XRD patterns of the prepared CoFe2O4 ferrites are shown in Fig.1. All the ferrite samples showed diffraction peaks at 2θ values 31.6°, 37.0°, 44.1°, 55.4°, 59.1° and 65.1° attributed to the reflection planes of (220), (311), (400), (422), (511) and (440), respectively, of the spinel crystal structure (JCPDS 22-1086). No other peaks are observed for all specimens indicating the purity of the ferrites. There is a slight shift of (311) peak towards lower angle side in the order CoFes> CoFep > CoFeg. The average lattice constant (a) for the (311) plane was determined using equation (1)  and the results obtained are listed in Table 1.
Where "h k l" are miller indices. It is noted that the lattice constant follows the order: CoFes > CoFep > CoFeg. This may be attributed to that a certain number of Co2+ ions (0.78 Å) transfer from octahedral sites, accompanied by opposite migrate of equivalent number of Fe3+ ions (0.645 Å) from tetrahedral to octahedral sites to relax the compressive strain. The average crystallite sizes (DXRD) of the investigated samples were calculated (based on 3 different peaks) from the widening of reflection peaks using the Scherrer formula :
DXRD = 0.9 λ / β cosθ (2)
Where λ is the X-ray wavelength and β is the half peak width of the diffraction peak in radiant. The results are listed in Table 1 and found to lie in the range of 21 – 33 nm. The average X-ray density (ρx) of the cobalt ferrite nanoparticles was determines using the following equation:
ρx = 8M/Na3 (3)
Where M is the molecular weight of cobalt ferrite, N is Avogadro's number. The results are also listed in Table 1.
The formation of the CoFe2O4 spinel structure was also supported by infrared spectra shown in Fig.1S. The spectra of all samples demonstrate two specific bands for spinel structure, ν1 in the range 581–563 cm−1, relates to Mtetr -O vibration at the tetrahedral site, and ν2 in the range 428–402 cm−1 attributed to Mocta -O vibration at the octahedral sites [9-11]. The mean feature bands observed are recorded in Table 1. The variation in the band positions is due to the difference in the metal–O distances for the octahedral and tetrahedral complexes referring to a change in cation distribution in the spinel structure by changing the preparation method. These results strongly support the results of XRD data. The broad bands focused at 1640-1625 cm−1 are appointed to the δ H-O-H bonding mode of the adsorbed water .
The SEM and TEM micrographs of the as-synthesized CoFe2O4 particles are given in Figs.2 (a and b). As seen, depending on the preparation method the nano-particles have been developed in different orders and clusters. SEM and TEM images of CoFes show morphology nanoparticles predominated by nanorods structure. The other cobalt ferrite specimens, CoFep and CoFeg, demonstrates aggregated spherical particles besides particles with polygon morphologies. The variation in the morphology of the NPS shown in Figure 2.b denotes that the crystal growth of CoFe2O4 depends to a large extent on the preparation method. The nanoparticle sizes were obtained using histograms of 100 particles observed in the TEM image are listed in Table 1. The slight contrast found in the results of XRD and TEM is due to the different handles of the two tools.TEM analysis offers a number-average size distribution, XRD manifests a volume ‐averaged median size. In XRD, the correctness of Scherrer's formula is influenced by numerous variables, for example, diffraction line width, and surface tension. Thus, Scherrer's equation might cause particular errors in determining the fixed value of the crystallite size .
XPS results of the CoFe2O4 samples are given in Fig. 3, where binding energy (B.E.) ranges from 0 to 1200 eV. The spectra show that the samples contain only the main elements: Fe, Co, and O beside the contaminated C element coming from the environment. The binding energy (B.E.) values obtained attached well with the literature data for CoFe2O4, Table 2,  proving that pure ferrite had been produced.
To investigate the cation valence states and their distribution in the CoFe2O4 spinel, the high-resolution XPS spectra of Fe 2p, Co 2p, and O1s peaks of the CoFe2O4 specimen were studied and given for CoFeg in Fig. 4. The integrated intensities of the fitted peaks of Co2+ and Fe3+ ions were used to determine their distributions in both octahedral and tetrahedral positions. The table also shows an increase in the concentration of Fe3+ cations on octahedral sites with the increase in the particle size of the sample, which agrees well with XRD data. In conclusion, it can be said that the selection of the preparation route is effective in controlling the cation distribution within the spinel lattice. The presence of high intense satellite structure on the high binding energy side of the Co 2p3/2 and Fe 2p3/2 might be attributed to the band structure related to octahedral Co 2p in the oxide lattice.
3.2. Magnetic Study
In order to investigate the effect of synthetic method on the magnetic properties, the VSM test was done at room temperature in an applied field of 10 kOe. The results are represented in Fig. 5, which shows hysteresis loops referring to the ferromagnetic nature of all samples. The M(H) curves also show a linear part at higher magnetic fields signifying a meaningful paramagnetic contribution to the magnetization. The saturation magnetizations (Ms) are evaluated by extrapolating the plots of M vs. 1/H employing data at high magnetic fields . The magnetic parameters are extracted from M-H plots Fig.5 and listed in Table 3. The coercivity (Hc), remanent magnetization (Mr) are extracted from M-H plots (Fig. 5) and listed with the saturation magnetizations (Ms) and the squareness values of the hysteresis loops for all specimens in Table 3. The small values of the coercivity (Hc) of CoFe2O4 nanoparticles denote that the studied samples lie near the superparamagnetic limit.
For the ideal inverse spinel crystal structure of CoFe2O4, with all the Co2+ ions located at the octahedral site, the magnetization per formula unit can be theoretically evaluated using Neel’s two sub lattice model by considering the difference of total magnetic moments in octahedral and tetrahedral sites [15-18] M=Moctahedral – Mtetrahedral. The magnetic moment of Fe3+ and Co2+ cations are 5.0 and 3.8 µB respectively, a theoretical magnetic moment of CoFe2O4 is 3.8 µB per formula unit. Based on the cation distribution obtained from XPS, the magnetization per formula units were also calculated and listed in Table 3, which shows that the magnetic moments changes with the preparation methods. The magnetic moment values of the investigated samples can be also determined experimentally by the following equation in Bohr magneton;
µB = Mol.wt×Ms/5585 (4)
However, the evaluated data from XPS and VSM are not equal, which can be associated to the finite size of nanoparticles conducting to the noncollinearity of magnetic moments on the surface of the nanoparticles. The disordered moments are developed due to the broken exchange bonds at the outer layer. On the other hand, the competition antiferromagnetic interactions precedes to a noncollinear arrangement of magnetic moments within interstitial sublattices which is induced because of the non-equilibrium cation distribution among tetrahedral and octahedral sites [15,17,18]. Shifting of larger Co2+ (0.78 Å) to substitute the smaller Fe3+ (0.645 Å) cations in octahedral sites produce strains on the surface due to the smaller space between the octahedral site cations comparable to the tetrahedral site cations in nanoparticles. The strains obtained can break the surface exchange bonds which cause the canted spin structure. This type of tetrahedral-octahedral interaction points to lower magnetization values in the ferrite nanoparticles compared with the bulk CoFe2O4 . The low Ms- value of the investigate ferrite samples compared with that of the bulk one (80.9 emu/g)  can be also explained on the basis of the core-shell model, which clarifies that the finite-size effects of the nanoparticles manage to canting or non-collinearity of spins on their surface, in that way reducing magnetization .
To sum up, it can be said that the change in saturation magnetization with the variation in the preparation methods is possibly due to the rearrangement of the cation distribution, i.e., the exchange of Co2+ and Fe3+ ions from octahedral and tetrahedral sites and vice versa. The low values of Ms for the investigated samples could be credited to surface distortion which destabilizes the collinear spin arrangement and producing various canted spin structures at the surface. This effect is especially noticeable for ultrafine particles owing to their large surface to volume ratio. The reduction in coercivity with increasing particle size could be accredited from the combination of surface anisotropy and thermal energies .
The values of the squareness ratio (Mr/Ms) of investigated samples, shown in Table 3, are below 0.5 refers to that these samples are multidomain and the particles interact by magnetostatic interaction .
The magnetic anisotropy (K/) has been also calculated using the following relation ,
Hc = 0.98 K//Ms (5)
and the results obtained showed high values of 16711, 13571, and 6181 emu.Oeg-1 for CoFep, CoFeg, and CoFes, respectively. The increase in K value is going parallel with increasing the presence of Co2+ ions in the octahedral sites, as shown in XPS results, Table 2.
The effect of the preparation method on the magnetic parameters of our CoFe2O4 nanoparticles is compared with other methods present in the literature and listed in Table 4.
3.3. Surface Properties
The surface properties of the spinels investigated were studied using the BET technique. The isothermal N2 adsorption-desorption plots of these samples Fig.2S can be classified as type IV for CoFep and type V for CoFeg and CoFes according to IUPAC, which is characteristic of the mesoporous material in which the adsorption proceeds via multilayer adsorption followed by capillary condensation. The hysteresis loops show the type H3 (aggregates of platelike particles forming slit-shaped pores) for all samples. The textural properties of the studied samples including BET-surface area, average pore diameter, pore-volume, and pore size distribution calculated from BJH method are derived from N2 nitrogen adsorption/desorption isotherms and listed in Table 5. The data obtained refer to that the surface properties depend to a large extent on the method of preparation and CoFeg exhibits the highest surface area.
3.4. Optical Properties
The prepared CoFe2O4 nanoparticles show still high magnetization, that photocatalyst appropriate for magnetically separable by a magnetic field and separation of photocatalyst from solution. Thus, the photocatalytic activity of the investigated samples has been studied. The optical absorption property related to the electronic structure characteristic is documented as the main factor in deciding the photocatalytic activity . The diffuse reflectance spectra of our ferrites were recorded and converted to the Kubelka-Munk function, K-M, Fig. 6, using the following equation:
K-M= (1−R)2/ 2R (6)
Where R is absolute reflectance. The results obtained are listed in Fig. 6A. The spectra show that all synthesized CoFe2O4 samples exhibited photo-absorption in the visible light region, which implies the probability of high photocatalytic efficiency of these materials under visible light. The absorption behavior in the visible region is originated from the electronic charge transformation of Co2+ and Fe3+ to their conduction level in the conduction band [7, 31, 32]. The CoFe2O4 stoichiometry is organized in an incompletely inverse structure , with the Co2+ ion at both tetrahedral and octahedral sites, as shown in our results (XRD, XPS, and magnetic data). Broad Co–O and Fe–O charge transfers, together with d–d electron moves of Co2+ and Fe3+ in numerous coordination, guarantee the full absorption of the visible spectrum. The bandgap energies of the investigated ferrites were estimated according to Tauc's  by plotting (KM. hν)1/n versus hn, where h is a Planck , s constant, n is the light frequency and n is a constant relating to a mode of transition ( n = ½ for allowed direct transition and n= 2 for indirect transition). Tauc's plots, shown in Fig.6-b, for every one of the specimens, demonstrated that the band-to-band direct transitions are more inclined to happen than the indirect transitions. The optical energy gaps, Eg, obtained from the intercept of the plot with the X-axis are recorded in Table 6, from which it can be seen that Eg value decreases with increasing the particle size and showed the smallest value for CoFeg sample.
3.4.1. Raman Spectroscopy
Raman spectra were used to acquire vision on the vibrational energy states within the spinel ferrite obtained, as well as to review the structural characteristics and compositional regularity throughout the samples . The spinel ferrite exhibit five active Raman vibration modes [35,36]. The Raman spectra of our samples showed only three bands, due to peak overlapping, at 284-297 cm-1, 460-470 cm-1, and 640-650 cm-1, as shown in Fig. 7, which was assigned as Eg, 3T2g and A1g (1), respectively . The A1g (1) band is related to symmetric stretching vibration mode at the tetrahedral (A) site. While A1g (1) band might be related to the vibration of Co-O bonds at the tetrahedral (A) site. The results obtained in Fig. 7; show that the frequency of Raman modes of the investigated spinels is slightly changed with the preparation method. This could be attributed to the variation in the cation distribution in the spinel lattice, as mentioned above.
3.4.2. Photoluminescence Study
Photoluminescence (PL) spectroscopy is an outstanding procedure to get valuable information concerning energy and the dynamics of charge carriers yielded during the exposure of light. The photoluminescence of the ferrite nanoparticles was studied using a 225 nm excitation wavelength source and the results obtained are presented in Fig. 8. The spectra of all samples show broad visible emission peaks at 434 - 442 nm, which are attributed to the charge transport between Co2+ at tetrahedral sites and Fe3+ at octahedral sites that are surrounded by O2- ions . The variation in the position and the intensity of luminescence can be explained on the basis that the PL-spectra are sensitive to the character of nanoparticles surface, due to the existence of gap surface disorders developing from surface non-stoichiometry and unsaturated bonds. The defects produced in the nanomaterial lattice during preparation are the base of luminescent properties . The emission intensity of CoFeg is lower than that of the rest samples. This indicates that this sample acted as traps for the photo-induced charge carriers. These outcomes confirm the previously mentioned results on the influence of the preparation methods on the surface and optical properties.
3.4.3. Surface Acidity:
Temperature programmed desorption of ammonia (TPD-NH3) is an appropriate procedure for measuring the quantity and the spreading of the acid sites on the surface of our samples. The acid site distribution results for the studied samples are summarized in Table 6. The ammonia desorbed at 100 oC contains some physisorbed ammonia as well as overstating the proportion of weak acid sites. Whereas, the ammonia desorbed at 220 – 370 oC and that at 450-600 oC are attributed to medium and strong acid sites, respectively . The results obtained show that the acidity varies with preparation methods. For all samples, the strength of acidic sites follows the order: Weak acid sites > medium sites > strong acid sites. CoFeg exhibits the highest total acidity than that of other samples.
3.5. Photocatalytic Activity of CoFe2O4 Samples
According to the above-mentioned optical properties, we studied the photocatalytic activity of the investigated CoFe2O4 nanoparticles under visible light irradiation using the degradation of Basic Red 18 (BR 18) aqueous solution, as a basic dye model. Before the irradiation process, the suspended solution of the dye and catalyst was stored in the dark for 45 min. to assurance adsorption/desorption equilibrium. The photocatalytic degradation results are illustrated in Fig. 9. From which it can be seen that the degradation of the dye is very slow in the absence of the catalyst and the CoFeg sample showed the highest photocatalytic efficiency due to the high optical absorptions in vis. Light region with lower bandgap energy and a larger surface area. Therefore, this sample was selected to test the impact of catalyst dosage, dye concentration, and pH of the solution on the dye degradation rate. The results obtained given in Fig. 3S show that the rate of dye degradation has the highest rate at 3 mg/L catalyst dosage and decreases with increasing dye concentration in the range of 10 -100 ppm and has the highest rate at pH= 7 of the solution.
In view of literature reports, the kinetic of photocatalytic reaction can be calculated according to:
- ln (C/Co) = kobs t (7)
Where Co and C are the concentrations of dye at zero time and time t, respectively, and kobs are the pseudo-first-order rate constant. The rate constant, kobs, evaluated from the slope of the straight line of plotting −ln(C/C0) vs. reaction time, Fig. 9-b showed a value of 0.1 min-1.
3.5.1. Mechanism of Photocatalysis
The major oxidative species in the photocatalytic progression are positive holes (h+/VB) and the OH- hydroxyl radical formed during the irradiation process. In the present work, the trapping experiments were used to determine which one of these species is active for organic degradation. EDTA-2Na and isopropyl alcohol were used as an h+, and isopropyl alcohol as an OH· scavenger, respectively . The results showed that the additive of isopropyl alcohol slightly changed the dye degradation indicating that OH. radicals were minor factors in the photocatalytic degradation process, whereas the addition of h+ capture (EDTA-2Na) caused a great decrease in the degradation efficiency as shown in Fig. 10. This foundation distinctly denoted that positive holes are the major active species of the dye dissociation.