which may bring about that the growth rate may exceed the nucleation rate, leading to the formation of larger cobalt ferrite particles in aqueous media.^38,39 We thus selected water solvent as the optimum solvent for this study due to the aforementioned reasons as well as it is the most environmentally friendly and inexpensive solvent compared to other solvents.³⁸ These results are compatible with those reported by Mousavi Ghahfarokhi et al. They investigated the effect of ethylene glycol as a surfactant on the magnetic, structural, and optical properties of SrFe₂O₄ NPs using ethylene glycol with different weight percentages. They concluded that by increasing the weight percentages of the applied ethylene glycol, the intensity of diffraction peaks and consequently the crystallinity of the prepared samples was reduced.³⁷ Besides, Rishikeshi et al. studied the effects of the use of varying chain length glycols (ethylene glycol (EG), diethylene glycol (DEG), and polyethylene glycol 400 (PEG)) as solvents. The crystallite size for ZnFe₂O₄ products decreased by increasing the chain length of the used ethylene glycol derivatives. They also concluded that the degree of crystallinity of the zinc ferrite products may be related to the structure of the intermediately formed complex as well.⁴⁰

Fig. 3 XRD patterns of the hydrothermal products: CF-W, CF-EG25, CF-EG50, CF-EG75, and CF-EG100, prepared using different solvents: 100% water, 25% EG, 50% EG, 75% EG, and 100% EG (v/v), respectively.

3.1.5. Effect of pH. The pH of the reaction medium was adjusted at different pH values (8, 9, and 9.6) by using the proper volume of ammonia solution 3 M. The XRD patterns of the products are depicted in Fig. 4. The results exhibited that the crystallinity of the products increased by increasing the pH value. The optimum pH value was chosen to be 9.6. These results can also be explained as follows. Cobalt ions [Co(H₂O)₆]²⁺ precipitated by raising the pH values and the major precipitated molecules might be Co(OH)_x^2−x which could be changed into more insoluble molecules (Co(OH)₂). In addition, cobalt salt has better solubility than iron salt, so cobalt ions require a higher pH value to precipitate.^32,41 The color of the supernatant after the hydrothermal reaction of prepared samples at 8 and 9 pH values was pale pink, evidence of the existence of the remaining cobalt, which was confirmed and detected by sodium hydroxide solution, leading to the formation of a blue precipitate. While at pH 9.6 value, the color of the supernatant was colorless, indicating the completion of the hydrothermal reaction. Similar results were reported by others.^32,41–44 For instance, Medeiros et al. investigated the influence of the pH factor on the crystallite size by means of the experimental design of CoFe₂O₄ NPs, which were prepared by the EDTA/citrate complexing method and the hydrothermal method. They indicated that the enhancement of the crystallinity of the products may be related to the hydrolysis step dependent on pH value. Where in the presence of excess sodium hydroxide (i.e. high pH), the formed cobalt hydroxide is surrounded by a large number of hydroxyl groups, leading to the production of large particles.³² Giri et al. optimized the pH parameter for the preparation of Co_1−xZn_xFe₂O₄, (0 ≤ x ≤ 0.8) NPs by the microwave refluxing method and concluded that the crystallinity of prepared samples increased with rising pH value.⁴³ Safi et al. synthesized cobalt ferrite nanoparticles by the co-precipitation method at different pH values. The XRD analysis of the products indicated that the prepared samples at higher pH values were pure CoFe₂O₄ phase, while the samples prepared at lower pH values showed α-Fe₂O₃ phase in addition to the CoFe₂O₄ phase.⁴¹

Fig. 4 XRD patterns of the hydrothermal products: CF-W-8, CF-W-9, and CF-W-9.6, prepared using different pH values: 8, 9, and 9.6, respectively.

3.1.6. Effect of using Arabic gum (AG) as surfactant. We investigated the influence of the presence of different amounts of Arabic gum as surfactant on the hydrothermal preparation of cobalt ferrite under the previously obtained optimum conditions. The XRD patterns of the obtained products are presented in Fig. 5. The results exhibited that on increasing the amount of AG in the hydrothermal treatment of interest the diffraction peaks of CoFe₂O₄ tend to be broader and the crystallite size gets lower. This is probably due to serving the AG agent as a surfactant in this reaction. Besides, cobalt hydroxide and ferric hydroxide can intercalate with the positive groups of AG and create ion pairs through electrostatic interactions that reduce the surface energy and surface tension of coated cobalt ferrite nanoparticles. Accordingly, the nucleation energy barrier decreased, and the nucleation rate became faster. As a result, uniform growth occurred and gave a narrow-size distribution.^45,46 By inspection of XRD patterns, there is an additional XRD diffraction peak appeared at the 2-theta position (24.4°) for the samples prepared using >0.5 g of Arabic gum due to the residue of Arabic gum. Therefore, the appropriate amount of arabic gum was chosen to be 0.5 g for the present hydrothermal preparation. Other research groups reported a similar trend as follows. Akbari et al. studied the influence of polyvinyl alcohol (PVA) on the characteristics of cobalt ferrite NPs prepared by the co-precipitation method and followed by calcination at 700 °C. By adding PVA, the peaks of XRD became broader, and the estimated crystallite sizes of the prepared samples were reduced by increasing the amount of polyvinyl alcohol.⁴⁵ Amanda et al. presented a hydrothermal route for the preparation of different metal ferrites using various types of surfactants. They demonstrated that varying the surfactant had a significant impact on the shape and particle size of the generated CoFe₂O₄ NPs.⁴⁶ Besides, Ansari et al. prepared tunable CoFe₂O₄ nanoparticles using an oleic acid-based solvothermal method. By increasing the oleic acid concentration, the average crystallite size was reduced.⁴⁷

Fig. 5 XRD patterns of the hydrothermal products: CF-AG-0, CF-AG-0.5, CF-AG-1, and CF-AG-2 prepared using different amounts of AG: 0, 0.5, 1.0, and 2.0 g, respectively.

3.2 The reaction mechanism

The mechanism of the hydrothermal formation of cobalt ferrite can be explained based on the dissolution, nucleation, and growth process mechanism (Scheme 2). These reactions can proceed as follows: (1) dissolution of cobalt and ferric nitrate salts according to eqn (1) and (2) (Scheme 2). Afterward, cobalt and iron aqua complexes underwent hydrolysis in the presence of ammonium hydroxide solution, forming cobalt and ferric hydroxides based on eqn (3) and (4) (Scheme 2). Finally, under hydrothermal conditions, cobalt and ferric hydroxides may undergo dehydration followed by the growth of cobalt ferrite (eqn (5), Scheme 2).⁴⁸

3.3 Fourier transform infrared investigation (FT-IR)

We investigated the hydrothermally prepared products by using FT-IR spectroscopy to identify their chemical composition and functional groups, as displayed in Fig. 6. The FT-IR spectra of all samples showed two bands at ca. 416 and 590 cm⁻¹, owing to the vibrational frequencies of Co–O bonds in octahedral sites and Fe–O bonds in tetrahedral sites, respectively, suggesting the formation of the cobalt ferrite NPs phase.^49–51 In addition, the bands appeared at ca. 3385 and 1616 cm⁻¹ are related to O–H stretching vibration and H–O–H bending vibration, respectively, originating from moisture and residue of ethylene glycol and Arabic gum.^14,43,52,53 Notably, the broad band at ca. 3385 cm⁻¹ is probably due to overlapped O–H and N–H stretching vibrations.⁵⁴ However, Fig. 6(e) exhibited vibrational bands at ca. 2925 and 2859 cm⁻¹ which might be attributed to asymmetric and symmetric stretching vibrations, respectively, of the CH₂ groups of the organic residue of Arabic gum.^20,43,55 This figure also showed stretching vibration of C=O (ca.1743 cm⁻¹),^56,57 bands of asymmetric and symmetric stretching vibrations of the COO⁻ (ca. 1555 and 1517 cm⁻¹, respectively),^20,58 and bending vibration of H–C–H (ca. 1463 cm⁻¹)⁴³ of the used excess Arabic gum in the hydrothermal treatment of interest.

Fig. 6 FTIR spectra of the hydrothermally prepared samples at different parameters (a) time reaction (0.5, 4, and 8 h); (b) temperature reaction (120, 150, and 180 °C); (c) solvent (100% water, 25% EG, 50% EG, 75% EG, and 100% EG); (d) pH value (8, 9, and 9.6); and (e) Arabic gum amount (0, 0.5, 1, and 2 g).

3.4 The morphology

The morphological characteristics of cobalt ferrite nanoparticles were examined by FE-SEM, HR-TEM, and selected area electron diffraction (SAED) techniques. The particle size of CoFe₂O₄ NPs was calculated using the corresponding size distribution histograms. Fig. 7(a–h) displays the FE-SEM images of CF-EG100, CF-EG50, CF-W, and CF-AG-0.5 samples at different magnifications. The FE-SEM images of the samples showed spherical agglomerated particles of sizes below 15 nm. This agglomeration is due to the magnetic interaction and van der Waals forces between the nanoparticles.^11,59,60 On the other hand, the HR-TEM images and the corresponding size distribution histograms were obtained for CF-AG-0 and CF-AG-0.5 samples and displayed in Fig. 8(a–f). The TEM images of both samples revealed that most particles are spherical in shape, but there are also some irregular shaped particles. The mean particle size was determined by calculating the diameter of 100 particles from several recorded images using Image j software and representing them in a histogram with normal distribution by the origin 2022 program. According to Sturges' criteria, the total number of bins (k) is calculated from this relation k = 1 + 3.322 log(N), where N is the total number of recorded particles and the bin width (W) is equal to W = (D_max − D_min)/k.⁶¹ The mean particle size was found to be ca. 10.8 and 10 nm for the CF-AG-0 and CF-AG-0.5 nanoparticles, respectively. These values are close to the crystallite size (D_XRD) values calculated from the XRD data. However, the disparity between the particle size values calculated by the two approaches could be attributed to only the largest particles are recorded in XRD, while in TEM, the size distribution with an average diameter is determined for a small number of particles.⁵⁰ Other research groups reported similar and different morphologies of CoFe₂O₄ nanoparticles. Gandhi et al. prepared spherical cobalt ferrite nanoparticles by co-precipitation method for magnetic hyperthermia,⁶² while Ratkovski et al. produced hexagonal cobalt ferrite nanoparticles by co-precipitation method for sensing phosphate ions in aqueous media and biological samples.⁶³

Fig. 7 FE-SEM images at different magnifications for CF-EG100 sample (a and b), CF-EG50 sample (c and d), CF-W sample (e and f), and CF-AG-0.5 sample (g and h).

Fig. 8 HR-TEM images at different magnifications for CF-AG-0 sample (a and b), and CF-AG-0.5 sample (d and e). The size distribution histograms for CF-AG-0 (c) and CF-AG-0.5 (f) samples.

In addition, the interplanar spacing was detected and presented in the insets of Fig. 9(a–c), which equaled 0.46 and 0.45 nm for CF-AG-0 and CF-AG-0.5, respectively. The corresponding SAED patterns are shown in Fig. 9(b–d) which display sharp and discrete rings can well be indexed to (111), (220), (311) and (400) planes (JCPDS card no. 022-1086)²⁶ for both samples. These results indicated a pure, crystalline spinel cubic structure of cobalt ferrite, which agreed well with the XRD results.

Fig. 9 HR-TEM images at high magnifications for CF-AG-0 sample (a); the inset presents the interplanar space for CF-AG-0 sample, and (c) CF-AG-0.5 sample; the inset is the interplanar space for CF-AG-0.5 sample. SAED patterns for CF-AG-0 sample (b), and (d) CF-AG-0.5 sample.

3.5 Energy-dispersive X-ray spectroscopy (EDS) analysis

EDS analysis was performed for CF-EG100, CF-W, and CF-AG-0.5 samples as depicted in Fig. 10 (a–c). The obtained data confirmed the successful preparation of cobalt ferrite since the samples contain only iron, cobalt, and oxygen elements in addition to traces of carbon element as impurities. The presence of carbon may be due to various factors, including the residue from ethylene glycol, arabic gum, or connection with any organic contaminants during the sample preparation for the EDS analysis. The Fe/Co molar ratios for the CF-EG100, CF-W, and CF-AG-0.5 were found to be 2.11, 1.93, and 2.14, respectively, as presented in the inset tables in Fig. 10, and these values are consistent with the corresponding theoretical molar ratios.

Fig. 10 EDS spectra of the CF-EG100 sample (a), CF-W sample (b), and CF-AG-0.5 sample (c).

3.6 The optical properties

The optical properties and band gap values for CF-AG-0 and CF-AG-0.5 samples were estimated by using UV-visible diffuse reflectance spectroscopy (DRS), Fig. 11. The diffuse reflectance spectroscopy (DRS) is a reliable technique for estimation of the band gap energy because this technique does not require special treatment for the samples.^64–66 The absorption profile is influenced by various factors such as synthetic method, lattice parameters, surface roughness, band gap, and impurity centers.⁶⁷ As depicted in the inset of Fig. 11, both CF-AG-0 and CF-AG-0.5 samples exhibited nearly the same absorption behavior with only one absorption edge in the visible region at wavelength of 765 and 735 nm, respectively. The optical band gap energy (E_g) can be determined by using the following relationship (2).⁶⁸ The estimated band gap values for CF-AG₀ and CF-AG_0.5 samples are 1.61 and 1.68 eV, respectively.where λ is the wavelength of the absorption edge, which is determined by the intercept on the wavelength axis as shown in the inset of Fig. 11.

Fig. 11 Tauc plot for CF-AG-0 sample (a), and CF-AG-0.5 sample (b); the insets of (a) and (b) are the UV-visible diffuse reflectance spectra (DRS) for CF-AG-0 and CF-AG-0.5 samples, respectively.

Besides, the band gap energy is also calculated by the Tauc method by using eqn (3).^69,70

αhν = A(hν − E_g)ⁿ (3)

where α, h, ν, A and n are absorption coefficient, Planck's constant, light frequency, proportionality constant, and the nature of optical transitions either direct allowed transitions (n = ½) or indirect allowed transitions (n = 2), respectively. In the literature, the kind of electronic transition for ferrite materials is not completely agreed, but it is clear that the direct allowed transition is more suitable for these materials.^69–72 The Schuster–Kubelka–Munk (SKM) function was applied to convert the reflectance data into the remission function F(R_∞), which gives the ratio between the absorption coefficient (K) and scattering coefficient (S) as seen in eqn (4).⁶⁹ Eqn (3) can be written as eqn (5) after substitution a = F(R) and n = 2.

(F(R)hν)² = A(hν − E_g) (5)

Through a plot of (F(R)hν)² versus E = hν, the band gap can be derived by extrapolating the linear section representing the absorption onset of the curve to intercept the energy axis at (F(R)hν)² = zero. The measured band gap values are 1.6 and 1.9 eV for CF-AG-0 (D_XRD = 12.9 nm) and CF-AG-0.5 (D_XRD = 12.4 nm), respectively. The band gap values increase as the crystallite size of the samples decreases, and these band gap values are greater than reported theoretical values for bulk cobalt ferrite (0.8 eV, 0.72 eV, and 0.63 eV).⁷³ This may be illustrated by the quantum confinement regime at nanoscale. Quantum confinement is an important phenomenon for semiconductor nanomaterials where electron and hole carriers are closer together at the nanoscale, enhancing interaction between them and confining them in a very narrow region, so more energy is required to separate them. As described in eqn (6), the shift in bulk band gap energy increases with size approximately to or less than the bulk exciton-Bohr radius.^68,71

where E_sh is the shift from bulk band gap energy, R is the crystallite radius, m_h and m_e are defined as the effective masses of the hole and electron carriers, respectively, S⃑ represents the position vector corresponding to the effective charge distribution of the system, and a_n is a quantity that is based on ε₁ and ε₂, where ε₁ and ε₂ are the dielectric coefficient of the material and surrounding medium, respectively. In the literature review, Andhare et al. demonstrated the influence of Zn doping on optical, structural, and magnetic properties of cobalt ferrite nanoparticles, which were synthesized by the co-precipitation method. The estimated optical band gap using the Tauc plot decreased from 2.831 eV to 2.258 eV related to increasing particle size from 12 nm to 17 nm with increasing Zn amount in Co_1−xZn_xFe₂O₄ where x = 0.0, 0.3, 0.5, 0.7, and 1.0. Additionally, Singh et al. studied the size-dependent behavior of CoFe₂O₄ NPs using synchrotron radiation-based techniques. For the optical behavior, the estimated optical band gaps were 1.90 ± 0.02, 1.85 ± 0.02, 1.80 ± 0.02, and 1.69 ± 0.02 eV for the annealed samples at 300, 500, 700, and 900 °C, respectively. It is clear that the band gap energy decreases with increasing the crystallite size. This observation may be attributed to the brass model.^68,71

4. Conclusions

In summary, tunable-size, pure, spherical cobalt ferrite nanoparticles were successfully synthesized by a facile, eco-friendly hydrothermal route within four hours using cobalt nitrate, ferric nitrate, ammonium hydroxide, and arabic gum. In this regard, we controlled the products' composition and crystallinity by changing the reaction conditions such as the reaction time (0.5–8 h), reaction temperature (120–180 °C), percentage of ethylene glycol (0–100% (v/v)), pH value (8–9.6), and amount of Arabic gum (0–2 g). The results exhibited that the crystallite size was enhanced from 3.6 to 8.5 nm by increasing the reaction time from 0.5 to 8 h; from 4.5 to 7.8 nm by raising the reaction temperature from 120 to 180 °C; and from 12.3 to 12.9 nm by increasing the pH value from 8 to 9.6, respectively. Whereas it was reduced from 12.9 to 7.8 nm by increasing the percentage of EG in aqueous solutions from 0% to 100% (v/v) and from 12.4 to 11.1 nm by increasing AG amount from 0 to 2 g, respectively. Based on the results, it could be deduced that the optimal hydrothermal treatment was carried out at 180 °C and pH 9.6 for 4 h in aqueous solutions. On the other hand, performing the same reaction using the usual co-precipitation method gave a semi-amorphous cobalt ferrite product along with other impurities such as hematite. The as-synthesized products were characterized by using XRD, FT-IR, EDS, DRS, FE-SEM, HR-TEM, and SAED techniques. The SAED patterns confirmed the formation of highly pure and crystalline spinel structure of CoFe₂O₄, which was consistent with the XRD results. The estimated optical band gap energy was in the range of 1.6–1.9 eV, indicating the semiconducting characteristics of the prepared cobalt ferrite nanoparticles. In addition, the results indicated that the developed hydrothermal procedure is a good candidate for the preparation of various metal ferrites in suitable crystallite sizes from aqueous solutions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We would like to express our appreciation and gratitude to Benha University and Zagazig University, Egypt for their support for this work.

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